Guizhong Liu

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

Wnt signalling has an important role in cell fate determination, tissue patterning and tumorigenesis. Secreted antagonists of Wnt include Frizzled (Fz)-related proteins (FRPs), Cerberus, Wnt inhibitory factor (WIF) and Dickkopf (Dkk). FRPs, Cerberus and WIF have all been shown to act by binding and sequestering Wnt. We report a novel mechanism of Wnt-signalling inhibition by human Dkk-1. Dkk-1 demonstrated no interaction with Wnt but bound a single cell surface site with high affinity (KD = 0.39 nM). Its receptor was detectable in a complex with a relative molecular mass of 240,000 (Mr 240K) with [125I] Dkk-1 by covalent affinity cross-linking. Wnt signalling through β-catenin is mediated by the Fz receptor and a recently identified low-density-lipoprotein-receptor-related co-receptor, LRP6/Arrow. Overproduction of the 200K LRP6 protein, but not of Fz, strikingly increased Dkk-1 binding as well as the amount of the 240K cross-linked complex, which was shown to be composed of Dkk-1 and LRP6. Moreover, Dkk-1 function was completely independent of Fz but LRP6 dramatically interfered with the Dkk-1 inhibition of Wnt signalling. Thus, unlike Wnt antagonists, which exert their effects by molecular mimicry of Fz or Wnt sequestration through other mechanisms, Dkk-1 specifically inhibits canonical Wnt signalling by binding to the LRP6 component of the receptor complex.

Canonical and noncanonical Wnts use a common mechanism to activate completely unrelated coreceptors

Wnt ligands signal through β-catenin and are critically involved in cell fate determination and stem/progenitor self-renewal. Wnts also signal through β-catenin-independent or noncanonical pathways that regulate crucial events during embryonic development. The mechanism of noncanonical receptor activation and how Wnts trigger canonical as opposed to noncanonical signaling have yet to be elucidated. We demonstrate here that prototype canonical Wnt3a and noncanonical Wnt5a ligands specifically trigger completely unrelated endogenous coreceptors-LRP5/6 and Ror1/2, respectively-through a common mechanism that involves their Wnt-dependent coupling to the Frizzled (Fzd) coreceptor and recruitment of shared components, including dishevelled (Dvl), axin, and glycogen synthase kinase 3 (GSK3). We identify Ror2 Ser 864 as a critical residue phosphorylated by GSK3 and required for noncanonical receptor activation by Wnt5a, analogous to the priming phosphorylation of low-density receptor-related protein 6 (LRP6) in response to Wnt3a. Furthermore, this mechanism is independent of Ror2 receptor Tyr kinase functions. Consistent with this model of Wnt receptor activation, we provide evidence that canonical and noncanonical Wnts exert reciprocal pathway inhibition at the cell surface by competition for Fzd binding. Thus, different Wnts, through their specific coupling and phosphorylation of unrelated coreceptors, activate completely distinct signaling pathways.

A Novel Mechanism for Wnt Activation of Canonical Signaling through the LRP6 Receptor

ABSTRACT LDL receptor-related protein 6 (LRP6) is a Wnt coreceptor in the canonical signaling pathway, which plays essential roles in embryonic development. We demonstrate here that wild-type LRP6 forms an inactive dimer through interactions mediated by epidermal growth factor repeat regions within the extracellular domain. A truncated LRP6 comprising its transmembrane and cytoplasmic domains is expressed as a constitutively active monomer whose signaling ability is inhibited by forced dimerization. Conversely, Wnts are shown to activate canonical signaling through LRP6 by inducing an intracellular conformational switch which relieves allosteric inhibition imposed on the intracellular domains. Thus, Wnt canonical signaling through LRP6 establishes a novel mechanism for receptor activation which is opposite to the general paradigm of ligand-induced receptor oligomerization.

p53 induction of heparin-binding EGF-like growth factor counteracts p53 growth suppression through activation of MAPK and PI3K/Akt signaling cascades

Tumor suppressor p53 induction in response to cellular stresses activates the mitogen‐activated protein kinase (MAPK) cascade through pathways involving Ras and Raf. p53s ability to activate this pathway is dependent on p53‐mediated transcription. In order to investigate potential p53 target gene(s) involved, we utilized expression array analysis and identified heparin‐binding epidermal growth factor‐like growth factor (HB‐EGF) as being markedly up‐regulated by p53. In response to DNA damage, HB‐EGF was induced in wild‐type, but not in mutant p53‐containing cells, implying its p53 dependence. HB‐EGF neutralizing antibody and inhibitors of EGF receptor signaling abrogated p53‐induced MAPK activation. Expression of HB‐EGF was shown to protect cells from H2O2‐induced apoptosis through MAPK activation. Additionally, the PI3K/Akt pathway was activated in response to p53 signaling through HB‐EGF induction, and inhibition of MAPK and Akt activation after DNA damage decreased cell survival in wild‐type p53‐containing cells. All these findings point to a novel aspect of p53 function. Namely, p53‐induced growth factors such as HB‐EGF, which activate MAPK and Akt signaling, may be involved in a compensatory mechanism to alleviate adverse effects of cellular stresses.

