High-level expression of RXRα and the presence of endogenous ligands contribute to expression of a peroxisome proliferator-activated receptor-responsive gene in hepatoma cells

Genes containing peroxisome proliferator-activated receptor (PPAR) binding sites are both inducible by peroxisome proliferators and expressed in a tissue-specific fashion. A PPAR-responsive reporter gene cotransfected with a PPAR alpha expression vector was highly expressed in H4IIEC3 hepatoma cells. Addition of clofibrate resulted in a modest further induction of the reporter gene, In CV-1 cells, high expression of the reporter required the addition of clofibrate. H4IIEC3 cells had higher levels of retinoid X receptor (RXR alpha) than CV-1 cells; cotransfection of CV-I cells with PPAR alpha plus RXR alpha! expression plasmids abolished the cell line difference in basal and clofibrate-stimulated expression of the reporter. Lipid extracts of hepatoma cells or of liver or kidney stimulated expression of the reporter; extracts of CV-1 cells were far less effective. Chromatographic analysis of these extracts revealed high levels of three fractions of lipid in liver and H4IIEC3 cells that were lower in CV-I cells. We conclude that (1) in cells expressing high levels of both RXRs and PPAR alpha, such as hepatocytes and kidney cells, these factors are constitutively active; (2) activators of PPAR alpha may increase its ability to form heterodimers with RXRs when the latter are limiting; and (3) hepatoma cells, liver, and kidney contain lipid-extractable compounds capable of activating PPAR alpha. (C) 1998 Academic Press.