Regulation of β-Catenin Signaling and Maintenance of Chondrocyte Differentiation by Ubiquitin-independent Proteasomal Degradation of α-Catenin

Abstract

Accumulation of β-catenin and subsequent stimulation of β-catenin-T cell-factor (Tcf)/lymphoid-enhancerfactor (Lef) transcriptional activity causes dedifferentiation of articular chondrocytes, which is characterized by decreased type II collagen expression and initiation of type I collagen expression. This study examined the mechanisms of α-catenin degradation, the role of α-catenin in β-catenin signaling, and the physiological significance of α-catenin regulation of β-catenin signaling in articular chondrocytes. We found that both α- and β-catenin accumulated during dedifferentiation of chondrocytes by escaping from proteasomal degradation. β-Catenin degradation was ubiquitination-dependent, whereas α-catenin was proteasomally degraded in a ubiquitination-independent fashion. The accumulated α- and β-catenin existed as complexes in the cytosol and nucleus. The complex formation between α- and β-catenin blocked proteasomal degradation of α-catenin and also inhibited β-catenin-Tcf/Lef transcriptional activity and the suppression of type II collagen expression associated with ectopic expression of β-catenin, the inhibition of proteasome, or Wnt signaling. Collectively, our results indicate that ubiquitin-independent degradation of α-catenin regulates β-catenin signaling and maintenance of the differentiated phenotype of articular chondrocytes. Accumulation of β-catenin and subsequent stimulation of β-catenin-T cell-factor (Tcf)/lymphoid-enhancerfactor (Lef) transcriptional activity causes dedifferentiation of articular chondrocytes, which is characterized by decreased type II collagen expression and initiation of type I collagen expression. This study examined the mechanisms of α-catenin degradation, the role of α-catenin in β-catenin signaling, and the physiological significance of α-catenin regulation of β-catenin signaling in articular chondrocytes. We found that both α- and β-catenin accumulated during dedifferentiation of chondrocytes by escaping from proteasomal degradation. β-Catenin degradation was ubiquitination-dependent, whereas α-catenin was proteasomally degraded in a ubiquitination-independent fashion. The accumulated α- and β-catenin existed as complexes in the cytosol and nucleus. The complex formation between α- and β-catenin blocked proteasomal degradation of α-catenin and also inhibited β-catenin-Tcf/Lef transcriptional activity and the suppression of type II collagen expression associated with ectopic expression of β-catenin, the inhibition of proteasome, or Wnt signaling. Collectively, our results indicate that ubiquitin-independent degradation of α-catenin regulates β-catenin signaling and maintenance of the differentiated phenotype of articular chondrocytes. β-Catenin interacts with cadherin to participate in cell-cell adhesion and regulates gene expression by acting as a transcriptional co-activator (1.Ben-Ze'ev A. Geiger B. Curr. Opin. Cell Biol. 1998; 10: 629-639Crossref PubMed Scopus (307) Google Scholar, 2.Willert K. Nusse R. Curr. Opin. Genet. Dev. 1998; 8: 95-102Crossref PubMed Scopus (666) Google Scholar). In the absence of extracellular stimuli, cytosolic β-catenin is phosphorylated by glycogen synthase kinase-3β, leading to its ubiquitination and subsequent degradation by the 26 S proteasome. However, extracellular stimuli, such as Wnt signaling, lead to the inhibition of glycogen synthase kinase-3β, escape of β-catenin from ubiquitin-dependent proteolytic degradation, and subsequent cytosolic accumulation of β-catenin (3.Aberle H. Bauer A. Stappert J. Kispert A. Kemler