The production of mammary carcinomas in rats by 9,10-dimethyl-1,2-benzanthracene and its relationship to the oestrous cycle

Estrogen synthesis in the stomach of Sprague-Dawley rats: comparison to Wistar rats

Aromatase, an estrogen synthase, exists in the gastric parietal cells of Wistar rats. The stomach synthesizes large amounts of estrogens and secretes them into the portal vein. We have been particularly studying gastric estrogen synthesis using Wistar rats. However, estrogen synthesis in the stomach of Sprague-Dawley (SD) rats, which are used as frequently as those of the Wistar strain, has not been clarified. We examined steroid synthesis in the stomach of SD rats using immunohistochemistry, in situ hybridization, Western blotting, real-time PCR, and LC-MS/MS. Aromatase also exists in the stomach of SD rats. Its distribution was not found to be different from that of Wistar rats. Results show that H+/K+-ATPase β-subunit and aromatase colocalized in double immunofluorescence staining. Each steroid synthase downstream from progesterone was present in the gastric mucosa. These results suggest that steroid hormones are synthesized in the parietal cells in the same pathway as Wistar rats. Although mRNA expression of steroid synthases were higher in SD, no significant difference was found in the amount of protein and each steroid hormone level in the portal vein. Although differences between strains might exist in steroid hormone synthesis, results show that SD rats are as useful as Wistar rats for gastric estrogen synthesis experimentation.

Inhibition of human breast cancer cell proliferation and delay of mammary tumorigenesis by flavonoids and citrus juices

Two citrus flavonoids, hesperetin and naringenin, found in oranges and grapefruit, respectively, and four noncitrus flavonoids, baicalein, galangin, genistein, and quercetin, were tested singly and in one-to-one combinations for their effects on proliferation and growth of a human breast carcinoma cell line, MDA-MB-435. The concentration at which cell proliferation was inhibited by 50% (IC50), based on incorporation of [3H]thymidine, varied from 5.9 to 140 micrograms/ml for the single flavonoids, with the most potent being baicalein. IC50 values for the one-to-one combinations ranged from 4.7 micrograms/ml (quercetin + hesperetin, quercetin + naringenin) to 22.5 micrograms/ml (naringenin + hesperetin). All the flavonoids showed low cytotoxicity (> 500 micrograms/ml for 50% cell death). Naringenin is present in grapefruit mainly as its glycosylated form, naringin. These compounds, as well as grapefruit and orange juice concentrates, were tested for their ability to inhibit development of mammary tumors induced by 7,12-dimethylbenz[a]anthracene (DMBA) in female Sprague-Dawley rats. Two experiments were conducted in which groups of 21 rats were fed a semipurified diet containing 5% corn oil and were given a 5-mg dose of DMBA intragastrically at approximately 50 days of age while in diestrus. One week later, individual groups were given double-strength grapefruit juice or orange juice or fed naringin or naringenin at levels comparable to that provided by the grapefruit juice; in the second experiment, the rats were fed a semipurified diet containing 20% corn oil at that time. As expected, rats fed the high-fat diet developed more tumors than rats fed the low-fat diet, but in both experiments tumor development was delayed in the groups given orange juice or fed the naringin-supplemented diet compared with the other three groups. Although tumor incidence and tumor burden (grams of tumor/rat) were somewhat variable in the different groups, rats given orange juice had a smaller tumor burden than controls, although they grew better than any of the other groups. These experiments provide evidence of anticancer properties of orange juice and indicate that citrus flavonoids are effective inhibitors of human breast cancer cell proliferation in vitro, especially when paired with quercetin, which is widely distributed in other foods.

Dietary fat and breast cancer

High-fat diets are thought to increase the risk of breast cancer because animals develop mammary cancer more readily when they are fed high-fat compared to low-fat diets, and breast cancer incidence and mortality are higher in countries with high-fat as compared to those with low-fat diets. Prospective cohort studies and case-control studies have failed to provide much support for this theory, but such studies are less capable of showing the relationship because of smaller differences in dietary fat intakes of the study populations; difficulties in assessing the diets of individuals over a period of time; and possible differences in genetic susceptibility of cases and controls to breast cancer. Studies on migrants have shown that breast cancer incidence and mortality increase in populations who move from countries with low-fat to those with high-fat diets, indicating that observed geographical differences in breast cancer are due to environmental rather than genetic factors. This is supported by time-trend studies showing that breast cancer increases in countries as the level of fat in the diet rises. Controlled, long-term dietary trials are needed to determine whether the converse is true: namely, that reduction of dietary fat can reduce the risk of breast cancer. Large groups are required to achieve statistical significance, but smaller numbers may be adequate for studies on high-risk individuals. Preliminary experiments already have demonstrated the feasibility of carrying out such dietary trials.

Effects of a long-chain fatty amine on mammary carcinogenesis induced in female Sprague-Dawley rats by DMBA

Reports that protein kinase C is inhibited by sphingosine and other long-chain amines and the suggestion that promotion of mammary carcinogenesis by dietary fat is mediated by protein kinase C prompted us to investigate the effects of a long-chain amine, 1-octadecylamine, on mammary carcinogenesis induced by 7,12-dimethylbenz[a]anthracene in rats fed a high-fat diet. Rats fed the amine sulfate at a level of 0.01% in a semipurified diet containing 20% corn oil developed more tumors than those fed the high-fat diet alone, although body weight gain was inhibited slightly. Rats fed the amine sulfate at 0.1% of the diet developed very few tumors compared with those fed either the high-fat diet or a low-fat diet containing 5% corn oil. At the higher level, the C18 amine also caused a marked inhibition of body weight gain.

Estrous cycle status alters N-methyl-N-nitrosourea (NMU)-induced rat mammary tumor growth and regression

The relationship between mammary carcinoma growth, ovariectomy-induced regression and estrogen receptor status were determined in Sprague-Dawley rats with 5-day estrous cycles after injection of N-methyl-N-nitrosourea (NMU) on metestrus (ME), diestrus-1 (DE-1), proestrus (PE) or estrus (E). Rats exposed to NMU on PE had a shorter tumor latency than those injected on ME and E, as well as more carcinomas per rat than those exposed on ME and DE-1. Mammary carcinomas grew faster in rats injected on ME (doubling time, 6.4 days) and DE-1 (6.9 days) compared with PE (15.2 days) and E (16.3 days). Tumor regression was also significantly faster in rats injected on ME (time to 50% vol., 5.5 days) and DE-1 (5.3 days) compared with PE (8.2 days) and E (8.5 days) following bilateral-ovariectomy during log phase growth. Significantly, total nuclear estrogen receptor (ERN) content was increased in carcinomas from rats injected on PE compared with DE-1 (70.8 +/- 11.3 vs. 32.9 +/- 7.3 fm/mg DNA) (P less than 0.05) and DE-1 and ME combined (P less than 0.01). These observations generalize the concept that estrous cycle stage at the time of NMU injection alters subsequent mammary carcinoma biology, and represents the first experimental evidence that slower growing and responding estrogen receptor positive rat mammary carcinomas may be associated with an increase in circulating estrogen prior to carcinogen exposure.