Differential interaction of plakoglobin and beta-catenin with the ubiquitin-proteasome system

Crosstalk Between β-CATENIN-Mediated Cell Adhesion and the WNT Signaling Pathway

Cell adhesion and stable signaling regulation are fundamental ways of maintaining homeostasis. Among them, the Wnt/β-CATENIN signaling plays a key role in embryonic development and maintenance of body dynamic homeostasis. At the same time, the key signaling molecule β-CATENIN in the Wnt signaling can also function as a cytoskeletal linker protein to regulate tissue barriers, cell migration, and morphogenesis. Dysregulation of the balance between Wnt signaling and adherens junctions can lead to disease. How β-CATENIN maintains the independence of these two functions, or mediates the interaction and balance of these two functions, has been explored and debated for a long time. In this study, we will focus on five aspects of β-CATENIN chaperone molecules, phosphorylation of β-CATENIN and related proteins, epithelial mesenchymal transition, β-CATENIN homolog protein γ-CATENIN and disease, thus deepening the understanding of the Wnt/β-CATENIN signaling and the homeostasis between cell adhesion and further addressing related disease problems.

Plakoglobin is involved in cytoskeletal rearrangement of podocytes under the regulation of UCH-L1

UCH-L1 is a de-ubiquitination enzyme comprehensively distributed in neural cells and podocytes, which is involved in several kinds of nervous system and kidney related diseases. Our previous studies have demonstrated the aberrant up-regulation of UCH-L1 in podocytes of renal diseases, but how dose podocytes are injured by up-regulated UCH-L1 is waiting to be elucidated. Here, we observed the cytoskeleton rearrangement in podocytes with over-expression of UCH-L1, accompanied with a down-regulation of synaptopodin and RhoA, which are closely related to cytoskeletal stabilization. However, we did not see any alteration of RhoA ubiquitination level under the stimulation of UCH-L1 in podocytes. Subsequently, mass spectrum was applied in UCH-L1-flag immunoprecipitation and plakoglobin was screened out, which was among the UCH-L1-combined proteins and most likely related to cytoskeleton rearrangement. Our experiment demonstrates UCH-L1 may not injure podocytes cytoskeleton through a direct regulation on RhoA/Synaptopodin, but through the regulation of plakoglobin, which could be a promising target for treatment of renal disease in the future.

Intercalated discs: cellular adhesion and signaling in heart health and diseases

Intercalated discs (ICDs) are highly orchestrated structures that connect neighboring cardiomyocytes in the heart. Three major complexes are distinguished in ICD: desmosome, adherens junction (AJ), and gap junction (GJ). Desmosomes are major cell adhesion junctions that anchor cell membrane to the intermediate filament network; AJs connect the actin cytoskeleton of adjacent cells; and gap junctions metabolically and electrically connect the cytoplasm of adjacent cardiomyocytes. All these complexes work as a single unit, the so-called area composita, interdependently rather than individually. Mutation or altered expression of ICD proteins results in various cardiac diseases, such as ARVC (arrhythmogenic right ventricular cardiomyopathy), dilated cardiomyopathy, and hypotrophy cardiomyopathy, eventually leading to heart failure. In this article, we first review the recent findings on the structural organization of ICD and their functions and then focus on the recent advances in molecular pathogenesis of the ICD-related heart diseases, which include two major areas: i) the ICD gene mutations in cardiac diseases, and ii) the involvement of ICD proteins in signal transduction pathways leading to myocardium remodeling and eventual heart failure. These major ICD-related signaling pathways include Wnt/β-catenin pathway, p38 MAPK cascade, Rho-dependent serum response factor (SRF) signaling, calcineurin/NFAT signaling, Hippo kinase cascade, etc., which are differentially regulated in pathological conditions.

The Barrier Molecules Junction Plakoglobin, Filaggrin, and Dystonin Play Roles in Melanoma Growth and Angiogenesis

OBJECTIVE

To understand role of barrier molecules in melanomas.

BACKGROUND

We have reported poor patient survival and low immune infiltration of melanomas that overexpress a set of genes that include filaggrin (FLG), dystonin (DST), junction plakoglobin (JUP), and plakophilin-3 (PKP3), and are involved in cell-cell adhesions. We hypothesized that these associations are causal, either by interfering with immune cell infiltration or by enhancing melanoma cell growth.

METHODS

FLG and DST were knocked out by CRISPR/Cas9 in human DM93 and murine B16-F1 melanoma cells. PKP3 and JUP were overexpressed in murine B16-AAD and human VMM39 melanoma cells by lentiviral transduction. These cell lines were evaluated in vitro for cell proliferation and in vivo for tumor burden, immune composition, cytokine expression, and vascularity.

RESULTS

Immune infiltrates were not altered by these genes. FLG/DST knockout reduced proliferation of human DM93 melanoma in vitro, and decreased B16-F1 tumor burden in vivo. Overexpression of JUP, but not PKP3, in B16-AAD significantly increased tumor burden, increased VEGF-A, reduced IL-33, and enhanced vascularity.

CONCLUSIONS

FLG and DST support melanoma cell growth in vitro and in vivo. Growth effects of JUP were only evident in vivo, and may be mediated, in part, by enhancing angiogenesis. In addition, growth-promoting effects of FLG and DST in vitro suggest that these genes may also support melanoma cell proliferation through angiogenesis-independent pathways. These findings identify FLG, DST, and JUP as novel therapeutic targets whose down-regulation may provide clinical benefit to patients with melanoma.

Physalis peruviana-Derived 4β-Hydroxywithanolide E, a Novel Antagonist of Wnt Signaling, Inhibits Colorectal Cancer In Vitro and In Vivo

Deregulation of the Wnt signaling pathway leads to colorectal cancer progression. Natural dietary compounds serve as promising candidates for development as chemopreventive agents by suppressing the Wnt/β-catenin signaling pathway. Physalis peruviana-derived 4βHWE showed a significant inhibitory activity with a calculated IC₅₀ of 0.09 μΜ in a screening of novel inhibitors of Wnt signaling with the dual-luciferase reporter assay. This study investigated the anti-tumor effect of 4βHWE and the potential Wnt signaling inhibitory mechanism. Both the western blot analysis and immunofluorescence assay showed that 4βHWE promoted the phosphorylation and degradation of β-catenin and the subsequent inhibition of its nuclear translocation to attenuate the endogenous Wnt target gene expression in colorectal cancer (CRC) cells. The cell viability assay indicated that 4βHWE preferentially inhibited the proliferation of CRC compared with CCD-841-CoN, a normal human colonic epithelial cell line. 4βHWE-mediated G0/G1 cell cycle arrest and apoptosis induction contributed to the suppression of the proliferation of CRC in the cell cycle and Annexin V-FITC/Propidium Iodide apoptosis analysis. Moreover, in vivo, 4βHWE dramatically inhibited tumor growth in HCT116 xenografts by attenuating the Wnt/β-catenin signaling pathway. In conclusion, our study suggested that 4βHWE could be of potential use in anti-tumor agent development as a novel Wnt signaling inhibitor.

