Identification and assessment of tumor-promoting and cocarcinogenic agents: state-of-the-art in vitro methods

Co-carcinogens of the initiation-(or tumor)promoter type as environmental risk factors of cancer in man, experimental analysis of an etiologic model situation of life style cancer and current problems of assessment of cancer risk in multifactorial carcinogenesis

The identification of so-called co-carcinogens i.e. "per se essentially non-carcinogenic amplifiers" of carcinogenesis - especially promoters of initiation(or tumors) - as a new, non-classical group of risk factors has added a new dimension to the etiology of cancer in man. Indeed, in real human life, multifactorial causation of cancer may be considered the rule and classical unifactorial causation the exception. This comprehensive and current concept of the etiology of cancer in man and certain prototype experimental models of multifactorial carcinogenesis allow for proposals of a standardized terminology which may be adequate for common use in both, experimental and epidemiological oncology. On the background of a recent experimental analysis of life style esophageal cancer the current situation regarding the scientific basis of classification and grading of carcinogenic risk factors will be reviewed. It may be helpful in legislation aiming at more environmental safety.

Free radicals as mediators of tissue injury and disease

A radical is any molecule that contains one or more unpaired electrons. Radicals are normally generated in many metabolic pathways. Some of these radicals can exist in a free form and subsequently interact with various tissue components resulting in dysfunction. The potential role of oxygen- or xenobiotic-derived free radicals in the pathology of several human diseases has stimulated extensive research linking the toxicity of numerous xenobiotics and disease processes to a free radical mechanism. However, because free radical-mediated changes are pervasive and often poorly understood, the question of whether such species are a major cause of tissue injury and human disease remains equivocal. This review discusses cellular sources of various radical species and their reactions with vital cellular constituents. Examples of purported free radical-mediated disorders are discussed in detail to provide insights into the controversy over whether free radicals are important mediators of tissue injury.

Comparison of the effects of auranofin and retinoic acid on plasminogen activator activity of peritoneal macrophages and Lewis lung carcinoma cells

Urokinase-type plasminogen activator, a neutral proteinase, seems to play a central role in the degradation of the extracellular matrix that accompanies a number of biological phenomena including inflammatory reactions and neoplasia. The effect of auranofin and retinoic acid on the plasminogen activator activity expressed by two cell types, i.e. murine macrophages and Lewis lung carcinoma cells, has been investigated. Low concentrations of both drugs (10(-6)-10(-7) M) can inhibit in vitro the induction of plasminogen activator in macrophages stimulated by phorbol 12-myristate 13-acetate. This action occurs rapidly (15 min), is irreversible and is independent of a global cytotoxic effect. Auranofin and retinoic acid remain without effect in macrophages when added after stimulation by the phorbol ester. Both drugs are thus potent inhibitors of the induction of plasminogen activator activity in macrophages, possibly through an interaction with the protein kinase C system. The plasminogen activator activity of Lewis lung carcinoma cells, which is apparently not dependent on a protein kinase C pathway, is not influenced by auranofin or retinoic acid. These observations may contribute to explain: (1) the activity of auranofin and retinoic acid in rheumatoid arthritis, and (2) the antitumor promoting activity of retinoic acid. It would be relevant to assess whether auranofin may exhibit, like retinoic acid, an antitumor-promoting activity.

Characterization of lymphocyte stimulation by phorbol related compounds

1. The stimulation of (3H)-thymidine incorporation by lymphocytes in response to phorbol and seven other phorbol related compounds was investigated. 2. Lymphocytes from each of a small group of individuals were treated with the test compounds over wide concentration ranges. 3. All the tested compounds, including the most active, 12-0-tetradecanoylphorbol-13-acetate, were far less effective lymphocyte mitogens than the plant lectin phytohaemagglutinin. 4. Inter-individual differences were detected in the maximum response to the phorbols but not in their stimulating potency, as estimated by the concentration producing a half maximal response. 5. The rank order of the lymphocyte stimulating potency of the tested compounds was similar to the rank order of both tumour promoting activity and irritant potency in mouse skin. 6. Lymphocyte stimulation was paralleled equally well by these two mouse skin responses.

