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Se-methylselenocysteine: a new compound for chemoprevention of breast cancer.

Selenium compounds have attracted renewed interest as chemopreventive agents for human cancer on the basis of the pioneering intervention study by Clark and co-workers. The rodent mammary gland has been used extensively as a model for examining the chemopreventive activities of inorganic and organic selenium compounds. This review summarizes the rationale and results for use of a new organic selenium compound, Se-methylselenocysteine, which exhibits greater efficacy as a chemopreventive agent than several previously used selenium compounds in experimental models of breast cancer and has potential for use in human populations.

Dietary selenium intake and growth of the MT-W9B transplantable rat mammary tumor.

The present study reports the effect of dietary selenium deficiency and supplementation on the growth of the transplantable MT-W9B mammary tumor in female Wistar-Furth rats. Supplementation of the diet with 2 ppm of selenium inhibited tumor growth and reduced the final tumor weight by approximately 50% compared to the control rats receiving 0.1 ppm of selenium. The inhibitory response was selective, without inducing any weight loss in the animals. On the other hand, selenium deficiency (less than 0.02 ppm) had no influence on the growth of this tumor.

Feasibility of using lower doses of chemopreventive agents in a combination regimen for cancer protection

In experimental cancer chemoprevention studies, the doses of the agent used to produce an inhibitory response are generally very high, sometimes bordering on levels that will result in toxicity to the test animals. The present study was designed to test the hypothesis that low doses of two or more agents may be just as effective as high doses of a single agent. An earlier report from our laboratory showed that vitamin E, although ineffective by itself, potentiates the anti-carcinogenic potency of selenite. Our objective was to complete a comprehensive set of dose titration experiments in the quantitative analysis of the synergism between selenium (Se) and vitamin E. Results indicated that the minimal amount of vitamin E required was around 500 ppm, which when combined with as little as 1 ppm Se, produced a significant protective effect in the 7,12-dimethylbenz[a]anthracene (DMBA)-induced rat mammary tumor model. This level of Se is 5 times below the marginal toxicity level of 5 ppm observed in our previous experience. With a 3-agent combination protocol involving Se (1 ppm), vitamin E (500 ppm) and vitamin A (66 ppm), it was found that vitamin A at this particular dose did not contribute any further chemoprevention than that provided by the Se/vitamin E duo. Both Se and vitamin E were present at a concentration 10 times, while vitamin A was present at 30 times their respective nutritional requirement. Our findings thus suggest that the minimum effective dose has to be systematically established for each micronutrient, and that it is feasible to use lower doses of a combination of agents if the threshold level of each agent can be determined under a prescribed set of conditions.

Comparative effects of antioxidants on enzymes involved in glutathione metabolism.

The present study was designed to examine changes in glutathione metabolism in the liver of mice as influenced by supplementation of their diet with 1 of 4 antioxidants: butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), vitamin E and selenium. In addition to determination of the acid-soluble thiol levels, 5 different enzymes involved with glutathione utilization and synthesis were measured: glutathione transferase, gamma-glutamyl transpeptidase, selenium-dependent glutathione peroxidase, gamma-glutamylcysteine synthetase and glutathione reductase. All 4 antioxidants produced significant increases in glutathione transferase activity, with BHA and BHT being much more effective than the other two. With the exception of vitamin E, BHA, BHT and selenium all resulted in a slight enhancement in the activity of glutathione reductase as well as in the acid-soluble thiol level. On the other hand, the induction of gamma-glutamyl transpeptidase and gamma-glutamylcysteine synthetase was responsive to only vitamin E and selenium supplementation, respectively. Although the influence of each of these antioxidants in glutathione metabolism appears to be specific and somewhat compartmentalized, the overall impression is that of an increased capacity for glutathione-conjugate formation and recovery of reduced glutathione. These biochemical changes in glutathione metabolism may be relevant to the anticarcinogenic effects observed with BHA, BHT and selenium.

