Effects of PPAR?? activation on the synthesis of apolipoproteins in HepG2 cells

The peroxisome proliferator activated receptor gamma (PP ARy) makes up a distinct subclass of the nuclear receptor family of transcription factors and is involved in the regulation of lipid and carbohydrate metabolism. PP ARy is the target receptor for a class of hypoglycemic pharmaceutical agents called Thiazolidinediones (TZDs), which are extensively used for the management of insulin resistance. TZDs reduce plasma glucose levels by increasing cellular glucose uptake and reducing hepatic glucose production. It is known that diabetic subjects taking TZDs show a decrease in plasma triglycerides and an increase in HDL lipoprotein levels. In spite of such an improvement in plasma lipid profile, a recent report suggests that subjects receiving a specific TZD treatment may be at a higher risk for cardiovascular disease. Several target genes of PPARy are relevant to the development of atherosclerosis. These include genes for the apolipoproteins apoE, apoAI, and ApoCI. These genes are highly expressed in the liver, a major organ for the assembly and secretion of very low density lipoproteins (VLDL). We have studied the regulation of apolipoproteins apoAI, apoCI, apo E, and apoBlOO by the TZD, Ciglitazone, in HepG2 liver cells. ApoE and apoCI are components ofVLDL, apoAI is the primary apoprotein of HDL particles, and apoB 100 is found in LD L particles. Our data show a significant increase in apoE, a po AI and apoCI transcripts after treatment of HepG2 cells with Ciglitazone. We confirmed that regulation of mRNA levels is extended to protein expression. Using specific immunoprecipitation and Western blot analysis, we found that Ciglitazone induces apoE, apoAI, and apoCI protein synthesis and secretion from HepG2 cells. Additionally, we performed metabolic labeling experiments to show an up-regulation in de nova synthesis of apoAI and apoE, but a down-regulation of apoB 100 synthesis, in Ciglitazone-treated HepG2 cells. Consistent with the physiologic effects of TZDs, we found that Ciglitazone treatment reduced the triglyceride content of HepG2 cells. Our studies about the effects of Ciglitazone on apolipoprotein mRNA and protein expression lead us to conclude that activation of PP ARy may influence the plasma lipoprotein profile by altering the expression of hepatic apolipoproteins. Our data show that Ciglitazone-mediated hepatic PPARy activation resulted in an increase in atheroprotective apolipoproteins apoAI and apoE, and a decrease in atherogenic apoBlOO. Thus, Ciglitazone's effect on hepatic apolipoprotein expression does not appear to contribute to cardiovascular risk