Tyrosine phosphorylation of LRP6 by Src and Fer inhibits Wnt/β-catenin signalling

Role of β-Catenin Activation in the Tumor Immune Microenvironment and Immunotherapy of Hepatocellular Carcinoma

Recently, the therapeutic combination of atezolizumab and bevacizumab was widely used to treat advanced hepatocellular carcinoma (HCC). According to recent clinical trials, immune checkpoint inhibitors (ICIs) and molecular target agents are expected to be key therapeutic strategies in the future. Nonetheless, the mechanisms underlying molecular immune responses and immune evasion remain unclear. The tumor immune microenvironment plays a vital role in HCC progression. The infiltration of CD8-positive cells into tumors and the expression of immune checkpoint molecules are key factors in this immune microenvironment. Specifically, Wnt/β catenin pathway activation causes "immune exclusion", associated with poor infiltration of CD8-positive cells. Some clinical studies suggested an association between ICI resistance and β-catenin activation in HCC. Additionally, several subclassifications of the tumor immune microenvironment were proposed. The HCC immune microenvironment can be broadly divided into inflamed class and non-inflamed class, with several subclasses. β-catenin mutations are important factors in immune subclasses; this may be useful when considering therapeutic strategies as β-catenin activation may serve as a biomarker for ICI. Various types of β-catenin modulators were developed. Several kinases may also be involved in the β-catenin pathway. Therefore, combinations of β-catenin modulators, kinase inhibitors, and ICIs may exert synergistic effects.

N-Glycosylation of LRP6 by B3GnT2 Promotes Wnt/β-Catenin Signalling

Reception of Wnt signals by cells is predominantly mediated by Frizzled receptors in conjunction with a co-receptor, the latter being LRP6 or LRP5 for the Wnt/β-catenin signalling pathway. It is important that cells maintain precise control of receptor activation events in order to properly regulate Wnt/β-catenin signalling as aberrant signalling can result in disease in humans. Phosphorylation of the intracellular domain (ICD) of LRP6 is well known to regulate Wntβ-catenin signalling; however, less is known for regulatory post-translational modification events within the extracellular domain (ECD). Using a cell culture-based expression screen for functional regulators of LRP6, we identified a glycosyltransferase, B3GnT2-like, from a teleost fish (medaka) cDNA library, that modifies LRP6 and regulates Wnt/β-catenin signalling. We provide both gain-of-function and loss-of-function evidence that the single human homolog, B3GnT2, promotes extension of polylactosamine chains at multiple N-glycans on LRP6, thereby enhancing trafficking of LRP6 to the plasma membrane and promoting Wnt/β-catenin signalling. Our findings further highlight the importance of LRP6 as a regulatory hub in Wnt signalling and provide one of the few examples of how a specific glycosyltransferase appears to selectively target a signalling pathway component to alter cellular signalling events.

Src heterodimerically activates Lyn or Fyn to serve as targets for the diagnosis and treatment of esophageal squamous cell carcinoma

Although Src is one of the oldest and most investigated oncoproteins, its function in tumor malignancy remains to be defined further. In this study, we demonstrated that the inhibition of Src activity by ponatinib effectively suppressed several malignant phenotypes of esophageal squamous cell carcinoma (ESCC) both in vitro and in vivo, whereas it did not produce growth-inhibitory effects on normal esophageal epithelial cells (NEECs). Importantly, we combined phosphoproteomics and several cellular and molecular biologic strategies to identify that Src interacted with the members of Src-family kinases (SFKs), such as Fyn or Lyn, to form heterodimers. Src interactions with Fyn and Lyn phosphorylated the tyrosine sites in SH2 (Fyn Tyr¹⁸⁵ or Lyn Tyr¹⁸³) and kinase domains (Fyn Tyr⁴²⁰ or Lyn Tyr³⁹⁷), which critically contributed to ESCC development. By contrast, Src could not form heterodimers with Fyn or Lyn in NEECs. We used RNA sequencing to comprehensively demonstrate that the inhibition of Src activity effectively blocked several critical tumor-promoting pathways, such as JAK/STAT, mTOR, stemness-related, and metabolism-related pathways. Results of the real-time polymerase chain reaction (RT-PCR) assay confirmed that Lyn and Fyn were critical effectors for the Src-mediated expression of tumor growth or metastasis-related molecules. Furthermore, results of the clinical ESCC samples showed that the hyperactivation of pSrc Tyr⁴¹⁹, Fyn Tyr¹⁸⁵ or Tyr⁴²⁰, and Lyn Tyr¹⁸³ or Tyr³⁹⁷ could be biomarkers of ESCC prognosis. This study illustrates that Src/Fyn and Src/Lyn heterodimers serve as targets for the treatment of ESCC.

circFBXO7/miR-96-5p/MTSS1 axis is an important regulator in the Wnt signaling pathway in ovarian cancer

BACKGROUND

CircRNAs are a novel class of evolutionarily conserved noncoding RNA molecules that form covalently closed continuous loop structures without 5' caps and 3' poly(A) tails. Accumulating evidence suggests that circRNAs play important regulatory roles in cancer and are promising biomarkers for cancer diagnosis and prognosis, as well as targets for cancer therapy. In this study, we identify and explore the role of a novel circRNA, circFBXO7, in ovarian cancer.

