Inhibition of colon carcinogenesis by post-initiation induction of NQO1 in Sprague-Dawley rats.

Inducers of phase II detoxifying enzymes have been studied as chemopreventive agents for a variety of cancers. Phase II detoxifying enzymes may play a significant role in preventing carcinogen-induced colon cancer at the initiation and post-initiation stage, but the contribution of NAD(P) H:quinone oxidoreductase 1 (NQO1) to this effect remains unclear. Using the carcinogen-induced colon cancer Sprague-Dawley rat model, we previously showed that oltipraz selectively induces NQO1 in the colons of these rats without inducing other phase II detoxifying enzymes. We demonstrated that selective induction of NQO1 in the rat colon prior to treatment with a carcinogen significantly inhibited the formation of aberrant crypt foci (ACF). Using the same rat model, we found that rats fed oltipraz containing diet following treatment with the colon carcinogen, azoxymethane (AOM), had 60% fewer ACF after 12 weeks compared with rats fed a control diet. In addition, rats fed oltipraz containing diet after AOM treatment developed 40% fewer colon adenomas and fewer colon tumors than rats fed a control diet. There was also a 60% increase in the percentage of apoptotic cells in ACF from oltipraz fed rats compared with ACF from control fed rats. Together, these results suggest that NQO1 can contribute to inhibition of colon carcinogenesis at the post-initiation stage. A possible mechanism for this effect may be that induction of NQO1 increases apoptosis in carcinogen initiated colonic epithelial cells that prevents these cells from progressing to a neoplastic state. Thus, NQO1 may be an important target for chemoprevention of colon cancer.

Induction of NAD(P)H quinone: oxidoreductase1 inhibits carcinogen-induced aberrant crypt foci in colons of Sprague-Dawley rats.

Phase II detoxifying enzymes like NAD(P)H (quinone acceptor)oxidoreductase1 (NQO1), glutathione S-transferases (GST), and UDP-glucuronyltransferases (UGT) may play an important role in preventing carcinogen-induced cancers. Inducers of these enzymes have been shown to inhibit carcinogen-induced colon tumors in rat and mouse models. However, it has not been clearly demonstrated that NQO1 contributes to this effect. We examined the effect of NQO1 inducers on colon carcinogenesis using an aberrant crypt foci (ACF) rat model. Sprague-Dawley rats were fed control diet or diet containing 400 ppm dimethyl fumarate or 200 ppm oltipraz for 7 days, and Phase II enzymes in rat colon and liver were measured. Dimethyl fumarate significantly increased NQO1 and GST activities in colon and liver but did not increase UGT activities in these tissues. In contrast, oltipraz significantly increased NQO1 activities in colon and liver and produced a small increase in GST activity in the liver but did not increase GST activity in the colon or UGT activities in the liver or colon. Sprague Dawley rats were fed control diet or diet containing 200 ppm oltipraz and then treated with the carcinogens azoxymethane or methyl nitrosourea. Both carcinogens produced ACF in all of the rat colons, but rats fed oltipraz diet had significantly fewer ACF than those fed control diet. This protective effect was reversed in rats treated with the NQO1 inhibitor, dicoumarol. However, treatment with oltipraz did not alter the distribution of crypt multiplicities in the ACF. These studies demonstrated that induction of NQO1 plays a significant role in inhibiting initiation of carcinogen-induced ACF in Sprague-Dawley rats. This provides the first direct evidence that NQO1 may play a role in preventing colon cancer. The study also found that oltipraz added to the diet of Sprague-Dawley rats selectively increased NQO1 activity in colon mucosa with no increase in GST and UGT activities in these tissues. Thus, this model will be useful for further investigating the role of NQO1 in prevention of colon cancer.