Beta-catenin mutations in cell lines established from human colorectal cancers

Enhanced 4-hydroxynonenal resistance in KEAP1 silenced human colon cancer cells

Nuclear factor erythroid 2-related factor 2 (NRF2) is the transcription factor that regulates an array of antioxidant/detoxifying genes for cellular defense. The conformational changes of Kelch-like ECH-associated protein 1 (KEAP1), a cytosolic repressor protein of NRF2, by various stimuli result in NRF2 liberation and accumulation in the nucleus. In the present study, we aimed to investigate the effect of KEAP1 knockdown on NRF2 target gene expression and its toxicological implication using human colon cancer cells. The stable KEAP1-knockdown HT29 cells exhibit elevated levels of NRF2 and its target gene expressions. In particular, the mRNA levels of aldo-keto reductases (AKR1C1, 1C2, 1C3, 1B1, and 1B10) were substantially increased in KEAP1 silenced HT29 cells. These differential AKRs expressions appear to contribute to protection against oxidative stress. The KEAP1-knockdown cells were relatively more resistant to hydrogen peroxide (H₂O₂) and 4-hydroxynonenal (4HNE) compared to the control cells. Accordantly, we observed accumulation of 4HNE protein adducts in H₂O₂- or 4HNE-treated control cells, whereas KEAP1-knockdown cells did not increase adduct formation. The treatment of KEAP1-silenced cells with AKR1C inhibitor flufenamic acid increased 4HNE-induced cellular toxicity and protein adduct formation. Taken together, these results indicate that AKRs, which are NRF2-dependent highly inducible gene clusters, play a role in NRF2-mediated cytoprotection against lipid peroxide toxicity.

Reciprocal Negative Regulation between the Guanine Nucleotide Exchange Factor C3G and β-Catenin

The guanine nucleotide exchange factor C3G (RAPGEF1) regulates proliferation, migration, and differentiation of cells and is essential for mammalian embryonic development. The molecular effectors of C3G dependent functions are poorly understood. Here we report that C3G functions as a negative regulator of β-catenin, a major player in pathways known to be deregulated in human cancers. In mammalian cells, C3G is present in a complex with cellular β-catenin. The proline rich Crk binding region of C3G and residues 90-525 of β-catenin are sufficient for the interaction. Knockdown of cellular C3G stimulated, and its overexpression repressed, β-catenin/TCF transcription activity. C3G acts by destabilizing β-catenin protein and inhibiting its nuclear accumulation. Nuclear extracts of C3G overexpressing cells showed reduced binding to TCF consensus oligos. C3G exerts its effects independent of its function as an exchange factor. It also inhibits stability and activity of an N-terminal deletion construct of β-catenin that is not subject to GSK3β dependent phosphorylation, suggesting that C3G exerts its effect independent of GSK3β. β-catenin repression by C3G was not significantly altered in the presence of proteasome inhibitors, MG132 or lactacystin, suggesting that alternate mechanisms are engaged by C3G to cause β-catenin turnover. C3G expression represses β-catenin target gene expression, and stable clones of MCF-7 breast cancer cells expressing C3G showed reduced migration. Activation of cellular β-catenin or expression of constitutively active β-catenin resulted in reduced C3G expression, indicating that C3G gene expression is negatively regulated by β-catenin. Our results identify a novel property of C3G in functioning as a negative regulator of β-catenin signaling by promoting its degradation. In addition, we show that β-catenin inhibits C3G expression, forming a feedback loop.

Recurrent somatic mutation of FAT1 in multiple human cancers leads to aberrant Wnt activation

Aberrant Wnt signaling can drive cancer development. In many cancer types, the genetic basis of Wnt pathway activation remains incompletely understood. Here, we report recurrent somatic mutations of the Drosophila melanogaster tumor suppressor-related gene FAT1 in glioblastoma (20.5%), colorectal cancer (7.7%), and head and neck cancer (6.7%). FAT1 encodes a cadherin-like protein, which we found is able to potently suppress cancer cell growth in vitro and in vivo by binding β-catenin and antagonizing its nuclear localization. Inactivation of FAT1 via mutation therefore promotes Wnt signaling and tumorigenesis and affects patient survival. Taken together, these data strongly point to FAT1 as a tumor suppressor gene driving loss of chromosome 4q35, a prevalent region of deletion in cancer. Loss of FAT1 function is a frequent event during oncogenesis. These findings address two outstanding issues in cancer biology: the basis of Wnt activation in non-colorectal tumors and the identity of a 4q35 tumor suppressor.

A reporter gene system for screening inhibitors of Wnt signaling pathway

Abnormal activation of canonical Wnt signaling has been associated with various types of cancer. Inhibitory reagents targeting the Wnt signaling have great potential to inhibit the growth of relevant tumors. Here we generated a cell-based screening strategy for identification of antagonists of the Wnt/β-catenin signaling pathway. Stable expression wnt3a was generated in HEK293 cell line, which harbors dual-luciferase reporters. The Wnt signaling in the stably transfected cell line was proved to be very sensitive to (−)-epigallocatechin-3-gallate (EGCG) and lithium chloride (LiCl) treatment, respectively. Natural compounds were screened and a couple of novel inhibitory modulators of the Wnt signaling pathway were obtained.

Synergistic effects of combined Wnt/KRAS inhibition in colorectal cancer cells

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.

Human SMC2 protein, a core subunit of human condensin complex, is a novel transcriptional target of the WNT signaling pathway and a new therapeutic target

Human SMC2 is part of the condensin complex, which is responsible for tightly packaging replicated genomic DNA prior to segregation into daughter cells. Engagement of the WNT signaling pathway is known to have a mitogenic effect on cells, but relatively little is known about WNT interaction with mitotic structural organizer proteins. In this work, we described the novel transcriptional regulation of SMC2 protein by direct binding of the β-catenin·TCF4 transcription factor to the SMC2 promoter. Furthermore, we identified the precise region in the SMC2 promoter that is required for β-catenin-mediated promoter activation. Finally, we explored the functional significance of down-regulating SMC2 protein in vivo. Treatment of WNT-activated intestinal tumor cells with SMC2 siRNA significantly reduced cell proliferation in nude mice, compared with untreated controls (p = 0.02). Therefore, we propose that WNT signaling can directly activate SMC2 transcription as a key player in the mitotic cell division machinery. Furthermore, SMC2 represents a new target for oncological therapeutic intervention.

β-catenin and K-ras mutations and RASSF1A promoter methylation in Taiwanese colorectal cancer patients

AIMS

The purpose of this study was to investigate the associations of β-catenin mutations, K-ras mutations, methylations of the RASSF1A promoter, and the survival of Taiwanese colorectal cancer (CRC) subjects who received 5-fluorouracil (5-FU) adjuvant chemotherapy.

