Action of metformin on the insulin-signaling pathway and on glucose transport in human granulosa cells.
Background: Hyperinsulinemia in polycystic ovary syndrome is widely treated with the insulin sensitizer metformin, which, in addition to its systemic effects, directly affects the ovarian insulin-stimulated steroidogenesis pathway.
Objective: Our aim was to investigate the interaction of metformin with the other insulin-stimulated ovarian pathway, namely that leading to glucose uptake.
Methods: Human granulosa-luteal cells were cultured with metformin (10(-7) M), insulin (10 ng/ml) or metformin and insulin (met + ins) combined. Insulin receptor (IR) involvement was assessed by culture with an (anti)-insulin receptor (IR) antibody. Methods: The effect of metformin on insulin-receptor substrate proteins 1 and 2 (IRS-1 and -2) mRNA and protein expression was determined. The KGN granulosa-cell line was used to investigate the effect of insulin and metformin on Akt activation and glucose transporter-4 (Glut-4) expression. Glut-4 translocation from the cytosol to the membrane was determined in cytoplasmic and membrane-enriched fractions of protein lysates.
Results: IRS-1 mRNA and protein increased with all treatments. In contrast, basal IRS-2 mRNA levels were barely detectable, but transcription was up-regulated by metformin. The anti-IR antibody reduced treatment-stimulated IRS-1 to basal levels and IRS-2 expression to an even greater extent than IRS-1, showing greater dependence on the IR than IRS-1. Metformin in the presence of insulin activated Akt and this was dependent on phosphoinositide-3 kinase, as was translocation of Glut-4 to the membrane. Metformin was able to substantially enhance the insulin-stimulated translocation of Glut-4 transporters from the cytosol to the membrane.
Conclusions: This net increase in Glut-4 transporters in the plasma membrane has the potential to increase glucose uptake and metabolism by granulosa cells of the insulin-resistant polycystic ovary, thereby facilitating follicle growth.