v-Ha-ras transgene abrogates the initiation step in mouse skin tumorigenesis: effects of phorbol esters and retinoic acid

Murine mentors: transgenic and knockout models of surgical disease

OBJECTIVE

Transgenic and knockout technologies have emerged from the "molecular biology revolution" as unprecedented techniques for manipulating gene function in intact mice. The goals of this review are to outline the techniques of creating transgenic and knockout mice, and to demonstrate their use in elucidation of the molecular mechanisms underlying common surgical diseases.

SUMMARY BACKGROUND DATA

Gain of gene function is created by transgenic technology, whereas gene function is ablated using gene knockouts. Each technique has distinctive applications and drawbacks. A unique feature of genetically manipulated mice is that combinatorial genetic experiments can be executed that precisely define the functional contribution of a gene to disease progression. Transgenic and knockout mouse models of wound healing, cardiovascular disease, transplant immunology, gut motility and inflammatory bowel disease, and oncology are beginning to illuminate the precise molecular regulation of these diseases. Transgenic technology has also been extended to larger mammals such as pigs, with the goal of using genetic manipulation of the xenogenic immune response to increase the availability of transplant organs. Continual refinements in gene manipulation technology in mice offer the opportunity to turn genes on or off at precise time intervals and in particular tissues, according to the needs of the investigator. Ultimately, investigation of disease development and progression in genetically manipulated mammals may delineate new molecular targets for drug discovery and provide novel platforms for drug efficacy screens.

CONCLUSIONS

Emulation of human disease and therapy using genetically manipulated mammals fulfills a promise of molecular medicine: fusion of molecular biochemistry with "classical" biology and physiology. Surgeons have unique skills spanning both worlds that can facilitate their success in this expanding arena.

The transgenic Tg.AC mouse model for identification of chemical carcinogens

The Tg.AC (zetaglobin promoted v-Ha-ras) transgenic mouse is being evaluated as a short-term carcinogenicity bioassay. In order to harmonize the evaluation effort in diverse laboratories, an operational bioassay protocol has been established. Data, based principally on retrospective assay of known carcinogens or tumor promoters and non-carcinogens, are presented that support the operational protocol. The Laboratory of Environmental Carcinogenesis and Mutagenesis at the NIEHS has been evaluating transgenic rodent models for utility in differentiating carcinogens from non-carcinogens. Our main approach in this method development effort has been to retrospectively study responses of the models to chemicals of known rodent carcinogenic potential. To this end we have tested mainly chemicals that have been previously studied in chronic rat and/or mouse bioassays by the National Toxicology Program. Development of the data base and assessment of the utility of the models will be immeasurably aided by the availability of a standardized experimental protocol. The purpose of this communication is to present the elements of the Laboratory of Environmental Carcinogenesis and Mutagenesis Tg.AC mouse bioassay protocol and to show experimental results that led to the development of our study design.

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.

Use of DNA repair-deficient XPA transgenic mice in short-term carcinogenicity testing

At present (putative) human carcinogens are identified via epidemiological studies and testing using the chronic 2-yr rodent bioassay. Both methods have severe limitations in that they are slow, insensitive, expensive, and are also hampered by many uncertainties. The development of methods to modify specific genes in the mammalian genome has provided promising new tools for use in identifying carcinogens and characterizing their (qualitative) risk. Several transgenic mouse lines are currently under study to test their possible use in short-term carcinogenicity testing. One such candidate alternative transgenic model is the XPA knock-out mouse. These mice have an almost complete deficiency in DNA nucleotide excision repair (NER). Nevertheless, XPA-deficient mice are viable and have a background of a low incidence of spontaneous development of cancers. Approximately 15% of the mice develop hepatocellular adenomas (only after 1.5 yr). After treatment with ultraviolet-B radiation or 7,12-dimethylbenz(a)anthracene, the XPA-deficient mice developed squamous cell carcinomas and papillomas, respectively, on their skin. Oral treatment of XPA-deficient mice with benzo[a]pyrene (B[a]P), 2-acetylaminofluorene (2-AAF), and 2-amino-1-methyl-6-phenylimidazo [4,5-b]-pyridine (PhIP) resulted in lymphomas (B[a]P), liver and bladder tumors (2-AAF), and intestinal adenomas plus lymphomas (PhIP). These results look encouraging, but it should be noted that the compounds and agents tested thus far have all been substrate for nucleotide excision repair. Animal studies with different genotoxic or nongenotoxic compounds, as organized for instance within the framework of the International Life Sciences Institute/Health and Environmental Sciences Institute program, are needed to further evaluate the suitability of the XPA model for short-term carcinogenicity testing.

Photocarcinogenesis and susceptibility to UV radiation in the v-Ha-ras transgenic Tg.AC mouse

The v-Ha-ras transgenic Tg.AC mouse line has proven to be a useful model for the study of chemical carcinogenic potential. We undertook experiments designed to study the effect of the physical carcinogen, UV radiation, on tumorigenesis in this mouse strain. Following a total of three exposures on alternating days to a radiation source covering a cumulative UVR exposure range of 2.6-42.6 kJ per m2, squamous papillomas developed by 4 wk after initial exposure in a dose-dependent manner. Malignancies developed within 18-30 wk following the initial UVR exposure and were all diagnosed as squamous cell carcinoma or spindle cell tumors. In contrast to other mouse stains used in photocarcinogenesis studies, few p53 mutations were found in Tg.AC malignancies upon polymerase chain reaction-single stranded conformational polymorphism analysis of exons 4-8 followed by sequencing of suspicious bands; however, all tumors analyzed by in situ hybridization expressed the v-Ha-ras transgene. Immunohistochemical analysis of UVR-exposed skin taken 24 h after the last of three exposures (13.1 kJ per m2 total UVR) showed expression of p53 in hair follicles and in interfollicular epidermis, which indicates that the gene was functional. Thus, although there are some differences between the Tg.AC and other mouse models, these results suggest that the Tg.AC mouse may be a useful model for the study of acute exposure photocarcinogenesis.

Dermal carcinogenicity in transgenic mice: relative responsiveness of male and female hemizygous and homozygous Tg.AC mice to 12-O-tetradecanoylphorbol 13-acetate (TPA) and benzene

Assessment of the carcinogenic potential of chemical agents continues to rely primarily upon the chronic rodent bioassay, a resource-intensive exercise. Recent advances in transgenic technology offer a potential resource conserving approach to carcinogen detection. Incorporation of oncogenes with known roles in the development of neoplasms into the genomes of laboratory rodents may provide new models with the potential of quickly and accurately separating carcinogenic from noncarcinogenic chemicals. The insertion of the v-Ha-ras oncogene into the genome of FVB/N mice imparts the qualities of genetically initiated skin in the transgenic mouse line designated as Tg.AC. The skin of either hemizygous (animals carrying the transgene on 1 allele) or homozygous (transgene copies on both alleles) Tg.AC mice promptly responds to the application of nongenotoxic carcinogens, such as the classical tumor promoting phorbol esters, with the development of squamous papillomas. Tumor production generally begins after 8-10 applications of 2.5 micrograms/mouse (3 times/wk) of 12-O-tetradecanoylphorbol 13-acetate (TPA). Maximal tumor response is usually in evidence within 20 wk. If this transgenic mouse line is to be useful in the identification of carcinogenic chemicals, experimental protocols must be systematically optimized. Experiments were conducted to compare the relative responsiveness of male and female hemizygous and homozygous Tg.AC mice to the dermal application of TPA and the known human leukemogen, benzene. Results revealed shipment-related variabilities in the relative responsiveness of hemizygous male and female mice to the application of the proliferative agent. Homozygous mice of both sexes were more reliable and uniform in responsiveness to both TPA and benzene. Therefore, our standard protocol for the conduct of bioassays with the Tg.AC mouse line specifies the use of homozygous males and/or females.

