Low zinc intake suppressed N-methyl-N-nitrosourea-induced mammary tumorigenesis in Sprague-Dawley rats

Zinc has been shown to be accumulated in N-methyl-N-nitrosourea (MNU)-induced rat mammary tumors. Zinc is required for cell proliferation and tumorigenesis is characterized by dysregulation of cell proliferation. An accumulation of zinc in mammary tumors perhaps indicates a reliance on zinc to sustain tumor growth. Limiting zinc supply by means such as reduced zinc intake should negatively modulate mammary tumorigenesis. Our objective was to determine the effects of zinc status on MNU-induced mammary tumorigenesis in sexually mature female rats. Twenty-one-day-old Sprague-Dawley rats were assigned to low-zinc (3 mg zinc/kg diet) or adequate-zinc (12 mg zinc/kg diet) ad libitum or pair-fed control group (n = 25-33 rats/group). On day 50 of age, all rats were intraperitoneally injected with MNU (50 mg/kg body wt) to induce mammary tumorigenesis. Rats were further maintained on their assigned diet until 14 weeks post-MNU injection. Total food intake and overall body weight gain were lower in low-zinc rats than in adequate-zinc ad libitum control rats, but were similar to adequate-zinc pair-fed control rats. Plasma zinc concentration was lower in low-zinc rats than in adequate-zinc ad libitum and pair-fed control rats, confirming moderately low-zinc status in low-zinc rats. Tumor incidence (46 versus 84 and 80%; P < 0.05) and tumor multiplicity (0.8 versus 5.0 and 2.6 tumors/rat; P < 0.05) and tumor number (28 versus 123 and 66 tumors) were reduced in low-zinc rats compared with that in adequate-zinc ad libitum and pair-fed control rats, respectively. Tumor latency in low-zinc and adequate-zinc pair-fed control rats was not significantly different, but was longer than in adequate-zinc ad libitum control rats (P < 0.05), suggesting that reduced food intake associated with low-zinc intake prolonged tumor latency. Tumor burden and size were not affected by zinc intake. Overall, our observations showed that moderately low-zinc status suppressed MNU-induced rat mammary tumorigenesis.     

Dietary effects of conjugated docosahexaenoic acid on N-methyl-N-nitrosourea-induced mammary carcinogenesis in female Sprague-Dawley rats

The dietary effects of conjugated docosahexaenoic acid (CDHA) were examined in an N-methyl-N-nitrosourea (MNU)-induced rat mammary carcinogenesis model. Female Sprague-Dawley rats were administered 50 mg/kg MNU intraperitoneally at 49 days of age. A powdered AIN-76A diet containing 0, 0.2 or 1.0% CDHA was fed to the rats from 21 to 49 days of age (before MNU; pre-initiation phase) or from 49 days to 40 weeks of age (after MNU; post-initiation phase). Rats were sacrificed when their largest mammary tumor was > or =1 cm in size or when they reached 40 weeks of age. All histologically detected mammary carcinomas were evaluated. In rats that received CDHA after MNU, development of mammary carcinoma > or =1 cm was inhibited, and there was a significant decrease in the final mammary cancer incidence and multiplicity, compared with rats that did not receive CDHA. Consumption of the 0.2% CDHA diet after MNU significantly prolonged latency. Suppression of mammary cancer yield by consumption of a CDHA diet after MNU administration was not dose-dependent. In rats that received CDHA before MNU, suppression of mammary cancer was not observed. These results indicate that CDHA administration in the post-initiation period suppressed mammary carcinogenesis, whereas CDHA administration in the pre-initiation period was ineffective.     