R. EMBO J. 1997; 16: 3797-3804Crossref PubMed Scopus (2160) Google Scholar). The accumulated β-catenin translocates into the nucleus in association with members of the T cell-factor (Tcf) 1The abbreviations used are: Tcf, T cell-factor; Lef, lymphoid-enhancer-factor; ALLnL, N-acetyl-leucyl-leucyl-norleucinal; ALLM, N-acetylleucyl-leucyl-methioninal; siRNA, small interference RNA; RT, reverse transcription; GFP, green fluorescent protein; ERK, extracellular signal-regulated kinase; Z, benzyloxycarbonyl; fmk, fluoromethyl ketone; CBZ, carbamazepine; Suc, succinic acid; MCA, 3-methylcholanthrene./lymphoid-enhancer-factor (Lef) family of transcription factors, leading to stimulation or suppression of target gene transcription (2.Willert K. Nusse R. Curr. Opin. Genet. Dev. 1998; 8: 95-102Crossref PubMed Scopus (666) Google Scholar). As a transcriptional co-activator, β-catenin is involved in the regulation of several biological functions. Our group previously has shown that β-catenin regulates maintenance of differentiated phenotypes (4.Hwang S.-G. Ryu J.-H. Kim I.-C. Jho E.-H. Kim K. Kim S.-J. Chun J.-S. J. Biol. Chem. 2004; 279: 26597-26604Abstract Full Text Full Text PDF PubMed Scopus (101) Google Scholar, 5.Ryu J.-H. Kim S.-J. Kim S.-H. Oh C.-D. Hwang S.-G. Chun C.-H. Oh S.-H. Seong J.-K. Huh T.-L. Chun J.-S. Development. 2002; 129: 5541-5550Crossref PubMed Scopus (131) Google Scholar) and expression of cyclooxygenase-2 (6.Kim S.-J. Im D.-S. Kim S.-H. Ryu J.-H. Hwang S.-G. Seong J.-K. Chun C.-H. Chun J.-S. Biochem. Biophys. Res. Commun. 2002; 296: 221-226Crossref PubMed Scopus (65) Google Scholar) in chondrocytes by acting as a transcriptional co-activator. We have also shown that Wnt-7a causes dedifferentiation of chondrocytes characterized by suppression of type II collagen expression and the onset of type I collagen expression. Accumulation and stimulation of β-catenin transcriptional activity by Wnt-7a signaling is sufficient to cause chondrocyte dedifferentiation (4.Hwang S.-G. Ryu J.-H. Kim I.-C. Jho E.-H. Kim K. Kim S.-J. Chun J.-S. J. Biol. Chem. 2004; 279: 26597-26604Abstract Full Text Full Text PDF PubMed Scopus (101) Google Scholar, 5.Ryu J.-H. Kim S.-J. Kim S.-H. Oh C.-D. Hwang S.-G. Chun C.-H. Oh S.-H. Seong J.-K. Huh T.-L. Chun J.-S. Development. 2002; 129: 5541-5550Crossref PubMed Scopus (131) Google Scholar). β-Catenin signaling can be regulated by a variety of proteins. Cadherins regulate the transcriptional activity of β-catenin in a cell adhesion-independent manner (7.Sadot E. Simcha I. Shtutman M. Ben-Ze'ev A. Geiger B. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 15339-15344Crossref PubMed Scopus (196) Google Scholar, 8.Stockinger A. Eger A. Wolf J. Beug H. Foisner R. J. Cell Biol. 2001; 154: 1185-1196Crossref PubMed Scopus (277) Google Scholar) via sequestration of β-catenin in the cytoplasm (9.Simcha I. Shtutman M. Salomon D. Zhurinsky J. Sadot E. Geiger B. Ben-Ze'ev A. J. Cell Biol. 1998; 141: 1433-1448Crossref PubMed Scopus (239) Google Scholar), whereas γ-catenin increases the turnover of β-catenin protein (10.Salomon D. Sacco P.A. Roy S.G. Simcha I. Johnson K.R. Wheelock M.J. Ben-Ze'ev A. J. Cell Biol. 1997; 139: 1325-1335Crossref PubMed Scopus (131) Google Scholar). The NEMO-like kinase does not regulate β-catenin stability but rather inhibits the interaction between DNA and the β-catenin-Tcf/Lef complex by phosphorylating Tcf/Lef proteins (11.Ishitani T. Ninomiya-Tsuji J. Nagai S. Nishita M. Meneghini M. Barker N. Waterman M. Bowerman B. Clevers H. Shibuya H. Matsumoto K. Nature. 