Skin carcinogenesis as a model system: observations on species, strain and tissue sensitivity to 7,12-dimethylbenz(a)anthracene with or without promotion from croton oil

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.

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.

Multidirectional tumor-suppressive activity of AIMP2/p38 and the enhanced susceptibility of AIMP2 heterozygous mice to carcinogenesis

Aminoacyl-transfer ribonucleic acid (tRNA) synthetases-interacting multifunctional protein (AIMP) 2 is a factor associated with the macromolecular protein synthesis machinery consisting of nine different aminoacyl-tRNA synthetases and three non-enzymatic factors. However, it was shown to work as a multifaceted regulator through the versatile interactions with diverse signal mediators. For instance, it can mediate pro-apoptotic response to DNA damage and tumor necrosis factor-alpha (TNF-alpha) stimulus and growth-arresting signal by transforming growth factor (TGF)-beta. Considering that these pathways are critically implicated in the control of tumorigenesis, AIMP2 is expected to work as a potent tumor suppressor with broad coverage against different cancer types. Here we investigated whether AIMP2 would give gene dosage effect on its pro-apoptotic and anti-proliferative activities using the wild-type, hetero- and homozygous AIMP2 cells and whether AIMP2 would be critical in preventing tumorigenesis using different in vivo tumor models. Both the apoptotic responses to DNA damage and TNF-alpha and sensitivity to growth arresting TGF-beta signal were reduced in AIMP2 hetero- and homozygous cells compared with the wild-type cells in dose-dependent manner. In all the in vivo carcinogenesis experiments, reduction of AIMP2 level in heterozygous AIMP2 mice provided higher susceptibility to tumor formation. Thus, this work proves the functional significance of AIMP2 in determination of cell proliferation and death, and as a haploinsufficient tumor suppressor.

The extracellular matrix in skin tumor development-a morphological study

The development of cancer involves epithelial-stromal interactions. Alterations in the synthesis and deposition of type I and III collagens are related to the tumor morphology. Skin carcinogenesis in experimental animals provides a reliable model for the development of neoplasia. Ultraviolet (UV) irradiation is the main etiological factor for epidermal dysplasia and malignant tumors in man, but also for dermal degeneration. Non-neoplastic dermal changes and skin tumors induced by ultraviolet irradiation and 7,12-dimethylbenz(a)anthracene were investigated in various mouse strains with different susceptibilities to tumor formation. UVB irradiation resulted in an increased immunoreactivity of collagens in the dermis, in comparison with 7,12-dimethylbenz(a)anthracene. Increased synthesis and deposition of type I and III collagens were found in the stroma adjacent to benign alterations. In well-differentiated squamous cell carcinomas, a similar induction of collagen synthesis and deposition was observed. The destruction of fibrillary structures was more pronounced during the decrease of differentiation from moderately to poorly differentiated squamous cell carcinomas. Anaplastic carcinomas with spindle cell morphology displayed a delicate meshwork of reticular fibers and collagen III, and abnormal expression of mRNA for collagens in some malignant cells with epithelial characteristics. The underlying stroma reacts to the development of epithelial tumors in a reproducible way, which is related to the carcinogenic agent involved.

Studies of Strain Differences in the Spontaneous and Induced Frequencies of Cells Resistant to Calcium-Induced Terminal Differentiation

C3H mice were found to be much less responsive to tumor induction in an initiation/promotion protocol than CD-I mice. To explore the difference in strain sensitivity, the frequencies of cells resistant to calcium-induced terminal differentiation(calcium resistant cells, CRCs) were measured in C3H, CD-I, and SENCAR mice. The induced frequencies after 7,12-dimethylbenz(a)anthracene(DMBA) treatment were similar in all three strains, ranging from 0.70 to 0.83 × 10-6, and suggested that the differences in tumor outcome were more likely related to promotion than initiation. Spontaneous frequencies ranged from 0.73 to 3.75 CRCs per mouse or 0.07 to 0.22 CRC/106cells. The spontaneous frequency in the C3H mouse was significantly less than that in the other two strains. However, the spontaneous mutation rates were found to be similar in all three strains, ranging from 0.05 to 0.07 × 10-6and it was found that the keratinocytes, which were resistant to terminal differentiation, were independently distributed in C3H but not in CD-I and SENCAR mice. These data are consistent with the hypothesis that clonal expansion occurs in CD-I and SENCAR but not C3H mice. Conceivably the greater propensity for clonal expansion in CD-I and SENCAR than in C3H mice is related to the greater likelihood of tumor formation as well as the more rapid development of tumors in the former strains.

