Analysis of beta-catenin aggregation and localization using GFP fusion proteins: nuclear import of alpha-catenin by the beta-catenin/Tcf complex

The Flavonol Quercitrin Hinders GSK3 Activity and Potentiates the Wnt/β-Catenin Signaling Pathway

The Wnt/β-catenin signaling pathway dictates cell proliferation and differentiation during embryonic development and tissue homeostasis. Its deregulation is associated with many pathological conditions, including neurodegenerative disease, frequently downregulated. The lack of efficient treatment for these diseases, including Alzheimer’s disease (AD), makes Wnt signaling an attractive target for therapies. Interestingly, novel Wnt signaling activating compounds are less frequently described than inhibitors, turning the quest for novel positive modulators even more appealing. In that sense, natural compounds are an outstanding source of potential drug leads. Here, we combine different experimental models, cell-based approaches, neuronal culture assays, and rodent behavior tests with Xenopus laevis phenotypic analysis to characterize quercitrin, a natural compound, as a novel Wnt signaling potentiator. We find that quercitrin potentiates the signaling in a concentration-dependent manner and increases the occurrence of the Xenopus secondary axis phenotype mediated by Xwnt8 injection. Using a GSK3 biosensor, we describe that quercitrin impairs GSK3 activity and increases phosphorylated GSK3β S9 levels. Treatment with XAV939, an inhibitor downstream of GSK3, impairs the quercitrin-mediated effect. Next, we show that quercitrin potentiates the Wnt3a-synaptogenic effect in hippocampal neurons in culture, which is blocked by XAV939. Quercitrin treatment also rescues the hippocampal synapse loss induced by intracerebroventricular injection of amyloid-β oligomers (AβO) in mice. Finally, quercitrin rescues AβO-mediated memory impairment, which is prevented by XAV939. Thus, our study uncovers a novel function for quercitrin as a Wnt/β-catenin signaling potentiator, describes its mechanism of action, and opens new avenues for AD treatments.

Allele-specific endogenous tagging and quantitative analysis of β-catenin in colorectal cancer cells

Wnt signaling plays important roles in development, homeostasis, and tumorigenesis. Mutations in β-catenin that activate Wnt signaling have been found in colorectal and hepatocellular carcinomas. However, the dynamics of wild-type and mutant forms of β-catenin are not fully understood. Here, we genome-engineered fluorescently tagged alleles of endogenous β-catenin in a colorectal cancer cell line. Wild-type and oncogenic mutant alleles were tagged with different fluorescent proteins, enabling the analysis of both variants in the same cell. We analyzed the properties of both β-catenin alleles using immunoprecipitation, immunofluorescence, and fluorescence correlation spectroscopy approaches, revealing distinctly different biophysical properties. In addition, activation of Wnt signaling by treatment with a GSK3β inhibitor or a truncating APC mutation modulated the wild-type allele to mimic the properties of the mutant β-catenin allele. The one-step tagging strategy demonstrates how genome engineering can be employed for the parallel functional analysis of different genetic variants.

Comparative stemness and differentiation of luminal and basal breast cancer stem cell type under glutamine-deprivation

Glutamine (gln) metabolism has emerged as a cancer therapeutic target in past few years, however, the effect of gln-deprivation of bCSCs remains elusive in breast cancer. In this study, effect of glutamine on stemness and differentiation potential of bCSCs isolated from MCF-7 and MDAMB-231 were studied. We have shown that bCSCs differentiate into CD24+ epithelial population under gln-deprivation and demonstrated increased expression of epithelial markers such as e-cadherin, claudin-1 and decreased expression of mesenchymal protein n-cadherin. MCF-7-bCSCs showed a decrease in EpCAMhigh population whereas MDAMB-231-bCSCs increased CD44high population in response to gln-deprivation. The expression of intracellular stem cell markers such sox-2, oct-4 and nanog showed a drastic decrease in gene expression under gln-deprived MDAMB-231-bCSCs. Finally, localization of β-catenin in MCF-7 and MDAMB-231 cells showed its accumulation in cytosol or perinuclear space reducing its efficiency to transcribe downstream genes. Conclusively, our study demonstrated that gln-deprivation induces differentiation of bCSCs into epithelial subtypes and also reduces stemness of bCSCs mediated by reduced nuclear localization of β-catenin. It also suggests that basal and luminal bCSCs respond differentially towards changes in extracellular and intracellular gln. This study could significantly affect the gln targeting regimen of breast cancer therapeutics.

Nanoscale architecture of cadherin-based cell adhesions

Multicellularity in animals requires dynamic maintenance of cell-cell contacts. Intercellularly ligated cadherins recruit numerous proteins to form supramolecular complexes that connect with the actin cytoskeleton and support force transmission. However, the molecular organization within such structures remains unknown. Here we mapped protein organization in cadherin-based adhesions by super-resolution microscopy, revealing a multi-compartment nanoscale architecture, with the plasma-membrane-proximal cadherin-catenin compartment segregated from the actin cytoskeletal compartment, bridged by an interface zone containing vinculin. Vinculin position is determined by α-catenin, and following activation, vinculin can extend ∼30 nm to bridge the cadherin-catenin and actin compartments, while modulating the nanoscale positions of the actin regulators zyxin and VASP. Vinculin conformational activation requires tension and tyrosine phosphorylation, regulated by Abl kinase and PTP1B phosphatase. Such modular architecture provides a structural framework for mechanical and biochemical signal integration by vinculin, which may differentially engage cadherin-catenin complexes with the actomyosin machinery to regulate cell adhesions.

Transactivation activity and nucleocytoplasmic transport of β-catenin are independently regulated by its C-terminal end

The key protein in the canonical Wnt pathway is β-catenin, which is phosphorylated both in absence and presence of Wnt signals by different kinases. Upon activation in the cytoplasm, β-catenin can enter into the nucleus to transactivate target gene expression, many of which are cancer-related genes. The mechanism governing β-catenin's nucleocytoplasmic transport has been recently unvealed, although phosphorylation at its C-terminal end and its functional consequences are not completely understood. Serine 646 of β-catenin is a putative CK2 phosphorylation site and lies in a region which has been proposed to be important for its nucleocytoplasmic transport and transactivation activity. This residue was mutated to aspartic acid mimicking CK2-phosphorylation and its effects on β-catenin activity as well as localization were explored. β-Catenin S6464D did not show significant differences in both transcriptional activity and nuclear localization compared to the wild-type form, but displayed a characteristic granular nuclear pattern. Three-dimensional models of nuclei were constructed which showed differences in number and volume of granules, being those from β-catenin S646D more and smaller than the wild-type form. FRAP microscopy was used to compare nuclear export of both proteins which showed a slightly higher but not significant retention of β-catenin S646D. Altogether, these results show that C-terminal phosphorylation of β-catenin seems to be related with its nucleocytoplasmic transport but not transactivation activity.

