AKT signalling in the failing heart

AKT is a serine/threonine protein kinase, also known as protein kinase B, which regulates cardiac growth, myocardial angiogenesis, glucose metabolism, and cell death in cardiac myocytes. AKT is activated by its phosphorylation at Thr 308 and ser 473 by PDK1 and mTORC2, respectively, in response to trophic stimuli such as insulin and insulin growth factor. c-Jun N-Terminal Kinases (JNKs) phosphorylate AKT at Thr 450 and potentiate its interaction with its downstream effectors. The short-term activation of AKT promotes physiological hypertrophy and protection from myocardial injury; whereas, its long-term activation causes pathological hypertrophy and heart failure. In this review we will discuss the role of AKT in regulating signalling pathways in the heart with special emphasis on the role of AKT in modulating stress induced autophagic cell death in cardiomyocytes in vitro.     

Cell transplantation and cardiac recovery:myocardial regeneration from endogenous cardiac progenitor cells

Following the bone marrow or cardiac derived progenitor cells transplantation,improved left ventricular(LV) function,decreased LV remodeling, and decreased fibrosis of non-infarcted LV regions,and in some cases,the reduction of infarct scar size have been reported to occur in animal mycardial infarction(MI) models.In clinical trials, stem cell transplantation has also been associated with significant,but modest improvements of LV functional parameters.These beneficial effects do occur although in many animal studies there is often very low long term engraftment or transdifferentiation of transplanted cells into myocytes and vascular cells.Importantly,paracrine signals generated by the implanted progenitor cells seem to play an important role in limiting or reversing myocardial damage as- sociated with acute MI.Paracrine signaling effects include increased myocardial vascularization and reduced apoptosis of native cardiomyocytes;these responses are most prominent in peri-myocardial infarction (MI) boarder zone(BZ) of the heart.Although much data supports the possibility that engrafted progenitor cells can mobilize endogenous cardiac progenitor cells(CPC) to the cardiac injury site and also stimulate them to propagate and transd-ifferentiate into cardiomyocytes and vascular cells, this concept remains controversial.We and others have reported evidences supporting the view that endogenous CPC can be stimulated to differentiate and partially replace cardiomyocytes destroyed during an MI.Data from our laboratory will be reviewed.     

Angiotensin-II activates apoptosis, proliferation and protein synthesis in the left heart ventricle of newborn albino rats

Angiotensin II (ANG-II) is a critical regulator of various signaling pathways involved in growth and remodeling of the vascular, cardiac, and renal cells and tissues. Although it contributes to several physiologic and pathologic events in the cardiovascular system, its role in growth and differentiation of the newborn heart is still unclear. We analyzed the effect of ANG-II treatment on apoptosis, DNA synthesis, and nucleolar organizer regions (AgNORs) activity in newborn rat myocardium. Injections of ANG-II for 5-days caused significant increase of the 3H-thymidine labeling index (M+/-m) in the myocardium of 7-day-old rats (from 6.95+/-0.32% to 8.53+/-0.22%, p<0.05). There was also significant increase in the cross sectional surface area of cardiomyocytes (from 686+/-57 to 872+/-54 microm2, p<0.05), number of nucleoli (from 2.5+/-0.05 to 2.8+/-0.1, p<0.05), and nucleolar surface area (from 2.6+/-0.09 to 3.2+/-0.22 microm2, p<0.05). These changes were accompanied by significant increase in the apoptotic indices analyzed by TDT-mediated dUTP-biotin nick end-labeling (TUNEL) (from 0.044+/-0.01% to 0.093+/-0.01%, p<0.05). Interestingly, we found no differences in cell proliferation between the test and control animals after 21-45 days of age, which were injected with ANG-II in the first postnatal week. However, the area of cardiomyocytes and the number of nucleoli in 21-day-old rats continued to increase significantly. Our results indicate that ANG-II modulates cardiac growth during the neonatal period via stimulation of apoptosis, cell cycle events and cellular growth of cardiomyocytes and that these effects can persist up to 15 days after injection of ANG-II has been completed.     

