Circadian Rhythms and Metabolic Syndrome
From Experimental Genetics to Human Disease
Abstract: The incidence of the metabolic syndrome represents a spectrum of disorders that continue to increase across the industrialized world. Both genetic and environmental factors contribute to metabolic syndrome and recent evidence has emerged to suggest that alterations in circadian systems and sleep participate in the pathogenesis of the disease. In this review, we highlight studies at the intersection of clinical medicine and experimental genetics that pinpoint how perturbations of the internal clock system, and sleep, constitute risk factors for disorders including obesity, diabetes mellitus, cardiovascular disease, thrombosis and even inflammation. An exciting aspect of the field has been the integration of behavioral and physiological approaches, and the emerging insight into both neural and peripheral tissues in disease pathogenesis. Consideration of the cell and molecular links between disorders of circadian rhythms and sleep with metabolic syndrome has begun to open new opportunities for mechanism-based therapeutics.
This Review is in a thematic series on Circadian Rhythm and Cardiovascular Function, which includes the following articles:
Molecular Time: An Often Overlooked Dimension to Cardiovascular Disease [2009;105:1047–1061]
Circadian Proteins and Genotoxic Stress Response [2010;106:68–78]
Circadian Rhythms and Metabolic Syndrome: From Experimental Genetics to Human Disease
The Cardiomyocyte Circadian Clock: Emerging Roles in Health and Disease Circadian Clocks and Vascular Function
Martin Young Guest Editor
The metabolic syndrome (MS) is comprised of several metabolic abnormalities, including central (intra-abdominal) obesity, dyslipidemia, hyperglycemia, and hypertension. This syndrome has become a major public health challenge worldwide; an estimated 25% to 40% of individuals between the ages of 25 and 64 years of age have MS (San Antonio Heart Study).1–4 MS is further defined by the presence of other components, including elevated circulating levels of triglycerides, reduced levels of high-density lipoprotein cholesterol, high blood pressure, and impaired fasting glycemia. Elevated circulating inflammatory and/or thrombotic markers (C-reactive protein, tumor necrosis factor-α, interleukin-6, and plasminogen activator inhibitor type 1) or reduced levels of antiinflammatory molecules such as adiponectin are further markers of MS.2,4
Excess food intake and physical inactivity underlie the growing worldwide epidemic of obesity and MS, not only in industrialized nations but also in developing countries. In addition, mounting evidence from clinical epidemiological studies has led to the hypothesis that one of the major changes in the industrialized world that contributes to the pathogenesis of the MS involves the introduction of artificial light and work into the night-time, in addition to the pervasive rise in voluntary sleep curtailment.5 Indeed, these common disorders of circadian behavior and sleep are associated with increased hunger, decreased glucose and lipid metabolism, and broad changes in the hormonal signals involved in satiety.6 Recently, Sheer et al demonstrated adverse cardiometabolic end points in human subjects who underwent forced misalignment of behavioral and circadian cycles, simulating the conditions of jet lag and shift work within a controlled clinical setting.7 Against this backdrop of human studies, advances in the field of experimental genetics have uncovered the fundamental molecular mechanism governing these 24-hour circadian rhythms of physiology, revealing that all circadian processes are programmed by a conserved transcription-translation feedback loop that oscillates with a periodicity of 24 hours.8
Remarkably, obesity and high-fat feeding also reciprocally affect the circadian system in mice, indicating that metabolism, circadian rhythms, and possibly sleep are interconnected through complex behavioral and molecular pathways.9 Thus, alterations in energy homeostasis associated with obesity may set in motion a “vicious cycle” of circadian disruption, in turn leading to exacerbation of the original metabolic disturbance.
The following are terms discussed in this review:
Core molecular clock: molecular machinery of the clock within all cells. The circadian gene network in mammals is controlled by a network of autoregulatory transcription–translation feedback loops.
Circadian rhythms: biochemical, physiological, or behavioral processes that persist under constant conditions with a period length of ≈24 hours.
Clock: a central mechanism controlling circadian rhythms.
Clock-controlled gene: a gene whose expression is rhythmically regulated by a clock.
Entraining agent or external cues or zeitgeber or inputs: extrinsic stimuli able to reset the rhythms (ie, daylight or food).
Oscillator: a system of components that produces a circadian rhythm.
Outputs: circadian rhythmicity of most physiological and behavioral functions, such as feeding, sleep-wakefulness, hormone secretion, and metabolic homeostasis.
Period: duration of one complete cycle in a rhythmic variation.
Adverse Effects of Alterations in Circadian Rhythms: Clinical Evidence
The decrease in sleep duration in the United States has occurred over the same time period as the increase in the prevalence of metabolic disease (reviewed previously5). Numerous cross-sectional, as well as prospective clinical, studies have demonstrated that short-duration and poor-quality sleep predicts the development of type 2 diabetes and obesity after age, body mass index and various other confounding variables are taken into account.10–13 For instance, reduced sleep duration in children is associated with increased risk of being overweight.14 The gradual decline in the amount of time spent asleep and also the routine extension of normal activity during the night may disrupt synchrony between the periods of sleep/activity with alternating periods of feeding/fasting and energy storage/utilization. Indeed, the relationship between sleep restriction, weight gain and diabetes risk may involve, at least in part, alterations in glucose metabolism, stimulation of appetite, and decreased energy expenditure (reviewed previously5). For example, healthy subjects who underwent six consecutive nights of sleep restricted to 4 hour exhibited impaired insulin sensitivity following a glucose challenge.15,16 Moreover, the induction of hunger may be partially related to reduced circulating levels of leptin (an adipose tissue–specific hormone which promotes satiety) and increased levels of the orexigenic hormone ghrelin (a peptide released primarily from the stomach) induced by sleep deprivation.17 Both hormones may also impact energy expenditure (reviewed previously18). Curiously, individuals diagnosed with night eating syndrome appear to have greater propensity toward obesity.19 Diseases related to changes in time and/or quality-sleep duration are also associated with metabolic disorders. For example, sleep apnoea syndrome, a sleep disorder that is highly prevalent in metabolic disorders,20 was proposed to cause clock gene dysfunction,21 and effective treatments of sleep apnoea have been found to improve glucose metabolism and energy balance.11 In addition, the circadian oscillation of leptin was found to be disrupted in narcoleptic patients, which may predispose them to weight gain.22 Understanding the molecular pathophysiology of metabolic disorders in states of disrupted sleep remains a major challenge.
It has also long been recognized that serious adverse cardiovascular events, including myocardial infarction, sudden cardiac death, pulmonary embolism, limb ischemia, and aortic aneurysm rupture all have pronounced circadian rhythmicity, reaching a peak during the morning.23 More recent evidence has accumulated to suggest that chronic circadian disruption may also increase susceptibility to such disorders. For example, shift work is associated with a 1.6- and 3.0-fold increased risk of cardiovascular disease for 45 to 55 years old men and women, respectively.24 Cardiovascular disease and hypertension are also associated with sleep loss: the risk of a fatal heart attack increases 45% in individuals who chronically sleep 5 hour per night or less.25 Interestingly, the incidence of acute myocardial infarction was also significantly increased for the first 3 weekdays after the transition to daylight saving time in the spring.26 This observation underscores the deleterious effects of transitions involved in daylight saving time on the disruption of chronobiologic rhythms. Another adverse aspect of sleep perturbation is its impact on the human immune system.27–29 For instance, sleep deprivation dysregulates monocyte production of several proinflammatory cytokines, including interleukin-6 and tumor necrosis factor-α.30,31 This point is of interest because obesity is recognized to involve a low-grade inflammatory state (reviewed previously32). Conversely, the inflammatory process can induce sleep disturbances.27 Other metabolic disorders may be induced by a phase shift, such as altered postprandial lipid excursion, thereby providing a partial explanation for the increased occurrence of cardiovascular disease reported in shift workers.33
Recently, Sheer et al investigated the causal link between circadian misalignment and metabolic homeostasis using a controlled simulation of “shift-work” in the clinical laboratory.7 In this study, 10 subjects underwent a progressive misalignment of behavioral and circadian cycles. Their behavioral cycle was extended to a 28-hour day, under dim light, with 14-hour rest, and fasting alternated with 14 hours of wakefulness, interspersed with 4 evenly spaced and isocaloric meals. When subjects ate and slept ≈12 hours out of phase from their habitual times, circadian desynchrony decreased leptin levels and resulted in hyperglycemia and hyperinsulinemia. In addition, their daily cortisol rhythm was reversed, arterial pressure was elevated, and sleep efficiency was decreased. Interestingly, some of the subjects also exhibited postprandial glucose responses comparable to those of a prediabetic state.7 Thus, this study suggests that synchrony between behavioral and physiological rhythms is advantageous to maintain normal glucose metabolism in otherwise healthy persons.34 An important question for future clinical studies will be to determine the impact of short sleep and/or circadian misalignment on molecular clock function, especially within metabolic tissues.
In addition to environmental sleep disruption (eg, shift work disorders), genetic polymorphisms in several clock genes have also been linked to sleep disorders.35–37 For instance, genetic variation in circadian clock genes has been associated with psychiatric diseases, such as bipolar disorders and schizophrenia,35 whereas many depressed patients, particularly bipolar patients, show delayed sleep phase,38 and depression is also a comorbidity of obesity.39 Interestingly, polymorphisms in Clock and Bmal1, whose proteins form the core mammalian clock, have been linked to some features of the metabolic syndrome. In small sample populations, polymorphisms in the Clock gene have been correlated with predisposition to obesity,40,41 and 2 Bmal1 haplotypes are associated with type 2 diabetes and hypertension.42 Polymorphisms within other clock core genes (ie, Per2 and Npas2) have also been associated with hypertension and high fasting blood glucose in studies of similar sample size.43 Interestingly, a rare variant in Nampt (Visfatin/Pbef1 [pre–B-cell colony enhancing factor 1]), which is involved in a negative clock feedback loop,44,45 is associated with protection from obesity.46
Recently, several genome-wide association studies led to the unexpected discovery that melatonin, a hormone implicated in seasonal and circadian rhythms, may be important in the regulation of mammalian glucose levels.47,48 Indeed, genetic variants of the melatonin 1B receptor gene (mtnr1b) increase type 2 diabetes risk.47,48 In agreement, mtnr1b is expressed in pancreatic β-cells, and melatonin modulates glucose-stimulated insulin secretion.49 Interestingly, melatonin secretion is reported to be impaired in type 2 diabetic patients,50 and the melatonin profile relative to the feeding/fasting cycle is reversed when individuals are subjected to forced desynchrony.7 Taken together, these recent findings raise the possibility that disruption of circadian systems, either directly at the level of altered clock gene expression, or indirectly through effects on melatonin, may contribute to human metabolic syndrome and cardiovascular complications.
