Milestones in skin carcinogenesis: the biology of multistage carcinogenesis

A common mechanism links Epstein-Barr virus infections and autoimmune diseases

Epstein-Barr virus (EBV) infection is associated with a variety of the autoimmune diseases. There is apparently no unified model for the role of EBV in autoimmune diseases. In this article, the development of autoimmune diseases is proposed as a simple two-step process: specific autoimmune initiators may cause irreversible changes to genetic materials that increase autoimmune risks, and autoimmune promoters promote autoimmune disease formation once cells are susceptible to autoimmunity. EBV has several types of latencies including type III latency with higher proliferation potential. EBV could serve as autoimmune initiators for some autoimmune diseases. At the same time, EBV may play a promotional role in majority of the autoimmune diseases by repeated replenishment of EBV type III latency cells and inflammatory cytokine productions in persistent stage. The type III latency cells have enhanced capacity as antigen-presenting cells that would facilitate the development of both B and T cell-mediated autoimmunity. The repeated cytokine productions are achieved by the repeated infection of naive B-lymphocytes and proliferation of type III latency cells that produce inflammatory cytokines. Presentation of viral or self-antigens by EBV type III latency B lymphocytes may promote autoreactive B cell and T cell proliferation, which can be amplified by type III latency cells-mediated cytokines productions. Different autoimmune diseases may require different kinds of pathogenic immune cells and/or specific cytokines. Frequency of the replenishment of EBV type III latency cells may determine the specific effect of the promoter functions. A specific initiator plus EBV-mediated common promoter function may lead to development of a specific autoimmune disease and link EBV-infection to a variety of autoimmunity.

RAS oncogene signal strength regulates matrisomal gene expression and tumorigenicity of mouse keratinocytes

Environmental and molecular carcinogenesis are linked by the discovery that chemical carcinogen induced-mutations in the Hras or Kras genes drives tumor development in mouse skin. Importantly, enhanced expression or allele amplification of the mutant Ras gene contributes to selection of initiated cells, tumor persistence, and progression. To explore the consequences of Ras oncogene signal strength, primary keratinocytes were isolated and cultured from the LSL-HrasG12D and LSL-KrasG12D C57BL/6J mouse models and the mutant allele was activated by adeno-Cre recombinase. Keratinocytes expressing one (H) or two (HH) mutant alleles of HrasG12D, one KrasG12D allele (K), or one of each (HK) were studied. All combinations of activated Ras alleles stimulated proliferation and drove transformation marker expression, but only HH and HK formed tumors. HH, HK, and K sustained long-term keratinocyte growth in vitro, while H and WT could not. RNA-Seq yielded two distinct gene expression profiles; HH, HK, and K formed one cluster while H clustered with WT. Weak MAPK activation was seen in H keratinocytes but treatment with a BRAF inhibitor enhanced MAPK signaling and facilitated tumor formation. K keratinocytes became tumorigenic when they were isolated from mice where the LSL-KrasG12D allele was backcrossed from the C57BL/6 onto the FVB/N background. All tumorigenic keratinocytes but not the non-tumorigenic precursors shared a common remodeling of matrisomal gene expression that is associated with tumor formation. Thus, RAS oncogene signal strength determines cell-autonomous changes in initiated cells that are critical for their tumor-forming potential.

Inactivation of Autophagy in Keratinocytes Reduces Tumor Growth in Mouse Models of Epithelial Skin Cancer

Autophagy is a ubiquitous degradation mechanism, which plays a critical role in cellular homeostasis. To test whether autophagy suppresses or supports the growth of tumors in the epidermis of the skin, we inactivated the essential autophagy gene Atg7 specifically in the epidermal keratinocytes of mice (Atg7^∆ep) and subjected such mutant mice and fully autophagy-competent mice to tumorigenesis. The lack of epithelial Atg7 did not prevent tumor formation in response to 7, 12-dimethylbenz(a)anthracene (DMBA) as the initiator and 12-O tetradecanoylphorbol-13-acetate (TPA) as the promoter of tumor growth. However, the number of tumors per mouse was reduced in mice with epithelial Atg7 deficiency. In the K5-SOS EGFR^wa2/wa2 mouse model, epithelial tumors were initiated by Son of sevenless (SOS) in response to wounding. Within 12 weeks after tumor initiation, 60% of the autophagy-competent K5-SOS EGFR^wa2/wa2 mice had tumors of 1 cm diameter and had to be sacrificed, whereas none of the Atg7^∆ep K5-SOS EGFR^wa2/wa2 mice formed tumors of this size. In summary, the deletion of Atg7 reduced the growth of epithelial tumors in these two mouse models of skin cancer. Thus, our data show that the inhibition of autophagy limits the growth of epithelial skin tumors.

