Functional and structural diversity of the human Dickkopf gene family

Binding of Dickkopf-3 to CXCR7 Enhances Vascular Progenitor Cell Migration and Degradable Graft Regeneration

Supplemental Digital Content is available in the text.

Rationale:

Vascular progenitor cells play key roles in physiological and pathological vascular remodeling—a process that is crucial for the regeneration of acellular biodegradable scaffolds engineered as vital strategies against the limited availability of healthy autologous vessels for bypass grafting. Therefore, understanding the mechanisms driving vascular progenitor cells recruitment and differentiation could help the development of new strategies to improve tissue-engineered vessel grafts and design drug-targeted therapy for vessel regeneration.

Objective:

In this study, we sought to investigate the role of Dkk3 (dickkopf-3), recently identified as a cytokine promotor of endothelial repair and smooth muscle cell differentiation, on vascular progenitor cells cell migration and vascular regeneration and to identify its functional receptor that remains unknown.

Methods and Results:

Vascular stem/progenitor cells were isolated from murine aortic adventitia and selected for the Sca-1 (stem cell antigen-1) marker. Dkk3 induced the chemotaxis of Sca-1+ cells in vitro in transwell and wound healing assays and ex vivo in the aortic ring assay. Functional studies to identify Dkk3 receptor revealed that overexpression or knockdown of chemokine receptor CXCR7 (C-X-C chemokine receptor type 7) in Sca-1+ cells resulted in alterations in cell migration. Coimmunoprecipitation experiments using Sca-1+ cell extracts treated with Dkk3 showed the physical interaction between DKK3 and CXCR7, and specific saturation binding assays identified a high-affinity Dkk3-CXCR7 binding with a dissociation constant of 14.14 nmol/L. Binding of CXCR7 by Dkk3 triggered the subsequent activation of ERK1/2 (extracellular signal-regulated kinases 1/2)-, PI3K (phosphatidylinositol 3-kinase)/AKT (protein kinase B)-, Rac1 (Ras-related C3 botulinum toxin substrate 1)-, and RhoA (Ras homolog gene family, member A)-signaling pathways involved in Sca-1+ cell migration. Tissue-engineered vessel grafts were fabricated with or without Dkk3 and implanted to replace the rat abdominal aorta. Dkk3-loaded tissue-engineered vessel grafts showed efficient endothelization and recruitment of vascular progenitor cells, which had acquired characteristics of mature smooth muscle cells. CXCR7 blocking using specific antibodies in this vessel graft model hampered stem/progenitor cell recruitment into the vessel wall, thus compromising vascular remodeling.

Conclusions:

We provide a novel and solid evidence that CXCR7 serves as Dkk3 receptor, which mediates Dkk3-induced vascular progenitor migration in vitro and in tissue-engineered vessels, hence harnessing patent grafts resembling native blood vessels.

Negative regulators of Wnt signaling in non-small cell lung cancer: Theoretical basis and therapeutic potency

Significant advances in the treatment of non-small cell lung cancer (NSCLC) have been made over the past decade, and they predominantly involve molecular targets such as epidermal growth factor receptor (EGFR) mutations and anaplastic lymphoma kinase (ALK) rearrangements. However, despite the initial good response, drug resistance eventually develops. The Wnt signaling pathway has recently been considered important in embryonic development and tumorigenesis in many cancers, particularly NSCLC. Moreover, the aberrant Wnt pathway plays a significant role in NSCLC and is associated with cancer cell proliferation, metastasis, invasion and drug resistance, and the suppression of canonical or noncanonical Wnt signaling through various biological or pharmacological negative regulators has been proven to produce specific anticancer effects. Thus, blocking the Wnt pathway via its negative regulators may overcome the resistance of current treatment methods and lead to new treatment strategies for NSCLC. Therefore, in this review, we summarize recent studies on the role of negative regulators in Wnt signaling in NSCLC and the therapeutic potency of these molecules as agents and targets for NSCLC treatments.

Downregulated MEG3 contributes to tumour progression and poor prognosis in oesophagal squamous cell carcinoma by interacting with miR-4261, downregulating DKK2 and activating the Wnt/β-catenin signalling

Long noncoding RNA (lncRNA) MEG3 has been widely reported to be decreased in a growing list of primary human tumours and play a key role in tumour suppression. However, there are few reports about MEG3 expression and function in oesophagal squamous cell carcinoma (ESCC). Here, we found that MEG3 expression was significantly downregulated in tumour tissues, and its low expression was associated with large tumour size, lymph node metastasis and advanced clinical stage in ESCC patients. Univariate and multivariate analyses revealed low expression of MEG3 as an independent predictor for disease-free survival and overall survival. Cell experiments showed that MEG3 inhibited ESCC cell proliferation, migration and invasion. Subsequently, miR-4261 was identified and confirmed to be the target of MEG3, and MEG3 functions, at least in part, by targeting miR-4261. Additionally, Dickkopf-2 (DKK2), a Wnt/β-catenin signalling inhibitor, was identified to be a target of miR-4261. MEG3 interacted with miR-4261, derepressed DKK2 and blocked the Wnt/β-catenin signalling, thereby inhibiting tumourigenesis and progression in ESCC. In vivo experiments also confirmed this conclusion. Our study for the first time elaborated the critical role of MEG3-miR-4261-DKK2-Wnt/β-catenin signalling axis in ESCC, and MEG3 could represent a novel diagnostic and prognostic biomarker and therapeutic target in ESCC.

Aberrant accumulation of Dickkopf 4 promotes tumor progression via forming the immune suppressive microenvironment in gastrointestinal stromal tumor

Background

Drug resistance and tumor recurrence are the major concerns in clinical practices of gastrointestinal stromal tumor (GIST), with the urgent requirement for exploring undiscovered pathways driving malignancy. To deal with these, recent studies have made many efforts to explore prognosis indicators and establish potential therapeutic targets.

Methods

Expression profiles of different risks of GISTs were described and abundant clinical evidences supported our findings in this study. Following exploration in vitro by cell experiments and verification in vivo using tumor microarray were taken to elucidate the underlying mechanism, which drove the malignancy in GIST.

Results

Dickkopf 4 (DKK4), as the canonical Wnt pathway antagonist, was unexpectedly and universally upregulated in high‐risk GISTs, and aberrant accumulation of DKK4 was closely correlated with poor prognosis. In addition, tumor‐derived DKK4 could decrease immune cells infiltration and activation in the tumor microenvironment, which decreased the antitumor effects in return. And this phenomenon was recurrent in human tumor specimens.

Conclusions

Our findings identified DKK4 as a proper tumor biomarker for prognosis predicting and recurrence monitoring, and suggested a novel immune‐escape mechanism driving malignancy in GIST, which might be a potential therapeutic target to improve the effects of canonical RTK therapy and combined immunotherapy.

Stabilization of Intrinsically Disordered DKK2 Protein by Fusion to RNA-Binding Domain

Intrinsic disorders are a common feature of hub proteins in eukaryotic interactomes controlling the signaling pathways. The intrinsically disordered proteins (IDPs) are prone to misfolding, and maintaining their functional stability remains a major challenge in validating their therapeutic potentials. Considering that IDPs are highly enriched in RNA-binding proteins (RBPs), here we reasoned and confirmed that IDPs could be stabilized by fusion to RBPs. Dickkopf2 (DKK2), Wnt antagonist and a prototype IDP, was fused with lysyl-tRNA synthetase (LysRS), with or without the fragment crystallizable (Fc) domain of an immunoglobulin and expressed predominantly as a soluble form from a bacterial host. The functional competence was confirmed by in vitro Wnt signaling reporter and tube formation in human umbilical vein endothelial cells (HUVECs) and in vivo Matrigel plug assay. The removal of LysRS by site-specific protease cleavage prompted the insoluble aggregation, confirming that the linkage to RBP chaperones the functional competence of IDPs. While addressing to DKK2 as a key modulator for cancer and ischemic vascular diseases, our results suggest the use of RBPs as stabilizers of disordered proteinaceous materials for acquiring and maintaining the structural stability and functional competence, which would impact the druggability of a variety of IDPs from human proteome.

