25-hydroxyvitamin D3 is an active hormone in human primary prostatic stromal cells

According to the present paradigm, 1alpha,25-dihydroxyvitamin D-3 [1alpha,25-(OH)(2)D-3] is a biologically active hormone; whereas 25-hydroxyvitamin D-3 (25OHD(3)) is regarded as a prohormone activated through the action of 25-hydroxyvitamin D-3 1alpha-hydroxylase (1alpha-hydroxylase). Although the role of vitamin D-3 in the regulation of growth and differentiation of prostatic epithelial cells has been well studied, its action and metabolism in prostatic stroma are still largely unknown. We investigated the effects of 25OHD(3) and 1alpha,25-(OH)(2)D-3 on two human stromal primary cultures termed P29SN and P32S. In a cell proliferation assay, 25OHD(3) was found at physiological concentrations of 100-250 nM to inhibit the growth of both primary cultures, whereas 1alpha,25-(OH)(2)D-3 at a pharmacological concentration of 10 nM exhibited the growth-inhibitory effects on P29SN cells but not on P32S cells. Quantitative real-time RT-PCR analysis revealed that both 25OHD(3) and 1alpha,25-(OH)(2)D-3 induced 25-hydroxyvitamin D-3 24-hydroxylase (24-hydroxylase) mRNA in a dose- and time-dependent manner. By inhibiting 1alpha-hydroxylase and/or 24-hydroxylase enzyme activities, the induction of 24-hydroxylase mRNA by 250 nM 25OHD(3) was clearly enhanced, suggesting that 1alpha-hydroxylation is not a prerequisite for the hormonal activity of 25OHD(3). Altogether our results suggest that 25OHD(3) at a high but physiological concentration acts as an active hormone with respect to vitamin D-3 responsive gene regulation and suppression of cell proliferation.