Tobacco smoke is a major risk factor for lung cancer but also for liver, bladder, and gastrointestinal tract cancers. The combustion of tobacco produces many classes of carcinogens, including polycyclic aromatic hydrocarbons, N-nitrosamines, aromatic amines (AAs), and heterocyclic aromatic amines (HAAs). 2-Amino-9H-pyrido[2,3-b]indole (AαC) is the most abundant HAA formed in tobacco smoke with levels ranging from 37 to 258 ng/cigarette. These amounts are 25−100-fold higher than those levels of 4-aminobiphenyl (4-ABP) and 2-naphthylamine, two AAs implicated in bladder cancer of smokers. AαC is a liver carcinogen, a transgene colon mutagen, and an inducer of colonic aberrant crypt foci, an early biomarker of colon neoplasia, in mice. The carcinogenic potential of AαC is unknown in humans. However, AαC damages DNA in human lymphocytes and lymphoblastic cells, based on the Comet assay. Moreover, AαC is bioactivated in human hepatocytes and T lymphocytes with greater efficiency than other HAAs or 4-ABP, leading to the formation of high levels of DNA adducts. AαC is bioactivated by cytochrome P450s (CYPs), to form 2-hydroxyamino-9H-pyrido[2,3-b]indole (HONH-AαC), which undergoes esterification by sulfotransferase (SULTs) or N-acetyltransferase (NATs) to form reactive intermediates that bind to DNA. We examined the capacity of 4-ABP, AαC, but also 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) and 2-amino-3,8-dimethylmidazo[4,5-f]quinoxaline (MeIQx), carcinogenic HAAs formed also formed in tobacco smoke or cooked meats, to undergo bioactivation in human bladder. DNA adducts levels were measured is RT4 cells, an epithelial human bladder cell line. AαC and 4-ABP form DNA adducts in RT4 cells, whereas PhIP and MeIQx do not. Thus, bladder cells can directly bioactivate AαC and 4-ABP and prior hepatic N-oxidation is not required. The inhibition of CYP1 activity, by α-naphthoflavone, resulted in a strong decrease DNA adduct formation of AαC but had no effect of the level of 4-ABP adducts, indicating the involvement of other CYPs or oxidases in the bioactivation of 4-ABP. Our current work is to identify the major CYPs involved in the bioactivation of 4-ABP and the role of NATs and UDP-Glucuronosyl-Transferases (UGTs) in the metabolism of 4-ABP and AαC in bladder cells. Our data provide support to the epidemiological observations implicating AAs and possibly AαC in tobacco smoke as risk factors for bladder cancer.

Mechanisms of bioactivation of the tobacco carcinogens and 2-amino-9H-pyrido[2,3-b]indole (AαC) and 4-aminobiphenyl (4-ABP) in human bladder

256th National Meeting (2018)

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