Two-stage Carcinogenesis Protocol Research Articles

EGFR and HER-2 oncogenes expression in an experimental model of two-stage chemically induced carcinogenesis in mouse skin

The aim of the study was to investigate the expression of EGFR and HER-2 oncogenes using an experimental two stage chemically induced carcinogenesis protocol on the dorsal skin in FVB/N mice.Forty female FVB/N mice 4 weeks old, were grouped into one control (n = 8) and two experimental groups (Group A: n = 16, Group B: n = 16) following a randomization process. Two-stage carcinogenesis protocol, was implicated, including an initial treatment with 97.4 nmol DMBA on their shaved dorsal skin and subsequent treatments of 32.4 nmol TPA applications after 13 weeks for Group A and after 20 weeks for Group B. The control group C, received no treatment. Skin was examined weekly for tumor development. Post-experiment, animals were euthanized for tissue analysis.The histological status of the skin lesions in the experimental groups corresponded well with tumour advancement (from dysplasia to poorly-differentiated carcinoma). Tumour sections were evaluated histologically and immunohistochemically. EGFR expression was found significantly higher in precancerous and malignant tumours (p = 042 and p = 008 respectively), while tended to be higher in benign tumours (p = 079), compared to normal histology. Moreover, mean percentage of EGFR positive expression in malignant tumours was significantly higher than in benign tumours (p < 001). HER-2 expression was found significantly higher in precancerous and malignant tumours (p = 042 and p = 015 respectively), while tended to be higher in benign tumours (p = 085), compared to normal histology. Furthermore, mean percentage of HER-2 positive expression in malignant tumours was significantly higher than in benign tumours (p = 005).The study demonstrated that in FVB/N mice subjected to a two-stage chemically induced carcinogenesis protocol, there was a significant increase in the expression of EGFR and HER-2 oncogenes in precancerous and malignant skin lesions compared to normal tissue. This suggests a potentially early role of these oncogenes in the progression of skin tumours in this model.

Keratinocyte Regnase-1, a Downregulator of Skin Inflammation, Contributes to Protection against Tumor Promotion by Limiting Cyclooxygenase-2 Expression

We previously showed that the ribonuclease Regnase-1 (Reg1) in keratinocytes plays a role in mitigating skin inflammation by downregulating proinflammatory cytokines. In this study, we explored whether Reg1 also has a protective role against skin carcinogenesis. The chemically induced two-stage carcinogenesis protocol revealed that epidermis-specific Reg1-deficient (Reg1-knockout [Reg1-cKO]) mice developed skin tumors with shorter latency and more multiplicity than control mice. In addition, repeated UVB irradiation readily provoked solar keratosis-like lesions in Reg1-cKO mice. Increased levels of cyclooxygenase 2, whose mRNA (Ptgs2) is reportedly a target of Reg1, have been known to be associated with the development of squamous cell carcinomas. Indeed, Ptgs2 mRNA levels were upregulated in the skin of Reg1-cKO mice after treatment with the tumor promoter 12-O-tetradecanoylphorbol-13-acetate. The level of prostaglandin E2 was higher in 12-O-tetradecanoylphorbol-13-acetate‒treated Reg1-cKO mouse skin than in control mice skin. Moreover, invivo inhibition of cyclooxygenase 2 attenuated the 12-O-tetradecanoylphorbol-13-acetate‒induced epidermal thickening in Reg1-cKO mice. Finally, REG1 knockdown in human squamous cell carcinomas lines enhanced PTGS2 mRNA levels after 12-O-tetradecanoylphorbol-13-acetate treatment. In conclusion, epidermal Reg1 plays a regulatory role not only in skin inflammation but also in tumor promotion through the downregulation of cyclooxygenase 2. Therefore, forced expression of Reg1 under inflammatory conditions may be relevant to preventing skin cancer.

436 Keratinocyte Regnase-1, a down-modulator of skin inflammation, contributes to protection from carcinogenesis through regulating COX-2

The relationship between chronic inflammation and carcinogenesis has been controversial, despite known clinical evidence such as development of squamous cell carcinoma (SCC) from chronic skin ulcer. We previously demonstrated that a ribonuclease Regnase-1 (Reg1) in keratinocytes, played a down-modulator of skin inflammation in a cross-regulatory fashion with proinflammatory signals. Here, we addressed whether Reg1 had a protective role in skin carcinogenesis. The two-stage carcinogenesis protocol allowed keratinocyte-specific Reg1 KO mice (K5.Cre+Reg1fl/fl: Reg1-cKO) mice developed 10 or more papillomas and several SCCs per head, while almost no tumor emerged in controls (Reg1fl/fl).

