Novel mechanism of Wnt signalling inhibition mediated by Dickkopf-1 interaction with LRP6/Arrow

Directed induction of anterior and posterior primitive streak by Wnt from embryonic stem cells cultured in a chemically defined serum-free medium

Formation of the primitive streak (PS) is the initial specification step that generates all the mesodermal and endodermal tissue lineages during early differentiation. Thus, a therapeutically compatible and efficient method for differentiation of the PS is crucial for regenerative medicine. In this study, we developed chemically defined serum-free culture conditions for the differentiation of embryonic stem (ES) cells into the PS-like cells. Cultures supplemented with Wnt showed induction of expression of a PS marker, the brachyury gene, followed by induction of the anterior PS markers goosecoid and foxa2, a posterior PS marker, evx1, and the endoderm marker sox17. Similar differentiation of PS by Wnt was also observed in human ES cells. Moreover, we revealed that the activation of the Wnt canonical pathway is essential for PS differentiation in mouse ES cells. These results demonstrated that Wnt is an essential and sufficient factor for the induction of the PS-like cells in vitro. These conditions of induction could constitute the initial step in generating therapeutically useful cells of the definitive endoderm lineage, such as hepatocytes and pancreatic endocrine cells, under chemically defined conditions.

Caprin-2 enhances canonical Wnt signaling through regulating LRP5/6 phosphorylation

The low-density lipoprotein receptor–related proteins 5 and 6 (LRP5/6) are coreceptors for Frizzled and transmit signals from the plasma membrane to the cytosol. However, the mechanism for LRP5/6 signal transmission remains undefined. Here, we identify cytoplasmic activation/proliferation-associated protein 2 (Caprin-2) as a LRP5/6-binding protein. Our data show that Caprin-2 stabilizes cytosolic β-catenin and enhances lymphoid enhancer-binding factor 1/T cell factor–dependent reporter gene activity as well as the expression of Wnt target genes in mammalian cells. Morpholino-mediated knockdown of Caprin-2 in zebrafish embryos inhibits Wnt/β-catenin signaling and results in a dorsalized phenotype. Moreover, Caprin-2 facilitates LRP5/6 phosphorylation by glycogen synthase kinase 3, and thus enhances the interaction between Axin and LRP5/6. Therefore, Caprin-2 promotes activation of the canonical Wnt signaling pathway by regulating LRP5/6 phosphorylation.

Characterization of the Kremen-binding site on Dkk1 and elucidation of the role of Kremen in Dkk-mediated Wnt antagonism

Wnt signaling is involved in a wide range of developmental, physiological, and pathophysiological processes and is negatively regulated by Dickkopf1 (Dkk1). Dkk1 has been shown to bind to two transmembrane proteins, the low density lipoprotein receptor-related proteins (LRP) 5/6 and Kremen. Here, we show that Dkk1 residues Arg(197), Ser(198), and Lys(232) are specifically involved in its binding to Kremen rather than to LRP6. These residues are localized at a surface that is at the opposite side of the LRP6-binding surface based on a three-dimensional structure of Dkk1 deduced from that of Dkk2. We were surprised to find that the Dkk1 mutants carrying a mutation at Arg(197), Ser(198), or Lys(232), the key Kremen-binding residues, could antagonize Wnt signaling as well as the wild-type Dkk1. These mutations only affected their ability to antagonize Wnt signaling when both LRP6 and Kremen were coexpressed. These results suggest that Kremen may not be essential for Dkk1-mediated Wnt antagonism and that Kremen may only play a role when cells express a high level of LRP5/6.

Structural insight into the mechanisms of Wnt signaling antagonism by Dkk

Dickkopf (Dkk) proteins are antagonists of the canonical Wnt signaling pathway and are crucial for embryonic cell fate and bone formation. Wnt antagonism of Dkk requires the binding of the C-terminal cysteine-rich domain of Dkk to the Wnt coreceptor, LRP5/6. However, the structural basis of the interaction between Dkk and low density lipoprotein receptor-related protein (LRP) 5/6 is unknown. In this study, we examined the structure of the Dkk functional domain and elucidated its interactions with LRP5/6. Using NMR spectroscopy, we determined the solution structure of the C-terminal cysteine-rich domain of mouse Dkk2 (Dkk2C). Then, guided by mutagenesis studies, we docked Dkk2C to the YWTD beta-propeller domains of LRP5/6 and showed that the ligand binding site of the third LRP5/6 beta-propeller domain matches Dkk2C best, suggesting that this domain binds to Dkk2C with higher affinity. Such differential binding affinity is likely to play an essential role in Dkk function in the canonical Wnt pathway.

Myeloma-derived Dickkopf-1 disrupts Wnt-regulated osteoprotegerin and RANKL production by osteoblasts: a potential mechanism underlying osteolytic bone lesions in multiple myeloma

Multiple myeloma (MM) is characterized by osteolytic bone lesions (OBL) that arise as a consequence of osteoblast inactivation and osteoclast activation adjacent to tumor foci within bone. Wnt signaling in osteoblasts regulates osteoclastogenesis through the differential activation and inactivation of Receptor Activator of Nuclear factor Kappa B Ligand (RANKL) and osteoprotegerin (OPG), positive and negative regulators of osteoclast differentiation, respectively. We demonstrate here that MM cell-derived DKK1, a soluble inhibitor of canonical Wnt signaling, disrupted Wnt3a-regulated OPG and RANKL expression in osteoblasts. Confirmed in multiple independent assays, we show that pretreatment with rDKK1 completely abolished Wnt3a-induced OPG mRNA and protein production by mouse and human osteoblasts. In addition, we show that Wnt3a-induced OPG expression was diminished in osteoblasts cocultured with a DKK1-expressing MM cell line or primary MM cells. Finally, we show that bone marrow sera from 21 MM patients significantly suppressed Wnt3a-induced OPG expression and enhanced RANKL expression in osteoblasts in a DKK1-dependent manner. These results suggest that DKK1 may play a key role in the development of MM-associated OBL by directly interrupting Wnt-regulated differentiation of osteoblasts and indirectly increasing osteoclastogenesis via a DKK1-mediated increase in RANKL-to-OPG ratios.

DKK1 antagonizes Wnt signaling without promotion of LRP6 internalization and degradation

DKK1 is a secreted protein that antagonizes Wnt signaling and plays essential roles in vertebrate embryogenesis including head induction, skeletal development, and limb patterning. DKK1 is also implicated in osteoporosis, arthritis, and cancer and represents a potential therapeutic target for the treatment of these diseases. DKK1 is a high affinity antagonistic ligand for LRP6, which is a Wnt coreceptor that acts together with the Frizzled serpentine receptor to initiate Wnt signal transduction. Two different models have been proposed to account for the mechanism by which DKK1 antagonizes LRP6 function. One model suggests that DKK1 binding to LRP6 disrupts Wnt-induced Frizzled-LRP6 complex formation, whereas the other model proposes that DKK1 interaction with LRP6 promotes LRP6 internalization and degradation, thereby reducing the cell surface LRP6 level. To clarify the molecular basis of DKK1 action, we examined how DKK1 affects the endogenous LRP6 in several mammalian cell lines including mouse embryonic fibroblasts. Here we show that DKK1 inhibits Wnt signaling but induces neither LRP6 down-regulation from the cell surface nor reduction of total LRP6 protein level and that DKK1 has no effect on the rate of continuous internalization of LRP6 and the half-life (about 4.7 h) of LRP6. We conclude that DKK1 inhibition of LRP6 is independent of LRP6 internalization and degradation.

