Dapper 1 antagonizes Wnt signaling by promoting dishevelled degradation

Wnt signaling plays pivotal roles in the regulation of embryogenesis and cancer development. Xenopus Dapper (Dpr) was identified as an interacting protein for Dishevelled (Dvl), a Wnt signaling mediator, and modulates Wnt signaling. However, it is largely unclear how Dpr regulates Wnt signaling. Here, we present evidence that human Dpr1, the ortholog of Xenopus Dpr, inhibits Wnt signaling. We have identified the regions responsible for the Dpr-Dvl interaction in both proteins and found that the interaction interface is formed between the DEP (Dishevelled, Egl-10, and pleckstrin) domain of Dvl and the central and the C-terminal regions of Dpr1. The inhibitory function of human Dpr1 requires both its N and C terminus. Overexpression of the C-terminal region corresponding to the last 225 amino acids of Dpr1, in contrast to wild-type Dpr1, enhances Wnt signaling, suggesting a dominant negative function of this region. Furthermore, we have shown that Dpr1 induces Dvl degradation via a lysosome inhibitor-sensitive and proteasome inhibitor-insensitive mechanism. Knockdown of Dpr1 by RNA interference up-regulates endogenous Dvl2 protein. Taken together, our data indicate that the inhibitory activity of Dpr on Wnt signaling is conserved from Xenopus to human and that Dpr1 antagonizes Wnt signaling by inducing Dvl degradation.     

Wnt proteins induce dishevelled phosphorylation via an LRP5/6- independent mechanism, irrespective of their ability to stabilize beta-catenin

Wnt glycoproteins play essential roles in the development of metazoan organisms. Many Wnt proteins, such as Wnt1, activate the well-conserved canonical Wnt signaling pathway, which results in accumulation of beta-catenin in the cytosol and nucleus. Other Wnts, such as Wnt5a, activate signaling mechanisms which do not involve beta-catenin and are less well characterized. Dishevelled (Dvl) is a key component of Wnt/beta-catenin signaling and becomes phosphorylated upon activation of this pathway. In addition to Wnt1, we show that several Wnt proteins, including Wnt5a, trigger phosphorylation of mammalian Dvl proteins and that this occurs within 20 to 30 min. Unlike the effects of Wnt1, phosphorylation of Dvl in response to Wnt5a is not concomitant with beta-catenin stabilization, indicating that Dvl phosphorylation is not sufficient to activate canonical Wnt/beta-catenin signaling. Moreover, neither Dickkopf1, which inhibits Wnt/beta-catenin signaling by binding the Wnt coreceptors LRP5 and -6, nor dominant-negative LRP5/6 constructs could block Wnt-mediated Dvl phosphorylation. We conclude that Wnt-induced phosphorylation of Dvl is independent of LRP5/6 receptors and that canonical Wnts can elicit both LRP-dependent (to beta-catenin) and LRP-independent (to Dvl) signals. Our data also present Dvl phosphorylation as a general biochemical assay for Wnt protein function, including those Wnts that do not activate the Wnt/beta-catenin pathway.     

Protein kinase A phosphorylation of human phosphodiesterase 3B promotes 14-3-3 protein binding and inhibits phosphatase-catalyzed inactivation

Recent studies confirm that intracellular cAMP concentrations are nonuniform and that localized subcellular cAMP hydrolysis by cyclic nucleotide phosphodiesterases (PDEs) is important in maintaining these cAMP compartments. Human phosphodiesterase 3B (HSPDE3B), a member of the PDE3 family of PDEs, represents the dominant particulate cAMP-PDE activity in many cell types, including adipocytes and cells of hematopoietic lineage. Although several previous reports have shown that phosphorylation of HSPDE3B by either protein kinase A (PKA) or protein kinase B (PKB) activates this enzyme, the mechanisms that allow cells to distinguish these two activated forms of HSPDE3B are unknown. Here we report that PKA phosphorylates HSPDE3B at several distinct sites (Ser-73, Ser-296, and Ser-318), and we show that phosphorylation of HSPDE3B at Ser-318 activates this PDE and stimulates its interaction with 14-3-3 proteins. In contrast, although PKB-catalyzed phosphorylation of HSPDE3B activates this enzyme, it does not promote 14-3-3 protein binding. Interestingly, we report that the PKA-phosphorylated, 14-3-3 protein-bound, form of HSPDE3B is protected from phosphatase-dependent dephosphorylation and inactivation. In contrast, PKA-phosphorylated HSPDE3B that is not bound to 14-3-3 proteins is readily dephosphorylated and inactivated. Our data are presented in the context that a selective interaction between PKA-activated HSPDE3B and 14-3-3 proteins represents a mechanism by which cells can protect this enzyme from deactivation. Moreover, we propose that this mechanism may allow cells to distinguish between PKA- and PKB-activated HSPDE3B.     

