Concepts in multistage carcinogenesis

Relevance of Thymic Stromal Lymphopoietin on the Pathogenesis of Glioblastoma: Role of the Neutrophil

Glioblastoma multiforme (GBM) is the most predominant and malignant primary brain tumor in adults. Thymic stromal lymphopoietin (TSLP), a cytokine primarily generated by activated epithelial cells, has recently garnered attention in cancer research. This study was aimed to elucidate the significance of TSLP in GBM cells and its interplay with the immune system, particularly focused on granulocyte neutrophils. Our results demonstrate that the tumor produces TSLP when stimulated with epidermal growth factor (EGF) in both the U251 cell line and the GBM biopsy (GBM-b). The relevance of the TSLP function was evaluated using a 3D spheroid model. Spheroids exhibited increased diameter, volume, and proliferation. In addition, TSLP promoted the generation of satellites surrounding the main spheroids and inhibited apoptosis in U251 treated with temozolomide (TMZ). Additionally, the co-culture of polymorphonuclear (PMN) cells from healthy donors with the U251 cell line in the presence of TSLP showed a reduction in apoptosis and an increase in IL-8 production. TSLP directly inhibited apoptosis in PMN from GBM patients (PMN-p). Interestingly, the vascular endothelial growth factor (VEGF) production was elevated in PMN-p compared with PMN from healthy donors. Under these conditions, TSLP also increased VEGF production, in PMN from healthy donors. Moreover, TSLP upregulated programed death-ligand 1 (PDL-1) expression in PMN cultured with U251. On the other hand, according to our results, the analysis of RNA-seq datasets from Illumina HiSeq 2000 sequencing platform performed with TIMER2.0 webserver demonstrated that the combination of TSLP with neutrophils decreases the survival of the patient. In conclusion, our results position TSLP as a possible new growth factor in GBM and indicate its modulation of the tumor microenvironment, particularly through its interaction with PMN.

Past, present, and future of chemically induced hepatocarcinogenesis rodent models: Perspectives concerning classic and new cancer hallmarks

Hepatocellular carcinoma (HCC), the main primary liver cancer, accounts for 5 % of all incident cases and 8.4 % of all cancer-related deaths worldwide. HCC displays a spectrum of environmental risk factors (viral chronic infections, aflatoxin exposure, alcoholic- and nonalcoholic fatty liver diseases) that result in molecular complexity and heterogeneity, contributing to a rising epidemiological burden, poor prognosis, and non-satisfactory treatment options. The emergence of HCC (i.e., hepatocarcinogenesis) is a multistep and complex process that addresses many (epi)genetic alterations and phenotypic traits, the so-called cancer hallmarks. "Polymorphic microbiomes", "epigenetic reprogramming", "senescent cells" and "unlocking phenotypic plasticity" are trending hallmarks/enabling features in cancer biology. As the main molecular drivers of HCC are still undruggable, chemically induced in vivo models of hepatocarcinogenesis are useful tools in preclinical research. Thus, this narrative review aimed at recapitulating the basic features of chemically induced rodent models of hepatocarcinogenesis, eliciting their permanent translational value regarding the "classic" and the "new" cancer hallmarks/enabling features. We gathered state-of-art preclinical evidence on non-cirrhotic, inflammation-, alcoholic liver disease- and nonalcoholic fatty liver-associated HCC models, demonstrating that these bioassays indeed express the recently added hallmarks, as well as reflect the interplay between classical and new cancer traits. Our review demonstrated that these protocols remain valuable for translational preclinical application, as they recapitulate trending features of cancer science. Further "omics-based" approaches are warranted while multimodel investigations are encouraged in order to avoid "model-biased" responses.

Glioblastoma cells potentiate the induction of the Th1-like profile in phosphoantigen-stimulated γδ T lymphocytes

PURPOSE

γδ T lymphocytes are non-conventional T cells that participate in protective immunity and tumor surveillance. In healthy humans, the main subset of circulating γδ T cells express the TCRVγ9Vδ2. This subset responds to non-peptide prenyl-pyrophosphate antigens such as (E)-4-hydroxy-3-methyl-but-enyl pyrophosphate (HMBPP). This unique feature of Vγ9Vδ2 T cells makes them a candidate for anti-tumor immunotherapy. In this study, we investigated the response of HMBPP-activated Vγ9Vδ2 T lymphocytes to glioblastoma multiforme (GBM) cells.

