Constitutive activation and targeted disruption of signal transducer and activator of transcription 3 (Stat3) in mouse epidermis reveal its critical role in UVB-induced skin carcinogenesis

EPHA5 promotes cell proliferation and inhibits apoptosis in Follicular Thyroid Cancer via the STAT3 signaling pathway

Follicular thyroid carcinoma (FTC) is a common endocrine malignancy characterized by a higher propensity for invasion and metastasis compared to papillary thyroid carcinoma (PTC). Ephrin type A receptor 5 (EPHA5) is a crucial receptor tyrosine kinase involved in orchestrating diverse physiological processes, including apoptosis and proliferation. However, the mechanism of EPHA5 in FTC remains unclear. This study identified significant overexpression of EPHA5 in FTC. In vitro experiments showed that increased expression of EPHA5 promotes proliferation and inhibits apoptosis in FTC. Furthermore, EPHA5 activates the STAT3 signaling pathway. To explore the interaction between EPHA5 and the STAT3 signaling pathway, we used SH-4-54 (a STAT3-specific inhibitor). Interestingly, the influence of EPHA5 on proliferation and apoptosis was reduced upon combination with SH-4-54. In summary, this study unveils the involvement of the EPHA5-STAT3 signaling pathway in FTC and implies that the function of EPHA5 in FTC may partly depend on the STAT3 signaling pathway.

Loss of TC-PTP in keratinocytes leads to increased UVB-induced autophagy

Ultraviolet B (UVB) radiation can distort cellular homeostasis and predispose the skin to carcinogenesis. Amongst the deteriorating effects of the sun's UVB radiation on cellular homeostasis is the formation of DNA photoproducts. These photoproducts can cause significant changes in the structure and conformation of DNA, inducing gene mutations which may accumulate to trigger the formation of skin cancer. Photoproducts are typically repaired by nucleotide excision repair. Notwithstanding, when the repair mechanism fails, apoptosis ensues to prevent the accumulation of mutations and to restore cellular homeostasis. This present study reports that T-cell protein tyrosine phosphatase (TC-PTP) can increase UVB-induced apoptosis by inhibiting autophagy-mediated cell survival of damaged keratinocytes. TC-PTP deficiency in 3PC mouse keratinocytes led to the formation of autophagic vacuoles and increased expression of LC3-II. We established human TC-PTP-deficient (TC-PTP/KO) HaCaT cells using the CRISPR/Cas9 system. TC-PTP/KO HaCaT cells exhibited increased cell survival upon UVB exposure, which was accompanied by increased expression of LC3-II and decreased expression of p62 compared to control cells. Pretreatment of TC-PTP/KO HaCaT cells with early-phase autophagy inhibitor, 3-methyladenine significantly decreased the expression of LC3-II and reduced cell survival in response to UVB irradiation in comparison with untreated TC-PTP/KO cells. Pretreatment of TC-PTP/KO HaCaT cells with late-phase inhibitor, chloroquine also significantly reduced cell viability with increased accumulation of LC3-II after UVB irradiation compared to untreated counterpart cells. While UVB significantly increased apoptosis in the engineered (Mock) cells, this was not observed in similarly treated TC-PTP/KO HaCaT cells. However, chloroquine treatment increased apoptosis in TC-PTP/KO HaCaT cells. Examination of human squamous cell carcinomas (SCCs) revealed that TC-PTP expression was inversely correlated with LC3 expression. Our findings suggest that TC-PTP negatively regulates autophagy-mediated survival of damaged cells following UVB exposure, which can contribute to remove damaged keratinocytes via apoptosis.

Inducible Keratinocyte Specific FGFR2 Deficiency Inhibits UVB-Induced Signaling, Proliferation, Inflammation, and Skin Carcinogenesis

A potential role for fibroblast growth factor receptor 2 (FGFR2) in cutaneous squamous cell carcinoma (cSCC) has been reported. To demonstrate the specific role of FGFR2 in UVB-induced skin carcinogenesis and development of cSCC, we generated a keratinocyte specific, tamoxifen inducible mouse model of FGFR2 deficiency. In this mouse model, topical application of 4-hydroxy tamoxifen led to the induction of Cre recombinase to delete FGFR2 in epidermal keratinocytes of both male and female transgenic mice. Analysis of epidermal protein lysates isolated from FGFR2 deficient mice exposed to UVB showed significant reductions of phospho-FGFR (pFGFR; Y653/654) and phospho-fibroblast growth factor receptor substrate 2α as well as downstream effectors of mTORC1 signaling. Phosphorylation of signal transducer and activators of transcription 1/3 was significantly reduced as well as levels of IRF-1, DUSP6, early growth response 1, and PD-L1 compared to the control groups. Keratinocyte-specific ablation of FGFR2 also significantly inhibited epidermal hyperproliferation, hyperplasia, and inflammation after exposure to UVB. Finally, keratinocyte-specific deletion of FGFR2 significantly inhibited UVB-induced cSCC formation. Collectively, the current data demonstrate an important role of FGFR2 in UVB-induced oncogenic signaling as well as development of cSCC. In addition, the current preclinical findings suggest that inhibition of FGFR2 signaling may provide a previously unreported strategy to prevent and/or treat UVB-induced cSCC.

Targeting receptor tyrosine kinase signaling: Avenues in the management of cutaneous squamous cell carcinoma

Non-melanoma skin cancer (NMSC) is the most frequently diagnosed cancer worldwide. Among the various types of NMSCs, cutaneous squamous cell carcinoma (cSCC) exhibits more aggressive phenotype and is also the second-most prevalent type. Receptor tyrosine kinases (RTK) triggers key signaling events that play critical roles in the development of various cancers including cSCC. Unsurprisingly, for this reason, this family of proteins has become the cynosure of anti-cancer drug discovery pipelines and is also being considered as attractive targets against cSCC. Though inhibition of RTKs in cSCC has yielded favourable results, there is still scope for bettering the therapeutic outcome. In this review, we discuss the relevance of RTK signaling in the progression of cutaneous squamous cell carcinoma, and observations from clinical trials that used RTK inhibitors against cSCC. Backed by results from preclinical studies, including those from our lab, we also give insights into the scope of using some natural products as effective suppressors of RTK signaling and skin carcinogenesis.

Twist1 as a target for prevention of cutaneous squamous cell carcinoma

Cutaneous squamous cell carcinoma (cSCC) represents an important clinical problem requiring novel approaches for both prevention and treatment. The transcription factor, Twist-related protein 1 (Twist1), has been identified as having a key mechanistic role in the development and progression of cSCC. Studies in relevant mouse models of cSCC have shown that Twist1 regulates epithelial-mesenchymal transition (EMT) and stemness driving progression and metastasis of cSCC. In addition, further research has shown that Twist1 regulates the balance between keratinocyte proliferation and differentiation and therefore impacts earlier stages of cSCC development. Through use of keratinocyte specific Twist1 knockout models, a role for this gene in keratinocyte stem cell homeostasis has been revealed. As a transcription factor, Twist1 regulates a large number of genes both in a positive, as well as a negative manner across several interdependent pathways. Studies in keratinocyte specific knockout models have shown that Twist1 upregulates the expression of genes involved in proliferation, stemness, and EMT while downregulating the expression of genes associated with differentiation. Furthermore, a number of compounds, including naturally occurring compounds, have been identified that target Twist1 and can block its effects in cancer cells and in keratinocytes in vivo. Collectively, the current understanding of Twist1 function in cSCC development and progression suggests that it represents a potential target for prevention and treatment of cSCC.

