RhoA Stimulates p27 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 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 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 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. Cotransfection of dominant-negative CDK2 and RhoA(63) inhibited RhoA(63)-induced cyclin E/CDK2 activity and p27 degradation. In addition, expression of dominant-negative RhoA blocked seruminduced cyclin E/CDK2 activity and p27 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 degradation through its regulation of cyclin E/CDK2 activity.