Published online before print January 15, 2004, doi: 10.1161/01.RES.0000117306.10142.50
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© 2004 American Heart Association, Inc.
Integrative Physiology |
Cardiac Stem Cell and Myocyte Aging, Heart Failure, and Insulin-Like Growth Factor-1 Overexpression
From the Cardiovascular Research Institute (D.T., M.R., D.N., E.M., A.M., E.Z., K.U., B.N.-G., J.K., P.A., A.L.), New York Medical College, Valhalla, NY; Cardiology Division (A.R.), Massachusetts General Hospital, Harvard Medical School, Boston, Mass; Molecular Cardiology (K.W.), Boston University, Boston, Mass; SDSU Heart Institute and Department of Biology (I.S., M.A.S.), San Diego State University, San Diego, Calif.
Correspondence to Annarosa Leri, MD, Cardiovascular Research Institute, Department of Medicine, Vosburgh Pavilion, Room 318, New York Medical College, Valhalla, NY 10595. E-mail annarosa_leri{at}nymc.edu
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Abstract |
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To determine whether cellular aging leads to a cardiomyopathy and heart failure, markers of cellular senescence, cell death, telomerase activity, telomere integrity, and cell regeneration were measured in myocytes of aging wild-type mice (WT). These parameters were similarly studied in insulin-like growth factor-1 (IGF-1) transgenic mice (TG) because IGF-1 promotes cell growth and survival and may delay cellular aging. Importantly, the consequences of aging on cardiac stem cell (CSC) growth and senescence were evaluated. Gene products implicated in growth arrest and senescence, such as p27Kip1, p53, p16INK4a, and p19ARF, were detected in myocytes of young WT mice, and their expression increased with age. IGF-1 attenuated the levels of these proteins at all ages. Telomerase activity decreased in aging WT myocytes but increased in TG, paralleling the changes in Akt phosphorylation. Reduction in nuclear phospho-Akt and telomerase resulted in telomere shortening and uncapping in WT myocytes. Senescence and death of CSCs increased with age in WT impairing the growth and turnover of cells in the heart. DNA damage and myocyte death exceeded cell formation in old WT, leading to a decreased number of myocytes and heart failure. This did not occur in TG in which CSC-mediated myocyte regeneration compensated for the extent of cell death preventing ventricular dysfunction. IGF-1 enhanced nuclear phospho-Akt and telomerase delaying cellular aging and death. The differential response of TG mice to chronological age may result from preservation of functional CSCs undergoing myocyte commitment. In conclusion, senescence of CSCs and myocytes conditions the development of an aging myopathy.
Key Words: telomerase • telomere dysfunction • cellular senescence
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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The accepted but never proven paradigm is that the heart is a postmitotic organ characterized by a predetermined number of myocytes, which is defined shortly after birth and is preserved throughout life till death of the organism.1 According to this view, age of cardiomyocytes corresponds to the age of the organ and organism, ie, without exception, cellular, organ, and organism age coincide. Myocytes must age at the same pace and, at any given time, the heart should be composed of a homogeneous population of myocytes of identical age. Therefore, myocardial aging has been interpreted as a time-dependent biological process that interacts with ischemic heart disease, hypertension, diabetes, and other pathological conditions, which together define the clinical phenotype.2 The possibility that cardiac aging is an independent determinant of morbidity and mortality has faced opposition and emphasis has been placed on age-associated changes, which increase the chances of cardiovascular events in the elderly. Treatment of cardiac diseases in old patients has resulted in a prolongation of average lifespan. However, maximum lifespan has not increased in the last 70 years,3 suggesting that cellular aging may play a more important role than generally expected.