Neoplastic growth of carcinogen-treated mammary transplants as influenced by fat intake of donor and host.

Female Wistar-Furth rats were fed either a high-fat (HF) or a low fat (LF) diet from weaning. The HF and LF diets contained 20% and 0.5% corn oil, respectively. Mammary gland explants from 50-day-old rats of both dietary groups were exposed to dimethylbenz[a]anthracene in organ culture before grafting to HF- or LF-hosts. A total of 4 groups were involved: LF leads to LF, HF leads to LF, LF leads to HF, and HF leads to HF, where the designations before and after the arrow describe the dietary treatment of the donor and host, respectively. Final tumor incidences were as follows: 28%, 20%, 72% and 76%, presented in the order of the above 4 groups. Tumors also appeared earlier in the HF-hosts. Results of this experiment indicate that regardless of the nutritional status of the donor, it was the fat intake of the host that governed the subsequent neoplastic growth of transformed cells, suggesting that the action of fat is primarily exerted at the promotional stage of carcinogenesis.

Inhibition of mammary tumorigenesis by a reduction of fat intake after carcinogen treatment in young versus adult rats

The present study demonstrates that a reduction of fat intake after dimethylbenz[alpha]anthracene (DMBA) administration to female Sprague--Dawley rats leads to an inhibition of mammary tumorigenesis. Animals were fed a 20% fat diet from weaning and were transferred to a 0.5% fat diet 0, 2, 4, and 6 weeks after carcinogen treatment. In rats given DMBA at 50 days of age, the following observations were obtained: (a) tumor incidence, as well as tumor yield, was decreased when the transfer to a low fat diet was initiated up to 4 weeks after DMBA; (b) regardless of fat intake, over 90% of tumors developed in all dietary groups were adenocarcinomas. This was in contrast to rats given DMBA at 150 days of age. In this case (a) a 50% reduction in tumor incidence was apparent when the low fat diet was introduced even 6 weeks after DMBA intubation; and (b) more benign lesions were found and an association between a reduced risk of carcinogenesis and a lower ratio of adenocarcinoma to fibroadenoma seems to exist. Thus, the data demonstrate that the rats were less vulnerable to delays in reduction of fat intake on subsequent inhibition of mammary tumorigenesis than if they were exposed to the carcinogen at an older age.

Isomeric fatty acids and tumorigenesis: A commentary on recent work

This article critically reviews the existing, although limited, literature concerning trans fatty acids and tumorigenesis. Neither epidemiological nor experimental studies published to date have demonstrated any valid association between trans fatty acid ingestion and tumorigenesis. A recent study showed that under controlled conditions, a fat with a high content of trans fatty acids did not promote the development of mammary tumors induced in rats by 7,12-dimethylbenz[a]anthracene to any greater extent than did a comparable fat with a high content of cis fatty acids. In addition, in this study a high trans fat was less tumor promoting than was a blend of fats that simulated the dietary fat composition of the United States and had a lower level of trans fatty acids. Another study using comparable cis and trans fats demonstrated that the high trans fat did not affect the growth and metastasis of implanted mammary tumors in mice relative to the high cis fat. Also, two recent studies reported no significant difference in the development of induced colon tumors in rats fed diets high in cis or trans fatty acids. The results of these and other studies are consistent with the conclusion that trans fatty acids are not uniquely related to tumor development.

