ToxSci Advance Access originally published online on August 21, 2009
Toxicological Sciences 2009 112(1):59-67; doi:10.1093/toxsci/kfp201
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3-Methylindole is Mutagenic and a Possible Pulmonary Carcinogen
Department of Pharmacology and Toxicology, University of Utah, Salt Lake City, Utah 84112
1 To whom correspondence should be addressed at Department of Pharmacology and Toxicology, University of Utah, 30 S 2000 E, Room 201, Salt Lake City, UT 84112. Fax: (801) 585-3945. E-mail: gyost{at}pharm.utah.edu.
Received April 18, 2009; accepted August 18, 2009
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Abstract |
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Previous work has shown that bioactivation of the cigarette smoke pneumotoxicant 3-methylindole (3MI) by pulmonary cytochrome P450 enzymes is directly associated with formation of DNA adducts. Here, we present evidence that normal human lung epithelial cells, exposed to low micromolar concentrations of 3MI, showed extensive DNA damage, as measured by the comet assay, with similar potency to the prototypical genotoxic agents, doxorubicin and irinotecan. The DNA damage caused by 3MI was predominantly caused by single-strand breaks. Furthermore, we show that this damage decreased with time, given a subtoxic concentration, with detectable DNA fragmentation peaking 4 h after exposure and diminishing to untreated levels within 24 h. Pretreatment with an inhibitor of poly(ADP-ribose) polymerase 1 (PARP1), NU1025, nearly doubled the DNA damage produced by 5µM 3MI, implying that PARP1, which among other activities, functions to repair single-strand breaks in DNA, repaired at least some of the 3MI-induced DNA fragmentation. A key cellular response to DNA damage, phosphorylation, and nuclear localization of p53 was seen at subtoxic levels of 3MI exposure. 3MI was highly mutagenic, with essentially the same potency as the prototype carcinogen, benzo[a]pyrene, only when a lung-expressed CYP2F3 enzyme was used to dehydrogenate 3MI to its putative DNA-alkylating intermediate. Conversely, a rat liver S9 metabolic system did not bioactivate 3MI to its mutagenic intermediate(s). Concentrations higher than 25µM caused apoptosis, which became extensive at 100µM, similar to the response seen with 10µM doxorubicin. Our findings indicate that there is a low concentration window in which 3MI can cause extensive DNA damage and mutation, without triggering apoptotic defenses, reinforcing the hypothesis that inhaled 3MI from cigarette smoke may be a potent lung-selective carcinogen.
Key Words: 3-methylindole; DNA damage; DNA repair mechanisms; chemical carcinogenesis; mutagen; p53; lung cancer; cytochrome P450.