In vivo modelling of cutaneous T-cell lymphoma: The role of SOCS1

Introduction

Mycosis fungoides (MF), the most common type of Cutaneous T cell Lymphoma (CTCL), is characterized by an inflamed skin intermixed with proliferating malignant mature skin-homing CD4+ T cells. Detailed genomic analyses of MF skin biopsies revealed several candidate genes possibly involved in genesis of these tumors and/or potential targets for therapy. These studies showed, in addition to common loss of cell cycle regulator CDKN2A, activation of several oncogenic pathways, most prominently and consistently involving JAK/STAT signaling. SOCS1, an endogenous inhibitor of the JAK/STAT signaling pathway, was identified as a recurrently deleted gene in MF, already occurring in the earliest stages of the disease.

Methods

To explore the mechanisms of MF, we create in vivo mouse models of autochthonous CTCLs and these genetically engineered mouse models (GEMMS) can also serve as valid experimental models for targeted therapy. We describe the impact of allelic deletion of Socs1 in CD4 T cells of the skin. To achieve this, we crossed inducible Cre-transgenic mice in the CD4 lineage with transgenic mice carrying floxed genes of Socs1. We first determined optimal conditions for Socs1 ablation with limited effects on circulating CD4 T-cells in blood. Next, we started time-course experiments mimicking sustained inflammation, typical in CTCL. FACS analysis of the blood was done every week. Skin biopsies were analyzed by immunocytochemical staining at the end of the experiment.

Results

We found that the Socs1 knockout transgenic group had thicker epidermis of treated skin compared with the control group and had more CD3 and CD4 in the skin of the transgenic group compared to the control group. We also noted more activation of Stat3 by staining for P-Stat3 in Socs1 knockout compared to wt CD4+T cells in the skin. The results also indicated that single copy loss of Socs1 in combination with sustained inflammation is insufficient to start a phenotype resembling early stage mycosis fungoides within eight weeks in these mice.

Conclusion

In sum, we developed and optimized an autochthonous murine model permitting selective knockout of Socs1 in skin infiltrating CD4 T-cells. This paves the way for more elaborate experiments to gain insight in the oncogenesis of CTCL.

Ultraviolet Radiation Exposure and its Impacts on Cutaneous Phosphorylation Signaling in Carcinogenesis: Focusing on Protein Tyrosine Phosphatases†

Sunlight exposure is a significant risk factor for UV-induced deteriorating transformations of epidermal homeostasis leading to skin carcinogenesis. The ability of UVB radiation to cause melanoma, as well as basal and squamous cell carcinomas, makes UVB the most harmful among the three known UV ranges. UVB-induced DNA mutations and dysregulation of signaling pathways contribute to skin cancer formation. Among various signaling pathways modulated by UVB, tyrosine phosphorylation signaling which is mediated by the action of protein tyrosine kinases (PTKs) on specific tyrosine residues is highly implicated in photocarcinogenesis. Following UVB irradiation, PTKs get activated and their downstream signaling pathways contribute to photocarcinogenesis by promoting the survival of damaged keratinocytes and increasing cell proliferation. While UVB activates oncogenic signaling pathways, it can also activate tumor suppressive signaling pathways as initial protective mechanisms to maintain epidermal homeostasis. Tyrosine dephosphorylation is one of the protective mechanisms and is mediated by the action of protein tyrosine phosphatases (PTPs). PTP can counteract UVB-mediated PTK activation and downregulate oncogenic signaling pathways. However, PTPs have not been studied extensively in photocarcinogenesis with previous studies regarding their inactivation induced by UVB. This current review will summarize the recent progress in the protective function of PTPs in epidermal photocarcinogenesis.

Inhibition of Fibroblast Growth Factor Receptor Attenuates Ultraviolet B-Induced Skin Carcinogenesis

Altered FGFR signaling has been shown to play a role in a number of cancers. However, the role of FGFR signaling in the development and progression of ultraviolet B-induced (UVB) induced cutaneous squamous cell carcinoma (cSCC) remains unclear. In the current study, the effect of UVB radiation on FGFR activation and its downstream signaling in mouse skin epidermis was examined. In addition, the impact of FGFR inhibition on UVB-induced signaling and skin carcinogenesis was also investigated. Exposure of mouse dorsal skin to UVB significantly increased phosphorylation of FGFRs in the epidermis as well as activation of downstream signaling pathways, including AKT/mTOR, STATs and MAPK. Topical application of the pan-FGFR inhibitor AZD4547 to mouse skin prior to exposure to UVB significantly inhibited FGFR phosphorylation as well as mTORC1, STAT3 and MAPK activation (i.e., phosphorylation). Moreover, AZD4547 pretreatment significantly inhibited UVB-induced epidermal hyperplasia and hyperproliferation and reduced infiltration of mast cells and macrophages into the dermis. AZD4547 treatment also significantly inhibited mRNA expression of inflammatory genes in the epidermis. Finally, mice treated topically with AZD4547 prior to UVB exposure showed decreased cSCC incidence and increased survival rate. Collectively, the current data supports the hypothesis that inhibition of FGFR in epidermis may provide a new strategy to prevent and/or treat UVB-induced cSCC.

The Role of p53 in Progression of Cutaneous Squamous Cell Carcinoma

Skin cancers are the most common types of cancer worldwide, and their incidence is increasing. Melanoma, basal cell carcinoma (BCC), and cutaneous squamous cell carcinoma (cSCC) are the three major types of skin cancer. Melanoma originates from melanocytes, whereas BCC and cSCC originate from epidermal keratinocytes and are therefore called keratinocyte carcinomas. Chronic exposure to ultraviolet radiation (UVR) is a common risk factor for skin cancers, but they differ with respect to oncogenic mutational profiles and alterations in cellular signaling pathways. cSCC is the most common metastatic skin cancer, and it is associated with poor prognosis in the advanced stage. An important early event in cSCC development is mutation of the TP53 gene and inactivation of the tumor suppressor function of the tumor protein 53 gene (TP53) in epidermal keratinocytes, which then leads to accumulation of additional oncogenic mutations. Additional genomic and proteomic alterations are required for the progression of premalignant lesion, actinic keratosis, to invasive and metastatic cSCC. Recently, the role of p53 in the invasion of cSCC has also been elucidated. In this review, the role of p53 in the progression of cSCC and as potential new therapeutic target for cSCC will be discussed.

JAK/STAT of all trades: Linking inflammation with cancer development, tumor progression, and therapy resistance

Inflammation is at the forefront of carcinogenesis, tumor progression, and resistance to therapy. The JAK/STAT signaling axis is a central pathway that mediates the cellular response to inflammation and contributes to carcinogenesis. The JAK/STAT pathway coordinates intercellular communication between tumor cells and their immune microenvironment, and JAK/STAT activation leads to the expression of a variety of proteins involved in cell proliferation, cell survival, stemness, self-renewal, evasion of immunosurveillance mechanisms, and overall tumor progression. Activation of JAK/STAT signaling also mediates resistance to radiation therapy or cytotoxic agents, and modulates tumor cell responses to molecularly targeted and immune modulating drugs. Despite extensive research focused on understanding its signaling mechanisms and downstream phenotypic and functional consequences in hematological disorders, the importance of JAK/STAT signaling in solid tumor initiation and progression has been underappreciated. We highlight the role of chronic inflammation in cancer, the epidemiological evidence for contribution of JAK/STAT to carcinogenesis, the current cancer prevention measures involving JAK/STAT inhibition, and the impact of JAK/STAT signaling activity on cancer development, progression, and treatment resistance. We also discuss recent therapeutic advances in targeting key factors within the JAK/STAT pathway with single agents, and the use of these agents in combination with other targeted therapies and immune checkpoint inhibitors.

