Peroxisome Proliferator Activated Receptor-α Expression in Human Liver
- 1Department of Molecular and Experimental Medicine, The Scripps Research Institute, La Jolla, California 92037 (C.N.A.P., M.-H.H., K.J.G., E.F.J.), and 2Agouron Institute, La Jolla, California 92037 (J.L.R.)
Abstract
The peroxisome proliferator activated receptor α (PPAR) is a member of the steroid/hormone receptor superfamily that mediates the peroxisome proliferator-dependent transcriptional activation of genes encoding several peroxisomal and microsomal enzymes as well as peroxisome proliferation. Human liver is refractory to the pathological effects of peroxisome proliferators that are seen in mice. With the use of RNase protection assays, the ratio of hepatic PPARα mRNA to β-actin mRNA was found to be 1 order of magnitude lower in humans than that observed in mice. In addition, the isolation of human cDNA for PPARα that does not encode a functional PPAR because it lacks exon 6 as a result of alternate RNA splicing suggested that this process might also diminish the expression of PPARα. RNase protection analysis of total RNA revealed the presence of splice variants lacking exon 6 at significant levels in all 10 human liver samples examined. Supershift analysis using the CYP4A6-Z peroxisome proliferator response element and antisera specific for PPARα revealed easily detectable amounts of PPARα DNA binding activity in mouse liver lysates, whereas human liver lysates contained >10-fold lower amounts of PPARα DNA binding activity. In contrast to mouse lysates, the amount of PPARα binding in human lysates was generally less than that of other unidentified proteins. These results suggest that although humans retain the coding potential for a functional receptor, the low levels of PPARα expression in liver may be insufficient to compete effectively with other proteins that bind to peroxisome proliferator response elements.
Footnotes
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Send reprint requests to: Eric F. Johnson, Ph.D., Division of Biochemistry, Dept. of Molecular and Experimental Medicine, The Scripps Research Institute, 10550 N. Torrey Pines Road, NX-4, La Jolla, CA 92037-9701. E-mail: johnson{at}scripps.edu
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↵1 Current affiliation: Molecular Pharmacology Unit, Biomedical Research Center, Ninewells Hospital and Medical School, Dundee, Scotland.
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↵2 A preliminary report of the characterization of this cDNA was presented at the 10th International Symposium on Microsomes and Drug Oxidations, Toronto, Canada, 1994.
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This work was supported by United States Public Health Service Grants HD04445 (E.F.J.) and AA08990 (J.R.) and by the American Heart Association, California Affiliate, Postdoctoral Fellowship 93–96 (C.N.A.P.). Facilities for computer-assisted analysis and the synthesis of oligonucleotides are supported in part by General Clinical Research Center Grant M01-RR00833 and by the Sam and Rose Stein Charitable Foundation, respectively.
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C.N.A.P. and M.-H.H. contributed equally to this work.
- Abbreviations:
- CoA
- coenzyme A
- ARP-1
- apolipoprotein regulatory protein 1
- PPRE
- peroxisome proliferator response element
- PPAR
- peroxisome proliferator activated receptor
- RXR
- retinoid X receptor
- THR
- thyroid hormone receptor
- DMEM
- Dulbecco’s modified Eagle’s medium
- DMSO
- dimethylsulfoxide
- EMSA
- electrophoretic mobility shift assay
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- Received June 20, 1997.
- Accepted September 17, 1997.
- The American Society for Pharmacology and Experimental Therapeutics
