Dysregulated ΔNp63α inhibits expression of Ink4a/arf, blocks senescence, and promotes malignant conversion of keratinocytes

Oncogenic Ras and ΔNp63α cooperate to recruit immunosuppressive polymorphonuclear myeloid-derived suppressor cells in a mouse model of squamous cancer pathogenesis

Introduction

Amplification of human chromosome 3q26-29, which encodes oncoprotein ΔNp63 among other isoforms of the p63 family, is a feature common to squamous cell carcinomas (SCCs) of multiple tissue origins. Along with overexpression of ΔNp63, activation of the protooncogene, RAS, whether by overexpression or oncogenic mutation, is frequently observed in many cancers. In this study, analysis of transcriptome data from The Cancer Genome Atlas (TCGA) demonstrated that expression of TP63 mRNA, particularly ΔNp63 isoforms, and HRAS are significantly elevated in advanced squamous cell carcinomas of the head and neck (HNSCCs), suggesting pathological significance. However, how co-overexpressed ΔNp63 and HRAS affect the immunosuppressive tumor microenvironment (TME) is incompletely understood.

Methods

Here, we established and characterized an immune competent mouse model using primary keratinocytes with retroviral-mediated overexpression of ΔNp63α and constitutively activated HRAS (v-rasHa G12R) to evaluate the role of these oncogenes in the immune TME.

Results

In this model, orthotopic grafting of wildtype syngeneic keratinocytes expressing both v-rasHa and elevated levels of ΔNp63α consistently yield carcinomas in syngeneic hosts, while cells expressing v-rasHa alone yield predominantly papillomas. We found that polymorphonuclear (PMN) myeloid cells, experimentally validated to be immunosuppressive and thus representing myeloid-derived suppressor cells (PMN-MDSCs), were significantly recruited into the TME of carcinomas arising early following orthotopic grafting of ΔNp63α/v-rasHa-expressing keratinocytes. ΔNp63α/v-rasHa-driven carcinomas expressed higher levels of chemokines implicated in recruitment of MDSCs compared to v-rasHa-initiated tumors, providing a heretofore undescribed link between ΔNp63α/HRAS-driven carcinomas and the development of an immunosuppressive TME.

Conclusion

These results support the utilization of a genetic carcinogenesis model harboring specific genomic drivers of malignancy to study mechanisms underlying the development of local immunosuppression.

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.

DNA Demethylation Switches Oncogenic ΔNp63 to Tumor Suppressive TAp63 in Squamous Cell Carcinoma

The TP63 gene encodes two major protein variants; TAp63 contains a p53-like transcription domain and consequently has tumor suppressor activities whereas ΔNp63 lacks this domain and acts as an oncogene. The two variants show distinct expression patterns in normal tissues and tumors, with lymphocytes and lymphomas/leukemias expressing TAp63, and basal epithelial cells and some carcinomas expressing high levels of ΔNp63, most notably squamous cell carcinomas (SCC). Whilst the transcriptional functions of TAp63 and ΔNp63 isoforms are known, the mechanisms involved in their regulation are poorly understood. Using squamous epithelial cells that contain high levels of ΔNp63 and low/undetectable TAp63, the DNA demethylating agent decitabine (5-aza-2’-deoxycytidine, 5-dAza) caused a dose-dependent increase in TAp63, with a simultaneous reduction in ΔNp63, indicating DNA methylation-dependent regulation at the isoform-specific promoters. The basal cytokeratin KRT5, a direct ΔNp63 transcriptional target, was also reduced, confirming functional alteration of p63 activity after DNA demethylation. We also showed high level methylation of three CpG sites in the TAP63 promoter in these cells, which was reduced by decitabine. DNMT1 depletion using inducible shRNAs partially replicated these effects, including an increase in the ratio of TAP63:ΔNP63 mRNAs, a reduction in ΔNp63 protein and reduced KRT5 mRNA levels. Finally, high DNA methylation levels were found at the TAP63 promoter in clinical SCC samples and matched normal tissues. We conclude that DNA methylation at the TAP63 promoter normally silences transcription in squamous epithelial cells, indicating DNA methylation as a therapeutic approach to induce this tumor suppressor in cancer. That decitabine simultaneously reduced the oncogenic activity of ΔNp63 provides a “double whammy” for SCC and other p63-positive carcinomas. Whilst a variety of mechanisms may be involved in producing the opposite effects of DNA demethylation on TAp63 and ΔNp63, we propose an “either or” mechanism in which TAP63 transcription physically interferes with the ability to initiate transcription from the downstream ΔNP63 promoter on the same DNA strand. This mechanism can explain the observed inverse expression of p63 isoforms in normal cells and cancer.

