Photocarcinogenesis in the Tg.AC mouse: lomefloxacin and 8-methoxypsoralen

The Tg.AC mouse is a good predictor of carcinogenic potential when the test article is administered by dorsal painting (Tennant et al. (1995) Environ. Health Perspect. 103, 942). We have used lomefloxacin (LOME) and 8-methoxypsoralen (8-MOP) in combination with UVA to determine whether the Tg.AC transgenic mouse also responds to parenterally administered photocarcinogens. Female Tg.AC mice were given LOME (25 mg/kg intraperitoneal in normal saline) followed by UVA (25 J/cm2) 1-2 h later, five times every 2 weeks on a repetitive schedule. Other groups received LOME, UVA or vehicle alone. After 16 weeks, the mean numbers of papillomas/mouse +/- SD (% responding) were: saline, 0.3 +/- 0.5 (33%); UVA + saline, 1.3 +/- 0.6 (100%); LOME, 1.9 +/- 1.6 (86%) and LOME-UVA, 1.5 +/- 1.9 (64%). Only the 100% incidence of tumors in the UVA group and the maximum tumor yields in the LOME and UVA groups are significant (P < 0.05) when compared with the control. In a second study, Tg.AC mice were administered the classical photocarcinogen 8-MOP (8 mg/kg intragastric in corn oil) followed by 2 J/cm2 UVA 1-2 h later, five times every 2 weeks on a repetitive schedule. The second group received 8-MOP, whereas the third was exposed to UVA alone. Papillomas began to appear at 2 weeks in the 8-MOP-UVA group, and after 17 weeks the mean numbers of papillomas/mouse +/- SD (% responding) were: 8-MOP-UVA, 6.9 +/- 8.6 (93%); UVA + corn oil, 1.1 +/- 1.2 (69%) and 8-MOP, 1.1 +/- 1.6 (50%). The maximum tumor yield in the 8-MOP-UVA group was significantly higher (P < 0.01) than that in the other two groups. Our findings suggest that more studies need to be done before the Tg.AC mouse can be used with confidence to identify parenterally administered photocarcinogens.

Age-dependent skin tumorigenesis and transgene expression in the Tg.AC (v-Ha-ras) transgenic mouse

Transgenic Tg.AC (v-Ha-ras ) mice develop skin tumors in response to specific carcinogens and tumor promoters. The Tg.AC mouse carries the coding sequence of v-Ha ras, linked to a zeta-globin promoter and an SV40 polyadenylation signal sequence. The transgene confers on these mice the property of genetically initiated skin. This study examines the age-dependent sensitivity of the incidence of skin papillomas in Tg.AC mice exposed to topically applied 12-O:-tetradecanoylphorbol-13-acetate (TPA) treatment, full thickness skin wounding or UV radiation. Skin tumor incidence and multiplicity were strongly age-dependent, increasing with increasing age of the animal when first treated at 5, 10, 21 or 32 weeks of age. Furthermore, the temporal induction of transgene expression in keratinocytes isolated from TPA-treated mouse skin was also influenced by the age of the mice. Transgene expression was seen as early as 14 days after the start of TPA treatment in mice that were 10-32 weeks of age, but was not detected in similarly treated 5-week old mice. When isolated keratinocytes were fractionated by density gradient centrifugation the highest transgene expression was found in the denser basal keratinocytes. Transgene expression could be detected in the denser keratinocyte fraction as early as 9 days from start of TPA treatment in 32-week old mice. Using flow cytometry, a positive correlation was observed between expression of the v-Ha-ras transgene and enriched expression of the cell surface protein beta1-integrin, a putative marker of epidermal stem cells. This result suggests that, in the Tg.AC mouse, an age-dependent sensitivity to tumor promotion and the correlated induction of transgene expression are related to changes in cellular development in the follicular compartment of the skin.

Oral administration of dimethylvinyl chloride increases frequency of forestomach papillomas in Tg.AC mice