Investigating Cutaneous Squamous Cell Carcinoma in vitro and in vivo: Novel 3D Tools and Animal Models

Cutaneous Squamous Cell Carcinoma (cSCC) represents the second most common type of skin cancer, which incidence is continuously increasing worldwide. Given its high frequency, cSCC represents a major public health problem. Therefore, to provide the best patients’ care, it is necessary having a detailed understanding of the molecular processes underlying cSCC development, progression, and invasion. Extensive efforts have been made in developing new models allowing to study the molecular pathogenesis of solid tumors, including cSCC tumors. Traditionally, in vitro studies were performed with cells grown in a two-dimensional context, which, however, does not represent the complexity of tumor in vivo. In the recent years, new in vitro models have been developed aiming to mimic the three-dimensionality (3D) of the tumor, allowing the evaluation of tumor cell-cell and tumor-microenvironment interaction in an in vivo-like setting. These models include spheroids, organotypic cultures, skin reconstructs and organoids. Although 3D models demonstrate high potential to enhance the overall knowledge in cancer research, they lack systemic components which may be solved only by using animal models. Zebrafish is emerging as an alternative xenotransplant model in cancer research, offering a high-throughput approach for drug screening and real-time in vivo imaging to study cell invasion. Moreover, several categories of mouse models were developed for pre-clinical purpose, including xeno- and syngeneic transplantation models, autochthonous models of chemically or UV-induced skin squamous carcinogenesis, and genetically engineered mouse models (GEMMs) of cSCC. These models have been instrumental in examining the molecular mechanisms of cSCC and drug response in an in vivo setting. The present review proposes an overview of in vitro, particularly 3D, and in vivo models and their application in cutaneous SCC research.

Loss of Tpl2 activates compensatory signaling and resistance to EGFR/MET dual inhibition in v-RAS transduced keratinocytes

Cutaneous squamous cell carcinoma (cSCC) is the second most common form of skin cancer in the United States, affecting one million people per year. Patients with aggressive disease have limited treatment options and high mortality, highlighting the need to identify new biomarkers linked to poor clinical outcome. HRAS mutations are found in skin papillomas and cSCCs and increase in frequency when MAP3K family members are inhibited, suggesting a link between blockade of mitogen-activated protein kinase (MAPK) signaling and initiation of RAS-primed cells. Tpl2, a MAP3K gene, can serve as a tumor suppressor gene in cSCC. We have previously shown that upon Tpl2 ablation, mice have heightened sensitivity to aberrant RAS signaling. Tpl2-/- mice display significantly higher numbers of papillomas and cSCCs in two-stage chemical carcinogenesis studies and increased tumorigenicity of keratinocytes expressing oncogenic v-rasHa in nude mouse skin grafts. In part, this is mediated through increased mesenchymal-epithelial transition factor (MET) receptor activity. Epidermal Growth Factor Receptor (EGFR) is reported to be an essential factor for MET-driven carcinogenesis and MET activation may confer resistance to EGFR therapies, suggesting that the concurrent use of both an EGFR inhibitor and a MET inhibitor may show promise in advanced cSCCs. In this study we assessed whether normal or Ras-transformed Tpl2-/- keratinocytes have aberrant EGFR signaling and whether concomitant treatment with EGFR/MET tyrosine kinase inhibitors was more effective than single agents in reducing growth and angiogenic potential of Ras-transformed keratinocytes. Tpl2-/- keratinocytes exhibited increased HER-2 and STAT-3 under basal conditions and elevated p-MET and p-EGFR when transduced with oncogenic RAS. Inhibition of MET by Capmatinib increased p-EGFR in Tpl2-/- keratinocytes and papillomas, and inhibition of EGFR by Gefitinib increased HER2 and HER3 signaling in both genotypes. Treatment of keratinocytes with EGFR and MET inhibitors, in combination, significantly enhanced endothelial tube formation, MMP-9 activity and activation of other RTKs, with more pronounced effects when Tpl2 was ablated. These data indicate that Tpl2 cross-talks with both EGFR and MET signaling pathways. Upon inhibition of EGFR/MET signaling, a myriad of escape mechanisms exists in keratinocytes to overcome targeted drug effects.

