DEXAMETHASONE POTENTIATES IGF-I ACTIONS IN PORCINE GRANULOSA-CELLS

To understand better interactions between glucocorticoids and insulin- like growth factor I (IGF-I) in the ovary, we studied the effects of d examethasone on IGF-I stimulation of P-450 cholesterol side-chain clea vage enzyme (P-450(scc)) mRNA concentrations in porcine granulosa cell s. Dexamethasone potentiated IGF-I-stimulated P-450(scc), mRNA concent rations and progesterone production in granulosa cell cultures. Time-c ourse and dose-response studies showed that maximal enhancement occurr ed at a 1-mu M dexamethasone concentration after 48 h of treatment. Th is potentiation was prevented by the glucocorticoid receptor antagonis t RU-38486, 17 beta-hydroxy-11 eta-[-4-dimethyl-aminophenyl]estra-4,9, -dien-3-one (RU-486). We investigated mechanisms for this potentiation by performing IGF-I binding studies in porcine granulosa cells. Dexam ethasone increased IGF-I binding, and Scatchard analysis showed this e nhanced binding was caused by an increase in receptor concentration. N orthern blot hybridization using a rat type I IGF-I receptor gene ribo probe showed that although dexamethasone alone did not increase IGF-I receptor mRNA concentrations, it did prevent a decrease in receptor mR NA concentrations caused by IGF-I. In addition, we used synthetic prim ers from conserved regions of the rat type I IGF-I receptor gene with total RNA from porcine granulosa cells and polymerase chain reaction t o isolate a 615-base pair porcine type I IGF-I receptor cDNA clone. Ri bonuclease protection assay results were similar to those found with t he rat IGF-I receptor riboprobe. We conclude that dexamethasone potent iates IGF-I actions on steroidogenesis in the porcine ovary. Possible mechanisms for this potentiation include the ability of dexamethasone to enhance IGF-I binding to porcine granulosa cells and prevent decrea ses in type I IGF-I receptor mRNA concentrations caused by IGF-I.