1999; 399: 798-802Crossref PubMed Scopus (517) Google Scholar). During cell-cell adhesion, α-catenin associates directly with β-catenin, acting to link cadherins to the actin cytoskeleton. However, α-catenin is localized not only to areas of cell-cell contact; it is also found in other parts of the cell, such as the cytoplasm and nucleus, suggesting that it participates in the regulation of other cellular functions. Indeed, recent reports indicated that α-catenin inhibits β-catenin signaling in various cell types. For instance, when α-catenin is overexpressed in Xenopus embryo, Wnt signaling is inhibited through direct binding of α- and β-catenin (9.Simcha I. Shtutman M. Salomon D. Zhurinsky J. Sadot E. Geiger B. Ben-Ze'ev A. J. Cell Biol. 1998; 141: 1433-1448Crossref PubMed Scopus (239) Google Scholar, 12.Sehgal R.N. Gumbiner B.M. Reichardt L.F. J. Cell Biol. 1997; 139: 1033-1046Crossref PubMed Scopus (53) Google Scholar), perhaps due to the sequestration of β-catenin in the cytoplasm (9.Simcha I. Shtutman M. Salomon D. Zhurinsky J. Sadot E. Geiger B. Ben-Ze'ev A. J. Cell Biol. 1998; 141: 1433-1448Crossref PubMed Scopus (239) Google Scholar, 13.Takahashi N. Ishihara S. Takada S. Tsukita S. Nagafuchi A. Biochem. Biophys. Res. Commun. 2000; 277: 691-698Crossref PubMed Scopus (26) Google Scholar) or inhibition of the interaction between β-catenin-Tcf/Lef complex and target DNA (14.Giannini A.L. Vivanco M. Kypta R.M. J. Biol. Chem. 2000; 275: 21883-21888Abstract Full Text Full Text PDF PubMed Scopus (78) Google Scholar). In addition to regulating β-catenin signaling, the α-catenin protein is actively degraded in cadherin-deficient L cells in a proteasome-independent manner, although α-catenin contains the PEST sequence, which is a common target for rapid degradation by calpain- or proteasome-mediated proteolysis (13.Takahashi N. Ishihara S. Takada S. Tsukita S. Nagafuchi A. Biochem. Biophys. Res. Commun. 2000; 277: 691-698Crossref PubMed Scopus (26) Google Scholar). It appears likely that α-catenin regulates β-catenin signaling during various cellular processes. However, the regulatory mechanisms underlying the modulation of α-catenin protein stability and the physiological function of α-catenin regulation of β-catenin signaling remain to be elucidated. We, therefore, investigated the mechanisms of α-catenin degradation, the role of α-catenin in β-catenin signaling, and the physiological significance of α-catenin regulation of β-catenin signaling in articular chondrocytes. Reagents—The proteasome inhibitors, Z-Leu-Leu-Leu-CHO (MG132) and N-acetyl-leucyl-leucyl-norleucinal (ALLnL), and the calpain inhibitor, N-acetyl-leucyl-leucyl-methioninal (ALLM), were purchased from Sigma. The peptide caspase inhibitors, CBZ-Val-Ala-Asp-fluoromethylketone (Z-VAD-fmk) and CBZ-Asp-Glu-Val-Asp-fluoromethylketone (Z-DEVD-fmk), were purchased from Enzyme Systems Products (Dublin, CA). The synthetic peptide substrate for proteasomes (Suc-Leu-Leu-Val-Tyr-MCA) was purchased from Biomol Research Laboratory Inc. (Plymouth Meeting, PA). Control- or Wnt-7a-conditioned medium were prepared as described previously by Hwang et al. (4.Hwang S.-G. Ryu J.-H. Kim I.-C. Jho E.-H. Kim K. Kim S.-J. Chun J.-S. J. Biol. Chem. 2004; 279: 26597-26604Abstract Full Text Full Text PDF PubMed Scopus (101) Google Scholar), by culturing Wnt-expressing L929 cells in Dulbecco's modified Eagle's medium. Cell Culture—Individual articular chondrocytes were isolated from joint cartilage slices from 2-week-old New Zealand White rabbits as described previously (15.Kim S.-J. Kim H.-G. Oh C.-D. Hwang S.-G. Song W.-K. Chun J.-S. J. Biol. Chem. 2002; 277: Full Text Full Text PDF PubMed Scopus (53) Google Scholar, Kim S.-J. Oh C.-D. Song W.-K. Huh T.-L. Chun J.