Genetics and molecular biology in cardiology (IV). Myocardial response to biomechanical stress

Biomechanical stress of the myocardium is the situation resulting from hypoxia, hypertension, and other forms of myocardial injury, that invariably lead to increased demands for cardiac work and/or loss of functional myocardium. As a consequence of biomechanical stress a number of responses develop involving all the myocardial cells, namely cardiomyocytes. As a result some myocardial phenotypic changes develop that are initially compensatory (i.e., hypertrophy) but which may mediate the eventual decline in myocardial function that occurs with the transition from hypertrophy to failure in conditions of persistent stress (i.e., apoptosis and fibrosis). This review focuses on the steps involved in the response of the myocardium to biomechanical stress and highlights the most recent developments in the molecular mechanisms involved in the development of heart failure.     

The fibrosis-cell death axis in heart failure

Cardiac stress can induce morphological, structural and functional changes of the heart, referred to as cardiac remodeling. Myocardial infarction or sustained overload as a result of pathological causes such as hypertension or valve insufficiency may result in progressive remodeling and finally lead to heart failure (HF). Whereas pathological and physiological (exercise, pregnancy) overload both stimulate cardiomyocyte growth (hypertrophy), only pathological remodeling is characterized by increased deposition of extracellular matrix proteins, termed fibrosis, and loss of cardiomyocytes by necrosis, apoptosis and/or phagocytosis. HF is strongly associated with age, and cardiomyocyte loss and fibrosis are typical signs of the aging heart. Fibrosis results in stiffening of the heart, conductivity problems and reduced oxygen diffusion, and is associated with diminished ventricular function and arrhythmias. As a consequence, the workload of cardiomyocytes in the fibrotic heart is further augmented, whereas the physiological environment is becoming less favorable. This causes additional cardiomyocyte death and replacement of lost cardiomyocytes by fibrotic material, generating a vicious cycle of further decline of cardiac function. Breaking this fibrosis-cell death axis could halt further pathological and age-related cardiac regression and potentially reverse remodeling. In this review, we will describe the interaction between cardiac fibrosis, cardiomyocyte hypertrophy and cell death, and discuss potential strategies for tackling progressive cardiac remodeling and HF.     

Inflammasome activation of cardiac fibroblasts is essential for myocardial ischemia/reperfusion injury

Background- Inflammation plays a key role in the pathophysiology of myocardial ischemia/reperfusion (I/R) injury; however, the mechanism by which myocardial I/R induces inflammation remains unclear. Recent evidence indicates that a sterile inflammatory response triggered by tissue damage is mediated through a multiple-protein complex called the inflammasome. Therefore, we hypothesized that the inflammasome is an initial sensor for danger signal(s) in myocardial I/R injury. Methods and Results- We demonstrate that inflammasome activation in cardiac fibroblasts, but not in cardiomyocytes, is crucially involved in the initial inflammatory response after myocardial I/R injury. We found that inflammasomes are formed by I/R and that its subsequent activation of inflammasomes leads to interleukin-1β production, resulting in inflammatory responses such as inflammatory cell infiltration and cytokine expression in the heart. In mice deficient for apoptosis-associated speck-like adaptor protein and caspase-1, these inflammatory responses and subsequent injuries, including infarct development and myocardial fibrosis and dysfunction, were markedly diminished. Bone marrow transplantation experiments with apoptosis-associated speck-like adaptor protein-deficient mice revealed that inflammasome activation in bone marrow cells and myocardial resident cells such as cardiomyocytes or cardiac fibroblasts plays an important role in myocardial I/R injury. In vitro experiments revealed that hypoxia/reoxygenation stimulated inflammasome activation in cardiac fibroblasts, but not in cardiomyocytes, and that hypoxia/reoxygenation-induced activation was mediated through reactive oxygen species production and potassium efflux. Conclusions- Our results demonstrate the molecular basis for the initial inflammatory response after I/R and suggest that the inflammasome is a potential novel therapeutic target for preventing myocardial I/R injury.     