The role of DKK1 in Alzheimer's disease: A potential intervention point of brain damage prevention?

Dickkopf-1 (DKK1), a secretory glycoprotein discovered for 'inducing generation of head', is an endogenous inhibitor of the canonical Wnt/β-catenin signaling pathway. It was found to be involved in many pathophysiological processes in vivo. Abnormal expression of DKK1 will alter expressions of related proteins and genes not only in canonical Wnt/β-catenin signaling pathway but also in other signaling pathways. Previous studies of DKK1 focused on its function in tumors. In recent years, a large number of studies have shown that it plays an important role in embryonic development, neural regeneration, synaptogenesis and so on. Therefore, its role in neuropsychiatric disorders, such as neurodysplasia, cognitive impairment and emotional disorder, has attracted increasing attention. At present, the role of DKK1 in Alzheimer's disease (AD) is one of the research hot topics. This article reviewed the research progress of its role in AD in order to provide new ideas and directions for further studies on the pathogenesis and treatment of AD.

MiR-92a modulates proliferation, apoptosis, migration, and invasion of osteosarcoma cell lines by targeting Dickkopf-related protein 3

Osteosarcoma (OS) is recognized as a common malignant tumor with a high trend of metastasis and diffusion. Despite the progresses that have been made in surgery, chemotherapy, and radiotherapy in the recent decades, the prognosis of patients with OS still remains poor. MiRNAs are being increasingly considered as new therapeutic targets for OS treatment. Our research aims to investigate the regulatory impact of miR-92a in the development of OS. Quantitative real-time PCR (qRT-PCR) results revealed that the expression of miR-92a was aberrantly overexpressed in human OS cell lines. By using cell counting kit-8 (CCK-8) assays, colony formation assays, flow cytometric analyses and Transwell assays, our data suggested that up-regulation of miR-92a promoted the proliferation, migration, and invasion of MNNG and U2OS cells, while inhibiting their apoptosis. In contrast, the knockdown of miR-92a effectively reversed these cellular biological behaviors. Furthermore, bioinformatics analysis indicated that Dickkopf-related protein 3 (DKK3) was a possible target of miR-92a. Subsequently, negative regulation of miR-92a on DKK3 was observed, which further supported the direct binding between them. In addition, silencing DKK3 rescued the inhibitory effect of miR-92a inhibitor on the development of OS. To sum up, our study revealed that miR-92a played a carcinogenic role in the growth of OS by promoting the tumorigenesis of OS cells via targeting of DKK3, thus revealing a new therapeutic target for OS.

Dynamics of Wnt activity on the acquisition of ectoderm potency in epiblast stem cells

During embryogenesis, the stringent regulation of Wnt activity is crucial for the morphogenesis of the head and brain. The loss of function of the Wnt inhibitor Dkk1 results in elevated Wnt activity, loss of ectoderm lineage attributes from the anterior epiblast, and the posteriorisation of anterior germ layer tissue towards the mesendoderm. The modulation of Wnt signalling may therefore be crucial for the allocation of epiblast cells to ectoderm progenitors during gastrulation. To test this hypothesis, we examined the lineage characteristics of epiblast stem cells (EpiSCs) that were derived and maintained under different signalling conditions. We showed that suppression of Wnt activity enhanced the ectoderm propensity of the EpiSCs. Neuroectoderm differentiation of these EpiSCs was further empowered by the robust re-activation of Wnt activity. Therefore, during gastrulation, the tuning of the signalling activities that mediate mesendoderm differentiation is instrumental for the acquisition of ectoderm potency in the epiblast.

The Wnt inhibitor Dkk1 is required for maintaining the normal cardiac differentiation program in Xenopus laevis

Wnt proteins can activate different intracellular signaling pathways. These pathways need to be tightly regulated for proper cardiogenesis. The canonical Wnt/β-catenin inhibitor Dkk1 has been shown to be sufficient to trigger cardiogenesis in gain-of-function experiments performed in multiple model systems. Loss-of-function studies however did not reveal any fundamental function for Dkk1 during cardiogenesis. Using Xenopus laevis as a model we here show for the first time that Dkk1 is required for proper differentiation of cardiomyocytes, whereas specification of cardiomyocytes remains unaffected in absence of Dkk1. This effect is at least in part mediated through regulation of non-canonical Wnt signaling via Wnt11. In line with these observations we also found that Isl1, a critical regulator for specification of the common cardiac progenitor cell (CPC) population, acts upstream of Dkk1.

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Dkk1 is required for cardiac development in Xenopus laevis.

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The Wnt inhibitor Dkk1 acts downstream of Isl1 during cardiac development in vivo.

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Loss of Dkk1 has no impact on cardiac specification in Xenopus.

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Normal cardiac differentiation is impaired upon Dkk1 inhibition in Xenopus.

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Dkk1 regulates canonical Wnt/β-catenin signaling during Xenopus cardiogenesis.

Pharmacokinetics, Tissue Distribution, Plasma Protein Binding Studies of 10-Dehydroxyl-12-Demethoxy-Conophylline, a Novel Anti-Tumor Candidate, in Rats

10-Dehydroxyl-12-demethoxy-conophylline is a natural anticancer candidate. The motivation of this study was to explore the pharmacokinetic profiles, tissue distribution, and plasma protein binding of 10-dehydroxyl-12-demethoxy-conophylline in Sprague Dawley rats. A rapid, sensitive, and specific ultra-performance liquid chromatography (UPLC) system with a fluorescence (FLR) detection method was developed for the determination of 10-dehydroxyl-12-demethoxy-conophylline in different rat biological samples. After intravenous (i.v.) dosing of 10-dehydroxyl-12-demethoxy-conophylline at different levels (4, 8, and 12 mg/kg), the half-life t_1/2α of intravenous administration was about 7 min and the t_1/2β was about 68 min. The AUC_0→∞ increased in a dose-proportional manner from 68.478 μg/L·min for 4 mg/kg to 305.616 mg/L·min for 12 mg/kg. After intragastrical (i.g.) dosing of 20 mg/kg, plasma levels of 10-dehydroxyl-12-demethoxy-conophylline peaked at about 90 min. 10-dehydroxyl-12-demethoxy-conophyllinea absolute oral bioavailability was only 15.79%. The pharmacokinetics process of the drug was fit to a two-room model. Following a single i.v. dose (8 mg/kg), 10-dehydroxyl-12-demethoxy-conophylline was detected in all examined tissues with the highest in kidney, liver, and lung. Equilibrium dialysis was used to evaluate plasma protein binding of 10-dehydroxyl-12-demethoxy-conophylline at three concentrations (1.00, 2.50, and 5.00 µg/mL). Results indicated a very high protein binding degree (over 80%), reducing substantially the free fraction of the compound.

