Cutaneous chemical carcinogenesis: past, present, and future

Efeito de promotores tumorais em pele de gerbil, Meriones unguiculatus

Supondo que a resistência relativa da pele de gerbil adulto à carcinogênese química estaria relacionada a um fenômeno de adaptação ao processo de promoção tumoral, foi caracterizado o efeito de óleo de cróton (OC) e peróxido de benzoíla (PB) sobre a pele de gerbil, através do estudo das alterações morfológicas, correlatas à atividade promotora e induzidas em função da dose administrada, da freqüência do tratamento e da associação ao iniciador metilcolantreno (MC) em modelos bifásicos e trifásicos de carcinogênese. Verificou-se que uma única aplicação tópica de 00 (0,94 mg e 1,88 mg) ou PB (20 mg e 40 mg) induz, na epiderme interfolicular, grau similar de hiperceratose e hiperplasia dose-dependente; outros efeitos, como espessamento da epiderme, hipertrofia celular e inflamação, eram mais acentuados pelo tratamento com OC. O efeito hiperplásico, também mais acentuado com OC, decorreria do estímulo proliferativo e do desequilíbrio entre proliferação e diferenciação epidérmica. O tratamento tópico repetido, com OC (1,41 mg) ou PB (30 mg), independente da freqüência semanal, bi-semanal ou tri-semanal, determinou diminuição do estímulo proliferativo inicial, além de intensa hiperceratose e regressão da hiperplasia, no caso do OC conseqüente ao desequilíbrio entre proliferação e diferenciação, favorecendo a perda celular. Entretanto, o PB, quando aplicado 2 ou 3 vezes/semana, determinou discreta progressão da hiperplasia inicial, decorrente de ligeiro desequilíbrio entre proliferação e diferenciação, prevalecendo discreto ganho celular. Ambas substâncias diferiram também no efeito inflamatório, ocorrendo regressão com OC e progressão com PB. A divergência nos efeitos hiperplásico e inflamatório estaria refletindo mecanismos de ação distintos destas duas substâncias. Ao ser aplicado OC ou PB após MC (0,2 mg), segundo protocolos de carcinogênese bifásica ou trifásica, comprovou-se que apenas o PB tem discreto efeito promotor - propagador – de papilomas na pele de gerbil adulto. Considerando a diminuição do estímulo proliferativo, que ocorre durante o tratamento repetido com OC e PB, associada ao efeito destas substâncias sobre a diferenciação celular - hiperceratose - entende-se que a expansão clonal seletiva de células epidérmicas estaria prejudicada, principalmente com OC, sendo talvez esta a base biológica da resistência relativa da pele de gerbil adulto à indução química de tumores de linhagem epidérmica.

Bioactive compound 1,8-Cineole selectively induces G2/M arrest in A431 cells through the upregulation of the p53 signaling pathway and molecular docking studies

BACKGROUND

Callistemon citrinus has been traditionally known for its medicinal property. Recently, our research group identified 1,8-Cineole, as one of the predominant compound present in the hexane extract (HE-C), whose leaves have potent anticancer activity.

HYPOTHESIS/PURPOSE

The present study was designed to isolate 1,8-Cineole from Callistemon citrinus plant and to determine their role in anticancer effects in in vitro using skin carcinoma cells. Moreover, the molecular mechanism of apoptosis and molecular docking studies were also investigated.

STUDY DESIGN/METHODS

In vitro cytotoxicity test was performed with HE-C fractionates 1F, 2F, and 3F against A431 and HaCaT cell lines. MTT and AB assay demonstrated that 1F was toxic to cancer cells with no adverse effect to non-malignant cells and it was subjected to ¹H NMR, ¹³C NMR spectroscopy and further characterized by FTIR and GC-MS analysis. On the basis of spectroscopic data, the metabolite was confirmed as 1,8-Cineole.

RESULTS

Based on the cytotoxicity results, the well-characterized metabolite 1,8-Cineole was investigated upon to understand the mechanism that caused cancer cell death. In this process, the changes in mitochondrial membrane potential (ΔΨ_m) were confirmed by Rh-123/DAPI staining; the ultra structure was observed by TEM and quantified by flow cytometric analysis. These results proved that the compound effectively induced the apoptosis and G2/M phase arrest in A431 cells by increasing the expression of p53 and that it was monitored by FACS. Further, the expression of apoptotic proteins, such as Bax/Bcl-2, Cyt-c, caspase-9, and caspase-3 was confirmed by western blot. The molecular docking simulations predicted the hydrophobic interaction between 1,8-cineole with Bcl-2 and PARP1 receptor.