Regulation of epithelial branching morphogenesis and cancer cell growth of the prostate by Wnt signaling

Although Wnt signaling has been shown to be important for embryonic morphogenesis and cancer pathogenesis of several tissues, its role in prostatic development and tumorigenesis is not well understood. Here we show that Wnt signaling regulated prostatic epithelial branching morphogenesis and luminal epithelial cell differentiation in developing rat prostate organ cultures. Specifically, Wnt signaling regulated the proliferation of prostate epithelial progenitor cells. Assessment of the expression levels of a Wnt pathway transcriptional target gene, Axin2, showed that the Wnt pathway was activated in the developing prostate, but was down-regulated in the adult. Castration resulted in an upregulation of Axin2 whereas androgen replacement resulted in a down regulation of Axin2. Such dynamic changes of Wnt activity was also confirmed in a BAT-gal transgenic mouse line in which β-galactosidase reporter is expressed under the control of β-catenin/T cell factor responsive elements. Furthermore, we evaluated the role of Wnt signaling in prostate tumorigenesis. Axin2 expression was found upregulated in the majority of human prostate cancer cell lines examined. Moreover, addition of a Wnt pathway inhibitor, Dickkopf 1 (DKK1), into the culture medium significantly inhibited prostate cancer cell growth and migration. These findings suggest that Wnt signaling regulates prostatic epithelial ductal branching morphogenesis by influencing cell proliferation, and highlights a role for Wnt pathway activation in prostatic cancer progression.

Rac1 activation controls nuclear localization of beta-catenin during canonical Wnt signaling

Canonical Wnt signaling critically regulates cell fate and proliferation in development and disease. Nuclear localization of beta-catenin is indispensable for canonical Wnt signaling; however, the mechanisms governing beta-catenin nuclear localization are not well understood. Here we demonstrate that nuclear accumulation of beta-catenin in response to Wnt requires Rac1 activation. The role of Rac1 depends on phosphorylation of beta-catenin at Ser191 and Ser605, which is mediated by JNK2 kinase. Mutations of these residues significantly affect Wnt-induced beta-catenin nuclear accumulation. Genetic ablation of Rac1 in the mouse embryonic limb bud ectoderm disrupts canonical Wnt signaling and phenocopies deletion of beta-catenin in causing severe truncations of the limb. Finally, Rac1 interacts genetically with beta-catenin and Dkk1 in controlling limb outgrowth. Together these results uncover Rac1 activation and subsequent beta-catenin phosphorylation as a hitherto uncharacterized mechanism controlling canonical Wnt signaling and may provide additional targets for therapeutic intervention of this important pathway.

Wnt/Frizzled signaling in angiogenesis

The roles of growth factors such as angiopoietin (Ang) and vascular endothelial growth factor (VEGF) in angiogenesis have been known for some time, yet we have just an incipient appreciation for the contribution of Wnts to this process. Cellular proliferation and polarity, apoptosis, branching morphogenesis, inductive processes, and the maintenance of stem cells in an undifferentiated, proliferative state are all regulated by Wnt signaling. The development and maintenance of vascular structures are dependent on all these processes, and their orchestration has, to some extent, been revealed in studies of VEGF and Ang receptors. Recent evidence links the Wnt/Frizzled signaling pathway to proper vascular growth in mammals but our knowledge of Wnt function in the vasculature is rudimentary. Further insight into vascular development and the process of angiogenesis depends on evaluating the function of novel endothelial regulatory pathways such as Wnt/Frizzled signaling.

Inactivation of nuclear Wnt-beta-catenin signaling limits blastocyst competency for implantation

The activation of the blastocyst, a process by which it gains competency to attach with the receptive uterus, is a prerequisite for successful implantation. However, the molecular basis of blastocyst activation remains largely unexplored. Combining molecular, pharmacological and physiological approaches, we show here that silencing of Wnt-beta-catenin signaling in mice does not adversely affect the development of preimplantation embryos to blastocysts and uterine preparation for receptivity, but, remarkably, blocks blastocyst competency to implantation. Using the physiologically relevant delayed implantation model and trophoblast stem cells in culture, we further demonstrate that a coordinated activation of canonical Wnt-beta-catenin signaling with attenuation of the non-canonical Wnt-RhoA signaling pathway ensures blastocyst competency to implantation. These findings constitute novel evidence that Wnt signaling is at least one pathway that determines blastocyst competency for implantation.

R-spondin1 synergizes with Wnt3A in inducing osteoblast differentiation and osteoprotegerin expression

R-spondins are a new group of Wnt/beta-catenin signaling agonists, however, the role of these proteins in bone remains unclear. We reported herein that R-sponin1 (Rspo1) acted synergistically with Wnt3A to activate Wnt/beta-catenin signaling in the uncommitted mesenchymal C2C12 cells. Furthermore, we found that Rspo1 at concentrations as low as 10 ng/ml synergized strongly with Wnt3A to induce C2C12 osteoblastic differentiation and osteoprotegerin expression. These events were blocked by Wnt/beta-catenin signaling antagonist Dickkopf-1. Finally, we demonstrated that Rspo1 synergized with Wnt3A to induce primary mouse osteoblast differentiation. Together, these findings suggest that Rpos1 may play an important role in bone remodeling.

Initiation of Wnt signaling: control of Wnt coreceptor Lrp6 phosphorylation/activation via frizzled, dishevelled and axin functions

Canonical Wnt/beta-catenin signaling has central roles in development and diseases, and is initiated by the action of the frizzled (Fz) receptor, its coreceptor LDL receptor-related protein 6 (Lrp6), and the cytoplasmic dishevelled (Dvl) protein. The functional relationships among Fz, Lrp6 and Dvl have long been enigmatic. We demonstrated previously that Wnt-induced Lrp6 phosphorylation via glycogen synthase kinase 3 (Gsk3) initiates Wnt/beta-catenin signaling. Here we show that both Fz and Dvl functions are critical for Wnt-induced Lrp6 phosphorylation through Fz-Lrp6 interaction. We also show that axin, a key scaffolding protein in the Wnt pathway, is required for Lrp6 phosphorylation via its ability to recruit Gsk3, and inhibition of Gsk3 at the plasma membrane blocks Wnt/beta-catenin signaling. Our results suggest a model that upon Wnt-induced Fz-Lrp6 complex formation, Fz recruitment of Dvl in turn recruits the axin-Gsk3 complex, thereby promoting Lrp6 phosphorylation to initiate beta-catenin signaling. We discuss the dual roles of the axin-Gsk3 complex and signal amplification by Lrp6-axin interaction during Wnt/beta-catenin signaling.

Positional pathway screen of wnt signaling genes in schizophrenia: association with DKK4

BACKGROUND

Wnt signaling has been implicated in schizophrenia from studies of gene expression in patients, from an understanding of the function of reported susceptibility genes and from experimental studies of psychoactive drugs. This diverse evidence suggests that wnt signaling genes, defined as pathway participants, modifiers or targets, are good candidates as susceptibility factors.

