Activated Wnt/beta-catenin signaling in melanoma is associated with decreased proliferation in patient tumors and a murine melanoma model

Microphthalmia-associated transcription factor in melanoma development and MAP-kinase pathway targeted therapy

Malignant melanoma is a neoplasm of melanocytes, and the microphthalmia-associated transcription factor (MITF) is essential for the existence of melanocytes. MITF's relevance for this cell lineage is maintained in melanoma, where it is an important regulator of survival and balances melanoma cell proliferation with terminal differentiation (pigmentation). The MITF gene is amplified in ~20% of melanomas and MITF mutation can predispose to melanoma development. Furthermore, the regulation of MITF expression and function is strongly linked to the BRAF/MEK/ERK/MAP-kinase (MAPK) pathway, which is deregulated in >90% of melanomas and central target of current therapies. MITF expression in melanoma is heterogeneous, and recent findings highlight the relevance of this heterogeneity for the response of melanoma to MAPK pathway targeting drugs, as well as for MITF's role in melanoma progression. This review aims to provide an updated overview on the regulation of MITF function and plasticity in melanoma with a focus on its link to MAPK signaling.

Daple is a novel non-receptor GEF required for trimeric G protein activation in Wnt signaling

Wnt signaling is essential for tissue homeostasis and its dysregulation causes cancer. Wnt ligands trigger signaling by activating Frizzled receptors (FZDRs), which belong to the G-protein coupled receptor superfamily. However, the mechanisms of G protein activation in Wnt signaling remain controversial. In this study, we demonstrate that FZDRs activate G proteins and trigger non-canonical Wnt signaling via the Dishevelled-binding protein, Daple. Daple contains a Gα-binding and activating (GBA) motif, which activates Gαi proteins and an adjacent domain that directly binds FZDRs, thereby linking Wnt stimulation to G protein activation. This triggers non-canonical Wnt responses, that is, suppresses the β-catenin/TCF/LEF pathway and tumorigenesis, but enhances PI3K-Akt and Rac1 signals and tumor cell invasiveness. In colorectal cancers, Daple is suppressed during adenoma-to-carcinoma transformation and expressed later in metastasized tumor cells. Thus, Daple activates Gαi and enhances non-canonical Wnt signaling by FZDRs, and its dysregulation can impact both tumor initiation and progression to metastasis.

Dietary phytochemicals alter epigenetic events and signaling pathways for inhibition of metastasis cascade: phytoblockers of metastasis cascade

Cancer metastasis is a multistep process in which a cancer cell spreads from the site of the primary lesion, passes through the circulatory system, and establishes a secondary tumor at a new nonadjacent organ or part. Inhibition of cancer progression by dietary phytochemicals (DPs) offers significant promise for reducing the incidence and mortality of cancer. Consumption of DPs in the diet has been linked to a decrease in the rate of metastatic cancer in a number of preclinical animal models and human epidemiological studies. DPs have been reported to modulate the numerous biological events including epigenetic events (noncoding micro-RNAs, histone modification, and DNA methylation) and multiple signaling transduction pathways (Wnt/β-catenin, Notch, Sonic hedgehog, COX-2, EGFR, MAPK-ERK, JAK-STAT, Akt/PI3K/mTOR, NF-κB, AP-1, etc.), which can play a key role in regulation of metastasis cascade. Extensive studies have also been performed to determine the molecular mechanisms underlying antimetastatic activity of DPs, with results indicating that these DPs have significant inhibitory activity at nearly every step of the metastatic cascade. DPs have anticancer effects by inducing apoptosis and by inhibiting cell growth, migration, invasion, and angiogenesis. Growing evidence has also shown that these natural agents potentiate the efficacy of chemotherapy and radiotherapy through the regulation of multiple signaling pathways. In this review, we discuss the variety of molecular mechanisms by which DPs regulate metastatic cascade and highlight the potentials of these DPs as promising therapeutic inhibitors of cancer.

Excluding T Cells: Is β-Catenin the Full Story?

Spranger and colleagues reported recently in Nature an inverse relationship between melanoma intrinsic β-catenin signaling and intratumoral T cell infiltration, providing an explanation for potential mechanisms of T cell exclusion. Further insights are needed into the mechanisms leading to a lack of T cell infiltration of cancers and primary immune resistance.

UV-Induced Wnt7a in the Human Skin Microenvironment Specifies the Fate of Neural Crest-Like Cells via Suppression of Notch

Multipotent stem cells with neural crest-like properties have been identified in the dermis of human skin. These neural crest stem cell (NCSC)-like cells display self-renewal capacity and differentiate into neural crest derivatives, including epidermal pigment-producing melanocytes. NCSC-like cells share many properties with aggressive melanoma cells, such as high migratory capabilities and expression of the neural crest markers. However, little is known about which intrinsic or extrinsic signals determine the proliferation or differentiation of these neural crest-like stem cells. Here we show that, in NCSC-like cells, Notch signaling is highly activated, similar to melanoma cells. Inhibition of Notch signaling reduced the proliferation of NCSC-like cells, induced cell death, and downregulated noncanonical Wnt5a, suggesting that the Notch pathway contributes to the maintenance and motility of these stem cells. In three-dimensional skin reconstructs, canonical Wnt signaling promoted the differentiation of NCSC-like cells into melanocytes. This differentiation was triggered by the endogenous Notch inhibitor Numb, which is upregulated in the stem cells by Wnt7a derived from UV-irradiated keratinocytes. Together, these data reveal a cross talk between the two conserved developmental pathways in postnatal human skin, and highlight the role of the skin microenvironment in specifying the fate of stem cells.

Interplay between microRNAs and WNT/β-catenin signalling pathway regulates epithelial-mesenchymal transition in cancer

The WNT/β-catenin signalling implies its significance in maintaining an epithelial cell phenotype, proper cell-cell junctions, and tissue homeostasis. Dysregulation of the members of this pathway involves in the development of cancer and an epithelial-mesenchymal transition (EMT) required for metastasis. Loss of E-cadherin is the major contributor to an EMT process and is largely influenced by the WNT/β-catenin signalling. An E-cadherin/β-catenin complex maintains epithelial integrity and disturbance of this complex and WNT/β-catenin pathway will ultimately lead to the nuclear translocation of β-catenin and transcription of EMT-promoting genes. WNT/β-catenin signalling is controlled by microRNAs (miRNAs), several of which are either up- or downregulated during EMT. The strong association between the expression of the WNT signalling components with miRNAs in the initiation and achievement of an EMT phenotype is suggestive of introducing these miRNAs as therapeutic targets against metastatic tumours. Therefore, this review aims to describe these putative miRNAs in altering the WNT/β-catenin signalling in EMT, and whether targeting them is a useful therapeutic option for human invasive tumours.