Morphological characterization of spindle cell tumors induced in transgenic Tg.AC mouse skin

Transgenic Tg.AC mice carry a v-Ha-ras coding region flanked by a zeta-globin promoter and an SV40 polyadenylation signal sequence. These mice respond to carcinogens by developing epidermal papillomas. In some cases, malignancies develop at the sites of these papillomas. Various patterns of squamous cell differentiation were observed in these malignancies. One malignancy that developed at the site of the papillomas was composed of bundles of spindle cells. This lesion is difficult to distinguish from fibrosarcomas by light microscopy. We characterized 16 of these malignancies (tentatively classified as spindle cell tumors) to determine if they were of epithelial or mesenchymal origin. Papillomas were induced in Tg.AC mice by full thickness wounding, 12-O-tetradecanoyl-13-phorbol acetate treatment, or ultraviolet radiation. With time, some papillomas became broad-based, downwardly invading lesions. These lesions were examined by light microscopy with immunohistochemical analysis for cytokeratins and by electron microscopy. Immunohistochemical examination with a polyclonal anti-cytokeratin antibody demonstrated various degrees of keratin staining in all tumors examined. Attenuated desmosomes were also observed in these lesions by electron microscopy. These results indicate an epithelial origin for these malignancies; therefore, they should be classified as spindle cell carcinomas.

An evaluation of the hemizygous transgenic Tg.AC mouse for carcinogenicity testing of pharmaceuticals. II. A genotypic marker that predicts tumorigenic responsiveness

The Tg.AC transgenic mouse skin paint assay is one of the short-term carcinogenesis models that has been proposed as a replacement for 1 species in the conventional 2-yr bioassay required for safety testing of pharmaceuticals. In our initial efforts to evaluate the sensitivity and specificity of this model for human pharmaceuticals, 61% of the hemizygous Tg.AC mice in the positive control groups were refractory to treatment with 12-O-tetradecanoylphorbol 13-acetate (TPA). Tg.AC mice are reported to carry < or = 10 copies of a transgene consisting of a zeta-globin promoter fused to the v-Ha-ras structural gene with a terminal simian virus 40 (SV40) polyadenylation signal. Southern blot hybridization of genomic DNA from 66 tail biopsies using a zeta-globin probe revealed that all of the hemizygous. Tg.AC mice screened contained approximately 40 copies of the transgene and that mice unresponsive to TPA had lost a 2-kb BamHI fragment containing zeta-globin promoter sequence. By systematic screening of Tg.AC breeder mice for this diagnostic marker of phenotypic responsiveness, it should be possible to selectively enrich the Tg.AC mouse colony to consist exclusively of responders and to guard against further genetic instability.

Tripropylene glycol diacrylate but not ethyl acrylate induces skin tumors in a twenty-week short-term tumorigenesis study in Tg.AC (v-Ha-ras) mice

The toxicity of the esters of acrylic acid are poorly understood even though significant human exposure occurs. To conduct rapid comparative short-term bioassays, we used the Tg.AC (v-Ha-ras) transgenic mouse model to determine the toxicity and potential carcinogenicity of tripropylene glycol diacrylate (TPGDA) alone and in a reference formulated ultraviolet radiation curable lacquer (Lacquer A), which is used in the ultraviolet radiation curable surface coatings. For comparison, ethyl acrylate (EA) was used as a reference acrylate. Insertion of the zeta-globin promoted v-Ha-ras transgene into the FVB mouse genome (Tg.AC) introduced a defined genetic lesion, which is critical but insufficient by itself to induce benign or malignant tumors in the skin unless activated. Activation and expression of the transgenic ras oncoprotein in this mouse line induces a dose-related increase in papillomas (skin reporter phenotype) within weeks. Based on dose-related increases in skin hyperplasia following dermal exposure to EA, TPGDA, or Lacquer A (applied equimolar for TPGDA concentration), the dosing regimen was selected. Starting at 12 wk of age, the agents were administered topically (200 microliters of acetone vehicle) 3 times/wk for 20 wk to the shaved dorsal skin of female Tg.AC mice (n = 10/group). TPGDA and reference Lacquer A (equimolar for TPGDA) at 5 or 10 mumoles/mouse but not EA (60, 300, or 600 mumoles/mouse) or TPGDA or Lacquer A at 1 mumole/mouse induced a dose-related increase in papillomas between 6 and 12 wk of treatment that reached a maximum number of papillomas per mouse between 19 and 20 wk of treatment. These results indicate that TPGDA is significantly more potent than EA for inducing the skin reporter phenotype and may be predicted to be carcinogenic in long-term cancer bioassays at the site of contact.

An evaluation of the hemizygous transgenic Tg.AC mouse for carcinogenicity testing of pharmaceuticals. I. Evidence for a confounding nonresponder phenotype

We have completed 2 26-wk studies to evaluate the hemizygous transgenic Tg.AC mouse, which has been proposed as an alternative short term model for testing carcinogenicity. We attempted to evaluate the response to the known rodent carcinogens cyclophosphamide, phenolphthalein, and tamoxifen and to the noncarcinogen chlorpheniramine following topical application. In the first study, a weak response (2/17 animals) was observed to the positive control 12-O-tetradecanoylphorbol 13-acetate (TPA in ethanol, 1.25 micrograms), and no response was observed to cyclophosphamide, phenolphthalein, or chlorpheniramine, despite evidence for skin penetration. The second study compared 1.25 micrograms and 6.25 micrograms of TPA in ethanol and acetone solutions. Tamoxifen was also evaluated in both solvents and orally. No significant response was observed to tamoxifen by skin paint or oral routes. Over 60% of the high dose TPA-treated animals showed no (0 or 1) papilloma response, and 30% of the animals each developed more than 32 papillomas. The heterogenous response to high dose TPA may be related to variability in the responsiveness of hemizygous animals. In light of these findings, further Tg.AC studies should employ homozygous animals, and the underlying cause for heterogeneity in the tumorigenic response of Tg.AC mice should be identified and eliminated.

TGF alpha is dispensable for skin tumorigenesis in Tg.AC mice

Alterations in growth factor signaling pathways frequently accompany the development and maintenance of epithelial neoplasia. Transforming growth factor alpha (TGF alpha) and its epidermal growth factor receptor have been thought to play an especially important role in epithelial neoplasia. In this study, mice were derived genetically deficient (null) in functional TGF alpha expression and carrying the Tg.AC/v-Ha-ras transgene. The goals were to determine if (a) papillomagenesis was dependent on TGF alpha and (b) progression to malignancy was dependent on TGF alpha expression. Groups of male and female mice heterozygous or homozygous for the TGF alpha null allele and hemizygous for the Tg.AC transgene were treated twice weekly for 10 or 15 wk with doses of 12-O-tetradecanoylphorbol-13-acetate (TPA) known to produce papillomas in Tg.AC mice. Papillomas were readily induced in both male and female TGF alpha null mice. Malignant progression of papillomas was observed in all TGF alpha null treatment groups. Additionally, we examined the response of TGF alpha null mice to full thickness dorsal wounds, a stimulus known to promote papillomagenesis in Tg.AC mice. As in the TPA study, papillomas were induced in both male and female TGF alpha null mice. These studies indicate that TGF alpha is not required for the induction and maintenance of papillomas nor is it essential for the malignant conversion of papillomas in Tg.AC mice.