Several lines of evidence have been obtained in favor of the regeneration of the adult and aged myocardium4 through the activation of cardiac stem cells (CSCs).5,6 A mitotic clock regulates cellular lifespan that is not related to organ and organism age and lifespan,7 raising the possibility that CSC aging conditions myocyte aging and the development of an aging myopathy. Although this may be seen as an oversimplification of a complex problem, myocardial aging may result from senescence of myocytes due to attenuation in their turnover dictated by CSC aging and the inevitable accumulation of older cells. Therefore, the hypothesis being tested is whether an imbalance between myocyte growth and death induced by alterations in CSC function occurs with age resulting in a decompensated myopathy. Conversely, growth factors enhancing cell replication and attenuating CSC and myocyte senescence may delay the appearance of a cardiomyopathic old heart.
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Materials and Methods |
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An expanded Materials and Methods can be found in an online data supplement available at http://circres.ahajournals.org.
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Myocyte Aging
Cellular aging is characterized by the expression of nuclear proteins involved in cell cycle inhibition and irreversible growth arrest. Growth factors promoting cell division may interfere with cellular senescence.8 A most likely candidate is insulin-like growth factor-1 (IGF-1) whose overexpression in myocytes potentiates new myocyte formation in the adult heart.9 Significant differences may exist in the phenotype of aging myocytes in wild-type (WT) and IGF-1 transgenic (TG) homozygous mice at 4, 10 to 12, and 20 to 22 months of age. This likelihood would strengthen the notion that senescence of parenchymal cells results in an aging myopathy, distinct from age-associated diseases.2
During aging, fibroblasts10 are arrested at G0-G1 by transient expression of p21Cip1 followed by a temporary upregulation of p27Kip1 and then by persistent elevation of p16INK4a. A comparable role for p19ARF and p53 was recognized in cellular aging.11 The regulation of p21Cip1 can be p53-dependent or -independent12 and p19ARF increases p53 quantity and activity13 by decreasing Mdm2 expression.
A similar pattern of gene expression was found in vivo in myocytes from aging WT: p21Cip1 decreased from 4 to 20 to 22 months, whereas p27Kip1, p16INK4a, and p19ARF increased. The increase in p19ARF was coupled with an upregulation of total and free p53 protein, which was paralleled by a decrease in the formation of Mdm2-p53 inactive complexes (Figure 1). IGF-1 opposed these age-dependent alterations in myocytes. With respect to WT, p21Cip1 was detected in lower quantities in TG myocytes at 4 and 10 to 12 months and was undetectable at 20 to 22 months. p27Kip1, p16INK4a, and p19ARF were absent at 4 months and present in modest levels at 10 to 12 and 20 to 22 months. p53 increased moderately with age and Mdm2-p53 complexes remained high in TG myocytes.
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Myocyte Size, Oxidative Stress, p53, p16INK4a, and Myocyte Death
In the aging failing heart, a subpopulation of myocytes undergoes DNA replication and mitosis, another undergoes hypertrophy, and another cell death.14 In animals and humans, small myocytes are the only myocytes capable of replicating DNA.5,14,15 There is an upper limit in myocyte volume, 
90 000 µm3, and cross-sectional area, 600 to 900 µm2, beyond which no further hypertrophy occurs.5,14 They are old cells positive for p16INK4a and p53.15 The high proficiency of these myocytes to die might be linked to accumulation of oxidative products that exceeds cellular repair mechanisms. The interdependence of oxidative damage and cell death is well established in several systems including myocytes.15,16 However, the role of cell dimension and age in this process remains to be determined.
Oxidative stress, identified by 8-hydroxy-2'-deoxyguanosine (8-OH-dG), varied from 200 to 1000 pixels (pixelsxfluorescence/fluorescence of propidium iodide per nucleus) in apoptotic myocytes with cross-sectional area 400 to 600 µm2. Higher signals were detected in apoptotic myocytes with cross-sectional area 700 to 900 µm2 (Figures 2A and 2B). With myocyte necrosis (Figures 2C and 2D), the oxidative challenge was higher ranging from 600 to 5000 pixels. In both forms of death, the intensity of the 8-OH-dG signal correlated directly with myocyte size, p16INK4a, p53, and age (Figure 2E).
