Mechanisms of Sertoli cell insulin-like growth factor (IGF)-binding protein-3 regulation by IGF-I and adenosine 3',5'-monophosphate

Abstract

FSH, which stimulates cAMP in the Sertoli cell, markedly lowers the concentration of insulin-like growth factor-binding protein-3 (IGFBP-3) in Sertoli cell-conditioned medium; in contrast, insulin-like growth factor-I (IGF-I) increases BP-3 expression. In this study, the mechanisms controlling the contrasting effects of cAMP and IGF-I were investigated. The abundance of BP-3 mRNA was dramatically lowered by (Bu) 2cAMP, but was unaffected by IGF- I. Analyzed by ligand blot of conditioned medium, coincubation of (Bu) 2cAMP and IGF-I largely eliminated the increase observed with IGF-I alone. Based on the following evidence, the effect of IGF-I appeared to be solely related to the capacity of IGF-I to interact directly with BP-3. 1) Insulin at micromolar concentrations failed to increase BP-3 abundance despite documentation by affinity cross-linking that insulin displaced [ 125I]IGF- I from the IGF-I receptor. 2) A synthetic IGF-I analog, (Leu 24,1-62]IGF-I, which has reduced binding affinity for rat IGF-I receptor but displays high affinity for rat Sertoli cell-conditioned medium BPs, increased BP-3 abundance. 3) A synthetic IGF-I analog, B-chain mutant, which has reduced affinity for rat Sertoli cell BPs but displays normal affinity for the rat IGF-I receptor, failed to increase BP-3 abundance. 4) Human recombinant glycosylated [ 125I]BP-3 when added to cultured Sertoli cells was preserved in the medium when IGF-I or analogs with BP-3 affinity were present. 5) IGF- I, in dose-responsive manner, both retarded the disappearance from the medium of exogenously added human recombinant nonglycosylated BP-3 and decreased the amount of membrane-associated BP-3. These results indicate that whereas cAMP lowers BP-3 abundance in medium, most likely by markedly decreasing synthesis, IGF-I increases BP-3 accumulation by retarding its clearance by the Sertoli cell.