beta-Trcp couples beta-catenin phosphorylation-degradation and regulates Xenopus axis formation

Wnt/beta-catenin signaling in oral tissue development and disease

The Wnt/beta-catenin signaling pathway is one of several key conserved intercellular signaling pathways in animals, and plays fundamental roles in the proliferation, regeneration, differentiation, and function of many cell and tissue types. This pathway is activated in a dynamic manner during the morphogenesis of oral organs, including teeth, taste papillae, and taste buds, and is essential for these processes to occur normally. Conversely, forced activation of Wnt/beta-catenin signaling promotes the formation of ectopic teeth and taste papillae. In this review, we discuss our current understanding of the roles of Wnt/beta-catenin signaling in oral tissue development and in related human diseases, and the potential of manipulating this pathway for therapeutic purposes.

Global regulator SATB1 recruits beta-catenin and regulates T(H)2 differentiation in Wnt-dependent manner

Chromatin organizer SATB1 and Wnt transducer β-catenin form a complex and regulate expression of GATA3 and T_H2 cytokines in Wnt-dependent manner and orchestrate T_H2 lineage commitment.

In vertebrates, the conserved Wnt signalling cascade promotes the stabilization and nuclear accumulation of β-catenin, which then associates with the lymphoid enhancer factor/T cell factor proteins (LEF/TCFs) to activate target genes. Wnt/β -catenin signalling is essential for T cell development and differentiation. Here we show that special AT-rich binding protein 1 (SATB1), the T lineage-enriched chromatin organizer and global regulator, interacts with β-catenin and recruits it to SATB1's genomic binding sites. Gene expression profiling revealed that the genes repressed by SATB1 are upregulated upon Wnt signalling. Competition between SATB1 and TCF affects the transcription of TCF-regulated genes upon β-catenin signalling. GATA-3 is a T helper type 2 (T_H2) specific transcription factor that regulates production of T_H2 cytokines and functions as T_H2 lineage determinant. SATB1 positively regulated GATA-3 and siRNA-mediated knockdown of SATB1 downregulated GATA-3 expression in differentiating human CD4⁺ T cells, suggesting that SATB1 influences T_H2 lineage commitment by reprogramming gene expression. In the presence of Dickkopf 1 (Dkk1), an inhibitor of Wnt signalling, GATA-3 is downregulated and the expression of signature T_H2 cytokines such as IL-4, IL-10, and IL-13 is reduced, indicating that Wnt signalling is essential for T_H2 differentiation. Knockdown of β-catenin also produced similar results, confirming the role of Wnt/β-catenin signalling in T_H2 differentiation. Furthermore, chromatin immunoprecipitation analysis revealed that SATB1 recruits β-catenin and p300 acetyltransferase on GATA-3 promoter in differentiating T_H2 cells in a Wnt-dependent manner. SATB1 coordinates T_H2 lineage commitment by reprogramming gene expression. The SATB1:β-catenin complex activates a number of SATB1 regulated genes, and hence this study has potential to find novel Wnt responsive genes. These results demonstrate that SATB1 orchestrates T_H2 lineage commitment by mediating Wnt/β-catenin signalling. This report identifies a new global transcription factor involved in β-catenin signalling that may play a major role in dictating the functional outcomes of this signalling pathway during development, differentiation, and tumorigenesis.

In vertebrates the canonical Wnt signalling culminates in β-catenin moving into the nucleus where it activates transcription of target genes. Wnt/β-catenin signalling is essential for the thymic maturation and differentiation of naïve T cells. Here we show that SATB1, a T cell lineage-enriched chromatin organizer and global regulator, binds to β-catenin and recruits it to SATB1's genomic binding sites so that genes formerly repressed by SATB1 are upregulated by Wnt signalling. Some of the genes known to be regulated by SATB1 (such as genes encoding cytokines and the transcription factor GATA3) are required for differentiation of Th2 cells, an important subset of helper T cells. Specifically we show that siRNA-mediated knockdown of SATB1 downregulated GATA-3 expression in differentiating human CD4⁺ T cells. Inhibiting Wnt signalling led to downregulation of GATA-3 and of signature T_H2 cytokines such as IL-4, IL-10, and IL-13. Knockdown of β-catenin also produced similar results, thus together these data confirm the role of Wnt/β-catenin signalling in T_H2 differentiation. Our data demonstrate that SATB1 orchestrates T_H2 lineage commitment by modulating Wnt/β-catenin signalling.

Identification of clusterin domain involved in NF-kappaB pathway regulation

Clusterin (CLU) is a ubiquitous protein that has been implicated in tumorigenesis, apoptosis, inflammation, and cell proliferation. We and others have previously shown that CLU is an inhibitor of the NF-kappaB pathway. However, the exact form of CLU and the region(s) of CLU involved in this effect were unknown. Using newly generated molecular constructs encoding for CLU and various regions of the molecule, we demonstrated that the presecretory form of CLU (psCLU) form bears the NF-kappaB regulatory activity. Sequence comparison analysis showed sequence motif identity between CLU and beta-transducin repeat-containing protein (beta-TrCP), a main E3 ubiquitin ligase involved in IkappaB-alpha degradation. These homologies were localized in the disulfide constraint region of CLU. We generated a specific molecular construct of this region, named DeltaCLU, and showed that it has the same NF-kappaB regulatory activity as CLU. Neither the alpha-chain nor the beta-chain of CLU had any NF-kappaB regulatory activity. Furthermore, we showed that following tumor necrosis factor-alpha stimulation of transfected cells, we could co-immunoprecipitate phospho-IkappaB-alpha with DeltaCLU. Moreover, we showed that DeltaCLU could localize both in the cytoplasm and in the nucleus. These results demonstrate the identification of a new CLU activity site involved in NF-kappaB pathway regulation.

Murrayafoline A attenuates the Wnt/beta-catenin pathway by promoting the degradation of intracellular beta-catenin proteins

Molecular lesions in Wnt/beta-catenin signaling and subsequent up-regulation of beta-catenin response transcription (CRT) occur frequently during the development of colon cancer. To identify small molecules that suppress CRT, we screened natural compounds in a cell-based assay for detection of TOPFalsh reporter activity. Murrayafoline A, a carbazole alkaloid isolated from Glycosmis stenocarpa, antagonized CRT that was stimulated by Wnt3a-conditioned medium (Wnt3a-CM) or LiCl, an inhibitor of glycogen synthase kinase-3beta (GSK-3beta), and promoted the degradation of intracellular beta-catenin without altering its N-terminal phosphorylation at the Ser33/37 residues, marking it for proteasomal degradation, or the expression of Siah-1, an E3 ubiquitin ligase. Murrayafoline A repressed the expression of cyclin D1 and c-myc, which is known beta-catenin/T cell factor (TCF)-dependent genes and thus inhibited the proliferation of various colon cancer cells. These findings indicate that murrayafoline A may be a potential chemotherapeutic agent for use in the treatment of colon cancer.

beta-TrCP-mediated ubiquitination and degradation of PHLPP1 are negatively regulated by Akt

PHLPP1 belongs to a novel family of Ser/Thr protein phosphatases that serve as tumor suppressors by negatively regulating Akt signaling. Our recent studies have demonstrated that loss of PHLPP expression occurs at high frequency in colorectal cancer. In this study, we identified PHLPP1 as a proteolytic target of a beta-TrCP-containing Skp-Cullin 1-F-box protein (SCF) complex (SCF(beta-TrCP)) E3 ubiquitin ligase in a phosphorylation-dependent manner. Overexpression of wild-type but not DeltaF-box mutant beta-TrCP leads to decreased expression and increased ubiquitination of PHLPP1, whereas knockdown of endogenous beta-TrCP has the opposite effect. In addition, we show that the beta-TrCP-mediated degradation requires phosphorylation of PHLPP1 by casein kinase I and glycogen synthase kinase 3beta (GSK-3beta), and activation of the phosphatidylinositol 3-kinase/Akt pathway suppresses the degradation of PHLPP1 by inhibiting the GSK-3beta activity. Furthermore, expression of a degradation-deficient PHLPP1 mutant in colon cancer cells results in a more effective dephosphorylation of Akt and inhibition of cell growth. Taken together, our findings demonstrate a key role for beta-TrCP in controlling the level of PHLPP1, and activation of Akt negatively regulates this degradation process.