Embryonic stem cells: differentiation into cardiomyocytes and potential for heart repair and regeneration

Many forms of heart disease are associated with the loss of cardiomyocytes both via apoptosis or necrosis, and despite the recent identification of resident cardiac stem cells, the native capacity for renewal and repair is inadequate. Cell transplantation strategies have emerged as a potential therapeutic approach for repairing injured myocardium. Many different cell types including embryonic stem cells have been transplanted in myocardial infarction (MI) models with resulting improvement in myocardial function. Here, we review the current state of knowledge with regard to the potential of embryonic stem (ES) cells to differentiate into cardiomyocytes in the embryonic stem cell derived-embryoid body (EB) in vitro system as well as for myocardial regeneration following myocardial infarction.     

Diabetes promotes cardiac stem cell aging and heart failure, which are prevented by deletion of the p66shc gene

Diabetes leads to a decompensated myopathy, but the etiology of the cardiac disease is poorly understood. Oxidative stress is enhanced with diabetes and oxygen toxicity may alter cardiac progenitor cell (CPC) function resulting in defects in CPC growth and myocyte formation, which may favor premature myocardial aging and heart failure. We report that in a model of insulin-dependent diabetes mellitus, the generation of reactive oxygen species (ROS) leads to telomeric shortening, expression of the senescent associated proteins p53 and p16INK4a, and apoptosis of CPCs, impairing the growth reserve of the heart. However, ablation of the p66shc gene prevents these negative adaptations of the CPC compartment, interfering with the acquisition of the heart senescent phenotype and the development of heart failure with diabetes. ROS elicit 3 cellular reactions: low levels activate cell growth, intermediate quantities trigger cell apoptosis, and high amounts initiate cell necrosis. CPC replication predominates in diabetic p66shc-/-, whereas CPC apoptosis and myocyte apoptosis and necrosis prevail in diabetic wild type. Expansion of CPCs and developing myocytes preserves cardiac function in diabetic p66shc-/-, suggesting that intact CPCs can effectively counteract the impact of uncontrolled diabetes on the heart. The recognition that p66shc conditions the destiny of CPCs raises the possibility that diabetic cardiomyopathy is a stem cell disease in which abnormalities in CPCs define the life and death of the heart. Together, these data point to a genetic link between diabetes and ROS, on the one hand, and CPC survival and growth, on the other.     

A potential cardioprotective role of hepatocyte growth factor in myocardial infarction in rats

OBJECTIVE: Cardiotrophic growth factors with anti-cell death actions on cardiac myocytes have gained attention for treatment of patients with myocardial infarction. Hepatocyte growth factor (HGF) plays a role in tissue repair and protection from injuries, however, the physiological role of HGF in the myocardium has not been well defined. We asked if HGF would afford to the infarcted myocardium. METHODS AND RESULTS: Mature cardiac myocytes prepared from adult rats expressed barely detectable levels of the c-Met/HGF receptor, however, c-Met receptor expression increased during cultivation, which meant that cardiac myocytes are potential targets of HGF. Addition of hydrogen peroxide remarkably decreased the number of viable mature cardiac myocytes in primary culture, whereas treatment with HGF enhanced survival of the cells subjected to the oxidant stress. Although very low levels of c-Met/HGF receptor and HGF mRNA expression were seen in normal rat hearts, both c-Met/HGF receptor and HGF mRNA levels rapidly increased to much higher levels than normal, when the rats were subjected to myocardial infarction. Immunohistochemical analysis of the c-Met receptor indicated that this receptor was expressed in cardiomyocytes localized in the border regions of the viable myocardium and in non-infarcted regions following myocardial infarction. CONCLUSION: The c-Met/HGF receptor is induced in cardiomyocytes following myocardial infarction and HGF exhibits protective effect on cardiomyocytes against oxidative stress. Our working hypothesis is that HGF may afford myocardial protection from myocardial infarction.     

