Functional and structural diversity of the human Dickkopf gene family

Dickkopf3 overexpression inhibits pancreatic cancer cell growth in vitro

AIM: To elucidate the role of dickkopf3 (Dkk3) in human pancreatic cancer cell growth.

METHODS: Dkk3 mRNA and protein expression in human pancreatic cancer cell lines were detected by real-time reverse transcription polymerase chain reaction (real-time RT-PCR), Western blotting and immunofluorescence. Methylation of the Dkk3 promoter sequence was examined by methylation-specific polymerase chain reaction (MSP) and Dkk3 mRNA expression was determined by real-time RT-PCR after 5-aza-2’-deoxycytidine (5-aza-dC) treatment. The effects of Dkk3 on cancer cell proliferation and in vitro sensitivity to gemcitabine were investigated by CellTiter 96^® AQueous One Solution Cell Proliferation Assay (MTS) after transfecting the Dkk3 expression plasmid into human pancreatic cancer cells. The expression of β-catenin, phosphorylated extracellular signal-regulated protein kinases (pERK) and extracellular signal-regulated protein kinases (ERK) was also examined by real-time RT-PCR and Western blotting after upregulating Dkk3 expression in human pancreatic cancer cells.

RESULTS: The results show that the expression levels of both Dkk3 mRNA and protein were low in all pancreatic cancer cell lines tested. The Dkk3 promoter sequence was methylated in the MIA PaCa-2 and AsPC-1 cell lines, which showed reduced Dkk3 expression. These two cell lines, which initially had a methylated Dkk3 promoter, showed increased Dkk3 mRNA expression that was dependent upon the dosage and timing of the DNA demethylating agent, 5-aza-dC, treatment (P < 0.05 or P < 0.01). When Dkk3 expression was upregulated following the transfection of a Dkk3 expression plasmid into MIA PaCa-2 cells, the ability of cells to proliferate decreased (P < 0.01), and the expression of β-catenin and pERK was downregulated (P < 0.01). Sensitivity to gemcitabine was enhanced in Dkk3 expression plasmid-transfected cells.

CONCLUSION: Our findings, for the first time, implicate Dkk3 as a tumor suppressor in human pancreatic cancer, through the downregulation of β-catenin expression via the ERK-mediated pathway.

Distinct expression patterns of dickkopf genes during late embryonic development of Danio rerio

Dickkopf (dkk) genes belong to the family of secreted wnt-inhibitors with conserved cysteine-rich domains. In contrast to the prototype dkk1, dkk3 does not modulate canonical Wnt/β-catenin signalling. Until now, neither functions nor interaction partners of dkk3 in lower vertebrates have been described. In this study we cloned two dkk3 homologues dkk3a(dkk3l) and dkk3b(dkk3) and a dkk1 homologue dkk1a of the zebrafish and studied their expression patterns during embryonic development in comparison to the known dkk1b gene. Moreover, mutants with defects in hedgehog signalling (smo), notch (mib) signalling, nodal signalling (Zoep) or retinoic acid synthesis (neckless) were analyzed for changes in dkk3 gene expression. In situ hybridization analyses showed a dynamic expression of dkk1a and dkk1b primarily in epidermal structures of the otic vesicle, lens, branchial arches and fin folds. While dkk1a was expressed mainly in deep tissues, dkk1b expression was mainly found in protrusions at the outer surface of the branchial arch epidermis. In contrast, dkk3 genes showed expression in different tissues. Strong signals for dkk3a(dkk3l) were present in various neuronal structures of the head, whereas dkk3b(dkk3) expression was restricted mainly to endocrine cells of the pancreas and to the brachial arches. In summary, both dkk3 genes display a unique and distinct expression pattern in late embryonic development, pointing to a specific role during neuronal and pancreatic cell differentiation.

Dickkopfs and Wnt/β-catenin signalling in liver cancer

Liver cancer is the fifth and seventh most common cause of cancer in men and women, respectively. Wnt/β-catenin signalling has emerged as a critical player in both the development of normal liver as well as an oncogenic driver in hepatocellular carcinoma (HCC). Based on the current understanding, this article summarizes the possible mechanisms for the aberrant activation of this pathway with specific focus on HCC. Furthermore, we will discuss the role of dickkopfs (DKKs) in regulating Wnt/β-catenin signalling, which is poorly understood and understudied. DKKs are a family of secreted proteins that comprise at least four members, namely DKK1-DKK4, which act as inhibitors of Wnt/β-catenin signalling. Nevertheless, not all members antagonize Wnt/β-catenin signalling. Their functional significance in hepatocarcinogenesis remains to be further characterized for which these studies should provide new insights into the regulatory role of DKKs in Wnt/β-catenin signalling in hepatic carcinogenesis. Because of the important oncogenic roles, there are an increasing number of therapeutic molecules targeting β-catenin and the Wnt/β-catenin pathway for potential therapy of HCC.

A rate-limiting role for Dickkopf-1 in bone formation and the remediation of bone loss in mouse and primate models of postmenopausal osteoporosis by an experimental therapeutic antibody

Genetic studies have linked both osteoporotic and high bone mass phenotypes to low-density lipoprotein receptor-related proteins (LRP4, LRP5, and LRP6). LRPs are receptors for inhibitory Dickkopf-1 (DKK1) protein, and treatment modalities that modulate LRP/DKK1 binding therefore may act as stimulators of bone mass accrual. Here, we report that RH2-18, a fully human monoclonal anti-DKK1 antibody elicits systemic pharmacologic bone efficacy and new bone formation at endosteal bone surfaces in vivo in a mouse model of estrogen-deficiency-induced osteopenia. This was paralleled by partial-to-complete resolution of osteopenia (bone mineral density) at all of the skeletal sites investigated in femur and lumbar-vertebral bodies and the restoration of trabecular bone microarchitecture. More importantly, testing of RH2-18 in adult, osteopenic rhesus macaques demonstrated a rate-limiting role of DKK1 at multiple skeletal sites and responsiveness to treatment. In conclusion, this study provides pharmacologic evidence for the modulation of DKK1 bioactivity in the adult osteopenic skeleton as a viable approach to resolve osteopenia in animal models. Thus, data described here suggest that targeting DKK1 through means such as a fully human anti-DKK1-antibody provides a potential bone-anabolic treatment for postmenopausal osteoporosis.

Expression of REIC/Dkk-3 in normal and hyperproliferative epidermis

Dickkopf (Dkk) family members are known as Wnt modulators involved in the development, cell growth/differentiation and cancer. REIC/Dkk-3, which does not interfere with Wnt signalling, has been proposed to be a tumor suppressor gene, but its physiological function has remained unclear. In this study, we analysed the expression of REIC/Dkk-3 in normal interfollicular epidermis (IFE) and hyperproliferative epidermis. REIC/Dkk-3 was expressed in human and mouse IFE, being localized at the interface of upper spinous layer and granular layer. Skin cancer cell lines lost REIC/Dkk-3 expression as reported previously. When we analysed patient samples, REIC/Dkk-3 expression was down-regulated in the hyperproliferative epidermis including skin cancers and non-cancerous proliferative diseases. REIC/Dkk-3 expression was also suppressed in the regenerative and inflammative epidermis of model mice. These findings will certainly contribute to the extension of studies on REIC/Dkk-3.