METHODS

rRNA-depleted RNA-sequencing was performed to identify differentially expressed circRNAs between ovarian cancerous and normal tissues. qRT-PCR and single-molecule RNA in-situ hybridization was used to quantify circFBXO7 expression in tumor tissues. The association of circFBXO7 expression with patient prognosis was evaluated by Kaplan-Meier survival analysis. The biological function of circFBXO7 was also investigated using loss-of-function and gain-of-function assays in vivo and in vitro. Luciferase reporter and TOP/FOP-Flash reporter assays were then conducted together with RNA immunoprecipitation and western blot to assess the circFBXO7/miR-96-5p/MTSS1/Wnt/β-catenin axis.

RESULTS

circFBXO7 was downregulated in ovarian cancer which was associated with poor prognosis. Biologically, circFBXO7 overexpression significantly suppressed ovarian cancer cell proliferation, migration, and invasion in vitro, and inhibited tumor growth and metastasis in vivo, whereas its knockdown exerted an opposite role. Mechanistically, circFBXO7 functioned as a competing endogenous RNA for miR-96-5p to regulate the expression of MTSS1. Consequently, downregulation of MTSS1 led to excessive accumulation of β-catenin and increased phosphorylation of GSK3β, leading to the translocation of β-catenin to the nucleus, thereby activating the Wnt/β-catenin signaling pathway and ultimately promoting ovarian cancer progression.

CONCLUSIONS

Our findings indicate that circFBXO7 acts as a bone fide tumor suppressor in ovarian cancer and that the circFBXO7/miR-96-5p/MTSS1 axis is an important regulator in the Wnt/β-catenin signaling pathway which may provide a promising target for ovarian cancer therapy.

Cross-Talk between Wnt Signaling and Src Tyrosine Kinase

Src, a non-receptor tyrosine kinase, was first discovered as a prototype oncogene and has been shown to critical for cancer progression for a variety of tissues. Src activity is regulated by a number of post-translational modifications in response to various stimuli. Phosphorylations of Src Tyr419 (human; 416 in chicken) and Src Tyr530 (human; 527 in chicken) have been known to be critical for activation and inactivation of Src, respectively. Wnt signaling regulates a variety of cellular functions including for development and cell proliferation, and has a role in certain diseases such as cancer. Wnt signaling is carried out through two pathways: β-catenin-dependent canonical and β-catenin-independent non-canonical pathways as Wnt ligands bind to their receptors, Frizzled, LRP5/6, and ROR1/2. In addition, many signaling components including Axin, APC, Damm, Dishevelled, JNK kinase and Rho GTPases contribute to these canonical and non-canonical Wnt pathways. However, the communication between Wnt signaling and Src tyrosine kinase has not been well reviewed as Src regulates Wnt signaling through LRP6 tyrosine phosphorylation. GSK-3β phosphorylated by Wnt also regulates Src activity. As Wnt signaling and Src mutually regulate each other, it is noted that aberrant regulation of these components give rise to various diseases including typically cancer, and as such, merit a closer look.

Src acts with WNT/FGFRL signaling to pattern the planarian anteroposterior axis

Tissue identity determination is crucial for regeneration, and the planarian anteroposterior (AP) axis uses positional control genes expressed from body wall muscle to determine body regionalization. Canonical Wnt signaling establishes anterior versus posterior pole identities through notum and wnt1 signaling, and two Wnt/FGFRL signaling pathways control head and trunk domains, but their downstream signaling mechanisms are not fully understood. Here, we identify a planarian Src homolog that restricts head and trunk identities to anterior positions. src-1(RNAi) animals formed enlarged brains and ectopic eyes and also duplicated trunk tissue, similar to a combination of Wnt/FGFRL RNAi phenotypes. src-1 was required for establishing territories of positional control gene expression in Schmidtea mediterranea, indicating that it acts at an upstream step in patterning the AP axis. Double RNAi experiments and eye regeneration assays suggest src-1 can act in parallel to at least some Wnt and FGFRL factors. Co-inhibition of src-1 with other posterior-promoting factors led to dramatic patterning changes and a reprogramming of Wnt/FGFRLs into controlling new positional outputs. These results identify src-1 as a factor that promotes robustness of the AP positional system that instructs appropriate regeneration.

Metastasis suppressor 1 controls osteoblast differentiation and bone homeostasis through regulating Src-Wnt/β-catenin signaling

Metastasis suppressor 1 (MTSS1) plays an inhibitory role in tumorigenesis and metastasis of a variety of cancers. To date, the function of MTSS1 in the differentiation of marrow stromal progenitor cells remains to be explored. In the current study, we investigated whether and how MTSS1 has a role in osteoblast differentiation and bone homeostasis. Our data showed that MTSS1 mRNA was upregulated during osteoblast differentiation and downregulated in the osteoblastic lineage cells of ovariectomized and aged mice. Functional studies revealed that MTSS1 promoted the osteogenic differentiation from marrow stromal progenitor cells. Mechanistic explorations uncovered that the inactivation of Src and afterward activation of canonical Wnt signaling were involved in osteoblast differentiation induced by MTSS1. The enhanced osteogenic differentiation induced by MTSS1 overexpression was attenuated when Src was simultaneously overexpressed, and conversely, the inhibition of osteogenic differentiation by MTSS1 siRNA was rescued when the Src inhibitor was supplemented to the culture. Finally, the in vivo transfection of MTSS1 siRNA to the marrow of mice significantly reduced the trabecular bone mass, along with the reduction of trabecular osteoblasts, the accumulation of marrow adipocytes, and the increase of phospho-Src-positive cells on the trabeculae. No change in the number of osteoclasts was observed. This study has unraveled that MTSS1 contributes to osteoblast differentiation and bone homeostasis through regulating Src-Wnt/β-catenin signaling. It also suggests the potential of MTSS1 as a new target for the treatment of osteoporosis.

