DMBA-induced Rat Mammary Tumors Research Articles

Chemopreventive effect of celecoxib and expression of cyclooxygenase-1 and cyclooxygenase-2 on chemically-induced rat mammary tumours.

We investigated the chemopreventive effect of celecoxib on 7,12-dimethylbenz[a]anthracene (DMBA)-induced rat mammary tumours and also the expression and immunolocalization of cyclooxygenase-1 (COX-1) and COX-2 in the various stages of rat mammary carcinogenesis. Rats were divided into normal control group, DMBA-control group, 500 p.p.m. celecoxib-treated group, and 1500 p.p.m. celecoxib-treated group. Both incidence and multiplicity values of tumour for rats treated with celecoxib were less than those in rats of the DMBA-control group. The level of prostaglandin E2 was higher in tumours of the DMBA-control and both celecoxib-treated groups compared to normal mammary glands of each group. In Western blot analysis, all tumours of the DMBA-control group expressed COX-1, whereas normal mammary glands showed insignificant expression. COX-2 expression was observed in 67% of the DMBA-control group and 20% of both celecoxib-treated groups and was absent in normal mammary glands. COX-1 protein was localized in the nuclear membrane and cytoplasm of epithelial tumour cells abutting on glandular lumen, stromal cells, and endothelial cells. COX-2 protein was detected in the perinuclear cytoplasm of tumour cells bordering on glandular lumen and surrounding stroma, stromal cells, and vascular smooth muscle. In the DMBA-control group, invasive carcinoma cells showed higher positive immunoreactivity of COX-2 than carcinomas in situ and atypical tumours. Tumours displayed an increased number of mast-like cells with COX-2 expression in comparison to carcinomas in situ. Our results suggest that COX-1 and COX-2 expression in tumour cells and stromal cells play an important role in the various stages of DMBA-induced rat mammary carcinogenesis. In addition, we reconfirm that celecoxib reduces the growth of DMBA-induced rat mammary tumours.

Inducible nitric oxide synthase expression of tumor and stromal cells is associated with the progression of 7,12-dimethylbenz[ a]anthracene-induced rat mammary tumors

The purpose of this study was to investigate the alteration of inducible nitric oxide synthase (iNOS) expression in rodent mammary tumors. We examined iNOS expression by immunohistochemistry and Western blot analysis in 7,12-dimethylbenz[ a]anthracene (DMBA)-induced rat mammary tumors. In Western blot analysis, invasive carcinomas showed strong expression of iNOS; however, carcinomas in situ, atypical tumors and normal mammary tissue revealed insignificant expression. In immunohistochemistry, tumors revealed positive immunoreactivity in either tumor epithelial, stromal or endothelial cells. In particular, invasive carcinomas showed strong expression at the tumor cells bordering on glandular lumen containing necrotic or apoptotic nuclear debris. Invasive carcinomas showed higher positive immunoreactivity of iNOS compared with normal mammary tissue, atypical tumors, and carcinomas in situ. Stromal iNOS expression was correlated with apoptotic count. These results suggest that iNOS expression of tumor and stromal cells is associated with the progression of DMBA-induced rat mammary tumors.

The anticancer activity of a new antiestrogenic agent, TAS-108 (SR16234), against DMBA-induced mammary tumors in rats is partly mediated by de Et-TAS-108, its potent metabolite exerting a strong antagonistic properties

The anticancer activity of a new antiestrogenic agent, TAS-108 (SR16234), against DMBA-induced mammary tumors in rats is partly mediated by de Et-TAS-108, its potent metabolite exerting a strong antagonistic properties

Chemoprevention of DMBA-Induced Mammary Tumors in Rats by a Combined Regimen of Alpha-Tocopherol, Selenium, and Ascorbic Acid

