Prolactin concentration in plasma and susceptibility to mammary tumors in female rats from different strains treated chronically with estradiol-17β

Summary Prolactin is associated with the development of mammary tumors in rats. The aim of the present study was to evaluate whether strain differences in susceptibility to the development of mammary tumors could be explained by genetic differences in the response of the pituitary to chronic stimulation by estrogens. Prolactin levels were measured in plasma from rats of the Sprague-Dawley, Wistar WAG/Rij and Brown Norway BN/BiRij strains before and at different times after subcutaneous implantation of estradiol-17 in cholesterol/paraffin pellets. In all strains plasma prolactin was elevated from the second week after implantation of the pellet, although there were quantitative differences between the responses. At 32 weeks after implantation of the pellets the plasma level of prolactin in Sprague-Dawley rats was 1247 ± 367 ng NIAMDD prolactin RP-1/ml (mean ± S.E.M), whereas Wistar WAG/Rij and Brown Norway BN/BiRij had plasma prolactin levels of 679 ± 211 and 182 ± 19 ng/ml respectively. Between 52 and 104 weeks after implantation these values rose to 4016 ± 1116, 5004 ± 1053 and 808 ± 129 ng/ml respectively. The plasma concentration of prolactin of rats in this age group was strongly associated with the occurrence of pituitary adenomas in all three strains. In untreated rats, the concentration of prolactin in the plasma increased with age to only 200–400 ng/ml at 12–24 months of age but no significant differences were observed between the three rat strains. It is concluded that observed differences in spontaneous and estrogen-mediated mammary tumor development in these rat strains cannot be explained by genetic differences in the plasma concentration of prolactin. The development of malignant mammary tumors after estrogen treatment appears to be associated with the extent of the increase in plasma prolactin induced by the estrogen.     

Post-initiation treatment of rats with indole-3-carbinol or beta-naphthoflavone does not suppress 7, 12-dimethylbenz[a]anthracene-induced mammary gland carcinogenesis

Indole-3-carbinol (I3C) and β-naphthoflavone (β-NF), blocking agents of 7,12-dimethylbenz[a]anthracene (DMBA)-initiated mammary gland carcinogenesis, were examined as potential post-initiation suppressing agents. Treatment of female Sprague–Dawley rats with I3C (250 mg/kg body weight (b.w.)), β-NF (20 mg/kg b.w.) or the vehicle ethanol:corn oil (2:3) (2.5 ml/kg b.w.), three times weekly by gavage, started 3 weeks after the initiation with one oral dose of DMBA (20 mg/rat at 7 weeks of age) and continued for up to 12 weeks. I3C- or β-NF- or vehicle-treated groups did not differ significantly in the overall outcome of mammary tumorigenesis including cumulative mammary tumor incidences and multiplicities, latent periods and number and weight of mammary tumors per tumor-bearing rat for malignant, benign and/or malignant + benign tumors. A tendency of the I3C-treated rats to develop fewer mammary adenocarcinomas with a greater average weight per tumor per rat (2.32±1.50 g) than in the β-NF- (1.52±1.58 g) or vehicle- (1.55±1.53 g) treated groups suggests an effect, yet to be confirmed, of I3C on tumor development and growth. A 12-week treatment with I3C or β-NF significantly increased the P450-dependent activities of ethoxy-, methoxy-, benzyloxy- and pentoxy-(with I3C only) resorufin O-dealkylase in hepatic microsomes indicating induction of several P450s. The alterations in the P450 complement may affect endogenous estrogen metabolism and mammary gland and tumor characteristics at the molecular level, e.g. estrogen receptor status and/or proliferative activity, which require further studies.     