Activating the Wnt/β-Catenin Pathway for the Treatment of Melanoma--Application of LY2090314, a Novel Selective Inhibitor of Glycogen Synthase Kinase-3

It has previously been observed that a loss of β-catenin expression occurs with melanoma progression and that nuclear β-catenin levels are inversely proportional to cellular proliferation, suggesting that activation of the Wnt/β-catenin pathway may provide benefit for melanoma patients. In order to further probe this concept we tested LY2090314, a potent and selective small-molecule inhibitor with activity against GSK3α and GSK3β isoforms. In a panel of melanoma cell lines, nM concentrations of LY2090314 stimulated TCF/LEF TOPFlash reporter activity, stabilized β-catenin and elevated the expression of Axin2, a Wnt responsive gene and marker of pathway activation. Cytotoxicity assays revealed that melanoma cell lines are very sensitive to LY2090314 in vitro (IC50 ~10 nM after 72hr of treatment) in contrast to other solid tumor cell lines (IC50 >10 uM) as evidenced by caspase activation and PARP cleavage. Cell lines harboring mutant B-RAF or N-RAS were equally sensitive to LY2090314 as were those with acquired resistance to the BRAF inhibitor Vemurafenib. shRNA studies demonstrated that β-catenin stabilization is required for apoptosis following treatment with the GSK3 inhibitor since the sensitivity of melanoma cell lines to LY290314 could be overcome by β-catenin knockdown. We further demonstrate that in vivo, LY2090314 elevates Axin2 gene expression after a single dose and produces tumor growth delay in A375 melanoma xenografts with repeat dosing. The activity of LY2090314 in preclinical models suggests that the role of Wnt activators for the treatment of melanoma should be further explored.

Wnt signaling regulates the stemness of lung cancer stem cells and its inhibitors exert anticancer effect on lung cancer SPC-A1 cells

Wnt signaling plays an important role in regulating the activity of cancer stem cells (CSCs) in a variety of cancers. In this study, we explored the role of Wnt signaling in the lung cancer stem cells (LCSCs). LCSCs were obtained by sphere culture, for which human lung adenocarcinoma cell line SPC-A1 was treated with IGF, EGF and FGF-10. The stemness of LCSCs was confirmed by immunofluorescence, and pathway analysis was performed by functional genome screening and RT-PCR. The relationship between the identified signaling pathway and the expression of the stemness genes was explored by agonist/antagonist assay. Moreover, the effects of different signaling molecule inhibitors on sphere formation, cell viability and colony formation were also analyzed. The results showed that LCSCs were successfully generated as they expressed pluripotent stem cell markers Nanog and Oct 4, and lung distal epithelial markers CCSP and SP-C, by which the phenotype characterization of stem cells can be confirmed. The involvement of Wnt pathway in LCSCs was identified by functional genome screening and verified by RT-PCR. The expression of Wnt signaling components was closely related to the expression of the Nanog and Oct 4. Furthermore, targeting Wnt signaling pathway by using different signaling molecule inhibitors can exert anticancer effects. In conclusion, Wnt signaling pathway is involved in the stemness regulation of LCSCs and might be considered as a potential therapeutic target in lung adenocarcinoma.

Control of the interface between heterotypic cell populations reveals the mechanism of intercellular transfer of signaling proteins

Direct intercellular transfer of cellular components is a recently described general mechanism of cell–cell communication. It is a more non-specific mode of intercellular communication that is not actively controlled by the participating cells. Though membrane bound proteins and small non-protein cytosolic components have been shown to be transferred between cells, the possibility of transfer of cytosolic proteins has not been clearly established, and its mechanism remains unexplained. Using a cell–cell pair of metastatic melanoma and endothelial cells, known to interact at various stages during cancer progression, we show that cytosolic proteins can indeed be transferred between heterotypic cells. Using precise relative cell patterning we provide evidence that this transfer depends on extent of the interface between heterotypic cell populations. This result is further supported by a mathematical model capturing various experimental conditions. We further demonstrate that cytosolic protein transfer can have important functional consequences for the tumor–stroma interactions, e.g., in heterotypic transfer of constitutively activated BRAF, a common melanoma associated mutation, leading to an enhanced activation of the downstream MAPK pathway. Our results suggest that cytosolic protein transfer can have important consequences for regulation of processes involving physical co-location of heterotypic cell types, particularly in invasive cancer growth.

Cancer stem cells in basic science and in translational oncology: can we translate into clinical application?

Since their description and identification in leukemias and solid tumors, cancer stem cells (CSC) have been the subject of intensive research in translational oncology. Indeed, recent advances have led to the identification of CSC markers, CSC targets, and the preclinical and clinical evaluation of the CSC-eradicating (curative) potential of various drugs. However, although diverse CSC markers and targets have been identified, several questions remain, such as the origin and evolution of CSC, mechanisms underlying resistance of CSC against various targeted drugs, and the biochemical basis and function of stroma cell-CSC interactions in the so-called ‘stem cell niche.’ Additional aspects that have to be taken into account when considering CSC elimination as primary treatment-goal are the genomic plasticity and extensive subclone formation of CSC. Notably, various cell fractions with different combinations of molecular aberrations and varying proliferative potential may display CSC function in a given neoplasm, and the related molecular complexity of the genome in CSC subsets is considered to contribute essentially to disease evolution and acquired drug resistance. In the current article, we discuss new developments in the field of CSC research and whether these new concepts can be exploited in clinical practice in the future.

Soma influences GSC progeny differentiation via the cell adhesion-mediated steroid-let-7-Wingless signaling cascade that regulates chromatin dynamics

It is known that signaling from the germline stem cell niche is required to maintain germline stem cell identity in Drosophila. However, it is not clear whether the germline stem-cell daughters differentiate by default (because they are physically distant from the niche) or whether additional signaling is necessary to initiate the differentiation program. Previously, we showed that ecdysteroid signaling cell non-autonomously regulates early germline differentiation via its soma-specific co-activator and co-repressor, Taiman and Abrupt. Now, we demonstrate that this regulation is modulated by the miRNA let-7, which acts in a positive feedback loop to confer ecdysone signaling robustness via targeting its repressor, the transcription factor Abrupt. This feedback loop adjusts ecdysteroid signaling in response to some stressful alterations in the external and internal conditions, which include temperature stress and aging, but not nutritional deprivation. Upon let-7 deficit, escort cells fail to properly differentiate: their shape, division, and cell adhesive characteristics are perturbed. These cells have confused cellular identity and form columnar-like rather than squamous epithelium and fail to send protrusions in between differentiating germline cysts, affecting soma-germline communication. Particularly, levels of the homophilic cell adhesion protein Cadherin, which recruits Wg signaling transducer β-catenin, are increased in mutant escort cells and, correspondingly, in the adjacent germline cells. Readjustment of heterotypic (soma-germline) cell adhesion modulates Wg signaling intensity in the germline, which in turn regulates histone modifications that promote expression of the genes necessary to trigger early germline differentiation. Thus, our data first show the intrinsic role for Wg signaling in the germline and support a model where the soma influences the tempo of germline differentiation in response to external conditions.

