Metabolism of Aflatoxin B₁ by Normal Human Bronchial Epithelial Cells

Authors

Terry R. Van Vleet, Utah State University
Patrick J. Klein, Utah State University
Roger A. Coulombe Jr., Utah State UniversityFollow

Document Type

Article

Journal/Book Title/Conference

Journal of Toxicology and Environmental Health, Part A

Volume

63

Issue

7

Publisher

Taylor & Francis

Publication Date

2001

First Page

525

Last Page

540

Abstract

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.

Comments

Originally published by Taylor & Francis. Publisher's PDF available through remote link.

Recommended Citation

Van Vleet, T.R., Klein, P.J., and R.A. Coulombe, Jr. (2001). Metabolism of Aflatoxin B1 by Normal Human Bronchial Epithelial Cells. J. Toxicology and Environmental Health , Part A, 63: 525-40.