Association of v-Ha-ras transgene expression with development of erythroleukemia in Tg.AC transgenic mice

A transgenic mouse line (Tg.AC) carrying an activated v-Ha-ras oncogene fused to the embryonic zeta-globin promoter develops an array of spontaneous epithelial and mesenchymal neoplasms. In this report we describe the morphological, immunophenotypic, and molecular features of a unique hematopoietic neoplasm in these mice. The cardinal lesion of this disease is marked hepatomegaly due to leukemic proliferation and infiltration. In the peripheral blood, there is a marked increase in the number of metarubricytes and other less differentiated erythroid progenitor cells. Leukemic cells stain positively with an erythroid-associated nuclear transcription factor (GATA-1). Using a reverse transcription polymerase chain reaction assay, co-expression of GATA-1 and endogenous zeta-globin genes is detected in hematopoietic tissues of nonleukemic transgenic and nontransgenic mice. ras transgene expression is, however, detected only in normal bone marrow and leukemic tissues of transgenic mice, and 5' mapping experiments using S1 protection analysis of total RNA from leukemic tissue indicates that transcription of the transgene mRNA is initiated from the natural zeta-globin promoter start site, supporting the belief that the zeta-globin promoter directs v-Ha-ras expression in erythroid progenitor cells, ultimately leading to leukemic transformation.

The National Toxicology Program evaluation of genetically altered mice as predictive models for identifying carcinogens

National Institute of Environmental Health Sciences researchers are exploring the utility of genetically altered mice to study mechanisms of carcinogenesis. Two of these mouse models, the Tg.AC (carrier of an activated mouse H-ras oncogene) and the p53+/- (heterozygous for the wild-type tumor suppressor gene Trp53), have genetic alterations that appear to hasten their expression of chemically induced tumors. These 2 models have been proposed as a basis for new strategies for identifying chemical carcinogens and for assessing risk. The National Toxicology Program (NTP) is conducting a series of studies with these 2 genetically altered strains to further examine their strengths and weaknesses for identification of documented rodent and human carcinogens. In this first evaluation, candidates for study were drawn from the NTP historical database of 2-yr rodent carcinogenicity studies and the open literature (primarily for drugs). Results with this first set of 11 chemicals tested in genetically altered mice, compared with previous findings in the traditional 2-yr rodent assays and literature on human tumor findings, appear to support the premise advanced by Tennant et al that these models have the potential to serve as more rapid and less expensive test systems to identify carcinogens.

Comparative expression of novel vascular endothelial growth factor/vascular permeability factor transcripts in skin, papillomas, and carcinomas of v-Ha-ras Tg.AC transgenic mice and FVB/N mice

One of the most frequently detected changes in human solid tumors is the mutation of the ras oncogene, which has been associated with production of angiogenic growth factors such as vascular endothelial growth factor/vascular permeability factor (VEGF/VPF). Using the v-Ha-ras Tg-AC transgenic mice and the background FVB/N strain of inbred mice, the pattern of expression of specific VEGF/VPF transcripts was characterized in major organs and in skin, papillomas, and carcinomas during multi-stage skin carcinogenesis. Three VEGF/VPF transcripts were found to be constitutively expressed in skin as well as the major organs in both mouse strains, which corresponded in size and sequence to previously reported murine VEGF120 with a bp size of 331, VEGF164 with a bp size of 333, and VEGF188 with a bp size of 407. A previously unreported fourth murine transcript was also detected in skin and major tissues from both mouse strains which corresponded to rat VEGF144, with a bp size of 404. In addition, a unique 425 bp VEGF transcript which corresponded to human VEGF205 was present in highly vascularized tissues including heart, lung, liver, kidney, brain, as well in papillomas and carcinomas isolated from v-Ha-ras Tg.AC mice. In contrast, VEGF205 was present only in carcinomas derived from FVB/N mice. An antibody generated from a peptide sequence designed to detect each of the five VEGF/VPF peptides defined by RT-PCR analysis confirmed the existence of these five peptides and confirmed that the murine VEGF205 peptide was selectively expressed in papillomas and carcinomas derived from v-Ha-ras Tg.AC mice. These results demonstrate that there is significant alternative splicing of the murine VEGF/VPF gene during multi-stage carcinogenesis, which results in four commonly expressed VEGF transcripts. In addition, these studies identified a fifth VEGF transcript and peptide at the later stages of tumor promotion and in progression which appears to be linked to the presence of v-Ha-ras.

Transgenic assays for mutations and cancer: current status and future perspectives

Transgenic mouse modelling has proved to be a powerful approach to explore the various steps involved in spontaneous and induced carcinogenesis. Some of the multitude of models currently available have the potential to become a substitute for the expensive, long-term rodent bioassay to predict carcinogenicity of environmental compounds. Here, we review the progress in the development and use of transgenic mouse models specifically for the purpose of carcinogenicity and mutagenicity testing.

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 zeta-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.

Evaluation and validation issues in the development of transgenic mouse carcinogenicity bioassays

Transgenic mouse models have emerged as plausible alternatives to long-term bioassays for carcinogenicity. Three transgenic lines evaluated to date have shown a clear capability to discriminate between carcinogens and noncarcinogens, using long-term bioassay results as the standard. The data also suggest that the transgenic lines will not fully duplicate long-term bioassay results. It is proposed that these models do not respond to chemicals that have induced highly restricted species or strain-specific tumor responses in mice or rats. Rather, the value of the transgenic models is predicated on a preferential response to transspecies carcinogens (i.e., those positive in both rats and mice, often including tumors in the same tissues). Thus, although results in transgenic models may not be completely concordant with long-term bioassays, the data can be used effectively in chemical and drug safety assessments. Further, it is proposed that validation of the models is readily achievable via ongoing studies. Validation of any alternative model is best achieved by sufficient mechanistic understanding of the model to reasonably predict the outcome of bioassays conducted in the models and use all available information on the drug or chemical. This goal can now be met with the transgenic mouse lines.

Hepatocarcinogenesis (Z#2)/mutagenesis during initiation stage

Previously, we developed a model for high incidence, endogenously generated hepatocellular carcinoma (HCC), the human alpha-1-antitrypsin (alpha1AT) Z gene transgenic mouse (Z#2). We now examine the potential utility of a model for endogenous carcinogenesis utilizing the Z#2 mouse also transgenic for the lacI gene, a convenient target for in vivo mutagenesis studies. We crossed the Z#2 line and mice transgenic for lambda/lacI shuttle vector (Big Blue), for determination of lacI mutant frequency during initiation of endogenous carcinogenesis. Five month old double transgenic mice (Z#2+/lacI+) successfully displayed: (1) the expected post-inflammatory stage of Z#2 carcinogenesis; and (2) hepatic lacI mutants measured at frequencies (10(-5)-10(-4)) useful to mutagenesis studies. In this study, hepatic lacI mutation frequencies in Z#2 transgenic mice appeared to be only slightly increased (< 2x) when compared to age matched negative controls. In the future, it may be important to reconcile possibly limited lacI mutagenesis at the time of initiation and demonstrated high incidence of hepatocarcinogenesis.

The U.S. National Toxicology Program evaluation of transgenic mice as predictive models for identifying carcinogens

National Institute of Environmental Health Sciences researchers have invested considerable effort in exploring the utility of transgenic mice to detect carcinogens and study mechanisms of carcinogenesis. Work has assessed several mouse models genetically altered to enhance their expression of chemically induced tumors. Results with the p53def (hemizygous for the tumor-suppressor gene) and the Tg.AC (carrier of an activated H-ras oncogene) mice have been used as a basis for a proposed new strategy for identifying chemical carcinogens and assessing risk. The U.S. National Toxicology Program is conducting a series of studies with these two transgenic strains to further examine their strengths and weaknesses for identification of documented rodent and human carcinogens and to explore their ability to provide information concerning the effective dosimetry for target organ mutation.