Wnt pathway aberrations including autocrine Wnt activation occur at high frequency in human non-small-cell lung carcinoma

Lung cancer is the most common cause of cancer mortality worldwide. Non-small-cell lung carcinomas (NSCLCs), which represent around 80% of lung tumors, exhibit poor prognosis and are usually refractory to conventional chemotherapy. Elucidating the molecular and cellular mechanisms that are dysregulated in NSCLCs may lead to new possibilities for targeted therapy or enhanced efficacy of current therapies. Here we demonstrate Wnt pathway activation in around 50% of human NSCLC cell lines and primary tumors, through different mechanisms, including autocrine Wnt pathway activation involving upregulation of specific Wnt ligands. Downregulation of activated Wnt signaling inhibited NSCLC proliferation and induced a more differentiated phenotype. Together, our findings establish importance of activated Wnt signaling in human NSCLCs and offer the possibility of targeting upregulated Wnt signaling as a new therapeutic modality for this disease.

The Mechanism of Endogenous Receptor Activation Functionally Distinguishes Prototype Canonical and Noncanonical Wnts

ABSTRACT Wnt glycoproteins are developmentally essential signaling molecules, and lesions afflicting Wnt pathways play important roles in human diseases. Some Wnts signal to the canonical pathway by stabilizing β-catenin, while others lack this activity. Frizzled serpentine receptors mediate distinct signaling pathways by both classes of Wnts. Here, we tandemly linked noncanonical Wnt5a with the C-terminal half of Dickkopf-2 (Dkk2C), a distinct ligand of the Wnt coreceptor LRP5/6. Whereas Wnt5a, Dkk2C, or both together were incapable of stimulating endogenous canonical signaling, the Wnt5a/Dkk2C chimera efficiently activated this pathway in a manner inhibitable by specific antagonists of either frizzled or LRP receptors. Thus, activation of the canonical pathway requires ligand coupling of an endogenous frizzled/LRP coreceptor complex, rather than Wnt triggering each receptor independently. Moreover, fusion of Wnt5a with Dkk2C unmasked its ability to signal to Dishevelled through multiple frizzleds, indicating that the lack of functional interaction with LRP distinguishes noncanonical Wnt5a from canonical Wnts in mammalian cells. These findings provide a novel mechanism by which the same receptor can be switched between distinct signaling pathways depending on the differential recruitment of a coreceptor by members of the same ligand family.

High-Frequency Canonical Wnt Activation in Multiple Sarcoma Subtypes Drives Proliferation through a TCF/β-Catenin Target Gene, CDC25A

Wnt canonical signaling is critical for normal development as well as homeostasis of several epithelial tissues, and constitutive activation of this pathway is commonly observed in carcinomas. We show here that 50% of human sarcomas (n = 45) and 65% of sarcoma cell lines (n = 23) of diverse histological subtypes exhibit upregulated autocrine canonical Wnt signaling. Furthermore, in Wnt autocrine cell lines, we identify alterations including overexpression or gene amplification of Wnt ligands and/or LRP5/6 coreceptors and epigenetic silencing of different cell surface Wnt antagonists. Mutations in adenomatous polyposis coli (APC) gene were observed in two nonautocrine Wnt-positive sarcoma cell lines. Finally, downregulation of the activated Wnt pathway inhibited sarcoma cell proliferation both in vitro and in vivo by a mechanism involving the downregulation of CDC25A.

