INSULIN-LIKE GROWTH-FACTOR (IGF) BINDING-PROTEIN-3 IN THE RAT TESTIS - FOLLICLE-STIMULATING-HORMONE DEPENDENCE OF MESSENGER-RNA EXPRESSION AND INHIBITION OF IGF-I ACTION ON CULTURED SERTOLI CELLS

Insulin-like growth factor binding protein 3 (IGFBP-3) is the predomin ant IGF binding protein produced by cultured rat Sertoli cells. Previo us studies have shown that FSH lowers the abundance of IGFBP-3 protein in Sertoli cell culture medium, suggesting that this binding protein has a physiological role in modulating insulin-like growth factor-I (I GF-I) activity in the testis. To characterize the physiological releva nce of the FSH regulation of IGFBP-3, in this study we examined effect s of FSH on IGFBP-3 mRNA expression in testes from hypophysectomized r ats and in Sertoli cell culture. We then examined whether or not IGFBP -3 could alter the effects of IGF-I on cultured Sertoli cells. FSH (1- 300 ng/ml) treatment of rat Sertoli cells dose-dependently decreased I GFBP-3 mRNA, with near-complete inhibition by 12 h of treatment. To ev aluate this effect in the whole animal, male rats were hypophysectomiz ed at 20 days of age and injected with FSH 10 days later. Testis IGFBP -3 mRNA increased following hypophysectomy relative to the value in sh am animals, while FSH treatment decreased IGFBP-3 mRNA to sham levels within 6 h. To determine whether or not IGFBP-3 modulates IGF-I activi ty in the Sertoli cell, recombinant human non-glycosylated rIGFBP-3 wa s added to Sertoli cell cultures in the presence or absence of IGF-I. The rIGFBP-3 dose-dependently inhibited IGF-I-stimulated lactate produ ction with a half-maximal dose of 100 ng/ml. Neither basal lactate pro duction nor FSH-stimulated lactate was altered by addition of rIGFBP-3 . Recombinant N-glycosylated rgIGFBP-3 inhibited IGF-I-stimulated lact ate production in an identical manner to that of the non-glycosylated binding protein and was without effect on FSH activity. Preincubation of Sertoli cells with either glycosylated or non-glycosylated IGFBP-3 (1000 ng/ml) for 24 or 72 h followed by IGF-I treatment showed neither inhibition nor stimulation of the IGF-I effect on lactate. The result s demonstrate that IGFBP-3 inhibits IGF-I action in the Sertoli cell. Additionally, although FSH action on lactate appears to be independent of changes in the autocrine expression of IGFBP-3 in the cultured Ser toli cell environment, the potent inhibition of IGFBP-3 by FSH suggest s that IGFBP-3 may modulate IGF-I activity in the testis.