Early spontaneous deficiency of calcitonin renal binding sites in rats with a high incidence of calcitonin-secreting tumors (WAG/Rij)

Old rats of the WAG/Rij strain have a high incidence (50%) of medullary thyroid carcinoma, a calcitonin (CT)-secreting tumor. We have characterized and quantified the topographical distribution of [125I]salmon calcitonin (sCT) binding sites in the kidneys of this strain, as compared to Wistar CF rats (2% incidence of spontaneous medullary thyroid carcinoma). We report here that, up to 15 days of postnatal development, the distribution of CT-binding sites in the kidney of the WAG/Rij strain was quite similar to that found in developing and adult Wistar CF rats. However, from the age of 1 month, sCT-binding sites were dramatically reduced in both the medulla and the inner part of the kidney cortex, though plasma CT levels were not significantly different in both strains. Adult WAG/Rij rats bearing a transplanted tumor for 12 weeks had a high level of plasma calcitonin and exhibited an even greater reduction of both medullary and cortical sCT-binding sites. These results suggest that the modification in the CT-binding sites in WAG/Rij rats is not a consequence of a possible down regulation due to elevated circulating hormonal level but could be inherited and possibly associated with the later development of the tumor in this strain.     

Role of alveolar macrophages in tumor-bearing rats: tumoricidal properties of carrageenan-activated macrophages

Tumor BN472, a malignant mammary adenocarcinoma, was subcutaneously transplanted into syngeneic female Brown Norway rats. Seven days after tumor inoculation, carrageenan-impregnated synthetic sponges were subcutaneously implanted in control and tumor-bearing rats. Another week later the animals were sacrificed and alveolar macrophages were harvested and tested for tumoricidal activity against a tissue culture line of BN472 cells and their capacity to phagocytose formaldehyde-treated sheep erythrocytes. The data demonstrate that carrageenan statistically significantly enhances the tumoricidal activity of alveolar macrophages in tumor-bearing rats. Phagocytic activity of the macrophages in these animals is not different from sham-operated control animals, whereas the phagocytic activity of tumor-bearing rats is statistically significantly decreased.     

Post-initiation treatment of rats with indole-3-carbinol or β-naphthoflavone does not suppress 7,12-dimethylbenz[a]anthracene-induced mammary gland carcinogenesis

Indole-3-carbinol (I3C) and β-naphthoflavone (β-NF), blocking agents of 7,12-dimethylbenz[a]anthracene (DMBA)-initiated mammary gland carcinogenesis, were examined as potential post-initiation suppressing agents. Treatment of female Sprague–Dawley rats with I3C (250 mg/kg body weight (b.w.)), β-NF (20 mg/kg b.w.) or the vehicle ethanol:corn oil (2:3) (2.5 ml/kg b.w.), three times weekly by gavage, started 3 weeks after the initiation with one oral dose of DMBA (20 mg/rat at 7 weeks of age) and continued for up to 12 weeks. I3C- or β-NF- or vehicle-treated groups did not differ significantly in the overall outcome of mammary tumorigenesis including cumulative mammary tumor incidences and multiplicities, latent periods and number and weight of mammary tumors per tumor-bearing rat for malignant, benign and/or malignant + benign tumors. A tendency of the I3C-treated rats to develop fewer mammary adenocarcinomas with a greater average weight per tumor per rat (2.32±1.50 g) than in the β-NF- (1.52±1.58 g) or vehicle- (1.55±1.53 g) treated groups suggests an effect, yet to be confirmed, of I3C on tumor development and growth. A 12-week treatment with I3C or β-NF significantly increased the P450-dependent activities of ethoxy-, methoxy-, benzyloxy- and pentoxy-(with I3C only) resorufin O-dealkylase in hepatic microsomes indicating induction of several P450s. The alterations in the P450 complement may affect endogenous estrogen metabolism and mammary gland and tumor characteristics at the molecular level, e.g. estrogen receptor status and/or proliferative activity, which require further studies.     