METHODS

Human purified γδ T cells were stimulated with HMBPP (1 µM) and incubated with GBM cells (U251, U373 and primary GBM cultures) or their conditioned medium. After overnight incubation, expression of CD69 and perforin was evaluated by flow cytometry and cytokines production by ELISA. As well, we performed a meta-analysis of transcriptomic data obtained from The Cancer Genome Atlas.

RESULTS

HMBPP-stimulated γδ T cells cultured with GBM or its conditioned medium increased CD69, intracellular perforin, IFN-γ, and TNF-α production. A meta-analysis of transcriptomic data showed that GBM patients display better overall survival when mRNA TRGV9, the Vγ9 chain-encoding gene, was expressed in high levels. Moreover, its expression was higher in low-grade GBM compared to GBM. Interestingly, there was an association between γδ T cell infiltrates and TNF-α expression in the tumor microenvironment.

CONCLUSION

GBM cells enhanced Th1-like profile differentiation in phosphoantigen-stimulated γδ T cells. Our results reinforce data that have demonstrated the implication of Vγ9Vδ2 T cells in the control of GBM, and this knowledge is fundamental to the development of immunotherapeutic protocols to treat GBM based on γδ T cells.

Classification or non-classification of substances with positive tumor findings in animal studies: Guidance by the German MAK commission

One of the important tasks of the German Senate Commission for the Investigation of Health Hazards of Chemical Compounds in the Work Area (known as the MAK Commission) is in the evaluation of a potential for carcinogenicity of hazardous substances at the workplace. Often, this evaluation is critically based on data on carcinogenic responses seen in animal studies and, if positive tumor responses have been observed, this will mostly lead to a classification of the substance under investigation into one of the classes for carcinogens. However, there are cases where it can be demonstrated with a very high degree of confidence that the tumor findings in the experimental animals are not relevant for humans at the workplace and, therefore, the MAK Commission will not classify the respective substance into one of the classes for carcinogens. This paper will summarize the general criteria used by the MAK Commission for the categorization into "carcinogen" and "non-carcinogen" and compare this procedure with those used by other national and international organizations.

Thrombospondin-2 overexpression in the skin of transgenic mice reduces the susceptibility to chemically induced multistep skin carcinogenesis

BACKGROUND

We have previously reported stromal upregulation of the endogenous angiogenesis inhibitor thrombospondin-2 (TSP-2) during multistep carcinogenesis, and we found accelerated and enhanced skin angiogenesis and carcinogenesis in TSP-2 deficient mice.

GOALS

To investigate whether enhanced levels of TSP-2 might protect from skin cancer development.

METHODS

We established transgenic mice with targeted overexpression of TSP-2 in the skin and subjected hemizygous TSP-2 transgenic mice and their wild-type littermates to a chemical skin carcinogenesis regimen.

RESULTS

TSP-2 transgenic mice showed a significantly delayed onset of tumor formation compared to wild-type mice, whereas the ratio of malignant conversion to squamous cell carcinomas was comparable in both genotypes. Computer-assisted morphometric analysis of blood vessels revealed pronounced tumor angiogenesis already in the early stages of carcinogenesis in wild type mice. TSP-2 overexpression significantly reduced tumor blood vessel density in transgenic mice but had no overt effect on LYVE-1 positive lymphatic vessels. The percentage of desmin surrounded, mature tumor-associated blood vessels and the degree of epithelial differentiation remained unaffected. The antiangiogenic effect of transgenic TSP-2 was accompanied by a significantly increased number of apoptotic tumor cells in transgenic mice.

CONCLUSION

Our results demonstrate that enhanced levels of TSP-2 in the skin result in reduced susceptibility to chemically-induced skin carcinogenesis and identify TSP-2 as a new target for the prevention of skin cancer.