IL-13Rα1 Suppresses Tumor Progression in Two-Stage Skin Carcinogenesis Model by Regulating Regulatory T Cells

Type 2 inflammation‒related cytokine IL-13 plays a protective role in experimental papilloma induction in mice. To understand the mechanisms by which IL-13 contributes to papilloma formation, we utilized Il13rα1-knockout (KO) mice in a widely used 7,12-dimethylbenz[a]anthracene/12-O-tetradecanoyl phorbol-13-acetate two-stage skin carcinogenesis protocol that mimics the development of squamous cell carcinoma. KO mice developed more papillomas and significantly faster than wild-type mice. Papilloma development reduced regulatory T cells in wild-type mice but substantially less in KO mice. In line with this, IL-2 and IL-10 levels decreased in wild-type mice but not in KO mice. Furthermore, systemic IL-5 and TSLP levels were elevated, whereas IL-22 was decreased during papilloma formation in the skin of KO mice. Polymorphonuclear myeloid‒derived suppressor cells were decreased in the KO mice at the early phase of papilloma induction. We show that IL-13Rα1 protects from papilloma development in chemically induced skin carcinogenesis, and our results provide further insights into the protective role of functional IL-4 and IL-13 signaling through type II IL-4 receptor in tumor development.

Effect of transgenic overexpression of BMP antagonist noggin on chemically-induced skin carcinogenesis

To study the role of BMP signaling pathway in the development of skin tumors, we employed two-stage chemical carcinogenesis protocol on K14-Noggin mice expressing BMP antagonist noggin in the epidermis. A comparative analysis showed an earlier appearance and a significant increase in the number of skin papillomas in K14-Noggin mice compared to the wild-type control (FVB). In contrast to control mice, the transgenic K14-Noggin mice also developed squamous cell carcinoma with local metastasis of malignant cells into the dermis of the skin. The results of this work indicate the tumor suppressive role of BMP signaling pathway in the skin epithelium.

Inhibition of HGF/MET signaling decreases overall tumor burden and blocks malignant conversion in Tpl2-related skin cancer

Tumor progression locus 2 (Tpl2) is a member of the mitogen-activated protein kinase kinase kinase (MAP3K) family of serine/threonine kinases. Deletion of the Tpl2 gene is associated with a significantly higher number of papillomas and cutaneous squamous cell carcinomas (cSCCs). Overexpression of hepatocyte growth factor (HGF) and its receptor MET is abundant in cSCC and can lead to increased proliferation, migration, invasion or resistance to epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors. The aim of this study was to address whether the increased tumor burden in Tpl2−/− mice is due to aberrant HGF/MET signaling. C57Bl/6 wild type (WT) and Tpl2−/− mice were subjected to a two-stage chemical carcinogenesis protocol for one year. At the time of promotion half of the mice received 44 mg/kg capmatinib (INC 280), a pharmacological inihibitor of MET, in their diet. Tpl2−/− mice had signficantly higher tumor incidence and overall tumor burden compared to WT mice. Further, carcinogen-intiated Tpl2−/− mice could bypass the need for promotion, as 89% of Tpl2−/− mice given only DMBA developed papillomas. v-rasHa -transduced keratinocytes and SCCs from Tpl2−/− mice revealed an upregulation in HGF and p-MET signaling compared to WT animals. Long-term capmatinib treatment had no adverse effects in mice and capmatinib-fed Tpl2−/− mice had a 60% reduction in overall tumor burden. Further, no tumors from Tpl2−/− mice fed capmatinib underwent malignant conversion. In summary targeting MET may be a potential new strategy to combat cutaneous squamous cell carcinomas that result from dysregulation in MAPK signaling.

