Judson W. Spalding

Arsenic Enhancement of Skin Neoplasia by Chronic Stimulation of Growth Factors

Although numerous epidemiological studies have shown that inorganic arsenicals cause skin cancers and hyperkeratoses in humans, there are currently no established mechanisms for their action or animal models. Previous studies in our laboratory using primary human keratinocyte cultures demonstrated that micromolar concentrations of inorganic arsenite increased cell proliferation via the production of keratinocyte-derived growth factors. As recent reports demonstrate that overexpression of keratinocyte-derived growth factors, such as transforming growth factor (TGF)-alpha, promote the formation of skin tumors, we hypothesized that similar events may be responsible for those associated with arsenic skin diseases. Thus, the influence of arsenic in humans with arsenic skin disease and on mouse skin tumor development in transgenic mice was studied. After low-dose application of tetradecanoyl phorbol acetate (TPA), a marked increase in the number of skin papillomas occurred in Tg.AC mice, which carry the v-Ha-ras oncogene, that received arsenic in the drinking water as compared with control drinking water, whereas no papillomas developed in arsenic-treated transgenic mice that did not receive TPA or arsenic/TPA-treated wild-type FVB/N mice. Consistent with earlier in vitro findings, increases in granulocyte/macrophage colony-stimulating factor (GM-CSF) and TGF-alpha mRNA transcripts were found in the epidermis at clinically normal sites within 10 weeks after arsenic treatment. Immunohistochemical staining localized TGF-alpha overexpression to the hair follicles. Injection of neutralizing antibodies to GM-CSF after TPA application reduced the number of papillomas in Tg.AC mice. Analysis of gene expression in samples of skin lesions obtained from humans chronically exposed to arsenic via their drinking water also showed similar alterations in growth factor expression. Although confirmation will be required in nontransgenic mice, these results suggest that arsenic enhances development of skin neoplasias via the chronic stimulation of keratinocyte-derived growth factors and may be a rare example of a chemical carcinogen that acts as a co-promoter.

Evaluation of transgenic mouse bioassays for identifying carcinogens and noncarcinogens.

Data supporting the use of transgenic lines to identify carcinogens and noncarcinogens are thus far based on a limited number of chemicals for which there are also long-term bioassay results in rats and/or mice. Six chemicals have been tested in the heterozygous p53-deficient mice and 13 in the Tg.AC line. The results show that the p53def responds rapidly to mutagenic carcinogens and the Tg.AC responds rapidly to both mutagenic and nonmutagenic carcinogens. Neither transgenic line responded to the noncarcinogens that were tested. The p53def line failed to respond to two nonmutagenic carcinogens (N-methyloacrylamide and reserpine), the Tg.AC line failed to respond to ethyl acrylate, a nonmutagenic chemical that induced tumors of the forestomach when administered by gavage, and to triethanolamine that caused an increase in hepatocellular tumors in B6C3F1 mice via skin painting. Both of the latter chemicals are examples of highly specific responses related to either route of administration or to strain susceptibility. Further efforts to evaluate the range of chemicals to which these transgenic lines respond are currently in progress.

Kinetics of wound-induced v-Ha-ras transgene expression and papilloma development in transgenic Tg.AC mice

The Tg.AC transgenic mouse, which harbors an activated v‐Ha‐ras coding region that is fused to an embryonic ζ globin transcriptional control region and a 3′ simian virus 40 polyadenylation sequence, rapidly develops epidermal papillomas in response to topical application of chemical carcinogens or tumor promoters or to full‐thickness wounding of the dorsal skin. In this report, we investigated the localization and temporal induction of v‐Ha‐ras transgene expression after full‐thickness wounding of Tg.AC mouse skin. Surgically inflicted full‐thickness incisions 3 cm long yielded four to six papillomas per Tg.AC mouse by 5 wk after wounding. Similar wounding of the FVB/N isogenic host strain did not produce tumors, which implicates a causal role for the v‐Ha‐ras transgene. Reverse transcription–polymerase chain reaction assays detected the v‐Ha‐ras transgene transcript in total RNA samples isolated from wound‐associated tissue 3 and 4 wk after wounding. Tissues 1–2 wk after wounding and all non‐wound–associated tissues were negative for transgene expression. In situ hybridization experiments using transgene‐specific 35S‐labeled antisense RNA probes localized transgene expression to the basal epidermal cells in wound‐induced papillomas. Adjacent normal and hyperplastic skin tissues were negative for transgene expression by this assay. This work supports the hypothesis that the wound repair response leads to the transcriptional activation and continued expression of the v‐Ha‐ras transgene in specific cells in the skin, which alters normal epithelial differentiation and ultimately results in neoplastic growth. Mol. Carcinog. 20:108–114, 1997.

The Tg.AC (v-Ha-ras) transgenic mouse: nature of the model.

