Previously, we demonstrated that bone marrow-derived mesenchymal stem cells (MSCs) differentiate into steroidogenic cells such as Leydig and adrenocortical cells by the introduction of steroidogenic factor-1 (SF-1) and treatment with cAMP. In this study, we employed the same approach to differentiate umbilical cord blood (UCB)-derived MSCs. Despite UCB-MSCs differentiating into steroidogenic cells, they exhibited characteristics of granulosa-luteal-like cells. We found that peroxisome proliferator-activated receptor- coactivator-1 (PGC-1) was expressed and further induced by cAMP stimulation in UCB-MSCs. Consistent with these results, tissue-specific expression of Pgc-1 was observed in rat ovarian granulosa cells. PGC-1 binds to the NR5A family [SF-1 and liver receptor homolog-1 (LRH-1)] of proteins and markedly enhances their transcriptional activities. Reporter assays revealed that PGC-1 activated the promoter activities of SF-1 and LRH-1 target genes. Infection of KGN cells (a human cell line derived from granulosa cells) with adenoviruses expressing PGC-1 resulted in the induction of steroidogenesis-related genes and stimulation of progesterone production. PGC-1 also induced SF-1 and LRH-1, with the latter induced to a greater extent. Knockdown of Pgc-1 in cultured rat granulosa cells resulted in attenuation of gene expression as well as progesterone production. Transactivation of the NR5A family by PGC-1 was repressed by Dax-1. PGC-1 binds to the activation function 2 domain of NR5A proteins via its consensus LXXLL motif. These results indicate that PGC-1 is involved in progesterone production in ovarian granulosa cells by potentiating transcriptional activities of the NR5A family proteins.

Caveolin, a member of the membrane-anchoring protein family, accumulates various growth receptors in caveolae and inhibits their function. Upregulation of caveolin attenuates cellular proliferation and growth. However, the role of caveolin in regulating insulin signals remains controversial. Here, we demonstrate that caveolin potently enhances insulin receptor (IR) signaling when overexpressed in the liver in vivo. Adenovirus-mediated gene transfer was used to overexpress caveolin specifically in the liver of diabetic obese mice, which were generated with a high-fat diet. Expression of molecules involved in IR signaling, such as IR or Akt, remained unchanged after gene transfer. However, hepatic glycogen synthesis was markedly increased with a decrease in phosphoenolpyruvate carboxykinase protein expression. Insulin sensitivity was increased after caveolin gene transfer as determined by decreased blood glucose levels in response to insulin injection and fasting blood glucose levels. Glucose tolerant test performance was also improved. Similar improvements were obtained in KKA^y genetically diabetic mice. Adenovirus-mediated overexpression of caveolin-3 in hepatic cells also enhanced IR signaling, as shown by increased phosphorylation of IR in response to insulin stimulation and higher glycogen synthesis at baseline. These effects were attributed mostly to increased insulin receptor activity and caveolin-mediated, direct inhibition of protein tyrosine phosphatase 1B, which was increased in obese mouse livers. In conclusion, our results suggest that caveolin is an important regulator of glucose metabolism that can enhance insulin signals.