This experimental study was designed to evaluate the efficacy of associated naturally occuring antioxidants in the prevention of chemically induced breast cancer using DMBA in virgin female Wistar rats. Rats were randomly allocated to three groups: control group (CG; n = 20), induction group (IG; n = 100), and prevention group (PG; n = 70). A single dose (65 mg/kg) of DMBA was administered in the IG and PG animals at 50 days of age. PG animals also received a single dose of alpha-tocopherol (200 mg/rat) 1 hour after DMBA administration and an association of selenium (p-XSC, 40 ppm/day/rat) and ascorbic acid (540 mg/day/rat) in drinking water, daily, from carcinogenic induction until necropsy. Macroscopic study and pathology revealed a significantly lower development of neoplasms in the PG animals (p < 0.05); the number of rats with mammary tumors, breast cancer incidence, and the number of malignant breast tumors per rat as well as per tumor-bearing rat were significantly decreased in the PG animals. Other types of primary neoplasms existing in the IG animals totally disappeared in the PG animals. Immunostaining to hormone steroid receptors (ER and PR) and cathepsin D was similar in both groups. Overexpression of p53 and metallothioneine was significantly increased in the PG animals (p < 0.05) and immunostaining to bromodeoxiuridin and Ki-67 was also stronger in the remaining tumors in the PG animals. These data thus add to the accumulating evidence that those micronutrients in combination seem to be effective in reducing the incidence of malignant tumors. Nevertheless, remaining tumors seem to present more aggressive behavior and characteristics of drug resistance.

Regression of DMBA-induced breast carcinoma following ovariectomy is associated with increased expression of genes encoding insulin-like growth factor binding proteins.

Insulin-like growth factor I (IGF-I) has well characterized mitogenic and antiapoptotic effects in both normal and neoplastic mammary epithelial cells which are mediated through the IGF-I receptor. IGF activity is modulated by a family of high affinity IGF binding proteins (IGFBPs 1-6). Here we show that gene expression of IGFBP-2, -3, -4 and -5 increases rapidly in DMBA-induced rat mammary tumors following ovariectomy. We observed a 90% reduction in volume of DMBA-induced rat mammary tumors by 14 days post-ovariectomy. The time course of maximal tumor regression was associated with increased gene expression of IGFBPs, as detected by Northern blot analysis. IGFBP-2, -3, -4 and -5 mRNA levels increased from 2- to 16-fold of control by 14 days of estrogen-ablation. These results are compatible with the hypothesis that response of DMBA-induced mammary tumors to estrogen deprivation therapies involves reduction of IGF bioactivity secondary to upregulation of expression of IGF binding proteins.

A novel aromatase inhibitor, vorozole, shows antitumor activity and a decrease of tissue insulin-like growth factor-I level in 7, 12-dimethylbenz[a]anthracene-induced rat mammary tumors.

Effects of vorozole, a potent and specific non-steroidal aromatase inhibitor, were evaluated on female Sprague-Dawley (SD) rats with 7, 12-dimethylbenz[a]anthracene (DMBA)-induced mammary tumors. Vorozole at a dose of 0.25, 1.0 and 4.0 mg/kg was orally administered once a day for 28 consecutive days. A significant regression in tumor size was observed in each treated group at 1, 2, 3 and 4 weeks after the start of treatment compared with control group. Tissue insulin-like growth factor I (IGF-I) in the DMBA-induced tumors in each treated group significantly decreased in a dose-dependent fashion compared with control group. These results show the mechanism of vorozole in DMBA-induced rat mammary tumors.

Effect of dietary soy protein isolate, genistein, and 1,4-phenylenebis(methylene)selenocyanate on DNA binding of 7,12-dimethylbenz[a]anthracene in mammary glands of CD rats.

