CONDITIONAL NEOPLASMS AND SUBTHRESHOLD NEOPLASTIC STATES : A STUDY OF THE TAR TUMORS OF RABBITS

New insights into the causes of cancer

Recent advances in molecular biologic analysis have led to major new insights concerning the genetic mechanisms underlying the development of cancer. This article examines the current state of our understanding of the genetic basis underlying the possible mechanisms of carcinogenesis and metastasis. The nature of the genetic lesions found in some cancer-causing genes, cancer-inhibiting genes, growth factor genes, and metastasis genes is discussed, as is the impact that these may have on clinical oncology.

International Commission for Protection Against Environmental Mutagens and Carcinogens. ICPEMC publication No. 19. The classification of carcinogens identified in the rodent bioassay as potential risks to humans: what type of substance should be tested next?

The relevance of rodent cancer bioassay data to humans is discussed in relation to the needs of regulatory agencies. The usefulness of in vivo and in vitro genotoxicity testing in this connection is also discussed. In the case of rodent carcinogens that do not elicit genotoxicity, it is suggested that homeostatic imbalance, cell proliferation, and other processes may play a major role in tumor development and its importance to the possible ability of the test agent to induce human cancer. These possibilities need to be evaluated on a case by case basis. The methods by which chemicals are selected for the rodent cancer bioassay are also discussed and it is pointed out that naturally-occurring constituents of human foods should in future receive greater priority as a consequence of anticipated changes resulting from biotechnology.

Why the incidence of cancer is increasing: the role of 'light pollution'

At present, cancer is responsible for almost half of all deaths among women 45-64 years of age, and about 30% of all deaths among men in the same age group (1). This high rate represents a marked increase from the end of the last century (2), and it probably has a multifactorial etiology. Air pollution, smoking, diet, alcohol, occupational exposures and stress are all considered as possible etiologic and risk factors (3). We put forward a hypothesis that one of the most important etiologic factors in the rapid growth rate of cancer is the change of light exposure that took place in the last 100 years, especially in the developed countries. Increased light exposure acting through the pineal gland reduces melatonin production, thereby diminishing the non-specific oncostatic effects of the pineal gland.

Carcinogenicity of polyhalogenated biphenyls: PCBs and PBBs

Polychlorinated biphenyls (PCBs) and polybrominated biphenyls (PBBs) are compounds whose physical/chemical properties led to their widespread commercial use. Although their production has been banned or severely limited in most countries since the 1970s, the persistence and stability of these compounds have resulted in a worldwide distribution, especially of PCBs. PBB contamination is limited principally to the state of Michigan, where a series of tragic errors eventually resulted in the accumulation of residues in livestock and the general human population. Long-term exposure to PCBs and PBBs in animals has been associated with the induction of neoplastic nodules in the liver and in some cases hepatocellular carcinoma. This review discusses the role of PCBs and PBBs in the process of carcinogenesis. The mutagenicity/genotoxicity of these compounds, as well as their initiation/promotion potential is discussed. The epidemiology of PCB and PBB exposure is reported along with an estimation of the risk of cancer to humans. Finally, possible molecular mechanisms of action are suggested for polyhalogenated biphenyls in cancer development.

Tumor-promoting activity and cytotoxicity of 3,4,3',4'-tetrachlorobiphenyl on N-nitrosomorpholine-induced murine liver foci

Effects of 3,4,3',4'-tetrachlorobiphenyl (TCB) on glucose-6-phosphatase (G6Pase)-altered hepatic foci of N-nitrosomorpholine (NNM)-treated B6C3F1 mice were investigated. TCB was chosen as a selective 3-methylcholanthrene-type inducer and tumor promoter. To initiate hepatocarcinogenesis, mice were treated with NNM (160 mg/l, in drinking water for 7 weeks), as in previous studies with the rat model. After a treatment-free interval of 22 weeks, TCB was administered (5 x 50 mg/kg, every 3 days), and liver foci were analysed 10 weeks after the start of TCB treatment. Unexpectedly, the number of G6Pase-negative and -positive foci per liver was markedly diminished following TCB treatment (to 32% and 57%, respectively). On the other hand, the mean volume of the remaining G6Pase-altered foci was enhanced, owing to an increase in the percentage of foci of large size (greater than 0.5 mm2). Throughout the experimental period of 39 weeks prolonged liver injury due to NNM and TCB treatment was demonstrated by histology and by elevated serum levels of glutamate-oxaloacetate transaminase. The results suggest that (in contrast to the rat system) TCB exhibited opposing effects on liver foci in the mouse model: (a) moderate tumor-promoting effects and (b) cytotoxic effects in NNM-injured liver, leading to decreased numbers of liver foci.

