Oligomeric assemblies of Amyloid-β (Aβ) are suggested to be central in the pathogenesis of Alzheimer’s disease, since levels of soluble Aβ much better correlate with the extent of cognitive dysfunctions than senile plaque counts do. Moreover, such Aβ species have been shown to be neurotoxic, to interfere with learned behavior and to inhibit maintenance of hippocampal long term potentiation. The tg-ArcSwe model, transgenic mice with the Arctic and Swedish Alzheimer mutations, expresses elevated levels of Aβ protofibrils in the brain, making tg-ArcSwe a highly suitable model to investigate the pathogenic role of these Aβ assemblies. In the present study, we estimated Aβ protofibril levels in the brain and cerebrospinal fluid of tg-ArcSwe mice, and also assessed their role with respect to cognitive functions. Protofibril levels, specifically measured with a sandwich ELISA, were found to be elevated in young tg-ArcSwe mice, as compared to several transgenic models lacking the Arctic mutation. In aged tg-ArcSwe mice with considerable plaque deposition, Aβ protofibrils were approximately 50 percent higher than in younger mice, whereas levels of total Aβ were exponentially increased. Young tg-ArcSwe mice showed deficits in spatial learning and individual performance in Morris water maze correlated inversely with levels of Aβ protofibrils, but not with total Aβ levels. We conclude that Aβ protofibrils accumulate in an age-dependent manner in tg-ArcSwe mice, although to a far less extent than total Aβ. Our findings suggest that increased levels of Aβ protofibrils could result in spatial learning impairment.
Alzheimer’s disease; amyloid-β protofibrils; Arctic mutation; transgenic mice; spatial learning
The Wnt/β-catenin signaling pathway controls cellular proliferation in the intestines. In response to Wnt, β-catenin transits into the nucleus and associates with members of the T-cell factor (TCF) family of transcription factors. β-Catenin/TCF complexes bind Wnt responsive DNA elements (WREs) to activate target gene expression. The c-MYC proto-oncogene (MYC) is a direct target of β-catenin/TCF complexes. We recently identified the MYC 3′ WRE, which maps 1.4-kb downstream from the MYC transcription stop site. To investigate the role of the Myc 3′ WRE in the intestines, we generated a mouse model with a germ line deletion of this element. The intestinal architecture was largely preserved in knockout mice; however, removal of the Myc 3′ WRE compromised the crypt microenvironment. In comparison to wild-type intestines, knockout intestines contained an increased number of proliferative cells and a reduced number of differentiated cells comprising both absorptive and secretory lineages. Using a model of colitis, we found that knockout colons repaired more rapidly during the recovery period of the protocol. These results indicate that regulation of MYC expression through the Myc 3′ WRE contributes to intestinal homeostasis. Furthermore, our study implicates MYC as an important regulator of intestinal regeneration following injury.
β-Catenin is an oncogenic protein involved in regulation of cell-cell adhesion and gene expression. Accumulation of cellular β-catenin occurs in many types of human cancers. Four mechanisms are known to cause increases in β-catenin: mutations of β-catenin, adenomatous polyposis coli, or axin genes and activation of Wnt signaling. We report a new cause of β-catenin accumulation involving oncogenic mutants of RON and MET receptor tyrosine kinases (RTKs). Cells transfected with oncogenic RON or MET were characterized by β-catenin tyrosine phosphorylation and accumulation; constitutive activation of a Tcf transcriptional factor; and increased levels of β-catenin/Tcf target oncogene proteins c-myc and cyclin D1. Interference with the β-catenin pathway reduced the transforming potential of mutated RON and MET. Activation of β-catenin by oncogenic RON and MET constitutes a new pathway, which might lead to cell transformation by these and other mutant growth factor RTKs.
Wnt3a stimulates cellular trafficking of key signaling elements (e.g., Axin, Dishevelled-2, β-catenin, and glycogen synthase kinase-3β) and primitive endoderm formation in mouse F9 embryonic teratocarcinoma cells.