RAS induced senescence of skin keratinocytes is mediated through Rho-associated protein kinase (ROCK)

Cellular senescence is a well-documented response to oncogene activation in many tissues. Multiple pathways are invoked to achieve senescence indicating its importance to counteract the transforming activities of oncogenic stimulation. We now report that the Rho-associated protein kinase (ROCK) signaling pathway is a critical regulator of oncogene-induced senescence in skin carcinogenesis. Transformation of mouse keratinocytes with oncogenic RAS upregulates ROCK activity and initiates a senescence response characterized by cell enlargement, growth inhibition, upregulation of senescence associated β-galactosidase (SAβgal) expression, and release of multiple pro-inflammatory factors comprising the senescence-associated secretory phenotype (SASP). The addition of the ROCK inhibitor Y-27632 and others prevents these senescence responses and maintains proliferating confluent RAS transformed keratinocyte cultures indefinitely. Mechanistically, oncogenic RAS transformation is associated with upregulation of cell cycle inhibitors p15Ink4b , p16Ink4a , and p19Arf and downregulation of p-AKT, all of which are reversed by Y-27632. RNA-seq analysis of Y-27632 treated RAS-transformed keratinocytes indicated that the inhibitor reduced growth-inhibitory gene expression profiles and maintained expression of proliferative pathways. Y-27632 also reduced the expression of NF-κB effector genes and the expression of IκBζ downstream mediators. The senescence inhibition from Y-27632 was reversible, and upon its removal, senescence reoccurred in vitro with rapid upregulation of cell cycle inhibitors, SASP expression, and cell detachment. Y-27632 treated cultured RAS-keratinocytes formed tumors in the absence of the inhibitor when placed in skin orthografts suggesting that factors in the tumor microenvironment can overcome the drive to senescence imparted by overactive ROCK activity.

Cutaneous Squamous Cell Carcinoma

Cutaneous squamous cell carcinoma represents 20% of all skin cancers, resulting in 1 million cases in the United States each year. The lifetime risk of developing squamous cell carcinoma continues to increase annually and will likely continue to increase because of the aging population. Most cutaneous squamous cell carcinoma are treated locally, with a subset leading to recurrence, metastasis, and death. This review of cutaneous squamous cell carcinoma covers incidence, recurrence rates, mortality rates, risk factors, staging systems, treatment, prevention, and monitoring.

New insights into the functions of Cox-2 in skin and esophageal malignancies

Understanding the cellular and molecular mechanisms of tumor initiation and progression for each cancer type is central to making improvements in both prevention and therapy. Identifying the cancer cells of origin and the necessary and sufficient mechanisms of transformation and progression provide opportunities for improved specific clinical interventions. In the last few decades, advanced genetic manipulation techniques have facilitated rapid progress in defining the etiologies of cancers and their cells of origin. Recent studies driven by various groups have provided experimental evidence indicating the cellular origins for each type of skin and esophageal cancer and have identified underlying mechanisms that stem/progenitor cells use to initiate tumor development. Specifically, cyclooxygenase-2 (Cox-2) is associated with tumor initiation and progression in many cancer types. Recent studies provide data demonstrating the roles of Cox-2 in skin and esophageal malignancies, especially in squamous cell carcinomas (SCCs) occurring in both sites. Here, we review experimental evidence aiming to define the origins of skin and esophageal cancers and discuss how Cox-2 contributes to tumorigenesis and differentiation.

Unraveling cancer lineage drivers in squamous cell carcinomas

Cancer hijacks embryonic development and adult wound repair mechanisms to fuel malignancy. Cancer frequently originates from de-regulated adult stem cells or progenitors, which are otherwise essential units for postnatal tissue remodeling and repair. Cancer genomics studies have revealed convergence of multiple cancers across organ sites, including squamous cell carcinomas (SCCs), a common group of cancers arising from the head and neck, esophagus, lung, cervix and skin. In this review, we summarize our current knowledge on the molecular drivers of SCCs, including these five major organ sites. We especially focus our discussion on lineage dependent driver genes and pathways, in the context of squamous development and stratification. We then use skin as a model to discuss the notion of field cancerization during SCC carcinogenesis, and cancer as a wound that never heals. Finally, we turn to the idea of context dependency widely observed in cancer driver genes, and outline literature support and possible explanations for their lineage specific functions. Through these discussions, we aim to provide an up-to-date summary of molecular mechanisms driving tumor plasticity in squamous cancers. Such basic knowledge will be helpful to inform the clinics for better stratifying cancer patients, revealing novel drug targets and providing effective treatment options.

A Multistage Murine Breast Cancer Model Reveals Long-Lived Premalignant Clones Refractory to Parity-Induced Protection

Breast cancers evolve in a multistage process that can span decades after a carcinogenic exposure. It follows that long-lived precursor breast lesions persist in a subclinical state prior to completing malignant transformation, yet widely used breast cancer models lack an experimental framework for targeting premalignant disease. Inspired by classic multistage skin carcinogenesis protocols, we combined chemical carcinogenesis with transgenic mouse modeling to resolve mouse mammary carcinogenesis into discrete initiation and progression stages. At the initiation stage, exposure to the carcinogen 7,12-dimethylbenzanthracene (DMBA) generated "initiated mammary epithelial cells" (iMEC) by introducing a stereotyped Hras^Q61L driver mutation. Whether DMBA exposure occurred during puberty or adulthood, mice efficiently acquired iMEC clones that eluded detection by conventional histology, yet were long lived, persisting in a clinically silent state for months in the absence of a cooperating event. At the progression stage, inducible activation of oncogenic Wnt signaling drove rapid and synchronous transformation of latent iMECs into overt mammary carcinomas, while Wnt activation in neighboring normal mammary epithelium yielded only benign hyperplasia over this same time period. Although early parity (completion of a full-term pregnancy) reduces breast cancer risk in some contexts, standard parity-induced protection schemes failed to eliminate iMECs in our multistage model, suggesting Wnt-responsive iMECs are maintained by hormone-independent mechanisms. Variations on our multistage modeling strategy may help to identify and validate cellular and molecular targets for breast cancer chemoprevention.