In vivo transgenic bioassays and assessment of the carcinogenic potential of pharmaceuticals

There is general agreement in the scientific community on the need to improve carcinogenicity testing and the assessment of human carcinogenic risk and to incorporate more information on mechanisms and modes of action into the risk assessment process. Advances in molecular biology have identified a growing number of genes such as protooncogenes and tumor-suppressor genes that are highly conserved across species and are associated with a wide variety of human and animal cancers. In vivo transgenic rodent models incorporating such mechanisms are used to identify mechanisms involved in tumor formation and as selective tests for carcinogens. Transgenic methods can be considered an extension of genetic manipulation by selective breeding, which long has been employed in science and agriculture. The use of two rodent species in carcinogenicity testing is especially important for identifying transspecies carcinogens. The capacity of a substance to induce neoplasia across species suggests that the mechanism(s) involved in the induction of the neoplasia are conserved and therefore may have significance for humans. Based on available information there is sufficient experience with some in vivo transgenic rodent carcinogenicity models to support their application as complementary second species studies in conjunction with a single 2-year rodent carcinogenicity study. The optional substitution of a second 2-year rodent carcinogenicity study with an alternative study such as an in vivo transgenic carcinogenicity study is part of the International Conference on Harmonization guidance S1B: Testing for Carcinogenicity of Pharmaceuticals. This guidance is intended to be flexible enough to accommodate a wide range of possible carcinogenicity assessment models currently under consideration or models that may be developed in the future. The use of an in vivo transgenic mouse model in place of a second 2-year mouse study will improve the assessment of carcinogenic risk by contributing insights into the mechanisms of tumorigenesis and potential human relevance not available from a standard 2-year bioassay. It is envisioned that this will stimulate the further development of more efficient and relevant methods for identifying and assessing potential human carcinogenic risk, which will benefit public health.

Inhibitory effect of soybean hypocotyls on Epstein-Barr virus early antigen induction and skin tumor promotion

The in vitro anti-tumor promoting effect of hypocotyls from fresh soybeans was evaluated. The dimethyl sulfoxide extracts of hypocotyls showed a stronger inhibitory effect than that of soybeans on Epstein-Barr virus early antigen activation induced by 12-O-tetradecanoylphorbol-13-acetate. Daidzin, an isoflavone with the highest content of hypocotyls, was also inhibitory. An in vivo evaluation of anti-tumor promoting activity of hypocotyls against the skin of mice also revealed a significant inhibitory effect on tumor formation.

Evidence that initiated keratinocytes clonally expand into multiple existing hair follicles during papilloma histogenesis in SENCAR mouse skin

We have previously shown that the precursors of cutaneous papillomas in SENCAR mice initiated with 7,12-dimethylbenz[a]anthracene and promoted with 12-O-tetradecanoylphorbol-13-acetate are focal hyperplastic lesions that we refer to as squamous cell hyperplastic foci (SCHF). Ha-ras gene codon 61 mutations were frequently found in SCHF, providing evidence that these lesions represent clones of initiated cells. We report here the pathogenesis of multiple hair follicle involvement in more advanced SCHF and describe the role of the hair follicle in papilloma histogenesis. Detailed histological evaluation of 83 SCHF and 25 early papillomas revealed a morphological continuum from the least developed SCHF, involving only one hair follicle, to advanced SCHF and early papillomas, which involved more than 10 hair follicles. These results provide evidence of the recruitment of additional hair follicles as SCHF progress. In advanced SCHF and early papillomas the bulk of the epithelial component in all cases consisted of several markedly hyperplastic adjacent hair follicles, whereas the involved interfollicular epidermis (IFE) was generally less hyperplastic. All of the hair follicles involved in SCHF appeared to have been preexisting, based on their pattern of spacing, that they were consistently normal appearing below the level of the sebaceous glands, and that they were in the same phase of the hair cycle as surrounding, uninvolved hair follicles. Also, no evidence of follicular neogenesis was observed in serially sectioned SCHF, and coalescence of smaller lesions was rare. To investigate whether the involvement of multiple hair follicles in SCHF was due to expansion of initiated cells into existing hair follicles or, possibly, to a paracrine mechanism, we analyzed different levels of three serially sectioned SCHF and one early papilloma for Ha-ras mutations. These analyses revealed cells with Ha-ras gene codon 61 mutations at multiple levels that involved different hair follicles. Overall, our results provide evidence that as initiated cells clonally expand, they spread across the IFE and populate the upper permanent portions of existing hair follicles. The abnormal proliferation of the infundibula of the hair follicles involved in SCHF appears to give rise to most of the epithelial component of papillomas.

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 zeta 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.

Delays in malignant tumor development in transgenic mice by forced epidermal keratin 10 expression in mouse skin carcinomas

The keratin cytoskeleton is formed in different epidermal compartments by distinct polypeptides. Basal, proliferative keratinocytes express keratin (K) 5 and K14, whereas, suprabasal, post-mitotic keratinocytes express K1 and K10. Changes in this keratin pattern have been found to occur in hyperproliferative skin disorders and, in particular, throughout mouse epidermal carcinogenesis. Whereas some keratins not found in normal epidermis (K6, K16, K13, and K8) are induced at different stages of tumor development, K1 and K10 expression is lost. To determine whether K1 and K10 loss is just a consequence of the altered differentiation program or an event required for tumor progression, we generated transgenic mice carrying the human keratin 10 gene (hK10) under the control of a bovine keratin 6 gene regulatory region, which is silent in normal skin but is induced and drives transgene expression in hyperproliferative skin keratinocytes and, therefore, in skin tumors. Transgenic animals subjected to a complete carcinogenesis protocol developed tumors that contained various amounts of transgenic hK10. Although no significant difference was found in tumor number or malignancy, tumor onset was significantly delayed in transgenic mice, indicating that the presence of K10 actually impairs tumor development.

Allergic contact sensitizing chemicals as environmental carcinogens

Chemicals that were bioassayed by the National Toxicology Program (NTP) and that also produce allergic dermatitis (ACD) in humans were evaluated for their tumorigenic characteristics. The impetus for the study was that most contact sensitizers, i.e., those that produce ACD, and genotoxic carcinogens are chemically similar in that they are electrophilic, thereby producing adducts on macromolecules including protein and DNA. This similarity in chemical behavior suggests that many contact sensitizers might be environmental carcinogens. All of the published NTP bioassays by early 1996 that had both genotoxicity and carcinogenicity studies were included in this analysis. The NTP chemicals had been chosen for bioassay without regard to their ability to produce ACD. Of the 209 chemicals that were bioassayed, there were 36 (17%) that were known to be human contact sensitizers; about half of these were positive on tumor bioassays. The contact sensitizers differed from the NTP sample as a whole by having a proportionately larger number of nongenotoxic chemicals by the Ames Salmonella assay, presumably because more of them were selected on the basis of widespread usage rather than structural resemblance to known carcinogens. Compared to the nongenotoxic chemicals, the genotoxics were stronger carcinogens in that they had a higher incidence of positive tumor bioassays, with twice the number of organs in which tumors were induced. The nongenotoxic chemicals had a preference for tumor induction in parenchymal tissues in contrast to epithelial tissues. The contact sensitizers showed essentially the same characteristics as the whole NTP sample when stratified according to genotoxicity. Judging by the chemicals that were chosen primarily for their widespread use rather than for their structural resemblance to carcinogens, the addition of a test for contact sensitization to the Ames test as a screening tool would increase the tumorigenic detection efficiency by about 40% because of the nongenotoxic tumorigens. A ballpark estimate suggests that there could be several thousand contact sensitizers for humans in commercial use that are rodent tumorigens.

