Published online before print January 17, 2008, doi: 10.1161/CIRCRESAHA.107.165464
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© 2008 American Heart Association, Inc.
Integrative Physiology |
Activation of Cardiac Progenitor Cells Reverses the Failing Heart Senescent Phenotype and Prolongs Lifespan
From the Departments of Anesthesia and Medicine and Division of Cardiology (A.G., M.R., D.N., Y.M., J.T., M.E.P.-I., P.M., G.E., C.B., S.V., M.B., K.U., T.H., P.A., J.K., A.L.), Brigham and Women’s Hospital, Harvard Medical School, Boston, Mass; Department of Physiology (C.O., T.H.H.), New York Medical College, Valhalla, New York 10595; and Institute of Molecular Cardiology (B.D., S.K.S., R.B.), University of Louisville, Ky.
Correspondence to Annarosa Leri, MD, Departments of Anesthesia and Medicine and Division of Cardiology, Brigham and Women’s Hospital, Harvard Medical School, Boston, MA. E-mail aleri{at}zeus.bwh.harvard.edu
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Abstract |
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Heart failure is the leading cause of death in the elderly, but whether this is the result of a primary aging myopathy dictated by depletion of the cardiac progenitor cell (CPC) pool is unknown. Similarly, whether current lifespan reflects the ineluctable genetic clock or heart failure interferes with the genetically determined fate of the organ and organism is an important question. We have identified that chronological age leads to telomeric shortening in CPCs, which by necessity generate a differentiated progeny that rapidly acquires the senescent phenotype conditioning organ aging. CPC aging is mediated by attenuation of the insulin-like growth factor-1/insulin-like growth factor-1 receptor and hepatocyte growth factor/c-Met systems, which do not counteract any longer the CPC renin–angiotensin system, resulting in cellular senescence, growth arrest, and apoptosis. However, pulse-chase 5-bromodeoxyuridine–labeling assay revealed that the senescent heart contains functionally competent CPCs that have the properties of stem cells. This subset of telomerase-competent CPCs have long telomeres and, following activation, migrate to the regions of damage, where they generate a population of young cardiomyocytes, reversing partly the aging myopathy. The senescent heart phenotype and heart failure are corrected to some extent, leading to prolongation of maximum lifespan.
Key Words: aging cardiomyopathy • cardiac stem cells • telomere–telomerase system
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Heart failure is the leading cause of death in the elderly, but whether this is the result of a primary aging myopathy or the consequence of coronary artery disease is unknown. In humans, it is difficult to separate the inevitable pathology of the coronary circulation with age from the intrinsic mechanisms of myocardial aging and heart failure. The aging heart typically shows a decreased functional reserve and limited capacity to adapt to cardiac diseases. Although the old heart cannot accommodate sudden increases in pressure and volume loads,1 the critical question is whether the etiology of ventricular decompensation in the elderly is the product of aging-associated events or the result of primary aging effects on the pool and function of cardiac progenitor cells (CPCs). A related question is whether average lifespan reflects the ineluctable genetic clock2 or whether heart failure interferes with the programmed death of the organ and organism, negatively affecting lifespan.
With the exception of a few hematological disorders,3 stem cell failure does not occur in self-renewing organs, including the human heart. Functionally competent CPCs are present in the hearts of patients who die acutely after a large myocardial infarct or undergo cardiac transplantation for end-stage ischemic cardiomyopathy.4 Similarly, cycling CPCs with long telomeres have been identified in the old decompensated human heart.5 Functional CPCs may not sense signals from the regions of damage, or their activation, growth, and migration may be impaired, and these variables interfere with the ability of resident CPCs to repair the old heart and preserve its youth. These issues have been addressed in the current study, in which an animal model of normal aging has been used to determine the role that CPCs have in physiological aging and establish whether chronological age impacts on the number and properties of CPCs. Additionally, we tested whether myocardial aging can be reversed by repopulating the senescent heart with new myocytes derived from differentiation of locally activated CPCs. Ultimately, the profound restructuring of the old heart was expected to prolong animal lifespan.