The role of ubiquitination and deubiquitination in the regulation of cell junctions

Maintenance of cell junctions plays a crucial role in the regulation of cellular functions including cell proliferation, permeability, and cell death. Disruption of cell junctions is implicated in a variety of human disorders, such as inflammatory diseases and cancers. Understanding molecular regulation of cell junctions is important for development of therapeutic strategies for intervention of human diseases. Ubiquitination is an important type of post-translational modification that primarily regulates endogenous protein stability, receptor internalization, enzyme activity, and protein-protein interactions. Ubiquitination is tightly regulated by ubiquitin E3 ligases and can be reversed by deubiquitinating enzymes. Recent studies have been focusing on investigating the effect of protein stability in the regulation of cell-cell junctions. Ubiquitination and degradation of cadherins, claudins, and their interacting proteins are implicated in epithelial and endothelial barrier disruption. Recent studies have revealed that ubiquitination is involved in regulation of Rho GTPases’ biological activities. Taken together these studies, ubiquitination plays a critical role in modulating cell junctions and motility. In this review, we will discuss the effects of ubiquitination and deubiquitination on protein stability and expression of key proteins in the cell-cell junctions, including junction proteins, their interacting proteins, and small Rho GTPases. We provide an overview of protein stability in modulation of epithelial and endothelial barrier integrity and introduce potential future search directions to better understand the effects of ubiquitination on human disorders caused by dysfunction of cell junctions.

Molecular and cytoskeletal regulations in epidermal development

At the surface of the body, the epidermis covers great depth in its developmental regulation. While many genes have been shown to be important for skin development through their associations with disease phenotypes in mice and human, it is in the past decade that the intricate interplay between various molecules become gradually revealed through sophisticated genetic models and imaging analyses. In particular, there is increasing evidence suggesting that cytoskeleton-associated proteins, including adhesion proteins and the crosslinker proteins may play critical roles in regulating epidermis development. We here provide a broad overview of the various molecules involved in epidermal development with special emphasis on the cytoskeletal components.

Novel β-carbolines against colorectal cancer cell growth via inhibition of Wnt/β-catenin signaling

Wnt signaling pathway is aberrantly activated in a variety of cancers, especially in colorectal cancer (CRC), because of mutations in the genes encoding adenomatous polyposis coli (APC), β-catenin and Axin. Small-molecule antagonists of Wnt/β-catenin signaling are attractive candidates for developing effective therapeutics for CRC. In this study, we have identified a novel Wnt signaling inhibitor, isopropyl 9-ethyl-1- (naphthalen-1-yl)-9H-pyrido[3,4-b]indole-3- carboxylate (Z86). Z86 inhibited Wnt reporter activities and the expression of endogenous Wnt signaling target genes in mammalian cells and antagonized the second axis formation of Xenopus embryos induced by Wnt8. We showed that Z86 treatment inhibits GSK3β (Ser9) phosphorylation, leading to its overactivation and promoting the phosphorylation and degradation of β-catenin. In vitro, Z86 selectively inhibited the growth of CRC cells with constitutive Wnt signaling and caused obvious G1-phase arrest of the cell cycle. Notably, in a nude mouse model, Z86 inhibited dramatically the xenografted tumor growth of CRC. Daily intraperitoneal injection of Z86 at 5 mg/kg resulted in >70% reduction in the tumor weight of HCT116 cell origin that was associated with decreased GSK3β (Ser9) phosphorylation and increased β-catenin phosphorylation. Taken together, our findings provide a novel promising chemotype for CRC therapeutics development targeting the canonical Wnt signaling.

Pathogenesis of Arrhythmogenic Cardiomyopathy

Arrhythmogenic cardiomyopathy (ACM) is a primary myocardial disease. It is characterized by frequent ventricular arrhythmias and increased risk of sudden cardiac death typically arising as an early manifestation before the onset of significant myocardial remodelling. Myocardial degeneration, often confined to the right ventricular free wall, with replacement by fibrofatty scar tissue, develops in many patients. ACM is a familial disease but genetic penetrance can be low and disease expression is highly variable. Inflammation might promote disease progression. It also appears that exercise increases disease penetrance and accelerates its development. More than 60% of probands harbour mutations in genes that encode desmosomal proteins, which has raised the possibility that defective cell-cell adhesion might play a role in disease pathogenesis. Recent advances have implicated changes in the canonical wingless-type mouse mammary tumour virus integration site (Wnt)/β-catenin and Hippo signalling pathways and defects in forwarding trafficking of ion channels and other proteins to the intercalated disk in cardiac myocytes. In this review we summarize the current understanding of the pathogenesis of ACM and highlight future research directions.

Remodeling of cell-cell junctions in arrhythmogenic cardiomyopathy

Arrhythmogenic cardiomyopathy (AC) is a primary myocardial disorder characterized by a high incidence of ventricular arrhythmias often preceding the onset of ventricular remodeling and dysfunction. Approximately 50% of patients diagnosed with AC have one or more mutations in genes encoding desmosomal proteins, although non-desmosomal genes have also been associated with the disease. Increasing evidence implicates remodeling of intercalated disk proteins reflecting abnormal responses to mechanical load and aberrant cell signaling pathways in the pathogenesis of AC. This review summarizes recent advances in understanding disease mechanisms in AC that have come from studies of human myocardium and experimental models.

Diabetes-induced superoxide anion and breakdown of the blood-retinal barrier: role of the VEGF/uPAR pathway

Diabetes-induced breakdown of the blood-retinal barrier (BRB) has been linked to hyperglycemia-induced expression of vascular endothelial growth factor (VEGF) and is likely mediated by an increase in oxidative stress. We have shown that VEGF increases permeability of retinal endothelial cells (REC) by inducing expression of urokinase plasminogen activator receptor (uPAR). The purpose of this study was to define the role of superoxide anion in VEGF/uPAR expression and BRB breakdown in diabetes. Studies were performed in streptozotocin diabetic rats and mice and high glucose (HG) treated REC. The superoxide dismutase (SOD) mimetic tempol blocked diabetes-induced permeability and uPAR expression in rats and the cell permeable SOD inhibited HG-induced expression of uPAR and VEGF in REC. Inhibiting VEGFR blocked HG-induced expression of VEGF and uPAR and GSK-3β phosphorylation in REC. HG caused β-catenin translocation from the plasma membrane into the cytosol and nucleus. Treatment with HG-conditioned media increased REC paracellular permeability that was blocked by anti-uPA or anti-uPAR antibodies. Moreover, deletion of uPAR blocked diabetes-induced BRB breakdown and activation of MMP-9 in mice. Together, these data indicate that diabetes-induced oxidative stress triggers BRB breakdown by a mechanism involving uPAR expression through VEGF-induced activation of the GSK3β/β-catenin signaling pathway.

c-Cbl, a ubiquitin E3 ligase that targets active β-catenin: a novel layer of Wnt signaling regulation

Regulation of transcriptionally active nuclear β-catenin during the Wnt-on phase is crucial to ensure controlled induction of Wnt target genes. Several ubiquitin E3 ligases are known to regulate cytosolic β-catenin during the Wnt-off phase, but little is known about the fate of active nuclear β-catenin in the Wnt-on phase. We now describe ubiquitination of active β-catenin in the Wnt-on phase by a RING finger ubiquitin E3 ligase, Casitas B-lineage lymphoma (c-Cbl) in endothelial cells. c-Cbl binds preferentially to nuclearly active β-catenin in the Wnt-on phase via the armadillo repeat region. Wild-type c-Cbl suppresses and E3 ligase-deficient c-Cbl-70Z increases Wnt signaling. Wnt induces nuclear translocation of c-Cbl where it ubiquitinates nuclear β-catenin. Deletion of the c-Cbl UBA domain abrogates its dimerization, binding to β-catenin, Wnt-induced c-Cbl nuclear translocation, and ubiquitination of nuclear β-catenin. c-Cbl activity inhibits pro-angiogenic Wnt targets IL-8 and VEGF levels and angiogenesis in a β-catenin-dependent manner. This study defines for the first time c-Cbl as a ubiquitin E3 ligase that targets nuclearly active β-catenin in the Wnt-on phase and uncovers a novel layer of regulation of Wnt signaling.