Molecular and Hierarchical Organization of the Clock
The Core Molecular Clock Network
Forward genetics and positional cloning enabled identification of the first mammalian circadian clock gene and provided an entry point into a molecular understanding of the clock mechanism.51,52 The core molecular clock is composed of a transcription–translation feedback loop that oscillates with 24-hour rhythmicity (Figure 1). The driving force is the positive limb of the clock comprised of the bHLH-PAS (basic helix-loop-helix–Period-Arnt-Single-minded) transcription factors CLOCK (circadian locomotor output cycles kaput), and its paralog NPAS2 (neuronal PAS domain protein 2), and BMAL1/ARNTL (brain and muscle aryl-hydrocarbon receptor nuclear translocator-like 1/aryl-hydrocarbon receptor nuclear translocator-like). CLOCK or NPAS2 and BMAL1 heterodimerize and activate the rhythmic transcription of downstream target genes that contain E-box cis-regulatory enhancer sequences, including the Period (Per1, Per2, and Per3) and cryptochrome (Cry1 and Cry2) genes.51,53–56 Following translation, PER/CRY dimerize and translocate back to the nucleus where they directly inhibit the CLOCK/BMAL1 complex, effectively repressing their own transcription.57–59
Additional regulatory loops are interconnected with the core loop described above, providing multiple layers of control of the core circadian clock.60,61 In addition to the Per and Cry genes, CLOCK/BMAL1 also activate transcription of the retinoic acid-related orphan nuclear receptors Rev-erbα and Rorα.62–65 REV-ERBα binds to the retinoic acid–related orphan receptor (ROR) response element (RORE) in the Bmal1 promoter resulting in inhibition of transcription and this action is opposed by RORα, which activates the RORE.62–64,66 In addition to the nuclear hormone receptor feedback loop, PAR domain basic leucine zipper transcription factors (PAR bZIP), including DBP (D-site binding protein), TEF (thyrotroph embryonic factor), HLF (hepatic leukemia factor), and the cAMP pathway (CREB-ATF-CREM) also feedback on the clock, acting through cognate D box and CREB elements respectively.60,61,67–70 Posttranslational modification, including phosphorylation and ubiquitination, provide further regulation of the clock network. Casein kinase 1ε and -δ (CK1ε and CK1δ) phosphorylates PER and CRY, tagging them for polyubiquitylation by the E3 ubiquitin ligase complexes β-TrCP1 (β-transducin repeat containing protein 1) and FBXL3 (F-box and leucine-rich repeat protein 3), respectively, ultimately leading to their degradation by the 26S proteosome.71–78 In addition to phosphorylation mediated by the casein kinase family, in Drosophila and mammalian cells, a role for GSK3-β signaling has been established.78a,79 As a result of this degradation, CLOCK/BMAL1 is released from repression, activating the forward limb of the 24-hour cycle. Lastly, epigenetic regulation has emerged as an additional node in circadian systems (discussed further below), including the possibility that CLOCK participates directly in protein acetylation and chromatin modification.80
Genetic mouse models have revealed key roles for each of the core clock genes in the generation and maintenance of circadian rhythms. Bmal1 knockout mice display a complete loss of circadian rhythmicity in constant darkness.53 Mice with a dominant-negative Clock mutation have an approximately 4-hour lengthening of their free-running period and become arrhythmic in constant darkness.52 Of note, Clock knockout mice have normal locomotor activity rhythms, because of developmental compensation by NPAS2.81,82 Compensation can also be demonstrated within the negative limb of the clock, as both Per1/Per2 and Cry1/Cry2 knockout mice display a much more pronounced loss of circadian rhythmicity compared to their single mutant counterparts.56,83–86 Although the aforementioned studies have focused on overt locomotor activity rhythms, it remains uncertain as to whether compensation extends to functions of the clock within peripheral tissues. It is likely that future genetic studies will continue to identify additional regulators and modifying loops of the core clock mechanism.
Central Clock Organization
Many metabolic functions occur at specific times of the day. Indeed, understanding the effects of molecular clock gene disruption on organismal physiology can be advanced through consideration of the molecular and hierarchical organization of the clock. The location of the master neural clock in mammals was originally discovered through classical lesioning studies within pacemaker neurons of the brain: the suprachiasmatic nucleus (SCN) of the hypothalamus (for more complete review, see87,88). The SCN controls physiological and behavioral circadian rhythms and coordinates peripheral clocks through hormonal and neural signals.87 The master role played by SCN was demonstrated by transplantation experiments in hamster. Neural grafts from the suprachiasmatic region restored circadian locomotor and feeding activity to arrhythmic animals whose own SCN had been ablated.89 The restored rhythms of the host always matched the rhythms of the donor, demonstrating the strong impact of this nucleus in circadian activity. However, despite the restoration of locomotor rhythmicity, melatonin and glucocorticoids remained arrhythmic, suggesting that neural connections must be critical for the generation of certain circadian rhythms.90 Interestingly, this master pacemaker has anatomic connections with several regions of the CNS involved in the control of appetite, energy expenditure regulation and behavioral activity, namely with the supraventricular area, the arcuate nucleus and the lateral hypothalamic area.91
The SCN clock is entrained by light through the retinohypothalamic tract (Figure 2). Photic input provides a dominant time-keeping signal (zeitgeber), orienting the animal each day to geophysical time. Endogenous period length is not precisely 24 hour (humans are longer, whereas mice are shorter), thus the daily entrainment to light is a critical mechanism to maintain organismal synchrony with the external environment (Figure 2). Perception of light occurs through activation of a population of directly light-sensitive ganglion cells within the eye, the melanopsin cells; these regulate both circadian rhythms and melatonin synthesis.92,93 Direct output of the SCN, and the entrainment of the SCN axis to light, plays a key role in synchronizing endogenous hormonal rhythms including the glucocorticoid rhythm.94 In turn, light-induced entrainment of the glucocorticoid rhythm may maintain phase coherence of multiple cellular oscillators, such as those in fibroblasts and liver.95,96
In addition to photic entrainment, food also entrains circadian processes in neural and peripheral cells (Figure 2). Food restriction to the normal rest period in rodents also induces a burst of anticipatory activity, an effect that is altered in some experimental systems following lesioning of the dorsomedial nucleus.97–99 However, there remains controversy concerning the involvement of circadian oscillators in food anticipatory activity (FAA) because the behavior persists in Bmal1 nullizygous mice.97,100,101 Interestingly, FAA appears to involve the melanocortin signaling pathway, because restriction failed to increase wakefulness before food presentation in melanocortin-3 receptor–null mice.102 Rather than localization to a single nucleus of hypothalamus, the food entrainable oscillator may in fact involve a more dispersed network of cell groups.103 Furthermore, the FAA may constitute a metabolic oscillator responsive to peripheral neural or circulating signals elicited by food ingestion.104,105 An interesting question remains concerning whether macronutrient flux in the postprandial state may participate in establishing FAA (reviewed in104,105). A related question is whether nutrient signaling per se may affect core properties of the SCN pacemaker.
Molecular analyses have revealed that the clock network is also widely expressed throughout nearly every tissue/cell type in vertebrates (Figure 3).106,107 Original studies by Schibler and colleagues demonstrated cell autonomous clock gene oscillation within fibroblasts ex vivo.108 Following this discovery, in addition to the master clock in the SCN, independent circadian oscillators have been found in a number of peripheral tissues in mammals. Gene expression profiling has shown that 3% to 20% of genes display a 24-hour rhythmic expression, and a large proportion of these genes have a role in metabolic processes (reviewed previously8). The circadian rhythms of peripheral organs are also self-sustained, as demonstrated using a mouse line in which luciferase expression is driven from the endogenous Per1 or Per2 loci.106,107 Variation in temporal gene expression was reported to play an important role in tissues implicated in glucose and lipid metabolism, such as fat, liver, cardiac, and skeletal muscle.109–117 Many nuclear receptors expressed in liver and white and brown adipose tissues also display rhythmic patterns of expression.118–120 Therefore, the nuclear receptors may link clock genes to metabolism by integrating energy flux with varying physiological demands across the light-dark cycle. In this way, circadian patterns of metabolic gene expression may optimize the switch between daily anabolic and catabolic states corresponding with periods of feeding and fasting. For example, the cyclic expression of gastrointestinal tract enzymes may ensure that factors involved in nutrient absorption are expressed in anticipation of daily episodes of food ingestion.105 In addition, adipose enzymes involved in fatty acid storage peak coincident with feeding (reviewed previously121). Moreover, components of gluconeogenesis, glycolysis, and fatty acid metabolism cycle with a peak during the subjective night in mouse liver.112 Coordinating gene expression patterns according to the varying metabolic demands across the active and rest period is also important in muscle, where elaboration of aerobic and anaerobic enzymes varies during the sleep-wake cycle.111,122 Thus, peripheral oscillators are self-sustained, cell autonomous and tissue-specific, yet a major question is: what are the mechanisms involved in maintaining synchrony within and between peripheral tissue clocks? A related question is whether misalignment of local circadian oscillation within and between peripheral tissues contributes to cardiovascular and metabolic pathologies.