The role of phosphoprotein phosphatase-2A in signaling of the Wnt/β-catenin/Lef-Tcf-sensitive gene activation pathway was investigated. Wnt3a action attenuates phosphoprotein phosphatase-2A activity and stimulates the Lef/Tcf-sensitive gene transcription. Inhibiting phosphoprotein phosphatase-2A by okadaic acid, by treatment with siRNA (targeting the C-subunit of the enzyme), or by expression of SV40 small t antigen mimics Wnt3a action, increasing the cellular abundance of Axin and phospho-glycogen synthase kinase-3β as well as the trafficking of signaling elements in the Wnt/β-catenin pathway. Although mimicking effects of Wnt3a on the cellular abundance and trafficking of key signaling elements in the Wnt canonical pathway, suppression of phosphatase-2A alone did not provoke activation of the Lef/Tcf-sensitive transcriptional response, but did potentiate its activation by Wnt3a. Phosphoprotein phosphatase-2A and the scaffold phosphoprotein Dishevelled-2 display similarities in cellular trafficking in response to either Wnt3a or suppression of the phosphatase. A docking site for phosphoprotein phosphatase-2A in the DEP domain of Dishevelled-2 was identified.
In current study, we showed new roles of phosphoprotein phosphatase-2A in Wnt/β-catenin signaling pathway: effect on protein expression, effect on protein trafficking, retention of molecules in subcellular compartments, and regulation of enzymatic activity of several key players. Docking of phosphoprotein phosphatase-2A by Dishevelled-2 suppresses phosphatase activity and explains in part the central role of this phosphatase in the counterregulation of the Wnt/β-catenin signaling pathway.
The Wnt pathway effector gene TCF7L2 has been linked to type II diabetes, making it important to study the role of Wnt signaling in diabetes pathogenesis. We examined the expression of multiple Wnt pathway components in pancreases from normal individuals and type II diabetic individuals. Multiple members of the Wnt signaling pathway, including TCF7L2, Wnt2b, β-catenin, pGSK3β, TCF3, cyclinD1, and c-myc, were undetectable or expressed at low levels in islets from nondiabetic individuals, but were also upregulated specifically in islets of type II diabetic patients. Culture of pancreatic tissue and islet isolation led to Wnt activation that was reversed by the Wnt antagonist sFRP, demonstrating that Wnt activation in that setting was due to soluble Wnt factors. These data support a model in which the Wnt pathway plays a dynamic role in the pathogenesis of type II diabetes and suggest manipulation of Wnt signaling as a new approach to β-cell-directed diabetes therapy.
TCF7L2 transcription factor is a downstream effector of the canonical Wnt/β-catenin signaling, which controls cell fate and homeostasis. However, the complexity of TCF7L2 expression with numerous mRNA isoforms coding for proteins with distinct N- and C-termini allows variability in TCF7L2 functions and regulations. Here, we show that although TCF7L2 mRNA isoforms distinguish fetal, immortalized and adult differentiated endothelial cells (EC), they cannot explain the lack of significant β-catenin/TCF7 activities in ECs. Lithium, a Wnt-signaling activator, increases TCF7L2 mRNA levels and induces an RNA isoform switch favoring the expression of TCF7L2-short forms lacking the C-termini domains. Although the latter occurs in different cell types, its extent depends on the overall increase of TCF7L2 transcription, which correlates with cell-responsiveness to Wnt/β-catenin signaling. While GSK3β down-regulation increases TCF7L2 expression, there is no concomitant change in TCF7L2 mRNA isoforms, which demonstrate the dual effects of lithium on TCF7L2 expression via a GSK3β-dependent up-regulation and a GSK3β-independent modulation of RNA-splicing. TCF7L2E-long forms display a repressor activity on TCF7L2-promoter reporters and lithium induces a decrease of the endogenous TCF7L2 forms bound to native TCF7L2-promoter chromatin at two novel distal TCF7-binding sites. Altogether our data reveal a lithium-induced RNA switch favoring the expression of TCF7L2-short forms, which results in a transcriptional de-repression of lithium-target genes negatively regulated by TCF7L2-long forms, like TCF7L2, and thus to an amplification of Wnt-signaling in responsive cells.
β-catenin; alternative-splicing; endothelial cells; immortalization
Inappropriate activation of c-Myc (MYC) gene expression by the Wnt/ß-catenin signaling pathway is required for colorectal carcinogenesis. The elevated MYC levels in colon cancer cells are attributed in part to ß-catenin/TCF4 transcription complexes that are assembled at proximal Wnt/ß-catenin responsive enhancers (WREs). Recent studies suggest that additional WREs that control MYC expression reside far upstream of the MYC transcription start site. Here, I report the characterization of five novel WREs that localize to a region over 400 kb upstream from MYC. These WREs harbor nucleosomes with post-translational histone modifications that demarcate enhancer and gene promoter regions. Using quantitative chromatin conformation capture, I show that the distal WREs are aligned with the MYC promoter through large chromatin loops. The chromatin loops are not restricted to colon cancer cells, but are also found in kidney epithelial and lung fibroblast cell lines that lack de-regulated Wnt signaling and nuclear ß-catenin/TCF4 complexes. While each chromatin loop is detected in quiescent cells, the positioning of three of the five distal enhancers with the MYC promoter is induced by serum mitogens. These findings suggest that the architecture of the MYC promoter is comprised of distal elements that are juxtaposed through large chromatin loops and that ß-catenin/TCF4 complexes utilize this conformation to activate MYC expression in colon cancer cells.
