Processing of the human protocadherin Fat1 and translocation of its cytoplasmic domain to the nucleus

Recent advances in the role of atypical cadherin FAT1 in tumorigenesis (Review)

The FAT atypical cadherin 1 (FAT1) gene is the ortholog of the Drosophila fat gene and encodes the protocadherin FAT1. FAT1 belongs to the cadherin superfamily, a group of full-length membrane proteins that contain cadherin-like repeats. In various types of human cancer, FAT1 is one of the most commonly mutated genes, and is considered to be an emerging cancer biomarker and a potential target for novel therapies. However, the biological functions of FAT1 and the precise downstream signaling pathways that it mediates have remained to be fully elucidated. The present review discussed the current literature on FAT1, focusing on FAT1 mutations and expression levels, and their impact on signaling pathways and mechanisms in various types of cancer, including both solid tumors and hematological malignancies, such as esophageal squamous cell carcinoma, head and neck squamous cell carcinoma, lung squamous cell carcinoma, hepatocellular carcinoma, glioma, breast cancer, acute lymphoblastic leukemia, acute myeloid leukemia, lymphoma and myeloma. The present review aimed to provide further insights and research directions for future studies on FAT1 as an oncogenic factor or tumor suppressor.

Clinical value of FAT1 mutations to indicate the immune response in colorectal cancer patients

Immunotherapy is currently approved for CRC whose tumors have high MSI-H. To find additional biomarkers for immunotherapy in CRC, targeted sequencing was performed on tumor tissues from a discovery cohort of 161 CRC patients. Validation cohorts from the cBioPortal were also used for survival and tumor cell infiltration analyses. The FAT1-mutated CRC group often co-occurred with MSI events and displayed a higher tumor mutational burden compared to the FAT1 wild-type CRC. Overall survival was higher in patients with FAT1 mutations than in patients with wild type FAT1. The altered PI3K-AKT pathway and immune pathways were enriched in the FAT1-mutated CRC. A higher infiltration rate of immune cells including CD4+ T cells, CD8+ T cells, macrophages M1 and regulatory T cells were also observed in the colorectal tumors with FAT1 mutation compared to tumors with wild type FAT1. The results showed that CRC patients with FAT1 mutations exhibited an immunotherapy-favorable profile.

The FAT1 Cadherin Drives Vascular Smooth Muscle Cell Migration

Vascular smooth muscle cells (VSMCs) are normally quiescent and non-migratory, regulating the contraction and relaxation of blood vessels to control the vascular tone. In response to arterial injury, these cells become active; they proliferate, secrete matrix proteins, and migrate, and thereby contribute importantly to the progression of several cardiovascular diseases. VSMC migration specifically supports atherosclerosis, restenosis after catheter-based intervention, transplant vasculopathy, and vascular remodeling during the formation of aneurysms. The atypical cadherin FAT1 is expressed robustly in activated VSMCs and promotes their migration. A positive role of FAT1 in the migration of other cell types, including neurons, fibroblasts, podocytes, and astrocyte progenitors, has also been described. In cancer biology, however, the effect of FAT1 on migration depends on the cancer type or context, as FAT1 either suppresses or enhances cancer cell migration and invasion. With this review, we describe what is known about FAT1's effects on cell migration as well as the factors that influence FAT1-dependent migration. In VSMCs, these factors include angiotensin II, which activates FAT1 expression and cell migration, and proteins of the Atrophin family: Atrophin-1 and the short isoform of Atrophin-2, which promote VSMC migration, and the long isoform of Atrophin-2, which exerts negative effects on FAT1-dependent VSMC migration.

A pan-cancer analysis of the FAT1 in human tumors

FAT atypical cadherin 1 (FAT1) is one of the most mutagenic genes in tumors, and several critical studies have revealed its role in tumors, although no pan-cancer studies are currently available. Therefore, we explored the potential oncogenic role of FAT1 in 33 tumors based on The Cancer Genome Atlas and Gene Expression Omibus datasets. We found that FAT1 was strongly expressed in most tumors and significantly correlated with their prognosis. Additionally, we analyzed the association of FAT1 with tumors from multiple perspectives, including single-cell sequencing, mutations, high tumor mutational burden, microsatellite instability, immune cell infiltration, and immune microenvironment. Our first pan-cancer study provided a relatively comprehensive understanding of the oncogenic role of FAT1 in tumors.

Integrating Genetic Alterations and the Hippo Pathway in Head and Neck Squamous Cell Carcinoma for Future Precision Medicine

Genetic alterations and dysregulation of signaling pathways are indispensable for the initiation and progression of cancer. Understanding the genetic, molecular, and signaling diversities in cancer patients has driven a dynamic change in cancer therapy. Patients can select a suitable molecularly targeted therapy or immune checkpoint inhibitor based on the driver gene alterations determined by sequencing of cancer tissue. This “precision medicine” approach requires detailed elucidation of the mechanisms connecting genetic alterations of driver genes and aberrant downstream signaling pathways. The regulatory mechanisms of the Hippo pathway and Yes-associated protein/transcriptional co-activator with PDZ binding motif (YAP/TAZ) that have central roles in cancer cell proliferation are not fully understood, reflecting their recent discovery. Nevertheless, emerging evidence has shown that various genetic alterations dysregulate the Hippo pathway and hyperactivate YAP/TAZ in cancers, including head and neck squamous cell carcinoma (HNSCC). Here, we summarize the latest evidence linking genetic alterations and the Hippo pathway in HNSCC, with the aim of contributing to the continued development of precision medicine.