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Materials and Methods |
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Male Fischer rats (Harlan, Indianapolis, Ind) of different ages were characterized physiologically, structurally, and biochemically. Subsequently, animals were injected intramyocardially with hepatocyte growth factor (HGF) and insulin-like growth factor (IGF)-1 to establish the effects of this intervention on myocardial regeneration, organ aging, and animal lifespan. Experimental protocols are described in the online data supplement, available at http://circres.ahajournals.org.
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Results |
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CPC Aging and Death
To define whether myocardial aging is conditioned by alterations in CPC function, we measured the number of CPCs together with the expression of the aging-associated protein p16INK4a6 in male rats at 4, 12, 20, and 28 months. CPCs are lineage-negative (Linneg) cells that express the stem cell antigens c-kit, MDR-1, and Sca-1, alone or in combination.7 Linneg CPCs are clustered in cardiac niches,8 which are located predominantly in the atria and apex. This study is mostly restricted to c-kit–positive CPCs because of their superior growth and differentiation behavior.9
From 4 to 28 months, CPCs increased 
2.9-fold and p16INK4a-positive CPCs 12-fold. Similarly, apoptosis increased with age and was restricted to p16INK4a-positive CPCs. As a result, the number of functionally competent CPCs remained constant up to 20 months and decreased sharply at 28 months (Figure I in the online data supplement).
CPC Activation and Differentiation
The aging myopathy is characterized by an increased myocyte mitotic index5 that reflects CPC activation and lineage commitment.10 Myocyte progenitors/precursors correspond to differentiating CPCs that express c-kit and transcription factors and cytoplasmic proteins specific to myocytes. This documents the linear relationship between CPCs and forming myocytes. CPCs positive for the myocyte transcription factor MEF2C, ie, myocyte progenitors, or both MEF2C and the sarcomeric protein cardiac myosin heavy chain (MHC), ie, myocyte precursors, increased with age (supplemental Figure I).
Pulse-chase 5-bromodeoxyuridine (BrdUrd)-labeling assay was performed8 because this protocol, together with the identification of c-kit, allowed us to assess whether the senescent heart contained functionally competent CPCs with the properties of stem cells.11 Rats at 4 and 27 months were exposed to BrdUrd for 7 days, and BrdUrd-positive CPCs were measured at 7 days and after 12 weeks of chasing. Bright and dim BrdUrd-labeled CPCs were discerned by fluorescence intensity to score long-term label-retaining CPCs; this provides a functional identification of resident stem cells.11
Recently, the long-term BrdUrd-retaining assay was challenged. The argument was made that isolation of hematopoietic stem cells (HSCs) by fluorescence-activated cell sorting is superior in specificity and sensitivity to the analysis of stem cells at the single-cell level within tissues.12 Unfortunately, the intensity of BrdUrd fluorescence after chasing was not evaluated. Instead, the number of cells with "detectable" BrdUrd levels was measured. Arbitrarily, the threshold of BrdUrd labeling was set at 6% of the signal of BrdUrd-positive cells at the end of the pulse period; the protocol was arranged to identify BrdUrd-positive cells not to distinguish the distribution of BrdUrd levels in HSCs. It is unrealistic that all examined HSCs experienced the same number of divisions and had identical levels of BrdUrd incorporation. By design, preservation of the BrdUrd signal or its dilution with time was not determined, defeating the purpose of the BrdUrd-retaining assay.
In the current study, after 12 weeks of chasing, the number of BrdUrd-bright CPCs detected at 7 days decreased 86% and 93% in young and old animals, respectively. At 7 days, 532 and 2012 BrdUrd-bright CPCs were found in young and old hearts, respectively. Corresponding values at 12 weeks were 73 and 140; they constituted slow-cycling stem cells (Figure 1A and 1B). From 7 days to 12 weeks, BrdUrd-dim CPCs increased 10-fold in young and 32-fold in old hearts. The number of BrdUrd-bright and BrdUrd-dim CPCs did not change in 12 weeks in young but increased in old animals. Thus, the growth kinetics of CPCs preserves the pool of primitive cells in the young heart but expands this compartment in the old myocardium. The number of BrdUrd-bright CPCs in other self-renewing organs, including the skin and ocular bulb, is several orders of magnitude higher than in the heart.13,14 However, the heart and bone marrow15 have similar values.