We examined whether a soy protein isolate or one of its major components (genistein) influences the initiation stage of carcinogenesis via DNA binding studies of 7,12-dimethylbenz[a]anthracene (DMBA) in liver and mammary tissue of female CD rats. A semipurified high-fat diet (23.5% corn oil) containing the soy protein isolate (10%), genistein (111 ppm), or 1,4-phenylenebis(methylene)selenocyanate (p-XSC) (5 ppm as selenium) as a positive control was fed to 6-week-old virgin female CD rats for 1 week before carcinogen treatment. Neither soy nor genistein affected the extent of DMBA-DNA binding in liver. In mammary tissue, 111 ppm genistein in the diet was more effective than the soy protein isolate, although the latter contains the same amount of genistein, mainly present as a glucoside conjugate. As shown before, p-XSC inhibited DMBA-DNA binding in mammary tissue. Total binding was inhibited because of reduced formation of three major adducts: anti-diol epoxide deoxyguanosine, syn-diol epoxide deoxyadenosine, and anti-diolepoxide deoxyadenosine. Thus, an additional experiment with 111 and 222 ppm of genistein was performed; 222 ppm genistein had a weaker effect than that observed for 111 ppm. Nevertheless, 111 ppm of genistein in the diet appears to inhibit the initiation phase of DMBA-induced rat mammary tumors and may partially account for the reported inhibitory effect of soy against DMBA-induced rat mammary tumors.

Dehydroepiandrosterone inhibits the growth of DMBA-induced rat mammary carcinoma via the androgen receptor.

Dehydroepiandrosterone (DHEA) exerts opposite effects on the growth of mammary carcinoma. A stimulatory effect is observed in absence of estrogens, due to interaction of DHEA metabolite(s) with the estrogen receptor (ER); by contrast, in presence of estrogens DHEA inhibits tumor growth. The mechanism underlying the latter DHEA effect, which might be related to activation of the androgen receptor (AR), is poorly understood. Focusing on this point, we measured over a 20 days period the areas of DMBA-induced mammary tumors in rats given DHEA and/or the anti-androgen flutamide (FLU). Results show that DHEA inhibitory effect on the growth of mammary carcinoma is no longer observed when the ARs are blocked by FLU. Data are consistent with an involvement of ARs in the inhibitory effect of DHEA.

Inhibition of 7,12-dimethylbenz[ a]anthracene-induced rat mammary tumor growth by aryl hydrocarbon receptor agonists

The antitumorigenic activities of 6-methyl-1,3,8-trichlorodibenzofuran (6-MCDF), 8-methyl-1,3,6-trichlorodibenzofuran (8-MCDF) and 6-cyclohexyl-1,3,8-trichlorodibenzofuran (6-CHDF) were investigated in the 7,12-dimethylbenz[ a]anthracene (DMBA) rat mammary tumor model. At doses of 5, 10 or 25 mg/kg per week, both 6-MCDF and 8-MCDF significantly inhibited mammary tumor growth and at the 5 mg/kg per week dose >50% growth inhibition was observed. In contrast, 6-CHDF was inactive at the 5 mg/kg per week dose and the structure-antitumorigenicity relationships (6-MCDF/8-MCDF>6-CHDF) correlated with structure-antiestrogenicity (rat uterus) studies and the relative binding affinities of these compounds for the aryl hydrocarbon receptor (AhR). The antitumorigenic activity of 6-MCDF or 8-MCDF in the mammary was not accompanied by any significant changes in liver/body weight ratios, liver morphology or induction of hepatic CYP1A1-dependent activity which is one of the most sensitive indicators of exposure to AhR agonists. RT-PCR and Western blot analysis of mammary tumor mRNA and protein extracts, respectively, confirmed the presence of AhR suggesting that AhR-mediated signaling pathways are functional in rat mammary tumors. These results define a relatively non-toxic group of AhR agonists which exhibit potent antitumorigenic activity in the DMBA-induced rat mammary tumor model (<1 mg/kg per day), and therefore represent a new class of indirect-acting antiestrogens which have potential for clinical treatment of mammary cancer.

