Search

This study was undertaken to elucidate the mechanism of organ specificity and differential efficacy of garlic organosulfides (OSCs) [diallyl sulfide (DAS), diallyl disulfide (DADS), diallyl trisulfide (DATS), dipropyl sulfide (DPS) and dipropyl disulfide (DPDS)] in preventing benzo(a)pyrene (BP)-induced tumorigenesis in mice. The results of the present study reveal a good correlation between chemopreventive efficacies of garlic OSCs and their inductive effects on the expression of NAD(P)H:quinone oxidoreductase (NQO), an enzyme implicated in the detoxification of activated quinone metabolites of BP. Treatment of mice with DADS and DATS, which are potent inhibitors of BP-induced forestomach tumorigenesis, resulted in a statistically significant increase (2.4- and 1.5-fold, respectively) in forestomach NQO activity. In addition, DADS and DATS were much more potent inducers of forestomach NQO activity than DAS, which is a weak inhibitor of BP-induced forestomach tumorigenesis than the former compounds. Propyl-group containing OSCs (DPS and DPDS), which do not inhibit BP-induced tumorigenesis, did not affect forestomach NQO activity. Similar to forestomach, a good correlation was also observed between effects of these OSCs against BP-induced pulmonary tumorigenesis and their effects on NQO expression in the lung. For example, treatment of mice with DAS, which is a potent inhibitor of BP-induced pulmonary tumorigenesis, resulted in about 3.2-fold increase in pulmonary NQO activity. On the other hand, this activity was increased by about 1.5-fold upon DATS administration, which does not inhibit BP-induced cancer of the lung. In conclusion, our results suggest that induction of NQO may be important in anti-cancer effects of garlic OSCs.

There is a growing need for short-term and cost-effective bioassay to assess the efficacy of potential chemo-preventive agents. We report that the induction of glutathione (GSH) S-transferase pi (mGSTP1-1) by a chemo-preventive agent can be used as a reliable marker to assess its efficacy in retarding chemical carcinogenesis induced by benzo(a)pyrene (BP), which is a widespread environmental pollutant and believed to be a risk factor in human chemical carcinogenesis. This conclusion is based on 1) the relative contribution of mGSTP1-1 of the liver and forestomach of female A/J mice in the detoxification of the ultimate carcinogenic metabolite of BP, (+)-anti-7,8-dihydroxy-9, 10-oxy-7,8,9, 10-tetrahydrobenzo(a)pyrene [(+)-anti-BPDE]; and 2) a positive correlation between the induction of hepatic and forestomach mGSTP1-1 by 5 naturally occurring organosulfides (OSCs) from garlic (diallyl sulfide, diallyl disulfide, diallyl trisulfide, dipropyl sulfide and dipropyl disulfide) and their effectiveness in preventing BP-induced forestomach neoplasia in mice. In the liver, the combined contribution of other GSTs in the detoxification of (+)-anti-BPDE was far less than the contribution of mGSTP1-1 alone. Likewise, in the forestomach, the contribution of mGSTP1-1 far exceeded the combined contribution of other GSTs. Studies on the effects of OSCs against BP-induced forestomach neoplasia revealed a good correlation between their chemo-preventive efficacy and their ability to induce mGSTP1-1 expression in the liver (r = -0.89; p < 0.05) as well as in the forestomach (r = -0.97; p < 0.05). Our results suggest that the induction of mGSTP1-1 may be a reliable marker for evaluating the efficacy of potential inhibitors of BP-induced cancer in a murine model.

The mechanism of differential efficacies of diallyl sulfide (DAS), diallyl disulfide (DADS), diallyl trisulfide (DATS), dipropyl sulfide (DPS) and dipropyl disulfide (DPDS) in preventing benzo(a)pyrene (BP)-induced cancer in mice has been investigated by determining their effects on the enzymes of BP activation/inactivation pathways. With the exception of DATS, treatment of mice with other organosulfides (OSCs) caused a small but significant increase (37-44%) in hepatic ethoxyresorufin O-deethylase (EROD) activity. However, the forestomach EROD activity did not differ significantly between control and treated groups. Only DAS treatment caused a modest but statistically significant reduction (about 25%) in pulmonary EROD activity. These results suggest that while reduction of EROD activity may, at least in part, contribute to the DAS-mediated inhibition of BP-induced lung cancer, anticarcinogenic effects of OSCs against BP-induced forestomach carcinogenesis seems to be independent of this mechanism. Treatment of mice with DAS, DADS and DATS resulted in a significant increase, as compared with control, in both hepatic (3.0-, 3.2- and 4.4-fold, respectively) and forestomach (1.5-, 2.7- and 2.7-fold, respectively) glutathione transferase (GST) activity toward anti-7beta,8alpha-dihydroxy-9alpha,10alpha-oxy-7,8,9,10-tetrahydrobenzo(a)pyrene (anti-BPDE), which is the ultimate carcinogen of BP. The pulmonary GST activity was not increased by any of the OSCs. Even though epoxide hydrolase (EH) activity was differentially altered by these OSCs, a correlation between chemopreventive efficacy of OSCs and their effects on EH activity was not apparent. The results of the present study suggest that differences in the ability of OSCs to modulate GST activity toward anti-BPDE may, at least in part, account for their differential chemopreventive efficacy against BP-induced cancer in mice.