Topically Applied Taurine Chloramine Protects against UVB-Induced Oxidative Stress and Inflammation in Mouse Skin

Excessive exposure to solar light, especially its UV component, is a principal cause of photoaging, dermatitis, and photocarcinogenesis. In searching for candidate substances that can effectively protect the skin from photodamage, the present study was conducted with taurine chloramine (TauCl), formed from taurine in phagocytes recruited to inflamed tissue. Irradiation with ultraviolet B (UVB) of 180 mJ/cm² intensity caused oxidative damage and apoptotic cell death in the murine epidermis. These events were blunted by topically applied TauCl, as evidenced by the lower level of 4-hydroxynonenal-modified protein, reduced proportions of TUNEL-positive epidermal cells, and suppression of caspase-3 cleavage. In addition, the expression of two prototypic inflammatory enzymes, cyclooxygenase-2 and inducible nitric oxide synthase, and transcription of some pro-inflammatory cytokines (Tnf, Il6, Il1b, Il10) were significantly lower in TauCl-treated mice than vehicle-treated control mice. The anti-inflammatory effect of TauCl was associated with inhibition of STAT3 activation and induction of antioxidant enzymes, such as heme oxygenase-1 and NAD(P)H:quinone oxidoreductase 1, through activation of nuclear factor erythroid 2-related factor 2.

Twist1 is required for the development of UVB-induced squamous cell carcinoma

The transcription factor Twist1 has been reported to be essential for the formation and invasiveness of chemically induced tumors in mouse skin. However, the impact of keratinocyte-specific Twist1 deletion on skin carcinogenesis caused by UVB radiation has not been reported. Deletion of Twist1 in basal keratinocytes of mouse epidermis using K5.Cre × Twist1^flox/flox mice led to significantly reduced UVB-induced epidermal hyperproliferation. In addition, keratinocyte-specific deletion of Twist1 significantly suppressed UVB-induced skin carcinogenesis. Further analyses revealed that deletion of Twist1 in cultured keratinocytes or mouse epidermis in vivo led to keratinocyte differentiation. In this regard, deletion of Twist1 in epidermal keratinocytes showed significant induction of early and late differentiation markers, including TG1, K1, OVOL1, loricrin, and filaggrin. Similar results were obtained with topical application of harmine, a Harmala alkaloid that leads to degradation of Twist1. In contrast, overexpression of Twist1 in cultured keratinocytes suppressed calcium-induced differentiation. Further analyses using both K5.Cre × Twist1^flox/flox mice and an inducible system where Twist1 was deleted in bulge region keratinocytes showed loss of expression of hair follicle stem/progenitor markers, including CD34, Lrig1, Lgr5, and Lgr6. These data support the conclusion that Twist1 has a direct role in maintaining the balance between proliferation and differentiation of keratinocytes and keratinocyte stem/progenitor populations. Collectively, these results demonstrate a critical role for Twist1 early in the process of UVB skin carcinogenesis, and that Twist1 may be a novel target for the prevention of cutaneous squamous cell carcinoma.

Spiny mice (Acomys) exhibit attenuated hallmarks of aging and rapid cell turnover after UV exposure in the skin epidermis

The study of long-lived and regenerative animal models has revealed diverse protective responses to stressors such as aging and tissue injury. Spiny mice (Acomys) are a unique mammalian model of skin wound regeneration, but their response to other types of physiological skin damage has not been investigated. In this study, we examine how spiny mouse skin responds to acute UVB damage or chronological aging compared to non-regenerative C57Bl/6 mice (M. musculus). We find that, compared to M. musculus, the skin epidermis in A. cahirinus experiences a similar UVB-induced increase in basal cell proliferation but exhibits increased epidermal turnover. Notably, A. cahirinus uniquely form a suprabasal layer co-expressing Keratin 14 and Keratin 10 after UVB exposure concomitant with reduced epidermal inflammatory signaling and reduced markers of DNA damage. In the context of aging, old M. musculus animals exhibit typical hallmarks including epidermal thinning, increased inflammatory signaling and senescence. However, these age-related changes are absent in old A. cahirinus skin. Overall, we find that A. cahirinus have evolved novel responses to skin damage that reveals new aspects of its regenerative phenotype.

UVB-induced DHODH upregulation, which is driven by STAT3, is a promising target for chemoprevention and combination therapy of photocarcinogenesis

The leading cause of cutaneous squamous cell carcinomas (cSCCs) is exposure to ultraviolet radiation (UV). Unlike most other cancers, the incidence rates of cSCCs are still on the rise and the treatment options currently available are limited. We have recently found that dihydroorotate dehydrogenase (DHODH), which is the rate-limiting enzyme in the de novo pyrimidine synthesis pathway, plays a critical role in UVB-induced energy metabolism reprogramming. Using a multistage model of UVB radiation-induced skin cancer, we show that UVB-induced DHODH upregulation is mainly regulated transcriptionally by STAT3. Our results indicate that chronic inhibition of DHODH by leflunomide (LFN) blocks UVB-induced tumor initiation. Human tumor xenograft studies showed that LFN treatment reduces growth of established tumors when used in combination with a genotoxic agent, 5-fluorouracil (5-FU). Our data suggest that DHODH is a promising target for chemoprevention and combination therapy of UVB-induced cSCCs.

Activation of a protumorigenic IFNγ/STAT1/IRF-1 signaling pathway in keratinocytes following exposure to solar ultraviolet light

In this study, we evaluated the role of signal transducer and activator of transcription 1 (STAT1) in response to acute solar ultraviolet (SUV) radiation in mouse epidermis. Analysis of the epidermis from SUV-irradiated mice revealed rapid phosphorylation of STAT1 (pSTAT1) on both tyrosine (tyr701) and serine (ser727) residues and increased levels of IRF-1 while later timepoints showed increased levels of unphosphorylated STAT1 (uSTAT1). STAT1 activation led to upregulation of several proinflammatory chemokine mRNAs in epidermis including Cxcl9, Cxcl10, and Ccl2, as well as, the immune checkpoint inhibitor Pd-l1. In addition, mRNA and protein levels of cyclooxygenase-2 (Cox-2/COX2) were upregulated in epidermis following exposure to SUV. Mice with keratinocyte-specific STAT1 deletion did not exhibit increased IRF-1 or proinflammatory gene expression in epidermis. Furthermore, epidermal COX-2 induction after SUV exposure was significantly reduced in mice with keratinocyte-specific deletion of STAT1. Additionally, SUV irradiation rapidly upregulated interferon gamma (IFNγ) mRNA in the epidermis and that skin resident epidermal CD3 + T-cells were the source of IFNγ production. IFNγ receptor-deficient mice confirmed dependency of STAT1 activation, proinflammatory gene expression and COX-2 upregulation in the epidermis on paracrine IFNγ signaling. Furthermore, keratinocyte-specific STAT1-deficiency reduced proliferation and hyperplasia due to SUV irradiation and this was associated with decreased immune infiltration of mast cells in the dermis. Collectively, the current results demonstrate that exposure to SUV leads to upregulation of IFNγ and downstream pSTAT1/IRF-1/uSTAT1 signaling in the epidermis. Further study of this pathway could lead to identification of novel targets for the prevention of nonmelanoma skin cancer.