An approach to the determination of the relative potencies of chemical agents during the stages of initiation and promotion in multistage hepatocarcinogenesis in the rat

The potency of carcinogenic agents in eliciting neoplastic lesions has long been a concern of investigators in the field of oncology. This paper describes a method, based on quantitative stereologic calculations, to estimate the relative potency of chemicals as initiating and/or promoting agents. The parameters defined in this paper are: (a) Initiation index = no. foci induced X liver-1 X [mmole/kg body weight]-1; and (b) Promotion index = Vf/Vc X mmol-1 X wk-1. These parameters have been calculated for a number of chemical agents, based both on data from this laboratory and others published in the literature. Neither parameter varied significantly with the dose of two different initiating agents used in this study. The range of promotion indices extended over more than eight orders of magnitude, whereas that of the initiation indices was much less variable. Such parameters may be useful as quantitative estimates of the potency of hepatocarcinogenic agents not only in rodents, but potentially in quantitative risk estimations in the human.

Lung organoid culture

An organoid culture system for lung cells is described in which morphogenesis of lung histotypic structures and differentiation of both pneumocytes type II and mesenchyme occur. The principle of this technique is the culture of mouse fetal lung cells at high density on a membrane filter at the medium/air interface. In the course of cultivation, cell sorting-out, epithelial cell aggregation, formation of an alveolar-like lumen in the organoids and formation of a basal lamina occur. Epithelial differentiation culminates in the production of lamellar bodies, and the mesenchyme develops into mature connective tissue. Morphogenesis and differentiation depend on the stage of fetal development from which the lung cells were derived but appear independent of the formation of a basal lamina. Various drugs have been tested for their effects on morphogenesis and differentiation in this lung organoid culture: some of them inhibit differentiation or damage the mesenchyme, others stimulate surfactant production. Due to the quite complex morphogenetic and cellular events occurring in lung organoid culture, it may be an applicable tool for alternative in vitro screening methods.

Regeneration and the mechanism of epidermal tumor promotion

Chemically induced epidermal carcinogenesis is often divided into two stages: initiation, which involves the conversion of some epidermal cells into latent neoplastic cells, and promotion, which allows the evolution of this neoplastic change into the formation of a neoplasm. The hallmark of epidermal tumor promotion is the transformation of the normal epidermis into a hyperplastic epidermis. A major unanswered question about epidermal tumor promotion is whether the epidermal hyperplasia that characterizes promoted skin is a regenerative epidermal hyperplasia resulting from damage produced by the promoter. The opinion currently held is that the epidermal hyperplasia produced by tumor promoters is not simply a regenerative epidermal hyperplasia and possesses characteristics which a regenerative hyperplasia does not have, enabling it to evolve into an epidermal neoplasm. The purpose of this review is to present recent evidence which strongly suggests that promoter-induced epidermal hyperplasia is a regenerative hyperplasia. Three principal lines of evidence are reviewed. The first demonstrates that an epidermal regenerative hyperplasia repeatedly produced by wounding or abrasion can promote epidermal carcinogenesis in the initiated skin of mice. The second line of evidence demonstrates that the epidermal hyperplasia produced by the application of 12-O-tetradecanoyl-phorbol-13-acetate (TPA), the most powerful and widely used promoter of skin carcinogenesis, is preceded by damage to the epidermis. This strongly suggests that the epidermal hyperplasia which ensues is a regenerative hyperplasia. Thirdly, evidence is presented which demonstrates that hyperplasia-producing agents which do not promote, produce an epidermal hyperplasia which is different from that produced by tumor promoters. Finally, the review discusses the evidence which suggests that the production of a hyperplasia may be the mechanism for tumor promotion in other organs, such as the liver, bladder, and intestine.