TISSUE DISTRIBUTION AND REGULATION OF PLASMINOGEN-ACTIVATOR INHIBITOR-1 IN OBESE MICE

Background: Although elevated plasminogen activator inhibitor-1 (PAI-1 ) is associated with obesity and may be a risk factor for cardiovascul ar disease, the mechanism(s) that lead to this elevation, and the tiss ue/cellular origins of this increase, remain to be defined. In this re port, we have addressed these questions using genetically obese mice ( ob/ob) and their lean counterparts (+/?). Materials and Methods: PAI-1 activity and antigen levels were determined using a tissue-type plasm inogen activator (t-PA) binding assay and Western blotting. The concen tration of PAI-1 mRNA in tissues was determined by quantitative revers e transcriptase-polymerase chain reaction (RT-PCR), and the cellular l ocalization of PAI-1 was evaluated using in situ hybridization, immuno histochemistry, and cell fractionation. Results: PAI-1 activity was ap proximately 4-fold higher in plasma from ob/ob mice than in that obtai ned from their lean counterparts, and this difference increased furthe r with age (i.e., 6-fold at 3 months). PAI-1 mRNA levels were elevated 4- to 5-fold in the adipose tissues of obese mice, and these differen ces in mRNA also increased with age. The elevated PAI-1 mRNA in the ad ipose tissues of obese mice was localized to mature adipocytes as well as to vascular smooth muscle cells and occasional endothelial cells. Obesity is often associated with hyperinsulinemia, and acute injection of insulin into lean mice increased PAI-1 mRNA 6- to 8-fold in the ep ididymal fat in cells that morphologically resembled adipocytes. Insul in did not increase PAI-1 in large vessel endothelial or smooth muscle cells. The adipocyte response to insulin was confirmed in cell cultur e studies where PAI-1 synthesis by mature 3T3-L1 adipocytes was increa sed 5- to 6-fold by insulin. Conclusions: Our results suggest that ele vated PAI-1 associated with obesity may result in part from insulin-in duced induction of PAI-1 specifically by adipocytes within the fat its elf.