Monitoring interactions and dynamics of endogenous beta-catenin with intracellular nanobodies in living cells

β-catenin is the key component of the canonical Wnt pathway and plays a crucial role in a multitude of developmental and homeostatic processes. The different tasks of β-catenin are orchestrated by its subcellular localization and participation in multiprotein complexes. To gain a better understanding of β-catenin's role in living cells we have generated a new set of single domain antibodies, referred to as nanobodies, derived from heavy chain antibodies of camelids. We selected nanobodies recognizing the N-terminal, core or C-terminal domain of β-catenin and applied these new high-affinity binders as capture molecules in sandwich immunoassays and co-immunoprecipitations of endogenous β-catenin complexes. In addition, we engineered intracellularly functional anti-β-catenin chromobodies by combining the binding moieties of the nanobodies with fluorescent proteins. For the first time, we were able to visualize the subcellular localization and nuclear translocation of endogenous β-catenin in living cells using these chromobodies. Moreover, the chromobody signal allowed us to trace the accumulation of diffusible, hypo-phosphorylated β-catenin in response to compound treatment in real time using High Content Imaging. The anti-β-catenin nanobodies and chromobodies characterized in this study are versatile tools that enable a novel and unique approach to monitor the dynamics of subcellular β-catenin in biochemical and cell biological assays.

Distinct roles for Wnt-4 and Wnt-11 during retinoic acid-induced neuronal differentiation

Retinoic acid and Wnt/β-catenin signals play important roles during neuronal differentiation but less is known about noncanonical Wnt signals in this context. We examined retinoic acid and Wnt signaling in two human embryonal carcinoma cell lines, NTERA-2 (clone D1), which undergoes neuronal differentiation in response to retinoic acid, and 2102Ep, which does not. Retinoic acid treatment inhibited β-catenin/Tcf activity in NTERA-2 cells but not in 2102Ep cells. Inhibition occurred downstream of β-catenin but did not involve competition between retinoic acid receptors and β-catenin for binding to p300 or Tcf-4. Ectopic expression of FZD1 partially restored inhibition in 2102Ep cells, suggesting the involvement of Wnt ligands. Retinoic acid treatment of NTERA-2 cells induced the expression of Wnt-4 and Wnt-11, both of which were able to inhibit β-catenin/Tcf activity. Wnt-4 and Wnt-11 were found at cell borders in islands of cells that expressed OCT4 and GFAP and were predominantly negative for Nestin, PAX6, and GATA6. Gene silencing of Wnt-4, but not Wnt-11, reduced retinoic acid downregulation of OCT4 and Nanog and upregulation of PAX6, ASCL1, HOXC5, and NEUROD1, suggesting that Wnt-4 promotes early neuronal differentiation. Gene expression analysis of NTERA-2 cells stably overexpressing Wnt-11 suggested that Wnt-11 potentiates retinoic acid induction of early neurogenesis. Consistent with this, overexpression of Wnt-11 maintained a population of proliferating progenitor cells in cultures treated with retinoic acid for several weeks. These observations highlight the distinct roles of two noncanonical Wnts during the early stages of retinoic acid-induced neuronal differentiation.

PAF-AH Catalytic Subunits Modulate the Wnt Pathway in Developing GABAergic Neurons

Platelet-activating factor acetylhydrolase 1B (PAF-AH) inactivates the potent phospholipid platelet-activating factor (PAF) and is composed of two catalytic subunits (α1 and α2) and a dimeric regulatory subunit, LIS1. The function of the catalytic subunits in brain development remains unknown. Here we examined their effects on proliferation in the ganglionic eminences and tangential migration. In α1 and α2 catalytic subunits knockout mice we noticed an increase in the size of the ganglionic eminences resulting from increased proliferation of GABAergic neurons. Our results indicate that the catalytic subunits act as negative regulators of the Wnt signaling pathway. Overexpression of each of the PAF-AH catalytic subunits reduced the amount of nuclear beta-catenin and provoked a shift of this protein from the nucleus to the cytoplasm. In the double mutant mice, Wnt signaling increased in the ganglionic eminences and in the dorsal part of the cerebral cortex. In situ hybridization revealed increased and expanded expression of a downstream target of the Wnt pathway (Cyclin D1), and of upstream Wnt components (Tcf4, Tcf3 and Wnt7B). Furthermore, the interneurons in the cerebral cortex were more numerous and in a more advanced position. Transplantation assays revealed a non-cell autonomous component to this phenotype, which may be explained in part by increased and expanded expression of Sdf1 and Netrin-1. Our findings strongly suggest that PAF-AH catalytic subunits modulate the Wnt pathway in restricted areas of the developing cerebral cortex. We hypothesize that modulation of the Wnt pathway is the evolutionary conserved activity of the PAF-AH catalytic subunits.

Cell-cell contact formation governs Ca2+ signaling by TRPC4 in the vascular endothelium: evidence for a regulatory TRPC4-beta-catenin interaction

TRPC4 is well recognized as a prominent cation channel in the vascular endothelium, but its contribution to agonist-induced endothelial Ca²⁺ entry is still a matter of controversy. Here we report that the cellular targeting and Ca²⁺ signaling function of TRPC4 is determined by the state of cell-cell adhesions during endothelial phenotype transitions. TRPC4 surface expression in human microvascular endothelial cells (HMEC-1) increased with the formation of cell-cell contacts. Epidermal growth factor recruited TRPC4 into the plasma membrane of proliferating cells but initiated retrieval of TRPC4 from the plasma membrane in quiescent, barrier-forming cells. Epidermal growth factor-induced Ca²⁺ entry was strongly promoted by the formation of cell-cell contacts, and both siRNA and dominant negative knockdown experiments revealed that TRPC4 mediates stimulated Ca²⁺ entry exclusively in proliferating clusters that form immature cell-cell contacts. TRPC4 co-precipitated with the junctional proteins β-catenin and VE-cadherin. Analysis of cellular localization of fluorescent fusion proteins provided further evidence for recruitment of TRPC4 into junctional complexes. Analysis of TRPC4 function in the HEK293 expression system identified β-catenin as a signaling molecule that enables cell-cell contact-dependent promotion of TRPC4 function. Our results place TRPC4 as a Ca²⁺ entry channel that is regulated by cell-cell contact formation and interaction with β-catenin. TRPC4 is suggested to serve stimulated Ca²⁺ entry in a specific endothelial state during the transition from a proliferating to a quiescent phenotype. Thus, TRPC4 may adopt divergent, as yet unappreciated functions in endothelial Ca²⁺ homeostasis and emerges as a potential key player in endothelial phenotype switching and tuning of cellular growth factor signaling.

