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Signal Transduction from N-cadherin Increases Bcl-2

REGULATION OF THE PHOSPHATIDYLINOSITOL 3-KINASE/Akt PATHWAY BY HOMOPHILIC ADHESION AND ACTIN CYTOSKELETAL ORGANIZATION*
  • Nhan L. Tran
    Affiliations
    Department of Cancer Biology Graduate Program, University of Arizona Health Sciences Center, Tucson, Arizona 85724
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  • Deanna G. Adams
    Affiliations
    Department of Pharmacology and Toxicology, University of Arizona Health Sciences Center, Tucson, Arizona 85724
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  • Richard R. Vaillancourt
    Affiliations
    Department of Pharmacology and Toxicology, University of Arizona Health Sciences Center, Tucson, Arizona 85724
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  • Ronald L. Heimark
    Correspondence
    To whom correspondence should be addressed: University of Arizona, Health Sciences Center, P.O. Box 245084, 1501 N. Campbell Ave, Tucson, AZ 85724. Tel.: 520-626-1913; Fax: 520-626-2890
    Affiliations
    Department of Cancer Biology Graduate Program, University of Arizona Health Sciences Center, Tucson, Arizona 85724

    Department of Surgery, and Cell Biology and Anatomy, University of Arizona Health Sciences Center, Tucson, Arizona 85724
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  • Author Footnotes
    * This work was supported by National Institutes of Health Grants CA5666 (to R. L. H.) and AG19710 (to R. R. V.), Cancer Biology Training Grant T32CA09213, a grant from the Achievement Rewards for College Scientists Foundation (to N. L. T.), and a Pharma Pre-doctoral Fellowship (to D. G. A.).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Open AccessPublished:July 02, 2002DOI:https://doi.org/10.1074/jbc.M200300200
      Associated with the metastatic progression of epithelial tumors is the dynamic regulation of cadherins. Whereas E-cadherin is expressed in most epithelium and carcinomas, recent studies suggest that the up-regulation of other cadherin subtypes in carcinomas, such as N-cadherin, may function in cancer progression. We demonstrate that a signal transduction cascade links the N-cadherin·catenin adhesion complex to up-regulation of the anti-apoptotic protein Bcl-2. In suspension, aggregates of DU-145 cells, an E-cadherin expressing human prostate carcinoma line, survive loss of integrin-dependent adhesion by a different anti-apoptotic signaling pathway than the N-cadherin expressing lines PC3 and PC3N. N-cadherin intercellular adhesion mediates a 3.5-fold increase in Bcl-2 protein expression, whereas the level of the proapoptotic protein Bax remains constant. Only N-cadherin ligation in PC3 cells, which express both N-cadherin and E-cadherin, is sufficient to induce activation of Akt/protein kinase B. N-cadherin homophilic ligation initiates phosphatidylinositol 3-kinase-dependent activation of Akt resulting in Akt phosphorylation of Bad on serine 136. Following N-cadherin homophilic adhesion phosphatidylinositol 3-kinase was identified in immunoprecipitates of the N-cadherin·catenin complex. The recruitment of phosphatidylinositol 3-kinase to the adhesion complex is dependent on ligation of N-cadherin and an organized actin cytoskeleton because cytochalasin D blocks the recruitment. We propose that N-cadherin homophilic adhesion can initiate anti-apoptotic signaling, which enhances the Akt cell survival pathway in metastatic cancer.
      PI 3-kinase
      phosphatidylinositol 3-kinase
      GST
      glutathioneS-transferase
      PARP
      poly(ADP-ribose)polymerase
      HA
      hemagglutinin
      EGF
      epidermal growth factor
      DOTAP
      N-[1-(2,3-dioleoyloxy)propyl]-N,N,N-trimethylammonium salt
      BSA
      bovine serum albumin
      PBS
      phosphate-buffered saline
      PIPES
      1,4-piperazinediethanesulfonic acid
      Cadherins are transmembrane cell-cell adhesion receptors, which have important functional roles in mediating cell segregation during embryonic development and in maintenance of adult tissue integrity (
      • Takeichi M.
      ). Functional cell adhesion by cadherin subtypes requires both the coordinated homophilic binding of the extracellular domain and the cadherin cytoplasmic domain with the actin cytoskeleton (
      • Yap A.S.
      • Brieher W.M.
      • Gumbiner B.M.
      ). Calcium binds to the cadherin extracellular domain inducing a conformation that initiates and stabilizes the homophilic binding of cadherin subtypes on adjacent cells (
      • Troyanovsky R.B.
      • Klingelhofer J.
      • Troyanovsky S.
      ,
      • Volberg T.
      • Geiger B.
      • Kartenbeck J.
      • Franke W.W.
      ). The conserved cadherin cytoplasmic domain is associated with a lateral network of actin filaments through the catenins (
      • Ozawa M.