METHODS

We performed a combined positional and candidate association screen by identifying known wnt signaling genes in regions linked to schizophrenia. In a staged study we examined over 50 single nucleotide polymorphisms (SNPs) in 28 wnt signaling genes, first in trios of Chinese origin and then in a case-control series from Hong Kong.

RESULTS

In both sets, Dickkopf 4 (DKK4) was associated with schizophrenia - with an odds ratio of 3.9 (p < .01, CI = 1.3-11.1) in the combined sample.

CONCLUSIONS

As DKK family members have previously been found to show altered expression in schizophrenia brain and to bind to neuregulin, this finding suggests that DKK4 may play a role in schizophrenia pathogenesis.

Dickkopf-1 mediated tumor suppression in human breast carcinoma cells

Dickkopf-1 (DKK-1) is a secreted inhibitor of the Wnt signaling pathway. We previously identified DKK-1 as a candidate tumor suppressor and demonstrated that ectopic expression of the DKK-1 suppressed the tumorigenicity of HeLa cells in vitro and in vivo. Since suppression of tumorigenicity of HeLa cells by DKK-1 overexpression was not mediated by effects on beta-catenin dependent transcription, we hypothesized that DKK-1 might also inhibit tumorigenicity of breast carcinoma cell lines lacking an activated canonical Wnt pathway. In the present study we show that ectopic expression of DKK-1 in various breast cancer cell lines resulted in a change in the cell phenotype, increased sensitivity to apoptosis, inhibition of anchorage independent growth in vitro, and suppression of tumorigenicity in vivo. Consistent with known effects of DKK-1 on the canonical Wnt signaling pathway, ectopic expression of DKK-1 in breast carcinoma cells was associated with increased phosphorylation and degradation of beta-catenin. However, none of the breast tumor cells used in this study showed detectable levels of beta-catenin dependent activation of TCF/Lef promoter activity measured by reporter constructs. Consistent with the results of these transient transfection assays, we were unable to demonstrate the expected beta-catenin dependent, TCF/Lef mediated inhibition of cyclin D1 and c-myc gene transcription in breast cells overexpressing DKK-1. However, we found that cells with DKK-1 overexpression have increased activity of CamKII pathway. Overexpression of the constitutively active form of CamKII (T286D) resulted in inhibition of breast cancer cell tumorigenicity. Thus, our study supports the hypothesis that DKK-1 mediated tumor suppressor effect is independent of beta-catenin dependent transcription and identified the CamKII pathway that contributes into DKK-1 signaling.

Kremen is required for neural crest induction in Xenopus and promotes LRP6-mediated Wnt signaling

Kremen 1 and 2 (Krm1/2) are transmembrane receptors for Wnt antagonists of the Dickkopf (Dkk) family and function by inhibiting the Wnt co-receptors LRP5/6. Here we show that Krm2 functions independently from Dkks during neural crest (NC) induction in Xenopus. Krm2 is co-expressed with, and regulated by, canonical Wnts. Krm2 is differentially expressed in the NC, and morpholino-mediated Krm2 knockdown inhibits NC induction, which is mimicked by LRP6 depletion. Conversely, krm2 overexpression induces ectopic NC. Kremens bind to LRP6, promote its cell-surface localization and stimulate LRP6 signaling. Furthermore, Krm2 knockdown specifically reduces LRP6 protein levels in NC explants. The results indicate that in the absence of Dkks, Kremens activate Wnt/beta-catenin signaling through LRP6.

Wnt10b increases postnatal bone formation by enhancing osteoblast differentiation

Overexpression of Wnt10b from the osteocalcin promoter in transgenic mice increases postnatal bone mass. Increases in osteoblast perimeter, mineralizing surface, and bone formation rate without detectable changes in pre-osteoblast proliferation, osteoblast apoptosis, or osteoclast number and activity suggest that, in this animal model, Wnt10b primarily increases bone mass by stimulating osteoblastogenesis.

INTRODUCTION

Wnt signaling regulates many aspects of development including postnatal accrual of bone. Potential mechanisms for how Wnt signaling increases bone mass include regulation of osteoblast and/or osteoclast number and activity. To help differentiate between these possibilities, we studied mice in which Wnt10b is expressed specifically in osteoblast lineage cells or in mice devoid of Wnt10b.

MATERIALS AND METHODS

Transgenic mice, in which mouse Wnt10b is expressed from the human osteocalcin promoter (Oc-Wnt10b), were generated in C57BL/6 mice. Transgene expression was evaluated by RNase protection assay. Quantitative assessment of bone variables was done by radiography, muCT, and static and dynamic histomorphometry. Mechanisms of bone homeostasis were evaluated with assays for BrdU, TUNEL, and TRACP5b activity, as well as serum levels of C-terminal telopeptide of type I collagen (CTX). The endogenous role of Wnt10b in bone was assessed by dynamic histomorphometry in Wnt10b(-/-) mice.

RESULTS

Oc-Wnt10b mice have increased mandibular bone and impaired eruption of incisors during postnatal development. Analyses of femoral distal metaphyses show significantly higher BMD, bone volume fraction, and trabecular number. Increased bone formation is caused by increases in number of osteoblasts per bone surface, rate of mineral apposition, and percent mineralizing surface. Although number of osteoclasts per bone surface is not altered, Oc-Wnt10b mice have increased total osteoclast activity because of higher bone mass. In Wnt10b(-/-) mice, changes in mineralizing variables and osteoblast perimeter in femoral distal metaphyses were not observed; however, bone formation rate is reduced because of decreased total bone volume and trabecular number.

CONCLUSIONS

High bone mass in Oc-Wnt10b mice is primarily caused by increased osteoblastogenesis, with a minor contribution from elevated mineralizing activity of osteoblasts.

Preferential hypermethylation of the Dickkopf-1 promoter in core-binding factor leukaemia

The Dickkopf-1 (DKK1) gene product is an extracellular Wnt inhibitor. Hypermethylation of the DKK1 promoter results in transcriptional silencing and may play an important role in cancer development. Here, we investigated hypermethylation of the DKK1 promoter in patients with acute myeloid leukaemia (AML), especially core-binding factor (CBF) leukaemia. The methylation status of DKK1 was analysed using methylation-specific polymerase chain reaction in 47 patients with AML. DKK1 methylation was found in 14 (29.8%) patients, and more frequently in those with CBF leukaemia (6 of 12 patients), than in those with acute promyelocytic leukaemia (APL) (0 of 6 patients) (P = 0.03). In contrast, Wnt inhibitory factor-1 methylation was found in APL (4 of 6 patients) but not in CBF leukaemia (0 of 12 patients) (P = 0.001). Multivariate analyses suggested that DKK1 methylation was a risk factor for poorer overall survival. Sequential analysis using four paired samples obtained at diagnosis and relapse suggested that DKK1 methylation was involved in the progression of leukaemia. Therefore, DKK1 methylation may be involved in leukaemogenesis, especially in CBF leukaemia, and may be a useful prognostic marker in AML.

