Two-stage Chemical Carcinogenesis Research Articles

Abstract 2462: Overexpression of prostaglandin E2 EP4 receptor enhances mouse skin tumorigenesis

Abstract Chronic inflammation has recently been suggested to be involved in cancer development, and cyclooxygenase-2 (COX-2), a key mediator of the inflammatory response, is known to be upregulated in several cancers. COX-2 is responsible for the production of prostaglandins, and prostaglandin E2 (PGE2) is the major prostaglandin involved in inflammation. PGE2 binds to four receptors (EP1-EP4), and the EP4 receptor has recently been shown to be important in carcinogenesis depending on the tissue in which it is expressed. To determine the role of EP4 in skin carcinogenesis, transgenic mice were generated that overexpress EP4 in the basal layer of the skin under the control of the bovine keratin 5 promoter (BK5.EP4). Using a two-stage chemical carcinogenesis protocol, the BK5.EP4 mice were shown to develop both papillomas and squamous cell carcinomas earlier than wild type (WT) mice. However, a tumor observed before 12-O-tetradecanoylphorbol-13-acetate (TPA) treatment on one BK5.EP4 mouse suggested that TPA was not necessary for the promotion of the tumors. Therefore a 7,12-dimethylbenz[a]anthracene (DMBA)-only carcinogenesis protocol was performed, and by 30 weeks, the BK5.EP4 mice again developed more carcinomas compared to none on the WT mice. Immunohistochemistry performed on DMBA-treated mice demonstrated that the BK5.EP4 epidermis began to detach 48 hours after treatment with total detachment occurring 5 days after treatment, while WT mice had only increased epidermal thickening. By 7 to 10 days after DMBA treatment, patches were observed on the BK5.EP4 skin where it appeared the hair and epidermis were detaching, which were not seen on the WT mice. MMP-9, MMP-7, and E-cadherin expression also were upregulated in the BK5.EP4 mice at the same time detachment of the epidermis was occurring. However, no differences in proliferation, stem cell numbers, or CYP1A1 and CYP1B1 expression have been observed. These data suggest that overexpression of EP4 in the mouse skin can increase tumorigenesis, possibly through the wounding of the skin and the subsequent inflammation and epidermal regeneration acting as a tumor promoter. Supported by CA100140. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2462.

Abstract 2463: Overexpression of galectin-7 in mouse skin inhibits tumorigenesis

Abstract Galectins are a family of proteins that contain conserved sequences in the carbohydrate-binding region that bind to β-galactosides. Galectin expression differs between tissues in addition to different stages of development and also has altered expression during tumor development. Galectin-7 (Gal-7) has been linked to both apoptosis and reduced cell growth in vitro, but conversely, upregulation of Gal-7 in skin has been reported in many transgenic mice that have endogenous skin tumor promoting activity in addition to being associated with an aggressive phenotype in lymphoma cells. To further investigate this conflicting evidence, the role of Gal-7 in skin carcinogenesis was examined. Gal-7 mRNA and protein levels were elevated in mouse skin after 12-O-tetradecanoylphorbol-13-acetate (TPA) treatment, while Gal-7 stably-transfected cells showed a decrease in tumor growth when compared to vector stably-transfected cells after injection into nude mice, although no difference in cell growth in vitro was observed. Transgenic mice were then generated that overexpress a flag-tagged Gal-7 protein in the basal layer of the skin under the control of the bovine keratin 5 promoter (K5. Gal-7). These mice display no visible phenotype, but immunohistochemistry shows the transgene is present and expressed. Using a two-stage chemical carcinogenesis protocol, the K5. Gal-7 mice were shown to develop significantly fewer papillomas and squamous cell carcinomas than wild type (WT) mice, though no difference in proliferation or apoptosis was seen in response to TPA treatment. No difference in the inflammatory response to TPA was observed, with the exception of a spike in B cells 36 hours after TPA. However, untreated K5. Gal-7 mice showed a decrease in T and B cells compared to WT mice. This supports data from a reverse phase protein array that indicates annexin, which suppresses phospholipase A2 and inhibits inflammation, is upregulated in the K5. Gal-7 mice. These data suggest that overexpression of Gal-7 in the mouse skin inhibits tumorigenesis, possibly through the inhibition of inflammation. Supported by CA100140. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2463.