The terminal region of beta-catenin promotes stability by shielding the Armadillo repeats from the axin-scaffold destruction complex

Post-translational stabilization of beta-catenin is a key step in Wnt signaling, but the features of beta-catenin required for stabilization are incompletely understood. We show that forms of beta-catenin lacking the unstructured C-terminal domain (CTD) show faster turnover than full-length or minimally truncated beta-catenins. Mutants that exhibit faster turnover show enhanced association with axin in co-transfected cells, and excess CTD polypeptide can compete binding of the beta-catenin armadillo (arm) repeat domain to axin in vitro, indicating that the CTD may restrict beta-catenin binding to the axin-scaffold complex. Fluorescent resonance energy transmission (FRET) analysis of cyan fluorescent protein (CFP)-arm-CTD-yellow fluorescent protein beta-catenin reveals that the CTD of beta-catenin can become spatially close to the N-terminal arm repeat region of beta-catenin. FRET activity is strongly diminished by the coexpression of beta-catenin binding partners, indicating that an unliganded groove is absolutely required for an orientation that allows FRET. Amino acids 733-759 are critical for beta-catenin FRET activity and stability. These data indicate that an N-terminal orientation of the CTD is required for beta-catenin stabilization and suggest a model where the CTD extends toward the N-terminal arm repeats, shielding these repeats from the beta-catenin destruction complex.

Overexpression of alpha-catenin increases osteoblastic differentiation in mouse mesenchymal C3H10T1/2 cells

alpha- and beta-Catenin link cadherins to the actin-based cytoskeleton at adherens junctions and regulate cell-cell adhesion. Although roles of cadherins and canonical Wnt-/beta-catenin-signaling in osteoblastic differentiation have been extensively studied, the role of alpha-catenin is not known. Murine embryonic mesenchymal stem cells, C3H10T1/2 cells, were transduced with retrovirus encoding alpha-catenin (MSCV-alpha-catenin-HA-GFP). In the presence of Wnt-3A conditioned medium or osteogenic medium (beta-glycerol phosphate and ascorbic acid), cells overexpressing alpha-catenin showed enhanced osteoblastic differentiation as measured by alkaline phosphatase (ALP) staining and ALP activity assay compared to cells transduced with empty virus (MSCV-GFP). In addition, mRNA expression of osteocalcin and Runx2 was significantly increased compared to control. Cell aggregation assay revealed that alpha-catenin overexpression has significantly increased cell-cell aggregation. However, cellular beta-catenin levels (total, cytoplasmic-nuclear ratio) and beta-catenin-TCF/LEF transcriptional activity did not change by overexpression of alpha-catenin. Knock-down of alpha-catenin using siRNA decreased osteoblastic differentiation as measured by ALP assay. These results suggest that alpha-catenin overexpression increases osteoblastic differentiation by increasing cell-cell adhesion rather than Wnt-/beta-catenin-signaling.

alpha-Catenin overrides Src-dependent activation of beta-catenin oncogenic signaling

Loss of alpha-catenin is one of the characteristics of prostate cancer. The catenins (alpha and beta) associated with E-cadherin play a critical role in the regulation of cell-cell adhesion. Tyrosine phosphorylation of beta-catenin dissociates it from E-cadherin and facilitates its entry into the nucleus, where beta-catenin acts as a transcriptional activator inducing genes involved in cell proliferation. Thus, beta-catenin regulates cell-cell adhesion and cell proliferation. Mechanisms controlling the balance between these functions of beta-catenin invariably are altered in cancer. Although a wealth of information is available about beta-catenin deregulation during oncogenesis, much less is known about how or whether alpha-catenin regulates beta-catenin functions. In this study, we show that alpha-catenin acts as a switch regulating the cell-cell adhesion and proliferation functions of beta-catenin. In alpha-catenin-null prostate cancer cells, reexpression of alpha-catenin increased cell-cell adhesion and decreased beta-catenin transcriptional activity, cyclin D1 levels, and cell proliferation. Further, Src-mediated tyrosine phosphorylation of beta-catenin is a major mechanism for decreased beta-catenin interaction with E-cadherin in alpha-catenin-null cells. alpha-Catenin attenuated the effect of Src phosphorylation by increasing beta-catenin association with E-cadherin. We also show that alpha-catenin increases the sensitivity of prostate cancer cells to a Src inhibitor in suppressing cell proliferation. This study reveals for the first time that alpha-catenin is a key regulator of beta-catenin transcriptional activity and that the status of alpha-catenin expression in tumor tissues might have prognostic value for Src targeted therapy.

alphaE-catenin is not a significant regulator of beta-catenin signaling in the developing mammalian brain

beta-Catenin is a crucial mediator of the canonical Wnt-signaling pathway. alpha-catenin is a major beta-catenin-binding protein, and overexpressed alpha-catenin can negatively regulate beta-catenin activity. Thus, alpha-catenin may be an important modulator of the Wnt pathway. We show here that endogenous alpha-catenin has little impact on the transcriptional activity of beta-catenin in developing mammalian organisms. We analyzed beta-catenin signaling in mice with conditional deletion of alphaE-catenin (Ctnna1) in the developing central nervous system. This mutation results in brain hyperplasia and we investigated whether activation of beta-catenin signaling may be at least partially responsible for this phenotype. To reveal potential quantitative or spatial changes in beta-catenin signaling, we used mice carrying a beta-catenin-signaling reporter transgene. In addition, we analyzed the expression of known endogenous targets of the beta-catenin pathway and the amount and localization of beta-catenin in mutant progenitor cells. We found that although loss of alphaE-catenin resulted in disruption of intercellular adhesion and hyperplasia in the developing brain, beta-catenin signaling was not altered. We conclude that endogenous alphaE-catenin has no significant impact on beta-catenin transcriptional activities in the developing mammalian brain.