PR55 alpha, a regulatory subunit of PP2A, specifically regulates PP2A-mediated beta-catenin dephosphorylation

A central question in Wnt signaling is the regulation of beta-catenin phosphorylation and degradation. Multiple kinases, including CKI alpha and GSK3, are involved in beta-catenin phosphorylation. Protein phosphatases such as PP2A and PP1 have been implicated in the regulation of beta-catenin. However, which phosphatase dephosphorylates beta-catenin in vivo and how the specificity of beta-catenin dephosphorylation is regulated are not clear. In this study, we show that PP2A regulates beta-catenin phosphorylation and degradation in vivo. We demonstrate that PP2A is required for Wnt/beta-catenin signaling in Drosophila. Moreover, we have identified PR55 alpha as the regulatory subunit of PP2A that controls beta-catenin phosphorylation and degradation. PR55 alpha, but not the catalytic subunit, PP2Ac, directly interacts with beta-catenin. RNA interference knockdown of PR55 alpha elevates beta-catenin phosphorylation and decreases Wnt signaling, whereas overexpressing PR55 alpha enhances Wnt signaling. Taken together, our results suggest that PR55 alpha specifically regulates PP2A-mediated beta-catenin dephosphorylation and plays an essential role in Wnt signaling.

Aryl hydrocarbon receptor suppresses intestinal carcinogenesis in ApcMin/+ mice with natural ligands

Intestinal cancer is one of the most common human cancers. Aberrant activation of the canonical Wnt signaling cascade, for example, caused by adenomatous polyposis coli (APC) gene mutations, leads to increased stabilization and accumulation of beta-catenin, resulting in initiation of intestinal carcinogenesis. The aryl hydrocarbon receptor (AhR) has dual roles in regulating intracellular protein levels both as a ligand-activated transcription factor and as a ligand-dependent E3 ubiquitin ligase. Here, we show that the AhR E3 ubiquitin ligase has a role in suppression of intestinal carcinogenesis by a previously undescribed ligand-dependent beta-catenin degradation pathway that is independent of and parallel to the APC system. This function of AhR is activated by both xenobiotics and natural AhR ligands, such as indole derivatives that are converted from dietary tryptophan and glucosinolates by intestinal microbes, and suppresses intestinal tumor development in Apc(Min/+) mice. These findings suggest that chemoprevention with naturally-occurring and chemically-designed AhR ligands can be used to successfully prevent intestinal cancers.

Aryl hydrocarbon receptor suppresses intestinal carcinogenesis in ApcMin/+ mice with natural ligands

Intestinal cancer is one of the most common human cancers. Aberrant activation of the canonical Wnt signaling cascade, for example, caused by adenomatous polyposis coli (APC) gene mutations, leads to increased stabilization and accumulation of beta-catenin, resulting in initiation of intestinal carcinogenesis. The aryl hydrocarbon receptor (AhR) has dual roles in regulating intracellular protein levels both as a ligand-activated transcription factor and as a ligand-dependent E3 ubiquitin ligase. Here, we show that the AhR E3 ubiquitin ligase has a role in suppression of intestinal carcinogenesis by a previously undescribed ligand-dependent beta-catenin degradation pathway that is independent of and parallel to the APC system. This function of AhR is activated by both xenobiotics and natural AhR ligands, such as indole derivatives that are converted from dietary tryptophan and glucosinolates by intestinal microbes, and suppresses intestinal tumor development in Apc(Min/+) mice. These findings suggest that chemoprevention with naturally-occurring and chemically-designed AhR ligands can be used to successfully prevent intestinal cancers.

Wnt/Fz signaling and the cytoskeleton: potential roles in tumorigenesis

Wnt/beta-catenin regulates cellular functions related to tumor initiation and progression, cell proliferation, differentiation, survival, and adhesion. Beta-catenin-independent Wnt pathways have been proposed to regulate cell polarity and migration, including metastasis. In this review, we discuss the possible roles of both beta-catenin-dependent and -independent signaling pathways in tumor progression, with an emphasis on their regulation of Rho-family GTPases, cytoskeletal remodeling, and relationships with cell-cell adhesion and cilia/ciliogenesis.

Inhibition of GSK3 phosphorylation of beta-catenin via phosphorylated PPPSPXS motifs of Wnt coreceptor LRP6

The Wnt/β-catenin signaling pathway plays essential roles in cell proliferation and differentiation, and deregulated β-catenin protein levels lead to many types of human cancers. On activation by Wnt, the Wnt co-receptor LDL receptor related protein 6 (LRP6) is phosphorylated at multiple conserved intracellular PPPSPXS motifs by glycogen synthase kinase 3 (GSK3) and casein kinase 1 (CK1), resulting in recruitment of the scaffolding protein Axin to LRP6. As a result, β-catenin phosphorylation by GSK3 is inhibited and β-catenin protein is stabilized. However, how LRP6 phosphorylation and the ensuing LRP6-Axin interaction lead to the inhibition of β-catenin phosphorylation by GSK3 is not fully understood. In this study, we reconstituted Axin-dependent β-catenin phosphorylation by GSK3 and CK1 in vitro using recombinant proteins, and found that the phosphorylated PPPSPXS peptides directly inhibit β-catenin phosphorylation by GSK3 in a sequence and phosphorylation-dependent manner. This inhibitory effect of phosphorylated PPPSPXS motifs is direct and specific for GSK3 phosphorylation of β-catenin at Ser33/Ser37/Thr41 but not for CK1 phosphorylation of β-catenin at Ser45, and is independent of Axin function. We also show that a phosphorylated PPPSPXS peptide is able to activate Wnt/β-catenin signaling and to induce axis duplication in Xenopus embryos, presumably by inhibition of GSK3 in vivo. Based on these observations, we propose a working model that Axin recruitment to the phosphorylated LRP6 places GSK3 in the vicinity of multiple phosphorylated PPPSPXS motifs, which directly inhibit GSK3 phosphorylation of β-catenin. This model provides a possible mechanism to account, in part, for inhibition of β-catenin phosphorylation by Wnt-activated LRP6.

Accumulation of phosphorylated beta-catenin enhances ROS-induced cell death in presenilin-deficient cells

Presenilin (PS) is involved in many cellular events under physiological and pathological conditions. Previous reports have revealed that PS deficiency results in hyperproliferation and resistance to apoptotic cell death. In the present study, we investigated the effects of PS on β-catenin and cell mortality during serum deprivation. Under these conditions, PS1/PS2 double-knockout MEFs showed aberrant accumulation of phospho-β-catenin, higher ROS generation, and notable cell death. Inhibition of β-catenin phosphorylation by LiCl reversed ROS generation and cell death in PS deficient cells. In addition, the K19/49R mutant form of β-catenin, which undergoes normal phosphorylation but not ubiquitination, induced cytotoxicity, while the phosphorylation deficient S37A β-catenin mutant failed to induce cytotoxicity. These results indicate that aberrant accumulation of phospho-β-catenin underlies ROS-mediated cell death in the absence of PS. We propose that the regulation of β-catenin is useful for identifying therapeutic targets of hyperproliferative diseases and other degenerative conditions.