Enhancement by chronic ethanol intake of N-methyl-N-nitrosourea-induced rat mammary tumorigenesis

A majority of epidemiological investigations report an associationbetween risk for breast cancer in women and alcohol consumption.However, evidence for an enhancing effect of ethanol on chemicallyinduced rat mammary tumorigenesis is limited and inconsistent.The present study was conducted to evaluate the influence oflow to high ethanol intakes (15, 20 or 30% of calories) as partof a defined, liquid diet on both the initiation and promotionstages of N-methyl-N-nitrosourea (MNU)-induced rat mammary tumorigenesis.Ethanol consumed by rats at 15% of calories during either theinitiation or promotion stages increased the number of mammaryadenocarcinomas compared to isocaloric controls. Ethanol intakeat 20% of calories only during the promotion stage resultedin an increase in the number of mammary adenocarcinomas comparedto controls. No enhancing effect of dietary ethanol at 30% ofcalories on either stage of tumorigenesis was observed in comparisonto isocaloric controls. Therefore, ethanol at specific intakescan enhance the initiation and promotion stages of MNU-inducedrat mammary tumorigenesis. There was not, however, a correspondingincrease in mammary tumor development at the highest intakeof ethanol evaluated. Possible reasons for this latter lackof effect of ethanol are discussed.     

Genistein suppresses mammary cancer in rats

Female Sprague-Dawley CD rats were injected s.c. with 5 mg genistein,a soy phytoestrogen, or 20 µl of the vehicle, dimethylsulfoxide(DMSO), on days 2, 4 and 6 postpartum. At day 50, they wereexposed to 80 µg dimethyl-benz[a]anthracene (DMBA)/g bodywt. Animals treated neonatally with genistein as compared toDMSO had increased latency and reduced incidence and multiplicityof DMBA-induced mammary adenocarcinomas. Mammary whole mountanalysis showed that 50 day old female rats treated neonatallywith genistein had fewer terminal end buds. Cell proliferationstudies revealed that 50 day old genistein-treated rats hadlower percentages and total numbers of cells in the S-phaseof the cell cycle in terminal end buds, terminal ducts, lobulesI and lobules II. In genistein-treated as compared to vehicle-treatedfemale rats, vaginal openings occurred earlier, the estrus cyclewas disrupted and the uterine-ovarian weights were smaller.In 50 day old genistein-treated females there were atretic antralfollicles, fewer corpora lutea, and lower circulating progesteronebut not estradiol-17ß concentrations. In 21 day oldrats treated neonatally with genistein, mammary glands werelarger and there were more terminal end buds and terminal ducts,and more proliferative activity in all terminal ductal structures.It appears that neonatal genistein-treatment exerted its chemopreventionaction by acting directly to enhance maturation of terminalductal structures and by altering the endocrine system to reducecell proliferation in the mammary gland.     

Dietary retinoids and carotenoids in rodent models of mammary tumorigenesis

In this review of the scientific literature the relationship between retinoids, carotenoids, and mammary carcinogenesis is examined. Several retinoids have shown promise as chemopreventive agents against chemically induced mammary carcinogenesis in mice and especially in rats. The most promising retinoids are retinyl acetate (RA) and N-(4-hydroxyphenyl)retinamide (4-HPR, fenretinide). In rats, dietary administration of these retinoids reduced tumor incidence and multiplicity, and increased the latency of DMBA or MNU-induced mammary cancers. In mice, 4-HPR reduced the number of hyperplastic alveolar nodules and the number of tumors in MTV- and MTV+ mice, respectively. Among retinoids, 4-HPR is at present the most promising analogue, due to its ability to concentrate in the mammary gland. The combination of 4-HPR with tamoxifen not only is more effective in suppressing breast cancer than either agent alone, but also inhibits the appearance of subsequent cancers following the surgical removal of the first tumor. These studies suggest that retinoids, like tamoxifen, may be applicable to the prevention of contralateral breast cancer in women who underwent breast cancer surgery. It is also becoming evident that differentiation therapy and chemoprevention can become attractive alternative approaches to intensive cytotoxic chemotherapy. The role of carotenoids in the prevention of mammary carcinogenesis, however, is ambiguous. Poor absorption and low levels of carotenoids that reach the target tissues complicate interpretation of data in rodent models of mammary carcinogenesis. Very few animal studies are presently available in which purified carotenoids were found effective against mammary carcinogenesis. These results do not justify undertaking clinical evaluation of individual carotenoids against breast cancer at this time.     