-S. J. Biol. Chem. 2002; 277: Full Text Full Text PDF PubMed Scopus Google Scholar). The cells were a of and in Dulbecco's modified Eagle's medium with and The medium was the were with the indicated in cell cell was from and in with and Cell and were prepared as described previously J.-H. Kim S.-J. Kim S.-H. Oh C.-D. Hwang S.-G. Chun C.-H. Oh S.-H. Seong J.-K. Huh T.-L. Chun J.-S. Development. 2002; 129: 5541-5550Crossref PubMed Scopus (131) Google Scholar). were prepared by chondrocytes in and and and the addition of the cells were for and for The was in and and and for The was for and of for α-catenin was prepared by reverse from cells with and to and the and The was into The β-catenin was as described previously J.-H. Kim S.-J. Kim S.-H. Oh C.-D. Hwang S.-G. Chun C.-H. Oh S.-H. Seong J.-K. Huh T.-L. Chun J.-S. Development. 2002; 129: 5541-5550Crossref PubMed Scopus (131) Google Scholar). chondrocytes were with expression the as described previously S.-J. Oh C.-D. Song W.-K. Kim J.-H. Chun J.-S. J. Biol. Chem. 2002; 277: Full Text Full Text PDF PubMed Scopus Google Scholar, Kim S.-J. Hwang S.-G. S. Chun J.-S. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). of by examined to α-catenin in chondrocytes and found that of of a the of was to the CA). and indicate α-catenin and indicate the were α-catenin was used as a which to gene and not regulate α-catenin expression. of and was in for and the in a was into was with as described β-Catenin transcriptional activity of the β-catenin-Tcf/Lef complex was the previously described gene (4.Hwang S.-G. Ryu J.-H. Kim I.-C. Jho E.-H. Kim K. Kim S.-J. Chun J.-S. J. Biol. Chem. 2004; 279: 26597-26604Abstract Full Text Full Text PDF PubMed Scopus (101) Google Scholar, 5.Ryu J.-H. Kim S.-J. Kim S.-H. Oh C.-D. Hwang S.-G. Chun C.-H. Oh S.-H. Seong J.-K. Huh T.-L. Chun J.-S. Development. 2002; 129: 5541-5550Crossref PubMed Scopus (131) Google Scholar). cells were with of the Tcf/Lef or and of activity was and for was isolated and with as described previously S.-J. Hwang S.-G. Chun J.-S. J. Biol. Chem. 2002; 277: Full Text Full Text PDF PubMed Scopus Google Scholar). The and were used for α-catenin and with of β-catenin and with of and with of The were the of the α- and β-catenin of the for α-catenin and β-catenin that gene were and to α- and β-catenin, not were with in for The cells were and blocked with and in for The cells were and for with α-catenin or β-catenin CA). The cells were with or for and a green protein α-catenin was used to the of activity in the cell was by of from a peptide substrate cell were by and the were used for of were with of peptide substrate in and The were for and with of was in a of and of and was as described by et al. D. M. J. Biol. Chem. Full Text PDF PubMed Google Scholar). were of a and a were in and and for the proteasome S and 26 were by the with and peptide and the for The 26 S proteasome were and the proteins were by and by were in and as described The were for to cell and the was with and protein for and with of α-catenin or The were by with protein for and by and were for in a and with and as described were by to a and the II collagen and from and and For the of were between several of and in and was by for of in reports that β-catenin were regulated by proteasomal degradation (3.Aberle H. Bauer A. Stappert J. Kispert A. Kemler R. EMBO J. 1997; 16: 3797-3804Crossref PubMed Scopus (2160) Google Scholar), examined the proteasomal degradation regulates α-catenin protein in articular chondrocytes. As β-catenin protein were by 26 S proteasome and but not by caspase and or calpain of α-catenin were also by the inhibition of the 26 S proteasome suggesting that both α- and degraded via proteasomal with a of α- and was blocked by the inhibition of the 26 S proteasome