Coordinated post-transcriptional control of oncogene-induced senescence by UNR/CSDE1

Oncogene-induced senescence (OIS) is a form of stable cell-cycle arrest arising in response to oncogenic stimulation. OIS must be bypassed for transformation, but the mechanisms of OIS establishment and bypass remain poorly understood, especially at the post-transcriptional level. Here, we show that the RNA-binding protein UNR/CSDE1 enables OIS in primary mouse keratinocytes. Depletion of CSDE1 leads to senescence bypass, cell immortalization, and tumor formation, indicating that CSDE1 behaves as a tumor suppressor. Unbiased high-throughput analyses uncovered that CSDE1 promotes OIS by two independent molecular mechanisms: enhancement of the stability of senescence-associated secretory phenotype (SASP) factor mRNAs and repression of Ybx1 mRNA translation. Importantly, depletion of YBX1 from immortal keratinocytes rescues senescence and uncouples proliferation arrest from the SASP, revealing multilayered mechanisms exerted by CSDE1 to coordinate senescence. Our data highlight the relevance of post-transcriptional control in the regulation of senescence.

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.

Oh, the Mutations You'll Acquire! A Systematic Overview of Cutaneous Squamous Cell Carcinoma

Nearly two million cases of cutaneous squamous cell carcinoma (cSCC) are diagnosed every year in the United States alone. cSCC is notable for both its prevalence and its propensity for invasion and metastasis. For many patients, surgery is curative. However, patients experiencing immunosuppression or recurrent, advanced, and metastatic disease still face limited therapeutic options and significant mortality. cSCC forms after decades of sun exposure and possesses the highest known mutation rate of all cancers. This mutational burden complicates efforts to identify the primary factors driving cSCC initiation and progression, which in turn hinders the development of targeted therapeutics. In this review, we summarize the mutations and alterations that have been observed in patients’ cSCC tumors, affecting signaling pathways, transcriptional regulators, and the microenvironment. We also highlight novel therapeutic opportunities in development and clinical trials.

Cutaneous Squamous Cell Carcinoma: From Pathophysiology to Novel Therapeutic Approaches

Cutaneous squamous cell carcinoma (cSCC), a non-melanoma skin cancer, is a keratinocyte carcinoma representing one of the most common cancers with an increasing incidence. cSCC could be in situ (e.g., Bowen’s disease) or an invasive form. A significant cSCC risk factor is advanced age, together with cumulative sun exposure, fair skin, prolonged immunosuppression, and previous skin cancer diagnoses. Although most cSCCs can be treated by surgery, a fraction of them recur and metastasize, leading to death. cSCC could arise de novo or be the result of a progression of the actinic keratosis, an in situ carcinoma. The multistage process of cSCC development and progression is characterized by mutations in the genes involved in epidermal homeostasis and by several alterations, such as epigenetic modifications, viral infections, or microenvironmental changes. Thus, cSCC development is a gradual process with several histological- and pathological-defined stages. Dermoscopy and reflectance confocal microscopy enhanced the diagnostic accuracy of cSCC. Surgical excision is the first-line treatment for invasive cSCC. Moreover, radiotherapy may be considered as a primary treatment in patients not candidates for surgery. Extensive studies of cSCC pathogenic mechanisms identified several pharmaceutical targets and allowed the development of new systemic therapies, including immunotherapy with immune checkpoint inhibitors, such as Cemiplimab, and epidermal growth factor receptor inhibitors for metastatic and locally advanced cSCC. Furthermore, the implementation of prevention measures has been useful in patient management.