Direct binding of the PDZ domain of Dishevelled to a conserved internal sequence in the C-terminal region of Frizzled

The cytoplasmic protein Dishevelled (Dvl) and the associated membrane-bound receptor Frizzled (Fz) are essential in canonical and noncanonical Wnt signaling pathways. However, the molecular mechanisms underlying this signaling are not well understood. By using NMR spectroscopy, we determined that an internal sequence of Fz binds to the conventional peptide binding site in the PDZ domain of Dvl; this type of site typically binds to C-terminal binding motifs. The C-terminal region of the Dvl inhibitor Dapper (Dpr) and Frodo bound to the same site. In Xenopus, Dvl binding peptides of Fz and Dpr/Frodo inhibited canonical Wnt signaling and blocked Wnt-induced secondary axis formation in a dose-dependent manner, but did not block noncanonical Wnt signaling mediated by the DEP domain. Together, our results identify a missing molecular connection within the Wnt pathway. Differences in the binding affinity of the Dvl PDZ domain and its binding partners may be important in regulating signal transduction by Dvl.     

Dishevelled: The hub of Wnt signaling

Wnt signaling controls a variety of developmental and homeostatic events. As a key component of Wnt signaling, Dishevelled (Dvl/Dsh) protein relays Wnt signals from receptors to downstream effectors. In the canonical Wnt pathway that depends on the nuclear translocation of β-catenin, Dvl is recruited by the receptor Frizzled and prevents the constitutive destruction of cytosolic β-catenin. In the non-canonical Wnt pathways such as Wnt-Frizzled/PCP (planar cell polarity) signaling, Dvl signals via the Daam1-RhoA axis and the Rac1 axis. In addition, Dvl plays important roles in Wnt-GSK3β-microtubule signaling, Wnt-calcium signaling, Wnt-RYK signaling, Wnt-atypical PKC signaling, etc. Dvl also functions to mediate receptor endocytosis. To fulfill its multifaceted functions, it is not surprising that Dvl associates with various kinds of proteins. Its activity is also modulated dynamically by phosphorylation, ubiquitination and degradation. In this review, we summarize the current understanding of Dvl functions in Wnt signal transduction and its biological functions in mouse development, and also discuss the molecular mechanisms of its actions.     

HECT Domain-containing E3 Ubiquitin Ligase NEDD4L Negatively Regulates Wnt Signaling by Targeting Dishevelled for Proteasomal Degradation

Wnt signaling plays a pivotal role in embryogenesis and tissue homeostasis. Dishevelled (Dvl) is a central mediator for both Wnt/β-catenin and Wnt/planar cell polarity pathways. NEDD4L, an E3 ubiquitin ligase, has been shown to regulate ion channel activity, cell signaling, and cell polarity. Here, we report a novel role of NEDD4L in the regulation of Wnt signaling. NEDD4L induces Dvl2 polyubiquitination and targets Dvl2 for proteasomal degradation. Interestingly, the NEDD4L-mediated ubiquitination of Dvl2 is Lys-6, Lys-27, and Lys-29 linked but not typical Lys-48-linked ubiquitination. Consistent with the role of Dvl in both Wnt/β-catenin and Wnt/planar cell polarity signaling, NEDD4L regulates the cellular β-catenin level and Rac1, RhoA, and JNK activities. We have further identified a hierarchical regulation that Wnt5a induces JNK-mediated phosphorylation of NEDD4L, which in turn promotes its ability to degrade Dvl2. Finally, we show that NEDD4L inhibits Dvl2-induced axis duplication in Xenopus embryos. Our work thus demonstrates that NEDD4L is a negative feedback regulator of Wnt signaling.     