Time-Series Analysis of Tumorigenesis in a Murine Skin Carcinogenesis Model

Recent years have witnessed substantial progress in understanding tumor heterogeneity and the process of tumor progression; however, the entire process of the transition of tumors from a benign to metastatic state remains poorly understood. In the present study, we performed a prospective cancer genome-sequencing analysis by employing an experimental carcinogenesis mouse model of squamous cell carcinoma to systematically understand the evolutionary process of tumors. We surgically collected a part of a lesion of each tumor and followed the progression of these tumors in vivo over time. Comparative time-series analysis of the genomes of tumors with different fates, i.e., those that eventually metastasized and regressed, suggested that these tumors acquired and inherited different mutations. These findings suggest that despite the occurrence of an intra-tumor selection event for malignant alteration during the transformation from early- to late-stage papilloma, the fate determination of tumors might be determined at an even earlier stage.

Pathogenesis of Skin Carcinomas and a Stem Cell as Focal Origin

UV radiation in sunlight has long been recognized as the main exogenous cause of skin carcinomas. We present a brief historical perspective on the progress in understanding the pathogenesis of skin carcinomas, and recent advances. Sun-exposed skin carries numerous UV-related mutations, and skin carcinomas rank among the tumors with the highest mutational loads. In this multitude of mutations only a few are crucial in driving the tumor. Some are known from hereditary (skin) cancer syndromes and other recurrent ones have been validated in transgenic mice. Considering the continuous renewal of the epidermis, the question arises whether the lifelong residing stem cells are the main targets in skin carcinogenesis, a multistep process that would require ample time to evolve. Therefore, classic quiescent stem cells have been studied as potential tumor-initiating cells, as well as more recently discovered actively dividing stem cells (either Lgr5+ or Lgr6+). Interesting differences have emerged between experimental UV and two-stage chemical carcinogenesis, e.g., the latter appears to originate from follicular stem cells, in contrast to the former.

Preclinical mouse solid tumour models: status quo, challenges and perspectives

Oncology research in humans is limited to analytical and observational studies for obvious ethical reasons, with therapy-focused clinical trials being the one exception to this rule. Preclinical mouse tumour models therefore serve as an indispensable intermediate experimental model system bridging more reductionist in vitro research with human studies. Based on a systematic survey of preclinical mouse tumour studies published in eight scientific journals in 2016, this Analysis provides an overview of how contemporary preclinical mouse tumour biology research is pursued. It thereby identifies some of the most important challenges in this field and discusses potential ways in which preclinical mouse tumour models could be improved for better relevance, reproducibility and translatability.

Lgr6 is a stem cell marker in mouse skin squamous cell carcinoma

The G-protein-coupled receptors Lgr4/5/6 are Wnt signalling mediators, but their functions in squamous carcinomas (SCCs) are unclear. Using lineage tracing in Lgr5-EGFP-CreERT2- and Lgr6-EGFP-CreERT2- Rosa26/Tomato reporter mice, we demonstrate that Lgr6, but not Lgr5, acts as an epithelial stem cell marker in vivo in SCCs. We identify, by single molecule in situ hybridisation and cell sorting, rare Lgr6-positive cells in immortalised keratinocytes, and show that their frequency increases in advanced SCCs. Lgr6 expression is enriched in cells with stem cell characteristics, and Lgr6 downregulation in vivo causes increased epidermal proliferation, with expanded lineage tracing from Lgr6⁺ epidermal stem cells. Surprisingly, Lgr6 germline knockout mice are predisposed to SCC development, by a mechanism that includes compensatory upregulation of Lgr5. These data provide a model for human patients with germline loss of function mutations in WNT pathway genes RSPO1 or LGR4, who show increased susceptibility to squamous tumour development.

Stem Cell Lineage Infidelity Drives Wound Repair and Cancer

Tissue stem cells contribute to tissue regeneration and wound repair through cellular programs that can be hijacked by cancer cells. Here, we investigate such a phenomenon in skin, where during homeostasis, stem cells of the epidermis and hair follicle fuel their respective tissues. We find that breakdown of stem cell lineage confinement-granting privileges associated with both fates-is not only hallmark but also functional in cancer development. We show that lineage plasticity is critical in wound repair, where it operates transiently to redirect fates. Investigating mechanism, we discover that irrespective of cellular origin, lineage infidelity occurs in wounding when stress-responsive enhancers become activated and override homeostatic enhancers that govern lineage specificity. In cancer, stress-responsive transcription factor levels rise, causing lineage commanders to reach excess. When lineage and stress factors collaborate, they activate oncogenic enhancers that distinguish cancers from wounds.