SMK-17, a MEK1/2-specific inhibitor, selectively induces apoptosis in β-catenin-mutated tumors

Although clinical studies have evaluated several MEK1/2 inhibitors, it is unlikely that MEK1/2 inhibitors will be studied clinically. BRAF mutations have been proposed as a responder marker of MEK1/2 inhibitors in a preclinical study. However, current clinical approaches focusing on BRAF mutations have shown only moderate sensitivity of MEK1/2 inhibitors. This has led to insufficient support for their promoted clinical adoption. Further characterization of tumors sensitive to MEK inhibitors holds great promise for optimizing drug therapy for patients with these tumors. Here, we report that β-catenin mutations accelerate apoptosis induced by MEK1/2 inhibitor. SMK-17, a selective MEK1/2 inhibitor, induced apoptosis in tumor cell lines harboring β-catenin mutations at its effective concentration. To confirm that β-catenin mutations and mutant β-catenin-mediated TCF7L2 (also known as TCF4) transcriptional activity is a predictive marker of MEK inhibitors, we evaluated the effects of dominant-negative TCF7L2 and of active, mutated β-catenin on apoptosis induced by MEK inhibitor. Indeed, dominant-negative TCF7L2 reduced apoptosis induced by MEK inhibitor, whereas active, mutated β-catenin accelerated it. Our findings show that β-catenin mutations are an important responder biomarker for MEK1/2 inhibitors.

MITF drives endolysosomal biogenesis and potentiates Wnt signaling in melanoma cells

Canonical Wnt signaling plays an important role in development and disease, regulating transcription of target genes and stabilizing many proteins phosphorylated by glycogen synthase kinase 3 (GSK3). We observed that the MiT family of transcription factors, which includes the melanoma oncogene MITF (micropthalmia-associated transcription factor) and the lysosomal master regulator TFEB, had the highest phylogenetic conservation of three consecutive putative GSK3 phosphorylation sites in animal proteomes. This finding prompted us to examine the relationship between MITF, endolysosomal biogenesis, and Wnt signaling. Here we report that MITF expression levels correlated with the expression of a large subset of lysosomal genes in melanoma cell lines. MITF expression in the tetracycline-inducible C32 melanoma model caused a marked increase in vesicular structures, and increased expression of late endosomal proteins, such as Rab7, LAMP1, and CD63. These late endosomes were not functional lysosomes as they were less active in proteolysis, yet were able to concentrate Axin1, phospho-LRP6, phospho-β-catenin, and GSK3 in the presence of Wnt ligands. This relocalization significantly enhanced Wnt signaling by increasing the number of multivesicular bodies into which the Wnt signalosome/destruction complex becomes localized upon Wnt signaling. We also show that the MITF protein was stabilized by Wnt signaling, through the novel C-terminal GSK3 phosphorylations identified here. MITF stabilization caused an increase in multivesicular body biosynthesis, which in turn increased Wnt signaling, generating a positive-feedback loop that may function during the proliferative stages of melanoma. The results underscore the importance of misregulated endolysosomal biogenesis in Wnt signaling and cancer.

Sox5 controls dorsal progenitor and interneuron specification in the spinal cord

The basic organization of somatosensory circuits in the spinal cord is already setup during the initial patterning of the dorsal neural tube. Extrinsic signals, such as Wnt and TGF-β pathways, activate combinatorial codes of transcription factors that are responsible for generating a pattern of discrete domains of dorsal progenitors (dp). These progenitors will give rise to distinct dorsal interneurons (dI). The Wnt/ βcatenin signaling pathway controls specification of dp/dI1-3 progenitors and interneurons. According to the current model in the field, Wnt/βcatenin activity seems to act in a graded fashion in the spinal cord, as different relative levels determine the identity of adjacent progenitors. However, it is not clear how this activity gradient is controlled and how the identities of dI1-3 are differentially regulated by Wnt signalling. We have determined that two SoxD transcription factors, Sox5 and Sox6, are expressed in restricted domains of dorsal progenitors in the neural tube. Using gain- and loss-of function approaches in chicken embryos, we have established that Sox5 controls cell fate specification of dp2 and dp3 progenitors and, as a result, controls the correct number of the corresponding dorsal interneurons (dI2 and dI3). Furthermore, Sox5 exerts its function by restricting dorsally Wnt signaling activity via direct transcriptional induction of the negative Wnt pathway regulator Axin2. By that way, Sox5 acts as a Wnt pathway modulator that contributes to sharpen the dorsal gradient of Wnt/βcatenin activity to control the distinction of two functionally distinct types of interneurons, dI2 and dI3 involved in the somatosensory relay.

WNT signaling: an emerging mediator of cancer cell metabolism?

WNT signaling was discovered in tumor models and has been recognized as a regulator of cancer development and progression for over 3 decades. Recent work has highlighted a critical role for WNT signaling in the metabolic homeostasis of mammals, where its misregulation has been heavily implicated in diabetes. While the majority of WNT metabolism research has focused on nontransformed tissues, the role of WNT in cancer metabolism remains underinvestigated. Cancer is also a metabolic disease where oncogenic signaling pathways regulate energy production and macromolecular synthesis to fuel rapidly proliferating tumors. This review highlights the emerging evidence for WNT signaling in the reprogramming of cancer cell metabolism and examines the role of these signaling pathways as mediators of tumor bioenergetics.

Updating the Wnt pathways

In the three decades since the discovery of the Wnt1 proto-oncogene in virus-induced mouse mammary tumours, our understanding of the signalling pathways that are regulated by the Wnt proteins has progressively expanded. Wnts are involved in an complex signalling network that governs multiple biological processes and cross-talk with multiple additional signalling cascades, including the Notch, FGF (fibroblast growth factor), SHH (Sonic hedgehog), EGF (epidermal growth factor) and Hippo pathways. The Wnt signalling pathway also illustrates the link between abnormal regulation of the developmental processes and disease manifestation. Here we provide an overview of Wnt-regulated signalling cascades and highlight recent advances. We focus on new findings regarding the dedicated Wnt production and secretion pathway with potential therapeutic targets that might be beneficial for patients with Wnt-related diseases.

Biological functions of casein kinase 1 isoforms and putative roles in tumorigenesis

Isoforms of the casein kinase 1 (CK1) family have been shown to phosphorylate key regulatory molecules involved in cell cycle, transcription and translation, the structure of the cytoskeleton, cell-cell adhesion and receptor-coupled signal transduction. They regulate key signaling pathways known to be critically involved in tumor progression. Recent results point to an altered expression or activity of different CK1 isoforms in tumor cells. This review summarizes the expression and biological function of CK1 family members in normal and malignant cells and the evidence obtained so far about their role in tumorigenesis.

Receptor heterodimerization as a novel mechanism for the regulation of Wnt/β-catenin signaling

The Wnt pathway plays important roles in multiple physiological and pathophysiological processes. Here, we report a novel mechanism that regulates the Wnt pathway through heterodimerization of the Wnt co-receptor low-density lipoprotein-receptor-related protein 6 (LRP6) and very low-density lipoprotein receptor (VLDLR); the latter belongs to the same protein family as LRP6 and was originally known as a receptor for lipoproteins. Knockdown of Vldlr expression elevated LRP6 protein levels and activated Wnt/β-catenin signaling, whereas overexpression of Vldlr suppressed Wnt signaling. Moreover, we demonstrate that the VLDLR ectodomain is essential and sufficient for inhibition of Wnt signaling. The VLDLR ectodomain accelerated internalization and degradation of LRP6 through heterodimerization with the LRP6 extracellular domain. Monoclonal antibodies specific for the VLDLR ectodomain blocked VLDLR-LRP6 heterodimerization, resulting in enhanced Wnt/β-catenin signaling in vitro and in vivo. Taken together, these findings suggest that heterodimerization of receptors in the membrane accelerates the turnover of LRP6, and represent a new mechanism for the regulation of Wnt/β-catenin signaling.