Modulation of the oncogenic potential of beta-catenin by the subcellular distribution of plakoglobin

Plakoglobin (Pg) and beta-catenin are homologous proteins that function in cell-cell adhesion and signaling. The cadherin-associated form of these proteins mediates adhesion, whereas the cytosolic/nuclear form has a signaling role. Despite their interactions with common cellular partners, beta-catenin has a well-documented oncogenic potential while Pg has a less characterized tumor suppressor activity. We showed previously that Pg overexpression in Pg-deficient SCC9 cells (SCC9-Pg-WT) induced Bcl-2 expression and inhibited apoptosis. To assess the exact role of Pg in Bcl-2 expression, we generated and characterized SCC9 transfectants expressing Pg with a restricted cytoplasmic (Pg-NES) or nuclear (Pg-NLS) distribution. We show that Bcl-2 was expressed regardless of Pg localization, although its level was substantially lower in SCC9-Pg-NLS cells. Bcl-2 expression coincided with increased nuclear beta-catenin levels (Pg-NES) or a decrease in the level of total and nuclear beta-catenin associated with N-cadherin and alpha-catenin (Pg-WT and -NLS) cells. Bcl-2 expression also was induced in SCC9 cells overexpressing beta-catenin. In contrast, SCC9 cells expressing mutant Pg proteins, unable to interact with N-cadherin and alpha-catenin, had noticeably lower Bcl-2 levels. Our data suggest that Bcl-2 expression is induced by beta-catenin and modulated by Pg. We show that the inhibition of beta-catenin-dependent TCF transactivation had no effect on Bcl-2 levels, suggesting that induction of Bcl-2 expression by beta-catenin and its modulation by Pg may involve factors other than, or in addition, to, TCF. These results provide a possible mechanism for the tumor suppressor activity of Pg via its role as a regulator of the oncogenic potential beta-catenin.

Nuclear translocation of alphaN-catenin by the novel zinc finger transcriptional repressor ZASC1

Alpha-catenins anchor the transmembrane cell-cell adhesion molecule E-cadherin indirectly to the actin cytoskeleton through interaction with beta-catenin or plakoglobin. Three different alpha-catenins are known at present: alphaE-, alphaT-, and alphaN-catenin. Despite their different expression patterns, no functional differences between the alpha-catenins are known. In a yeast two-hybrid screening with alphaN-catenin as bait, we identified the Cys(2)-His2 zinc finger protein ZASC1. The mRNA and protein of ZASC1 were ubiquitously expressed in various cell lines and human tissues. Our results suggest an association of the ZASC1 protein with DNA, and luciferase reporter assays revealed that ZASC1 is a transcriptional repressor. Upon transient overexpression, the ZASC1 protein localized in the nucleus, to where it was able to recruit cytoplasmic alphaN-catenin. Neither the highly related alphaE-catenin nor alphaT-catenin interacted with ZASC1. By interchanging parts of alphaN-catenin and alphaE-catenin cDNAs, we were able to narrow down the interaction region of alphaN-catenin to two limited amino-terminal regions. On the other hand, the interaction of ZASC1 with alphaN-catenin can be mediated by the domain comprising zinc fingers six to eight of ZASC1. The interaction and nuclear cotranslocation of a neural alpha-catenin with a putative proto-oncogene product as reported here provides novel insights into the signaling functions of alpha-catenins.

Lysosomal trafficking of beta-catenin induced by the tea polyphenol epigallocatechin-3-gallate

beta-Catenin is a cadherin-binding protein involved in cell-cell adhesion, which also functions as a transcriptional activator when complexed in the nucleus with members of the T-cell factor (TCF)/lymphoid enhancer factor (LEF) family of proteins. There is considerable interest in mechanisms that down-regulate beta-catenin, since this provides an avenue for the prevention of colorectal and other cancers in which beta-catenin is frequently over-expressed. We show here that physiologically relevant concentrations of the tea polyphenol epigallocatechin-3-gallate (EGCG) inhibited beta-catenin/TCF-dependent reporter activity in human embryonic kidney 293 cells transfected with wild type or mutant beta-catenins, and there was a corresponding decrease in beta-catenin protein levels in the nuclear, cytosolic and membrane-associated fractions. However, beta-catenin accumulated as punctate aggregates in response to EGCG treatment, including in human colon cancer cells over-expressing beta-catenin endogenously. Confocal microscopy studies revealed that the aggregated beta-catenin in HEK293 cells was extra-nuclear and co-localized with lysosomes, suggesting that EGCG activated a pathway involving lysosomal trafficking of beta-catenin. Lysosomal inhibitors leupeptin and transepoxysuccinyl-l-leucylamido(4-guanido)butane produced an increase in beta-catenin protein in total cell lysates, without a concomitant increase in beta-catenin transcriptional activity. These data provide the first evidence that EGCG facilitates the trafficking of beta-catenin into lysosomes, presumably as a mechanism for sequestering beta-catenin and circumventing further nuclear transport and activation of beta-catenin/TCF/LEF signaling.

Regulation of β-Catenin Signaling and Maintenance of Chondrocyte Differentiation by Ubiquitin-independent Proteasomal Degradation of α-Catenin

Accumulation of beta-catenin and subsequent stimulation of beta-catenin-T cell-factor (Tcf)/lymphoid-enhancerfactor (Lef) transcriptional activity causes dedifferentiation of articular chondrocytes, which is characterized by decreased type II collagen expression and initiation of type I collagen expression. This study examined the mechanisms of alpha-catenin degradation, the role of alpha-catenin in beta-catenin signaling, and the physiological significance of alpha-catenin regulation of beta-catenin signaling in articular chondrocytes. We found that both alpha- and beta-catenin accumulated during dedifferentiation of chondrocytes by escaping from proteasomal degradation. Beta-catenin degradation was ubiquitination-dependent, whereas alpha-catenin was proteasomally degraded in a ubiquitination-independent fashion. The accumulated alpha- and beta-catenin existed as complexes in the cytosol and nucleus. The complex formation between alpha- and beta-catenin blocked proteasomal degradation of alpha-catenin and also inhibited beta-catenin-Tcf/Lef transcriptional activity and the suppression of type II collagen expression associated with ectopic expression of beta-catenin, the inhibition of proteasome, or Wnt signaling. Collectively, our results indicate that ubiquitin-independent degradation of alpha-catenin regulates beta-catenin signaling and maintenance of the differentiated phenotype of articular chondrocytes.