The Tg.AC (v-Ha-ras) transgenic mouse model provides a reporter phenotype of skin papillomas in response to either genotoxic or nongenotoxic carcinogens. In common with the conventional bioassay, the Tg.AC model responds to known human carcinogens and does not respond to noncarcinogens. It also does not respond to most chemicals that are positive in conventional bioassays principally at sites of high spontaneous tumor incidence. The mechanism of response of the Tg.AC model is related to the structure and genomic position of the transgene and the induction of transgene expression through specific mediated interactions between the chemicals and target cells in the skin.

Role of keratinocyte-derived cytokines in chemical toxicity.

Following appropriate stimulation, such as with tumor promoters, ultraviolet light or various chemical agents, keratinocytes synthesize and secrete cytokines which can mediate or participate in dermatotoxic responses such as inflammation, hyperkeratosis, hypersensitivity and skin cancer. We have determined the qualitative and quantitative cytokine response in primary human keratinocyte cultures following exposure to several non-sensitizing contact irritants, sensitizers and ulcerative agents as well as a skin carcinogen. The chemicals were also administered to mice to assess whether the dermatotoxic response correlated with the in vitro production of keratinocyte-derived cytokines. Due to the complex cellular interactions that occur in the skin, it was not possible to identify specific cytokine profiles for most of the classes of dermatotoxic agents studied. However, the non-sensitizing contact irritants produced relative increases in the synthesis and secretion of the proinflammatory cytokines, interleukin-1 and tumor necrosis factor-alpha, as well as the neutrophil chemotactic cytokine, interleukin-8 compared to the other chemical agents. While ulcerative compounds as well as irritants elicited neutrophils to the site of chemical application when applied to the mouse skin, time-dependent and chemical-specific patterns of inflammation were detected. Treatment of human keratinocyte cultures with arsenic, a human skin carcinogen, resulted in a unique cytokine profile characterized by induction of growth factors, including transforming growth factor-alpha and granulocyte-macrophage colony stimulating factor. Treatment of v-Ha-ras transgenic mice, an animal model for skin cancer, with arsenic caused an increase in the number of papillomas as well as overexpression of these growth factors suggesting that they participate in arsenic-induced skin papilloma development. These studies indicate a diverse role exists for keratinocyte-derived cytokines in dermatotoxic actions.

Induction of transgene expression in Tg.AC (v-Ha-ras) transgenic mice concomitant with DNA hypomethylation

Tg.AC transgenic mice have a transgene composed of a ζ‐globin transcriptional control region, a v‐Ha‐ras coding region, and a simian virus 40 3′ polyadenylation signal sequence. Induced ectopic expression of the transgene by chemical treatment or full‐skin‐thickness wounding leads to the development of skin papillomas. Reverse transcription–polymerase chain reaction assays and protein blotting indicated that the transgene was expressed 16–28 d after full‐skin‐thickness surgical wounding. Normal unwounded skin did not express the transgene. DNA blotting indicated that the position of the transgene remained stable during wound‐induced tumorigenesis. Concomitant with the v‐Ha‐ras mRNA and protein expression was the hypomethylation of specific MspI/HpaII sites within the transgene. These results are consistent with the hypothesis that hypomethylation is required for the induced and sustained expression of the Tg.AC v‐Ha‐ras transgene in spontaneous and induced tumors in Tg.AC mice. Mol. Carcinog. 21:244–250, 1998.

Biological, cellular, and molecular characteristics of an inducible transgenic skin tumor model : a review

The genetically initiated Tg.AC transgenic mouse carries a transgene consisting of an oncogenic v-Ha-ras coding region flanked 5′ by a mouse ζ-globin promoter and 3′ by an SV-40 polyadenylation sequence. Located on chromosome 11, the transgene is transcriptionally silent until activated by chemical carcinogens, UV light, or full-thickness wounding. Expression of the transgene is an early event that drives cellular proliferation resulting in clonal expansion and tumor formation, the unique characteristics now associated with the Tg.AC mouse. This ras-dependent phenotype has resulted in the widespread interest and use of the Tg.AC mouse in experimental skin carcinogenesis and as an alternative carcinogenesis assay. This review examines the general biology of the tumorigenic responses observed in Tg.AC mice, the genetic interactions of the ras transgene, and explores the cellular and molecular regulation of ζ-globin promoted transgene expression. As a prototype alternative model to the current long-term rodent bioassays, the Tg.AC has generated a healthy discussion on the future of transgenic bioassays, and opened the doors for subsequent models for toxicity testing. The further exploration and elucidation of the molecular controls of transgene expression will enhance the usefulness of this mouse and enable a better understanding of the Tg.ACs discriminate response to chemical carcinogens.

