1,25(dihydroxy) vitamin D(3)-mediated growth inhibition of prostate cancer cells

1,25(OH)2 vitamin D3 (1,25(OH) 2 D) exerts important actions on cellular growth and differentiation in prostate. These growth effects occur mostly via regulation of the cell cycle. LNCaP, an androgen receptor (AR) positive, androgen-dependent prostate cancer cell line, exhibits the greatest antiproliferative effect upon 1,25(OH) 2 D treatment. Consistent with this growth inhibition, 1,25(OH) 2 D persistently upregulates the cyclin-dependent kinase inhibitor (CKI) p27Kip1, increases the association of p27 with cyclin-dependent kinase 2 (Cdk2), and inhibits phosphorylation of the retinoblastoma protein, which ultimately leads to accumulation of cells in the G1 phase of the cell cycle. In contrast, ALVA31, an AR negative, androgen-independent prostate cancer cell line, are minimally inhibited with 1,25(OH)2 D despite expressing nearly twice the levels of functional vitamin D receptor (VDR) as LNCaP. Because ALVA31 cells lack AR and are resistant to 1,25(OH) 2 D-mediated growth inhibition while LNCaP cells express AR and are the most sensitive to growth inhibition of the prostate cancer cell lines, it has been proposed that the antiproliferative effects of 1,25(OH)2 D involve androgen/AR.In this study, we investigated the mechanism of 1,25(OH)2 D-mediated upregulation of p27 and the possible role of androgens in 1,25(OH) 2 D mediated growth inhibition. 1,25(OH)2 D stabilizes p27 protein by decreasing Cdk2-mediated Thr187-phosphorylation of p27, the critical event that targets p27 for ubiquitin-dependent proteolysis. Cdk2 activation occurs in the nucleus. 1,25(OH)2 D treatment also results in decreased nuclear Cdk2, suggesting that 1,25(OH)2 D may be decreasing Cdk2 activation by keeping it in the cytoplasm.In addition, we found that 1,25(OH)2 D resistance in ALVA31 cells is not due to lack of AR. Heterologous stable expression of AR in ALVA31 cells does not confer enhanced growth inhibition by 1,25(OH)2 D even in the presence of androgens. Also, LNCaP-104R1 cells, an androgen-independent derivative of LNCaP, are profoundly growth inhibited by 1,25(OH)2 D in the absence of androgens or in the presence of antiandrogen Casodex. Similar to LNCaP, 1,25(OH)2 D-mediated antiproliferative effects in LNCaP-104R1 cells involve G1 accumulation and increased p27 associated with Cdk2. Thus, a critical determinant of 1,25(OH)2 D-mediated growth inhibition may be the capacity to regulate Cdk2 localization