This study characterizes glutathione (GSH) S-transferase (GST) isoenzymes of the liver and forestomach of the female A/J mouse and compares their specificities in catalyzing the conjugation of GSH with 7beta,8alpha-dihydroxy-9alpha,10alpha-oxy-7,8,9, 10-tetrahydrobenzo[a] pyrene (anti-BPDE), the ultimate carcinogenic metabolite of benzo[a]pyrene (BP). The GST activity in female A/J mouse liver was expressed by a minimum of seven isoenzymes which arose from different homo- or heterodimeric combinations of at least two alpha class (designated as alpha1 and alpha4), four micro class (micro1 to micro4), and one pi class GST subunit. The GST isoenzyme composition of A/J mouse forestomach appeared to be different from that of the liver. For example, while GST isoenzymes containing micro3 and micro4 type subunits were selectively expressed in the liver, an alpha class heterodimeric GST isoenzyme (containing alpha2 and alpha3 subunits) was expressed in the forestomach but could not be detected in the liver. The (+)-anti-BPDE appeared to be a better substrate than the (-)-enantiomer for all GSTs, except for isoenzymes containing the alpha4 type GST subunit. The murine pi class GST isoenzyme displayed relativey higher specific activity toward (+)-anti-BPDE compared to other GSTs. The specific activities of mouse GSTs toward (+)-anti-BPDE were in the order of pi > micro > alpha. These results suggest that the pi class GST isoenzyme may play an important role in providing protection against BP-induced cancer. Therefore, it seems logical to postulate that the ability of a chemoprotector to increase the expression of GST pi may be an important determinant of its effectiveness against BP-induced cancer. To test the validity of this contention, we have determined the effects on hepatic and forestomach GST isoenzyme/subunit expression of three naturally occurring organosulfides (OSCs) from garlic, which significantly differ in their effectiveness against BP-induced forestomach cancer. Treatment of mice with diallyl sulfide (DAS) and diallyl trisulfide (DATS), which are potent inhibitors of BP-induced fore- stomach cancer in mice, resulted in a significant increase in hepatic and forestomach GST activity toward anti-BPDE. On the contrary, this activity was not increased in either organ by dipropyl sulfide (DPS), which is ineffective against BP-induced forestomach cancer. The chemopreventive efficacy of these OSCs correlated with their ability to increase the expression of GST pi. For example, DAS treatment resulted in approximate increases of 1.7- and 2.2-fold in hepatic and forestomach GST pi expression, respectively, over the control. Treatment of mice with DATS, which is a relatively more potent inhibitor of BP-induced forestomach cancer than DAS, resulted in about 3.8- and 3.2-fold increases, respectively, in hepatic and forestomach GST pi expression over the control. On the contrary, the expression of hepatic and forestomach GST pi was increased only marginally (10-20%) upon DPS administration. In conclusion, the results of the present study suggest that induction of GST pi can be used as a bioassay for screening potential inhibitors of BP-induced cancer.

In this study, we report a novel anticarcinogenic activity of an organosulfur compound from garlic, diallyl disulfide (DADS). DADS treatment significantly inhibited the growth of H-ras oncogene transformed tumors in nude mice. As compared to controls, the appearance of tumors was also delayed markedly by oral administration of DADS. The inhibition of tumor growth by DADS treatment correlated with the inhibition of p21H-ras membrane association in the tumor tissue. The levels of membrane associated p21H-ras were markedly lower in the tumor tissues of DADS treated mice as compared to controls. An opposite trend, however, was evident for cytosolic p21H-ras. Furthermore, DADS treatment resulted in a significant inhibition of hepatic as well as tumoral 3-hydroxy-3-methylglutaryl coenzyme A reductase activity. These results indicate that DADS suppresses the growth of H-ras oncogene transformed tumors in nude mice by inhibiting the membrane association of tumoral p21H-ras.