RESULTS

The complete coding region of the K-ras gene and exon 3 and exon 4 of the β-catenin gene isolated from tumor tissues and adjacent normal colon tissues from 117 CRC subjects were sequenced, respectively. Methylations in the RASSF1A promoter region were also investigated. Various characteristics of the 117 subjects were recorded and used in the Cox proportional-hazard model analyses. Three missense mutations, one nonsense mutation, and one deletion were identified in the β-catenin gene. A 2 bp deletion was identified in the K-ras gene. We found that the frequencies of mutations in the β-catenin and K-ras genes were less pronounced in Taiwanese CRC subjects as compared with other populations. Methylations in the RASSF1A promoter region were detected in 73.5% (n=86/117) of the subjects, which was higher than in other studies. Methylations in the RASSF1A promoter have no significant effect on hazards for all CRC deaths caused in Taiwanese CRC patients. No interaction between 5-FU adjuvant chemotherapy and methylations of the RASSF1A promoter was observed.

CONCLUSIONS

The mutation frequencies of β-catenin and K-ras genes in Taiwanese CRC patients are very low, which may suggest that they are not the dominant factors for CRC occurrence and prognosis in Taiwanese CRC patients. Methylation of RASSF1A promoter is independent of the prognosis for Taiwanese CRC patients. Taiwanese subjects differ from subjects of other populations with regard to β-catenin, K-ras, and RASSF1A presentations for CRC.

The Wnt/β-catenin/T-cell factor 4 pathway up-regulates high-mobility group A1 expression in colon cancer

High-mobility group A1 (HMGA1) encodes proteins that act as mediators in viral integration, modification of chromatin structure, neoplastic transformation and metastatic progression. Because HMGA1 is overexpressed in most cancers and has transcriptional relationships with several Wnt-responsive genes, we explored the involvement of HMGA1 in Wnt/β-catenin/TCF-4 signalling. In adenomatous polyposis coli (APC(Min/+)) mice, we observed significant up-regulation of HMGA1 mRNA and protein in intestinal tumours when compared with normal intestinal mucosa. Conversely, restoration of Wnt signalling by the zinc induction of wild-type APC resulted in HMGA1 down-regulation in HT-29 cells. Because APC mutations are associated with mobilization of the β-catenin/TCF-4 transcriptional complex and subsequent activation of downstream oncogenic targets, we analyzed the 5'-flanking sequence of HMGA1 for putative TCF-4 binding elements. We identified two regions that specifically bind the β-catenin/TCF-4 complex in vitro and in vivo, identifying HMGA1 as an immediate target of the β-catenin/TCF-4 signalling pathway in colon cancer. Collectively, these findings strongly implicate Wnt/β-catenin/TCF-4 signalling in regulating HMGA1 to further expand the extensive regulatory network affected by Wnt/β-catenin/TCF-4 signalling.

The E3 ubiquitin ligase ITCH negatively regulates canonical Wnt signaling by targeting dishevelled protein

Dishevelled (Dvl) is a key component in the canonical Wnt signaling pathway and becomes hyperphosphorylated upon Wnt stimulation. Dvl is required for LRP6 phosphorylation, which is essential for subsequent steps of signal transduction, such as Axin recruitment and cytosolic β-catenin stabilization. Here, we identify the HECT-containing Nedd4-like ubiquitin E3 ligase ITCH as a new Dvl-binding protein. ITCH ubiquitinates the phosphorylated form of Dvl and promotes its degradation via the proteasome pathway, thereby inhibiting canonical Wnt signaling. Knockdown of ITCH by RNA interference increased the stability of phosphorylated Dvl and upregulated Wnt reporter gene activity as well as endogenous Wnt target gene expression induced by Wnt stimulation. In addition, we found that both the PPXY motif and the DEP domain of Dvl are critical for its interaction with ITCH, as mutation in the PPXY motif (Dvl2-Y568F) or deletion of the DEP domain led to reduced affinity for ITCH. Consistently, overexpression of ITCH inhibited wild-type Dvl2-induced, but not Dvl2-Y568F mutant-induced, Wnt reporter activity. Moreover, the Y568F mutant, but not wild-type Dvl2, can reverse the ITCH-mediated inhibition of Wnt-induced reporter activity. Collectively, these results indicate that ITCH plays a negative regulatory role in modulating canonical Wnt signaling by targeting the phosphorylated form of Dvl.

The insulin receptor substrate 1 (IRS1) in intestinal epithelial differentiation and in colorectal cancer

Colorectal cancer (CRC) is associated with lifestyle factors that affect insulin/IGF signaling, of which the insulin receptor substrate 1 (IRS1) is a key transducer. We investigated expression, localization and pathologic correlations of IRS1 in cancer-uninvolved colonic epithelium, primary CRCs with paired liver metastases and in vitro polarizing Caco2 and HT29 cells. IRS1 mRNA and protein resulted higher, relative to paired mucosa, in adenomas of familial adenomatous polyposis patients and in CRCs that overexpressed c-MYC, ß-catenin, InsRß, and IGF1R. Analysis of IRS1 immunostaining in 24 cases of primary CRC with paired colonic epithelium and hepatic metastasis showed that staining intensity was significantly higher in metastases relative to both primary CRC (P<0.01) and colonic epithelium (P<0.01). Primary and metastatic CRCs, compared to colonic epithelium, contained significantly higher numbers of IRS1-positive cells (P = 0.013 and P = 0.014, respectively). Pathologic correlations in 163 primary CRCs revealed that diffuse IRS1 staining was associated with tumors combining differentiated phenotype and aggressive markers (high Ki67, p53, and ß-catenin). In Caco 2 IRS1 and InsR were maximally expressed after polarization, while IGF1R was highest in pre-polarized cells. No nuclear IRS1 was detected, while, with polarization, phosphorylated IRS1 (pIRS1) shifted from the lateral to the apical plasma membrane and was expressed in surface cells only. In HT29, that carry mutations constitutively activating survival signaling, IRS1 and IGF1R decreased with polarization, while pIRS1 localized in nuclear spots throughout the course. Overall, these data provide evidence that IRS1 is modulated according to CRC differentiation, and support a role of IRS1 in CRC progression and liver metastatization.

Impact of mutant β-catenin on ABCB1 expression and therapy response in colon cancer cells

Background:

Colorectal cancers are often chemoresistant toward antitumour drugs that are substrates for ABCB1-mediated multidrug resistance (MDR). Activation of the Wnt/β-catenin pathway is frequently observed in colorectal cancers. This study investigates the impact of activated, gain-of-function β-catenin on the chemoresistant phenotype.

Methods:

The effect of mutant (mut) β-catenin on ABCB1 expression and promoter activity was examined using HCT116 human colon cancer cells and isogenic sublines harbouring gain-of-function or wild-type β-catenin, and patients’ tumours. Chemosensitivity towards 24 anticancer drugs was determined by high throughput screening.

Results:

Cell lines with mut β-catenin showed high ABCB1 promoter activity and expression. Transfection and siRNA studies demonstrated a dominant role for the mutant allele in activating ABCB1 expression. Patients’ primary colon cancer tumours shown to express the same mut β-catenin allele also expressed high ABCB1 levels. However, cell line chemosensitivities towards 24 MDR-related and non-related antitumour drugs did not differ despite different β-catenin genotypes.

Conclusion:

Although ABCB1 is dominantly regulated by mut β-catenin, this did not lead to drug resistance in the isogenic cell line model studied. In patient samples, the same β-catenin mutation was detected. The functional significance of the mutation for predicting patients’ therapy response or for individualisation of chemotherapy regimens remains to be established.

