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-1 30 ± 2 ng/ml, PAI-1 with 400 mg/dl VLDL-TG 97 ± 6 ng/ml; with 4 nmol/l insulin 45 ± 6 ng/ml, with 400 mg/dl VLDL-TG plus 4 nmol/l 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.
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