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 PPARγ agonist 15-deoxy-delta(12,14)-prostaglandin J(2) (15d-PGJ(2)) in dose- and time-dependant manners.
Knockdown of peroxisome proliferator-activated receptor-γ (PPARγ) with siRNA abolished pioglitazone- and 15d-PGJ(2)-induced VLDLR expression and simultaneously reduced VLDL uptake in adipocytes.
In addition, PPARγ-agonist treatment of control mouse adipocytes (vldlr(+/+)) enhanced adipogenesis and VLDL uptake concurrently with the induction of VLDLR expression. However, vldlr deficiency (vldlr(-/-)) significantly blunted the proadipogenic effects of PPARγ agonists.
Sequence analysis revealed the presence of a putative PPARγ responsive sequence (PPRE) within the vldlr promoter, which is responsive to natural (15d-PGJ(2)) and synthetic (pioglitazone) PPARγ 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 PPARγ to the PPRE sequence. Together, these results support a crucial function for VLDLR in adipocyte differentiation and mediation of the proadipogenic effect of PPARγ.