The integrated regulation of different intracellular signaling pathways is fundamental to ensure appropriate timing of cell division. One critical mediator of this regulation is the CDK inhibitor p27kip1, via a mechanism that still presents many controversial aspects. p27kip1 (hereafter p27) is a tumor suppressor protein, implicated in the regulation of several biological activities, thus acting as a relay that integrates different signals and ensuring appropriate timing of cellular behaviors. Knock out of p27 (p27KO) in mice causes an increase in animal size due to increased proliferation and hypercellularity in all tissues. This phenotype seems not to be due to CDK-dependent activity of p27, since it is not rescued by concomitant genetic ablation of the major p27 target, CDK2. Mice double KO (DKO) for p27 and CDK2 still display multiorgan hyperplasia and gigantism. We recently demonstrated that p27 also controls cell morphology and motility, by interacting with stathmin in the cytoplasm. Stathmin is a cytosolic phospho-protein that plays an important role in the regulation of microtubule dynamics, particularly during mitosis. It also participates in many important biological functions during interphase, and human tumors frequently overexpress stathmin, especially in advanced invasive and metastatic stages. Thus, we decided to generate mice DKO for p27 and stathmin, to highlight the possible in vivo consequences of such interaction characterized in vitro. Unexpectedly, the characterization of mice WT, p27 KO and p27/stathmin DKO revealed an important rescue of the most significant phenotypes of p27 KO mice by the DKO ones. Not only body size and organomegalia, but also development of pituitary tumors and outgrowth of retina basal layer were reverted by loss of stathmin in p27 null background. These unpredictable findings clearly indicated that p27/stathmin interaction importantly affected cell proliferation, as well as migration, and we thus focused on the mechanism underlying these phenotypes. We here present evidences that p27kip1 is implicated in the control of the MAPK pathway, through a CDK-independent mechanism. Acting on the subcellular localization and activity of stathmin, p27kip1 controls Ras full activation, and as a consequence, the downstream MAPK cascade. In this way p27kip1 restrains the G1-S phase transition, eventually influencing cell proliferation in vitro, mice growth in vivo and tumorigenesis in humans. These data led us to hypothesize that p27 controls cell proliferation by two different mechanisms: one directly blocking CDK activity and the other interacting with stathmin and thus controlling MAPK pathway activation. Overall our work unveils a new mechanism that in mammalian cells contributes to the proper regulation of cell proliferation, whose alteration may contribute to tumor onset and progression, and that could be of great value in the design of innovative therapeutic approaches.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3043. doi:1538-7445.AM2012-3043