Abstract
Dehydroepiandrosterone (DHEA) is the only known naturally occurring compound that promotes peroxisome proliferation in rodent liver, and stimulates transcriptional induction of genes involved in lipid metabolism and peroxisomal β-oxidation. Therefore, we examined mRNA for several such genes in rat liver, specifically acyl-CoA oxidase and the cytochromes P-450 (CYP4A1, CYP4A3, and CYP3A23), after 5 to 6 day treatments with either DHEA, or nafenopin, a known peroxisome proliferator. Acyl-CoA oxidase and CYP4A1 were induced nearly identically by DHEA and nafenopin, with induction being more pronounced in female rats. However, CYP3A23 was induced only by DHEA, suggesting an induction mechanism independent of the peroxisome proliferator activated receptor. Previously, we observed triiodothyronine (T3) suppression of peroxisome proliferator induced CYP4As and we sought to determine whether CYP3A23 might be regulated in a different manner. T3 was found to also suppress DHEA-dependent induction of CYP3A23. CYP4A2 expression in kidney was also negatively regulated by T3. To characterize a putative negative thyroid hormone response element (nTRE) in the 5′ flanking region of this gene, a luciferase reporter gene containing a rat CYP4A2 flanking sequence extending to −1865 bp was transfected into HepG2 cells along with human thryroid hormone receptor expression vector. Expression of luciferase activity was unaffected by T3, suggesting the absence of a functional nTRE within this portion of CYP4A2. These data demonstrate gene regulatory activity by DHEA different from that of nafenopin, and a suppressive effect of T3, consistent with indirect regulatory mechanisms not involving an nTRE.
Footnotes
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Send reprint requests to: Dr. Thomas E. Geoghegan, Department of Biochemistry and Molecular Biology, University of Louisville, School of Medicine, Louisville, KY 40292.
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This research was supported by National Cancer Institute Grant CA43839. X.D.L. was supported in part by funds from the Humana Endowment for Excellence, administered by the J. Graham Brown Cancer Center, University of Louisville.
- Abbreviations used are::
- DHEA
- dehydroepiandrosterone
- ACO1
- Acyl-CoA oxidase
- T3
- triiodothyronine
- TR
- thyroid hormone receptor
- TRβ
- thyroid hormone receptor β
- nTRE
- negative thyroid hormone response element
- PPAR
- peroxisome proliferator-activated receptor
- RAR
- retinoic acid receptor
- RXR
- retinoid X receptor
- EDTA
- ethylenediamine tetraacetic acid
- GAPDH
- glyceraldehyde-3-phosphate dehydrogenase
- CMV
- cytomegalovirus
- SV40
- simian virus 40
- SSPE
- standard saline phosphate-EDTA
- PXR
- pregnane X receptor
- Received February 27, 1998.
- Accepted September 22, 1998.
- The American Society for Pharmacology and Experimental Therapeutics
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