Chemoprevention of familial adenomatous polyposis by bromo-noscapine (EM011) in the Apc(Min/+) mouse model

Germline mutation of the tumor suppressor gene, adenomatous polyposis coli (APC), is responsible for familial adenomatous polyposis (FAP) with nearly 100% risk for colon cancer at an early age. Although FAP is involved in only 1% of all colon cancer cases, over 80% of sporadic cancers harbor somatic mutations of APC. We show here that bromo-noscapine (EM011), a rationally designed synthetic derivative of a natural nontoxic tubulin-binding alkaloid-noscapine, that reduces the dynamics of microtubules, causes a reversible G(2) /M arrest in wild type murine embryonic fibroblasts (MEFs), but an aberrant exit from a brief mitotic block, followed by apoptosis in MEFs after APC deletion with small interfering RNA. Furthermore, both β-catenin levels and activity fell to half the original levels with a concomitant reduction of cell proliferation-inducing cyclin D1, c-Myc, and induction of cytostatic protein p21 before caspase-3 activation. Additionally, we show a statistically significant reduction in the number of newly emerging intestinal polyps (to 35% compared with untreated mice) as well as the mean size of polyps (to 42% compared with untreated mice) in EM011-treated Apc(Min/+) mice as compared to their sham-treated control littermates. The remaining polyps in the EM011 treated group of Apc(Min/+) mice showed evidence of elevated apoptosis as revealed by immunohistochemistry. We failed to detect any evidence of histopathological and hematological toxicities following EM011 treatment. Taken together, our data are persuasive that a clinical trial of EM011 is possible for the prevention/amelioration of polyposis in FAP patients.

Differential WNT activity in colorectal cancer confers limited tumorigenic potential and is regulated by MAPK signaling

Colorectal cancers (CRC) express the WNT effector protein β-catenin in a heterogeneous subcellular pattern rather than uniformly in the nucleus. In this study, we investigated this important aspect of molecular heterogeneity in CRCs by analyzing its basis and relationship with tumor-initiating capability. CRC cells with the highest WNT levels showed only a marginal increase in tumor initiation capacity. Notably, high WNT activity correlated with a coincident activation of robust mitogen-activated protein kinase (MAPK) signaling, which when upregulated by KRAS expression or downregulated by epidermal growth factor receptor inhibition elicited parallel effects on WNT activity. These findings suggested that on its own high WNT activity may not be a reliable signifier of tumor-initiating potential or stem-like potential. Furthermore, they suggest that MAPK signaling is a critical modifier of intratumoral heterogeneity that contributes significantly to determining the impact of WNT activity on stemness phenotypes in colon cancer cells.

Ovarian steroid cell tumor with biallelic adenomatous polyposis coli inactivation in a patient with familial adenomatous polyposis

Familial adenomatous polyposis (FAP) is an autosomal dominant cancer predisposition syndrome that accounts for approximately 0.5-1% of all colorectal cancer cases. It is caused by germline mutations in the gene encoding the adenomatous polyposis coli (APC) tumor suppressor. Somatic APC inactivation due to mutation or loss of heterozygosity (LOH) promotes the development of adenomatous polyps by stabilizing the transcriptional coactivator β-catenin. Although colorectal cancer is by far the most common malignancy seen in FAP patients, the widespread use of prophylactic colectomy in these patients has increased the clinical importance of extracolonic tumors that are part of the neoplastic spectrum in FAP. Many of these tumors exhibit LOH or somatic APC mutation, strongly supporting a causative role of APC inactivation in their pathogenesis. Here we describe a 47-year-old female FAP patient with clinical manifestations of virilization who was found to have an ovarian steroid cell tumor, a rare neoplasm not known to be associated with FAP. Immunohistochemical analysis of the ovarian tumor demonstrated strong nuclear β-catenin staining consistent with somatic APC inactivation, and molecular analysis confirmed biallelic APC inactivation in the tumor. Our findings provide the first evidence that ovarian steroid cell tumors may be an extracolonic manifestation of FAP and implicate β-catenin activation as an oncogenic mechanism in ovarian steroid cell tumorigenesis.

Vectors bicistronically linking a gene of interest to the SV40 large T antigen in combination with the SV40 origin of replication enhance transient protein expression and luciferase reporter activity

The simian virus 40 large T antigen (SVLT) induces replication of plasmids bearing the SV40 origin of replication (SV40 ori) within mammalian cells. The internal ribosomal entry site (IRES) is an element that allows for the cotranslation of proteins from one polycistronic mRNA. Through the combination of these elements, IRES-dependent coexpression of a protein of interest and the SVLT, either constitutive or regulated, on plasmids bearing the SV40 ori generates a positive feedback loop, resulting in enhanced expression. A vector linking red fluorescent protein (RFP) to the IRES-SVLT element enhances fluorescence ~10-fold over that demonstrated from a vector lacking this element. In transfection-resistant CV-1 cells, the RFP-IRES-SVLT vector substantially increases the number of cells expressing detectable levels of RFP. Furthermore, inclusion of the IRES-SVLT/SV40 ori elements in standard luciferase-based reporter gene constructs and associated effectors results in marked increases in luminescent output and sensitivity, using the β-catenin/TCF pathway and the mammalian two-hybrid assay as models. Ultimately, vector systems combining these well-established elements (IRES-SVLT/SV40 ori) will increase the utility of transient transfection for the production of recombinant proteins, the use of transfection-resistant cell lines, and the effectiveness of luciferase-based high-throughput screening assays.

Inhibition of β-catenin signaling by nongenomic action of orphan nuclear receptor Nur77

Dysregulation of β-catenin turnover due to mutations of its regulatory proteins including adenomatous polyposis coli (APC) and p53 is implicated in the pathogenesis of cancer. Thus, intensive effort is being made to search for alternative approaches to reduce abnormally activated β-catenin in cancer cells. Nur77, an orphan member of the nuclear receptor superfamily, has a role in the growth and apoptosis of cancer cells. Here, we reported that Nur77 could inhibit transcriptional activity of β-catenin by inducing β-catenin degradation via proteasomal degradation pathway that is glycogen synthase kinase 3β and Siah-1 independent. Nur77 induction of β-catenin degradation required both the N-terminal region of Nur77, which was involved in Nur77 ubiquitination, and the C-terminal region, which was responsible for β-catenin binding. Nur77/ΔDBD, a Nur77 mutant lacking its DNA-binding domain, resided in the cytoplasm, interacted with β-catenin, and induced β-catenin degradation, demonstrating that Nur77-mediated β-catenin degradation was independent of its DNA binding and transactivation, and might occur in the cytoplasm. In addition, we reported our identification of two digitalis-like compounds (DLCs), H-9 and ATE-i2-b4, which potently induced Nur77 expression and β-catenin degradation in SW620 colon cancer cells expressing mutant APC protein in vitro and in animals. DLC-induced Nur77 protein was mainly found in the cytoplasm, and inhibition of Nur77 nuclear export by the CRM1-dependent nuclear export inhibitor leptomycin B or Jun N-terminal kinase inhibitor prevented the effect of DLC on inducing β-catenin degradation. Together, our results demonstrate that β-catenin can be degraded by cytoplasmic Nur77 through their interaction and identify H-9 and ATE-i2-b4 as potent activators of the Nur77-mediated pathway for β-catenin degradation.

