OVEREXPRESSION OF CYCLIN-E IN THE HC11 MOUSE MAMMARY EPITHELIAL-CELL LINE IS ASSOCIATED WITH GROWTH-INHIBITION AND INCREASED EXPRESSION OF P27(KIP1)

To elucidate the role of cyclin E in cell growth and tumorigenesis in mammary epithelial cells, we have used retrovirus-mediated transductio n to generate derivatives of the nontransformed HC11 mouse mammary epi thelial cell line that stably express a human cyclin E cDNA (HU4). The se derivatives expressed two distinct forms of the exogenous cyclin E protein, which were about M(r) 50,000 and M(r) 42,000, thus correspond ing to endogenous cyclin E proteins found in human cells. In contrast to results obtained previously in fibroblasts, overexpression of the H U4 cyclin E cDNA in HC11 cells was associated with an increase in cell size, lengthening of G(1), and inhibition of both anchorage-dependent and -independent growth. Furthermore, when quiescent serum-starved ce lls were restimulated with serum, entry into the S-phase was delayed i n the overexpressor cells. Under these conditions, there was also dela yed induction in the expression of the endogenous cyclin E protein and in other events involved in the G(1) transition. Despite the high lev el of expression of the exogenous cyclin E, the derivatives did not di splay increased cyclin E-associated irt vitro kinase activity. The HC1 1 cells that overexpressed the exogenous cyclin E displayed an increas e in the cyclin/cyclin-dependent kinase inhibitor p27(Kip1) in both as ynchronous exponentially dividing and synchronous cell populations. Th ese findings indicate that increased expression of this cyclin E cDNA in HC11 cells inhibits rather than stimulates growth and that this may be due to increased expression of the inhibitor p27(Kip1).