Eosinophils in glioblastoma biology

Glioblastoma multiforme (GBM) is the most common primary brain tumor in adults. The development of this malignant glial lesion involves a multi-faceted process that results in a loss of genetic or epigenetic gene control, un-regulated cell growth, and immune tolerance. Of interest, atopic diseases are characterized by a lack of immune tolerance and are inversely associated with glioma risk. One cell type that is an established effector cell in the pathobiology of atopic disease is the eosinophil. In response to various stimuli, the eosinophil is able to produce cytotoxic granules, neuromediators, and pro-inflammatory cytokines as well as pro-fibrotic and angiogenic factors involved in pathogen clearance and tissue remodeling and repair. These various biological properties reveal that the eosinophil is a key immunoregulatory cell capable of influencing the activity of both innate and adaptive immune responses. Of central importance to this report is the observation that eosinophil migration to the brain occurs in response to traumatic brain injury and following certain immunotherapeutic treatments for GBM. Although eosinophils have been identified in various central nervous system pathologies, and are known to operate in wound/repair and tumorstatic models, the potential roles of eosinophils in GBM development and the tumor immunological response are only beginning to be recognized and are therefore the subject of the present review.

Reviewing once more the c-myc and Ras collaboration: converging at the cyclin D1-CDK4 complex and challenging basic concepts of cancer biology

The c-myc is a proto-oncogene that manifests aberrant expression at high frequencies in most types of human cancer. C-myc gene amplifications are often observed in various cancers as well. Ample studies have also proved that c-myc has a potent oncogenicity, which can be further enhanced by collaborations with other oncogenes such as Bcl-2 and activated Ras. Studies on the collaborations of c-myc with Ras or other genes in oncogenicity have established several basic concepts and have disclosed their underlying mechanisms of tumor biology, including "immortalization" and "transformation". In many cases, these collaborations may converge at the cyclin D1-CDK4 complex. In the meantime, however, many results from studies on the c-myc, Ras and cyclin D1-CDK4 also challenge these basic concepts of tumor biology and suggest to us that the immortalized status of cells should be emphasized. Stricter criteria and definitions for a malignantly transformed status and a benign status of cells in culture also need to be established to facilitate our study of the mechanisms for tumor formation and to better link up in vitro data with animal results and eventually with human cancer pathology.

Identification of novel tumour-associated genes differentially expressed in the process of squamous cell cancer development

Chemically induced mouse skin carcinogenesis represents the most extensively utilized animal model to unravel the multistage nature of tumour development and to design novel therapeutic concepts of human epithelial neoplasia. We combined this tumour model with comprehensive gene expression analysis and could identify a large set of novel tumour-associated genes that have not been associated with epithelial skin cancer development yet. Expression data of selected genes were confirmed by semiquantitative and quantitative RT-PCR as well as in situ hybridization and immunofluorescence analysis on mouse tumour sections. Enhanced expression of genes identified in our screen was also demonstrated in mouse keratinocyte cell lines that form tumours in vivo. Self-organizing map clustering was performed to identify different kinetics of gene expression and coregulation during skin cancer progression. Detailed analysis of differential expressed genes according to their functional annotation confirmed the involvement of several biological processes, such as regulation of cell cycle, apoptosis, extracellular proteolysis and cell adhesion, during skin malignancy. Finally, we detected high transcript levels of ANXA1, LCN2 and S100A8 as well as reduced levels for NDR2 protein in human skin tumour specimens demonstrating that tumour-associated genes identified in the chemically induced tumour model might be of great relevance for the understanding of human epithelial malignancies as well.

Cancer prevention by natural compounds

Increasing attention is being paid to the possibility of applying cancer chemopreventive agents for individuals at high risk of neoplastic development. For this purpose by natural compounds have practical advantages with regard to availability, suitability for oral application, regulatory approval and mechanisms of action. Candidate substances such as phytochemicals present in foods and their derivatives have been identified by a combination of epidemiological and experimental studies. Plant constituents include vitamin derivatives, phenolic and flavonoid agents, organic sulfur compounds, isothiocyanates, curcumins, fatty acids and d-limonene. Examples of compounds from animals are unsaturated fatty acids and lactoferrin. Recent studies have indicated that mechanisms underlying chemopreventive potential may be combinations of anti-oxidant, anti-inflammatory, immune-enhancing, and anti-hormone effects, with modification of drug-metabolizing enzymes, influence on the cell cycle and cell differentiation, induction of apoptosis and suppression of proliferation and angiogenesis playing roles in the initiation and secondary modification stages of neoplastic development. Accordingly, natural agents are advantageous for application to humans because of their combined mild mechanism. Here we review naturally occurring compounds useful for cancer chemprevention based on in vivo studies with reference to their structures, sources and mechanisms of action.