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-sarcomeric actin, BrdUrd-positive myocytes were measured at 7 days (supplemental Figure IIA) and 12 weeks. BrdUrd-bright myocytes at 12 weeks were cells that experienced a limited number of divisions, whereas BrdUrd-dim myocytes were considered the progeny of CPCs, which became BrdUrd-positive at the time of exposure and formed a large number of committed cells. Cells with intermediate BrdUrd levels were assumed to represent amplifying myocytes that incorporated BrdUrd at the time of exposure and continued to divide and differentiate. Scattered BrdUrd-positive myocytes were observed at 7 days. Following chasing, clusters of BrdUrd-dim myocytes together with BrdUrd-bright myocytes were detected predominantly in old animals. The percentage of BrdUrd-positive myocytes was 5.3-fold higher in old than in young hearts (Figure 1C and 1D). In both cases, the number of BrdUrd-bright myocytes decreased markedly with chasing, whereas BrdUrd-dim myocytes increased.
The high level of myocyte formation in old hearts was confirmed by the myocyte mitotic index measured in situ and in isolated cells (Figure 1E and 1F and supplemental Figure II). It is remarkable that 45% of myocytes were replaced in 12 weeks in senescent animals; this value was 5-fold larger than in young rats. However, in spite of this dramatic increase in new myocytes, cell death exceeds myocyte regeneration and the aging myopathy cannot be prevented.5,16 Additionally, the increased number of cycling cells in the senescent heart is consistent with the enhanced toxicity of anticancer drugs in the elderly.17
CPC and Myocyte Senescence
The accumulation of p16INK4a-positive myocytes with age16 may be mediated by differentiation of CPCs with short telomeres, which form a myocyte progeny that rapidly reaches cellular senescence. Therefore, telomere length was measured in c-kit–positive cells (Figure 2A through 2C) and developing myocytes expressing the cell cycle protein Ki67 (supplemental Figure III). Telomere length in CPCs, myocyte progenitors/precursors and developing myocytes was 30%, 35%, and 51% shorter in old than young cells, respectively (Figure 2D); 50% of old and 15% of young CPCs had telomeres less than 12 kbp and were p16INK4a-positive. However, 20% of old CPCs had telomeres >18 kbp, pointing to a relevant growth reserve of the senescent myocardium. Thus, telomere attrition in CPCs with age leads to the generation of a myocyte progeny that acquires quickly the senescent phenotype conditioning organ aging.
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12 kbp and 
18 kbp. Fractions of cycling cells (green solid bars) and senescent p16INK4a-positive cells (red solid bars) are shown.
CPC Aging and Growth Factor Receptor Systems
The IGF-1/IGF-1R pathway preserves telomere length and promotes CPC growth and survival,16 whereas CPC migration and homing are predominantly modulated by the HGF/c-Met receptor system.9 Although the consequences of angiotensin (Ang) II on CPCs are unknown, this growth factor decreases the number and function of endothelial progenitor cells, triggers apoptosis, and is implicated in the progression of heart failure.18 Therefore, we determined whether aging alters IGF-1/IGF-1R and HGF/c-Met in CPCs and whether CPCs possess a local renin–angiotensin system (RAS), which is influenced by age.
