Comparison of the skin tumor-promoting potential of different organic peroxides in SENCAR mice

Relevance of the mouse skin initiation-promotion model for the classification of carcinogenic substances encountered at the workplace

The Permanent Senate Commission for the Investigation of Health Hazards of Chemical Compounds in the Work Area (MAK Commission of the Deutsche Forschungsgemeinschaft) evaluates chemical substances using scientific criteria to prevent adverse effects on health at the work place. As part of this task there is a need to evaluate tumor promoting activity of chemicals (enhancement of formation of squamous cell carcinomas via premalignant papillomas) obtained from two-stage initiation/promotion experiments using the mouse skin model. In the present communication we address this issue by comparing responses seen in mouse skin with those in humans. We conclude that tumor promotional effects seen in such animal models be carefully analyzed on a case by case basis. Substances that elicit a rather non-specific effect that is restricted to the high dose range are considered to be irrelevant to humans and thus do not require classification as carcinogens. In contrast, substances that might have both a mode of action and a potency similar to the specific effects seen with TPA (12-O-tetradecanoylphorbol-13-acetate), the prototype tumor promoter in mouse skin, which triggers receptor-mediated signal cascades in the very low dose range, have to be classified in a category for carcinogens.

Chemoprevention of two-stage skin cancer in vivo by Saraca asoca

Saraca asoca (Family Caesalpiniaceae) has been widely used in traditional Indian medicine especially due to its wound-healing property. The present study investigates the chemopreventive property of flavonoids from Saraca asoca (flowers) on 2-stage skin carcinogenesis in mice models. Skin cancer was induced in Swiss albino mice by single topical application of 7,12-dimethyl benzanthracene (100 µg/50 µL of acetone) followed by thrice a week treatment of croton oil for 20 weeks. The topical pretreatment of the flavonoid fraction from S asoca (FF S asoca) was 30 minutes prior to the application of croton oil thrice weekly for 20 weeks. At the end of the experimental period the animals were sacrificed, and the tumor statistics and various marker parameters were studied (enzyme assays, Western blotting). The pretreatment of the FF of S asoca caused significant reduction in the number of tumors per mouse and the percentage of tumor-bearing mice. Also, the latency period for the appearance of the first tumor was delayed by S asoca pretreatment. In plant-treated animals there was a significant increase in the levels of reduced glutathione, catalase, and protein in skin when compared with the untreated animals. Conversely, there was a significant decrease in the lipid peroxidation levels. A significant reduction in the expression of ornithine decarboxylase, a key enzyme in the promotion stage of 2-stage skin cancer, in the plant-treated group was also observed. These findings suggest the chemopreventive activity of flavonoids from S asoca on 2-stage skin carcinogenesis.

Oxidative-dependent integration of signal transduction with intercellular gap junctional communication in the control of gene expression

Research on oxidative stress focused primarily on determining how reactive oxygen species (ROS) damage cells by indiscriminate reactions with their macromolecular machinery, particularly lipids, proteins, and DNA. However, many chronic diseases are not always a consequence of tissue necrosis, DNA, or protein damage, but rather to altered gene expression. Gene expression is highly regulated by the coordination of cell signaling systems that maintain tissue homeostasis. Therefore, much research has shifted to the understanding of how ROS reversibly control gene expression through cell signaling mechanisms. However, most research has focused on redox regulation of signal transduction within a cell, but we introduce a more comprehensive-systems biology approach to understanding oxidative signaling that includes gap junctional intercellular communication, which plays a role in coordinating gene expression between cells of a tissue needed to maintain tissue homeostasis. We propose a hypothesis that gap junctions are critical in modulating the levels of second messengers, such as low molecular weight reactive oxygen, needed in the transduction of an external signal to the nucleus in the expression of genes. Thus, any comprehensive-systems biology approach to understanding oxidative signaling must also include gap junctions, in which aberrant gap junctions have been clearly implicated in many human diseases.

