Suppressive Role of Indole on 2-Acetylaminofluorene Hepatotoxicity

Indole is known to suppress the hepatotoxicity and carcinogenicity of 2-acetylaminofluorene (AAF) in rats and hamsters. For elucidation of the mechanism of its protective role, 2 experiments were conducted using young male rats. In the 1st experiment, the 24-hr biliary excretion of N-hydroxy-2-acetylaminofluorene (N-OH-AAF)-glucuronide was measured after 2 and 4 weeks of dietary administration of 0.03% AAF with or without 1.6% indole. The amount of [9-^,4C]N-OH-AAF that was excreted as the glucuronide following a single i.p. injection of [9-¹⁴C]AAF was lower after 2 weeks in animals fed AAF and indole, as compared to those fed AAF alone [1.5 ± 1.2% versus 19.6 ± 3.6% S.E. (p < 0.001)]. After 4 weeks of AAF administration without indole, the biliary excretion fell to 4.8 ± 2.1%. This was also significantly higher than that of the animals fed both AAF and indole [1.8 ± 1.2% (p < 0.025)]. The suppressive role of indole on the conjugate excretion was also reflected in a decreased biliary excretion of all [9-¹⁴C]AAF metabolites in animals treated with indole alone. In the 2nd experiment, the protective action of indole was assessed by survival following daily i.p. injections of N-OH-AAF and Na₂SO₄solution. Na₂SO₄increased the hepatotoxicity of N-OH-AAF. Indole suppressed the toxicity of N-OH-AAF even in the presence of Na₂SO₄. This protective role of indole was partially overcome only when excess sulfate was coadministered. These results indicate that indole suppresses the biliary excretion of the O-glucuronide of N-OH-AAF during the initial exposure of the animal to the carcinogen, possibly reflecting decreased N-OH-AAF formation. Indole also modifies the metabolism of AAF following N-hydroxylation, perhaps activating N-OH-AAF, depending upon the concentration of sulfate available.