Insulin-like growth factor I (IGF-I) and the gonadotropin, FSH, can synergize to stimulate progesterone production in primary cultures of maturing human, rat, and pig granulosa cells. These trophic hormones act by increasing the activity and production of proteins and their gene transcripts essential to sterol uptake, delivery, and utilization in steroidogenesis. We previously observed that FSH and IGF-I interact synergistically to promote the accumulation of steroidogenic acute regulatory protein (STAB) messenger RNA and protein in granulosa cells. Here we investigate potential mechanisms of IGF-I synergy with FSH and the protein kinase A (PKA) pathway in activating the porcine StAR gene promoter. To this end, we first cloned 1423 bp of the porcine StAR promoter upstream of the transcriptional start site using PCR and created 5'-deletional constructs coupled to a cytoplasmically targeted firefly luciferase reporter gene. FSH, 8-bromo-cAMP, and transient transfection of the protein kinase A (PKA) catalytic subunit (driven by the Rous sarcoma virus promoter) were used to activate the PKA effector pathway. All three agonists alone stimulated StAR promoter-driven luciferase activity in primary cultures of granulosa cells after 4-h treatment. IGF-I significantly augmented PKA pathway agonist activation of the StAR promoter, whereas IGF-I had no effect alone. Binding experiments with 125I-labeled ovine FSH-20 in IGF-I (100 ng/ml)-treated granulosa cells showed that FSH binding affinity and receptor number were unchanged by IGF-I treatment. However, IGF-I augmented FSH-stimulated, but not forskolin-stimulated, cAMP accumulation. Analysis of 5'-deletion constructs of the StAR promoter revealed three regions of stimulatory activity within the -139-bp fragment upstream of the transcriptional start site as well as another potentially inhibitory region upstream (-1115 to -905). Elimination of the putative SF-1 site (-48 to -41) virtually abolished StAR promoter responsiveness. In summary, our data indicate that IGF-I can act via two post FSH-binding mechanisms to augment FSH/PKA pathway-mediated StAR gene promoter transactivation: at the level of cAMP accumulation and distal to cAMP production and PKA activation.
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