Effects of hypothyroidism and progesterone on mammary tumours induced by 7,12-dimethylbenz(a)anthracene in Sprague-Dawley rats

Hypothyroidism, alone or combined with progesterone, significantly decreased 7,12-dimethylbenz(a)anthracene (DMBA) mammary tumorigenesis relative to controls. However, the decrease was less in the progesterone-treated group, and statistical analysis showed that progesterone enhanced tumorigenesis to the same extent in hypothyroid animals as in the controls. Most tumours in hypothyroid progesterone-treated rats were adenocarcinomata; in the absence of the hormone most tumours were benign. However, the difference between the tumour types in the 2 groups was not statistically significant.     

Effects of time and duration of progesterone administration on mammary tumours induced by 7,12-dimethylbenz(a)anthracene in Sprague-Dawley rats

A reduction in tumour yield was apparent when progesterone administration was begun 25 days before feeding 7,12-dimethylbenz(a)anthracene (DMBA). This effect was most obvious when the duration of hormone administration was brief. Continuation of progesterone for some time after feeding DMBA caused a progressive diminution of the inhibitory effect, and 135 days of continuous hormone treatment entirely abolished the effects of 25 days pretreatment with the hormone.     

Specific estrogen binding in rat mammary tumors induced by 7,12-dimethylbenz(a)anthracene.

Experiments were undertaken with 7,12-dimethylbenz(q)anthracene-induced mammary tumors of the rat to determine whether ovarian-dependent and ovarian-independent tumors could be distinguished on the basis of differences in the estrogen binding capacity of the tumors in vitro and in vivo. Our results confirm reports showing that ovarian-depentent tumors undergo interaction between ovarian-dependent tumors undergo interaction between 17beta-(3H)estradiol and specific estrogen binding components both in vivo and in vitro, as described for other estrogen target tissues. However, our results also demonstrated that certain 7,12-dimethylbenz(a)anthracene-induced tumors, which continue to grow after ovariectomy of the host, contained significant amounts of 17beta-(3H)estradiol bound to cytoplasmic as well as nuclear components. The sedimentation properties of these components were indistinguishable from those of either ovarian-dependent 7,12-dimethylbenz(a)anthracene-induced tumors or rat uterus. The cytoplasmic binding components of both classes of tumors exhibited similar specificities for estrogens. There did not appear to be an absolute correlation between estrogen-binding capacity of a tumor and its growth response to ovariectomy.     

Hepatic metabolism of 7,12-dimethylbenz(a)anthracene in male, female, and ovariectomized Sprague-Dawley rats

Dimethylbenz(a)anthracene (DMBA) is a potent inducer of mammary tumors in intact female Sprague-Dawley rats, but not in males or ovariectomized females (OVX). Qualitative and quantitative aspects of hepatic metabolism of DMBA were examined in these three groups of rats, using the nonrecirculating perfused liver, to determine whether the production of proximate carcinogenic metabolites of DMBA by the liver differed among these groups in the same manner as does sensitivity to tumor induction. DMBA was infused into the liver at a constant rate for 60 min. Rates of appearance of DMBA and its metabolites were measured in perfusate and bile during the infusion period and the first 60 min thereafter. The maximum rate of appearance of total metabolites in the perfusate, seen at the end of the infusion period, was highest in the intact female [2.6 +/- 0.3 nmol/(g x min)], slightly lower in the OVX [2.3 +/- 0.2 nmol/(g x min)] and significantly lower in the male [1.0 +/- 0.1 nmol/(g x min)]. The rates of appearance of metabolites in the bile showed the same order as those seen in the perfusate. The major metabolites extracted from the perfusate in all three groups were dihydrodiols, hydroxymethyl metabolites, and several unidentified metabolites. The 3,4-dihydrodiol, a proximate carcinogenic metabolite, appeared in the perfusate at higher rates in the intact female and OVX than in the male. Hydrolysis of bile samples showed that glucuronidation was a major pathway in the excretion of DMBA metabolites in bile. High performance liquid chromatographic analysis indicated that hydrolysis of DMBA glucuronides yielded the 7- and 12-hydroxymethyl metabolites and an unidentified metabolite designated X. The major hydrolysis product in the male was 12-hydroxymethyl while X was found to be the major product in the intact female and OVX. Under the conditions of this study, there were differences in the metabolic activation of DMBA by male and female rat liver. Ovariectomy, followed by DMBA perfusion 7 days later, did not result in significant changes in DMBA metabolism relative to the intact female, except for a decreased rate of excretion of metabolites in bile.     