Differential effect of oxidative or excitotoxic stress on the transcriptional profile of amyotrophic lateral sclerosis-linked mutant SOD1 cultured neurons

Amyotrophic lateral sclerosis (ALS) is a progressive, lethal, degenerative disorder of motor neurons. The causes of most cases of ALS are as yet undefined. In a previous study, it was shown that N-methyl-D-aspartate (NMDA) and H(2)O(2) stimuli reduce neuronal survival in cortical neurons in culture (Boutahar et al., 2008). To identify variations in gene expression in response to these neurotoxins in transgenic vs. control cortical neurons cultures, both microarray and RT-PCR analysis were performed. High-density oligonucleotide microarrays showed changes in the expression of about 600 genes involved in protein degradation, neurotrophic factors pathway, cell cycle, inflammation, cytoskeleton, cell adhesion, transcription, or signalling. The most up-regulated genes following H(2)O(2) treatment were involved in cytoskeletal organization and axonal transport, such as ARAP2, KIF17, and DKK2, or in trophic factors pathways, such as insulin-like growth factor-binding protein 4 (IGFBP4), FGF17, and serpin2. The most down-regulated genes were involved in ion transport, such as TRPV1. After NMDA treatment, the most up-regulated genes were involved in protein degradation, such as ubiquitin-conjugating enzyme E2I and cathepsin H, and the most down-regulated genes were involved in ion transport, such as SCN7A. We conclude that these neurotoxins act through different transcriptional inductions, and these changes may reflect an adaptative cellular response to the cellular stress induced by the neurotoxins involved in ALS in the presence of mutant human SOD1.

Dickkopf-1 is frequently overexpressed in ovarian serous carcinoma and involved in tumor invasion

Dickkopf-1 (DKK1) was known as a negative regulator of the Wnt signaling pathway, which played a crucial role in carcinogenesis. However, its function in human cancers remained elusive. In this study, we aimed to investigate the role of DKK1 in ovarian serous papillary adenocarcinoma (OSC) tumor progression and invasion. Quantitative real-time RT-PCR and Western blot analysis showed that the expression of DKK1 mRNA and protein in 32 OSC tissues were elevated as compared with those in 10 normal ovarian tissues (P = 0.005, P = 0.003, respectively). Immunohistochemical analysis in 178 clinical OSC samples showed that the expression of DKK1 was positively associated with FIGO stage (P = 0.016). Furthermore, the expression of DKK1 protein was positively associated with P-JNK1 protein expression in 178 OSC tissues. DKK1, P-JNK1 and the co-expression of DKK1 and P-JNK1 were all unfavorable prognosis factors for OSC patients (P < 0.0001). DKK1, alone or combined with P-JNK1, was an independent predictor for the 5 year survival (P = 0.015, P < 0.0001, respectively). DKK1 could promote OSC cells invasion and the growth of OSC nude mice xenograft. DKK1 in OSC cells could activate P-JNK1 expression and significantly promote formations of actin filaments and filopodia. Thus, DKK1, alone or combined with P-JNK1, was a novel prognostic predictor for OSC patients and contributed to the invasion of OSC.

Circulating Dickkopf-1 is correlated with bone erosion and inflammation in rheumatoid arthritis

OBJECTIVE

To explore the potential role of Dickkopf-1 (DKK-1) in rheumatoid arthritis (RA) and to evaluate the effect of a tumor necrosis factor-α (TNF-α) inhibitor (infliximab) and an interleukin 1 receptor antagonist (IL-1Ra; anakinra) on DKK-1 secretion in patients with RA.

METHODS

Serum samples were collected from 100 patients with RA, 100 patients with other rheumatic diseases (e.g., osteoarthritis and ankylosing spondylitis), and 40 healthy controls. DKK-1 and osteoprotegerin (OPG) levels in serum were detected by ELISA. Serum C-reactive protein (CRP) levels, erythrocyte sedimentation rates (ESR), rheumatoid factor (RF) titers, and anti-cyclic citrullinated peptide antibody were also measured in patients with RA.

RESULTS

The serum level of DKK-1 was significantly higher in patients with RA than in healthy controls and those with other rheumatic diseases (p < 0.01); the serum DKK-1 level was correlated with levels of CRP (r = 0.488, p = 0.003) and ESR (r = 0.458, p = 2.4 x 10(-4)) and the Sharp score of radiologic change (r = 0.449, p = 0.001) in RA. In contrast to the increasing level of OPG, DKK-1 was significantly decreased in RA patients treated with TNF-α inhibitor (p < 0.01). DKK-1 was significantly decreased in RA patients treated with IL-1Ra (p < 0.01).

CONCLUSION

DKK-1, as an important mediator, was correlated with bone erosion and inflammation in RA. The change of DKK-1 level may serve as a biomarker of disease activity and bone erosion.

Dkk1 regulates ventral midbrain dopaminergic differentiation and morphogenesis

Dickkopf1 (Dkk1) is a Wnt/β-catenin inhibitor that participates in many processes during embryonic development. One of its roles during embryogenesis is to induce head formation, since Dkk1-null mice lack head structures anterior to midbrain. The Wnt/β-catenin pathway is also known to regulate different aspects of ventral midbrain (VM) dopaminergic (DA) neuron development and, in vitro, Dkk1-mediated inhibition of the Wnt/β-catenin pathway improves the DA differentiation in mouse embryonic stem cells (mESC). However, the in vivo function of Dkk1 on the development of midbrain DA neurons remains to be elucidated. Here we examined Dkk1^+/− embryos and found that Dkk1 is required for the differentiation of DA precursors/neuroblasts into DA neurons at E13.5. This deficit persisted until E17.5, when a defect in the number and distribution of VM DA neurons was detected. Furthermore, analysis of the few Dkk1^−/− embryos that survived until E17.5 revealed a more severe loss of midbrain DA neurons and morphogenesis defects. Our results thus show that Dkk1 is required for midbrain DA differentiation and morphogenesis.

dickkopf-3-related gene regulates the expression of zebrafish myf5 gene through phosphorylated p38a-dependent Smad4 activity

Myf5 is a myogenic regulatory factor that functions in myogenesis. An intronic microRNA, miR-In300, located within zebrafish myf5 intron I, has been reported to silence myf5 through the targeting of dickkopf-3-related gene (dkk3r). However, the molecular mechanism underlying the control of myf5 expression by dkk3r is unknown. By injecting dkk3r-specific morpholino-oligonucleotide (dkk3r-MO) to knock down Dkk3r, we found that the phosphorylated p38a protein was reduced. Knockdown of p38a resulted in malformed somites and reduced myf5 transcripts, which photocopied the defects induced by injection of dkk3r-MO. To block the MAPK pathway, phosphorylation of p38 was inhibited by introduction of SB203580, which caused the down-regulation of myf5 expression. The GFP signal was dramatically decreased in somites when we injected p38a-MO into embryos derived from transgenic line Tg(myf5(80K):GFP), in which the GFP was driven by the myf5 promoter. Although these p38a-MO-induced defects were rescued by co-injection with p38a mRNA, they were not rescued with p38a mRNA containing a mutation at the phosphorylation domain. Moreover, overexpression of Smad2 or Smad3a enhanced myf5 expression, but the defects induced by the dominant negative form of either Smad2 or Smad3a equaled those of embryos injected with either dkk3r-MO or p38a-MO. These results support the involvement of Smad2·Smad3a in p38a mediation. Overexpression of Smad4 enabled the rescue of myf5 defects in the dkk3r-MO-injected embryos, but knockdown of either dkk3r or p38a caused Smad4 protein to lose stability. Therefore, we concluded that Dkk3r regulates p38a phosphorylation to maintain Smad4 stability, in turn enabling the Smad2·Smad3a·Smad4 complex to form and activate the myf5 promoter.