Loss of oral mucosal stem cell markers in oral submucous fibrosis and their reactivation in malignant transformation

The integrity of the basal stem cell layer is critical for epithelial homoeostasis. In this paper, we review the expression of oral mucosal stem cell markers (OM-SCMs) in oral submucous fibrosis (OSF), oral potentially malignant disorders (OPMDs) and oral squamous cell carcinoma (OSCC) to understand the role of basal cells in potentiating cancer stem cell behaviour in OSF. While the loss of basal cell clonogenicity triggers epithelial atrophy in OSF, the transition of the epithelium from atrophic to hyperplastic and eventually neoplastic involves the reactivation of basal stemness. The vacillating expression patterns of OM-SCMs confirm the role of keratins 5, 14, 19, CD44, β1-integrin, p63, sex-determining region Y box (SOX2), octamer-binding transcription factor 4 (Oct-4), c-MYC, B-cell-specific Moloney murine leukaemia virus integration site 1 (Bmi-1) and aldehyde dehydrogenase 1 (ALDH1) in OSF, OPMDs and OSCC. The downregulation of OM-SCMs in the atrophic epithelium of OSF and their upregulation during malignant transformation are illustrated with relevant literature in this review.

Knockdown of Long Non-Coding RNA (lncRNA) Colon Cancer-Associated Transcript-1 (CCAT1) Suppresses Oral Squamous Cell Carcinoma Proliferation, Invasion, and Migration by Inhibiting the Discoidin Domain Receptor 2 (DDR2)/ERK/AKT Axis

BACKGROUND Emerging evidence shows that lncRNAs are involved in carcinogenesis or suppression in diverse cancers. This study assessed the biological role of lncRNA CCAT1 in OSCC and explored the underlying molecule mechanism. MATERIAL AND METHODS CCAT1 and DDR2 expression was measured by qRT-PCR. Colony formation assay and CCK-8 assay were performed to evaluate cell proliferation. Cell cycle was determined by flow cytometric analysis and Western blot analysis. In addition, wound healing and Transwell assay were used to assess cell migration and invasion, respectively. RNA immunoprecipitation (RIP) assay were employed to identify the interaction between DDR2 and CCAT1. Protein levels involved in DDR2/ERK/AKT pathway were estimated by Western blot assay. RESULTS The findings revealed that CCAT1expression was upregulated in OSCC cell lines. Knockdown of CCAT1 repressed cell proliferation, blocked the cell cycle, and suppressed the invasion and migration of TCA-8113 cells. Moreover, DDR2 expression in OSCC cell lines was downregulated and CCAT1 silencing repressed the expression of DDR2. RIP assays validated the binding of CCAT1 and DDR2 protein. Moreover, CCAT1 silencing suppressed the ERK/AKT signaling through DDR2 in TCA-8113 cells. CONCLUSIONS Downregulation of CCAT1 suppressed TCA-8113 cell proliferation, invasion, and migration by inactivation of the ERK/AKT pathway via inhibition of DDR2, suggesting the value of CCAT1 in diagnosis and treatment of patients with OSCC.

The Simplest Protocol for Rapid and Long-Term Culture of Primary Epidermal Keratinocytes from Human and Mouse

Although mouse models have been used as an essential tool for studying the physiology and diseases of the skin, propagation of mouse primary epidermal keratinocytes remains challenging. In this chapter, we introduce the simplest, at least to our knowledge, protocol that enables long-term expansion of p63⁺ mouse epidermal keratinocytes in low Ca²⁺ media without the need of progenitor cell-purification steps or support by a feeder cell layer. Pharmacological inhibition of TGF-β signaling in crude preparations of mouse epidermis robustly increases proliferative capacity of p63⁺ epidermal progenitor cells, while preserving their ability to differentiate. Suppression of TGF-β signaling also permits p63⁺ epidermal keratinocytes to form macroscopically large clones in 3T3-J2 feeder cell co-culture. Suppression of TGF-β signaling also enhances the clonal growth of human keratinocytes in co-culture with a variety of feeder cells. This simple and efficient approach will not only facilitate the use of mouse models by providing p63⁺ primary epidermal keratinocytes in quantity but also significantly reduce the time needed for preparing the customized skin grafts in Green method.

p63 Is a Promising Marker in the Diagnosis of Unusual Skin Cancer

Skin cancer is the most common type of cancer worldwide. Ozone depletion and climate changes might cause a further increase in the incidence rate in the future. Although the early detection of skin cancer enables it to be treated successfully, some tumours can evolve and become more aggressive, especially in the case of melanoma. Therefore, good diagnostic and prognostic markers are needed to ensure correct detection and treatment. Transcription factor p63, a member of the p53 family of proteins, plays an essential role in the development of stratified epithelia such as skin. In this paper, we conduct a comprehensive review of p63 expression in different types of skin cancer and discuss its possible use in the diagnosis and prognosis of cutaneous tumours.

