Crème de la Kremen of Wnt signalling inhibition

ACE2-Independent Alternative Receptors for SARS-CoV-2

Severe acute respiratory syndrome-related coronavirus (SARS-CoV-2), the causative agent of coronavirus disease 2019 (COVID-19), is highly contagious and remains a major public health challenge despite the availability of effective vaccines. SARS-CoV-2 enters cells through the binding of its spike receptor-binding domain (RBD) to the human angiotensin-converting enzyme 2 (ACE2) receptor in concert with accessory receptors/molecules that facilitate viral attachment, internalization, and fusion. Although ACE2 plays a critical role in SARS-CoV-2 replication, its expression profiles are not completely associated with infection patterns, immune responses, and clinical manifestations. Additionally, SARS-CoV-2 infects cells that lack ACE2, and the infection is resistant to monoclonal antibodies against spike RBD in vitro, indicating that some human cells possess ACE2-independent alternative receptors, which can mediate SARS-CoV-2 entry. Here, we discuss these alternative receptors and their interactions with SARS-CoV-2 components for ACE2-independent viral entry. These receptors include CD147, AXL, CD209L/L-SIGN/CLEC4M, CD209/DC-SIGN/CLEC4L, CLEC4G/LSECtin, ASGR1/CLEC4H1, LDLRAD3, TMEM30A, and KREMEN1. Most of these receptors are known to be involved in the entry of other viruses and to modulate cellular functions and immune responses. The SARS-CoV-2 omicron variant exhibits altered cell tropism and an associated change in the cell entry pathway, indicating that emerging variants may use alternative receptors to escape the immune pressure against ACE2-dependent viral entry provided by vaccination against RBD. Understanding the role of ACE2-independent alternative receptors in SARS-CoV-2 viral entry and pathogenesis may provide avenues for the prevention of infection by SARS-CoV-2 variants and for the treatment of COVID-19.

Perspectives on miRNAs Targeting DKK1 for Developing Hair Regeneration Therapy

Androgenetic alopecia (AGA) remains an unsolved problem for the well-being of humankind, although multiple important involvements in hair growth have been discovered. Up until now, there is no ideal therapy in clinical practice in terms of efficacy and safety. Ultimately, there is a strong need for developing a feasible remedy for preventing and treating AGA. The Wnt/β-catenin signaling pathway is critical in hair restoration. Thus, AGA treatment via modulating this pathway is rational, although challenging. Dickkopf-related protein 1 (DKK1) is distinctly identified as an inhibitor of canonical Wnt/β-catenin signaling. Thus, in order to stimulate the Wnt/β-catenin signaling pathway, inhibition of DKK1 is greatly demanding. Studying DKK1-targeting microRNAs (miRNAs) involved in the Wnt/β-catenin signaling pathway may lay the groundwork for the promotion of hair growth. Bearing in mind that DKK1 inhibition in the balding scalp of AGA certainly makes sense, this review sheds light on the perspectives of miRNA-mediated hair growth for treating AGA via regulating DKK1 and, eventually, modulating Wnt/β-catenin signaling. Consequently, certain miRNAs regulating the Wnt/β-catenin signaling pathway via DKK1 inhibition might represent attractive candidates for further studies focusing on promoting hair growth and AGA therapy.

WNT Signalling in Osteoarthritis and Its Pharmacological Targeting

Osteoarthritis (OA) is a highly disabling musculoskeletal condition affecting millions of people worldwide. OA is characterised by progressive destruction and irreversible morphological changes of joint tissues and architecture. At molecular level, de-regulation of several pathways contributes to the disruption of tissue homeostasis in the joint. Overactivation of the WNT/β-catenin signalling pathway has been associated with degenerative processes in OA. However, the multiple layers of complexity in the modulation of the signalling and the still insufficient knowledge of the specific molecular drivers of pathogenetic mechanisms have made difficult the pharmacological targeting of this pathway for therapeutic purposes. This review aims to provide an overview of the WNT/β-catenin signalling in OA with a particular focus on its role in the articular cartilage. Pathway components whose targeting showed therapeutic potential will be highlighted and described. A specific section will be dedicated to Lorecivivint, the first inhibitor of the β-catenin-dependent pathway currently in phase III clinical trial as OA-modifying agent.