Metabolism of retinol and retinoic acid in N-methyl-N-nitrosourea-induced mammary carcinomas in rats

This study was conducted to examine the in vivo uptake and metabolism of natural retinoids by N-methyl-N-nitrosourea-induced mammary carcinomas. In this study, endogenous retinol and retinyl esters were present in normal mammary epithelial cells, but were undetectable in N-methyl-N-nitrosourea-induced mammary carcinomas in rats as determined by high-pressure liquid chromatography. No differences were found in plasma levels of retinol, in liver retinyl esters, or total content of vitamin A between tumor-bearing and control animals. Administered labeled retinol was taken up and esterified by normal mammary epithelial cells. Tumor-bearing rats were given injections i.p. of either [3H]retinol or [3H]retinoic acid. Radioactivity increased progressively with time in liver and other tissues except in breast tumor, where the uptake fluctuated over the 8 days after the injection of [3H]retinol; in mammary tumors practically no metabolism of [3H]retinol occurred, while in other tissues extensive esterification was detectable. In contrast, in animals given injections of [3H]retinoic acid, the uptake and metabolism of the label in the breast tumors paralleled with those found in other tissues. Neither the activity of acyl coenzyme A:retinol acyl transferase nor the activity of retinyl ester hydrolase was altered in the mammary tumor compared to the normal mammary gland. On the other hand, a significant decrease in the retinal oxidase activity was found in tumor tissue compared to normal mammary tissue. Since no esterification of [3H]retinol occurred in vivo despite the presence of acyl coenzyme A:retinol acyl transferase activity, it is possible that a specific defect in the cellular uptake of retinol may exist in N-methyl-N-nitrosourea-induced mammary carcinomas.     

Suppression of mammary gland tumorigenesis in diabetic rats

The aim of this study was to compare mammary gland tumorigenesis in diabetic and non-diabetic rats. Streptozotocin and N-nitroso-N-methylurea were used to induce diabetes and mammary tumors, respectively. A suppression of mammary carcinogenesis in diabetic rats was shown by a longer latency period, a lower number of tumors per animal and a smaller final tumor volume. An 84% of the lesions developed in diabetic animals were benign tumors. Eighty day-old diabetic rats had significantly lower plasma levels of total-IGF-I and insulin versus non-diabetic rats. We postulate that the decrease in the total IGF-I and insulin levels during the promotion phase of carcinogenesis in this model plays an important role in retarding the tumor development in diabetic animals and in favoring the development of benign mammary lesions.     

High birth weight increases mammary tumorigenesis in rats

Epidemiological studies have investigated whether a high birth weight is associated with increased breast cancer risk, but the results remain inconclusive. This study was designed to determine whether high birth weight increases later susceptibility to carcinogen-induced mammary tumorigenesis in an animal model and to determine mechanisms mediating this association. Pregnant female Sprague Dawley rats were fed either a control or a high-fat diet during the extent of gestation. Maternal exposure to the high-fat diet increased pregnancy leptin levels and offspring's birth weight, but had no effect on pregnancy estradiol or insulin-like growth factor 1 levels. Changes in the offspring's mammary gland morphology and protein expression were assessed. The mammary epithelial tree of the high-birth-weight offspring was denser, contained more terminal end buds and exhibited higher number of proliferating cells. Further, their mammary glands expressed lower levels of ER-alpha, but higher levels of activated MAPK. No alterations in apoptosis were noted. High-birth-weight rats developed 7,12-dimethylbenz[a]anthracene-induced mammary tumors significantly earlier, and tumors grew larger than in the controls. The tumors in this group expressed higher levels of leptin receptor and activated Akt, and contained fewer apoptotic cells than those in the controls. Our results indicate that high birth weight is related to shortened latency to develop mammary tumors--perhaps reflecting an increase in activated MAPK levels and increased tumor growth--perhaps caused by a lower apoptotic response due to higher leptin receptor and activated Akt levels in the tumors.     

