Aberrant accumulation of intracellular β-catenin is a well recognized characteristic of several cancers, including prostate, colon, and liver cancers, and is a potential target for development of anticancer therapeutics. Here, we used cell-based small molecule screening to identify CGK062 as an inhibitor of Wnt/β-catenin signaling. CGK062 promoted protein kinase Cα (PKCα)-mediated phosphorylation of β-catenin at Ser33/Ser37, marking it for proteasomal degradation. This reduced intracellular β-catenin levels and consequently antagonized β-catenin response transcription (CRT). Pharmacological inhibition or depletion of PKCα abrogated CGK062-mediated phosphorylation and degradation of β-catenin. In addition, CGK062 repressed the expression of the genes encoding cyclin D1, c-myc, and axin-2, β-catenin target genes, and thus inhibited the growth of CRT-positive cancer cells. Furthermore, treatment of nude mice bearing PC3 xenograft tumors with CGK062 at doses of 50 mg/kg and 100 mg/kg (i.p.) significantly suppressed tumor growth. Our findings suggest that CGK062 exerts its anticancer activity by promoting PKCα-mediated β-catenin phosphorylation/degradation. Therefore, CGK062 has significant therapeutic potential for the treatment of CRT-positive cancers.
Epitope-tagged active-site-directed probes are widely used to visualize the activity of deubiquitinases (DUBs) in cell extracts, to investigate the specificity and potency of small-molecule DUB inhibitors, and to isolate and identify DUBs by mass spectrometry. With DUBs arising as novel potential drug targets, probes are required that can be produced in sufficient amounts and to meet the specific needs of a given experiment. The established method for the generation of DUB probes makes use of labor-intensive intein-based methods that have inherent limitations concerning the incorporation of unnatural amino acids and the amount of material that can be obtained. Here, we describe the total chemical synthesis of active-site-directed probes and their application to activity-based profiling and identification of functional DUBs. This synthetic methodology allowed the easy incorporation of desired tags for specific applications, for example, fluorescent reporters, handles for immunoprecipitation or affinity pull-down, and cleavable linkers. Additionally, the synthetic method can be scaled up to provide significant amounts of probe. Fluorescent ubiquitin probes allowed faster, in-gel detection of active DUBs, as compared to (immuno)blotting procedures. A biotinylated probe holding a photocleavable linker enabled the affinity pull-down and subsequent mild, photorelease of DUBs. Also, DUB activity levels were monitored in response to overexpression or knockdown, and to inhibition by small molecules. Furthermore, fluorescent probes revealed differential DUB activity profiles in a panel of lung and prostate cancer cells.
activity-based protein profiling; deubiquitinating enzymes; fluorescent probes; solid-phase synthesis; ubiquitin
The Wnt/ß-catenin signaling pathway controls important cellular events during development and often contributes to disease when dysregulated. Using high throughput screening we have identified a new small molecule inhibitor of Wnt/ß-catenin signaling, WIKI4. WIKI4 inhibits expression of ß-catenin target genes and cellular responses to Wnt/ß-catenin signaling in cancer cell lines as well as in human embryonic stem cells. Furthermore, we demonstrate that WIKI4 mediates its effects on Wnt/ß-catenin signaling by inhibiting the enzymatic activity of TNKS2, a regulator of AXIN ubiquitylation and degradation. While TNKS has previously been shown to be the target of small molecule inhibitors of Wnt/ß-catenin signaling, WIKI4 is structurally distinct from previously identified TNKS inhibitors.