Interactions of varying levels of dietary fat, carbohydrate, and fiber on food consumption and utilization, weight gain and fecal fat contents in female sprague-dawley rats

The purpose of this study was to better define a nutritional regime for the DMBA-induced rat mammary tumor model for studying the interdependent effects of fats, carbohydrates, and fiber on mammary tumor development. Female Sprague-Dawley rats were administered ( ad libitum) a modified AIN-76A diet containing different ratios of dietary fat, carbohydrate, and fiber for 6 weeks. Food consumption was decreased in the higher fat groups, but the decrease did not compensate completely for the higher caloric density of the diets. Calorie consumption and body weight gain were greater in both the mid and high than in the low fat groups. Dietary fiber had only a marginally significant effect on food consumption but did decrease both calorie consumption and body weight gain in the high fat groups. There was no significant interaction between fat and fiber with respect to food or calorie consumption, but there was a marginally significant interaction between these two dietary components on body weight gain. Food utilization, with respect to weight gain, was increased by fat and decreased by fiber, with a significant antagonistic interaction between the two. Calorie utilization (g weight gain/100 kcal) was increased by increased fat but the effect of fiber was not significant. Dry fecal weight was increased by dietary fiber and decreased by dietary fat. The amount of fecal fat excreted was increased by dietary fiber but the effects of dietary fat on fecal fat excretion were inconsistent. The results indicate a complex interaction of dietary fat and fiber. Animals administered the low fat/high fiber diet consumed the least number of calories and thus would be the control group in a pair fed, isocaloric study.

Effects of gamma-linolenic and dihomo-gamma-linolenic acids on 7,12-dimethylbenz(α)anthracene-induced mammary tumors in rats

The effects of pure gamma-linolenic acid (GLA; C18:3, n-6) and dihomo-gamma-linolenic acid (DGLA; C20:3, n-6) were investigated in 7,12-dimethylbenz(α)anthracene (DMBA) (10 mg/rat)-induced mammary tumors in Sprague-Dawley rats. 0.15 g of GLA, DGLA, or corn oil (CO) were administered (two times per week) by oral intubation, for 12 weeks to rats maintained on a 5% (w/w) CO diet. Tumor incidence, tumor multiplicity, and percent of tumor-bearing rats were highest in the CO group. Tumor multiplicity was significantly reduced in the GLA group ( p = 0.015). Feeding of GLA and DGLA resulted in significant alterations in levels of these fatty acids in phospholipids of mammary tissue, thymus, colon, liver, stomach, and ovary. These results suggest that GLA may have a small, but significant, inhibitory effect on the development of DMBA-induced mammary tumors in rats.

Inositol hexaphosphate and inositol inhibit DMBA-induced rat mammary cancer.

Because inositol hexaphosphate (InsP6) and inositol (Ins), contained in plants and most mammalian cells, have been demonstrated to have anti-cancer and anti-cell proliferative action in several experimental models of carcinogenesis we have examined the effect of InsP6 +/- Ins on DMBA-induced rat mammary tumor model. Starting two weeks prior to induction with DMBA, the drinking water of female Sprague-Dawley rats was supplemented with either: 15 mM InsP6, 15 mM Ins, or 15 mM InsP6 + 15 mM Ins; a control group received no inositol compounds. Animals (49-day-old) were given a single intragastric dose of DMBA (5 mg/rat) in 1 ml of corn oil administered by oral intubation. After 45 weeks of treatment, the animals in all the three treatment regimens showed a significant reduction (P < 0.05) in tumor incidence. Tumor number, multiplicity and tumor burden were also significantly (P < 0.05) reduced by InsP6 +/- Ins. When all the parameters were taken into consideration, the best results were obtained by the combination treatment of InsP6 + Ins. Four additional groups not receiving DMBA, but drinking tap water, InsP6, Ins, or InsP6 + Ins of the same molarity as experimental groups were observed for the duration of the study to monitor for any toxicity following this long-term treatment; no significant toxicity as evaluated by body weight gain, serum and bone mineral levels was detected. We demonstrate that InsP6 +/- Ins reproducibly inhibits experimental mammary carcinoma, therefore having great potential as a chemopreventive and adjuvant therapeutic agent for this disease as well.