The Atypical Cadherin FAT1 Limits Mitochondrial Respiration and Proliferation of Vascular Smooth Muscle Cells

Smooth muscle cells contribute to cardiovascular disease, the leading cause of death worldwide. The capacity of these cells to undergo phenotypic switching in mature arteries of the systemic circulation underlies their pathogenic role in atherosclerosis and restenosis, among other vascular diseases. Growth factors and cytokines, extracellular matrix components, regulation of gene expression, neuronal influences, and mechanical forces contribute to smooth muscle cell phenotypic switching. Comparatively little is known about cell metabolism in this process. Studies of cancer and endothelial cell biology have highlighted the importance of cellular metabolic processes for phenotypic transitions that accompany tumor growth and angiogenesis. However, the understanding of cell metabolism during smooth muscle cell phenotypic modulation is incipient. Studies of the atypical cadherin FAT1, which is strongly upregulated in smooth muscle cells in response to arterial injury, suggest that it has important and distinctive functions in this context, mediating control of both smooth muscle cell mitochondrial metabolism and cell proliferation. Here we review the progress made in understanding how FAT1 affects the smooth muscle cell phenotype, highlighting the significance of FAT1 as a processed protein and unexpected regulator of mitochondrial respiration. These mechanisms suggest how a transmembrane protein may relay signals from the extracellular milieu to mitochondria to control metabolic activity during smooth muscle cell phenotypic switching.

Cell-cell adhesion regulates Merlin/NF2 interaction with the PAF complex

The PAF complex (PAFC) coordinates transcription elongation and mRNA processing and its CDC73/parafibromin subunit functions as a tumour suppressor. The NF2/Merlin tumour suppressor functions both at the cell cortex and nucleus and is a key mediator of contact inhibition but the molecular mechanisms remain unclear. In this study we have used affinity proteomics to identify novel Merlin interacting proteins and show that Merlin forms a complex with multiple proteins involved in RNA processing including the PAFC and the CHD1 chromatin remodeller. Tumour-derived inactivating mutations in both Merlin and the CDC73 PAFC subunit mutually disrupt their interaction and growth suppression by Merlin requires CDC73. Merlin interacts with the PAFC in a cell density-dependent manner and we identify a role for FAT cadherins in regulating the Merlin-PAFC interaction. Our results suggest that in addition to its function within the Hippo pathway, Merlin is part of a tumour suppressor network regulated by cell-cell adhesion which coordinates post-initiation steps of the transcription cycle of genes mediating contact inhibition.

Protocadherins at the Crossroad of Signaling Pathways

Protocadherins (Pcdhs) are cell adhesion molecules that belong to the cadherin superfamily, and are subdivided into clustered (cPcdhs) and non-clustered Pcdhs (ncPcdhs) in vertebrates. In this review, we summarize their discovery, expression mechanisms, and roles in neuronal development and cancer, thereby highlighting the context-dependent nature of their actions. We furthermore provide an extensive overview of current structural knowledge, and its implications concerning extracellular interactions between cPcdhs, ncPcdhs, and classical cadherins. Next, we survey the known molecular action mechanisms of Pcdhs, emphasizing the regulatory functions of proteolytic processing and domain shedding. In addition, we outline the importance of Pcdh intracellular domains in the regulation of downstream signaling cascades, and we describe putative Pcdh interactions with intracellular molecules including components of the WAVE complex, the Wnt pathway, and apoptotic cascades. Our overview combines molecular interaction data from different contexts, such as neural development and cancer. This comprehensive approach reveals potential common Pcdh signaling hubs, and points out future directions for research. Functional studies of such key factors within the context of neural development might yield innovative insights into the molecular etiology of Pcdh-related neurodevelopmental disorders.

FAT1 somatic mutations in head and neck carcinoma are associated with tumor progression and survival

In recent years, the incidence and mortality rates of head and neck squamous cell carcinoma (HNSCC) have increased worldwide. Therefore, understanding genomic alterations in HNSCC carcinogenesis is crucial for appropriate diagnosis and therapy. Protocadherin FAT1, which encodes 4588 amino acid residues, regulates complex mechanisms to promote oncogenesis or suppression of malignancies. Multiplex PCR-based next-generation sequencing (NGS) revealed FAT1 somatic mutations. The clinicopathologic implications of FAT1 in HNSCC were investigated using expression assays, and the functional role of FAT1 in HNSCC pathogenesis was determined using ectopic expression and knockdown experiments. Approximately 29% patients with HNSCC harbored damaging FAT1 mutations. InVEx algorithm identified FAT1 as a significant functional mutation burden. Each type of mutation (missense, nonsense and frameshift) accounted for nearly one-third of deleterious mutations. FAT1 mutations correlated with lower FAT1 expression in tumors. The knockdown of the endogenous expression of FAT1 and exogenous expression of crucial FAT1 domains unequivocally indicated that FAT1 suppressed the migration and invasion capability of HNSCC cells. Functional analysis suggested that nonsense mutations in FAT1 result in the loss of the suppression of tumor progression. FAT1 mutations and downregulation defined nodal involvement, lymphovascular permeation and tumor recurrence. In addition, FAT1 mutations and downregulation are independent predictors of poor disease-free survival in patients with HNSCC. This study is the first to perform multiplex PCR-based NGS to indicate marked non-synonymous FAT1 mutations in HNSCC, which are prognostic indicators. The gene analysis strategy proposed for detecting FAT1 mutations may be a valid method for mutation screening.

