INDUCTION OF DNA-SYNTHESIS AND APOPTOSIS IN CARDIAC MYOCYTES BY E1A ONCOPROTEIN

Beginning during the second half of gestation, increasing numbers of c ardiac myocytes withdraw from the cell cycle such that DNA synthesis i s no longer detectable in these cells by neonatal day 17 in vivo. The mechanisms that exclude these and other terminally differentiated cell s from the cell division cycle are poorly understood, To begin to expl ore the molecular basis of the barrier to G1/S progression in cardiac myocytes, we used adenoviruses to express wild-type and mutant E1A pro teins in primary cultures from embryonic day 20 rats. While most of th ese cardiac myocytes are ordinarily refractory to DNA synthesis, even in the presence of serum growth factors, expression of wild-type E1A s timulates DNA synthesis in up to 94% or almost all successfully transd uced cells, Rather than complete the cell cycle, however, these cells undergo apoptosis. Apoptosis is limited to those cells that engage in DNA synthesis, and the kinetics of the two processes suggest that DNA synthesis precedes apoptopsis, Mutations in E1A that disable it from b inding Rb and related Pocket proteins have little effect on its abilit y to stimulate DNA synthesis in cardiac myocytes, In contrast, mutants that are defective in binding the cellular protein p300 stimulate DNA synthesis 2.4-4.1-fold less efficiently, even in the context of retai ned E1A pocket protein binding, In the absence of E1A pocket protein b inding, the usual situation in the cell, loss of p300 binding severely decreases the ability of E1A to stimulate DNA synthesis, These result s suggest that the barrier to G1/S progression in cardiac myocytes is mediated, at least in part, by the same molecules that gate the G1/S t ransition in actively cycling cells, and that p300 or related family m embers play an important role in this process.