Ubiquitin (Ub) is a vital regulatory component in various cellular processes, including cellular responses to viral infection. As obligate intracellular pathogens, viruses have the capacity to manipulate the ubiquitin (Ub) cycle to their advantage by encoding Ub-modifying proteins including deubiquitinases (DUBs). However, how cellular DUBs modulate specific viral infections, such as norovirus, is poorly understood. To examine the role of DUBs during norovirus infection, we used WP1130, a small molecule inhibitor of a subset of cellular DUBs. Replication of murine norovirus in murine macrophages and the human norovirus Norwalk virus in a replicon system were significantly inhibited by WP1130. Chemical proteomics identified the cellular DUB USP14 as a target of WP1130 in murine macrophages, and pharmacologic inhibition or siRNA-mediated knockdown of USP14 inhibited murine norovirus infection. USP14 is a proteasome-associated DUB that also binds to inositol-requiring enzyme 1 (IRE1), a critical mediator of the unfolded protein response (UPR). WP1130 treatment of murine macrophages did not alter proteasome activity but activated the X-box binding protein-1 (XBP-1) through an IRE1-dependent mechanism. In addition, WP1130 treatment or induction of the UPR also reduced infection of other RNA viruses including encephalomyocarditis virus, Sindbis virus, and La Crosse virus but not vesicular stomatitis virus. Pharmacologic inhibition of the IRE1 endonuclease activity partially rescued the antiviral effect of WP1130. Taken together, our studies support a model whereby induction of the UPR through cellular DUB inhibition blocks specific viral infections, and suggest that cellular DUBs and the UPR represent novel targets for future development of broad spectrum antiviral therapies.
Deubiquitinases (DUBs) are enzymes, which are implicated in many cellular processes but their functions during virus infection are not well understood. We used WP1130, a small molecule inhibitor of a subset of DUBs, as a probe to unravel the functions of DUBs during norovirus infections. We identified USP14 as a cellular DUB target of WP1130 that is required for optimal norovirus infection. Furthermore, we demonstrated that chemical induction of the unfolded protein response can significantly inhibit viral progeny production of several RNA viruses, including noroviruses. These results suggest that chemical inhibition of cellular DUBs and/or modulation of the unfolded protein response could represent novel targets for therapy against a variety of viral pathogens.
Macrophages are key mediators of antimicrobial defense and innate immunity. Innate intracellular defense mechanisms can be rapidly regulated at the posttranslational level by the coordinated addition and removal of ubiquitin by ubiquitin ligases and deubiquitinases (DUBs). While ubiquitin ligases have been extensively studied, the contribution of DUBs to macrophage innate immune function is incompletely defined. We therefore employed a small molecule DUB inhibitor, WP1130, to probe the role of DUBs in the macrophage response to bacterial infection. Treatment of activated bone marrow-derived macrophages (BMM) with WP1130 significantly augmented killing of the intracellular bacterial pathogen Listeria monocytogenes. WP1130 also induced killing of phagosome-restricted bacteria, implicating a bactericidal mechanism associated with the phagosome, such as the inducible nitric oxide synthase (iNOS). WP1130 had a minimal antimicrobial effect in macrophages lacking iNOS, indicating that iNOS is an effector mechanism for WP1130-mediated bacterial killing. Although overall iNOS levels were not notably different, we found that WP1130 significantly increased colocalization of iNOS with the Listeria-containing phagosome during infection. Taken together, our data indicate that the deubiquitinase inhibitor WP1130 increases bacterial killing in macrophages by enhancing iNOS localization to the phagosome and suggest a potential role for ubiquitin regulation in iNOS trafficking.
The translation initiation factor complex eIF3f has an intrinsic deubiquitinase activity and regulates the Notch signaling pathway.
Activation of the mammalian Notch receptor after ligand binding relies on a succession of events including metalloprotease-cleavage, endocytosis, monoubiquitination, and eventually processing by the gamma-secretase, giving rise to a soluble, transcriptionally active molecule. The Notch1 receptor was proposed to be monoubiquitinated before its gamma-secretase cleavage; the targeted lysine has been localized to its submembrane domain. Investigating how this step might be regulated by a deubiquitinase (DUB) activity will provide new insight for understanding Notch receptor activation and downstream signaling. An immunofluorescence-based screening of an shRNA library allowed us to identify eIF3f, previously known as one of the subunits of the translation initiation factor eIF3, as a DUB targeting the activated Notch receptor. We show that eIF3f has an intrinsic DUB activity. Knocking down eIF3f leads to an accumulation of monoubiquitinated forms of activated Notch, an effect counteracted by murine WT eIF3f but not by a catalytically inactive mutant. We also show that eIF3f is recruited to activated Notch on endocytic vesicles by the putative E3 ubiquitin ligase Deltex1, which serves as a bridging factor. Finally, catalytically inactive forms of eIF3f as well as shRNAs targeting eIF3f repress Notch activation in a coculture assay, showing that eIF3f is a new positive regulator of the Notch pathway. Our results support two new and provocative conclusions: (1) The activated form of Notch needs to be deubiquitinated before being processed by the gamma-secretase activity and entering the nucleus, where it fulfills its transcriptional function. (2) The enzyme accounting for this deubiquitinase activity is eIF3f, known so far as a translation initiation factor. These data improve our knowledge of Notch signaling but also open new avenues of research on the Zomes family and the translation initiation factors.