This work was initiated to determine the potential for the Tg.AC mouse model to identify chemical carcinogens by an oral route of administration. Tg.AC v-Ha-ras transgenic mice were exposed to dimethyvinyl chloride (DMVC; 1-chloro-2-methylpropene), a structural analog of the human carcinogen vinyl chloride. In the National Toxicology Program 2-yr bioassay, DMVC induced tumors in the oral, nasal, and gastric epithelia of rats and mice. Initial studies were performed in female Tg.AC mice to determine an appropriate oral dose of DMVC to evaluate the potential for stratified gastric or oral epithelia of Tg.AC mice to serve as a target tissue for a transgene-dependent induced tumorigenic response. DMVC was administered to 13- to14-wk-old Tg.AC mice by gavage at doses of 0, 50, 100, and 200 mg/kg five times a week for 20 wk. The forestomachs of DMVC-treated Tg.AC mice had an increasing number of papillomas, which were associated with an increase in the dose of DMVC. The average numbers of papillomas per mouse per dose were 2.4, 7.6, 14.1, and 12.6 for the 0, 50, 100, and 200-mg/kg dose groups, respectively. The optimum papillomagenic dose of 100 mg/kg DMVC was established and administered for 5, 10, and 15/wk to investigate the kinetics of papilloma induction in Tg.AC mice. The average numbers of papillomas per animal were 1.8, 8.8, and 19.0 at 5, 10, and 15 wk, respectively. Reverse transcription-polymerase chain reaction assays determined that the v-Ha-ras transgene was transcriptionally active in all tumor tissues but not in nontumor tissues. In situ hybridization assays performed in conjunction with bromodeoxyuridine in vivo labeling localized the transgene-expressing cells of the forestomach papillomas to the proliferating cellular component of the tumors, as previously seen in skin papillomas of Tg.AC mice. The present results confirm that DMVC is tumorigenic and that oral routes of administration can be used to rapidly elicit a transgene-associated tumor response in the forestomach of Tg.AC mice.

A farnesyl transferase inhibitor suppresses TPA-mediated skin tumor development without altering hyperplasia in the ras transgenic Tg.AC mouse

The Tg.AC mouse carries an activated v-Ha-ras oncogene fused to an embryonic zeta-globin promoter and develops cutaneous papillomas in response to specific chemicals, full thickness wounding, and ultraviolet radiation. Papilloma development in these mice has been suggested to be dependent upon activation of ras transgene expression, thus providing a potential model for studying ras-inhibitory compounds. Farnesyl transferase inhibitors (FTIs) prevent a critical posttranslational modification step necessary for activation of ras proteins. Our studies demonstrated that a tricyclic FTI (SCH 56582) applied directly to the skin of homozygous Tg.AC mice 1 h prior to administration of the tumor promoter TPA decreased tumor multiplicity compared to TPA-only controls. In addition, a reduction of TPA-induced tumor development was seen in similarly treated hemizygous Tg.AC mice either on an FVB/N strain background or 50% C57BL/6. Histological examination of skin from Tg. AC(+/-):FVB/N mice revealed no differences with respect to 12-O-tetradecamoylpharbol-13-acetate (TPA)-mediated hyperplasia. Keratinocytes isolated from treated and control skin were assayed for ras transgene expression by reverse transcription-polymerase chain reaction, and expression was detected in both TPA- and FTI+TPA-treated tissue, although the appearance of transgene positive pre-papillomas was observed only in histological sections taken 21 d after the first treatment. In summary, we have used a regimen of topical application of an FTI (SCH 56582) to suppress TPA-mediated papillomagenesis in v-Ha-ras transgenic Tg.AC mice. These studies demonstrate that TPA-induced epidermal hyperplasia is a ras-independent process, while papilloma development in response to TPA treatment requires the function of activated ras.

Effects of fixation on RNA extraction and amplification from laser capture microdissected tissue

One of the key end points for understanding the molecular basis of carcinogenesis is the quantitation of gene expression in specific cell populations. Microdissection techniques allow extraction of morphologically distinct cells for molecular analysis. A recent advance in microdissection uses the PixCell laser capture microdissection (LCM) system, which allows for precise removal of pure cell populations from morphologically preserved tissue sections. The objective of this study was to determine the optimal fixation protocol for analyzing RNA from tissue samples using LCM. Optimal fixation must provide acceptable morphology, allow proper laser capture of selected cells, and preserve the integrity of mRNA. We evaluated the effects of both cross-linking and precipitive-type fixatives on frozen and paraffin-embedded mouse liver tissue. For assessment of the quality of the mRNA in LCM samples generated from various fixed tissues, reverse transcription-polymerase chain reaction (RT-PCR)-amplified mouse liver beta2-microglobulin mRNA was detected with ethidium bromide. We also examined mouse glyceraldehyde-3-phosphate-dehydrogenase by using the fluorogenic TaqMan system for real-time quantitative detection of RT-PCR products. Frozen tissues yielded more RT-PCR product than did paraffin-embedded tissues. In both frozen and paraffin-embedded tissues, differences were observed between the fixatives. Precipitive fixatives, such as ethanol and acetone, consistently produced more RT-PCR amplification product than did cross-linking fixatives such as formalin. Optimal fixation protocols for LCM analysis will facilitate the examination of gene expression in specific cell populations, accelerating investigations of the molecular differences responsible for the phenotypic changes observed during carcinogenesis.