Dkk1 involvement and its potential as a biomarker in pancreatic ductal adenocarcinoma

Dickkopf-1 (Dkk1)'s dysregulation has been implicated in the pathogenesis of a variety of cancers. It is part of the Dkk family of proteins that includes Dkk2, Dkk3 and Dkk4. This family of secreted proteins shares similar conserved cysteine domains and inhibits the Wnt/b-catenin pathway by causing proteasomal B-catenin degradation, inducing apoptosis, and preventing cell proliferation. Pancreatic ductal adenocarcinoma (PDAC) is the 4th leading cause of cancer mortality in the United States due to the late stage of diagnosis and the limited effectiveness of current therapy. Dkk1 is found increased in PADC patients' specimens and serum. Dkk1 can be a promising biomarker specific to PDAC, which has the potential to increase PDAC survival rates through improving early stage detection and monitoring progression compared to current biomarker gold standards. In addition, recent studies suggest that Dkk1 could be an excellent target for cancer immunotherapy. Interestingly, Dkk1-CKAP4-PI3K/AKT signal pathway also plays role in pancreatic cancer cell proliferation. In this review, we present the multiple mechanisms of Dkk1 in PDAC studied thus far and explore its function, regulation, and clinical applications in gynecological cancers including pancreatic ductal adenocarcinoma (PDAC), breast, ovarian, cervical, and endometrial cancer. Further research into Dkk1's mechanism and use as a diagnostic tool, alone or in combination with other biomarkers, could prove clinically useful for better understanding the pathology of PDAC and improving its early detection and treatment.

Production and Characterization of Monoclonal and Polyclonal Antibodies Against Truncated Recombinant Dickkopf-1 as a Candidate Biomarker

Several studies have reported an increased serum level of Dickkopf (DKK-1) protein in a variety of cancers, including multiple myeloma, lung, colorectal, bone loss, and Alzheimer's disease. This protein has potential to be used as a biomarker for the diagnosis of some cancers, especially bone loss in multiple myeloma. In the present study, to measure the concentration level of DKK-1 protein, rabbit polyclonal antibody (pAb) and mouse monoclonal antibodies (mAbs) were produced against this protein. New Zealand white rabbits and BALB/c mice were immunized with the chimeric recombinant DKK-1 antigen. Immunized mouse spleen cells were fused with SP2/0 cells to generate anti-rDKK-1 antibody-producing hybridoma cells. Antibodies were purified by protein A affinity chromatography and assessed using sodium dodecyl sulfate polyacrylamide gel, western blotting and enzyme-linked immunosorbent assay. These results implied that the pAb and mAb were produced against the DKK-1 protein. The Kd value of 5 × 10^-9 M was recorded for the mAb MR6F3 toward native DKK-1, and the Ig isotype was identified as IgG2b. No cross-reactivity was shown with DKK-2 by MR6F3. Collectively, our results revealed that the produced pAb and mAb could be used in the measurement of DKK-1 protein.

Rethinking Bone Disease in Kidney Disease

Renal osteodystrophy (ROD) is the bone component of chronic kidney disease mineral and bone disorder (CKD-MBD). ROD affects bone quality and strength through the numerous hormonal and metabolic disturbances that occur in patients with kidney disease. Collectively these disorders in bone quality increase fracture risk in CKD patients compared with the general population. Fractures are a serious complication of kidney disease and are associated with higher morbidity and mortality compared with the general population. Furthermore, at a population level, fractures are at historically high levels in patients with end-stage kidney disease (ESKD), whereas in contrast the general population has experienced a steady decline in fracture incidence rates. Based on these findings, it is clear that a paradigm shift is needed in our approach to diagnosing and managing ROD. In clinical practice, our ability to diagnose ROD and initiate antifracture treatments is impeded by the lack of accurate noninvasive methods that identify ROD type. The past decade has seen advances in the noninvasive measurement of bone quality and strength that have been studied in kidney disease patients. Below we review the current literature pertaining to the epidemiology, pathology, diagnosis, and management of ROD. We aim to highlight the pressing need for a greater awareness of this condition and the need for the implementation of strategies that prevent fractures in kidney disease patients. Research is needed for more accurate noninvasive assessment of ROD type, clinical studies of existing osteoporosis therapies in patients across the spectrum of kidney disease, and the development of CKD-specific treatments. © 2018 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of the American Society for Bone and Mineral Research.

Dkk-3 as a potential biomarker for diagnosis and prognosis of colorectal cancer

Background: The Dickkopf 3 (Dkk-3) protein is a member of the Dkk family known as Wnt signaling inhibitor. The level of DKk-3 changes in a wide range of cancers, such as colorectal cancer, lung cancer, prostate cancer, and bladder cancer, is proposed as a biomarker for diagnosis and prognosis of many cancers. The present study was conducted to evaluate the serum level of Dkk-3 as a cancer biomarker and to determine their prognostic value in colorectal cancer (CRC) patients and healthy matched controls.

Methods: A total of 30 colorectal cancer patients at different stages of the disease and healthy matched controls with no history of inflammatory and autoimmune disease or cancer were enrolled in the study. The level of Dkk-3 was assessed serologically using enzymelinked immunosorbent assay (ELISA) method, moreover, relevance of these markers with patients’ clinicopathological features was subsequently assessed. Means comparison and ROC curves analysis were done using SPSS software. P-value ˂0.05 was considered significant in all the tests.

Results: In this study, it was revealed that serum level of Dkk-3 was significantly (p<0.001) lower in patients compared to the healthy controls. Statistical analysis showed that serum level of Dkk-3 has 78% specificity and 77% sensitivity (AUC= 0.782, 95% CI) for diagnosis of colorectal cancer.

Conclusion: Dkk-3 protein can be considered as a potential biomarker for diagnosis and possibly the prognosis of colorectal cancer.

Attenuation of Intestinal Epithelial Cell Migration During Cryptosporidium parvum Infection Involves Parasite Cdg7_FLc_1030 RNA-Mediated Induction and Release of Dickkopf-1

Intestinal infection by Cryptosporidium is known to cause epithelial cell migration disorder but the underlying mechanisms are unclear. Previous studies demonstrated that a panel of parasite RNA transcripts of low protein-coding potential are delivered into infected epithelial cells. Using multiple models of intestinal cryptosporidiosis, we report here that C. parvum infection induces expression and release of the dickkopf protein 1 (Dkk1) from intestinal epithelial cells. Delivery of parasite Cdg7_FLc_1030 RNA to intestinal epithelial cells triggers transactivation of host Dkk1 gene during C. parvum infection. Release of Dkk1 is involved in C. parvum-induced inhibition of cell migration of epithelial cells, including noninfected bystander cells. Moreover, Dkk1-mediated suppression of host cell migration during C. parvum infection involves inhibition of Cdc42/Par6 signaling. Our data support the hypothesis that attenuation of intestinal epithelial cell migration during Cryptosporidium infection involves parasite Cdg7_FLc_1030 RNA-mediated induction and release of Dkk1 from infected cells.

