p27kip1 controls H-Ras/MAPK activation and cell cycle entry via modulation of MT stability.

The cyclin-dependent kinase (CDK) inhibitor p27(kip1) is a critical regulator of the G1/S-phase transition of the cell cycle and also regulates microtubule (MT) stability. This latter function is exerted by modulating the activity of stathmin, an MT-destabilizing protein, and by direct binding to MTs. We recently demonstrated that increased proliferation in p27(kip1)-null mice is reverted by concomitant deletion of stathmin in p27(kip1)/stathmin double-KO mice, suggesting that a CDK-independent function of p27(kip1) contributes to the control of cell proliferation. Whether the regulation of MT stability by p27(kip1) impinges on signaling pathway activation and contributes to the decision to enter the cell cycle is largely unknown. Here, we report that faster cell cycle entry of p27(kip1)-null cells was impaired by the concomitant deletion of stathmin. Using gene expression profiling coupled with bioinformatic analyses, we show that p27(kip1) and stathmin conjunctly control activation of the MAPK pathway. From a molecular point of view, we observed that p27(kip1), by controlling MT stability, impinges on H-Ras trafficking and ubiquitination levels, eventually restraining its full activation. Our study identifies a regulatory axis controlling the G1/S-phase transition, relying on the regulation of MT stability by p27(kip1) and finely controlling the spatiotemporal activation of the Ras-MAPK signaling pathway.