CPCs express IGF-1R, c-Met, and Ang II type 1 (AT1) receptors, together with IGF-1, HGF, and Ang II (supplemental Figure IV). Detection of Ang II, IGF-1, and HGF in freshly isolated CPCs and tissue sections cannot discriminate whether growth factors are formed within cells or sequestered from the circulation. However, transcripts for renin, angiotensinogen (Aogen), and AT1 receptor were detected by real-time RT-PCR in CPCs isolated from hearts at 3, 12, 16, and 24 months (Figure 3A and supplemental Figure V). CPC aging resulted in downregulation of Aogen and AT1 receptors, whereas renin mRNA increased at 12 and 16 months, returning to baseline at 24 months. Although changes in mRNAs occurred with age, the protein levels of Aogen and AT1 receptors did not vary (Figure 3B), suggesting that RAS function remained intact in old CPCs.
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The CPC IGF-1/IGF-1R system was characterized by a significant decrease in IGF-1R mRNA with aging, whereas IGF-1 expression was variable and tended to be reduced only at 24 months. c-Met transcripts were modestly affected in aging CPCs, but HGF mRNA was attenuated at 24 months (Figure 3C and supplemental Figure V).
The ability of CPCs to synthesize IGF-1, HGF, and Ang II was measured. Baseline values for IGF-1 were similar in young and old CPCs, but HGF tended to be lower in old cells. Ang II levels, however, were 3-fold higher in old than in young CPCs (Figure 3D). Following ligand stimulation, IGF-1 formation was 8-fold higher in cells at 3 than at 27 months, whereas HGF synthesis increased 4-fold in young and 3.5-fold in old CPCs. Ang II stimulation did not increase octapeptide synthesis in either cell population. Thus, aging negatively affects regulatory systems involved in CPC growth, survival, and migration, potentiating the detrimental consequences of the local RAS.
CPC Division and Apoptosis
CPCs were cultured in serum-free medium and stimulated with Ang II, IGF-1, HGF, or IGF-1 and HGF together (IGF-1/HGF). The ability of IGF-1, HGF, and IGF-1/HGF to induce CPC proliferation was attenuated but not abolished in old CPCs. Ang II had no growth-promoting effects on CPCs (Figure 4A). Ang II stimulated CPC apoptosis, and IGF-1 decreased the extent of Ang II–mediated CPC death. Conversely, HGF did not decrease apoptosis or enhanced the effects of IGF-1 on CPC survival. Although the inhibitory role of IGF-1 in CPC apoptosis was higher in young than in old cells, a 40% reduction in apoptosis was measured in old CPCs.
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P<0.05 vs serum-free medium.A question concerned the impact of Ang II on young and old CPCs because the rate of apoptosis triggered by Ang II was comparable in the 2 cell populations. Ang II leads to the generation of hydroxyl radical, which promotes deoxyguanosine (dG) oxidation, a process that may vary in young and old CPCs. In the presence of hydroxyl radical, the formation of 8-OH-dG lesions is 5-fold higher in telomeric than in nontelomeric DNA.19 Importantly, 8-OH-dG was detected in a larger fraction of old than young CPCs (Figure 4B through 4D) and Ang II further enhanced this phenomenon, providing a mechanism for Ang II–mediated DNA damage with age.
Aging, CPC Heterogeneity, and Growth
We then established whether these growth factor receptor systems were uniformly affected in aging CPCs or aging progressively involved a larger number of CPCs leaving intact a subset of progenitor cells. The fraction of CPCs positive for IGF-1, IGF-1R, HGF, and c-Met decreased from 3 to 28 months, and the percentage of CPCs expressing Ang II and AT1 receptors increased (Figure 5A). CPCs expressing IGF-1/IGF-1R and HGF/c-Met were consistently negative for p16INK4a, whereas p16INK4a was detected in CPCs positive for Ang II and AT1 receptors.
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To test whether a small proportion of old CPCs possessed a growth potential similar to young CPCs, CPCs at 3 and 27 months were serially passaged to reach 20 population doublings. Although the lag-growth phase was longer in old CPCs, the exponential-growth phase was similar in both cell classes (Figure 5B). Similarly, BrdUrd labeling at passage P7 to P8 resulted in comparable levels of BrdUrd-positive CPCs, and telomerase activity in old CPCs decreased only 33% (Figure 5C). Thus, myocardial aging does not deplete the pool of functionally competent CPCs.
