C-MYC-ENHANCED S-PHASE ENTRY IN KERATINOCYTES IS ASSOCIATED WITH POSITIVE AND NEGATIVE EFFECTS ON CYCLIN-DEPENDENT KINASES

The function of the c-myc proto-oncogene in cell cycle progression rem ains unclear. In order to examine the role c-myc may play in cell cycl e progression, we have expressed the hormone inducible MycER protein i n the nontransformed, EGF-dependent mouse keratinocyte cell line BALB/ MK. We have found that activation of MycER, but not a mutant MycER, Ga l4ER, or FosER, leads to an EGF-dependent and hormone-dependent increa sed incorporation of labeled thymidine only during the S phase of the cell cycle in BALB/MK cells. A possible Explanation for the increase i n thymidine incorporation comes from flow cytometric analyses that rev eal that activation of MycER leads to an increase in the total number of cells that enter S phase after EGF restimulation. Investigation of the intracellular effects of Myc activation shows that the expression of several putative Myc-sensitive proteins, cyclins A, E, and D1, and the E2F-1 protein are unaffected by Myc induction, Interestingly, we f ind that the histone H1 kinase activity associated with an E2F-1 compl ex containing Cyclin A and Cdk-2, but not that associated with Cyclin E, in late G(1) and early S phases is increased in cells containing ho rmone-activated MycER but not FosER. Although the mechanism for this M yc-dependent effect on E2F-1-associated kinase activity is still unkno wn, it does not appear to involve dissociation of the Cdk inhibitor p2 7(Kip1) from the complexes as suggested by others. However, we have al so found that hormone-treated cells actually show more p16(INK4A) inhi bitor associated with another kinase, Cdk-4, as the cells are entering S phase. Altogether, the data suggest that the presence of excessive Myc protein in keratinocytes can stimulate otherwise noncycling cells to enter the cell cycle, and that this effect of Myc involves both pos itive effects on E2F-1-associated Cdk-2 and negative effects on Cdk-4 in late G(1). J. Cell Biochem. 70:528-542, 1998. (C) 1998 Wiley-Liss, Inc.