Sox9 inhibits Wnt signaling by promoting beta-catenin phosphorylation in the nucleus

Chondrocyte fate determination and maintenance requires Sox9, an intrinsic transcription factor, but is inhibited by Wnt/beta-catenin signaling activated by extrinsic Wnt ligands. Here we explored the underlying molecular mechanism by which Sox9 antagonizes the Wnt/beta-catenin signaling in chondrocyte differentiation. We found that Sox9 employed two distinct mechanisms to inhibit Wnt/beta-catenin signaling: the Sox9 N terminus is necessary and sufficient to promote beta-catenin degradation, whereas the C terminus is required to inhibit beta-catenin transcriptional activity without affecting its stability. Sox9 binds to beta-catenin and components of the beta-catenin "destruction complex," glycogen synthase kinase 3 and beta-transducin repeat containing protein, to promote their nuclear localization. Independent of its DNA binding ability, nuclear localization of Sox9 is both necessary and sufficient to enhance beta-catenin phosphorylation and its subsequent degradation. Thus, one mechanism whereby Sox9 regulates chondrogenesis is to promote efficient beta-catenin phosphorylation in the nucleus. This mechanism may be broadly employed by other intrinsic cell fate determining transcription factors to promptly turn off extrinsic inhibitory Wnt signaling mediated by beta-catenin.

Wnt and Frizzled RNA expression in human mesenchymal and embryonic (H7) stem cells

Background

Wnt signals are important for embryonic stem cells renewal, growth and differentiation. Although 19 Wnt, 10 Frizzled genes have been identified in mammals, their expression patterns in stem cells were largely unknown.

Results

We conducted RNA expression profiling for the Wnt ligands, their cellular receptors "Frizzleds" and co-receptors LRP5/6 in human embryonic stem cells (H7), human bone marrow mesenchymal cells, as well as mouse totipotent F9 teratocarcinoma embryonal cells. Except failing to express Wnt2 gene, totipotent F9 cells expressed RNA for all other 18 Wnt genes as well as all 10 members of Frizzled gene family. H7 cells expressed RNA for each of the 19 Wnt genes. In contrast, human mesenchymal cells did not display detectable RNA expression of Wnt1, Wnt8a, Wnt8b, Wnt9b, Wnt10a, and Wnt11. Analysis of Frizzled RNAs in H7 and human mesechymal cells revealed expression of 9 members of the receptor gene family, except Frizzled8. Expression of the Frizzled co-receptor LRP5 and LRP6 genes were detected in all three cell lines. Human H7 and mouse F9 cells express nearly a full complement of both Wnts and Frizzleds genes. The human mesenchymal cells, in contrast, have lost the expression of six Wnt ligands, i.e. Wnt1, 8a, 8b, 9b, 10a and 11.

Conclusion

Puripotent human H7 and mouse F9 embryonal cells express the genes for most of the Wnts and Frizzleds. In contrast, multipotent human mesenchymal cells are deficient in expression of Frizzled-8 and of 6 Wnt genes.

Large-scale N-terminal deletions but not point mutations stabilize beta-catenin in small bowel carcinomas, suggesting divergent molecular pathways of small and large intestinal carcinogenesis

Small intestinal adenocarcinoma is rare and its molecular pathogenesis is incompletely understood. Stabilization of beta-catenin, a mediator of wnt/wingless signalling, can be detected in 50% of sporadic carcinomas but, in contrast to colorectal cancer, this finding can not be explained by the inactivation of adenomatous polyposis coli (APC). In order to elucidate the molecular background of beta-catenin stabilization in small intestinal adenocarcinoma, we investigated 20 non-familial adenomatous polyposis coli (FAP)-associated tumours, including five microsatellite-unstable carcinomas for beta-catenin alterations, by immunohistochemistry, western blot analysis and sequence analysis on the RNA and DNA levels. Nuclear accumulation of beta-catenin was found in 50% of carcinomas. In 30%, nuclear stabilization was restricted to tumour cells at the invasion front, while 20% of tumours displayed intense homogeneous nuclear stabilization throughout all areas. Large deletions and insertions in the beta-catenin gene (CTNNB1) resulting in a partial or complete in-frame loss of exons 3 and 4 on the RNA-transcript level were found in the latter, exclusively microsatellite-stable carcinomas. The mutations resulted in the stabilization of aberrant beta-catenin lacking large parts of N-terminal protein domains. No point mutations in CTNNB1 were observed. Our data show that large intragenic CTNNB1 mutations stabilize beta-catenin in small intestinal adenocarcinomas and influence the subcellular distribution of the protein. In contrast to colon carcinomas, neither APC nor CTNNB1 point mutations seem to play a significant role in carcinogenesis, indicating divergent mechanisms of wnt/wingless control in the small and the large intestine.

Association of genetic variation in genes implicated in the beta-catenin destruction complex with risk of breast cancer

Aberrant Wnt/beta-catenin signaling leading to nuclear accumulation of the oncogene product beta-catenin is observed in a wide spectrum of human malignancies. The destruction complex in the Wnt/beta-catenin pathway is critical for regulating the level of beta-catenin in the cytoplasm and in the nucleus. Here, we report a comprehensive study of the contribution of genetic variation in six genes encoding the beta-catenin destruction complex (APC, AXIN1, AXIN2, CSNK1D, CSNK1E, and GSK3B) to breast cancer using a Mayo Clinic Breast Cancer Case-Control Study. A total of 79 candidate functional and tagging single nucleotide polymorphisms (SNP) were genotyped in 798 invasive cases and 843 unaffected controls. Of these, rs454886 in the APC tumor suppressor gene was associated with increased breast cancer risk (per allele odds ratio, 1.23; 95% confidence intervals, 1.05-1.43; P(trend) = 0.01). In addition, five SNPs in AXIN2 were associated with increased risk of breast cancer (P(trend) < 0.05). Haplotype-based tests identified significant associations between specific haplotypes in APC and AXIN2 (P < or = 0.03) and breast cancer risk. Further characterization of the APC and AXIN2 variants suggested that AXIN2 rs4791171 was significantly associated with risk in premenopausal (P(trend) = 0.0002) but not in postmenopausal women. The combination of our findings and numerous genetic and functional studies showing that APC and AXIN2 perform crucial tumor suppressor functions suggest that further investigation of the contribution of AXIN2 and APC SNPs to breast cancer risk are needed.

Wnt signaling: the good and the bad

Since the first Wnt gene was identified in 1982, the functions and mechanisms of Wnt signaling have been extensively studied. Wnt signaling is conserved from invertebrates to vertebrates and regulates early embryonic development as well as the homeostasis of adult tissues. In addition, both embryonic stem cells and adult stem cells are regulated by Wnt signaling. Deregulation of Wnt signaling is associated with many human diseases, particularly cancers. In this review, we will discuss in detail the functions of many components involved in the Wnt signal transduction pathway. Then, we will explore what is known about the role of Wnt signaling in stem cells and cancers.

Deregulated proteolysis by the F-box proteins SKP2 and beta-TrCP: tipping the scales of cancer

The maintenance and preservation of distinct phases during the cell cycle is a highly complex and coordinated process. It is regulated by phosphorylation--through the activity of cyclin-dependent kinases (CDKs)--and protein degradation, which occurs through ubiquitin ligases such as SCF (SKP1-CUL1-F-box protein) complexes and APC/C (anaphase-promoting complex/cyclosome). Here, we explore the functionality and biology of the F-box proteins, SKP2 (S-phase kinase-associated protein 2) and beta-TrCP (beta-transducin repeat-containing protein), which are emerging as important players in cancer biogenesis owing to the deregulated proteolysis of their substrates.