CPC Aging and Cell Mobilization
The presence of a compartment of nonsenescent CPCs in the aged heart raised the possibility that these cells may be activated and induced to translocate from their sites of storage in the atria and apex to the base/midregion of the left ventricle (LV). A retroviral vector encoding enhanced green fluorescent protein (EGFP) was injected into the atrioventricular groove to label replicating cells in animals at 4 and 27 months (supplemental Figure VI). A retrovirus was used to infect only replicating cells and avoid cell cycle activation in terminally differentiated myocytes. In both cases, 9% to 12% c-kit–positive CPCs were infected with EGFP; this value was consistent with the fraction of Ki67-positive CPCs in this region (data not shown). Two days later, 3 increasing concentrations of HGF were administered from the site of CPC accumulation in proximity of the atrioventricular groove to the LV midregion (supplemental Figure VI). The migration of EGFP-positive cells was then determined by 2-photon microscopy (supplemental Figures VII through X).
The myocardial area examined by 2-photon microscopy was analyzed subsequently by confocal microscopy to characterize the identity of the migrated EGFP-positive cells. Translocated EGFP-positive cells expressed c-kit, MDR1, or Sca-1 together with c-Met. Moreover, GATA-4 was detected in some cells, documenting their commitment to the myocyte lineage. Ki67 was present in a subset of EGFP-positive cells (supplemental Figures VIII and X).
To determine whether translocation of EGFP-positive cells occurred through the coronary circulation, myocardial interstitium, or both, the coronary vasculature was perfused with rhodamine-labeled dextran, and HGF was injected at the time of observation. Over 5 to 6 hours, none of the moving EGFP-positive cells was found within the lumen of coronary vessels (Figure 6A and supplemental Figure XI); CPCs migrated through the interstitium within tunnels defined by fibronectin.
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HGF mobilized and translocated CPCs from the atrioventricular groove toward the LV midregion. Two aging effects were observed: the speed of migration and the number of migrating CPCs were significantly higher in young than in old hearts (supplemental Figure XII). Because of the growth-promoting effects of IGF-1 on CPCs,9 IGF-1 was injected alone or in combination with HGF, and the number and rate of migration of EGFP-positive cells was determined together with the fraction of cycling EGFP-positive cells. In young and old hearts, IGF-1 failed to stimulate the locomotion of CPCs and to increase the migratory ability of HGF. However, IGF-1 significantly increased the number of dividing CPCs in both young and old hearts (supplemental Figure XII).
Migrating CPCs and their early committed progeny had long telomeres and were p16INK4a-negative. This was in contrast to the properties of nontranslocated CPCs in the LV midregion of control hearts. These cells had short telomeres and frequently expressed p16INK4a (supplemental Figure XII). These results suggest that functionally competent CPCs were stored in atrial niches, whereas aging effects were more prominent in the midregion of the LV myocardium.
CPC Aging and Myocardial Regeneration
To test whether the negative effects of aging on the heart could be reversed by activation of resident CPCs, HGF and IGF-1 were injected intramyocardially into rats at 15, 20, and 27 months of age (supplemental Figure XIII). The animals were euthanized 45 days later. Two days before growth factor administration, the atrioventricular groove and the apex were injected with the EGFP–retrovirus to label cycling CPCs.
In all treated rats, EGFP-positive myocytes, coronary arterioles, and capillaries were identified in the LV midregion (Figure 6B and 6C and supplemental Figure XIV). Conversely, EGFP-positive myocytes and vessels were absent in untreated animals. Frequently, clusters of regenerated myocytes replaced foci of myocardial damage (Figure 6D through 6G). Quantitatively, the number of EGFP-positive myocytes was significantly higher in rats at 28 to 29 months than at 16 to 17 and 21 to 22 months. A similar response was observed for coronary vessels (supplemental Figure XIV).