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Indicate statistical difference (P<0.05) versus WT and animals at 4 months, respectively. F, Three nuclei express p16INK4a (green dots; arrowheads) and two of the three (G) are labeled by hairpin 1 (white dots, arrowheads). Apoptotic nuclei belong to large myocytes. H and I, Myocyte necrosis: yellow dots (I, arrowheads) correspond to hairpin 2.Myocyte apoptosis increased 4-fold from 4 (149±60/106) to 10 to 12 (622±174/106) months and 12-fold from 4 to 20 to 22 (1804±352/106) months (P<0.001). Corresponding increases in myocyte necrosis were 4-fold (317±70/106 versus 1318±352/106) and 30-fold (317±70/106 versus 9510±2167/106) (P<0.001). Cell necrosis was 2-fold, 2-fold, and 5-fold greater than apoptosis at 4, 10 to 12, and 20 to 22 months, respectively (P<0.001). Dying myocytes were p16INK4a and p53 positive (Figures 2F and 2G), but not all myocytes expressing these proteins were dying: p16INK4a-positive myocytes were 25±6%, 42±14%, and 82±19%, and p53-positive myocytes were 5±4%, 9±6%, and 15±8% in WT mice at 4, 10 to 12, and 20 to 22 months, respectively.
IGF-1 decreased myocyte death. In comparison with WT, myocyte apoptosis in TG was 11% (132±33/106; NS), 68% (197±44/106), and 78% (396±96/106; P<0.001) lower at 4, 10 to 12, and 20 to 22 months, respectively. Decreases in myocyte necrosis in TG were 7% (296±50/106; NS), 71% (377±87/106), and 93% (666±121/106; P<0.001). Again, cell death correlated directly with 8-OH-dG, cell size, p16INK4a, p53, and age (Figures 2H and 2I). TG mice had lower percentages of p16INK4a (9±5%, 15±7% and 35±13%) and p53 (3±1%, 5±2% and 9±4%) positive myocytes.
Telomere Dysfunction, Senescence, and Death of Myocytes
Alterations in telomeres result in inhibition of cell replication, myocyte senescence, and death.5,14,15 Telomeres are protected by a complex of proteins including TRF1, TRF2, uncleaved poly(ADP-ribose) polymerase (PARP) and DNA-dependent protein kinase (DNA-PK). Telomere-binding proteins have not been yet identified in myocytes; if present, a decrease in their levels would indicate defects in telomere integrity17,18 of noncycling myocytes. This could provide a link between telomere shortening or uncapping and senescence and death of these non-cycling myocytes. In fact, TRF2 decreased in WT myocytes from 4 to 10 to 12 months becoming undetectable at 20 to 22 months. Cleaved PARP increased at 20 to 22 months, exceeding the uncleaved form. Telomere dysfunction was aggravated by downregulation of the components of DNA-PK with age: heterodimeric complex Ku86/Ku70 and DNA-PK catalytic subunit. TRF1 was not influenced by aging. The impact of aging on telomeric proteins was attenuated in TG myocytes (Figure 3).
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Hypophosphorylated RB was predominant in aging WT myocytes reflecting the upregulation of p16INK4a. IGF-1 produced an opposite effect; phosphorylated RB was several-fold higher than hypophosphorylated RB and the phosphorylated-to-hypophosphorylated RB ratio increased with age (Figure 3). This condition may delay myocyte hypertrophy, senescence, and death.
Telomere length is another parameter that regulates cell division and survival in cycling cells17,18 and might regulate survival of differentiated myocytes. Telomere length in myocyte nuclei, cell cross section, and p16INK4a and p53 expression were plotted as a function of age. There was an inverse relationship between myocyte size and telomere length in WT and TG at 4 and 20 to 22 months of age; larger cells with shorter telomeres were p16INK4a and p53 positive (Figures 4A through 4E; online data supplement). IGF-1 interfered with the age-dependent increases in myocyte size, telomeric shortening and p16INK4a and p53 proteins. In comparison with young cells, telomere length decreased more in old WT than in TG myocytes.