CDH13 promoter methylation regulates cisplatin resistance of non-small cell lung cancer cells

Reversing cisplatin resistance of lung cancer cell line A549/DDP through recovering cadherin 13 (CDH13) expression by demethylation was investigated in the current study. RT-PCR was used to measure CDH13 expression in lung cancer A549 and A549/DDP cells with or without 5-Aza-CdR intervention. Methylation-specific PCR was used to detect CDH13 methylation. MTT assay and flow cytometry were used to measure the effects of cisplatin on inhibiting cell proliferation, apoptosis, and the reversal of cisplatin resistance. The IC₅₀ value of cisplatin for A549 and A549/DDP cells was 3.278±0.532 and 28.341±1.435 µmol/l, respectively (P<0.05). The cisplatin-resistance index of A549/DDP cells was up to 8.65. After 2.5, 10, or 40 µmol/l 5-Aza-CdR treatment, the apoptotic rates of A549/DDP cells were 9.4±0.86, 18.1±1.42 and 42±2.01%, respectively, which were significantly different to those of the control group (P<0.05). Methylation-specific PCR detected both methylation (M) and unmethylation (U) bands at CDH13 promoter region before 5-Aza-CdR intervention while it only detected an unmethylation band after the treatment with a higher concentration of 5-Aza-CdR, which indicates the transformation to unmethylation state. When 10 µmol/l 5-Aza-CdR was added, the IC₅₀ of cisplatin to A549/DDP cells was 8.472±0.415 µmol/l, and cisplatin resistance was reversed by 3.35-fold. CDH13 methylation is related to the cisplatin resistance of A549/DDP cells. 5-Aza-CdR can inhibit CDH13 methylation and recover CDH13 expression. With the increase in 5-Aza-CdR concentration, the unmethylation state of CDH13 is enhanced, which can strengthen the function of cisplatin inhibiting proliferation and apoptosis in A549/DDP cells.

Exome sequencing and network analysis identifies shared mechanisms underlying spinocerebellar ataxia

The autosomal dominant cerebellar ataxias, referred to as spinocerebellar ataxias in genetic nomenclature, are a rare group of progressive neurodegenerative disorders characterized by loss of balance and coordination. Despite the identification of numerous disease genes, a substantial number of cases still remain without a genetic diagnosis. Here, we report five novel spinocerebellar ataxia genes, FAT2, PLD3, KIF26B, EP300, and FAT1, identified through a combination of exome sequencing in genetically undiagnosed families and targeted resequencing of exome candidates in a cohort of singletons. We validated almost all genes genetically, assessed damaging effects of the gene variants in cell models and further consolidated a role for several of these genes in the aetiology of spinocerebellar ataxia through network analysis. Our work links spinocerebellar ataxia to alterations in synaptic transmission and transcription regulation, and identifies these as the main shared mechanisms underlying the genetically diverse spinocerebellar ataxia types.

Configuring a robust nervous system with Fat cadherins

Atypical Fat cadherins represent a small but versatile group of signaling molecules that influence proliferation and tissue polarity. With huge extracellular domains and intracellular domains harboring many independent protein interaction sites, Fat cadherins are poised to translate local cell adhesion events into a variety of cell behaviors. The need for such global coordination is particularly prominent in the nervous system, where millions of morphologically diverse neurons are organized into functional networks. As we learn more about their biological functions and molecular properties, increasing evidence suggests that Fat cadherins mediate contact-induced changes that ultimately impose a structure to developing neuronal circuits.

SheddomeDB: the ectodomain shedding database for membrane-bound shed markers

BACKGROUND

A number of membrane-anchored proteins are known to be released from cell surface via ectodomain shedding. The cleavage and release of membrane proteins has been shown to modulate various cellular processes and disease pathologies. Numerous studies revealed that cell membrane molecules of diverse functional groups are subjected to proteolytic cleavage, and the released soluble form of proteins may modulate various signaling processes. Therefore, in addition to the secreted protein markers that undergo secretion through the secretory pathway, the shed membrane proteins may comprise an additional resource of noninvasive and accessible biomarkers. In this context, identifying the membrane-bound proteins that will be shed has become important in the discovery of clinically noninvasive biomarkers. Nevertheless, a data repository for biological and clinical researchers to review the shedding information, which is experimentally validated, for membrane-bound protein shed markers is still lacking.