Therapeutic potency of Wnt signaling antagonists in the pathogenesis of prostate cancer, current status and perspectives

Prostate cancer is a major cause of cancer-related death in males. Wnt/β-catenin signaling plays a critical role in the pathogenesis of this disease by regulating angiogenesis, drug resistance, cell proliferation, and apoptosis. Suppression of Wnt canonical or noncanonical signaling pathways via Wnt biological or pharmacological antagonists is a potentially novel therapeutic approach for patients with prostate cancer. This review summarizes the role of Wnt signaling inhibitors in the pathogenesis of prostate cancer for a better understanding and hence a better management of this disease.

p63-Dependent Dickkopf3 Expression Promotes Esophageal Cancer Cell Proliferation via CKAP4

Dickkopf3 (DKK3) is a secretory protein that belongs to the DKK family, but exhibits structural divergence from other family members, and its corresponding receptors remain to be identified. Although DKK3 has been shown to have oncogenic functions in certain cancer types, the underlying mechanism by which DKK3 promotes tumorigenesis remains to be clarified. We show here that DKK3 stimulates esophageal cancer cell proliferation via cytoskeleton-associated protein 4 (CKAP4), which acts as a receptor for DKK3. DKK3 was expressed in approximately 50% of tumor lesions of esophageal squamous cell carcinoma (ESCC) cases; simultaneous expression of DKK3 and CKAP4 was associated with poor prognosis. Anti-CKAP4 antibody inhibited both binding of DKK3 to CKAP4 and xenograft tumor formation induced by ESCC cells. p63, a p53-related transcriptional factor frequently amplified in ESCC, bound to the upstream region of the DKK3 gene. Knockdown of p63 decreased DKK3 expression in ESCC cells, and reexpression of DKK3 partially rescued cell proliferation in p63-depleted ESCC cells. Expression of ΔNp63α and DKK3 increased the size of tumor-like esophageal organoids, and anti-CKAP4 antibody inhibited growth of esophageal organoids. Taken together, these results suggest that the DKK3-CKAP4 axis might serve as a novel molecular target for ESCC.Significance: In esophageal cancer, findings identify DKK3 as a poor prognostic indicator and demonstrate CKAP4 inhibition as an effective therapeutic strategy. Cancer Res; 78(21); 6107-20. ©2018 AACR.

Part I: Development and Physiology of the Temporomandibular Joint

PURPOSE OF REVIEW

Investigate the developmental physiology of the temporomandibular joint (TMJ), a unique articulation between the cranium and the mandible.

RECENT FINDINGS

Principal regulatory factors for TMJ and disc development are Indian hedgehog (IHH) and bone morphogenetic protein (BMP-2). The mechanism is closely associated with ear morphogenesis. Secondary condylar cartilage emerges as a subperiosteal blastema on the medial surface of the posterior mandible. The condylar articular surface is immunoreactive for tenascin-C, so it is a modified fibrous periosteum with an underlying proliferative zone (cambrium layer) that differentiates into fibrocartilage. The latter cushions high loads and subsequently produces endochondral bone. The TMJ is a heavily loaded joint with three cushioning layers of fibrocartilage in the disc, as well as in subarticular zones in the fossa and mandibular condyle. The periosteal articular surface produces fibrocartilage to resist heavy loads, and has unique healing and adaptive properties for maintaining life support functions under adverse environmental conditions.

Dkk2 promotes neural crest specification by activating Wnt/β-catenin signaling in a GSK3β independent manner

Neural crest progenitors are specified through the modulation of several signaling pathways, among which the activation of Wnt/β-catenin signaling by Wnt8 is especially critical. Glycoproteins of the Dickkopf (Dkk) family are important modulators of Wnt signaling acting primarily as Wnt antagonists. Here we report that Dkk2 is required for neural crest specification functioning as a positive regulator of Wnt/β-catenin signaling. Dkk2 depletion in Xenopus embryos causes a loss of neural crest progenitors, a phenotype that is rescued by expression of Lrp6 or β-catenin. Dkk2 overexpression expands the neural crest territory in a pattern reminiscent of Wnt8, Lrp6 and β-catenin gain-of-function phenotypes. Mechanistically, we show that Dkk2 mediates its neural crest-inducing activity through Lrp6 and β-catenin, however unlike Wnt8, in a GSK3β independent manner. These findings suggest that Wnt8 and Dkk2 converge on β-catenin using distinct transduction pathways both independently required to activate Wnt/β-catenin signaling and induce neural crest cells.

Elevated blood levels of Dickkopf-1 are associated with acute infections

INTRODUCTION

Dickkopf-1 (DKK1) is a soluble protein and antagonist of the Wnt/β-catenin signaling pathway. DKK1 is found elevated in serum from patients affected with various types of cancers and in some instances, it is considered a diagnostic and prognostic biomarker. Elevated serum levels of DKK1 have also been detected in animal models of chronic inflammatory diseases. Previous work from our laboratory has demonstrated upregulation of DKK1 in cells and mouse models of the bone marrow failure (BMF) and cancer-prone disease Fanconi anemia (FA). The present study aimed to investigate whether DKK1 blood levels in patients are associated with FA or inflammatory responses to acute infections.

METHODS

Plasma samples were collected from 58 children admitted to the Centre Mère-Enfant Soleil du Centre Hospitalier de Québec-Université Laval with signs of acute infections. Blood plasma specimens were also collected from healthy blood donors at the Héma-Québec blood donor clinic. Plasmas from patients diagnosed with FA were also included in the study. DKK1 levels in blood plasmas were assessed by standard ELISA.

RESULTS

Patients with acute infections showed dramatically high levels of DKK1 (6072 ± 518 pg/ml) in their blood compared to healthy blood donors (1726 ± 95 pg/ml). No correlations were found between DKK1 levels and C reactive protein (CRP) concentration, platelet numbers, or white blood cell counts. Patients with FA showed higher DKK1 plasma levels (3419 ± 147.5 pg/ml) than healthy blood donors (1726 ± 95 pg/ml) but significantly lower than patients with acute infections.

CONCLUSION

These findings suggest that blood DKK1 is elevated in response to infections and perhaps to inflammatory responses.

DKK4-knockdown enhances chemosensitivity of A549/DTX cells to docetaxel

Drug resistance greatly limits docetaxel efficiency in the treatment of non-small cell lung cancer (NSCLC). Dickkopf 4 (DKK4), a negative regulator of Wnt/β-catenin pathway, is believed to be involved in various human cancers; whereas the association of DKK4 with acquired docetaxel resistance in NSCLC remains unclear. In the present study, we investigated the involvement of DKK4 in the docetaxel-resistant human lung adenocarcinoma A549 (A549/DTX) cells. Our results showed that DKK4 expression was significantly increased in the A549/DTX cells compared with in the A549 cells, as well as in the culture supernatant of A549/DTX cells. DKK4 overexpression increased the resistance of A549 cells to docetaxel. DKK4-knockdown promoted inhibition of A549/DTX cell growth, and reduced the colony formation and invasion capacity of A549/DTX cells. Moreover, DKK4-knockdown promoted the pro-apoptotic effect of docetaxel characterized with caspase 3 activation and inhibition of BCL-2 expression in A549/DTX cells, which was possibly mediated by inducing the activation of c-Jun N-terminal kinase (JNK)-related signaling pathway. Thus, our results indicated that DKK4-knockdown promoted the cytotoxic and pro-apoptotic activity of A549/DTX cells, which suggests a critical role of DKK4 in docetaxel resistance of the A549 cells and provides the potential to combine docetaxel therapy with DKK4 depletion in treating NSCLC.