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Telomerase Activity and Aging
Telomerase protects chromosomes from telomere erosion, maintains cell replication, and opposes cell death.19,20 Telomerase competent myocytes are cycling and are p16INK4a and p53 negative. Telomerase identifies young myocytes, actively growing and replacing old dying myocytes.5,14,19 The telomere-telomerase system is a cell-autonomous determinant of organ aging. The catalytic subunit of telomerase was higher in TG than WT myocytes (Figure 5
A). Telomerase activity in WT myocytes decreased from 4 to 20 to 22 months, but increased in TG myocytes. Moreover, enzyme activity was higher in TG than in WT myocytes at each age (Figures 5B through 5D).
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IGF-1 activates the PI3K-Akt pathway, and this signaling mechanism may phosphorylate and activate telomerase in TG myocytes. We identified a consensus site for Akt phosphorylation, RVRLRELSQE (amino acids 585 to 594), in the mouse telomerase, suggesting that Akt potentiates telomerase function.21 Thus, the amino acid sequence was labeled with fluorescein and used in an Akt-kinase assay. Noninfected myocytes and myocytes infected with an adenovirus carrying a nuclear (Akt-nuc) or membrane (Akt-myr) targeted Akt or dominant-negative form of Akt (Akt-DN) were used. Four observations were made (Figures 5E and 5
F): (1) At 20 to 22 months, noninfected TG myocytes had higher levels of phosphorylated telomerase sequence than WT myocytes; (2) Akt-nuc infection was involved in phosphorylation of telomerase sequence of WT myocytes; (3) Akt-DN decreased substrate phosphorylation in TG myocytes; and (4) peptide phosphorylation was proportional to Akt-nuc–infected WT myocyte proteins.
Phosphorylation of telomerase by Akt was determined in vivo to identify the role of Akt in telomerase function. Telomerase protein was immunoprecipitated from freshly isolated WT and TG myocytes and was then exposed to an antibody against its phosphorylated Akt consensus site (Figure 5G). Phosphotelomerase level decreased in WT myocytes, with age becoming barely detectable at 20 to 22 months. Conversely, phosphotelomerase increased progressively as a function of age in TG myocytes. At all intervals, phosphotelomerase was higher in TG than in WT myocytes, paralleling the results of telomerase activity described earlier (see Figures 5B through 5 D).
The expression of Akt was measured in aging WT and TG myocytes. Akt protein (total) remained constant and did not differ in WT and TG myocytes. However, phospho-Akt levels decreased in WT and increased in TG myocytes from 4 to 20 to 22 months (Figures 5H through 5 J). These results are consistent with attenuation and increase in telomerase activity in aging WT and TG myocytes, respectively. To confirm phospho-Akt function in the regulation of telomerase, a TRAP assay was performed in old WT myocytes infected with Akt-nuc or Akt-myr. Only Akt-nuc expression resulted in a marked elevation in telomerase activity. Increasing degrees of Akt-nuc infection of WT myocytes were accompanied by increases in telomerase activity. Akt-myr infection did not modify baseline telomerase in WT myocytes. Telomerase activity in old TG myocytes was severely depressed after infection with the Akt-DN adenovirus (Figures 5K through 5M).
Aging, Myocyte Growth, Number, and Contractile Performance
Myocyte aging in WT mice typically shows alterations in telomeres, telomerase activity, oxidative stress response, and expression of p16INK4a and p53. These intrinsic determinants of cellular senescence promote cell death and impair cell growth, compromising cardiac structure and function. Myocyte loss increases with age more than cell regeneration and the excess in cell dropout leads to ventricular decompensation, suggesting that the aging myopathy is mediated by molecular defects in myocytes. If this were the case, attenuation of cell death and/or potentiation of cell growth should preserve ventricular performance. These conditions are operative in IGF-1 TG mice.
