Some aspects of the basic mechanisms of chemical carcinogenesis

Suppression of hepatic prostaglandin F2 alpha in rats by dietary alpha-tocopherol acetate is independent of total hepatic alpha-tocopherol

Groups of eight weanling female F344/N rats were fed semipurified diets that supplied 0, 50, 500, 5000, or 15,000 mg alpha-tocopherol acetate/kg diet, with and without 0.05% phenobarbital (PB) for 9 weeks. Both plasma and hepatic alpha-tocopherol levels, measured by HPLC, strongly correlated with alpha-tocopherol intake (r greater than 0.73, p less than 0.0001). Phenobarbital both depleted hepatic alpha-tocopherol and increased plasma alpha-tocopherol significantly. Although treatment with PB for 9 weeks significantly increased GST activity, PB did not affect hepatic prostaglandin (PG)F2 alpha status, as determined by radioimmunoassay. PGF2 alpha was significantly greater (by 52%) in rats fed no alpha-tocopherol than in rats fed 15,000 mg alpha-tocopherol acetate/kg diet. Hepatic PGF2 alpha status was correlated inversely but weakly with dietary alpha-tocopherol (r = -0.24, p less than 0.05). Hepatic PGF2 alpha status was not correlated with hepatic or plasma alpha-tocopherol status. This finding suggests either that there is a small depletion-resistant subcellular alpha-tocopherol pool which regulates PGF2 alpha production or that alpha-tocopherol alters PGF2 alpha production in vivo by an indirect mechanism.

The effects of dietary brussels sprouts and Schizandra chinensis on the xenobiotic-metabolizing enzymes of the rat small intestine

After an initial equilibration period of 7 days on a semi-synthetic basal diet, male Sprague-Dawley rats were fed for 2 wk on either the basal diet (controls), the basal diet containing 5% Schizandra chinensis or 25% Brussels sprouts, or on rat chow. One group of chow-fed rats was pretreated with 20 mg 3-methylcholanthrene (3-MC)/kg body weight, 24 hr before they were killed. Microsomal and cytosolic fractions were prepared from small intestine mucosa. Microsomes were assayed for cytochrome P-450, aryl hydrocarbon hydroxylase (AHH), ethoxycoumarin O-deethylase (ECD) and epoxide hydrolase (EH) activities, and for metabolism of benzo[a]pyrene (BaP), Cytosols were assayed for glutathione S-transferase (GST) activity. The largest increase in intestinal mixed-function oxidase activity over levels in the controls was seen in the 3-MC-treated group. However, EH and GST activities in these animals were not significantly increased. Increases in cytochrome P-450 levels and significant increases in AHH, ECD, EH and GST activities occurred in the rats fed Brussels sprouts. Rats in the S. chinensis group showed inhibition of AHH activity relative to controls, but increased activity of ECD, EH and GST. In the rats fed chow there were significant increases in the activities of all the enzymes assayed except GST. The percentage conversion of BaP to metabolites reflected the results of the AHH assay and the groups were ranked in the following order: 3-MC greater than Brussels sprouts greater than rat chow greater than basal diet greater than S. chinensis. The profile of BaP metabolites showed a larger proportion of the BaP diols and 3-hydroxybenzo[a]pyrene, and a smaller proportion of BaP-4,5-epoxide and the BaP quinones, for the Brussels sprouts- and S. chinensis-fed groups. The significance of these results is discussed in regard to the role of the small intestine as a mediator of toxicity induced by ingested chemicals.

The relationship between transformation and somatic mutation in human and Chinese hamster cells

The frequencies of transformations of primary human and Chinese hamster fibroblasts have been compared with the spontaneous and induced frequencies of mutation for resistance to thioguanine and ouabain, and for ability to use fructose, using the carcinogens benzo (alpha) pyrene and urethane. Whereas the rates and frequencies of mutation were similar in the two cell systems, transformations to morphologically altered cells was observed only in hamster cells. The frequency of this latter transformation event in hamster cells was abour 10(3) greater than the frequencies of mutation in these cells. The morphologically altered cells formed in the above transformation process cannot grow in agar (aga-) and do not produce tumors when injected into animals. The frequency of transition of these latter cells to aga+ cells which produce tumors in animals is similar to the mutation-like events.

Mutational approaches to the study of carcinogenesis

A number of circumstantial lines of evidence are consistent with the somatic mutation theory of carcinogenesis, but there has been a paucity of experimental data that either support or contradict the genetic theory. In this paper, we summarize the predictions, the recent experimental approaches, and the problems involved in testing the theory. Results are presented that define the conditions and demonstrate the existence of two-stage processes of mutagenesis and carcinogenesis in vitro. We conclude that mutagenesis is responsible for the initiation of carcinogenesis and an epigenetic mechanism is responsible for its promotion. Carcinogenic agents can induce a stable transformation of a cell by either mutation or epigenetic alteration in gene expression. This conclusion has led us to propose a new integrative theory of carcinogenesis, encompassing the tenets of four main theories: (1) the mutation and epigenetic basis for carcinogenesis, (2) the two-stage theory of carcinogenesis, (3) a general theory of carcinogenesis, and (4) the somatic deletion mutation theory of carcinogenesis.