Michiko Kishida

Axin, a negative regulator of the wnt signaling pathway, directly interacts with adenomatous polyposis coli and regulates the stabilization of beta-catenin

The regulators of G protein signaling (RGS) domain of Axin, a negative regulator of the Wnt signaling pathway, made a complex with full-length adenomatous polyposis coli (APC) in COS, 293, and L cells but not with truncated APC in SW480 or DLD-1 cells. The RGS domain directly interacted with the region containing the 20-amino acid repeats but not with that containing the 15-amino acid repeats of APC, although both regions are known to bind to β-catenin. In the region containing seven 20-amino acid repeats, the region containing the latter five repeats bound to the RGS domain of Axin. Axin and β-catenin simultaneously interacted with APC. Furthermore, Axin stimulated the degradation of β-catenin in COS cells. Taken together with our recent observations that Axin directly interacts with glycogen synthase kinase-3β (GSK-3β) and β-catenin and that it promotes GSK-3β-dependent phosphorylation of β-catenin, these results suggest that Axin, APC, GSK-3β, and β-catenin make a tetrameric complex, resulting in the regulation of the stabilization of β-catenin.

DIX Domains of Dvl and Axin Are Necessary for Protein Interactions and Their Ability To Regulate β-Catenin Stability

ABSTRACT The N-terminal region of Dvl-1 (a mammalian Dishevelled homolog) shares 37% identity with the C-terminal region of Axin, and this related region is named the DIX domain. The functions of the DIX domains of Dvl-1 and Axin were investigated. By yeast two-hybrid screening, the DIX domain of Dvl-1 was found to interact with Dvl-3, a second mammalian Dishevelled relative. The DIX domains of Dvl-1 and Dvl-3 directly bound one another. Furthermore, Dvl-1 formed a homo-oligomer. Axin also formed a homo-oligomer, and its DIX domain was necessary. The N-terminal region of Dvl-1, including its DIX domain, bound to Axin directly. Dvl-1 inhibited Axin-promoted glycogen synthase kinase 3β-dependent phosphorylation of β-catenin, and the DIX domain of Dvl-1 was required for this inhibitory activity. Expression of Dvl-1 in L cells induced the nuclear accumulation of β-catenin, and deletion of the DIX domain abolished this activity. Although expression of Axin in SW480 cells caused the degradation of β-catenin and reduced the cell growth rate, expression of an Axin mutant that lacks the DIX domain did not affect the level of β-catenin or the growth rate. These results indicate that the DIX domains of Dvl-1 and Axin are important for protein-protein interactions and that they are necessary for the ability of Dvl-1 and Axin to regulate the stability of β-catenin.

PHOSPHORYLATION OF AXIN, A WNT SIGNAL NEGATIVE REGULATOR, BY GLYCOGEN SYNTHASE KINASE-3BETA REGULATES ITS STABILITY

Axin forms a complex with glycogen synthase kinase-3β (GSK-3β) and β-catenin and promotes GSK-3β-dependent phosphorylation of β-catenin, thereby stimulating the degradation of β-catenin. Because GSK-3β also phosphorylates Axin in the complex, the physiological significance of the phosphorylation of Axin was examined. Treatment of COS cells with LiCl, a GSK-3β inhibitor, and okadaic acid, a protein phosphatase inhibitor, decreased and increased, respectively, the cellular protein level of Axin. Pulse-chase analyses showed that the phosphorylated form of Axin was more stable than the unphosphorylated form and that an Axin mutant, in which the possible phosphorylation sites for GSK-3β were mutated, exhibited a shorter half-life than wild type Axin. Dvl-1, which was genetically shown to function upstream of GSK-3β, inhibited the phosphorylation of Axin by GSK-3β in vitro. Furthermore, Wnt-3a-containing conditioned medium down-regulated Axin and accumulated β-catenin in L cells and expression of Dvl-1ΔPDZ, in which the PDZ domain was deleted, suppressed this action of Wnt-3a. These results suggest that the phosphorylation of Axin is important for the regulation of its stability and that Wnt down-regulates Axin through Dvl.