RESULTS

In this study, the database SheddomeDB was developed to integrate publicly available data of the shed membrane proteins. A comprehensive literature survey was performed to collect the membrane proteins that were verified to be cleaved or released in the supernatant by immunological-based validation experiments. From 436 studies on shedding, 401 validated shed membrane proteins were included, among which 199 shed membrane proteins have not been annotated or validated yet by existing cleavage databases. SheddomeDB attempted to provide a comprehensive shedding report, including the regulation of shedding machinery and the related function or diseases involved in the shedding events. In addition, our published tool ShedP was embedded into SheddomeDB to support researchers for predicting the shedding event on unknown or unrecorded membrane proteins.

CONCLUSIONS

To the best of our knowledge, SheddomeDB is the first database for the identification of experimentally validated shed membrane proteins and currently may provide the most number of membrane proteins for reviewing the shedding information. The database included membrane-bound shed markers associated with numerous cellular processes and diseases, and some of these markers are potential novel markers because they are not annotated or validated yet in other databases. SheddomeDB may provide a useful resource for discovering membrane-bound shed markers. The interactive web of SheddomeDB is publicly available at http://bal.ym.edu.tw/SheddomeDB/ .

FAT1: a potential target for monoclonal antibody therapy in colon cancer

Background:

Colorectal cancer (CRC) is one of the major causes of cancer-associated mortality worldwide. The currently approved therapeutic agents have limited efficacy.

Methods:

The atypical cadherin FAT1 was discovered as a novel CRC-associated protein by using a monoclonal antibody (mAb198.3). FAT1 expression was assessed in CRC cells by immunohistochemistry (IHC), immunoblots, flow cytometry and confocal microscopy. In addition, in vitro and in vivo tumour models were done to assess FAT1 potential value for therapeutic applications.

Results:

The study shows that FAT1 is broadly expressed in primary and metastatic CRC stages and detected by mAb198.3, regardless of KRAS and BRAF mutations. FAT1 mainly accumulates at the plasma membrane of cancer cells, whereas it is only marginally detected in normal human samples. Moreover, the study shows that FAT1 has an important role in cell invasiveness while it does not significantly influence apoptosis. mAb198.3 specifically recognises FAT1 on the surface of colon cancer cells and is efficiently internalised. Furthermore, it reduces cancer growth in a colon cancer xenograft model.

Conclusions:

This study provides evidence that FAT1 and mAb198.3 may offer new therapeutic opportunities for CRC including the tumours resistant to current EGFR-targeted therapies.

FAT4 functions as a tumour suppressor in gastric cancer by modulating Wnt/β-catenin signalling

Background:

FAT4, a cadherin-related protein, was shown to function as a tumour suppressor; however, its role in human gastric cancer remains largely unknown. Here, we investigated the role of FAT4 in gastric cancer and examined the underlying molecular mechanisms.

Methods:

The expression of FAT4 was evaluated by immunohistochemistry, western blotting, and qRT–PCR in relation to the clinicopathological characteristics of gastric cancer patients. The effects of FAT4 silencing on cell proliferation, migration, and invasion were assessed by the MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium) assay, and migration and invasion assays in gastric cancer cell lines in vitro and in a mouse xenograft model in vivo.

Results:

Downregulation of FAT4 expression in gastric cancer tissues compared with adjacent normal tissues was correlated with lymph-node metastasis and poor survival. Knockdown of FAT4 promoted the growth and invasion of gastric cancer cells via the activation of Wnt/β-catenin signalling, and induced epithelial-to-mesenchymal transition (EMT) in gastric cancer cells, as demonstrated by the upregulation and downregulation of mesenchymal and epithelial markers. Silencing of FAT4 promoted tumour growth and metastasis in a gastric cancer xenograft model in vivo.

Conclusions:

FAT4 has a tumour suppressor role mediated by the modulation of Wnt/β-catenin signalling, providing potential novel targets for the treatment of gastric cancer.

[Urinary protein detection by iTRAQ® associated with renal transplant complications and its modification with therapy]

BACKGROUND

After renal transplant, surgical, infection complications, as well as graft rejection may occur; early detection through non-invasive markers is the key to change therapy and avoid biopsy.

OBJECTIVE

The aime of the study is to determine urine protein profiles in patients undergoing renal transplant with complications and detect its variation when therapy is modified.

MATERIAL AND METHODS

Urine samples were collected from patients prior the transplant and various postoperative stages. Urinary protein profiles were obtained by peptide labeling using isobaric isotopes for relative quantification (iTRAQ(®)).

RESULTS

A total of 22 patients were included, of whom 12 developed post-transplant complication: 2 with graft rejection (one male and one female) and 10 (6 males and 4 females) in the group of post-transplant infections. Using iTRAQ(®) 15/345 and 28/113 proteins were identified and fulfilled the acceptance criteria, in graft rejection and post-transplant infections group, respectively.