Homozygous deletion of glycogen synthase kinase 3beta bypasses senescence allowing Ras transformation of primary murine fibroblasts

In primary mammalian cells, expression of oncogenes such as activated Ras induces premature senescence rather than transformation. We show that homozygous deletion of glycogen synthase kinase (GSK) 3beta (GSK3beta-/-) bypasses senescence induced by mutant Ras(V12) allowing primary mouse embryo fibroblasts (MEFs) as well as immortalized MEFs to exhibit a transformed phenotype in vitro and in vivo. Both catalytic activity and Axin-binding of GSK3beta are required to optimally suppress Ras transformation. The expression of Ras(V12) in GSK3beta-/-, but not in GSK3beta+/+ MEFs results in translocation of beta-catenin to the nucleus with concomitant up-regulation of cyclin D1. siRNA-mediated knockdown of beta-catenin decreases both cyclin D1 expression and anchorage-independent growth of transformed cells indicating a causal role for beta-catenin. Thus Ras(V12) and the lack of GSK3beta act in concert to activate the beta-catenin pathway, which may underlie the bypass of senescence and tumorigenic transformation by Ras.

Regulation of hematopoiesis and the hematopoietic stem cell niche by Wnt signaling pathways

Hematopoietic stem cells (HSCs) are a rare population of cells that are responsible for life-long generation of blood cells of all lineages. In order to maintain their numbers, HSCs must establish a balance between the opposing cell fates of self-renewal (in which the ability to function as HSCs is retained) and initiation of hematopoietic differentiation. Multiple signaling pathways have been implicated in the regulation of HSC cell fate. One such set of pathways are those activated by the Wnt family of ligands. Wnt signaling pathways play a crucial role during embryogenesis and deregulation of these pathways has been implicated in the formation of solid tumors. Wnt signaling also plays a role in the regulation of stem cells from multiple tissues, such as embryonic, epidermal, and intestinal stem cells. However, the function of Wnt signaling in HSC biology is still controversial. In this review, we will discuss the basic characteristics of the adult HSC and its regulatory microenvironment, the "niche", focusing on the regulation of the HSC and its niche by the Wnt signaling pathways.

Regulation of the Arabidopsis GSK3-like kinase BRASSINOSTEROID-INSENSITIVE 2 through proteasome-mediated protein degradation

Glycogen synthase kinase 3 (GSK3) is a unique serine/threonine kinase that is implicated in a variety of cellular processes and is regulated by phosphorylation or protein-protein interaction in animal cells. BIN2 is an Arabidopsis GSK3-like kinase that negatively regulates brassinosteroid (BR) signaling. Genetic studies suggested that BIN2 is inhibited in response to BR perception at the cell surface to relieve its inhibitory effects on downstream targets; however, little is known about biochemical mechanisms of its inhibition. Here, we show that BIN2 is regulated by proteasome-mediated protein degradation. Exogenous application of a BR biosynthesis inhibitor and an active BR increased and decreased the amount of BIN2 proteins, respectively. Interestingly, the gain-of-function bin2-1 mutation significantly stabilizes BIN2, making it unresponsive to BR-induced BIN2 depletion. Exogenous application of different plant growth hormones revealed that BIN2 depletion is specifically induced by BR through a functional BR receptor, while treatment of a proteasome inhibitor, MG132, not only prevented the BR-induced BIN2 depletion but also nullified the inhibitory effect of BR on the BIN2 kinase activity. Taken together, our results strongly suggest that proteasome-mediated protein degradation constitutes an important regulatory mechanism for restricting the BIN2 activity.

Agelastatin A: a novel inhibitor of osteopontin-mediated adhesion, invasion, and colony formation

Effective inhibitors of osteopontin (OPN)-mediated neoplastic transformation and metastasis are still lacking. (-)-Agelastatin A is a naturally occurring oroidin alkaloid with powerful antitumor effects that, in many cases, are superior to cisplatin in vitro. In this regard, past comparative assaying of the two agents against a range of human tumor cell lines has revealed that typically (-)-agelastatin A is 1.5 to 16 times more potent than cisplatin at inhibiting cell growth, its effects being most pronounced against human bladder, skin, colon, and breast carcinomas. In this study, we have investigated the effects of (-)-agelastatin A on OPN-mediated malignant transformation using mammary epithelial cell lines. Treatment with (-)-agelastatin A inhibited OPN protein expression and enhanced expression of the cellular OPN inhibitor, Tcf-4. (-)-Agelastatin A treatment also reduced beta-catenin protein expression and reduced anchorage-independent growth, adhesion, and invasion in R37 OPN pBK-CMV and C9 cell lines. Similar effects were observed in MDA-MB-231 and MDA-MB-435s human breast cancer cell lines exposed to (-)-agelastatin A. Suppression of Tcf-4 by RNA interference (short interfering RNA) induced malignant/invasive transformation in parental benign Rama 37 cells; significantly, these events were reversed by treatment with (-)-agelastatin A. Our study reveals, for the very first time, that (-)-agelastatin A down-regulates beta-catenin expression while simultaneously up-regulating Tcf-4 and that these combined effects cause repression of OPN and inhibition of OPN-mediated malignant cell invasion, adhesion, and colony formation in vitro. We have also shown that (-)-agelastatin A inhibits cancer cell proliferation by causing cells to accumulate in the G(2) phase of cell cycle.

Role of Wnt/beta-catenin signaling pathway in epithelial-mesenchymal transition of human prostate cancer induced by hypoxia-inducible factor-1alpha

OBJECTIVES

Epithelial-mesenchymal transition (EMT) is an important process in tumor development, and several studies suggest that the Wnt/beta-catenin signal pathway may play an important role in EMT. However, there is no direct evidence showing that the Wnt/beta-catenin pathway actually determines the EMT induced by an exogenous signal. Our previous research has successfully proved that overexpression of hypoxia-inducible factor-1alpha (HIF-1alpha) could induce EMT in LNCaP cells, but not in PC-3. The present study aims to determine whether the signal of HIF-1alpha for inducing prostate cancer cells to undergo EMT might possibly pass through the Wnt/beta-catenin pathway.

METHODS

Epithelial-mesenchymal transition associated proteins were detected in several human prostate carcinoma cell lines by Western blot, and then we distinguished the EMT positive cell lines from the EMT negative cell lines. Furthermore, we evaluated the possible correlation between potency of invasiveness and proliferation among these cell lines with different characteristics of EMT using Matrigel transwell and thiazolyl blue tetrazolium bromide (MTT) assays. Finally, the different expression of some critical proteins and genes in Wnt/beta-catenin signaling pathway were analyzed by Western blot and reverse transcription-polymerase chain reaction (RT-PCR) in these cells with different characteristics of EMT.

RESULTS

Among several prostate cancer cell lines, PC-3, LNCaP and PC-3/HIF-1alpha are EMT negative cell lines, whereas LNCaP/HIF-1alpha and IA8 have undergone the EMT process. EMT positive cells (LNCaP/HIF-1alpha and IA8) exhibit much stronger potency of invasiveness and proliferation than those of PC-3 and LNCaP, which belong to EMT negative cells. Interestingly, although PC-3/HIF-1alpha had not completed the EMT process, it still displayed stronger potency of invasion and proliferation, resembling EMT positive cells. The protein expression level of total glycogensynthase kinase 3beta (GSK-3beta) and phospho-GSK-3beta in LNCaP/HIF-1alpha, IA8 and PC-3/HIF-1alpha cells significantly decreased; however, the relative ratios of p-GSK3beta/t-GSK3beta in LNCaP/HIF-1alpha, IA8 and PC-3/HIF-1alpha cells were significantly higher than PC-3 and LNCaP. Consistently, beta-catenin protein expression increased in LNCaP/HIF-1alpha and IA8 cells, but not in PC-3/HIF-1alpha; RT-PCR confirmed these results, except for the enhanced transcription activity of beta-catenin mRNA in PC-3/HIF-1alpha.

CONCLUSION

Our data suggests that activation of the Wnt/beta-catenin signaling pathway correlates with the characteristic of EMT and potency of invasiveness and proliferation. This may be the critical factor that directly controls the process of EMT induced by HIF-1alpha in prostate cancer cells.