Regulation of the Low-Density Lipoprotein Receptor-Related Protein LRP6 and Its Association With Disease: Wnt/β-Catenin Signaling and Beyond

Wnt signaling plays crucial roles in development and tissue homeostasis, and its dysregulation leads to various diseases, notably cancer. Wnt/β-catenin signaling is initiated when the glycoprotein Wnt binds to and forms a ternary complex with the Frizzled and low-density lipoprotein receptor-related protein 5/6 (LRP5/6). Despite being identified as a Wnt co-receptor over 20 years ago, the molecular mechanisms governing how LRP6 senses Wnt and transduces downstream signaling cascades are still being deciphered. Due to its role as one of the main Wnt signaling components, the dysregulation or mutation of LRP6 is implicated in several diseases such as cancer, neurodegeneration, metabolic syndrome and skeletal disease. Herein, we will review how LRP6 is activated by Wnt stimulation and explore the various regulatory mechanisms involved. The participation of LRP6 in other signaling pathways will also be discussed. Finally, the relationship between LRP6 dysregulation and disease will be examined in detail.

A MET-PTPRK kinase-phosphatase rheostat controls ZNRF3 and Wnt signaling

Zinc and ring finger 3 (ZNRF3) is a transmembrane E3 ubiquitin ligase that targets Wnt receptors for ubiquitination and lysosomal degradation. Previously, we showed that dephosphorylation of an endocytic tyrosine motif (4Y motif) in ZNRF3 by protein tyrosine phosphatase receptor-type kappa (PTPRK) promotes ZNRF3 internalization and Wnt receptor degradation (Chang et al 2020). However, a responsible protein tyrosine kinase(s) (PTK) phosphorylating the 4Y motif remained elusive. Here we identify the proto-oncogene MET (mesenchymal-epithelial transition factor) as a 4Y kinase. MET binds to ZNRF3 and induces 4Y phosphorylation, stimulated by the MET ligand HGF (hepatocyte growth factor, scatter factor). HGF-MET signaling reduces ZNRF3-dependent Wnt receptor degradation thereby enhancing Wnt/β-catenin signaling. Conversely, depletion or pharmacological inhibition of MET promotes the internalization of ZNRF3 and Wnt receptor degradation. We conclude that HGF-MET signaling phosphorylates- and PTPRK dephosphorylates ZNRF3 to regulate ZNRF3 internalization, functioning as a rheostat for Wnt signaling that may offer novel opportunities for therapeutic intervention.

Identification of Novel Variants in Cleft Palate-Associated Genes in Brazilian Patients With Non-syndromic Cleft Palate Only

The identification of genetic risk factors for non-syndromic oral clefts is of great importance for better understanding the biological processes related to this heterogeneous and complex group of diseases. Herein we applied whole-exome sequencing to identify potential variants related to non-syndromic cleft palate only (NSCPO) in the multiethnic Brazilian population. Thirty NSCPO samples and 30 sex- and genetic ancestry-matched healthy controls were pooled (3 pools with 10 samples for each group) and subjected to whole-exome sequencing. After filtering, the functional affects, individually and through interactions, of the selected variants and genes were assessed by bioinformatic analyses. As a group, 399 variants in 216 genes related to palatogenesis/cleft palate, corresponding to 6.43%, were exclusively identified in the NSCPO pools. Among those genes are 99 associated with syndromes displaying cleft palate in their clinical spectrum and 92 previously related to cleft lip palate. The most significantly biological processes and pathways overrepresented in the NSCPO-identified genes were associated with the folic acid metabolism, highlighting the interaction between LDL receptor-related protein 6 (LRP6) and 5-methyltetrahydrofolate-homocysteine methyltransferase (MTR) that interconnect two large networks. This study yields novel data on characterization of specific variants and complex processes and pathways related to NSCPO, including many variants in genes of the folate/homocysteine pathway, and confirms that variants in genes related to syndromic cleft palate and cleft lip-palate may cause NSCPO.

Dysregulation of Wnt/β-catenin signaling by protein kinases in hepatocellular carcinoma and its therapeutic application

Wnt/β-catenin signaling is indispensable for many biological processes, including embryonic development, cell cycle, inflammation, and carcinogenesis. Aberrant activation of the Wnt/β-catenin signaling can promote tumorigenicity and enhance metastatic potential in hepatocellular carcinoma (HCC). Targeting this pathway is a new opportunity for precise medicine for HCC. However, inhibiting Wnt/β-catenin signaling alone is unlikely to significantly improve HCC patient outcome due to the lack of specific inhibitors and the complexity of this pathway. Combination with other therapies will be an important next step in improving the efficacy of Wnt/β-catenin signaling inhibitors. Protein kinases play a key and evolutionarily conserved role in the Wnt/β-catenin signaling and have become one of the most important drug targets in cancer. Targeting Wnt/β-catenin signaling and its regulatory kinase together will be a promising HCC management strategy. In this review, we summarize the kinases that modulate the Wnt/β-catenin signaling in HCC and briefly discuss their molecular mechanisms. Furthermore, we list some small molecules that target the kinases and may inhibit Wnt/β-catenin signaling, to offer new perspectives for preclinical and clinical HCC studies.