Retrotransposon-like VL30 elements are efficiently induced in anoxic rat fibroblasts

VL30 elements are a multigene family within the class of retroviruses and retrotransposons. We have characterized the response of normal rat fibroblasts to anoxia, in which endogenous VL30 element expression is strongly induced. Optimal induction up to 500-fold occurs under complete anoxia, although a lesser response is seen under atmospheres up to 2% oxygen. Phorbol esters and diacylglycerol, which induce mouse VL30 RNA approximately eightfold, show no effect on the rat VL30 system. The hypoxic conditions optimal for rat VL30 induction represent a mild cellular stress, with no reduction in cell viability during the induction period. Although the precise physiological role of this fibroblast response to temporary anoxia is unknown, it may occur during wound healing. The induction of VL30 by anoxia provides a unique model system wherein a member of the mammalian retrovirus/retrotransposon family is highly responsive to a common physiological signal.

Mechanisms of carcinogenesis with particular reference to the oral mucosa

Three types of stimulus, chemical, physical and viral, are known to be carcinogenic to susceptible animals. This review considers these stimuli and their possible mechanisms in general terms and their possible relevance to the induction of oral mucosal carcinoma in particular.

Studies of ionizing radiation as a promoter of neoplastic transformation in vitro

The induction of malignant transformation was examined in a standard promotion protocol in which BALB/3T3 cells were incubated continuously with tritiated water (3HOH) following acute treatment with various doses of either X-rays or benzo(a)pyrene (BP). In no case was there any evidence that protracted exposure to ionizing radiation from 3HOH enhanced the yield of transformants induced by the primary carcinogen over that predicted if the effects of the two agents were additive.

International Commission for Protection against Environmental Mutagens and Carcinogens. ICPEMC publication No. 13. The need for biological risk assessment in reaching decisions about carcinogens

The prudent assumption that carcinogen bioassays in rodents predict for human carcinogenicity is examined. It is suggested that in certain cases, as for example the induction of tumors against a high incidence in controls, or in situations in which high dose toxicity may be a critical factor in the induction of cancer, the probability that animal bioassays predict for humans may be low. The term 'biological risk assessment' is introduced to describe that part of risk assessment concerned with the relevance of specific animal results to the induction of human cancer. Biological risk assessment, which is almost entirely dependent on an understanding of carcinogenesis mechanisms, is an important addition to present mathematical modeling used to predict the effects of animal carcinogens that have been demonstrated after high dose exposure, to the effects of the much smaller doses to which humans are perceived to be exposed. Evidence for the conclusions reached by biological risk assessment may sometimes be supported by a careful review of human epidemiological data.

A nonmathematical view of mathematical models for cancer

A qualitative view of mathematical models for cancer is presented. The Armitage and Doll multistage and Moolgavkar two-stage models of cancer are discussed in terms of their physical models. Time-related factors for these models as well as some of their characteristics are presented. The effect of age at first exposure, duration of exposure, time since last exposure and stage of carcinogenic effect on risk are detailed.