For this purpose, the number of dying myocytes (hairpin 1 and hairpin 2) in the left ventricle was compared with the number of forming cells (BrdU). In WT, the number of dying myocytes was significantly higher than that of replicating cells and this difference increased from 4 to 20 to 22 months. Conversely, in TG, the values of cell death and cell proliferation were comparable at all ages. The rate of myocyte formation was 3-fold higher in TG, and the rate of cell death was 4-fold higher in WT. Therefore, the total number of ventricular myocytes decreased by 33% in WT at 20 to 22 months, whereas myocyte number remained constant in TG mice (Figure 6). Myocyte loss in aging WT was associated with elevation in LVEDP and a significant decrease in LVDevP and + and - dP/dt (Figure 6). Ventricular dysfunction was absent in TG mice; cell regeneration maintained constant myocyte number in the old heart. The cellular and functional indices of TG hearts at 20 to 22 months were comparable with those of WT hearts at 10 to 12 months.
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At all ages, TG myocytes had higher peak shortening, velocity of shortening, and relengthening than WT. Myocyte contractility decreased with age in WT and remained constant in TG, amplifying the difference between old WT and TG. Developed Ca2+ was higher in TG than WT myocytes at high rate of stimulation. L-type Ca2+ current differed in amplitude being higher in young and old TG than in WT, suggesting that TG myocytes have a more efficient SR with enhanced reuptake and improved diastolic relaxation. WT myocytes manifested a longer recovery from inactivation than TG with age (Figures 7A and 7B; online data supplement). Thus, the mechanical behavior of myocytes contributed to the preservation of ventricular hemodynamics in TG with age.
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CSC Aging
Clusters of primitive and early committed cells expressing the stem cell antigen c-kit were found in WT and TG myocardium (Figure 8A). These cells were lineage negative (see expanded Materials and Methods) or expressed the transcription factors GATA-4 and MEF2C, which are implicated in cardiac and myocyte differentiation, respectively.5,6,14 Moreover, 
-sarcomeric actin, cardiac myosin heavy chain, connexin43, and N-cadherin were detected. New small myocytes were telomerase competent and Ki67 positive (Figures 8B and 8C), representing amplifying cells that divided rapidly and differentiated.5,6,14 The recognition of cells positive for c-kit, GATA-4, and cardiac myosin provided an indisputable link between primitive cells, early differentiating myocytes, and mature parenchymal cells (Figure 8D).
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The number of c-kitPOS cells in the left ventricle was 47% lower in WT than in TG at 4 months and this difference increased to 56% at 20 to 22 months (Figure 8E). From 4 to 20 to 22 months, there was an increase of 1.9- and 2.3-fold in the number of c-kitPOS cells in WT and TG, respectively. However, the expression of p16INK4a was detected in 70% of WT c-kitPOS cells, indicating a block in their ability to reenter the cell cycle and divide. This phenomenon involved only 16% of TG c-kitPOS cells (Figure 8F). Importantly, apoptosis was 3-fold and 5-fold higher in WT than in TG c-kitPOS cells at 4 and 20 to 22 months, respectively. These results on cell death amplified the difference in the pool size of functionally competent CSCs between WT and TG. As in myocytes, all apoptotic c-kitPOS cells expressed p16INK4a (Figures 8G and 8H).
To estimate the length of telomeres in c-kitPOS cells, a Q-FISH with a probe complementary to the telomere sequence was performed. Nuclei with short telomeres expressed p16INK4a in both WT and TG c-kitPOS cells. The combination of short telomeres and p16INK4a was more prominent in WT than in TG c-kitPOS cells at 4 and 20 to 22 months. Additionally, the largest fraction of c-kitPOS cells labeled by p16INK4a was detected in old WT that had the shortest telomeres (Figure 8I).
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Discussion |
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Decompensated Aging Heart
The objective of this study was to establish whether aging of the heart leads to molecular modifications of myocytes that profoundly alter the characteristics and performance of the old heart resulting in a failing myopathy. These cellular age-dependent effects are dictated by senescence of CSCs, which become largely unable to divide and generate new functionally competent myocytes. CSC apoptosis further attenuates the growth reserve of the myocardium as well as its ability to replace dying myocytes and respond adequately to the increases in physiological loads imposed by myocyte dropout. Myocytes lose with time their youth; the myocyte compartment becomes progressively less heterogeneous with large cells comprising the majority of the myocardium. The normal spectrum of developing, maturing, and senescent myocytes, typical of the adult and middle-aged heart, is no longer apparent and hypertrophied myocytes predominate.