The role of T-cell protein tyrosine phosphatase in epithelial carcinogenesis

T-cell protein tyrosine phosphatase (TC-PTP, encoded by PTPN2) is a nonreceptor PTP that is most highly expressed in hematopoietic tissues. TC-PTP modulates a variety of physiological functions including cell cycle progression, cell survival and proliferation, and hematopoiesis through tyrosine dephosphorylation of its target substrates, such as EGFR, JAK1, JAK3, STAT1, and STAT3. Studies with whole or tissue-specific loss of TC-PTP function transgenic mice have shown that TC-PTP has crucial roles in the regulation of the immune response, insulin signaling, and oncogenic signaling. More recently, the generation of epidermal-specific TC-PTP-deficient mice for use in multistage skin carcinogenesis bioassays demonstrated that TC-PTP suppresses skin tumor formation by negatively regulating STAT3 and AKT signaling. Further investigation showed that TC-PTP also minimizes UVB-induced epidermal cell damage by promoting apoptosis through the negative regulation of Flk-1/JNK signaling. These findings provide major evidence for a tumor suppressive function for TC-PTP against environment-induced skin cancer. Here, we will discuss TC-PTP, its substrates, and its functions with an emphasis on its role in skin carcinogenesis.

STAT3, STAT5A, STAT5B and STAT6 proteins are overexpressed in human basal cell carcinoma

BACKGROUND

The molecular pathogenesis of basal cell carcinoma (BCC) is still not precisely described and is the subject of ongoing studies. The role of signal transducers and activators of transcription (STATs) in human epithelial carcinogenesis has been poorly investigated, but in the era of studies on inhibitors targeting STAT proteins this topic seems worth exploring. Increased expression of STAT3 in human nonmelanoma skin cancer (NMSC) has been confirmed in a few studies, but to our knowledge, expression of STAT5A, STAT5B and STAT6 in BCC has not been previously evaluated.

AIM

To measure expression of STAT3, STAT5A, STAT5B and STAT6 expression in different histopathological subtypes of human BCC and its correlation with selected clinical variables.

METHODS

Immunohistochemistry was used to assess 60 BCC tumour specimens [20 superficial (s)BCCs, 20 nodular (n)BCCs and 20 infiltrative (i)BCCs] and to compare with specimens of healthy skin. There was no significant difference in age or sex between the three groups of patients with BCC. As many tumours showed heterogeneity of staining, the H-score system was applied to calculate the intensity of immunoexpression.

RESULTS

Expression of STAT3, STAT5A, STAT5B and STAT6 was observed in all histopathological subtypes of BCC, and was stronger than the expression within the adjacent epidermis and also stronger than the expression within the epidermis in the healthy control group. Statistical analysis revealed no significant differences in mean H-scores calculated for sBCCs, nBCCs and iBCCs. There were no statistically significant associations between STAT3, STAT5A, STAT5B and STAT6 expression and patient sex/age, and tumour size/site.

CONCLUSION

Our results confirm a possible role of STATs in the pathogenesis of BCC and should encourage future investigations on the possible therapeutic implications of this finding.

Silibinin inhibits ultraviolet B radiation-induced mast cells recruitment and bone morphogenetic protein 2 expression in the skin at early stages in Ptch(+/-) mouse model of basal cell carcinoma

Around 80% of nonmelanoma skin cancers (NMSCs) are basal cell carcinoma (BCC), still studies evaluating the efficacy of chemopreventive agents during early stage/s of BCC development are lacking. Accordingly, utilizing the well-established patched (Ptch)+/- mouse model of ultraviolet B (UVB) radiation-induced BCC formation, we excised skin samples from UVB exposed Ptch+/- and Ptch+/+ mice before tumor formation to study the promotion/progression of BCC and to determine the efficacy and target/s of silibinin, a well-known skin cancer chemopreventive agent. UVB exposure for 1 month increased the number of mast cells in Ptch+/- mice by ~48% (P < 0.05), which was completely inhibited by silibinin. Polymerase chain reaction profiler array analysis of skin samples showed strong molecular differences between Ptch+/+ and Ptch+/- mice which were either unexposed or UVB irradiated+/- silibinin treatment. Most notably, silibinin treatment significant decreased the expression of BMP-2, Bbc3, PUMA, and Ccnd1 in Ptch+/- mice irradiated with silibinin + UVB. Additional studies showed that silibinin targets UVB-induced expression of bone morphogenetic protein 2 (BMP-2) in Ptch+/- mouse skin. Last, our studies found that silibinin strongly attenuates UVB-induced BMP-2 expression and DNA damage in Ptch+/- mouse skin ex vivo only after single UVB exposure. Together, our results suggest a possible role of mast cell recruitment and BMP-2 activation in the early stages of BCC development; these are strongly inhibited by silibinin suggesting its possible chemopreventive efficacy against BCC formation in long-term UVB exposure regimen.

Inhibitory effect of Carnosol on UVB-induced inflammation via inhibition of STAT3

Ultraviolet B (UVB) irradiation causes sunburn, inflammatory responses, dysregulation of immune function, oxidative stress, DNA damage and photocarcinogenesis on skin. Rosemary (Rosmarinus officinalis L.) has been reported to inhibit inflammation. Carnosol, a major component of Rosemary, has prominent anti-inflammatory effects. However, its protective effect on UVB-induced inflammatory skin responses has not yet been reported. Here, we investigated the effectiveness of carnosol on UVB-induced inflammation. We examined the anti-inflammation effect of topical application of carnosol (0.05 µg/cm²) on UVB (540 mJ/cm², for 3 successive days)-induced skin inflammation in HR1 mice. Topical application of carnosol inhibited UVB-induced erythema, epidermal thickness, inflammatory responses in HR1 mice. Carnosol reduced the level of Immunoglobulin-E and IL-1β in blood serum of UVB-induced mice. Carnosol also significantly inhibited the UVB-induced expression of inflammatory marker protein (iNOS and COX-2) in back skin of mice. In addition, carnosol treated skin decreased activation of STAT3, a transcriptional factor regulating inflammatory genes. Our study suggested that carnosol has protective effects on skin inflammatory skin damages by UVB.

Protein Tyrosine Phosphatases as Potential Regulators of STAT3 Signaling

The signal transducer and activator of transcription 3 (STAT3) protein is a major transcription factor involved in many cellular processes, such as cell growth and proliferation, differentiation, migration, and cell death or cell apoptosis. It is activated in response to a variety of extracellular stimuli including cytokines and growth factors. The aberrant activation of STAT3 contributes to several human diseases, particularly cancer. Consequently, STAT3-mediated signaling continues to be extensively studied in order to identify potential targets for the development of new and more effective clinical therapeutics. STAT3 activation can be regulated, either positively or negatively, by different posttranslational mechanisms including serine or tyrosine phosphorylation/dephosphorylation, acetylation, or demethylation. One of the major mechanisms that negatively regulates STAT3 activation is dephosphorylation of the tyrosine residue essential for its activation by protein tyrosine phosphatases (PTPs). There are seven PTPs that have been shown to dephosphorylate STAT3 and, thereby, regulate STAT3 signaling: PTP receptor-type D (PTPRD), PTP receptor-type T (PTPRT), PTP receptor-type K (PTPRK), Src homology region 2 (SH-2) domain-containing phosphatase 1(SHP1), SH-2 domain-containing phosphatase 2 (SHP2), MEG2/PTP non-receptor type 9 (PTPN9), and T-cell PTP (TC-PTP)/PTP non-receptor type 2 (PTPN2). These regulators have great potential as targets for the development of more effective therapies against human disease, including cancer.