The wnt pathway regulates the steady state level of β-catenin, a transcriptional coactivator for the Tcf3/Lef1 family of DNA binding proteins. We demonstrate that Tcf3 can inhibit β-catenin turnover via its competition with axin and adenomatous polyposis for β-catenin binding. A mutant of β-catenin that cannot bind Tcf3 is degraded faster than the wild-type protein in Xenopus embryos and extracts. A fragment of β-catenin and a peptide encoding the NH2 terminus of Tcf4 that block the interaction between β-catenin and Tcf3 stimulate β-catenin degradation, indicating this interaction normally plays an important role in regulating β-catenin turnover. Tcf3 is a substrate for both glycogen synthase kinase (GSK) 3 and casein kinase (CK) 1ε, and phosphorylation of Tcf3 by CKIε stimulates its binding to β-catenin, an effect reversed by GSK3. Tcf3 synergizes with CK1ε to inhibit β-catenin degradation, whereas CKI-7, an inhibitor of CK1ε, reduces the inhibitory effect of Tcf3. Finally, we provide evidence that CK1ε stimulates the binding of dishevelled (dsh) to GSk3 binding protein (GBP) in extracts. Along with evidence that a significant amount of Tcf protein is nonnuclear, these findings suggest that CK1ε can modulate wnt signaling in vivo by regulating both the β-catenin-Tcf3 and the GBP-dsh interfaces.
β-catenin; Tcf; wnt; casein kinase; Xenopus
Inactivation of the gene encoding the adenomatous polyposis coli (APC) tumour suppressor protein is recognized as the key early event in the development of colorectal cancers (CRC). Apc loss leads to nuclear localization of beta-catenin and constitutive activity of the beta-catenin-Tcf4 transcription complex. This complex drives the expression of genes involved in cell cycle progression such as c-Myc and cyclin D2. Acute loss of Apc in the small intestine leads to hyperproliferation within the intestinal crypt, increased levels of apoptosis, and perturbed differentiation and migration. It has been demonstrated that c-Myc is a critical mediator of the phenotypic abnormalities that follow Apc loss in the intestine. As it may be difficult to pharmacologically inhibit transcription factors such as c-Myc, investigating more druggable targets of the Wnt-c-Myc pathway within the intestine may reveal potential therapeutic targets for CRC. Recent work in our laboratory has shown that the cyclin D2-cyclin-dependent kinase 4/6 (CDK4/6) complex promotes hyperproliferation in Apc deficient intestinal tissue and ApcMin/+ adenomas. We showed that the hyperproliferative phenotype associated with Apc loss in vivo was partially dependent on the expression of cyclin D2. Most importantly, tumour growth and development in ApcMin/+ mice was strongly perturbed in mice lacking cyclin D2. Furthermore, pharmacological inhibition of CDK4/6 suppressed the proliferation of adenomatous cells. This commentary discusses the significance of this work in providing evidence for the importance of the cyclin D2-CDK4/6 complex in colorectal adenoma formation. It also argues that inhibition of this complex may be an effective chemopreventative strategy in CRC.
Clusterin (CLU) is an enigmatic molecule associated with various physiological processes and disease states. Different modes of cellular stress lead to increased CLU levels, and additionally numerous growth factors and cytokines affect the expression of the CLU gene. APC and c-MYC, both intimately linked to the Wnt signaling pathway have previously been shown to influence CLU levels, and we therefore investigated if changes in Wnt signaling activity in vitro could regulate the expression of one, or more, of several CLU mRNA and protein variants.