The highly conserved signaling pathway involving the transmembrane receptor Notch is essential for development, and misregulation of this pathway is linked to many diseases. We previously proposed that the Notch1 receptor is monoubiquitinated during its activation. With the aim of identifying a deubiquinating enzyme that could regulate Notch activation, we demonstrated that eIF3f, known previously as part of the multiprotein translation initiation factor eIF3 complex, harbors an enzymatic activity that acts on Notch. The activated form of Notch is able to interact with eIF3f only in the presence of the E3 ubiquitin ligase Deltex, and Notch needs to be deubiquitinated before it can be cleared and its intracellular domain can enter the nucleus and fulfill its transcriptional function. Our results further decipher the molecular mechanisms of Notch signaling activation, showing that ubiquitination and deubiquitination events are required. Additionally, we show that beyond acting as a translation initiation factor, eIF3f fulfills other functions and has an intrinsic enzymatic activity.
The Wnt/β-catenin signaling pathway is a highly conserved pathway in organism evolution and regulates many biological processes. Aberrant activation of the Wnt/β-catenin signaling pathway is closely related to tumorigenesis. In order to identify potent small molecules to treat the over-activated Wnt signaling-mediated cancer, such as colon cancer, we established a mammalian cell line-based reporter gene screening system. The screen revealed a diterpenoid derivative, 15-oxospiramilactone (NC043) that inhibits Wnt3a or LiCl-stimulated Top-flash reporter activity in HEK293T cells and growth of colon cancer cells, SW480 and Caco-2. Treatment of SW480 cells with NC043 led to decreases in the mRNA and/or protein expression of Wnt target genes Axin2, Cyclin D1 and Survivin , as well as decreases in the protein levels of Cdc25c and Cdc2. NC043 did not affect the cytosol-nuclear distribution and protein level of soluble β-catenin, but decreased β-catenin/TCF4 association in SW480 cells. Moreover, NC043 inhibited anchorage-independent growth and xenograft tumorigenesis of SW480 cells. Collectively these results demonstrate that NC043 is a novel small molecule that inhibits canonical Wnt signaling downstream of β-catenin stability and may be a potential compound for treating colorectal cancer.
Wnt; small molecule; inhibitor; tumorigenesis
Aberrant activation of Wnt/β-catenin signaling, resulting in the expression of Wnt regulated oncogenes, is recognized as a critical factor in the etiology of colorectal cancer. Occupancy of β-catenin at promoters of Wnt target genes drives transcription, but the mechanism of β-catenin action remains poorly understood. Here, we show that CARM1 (coactivator associated protein arginine methyltransferase 1) interacts with β-catenin and positively modulates β-catenin-mediated gene expression. In colorectal cancer cells with constitutively high Wnt/β-catenin activity, depletion of CARM1 inhibits expression of endogenous Wnt/β-catenin target genes and suppresses clonal survival and anchorage-independent growth. We also identified a colorectal cancer cell line (RKO) with a low basal level of β-catenin, which is dramatically elevated by treatment with Wnt3a. Wnt3a also increased expression of a subset of endogenous Wnt target genes, and CARM1 was required for the Wnt-induced expression of these target genes and the accompanying dimethylation of arginine 17 of histone H3. Depletion of β-catenin from RKO cells diminished the Wnt-induced occupancy of CARM1 on a Wnt target gene, indicating that CARM1 is recruited to Wnt target genes through its interaction with β-catenin and contributes to transcriptional activation by mediating events (including histone H3 methylation) which are downstream from the actions of β-catenin. Therefore, CARM1 is an important positive modulator of Wnt/β-catenin transcription and neoplastic transformation, and may thereby represent a novel target for therapeutic intervention in cancers involving aberrantly activated Wnt/β-catenin signaling.
