Aflatoxin B1 alters the expression of p53 in cytochrome P450-expressing human lung cells

Aflatoxin B-1 (AFB(1)) is a potent dietary hepatocarcinogen in animals and probably in humans. Mutations (and altered expression) of the tumor suppresser gene p53 have been observed in liver tumors from patients exposed to high dietary AFB(1). Inhalation of AFB(1)-laden grain dusts has been associated with an increased incidence of lung cancer in humans as well. We examined the effects of low concentrations of AFB(1) on the expression of p53 and MDM2 in human bronchial epithelial cells (BEAS-2B) transfected with cDNA for either cytochrome P450 (CYP) 1A2 (B-CMV1A2) or CYP 3A4 (B3A4), two isozymes that are responsible for AFB(1) activation in human liver and possibly the lung. Untreated B-CMV1A2 and B3A4 cells constitutively expressed p53. Exposure to a range (0.015-15 mu M for 30 min) of AFB(1) concentrations caused a concentration-dependent decline in p53 expression in B-CMV1A2 cells, and to a lesser extent, in B3A4 cells. The AFB(1)-mediated decrease in p53 continued for at least 12 h after 30-min exposures to 1.5 mu M AFB(1). Mirroring the decrease in p53 expression was a concentration-dependent increase in the expression of the 76-kDa MDM2 isoform in B-CMV1A2 and B-3A4 cells. Interestingly, AFB(1) did not induce DNA laddering, an indicator of apoptotic cell death, but proteolytic activation of caspase-3 was detected in AFB(1)-treated B-CVM1A2 cells. In total, these data show that low, environmentally-relevant concentrations of AFB(1) alter the expression of p53 and MDM2 in these human lung cells, and that cells that stably express CYP 1A2 were more susceptible to this effect than nontransfected, or 3A4-expressing cells.