with that both α- and regulated via the proteasomal degradation regulation of α- and β-catenin was by our that were not by proteasome inhibition the proteasomal degradation of examined its association with the 26 S proteasome. For chondrocytes were with or to proteasome activity and cell were by The 26 S proteasome was by substrate localized in the 26 S proteasome were by indicated that both α- and were in the 26 S proteasome suggesting that α- and with the 26 S proteasome. inhibition with and accumulation of α- and in a and β-catenin was in the as a with ubiquitination (3.Aberle H. Bauer A. Stappert J. Kispert A. Kemler R. EMBO J. 1997; 16: 3797-3804Crossref PubMed Scopus (2160) Google Scholar). not of α-catenin proteasome inhibition and the of α-catenin for ubiquitination of α- and β-catenin by and As shown in indicated that β-catenin, but not was in chondrocytes, suggesting that α-catenin is degraded by the 26 S proteasome in a ubiquitin-independent fashion. β-Catenin examined accumulation of α- and β-catenin proteins is associated with other in chondrocytes. For used and to the expression of α- and in chondrocytes with α-catenin or β-catenin be due to the of its of chondrocytes and that cells β-catenin of α-catenin whereas of α-catenin not β-catenin protein the results that accumulation of α-catenin is associated with the increases in β-catenin protein the association of α- and β-catenin accumulation was characterized by the degradation of α-catenin from that of β-catenin a cell As shown in α-catenin was accumulated in the cell by the inhibition of proteasome, that α-catenin degradation via the proteasomal is of the of β-catenin The role of β-catenin accumulation in α-catenin degradation was examined by ectopic expression of β-catenin in β-catenin to α-catenin and the in α-catenin protein suggesting that the of β-catenin protein and its binding to α-catenin proteasomal degradation of This also that the in α-catenin protein in chondrocytes is due to the inhibition of α-catenin degradation by The role of β-catenin in α-catenin degradation was examined by β-catenin accumulation in chondrocytes. with our that accumulation of β-catenin and subsequent stimulation of β-catenin-Tcf/Lef complex transcriptional activity dedifferentiation of articular chondrocytes (4.Hwang S.-G. Ryu J.-H. Kim I.-C. Jho E.-H. Kim K. Kim S.-J. Chun J.-S. J. Biol. Chem. 2004; 279: 26597-26604Abstract Full Text Full Text PDF PubMed Scopus (101) Google Scholar, 5.Ryu J.-H. Kim S.-J. Kim S.-H. Oh C.-D. Hwang S.-G. Chun C.-H. Oh S.-H. Seong J.-K. Huh T.-L. Chun J.-S. Development. 2002; 129: 5541-5550Crossref PubMed Scopus (131) Google Scholar, S.-J. Im D.-S. Kim S.-H. Ryu J.-H. Hwang S.-G. Seong J.-K. Chun C.-H. Chun J.-S. Biochem. Biophys. Res. Commun. 2002; 296: 221-226Crossref PubMed Scopus (65) Google Scholar), of chondrocytes with Wnt-7a-conditioned a glycogen synthase or a accumulation of β-catenin stimulation of β-catenin-Tcf/Lef complex transcriptional activity and suppression of type II collagen expression The accumulation of β-catenin is by the in α-catenin protein of α- and β-catenin and proteasome activity were not by the of chondrocytes with or suggesting regulation of α- and β-catenin protein β-Catenin and of examined the significance of the proteasomal degradation of α-catenin in articular chondrocytes. examined the of α- and that α-catenin was localized whereas β-catenin was areas of cell-cell However, when proteasome activity was blocked by of α- and were in the nucleus Cell indicated that proteasome accumulated α- and were localized in both the cytosolic and the of α- and that proteins as complexes in both the cytosolic and the The significance of α-catenin degradation by the proteasomal was examined in of its role in and β-catenin signaling. As accumulation of β-catenin proteasome inhibition by to stimulation of β-catenin-Tcf/Lef transcriptional activity β-Catenin accumulation also inhibited type II collagen expression which is with our (4.Hwang S.