Carcinogenic effects of intragastric 3-methylcholanthrene and 7,12-dimethylbenz[a]anthracene in Wistar and Sprague-Dawley rats

In 2 experiments involving 384 rats of the Wistar (W) and Sprague-Dawley (SD) strains, the carcinogenic effects of repeated intragastric instillations of 3-methylcholanthrene (MCA) were compared with those of single intragastric doses of 7,12-dimethylbenz[a]anthracene (DMBA). Mammary carcinomas appeared in a higher proportion and earlier in SD than in W rats exposed to either carcinogen (during 36 weeks, 90 vs. 50% with MCA in SD and W rats, at an average of 12 and 15 weeks, respectively, and 80 vs. 64% with DMBA in SD and W rats, at an average of 16 and 22 weeks, respectively). Fibroadenomas of the breast were seen in 20 percent of SD rats by 36 weeks. In W rats observed for 1 year following DMBA administration, Fibroadenomas occurred in 64 percent and adenosis in 49 percent, at an average of 45 to 51 weeks, whereas after MCA administration these reactions occurred in less than 10 percent. Intraluminal secretion indicative of lactation was noted in approximately 20 percent of fibroadenomas and adenoses. Nine W rats developed lymphoma, 5 following DMBA and 4 following MCA administration. Uterine papillary formations were observed in 1 W and 2 SD rats, and uterine carcinomas in 2 SD rats; 3 animals had received DMBA and 2, MCA.     

Further studies on mammary tumours induced in rats by 7,12-dimethylbenz(A)anthracene (DMBA)

Mammary tumours were induced in female Sprague-Dawley rats by a single oral dose of 20 mg of 7,12-dimethylbenz(a)anthracene (DMBA) given when the rats were 50 days old. The tumours were of three main types: non-secreting adenomata, milk-secreting adenomata and fibro-adenomata. The histological appearance of the nonsecreting adenomata was similar to that of tumours induced by 3-methylcholanthrene. The appearance of the milk-secreting adenomata, with prominent vacuoles in the epithelial cells and milk filling the lumen of the acini, resembled the picture of the normal mammary gland of the rat in late pregnancy. The tumours which appeared earliest were the non-secreting adenomata, and most of these tumours developed during the first 6 months after administration of DMBA. In the first 5 months milk-secreting adenomata and fibro-adenomata were rare, but after this time nearly all the tumours which developed were either milk-secreting adenomata or fibro-adenomata. The milk-secreting adenomata, though late in appearing, were the most numerous of all the tumours induced. The reasons for describing the tumours as adenomata rather than carcinomata are given. Hypophysectomy was very effective in causing regression of the non-secreting adenomata. Less successful results were obtained by removal of the ovaries and adrenal glands.     

Effect of dietary palm oils on mammary carcinogenesis in female rats induced by 7,12-dimethylbenz(a)anthracene

Female Sprague-Dawley rats, 50 days of age, were treated with a single dose of 5 mg of 7,12-dimethylbenz(a)anthracene intragastrically. 3 days after carcinogen treatment, the rats were put on semisynthetic diets containing 20% by weight of corn oil (CO), soybean oil (SBO), crude palm oil (CPO), refined, bleached, deodorized palm oil (RBD PO) and metabisulfite-treated palm oil (MCPO) for 5 months. During the course of experiments, rats fed on different dietary fats had similar rate of growth. Rats fed 20% CO or SBO diet have higher tumor incidence than rats fed on palm oil (PO) diets; however differences of mean tumor latency periods among the groups were not statistically significant. At autopsy, rats fed on high CO or SBO diets had significantly more tumors than rats fed on the three PO diets. Our results showed that high PO diets did not promote chemically induced mammary tumorigenesis in female rats when compared to high CO or SBO diets. CO and SBO differ greatly from the palm oils in their contents of tocopherols, tocotrienols, and carotenes. But further experiments would be required to determine whether the observed differences in tumor incidence and tumor numbers were due to the differences in these minor components or due to the unique triglyceride structure of the palm oils. Analysis of the fatty acid profiles of plasma total lipids of tumor-bearing rats and of the tumor total lipids showed that, with the exception of arachidonic acid, the fatty acid profiles reflect the nature of the dietary fats. At autopsy, there were no differences in the plasma total cholesterol contents among rats fed on different dietary fats, but rats fed on palm oil diets had a significantly higher plasma triglyceride level than that of rats fed CO or SBO diets. As for the tumor lipids, there were no significant differences in the triglyceride, diglyceride, and phospholipid levels when the CO or SBO groups were compared to the palm oil groups.     

