Abstract
Environmental polycyclic aromatic hydrocarbons (PAHs) and metals coexist, and such mixtures could affect the carcinogenicity of PAHs, possibly by modification of PAH induction of the PAH-bioactivating CYP1A. The effect on PAH-mediated CYP1A induction of arsenic, lead, mercury, or cadmium (ranked as the most hazardous environmental metals by the Environmental Protection Agency and the Agency for Toxic Substances and Disease Registry) has thus been investigated. Induction of CYP1A1 by benzo[a]pyrene (BAP), benzo[b]fluoranthene (BBF), dibenzo[a,h]anthracene (DBAHA), benzo[a]anthracene (BAA), or benzo[k]fluoranthene (BKF) was probed by ethoxyresorufin-O-deethylase activity (EROD) in 96-well plates of human HepG2 cells, by immunoblot analysis, and by reverse transcription-polymerase chain reaction. Cells rapidly took up PAHs (BAP, BKF) from medium; by 24 h only 14% remained in the medium, and no detectable PAH bound to well walls. Induction efficiency (relative to dimethyl sulfoxide controls) was in the order BKF (16-fold) > DBAHA (14-fold) > BAA (4-fold) > BAP (3-fold) > BBF (1-fold), all at 5 μM PAH. The metals did not markedly affect cell viability at concentrations of arsenic, 5 μM; lead, 50 μM; mercury, 5 μM; and cadmium, 5 μM. At 5 μM PAH concentration, all of the metals decreased levels of PAH-induced CYP1A1 activities (direct inhibition of EROD activity was excluded) by variable extents and in a PAH-dependent manner. With BAP as inducer decreases in induction were arsenic, 57%; cadmium, 82%; mercury, 4%; and lead, 20%. The decreases were not a consequence of transcriptional down-regulation. One possible conclusion is that these metals could diminish PAH carcinogenic potential by decreasing PAH-mediated induction of their bioactivation by CYP1A1.
Footnotes
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Although the research described in this article has been funded in part by the United States Environmental Protection Agency through Grant R827180010 to L.S.K., it has not been subjected to the Agency's required peer and policy review and therefore does not necessarily reflect the views of the Agency and no official endorsement should be inferred. N.L., who is affiliated with the Division of Environmental Health, Department of Preventive Medicine, Jiangxi Medical College, Nanchang, China, is grateful to The World Health Organization for support.
- Abbreviations used are::
- PAH
- polycyclic aromatic hydrocarbon
- CYP or P450
- cytochrome P450
- BAP
- benzo[a]pyrene
- BBF
- benzo[b]fluoranthene
- DBAHA
- dibenzo[a,h]anthracene
- BAA
- benzo[a]anthracene
- BKF
- benzo[k]fluoranthene
- DMSO
- dimethyl sulfoxide
- αEW
- α-modified Eagle's and Waymouth medium
- EROD
- ethoxyresorufin-O-deethylase
- PBS
- phosphate-buffered saline
- MTT
- 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide
- RT-PCR
- reverse transcription-polymerase chain reaction
- PCR
- polymerase chain reaction
- Received August 11, 2000.
- Accepted March 22, 2001.
- The American Society for Pharmacology and Experimental Therapeutics
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