Analysis of Wnt gene expression in prostate cancer: mutual inhibition by WNT11 and the androgen receptor

The Wnt signaling pathway is aberrantly activated in many tumor types, including those of the prostate, in which beta-catenin accumulates in cell nuclei and acts as a transcriptional coregulator for the androgen receptor. Because activating mutations in the beta-catenin gene are rare in prostate cancer, we have looked for altered expression of other components of the Wnt signaling pathway in prostate cancer cells. Here we determined the expression levels of Wnt family genes in cultured human prostate cells and prostate cancer cell lines. We found that WNT11 expression is elevated in hormone-independent prostate cancer cell lines. Additional analysis indicated that WNT11 expression is also elevated in high-grade prostatic tumors and in hormone-independent xenografts. Growth of hormone-dependent LNCaP cells in hormone-depleted media led to increased WNT11 expression, which was repressed by the synthetic androgen R1881. This repression was inhibited by the antiandrogen bicalutamide, suggesting that androgens negatively regulate WNT11 expression through the androgen receptor. Expression of WNT11 inhibited androgen receptor transcriptional activity and cell growth in androgen-dependent cells but not in androgen-independent cells. WNT11 inhibited activation of the canonical Wnt pathway by WNT3A in HEK 293 cells and inhibited basal beta-catenin/Tcf transcriptional activity in LNCaP cells. However, expression of stabilized beta-catenin did not prevent the inhibition of androgen receptor transcriptional activity by WNT11. Our observations are consistent with a model in which androgen depletion activates WNT11-dependent signals that inhibit androgen-dependent but not androgen-independent cell growth.

A divergent canonical WNT-signaling pathway regulates microtubule dynamics: dishevelled signals locally to stabilize microtubules

Dishevelled (DVL) is associated with axonal microtubules and regulates microtubule stability through the inhibition of the serine/threonine kinase, glycogen synthase kinase 3beta (GSK-3beta). In the canonical WNT pathway, the negative regulator Axin forms a complex with beta-catenin and GSK-3beta, resulting in beta-catenin degradation. Inhibition of GSK-3beta by DVL increases beta-catenin stability and TCF transcriptional activation. Here, we show that Axin associates with microtubules and unexpectedly stabilizes microtubules through DVL. In turn, DVL stabilizes microtubules by inhibiting GSK-3beta through a transcription- and beta-catenin-independent pathway. More importantly, axonal microtubules are stabilized after DVL localizes to axons. Increased microtubule stability is correlated with a decrease in GSK-3beta-mediated phosphorylation of MAP-1B. We propose a model in which Axin, through DVL, stabilizes microtubules by inhibiting a pool of GSK-3beta, resulting in local changes in the phosphorylation of cellular targets. Our data indicate a bifurcation in the so-called canonical WNT-signaling pathway to regulate microtubule stability.

Distinct activities of the alpha-catenin family, alpha-catulin and alpha-catenin, on beta-catenin-mediated signaling

Alpha-catenin, an integral part of cadherin-catenin adhesion complexes, is a major binding partner of beta-catenin, a key component of the Wnt pathway, which activates T-cell factor (TCF)/lymphoid enhancer factor (LEF) transcription and is often upregulated in cancers. Recently, we identified an alpha-catenin-related protein, alpha-catulin, whose function is poorly understood, as part of a Rho GTPase signaling complex. Here, based on evidence suggesting that alpha-catulin may associate with a beta-catenin fraction, we investigated the role of alpha-catenin family members in beta-catenin-mediated signals. Expression of the full length or a 103-residue region of alpha-catenin strongly inhibits the induction of the TCF/LEF-responsive TOPFLASH reporter in HEK293T cells expressing activated beta-catenin or in cancer cells with constitutively upregulated Wnt signaling, whereas alpha-catulin expression had no effect. Interestingly, alpha-catulin expression attenuates the activation of the cyclin D1 promoter, a target of Wnt pathway signals. Alpha-catulin appears to inhibit Ras-mediated signals to the cyclin D1 promoter, rather than beta-catenin signals, and the synergy between Ras and beta-catenin required to fully activate this promoter. Data suggesting the involvement of Rho in this response are presented and discussed. These results suggest a novel function for alpha-catulin and imply that alpha-catenin and alpha-catulin have distinct activities that downregulate, respectively, beta-catenin and Ras signals converging on the cyclin D1 promoter.

Translocation of β-catenin into the nucleus independent of interactions with FG-rich nucleoporins

beta-Catenin nuclear import has been found to be independent of classical nuclear localization signal (NLS) nuclear import factors. Here, we test the hypothesis that beta-catenin interacts directly with nuclear pore proteins to mediate its own transport. We show that beta-catenin, unlike importin-beta, does not interact detectably with Phe/Gly(FG)-repeat-rich nuclear pore proteins or nucleoporins (Nups). Moreover, unlike NLS-containing proteins, beta-catenin nuclear import is not inhibited by wheat germ agglutinin (WGA) or excess importin-beta. These results suggest beta-catenin nuclear translocation does not involve direct interactions with FG-Nups. However, beta-catenin has two regions that can target it to the nucleus, and its import is cold sensitive, indicating that beta-catenin nuclear import is still an active process. Transport is blocked by a soluble form of the C-cadherin cytoplasmic domain, suggesting that masking of the nuclear targeting signal may be a mechanism of regulating beta-catenin subcellular localization.

Glycogen synthase kinase-3 and Axin function in a β-catenin-independent pathway that regulates neurite outgrowth in neuroblastoma cells

We have sought to determine the roles of beta-catenin and the Wnt signaling pathway in neurite outgrowth using a model cell system, the Neuro-2a neuroblastoma cell line. Activation of the Wnt signaling pathway disrupts a multiprotein complex that includes beta-catenin, Axin, and glycogen synthase kinase-3 (GSK-3), which would otherwise promote the phosphorylation and degradation of beta-catenin. Stabilized beta-catenin accumulates in the cytosol and in the nucleus; in the nucleus it binds to TCF family transcription factors, forming a bipartite transcriptional activator of Wnt target genes. These events can be mimicked by lithium (Li(+)), which inhibits GSK-3 activity. Both Li(+) and the GSK-3 inhibitor SB415286 induced neurite outgrowth of Neuro-2a cells. Li(+)-induced neurite outgrowth did not require beta-catenin-/TCF-dependent transcription, and increasing levels of beta-catenin either by transfection or using Wnt-3A was not sufficient to induce neurite outgrowth. Interestingly, Axin, which is also a substrate for GSK-3, was destabilized by Li(+) and ectopic expression of Axin inhibited Li(+)-induced neurite outgrowth. Deletion analysis of Axin indicated that this inhibition required the GSK-3 binding site, but not the beta-catenin binding site. Our results suggest that a signaling pathway involving Axin and GSK-3, but not beta-catenin, regulates Li(+)-induced neurite outgrowth in Neuro-2a cells.

