Rac and Cdc42 are potent stimulators of E2F-dependent transcription capable of promoting retinoblastoma susceptibility gene product hyperphosphorylation.

The Rho family of GTPases plays an important and diverse role in reorganization of the actin cytoskeleton, transcriptional regulation, and multiple aspects of cell growth. Our study has examined their potential links to the cell cycle machinery. We find that constitutively active mutants of Rac and Cdc42, but not Rho, are potent inducers of E2F transcriptional activity in NIH 3T3 fibroblasts. Furthermore, activated Rac and Cdc42, but again not Rho, are capable of inducing cyclin D1 accumulation and pRB hyperphosphorylation in serum-deprived cells, outlining one route leading to enhanced E2F-mediated transcription. The inhibitory effect of the cyclin-dependent kinase inhibitors, p16(ink4), p21(cip1), and p27(cip) on Rac/Cdc42-mediated E2F transcription corroborates a role for pRB family members and their functional inactivation by cyclin-dependent kinases in generating E2F activity. While the up-regulation of E2F transcriptional activity by Rac or Cdc42, not Rho, suffices for entry into S phase and DNA synthesis in Rat-1 R12 cells, this is clearly not the case in NIH 3T3, where additional requirements must exist.