Promotion of aflatoxin B1 carcinogenesis by the natural tumor modulator indole-3-carbinol: influence of dose, duration, and intermittent exposure on indole-3-carbinol promotional potency.

Indole-3-carbinol (13C), a secondary metabolite from cruciferous vegetables, inhibits aflatoxin B1 (AFB1) hepatocarcinogenesis in trout (Bailey et al., J. Natl. Cancer Inst., 78: 931-934, 1987) and rats (Selivonchick et al., unpublished results) when given prior to and with carcinogen but promotes carcinogenesis in both species when given continuously following AFB1 initiation. Since human 13C intake may not be continuous, and the promotional stimulation may be reversible, we have assessed 13C promotion using delayed and discontinuous exposure protocols. Following initiation with AFB1, 13C was fed to trout for varying periods of time, with varying lengths of delay after initiation and continuous or intermittent patterns of 13C treatment. Promotional enhancement of tumor incidence by 13C was found to be significant when 13C treatment was delayed for several weeks or months after the initial AFB1 challenge. Promotion also was found to increase with length of exposure to 13C treatment and to be decreased but still evident when 13C was given in alternating months or weeks, or twice per week only. These results do not support the idea that promotional stimulation in hepatocarcinogenesis is a reversible phenomenon. To quantify 13C promotional potency in terms of its dietary concentration, a series of AFB1 tumor dose-response curves was established, each with a different level of 13C fed continuously following AFB1 initiation. The resultant tumor dose-response curves, plotted as logit percentage of incidence versus log AFB1 dose, were displaced parallel toward lower AFB1 50% tumor take (TD50) values with increasing 13C concentration. The level of 13C that halves the AFB1 dose for 50% tumor incidence was calculated to be approximately 1000 ppm 13C, fed continuously, with no substantial threshold for promotion. By comparison, 13, when fed before and with AFB1, shows a 50% inhibitory value (13C concentration that doubles the dose of AFB1 for 50% tumor incidence) in trout of 1400 ppm 13C [Dashwood et al., Carcinogenesis (Lond.), 10: 175-181, 1989]. Thus the potential for 13C as a dietary additive to promote prior hepatic initiation events when fed continuously is approximately as great as its potential to inhibit concurrent AFB1 initiation.