Antitumor-Promoting Effect of Ethanolic Extract from Leaves of Perilla frutescens var. crispa (Benth.) In Mouse Skin

This study aimed to investigate the antitumor-promoting effects of total ethanolic extract from Perilla leaf (PLE) in a two-stage chemical carcinogenesis protocol in mice. Quantitative analysis active compounds in PLE were performed by thin layer chromatography with normal phase of silica gel F254 and HPLC-PDA with reversed phase of C18 column at UV 300 nm. The cutaneous tumors were initiated by a single application by 7,12-dimethylbenz[a]anthracene (DMBA) on the dorsal shaved skin, then promoted by repeated applications of croton oil during 20 weeks. PLE was topically applied daily until the end of the experiment. The results observed from thin layer chromatogram highlighted the presence of rosmarinic acid and luteolin in PLE. By means of HPLC-PDA analysis, the total content of rosmarinic acid and luteolin were found in amounts of 1.178 ± 0.011 % (w/w) and 0.105 ± 0.001 % (w/w), respectively. The PLE-treated group prolonged the latency period of tumor appearance up to 2 weeks and also signifi reduced tumor volume in dorsal skin of mice as compared to untreated group. In conclusion, the total ethanolic extract from leaves of Perilla frutescens demonstrated promising antitumor-promotion activity in mouse skin.

Mapping of new skin tumor susceptibility loci by a phenotype-driven congenic approach.

As cancer susceptibility varies among mouse strains, mouse models are powerful tools for the identification of genes responsible for cancer development. Several cancer susceptibility loci have been mapped by genetic analysis using cancer-resistant and cancer-susceptible mouse strains. However, only a few corresponding genes for these loci have been identified, because most of the cancer susceptibility loci are low-penetrance alleles. We reported previously that wild-derived PWK mice showed no tumor development on treatment with the two-stage skin carcinogenesis protocol [induced by 7.12-dimethylbenz(a)anthracene (DMBA)/12-O-tetradecanoylphorbol-13-acetate (TPA)], and that this phenotype is dominant-resistant when crossed with the highly susceptible strain FVB. From the analysis of the F1 backcross generation between PWK and FVB, we have mapped the new significant locus Skts-fp1 on chromosome 4. In the present study, congenic strains were generated with the PWK resistance allele in the FVB background using a phenotype-driven approach, and sought to narrow down the candidate loci and find the responsible gene(s). One of the resistant mice in the N6 generation carried the remaining PWK allele on chromosomes 4, 7 and 11, and an association study using the progeny of this mouse suggested that the locus on chromosome 11 may affect the cancer susceptibility locus on chromosome 7. On the other hand, no skin tumor susceptibility locus was mapped on chromosome 11 as examined in N2 progeny. These findings suggest that there is at least one tumor-resistance gene on chromosome 7, the function of which could be regulated by gene(s) located on chromosome 11.

Abstract 2809: Role of c-Met in the development and progression of Tpl2-related skin cancer

Abstract Cutaneous squamous cell carcinoma (cSCC), a form of non-melanoma skin cancer, is the second most common form of cancer in the United States, with over one million cases diagnosed annually. Tpl2 (MAP3K8) is a serine threonine protein kinase in the mitogen-activated protein kinase (MAPK) signal transduction cascade. Tpl2 was identified by our laboratory as having a tumor suppressive function in skin carcinogenesis, with Tpl2 knockout mice developing significantly more papillomas and squamous cell carcinomas than matched wildtype (WT) controls. Cellular mesenchymal to epithelial transition factor (c-Met) is a receptor heightened in malignant skin cancers and involved in Tpl2 signaling. When bound to its ligand, hepatocyte growth factor (HGF), c-Met can regulate cell proliferation, survival, angiogenesis and invasion. The aim of this study was to ascertain whether c-Met signaling contributes to the heightened skin tumorigenesis in Tpl2-/- mice. Forty-four WT and Tpl2 -/- mice were subjected to a two-stage chemical carcinogenesis protocol for one year to induce skin tumors. Starting at the time of promotion, half of the WT and Tpl2 -/- mice were placed on normal diet and half received diet plus 30mg/kg Capmatinib, a pharmacological inihibitor of c-Met. This diet continued for the duration of the study. No adverse health effects were found in Capmatinib-treated mice. Similar to what we previously reported, Tpl2 -/- mice developed tumors earlier than WT controls and had statistically higher tumor incidence and overall tumor burden than WT mice. The number of tumor bearing Tpl2 -/- and Tpl2 -/- + Capmatinib fed mice was similar, although Capmatinib fed mice had a 60% reduction in overall tumor burden. Studies are underway to determine if Capmatinib affected the frequency of malignant conversion in Tpl2 -/- mice. In summary, our studies demonstrate that heightened c-Met signaling contributes to the higher tumor formation in Tpl2-/- mice and should be evaluated further as a possible therapeutic target in Tpl2-related skin cancers. Citation Format: Nicole F. Bonan, David Kowalski, Kaitlin Kudlac, Lauren G. Falkenberg, Erik Maradiaga, Kira Flaherty, Jonathan S. Wiest, Katie L. DeCicco-Skinner. Role of c-Met in the development and progression of Tpl2-related skin cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2809. doi:10.1158/1538-7445.AM2017-2809

