Interactions between 4-HPR and diet in NMU-induced mammary tumorigenesis

Terpenoids and breast cancer chemoprevention

Cancer chemoprevention is defined as the use of natural or synthetic agents that reverse, suppress or arrest carcinogenic and/or malignant phenotype progression towards invasive cancer. Phytochemicals obtained from vegetables, fruits, spices, herbs and medicinal plants, such as terpenoids, carotenoids, flavanoids, phenolic compounds, and other groups of compounds have shown promise in suppressing experimental carcinogenesis in various organs. Recent studies have indicated that mechanisms underlying chemopreventive action may include combinations of anti-oxidant, anti-inflammatory, immune-enhancing, and anti-hormone effects. Further, modification of drug-metabolizing enzymes, and influences on cell cycling and differentiation, induction of apoptosis, and suppression of proliferation and angiogenesis that play a role in the initiation and secondary modification of neoplastic development, have also been under investigation as possible mechanisms. This review will highlight the biological effects of terpenoids as chemopreventive agents on breast epithelial carcinogenesis, and the utility of intermediate biomarkers as indicators of premalignancy. Selected breast chemoprevention trials are discussed with a focus on strategies for trial design, and clinical outcomes. Future directions in the field of chemoprevention are proposed based on recently acquired mechanistic insights into breast carcinogenesis.

Correlation of in vitro chemopreventive efficacy data from the human epidermal cell assay with animal efficacy data and clinical trial plasma levels

The human epidermal cell (HEC) assay, which uses carcinogen exposed normal skin keratinocytes to screen for cancer prevention efficacy, was used to screen possible preventive agents. The endpoints measured were inhibition of carcinogen-induced growth and induction of involucrin, an early marker of differentiation. Sixteen of twenty agents (apigenin, apomine, budesonide, N-(2-carboxyphenyl)retinamide, ellagic acid, ibuprofen, indomethacin, melatonin, (-)-2-oxo-4-thiazolidine carboxylic acid, polyphenon E, resveratrol, beta-sitosterol, sulfasalazine, vitamin E acetate, and zileuton) were positive in at least one of the two assay endpoints. Four agents (4-methoxyphenol, naringenin, palmitoylcarnitine chloride, and silymarin) were negative in the assay. Nine of the sixteen agents were positive for both endpoints. Agents that showed the greatest response included: ellagic acid > budesonide, ibuprofen > apigenin, and quinicrine dihydrochloride. Fifty-eight of sixty-five agents that have been evaluated in the HEC assay have also been evaluated in one or more rodent bioassays for cancer prevention and several are in clinical trials for cancer prevention. The assay has an overall predictive accuracy of approximately 91.4% for efficacy in rodent cancer prevention irrespective of the species used, the tissue model, or the carcinogen used. Comparison of the efficacious concentrations in vitro to plasma levels in clinical trials show that concentrations that produced efficacy in the HEC assay were achieved in clinical studies for 31 of 33 agents for which plasma levels and/or C(max) levels were available. For two agents, 9-cis-retinoic acid (RA) and dehydroepiandrosterone (DHEA), the plasma levels greatly exceeded the highest concentration (HC) found to have efficacy in vitro. Thus, the HEC assay has an excellent predictive potential for animal efficacy and is responsive at clinically achievable concentrations.

Effect of early vs. late administration of 4-hydroxyphenylretinamide (4-HPR) on N-methyl-N-nitrosourea (MNU)-induced mammary tumorigenesis

Mammary tumors were induced in 48-52-day-old female Sprague-Dawley rats in metestrus or diestrus with a single jugular injection of MNU (50 mg/kg). Control rats received the saline vehicle (Group 4 n = 9). Rats were fed 4% Teklad diet containing either 0 (Group 3, n = 20) or 782 mg 4-HPR/kg diet. 4-HPR supplementation was initiated either 1 week prior to (Group 1, n = 14) or 4 weeks following MNU administration (Group 2, n = 19). Neither body weight nor food intake differed significantly between treatment groups. Feeding of 4-HPR 1 week prior to tumor induction reduced the number of tumors (0.8 +/- .2) when compared to MNU control rats (2.1 +/- .4). Immunohistochemical staining of mammary tumor sections for PCNA was quantitated by microdensitometry and expressed as an HSCORE. No differences in HSCORE were observed between tumor groups although the percentage of nuclear area occupied by intermediate and darkly stained nuclei was reduced in the late 4-HPR group. GC-->AT transitions in codon 12 of the H-ras gene were detected in 50% (12/24) of MNU control tumors, 60% (6/10) of early 4-HPR tumors, and 38% (6/16) of late 4-HPR tumors. Mutation rates did not differ significantly between groups. 4-HPR appears to be a more effective chemopreventive when fed during the initiation period.