Profile of selumetinib and its potential in the treatment of melanoma

The mitogen-activated protein kinase (MAPK) pathway is a critical oncogenic driver signal in a number of malignancies. The discovery of activating mutations in the MAPK pathway has led to the development of MAPK pathway inhibitors. Selumetinib is a potent and selective inhibitor of MEK1 and MEK2, which are essential downstream molecules in the MAPK pathway. Several preclinical and clinical studies have demonstrated the promising antitumor activity of selumetinib. In this review, we discuss the MAPK pathway in melanoma and summarized data from preclinical and clinical studies of selumetinib for advanced melanoma.

Targeting the Wnt pathways for therapies

The Wnt/β-catenin signaling pathway is crucial in animal development from sponges to humans. Its activity in the adulthood is less general, with exceptions having huge medical importance. Namely, improper activation of this pathway is carcinogenic in many tissues, most notably in the colon, liver and the breast. On the other hand, the Wnt/β-catenin signaling must be re-activated in cases of tissue damage, and insufficient activation results in regeneration failure and degeneration. These both medically important implications are unified by the emerging importance of this signaling pathway in the control of proliferation of various types of stem cells, crucial for tissue regeneration and, in case of cancer stem cells – cancer progression and relapse. This article aims at briefly reviewing the current state of knowledge in the field of Wnt signaling, followed by a detailed discussion of current medical developments targeting distinct branches of the Wnt pathway for anti-cancer and pro-regeneration therapies.

Targeting the Wnt pathways for therapies

The Wnt/β-catenin signaling pathway is crucial in animal development from sponges to humans. Its activity in the adulthood is less general, with exceptions having huge medical importance. Namely, improper activation of this pathway is carcinogenic in many tissues, most notably in the colon, liver and the breast. On the other hand, the Wnt/β-catenin signaling must be re-activated in cases of tissue damage, and insufficient activation results in regeneration failure and degeneration. These both medically important implications are unified by the emerging importance of this signaling pathway in the control of proliferation of various types of stem cells, crucial for tissue regeneration and, in case of cancer stem cells - cancer progression and relapse. This article aims at briefly reviewing the current state of knowledge in the field of Wnt signaling, followed by a detailed discussion of current medical developments targeting distinct branches of the Wnt pathway for anti-cancer and pro-regeneration therapies.

Developmental pathways activated in melanocytes and melanoma

Cutaneous malignant melanomas originate primarily within epidermal melanocytic cells. Melanoma cells share many characteristics with melanocyte precursors, suggesting that melanoma cells utilize the developmental programs of their normal counterpart for their own progression. The pigmentation system provides an advantageous model to assess survival pathway interactions in the melanocytic lineage, as genetic alterations controlling melanocyte development can be easily detectable by coat color phenotype that do not affect the viability of an animal. By integrating combinatorial gene knockout approaches, cell-based assays and immunohistochemical observations, recent studies have illustrated several genes and pathways that play important roles both in melanocyte specification and maintenance and in melanoma formation and progression. We are reviewing those genes and pathways to understand the connection between normal and cancerous development and to reveal therapeutic potential of targeting developmental pathways for melanoma therapy.

6-Bromoindirubin-3'oxime (BIO) decreases proliferation and migration of canine melanoma cell lines

Despite recent therapeutic advances, malignant melanoma is an aggressive tumor in dogs and is associated with a poor outcome. Novel, targeted agents are necessary to improve survival. In this study, 6-bromoindirubin-3'-oxime (BIO), a serine/threonine kinase inhibitor with reported specificity for glycogen synthase kinase-3 beta (GSK-3β) inhibition, was evaluated in vitro in three canine melanoma cell lines (CML-10C2, UCDK9M2, and UCDK9M3) for β-catenin-mediated transcriptional activity, Axin2 gene and protein expression levels, cell proliferation, chemotoxicity, migration and invasion assays. BIO treatment of canine malignant melanoma cell lines at 5 µM for 72 h enhanced β-catenin-mediated transcriptional activity, suggesting GSK-3β inhibition, and reduced cell proliferation and migration. There were no significant effects on invasion, chemotoxicity, or apoptosis. The results suggest that serine/threonine kinases may be viable therapeutic targets for the treatment of canine malignant melanoma.

Can we safely target the WNT pathway?

WNT-β-catenin signalling is involved in a multitude of developmental processes and the maintenance of adult tissue homeostasis by regulating cell proliferation, differentiation, migration, genetic stability and apoptosis, as well as by maintaining adult stem cells in a pluripotent state. Not surprisingly, aberrant regulation of this pathway is therefore associated with a variety of diseases, including cancer, fibrosis and neurodegeneration. Despite this knowledge, therapeutic agents specifically targeting the WNT pathway have only recently entered clinical trials and none has yet been approved. This Review examines the problems and potential solutions to this vexing situation and attempts to bring them into perspective.

The genetic basis for inactivation of Wnt pathway in human osteosarcoma

BACKGROUND

Osteosarcoma is a highly genetically unstable tumor with poor prognosis. We performed microarray-based comparative genomic hybridization (aCGH), transcriptome sequencing (RNA-seq), and pathway analysis to gain a systemic view of the pathway alterations of osteosarcoma.

METHODS

aCGH experiments were carried out on 10 fresh osteosarcoma samples. The output data (Gene Expression Omnibus Series accession number GSE19180) were pooled with published aCGH raw data (GSE9654) to determine recurrent copy number changes. These were analyzed using Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis to identify altered pathways in osteosarcoma. Transcriptome sequencing of six osteosarcomas was performed to detect the expression profile of Wnt signaling pathway genes. Protein expression of WNT1, β-catenin, c-myc, and cyclin D1 in the Wnt pathway was detected by immunohistochemistry (IHC) in an independent group of 46 osteosarcoma samples.

RESULTS

KEGG pathway analysis identified frequent deletions of Wnt and other Wnt signaling pathway genes. At the mRNA level, transcriptome sequencing found reduced levels of mRNA expression of Wnt signaling pathway transcripts. While WNT1 protein expression was detected by IHC in 69.6% (32/46) of the osteosarcomas, no β-catenin protein was detected in the nucleus. β-catenin protein expression was, however, detected in the membrane and cytoplasm of 69.6% (32/46) of the osteosarcomas. c-myc protein expression was detected in only 47.8% (22/46) and cyclin D1 protein expression in 52.2% (24/46) of osteosarcoma samples. Kaplan-Meier survival analysis showed that WNT1-negative patients had a trend towards longer disease free survival than WNT1-positive patients. Interestingly, in WNT1-negative patients, those who were also cyclin D1-negative had significantly longer disease free survival than cyclin D1-positive patients. However, there was no significant association between any of the investigated proteins and overall survival of human osteosarcoma patients.

CONCLUSIONS

Frequent deletions of Wnt and other Wnt signaling pathway genes suggest that the Wnt signaling pathway is genetically inactivated in human osteosarcoma.