Stromal expression of tenascin is inversely correlated to epithelial differentiation of hormone dependent tissues

We were previously investigating the expression of the extracellular matrix glycoprotein tenascin in normal and malignant endometrial tissues of humans and rodents. These studies suggested that the expression of tenascin was induced by proliferating epithelia (normal and particularly malignant) and was downregulated with their differentiation. The aim of this study was to investigate the hormone dependency of tenascin expression in (a) the transplantable EnDA endometrial tumor model with or without estrogen deprivation (overiectomy) of the animals, (b) DMBA-induced rat mammary tumors with or without a hormonal treatment of the animals [ovariectomy, antiestrogen (tamoxifen) or antiprogestin (ZK 98299) treatment] and (c) in the rat prostate of untreated or androgen deprived animals (orchiectomy, flutamide-, casodex- or cyproterone acetate (CPA)- treatment). 1. Estrogen withdrawal by ovariectomy did not affect tenascin expression in transplantable EnDA endometrial adenocarcinoma, meaning the entire extracellular space of the stromal mesenchyme was decorated by tenascin immunoreactivity. 2. In untreated DMBA-induced rat mammary tumors almost the entire extracellular space of the stroma was stained by tenascin immunoreactivity. Ovariectomy and antiestrogen treatment did not affect tenascin expression. In contrast, antiprogestin treatment induced terminal differentiation of mammary tumor cells and in parallel downregulated tenascin expression. 3. In the normal rat prostate no tenascin was detectable by immunocytochemistry. However, following androgen deprivation we found tenascin expression in the stroma of the prostate. The most prominent expression was observable after CPA-treatment, possibly due to its progestagenic potency. In conclusion, the hormones and antihormones tested show no direct effect on the stromal expression of tenascin. However, proliferative activity and a low degree of differentiation of the epithelium induces tenascin expression, whereas epithelial differentiation apparently shuts down tenascin expression. Preliminary in vitro studies suggest that paracrine acting growth factors trigger the hormonal regulation of tenascin expression.

Cell proliferation and prevalence of ras gene mutations in 7,12-dimethylbenz(a)anthracene (DMBA)-induced rat mammary tumors.

Exposure of female inbred Sprague-Dawley Curl rats to intragastric 7,12-dimethylbenz(a)anthracene (DMBA) resulted in a variety of benign and malignant mammary tumors exhibiting a wide spectrum of proliferative activity as measured by [3H]-thymidine autoradiography. On the basis of earlier observations of 21% H-ras codon 61 mutations in DMBA-induced rat mammary tumors reported by Zarbl et al., the prevalence of this type of ras gene mutation was studied in relation to the rate of proliferation in individual benign and malignant tumors. In a series of 50 mammary tumors exhibiting highly different proliferation activities we did not detect any H-ras codon 61 mutations after allele-specific dot blot hybridization of PCR-amplified material obtained from paraffin sections. Neither mode of proliferation nor type of differentiation appears related to H-ras codon 61 mutations. The relevance of activated ras genes for rat mammary carcinogenesis appears restricted to model systems using N-methyl-N-nitro-sourea (MNU) as initiating carcinogen.

ANTITUMOR EFFECT OF MEDROXYPROGESTERONE ACETATE (MPA) ON 7,12-DIMETHYLBENZ(A)ANTHRACENE-INDUCED RAT MAMMARY-CARCINOMA DEPENDS UPON THE MPA CONCENTRATION IN TUMORS OF RATS

Medroxyprogesterone acetate (MPA) in doses of 1 mg, 5 mg or 10 mg, was orally administered daily for four weeks to rats with 7,12-dimethylbenz(a)anthrace (DMBA)-induced mammary carcinoma. All rats were sacrificed after four weeks and MPA concentration was determined in serum, tumor and muscle. MPA concentration in the tumor was much higher compared with that in the serum or in the muscle in each treatment group. There was no difference between responder group and non-responder group in MPA concentration in the serum, however, MPA concentration in the responder group was significantly higher than that in the non-responder group in all the tumors of the treatment groups. These results suggest that antitumor effect of MPA on DMBA-induced rat mammary tumors depends upon the MPA concentration in the tumor of the rat.