Protein Tyrosine Signaling and its Potential Therapeutic Implications in Carcinogenesis

Protein tyrosine phosphorylation is a crucial signaling mechanism that plays a role in epithelial carcinogenesis. Protein tyrosine kinases (PTKs) control various cellular processes including growth, differentiation, metabolism, and motility by activating major signaling pathways including STAT3, AKT, and MAPK. Genetic mutation of PTKs and/or prolonged activation of PTKs and their downstream pathways can lead to the development of epithelial cancer. Therefore, PTKs became an attractive target for cancer prevention. PTK inhibitors are continuously being developed, and they are currently used for the treatment of cancers that show a high expression of PTKs. Protein tyrosine phosphatases (PTPs), the homeostatic counterpart of PTKs, negatively regulate the rate and duration of phosphotyrosine signaling. PTPs initially were considered to be only housekeeping enzymes with low specificity. However, recent studies have demonstrated that PTPs can function as either tumor suppressors or tumor promoters, depending on their target substrates. Together, both PTK and PTP signal transduction pathways are potential therapeutic targets for cancer prevention and treatment.

The AHR represses nucleotide excision repair and apoptosis and contributes to UV-induced skin carcinogenesis

Ultraviolet B (UVB) radiation induces mutagenic DNA photoproducts, in particular cyclobutane pyrimidine dimers (CPDs), in epidermal keratinocytes (KC). To prevent skin carcinogenesis, these DNA photoproducts must be removed by nucleotide excision repair (NER) or apoptosis. Here we report that the UVB-sensitive transcription factor aryl hydrocarbon receptor (AHR) attenuates the clearance of UVB-induced CPDs in human HaCaT KC and skin from SKH-1 hairless mice. Subsequent RNA interference and inhibitor studies in KC revealed that AHR specifically suppresses global genome but not transcription-coupled NER. In further experiments, we found that the accelerated repair of CPDs in AHR-compromised KC depended on a modulation of the p27 tumor suppressor protein. Accordingly, p27 protein levels were increased in AHR-silenced KC and skin biopsies from AHR^−/− mice, and critical for the improvement of NER. Besides increasing NER activity, AHR inhibition was accompanied by an enhanced occurrence of DNA double-strand breaks triggering KC apoptosis at later time points after irradiation. The UVB-activated AHR thus acts as a negative regulator of both early defense systems against carcinogenesis, NER and apoptosis, implying that it exhibits tumorigenic functions in UVB-exposed skin. In fact, AHR^−/− mice developed 50% less UVB-induced cutaneous squamous cell carcinomas in a chronic photocarcinogenesis study than their AHR^+/+ littermates. Taken together, our data reveal that AHR influences DNA damage-dependent responses in UVB-irradiated KC and critically contributes to skin photocarcinogenesis in mice.

Amelioration of UVB-induced oxidative stress and inflammation in fat-1 transgenic mouse skin

ω-3 polyunsaturated fatty acids (PUFAs), which are abundant in fish oils, are known to scavenge lipid peroxyl radicals and potentiate host immune defence. As UVB-induced oxidative stress and inflammation have been implicated in apoptotic cell death, this study was aimed to investigate the anti-inflammatory, anti-oxidative, and anti-apoptotic capacity of fat-1 transgenic mice capable of converting ω-6 to ω-3 PUFAs. Wild-type (WT) C57BL/6 mice and fat-1 mice were maintained on the AIN-93 diet supplemented with 10% safflower oil rich in ω-6 PUFAs for 5 weeks. The ω-3/ω-6 PUFA ratio was significantly higher in the dorsal skin of fat-1 mice than that in the WT mice. Upon single exposure to UVB (5.0 kJ/m²) radiation, fat-1 mice showed inflammatory as well as oxidative tissue damage and the expression of pro-inflammatory enzymes, cyclooxygenases-2 and inducible nitric oxide synthase in the skin to a much lesser extent than the WT mice. The protection of fat-1 mice from UVB-induced skin inflammation was associated with decreased phosphorylation of STAT3. Moreover, UVB-induced apoptosis was attenuated in fat-1 mouse skin. In comparison to WT animals, higher levels of Nrf2 and its target proteins, such as heme oxygenase-1, NAD(P)H:quinone oxidoreductase-1 and thioredoxin-1, were found in the skin of fat-1 mice.

Targeting the IL-6/JAK/STAT3 signalling axis in cancer

The IL-6/JAK/STAT3 pathway is aberrantly hyperactivated in many types of cancer, and such hyperactivation is generally associated with a poor clinical prognosis. In the tumour microenvironment, IL-6/JAK/STAT3 signalling acts to drive the proliferation, survival, invasiveness, and metastasis of tumour cells, while strongly suppressing the antitumour immune response. Thus, treatments that target the IL-6/JAK/STAT3 pathway in patients with cancer are poised to provide therapeutic benefit by directly inhibiting tumour cell growth and by stimulating antitumour immunity. Agents targeting IL-6, the IL-6 receptor, or JAKs have already received FDA approval for the treatment of inflammatory conditions or myeloproliferative neoplasms and for the management of certain adverse effects of chimeric antigen receptor T cells, and are being further evaluated in patients with haematopoietic malignancies and in those with solid tumours. Novel inhibitors of the IL-6/JAK/STAT3 pathway, including STAT3-selective inhibitors, are currently in development. Herein, we review the role of IL-6/JAK/STAT3 signalling in the tumour microenvironment and the status of preclinical and clinical investigations of agents targeting this pathway. We also discuss the potential of combining IL-6/JAK/STAT3 inhibitors with currently approved therapeutic agents directed against immune-checkpoint inhibitors.

LGR5 expression is controled by IKKα in basal cell carcinoma through activating STAT3 signaling pathway

Basal cell carcinomas (BCC) of the skin are the most common of human cancers. The noncanonical NF-κB pathway is dependent on IKKα. However, the role of IKKα in BCC has not been elucidated. We show here that IKKα is expressed in the nucleus in BCC and non-malignant diseases. Nuclear IKKα could directly bind to the promoters of inflammation factors and LGR5, a stem cell marker, in turn, upregulating LGR5 expression through activation of STAT3 signaling pathway during cancer progression. Activation of STAT3 signaling pathway contributes LGR5 expression in dependent of IKKα after the interplay between STAT3 and IKKα. Meanwhile knockdown of IKKα inhibits tumor growth and transition of epithelial stage to mescheme stage. Taken together, we demonstrate that IKKα functions as a bone fide chromatin regulator in BCC, whose promoted expression contributes to oncogenic transformation via promoting expression stemness- and inflammatory- related genes. Our finding reveals a novel viewpoint for how IKKα may involve in BCCs tumor progression in the inflammatory microenvironment.

UVB-induced nuclear translocation of TC-PTP by AKT/14-3-3σ axis inhibits keratinocyte survival and proliferation

Understanding protein subcellular localization is important to determining the functional role of specific proteins. T-cell protein tyrosine phosphatase (TC-PTP) contains bipartite nuclear localization signals (NLSI and NLSII) in its C-terminus. We previously have demonstrated that the nuclear form of TC-PTP (TC45) is mainly localized to the cytoplasm in keratinocytes and it is translocated to the nucleus following UVB irradiation. Here, we report that TC45 is translocated by an AKT/14-3-3σ-mediated mechanism in response to UVB exposure, resulting in increased apoptosis and decreased keratinocyte proliferation. We demonstrate that UVB irradiation increased phosphorylation of AKT and induced nuclear translocation of 14-3-3σ and TC45. However, inhibition of AKT blocked nuclear translocation of TC45 and 14-3-3σ. Site-directed mutagenesis of 14-3-3σ binding sites within TC45 showed that a substitution at Threonine 179 (TC45/T179A) effectively blocked UVB-induced nuclear translocation of ectopic TC45 due to the disruption of the direct binding between TC45 and 14-3-3σ. Overexpression of TC45/T179A in keratinocytes resulted in a decrease of UVB-induced apoptosis which corresponded to an increase in nuclear phosphorylated STAT3, and cell proliferation was higher in TC45/T179A-overexpressing keratinocytes compared to control keratinocytes following UVB irradiation. Furthermore, deletion of TC45 NLSII blocked its UVB-induced nuclear translocation, indicating that both T179 and NLSII are required. Taken together, our findings suggest that AKT and 14-3-3σ cooperatively regulate TC45 nuclear translocation in a critical step of an early protective mechanism against UVB exposure that signals the deactivation of STAT3 in order to promote keratinocyte cell death and inhibit keratinocyte proliferation.