CARM1; Wnt/β-catenin; coactivators; transcriptional regulation; colorectal cancers
Activation of Wnt signalling due to inability to degrade β-catenin is found in >85% of colorectal cancers. Approximately half of colon cancers express a constitutively active KRAS protein. A significant fraction of patients show both abnormalities. We previously reported that simultaneous down-regulation of both β-catenin and KRAS was necessary to induce significant cell death and tumor growth inhibition of colorectal cancer cells. Although attractive, an RNAi-based therapeutic approach is still far from being employed in the clinical setting. Therefore, we sought to recapitulate our previous findings by the use of small-molecule inhibitors of β-catenin and KRAS. We show here that the β-catenin inhibitors PKF115-584 and pyrvinium pamoate block β-catenin-dependent transcriptional activity and synergize with the KRAS inhibitor S-trans, trans-farnesylthiosalicylic acid (FTS, salirasib) in colon cancer cells driven by Wnt and KRAS oncogenic signals, but not in cells carrying BRAF mutations. The combined use of these compounds was superior to the use of any drug alone in inducing cell growth arrest, cell death, MYC and survivin down-modulation, and inhibition of anchorage-independent growth. Expression analysis of selected cancer-relevant genes revealed down-regulation of CD44 as a common response to the combined treatments. These data provide a proof of principle for a combination therapeutic strategy in colorectal cancer.
The Wnt/β-Catenin signaling pathway is central for liver functions and frequently deregulated in hepatocellular carcinoma (HCC). Analysis of the early phenotypes and molecular events following β-Catenin activation is therefore essential for better understanding HCC pathogenesis. The AP-1 transcription factor c-Jun is a putative β-Catenin target gene and promotes hepatocyte survival, proliferation, and liver tumorigenesis, suggesting that c-Jun may be a key target of β-Catenin signaling in the liver.
To address this issue, the immediate hepatic phenotypes following deletion of the tumor suppressor Apc and subsequent β-Catenin activation were analyzed in mice. The contribution of c-Jun to these phenotypes was dissected in double mutant animals lacking both, Apc and c-Jun. β-Catenin was rapidly activated in virtually all Apc mutant hepatocytes while c-Jun was induced only after several days, suggesting that its expression was rather a secondary event following Apc deletion in the liver. Loss of Apc resulted in increased hepatocyte proliferation, hepatomegaly, deregulated protein metabolism, and premature death. Interestingly, additional deletion of c-Jun did not affect hepatocyte proliferation but resulted in increased liver damage and mortality. This phenotype correlated with impaired expression of hepatoprotective genes such as Birc5, Egfr Igf1 and subsequently deregulated Akt signaling.
These data indicate that c-Jun is not a primary target of β-Catenin signaling in the liver, but rather protects against liver damage, which in turn may promote liver tumorigenesis.
Wnt/β-catenin signalling controls development and tissue homeostasis. Moreover, activated β-catenin can be oncogenic and, notably, drives colorectal cancer. Inhibiting oncogenic β-catenin has proven a formidable challenge. Here we design a screen for small-molecule inhibitors of β-catenin's binding to its cofactor BCL9, and discover five related natural compounds, including carnosic acid from rosemary, which attenuates transcriptional β-catenin outputs in colorectal cancer cells. Evidence from NMR and analytical ultracentrifugation demonstrates that the carnosic acid response requires an intrinsically labile α-helix (H1) amino-terminally abutting the BCL9-binding site in β-catenin. Similarly, in colorectal cancer cells with hyperactive β-catenin signalling, carnosic acid targets predominantly the transcriptionally active ('oncogenic') form of β-catenin for proteasomal degradation in an H1-dependent manner. Hence, H1 is an 'Achilles' Heel' of β-catenin, which can be exploited for destabilization of oncogenic β-catenin by small molecules, providing proof-of-principle for a new strategy for developing direct inhibitors of oncogenic β-catenin.
β-Catenin can be oncogenic but finding inhibitors has been a challenge. Here, five compounds are identified, which attenuate transcriptional β-catenin outputs in colorectal cancer cells, and the response to one of them is shown to require an intrinsically labile α-helix next to the BCL9-binding site in β-catenin.
