Elaboration of Type-1 Plasminogen Activator Inhibitor From Adipocytes

Abstract

Background Obesity is known to predispose to attenuated fibrinolysis attributable to increased concentrations in plasma of type-1 plasminogen activator inhibitor (PAI-1), the primary physiological inhibitor of endogenous fibrinolysis. PAI-1 is present in neointimal vascular smooth muscle cells and lipid-laden macrophages. Methods and Results The present study was designed to determine whether PAI-1 expression occurs in adipose tissue as well, thereby potentially contributing to increased cardiovascular risk associated with obesity. 3T3-L1 preadipocytes were differentiated into adipocytes by exposing them to isobutylxanthine (0.5 mmol/L) and dexamethasone (0.25 μmol/L) over 7 days and incubated for 24 hours with transforming growth factor-β (TGF-β), known to augment PAI-1 synthesis in several cell types and to be released from platelets when they are activated. TGF-β increased PAI-1 activity in the conditioned media of the 3T3-L1–derived cells in a concentration-dependent fashion without significantly affecting cell proliferation. Western blotting and immunoprecipitation of 35 S-labeled PAI-1 showed that the increased PAI-1 activity paralleled increased PAI-1 protein. Northern blotting showed that increased PAI-1 mRNA preceded increased accumulation of PAI-1 activity and protein in the conditioned media. Furthermore, TGF-β (10 ng/g body wt) administered in vivo increased PAI-1 activity in mouse plasma and PAI-1 mRNA expression in mouse adipose tissue. Conclusions Increased plasma PAI-1 activity in obese human subjects may result from PAI-1 release from an increased mass of adipose tissue, particularly in association with thrombosis and elaboration of TGF-β from platelet α-granules into the circulation. The increased PAI-1 may exacerbate vascular disease by shifting the balance between thrombosis and thrombolysis toward thrombosis and consequently exposing luminal surfaces of vessels to mitogens associated with microthrombi over protracted intervals.