Genome-wide methylation analyses identify a subset of mantle cell lymphoma with a high number of methylated CpGs and aggressive clinicopathological features

Mantle cell lymphoma (MCL) is a B-cell neoplasm with an aggressive clinical behavior characterized by the t(11;14)(q13;q32) and cyclin D1 overexpression. To clarify the potential contribution of altered DNA methylation in the development and/or progression of MCL, we performed genome-wide methylation profiling of a large cohort of primary MCL tumors (n = 132), MCL cell lines (n = 6) and normal lymphoid tissue samples (n = 31), using the Infinium HumanMethylation27 BeadChip. DNA methylation was compared to gene expression, chromosomal alterations and clinicopathological parameters. Primary MCL displayed a heterogeneous methylation pattern dominated by DNA hypomethylation when compared to normal lymphoid samples. A total of 454 hypermethylated and 875 hypomethylated genes were identified as differentially methylated in at least 10% of primary MCL. Annotation analysis of hypermethylated genes recognized WNT pathway inhibitors and several tumor suppressor genes as frequently methylated, and a substantial fraction of these genes (22%) showed a significant downregulation of their transcriptional levels. Furthermore, we identified a subset of tumors with extensive CpG methylation that had an increased proliferation signature, higher number of chromosomal alterations and poor prognosis. Our results suggest that a subset of MCL displays a dysregulation of DNA methylation characterized by the accumulation of CpG hypermethylation highly associated with increased proliferation that may influence the clinical behavior of the tumors.

Increased Dickkopf-1 expression in patients with unexplained recurrent spontaneous miscarriage

Wnt pathways play an important role in pre-implantation embryo development, blastocyst implantation, and post-implantation uterine decidualisation. However, little is known about the potential role that Wnt signaling plays in patients with unexplained recurrent spontaneous miscarriage (URSM), and no single biomarker with a high predictive value of maternally caused URSM has been identified. We aim to study the molecular mechanisms by which the Wnt pathway controls the progression of early pregnancy by investigating the expression of Dickkopf-1 (DKK1), one of the Wnt agonists, in URSM patients. Plasma and fresh decidual tissues samples were collected from 59 subjects (29 patients with URSM and 30 patients with normal, early pregnancy). Time-resolved immunofluorometric assay system and quantitative real-time RT-PCR were used to determine the serum levels of DKK1 and DKK1 mRNA in the deciduas, respectively. Western blot and immunohistochemistry were used to measure DKK1 protein levels in the deciduas. Serum DKK1 levels were significantly higher in URSM patients compared to the control group (P < 0·001); the expression of DKK1 mRNA and protein in URSM patients were higher relative to healthy controls (P = 0·013). Glandular epithelium from decidual tissues demonstrated cytoplasmic signals for DKK1 in URSM patients, and DKK1 did not stain in healthy controls. Furthermore, serum DKK1 levels significantly correlated with those in the decidual tissues. Our study suggests that DKK1 may be a valuable biomarker of URSM; it can be reliably and conveniently detected in serum, thus obviating the need for decidual tissue analysis.

Dickkopf-1 (DKK1) promotes invasion and metastasis of hepatocellular carcinoma

BACKGROUND

Dickkopf-1 (DKK1) recently has been reported to be involved in the progress of hepatocellular carcinoma, but its concrete role is not clear. The objective of this study is to investigate the clinical significance, biological function and molecular mechanism of DKK1 in the invasion and metastasis of hepatocellular carcinoma.

METHODS

The mRNA and protein expression levels of DKK1 in hepatocellular carcinoma were detected and its prognostic significance was assessed. The biological function of DKK1 in hepatocellular carcinoma was investigated by using wound healing, transwell invasion assay and hepatocellular carcinoma metastatic mouse model.

RESULTS

DKK1 was predominantly elevated in hepatocellular carcinoma tissues, especially in tissue with vascular invasion. The increased DKK1 expression was correlated with multiple tumour nodes, high Edmondson-Steiner grade and vein invasion, as well as poor overall and disease-free survival of hepatocellular carcinoma. DKK1 could promote hepatocellular carcinoma cell invasion and metastasis in vitro and in vivo. Finally, a positive relationship of DKK1 expression with RhoA and JNK levels was found in both hepatocellular carcinoma cell lines and tissues, suggesting that DKK1 promotes invasion and metastasis of hepatocellular carcinoma possibly through the non-canonical Wnt pathway.