The Wnt signaling pathway in familial exudative vitreoretinopathy and Norrie disease

The Wnt signaling pathway is highly conserved among species and has an important role in many cell biological processes throughout the body. This signaling cascade is involved in regulating ocular growth and development, and recent findings indicate that this is particularly true in the retina. Mutations involving different aspects of the Wnt signaling pathway are being linked to several diseases of retinal development. The aim of this article is to first review the Wnt signaling pathway. We will then describe two conditions, familial exudative vitreoretinopathy (FEVR) and Norrie disease (ND), which have been shown to be caused in part by defects in the Wnt signaling cascade.

Mechanistic insights from structural studies of β-catenin and its binding partners

β-catenin is both a crucial regulator of cell adhesion and the central effector of the canonical Wnt signaling pathway. It functions as a protein organizer by interacting with numerous partners at the membrane, in the cytosol, and in the nucleus. Recent structural and biochemical studies have revealed how β-catenin engages in critical protein-protein interactions by using its armadillo repeat region and its N- and C-terminal domains. The groove in the armadillo repeat region is a particularly interesting feature of β-catenin, since it serves as a common binding site for several β-catenin-binding partners, with steric hindrance limiting which partners can be bound at a specific time. These studies provide important insights into β-catenin-mediated mechanisms of cell adhesion and Wnt signaling and suggest potential approaches for the design of therapeutic agents to treat diseases caused by misregulated β-catenin expression.

Thiazolidinediones modulate the expression of beta-catenin and other cell-cycle regulatory proteins by targeting the F-box proteins of Skp1-Cul1-F-box protein E3 ubiquitin ligase independently of peroxisome proliferator-activated receptor gamma

Considering the role of aberrant beta-catenin signaling in tumorigenesis, we investigated the mechanism by which the peroxisome proliferator-activated receptor gamma (PPARgamma) agonist troglitazone facilitated beta-catenin down-regulation. We demonstrate that troglitazone and its more potent PPARgamma-inactive analogs Delta2TG and STG28 mediated the proteasomal degradation of beta-catenin in prostate cancer cells by up-regulating the expression of beta-transducin repeat-containing protein (beta-TrCP), an F-box component of the Skp1-Cul1-F-box protein E3 ubiquitin ligase. Evidence indicates that although small interfering RNA-mediated beta-TrCP knockdown protected cells against STG28-facilitated beta-catenin ablation, ectopic beta-TrCP expression enhanced the degradation. The involvement of beta-TrCP in beta-catenin degradation was also corroborated by the pull-down analysis and the concurrent down-regulation of known beta-TrCP substrates examined, including Wee1, Ikappabetaalpha, cdc25A, and nuclear factor-kappaB/p105. Furthermore, glycogen synthase kinase-3beta represented a key regulator in the effect of these thiazolidinedione derivatives on beta-catenin proteolysis even though these agents increased its phosphorylation level. It is noteworthy that this drug-induced beta-TrCP up-regulation was accompanied by the concomitant down-regulation of Skp2 and Fbw7, thereby affecting many of the target proteins of these two F-box proteins (such as p27 and cyclin E). As a consequence, the ability of troglitazone to target these F-box proteins provides a molecular basis to account for its reported effect on modulating the expression of aforementioned cell-cycle regulatory proteins. Despite this complicated mode of pharmacological actions, normal prostate epithelial cells, relative to LNCaP cells, were less susceptible to the effects of STG28 on modulating the expression of beta-catenin and beta-TrCP, suggesting the translation potential of using STG28 as a scaffold to develop more potent chemopreventive agents.

Analysis of dishevelled localization and function in the early sea urchin embryo

Dishevelled (Dsh) is a key signaling molecule in the canonical Wnt pathway. Although the mechanism by which Dsh transduces a Wnt signal remains elusive, the subcellular localization of Dsh may be critical for its function. In the early sea urchin embryo, Dsh is concentrated in punctate structures within the cytoplasm of vegetal blastomeres. In these cells, Dsh stabilizes beta-catenin and causes it to accumulate in nuclei, resulting in the activation of transcriptional gene regulatory networks that drive mesoderm and endoderm formation. Here, we present a systematic mutational analysis of Lytechinus variegatus Dsh (LvDsh) that identifies motifs required for its vegetal cortical localization (VCL). In addition to a previously identified lipid-binding motif near the N-terminus of Dsh (Weitzel, H.E., Illies, M.R., Byrum, C.A., Xu, R., Wikramanayake, A.H., Ettensohn, C.A., 2004. Differential stability of beta-catenin along the animal-vegetal axis of the sea urchin embryo mediated by dishevelled. Development 131, 2947-56), we identify a short (21 amino acid) motif between the PDZ and DEP domains that is required for VCL. Phosphorylation of threonine residues in this region regulates both the targeting and stability of LvDsh. We also identify functional nuclear import and export signals within LvDsh. We provide additional evidence that LvDsh is active locally in the vegetal region of the embryo but is inactive in animal blastomeres and show that the inability of LvDsh to function in animal cells is not a consequence of impaired nuclear import. The DIX domain of LvDsh functions as a potent dominant negative when overexpressed (Weitzel, H.E., Illies, M.R., Byrum, C.A., Xu, R., Wikramanayake, A.H., Ettensohn, C.A., 2004. Differential stability of beta-catenin along the animal-vegetal axis of the sea urchin embryo mediated by dishevelled. Development 131, 2947-56). Here, we show that the dominant negative effect of DIX is dependent on a highly conserved, lipid-binding motif that includes residues K57 and E58. The dominant negative effect of DIX is not a consequence of blocking VCL or the nuclear import of LvDsh. We provide evidence that isolated DIX domains interact with full-length LvDsh in vivo. In addition, we show that the K57/E58 lipid-binding motif of DIX is essential for this interaction. We propose that binding of the isolated DIX domain to full-length Dsh may be facilitated by interactions with lipids, and that this interaction may inhibit signaling by a) preventing endogenous Dsh from interacting with Axin, or b) blocking the ability of Dsh to recruit other proteins, such as GBP/Frat1, to the beta-catenin degradation complex.

β-Catenin (CTNNB1) in the Mouse Uterus During Decidualization and the Potential Role of Two Pathways in Regulating Its Degradation

β-catenin plays a role in cell adhesion and as a transcriptional coactivator. Its levels are regulated in cells by controlling its degradation through ubiquitination by two different E3 ligase complexes. One complex contains β-transducing repeat containing (BTRC) protein, which binds to β-catenin when phosphorylated on specific (S33 and S37) residues, whereas the other involves calcyclin-binding protein (CACYBP). The aim of this study was to determine the localization and levels of total and active (S33/S37-dephosphorylated) β-catenin in the pregnant mouse uteri and those undergoing artificially stimulated decidualization. These two forms of β-catenin were localized almost exclusively to the endometrial epithelia just prior to the onset of implantation. Although this localization continued after the onset of implantation, there were less epithelial cells present in areas of the uterus undergoing decidualization. Rather, there was a progressive increase in β-catenin localization in endometrial stromal cells undergoing decidualization in the anti-mesometrial and, to a lesser extent, in the mesometrial regions. The presence of a conceptus was not required for the changes in localization seen in the pregnant uterus because similar findings were also seen in uteri undergoing artificially stimulated decidualization. Finally, overall levels of total, active (S33 and S37 dephosphorylated), and phosphorylated (S33/S37/T42) β-catenin protein and the steady-state levels of calcyclin-binding protein mRNA changed in the uterus during decidualization. The result of this study shows the changing localization and levels of β-catenin in the mouse uterus during decidualization. Further, the results suggest potential roles for both the BTRC and CACYBP E3 ligase mechanisms of β-catenin ubiquitination in the uterus during decidualization.