The Wnt Signaling Antagonist Dapper1 Accelerates Dishevelled2 Degradation via Promoting Its Ubiquitination and Aggregate-induced Autophagy

Autophagy is a regulated process that sequesters and transports cytoplasmic materials such as protein aggregates via autophagosomes to lysosomes for degradation. Dapper1 (Dpr1), an interacting protein of Dishevelled (Dvl), antagonizes Wnt signaling by promoting Dishevelled degradation via lysosomes. However, the mechanism is unclear. Here, we show that Dpr1 promotes the von Hippel-Lindau tumor suppressor (VHL)-mediated ubiquitination of Dvl2 and its autophagic degradation. Knockdown of Dpr1 decreases the interaction between Dvl2 and pVHL, resulting in reduced ubiquitination of Dvl2. Dpr1-mediated autophagic degradation of Dvl2 depends on Dvl2 aggregation. Moreover, the aggregate-prone proteins Dvl2, p62, and the huntingtin mutant Htt103Q promote autophagy in a Dpr1-dependent manner. These protein aggregates enhance the Beclin1-Vps34 interaction and Atg14L puncta formation, indicating that aggregated proteins stimulate autophagy initiation. Ubiquitination is not essential for the aggregate-induced autophagy initiation as inhibition of the ubiquitin-activation E1 enzyme activity did not block the aggregate-induced Atg14L puncta formation. Our findings suggest that Dpr1 promotes the ubiquitination of Dvl2 by pVHL and mediates the protein aggregate-elicited autophagy initiation.     

Mechanical regulation of glycogen synthase kinase 3β (GSK3β) in mesenchymal stem cells is dependent on Akt protein serine 473 phosphorylation via mTORC2 protein

Mechanical signals can inactivate glycogen synthase kinase 3β (GSK3β), resulting in stabilization of β-catenin. This signaling cascade is necessary for the inhibition of adipogenesis in mesenchymal stem cells (MSC) that is produced by a daily strain regimen. We investigated whether Akt is the mechanically activated kinase responsible for phosphorylation and inactivation of GSK3β in MSC. Mechanical strain (2% magnitude, 0.17 Hz) induced phosphorylation of Akt at Ser-473 and Thr-308 in parallel with phosphorylation of GSK3β at Ser-9. Inhibiting Akt (Akt1/2 kinase inhibitor treatment or Akt knockdown) prevented strain-induced phosphorylation of GSK3β at Ser-9. Inhibition of PI3K prevented Thr-308 phosphorylation, but strain-induced Ser-473 phosphorylation was measurable and induced phosphorylation of GSK3β, suggesting that Ser-473 phosphorylation is sufficient for the downstream mechanoresponse. As Rictor/mTORC2 (mammalian target of rapamycin complex 2) is known to transduce phosphorylation of Akt at Ser-473 by insulin, we investigated whether it contributes to strain-induced Ser-473 phosphorylation. Phosphorylation of Ser-473 by both mechanical and insulin treatment in MSC was prevented by the mTOR inhibitor KU0063794. When mTORC2 was blocked, mechanical GSK3β inactivation was prevented, whereas insulin inhibition of GSK3β was still measured in the absence of Ser-473 phosphorylation, presumably through phosphorylation of Akt at Thr-308. In sum, mechanical input initiates a signaling cascade that is uniquely dependent on mTORC2 activation and phosphorylation of Akt at Ser-473, an effect sufficient to cause inactivation of GSK3β. Thus, mechanical regulation of GSK3β downstream of Akt is dependent on phosphorylation of Akt at Ser-473 in a manner distinct from that of growth factors. As such, Akt reveals itself to be a pleiotropic signaling molecule whose downstream targets are differentially regulated depending upon the nature of the activating input.     

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

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

Oncogenic function of DACT1 in colon cancer through the regulation of β-catenin

The Wnt/β-catenin signaling pathway plays important roles in the progression of colon cancer. DACT1 has been identified as a modulator of Wnt signaling through its interaction with Dishevelled (Dvl), a central mediator of both the canonical and noncanonical Wnt pathways. However, the functions of DACT1 in the WNT/β-catenin signaling pathway remain unclear. Here, we present evidence that DACT1 is an important positive regulator in colon cancer through regulating the stability and sublocation of β-catenin. We have shown that DACT1 promotes cancer cell proliferation in vitro and tumor growth in vivo and enhances the migratory and invasive potential of colon cancer cells. Furthermore, the higher expression of DACT1 not only increases the nuclear and cytoplasmic fractions of β-catenin, but also increases its membrane-associated fraction. The overexpression of DACT1 leads to the increased accumulation of nonphosphorylated β-catenin in the cytoplasm and particularly in the nuclei. We have demonstrated that DACT1 interacts with GSK-3β and β-catenin. DACT1 stabilizes β-catenin via DACT1-induced effects on GSK-3β and directly interacts with β-catenin proteins. The level of phosphorylated GSK-3β at Ser9 is significantly increased following the elevated expression of DACT1. DACT1 mediates the subcellular localization of β-catenin via increasing the level of phosphorylated GSK-3β at Ser9 to inhibit the activity of GSK-3β. Taken together, our study identifies DACT1 as an important positive regulator in colon cancer and suggests a potential strategy for the therapeutic control of the β-catenin-dependent pathway.     