To discern whether the rhythmic expression of genes in peripheral organs is driven by local (cell autonomous) oscillators or by circadian systemic signals, Schibler and colleagues have recently exploited the tetracycline-inducible system of Bujard, enabling conditional Rev-erbα overexpression within liver.123 In this model, REV-ERBα represses the transcription of the essential core clock gene Bmal1 in a doxycycline-dependent manner. Among 351 genes with rhythmic expression revealed in the doxycycline-fed mice, only 31 genes, including the core clock gene mPer2, oscillated robustly irrespective of whether the liver clock was running or not. These studies suggest that the rhythmicity of metabolic liver genes is driven by both cell-autonomous and nonautonomous signals.123
Multiple signals related to feeding, and even fasting, may entrain peripheral clocks. Indeed, in in vitro experiments, a bewildering variety of stimuli can induce or reset circadian gene expression. These factors include chemical activators of protein kinase A (forskolin, butyryl cAMP), protein kinase C, and/or mitogen-activated protein kinase (phorbol esters, fibroblast growth factor, endothelin) and glucocorticoid receptors (dexamethasone) and even glucose; dissecting how these signaling pathways converge on the clock remains an area of intensive investigation (reviewed previously108).
Evidence for a Molecular Link Between Circadian and Metabolism Systems
The availability of genetic models of circadian disruption has provided new opportunities to dissect the interrelationship of circadian and metabolic systems. Early studies indicated the cellular redox status, represented by the nicotinamide adenine dinucleotide cofactors NAD(H) and NADP(H), regulate the transcriptional activity of CLOCK/BMAL1 and NPAS2/BMAL1.124 The reduced forms of these cofactors increase DNA binding, whereas the oxidized forms decrease binding, thus coupling activity of these core clock components with the metabolic state of the cell. Two recent studies have further linked the biology of NAD production with the core molecular clock.44,45 The gene encoding the rate-limiting enzyme in NAD biosynthesis, nicotinamide phosphoribosyltransferase (NAMPT), displays circadian rhythmicity in peripheral tissues, including liver and white adipose tissue, and is under the direct control of CLOCK/BMAL1. Such rhythmicity translates to daily oscillations in NAD levels in liver. Both Nampt RNA and NAD levels are reduced in liver from ClockΔ19/Δ19 and Bmal1−/− mice, whereas they are increased in liver from mice deficient for both CRY1 and CRY2, suggesting that Nampt, and therefore NAD production, is a downstream target of CLOCK/BMAL1. Not only is NAD important in cellular redox reactions, but it also serves as a substrate for sirtuin (SIRT)1, an NAD-dependent and nutrient responsive deacetylase, which has also recently been described as a novel regulator of circadian clock function.125,126 Of note, the timing of the peak in NAMPT and NAD levels corresponds with the peak in SIRT1 activity. SIRT1 then physically associates with components of the positive limb of the core clock machinery (CLOCK and BMAL1) and is recruited to clock target genes. Genetic and pharmacological manipulation of SIRT1 and the NAD biosynthesis pathway reveal that SIRT1 negatively regulates CLOCK and BMAL1. Together, these studies demonstrate the existence of a negative feedback loop whereby CLOCK/BMAL1 positively regulate both NAD production and SIRT1 activity, whereas, in turn, SIRT1 negatively regulates the activity of CLOCK/BMAL1.
The existence of this pathway is particularly intriguing in light of the fact that NAMPT and SIRT1 are regulated not only by the clock, but also by the nutritional status of the organism. For example, Nampt is upregulated in response to glucose restriction in skeletal muscle in an AMP-activated protein kinase–dependent manner,127,128 and SIRT1 has been demonstrated in numerous tissues to be increased during fasting or caloric restriction.129–132 Thus, regulation of the clock by NAD and SIRT1 allows for coordination and fine-tuning of the core clock machinery with the daily cycles of fasting/feeding and rest/activity. Furthermore, NAD and SIRT1 are also involved in the regulation of a myriad of metabolic processes, including regulation of glucose-stimulated insulin secretion, adipocyte differentiation, and gluconeogenesis.133 Regulation of NAD and SIRT1 by the clock likely has a cascade of effects on downstream metabolic pathways, and it is tempting to speculate that the reduction in NAD and SIRT1 activity in the circadian mutant mice contributes to some of their metabolic phenotypes. It has also recently been demonstrated that NAMPT is secreted and is present in the circulation,134 though it is not yet known whether extracellular NAMPT is regulated in a circadian manner, thereby influencing downstream processes on a systemic level.135 Recent evidence has also implicated the other NAD-dependent sirtuin family members (SIRT2–7) in a variety of metabolic processes136; it will therefore be of great interest to determine whether any of these other sirtuins are also involved in the crosstalk between the core circadian clock and metabolism.
Additional key nutrient sensors that have been implicated in the cross-talk between circadian rhythms and metabolism are the nuclear receptor peroxisome proliferator-activated receptor (PPAR)γ and the coactivator PGC1α (PPARγ coactivator 1-α). PPARγ is rhythmically expressed and directly regulates Bmal1 transcription, and mice lacking PPARγ exhibit reduced rhythmicity of clock gene expression, blood pressure, and heart rate.137 It is interesting to note that SIRT1 promotes fat mobilization during fasting by binding to and repressing PPARγ.138 PGC1α also displays circadian oscillations in liver and skeletal muscle and upregulates the transcription of Bmal1 and Rev-erbα. Mice lacking PGC1α have abnormal diurnal locomotor activity rhythms, body temperature, and metabolic rate, along with altered expression of clock and metabolic genes.139 PGC1α levels are elevated in response to cold exposure, starvation, and physical activity, and hence may also help coordinate the circadian clock with the nutritional status of the organism. Of note, SIRT1 also deacetylates and activates PGC1,132 indicating an additional mechanism linking molecular clock function and energy utilization. A more detailed understanding of the molecular links between the core molecular clock machinery and metabolism will be necessary to develop therapies targeting disease states involving disruption of both rhythms and metabolism, such as type 2 diabetes.
From Circadian Disruption to Metabolic Disease
What Have We Learned From the Experimental Models?
How might circadian misalignment impact the metabolic comorbidities of obesity, diabetes, and cardiovascular disease? Several lines of evidence suggest that circadian dysregulation may exert a broad impact not only on glucose control, but also on inflammation, fibrinolysis, fluid balance, and vascular reactivity. A central node linking metabolic and circadian pathways involves the nuclear receptor superfamily, including those downstream of REV-ERBα and the RORs that modulate the core clock and diverse metabolic processes ranging from adipogenesis to inflammation and thrombosis (reviewed previously140). Experimental models have helped to demonstrate the impact of the clock network in metabolic gene expression and provide evidence that this clock disruption leads to metabolic abnormalities. Homozygous ClockΔ19 mutant mice, which express a loss of function mutation in Clock, have yielded new insight in this field.141 In addition to disruptions in sleep and circadian behavior, these mice also develop hyperphagia early in life, with subsequent development of hyperlipidemia, hyperleptinemia, and hypoinsulinemic hyperglycemia, indicating that this animal exhibits features of the metabolic syndrome.141 The feeding rhythm in these mice is damped, with increased food intake during the day, and, in addition, these mice have significantly increased food intake overall. High-fat feeding studies revealed exaggerated weight gain of ClockΔ19 mutant mice, and dual-energy X-ray absorptiometry scanning and fat pad weight both demonstrated significant increases in fat and lean mass relative to controls following high fat feeding.141 It is likely that the obese phenotype results, at least in part, from altered rhythms of neuropeptides in the hypothalamus, because ghrelin, CART (cocaine- and amphetamine-regulated transcript), and orexin are all expressed at constitutively low levels in the ClockΔ19 mutant mice.141 In addition, the anorectic neuropeptide POMC was decreased throughout the entire light/dark cycle in hypothalami of young ClockΔ19 mutant before the onset of weight gain and overt diabetes and is consistent with a deficit in the central homeostatic regulation of weight constancy. Because the original ClockΔ19 mutant was developed in a melatonin-deficient strain (C57BL/6J), Kennaway et al evaluated the contribution of melatonin deficiency on glucose metabolism by crossing ClockΔ19 mutant mouse with the melatonin-producing CBA strain to produce the “ClockΔ19+MEL” mouse.142 Interestingly, in this model, the restoration of melatonin did not rescue gene expression rhythms in liver or muscle.143 Such studies underscore the importance of strain background in the evaluation of metabolic phenotype. For example, when introgressed onto the ICR strain, the ClockΔ19 mutation results in malabsorption of lipid and thus resistance to diet-induced obesity, thus primary effects of the Clock mutation on energy balance and fuel homeostasis cannot be evaluated in the ICR strain.144 Disruption of other circadian clock genes also leads to metabolic alterations. For example, gene disruption in Bmal1 induces an abnormal metabolic phenotype characterized by impaired gluconeogenesis, hyperleptinemia, glucose intolerance, and dyslipidemia.145–147 In addition, Per2 knock-out mice develop increased weight gain on high-fat diet (HFD).148 Conversely, mice deficient in the circadian deadenylase nocturin remain lean and resistant to hepatic steatosis when fed a HFD despite equivalent caloric intake, similar metabolic rates, and reduced activity compared with control mice.149
Although clock genes impact metabolic homeostasis, a reciprocal effect of metabolic disruption on circadian rhythms also exists, because diet-induced obesity per se alters circadian behavioral and molecular rhythms in C57BL/6J mice.9 Indeed, HFD also attenuates the amplitude of diurnal rhythms of feeding and locomotor activity, as high fat fed mice increase their food intake during their normal rest (light) period.9 Interestingly, genetically obese animals are resistant to weight gain when feeding is restricted to the active (dark) phase.150 In agreement with these observations, recent evidence demonstrated that circadian timing of food intake contributes to weight gain.151 Indeed, mice fed a HFD only during the 12-hour light phase gain significantly more weight compared to isocalorically fed mice provided food only during the 12-hour dark phase.151 Further studies are necessary to understand how the timing of food intake impacts energy constancy. Interestingly, a recent study demonstrated that treatment with an antagonist of T-type calcium channel, which is involved in sleep-wake regulation, improved HFD-induced behavioral alterations, including both a decrease in inactive phase activity, core body temperature, feeding and adiposity.152 Taken together, these observations largely based on animal studies, raise important questions concerning the impact of circadian misalignment and clock gene disruption on obesity and its metabolic complications, and suggest avenues for future investigation in human subjects.