The pervasive influence of secreted Wnt signaling proteins in tissue homeostasis and tumorigenesis has galvanized efforts to identify small molecules that target Wnt-mediated cellular responses. By screening a diverse synthetic chemical library, we have discovered two novel classes of small molecules that disrupt Wnt pathway responses - whereas one class inhibits the activity of Porcupine (Porcn), a membrane-bound acyltransferase that is essential to the production of Wnt proteins, the other abrogates destruction of Axin proteins, suppressors of Wnt/β-catenin pathway activity. With these small molecules we establish a chemical genetic approach for studying Wnt pathway responses and stem cell function in adult tissue. We achieve transient, reversible suppression of Wnt/β-catenin pathway response in vivo, and establish a mechanism-based approach to target cancerous cell growth. The signal transduction mechanisms shown here to be chemically tractable additionally contribute to Wnt-independent signal transduction pathways and thus could be broadly exploited for chemical genetics and therapeutic goals.
Deregulation of the Wnt signalling pathway by mutations in the Apc or β-catenin genes underlies colorectal carcinogenesis. As a result, β-catenin stabilises, translocates to the nucleus, and activates gene transcription. Intestinal tumours show a heterogeneous pattern of nuclear β-catenin, with the highest levels observed at the invasion front. Activation of receptor tyrosine kinases in these tumour areas by growth factors expressed by surrounding stromal cells phosphorylate β-catenin at tyrosine residues, which is thought to increase β-catenin nuclear translocation and tumour invasiveness. This study investigates the relevance of β-catenin tyrosine phosphorylation for Wnt signalling and intestinal tumorigenesis in vivo.
A conditional knock-in mouse model was generated into which the phospho-mimicking Y654E modification in the endogenous β-catenin gene was introduced.
This study provided in vivo evidence that β-cateninE654 is characterised by reduced affinity for cadherins, increased signalling and strongly increased phosphorylation at serine 675 by protein kinase A (PKA). In addition, homozygosity for the β-cateninE654 targeted allele caused embryonic lethality, whereas heterozygosity predisposed to intestinal tumour development, and strongly enhanced Apc-driven intestinal tumour initiation associated with increased nuclear accumulation of βcatenin. Surprisingly, the expression of β-cateninE654 did not affect histological grade or induce tumour invasiveness.
A thus far unknown mechanism was uncovered in which Y654 phosphorylation of β-catenin facilitates additional phosphorylation at serine 675 by PKA. In addition, in contrast to the current belief that β-catenin Y654 phosphorylation increases tumour progression to a more invasive phenotype, these results show that it rather increases tumour initiation by enhancing Wnt signalling.
β-Catenin; colorectal cancer; molecular biology; mouse model; tyrosine 654 phosphorylation; Wnt signalling
Wnt/β-catenin signaling plays a major role in embryonic development and adult stem cell maintenance. Reduced activation of the Wnt/β-catenin pathway underlies neurodegenerative disorders and aberrations in bone formation. Screening of a small molecule compound library with a β-galactosidase fragment complementation assay measuring β-catenin nuclear entry revealed bona fide activators of β-catenin signaling. The compounds stabilized cytoplasmic β-catenin and activated β–catenin-dependent reporter gene activity. Although the mechanism through which the compounds activate β-catenin signaling has yet to be determined, several key regulators of Wnt/β-catenin signaling, including glycogen synthase kinase 3 and Frizzled receptors, were excluded as the molecular target. The compounds displayed remarkable selectivity, as they only induced β-catenin signaling in a human osteosarcoma U2OS cell line and not in a variety of other cell lines examined. Our data indicate that differences in cellular Wnt/β-catenin signaling machinery can be exploited to identify cell type-specific activators of Wnt/β-catenin signaling.
Calcimycin restricts Wnt/β-catenin–regulated S100A4 expression, leading to reduced S100A4-mediated cell migration and invasion in colon cancer cells, as well as to inhibition of metastasis formation in xenografted mice.