Inhibition of Gap Junctional Intercellular Communication and Activation of Mitogen-Activated Protein Kinase by Tumor-Promoting Organic Peroxides and Protection by Resveratrol

Dicumyl peroxide (di-CuOOH) and benzoyl peroxide (BzOOH) act as tumor promoters in SENCAR mice, whereas di-tert-butylhydroperoxide does not. Tumor promotion requires the removal of growth suppression by inhibition of gap junctional intercellular communication (GJIC) and the induction of mitogenic intracellular pathways. We showed that di-CuOOH and BzOOH both reversibly inhibited GJIC and transiently activated mitogen-activated protein kinase, specifically, the extracellular receptor kinase at noncytotoxic conditions in WB-F344 rat liver epithelial cells, whereas the non-tumor-promoting di-tert-butylhydroperoxide did not inhibit GJIC or activate extracellular receptor kinase. di-CuOOH but not BzOOH inhibited GJIC through a phosphatidylcholine-specific phospholipase C-dependent mechanism. N-acetylcysteine (NAC) was needed to prevent a cytotoxic, glutathione-depleting effect of BzOOH, whereas di-CuOOH was noncytotoxic and did not alter glutathione levels at all doses and times tested. Pretreatment of WB-F344 cells with resveratrol, a polyphenolic antioxidant present in red wine, prevented at physiological doses the inhibition of GJIC by di-CuOOH but not from BzOOH and was effective in significantly preventing extracellular receptor kinase activation by both peroxides. NAC did not prevent any of the peroxide effects on either GJIC or extracellular receptor kinase, suggesting a specific antioxidant effect of resveratrol.

Specific induction of oxidative stress in terminal bronchiolar Clara cells during dimethylarsenic-induced lung tumor promoting process in mice

The induction of oxidative stress in pulmonary cells during the process of lung tumor promotion by dimethylarsinic acid (DMA), a main metabolite of inorganic arsenics in mammals, was examined by immunohistochemical analysis using a specific antibody against 4-hydroxy-2-nonenal (4HNE) adducts, which are major aldehydic metabolites of lipid peroxidation. We demonstrated that 4HNE-modified proteins existed specifically in the secretory granules in terminal bronchiolar Clara cells. Furthermore, the degree of positive staining increased with the duration of DMA administration. Transmission electron microscopy revealed morphological changes in the Clara cells of DMA-treated mice. These results suggest that Clara cells are the major target cell for DMA-induced oxidative stress and that the cells may play an important role in the lung tumor promotion process in mice.

The emperor wears no clothes in the field of carcinogen risk assessment: ignored concepts in cancer risk assessment

The following is a position paper challenging the paradigm that 'carcinogen = mutagen', and that the current rodent bioassay to predict risks to human cancers is relevant and useful. Specifically, we review current observations concerning carcinogenesis that might lead to another approach for assessing the identification of human carcinogenic hazards and the risk assessment that chemicals might pose. We give a brief review of the multistage and multimechanism process of cancer in a tissue that involves not only genotoxic but also epigenetic events, and the importance of stem and progenitor cells in the development of cancer. We focus on the often ignored 'epigenetic' effects of carcinogens and the role of cell communication systems in epigenetically altering gene expression that leads to an imbalance of cell proliferation, differentiation and apoptosis in a tissue that can contribute to the cancer process. To draw attention to the fact that the current paradigm and policy to test toxic chemicals is often misleading and incorrect, we discuss how oxidative stress, in spite of the DNA damaging data, most probably contributes to cancer at the epigenetic level. Additionally, we briefly review how this mutagenic concept has greatly diverted attention away from doing research on the lower molecular weight, non-genotoxic, polycyclic aromatic hydrocarbons (PAHs), and how these low molecular weight PAHs are etiologically more relevant to the disease potential of environmental mixtures such as cigarette smoke.

Responses of tumorigenic and non-tumorigenic mouse lung epithelial cell lines to electrophilic metabolites of the tumor promoter butylated hydroxytoluene

A model system to investigate the promotion phase of pulmonary carcinogenesis involves chronic exposure of carcinogen-initiated mice to the food additive, butylated hydroxytoluene (BHT). Previous studies strongly suggested that this activity is due to the cytochrome p450-catalyzed formation of quinone methides 2,6-di-tert-butyl-4-methylenecyclohexa-2,5-dienone (BHT-QM) and 6-tert-butyl-2-(1',1'-dimethyl-2'-hydroxy)ethyl-4-methylenecyclohexa-2,5-dienone (BHTOH-QM). The effects of these electrophiles on non-tumorigenic C10 and E10 epithelial cell lines derived from a normal mouse lung explant were compared with effects on their corresponding neoplastic siblings, the A5 and E9 spontaneous transformants, respectively. The tumorigenic cells were more resistant to cell killing, with LC(50) values of 165-180 microM for BHT-QM and 12-22 microM for BHTOH-QM, versus LC(50) values in the non-tumorigenic cells of 105-118 microM and 5.0-6.0 microM, respectively. Constitutive glutathione (GSH) concentrations were 12-20 nmol/10(6) cells, and BHT-QM toxicity was enhanced >2-fold by depleting GSH with buthionine sulfoximine (BSO). Formation of the GSH conjugate of BHT-QM accounted for a substantial fraction of the cellular GSH lost by quinone methide exposure. Enhanced lipid peroxidation and superoxide formation occurred in all cell lines treated with BHT-QM, but both tumorigenic lines contained higher levels of GSH S-transferase and superoxide dismutase (SOD) activities. These data suggest the possibility that BHT-derived quinone methides may exert their promoting effects by inducing oxidative stress; such stress is better tolerated by tumorigenic cells, which have higher levels of antioxidant enzymes. Normal cells are destroyed more readily which allows neoplastic cells to expand their proliferation.

