Lipid peroxidation in liver and colon of methylazoxymethanol treated rats.

Weanling male Sprague-Dawley rats were injected via the tail vein with methylazoxymethanol (MAM) acetate at a dose of 70 mg/kg body weight. Measurements of lipid peroxidation were carried out on mitochondrial and microsomal fractions of liver and colonic mucosa at various intervals over the first 24 h following delivery of the carcinogen. Significantly increased levels of peroxidation were observed 3-6 h after treatment in microsomal and mitochondrial fractions of both these tissues. A return to control levels was seen by the end of the first day. These results are discussed in relation to the role of lipid peroxidation in carcinogenesis and the proposed mechanism of tumor prevention by selenium.

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.

Selective distribution of selenium in colon parallels its antitumor activity.

Results of epidemiologic and experimental studies suggest that selenium can inhibit the development of tumors. In rats, the administration of selenium decreases the incidence of carcinogen-induced colon tumors; the inhibition is greater in the proximal colon that in the distal colon. We investigated the distribution of selenium in the different segments of rat colon and determined the uptake of selenium in the mucosa and in the muscle layers of each segment. The colon was perfused before removal of the segments to ensure complete removal of blood-borne selenium. We found that the concentration of selenium was greater in the proximal colon than in the distal colon and that within each segment the uptake was higher in mucosa than in muscle. In addition, we determined the level of selenium in blood, serum, and liver at different times after the administration of various doses of selenium. Though the mechanism by which selenium prevents tumor development is unknown, the data indicate a correlation between the uptake of selenium in different segments of colon and inhibition of tumorigenesis.

Selenium and the acute effects of the carcinogens, 2-acetylaminofluorene and methylazoxymethanol acetate.

Selenium inhibits the development of 2-acetylaminofluorene-induced hepatic tumors and methylazoxymethanol-induced colon tumors. It has been suggested that selenium exerts these protective effects by inhibiting metabolic activation of the carcinogen. We have studied the effects of selenium upon the acute inhibition of RNA and DNA synthesis induced by 2-acetylaminofluorene or methylazoxymethanol in intact liver, regenerating liver, and colon of weanling male Sprague-Dawley rats. Some animals received selenium in the drinking water (4 ppm) for 1 week, while others received a single injection of selenium (1 mg/kg i.p.) prior to being treated with the carcinogens. No protection against the effects of the carcinogens on RNA or DNA synthesis was noted with either treatment of selenium. Disulfiram did protect against the 2-acetylaminofluorene-induced inhibition of hepatic RNA synthesis, and pyrazole prevented the inhibition of RNA synthesis induced by methylazoxymethanol in both liver and colon. Serum selenium levels are reported. The data indicate that selenium does not influence the acute alterations induced by the carcinogens 2-acetylaminofluorene or methylazoxymethanol and suggest that the tumor-preventive effects of selenium are probably due to a mechanism other than interference with carcinogen activation and interaction with cellular macromolecules.

Inhibition of methylazoxymethanol-induced intestinal tumors in the rat by pyrazole with paradoxical effects on skin and kidney.

Methylazoxymethanol is a potent carcinogen and induces tumors predominantly of the small intestine and colon following a single injection. Previous data indicated that alcohol dehydrogenase could convert this carcinogen to a reactive alkylating agent. Rats were treated with an inhibitor of this enzyme, pyrazole, 2 hr prior to their receiving the carcinogen. The development of intestinal and colonic tumors was prevented. The rats did, however, develop numerous tumors of the skin and kidney. Analyses of the complete autopsies are presented. The data indicate that intestinal and colonic alcohol dehydrogenase plays a role in the tumorigenic effects of methylazoxymethanol and that other non-pyrazole-sensitive enzymes exist in other organs that can also activate this carcinogen.

Histochemical findings suggesting that methylazoxymethanol, a liver and kidney carcinogen, is a substrate for hepatic and renal choline dehydrogenase.

Methylazoxymethanol (MAM) is a potent carcinogen and induces tumors predominantly in rat liver, colon and kidney. The findings reported in this paper suggest that MAM is a substrate for the enzyme choline dehydrogenase located in rat hepatocytes and in the terminal portion of the renal proximal convoluted tubule. As with the natural substrate choline, this reaction with MAM did not require NAD+, was not inhibited by pyrazole and was dependent on the electron transfer reagent, phenazine methosulfate. The product of this reaction is probably the same as that obtained from the metabolism of MAM by alcohol dehydrogenase, namely, an unstable aldehydic derivative which decomposes rapidly to carbonium ions. The reaction with alcohol dehydrogenase offered an explanation for the organotropic effects of this carcinogen in liver and colon and the current report provides a mechanism for the induction of kidney tumors as well as another possible means for production of liver tumors.

Mechanism of inhibition of hepatic protein synthesis in rats by the carcinogen, methylazoxymethanol acetate.

