Aflatoxin B(1) metabolism and DNA adduct formation in the proliferating mouse liver

Hepatocellular carcinoma (HCC) was fifth in incidence among all cancers worldwide in the year 2000 (1). One of the major risk factors associated with the development of HCC is dietary exposure to the fungal metabolite and hepatocarcinogen aflatoxin B₁ (AFB₁). This molecule requires metabolic activation to an ultimate carcinogenic form in order to assert its maximum carcinogenic potential. Metabolism of AFB₁ occurs in the liver where it undergoes an initial enzymatic oxidation, mainly by members of the cytochrome P450 superfamily, to the electrophilic intermediate AFB₁-8,9-epoxide. The epoxide functional group is modified by glutathione-s-transferase (GST) mediated conjugation with glutathione (GSH) forming a water-soluble product (2,3). Unmodified AFB₁-epoxide has the potential to react with any cellular macromolecule, including DNA, forming a covalently bound adduct. AFB₁-DNA adduction is believed to be the source of point mutations that initiate AFB₁ induced HCC (4,5). In the liver, GST level is the principal regulator of AFB₁ sensitivity and is responsible for interspecies variations in AFB₁ susceptibility (3,6–8). Previous studies in our laboratory and others demonstrate an increased sensitivity to AFB₁ in mice with hyperproliferative livers (9–14). There is also a strong synergistic relationship between hepatitis B virus (HBV) infection and dietary AFB₁ exposure (10,15–18). We hypothesize that hepatic GST is decreased during periods of hepatocyte proliferation rendering the liver more susceptible to AFB₁-DNA adduction. The increased AFB₁-DNA adduct load results in a greater number of tumor initiating point mutations. Herein, we examine hepatic GST levels and AFB₁-DNA adduct formation in the context of proliferating liver associated with several mouse models of hepatocyte proliferation. We first look at two models for a rapidly proliferating liver, neonatal mouse liver and the proliferative response to 2/3 partial hepatectomy. We then examine two models of chronic hepatocyte proliferation, p53 deficient and hepatitis B surface antigen (HBsAg) expressing mice. We conclude that rapid hepatocyte proliferation associated with neonatal liver or 2/3 partial hepatectomy greatly reduces hepatic GST levels and increases AFB₁-DNA adduct formation upon exposure to AFB₁. This likely contributes to the increased hepatocarcinogenic potential of AFB₁ reported in these systems. Chronic hepatocyte proliferation associated with loss of p53 also decreases hepatic GST and increase AFB₁-DNA adduct formation whereas these parameters appear to be unaffected in HBsAg expressing mice.