Growth and characterization of N-methyl-N-nitrosourea-induced mammary tumors in intact and ovariectomized rats.

It is well established that 85-90% of chemically induced mammary tumors in rats will disappear or diminish significantly in size after the ovaries are removed from the animal. However, it is less well established whether a high percentage of these mammary tumors will grow back with prolonged time after ovariectomy. It is also not known what changes in gene expression take place in the tumors as they develop an independence from hormones for growth. This study was carried out to investigate this. Virgin, 50-day-old female Sprague-Dawley rats were injected with N-methyl-N-nitrosourea (MNU) at the dose of 50 mg MNU/kg body wt. When at least one mammary tumor had grown to 1.0-1.5 cm in one dimension, the animal was bilaterally ovariectomized and reduction and then re-growth of the tumors monitored. Control animals were treated identically except they were not ovariectomized when tumors appeared. Re-growths and new tumors and tumors that developed in the control rats were removed when they reached 1.0-1.5 cm in diameter and all animals were killed 25 weeks after the MNU injection. All the animals in the study (100%) developed mammary tumors after MNU injection with an average latency of 56.5 days. After ovariectomy, 93% of the tumors showed 50% or more reduction in size and 76% of the tumors could not be detected by palpation. However, in 96% of the animals where tumor reduction or disappearance occurred, a re-growth or new mammary tumor development took place with an average latency period of 52.8 days from the day of ovariectomy. Of these post-ovariectomy tumors, 36% occurred at a location where tumors had developed prior to ovariectomy, but 64% appeared at new locations. The circulating levels of 17beta-estradiol (E2) was undetectable in the ovariectomized (OVX) rats and significant reduction was seen in the serum concentrations of progesterone (P4), prolactin (PRL), growth hormone (GH) and insulin-like growth factor-I (IGF-I). The tumors from the OVX rats showed indications of progression as evident from loss of differentiation and invasive characteristics. Comparison between tumors from OVX and intact rats revealed a significantly increased expression of P450 aromatase and elevated activation of extracellular signal-regulated kinase 1 and 2, but reduced levels of the progesterone receptor and cyclin D1 in OVX rats. However, the estrogen receptor (ER) content remained similar in tumors from both groups, at least at the protein level, and so did the expression of IGF-I, IGF-II, insulin receptor substrate-1 (IRS1), IRS-2 and epidermal growth factor receptor. IGF-I receptor (IGF-IR) and ErbB-2 were expressed, respectively, in 50 and 70% of the tumors from the OVX animals, whereas these genes were expressed in 100% of the tumors from the intact rats. It is concluded that chemically induced rat mammary tumors may still depend on the ER and local syntheses of E2 and growth factors for growth initially after ovariectomy. However, as these tumors progress, they develop a more aggressive phenotype and lose their dependency on the ER and possibly growth factors.     