Differential activation of the calcium/protein kinase C and the canonical beta-catenin pathway by Wnt1 and Wnt7a produces opposite effects on cell proliferation in PC12 cells

We examined the effect of Wnt1 and Wnt7a on cell proliferation using undifferentiated PC12 cells, which originate from the neural crest and are widely employed as a neuronal cell model. Heterologous expression of Wnt1 enhanced [3H]thymidine incorporation and expression of cyclin D1 and cylin E in PC12 cells. Opposite effects were observed in PC12 cells expressing Wnt7a. Searching for the mechanisms underlying the opposite effects of Wnt1 and Wnt7a on PC12 cell proliferation, we examined the activation of the canonical beta-catenin/T-cell-lymphoid enhancer-binding protein transcription factor pathway and the 'calcium pathway' by co-transfecting the cells with a reporter gene controlled by either T-cell-lymphoid enhancer-binding protein transcription factor or the calcium-activated transcription factor, NFAT. Wnt1 and Wnt7a activated both pathways, but to a different extent. While Wnt1 preferentially activated the calcium pathway, Wnt7a mainly activated the canonical pathway. Pharmacological inhibition of protein kinase C, which is a component of the calcium pathway, abrogated the increase in cell proliferation induced by Wnt1 without affecting the antiproliferative action of Wnt7a. The action of Wnt7a was instead occluded by lithium ions, which mimic the activation of the canonical pathway, and was largely reduced by Dickkopf-1, which acts as an inhibitor of the canonical pathway. In addition, expression of a constitutively active mutant of beta-catenin potently activated the canonical Wnt pathway and reduced [3H]thymidine incorporation. These data challenge the view that the canonical Wnt pathway invariably supports cell growth and suggest that, at least in PC12 cells, cell proliferation is regulated by the balance between the calcium/protein kinase C pathway and the canonical pathway.

Essential role of Wnt3a-mediated activation of mitogen-activated protein kinase p38 for the stimulation of alkaline phosphatase activity and matrix mineralization in C3H10T1/2 mesenchymal cells

Signaling pathways involved in the development of osteoprogenitors induced by Wnts remain poorly understood. In this study, we investigated the role of MAPKs in the development of mesenchymal cells into osteoprogenitors. In C3H10T1/2 mesenchymal cells, Wnt3a induced a rapid and transient activation of MAPKs p38 and ERK. Dickkopf 1, a selective antagonist of Wnt proteins binding to low-density lipoprotein-receptor-related protein-5/6 did not influence activation of p38 and ERK induced by Wnt3a. A MAPK kinase-1/2 (MEK1/2) inhibitor blocked, whereas a p38 inhibitor had no effect on, Wnt3a-induced cell proliferation. In contrast, both inhibitors significantly reduced alkaline phosphatase stimulation with a more pronounced effect of the p38 inhibitor. The p38 inhibitor also blunted nodule mineralization induced by Wnt3a. Associated with these effects, beta-catenin transcriptional activity, assessed with the TOPflash system, was dose-dependently decreased by the p38 but not by the ERK inhibitor. Both the reduced alkaline phosphatase stimulation and blunting of beta-catenin transcriptional activity were mimicked by expression of dominant-negative (dn) p38 and dnMEK 3/6. Inhibition of beta-catenin transcriptional activity by the p38 inhibitor as well as by dnp38 and dnMEK 3/6 molecules were not associated with changes in cytosolic and nuclear beta-catenin levels induced by Wnt3a. In conclusion, Wnt3a activates ERK and p38 in mesenchymal C3H10T1/2 cells by a low-density lipoprotein-receptor-related protein-5/6-independent mechanism. Activation of p38 regulates alkaline phosphatase activity and nodule mineralization induced by Wnt3a probably by interacting with beta-catenin transcriptional activity. These observations suggest that MAPKs ERK and p38 are probably essential pathways activated by Wnt proteins for the development of mesenchymal cells into osteoprogenitors.

Proliferation and migration mediated by Dkk-1/Wnt/beta-catenin cascade in a model of hepatocellular carcinoma cells

Beta-catenin is a multifunctional protein acting as a key factor in the cadherin-mediated cell-cell adhesion system and in the Wnt signaling pathway. To demonstrate the molecular mechanisms of metastasis of hepatocellular carcinoma (HCC) cells, we established a metastatic subclone of human HCC H7402 cells, termed M-H7402, by isolating from transplantation of H7402 cells into severe combined immunodeficient (SCID) mice. Based on the 2 parallel cell lines, we investigated the roles of dickkopf-1 (Dkk-1) and Wnt/beta-catenin pathway in proliferation and migration of HCC cells. cDNA microarray showed that 24 genes were related to tumor metastasis differentially expressed between H7402 and M-H7402 cells. Western blot analysis revealed that the expression levels of beta-catenin, c-Myc, and cyclin D1 were upregulated, but Dkk-1 and nm23 were dramatically downregulated in M-H7402 cells, which suggests that the 2 cell lines were remarkably different in molecular events associated with metastasis. Furthermore, we found that overexpression of Dkk-1 by transfection was able to downregulate the expression of c-Myc and cyclin D1, and it also inhibited the growth and migration in M-H7402 cells. Although reduction of Dkk-1 expression by RNA interference was able to upregulate the expression of beta-catenin, c-Myc, and cyclin D1 in H7402 cells, it also promoted beta-catenin translocation from cytoplasm into nuclei and increased the migration of the cells. Therefore, we conclude that Dkk-1/Wnt/beta-catenin cascade may mediate the proliferation and migration of HCC cells during the metastasis process.

Joint remodelling in inflammatory disease

Bone and the immune system share multiple interactions. The skeleton harbours the bone marrow and provides the niche for development of haematopoietic cells including the immune system. The immune system provides cells as well as molecular signals, which regulate bone homeostasis. Understanding the cellular and molecular regulation of the tight interaction between bone and the immune system is crucial for understanding the changes of skeletal architecture during inflammation. Whereas a short and self-limited activation of the immune system has no clinically meaningful effect on bone, prolonged immune activation as found in chronic inflammatory disease inevitably leads to bone wasting.

Recombinant adenovirus as a methodology for exploration of physiologic functions of growth factor pathways

The use of recombinant adenoviruses (Ad) to express secreted antagonists of growth factors represents a powerful strategy for studying physiologic functions of growth factor pathways in experimental animals. Indeed, a single adenoviral injection can produce characteristic high-level and persistent plasma expression of soluble receptor ectodomains or secreted protein antagonists, allowing highly stringent conditional inactivation of target pathways in vivo. In this review, we describe our experience using recombinant Ad to inactivate growth factor pathways in vivo and discuss their advantages and limitations. Using our studies on vascular endothelial growth factor and Wnt systems as examples, we further describe how recombinant Ad can unveil previously unknown physiological roles of signaling pathways. Finally, we discuss the potential physiological and therapeutic relevance of our findings.

Laser capture-microarray analysis of Lim1 mutant kidney development

The Lim1 gene has essential functions during several stages of kidney development. In particular, a tissue-specific knockout in the early metanephric mesenchyme results in the formation of the earliest nephron precursor, the renal vesicle, but failure of this structure to progress to the next stage, the comma-shaped body. To better understand the molecular nature of this developmental arrest, we used a laser capture microdissection-microarray strategy to examine the perturbed gene expression pattern of the mutant renal vesicles. Among the genes found differently expressed were Chrdl2, an inhibitor of BMP signaling, the proapoptotic factor Bmf, as well as myob5, an atypical myosin that modulates chemokine signaling, and pdgfrl, which is important in epithelial folding. Of particular interest, the microarray data indicated that the Dkk1 gene, which encodes an inhibitor of Wnt signaling, was downregulated ninefold in mutants. This was confirmed by in situ hybridizations. It is interesting to note that Lim1 and Dkk1 mutant mice have striking similarities in phenoytpe. These results suggest that the Dkk1 gene might be a key downstream effector of Lim1 function.

