Metabolism of Aflatoxin B1 by Normal Human Bronchial Epithelial Cells

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Journal of Toxicology and Environmental Health, Part A






Taylor & Francis

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Aflatoxin B1 (AFB1) is a potent hepatocarcinogen in animal models and a suspected carcinogen in humans. High concentrations of AFB1 have been found in respirable grain dusts, and may therefore be a risk factor for human lung cancer in certain occupations. To study the potential for AFB activation in human lung, cytochrome P-450 (CYP)-mediated activa1 tion and glutathione S-transferase (GST)-mediated detoxification of AFB1 were examined in cultured normal human bronchial epithelial (NHBE) cells. Cells were exposed to 0.15 µM or 1.5 µM AFB1 for 48 h and media was collected for metabolite analysis by high-performance liquid chromatography (HPLC). At 0.15 µM, AFB1 was metabolized only to the detoxified metabolite aflatoxin Q1(AFQ1). At 1.5 µM AFB1, both aflatoxin M1(AFM1), and AFQ1 were produced. Cells pretreated with 50 µM 3-methylcholanthrene (3MC), a CYP 1A inducer, for 72 h prior to 0.15 µ M AFB1, produced the activated AFB1 8,9-epoxide (AFBO). Similarly, microsomes prepared from 3MC-pretreated cells formed AFBO, but microsomes from noninduced cells did not. While AFB1-DNA adducts were not detected at low AFB1 concentrations in untreated NHBE, 3MC induction caused the production of AFB1-DNA adducts at 0.015 and 0.15 µM AFB1. Western immunoblots showed that the primary CYP isoforms responsible for AFB1 activation in the liver, 1A and 3A4, to be constitutively expressed in NHBE cells. Expression of CYP 1A was significantly increased in 3MC-pretreated cells, while CYP 3A4 expression increased slightly, but not to the extent of the 1A isoforms. The principal AFBO detoxifying enzyme, glutathione S -transferase (GST), was constitutively expressed in NHBE cells, and was increased approximately twofold by 3MC pretreatment. Cytosolic fractions from neither control nor 3MC-induced NHBE had measurable AFBO conjugating activity, indicating that these cells may lack AFB1-relevant GST activity. From these data, it appears that NHBE cells activate AFB1 inefficiently, but possess CYPs reportedly responsible for metabolism of AFB1. These data support earlier findings showing modest CYP-mediated AFB1 activation in human airways, but indicate that exposure to polycyclic aromatic hydrocarbons (PAHs), such as 3MC, which induce CYP(s) that specifically activate AFB1 may increase the harmful effects of AFB1 exposures in human airways.


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