Endogenous ω-3 Fatty Acid Production by fat-1 Transgene and Topically Applied Docosahexaenoic Acid Protect against UVB-induced Mouse Skin Carcinogenesis

The present study was intended to explore the effects of endogenously produced ω-3 polyunsaturated fatty acids (PUFAs) on ultraviolet B (UVB)-induced skin inflammation and photocarcinogenesis using hairless fat-1 transgenic mice harboring ω-3 desaturase gene capable of converting ω-6 to ω-3 PUFAs. Upon exposure to UVB irradiation, fat-1 transgenic mice exhibited a significantly reduced epidermal hyperplasia, oxidative skin damage, and photocarcinogenesis as compared to wild type mice. The transcription factor, Nrf2 is a master regulator of anti-inflammatory and antioxidant gene expression. While the protein expression of Nrf2 was markedly enhanced, the level of its mRNA transcript was barely changed in the fat-1 transgenic mouse skin. Topical application of docosahexaenoic acid (DHA), a representative ω-3 PUFA, in wild type hairless mice induced expression of the Nrf2 target protein, heme oxygenase-1 in the skin and protected against UVB-induced oxidative stress, inflammation and papillomagenesis. Furthermore, transient overexpression of fat-1 gene in mouse epidermal JB6 cells resulted in the enhanced accumulation of Nrf2 protein. Likewise, DHA treated to JB6 cells inhibited Nrf2 ubiquitination and stabilized it. Taken together, our results indicate that functional fat-1 and topically applied DHA potentiate cellular defense against UVB-induced skin inflammation and photocarcinogenesis through elevated activation of Nrf2 and upregulation of cytoprotective gene expression.

The mechanism of CIRP in inhibition of keratinocytes growth arrest and apoptosis following low dose UVB radiation

UV induces CIRP expression and subsequent Stat3 activation, but the biological function and mechanism of CIRP and Stat3 in mediating UVB-induced skin carcinogenesis have not been fully elucidated. In this study, we demonstrate that CIRP is elevated in all tested melanoma and non-melanoma skin cancer cell lines; and the expression of CIRP is upregulated in keratinocytes after being irradiated with relatively low dose (<5 mJ/cm2 ), but not high dose (50 mJ/cm2 ), UVB acutely and chronically. The increased expression of CIRP, either induced by UVB or through overexpression, leads to resistance of keratinocytes to UVB-induced growth arrest and death; and reduced expression of CIRP by RNA knockdown sensitizes keratinocyte cells to the low dose UVB radiation. We also demonstrated that CIRP expression is required for the low dose UVB-induced Tyr705-phosphorylation, but not total amount, of Stat3. The p-Stat3 level is correlated with the expression levels of cyclin D1 and VEGF, two known downstream cell growth regulators of Stat3, as well as Bag-1/S, an apoptosis regulator. Inhibition of Stat3 DNA-binding activity by S3I-201 leads to a reduction of the p-Stat3 and Bag-1/S along with growth and survival of keratinocytes post-UVB; and the effect of S3I-201 on the UVB-irradiated cells can be partially inhibited by overexpression of CIRP or Bag-1/S. Furthermore, the overexpression of Bag-1/S can totally inhibit UVB-induced PARP cleavage and caspase 3 activation. The results presented above led us to propose that CIRP-p(705)Stat3 cascade promotes cell proliferation and survival post-UVB via upregulating the expression of cyclin D1 and Bag-1/S, respectively. Published 2017. This article is a U.S. Government work and is in the public domain in the USA.

Multifaceted role of TREX2 in the skin defense against UV-induced skin carcinogenesis

TREX2 is a 3′-DNA exonuclease specifically expressed in keratinocytes. Here, we investigated the relevance and mechanisms of TREX2 in ultraviolet (UV)-induced skin carcinogenesis. TREX2 expression was up-regulated by chronic UV exposure whereas it was de-regulated or lost in human squamous cell carcinomas (SCCs). Moreover, we identified SNPs in the TREX2 gene that were more frequent in patients with head and neck SCCs than in healthy individuals. In mice, TREX2 deficiency led to enhanced susceptibility to UVB-induced skin carcinogenesis which was preceded by aberrant DNA damage removal and degradation as well as reduced inflammation. Specifically, TREX2 loss diminished the up-regulation of IL12 and IFNγ, key cytokines related to DNA repair and antitumor immunity. In UV-treated keratinocytes, TREX2 promoted DNA repair and passage to late apoptotic stages. Notably, TREX2 was recruited to low-density nuclear chromatin and micronuclei, where it interacted with phosphorylated H2AX histone, which is a critical player in both DNA repair and cell death. Altogether, our data provide new insights in the molecular mechanisms of TREX2 activity and establish cell autonomous and non-cell autonomous functions of TREX2 in the UVB-induced skin response.

Overexpression of PRAS40(T246A) in the Proliferative Compartment Suppresses mTORC1 Signaling, Keratinocyte Migration, and Skin Tumor Development

The proline-rich Akt (v-akt murine thymoma viral oncogene homolog 1) substrate of 40 kDa (PRAS40), an inhibitory component of the mTORC1 complex, was identified as an Akt substrate through phosphorylation at Thr246. Phosphorylation at this site releases PRAS40 from the mammalian/mechanistic target of rapamycin complex 1 (mTORC1) complex allowing increased activity. Targeted expression of a mutant form of PRAS40 (PRAS40(T246A)) in basal keratinocytes of mouse epidermis (BK5.PRAS40(T246A) mice) has allowed further examination of mTORC1-specific signaling in epithelial carcinogenesis. BK5.PRAS40(T246A) mice were resistant to 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced epidermal hyperproliferation and skin tumor development. In transgenic mice, PRAS40(T246A) remained bound to raptor in keratinocytes even after treatment with TPA, consistent with reduced mTORC1 signaling and altered levels of cell cycle proteins. BK5.PRAS40(T246A) mice also displayed attenuated skin inflammation in response to TPA. Inhibition of mTORC1 in keratinocytes significantly inhibited their migration in vitro and, in addition, inhibited 12-O-tetradecanoylphorbol-13-acetate-induced proliferation and migration of bulge-region stem cells in vivo. Furthermore, targeted inhibition of mTORC1 in BK5.PRAS40(T246A) mice resulted in delayed wound healing. Decreased keratinocyte migration and impaired wound healing correlated with altered expression of epithelial-mesenchymal transition (EMT) markers and reduced smad signaling. Collectively, the current data using this unique mouse model provide further evidence that mTORC1 signaling in keratinocytes regulates key events in keratinocyte function and epithelial cancer development.

p62 modulates the intrinsic signaling of UVB-induced apoptosis

BACKGROUND

UVB radiation is the main source of sunburn and skin cancers. Apoptosis eliminates photodamaged cells, and is thus important for preventing epidermal carcinogenesis. The cytoplasmic regulatory protein p62/A170/sequestosome 1 (p62) molecule is involved in a variety of cellular and signaling pathways. p62 is known to be and important in autophagy, but its role in UVB-induced apoptosis remains to be clarified.

