Induction of Plasminogen Activator Inhibitor Type-1 (PAI-1) by Proinsulin and Insulin In Vivo

Abstract

Background Fasting hyperinsulinemia (reflected by elevations in immunoreactive “insulin”) is typical of patients with non–insulin-dependent diabetes mellitus (NIDDM) and is often associated with obesity and hypertension. The elevated concentrations detected are indicative not only of insulin but also of its immunologically cross-reactive precursors, including proinsulin. Fasting hyperinsulinemia appears to be associated with decreased fibrinolytic activity in blood, which results from increased activity of plasminogen activator inhibitor type-1 (PAI-1), a potential independent risk factor for coronary artery disease. Patients who were given proinsulin in a previous clinical study by others exhibited an increased incidence of cardiovascular events. Thus, a “proinsulin–PAI-1 axis” may predispose to coronary thrombosis. To define the possible presence of such an axis, this study was designed to determine whether insulin, its precursors, or both increase the concentrations of PAI-1 in rabbits in vivo. Methods and Results Equimolar proinsulin (n=10), insulin (n=11), C-peptide (n=4), or vehicle alone (n=10) was administered intravenously over 1 hour to euglycemic, conscious rabbits. Plasma PAI-1 activity increased 3.8-fold with proinsulin ( P =.002) and 3.6-fold with insulin ( P =.002). By contrast, no increase occurred after C-peptide or vehicle was administered. The increased PAI-1 activity was shown to be attributable to PAI-1 protein by reverse fibrin autography. As judged from changes in mRNA in tissues, proinsulin and insulin increased PAI-1 gene expression within 3 hours by 2.1- and 2.1-fold, respectively, in aorta ( P =.025 each) and by 1.9- and 2.4-fold in liver ( P =.015 and P =.001), with return of values to baseline within 24 hours (n=4 experiments in each case). Conclusions These results extend our previous observations from studies in vitro and suggest that hyperinsulinemia attributable to augmented concentrations of proinsulin and insulin in plasma increase plasma PAI-1 activity and may contribute to acceleration of atherosclerosis and impairment of coronary thrombolysis in patients with NIDDM.