Experimental induction of renal tumors

Renal tumors have been reported to be induced by natural products, chemical carcinogens, viruses, or radiation. Species or strain specificity and sex also appear to play significant roles in their development. In man, it is also likely that the heredity existence of other diseases, smoking, food habits, and irradiation may be etiological factors. It also appears that hormonal, chemical, and other environmental factors can play a role. Nephrotoxin modifies two-stage chemical carcinogenesis in rat kidney. Some nephrotoxins without carcinogenicity promote the development of renal tumors in rats pretreated with subcarcinogenic doses of chemical carcinogens. The importance of nephrotoxin in development of renal adenocarcinomas needs elucidation. Preneoplastic lesions in the kidneys can be recognized by histochemical methods with specific antibodies. It is hoped that further research will be continued, so that data obtained from experimental work will provide a better understanding of the etiology and induction of renal cancer in man.

Initiation and promotion of experimental oral mucosal carcinogenesis in mice

While there is considerable evidence that skin carcinogenesis proceeds as a step-wise series of changes, little evidence is available to indicate that a similar mechanism applies to oral mucosal carcinogenesis. In the current study, a mouse model of mucosal carcinogenesis was used to examine the effects of repeated applications of the phorbol ester phorbol-12,13-didecanoate (PDD) following various periods of treatment with the carcinogen 4-nitroquinoline-1-oxide (4NQO). All animals were histologically examined at 50 weeks. Oral epithelial dysplasia was seen in animals treated with 4NQO for as little as 2 weeks, and oral squamous cell carcinomas developed in all animals treated with 4NQO for 16 weeks. In those mice treated with PDD as well, carcinomas developed in mice receiving as little as 2 weeks treatment with 4NQO, and 100% of mice treated with 4NQO for 12 weeks, followed by PDD treatment, developed carcinomas. The results indicate that irreversible changes in the oral mucosa of mice occur relatively early during treatment with 4NQO, and the development of carcinomas can be enhanced with subsequent PDD treatment, giving an indication of staged carcinogenesis in this model system.

Models of hepatocarcinogenesis in the rat--contrasts and comparisons

A variety of approaches for the induction of altered hepatic foci, hyperplastic nodules, and hepatocellular carcinomas in rat liver have been developed. These protocols, aided by the appearance of preneoplastic lesions during the carcinogenic process, have proven to be very useful for examining many of the characteristics of events involved in rat hepatocarcinogenesis. A number of models have demonstrated distinct steps or stages in the progression of the carcinogenic process. These protocols are currently employed in the classification and distinction of agents effecting hepatocarcinogenesis at one or more of its stages. This review presents an overview of the present-day multistage hepatocarcinogenesis model systems in the rat, with contrasts and comparisons of these systems. The potential uses of the model systems in the identification of complete carcinogens, initiating agents, and promoting agents devoid of initiating action are discussed.

Wounding and its role in RSV-mediated tumor formation

Tumors induced in chickens by Rous sarcoma virus remain localized at the site of injection even though the animals become viremic. Tumors have now been shown to be inducible at other sites if a wound is inflicted or if the tissue is injured by administration of tumor promoters. These findings indicate that local wounding plays a role in the spread of tumorigenicity of Rous sarcoma virus.

Effects of phorbol myristate acetate, phorbol dibutyrate, ethanol, dimethylsulfoxide, phenol, and seven metabolites of phenol on metabolic cooperation between Chinese hamster V79 lung fibroblasts

The effect of phorbol myristate acetate, phorbol dibutyrate, ethanol, dimethylsulfoxide, phenol, and seven metabolites of phenol on metabolic cooperation were assessed as a function of mutant cell recovery from populations of cocultivated hypoxanthine-guanine phosphoribosyl transferase-deficient mutant (HGPRT-) and wild-type (HGPRT+) Chinese hamster V79 lung fibroblasts. Phorbol myristate acetate and phorbol dibutyrate, two established tumor promoters, were potent inhibitors of metabolic cooperation. Ethanol and dimethylsulfoxide, solvents commonly used to prepare chemicals for testing, weakly inhibited metabolic cooperation. Phenol and phenylglucuronide had no effect on metabolic cooperation. Four oxidative metabolites (1,4-benzoquinone, catechol, hydroxyquinol and quinol) inhibited metabolic cooperation. Phenylsulfate weakly inhibited metabolic cooperation. Conversely, 2-methoxyphenol, a methylated derivative of catechol, appeared to enhance metabolic cooperation. These results generally support the hypothesis that tumor promoters inhibit metabolic cooperation and illustrate the importance of considering metabolites when testing this hypothesis. The weak capacity of five metabolites of phenol to inhibit metabolic cooperation correlates with the weakness of phenol as a tumor promoter. Interpretation of these results is complicated because two metabolic cooperation-inhibiting metabolites (catechol and quinol) are nonpromoting when tested individually in the same assay where phenol shows promoting activity. Such metabolites may be incomplete (stage) promoters, and exposure to two or more may be required for a promoting effect. The significance of enhanced metabolic cooperation requires further investigation, particularly in relation to antipromoting effects.