Vangl2 promotes the formation of long cytonemes to enable distant Wnt/β-catenin signaling

Wnt signaling regulates cell proliferation and cell differentiation as well as migration and polarity during development. However, it is still unclear how the Wnt ligand distribution is precisely controlled to fulfil these functions. Here, we show that the planar cell polarity protein Vangl2 regulates the distribution of Wnt by cytonemes. In zebrafish epiblast cells, mouse intestinal telocytes and human gastric cancer cells, Vangl2 activation generates extremely long cytonemes, which branch and deliver Wnt protein to multiple cells. The Vangl2-activated cytonemes increase Wnt/β-catenin signaling in the surrounding cells. Concordantly, Vangl2 inhibition causes fewer and shorter cytonemes to be formed and reduces paracrine Wnt/β-catenin signaling. A mathematical model simulating these Vangl2 functions on cytonemes in zebrafish gastrulation predicts a shift of the signaling gradient, altered tissue patterning, and a loss of tissue domain sharpness. We confirmed these predictions during anteroposterior patterning in the zebrafish neural plate. In summary, we demonstrate that Vangl2 is fundamental to paracrine Wnt/β-catenin signaling by controlling cytoneme behaviour.

Wnt/β-catenin signaling: Structure, assembly and endocytosis of the signalosome

Wnt/β‐catenin signaling is an ancient pathway that regulates key aspects of embryonic development, cell differentiation, proliferation, and adult stem cell homeostasis. Work from different laboratories has shed light on the molecular mechanisms underlying the Wnt pathway, including structural details of ligand–receptor interactions. One key aspect that has emerged from multiple studies is that endocytosis of the receptor complex plays a crucial role in fine‐tuning Wnt/β‐catenin signaling. Endocytosis is a key process involved in both activation as well as attenuation of Wnt signaling, but how this is regulated is still poorly understood. Importantly, recent findings show that Wnt also regulates central metabolic pathways such as the acquisition of nutrients through actin‐driven endocytic mechanisms. In this review, we propose that the Wnt pathway displays diverse characteristics that go beyond the regulation of gene expression, through a connection with the endocytic machinery.

LRPs in WNT Signalling

The WNT/β-catenin signalling pathway is a rich and complex network of cellular proteins that orchestrates diverse short-range cell-to-cell communication in metazoans and is essential for both embryonic development and adult homeostasis. Due to its fundamental importance in controlling cell behaviour at multiple levels, its deregulation is associated with a wide range of diseases in humans and identification of drugs targeting the pathway has attracted strong interest in the pharmaceutical sector. Transduction of WNT signals across the plasma membrane of cells involves a staggering degree of complexity and variety with respect to ligand-receptor, receptor-receptor and receptor-co-receptor interactions (Niehrs, Nat Rev Mol Cell Biol 13:767-779, 2012). Although the low-density-lipoprotein-receptor-related-protein (LRP) family is best known for its role in binding and endocytosis of lipoproteins, specific members appear to have additional roles in cellular communication. Indeed, for WNT/β-catenin signalling one apparently universal requirement is the presence of either LRP5 or LRP6 in combination with one of the ten Frizzled (FZD) WNT receptors (FZD_1-10). In the 20 years since their discovery as WNT/FZD co-receptors, research on the LRP family has contributed greatly to our understanding of WNT signalling and LRPs have emerged as central players in WNT/β-catenin signalling. LRP5/6 are highly similar and represent the least redundant class of WNT receptor that transduce WNT/β-catenin signalling from a wide range of different WNT and FZD subtypes. This apparent simplicity however belies the complex arrangement of binding sites in the extracellular domain (ECD) of LRP5/6, which regulate interaction not only with WNTs but also with several inhibitors of WNT signalling. This chapter provides a historical overview, chronologically charting this remarkable progress in the field during the last 20 years of research on LRPs and their role in WNT/-catenin signalling. A more focused overview of the structural, functional and mechanistic aspects of LRP biology is also provided, together with the implications this has for pharmacological targeting of this notoriously intractable pathway.

RhoA GTPase phosphorylated at tyrosine 42 by src kinase binds to β-catenin and contributes transcriptional regulation of vimentin upon Wnt3A

In the Wnt canonical pathway, Wnt3A has been known to stabilize β-catenin. In the non-canonical Wnt signaling pathway, Wnt is known to activate Rho GTPases. The correlation between canonical and non-canonical pathways by Wnt signaling, however, has not been well elucidated. Here, we identified that Wnt3A promoted superoxide generation, leading to Tyr42 phosphorylation of RhoA through activations of c-Src and Rho-dependent coiled coil kinase 2 (ROCK2) and phosphorylation of p47phox, a component of NADPH oxidase. Wnt3A also induced accumulation of β-catenin along with activations of RhoA and ROCK1. Concurrently, ROCK1 was able to phosphorylate GSK-3β at Ser9, which phosphorylated Src at Ser51 and Ser492 residues, leading to Src inactivation through dephosphorylation of Tyr416 during the late period of Wnt3A treatment. Meanwhile, p-Tyr42 RhoA bound to β-catenin via the N-terminal domain of β-catenin, thereby leading to the nuclear translocation of p-Tyr42 RhoA/β-catenin complex. Notably, p-Tyr42 RhoA as well as β-catenin was associated with the promoter of Vim, leading to increased expression of vimentin. In addition, stomach cancer patients harboring higher expressed p-Tyr42 Rho levels revealed the much poorer survival probability. Therefore, we propose that p-Tyr42 RhoA is crucial for transcriptional regulation of specific target genes in the nucleus by binding to their promoters and involved in tumorigenesis.