Bone mass is inversely proportional to Dkk1 levels in mice

The Wnt/beta-catenin signaling pathway has emerged as a key regulator in bone development and bone homeostasis. Loss-of-function mutations in the Wnt co-receptor LRP5 result in osteoporosis and "activating" mutations in LRP5 result in high bone mass. Dickkopf-1 (DKK1) is a secreted Wnt inhibitor that binds LRP5 and LRP6 during embryonic development, therefore it is expected that a decrease in DKK1 will result in an increase in Wnt activity and a high bone mass phenotype. Dkk1-/- knockout mice are embryonic lethal, but mice with hypomorphic Dkk1d (doubleridge) alleles that express low amounts of Dkk1 are viable. In this study we generated an allelic series by crossing Dkk1+/- and Dkk1+/d mice resulting in the following genotypes with decreasing Dkk1 expression levels: +/+, +/d, +/- and d/-. Using muCT imaging we scanned dissected left femora and calvariae from 8-week-old mice (n=60). We analyzed the distal femur to represent trabecular bone and the femur diaphysis for cortical endochondral bone. A region of the parietal bones was used to analyze intramembranous bone of the calvaria. We found that trabecular bone volume is increased in Dkk1 mutant mice in a manner that is inversely proportional to the level of Dkk1 expression. Trabeculae number and thickness were significantly higher in the low Dkk1 expressing genotypes from both female and male mice. Similar results were found in cortical bone with an increase in cortical thickness and cross sectional area of the femur diaphysis that correlated with lower Dkk1 expression. No consistent differences were found in the calvaria measurements. Our results indicate that the progressive Dkk1 reduction increases trabecular and cortical bone mass and that even a 25% reduction in Dkk1 expression could produce significant increases in trabecular bone volume fraction. Thus DKK1 is a negative regulator of normal bone homeostasis in vivo. Our study suggests that manipulation of DKK1 function or expression may have therapeutic significance for the treatment of low bone mass disorders.

PGAP1 knock-out mice show otocephaly and male infertility

A palmitate linked to the inositol in glycosylphosphatidylinositol (GPI) is removed in the endoplasmic reticulum immediately after the conjugation of GPI with proteins in most cells. Previously, we identified PGAP1 (post GPI attachment to proteins 1) as a GPI inositoldeacylase that removes the palmitate from inositol. A defect in PGAP1 caused a delay in the transport of GPI-anchored proteins (GPI-APs) from the endoplasmic reticulum to the cell surface in Chinese hamster ovary cells, although the cell-surface expression of GPI-APs in the steady state was normal. Nevertheless, in most cells, GPI-APs undergo deacylation. To elucidate the biological significance of PGAP1 in vivo, we established PGAP1 knock-out mice. Most PGAP1 knock-out mice showed otocephaly, a developmental defect, and died right after birth. However, some survived with growth retardation. Male knock-out mice showed severely reduced fertility despite the capability of ejaculation. Their spermatozoa were normal in number, motility, and ability to ascend the uterus, but were unable to go into the oviduct. In vitro, PGAP1-deficient spermatozoa showed weak attachment to the zona pellucida and a severely diminished rate of fertilization. Therefore, an extra acyl chain in GPI anchors caused severe deleterious effects to development and sperm function.

Low-density lipoprotein receptor-related protein 5 (LRP5) mediates the prostate cancer-induced formation of new bone

The tendency of prostate cancer to produce osteoblastic bone metastases suggests that cancer cells and osteoblasts interact in ways that contribute to cancer progression. To identify factors that mediate these interactions, we compared gene expression patterns between two bone-derived prostate cancer cell lines that produce osteoblastic (MDA PCa 2b) or osteolytic lesions (PC-3). Both cell lines expressed Wnt ligands, including WNT7b, a canonical Wnt implicated in osteogenesis. PC-3 cells expressed 50 times more Dickkopf-1 (DKK1), an inhibitor of Wnt pathways, than did MDA PCa 2b cells. Evaluation of the functional role of these factors (in cocultures of prostate cancer cells with primary mouse osteoblasts (PMOs) or in bone organ cultures) showed that MDA PCa 2b cells activated Wnt canonical signaling in PMOs and that DKK1 blocked osteoblast proliferation and new bone formation induced by MDA PCa 2b cells. MDA PCa 2b cells did not induce bone formation in calvaria from mice lacking the Wnt co-receptor Lrp5. In human specimens, WNT7b was not expressed in normal prostate but was expressed in areas of high-grade prostate intraepithelial neoplasia, in three of nine primary prostate tumor specimens and in 16 of 38 samples of bone metastases from prostate cancer. DKK1 was not expressed in normal or cancerous tissue but was expressed in two of three specimens of osteolytic bone metastases (P=0.0119). We conclude that MDA PCa 2b induces new bone formation through Wnt canonical signaling, that LRP5 mediates this effect, and that DKK1 is involved in the balance between bone formation and resorption that determines lesion phenotype.

Analysis of endogenous LRP6 function reveals a novel feedback mechanism by which Wnt negatively regulates its receptor

The canonical Wnt pathway plays a crucial role in embryonic development, and its deregulation is involved in human diseases. The LRP6 single-span transmembrane coreceptor is essential for transmission of canonical Wnt signaling. However, due to the lack of immunological reagents, our understanding of LRP6 structure and function has relied on studies involving its overexpression, and regulation of the endogenous receptor by the Wnt ligand has remained unexplored. Using a highly sensitive and specific antibody to LRP6, we demonstrate that the endogenous receptor is modified by N-glycosylation and is phosphorylated in response to Wnt stimulation in a sustained yet ligand-dependent manner. Moreover, following triggering by Wnt, endogenous LRP6 is internalized and recycled back to the cellular membrane within hours of the initial stimulus. Finally, we have identified a novel feedback mechanism by which Wnt, acting through beta-catenin, negatively regulates LRP6 at the mRNA level. Together, these findings contribute significantly to our understanding of LRP6 function and uncover a new level of regulation of Wnt signaling. In light of the direct role that the Wnt pathway plays in human bone diseases and malignancies, our findings may support the development of novel therapeutic approaches that target Wnt signaling through LRP6.

Beta-catenin signaling promotes proliferation of progenitor cells in the adult mouse subventricular zone

The subventricular zone (SVZ) is the largest germinal zone in the mature rodent brain, and it continuously produces young neurons that migrate to the olfactory bulb. Neural stem cells in this region generate migratory neuroblasts via highly proliferative transit-amplifying cells. The Wnt/beta-catenin signaling pathway partially regulates the proliferation and neuronal differentiation of neural progenitor cells in the embryonic brain. Here, we studied the role of beta-catenin signaling in the adult mouse SVZ. beta-Catenin-dependent expression of a destabilized form of green fluorescent protein was detected in progenitor cells in the adult SVZ of Axin2-d2EGFP reporter mice. Retrovirus-mediated expression of a stabilized beta-catenin promoted the proliferation of Mash1+ cells and inhibited their differentiation into neuroblasts. Conversely, the expression of Dkk1, an inhibitor of Wnt signaling, reduced the proliferation of Mash1+ cells. In addition, an inhibitor of GSK3 beta promoted the proliferation of Mash1+ cells and increased the number of new neurons in the olfactory bulb 14 days later. These results suggest that beta-catenin signaling plays a role in the proliferation of progenitor cells in the SVZ of the adult mouse brain.