CONCLUSIONS

Albumin was the only protein found in both groups, the remaining proteins were different. The 5 proteins with higher scores in graft rejection were: alpha-1-microglobulin, 5'-nucleotidase cytosolic III, retinol-binding protein 4, membrane protein palmitoylated 4, and serine carboxypeptidase, while post-transplant infections were: mitochondrial acetyl-coenzyme A synthetase, putative adenosyl homocysteinase 2, zinc finger protein GLIS1, putative protein FAM157B, and zinc finger protein 615. It remains to elucidate the involvement of each of these in patients with renal transplantation.

A soluble form of the giant cadherin Fat1 is released from pancreatic cancer cells by ADAM10 mediated ectodomain shedding

In pancreatic cancer, there is a clear unmet need to identify new serum markers for either early diagnosis, therapeutic stratification or patient monitoring. Proteomic analysis of tumor cell secretomes is a promising approach to indicate proteins released from tumor cells in vitro. Ectodomain shedding of transmembrane proteins has previously been shown to contribute significant fractions the tumor cell secretomes and to generate valuable serum biomarkers. Here we introduce a soluble form of the giant cadherin Fat1 as a novel biomarker candidate. Fat1 expression and proteolytic processing was analyzed by mass spectrometry and Western blotting using pancreatic cancer cell lines as compared to human pancreatic ductal epithelial cells. RNA expression in cancer tissues was assessed by in silico analysis of publically available microarray data. Involvement of ADAM10 (A Disintegrin and metalloproteinase domain-containing protein 10) in Fat1 ectodomain shedding was analyzed by chemical inhibition and knockdown experiments. A sandwich ELISA was developed to determine levels of soluble Fat1 in serum samples. In the present report we describe the release of high levels of the ectodomain of Fat1 cadherin into the secretomes of human pancreatic cancer cells in vitro, a process that is mediated by ADAM10. We confirm the full-length and processed heterodimeric form of Fat1 expressed on the plasma membrane and also show the p60 C-terminal transmembrane remnant fragment corresponding to the shed ectodomain. Fat1 and its sheddase ADAM10 are overexpressed in pancreatic adenocarcinomas and ectodomain shedding is also recapitulated in vivo leading to increased Fat1 serum levels in some pancreatic cancer patients. We suggest that soluble Fat1 may find an application as a marker for patient monitoring complementing carbohydrate antigen 19-9 (CA19-9). In addition, detailed analysis of the diverse processed protein isoforms of the candidate tumor suppressor Fat1 can also contribute to our understanding of cell biology and tumor behavior.

Furin processing dictates ectodomain shedding of human FAT1 cadherin

Fat1 is a single pass transmembrane protein and the largest member of the cadherin superfamily. Mouse knockout models and in vitro studies have suggested that Fat1 influences cell polarity and motility. Fat1 is also an upstream regulator of the Hippo pathway, at least in lower vertebrates, and hence may play a role in growth control. In previous work we have established that FAT1 cadherin is initially cleaved by proprotein convertases to form a noncovalently linked heterodimer prior to expression on the cell surface. Such processing was not a requirement for cell surface expression, since melanoma cells expressed both unprocessed FAT1 and the heterodimer on the cell surface. Here we further establish that the site 1 (S1) cleavage step to promote FAT1 heterodimerisation is catalysed by furin and we identify the cleavage site utilised. For a number of other transmembrane receptors that undergo heterodimerisation the S1 processing step is thought to occur constitutively but the functional significance of heterodimerisation has been controversial. It has also been generally unclear as to the significance of receptor heterodimerisation with respect to subsequent post-translational proteolysis that often occurs in transmembrane proteins. Exploiting the partial deficiency of FAT1 processing in melanoma cells together with furin-deficient LoVo cells, we manipulated furin expression to demonstrate that only the heterodimer form of FAT1 is subject to cleavage and subsequent release of the extracellular domain. This work establishes S1-processing as a clear functional prerequisite for ectodomain shedding of FAT1 with general implications for the shedding of other transmembrane receptors.

Deregulation of the protocadherin gene FAT1 alters muscle shapes: implications for the pathogenesis of facioscapulohumeral dystrophy

Generation of skeletal muscles with forms adapted to their function is essential for normal movement. Muscle shape is patterned by the coordinated polarity of collectively migrating myoblasts. Constitutive inactivation of the protocadherin gene Fat1 uncoupled individual myoblast polarity within chains, altering the shape of selective groups of muscles in the shoulder and face. These shape abnormalities were followed by early onset regionalised muscle defects in adult Fat1-deficient mice. Tissue-specific ablation of Fat1 driven by Pax3-cre reproduced muscle shape defects in limb but not face muscles, indicating a cell-autonomous contribution of Fat1 in migrating muscle precursors. Strikingly, the topography of muscle abnormalities caused by Fat1 loss-of-function resembles that of human patients with facioscapulohumeral dystrophy (FSHD). FAT1 lies near the critical locus involved in causing FSHD, and Fat1 mutant mice also show retinal vasculopathy, mimicking another symptom of FSHD, and showed abnormal inner ear patterning, predictive of deafness, reminiscent of another burden of FSHD. Muscle-specific reduction of FAT1 expression and promoter silencing was observed in foetal FSHD1 cases. CGH array-based studies identified deletion polymorphisms within a putative regulatory enhancer of FAT1, predictive of tissue-specific depletion of FAT1 expression, which preferentially segregate with FSHD. Our study identifies FAT1 as a critical determinant of muscle form, misregulation of which associates with FSHD.