Functional localization and competition between the androgen receptor and T-cell factor for nuclear beta-catenin: a means for inhibition of the Tcf signaling axis

Recent reports suggest that the beta-catenin-T-cell factor (Tcf) (BCT) signaling pathway is important in the progression of prostate cancer. Evidence suggests that the androgen receptor (AR) can repress BCT-mediated transcription both in prostate cancer and colon cancer cells (Chesire and Isaacs, 2002). In this study, we validate such findings and show that repression of BCT signaling is facilitated by competition between the AR and Tcf. Measurements of the Tcf transcriptional reporter (TOPFLASH) indicated that AR+DHT-mediated repression can inhibit BCT transcription in the presence of WT and exogenous activating beta-catenin (Delta1-130 bp). Transient transfections in SW480 cells (APC(mut/mut)) showed that this mode of repression is functionally independent of APC-mediated beta-catenin ubiquitination. Using a recently developed red flourescent protein (HcRed), we demonstrate novel observations about the nuclear distribution of Tcf. Furthermore, with the use of red (HcRed-AR and HcRed-Tcf) and green fusion proteins (beta-catenin-EGFP), we provide morphological evidence of a reciprocal balance of nuclear beta-catenin-EGFP (BC-EGFP). By cotransfecting in LNCaP prostate tumor cells and using quantitative imaging software, we demonstrated a 62.0% colocalization of HcRed-AR and BC-EGFP in the presence of DHT and 63.3% colocalization of HcRed-Tcf/BC-EGFP in the absence of DHT. Costaining for activated RNA Pol II (phosphoserine 2) and HcRed-Tcf suggested that Tcf foci contain transcriptional 'hotspots' validating that these sites have the capacity for transcriptional activity. Given this apparent androgen-dependent competition for nuclear BC-EGFP, we chose to assess our hypothesis by in vivo and in vitro binding assays. SW480 cells transiently transfected with an AR expression construct, treated with DHT and immunoprecipitated for Tcf showed less associated beta-catenin when compared to Tcf precipitates from untreated cells. Furthermore, by treating cells with DHT+Casodex, we were able to abrogate the androgen-sensitive AR/beta-catenin interaction, in addition to relieving transcriptional repression of the TOPFLASH reporter. In vitro binding assays, with increasing amounts of AR(S35), resulted in decreased Tcf(S35) association with immunoprecipitated recombinant beta-catenin-HIS. These data suggest that in steady-state conditions, AR has the ability to compete out Tcf binding for beta-catenin. Finally, using SW480 cells, we show that AR-mediated repression of the BCT pathway has implications for cell cycle progression and in vitro growth. Using FACs analysis, we observed a 26.1% increase in accumulation of cells in the G1 phase of the cell cycle, while in vitro growth assays showed a 35% reduction in viable cells transfected with AR+DHT treatment. Together, our data strongly suggest that a reciprocal balance of nuclear beta-catenin facilitates AR-mediated repression of BCT-driven transcription and cell growth.

Changes in human bladder epithelial cell gene expression associated with interstitial cystitis or antiproliferative factor treatment

Explanted bladder epithelial cells from patients with interstitial cystitis (IC) have been shown to differ from explanted control cells in several ways, including production of an antiproliferative factor (APF), altered production of certain epithelial growth factors, and rate of proliferation. To better understand the role of the APF in abnormal bladder epithelial cell proliferation in IC, we studied gene expression patterns in normal bladder epithelial cells treated with APF vs. mock APF and compared them to expression patterns in IC vs. normal cells using microarray analysis. Oligo-dT-primed total cellular RNA was labeled with [(33)P]dCTP and hybridized to GeneFilter GF211 microarray membranes (Research Genetics) containing cDNA for 3,964 human genes. Thirteen genes that function in epithelial cell proliferation or differentiation were consistently differentially expressed in both IC (compared with control) and APF-treated (compared with mock APF-treated) normal bladder epithelial cells. The general pattern of gene expression in IC and APF-treated cells suggested a less proliferative phenotype, with increased expression of E-cadherin, phosphoribosylpyrophosphate synthetase-associated protein 39, and SWI/SNF complex 170-kDa subunit, and decreased expression of vimentin, alpha2-integrin, alpha1-catenin, cyclin D1, and jun N-terminal kinase 1; these findings were confirmed for the structural gene products (E-cadherin, vimentin, alpha2-integrin, and alpha-catenin) by immunohistochemistry. These results are compatible with the previously noted decreased proliferation rate of IC and APF-treated normal cells, and indicate that the mechanism whereby APF inhibits cell proliferation may involve both downregulation of genes that stimulate cell proliferation along with upregulation of genes that inhibit cell growth.

Association of Lbc Rho guanine nucleotide exchange factor with alpha-catenin-related protein, alpha-catulin/CTNNAL1, supports serum response factor activation

The Rho GTPase signaling pathway is required for actin cytoskeletal organization and serum response factor-dependent gene transcription. Lbc is a Rho-specific guanine nucleotide exchange factor that contains a modulatory C-terminal region. To elucidate Lbc regulatory mechanism(s), a yeast two-hybrid screen for proteins that interact with the Lbc C-terminal region was carried out, resulting in multiple isolation of cDNAs encoding the same 734-amino acid Lbc interacting protein. The Lbc interacting protein has homology with the alpha-catenin cell adhesion component and is identical to the alpha-catenin-like alpha-catulin protein of unknown function. The human alpha-catulin gene (CTNNAL1) maps to 9q31-32. Here we identify the predicted endogenous alpha-catulin product, document alpha-catulin and Lbc co-expression in multiple human cell lines, and show alpha-catulin and Lbc subcellular co-fractionation and intracellular localization. The required regions for Lbc and alpha-catulin interaction were mapped, and complex formation between Lbc and alpha-catulin in mammalian cells was detected. Functionally, alpha-catulin co-expression leads to increased Lbc-induced serum response factor activation in vivo as measured by a transcriptional reporter assay. Furthermore, alpha-catulin co-expression enhances Lbc-induced GTP-Rho formation in vivo. These results support the concept that the recently identified alpha-catulin protein may modulate Rho pathway signaling in vivo by providing a scaffold for the Lbc Rho guanine nucleotide exchange factor.

