Low-Density Lipoprotein Particle Size Is Inversely Related to Plasminogen Activator Inhibitor-1 Levels

High levels of plasminogen activator inhibitor-1 (PAI-1) and preponderance of small dense low-density lipoproteins (LDL) have both been associated with atherosclerotic disease and with the insulin resistance syndrome (IRS). In vitro studies have shown a stimulatory effect of various lipoproteins on PAI-1 release from different cells, including endothelial cells and adipocytes. The authors sought to investigate the relation of PAI-1 to LDL particle size in a large tri-ethnic population (n=1549) across different states of glucose tolerance. LDL size was determined by gradient gel electrophoresis, and PAI-1 was measured by a 2-site immunoassay, sensitive to free PAI-1. PAI-1 was inversely related to LDL size in the overall population (r=-0.21, P<0.0001), independent of gender and ethnicity. However, the authors found a significant interaction with glucose tolerance status (P=0.035). In univariate analysis, the association between PAI-1 and LDL size was most pronounced in subjects with normal glucose tolerance (NGT, r=-0.22, P<0.0001) and weaker in impaired glucose tolerance (IGT, r=-0.12, P=0.03) and type-2 diabetes (r=-0.10, P=0.02). After adjustment for demographic variables and metabolic variables known to influence PAI-1 levels (triglyceride and insulin sensitivity), a significant inverse relation of LDL size to PAI-1 levels was only present in NGT (P=0. 023). In subjects with IGT or overt diabetes, who usually have elevated PAI-1 levels, additional factors other than LDL size seem to contribute more importantly to PAI-1 levels. The demonstrated inverse relation of LDL size and PAI-1 levels provides one possible explanation for the atherogeneity of small dense LDL particles.