The inflammasome promotes adverse cardiac remodeling following acute myocardial infarction in the mouse

Acute myocardial infarction (AMI) initiates an intense inflammatory response that promotes cardiac dysfunction, cell death, and ventricular remodeling. The molecular events underlying this inflammatory response, however, are incompletely understood. In experimental models of sterile inflammation, ATP released from dying cells triggers, through activation of the purinergic P2X7 receptor, the formation of the inflammasome, a multiprotein complex necessary for caspase-1 activation and amplification of the inflammatory response. Here we describe the presence of the inflammasome in the heart in an experimental mouse model of AMI as evidenced by increased caspase-1 activity and cytoplasmic aggregates of the three components of the inflammasome--apoptosis speck-like protein containing a caspase-recruitment domain (ASC), cryopyrin, and caspase-1, localized to the granulation tissue and cardiomyocytes bordering the infarct. Cultured adult murine cardiomyocytes also showed the inducible formation of the inflammasome associated with increased cell death. P2X7 and cryopyrin inhibition (using silencing RNA or a pharmacologic inhibitor) prevented the formation of the inflammasome and limited infarct size and cardiac enlargement after AMI. The formation of the inflammasome in the mouse heart during AMI causes additional loss of functional myocardium, leading to heart failure. Modulation of the inflammasome may therefore represent a unique therapeutic strategy to limit cell death and prevent heart failure after AMI.     

Comparison of adriamycin-induced nucleolar segregation in skeletal muscle, cardiac muscle, and liver cells

Male Sprague-Dawley rats were treated with either 2.5, 3.5, or 5.0 mg/kg of adriamycin by iv injection. After 1 or 3 hours of treatment, samples of liver, cardiac muscle, and skeletal muscle cells were examined by electron microscopy. The changes in ultrastructure observed in these tissues after the first hour included nucleolar segregation and altered distribution of the perinucleolar chromatin. However, no alterations in the ultrastructure of either the nucleus or cytoplasm were observed in tissues examined 3 hours after a 5-mg/kg dose of adriamycin. The doses at which nucleolar alterations occurred varied between tissues. In skeletal and cardiac muscle cells, marked alterations in nucleolar ultrastructure were observed at doses of 3.5 and 5.0 mg/kg of adriamycin. Liver cell nucleoli, however, exhibited few structural aberrations at these doses. The similarities in response of skeletal and cardiac muscle suggest that ultrastructural analysis of skeletal muscle biopsies may be useful in evaluating adriamycin cardiotoxicity.     

Abstracts of the Sixth International Symposium on Myocardial Cytoprotection: From Basic Science to Clinical Perspectives: October 7 to 9, 2010, Pécs,Hungary

PURPOSE:

Pituitary adenylate cyclase activating polypeptide (PACAP) is a widely distributed endogenous neuropeptide, also occurring in the cardiovascular system. Among others, PACAP has been suggested as a cardioprotective factor. It has been shown that PACAP inhibits cardiac fibrosis and protects cardiomyocytes against oxidative stress and in vitro ischemia/reperfusion. The aim of the present study was to investigate whether PACAP is protective in doxorubicin-induced cell death of cardiomyocytes.

METHODS:

Primary culture of neonatal rat cardiomyocytes was prepared from ventricular slices of 2-4-day-old Wistar rats. Non-treated cells served as control (Group I). In Group II 1 μM of doxorubicin was added to the media while in Group III cells were treated with 1 μM of doxorubicin together with 20 nM PACAP1-38. In Group IV to antagonize the effect of PACAP1-38, 250 nM of the PACAP antagonist PACAP6-38 was added simultaneously with 1 μM doxorubicin. Cells were exposed to the mentioned concentration of chemicals for 24 h. Viability of cells were measured by MTT assay, the amount of apoptotic cells was assessed by flow cytometry following annexin V/propidium iodide double staining. The rate of apoptosis was further examined by measuring caspase-3 activity and phospho-Bad using flow cytometry.

