REGULATION OF LIPOPROTEIN METABOLISM BY THIAZOLIDINEDIONES OCCURS THROUGH A DISTINCT BUT COMPLEMENTARY MECHANISM RELATIVE TO FIBRATES

Thiazolidinediones are antidiabetic agents, which not only improve glu cose metabolism but also reduce blood triglyceride concentrations. The se compounds are synthetic ligands for PPAR gamma, a transcription fac tor belonging to the nuclear receptor subfamily of PPARs, which are im portant transcriptional regulators of lipid and lipoprotein metabolism . The goal of this study was to evaluate the influence of a potent thi azolidinedione, BRL49653, on serum lipoproteins and to determine wheth er its lipid-lowering effects are mediated by changes in the expressio n of key genes implicated in lipoprotein metabolism. Treatment of norm al rats for 7 days with BRL49653 decreased serum triglycerides in a do se-dependent fashion without affecting serum total and HDL cholesterol and apolipoprotein (apo) A-I and apo A-II concentrations. The decreas e in triglyceride concentrations after BRL49653 was mainly due to a re duction of the amount of VLDL particles of unchanged lipid and apo com position. BRL49653 treatment did not change triglyceride production in vivo as analyzed by injection of Triton WR-1339, indicating a primary action on triglyceride catabolism. Analysis of the influence of BRL49 653 on the expression of LPL and apo C-III, two key players in triglyc eride catabolism, showed a dose-dependent increase in mRNA levels and activity of LPL in epididymal adipose tissue, whereas liver apo C-III mRNA levels remained constant. Furthermore, addition of BRL49653 to pr imary cultures of differentiated adipocytes increased LPL mRNA levels, indicating a direct action of the drug on the adipocyte. Simultaneous administration of BRL49653 and fenofibrate, a hypolipidemic drug that acts primarily on liver through activation of PPAR alpha both decreas ed liver apo C-III and increased adipose tissue LPL mRNA levels, resul ting in a more pronounced lowering of serum triglycerides than each dr ug alone. In conclusion, both fibrates and thiazolidinediones exert a hypotriglyceridemic effect. While fibrates act primarily on the liver by decreasing apo C-III production, BRL49653 acts primarily on adipose tissue by increasing lipolysis through the induction of LPL expressio n. Drugs combining both PPAR alpha and gamma activation potential shou ld therefore display a more efficient hypotriglyceridemic activity tha n either compound alone and may provide a rationale for improved thera py for elevated triglycerides.