Treatment of rats with the carcinogen, methylazoxymethanol acetate, results in a rapid, marked inhibition of hepatic protein synthesis and disaggregation of polysomes. Studies were undertaken to learn the mechanism by which this carcinogen induces these effects in rat liver. The data show that the inhibition of endogenous protein synthesis is not due to an effect on the high speed supernatant 'factors' but rather at the level of the polysome, and that both free and membrane-bound polysomes are affected. Poly(U)-directed polyphenylalanine synthesis by native ribosomal subunits is greater in preparations isolated from rats treated with carcinogen than it is in controls. Moreover, the native ribosomal subunit fraction from treated livers in response to added rabbit globin mRNA is able to synthesize a protein similar in molecular weight to globin. These studies show that methylazoxymethanol acetate does not induce significant alterations of ribosomal subunits or of initiation factors and suggest that the inhibition of protein synthesis and disaggregation of polysomes may be the results of an alteration of cytoplasmic mRNA, or its association with ribosomes.

Effects of adriamycin on DNA synthesis in mouse and rat heart.

Adriamycin induces an inhibition of DNA synthesis in mouse tissues within one hr after treatment. While the effects are short-lived in liver and small intestine, DNA synthesis in heart remains below control values for up to 7 days. After this period DNA synthesis in hearts of treated mice is elevated and remains above control values for as long as 4 weeks. Both 1-beta-D-arabinofuranosylcytosine and actinomycin D also induce inhibition of cardiac DNA synthesis soon after treatment; the effects of 1-beta-D-arabinofuranosylcytosine are over by the end of 24 hr while the effects of actinomycin D persist for at least 4 days. Actinomycin D treatment also induces an "over-shoot" of DNA synthesis in mouse heart. Adriamycin can induce a loss of prelabeled DNA from heart, although no pathological alterations are immediately obvious. The small intestine, however, shows extensive karyorrhexis. The initial effects on cardiac DNA synthesis occur in adrenalectomized animals, indicating that the effects are not mediated via the adrenal gland. We did find, however, that DNA synthesis in heart was sensitive to the effects of starvation. The results of this study indicate that inhibition of mouse heart DNA synthesis is not specific for Adriamycin and that the effects of Adriamycin in heart following a single treatment are long-lived.

Induction of colon tumors in 1,2-dimethylhydrazine-resistant Lobund Wistar rats by methylazoxymethanol acetate.

Sprague-Dawley and Lobund Wistar rats, which were sensitive and resistant to induction of colon tumors by 1,2-dimethylhydrazine (DMH), respectively, were treated with methylazoxymethanol (MAM), the product of DMH metabolism by the microsomal mixed-function oxidase system. Although the colon tissue in both stocks of rats had similar NAD+-dependent dehydrogenase activities that are considered necessary to activate MAM to an ultimate carcinogen, still a sevenfold greater incidence of colon tumors was found in the Sprague-Dawley rats, and their tumors were more extensive. The results indicated that the difference in susceptibility to colon tumor induction between the rat stocks was partially related to metabolic activation of the DMH and to other, as yet undetermined, endogenous factors.

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.

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.

Differences in the acute response of the various segments ofrat intestine to treatment with the intestinal carcinogen, methylazoxymethanol acetate.

Previous work has shown that single injections of methylazoxymethanol acetate in rats induce tumors predominantly in the colon, occasionally in the duodenum, and rarely in the jejunum and ileum. These studies describe the acute pathological and biochemical, alterations induced by this carcinogen in the different segments of rat small intestine and colon. Karyorrhexis was found in crypts of duodenum, cecum, and all segments of colon at 6 hr after treatment. Much of the cellular debris was removed by 24 hr, although mitoses did not return to normal levels until the third day after treatment. No pathological alterations were found in jejunum or ileum, even as late as 24 hr after treatment. Studies of DNA synthesis at 24 hr after treatment indicated that jejunum and ileum were much less affected than were duodenum, cecum, or colon. In contrast, 5-fluorouracil and nitrogen mustard, agents that can inhibit proliferating cells but are not known to be intestinal carcinogens, affected all of the segments equally. The results indicate that a correlation exists between those segments of intestine acutely affected by methylazoxymethanol acetate and the sites of eventual tumor development. The level of deacetylase activity in the various intestinal segments did not correlate with sensitivity to methylazoxymethanol acetate-induced inhibition of DNA synthesis. We also found that methylazoxy-methanol acetate inhibited DNA synthesis in the duodenum and colon in rats with cannulated bile ducts. These data indicate that the carcinogen does not require biliary transport to the intestinal lumen to exert its biological effects. Mechanisms that might account for the observed selectivity in action of methylazoxymethanol acetate in the various rat intestinal segments are discussed.

Effect of methylazoxymethanol acetate on adenylate cyclase and 5'-nucleotidase in rat liver plasma membranes.

A single tumorigenic dose of methylazoxymethanol acetate increased adenylate cyclase activity in total liver homogenates 70-100% by the seventh day after treatment. The increased activity occurred in the plasma membranes rather than in the nuclei and was accompanied by a significant increase in 5'-nucleotidase activity. The data indicate that the carcinogen may alter the structure of the plasma membrane.