Activation of promutagens in a human bronchial epithelial cell line stably expressing human cytochrome P450 1A2.

Normal human bronchial epithelial (NHBE) cells are the putative progenitor cells of all types of lung cancer. NHBE cells immortalized by SV40 T-antigen retain many characteristics of the primary cells and are a useful model for investigating the role of oncogenes, tumor suppressor genes, and certain chemical carcinogens in the molecular pathogenesis of lung cancer. In this study, SV40 T-antigen-positive cells (BEAS-2B) were characterized for their metabolic functions and were shown to continue to express epoxide hydrolase, glutathione S-transferase pi, glutathione peroxidase, and catalase. To increase their metabolic activity towards human procarcinogens, human cytochrome P450 1A2 (CYP1A2) was stably expressed by introducing CYP1A2 cDNA into BEAS-2B cells either by infection with a high-titer recombinant retrovirus (pXT-1A2) or by transfection with a CYP1A2 expression vector (pCMV1A2), which produced the cell lines B-1A2 and B-CMV1A2, respectively. Cell lines established with either expression system expressed enzymatically active CYP1A2 protein and were 50- to 400-fold more sensitive to the cytotoxic effect of the carcinogen aflatoxin B1 (AFB1) than the corresponding control cell lines. The cytotoxic effects of AFB1 were paralleled by increased metabolism of AFB1 and enhanced formation of the AFB1-N7 guanine adduct in B-CMV1A2 cells. Cytotoxicity and adduct formation correlated with a significantly higher protein expression of CYP1A2 by the cytomegalovirus promoter-driven plasmid. Since this human epithelial cell line is the precursor cell type of lung cancer, has normal phase II enzymes, and exhibits highly reproducible expression of phase I enzymes, this in vitro model should aid in the evaluation of putative human carcinogens and anticarcinogens.