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 remains unclear. In order to examine the role c-myc may play in cell cycle progression, we have expressed the hormone-inducible MycER protein in the nontransformed, EGF-dependent mouse keratinocyte cell line BALB/MK. We have found that activation of MycER, but not a mutant MycER, Gal4ER, or FosER, leads to an EGF-dependent and hormone-dependent increased incorporation of labeled thymidine only during the S phase of the cell cycle in BALB/MK cells. A possible explanation for the increase in thymidine incorporation comes from flow cytometric analyses that reveal 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 find that the histone H1 kinase activity associated with an E2F-1 complex containing Cyclin A and Cdk-2, but not that associated with Cyclin E, in late G1 and early S phases is increased in cells containing hormone-activated MycER, but not FosER. Although the mechanism for this Myc-dependent effect on E2F-1-associated kinase activity is still unknown, it does not appear to involve dissociation of the Cdk inhibitor p27Kip1 from the complexes as suggested by others. However, we have also found that hormone-treated cells actually show more p16INK4A inhibitor 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 positive effects on E2F-1-associated Cdk-2 and negative effects on Cdk-4 in late G1.