Sleeping giants: emerging roles for the fat cadherins in health and disease

The vertebrate Fat cadherins comprise a small gene family of four members, Fat1-Fat4, all closely related in structure to Drosophila ft and ft2. Over the past decade, knock-out mouse studies, genetic manipulation, and large sequencing projects has aided our understanding of the function of vertebrate Fat cadherins in tissue development and disease. The majority of studies of this family have focused on Fat1, with evidence now showing it can bind enable (ENA)/Vasodilator-stimulated phosphoprotein (VASP), β-catenin and Atrophin proteins to influence cell polarity and motility; HOMER-1 and HOMER-3 proteins to regulate actin accumulation in neuronal synapses; and scribble to influence the Hippo signaling pathway. Fat2 and Fat3 can regulate cell migration in a tissue specific manner and Fat4 appears to influence both planar cell polarity and Hippo signaling recapitulating the activity of Drosophila ft. Knowledge about the exact downstream signaling pathways activated by each family member remains in its infancy, but it is becoming clearer that they have tissue specific and redundant roles in development and may be lost or gained in cancer. In this review, we summarize the recent progress on understanding the role of the Fat cadherin family, integrating the current knowledge of molecular interactions and tissue distributions, together with the accumulating evidence of their changed expression in human disease. The latter is now beginning to promote interest in these molecules as both biomarkers and new targets for therapeutic intervention.

Integrative genomic characterization of oral squamous cell carcinoma identifies frequent somatic drivers

The survival of patients with oral squamous cell carcinoma (OSCC) has not changed significantly in several decades, leading clinicians and investigators to search for promising molecular targets. To this end, we conducted comprehensive genomic analysis of gene expression, copy number, methylation, and point mutations in OSCC. Integrated analysis revealed more somatic events than previously reported, identifying four major driver pathways (mitogenic signaling, Notch, cell cycle, and TP53) and two additional key genes (FAT1, CASP8). The Notch pathway was defective in 66% of patients, and in follow-up studies of mechanism, functional NOTCH1 signaling inhibited proliferation of OSCC cell lines. Frequent mutation of caspase-8 (CASP8) defines a new molecular subtype of OSCC with few copy number changes. Although genomic alterations are dominated by loss of tumor suppressor genes, 80% of patients harbored at least one genomic alteration in a targetable gene, suggesting that novel approaches to treatment may be possible for this debilitating subset of head and neck cancers.

Function and cancer genomics of FAT family genes (review)

FAT1, FAT2, FAT3 and FAT4 are human homologs of Drosophila Fat, which is involved in tumor suppression and planar cell polarity (PCP). FAT1 and FAT4 undergo the first proteolytic cleavage by Furin and are predicted to undergo the second cleavage by γ-secretase to release intracellular domain (ICD). Ena/VAPS-binding to FAT1 induces actin polymerization at lamellipodia and filopodia to promote cell migration, while Scribble-binding to FAT1 induces phosphorylation and functional inhibition of YAP1 to suppress cell growth. FAT1 is repressed in oral cancer owing to homozygous deletion or epigenetic silencing and is preferentially downregulated in invasive breast cancer. On the other hand, FAT1 is upregulated in leukemia and prognosis of preB-ALL patients with FAT1 upregulation is poor. FAT4 directly interacts with MPDZ/MUPP1 to recruit membrane-associated guanylate kinase MPP5/PALS1. FAT4 is involved in the maintenance of PCP and inhibition of cell proliferation. FAT4 mRNA is repressed in breast cancer and lung cancer due to promoter hypermethylation. FAT4 gene is recurrently mutated in several types of human cancers, such as melanoma, pancreatic cancer, gastric cancer and hepatocellular carcinoma. FAT1 and FAT4 suppress tumor growth via activation of Hippo signaling, whereas FAT1 promotes tumor migration via induction of actin polymerization. FAT1 is tumor suppressive or oncogenic in a context-dependent manner, while FAT4 is tumor suppressive. Copy number aberration, translocation and point mutation of FAT1, FAT2, FAT3, FAT4, FRMD1, FRMD6, NF2, WWC1, WWC2, SAV1, STK3, STK4, MOB1A, MOB1B, LATS1, LATS2, YAP1 and WWTR1/TAZ genes should be comprehensively investigated in various types of human cancers to elucidate the mutation landscape of the FAT-Hippo signaling cascades. Because YAP1 and WWTR1 are located at the crossroads of adhesion, GPCR, RTK and stem-cell signaling network, cancer genomics of the FAT signaling cascades could be applied for diagnostics, prognostics and therapeutics in the era of personalized medicine.