P63 in Squamous Cell Carcinoma of the Skin: More Than a Stem Cell/Progenitor Marker

The p63 gene is often overexpressed in squamous cell carcinomas; however, how its overexpression contributes to tumor formation and expansion is still incompletely understood. Devos etal. report the development of a versatile mouse model demonstrating that p63 facilitates squamous cell carcinoma formation in skin and providing an excellent tool to dissect the relevance of its downstream signaling pathways in tumorigenesis.

T-cell-expressed proprotein convertase FURIN inhibits DMBA/TPA-induced skin cancer development

ABSTRACTProprotein convertases (PCSK) have a critical role in the body homeostasis as enzymes responsible for processing precursor proteins into their mature forms. FURIN, the first characterized member of the mammalian PCSK family, is overexpressed in multiple malignancies and the inhibition of its activity has been considered potential cancer treatment. FURIN has also an important function in the adaptive immunity, since its deficiency in T cells causes an impaired peripheral immune tolerance and accelerates immune responses. We addressed whether deleting FURIN from the immune cells would strengthen anticancer responses by subjecting mouse strains lacking FURIN from either T cells or macrophages and granulocytes to the DMBA/TPA two-stage skin carcinogenesis protocol. Unexpectedly, deficiency of FURIN in T cells resulted in enhanced and accelerated development of tumors, whereas FURIN deletion in macrophages and granulocytes had no effect. The epidermises of T-cell-specific FURIN deficient mice were significantly thicker with more proliferating Ki67+ cells. In contrast, there were no differences in the numbers of the T cells. The flow cytometric analyses of T-cell populations in skin draining lymph nodes showed that FURIN T-cell KO mice have an inherent upregulation of early activation marker CD69 as well as more CD4+CD25+Foxp3+ positive T regulatory cells. In the early phase of tumor promotion, T cells from the T-cell-specific FURIN knockout animals produced more interferon gamma, whereas at later stage the production of Th2- and Th17-type cytokines was more prominent than in wild-type controls. In conclusion, while PCSK inhibitors are promising therapeutics in cancer treatment, our results show that inhibiting FURIN specifically in T cells may promote squamous skin cancer development.

The oncogenic role of GASC1 in chemically induced mouse skin cancer.

Gene amplified in squamous cell carcinoma (SCC) 1 (GASC1), also known as KDM4C/JMJD2C, encodes a histone demethylase that specifically demethylates lysine residues (H3K9, H3K36, and H1.4K26) and plays a crucial role in the regulation of gene expression as well as in heterochromatin formation. GASC1 is located at human chromosome 9p23-24, where frequent genomic amplification is observed in human esophageal cancer, and its aberrant expression is detected in a variety of human cancers, such as breast, colon, and prostate. Therefore, it is highly likely that GASC1 contributes to the genesis and/or development of cancer. However, there is a lack of direct evidence of GASC1 having an oncogenic function. In this study, we aimed to clarify the role of GASC1 in the skin SCC carcinogenesis. For this purpose, we generated Gasc1-heterozygous mice (Gasc1+/-) with reduced expression of Gasc1. On the basis of our results, Gasc1+/- mice displayed a significantly lower incidence and multiplicity of both benign and malignant tumors induced by the two-stage skin carcinogenesis protocol than wild-type mice. In addition, the volume of carcinoma was significantly lower in Gasc1+/- mice. Consistent with these observations, knocking down of Gasc1 resulted in reduced cell viability of SCC cells in vitro. Our findings clearly demonstrated that GASC1 has an oncogenic role in skin carcinogenesis.

The loss of Tm7sf gene accelerates skin papilloma formation in mice.