The calcium-binding protein S100A4 is a central mediator of metastasis formation in colon cancer. S100A4 is a target gene of the Wnt/β-catenin pathway, which is constitutively active in the majority of colon cancers. In this study a high-throughput screen was performed to identify small-molecule compounds targeting the S100A4-promoter activity. In this screen calcimycin was identified as a transcriptional inhibitor of S100A4. In colon cancer cells calcimycin treatment reduced S100A4 mRNA and protein expression in a dose- and time-dependent manner. S100A4-induced cellular processes associated with metastasis formation, such as cell migration and invasion, were inhibited by calcimycin in an S100A4-specific manner. Calcimycin reduced β-catenin mRNA and protein levels despite the expression of Δ45-mutated β-catenin. Consequently, calcimycin inhibited Wnt/β-catenin pathway activity and the expression of prominent β-catenin target genes such as S100A4, cyclin D1, c-myc, and dickkopf-1. Finally, calcimycin treatment of human colon cancer cells inhibited metastasis formation in xenografted immunodeficient mice. Our results demonstrate that targeting the expression of S100A4 with calcimycin provides a functional strategy to restrict cell motility in colon cancer cells. Therefore calcimycin may be useful for studying S100A4 biology, and these studies may serve as a lead for the development of treatments for colon cancer metastasis.
Deciphering the epigenetic “code” remains a central issue in transcriptional regulation. Here, we report the identification of a MPN+/JAMM domain-containing histone H2A deubiquitinase (2A-DUB, or KIAA1915/MYSM1) specific for monoubiquitinated H2A (uH2A), that has permitted delineation of a strategy for specific regulatory pathways of gene activation. 2A-DUB regulates transcription by coordinating histone acetylation and deubiquitination, and destabilizing the association of linker histone H1 with nucleosomes. 2A-DUB interacts with p/CAF in a co-regulatory protein complex, with its deubiquitinase activity modulated by the status of acetylation of nucleosomal histones. Consistent with this mechanistic role, 2A-DUB participates in transcriptional regulation events in androgen receptor-dependent gene activation, and the levels of uH2A are dramatically decreased in prostate tumors, serving as a cancer-related mark. We suggest that H2A ubiquitination represents a widely-used mechanism for many regulatory transcriptional programs, and predict that various H2A ubiquitin ligases/deubiquitinases will be identified for specific cohorts of regulated transcription units.
Aberrant Wnt signal transduction is involved in many human diseases such as cancer and neurodegenerative disorders. The key effector protein of the canonical Wnt pathway is β-catenin, which functions with T-cell factor/lymphoid enhancer factor (TCF/LEF) to activate gene transcription that leads to expression of Wnt target genes. In this study we provide results obtained from a novel functional screen of a human brain cDNA library used to identify 63 genes that are putative negative Wnt regulators. These genes were divided into eight functional groups that include known canonical and noncanonical Wnt pathway components and genes that had not yet been assigned to the Wnt pathway. One of the groups, the presenilin-binding proteins, contains the modifier of cell adhesion (MOCA) gene. We show that MOCA is a novel inhibitor of Wnt/β-catenin signaling. MOCA forms a complex with β-catenin and inhibits transcription of known Wnt target genes. Epistasis experiments indicate that MOCA acts to reduce the levels of nuclear β-catenin, increase the levels of membrane-bound β-catenin, and enhances cell–cell adhesion. Therefore, our data indicate that MOCA is a novel Wnt negative regulator and demonstrate that this screening approach can be a rapid means for isolation of new Wnt regulators.
β-catenin is a critical mediator of the canonical Wnt signaling pathway. α-catenin is a major β-catenin binding protein and overexpressed α-catenin can negatively regulate β-catenin activity. Thus, α-catenin may be an important modulator of Wnt pathway. We show here that endogenous α-catenin has little impact on transcriptional activity of β-catenin in developing mammalian organism. We analyzed β-catenin signaling in mice with conditional deletion of αE-catenin in the developing central nervous system. This mutation results in brain hyperplasia and we investigated whether activation of β-catenin signaling may be at least partially responsible for this phenotype. To reveal potential quantitative or spatial changes in β-catenin signaling, we utilized mice carrying a β-catenin-signaling reporter transgene. In addition, we analyzed the expression of known endogenous targets of the β-catenin pathway and the amount and localization of β-catenin in mutant progenitor cells. We found that while loss of αE-catenin resulted in disruption of intercellular adhesion and hyperplasia in the developing brain, β-catenin signaling was not altered. We conclude that endogenous αE-catenin has no significant impact on β-catenin transcriptional activities in the developing mammalian brain.
brain development; β-catenin signaling; α-catenin
Aberrant activation of Wnt/β-catenin signaling promotes the development of several cancers. It has been demonstrated that the Wnt signaling pathway is activated in chronic lymphocytic leukemia (CLL) cells, and that uncontrolled Wnt/β-catenin signaling may contribute to the defect in apoptosis that characterizes this malignancy. Thus, the Wnt signaling pathway is an attractive candidate for developing targeted therapies for CLL.