Dual processing of FAT1 cadherin protein by human melanoma cells generates distinct protein products

The giant cadherin FAT1 is one of four vertebrate orthologues of the Drosophila tumor suppressor fat. It engages in several functions, including cell polarity and migration, and in Hippo signaling during development. Homozygous deletions in oral cancer suggest that FAT1 may play a tumor suppressor role, although overexpression of FAT1 has been reported in some other cancers. Here we show using Northern blotting that human melanoma cell lines variably but universally express FAT1 and less commonly FAT2, FAT3, and FAT4. Both normal melanocytes and keratinocytes also express comparable FAT1 mRNA relative to melanoma cells. Analysis of the protein processing of FAT1 in keratinocytes revealed that, like Drosophila FAT, human FAT1 is cleaved into a non-covalent heterodimer before achieving cell surface expression. The use of inhibitors also established that such cleavage requires the proprotein convertase furin. However, in melanoma cells, the non-cleaved proform of FAT1 is also expressed at the cell surface together with the furin-cleaved heterodimer. Moreover, furin-independent processing generates a potentially functional proteolytic product in melanoma cells, a persistent 65-kDa membrane-bound cytoplasmic fragment no longer in association with the extracellular fragment. In vitro localization studies of FAT1 showed that melanoma cells display high levels of cytosolic FAT1 protein, whereas keratinocytes, despite comparable FAT1 expression levels, exhibited mainly cell-cell junctional staining. Such differences in protein distribution appear to reconcile with the different protein products generated by dual FAT1 processing. We suggest that the uncleaved FAT1 could promote altered signaling, and the novel products of alternate processing provide a dominant negative function in melanoma.

Cadherin 13 in cancer

We review the evidence suggesting the involvement of Cadherin 13 (CDH13, T-cadherin, H-cadherin) in various cancers. CDH13 is an atypical member of the cadherin family, devoid of a transmembrane domain and anchored to the exterior surface of the plasma membrane via a glycosylphosphatidylinositol anchor. CDH13 is thought to affect cellular behavior largely through its signaling properties. It is often down-regulated in cancerous cells. CDH13 down-regulation has been associated with poorer prognosis in various carcinomas, such as lung, ovarian, cervical and prostate cancer. CDH13 re-expression in most cancer cell lines inhibits cell proliferation and invasiveness, increases susceptibility to apoptosis, and reduces tumor growth in in vivo models. These properties suggest that CDH13 may represent a possible target for therapy in some cancers. At the same time, CDH13 is up-regulated in blood vessels growing through tumors and promotes tumor neovascularization. In contrast to most cancer cell lines, CDH13 overexpression in endothelial cells promotes their proliferation and migration, and has a pro-survival effect. We also discuss molecular mechanisms that may regulate CDH13 expression and underlie its roles in cancer.

T-cadherin is located in the nucleus and centrosomes in endothelial cells

T-cadherin (H-cadherin, cadherin 13) is upregulated in vascular proliferative disorders and in tumor-associated neovascularization and is deregulated in many cancers. Unlike canonical cadherins, it lacks transmembrane and intracellular domains and is attached to the plasma membrane via a glycosylphosphatidylinositol anchor. T-cadherin is thought to function in signaling rather than as an adhesion molecule. Some interactive partners of T-cadherin at the plasma membrane have recently been identified. We examined T-cadherin location in human endothelial cells using confocal microscopy and subcellular fractionation. We found that a considerable proportion of T-cadherin is located in the nucleus and in the centrosomes. T-cadherin colocalized with a centrosomal marker gamma-tubulin uniformly throughout the cell cycle at least in human umbilical vein endothelial cells. In the telophase, T-cadherin transiently concentrated in the midbody and was apparently degraded. Its overexpression resulted in an increase in the number of multinuclear cells, whereas its downregulation by small interfering RNA led to an increase in the number of cells with multiple centrosomes. These findings indicate that deregulation of T-cadherin in endothelial cells may lead to disturbances in cytokinesis or centrosomal replication.

Atrophin proteins interact with the Fat1 cadherin and regulate migration and orientation in vascular smooth muscle cells

Fat1, an atypical cadherin induced robustly after arterial injury, has significant effects on mammalian vascular smooth muscle cell (VSMC) growth and migration. The related Drosophila protein Fat interacts genetically and physically with Atrophin, a protein essential for development and control of cell polarity. We hypothesized that interactions between Fat1 and mammalian Atrophin (Atr) proteins might contribute to Fat1 effects on VSMCs. Like Fat1, mammalian Atr expression increased after arterial injury and in VSMCs stimulated with growth and chemotactic factors including angiotensin II, basic fibroblast growth factor, and platelet-derived growth factor BB. Two distinct Atr2 transcripts, atr2L and atr2S, were identified by Northern analysis; in VSMCs, atr2S mRNA expression was more responsive to stimuli. By immunocytochemistry, Fat1 and Atrs colocalized at cell-cell junctions, in the perinuclear area, and in the nucleus. In coimmunoprecipitation studies, Fat1 interacted with both Atr1 and Atr2; these interactions required Fat1 amino acids 4300-4400 and an intact Atro-box in the Atrs. Knock-down of Atrs by small interfering RNA did not affect VSMC growth but had complex effects on migration, which was impaired by Atr1 knockdown, enhanced by Atr2L knockdown, and unchanged when both Atr2S and Atr2L were depleted. Enhanced migration caused by Atr2L knockdown required Fat1 expression. Similarly, orientation of cells after monolayer denudation was impaired in cells with Atr1 knockdown but enhanced in cells selectively depleted of Atr2L. Together these findings suggest that Fat1 and Atrs act in concert after vascular injury but show further that distinct Atr isoforms have disparate effects on VSMC directional migration.