CONCLUSIONS

1,8-Cineole is a potential candidate for skin carcinoma, which is possible by regulating the p53 apoptotic signaling pathway.

Panx3 links body mass index and tumorigenesis in a genetically heterogeneous mouse model of carcinogen-induced cancer

Background

Body mass index (BMI) has been implicated as a primary factor influencing cancer development. However, understanding the relationship between these two complex traits has been confounded by both environmental and genetic heterogeneity.

Methods

In order to gain insight into the genetic factors linking BMI and cancer, we performed chemical carcinogenesis on a genetically heterogeneous cohort of interspecific backcross mice ((Mus Spretus × FVB/N) F1 × FVB/N). Using this cohort, we performed quantitative trait loci (QTL) analysis to identify regions linked to BMI. We then performed an integrated analysis incorporating gene expression, sequence comparison between strains, and gene expression network analysis to identify candidate genes influencing both tumor development and BMI.

Results

Analysis of QTL linked to tumorigenesis and BMI identified several loci associated with both phenotypes. Exploring these loci in greater detail revealed a novel relationship between the Pannexin 3 gene (Panx3) and both BMI and tumorigenesis. Panx3 is positively associated with BMI and is strongly tied to a lipid metabolism gene expression network. Pre-treatment Panx3 gene expression levels in normal skin are associated with tumor susceptibility and inhibition of Panx function strongly influences inflammation.

Conclusions

These studies have identified several genetic loci that influence both BMI and carcinogenesis and implicate Panx3 as a candidate gene that links these phenotypes through its effects on inflammation and lipid metabolism.

Electronic supplementary material

The online version of this article (doi:10.1186/s13073-016-0334-8) contains supplementary material, which is available to authorized users.

Hydrodynamic cell delivery for simultaneous establishment of tumor growth in mouse lung, liver and kidney

To mimic advanced stage of cancer development involving multi-organ metastasis, hydrodynamic delivery commonly used in gene transfer was explored for establishing concurrent tumors in the lung, liver and kidney using B16-F1 melanoma cells, 4T1 breast cells and Renca renal carcinoma cells, as a model. The procedure involves a rapid injection into a mouse tail-vein of serum-free medium, containing tumor cells in a volume equal to approximately 7-9% of body weight. Compared to the conventional tail vein injection of tumor cells resulting in tumor growth only in the lung, hydrodynamic injection is highly effective in establishing tumor growth in the liver, kidney and lung. All tumor cells examined including melanoma, breast metastatic, and renal carcinoma cells showed significant tumor growth in these organs. These results suggest that the hydrodynamic delivery can be a valuable tool for modeling cancer in laboratory animals, especially in experimental mice.

The potential use of protein kinase D inhibitors for prevention/treatment of epidermal tumors

BACKGROUND

The serine/threonine kinase protein kinase D (PKD) has been proposed to be a pro-proliferative, anti-differentiative signal in epidermal keratinocytes. Indeed, the phorbol ester tumor promoter, 12-O-tetradecanoylphorbol 13-acetate (TPA) induces biphasic PKD activation, which mirrors the biphasic response of initial differentiation followed by proliferation and tumor promotion seen in TPA-treated keratinocytes in vitro and epidermis in vivo.

OBJECTIVE

Our objective was to test the idea that PKD's pro-proliferative and/or anti-differentiative effects in keratinocytes contribute to TPA-induced tumorigenesis.

METHODS

Using western analysis and assays of keratinocyte proliferation and differentiation, we investigated the effect of inhibitors of PKD on keratinocyte function.

RESULTS

We found that overexpression of a constitutively active PKD mutant increased, and of a dominant-negative PKD mutant decreased, keratinocyte proliferation. A recently described selective PKD inhibitor showed low potency to inhibit keratinocyte proliferation or PKD activation. Therefore, we tested the ability of known only relatively selective PKD inhibitors on keratinocyte function and protein kinase activation. H89 {N-[2-(p-bromocinnamylamino) ethyl]-5-isoquinoline-sulfonamide}, a reported inhibitor of PKD and cAMP-dependent protein kinase, enhanced the effect of a differentiating agent on a marker of keratinocyte differentiation. Another reported non-selective PKD inhibitor, resveratrol stimulated differentiation and inhibited proliferation. The protein kinase C/PKD inhibitor Gö6976 blocked the increase in proliferation (as measured by DNA specific activity) induced by chronic TPA without affecting the initial TPA-elicited differentiation.