International Union of Basic and Clinical Pharmacology. LXXX. The class Frizzled receptors

The receptor class Frizzled, which has recently been categorized as a separate group of G protein-coupled receptors by the International Union of Basic and Clinical Pharmacology, consists of 10 Frizzleds (FZD(1-10)) and Smoothened (SMO). The FZDs are activated by secreted lipoglycoproteins of the Wingless/Int-1 (WNT) family, whereas SMO is indirectly activated by the Hedgehog (HH) family of proteins acting on the transmembrane protein Patched (PTCH). Recent years have seen major advances in our knowledge about these seven-transmembrane-spanning proteins, including: receptor function, molecular mechanisms of signal transduction, and the receptor's role in embryonic patterning, physiology, cancer, and other diseases. Despite intense efforts, many question marks and challenges remain in mapping receptor-ligand interaction, signaling routes, mechanisms of specificity and how these molecular details underlie disease and also the receptor's important role in physiology. This review therefore focuses on the molecular aspects of WNT/FZD and HH/SMO signaling discussing receptor structure, mechanisms of signal transduction, accessory proteins, receptor dynamics, and the possibility of targeting these signaling pathways pharmacologically.

Regulation of TCF3 by Wnt-dependent phosphorylation during vertebrate axis specification

A commonly accepted model of Wnt/β-catenin signaling involves target gene activation by a complex of β-catenin with a T-cell factor (TCF) family member. TCF3 is a transcriptional repressor that has been implicated in Wnt signaling and plays key roles in embryonic axis specification and stem cell differentiation. Here we demonstrate that Wnt proteins stimulate TCF3 phosphorylation in gastrulating Xenopus embryos and mammalian cells. This phosphorylation event involves β-catenin-mediated recruitment of homeodomain-interacting protein kinase 2 (HIPK2) to TCF3 and culminates in the dissociation of TCF3 from a target gene promoter. Mutated TCF3 proteins resistant to Wnt-dependent phosphorylation function as constitutive inhibitors of Wnt-mediated activation of Vent2 and Cdx4 during anteroposterior axis specification. These findings reveal an alternative in vivo mechanism of Wnt signaling that involves TCF3 phosphorylation and subsequent derepression of target genes and link this molecular event to a specific developmental process.

Wnt pathway antagonists and angiogenesis

Dysregulation of the Wnt pathway has been extensively studied in multiple diseases, including some angiogenic disorders. Wnt signaling activation is a major stimulator in pathological angiogenesis and thus, Wnt antagonists are believed to have therapeutic potential for neovascular disorders. Actually, some Wnt antagonists have been identified directly from the anti-angiogenic factor family. This review summarizes the recent progress toward understanding of the roles of Wnt pathway antagonists in angiogenic regulation and their mechanism of action, and exploring their therapeutic potential.

Clinical significance of serum Dkk-3 in patients with gynecological cancer

AIM

The expression of Dkk-3, as a number of the Dkk family was different in different cancer. Because Dkk-3 encodes a secreted protein, we investigated whether the Dkk-3 protein is secreted into the sera of patients with gynecological cancer.

MATERIAL & METHODS

The levels of Dkk-3 protein were assessed by enzyme-linked immunosorbent assay in the sera of 104 patients with gynecological cancer: 36 with ovarian, 40 with cervical and 28 with endometrial cancers.

RESULTS

The serum levels of Dkk-3 protein in patients with ovarian cancer [25.54 (7.99) pg/mL] was lower than in normal individuals [42.08 (14.89) pg/mL] (P = 0.000). But serum levels of Dkk-3 protein in patients with cervical [166.39 (300.68) pg/mL] (P = 0.013) and endometrial cancers [73.64 (23.36) pg/mL] (P = 0.000) were higher than in normal individuals. The serum levels of Dkk-3 protein were associated with clinical stage in patients with cervical and endometrial cancers. In patients with ovarian cancer, the serum levels of Dkk-3 protein were associated with lymphatic metastasis. In patients with cervical cancer, the serum levels of Dkk-3 protein were associated with tumor diameters.

CONCLUSION

Dkk-3 protein detection using enzyme-linked immunosorbent assay as molecular markers can contribute to detection and diagnosis of gynecological cancer, especially for ovarian cancer and endometrial cancer.

Molecular actions and therapeutic potential of lithium in preclinical and clinical studies of CNS disorders

Lithium has been used clinically to treat bipolar disorder for over half a century, and remains a fundamental pharmacological therapy for patients with this illness. Although lithium's therapeutic mechanisms are not fully understood, substantial in vitro and in vivo evidence suggests that it has neuroprotective/neurotrophic properties against various insults, and considerable clinical potential for the treatment of several neurodegenerative conditions. Evidence from pharmacological and gene manipulation studies support the notion that glycogen synthase kinase-3 inhibition and induction of brain-derived neurotrophic factor-mediated signaling are lithium's main mechanisms of action, leading to enhanced cell survival pathways and alteration of a wide variety of downstream effectors. By inhibiting N-methyl-D-aspartate receptor-mediated calcium influx, lithium also contributes to calcium homeostasis and suppresses calcium-dependent activation of pro-apoptotic signaling pathways. In addition, lithium decreases inositol 1,4,5-trisphosphate by inhibiting phosphoinositol phosphatases, a process recently identified as a novel mechanism for inducing autophagy. Through these mechanisms, therapeutic doses of lithium have been demonstrated to defend neuronal cells against diverse forms of death insults and to improve behavioral as well as cognitive deficits in various animal models of neurodegenerative diseases, including stroke, amyotrophic lateral sclerosis, fragile X syndrome, as well as Huntington's, Alzheimer's, and Parkinson's diseases, among others. Several clinical trials are also underway to assess the therapeutic effects of lithium for treating these disorders. This article reviews the most recent findings regarding the potential targets involved in lithium's neuroprotective effects, and the implication of these findings for the treatment of a variety of diseases.

Analysis of the expression profile of Dickkopf-1 gene in human glioma and the association with tumor malignancy

BACKGROUND

Gliomas represent the most common primary malignant brain tumors, yet little is known about the molecular pathogenesis of these tumors. The highly-regulated Wnt signal transduction pathway is essential for normal developmental processes, and defects in the pathway are closely linked to oncogenesis. Dickkopf-1 (DKK-1) is a secreted protein that acts as a potent inhibitor of the Wnt pathway. The aim of this study was to examine the expression profile of DKK-1 gene in human glioma and its association with tumor malignancy.