Decreases in phorbol ester-induced papilloma development in v-Ha-ras transgenic TG.AC mice during reduced gene dosage of bcl-2

We have demonstrated that induction of transgene expression in the v-Ha-ras-transgenic TG.AC mouse is a critical event in skin tumorigenesis and that cutaneous papillomas arise from follicular epidermis after treatment with chemical carcinogens. The sensitivity of TG.AC mice to skin tumorigenesis, coupled with their low incidence of spontaneous skin tumors, makes this strain a good model for identifying carcinogens and for investigating the roles that other genes may play in the development of skin neoplasia. To investigate the possible involvement of the bcl-2 gene in skin tumorigenesis in the TG.AC mouse, we crossed heterozygous bcl-2-knockout mice (C57BI/6, 129 background) with TG.AC mice (FVB/N background). Female mice were genotyped by using a neo cassette to identify bcl-2-deficient mice. In addition, homozygous TG.AC mice were bred with FVB/N mice to generate hemizygous TG.AC mice on an FVB/N background to serve as a gene-dosage control. The F1 progeny consisted of FVB/N(v-Ha-ras+/-):C57BI/6,129(bcl-2+/+),FVB/N(v-Ha-ra s+/-):C57BI/6,129(bcl-2+/-), and FVB/N(v-Ha-ras+/-,bcl-2+/+). Ten-week-old mice were dosed twice weekly for 10 wk with acetone, 1.25 microg of 7,12-tetradecanoylphorbol-13-acetate (TPA), or 2.5 microg of TPA, and papillomas were counted weekly. Papillomas were analyzed for ras transgene and bcl-2 expression by reverse transcription-polymerase chain reaction, v-Ha-ras expression by in situ hybridization, and proliferating cell nuclear antigen expression by immunohistochemical analysis. Fewer papillomas (P < 0.05) were observed at the low dose of TPA (1.25 microg) in mice carrying the bcl-2 knockout allele than in the wild-type mice, suggesting that reduction of the bcl-2 gene product affects the susceptibility of TG.AC mice to TPA-induced papillomas. However, at the high dose of TPA (2.5 microg), there was no difference in papilloma response between knockout and wild-type mice, regardless of strain background. This suggests that at the higher dose of TPA, the effect of reduction in bcl-2 gene product was obscured. These results support the hypothesis that bcl-2 plays a limited role in skin tumorigenesis in the TG.AC mouse.

Methods for predicting carcinogenic hazards: new opportunities coming from recent developments in molecular oncology and SAR studies

Without epidemiological evidence, and prior to either short-term tests of genotoxicity or long-term tests of carcinogenicity in rodents, an initial level of information about the carcinogenic hazard of a chemical that perhaps has been designed on paper, but never synthesized, can be provided by structure-activity relationship (SAR) studies. Herein, we have reviewed the interesting strategies developed by human experts and/or computerized approaches for the identification of structural alerts that can denote the possible presence of a carcinogenic hazard in a novel molecule. At a higher level of information, immediately below epidemiological evidence, we have discussed carcinogenicity experiments performed in new types of genetically engineered small rodents. If a dominant oncogene is already mutated, or if an allele of a recessive oncogene is inactivated, we have a model animal with (n-1) stages in the process of carcinogenesis. Both genotoxic and receptor-mediated carcinogens can induce cancers in 20-40% of the time required for classical murine strains. We have described the first interesting results obtained using these new artificial animal models for carcinogenicity studies. We have also briefly discussed other types of engineered mice (lac operon transgenic mice) that are especially suitable for detecting mutagenic effects in a broad spectrum of organs and tissues and that can help to establish mechanistic correlations between mutations and cancer frequencies in specific target organs. Finally, we have reviewed two complementary methods that, while obviously also feasible in rodents, are especially suitable for biomonitoring studies. We have illustrated some of the advantages and drawbacks related to the detection of DNA adducts in target and surrogate tissues using the 32P-DNA postlabeling technique, and we have discussed the possibility of biomonitoring mutations in different human target organs using a molecular technique that combines the activity of restriction enzymes with polymerase chain reaction (RFLP/PCR). Prediction of carcinogenic hazard and biomonitoring are very wide-ranging areas of investigation. We have therefore selected five different subfields for which we felt that interesting innovations have been introduced in the last few years. We have made no attempt to systematically cover the entire area: such an endeavor would have produced a book instead of a review article.

Topics in cancer risk assessment

The estimation of carcinogenic risks from exposure to chemicals has become an integral part of the regulatory process in the United States within the past decade. With it have come considerable controversy and debate over the scientific merits and shortcomings of the methods and their impact on risk management decisions. In this paper we highlight selected topics of current interest in the debate. As an indication of the level of public concern, we note the major recent reports on risk assessment from the National Academy of Sciences and the U.S Environmental Protection Agency's proposed substantial revisions to its Guidelines for Carcinogen Risk Assessment. We identify and briefly frame several key scientific issues in cancer risk assessment, including the growing recognition of the importance of understanding the mode of action of carcinogenesis in experimental animals and in humans, the methodologies and challenges in quantitative extrapolation of cancer risks, and the question of how to assess and account for human variability in susceptibility to carcinogens. In addition, we discuss initiatives in progress that may fundamentally alter the carcinogenesis testing paradigm.

Cellular aging is a critical determinant of primary cell resistance to v-src transformation

Primary cell cultures are in general resistant to the transforming effect of a single oncogene, a finding considered consistent with the multistage theory of carcinogenesis. In the present studies, we examined whether cellular age, differentiation stage, and/or tissue origin of primary cells plays a role in determining their response to v-src transformation. To study the role of cellular age, rat mammary fibroblasts were isolated from a 50-day-old female rat and infected with a recombinant retrovirus carrying a v-src gene after 2, 7, 14, 21, and 28 days of continuous growth. To determine whether cellular differentiation is important, fibroblasts were isolated from embryos at 12 and 16 days of gestation, from newborns, and from a 30-day-old rat and similarly infected. Finally, the role of primary-cell histogenesis was assessed by infecting primary cultures of fibroblasts isolated from the mammary gland, dermis, and lungs of a mature rat. When compared to 3Y1 cells, all preparations of primary cultures exhibited considerable resistance to v-src transformation. However, whereas primary cells isolated from different tissues responded similarly to the transforming effect of the oncogene, major differences were observed when cells were transduced at different stages of their in vitro life span. v-src was capable of inducing formation of foci and growth in soft agar in early-passage cells but failed to do so in primary cultures infected after 14 days of continuous passaging. Similarly, both the number of foci and the number of colonies in soft agar decreased with tissue donor age. The differential response of young and senescing cells could not be explained by mutations in v-src provirus, by differences in functional v-src expression, or by growth stimulation or suppression via paracrine mechanisms. Furthermore, v-src cooperated with an immortalizing gene, like simian virus 40 large T, polyomavirus large T, E6 and E7 of human papillomavirus, or an activated p53 mutant, to induce anchorage-independent growth of primary cultures but failed to do so with cytoplasmic transforming genes, like v-abl, v-ras, or v-raf, which did not confer indefinite division potential. These studies indicate that cellular aging is a critical determinant of primary-cell resistance to v-src transformation. It is suggested that v-src requires a nuclear auxiliary function for transformation which is present in early-passage cells, particularly when these cells are derived from embryonic tissue, but is lost as cells approach replicative senescence. This auxiliary function is provided by nuclear oncogenes but not cytoplasmic transforming genes.

