Increased plasminogen activator inhibitor-1 (PAI-1) is linked to obesity and insulin resistance. However, the functional role of PAI-1 in adipocytes is unknown. This study was designed to investigate effects and underlying mechanisms of PAI-1 on glucose uptake in adipocytes and on adipocyte differentiation. Using primary cultured adipocytes from PAI-1+/+ and PAI-1−/− mice, we found that PAI-1 deficiency promoted adipocyte differentiation, enhanced basal and insulin-stimulated glucose uptake, and protected against tumor necrosis factor-α-induced adipocyte dedifferentiation and insulin resistance. These beneficial effects were associated with upregulated glucose transporter 4 at basal and insulin-stimulated states and upregulated peroxisome proliferator-activated receptor-γ (PPARγ) and adiponectin along with downregulated resistin mRNA in differentiated PAI-1−/− vs. PAI-1+/+ adipocytes. Similarly, inhibition of PAI-1 with a neutralizing anti-PAI-1 antibody in differentiated 3T3-L1 adipocytes further promoted adipocyte differentiation and glucose uptake, which was associated with increased expression of transcription factors PPARγ, CCAAT enhancer-binding protein-α (C/EBPα), and the adipocyte-selective fatty acid-binding protein aP2, thus mimicking the phenotype in PAI-1−/− primary adipocytes. Conversely, overexpression of PAI-1 by adenovirus-mediated gene transfer in 3T3-L1 adipocytes inhibited differentiation and reduced PPARγ, C/EBPα, and aP2 expression. This was also associated with a decrease in urokinase-type plasminogen activator mRNA expression, decreased plasmin activity, and increased collagen I mRNA expression. Collectively, these results indicate that absence or inhibition of PAI-1 in adipocytes protects against insulin resistance by promoting glucose uptake and adipocyte differentiation via increased PPARγ expression. We postulate that these PAI-1 effects on adipocytes may, at least in part, be mediated via modulation of plasmin activity and extracellular matrix components.
Induction of insulin resistance by the adipokines resistin, leptin, plasminogen activator inhibitor-1 and retinol binding protein 4 in human megakaryocytes