WNT5A enhances resistance of melanoma cells to targeted BRAF inhibitors

About half of all melanomas harbor a mutation that results in a constitutively active BRAF kinase mutant (BRAF(V600E/K)) that can be selectively inhibited by targeted BRAF inhibitors (BRAFis). While patients treated with BRAFis initially exhibit measurable clinical improvement, the majority of patients eventually develop drug resistance and relapse. Here, we observed marked elevation of WNT5A in a subset of tumors from patients exhibiting disease progression on BRAFi therapy. WNT5A transcript and protein were also elevated in BRAFi-resistant melanoma cell lines generated by long-term in vitro treatment with BRAFi. RNAi-mediated reduction of endogenous WNT5A in melanoma decreased cell growth, increased apoptosis in response to BRAFi challenge, and decreased the activity of prosurvival AKT signaling. Conversely, overexpression of WNT5A promoted melanoma growth, tumorigenesis, and activation of AKT signaling. Similarly to WNT5A knockdown, knockdown of the WNT receptors FZD7 and RYK inhibited growth, sensitized melanoma cells to BRAFi, and reduced AKT activation. Together, these findings suggest that chronic BRAF inhibition elevates WNT5A expression, which promotes AKT signaling through FZD7 and RYK, leading to increased growth and therapeutic resistance. Furthermore, increased WNT5A expression in BRAFi-resistant melanomas correlates with a specific transcriptional signature, which identifies potential therapeutic targets to reduce clinical BRAFi resistance.

Wnt inhibitory factor 1 suppresses cancer stemness and induces cellular senescence

Hyperactivation of the Wingless-type (Wnt)/β-catenin pathway promotes tumor initiation, tumor growth and metastasis in various tissues. Although there is evidence for the involvement of Wnt/β-catenin pathway activation in salivary gland tumors, the precise mechanisms are unknown. Here we report for the first time that downregulation of the Wnt inhibitory factor 1 (WIF1) is a widespread event in salivary gland carcinoma ex-pleomorphic adenoma (CaExPA). We also show that WIF1 downregulation occurs in the CaExPA precursor lesion pleomorphic adenoma (PA) and indicates a higher risk of progression from benign to malignant tumor. Our results demonstrate that diverse mechanisms including WIF1 promoter hypermethylation and loss of heterozygosity contribute to WIF1 downregulation in human salivary gland tumors. In accordance with a crucial role in suppressing salivary gland tumor progression, WIF1 re-expression in salivary gland tumor cells inhibited cell proliferation, induced more differentiated phenotype and promoted cellular senescence, possibly through upregulation of tumor-suppressor genes, such as p53 and p21. Most importantly, WIF1 significantly diminished the number of salivary gland cancer stem cells and the anchorage-independent cell growth. Consistent with this observation, WIF1 caused a reduction in the expression of pluripotency and stemness markers (OCT4 and c-MYC), as well as adult stem cell self-renewal and multi-lineage differentiation markers, such as WNT3A, TCF4, c-KIT and MYB. Furthermore, WIF1 significantly increased the expression of microRNAs pri-let-7a and pri-miR-200c, negative regulators of stemness and cancer progression. In addition, we show that WIF1 functions as a positive regulator of miR-200c, leading to downregulation of BMI1, ZEB1 and ZEB2, with a consequent increase in downstream targets such as E-cadherin. Our study emphasizes the prognostic and therapeutic potential of WIF1 in human salivary gland CaExPA. Moreover, our findings demonstrate a novel mechanism by which WIF1 regulates cancer stemness and senescence, which might have major implications in the field of cancer biology.

Rad6 is a Potential Early Marker of Melanoma Development

Melanoma is the leading cause of death from skin cancer in industrialized countries. Several melanoma-related biomarkers and signaling pathways have been identified; however, their relevance to melanoma development/progression or to clinical outcome remains to be established. Aberrant activation of Wnt/β-catenin pathway is implicated in various cancers including melanoma. We have previously demonstrated Rad6, an ubiquitin-conjugating enzyme, as an important mediator of β-catenin stability in breast cancer cells. Similar to breast cancer, β-catenin-activating mutations are rare in melanomas, and since β-catenin signaling is implicated in melanoma, we examined the relationship between β-catenin levels/activity and expression of β-catenin transcriptional targets Rad6 and microphthalmia-associated transcription factor-M (Mitf-M) in melanoma cell models, and expression of Rad6, β-catenin, and Melan-A in nevi and cutaneous melanoma tissue specimens. Our data show that Rad6 is only weakly expressed in normal human melanocytes but is overexpressed in melanoma lines. Unlike Mitf-M, Rad6 overexpression in melanoma lines is positively associated with high molecular weight β-catenin protein levels and β-catenin transcriptional activity. Double-immunofluorescence staining of Rad6 and Melan-A in melanoma tissue microarray showed that histological diagnosis of melanoma is significantly associated with Rad6/Melan-A dual positivity in the melanoma group compared to the nevi group (P=.0029). In contrast to strong β-catenin expression in normal and tumor areas of superficial spreading malignant melanoma (SSMM), Rad6 expression is undetectable in normal areas and Rad6 expression increases coincide with increased Melan-A in the transformed regions of SSMM. These data suggest a role for Rad6 in melanoma pathogenesis and that Rad6 expression status may serve as an early marker for melanoma development.

Melanoma Development and Progression Are Associated with Rad6 Upregulation and β -Catenin Relocation to the Cell Membrane

We have previously demonstrated that Rad6 and β -catenin enhance each other's expression through a positive feedback loop to promote breast cancer development/progression. While β -catenin has been implicated in melanoma pathogenesis, Rad6 function has not been investigated. Here, we examined the relationship between Rad6 and β -catenin in melanoma development and progression. Eighty-eight cutaneous tumors, 30 nevi, 29 primary melanoma, and 29 metastatic melanomas, were immunostained with anti- β -catenin and anti-Rad6 antibodies. Strong expression of Rad6 was observed in only 27% of nevi as compared to 100% of primary and 96% of metastatic melanomas. β -Catenin was strongly expressed in 97% of primary and 93% of metastatic melanomas, and unlike Rad6, in 93% of nevi. None of the tumors expressed nuclear β -catenin. β -Catenin was exclusively localized on the cell membrane of 55% of primary, 62% of metastatic melanomas, and only 10% of nevi. Cytoplasmic β -catenin was detected in 90% of nevi, 17% of primary, and 8% of metastatic melanoma, whereas 28% of primary and 30% of metastatic melanomas exhibited β -catenin at both locations. These data suggest that melanoma development and progression are associated with Rad6 upregulation and membranous redistribution of β -catenin and that β -catenin and Rad6 play independent roles in melanoma development.