Prevention by dehydroepiandrosterone of the development of mammary carcinoma induced by 7,12-dimethylbenz(a)anthracene (DMBA) in the rat

Summary The concentration of serum dehydroepiandrosterone sulfate (DHEA-S) and DHEA decreases markedly during aging, and low circulating levels of DHEA have been associated with a higher incidence of breast cancer in women. Using 7,12-dimethylbenz(a)anthracene (DMBA)-induced mammary carcinoma in the rat as model, we have studied the effect of increasing serum levels of DHEA released from Silastic implants on the incidence of these tumors in the rat. Treatment with increasing doses of DHEA leading to serum DHEA levels comparable to those observed in normal adult women (7.1 ± 0.6 nM and 17.5 ± 1.1 nM) caused a progressive inhibition of tumor development from 68% bearing tumors in control animals to 22% and 11%, respectively. The average tumor area per rat decreased from 2.81 cm² in intact control animals to 0.96 and 0.09 cm² in the groups treated with the same doses of DHEA, respectively. The present data indicate that circulating levels of DHEA similar to those found in normal adult premenopausal women exert a potent inhibitory effect on the development of DMBA-induced mammary tumors in the rat, thus suggesting the possibility of a new and more physiological approach for the prevention of breast cancer in women.     

Effects of dietary fat, calcium, and vitamin D on growth and mammary tumorigenesis induced by 7,12-dimethylbenz(a)anthracene in female Sprague-Dawley rats

This study was designed to test the influence of dietary calcium and vitamin D levels on the promotional effect of high-fat diets on chemically induced mammary carcinogenesis. In a small preliminary experiment (Experiment A), 40 female Sprague-Dawley rats, 43 days old, were randomly divided into 5 groups (8 rats/group) and fed a semipurified diet containing 3% sunflower seed oil (SF) by weight, 1.5 mg of calcium/kcal and 0.5 IU vitamin D/kcal of diet. After 1 week, each rat was given 2.5 mg of dimethylbenz(a)anthracene by gastric gavage. One week later, the animals were switched to 1 of 4 diets varying in fat (3 or 20% SF by weight), calcium (1.5 or 0.25 mg/kcal), vitamin D (0.5 or 0.05 IU/kcal), and phosphate or to a fifth diet containing 3% SF by weight, 0.1 mg of calcium/kcal and 0.05 IU of vitamin D/kcal. In all 5 diets, calcium:phosphate weight ratios were maintained at 1.2:1. In animals fed the high-fat diet, reduction of dietary calcium (1.5 to 0.25 mg/kcal) and vitamin D (0.5 to 0.05 IU/kcal) increased the incidence of mammary lesions from 37 to 75% and the total number of lesions from 4 to 16. A trend toward an increase in lesion weight and total lesion burden was also seen. To confirm these results, the experiment was repeated using the same protocol; 126 rats were divided into 6 groups, treated with dimethylbenz(a)anthracene, and fed the diets as described. A sixth diet was included that contained 20% SF by weight, 0.01 mg of calcium/kcal, and 0.05 IU of vitamin D/kcal. As for Experiment A, in animals fed the high-fat diet, reduction of dietary calcium (1.5 to 0.25 mg/kcal) and vitamin D (0.5 to 0.05 IU/kcal) resulted in an increase in total mammary lesions from 31 to 55, a significant increase in average lesion burden/rat with lesions (1.6 +/- 0.6 to 12 +/- 3 g), and a trend toward increasing weight of lesions. The effect was less obvious in animals fed the low-fat diet where, in both experiments, an increase in the incidence of mammary lesions was observed only when the dietary calcium was reduced from 1.5 to 0.1 mg/kcal. These data suggest that decreasing calcium and vitamin D increase the promoting effects of a high-fat diet on mammary tumorigenesis in the rat.     

Induction of mammary dysplasia and mammary carcinoma in neonatally androgenized female rats by 7,12-dimethylbenz[a]anthracene

Effects of neonatal androgenization on 7,12-dimethylbenz[a]anthracene (DMBA)-induced mammary tumorigenesis infemale noninbred Sprague-Dawley rats are reported. Testosterone propionate (1.25 mg) was injected sc into 29 2-day-old rats. At 50 days of age, all rats were given 20 mg of DMBA through a stomach tube. In these androgenized rats, no corpora lutea were found in the ovaries and the induction of mammary carcinoma by DMBA was strongly suppressed, whereas the induction of mammary dysplasia was significantly accelerated in comparison with the neonatally intact controls. Mammary dysplasia in the androgenized group varied widely, with two kinds of macroscopically detectable tumor-forming lesions (solid and cystic) and with microscopic characteristics of acinar adenosis, fibrotic adenosis, fibrosis, intraductal papillary proliferative lesions, and epithelial cysts. The earliest lesions of mammary dysplasia observed were acinar adenosis nodules and microcysts, both of which appeared as multifocal phenomena as early as 25 days after administration of DMBA.