The biologically active form of vitamin D, 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) has been shown to regulate the proliferation of human prostate epithelial cell lines. Since the insulin-like growth factor (IGF) system is involved in the transformation process of epithelial cells, the following study was undertaken to determine if the IGF system, in particular IGF binding protein-3 (IGFBP-3), is altered by 1,25-(OH)2D3 in normal prostate epithelial cells as part of a mechanism for inhibition of transformation. Two cell systems were used in this study: (1) primary cultures of benign human prostate epithelial cells (PECs) and (2) an SV40-T immortalized prostate epithelial cell line (P153) that is non-tumorigenic. 1,25-(OH)2D3 was added to parallel sets of PECs and P153 cells in addition to the presence or absence of IGF-I or des(1-3)IGF-I. Treatment with 1,25-(OH)2D3 resulted in significant growth inhibition of both PECs and P153 cells. Furthermore, 1,25-(OH)2D3 inhibited IGF-induced proliferation, but this was partially reversed by high concentrations of IGF-I. Western ligand blots of condition media demonstrated a significant increase in IGFBP-3; likewise Northern blots demonstrated an increase in mRNA for IGFBP-3. Proliferation assays using an antibody designed to block the IGF-independent effects of IGFBP-3 failed to reverse the inhibitory effect of 1,25-(OH)2D3. Thus, IGFBP-3 acts in an IGF-dependent manner to inhibit cell growth of benign prostate epithelial cells.

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