The cardiac cell cycle, pocket proteins, and p300

Striking a balance: autophagy, apoptosis, and necrosis in a normal and failing heart

Despite the progress that has been made over the past two decades in cardiovascular research, heart failure remains a major cause of morbidity and mortality worldwide. Insight into the cellular and molecular mechanisms that underlie the heart failure in individuals with ischemic heart disease have identified defects in cellular processes that govern autophagy, apoptosis and necrosis as a prevailing underlying cause. Indeed, programmed cell death of cardiac cells by apoptosis or necrosis is believed to involve the intrinsic mitochondrial pathway and/or extrinsic death receptor pathway by certain Bcl-2 family members as well as components of the TNFα signaling pathway. In this review, we discuss recent advances in the molecular signaling factors that govern cardiac cell fate under normal and disease conditions.

Interplay between the retinoblastoma protein and LEK1 specifies stem cells toward the cardiac lineage

The molecular mechanisms governing early cardiogenesis are still largely unknown. Interestingly, the retinoblastoma protein (Rb), a regulator of cell cycle, has recently emerged as a new candidate regulating cell differentiation. Rb-/- mice die at midgestation and mice lacking E2f1/E2f3, downstream components of the Rb-dependent transcriptional pathway, die of heart failure. To gain insight into the function of Rb pathway in early cardiogenesis, we used Rb-/- embryonic stem (ES) cells differentiating into cardiomyocytes. Rb-/- cells displayed a dramatic delay in expression of cardiac-specific transcription factors and in turn in the whole process of cardiac differentiation. The phenotype of Rb-/- ES cell-derived cardiomyocytes was rescued by reintroducing Rb in cardiac progenitors, by stimulating the BMP-dependent cardiogenic pathway or by overexpression of Nkx2.5. ES cells deficient in the recently identified factor LEK1, a murine homolog of the cardiomyogenic factor 1, or specific disruption of Rb-LEK1 interaction into the nucleus of differentiating ES cells recapitulated the delay in cardiac differentiation of Rb-/- ES cells. Thus, we provide evidence for a novel Rb/LEK1-dependent and BMP-independent transcriptional program, which plays a pivotal role in priming ES cells toward a cardiac fate.

Myocyte growth in the failing heart

Adult ventricular myocytes can undergo mitotic division, resulting in an increase in the aggregate number of cells in the heart. The improvement in the methodological approach to the analysis of tissue sections by immunostaining and confocal microscopy has defeated the dogma that myocyte regeneration cannot occur in the adult heart. Most importantly, primitive and progenitor cells have been identified in the human heart. These cells express telomerase and have the capability of undergoing lineage commitment and rapid cell division, expanding significantly the contracting ventricular myocardium. These cell populations possess all the molecular components regulating the entry and progression through the cell cycle, karyokinesis, and cytokinesis. The recognition that myocyte hypertrophy and regeneration, as well as myocyte necrosis and apoptosis, occur in cardiac diseases has contributed to enhancing our understanding of the plasticity of the human heart.

Myocyte growth and cardiac repair

Introduced several decades ago, the dogma persists that ventricular myocytes are terminally differentiated cells and cardiac repair by myocyte regeneration is completely inhibited shortly after birth. On the basis that cardiac myocytes are unable to divide in the adult heart, myocyte growth under physiologic and pathologic conditions is believed to be restricted to cellular hypertrophy. Evidence is presented to indicate that this old paradigm has to be changed to include myocyte replication as a significant component of the cellular processes of ventricular remodeling. Importantly, myocyte death, apoptotic and necrotic in nature, has to be regarded as an additional critical variable of the multifactorial events implicated in the alterations of cardiac anatomy and myocardial structure of the decompensated heart. Methodologies are currently available to recognize and measure quantitatively the contribution of myocyte size, number and death to the adaptation of the overloaded heart and its progression to cardiac failure.