METHODS

We determined the expression levels of DKK-1 transcript and protein in 12 glioblastoma cell lines, medulloblastoma cells, low-grade glioma cells, and human astrocyte cells by semiquantitative RT-PCR and ELISA. A total of 47 tumor biopsy specimens and 11 normal brain tissue samples from patients with cerebral trauma internal decompression were embedded in paraffin blocks and used for immunostaining. Twenty-six primary tumors and 7 corresponding brain samples were stored in liquid nitrogen and used for RT-PCR. We further examined serologic concentrations and cerebral fluid levels of DKK-1 in patients with tumors.

RESULTS

DKK-1 could only be detected in 12 human glioblastoma cell lines, not in a panel of other tumor and normal cell lines. The difference between glioma patients and healthy individuals was significant. Kendall's tau-c association analysis also revealed the increased DKK-1 protein expression in tumor tissues of higher pathologic classification. The levels of cerebral fluid DKK-1 protein were significantly higher in glioma patients than in healthy donors or in neuronal benign tumor patients, suggesting that the DKK-1 molecule in cerebral fluids can be applicable to detect the presence of glioma and be developed as a novel prognostic treatment.

CONCLUSION

The Wnt antagonist DKK-1 gene may have important roles in glioma tumorigenesis and act as a novel biomarker in human malignant glioblastoma.

Molecular architecture of the DNA replication origin activation checkpoint

Perturbation of DNA replication initiation arrests human cells in G1, pointing towards an origin activation checkpoint. We used RNAi against Cdc7 kinase to inhibit replication initiation and dissect this checkpoint in fibroblasts. We show that the checkpoint response is dependent on three axes coordinated through the transcription factor FoxO3a. In arrested cells, FoxO3a activates the ARF-∣Hdm2-∣p53 → p21 pathway and mediates p15(INK4B) upregulation; p53 in turn activates expression of the Wnt/β-catenin signalling antagonist Dkk3, leading to Myc and cyclin D1 downregulation. The resulting loss of CDK activity inactivates the Rb-E2F pathway and overrides the G1-S transcriptional programme. Fibroblasts concomitantly depleted of Cdc7/FoxO3a, Cdc7/p15, Cdc7/p53 or Cdc7/Dkk3 can bypass the arrest and proceed into an abortive S phase followed by apoptosis. The lack of redundancy between the checkpoint axes and reliance on several tumour suppressor proteins commonly inactivated in human tumours provides a mechanistic basis for the cancer-cell-specific killing observed with emerging Cdc7 inhibitors.

Elevated expression of Dickkopf-1 increases the sensitivity of human glioma cell line SHG44 to BCNU

BACKGROUND

Studies have shown that Dickkopf-1 (DKK-1) is involved in tumorigenesis. Recently, we found that 9 out of 12 human glioma cell lines had high level of DKK-1 protein while the other 3 had very low or non-detectable level of DKK-1. The aim of this study is to further examine the function of DKK-1 in glioma cells.

MATERIALS AND METHODS

The glioma cell line SHG44 was obtained from a patient with grade II-III astrocytoma. SHG44 cells were transfected with a human DKK-1 gene. Transfection of the empty vector pcDNA3.1 was used as negative control. Sensitivity to BCNU was measured by Annexin-V staining. Expression of bax, bcl-2 and caspase-3 of three groups was determined by immunohistochemistry.

RESULTS

The tranfection was confirmed by PCR, RT-PCR and Western blot. More apoptotic cell death was observed in the DKK-1 transfected cells, comparing to the non-transfected cells, or cells with empty vector. The expression of bax and caspase-3 of the SHG44 -DDK-1 increased, whereas the expression of bcl-2 decreased

CONCLUSION

Our results indicated that DKK-1 has a pro-apoptotic function of in glioma.

The overexpression of the putative gut stem cell marker Musashi-1 induces tumorigenesis through Wnt and Notch activation

The RNA-binding protein Musashi-1 (Msi1) has been proposed as a marker of intestinal epithelial stem cells. These cells are responsible for the continuous renewal of the intestinal epithelium. Although the function of Msi1 has been studied in several organs from different species and in mammalian cell lines, its function and molecular regulation in mouse intestinal epithelium progenitor cells are still undefined. We describe here that, in these cells, the expression of Msi1 is regulated by the canonical Wnt pathway, through a mechanism involving a functional Tcf/Lef binding site on its promoter. An in vitro study in intestinal epithelium primary cultures showed that Msi1 overexpression promotes progenitor proliferation and activates Wnt and Notch pathways. Moreover, Msi1-overexpressing cells exhibit tumorigenic properties in xenograft experiments. These data point to a positive feedback loop between Msi1 and Wnt in intestinal epithelial progenitors. They also suggest that Msi1 has oncogenic properties in these cells, probably through induction of both the Wnt and Notch pathways.

Targeting the Wnt signaling pathway for the development of novel therapies for osteoporosis

A number of anti-osteoporotic drugs, predominantly inhibitors of bone resorption, are currently used in the management of patients with osteoporosis to reduce the risk of fractures. While the management of the disease has improved significantly, there are still unmet needs, mainly due to a lack of agents able to replace bone that has already been lost. Human and animal genetics have identified the pivotal role of the Wnt signaling pathway in the regulation of bone formation by the osteoblasts and have made it a very attractive target for the development of novel treatments for osteoporosis. In this article, we review evidence that supports the targeting of components of the Wnt signaling pathway for the design of bone-forming treatments for osteoporosis.

Analysis of Dickkopf3 interactions with Wnt signaling receptors

Wnt signaling regulates essential biological processes ranging from embryogenesis to neurodegeneration. Recently, we demonstrated that Dickkopf3 (Dkk3) is a pro-survival glycoprotein that positively modulates Wnt signaling. An important step in understanding the mechanism of action of Dkk3 is identifying its interacting proteins in the Wnt pathway. In this study, we used a series of biochemical and functional assays to investigate the interaction between Dkk3 and the Wnt pathway receptors Kremen1 (Krm1), Kremen 2 (Krm2) and low-density lipoprotein receptor-related protein 6 (LRP6). Here, we report that, contrary to previous studies, Dkk3 interacts with Krm1 and Krm2. However, Dkk3 did not interact with, or alter expression of, LRP6. Blocking protein glycosylation did not alter the interaction between Dkk3 and Krm proteins. Additionally, Krm2 abolished Dkk3-mediated potentiation of Wnt signaling. Therefore, our data establish that Krm proteins are novel binding partners of Dkk3 and suggest a mechanism by which Dkk3 potentiates Wnt signaling.

Induction of neuro-protective/regenerative genes in stem cells infiltrating post-ischemic brain tissue

BACKGROUND

Although the therapeutic potential of bone marrow-derived stromal stem cells (BMSC) has been demonstrated in different experimental models of ischemic stroke, it remains unclear how stem cells (SC) induce neuroprotection following stroke. In this study, we describe a novel method for isolating BMSC that infiltrate postischemic brain tissue and use this method to identify the genes that are persistently activated or depressed in BMSC that infiltrate brain tissue following ischemic stroke.

METHODS

Ischemic strokes were induced in C57BL/6 mice by middle cerebral artery occlusion for 1 h, followed by reperfusion. BMSC were isolated from H-2 Kb-tsA58 (immortomouse) mice, and were administered (i.v.) 24 h after reperfusion. At the peak of therapeutic improvement (14 days after the ischemic insult), infarcted brain tissue was isolated, and the BMSC were isolated by culturing at 33 degrees C. Microarray analysis and RT-PCR were performed to compare differential gene expression between naïve and infiltrating BMSC populations.

