| Prostate cancer is the most commonly diagnosed cancer of American men and the second leading cause of male cancer death. 1alpha, 25 dihydroxyvitamin D3 (1alpha, 25-(OH)2D3) has been shown to have an antiproliferative effect on prostate cancer cells. We hypothesized that 1alpha, 25-(OH)2D3 treatment causes the upregulation of the cell cycle dependent kinase inhibitor p21 transcription, resulting in increased p21 protein levels which, in turn, mediate the antiproliferative effect by slowing progression through the cell cycle. Therefore, we examined the effect of 1alpha, 25-(OH)2D3 treatment on p21 expression in multiple prostate cancer cell lines. Of the cell lines studied, two are growth inhibited by treatment with 1alpha, 25-(OH)2D 3 (LNCaP and ALVA-31) and two (JCA1 and TSU-Pr1) are not. Only the growth inhibited lines showed upregulation of the p21 mRNA and protein. Stable transfection of p21 antisense cDNA into the ALVA-31 cell line abrogated the growth inhibitory effects of 1alpha, 25-(OH)2D3. It has been proposed that p21 mediates the antiproliferative effect of TGFbeta. Additionally, synergistic actions between the growth inhibitory properties of 1alpha, 25-(OH)2D3 and TGF-beta have been reported in several cell types. Thus, we further hypothesized that the growth inhibitory response to 1alpha, 25-(OH)2D3, as mediated by p21, is TGF-beta dependent. The ALVA-31 cell line shows both increased expression of p21 and growth inhibition in response to treatment with exogenous TGF-beta1. Furthermore, 1alpha,25(OH)D3 treatment of ALVA-31 cells increases TGF-beta1 mRNA and protein expression. Simultaneous addition of a panneutralizing TGF-beta antibody in a growth inhibition assay using 1alpha, 25-(OH)2D 3 ablates the growth inhibitory effects of 1alpha, 25-(OH)2D 3 in the ALVA-31 cell line. Conversely, similarly treated LNCaP cells maintained an antiproliferative response. We conclude that there are at least two separate pathways for 1alpha, 25-(OH)2D3 regulation of p21 expression levels, which ultimately leads to the slowing of the cell cycle. Because of the potential therapeutic implications, we investigated the interactions between 1alpha, 25-(OH)2D3 and platinum based chemotherapeutic agents. The antiproliferative effects of cisand carboplatin are synergistically increased in both androgen responsive and androgen unresponsive cell lines. These results hold promise for the treatment of patients with advanced stage prostate cancer. |