Involvement of the betaTrCP in the ubiquitination and stability of the HIV-1 Vpu protein

The human immunodeficiency virus type 1 (HIV-1) Vpu protein binds to the CD4 receptor and targets it to the proteasome for degradation. This process requires the recruitment of human betaTrCP, a component of the Skp1-Cullin-F box (SCF) ubiquitin ligase complex, that interacts with phosphorylated Vpu molecules. Vpu, unlike other ligands of betaTrCP, has never been reported to be degraded. We provide evidence that Vpu, itself, is ubiquitinated and targeted for degradation by the proteasome. We demonstrate that the mutant Vpu2.6, which cannot interact with betaTrCP, is stable and, unlike wild-type Vpu, is not polyubiquitinated. These results suggest that betaTrCP is involved in Vpu polyubiquitination.

R-spondin1 is a high affinity ligand for LRP6 and induces LRP6 phosphorylation and beta-catenin signaling

R-spondin proteins are newly identified secreted molecules that activate beta-catenin signaling. However, the mechanism of R-spondin action and its relationship with Wnt signaling remain unclear. Here we show that human R-spondin1 (hRspo1) is a high affinity ligand for the Wnt co-receptor LRP6 (K(d) = 1.2 nm). hRspo1 induces glycogen synthase kinase 3-dependent phosphorylation and activation of LRP6. DKK1, an LRP6 antagonist, inhibits hRspo1-induced LRP6 phosphorylation. We further demonstrate that hRspo1 synergizes with Frizzled5 in Xenopus axis induction assays and induces the phosphorylation of Dishevelled, a cytoplasmic component downstream of Frizzled function. Our study reveals interesting similarity and distinction between Wnt and R-spondin signaling.

Somatic mutations of the beta-TrCP gene in gastric cancer

Beta-TrCP is a component of the ubiquitin ligase complex targeting beta-catenin for proteasomal degradation, and is a negative regulator of Wnt/beta-catenin signaling. To determine whether genetic alterations of the beta-TrCP gene are involved in the development or progression of gastric cancer, we analyzed its somatic mutations in 95 gastric cancers by single-strand conformational polymorphism and sequencing. We found five missense mutations (5.3%): A99V, H342Y, H425Y, C206Y, and G260E. Tissue carrying mutations showed moderate to strong cytoplasmic and/or nuclear staining of beta-catenin by immunohistochemistry. Thus, somatic mutations of the beta-TrCP gene may contribute to the development of gastric cancer through beta-catenin stabilization.

Modulation of beta-catenin by cyclin-dependent kinase 6 in Wnt-stimulated cells

Beta-catenin is implicated in quite different cellular processes, which require a fine-tuned regulation of its function. Here we demonstrate that cyclin-dependent kinase 6 (CDK6), in association with cyclin D1 (CCND1), directly binds to beta-catenin. We showed that CCND1-CDK6 phosphorylates beta-catenin on serine 45 (S45). This phosphorylation creates a priming site for glycogen synthase kinase 3beta (GSK3beta) and is both necessary and sufficient to initiate the beta-catenin phosphorylation-degradation cascade. Moreover, co-immunoprecipitation assays using Wnt3a-conditioned medium reveals that while Wnt stimulation leads to the dissociation of beta-catenin from axin and casein kinase Ialpha (CKIalpha), Wnt treatment promotes an increase in CCND1 level and the association of beta-catenin with CCND1-CDK6. Furthermore, Wnt3a-stimulated cytosolic beta-catenin levels were higher in CDK6 knockout mouse embryonic fibroblasts (CDK6-/- MEFs) compared to wild-type MEFs. Thus, the CCND1-CDK6 complex is like to negatively regulate Wnt signaling by mediating beta-catenin phosphorylation and its subsequent degradation in Wnt-stimulated cells.

Protein-kinase-C-mediated β-catenin phosphorylation negatively regulates the Wnt/β-catenin pathway

Normally, the Wnt/β-catenin pathway controls developmental processes and homeostasis, but abnormal activation of this pathway is a frequent event during the development of cancer. The key mechanism in regulation of the Wnt/β-catenin pathway is the amino-terminal phosphorylation of β-catenin, marking it for proteasomal degradation. Here we present small-molecule-based identification of protein kinase C (PKC)-mediated β-catenin phosphorylation as a novel mechanism regulating the Wnt/β-catenin pathway. We used a cell-based chemical screen to identify A23187, which inhibits the Wnt/β-catenin pathway. PKC was activated by A23187 treatment and subsequently phosphorylated N-terminal serine (Ser) residues of β-catenin, which promoted β-catenin degradation. Moreover, the depletion of PKCα inhibited the phosphorylation and degradation of β-catenin. Therefore, our findings suggest that the PKC pathway negatively regulates the β-catenin level outside of the Wnt/β-catenin pathway.

STD and TRNOESY NMR studies for the epitope mapping of the phosphorylation motif of the oncogenic protein beta-catenin recognized by a selective monoclonal antibody

The interaction of the P-beta-Cat(19-44) peptide, a 26 amino acid peptide (K(19)AAVSHWQQQSYLDpSGIHpSGATTTAP(44)) that mimics the phosphorylated beta-Catenin antigen, has been studied with its monoclonal antibody BC-22, by transferred nuclear Overhauser effect NMR spectroscopy (TRNOESY) and saturation transfer difference NMR (STD NMR) spectroscopy. This antibody is specific to diphosphorylated beta-Catenin and does not react with the non-phosphorylated protein. Phosphorylation of beta-Catenin at sites Ser33 and Ser37 on the DSGXXS motif is required for the interaction of beta-Catenin with the ubiquitin ligase SCF(beta-TrCP). beta-TrCP is involved in the ubiquitination and proteasome targeting of the oncogenic protein beta-Catenin, the accumulation of which has been implicated in various human cancers. The three-dimensional structure of the P-beta-Cat(19-44) in the bound conformation was determined by TRNOESY NMR experiments; the peptide adopts a compact structure in the presence of mAb with formation of turns around Trp25 and Gln26, with a tight bend created by the DpS(33)GIHpS(37) motif; the peptide residues (D32-pS37) forming this bend are recognized by the antibody as demonstrated by STD NMR experiments. STD NMR studies provide evidence for the existence of a conformational epitope containing tandem repeats of phosphoserine motifs. The peptide's epitope is predominantly located in the large bend and in the N-terminal segment, implicating bidentate association. These findings are in excellent agreement with a recently published NMR structure required for the interaction of beta-Catenin with the SCF(beta-TrCP) protein.

Lysine residues Lys-19 and Lys-49 of beta-catenin regulate its levels and function in T cell factor transcriptional activation and neoplastic transformation

Wnt signaling regulates cell fate determination, proliferation, and survival, among other processes. Certain Wnt ligands stabilize the beta-catenin protein, leading to the ability of beta-catenin to activate T cell factor-regulated genes. In the absence of Wnts, beta-catenin is phosphorylated at defined serine and threonine residues in its amino (N) terminus. The phosphorylated beta-catenin is recognized by a beta-transducin repeat-containing protein (betaTrCP) and associated ubiquitin ligase components. The serine/threonine residues and betaTrCP-binding site in the N-terminal region of beta-catenin constitute a key regulatory motif targeted by somatic mutations in human cancers, resulting in constitutive stabilization of the mutant beta-catenin proteins. Structural studies have implicated beta-catenin lysine 19 as the major target for betaTrCP-dependent ubiquitination, but Lys-19 mutations in cancer have not been reported. We studied the consequences of single amino acid substitutions of the only 2 lysine residues in the N-terminal 130 amino acids of beta-catenin. Mutation of Lys-19 minimally affected beta-catenin levels and functional activity, and mutation of Lys-49 led to reduced beta-catenin levels and function. In contrast, beta-catenin proteins with substitutions at both Lys-19 and Lys-49 positions were present at elevated levels and had the ability to potently activate T cell factor-dependent transcription and promote neoplastic transformation. We furthermore demonstrate that the K19/K49 double mutant forms of beta-catenin are stabilized as a result of reduced betaTrCP-dependent ubiquitination. Our findings suggest that Lys-19 is a primary in vivo site of betaTrCP-dependent ubiquitination and Lys-49 may be a secondary or cryptic site. Moreover, our results inform understanding of why single amino acid substitutions at lysine 19 or 49 have not been reported in human cancer.

