Abstract 1372: p300-mediated Inhibition of Doxorubicin-induced Myocardial Cell Apoptosis Involves Ubiquitin-dependent p53 Degradation
An intrinsic histone acetyltransferase, p300, serves as a coactivator of hypertrophy-responsive transcription factors such as MEF-2 and GATA-4 in cardiac myocytes. While an excess of p300 results in overgrowth of cardiomyocytes, p300 depletion by doxorubicin results in apoptosis of these cells. Our previous study demonstrated that overexpression of p300 in the heart rescues cardiac myocytes from doxorubicin-induced depletion of p300 and apoptosis. However, the precise mechanisms of the p300-mediated inhibition of apoptosis are unclear. p300 protein possesses ubiquitin ligase activity for the p53 tumor suppressor gene product, catalyzes p53 polyubiqutination, and facilitates p53 degradation in an ubiquitin-dependent manner. Thus, p300 is able to inhibit p53-mediated apoptosis. The present study investigated the regulation of ubiquitin-dependent p53 degradation in cardiac myocytes. We first exposed primary cardiac myocytes from neonatal rats to a proteasome inhibitor, MG132, in culture. MG132 increased both p300 and p53 protein levels in these cells, suggesting that ubiquitin-dependent degradation is involved in the homeostasis of these proteins. Notably, treatment of myocytes with doxorubicin decreased the protein levels of p300 but markedly increased those of p53. By immunoprecipitation-Western blotting in MG132-treated myocytes, we show here that treatment with doxorubicin decreases poly-ubiquitinated p53 in cardiac myocytes. In addition, we have found that doxorubicin reduces the binding between p53 and p300 in cardiomyocytes. Thus, doxorubicin may suppress ubiquitin-dependent p53 degradation in part by dissociating p53 from p300 as well as by decreasing p300 protein levels. Finally, overexpression of p300 in cardiomyocytes suppressed the doxorubicin-mediated increase in the p53 level in addition to inhibiting doxorubicin-induced apoptosis. These findings suggest that p300, by its ligase activity, promotes ubiquitin-dependent degradation of p53, providing a novel mechanism of p300-mediated inhibition of doxorubicin-induced apoptosis in cardiac myocytes.