Clock Disruption in Adipose Tissue
Excess adipose tissue and altered body fat distribution, rather than adiposity per se, is an important risk factor for obesity-related disorders. Excess intra-abdominal fat rather than subcutaneous fat (central versus peripheral obesity) is associated with MS and cardiovascular disease.3 However, the mechanisms responsible for this association, and its causality, remain uncertain. Emerging evidence from both cell-based and human studies suggests that expression of the circadian clock transcription network within adipose tissue may influence both adipogenesis and the relative distribution of subcutaneous versus visceral depots.121,153,154
In adipose tissue, the clock machinery controls the expression of a large array of enzymes involved in lipid metabolism. Indeed, adenovirus-mediated expression of BMAL1 in 3T3-L1 adipocytes resulted in induction of several factors involved in lipogenesis, whereas BMAL1 deletion in adipose cell lines resulted in impaired adipogenesis.147 Furthermore, heme, the REV-ERBα/β natural ligand, has long been known to enhance adipocyte differentiation in vitro.155 Activation of SIRT1, which regulates the clock network, may increase insulin sensitivity and reduce the inflammatory response in adipocytes,156,157 however it is unclear whether the effect is direct or not.
Experiments in mice have revealed that temporally restricted feeding causes a coordinated phase-shift in circadian expression of core clock genes and their downstream targets in adipose tissues.117 In addition, HFD also alters the cyclic expression and function of core clock genes and clock-controlled genes in adipose tissue, resulting in disrupted fuel use.9,158 Of further interest, clock gene disruption targeted to the fat body in flies is sufficient to induce increased food consumption, decreased glycogen levels, and increased sensitivity to starvation.159 At least in flies, these findings suggest involvement of a peripheral tissue clock in neural energy homeostasis.159
Several teams have recently started to examine the potential relationship between clock gene expression and metabolic syndrome parameters in humans. Expression levels of the core molecular clock genes in cultured visceral and subcutaneous fat explants obtained from morbidly obese subjects correlated with certain metabolic syndrome parameters, such as waist circumference, sagittal diameter, and body mass index.153,154,160 However, biopsies from human fat likely represent a heterogenous mixture of adipose cells in addition to macrophages, thus conclusions must be viewed with caution regarding the contribution of adipose tissue to the observed circadian patterns of gene expression. Interestingly, circadian rhythms of gene expression are sustained ex vivo in human fat explants,160,161 including the rhythmic oscillation of genes involved in glucocorticoid turnover.161 In agreement, human adipose biopsies removed at different zeitgeber times reflect different levels of gene expression consistent with the observed circadian rhythmicity found in cell-culture studies.162 Further studies are necessary to gain more detailed insight into the relationship between temporal patterns of gene expression in adipose tissue and development of MS.
In addition to effects of circadian transcription on intracellular metabolic pathways, clock dysregulation in adipose tissue and/or misalignment with meal times may lead to inappropriate expression patterns of enzymes involved in lipid metabolism such as lipoprotein lipase.121 For example, misalignment between the fasting/feeding cycle and lipogenic and/or lipid catabolic gene expression pathways may perturb fatty acid flux and contribute to lipotoxicity. Indeed, circadian synchrony may play a distinct role not only within different tissue types (liver versus muscle) but also within distinct adipose depots (visceral versus subcutaneous).160 It is further possible that differences of circadian gene expression patterns within visceral adipose tissue and subcutaneous adipose tissue depots may contribute to cell-autonomous differences in inflammatory, lipogenic, and/or lipolytic pathways within these locales.163,164 The limited storage-capacity of fat and/or increased lipolysis results in an overflow of fatty acids to ectopic sites such as liver, muscle, and islets (reviewed previously165,166). Interestingly, both have been proposed to be involved in the etiology of the MS.3,167
Another important function of adipose tissue is its secretion of numerous bioactive peptides or proteins, collectively named “adipokines.” These may play a central role in energy and vascular homeostasis, as well as immunity, and are fundamental to the pathogenesis of the MS (reviewed previously168). Because obesity-related inflammation is receiving increased attention for its potential role in the pathogenesis of MS, steatosis and cardiovascular disease, it may be opportune to consider the impact of circadian systems at the level of adipokine regulation. In mice, leptin exhibits rhythmic production across the light-dark cycle that appears to be dependent of the feeding rhythm.169 Interestingly, in obese humans, disruption of the 24 hour profiles of leptin and adiponectin was observed compared to healthy lean subjects.170,171 These adipokines play major roles in fuel partitioning and insulin sensitivity but also regulate immunity.168,172 Indeed, leptin was the first adipokine found to control energy balance.173 The metabolic effects of leptin are thought to primarily involve its actions within brain,174–176 whereas adiponectin functions primarily within peripheral target tissues. Many of the metabolic effects of leptin and adiponectin involve activation of AMP-activated protein kinase signaling in muscle/liver.177,178 Following the discovery of leptin, a growing list of adipokines has been identified, some of which also exert proinflammatory roles. Because many adipokines are expressed in a circadian fashion in humans,162 it is tempting to speculate that regulation of adipokine oscillation may be important in metabolic homeostasis. In addition, certain adipokines may also interact with or modulate sleep. For instance, leptin is involved in sleep regulation as demonstrated by EEG monitoring of sleep in leptin deficient and leptin resistant mice.179,180 Interleukin-6 and tumor necrosis factor-α plasma levels, which are increased in obesity181,182 may also impair circadian clock gene oscillations and promote sleep.183,184
Circadian regulation may extend to effects within adipose tissue on endoplasmic reticulum (ER) stress, an important component of the inflammatory response in this tissue.185 Obesity results in conditions that increase demand on the ER in metabolic tissues including liver, adipocytes and pancreas, resulting in a persistent inflammatory state.186 For example, accumulation of reactive oxygen species, which are abundantly produced by both the ER and the mitochondria during conditions of stress are increased in metabolic organs in MS.185,186 In adipose tissue, ER stress is involved in adipogenesis and adipokine oversecretion.187,188 Interestingly, the endoplasmic reticulum chaperone protein BiP, a key protein involved in the ER stress response, is expressed in a circadian manner in flies.189 It has also been reported that clock genes may influence the production of reactive oxygen species.8,190,191 Thus, disrupted synchrony of stress response gene expression may alter adipose function and thereby directly contribute to insulin resistance. ER stress may also be induced in brain following high fat feeding, thereby contributing to leptin resistance192 and perhaps circadian and sleep disturbances (reviewed previously193).
In addition to the impact of white adipose tissue excess in MS pathogenesis, several independent groups recently demonstrated that brown adipose tissue is present and active in adult humans, and its presence and activity are inversely associated with adiposity and indexes of the metabolic syndrome.194–196 As numerous genes including nuclear receptors exert circadian expression profiles in brown adipose tissue,116,117 alterations of circadian oscillator genes in fat may have significant metabolic implications.
Clock Disruption and Impaired Glucose Tolerance
Disruption in the normal cyclic pattern of glucose tolerance is a hallmark of type 2 diabetes,197 and as such, understanding the circadian control of glucose metabolism is critical for delivering the best clinical diabetes management. Strong evidence from human studies demonstrates rhythmic variation in glucose tolerance and insulin action across the day.198–202 For example, oral glucose tolerance is impaired in the evening compared to the morning,203,204 an effect which is believed to be attributable to a combination of both decreased insulin secretion and altered insulin sensitivity in the evening.200,205–210 The ‘dawn phenomenon’ is also a well-described phenomenon where glucose levels are known to peak before the onset of the active period.211,212 Furthermore, studies in rats have revealed that the SCN is critical for the maintenance of diurnal variations in glucose metabolism.207
Although these studies indicate a role for circadian systems in the control of glucose metabolism, the molecular mechanisms underlying these phenomena are not well understood. Recently, human genome-wide association studies and experimental mouse model systems have begun to provide clues as to the nature of the molecular links between rhythms and glucose metabolism. As described above, data from several independent groups has now demonstrated that genetic variants of the melatonin receptor may be involved in abnormal glucose homeostasis47,48 and that melatonin treatment of pancreatic β-cells inhibits glucose-induced insulin release.49 Furthermore, mice that have a mutation in the Clock gene develop hypoinsulinemic hyperglycemia, and mice nullizygous for Bmal1 have impaired glucose tolerance,141,145,146 suggesting that a functional clock network is required for the maintenance of glucose homeostasis. The cellular etiology of impaired glucose tolerance in circadian mutant animals remains an important yet unresolved area of research.
Finally, it is interesting to speculate that the NAD biosynthetic enzyme NAMPT and SIRT1 may play an important role in the circadian control of glucose metabolism. As described above, NAMPT and SIRT1 are regulated by CLOCK/BMAL1 and constitute a negative feedback loop within the core circadian network. NAMPT and SIRT1 have been demonstrated to be involved in a myriad of metabolic functions, including regulation of gluconeogenesis in liver and glucose-stimulated insulin secretion in islets.44,45 Indeed Nampt-deficient (Nampt+/−) mice showed impaired glucose tolerance attributable to a defect in glucose-stimulated insulin secretion, which was corrected by intraperitoneal administration of NMN,134 and mice overexpressing SIRT1 specifically in their β-cells displayed improved glucose tolerance and increased glucose-stimulated insulin secretion.213 As NAMPT and SIRT1 function are impaired in circadian mutant mice, these data suggest that circadian rhythms of NAMPT and SIRT1 may act in β cells to regulate the daily cycles of insulin secretion and that NAD+ might function as an oscillating metabolite linking circadian and metabolic cycles.