Protocadherin 12 deficiency alters morphogenesis and transcriptional profile of the placenta

Protocadherins are transmembrane proteins exhibiting homophilic adhesive activities through their extracellular domain. Protocadherin 12 (Pcdh12) is expressed in angiogenic endothelial cells, mesangial cells of kidney glomeruli, and glycogen cells of the mouse placenta. To get insight into the role of this protein in vivo, we analyzed PCDH12-deficient mice and investigated their placental phenotype. The mice were alive and fertile; however, placental and embryonic sizes were reduced compared with wild-type mice. We observed defects in placental layer segregation and a decreased vascularization of the labyrinth associated with a reduction in cell density in this layer. To understand the molecular events responsible for the phenotypic alterations observed in Pcdh12(-/-) placentas, we analyzed the expression profile of embryonic day 12.5 mutant placentas compared with wild-type placentas, using pangenomic chips: 2,289 genes exhibited statistically significant changes in expressed levels due to loss of PCDH12. Functional grouping of modified genes was obtained by GoMiner software. Gene clusters that contained most of the differentially expressed genes were those involved in tissue morphogenesis and development, angiogenesis, cell-matrix adhesion and migration, immune response, and chromatin remodeling. Our data show that loss of PCDH12 leads to morphological alterations of the placenta and to notable changes in its gene expression profile. Specific genes emerging from the microarray screen support the biological modifications observed in PCDH12-deficient placentas.

The intracellular domain of the human protocadherin hFat1 interacts with Homer signalling scaffolding proteins

The cadherin superfamily protein Fat1 is known to interact with the EVH1 domain of mammalian Ena/VASP. Here we demonstrate that: (i) the scaffolding proteins Homer-3 and Homer-1 also interact with the EVH1 binding site of hFat1 in vitro, and (ii) binding of Homer-3 and Mena to hFat1 is mutually competitive. Endogenous Fat1 binds to immobilised Homer-3 and endogenous Homer-3 binds to immobilised Fat1. Both, endogenous and over-expressed Fat1 exhibit co-localisation with Homer-3 in cellular protrusions and at the plasma membrane of HeLa cells. As Homer proteins and Fat1 have been both linked to psychic disorders, their interaction may be of patho-physiological importance.

The Fat1 cadherin integrates vascular smooth muscle cell growth and migration signals

The significance of cadherin superfamily proteins in vascular smooth muscle cell (VSMC) biology is undefined. Here we describe recent studies of the Fat1 protocadherin. Fat1 expression in VSMCs increases significantly after arterial injury or growth factor stimulation. Fat1 knockdown decreases VSMC migration in vitro, but surprisingly, enhances cyclin D1 expression and proliferation. Despite limited similarity to classical cadherins, the Fat1 intracellular domain (Fat1(IC)) interacts with beta-catenin, inhibiting both its nuclear localization and transcriptional activity. Fat1 undergoes cleavage and Fat1(IC) species localize to the nucleus; however, inhibition of the cyclin D1 promoter by truncated Fat1(IC) proteins corresponds to their presence outside the nucleus, which argues against repression of beta-catenin-dependent transcription by nuclear Fat1(IC). These findings extend recent observations about Fat1 and migration in other cell types, and demonstrate for the first time its anti-proliferative activity and interaction with beta-catenin. Because it is induced after arterial injury, Fat1 may control VSMC functions central to vascular remodeling by facilitating migration and limiting proliferation.

A requirement for NF-protocadherin and TAF1/Set in cell adhesion and neural tube formation

Neurulation in vertebrates is an intricate process requiring extensive alterations in cell contacts and cellular morphologies as the cells in the neural ectoderm shape and form the neural folds and neural tube. Despite these complex interactions, little is known concerning the molecules that mediate cell adhesion within the embryonic neural plate and neural folds. Here, we demonstrate the requirement for NF-protocadherin (NFPC) and its cytosolic partner TAF1/Set for proper neurulation in Xenopus. Both NFPC and TAF1 function in cell-cell adhesion in the neural ectoderm, and disruptions in either NFPC or TAF1 result in a failure of the neural tube to close. This neural tube defect can be attributed to a lack of proper organization of the cells in the dorsal neural folds, manifested by a loss in the columnar epithelial morphology and apical localization of F-actin. However, the epidermal ectoderm is still able to migrate and cover the open neural tube, indicating that the fusions of the neural tube and epidermis are separate events. These studies demonstrate that NFPC and TAF1 function to maintain proper cell-cell interactions within the neural folds and suggest that NFPC and TAF1 participate in novel adhesive mechanisms that contribute to the final events of vertebrate neurulation.