A rapid dual organ rat carcinogenesis bioassay for evaluating the chemoprevention of breast and colon cancer

In this study we evaluated the effect of dietary administration of a high fat, low fiber diet (HRD) with or without 2% phytic acid (PA) on the development of mammary cancer and/or colon cancer in rats exposed to methylnitrosourea (MNU), azoxymethane (AOM) or MNU + AOM. The rats were fed a HRD alone or a HRD + 2% PA. At the end of week 2, the rats were given either a s.c. injection of MNU (50 mg/kg body wt) or one of normal saline (vehicle). At the end of weeks 3 and 4, the rats were given either a s.c. injection of AOM (15 mg/kg body wt per week) or one of normal saline (vehicle). Nine weeks after the injection of MNU or saline, 10 rats from each group were sacrificed and the mammary tumor incidence and the number of colonic aberrant crypt foci (ACF) were compared between different groups. The administration of different diets was continued for an additional 21 weeks and the mammary tumor and colon tumor incidence between different groups were compared. Results showed that rats injected with MNU alone did not develop ACF or colon tumors while those injected with AOM alone did not develop mammary tumors. Linear regression analysis of the number of ACF at 11 weeks versus colonic tumor incidence at 32 weeks, and the linear regression analysis of mammary tumor incidence at 11 weeks versus mammary tumor incidence at 32 weeks, both showed good linear correlation. These results demonstrate the potential value of the short term dual organ carcinogenesis bioassay for screening chemopreventive agents for their relative ability to inhibit the development of mammary cancer and/or colon cancer while on high risk diet.

The effect of dietary fat on the promotion of mammary and colon cancer in a dual-organ rat carcinogenesis model

Female Sprague-Dawley rats (n = 270) were randomly divided into nine subgroups (30 rats/group) and fed diets with fat contents ranging from 5% to 45% of total energy, in which fat was isocalorically substituted for carbohydrates. At Week 3, the rats were given a single injection of methylnitrosourea (MNU, 50 mg/kg body wt). At Weeks 4 and 5, rats were given an injection of azoxymethane (AOM, 15 mg/kg body wt sc). The rats were maintained on their respective experimental diets until they were sacrificed. Nine weeks after the injection of MNU, 10 rats from each group were killed and their mammary tissue was examined for tumors and their colons for aberrant crypt foci (ACF) formation. The results showed no significant increase in the number of colonic ACF or mammary tumors at 11 weeks among individual groups consuming 5-25% energy from fat or among groups consuming 30-45% energy from fat. Data analysis indicated that the effects of the two diets (5-25% energy from fat and 30-45% energy from fat) on ACF formation as well mammary tumor development were statistically different. Continuation of the diets for 32 weeks led to the development of colonic and mammary tumors. Analysis of the combined group tumor incidence data clearly showed that the colonic and mammary tumor incidence in groups fed 30-45% of energy from fat was significantly higher than that observed among groups fed 5-25% of energy from fat. Linear regression analysis of the colonic ACF at 11 weeks, colonic tumor incidence at 32 weeks, and mammary tumor incidence at 11 and 32 weeks showed good linear correlation, demonstrating the potential value of ACF and mammary tumors at 11 weeks for evaluating the carcinogenic risk associated with different diets. The main conclusion of the experiment is that the incidence of colon and mammary cancers increased rapidly when the dietary levels of fat were increased from 15% to 30% of calories and that there was no significant influence on the colon and mammary cancer risk beyond 30% of calories from fat.

Preclinical efficacy evaluation of potential chemopreventive agents in animal carcinogenesis models: methods and results from the NCI Chemoprevention Drug Development Program

In the NCI, Chemoprevention Branch drug development program, potential chemopreventive agents are evaluated for efficacy against chemical carcinogen-induced tumors in animal models. This paper summarizes the results of 144 agents in 352 tests using various animal efficacy models. Of these results, 146 were positive, representing 85 different agents. The target organs selected for the animals model are representative of high-incidence human cancers. The assays include inhibition of tumors induced by MNU in hamster trachea, DEN in hamster lung, AOM in rat colon (including inhibition of AOM-induced aberrant crypts), MAM in mouse colon, DMBA and MNU in rat mammary glands, DMBA promoted by TPA in mouse skin, and OH-BBN in mouse bladder. The agents tested may be classified into various pharmacological and chemical structural categories that are relevant to their chemopreventive potential. These categories include antiestrogens, antiinflammatories (e.g., NSAIDs), antioxidants, arachidonic acid metabolism inhibitors, GST and GSH enhancers, ODC inhibitors, protein kinase C inhibitors, retinoids and carotenoids, organosulfur compounds, calcium compounds, vitamin D3 and analogs, and phenolic compounds (e.g., flavonoids). The various categories of compounds have different spectra of efficacy in animal models. In hamster lung, GSH-enhancing agents and antioxidants appear to have high potential for inhibiting carcinogenesis. In the colon, NSAIDs and other antiinflammatory agents appear particularly promising. Likewise, NSAIDs are very active in mouse bladder. In rat mammary glands, retinoids and antiestrogens (as would be expected) are efficacious. Several of the chemicals evaluated also appear to be promising chemopreventive agents based on their activity in several of the animal models. Particularly, the ODC inhibitor DFMO was active in the colon, mammary glands, and bladder models, while the dithiolthione, oltipraz, was efficacious in all the models listed above (i.e., lung, colon, mammary glands, skin, and bladder).