Very low density lipoprotein receptor promotes adipocyte differentiation and mediates the proadipogenic effect of peroxisome proliferator-activated receptor gamma agonists

Very low density lipoprotein receptor (VLDLR) is a member of the low density receptor family, expressed mostly in adipose tissue, heart, and skeletal muscles. VLDLR binds apolipoprotein-E-triglyceride-rich lipoproteins and plays a key role in lipid metabolism. In adipocytes, VLDLR expression increases with differentiation but it is not known whether it plays a role in the adipogenesis. Here we report that VLDLR expression in 3T3-L1 adipocytes is upregulated by PPARy agonist 15-deoxy-delta(12,14)-prostaglandin J(2) (15d-PGJ(2)) in dose- and time-dependant manners. Knockdown of peroxisome proliferator-activated receptor-gamma (PPAR gamma) with siRNA abolished pioglitazone- and 15d-PGJ(2)-induced VLDLR expression and simultaneously reduced VLDL uptake in adipocytes. In addition, PPAR gamma-agonist treatment of control mouse adipocytes (vldlr(+/+)) enhanced adipogenesis and VLDL uptake concurrently with the induction of VLDLR expression. However, vldir deficiency (vldlr(-/-)) significantly blunted the proadipogenic effects of PPAR gamma agonists. Sequence analysis revealed the presence of a putative PPAR gamma responsive sequence (PPRE) within the vldlr promoter, which is responsive to natural (15d-PGJ(2)) and synthetic (pioglitazone) PPAR gamma agonists. Reporter gene assays using serial deletion of the 5'-flanking region showed that this putative PPRE site induced promoter transactivation, while a site-targeted mutation abolished transactivation. Moreover, electrophoresis mobility shift assay (EMSA) and chromatic immunoprecipitation (ChIP) assays showed the specific binding of PPARy to the PPRE sequence. Together, these results support a crucial function for VLDLR in adipocyte differentiation and mediation of the proadipogenic effect of PPAR gamma. (C) 2011 Elsevier Inc. All rights reserved.