Profile of gene expression induced by the tumour promotor TPA in murine epithelial cells

Malignant transformation of mouse skin by chemical carcinogens and tumour promoters, such as the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA), is a multistage process that leads to squamous cell carcinoma (SCC) formation. In an effort to identify tumour-associated genes, we studied the influence of short-term TPA-treatment on the gene expression profile of murine skin. A comprehensive microarray with some 5,000 murine gene specific cDNA fragments was established and hybridised with pooled RNA derived from control and TPA-treated dorsal skin samples. Of these genes, 54 were up- and 35 were down-regulated upon TPA application. Additionally, we performed suppression subtractive hybridisation (SSH) with respective RNA pools to generate and analyse a cDNA library enriched for TPA-inducible genes. Expression data of selected genes were confirmed by quantitative real-time PCR and Northern blot analysis. Comparison of microarray and SSH data revealed that 26% of up-regulated genes identified by expression profiling matched with those present in the SSH library. Besides numerous known genes, we identified a large set of unknown cDNAs that represent previously unrecognised TPA-regulated genes in murine skin with potential function in tumour promotion. Additionally, some TPA-induced genes, such as Sprr1A, Saa3, JunB, Il4ralpha, Gp38, RalGDS and Slpi exhibit high basal level in advanced stages of skin carcinogenesis, suggesting that at least a subgroup of the identified TPA-regulated genes may contribute to tumour progression and metastasis.

Thrombospondin-1 selectively inhibits early-stage carcinogenesis and angiogenesis but not tumor lymphangiogenesis and lymphatic metastasis in transgenic mice

The roles played by the endogenous angiogenesis inhibitor thrombospondin-1 (TSP-1) in the early stages of multi-step carcinogenesis and in the control of hematogenous versus lymphatic metastasis are unknown. To investigate these issues we compared tumor development in normal mice and in transgenic mice with targeted overexpression of TSP-1 in the epidermis following a standard two-step chemical skin carcinogenesis regimen. Overexpression of TSP-1 resulted in delayed and reduced development of premalignant epithelial hyperplasias, but did not inhibit the malignant conversion to squamous cell carcinomas. TSP-1 overexpression also suppressed tumor angiogenesis and distant organ metastasis, but failed to inhibit tumor-associated lymphangiogenesis or lymphatic tumor spread to regional lymph nodes. Concomitant with these results, we found that the endothelial TSP-1 receptor CD36 was mostly absent from cutaneous lymphatic vessels. Our findings indicate the potential use of TSP-1 for the prevention of premalignant stages of tumorigenesis and are likely to have implications for the further development of anti-angiogenic cancer therapies.

Delay of DNA-adduct repair and severe toxicity in xeroderma pigmentosum group A gene (XPA) deficient mice treated with 2-amino-1-methyl-6-phenyl-imidazo [4,5-b] pyridine (PhIP)