Targeted BRAF inhibition impacts survival in melanoma patients with high levels of Wnt/β-catenin signaling

Unprecedented clinical responses have been reported in advanced stage metastatic melanoma patients treated with targeted inhibitors of constitutively activated mutant BRAF, which is present in approximately half of all melanomas. We and others have previously observed an association of elevated nuclear β-catenin with improved survival in molecularly-unselected melanoma patients. This study sought to determine whether levels of Wnt/β-catenin signaling in melanoma tumors prior to treatment might predict patient responses to BRAF inhibitors (BRAFi). We performed automated quantification of β-catenin immunohistochemical expression in pretreatment BRAF-mutant tumors from 32 BRAFi-treated melanoma patients. Unexpectedly, patients with higher nuclear β-catenin in their tumors did not exhibit the survival advantage previously observed in molecularly-unselected melanoma patients who did not receive BRAFi. In cultured melanoma cells treated with long-term BRAFi, activation of Wnt/β-catenin signaling is markedly inhibited, coinciding with a loss of the enhancement of BRAFi-induced apoptosis by WNT3A observed in BRAFi-naïve cells. Together, these observations suggest that long-term treatment with BRAFi can impact the interaction between BRAF/MAPK and Wnt/β-catenin signaling to affect patient outcomes. Studies with larger patient cohorts are required to determine whether nuclear β-catenin expression correlates with clinical responses to BRAFi and to specific mechanisms of acquired resistance to BRAFi. Understanding these pathway interactions will be necessary to facilitate efforts to individualize therapies for melanoma patients.

Potential of long-chain n-3 polyunsaturated fatty acids in melanoma prevention

The possible antineoplastic activity of dietary long-chain n-3 polyunsaturated fatty acids (LC n-3 PUFAs) has been supported by ample preclinical studies that have identified a number of molecular factors and pathways affected by these fatty acids and involved in cell growth, apoptosis, invasion, and angiogenesis. The aim of this critical review is to assess the current state of knowledge on the potential anticancer effects of LC n-3 PUFAs against malignant melanoma, one of the most common cancers among Western populations. The results of preclinical as well as human observational and interventional studies investigating the effects of LC n-3 PUFAs in melanoma were examined. Overall, the analysis of the literature reveals that, even though a large body of information is available, further effort is needed to identify the main molecular targets of LC n-3 PUFAs in melanoma. Moreover, additional well-designed human observational studies are essential to shed further light on the issue. The results of these studies could provide support and specific information for the development of clinical studies, especially those performed in combination with conventional or innovative antineoplastic therapies.

Survivin and Related Proteins in Canine Mammary Tumors

Survivin is reexpressed in most human breast cancers, where its expression has been associated with tumor aggressiveness, poor prognosis, and poor response to therapy. Survivin expression was evaluated in 41 malignant canine mammary tumors (CMTs) by immunohistochemistry, in relation to histological grade and stage, and correlated with that of some related molecules (β-catenin, caspase 3, heat shock proteins) to understand their possible role in canine mammary tumorigenesis. An increase in nuclear survivin expression, compared with healthy mammary glands, was observed in CMTs, where nuclear immunolabeling was related to the presence of necrosis. No statistically significant relation was found between the expression of the investigated molecules and the histological grade or stage. The present study may suggest an important involvement of survivin in CMT tumorigenesis. Its overexpression in most of the cases evaluated might suggest that targeting survivin in CMTs may be a valid anticancer therapy.

Glycogen synthase kinase 3 inhibitors induce the canonical WNT/β-catenin pathway to suppress growth and self-renewal in embryonal rhabdomyosarcoma

Embryonal rhabdomyosarcoma (ERMS) is a common pediatric malignancy of muscle, with relapse being the major clinical challenge. Self-renewing tumor-propagating cells (TPCs) drive cancer relapse and are confined to a molecularly definable subset of ERMS cells. To identify drugs that suppress ERMS self-renewal and induce differentiation of TPCs, a large-scale chemical screen was completed. Glycogen synthase kinase 3 (GSK3) inhibitors were identified as potent suppressors of ERMS growth through inhibiting proliferation and inducing terminal differentiation of TPCs into myosin-expressing cells. In support of GSK3 inhibitors functioning through activation of the canonical WNT/β-catenin pathway, recombinant WNT3A and stabilized β-catenin also enhanced terminal differentiation of human ERMS cells. Treatment of ERMS-bearing zebrafish with GSK3 inhibitors activated the WNT/β-catenin pathway, resulting in suppressed ERMS growth, depleted TPCs, and diminished self-renewal capacity in vivo. Activation of the canonical WNT/β-catenin pathway also significantly reduced self-renewal of human ERMS, indicating a conserved function for this pathway in modulating ERMS self-renewal. In total, we have identified an unconventional tumor suppressive role for the canonical WNT/β-catenin pathway in regulating self-renewal of ERMS and revealed therapeutic strategies to target differentiation of TPCs in ERMS.

High expression of the secreted protein dickkopf homolog 4: roles in invasion and metastasis of renal cell carcinoma and its association with Von Hippel-Lindau gene

The aim of this study was to investigate the effects of the dickkopf homolog 4 (DKK4)/Wnt/β-catenin signaling pathway on tumorigenesis and metastasis in clear cell renal cell carcinoma (ccRCC), as well as to elucidate the underlying mechanisms. We examined the expression of DKK4 in 30 cases of ccRCC and matched adjacent normal tissues, and investigated its correlation with clinicopathological characteristics. Stable DKK4-transfected cells were established, and DKK4 functional analyses were performed, including a T-cell factor/lymphoid enhancer factor (TCF/LEF) reporter assay, and experiments on cell viability, apoptosis, invasive capability and tumor growth in vivo. Finally, western blot analysis was performed to detect Von Hippel-Lindau (VHL) expression in 50 clinical specimens. The expression levels of the DKK4, β-catenin and β-catenin downstream target genes, cyclin D1 and c-myc, were determined in the these specimens, as well as in RCC4(-), T3-14(+) cell lines by qRT-PCR and western blot analysis. The same tests were also performed in human embryonic kidney (HEK)293 cells which were transfected with the pCDH-DKK4 plasmid. After 6 weeks the tumor weight significantly increased in the mice transfected with the tumor cells. DKK4 mRNA and protein expression levels were significantly upregulated (p<0.001). DKK4 was distinctly overexpressed (68.0%) in all patient tissues. VHL(-) samples accounted for 60.0% of all samples, while DKK4 expression was significantly upregulated in 50% of these samples, indicating a correlation with VHL(-) expression (r=0.403, p<0.05). We also observed reduced expression levels of cyclin D1, c-myc and β-catenin (to a greater extent) in the VHL(-), RCC4(-) and T3-14(+) cells, as well as in the stably transfected HEK293 cells. DKK4 may be an oncogene, and its upregulated expression may be involved in the pathogenesis of ccRCC as a downstream gene of VHL. By activating other pathways apart from the Wnt/β-catenin pathway, DKK4 may play an important role in ccRCC tumorigenesis and metastasis.

Wnt modulates MCL1 to control cell survival in triple negative breast cancer

BACKGROUND

Triple negative breast cancer (TNBC) has higher rates of recurrence and distant metastasis, and poorer outcome as compared to non-TNBC. Aberrant activation of WNT signaling has been detected in TNBC, which might be important for triggering oncogenic conversion of breast epithelial cell. Therefore, we directed our focus on identifying the WNT ligand and its underlying mechanism in TNBC cells.