RESULTS

Z-scoring revealed dramatic differences in the expression of extracellular genes between naïve and infiltrating BMSC. Pair-wise analysis detected 80 extracellular factor genes that were up-regulated (>/= 2 fold, P < 0.05, Benjamini-Hochberg correction) between naïve and infiltrated BMSC. Although several anticipated neuroregenerative, nerve guidance and angiogenic factor (e.g., bFGF, bone morphogenetic protein, angiopoietins, neural growth factor) genes exhibited an increased expression, a remarkable induction of genes for nerve guidance survival (e.g., cytokine receptor-like factor 1, glypican 1, Dickkopf homolog 2, osteopontin) was also noted.

CONCLUSIONS

BMSC infiltrating the post-ischemic brain exhibit persistent epigenetic changes in gene expression for numerous extracellular genes, compared to their naïve counterparts. These genes are relevant to the neuroprotection, regeneration and angiogenesis previously described following stem cell therapy in animal models of ischemic stroke.

A systematic gene-based screen of chr4q22-q32 identifies association of a novel susceptibility gene, DKK2, with the quantitative trait of alcohol dependence symptom counts

Studies of alcohol dependence (AD) have consistently found evidence of linkage on chromosome 4q21-q32. A genome-wide linkage scan in the Irish Affected Sib Pair Study of Alcohol Dependence (IASPSAD) sample also provided its strongest evidence of linkage on chromosome 4q22-q32 using an index of AD severity based on the count of DSM-IV AD symptoms (ADSX; LOD = 4.59). We conducted a systematic, gene-centric association study using 518 LD-tagging single nucleotide polymorphisms (SNPs) in the 65 known and predicted genes within the 1-LOD interval surrounding the linkage peak. Case-only regression analysis with the quantitative variable of ADSX was performed in the 562 genetically independent cases; nominal support for association was demonstrated by 32 tagging SNPs in 14 genes. We did not observe study-wide significance, but gene-wise correction for multiple testing with the Nyholt procedure yielded empirical evidence of association with two genes, DKK2 (dickkopf homolog 2) (P = 0.007) and EGF (epidermal growth factor) (P = 0.025) in the IASPSAD sample. Three SNPs in DKK2 (rs427983; rs419558; rs399087) demonstrated empirical significance. Assessment of possible replication in 847 cases of European descent from a large independent sample, the Collaborative Study of the Genetics of Alcoholism, yielded replication for DKK2 but not EGF. We observed genotypic and phenotypic replication for DKK2 with the three SNPs yielding significant association with ADSX in the IASPSAD sample. Haplotype-specific expression measurements in post-mortem tissue samples suggested a functional role for DKK2. This evidence notwithstanding, replication is needed before confidence can be placed in these findings.

Multiplex N-terminome analysis of MMP-2 and MMP-9 substrate degradomes by iTRAQ-TAILS quantitative proteomics

Proteolysis is a major protein posttranslational modification that, by altering protein structure, affects protein function and, by truncating the protein sequence, alters peptide signatures of proteins analyzed by proteomics. To identify such modified and shortened protease-generated neo-N-termini on a proteome-wide basis, we developed a whole protein isobaric tag for relative and absolute quantitation (iTRAQ) labeling method that simultaneously labels and blocks all primary amines including protein N- termini and lysine side chains. Blocking lysines limits trypsin cleavage to arginine, which effectively elongates the proteolytically truncated peptides for improved MS/MS analysis and peptide identification. Incorporating iTRAQ whole protein labeling with terminal amine isotopic labeling of substrates (iTRAQ-TAILS) to enrich the N-terminome by negative selection of the blocked mature original N-termini and neo-N-termini has many advantages. It enables simultaneous characterization of the natural N-termini of proteins, their N-terminal modifications, and proteolysis product and cleavage site identification. Furthermore, iTRAQ-TAILS also enables multiplex N-terminomics analysis of up to eight samples and allows for quantification in MS2 mode, thus preventing an increase in spectral complexity and extending proteome coverage by signal amplification of low abundance proteins. We compared the substrate degradomes of two closely related matrix metalloproteinases, MMP-2 (gelatinase A) and MMP-9 (gelatinase B), in fibroblast secreted proteins. Among 3,152 unique N-terminal peptides identified corresponding to 1,054 proteins, we detected 201 cleavage products for MMP-2 and unexpectedly only 19 for the homologous MMP-9 under identical conditions. Novel substrates identified and biochemically validated include insulin-like growth factor binding protein-4, complement C1r component A, galectin-1, dickkopf-related protein-3, and thrombospondin-2. Hence, N-terminomics analyses using iTRAQ-TAILS links gelatinases with new mechanisms of action in angiogenesis and reveals unpredicted restrictions in substrate repertoires for these two very similar proteases.

Novel intronic microRNA represses zebrafish myf5 promoter activity through silencing dickkopf-3 gene

A strong, negative cis-element located at the first intron +502/+835 (I300) of zebrafish myf5 has been reported. To elucidate the molecular mechanism underlying this repression network, we microinjected zebrafish single-cell embryos with I300 RNA, resulting in the dramatic reduction of luciferase activity driven by the myf5 promoter. Within this I300 segment, we identified an intronic microRNA (miR-In300) located at +609/+632 and found that it was more highly expressed in the older mature somites than those newly formed, which negatively correlated with the distribution of zebrafish myf5 transcripts. We proved that miR-In300 suppressed the transcription of myf5 through abolishing myf5 promoter activity, and we subsequently identified the long isoform of the Dickkopf-3 gene (dkk3) as the target gene of miR-In300. We further found that injection of the dkk3-morpholinos (MOs) resulted in downregulation of myf5 transcripts in somites, whereas co-injection of myf5 mRNA with dkk3-MO1 enabled rescue of the defects induced by dkk3-MO1 alone. Finally, injection of miR-In300-MO enhanced both myf5 transcripts in somites and the level of Dkk3 protein in zebrafish embryos. Based on these findings, we concluded that miR-In300 binds to its target gene dkk3, which inhibits the translation of dkk3 mRNA and, in turn, suppresses zebrafish myf5 promoter activity.

MYCN oncoprotein targets and their therapeutic potential

The MYCN oncogene encodes a transcription factor which is amplified in up to 40% of high risk neuroblastomas. MYCN amplification is a well-established poor prognostic marker in neuroblastoma, however the role of MYCN expression and the mechanisms by which it acts to promote an aggressive phenotype remain largely unknown. This review discusses the current evidence identifying the direct and indirect downstream transcriptional targets of MYCN from recent studies, with particular reference to how MYCN affects the cell cycle, DNA damage response, differentiation and apoptosis in neuroblastoma.

Hairless is a nuclear receptor corepressor essential for skin function

The activity of nuclear receptors is modulated by numerous coregulatory factors. Corepressors can either mediate the ability of nuclear receptors to repress transcription, or can inhibit transactivation by nuclear receptors. As we learn more about the mechanisms of transcriptional repression, the importance of repression by nuclear receptors in development and disease has become clear. The protein encoded by the mammalian Hairless (Hr) gene was shown to be a corepressor by virtue of its functional similarity to the well-established corepressors N-CoR and SMRT. Mutation of the Hr gene results in congenital hair loss in both mice and men. Investigation of Hairless function both in vitro and in mouse models in vivo has revealed a critical role in maintaining skin and hair by regulating the differentiation of epithelial stem cells, as well as a putative role in regulating gene expression via chromatin remodeling.