Over-expression of deubiquitinating enzyme USP14 in lung adenocarcinoma promotes proliferation through the accumulation of β-catenin

The deubiquitinating enzyme USP14 has been identified and biochemically studied, but its role in lung cancer remains to be elucidated. The aim of this study was to evaluate the prognostic significance of USP14 in patients with lung adenocarcinoma and to define its role in lung cancer cell proliferation. USP14 mRNA levels in different non-small cell lung cancer (NSCLC) cell lines were detected by real-time qPCR. USP14 protein levels in surgically resected samples from NSCLC patients, and in NSCLC cell lines, were detected by immunohistochemistry or Western blot. The correlation of USP14 expression with clinical characteristics and prognosis was determined by survival analysis. After silencing USP14, cell proliferation was assessed by MTT assay and the cell cycle was measured by FACS assay. It was found that USP14 expression was upregulated in NSCLC cells, especially in adenocarcinoma cells. Over-expression of USP14 was associated with shorter overall survival of patients. Downregulation of USP14 expression arrested the cell cycle, which may be related to β-catenin degradation. Over-expression of USP14 was associated with poor prognosis in NSCLC patients and promoted tumor cell proliferation, which suggests that USP14 is a tumor-promoting factor and a promising therapeutic target for NSCLC.

Downregulation of γ-catenin inhibits CML cell growth and potentiates the response of CML cells to imatinib through β-catenin inhibition

γ-catenin plays different roles in different types of tumors, and its role in chronic myeloid leukemia (CML) cells has yet to be identified. In our study, two CML cell lines (K562, KU812) had higher γ-catenin expression levels compared to five types of BCR-ABL-negative leukemia cells. Knockdown of the expression of BCR-ABL resulted in downregulation of γ-catenin. Furthermore, downregulation of γ-catenin by siRNA inhibited the proliferation and colony formation of CML cells and the expression of the c-Myc and cyclin D1 genes; downregulation of γ-catenin also potentiated the effects of imatinib (inhibiting CML cell proliferation and inducing apoptosis) and suppressed the anti-apoptotic genes Bcl-xL and survivin. We also showed that downregulation of γ-catenin suppressed the phosphorylation of STAT5, promoted the phosphorylation of β-catenin and reduced the translocation of β-catenin into the nucleus, although there were no effects on the total level of β-catenin expression in the whole cells. Furthermore, downregulation of γ-catenin was found to promote glycogen synthase kinase-3β (GSK3β) and inhibit its phosphorylation. Collectively, our results suggest that γ-catenin is an oncogene protein in CML that can be regulated by BCR-ABL and that suppression of γ-catenin inhibits CML cell growth and potentiates the effects of imatinib on CML cells through inhibition of the activation of STAT5 and suppression of β-catenin by activating GSK3β.

Beyond cell adhesion: the role of armadillo proteins in the heart

Plakoglobin (PG, γ-Catenin, JUP), a member of the armadillo protein family and close homolog of β-catenin, functions to link cell surface cadherin molecules with the cytoskeleton. PG is the only junctional component found in both desmosomes and adherens junctions and thus plays a critical role in the regulation of cell-cell adhesion. Similar to β-catenin, PG is able to interact with components of the Wnt signaling pathway and directly affect gene expression by binding with LEF/TCF transcription factors. In addition, it has been proposed that PG functions primarily as a competitive inhibitor of β-catenin transcriptional activity by sequestering LEF/TCF. Compared to β-catenin, the contribution of PG as a transcriptional regulator in either physiological or pathological conditions is poorly understood. There is increasing clinical interest in PG as both a structural protein as well as a signaling molecule as mutations have been identified in the human PG gene that cause Arrhythmogenic Right Ventricular Cardiomyopathy (ARVC) and cutaneous syndromes. This review will discuss the connection between altered cell adhesion and gene expression and its contribution to disease pathogenesis.

Deregulation of DNA damage response pathway by intercellular contact

Deregulation of the DNA damage response (DDR) pathway could compromise genomic integrity in normal cells and reduce cancer cell sensitivity to anticancer treatments. We found that intercellular contact stabilizes histone H2AX and γH2AX (H2AX phosphorylated on Ser-139) by up-regulating N/E-cadherin and γ-catenin. γ-catenin and its DNA-binding partner LEF-1 indirectly increase levels of H2AX by suppressing the promoter of the RNF8 ubiquitin ligase, which decreases levels of H2AX protein under conditions of low intercellular contact. Hyperphosphorylation of DDR proteins is induced by up-regulated H2AX. Constitutive apoptosis is caused in confluent cells but is not further induced by DNA damage. This is conceivably due to insufficient p53 activation because ChIP assay shows that its DNA binding ability is not induced in those cells. Together, our results illustrate a novel mechanism of the regulation of DDR proteins by the cadherin-catenin pathway.

The noncanonical NF-κB pathway

The noncanonical nuclear factor-κB (NF-κB) signaling pathway mediates activation of the p52/RelB NF-κB complex and, thereby, regulates specific immunological processes. This NF-κB pathway relies on the inducible processing of NF-κB2 precursor protein, p100, as opposed to the degradation of IκBα in the canonical NF-κB pathway. A central signaling component of the noncanonical NF-κB pathway is NF-κB-inducing kinase (NIK), which functions together with a downstream kinase, IKKα (inhibitor of NF-κB kinase α), to induce phosphorylation-dependent ubiquitination and processing of p100. Under normal conditions, NIK is targeted for continuous degradation by a tumor necrosis factor (TNF) receptor-associated factor-3 (TRAF3)-dependent E3 ubiquitin ligase. In response to signals mediated by a subset of TNF receptor superfamily members, NIK becomes stabilized as a result of TRAF3 degradation, leading to the activation of noncanonical NF-κB. This review discusses both the historical perspectives and the recent progress in the regulation and biological function of the noncanonical NF-κB pathway.

Non-canonical NF-κB signaling pathway

The non-canonical NF-κB pathway is an important arm of NF-κB signaling that predominantly targets activation of the p52/RelB NF-κB complex. This pathway depends on the inducible processing of p100, a molecule functioning as both the precursor of p52 and a RelB-specific inhibitor. A central signaling component of the non-canonical pathway is NF-κB-inducing kinase (NIK), which integrates signals from a subset of TNF receptor family members and activates a downstream kinase, IκB kinase-α (IKKα), for triggering p100 phosphorylation and processing. A unique mechanism of NIK regulation is through its fate control: the basal level of NIK is kept low by a TRAF-cIAP destruction complex and signal-induced non-canonical NF-κB signaling involves NIK stabilization. Tight control of the fate of NIK is important, since deregulated NIK accumulation is associated with lymphoid malignancies.

Interaction and co-localization of JC virus large T antigen and the F-box protein β-transducin-repeat containing protein

Lytic infection and transformation of cultured cells by JC virus (JCV) require five tumor proteins, which interact with factors regulating critical cellular processes. We demonstrate that JCV large T antigen (TAg) binds the F-box proteins β-transducin-repeat containing protein-1 and 2 (βTrCP1/2). These interactions involve a phosphodegron (DpSGX(2-4)pS) found in βTrCP substrates. TAg stability is unaltered, suggesting TAg is a pseudo-substrate. βTrCP and TAg co-localize in the cytoplasm, and a functional SCF complex is required. We examined whether TAg influences the levels of β-catenin, a βTrCP substrate. We were unable to demonstrate that TAg elevates β-catenin as previously reported, and a mutant TAg unable to bind βTrCP also had no detectable effect on β-catenin stability. Results presented in this study link JCV TAg to the cellular degradation complex, SCF(βTrCP1/2). Proteasomal degradation is essential for proper regulation of cellular functions, and interference with proteasomal pathways highlights possible JCV pathogenic and oncogenic mechanisms.