Novel cross talk of Kruppel-like factor 4 and beta-catenin regulates normal intestinal homeostasis and tumor repression

Epithelial cells of the intestinal mucosa undergo a continual process of proliferation, differentiation, and apoptosis which is regulated by multiple signaling pathways. The Wnt/beta-catenin pathway plays a critical role in this process. Mutations in the Wnt pathway, however, are associated with colorectal cancers. Krüppel-like factor 4 (KLF4) is an epithelial transcriptional factor that is down-regulated in many colorectal cancers. Here, we show that KLF4 interacts with beta-catenin and represses beta-catenin-mediated gene expression. Moreover, KLF4 inhibits the axis formation of Xenopus embryos and inhibits xenograft tumor growth in athymic nude mice. Our findings suggest that the cross talk of KLF4 and beta-catenin plays a critical role in homeostasis of the normal intestine as well as in tumorigenesis of colorectal cancers.

Adenomatous polyposis coli (APC) differentially regulates beta-catenin phosphorylation and ubiquitination in colon cancer cells

Most colorectal cancers have mutations of the adenomatous polyposis coli (APC) gene or the beta-catenin gene that stabilize beta-catenin and activate beta-catenin target genes, leading ultimately to cancer. The molecular mechanisms of APC function in beta-catenin degradation are not completely known. APC binds beta-catenin and is involved in the Axin complex, suggesting that APC regulates beta-catenin phosphorylation. Some evidence also suggests that APC regulates beta-catenin nuclear export. Here, we examine the effects of APC mutations on beta-catenin phosphorylation, ubiquitination, and degradation in the colon cancer cell lines SW480, DLD-1, and HT29, each of which contains a different APC truncation. Although the current models suggest that beta-catenin phosphorylation should be inhibited by APC mutations, we detected significant beta-catenin phosphorylation in these cells. However, beta-catenin ubiquitination and degradation were inhibited in SW480 but not in DLD-1 and HT29 cells. The ubiquitination ofbeta-catenin in SW480 cells can be rescued by exogenous expression of APC. The APC domains required for beta-catenin ubiquitination were analyzed. Our results suggest that APC regulates beta-catenin phosphorylation and ubiquitination by distinct domains and by separate molecular mechanisms.

The role of the ubiquitination machinery in dislocation and degradation of endoplasmic reticulum proteins

Ubiquitination is essential for the dislocation and degradation of proteins from the endoplasmic reticulum (ER). How exactly this is regulated is unknown at present. This review provides an overview of ubiquitin-conjugating enzymes (E2s) and ubiquitin ligases (E3s) with a role in the degradation of ER proteins. Their structure and functions are described, as well as their mutual interactions. Substrate specificity and functional redundancy of E3 ligases are discussed, and other components of the ER degradation machinery that may associate with the ubiquitination system are reviewed.

E-cadherin regulates the association between beta-catenin and actinin-4

The E-cadherin/catenin system acts as an invasion suppressor of epithelial malignancies. This invasion suppressive activity seems be mediated not only by the cell adhesive activity of E-cadherin but by other undetermined signaling pathways elicited by beta-catenin. In fact, cancer cells that have infiltrated the stroma reduce the expression of E-cadherin and accumulate beta-catenin. We attempted to identify the alternative partner proteins that make complexes with beta-catenin in the absence of E-cadherin. An approximately 100-kDa protein was constantly coimmunoprecipitated with beta-catenin from SW480 colorectal cancer cells, which lack the expression of E-cadherin, and was identified as actinin-4 by mass spectrometry. Transfection of E-cadherin cDNA suppressed the association between beta-catenin and actinin-4. Inhibition of E-cadherin by RNA interference transferred the beta-catenin and actinin-4 proteins into the membrane protrusions of DLD-1 cells. Immunofluorescence histochemistry of clinical colorectal cancer specimens showed that the beta-catenin and actinin-4 proteins were colocalized in colorectal cancer cells infiltrating the stroma. We reported previously that overexpression of actinin-4 induces cell motility and specifically promotes lymph node metastasis by colorectal cancer. The association between beta-catenin and actinin-4 and its regulation by E-cadherin may represent a novel molecular link connecting cell adhesion and motility. Shutting down the signals mediating this association may be worth considering as a therapeutic approach to cancer invasion and metastasis.

Combretastatin A4 phosphate induces rapid regression of tumor neovessels and growth through interference with vascular endothelial-cadherin signaling

The molecular and cellular pathways that support the maintenance and stability of tumor neovessels are not well defined. The efficacy of microtubule-disrupting agents, such as combretastatin A4 phosphate (CA4P), in inducing rapid regression of specific subsets of tumor neovessels has opened up new avenues of research to identify factors that support tumor neoangiogenesis. Herein, we show that CA4P selectively targeted endothelial cells, but not smooth muscle cells, and induced regression of unstable nascent tumor neovessels by rapidly disrupting the molecular engagement of the endothelial cell-specific junctional molecule vascular endothelial-cadherin (VE-cadherin) in vitro and in vivo in mice. CA4P increases endothelial cell permeability, while inhibiting endothelial cell migration and capillary tube formation predominantly through disruption of VE-cadherin/beta-catenin/Akt signaling pathway, thereby leading to rapid vascular collapse and tumor necrosis. Remarkably, stabilization of VE-cadherin signaling in endothelial cells with adenovirus E4 gene or ensheathment with smooth muscle cells confers resistance to CA4P. CA4P synergizes with low and nontoxic doses of neutralizing mAbs to VE-cadherin by blocking assembly of neovessels, thereby inhibiting tumor growth. These data suggest that the microtubule-targeting agent CA4P selectively induces regression of unstable tumor neovessels, in part through disruption of VE-cadherin signaling. Combined treatment with anti-VE-cadherin agents in conjunction with microtubule-disrupting agents provides a novel synergistic strategy to selectively disrupt assembly and induce regression of nascent tumor neovessels, with minimal toxicity and without affecting normal stabilized vasculature.

xBtg-x regulates Wnt/beta-Catenin signaling during early Xenopus development

In Xenopus, two signaling systems, maternal beta-Catenin and Nodal-related, are required for induction of the Spemann organizer and establishment of the body plan. By screening cDNA macroarrays for genes activated by these two signaling pathways, we identified Xenopus xBtg-x, a novel member of the Btg/Tob gene family of antiproliferative proteins. We show that xBtg-x is expressed in the dorsal mesendoderm (Spemann organizer tissue) of gastrula stage embryos and that its expression is regulated by both beta-Catenin and Nodal-related signals. Microinjection of synthetic xBtg-x mRNA into Xenopus embryos induced axis duplication and completely rescued the ventralizing effects of UV irradiation through the activation of the canonical Wnt/beta-Catenin signaling pathway. Interestingly, xBtg-x stimulated beta-Catenin-dependent transcription without affecting the stability of beta-Catenin protein. These data suggest that xBtg-x is a novel component of the Wnt/beta-Catenin signaling pathway regulating early embryonic patterning.

Wnt signaling in disease and in development

The highly conserved Wnt secreted proteins are critical mediators of cell-to-cell signaling during development of animals. Recent biochemical and genetic analyses have led to significant insight into understanding how Wnt signals work. The catalogue of Wnt signaling components has exploded. We now realize that multiple extracellular, cytoplasmic, and nuclear components modulate Wnt signaling. Moreover, receptor-ligand specificity and multiple feedback loops determine Wnt signaling outputs. It is also clear that Wnt signals are required for adult tissue maintenance. Perturbations in Wnt signaling cause human degenerative diseases as well as cancer.