CONCLUSIONS

Collectively, our findings suggest that DKK1 could serve as a novel prognostic marker and therapeutic target for hepatocellular carcinoma.

Wnt signaling influences the development of murine epidermal Langerhans cells

Langerhans cells (LC) are distinct dendritic cells (DC) that populate stratified squamous epithelia. Despite extensive studies, our understanding of LC development is incomplete. TGFβ1 is required for LC development, but other epidermis-derived influences may also be important. Recently, EpCAM (CD326) has been identified as cell surface protein discriminating LC from Langerin⁺ dermal and other DC in skin. EpCAM is a known transcriptional target of the Wnt signaling pathway. We hypothesized that intraepidermal Wnt signaling might influence LC development. Addition of Wnt3A into cultures of bone marrow-derived cells in combination with TGFβ1, GM-CSF, M-CSF resulted in increased (33%; p<0.05) accumulation of EpCAM⁺ DC. In contrast, addition of the Wnt antagonist Dkk1 decreased numbers of EpCAM⁺ DC (21%; p<0.05). We used K14-KRM1; K5-rtTA; tetO-Dkk1 triple transgenic and K5-rtTA; tetO-Dkk1 double transgenic mice to test the in vivo relevance of our in vitro findings. Feeding doxycycline to nursing mothers induced expression of Dkk1 in skin of transgenic pups causing an obvious hair phenotype. Expression of Dkk1 reduced LC proliferation (40%; p<0.01) on P7, decreased LC densities (26%; p<0.05) on P14, and decreased EpCAM expression intensities on LC as well (33%). In aggregate, these data suggest that Wnt signaling in skin influences LC development.

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.

Networking of differentially expressed genes in human cancer cells resistant to methotrexate

Background

The need for an integrated view of data obtained from high-throughput technologies gave rise to network analyses. These are especially useful to rationalize how external perturbations propagate through the expression of genes. To address this issue in the case of drug resistance, we constructed biological association networks of genes differentially expressed in cell lines resistant to methotrexate (MTX).

Methods

Seven cell lines representative of different types of cancer, including colon cancer (HT29 and Caco2), breast cancer (MCF-7 and MDA-MB-468), pancreatic cancer (MIA PaCa-2), erythroblastic leukemia (K562) and osteosarcoma (Saos-2), were used. The differential expression pattern between sensitive and MTX-resistant cells was determined by whole human genome microarrays and analyzed with the GeneSpring GX software package. Genes deregulated in common between the different cancer cell lines served to generate biological association networks using the Pathway Architect software.

Results

Dikkopf homolog-1 (DKK1) is a highly interconnected node in the network generated with genes in common between the two colon cancer cell lines, and functional validations of this target using small interfering RNAs (siRNAs) showed a chemosensitization toward MTX. Members of the UDP-glucuronosyltransferase 1A (UGT1A) family formed a network of genes differentially expressed in the two breast cancer cell lines. siRNA treatment against UGT1A also showed an increase in MTX sensitivity. Eukaryotic translation elongation factor 1 alpha 1 (EEF1A1) was overexpressed among the pancreatic cancer, leukemia and osteosarcoma cell lines, and siRNA treatment against EEF1A1 produced a chemosensitization toward MTX.

Conclusions

Biological association networks identified DKK1, UGT1As and EEF1A1 as important gene nodes in MTX-resistance. Treatments using siRNA technology against these three genes showed chemosensitization toward MTX.