Cardiac aging may be viewed as a limitation in the replication and commitment of CSCs that affects the turnover of myocytes favoring cell loss and the accumulation of old poorly contracting cells. These cells cannot be adequately replaced by new amplifying myocytes undergoing cell division and maturation5,14 in order to meet the functional needs of the aged heart.2,22 A similar phenomenon is operative in other organs.11,18 Cell death coupled with an increased number of myocytes with depressed contractile behavior underlies the onset of cardiac decompensation. The results in WT are consistent with observations in rats4 and in large animals,23 although not all studies are in agreement.24 Heart failure in individuals 66 years of age and older, up to 103 years, has no other confounding variables in nearly 50% of the cases, suggesting that age may be a primary cause of cardiac decompensation and diastolic dysfunction.25
In TG mice, CSCs are more numerous and actively growing enhancing myocyte turnover, preserving myocyte number, attenuating the accumulation of hypertrophied myocytes, and increasing the number of new functionally efficient young cells. Together, these factors maintain ventricular function and prevent heart failure, strengthening the contention that cardiac aging occurs at the cellular level. These findings are at variance with the view that IGF-1 promotes premature aging in fruit flies, nematodes, and mice26 but are consistent with the notion that IGF-1 delays aging and heart failure in humans.27 However, it is difficult to discriminate whether changes in CSCs are primary or secondary events of the aging cardiomyopathy.
Myocyte Senescence
Cellular senescence is characterized by biochemical events that occur within the cell leading to growth arrest and loss of specialized cellular functions.11,18 These defects, such as repeated oxidative stress, may evolve into the activation of cell death or result in the development of a senescent phenotype. The old cells in the heart do not differ from those in other organs.28 Senescent cells cannot divide, their ability to synthesize proteins or secrete autacoids and hormones is reduced, and the antioxidant defense mechanisms and the DNA repair system are attenuated.29 In myocytes, in which cell replication is already lost, aging leads to a block in the capacity to hypertrophy.5,13 Myocyte mechanics and Ca2+ transients are altered indicating that the systems regulating activation, contraction and relaxation are defective. A preservation of myocyte mechanical behavior coupled with a reduced response to norepinephrine has been seen in aging Wistar rats.30,31 Importantly, there is an upper limit to myocyte growth; cells with cross-sectional areas 300 to 500 µm2 do not replicate but can hypertrophy, whereas myocytes 600 to 900 µm2 do not incorporate BrdU, do not hypertrophy and express p16INK4a and/or p53.5,14,15
p53 modulates apoptosis and senescence by increasing the expression of specific proteins, including Bax, Bad, and p21Cip1. Bax and Bad are implicated in apoptosis, whereas high levels of p21Cip1 are involved in irreversible growth arrest and cellular senescence.18 Low quantities of p21Cip1, however, result in transient inhibition of cell growth. p16INK4a blocks the cell cycle by maintaining RB in its hypophosphorylated state; RB interferes with the cell cycle by repressing the E2F transcription factors. Therefore, p16 INK4a is an ideal inducer of cellular senescence. The p16INK4a/RB pathway promotes also apoptosis by increasing p53 through Mdm2 downregulation.13,32 Why a cell undergoes apoptosis or senescence in response to identical stimuli is unclear. However, apoptosis and senescence are influenced by the intensity and duration of the inciting stimulus, the past history of the cell, and environmental cues.18 IGF-1 in myocytes counteracts these variables, limiting the expression of age-related genes and cell death inducers.