Construction of a zebrafish cDNA microarray: gene expression profiling of the zebrafish during development

Vertebrate embryogenesis is a complex process controlled by a transcriptional hierarchy that coordinates the action of thousands of genes. To identify and analyze the expression patterns of these genes, we constructed a zebrafish cDNA microarray containing 4512 unique genes identified from zebrafish embryonic heart, adult hearts, and skeletal muscle cDNA libraries. We examined the patterns of gene expression during development in the zebrafish between five time points relative to 12h post-fertilization (hpf). Differentially expressed genes can be grouped into two categories, early genes that are expressed at 5hpf and genes expressed at 48/72/120hpf. Furthermore, we report the utilization of cDNA microarray technology to investigate the adaptive molecular responses of zebrafish to hypoxia during development. Our study provides the first utilization of cDNA microarray in the zebrafish and reveals dynamic changes in levels of gene expression in relation to development and survival of the zebrafish embryos under hypoxic stress.

Protection against anoikis and down-regulation of cadherin expression by a regulatable beta-catenin protein

beta-Catenin signaling plays a key role in a variety of cellular contexts during embryonic development and tissue differentiation. Aberrant beta-catenin signaling has also been implicated in promoting human colorectal carcinomas as well as a variety of other cancers. To study the molecular and cellular biological functions of beta-catenin in a controlled fashion, we created a regulatable form of activated beta-catenin by fusion to a modified estrogen receptor (ER) ligand binding domain (G525R). Transfection of tissue culture cells with expression vectors encoding this hybrid protein allows the signal transduction function of beta-catenin to be induced by the synthetic estrogen, 4-hydroxytamoxifen, leading to regulated activation of a beta-catenin-lymphocyte enhancer-binding factor-dependent reporter gene as well as induction of endogenous cyclin D1 expression. The activation of ER-beta-catenin signaling rescues RK3E cells from anoikis and correlates with an increased phosphorylation of mitogen-activated protein kinase. The inhibition of anoikis by ER-beta-catenin can be abolished by a mitogen-activated protein kinase pathway inhibitor, PD98059. Evidence is also provided to show that ER-beta-catenin down-regulates cadherin protein levels. These findings support a key role for activated beta-catenin signaling in processes that contribute to tumor formation and progression.

Roles of cytoskeletal and junctional plaque proteins in nuclear signaling

Cytoplasmic junctional plaque proteins play an important role at intercellular junctions. They link transmembrane cell adhesion molecules to components of the cytoskeleton, thereby playing an important role in the control of many cellular processes. Recent studies on the subcellular distribution of some plaque proteins have revealed that a number of these proteins are able to localize in the nucleus. This dual location indicates that in addition to promoting adhesive interactions, plaque proteins may also play a direct role in nuclear processes, and in particular in the transfer of signals from the membrane to the nucleus. Therefore, translocation of plaque proteins into the nucleus in response to extracellular signals could represent a novel and direct mechanism by which signals can be transmitted from the plasma membrane to the nucleus. This could allow cells to respond to changing environmental conditions in a rapid and efficient way. In addition, conditional sequestration of karyophilic proteins at the sites of cell-cell and cell-substratum adhesion may represent a general mechanism for the regulation of nucleocytoplasmic transport.

Gene expression profiling following BMP-2 induction of mesenchymal chondrogenesis in vitro

OBJECTIVE

This study aims to apply gene expression profiling technology to gain insight into the molecular regulation of mesenchymal chondrogenesis.

METHODS

The experimental system consists of micromass cultures of C3H10T1/2 cells, a murine multipotential embryonic cell line, treated with the chondroinductive growth factor, bone morphogenetic factor-2 (BMP-2). In this system, chondrogenic differentiation characterized by both morphological changes and cartilage matrix gene expression has been shown to be completely dependent upon BMP-2 treatment and the high cell plating density of micromass cultures. To identify candidate genes that may have key functional roles in chondrogenesis, we have applied subtractive hybridization to isolate genes whose expression is significantly up- or down-regulated during chondrogenesis. RNA was isolated from micromass cultures treated with BMP-2 for 24 h and analysed for representational differences by means of a subtractive hybridization screening method.

RESULTS

Sixteen different genes were identified whose expression was up-regulated between two- and 12-fold by B,P-2, and twelve different genes were identified whose expression was down-regulated between two- and seven-fold by BMP-2.

CONCLUSIONS

The potential of this screening methodology to identify new BMP-2 regulated genes is suggested by the fact that a majority of the identified genes are indeed novel. Identification and characterization of these genes should provide insight as to how chondrogenesis is regulated and also should provide important new markers for the study of osteoarthritis.

Adenomatous polyposis coli (APC), beta-catenin, and cadherin are expressed in human bone and cartilage

AIMS

Members of the cadherin and catenin families are involved in chondrogenesis and catenin gene mutations have been detected in malignant tumours of bone. This study was undertaken to assess in detail expression of cadherin, beta-catenin and the associated tumour suppressor gene product APC in bone and cartilage at different stages of human skeletal maturity and in non-neoplastic and neoplastic osteoarticular disease.

METHODS AND RESULTS

Immunohistochemical staining of formalin-fixed paraffin-embedded normal and osteoarthritic adult articular cartilage, fetal growth plate and a series of tumours of bone and cartilage was undertaken with a panel of antibodies against APC, beta-catenin, and pan-cadherin. This study demonstrated expression of APC, beta-catenin and cadherin in normal and diseased bone and cartilage. APC was present both in osteoblasts and osteoclasts but not in osteocytes. Although only weak APC staining of occasional growth plate hypertrophic chondrocytes and normal articular chondrocytes was seen, APC staining was increased in osteoarthritic articular cartilage. beta-catenin and pan-cadherin staining was strongly positive in osteoclasts and osteoblasts, with expression being lost when bone cells differentiated into osteocytes. Expression of APC, beta-catenin and pan-cadherin in bone tumours was similar to that of non-neoplastic adult tissues.