Androgen-Induced Wnt Signaling in Preosteoblasts Promotes the Growth of MDA-PCa-2b Human Prostate Cancer Cells

The high morbidity and mortality associated with prostate cancer (PCa) result from its tendency to metastasize to bone where it produces predominantly osteoblastic lesions. The Wnt signaling pathway plays an important role in embryogenesis, tumorigenesis, osteoblast development, and bone formation. Androgen signaling via the androgen receptor (AR) is critical in both PCa and bone cell growth. We examined the effects of androgens on cell growth and Wnt signaling in the AR-positive MDA-PCa-2b cell line and MC3T3 preosteoblasts, grown alone and in coculture. We show that the potent androgen dihydrotestosterone increases AR expression and transcriptional activity only in the preosteoblasts. Although dihydrotestosterone induced an 80% increase in PCa cell growth when the cells were grown alone, dihydrotestosterone had a more significant effect on MDA-PCa-2b cell proliferation (3.2-fold increase) when the PCa cells were cocultured with preosteoblasts. Dihydrotestosterone addition to preosteoblasts promoted Wnt-dependent transcriptional reporter activity associated with GSK3beta(S-9) phosphorylation and accumulation of nuclear beta-catenin as well as elevated Runx2 expression. In addition, the increased proliferation of PCa cells in coculture with MC3T3 cells in response to dihydrotestosterone was abrogated by the addition of either exogenous DKK-1 or sFRP-1 protein, two naturally occurring Wnt antagonists. Finally, we show that the paracrine growth-promoting effect of androgens is limited to MDA-PCa-2b cells. These data imply that Wnt signaling is involved in the androgen-regulated crosstalk between preosteoblasts and PCa cells and suggest that androgens may stimulate growth of some prostate tumor cells indirectly, via up-regulation of Wnt signaling in bone cells.

β-Catenin-Independent Activation of TCF1/LEF1 in Human Hematopoietic Tumor Cells through Interaction with ATF2 Transcription Factors

The role of Wnt signaling in embryonic development and stem cell maintenance is well established and aberrations leading to the constitutive up-regulation of this pathway are frequent in several types of human cancers. Upon ligand-mediated activation, Wnt receptors promote the stabilization of β-catenin, which translocates to the nucleus and binds to the T-cell factor/lymphoid enhancer factor (TCF/LEF) family of transcription factors to regulate the expression of Wnt target genes. When not bound to β-catenin, the TCF/LEF proteins are believed to act as transcriptional repressors. Using a specific lentiviral reporter, we identified hematopoietic tumor cells displaying constitutive TCF/LEF transcriptional activation in the absence of β-catenin stabilization. Suppression of TCF/LEF activity in these cells mediated by an inducible dominant-negative TCF4 (DN-TCF4) inhibited both cell growth and the expression of Wnt target genes. Further, expression of TCF1 and LEF1, but not TCF4, stimulated TCF/LEF reporter activity in certain human cell lines independently of β-catenin. By a complementary approach in vivo, TCF1 mutants, which lacked the ability to bind to β-catenin, induced Xenopus embryo axis duplication, a hallmark of Wnt activation, and the expression of the Wnt target gene Xnr3. Through generation of different TCF1-TCF4 fusion proteins, we identified three distinct TCF1 domains that participate in the β-catenin-independent activity of this transcription factor. TCF1 and LEF1 physically interacted and functionally synergized with members of the activating transcription factor 2 (ATF2) family of transcription factors. Moreover, knockdown of ATF2 expression in lymphoma cells phenocopied the inhibitory effects of DN-TCF4 on the expression of target genes associated with the Wnt pathway and on cell growth. Together, our findings indicate that, through interaction with ATF2 factors, TCF1/LEF1 promote the growth of hematopoietic malignancies in the absence of β-catenin stabilization, thus establishing a new mechanism for TCF1/LEF1 transcriptional activity distinct from that associated with canonical Wnt signaling.

Cdo suppresses canonical Wnt signalling via interaction with Lrp6 thereby promoting neuronal differentiation

Canonical Wnt signalling regulates expansion of neural progenitors and functions as a dorsalizing signal in the developing forebrain. In contrast, the multifunctional co-receptor Cdo promotes neuronal differentiation and is important for the function of the ventralizing signal, Shh. Here we show that Cdo negatively regulates Wnt signalling during neurogenesis. Wnt signalling is enhanced in Cdo-deficient cells, leading to impaired neuronal differentiation. The ectodomains of Cdo and Lrp6 interact via the Ig2 repeat of Cdo and the LDLR repeats of Lrp6, and the Cdo Ig2 repeat is necessary for Cdo-dependent Wnt inhibition. Furthermore, the Cdo-deficient dorsal forebrain displays stronger Wnt signalling activity, increased cell proliferation and enhanced expression of the dorsal markers and Wnt targets, Pax6, Gli3, Axin2. Therefore, in addition to promoting ventral central nervous system cell fates with Shh, Cdo promotes neuronal differentiation by suppression of Wnt signalling and provides a direct link between two major dorsoventral morphogenetic signalling pathways.