Molecular Mechanisms of p63-Mediated Squamous Cancer Pathogenesis

The p63 gene is a member of the p53/p63/p73 family of transcription factors and plays a critical role in development and homeostasis of squamous epithelium. p63 is transcribed as multiple isoforms; ΔNp63α, the predominant p63 isoform in stratified squamous epithelium, is localized to the basal cells and is overexpressed in squamous cell cancers of multiple organ sites, including skin, head and neck, and lung. Further, p63 is considered a stem cell marker, and within the epidermis, ΔNp63α directs lineage commitment. ΔNp63α has been implicated in numerous processes of skin biology that impact normal epidermal homeostasis and can contribute to squamous cancer pathogenesis by supporting proliferation and survival with roles in blocking terminal differentiation, apoptosis, and senescence, and influencing adhesion and migration. ΔNp63α overexpression may also influence the tissue microenvironment through remodeling of the extracellular matrix and vasculature, as well as by enhancing cytokine and chemokine secretion to recruit pro-inflammatory infiltrate. This review focuses on the role of ΔNp63α in normal epidermal biology and how dysregulation can contribute to cutaneous squamous cancer development, drawing from knowledge also gained by squamous cancers from other organ sites that share p63 overexpression as a defining feature.

Intersection of the p63 and NF-κB pathways in epithelial homeostasis and disease

Overexpression of ΔNp63α, a member of the p53/p63/p73 family of transcription factors, is a molecular attribute of human squamous cancers of the head and neck, lung and skin. The TP63 gene plays important roles in epidermal morphogenesis and homeostasis, regulating diverse biological processes including epidermal fate decisions and keratinocyte proliferation and survival. When overexpressed experimentally in primary mouse keratinocytes, ΔNp63α maintains a basal cell phenotype including the loss of normal calcium-mediated growth arrest, at least in part through the activation and enhanced nuclear accumulation of the c-rel subunit of NF-κB (Nuclear Factor-kappa B). Initially identified for its role in the immune system and hematopoietic cancers, c-Rel has increasingly been associated with solid tumors and other pathologies. ΔNp63α and c-Rel have been shown to be associated in the nuclei of ΔNp63α overexpressing human squamous carcinoma cells. Together, these transcription factors cooperate in the transcription of genes regulating intrinsic keratinocyte functions, as well as the elaboration of factors that influence the tumor microenvironment (TME). This review provides an overview of the roles of ΔNp63α and c-Rel in normal epidermal homeostasis and elaborates on how these pathways may intersect in pathological conditions such as cancer and the associated TME.

Long-Term Expansion of Mouse Primary Epidermal Keratinocytes Using a TGF-β Signaling Inhibitor

Mouse models have been used to study the physiology and pathogenesis of the skin. However, propagation of mouse primary epidermal keratinocytes remains challenging. In this chapter, we introduce a newly developed protocol that enables long-term expansion of p63⁺ mouse epidermal keratinocytes in low-Ca²⁺ media without the need of progenitor cell purification steps or support by a feeder cell layer. Pharmacological inhibition of TGF-β signaling in crude preparations of mouse epidermis robustly increases proliferative capacity of p63⁺ epidermal progenitor cells while preserving their ability to differentiate. Suppression of TGF-β signaling also permits p63⁺ epidermal keratinocytes to form macroscopically large clones in 3T3-J2 feeder cell co-culture. This simple and efficient approach will facilitate the use of mouse models by providing p63⁺ primary epidermal keratinocytes in quantity.

PKK deletion in basal keratinocytes promotes tumorigenesis after chemical carcinogenesis

Squamous cell carcinoma (SCC) of the skin is a keratinocyte malignancy characterized by tumors presenting on sun-exposed areas with surgery being the mainstay treatment. Despite advances in targeted therapy in other skin cancers, such as basal cell carcinoma and melanoma, there have been no such advances in the treatment of SCC. This is partly due to an incomplete knowledge of the pathogenesis of SCC. We have recently identified a protein kinase C-associated kinase (PKK) as a potential tumor suppressor in SCC. We now describe a novel conditional PKK knockout mouse model, which demonstrates that PKK deficiency promotes SCC formation during chemically induced tumorigenesis. Our results further support that PKK functions as a tumor suppressor in skin keratinocytes and is important in the pathogenesis of SCC of the skin. We further define the interactions of keratinocyte PKK with TP63 and NF-κB signaling, highlighting the importance of this protein as a tumor suppressor in SCC development.