Group-A xeroderma pigmentosum (XPA) gene-deficient mice are defective in nucleotide-excision repair and highly susceptible to ultraviolet-B-, and 9,10-dimethyl-1,2-benz[a]anthracene (DMBA)-induced skin carcinogenesis. In this study, changes of 2-amino-1-methyl-6-phenylimidazo[4,5-b] pyridine (PhIP)-DNA adduct formations in the liver, colon and lung, as assessed by the 32P-postlabeling method and immunohistochemical analysis, and carcinogenic and/or toxic susceptibility of both sexes of XPA-deficient mice (XPA-/-) to PhIP, which is a carcinogenic heterocyclic amine, was examined. Levels of PhIP-DNA adduct formations in the liver, colon and lung, were almost twice as high in XPA-/- as in wild type mice (XPA+/+) mice, 7 days after a single i.g. administration of PhIP, and their delay in recovery was observed in XPA-/- mice. For the long-term experiment, XPA-/- and XPA+/+ type mice were treated with 80 ppm PhIP in the diet for the first 4 weeks followed by 40 ppm after a 2-week recovery period (long-term experiment I), or 40 ppm PhIP throughout the experiment (long-term experiment II). Severe toxicity, as evidenced by body weight retardation and poor survival, was observed in the PhIP treated XPA-/- mice of both sexes, but not in the XPA+/+. At week 40 the experiments were terminated and histopathological examinations were performed after complete autopsy. Only lymphomas/leukemias were observed as neoplastic lesions, but no significant differences were observed between the groups. As non-neoplastic lesions, degenerating changes, for example in the pancreatic acinar cells, were observed with XPA-/- mice tending to be more sensitive than XPA+/+ mice. The present study demonstrated that PhIP-DNA adduct formations in the liver, colon and lung of XPA-/- mice were demonstrated and their recovery rate was more delayed than XPA+/+ mice, and furthermore, more severe toxicity to PhIP in XPA-deficient mice was observed, but they were not susceptible to PhIP carcinogenicity under the conditions of the experiment.

Cancer chemoprevention through interruption of multistage carcinogenesis. The lessons learnt by comparing mouse skin carcinogenesis and human large bowel cancer

Whilst in the early stages, neoplastic development is predominantly triggered by environmental genotoxic and non-genotoxic carcinogens, tumour progression becomes more and more autonomous at later stages. In this context a dysregulation of arachidonic acid metabolism seems to play a disastrous role. Conversely, non-steroidal anti-inflammatory drugs (NSAIDs) rank among the most potent and most promising agents for cancer chemoprevention probably because of their ability to inhibit prostaglandin biosynthesis, in particular, at the level of the 'pro-inflammatory' enzyme cyclooxygenase-2 (COX-2). A pathological overexpression of COX-2 resulting in excessive prostaglandin production has been found already in early stages of carcinogenesis and seems to be a consistent feature of neoplastic development in a wide variety of tissues. COX-2 overexpression is thought to occur along signalling pathways of inflammation and tissue repair which become activated in the course of tumour promotion and, due to autocrine and auto-stimulatory mechanisms, finally lead to some autonomy of tumour development (self-promotion). Prostaglandins formed along a dysregulated COX pathway have been shown to mediate tumour promotion in animal experiments and may play a role, in addition, in other processes involved in tumour growth such as angiogenesis, metastasis and immunosuppression. Moreover, genotoxic byproducts such as organic free radicals, reactive oxygen species and malondialdehyde produced in the course of prostanoid biosynthesis may contribute to genetic instability (mutator phenotype) of neoplastic cells thereby promoting malignant progression. Such mixtures of physiologically highly active mediators and genotoxic byproducts are, in addition, formed along the various lipoxygenase-catalysed pathways of arachidonic acid metabolism some of which also become dysregulated during tumour development and, therefore, provide novel targets of future chemopreventive approaches.

In vitro analysis of transformation potential associated with retroviral vector insertions

While replication-defective retroviral vectors provide excellent vehicles for the long-term expression of therapeutic genes, they also harbor the potential to induce undesired genetic changes by random insertions into the host genome. The rate of insertional mutagenesis for retroviral vectors has been determined in several different assay systems; however, the rate at which such events induce cellular transformation has not been directly determined. Such measurements are critical to determining the actual risk of carcinogenesis resulting from retroviral gene therapy. In this study, the ability of a replication-defective retroviral vector, GlnBgSvNa, to induce cellular transformation in the BALB/c-3T3 in vitro transformation assay was assessed. The transformation frequency observed in vector-transduced BALB/c-3T3 cells, which contained one to six copies of integrated provirus, was not significantly different from that of untreated control cells. The finding that GlnBgSvNa was nontransforming in this assay indicates that the rate of transformation induced by retroviral insertions is less than the spontaneous rate of cellular transformation by BALB/c-3T3 cells, or less than 1.1 x 10(-5). These results are the first to define an upper limit for the rate of transformation induced by retroviral vectors.