Telomere shortening is an in vivo marker of myocyte replication and aging

To determine whether adult cardiac myocytes are capable of multiple divisions and whether this form of growth is restricted to a subpopulation of cells that retain this capacity with age, telomere lengths were measured in myocyte nuclei isolated from the left ventricle of fetal and neonatal Fischer 344 rats and rats at 4, 12, and 27 months after birth. Two independent methodologies were used for this analysis: laser scanning cytometer and confocal microscopy. In each case, fluorescence intensity of a peptide nucleic acid probe specific for telomeric sequence was evaluated. The two techniques yielded comparable results. Telomeric shortening increased with age in a subgroup of myocytes that constituted 16% of the entire cell population. In the remaining nondividing cells, progressive accumulation of a senescent associated nuclear protein, p16(INK4), was evidenced. In conclusion, a significant fraction of myocytes divides repeatedly from birth to senescence, counteracting the continuous death of cells in the aging mammalian rat heart.

Angiotensin II and serum differentially regulate expression of cyclins, activity of cyclin-dependent kinases, and phosphorylation of retinoblastoma gene product in neonatal cardiac myocytes

The hypertrophic response in cardiac myocytes and the mitogenic response in other cell types share various early cellular responses. However, how the subsequent cell growth response, such as cell cycle machinery, is regulated in cardiac hypertrophy is not understood. Using cultured neonatal rat cardiac myocytes, we examined the effect of angiotensin II (Ang II), a hypertrophic stimulus, on mRNA and protein expression of cyclins and cyclin-dependent protein kinases (cdks), activity of cdks, and phosphorylation of retinoblastoma gene product (pRb). The effect of FCS, a stimulus that was previously reported to initiate both protein and DNA synthesis in cardiac myocytes, was also examined for comparison. Ang II activated cdk4 and caused phosphorylation of pRb, peaking at 12 hours, but subsequently downregulated cyclin D1, D3, and A expression and cdk2 activity. FCS increased the expression of G1-S cyclins, caused activation of cdk4, cdk2, and cdc2, and strongly phosphorylated pRb but failed to significantly stimulate DNA synthesis in neonatal cardiac myocytes. These results suggest that Ang II transiently activates but subsequently downregulates cell cycle regulators. Induction of G1 and G1-S cyclins and activation of cdks by FCS are not sufficient to drive cardiac myocytes into S phase. The functional role of pRb phosphorylation by Ang II and serum stimulation and, by inference, the subsequent liberation of E2F in terminally differentiated myocytes remain to be elucidated.

An effector-like function of Ras GTPase-activating protein predominates in cardiac muscle cells

In contrast to familiar role for Ras in proliferation, we and others previously suggested that Ras also mediates hypertrophy, the increase in cell mass characteristic of post-natal ventricular muscle. We showed that activated (G12R) and dominant-negative (S17N) Ha-Ras regulate "constitutive" and growth factor-responsive genes equivalently, in both cardiac myocytes and non-cardiac, Mv1Lu cells. Here, we attempt to delineate pathways by which Ras exerts this global effect. The E63K mutation, which impairs binding of guanine nucleotide releasing factor to Ras, alleviated suppression by S17N, consistent with sequestration of exchange factors as the mechanism for inhibition. To compare potential Ras effector proteins, we first engineered G12R/D38N, to abolish binding of Raf and phosphatidylinositol-3-kinase and established that this site was indispensable for augmenting gene expression. To distinguish between inhibition of Ras by Ras GTPase-activating protein (GAP) versus a potential effector function of GAP, we tested the effector domain substitution P34R: this mutation, which abolishes GAP binding, enhanced Ras-dependent transcription in Mv1Lu cells, yet interfered with Ras-dependent expression in ventricular myocytes. To examine the dichotomous role of Ras-GAP predicted from these P34R results, we transfected both cell types with full-length GAP, the C-terminal catalytic domain (cGAP), or N-terminal Src homology domains (nGAP). In Mv1Lu cells, cGAP markedly inhibited both reporter genes, whereas GAP and nGAP had little effect. Antithetically, in ventricular myocytes, GAP and nGAP activated gene expression, whereas cGAP was ineffective. Thus, Ras activates gene expression through differing effectors contingent on cell type, and an effector-like function of GAP predominates in ventricular muscle.