In vivo induction of rat hepatic ornithine decarboxylase and plasminogen activator by 12-O-tetradecanoylphorbol 13-acetate

Rapid and substantial elevations in ornithine decarboxylase and plasminogen activator have been linked to tumor promotion in mouse epidermis and in vitro. Systemic administration of 12-O-tetradecanoylphorbol 13-acetate (TPA) rapidly increased both enzymic activities in rat liver. Pretreatment with either cycloheximide or actinomycin D attenuated both enzyme inductions. It is concluded that: (1) systemic TPA rapidly induces plasminogen activator and ornithine decarboxylase activities in rat liver; and (2) both inductions reflect de novo enzyme synthesis.

Promotion of epidermal carcinogenesis by repeated damage to mouse skin

Chemically induced epidermal carcinogenesis is usually divided into two stages: initiation, which involves the conversion of some epidermal cells into latent neoplastic cells; and promotion, which results in tumors. The hallmark of chemical promoters is epidermal hyperplasia. The hyperplasia caused by a strong promoter, such as 12-O-tetradecanoylphorbol-13-acetate (TPA), differs morphologically from that caused by weak promoters, such as acetic acid and mezerin. The epidermal regeneration that follows abrasion results in a hyperplastic epidermis that resembles the effects of strong promoters. Repeated mechanical injuries are capable of enhancing papillomas and carcinomas in mouse skin initiated with 7,12-dimethylbenzanthracene (DMBA). Thus, a regenerative epidermal hyperplasia appears to be a precondition for tumor promotion. It is highly probable that many epidermal cells are initiated during the lifetime of man. In the work place, repeated mechanical injury could predispose to epidermal neoplasms.

Genetic mode of action of cocarcinogens and tumor promoters in yeast and mice

In experiments with yeast, cocarcinogens were found to be comutagenic and antirecombinogenic , tumor promoters to be corecombinogenic and antimutagenic. Substances that were cocarcinogens as well as tumor promoters had an intermediary effect. These results were confirmed in the mammalian spot test: By in vivo treatment of mice with the cocarcinogen catechol and the tumor promoter limonene carcinogen-induced recombination due to mitotic crossing over and gene mutations was reduced and enhanced, respectively. Our results support the hypothesis that mutagenesis is the mechanism by which chemicals induce malignancy, and that cocarcinogens modify the process by enhancement of mutagenicity whereas tumor promoters effect carcinogenesis by increase of the spontaneous frequency of recombination. In addition, induced mitotic recombination in mammals in vivo has been demonstrated for the first time.

Role of oxygen radicals in tumor promotion

Tumor promoters provoke the elaboration of oxygen radicals by direct chemical generation and through the indirect activation or alteration of cellular sources including membrane oxidases, peroxisomes, and electron transport chains in mitochondria and endoplasmic reticulum. Although direct measurement of amplified oxygen radical production in response to tumor promoters in target tissues remains problematic, studies with scavengers of reactive oxygen species demonstrate inhibition of biochemical and biological sequelae of tumor promoter exposure and provide strong presumptive evidence for oxygen radical involvement in this late stage of carcinogenesis. The critical macromolecular targets for these oxygen radicals remain undefined; however, they may include lipids, DNA, DNA repair systems, and other enzymes.