Skin tumor development and keratin expression in different experimental models. Relation to inducing agent and target tissue structure

The applicability of the experimental skin carcinogenesis model for studies of tumor development was examined by exposing the skin of various mouse strains to different chemical carcinogens and UV radiation regimens, in order to analyze the development and progression of the neoplastic process and the role of differentiation markers such as keratins. In tumor-sensitive hairy NMRI mouse skin, the chemical carcinogen, 7,12-dimethylbenz(a)-anthracene (DMBA) induced an abnormal epidermal cell differentiation and structural irregularities associated with an altered keratin expression, as well as numerous papillomas and squamous cell carcinomas. A suboptimal dose of UVB irradiation increased the number of DMBA-induced benign squamous neoplasms. Low doses of benzo(a)pyrene resulted in mild epidermal alterations, but only in one tumor. High doses of UVB induced a large number of undifferentiated spindle cell tumors with few keratinpositive cells in NMRI mice, similar though fewer tumors in hairy, heavily pigmented C57BL/6 mice, numerous papillomas and squamous cell carcinomas in hairless hr/hr mice but only two papillomas in hairy, moderately pigmented DBA/2 mice while UVA exposure produced only two papillomas in hairless SKH-1 mice. In conclusion, the extent and type of skin tumor development depended upon the induction regimen: physical, chemical, dose and duration, as well as on the skin structure: pigmentation and adnexal development, all of which have to be taken into account when relating experimental results to human conditions.

Dithranol (anthralin)-induced skin irritation in C57BL/6, NMRI and SENCAR mice

Dithranol-induced skin irritation was compared in C57BL/6, NMRI and SENCAR mice, the strains representing different sensitivity to tumour promotion. Skin irritation was assessed using ear thickness and skin weight measurements, visual estimation of back skin irritation and histopathology. Both single and repeated applications of dithranol caused a delayed skin irritation resulting in the maximal response between 7-11 days after the beginning of the treatment. Contrary to the findings with 12-O-tetradecanoyl-phorbol-13-acetate (TPA), C57BL/6 mice were the most sensitive and SENCAR mice the most resistant to the dithranol-induced skin irritation up to 30 days from the beginning of the treatment. NMRI mice were intermediate. Differences were found in the ear swelling, epidermal hyperplasia, amount of inflammatory cell infiltrate and skin ulceration. During repeated treatment of about 40 days, however, the responsiveness of SENCAR mice increased over that of C57BL/6 and NMRI mice. SENCAR mice had also more epidermal hyperplasia than the other strains at the end of the 74 day period of 3 times weekly applications. The magnitude of epidermal hyperplasia after long term treatment seems to correlate with the sensitivity to tumor promotion in the different mouse strains.

Chemical carcinogenesis: from animal models to molecular models in one decade

During the last decade, progress in chemical carcinogenesis research has been substantial, and understanding the cellular changes and molecular causes of initiation, promotion, and malignant conversion appears to be within reach. Cancer begins as a carcinogen-induced genetic change in a single cell. The interaction of a particular carcinogen with specific genetic sites results, in part, from selectivity of metabolically activated carcinogens for particular nucleosides or gene sequences. In turn, modification of the molecular structure at specific genetic loci will have tissue-specific and species-specific consequences dependent on the expression of a particular gene, its sequence, and the function of the gene product in the target cell. It is likely that inactivation of regulatory regions, genomic rearrangements, and point mutations in coding sequences all can result in an altered cell phenotype. The rasH gene (and perhaps other members of the ras gene family) appears to be a common target for coding sequence mutations in the initiation of carcinogenesis in several organ sites and species by specific carcinogens. Whatever genetic mechanisms are involved, an initiated cell phenotype common to many epithelial cell types is observed. Initiated cells have an altered program of terminal differentiation, are resistant to cytotoxic substances or show altered requirements for specific growth factors or nutrients. These cells would have a selective growth advantage in cytostatic or cytotoxic situations or under conditions favoring terminal differentiation. Tumor promoters, some acting through specific cellular receptors, produce a tissue environment conductive to the selective clonal outgrowth of the initiated cell population resulting in a clinically evident premalignant lesion. The tissue specificity for most promoters depends on the ability of a particular agent to produce the selective conditions required for the initiated phenotype of that organ. At the molecular level, phorbol ester tumor promoters bind to and activate protein kinase C and transduce signals through this second-messenger pathway. Heterogeneity in the species of protein kinase C molecule expressed by normal and initiated epidermal cells could account for the differential response pattern observed in these cell types during skin tumor promotion. Malignant conversion of benign tumors requires further genetic changes in the tumor cell. Such changes could result from inherent instability in the genome of initiated cells, from spontaneous mutations more likely to occur in the expanding population of proliferating benign tumor cells, or by additional exposure to exogenous genotoxic agents.(ABSTRACT TRUNCATED AT 400 WORDS)