Shared molecular mechanisms regulate multiple catenin proteins: canonical Wnt signals and components modulate p120-catenin isoform-1 and additional p120 subfamily members

Wnt signaling pathways have fundamental roles in animal development and tumor progression. Here, employing Xenopus embryos and mammalian cell lines, we report that the degradation machinery of the canonical Wnt pathway modulates p120-catenin protein stability through mechanisms shared with those regulating β-catenin. For example, in common with β-catenin, exogenous expression of destruction complex components, such as GSK3β and axin, promotes degradation of p120-catenin. Again in parallel with β-catenin, reduction of canonical Wnt signals upon depletion of LRP5 and LRP6 results in p120-catenin degradation. At the primary sequence level, we resolved conserved GSK3β phosphorylation sites in the amino-terminal region of p120-catenin present exclusively in isoform-1. Point-mutagenesis of these residues inhibited the association of destruction complex components, such as those involved in ubiquitylation, resulting in stabilization of p120-catenin. Functionally, in line with predictions, p120 stabilization increased its signaling activity in the context of the p120-Kaiso pathway. Importantly, we found that two additional p120-catenin family members, ARVCF-catenin and δ-catenin, associate with axin and are degraded in its presence. Thus, as supported using gain- and loss-of-function approaches in embryo and cell line systems, canonical Wnt signals appear poised to have an impact upon a breadth of catenin biology in vertebrate development and, possibly, human cancers.

beta-TrCP inhibition reduces prostate cancer cell growth via upregulation of the aryl hydrocarbon receptor

Background

Prostate cancer is a common and heterogeneous disease, where androgen receptor (AR) signaling plays a pivotal role in development and progression. The initial treatment for advanced prostate cancer is suppression of androgen signaling. Later on, essentially all patients develop an androgen independent stage which does not respond to anti hormonal treatment. Thus, alternative strategies targeting novel molecular mechanisms are required. β-TrCP is an E3 ligase that targets various substrates essential for many aspects of tumorigenesis.

Methodology/Principal Findings

Here we show that β-TrCP depletion suppresses prostate cancer and identify a relevant growth control mechanism. shRNA targeted against β-TrCP reduced prostate cancer cell growth and cooperated with androgen ablation in vitro and in vivo. We found that β-TrCP inhibition leads to upregulation of the aryl hydrocarbon receptor (AhR) mediating the therapeutic effect. This phenomenon could be ligand independent, as the AhR ligand 2,3,7,8-Tetrachlorodibenzo-p-Dioxin (TCDD) did not alter prostate cancer cell growth. We detected high AhR expression and activation in basal cells and atrophic epithelial cells of human cancer bearing prostates. AhR expression and activation is also significantly higher in tumor cells compared to benign glandular epithelium.

Conclusions/Significance

Together these observations suggest that AhR activation may be a cancer counteracting mechanism in the prostate. We maintain that combining β-TrCP inhibition with androgen ablation could benefit advanced prostate cancer patients.

Anchoring junctions as drug targets: role in contraceptive development

In multicellular organisms, cell-cell interactions are mediated in part by cell junctions, which underlie tissue architecture. Throughout spermatogenesis, for instance, preleptotene leptotene spermatocytes residing in the basal compartment of the seminiferous epithelium must traverse the blood-testis barrier to enter the adluminal compartment for continued development. At the same time, germ cells must also remain attached to Sertoli cells, and numerous studies have reported extensive restructuring at the Sertoli-Sertoli and Sertoli-germ cell interface during germ cell movement across the seminiferous epithelium. Furthermore, the proteins and signaling cascades that regulate adhesion between testicular cells have been largely delineated. These findings have unveiled a number of potential "druggable" targets that can be used to induce premature release of germ cells from the seminiferous epithelium, resulting in transient infertility. Herein, we discuss a novel approach with the aim of developing a nonhormonal male contraceptive for future human use, one that involves perturbing adhesion between Sertoli and germ cells in the testis.

Structure and function of desmosomes

Desmosomes are prominent adhesion sites that are tightly associated with the cytoplasmic intermediate filament cytoskeleton providing mechanical stability in epithelia and also in several nonepithelial tissues such as cardiac muscle and meninges. They are unique in terms of ultrastructural appearance and molecular composition with cell type-specific variations. The dynamic assembly properties of desmosomes are important prerequisites for the acquisition and maintenance of tissue homeostasis. Disturbance of this equilibrium therefore not only compromises mechanical resilience but also affects many other tissue functions as becomes evident in various experimental scenarios and multiple diseases.

A novel dominant mutation in plakoglobin causes arrhythmogenic right ventricular cardiomyopathy

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is an inherited disorder associated with arrhythmias and sudden death. A recessive mutation in the gene encoding plakoglobin has been shown to cause Naxos disease, a cardiocutaneous syndrome characterized by ARVC and abnormalities of hair and skin. Here, we report, for the first time, a dominant mutation in the gene encoding plakoglobin in a German family with ARVC but no cutaneous abnormalities. The mutation (S39_K40insS) is predicted to insert an extra serine residue at position 39 in the N-terminus of plakoglobin. Analysis of a biopsy sample of the right ventricle from the proband showed markedly decreased localization of plakoglobin, desmoplakin, and connexin43 at intercalated discs in cardiac myocytes. A yeast-two-hybrid screen revealed that the mutant protein established novel interactions with histidine-rich calcium-binding protein and TGF beta induced apoptosis protein 2. Immunoblotting and confocal microscopy in human embryonic kidney 293 (HEK293) cell lines transfected to stably express either wild-type or mutant plakoglobin protein showed that the mutant protein was apparently ubiquitylated and was preferentially located in the cytoplasm, suggesting that the S39_K40insS mutation may increase plakoglobin turnover via proteasomal degradation. HEK293 cells expressing mutant plakoglobin also showed higher rates of proliferation and lower rates of apoptosis than did cells expressing the wild-type protein. Electron microscopy showed smaller and fewer desmosomes in cells expressing mutant plakoglobin. Taken together, these observations suggest that the S39_K40insS mutation affects the structure and distribution of mechanical and electrical cell junctions and could interfere with regulatory mechanisms mediated by Wnt-signaling pathways. These results implicate novel molecular mechanisms in the pathogenesis of ARVC.

Functional properties and intracellular signaling of CCN1/Cyr61

CCN1/Cyr61 is a member of the protein family that can be promptly induced by growth factors. CCN1/Cyr61 promotes cell proliferation, adhesion, and differentiation. It plays important roles in angiogenesis and extracellular matrix production. In addition, CCN1/Cyr61 has many potential functions in tumorigenesis, development, embryo implantation, as well as formation of endometriotic lesions. Expression of CCN1/Cyr61 is regulated by a variety of agents including cytokines, growth factors, steroid hormones, and some drugs. These inducers regulate the transcription of CCN1/Cyr61 through several signaling transduction pathways. CCN1/Cyr61 is able to interact either with the cell itself or the surrounding cells through an autocrine-paracrine mechanism. It has been reported that CCN1/Cyr61 exerts its functions via interacting with at least five integrins as well as heparan sulfate proteoglycan. By activating Wnt, NF-kappaB, or tyrosine kinase signaling pathways, CCN1/Cyr61 is not only able to control the growth of epithelial cells and fibroblasts, but also induce or suppress apoptosis in a cell type-specific manner.