Protective effect of stromal Dickkopf-3 in prostate cancer: opposing roles for TGFBI and ECM-1

Aberrant transforming growth factor-β (TGF-β) signaling is a hallmark of the stromal microenvironment in cancer. Dickkopf-3 (Dkk-3), shown to inhibit TGF-β signaling, is downregulated in prostate cancer and upregulated in the stroma in benign prostatic hyperplasia, but the function of stromal Dkk-3 is unclear. Here we show that DKK3 silencing in WPMY-1 prostate stromal cells increases TGF-β signaling activity and that stromal cell-conditioned media inhibit prostate cancer cell invasion in a Dkk-3-dependent manner. DKK3 silencing increased the level of the cell-adhesion regulator TGF-β-induced protein (TGFBI) in stromal and epithelial cell-conditioned media, and recombinant TGFBI increased prostate cancer cell invasion. Reduced expression of Dkk-3 in patient tumors was associated with increased expression of TGFBI. DKK3 silencing reduced the level of extracellular matrix protein-1 (ECM-1) in prostate stromal cell-conditioned media but increased it in epithelial cell-conditioned media, and recombinant ECM-1 inhibited TGFBI-induced prostate cancer cell invasion. Increased ECM1 and DKK3 mRNA expression in prostate tumors was associated with increased relapse-free survival. These observations are consistent with a model in which the loss of Dkk-3 in prostate cancer leads to increased secretion of TGFBI and ECM-1, which have tumor-promoting and tumor-protective roles, respectively. Determining how the balance between the opposing roles of extracellular factors influences prostate carcinogenesis will be key to developing therapies that target the tumor microenvironment.

Dkk3 dependent transcriptional regulation controls age related skeletal muscle atrophy

Age-related muscle atrophy (sarcopenia) is the leading cause for disability in aged population, but the underlying molecular mechanisms are poorly understood. Here we identify a novel role for the secreted glycoprotein Dickkopf 3 (Dkk3) in sarcopenia. Forced expression of Dkk3 in muscles in young mice leads to muscle atrophy. Conversely, reducing its expression in old muscles restores both muscle size and function. Dkk3 induces nuclear import of β-catenin and enhances its interaction with FoxO3, which in turn activates the transcription of E3 ubiquitin ligase Fbxo32 and Trim63, driving muscle atrophy. These findings suggest that Dkk3 may be used as diagnostic marker and as therapeutic target for age-related muscle atrophy, and reveal a distinct transcriptional control of Fbxo32 and Trim63.

Cardiac Remodeling: Endothelial Cells Have More to Say Than Just NO

The heart is a highly structured organ consisting of different cell types, including myocytes, endothelial cells, fibroblasts, stem cells, and inflammatory cells. This pluricellularity provides the opportunity of intercellular communication within the organ, with subsequent optimization of its function. Intercellular cross-talk is indispensable during cardiac development, but also plays a substantial modulatory role in the normal and failing heart of adults. More specifically, factors secreted by cardiac microvascular endothelial cells modulate cardiac performance and either positively or negatively affect cardiac remodeling. The role of endothelium-derived small molecules and peptides—for instance NO or endothelin-1—has been extensively studied and is relatively well defined. However, endothelial cells also secrete numerous larger proteins. Information on the role of these proteins in the heart is scattered throughout the literature. In this review, we will link specific proteins that modulate cardiac contractility or cardiac remodeling to their expression by cardiac microvascular endothelial cells. The following proteins will be discussed: IL-6, periostin, tenascin-C, thrombospondin, follistatin-like 1, frizzled-related protein 3, IGF-1, CTGF, dickkopf-3, BMP-2 and−4, apelin, IL-1β, placental growth factor, LIF, WISP-1, midkine, and adrenomedullin. In the future, it is likely that some of these proteins can serve as markers of cardiac remodeling and that the concept of endothelial function and dysfunction might have to be redefined as we learn more about other factors secreted by ECs besides NO.

Effects of DKK1 overexpression on bone metastasis of SBC-3 cells

Among all malignancies, lung cancer is the leading cause of cancer-related deaths in China. Bone metastasis is one of the most common complications and one of the most important factors affecting the prognosis of lung cancer patients, which resulting in very poor therapeutic effects. Previously, we have demonstrated that the expression levels of Dickkopf1 (DKK1), a protein involved in cell regulation and proliferation, was dramatically higher in cells that have a tendency to metastasize and invade the bone tissue (SBC-5 cells) compared with cells that do not (SBC-3 cells). Downregulation of DKK1 in SBC-5 cells inhibited cell malignancy in vitro, and the formation of bone metastasis in vivo. However, whether upregulating DKK1 would be sufficient to induce aggressive tumor behavior (proliferation, migration, invasion and metastasis) in SBC-3 cells remained to be investigated. The present study aimed to examine the role of DKK1 in SBC-3 cells, as well as to investigate the SBC-3 ability to metastasize and invade the bone tissue. The results demonstrated that upregulation of DKK1 in SBC-3 cells enhanced cell proliferation, colony formation, cell migration and invasion in vitro, as well as bone metastasis in vivo. These results indicate that DKK1 may be an important regulator in the development of small cell lung cancer (SCLC), and targeting DKK1 may be an effective method for preventing and/or treating skeletal metastases in SCLC cases.

Knockdown of dickkopf2 inhibits vascular endothelia growth factor expression through the Wnt/β-catenin signaling pathway in human retinal pigment epithelial cells under hypoxic conditions

Hypoxia has been demonstrated to be a proangiogenic factor that induces vascular endothelial growth factor (VEGF) in retinal pigment epithelial (RPE) cells. Dickkopf2 (DKK2), originally known as Wnt antagonist, has recently been demonstrated to have an important regulatory role in angiogenesis; however, the specific role of DKK2 in RPE cells is not known. In the present study, the effects of DKK2 on VEGF expression under hypoxic conditions were investigated, as well as the molecular mechanisms involved. The results demonstrated that the expression of DKK2 was markedly increased under hypoxic conditions compared with normoxic conditions. Knockdown of DKK2 markedly attenuated the CoCl₂-induced expression of hypoxia-inducible factor (HIF)-1α and VEGF in RPE cells. Furthermore, knockdown of DKK2 markedly inhibited the expression of β-catenin induced by hypoxia. In conclusion, the findings of the present study demonstrate that knockdown of DKK2 inhibits the hypoxia-induced production of VEGF by suppressing the activation of the Wnt/β-catenin signaling pathway.

The Pleiotropic Effects of the Canonical Wnt Pathway in Early Development and Pluripotency

The technology to derive embryonic and induced pluripotent stem cells from early embryonic stages and adult somatic cells, respectively, emerged as a powerful resource to enable the establishment of new in vitro models, which recapitulate early developmental processes and disease. Additionally, pluripotent stem cells (PSCs) represent an invaluable source of relevant differentiated cell types with immense potential for regenerative medicine and cell replacement therapies. Pluripotent stem cells support self-renewal, potency and proliferation for extensive periods of culture in vitro. However, the core pathways that rule each of these cellular features specific to PSCs only recently began to be clarified. The Wnt signaling pathway is pivotal during early embryogenesis and is central for the induction and maintenance of the pluripotency of PSCs. Signaling by the Wnt family of ligands is conveyed intracellularly by the stabilization of β-catenin in the cytoplasm and in the nucleus, where it elicits the transcriptional activity of T-cell factor (TCF)/lymphoid enhancer factor (LEF) family of transcription factors. Interestingly, in PSCs, the Wnt/β-catenin–TCF/LEF axis has several unrelated and sometimes opposite cellular functions such as self-renewal, stemness, lineage commitment and cell cycle regulation. In addition, tight control of the Wnt signaling pathway enhances reprogramming of somatic cells to induced pluripotency. Several recent research efforts emphasize the pleiotropic functions of the Wnt signaling pathway in the pluripotent state. Nonetheless, conflicting results and unanswered questions still linger. In this review, we will focus on the diverse functions of the canonical Wnt signaling pathway on the developmental processes preceding embryo implantation, as well as on its roles in pluripotent stem cell biology such as self-renewal and cell cycle regulation and somatic cell reprogramming.

