Hypoxia-inducible factor 1 mediates hypoxia-enhanced synthesis of progesterone during luteinization of granulosa cells.

Hypoxia has been suggested to enhance progesterone (P4) synthesis in luteinizing granulosa cells (GCs), but the mechanism is unclear. The present study was designed to test the hypothesis that the hypoxia-induced increase in P4 synthesis during luteinization in bovine GCs is mediated by hypoxia-inducible factor 1 (HIF-1). GCs obtained from small antral follicles were cultured with 2 µg/ml insulin in combination with 10 µM forskolin for 24 h as a model of luteinizing GCs. To examine the influence of HIF-1 on P4 synthesis, we determined the effect of changes in protein expression of the α-subunit of HIF-1 (HIF1A) on P4 production and on the expression levels of StAR, P450scc, and 3β-HSD. CoCl2 (100 µM), a hypoxia-mimicking chemical, increased HIF-1α protein expression in luteinizing GCs. After the upregulation of HIF-1α, we observed an increase in P4 production and in the gene and protein expression levels of StAR in CoCl2-treated luteinizing GCs. In contrast, CoCl2 did not affect the expression of either P450scc or 3β-HSD. Echinomycin, a small-molecule inhibitor of HIF-1's DNA-binding activity, attenuated the effects of CoCl2 and of low oxygen tension (10% O2) on P4 production and StAR expression in luteinizing GCs. Overall, these findings suggest that HIF-1 is one of the factors that upregulate P4 in GCs during luteinization.