Impact of Circadian Systems on Cardiovascular Function
Circadian variation in endogenous factors such as autonomic nervous system function, blood catecholamine concentrations, coagulability, heart rate, blood pressure regulation, and platelet aggregability have been suggested to explain the morning onset of myocardial infarction.23 Conversely, pressure overload–induced hypertrophy and diabetes mellitus result in alterations in the circadian clock within the heart.214,215 In this regard, it was suggested that alterations in circadian control of cardiac fuel handling may contribute to myocardial contractile dysfunction (reviewed previously216). The identification of genes that exhibit circadian regulation within large vessels of the mouse217–220 has provided clues as to the impact of the circadian system within vasculature. Indeed, clock gene expression within vasculature has been shown to impact blood pressure and thromboocclusive response.219,221,222 Clock genes may influence the temporal incidence of clinical cardiovascular events by regulating the magnitude of the early morning rise in blood pressure.219 Indeed, the circadian variation in blood pressure and heart rate is disrupted in mice with deleted or mutated core clock genes, a phenomenon that may be partially explained by the altered diurnal variation in epinephrine and norepinephrine in these mice.219 Interestingly, the mutated mice also showed a reduced response to immobilization stress compared to wild-type mice. Thus, expression of the core clock within peripheral vasculature may modulate the capacity to respond to environmental stressors at different times of day.219 Effects of the clock system on blood pressure may also involve the modulation of aldosterone biosynthesis by Per1.223 Furthermore, Anea et al demonstrated that Bmal1-knockout and Clock mutant mice present a loss of vascular adaptation and predisposition to thrombosis,222 both hallmarks of endothelial dysfunction.224 The endothelial dysfunction in Bmal1-knockout mice has been related to defects in Akt and nitric oxide signaling.222 Interestingly, the defects in endothelium-dependent arterial relaxation of Clock mutant mice were normalized by entrainment to light, indicating that the vascular phenotype is not simply a consequence of Clock mutation or Bmal1 deficiency but rather the result of behavioral disruption in these animals.222
As noted above, NAD+ regulation has recently emerged as a major factor coupling circadian rhythms and metabolic signaling pathways. Because NAMPT-mediated NAD biosynthesis has also been shown to impact cardiomyocyte survival pathways, it will be important to ascertain whether dysregulation of NAD contributes to the adverse cardiovascular consequences of circadian disruption.225
Cardiomyocytes must adapt rapidly to changes in circulating fatty acid, the primary fuel source for contraction. In this regard, PPAR signaling is important in the control of cardiac energy metabolism.226 Of note, it was also demonstrated that the circadian clock within the cardiomyocyte is essential for responsiveness of the heart to fatty acids.227–229 To address the circadian function of vascular tissue and the role of PPARγ in the vascular clock, conditional deletion of PPARγ targeted to this tissue was performed.230 These mice developed abnormalities in blood pressure and heart rate in parallel with a reduction of diurnal variation in the sympathetic nerve activity.230 Furthermore, vascular PPARγ exhibits a robust cyclic expression, whose rhythmic phase may be reset by changes in feeding time as well as changes in the photoperiod.230 Thus, the temporal environment may be integrated within the heart by PPARγ.230 In agreement, PPARγ agonists were found to shift the circadian fluctuation of blood pressure in patients with type 2 diabetes, indicating that vasculoprotective actions of thiazolidinediones may in part involve effects on the clock transcription network.231
Emerging clinical evidence has uncovered unique actions of the PPARα agonist fenofibrate in the circadian control of blood pressure and heart rate in diabetic subjects. In particular, fenofibrate exerted its most marked antihypertensive effects at night.232 In contrast, only modest decrease antihypertensive effects were detected in studies involving a single measurement during the day.233,234 In addition, fenofibrate lowered heart rate throughout the 24-hour period.232 Taking together, these data suggest an interaction between PPARα, blood pressure control and circadian rhythms in diabetes.
A comprehensive temporal map of the nuclear receptor transcriptome provides additional clues concerning the circadian control of cardiovascular physiology.120 For instance, RARα and RXRα interact with CLOCK and MOP4 resulting in repression of CLOCK/MOP4: BMAL1 activity in vascular cells.218 Indeed, the ligation of retinoic acid, the oxidized form of vitamin A, to its receptors can phase shift Per2 mRNA rhythm in vivo and in smooth muscle cells in vitro.218 Whether additional nuclear hormone receptor agonists impact circadian regulation of vascular tone remains a question for future investigation.
Finally, the role of circadian rhythms in the time of onset of thrombotic events has been recognized for many years. Numerous coagulation/ fibrinolytic factors, such as protein C, antithrombin, factor VII, protein S, and fibrinogen, have been demonstrated to fluctuate in a circadian manner in humans. Among these factors, plasminogen activator inhibitor type 1, the most important physiological inhibitor of plasminogen activation, peaks in the early morning, explaining, at least in part, the occurrence of hypofibrinolysis and of prothrombotic state.235 Circadian control of plasminogen activator inhibitor type 1 gene expression by the REV-ERBα in liver may contribute to the circadian variation in fibrinolysis,236 an effect that may also involve interactions between the cycle-like factor (CLIF) and CLOCK.237 Thus, further studies on the circadian gene control of fibrinolysis may shed new light on factors contributing to the prothrombotic state.
Circadian Rhythms and Hepatic Function
Liver also plays a key role in the development of metabolic syndrome (reviewed previously238). BMAL1 and CLOCK control gene expression of enzymes critical in liver and influence both glucose and lipid homeostasis (reviewed previously239). A recent study reported that mice with a liver-specific deletion of Bmal1 exhibited hypoglycemia during fasting, indicating a role for the liver clock in maintaining euglycemia during rest.146 Some of the effects of BMAL1 and CLOCK in liver may involve direct regulation of phosphoenolpyruvate carboxykinase (Pepck).145 In addition, circadian gene expression in hepatocytes is altered in mouse models of type 2 diabetes240 and by high fat feeding.9,241 Moreover, HFD induced a phase delay of components of the adiponectin signaling pathway.241 Alterations in circadian control of adiponectin signaling may reduce its protective effects241 and thereby increase susceptibility to steatosis, a major risk factor in cardiovascular disease.242
Hepatic clock gene expression also modulates both bile acid and apolipoprotein biosynthesis, raising the possibility that clock disruption may impact multiple components of hepatic lipid homeostasis.243 For example, several proteins involved in lipid metabolism (such as hepatic cytochrome P450 cholesterol 7 α-hydroxylase, 3-hydroxy-3-methylglutaryl coenzyme A [HMG-CoA] reductase, or apolipoprotein AIV) show diurnal variation in both humans and rodents.105 Interestingly, Rev-erbα was recently found to play an important role in the control of bile acid metabolism via the regulation of the neutral bile acid synthesis pathway.244 In mouse liver, Rev-erbα expression levels are high during the late light phase, leading to the repression of both small heterodimer partner (SHP) and E4 promoter binding protein 4 (E4BP4) hepatic expression. Reduced levels of SHP and E4BP4 may counter the suppressive effects of bile acids on the cholesterol 7α-hydroxylase (CYP7A1) gene transcription, thereby contributing to the circadian regulation of bile acid and cholesterol homeostasis.244 In addition, Rev-erbα also controls the daily expression of genes involved in cholesterol and lipid homeostasis through circadian modulation of SREBP (sterol regulatory element-binding protein) signaling.245
Clock Dysfunction in the Immune System
In addition to effects on lipogenesis, lipid catabolism and thrombosis, the circadian system may also promote inflammatory pathways that contribute to the development of cardiovascular disease. At the molecular level, the circadian transcription factor Rev-erbα, which is expressed in cells from the immune system such as macrophages and other cell types, may impact the inflammatory response140 (also see elsewhere246). Intriguingly, REV-ERBα increases the tumor necrosis factor-α–induced nuclear factor-κB response, whereas RORα impedes it.140 As rhythmic mRNA expression of the clock genes is dampened in peripheral leukocytes of patients with type 2 diabetes, this impairment might be involved in its pathogenesis.247
Both inter- and intraorgan desynchrony may be involved in the pathogenesis of cardiometabolic disease attributable to effects in brain and multiple metabolic tissues including heart, liver, fat, muscle, pancreas, and gut. In this context, strategies to improve alignment between the cycles of sleep/wakefulness and feeding/fasting may ameliorate physiological processes including appetitive behavior, carbohydrate and lipid metabolism, inflammation, thrombosis, and sodium handling. Efforts to dissect the molecular mediators that coordinate circadian, metabolic, and cardiovascular systems may ultimately lead to both improved therapeutics and preventive interventions.
We thank members of the Bass, Takahashi, Turek and Allada laboratories for helpful discussions.
Sources of Funding
This work was supported by Alfediam (to E.M.), National Institute of Diabetes and Digestive and Kidney Diseases (T32 DK007169 to K.M.R.), NIH (P01 AG011412 and R01HL097817-01 to J.B.), American Diabetes Association (to J.B.), Chicago Biomedical Consortium Searle Funds (to J.B.), Juvenile Diabetes Research Foundation (to J.B.), and the University of Chicago Diabetes Research and Training Center (P60 DK020595).
J.B. is a member of the scientific advisory board of ReSet Therapeutics Inc and is an advisor and receives support from Amylin Pharmaceuticals.
Original received September 1, 2009; revision received October 25, 2009; accepted November 9, 2009.