Abstract 4200: Transgenic mice overexpressing a Claudin-6 tail deletion mutant are resistant to tumor formation

Abstract Morphological and physiological observations suggest that the tight junction (TJ)-based permeability barrier is modified/disrupted during tumorigenesis. It is now also widely recognized that the Claudin (Cldn) family of four tetraspan transmembrane proteins is crucial for tight junction assembly and permeability barrier function. However, the detailed roles of the Cldn cytoplasmic tail and extracellular loop domains in these processes or their dysfunction in tumorigenesis are not yet understood. We recently demonstrated that the tail domain of Cldn6 is crucial for membrane targeting and epidermal permeability barrier (EPB) formation, by forced expression of a Cldn6 tail deletion mutant (CDelta187) via the involucrin (Inv) promoter in the suprabasal compartment of the mouse epidermis. Even though a functional barrier formed, Inv-CDelta187 mice displayed histological and biochemical abnormalities in the epidermal differentiation program leading to a thickening of the epidermis after 1 week of age that persisted throughout life. Notably, a significant amount of not only Cldn6, but also Cldn10, Cldn11, and Cldn18 remained cytoplasmically localized and the protein-unfolding pathway was activated in transgenic epidermal cells. Using a well-established two-stage chemical carcinogenesis model, we are now investigating the temporal and spatial changes in expression of these and other (Cldn1, Cldn12) Cldns that we have previously demonstrated to be important in epidermal differentiation and tumorigenesis in CDelta187 versus wild type (WT) mice. METHODS: The lower dorsal backskin of mice was treated topically with 7,12-dimethylbenz(a)anthracene (DMBA; 0.25 mg/ml in acetone) and following a 10-day incubation period, 12-O-tetradecanoyl-phorbol-13-acetate (TPA; 25 microg/ml in acetone) was applied three times a week to the same area. Backskin samples were dissected 2, 4, 6, 8 and 12 weeks after the initiation of the experimental protocol and immunohistochemistry was performed on sections using antibodies against the following: Cldn1, Cldn6, Cldn11, Cldn12, Cldn18, Ki67 (proliferation marker) and CD3 (immune infiltration marker). Strikingly, epidermal tumor formation was inhibited in CDelta187 as compared to WT animals. This is the first demonstration that alterations in Cldn structure and homeostasis may play a protective role against DMBA/TPA-induced skin tumors. Further investigation into the molecular mechanisms underlying this protective function is underway. Supported by CIHR Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4200.

Enhancement factor in low-coherence enhanced backscattering and its applications for characterizing experimental skin carcinogenesis

We experimentally study potential mechanisms by which the enhancement factor in low-coherence enhanced backscattering (LEBS) can probe subtle variations in radial intensity distribution in weakly scattering media. We use enhanced backscattering of light by implementing either (1) low spatial coherence illumination or (2) multiple spatially independent detections using a microlens array under spatially coherent illumination. We show that the enhancement factor in these configurations is a measure of the integrated intensity within the localized coherence or detection area, which can exhibit strong dependence on small perturbations in scattering properties. To further evaluate the utility of the LEBS enhancement factor, we use a well-established animal model of cutaneous two-stage chemical carcinogenesis. In this pilot study, we demonstrate that the LEBS enhancement factor can be substantially altered at a stage of preneoplasia. Our animal result supports the idea that early carcinogenesis can cause subtle alterations in the scattering properties that can be captured by the LEBS enhancement factor. Thus, the LEBS enhancement factor has the potential as an easily measurable biomarker in skin carcinogenesis.

