The insulin-like growth factors (IGFs) are found in extracellular fluids bound to carrier proteins which influence the biological activity of the IGFs. Three structurally different binding proteins (BPs) have been isolated and cloned; each has distinct tissue specific expression and unique properties. We report here that testicular cells synthesize a specific subset of these binding proteins. Ligand blot analysis and RNA blot hybridization indicates that cultured peritubular cells synthesize primarily IGFBP-2. In contrast, as determined by ligand blot, RNA blot hybridization and N-linked deglycosylated studies, IGFBP-3 is predominantly synthesized by the Sertoli cell. In a dose dependent fashion, FSH markedly reduces the levels of IGFBP-3 in Sertoli cell conditioned medium. Similarly, isoproterenol, (Bu)2cAMP and cholera toxin also markedly reduce the abundance of IGFBP-3 in conditioned media. In contrast, IGFI increases the concentrations of IGFBP-3 with the concentration required for half-maximal stimulation, approximately 20 ng/ml. Consistent with a peritubular cell origin, IGFBP-2 may be the predominant species found in interstitial fluid. In summary, our data reveal that the IGFBPs are expressed in a cell type specific manner in the testis. The opposing effects of FSH and IGF-I on Sertoli cell IGFBP-3 expression suggests a mechanism by which the IGF-I biological activity on Sertoli cell might be influenced.
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