Background: Plasminogen activator inhibitor type-1 (PAI-1) implicated as a determinant of thrombosis, and possibly of atherosclerosis, is increased approximately four-fold in the blood of subjects with type II diabetes, and the increase is closely correlated with concentrations of insulin. Insulin can stimulate expression of PAI-1 in vitro, but infusions of insulin in human subjects have not increased PAI-1 in blood in acute, euglycemic clamp studies.
Objective: To determine whether the dichotomy reflects interactions between insulin and both very-low-density lipoprotein (VLDL)-bound triglycerides (VLDL-TG) and albumin-bound nonesterified (free) fatty acids (FFA) affecting PAI-1 elaboration.
Methods: HepG2 cells (human hepatoma cell line) were exposed to insulin, VLDL, and FFA alone and in selected combinations for 24 h.
Results: Striking synergistic effects were observed. Thus, compared with control, pathophysiologic concentrations of insulin increased PAI-1 accumulation in conditioned media modestly, as did VLDL, but the combination elicited a marked nine-fold increase (control PAI-130 +/- 2 ng/ml, PAI-1 with 400 mg/dl VLDL-TG 97 +/- 6 ng/ml; with 4 nmol/1 insulin 45 +/- 6 ng/ml, with 400 mg/dl VLDL-TG plus 4 nmol/1 insulin 276 +/- 47 ng/ml; P < 0.01 for the combination compared with control or with either agent alone). Similarly, a modest increase was found either with insulin or with FFA alone, but a synergistic increase was evident when they were combined (control 27 +/- 3 ng/ml; 1 mmol/l FFA 36 +/- 2 ng/ml; 10 nmol/l insulin 59 +/- 6 ng/ml; 1 mmol/l FFA plus 10 nmol/l insulin 82 +/- 1 ng/ml; P < 0.01 for the combination compared with control or with either agent alone).
Conclusion: The combination of increased insulin with elevated VLDL-TG and increased FFA appears to cause the increase in PAI-1 in blood of subjects with type II diabetes mellitus and other insulin-resistant states, providing novel and promising targets for normalizing altered fibrinolytic system potential and thereby ameliorating the associated predisposition to persistent thrombosis and possibly atherosclerosis.