MK2 regulates the early stages of skin tumor promotion

The association between inflammation and tumorigenesis is well recognized. Mitogen-activated protein kinase-activated protein kinase-2 (MK2) is known to play a pivotal role in inflammatory processes. Here, we studied the effect of MK2-deficiency and tumor necrosis factor (TNF)-alpha-deficiency on skin tumor development in mice using the two-stage chemical carcinogenesis model. We found that MK2(-/-) mice developed significantly fewer skin tumors compared with both TNF-alpha(-/-) and wild-type mice when induced by initiation with 7,12-dimethylbenz[a]anthracene (DMBA) and by promotion with 12-O-tetradecanoylphorbol-13-acetate (TPA). The TPA-induced inflammatory response was reduced in both, TNF-alpha(-/-) mice and MK2(-/-) mice, but most pronounced in TNF-alpha(-/-) mice, indicating that a reduced inflammatory response was not the only explanation for the inhibited tumorigenesis seen in MK2(-/-) mice. Interestingly, increased numbers of apoptotic cells were detected in the epidermis of MK2(-/-) mice compared with TNF-alpha(-/-) and wild-type mice, suggesting an additional role of MK2 in the regulation of apoptosis. This was further supported by: (i) increased levels of the tumor suppressor protein p53 in MK2(-/-) mice after DMBA/TPA treatment compared with controls, (ii) reduced phosphorylation (activation) of the negative p53 regulator, murine double minute 2 in MK2(-)(/-) mouse keratinocytes in vitro and (iii) a significant decrease in the DMBA/TPA induced apoptosis in cultured MK2(-/-) keratinocytes transfected with p53 small interfering RNA. Taken together, these findings demonstrate a dual role of MK2 in the early stages of tumor promotion through regulation of both the inflammatory response and apoptosis of DNA-damaged cells. These results also identify MK2 as a putative target for future skin carcinoma therapy.

IL-12 deficiency suppresses 12- O -tetradecanoylphorbol-13-acetate-induced skin tumor development in 7,12-dimethylbenz( a )anthracene-initiated mouse skin through inhibition of inflammation

Interleukin (IL)-12 deficiency exacerbates tumorigenesis in ultraviolet (UV) radiation-induced skin. Here, we assessed the effects of IL-12 deficiency on 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced tumor promotion in 7,12-dimethylbenz(a)anthracene (DMBA)-initiated mouse skin. Using this two-stage chemical carcinogenesis protocol, we found that the development of DMBA/TPA-induced skin tumors was diminished in IL-12p40-knockout mice than in their wild-type counterparts. At the termination of the experiment (at 24 weeks), the skin tumor incidence and tumor multiplicity were significantly lower (P < 0.005) in interleukin-12-knockout (IL-12 KO) mice than in their wild-type counterparts, as was the malignant transformation of DMBA/TPA-induced papillomas to carcinomas (P < 0.01). Analysis of samples collected at the termination of the experiments for biomarkers of inflammation by immunohistochemical analysis, western blotting, enzyme-linked immunosorbent assay and real-time polymerase chain reaction revealed significantly lower levels of cyclooxygenase-2 (COX-2), prostaglandin (PG) E(2), proliferating cell nuclear antigen, cyclin D1 and the proinflammatory cytokines (tumor necrosis factor-alpha, IL-1beta and IL-6) in the DMBA/TPA-treated tumors and tumor-uninvolved skin of IL-12 KO mice than the skin and tumors of DMBA/TPA-treated wild-type mice. Analysis of the skin 6 h after TPA treatment showed that the TPA-induced promotion of skin edema, inflammatory leukocyte infiltration, COX-2 expression and PGE(2) production was significantly lower in the skin of the IL-12-KO mice than their wild-type counterparts. These results indicate that DMBA/TPA-induced skin tumor development differs from UVB-induced skin tumor development in that endogenous IL-12 acts to inhibit UVB-induced skin tumor development and malignant progression of the skin tumors to carcinoma. In the case of DMBA/TPA-induced skin tumor development, the endogenous IL-12 modulates the tumor promoter stimulation of inflammatory responses.

Ligand Activation of Peroxisome Proliferator–Activated Receptor-β/δ and Inhibition of Cyclooxygenase-2 Enhances Inhibition of Skin Tumorigenesis