RESULTS:

In doxorubicin-treated group (II) a lower number of living cells was observed with an increase of apoptotic cells while PACAP administration (Group III) led to a significant increase in the percentage of living cells and reproducible decrease in the rate of apoptosis. This beneficial effect of PACAP1-38 was diminished by PACAP6-38. Furthermore, doxorubicin increased the activation of pro-apoptotic caspase-3 and decreased the phosphorylation of Bad, while simultaneous PACAP treatment reduced the caspase-3 activation and increased the level of phospho-Bad. PACAP antagonist abolished the advantageous effect of PACAP1-38 on caspase-3 and phospho-BAD activation as compared to the control group and the group receiving co-treatment with PACAP1-38 and doxorubicin.

CONCLUSION:

In summary, our present results show that PACAP effectively counteracts the apoptosis-inducing action of the cardiotoxic doxorubicin in vitro, involving caspase-3 and Bad inactivation and it has a clinical importance at the myocardial complications of the doxorubicin-treated patients.     

Notch signaling as an important mediator of cardiac repair and regeneration after myocardial infarction

Through local cell-cell interactions, the Notch signaling pathway controls tissue formation and homeostasis during embryonic and adult life. In the heart, Notch1 is expressed in a variety of cell types, such as cardiomyocytes, smooth muscle cells, and endothelial cells. In cardiomyocytes, Notch1 is activated in proliferating embryonic and immature cardiomyocytes, and it is downregulated in the myocardium during postnatal development. However, Notch signaling in the adult myocardium could be activated transiently in response to myocardial injury, suggesting that Notch signaling may contribute to cardiac repair. Indeed, activation of Notch1 intracellular domain blunts the severity of myocardial injury and improves myocardial hemodynamic function. Conversely, genetic ablation of the Notch1 gene, either systemically or in bone marrow-derived cells, leads to impaired cardiac repair following myocardial infarction. In this review, we discuss the complex mechanisms of Notch signaling and its role in cardiac repair and regeneration after myocardial infarction.     

A signature pattern of stress-responsive microRNAs that can evoke cardiac hypertrophy and heart failure

Diverse forms of injury and stress evoke a hypertrophic growth response in adult cardiac myocytes, which is characterized by an increase in cell size, enhanced protein synthesis, assembly of sarcomeres, and reactivation of fetal genes, often culminating in heart failure and sudden death. Given the emerging roles of microRNAs (miRNAs) in modulation of cellular phenotypes, we searched for miRNAs that were regulated during cardiac hypertrophy and heart failure. We describe >12 miRNAs that are up- or down-regulated in cardiac tissue from mice in response to transverse aortic constriction or expression of activated calcineurin, stimuli that induce pathological cardiac remodeling. Many of these miRNAs were similarly regulated in failing human hearts. Forced overexpression of stress-inducible miRNAs was sufficient to induce hypertrophy in cultured cardiomyocytes. Similarly, cardiac overexpression of miR-195, which was up-regulated during cardiac hypertrophy, resulted in pathological cardiac growth and heart failure in transgenic mice. These findings reveal an important role for specific miRNAs in the control of hypertrophic growth and chamber remodeling of the heart in response to pathological signaling and point to miRNAs as potential therapeutic targets in heart disease.     

