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.

Hypoxia promotes progesterone synthesis during luteinization in bovine granulosa cells

To determine whether hypoxia has an effect on luteinization, we examined the influence of hypoxia on a model of bovine luteinizing and non-luteinizing granulosa cell culture. The granulosa cells were obtained from small antral follicles (≤ 6 mm in diameter). To induce luteinization, the cells were treated for 24 h with insulin (2 µg/ml), forskolin (10 µM) or insulin in combination with forskolin at 20% O2. After 24 h, progesterone (P4) production was higher in the treated cells, which we defined as luteinizing granulosa cells, than in non-treated cells, which we defined as non-luteinizing granulosa cells. P4 production by non-luteinizing granulosa cells was not affected by hypoxia (24 h at 10% and 5% O2), while P4 production by granulosa cells treated with insulin in combination with forskolin was significantly increased under hypoxia (24 h at 10% and 5% O2). Because hypoxia affected P4 production by the luteinizing granulosa cells but not by the non-luteinizing granulosa cells, hypoxia seems to promote P4 production during, rather than before, luteinization. In the cells treated with insulin in combination with forskolin, mRNA and protein expression of steroidogenic acute regulatory protein (StAR) and protein expression of 3β-hydroxysteroid dehydrogenase (3β-HSD) increased under 10% O2, while mRNA and protein expressions of key protein and enzymes in P4 biosynthesis did not increase under 5% O2. The overall results suggest that hypoxia plays a role in progressing and completing the luteinization by enhancing P4 production through StAR as well as 3β-HSD expressions in the early time of establishing the corpus luteum.

Effects of 17.BETA.-Estradiol on In Vitro Maturation of Pig Oocytes in Protein-Free Medium

The present study examined the effects of 17 beta-estradiol (E(2)) on in vitro maturation and subsequent in vitro fertilization of pig oocytes matured with or without cumulus cells. When E(2) (10 ng/ml) was added to the protein-free maturation medium, the proportions of cumulus-enclosed oocytes that underwent germinal vesicle breakdown and reached metaphase II were significantly reduced (P<0.05), and cumulus expansion was also significantly inhibited (P<0.05) compared with the control (no E(2) added). Although oocytes matured in the presence of E(2) were penetrated by sperm in vitro at the same level as the control, the incidences of male pronuclear (MPN) formation and activated oocytes were significantly lower (P<0.05) than the control. These inhibitory effects of E(2) were prevented when the medium was supplemented with E(2) together with its antagonist, ICI 182,780 (1 microg/ml), although the presence of the antagonist alone in the medium had no effect on the maturation and fertilization in vitro of oocytes. In cumulus-free oocytes, E(2) had no effect on nuclear maturation and penetration in vitro, but low MPN formation was observed in oocytes matured in the presence and absence of E(2). When cumulus-enclosed oocytes were cultured in the presence of progesterone (P(4); 600 ng/ml) alone or together with E(2), no significant differences in nuclear maturation, cumulus expansion or penetration in vitro were observed compared with control oocytes. The concentration of P(4) in maturation medium was significantly (P<0.01) lower when cumulus-enclosed oocytes were cultured for 44 h in the medium with E(2) than in medium without E(2). These results indicate that E(2) inhibits both nuclear and cytoplasmic maturation of cumulus-enclosed pig oocytes, and that this inhibition can be prevented by an E(2) antagonist or P(4). This E(2) inhibition may occur indirectly via the cumulus cells and inhibition of P(4) synthesis.