Ligand activation of peroxisome proliferator-activated receptor (PPAR)-beta/delta and inhibition of cyclooxygenase-2 (COX-2) activity by nonsteroidal anti-inflammatory drugs can attenuate skin tumorigenesis. There is also evidence that attenuation of skin tumorigenesis by inhibition of COX-2 activity occurs through PPARbeta/delta-independent mechanisms. The present study examined the hypothesis that combining ligand activation of PPARbeta/delta with inhibition of COX-2 activity will cooperatively inhibit chemically induced skin tumor progression using both in vivo and ex vivo models. A two-stage chemical carcinogenesis bioassay was performed in wild-type and Pparbeta/delta-null mice. After 22 weeks, cohorts of both mouse lines were divided into four experimental groups: (1) control, (2) topical application of the PPARbeta/delta ligand GW0742, (3) dietary administration of the COX-2 inhibitor nimesulide, or (4) both GW0742 and nimesulide. Ligand activation of PPARbeta/delta did not influence skin tumor progression, while a modest decrease in skin tumor multiplicity was observed with dietary nimesulide. Interestingly, the combined treatment of GW0742 and nimesulide increased the efficacy of the decrease in papilloma multiplicity for 6 weeks in wild-type mice, but this effect was not found at later time points and was not found in similarly treated Pparbeta/delta-null mice. Neoplastic keratinocyte lines cultured with GW0742 and nimesulide also exhibited enhanced inhibition of cell proliferation coincident with increased expression of Keratin messenger RNAs. Results from these studies support the hypothesis that combining ligand activation of PPARbeta/delta with inhibition of COX-2 activity can inhibit chemically induced skin tumor progression by modulating differentiation.

Ligand activation of peroxisome proliferator-activated receptor β/δ (PPARβ/δ) inhibits chemically induced skin tumorigenesis

Peroxisome proliferator-activated receptor (PPAR)beta/delta-null mice exhibit enhanced tumorigenesis in a two-stage chemical carcinogenesis model as compared with wild-type mice. Previous work showed that ligand activation of PPARbeta/delta induces terminal differentiation and inhibits proliferation of primary keratinocytes, and this effect does not occur in the absence of PPARbeta/delta expression. In the present studies, the effect of ligand activation of PPARbeta/delta on skin tumorigenesis was examined using both in vivo and ex vivo skin carcinogenesis models. Inhibition of chemically induced skin tumorigenesis was observed in wild-type mice administered GW0742, and this effect was likely the result of ligand-induced terminal differentiation and inhibition of replicative DNA synthesis. These effects were not found in similarly treated PPARbeta/delta-null mice. Ligand activation of PPARbeta/delta also inhibited cell proliferation and induced terminal differentiation in initiated/neoplastic keratinocyte cell lines representing different stages of skin carcinogenesis. These studies suggest that topical administration of PPARbeta/delta ligands may be useful as both a chemopreventive and/or a chemotherapeutic approach to inhibit skin cancer.

Susceptibility of pRb‐deficient epidermis to chemical skin carcinogenesis is dependent on the p107 allele dosage

Functional inactivation of the pRb-dependent pathway is a general feature of human cancer. However, only a reduced spectrum of tumors displays inactivation of the Rb gene. This can be attributed, at least partially, to the possible overlapping functions carried out by the related retinoblastoma family members p107 and p130. We observed that loss of pRb in epidermis, using the Cre/LoxP technology, results in proliferation and differentiation defects. These alterations are partially compensated by the elevation in the levels of p107. Moreover, epidermis lacking pRb and p107, but not pRb alone, develops spontaneous tumors, and double deficient primary keratinocytes are highly susceptible to Ha-ras-induced transformation. Two-stage chemical carcinogenesis experiments in mice lacking pRb in epidermis revealed a reduced susceptibility in papilloma formation and an increase in the malignant conversion. We have now explored whether the loss of one p107 allele, inducing a decrease in the levels of p107 up to normal levels could restore the susceptibility of pRb-deficient skin to two-stage protocol. We observed partial restoration in the incidence, number, and size of tumors. However, there is no increased malignancy despite sustained p53 activation. We also observed a partial reduction in the levels of proapoptotic proteins in benign papillomas. These data confirm our previous suggestions on the role of p107 as a tumor suppressor in epidermis in the absence of pRb.

Hmga1 null mice are less susceptible to chemically induced skin carcinogenesis

The HMGA1 proteins have a critical role in the process of carcinogenesis. They are overexpressed in most human malignant neoplasias, and the inhibition of their expression has been shown to prevent cell transformation and results in malignant cell death. To determine whether HMGA1 proteins are also required for in vivo carcinogenesis, we compared the tumour susceptibility of mice wild-type or knockout for the Hmga1-null allele using a two-stage chemical skin carcinogenesis protocol. Hmga1 −/− mice exhibited a decreased number and a delayed onset of skin papillomas in comparison with wild-type mice. Moreover, the progression of skin papillomas to carcinomas was observed in only 5% of Hmga1 −/− compared to 18% of wild-type mice. These results suggest a lower susceptibility of Hmga1 −/− mice to skin carcinogenesis induced by chemical agents.