Dickkopf-1 secreted by decidual cells promotes trophoblast cell invasion during murine placentation

Dickkopf-1 (Dkk1) is one of the secreted antagonists in the canonical Wnt signaling pathway. It plays important roles in diverse developmental processes. However, the role of Dkk1 in trophoblast cell invasion during placentation remains unclear. In this study, we found that Dkk1 was mainly expressed in maternal decidual tissue but trivially in ectoplacental cones (EPCs) in day 8 post coitum (p.c.) pregnant mouse uterus and that the efficiency of EPC attachment and outgrowth was increased when co-cultured with decidual cells, which secreted Dkk1, and this enhancement was abolished by pretreating decidual cells with Dkk1 blocking antibody before co-culture experiment. This indicates that Dkk1 secreted by decidual cells plays an important role in trophoblast cell invasion. Indeed, when recombinant mouse Dkk1 was added to EPCs in vitro, the efficiency of attachment and outgrowth was increased. Migration of EPCs toward the decidua was retarded when antisense Dkk1 oligonucleotide (ODN) was administered via intrauterine injection in vivo. Furthermore, the active beta-catenin nuclear location was lost when we treated cultured EPCs with recombinant mouse Dkk1, and the efficiency of EPCs attachment and outgrowth was obviously increased when we treated cultured EPCs with antisense beta-catenin ODN. Taken together, Dkk1 secreted by decidual cells may induce trophoblast cell invasion in the mouse and beta-catenin may be involved in such functions of Dkk1.

Roles of Dickkopf-1 and its receptor Kremen1 during embryonic implantation in mice

OBJECTIVE

To determine the roles of Dickkopf-1 (Dkk1) in mouse embryo implantation.

DESIGN

Experimental prospective study.

SETTING

Animal research and institute laboratory facility.

PATIENT(S)

Virgin Kunming female mice and adult male mice.

INTERVENTION(S)

The expression of Dkk1 and its receptor Kremen1 in embryos and uteri was observed by immunofluorescence or immunohistochemistry. Then, Dkk1 or Kremen1 antisense oligodeoxynucleotides (ODNs) were used to assess their effects on embryo implantation in in vitro or in vivo assays.

MAIN OUTCOME MEASURE(S)

Dynamic changes of Dkk1 and Kremen1 in embryos and uterine stroma during the window of implantation.

RESULT(S)

Dickkopf-1 and Kremen1 are expressed dynamically in both embryos and uterine stroma during embryonic implantation. Dickkopf-1 or Kremen1 antisense ODNs significantly inhibited the adhesion and outgrowth of hatched blastocysts on fibronectin. The expressional patterns of Dkk1 and Kremen1 proteins in the uterine stroma of pseudopregnant, implantation-delayed, and artificially decidualized mice imply the roles of these proteins in uterine receptivity and decidualization. Time-dependent increases of Dkk1 and Kremen1 in uterine stromal cells of ovariectomized mice treated with steroids further suggest that their expression was under the control of maternal steroids E(2) and P. Embryo implantation also was inhibited when Dkk1 antisense ODNs were injected into mouse uterine horns on day 3 of pregnancy.

CONCLUSION(S)

These results suggest an important role of Dkk1 and Kremen1 in blastocyst activation and uterine receptivity during the window of implantation.

The Wnt signaling pathway and bone metabolism

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

Dickkopf homolog 1 mediates endothelin-1-stimulated new bone formation

Tumor-produced endothelin-1 (ET-1) stimulates osteoblasts to form new bone and is an important mediator of osteoblastic bone metastasis. The anabolic actions of ET-1 in osteoblasts were investigated by gene microarray analyses of murine neonatal calvarial organ cultures. Targets of ET-1 action were validated by real-time RT-PCR in murine primary osteoblast cultures. IL-6, IL-11, the CCN (CYR61, CTGF, NOV) family members cysteine-rich protein 61 and connective tissue growth factor, inhibin beta-A, serum/glucocorticoid regulated kinase, receptor activator of nuclear factor kappaB ligand, snail homolog 1, tissue inhibitor of metalloproteinase 3, and TG-interacting factor transcripts were increased by ET-1. ET-1 decreased the transcript for the Wnt signaling pathway inhibitor, dickkopf homolog 1 (Dkk1). Calvarial organ cultures treated with ET-1 had lower concentrations of DKK1 protein in conditioned media than control cultures. High DKK1 concentrations in bone marrow suppress bone formation in multiple myeloma. We hypothesized that the converse occurs in osteoblastic bone metastasis, where ET-1 stimulates osteoblast activity by reducing autocrine production of DKK1. Recombinant DKK1 blocked ET-1-mediated osteoblast proliferation and new bone formation in calvarial organ cultures, whereas a DKK1-neutralizing antibody increased osteoblast numbers and new bone formation. ET-1 directed nuclear translocation of beta-catenin in osteoblasts, indicating activation of the Wnt signaling pathway. The data suggest that ET-1 increases osteoblast proliferation and new bone formation by activating the Wnt signaling pathway through suppression of the Wnt pathway inhibitor DKK1.