Microfluidics for secretome analysis under enhanced endogenous signaling

Cell secretome, the complex set of proteins that are secreted by the cells, is a fundamental mechanism of cell-cell communication both in vitro and in vivo. In vivo, the analysis of proteins secreted into body fluids can bring to the identification of biomarkers for important physiopathological conditions. However, due to the complexity of the protein content of body fluids, a better understanding of the secreted proteins by different cell types is highly desirable and can be performed in vitro for dissection. To this aim, microfluidic culture systems could be particularly relevant because of the accumulation of extrinsic endogenous signals at microliter scale, which better preserves the self-regulation occurring in the small interstitial spaces in vivo. In this work, we perform a quantitative study to compare the secretome in microfluidics and in a standard well plate. Human foreskin fibroblasts are used as a case study. This work also represents an important technological advance in terms of feasibility of high-throughput quantitative protein analyses in microfluidics.

Dickkopf homolog 3 (DKK3): A candidate for detection and treatment of cancers?

Wnt signaling is an evolutionary highly conserved pathway that is modulated by several inhibitors and activators, and plays a key role in numerous physiological processes. One of the extracellular Wnt inhibitors is the DKK (Dickkopf Homolog) family which has four members (Dkk1-4) and a unique Dkk3-related gene, Dkkl1 (soggy). DKK3 is a divergent member of the DKK protein family. Evidence suggests that DKK3 may serve as a potential therapeutic target in several types of human cancers. We review here the biological role of DKK3 as a tumor suppressor gene (TSG) or oncogene, and its correlation with various miRNAs. In addition, we discuss the role of polymorphisms and promoter methylation of the DKK3 gene, and of its expression in regulating cancer cell proliferation. Finally, we propose that DKK3 may be considered as both a biomarker and a therapeutic target in different cancers.

High-frequency deregulated expression of Wnt signaling pathway members in breast carcinomas

Purpose

Breast carcinoma is the most common malignancy and leading cause of cancer-related deaths in women worldwide including Saudi Arabia. Breast cancer in Saudi women develops at a much early age with median age of onset of 49 years compared to 62 years observed in patients from USA. Aberrations in wingless and integration site growth factor (Wnt) signaling pathway have been pathologically implicated in development of breast cancers and hence its role was examined in Saudi patients.

Materials and methods

We immunohistochemically examined various components of Wnt signaling pathway including β-catenin, tumor suppressor proteins, adenomatous polyposis coli (APC), and Axin, expression of naturally occurring pathway antagonists such as Dickkopf Wnt signaling pathway inhibitor 3 (DKK3), FRP2, and WIF1, as well as Wnt target cyclin D1 and c-Myc to establish if the pathway is constitutively activated in breast cancers arising in Saudi women.

Results

Cytoplasmic β-catenin, indicative of activation of the pathway, was observed in 24% of cases. Expression of APC and Axin, which are components of β-catenin destruction complex, was lost in 5% and 10% of tumors, respectively. Additionally, Wnt signaling inhibitors DKK3, FRP2, and Wnt inhibitory factor 1 (WIF1) were not expressed in 8%, 14%, and 5% breast tumors, respectively. Overall, accumulation of cytoplasmic β-catenin and downregulation of other Wnt pathway proteins (APC/Axin/DKK3/FRP2/WIF1) were found in approximately half of the breast cancers (47%) in our cohort. Consistent with this, analysis of Wnt target genes demonstrated moderate-to-strong expression of c-Myc in 58% and cyclin D1 in 50% of breast cancers. Deregulation of Wnt pathway was not associated with age of onset of the disease, tumor grade, and triple-negative status of breast cancers.

Conclusions

High level of deregulated expression of Wnt pathway proteins suggests its important role in pathogenesis of breast cancers arising in Saudi women who may benefit from development of therapeutic drugs targeting this pathway.

Neural Stem Cells and Its Derivatives as a New Material for Melanin Inhibition

The pigment molecule, melanin, is produced from melanosomes of melanocytes through melanogenesis, which is a complex process involving a combination of chemical and enzymatically catalyzed reactions. The synthesis of melanin is primarily influenced by tyrosinase (TYR), which has attracted interest as a target molecule for the regulation of pigmentation or depigmentation in skin. Thus, direct inhibitors of TYR activity have been sought from various natural and synthetic materials. However, due to issues with these inhibitors, such as weak or permanent ability for depigmentation, allergy, irritant dermatitis and rapid oxidation, in vitro and in vivo, the development of new materials that inhibit melanin production is essential. A conditioned medium (CM) derived from stem cells contains many cell-secreted factors, such as cytokines, chemokines, growth factors and extracellular vesicles including exosomes. In addition, the secreted factors could negatively regulate melanin production through stimulation of a microenvironment of skin tissue in a paracrine manner, which allows the neural stem cell CM to be explored as a new material for skin depigmentation. In this review, we will summarize the current knowledge regulating depigmentation, and discuss the potential of neural stem cells and their derivatives, as a new material for skin depigmentation.

Dickkopf2 rescues erectile function by enhancing penile neurovascular regeneration in a mouse model of cavernous nerve injury

Penile erection is a neurovascular event and neurologic or vascular disturbances are major causes of erectile dysfunction (ED). Radical prostatectomy for prostate cancer not only induces cavernous nerve injury (CNI) but also results in cavernous angiopathy, which is responsible for poor responsiveness to oral phosphodiesterase-5 inhibitors. Dickkopf2 (DKK2) is known as a Wnt signaling antagonist and is reported to promote mature and stable blood vessel formation. Here, we demonstrated in CNI mice that overexpression of DKK2 by administering DKK2 protein or by using DKK2-Tg mice successfully restored erectile function: this recovery was accompanied by enhanced neural regeneration through the secretion of neurotrophic factors, and restoration of cavernous endothelial cell and pericyte content. DKK2 protein also promoted neurite outgrowth in an ex vivo major pelvic ganglion culture experiment and enhanced tube formation in primary cultured mouse cavernous endothelial cells and pericytes co-culture system in vitro. In light of critical role of neuropathy and angiopathy in the pathogenesis of radical prostatectomy-induced ED, reprogramming of damaged erectile tissue toward neurovascular repair by use of a DKK2 therapeutic protein may represent viable treatment option for this condition.

WNT signalling events near the cell membrane and their pharmacological targeting for the treatment of cancer

WNT signalling is an essential signalling pathway for all multicellular animals. Although first described more than 30 years ago, new components and regulators of the pathway are still being discovered. Considering its importance in both embryonic development and adult homeostasis, it is not surprising that this pathway is often deregulated in human diseases such as cancer. Recently, it became clear that in addition to cytoplasmic components such as β-catenin, other, membrane-bound or extracellular, components of the WNT pathway are also altered in cancer. This review gives an overview of the recent discoveries on WNT signalling events near the cell membrane. Furthermore, membrane-associated components of the WNT pathway, which are more accessible for therapeutic intervention, as well therapeutic approaches that already target those components will be discussed. In this way, we hope to stimulate the development of effective anti-cancer therapies that target this fascinating pathway.

LINKED ARTICLES

This article is part of a themed section on WNT Signalling: Mechanisms and Therapeutic Opportunities. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.24/issuetoc.

