Induction of Hepatic CYPIA by Indole-3-Carbinol in Protection Against Aflatoxin B1 Hepatocarcinogenesis in Rainbow Trout

This study examined the significance of hepatic cytochrome P4501A (CYPIA) induction in the inhibition of aflatoxin B1 (AFB1)-DNA adduction by indole-3-carbinol (13C) in rainbow trout. I3C, fed prior to [3H]AFB1, exposure, provided dose-dependent inhibition of hepatic AFB1,-DNA binding, which appeared to vary inversely with hepatic CYPIA-mediated ethoxyresorufin O-deethylase (EROD) activity (r = -0.81, P = 0.051). However, 1000 ppm dietary I3C inhibited AFB1,-DNA adduction without detectably inducing CYPIA protein or EROD activity. Dietary I3C was found to inhibit AFB1-DNA adduction by approximately 50%, whether[3H]AFB1, was injected ip 1, 2, 3, 5 or 7 days after the onset of 13C feeding, yet hepatic EROD activity was only transiently induced over this period and was not correlated with AFB1,-DNA inhibition. Microsome-catalysed AFB1,-DNA binding in vitro did correlate inversely with EROD activity in microsomes from control- and 13C-treated trout (r = -0.955, P = 0.01), but data obtained using microsomes from β-naphthoflavone-treated trout suggest that this observation may not be indicative of a cause-and-effect relationship. 13C-mediated reduction in covalent binding was not due to I3C derivatives in the microsomal preparation or to reduced CYP protein levels, but may reflect a lower microsomal catalytic capacity for AFB1, epoxidation as a result of enzyme inactivation. In addition, the major 13C derivative found in liver, 3,3ʹ-diindolylmethane, has been shown to be a non-competitive inhibitor of EROD, and of enzymes that catalyse AFB1, epoxidation. These findings indicate little, if any, role for CYPIA induction in the inhibition of AFB1, carcinogenicity in rainbow trout by levels of 13C likely to be encountered in cruciferous vegetables.