Mechano-transduction in osteoblastic cells involves strain-regulated estrogen receptor alpha-mediated control of insulin-like growth factor (IGF) I receptor sensitivity to Ambient IGF, leading to phosphatidylinositol 3-kinase/AKT-dependent Wnt/LRP5 receptor-independent activation of beta-catenin signaling

The capacity of bones to adjust their mass and architecture to withstand the loads of everyday activity derives from the ability of their resident cells to respond appropriately to the strains engendered. To elucidate the mechanisms of strain responsiveness in bone cells, we investigated in vitro the responses of primary mouse osteoblasts and UMR-106 osteoblast-like cells to a single period of dynamic strain. This stimulates a cascade of events, including activation of insulin-like growth factor I receptor (IGF-IR), phosphatidylinositol 3-kinase-mediated phosphorylation of AKT, inhibition of GSK-3β, increased activation of β-catenin, and associated lymphoid-enhancing factor/T cell factor-mediated transcription. Initiation of this pathway does not involve the Wnt/LRP5/Frizzled receptor and does not culminate in increased IGF transcription. The effect of strain on IGF-IR is mimicked by exogenous des-(1–3)IGF-I and is blocked by the IGF-IR inhibitor H1356. Inhibition of strain-related prostanoid and nitric oxide production inhibits strain-related (and basal) AKT activity, but their separate ectopic administration does not mimic it. Strain-related IGF-IR activation of AKT requires estrogen receptor α (ERα) with which IGF-1R physically associates. The ER blocker ICI 182,780 increases the concentration of des-(1–3)IGF-I necessary to activate this cascade, whereas estrogen inhibits both basal AKT activity and its activation by des-(1–3)IGF-I. These data suggest an initial cascade of strain-related events in osteoblasts in which strain activates IGF-IR, in association with ERα, so initiating phosphatidylinositol 3-kinase/AKT-dependent activation of β-catenin and altered lymphoid-enhancing factor/T cell factor transcription. This cascade requires prostanoid/nitric oxide production and is independent of Wnt/LRP5.

Is Wnt signalling the final common pathway leading to bone formation?

Since the discovery of the link between mutations in the LRP5 gene and human bone mass, considerable progress has been made in our understanding of Wnt signalling and bone formation. The connection between canonical Wnt signalling and bone formation is convincing, and there is evidence of interaction between the Wnt signalling pathway and key growth factors, transcriptional factors and systemic hormones. More recently, the role of the non-canonical pathway in bone metabolism has also started to be explored as well as potential bone-gut interactions. This review focuses on the role of the Wnt pathway in osteoblast differentiation as well as the interplay between Wnt signalling and other pathways involved in bone formation.

Neucrin is a novel neural-specific secreted antagonist to canonical Wnt signaling

A gene encoding a novel secreted protein in mice and humans was identified, and named Neucrin. Mouse Neucrin consists of 343 amino acids with a cysteine-rich domain in its carboxyl terminal region. The positions of 10 cysteine residues in the cysteine-rich domain are similar to those of Dickkopfs (Dkks), secreted Wnt antagonists. However, whereas Dkks have two cysteine-rich domains, Neucrin has only one. Neucrin as well as Dkks bound to LDL receptor-related protein 6 and inhibited the stabilization of cytosolic beta-catenin, indicating that Neucrin is an antagonist of canonical Wnt signaling. Mouse Neucrin expression was not detected in any major tissues in the adult, but was detected in developing neural tissues, including the brain and spinal cord. The expression pattern of Neucrin is distinct from that of any Dkk. Neucrin is a unique secreted Wnt antagonist that is predominantly expressed in developing neural tissues.

Unconventional secretion of tubby and tubby-like protein 1

Tubby-like proteins (Tulps) with no signal peptide have been characterized as cytoplasmic proteins with various intracellular functions, including binding to phosphatidylinositol-4,5-bisphosphate [PI(4,5)P(2)]. PI(4,5)P(2) has been implicated in unconventional secretion of fibroblast growth factor-2 without a signal peptide. Here, we show that all Tulps are expressed intracellularly and extracellularly. Tubby secretion is partially dependent on its PI(4,5)P(2)-binding activity with an essential secretory signal in the N-terminus. Pathogenic mutation in Tubby mice has no impact on tubby extracellular trafficking. Moreover, unconventional secretion of tubby and Tulp1 is independent of endoplasmic reticulum-Golgi pathway. These data implicate that Tulps may function extracellularly as well.

Expression, purification and functional characterization of Wnt signaling co-receptors LRP5 and LRP6

Activation of the Wnt signaling cascade plays a pivotal role during development and in various disease states. Wnt signals are transduced by seven-transmembrane Frizzled (Fz) proteins and the single-transmembrane LDL-receptor-related proteins 5 or 6 (LRP5/6). Genetic mutations resulting in a loss or gain of function of LRP5 in humans lead to osteopenia and bone formation, respectively. These findings demonstrate the genetic link between LRP5 signaling and the regeneration of bone mass. Herein we describe for the first time the production and characterization of soluble ectodomains of LRP5 and LRP6, (EC-LRP5, EC-LRP6). We have produced these proteins in amounts that are compatible with both in vitro and cell-based assays to study their binding properties. Purified EC-LRP5 and EC-LRP6 were able to interact with Wnt signaling components Dkk1 and Dkk2 and their functionality was confirmed in cell-based Wnt signaling assays. Hence, tools are now available to explore LRP5/6 interaction with other proteins and to screen for synthetic or natural compounds and biologics that might be novel therapeutics targeting the Wnt pathway.

Dickkopf-1: a suitable target for the management of myeloma bone disease

Bone disease remains a major problem in the management of patients with multiple myeloma (MM) and is characterized by the presence of lytic lesions due to increased osteoclastic activity and reduced osteoblast function. Wingless-type and integrase 1 (Wnt)/beta-catenin signaling is a central pathway for bone development and homeostasis. Dickkopf-1 (Dkk-1) is a soluble inhibitor of Wnt, which disrupts osteoblast differentiation and action. Dkk-1 is produced by myeloma cells and overexpressed in myeloma microenvironment of patients with extensive bone disease. In addition to its direct inhibitory effect of Dkk-1 on osteoblasts, Dkk-1 disrupts the Wnt3a-regulated osteoprotegerin and receptor activator of NF-kappaB ligand (RANKL) expression in osteoblasts and thus it indirectly enhances osteoclast function in MM. Dkk-1 serum and bone marrow plasma levels are increased in MM patients and correlated with advanced International Staging System stage and presence of osteolytic lesions. Preclinical studies in mouse myeloma models showed that targeting Dkk-1 with neutralizing anti-Dkk-1 antibodies resulted in increased numbers of osteoblasts, reduced numbers of multinucleated osteoclasts and increased bone volume. The bone anabolic effect of anti-Dkk-1 may also be associated with reduced myeloma burden. These data show that Dkk-1 has a pivotal role in bone health and disease and is a novel target for the management of myeloma patients with lytic bone disease.

