Possible Application of Human c-Ha-ras Proto-Oncogene Transgenic Rats in a Medium-Term Bioassay Model for Carcinogens

A high-fat diet increases the incidence of mammary cancer inc-Ha-ras proto-oncogene transgenic rats

Mammary cancer is the most common type of cancer and the fifth most common cause of cancer-related deaths among Japanese women. The recent sharp increase in the number of women diagnosed with mammary cancer per year is thought to be associated with increased fat intake resulting from changes in the dietary habits of contemporary Japanese citizens. In this study, human c-Ha-ras proto-oncogene transgenic (Hras128) rats, which are highly susceptible to mammary carcinogens, were fed high- or low-fat diets to examine the relationship between fat consumption and the development of mammary cancer. Female 7-week-old Hras128 rats and wild-type littermates were administered benzo[a]pyrene. A week later, the animals were randomly assigned to high-fat or low-fat diet groups (45% or 10% of calories from fat, respectively). After 12 weeks, the rats were sacrificed and autopsied, and mammary tumors were excised and processed for microscopic observation. Mammary tumors were found in 11 of the 12 animals in the high-fat diet group and in 5 of the 12 animals in the low-fat diet group, and the numbers of mammary gland tumors per animal in these groups were 1.7 and 0.7, respectively. Notably, the observed differences in incidence and multiplicity of mammary tumors between the two groups were statistically significant. These results suggest a positive relationship between the incidence of breast cancer and high fat intake.

Reactive oxygen species-mediated breast cell carcinogenesis enhanced by multiple carcinogens and intervened by dietary ergosterol and mimosine

Most breast cancers occur sporadically due to long-term exposure to low-dose carcinogens in the diet and the environment. Specifically, smoke, polluted air, and high-temperature cooked meats comprise multiple carcinogens, such as 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), benzo[α]pyrene (B[α]P), and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). We sought to determine if these carcinogens act together to induce breast cell carcinogenesis, and if so, whether noncytotoxic dietary agents could intervene. We demonstrated that coexposure to physiologically achievable doses of NNK, B[α]P, and PhIP (NBP) holistically enhanced initiation and progression of breast cell carcinogenesis. Reactive oxygen species (ROS) and activation of the ERK pathway were transiently induced by NBP in each exposure, and cross talk between reinforced ROS elevation and ERK activation played an essential role in increased DNA oxidation and damage. After cumulative exposures to NBP, this cross talk contributed to enhanced initiation of cellular carcinogenesis and led to enhanced acquisition of cancer-associated properties. Using NBP-induced transient changes, such as ROS elevation and ERK pathway activation, and cancer-associated properties as targeted endpoints, we revealed, for the first time, that two less-studied dietary compounds, ergosterol and mimosine, at physiologically achievable noncytotoxic levels, were highly effective in intervention of NBP-induced cellular carcinogenesis. Combined ergosterol and mimosine were more effective than individual agents in blocking NBP-induced transient endpoints, including ROS-mediated DNA oxidation, which accounted for their preventive ability to suppress progression of NBP-induced cellular carcinogenesis. Thus, dietary components, such as mushrooms containing ergosterol and legumes containing mimosine, should be considered for affordable prevention of sporadic breast cancer associated with long-term exposure to environmental and dietary carcinogens.

Chronic exposure to combined carcinogens enhances breast cell carcinogenesis with mesenchymal and stem-like cell properties

Breast cancer is the most common type of cancer affecting women in North America and Europe. More than 85% of breast cancers are sporadic and attributable to long-term exposure to small quantities of multiple carcinogens. To understand how multiple carcinogens act together to induce cellular carcinogenesis, we studied the activity of environmental carcinogens 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and benzo[a]pyrene (B[a]P), and dietary carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) using our breast cell carcinogenesis model. Our study revealed, for the first time, that combined NNK and B[a]P enhanced breast cell carcinogenesis chronically induced by PhIP in both non-cancerous and cancerous breast cells. Co-exposure was more potent than sequential exposure to combined NNK and B[a]P followed by PhIP in inducing carcinogenesis. Initiation of carcinogenesis was measured by transient endpoints induced in a single exposure, while progression of carcinogenesis was measured by acquisition of constitutive endpoints in cumulative exposures. Transient endpoints included DNA damage, Ras-Erk-Nox pathway activation, reactive oxygen species elevation, and increased cellular proliferation. Constitutive endpoints included various cancer-associated properties and signaling modulators, as well as enrichment of cancer stem-like cell population and activation of the epithelial-to-mesenchymal transition program. Using transient and constitutive endpoints as targets, we detected that a combination of the green tea catechins ECG and EGCG, at non-cytotoxic levels, was more effective than individual agents in intervention of cellular carcinogenesis induced by combined NNK, B[a]P, and PhIP. Thus, use of combined ECG and EGCG should be seriously considered for early intervention of breast cell carcinogenesis associated with long-term exposure to environmental and dietary carcinogens.

