EXPRESSION OF E1A IN TERMINALLY DIFFERENTIATED MUSCLE-CELLS REACTIVATES THE CELL-CYCLE AND SUPPRESSES TISSUE-SPECIFIC GENES BY SEPARABLE MECHANISMS

Terminally differentiated cells are characterized by permanent withdra wal from the cell cycle; they do not enter S phase even when stimulate d by growth factors or retroviral oncogenes, We have shown, however, t hat the adenovirus EIA oncogene can reactivate the cell cycle in termi nally differentiated cells, In this report, we describe the molecular events triggered by ELA in terminally differentiated skeletal muscle c ells, We found that in myotubes infected with the adenovirus mutant d/ 520, 12S E1A bypasses the early G(1) phase and activates the expressio n of late-G, genes, such as the cyclin E and cyclin A genes, cdR2, PCN A, and B-myb. Of these, the cyclin E gene and cdk2 were significantly overexpressed in comparison with levels in proliferating, undifferenti ated myoblasts, p130 and pRb were phosphorylated before the infected m yotubes entered S phase, despite the high expression of the cyclin-dep endent kinase inhibitor p21, and E2F was released, Our results suggest that one of the mechanisms that E1A uses to overcome the proliferativ e block of terminally differentiated cells involves coordinated overex pression of cyclin E and cdk2, Following EIA expression, the myogenic transcription factors MyoD and myogenin and the muscle-specific struct ural genes encoding muscle creatine kinase and myosin heavy chain were downregulated. The muscle regulatory factors were also silenced in my otubes infected with adenovirus E1A mutants incapable of reactivating the cell cycle in terminally differentiated muscle cells, Thus, the su ppression of the differentiation program is not a consequence of cell cycle reactivation in myotubes, and it is induced by an independent me chanism. Our results show that E1A reactivates the cell cycle and supp resses tissue-specific gene expression in terminally differentiated mu scle sells, thus causing dedifferentiation.