Co-activation of super-enhancer-driven CCAT1 by TP63 and SOX2 promotes squamous cancer progression

Squamous cell carcinomas (SCCs) are aggressive malignancies. Previous report demonstrated that master transcription factors (TFs) TP63 and SOX2 exhibited overlapping genomic occupancy in SCCs. However, functional consequence of their frequent co-localization at super-enhancers remains incompletely understood. Here, epigenomic profilings of different types of SCCs reveal that TP63 and SOX2 cooperatively and lineage-specifically regulate long non-coding RNA (lncRNA) CCAT1 expression, through activation of its super-enhancers and promoter. Silencing of CCAT1 substantially reduces cellular growth both in vitro and in vivo, phenotyping the effect of inhibiting either TP63 or SOX2. ChIRP analysis shows that CCAT1 forms a complex with TP63 and SOX2, which regulates EGFR expression by binding to the super-enhancers of EGFR, thereby activating both MEK/ERK1/2 and PI3K/AKT signaling pathways. These results together identify a SCC-specific DNA/RNA/protein complex which activates TP63/SOX2-CCAT1-EGFR cascade and promotes SCC tumorigenesis, advancing our understanding of transcription dysregulation in cancer biology mediated by master TFs and super-enhancers.

Master regulator transcription factors TP63 and SOX2 have been reported to overlap in genomic occupancy in squamous cell carcinomas (SCCs). Here, the authors demonstrate that TP63 and SOX2 promote co-operatively long non-coding RNA CCAT1 expression through activating its super-enhancer, and CCAT1 forms a complex with TP63 and SOX2, which regulates EGFR super-enhancers and enhances both the MEK/ERK1/2 and PI3K/AKT signaling pathways in SCC.

Inhibition of TGF-β signaling supports high proliferative potential of diverse p63+ mouse epithelial progenitor cells in vitro

Mouse models have been used to provide primary cells to study physiology and pathogenesis of epithelia. However, highly efficient simple approaches to propagate mouse primary epithelial cells remain challenging. Here, we show that pharmacological inhibition of TGF-β signaling enables long-term expansion of p63⁺ epithelial progenitor cells in low Ca²⁺ media without the need of progenitor cell-purification steps or support by a feeder cell layer. We find that TGF-β signaling is operative in mouse primary keratinocytes in conventional cultures as determined by the nuclear Smad2/3 localization. Accordingly, TGF-β signaling inhibition in crude preparations of mouse epidermis robustly increases proliferative capacity of p63⁺ epidermal progenitor cells, while preserving their ability of differentiation in response to Ca²⁺ stimulation. Notably, inhibition of TGF-β signaling also enriches and expands other p63⁺ epithelial progenitor cells in primary crude cultures of multiple epithelia, including the cornea, oral and lingual epithelia, salivary gland, esophagus, thymus, and bladder. We anticipate that this simple and efficient approach will facilitate the use of mouse models for studying a wide range of epithelia by providing highly enriched populations of diverse p63⁺ epithelial progenitor cells in quantity.

ACTL6A Is Co-Amplified with p63 in Squamous Cell Carcinoma to Drive YAP Activation, Regenerative Proliferation, and Poor Prognosis

Loss-of-function mutations in SWI/SNF chromatin-remodeling subunit genes are observed in many cancers, but an oncogenic role for SWI/SNF is not well established. Here, we reveal that ACTL6A, encoding an SWI/SNF subunit linked to stem cell and progenitor cell function, is frequently co-amplified and highly expressed together with the p53 family member p63 in head and neck squamous cell carcinoma (HNSCC). ACTL6A and p63 physically interact, cooperatively controlling a transcriptional program that promotes proliferation and suppresses differentiation, in part through activation of the Hippo-YAP pathway via regulators including WWC1. Ectopic ACTL6A/p63 expression promotes tumorigenesis, while ACTL6A expression and YAP activation are highly correlated in primary HNSCC and predict poor patient survival. Thus, ACTL6A and p63 collaborate as oncogenic drivers in HNSCC.