Both carboxy-terminus NES motif and mutated tryptophan(s) are crucial for aberrant nuclear export of nucleophosmin leukemic mutants in NPMc+ AML

We recently identified aberrant cytoplasmic expression of nucleophosmin (NPM) as the immunohistochemical marker of a large subgroup of acute myeloid leukemia (AML) (about one-third of adult AML) that is characterized by normal karyotype and mutations occurring at the exon-12 of the NPM gene. In this paper, we have elucidated the molecular mechanism underlying the abnormal cytoplasmic localization of NPM. All 29 AML-associated mutated NPM alleles so far identified encode abnormal proteins which have acquired at the C-terminus a nuclear export signal (NES) motif and lost both tryptophan residues 288 and 290 (or only the residue 290) which determine nucleolar localization. We show for the first time that both alterations are crucial for NPM mutant export from nucleus to cytoplasm. In fact, the cytoplasmic accumulation of NPM is blocked by leptomycin-B and ratjadones, specific exportin-1/Crm1-inhibitors, and by reinsertion of tryptophan residues 288 and 290, which respectively relocate NPM mutants in the nucleoplasm and nucleoli. NPM leukemic mutants in turn recruit the wild-type NPM from nucleoli to nucleoplasm and cytoplasm. These findings indicate that potential therapeutic strategies aimed to retarget NPM to its physiological sites will have to overcome 2 obstacles, the new NES motif and the mutated tryptophan(s) at the NPM mutant C-terminus.     

An emerging role for the miR-26 family in cardiovascular disease

In response to acute myocardial infarction (MI), a complex series of cellular and molecular signaling events orchestrate the myocardial remodeling that ensues weeks to months after injury. Clinical, epidemiological, and pathological studies demonstrate that inadequate or impaired angiogenesis after myocardial injury is often associated with decreased left ventricular (LV) function and clinical outcomes. The microRNA family, miR-26, plays diverse roles in regulating key aspects of cellular growth, development, and activation. Recent evidence supports a central role for the miR-26 family in cardiovascular disease by controlling critical signaling pathways, such as BMP/SMAD1 signaling, and targets relevant to endothelial cell growth, angiogenesis, and LV function post-MI. Emerging studies of the miR-26 family in other cell types including vascular smooth muscle cells, cardiac fibroblasts, and cardiomyocytes suggest that miR-26 may bear important implications for a range of cardiovascular repair mechanisms. This review examines the current knowledge of the miR-26 family’s role in key cell types that critically control cardiovascular disease under pathological and physiological stimuli.     

Insulin-like growth factor binding protein-3 mediates serum starvation- and doxorubicin-induced apoptosis in H9c2 cardiac cells

Insulin-like growth factor binding protein 3 (IGFBP-3) modulates the activity of IGF-I, which exerts antiapoptotic action upon the myocardiocyte. IGFBP-3 also exerts IGF-independent actions to inhibit cell growth and induce apoptosis, mediating the effects of several antiproliferative agents. We hypothesized that IGFBP-3 mediates cardiomyocyte apoptosis. IGFBP-3 expression was studied in H9c2 rat cardiac cells cultured in serum-deprived medium in the absence or presence of 1 microM doxorubicin during a 72 h time-span. To a greater degree than serum withdrawal, doxorubicin induced IGFBP-3 up-regulation that was time-dependent. IGFBP-3 mRNA levels positively correlated with the degree of apoptosis. Exogenous IGFBP-3 decreased cell viability and induced apoptosis in serum-starved cells exposed to doxorubicin. IGFBP-3 antisense oligonucleotides markedly decreased apoptosis induced by either serum withdrawal or doxorubicin. Binding studies revealed specific high-affinity sites for IGFBP-3 in H9c2 cardiomyocytes, with binding characteristics typical of receptor-ligand interactions. These findings indicate that IGFBP-3 could play proapoptotic action at the myocardial level and suggest a novel role for this protein in cardiovascular dysfunction.     

