Morris S. Zedeck

Methylation of intestinal and hepatic DNA in rats treated with methylazoxymethanol acetate.

Descending colon is the most sensitive segment of rat intestine and is at least as sensitive as liver to the carcinogenic effects of methylazoxymethanol acetate. To determine whether a relationship exists between tumor induction and level of DNA methylation, we measured the levels of 7‐methylguanine in the DNA isolated from duodenum, descending colon, and liver of rats treated with this carcinogen. Because radiolabeled methylazoxymethanol acetate is not available, we utilized high pressure liquid chromatography whereby methylated purines could be detected in amounts as little as 100‐300 pmoles. DNA isolated from liver of carcinogen‐treated rats had significant amounts of 7‐methylguanine. On the contrary, DNA isolated from descending colon of rats treated with methylazoxymethanol acetate had minimal amounts of 7‐methylguanine; these data suggest that the level of 7‐methylguanine does not correlate with sensitivity to tumor induction by methylazoxymethanol acetate.

Colon carcinogenesis and the role of dehydrogenase enzyme activity: Inhibition of tumorigenesis by pyrazole

Abstract Dimethylhydrazine and the products of its metabolism, azoxymethane and methylazoxymethanol, are potent inducers of colon tumors in experimental animals. The marked sensitivity of the colon to these substances, as compared with the other segments of intestine, appears to be related to the presence of dehydrogenase enzymes in colonic epithelium able to activate methylazoxymethanol. Results of several studies indicate that the organotropic effects of this carcinogen are related to NAD+/NADP+-dependent dehydrogenase enzyme activity and that the product of this reaction, the aldehydic form of methylazoxymethanol, is the agent responsible for the observed biologic effects. Rats treated with pyrazole, an inhibitor of NAD+-dependent alcohol dehydrogenase, prior to their receiving methylazoxymethanol, are protected against the acute toxicity induced by this agent and do not develop any intestinal tumors. In related studies aimed at determining the mechanism of resistance to colon tumor induction in Lobund Wistar rats, it was found that this phenomenon is not due to differences in metabolic activation of carcinogen.

Tumor induction in intact and regenerating liver of adult rats by a single treatment with methylazoxymethanol acetate.

Methylazoxymethanol (MAM) acetate was given once i.p. to intact adult rats and to rats at 24 h after partial hepatectomy. In each group, tumors developed as early as 10 months. The incidence of neoplastic nodules and of hepatocellular carcinomas in the carcinogen-treated intact adult rats was approximately 70% and, in the rats treated after partial hepatectomy, it was approximately 80%. The data suggest that both dividing and resting liver cells are sensitive to the tumor-initiating effects of methylazoxymethanol acetate.

The effect of selenium on cell proliferation in liver and colon

Epidemiologic and experimental evidence support a chemoprotective role for selenium (Se) in malignancy. Many mechanisms have been proposed to explain this phenomenon. In this study, the effect of Se intake on proliferation of hepatocytes and normal colonic epithelial cells in rats was determined using autoradiographic analysis of thymidine incorporation into DNA. Hepatocyte proliferation was measured 24 h after partial hepatectomy. Selenium-dosed animals demonstrated a significant reduction in hepatocyte labeling compared to the control group (6.1±2.6 vs 29.2±15.6,p=0.003). However, Se dosing did not affect the thymidine-labeling indices or distribution of labeling in colonic epithelium. Selenium may inhibit cell proliferation when it is the result of an unusually intense stimulus. This finding could explain in part the inhibitory effect of Se in some experimental cancer models.

Experimental Colon Carcinogenesis

Probably the most significant contribution in recent years toward the understanding of colon carcinogenesis has been the introduction of several compounds having marked propensity for induction of colon tumors in laboratory animals. Single or relatively few doses of MAM,* DMH, AOM, MNNG, or MNU result in the induction of large numbers of colon tumors in most, if not all, of the treated animals within a relatively short time. The intestinal distribution and the histological characteristics of the tumors are remarkably similar to what is found in humans. Animal model systems have been developed with the use of these agents and are invaluable for studying the mechanism(s) of initiation of tumor development as well as, in the case of those agents effective after a single treatment, the sequential changes from the moment of initiation until the formation of a tumor. In addition, epidemiological studies of various populations of the world have led to the suggestion that dietary fat, intestinal flora, and bile acids may influence the induction of colon tumors. The modifying effects of these factors in the formation of colon tumors can now also be evaluated in reliable animal model systems. The basic aims of this chapter are to present data of studies directed at understanding why colonic epithelium is especially sensitive to induction of tumors by these compounds and to present and consider the evidence obtained thus far of the role of dietary factors, intestinal flora, and bile acids in the induction of colon cancer. Other experimental systems that may prove useful in understanding colon carcinogenesis are also discussed.

Summation and Future Challenges

The foregoing chapters have described a variety of studies on the prevention of tumor induction in experimental animals using a wide spectrum of naturally occurring and synthetic compounds. These substances act in different ways to inhibit the actions of carcinogenic agents in many organ systems. Studies of immunoprevention have also been described.