Suramin, hydrocortisone, and retinoic acid modify inhibitory effects of 1,25-dihydroxyvitamin D3 on prostatic epithelial cells

The proliferation of prostatic epithelial cells is regulated by the complex interplay of numerous growth-stimulatory and growth-inhibitory factors. 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] has recently been identified as a potent inhibitor of the growth of prostatic epithelial cells. Epidemiologic studies indicate that vitamin D deficiency may be a risk factor for the development of clinical prostate cancer, possibly due to increased growth and reduced differentiation of prostatic cells in an environment with decreased 1,25(OH)2D3. The application of vitamin D or analogs in chemotherapy against prostate and other cancers is being explored by several investigators. In order to use vitamin D most efficaciously in a clinical setting, it may be beneficial to learn more about the interaction of 1,25(OH)2D3 with other factors that regulate prostatic epithelial cellular growth. In this study, we examined the effect of the proliferative status of cultured cells on their ability to respond to 1,25(OH)2D3, and found that minimally proliferative cells were equally as responsive to 1,25(OH)2D3 as actively dividing cells. We noted no apparent interaction of 1,25(OH)2D3 with epidermal growth factor, insulin-like growth factor, cholera toxin, or transforming growth factor-β, but we did find synergistic inhibitory effects of 1,25(OH)2D3 with suramin and retinoic acid. Perhaps most noteworthy was the dramatic increase in potency of 1,25(OH)2D3 that occurred upon deletion of hydrocortisone from the culture medium. Our in vitro studies indicate that combination therapy of vitamin D analogs with suramin, vitamin A analogs, or anti-glucocorticoids might be considered for prostate cancer.