CONCLUSION

Our results support the idea that relatively selective PKD inhibitors, such as Gö6976, H89 and resveratrol, might be useful for preventing/treating epidermal tumorigenesis without affecting keratinocyte differentiation.

A definitive role of ornithine decarboxylase in photocarcinogenesis

Excessive exposure of solar ultraviolet (UV) radiation, particularly its UVB component, to human skin is the major cause for more than a million new cases of cutaneous malignancies diagnosed annually in the United States. Photocarcinogenesis, like other cancers, is a multistep process that includes initiation and promotion. A proper understanding of the molecular events occurring during the tumor promotion phase of photocarcinogenesis could lead to the development of novel approaches for the management of skin cancer. Using a transgenic mouse model (K5/ODC mice), which overexpresses the enzyme ornithine decarboxylase (ODC) in hair follicle keratinocytes, we studied the role of this gene in photocarcinogenesis. A single UVB-exposure of 180 mJ/cm(2) to the transgenic mice resulted in a minimal increase in bifold skin thickness and ODC activity. However, in SKH-1 hairless mice, the most common and highly sensitive model for photocarcinogenesis, and in littermate nontransgenic mice, increases in skin thickness and ODC activity were substantial. In long-term experiments, mice were exposed to 180 mJ/cm(2) of UVB radiation three times a week for 2 weeks (tumor-initiating dose). At 30 weeks after this treatment, in two independent experiments, 40% of the K5/ODC transgenic mice exposed to UVB were found to develop epidermal tumors. The tumors were histologically verified as benign papillomas and squamous cell carcinomas. Interestingly, 100% of the transgenic mice also developed >20 pigmented cysts/mouse, which contained keratinocyte material with increased keratinocytic melanization. Under similar UVB-exposure protocol, the nontransgenic littermates or SKH-1 hairless mice did not develop tumors or pigmented cysts for up to 50 weeks. Oral consumption of alpha-difluoromethylornithine, an irreversible specific inhibitor of ODC, in the drinking water (1% w/v) to the transgenic mice resulted in complete prevention of UVB-mediated tumorigenesis and a substantial decrease in the formation of pigmented cysts (<10 per mouse). These data establish a definitive role of ODC in the promotion phase of photocarcinogenesis.

Promotion of skin carcinogenesis by dimethylarsinic acid in keratin (K6)/ODC transgenic mice

Dimethylarsinic acid (DMA) is a major metabolite of inorganic arsenicals in mammals, and arsenic exposure is associated with tumor development in a wide variety of human tissues, particularly the skin. Transgenic mice with ornithine decarboxylase (ODC) targeted to hair follicle keratinocytes are much more sensitive than littermate controls to carcinogens. In this study we investigated the promoting effect of DMA on skin carcinogenesis in such K6 / ODC transgenic mice. The back skin of female C57BL / 6J K6 / ODC transgenic mice, 10 to 14 weeks old, was initiated with topical application of 7, 12-dimethylbenz[alpha]anthracene (DMBA) at a dose of 50 microg or acetone alone on day 1 of the experiment, followed by treatment with 3.6 mg of DMA, 5 microg of 12-O-tetradecanoylphorbol-13-acetate (TPA) or neutral vehicle (control) twice a week for 18 weeks. Mice were killed 1 week after the end of the treatment. Induction of skin tumors was significantly accelerated in the DMA-treated group, as well as in the TPA-treated group, indicating that DMA has a promoting effect on skin tumorigenesis in K6 / ODC transgenic mice.

