Biotransformation of aflatoxin B1 in human lung

2Surgery, and 'Medicine, Queen's University, Kingston, ON, Canada, K7L 3N6 ''To whom correspondence should be addressed In addition to being a potent hepatocarcinogen, aflatoxin Bi (AFBi) is a pulmonary carcinogen in experimental animals, and epidemiological studies have shown an association between AFT*! exposure and lung cancer in humans. This study investigated AFBj bioactivation and detoxification in human lung tissue obtained from patients undergoing clinically indicated lobectomy. [3H]AFB! was bioactivated to a DNA binding metabolite by human whole lung cytosols in a time-, protein concentration-, and AFT*! concentration-dependent manner. Cytosolic activation of [3H]AFB, correlated with lipoxygenase (LOX) activity and was inhibited by the LOX inhibitor nordihydroguaiaretic acid (NDGA; 100 uM), indicating that LOXs were largely responsible for the observed cytosolic activation of AFBi. In whole lung microsomes, low levels of indomethacin inhibitable prostaglandin H synthase (PHS)-mediated [3H]AFB,-DNA binding and cytochrome P-450 (P450)mediated pHJAFB^DNA binding were observed. Cytosolic glutathione S-transferase (GST)-catalyzed detoxification of AFBi-8,9-epoxide, produced by rabbit liver microsomes, was minimal at 1 and 10 ^iM pHJAFB,. With 100 pM [3H]AFB,, [3H]AFB!-8^-epoxide conjugation with reduced glutathione was 034 ± 0.26 pmol/mg/h (n = 10). In intact, isolated human lung cells, [3H]AFB, binding to cellular DNA was higher in cell fractions enriched in macrophages than in either type II cell-enriched fractions or fractions containing unseparated cell types. Indomethacin produced a 63-100% decrease in [ 3H]AFB,-DNA binding in macrophages from five of seven patients, while NDGA inhibited [3H]AFB,-DNA adduct formation by 19, 40 and 56% in macrophages from three of seven patients. In alveolar type II cells, NDGA decreased [3H]AFB,-DNA binding by 30100% in cells from three patients and indomethacin had little effect SKF525A, an isozyme non-selective P4S0 inhibitor, enhanced [3H]AFB, binding to cellular DNA in unseparated cells, macrophages, and type II cells, suggesting that P450-mediated bioactivation of AFBi is n«t a major pathway by which AFB^^-epoxide is formed in human lung cells. Overall, these studies suggest that P450 has a minor role in the bioactivation of AFB, in human lung. Rather, LOXs and PUS appear to be important bioactivation enzymes. Co-oxidative bioactivation of AFB 1; in combination with the low conjugating activity displayed