Genetic predisposition and parameters of malignant progression in K14-HPV16 transgenic mice

Reproducible multi-stage progression to invasive squamous carcinoma of the epidermis has been achieved in transgenic mice expressing the HPV16 early-region genes, including the E6/E7 oncogenes, under the control of the human keratin-14 promoter/enhancer. Although 100% of K14-HPV16 transgenic animals develop hyperplastic and/or dysplastic lesions in several inbred backgrounds, including C57BL/6, BALB/c, and SSIN/SENCAR, only mice backcrossed into the FVB/n background progress to malignant squamous cell carcinomas of two pathological grades, well differentiated and moderate/poorly differentiated (WDSC or MPDSC, respectively), each displaying characteristic patterns of malignant behavior. WDSCs typically arise within the epidermis of the ear and invade deeply into the underlying dermis but fail to metastasize, whereas MPDSCs develop on the chest and truncal skin and invariably metastasize to regional lymph nodes. The transition to the malignant state, in 21% of FVB/n transgenic mice, is characterized by alteration of the repertoire of keratin intermediate filament proteins expressed within neoplastic epidermis, such that WDSCs maintain expression of keratins common to terminally differentiating stratified keratinocytes (K10), whereas MPDSCs are distinguished from WDSCs by activation of embryonic and mucosal keratins (K13, K8, and K19). Precursor hyperplastic and dysplastic lesions are characterized by a progressively increased proliferative index, striking morphological alterations in keratinocyte cell-cell and cell-matrix interactions, and extensive remodeling of the underlying dermal stroma. Remarkably, this extensive stromal remodeling, which may facilitate both angiogenesis and eventual tumor cell invasion, develops early at the dysplastic stage in all animals well before malignant conversion.

Spontaneous lesions in aging FVB/N mice

The FVB/N mouse strain was created in the early 1970s and has since been used extensively in transgenic research because of its well-defined inbred background, superior reproductive performance, and prominent pronuclei of fertilized zygotes, which facilitate microinjection of DNA. Little is known, however, about the survivability and spontaneous disease of nontransgenic FVB/N mice. Therefore, the purpose of this study was to determine survival to 24 mo of age and the incidence of neoplastic and nonneoplastic disease at 14 and 24 mo of age. At 14 mo of age, the incidence of tumor-bearing mice was 13% in males (n = 45) and 26% in females (n = 98). All tumors in males and most in females at this time were alveolar-bronchiolar (AB) neoplasms of the lung. Survival to 24 mo of age was approximately 60% in both sexes (29/50 males, 71/116 females), and the incidence of mice with tumors at this time was 55% in males and 66% in females. In decreasing order of frequency, the following neoplasms were observed in > 5% of subjects: in males, lung AB tumors, liver hepatocellular tumors, subcutis neural crest tumors, and Harderian gland adenomas; in females, lung AB tumors, pituitary gland adenomas, ovarian tumors (combined types), lymphomas, histiocytic sarcomas, Harderian gland adenomas, and pheochromocytomas. Compared with other mouse strains, the observed incidences of tumors in FVB/N mice suggest a higher than usual rate of lung tumors and a lower than usual incidence of liver tumors and lymphomas. This tumor profile should be considered in the interpretation of neoplastic phenotypes in FVB/N-derived transgenic lines.

Mechanism of skin tumorigenesis by contact sensitizers: the effect of the corticosteroid fluocinolone acetonide on inflammation and tumor induction by 2,4 dinitro-1-fluorobenzene in the skin of the TG.AC (v-Ha-ras) mouse

The effect of the corticosteroid fluocinolone acetonide (FA) on skin tumor induction and inflammation by the contact sensitizer dinitrofluorobenzene (DNFB) was examined. This study broadly relates to the question of whether contact sensitizers, as electrophilic chemicals that produce protein adduction, may constitute an environmental cancer hazard. The specific aim of this study was to evaluate the extent to which the immunogenic inflammatory response to DNFB, in contrast to DNFB cytotoxicity, might be responsible for tumor induction. Experiments were conducted on a transgenic (TG.AC) mouse, incorporating a mutated ras oncogene (v-Ha-ras) that responds rapidly and profusely with skin papillomas to tumor promoters as if it were genetically initiated. Various doses and patterns of DNFB and FA were applied to the skin in a 2-week period; DNFB was given four times and FA was given either with the DNFB or daily. The tumor response to DNFB was completed by 8 weeks from the first dose and was consistent with a dose-squared relationship. FA was not tumorigenic alone; when given with DNFB, it caused only a small reduction in inflammation and tumor yield. When given daily, FA increased ulcerative skin damage, inflammation, and the yield tumors. The results suggest that tumorigenesis by DNFB, in the high-dose short-term regimen used here, is mainly due to its cytotoxicity and not contact sensitization.

Identification of a cytoplasmic, phorbol ester-inducible isoform of protein tyrosine phosphatase epsilon

The protein-tyrosine phosphatase epsilon (PTP epsilon) is a transmembranal, receptor-type protein that possesses two phosphatase catalytic domains characteristic of transmembranal phosphatases. Here we demonstrate the existence of a nontransmembranal isoform of PTP epsilon, PTP epsilon-cytoplasmic. PTP epsilon-cytoplasmic and the transmembranal isoform of PTP epsilon have separate, nonoverlapping expression patterns. Further, the data clearly indicate that control of which of the two isoforms is to be expressed is initiated at the transcriptional level, suggesting that they have distinct physiological roles. PTP epsilon-cytoplasmic mRNA is the product of a delayed early response gene in NIH 3T3 fibroblasts, and its transcription is regulated through a pathway that requires protein kinase C. The human homologue of PTP epsilon-cytoplasmic has also been cloned and is strongly up-regulated in the early stages of phorbol 12-tetradecanoate 13-acetate-induced differentiation of HL-60 cells. Sequence analysis indicates and cellular fractionation experiments confirm that this isoform is a cytoplasmic molecule. PTP epsilon-cytoplasmic is therefore the initial example to our knowledge of a nontransmembranal protein-tyrosine phosphatase that contains two tandem of catalytic domains.

Protein-tyrosine phosphatase epsilon. An isoform specifically expressed in mouse mammary tumors initiated by v-Ha-ras OR neu

Transgenic mice that overexpress v-Ha-ras, c-myc, c-neu or int-2 proto-oncogenes in the mammary epithelium develop breast tumors with morphologies that are characteristic of each initiating oncogene. Since these morphological differences reflect distinctive patterns of tumor-specific gene expression, the identification of the products of these genes might shed light on the mechanisms of transformation and/or the identity of target cells that are transformed by specific classes of oncogenes. By focusing on the tyrosine phosphorylation pathway, we have found that the transmembranal protein-tyrosine phosphatase epsilon (PTP epsilon) is highly expressed in murine mammary tumors initiated by c-neu and v-Haras, but not in mammary tumors initiated by c-myc or int-2. This difference is striking and occurs both in primary tumors and in epithelial cells cultured from them. Moreover, PTP epsilon overexpression appears to be mammary tumor-specific in that it is not found in other ras-based tumors and cell lines. These observations suggest that PTP epsilon either plays a role in ras- and neu-mediated transformation of mammary epithelium or marks mammary epithelial cells particularly susceptible to transformation by these oncogenes. Because of its distinctive expression in these mammary tumors, we have further characterized murine PTP epsilon, cloning and determining the complete structures of its cDNAs and showing that it is a glycoprotein that is N-glycosylated in a tissue-specific manner.

Expression of Brca1 is associated with terminal differentiation of ectodermally and mesodermally derived tissues in mice

We have isolated genomic and cDNA clones of Brca1, a mouse homolog of the recently cloned breast cancer-associated gene, BRCA1. Brca1 encodes an 1812-amino-acid protein with a conserved zinc finger domain and significant homology to the human protein. Brca1 maps to Chromosome 11 within a region of conserved synteny with human chromosome 17, consistent with the mapping of the human gene to 17q21. Brca1 transcripts are expressed in a variety of cultured cells but reveal a specific and dynamic expression pattern during embryonic development. For example, expression is observed first in the otic vesicle of embryonic day 9.5 (E9.5) embryos. This expression diminishes and is replaced by expression in the neuroectoderm at E10.5. By E11-12.5, higher levels are observed in differentiating keratinocytes and in whisker pad primordia. Transcripts also become evident in epithelial cells of the E14-17 kidney. Brca1 expression occurs in differentiating epithelial cells of several adult organs as well, suggesting a general role in the functional maturation of these tissues. Consistent with this, Brca1 transcripts are expressed in both alveolar and ductal epithelial cells of the mammary gland. During pregnancy, there is a large increase in Brca1 mRNA in mammary epithelial cells, an increase that parallels their functional differentiation. Because high rates of breast cancer are associated with loss of BRCA1 in humans, it is possible that this gene provides an important growth regulatory function in mammary epithelial cells. In addition, increased transcription of mammary Brca1 during pregnancy might contribute, in part, to the reduced cancer risk associated with exposure to pregnancy and lactation.