Elevated ΔNp63α Levels Facilitate Epidermal and Biliary Oncogenic Transformation

Unlike its family member p53, TP63 is rarely mutated in human cancer. However, ΔNp63α protein levels are often elevated in tumors of epithelial origin, such as squamous cell carcinoma and cholangiocarcinoma. To study the oncogenic properties of ΔNp63α in vivo, we generated transgenic mice overexpressing ΔNp63α from the Rosa26 locus promoter controlled by keratin 5-Cre. We found that these mice spontaneously develop epidermal cysts and ectopic ΔNp63α expression in the bile duct epithelium that leads to dilatation of the intrahepatic biliary ducts, to hepatic cyst formation and bile duct adenoma. Moreover, when subjected to models of 7,12-dimethylbenz[a]anthracene-based carcinogenesis, tumor initiation was increased in ΔNp63α transgenic mice in a gene dosage-dependent manner although ΔNp63α overexpression did not alter the sensitivity to 7,12-dimethylbenz[a]anthracene-induced cytotoxicity in vivo. However, keratinocytes isolated from ΔNp63α transgenic mice displayed increased survival and delayed cellular senescence compared with wild-type keratinocytes, marked by decreased p16^Ink4a and p19^Arf expression. Taken together, we show that increased ΔNp63α protein levels facilitate oncogenic transformation in the epidermis as well as in the bile duct.

Fibroblast spheroids as a model to study sustained fibroblast quiescence and their crosstalk with tumor cells

Stromal fibroblasts have an important role in regulating tumor progression. Normal and quiescent fibroblasts have been shown to restrict and control cancer cell growth, while cancer-associated, i. e. activated fibroblasts have been shown to enhance proliferation and metastasis of cancer cells. In this study we describe generation of quiescent fibroblasts in multicellular spheroids and their effects on squamous cell carcinoma (SCC) growth in soft-agarose and xenograft models. Quiescent phenotype of fibroblasts was determined by global down-regulation of expression of genes related to cell cycle and increased expression of p27. Interestingly, microarray analysis showed that fibroblast quiescence was associated with similar secretory phenotype as seen in senescence and they expressed senescence-associated-β-galactosidase. Quiescent fibroblasts spheroids also restricted the growth of RT3 SCC cells both in soft-agarose and xenograft models unlike proliferating fibroblasts. Restricted tumor growth was associated with marginally increased tumor cell senescence and cellular differentiation, showed with senescence-associated-β-galactosidase and cytokeratin 7 staining. Our results show that the fibroblasts spheroids can be used as a model to study cellular quiescence and their effects on cancer cell progression.

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.

ΔNp63 regulates IL-33 and IL-31 signaling in atopic dermatitis

Atopic dermatitis (AD) is the most common inflammatory skin disease with no well-delineated cause or effective cure. Here we show that the p53 family member p63, specifically the ΔNp63, isoform has a key role in driving keratinocyte activation in AD. We find that overexpression of ΔNp63 in transgenic mouse epidermis results in a severe skin phenotype that shares many of the key clinical, histological and molecular features associated with human AD. This includes pruritus, epidermal hyperplasia, aberrant keratinocyte differentiation, enhanced expression of selected cytokines and chemokines and the infiltration of large numbers of inflammatory cells including type 2  T-helper cells – features that are highly representative of AD dermatopathology. We further demonstrate several of these mediators to be direct transcriptional targets of ΔNp63 in keratinocytes. Of particular significance are two p63 target genes, IL-31 and IL-33, both of which are key players in the signaling pathways implicated in AD. Importantly, we find these observations to be in good agreement with elevated levels of ΔNp63 in skin lesions of human patients with AD. Our studies reveal an important role for ΔNp63 in the pathogenesis of AD and offer new insights into its etiology and possible therapeutic targets.

The homeoprotein DLX3 and tumor suppressor p53 co-regulate cell cycle progression and squamous tumor growth

Epidermal homeostasis depends on the coordinated control of keratinocyte cell cycle. Differentiation and the alteration of this balance can result in neoplastic development. Here we report on a novel DLX3-dependent network that constrains epidermal hyperplasia and squamous tumorigenesis. By integrating genetic and transcriptomic approaches, we demonstrate that DLX3 operates through a p53-regulated network. DLX3 and p53 physically interact on the p21 promoter to enhance p21 expression. Elevating DLX3 in keratinocytes produces a G1-S blockade associated with p53 signature transcriptional profiles. In contrast, DLX3 loss promotes a mitogenic phenotype associated with constitutive activation of ERK. DLX3 expression is lost in human skin cancers and is extinguished during progression of experimentally induced mouse squamous cell carcinoma (SCC). Reinstatement of DLX3 function is sufficient to attenuate the migration of SCC cells, leading to decreased wound closure. Our data establish the DLX3-p53 interplay as a major regulatory axis in epidermal differentiation and suggest that DLX3 is a modulator of skin carcinogenesis.

