Effects of luteinizing hormone on peroxisome proliferator-activated receptor γ in the rat ovary before and after the gonadotropin surge

Abstract

We have shown previously that mRNA for peroxisome proliferator-activated receptor γ (PPARγ) is expressed in granulosa cells and downregulated by the luteinizing hormone (LH) surge. The current studies were undertaken to test the hypothesis that LH stimulates a decrease in the expression of PPARγ, as well as its activity, in granulosa cells. Ovaries were collected from immature rats 0 and 48 h after they received pregnant mares’ serum gonadotropin (PMSG), and 4 and 24 h after administration of human chorionic gonadotropin (hCG), and used for protein isolation or processed for immunolocalization of PPARγ. The amount of phosphorylated PPARγ was measured by immunoblot analysis to determine how LH affects the phosphorylation status, and therefore the activity, of PPARγ. Granulosa cells were also collected from immature rats 48 h after PMSG. Cells were cultured with LH in the absence and presence of H89 and cycloheximide to investigate the role of PKA and protein synthesis in the LH-mediated decline in mRNA for PPARγ respectively. Protein corresponding to PPARγ was localized to nuclei of granulosa cells 0 and 48 h after PMSG. Expression was greatly reduced by 4 h after hCG, with expression in mural granulosa cells lost before that in cumulus cells. The amount of phosphorylated PPARγ did not change during the periovulatory period. Blocking PKA activity had no effect on levels of mRNA for PPARγ. However, levels of mRNA for PPARγ were significantly increased in cells treated with cycloheximide (P< 0.05, ANOVA followed by Tukey’s Hsd). These data suggest that PPARγ is tightly regulated in the ovary and that its expression is the primary mechanism by which LH influences the activity of PPARγ. In addition, protein synthesis may be involved in modulating levels of PPARγ in granulosa cells.