Mask, a component of the Hippo pathway, is required for Drosophila eye morphogenesis

Hippo signaling is an important regulator of tissue size, but it also has a lesser-known role in tissue morphogenesis. Here we use the Drosophila pupal eye to explore the role of the Hippo effector Yki and its cofactor Mask in morphogenesis. We found that Mask is required for the correct distribution and accumulation of adherens junctions and appropriate organization of the cytoskeleton. Accordingly, disrupting mask expression led to severe mis-patterning and similar defects were observed when yki was reduced or in response to ectopic wts. Further, the patterning defects generated by reducing mask expression were modified by Hippo pathway activity. RNA-sequencing revealed a requirement for Mask for appropriate expression of numerous genes during eye morphogenesis. These included genes implicated in cell adhesion and cytoskeletal organization, a comprehensive set of genes that promote cell survival, and numerous signal transduction genes. To validate our transcriptome analyses, we then considered two loci that were modified by Mask activity: FER and Vinc, which have established roles in regulating adhesion. Modulating the expression of either locus modified mask mis-patterning and adhesion phenotypes. Further, expression of FER and Vinc was modified by Yki. It is well-established that the Hippo pathway is responsive to changes in cell adhesion and the cytoskeleton, but our data indicate that Hippo signaling also regulates these structures.

CDK11 negatively regulates Wnt/β-catenin signaling in the endosomal compartment by affecting microtubule stability

Objectives: Improper activation of Wnt/β-catenin signaling has been implicated in human diseases. Beyond the well-studied glycogen synthase kinase 3β (GSK3β) and casein kinase 1 (CK1), other kinases affecting Wnt/β-catenin signaling remain to be defined. Methods:To identify the kinases that modulate Wnt/β-catenin signaling, we applied a kinase small interfering RNA (siRNA) library screen approach. Luciferase assays, immunoblotting, and real-time polymerase chain reaction (PCR) were performed to confirm the regulation of the Wnt/β-catenin signaling pathway by cyclin-dependent kinase 11 (CDK11) and to investigate the underlying mechanism. Confocal immunofluorescence, coimmunoprecipitation (co-IP), and scratch wound assays were used to demonstrate colocalization, detect protein interactions, and explore the function of CDK11. Results: CDK11 was found to be a significant candidate kinase participating in the negative control of Wnt/β-catenin signaling. Down-regulation of CDK11 led to the accumulation of Wnt/β-catenin signaling receptor complexes, in a manner dependent on intact adenomatosis polyposis coli (APC) protein. Further analysis showed that CDK11 modulation of Wnt/β-catenin signaling engaged the endolysosomal machinery, and CDK11 knockdown enhanced the colocalization of Wnt/β-catenin signaling receptor complexes with early endosomes and decreased colocalization with lysosomes. Mechanistically, CDK11 was found to function in Wnt/β-catenin signaling by regulating microtubule stability. Depletion of CDK11 down-regulated acetyl-α-tubulin. Moreover, co-IP assays demonstrated that CDK11 interacts with the α-tubulin deacetylase SIRT2, whereas SIRT2 down-regulation in CDK11-depleted cells reversed the accumulation of Wnt/β-catenin signaling receptor complexes. CDK11 was found to suppress cell migration through altered Wnt/β-catenin signaling. Conclusions: CDK11 is a negative modulator of Wnt/β-catenin signaling that stabilizes microtubules, thus resulting in the dysregulation of receptor complex trafficking from early endosomes to lysosomes.

Src and Fyn define a new signaling cascade activated by canonical and non-canonical Wnt ligands and required for gene transcription and cell invasion

Wnt ligands signal through canonical or non-canonical signaling pathways. Although both routes share common elements, such as the Fz2 receptor, they differ in the co-receptor and in many of the final responses; for instance, whereas canonical Wnts increase β-catenin stability, non-canonical ligands downregulate it. However, both types of ligands stimulate tumor cell invasion. We show here that both the canonical Wnt3a and the non-canonical Wnt5a stimulate Fz2 tyrosine phosphorylation, Fyn binding to Fz2, Fyn activation and Fyn-dependent Stat3 phosphorylation. Wnt3a and Wnt5a require Src for Fz2 tyrosine phosphorylation; Src binds to canonical and non-canonical co-receptors (LRP5/6 and Ror2, respectively) and is activated by Wnt3a and Wnt5a. This Fz2/Fyn/Stat3 branch is incompatible with the classical Fz2/Dvl2 pathway as shown by experiments of over-expression or depletion. Fyn is necessary for transcription of genes associated with invasiveness, such as Snail1, and for activation of cell invasion by both Wnt ligands. Our results extend the knowledge about canonical Wnt pathways, demonstrating additional roles for Fyn in this pathway and describing how this protein kinase is activated by both canonical and non-canonical Wnts.