Treatment of PC12 cells with nerve growth factor induces proteasomal degradation of T-cadherin that requires tyrosine phosphorylation of its cadherin domain

T-cadherin (T-Cad), a unique member of the cadherin family of proteins, plays an important role in cell adhesion and cell signaling. Recently, we demonstrated that T-Cad is transcriptionally repressed by DNA methyltransferase 3b during nerve growth factor (NGF)-induced neuronal differentiation of PC12 cells. Here, we show that T-Cad expression is also regulated at the post-translational level by the proteasomal pathway in these cells, which is facilitated upon NGF treatment. Pulse-chase experiments demonstrated that NGF treatment significantly reduced the half-life of T-Cad. Degradation of T-Cad was blocked upon treatment of PC12 cells with the proteasomal inhibitor ZLLL or lactacystin. Ectopic expression of Cdh1 (CDC20 homolog 1), one of the substrate recognition components of anaphase promoting complex (E3 ligase), stimulated T-Cad degradation. Deletion of CD1, one of the five extracellular cadherin domains (CD), promoted degradation of T-Cad, especially in the presence of NGF. On the contrary, deletion of CD2 stabilized this protein maximally. Ubiquitination of different deletion mutants indicates that T-Cad harbors multiple ubiquitination signals. Furthermore, genistein, a protein-tyrosine kinase inhibitor, impeded T-Cad degradation in PC12 cells, implicating requirement of tyrosine phosphorylation in this process. Mutation at tyrosine 327 (Y327F) markedly increased the half-life of T-Cad, suggesting that phosphorylation of this tyrosine residue located within CD2 is critical for this process. These results show that T-cadherin is subject to dual regulation during NGF-induced differentiation of PC12 cells, namely transcriptional repression mediated by Dnmt3b and post-translational degradation through the proteasomal pathway. These data, together with the inhibitory role of T-Cad in neurite outgrowth of PC12 cells upon NGF treatment, underscore the significance of its stringent regulation during this differentiation process.

The ubiquitin ligase SCF(betaTrCP) regulates the degradation of the growth hormone receptor

SCF ubiquitin ligases play a pivotal role in the regulation of cell division and various signal transduction pathways, which in turn are involved in cell growth, survival, and transformation. SCF(TrCP) recognizes the double phosphorylated DSGPhiXS destruction motif in beta-catenin and IkappaB. We show that the same ligase drives endocytosis and degradation of the growth hormone receptor (GHR) in a ligand-independent fashion. The F-box protein beta-TrCP binds directly and specifically with its WD40 domain to a novel recognition motif, previously designated as the ubiquitin-dependent endocytosis motif. Receptor degradation requires an active neddylation system, implicating ubiquitin ligase activity. GHR-TrCP binding, but not GHR ubiquitination, is necessary for endocytosis. TrCP2 silencing is more effective on GHR degradation and endocytosis than TrCP1, although overexpression of either isoform restores TrCP function in silenced cells. Together, these findings provide direct evidence for a key role of the SCF(TrCP) in the endocytosis and degradation of an important factor in growth, immunity, and life span regulation.

Cdc42 controls progenitor cell differentiation and beta-catenin turnover in skin

Differentiation of skin stem cells into hair follicles (HFs) requires the inhibition of beta-catenin degradation, which is controlled by a complex containing axin and the protein kinase GSK3beta. Using conditional gene targeting in mice, we show now that the small GTPase Cdc42 is crucial for differentiation of skin progenitor cells into HF lineage and that it regulates the turnover of beta-catenin. In the absence of Cdc42, degradation of beta-catenin was increased corresponding to a decreased phosphorylation of GSK3beta at Ser 9 and an increased phosphorylation of axin, which is known to be required for binding of beta-catenin to the degradation machinery. Cdc42-mediated regulation of beta-catenin turnover was completely dependent on PKCzeta, which associated with Cdc42, Par6, and Par3. These data suggest that Cdc42 regulation of beta-catenin turnover is important for terminal differentiation of HF progenitor cells in vivo.

Critical function for SIP, a ubiquitin E3 ligase component of the beta-catenin degradation pathway, for thymocyte development and G1 checkpoint

Beta-catenin has been implicated in thymocyte development because of its function as a coactivator of Tcf/LEF-family transcription factors. Previously, we discovered a novel pathway for p53-induced beta-catenin degradation through a ubiquitin E3 ligase complex involving Siah1, SIP (CacyBP), Skp1, and Ebi. To gain insights into the physiological relevance of this new degradation pathway in vivo, we generated mutant mice lacking SIP. We demonstrate here that SIP-/- thymocytes have an impaired pre-TCR checkpoint with failure of TCRbeta gene rearrangement and increased apoptosis, resulting in reduced cellularity of the thymus. Moreover, the degradation of beta-catenin in response to DNA damage is significantly impaired in SIP-/- cells. SIP-/- embryonic fibroblasts show a growth-rate increase resulting from defects in G1 arrest. Thus, the beta-catenin degradation pathway mediated by SIP defines an essential checkpoint for thymocyte development and cell-cycle progression.

Desmosomal cadherin misexpression alters beta-catenin stability and epidermal differentiation

Desmosomal adhesion is important for the integrity and protective barrier function of the epidermis and is disregulated during carcinogenesis. Strong adhesion between keratinocytes is conferred by the desmosomal cadherins, desmocollin (Dsc) and desmoglein. These constitute two gene families, members of which are differentially expressed in epidermal strata. It has been suggested that this stratum-specific expression regulates keratinocyte differentiation. We tested this hypothesis by misdirecting the expression of the basally abundant desmosomal cadherins Dsc3a and Dsc3b to suprabasal differentiating keratinocytes in transgenic mice. No phenotype was apparent until adulthood, when mice developed variable ventral alopecia and had altered keratinocyte differentiation within affected areas. The follicular changes were reminiscent of changes in transgenic mice with an altered beta-catenin stability. Stabilized beta-catenin and increased beta-catenin transcriptional activity were demonstrated in transgenic mice prior to the phenotypic change and in transgenic keratinocytes as a consequence of transgene expression. Hence, a link between desmosomal cadherins and beta-catenin stability and signaling was demonstrated, and it was shown that desmocollin cadherin expression can affect keratinocyte differentiation. Furthermore, the first function for a "b-type" desmocollin cadherin was demonstrated.

Overexpression of human βTrCP1 deleted of its F box induces tumorigenesis in transgenic mice

Genetic alterations affecting beta-catenin, adenomatous polyposis coli or axin proteins are associated with the pathogenesis of numerous human tumors. All these mutations result in the synthesis of unphosphorylated beta-catenin that escapes recognition by the beta transducin repeat protein (beta TrCP1), the receptor of an ubiquitin. The stabilized beta-catenin translocates to the nucleus and activates the transcription of genes crucial for tumorigenesis. Recent evidence implicates mutations and overexpresssion of beta TrCP1 in human prostate and colon tumors, respectively, suggesting that deregulated beta TrCP1 may be involved in tumorigenesis. To explore this possibility further, we generated transgenic mice that specifically express a dominant-negative mutant of beta TrCP1 (Delta F beta TrCP1) or full-length beta TrCP1 in intestine, liver and kidney. We found that 46% (16/35) of the transgenic mice that overexpressed the transgenes developed either intestinal adenomas (10/35) or hepatic (4/35) or urothelial (2/35) tumors. Immunohistological analysis of the tumors revealed that upregulation of cyclin D1, glutamine synthetase and chemotaxin 2 was associated with nuclear accumulation of beta-catenin. These results show that the overexpression of Delta F beta TrCP1 or beta TrCP1 in vivo induce tumors through beta-catenin activation.