Exposure of Fischer 344 rats to a weak power frequency magnetic field facilitates mammary tumorigenesis in the DMBA model of breast cancer

The possibility that long-term exposure to relatively weak power frequency magnetic fields (MFs) emanating from the generation, transmission and use of electricity could increase the risk of breast cancer is a matter of ongoing debate. Laboratory studies using well-defined exposure conditions are useful to examine whether exposure to MF affects mammary tumorigenesis. Previous studies from different laboratories using the 7,12-dimethylbenz[a]anthracene (DMBA) model of breast cancer in female Sprague-Dawley (SD) rats have been inconclusive, which has been related to differences in MF sensitivity between SD substrains used in these studies. When we compared the effects of MF exposure on cell proliferation in the mammary gland of various outbred and inbred rat strains, Fischer 344 was the only inbred strain that exhibited a marked increase in cell proliferation. Based on these data, we suggested that MF exposure should significantly facilitate development and growth of mammary tumors in Fischer 344 rats, which was tested in the present study. Groups of 108 DMBA-treated rats were either MF exposed (100 muT, 50 Hz) or sham exposed for 26 weeks. MF exposure significantly facilitated mammary tumorigenesis. The incidence of rats with grossly recorded, histologically verified adenocarcinomas was increased by 45% (P = 0.0095). The most pronounced MF effect on tumor incidence was seen in the cranial inguinal complexes (L/R5). These data indicate that Fischer 344 rats are a suitable inbred strain to study the mechanisms underlying the effects of MF exposure on mammary tumorigenesis.     

Mammary tumors in splenectomized rats.

The objective of this study was to examine how splenectomy affects the immune response, particularly T cells, in chemically-induced mammary tumors. Female rats were splenectomized and then exposed to 9,10-dimethyl-1,2-benz(a)anthracene (DMBA) to induce mammary tumors. Splenectomy significantly decreased the rate of tumor appearance and their malignant transformation. The tumor latency period in splenectomized rats was 12.0+/-0.9 weeks compared to 9.7+/-0.5 wk in intact controls, and malignancy appeared in 45% of splenectomized rats, compared to 70% in controls. By the end of the experiment, the total number of tumors and their size were similar in both groups. Blood CD4+ and CD8+ T cell concentrations were similar in tumor-bearing and tumor-free splenectomized animals, but in both groups CD4- and CD8- lymphocytes decreased sharply compared to control animals. In tumor-bearing rats, splenectomy also resulted in significantly more circulating natural killer cells. The spleens of tumor-bearing control rats had significantly fewer CD4+ and CD8+ lymphocytes and more CD4- and CD8- lymphocytes and natural killer cells than did their blood. In conclusion, splenectomy inhibits the early stages of tumorigenesis and reduces the rate of malignant transformation of benign tumors, but does not prevent the progress of carcinogenesis. Differences between splenectomized (operated) and intact rats to the effect of DMBA can be explained by an increase in non-specific resistance of splenectomized rats as a result of operation.     

Transplacental effects of diethylstilbestrol on mammary development and tumorigenesis in female ACI rats