The 3β-hydroxysterol Δ14-reductase, encoded by the Tm7sf2 gene, is an enzyme involved in cholesterol biosynthesis. Cholesterol and its derivatives control epidermal barrier integrity and are protective against environmental insults. To determine the role of the gene in skin cholesterol homeostasis, we applied 12-o-tetradecanoylphorbol-13-acetate (TPA) to the skin of Tm7sf2+/+ and Tm7sf2-/- mice. TPA increased skin cholesterol levels by inducing de novo synthesis and up-take only in Tm7sf2+/+ mouse, confirming that the gene maintains cholesterol homeostasis under stress conditions. Cholesterol sulfate, one of the major players in skin permeability, was doubled by TPA treatment in the skin of wild-type animals but this response was lost in Tm7sf2-/- mice. The expression of markers of epidermal differentiation concomitant with farnesoid-X-receptor and p38 MAPK activation were also disrupted in Tm7sf2-/- mice. We then subjected Tm7sf2+/+ and Tm7sf2-/- mice to a classical two-stage skin carcinogenesis protocol. We found that the loss of Tm7sf2 increased incidence and multiplicity of skin papillomas. Interestingly, the null genotype showed reduced expression of nur77, a gene associated with resistance to neoplastic transformation. In conclusion, the loss of Tm7sf2 alters the expression of proteins involved in epidermal differentiation by reducing the levels of cholesterol sulfate.

Loss of Snail2 favors skin tumor progression by promoting the recruitment of myeloid progenitors

Snail2 is a zinc finger transcription factor involved in driving epithelial to mesenchymal transitions. Snail2 null mice are viable, but display defects in melanogenesis, gametogenesis and hematopoiesis, and are markedly radiosensitive. Here, using mouse genetics, we have studied the contributions of Snail2 to epidermal homeostasis and skin carcinogenesis. Snail2 (-/-) mice presented a defective epidermal terminal differentiation and, unexpectedly, an increase in number, size and malignancy of tumor lesions when subjected to the two-stage mouse skin chemical carcinogenesis protocol, compared with controls. Additionally, tumor lesions from Snail2 (-/-) mice presented a high inflammatory component with an elevated percentage of myeloid precursors in tumor lesions that was further increased in the presence of the anti-inflammatory agent dexamethasone. In vitro studies in Snail2 null keratinocytes showed that loss of Snail2 leads to a decrease in proliferation indicating a non-cell autonomous role for Snail2 in the skin carcinogenic response observed in vivo. Bone marrow (BM) cross-reconstitution assays between Snail2 wild-type and null mice showed that Snail2 absence in the hematopoietic system fully reproduces the tumor behavior of the Snail2 null mice and triggers the accumulation of myeloid precursors in the BM, blood and tumor lesions. These results indicate a new role for Snail2 in preventing myeloid precursors recruitment impairing skin chemical carcinogenesis progression.

Topical application of a platelet activating factor receptor agonist suppresses phorbol ester-induced acute and chronic inflammation and has cancer chemopreventive activity in mouse skin.

Platelet activating factor (PAF) has long been associated with acute edema and inflammatory responses. PAF acts by binding to a specific G-protein coupled receptor (PAF-R, Ptafr). However, the role of chronic PAF-R activation on sustained inflammatory responses has been largely ignored. We recently demonstrated that mice lacking the PAF-R (Ptafr-/- mice) exhibit increased cutaneous tumorigenesis in response to a two-stage chemical carcinogenesis protocol. Ptafr-/- mice also exhibited increased chronic inflammation in response to phorbol ester application. In this present study, we demonstrate that topical application of the non-hydrolysable PAF mimetic (carbamoyl-PAF (CPAF)), exerts a potent, dose-dependent, and short-lived edema response in WT mice, but not Ptafr -/- mice or mice deficient in c-Kit (c-Kit W-sh/W-sh mice). Using an ear inflammation model, co-administration of topical CPAF treatment resulted in a paradoxical decrease in both acute ear thickness changes associated with a single PMA application, as well as the sustained inflammation associated with chronic repetitive PMA applications. Moreover, mice treated topically with CPAF also exhibited a significant reduction in chemical carcinogenesis. The ability of CPAF to suppress acute and chronic inflammatory changes in response to PMA application(s) was PAF-R dependent, as CPAF had no effect on basal or PMA-induced inflammation in Ptafr-/- mice. Moreover, c-Kit appears to be necessary for the anti-inflammatory effects of CPAF, as CPAF had no observable effect in c-Kit W-sh/W-sh mice. These data provide additional evidence that PAF-R activation exerts complex immunomodulatory effects in a model of chronic inflammation that is relevant to neoplastic development.