CONCLUSIONS

These findings suggest previously unrecognized roles for APC in regulation of function of chondrocytes, osteoblasts and osteoclasts and support the view that catenin-cadherin interactions are important in regulation of bone cell activity. Abnormalities of expression or function of these molecules may be important in formation of bone tumours and their clinical behaviour.

Pin1 regulates turnover and subcellular localization of beta-catenin by inhibiting its interaction with APC

Phosphorylation on a serine or threonine residue preceding proline (Ser/Thr-Pro) is a key regulatory mechanism, and the conformation of certain phosphorylated Ser/Thr-Pro bonds is regulated specifically by the prolyl isomerase Pin1. Whereas the inhibition of Pin1 induces apoptosis, Pin1 is strikingly overexpressed in a subset of human tumours. Here we show that Pin1 regulates beta-catenin turnover and subcellular localization by interfering with its interaction with adenomatous polyposis coli protein (APC). A differential-display screen reveals that Pin1 increases the transcription of several beta-catenin target genes, including those encoding cyclin D1 and c-Myc. Manipulation of Pin1 levels affects the stability of beta-catenin in vitro. Furthermore, beta-catenin levels are decreased in Pin1-deficient mice but are increased and correlated with Pin1 overexpression in human breast cancer. Pin1 directly binds a phosphorylated Ser-Pro motif next to the APC-binding site in beta-catenin, inhibits its interaction with APC and increases its translocation into the nucleus. Thus, Pin1 is a novel regulator of beta-catenin signalling and its overexpression might contribute to the upregulation of beta-catenin in tumours such as breast cancer, in which APC or beta-catenin mutations are not common.

Isolation and characterization of the TERE1 gene, a gene down-regulated in transitional cell carcinoma of the bladder

We have identified a novel cDNA product designated transitional epithelial response gene (TERE1), which was localized to chromosome 1p36. The TERE1 transcript (1.5 and 3.5 kb) is present in most normal human tissues including urothelium, but was reduced or absent in the majority of muscle invasive TCC tumors (22 out of 29 cases). The open reading frame encodes a protein of 338 amino acids (MW 36.8 KD). This protein is 57% homologous to a Drosophila protein called heix. We have shown by Western blotting and immuno-histochemistry with a polyclonal antibody to a specific TERE1 peptide, reduced or absent staining in muscle invasive tumors. Transfection of a sense TERE1 construct resulted in an 80-90% inhibition of cellular proliferation in two TCC cell lines and a lack of aneuploidy in the TERE1-transduced J82 cell line. These data suggest a potential role for this gene product in the progression of bladder cancer.

Posttranscriptional regulation of alpha-catenin expression is required for Wnt signaling in L cells

alpha-Catenin is an essential component of the cadherin-catenin cell-cell adhesion complex. An excess amount of alpha-catenin also affects the Wnt signaling pathway probably through its direct binding to beta-catenin. Here, we examined the molecular mechanisms of the posttranscriptional regulation of alpha-catenin expression. We constructed an expression vector with alpha-catenin cDNA lacking the 5'-untranslated sequence. In L cell transfectants stably expressing mRNA derived from this vector, the amount of exogenous alpha-catenin protein was about 10-fold higher than that of the endogenous protein. The expression level of the exogenously expressed alpha-catenin mRNA, however, was about 80% of that of endogenous molecule. Most of the endogenous and exogenous alpha-catenin protein in cadherin-negative cells was degraded 5 h after inhibition of protein synthesis. Although alpha-catenin contains the PEST sequence, various proteasome and calpain inhibitors did not affect the level of expression of endogenous alpha-catenin protein in L cells. Overexpressed alpha-catenin showed cytoplasmic localization, disturbed the nuclear localization of stabilized beta-catenin, and inhibited TCF-4-responsive transactivation after Wnt-3a treatment. These results suggested that the low-efficiency of translation and unidentified degradation mechanisms maintained the low levels of alpha-catenin expression in the cytoplasm as a necessary condition for the Wnt signaling pathway.

Expression of green fluorescent protein under beta-actin promoter in living spermatogenic cells of the mouse: stage-specific regulation by FSH

Spermatogenic cells from a mouse strain expressing enhanced green fluorescent protein (EGFP) under chicken beta-actin promoter were studied under living conditions to analyse stage- and cell-specific expression and hormonal regulation of the transgene. The isolated seminiferous tubules were examined by transillumination and the live cell squashes by phase contrast and fluorescence microscopy. FSH effects were measured in whole seminiferous tubules comparing stages I-VI, VII-VIII and IX-XII of the cycle. Beta-actin was highly expressed in spermatogonia, but almost no expression was found at early meiosis (leptotene spermatocytes). A gradual increase in translation of beta-actin was found during later stages of meiosis and early spermiogenesis, with a maximum in elongating spermatids. FSH increased the translation of beta-actin after 4 h and 24 h of incubation at stages I-VI, after 24 h at stages VII-VIII but not at stages IX-XII of the cycle. The results support the view that beta-actin plays a role in the nuclear elongation of spermatids and that its expression is regulated by FSH in a stage-specific fashion. Techniques used in this study give us new insight to study temporal and hormonal regulation of gene products in living spermatogenic cells.

alpha-catenin inhibits beta-catenin signaling by preventing formation of a beta-catenin*T-cell factor*DNA complex

alpha-Catenin and beta-catenin link cadherins to the cytoskeleton at adherens junctions. beta-Catenin also associates with members of the T-cell factor (Tcf) family of transcription factors, and mutations in beta-catenin lead to activation of Tcf-dependent transcription and increased cell growth. Although the loss of alpha-catenin expression can also promote cell growth, the role of endogenous alpha-catenin in beta-catenin signaling is unclear. Here we show that loss of alpha-catenin expression in a colon cancer cell line correlates with increased Tcf-dependent transcription. The presence of alpha-catenin in colon cancer cell nuclei suggests that it inhibits transcription directly, and, in agreement with this, ectopic expression of alpha-catenin in the nucleus represses Tcf-dependent transcription. Furthermore, recombinant alpha-catenin disrupts the interaction between the beta-catenin.Tcf complex and DNA. We conclude that alpha-catenin inhibits beta-catenin signaling in the nucleus by interfering with the formation of a beta-catenin. Tcf.DNA complex.