Female ACI rats were exposed to diethylstilbestrol (DES) transplacentally and followed to 10 months of age to assess the effect of the drug on mammary development and tumorigenesis. Pregnant rats were given injections of vehicle (sesame oil) or DES (total dose, 0.8 micrograms = low DES or 8.0 micrograms = high DES) on days 15 and 18 of gestation. Pellets containing 2.5 mg DES + 17.5 mg cholesterol (DES pellet) or 20 mg cholesterol (chol pellet) were implanted s.c. into 12-week-old female offspring, creating 6 experimental groups: vehicle exposure + chol pellet (1) or + DES pellet (2); low DES exposure + chol pellet (3) or + DES pellet (4); high DES exposure + chol pellet (5) or + DES pellet (6). At sacrifice, representative mammary tissue and all palpable mammary tumors were removed for histopathological analysis. Each of the 6 experimental groups contained a minimum of 32 rats from at least 14 litters. In computation of data, the unit of analysis was the litter. Groups which had received any DES (prenatally or postnatally) were found to have elongated nipples and enlarged pituitaries. The mammary gland whole mounts from all rats in groups 4 and 6 displayed extensive lobuloalveolar proliferation comparable to that seen in DES pellet controls (group 2). Mammary glands of approximately 75% of rats in groups 3 and 5 were categorized as showing the lowest grade of differentiation while this undifferentiated condition was seen in only 36% of group I controls. No palpable mammary tumors were found in rats exposed to vehicle in utero (group 1). But in group 5, a total of 6 tumors in 5 animals derived from 4 different litters were obtained, a difference shown to be statistically significant. Group 3 had 1 rat with 8 tumors. Among rats bearing the DES pellet, tumor latency was shortened significantly in both groups exposed to DES in utero. By 22 weeks after pellet implantation, 100% of the DES-exposed litters (groups 4 and 6) contained at least 1 tumor-bearing rat compared to about 50% of the tumor-bearing litters in group 2. Tumor multiplicity at sacrifice was increased significantly in the group exposed prenatally to the higher dose of DES. Histologically, the overwhelming majority of palpable mammary tumors from all tumor-bearing treatment groups were classified as adenocarcinomas. Prenatal exposure to DES did not alter the ratio of malignant to benign lesions observed, nor did it affect the degree of differentiation noted in the adenocarcinomas.(ABSTRACT TRUNCATED AT 400 WORDS)     

Transplacental effects of a 15% olive-oil diet on chemically-induced tumorigenesis in offspring

We evaluated whether feeding pregnant female rats a diet high in olive-oil, that showed a tumor-preventive effect in adults, has a similar preventive effect on chemically-induced cancer in offspring (i.e. mammary glands and colon cancer in rats). The control group was fed the same 7% corn-oil diet as their mothers. Experimental group I was fed a 7% corn-oil diet while their mothers received a 15% olive-oil diet. Experimental group II was fed the same 15% olive-oil diet as their mothers. Female offspring were twice administered 7,12-dimethylbenz(a)antracene (DMBA) in doses of 10 mg/rat. Male offspring were injected 6 times with 1, 2-dimethylhydrazine (DMH) in doses of 20 mg/kg body weight. Effect of DMBA was manifested in a high rate of tumorigenesis: the number of tumor-bearing rats in control offspring reached 52.0%. This effect increased to 60.6% among offspring of experimental group II and to 67.7% in offspring of experimental group I. The mean tumor size increased significantly in control offspring. Following administration of DMH number of tumor-bearing rats was similar in all groups of offspring: 36.7%, 40.7% and 42.8%. Tumor types differed: the majority of tumors in the control group were benign polyps and adenomas (72.1%) and the number of adenocarcinomas was low (27.9%). The number of malignant tumors increased to 37.5% in offspring of experimental group II and to 45.5% in offspring of experimental group I. In control group offspring, a distinct tendency to increased body weight and a significant increase in spleen weight were seen. The findings indicate that feeding mothers a diet high in fat concentrations, even those with known tumor preventive significance in adults, lose this cancer-inhibiting role in offspring.     

Inhibition of NMU-induced mammary tumorigenesis by dietary soy

We previously demonstrated that female Sprague-Dawley rats fed AIN-93G diets containing soy protein isolate (SPI+) had lower DMBA-induced mammary tumor incidence than those fed diets containing casein (CAS), due partly to altered Phase I metabolism with soy. Here, we evaluated the tumor protective effects of these same diets to the direct-acting carcinogen N-methyl-nitrosourea (NMU). Tumor incidence was reduced and tumor latency was enhanced, in NMU-administered female rats lifetime exposed to SPI+, relative to the CAS group. Tumor multiplicity did not differ with diet, while tumor grade tended to be more advanced with SPI+. Normal mammary glands of CAS and SPI+ tumor-bearing rats had comparable proliferative and apoptotic status. However, mammary expression of HER-2/neu and progesterone receptor (PR) genes was higher for SPI+ rats. Moreover, tumored SPI+ rats had lower serum progesterone levels than those fed CAS, while serum estrogen did not differ. Serum from tumored SPI+ rats had higher apoptotic activity towards mammary epithelial MCF-7 cells, than CAS serum. Thus, dietary soy protects against mammary tumorigenesis induced by a direct-acting carcinogen and alters signaling pathways involving PR and HER-2/neu.     