-G. Ryu J.-H. Kim I.-C. Jho E.-H. Kim K. Kim S.-J. Chun J.-S. J. Biol. Chem. 2004; 279: 26597-26604Abstract Full Text Full Text PDF PubMed Scopus (101) Google Scholar, 5.Ryu J.-H. Kim S.-J. Kim S.-H. Oh C.-D. Hwang S.-G. Chun C.-H. Oh S.-H. Seong J.-K. Huh T.-L. Chun J.-S. Development. 2002; 129: 5541-5550Crossref PubMed Scopus (131) Google Scholar). The significance of α-catenin accumulation and association with β-catenin was by α-catenin by proteasome As shown in ectopic expression of α-catenin blocked the of β-catenin-Tcf/Lef transcriptional activity and also blocked the inhibition of type II collagen expression. The of type II collagen expression be due to α-catenin whereas the inhibition of β-catenin-Tcf/Lef transcriptional activity be due to our of the gene and the results that α-catenin β-catenin signaling, the role of α-catenin in β-catenin signaling was directly examined by α- As shown in of α-catenin to decreased of β-catenin-Tcf/Lef transcriptional activity and type II collagen expression expression of β-catenin to β-catenin transcriptional activity and suppression of type II collagen expression of α-catenin and β-catenin blocked the in β-catenin-Tcf/Lef complex transcription activity and the inhibition of type II collagen expression that α-catenin inhibits β-catenin signaling and subsequent dedifferentiation of chondrocytes. The of α-catenin in β-catenin signaling was by the role of α-catenin in Wnt signaling. of cells with or of cells with Wnt-7a-conditioned medium to β-catenin-Tcf/Lef transcriptional activity and suppression of type II collagen expression expression of α-catenin blocked the of transcriptional activity and type II collagen expression that α-catenin inhibits Wnt signaling by the transcriptional activity of inhibits Wnt signaling and inhibition of type II collagen chondrocytes were with and or with a for the cells were with or with The transcriptional activity of β-catenin was by gene and expression of β-catenin, type II and with were by chondrocytes were with or α-catenin expression for the cells were with of Wnt-7a-conditioned medium for The transcriptional activity of β-catenin was by gene and of α-catenin with β-catenin, and type II collagen were by was as a In to the physiological significance of α-catenin degradation and its regulation of β-catenin signaling, examined α-catenin can regulate β-catenin signaling and chondrocyte by to the α-catenin in chondrocytes. As shown in of α-catenin the of β-catenin-Tcf/Lef transcriptional activity and in type II collagen expression of α-catenin also of β-catenin-Tcf/Lef transcriptional activity and the inhibition of type II collagen expression by Wnt-7a We previously that stimulation of β-catenin-Tcf/Lef transcriptional activity by ectopic expression of β-catenin or Wnt-7a dedifferentiation of articular chondrocytes (4.Hwang S.-G. Ryu J.-H. Kim I.-C. Jho E.-H. Kim K. Kim S.-J. Chun J.-S. J. Biol. Chem. 2004; 279: 26597-26604Abstract Full Text Full Text PDF PubMed Scopus (101) Google Scholar, 5.Ryu J.-H. Kim S.-J. Kim S.-H. Oh C.-D. Hwang S.-G. Chun C.-H. Oh S.-H. Seong J.-K. Huh T.-L. Chun J.-S. Development. 