The diuretic agent ethacrynic acid (EA) was identified as a Wnt inhibitor using a cell-based Wnt reporter assay. In vitro assays further confirmed the inhibitory effect of EA on Wnt/β-catenin signaling. Cell viability assays showed that EA selectively induced cell death in primary CLL cells. Exposure of CLL cells to EA decreased the expression of Wnt/β-catenin target genes, including LEF-1, cyclin D1 and fibronectin. Immune co-precipitation experiments demonstrated that EA could directly bind to LEF-1 protein and destabilize the LEF-1/β-catenin complex. N-acetyl-L-cysteine (NAC), which can react with the α, β-unsaturated ketone in EA, but not other anti-oxidants, prevented the drug's inhibition of Wnt/β-catenin activation and its ability to induce apoptosis in CLL cells.
Our studies indicate that EA selectively suppresses CLL survival due to inhibition of Wnt/β-catenin signaling. Antagonizing Wnt signaling in CLL with EA or related drugs may represent an effective treatment of this disease.
Constitutive Wnt signalling is characterized by excessive levels of β-catenin protein and is a frequent occurrence in cancer. APC and Axin are key components of the β-catenin destruction complex that acts to promote β-catenin degradation. The levels of Axin are in turn controlled by tankyrases, members of the PARP-family of poly-ADP-ribosylation enzymes. In colorectal cancer cells, which typically harbor APC mutations, inhibition of tankyrase activity promotes Axin stabilization and attenuates Wnt signalling. Here, we examined the effect of inhibiting tankyrases in breast cancer cells with normal APC. We show that application of the small molecule tankyrase inhibitor, XAV939 or siRNA-mediated abrogation of tankyrase expression increases Axin1 and Axin2 protein levels and attenuates Wnt-induced transcriptional responses in several breast cancer lines. In MDA-MB-231 cells, inhibiton of tankyrase activity also attenuate Wnt3a induced cell migration. Moreover, in both MDA-MB-231 and colorectal cancer cells, XAV939 inhibits cell growth under conditions of serum-deprivation. However, the presence of serum prevents this growth inhibitory effect, although inhibition of Wnt-induced transcriptional and migratory responses was maintained. These results indicate that stabilization of Axin by inhibition of tankyrases alone, may not be an effective means to block tumor cell growth and that combinatorial therapeutic approaches should be considered.
During embryonic development, β-catenin is central both to the transcriptional activation of Wnt [wingless-type MMTV (murine-mammary-tumour virus) integration site family] target genes and as a mediator of cell–cell adhesion. Signals that regulate its levels and subcellular localization are critical. One mechanism of Wnt signalling results in stabilization of β-catenin protein, which leads to its translocation into the nucleus, where it interacts with TCF (T-cell factor, HMG box) and activates transcription of target genes. Less well understood are mechanisms of Wnt signalling that do not involve β-catenin stabilization and result in inhibition of β-catenin-mediated transcription.
Here, we show that a member of the Wnt protein family, Wnt4 (Wnt, member 4), regulates the subcellular localization of β-catenin, redirecting it to the cell membrane. Unique among Wnts, this action does not affect the stability of β-catenin but does prohibit its involvement in TCF gene transactivation.
This novel mechanism suggests that Wnt4 acts as a switch between the two modes of β-catenin function, transcriptional activation and cell–cell adhesion.
β-catenin; murine-mammary-tumour virus (MMTV); subcellular localization; T-cell factor (TCF); wingless-type MMTV integration site family; member 4 (Wnt4)
Multiple myeloma (MM) is a malignant neoplasm of plasma cells. Although new molecular targeting agents against MM have been developed based on the better understanding of the underlying pathogenesis, MM still remains an incurable disease. We previously demonstrated that β-catenin, a downstream effector in the Wnt pathway, is a potential target in MM using RNA interference in an in vivo experimental mouse model. In this study, we have screened a library of more than 100 000 small-molecule chemical compounds for novel Wnt/β-catenin signaling inhibitors using a high-throughput transcriptional screening technology. We identified AV-65, which diminished β-catenin protein levels and T-cell factor transcriptional activity. AV-65 then decreased c-myc, cyclin D1 and survivin expression, resulting in the inhibition of MM cell proliferation through the apoptotic pathway. AV-65 treatment prolonged the survival of MM-bearing mice. These findings indicate that this compound represents a novel and attractive therapeutic agent against MM. This study also illustrates the potential of high-throughput transcriptional screening to identify candidates for anticancer drug discovery.
