In cardiomyocyte hypoxia, insulin-like growth factor-I-induced antiapoptotic signaling requires phosphatidylinositol-3-OH-kinase-dependent and mitogen-activated protein kinase-dependent activation of the transcription factor cAMP response element-binding protein.

BACKGROUND: A variety of pathologic stimuli lead to apoptosis of cardiomyocytes. Survival factors like insulin-like growth factor-I (IGF-I) exert anti-apoptotic effects in the heart. Yet the underlying signaling pathways are poorly understood. METHODS AND RESULTS: In a model of hypoxia-induced apoptosis of cultured neonatal cardiomyocytes, IGF-I prevented cell death in a dose-dependent manner. Antiapoptotic signals induced by IGF-I are mediated by more than one signaling pathway, because pharmacological inhibition of the phosphatidylinositol-3-OH-kinase (PI3K) or the mitogen-activated protein kinase kinase (MEK1) signaling pathway both antagonize the protective effect of IGF-I in an additive manner. IGF-I-stimulation was followed by a PI3K-dependent phosphorylation of AKT and BAD and an MEK1-dependent phosphorylation of extracellular signal-regulated kinase (ERK) 1 and ERK2. IGF-I also induced phosphorylation of cAMP response element-binding protein (CREB) in a PI3K- and MEK1-dependent manner. Ectopic overexpression of a dominant-negative mutant of CREB abolished the antiapoptotic effect of IGF-I. Protein levels of the antiapoptotic factor bcl-2 increased after longer periods of IGF-I-stimulation, which could be reversed by pharmacological inhibition of PI3K as well as MEK1 and also by overexpression of dominant-negative CREB. CONCLUSIONS: In summary, our data demonstrate that in cardiomyocytes, the antiapoptotic effect of IGF-I requires both PI3K- and MEK1-dependent pathways leading to the activation of the transcription factor CREB, which then induces the expression of the antiapoptotic factor bcl-2.

E2F-1 overexpression in cardiomyocytes induces downregulation of p21CIP1 and p27KIP1 and release of active cyclin-dependent kinases in the presence of insulin-like growth factor I.

The heart is a postmitotic organ unable to regenerate after injury. The mechanisms controlling cell cycle arrest in cardiomyocytes are still unknown. Adenoviral delivery of E2F-1 to primary rat cardiomyocytes resulted in an increase in the expression of key cell cycle activators and apoptosis in >90% of the cells. However, insulin-like growth factor I (IGF-I) rescued cardiomyocytes from E2F-1-induced apoptosis. Furthermore, overexpression of E2F-1 in the presence of IGF-I induced the specific downregulation of total p21(CIP1) and p27(KIP1) protein levels and their dissociation from cyclin-dependent kinases (cdks). In contrast, p16(INK4) and p57(KIP2) protein levels and their association with cdks remained unaltered. The dissociation of p21(CIP1) and p27(KIP1) from their cdk complexes correlated well with the activation of cdk2, cdk4, and cdk6 and the release from cell cycle arrest. Under these circumstances, the number of cardiomyocytes in S phase rose from 1.2% to 23%. These results indicate that IGF-I renders cardiomyocytes permissive for cell cycle reentry. Finally, the specific downregulation of p21(CIP1) and p27(KIP1) further suggests their key role in the maintenance of cell cycle arrest in cardiomyocytes.