Upregulation of glycogen synthase kinase 3β in human colorectal adenocarcinomas correlates with accumulation of CTNNB1

INTRODUCTION

Mutations of the adenomatous polyposis coli (APC) tumor suppressor gene or the CTNNB1 protooncogene have been implicated in the initiation of most human colorectal epithelial neoplasms. Glycogen synthase kinase 3β (GSK3B) serves a critical role in regulating their functions by phosphorylating both APC and CTNNB1 to facilitate CTNNB1 degradation. The current studies were performed to investigate whether GSK3B itself is regulated during the process of colorectal tumorigenesis.

PATIENTS AND METHODS

We examined the expression of GSK3B and CTNNB1 in tissue samples from 24 human colorectal adenocarcinomas by Western immunoblotting analysis, kinase activity assays and immunohistochemistry. Normal colonic mucosa from the same colectomy specimens were used as a reference for comparison.

RESULTS

We demonstrated that GSK3B expression levels and kinase activities were markedly and significantly increased in colorectal adenocarcinomas in all 24 cases compared with paired adjacent normal-appearing colonic mucosa. These increases correlated with significantly increased expression of CTNNB1 in the same tumors. Similar results were obtained in several cultured human colon cancer cell lines, demonstrating GSK3B levels correlated with CTNNB1 expression.

CONCLUSION

Though APC and CTNNB1 regulation by GSK3B are frequently disrupted by mutations in colon cancers, our observations suggest that increased functional GSK3B might drive other growth-promoting signals in colorectal tumorigenesis.

BCL9-2 promotes early stages of intestinal tumor progression

BACKGROUND & AIMS

The roles of the 2 BCL9 and 2 Pygopus genes in Wnt to β-catenin signaling are not clear in vertebrates. We examined their expression and function in normal and tumor intestinal epithelia in mice and humans.

METHODS

Specific antibodies were generated to characterize the BCL9 and Pygopus proteins in normal intestine and in colon tumors. Targets of BCL9 and Pygopus in colon cancer cells were analyzed using small interfering RNA analysis. Transgenic mice were created that overexpressed BCL9-2 in intestine; these were crossed with APCMin/+ mice to create BCL9-2;APCMin/+ mice.

RESULTS

BCL9 and Pygopus2 were expressed in all normal intestinal and colon cancer cells. BCL9-2 was detectable only in the villi, not in the crypts of normal intestine. BCL9-2 was up-regulated in adenomas and in almost all colon tumors, with a concomitant increase of Pygopus2, whereas levels of BCL9 were similar between normal and cancer cells. Transgenic overexpression of BCL9-2 in the intestine of BCL9-2; APCMin/+ mice increased formation of adenomas that progressed to invasive tumors, resulting in reduced survival time. Using small interfering RNA analysis, we found that BCL9s and Pygopus are not targets of Wnt in colon cancer cells, but Wnt signaling correlated with levels of BCL9-2. BCL9-2 regulated expression of β-catenin-dependent and -independent target genes that have been associated with early stages of intestinal tumorigenesis.

CONCLUSIONS

BCL9-2 promotes early phases of intestinal tumor progression in humans and in transgenic mice. BCL9-2 increases the expression of a subset of canonical Wnt target genes but also regulates genes that are required for early stages of tumor progression.

PTEN regulates colorectal epithelial apoptosis through Cdc42 signalling

Background:

Phosphatase and tensin homologue deleted on chromosome 10 (PTEN) regulation of the Rho-like GTPase Cdc42 has a central role in epithelial polarised growth, but effects of this molecular network on apoptosis remain unclear.

Methods:

To investigate the role of Cdc42 in PTEN-dependent cell death, we used flow cytometry, in vitro pull-down assays, poly(ADP ribose) polymerase (PARP) cleavage and other immunoblots in isogenic PTEN-expressing and -deficient colorectal cells (HCT116PTEN^+/+, HCT116PTEN^−/−, Caco2 and Caco2 ShPTEN cells) after transfection or treatment strategies.

Results:

The PTEN knockout or suppression by short hairpin RNA or small interfering RNA (siRNA) inhibited Cdc42 activity, PARP cleavage and/or apoptosis in flow cytometry assays. Transfection of cells with wild-type or constitutively active Cdc42 enhanced PARP cleavage, whereas siRNA silencing of Cdc42 inhibited PARP cleavage and/or apoptosis. Pharmacological upregulation of PTEN by sodium butyrate (NaBt) treatment enhanced Cdc42 activity, PARP cleavage and apoptosis, whereas Cdc42 siRNA suppressed NaBt-induced PARP cleavage. Cdc42-dependent signals can suppress glycogen synthase kinase-β (GSK3β) activity. Pharmacological inhibition of GSK3β by lithium chloride treatment mimicked effects of Cdc42 in promotion of PARP cleavage and/or apoptosis.

Conclusion:

Phosphatase and tensin homologue deleted on chromosome 10 may influence apoptosis in colorectal epithelium through Cdc42 signalling, thus providing a regulatory framework for both polarised growth and programmed cell death.

A Rac1/PAK1 cascade controls β-catenin activation in colon cancer cells

P21-activated kinase 1 (PAK1) is associated with colon cancer progression and metastasis, whereas the molecular mechanism remains elusive. Here, we show that downregulation of PAK1 in colon cancer cells reduces total β-catenin level, as well as cell proliferation. Mechanistically, PAK1 directly phosphorylates β-catenin proteins at Ser675 site and this leads to more stable and transcriptional active β-catenin. Corroborating these results, PAK1 is required for full Wnt signaling, and superactivation of β-catenin is achieved by simultaneous knockdown of adenomatous polyposis coli protein and activation of PAK1. Moreover, we show that Rac1 functions upstream of PAK1 in colon cancer cells and contributes to β-catenin phosphorylation and accumulation. We conclude that a Rac1/PAK1 cascade controls β-catenin S675 phosphorylation and full activation in colon cancer cells. Supporting this conclusion, overexpression of PAK1 is observed in 70% of colon cancer samples and is correlated with massive β-catenin accumulation.

Ubiquitin ligase RNF146 regulates tankyrase and Axin to promote Wnt signaling

Canonical Wnt signaling is controlled intracellularly by the level of β-catenin protein, which is dependent on Axin scaffolding of a complex that phosphorylates β-catenin to target it for ubiquitylation and proteasomal degradation. This function of Axin is counteracted through relocalization of Axin protein to the Wnt receptor complex to allow for ligand-activated Wnt signaling. AXIN1 and AXIN2 protein levels are regulated by tankyrase-mediated poly(ADP-ribosyl)ation (PARsylation), which destabilizes Axin and promotes signaling. Mechanistically, how tankyrase limits Axin protein accumulation, and how tankyrase levels and activity are regulated for this function, are currently under investigation. By RNAi screening, we identified the RNF146 RING-type ubiquitin E3 ligase as a positive regulator of Wnt signaling that operates with tankyrase to maintain low steady-state levels of Axin proteins. RNF146 also destabilizes tankyrases TNKS1 and TNKS2 proteins and, in a reciprocal relationship, tankyrase activity reduces RNF146 protein levels. We show that RNF146, tankyrase, and Axin form a protein complex, and that RNF146 mediates ubiquitylation of all three proteins to target them for proteasomal degradation. RNF146 is a cytoplasmic protein that also prevents tankyrase protein aggregation at a centrosomal location. Tankyrase auto-PARsylation and PARsylation of Axin is known to lead to proteasome-mediated degradation of these proteins, and we demonstrate that, through ubiquitylation, RNF146 mediates this process to regulate Wnt signaling.