Selective peroxidation and externalization of phosphatidylserine in normal human epidermal keratinocytes during oxidative stress induced by cumene hydroperoxide

Reactive oxygen species not only modulate important signal transduction pathways, but also induce DNA damage and cytotoxicity in keratinocytes. Hydrogen peroxide and organic peroxides are particularly important as these chemicals are widely used in dermally applied cosmetics and pharmaceuticals, and also represent endogenous metabolic intermediates. Lipid peroxidation is of fundamental interest in the cellular response to peroxides, as lipids are extremely sensitive to oxidation and lipid-based signaling systems have been implicated in a number of cellular processes, including apoptosis. Oxidation of specific phospholipid classes was measured in normal human epidermal keratinocytes exposed to cumene hydroperoxide after metabolic incorporation of the fluorescent oxidation-sensitive fatty acid, cis-parinaric acid, using a fluorescence high-performance liquid chromatography assay. In addition, lipid oxidation was correlated with changes in membrane phospholipid asymmetry and other markers of apoptosis. Although cumene hydroperoxide produced significant oxidation of cis-parinaric acid in all phospholipid classes, one phospholipid, phosphatidylserine, appeared to be preferentially oxidized above all other species. Using fluorescamine derivatization and annexin V binding it was observed that specific oxidation of phosphatidylserine was accompanied by phosphatidylserine translocation from the inner to the outer plasma membrane surface where it may serve as a recognition signal for interaction with phagocytic macrophages. These effects occurred much earlier than any detectable changes in other apoptotic markers such as caspase-3 activation, DNA fragmentation, or changes in nuclear morphology. Thus, normal human epidermal keratinocytes undergo profound lipid oxidation with preference for phosphatidylserine followed by phosphatidylserine externalization upon exposure to cumene hydroperoxide. It is therefore likely that normal human epidermal keratinocytes exposed to similar oxidative stress in vivo would under go phosphatidylserine oxidation/translocation. This would make them targets for macrophage recognition and phagocytosis, and thus limit their potential to invoke inflammation or give rise to neoplastic transformations.

Biodegradable injectable in situ forming drug delivery systems

The ability to inject a drug incorporated into a polymer to a localized site and have the polymer form a semi-solid drug depot has a number of advantages. Among these advantages is ease of application and localized, prolonged drug delivery. For these reasons a large number of in situ setting polymeric delivery systems have been developed and investigated for use in delivering a wide variety of drugs. In this article we introduce the various strategies that have been used to prepare in situ setting systems, and outline their advantages and disadvantages as localized drug delivery systems.

Toxicant-induced oxidative stress in cancer

The article highlighted in this issue is "The Role of Oxidative Stress in Indium Phosphide-Induced Lung Carcinogenesis in Rats" by Barbara C. Gottschling, Robert R. Maronpot, James R. Hailey, Shyamal Peddada, Cindy R. Moomaw, James E. Klaunig, and Abraham Nyska (pp. 28-40). The article integrates a traditional pathologic study of toxicant-induced pulmonary carcinogenesis with an immunohistologic assessment of oxidative stress, thereby determining a potential mechanism of action of a toxicant, specifically indium phosphide.