In utero alcohol exposure increases mammary tumorigenesis in rats

Findings in humans and animal models suggest that in utero hormonal and dietary exposures increase later breast cancer risk. Since alcohol intake by adult women consistently increases their breast cancer risk, we wondered whether maternal alcohol consumption during pregnancy increases female offspring's mammary tumorigenesis. In our study, pregnant female rats were pair-fed isocaloric diets containing either 0 (control), 16 or 25 g alcohol kg(-1) feed between days 7 and 19 of gestation. These alcohol exposures generate blood alcohol levels that correspond to low and moderate alcohol consumption and are lower than those that induce foetal alcohol syndrome. Serum oestradiol levels were elevated in pregnant rats exposed to alcohol (P<0.003). When adult, female offspring of alcohol-exposed dams developed significantly more 7,12-dimethylbenz[a]anthracene -induced mammary tumours, compared to the controls (tumour multiplicity; mean+/-s.e.m., controls: 2.0+/-0.3, 16 g alcohol: 2.7+/-0.4 and 25 g alcohol: 3.7+/-0.4; P<0.006). In addition, the mammary epithelial tree of the alcohol-exposed offspring was denser (P<0.004) and contained more structures that are susceptible for the initiation of breast cancer (P<0.001). Immunohistochemical assessment indicated that the mammary glands of 22-week-old in utero alcohol-exposed rats contained elevated levels of oestrogen receptor-alpha (P<0.04) that is consistent with the changes in mammary gland morphology. In summary, maternal alcohol intake during pregnancy increases female offspring's mammary tumorigenesis, perhaps by programming the foetal mammary gland to exhibit persistent alterations in morphology and gene expression. It remains to be determined whether an increase in pregnancy oestradiol levels mediated alcohol's effects on offspring's mammary tumorigenesis.     

Dietary folate deficiency blocks prostate cancer progression in the TRAMP model

Dietary folate is essential in all tissues to maintain several metabolite pools and cellular proliferation. Prostate cells, due to specific metabolic characteristics, have increased folate demand to support proliferation and prevent genetic and epigenetic damage. Although several studies have found that dietary folate interventions can affect colon cancer biology in rodent models, its impact on prostate is unknown. The purpose of this study was to determine whether dietary folate manipulation, possibly being of primary importance for prostate epithelial cell metabolism, could significantly affect prostate cancer progression. Strikingly, mild dietary folate depletion arrested prostate cancer progression in 25 of 26 transgenic adenoma of the mouse prostate (TRAMP) mice, in which tumorigenesis is prostate-specific and characteristically aggressive. The significant effect on prostate cancer growth was characterized by size, grade, proliferation, and apoptosis analyses. Folate supplementation had a mild, nonsignificant, beneficial effect on grade. In addition, characterization of folate pools (correlated with serum), metabolite pools (polyamines and nucleotides), genetic and epigenetic damage, and expression of key biosynthetic enzymes in prostate tissue revealed interesting correlations with tumor progression. These findings indicate that prostate cancer is highly sensitive to folate manipulation and suggest that antifolates, paired with current therapeutic strategies, might significantly improve treatment of prostate cancer, the most commonly diagnosed cancer in American men.     

Protective effects of pregnancy and lactation against N-methyl-N-nitrosourea-induced mammary carcinomas in female Lewis rats

The role of parity before and after N-methyl-N-nitrosourea (MNU) treatment in protection against mammary carcinogenesis was investigated. The effect of lactation on reduction in the incidence of mammary carcinoma was also examined. Parous rats were compared with respective age-matched virgins (AMVs). Pregnancy and lactation prior to MNU exposure significantly reduced both the incidence of mammary carcinoma (22 versus 72%) and the average number of mammary carcinomas per rat (0.22 versus 0.86) and significantly prolonged the latency of the carcinomas (247 versus 215 days). Pregnancy and lactation following MNU exposure also significantly reduced both the incidence of mammary carcinoma (25 versus 94%) and the average number of mammary carcinomas per rat (0.25 versus 1.50) and significantly prolonged the latency (240 versus 155 days). Lactation showed an additive effect on the reduction in mammary cancer. Pregnancy suppressed the number of estrogen receptor (ER)- and progesterone receptor (PgR)-positive cells and lowered the cell proliferation rate in the non-tumoral mammary glands. Since the majority (>76%) of the mammary carcinomas was hormone dependent in both the parous and AMV rats, pregnancy and lactation appear to decrease the ER- and/or PgR-positive cells presumed to be the progenitors of hormone-dependent carcinomas and they lowered the cell turnover necessary for tumor promotion in parous rats, resulting in a lower mammary carcinoma yield.     