Intervening in β-catenin signaling by sulindac inhibits S100A4-dependent colon cancer metastasis

Colon cancer metastasis is often associated with activation of the Wnt/β-catenin signaling pathway and high expression of the metastasis mediator S100A4. We previously demonstrated the transcriptional regulation of S100A4 by β-catenin and the importance of the interconnection of these cellular programs for metastasis. Here we probe the hypothesis that the nonsteroidal anti-inflammatory drug sulindac sulfide can inhibit colon cancer metastasis by intervening in β-catenin signaling and thereby interdicting S100A4. We treated colon cancer cell lines heterozygous for gain-of-function and wild-type β-catenin with sulindac. We analyzed sulindac's effects on β-catenin expression and subcellular localization, β-catenin binding to the T-cell factor (TCF)/S100A4 promoter complex, S100A4 promoter activity, S100A4 expression, cell motility, and proliferation. Mice intrasplenically transplanted with S100A4-overexpressing colon cancer cells were treated with sulindac. Tumor growth and metastasis, and their β-catenin and S100A4 expressions, were determined. We report the expression knockdown of β-catenin by sulindac, leading to its reduced nuclear accumulation. The binding of β-catenin to TCF was clearly lowered, resulting in reduced S100A4 promoter activity and expression. This correlated well with the inhibition of cell migration and invasion, which could be rescued by ectopic S100A4 expression. In mice, sulindac treatment resulted in reduced tumor growth in the spleen (P = .014) and decreased liver metastasis in a human colon cancer xenograft model (P = .025). Splenic tumors and liver metastases of sulindac-treated mice showed lowered β-catenin and S100A4 levels. These results suggest that modulators of β-catenin signaling such as sulindac offer potential as antimetastatic agents by interdicting S100A4 expression.

Reduced tumour progression and angiogenesis in 1,2-dimethylhydrazine mice treated with NS-398 is associated with down-regulation of cyclooxygenase-2 and decreased beta-catenin nuclear localisation

Cyclooxygenase (COX)-2 is a key molecular target of colon cancer prevention. However, the mechanisms by which COX-2 inhibitors confer protective effects against tumour development are not completely understood. The aim of this study was to elucidate the effects of NS-398 in the 1,2-dimethylhydrazine (DMH) mouse model with respect to alteration in the expression of COX-2 and E-cadherin-catenin complex. Alterations in cell proliferation, apoptosis, and vascular density were investigated. NS-398 showed reduced COX-2 immunoreactivity in adenomas with a decrease in vascular density in non-dysplastic mucosa. Adenomas revealed increased E-cadherin and beta-catenin reactivity. NS-398 reduced the percentages of tumour cells with nuclear localisation of beta-catenin and cyclin D1. Bromodeoxyuridine (BrdUrd) index in adenomas was significantly higher in untreated animals. NS-398 resulted in significant increase in apoptosis in adenomas. Our results suggest a protective role of NS-398 on tumour development associated with reduced COX-2 expression, reduced vascular density and perturbation of beta-catenin signalling pathway.

Protein nitration and nitrosylation by NO-donating aspirin in colon cancer cells: Relevance to its mechanism of action

Nitric oxide-donating aspirin (NO-ASA) is a promising agent for cancer prevention. Although studied extensively, its molecular targets and mechanism of action are still unclear. S-nitrosylation of signaling proteins is emerging as an important regulatory mechanism by NO. Here, we examined whether S-nitrosylation of the NF-κB, p53, and Wnt signaling proteins by NO-ASA might explain, in part, its mechanism of action in colon cancer. NO-ASA releases significant amounts of NO detected intracellularly in HCT116 and HT-29 colon cells. Using a modified biotin switch assay we demonstrated that NO-ASA S-nitrosylates the signaling proteins p53, β-catenin, and NF-κB, in colon cancer cells in a time- and concentration-dependent manner. NO-ASA suppresses NF-κB binding to its cognate DNA oligonucleotide, which occurs without changes in the nuclear levels of the NF-κB subunits p65 and p50 and is reversed by dithiothreitol that reduces -S-NO to -SH. In addition to S-nitrosylation, we documented both in vitro and in vivo widespread nitration of tyrosine residues of cellular proteins in response to NO-ASA. Our results suggest that the increased intracellular NO levels following treatment with NO-ASA modulate cell signaling by chemically modifying key protein members of signaling cascades. We speculate that S-nitrosylation and tyrosine nitration are responsible, at least in part, for the inhibitory growth effect of NO-ASA on cancer cell growth and that this may represent a general mechanism of action of NO-releasing agents.

Aberrant β-catenin expression in the histologic differentiation of oral squamous cell carcinoma and verrucous carcinoma: an immunohistochemical study

β-Catenin acts as a structural protein at cell-cell adherens junctions and as a transcription activator mediating Wnt signal transduction. Altered β-catenin expression has been associated with loss of cell differentiation and acquisition of an invasive phenotype. In the present study, β-catenin expression was compared immunohistochemically between oral squamous cell carcinoma (30 cases) and verrucous carcinoma (30 cases), and correlated with different histological grades of oral squamous cell carcinoma. Positivity for β-catenin was seen in 17 cases (56.6%) of oral squamous cell carcinoma and 25 cases (83.3%) of verrucous carcinoma, and was significantly correlated with the grade of oral squamous cell carcinoma, whereas no significant correlation of β-catenin expression was observed between oral squamous cell carcinoma and verrucous carcinoma. In oral squamous cell carcinoma, the number of β-catenin-positive cases and the intensity of expression decreased as cancers became more poorly differentiated. Decreased membranous localization and intense cytoplasmic staining were observed in poorly differentiated squamous cell carcinoma. In verrucous carcinoma, β-catenin was demonstrable mainly in the membrane. Down-regulation of β-catenin was significantly correlated with lack of differentiation in oral squamous cell carcinoma. Reduced membranous expression and predominant cytoplasmic localization were prominent among higher-grade tumors, suggesting stabilization of β-catenin and its role as a signaling molecule. Predominant membranous expression in verrucous carcinoma was similar to that observed in well differentiated squamous cell carcinoma, thus corroborating its role in cell adhesion in these subgroups.