Lorenzo C, Williams K, Hunt KJ, Haffner SM. The National Cholesterol Education Program - Adult Treatment Panel III, International Diabetes Federation, and World Health Organization definitions of the metabolic syndrome as predictors of incident cardiovascular disease and diabetes. Diabetes Care. 2007; 30: 8–13.
Klein S, Allison DB, Heymsfield SB, Kelley DE, Leibel RL, Nonas C, Kahn R. Waist circumference and cardiometabolic risk: a consensus statement from shaping America’s health: Association for Weight Management and Obesity Prevention; NAASO, the Obesity Society; the American Society for Nutrition; and the American Diabetes Association. Diabetes Care. 2007; 30: 1647–1652.
Scheer FA, Hilton MF, Mantzoros CS, Shea SA. Adverse metabolic and cardiovascular consequences of circadian misalignment. Proc Natl Acad Sci U S A. 2009; 106: 4453–4458.
Kawakami N, Takatsuka N, Shimizu H. Sleep disturbance and onset of type 2 diabetes. Diabetes Care. 2004; 27: 282–283.
Yaggi HK, Araujo AB, McKinlay JB. Sleep duration as a risk factor for the development of type 2 diabetes. Diabetes Care. 2006; 29: 657–661.
Lumeng JC, Somashekar D, Appugliese D, Kaciroti N, Corwyn RF, Bradley RH. Shorter sleep duration is associated with increased risk for being overweight at ages 9 to 12 years. Pediatrics. 2007; 120: 1020–1029.
Spiegel K, Knutson K, Leproult R, Tasali E, Van Cauter E. Sleep loss: a novel risk factor for insulin resistance and Type 2 diabetes. J Appl Physiol. 2005; 99: 2008–2019.
Ramsey KM, Bass J. Lean gene and the clock machine. Proc Natl Acad Sci U S A. 2007; 104: 9553–9554.
Burioka N, Koyanagi S, Endo M, Takata M, Fukuoka Y, Miyata M, Takeda K, Chikumi H, Ohdo S, Shimizu E. Clock gene dysfunction in patients with obstructive sleep apnoea syndrome. Eur Respir J. 2008; 32: 105–112.
Knutsson A. Health disorders of shift workers. Occup Med. 2003; 53: 103–108.
Ryan S, Taylor CT, McNicholas WT. Systemic inflammation: a key factor in the pathogenesis of cardiovascular complications in obstructive sleep apnoea syndrome? Thorax. 2009; 64: 631–636.
Ribeiro DC, Hampton SM, Morgan L, Deacon S, Arendt J. Altered postprandial hormone and metabolic responses in a simulated shift work environment. J Endocrinol. 1998; 158: 305–310.
Ramsey KM, Bass J. Obeying the clock yields benefits for metabolism. Proc Natl Acad Sci U S A. 2009; 106: 4069–4070.
He Y, Jones CR, Fujiki N, Xu Y, Guo B, Holder JL Jr, Rossner MJ, Nishino S, Fu YH. The transcriptional repressor DEC2 regulates sleep length in mammals. Science. 2009; 325: 866–870.
Ptacek LJ, Jones CR, Fu YH. Novel insights from genetic and molecular characterization of the human clock. Cold Spring Harb Symp Quant Biol. 2007; 72: 273–277.
Sookoian S, Gemma C, Gianotti TF, Burgueno A, Castano G, Pirola CJ. Genetic variants of Clock transcription factor are associated with individual susceptibility to obesity. Am J Clin Nutr. 2008; 87: 1606–1615.
Woon PY, Kaisaki PJ, Braganca J, Bihoreau MT, Levy JC, Farrall M, Gauguier D. Aryl hydrocarbon receptor nuclear translocator-like (BMAL1) is associated with susceptibility to hypertension and type 2 diabetes. Proc Natl Acad Sci U S A. 2007; 104: 14412–14417.
Nakahata Y, Sahar S, Astarita G, Kaluzova M, Sassone-Corsi P. Circadian control of the NAD+ salvage pathway by CLOCK-SIRT1. Science. 2009; 324: 654–657.
Ramsey KM, Yoshino J, Brace CS, Abrassart D, Kobayashi Y, Marcheva B, Hong HK, Chong JL, Buhr ED, Lee C, Takahashi JS, Imai S, Bass J. Circadian clock feedback cycle through NAMPT-mediated NAD+ biosynthesis. Science. 2009; 324: 651–654.
Blakemore AI, Meyre D, Delplanque J, Vatin V, Lecoeur C, Marre M, Tichet J, Balkau B, Froguel P, Walley AJ. A rare variant in the visfatin gene (NAMPT/PBEF1) is associated with protection from obesity. Obesity (Silver Spring). 2009; 7: 1549–1553.
Bouatia-Naji N, Bonnefond A, Cavalcanti-Proenca C, Sparso T, Holmkvist J, Marchand M, Delplanque J, Lobbens S, Rocheleau G, Durand E, De Graeve F, Chevre JC, Borch-Johnsen K, Hartikainen AL, Ruokonen A, Tichet J, Marre M, Weill J, Heude B, Tauber M, Lemaire K, Schuit F, Elliott P, Jorgensen T, Charpentier G, Hadjadj S, Cauchi S, Vaxillaire M, Sladek R, Visvikis-Siest S, Balkau B, Levy-Marchal C, Pattou F, Meyre D, Blakemore AI, Jarvelin MR, Walley AJ, Hansen T, Dina C, Pedersen O, Froguel P. A variant near MTNR1B is associated with increased fasting plasma glucose levels and type 2 diabetes risk. Nat Genet. 2009; 41: 89–94.
Prokopenko I, Langenberg C, Florez JC, Saxena R, Soranzo N, Thorleifsson G, Loos RJ, Manning AK, Jackson AU, Aulchenko Y, Potter SC, Erdos MR, Sanna S, Hottenga JJ, Wheeler E, Kaakinen M, Lyssenko V, Chen WM, Ahmadi K, Beckmann JS, Bergman RN, Bochud M, Bonnycastle LL, Buchanan TA, Cao A, Cervino A, Coin L, Collins FS, Crisponi L, de Geus EJ, Dehghan A, Deloukas P, Doney AS, Elliott P, Freimer N, Gateva V, Herder C, Hofman A, Hughes TE, Hunt S, Illig T, Inouye M, Isomaa B, Johnson T, Kong A, Krestyaninova M, Kuusisto J, Laakso M, Lim N, Lindblad U, Lindgren CM, McCann OT, Mohlke KL, Morris AD, Naitza S, Orru M, Palmer CN, Pouta A, Randall J, Rathmann W, Saramies J, Scheet P, Scott LJ, Scuteri A, Sharp S, Sijbrands E, Smit JH, Song K, Steinthorsdottir V, Stringham HM, Tuomi T, Tuomilehto J, Uitterlinden AG, Voight BF, Waterworth D, Wichmann HE, Willemsen G, Witteman JC, Yuan X, Zhao JH, Zeggini E, Schlessinger D, Sandhu M, Boomsma DI, Uda M, Spector TD, Penninx BW, Altshuler D, Vollenweider P, Jarvelin MR, Lakatta E, Waeber G, Fox CS, Peltonen L, Groop LC, Mooser V, Cupples LA, Thorsteinsdottir U, Boehnke M, Barroso I, Van Duijn C, Dupuis J, Watanabe RM, Stefansson K, McCarthy MI, Wareham NJ, Meigs JB, Abecasis GR. Variants in MTNR1B influence fasting glucose levels. Nat Genet. 2009; 41: 77–81.
Lyssenko V, Nagorny CL, Erdos MR, Wierup N, Jonsson A, Spegel P, Bugliani M, Saxena R, Fex M, Pulizzi N, Isomaa B, Tuomi T, Nilsson P, Kuusisto J, Tuomilehto J, Boehnke M, Altshuler D, Sundler F, Eriksson JG, Jackson AU, Laakso M, Marchetti P, Watanabe RM, Mulder H, Groop L. Common variant in MTNR1B associated with increased risk of type 2 diabetes and impaired early insulin secretion. Nat Genet. 2009; 41: 82–88.
Vitaterna MH, King DP, Chang AM, Kornhauser JM, Lowrey PL, McDonald JD, Dove WF, Pinto LH, Turek FW, Takahashi JS. Mutagenesis and mapping of a mouse gene, Clock, essential for circadian behavior. Science. 1994; 264: 719–725.
Gekakis N, Staknis D, Nguyen HB, Davis FC, Wilsbacher LD, King DP, Takahashi JS, Weitz CJ. Role of the CLOCK protein in the mammalian circadian mechanism. Science. 1998; 280: 1564–1569.
Shearman LP, Jin X, Lee C, Reppert SM, Weaver DR. Targeted disruption of the mPer3 gene: subtle effects on circadian clock function. Mol Cell Biol. 2000; 20: 6269–6275.
Ueda HR. Systems biology of mammalian circadian clocks. Cold Spring Harb Symp Quant Biol. 2007; 72: 365–380.
Triqueneaux G, Thenot S, Kakizawa T, Antoch MP, Safi R, Takahashi JS, Delaunay F, Laudet V. The orphan receptor Rev-erbalpha gene is a target of the circadian clock pacemaker. J Mol Endocrinol. 2004; 33: 585–608.
Guillaumond F, Dardente H, Giguere V, Cermakian N. Differential control of Bmal1 circadian transcription by REV-ERB and ROR nuclear receptors. J Biol Rhythms. 2005; 20: 391–403.
Maldonado R, Smadja C, Mazzucchelli C, Sassone-Corsi P. Altered emotional and locomotor responses in mice deficient in the transcription factor CREM. Proc Natl Acad Sci U S A. 1999; 96: 14094–14099.
O'Neill JS, Maywood ES, Chesham JE, Takahashi JS, Hastings MH. cAMP-dependent signaling as a core component of the mammalian circadian pacemaker. Science. 2008; 320: 949–953.