Global profiling of O-GlcNAcylated and/or phosphorylated proteins in hepatoblastoma

O-linked-β-N-acetylglucosamine (O-GlcNAc) glycosylation (O-GlcNAcylation) and phosphorylation are critical posttranslational modifications that are involved in regulating the functions of proteins involved in tumorigenesis and the development of various solid tumors. However, a detailed characterization of the patterns of these modifications at the peptide or protein level in hepatoblastoma (HB), a highly malignant primary hepatic tumor with an extremely low incidence in children, has not been performed. Here, we examined O-GlcNAc-modified or phospho-modified peptides and proteins in HB through quantitative proteomic analysis of HB tissues and paired normal liver tissues. Our results identified 114 O-GlcNAcylated peptides belonging to 78 proteins and 3494 phosphorylated peptides in 2088 proteins. Interestingly, 41 proteins were modified by both O-GlcNAcylation and phosphorylation. These proteins are involved in multiple molecular and cellular processes, including chromatin remodeling, transcription, translation, transportation, and organelle organization. In addition, we verified the accuracy of the proteomics results and found a competitive inhibitory effect between O-GlcNAcylation and phosphorylation of HSPB1. Further, O-GlcNAcylation modification of HSPB1 promoted proliferation and enhanced the chemotherapeutic resistance of HB cell lines in vitro. Collectively, our research suggests that O-GlcNAc-modified and/or phospho-modified proteins may play a crucial role in the pathogenesis of HB.

Intracellular Signals Activated by Canonical Wnt Ligands Independent of GSK3 Inhibition and β-Catenin Stabilization

In contrast to non-canonical ligands, canonical Wnts promote the stabilization of β-catenin, which is a prerequisite for formation of the TCF4/β-catenin transcriptional complex and activation of its target genes. This pathway is initiated by binding of Wnt ligands to the Frizzled/LRP5/6 receptor complex, and it increases the half-life of β-catenin by precluding the phosphorylation of β-catenin by GSK3 and its binding to the βTrCP1 ubiquitin ligase. Other intercellular signals are also activated by Wnt ligands that do not inhibit GSK3 and increase β-catenin protein but that either facilitate β-catenin transcriptional activity or stimulate other transcriptional factors that cooperate with it. In this review, we describe the layers of complexity of these signals and discuss their crosstalk with β-catenin in activation of transcriptional targets.

A Role for the WNT Co-Receptor LRP6 in Pathogenesis and Therapy of Epithelial Cancers

The WNT/β-catenin signaling pathway controls stem and progenitor cell proliferation, survival and differentiation in epithelial tissues. Aberrant stimulation of this pathway is therefore frequently observed in cancers from epithelial origin. For instance, colorectal and hepatic cancers display activating mutations in the CTNNB1 gene encoding β-catenin, or inactivating APC and AXIN gene mutations. However, these mutations are uncommon in breast and pancreatic cancers despite nuclear β-catenin localization, indicative of pathway activation. Notably, the low-density lipoprotein receptor-related protein 6 (LRP6), an indispensable co-receptor for WNT, is frequently overexpressed in colorectal, liver, breast and pancreatic adenocarcinomas in association with increased WNT/β -catenin signaling. Moreover, LRP6 is hyperphosphorylated in KRAS-mutated cells and in patient-derived colorectal tumours. Polymorphisms in the LRP6 gene are also associated with different susceptibility to developing specific types of lung, bladder and colorectal cancers. Additionally, recent observations suggest that LRP6 dysfunction may be involved in carcinogenesis. Indeed, reducing LRP6 expression and/or activity inhibits cancer cell proliferation and delays tumour growth in vivo. This review summarizes current knowledge regarding the biological function and regulation of LRP6 in the development of epithelial cancers—especially colorectal, liver, breast and pancreatic cancers.

Functional link between plasma membrane spatiotemporal dynamics, cancer biology, and dietary membrane-altering agents

The cell plasma membrane serves as a nexus integrating extra- and intracellular components, which together enable many of the fundamental cellular signaling processes that sustain life. In order to perform this key function, plasma membrane components assemble into well-defined domains exhibiting distinct biochemical and biophysical properties that modulate various signaling events. Dysregulation of these highly dynamic membrane domains can promote oncogenic signaling. Recently, it has been demonstrated that select membrane-targeted dietary bioactives (MTDBs) have the ability to remodel plasma membrane domains and subsequently reduce cancer risk. In this review, we focus on the importance of plasma membrane domain structural and signaling functionalities as well as how loss of membrane homeostasis can drive aberrant signaling. Additionally, we discuss the intricacies associated with the investigation of these membrane domain features and their associations with cancer biology. Lastly, we describe the current literature focusing on MTDBs, including mechanisms of chemoprevention and therapeutics in order to establish a functional link between these membrane-altering biomolecules, tuning of plasma membrane hierarchal organization, and their implications in cancer prevention.

Targeting FER Kinase Inhibits Melanoma Growth and Metastasis

Melanoma is one of the most aggressive types of tumors and exhibits high metastatic potential. Fes-related (FER) kinase is a non-receptor tyrosine kinase that has been implicated in growth and metastasis of various epithelial tumors. In this study, we have examined the role that FER kinase plays in melanoma at the molecular level. FER-depleted melanoma cells exhibit impaired Wnt/β-catenin pathway activity, as well as multiple proteomic changes, which include decreased abundance of L1-cell adhesion molecule (L1-CAM). Consistent with the pro-metastatic functions of these pathways, we demonstrate that depletion of FER kinase decreases melanoma growth and formation of distant metastases in a xenograft model. These findings indicate that FER is an important positive regulator of melanoma metastasis and a potential target for innovative therapies.