2002; 129: 5541-5550Crossref PubMed Scopus (131) Google Scholar). investigated the regulatory mechanisms of α-catenin degradation in articular chondrocytes and examined the role of α-catenin in β-catenin signaling and its physiological significance in chondrocyte We found that α-catenin was actively degraded by a ubiquitin-independent proteasomal and that accumulated α-catenin inhibited the transcriptional activity of the β-catenin-Tcf/Lef or dedifferentiation of articular chondrocytes. of is the that α-catenin is degraded via a proteasomal as by our that the inhibition of proteasome accumulation of This is by our that α-catenin was associated with the proteasome role in the rapid of regulatory proteins. for protein is the of degradation such as the degradation the PEST sequence, and the (2.Willert K. Nusse R. Curr. Opin. Genet. Dev. 1998; 8: 95-102Crossref PubMed Scopus (666) Google Scholar). It was previously shown that which contains a PEST was actively degraded in L cells (13.Takahashi N. Ishihara S. Takada S. Tsukita S. Nagafuchi A. Biochem. Biophys. Res. Commun. 2000; 277: 691-698Crossref PubMed Scopus (26) Google Scholar). However, the found that various proteasome and calpain not the of α-catenin proteins in L cells (13.Takahashi N. Ishihara S. Takada S. Tsukita S. Nagafuchi A. Biochem. Biophys. Res. Commun. 2000; 277: 691-698Crossref PubMed Scopus (26) Google Scholar). In our proteasome inhibition α-catenin protein not only in chondrocytes but also in and cells not suggesting that proteasomal degradation of α-catenin is not to articular chondrocytes. in study is that α-catenin degradation is For proteasomal protein degradation, the is the of the which a to the substrate 2002; PubMed Scopus Google Scholar). 26 S proteasomes proteins N. E. A.L. 2001; PubMed Scopus Google Scholar). However, not for the ubiquitination of α-catenin in articular chondrocytes, which that α-catenin is degraded via the 26 S proteasome in a ubiquitination-independent This is not several other proteins degraded by proteasomes in a ubiquitination-independent fashion. For instance, is degraded by the 26 S proteasome via a ubiquitin-independent S. S. N. K. Biochem. Biophys. Res. Commun. 2000; PubMed Scopus (78) Google Scholar), and is between and M. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). proteins such as N. E. A.L. 2001; PubMed Scopus Google Scholar), and R. 2001; PubMed Scopus Google Scholar) also degraded by 26 S proteasomes The protein can proteasomal degradation by is ubiquitin-dependent and regulated by whereas the other is ubiquitin-independent and regulated by J. Proc. Natl. Acad. Sci. U. S. A. 2002; PubMed Scopus Google Scholar). it appears that ubiquitin-independent degradation by 26 S proteasomes be has However, not in the of ubiquitin-independent degradation of α-catenin in chondrocytes. is that α-catenin degradation is to the degradation of β-catenin and that β-catenin α-catenin into proteasomal degradation. This is the that of β-catenin or accumulation of β-catenin accumulation of whereas of α-catenin not β-catenin However, is to in chondrocytes α-catenin degradation is also in the cell our the degradation of α-catenin from that of β-catenin the cell indicate that α-catenin degradation is to the β-catenin degradation. Our results indicate that accumulation of β-catenin inhibits degradation of as by the that β-catenin in cells to α-catenin and causes the in α-catenin protein This also the the accumulation of β-catenin causes accumulation of α-catenin in chondrocytes. of β-Catenin and results indicated that α-catenin inhibits signaling of β-catenin and or dedifferentiation of chondrocytes. The that α-catenin inhibits β-catenin signaling and that Wnt signaling not only β-catenin but also α-catenin to be for the of Wnt signaling, stimulation of β-catenin-Tcf/Lef transcriptional activity and dedifferentiation of chondrocytes. However, our results that the in α-catenin by Wnt signaling is to of the transcriptional activity of β-catenin-Tcf/Lef This is the that of α-catenin by the of β-catenin-Tcf/Lef transcriptional activity and of β-catenin-Tcf/Lef transcriptional activity and binding of α-catenin to β-catenin inhibited degradation of α-catenin and inhibited β-catenin signaling. Our study Kim S.