OBJECTIVE

To investigate the role of p62 against UVB-induced apoptotic changes, using mouse embryonic fibroblasts (MEFs) derived from p62 homozygous knockout (p62(-/-)) mice.

METHODS

p62(-/-) and wild-type (p62(+/+)) mice and MEFs were subjected to UVB irradiation, and the resultant apoptosis was analyzed using flow cytometry, quantitative real-time PCR, and western blots.

RESULTS

Apoptosis was decreased in the p62(-/-) MEFs compared to p62(+/+) MEFs in response to UVB treatment. Compared with p62(+/+) MEFs, p62(-/-) MEFs expressed significantly more Bcl-2 and less Bax, and showed increased Src and Stat3 phosphorylation. Our results show that p62 regulates apoptotic pathways by modifying critical signaling intermediates such as Src and Stat3.

CONCLUSION

p62 deficiency [corrected] reduces UVB-induced apoptosis by modulating intrinsic apoptotic signaling through Src phosphorylation.

Chronic Inflammation in Skin Malignancies

Chronic inflammation is linked to the development and progression of multiple cancers, including those of the lung, stomach, liver, colon, breast and skin. Inflammation not only drives the oncogenic transformation of epithelial cells under the stress of chronic infection and autoimmune diseases, but also promotes the growth, progression and metastatic spread of cancers. Tumor-infiltrating inflammatory cells are comprised of a diverse population of myeloid and immune cell types, including monocytes, macrophages, dendritic cells, T and B cells, and others. Different myeloid and lymphoid cells within tumor microenvironment exert diverse, often contradicting, effects during skin cancer development and progression. The nature of tumor-immune interaction determines the rate of cancer progression and the outcome of cancer treatment. Inflammatory environment within skin tumor also inhibits naturally occurring anti-tumor immunity and limits the efficacy of cancer immunotherapy. In this article we aim to give an overview on the mechanism by which inflammation interferes with the development and therapeutic intervention of cancers, especially those of the skin.

Endogenous Sterol Metabolites Regulate Growth of EGFR/KRAS-Dependent Tumors via LXR

Meiosis-activating sterols (MAS) are substrates of SC4MOL and NSDHL in the cholesterol pathway and are important for normal organismal development. Oncogenic transformation by epidermal growth factor receptor (EGFR) or RAS increases the demand for cholesterol, suggesting a possibility for metabolic interference. To test this idea in vivo, we ablated Nsdhl in adult keratinocytes expressing KRAS(G12D). Strikingly, Nsdhl inactivation antagonized the growth of skin tumors while having little effect on normal skin. Loss of Nsdhl induced the expression of ATP-binding cassette (ABC) transporters ABCA1 and ABCG1, reduced the expression of low-density lipoprotein receptor (LDLR), decreased intracellular cholesterol, and was dependent on the liver X receptor (LXR) α. Importantly, EGFR signaling opposed LXRα effects on cholesterol homeostasis, whereas an EGFR inhibitor synergized with LXRα agonists in killing cancer cells. Inhibition of SC4MOL or NSDHL, or activation of LXRα by sterol metabolites, can be an effective strategy against carcinomas with activated EGFR-KRAS signaling.

Protein Tyrosine Kinase 6 Regulates UVB-Induced Signaling and Tumorigenesis in Mouse Skin

Protein Tyrosine Kinase 6 (PTK6, also called BRK) is an intracellular tyrosine kinase expressed in the epithelial linings of the gastrointestinal tract and skin, where it is expressed in nondividing differentiated cells. We found PTK6 expression increases in the epidermis following UVB treatment. To evaluate the roles of PTK6 in the skin following UVB-induced damage, we exposed back skin of Ptk6 +/+ and Ptk6−/− SENCAR mice to incremental doses of UVB for thirty weeks. Wild type mice were more sensitive to UVB and exhibited increased inflammation and greater activation of STAT3 than Ptk6−/− mice. Disruption of Ptk6 did not have an impact on proliferation, although PTK6 was expressed and activated in basal epithelial cells in wild type mice following UVB treatment. However, wild type mice exhibited shortened tumor latency and increased tumor load compared with Ptk6−/− mice, and STAT3 activation was increased in these tumors. PTK6 activation was detected in UVB-induced tumors, and this correlated with increased activating phosphorylation of FAK and BCAR1. Activation of PTK6 was also detected in human squamous cell carcinomas of the skin. Although PTK6 plays roles in normal differentiation, it also contributes to UVB induced injury and tumorigenesis in vivo.

Activation of T-cell protein-tyrosine phosphatase suppresses keratinocyte survival and proliferation following UVB irradiation

Chronic exposure to UV radiation can contribute to the development of skin cancer by promoting protein-tyrosine kinase (PTK) signaling. Studies show that exposure to UV radiation increases the ligand-independent activation of PTKs and induces protein-tyrosine phosphatase (PTP) inactivation. In the present work, we report that T-cell PTP (TC-PTP) activity is stimulated during the initial response to UVB irradiation, which leads to suppression of keratinocyte cell survival and proliferation via the down-regulation of STAT3 signaling. Our results show that TC-PTP-deficient keratinocyte cell lines expressed a significantly increased level of phosphorylated STAT3 after exposure to low dose UVB. This increase corresponded with increased cell proliferation in TC-PTP-deficient keratinocytes following UVB irradiation. Loss of TC-PTP also reduced UVB-induced apoptosis. Corroborating with these results, overexpression of TC-PTP in keratinocyte cell lines yielded a decrease in phosphorylated STAT3 levels, which corresponded with a significant decrease in cell proliferation in response to low dose UVB. We demonstrate that TC-PTP activity was increased upon UVB exposure, and overexpression of TC-PTP in keratinocyte cell lines further increased its activity in the presence of UVB. Treatment of TC-PTP-deficient keratinocytes with the STAT3 inhibitor STA21 significantly reduced cell viability following UVB exposure in comparison with untreated TC-PTP-deficient keratinocytes, confirming that the effect of TC-PTP on cell viability is mediated by STAT3 dephosphorylation. Combined, our results indicate that UVB-mediated activation of TC-PTP plays an important role in the STAT3-dependent regulation of keratinocyte cell proliferation and survival. Furthermore, these results suggest that TC-PTP may be a novel potential target for the prevention of UVB-induced skin cancer.

Targeting the tumor microenvironment: JAK-STAT3 signaling

Persistent JAK-STAT3 signaling is implicated in many aspects of tumorigenesis. Apart from its tumor-intrinsic effects, STAT3 also exerts tumor-extrinsic effects, supporting tumor survival and metastasis. These involve the regulation of paracrine cytokine signaling, alterations in metastatic sites rendering these permissive for the growth of cancer cells and subversion of host immune responses to create an immunosuppressive environment. Targeting this signaling pathway is considered a novel promising therapeutic approach, especially in the context of tumor immunity. In this article, we will review to what extent JAK-STAT3-targeted therapies affect the tumor microenvironment and whether the observed effects underlie responsiveness to therapy.

Antagonizing STAT3 dimerization with a rhodium(III) complex

Kinetically inert metal complexes have arisen as promising alternatives to existing platinum and ruthenium chemotherapeutics. Reported herein, to our knowledge, is the first example of a substitutionally inert, Group 9 organometallic compound as a direct inhibitor of signal transducer and activator of transcription 3 (STAT3) dimerization. From a series of cyclometalated rhodium(III) and iridium(III) complexes, a rhodium(III) complex emerged as a potent inhibitor of STAT3 that targeted the SH2 domain and inhibited STAT3 phosphorylation and dimerization. Significantly, the complex exhibited potent anti-tumor activities in an in vivo mouse xenograft model of melanoma. This study demonstrates that rhodium complexes may be developed as effective STAT3 inhibitors with potent anti-tumor activity.