ROCK2/rasHa co-operation induces malignant conversion via p53 loss, elevated NF-κB and tenascin C-associated rigidity, but p21 inhibits ROCK2/NF-κB-mediated progression

To study ROCK2 activation in carcinogenesis, mice expressing 4-hydroxytamoxifen (4HT)-activated ROCK2 (K14.ROCKer) were crossed with mice expressing epidermal-activated rasHa (HK1.ras1205). At 8 weeks, 4HT-treated K14.ROCKer/HK1.ras1205 cohorts exhibited papillomas similar to HK1.ras1205 controls; however, K14.ROCKer/HK1.ras1205 histotypes comprised a mixed papilloma/well-differentiated squamous cell carcinoma (wdSCC), exhibiting p53 loss, increased proliferation and novel NF-κB expression. By 12 weeks, K14.ROCKer/HK1.ras1205 wdSCCs exhibited increased NF-κB and novel tenascin C, indicative of elevated rigidity; yet despite continued ROCK2 activities/p-Mypt1 inactivation, progression to SCC required loss of compensatory p21 expression. K14.ROCKer/HK1.ras1205 papillomatogenesis also required a wound promotion stimulus, confirmed by breeding K14.ROCKer into promotion-insensitive HK1.ras1276 mice, suggesting a permissive K14.ROCKer/HK1.ras1205 papilloma context (wound-promoted/NF-κB+/p53-/p21+) preceded K14.ROCKer-mediated (p-Mypt1/tenascin C/rigidity) malignant conversion. Malignancy depended on ROCKer/p-Mypt1 expression, as cessation of 4HT treatment induced disorganized tissue architecture and p21-associated differentiation in wdSCCs; yet tenascin C retention in connective tissue extracellular matrix suggests the rigidity laid down for conversion persists. Novel papilloma outgrowths appeared expressing intense, basal layer p21 that confined endogenous ROCK2/p-Mypt1/NF-κB to supra-basal layers, and was paralleled by restored basal layer p53. In later SCCs, 4HT cessation became irrelevant as endogenous ROCK2 expression increased, driving progression via p21 loss, elevated NF-κB expression and tenascin C-associated rigidity, with p-Mypt1 inactivation/actinomyosin-mediated contractility to facilitate invasion. However, p21-associated inhibition of early-stage malignant progression and the intense expression in papilloma outgrowths, identifies a novel, significant antagonism between p21 and rasHa/ROCK2/NF-κB signalling in skin carcinogenesis. Collectively, these data show that ROCK2 activation induces malignancy in rasHa-initiated/promoted papillomas in the context of p53 loss and novel NF-κB expression, whereas increased tissue rigidity and cell motility/contractility help mediate tumour progression.

Chemical Carcinogenesis Models of Cancer: Back to the Future

Over a century has elapsed since the first demonstration that exposure to chemicals in coal tar can cause cancer in animals. These observations provided an essential causal mechanistic link between environmental chemicals and increased risk of cancer in human populations. Mouse models of chemical carcinogenesis have since led to the concept of multistage tumor development through distinct stages of initiation, promotion, and progression and identified many of the genetic and biological events involved in these processes. Recent breakthroughs in DNA sequencing have now given us tools to dissect complete tumor genome architectures and revealed that chemically induced cancers in the mouse carry a high point mutation load and mutation signatures that reflect the causative agent used for tumor induction. Chemical carcinogenesis models may therefore provide a route to identify the causes of mutation signatures found in human cancers and further inform studies of therapeutic drug resistance and responses to immunotherapy, which are dependent on mutation load and genetic heterogeneity.

Linking DNA adduct formation and human cancer risk in chemical carcinogenesis

Over two centuries ago, Sir Percival Pott, a London surgeon, published a pioneering treatise showing that soot exposure was the cause of high incidences of scrotal cancers occurring in young men who worked as chimney sweeps. Practicing at a time when cellular pathology was not yet recognized, Sir Percival nonetheless observed that the high incidence and short latency of the chimney sweep cancers, was fundamentally different from the rare scrotal cancers typically found in elderly men. Furthermore, his diagnosis that the etiology of these cancers was related to chimney soot exposure, was absolutely accurate, conceptually novel, and initiated the field of "occupational cancer epidemiology." After many intervening years of research focused on mechanisms of chemical carcinogenesis, briefly described here, it is clear that DNA damage, or DNA adduct formation, is "necessary but not sufficient" for tumor induction, and that many additional factors contribute to carcinogenesis. This review includes a synopsis of carcinogen-induced DNA adduct formation in experimental models and in the human population, with particular attention paid to molecular dosimetry and molecular cancer epidemiology. Environ. Mol. Mutagen. 57:499-507, 2016. © 2016 Wiley Periodicals, Inc.