Wnt/beta-catenin signaling activates growth-control genes during overload-induced skeletal muscle hypertrophy

Beta-catenin is a transcriptional activator shown to regulate the embryonic, postnatal, and oncogenic growth of many tissues. In most research to date, beta-catenin activation has been the unique downstream function of the Wnt signaling pathway. However, in the heart, a Wnt-independent mechanism involving Akt-mediated phosphorylation of glycogen synthase kinase (GSK)-3beta was recently shown to activate beta-catenin and regulate cardiomyocyte growth. In this study, results have identified the activation of the Wnt/beta-catenin pathway during hypertrophy of mechanically overloaded skeletal muscle. Significant increases in beta-catenin were determined during skeletal muscle hypertrophy. In addition, the Wnt receptor, mFrizzled (mFzd)-1, the signaling mediator disheveled-1, and the transcriptional co-activator, lymphocyte enhancement factor (Lef)-1, are all increased during hypertrophy of the overloaded mouse plantaris muscle. Experiments also determined an increased association between GSK-3beta and the inhibitory frequently rearranged in advanced T cell-1 protein with no increase in GSK-3beta phosphorylation (Ser9). Finally, skeletal muscle overload resulted in increased nuclear beta-catenin/Lef-1 expression and induction of the transcriptional targets c-Myc, cyclin D1, and paired-like homeodomain transcription factor 2. Thus this study provides the first evidence that the Wnt signaling pathway induces beta-catenin/Lef-1 activation of growth-control genes during overload induced skeletal muscle hypertrophy.

A Vertebrate Homolog of the Cell Cycle Regulator Dbf4 Is an Inhibitor of Wnt Signaling Required for Heart Development

Early stages of vertebrate heart development have been linked to Wnt signaling. Here we show in both gain- and loss-of-function experiments that XDbf4, a known regulator of Cdc7 kinase, is an inhibitor of the canonical Wnt signaling pathway. Depletion of endogenous XDbf4 protein did not disturb gastrulation movements or early organizer genes but resulted in embryos with morphologically defective heart and eyes and suppressed cardiac markers. These markers were restored by overexpressed XDbf4, or an XDbf4 mutant that inhibits Wnt signaling but lacks the ability to regulate Cdc7. This indicates that the function of XDbf4 in heart development is independent of its role in the cell cycle. Moreover, our data suggest that XDbf4 acts through the physical and functional interaction with Frodo, a context-dependent regulator of Wnt signaling. These findings establish an unexpected function for a vertebrate Dbf4 homolog and demonstrate the requirement for Wnt inhibition in early cardiac specification.

Polarized distribution of mRNAs encoding a putative LDL receptor adaptor protein, xARH (autosomal recessive hypercholesterolemia) in Xenopus oocytes

The Xenopus homologue of hARH (human autosomal recessive hypercholesterolemia) was identified in a screen for vegetally localized RNAs. xARH contains a N-terminal phosphotyrosine binding (PTB) domain that is 91% identical to that of the human gene, a domain previously shown to bind the LDL receptor family members. Maternal xARH, unlike hARH, is present as two transcripts that differ in their 3' UTRs. The large transcript, xARH-alpha, primarily localizes to the oocyte vegetal cortex. The small transcript, xARH-beta, is not localized. During embryogenesis, xARH RNA is found redistributed in a perinuclear pattern. Similar to hARH, xARH is found in the adult liver, but at low levels compared to oocytes. Downstream of the PTB domain is a conserved clathrin box and a C terminal region 50% identical to that of hARH. Previous in vitro studies from this lab have shown xARH can bind the LDLR as well as the vitellogenin (VTG) receptor. We find that injection of the C terminal region missing the PTB domain significantly reduces the internalization of VTG in early stage oocytes, an event that requires the VTG receptor. The data strongly suggest that xARH encodes an adaptor protein that functions in the essential receptor-mediated endocytosis of nutrients during oogenesis. Because xARH protein is found uniformly distributed along the animal/vegetal axis in oocytes, we propose that the localization of xARH-alpha to the vegetal cortex while xARH-beta remains unlocalized, facilitates the uniform distribution of the protein in this extraordinarily large cell.

Zebrafish Dapper1 and Dapper2 play distinct roles in Wnt-mediated developmental processes

Wnt signaling pathways in vertebrates use the phosphoprotein Dishevelled (Dvl). The cellular responses to Wnt signaling may in part be modulated by Dvl-associated proteins, including Dapper (Dpr). We have cloned and characterized the zebrafish Dpr paralogs Dpr1 and Dpr2. Loss-of-function studies reveal that endogenous Dpr1 but not Dpr2 is required to enhance Wnt/beta-catenin activity in zebrafish embryos that are hypomorphic for Wnt8. Conversely, Dpr2 but not Dpr1 is required for normal convergence extension movements in embryos that are hypomorphic for Stbm or Wnt11, supporting a functional interaction of Dpr2 with Wnt/Ca2+-PCP signaling. In gain-of-function experiments, Dpr1 but not Dpr2 induces Wnt/beta-catenin target genes. Dpr1 synergizes with zebrafish Dvl2, and with the Dvl-interacting kinases CK1epsilon, Par1 and CK2, in activating target genes. We conclude that two Dvl-associated paralogs, Dpr1 and Dpr2, participate in distinct Wnt-dependent developmental processes.

Dishevelled and Wnt signaling: is the nucleus the final frontier?

The phosphoprotein Dishevelled (Dsh) is an essential component of Wnt signaling pathways and transduces signals into three separate branches, the canonical, non-canonical and Ca²⁺ pathways. How Dsh focuses signaling into these branches remains mysterious, but a new study reveals the importance of nuclear localization of Dsh for pathway-specific activation.

Nuclear localization is required for Dishevelled function in Wnt/beta-catenin signaling

Background

Dishevelled (Dsh) is a key component of multiple signaling pathways that are initiated by Wnt secreted ligands and Frizzled receptors during embryonic development. Although Dsh has been detected in a number of cellular compartments, the importance of its subcellular distribution for signaling remains to be determined.

Results

We report that Dsh protein accumulates in cell nuclei when Xenopus embryonic explants or mammalian cells are incubated with inhibitors of nuclear export or when a specific nuclear-export signal (NES) in Dsh is disrupted by mutagenesis. Dsh protein with a mutated NES, while predominantly nuclear, remains fully active in its ability to stimulate canonical Wnt signaling. Conversely, point mutations in conserved amino-acid residues that are essential for the nuclear localization of Dsh impair the ability of Dsh to activate downstream targets of Wnt signaling. When these conserved residues of Dsh are replaced with an unrelated SV40 nuclear localization signal, full Dsh activity is restored. Consistent with a signaling function for Dsh in the nucleus, treatment of cultured mammalian cells with medium containing Wnt3a results in nuclear accumulation of endogenous Dsh protein.

Conclusions

These findings suggest that nuclear localization of Dsh is required for its function in the canonical Wnt/β-catenin signaling pathway. We discuss the relevance of these findings to existing models of Wnt signal transduction to the nucleus.

The Wnt signaling pathway in development and disease

Tight control of cell-cell communication is essential for the generation of a normally patterned embryo. A critical mediator of key cell-cell signaling events during embryogenesis is the highly conserved Wnt family of secreted proteins. Recent biochemical and genetic analyses have greatly enriched our understanding of how Wnts signal, and the list of canonical Wnt signaling components has exploded. The data reveal that multiple extracellular, cytoplasmic, and nuclear regulators intricately modulate Wnt signaling levels. In addition, receptor-ligand specificity and feedback loops help to determine Wnt signaling outputs. Wnts are required for adult tissue maintenance, and perturbations in Wnt signaling promote both human degenerative diseases and cancer. The next few years are likely to see novel therapeutic reagents aimed at controlling Wnt signaling in order to alleviate these conditions.