Mesenchymal and stem-like cell properties targeted in suppression of chronically-induced breast cell carcinogenesis

Stem-like cells and the epithelial-to-mesenchymal transition (EMT) program are postulated to play important roles in various stages of cancer development, but their roles in breast cell carcinogenesis and intervention remain to be clarified. We investigated stem-like cell- and EMT-associated properties and markers in breast epithelial cells chronically exposed to low-dose 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone and benzo[a]pyrene in the presence and absence of the preventive agents green tea catechins and grape seed extract. Our results indicate that stem-like cell- and EMT-associated properties and markers should be seriously considered as new cancer-associated indicators for detecting breast cell carcinogenesis and as endpoints for intervention of carcinogenesis.

Green tea catechin intervention of reactive oxygen species-mediated ERK pathway activation and chronically induced breast cell carcinogenesis

Long-term exposure to low doses of environmental carcinogens contributes to sporadic human breast cancers. Epidemiologic and experimental studies indicate that green tea catechins (GTCs) may intervene with breast cancer development. We have been developing a chronically induced breast cell carcinogenesis model wherein we repeatedly expose non-cancerous, human breast epithelial MCF10A cells to bioachievable picomolar concentrations of environmental carcinogens, such as 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and benzo[a]pyrene (B[a]P), to progressively induce cellular acquisition of cancer-associated properties, as measurable end points. The model is then used as a target to identify non-cytotoxic preventive agents effective in suppression of cellular carcinogenesis. Here, we demonstrate, for the first time, a two-step strategy that initially used end points that were transiently induced by short-term exposure to NNK and B[a]P as targets to detect GTCs capable of blocking the acquisition of cancer-associated properties and subsequently used end points constantly induced by long-term exposure to carcinogens as targets to verify GTCs capable of suppressing carcinogenesis. We detected that short-term exposure to NNK and B[a]P resulted in elevation of reactive oxygen species (ROS), leading to Raf-independent extracellular signal-regulated kinase (ERK) pathway activation and subsequent induction of cell proliferation and DNA damage. These GTCs, at non-cytotoxic levels, were able to suppress chronically induced cellular carcinogenesis by blocking carcinogen-induced ROS elevation, ERK activation, cell proliferation and DNA damage in each exposure cycle. Our model may help accelerate the identification of preventive agents to intervene in carcinogenesis induced by long-term exposure to environmental carcinogens, thereby safely and effectively reducing the health risk of sporadic breast cancer.

Identification of long stress-induced non-coding transcripts that have altered expression in cancer

It has recently become clear that the transcriptional output of the human genome is far more abundant than previously anticipated, with the vast majority of transcripts not coding for protein. Utilizing whole-genome tiling arrays, we analyzed the transcription across the entire genome in both normal human bronchial epithelial cells (NHBE) and NHBE cells exposed to the tobacco carcinogen NNK. Our efforts focused on the characterization of non-coding transcripts that were greater than 300 nucleotides in length and whose expression was increased in response to NNK. We identified 12 Long Stress-Induced Non-coding Transcripts that we term LSINCTs. Northern blot analysis revealed that these transcripts were larger than predicted from the tiling array data. Quantitative real-time RT-PCR performed across a panel of normal cell lines indicates that these transcripts are more abundantly expressed in rapidly growing tissues or in tissues that are more prone to cellular stress. These transcripts that have increased expression after exposure to NNK also had increased expression in a number of lung cancer cell lines and also in many breast cancer cell lines. Collectively, our results identified a new class of long stress responsive non-coding transcripts, LSINCTs, which have increased expression in response to DNA damage induced by NNK. LSINCTs interestingly also have increased expression in a number of cancer-derived cell lines, indicating that the expression is increased in both, correlating cellular stress and cancer.

Cancer research in rat models

Rat has been the major model species used in several biomedical fields, notably in drug development and toxicology, including carcinogenicity testing. Rat is also a useful model in basic cancer research. Several rat models of monogenic (Mendelian) human hereditary cancers are available. Some were obtained spontaneously, while others were generated either by mutagenesis of tumor suppressor genes or by transgenesis of activated oncogenes (transgenesis can be performed efficiently in the rat). In addition, among the hundreds of inbred rat strains that have been isolated, some are highly susceptible or resistant to certain types of cancer, and these divergent phenotypes were shown to be polygenic. Numerous quantitative trait loci (QTLs) controlling cancer susceptibility/resistance have been defined in linkage analyses, and several of these QTLs were physically demonstrated in congenic strains. These studies led, in particular, to rapid translation to the human, with the identification of loci controlling susceptibility to a form of multiple endocrine neoplasia (monogenic trait) and to breast cancer (polygenic disease). The biology of cancer resistance has also been analyzed, and in some (but not all) cases, it was linked to regression of preneoplasic lesions. Rat tumors have been the subject of various types of analyses, and these studies led to important conclusions, including that tumors can be classified on the basis of the identity of the inducing agent, thereby suggesting that analyses of human tumors may be valuable in determining retrospectively the role of specific carcinogens in the formation of human cancers, and of human breast cancer in particular.