Phytol is a novel tumor promoter on ICR mouse skin

Phytol is a branched, long-chain aliphatic alcohol which has various biological effects. In this study, we examined phytol as a tumor promoter in a mouse skin initiation-promotion model, and compared its promotion activity with that of 12-O-tetradecanoyl phorbol-13-acetate (TPA). Female ICR mice, 7 weeks of age, were initiated with 100 microg of 7,12-dimethylbenz(a)anthracene, and were then topically promoted twice a week for 16 weeks with 100 mg of phytol or with 2.5 microg of TPA. In this model 95% of animals treated with phytol developed skin tumors within 16 weeks. The average number of lesions per mouse treated with phytol was significantly lower than that in mice treated with TPA, and this significant difference continued up to 16 weeks after the end of promotion treatment. Characterization of hyperplasia 48 h after topical application of agents showed that epidermal thickness and vertical thickness following topical application of phytol were significantly increased compared with vehicle controls, but were significantly smaller than in animals treated with TPA. Ornithine decarboxylase (ODC) activity following topical application of phytol was increased in a dose-dependent manner and showed a weak, delayed induction (which was maximal 11-12 h after treatment) as compared with the case of TPA. The specific binding of [3H]phorbol-12,13-dibutyrate (PDBU) by JB6 cells was not inhibited by phytol at concentrations up to 1 mM. These results indicate that phytol has a weak tumor promoter activity compared to TPA and is a non-TPA-type tumor promoter in this model of mouse skin carcinogenesis.

Short- and intermediate-term carcinogenicity testing--a review. Part 1: the prototypes mouse skin tumour assay and rat liver focus assay

Carcinogenicity testing is by far the most expensive and time-consuming study type of toxicology. For many years, the lifetime exposure with the maximum tolerated dose in two rodent species has been the gold standard of carcinogenicity testing of pharmaceuticals. Major change was introduced by the Fourth International Conference on Harmonization in July 1997; a chronic rodent bioassay in one species and a short-term carcinogenicity assay are regarded as sufficient for registration. Such requirements provide the opportunity to redirect the vast resources previously spent on the lifetime study in the second species. Numerous experimental protocols for short- and intermediate-term carcinogenicity testing in many target tissues have been available for years. The first part of this review describes the basic principles of short- and intermediate-term carcinogenicity testing using the examples of the widely used mouse skin tumour assay and the rat liver foci assay. In the context of these experimental models, the discrimination and quantification of initiating and promoting activity and the use of preneoplastic lesions as endpoints in carcinogenicity testing are described. The review includes the limitations of the models with regard to the extrapolation from effects observed in animal experiments to a potential exposure of humans.

Antioxidants and multistage carcinogenesis in mouse skin

The two-step initiation-promotion protocol for the induction of skin tumors in mice is a convenient model to elucidate what molecular events are involved in the multistage process of carcinogenesis and how they can be modulated. The current theories concerning the mechanisms of skin tumor initiation, stages 1 and 2 of tumor promotion, and tumor progression are reviewed. Because chemical carcinogens and tumor promoters may, directly or indirectly, generate reactive oxygen species (ROS) and because various antioxidants inhibit effectively some of the biochemical and biological events linked to tumor initiation, promotion and/or progression, it is conceivable that different sequences and levels of free radical-induced macromolecule damage may contribute to the evolution of the epidermal target cells from the preneoplastic stage to the malignant stage.

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.

Oxidised glutathione reductase activity in mouse epidermis: TPA induced change and its modulation by vitamin A

Single cutaneous application of 12-O-tetradecanoyl-phorbol-13-acetate (TPA) increased epidermal oxidised glutathione reductase activity in adult mouse by almost 100%. Pretreatment of animals with vitamin A for a week resulted in 75% inhibition of TPA induced change in the enzyme activity which remained unaffected in skin treated with vitamin A alone. This biochemical change in skin induced by TPA and modulated by vitamin A has been discussed in relation to epidermal hyperplasia.

Mouse epidermal keratinocytes. Clonal proliferation and response to hormones and growth factors in serum-free medium

A serum-free medium (LEP-1) has been developed for mouse epidermal keratinocytes. LEP-1 consists of "Ca2+-free" Eagle's MEM with non-essential amino acids and seven added supplements (transferrin, 5 micrograms/ml; epidermal growth factor (EGF), 5 ng/ml; hydrocortisone, 0.5 microM; insulin, 5 micrograms/ml; phosphoethanolamine and ethanolamine, each 50 microM; bovine pituitary extract, 180 micrograms of protein/ml). Although serum-free the culture system was dependent for growth on bovine pituitary extract as the only still undefined supplement. LEP-1 supports sustained multiplication of mouse keratinocytes for 25 or more population doublings. A clonal growth assay was developed to investigate the action of growth factors, hormones and other supplements on keratinocytes. Cells grown in LEP-1 (calcium concentration was 0.03 mM) maintained a high proliferative rate and presented the typical morphology of basal epidermal cells. When the calcium concentration of the medium was raised to 1.0 mM, the cells were triggered to differentiate terminally. The epithelial nature of the cells was demonstrated both by electron microscopy and by immunostaining with anti-keratin antibody. The maturation stage of the keratinocytes was defined by several morphological features during the proliferative phase and in terminally differentiating cultures. This serum-free system supported a useful number of cell divisions while keratinocytes retained the capacity to undergo terminal differentiation when given the appropriate stimulus. It provides, therefore, provides a useful model for investigations on growth, differentiation and malignant transformation of epidermal cells in culture.