Myocytes, Telomeres, and Telomerase
Chromosomal ends contain long stretches of hexameric sequences, the telomeres, which bind specific proteins. Telomerase and telomere-binding proteins maintain telomere integrity and cell viability.15,19,20 The presence of short or uncapped telomeres discriminates old from young cells and conditions cell fate.19 In WT, enlarged cells with shortened and uncapped telomeres characterize myocyte senescence. Telomere attrition initiating the senescence program or cell death is identical in young and old myocytes but the number of cells involved differs in magnitude. Senescent WT myocytes have telomeres of 15 kbp instead of 30 kbp, express p16INK4a, p53, and show modest levels of 8-OH-dG in the nucleus. However, cell death is coupled with high levels of 8-OH-dG. IGF-1 overexpression does not change the properties of senescent and dying myocytes in terms of telomere length and expression of p16INK4a and p53. IGF-1 reduces myocyte senescence and death; telomerase activity is enhanced in younger myocytes and this enzyme protects from telomeric shortening, oxidative injury, growth arrest, and cell death. The expression of proteins implicated in DNA repair and telomere integrity, Ku86/Ku70, PARP, and TRF2, decreases with age in WT but remains constant in TG, pointing to an antiaging effect of IGF-1. Telomere erosion at each round of cell division, oxidative stress, and downregulation of telomeric proteins define myocyte senescence.
Cardiac Stem Cells
The process of CSC senescence follows a pattern similar to that identified in this study in myocytes of rodents and previously in humans.14 This process is characterized by the expression of p16INK4a and telomeric shortening. Because of this CDK inhibitor, a large number of c-kitPOS cells cannot divide symmetrically or asymmetrically and maintain the CSC pool. The increase in c-kitPOS cells undergoing senescence and death interferes with the needs for myocyte replacement in the old heart. This deficiency promotes differentiation of the remaining p16INK4a-negative c-kitPOS cells into amplifying myocytes. The higher fraction of CSCs undergoing myocyte commitment is consistent with the increase in BrdU labeling of WT myocytes with age. CSC senescence, on the one hand, and differentiation, on the other, leads to a progressive depletion of the growth reserve of the heart. Although transgene expression was myocyte restricted, overexpression of the secreted form of IGF-1 had an impact on CSC function. IGF-1 accumulates at high concentration in myocytes where it is then secreted in the interstitium.9 The released IGF-1 binds to IGF-1 receptors present on CSCs (unpublished data, 2004). The IGF-1/IGF-1 receptor system induces CSC division, enhances telomerase activity, delays senescence, and preserves the reservoir of functionally competent CSCs (unpublished data, 2004). But whether IGF-1–mediated attenuation of p53, angiotensin II formation, oxidative stress, and myocyte death14,16,32 is operative in CSCs remains to be determined. Importantly, IGF-1 suppresses the ASK-1–induced activation of JNK/p38 kinase and apoptosis.33 In fact, the expression of IGF-1 increases in the surviving myocytes after infarction and this response prevents the activation of myocyte apoptosis in the overloaded spared myocardium positively interfering with side-side slippage, ventricular dilation, and wall thinning of the postinfarcted heart.34,35 Thus, CSCs and IGF-1 protect myocardial regeneration and delay organ aging and heart dysfunction.
Akt and Myocardial Aging
On the basis of this discussion, cardiac aging is a time-dependent process that consistently attenuates myocyte turnover mediated by growth limitation at the level of the controlling cell, the CSC. In the old heart, the myocyte commitment of CSCs is severely reduced by senescence and death of this cell compartment and the postulated decrease in the number of rounds of division of amplifying myocytes. Telomerase activity is markedly decreased in amplifying myocytes leading to premature telomeric shortening and growth arrest. The principal defect seems to be linked to diminished nuclear expression of phospho-Akt. This kinase has the ability to promote phosphorylation of telomerase enhancing enzyme activity and, thereby, cell growth and survival delaying the onset of the aging myopathy. The PI3K-Akt signaling pathway is partially rescued by IGF-1 overexpression that upregulates nuclear phospho-Akt function and telomerase.
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Acknowledgments |
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This work was supported by NIH Grants HL-38132, AG-15756, HL-65577, HL-66923, HL-65573, AG-17042, and AG-023071.
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Footnotes |
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Original received November 20, 2003; revision received January 5, 2004; accepted January 7, 2004.
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References |
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