Increased Expression of p 27 Epithelial Cell Line Is Associated with Growth Inhibition and Overexpression of Cyclin E in the HC 11 Mouse Mammary Updated

To elucidate the role of cyclin E ¡ncell growth and tumorigenesis in mammary epithelial cells, we have used retrovirus-mediated transduction to generate derivatives of the nontransformed HC11 mouse mammary epithelial cell line that stably express a human cyclin E cDNA i 1114). These derivatives expressed two distinct forms of the exogenous cyclin E protein, which were about Mr 50,000 and Mr 42,000, thus corresponding to endogenous cyclin E proteins found in human cells. In contrast to results obtained previously in fibroblasts, overexpression of the HU4 cyclin E cDNA in HC11 cells was associated with an increase in cell size, lengthening <>!'<;,. and inhibition of both anchorage-dependent and -independent growth. Furthermore, when quiescent serum-starved cells were restimulated with serum, entry into the S-phase was delayed in the overexpressor cells. Under these conditions, there was also delayed induc tion in the expression of the endogenous cyclin E protein and in other events involved in the G, transition. Despite the high level of expression of the exogenous cyclin E, the derivatives did not display increased cyclin E-associated in vitro kinase activity. The 11C11 cells that overexpressed the exogenous cyclin E displayed an increase in the cyclin/cyclin-dependent kinase inhibitor p27Kip> in both asynchronous exponentially dividing and synchronous cell populations. These findings indicate that increased ex pression of this cyclin E cDNA in HC11 cells inhibits rather than stimu lates growth and that this may be due to increased expression of the inhibitor p27KlpI.