Over-expression of the cytoplasmic domain of E-cadherin tagged with GFP was used to abrogate Wnt signaling activity in LS174T and HCT116 colon carcinoma cells. This fusion construct sequestered signaling competent β-catenin whereby Wnt signaling was abrogated, and consequently cytoplasmic CLU protein levels increased as demonstrated by immunofluorescence. To determine which branch of the Wnt pathway was mediating the CLU response, we over-expressed dominant negative (dn) TCF1 and TCF4 transcription factors in stably transfected LS174T cells. We observed both intra- and extracellular levels of CLU protein to be induced by dnTCF1 but not dnTCF4. Subsequent analysis of the expression levels of three CLU mRNA variants by real time RT-PCR revealed only one CLU mRNA variant to be responsive to dnTCF1 over-expression. 5'-end RACE indicated that this CLU mRNA variant was shorter at the 5'-end than previously reported, and accordingly the translated protein was predicted to be shorter at the N-terminus and destined to the secretory pathway which fit our observations. Examination of the immediate expression kinetics of CLU after dnTCF1 over-expression using real time RT-PCR indicated that CLU might be a secondary Wnt target.
In conclusion, we have demonstrated that the Wnt signaling pathway specifically regulates one out of three CLU mRNA variants via TCF1. This CLU transcript is shorter at the 5' end than reported by the RefSeq database, and produces the intracellular 60 kDa CLU protein isoform which is secreted as a ~80 kDa protein after post-translational processing.
The vitamin D receptor (VDR) pathway is important in the prevention and potentially in the treatment of many cancers. One important mechanism of VDR action is related to its interaction with the Wnt/β-catenin pathway. Agonist-bound VDR inhibits the oncogenic Wnt/β-catenin/TCF pathway by interacting directly with β-catenin and in some cells by increasing cadherin expression which, in turn, recruits β-catenin to the membrane. Here we identify TCF-4, a transcriptional regulator and β-catenin binding partner as an indirect target of the VDR pathway.
In this work, we show that TCF-4 (gene name TCF7L2) is decreased in the mammary gland of the VDR knockout mouse as compared to the wild-type mouse. Furthermore, we show 1,25(OH)2D3 increases TCF-4 at the RNA and protein levels in several human colorectal cancer cell lines, the effect of which is completely dependent on the VDR. In silico analysis of the human and mouse TCF7L2 promoters identified several putative VDR binding elements. Although TCF7L2 promoter reporters responded to exogenous VDR, and 1,25(OH)2D3, mutation analysis and chromatin immunoprecipitation assays, showed that the increase in TCF7L2 did not require recruitment of the VDR to the identified elements and indicates that the regulation by VDR is indirect. This is further confirmed by the requirement of de novo protein synthesis for this up-regulation.
Although it is generally assumed that binding of β-catenin to members of the TCF/LEF family is cancer-promoting, recent studies have indicated that TCF-4 functions instead as a transcriptional repressor that restricts breast and colorectal cancer cell growth. Consequently, we conclude that the 1,25(OH)2D3/VDR-mediated increase in TCF-4 may have a protective role in colon cancer as well as diabetes and Crohn's disease.
Self-renewal of rodent embryonic stem (ES) cells is enhanced by partial inhibition of glycogen synthase kinase-3 (Gsk3)1
2. This effect has variously been attributed to stimulation of Wnt signalling via β-catenin1, stabilisation of cMyc3, and global de-inhibition of anabolic processes4. Here we demonstrate that β-catenin is not necessary for ES cell identity or expansion, but its absence eliminates the self-renewal response to Gsk3 inhibition. Responsiveness is fully restored by truncated β-catenin lacking the C-terminal transactivation domain5. However, requirement for Gsk3 inhibition is dictated by expression of Tcf3 and mediated by direct interaction with β-catenin. Tcf3 localises to many pluripotency genes6 in ES cells. Our findings confirm that Tcf3 acts as a transcriptional repressor and reveal that β-catenin directly abrogates Tcf3 function. We conclude that Gsk3 inhibition stabilises the ES cell state primarily by reducing repressive influence on the core pluripotency network.
Aberrantly activated Wnt/β-catenin signaling is important in hepatocellular carcinoma (HCC) development. Downstream gene expressions involving the Wnt/β-catenin cascade occur through T-cell factor (TCF) proteins. Here, we show the oncogenic potential of human TCF-4 isoforms based on the expression of a single conserved SxxSS motif.
We investigated the TCF-4J and K isoform pair characterized by the presence (K) or absence (J) of the SxxSS motif. The mRNA expression profiles were examined in 47 pairs of human HCCs and adjacent non-cancerous liver tissues by RT-PCR. Proliferation, sphere assays and immunoblot analysis were performed under normoxia and hypoxia conditions. The ability of HCC cells overexpressing TCF-4J (J cells) and K (K cells) to grow as solid tumors in nude mice was explored.
