RhoA stimulates p27(Kip) degradation through its regulation of cyclin E/CDK2 activity.

RhoA has been identified as an important regulator of cell proliferation. We recently showed that the Ras/RhoA pathway regulates the degradation of p27(Kip) and the progression of Chinese hamster embryo fibroblasts (IIC9 cells) through G1 into S phase (Weber, J. D., Hu, W., Jefcoat, S. C., Raben, D. M., and Baldassare, J. J. (1997) J. Biol. Chem. 272, 32966-32971). In this report, we have demonstrated that, in IIC9 cells, RhoA regulates cyclin E/CDK2 activity, which is required for p27(Kip) degradation. As previously shown in several fibroblasts cell lines, expression of dominant-negative CDK2 in IIC9 cells blocked serum-induced cyclin E/CDK2 activity and p27(Kip) degradation. In the absence of serum, expression of constitutively active RhoA(63) resulted in significant stimulation of cyclin E/CDK2 activity and degradation of p27(Kip). Cotransfection of dominant-negative CDK2 and RhoA(63) inhibited RhoA(63)-induced cyclin E/CDK2 activity and p27(Kip) degradation. In addition, expression of dominant-negative RhoA blocked serum-induced cyclin E/CDK2 activity and p27(Kip) degradation. Finally, expression of catalytically active cyclin E/CDK2 rescued the effect of expression of dominant-negative RhoA. Taken together, these data show that RhoA regulates p27(Kip) degradation through its regulation of cyclin E/CDK2 activity.