METHODS

We performed large-scale analysis of public microarray data to screen the WNT ligands and the clinical significance of the responsible ligand in TNBC. WNT5B was identified and its overexpression in TNBC was confirmed by immunohistochemistry staining, Western blot and ELISA. ShRNA was used to knockdown WNT5B expression (shWNT5B). Cellular functional alteration with shWNT5B treatment was determined by using wound healing assay, mammosphere assay; while cell cycle and apoptosis were examined by flowcytometry. Mitochondrial morphology was photographed by electron microscope. Biological change of mitochondria was detected by RT-PCR and oxygen consumption assay. Activation of WNT pathway and its downstream targets were evaluated by liciferase assay, immunohistochemistry staining and immunoblot analysis. Statistical methods used in the experiments besides microarray analysis was two-tailed t-test.

RESULTS

WNT5B was elevated both in the tumor and the patients' serum. Suppression of WNT5B remarkably impaired cell growth, migration and mammosphere formation. Additionally, G0/G1 cell cycle arrest and caspase-independent apoptosis was observed. Study of the possible mechanism indicated that these effects occurred through suppression of mitochondrial biogenesis, as evidenced by reduced mitochondrial DNA (MtDNA) and compromised oxidative phosphorylation (OXPHOS). In Vivo and in vitro data uncovered that WNT5B modulated mitochondrial physiology was mediated by MCL1, which was regulated by WNT/β-catenin responsive gene, Myc. Clinic data analysis revealed that both WNT5B and MCL1 are associated with enhanced metastasis and decreased disease-free survival.

CONCLUSIONS

All our findings suggested that WNT5B/MCL1 cascade is critical for TNBC and understanding its regulatory apparatus provided valuable insight into the pathogenesis of the tumor development and the guidance for targeting therapeutics.

β-catenin inhibitor ICAT modulates the invasive motility of melanoma cells

Inhibitor of β-catenin and TCF (ICAT) inhibits β-catenin transcriptional activity by competing with T-cell factor/lymphoid enhancer factor. We documented high ICAT levels in human melanoma cells, in which β-catenin signaling is frequently deregulated, finding a correlation with the capacity to form metastases in nude mice. Ectopic expression of ICAT in melanoma cells did not affect their proliferation but increased cell motility and Matrigel invasion of metastatic cells in a manner relying upon stable ICAT-β-catenin interaction. This effect was associated with conversion of an elongated/mesenchymal phenotype to a round/amoeboid phenotype in the absence of similar effects on elongated morphology of nonmetastatic melanoma cells. Transition from mesenchymal to amoeboid movement was associated with decreased levels of NEDD9 and activated Rac1, a positive regulator of mesenchymal movement. Ectopic ICAT promoted colonization of melanoma cells in the lungs of nude mice, suggesting an increase in metastatic potential. Together, our results showed that by downregulating Rac signaling in metastatic melanoma cells, ICAT increased their invasive motility by promoting a morphologic variation that facilitates a favorable adaptation to their microenvironment.

Melanoma: from melanocyte to genetic alterations and clinical options

Metastatic melanoma remained for decades without any effective treatment and was thus considered as a paradigm of cancer resistance. Recent progress with understanding of the molecular mechanisms underlying melanoma initiation and progression revealed that melanomas are genetically and phenotypically heterogeneous tumors. This recent progress has allowed for the development of treatment able to improve for the first time the overall disease-free survival of metastatic melanoma patients. However, clinical responses are still either too transient or limited to restricted patient subsets. The complete cure of metastatic melanoma therefore remains a challenge in the clinic. This review aims to present the recent knowledge and discoveries of the molecular mechanisms involved in melanoma pathogenesis and their exploitation into clinic that have recently facilitated bench to bedside advances.

Hypoxia induces phenotypic plasticity and therapy resistance in melanoma via the tyrosine kinase receptors ROR1 and ROR2

An emerging concept in melanoma biology is that of dynamic, adaptive phenotype switching, where cells switch from a highly proliferative, poorly invasive phenotype to a highly invasive, less proliferative one. This switch may hold significant implications not just for metastasis, but also for therapy resistance. We demonstrate that phenotype switching and subsequent resistance can be guided by changes in expression of receptors involved in the noncanonical Wnt5A signaling pathway, ROR1 and ROR2. ROR1 and ROR2 are inversely expressed in melanomas and negatively regulate each other. Furthermore, hypoxia initiates a shift of ROR1-positive melanomas to a more invasive, ROR2-positive phenotype. Notably, this receptor switch induces a 10-fold decrease in sensitivity to BRAF inhibitors. In patients with melanoma treated with the BRAF inhibitor vemurafenib, Wnt5A expression correlates with clinical response and therapy resistance. These data highlight the fact that mechanisms that guide metastatic progression may be linked to those that mediate therapy resistance.

SIGNIFICANCE

These data show for the fi rst time that a single signaling pathway, the Wnt signaling pathway, can effectively guide the phenotypic plasticity of tumor cells, when primed to do so by a hypoxic microenvironment. Importantly, this increased Wnt5A signaling can give rise to a subpopulation of highly invasive cells that are intrinsically less sensitive to novel therapies for melanoma, and targeting the Wnt5A/ROR2 axis could improve the efficacy and duration of response for patients with melanoma on vemurafenib.

WNT5A-mediated β-catenin-independent signalling is a novel regulator of cancer cell metabolism

WNT5A has been identified as an important ligand in the malignant progression of a number of tumours. Although WNT5A signalling is often altered in cancer, the ligand’s role as either a tumour suppressor or oncogene varies between tumour types and is a contemporary issue for investigators of β-catenin-independent WNT signalling in oncology. Here, we report that one of the initial effects of active WNT5A signalling in malignant melanoma cells is an alteration in cellular energy metabolism and specifically an increase in aerobic glycolysis. This was found to be at least in part due to an increase in active Akt signalling and lactate dehydrogenase (LDH) activity. The clinical relevance of these findings was strengthened by a strong correlation (P < 0.001) between the expression of WNT5A and LDH isoform V in a cohort of melanocytic neoplasms. We also found effects of WNT5A on energy metabolism in breast cancer cells, but rather than promoting aerobic glycolysis as it does in melanoma, WNT5A signalling increased oxidative phosphorylation rates in breast cancer cells. These findings support a new role for WNT5A in the metabolic reprogramming of cancer cells that is a context- dependent event.

Essential role of the molecular chaperone gp96 in regulating melanogenesis

Through a process known as melanogenesis, melanocyte produces melanin in specialized organelles termed melanosomes, which regulates pigmentation of the skin, eyes, and hair. Gp96 is a constitutively expressed heat shock protein in the endoplasmic reticulum whose expression is further upregulated upon ultraviolet irradiation. However, the roles and mechanisms of this chaperone in pigmentation biology are unknown. In this study, we found that knockdown of gp96 by RNA interference significantly perturbed melanin synthesis and blocked late melanosome maturation. Gp96 knockdown did not impair the expression of tyrosinase, an essential enzyme in melanin synthesis, but compromised its catalytic activity and melanosome translocation. Further, mice with melanocyte-specific deletion of gp96 displayed decreased pigmentation. A mechanistic study revealed that the defect in melanogenesis can be rescued by activation of the canonical Wnt pathway, consistent with the critical roles of gp96 in chaperoning Wnt-coreceptor LRP6. Thus, this work uncovered the essential role of gp96 in regulating melanogenesis.