EGFP-labeled structures reflected only partly the extent of tissue regeneration because 9% to 12% CPCs were infected by the EGFP–retrovirus. Thus, BrdUrd was given after the delivery of growth factors or vehicle and was continued throughout to assess cumulative myocyte and vessel formation. Without treatment, myocyte and vessel growth increased with age, pointing to the ability of the old heart to react to tissue injury. Growth factor administration increased cardiomyocyte formation by 55%, 66%, and 88% at 16 to 17, 21 to 22, and 28 to 29 months, respectively (Figure 7A). Vessel regeneration also occurred (supplemental Figure XV). In treated hearts at 28 to 29 months, new myocytes decreased by 20% the number of p16INK4a-positive cells (Figure 7B), and this change reflected the increase in BrdUrd-labeled cells.
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Remodeling of the Aging Heart and Mortality
Myocardial regeneration mediated by CPC activation attenuated ventricular dilation and the decrease in ventricular mass-to-chamber volume ratio (supplemental Figure XVI), resulting in improvement of cardiac function in animals at 28 to 29 months. Without treatment, heart failure at 27 months deteriorated further at 28 to 29 months. Following treatment, the alterations in ventricular pressures, dP/dt, and diastolic stress at 27 months were no longer apparent at 28 to 29 months (Figure 7C). The anatomy and function of treated hearts at 28 to 29 months became similar to the anatomy and function of untreated hearts at 16 to 17 months (Figure 8A).
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Echocardiography was performed in rats at 27 months, 1 day before treatment, and 45 days later, before euthanasia, at 28 to 29 months. Therapy significantly decreased end-diastolic and end-systolic LV diameters, whereas ejection fraction increased 12 percentage points, from 67±7% to 79±7%. The improvement in cardiac function was apparent when early and late echocardiograms were compared (supplemental Figure XVII and Movies I and II). In untreated rats, ventricular hemodynamics deteriorated with time (Figure 8B).
A mortality study was conducted in a cohort of 32 untreated and 48 treated rats at 27 months. By 31 months, all untreated rats were dead. However, 28% of treated rats were alive at 31 months and the last animal died at 33 months (Figure 8C). Growth factor treatment increased life expectancy at 27 months by 44%, from 57 to 82 days.
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Data in the present study indicate that CPC aging conditions myocardial aging and heart failure. Chronological age leads to telomeric shortening in CPCs, which generate a progeny that rapidly acquires the senescent phenotype. Daughter cells inherit the shortened telomeres of maternal CPCs and, after a few rounds of division, express the senescence-associated protein p16INK4a. The pool of old cardiomyocytes progressively increases, and ventricular function is impaired. These observations in rats are consistent with findings in humans experiencing aging myopathy.5,20 Progenitor cell aging affects the function of the brain, pancreas, and bone marrow,6 which, together with current results in the heart, point to stem cell dysfunction as a critical determinant of organ and organism aging. However, telomerase-competent CPCs with long telomeres are present in regions of storage in the atria and apex, and following activation by growth factors, migrate to areas of damage, where they create a population of young myocytes, reversing, to some extent, the aging myopathy structurally and functionally. The senescent heart phenotype is partially corrected and the improvement in cardiac performance results in prolongation of maximum lifespan.