Stat3 binds to mtDNA and regulates mitochondrial gene expression in keratinocytes

The nuclear transcription factor Stat3 has recently been reported to have a localized mitochondrial regulatory function. Current data suggest that mitochondrial Stat3 (mitoStat3) is necessary for maximal mitochondrial activity and for Ras-mediated transformation independent of Stat3 nuclear activity. We have previously shown that Stat3 plays a pivotal role in epithelial carcinogenesis. Therefore, the aim of the current study was to determine the role of mitoStat3 in epidermal keratinocytes. Herein, we show that normal and neoplastic keratinocytes contain a pool of mitoStat3. EGF and TPA induce Stat3 mitochondrial translocation mediated through phosphorylation of Stat3 at Ser⁷²⁷. In addition, we report that mitoStat3 binds mitochondrial DNA (mtDNA) and associates with the mitochondrial transcription factor TFAM. Furthermore, Stat3 ablation resulted in an increase of mitochondrial encoded gene transcripts. An increase in key nuclear-encoded metabolic genes, PGC-1α and NRF-1, was also observed in Stat3 null keratinocytes, however no changes in nuclear-encoded ETC gene transcripts or mtDNA copy number were observed. Collectively, our findings suggest a heretofore-unreported function for mitoStat3 as a potential mitochondrial transcription factor in keratinocytes. This mitoStat3-mtDNA interaction may represent an alternate signaling pathway that could alter mitochondrial function and biogenesis and play a role in tumorigenesis.

Loss of STAT3 in Lymphoma Relaxes NK Cell-Mediated Tumor Surveillance

The transcription factors and proto-oncogenes STAT3 and STAT5 are highly activated in hematological malignancies and represent promising therapeutic targets. Whereas the importance of STAT5 as tumor promoter is beyond doubt, the role of STAT3 in hematological cancers is less well understood. Both, enforced as well as attenuated expression of STAT3 were reported in hematopoietic malignancies. Recent evidence implicates STAT3 as key player for tumor immune surveillance as it both mediates the production of and response to inflammatory cytokines. Here we investigated the effects of STAT3 deletion in a BCR/ABL-induced lymphoma model, which is tightly controlled by natural killer (NK) cells in vivo. Upon STAT3 deletion tumor growth is significantly enhanced when compared to STAT3-expressing controls. The increased tumor size upon loss of STAT3 was accompanied by reduced NK cell infiltration and decreased levels of the cytokine IFN-γ and the chemokine RANTES. Upon transplantation into NK cell-deficient mice differences in lymphoma size were abolished indicating that STAT3 expression in the tumor cells controls NK cell-dependent tumor surveillance. Our findings indicate that STAT3 inhibition in lymphoma patients will impair NK cell-mediated tumor surveillance, which needs to be taken into account when testing STAT3 inhibitors in preclinical or clinical trials.

A novel mechanism of skin tumor promotion involving interferon-gamma (IFNγ)/signal transducer and activator of transcription-1 (Stat1) signaling

The current study was designed to explore the role of signal transducer and activator of transcription 1 (Stat1) during tumor promotion using the mouse skin multistage carcinogenesis model. Topical treatment with both 12-O-tetradecanoylphorbol-13-acetate (TPA) and 3-methyl-1,8-dihydroxy-9-anthrone (chrysarobin or CHRY) led to rapid phosphorylation of Stat1 on both tyrosine (Y701) and serine (S727) residues in epidermis. CHRY treatment also led to upregulation of unphosphorylated Stat1 (uStat1) at later time points. CHRY treatment also led to upregulation of interferon regulatory factor 1 (IRF-1) mRNA and protein, which was dependent on Stat1. Further analyses demonstrated that topical treatment with CHRY but not TPA upregulated interferon-gamma (IFNγ) mRNA in the epidermis and that the induction of both IRF-1 and uStat1 was dependent on IFNγ signaling. Stat1 deficient (Stat1(-/-) ) mice were highly resistant to skin tumor promotion by CHRY. In contrast, the tumor response (in terms of both papillomas and squamous cell carcinomas) was similar in Stat1(-/-) mice and wild-type littermates with TPA as the promoter. Maximal induction of both cyclooxygenase-2 and inducible nitric oxide synthase in epidermis following treatment with CHRY was also dependent on the presence of functional Stat1. These studies define a novel mechanism associated with skin tumor promotion by the anthrone class of tumor promoters involving upregulation of IFNγ signaling in the epidermis and downstream signaling through activated (phosphorylated) Stat1, IRF-1 and uStat1.

Reconstructing skin cancers using animal models

The American Cancer Society estimates that skin cancer is the most prevalent of all cancers with over 2 million cases of nonmelanoma skin cancer each year and 75,000 melanoma cases in 2012. Representative animal cancer models are important for understanding the underlying molecular pathogenesis of these cancers and the development of novel targeted anticancer therapeutics. In this review, we will discuss some of the important animal models that have been useful to identify important pathways involved in basal cell carcinoma, squamous cell carcinoma, and melanoma.

Constitutive Stat3 activation alters behavior of hair follicle stem and progenitor cell populations

STATs play crucial roles in a wide variety of biological functions, including development, proliferation, differentiation and migration as well as in cancer development. In the present study, we examined the impact of constitutive activation of Stat3 on behavior of keratinocytes, including keratinocyte stem cells (KSC) in vivo. BK5.Stat3C transgenic (Tg) mice, which express a constitutively active form of Stat3 (Stat3C) in the basal layer of the epidermis and in the bulge region KSCs exhibited a significantly reduced number of CD34+/α6 integrin+ cells compared to non-transgenic (NTg) littermates. There was a concomitant increase in the Lgr-6, Lrig-1, and Sca-1 populations in the Tg mice in contrast to the CD34 and Keratin-15 positive population. In addition, increased expression of c-myc, β-catenin, and epithelial-mesenchymal transition (EMT)-related genes as well as decreased expression of α6-integrin was observed in the hair follicles of Tg mice. Notably, Sca-1 was found to be a direct transcriptional target of Stat3 in keratinocytes. The current data suggest that elevated Stat3 activity leads to depletion of hair follicle KSCs along with a concomitant increase of stem/progenitor cells above the bulge region. Overall, the current data indicate that Stat3 plays an important role in keratinocyte stem/progenitor cell homeostasis.

A green approach toward quinoxalines and bis-quinoxalines and their biological evaluation against A431, human skin cancer cell lines

Background: The objective of this study was to develop a practical green procedure to synthesize quinoxalines and bis-quinoxalines and evaluate their inhibitory effects on the viability of A431 human epidermoid carcinoma cells. Method: A series of quinoxaline and bis-quinoxaline derivatives have been designed and synthesized following a microwave-assisted and bismuth nitrate-catalyzed eco-friendly route. A detailed comparison has been made between microwave-induced protocol with the reactions occurred at room temperature. The structure of the compounds have been elucidated by various spectroscopic methods and finally confirmed by x-ray crystallographic analyses. Results: Two quinoxaline derivatives, compounds 6 and 12 have demonstrated inhibitory effects on the viability of A431 human epidermoid carcinoma cells when compared with HaCaT nontumorigenic human keratinocyte cells. Conclusion: Notably, compound 6 inhibits Stat3 phosphorylation/activation in A431 skin cancer cells.