Changes in the distribution pattern of Claudin tight junction proteins during the progression of mouse skin tumorigenesis

BackgroundDespite the fact that morphological and physiological observations suggest that the tight junction (TJ)-based permeability barrier is modified/disrupted in tumorigenesis, the role of members of the Claudin (Cldn) family of TJ proteins is not well-understood. Using a well-established two-stage chemical carcinogenesis model, we investigated the temporal and spatial changes in expression of those Cldns that we have previously demonstrated to be important in epidermal differentiation and the formation of the epidermal permeability barrier, i.e., Cldn1, Cldn6, Cldn11, Cldn12 and Cldn18.MethodsThe lower dorsal backskin of mice was treated topically with 7,12-dimethylbenz(a)anthracene (DMBA; 0.25 mg/ml in acetone) and following a 10-day incubation period, 12-O-tetradecanoyl-phorbol-13-acetate (TPA; 25 μg/ml in acetone) was applied three times a week to the same area. Backskin samples were dissected 2, 4, 6, 8 and 12 weeks after the initiation of the experimental protocol and immunohistochemistry was performed on sections using antibodies against the following: Cldn1, Cldn6, Cldn11, Cldn12, Cldn18, Ki67 and CD3.ResultsOur data indicate that along with the changes in epidermal cell morphology and differentiation that occur during tumor formation, there is a dramatic change in Cldn distribution consistent with cell polarity and barrier selectivity changes. Specifically, in the early stages of DMBA/TPA treatment, the suprabasal-specific Cldns occupy an expanded zone of expression corresponding to an increased number of suprabasal epidermal cell layers. As tumorigenesis progressed, the number of suprabasal epidermal layers positive for Cldn6, Cldn11, Cldn12 and Cldn18 was reduced, especially in the lower strata of the expanded suprabasal zone. In addition, a variably reduced cell membrane association of those differentiation-specific Cldns was observed, especially within the infiltrating epidermal structures. In contrast, Cldn1 (which is normally expressed in all the living layers of the epidermis) remained restricted to the cell membrane throughout the tumorigenesis protocol. However commencing 2 weeks after treatment there was a marked decrease in the number of Cldn1-positive basal cells, and the zone of Cldn1-null epidermal cells was expanded up into the lower stratified epidermis throughout the progression of DMBA/TPA treatment. In addition, there was no Cldn1 localization in the infiltrating epidermal structures of the tumorigenic epidermis.ConclusionThis is the first demonstration of the changes in Cldn expression in the progression of DMBA/TPA-induced skin tumors; however further investigation into the molecular mechanisms regulating the observed changes in barrier selectivity during tumorigenesis is required.

Cutaneous Two-Stage Chemical Carcinogenesis

INTRODUCTIONThe induction of cutaneous carcinogenesis in mice by topical administration of chemicals enables the local, systemic, and environmental factors that influence tumor susceptibility, growth, and progression to be studied in the laboratory. Chemical carcinogenesis in mouse skin has been used for several decades, and continues to help in the identification of important molecular and immunological pathways involved in cutaneous malignancy. This protocol describes the two-stage application of chemicals to the skin for the initiation and promotion of cutaneous tumors. In the two-stage model of induction, a single application of the chemical initiator mutagen 7,12-dimethylbenz[a]anthracene (DMBA) is followed by repeated applications of a pro-inflammatory phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA; otherwise known as phorbol 12-myristate 13-acetate [PMA]). Resulting tumors may develop in the form of benign papillomas that regress or progress to SCC, or SCC may arise without an apparent precursor lesion. Thus, two-stage chemical carcinogenesis provides an opportunity to monitor early and late events in cancer development and progression.