[Carcinogenic effects of low radiation doses]

The carcinogenic effect of low doses of ionizing radiation has been a matter of much debate over the last few years. The French Academy of Science published a report on the subject in 1995. The central point of discussion concerns the validity of the linear no threshold relationship for estimating, by extrapolation, the carcinogenic effect of low doses. The aim of this article is to analyze the epidemiological data on the effect of low doses and the biological data on the mechanisms of carcinogenesis that have been obtained since 1995. These data strengthen doubts concerning the validity of current risk evaluations at low doses.

Contribution of human data to the analysis of human carcinogenesis

Human data strongly suggest that small doses or low concentrations of genotoxic agents cause only a relatively small number of human cancers. They emphasize the role of promotion, in particular that associated with cell proliferation. There is therefore a qualitative difference between high doses of genotoxic agents which provoke cell death and a compensatory increase in cell division, and low doses which do not. During the promotion phase, human data demonstrate the importance of induced genetic instability and defects in apoptosis as well as that of cell immortalization which play a main role for the accumulation in a cell genome of several specific lesions. Carcinogenesis is a complex process in which initial mutations do not appear to be a limiting or crucial step. This view is supported by the paramount influence of age on the induction by radiation of thyroid and breast cancer. It is also compatible with practical thresholds observed in subjects whose bones or liver were exposed to alpha-emitters, as well as with the curvilinearity in the leukemia incidence dose-response in the Japanese atomic bomb survivors. The linear no threshold model assumes that: 1) the probability of DNA lesion repair is constant whatever the dose and, hence, the number of lesions provoked in the same cell and the surrounding cells; 2) the probability for a damaged cell to evolve toward an invasive cancer is not influenced by the possible promotional effect of further irradiation or induced tissue proliferation, nor the control exerted by surrounding cells. These assumptions deserve a critical analysis.

The 1.5 GHz electromagnetic near-field used for cellular phones does not promote rat liver carcinogenesis in a medium-term liver bioassay

We have recently established that local exposure to a 929.2 MHz electromagnetic near-field, used for cellular phones, does not promote rat liver carcinogenesis in a medium-term bioassay system. In the present study, a 1.439 GHz electromagnetic near-field (EMF), another microwave band employed for cellular phones in Japan, was similarly investigated. Time division multiple access (TDMA) signals for the Personal Digital Cellular (PDC) Japanese cellular telephone standard system were directed to rats through a quarter-wavelength monopole antenna. Numerical dosimetry showed that the peak SARs within the liver were 1.91-0.937 W/kg, while the whole-body average specific absorption rates (SARs) were 0.680-0.453 W/kg, when the time-averaged antenna radiation power was 0.33 W. Exposure was for 90 min a day, 5 days a week, over 6 weeks, to male F344 rats given a single dose of diethylnitrosamine (200 mg/kg, i.p.) 2 weeks previously. At week 3, all rats were subjected to a two-thirds partial hepatectomy. At week 8, the experiment was terminated and the animals were killed. Carcinogenic potential was scored by comparing the numbers and areas of the induced glutathione S-transferase placental form (GST-P)-positive foci in the livers of exposed (48) and sham-exposed rats (48). Despite increased serum levels of corticosterone, adrenocorticotropic hormone (ACTH) and melatonin, the numbers and the areas of GST-P-positive foci were not significantly altered by the exposure. These findings clearly indicated that local body exposure to a 1.439 GHz EMF, as in the case of a 929.2 MHz field, has no promoting effect on rat liver carcinogenesis in the present model.

