1 alpha,25-Dihydroxyvitamin D-3 Inhibits C4-2 Prostate Cancer Cell Growth via a Retinoblastoma Protein (Rb)-Independent G(1) Arrest

BACKGROUND. The active metabolite of vitamin D, 1 alpha,25-dihydroxyvitamin D-3 (1,25D) reduces the growth of several prostate cancer cell lines, most commonly by inducing a cell-cycle arrest in G(1). This is mediated, in part, through down-regulation of c-Myc, a positive regulator of the transcription factor, E2F. There is evidence that prostate cancer cells lacking functional retinoblastoma protein (Rb), a negative regulator of E2F activity, are poorly responsive to 1,25D treatment. Since up to 60% of prostate cancers demonstrate a loss of heterozygosity for Rb, we sought to determine whether Rb is required for the growth inhibitory effects of 1,25D. METHODS. Using siRNA, Rb was reduced in C4-2 prostate cancer cells, and the response of cells to 1,25D treatment or depletion of c-myc measured by [H-3]-thymidine incorporation and flow cytometry. The effects of 1,25D treatment on E2F levels and activity, and E2F target gene expression were also measured. RESULTS. 1,25D treatment and c-Myc depletion both cause a G(1) arrest inhibiting C4-2 cell proliferation independently of Rb. 1,25D reduces c-Myc expression and causes a decrease in E2F and E2F target genes. Bcl-2, an E2F target and positive regulator of C4-2 cell growth, also is down-regulated by 1,25D independently of Rb. CONCLUSIONS. Redundant growth inhibitory pathways compensate for the loss of Rb, and tumors lacking functional Rb may be responsive to 1,25D. Prostate 71: 98-110, 2011. (C) 2010 Wiley-Liss, Inc.