Protection against 3-methylcholanthrene-induced skin tumorigenesis in Balb/C mice by ellagic acid

Topical application of ellagic acid, a naturally occurring dietary plant phenol, to Balb/C mice resulted in significant protection against 3-methylcholanthrene (MCA)-induced skin tumorigenesis. Ellagic acid was found to be an effective inhibitor of tumor formation whether the tumor data are considered as percent mice with tumors, cumulative number of tumors, tumors per mouse or tumors per tumor bearing animal as a function of the number of weeks on test. By 8, 10, 12, 14, and 16 weeks of testing, the number of tumors per mouse in the group receiving MCA alone was 2.0, 3.4, 4.0, 4.9 and 5.3, respectively, whereas the corresponding numbers in the group receiving MCA plus 2 mumol ellagic acid were 0, 0.3, 0.4, 0.6 and 1.2, respectively. At the termination of the experiment (16 weeks) aryl hydrocarbon hydroxylase (AHH) activity in skin and liver and the extent of 3H-BP-binding to skin, liver and lung DNA were determined and both of these parameters were found to be significantly inhibited in the animals treated with ellagic acid. These results indicate that ellagic acid can inhibit the metabolism of polyaromatic hydrocarbons and modulate skin carcinogenesis induced by these chemicals.

A unifying concept for carcinogenic risk assessments

Carcinogens influence both the initiation of abnormal cells and the subsequent promotion of such cells into neoplasia. Certain other insults seem limited to the stimulation of cellular proliferation and of carcinogenic potentiation. Common examples include surgical, mechanical, chemical, and temperature wounding of tissue followed by healing. In addition, certain hyperplastic growth induced by some chemicals may also enhance tumorigenesis. We propose that the quantification of carcinogenic potentiation may derive from a common-index-quantity estimated according to enhanced cell proliferation resulting from cytotoxicity or toxic hyperplasia induced by a specific exposure. At this time, it is not possible to define, in a restrictive sense, the molecular events which are critical to potentiation but the processes of cell proliferation resulting from cytotoxicity/hyperplasia seem to serve as indices which contain the necessary (and perhaps several secondary) biological responses. The unique advantage is that cell-culture, animal, and human-level studies can be used to evaluate certain parameters of the mathematical model for an untested treatment protocol or chemical insult suspected to be a cofactor in tumorigenesis. The main thrust of this paper is to propose that tumorigenesis should be studied in terms of cellular-population kinetics in response to a biological challenge rather than according to chemical or energetic parameters of that challenge. This approach leads to mathematical equations which can serve as a unifying concept for carcinogenic risk assessments. Sample results, to illustrate the utility of this model, are given for polynuclear aromatic hydrocarbons, trace metals, ionizing radiations, CO, NO, SO2, O3, and NO2. Treatment, here, is for acute exposure conditions, but because the model is mechanistic, other exposure protocols can be addressed by simply adjusting some of the mathematical parameters according to factors estimated from a relative potency comparison of in vitro and in vivo studies best suited to the particular application of interest.

Cell population kinetics in hairless mouse epidermis following a single topical application of 12-O-tetradecanoylphorbol-13-acetate II

It is known that a single application of 17 nmol 12-O-tetradecanoylphorbol-13-acetate (TPA) to hairless mouse skin first induces (from 0-12 h) a short block in mitotic activity followed by a transient stimulation of proliferation (from 12-96 h) characterized by multiple waves of rapid proliferation in partly synchronized basal cells, leading to hyperplasia. The replication rate of basal cells, the number of basal and suprabasal (maturing) cells per unit of interfollicular epidermis and the number of squamous layers in the stratum corneum were recorded. Changes in the nuclear area of living cells were monitored by morphometry. The changes in the rate of basal cell proliferation are accompanied by concomitant waves of increased rates of cell maturation and cell loss, with a considerable reduction in epidermal cell transit time. When 14-15 squamous layers were observed, this resulted in an increased cell loss, which was visible as scaling from 24 h onwards. The total cell mass lost from 16 to 72 h after TPA application amounted to about 67% of the newborn cell mass. Thus the hyperplasia occurring after a single TPA treatment results from a considerably enhanced cell proliferation that exceeds concomitant increases in the rates of cell maturation and loss. There is no delayed maturation. The results are also consistent with the chalone theory of epidermal growth regulation, assuming that the G1 chalone is produced during cell maturation.