The loss of CPC function with aging is mediated partly by an imbalance between factors promoting growth, migration, and survival and factors enhancing oxidative stress, telomere attrition, and death. Three growth-factor receptor systems appear to play a role in the development of CPC senescence and myocardial aging: IGF-1/IGF-1R, HGF/c-Met, and RAS. The IGF-1/IGF-1R induces CPC division, upregulates telomerase activity, hinders replicative senescence, and preserves the pool of functionally competent CPCs.16,21 The expression of IGF-1R and the synthesis of IGF-1 are attenuated in aging CPCs, diminishing the ability of IGF-1 to activate cell growth and interfere with oxidative damage and telomeric shortening.22 8-OH-dG accumulates in old CPCs, favoring telomere dysfunction, cellular senescence, and growth arrest. Additionally, the expression and secretion of HGF in CPCs decreases with age, impacting on their ability to translocate to areas of damage and promote cardiac repair.23 Defects in these 2 autocrine–paracrine effector pathways may have profound consequences on CPC function and may account for myocyte death, myocardial scarring, and depressed performance of the old heart.20
The recognition that a local RAS is present in CPCs and the formation of Ang II is enhanced in old cells provides evidence in favor of the role of this octapeptide in CPC senescence and death. Ang II may contribute to the age-dependent accumulation of oxidative damage in the heart.24 Inhibition of Ang II function positively interferes with heart failure and prolongs life in humans.18 Ang II generates reactive oxygen species, and the most prominent form of DNA damage induced by free radicals is 8-OH-dG. In the presence of Ang II, 8-OH-dG increases more in old than in young CPCs. 8-OH-dG accumulates at the GGG triplets of telomeres, resulting in telomeric shortening and uncapping,19 and loss of telomere integrity is the major determinant of cellular senescence and death. Conversely, IGF-1 interferes with reactive oxygen species generation,24 decreases oxidative stress with age,16 and can repair DNA damage by homologous recombination.25
The recognition that IGF-1 and HGF modify the effects of aging on CPC behavior, myocardial composition, ventricular performance, and maximum lifespan points to IGF-1 and HGF as potential therapeutic targets of the aging myopathy. Although it is impossible to discriminate the contribution of each growth factor to the reversal of myocardial aging in the rat model, cardiac-restricted overexpression of IGF-1 delays the aging myopathy and the manifestations of heart failure in transgenic mice.16 These findings are at variance with the notion that IGF-1 promotes premature aging in fruit flies and nematodes.26 In these lower organisms, attenuation of insulin/IGF-1 signaling prolongs lifespan. However, with adulthood, Caenorhabditis elegans and Drosophila lose the regenerative capacity of somatic tissues, which makes comparisons with mammals hardly feasible.27 Dying cells cannot be replaced, and this results in a rapid and progressive decline in organ function. Conversely, cell turnover by activation and commitment of resident progenitor cells remains active in mammals throughout life and IGF-1 potentiates regeneration in susceptible cells. The impact of IGF-1 on terminally differentiated cells is complex and only partially understood. The growth-promoting effects of IGF-1 may be repressed, and metabolic insulin signals may predominate,28 enhancing oxidative stress and apoptosis.
The modest increase in lifespan of mice heterozygous for the deletion of IGF-1R is restricted to the female cohort, and the lack of evidence in males remains unexplained.29 Animal models with deficiencies in IGF-1 are complicated by the presence of hypopituitarism, which results in multiple developmental abnormalities.30 These confounding factors make the dwarf mice inappropriate for the assessment of the role of IGF-1 in biological aging and lifespan.30 In fact, women live several years more than men, have a reduced incidence of heart failure, and have a better prognosis in the presence of heart failure.18 The female heart is more resistant to aging than the male heart and has an enhanced expression and nuclear localization of Akt,31 powerful survival factor and distal effector of IGF-1. Increased levels of IGF-1 characteristically decrease the incidence of heart failure and mortality in the elderly population.32 Hormone-replacement therapy with restoration of IGF-1 levels has significant health benefits.30 As suggested by the current results, the effects of IGF-1 on the human heart may involve the CPC compartment potentiating their ability to divide and differentiate. Translation of results in simple postmitotic organisms to mammals in which the life and death of somatic organs is regulated by a stem cell compartment is open to question.
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Acknowledgments |
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Sources of Funding
This study was supported by NIH grants. A.G. is the recipient of a Norman R. Alpert Visiting Scientist Award from the American Heart Association and the European Society of Cardiology.
Disclosures
None.
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Footnotes |
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This manuscript was sent to James T. Willerson, Consulting Editor, for review by expert referees, editorial decision, and final disposition.
*These authors contributed equally to this work. 
Original received October 6, 2007; revision received December 8, 2007; accepted January 7, 2008.
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