Role of the F-box protein Skp2 in adhesion-dependent cell cycle progression

Cell adhesion to the extracellular matrix (ECM) is a requirement for prolif eration that is typically lost in malignant cells. In the absence of adhesi on, nontransformed cells arrest in G1 with increased levels of the cyclin-d ependent kinase inhibitor p27. We have reported previously that the degrada tion of p27 requires its phosphorylation on Thr-187 and is mediated by Skp2 , an F-box protein that associates with Skp1, Cull, and Roc1/Rbx1 to form t he SCFSkp2 ubiquitin ligase complex. Here, we show that the accumulation of Skp2 protein is dependent on both cell adhesion and growth factors but tha t the induction of Skp2 mRNA is exclusively dependent on cell adhesion to t he ECM, Conversely, the expression of the other three subunits of the SCFSk p2 complex is independent of cell anchorage. Phosphorylation of p27 on Thr- 187 is also not affected significantly by the loss of cell adhesion, demons trating that increased p27 stability is not dependent on p27 dephosphorylat ion. Significantly, ectopic expression of Skp2 in nonadherent G1 cells resu lted in p27 downregulation, entry into S phase, and cell division. The abil ity to induce adhesion-independent cell cycle progression was potentiated b y coexpressing Skp2 with cyclin D1 but not with cyclin E, indicating that S kp2 and cyclin D1 cooperate to rescue proliferation in suspension cells. Ou r study shows that Skp2 is a key target of ECM signaling that controls cell proliferation.