Pleiotropic function of SRY-related HMG box transcription factor 4 in regulation of tumorigenesis

In addition to their critical roles in embryonic development, cell fate decision, and differentiation, members of Sox (Sry-related high-mobility group box) family of transcription factors including Sox4 have been implicated in various cancers. Multiple studies have revealed an increased expression along with specific oncogenic function of Sox4 in tumors, while others observed a reduced expression of Sox4 in different types of malignancies and suppression of tumor initiation or progression by this protein. More interestingly, the prognostic value of Sox4 is debated due to obvious differences between various reports as well as inconsistencies within specific studies. This review summarizes our current understanding of Sox4 expression pattern and its transcription-dependent, as well as transcription-independent, functions in tumor initiation or progression and its correlation with patient survival. We also discuss the existing discrepancies between different reports and their possible explanations.

MITF, the Janus transcription factor of melanoma

Current models of melanoma propose that transition from the proliferative to the invasive stages of tumor development involves a dynamic and reversible switch in cell phenotype. The almost mutually exclusive proliferative and invasive phenotypes are defined by distinct gene expression signatures, which are themselves controlled by the level of functional MITF protein present in the cell. Recently, new signaling pathways and transcription factors that regulate MITF expression have been defined, and high throughput genomics have identified novel MITF target genes. MITF acts both as a transcription activator to promote expression of genes involved in cell cycle, but also as a transcriptional repressor of genes involved in invasion. A novel human germline mutation in MITF has been identified that blocks its sumoylation, thereby altering its transcriptional properties and conferring an increased risk of melanoma. These new studies depict an ever more complex function for MITF in melanoma.

Characterization of xanthatin: anticancer properties and mechanisms of inhibited murine melanoma in vitro and in vivo

Anti-cancer investigations on Xanthatin mainly focus on in vitro experiments. We herein reported the anti-tumor effects of Xanthatin both in vitro and in vivo. MTS assay results showed that Xanthatin had a remarkable anti-proliferative effect on B16-F10 cells. Moreover, the expression of β-catenin was up-regulated both in vitro and in vivo. Animal studies further revealed that Xanthatin killed the tumor cells around the blood vessels which contributes to reduce microvascular density extremely. All these results indicate that Xanthatin inhibited murine melanoma B16-F10 cell proliferation possibly associated with activation of Wnt/β-catenin pathway and its activity against melanoma tumor might also be relevant to inhibition of angiogenesis.

Activation of Wnt/β-catenin signaling increases apoptosis in melanoma cells treated with trail

While the TRAIL pathway represents a promising therapeutic target in melanoma, resistance to TRAIL-mediated apoptosis remains a barrier to its successful adoption. Since the Wnt/β-catenin pathway has been implicated in facilitating melanoma cell apoptosis, we investigated the effect of Wnt/β-catenin signaling on regulating the responses of melanoma cells to TRAIL. Co-treatment of melanoma cell lines with WNT3A-conditioned media and recombinant TRAIL significantly enhanced apoptosis compared to treatment with TRAIL alone. This apoptosis correlates with increased abundance of the pro-apoptotic proteins BCL2L11 and BBC3, and with decreased abundance of the anti-apoptotic regulator Mcl1. We then confirmed the involvement of the Wnt/β-catenin signaling pathway by demonstrating that siRNA-mediated knockdown of an intracellular β-catenin antagonist, AXIN1, or treating cells with an inhibitor of GSK-3 also enhanced melanoma cell sensitivity to TRAIL. These studies describe a novel regulation of TRAIL sensitivity in melanoma by Wnt/β-catenin signaling, and suggest that strategies to enhance Wnt/β-catenin signaling in combination with TRAIL agonists warrant further investigation.

FTY720 induces apoptosis in B16F10-NEX2 murine melanoma cells, limits metastatic development in vivo, and modulates the immune system

OBJECTIVE

Available chemotherapy presents poor control over the development of metastatic melanoma. FTY720 is a compound already approved by the Food and Drug Administration for the treatment of patients with multiple sclerosis. It has also been observed that FTY720 inhibits tumor growth in vivo (experimental models) and in vitro (animal and human tumor cells). The aim of this study was to evaluate the effects of FTY720 on a metastatic melanoma model and in tumor cell lines.

METHODS

We analyzed FTY720 efficacy in vivo in a syngeneic murine metastatic melanoma model, in which we injected tumor cells intravenously into C57BL/6 mice and then treated the mice orally with the compound for 7 days. We also treated mice and human tumor cell lines with FTY720 in vitro, and cell viability and death pathways were analyzed.

RESULTS

FTY720 treatment limited metastatic melanoma growth in vivo and promoted a dose-dependent decrease in the viability of murine and human tumor cells in vitro. Melanoma cells treated with FTY720 exhibited characteristics of programmed cell death, reactive oxygen species generation, and increased β-catenin expression. In addition, FTY720 treatment resulted in an immunomodulatory effect in vivo by decreasing the percentage of Foxp3+ cells, without interfering with CD8+ T cells or lymphocyte-producing interferon-gamma.

CONCLUSION

Further studies are needed using FTY720 as a monotherapy or in combined therapy, as different types of cancer cells would require a variety of signaling pathways to be extinguished.

Frizzled-8 receptor is activated by the Wnt-2 ligand in non-small cell lung cancer

Background

Wnt-2 plays an oncogenic role in cancer, but which Frizzled receptor(s) mediates the Wnt-2 signaling pathway in lung cancer remains unclear. We sought to (1) identify and evaluate the activation of Wnt-2 signaling through Frizzled-8 in non-small cell lung cancer, and (2) test whether a novel expression construct dominant negative Wnt-2 (dnhWnt-2) reduces tumor growth in a colony formation assay and in a xenograft mouse model.

Methods

Semi-quantitative RT-PCR was used to identify the expression of Wnt-2 and Frizzled-8 in 50 lung cancer tissues from patients. The TCF reporter assay (TOP/FOP) was used to detect the activation of the Wnt canonical pathway in vitro. A novel dnhWnt-2 construct was designed and used to inhibit activation of Wnt-2 signaling through Frizzled-8 in 293T, 293, A549 and A427 cells and in a xenograft mouse model. Statistical comparisons were made using Student’s t-test.

Results

Among the 50 lung cancer samples, we identified a 91% correlation between the transcriptional increase of Wnt-2 and Frizzled-8 (p<0.05). The Wnt canonical pathway was activated when both Wnt-2 and Frizzled-8 were co-expressed in 293T, 293, A549 and A427 cells. The dnhWnt-2 construct we used inhibited the activation of Wnt-2 signaling in 293T, 293, A549 and A427 cells, and reduced the colony formation of NSCLC cells when β-catenin was present (p<0.05). Inhibition of Wnt-2 activation by the dnhWnt-2 construct further reduced the size and mass of tumors in the xenograft mouse model (p<0.05). The inhibition also decreased the expression of target genes of Wnt signaling in these tumors.

Conclusions

We demonstrated an activation of Wnt-2 signaling via the Frizzled-8 receptor in NSCLC cells. A novel dnhWnt-2 construct significantly inhibits Wnt-2 signaling, reduces colony formation of NSCLC cells in vitro and tumor growth in a xenograft mouse model. The dnhWnt-2 construct may provide a new therapeutic avenue for targeting the Wnt pathway in lung cancer.