On the long-term toxic risk of dinaline

The long-term toxic risk of the cytostatic agent dinaline (4-amino-N-(2′-aminophenyl)-benzamide) was assessed in a rat bioassay. Regular administration of 9, 3 and 1 mg/kg for 106 weeks was associated with significantly increased survival of female rats receiving the median and low doses. Dinaline significantly reduced the occurrence of malignant tumors in male rats and prolonged the manifestation time of malignancies in female rats. Unlike malignant tumors, benign neoplasms were increased in male rats and were not significantly different from controls in female rats. Analysis of organ distribution of neoplastic lesions revealed a dose-dependently decreased tumor incidence in the hematopoietic and lymphatic tissue, the mammary gland (females only) and the pituitary gland and a not dose dependently reduced incidence of liver tumors. This contrasted with dose dependently increased tumor incidences in the adrenal gland, the gonads and the vagina. Considering these findings, dinaline has to be assessed as a modulator of carcinogenesis in rats. The observed decreased and increased tumor incidences suggest a hormone-related mechanism of action.     

Genetic control of resistance to chemically induced mammary adenocarcinogenesis in the rat

Fifty-day-old female rats of a series of outbred (i.e., SD) and inbred (i.e., NSD, WF, LEW, F344, ACI, and COP) strains were exposed to a single dose of either of two highly effective mammary chemical carcinogens, 7,12-dimethylbenz[a]anthracene (DMBA) or 1-methyl-1-nitrosourea (MNU), to determine the characteristic number of mammary adenocarcinomas induced/rat for each strain. Female rats of the inbred NSD, WF, and LEW strains were found to be as highly susceptible to DMBA exposure as the randomly outbred SD strain (i.e., greater than 2 mammary adenocarcinomas/rat develop). Inbred female F344 and ACI rats were found to be much less susceptible to DMBA induced mammary adenocarcinogenesis (i.e., less than 1.2 mammary adenocarcinomas/rat). In contrast to all the other inbred strains, the female COP rat was unique in that it is essentially completely resistant to all attempts to induce mammary adenocarcinomas by either DMBA or MNU exposure. Genetic breeding analysis demonstrated that the resistance of the mammary epithelium of the female COP rat to DMBA and MNU is due to the mendelian inheritance of a dominant, autosomal genetic allele. The inheritance of a single copy of this resistance allele is able to prevent both the DMBA and MNU induced development of mammary adenocarcinomas in F1 hybrids produced by cross-breeding COP to the highly susceptible NSD animal.     

Enhancing Effects of β-Estradiol 3–Benzoate but Not Methoxychlor on the Promotion/Progression Stage of Chemically-induced Mammary Carcinogenesis in Ovariectomized Rats

Modifying effects of β-estradiol 3–benzoate (EB) and methoxychlor (MXC), a pesticide which possesses weak estrogenic activity, on 7,12–dimethylbenz( a )anthracene (DMBA)-induced mammary carcinogenesis were investigated in ovariectomized or intact female Sprague-Dawley rats. Twenty-eight weeks after a single DMBA (100 mg/kg body weight) initiation, when the incidence of mammary tumor-bearing rats had reached 75%, a number of the animals were subjected to ovariectomy in order to obtain 3 groups: i) tumor-bearing, ovariectomized group; ii) tumor-bearing, intact group; iii) no-tumor, ovariectomized group. Subsequently animals of each group were subjected to subcutaneous implantation of 0.5 mg EB or given diet containing 1000 ppm MXC for 13 weeks. Although the incidences, multiplicities and volumes of the palpable tumors gradually decreased after ovariectomy, EB treatment stimulated tumor growth in the tumor-bearing, ovariectomized group thereafter. A similar effect of EB treatment was also observed in the no-tumor, ovariectomized group. However, MXC did not show any effect in the tumor-bearing, or no-tumor ovariectomized groups, except that the multiplicity of tumors was significantly decreased by MXC treatment in the tumor-bearing, intact group. The results of our study suggest that MXC has no promotion/progression effect, but rather possesses a weak inhibitory effect, whereas the strongly estrogenic substance EB clearly enhanced DMBA-induced mammary tumorigenesis.     

