E2F activates late-G1 events but cannot replace E1A in inducing S phase interminally differentiated skeletal muscle cells

We have previously shown that the adenovirus E1A oncogene can reactivate th e cell cycle in terminally differentiated cells. Current models imply that much or all of this E1A activity is mediated by the release of the E2F tran scription factors from pocket-protein control. In contrast, me show here th at overexpression of E2F-1, E2F-2 and E2F-4, or a chimeric E2F-4 tethered t o a nuclear localization signal cannot reactivate postmitotic skeletal musc le cells (myotubes). This is not due to lack of transcriptional activity, a s demonstrated on both a reporter construct and a number of endogenous targ et genes. Although cyclin E was strongly overexpressed in E2F-transduced my otubes, it lacked associated kinase activity, possibly explaining the inabi lity of the myotubes to enter S phase and accumulate cyclin A. Although E2F is not sufficient to trigger DNA synthesis in myotubes, its activity is ne cessary even in the presence of E1A, as dominant-negative DP-1 mutants inhi bit E1A-mediated cell cycle reentry. Our data show that, to reactivate myot ubes, E1A must exert other functions, in addition to releasing E2F. They al so establish mouse myotubes as an experimental system uniquely suited to st udy the most direct E2F functions in the absence of downstream cell cycle e ffects.