Prevention of N-methyl-N-nitrosourea-induced mammary carcinogenesis in rats by 1alpha-hydroxyvitamin D(5)

BACKGROUND: Although the active form of vitamin D, i.e., 1,25-dihydroxyvitamin D(3), is a potent cell-differentiating agent, its use in cancer prevention or therapy is precluded because it induces excessive blood calcium levels (hypercalcemia). However, less calcemic or noncalcemic synthetic analogues of vitamin D(3) are poorly effective against mammary carcinogenesis. We synthesized an analogue of vitamin D(5), 1alpha-hydroxy-24-ethylcholecalciferol (1alpha-hydroxyvitamin D(5)), which was less calcemic than 1,25-dihydroxyvitamin D(3) and prevented the development of precancerous lesions in mammary glands. Here, we evaluate its efficacy in an experimental rat mammary carcinogenesis model. METHODS: Sprague-Dawley rats were treated with 1alpha-hydroxyvitamin D(5) beginning 2 weeks before carcinogen treatment. Animals received an intravenous injection of N-methyl-N-nitrosourea at 80 days of age and continued to receive dietary 1alpha-hydroxyvitamin D(5) for an additional 105 days. Tumor incidence and multiplicity were determined, and plasma concentrations of calcium and phosphorus were measured. The efficacy of 1alpha-hydroxyvitamin D(5) at different stages of carcinogenesis was determined in mouse mammary gland organ culture. All statistical tests were two-sided. RESULTS: The tumor incidence was reduced from 80% (95% confidence interval [CI] = 51.9%-95.7%) in control rats to 53.3% (95% CI = 26.6%-78.8%) and 46.6% (95% CI = 21.3%-73.4%) in rats treated with 1alpha-hydroxyvitamin D(5) at 25 microg/kg diet and 50 microg/kg diet, respectively. The tumor multiplicity was reduced from 1.6 tumors per rat to 1.2 (95% CI for the difference = -0.45 to 1.25; P=.34) and 0.8 (95% CI for the difference = 0.14-1.46; P =.02), respectively. There was no statistically significant increase in the plasma calcium or phosphorus concentration at either dose level. The vitamin D(5) analogue was effective during both the initiation and the promotion stages of mammary lesion formation in organ culture. CONCLUSION: Our findings indicate that 1alpha-hydroxyvitamin D(5) reduces the incidence of mammary carcinogenesis in vivo. This analogue appears to be a good candidate for further development as a chemopreventive agent.     

Promoting effect of 5 beta-chol-3-en-24-oic acid on N-methyl-N-nitrosourea-induced colonic tumorigenesis in rats

The effect of 5 beta-chol-3-en-24-oic acid (delta 3) on N-methyl-N-nitrosourea (MNU)-induced colonic tumorigenesis was studied by intrarectal injection of these compounds in rats. Female Fischer rats received 0.5 ml of distilled water (DW) alone or DW containing 2.5 mg of MNU twice in one week followed by 0.5 ml of peanut oil (PO) alone or PO containing 1 mg of delta 3, or 1 mg of lithocholic acid (LC) thrice weekly for 48 weeks. Thus, 6 groups were employed as follows: group I, DW + PO (n = 12); group II, DW + delta 3 (n = 30); group III, DW + LC (n = 30); group IV, MNU + PO (n = 30); group V, MNU + delta 3 (n = 30) and group VI, MNU + LC (n = 37). Fecal bile acid profiles were analyzed before and during the treatment. Numbers of rats bearing colonic tumor were none in the groups without MNU, but 5 (17%) in group IV, 15 (50%) in group V and 12 (32%) in group VI (corrected X2 = 6.07, P less than 0.025 for group IV vs V, and 1.42, P less than 0.3 for group IV vs VI). Total numbers of tumors were 7, 17 and 15 in group IV, V and VI, respectively, and they were mostly adenomas and adenocarcinomas. A breast fibroadenoma in one rat of group VI was the sole extracolonic neoplasm in these rats. Total fecal bile acids ranged from 7.7 to 10.5 mumol/g dry feces during the study without any significant quantitative or qualitative difference with respect to with or without bile acids and MNU treatment. These results indicated that delta 3 promoted MNU-induced colonic tumorigenesis in rats without alteration in bile acid metabolism.     