Oral exposure of dimethylarsinic acid, a main metabolite of inorganic arsenics, in mice leads to an increase in 8-Oxo-2'-deoxyguanosine level, specifically in the target organs for arsenic carcinogenesis

We have proposed that oral administration of dimethylarsinic acid (DMA), a metabolite of inorganic arsenics in mammals, rather than inorganic arsenics themselves, promotes lung and skin tumors by way of the metabolic production of free radicals such as dimethylarsenic peroxy radical [(CH(3))(2)AsOO*]. The purpose of the present study was to examine if dimethylarsenic has the ability to induce oxidative damage. 8-oxo-2'-deoxyguanosine (8-oxodG) was used as a biomarker of DNA oxidation. The oral administration of DMA enhanced significantly the amounts of 8-oxodG specifically in the target organs (skin, lung, liver, and urinary bladder) of arsenic carcinogenesis and also in urine, whereas arsenite did not. The dimethylarsenics thus may play an important role in arsenic carcinogenesis through the induction of oxidative damage, particularly of base oxidation.

Oral administration of dimethylarsinic acid, a main metabolite of inorganic arsenic, in mice promotes skin tumorigenesis initiated by dimethylbenz(a)anthracene with or without ultraviolet B as a promoter

Concerning arsenic-induced tumorigenesis, an animal model must be developed for understanding the mechanism of human carcinogenesis by arsenics. To determine whether orally administered dimethylarsinic acid (DMA) promotes and causes the progression of skin tumorigenesis, an animal protocol by topical application of dimethylbenz(a)anthracene (DMBA) with or without UVB, a tumor promoter, in hairless mice was used. The administration of DMA by the oral route promoted not only the formation of papillomas induced by DMBA alone but also the formation of malignant tumors induced by way of the formation of atypical keratoses by treatment with DMBA and UVB. A phenomenon, the progression of keratoses-->atypical keratoses-->squamous cell carcinomas (SCCs), observed in the present study may resemble the development of tumors in arsenic-exposed humans. We also discussed the involvement of a reactive oxygen species (ROS), e.g., the dimethylarsenic peroxy radical [(CH3)2AsOO.], produced during the metabolic processing of DMA, in skin and in multi-organ tumorigenesis.

Analysis of c-Ha-ras gene mutations in skin tumors induced in carcinogenesis-susceptible and carcinogenesis-resistant mice by different two-stage protocols or tumor promoter alone

In the present study we describe the molecular analysis of c-Ha-ras gene mutations in 47 papillomas and 17 carcinomas developed in two lines of mice, carcinogenesis-susceptible (Car-S) and carcinogenesis-resistant (Car-R), selectively bred for extreme susceptibility or resistance to chemical skin carcinogenesis initiated and promoted with different doses of 7,12-dimethylbenz[a]anthracene (DMBA) and 12-O-tetradecanoylphorbol-13-acetate (TPA). This study also presents the analysis of c-Ha-ras gene mutations in 22 papillomas and 22 carcinomas in Car-S mice initiated with DMBA and promoted with benzoyl peroxide (BzPo) and in seven papillomas and one carcinoma from a group of uniniated Car-S mice that received only BzPo treatment. The data showed that a A(182)-->T transversion in the c-Ha-ras gene was present in 100% and 81% of the skin tumors developed in Car-S and Car-R mice, respectively, after DMBA initiation and TPA promotion, suggesting that differences in genetic susceptibility can influence the frequency of c-Ha-ras mutations in the skin tumors produced. The same A(182)-->T mutation with an incidence of 68% was found in papillomas from DMBA-initiated and BzPo-promoted Car-S mice. The difference in the mutation frequency between DMBA/BzPo and DMBA/TPA papillomas suggested that the promotion step contributes to the final mutation pattern. The tumor induction experiment with BzPo alone showed that this compound can induce tumor development in 26% of Car-S mice, and the molecular analysis of the tumors showed a broad mutation spectrum, including mutations in codons 12, 13, and 61 of the c-Ha-ras gene. Mol. Carcinog. 30:111-118, 2001.

The role of orally administered dimethylarsinic acid, a main metabolite of inorganic arsenics, in the promotion and progression of UVB-induced skin tumorigenesis in hairless mice

The effect of dimethylarsinic acid (DMA) on skin tumorigenesis by UVB irradiation was examined. Hairless mice (Hos: HR-1) irradiated with UVB at a dose of 2 kJ/m(2) twice weekly, were fed with drinking water containing 1000 ppm DMA, a main metabolite of inorganic arsenics, produced more skin tumors than DMA-untreated mice. Histopathological examination revealed that the mouse malignant tumors with severe atypism appeared only in the treatment group of UVB plus 1000 ppm DMA. These positive results point out the importance of dimethylated metabolites of inorganic arsenic in the process of skin carcinogenesis.