UNCOUPLING OF P21 INDUCTION AND MYOD ACTIVATION RESULTS IN THE FAILURE OF IRREVERSIBLE CELL-CYCLE ARREST IN DOXORUBICIN-TREATED MYOCYTES

Doxorubicin (Dox, Adriamicin), a potent broad spectrum anthracycline a nticancer drug, selectively inhibits muscle specific gene expression i n cardiac cells in vivo and prevents terminal differentiation of skele tal muscle cells in vitro. By inducing the expression of the helix-loo p-helix (HLH) transcriptional inhibitor ld2, Dox represses the myogeni c function of the MyoD family of muscle regulatory factors (MRFs). In many cell types, terminal differentiation is coupled to an irreversibl e exit from the cell cycle and MyoD plays a critical role in the perma nent cell cycle arrest of differentiating myocytes by upregulating the cyclin dependent kinase inhibitor (cdki) p21. Here, we correlate Dox effects on cell cycle with changes of E2F/DP complexes and activity in differentiating C2C12 myocytes. In Dox-treated quiescent myoblasts, w hich fail to differentiate into myotubes under permissive culture cond itions, serum re-stimulation induces cyclin/cdk re-association on the E2F/DP complexes and this correlates with an evident increase in E2F/D P driven transcription and re-entry of myoblasts into the cell cycle. Despite Dox ability to activate the DNA-damage dependent p53/p21 pathw ay when induced in the absence of MyoD or other MRFs, p21 fails to mai ntain the postmitotic state in Dox-treated myocytes induced to differe ntiate. Thus, uncoupling p21 induction and MyoD activity results in a serum-reversible cell cycle arrest, indicating that MRF specific activ ation of cdki(s) is required for permanent cell cycle arrest in differ entiating muscle cells. (C) 1997 Wiley-Liss, Inc.