Forced expression of a constitutively active form of Stat3 in mouse epidermis enhances malignant progression of skin tumors induced by two-stage carcinogenesis

Recently, our laboratory demonstrated that Stat3 is required for the de novo development of chemically-induced skin tumors. We have further investigated the role of Stat3 in epithelial carcinogenesis using mice in which the expression of a constitutively active/dimerized form of Stat3 (Stat3C) is targeted to the proliferative compartment of epidermis (referred to as K5.Stat3C transgenic mice). Keratinocytes from K5.Stat3C mice showed increased survival following exposure to 7,12-dimethylbenz[a]anthracene (DMBA) and enhanced proliferation following exposure to 12-O-tetradecanoylphorbol-13-acetate (TPA). In two-stage chemical carcinogenesis experiments using DMBA as the tumor initiator and TPA as the promoter, K5.Stat3C mice developed skin tumors with a shorter latency and in much greater number compared to non-transgenic littermates. Remarkably, 100% of the skin tumors that developed in K5.Stat3C transgenic mice bypassed the premalignant stage and were initially diagnosed as carcinoma in situ which rapidly progressed to squamous cell carcinoma (SCC). These tumors were highly vascularized, poorly differentiated and invasive and loss of expression of K10, filaggrin and E-cadherin was observed by 20 weeks. Finally, overexpression of Stat3C in a papilloma cell line led to enhanced cell migration and enhanced invasion through Matrigel in both the absence and presence of growth factors. In addition to its critical role in early stages of epithelial carcinogenesis, the current study reveals a novel role for Stat3 in driving malignant progression of skin tumors in vivo.

Cooperation of Ha‐ras and Bcl‐2 during multistep skin carcinogenesis

Nonmelanoma skin cancer (NMSC) is the most frequently diagnosed cancer in the United States. Deregulation of bcl-2 and ras family members is commonly observed in NMSC. It has been previously demonstrated that simultaneous bcl-2 and Ha-ras gene expression in keratinocytes results in disordered differentiation and resistance to cell death induced by ultraviolet (UV) radiation. It was, therefore, interest to assess the extent of cooperation between bcl-2 and Ha-ras during multistep skin carcinogenesis. The keratin 1 promoter was used to generate HK1.ras and HK1.bcl-2 transgenic mice, which were subsequently crossed to generate HK1.ras/bcl-2 double transgenic mice. The apoptotic index (AI) following UV-irradiation was significantly lower in HK1.bcl-2 and HKI.ras/bcl-2 epidermis compared to control littermates. Interestingly, the AI of HK1.ras/bcl-2 mice was significantly lower than even HK1.bcl-2 mice following UV-irradiation. To investigate the interaction of these oncogenes in skin tumorigenesis, a two-stage chemical carcinogenesis protocol was used to induce tumors. The individual contributions of Ha-ras and bcl-2 to papilloma latency, incidence, and growth rate in HK1.ras/bcl-2 double transgenic mice was marginally additive. Papillomas arising in HK1.ras transgenic mice exhibited the highest rate of apoptosis whereas papillomas arising in the HK1.ras/bcl-2 double transgenic mice exhibited rates of apoptosis that were significantly lower than papillomas arising in either control littermate or HK1.ras mice. Constitutive expression of either Ha-ras or bcl-2 exhibited similar rates of malignant tumor progression and they were not significantly different than control littermates. Importantly, when these two oncoproteins were coexpressed, a significant, and synergistic, increase in malignant transformation was observed.

Dual Role of Inactivating Lef1 Mutations in Epidermis: Tumor Promotion and Specification of Tumor Type

The NH(2) terminus of LEF1 is frequently mutated in human sebaceous tumors. To investigate how this contributes to cancer, we did two-stage chemical carcinogenesis on K14DeltaNLef1 transgenic mice, which express NH(2)-terminally truncated Lef1 in the epidermal basal layer. Transgenic mice developed more tumors, more rapidly than littermate controls, even without exposure to tumor promoter. They developed sebaceous tumors, whereas controls developed squamous cell carcinomas. K14DeltaNLef1 epidermis failed to up-regulate p53 and p21 proteins during tumorigenesis or in response to UV irradiation, and this correlated with impaired p14ARF induction. We propose that LEF1 NH(2)-terminal mutations play a dual role in skin cancer, specifying tumor type by inhibiting Wnt signaling and acting as a tumor promoter by preventing induction of p53.

KERATINOCYTE-SPECIFIC SMAD2 ABLATION PROMOTES SKIN CARCINOGENESIS VIA AFFECTING GENES RELATED TO TUMOR INVASION AND ANGIOGENESIS.

