Insulin-like growth factors (IGFs), IGF receptors, and IGF-binding proteins in primary cultures of prostate epithelial cells

Insulin-like growth factors (IGFs) are potent mitogens that bind with high affinity and specificity to IGF receptors and IGF-binding proteins (IGFBPs). We studied the roles of these three groups of proteins in prostate epithelial cells (PEC) in primary culture grown under serum-free conditions. Affinity cross-linking of IGF-I and IGF-II to crude membranes prepared from PEC revealed an abundance of type 1 IGF receptors and no evidence of type 2 IGF receptors. Western ligand blots of conditioned media (CM) from PEC demonstrated the presence of two specific IGFBP bands similar to those previously demonstrated in seminal plasma, with approximate mol wt of 31 and 24 kDa. The 31-kDa band was immunoprecipitable with an antibody to IGFBP-2, and neither band could be deglycosylated with endoglycosidase-F. Northern blot analysis of poly (A)⁺ RNA prepared from PEC with cDNAs for hIGFBP-1, -2, and - 3 documented the expression of mRNA for hIGFBP-2 only. Modifications of the serum-free conditions of PEC did not significantly alter the IGFBP profile of PEC CM. The ability of IGF-I, IGF-II, and insulin to stimulate clonal growth of PEC was examined. IGF-I stimulated PEC growth with an ED₅₀ of 0.1 ng/mL. IGF-II and insulin, respectively, were 1 and 3 orders of magnitude less effective than IGF-I in stimulating the growth of PEC. Radioimmunoassayable IGF-I and IGF-II levels in PEC CM were below the assay detection levels. In conclusion, we suggest that IGFs are important growth stimulators of PEC in culture, that their actions are mediated through the type 1 IGF receptor, and that PEC produce hIGFBP-2 and a 24-kDa IGFBP which may modulate IGF action in these cells.