Effects of D,L-2-difluoromethylornithine and indomethacin on mammary tumor promotion in rats fed high n-3 and/or n-6 fat diets

Virgin female Sprague-Dawley rats (50 days of age) were administered a single intragastric 10-mg dose of 7,12-dimethylbenz(a)anthracene (DMBA). Twenty-one days later they were placed on diets containing either 20% corn oil (CO), 15% menhaden oil plus 5% corn oil (MO + CO), 20% CO plus 0.5% w/w of the irreversible ornithine decarboxylase inhibitor, D,L-2-difluoromethylornithine (CO + DFMO), 20% CO plus 0.004% w/w of the cyclooxygenase inhibitor indomethacin (CO + INDO), 20% CO + 0.004% INDO + 0.5% DFMO (CO + INDO + DFMO), or 15% MO + 5% CO + 0.5% DFMO (MO + CO + DFMO). The incidence of DMBA-induced mammary tumors was significantly reduced in rats fed diets containing DFMO but not in rats fed the diet containing indomethacin. Incidences of mammary tumors at 16 weeks post-DMBA were 86% in rats fed the CO diet, 83% in rats ingesting the diet containing CO + INDO, 28% in rats fed CO + DFMO, 32% in rats fed diet containing CO + INDO + DFMO, 59% in rats fed the MO + CO diet, and 24% in rats fed the MO + CO + DFMO diet. The average number of tumors and tumor burden per tumor-bearing rat were reduced and tumor latency was increased in all rats fed diets containing DFMO. Body weight gain, but not food intake, of rats fed the 20% fat + 0.5% DFMO diets was significantly less than in rats fed the 20% fat diets. Prostaglandin E and leukotriene (LTB4) syntheses, ODC activity and mammary tumorigenesis were significantly inhibited by feeding the diet containing menhaden oil or by adding 0.5% DFMO to any of the high fat diets. Feeding a 20% CO diet containing 0.004% INDO significantly reduced prostaglandin synthesis and ODC activity and increased LTB4 synthesis of mammary tumors but did not inhibit mammary tumorigenesis. This study suggests that the 5-lipoxygenase product LTB4 may be involved in mammary tumor production. Whereas a decrease in LTB4 appears to be associated with a decrease in tumorigenesis, an increase (as seen in the indomethacin group) was not associated with any change in the tumorigenic response.     

Enhancing effects of beta-estradiol 3-benzoate but not methoxychlor on the promotion/progression stage of chemically-induced mammary carcinogenesis in ovariectomized rats.

Modifying effects of beta-estradiol 3-benzoate (EB) and methoxychlor (MXC), a pesticide which possesses weak estrogenic activity, on 7,12-dimethylbenz(a)anthracene (DMBA)-induced mammary carcinogenesis were investigated in ovariectomized or intact female Sprague-Dawley rats. Twenty-eight weeks after a single DMBA (100 mg / kg body weight) initiation, when the incidence of mammary tumor-bearing rats had reached 75%, a number of the animals were subjected to ovariectomy in order to obtain 3 groups: i) tumor-bearing, ovariectomized group; ii) tumor-bearing, intact group; iii) no-tumor, ovariectomized group. Subsequently animals of each group were subjected to subcutaneous implantation of 0.5 mg EB or given diet containing 1000 ppm MXC for 13 weeks. Although the incidences, multiplicities and volumes of the palpable tumors gradually decreased after ovariectomy, EB treatment stimulated tumor growth in the tumor-bearing, ovariectomized group thereafter. A similar effect of EB treatment was also observed in the no-tumor, ovariectomized group. However, MXC did not show any effect in the tumor-bearing, or no-tumor ovariectomized groups, except that the multiplicity of tumors was significantly decreased by MXC treatment in the tumor-bearing, intact group. The results of our study suggest that MXC has no promotion / progression effect, but rather possesses a weak inhibitory effect, whereas the strongly estrogenic substance EB clearly enhanced DMBA-induced mammary tumorigenesis.     