The E3 Ubiquitin Ligase ITCH Negatively Regulates Canonical Wnt Signaling by Targeting Dishevelled Protein

Dishevelled (Dvl) is a key component in the canonical Wnt signaling pathway and becomes hyperphosphorylated upon Wnt stimulation. Dvl is required for LRP6 phosphorylation, which is essential for subsequent steps of signal transduction, such as Axin recruitment and cytosolic β-catenin stabilization. Here, we identify the HECT-containing Nedd4-like ubiquitin E3 ligase ITCH as a new Dvl-binding protein. ITCH ubiquitinates the phosphorylated form of Dvl and promotes its degradation via the proteasome pathway, thereby inhibiting canonical Wnt signaling. Knockdown of ITCH by RNA interference increased the stability of phosphorylated Dvl and upregulated Wnt reporter gene activity as well as endogenous Wnt target gene expression induced by Wnt stimulation. In addition, we found that both the PPXY motif and the DEP domain of Dvl are critical for its interaction with ITCH, as mutation in the PPXY motif (Dvl2-Y568F) or deletion of the DEP domain led to reduced affinity for ITCH. Consistently, overexpression of ITCH inhibited wild-type Dvl2-induced, but not Dvl2-Y568F mutant-induced, Wnt reporter activity. Moreover, the Y568F mutant, but not wild-type Dvl2, can reverse the ITCH-mediated inhibition of Wnt-induced reporter activity. Collectively, these results indicate that ITCH plays a negative regulatory role in modulating canonical Wnt signaling by targeting the phosphorylated form of Dvl.     

Phosphorylation-dependent 14-3-3 protein interactions regulate CFTR biogenesis

Cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP/protein kinase A (PKA)-regulated chloride channel whose phosphorylation controls anion secretion across epithelial cell apical membranes. We examined the hypothesis that cAMP/PKA stimulation regulates CFTR biogenesis posttranslationally, based on predicted 14-3-3 binding motifs within CFTR and forskolin-induced CFTR expression. The 14-3-3β, γ, and ε isoforms were expressed in airway cells and interacted with CFTR in coimmunoprecipitation assays. Forskolin stimulation (15 min) increased 14-3-3β and ε binding to immature and mature CFTR (bands B and C), and 14-3-3 overexpression increased CFTR bands B and C and cell surface band C. In pulse-chase experiments, 14-3-3β increased the synthesis of immature CFTR, reduced its degradation rate, and increased conversion of immature to mature CFTR. Conversely, 14-3-3β knockdown decreased CFTR B and C bands (70 and 55%) and elicited parallel reductions in cell surface CFTR and forskolin-stimulated anion efflux. In vitro, 14-3-3β interacted with the CFTR regulatory region, and by nuclear magnetic resonance analysis, this interaction occurred at known PKA phosphorylated sites. In coimmunoprecipitation assays, forskolin stimulated the CFTR/14-3-3β interaction while reducing CFTR's interaction with coat protein complex 1 (COP1). Thus 14-3-3 binding to phosphorylated CFTR augments its biogenesis by reducing retrograde retrieval of CFTR to the endoplasmic reticulum. This mechanism permits cAMP/PKA stimulation to make more CFTR available for anion secretion.     

Phosphorylation of transglutaminase 2 by PKA at Ser216 creates 14-3-3 binding sites

Transglutaminase 2 (TG2) is a multifunctional ubiquitous enzyme which is present in various cellular compartments and is subject to phosphorylation by PKA. To better understand the relevance of PKA induced phosphorylation of TG2, we performed pull-down assays using phosphorylated biotinylated-TG2(209-223) peptides spanning PKA induced phosphorylation sites as a bait. Subsequent analysis of pull-down protein by SDS-PAGE and LC/MS identified 14-3-3epsilon as the binding partner for TG2 which was further confirmed by immunoblotting with 14-3-3 specific antiserum. In contrast, non-phosphorylated and/or phosphorylation site substituted peptides fail to pull-down 14-3-3. Furthermore, we demonstrate that 14-3-3 co-immunoprecipitated with TG2 antiserum after activation of PKA from mouse embryonic fibroblasts (MEF)(TG2+/+) cells but not from MEF(TG2-/-) cells. In summary, we provide convincing evidence that phosphorylation of TG2 by PKA creates binding site(s) for 14-3-3 both in vitro and in vivo.