-J. Oh C.-D. Song W.-K. Huh T.-L. Chun J.-S. J. Biol. Chem. 2002; 277: Full Text Full Text PDF PubMed Scopus Google Scholar) indicated that of cells decreased expression of α- and We also that of β-catenin is for and accumulation of β-catenin inhibited by cell-cell adhesion J.-H. Kim S.-J. Kim S.-H. Oh C.-D. Hwang S.-G. Chun C.-H. Oh S.-H. Seong J.-K. Huh T.-L. Chun J.-S. Development. 2002; 129: 5541-5550Crossref PubMed Scopus (131) Google Scholar). β-catenin inhibits by acting as a whereas it causes dedifferentiation of chondrocytes by acting as a transcriptional co-activator. it is likely that of both α- and β-catenin during is for the of cell-cell adhesion, which is for chondrocyte and the of α-catenin during dedifferentiation is to of the β-catenin-Tcf/Lef transcriptional with our several have indicated that the inhibition of β-catenin signaling by α-catenin via direct binding to β-catenin (9.Simcha I. Shtutman M. Salomon D. Zhurinsky J. Sadot E. Geiger B. Ben-Ze'ev A. J. Cell Biol. 1998; 141: 1433-1448Crossref PubMed Scopus (239) Google Scholar, 12.Sehgal R.N. Gumbiner B.M. Reichardt L.F. J. Cell Biol. 1997; 139: 1033-1046Crossref PubMed Scopus (53) Google Scholar, 13.Takahashi N. Ishihara S. Takada S. Tsukita S. Nagafuchi A. Biochem. Biophys. Res. Commun. 2000; 277: 691-698Crossref PubMed Scopus (26) Google Scholar, A.L. Vivanco M. Kypta R.M. J. Biol. Chem. 2000; 275: 21883-21888Abstract Full Text Full Text PDF PubMed Scopus (78) Google Scholar, A.L. Vivanco Kypta R.M. Cell Res. 2000; PubMed Scopus Google Scholar). It is that α-catenin inhibits transcription by β-catenin in the cytoplasm (9.Simcha I. Shtutman M. Salomon D. Zhurinsky J. Sadot E. Geiger B. Ben-Ze'ev A. J. Cell Biol. 1998; 141: 1433-1448Crossref PubMed Scopus (239) Google Scholar, 12.Sehgal R.N. Gumbiner B.M. Reichardt L.F. J. Cell Biol. 1997; 139: 1033-1046Crossref PubMed Scopus (53) Google Scholar, 13.Takahashi N. Ishihara S. Takada S. Tsukita S. Nagafuchi A. Biochem. Biophys. Res. Commun. 2000; 277: 691-698Crossref PubMed Scopus (26) Google Scholar). In or α-catenin is found in the of various cell such as and cells but not in the of cells (14.Giannini A.L. Vivanco M. Kypta R.M. J. Biol. Chem. 2000; 275: 21883-21888Abstract Full Text Full Text PDF PubMed Scopus (78) Google Scholar, A.L. Vivanco Kypta R.M. Cell Res. 2000; PubMed Scopus Google Scholar). The for of α-catenin is but that β-catenin is not the only involved in of It was that α-catenin binding to β-catenin does not of α-catenin into the nucleus but rather inhibits the interaction of the β-catenin-Tcf/Lef complex with target DNA A.L. Vivanco Kypta R.M. Cell Res. 2000; PubMed Scopus Google Scholar). In articular chondrocytes, α- and β-catenin complexes localized in both the cytosol and the nucleus, that sequestration of β-catenin in the cytoplasm is not the by which β-catenin signaling is inhibited by it is likely that association of α-catenin with β-catenin interaction with target However, In have that both α- and actively degraded by 26 S proteasomes in articular chondrocytes. The degradation of β-catenin is ubiquitination-dependent, whereas α-catenin is degraded of We also that accumulated α-catenin associates with β-catenin, in the inhibition of β-catenin-Tcf/Lef transcriptional activity and subsequent of or the inhibition of type II collagen expression. Our results that α-catenin in chondrocytes not only as a but also as a of β-catenin signaling.