Axl as a potential therapeutic target in cancer: role of Axl in tumor growth, metastasis and angiogenesis

Dysregulation of Axl and its ligand growth arrest-specific 6 is implicated in the pathogenesis of several human cancers. In this study, we have used RNAi and monoclonal antibodies to assess further the oncogenic potential of Axl. Here we show that Axl knockdown reduces growth of lung and breast cancer xenograft tumors. Inhibition of Axl expression attenuates breast cancer cell migration and inhibits metastasis to the lung in an orthotopic model, providing the first in vivo evidence that links Axl directly to cancer metastasis. Axl knockdown in endothelial cells impaired tube formation and this effect was additive with anti-vascular endothelial growth factor (VEGF). Further analysis demonstrated that Axl regulates endothelial cell functions by modulation of signaling through angiopoietin/Tie2 and Dickkopf (DKK3) pathways. We have developed and characterized Axl monoclonal antibodies that attenuate non-small cell lung carcinoma xenograft growth by downregulation of receptor expression, reducing tumor cell proliferation and inducing apoptosis. Our data demonstrate that Axl plays multiple roles in tumorigenesis and that therapeutic antibodies against Axl may block Axl functions not only in malignant tumor cells but also in the tumor stroma. The additive effect of Axl inhibition with anti-VEGF suggests that blocking Axl function could be an effective approach for enhancing antiangiogenic therapy.

A Wnt survival guide: from flies to human disease

It has been two decades since investigators discovered the link between the Drosophila wingless (Wg) gene and the vertebrate oncogene int-1, thus establishing the family of signaling proteins known as Wnts. Since the inception of the Wnt signaling field, there have been 19 Wnt isoforms identified in humans. These secreted glycoproteins can activate at least two distinct signaling pathways in vertebrate cells, leading to cellular changes that regulate a vast array of biological processes, including embryonic development, cell fate, cell proliferation, cell migration, stem cell maintenance, tumor suppression, and oncogenesis. In certain contexts, one subset of Wnt isoforms activates the canonical Wnt/beta-catenin pathway that is characterized by the activation of certain beta-catenin-responsive target genes in response to the binding of Wnt ligand to its cognate receptors. Similarly, a second subset of Wnt isoforms activates beta-catenin-independent pathways, including the Wnt/calcium (Wnt/Ca) pathway and the Wnt/planar cell polarity (Wnt/PCP) pathway, in certain cellular contexts. In addition, research has identified several secreted proteins known to regulate Wnt signaling, including the Dickkopf (DKK) family, secreted Frizzled-related proteins (sFRPs), and Wnt inhibitory factor-1 (WIF-1). The advent of technologies that can provide genome-wide expression data continues to implicate Wnts and proteins that regulate Wnt signaling pathways in a growing number of disease processes. The aim of this review is to provide a context on the Wnt field that will facilitate the interpretation and study of Wnt signaling in the context of human disease.

Dkk-3 is elevated in CSF and plasma of Alzheimer's disease patients

Biomarkers in CSF can offer improved diagnostic accuracy for Alzheimer's disease (AD). The present study investigated whether the glycoprotein and putative tumor suppressor Dickkopf homolog 3 (Dkk-3) is secreted into CSF and evaluated its applicability as a diagnostic marker for AD. Using our highly specific immunoenzymometric assay, Dkk-3 levels were measured in plasma and/or CSF of patients suffering from depression, mild cognitive impairment (MCI), or AD and compared with healthy subjects. Dkk-3 identity was verified by western blot and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry (MS)/MS. High concentrations of Dkk-3 were detected in CSF compared with plasma (28.2 +/- 1.3 vs. 1.22 +/- 0.04 nmol/L, respectively). Consistently Dkk-3 expression was demonstrated in neurons of the cortex and epithelial cells of the choroid plexus, the major source of CSF. Significantly increased Dkk-3 levels in plasma and CSF were observed for AD patients compared with healthy subjects but not patients suffering from MCI or depression. In summary, our data indicate that elevated Dkk-3 levels are specifically associated with AD and might serve as a potential non-invasive AD biomarker in plasma.

Wnt signaling in Alzheimer's disease: up or down, that is the question

Alzheimer's disease (AD) is a progressive neurodegenerative disorder, neuropathologically characterized by amyloid-beta (Abeta) plaques and hyperphosphorylated tau accumulation. AD occurs sporadically (SAD), or is caused by hereditary missense mutations in the amyloid precursor protein (APP) or presenilin-1 and -2 (PSEN1 and PSEN2) genes, leading to early-onset familial AD (FAD). Accumulating evidence points towards a role for altered Wnt/beta-catenin-dependent signaling in the etiology of both forms of AD. Presenilins are involved in modulating beta-catenin stability; therefore FAD-linked PSEN-mediated effects can deregulate the Wnt pathway. Genetic variations in the low-density lipoprotein receptor-related protein 6 and apolipoprotein E in AD have been associated with reduced Wnt signaling. In addition, tau phosphorylation is mediated by glycogen synthase kinase-3 (GSK-3), a key antagonist of the Wnt pathway. In this review, we discuss Wnt/beta-catenin signaling in both SAD and FAD, and recapitulate which of its aberrant functions may be critical for (F)AD pathogenesis. We discuss the intriguing possibility that Abeta toxicity may downregulate the Wnt/beta-catenin pathway, thereby upregulating GSK-3 and consequent tau hyperphosphorylation, linking Abeta and tangle pathology. The currently available evidence implies that disruption of tightly regulated Wnt signaling may constitute a key pathological event in AD. In this context, drug targets aimed at rescuing Wnt signaling may prove to be a constructive therapeutic strategy for AD.

Comparative expression pathway analysis of human and canine mammary tumors

BACKGROUND

Spontaneous tumors in dog have been demonstrated to share many features with their human counterparts, including relevant molecular targets, histological appearance, genetics, biological behavior and response to conventional treatments. Mammary tumors in dog therefore provide an attractive alternative to more classical mouse models, such as transgenics or xenografts, where the tumour is artificially induced. To assess the extent to which dog tumors represent clinically significant human phenotypes, we performed the first genome-wide comparative analysis of transcriptional changes occurring in mammary tumors of the two species, with particular focus on the molecular pathways involved.

RESULTS

We analyzed human and dog gene expression data derived from both tumor and normal mammary samples. By analyzing the expression levels of about ten thousand dog/human orthologous genes we observed a significant overlap of genes deregulated in the mammary tumor samples, as compared to their normal counterparts. Pathway analysis of gene expression data revealed a great degree of similarity in the perturbation of many cancer-related pathways, including the 'PI3K/AKT', 'KRAS', 'PTEN', 'WNT-beta catenin' and 'MAPK cascade'. Moreover, we show that the transcriptional relationships between different gene signatures observed in human breast cancer are largely maintained in the canine model, suggesting a close interspecies similarity in the network of cancer signalling circuitries.

CONCLUSION

Our data confirm and further strengthen the value of the canine mammary cancer model and open up new perspectives for the evaluation of novel cancer therapeutics and the development of prognostic and diagnostic biomarkers to be used in clinical studies.