Mouse skin tumor initiation-promotion and complete carcinogenesis bioassays: mechanisms and biological activities of emission samples

Extracts of soots obtained from various sources were applied to the skin of mice in an effort to identify carcinogens in these mixtures and to link these materials to the etiology of human cancer. Samples of coal chimney soot, coke oven materials, industrial carbon black, oil shale soot, and gasoline vehicle exhaust materials have been examined by this method. The studies reported here have been constructed to compare the carcinogenic and tumorigenic potency of extracts from various particulate emissions: coke ovens, diesel and gasoline vehicles and a roofing tar pot. Automobile emission samples were obtained by collecting the diluted and cooled exhaust on Teflon-coated glass fiber filters. Coke oven and roofing tar samples were particulate emission samples collected by impaction and filtration. The organic components associated with each of the particles were extracted with dichloromethane and dermally applied to SENCAR mice. All agents were applied as tumor initiators by using a five-dose protocol. Selected extracts were also applied as complete carcinogens and as tumor promotors. Statistical analyses of the resulting tumor data were performed by using nonlinear Poisson and probit models. The results from these experiments provide a suitable data base for comparative potency estimation of complex mixtures.

Early skin responses of hibernating and nonhibernating ground squirrels to topical applications of DMBA

Skin patches on hibernating and nonhibernating ground squirrels were treated with multiple topical applications of the carcinogen 7,12-dimethylbenza(a)anthracene. Nonhibernators showed blistering, peeling, drying, hair loss, increased vascularization and hyperpigmentation in proportion to DMBA concentration. The latter was apparently due to a) an increased number of dermal and epidermal melanocytes and b) the appearance of melanocytes with large coarse cytoplasmic granules. Notably, hibernators remained free of gross skin changes and were histologically similar to untreated controls.

Effect of 7,12-dimethylbenz(a)anthracene on the integument of the hibernating and nonhibernating 13-lined ground squirrel

1. DMBA, a chemical carcinogen, was topically applied to skin patches of hibernating and nonhibernating ground squirrels (Spermophilus tridecemlineatus). 2. Macroscopically, hyperpigmentation, hair loss and excessive skin sloughing were evident in all treated nonhibernator skin patches. 3. Histological sections of skin revealed hyperkeratosis, epidermal vesicles, acanthosis, an indistinct basal layer and increased vascularization in nonhibernators. 4. Skin patches on hibernators were unaffected by treatment showing hibernation confers protection from the pathological effects of DMBA.

In vivo formation of benzo(alpha)pyrene diol epoxide-deoxyadenosine adducts in the skin of mice susceptible to benzo(alpha)pyrene-induced carcinogenesis

The hydrocarbon-deoxyribonucleoside adducts formed in mouse skin DNA have been determined following topical application of an initiating dose of benzo(a)pyrene to Swiss mice, a strain shown to be susceptible to benzo(a)pyrene-induced skin carcinogenesis. Several DNA-bound products were formed, of which the major one (60% of total adducts), in agreement with other workers' findings, was derived from reaction of (+/-) 7 beta, 8 alpha-dihydroxy-9 alpha, 10 alpha-epoxy-7,8,9,10-tetrahydrobenzo(alpha)pyrene (BDE) with the exocyclic aminogroup of deoxyguanosine. A further product (9-10% of total adducts), previously observed only after microsomal activation of benzo(a)pyrene, was observed and co-chromatographed with a further metabolite of 9-hydroxybenzo(alpha)pyrene bound to an uncharacterized base in the DNA. Two otherr products (2-3% of total adducts) were also found in the in vivo studies which co-chromatographed with BPDE-deoxyadenosine adducts and arose from cis and trans addition of the exocyclic amino group of deoxyadenosine to the 7R form, but not the 7S form, of BPDE. In contrast to this, the major in vitro deoxyadenosine-bound products, formed following reaction of BPDE with calf-thymus DNA, were derived from the 7S form of BPDE, suggesting either stereoselective formation or reaction of the 7R form of BPDE in mouse skin in vivo. Similar amounts of BPDE-deoxyguanosine and BPDE-deoxyadenosine adducts, as well as those derived from further metabolism of 9-hydroxybenzo(alpha)pyrene were formed in three strains of mice reported to have widely differing susceptibilities to polycyclic hydrocarbon-induced skin carcinogenesis. The relevance of these different hydrocarbon-DNA adducts to carcinogenesis requires further investigation.