Chromosomal instability confers intrinsic multidrug resistance

Aneuploidy is associated with poor prognosis in solid tumors. Spontaneous chromosome missegregation events in aneuploid cells promote chromosomal instability (CIN) that may contribute to the acquisition of multidrug resistance in vitro and heighten risk for tumor relapse in animal models. Identification of distinct therapeutic agents that target tumor karyotypic complexity has important clinical implications. To identify distinct therapeutic approaches to specifically limit the growth of CIN tumors, we focused on a panel of colorectal cancer (CRC) cell lines, previously classified as either chromosomally unstable (CIN(+)) or diploid/near-diploid (CIN(-)), and treated them individually with a library of kinase inhibitors targeting components of signal transduction, cell cycle, and transmembrane receptor signaling pathways. CIN(+) cell lines displayed significant intrinsic multidrug resistance compared with CIN(-) cancer cell lines, and this seemed to be independent of somatic mutation status and proliferation rate. Confirming the association of CIN rather than ploidy status with multidrug resistance, tetraploid isogenic cells that had arisen from diploid cell lines displayed lower drug sensitivity than their diploid parental cells only with increasing chromosomal heterogeneity and isogenic cell line models of CIN(+) displayed multidrug resistance relative to their CIN(-) parental cancer cell line derivatives. In a meta-analysis of CRC outcome following cytotoxic treatment, CIN(+) predicted worse progression-free or disease-free survival relative to patients with CIN(-) disease. Our results suggest that stratifying tumor responses according to CIN status should be considered within the context of clinical trials to minimize the confounding effects of tumor CIN status on drug sensitivity.

A novel conditionally replicating "armed" adenovirus selectively targeting gastrointestinal tumors with aberrant wnt signaling

Using conditionally replicating adenoviral vectors (CRAds) is a promising strategy in the treatment of solid tumors. The prospective of this study was to design a novel CRAd for the treatment of gastrointestinal cancer and show its efficacy in vitro, as well as in vivo. To determine if aberrant wnt signaling in tumor cells can be used to selectively drive viral replication, we analyzed six colorectal and hepatocellular cell lines, as well as 13 colorectal tumors and 17 gastric tumors, for β-catenin mutation status or aberrant wnt signaling, both of which were found frequently. Based on these findings, a novel CRAd (Ad5F11.wnt-E1A-hIL24) containing an E1A expression cassette driven by an artificial wnt promoter and delivering an apoptosis-inducing gene, interleukin-24 (IL24), was engineered. To enhance infection efficiency, the virus was pseudotyped by replacing adenovirus serotype 5 (Ad5) with Ad11 fiber. Ad5F11.wnt-E1A-hIL24 virus exhibited high selectivity toward cells with aberrant wnt signaling both in vitro and in mouse xenograft tumors. Transduction efficiency was significantly improved compared with that of nonpseudotyped control viruses. The proliferation of tumor cell lines, as well as tumor growth, in mouse xenografts could be profoundly inhibited by viral infection with Ad5F11.wnt-E1A-hIL24. The therapeutic effect was associated with increased apoptosis through caspase-3 activation. In addition, Ad5F11b vector exhibited a more favorable biodistribution, blood clearance, and transgene expression compared with conventional Ad5 vector after systemic or intratumoral injection in human gastrointestinal cancer xenografts. We think that our approach is a promising strategy in the treatment of gastrointestinal cancer, warranting further clinical investigation.

A diterpenoid derivative 15-oxospiramilactone inhibits Wnt/β-catenin signaling and colon cancer cell tumorigenesis

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.

Defining the cancer master switch

BACKGROUND

Recent research has focused on signaling cascades and their interactions yielding considerable insight into which genetic pathways are targeted and how they tend to be altered in tumors. Therapeutic interventions now can be designed based on the knowledge of pathways vital to tumor growth and survival. These critical targets for intervention, master switches for cancer, are termed so because the tumor attempts to "flip the switch" in a way that promotes its survival, whereas molecular therapy aims to "switch off" signals important for tumor-related processes.

METHODS

Literature review.

CONCLUSIONS

Defining useful targets for therapy depends on identifying pathways that are crucial for tumor growth, survival, and metastasis. Because not all signaling cascades are created equal, selecting master switches or targets for intervention needs to be done in a systematic fashion. This discussion proposes a set of criteria to define what it means to be a cancer master switch and provides examples to illustrate their application.

Protein kinase A-mediated 14-3-3 association impedes human Dapper1 to promote dishevelled degradation

Wnt signaling regulates embryo development and tissue homeostasis, and its deregulation leads to an array of diseases, including cancer. Dapper1 has been shown to be a key negative regulator of Wnt signaling. However, its function and regulation remain poorly understood. In this study, we report that 14-3-3β interacts with human Dapper1 (hDpr1). The interaction is dependent on protein kinase A (PKA)-mediated phosphorylation of hDpr1 at Ser-237 and Ser-827. 14-3-3β binding attenuates the ability of hDpr1 to promote Dishevelled (Dvl) degradation, thus enhancing Wnt signaling. We further provide evidence that PKA-mediated Dpr1 phosphorylation may contribute to growth and tumor formation of colon cancer Caco2 cells. Finally, we show that cyclooxygenase-2 expression and PKA activation are positively correlated with Dvl protein levels in colon cancer samples. Together, our findings establish a novel layer of regulation of Wnt signaling by PKA via the 14-3-3-Dpr1-Dvl axis.

Silibinin suppresses growth of human colorectal carcinoma SW480 cells in culture and xenograft through down-regulation of beta-catenin-dependent signaling

Mutations in APC/beta-catenin resulting in an aberrant activation of Wnt/beta-catenin pathway are common in colorectal cancer (CRC), suggesting that targeting the beta-catenin pathway with chemopreventive/anticancer agents could be a potential translational approach to control CRC. Using human CRC cell lines harboring mutant (SW480) versus wildtype (HCT116) APC gene and alteration in beta-catenin pathway, herein we performed both in vitro and in vivo studies to examine for the first time whether silibinin targets beta-catenin pathway in its efficacy against CRC. Silibinin treatment inhibited cell growth, induced cell death, and decreased nuclear and cytoplasmic levels of beta-catenin in SW480 but not in HCT116 cells, suggesting its selective effect on the beta-catenin pathway and associated biologic responses. Other studies, therefore, were performed only in SW480 cells where silibinin significantly decreased beta-catenin-dependent T-cell factor-4 (TCF-4) transcriptional activity and protein expression of beta-catenin target genes such as c-Myc and cyclin D1. Silibinin also decreased cyclin-dependent kinase 8 (CDK8), a CRC oncoprotein that positively regulates beta-catenin activity, and cyclin C expression. In a SW480 tumor xenograft study, 100- and 200-mg/kg doses of silibinin feeding for 6 weeks inhibited tumor growth by 26% to 46% (P < .001). Analyses of xenografts showed that similar to cell culture findings, silibinin decreases proliferation and expression of beta-catenin, cyclin D1, c-Myc, and CDK8 but induces apoptosis in vivo. Together, these findings suggest that silibinin inhibits the growth of SW480 tumors carrying the mutant APC gene by down-regulating CDK8 and beta-catenin signaling and, therefore, could be an effective agent against CRC.