Expression of Wnt-5a Is Correlated with Aggressiveness of Gastric Cancer by Stimulating Cell Migration and Invasion

Wnt-5a is a representative ligand that activates a beta-catenin-independent pathway in the Wnt signaling. Although abnormal activation of beta-catenin-dependent pathway is often observed in human cancer, the relationship between beta-catenin-independent pathway and tumorigenesis is not clear. We sought to clarify how Wnt-5a is involved in aggressiveness of gastric cancer. Abnormal expression of Wnt-5a was observed in 71 of 237 gastric cancer cases by means of immunohistochemistry. The positivity of Wnt-5a expression was correlated with advanced stages and poor prognosis of gastric cancer. Wnt-5a had the abilities to stimulate cell migration and invasion in gastric cancer cells. Wnt-5a activated focal adhesion kinase and small GTP-binding protein Rac, both of which are known to play a role in cell migration. Cell migration, membrane ruffling, and turnover of paxillin were suppressed in Wnt-5a knockdown cells. Furthermore, anti-Wnt-5a antibody suppressed gastric cancer cell migration. These results suggest that Wnt-5a stimulates cell migration by regulating focal adhesion complexes and that Wnt-5a is not only a prognostic factor but also a good therapeutic target for gastric cancer.

Small G protein Ral and its downstream molecules regulate endocytosis of EGF and insulin receptors.

The involvement of Ral and its downstream molecules in receptor‐mediated endocytosis was examined. Expression of either RalG23V or RalS28N, which are known to be constitutively active and dominantnegative forms, respectively, in A431 cells blocked internalization of epidermal growth factor (EGF). Stable expression of RalG23V or RalS28N in CHO‐IR cells also inhibited internalization of insulin. Internalization of EGF and insulin was not affected by full‐length RalBP1 which is an effector protein of Ral, but was inhibited by its C‐terminal region which binds directly to Ral and POB1. POB1 is a binding protein of RalBP1 and has the Eps15 homology (EH) domain. Deletion mutants of POB1 inhibited internalization of EGF and insulin. However, internalization of transferrin was unaffected by Ral, RalBP1, POB1 and their mutants. Epsin and Eps15 have been reported to be involved in the regulation of endocytosis of the receptors for EGF and transferrin. The EH domain of POB1 bound directly to Epsin and Eps15. Taken together with the observation that EGF and insulin activate Ral, these results suggest that Ral, RalBP1 and POB1 transmit the signal from the receptors to Epsin and Eps15, thereby regulating ligand‐dependent receptor‐mediated endocytosis.

GSK-3β-dependent phosphorylation of adenomatous polyposis coli gene product can be modulated by β-catenin and protein phosphatase 2A complexed with Axin

Axin forms a complex with adenomatous polyposis coli gene product (APC), glycogen synthase kinase-3β (GSK-3β), and β-catenin through different binding sites and downregulates β-catenin. GSK-3β-dependent phosphorylation of APC-(1211-2075) which has the Axin-binding site was facilitated by Axin, but that of APC-(959-1338) which lacks the Axin-binding site was not. Axin-(298-506) or Axin-(298-832), which has the GSK-3β- and β-catenin- but not APC-binding sites, did not enhance GSK-3β-dependent phosphorylation of either APC-(1211-2075) or APC-(959-1338). Furthermore, β-catenin stimulated the phosphorylation of APC-(959-1338) and APC-(1211-2075) by GSK-3β in the presence of Axin. Consistent with these in vitro observations, expression of β-catenin or Axin in COS cells promoted an SDS gel band shift of APC. These results indicate that APC complexed with Axin is effectively phosphorylated by GSK-3β and that β-catenin may modulate this phosphorylation. In addition, the heterodimeric form of protein phosphatase 2A (PP2A) directly bound to Axin, and PP2A complexed with Axin dephosphorylated APC phosphorylated by GSK-3β. Taken together, these results suggest that GSK-3β-dependent phosphorylation of APC can be modulated by β-catenin and PP2A complexed with Axin.

Complex Formation of Adenomatous Polyposis Coli Gene Product and Axin Facilitates Glycogen Synthase Kinase-3β-dependent Phosphorylation of β-Catenin and Down-regulates β-Catenin

Adenomatous polyposis coli gene product (APC) functions as a tumor suppressor and its mutations in familial adenomatous polyposis and colorectal cancers lead to the accumulation of cytoplasmic β-catenin. The molecular mechanism by which APC regulates the stability of β-catenin was investigated. The central region of APC, APC-(1211–2075), has the β-catenin- and Axin-binding sites and down-regulates β-catenin. Glycogen synthase kinase-3β (GSK-3β) phosphorylated β-catenin slightly in the presence of either APC-(1211–2075) or AxinΔ β -catenin, in which the β-catenin-binding site is deleted, and greatly in the presence of both proteins. The enhancement of the GSK-3β-dependent phosphorylation of β-catenin was eliminated by the APC-binding site of Axin. Axin down-regulated β-catenin in SW480 cells, but not AxinΔ β -catenin. In L cells where APC is intact, AxinΔ β -catenin inhibited Wnt-dependent accumulation of β-catenin but not Axin-(298–832)Δ β -catenin in which the APC- and β-catenin-binding sites are deleted. These results indicate that the complex formation of APC and Axin enhances the phosphorylation of β-catenin by GSK-3β, leading to the down-regulation of β-catenin.