multiple myeloma; Wnt; β-catenin; high-throughput screening
Wnt signaling affects fundamental development pathways by regulating cell proliferation and differentiation. Aberrant activation of Wnt/β-catenin signaling promotes the development of several cancers and is an attractive target for chemopreventive and chemotherapeutic agents.
In order to identify the novel antagonists for the Wnt/β-catenin pathway, we employed a cell-based Wnt reporter system (TOPflash) to screen a library of 960 known drugs. We identified spiperone, a psychotropic drug, as a novel Wnt inhibitor, which specifically blocks canonical Wnt signaling prior to the activation of β-catenin. The Wnt inhibitory function of spiperone is not associated with its dopamine-, serotonin- and sigma-receptor antagonist properties. Instead, spiperone increases intracellular calcium levels in a similar manner to thapsigargin, that also impedes Wnt signal transduction. Inhibition of protein kinase C had no effect on spiperone-mediated antagonism of Wnt signaling.
Spiperone is a calcium regulator. It inhibits Wnt signaling by enhancing intracellular calcium levels.
Using embryos transgenic for the TOP-GFP reporter, we show that the two zebrafish β-catenins have different roles in the organizer and germ-ring regions of the embryo. β-catenin-activated transcription in the prospective organizer region specifically requires β-catenin-2, whereas the ventrolateral domain of activated transcription is abolished only when both β-catenins are inhibited. chordin expression during zebrafish gastrulation has been previously shown in both axial and paraxial domains, but is excluded from ventrolateral domains. We show that this gene is expressed in paraxial territories adjacent to the domain of ventrolateral β-catenin-activated transcription, with only slight overlap, consistent with the now well-known inhibitory effects of Wnt8 on dorsal gene expression. Eliminating both Wnt8/β-catenin signaling and organizer activity by inhibition of expression of the two β-catenins results in massive ectopic circumferential expression of chordin and later, by formation of a distinctive embryonic phenotype (‘ciuffo’) that expresses trunk and anterior neural markers with correct relative anteroposterior patterning. We show that chordin expression is required for this neural gene expression. The Nodal gene squint has been shown to be necessary for optimal expression of chordin and is sufficient in some contexts for its expression. However, chordin is not normally expressed in the ventrolateral germ-ring despite robust expression of squint in this domain. We show the ectopic circumferential expression of chordin and other dorsal genes to be completely dependent on Nodal and FGF signaling, and to be independent of a functional organizer. We propose that whereas the axial domain of chordin expression is formed by cells that are derived from the organizer, the paraxial domain is the result of axial-derived anti-Wnt signals, which relieve the repression that otherwise is set by the Wnt8/β-catenin/vox,vent pathway on latent germ-ring Nodal/FGF-activated expression.
β-catenin; Wnt8; Vox; Vent; Nodal signaling; FGF signaling; dorsoventral patterning; ichabod (ich)
Cross-regulation between the Wnt and nuclear factor (NF)-κB signaling pathways has emerged as an important area for the regulation of a diverse array of genes and pathways active in chronic inflammation, immunity, development, and tumorigenesis. The ligands, kinases, transcription factors, and products of their target gene expression are involved in cross-regulation of these two signaling pathways. Both β-catenin and NF-κB activate inducible nitric oxide synthase (iNOS) gene expression; however, β-catenin also exerts an inhibitory effect on NF-κB-mediated transcriptional activation, including iNOS. The recent discovery of functional cross-regulation between these two pathways has shown complex roles for Wnt/β-catenin and NF-κB signaling in the pathogenesis of certain cancers and other diseases. This review focuses on the molecular mechanisms of cross-regulation between Wnt/β-catenin and NF-κB signaling pathways in cancer cells.
Wnt signaling pathway; NF-κB signaling pathway; cross-regulation; β-catenin