Positive feedback regulation between phospholipase D and Wnt signaling promotes Wnt-driven anchorage-independent growth of colorectal cancer cells

BACKGROUND

Aberrant activation of the canonical Wnt/beta-catenin pathway occurs in almost all colorectal cancers and contributes to their growth, invasion and survival. Phopholipase D (PLD) has been implicated in progression of colorectal carcinoma However, an understanding of the targets and regulation of this important pathway remains incomplete and besides, relationship between Wnt signaling and PLD is not known.

METHODOLOGY/PRINCIPAL FINDINGS

Here, we demonstrate that PLD isozymes, PLD1 and PLD2 are direct targets and positive feedback regulators of the Wnt/beta-catenin signaling. Wnt3a and Wnt mimetics significantly enhanced the expression of PLDs at a transcriptional level in HCT116 colorectal cancer cells, whereas silencing of beta-catenin gene expression or utilization of a dominant negative form of T cell factor-4 (TCF-4) inhibited expression of PLDs. Moreover, both PLD1 and PLD2 were highly induced in colon, liver and stomach tissues of mice after injection of LiCl, a Wnt mimetic. Wnt3a stimulated formation of the beta-catenin/TCF complexes to two functional TCF-4-binding elements within the PLD2 promoter as assessed by chromatin immunoprecipitation assay. Suppressing PLD using gene silencing or selective inhibitor blocked the ability of beta-catenin to transcriptionally activate PLD and other Wnt target genes by preventing formation of the beta-catenin/TCF-4 complex, whereas tactics to elevate intracellular levels of phosphatidic acid, the product of PLD activity, enhanced these effects. Here we show that PLD is necessary for Wnt3a-driven invasion and anchorage-independent growth of colon cancer cells.

CONCLUSION/SIGNIFICANCE

PLD isozyme acts as a novel transcriptional target and positive feedback regulator of Wnt signaling, and then promotes Wnt-driven anchorage-independent growth of colorectal cancer cells. We propose that therapeutic interventions targeting PLD may confer a clinical benefit in Wnt/beta-catenin-driven malignancies.

Inhibition of cell growth and potentiation of tumor necrosis factor-α (TNF-α)-induced apoptosis by a phenanthroindolizidine alkaloid antofine in human colon cancer cells

Based on the potential of natural products as a source for the development of cancer chemotherapeutic agents, this study was performed to investigate the anti-proliferative and antitumor effects of antofine, a phenanthroindolizidine alkaloid derived from Cynanchum paniculatum. Antofine showed potent anti-proliferative effects in several human cancer cells with IC(50) values in the nanomolar range. Treatment with antofine for 24h did not result in the induction of apoptotic cell death but moderately induced cell cycle arrest at G0/G1 phase and inhibited the expression of cyclin D1, cyclin E, and CDK4. In addition, antofine inhibited the transcriptional activity of β-catenin/Tcf in human colon HCT 116 cells, and the expression level of β-catenin and cyclin D1 was also down-regulated by antofine in human colon SW480 cells. Moreover, antofine potentiated tumor necrosis factor-α (TNF-α)-induced apoptosis, which was demonstrated by the increase of Annexin V-positive cell population and of the cleavage of poly (ADP-ribose) polymerase (PARP) and caspase-8. Antofine also effectively suppressed tumor growth in the HCT 116 implanted xenograft nude mouse model. Taken together, these findings suggest that antofine might be a potential candidate for the development of cancer chemotherapeutic agents derived from natural products.

Phospholipase D1 drives a positive feedback loop to reinforce the Wnt/beta-catenin/TCF signaling axis

Activation of the Wnt signaling pathway occurs frequently in human cancers, but an understanding of the targets and regulation of this important pathway remains incomplete. In this study, we report that phospholipase D (PLD), a cell survival mediator that is upregulated in cancer, is an important target of the Wnt signaling pathway that functions in a positive feedback loop to reinforce pathway output. PLD1 expression and activity was enhanced by treatment with Wnt3a and glycogen synthase kinase-3 inhibitors, and the Wnt pathway-regulated transcription factors beta-catenin and TCF-4 were required for this effect. Three functional TCF-4-binding sites were identified within the PLD1 promoter. Interestingly, suppressing PLD1 blocked the ability of beta-catenin to transcriptionally activate PLD1 and other Wnt target genes by preventing beta-catenin/TCF-4 complex formation. Conversely, tactics to elevate intracellular levels of phosphatidic acid, the product of PLD1 enzyme activity, enhanced beta-catenin/TCF-4 complex formation as well as beta-catenin-dependent TCF transcriptional activity. In cell-based assays, PLD1 was necessary for the anchorage-independent growth driven by Wnt/beta-catenin signaling, whereas beta-catenin/TCF-4 was necessary for the anchorage-independent growth driven by PLD1 activation. Taken together, our findings define a function for PLD1 in a positive feedback loop of Wnt/beta-catenin/TCF-4 signaling that provides new mechanistic insights into cancer, with implications of novel strategies to disrupt Wnt signaling in cancer.

Striking the target in Wnt-y conditions: intervening in Wnt signaling during cancer progression

Wnt signaling can be divided into three pathways, namely the canonical Wnt/beta-catenin pathway, and the non-canonical (or heretical) Wnt/Ca(2+) and planar cell polarity (PCP) pathways. Although the canonical Wnt/beta-catenin pathway is the best described in cancer, increasing data points to the importance of the heretical Wnt pathways in several aspects of tumor progression. The recent advances in understanding the players and mechanisms by which these Wnt pathways contribute to cancer progression have led to the identification of numerous molecules that are already, or could be considered, targets for cancer therapy.

Insulin alters the expression of components of the Wnt signaling pathway including TCF-4 in the intestinal cells

BACKGROUND

Epidemiological and experimental evidence that support the correlation between Type 2 diabetes mellitus (T2D) and increased risks of colorectal cancer formation have led us to hypothesize the existence of molecular crosstalk between insulin and canonical Wnt signaling pathways. Insulin was shown to stimulate Wnt target gene expression, utilizing the effector of the Wnt signaling pathway. Whether insulin affects expression of components of Wnt pathway has not been extensively examined.

METHODS

cDNA microarray was utilized to assess the effect of insulin on gene expression profile in the rat intestinal non-cancer IEC-6 cell line, followed by real-time RT-PCR, Western blotting and reporter gene analyses in intestinal cancer and non-cancer cells.

RESULTS

Insulin was shown to alter the expression of a dozen of Wnt pathway related genes including TCF-4 (=TCF7L2) and frizzled- (Fzd-4). The stimulatory effect of insulin on TCF-4 expression was then confirmed by real-time RT-PCR, Western blotting and luciferase reporter analyses, while the activation on Fzd-4 was confirmed by real-time PCR.

GENERAL SIGNIFICANCE

Our observations suggest that insulin may crosstalk with the Wnt signaling pathway in a multi-level fashion, involving insulin regulation of the expression of Wnt target genes, a Wnt receptor, as well as mediators of the Wnt signaling pathway.