Selenium-mediated inhibition of mammary carcinogenesis

Using the dimethylbenz(a)anthracene-induced mammary tumor model in rats, our studies indicated that there was a dose-response relationship between dietary selenium supplementation and the inhibition of mammary carcinogenesis. The degree of inhibition was proportional to the level of dietary selenium up to 5 ppm, at which point toxicity in the form of a reduction in weight gain was evident. Moreover, it was observed that the chemopreventive efficacy of selenium was influenced by the dose of carcinogen as well as the fat intake of the animals. By supplementing selenium for defined periods of time, we concluded that selenium inhibited both the initiation and the promotion phases of chemical carcinogenesis, and that a continuous intake of selenium was necessary to achieve maximal suppression of tumor growth. In an attempt to improve the efficacy of lower levels of selenium, we conducted another series of experiments in which selenium and vitamin E were tested in combination. Results showed that although vitamin E alone had no prophylactic effect against tumorigenesis, it potentiated the ability of selenium to inhibit the development of mammary tumors. Further investigation suggested that the anticarcinogenic action of selenium could not be explained by its antioxidant function in lipid peroxidation. On the other hand, vitamin E might be able to provide a more favorable climate against oxidant stress to facilitate selenium in exerting its inhibitory effect through some other mechanisms.

Levels and 75Se-Labeling of Specific Proteins as a Consequence of Dietary Selenium Concentration in Mice and Rats

Abstract Selenium-labeled proteins (SLP) distinct from glutathione peroxidase (GSH-PX) recently have been purified and partially characterized. Antisera to two SLP, a 56-kDa and a 14-kDa protein, were generated in rabbits and used to examine expression of these proteins as a consequence of dietary selenium concentration (0.02, 0.2, 2.0 ppm) in mice and rats. Additionally, the kinetics of 75Se labeling in plasma, liver, kidney, and mammary gland were examined over a 40-hr time period as a function of dietary selenium concentration. A plasma 57-kDa protein was labeled by 30 min after 75Se injection and reached maximum labeling by 4 hr. The cellular 56-kDa and 14-kDa proteins, as well as GSH-Px, labeled progressively over 40 hr starting between 1 and 4 hr after injection. In general, the 56-kDa and GSH-Px followed similar labeling patterns, whereas the 14-kDa protein was labeled less and was not labeled in discernible quantities until 40 hr. The extent of labeling of all proteins was inversely proportional to the dietary selenium concentration and was probably a reflection of different endogenous selenium body pools. The most important observation was generated by the immunoblot data. The amount of 56-kDa and 14-kDa proteins as detected and measured on immunoblots was not a function of dietary selenium concentration. This result suggests that the synthesis and maintenance of the 56-kDa and 14-kDa proteins are not selenium dependent, a characteristic which distinguishes the two proteins from GSH-Px. The single exception to the above results was the 40% decrease of liver 14-kDa protein concentration in carcinogen-treated rats fed 2.0 ppm of selenium. An organic selenium compound, selenobetaine, did not lead to a decrease under similar conditions. In 15 rat mammary tumors induced by 7,12-dimethylbenzanthracene and analyzed on immunoblots, the SLP-56 was undetected in 5 cases and appeared as two bands (56,000 Da, 50,000 Da) in 10 cases. This latter result raises the possibility that the expression of SLP-56 may be altered in mammary tumors as compared with normal mammary gland.

BCG-modulated mammary carcinogenesis is dependent on the schedule of immunization but is not affected by dietary fat

The present study was designed to study the effect of dietary fat intake on the modulation of dimethylbenz[a]anthracene (DMBA)-induced mammary carcinogenesis in rats injected with the methanol extract residue of Bacillus Calmette-Guerin (MER-BCG). Rats were maintained on either a 5% or a 20% corn oil diet for the entire duration of the experiment. When MER-BCG was administered 2 and 3 weeks before DMBA, mammary tumorigenesis was suppressed in the 2 dietary groups with different levels of fat intake. This was in contrast to when MER-BCG was administered 3 and 5 weeks after DMBA; in this case the development of mammary tumors was noticeably enhanced regardless of the fat intake of the host. The magnitude of inhibition or increase by MER-BCG was similar in animals fed either fat level, although a high fat diet consistently stimulated mammary tumorigenesis in the 2 experiments. In vitro assays on T cell mitogen-induced blastogenesis and natural killer cell activity in splenocytes isolated from the untreated rats showed that dietary fat failed to elicit any differential response in these immune functions.