Akashi M, Tsuchiya Y, Yoshino T, Nishida E. Control of intracellular dynamics of mammalian period proteins by casein kinase I epsilon (CKIepsilon) and CKIdelta in cultured cells. Mol Cell Biol. 2002; 22: 1693–1703.
Eide EJ, Vielhaber EL, Hinz WA, Virshup DM. The circadian regulatory proteins BMAL1 and cryptochromes are substrates of casein kinase Iepsilon. J Biol Chem. 2002; 277: 17248–17254.
Eide EJ, Woolf MF, Kang H, Woolf P, Hurst W, Camacho F, Vielhaber EL, Giovanni A, Virshup DM. Control of mammalian circadian rhythm by CKIepsilon-regulated proteasome-mediated PER2 degradation. Mol Cell Biol. 2005; 25: 2795–2807.
Godinho SI, Maywood ES, Shaw L, Tucci V, Barnard AR, Busino L, Pagano M, Kendall R, Quwailid MM, Romero MR, O'Neill J, Chesham JE, Brooker D, Lalanne Z, Hastings MH, Nolan PM. The after-hours mutant reveals a role for Fbxl3 in determining mammalian circadian period. Science. 2007; 316: 897–900.
Reischl S, Vanselow K, Westermark PO, Thierfelder N, Maier B, Herzel H, Kramer A. Beta-TrCP1-mediated degradation of PERIOD2 is essential for circadian dynamics. J Biol Rhythms. 2007; 22: 375–386.
Shirogane T, Jin J, Ang XL, Harper JW. SCFbeta-TRCP controls clock-dependent transcription via casein kinase 1-dependent degradation of the mammalian period-1 (Per1) protein. J Biol Chem. 2005; 280: 26863–26872.
Hirota T, Lewis WG, Liu AC, Lee JW, Schultz PG, Kay SA. A chemical biology approach reveals period shortening of the mammalian circadian clock by specific inhibition of GSK-3beta. Proc Natl Acad Sci U S A. 2008; 105: 20746–20751.
Cermakian N, Monaco L, Pando MP, Dierich A, Sassone-Corsi P. Altered behavioral rhythms and clock gene expression in mice with a targeted mutation in the Period1 gene. EMBO J. 2001; 20: 3967–3974.
Vitaterna MH, Selby CP, Todo T, Niwa H, Thompson C, Fruechte EM, Hitomi K, Thresher RJ, Ishikawa T, Miyazaki J, Takahashi JS, Sancar A. Differential regulation of mammalian period genes and circadian rhythmicity by cryptochromes 1 and 2. Proc Natl Acad Sci U S A. 1999; 96: 12114–12119.
Okamura H. Suprachiasmatic nucleus clock time in the mammalian circadian system. Cold Spring Harb Symp Quant Biol. 2007; 72: 551–556.
Weaver DR. The suprachiasmatic nucleus: a 25-year retrospective. J Biol Rhythms. 1998; 13: 100–112.
Ralph MR, Foster RG, Davis FC, Menaker M. Transplanted suprachiasmatic nucleus determines circadian period. Science. 1990; 247: 975–978.
Sancar A. Regulation of the mammalian circadian clock by cryptochrome. J Biol Chem. 2004; 279: 34079–34082.
Balsalobre A, Brown SA, Marcacci L, Tronche F, Kellendonk C, Reichardt HM, Schutz G, Schibler U. Resetting of circadian time in peripheral tissues by glucocorticoid signaling. Science. 2000; 289: 2344–2347.
Fuller PM, Lu J, Saper CB. Differential rescue of light- and food-entrainable circadian rhythms. Science. 2008; 320: 1074–1077.
Mieda M, Williams SC, Richardson JA, Tanaka K, Yanagisawa M. The dorsomedial hypothalamic nucleus as a putative food-entrainable circadian pacemaker. Proc Natl Acad Sci U S A. 2006; 103: 12150–12155.
Landry GJ, Simon MM, Webb IC, Mistlberger RE. Persistence of a behavioral food-anticipatory circadian rhythm following dorsomedial hypothalamic ablation in rats. Am J Physiol Regul Integr Comp Physiol. 2006; 290: R1527–R1534.
Storch KF, Weitz CJ. Daily rhythms of food-anticipatory behavioral activity do not require the known circadian clock. Proc Natl Acad Sci U S A. 2009; 106: 6808–6813.
Sutton GM, Perez-Tilve D, Nogueiras R, Fang J, Kim JK, Cone RD, Gimble JM, Tschop MH, Butler AA. The melanocortin-3 receptor is required for entrainment to meal intake. J Neurosci. 2008; 28: 12946–12955.
Davidson AJ. Search for the feeding-entrainable circadian oscillator: a complex proposition. Am J Physiol Regul Integr Comp Physiol. 2006; 290: R1524–R1526.
Stephan FK. The “other” circadian system: food as a Zeitgeber. J Biol Rhythms. 2002; 17: 284–292.
Yamazaki S, Numano R, Abe M, Hida A, Takahashi R, Ueda M, Block GD, Sakaki Y, Menaker M, Tei H. Resetting central and peripheral circadian oscillators in transgenic rats. Science. 2000; 288: 682–685.
Yoo SH, Yamazaki S, Lowrey PL, Shimomura K, Ko CH, Buhr ED, Siepka SM, Hong HK, Oh WJ, Yoo OJ, Menaker M, Takahashi JS. PERIOD2::LUCIFERASE real-time reporting of circadian dynamics reveals persistent circadian oscillations in mouse peripheral tissues. Proc Natl Acad Sci U S A. 2004; 101: 5339–5346.
McCarthy JJ, Andrews JL, McDearmon EL, Campbell KS, Barber BK, Miller BH, Walker JR, Hogenesch JB, Takahashi JS, Esser KA. Identification of the circadian transcriptome in adult mouse skeletal muscle. Physiol Genomics. 2007; 31: 86–95.
Zvonic S, Ptitsyn AA, Conrad SA, Scott LK, Floyd ZE, Kilroy G, Wu X, Goh BC, Mynatt RL, Gimble JM. Characterization of peripheral circadian clocks in adipose tissues. Diabetes. 2006; 55: 962–970.
Gimble JM, Floyd ZE. Fat Circadian Biology. J Appl Physiol. 2009; 107: 1629–1637.
Miller BH, McDearmon EL, Panda S, Hayes KR, Zhang J, Andrews JL, Antoch MP, Walker JR, Esser KA, Hogenesch JB, Takahashi JS. Circadian and CLOCK-controlled regulation of the mouse transcriptome and cell proliferation. Proc Natl Acad Sci U S A. 2007; 104: 3342–3347.
Rutter J, Reick M, Wu LC, McKnight SL. Regulation of clock and NPAS2 DNA binding by the redox state of NAD cofactors. Science. 2001; 293: 510–514.
Cohen HY, Miller C, Bitterman KJ, Wall NR, Hekking B, Kessler B, Howitz KT, Gorospe M, de Cabo R, Sinclair DA. Calorie restriction promotes mammalian cell survival by inducing the SIRT1 deacetylase. Science. 2004; 305: 390–392.
Nemoto S, Fergusson MM, Finkel T. Nutrient availability regulates SIRT1 through a forkhead-dependent pathway. Science. 2004; 306: 2105–2108.
Haigis MC, Guarente LP. Mammalian sirtuins–emerging roles in physiology, aging, and calorie restriction. Genes Dev. 2006; 20: 2913–2921.
Imai SI. The NAD world: a new systemic regulatory network for metabolism and aging-Sirt1, systemic NAD biosynthesis, and their importance. Cell Biochem Biophys. 2009; 15: 20–28.
Wang J, Lazar MA. Bifunctional role of Rev-erbalpha in adipocyte differentiation. Mol Cell Biol. 2008; 28: 2213–2220.
Duez H, Staels B. The nuclear receptors Rev-erbs and RORs integrate circadian rhythms and metabolism. Diab Vasc Dis Res. 2008; 5: 82–88.
Turek FW, Joshu C, Kohsaka A, Lin E, Ivanova G, McDearmon E, Laposky A, Losee-Olson S, Easton A, Jensen DR, Eckel RH, Takahashi JS, Bass J. Obesity and metabolic syndrome in circadian Clock mutant mice. Science. 2005; 308: 1043–1045.
Kennaway DJ, Voultsios A, Varcoe TJ, Moyer RW. Melatonin and activity rhythm responses to light pulses in mice with the Clock mutation. Am J Physiol Regul Integr Comp Physiol. 2003; 284: R1231–R1240.
Kennaway DJ, Owens JA, Voultsios A, Boden MJ, Varcoe TJ. Metabolic homeostasis in mice with disrupted Clock gene expression in peripheral tissues. Am J Physiol Regul Integr Comp Physiol. 2007; 293: R1528–R1537.
Lamia KA, Storch KF, Weitz CJ. Physiological significance of a peripheral tissue circadian clock. Proc Natl Acad Sci U S A. 2008; 105: 15172–15177.
Shimba S, Ishii N, Ohta Y, Ohno T, Watabe Y, Hayashi M, Wada T, Aoyagi T, Tezuka M. Brain and muscle Arnt-like protein-1 (BMAL1), a component of the molecular clock, regulates adipogenesis. Proc Natl Acad Sci U S A. 2005; 102: 12071–12076.
Green CB, Douris N, Kojima S, Strayer CA, Fogerty J, Lourim D, Keller SR, Besharse JC. Loss of Nocturnin, a circadian deadenylase, confers resistance to hepatic steatosis and diet-induced obesity. Proc Natl Acad Sci U S A. 2007; 104: 9888–9893.
Masaki T, Chiba S, Yasuda T, Noguchi H, Kakuma T, Watanabe T, Sakata T, Yoshimatsu H. Involvement of hypothalamic histamine H1 receptor in the regulation of feeding rhythm and obesity. Diabetes. 2004; 53: 2250–2260.