Wnt/PCP controls spreading of Wnt/β-catenin signals by cytonemes in vertebrates

Signaling filopodia, termed cytonemes, are dynamic actin-based membrane structures that regulate the exchange of signaling molecules and their receptors within tissues. However, how cytoneme formation is regulated remains unclear. Here, we show that Wnt/planar cell polarity (PCP) autocrine signaling controls the emergence of cytonemes, and that cytonemes subsequently control paracrine Wnt/β-catenin signal activation. Upon binding of the Wnt family member Wnt8a, the receptor tyrosine kinase Ror2 becomes activated. Ror2/PCP signaling leads to the induction of cytonemes, which mediate the transport of Wnt8a to neighboring cells. In the Wnt-receiving cells, Wnt8a on cytonemes triggers Wnt/β-catenin-dependent gene transcription and proliferation. We show that cytoneme-based Wnt transport operates in diverse processes, including zebrafish development, murine intestinal crypt and human cancer organoids, demonstrating that Wnt transport by cytonemes and its control via the Ror2 pathway is highly conserved in vertebrates.

Communication helps the cells that make up tissues and organs to work together as a team. One way that cells share information with each other as tissues grow and develop is by exchanging signaling proteins. These interact with receptors on the surface of other cells; this causes the cell to change how it behaves.

The Wnt family of signaling proteins orchestrate organ development. Wnt proteins influence which types of cells develop, how fast they divide, and how and when they move. Relatively few cells, or small groups of cells, in developing tissues produce Wnt proteins, while larger groups nearby respond to the signals.

We do not fully understand how Wnt proteins travel between cells, but recent work revealed an unexpected mechanism – cells seem to hand-deliver their messages. Finger-like structures called cytonemes grow out of the cell membrane and carry Wnt proteins to their destination. If the cytonemes do not form properly the target cells do not behave correctly, which can lead to severe tissue malformation.

Mattes et al. have now investigated how cytonemes form using a combination of state-of-the-art genetic and high-resolution imaging techniques. In initial experiments involving zebrafish cells that were grown in the laboratory, Mattes et al. found that the Wnt proteins kick start their own transport; before they travel to their destination, they act on the cells that made them. A Wnt protein called Wnt8a activates the receptor Ror2 on the surface of the signal-producing cell. Ror2 then triggers signals inside the cell that begin the assembly of the cytonemes. The more Ror2 is activated, the more cytonemes the cell makes, and the more Wnt signals it can send out.

This mechanism operates in various tissues: Ror2 also controls the cytoneme transport process in living zebrafish embryos, the mouse intestine and human stomach tumors. This knowledge will help researchers to develop new ways to control Wnt signaling, which could help to produce new treatments for diseases ranging from cancers (for example in the stomach and bowel) to degenerative diseases such as Alzheimer’s disease.

Toward an integrated map of genetic interactions in cancer cells

Cancer genomes often harbor hundreds of molecular aberrations. Such genetic variants can be drivers or passengers of tumorigenesis and create vulnerabilities for potential therapeutic exploitation. To identify genotype-dependent vulnerabilities, forward genetic screens in different genetic backgrounds have been conducted. We devised MINGLE, a computational framework to integrate CRISPR/Cas9 screens originating from different libraries building on approaches pioneered for genetic network discovery in model organisms. We applied this method to integrate and analyze data from 85 CRISPR/Cas9 screens in human cancer cells combining functional data with information on genetic variants to explore more than 2.1 million gene-background relationships. In addition to known dependencies, we identified new genotype-specific vulnerabilities of cancer cells. Experimental validation of predicted vulnerabilities identified GANAB and PRKCSH as new positive regulators of Wnt/β-catenin signaling. By clustering genes with similar genetic interaction profiles, we drew the largest genetic network in cancer cells to date. Our scalable approach highlights how diverse genetic screens can be integrated to systematically build informative maps of genetic interactions in cancer, which can grow dynamically as more data are included.

Whole proteome analysis of human tankyrase knockout cells reveals targets of tankyrase-mediated degradation

Tankyrase 1 and 2 are poly(ADP-ribose) polymerases that function in pathways critical to cancer cell growth. Tankyrase-mediated PARylation marks protein targets for proteasomal degradation. Here, we generate human knockout cell lines to examine cell function and interrogate the proteome. We show that either tankyrase 1 or 2 is sufficient to maintain telomere length, but both are required to resolve telomere cohesion and maintain mitotic spindle integrity. Quantitative analysis of the proteome of tankyrase double knockout cells using isobaric tandem mass tags reveals targets of degradation, including antagonists of the Wnt/β-catenin signaling pathway (NKD1, NKD2, and HectD1) and three (Notch 1, 2, and 3) of the four Notch receptors. We show that tankyrases are required for Notch2 to exit the plasma membrane and enter the nucleus to activate transcription. Considering that Notch signaling is commonly activated in cancer, tankyrase inhibitors may have therapeutic potential in targeting this pathway.

Tankyrase 1 and 2 are poly(ADP-ribose) polymerases that mark proteins for degradation, but there is a current lack of knowledge about their distinct functions and substrates. Here, the authors elucidate the cellular roles and substrates of these polymerases using comparative functional and proteomics analyses of tankyrase knockout cell lines.