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.     

Morphological and immunological characteristics of a rat choriocarcinoma

A post-gestational rat choriocarcinoma has been studied for its immunological and morphological characteristics. The tumor can be transplanted in syngeneic (WKA/H rats) as well as in allogeneic rats (BN, R/A). Even after immunization of the allogeneic hosts with tissues of the WKA/H rats, the development of the tumor was not inhibited. This neoplasm is composed of giant cells, surrounding spaces filled with blood, and of proliferating cytotrophoblasts. Ultrastructurally, these cells are very similar to those found in normal rat placenta. In some rats lung metastases were also observed. The tumor is hormonally active as demonstrated by proliferation of and secretions from the mammary glands in tumor-bearing animals and by the presence of delta 5-3 beta-hydroxysteroid dehydrogenase in the giant cells of the neoplasms. These 2 findings indicate that the rat choriocarcinoma cells secrete lactogen and might produce progesterone, like cells of rat placenta. All the described features of this tumor point to its similarity to human choriocarcinoma.     

Caffeine inhibits development of benign mammary gland tumors in carcinogen-treated female Sprague-Dawley rats

Summary The purpose of this study was to assess the influence of caffeine on the incidence of benign mammary tumors in carcinogen (DMBA) treated female Sprague-Dawley rats. Four different animal models were used in these studies, i.e., the administration of DMBA to: [1] 55 day old virgin rats; [2] 53 day old ovariectomized, estrogen treated virgin rats; [3] 135 day old virgin rats and [4] 135 day old parous rats. A high incidence of benign mammary fibroadenomas was observed in each of the four animal models. In addition, in the estrogen treated ovariectomized animals, a high incidence of secretory mammary gland cysts was observed. Caffeine (500 mg/L drinking water) was administered daily throughout the study commencing 3–31 days after carcinogen treatment. Caffeine treatment significantly (P<0.05 to P<0.001) reduced the incidence of benign mammary fibroadenomas in the 55 day old virgin rat model (P<0.01), in the 53 day old estrogen treated ovariectomized virgin rat model (P<0.05 to P<0.001) and in the 135 day old virgin rat model (P<0.05). The number of benign mammary fibroadenomas was reduced by caffeine in the 135 day old parous rat model but this reduction was not significant (P<0.10). In addition, in the estrogen treated ovariectomized virgin rat model, caffeine significantly (P<0.05 to P<0.001) reduced the incidence of mammary gland cysts. Caffeine treatment either increased or had no significant effect on body weight gains, depending upon the animal model. Thus, caffeine consumption can influence the development of benign mammary tumors (fibroadenomas and cysts) in carcinogen treated female Sprague-Dawley rats, an influence that was shown to be consistently inhibitory.     

The gene transfer-misrepair hypothesis of radiation carcinogenesis tested for induction of mammary tumours in rats

Mammary tumour induction was studied in female WAG/Rij rats following exposure with single doses of 0.3 and 1.2 Gy gamma radiation and with the same total doses delivered in fractions of 2.5 and 10 mGy respectively at intervals of 12 h. All rats were implanted with pellets containing 2 mg of estradiol-17 beta prior to the irradiation. The occurrence of mammary carcinomas and of fibroadenomas was recorded. The relative excess hazard for tumour induction was lower for the fractionated regimens than for the single dose exposures. The results are compatible with the predictions of the gene transfer-misrepair hypothesis for radiation carcinogenesis.