In VivoRegulation of Granulosa Cell Somatomedin-C/ Insulin-Like Growth Factor I Receptors*

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Abstract
The characteristics and regulation of the murine granulosa cell type I insulin-like growth factor (IGF) receptor under in vivo conditions were studied. In vivo treatment of immature hypophysectomized diethylstilbestrol-treated rats with increasing doses (0.3-30 .mu.g/rat, twice daily) of FSH for 72 h resulted in dose-dependent increments in specific granulosa cell somatomedin-C (Sm-C)/IGF-I binding, peaking (5150 .+-. 350 cpm/3 .times. 105 cells) at the 10 .mu.g/rat (twice daiy) dose level to yield a 2.6-fold increase relative to that in untreated controls. This FSH (10 .mu.g/rat, twice daily) effect proved time dependent; the first significant (P < 0.05) increase in binding (3670 .+-. 150 cpm/3 .times. 105 cells) was noted after 48 h of treatment (1.6-fold increase). Significantly, little or no variation was observed for basal Sm-C/IGF-I binding over the course of the experiment, suggesting that this component of Sm-C/IGF-I receptor complement may be independent of the trophic influence(s) of the pituitary gland. Equilibrium competition studies carried out with granulosa cells derived from both control and FSH-treated rats revealed linear Scatchard plots consistent with a single class of noninteracting binding sites, a 2.8-fold increase in FSH-associated Sm-C/IGF-I-binding capacity, but not affinity (Kd control, 1.9 .+-. 0.3 nM; kd FSH, 2.6 .+-. 0.9 nM). Limited specificity studies of the FSH-induced receptor revealed related peptides to compete for Sm-C/IGF-I binding with a relative rank order of potency of Sm-C/IGF-I .mchgt. multiplication-stimulating activity > insulin, a pattern compatible with a type I IGF receptor. A series of other polypeptides, including porcine relaxin, porcine proinsulin, epidermal growth factor, basic fibroblast growth factor as well as transforming growth factor-.alpha. and -.beta. (TGF.beta.) were nonreactive. Significantly, the induced type I IGF receptor proved functionally coupled to granulosa cell proteoglycan biosynthesis. The ability of FSH (10 .mu.g/rat, twice daily) to enhance granulosa cell Sm-C/IGF-I binding was significantly (P < 0.05) up-regulated (1.53-fold amplification) by ovine GH (100 .mu.g/rat, twice daily); a down-regulatory effect (64% inhibition) was observed for a potent GnRH agonist ([D-Ala6,Des-Gly10]GnRH ethyl amide; 25 .mu.g/rat, twice daily). Once induced, the Sm-C/IGF-I receptor of the granuloma cell required the continued presence of either FSH or LH for its maintenance; the lactogenic receptor agonist PRL had no effect. Our findings indicate that the ability of FSH to up-regulate granulosa cell Sm-C/IGF-I receptors 1) is not strictly an in vitro phenomenon, but can be fully reproduced in vivo; 2) is due to enhancement of Sm-C/IGF-I-binding capacity rather than affinity; 3) may be subject to modulation by somatogenic and GnRH-like granulosa cell agonists; and 4) is best maintained by gonadotropins, but not PRL.