Differential effects on lung and bone metastasis of breast cancer by Wnt signalling inhibitor DKK1

Metastatic cancer is a systemic disease, and metastasis determinants might elicit completely different effects in various target organs. Here we show that tumour-secreted DKK1 is a serological marker of breast cancer metastasis organotropism and inhibits lung metastasis. DKK1 suppresses PTGS2-induced macrophage and neutrophil recruitment in lung metastases by antagonizing cancer cell non-canonical WNT/PCP-RAC1-JNK signalling. In the lungs, DKK1 also inhibits WNT/Ca²⁺-CaMKII-NF-κB signalling and suppresses LTBP1-mediated TGF-β secretion of cancer cells. In contrast, DKK1 promotes breast-to-bone metastasis by regulating canonical WNT signalling of osteoblasts. Importantly, targeting canonical WNT may not be beneficial to treatment of metastatic cancer, while combinatory therapy against JNK and TGF-β signalling effectively prevents metastasis to both the lungs and bone. Thus, DKK1 represents a class of Janus-faced molecules with dichotomous roles in organotropic metastasis, and our data provide a rationale for new anti-metastasis approaches.

Exogenous DKK-3/REIC inhibits Wnt/β-catenin signaling and cell proliferation in human kidney cancer KPK1

The third member of the Dickkopf family (DKK-3), also known as reduced expression in immortalized cells (REIC), is a tumor suppressor present in a variety of tumor cells. Regarding the regulation of the Wnt/β-catenin signaling pathway, exogenous DKK-1 and DKK-2 are reported to inhibit Wnt signaling by binding the associated effectors. However, whether exogenous DKK-3 inhibits Wnt signaling remains unclear. A recombinant protein of human full-length DKK-3 was used to investigate the exogenous effects of the protein in vitro in KPK1 human renal cell carcinoma cells. It was demonstrated that the expression of phosphorylated (p-)β-catenin (inactive form as the transcriptional factor) was increased in KPK1 cells treated with the exogenous DKK-3 protein. The levels of non-p-β-catenin (activated form of β-catenin) were consistently decreased. It was revealed that the expression of transcription factor (TCF) 1 and c-Myc, the downstream transcription factors of the Wnt/β-catenin signaling pathway, was inhibited following treatment with DKK-3. A cancer cell viability assay confirmed the anti-proliferative effects of exogenous DKK-3 protein, which was consistent with a suppressed Wnt/β-catenin signaling cascade. In addition, as low-density lipoprotein receptor-related protein 6 (LRP6) is a receptor of DKK-1 and DKK-2 and their interaction on the cell surface inhibits Wnt/β-catenin signaling, it was examined whether the exogenous DKK-3 protein affects LRP6-mediated Wnt/β-catenin signaling. The LRP6 gene was silenced and the effects of DKK-3 on the time course of the upregulation of p-β-catenin expression were subsequently analyzed. Notably, LRP6 depletion elevated the base level of p-β-catenin; however, there was no significant effect on its upregulation course or expression pattern. These findings indicate that exogenous DKK-3 upregulates p-β-catenin and inhibits Wnt/β-catenin signaling in an LRP6-independent manner. Therefore, exogenous DKK-3 protein may inhibit the proliferation of KPK1 cells via inactivating Wnt/β-catenin signaling.

The role of Dickkopf-1 as a potential prognostic marker in pancreatic ductal adenocarcinoma

Dickkopf-1(DKK-1), the downstream target of β-catenin/T-cell factor, participates in a negative feedback loop in the Wnt signaling and reported as an important biomarker in many tumors. In this study, we analyzed the expression of DKK-1 in pancreatic ductal adenocarcinoma (PDAC) patients at both mRNA and protein levels. We used real-time PCR to detect the expression of DKK-1 in 32 PDAC and paired adjacent non-tumor tissues, results suggested that the expression of DKK-1 was increased in PDAC tissues. We found the similar results in the analysis of 3 independent microarray data sets. Immunohistochemical staining of 311 pairs of PDAC tissues suggested that DKK-1 expression was significantly associated with T classification (P = 0.039) and lymph node metastasis (P = 0.035). Furthermore, Kaplan-Meier analysis for DKK-1 expression demonstrated that patients with higher DKK-1 level had shorter overall survival (OS) and relapse-free survival (RFS) time in Ren Ji cohort and online PDAC database at both mRNA and protein levels. Univariable and multivariable Cox regression analysis confirmed that DKK-1 as well as lymph node metastasis and histology were independent predictors of OS in patients with PDAC. This study demonstrated that DKK-1 may be a predictor for prognosis in PDAC patients.

The Dkk3 gene encodes a vital intracellular regulator of cell proliferation

Members of the Dickkopf (Dkk) family of Wnt antagonists interrupt Wnt-induced receptor assembly and participate in axial patterning and cell fate determination. One family member, DKK3, does not block Wnt receptor activation. Loss of Dkk3 expression in cancer is associated with hyperproliferation and dysregulated ß-catenin signaling, and ectopic expression of Dkk3 halts cancer growth. The molecular events mediating the DKK3-dependent arrest of ß-catenin-driven cell proliferation in cancer cells are unknown. Here we report the identification of a new intracellular gene product originating from the Dkk3 locus. This Dkk3b transcript originates from a second transcriptional start site located in intron 2 of the Dkk3 gene. It is essential for early mouse development and is a newly recognized regulator of ß-catenin signaling and cell proliferation. Dkk3b interrupts nuclear translocation ß-catenin by capturing cytoplasmic, unphosphorylated ß-catenin in an extra-nuclear complex with ß-TrCP. These data reveal a new regulator of one of the most studied signal transduction pathways in metazoans and provides a novel, completely untapped therapeutic target for silencing the aberrant ß-catenin signaling that drives hyperproliferation in many cancers.

Dickkopf-related protein 2 induces G0/G1 arrest and apoptosis through suppressing Wnt/β-catenin signaling and is frequently methylated in breast cancer

Dickkopf-related protein 2 (DKK2) is one of the antagonists of Wnt/β-catenin signaling, with its downregulation reported in multiple cancers. However, how DKK2 contributes to breast tumorigenesis remains unclear. We examined its expression and promoter methylation in 10 breast tumor cell lines, 98 primary tumors, and 21 normal breast tissues. Compared with normal tissues, DKK2 was frequently silenced in breast cell lines (7/8). DKK2 promoter methylation was detected in 77.8% of cell lines and 86.7% of breast tumors; while rarely detected in normal breast tissues (19%), indicating common DKK2 methylation in breast cancer. Ectopic expression of DKK2 changed breast tumor cell morphology, inhibited cell proliferation and colony formation by inducing G0/G1 cell cycle arrest and apoptosis, and suppressed tumor cell migration by reversing epithelial-mesenchymal transition (EMT) and downregulating stem cell markers. Moreover, restored expression of DKK2 in MCF7 cells disrupted the microtube formation of human umbilical vein endothelial cells on Matrigel®. In vivo, the growth of MDA-MB-231 cells in nude mice was markedly decreased after stable expression of DKK2. DKK2 suppressed canonical Wnt/β-catenin signaling by inhibiting β-catenin activity with decreased active β-catenin protein. Thus, our findings demonstrate that DKK2 functions as a tumor suppressor through inhibiting cell proliferation and inducing apoptosis via regulating Wnt signaling during breast tumorigenesis.