Effect of constant light on DMBA mammary tumorigenesis in rats

A study of light, and mammary tumorigenesis was conducted in rats. One-hundred female Sprague-Dawley rats were divided by weight into two groups. One group was exposed to constant light (LL) from 26 days of age, and the second group was exposed to 8 h light and 16 h dark per day (LD). Both groups received an 8 mg dose of a chemical carcinogen, dimethylben-zanthracene (DMBA) at 52 days of age. At 13 weeks post-DMBA, there were significantly fewer mammary tumors in the LL group compared with the LD group. Constant light was clearly demonstrated to have a profound effect on mammary tissue development. Although virgin, the majority of the LL rats (29/50) had gross evidence of lactation at 141 days of age. None of the LD rats (0/50) showed evidence of milk production. These results suggest that constant light not only substantially accelerated mammary gland development, but pushed development of the tissue past the stage normally observed in virgin animals (to the lactation stage).     

Dietary fat and DMBA mammary carcinogenesis in rats

While there is no convincing direct evidence of an influence of specific dietary factors on breast cancer in women, the overall geographic correlation between risk of breast cancer and food consumption patterns suggests a positive link. Epidemiologic studies have correlated breast cancer rates with fat consumption. Increased dietary fat, through intestinal microflora production of estrogens, might expose breast tissue to chronic, excessive stimulation and increase cancer risk. Laboratory animal studies have shown that dietary fat affects response to DMBA carcinogenesis. Studies in our own laboratory have shown that 20% corn oil or lard increased DMBA mammary tumorigenesis, compared to rats fed 5% fat; 20% corn oil accelerated sexual maturation, but 20% lard did not. The mechanisms of tumorigenesis under such circumstances are unknown.     

Exercise during puberty and NMU induced mammary tumorigenesis in rats

Previously we have shown that exercising rats prior to administration of 50 mg NMU/kg (high dose) significantly decreased tumor multiplicity, but not incidence or latency. We hypothesized that the dose of NMU saturated the system such that the exercise intervention was overshadowed. Therefore, the purpose of this investigation was to determine if exercise would have a more pronounced effect with a moderate dose of 37.5 mg NMU/kg. Female Sprague Dawley rats were divided into sedentary and exercised groups. The rats were exercised five times per week from 21 to 50 days of age on a progressive treadmill training program with a final workload of 18 m/min at 15% incline for 60 min a day. At 50 days of age all rats were given an i.p. injection of NMU at 37.5 mg/kg body weight. The experiment was terminated 22 weeks post carcinogen administration. Although there was no difference in terms of tumor incidence, multiplicity, or latency between the two groups, the tumor growth rate (0.107 ± 0.025 vs. 0.043 ± 0.009 g/day) and final tumor weight (3.2 ± 0.74 vs. 1.2 ± 0.34 grams) were significantly higher in the exercised animals. IGF-I receptor (9.4 ± 96 vs. 10.5 ± 57 per µg protein) and estrogen receptor (18.4 ± 1.14 vs. 19.6 ± 1.6 per µg protein) levels were not significantly different in tumors from exercised animals compared to those from sedentary animals. For both sets of tumors, correlation between estrogen receptor content and growth rate is positive, while the correlation between IGF-I receptor content and growth rate is negative. The results of this study suggest that exercise prior to NMU administration does not affect tumor burden but does alter tumor growth rate, which is not due to differences in estrogen receptor or IGF-I receptor content.     