Dynamic changes in DNA methylation during multistep rat lung carcinogenesis induced by 3-methylcholanthrene and diethylnitrosamine

3-methylcholanthrene (MCA) and diethylnitrosamine (DEN) are typical genotoxic carcinogens that can induce tumors in a variety of human and rodent tissues. However, the epigenetic mechanisms underlying their tumorigenesis are unclear. In this study we used a MCA/DEN-induced multistep lung carcinogenesis rat model to study the evolution of alterations in DNA methylation. Rats were treated with a single dose of MCA and DEN in iodized oil by left intra-bronchial instillation. The animals were killed on days 15, 35, 55, 65 and 75 and samples of various pathological phases during carcinogenesis were obtained on these days. The status of global methylation was analyzed for each sample using a monoclonal antibody specific for 5-methycytosine (5-mC) and quantified by image analysis software. We found that the degree of global methylation was, in general, higher in basal cells compared to luminal cells of normal, precancerous and tumor tissues. The combined 5-mC scores of different types of tissues decreased gradually during the progression of carcinogenesis. We also used methylation-sensitive arbitrarily primed PCR (MS-AP-PCR) to screen a total of eight differentially methylated DNA fragments in both precancerous and tumor tissues isolated using laser capture microdissection (LCM), and observed that both unique hypomethylation and hypermethylation fragments coexist after exposure to genotoxic carcinogens. Remarkably, epigenetic alterations in p16 (CDKN2A), but not in p15 (CDKN2B), were observed, and these correlated with the presence of pathologic lung lesions and loss of p16 protein expression. Moreover, defective expression of p16 in methylated primary tumor cell lines recovered markedly after treated with 5-aza-2'-deoxycytidine (5-aza-dC). These results suggest that DNA methylation alterations are an early event in tumorigenesis and play an important role during MCA/DEN-induced multistep rat lung carcinogenesis.

Risk of human health by particulate matter as a source of air pollution--comparison with tobacco smoking

Increased air pollution, containing carcinogenic particulate matter smaller than 2.5 microm (PM(2.5)), has gained particular attention in recent years as a causative factor in the increased incidence of respiratory diseases, including lung cancer. Extensive carcinogenicity studies conducted recently under Good Laboratory Practice conditions by National Toxicology Program in the USA, Ramazzini Foundation in Italy or Contract Research Organizations on numerous chemical compounds have demonstrated the importance of considering dose levels, times and duration of exposure in the safety evaluation of carcinogenic as well as classical toxic agents. Data on exposure levels to chemical carcinogens that produce tumor development have contributed to the evaluation of human carcinogens from extrapolation of animal data. A popular held misconception is that the risk from smoking is the result of inhaling assorted particulate matter and by products from burning tobacco rather than the very low ng levels of carcinogens present in smoke. Consider the fact that a piece of toasted bread contains ng levels of the carcinogen urethane (ethyl carbamate). Yet, no one has considered toast to be a human carcinogen. Future human carcinogenic risk assessment should emphasize consideration of inhalation exposure to higher levels of benzo (a) pyrene and other possible carcinogens and particulate matter present in polluted air derived from automobile exhaust, pitch and coal tar on paved roads and asbestos, in addition to other environmental contaminant exposure via the food and drinking water.

Purple corn color suppresses Ras protein level and inhibits 7,12-dimethylbenz[a]anthracene-induced mammary carcinogenesis in the rat

Anthocyanins belong to the class of phenolic compounds collectively named flavonoids. Many anthocyanins are reported to have inhibitory effects on carcinogenesis. Purple corn color (PCC), an anthocyanin containing extract of purple corn seeds, is used as a food colorant. The major anthocyanin in PCC is cyanidin 3-O-beta-D-glucoside (C3-G). The present study was conducted to assess the influence of dietary PCC on 7,12-dimethylbenz[a]anthracene (DMBA)-induced mammary carcinogenesis in rats. PCC significantly inhibited DMBA-induced mammary carcinogenesis in human c-Ha-ras proto-oncogene transgenic (Hras128) rats and in their non-transgenic counterparts. PCC and C3-G also inhibited cell viability and induced apoptosis in mammary tumor cells derived from Hras128 rat mammary carcinomas. At the molecular level, PCC and C3-G treatment resulted in a preferential activation of caspase-3 and reduction of Ras protein levels in tumor cells. It is proposed that C3-G could act as a chemopreventive and possibly chemotherapeutic agent for cancers with mutations in ras. Secondly, the in vitro-in vivo system used in this study can be utilized for screening for cancer preventive compounds that act via Ras down-regulation.