Nuclear targeting of cyclin-dependent kinase 2 reveals essential roles of cyclin-dependent kinase 2 localization and cyclin E in vitamin D-mediated growth inhibition

1,25-Dihydroxyvitamin D₃ (1,25-(OH)₂D₃), inhibits proliferation of a variety of cell types including adenocarcinoma of the prostate. We have previously shown that 1,25-(OH)₂D₃ increases the stability of the cyclin-dependent kinase inhibitor p27 ^KIP1, decreases cyclin-dependent kinase 2 (CDK2) activity, and promotes G₁ phase accumulation in human prostate cancer cells. These effects correlate with cytoplasmic relocalization of CDK2. In this study, we investigated the role of CDK2 cytoplasmic relocalization in the antiproliferative effects of 1,25-(OH)₂D₃. CDK2 was found to be necessary for prostate cancer cell proliferation. Although induced by 1,25-(OH)₂D₃, the cyclin-dependent kinase inhibitor p27^KIP1 was dispensable for 1,25-(OH)₂D ₃-mediated growth inhibition. Reduction in CDK2 activity by 1,25-(OH)₂D₃ was associated with decreased T160 phosphorylation, a residue whose phosphorylation in the nucleus is essential for CDK2 activity. Ectopic expression of cyclin E was sufficient to overcome 1,25-(OH)₂D₃-mediated cytoplasmic mislocalization of CDK2 and all antiproliferative effects of 1,25-(OH)₂D₃, yet endogenous levels of cyclin E or binding to CDK2 were not affected by 1,25-(OH)₂D₃. Similarly, knockdown of the CDK2 substrate retinoblastoma, which causes cyclin E up-regulation, resulted in resistance to 1,25-(OH)₂D₃-mediated growth inhibition. Human prostate cancer cells resistant to growth inhibition by 1,25-(OH)₂D ₃ but retaining fully functional vitamin D receptors were developed.Thesecells didnotexhibit 1,25-(OH)₂D₃-mediated cytoplasmic relocalization of CDK2. Targeting CDK2 to the nucleus of 1,25-(OH)₂D₃-sensitive cancer cells blocked G₁ accumulation and growth inhibition by 1,25-(OH)2D3. These data establish central roles for CDK2 nuclear-cytoplasmic trafficking and cyclin E in the mechanism of 1,25-(OH)₂D₃-mediated growth inhibition in prostate cancer cells.