Phorbol ester-stimulated murine myelopoiesis: role of colony-stimulating factors

Tumor promoting phorbol esters, such as 12-0-tetradecanoyl-phorbol-13-acetate (TPA), stimulate colony formation in vitro by murine granulocyte-macrophage progenitors (GM-CFC) without added colony stimulating factors (CSF). To determine whether TPA induces CSF production in vitro, marrow cells were cultured for 1 to 7 days in liquid medium with or without TPA. No CSF was detected in any sample by a double antibody radioimmunoassay (sensitivity = 2 units/0.1 ml), however, colony-stimulating activity was detected in supernatant fluid from all TPA containing cultures by bioassay. This activity appeared to result from a direct effect of TPA rather than from production of CSF, as equivalent activity was found in TPA-containing medium incubated in the absence of marrow cells. Rabbit antiserum to purified L-cell CSF inhibited colony formation stimulated by L-cell CSF and WEHI-3 CSF, but had no effect on colony formation induced by TPA. Cells from long-term marrow cultures responded to TPA with colony formation, despite culture conditions and cell fractionation procedures that reduced the frequency of CSF-producing macrophages to less than 1.0%. TPA inhibited binding of radioiodinated L-cell CSF to marrow cells, especially if the cells were first exposed to TPA. These results do not support induction of CSF production as the major mechanism of phorbol ester stimulation of myelopoiesis. Phorbol esters may directly stimulate GM-CFC and/or enhance their response to CSF by a mechanism involving CSF binding sites.

Transformation of epidermal cells in culture

Studies performed on mouse skin have indicated that chemical carcinogenesis can be subdivided into two distinct stages, initiation and promotion. Initiation results from exposure to a classical mutagenic carcinogen and is irreversible even after a single exposure. The permanently altered initiated cell and its progeny may never form a tumor or in any way be recognizable in the target tissue. Exposure to tumor promoters permits the expression of the neoplastic change in initiated cells, and tumors develop. In contrast to initiators, promoters must be given repeatedly to be effective; individual exposures are reversible. A similar biology is suggested by epidemiologic studies of certain human cancers, particularly lung, breast, colon, and uterine malignancies. Studies in mouse skin cell culture have provided new insights into the changes associated with initiation and promotion. Initiated cells appear to be resistant to signals for terminal differentiation and can proliferate under conditions where normal epidermal cells are obligated to cease proliferation and begin their maturation program. This change is essential for an epithelial tumor cell since it provides the ability to grow away from a basement-membrane attachment site. In cultured epidermal cells, tumor promoters are capable of selectively stimulating the growth of certain cells, including initiated cells, while simultaneously inducing terminal differentiation in other epidermal cells. The net effect of these responses to promoters is the clonal expansion of cells stimulated to proliferate. In this way, promoters are capable of increasing the clone size of initiated cells. These cell culture data provided a biological framework for understanding initiation and promotion in terminally differentiating epithelial tissues.

Nevi and melanoma induced by chemical carcinogens in laboratory animals: similarities and differences with human lesions

The isolation of benzo(a)pyrene (BP) from coal tar, and the synthesis of dibenzanthracene (DBA), methylcholanthrene (MCA), and dimethylbenzanthracene (DMBA) in the 1930's, facilitated the induction of epidermal tumors in experimental animals. It was soon found that chemical carcinogens affect not only keratinocytes, but also the melanocytes (Szabo 1953). "Painting" the skin of small laboratory animals, with carcinogens, became the principal method of studying the mechanisms of chemical carcinogenesis (Yuspa et al. 1976a). Pigmented nevi and melanoma are perhaps two of the most interesting problems in dermatology, oncology and pathology. Questions related to histogenesis, classification, diagnosis, treatment and eventual prevention remain unanswered. A substantial amount of our work is used to support most of the main points regarding the development of nevi and melanoma in laboratory animals.