Inhibitory effect of combined treatment with the aromatase inhibitor exemestane and tamoxifen on DMBA-induced mammary tumors in rats

The antitumor effect of exemestane (FCE 24304), an irreversible aromatase inhibitor, given alone or in combination with tamoxifen, was investigated in rats with 7, 12-dimethylbenzanthracene (DMBA)-induced mammary tumors. The compounds were given once daily, 6 days a week for 4 weeks. Exemestane, given at the dose of 20 mg/kg/day s.c., induced 26% complete (CR) and 18% partial (PR) tumor regressions, compared to 0% CR and 6% PR observed in controls. Tamoxifen, given at 1 mg/kg/day p.o., induced 16% CR and 13% PR. The combined treatment caused 41% CR and 16% PR, thus resulting in a higher antitumor effect than either single treatment. The apperance of new tumors was reduced by each single treatment and almost totally prevented by the combined treatment. Serum prolactin (PRL) levels, assayed 4 h after the last dose, were unchanged in the group treated with the combination, whereas tamoxifen alone caused a slight increase of serum PRL. These results indicate that estrogen deprivation through aromatase inhibition and estrogen receptor antagonism causes a better inhibition of DMBA-induced mammary tumors than either treatment modality alone.

Preclinical Studies on the Anticancer Activity of the Somatostatin Analog Octreotide (SMS 201-995)

The antiproliferative effect of somatostatin-14 and its analog octreotide on in vitro pancreatic and breast tumor cells has led to the suggestion that octreotide may have further oncological indications in addition to gastroenteropancreatic tumors. To extend these in vitro observations, we evaluated the effect of octreotide in rodent models of pancreatic and breast tumors. Octreotide of 5 or 50 micrograms b.i.d. in nude mice bearing solid MiaPaCa pancreatic tumors (subline 21) or ZR-75-1 breast tumors induced significant inhibition of tumor growth from week 2 until the end of treatment at week 5. After 5 weeks the mean volume of ZR-75-1 tumors in animals treated with the 50-micrograms regimen was 48% that of control. Autoradiographic studies showed a high percentage (71%) of ZR-75-1 tumors to be somatostatin receptor-positive. In addition, the growth of ZR-75-1 cells in vitro was significantly inhibited by octreotide. The drug was also tested in a second breast cancer model, DMBA-induced mammary tumors in rats, and continuous administration of 10 micrograms/kg/h over 6 weeks led to an approximately 50% reduction in the number of tumors arising in the rat mammary gland. These data suggest that pancreatic and breast cancer may be among the malignant diseases clinically susceptible to octreotide.

The effect of vitamin E tocotrienols from palm oil on chemically induced mammary carcinogenesis in female rats