Abstract 1323: Lack of cyclin D3 enhances the CDK6-dependent skin tumor susceptibility to malignant progression

Abstract The Cyclin-Dependent Kinases (CDKs) are a family of essential serine-threonine kinases that are cell cycle regulated by association with cyclins at specific time-points of the cell cycle. Specifically, D-type cyclins form complexes and activate CDK6 and CDK4. We have previously established that transgenic expression of CDK6 in mouse epidermis (K5CDK6 mice) alter an early stage of tumorigenesis leading to strong reduction of the multiplicity and incidence of skin tumors. Our previous results have shown that CDK6 preferentially binds to cyclin D3, which overexpression also decreases tumor development in mouse epidermis. Thus, we hypothesized that the association of CDK6/cyclin D3 and its enzymatic activity negatively impacts tumor development. To test this idea, we developed K5CDK6/Cyclin D3-/- mutant mice and subjected them to the two-stage carcinogenesis protocol. Contrary to our hypothesis, lack of cyclin D3 expression does not affect the tumor repression activity of CDK6 since both, K5CDK6 and K5CDK6/cyclin D3-/- mice, exhibit a severe reduction in the multiplicity and incidence of skin tumors compared to wild-type siblings. However, histopathological analysis of skin tumors revealed that 60% of the skin tumors in K5CDK6/cyclin D3-/- mice were classified as squamous-cell carcinomas I (SCC) with a variable degree of squamous differentiation; whereas wild type, K5CDK6 and cyclin D3-/- siblings exhibit benign tumors with no significant changes in the rate of malignant progression. Consistent with the malignant nature of the K5CDK6/cyclin D3-/- tumors, lack or reduction of cyclin D3 levels within the context of CDK6 overexpression (K5CDK6/D3-/- and K5CDK6/cyclin D3+/-) lead to 7-fold increase in tumor size compared to K5CDK6, cyclin D3-/- and wild-type tumors. Biochemical analysis of epidermal tumors shows that cyclin D3 ablation results in a rise of cyclin D1 protein levels. These data suggest that though cyclin D3 expression does not alter the tumor suppressive role of CDK6 at early stages of tumorigenesis, it does affect the expression of cyclin D1 resulting in the increased rate of malignant progression. Consistent with these studies, we have shown that elevated activity of CDK4/cyclin D1 complex results in an elevated number of SCC. We conclude that CDK6 activity can prevent tumor formation during the initial stages of tumorigenesis in a cyclin D3-independent manner and likely Rb-independent as well (Kollman et al, 2013). However, variations in the expression of cyclin D3 affects cyclin D2 (Rojas et al, 2007) and/or cyclin D1 levels, directly altering the susceptibility of CDK6 expressing tumors to malignant conversion. Therefore, therapies directed to reduce D-type cyclins expression should be considered within the context of the expression level of CDK6 and CDK4 to define the potential effect in the malignant progression of skin tumors. Supported by NIH grant CA116328. Note: This abstract was not presented at the meeting. Citation Format: Sung Hyun Lee, Xian Wang, Marcelo L. Rodriguez-Puebla. Lack of cyclin D3 enhances the CDK6-dependent skin tumor susceptibility to malignant progression. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1323. doi:10.1158/1538-7445.AM2014-1323

In vivo long‐term effects of retinoic acid exposure in utero on induced tumours in adult mouse skin

Retinoic acid (RA) and its analogues (retinoids) are promising agents in skin cancer prevention following either topical application or oral administration. However, long-term in vivo effects of RA on chemically induced hyperplastic epidermal foci in adult mouse skin have also been described, casting some doubt with regard to its chemopreventive activity. To characterize chemically induced skin tumours and to investigate the in vivo long-term action and preventive effect of RA on adult mouse skin carcinogenesis. Fifty-six adult Naval Medical Research Institute mice, exposed (n=28) or not exposed (n=28) to RA in utero. Mice were treated with a standard two-stage skin carcinogenesis protocol, which included an initiating application of 7,12-dimethylbenz(a)anthracene followed by promotion with 12-O-tetradecanoylphorbol 13-acetate. Retinoic acid administered to pregnant mice showed a long-term inhibitory action on cell differentiation and development of chemically induced tumours on the adult skin of their offspring, as well as a stimulatory effect on cell proliferation and expression of an early marker of malignant progression (keratin13). The results suggest that RA exposure in utero confers long-lasting effects on adult mouse skin carcinogenesis. These include chemopreventive activity (reduced number of tumours), as well as enhancement of squamous papilloma progression, which appears to be due to enhanced keratinocyte proliferation and suppression of epidermal maturation. The clinical significance of these findings is not known for other routes of RA administration at this time.