Purpose of Study Transforming growth factor β (TGF-β) has roles in both tumor suppression and promotion. Although Smad2 and Smad3 are TGF-β signaling mediators, their functions in mediating TGF-β signaling during cancer development remain to be determined. We have examined expression patterns of Smad2 and Smad3 in human skin SCCs and found that over 95% of tumor samples retain Smad3, whereas 70% of SCCs have lost the Smad2 protein. Previously, we have shown that Smad3 knockout mice are resistant to skin carcinogenesis due to abrogation of TGF-β-mediated tumor promotion effect. Methods Used In the current study, we assessed the role of Smad2 in skin carcinogenesis by generating mice with keratinocyte-specific Smad2 deletion. Summary of Results Although conditional Smad2 knockout mice did not develop spontaneous skin tumors, when exposed to a two-stage chemical carcinogenesis protocol, they exhibited accelerated tumor formation and malignant progression compared with wild-type mice. Smad2−/− tumors were more poorly differentiated in comparison with tumors from wild-type mice. At the papilloma stage, Smad2−/− tumors exhibited molecular alterations usually associated with late-stage SCC progression and epithelial-mesenchymal transition (EMT), eg, a significant elevation of keratin K8, snail, twist, tenascin C and vimentin. Additionally, Smad2−/− tumors displayed HGF-dependent vessel growth culminating in approximately three times the angiogenesis seen in Smad2+/+ tumors. Since these molecules are documented or potential TGF-β target genes, we examined TGF-β1 levels in Smad2−/− tumors. We found that neither the TGF-β1 level nor Smad3 activation was elevated in Smad2−/− tumors. Conclusions Reached Thus, our study suggests that Smad2 loss results in the elevation of tumor invasion-associated and angiogenesis genes via a TGF-β-independent mechanism.

A novel role of DNA fragmentation factor as a tumor suppressor through maintaining genomic stability

10023 Background: DNA fragmentation is a hallmark of apoptosis. However, the biological function of apoptotic DNA fragmentation remains unclear. Genes encoding the nuclease responsible for DNA fragmentation, DFF40/CAD and DFF45/ICAD, are deleted, mutated, or aberrantly expressed in many human tumor types. Abnormalities in this gene are associated with aggressive tumors and poor prognosis in cancer patients. Tumor-specific DFF45 gene mutations were identified in human tumors, indicating the involvement of this gene in tumor development. Methods: We studied genetic instability, cellular transformation and tumorigenesis in cells and mice deficient in DNA fragmentation. We applied two methods to inhibit DNA fragmentation, overexpressing a modified ICAD which inhibits DNA fragmentation during apoptosis in stable cell lines and disrupting cad gene in transgenic mice. Genetic instability was studied by gene amplification assay, mutation assay and cytogenetic analysis including chromosomal aberrations and translocations. Cellular transformation was studied by soft agar assay using CAD-/- MEF cells. Tumorigenesis in DFF/CAD-knockout mice was studied by radiation carcinogenesis and two-stage skin chemical carcinogenesis. Results: (1) Human and mouse cancer cell lines with their CAD activity inhibited exhibited significantly more genetic instability under spontaneous or mutagen (ionizing radiation) induced conditions. These are reflected as elevated frequencies of chromosomal aberrations, gene amplifications, and gene mutations; (2) Mouse cells with targeted disruption of the cad gene exhibit a similar, increased level of genetic instability when the host animal is irradiated; (3) Mechanistically, CAD maintains genetic instability through the removal of cells with DNA damage, a role that is similar to genomic “gatekeepers” like p53; (4) CAD-null mouse embryo fibroblasts exhibited enhanced cellular transformation; (5) Significantly enhanced susceptibility to chemical and radiation-induced tumor development was observed in mice with targeted disruption of the CAD gene, indicating that it plays a role in suppressing tumor development. Conclusions: CAD plays an important role in maitaining genetic stability and suppressing tumor development. No significant financial relationships to disclose.

Deregulated minichromosomal maintenance protein MCM7 contributes to oncogene driven tumorigenesis