In the DMBA induced rat mammary tumor model, palm oil appeared to be less tumourigenic than corn oil, soybean oil, beef tallow and lard. Palm oil is a rich source of tocotrienols which have been reported to have antitumour activity against certain types of transplantable murine tumors. Results of the present experiments showed that palm oil stripped of its vitamin E tocopherols and tocotrienols (EFPO) enhanced tumorigenesis compared to palm oil containing vitamin E. In view of this a second experiment was designed to determine whether addition of a tocotrienol rich fraction (TRF) of palm oil to corn oil was able to inhibit development of mammary tumors in the DMBA treated rat model. One hundred female Sprague Dawley rats at 50 days of age were treated with a single dose of 5 mg of DMBA. They were then divided into 5 groups and fed semi-synthetic diets containing 20% by weight (40 en%) of either corn oil (CO), refined bleached and deodorized palm oil (RBDPO), corn oil + 500 ppm TRF, corn oil + 1000 ppm TRF or CO + 135 ppm α-tocopherol. The experiment was terminated 5 months after DMBA treatment. The RBD palm oil group had the lowest tumor incidence, tumor count and tumors per rat compared to animals fed the CO diet. This suggests that CO has a greater tumor promoting effect than palm oil. Supplementation of TRF at 500 ppm to corn oil did not provide protection. However at the higher concentration of 1000 ppm of TRF, the corn oil fed animals had significantly greater median latency period, lower tumor incidence and tumor count than the corn oil fed group as well as the group supplemented with either 500 ppm of TRF or 135 ppm α-tocopherol. The results appear to indicate that supplementation of corn oil with adequate amounts of TRF may give some protection against the cancer promoting effect of corn oil. The results also suggest that the favourable effects observed on a palm oil diet maybe attributed to the presence of naturally occurring tocotrienols in palm oil.

Hormone control of total plasminogen activator activity is specific to malignant DMBA-induced rat mammary tumours.

Hormonal regulation of plasminogen activator expression in 7,12-dimethylbenz[a]anthracene (DMBA)--induced rat mammary carcinomas was studied both in vivo and in vitro and was compared to that in DMBA-mammary dysplasia induced in neonatally androgenised rats. The plasminogen activator activity in DMBA-mammary carcinomas, but not in DMBA-mammary dysplasia, was regulated by oestrogen. This suggests that expression of this enzyme is hormonally regulated in carcinoma cells. Furthermore, in two of six DMBA-mammary carcinoma groups classified in terms of hormonal treatment, plasminogen activator activity was not under the control of oestrogen. Thus, the present results suggest that at the time of carcinogenesis, the hormonal milieu determines the hormone sensitivities of the malignant cells.

The effect of vitamin e tocotrienols from palm oil on chemically-induced mammary carcinogenesis in female rats

In the DMBA induced rat mammary tumor model, palm oil appeared to be less tumourigenic than corn oil, soy bean oil, beef tallow and lard. Palm oil is a rich source of tocotrienols which have been reported to have antitumour activity against certain types of transplantable murine tumors. Results of the present experiments showed that palm oil stripped of its Vitamin E tocopherols and tocotrienols (EFPO) enhanced tumorigenesis compared to palm oil containing Vitamin E. In view of this, a second experiment was designed to determine whether addition of a tocotrienol rich fraction (TRF) of palm oil to corn oil was able to inhibit development of mammary tumors in the DMBA treated rat model One hundred female Sprague Dawley rats at 50 days of age were treated with a single dose of 5 mg of DMBA. They were then divided into 5 groups and fed semi-synthetic diets containing 20% by weight (40 en %) of either corn oil (CO), refined bleached and deodorized palm oil (RBDPO), corn oil + 500 ppm TRF, corn oil + 1000 ppm TRF or CO + 135 ppm α-tocopherol. The experiment was terminated 5 months after DMBA treatment. The RBD palm oil group had the lowest tumor incidence, tumor count and tumors per rat compared to animals fed the CO diet. This suggests that CO has a greater tumor promoting effect than palm oil. Supplementation of TRF at 500 ppm to corn oil did not provide protection. However at the higher concentration of 1000 ppm of TRF, the corn oil fed animals had significantly greater median latency period, lower tumor incidence and tumor count than the corn oil fed group as well as the group supplemented with either 500 ppm of TRF or 135 ppm α-tocopherol. The results appear to indicate that supplementation of corn oil with adequate amounts of TRF may give some protection against the cancer promoting effect of corn oil. The results also suggest that the favourable effects observed on a palm oil diet may be attributed to the presence of naturally occurring tocotrienols in palm oil.