Role of subtilisin-like convertases in cadherin processing or the conundrum to stall cadherin function by convertase inhibitors in cancer therapy

Cadherins are a family of intercellular adhesion receptors. Produced as inactive precursors, they become functional adhesion molecules after proteolytic cleavage by subtilisin-like pro-protein convertases (PCs). Owing to their activation and assembly into multiprotein adhesion complexes at sites of cell contacts, adhesion-competent cadherins are prerequisite for tissue integrity. In recent years evidence has accumulated that intercellular junctions not only provide mechanical linkage, but in addition are potent modulators of signalling cascades. This infers a biological role to intercellular adhesion complexes that is significantly more complex and powerful. Currently, the broad implications of disturbances in somatic tissue adhesion components are only just beginning to emerge. Prominent examples of adhesion defects include autoimmune diseases, or tumour invasion and metastasis and malignant transformation. This review reports on our current knowledge of cadherin function and their maturation by pro-protein convertases, and puts special emphasis on the consequences of pro-protein convertase inhibition for epithelial tissue homeostasis.

beta-catenin-dependent and -independent effects of DeltaN-plakoglobin on epidermal growth and differentiation

Both beta-catenin and plakoglobin can stimulate the expression of Lef/Tcf target genes in vitro. beta-Catenin is known to associate with Lef/Tcf factors and to participate directly in transactivation in vivo, whereas the role of plakoglobin in transcriptional regulation has been less studied. To analyze the functions of plakoglobin in vivo, we generated transgenic mice expressing in the epidermis N-terminally truncated plakoglobin (DeltaN122-PG) lacking the glycogen synthase kinase 3beta phosphorylation sites and therefore protected against degradation (transgenic line K5-DeltaN122-PG). The expression of DeltaN122-PG led to the formation of additional hair germs, hyperplastic hair follicles, and noninvasive hair follicle tumors, a phenotype reminiscent of that induced by expression of N-terminally truncated beta-catenin. However, if expressed in beta-catenin-null epidermis, DeltaN122-PG did not induce new hair follicle germs and follicular tumors. Thus, DeltaN122-PG cannot substitute for beta-catenin in its signaling functions in vivo and the phenotype observed in K5-DeltaN122-PG mouse skin must be due to the aberrant activation of beta-catenin signaling. On the other hand, the expression of DeltaN122-PG in beta-catenin-null skin significantly increased the survival rate of mutant mice, rescued differentiation, and limited excessive proliferation in the interfollicular epidermis, suggesting that plakoglobin may be involved in the intracellular signaling events essential for epidermal differentiation.

Cyr61 suppresses the growth of non-small-cell lung cancer cells via the beta-catenin-c-myc-p53 pathway

Cysteine-rich protein 61 (Cyr61) is a growth factor-inducible, immediate-early gene that has multifaceted activities in various cancers. In a previous study, we found that Cyr61 inhibited the growth of the H520 and H460 non-small-cell lung cancer (NSCLC) cell lines. In further studies, we now report that p53 plays a pivotal role in Cyr61-dependent cellular growth arrest. Blocking Cyr61 with a Cyr61 antibody resulted in the downregulation of expression of p53 and p21, as well as partially reversing the growth suppression of H520-Cyr61 cells. Proliferation of NSCLC cell lines (NCI-H157, H125, H1299), having a mutant p53, were not suppressed by Cyr61. Inhibition of wild-type p53, by either human papilloma virus type 16 E6 or a dominant-negative p53, resulted in the rescue of the growth suppression mediated by Cyr61 in the H520-Cyr61 cells. The enhanced levels of p21WAF1 and p130/RB2, in the Cyr61-expressing H520-Cyr61 cells, were also inhibited by blocking p53 showing that p21 and p130 were induced by p53 in these cells. In addition, levels of both c-myc and beta-catenin increased in Cyr61 stably transfected H520 cells. Moreover, beta-catenin was translocated into the nucleus in these cells. Inhibition of c-myc expression in the H520-Cyr61 cells with antisense c-myc resulted in their decreased levels of p53. Transfecting cells with a dominant-negative T-cell factor (TCF4), the specific inhibitor of the beta-catenin/TCF4 complex, downregulated the expression of c-myc. Taken together, the data suggest that Cyr61 suppressed the growth of NSCLC cells by triggering a signal transduction pathway through beta-catenin. In this pathway, Cyr61 activated the beta-catenin/TCF4 complex, which promoted the expression of c-myc and the latter induced expression of p53, and p53 upregulated p21WAF1 and p130/RB2, resulting in growth arrest.

Translocation products in acute myeloid leukemia activate the Wnt signaling pathway in hematopoietic cells

The acute myeloid leukemia (AML)-associated translocation products AML1-ETO, PML-retinoic acid receptor alpha (RARalpha), and PLZF-RARalpha encode aberrant transcription factors. Several lines of evidence suggest similar pathogenetic mechanisms for these fusion proteins. We used high-density oligonucleotide arrays to identify shared target genes in inducibly transfected U937 cells expressing AML1-ETO, PML-RARalpha, or PLZF-RARalpha. All three fusion proteins significantly repressed the expression of 38 genes and induced the expression of 14 genes. Several of the regulated genes were associated with Wnt signaling. One of these, plakoglobin (gamma-catenin), was induced on the mRNA and protein level by all three fusion proteins. In addition, primary AML blasts carrying one of the fusion proteins significantly overexpressed plakoglobin. The plakoglobin promoter was cloned and shown to be induced by AML1-ETO, with promoter activation depending on the corepressor and histone deacetylase binding domains. The induction of plakoglobin by AML fusion proteins led to downstream signaling and transactivation of TCF- and LEF-dependent promoters, including the c-myc promoter, which was found to be bound by plakoglobin in vivo after AML1-ETO expression. beta-Catenin protein levels and TCF and LEF target genes such as c-myc and cyclin D1 were found to be induced by the fusion proteins. On the functional level, a dominant negative TCF inhibited colony growth of AML1-ETO-positive Kasumi cells, whereas plakoglobin transfection into myeloid 32D cells enhanced proliferation and clonal growth. Injection of plakoglobin-expressing 32D cells into syngeneic mice accelerated the development of leukemia. Transduction of plakoglobin into primitive murine hematopoietic progenitor cells preserved the immature phenotype during colony growth, suggesting enhanced self-renewal. These data provide evidence that activation of Wnt signaling is a common feature of several balanced translocations in AML.

SCF(HOS) ubiquitin ligase mediates the ligand-induced down-regulation of the interferon-alpha receptor

Down-regulation of activated signaling receptors in response to their ligands plays a key role in restricting the extent and duration of the signaling. Mechanisms underlying down-regulation of the type I interferon receptor consisting of IFNAR1 and IFNAR2 subunits remain largely unknown. Here we show that IFNAR1 interacts with the Homolog of Slimb (HOS) F-box protein in a phosphorylation-dependent manner, and that this interaction is promoted by interferon alpha (IFNalpha). IFNAR1 is ubiquitinated by the Skp1-Cullin1-HOS-Roc1 (SCF(HOS)) ubiquitin ligase in vitro. HOS expression and activities are required for IFNalpha-stimulated ubiquitination of IFNAR1, endocytosis of the type I interferon receptor, down-regulation of IFNAR1 levels, and IFNAR1 proteolysis via the lysosomal pathway. Furthermore, modulations of HOS activities affect the extent of Stat1 phosphorylation and Stat-mediated transcriptional activities as well as the extent of antiproliferative effects of type I interferons. These findings characterize SCF(HOS) as an E3 ubiquitin ligase that is essential for ubiquitination, proteolysis and down-regulation of IFNAR1, and implicate HOS in the regulation of cellular responses to IFNalpha.

