Coordinated effects of insulin-like growth factor I on inhibitory pathways of cell cycle progression in cultured cardiac muscle cells.

Insulin-like growth factor I (IGF I) plays a key role in the regulation of cell proliferation. Progression of the cell cycle is regulated by stimulatory and inhibitory pathways. In order to understand the mechanisms through which IGF I regulates cardiac muscle growth, we have studied the effects of IGF I on inhibitory pathways involving p53 and WAF1 in cultured cardiac muscle cell line H9C2. The onset of DNA synthesis in response to IGF I stimulation was preceded by activation of p53 expression. In addition, IGF I increased p53-dependent and p53-independent induction of WAF1 in H9C2 cells. Dose-response studies showed that IGF I effects on p53-dependent and p53-independent induction of WAF1 occur at physiological concentrations of IGF I. These data indicate that IGF I coordinately regulates inhibitory pathways of cell cycle progression, and that p53-dependent and p53-independent induction of WAF1 may provide negative control mechanisms to regulate stimulatory pathways of cell cycle progression activated by IGF I.