In patients with non-insulin-dependent diabetes mellitus, concentrations in plasma of insulin and its precursors, proinsulin and split proinsulin, are increased. Because increased concentrations of plasminogen activator inhibitor type-1 (PAI-1) occur also, we hypothesized that proinsulin and split proinsulin may augment endothelial cell PAI-1 expression, thereby potentially attenuating endogenous fibrinolysis and accelerating atherosclerosis. Proinsulin increased PAI-1 activity in conditioned media of endothelial cells as did split proinsulin, paralleled by increased expression of PAI-1 mRNA. These effects of proinsulin were not dependent on its conversion to insulin nor on its interactions with the insulin receptor. The proinsulin stimulation of PAI-1 expression was not attenuated by either anti-insulin receptor antibodies or a 100-fold excess of insulin. Furthermore, proinsulin-mediated increases in PAI-1 expression were not inhibited by a 500-fold excess of insulinlike growth factor I. In addition, inhibition of tyrosine kinase, which mediates many of the diverse effects of insulin and insulinlike growth factor I, did not attenuate the effect of proinsulin. These results indicate that proinsulin augments PAI-1 expression, potentially contributing to vasculopathy in patients with non-insulin-dependent diabetes mellitus.

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