N-bromotaurine surrogates for loss of antiproliferative response and enhances cisplatin efficacy in cancer cells with impaired glucocorticoid receptor

Glucocorticoids (GCs) are frequently used in anticancer combination regimens; however, their continuous use adds selective pressure on cancer cells to develop GC-resistance via impairment of the glucocorticoid receptor (GR), therefore creating a need for GC-alternatives. Based on the drug repurposing approach and the commonalities between inflammation and neoplasia, drugs that are either in late-stage clinical trials and/or already marketed for GC-refractory inflammatory diseases could be evaluated as GC-substitutes in the context of cancer. Advantageously, unlike new molecular entities currently being de novo developed to restore GC-responsiveness of cancer cells, such drugs have documented safety and efficacy profile, which overall simplifies their introduction in clinical cancer trials. In this study, we estimated the potential of a well-established, multistage, cell line-based, mouse skin carcinogenesis model to be exploited as an initial screening tool for unveiling covert GC-substitutes. First, we categorized the cell lines of this model to GC-sensitive and GC-resistant, in correlation with their corresponding GR status, localization, and functionality. We found that GC-resistance starts in papilloma stages, due to a dysfunctional GR, which is overexpressed, DNA binding-competent, but transactivation-incompetent in papilloma, squamous, and spindle stages of the model. Then, aided by this tool, we evaluated the ability of N-bromotaurine, a naturally occurring, small-molecule, nonsteroid anti-inflammatory drug which is under consideration for use interchangeably/in replacement to GCs in skin inflammations, to restore antiproliferative response of GC-resistant cancer cells. Unlike GCs, N-bromotaurine inhibited cell-cycle progression in GC-resistant cancer cells and efficiently synergized with cisplatin, thus indicating a potential to be exploited instead of GCs against cancer.

MET signaling in keratinocytes activates EGFR and initiates squamous carcinogenesis

The receptor tyrosine kinase MET is abundant in many human squamous cell carcinomas (SCCs), but its functional significance in tumorigenesis is not clear. We found that the incidence of carcinogen-induced skin squamous tumors was substantially increased in transgenic MT-HGF (mouse metallothionein-hepatocyte growth factor) mice, which have increased abundance of the MET ligand HGF. Squamous tumors also erupted spontaneously on the skin of MT-HGF mice that were promoted by wounding or the application of 12-O-tetradecanoylphorbol 13-acetate, an activator of protein kinase C. Carcinogen-initiated tumors had Ras mutations, but spontaneous tumors did not. Cultured keratinocytes from MT-HGF mice and oncogenic RAS-transduced keratinocytes shared phenotypic and biochemical features of initiation that were dependent on autocrine activation of epidermal growth factor receptor (EGFR) through increased synthesis and release of EGFR ligands, which was mediated by the kinase SRC, the pseudoproteases iRhom1 and iRhom2, and the metallopeptidase ADAM17. Pharmacological inhibition of EGFR caused the regression of MT-HGF squamous tumors that developed spontaneously in orthografts of MT-HGF keratinocytes combined with dermal fibroblasts and implanted onto syngeneic mice. The global gene expression profile in MET-transformed keratinocytes was highly concordant with that in RAS-transformed keratinocytes, and a core RAS/MET coexpression network was activated in precancerous and cancerous human skin lesions. Tissue arrays revealed that many human skin SCCs have abundant HGF at both the transcript and protein levels. Thus, through the activation of EGFR, MET activation parallels a RAS pathway to contribute to human and mouse cutaneous cancers.

Carcinogen-specific mutations in preferred Ras-Raf pathway oncogenes directed by strand bias

