REGULATION OF 2 E2F-RELATED GENES IN PRESENESCENT AND SENESCENT HUMANFIBROBLASTS

Several mammalian genes expressed in late G, are positively regulated by E2F, a heterodimeric transcription factor, Genes encoding two E2F p roteins, E2F-1 and DP-1, were regulated differently during the cell cy cle and replicative senescence of normal human fibroblasts. In presene scent cells, E2F-1 mRNA was cell-cycle regulated, appearing a few hour s before S phase. Ey contrast, DP-1 mRNA was constitutively expressed, independent of position in the cell cycle. After a finite number of d ivisions, normal cells enter a state of irreversible growth arrest ter med senescence. Many genes remain mitogen-inducible in senescent cells ; there are, however, exceptions, including several late G(1) genes po tentially regulated by E2F. Senescent cells expressed DP-1 at the pres enescent level, but did not express E2F-1 mRNA. Senescent cells were a lso markedly deficient in E2F binding activity associated with the dih ydrofolate reductase promoter. E2F-1 and DP-1 expression vectors only weakly induced DNA synthesis in quiescent or senescent human cells and immortal murine NIH3T3 cells, although the E2F-1 vector stimulated DN A synthesis in immortal murine A31 cells, and transactivated E2F-respo nsive promoters in NIH3T3 cells. The results suggest that senescent ce lls may fail to express late G, genes due to repression of E2F-1, lead ing to a deficiency of E2F activity. Furthermore, although E2F-1 stimu lates DNA synthesis in some cells, other cells, including normal human fibroblasts, require additional factors.