P27Kip1 stabilization and G1 arrest by 1,25-dihydroxyvitamin D3 in ovarian cancer cells mediated through down-regulation of cyclin E/cyclin-dependent kinase 2 and Skp1-Cullin-F-box Protein/Skp2 ubiquitin ligase

p27(Kip1) (p27) is a tumor suppressor whose stability is controlled by proteasome-mediated degradation, a process directed in part by cyclin-dependent kinase 2 (CDK2)-mediated phosphorylation of p27 at Thr(187) and its subsequent interaction with the Skp1-Cullin-F-box protein/Skp2 (Skp2) ubiquitin ligase. The present study shows that 1,25-dihydroxyvitamin D-3 (1,25(OH)(2)D-3) arrests ovarian cancer cells in G(1) by stabilizing the p27 protein. 1,25( OH) 2D3 initiates a chain of events by decreasing the amounts of cyclin E and cyclin E-associated CDK2 activity. As a result, p27 phosphorylation at Thr187 and consequently the interaction with Skp2 are decreased. 1,25( OH) 2D3 also increases p27 stability by decreasing the abundance of Skp2. It is the combined effect of 1,25(OH)(2)D-3 on both the CDK2-dependent phosphorylation of p27, and thus its affinity for Skp2, and Skp2 expression that dramatically increases the stability of the p27 protein. Similar to its effects in ovarian cancer cells, 1,25( OH) 2D3 induces p27 accumulation in wild type mouse embryo fibroblasts and arrests wild type but not p27-null mouse embryo fibroblasts in G(1). Stable expression of Skp2 in OVCAR3 cells diminishes the G(1) arrest and decreases the growth response to 1,25( OH) 2D3. Taken together, the results of this study identify p27 as the key mediator of 1,25(OH)(2)D-3-induced growth suppression in G(1) and show that the hormone achieves this by decreasing the activity of CDK2 and reducing the abundance of Skp2, which act together to degrade p27.