Dihydrotestosterone-inducible dickkopf 1 from balding dermal papilla cells causes apoptosis in follicular keratinocytes

Recent studies suggest that androgen-driven alteration to the autocrine and paracrine factors produced by scalp dermal papilla (DP) cells may be a key to androgen-potentiated balding. Here, we screened dihydrotestosterone (DHT)-regulated genes in balding DP cells and found that dickkopf 1 (DKK-1) is one of the most upregulated genes. DKK-1 messenger RNA is upregulated in 3-6 hours after 50-100 nM DHT treatment and ELISA showed that DKK-1 is secreted from DP cells in response to DHT. A co-culture system using outer root sheath (ORS) keratinocytes and DP cells showed that DHT inhibits the growth of ORS cells, and neutralizing antibody against DKK-1 significantly reversed the growth inhibition of ORS cells. Analysis of co-cultured ORS cells showed a significant increment of sub-G1 apoptotic cells in response to DHT. Also, recombinant human DKK-1 inhibited the growth of ORS cells and triggered apoptotic cell death. In addition, DHT-induced epithelial cell death in cultured hair follicles was reversed by neutralizing DKK-1 antibody. Moreover, immunoblotting showed that the DKK-1 level is up in the bald scalp compared with the haired scalp of patients with androgenetic alopecia. Altogether, our data strongly suggest that DHT-inducible DKK-1 is involved in DHT-driven balding.

Canonical WNT signalling controls hair follicle spacing

Canonical WNT signals play an important role in hair follicle development. In addition to being crucial for epidermal appendage initiation, they control the interfollicular spacing pattern and contribute to the spatial orientation and largely parallel alignment of hair follicles. However, owing to the complexity of canonical WNT signalling and its interconnections with other pathways, many details of hair follicle formation await further clarification. Here, we discuss the recently suggested reaction-diffusion (RD) mechanism of spatial hair follicle arrangement in the light of yet unpublished data and conclusions. They clearly demonstrate that the observed hair follicle clustering in dickkopf (DKK) transgenic mice cannot be explained by any trivial process caused by protein overexpression, thereby further supporting our model of hair follicle spacing. Furthermore, we suggest future experiments to challenge the RD model of spatial follicle arrangement.

The Wnt signaling pathway and bone metabolism

PURPOSE OF REVIEW

The Wnt signaling pathway has been a major focus of effort in the bone field for the past 5 years. This review will examine some of the seminal findings that have brought us to our current understanding of the role of this pathway in bone metabolism.

RECENT FINDINGS

The Wnt/beta-catenin signaling pathway has been shown to play a major role in bone cell differentiation, proliferation and apoptosis. It is a critical component of bone mass regulation and required for bone to respond to mechanical loading. The pathway is tightly regulated by a number of modulator proteins. Mutations in pathway components that result in aberrant regulation are involved in a number of bone diseases.

SUMMARY

Understanding the role that the Wnt signaling pathways play in the regulation of bone metabolism offers great promise for the developmental of new paradigms and pharmaceutical strategies for the treatment of various diseases such as osteoporosis, rheumatoid arthritis and osteoarthritis. Progress in this regard has already been made.

WNTs in the ovine uterus: potential regulation of periimplantation ovine conceptus development

WNTs (Wingless-type MMTV integration site family member) are involved in critical developmental and growth processes in animals. These studies investigated WNT pathways in the ovine uterus and conceptus during the periimplantation period of pregnancy. WNT2 and WNT2B mRNAs were detected in endometrial stroma. WNT5A and WNT5B mRNAs were most abundant in the stroma and less so in the luminal epithelium, whereas WNT11 mRNA was detected primarily in the glands. WNT7A mRNA was present in the luminal epithelium on d 10, absent on d 12 and 14, and increased between d 16 and 20. Only WNT2, WNT2B, and WNT4 were detected in conceptus trophectoderm. FZD6/8 (frizzled receptor) and GSK3B (glycogen synthase kinase 3beta) mRNAs were detected primarily in endometrial epithelia and conceptus trophectoderm, whereas the LRP5/6 (low-density lipoprotein receptor-related proteins 5 and 6) coreceptor was present in all endometrial cells and the trophectoderm. DKK1 (Dickkopf), a WNT signaling inhibitor, increased in the endometrium from d 16-20. CTNNB1 [catenin (cadherin associated protein) beta1] and CDH1 (E-cadherin) mRNAs were most abundant in the endometrial epithelia and trophectoderm. LEF1 (lymphoid enhancer-binding factor 1) mRNA was expressed primarily in uterine epithelia, whereas TCF7L2 [(transcription factor 7-like 2 (T-cell specific, HMG-box)] was primarily in the conceptus. CTNNB1 and TCF7L2 proteins were both abundant in the nuclei of trophoblast giant binucleate cells. WNT7A stimulated a TCF/LEF-luciferase reporter activity in ovine trophectoderm cells that was inhibited by dominant-negative TCF and Sfrp2 (secreted FZD-related protein 2). WNT7A increased trophectoderm cell proliferation as well as MSX2 (msh homeobox 2) and MYC (myelocytomatosis oncogene) mRNA levels. Wnt5a increased trophectoderm cell migration in a Rho kinase-dependent manner. These results support the hypotheses that canonical and noncanonical WNT signaling pathways are conserved regulators of conceptus-endometrial interactions in mammals and regulate periimplantation ovine conceptus development.

Wnt signaling and the regulation of bone mass

Human genetic studies have firmly established a link between bone mass in humans and gain-of-function or loss-of-function mutations in a Wnt coreceptor, low-density lipoprotein receptor-related protein 5 (LRP5), or in the Wnt antagonist sclerostin, and several molecular genetic studies in mice have consistently confirmed the critical importance of the Wnt signaling pathway in skeletal biology and disease. In what may be a novel paradigm, the ubiquitous nature of LRP5/6 and Wnt signaling is counterbalanced by the bone-restricted and regulated expression of Wnt antagonists such as sclerostin and Dickkopf-1 (Dkk1) in adult tissues, offering new and potentially safe therapeutic means of intervention to stimulate bone formation.

Canonical WNT signaling during kidney development

The canonical WNT signaling pathway plays a crucial role in patterning of the embryo during development, but little is known about the specific developmental events which are under WNT control. To understand more about how the WNT pathway orchestrates mammalian organogenesis, we studied the canonical beta-catenin-mediated WNT signaling pathway in kidneys of mice bearing a beta-catenin-responsive TCF/betaGal reporter transgene. In metanephric kidney, intense canonical WNT signaling was evident in epithelia of the branching ureteric bud and in nephrogenic mesenchyme during its transition into renal tubules. WNT signaling activity is rapidly downregulated in maturing nephrons and becomes undetectable in postnatal kidney. Sites of TCF/betaGal activity are in proximity to the known sites of renal WNT2b and WNT4 expression, and these WNTs stimulate TCF reporter activity in kidney cell lines derived from ureteric bud and metanephric mesenchyme lineages. When fetal kidney explants from HoxB7/GFP mice were exposed to the canonical WNT signaling pathway inhibitor, Dickkopf-1, arborization of the ureteric bud was significantly reduced. We conclude that restricted zones of intense canonical WNT signaling drive branching nephrogenesis in fetal kidney.