Uebele VN, Gotter AL, Nuss CE, Kraus RL, Doran SM, Garson SL, Reiss DR, Li Y, Barrow JC, Reger TS, Yang ZQ, Ballard JE, Tang C, Metzger JM, Wang SP, Koblan KS, Renger JJ. Antagonism of T-type calcium channels inhibits high-fat diet-induced weight gain in mice. J Clin Invest. 2009; 119: 1659–1667.
Yoshizaki T, Milne JC, Imamura T, Schenk S, Sonoda N, Babendure JL, Lu JC, Smith JJ, Jirousek MR, Olefsky JM. SIRT1 exerts anti-inflammatory effects and improves insulin sensitivity in adipocytes. Mol Cell Biol. 2009; 29: 1363–1374.
Loboda A, Kraft WK, Fine B, Joseph J, Nebozhyn M, Zhang C, He Y, Yang X, Wright C, Morris M, Chalikonda I, Ferguson M, Emilsson V, Leonardson A, Lamb J, Dai H, Schadt E, Greenberg HE, Lum PY. Diurnal variation of the human adipose transcriptome and the link to metabolic disease. BMC Med Genomics. 2009; 2: 7.
Savage DB, Petersen KF, Shulman GI. Disordered lipid metabolism and the pathogenesis of insulin resistance. Physiol Rev. 2007; 87: 507–520.
Mingrone G, Manco M, Granato L, Calvani M, Scarfone A, Mora EV, Greco AV, Vidal H, Castagneto M, Ferrannini E. Leptin pulsatility in formerly obese women. FASEB J. 2005; 19: 1380–1382.
Yildiz BO, Suchard MA, Wong ML, McCann SM, Licinio J. Alterations in the dynamics of circulating ghrelin, adiponectin, and leptin in human obesity. Proc Natl Acad Sci U S A. 2004; 101: 10434–10439.
Laposky AD, Bradley MA, Williams DL, Bass J, Turek FW. Sleep-wake regulation is altered in leptin-resistant (db/db) genetically obese and diabetic mice. Am J Physiol Regul Integr Comp Physiol. 2008; 295: R2059–R2066.
Laposky AD, Shelton J, Bass J, Dugovic C, Perrino N, Turek FW. Altered sleep regulation in leptin deficient mice. Am J Physiol Regul Integr Comp Physiol. 2006; 290: R894–R903.
Bastard JP, Lagathu C, Caron M, Capeau J. Point-counterpoint: interleukin-6 does/does not have a beneficial role in insulin sensitivity and glucose homeostasis. J Appl Physiol. 2007; 102: 821–822;author reply 825.
Cavadini G, Petrzilka S, Kohler P, Jud C, Tobler I, Birchler T, Fontana A. TNF-alpha suppresses the expression of clock genes by interfering with E-box-mediated transcription. Proc Natl Acad Sci U S A. 2007; 104: 12843–12848.
Hosogai N, Fukuhara A, Oshima K, Miyata Y, Tanaka S, Segawa K, Furukawa S, Tochino Y, Komuro R, Matsuda M, Shimomura I. Adipose tissue hypoxia in obesity and its impact on adipocytokine dysregulation. Diabetes. 2007; 56: 901–911.
Shaw PJ, Cirelli C, Greenspan RJ, Tononi G. Correlates of sleep and waking in Drosophila melanogaster. Science. 2000; 287: 1834–1837.
Kondratov RV, Kondratova AA, Gorbacheva VY, Vykhovanets OV, Antoch MP. Early aging and age-related pathologies in mice deficient in BMAL1, the core component of the circadian clock. Genes Dev. 2006; 20: 1868–1873.
Holterhus PM, Odendahl R, Oesingmann S, Lepler R, Wagner V, Hiort O, Holl R. Classification of distinct baseline insulin infusion patterns in children and adolescents with type 1 diabetes on continuous subcutaneous insulin infusion therapy. Diabetes Care. 2007; 30: 568–573.
Aparicio NJ, Puchulu FE, Gagliardino JJ, Ruiz M, Llorens JM, Ruiz J, Lamas A, De Miguel R. Circadian variation of the blood glucose, plasma insulin and human growth hormone levels in response to an oral glucose load in normal subjects. Diabetes. 1974; 23: 132–137.
Carroll KF, Nestel PJ. Diurnal variation in glucose tolerance and in insulin secretion in man. Diabetes. 1973; 22: 333–348.
Jarrett RJ, Keen H. Diurnal variation of oral glucose tolerance: a possible pointer to the evolution of diabetes mellitus. BMJ. 1969; 2: 341–344.
la Fleur SE, Kalsbeek A, Wortel J, Fekkes ML, Buijs RM. A daily rhythm in glucose tolerance: a role for the suprachiasmatic nucleus. Diabetes. 2001; 50: 1237–1243.
Lee A, Ader M, Bray GA, Bergman RN. Diurnal variation in glucose tolerance. Cyclic suppression of insulin action and insulin secretion in normal-weight, but not obese, subjects. Diabetes. 1992; 41: 742–749.
Bolli GB, De Feo P, De Cosmo S, Perriello G, Ventura MM, Calcinaro F, Lolli C, Campbell P, Brunetti P, Gerich JE. Demonstration of a dawn phenomenon in normal human volunteers. Diabetes. 1984; 33: 1150–1153.
Young ME, Razeghi P, Taegtmeyer H. Clock genes in the heart: characterization and attenuation with hypertrophy. Circ Res. 2001; 88: 1142–1150.
Bray MS, Young ME. Diurnal variations in myocardial metabolism. Cardiovasc Res. 2008; 79: 228–237.
Rudic RD, McNamara P, Reilly D, Grosser T, Curtis AM, Price TS, Panda S, Hogenesch JB, FitzGerald GA. Bioinformatic analysis of circadian gene oscillation in mouse aorta. Circulation. 2005; 112: 2716–2724.
Curtis AM, Cheng Y, Kapoor S, Reilly D, Price TS, Fitzgerald GA. Circadian variation of blood pressure and the vascular response to asynchronous stress. Proc Natl Acad Sci U S A. 2007; 104: 3450–3455.
Curtis AM, Seo SB, Westgate EJ, Rudic RD, Smyth EM, Chakravarti D, FitzGerald GA, McNamara P. Histone acetyltransferase-dependent chromatin remodeling and the vascular clock. J Biol Chem. 2004; 279: 7091–7097.
Westgate EJ, Cheng Y, Reilly DF, Price TS, Walisser JA, Bradfield CA, FitzGerald GA. Genetic components of the circadian clock regulate thrombogenesis in vivo. Circulation. 2008; 117: 2087–2095.
Anea CB, Zhang M, Stepp DW, Simkins GB, Reed G, Fulton DJ, Rudic RD. Vascular disease in mice with a dysfunctional circadian clock. Circulation. 2009; 119: 1510–1517.
Hsu CP, Oka S, Shao D, Hariharan N, Sadoshima J. Nicotinamide phosphoribosyltransferase regulates cell survival through NAD+ synthesis in cardiac myocytes. Circ Res. 2009; 105: 481–491.
Durgan DJ, Trexler NA, Egbejimi O, McElfresh TA, Suk HY, Petterson LE, Shaw CA, Hardin PE, Bray MS, Chandler MP, Chow CW, Young ME. The circadian clock within the cardiomyocyte is essential for responsiveness of the heart to fatty acids. J Biol Chem. 2006; 281: 24254–24269.
Young ME. The circadian clock within the heart: potential influence on myocardial gene expression, metabolism, and function. Am J Physiol Heart Circ Physiol. 2006; 290: H1–H16.
Anan F, Masaki T, Fukunaga N, Teshima Y, Iwao T, Kaneda K, Umeno Y, Okada K, Wakasugi K, Yonemochi H, Eshima N, Saikawa T, Yoshimatsu H. Pioglitazone shift circadian rhythm of blood pressure from non-dipper to dipper type in type 2 diabetes mellitus. Eur J Clin Invest. 2007; 37: 709–714.
Chew GT, Watts GF, Davis TM, Stuckey BG, Beilin LJ, Thompson PL, Burke V, Currie PJ. Hemodynamic effects of fenofibrate and coenzyme Q10 in type 2 diabetic subjects with left ventricular diastolic dysfunction. Diabetes Care. 2008; 31: 1502–1509.
Keech A, Simes RJ, Barter P, Best J, Scott R, Taskinen MR, Forder P, Pillai A, Davis T, Glasziou P, Drury P, Kesaniemi YA, Sullivan D, Hunt D, Colman P, d'Emden M, Whiting M, Ehnholm C, Laakso M. Effects of long-term fenofibrate therapy on cardiovascular events in 9795 people with type 2 diabetes mellitus (the FIELD study): randomised controlled trial. Lancet. 2005; 366: 1849–1861.
Wang J, Yin L, Lazar MA. The orphan nuclear receptor Rev-erb alpha regulates circadian expression of plasminogen activator inhibitor type 1. J Biol Chem. 2006; 281: 33842–33848.
Maemura K, de la Monte SM, Chin MT, Layne MD, Hsieh CM, Yet SF, Perrella MA, Lee ME. CLIF, a novel cycle-like factor, regulates the circadian oscillation of plasminogen activator inhibitor-1 gene expression. J Biol Chem. 2000; 275: 36847–36851.
Marcheva B, Ramsey KM, Affinati A, Bass J. Clock genes and metabolic disease. J Appl Physiol. 2009; 107: 1638–1646.
Duez H, van der Veen JN, Duhem C, Pourcet B, Touvier T, Fontaine C, Derudas B, Bauge E, Havinga R, Bloks VW, Wolters H, van der Sluijs FH, Vennstrom B, Kuipers F, Staels B. Regulation of bile acid synthesis by the nuclear receptor Rev-erbalpha. Gastroenterology. 2008; 135: 689–698.
Albrecht U, Bordon A, Schmutz I, Ripperger J. The multiple facets of Per2. Cold Spring Harb Symp Quant Biol. 2007; 72: 95–104.