The Wnt pathway regulator DKK1 is preferentially expressed in hormone-resistant breast tumours and in some common cancer types

In addition to new tumour antigens, new prognostic and diagnostic markers are needed for common cancers. In this study, we report the expression of Dickkopf-1 (DKK1) in multiple common cancers. This constitutes a comprehensive analysis of the DKK1 expression profile. Dickkopf-1 expression was evaluated by classical and quantitative reverse transcriptase–polymerase chain reaction (RT–PCR) and enzyme-linked immunosorbant assay for protein determination, in cancer lines and clinical specimens of several cancer origins. For breast cancer, expression was correlated with clinicopathological parameters. Dickkopf-1 expression was confirmed in several cancer cell lines derived from breast and other common cancers. Dickkopf-1 protein secretion was documented in breast, prostate and lung cancer lines, but was negligible in melanoma. Analysis of DKK1 expression in human cancer specimens revealed DKK1 expression in breast (21 out of 73), lung (11 out of 23) and kidney cancers (six out of 20). Interestingly, DKK1 was preferentially expressed in oestrogen and progesterone receptor-negative tumours (ER⁻/PR⁻; P=0.005) and in tumours from women with a family history of breast cancer (P=0.024). Importantly, DKK1 protein production was confirmed in multiple breast cancer specimens that were positive by RT–PCR. This work establishes DKK1 as a potential prognostic and diagnostic marker for cohorts of breast cancer patients with poor prognosis. Dickkopf-1 may also become a relevant candidate target for immunotherapy of different cancers.

Dickkopf-1 induced apoptosis in human placental choriocarcinoma is independent of canonical Wnt signaling

Placental choriocarcinoma, a reproductive system carcinoma in women, has about 0.81% occurrence frequency in China, which leads to over 90% lethality due to indistinct pathogenesis and the absence of efficient therapeutic treatment. In the present study, using immunostaining and reverse transcription PCR, we reported that Dickkopf-1 (Dkk-1) is prominently expressed in human cytotrophoblast (CTB) cell, but absent in the human placental choriocarcinoma cell line JAR and JEG3, implicating an unknown correlation between Dkk-1 and carcinogenesis of placental choriocarcinoma. Further, through exogenous introduction of Dkk-1, we found repressed proliferation in JAR and JEG3, induced apoptosis in JAR, and discovered significant tumor suppression effects of Dkk-1 in placental choriocarcinoma. Moreover we found that this function of Dkk-1 is achieved through c-Jun N-terminal kinase (JNK), whereas the canonical Wnt pathway may not have a great role. This discovery is not symphonic to previous functional understanding of Dkk-1, a canonical Wnt signaling antagonist. Together, our data indicate the possible correlation between Dkk-1 and human placental choriocarcinoma and suggest potential applications of Dkk-1 in treatment of human placental choriocarcinomas.

Activation of Wnt/beta-catenin signaling in distinct histologic subtypes of human germ cell tumors