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.

Co-operation between follicular ornithine decarboxylase and v-Ha-ras induces spontaneous papillomas and malignant conversion in transgenic skin

Ornithine decarboxylase (ODC) is aberrantly regulated in tumor cells and results in high basal levels of ODC and polyamines in many epithelial tumors. To determine if elevated ODC/polyamine levels can co-operate with a mutant Ha-ras gene in mouse skin tumorigenesis, double transgenic mice were generated by breeding K6/ODC transgenic mice with TG.AC v-Ha-ras transgenic mice. A K6 keratin promoter drives the ODC transgene in K6/ ODC transgenic mice, which results in elevated ODC/ polyamine levels directed to the outer root sheath cells of hair follicles. TG.AC transgenic mice carry a v-Ha-ras transgene while still retaining two normal c-Ha-ras alleles. Transgenic mice that possess only the K6/ODC or the v-Ha-ras transgene did not develop tumors unless treated with either a carcinogen or a tumor promoter, respectively. However, a high percentage of double transgenic mice possessing both the K6/ODC and v-Ha-ras transgenes developed spontaneous tumors. All tumors were well-differentiated keratoacanthomas, some of which progressed to carcinomas within 2 months. The development and the maintenance of these ODC/ras tumors was ODC-dependent since alpha-difluoromethylornithine (DFMO), a specific ODC inhibitor, prevented the formation and caused the regression of these tumors. These findings indicate that ODC overexpression and an activated Ha-ras are sufficient to produce a high rate of malignant transformation in an animal model. The ODC/ras double transgenic mouse provides a simple in vivo model without the use of chemical carcinogens or tumor promoters in which to test downstream effectors that play a key role in mediating the development of epithelial tumors resulting from the cooperation between ODC and v-Ha-ras.

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.

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.

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.

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.

Expression of a prolactin-like factor in preneoplastic and neoplastic mouse mammary gland and cells

Prolactin is a member of the growth hormone family and is required for the growth and terminal differentiation of the mammary gland. Ectopic production of this hormone has been reported in several species, including rat, sheep, goat and human mammary tissues. In this study, mouse mammary cell lines, xenographs in the mammary gland from these cell lines and from hyperplastic alveolar nodules, spontaneous tumours, and normal tissues were studied for de novo production of this growth factor. Prolactin transcripts were found by reverse transcriptase PCR in some neoplastic and preneoplastic tissues and in mouse mammary cell lines, NOG8 and CDNR4, but were not detected in the normal mouse mammary gland. Northern analysis revealed a 1 kb transcript for both cell lines that co-migrated with the prolactin pituitary transcript. Conditioned medium from NOG8 cells was positive for prolactin bioactivity by the Nb2 rat lymphoma cell proliferation assay, and Western analysis revealed the presence of immunoreactive proteins at M(r) 14,000 and 60,000. Prolactin-like bioactivity was not detected in conditioned medium from CDNR4 cells, but an immunoreactive protein of M(r) 60,000 was detected by Western analysis. The mouse mammary cell line, Comma D, was negative for prolactin transcripts; however, adenocarcinomas derived from inoculation of Comma D cells into the cleared mammary fat pad were positive by reverse transcriptase PCR in two of four cases. Hyperplastic outgrowths maintained in the cleared mammary fat pad as well as spontaneous tumors were positive for prolactin transcripts in one of four cases. These results suggest that prolactin can be produced ectopically by the neoplastic mouse mammary gland.

Association of tumor development with increased cellular proliferation and transgene overexpression, but not c-Ha-ras mutations, in v-Ha-ras transgenic Tg.AC mice

The transgenic mouse line Tg.AC carries a v-Ha-ras gene fused to a fetal (zeta) globin promoter and uniquely responds to chemical carcinogens and tumor promoters by the induction of epidermal papillomas. Although the transgene was not constitutively expressed in non-tumor-bearing tissues, expression was induced by exposure to selected chemicals. Tg.AC transgenic mice on the FVB/N background developed occasional spontaneous tumors, including odontomas, squamous cell carcinoma of the salivary gland, leukemias and a rare ovarian yolk sac carcinoma. Both spontaneous and induced tumors are associated with expression of the transgene and, as determined by in situ hybridization, transgene expression is localized to proliferative areas of the tumors. Sequence analysis of the endogenous c-Ha-ras gene in both induced and spontaneous tumors revealed no mutations in codons 12, 59 or 61. These results suggest that expression of the v-Ha-ras transgene induces proliferation of specific cells in diverse tissues which then acquire neoplastic properties.