Identifying chemical carcinogens and assessing potential risk in short-term bioassays using transgenic mouse models

Cancer is a worldwide public health concern. Identifying carcinogens and limiting their exposure is one approach to the problem of reducing risk. Currently, epidemiology and rodent bioassays are the means by which putative human carcinogens are identified. Both methods have intrinsic limitations: they are slow and expensive processes with many uncertainties. The development of methods to modify specific genes in the mammalian genome has provided promising new tools for identifying carcinogens and characterizing risk. Transgenic mice may provide advantages in shortening the time required for bioassays and improving the accuracy of carcinogen identification; transgenic mice might now be included in the testing armamentarium without abandoning the two-year bioassay, the current standard. We show that mutagenic carcinogens can be identified with increased sensitivity and specificity using hemizygous p53 mice in which one allele of the p53 gene has been inactivated. Furthermore, the TG.AC transgenic model, carrying a v-Ha-ras construct, has developed papillomas and malignant tumors in response to a number of mutagenic and nonmutagenic carcinogens and tumor promoters, but not to noncarcinogens. We present a decision-tree approach that permits, at modest extra cost, the testing of more chemicals with improved ability to extrapolate from rodents to humans.

Retinoid-dependent pathways suppress myocardial cell hypertrophy

Utilizing an in vitro model system of cardiac muscle cell hypertrophy, we have identified a retinoic acid (RA)-mediated pathway that suppresses the acquisition of specific features of the hypertrophic phenotype after exposure to the alpha-adrenergic receptor agonist phenylephrine. RA at physiological concentrations suppresses the increase in cell size and induction of a genetic marker for hypertrophy, the atrial natriuretic factor (ANF) gene. RA also suppresses endothelin 1 pathways for cardiac muscle cell hypertrophy, but it does not affect the increase in cell size and ANF expression induced by serum stimulation. A trans-activation analysis using a transient transfection assay reveals that neonatal rat ventricular myocardial cells express functional RA receptors of both the retinoic acid receptor and retinoid X receptor (RAR and RXR) subtypes. Using synthetic agonists of RA, which selectively bind to RXR or RAR, our data indicate that RAR/RXR heterodimers mediate suppression of alpha-adrenergic receptor-dependent hypertrophy. These results suggest the possibility that a pathway for suppression of hypertrophy may exist in vivo, which may have potential therapeutic value.

Transgenic and gene knockout mice in cancer research

Transgenic animal technology, and the use of germline manipulation for the creation of targeted gene mutations, has resulted in a plethora of murine models for cancer research. Our understanding of some of the important issues regarding the mechanisms controlling cell division, differentiation and death has dramatically advanced in recent years through exploitation of these techniques to generate transgenic mice. In particular, the generation of mice with targeted mutations in genes encoding proteins of oncological interest has proved to be a useful way of elucidating the function of these gene products in vivo. Transgenic mouse models have provided some insight into the complex oncogenic events contributing to cellular dysregulation and the loss of growth control that can lead to tumorigenesis. These animal studies have highlighted the fact that there are many different stages at which the loss of cell cycle control can occur, as a result of mutations affecting proteins anywhere from the cell surface to the nucleus. Although mutations affecting growth factors, growth factor receptors, signal transduction molecules, cytoplasmic proteins or nuclear proteins might appear to be very distinct, the end result of these changes may be accelerated and unchecked cell growth ultimately leading to cancer. It is beyond the scope of this review to mention every animal model that has been developed for cancer research, especially since many of the early studies have been covered extensively in previous reviews. This article will instead focus on a small selection of transgenic and knockout animal models which exemplify how proteins from distinct localisations along multiple pathways can contribute to loss of cell cycle control and the pathogenesis of cancer.

Transgenic mice and squamous multistage skin carcinogenesis

The use of animals models of human cancers has proved useful in the elucidation of molecular events which occur during tumour development. Mouse skin has been used as a model for human squamous cancer for a number of decades, and analysis of this model has identified a number of changes important for the evolution of malignancy. Transgenic mice offer a further avenue of advancement, allowing refinement of the model, and the ability to examine the consequences of individual events in vivo in greater detail. This article reviews the impact of transgenic approaches to our understanding of multistage squamous carcinogenesis in mouse skin.

RAR-specific agonist/antagonists which dissociate transactivation and AP1 transrepression inhibit anchorage-independent cell proliferation

Using retinoic acid receptor (RAR) reporter cells specific for either RAR alpha, beta or gamma, we have identified synthetic retinoids which specifically induce transactivation by RAR beta, while antagonizing RA-induced transactivation by RAR alpha and RAR gamma. Like RA, these synthetic retinoids allow all three RAR types to repress AP1 (c-Jun/c-Fos) activity, demonstrating that the transactivation and transrepression functions of RARs can be dissociated by properly designed ligands. Using AP1 reporter cells, we also show that glucocorticoids or vitamin D3, together with either RA or these 'dissociating' synthetic retinoids, can synergistically repress phorbol ester-induced AP1 activity. RA, but not these 'dissociating' retinoids, induced transcription of an interleukin-6 promoter-based reporter gene transiently transfected into HeLa cells together with RARs. Using Ki-ras-transformed 3T3 cells as a model system, we show that both RA and the 'dissociating' retinoids inhibit anchorage-independent cell proliferation, suggesting that retinoid-induced growth inhibition may be related to AP1 transrepression.

c-Ha-rasEJ transfection of rat aortic smooth muscle cells induces epidermal growth factor responsiveness and characteristics of a malignant phenotype

Although the role of several protooncogenes, including sis, myc, and myb in the regulation of growth and differentiation of vascular cells has been examined in some detail, limited information is available on the contribution of ras genes to these processes. In the present studies the influence of oncogenic ras transfection on the phenotypic expression of rat aortic smooth muscle cells (SMCs) was examined. Cultured rat aortic SMCs during early passage (P4) were transfected by lipofection with c-Ha-rasEJ in a pSV2 neo vector or with pSV2 neo vector alone. Stable transfectants were selected in G418 over a 6-week period. Oncogene-transfected cells (ras-LF-1) exhibited differences in morphology and growth pattern relative to vector controls (neo-LF-1), or naive SMCs, including the development of prominent processes and the appearance of focal cellular arrangements giving rise to latticelike structures. Southern analysis revealed multiple integration of oncogenic ras in ras LF-1 cells. Transfection of c-Ha-rasEJ was associated with a twofold increase in p21 levels relative to pSV2 vector controls demonstrating that exogenous ras was expressed in these cells. Overexpression of ras p21 afforded SMCs a lower serum requirement for growth compared to vector controls, anchorage independent growth on soft agar, and acquisition of epidermal growth factor (EGF) responsiveness. Stimulation of serum-deprived SMCs with 5% fetal bovine serum (FBS) increased steady-state levels of c-Ha-ras mRNA in both ras-LF-1 and neo-LF-1 but ras induction was more pronounced in ras-transfected cells. alpha-smooth muscle (SM) actin gene expression was markedly reduced in ras-transfected cells relative to vector controls. These results show that transfection of c-Ha-rasEJ into aortic SMCs induces an altered phenotypic state characterized by alterations in growth factor-related signal transduction and tumorigenic potential.