Carcinogen exposures inscribe mutation patterns on cancer genomes and sometimes bias the acquisition of driver mutations toward preferred oncogenes, potentially dictating sensitivity to targeted agents. Whether and how carcinogen-specific mutation patterns direct activation of preferred oncogenes remains poorly understood. Here, mouse models of breast cancer were exploited to uncover a mechanistic link between strand-biased mutagenesis and oncogene preference. When chemical carcinogens were employed during Wnt1-initiated mammary tumorigenesis, exposure to either 7,12-dimethylbenz(a)anthracene (DMBA) or N-ethyl-N-nitrosourea (ENU) dramatically accelerated tumor onset. Mammary tumors that followed DMBA exposure nearly always activated the Ras pathway via somatic Hras(CAA61CTA) mutations. Surprisingly, mammary tumors that followed ENU exposure typically lacked Hras mutations, and instead activated the Ras pathway downstream via Braf(GTG636GAG) mutations. Hras(CAA61CTA) mutations involve an A-to-T change on the sense strand, whereas Braf(GTG636GAG) mutations involve an inverse T-to-A change, suggesting that strand-biased mutagenesis may determine oncogene preference. To examine this possibility further, we turned to an alternative Wnt-driven tumor model in which carcinogen exposures augment a latent mammary tumor predisposition in Apc(min) mice. DMBA and ENU each accelerated mammary tumor onset in Apc(min) mice by introducing somatic, "second-hit" Apc mutations. Consistent with our strand bias model, DMBA and ENU generated strikingly distinct Apc mutation patterns, including stringently strand-inverse mutation signatures at A:T sites. Crucially, these contrasting signatures precisely match those proposed to confer bias toward Hras(CAA61CTA) versus Braf(GTG636GAG) mutations in the original tumor sets. Our findings highlight a novel mechanism whereby exposure history acts through strand-biased mutagenesis to specify activation of preferred oncogenes.

Oncogenic Ras: A double-edged sword for human epidermal stem and transient amplifying cells

The human epidermal clonal evolution, i.e. the transition from stem cells (SCs) to transient amplifying (TA)-cells and post-mitotic cells, is a continuous and tightly regulated process that ensures physiologic tissue homeostasis. The Ras family of small GTPases has a key role in skin homeostasis and tumorigenesis. Indeed, activating mutations in Ras genes have been found in human cutaneous squamous cell carcinomas (cSCCs) and in experimentally-induced murine cSCCs. In mouse models, the Ras signaling might lead to hyperproliferative phenotypes, including the development of cSCCs, depending on the nature of the founding cells. Tumor-initiating cells or Cancer Stem Cells (CSCs) have been demonstrated in murine and human cSCCs even if the mechanism of their development from normal SCs or TA-cells is not completely elucidated. Here, the relation between the Ras expression outcome and the clonogenic potential of the target keratinocyte is discussed.

Optimizing mouse models for precision cancer prevention

As cancer has become increasingly prevalent, cancer prevention research has evolved towards placing a greater emphasis on reducing cancer deaths and minimizing the adverse consequences of having cancer. 'Precision cancer prevention' takes into account the collaboration of intrinsic and extrinsic factors in influencing cancer incidence and aggressiveness in the context of the individual, as well as recognizing that such knowledge can improve early detection and enable more accurate discrimination of cancerous lesions. However, mouse models, and particularly genetically engineered mouse (GEM) models, have yet to be fully integrated into prevention research. In this Opinion article, we discuss opportunities and challenges for precision mouse modelling, including the essential criteria of mouse models for prevention research, representative success stories and opportunities for more refined analyses in future studies.

The role of oncogenic Ras in human skin tumorigenesis depends on the clonogenic potential of the founding keratinocytes

The role of Ras in human skin tumorigenesis induction is still ambiguous. Overexpression of oncogenic Ras causes premature senescence in cultured human cells and hyperplasia in transgenic mice. Here, we investigated whether the oncogenic insult outcome might depend on the nature of the founding keratinocyte. We demonstrate that overexpression of the constitutively active Ras-V12 induces senescence in primary human keratinocyte cultures, but that some cells escape senescence and proliferate indefinitely. Ras overexpression in transient-amplifying- or stem-cell-enriched cultures shows that p16 (encoded by CDKN2A) levels are crucial for the final result. Indeed, transient-amplifying keratinocytes expressing high levels of p16 are sensitive to Ras-V12-induced senescence, whereas cells with high proliferative potential, but that do not display p16, are resistant. The subpopulation that sustains the indefinite culture growth exhibits stem cell features. Bypass of senescence correlates with inhibition of the pRb (also known as RB1) pathway and resumption of telomerase reverse transcriptase (TERT) activity. Immortalization is also sustained by activation of the ERK1 and ERK2 (ERK1/2, also known as MAPK3 and MAPK1) and Akt pathways. Moreover, only transduced cultures originating from cultures bearing stem cells induce tumors in nude mice. Our findings demonstrate that the Ras overexpression outcome depends on the clonogenic potential of the recipient keratinocyte and that only the stem cell compartment is competent to initiate tumorigenesis.