Involvement of casein kinase Iepsilon in cytokine-induced granulocytic differentiation

Two closely related casein kinase I (CKI) isoforms, CKIdelta and CKIepsilon, are ubiquitously expressed in many human tissues, but their specific biologic function remains to be clarified. Here, we provide the first evidence that CKIepsilon is involved in hematopoietic cell differentiation. CKIepsilon, but not CKIdelta, was down-regulated along with human granulocytic differentiation. The specific down-regulation was observed in granulocyte colony-stimulating factor (G-CSF)-induced cell differentiation of murine interleukin-3 (IL-3)-dependent myeloid progenitor 32D cells. Introduction of wild-type (WT)-CKIepsilon into 32D cells inhibited the G-CSF-induced cell differentiation, whereas kinase-negative (KN)-CKIepsilon promoted the differentiation. Neither WT- nor KN-CKIepsilon affected IL-3-dependent cell growth. Moreover, introduction of WT- or KN-CKIdelta did not affect the cytokine-induced cell growth and differentiation. While G-CSF-induced activation of signal transducers and activators of transcription 3 (STAT3) was sustained by KN-CKIepsilon, STAT3 activation was attenuated by WT-CKIepsilon. This may be explained by the fact that the suppressor of cytokine signaling 3 (SOCS3) was stabilized by its physical association with CKIepsilon. Such stabilization by CKIepsilon was also seen in IL-3-induced beta-catenin. The stabilization of downstream components of cytokine and Wnt signaling by CKIepsilon might be critical for integration of several intracellular signaling pathways to a cell-specific biologic response in hematopoietic cell self-renewal.

HIV-1 Vpu sequesters beta-transducin repeat-containing protein (betaTrCP) in the cytoplasm and provokes the accumulation of beta-catenin and other SCFbetaTrCP substrates

The human immunodeficiency virus type 1 Vpu protein acts as an adaptor for the proteasomal degradation of CD4 by recruiting CD4 and beta-transducin repeat-containing protein (betaTrCP), the receptor component of the multisubunit SCF-betaTrCP E3 ubiquitin ligase complex. We showed that the expression of a Vpu-green fluorescent fusion protein prevented the proteosomal degradation of betaTrCP substrates such as beta-catenin, IkappaBalpha, and ATF4, which are normally directly targeted to the proteasome for degradation. Beta-catenin was translocated into the nucleus, whereas the tumor necrosis factor-induced nuclear translocation of NFkappaB was impaired. Beta-catenin was also up-regulated in cells producing Vpu+ human immunodeficiency virus type 1 but not in cells producing Vpu-deficient viruses. The overexpression of ATF4 also provoked accumulation of beta-catenin, but to a lower level than that resulting from the expression of Vpu. Finally, the expression of Vpu induces the exclusion of betaTrCP from the nucleus. These data suggest that Vpu is a strong competitive inhibitor of betaTrCP that impairs the degradation of SCFbetaTrCP substrates as long as Vpu has an intact phosphorylation motif and can bind to betaTrCP.

Plakoglobin is O-glycosylated close to the N-terminal destruction box

Plakoglobin provides a key linkage in protein chains that connect desmosomal and classical cadherins to the cytoskeleton. It is also present in a significant cytosolic pool that has the capacity to impact on canonical Wnt signaling by competing for interaction with partner proteins of beta-catenin. The closely related protein, beta-catenin, is rapidly targeted for proteasomal degradation by phosphorylation of a "destruction box" within the N-terminal domain. Inhibition of this process forms the basis of Wnt signaling. This destruction box is also found in the N-terminal domain of plakoglobin. We report that plakoglobin is modified by the addition of O-GlcNAc at a single site in close proximity to the destruction box. O-GlcNAc modification has been proposed to counteract phosphorylation, provide protection from proteasomal degradation, mediate signal transduction, silence transcription, and regulate multimolecular protein assembly. This finding has potential implications for understanding the roles of plakoglobin.

Regulation of NF-kappaB2/p100 processing by its nuclear shuttling

Processing of the NF-kappaB2 precursor protein p100 to generate p52 is an important step of NF-kappaB regulation. This proteolytic event is tightly regulated by sequences located at the C-terminal portion of p100. Constitutive processing of p100 occurs in certain lymphoma cells due to the loss of its C-terminal regulatory domain, although the underlying mechanisms remain unknown. We show here that the constitutive processing of C-terminal truncation mutants of p100 is associated with their active nuclear translocation. Deletion of the C-terminal death domain of p100 triggers a low, but significant, level of nuclear translocation and processing. Disruption of the ankyrin-repeat domain of p100 further enhances its nuclear shuttling activity, which is again associated with elevated level of processing. More importantly, mutation of the nuclear localization signal (NLS) of p100 abolishes its processing, and this defect can be rescued by fusion of a heterologous NLS to the amino- or carboxyl-terminus of the p100 mutant. These results suggest that nuclear shuttling is a mechanism regulating the processing of NF-kappaB2/p100.

Real-time analysis of beta- and gamma-catenin mRNA expression in ret/PTC-1 activated and nonactivated thyroid tissues

Our group has previously demonstrated an association between ret/PTC-1 activation and decreased E-cadherin mRNA levels in papillary thyroid carcinoma. We also observed similarities in the E-cadherin expression profiles of Hashimoto thyroiditis and ret/PTC-1-positive papillary thyroid carcinomas and have hypothesized that ret/PTC-1 activation might cause not only the structural and nuclear peculiarities of PTC but also an immune reaction to thyroid epithelium. The objective of this study was to examine the expression of E-cadherin's ligands, beta- and gamma-catenin, in various thyroid tissue types in the context of ret/PTC-1 positivity using laser capture microdissection and TaqMan (Applied Biosystems, Foster City, CA). One-Step RT-PCR. Beta-catenin mRNA levels were found to be consistently decreased in both papillary and anaplastic carcinomas when compared with a normal/follicular adenoma group. A significant difference in expression levels was observed between papillary and follicular thyroid carcinomas with the latter having elevated mRNA levels of beta-catenin. Gamma-catenin mRNA was decreased in anaplastic carcinomas compared with normal/follicular adenoma groups. A similar expression profile of gamma-catenin as beta-catenin was observed in papillary and follicular carcinomas with the latter once again having higher mRNA levels. These results therefore suggest that although beta- and gamma-catenin may play a role in the progression of thyroid cancer in general, they do not appear to be associated with ret/PTC-1-modulated pathways.

Protein evolution: structure-function relationships of the oncogene beta-catenin in the evolution of multicellular animals

Beta-catenin functions as a cytoskeletal linker protein in cadherin-mediated adhesion and as a signal mediator in wnt-signal transduction pathways. We use a novel integrative approach, combining evolutionary, genomic, and three-dimensional structural data to analyze and trace the structural and functional evolution of beta-catenin genes. This approach also enabled us to examine the effects of gene duplication on the structure and function of beta-catenin genes in Drosophila, C. elegans, and vertebrates. By sampling a large number of different taxa, we identified both ancestral and derived motifs and residues within the different regions of the beta-catenin proteins. Projecting amino acid substitutions onto the three- dimensional structure established for mouse beta-catenin, we identified specific domains that exhibit loss and gain of selective constraints during beta catenin evolution. Structural changes, changes in the amino acid substitution rate, and the appearance of novel functional domains in beta-catenin can be mapped to specific branches on the metazoan tree. Together, our analyses suggest that a single, beta-catenin gene fulfilled both adhesion and signaling functions in the last common ancestor of metazoans some 700 million years ago. In addition, gene duplications facilitated the evolution of beta-catenins with novel functions and allowed the evolution of multiple, single-function proteins (cell adhesion or wnt-signaling) from the ancestral, dual-function protein. Integrative methods such as those we have applied here, utilizing the 'natural experiments' present in animal diversity, can be employed to identify novel and shared functional motifs and residues in virtually any protein among the proteomes of model systems and humans.

