PLASMINOGEN-ACTIVATOR INHIBITOR-1 SYNTHESIS IN THE HUMAN HEPATOMA-CELL LINE HEP G2 - METFORMIN INHIBITS THE STIMULATING EFFECT OF INSULIN

High plasma plasminogen activator inhibitor-1 (PAI-1) activity is asso ciated with insulin resistance and is correlated with hyperinsulinemia . The cellular origin of plasma PAI-1 in insulin resistance is not kno wn. The hepatoma cell line Hep G2 has been shown to synthesize PAI-1 i n response to insulin. The aim of this study was to analyze the insuli n-mediated response of PAI-1 and lipid synthesis in Hep G2 cells after producing an insulin-resistant state by decreasing insulin receptor n umbers. The effect of metformin, a dimethyl-substituted biguanide, kno wn to lower plasma insulin and PAI-1 levels in vivo was concomitantly evaluated. Preincubation by an 18-h exposure of Hep G2 cells to 10(-7) M insulin aimed at reducing the number of insulin receptors, was foll owed by a subsequent 24-h stimulation with 10(-9) M insulin. The decre ase in insulin receptors was accompanied as expected, by a reduction i n [C-14]acetate incorporation, an index of lipid synthesis, whereas PA I-1 secretion and PAI-1 mRNA expression were enhanced. The addition of metformin did not modify the effect of insulin on insulin receptors o r [C-14]acetate incorporation. In contrast, the drug (10(-4) M) inhibi ted insulin-mediated PAI-1 synthesis. The results indicate that PAI-1 synthesis in presence of insulin is markedly increased in down-regulat ed cells, and that metformin inhibits this effect by acting at the cel lular level. These in vitro data are relevant with those found in vivo in insulin-resistant patients. Hep G2 cells may be a suitable model t o study PAI-1 regulation in response to hyperinsulinemia.