Metformin enhances the action of insulin on porcine granulosa-lutein cells in vitro.

Metformin is an oral antidiabetic drug extensively used to treat the polycystic ovary syndrome in women. Metformin increases insulin-stimulated glucose uptake and has direct effects on ovarian steroidogenesis in humans. However, the molecular mechanisms of metformin' action on the ovary are not clear. To investigate the effects of this drug on the insulin-signaling pathway in porcine granulosa cells as an alternative model for human research, we examined the mRNA expressions of porcine insulin receptor (INSR), insulin-like growth factor-1 receptor (IGF-1R), insulin receptor substrate-1 (IRS-1), and the protein activity (activation and phosphorylation) of downstream targets including Raf, mitogen-activated protein kinase (MEK)1/2, extracellular signal regulated kinase (ERK), phosphoinositide-dependent 1 kinase (PDK1), mammalian target of rapamycin (TOR), p70, and nuclear factor-κB (NF-κB) in a primary culture system consisting of porcine granulosa-lutein cells (pGLs) incubated with 10(-5)M metformin and/or 100ng/ml insulin for 24h in a serum-free medium. We also investigated the luciferase activity of transcription factors activator protein-1 (AP-1) and NF-κB. Metformin with insulin significantly increased mRNA expressions of INSR, IGF-1R, and IRS-1, while metformin alone had no significant effect. And metformin with insulin had the significant effect on the protein activity (activation and phosphorylation) of downstream targets of INSR signaling pathway. Metformin with insulin significantly elicited an induction of luciferase activity in the transfection of AP-1 and NF-κBreporter, while metformin alone did not. In conclusion, we examined the activity of metformin and insulin on pGLS in vitro and metformin enhanced the action of insulin on the intracellular signaling pathways. These results suggest that metformin could change the function of ovarian granulosa cells.