Spontaneous proliferative lesions in the nasopharyngeal meatus of F344 rats

Spontaneous proliferative lesions in the nasopharyngeal meatus were identified as the cause of death in 12 of 1,600 male and 5 of 1,600 female Fisher 344 (F344) rats used in 2-yr carcinogenicity studies; none of the lesions were considered treatment related. All the rats showed dyspnea, abdominal distension, and clinical deterioration. Gross features were characterized by simultaneous occurrence of conspicuous gaseous distension of the intestinal tract, especially in the ileum and cecum, and focal nodular lesions in the nasopharyngeal meatus. Histopathologically, the nasopharyngeal meatus was partially obstructed by the following proliferative lesions: 3 areas of hyperplasia of the ectopic sebaceous glands in the soft and hard palate, 4 areas of squamous metaplasia (SM) with massive hyperkeratosis, 5 squamous cell papillomas (SCPs), and 5 squamous cell carcinomas (SCCs). No pathological changes were found in the distended portion of the intestinal tract. Formalin-fixed, paraffin-embedded samples of the proliferative lesions from the nasopharyngeal meatus were examined for the presence of mutations in the c-H-ras and c-K-ras genes. In vitro amplification of DNA using a polymerase chain reaction was combined with a nonisotopic method for selective oligonucleotide hybridization. Two of the 4 SM lesions, 3 of the 5 SCPs, and 5 of the 5 SCCs contained 1-3 point mutations in the c-H-ras and/or c-K-ras gene. Immunohistochemically, overexpression of p53 protein was found in 1 area of SM with a dysplastic lesion and 2 SCCs. These findings indicate that detailed examination of the upper respiratory system, including the nasopharyngeal meatus, may be particularly helpful for identifying primary occult lesions in F344 rats that show only gut distension at necropsy in long-term toxicity studies. In addition, mutations of the ras genes may be an important step in the early stages of carcinogenesis in the rat nasopharyngeal meatus, whereas p53 mutations could occur relatively late.

Studies of initiation and promotion of carcinogenesis by N-nitroso compounds

Complete carcinogens must possess both initiating and promoting properties. Most N-nitroso compounds are mutagens and are considered to be initiators, but some are not mutagenic and yet are complete carcinogens. To investigate the two activities, brief treatments of male F344 rats with each of three mutagens, nitrosodimethylurea, nitrosodiethylurea and nitrosobis-(2-oxopropyl)-amine, were followed by chronic treatment (40 weeks) with one of four non-mutagens, nitrosomethyl-3-carboxypropylamine, nitrosodiethanolamine, nitrosomethyl-2-hydroxypropylamine or phenobarbital, the last being a well-known promotor of liver tumors in rats. Each treatment group consisted of 18 animals and there were control groups of 15 animals without initiation and 15 animals without promotion. All surviving rats were sacrificed at week 78. There were almost no tumors in untreated controls or in groups treated with the promotors, other than bladder tumors in one group. Certain tumors were numerous in the initiated groups, but there were only a few instances of increased incidences after treatment with the promotors. The action of the initiators appeared to be the dominant factor and there was scant indication in this experiment of the induction of tumors by the promotors of promotion of initiated cells in most organs (e.g. the liver). This indicates that it is unlikely that non-genotoxic carcinogens induce tumors by promotion of already-initiated cells, but that some other mechanism prevails.

Demonstration of ras and p53 gene mutations in carcinomas in the forestomach and intestine and soft tissue sarcomas induced by N-methyl-N-nitrosourea in the rat

The presence of ras family and p53 gene mutations in rat forestomach, intestine and liver tumors and soft tissue sarcomas induced by N-methyl-N-nitrosourea (MNU) was examined using polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) followed by direct sequencing analysis. In the forestomach squamous cell carcinomas (SCC), Ha-ras and p53 mutations were detected in 2 (40%) and 4 (80%) of 5 cases, respectively. The figures for Ki-ras and p53 gene mutations in adenocarcinomas of the large and small intestines were 3 (18.8%) and 5 (31.3%) of 16 cases. Soft tissue sarcomas in different sites were found to have mutations of Ki-ras in 7 (23.3%) and of p53 in 9 (30%) of 30 cases. One forestomach SCC and 2 soft tissue sarcomas had double p53 mutations in different exons. Single cases of forestomach SCC and intestinal adenocarcinoma had mutations in both Ki-ras and p53 genes. No mutations were found in counterpart benign tumors or hepatocellular adenomas. The p53 mutation spectrum revealed preferential clustering within exon 8 for the forestomach SCCs, and exons 5 and 8 for the intestinal adenocarcinomas, whereas the distribution was evenly spread through exons 5 to 8 in soft tissue sarcomas. All the detected ras or p53 mutations were G:C to A:T transitions. These results indicate firstly that specific Ki-ras, Ha-ras and p53 gene mutations in MNU-induced lesions are related to particular alkylation sites (G:C to A:T transitions) and secondly, although not essential, Ki-ras, Ha-ras or p53 gene mutations may be involved in the progression stage of forestomach, intestine and soft tissue neoplasms induced by MNU.