Dickkopf 3 attenuates xanthine dehydrogenase expression to prevent oxidative stress-induced apoptosis

Dickkopf (DKK) 3 is a DKK glycoprotein family member that controls cell fate during embryogenesis and exerts opposing effects on survival in a cell type-dependent manner; however, the mechanisms governing its pro-apoptosis versus pro-survival functions remain unclear. Here, we investigated DKK3 function in Li21 hepatoma cells and tPH5CH immortalized hepatocytes. DKK3 knockdown by siRNA resulted in reactive oxygen species accumulation and subsequent apoptosis, which were abrogated by administration of the antioxidant N-acetyl-cysteine. Moreover, forced DKK3 over-expression induced resistance to hydrogen peroxide (H₂ O₂ )-induced apoptosis. Expression analysis by cDNA microarray showed that xanthine dehydrogenase (XDH) expression was significantly lower in Li21 and tPH5CHDKK3-over-expressing cells in response to H₂ O₂ treatment when compared to that in their respective mock-transfected controls, whereas a marked increase was observed in H₂ O₂ -treated DKK3 knockdown cells. Thus, these data suggest that DKK3 promotes cell survival during oxidative stress by suppressing the expression of the superoxide-producing enzyme XDH.

Dickkopf-3 Upregulates VEGF in Cultured Human Endothelial Cells by Activating Activin Receptor-Like Kinase 1 (ALK1) Pathway

Dkk-3 is a member of the dickkopf protein family of secreted inhibitors of the Wnt pathway, which has been shown to enhance angiogenesis. The mechanism underlying this effect is currently unknown. Here, we used cultured HUVECs to study the involvement of the TGF-β and VEGF on the angiogenic effect of Dkk-3. Addition of hrDkk-3 peptide (1 or 10 ng/ml) to HUVECs for 6 or 12 h enhanced the intracellular and extracellular VEGF protein levels, as assessed by RTPCR, immunoblotting, immunocytochemistry and ELISA. The increase in the extracellular VEGF levels was associated to the VEGFR2 activation. Pharmacological blockade of VEGFR2 abrogated Dkk-3-induced endothelial cell tubes formation, indicating that VEGF is a molecular player of the angiogenic effects of Dkk-3. Moreover, Dkk-3 enhanced Smad1/5/8 phosphorylation and recruited Smad4 to the VEGF gene promoter, suggesting that Dkk-3 activated ALK1 receptor leading to a transcriptional activation of VEGF. This mechanism was instrumental to the increased VEGF expression and endothelial cell tubes formation mediated by Dkk-3, because both effects were abolished by siRNA-mediated ALK1 knockdown. In summary, we have found that Dkk-3 activates ALK1 to stimulate VEGF production and induce angiogenesis in HUVECs.

Dickkopf-related protein 3 negatively regulates the osteogenic differentiation of rat dental follicle cells

The present study aimed to investigate the effect of Dickkopf-related protein 3 (DKK3) on osteogenic differentiation of rat dental follicle cells (DFCs). A PCR array analysis of Wnt pathway activation in DFCs identified genes dysregulated by mineral induction. Among them, DKK3expression levels were decreased, and further experiments were conducted to investigate its role in DFC osteogenesis. By comparing DFCs grown in normal growth and mineral-induction media for 4 weeks, the present study confirmed that DKK3 was a potential target gene of osteogenesis through reverse transcription-quantitative polymerase chain reaction (RT-qPCR) and western blotting (WB). A short hairpin RNA (shRNA) was introduced into DFCs using a lentiviral vector to inhibit DKK3 expression. An alkaline phosphatase (ALP) activity assay and Alizarin Red staining were performed to observe the DKK3-shRNA DFCs. In addition, the osteogenic differentiation of DKK3-shRNA DFCs was analyzed by RT-qPCR and WB. In vivo, DKK3-shRNA DFCs seeded on hydroxyapatite/β-tricalcium phosphate (HA/TCP) scaffolds were transplanted into the subcutaneous tissue of mice with severe combined immunodeficiency, followed by hematoxylin-eosin and Masson staining. The results confirmed that DKK3 expression was downregulated during mineral induction in rat DFCs. Lentivirus-mediated expression of DKK3 shRNA in DFCs promoted calcified-nodule formation, ALP activity and the expression of β-catenin, runt-related transcription factor 2 and osteocalcin, compared with control cells. In vivo, the implanted section presented the majority of newly formed osteoid matrices and collagen, with limited space between the HA/TCP scaffolds and matrices. In conclusion, DKK3 expression negatively regulates the osteogenic differentiation of DFCs and, conversely, downregulation of DKK3 may enhance DFC osteogenesis.

Association of polymorphisms in Dickopff (DKK) gene towards modulating risk for lung cancer in north Indians

AIM

To investigate the association between the genetic variants of DKK4 (rs2073664), DKK3 (rs2291599, rs3206824 and rs7391689) and DKK2 (rs447372, rs419558 and rs17037102) and lung cancer predisposition in north Indians.

MATERIALS & METHODS

A total of 600 subjects were genotyped using PCR restriction fragment length polymorphism technique. Association analysis was carried out using logistic regression approach.

RESULTS

DKK3 rs7396187 showed a protective effect (p = 0.01). Subjects with heterozygous genotype of DKK2 rs17037102 and rs419558 showed an increased risk. The variant genotype of the genotypic combination, DKK3 rs3206824 and DKK2 rs419558 showed a twofold increased risk (p = 0.008). Lung cancer risk increased four-times in subjects with variant genotype of all the three DKK2 variants.

CONCLUSION

DKK2 is certainly playing a crucial role in modulating cancer susceptibility.

Surveillance for hepatocellular carcinoma

Hepatocellular carcinoma (HCC) appears mainly in patients with underlying liver disease and it is recognized as one of the most important causes of death in this population. Early detection by surveillance has been suggested as an effective tool for reducing cancer-specific mortality and the most accepted strategy is semiannual abdominal ultrasound in those patients at risk of HCC development. The benefit of HCC surveillance is proven by a randomized-controlled study, several prospective or retrospective analyses, and multiple modeling studies and according to the current scientific evidence, surveillance of HCC should be recommended and widely implemented. Major efforts should be done for improving the diagnostic accuracy of the screening tools and for better identifying those patients at risk of HCC development in whom a surveillance program would be cost-effective.

Molecular dynamics of Dkk4 modulates Wnt action and regulates meibomian gland development

Secreted Dickkopf (Dkk) proteins are major Wnt pathway modulators during organ development. Dkk1 has been widely studied and acts as a general Wnt inhibitor. However, the molecular function of other Dkks remains largely unknown. Here, we show that Dkk4 selectively inhibits a subset of Wnts, but is further inactivated by proteolytic cleavage. Meibomian gland (MG) formation is employed as a model where Dkk4 and its Wnt targets are expressed. Skin-specific expression of Dkk4 arrests MG growth at early germ phase, which is similar to that observed in Eda-ablated Tabby mice. Consistent with transient Dkk4 action, intact Dkk4 inhibits MG extension but the cleaved form progressively increases during MG development with a concomitant upswing in Wnt activity. Furthermore, both Dkk4 and its receptor (and Wnt co-receptor) Lrp6 are direct Eda targets during MG induction. In cell and organotypic cultures, Dkk4 inhibition is eliminated by elevation of Lrp6. Also, Lrp6 upregulation restores MG formation in Tabby mice. Thus, the dynamic state of Dkk4 itself and its interaction with Lrp6 modulates Wnt function during MG development, with a novel limitation of Dkk4 action by proteolytic cleavage.