Hsp10 and Hsp60 modulate Bcl-2 family and mitochondria apoptosis signaling induced by doxorubicin in cardiac muscle cells

The development of doxorubicin cardiomyopathy involves apoptosis of cardiac muscle cells. This study was carried out to define the roles of two heat-shock proteins, Hsp10 and Hsp60, on doxorubicin-induced apoptosis in primary cardiomyocytes. Doxorubicin induces apoptosis of cardiomyocytes by activating mitochondria apoptosis signaling. Transducing cardiomyocytes with Hsp10 or Hsp60 with adenoviral vector suppressed the occurrence of apoptosis in the doxorubicin-treated cardiomyocytes. Overexpression of Hsp10 and Hsp60 increased the abundance of the anti-apoptotic Bcl-xl and Bcl-2, and reduced the protein content of the pro-apoptotic Bax. Hsp60 overexpression also significantly reduced doxorubicin induction of Bad, whereas overexpression of Hsp10 did not alter the expression of Bad in the doxorubicin-treated cells. Overexpression of Hsp10 and Hsp60, respectively, stabilized mitochondrial cross-membrane potential, inhibited Caspase 3, and suppressed PARP. These findings indicate that overexpression of Hsp10 and Hsp60 differentially modulated Bcl-2 family and in turn attenuate doxorubicin-induced cardiac muscle death. The effects of Hsp10 and Hsp60 on Bcl-2 family could not be explained by the abundance of Bcl-2 family mRNA levels. Hsp60 interacted with Bcl-xl and Bax in the cardiomyocytes in vivo. The effect of Hsp10 and Hsp60 on the abundance of Bcl-xl could not be blocked by cycloheximide. Moreover, Hsp10 and Hsp60 inhibited ubiquitination of Bcl-xl. These findings suggest that Hsp10 and Hsp60 modulated post-translational modification of Bcl-xl. Antisense Hsp60 reduced the abundance of endogenous Hsp60 in cardiomyocytes and amplified the cytotoxicity of doxorubicin. These data provide a novel link between Hsp10/Hsp60 and cardiac protection in doxorubicin cardiomyopathy.     

Generation of new cardiomyocytes in the adult heart: Prospects of myocardial regeneration as an alternative to cardiac transplantation]

The classic dogma, still prevalent in cardiology, that the adult myocardium is a terminally differentiated tissue unable to produce new cardiomyocytes needs to be revised in light of recent results. In human and experimental animals there is now incontrovertible evidence that new myocytes are continuously generated throughout life in response to physiological and pathological stimuli. Moreover, the elucidation of mechanisms responsible for the hypertrophic response indicate similarity and overlap with the mechanisms involved in cell death by apoptosis as well as cell growth.During cardiac development, from birth to adulthood, there is a balance between the stimuli induce cell growth -by hypertrophy and hyperplasia- on one hand and those that induce programmed cell death on the other. In human and experimental animals it has been well documented that pathological conditions, such as diabetes and hypertension, can increase dramatically the rate of cell death. Moreover, high rates of cell death have been measured in normal adult human hearts and those of mice and rats. No surprisingly, these values increase significantly with age and high in senescence. By themselves, these high rates of normal cell death provide a very compelling argument in favor of cardiomyocyte regeneration. Without cell renewal, these rates of cell death would be incompatible with survival because the heart would disappear before early adulthood. As expected, direct measurement of rates of new cell formation in adult hearts demonstrate high rates of cell renewal that compensate for cell death. Thus, the heart is in continuous cellular turnover with new myocardial cells replacing the older ones.Experiments with fetal mouse cardiocytes shows that the retinoblastoma protein is responsible for the cardiocyte withdrawal from the cell cycle during development. The identification in the adult heart of a subpopulation of quiescent cells that have many of the characteristics of stem cells able to rapidly enter the cell cycle and generate new cardiocytes is yet another evidence that the heart continuously produces new cardiocytes to replace those that disappear either by apoptosis or necrosis.Surprisingly, stem cells other that those from the heart are able to produce new cardiocytes and repopulate the myocardium. We have used bone marrow stem cells injected into the border zone of post-coronary occlusion necrosis. Remarkably, these cells have proven to be very effective in generating new myocardium in the necrotic zone that is integrated to the rest of the muscle and irrigated by new vessels. These results demonstrate that stem cells provide a new avenue for the generation of new contractile tissue. This approach could prove useful in the treatment of chronic cardiac failure and post-ischemic necrosis.