TCF-4J expression was significantly upregulated in HCC tumors compared to corresponding peritumor and normal liver and was preferentially expressed in poorly differentiated HCCs. In contrast, TCF-4K was downregulated in those same HCC tumors. TCF-4J-overexpressing HCC cells (J cells) revealed a survival advantage under hypoxic conditions, high proliferation rate and formation of aggregates/spheres compared to overexpression of TCF-4K (K cells). The hypoxic J cells had high expression levels of HIF-2α and EGFR as possible mechanisms to promote tumorigenesis. Increased stability of HIF-2α under hypoxia in J cells was associated with a decreased level of von Hippel-Lindau (VHL) protein, a known E3 ligase for HIF-αs. In a xenograft model, the J cells rapidly developed tumors compared to K cells. Tumor tissues derived from J cells exhibited high expression levels of HIF-2α and EGFR compared to the slow developing and small K cell derived tumors.
Our results suggest that the specific TCF-4J isoform, which lacks a regulatory SxxSS motif, has robust tumor-initiating potential under hypoxic conditions.
Transcription factor 7-like 2 (TCF7L2) is a high mobility group -box containing protein that is a critical member of the Wnt/β-catenin signaling pathway. In addition to its recently recognized role in diabetes, aberrant TCF7L2 expression has been implicated in cancer through regulation of cell proliferation and apoptosis by c-MYC and cyclin D. It has been hypothesized that germline variants within the TCF7L2 gene previously associated with diabetes may affect cancer risk through the Wnt/β-catenin signaling pathway. Specifically, the same risk allele of single nucleotide polymorphism (SNP) rs12255372 that is associated with diabetes (T allele) has recently been associated with an increased risk of breast cancer.
Here, we investigated associations between rs12255372 and prostate cancer risk among 597 cases and 548 controls from a population-based study. We also evaluated prostate cancer progression/recurrence and prostate-specific mortality among these cases under long-term surveillance. The risk allele was associated with cancer progression/recurrence after primary treatment [hazard ratio (HR) =1.48, 95% confidence interval (CI) =1.01-2.19), but this association was attenuated when the model was further adjusted for Gleason score and tumor stage. There were no associations between this SNP and the risk of developing disease or prostate cancer-specific mortality. Our findings suggest that this variant in the TCF7L2 gene may enhance tumor progression or metastasis, but it does not contribute significantly to tumor initiation. These findings are of clinical importance for identification of patients that may develop more aggressive/recurrent disease after primary treatment.
Wnt signaling is a fundamental pathway in embryogenesis which is evolutionary conserved from metazoans to humans. Much of our understanding of Wnt signaling events emerged from key developmental studies in drosophila, zebra fish, xenopus, and mice. Considerable data now exists on the role of Wnt signaling beyond these developmental processes and in particular its role in health and disease. The focus of this special issue is on Wnt/β-catenin and its diverse physiological cell signaling pathways in neurodegenerative and neuropsychiatric disorders. This special issue is composed of six reviews and two original articles selected to highlight recent advances in the role of Wnt signaling in CNS embryonic development, in adult brain function, in neurodegenerative conditions such as Alzheimer’s disease, schizophrenia, NeuroAIDS, and in gliomas. The finding that β-catenin can translocate to the nucleus where it binds to TCF/LEF transcription factors to regulate target gene expression was a seminal observation that linked β-catenin/LEF to T cell development and differentiation. We also provide a nostalgic look on recent advances in role of Wnts in T cell development and maturation. These reviews highlight the extensive body of work in these thematic areas as well as identify knowledge gaps, where appropriate. Understanding Wnt function under healthy and diseased conditions may provide a therapeutic resource, albeit it a challenging one, in diseases where dysfunctional and/or diminished Wnt signaling is a prominent player in the disease process.
