K-ras modulates the cell cycle via both positive and negative regulatory pathways.

The effect of activated human K-ras on cell cycle proteins was studied by use of a stable MCF-7 transfectant expressing inducible activated K-ras under the control of a tetracycline (Tet)-responsive promoter. Induction of activated K-ras by Tet withdrawal accelerated cell growth and entry into S-phase. To understand the mechanism(s) by which activated K-ras exerts its effect on the cell cycle, expression of both cell cycle stimulatory proteins as well as cell cycle inhibitors was examined. Upon induction of activated K-ras, several cell cycle stimulators were up-regulated, including cyclins A, D3, and E, and the E2F family of transcription factors, which was accompanied by increased cyclin A-associated kinase activity and E2F transcriptional activity, respectively. Up-regulation of cyclin A occurred at the transcriptional level and in a serum-dependent manner. Furthermore, induction of activated K-ras down-regulated p27Kip1 and up-regulated p53. Up-regulation of p53 was correlated with enhanced p53 transactivation and accompanied by up-regulation of p21Waf1 and Gadd 45, two p53 effectors and negative cell cycle regulators. In addition, activated K-ras up-regulates bcl-2 but has no effect on bax or bcl-x expression. Taken together, these data indicate that activated K-ras affects the cell cycle by modulating both positive and negative cell cycle regulatory pathways.