The power and interpretation of the carcinogenicity bioassay

Carcinogenicity is a major consideration in the assessment of risks due to environmental chemicals. The carcinogen bioassay therefore is a very important component of the battery of toxicological tests used in hazard evaluation. The strengths and limitations of this bioassay are discussed with emphasis upon the unresolved practical considerations, the interpretation of negative results, the significance of tumors induced in the presence of a high background incidence of naturally occurring tumors, and the difficulties in transspecies extrapolation. These factors, in combination with consideration of the biological mechanisms of chemical cancer induction, will be valuable in assessing the potential risk to man posed by individual chemicals.

Spontaneous regression of hepatocellular carcinoma: a case study

A Chinese patient with documented hepatocellular carcinoma (HCC) satisfied the criteria of Everson and Cole for spontaneous regression of malignant tumors. Subsequently he survived a tumor-free period of at least 13 years. During the period of regression, shrinkage of liver coincided with a rise of SGOT to a level comparable to that reported for patients with liver cancer during hepatic arterial ligation and cytotoxic therapy. Postregression liver biopsy from the site of the previous tumor revealed relatively uninflamed HBsAg-positive tissue without dysplasia. The case provided the positive end of the survival spectrum in HCC, evidence that regression of HCC might occur by involution rather than maturation, and histologic data suggesting that regressed HCC might be replaced by surrounding tissue instead of leaving behind dysplasia.

Spontaneous regression of hepatocellular carcinoma: a case study

A Chinese patient with documented hepatocellular carcinoma (HCC) satisfied the criteria of Everson and Cole for spontaneous regression of malignant tumors. Subsequently he survived a tumor-free period of at least 13 years. During the period of regression, shrinkage of liver coincided with a rise of SGOT to a level comparable to that reported for patients with liver cancer during hepatic arterial ligation and cytotoxic therapy. Postregression liver biopsy from the site of the previous tumor revealed relatively uninflamed HBsAg-positive tissue without dysplasia. The case provided the positive end of the survival spectrum in HCC, evidence that regression of HCC might occur by involution rather than maturation, and histologic data suggesting that regressed HCC might be replaced by surrounding tissue instead of leaving behind dysplasia.

The natural history of neoplastic development: the relation of experimental models to human cancer

The natural history of the development of neoplasia in experimental systems may be separated into at least three different stages, those of initiation, promotion and progression. Evidence for such distinct stages has been demonstrated in at least half a dozen different experimental systems. The stage of promotion is that stage most easily modulated. The actions of promoting agents are reversible and only effective above certain threshold levels of the promoting agent. This is in contrast to the stage of initiation, which can be induced at any dose of the carcinogenic agent in an irreversible manner. Agents exhibiting both initiating and promoting activities are termed complete carcinogens while incomplete carcinogens are those capable only of initiation. Promoting agents do not initiate but may promote cells initiated by ambient environmental means, giving the appearance of complete carcinogens in standard bioassay procedures. The stages of initiation and promotion have been extensively studied in skin and liver carcinogenesis and show almost identical characteristics. A variety of promoting agents in the human environment have been demonstrated both by experimental and epidemiologic methodologies. The importance of an understanding of tumor promotion in relation to the prevention of human cancer is emphasized.

The role of inhibited cell-cell communication in teratogenesis

A mechanistic link between teratogenesis and carcinogenesis has been suggested by a wide variety of scientific observations. This report attempts to provide a theoretical explanation for one of the several possible mechanisms which might be shared during carcinogenesis and teratogenesis. The initiation and promotion concept of carcinogenesis was briefly reviewed and the role of intercellular communication during the complex tumor promotion phase was discussed. Inhibition of intercellular communication by a wide variety of physical, chemical and biological factors was speculated to disrupt the regulation of proliferation and differentiation in stem cells. Chemicals, which interfered with intercellular communication during early organogenesis, have the potential of being teratogens, while if they are present in the developed, initiated organisms have the potential of being tumor promoters. Evidence was presented showing that known tumor promoters which inhibited intercellular communication also had been shown to be teratogens. It was concluded that in vitro assays, designed to measure intercellular communication, although having known limitations, might be used as an in vitro means to screen for potential teratogens.