Genome-wide association between YAP/TAZ/TEAD and AP-1 at enhancers drives oncogenic growth

YAP/TAZ are nuclear effectors of the Hippo pathway regulating organ growth and tumorigenesis. Yet, their function as transcriptional regulators remains underinvestigated. By ChIP-seq analyses in breast cancer cells, we discovered that the YAP/TAZ transcriptional response is pervasively mediated by a dual element: TEAD factors, through which YAP/TAZ bind to DNA, co-occupying chromatin with activator protein-1 (AP-1, dimer of JUN and FOS proteins) at composite cis-regulatory elements harbouring both TEAD and AP-1 motifs. YAP/TAZ/TEAD and AP-1 form a complex that synergistically activates target genes directly involved in the control of S-phase entry and mitosis. This control occurs almost exclusively from distal enhancers that contact target promoters through chromatin looping. YAP/TAZ-induced oncogenic growth is strongly enhanced by gain of AP-1 and severely blunted by its loss. Conversely, AP-1-promoted skin tumorigenesis is prevented in YAP/TAZ conditional knockout mice. This work highlights a new layer of signalling integration, feeding on YAP/TAZ function at the chromatin level.

Progression of mouse skin carcinogenesis is associated with the orchestrated deregulation of mir-200 family members, mir-205 and their common targets

MicroRNAs are small, non-coding RNAs which regulate post-transcriptionally hundreds of target mRNAs. Given that their expression is deregulated in several cancer types, they represent potential diagnostic, prognostic, and predictive biomarkers, as well as next-generation therapeutic targets. Nevertheless, the involvement of miRNAs in non-melanoma skin cancer, a cancer type with increasing prevalence, is not extensively studied, and their comprehensive characterization as regard to the initiation, promotion, and progression stages is missing. To this end, we exploited a well-established multistage mouse skin carcinogenesis model in order to identify miRNAs consistently implicated in different stages of skin carcinogenesis. The cell lines comprising this model were subjected to miRNA expression profiling using microarrays, followed by bioinformatics analysis and validation with Q-PCR, as well as treatment with miRNA modulators. We showed that among all deregulated miRNAs in our system, only a functionally coherent group consisting of the miR-200 family members and miR-205-5p displays a pattern of progressive co-downregulation from the early toward the most aggressive stages of carcinogenesis. Their overlapping, co-regulated putative targets are potentially inter-associated and, of these, the EMT-related Rap1a is overexpressed toward aggressive stages. Ectopic expression of miR-205-5p in spindle cancer cells reduces Rap1a, mitigates cell invasiveness, decreases proliferation, and delays tumor onset. We conclude that deregulation of this miRNA group is primarily associated with aggressive phenotypes of skin cancer cells. Restoration of the miR-205-5p member of this group in spindle cells reduces the expression of critical, co-regulated targets that favor cancer progression, thus reversing the EMT characteristics. © 2015 Wiley Periodicals, Inc.

Skin Carcinogenesis Studies Using Mouse Models with Altered Polyamines

Nonmelanoma skin cancer (NMSC) is a major health concern worldwide. With increasing numbers in high-risk groups such as organ transplant recipients and patients taking photosensitizing medications, the incidence of NMSC continues to rise. Mouse models of NMSC allow us to better understand the molecular signaling cascades involved in skin tumor development in order to identify novel therapeutic strategies. Here we review the models designed to determine the role of the polyamines in NMSC development and maintenance. Elevated polyamines are absolutely required for tumor growth, and dysregulation of their biosynthetic and catabolic enzymes has been observed in NMSC. Studies using mice with genetic alterations in epidermal polyamines suggest that they play key roles in tumor promotion and epithelial cell survival pathways, and recent clinical trials indicate that pharmacological inhibitors of polyamine metabolism show promise in individuals at high risk for NMSC.

A Method for the Immortalization of Newborn Mouse Skin Keratinocytes

Isolation and culture of mouse primary epidermal keratinocytes is a common technique that allows for easy genetic and environmental manipulation. However, due to their limited lifespan in culture, experiments utilizing primary keratinocytes require large numbers of animals, and are time consuming and expensive. To avoid these issues, we developed a method for the immortalization of primary mouse epidermal keratinocytes. Upon isolation of newborn epidermal keratinocytes according to established methods, the cells were cultured long-term in keratinocyte growth factor-containing medium. The cells senesced within a few weeks and eventually, small, slowly growing colonies emerged. After they regained confluency, the cells were passaged and slowly refilled the dish. With several rounds of subculture, the cells adapted to culture conditions, were easily subcultured, maintained normal morphology, and were apparently immortal. The immortalized cells retained the ability to differentiate with increased calcium concentrations, and were maintained to high passage numbers while maintaining a relatively stable karyotype. Analysis of multiple immortalized cell lines as well as primary keratinocyte cultures revealed increased numbers of chromosomes, especially in the primary keratinocytes, and chromosomal aberrations in most of the immortalized cultures and in the primary keratinocytes. Orthotopic grafting of immortalized keratinocytes together with fibroblasts onto nude mouse hosts produced skin while v-ras^Ha infection of the immortalized keratinocytes prior to grafting produced squamous cell carcinoma. In summary, this method of cell line generation allows for decreased use of animals, reduces the expense and time involved in research, and provides a useful model for cutaneous keratinocyte experimentation.