DMBA-induced tumors and their prevention by aromatic retinoid (Ro 10-9359)

Both auricles of 21 domestic rabbits were painted with dimethylbenzanthracene (DMBA). Eleven animals of this group were additionally fed aromatic retinoid (AR) by an esophageal tube. Two control animals were not treated at all. Eight or 9 weeks after the beginning of the study six of the seven remaining animals, which had only been painted with DMBA, developed a total of 25 keratoacanthoma-like tumors (KA). On the other hand, none of the seven animals left, which were painted with DMBA and fed AR showed any tumor by this time. The systemic effect of AR was studied in biopsies from the snout and the back. The epidermis of the snout showed 'mucous mataplasia' by histochemical and electron-microscopic criteria, whereas the epidermis of the back was not significantly altered. The production of intra- and extracellular lamellated material indicated an additional effect of AR on epidermal lipid metabolism. The effect of AR in the prevention of DMBA-induced tumors was characterized by 'mucoid cytolysis' and karyolysis.

Initiator and promoter induced specific changes in epidermal function and biological potential

Mouse epidermal basal cells can be selectively cultivated in medium with a calcium concentration of 0.01--9.09 mM. Terminal differentiation and sloughing of mature keratinocytes occur when the calcium concentration is increased to 1.2--1.4 mM. When basal cell cultures are exposed to chemical initiators of carcinogenesis, colonies of cells that resist calcium-induced differentiation evolve. Likewise, basal cells derived from mouse skin initiated in vivo yield foci that resist terminal differentiation. This defect in the commitment to terminal differentiation appears to be an essential change in initiated cells in skin and is also characteristic of malignant epidermal cells. This model system has also provided a means to determine if basal cells are more responsive to phorbol esters than other cells in epidermis and to explore the possibility that heterogeneity of response exists within subpopulations of basal cells. The induction of the enzyme ornithine decarboxylase (ODC) was used as a marker for responsiveness to phorbol esters. ODC induction after exposure to 12-0-tetradecanoylphorbol-13-acetate (TPA) in basal cells is enhanced 20-fold over the response of a culture population containing both differentiating and basal cells. When basal cells are induced to differentiate by increased calcium, responsiveness to TPA is lost within several hours. In basal cell cultures, two ODC responses can be distinguished. After exposure to low concentrations of TPA or to weak promoters of the phorbol ester series, ODC activity is maximal at 3 hr. With higher concentrations of TPA, the ODC maximum is at 9 hr. These results are consistent with the presence of subpopulations of basal cells with differing sensitivities to TPA. Other studies that use the enzyme epidermal transglutaminase as a marker for differentiation support this conclusion. In basal cell culture TPA exposure rapidly increases transglutaminase activity and cornified envelope development, reflecting induced differentiation in some cells. As differentiated cells are sloughed from the dish, the remaining basal cells proliferate and become resistant to induced differentiation by 1.2 mM calcium. These data provide additional evidence of basal cell heterogeneity in which TPA induces one subpopulation to differentiate while another is stimulated to proliferate and resists a differentiation signal. Tumor promoters, by their ability to produce heterogeneous responses with regard to terminal differentiation and proliferation, would cause redistribution of subpopulations of epidermal cells in skin. Cells that resist signals for terminal differentiation, such as initiated cells, would be expected to increase in number during remodeling, Clonal expansion of the initiated population could result in a benign tumor with an altered program of differentiation. In skin, benign tumors are the principal product of 2-stage carcinogenesis. Subsequent progression to malignancy may involve an additional step, probably a genetic alteration, that is independent of the tumor promoter.

Historical perspectives of occupational cancer

Three topics are discussed in this review, which is not intended to give even a short description of the history of occupational cancer. First, the present state and possible future trends of occupational cancer are examined. Such factors as rapid industrialization, increasing amounts of chemical compounds in the environment, and discoveries of new occupational carcinogens such as asbestos and vinyl chloride indicate that occupational cancer is likely to become more frequent in the future. The controversial issue of the proportion of cancers related to occupation is briefly considered. The upward trend of estimates of various authors during a quarter of a century indicates a growing proportion of occupational cancers in the overall incidence of cancer. Second, some lessons from the past are considered. Careful observations and alertness of physicians and proper documentation of occupational cancer cases are pointed out. Interdisciplinary teamwork and international cooperation have been useful in the past and continue to be desirable. Some details of the studies of skin cancer caused by mineral oil are informative. Individual susceptibility, whether genetically determined or due to pathological conditions, needs further study. As an example of the predictive value of animal experiments, skin cancer related to the oil shale industry in Estonia is discussed. The third topic--input from experimental cancer research--deals mainly with the problem of modifying factors. Experimental data on such factors could facilitate investigations of life-style effects, using the proposed classification of modifying factors. The problem of nasal cancer in woodworkers may be easier to solve by taking into account some experimental data on tannin-containing material. Some possibilities for future action and suggestions for further research are outlined.