Wnt Drug Discovery: Weaving Through the Screens, Patents and Clinical Trials

The Wnt signaling pathway is intricately involved in many aspects of development and is the root cause of an increasing number of diseases. For example, colorectal cancer is the second leading cause of death in the industrialized world and aberration of Wnt signaling within the colonic stem cell is the cause of more than 90% of these cancers. Despite our advances in successfully targeting other pathways, such as Human Epidermal Growth Factor Receptor 2 (HER2), there are no clinically relevant therapies available for Wnt-related diseases. Here, we investigated where research activities are focused with respect to Wnt signaling modulators by searching the United States Patent and Trade Office (USPTO) for patents and patent applications related to Wnt modulators and compared this to clinical trials focusing on Wnt modulation. We found that while the transition of intellectual property surrounding the Wnt ligand-receptor interface to clinical trials is robust, this is not true for specific inhibitors of β-catenin, which is constitutively active in many cancers. Considering the ubiquitous use of the synthetic T-cell Factor/Lymphoid Enhancer Factor (TCF/Lef) reporter system and its success in identifying novel modulators in vitro, we speculate that this model of drug discovery does not capture the complexity of in vivo Wnt signaling that may be required if we are to successfully target the Wnt pathway in the clinic. Notwithstanding, increasingly more complex models are being developed, which may not be high throughput, but more pragmatic in our pursuit to control Wnt signaling.

Wnt3a suppresses Wnt/β-catenin signaling and cancer cell proliferation following serum deprivation

Canonical Wnt/β-catenin signaling is often aberrantly activated in tumor cells and required for tumor growth. The internalization of Wnt co-receptor low-density lipoprotein receptor-related protein 6 (LRP6) induced by Wnt ligands is commonly thought to be critical for Wnt/β-catenin signaling activation. However, in contrast to theses previous studies, we here show that persistent excessive stimulation with a canonical Wnt ligand Wnt3a could induce a long-term decreased expression level of membrane LRP6, which prevented nuclear β-catenin accumulation and tumor cell;proliferation. Importantly, Wnt3a was robustly upregulated following serum deprivation. The upregulated Wnt3a under serum deprivation was responsible for LRP6 internalization, decreased accumulation of nuclear β-catenin, and further inhibition of tumor cell proliferation. It has well been known that insufficient blood supply during tumor development occurs frequently, causing a worsening environment for tumor growth. Therefore, these results reveal a novel inhibitory role of Wnt3a on canonical Wnt/β-catenin signaling and cancer cell proliferation when there is an insufficient blood supply during tumor development, which might be a potential mechanism for tumor evasion within a worsening environment.

Functional Crosstalk Between WNT Signaling and Tyrosine Kinase Signaling in Cancer

Extensive molecular characterization of tumors has revealed that the activity of multiple signaling pathways is often simultaneously dampened or enhanced in cancer cells. Aberrant WNT signaling and tyrosine kinase signaling are two pathways that are frequently up- or downregulated in cancer. Although signaling pathways regulated by WNTs, tyrosine kinases, and other factors are often conceptualized as independent entities, the biological reality is likely much more complex. Understanding the mechanisms of crosstalk between multiple signal transduction networks is a key challenge for cancer researchers. The overall goals of this review are to describe mechanisms of crosstalk between WNT and tyrosine kinase pathways in cancer and to discuss how understanding intersections between WNT and tyrosine kinase signaling networks might be exploited to improve current therapies.

Fresh WNT into the regulation of mitosis

Canonical Wnt signaling triggering β-catenin-dependent gene expression contributes to cell cycle progression, in particular at the G1/S transition. Recently, however, it became clear that the cell cycle can also feed back on Wnt signaling at the G2/M transition. This is illustrated by the fact that mitosis-specific cyclin-dependent kinases can phosphorylate the Wnt co-receptor LRP6 to prime the pathway for incoming Wnt signals when cells enter mitosis. In addition, there is accumulating evidence that various Wnt pathway components might exert additional, Wnt-independent functions that are important for proper regulation of mitosis. The importance of Wnt pathways during mitosis was most recently enforced by the discovery of Wnt signaling contributing to the stabilization of proteins other than β-catenin, specifically at G2/M and during mitosis. This Wnt-mediated stabilization of proteins, now referred to as Wnt/STOP, might on one hand contribute to maintaining a critical cell size required for cell division and, on the other hand, for the faithful execution of mitosis itself. In fact, most recently we have shown that Wnt/STOP is required for ensuring proper microtubule dynamics within mitotic spindles, which is pivotal for accurate chromosome segregation and for the maintenance of euploidy.

An aberrant nuclear localization of E-cadherin is a potent inhibitor of Wnt/β-catenin-elicited promotion of the cancer stem cell phenotype

Several studies suggest that Wnt signaling contributes to reprogramming and maintenance of cancer stem cell (CSC) states activated by loss of membranous E-cadherin expression. However, E-cadherin's exact role in Wnt/β-catenin-mediated promotion of the CSC phenotype remains unclear. Recently, a significant positive correlation has been observed between the expression of nuclear (an aberrant nuclear localization) E-cadherin and β-catenin in gastric and colorectal carcinomas. Here we conducted a series of in-vitro and in-vivo studies to show that the β-catenin/TCF4 interaction was abolished by E-cadherin and was correlated with its nuclear localization, and consequently decreased β-catenin/TCF4 transcriptional activity. Nuclear E-cadherin was a negative regulator of Wnt/β-Catenin-elicited promotion of the CSC phenotype. Using immunohistochemistry on lung cancer tissue microarrays, we found that changes in subcellular location of E-cadherin may be described by tumor grade and stage, suggesting cellular redistribution during lung tumorigenesis. Furthermore, nuclear E-cadherin expression was more significantly inversely correlated with CD133 (a lung CSC marker) expression (P<0.005) than total E-cadherin expression (P<0.05), suggesting that lung cancer as defined by nuclear E-cadherin^Low/nuclear β-catenin^High/CD133^High biomarkers has superior prognostic value over total E-cadherin^Low/nuclear β-catenin^High/CD133^High.