The activation of beta-catenin by Wnt signaling mediates the effects of histone deacetylase inhibitors

Most colorectal carcinomas (CRCs) exhibit constitutively active Wnt signaling. We have reported that (a) the histone deacetylase inhibitor (HDACi)(2) sodium butyrate (NaB) modulates the canonical Wnt transcriptional activity of CRC cells in vitro and (b) a linear relationship exists between the increase in Wnt transcriptional activity and the levels of apoptosis in ten CRC cell lines treated with NaB. Herein we report that structurally different HDACis modulate Wnt signaling in CRC cells and a mechanism involved in this action is an increase in beta-catenin that is dephosphorylated at Ser-37 and Thr-41 residues. The increase of active (Ser-37 and Thr-41 dephosphorylated) beta-catenin in CRC cells treated with HDACis is initiated at the ligand level and the inhibition of this increase suppresses Wnt signaling and lowers the levels of apoptosis. CRC cells that develop resistance to the apoptotic effects of HDACis exhibit lower levels of active beta-catenin compared to apoptosis-sensitive parental cells and this resistance is reversed by increasing the levels of active beta-catenin. Results from comparative studies between HDACi-resistant and HDACi-sensitive cells suggest that non-histone targets of HDACis mediate the effects on Wnt signaling and apoptosis.

How does joint remodeling work?: new insights in the molecular regulation of the architecture of joints

Remodeling of joints is a key feature of inflammatory and degenerative joint disease. Bone erosion, cartilage degeneration and growth of bony spurs termed osteophytes are key features of structural joint pathology in the course of arthritis, which lead to impairment of joint function. Understanding their molecular mechanisms is essential to tailor targeted therapeutic approaches to protect joint architecture from inflammatory and mechanical stress. This addendum summarizes the new insights in the molecular regulation of bone formation in the joint and its relation to bone resorption. It describes how inflammatory cytokines impair bone formation and block the repair response of joints towards inflammatory stimuli. It particularly points out the key role of Dickkopf-1 protein, a regulator of the Wingless signaling and inhibitor of bone formation. This new link between inflammation and bone formation is also crucial for explaining the generation of osteophytes, bony spurs along joints, which are characterized by new bone and cartilage formation. This mechanism is largely dependent on an activation of wingless protein signaling and can lead to complete joint fusion. This addendum summarized the current concepts of joint remodeling in the limelight of these new findings.

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

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

The role of Wnts in bone metastases

Wnts are a large family of secreted glycoproteins that mediate bone development in the embryo and promote bone production in the adult. Autocrine Wnt signaling within tumor cells has been shown to promote tumorigenesis by enhancing tumor cell proliferation and survival. We recently demonstrated that prostate cancer cells (CaP) produce Wnts which act in a paracrine fashion to induce osteoblastic activity in CaP bone metastases. The ability of tumor-derived Wnts to influence bone development is regulated by multiple families of secreted antagonists including soluble frizzled related receptors (sFrp) and dickkopfs (DKK). CaP cells appear to produce DKK-1 early in the development of skeletal metastases, which masks osteogenic Wnts and thus favors an osteolytic environment at the metastatic site. As the metastases progresses, DKK-1 expression is lost allowing for a Wnt mediated osteoblastic response which predominates CaP boney lesions. Interestingly, blocking DKK-1 expression early in CaP metastasis prevents tumor establishment within the bone suggesting that osteolysis is a required first step in the development of CaP bone metastases. In this review, we discuss our data on the Wnt inhibitor DKK-1 in CaP bone metastasis in the context of current literature evidence that demonstrate that Wnt inhibitors can function as both tumor suppressors and tumor promoters. We provide a model that the affect of Wnt inhibitors on tumor development is dependent on the tumor micro-environment and suggest that DKK-1 is a switch which transitions CaP bone metastases from osteolytic to osteoblastic.

Folate deficiency in normal human fibroblasts leads to altered expression of genes primarily linked to cell signaling, the cytoskeleton and extracellular matrix

The molecular basis linking folate deficiency to certain health conditions and developmental defects is not fully understood. We examined the consequences of folate deficiency on global gene expression by microarray and compared transcript levels in normal human fibroblast cells (GM03349) grown in folate-deficient and -sufficient medium. The largest represented groups from the selected genes functioned in cell signaling, the cytoskeleton and the extracellular matrix and included the Wnt pathway genes DKK1, WISP1 and WNT5A. Twelve selected genes were further validated by qRT-PCR. Analysis of six genes at 4, 7, 10 and 14 days indicated that the relative differences in transcript levels between folate-sufficient and -deficient cells increases with time. Transcripts for 7 of the 12 selected genes were detected in the human lymphoblast cell line GM02257, and of these, changes in 4 genes corresponded to the results with fibroblast cells. Fibroblast cells were treated with the compounds homocysteine, methotrexate and the MEK1/2 inhibitor U0126, and relative transcript levels of six genes were determined. U0126 caused changes that more closely mimicked those detected in folate-deficient cells. The response of the DKK1 and TAGLN gene promoters to folate deficiency and compounds was examined in NIH3T3 cells using luciferase reporter plasmids. Promoter activity for both genes was decreased by folate deficiency and methotrexate and unaffected by homocysteine. U0126 caused a decrease in DKK1 promoter activity at 50 microM and had no effect on TAGLN promoter activity. These findings suggest an alternative mechanism for how folate deficiency leads to changes in gene expression and altered cell function.

EGF receptor is involved in WNT3a-mediated proliferation and motility of NIH3T3 cells via ERK pathway activation

WNT3a stimulates proliferation of NIH3T3 cells via activation of the extracellular signal-regulated kinase (ERK) pathway. The RAF-1-->MEK-->ERK cascade was immediately increased by WNT3a treatment, however, the upstream event triggering ERK pathway activation by WNT3a is not clear. WNT3a activated RAS and WNT3a-induced ERK activation was blocked by dominant-negative RAS, indicating that WNT3a might act upstream of RAS. WNT3a-induced ERK pathway activations were blocked by AG1478, the epidermal growth factor receptor (EGFR) inhibitor, and EGFR siRNA. The WNT3a-induced ERK pathway activation was not observed in fibroblasts retaining defective EGFR, but the WNT3a effect was restored by EGFR reconstitution. These results indicate involvement of EGFR in the WNT3a-induced ERK pathway activation. WNT3a-induced motility and cytoskeletal rearrangement as well as proliferation of NIH3T3 cells were blocked by AG1478 and EGFR siRNA or abolished in EGFR knock-out fibroblasts, indicating involvement of EGFR in those cellular processes. WNT3a-induced ERK pathway activation was not affected by Dickkoff-1 (DKK-1), although WNT3a-induced activations of the WNT/beta-catenin pathway and proliferation were reduced by DKK-1. EGFR is involved in WNT3a-induced proliferation via both routes dependent on and independent of the WNT/beta-catenin pathway. These results indicate that WNT3a stimulates proliferation and motility of NIH3T3 fibroblasts via EGFR-mediated ERK pathway activation.