Dickkopf-like1 regulates postpubertal spermatocyte apoptosis and testosterone production

Dickkopf-like1 (Dkkl1) encodes a glycoprotein secreted by postmeiotic male germ cells. We report here that adult Dkkl1-deficient males have elevated sperm counts caused by a decrease in postpubertal spermatocyte apoptosis and display, upon aging, increased local production of testosterone. Molecular analyses identified the Fas death ligand (FasL) as a target for Dkkl1 pro-apoptotic activity in adult mice. Accordingly, adult FasL-deficient gld mice display an increased sperm count and decreased spermatocyte apoptosis phenotype similar to the one observed in Dkkl1-deficient mice. We also show that the elevated testosterone level observed in aging Dkkl1-deficient males is secondary to increased expression in Leydig cells of CYP11A and CYP17, two genes implicated in steroidogenesis. Furthermore, treatment of Leydig cells with Dkkl1 decreases DNA binding and transcriptional activity of steroidogenic factor 1, a pivotal regulator of gene expression in testis. Thus, this study establishes Dkkl1 as a negative regulator of adult testis homeostasis and identifies a novel, Dkkl1/FasL-dependent, regulation that specifically controls the number of postpubertal spermatocytes.

Dickkopf-4 is frequently down-regulated and inhibits growth of colorectal cancer cells

Like Dickkopf-1 (DKK1), DKK4 is a target of beta-catenin/Tcf-4 in colorectal cancer. However, as a negative regulator of Wnt signalling its function in colorectal cancer cells is not well understood. We report that DKK4 is frequently down-regulated in colorectal cancer cell lines with deregulated beta-catenin/Tcf-4 and in primary colorectal cancers. Exposure of cancer cells to DKK4 strongly inhibits basal beta-catenin/Tcf-4 signalling activity, cancer cell growth and cell cycle progression. Therefore, loss of this negative feed-back loop provides Wnt factor expressing cancer cells with a growth advantage. Our data demonstrate that DKK4 is an important negative regulator of colon cancer cell growth.

To beta or not to beta: canonical beta-catenin signaling pathway and ovarian development

The mammalian embryonic gonad is a unique organ primordium in that it can adopt two different developmental fates-namely, differentiate as either a testis or an ovary-with dramatic consequences for an individual. While a molecular cascade culminating in testis development is well characterized, the ovarian pathways still remain enigmatic. The canonical Wnt/beta-catenin signaling implements a conserved mechanism of regulating gene expression that is integral to development of all metazoans. In this review, we summarize the recent evidence that suggests a central role for this signaling pathway in the development of the mammalian female.

A novel activity of the Dickkopf-1 amino terminal domain promotes axial and heart development independently of canonical Wnt inhibition

The secreted Dickkopf-1 (Dkk1) protein mediates numerous cell fate decisions and morphogenetic processes. Its carboxyl terminal cysteine-rich region (termed C1) binds LRP5/6 and inhibits canonical Wnt signaling. Paradoxically, the isolated C1 domain of Dkk1 as well as Wnt antagonists that act by sequestering Wnts, such as Frz-B, WIF-1 and Crescent, are poor mimics of the inductive and patterning activities of Dkk1 critical for heart and axial development. To understand the basis for the unique properties of Dkk1, we investigated the function of its amino terminal cysteine-rich region (N1). N1 does not bind LRP or Kremen nor inhibit Wnt signaling and has had no known function. We show that it can synergize with BMP antagonism to induce prechordal and axial mesoderm when expressed as an independent protein in Xenopus embryos. Moreover, we show that it can function in trans to complement the activity of C1 protein to mediate two embryologic functions of Dkk1: induction of chordal and prechordal mesoderm and specification of heart tissue from non-cardiogenic mesoderm. Remarkably, N1 also synergizes with WIF-1 and Crescent, indicating that N1 signals independently of C1 and its interactions with LRP. Since cleavage of Dkk1 is not detected, these results define N1 as a novel signaling domain within the intact protein that is responsible for the potent effects of Dkk1 on the induction and patterning of the body axis and heart. We conclude that this new activity is also likely to synergize with canonical Wnt inhibitory in the numerous developmental and disease processes that involve Dkk1.

Ovarian development in mice requires the GATA4-FOG2 transcription complex

We have demonstrated previously that mammalian sexual differentiation requires both the GATA4 and FOG2 transcriptional regulators to assemble the functioning testis. Here we have determined that the sexual development of female mice is profoundly affected by the loss of GATA4-FOG2 interaction. We have also identified the Dkk1 gene, which encodes a secreted inhibitor of canonical β-catenin signaling, as a target of GATA4-FOG2 repression in the developing ovary. The tissue-specific ablation of theβ-catenin gene in the gonads disrupts female development. In Gata4ki/ki; Dkk1-/- or Fog2-/-;Dkk1-/- embryos, the normal ovarian gene expression pattern is partially restored. Control of ovarian development by the GATA4-FOG2 complex presents a novel insight into the cross-talk between transcriptional regulation and extracellular signaling that occurs in ovarian development.

Wnt signalling in human breast cancer: expression of the putative Wnt inhibitor Dickkopf-3 (DKK3) is frequently suppressed by promoter hypermethylation in mammary tumours

Introduction

Expression of the putative Wnt signalling inhibitor Dickkopf-3 (DKK3) is frequently lost in human cancer tissues because of aberrant 5'-cytosine methylation within the DKK3 gene promoter. Since other Wnt signalling inhibitors have been reported to be targets of epigenetic inactivation in human breast cancer, we questioned if DKK3 expression is also epigenetically silenced during breast carcinogenesis and therefore might contribute to oncogenic Wnt signalling commonly found in this disease.

Methods

DKK3 mRNA expression and DKK3 promoter methylation were determined by RT-PCR, realtime PCR and methylation-specific PCR in breast cell lines (n = 9), normal breast tissues (n = 19) and primary breast carcinomas (n = 150), respectively. In vitro DNA demethylation was performed by incubating breast cell lines with 5-aza-2'-deoxycytidine and trichostatin A. DKK3 protein expression was analysed by immunohistochemistry in breast carcinomas (n = 16) and normal breast tissues (n = 8). Methylation data were statistically correlated with clinical patient characteristics. All statistical evaluations were performed with SPSS 14.0 software.

Results

DKK3 mRNA was downregulated in 71% (five of seven) of breast cancer cell lines and in 68% of primary breast carcinomas (27 of 40) compared with benign cell lines and normal breast tissues, respectively. A DNA demethylating treatment of breast cell lines resulted in strong induction of DKK3 mRNA expression. In tumourous breast tissues, DKK3 mRNA downregulation was significantly associated with DKK3 promoter methylation (p < 0.001). Of the breast carcinomas, 61% (92 of 150) revealed a methylated DKK3 promoter, whereas 39% (58 of 150) retained an unmethylated promoter. Loss of DKK3 expression in association with DKK3 promoter methylation (p = 0.001) was also confirmed at the protein level (p < 0.001). In bivariate analysis, DKK3 promoter methylation was not associated with investigated clinicopathological parameters except patient age (p = 0.007).

Conclusions

DKK3 mRNA expression and consequently DKK3 protein expression become frequently downregulated during human breast cancer development due to aberrant methylation of the DKK3 promoter. Since DKK3 is thought to negatively regulate oncogenic Wnt signalling, DKK3 may be a potential tumour suppressor gene in normal breast tissue.