Adrenal regulation of mammary tumorigenesis in female Sprague-Dawley rats: histopathology of mammary tumors

Mammary tumors induced in female Sprague-Dawley rats by feeding 7,12-dimethylbenz(a)anthracene (DMBA; 20 mg/100 g body weight) were classified according to histological criteria of tissue differentiation, cellular atypia, and evidence of invasion. The 549 tumors could be placed in three categories, nodular hyperplasia, nodular hyperplasia with atypia, and carcinoma, and combinations of all three. Although tumors classified histologically as carcinomas did not metastasize, upon transplantation to the kidney capsule, a tumor classified as a carcinoma grew for eight generations and metastasized. Tumor heterogeneity was a common finding in DMBA-initiated tumors. Carcinomas were an early lesion. As the length of time between DMBA treatment and sacrifice increased, more tumors with areas of carcinoma were found. Therefore, DMBA-initiated tumors progressed to carcinomas either soon after initiation or later by development within nodular hyperplasias. In 4 separate groups of animals (74 adrenalectomized rats and 90 intact rats), postinitiation adrenalectomy increased the numbers of carcinomas compared to intact animals. This effect was consistently seen in the cervical and thoracic mammary glands. We propose that the mechanism for enhancement of progression to greater malignancy by adrenalectomy may be inhibition of differentiation of initiated cells in the absence of glucocorticoids.     

Dietary fat versus caloric content in initiation and promotion of 7,12-dimethylbenz(a)anthracene-induced mammary tumorigenesis in rats

Enhancement of mammary tumor formation by dietary fat may be mediated via increased caloric intake. Three experiments were performed to study this relationship in 7,12-dimethyl-benz(a)anthracene (DMBA)-treated female Sprague-Dawley rats: (a) high- or low-fat isocaloric diets were fed in a crossover design; (b) low-fat, high-calorie and high-fat, low-calorie diets were fed in a crossover design; (c) pair-fed rats were restricted to 60% of the calories of controls with ad libitum access to food beginning 10 days after DMBA administration. The pair-fed rats received daily 60% of calories, the same level of fiber, and 115% more fat than did rats fed ad libitum. Tumor yield but not tumor incidence was greater in rats fed high-fat rather than low-fat isocaloric diets prior to initiation of tumorigenesis. A low-fat, high-calorie diet led to more tumor incidence and yield than was associated with feeding of a high-fat, low-calorie diet. Caloric restriction (although with concomitant intake of more fat) led to complete inhibition of tumor formation. These results indicate that both high-fat and high-calorie diets exhibit cocarginogenic, not merely promotional, properties. Caloric intake may be a greater determinant than dietary fat of a tumor-enhancing regimen. Finally, restriction of caloric intake during promotion markedly suppresses tumor formation, despite the increased fat content of the restricted diet, suggesting a permissive role for calories in tumor formation. The possibility remains that alterations in levels of other dietary components could also have contributed to the observed effects.     

Dietary supplemented 2-mercaptoethanol prevents spontaneous and delays virally-induced murine mammary tumorigenesis

There seems to be little doubt that organosulfur compounds have enormous benefits for biological processes, especially those of diseases like cancer. The preliminary results herein define a cancer model in which benefits/mechanisms of multitudes of xenobiotic and nature’s organosulfurs could easily be compared. Mice from three strains with a high incidence for naturally occurring tumors were treated daily with 2-mercaptoethanol (2-Me) starting at weaning. The 100% tumor incidence of undefined etiology in untreated BXSB-Yaa⁺ males was completely prevented by 2-Me. In contrast, 2-Me treatment of female and male C3H.OL and C3H.OH congenic strains, did not change the 100% tumor incidence due to milk-borne retrovirus, MMTV(S), but did: (1) delay the appearance of tumors by 42%; (2) increase longevity 56%; and (3) increase longevity, post-tumor detection, 95%. The addition of these results to the increasingly impressive anti-cancer benefits of simple xenobiotic organosulfurs raise the question: Can they be adapted for use as a preventive modality for human cancer?