Axin prevents Wnt-3a-induced accumulation of β-catenin

When Axin, a negative regulator of the Wnt signaling pathway, was expressed in COS cells, it coeluted with glycogen synthase kinase-3β (GSK-3β), β-catenin, and adenomatous polyposis coli protein (APC) in a high molecular weight fraction on gel filtration column chromatography. In this fraction, GSK-3β, β-catenin, and APC were co-precipitated with Axin. Although β-catenin was detected in the high molecular weight fraction in L cells on gel filtration column chromatography, addition of conditioned medium expressing Wnt-3a to the cells increased β-catenin in the low molecular weight fraction. However, Wnt-3a-dependent accumulation of β-catenin was greatly inhibited in L cells stably expressing Axin. Axin also suppressed Wnt-3a-dependent activation of Tcf-4 which binds to β-catenin and acts as a transcription factor. These results suggest that Axin forms a complex with GSK-3β, β-catenin, and APC, resulting in the stimulation of the degradation of β-catenin and that Wnt-3a induces the dissociation of β-catenin from the Axin complex and accumulates β-catenin.

Wnt‐5a signaling is correlated with infiltrative activity in human glioma by inducing cellular migration and MMP‐2

Wnts are secreted ligands that consist of 19 members in humans, regulate cell proliferation, differentiation, motility and fate in many stages including the embryonic stage and tumorigenesis. Wnts bind to cell surface receptors named Frizzleds and LRPs, and transduce their signals through β‐catenin‐dependent and ‐independent intracellular pathways. Gliomas are one of the most common intracranial tumors. Gliomas exhibit a progression associated with widespread infiltration into surrounding neuronal tissues. However, the molecular mechanisms that stimulate the invasion of glioma cells are not fully understood. We established two cell lines from human glioma cases and analyzed the expression of all Wnt and Frizzled members in these cell lines and other well‐known glioma cell lines by real‐time PCR study. The mRNA of Wnt‐5a and ‐7b and Frizzled‐2, ‐6 and ‐7 were overexpressed in glioma cells. The elevation of Wnt‐5a expression was most remarkable. Although Wnt‐5a is reported to have oncogenic and antioncogenic activity in several cancers, the role of Wnt‐5a signaling in human glioma cells remains unclear. Immunohistochemical study also revealed high expression of Wnt‐5a in 26 (79%) of 33 human glioma cases. The positivity of Wnt‐5a expression was correlated with the clinical grade. Knockdown of Wnt‐5a expression suppressed migration, invasion and expression of matrix metalloproteinase‐2 of glioma cells. Reciprocally, treatment with purified Wnt‐5a ligand resulted in stimulation of cell migration and invasion. MMP‐2 inhibitor suppressed the Wnt‐5a‐dependent invasion of U251 cells. These results suggested that Wnt‐5a is not only a prognostic factor but also a therapeutic target molecule in gliomas for preventing tumor cell infiltration. (Cancer Sci 2011; 102: 540–548)

Axin Directly Interacts with Plakoglobin and Regulates Its Stability

Plakoglobin is homologous to β-catenin. Axin, a Wnt signal negative regulator, enhances glycogen synthase kinase (GSK)-3β-dependent phosphorylation of β-catenin and stimulates the degradation of β-catenin. Therefore, we examined the effect of Axin on plakoglobin stability. Axin formed a complex with plakoglobin in COS cells and SW480 cells. Axin directly bound to plakoglobin, and this binding was inhibited by β-catenin. Axin promoted GSK-3β-dependent phosphorylation of plakoglobin. Furthermore, overexpression of Axin down-regulated the level of plakoglobin in SW480 cells. These results suggest that Axin regulates the stability of plakoglobin by enhancing its phosphorylation by GSK-3β and that Axin may act on β-catenin and plakoglobin in similar manners.