Identification and definition of chemical carcinogens: review of criteria and research needs

Advances in carcinogenesis research have greatly improved the methods for detection, investigation, and definition of occupational carcinogens. A basis of scientific criteria has been developed for carcinogen identification and evaluation. Research, legislative, and regulatory initiatives in the United States are reviewed. The 1979 Report of the Working Group on Occupational Carcinogenesis, Occupational Cancer Task Force, National Cancer Institute, is added as an appendix. A detailed discussion is given of the 1979 report of the Interagency Regulatory Liaison Group entitled "Scientific bases for identification of potential carcinogens and estimation of risks." Two separate approaches are considered, i.e., the qualitative determination that a substance poses a carcinogenic hazard and the quantitative estimation of risk. The qualitative criteria are reviewed in the present paper. The need for critical data evaluation and the judgmental nature of the process are emphasized. Research needs arising from the analysis of these criteria are discussed. Laboratory research contributions to the identification and characterization of carcinogens are summarized, including development and study of organ target models for carcinogenesis in animals; studies of human target tissues and cells in culture; studies of transformation, initiation, and promotion of epidermal cells in culture; studies of transplacental carcinogenesis; and studies of molecular mechanisms.

Enhancement by Wy-14,643, a hepatic peroxisome proliferator, of diethylnitrosamine-initiated hepatic tumorigenesis in the rat

Diethylnitrosamine (DEN), at a concentration of 100 parts/10(6) in drinking water for 14 days, caused the development, by 48 weeks, of very few liver tumours in 5 of 18 (27%) male F=344 rats fed control diet. When the DEN treatment was followed one week later by continuous feeding of the hypolipidemic hepatic peroxisome proliferator, Wy-14,643, at 0.1% dietary level, all of 28 rats (100%) developed, between 38 and 48 weeks, a significantly higher number of liver tumours. Furthermore, laparotomy at 22 weeks revealed that several rats fed Wy-14,643 after DEN initiation had developed visible liver nodules, suggesting that Wy-14,643 also accelerates the appearance of these tumours. Administration of another peroxisome proliferator, clofibrate, at 0.5% level in the diet after DEN initiation, also caused a substantial enhancement of liver tumorigenesis. The enhancement of liver-tumour development by clofibrate, however, was less than that by Wy-14,643. The marked enhancing effect of Wy-14,643 may be due to its profound hepatomegalic and peroxisome proliferative properties.

Retinyl acetate modulation of cell growth kinetics and carcinogen--cellular interaction in mouse epidermal cell cultures

Exposure of mouse epidermal cell cultures to beta-retinyl acetate (RA) affects a number of parameters presumed to be important in chemical carcinogenesis. (1) RA alters the course of differentiation of the epidermal cells in culture resulting in a reduced rate of cell death which normally follows cellular maturation during the first two weeks in culture. The extended life span of the cultures appeared due to prolonged survival of cells and not to increased growth rate since RA inhibited the rate of cellular proliferation. This inhibition took place only after completion of a full cell cycle in the presence of RA. (2) DNA repair in response to physical and chemical agents was quantitatively unaffected in the presence of RA. (3) The activity of constitutive aryl hydrocarbon hydroxylase (AHH) was slightly decreased after exposure to RA but the level of enzyme induced by benz[a]anthracene was strongly reduced to 20% of the controls. (4) In the presence of RA, binding of 7,12-dimethylbenz[a]anthracene to epidermal cell DNA was markedly decreased. In contrast, binding to cellular protein was significantly increased by the retinoid.

Cutaneous oncology 1950-1975

Sociopolitical and economic climates between 1950 and 1975 favored the funding of cancer research, attracted imaginative policy makers, encouraged investigators, and generated new knowledge for clinical application. Important advances during this period include the discovery that DNA alteration is requisite for carcinogenesis and that DNA repair processes are important deterrents in the development of cancer. The better understanding of the histogenesis of epidermal cancer and melanoma has potential clinical value. More easily grasped is the progress made in the therapeutic control of mycosis fungoides and epidermal cancer. Complete control of epidermal cancer is now a foreseeable reality.