Inhibition of DES-induced DNA adducts by diallyl sulfide: Implications in liver cancer prevention

Diethylstilbesterol (DES) is known to cause cancer in humans and animals. Diallyl sulfide (DAS), a component of garlic, has been shown to prevent various types of cancer, presumably via metabolic modulation. Previously, we have demonstrated that DAS prevents the oxidation and reduction of DES in vitro. We hypothesize that DAS will inhibit the metabolism of DES in vivo thus preventing the formation of DES-induced DNA adducts. To test this hypothesis, five groups of five male Sprague-Dawley rats were treated as follows: the control received 0.5 ml of corn oil daily for four days. The second group received 50 mg/kg DAS daily for four days. The third group received 50 mg/kg DAS daily for four days followed by 150 mg/kg DES on day five. The fourth group received 400 mg/kg DAS on day five followed by 150 mg/kg DES. The fifth group received 150 mg/kg DES on day five. All of the rats were sacrificed on day five, 4 h after DES treatment. DNA was isolated from the liver and analyzed by 32P-post-labeling for DNA adducts. The in vitro study was performed utilizing four reactions described as follows: the control reaction contained 200 µg DNA, microsomes (346 µg protein/ml), and 10 mM DES; no oxidation co-factor (cumen hydroperoxide) was added. The second reaction, a complete oxidation system, contained 200 µg DNA, microsomes (346 µg protein/ml), 30 mM cumen hydroperoxide, and 10 mM DES. The third reaction contained 200 µg DNA, microsomes (346 µg protein/ml), 30 mM cumen hydroperoxide, 50 mM DAS, and 10 mM DES. The fourth reaction contained 200 µg DNA, microsomes (346 µg protein/ml), 30 mM cumen hydroperoxide, 100 mM DAS, and 10 mM DES. All of the in vitro reactions were buffered with 100 mM KPO4 pH 7.4 and incubated for 30 min at 37°C. DNA was extracted and analyzed by 32P-post-labeling. We found that DAS inhibited the formation of DES-induced DNA adducts in a dose-dependent fashion. We have shown that DES-induced DNA adducts were inhibited in rats that received DAS pre-treatment and co-treatment with DES. These results suggest that DAS directly inhibits the metabolism of DES thus preventing the formation of DNA adducts. In addition to directly inhibiting the metabolism of DES, DAS appears to alter the expression of the metabolic machinery such that DES-induced adducts are inhibited. The inhibition of DES-induced DNA adducts by DAS may prevent the initiation of estrogen-induced cancer.