Aryl Hydrocarbon Receptor Regulation of Cytochrome P4501B1 in Rat Mammary Fibroblasts: Evidence for Transcriptional Repression by Glucocorticoids
- 1Environmental Toxicology Center (P.B.B., C.R.J.) and 2Department of Pharmacology (L.Z., C.R.J.), University of Wisconsin Medical School, Madison, Wisconsin 53706.
Abstract
Cytochrome P450 1B1 (CYP1B1), which actively metabolizes polycyclic aromatic hydrocarbons, is regulated by the aryl hydrocarbon receptor (AhR) in primary cultures of rat mammary fibroblasts (RMF) and rat embryo fibroblasts (REF). 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) induced the 5.2-kilobase CYP1B1 mRNA in RMF (12-fold) and REF (14-fold) after a 6-hr treatment, with comparable increases in the microsomal protein. The synthetic glucocorticoid dexamethasone (DEX) suppresses TCDD-induced expression of CYP1B1 in RMF and REF. Suppression of CYP1B1 mRNA in RMF (maximal suppression, 70%) was observed when DEX was added 2 hr before TCDD, but was not observed with co-administration. The concentration dependence (EC50 ≈ 10 nm) and reversal by the antagonist, RU486, implicates the glucocorticoid receptor. DEX inhibition of TCDD-induced CYP1B1 protein needed more extensive preincubation (>6 hr). TCDD induction of CYP1B1-luciferase constructs in RMF was mediated by a 265-base-pair upstream region (−810 to −1075), which was similarly suppressed (50–70%) by a 2-hr preincubation with 10-7m DEX via this enhancer region. Expression of the AhR is suppressed by DEX (70% after 12 hr), but not after the 2-hr period that was sufficient for suppression of transcription. The AhR nuclear translocator is not affected by this treatment. We conclude that glucocorticoid receptor rapidly suppresses activity of the AhR/AhR nuclear translocator complex in the CYP1B1 enhancer region, even though lacking glucocorticoid responsive element(s). DEX inhibits proliferation of RMF in this same concentration range (35%, EC50 ≈ 5 nm), indicating additional effects on intracellular activity that may link to this suppression.
Footnotes
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Send reprint requests to: Dr. Colin R. Jefcoate, Department of Pharmacology, University of Wisconsin Medical School, 1300 University Avenue, Madison, WI 53706. E-mail:jefcoate{at}facstaff.wisc.edu
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↵1 Current affiliation: Reproductive Endocrinology Center, Department of Obstetrics and Gynecology, University of California-San Francisco, San Francisco, CA 94513.
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This work was supported by National Research Service Award T32 ES07015 from the National Institute of Environmental Health Sciences (P.B.B.), NIH Grant 144EN46 and DOD Breast Cancer Research Grant DAMD17–94-J-4054 (C.R.J.).
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Contribution 317, Environmental Toxicology Center, University of Wisconsin, Madison, WI 53706.
- Abbreviations:
- CYP
- cytochrome P450
- RMF
- rat mammary fibroblasts
- REF
- rat embryo fibroblasts
- PAH
- polycyclic aromatic hydrocarbon
- TCDD
- 3,7,8-tetrachlorodibenzo-p-dioxin
- AhR
- aryl hydrocarbon receptor
- Arnt
- aryl hydrocarbon nuclear translocator
- GC
- glucocorticoid
- GR
- glucocorticoid receptor
- DEX
- dexamethasone
- XRE
- xenobiotic response element
- bp
- base pair(s)
- kb
- kilobase pair(s)
- GRE
- glucocorticoid responsive element
- DME/F12
- Dulbecco’s modified Eagle’s/Ham’s F12 medium
- FBS
- fetal bovine serum
- PMSF
- phenylmethylsulfonyl fluoride
- MOPS
- 3-(N-morpholino)propanesulfonic acid
- DMSO
- dimethyl sulfoxide
- EGTA
- ethylene glycol bis(β-aminoethyl ether)-N,N,N′,N′-tetraacetic acid
- TBST
- Tris-buffered saline/Tween 20
- BES
- N,N-bis(2-hydroxyethyl)-2-aminoethanesulfonic acid
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- Received February 24, 1998.
- Accepted August 17, 1998.
- The American Society for Pharmacology and Experimental Therapeutics
