Persistent Expression of 3-Methylcholanthrene-Inducible Cytochromes P4501A in Rat Hepatic and Extrahepatic Tissues1
Abstract
We reported earlier that 3-methylcholanthrene (MC) persistently induces hepatic ethoxyresorufinO-deethylase activities (CYP1A1) in rats for up to 45 days. In this investigation, we tested the hypotheses that persistent expression of CYP1A1 activities is paralleled by sustained induction of CYP1A1/CYP1A2 apoproteins and their mRNAs and that this phenomenon is mediated by mechanisms other than retention of MC in the rat. Rats were given MC (93 μmol/kg) i.p., once daily for 4 days, and CYP1A1/1A2 parameters were measured in liver at selected time points. MC-elicited increases in CYP1A1/1A2 activities, apoprotein contents, and mRNA levels were sustained for several weeks after the last dose of MC treatment. MC also caused long-term induction of CYP1A1 in lungs and mammary glands. Rats treated with [3H]MC once daily for 4 days excreted 92.3% of the administered radioactivity in feces and urine by day 15. The intrahepatic concentration of MC at the 15-day time point was 270 pmol/g. Dose-response studies showed that administration of MC (2 μmol/kg), which produced an intrahepatic concentration of 271 pmol/g after 24 h, did not induce CYP1A1/1A2 activities, strongly suggesting that the sustained induction of CYP1A1/1A2 was not due to retention of the parent MC in the body. Electrophoretic mobility shift assays revealed that persistent CYP1A1 induction by MC involved Ah receptor-independent mechanisms. In conclusion, our results support the hypothesis that persistent expression of CYP1A1/1A2 by MC is mediated by mechanisms independent of the retention of the parent carcinogen.
Footnotes
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Send reprint requests to: Dr. Bhagavatula Moorthy, Ph.D., Department of Pediatrics, Baylor College of Medicine, One Baylor Plaza, Houston, TX 77030. E-mail: bmoorthy{at}neo.bcm.tmc.edu
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↵1 This work was supported in part by U.S. Public Health Service (USPHS) Grant ES09132 from the National Institute of Environmental Health Sciences and a grant-in-aid from the American Heart Association (Texas Affiliate) to B. Moorthy and by USPHS Grant CA32157 from the National Cancer Institute to Kurt Randerath.
- Abbreviations:
- MC
- 3-methylcholanthrene
- PAH
- polycyclic aromatic hydrocarbon
- TCDD
- 2,3,7,8-tetrachlorodibenzo-p-dioxin
- BP
- benzo[a]pyrene
- AHR
- Ah receptor
- AhRE
- Ah response elements
- ARNT
- Ah receptor nuclear translocator
- EROD
- ethoxyresorufinO-deethylase
- MROD
- methoxyresorufinO-demethylase
- AHH
- aryl hydrocarbon hydroxylase
- HRP
- horseradish peroxidase
- PAGE
- polyacrylamide gel electrophoresis
- CO
- corn oil
- GAPDH
- glyceraldehyde-3-phosphate dehydrogenase
- SSC
- sodium chloride sodium citrate
- DBA
- dibenz[ac]anthracene
- EMSA
- electrophoretic mobility shift assay
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- Received December 13, 1999.
- Accepted March 9, 2000.
- The American Society for Pharmacology and Experimental Therapeutics