The molecular signaling pathways mediating human germ cell tumor (GCT) formation and progression are poorly understood despite a large number of studies detailing recurrent cytogenetic abnormalities. Germ cell tumors consist of multiple histologic subtypes and can also be divided into infantile/childhood or adolescent/adult tumors as well as gonadal or nongonadal sites of origin. All of these parameters are important in defining clinical outcome and in understanding the pathogenesis of these tumors. We utilized complementary DNA (cDNA) microarray analysis to identify differences in signal transduction pathways between 2 histologic subtypes of malignant ovarian GCTs (dysgerminomas versus ovarian endodermal sinus tumors). Hierarchical cluster analysis using only the genes involved in Wnt/beta-catenin signaling was able to distinguish these 2 tumor subtypes from each other. Wnt13 and beta-catenin showed significant differential expression patterns between the 2 tumor subtypes, and the results were confirmed by semiquantitative reverse transcriptase-polymerase chain reaction. Additional GCTs were studied for the expression of other members of Wnt/beta-catenin signaling, including Wnt13, frizzled, disheveled, low-density lipoprotein receptor-related protein 6, and beta-catenin. Differential expression levels were identified for several histologic subtypes of human GCTs. Finally, we prepared tissue microarrays containing GCTs from 83 different patients and demonstrated high levels of beta-catenin protein expression in 100% and nuclear accumulation in approximately 50% to 70% of all endodermal sinus tumors and immature teratomas (ITs). This pattern was independent of the patient's age. No nuclear accumulation of beta-catenin was observed in germinomas, embryonal carcinomas, or choriocarcinomas. These results indicate that activation of Wnt/beta-catenin signaling plays an important role in the pathogenesis of 2 histologic subtypes of human GCTs.

Sharing of unrelated receptors and ligands by cognate partners: possible implications for ovarian and endometrial physiology

The majority of the recognized extracellular signalling molecules are known to participate in paracrine and autocrine functions. The classical model of signalling involves a ligand and its cognate receptor. A unique number of ligands activate two phylogenetically unrelated receptors; some receptors are activated by more than one unrelated polypeptide ligand, and some unrelated receptors share common co-receptors. Such a situation introduces a new dimension of complexity into the processes governed by these signalling mechanisms. These unique 'three-way partnerships' often involve signalling molecules that have key roles in the reproductive system. This review presents the known cases of three-way partnerships and examines their possible significance to the reproductive processes in the ovary and endometrium. Most notably present in the ovary are Wnt, Frizzled, Dickkopf (Dkk), low density lipoprotein receptor-related protein (LRP)5, RYK and Kremen system, and semaphorin, plexin, vascular endothelial growth factor and neuropilin system. In the endometrium one finds potential three-way partnerships in Wnt, Frizzled and RYK system, and ATP, P2X7, P2Y2 system. Three-way partnerships may explain previously enigmatic cases of biphasic effects of a ligand, or may reveal that a ligand thought to be pleiotrophic through the activation of one receptor is actually affecting two unrelated signalling receptors in the same tissue. The potential significance to new pharmacological developments is evident.

Three's company: two or more unrelated receptors pair with the same ligand

Intercellular communication relies on signal transduction mediated by extracellular ligands and their receptors. Although the ligand-receptor interaction is usually a two-player event, there are selective examples of one polypeptide ligand interacting with more than one phylogenetically unrelated receptor. Likewise, a few receptors interact with more than one polypeptide ligand, and sometimes with more than one coreceptor, likely through an interlocking of unique protein domains. Phylogenetic analyses suggest that for certain triumvirates, the matching events could have taken place at different evolutionary times. In contrast to a few polypeptide ligands interacting with more than one receptor, we found that many small nonpeptide ligands have been paired with two or more plasma membrane receptors, nuclear receptors, or channels. The observation that many small ligands are paired with more than one receptor type highlights the utilitarian use of a limited number of cellular components during metazoan evolution. These conserved ligands are ubiquitous cell metabolites likely favored by natural selection to establish novel regulatory networks. They likely possess structural features useful for designing agonistic and antagonistic drugs to target diverse receptors.

Dickkopf-1 antagonizes Wnt signaling independent of β-catenin in human mesothelioma

Dickkopf-1 (Dkk-1) is a secreted protein that acts as a potent inhibitor of the Wnt signal transduction pathway. It is thought that the antagonistic effect of Dkk-1 is specific to the canonical (Wnt/beta-catenin) pathway. In this study, we demonstrate that restoration of Dkk-1 expression suppresses cell growth and induces apoptotic cell death in beta-catenin-deficient mesothelioma cell lines H28 and MS-1. Furthermore, we found that a small-molecule inhibitor of JNK inhibited the apoptosis induced by Dkk-1 overexpression in these cells. Together, our data suggest that Dkk-1 may be able to antagonize Wnt signaling and exert its tumor suppressive effects through beta-catenin-independent non-canonical pathways (i.e., the Wnt/JNK pathway).