PPARalpha and PPARgamma activators direct a distinct tissue-specific transcriptional response via a PPRE in the lipoprotein lipase gene
Increased activity of lipoprotein lipase (LPL) may explain the hypotriglyceridemic effects of fibrates, thiazolidinediones and fatty acids, which are known activators (and/or ligands) of the various peroxisome proliferator-activated receptors (PPARs). Treatment with compounds which activate preferentially PPARalpha, such as fenofibrate, induced LPL expression exclusively in rat liver. In contrast, the antidiabetic thiazolidinedione BRL 49653, a high affinity ligand for PPARgamma, had no effect on liver, but induced LPL expression in rat adipose tissue. In the hepatocyte cell line AML-12, fenofibric acid, but not BRL 49653, induced LPL mRNA, whereas in 3T3-L1 preadipocytes, the PPARgamma ligand induced LPL mRNA levels much quicker and to a higher extent than fenofibric acid. In both the in vivo and in vitro studies, inducibility by either PPARalpha or gamma activators, correlated with the tissue distribution of the respective PPARs: an adipocyte-restricted expression of PPARgamma, whereas PPARalpha was expressed predominantly in liver. A sequence element was identified in the human LPL promoter that mediates the functional responsiveness to fibrates and thiazolidinediones. Methylation interference and gel retardation assays demonstrated that a PPARalpha or gamma and the 9-cis retinoic acid receptor (RXR) heterodimers bind to this sequence -169 TGCCCTTTCCCCC -157. These data provide evidence that transcriptional activation of the LPL gene by fibrates and thiazolidinediones is mediated by PPAR-RXR heterodimers and contributes significantly to their hypotriglyceridemic effects in vivo. Whereas thiazolidinediones predominantly affect adipocyte LPL production through activation of PPARgamma, fibrates exert their effects mainly in the liver via activation of PPARalpha.