Protein Kinase D1 Has a Key Role in Wound Healing and Skin Carcinogenesis

Protein kinase D (PKD) is a family of stress-responsive serine/threonine kinases implicated in the regulation of diverse cellular functions including cell growth, differentiation, apoptosis, and cell motility. Although all three isoforms are expressed in keratinocytes, their role in skin biology and pathology is poorly understood. We recently identified a critical role for PKD1 during reversal of keratinocyte differentiation in culture, suggesting a potential pro-proliferative role in epidermal adaptive responses. Here, we generated mice with targeted deletion of PKD1 in epidermis to evaluate the significance of PKD1 in normal and hyperplastic conditions. These mice displayed a normal skin phenotype indicating that PKD1 is dispensable for skin development and homeostasis. Upon wounding however, PKD1-deficient mice exhibited delayed wound re-epithelialization correlated with a reduced proliferation and migration of keratinocytes at the wound edge. In addition, the hyperplastic and inflammatory responses to topical phorbol ester were significantly suppressed suggesting involvement of PKD1 in tumor promotion. Consistently, when subjected to two-stage chemical skin carcinogenesis protocol, PKD1-deficient mice were resistant to papilloma formation when compared to control littermates. These results revealed a critical pro-proliferative role for PKD1 in epidermal adaptive responses, suggesting a potential therapeutic target in skin wound and cancer treatment.

Does increase in DNA repair allow “Tolerance‐to‐Insult” in chemical carcinogenesis? Skin tumor experiments with MGMT‐overexpressing mice

Several genotoxicity endpoints have been evaluated to define nonlinear dose-responses for SN 1 and SN 2 alkylating genotoxicants. Dose-response studies acknowledging the process of multistage tumorigenesis are important; however, data pertaining nonlinearity are not yet available. In this communication, the role of DNA repair in the dose-response relationship for benign papillomas was examined using the two-stage skin carcinogenesis protocol. The data obtained with O(6) -methylguanine-DNA methyltransferase (MGMT) overexpressing mice in which papillomas were induced by a single topical treatment with N-methyl-N-nitrosourea (MNU) followed by promotion with 12-O-tetradecanoylphorbol-13-acetate are reported. As MGMT efficiently protects cells from mutations by repairing O(6) -methylguanine, a miscoding lesion induced by MNU, the question whether MGMT is able to nullify carcinogenic lesions to an extent where they would be considered nonhazardous has been addressed. It is shown here that MGMT overexpression significantly protects against, but does not completely nullify, the effect of MNU in tumor initiation. The possible mechanisms involved have also been discussed.

Loss of integrin α3 prevents skin tumor formation by promoting epidermal turnover and depletion of slow-cycling cells

Progression through the various stages of skin tumorigenesis is correlated with an altered expression of the integrin α3β1, suggesting that it plays an important role in the tumorigenic process. Using epidermis-specific Itga3 KO mice subjected to the 7,12-dimethylbenzanthracene (DMBA)/12-O-tetradecanoylphorbol-13-acetate two-stage skin carcinogenesis protocol, we demonstrate that efficient tumor development is critically dependent on the presence of α3β1. In the absence of α3β1, tumor initiation is dramatically decreased because of increased epidermal turnover, leading to a loss of DMBA-initiated label-retaining keratinocytes. Lineage tracing revealed emigration of α3-deficient keratinocytes residing in the bulge of the hair follicle toward the interfollicular epidermis. Furthermore, tumor growth and cell proliferation were strongly reduced in mice with an epidermis-specific deletion of Itga3. However, the rate of progression of α3β1-null squamous cell carcinomas to undifferentiated, invasive carcinomas was increased. Therefore, α3β1 critically affects skin carcinogenesis with opposing effects early and late in tumorigenesis.