Searches for ultimate chemical carcinogens and their reactions with cellular macromolecules

Studies on a variety of chemical carcinogens have demonstrated that their ultimate reactive and carcinogenic forms are strong electrophiles. Some carcinogens, such as alkylating agents, are in their ultimate forms as administered, but most require metabolism to these active derivatives. The ultimate carcinogens react, usually non-enzymatically, with nucleophilic constituents in vivo. Of particular interest in regard to their possible importance in carcinogenesis have been the covalent interactions of these electrophilic reactants with cellular informational macromolecules, the DNAs, RNAs, and proteins. Current data are consistent with the idea that the initiation step of chemical carcinogenesis is a mutagenic event and is caused by alteration of DNA by the ultimate carcinogens. The nature of the carcinogen metabolite(s) involved in the promotion phase has not been determined, but there appears to be no requirement that they be electrophilic. The development of the concept of ultimate chemical carcinogens as strong electrophilic reactants is reviewed, especially with respect to the studies carried in the authors' laboratory.

Mechanisms of chemical carcinogenesis

Of the known carcinogenic agents (viruses, ultraviolet and ionizing radiations, and chemicals), chemicals appear to be of major importance in the induction of human cancers. The known chemical carcinogens include a wide range of structures. Their common feature is that their ultimate forms are electrophilic reactants; in most cases, these reactants arise through metabolism in vivo. Carcinogenesis by chemicals is a multistage process. The first stage, initiation, occurs rapidly and appears to be irreversible. The available data indicate that initiation generally results from one or more mutations of cellular DNA. Covalent reactions of electrophilic derivatives of carcinogens with DNA are the major cause of these mutations. The second stage, promotion, occurs over a longer period of time. Promotion is a complex process, for which the early stages are largely reversible. The critical events appear to be epigenetic. Complete carcinogens have both initiating and promoting activities, but the ratios of these two activities for various chemicals may differ greatly. This knowledge of the mechanisms of carcinogens by chemicals provides a useful basis for approaches to the prevention of human cancer.

Who is afraid of carcinogens?

The reevaluation of previously reported studies in carcinogenesis indicates the existence of a mechanism which protects mice from the carcinogenic hazard, Mice had been subjected to ultraviolet radiation (U.V.), applied at regular doses and intervals (10). The time from the first application to the appearance of a 100 mm3 sized tumor was defined as "development time" td. The td times had been found to be distributed lognormally. Such a distribution indicates that the magnitude of td is proportional to the carcinogenic exposure which means that the longer a mouse is exposed to U.V. the slower the progression of its neoplasms. This may be evident also from the lognormal risk function or conditional failure rate lambda (td) which initially rises in these animals to a maximum, whereupon it declines. The declining risk function supports the existence of a protective mechanism in the body which withstands carcinogenesis.

Induction of the deficient acid DNAse activity in mouse interfollicular epidermis by croton oil as a possible tumor promoting mechanism

Histochemical activity of acid DNAse, intensity of nucleic acid staining and histological alterations in mouse interfollicular epidermis (I.F.E.) were investigated after a single dose or after chronic topical administration of two hyperplastic agents, of which one (croton oil) was a potent tumor promotor, and the other one (podophyllin) did not promote skin carcinogenesis. Podophyllin induced intense uniform I.F.E. hyperplasia without any proliferation of poorly differentiated basal cells, without increased nucleic acid staining and without any appreciably decreased acid DNAse activity. On the other hand, croton oil (as well as TPA) produced almost immediate, distinct hyperplasia of poorly differentiated basal cells with increased intensity in the staining of both nucleic acids and nearly complete deficiency in acid DNAse activity. Similar histochemical and histological patterns were observed at the sites of wounding hyperplasia in untreated control mice. Such wounding hyperplasia was thought also to be a tumor promoting factor. It was suggested that the decrease in acid DNAse activity which occurred almost immediately after administration of potent tumor promoters and which could not be induced by a hyperplastic agent without tumor promoting action may have a particular importance in the mechanisms of tumor promotion.