Wnt signaling; β-catenin; Neurodegenerative diseases; T cell differentation; NeuroAIDS
Tcf transcription factors interact with β-catenin and Armadillo to mediate Wnt/Wingless signaling. We now report the characterization of genes encoding two murine members of the Tcf family, mTcf-3 and mTcf-4. mTcf-3 mRNA is ubiquitously present in embryonic day 6.5 (E6.5) mouse embryos but gradually disappears over the next 3 to 4 days. mTcf-4 expression occurs first at E10.5 and is restricted to di- and mesencephalon and the intestinal epithelium during embryogenesis. The mTcf-3 and mTcf-4 proteins bind a canonical Tcf DNA motif and can complex with the transcriptional coactivator β-catenin. Overexpression of Wnt-1 in a mammary epithelial cell line leads to the formation of a nuclear complex between β-catenin and Tcf proteins and to Tcf reporter gene transcription. These data demonstrate a direct link between Wnt stimulation and β-catenin/Tcf transcriptional activation and imply a role for mTcf-3 and -4 in early Wnt-driven developmental decisions in the mouse embryo.
In the canonical Wnt pathway, signaling results in the stabilization and increased levels of β-catenin in responding cells. β-catenin then enters the nucleus, functioning as a coactivator for the Wnt effector, TCF/LEF protein. In the absence of Wnt signaling, TCF is complexed with corepressors, together repressing Wnt target genes. In C. elegans, Wnt signaling specifies the E blastomere to become the endoderm precursor. Activation of endoderm genes in E requires not only an increase in β-catenin level, but a concomitant decrease in the nuclear level of POP-1, the sole C. elegans TCF. A decrease in nuclear POP-1 levels requires Wnt-induced phosphorylation of POP-1 and 14-3-3 protein-mediated nuclear export. Nuclear POP-1 levels remain high in the sister cell of E, MS, where POP-1 represses the expression of endoderm genes. Here we express three vertebrate TCF proteins (human TCF4, mouse LEF1 and Xenopus TCF3) in C. elegans embryos and compare their localization, repression and activation functions to POP-1. All three TCFs are localized to the nucleus in C. elegans embryos, but none undergoes Wnt-induced nuclear export. Although unable to undergo Wnt-induced nuclear export, human TCF4, but not mouse LEF1 or Xenopus TCF3, can repress endoderm genes in MS, in a manner very similar to POP-1. This repressive activity requires that human TCF4 recognize specific promoter sequences upstream of endoderm genes and interact with C. elegans corepressors. Domain swapping identified two regions of POP-1 that are sufficient to confer nuclear asymmetry to human TCF4 when swapped with its corresponding domains. Despite undergoing Wnt-induced nuclear export, the human TCF4/POP-1 chimeric protein continues to function as a repressor for endoderm genes in E, a result we attribute to the inability of hTCF4 to bind to C. elegans β-catenin. Our results reveal a higher degree of species specificity among TCF proteins for coactivator interactions than for corepressor interactions, and uncover a basic difference between how POP-1 and human TCF4 steady state nuclear levels are regulated.
C. elegans; Wnt signaling; POP-1; vertebrate TCF; A-P asymmetry
High mobility group (HMG) transcription factors of the T-cell-specific transcription factor/lymphoid enhancer binding factor (TCF/LEF) family are a class of intrinsic regulators that are dynamically expressed in the embryonic mouse retina. Activation of TCF/LEFs is a hallmark of the Wnt/β-catenin pathway, however, the requirement for Wnt/β-catenin and noncanonical Wnt signaling during mammalian retinal development remains unclear. Our goal was to characterize more fully a TCF/LEF-responsive retinal progenitor population in the mouse embryo, and to correlate this with Wnt/β-catenin signaling.
TCF/LEF activation was analyzed in the TOPgal reporter mouse (TOP: TCF optimal promoter) at embryonic ages and compared to Axin2 mRNA expression, an endogenous readout of Wnt/β-catenin signaling. Reporter expression was also examined in embryos with a retina-specific deletion of the/β-catenin gene (Ctnnb1), using Six3-Cre transgenic mice. Finally, the extent to which TOPgal cells coexpress cell cycle proteins, basic helix-loop–helix (bHLH) transcription factors and other retinal cell type markers was tested by double-immunohistochemistry.
TOPgal reporter activation occurs transiently in a subpopulation of embryonic retinal progenitor cells. Axin2 is not expressed in the central retina, and TOPgal reporter expression persists in the absence of β-catenin. Although a proportion of TOPgal-labeled cells are proliferative, most coexpress the cyclin-dependent kinase inhibitor p27/Kip1.
TOPgal cells give rise to the four earliest cell types: ganglion cells, amacrines, horizontals and photoreceptors. TCF/LEF activation in the central retina does not correlate with Wnt/β-catenin signaling, pointing to an alternate role for this transcription factor family during retinal development.
