High-level expression of RXRalpha 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 PPARalpha 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 (RXRalpha) than CV-1 cells; cotransfection of CV-1 cells with PPARalpha plus RXRalpha 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-1 cells. We conclude that (1) in cells expressing high levels of both RXRs and PPARalpha, such as hepatocytes and kidney cells, these factors are constitutively active; (2) activators of PPARalpha 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 PPARalpha.
Galli
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A
Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, Indianapolis, Indiana, 46202, USA.
Stewart
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M
Dorris
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Crabb
D
D
D006528:Carcinoma, Hepatocellular / Q000235:genetics; D002994:Clofibrate / Q000494:pharmacology; D005786:Gene Expression Regulation; D015972:Gene Expression Regulation, Neoplastic; D017930:Genes, Reporter; D008099:Liver / Q000378:metabolism*; D008156:Luciferases / Q000235:genetics; D018160:Receptors, Cytoplasmic and Nuclear / Q000502:physiology*; D018168:Receptors, Retinoic Acid / Q000096:biosynthesis*; D047488:Retinoid X Receptors; D014157:Transcription Factors / Q000096:biosynthesis* / Q000502:physiology*; D014212:Tretinoin / Q000494:pharmacology; D014407:Tumor Cells, Cultured
D018160:Receptors, Cytoplasmic and Nuclear; D018168:Receptors, Retinoic Acid; D047488:Retinoid X Receptors; D014157:Transcription Factors; D014212:Tretinoin; D008156:Luciferases; D002994:Clofibrate