Osteopontin provides early proliferative drive and may be dependent upon aberrant c-myc signalling in murine intestinal tumours

Osteopontin is thought to play an important role in tumour metastasis. In a previous expression profiling study of murine intestinal adenomas, we found that Opn was up-regulated. We also found beta-catenin binding motifs in the Opn promoter implying that, contrary to current beliefs, induction of Opn may occur during early tumourigenesis. We studied 59 murine intestinal adenomas for Opn expression and every tumour showed up-regulation compared to normal mucosa confirming early deregulation in these tumours. To determine whether Opn makes a functional contribution to tumourigenesis, Opn was knocked down in the murine colorectal cancer cell line CMT93. Inhibition of Opn expression resulted in decreased cell numbers. To determine the mechanism of Opn induction in these tumours, the Opn promoter was cloned and each of the putative beta-catenin binding motifs was mutated. No major change in Opn promoter activity was observed thereby excluding Opn as a direct beta-catenin target gene. However, mutation of one of two putative c-myc binding sites in the Opn promoter led to near complete loss of promoter activity whilst mutation of one of four PEA3 binding sites led to a 50% reduction in promoter activity. We conclude that Opn deregulation is an early event in intestinal tumourigenesis which may promote tumour development by altering either proliferation or apoptosis to increase tumour cell numbers. Opn expression in the intestine is dependent on c-myc binding sites in the promoter. Since c-myc is a known beta-catenin target gene, deregulation of Opn may be a secondary effect of aberrant Wnt signalling.

A beta-catenin/TCF-coordinated chromatin loop at MYC integrates 5' and 3' Wnt responsive enhancers

Aberrant MYC gene expression by the Wnt/beta-catenin pathway is implicated in colorectal carcinogenesis. Wnt/beta-catenin signaling stimulates association of the beta-catenin coactivator complex with two Wnt responsive enhancers (WREs) located in close proximity to MYC gene boundaries. Each enhancer directly binds members of the TCF/Lef family of transcription factors that, in turn, recruit beta-catenin. In a previous report, we showed that the downstream MYC enhancer (MYC 3' WRE) cooperated with the upstream enhancer (MYC 5' WRE) to activate expression of a heterologous reporter gene in response to Wnt/beta-catenin and mitogen signaling. Here we use chromatin conformation capture (3C) to show that the MYC 5' and 3' WREs are juxtaposed at the genomic MYC locus during active transcription. This MYC 5'3' chromatin loop is present in HCT116 human colorectal cancer cells that contain high levels of nuclear beta-catenin and is absent in HEK293 cells that contain trace amounts of nuclear beta-catenin. Depletion of functional beta-catenin/TCF complexes blocks formation of the MYC 5'3 chromatin loop. Furthermore, we find that the chromatin loop is absent in quiescent cells, but is rapidly and transiently induced by serum mitogens in a beta-catenin-dependent manner. Thus, we propose that a distinct chromatin architecture coordinated by beta-catenin/TCF-bound WREs accompanies transcriptional activation of MYC gene expression.

Frequent long-range epigenetic silencing of protocadherin gene clusters on chromosome 5q31 in Wilms' tumor

Wilms' tumour (WT) is a pediatric tumor of the kidney that arises via failure of the fetal developmental program. The absence of identifiable mutations in the majority of WTs suggests the frequent involvement of epigenetic aberrations in WT. We therefore conducted a genome-wide analysis of promoter hypermethylation in WTs and identified hypermethylation at chromosome 5q31 spanning 800 kilobases (kb) and more than 50 genes. The methylated genes all belong to α-, β-, and γ-protocadherin (PCDH) gene clusters (Human Genome Organization nomenclature PCDHA@, PCDHB@, and PCDHG@, respectively). This demonstrates that long-range epigenetic silencing (LRES) occurs in developmental tumors as well as in adult tumors. Bisulfite polymerase chain reaction analysis showed that PCDH hypermethylation is a frequent event found in all Wilms' tumor subtypes. Hypermethylation is concordant with reduced PCDH expression in tumors. WT precursor lesions showed no PCDH hypermethylation, suggesting that de novo PCDH hypermethylation occurs during malignant progression. Discrete boundaries of the PCDH domain are delimited by abrupt changes in histone modifications; unmethylated genes flanking the LRES are associated with permissive marks which are absent from methylated genes within the domain. Silenced genes are marked with non-permissive histone 3 lysine 9 dimethylation. Expression analysis of embryonic murine kidney and differentiating rat metanephric mesenchymal cells demonstrates that Pcdh expression is developmentally regulated and that Pcdhg@ genes are expressed in blastemal cells. Importantly, we show that PCDHs negatively regulate canonical Wnt signalling, as short-interfering RNA–induced reduction of PCDHG@ encoded proteins leads to elevated β-catenin protein, increased β-catenin/T-cell factor (TCF) reporter activity, and induction of Wnt target genes. Conversely, over-expression of PCDHs suppresses β-catenin/TCF-reporter activity and also inhibits colony formation and growth of cancer cells in soft agar. Thus PCDHs are candidate tumor suppressors that modulate regulatory pathways critical in development and disease, such as canonical Wnt signaling.

The development of tissues and organs in the human body requires carefully regulated production of proteins by cells. Proteins permit the growth and development of the many varied structures required for a healthy body. In many diseases, including some cancers, tissues and organs fail to develop as they should due to the normal production of proteins being changed. The work presented here shows that in Wilms' tumor, a childhood cancer of the kidney, a large group of related proteins that are likely necessary for growth and development of a normal kidney are not produced properly. This is due to their production being switched off within the cancer cells. We show how these proteins, known as protocadherins, can themselves alter the function of other proteins already known to be important in normal growth and cancer. Thus our study increases our understanding of how protocadherins are important in normal growth and of how altering protocadherins may lead to disease, such as cancer.

BRAF(V600E) efficient transformation and induction of microsatellite instability versus KRAS(G12V) induction of senescence markers in human colon cancer cells

In colorectal cancer, BRAF and KRAS oncogenes are mutated in about 15% and 35% respectively at approximately the same stage of the adenoma-carcinoma sequence. Since these two mutations rarely coexist, further analysis to dissect their function of transformation in colon cancer is required. Caco-2 human colon adenocarcinoma cells were stably transfected with BRAF(V600E) (Caco-BR cells) or KRAS(G12V) (Caco-K cells) oncogenes. BRAF(V600E) is more efficient in transforming Caco-2 cells and altering their morphology. The dominant nature of BRAF(V600E) is evident by its ability to render Caco-2 cells tumorigenic in vivo all be it through selective extracellular signal-related kinase (ERK) 2 phosphorylation and high levels of cyclin D1. As a consequence, the cell cycle distribution of parental cells is altered and microsatellite instability is introduced. Attenuated ERK activation observed correlated with KSR downregulation by BRAF(V600E) without further implications to signaling. Highly activated ERK in case of KRAS(G12V) (Caco-K cells) leads to mild transformation causing Caco-K cells to express premature senescence-related markers and acquire growth factor-dependent viability. Interestingly, BRAF(WT)gets equally activated by upstream KRAS mutations present in colon adenocarcinoma cells such as DLD-1 and SW620. Taken together, these results suggest that the two oncogenes have different transforming capability in colon cancer, although they both use the mitogen-activated protein (MAP) kinase pathway to carry out their effect. In general, BRAF(V600E) presents greater potential in mediating tumorigenic effect as compared to KRAS(G12V) both in vivo and in vitro. These findings may have implications in personalised diagnosis and targeted therapeutics.