Minichromosomal maintenance protein 7 (MCM7) is an essential component of the replication helicase complex (MCM2-7) required for DNA replication. Although this function is highly conserved among eukaryotes, additional functions for the MCM molecules continue to be described. Minichromosomal maintenance protein 7 is a marker for proliferation and is upregulated in a variety of tumors including neuroblastoma, prostate, cervical and hypopharyngeal carcinomas. To further investigate the general role of MCM7 in tumorigenesis, we generated a mouse model with deregulated MCM7 expression targeted to the basal layer of the epidermis using the keratin 14 (K14) promoter (K14.MCM7). When subjected to a two-stage chemical carcinogenesis protocol (dimethylbenz[alpha]anthracene (DMBA) initiation with 12-ortho-tetradecanoylphorbol-13-acetate promotion), K14.MCM7 mice showed significantly increased incidence and prevalence of tumor development relative to controls. Furthermore, within 40 weeks of treatment over 45% K14.MCM7 mice exhibited tumors that had converted to squamous cell carcinomas versus none in the control group. As predicted from previous skin carcinogenesis studies using DMBA as the initiating agent, Ras mutations where found in more than 90% of tumors isolated from K14.MCM7 mice. Whereas previous studies have shown that MCM7 is useful as a proliferation marker, our data suggest that deregulated MCM7 expression actively contributes to tumor formation, progression and malignant conversion.

Chemoprevention of Skin Carcinogenesis by Phenylretinamides: Retinoid Receptor–Independent Tumor Suppression

Fenretinide [N-(4-hydroxyphenyl)retinamide or 4-HPR] is a synthetic retinoid analogue with antitumor and chemopreventive activities. N-(4-Methoxyphenyl)retinamide (4-MPR) is the most abundant metabolite of 4-HPR detected in human serum following 4-HPR therapy. We have shown in in vitro studies that 4-HPR and 4-MPR can act independent of the classic nuclear retinoid receptor pathway and that 4-HPR, but not 4-MPR, can also activate nuclear retinoid receptors. In this study, we have compared the chemopreventive effects of topically applied 4-HPR and 4-MPR with the primary biologically active retinoid, all-trans retinoic acid (ATRA), in vivo in the mouse skin two-stage chemical carcinogenesis model. All three retinoids suppressed tumor formation but the effect of 4-HPR and 4-MPR, and not of ATRA, was sustained after their discontinuation. The tumor-suppressive effects of 4-HPR and 4-MPR were quantitatively and qualitatively similar, suggesting that the two may be acting through the same retinoid receptor-independent mechanism(s). We further explored this effect in vitro by analyzing primary cultures of mouse keratinocytes treated with the same retinoids. All three could induce apoptosis with a 48-hour treatment and only ATRA and 4-HPR induced an accumulation of cells in the G1 phase of the cell cycle. This finding is consistent with our previous results showing that the effects of phenylretinamides on the cell cycle are retinoid receptor dependent whereas apoptosis induction is not. A microarray-based comparison of gene expression profiles for mouse skin treated with 12-O-tetradecanoylphorbol-13-acetate (TPA) alone and TPA + 4-HPR or TPA + 4-MPR reveals a high degree of coincidence between the genes regulated by the two phenylretinamides. We propose that 4-HPR may exert therapeutic and chemopreventive effects by acting primarily through a retinoid receptor-independent mechanism(s) and that 4-MPR may contribute to the therapeutic effect of 4-HPR by acting through the same retinoid receptor-independent mechanism(s).

Unexpected Roles for pRb in Mouse Skin Carcinogenesis

The mouse skin carcinogenesis represents one of the best models for the understanding of malignant transformation, including the multistage nature of tumor development. The retinoblastoma gene product (pRb) plays a critical role in cell cycle regulation, differentiation, and inhibition of oncogenic transformation. In epidermis, Rb-/- deletion leads to proliferation and differentiation defects. Numerous evidences showed the involvement of the retinoblastoma pathway in this model. However, the actual role of pRb is still unknown. To study the possible involvement of pRb in keratinocyte malignant transformation, we have carried out two-stage chemical skin carcinogenesis on Rb(F19/F19) (thereafter Rb+/+) and Rb(F19/F19);K14Cre (thereafter Rb-/-) animals. Unexpectedly, we found that Rb-/- mice developed fewer and smaller papillomas than the Rb+/+ counterparts. Moreover, the small size of the pRb-deficient tumors is associated with an increase in the apoptotic index. Despite this, pRb-deficient tumors display an increased conversion rate to squamous cell carcinomas. Biochemical analyses revealed that these characteristics correlate with the differential expression and activity of different pathways, including E2F/p19arf/p53, PTEN/Akt, c-jun NH2-terminal kinase/p38, and nuclear factor-kappaB. Collectively, our findings show unexpected and hitherto nondescribed roles of pRb during the process of epidermal carcinogenesis.