A morphogenesis checkpoint in budding yeast delays nuclear division (and subsequent cell cycle progression) in cells that have failed to make a bud. We show that the ability of this checkpoint to delay nuclear division requires the SWE1 gene, encoding a protein kinase that inhibits the master cell cycle regulatory kinase Cdc28. The timing of nuclear division in cells that cannot make a bud is exquisitely sensitive to the dosage of SWE1 and MIH1 genes, which control phosphorylation of Cdc28 at tyrosine 19. In contrast, the timing of nuclear division in budded cells does not rely on Cdc28 phosphorylation, suggesting that the morphogenesis checkpoint somehow turns on this regulatory pathway. We show that SWE1 mRNA levels fluctuate during the cell cycle and are elevated in cells that cannot make a bud. However, regulation of SWE1 mRNA levels by the checkpoint is indirect, acting through a feedback loop requiring Swe1 activity. Further, the checkpoint is capable of delaying nuclear division even when SWE1 transcription is deregulated. We propose that the checkpoint delays nuclear division through post-translational regulation of Swe1 and that transcriptional feedback loops enhance the efficacy of the checkpoint.
A single nucleotide polymorphism (SNP) rs6983267, located within the 8q24 region, is strongly associated with risk of colorectal and prostate cancer. It has been suggested that the mechanism of this association is related to differential interaction of TCF7L2 protein (previously known as TCF-4) with alleles of rs6983267, influencing the expression of a well-known oncogene, MYC, located 335 Kb telomeric. Here, we tested the correlation between mRNA expression of MYC and several alternatively spliced forms of TCF7L2 in 117 non-cancer colon samples. We observed a strong correlation (r = 0.60, p < 10-6) between expression of MYC and a unique splicing form of TCF7L2. The level of MYC expression in these samples was associated with expression of some TCF7L2 splicing forms but not with genotypes of rs6983267, or interaction of rs6983267 with TCF7L2 expression. These findings suggest that some splicing forms of TCF7L2 may be functionally important for regulation of MYC expression in colon tissue but this regulation is not directly dependent on rs6983267.
The Wnt signaling pathway is deregulated in over 90% of human colorectal cancers. β-Catenin, the central signal transducer of the Wnt pathway, can directly modulate gene expression by interacting with transcription factors of the TCF/LEF family. In the present study we investigate the role of Wnt signaling in the homeostasis of intestinal epithelium by using tissue-specific, inducible β-catenin gene ablation in adult mice. Block of Wnt/β-catenin signaling resulted in rapid loss of transient-amplifying cells and crypt structures. Importantly, intestinal stem cells were induced to terminally differentiate upon deletion of β-catenin, resulting in a complete block of intestinal homeostasis and fatal loss of intestinal function. Transcriptional profiling of mutant crypt mRNA isolated by laser capture microdissection confirmed those observations and allowed us to identify genes potentially responsible for the functional preservation of intestinal stem cells. Our data demonstrate an essential requirement of Wnt/β-catenin signaling for the maintenance of the intestinal epithelium in the adult organism. This challenges attempts to target aberrant Wnt signaling as a new therapeutic strategy to treat colorectal cancer.
Wnt3a regulates a canonical signaling pathway in early development that controls the nuclear accumulation of β-catenin and its activation of Lef/Tcf-sensitive transcription of developmentally important genes.
Using totipotent mouse F9 teratocarcinoma cells expressing Frizzled-1 and biochemical analyses, we detail the influence of Wnt3a stimulation on the expression, complexation, and subcellular trafficking of key signaling elements of the canonical pathway, i.e., Dishevelled-2, Axin, glycogen synthase kinase-3β, and β-catenin. Cellular content of β-catenin and Axin, and phospho-glycogen synthase kinase-3β, but not Dishevelled-2, increases in response to Wnt3a. Subcellular localization of Axin in the absence of Wnt3a is symmetric, found evenly distributed among plasma membrane-, cytosol-, and nuclear-enriched fractions. Dishevelled-2, in contrast, is found predominately in the cytosol, whereas β-catenin is localized to the plasma membrane-enriched fraction. Wnt3a stimulates trafficking of Dishevelled-2, Axin, and glycogen synthase kinase-3β initially to the plasma membrane, later to the nucleus. Bioluminescence resonance energy transfer measurements reveal that complexes of Axin with Dishevelled-2, with glycogen synthase kinase-3β, and with β-catenin are demonstrable and they remain relatively stable in response to Wnt3a stimulation, although trafficking has occurred. Mammalian Dishevelled-1 and Dishevelled-2 display similar patterns of trafficking in response to Wnt3a, whereas that of Dishevelled-3 differs from the other two.