Topical and oral administration of the natural water-soluble antioxidant from spinach reduces the multiplicity of papillomas in the Tg.AC mouse model

The Tg.AC mouse carrying the v-Ha-ras structural gene is a useful model for the study of chemical carcinogens, especially those acting via non-genotoxic mechanisms. This study evaluated the efficacy of the non-toxic, water-soluble antioxidant from spinach, natural antioxidant (NAO), in reducing skin papilloma induction in female hemizygous Tg.AC mice treated dermally five times over 2.5 weeks with 2.5 microg 12-O-tetradecanoylphorbol-13-acetate (TPA). The TPA-only group was considered as a control; the other two groups received, additionally, NAO topically (2 mg) or orally (100 mg/kg), 5 days/week for 5 weeks. Papilloma counts made macroscopically during the clinical observations showed a significant decrease in multiplicity (P<0.01) in the NAO topically treated group. According to histological criteria, papilloma multiplicity were lower in both topical-NAO and oral-NAO groups, but significantly so only in the oral-NAO mice (P<0.01). The beneficial effect of NAO in the Tg.AC mouse is reported.

Odontogenic tumours in the v-Ha-ras (TG · AC) transgenic mouse

A line of homozygous transgenic mice (TG.AC) carrying a v-Ha-ras gene fused to the promoter of the zeta globin gene produces a variety of mesenchymal and epithelial neoplasms including odontogenic tumours. The 1-year incidence of odontogenic tumour formation in these mice was approx. 35%. Tumours formed more often in the mandible than maxilla. The various types of tumours frequently presented with: (1) primarily mesenchymal cells in a dense fibrous-like matrix, or (2) loose stroma surrounded by anastomosing cords of epithelial cells that exhibited squamous differentiation, or (3) odontomas forming mineralized tooth structures by well-differentiated odontoblasts and ameloblasts. Some tumours had areas with all three of these characteristics. Mineralized dentine and enamel in the odontomas were morphologically similar to those of normal murine teeth. Odontogenic tumours expressed the v-Ha-ras transgene that was primarily localized to the mesenchymal cells. Proliferating-cell nuclear antigen immunohistochemistry showed that the mesenchymal cells adjacent to the epithelial cords not only expressed the ras transgene but were also actively proliferating. The TG.AC mouse provides an excellent model for the study of odontogenic tumours and tooth development.

Hemizygous Tg.AC transgenic mouse as a potential alternative to the two-year mouse carcinogenicity bioassay: evaluation of husbandry and housing factors

The dermal Tg.AC transgenic mouse model has been proposed as a potential alternative to the conventional (e.g. oral, dermal, parenteral, inhalation, etc.) 2‐year rodent bioassay for detecting chemical carcinogenicity. The present study was designed to address a number of technical aspects of this model as well as to augment the database being developed with the Tg.AC system at the NIEHS. Hemizygous Tg.AC mice were implanted s.c. with microchips for identification and housed individually in polycarbonate (i.e. ‘plastic’) or suspended stainless‐steel wire‐bottom (i.e. ‘metal’) cages. Treatment consisted of dermal application of the test or control material in treatment volumes of 200 μl of acetone. Groups of 10 males and 10 females were treated as follows: G1—shaved, no treatment; G2—acetone control seven times a week; G3—100 μl of benzene three times a week; G4—150 μl of benzene three times a week; G5—1.25 μg of phorbol ester (PMA) twice a week. The G1–G5 mice were housed in plastic caging with Alpha‐dri® bedding. Three additional groups were housed in stainless‐steel wire‐bottom caging: G6—shaved, no treatment; G7—acetone control seven times a week; G8—1.25 μg of PMA twice a week. The PMA‐treated mice (G5 and G8) served as the positive controls. Mice were treated for 20 weeks followed by a 6‐week recovery period prior to necropsy. The incidence of dermal papillomas in the shaved area was recorded weekly. There were no spontaneous papillomas in the target area of any of the untreated (G1) or vehicle control (G2) animals in the polycarbonate cages. One papilloma was observed in the untreated mice (G6) and one in the vehicle control group (G7) in the steel cages. This suggests that the type of caging, the shaving process, microchip implantation and daily acetone treatment for 20 weeks are all consistent with a very low background incidence of papillomas in this model. Papillomas were observed in the positive control groups as early as 4 weeks of treatment and increased both in number per mouse and number of mice affected up to a maximum average of 3.5 papillomas per mouse and 55% (11/20) mice with papillomas in G5 and 2.7 and 80% (16/20) in G8. A plateau was reached at about week 13 and the numbers of papillomas remained stable through the rest of the treatment and recovery phases. The low dose of benzene (100 μl) showed no significant effect, whereas the higher dose (150 μl) produced a moderate number of papillomas beginning at about week 11. The results of this study are comparable with earlier studies at the NIEHS and indicate reproducibility between laboratories and that the Tg.AC transgenic mouse model is suitable for use in an industrial pre‐clinical safety evaluation context.