Nr-CAM is a target gene of the beta-catenin/LEF-1 pathway in melanoma and colon cancer and its expression enhances motility and confers tumorigenesis

beta-catenin and plakoglobin (gamma-catenin) are homologous molecules involved in cell adhesion, linking cadherin receptors to the cytoskeleton. beta-catenin is also a key component of the Wnt pathway by being a coactivator of LEF/TCF transcription factors. To identify novel target genes induced by beta-catenin and/or plakoglobin, DNA microarray analysis was carried out with RNA from cells overexpressing either protein. This analysis revealed that Nr-CAM is the gene most extensively induced by both catenins. Overexpression of either beta-catenin or plakoglobin induced Nr-CAM in a variety of cell types and the LEF/TCF binding sites in the Nr-CAM promoter were required for its activation by catenins. Retroviral transduction of Nr-CAM into NIH3T3 cells stimulated cell growth, enhanced motility, induced transformation, and produced rapidly growing tumors in nude mice. Nr-CAM and LEF-1 expression was elevated in human colon cancer tissue and cell lines and in human malignant melanoma cell lines but not in melanocytes or normal colon tissue. Dominant negative LEF-1 decreased Nr-CAM expression and antibodies to Nr-CAM inhibited the motility of B16 melanoma cells. The results indicate that induction of Nr-CAM transcription by beta-catenin or plakoglobin plays a role in melanoma and colon cancer tumorigenesis, probably by promoting cell growth and motility.

Regulation of S33/S37 phosphorylated β-catenin in normal and transformed cells

A novel phosphorylation-specific antibody (αpβ-catenin) was generated against a peptide corresponding to amino acids 33-45 of humanβ-catenin, which contained phosphorylated serines at positions 33 and 37. This antibody is specific to phosphorylated β-catenin and reacts neither with the non-phosphorylated protein nor with phosphorylated or non-phosphorylated plakoglobin. It weakly interacts with S33Y β-catenin but not with the S37A mutant. pβ-catenin is hardly detectable in normal cultured cells and accumulates (up to 55% of total β-catenin) upon overexpression of the protein or after blocking its degradation by the proteasome. Inhibition of both GSK-3β and the proteasome resulted in a rapid (t1/2=10 minutes) and reversible reduction in pβ-catenin levels, suggesting that the protein can undergo dephosphorylation in live cells, at a rate comparable to its phosphorylation by GSK-3β. pβ-catenin interacts with LEF-1, but fails to form a ternary complex with DNA, suggesting that it is transcriptionally inactive. Immunofluorescence microscopy indicated that pβ-catenin accumulates in the nuclei of MDCK and BCAP cells when overexpressed and is transiently associated with adherens junctions shortly after their formation. pβ-catenin only weakly interacts with co-transfected N-cadherin, although it forms a complex with the ubiquitin ligase component β-TrCP. SW480 colon cancer cells that express a truncated APC, at position 1338, contain high levels of pβ-catenin,whereas HT29 cells, expressing APC truncated at position 1555, accumulate non-phosphorylated β-catenin, suggesting that the 1338-1555 amino acid region of APC is involved in the differential regulation of the dephosphorylation and degradation of pβ-catenin.

Pseudosubstrate regulation of the SCF(beta-TrCP) ubiquitin ligase by hnRNP-U

beta-TrCP/E3RS (E3RS) is the F-box protein that functions as the receptor subunit of the SCF(beta-TrCP) ubiquitin ligase (E3). Surprisingly, although its two recognized substrates, IkappaB(alpha) and beta-catenin, are present in the cytoplasm, we have found that E3RS is located predominantly in the nucleus. Here we report the isolation of the major E3RS-associated protein, hnRNP-U, an abundant nuclear phosphoprotein. This protein occupies E3RS in a specific and stoichiometric manner, stabilizes the E3 component, and is likely responsible for its nuclear localization. hnRNP-U binding was abolished by competition with a pIkappaB(alpha) peptide, or by a specific point mutation in the E3RS WD region, indicating an E3-substrate-type interaction. However, unlike pI(kappa)Balpha, which is targeted by SCF(beta-TrCP) for degradation, the E3-bound hnRNP-U is stable and is, therefore, a pseudosubstrate. Consequently, hnRNP-U engages a highly neddylated active SCF(beta-TrCP), which dissociates in the presence of a high-affinity substrate, resulting in ubiquitination of the latter. Our study points to a novel regulatory mechanism, which secures the localization, stability, substrate binding threshold, and efficacy of a specific protein-ubiquitin ligase.

FAM deubiquitylating enzyme is essential for preimplantation mouse embryo development

FAM is a developmentally regulated substrate-specific deubiquitylating enzyme. It binds the cell adhesion and signalling molecules beta-catenin and AF-6 in vitro, and stabilises both in mammalian cell culture. To determine if FAM is required at the earliest stages of mouse development we examined its expression and function in preimplantation mouse embryos. FAM is expressed at all stages of preimplantation development from ovulation to implantation. Exposure of two-cell embryos to FAM-specific antisense, but not sense, oligodeoxynucleotides resulted in depletion of the FAM protein and failure of the embryos to develop to blastocysts. Loss of FAM had two physiological effects, namely, a decrease in cleavage rate and an inhibition of cell adhesive events. Depletion of FAM protein was mirrored by a loss of beta-catenin such that very little of either protein remained following 72h culture. The residual beta-catenin was localised to sites of cell-cell contact suggesting that the cytoplasmic pool of beta-catenin is stabilised by FAM. Although AF-6 levels initially decreased they returned to normal. However, the nascent protein was mislocalised at the apical surface of blastomeres. Therefore FAM is required for preimplantation mouse embryo development and regulates beta-catenin and AF-6 in vivo.

Down-regulation of beta-catenin by activated p53

beta-Catenin is a cytoplasmic protein that participates in the assembly of cell-cell adherens junctions by binding cadherins to the actin cytoskeleton. In addition, it is a key component of the Wnt signaling pathway. Activation of this pathway triggers the accumulation of beta-catenin in the nucleus, where it activates the transcription of target genes. Abnormal accumulation of beta-catenin is characteristic of various types of cancer and is caused by mutations either in the adenomatous polyposis coli protein, which regulates beta-catenin degradation, or in the beta-catenin molecule itself. Aberrant accumulation of beta-catenin in tumors is often associated with mutational inactivation of the p53 tumor suppressor. Here we show that overexpression of wild-type p53, by either transfection or DNA damage, down-regulates beta-catenin in human and mouse cells. This effect was not obtained with transcriptionally inactive p53, including a common tumor-associated p53 mutant. The reduction in beta-catenin level was accompanied by inhibition of its transactivation potential. The inhibitory effect of p53 on beta-catenin is apparently mediated by the ubiquitin-proteasome system and requires an active glycogen synthase kinase 3beta (GSK3beta). Mutations in the N terminus of beta-catenin which compromise its degradation by the proteasomes, overexpression of dominant-negative DeltaF-beta-TrCP, or inhibition of GSKbeta activity all rendered beta-catenin resistant to down-regulation by p53. These findings support the notion that there will be a selective pressure for the loss of wild-type p53 expression in cancers that are driven by excessive accumulation of beta-catenin.

Deregulated beta-catenin induces a p53- and ARF-dependent growth arrest and cooperates with Ras in transformation

Aberrant activation of beta-catenin contributes to the onset of a variety of tumors. We report that a tumor-derived beta-catenin mutant induces accumulation and activation of the p53 tumor suppressor protein. Induction is mediated through ARF, an alternative reading frame product of the INK4A tumor suppressor locus, in a manner partially dependent on the transcription factor E2F1. In wild-type mouse embryo fibroblasts, mutant beta-catenin inhibits cell proliferation and imposes a senescence-like phenotype. This does not occur in cells lacking either ARF or p53, where deregulated beta-catenin actually overrides density-dependent growth inhibition and cooperates with activated Ras in transformation. Thus, the oncogenic activity of deregulated beta-catenin is curtailed by concurrent activation of the p53 pathway, thereby providing a protective mechanism against cancer. When the p53 pathway is impaired, deregulated beta-catenin is free to manifest its oncogenic features. This can occur not only by p53 mutations, but also by ablation of ARF expression, as observed frequently in early stages of colorectal carcinogenesis.