Cytogenetic analysis of malignant skin tumors induced in chemically treated TG-AC transgenic mice

TG.AC mice (which carry a v-Ha-ras transgene) rapidly develop papillomas in response to 12-O-tetradecanoylphorbol-13-acetate (TPA). Approximately 30% of the papillomas are associated with subsequent development of malignancies. Early-passage spindle-shaped tumor cells arising from explant cultures of TPA-induced tumors in TG.AC mice were tumorigenic when transplanted to syngeneic recipients. The v-Ha-ras transgene in the transplanted tumors was expressed at a high level. To identify possible genetic changes associated with the development of malignant tumors, explanted cells were cultured in vitro and assessed for karyotypic changes between the second and third passages by analyzing G-banded metaphase chromosomes. For comparison, skin malignancies were induced in nontransgenic FVB/N mice (parent strain) by 7,12-dimethylbenz[a]anthracene (DMBA) initiation and TPA promotion, and their G-banded metaphase chromosomes were analyzed. Trisomy (in at least 50% of about 30 metaphases) of chromosome 15 (in five of 15 tumors) and chromosome 6 (four of 15 tumors) was observed in TG.AC mice, independent of chemical treatment or tumor type. Of six tumors from DMBA/TPA-treated FVB/N mice, three had trisomy 10 or 15 (or both), and two appeared normal. The absence of trisomy 7 is notable because c-Ha-ras maps to that chromosome. The absence of trisomy 7 in the six FVB/N DMBA/TPA-induced skin malignancies contrasts with DMBA/TPA-induced karyotypic effects in SENCAR mice. Expression of the v-Ha-ras transgene may have precluded the requirement for endogenous mutant ras and allelic imbalance involving chromosome 7 in TG.AC mice, but it could not have in FVB/N mice. These results suggest the possibility that the observed trisomies are consequential, rather than causal, events in the development of TG.AC or FVB/N skin malignancies. Molecular genetic analysis will be required to understand the changes associated with tumorigenesis in this transgenic line as well as in the parent mouse line.

Augmented expression of cytokines in mouse epidermal tumor cells and its possible involvement in the induction of hematopoietic alterations

Mice with skin tumors induced either by 7,12-dimethylbenz[a]anthracene complete carcinogenesis or subcutaneous injection of a carcinogenic keratinocyte cell line showed moderate to severe splenomegaly as a result of an increase in splenic granulocyte-macrophage and erythroid (erythroid burst-forming unit) progenitors. To test whether the observed alterations involve the release of soluble factors by the epidermal component of skin tumors, we used an in vitro approach. A series of mouse keratinocyte cell lines resembling progressive stages of skin carcinogenesis and carrying either normal or activated Ha-ras genes were assayed for their ability to produce the factors required for colony growth of hematopoietic-committed progenitors. Only the conditioned media of keratinocytes harboring activated Ha-ras genes were able to support the growth of granulocyte-macrophage colony-forming units. In addition, preincubation of normal bone-marrow cells with conditioned media from the transformed epidermal cell lines stimulated in vitro amplification of the hematopoietic granulocyte-macrophage progenitor compartment. To identify the possible factors responsible for the activities detected in the keratinocyte-conditioned media, we performed northern blot analysis using the cytokine probes granulocyte colony-stimulating factor, macrophage colony-stimulating factor, granulocyte-macrophage colony-stimulating factor, stem cell factor, interleukin-1 alpha, interleukin-3, and tumor necrosis factor-alpha. The cell lines expressed different cytokine mRNA combinations that positively correlated with the colony-stimulating activity detected in the corresponding conditioned medium. These results suggest that transformed epidermal tumor cells in vivo may alter normal hematopoiesis as a consequence of the production of cytokines that act in autocrine or paracrine loops probably related to tumor growth.

Follicular origin of epidermal papillomas in v-Ha-ras transgenic TG.AC mouse skin

A follicular origin for some skin tumors has been hypothesized in both humans and animal models. Because of its rapid and sensitive response to tumor promoter treatment, a v-Ha-ras transgenic (TG.AC) mouse line was used to determine the origins of epidermal papillomas. Using histological studies and transgene expression as a marker for papilloma development, we determined that pedunculated papillomas arose from focal hyperplasias of the permanent portion of the follicular epithelium in phorbol 12-myristate 13-acetate-treated TG.AC mouse skin. Damage to the hair follicle by depilation was also sufficient to induce papillomas that were histologically indistinguishable from those produced by chemical exposure. Identification of the cellular origins of papillomas in this transgenic mouse model will allow for an analysis of the role of the hair follicle and hair cycle-associated signaling in tumor development.

Progressive squamous epithelial neoplasia in K14-human papillomavirus type 16 transgenic mice

To model human papillomavirus-induced neoplastic progression, expression of the early region of human papillomavirus type 16 (HPV16) was targeted to the basal cells of the squamous epithelium in transgenic mice, using a human keratin 14 (K14) enhancer/promoter. Twenty-one transgenic founder mice were produced, and eight lines carrying either wild-type or mutant HPV16 early regions that did not express the E1 or E2 genes were established. As is characteristic of human cancers, the E6 and E7 genes remained intact in these mutants. The absence of E1 or E2 function did not influence the severity of the phenotype that eventually developed in the transgenic mice. Hyperplasia, papillomatosis, and dysplasia appeared at multiple epidermal and squamous mucosal sites, including ear and truncal skin, face, snout and eyelids, and anus. The ears were the most consistently affected site, with pathology being present in all lines with 100% penetrance. This phenotype also progressed through discernible stages. An initial mild hyperplasia was followed by hyperplasia, which further progressed to dysplasia and papillomatosis. During histopathological progression, there was an incremental increase in cellular DNA synthesis, determined by 5-bromo-2'-deoxyuridine incorporation, and a profound perturbation in keratinocyte terminal differentiation, as revealed by immunohistochemistry to K5, K14, and K10 and filaggrin. These K14-HPV16 transgenic mice present an opportunity to study the role of the HPV16 oncogenes in the neoplastic progression of squamous epithelium and provide a model with which to identify genetic and epigenetic factors necessary for carcinogenesis.

Focal transgene expression associated with papilloma development in v-Ha-ras-transgenic TG.AC mice

The homozygous transgenic mouse line TG.AC contains a v-Ha-ras transgene and rapidly develops epidermal papillomas in response to either wounding or treatment with tumor promoters such as 12-O-tetradecanoylphorbol-13-acetate (TPA). The transgenic v-Ha-ras protein product was detected in all papillomas removed from TPA-treated TG.AC mice but not in vehicle- or TPA-treated TG.AC skin without tumors. In situ hybridization demonstrated that focal expression of the transgene was limited to regions of papilloma development and further localized the expression of the transgene message to the epidermal component of the papillomas, with the strongest signal in the basal epidermoid cells. Cellular proliferation, as indicated by immunohistochemical staining for proliferating-cell nuclear antigen (PCNA), was similarly localized primarily to basal epidermoid cells and, to a lesser extent, stratum spinosum cells in all papillomas analyzed. Cells that stained positively for PCNA were much more common in the papillomas than in the surrounding, normal-appearing skin. The focal nature of papilloma development was also evidenced by protein kinase C activity and hyperplasia after TPA treatment. As early as 18 d after the start of TPA treatment, focal hyperplasias associated with the follicular epidermis were observed in TG.AC but not nontransgenic FVB/N skin; these hyperplasias were assumed to be the precursors of the epidermal papillomas. To explain the development of transgene-expressing tumors from apparently transgene-negative, normal-appearing skin, we hypothesize that the papillomas arise from the clonal expansion of focal areas of epidermal cells that overexpress the transgene. We also propose that the TG.AC line is an excellent model for studying very early events in papillomagenesis.