Delineating Molecular Mechanisms of Squamous Tissue Homeostasis and Neoplasia: Focus on p63

Mouse models have informed us that p63 is critical for normal epidermal development and homeostasis. The p53/p63/p73 family is expressed as multiple protein isoforms due to a combination of alternative promoter usage and C-terminal alternative splicing. These isoforms can mimic or interfere with one another, and their balance ultimately determines biological outcome in a context-dependent manner. While not frequently mutated, p63, and in particular the ΔNp63 subclass, is commonly overexpressed in human squamous cell cancers. In vitro keratinocytes and murine transgenic and transplantation models have been invaluable in elucidating the contribution of altered p63 levels to cancer development, and studies have identified the roles for ΔNp63 isoforms in keratinocyte survival and malignant progression, likely due in part to their transcriptional regulatory function. These findings can be extended to human cancers; for example, the novel recognition of NFκB/c-Rel as a downstream effector of p63 has identified a role for NFκB/c-Rel in human squamous cell cancers. These models will be critical in enhancing the understanding of the specific molecular mechanisms of cancer development and progression.

The p63 protein isoform ΔNp63α modulates Y-box binding protein 1 in its subcellular distribution and regulation of cell survival and motility genes

The Y-box binding protein 1 (YB-1) belongs to the cold-shock domain protein superfamily, one of the most evolutionarily conserved nucleic acid-binding proteins currently known. YB-1 performs a wide variety of cellular functions, including transcriptional and translational regulation, DNA repair, drug resistance, and stress responses to extracellular signals. Inasmuch as the level of YB-1 drastically increases in tumor cells, this protein is considered to be one of the most indicative markers of malignant tumors. Here, we present evidence that ΔNp63α, the predominant p63 protein isoform in squamous epithelia and YB-1, can physically interact. Into the nucleus, ΔNp63α and YB-1 cooperate in PI3KCA gene promoter activation. Moreover, ΔNp63α promotes YB-1 nuclear accumulation thereby reducing the amount of YB-1 bound to its target transcripts such as that encoding the SNAIL1 protein. Accordingly, ΔNp63α enforced expression was associated with a reduction of the level of SNAIL1, a potent inducer of epithelial to mesenchymal transition. Furthermore, ΔNp63α depletion causes morphological change and enhanced formation of actin stress fibers in squamous cancer cells. Mechanistic studies indicate that ΔNp63α affects cell movement and can reverse the increase of cell motility induced by YB-1 overexpression. These data thus suggest that ΔNp63α provides inhibitory signals for cell motility. Deficiency of ΔNp63α gene expression promotes cell mobilization, at least partially, through a YB-1-dependent mechanism.

Dysfunction of nucleus accumbens-1 activates cellular senescence and inhibits tumor cell proliferation and oncogenesis

Nucleus accumbens-1 (NAC1), a nuclear factor belonging to the BTB/POZ gene family, has emerging roles in cancer. We report here that NAC1 acts as a negative regulator of cellular senescence in transformed and nontransformed cells, and dysfunction of NAC1 induces senescence and inhibits its oncogenic potential. We show that NAC1 deficiency markedly activates senescence and inhibits proliferation in tumor cells treated with sublethal doses of γ-irradiation. In mouse embryonic fibroblasts from NAC1 knockout mice, following infection with a Ras virus, NAC1-/- cells undergo significantly more senescence and are either nontransformed or less transformed in vitro and less tumorigenic in vivo when compared with NAC1+/+ cells. Furthermore, we show that the NAC1-caused senescence blunting is mediated by ΔNp63, which exerts its effect on senescence through p21, and that NAC1 activates transcription of ΔNp63 under stressful conditions. Our results not only reveal a previously unrecognized function of NAC1, the molecular pathway involved and its impact on pathogenesis of tumor initiation and development, but also identify a novel senescence regulator that may be exploited as a potential target for cancer prevention and treatment.