This study provides a detailed biochemical analysis of signaling elements key to Wnt3a regulation of the canonical pathway. We quantify, for the first time, the Wnt-dependent regulation of cellular abundance and intracellular trafficking of these signaling molecules. In contrast, we observe little effect of Wnt3a stimulation on the level of protein-protein interactions among these constituents of Axin-based complexes themselves.
The transcription factor 7-like 2 (TCF7L2) is a critical component of the Wnt/β-catenin pathway. Aberrant TCF7L2 expression modifies Wnt signaling and mediates oncogenic effects through the upregulation of c-MYC and cyclin D. Genetic alterations in TCF7L2 may therefore affect cancer risk. Recently, TCF7L2 variants, including the microsatellite marker DG10S478 and the nearly perfectly linked SNP rs12233372, were identified to associate with type 2 diabetes.
We investigated the effect of the TCF7L2 rs12255372 variant on familial breast cancer (BC) risk by means of TaqMan allelic discrimination, analyzing BRCA1/2 mutation-negative index patients of 592 German BC families and 735 control individuals.
The T allele of rs12255372 showed an association with borderline significance (OR = 1.19, 95% C.I. = 1.01-1.42, P = 0.04), and the Cochran-Armitage test for trend revealed an allele dose-dependent association of rs12255372 with BC risk (Ptrend = 0.04).
Our results suggest a possible influence of TCF7L2 rs12255372 on the risk of familial BC.
Invasion of human trophoblasts is promoted through activation of wingless (Wnt) signaling, suggesting a role of the pathway in placental development and morphogenesis. However, details on the process such as involvement of canonical and/or noncanonical Wnt signaling cascades as well as their target genes are largely unknown. Hence, signal transduction via canonical Wnt signaling or phosphatidylinositide 3-kinase (PI3K)/AKT and their cross talk as well as trophoblast-specific protease expression were investigated in trophoblastic SGHPL-5 cells and primary extravillous trophoblasts purified from first-trimester placentas. Western blot analyses revealed that the recombinant Wnt ligand Wnt-3A increased phosphorylation of AKT and the downstream kinase glycogen synthase kinase (GSK)-3β as well as accumulation of activated, nuclear β-catenin. In accordance, luciferase expression of a canonical Wnt/TCF reporter and cell migration in first-trimester villous explant cultures and of SGHPL-5 cells were stimulated. Chemical inhibition of PI3K abolished Wnt-dependent phosphorylation of AKT and GSK-3β and trophoblast motility but did not affect appearance of activated β-catenin or Wnt/TCF reporter activity. In contrast, inhibition of the canonical pathway through soluble Dickkopf-1 did not influence AKT and GSK-3β phosphorylation but reduced Wnt reporter activity, accumulation of active β-catenin, and cell migration. Both inhibitors decreased Wnt-3A-induced secretion of pro- and active matrix metalloproteinase-2 from SGHPL-5 cells and pure EVT. The data suggest that Wnt-3A may activate canonical Wnt signaling and PI3K/AKT through distinct receptors. The two signaling cascades act independently in trophoblasts; however, both pathways promote Wnt-dependent migration and the release of matrix metalloproteinase-2, which has been identified as novel Wnt target in invasive trophoblasts.
The Wnt signaling pathway plays a major role in development, and upon deregulation it is implicated in neoplasia. The hallmark of the canonical Wnt signal is the protection of β-catenin from ubiquitination and proteasomal degradation induced by glycogen synthase kinase (GSK)-3β inhibition. The stabilized β-catenin translocates to the nucleus where it binds to T-cell factor/lymphoid enhancer factor (TCF/LEF) transcription factors, activating the expression of Wnt target genes. In the absence of Wnt signal, TCF/LEF bind to Groucho (Gro)/TLE corepressors and repress Wnt target genes. Gro/TLE bind also to Engrailed (En) transcription factors mediating En-repressive activity on En target genes. Here, we present data suggesting that En-1 serves also as a negative regulator of β-catenin transcriptional activity; however, its repressive effect is independent of Gro/TLE. Our data suggest that En-1 acts by destabilizing β-catenin via a proteasomal degradation pathway that is GSK-3β–independent. Moreover, because En-1-mediated β-catenin degradation is also Siah independent, our data imply that En-1 exerts its repressive effect by a novel mechanism negatively controlling the level of β-catenin.