Wnt signaling induces differentiation of progenitor cells in organotypic keratinocyte cultures

Background

Interfollicular skin develops normally only when the activity of the progenitor cells in the basal layer is counterbalanced by the exit of cells into the suprabasal layers, where they differentiate and cornify to establish barrier function. Distinct stem and progenitor compartments have been demonstrated in hair follicles and sebaceous glands, but there are few data to describe the control of interfollicular progenitor cell activity. Wnt signaling has been shown to be an important growth-inducer of stem cell compartments in skin and many other tissues.

Results

Here, we test the effect of ectopic Wnt1 expression on the behavior of interfollicular progenitor cells in an organotypic culture model, and find that Wnt1 signaling inhibits their growth and promotes terminal differentiation.

Conclusion

These results are consistent with the phenotypes reported for transgenic mice engineered to have gain or loss of function of Wnt signaling in skin, which would recommend our culture model as an accurate one for molecular analysis. Since it is known that canonical ligands are expressed in skin, it is likely that this pathway normally regulates the balance of growth and differentiation, and suggests it could be important to pathogenesis.

Noncanonical Wnt signaling through G protein-linked PKCdelta activation promotes bone formation

Wnt signaling regulates a variety of developmental processes in animals. Although the beta-catenin-dependent (canonical) pathway is known to control cell fate, a similar role for noncanonical Wnt signaling has not been established in mammals. Moreover, the intracellular cascades for noncanonical Wnt signaling remain to be elucidated. Here, we delineate a pathway in which Wnt3a signals through the Galpha(q/11) subunits of G proteins to activate phosphatidylinositol signaling and PKCdelta in the murine ST2 cells. Galpha(q/11)-PKCdelta signaling is required for Wnt3a-induced osteoblastogenesis in these cells, and PKCdelta homozygous mutant mice exhibit a deficit in embryonic bone formation. Furthermore, Wnt7b, expressed by osteogenic cells in vivo, induces osteoblast differentiation in vitro via the PKCdelta-mediated pathway; ablation of Wnt7b in skeletal progenitors results in less bone in the mouse embryo. Together, these results reveal a Wnt-dependent osteogenic mechanism, and they provide a potential target pathway for designing therapeutics to promote bone formation.

Dickkopf-1 is a master regulator of joint remodeling

Degenerative and inflammatory joint diseases lead to a destruction of the joint architecture. Whereas degenerative osteoarthritis results in the formation of new bone, rheumatoid arthritis leads to bone resorption. The molecular basis of these different patterns of joint disease is unknown. By inhibiting Dickkopf-1 (DKK-1), a regulatory molecule of the Wnt pathway, we were able to reverse the bone-destructive pattern of a mouse model of rheumatoid arthritis to the bone-forming pattern of osteoarthritis. In this way, no overall bone erosion resulted, although bony nodules, so-called osteophytes, did form. We identified tumor necrosis factor-alpha (TNF) as a key inducer of DKK-1 in the mouse inflammatory arthritis model and in human rheumatoid arthritis. These results suggest that the Wnt pathway is a key regulator of joint remodeling.

Function and biological roles of the Dickkopf family of Wnt modulators

Dickkopf (Dkk) genes comprise an evolutionary conserved small gene family of four members (Dkk1-4) and a unique Dkk3-related gene, Dkkl1 (soggy). They encode secreted proteins that typically antagonize Wnt/beta-catenin signaling, by inhibiting the Wnt coreceptors Lrp5 and 6. Additionally, Dkks are high affinity ligands for the transmembrane proteins Kremen1 and 2, which also modulate Wnt signaling. Dkks play an important role in vertebrate development, where they locally inhibit Wnt regulated processes such as antero-posterior axial patterning, limb development, somitogenesis and eye formation. In the adult, Dkks are implicated in bone formation and bone disease, cancer and Alzheimer's disease.

Wnt signaling antagonists are potential prognostic biomarkers for the progression of radiographic hip osteoarthritis in elderly Caucasian women

OBJECTIVE

To determine whether serum levels of 2 Wnt signaling antagonists, Frizzled-related protein (FRP) and Dkk-1, are associated with the development and progression of radiographic hip osteoarthritis (RHOA).

METHODS

Pelvic radiographs were obtained a mean of 8.3 years apart in 5,928 Caucasian women >or=65 years of age who were enrolled in the Study of Osteoporotic Fractures. Random sampling of this cohort was performed, with approximately 180 subjects per group assigned to 2 nested case-control studies on RHOA incidence and progression. Baseline serum levels of FRP and Dkk-1 were measured by capture enzyme-linked immunosorbent assay. Odds ratios (ORs) and 95% confidence intervals (95% CIs) were calculated using logistic regression analyses with adjustment for potential covariates.

RESULTS

There were no differences in serum levels of FRP and Dkk-1 between case subjects with incidence or progression of RHOA and their respective control subjects. There was a trend for higher baseline serum levels of FRP to be associated with a reduced risk of incident RHOA (age-adjusted OR 0.59 [95% CI 0.32-1.09], P = 0.09 for women in the highest quartile versus women in the lowest quartile). There was no association of serum levels of FRP with progression of RHOA. Serum levels of Dkk-1 did not correlate with incident RHOA. However, higher serum levels of Dkk-1 were associated with diminished risk of RHOA progression (age-adjusted OR 0.43 [95% CI 0.23-0.79], P = 0.007 for women in the highest quartile compared with women in the lowest quartile).

CONCLUSION

Elevated circulating levels of Dkk-1 appeared to be associated with reduced progression of RHOA in elderly women, whereas the highest quartile of serum FRP levels tended to be associated with a modest reduction in risk of incident RHOA.

Influence of an LRP5 cytoplasmic SNP on Wnt signaling and osteoblastic differentiation

The low density lipoprotein receptor-related protein 5 (LRP5) is a key determinant of bone mass, via the Wnt signaling pathway control of osteoblast function. This study examined human LRP5 signaling and the effects of an intracellular domain single nucleotide polymorphism (SNP: p.V1525A) on osteoblast differentiation and mineralization. Constitutively active LRP5 was constructed by deletion of the extracellular domain of LRP5 (LRP5DeltaN). Expression of LRP5DeltaN-V, which carries the allele p.1525V, induced higher beta-catenin/TCF-LEF activity compared to LRP5DeltaN-A, which carries the allele p.1525A. In a yeast two-hybrid assay, LRP5DeltaN-V also demonstrated a stronger interaction with AXIN than LRP5DeltaN-A. Expression of either of the alleles did not change cell proliferation. However, cells expressing LRP5DeltaN-V showed increased alkaline phosphatase activity and bone nodule formation compared to cells transfected with empty vector or LRP5DeltaN-A after osteogenic supplement (OS: beta-glycerophosphate and l-ascorbic acid) treatment. Cells expressing LRP5DeltaN-V revealed significantly increased bone sialoprotein (BSP) expression after 7 days of OS treatment and maintained elevated expression until day 21. Osteocalcin (OCN) mRNA levels were increased after 14-21 days of OS treatment in LRP5DeltaN-V expressing cells. LRP5DeltaN-V expressing cells demonstrated positive interaction with BMP-2 signaling of transcription at the SBE-luc promoter. LRP5 signaling is affected by the cytoplasmic SNP, p.V1525A. mRNA levels of Runx2 and Osterix were not affected by this SNP.