      • Kemler R.
      ). Either β-catenin or γ-catenin binds the complex to α-catenin, which either interacts directly with peripheral actin filaments, or indirectly through actin-associated proteins, such as vinculin or α-actinin (
      • Hazan R.B.
      • Kang L.
      • Roe S.
      • Borgen P.I.
      • Rimm D.L.
      ,
      • Knudsen K.A.
      • Soler A.P.
      • Johnson K.R.
      • Wheelock M.J.
      ). In addition, p120ctn binds to a separate site in the juxtamembrane region of cadherins and controls the strength of cadherin-mediated adhesion (
      • Yap A.S.
      • Niessen C.M.
      • Gumbiner B.M.
      ).
      Although the classical cadherin subtypes and the catenins lack enzymatic activity, they can associate with kinases and phosphatases, such as Fer and PTP1B, in adheren junctions (
      • Tsukita S.
      • Oishi K.
      • Akiyama T.
      • Yamanashi Y.
      • Yamamoto T.
      • Tsukita S.
      ,
      • Arregui C.
      • Pathre P.
      • Lilien J.
      • Balsamo J.
      ). Homophilic E-cadherin adhesion initiates the activation of phosphatidylinositol 3-kinase (PI 3-kinase)1followed by downstream activation of Akt/protein kinase B (
      • Pece S.
      • Chiariello M.
      • Murga C.
      • Gutkind J.S.
      ,
      • Kovacs E.M.
      • Ali R.G.
      • McCormack A.J.
      • Yap A.S.
      ). The serine/threonine kinase Akt is an important regulator of metabolic pathways, as well as, apoptotic pathways. Akt is activated in response to diverse extracellular stimuli, such as EGF and other growth factors and integrin adhesion to matrix (
      • Datta S.R.
      • Brunet A.
      • Greenberg M.E.
      ). Akt kinase activity is regulated by PI 3-kinase at the plasma membrane through the production of phosphatidylinositol 3,4-biphosphate and phosphatidylinositol 3,4,5-triphosphate, which bind to the pleckstrin homology domain of Akt altering its conformation. For full activation, Akt requires phosphorylation on threonine 308 (Thr-308) and serine 473 (Ser-473) by 3-phosphoinositide-dependent kinase 1 (
      • Alessi D.R.
      • James S.R.
      • Downes C.P.
      • Holmes A.B.
      • Gaffney P.R.
      • Reese C.B.
      • Cohen P.
      ) or the phosphoinositide-dependent kinase 2/integrin-linked kinase (
      • Persad S.
      • Attwell S.
      • Gray V.
      • Mawji N.
      • Deng J.T.
      • Leung D.
      • Yan J.
      • Sanghera J.
      • Walsh M.P.
      • Dedhar S.
      ). Phosphorylation of Thr-308 occurs when Akt is recruited to the membrane, but is not sufficient for activation of kinase activity, which requires phosphorylation of Ser-473 (
      • Alessi D.R.
      • Andjelkovic M.
      • Caudwell B.
      • Cron P.
      • Morrice N.
      • Cohen P.
      • Hemmings B.A.
      ). Activated Akt can phosphorylate several substrates that result in suppression of apoptosis (
      • Datta S.R.
      • Brunet A.
      • Greenberg M.E.
      ).
      Upon exposure to apoptotic signals, it is proposed that cell fate is determined by the relative balance between pro- and anti-apoptotic protein interactions of the Bcl-2 family (
      • Yin X.M.
      • Oltvai Z.N.
      • Korsmeyer S.J.
      ). Members of this family include the pro-apoptotic proteins Bad, Bik, Bid, and the anti-apoptotic cell survival proteins Bcl-2 and Bcl-xL (
      • Adams J.M.
      • Cory S.
      ). Whereas homodimers of Bcl-2 in the mitochondrial membrane prevent the activation of caspase-9, heterodimerization of Bcl-2 and Bad induces the activation of caspase-9 and initiates an irreversible pathway (
      • Green D.R.
      • Reed J.C.
      ). One mechanism by which Akt prevents apoptosis is through the phosphorylation of the pro-apoptotic protein Bad. Akt phosphorylates Bad on Ser-136, which inhibits the heterodimer formation of Bad and Bcl-2 in the mitochondrial membrane (
      • Datta S.R.
      • Dudek H.
      • Tao X.
      • Masters S., Fu, H.
      • Gotoh Y.
      • Greenberg M.E.
      ). Phosphorylated Bad is sequestered in the cytoplasm by interacting with 14-3-3 scaffolding proteins, which in turn suppresses apoptosis (
      • Masters S.C.
      • Yang H.
      • Datta S.R.
      • Greenberg M.E.
      • Fu H.
      ).
      Among the alterations that facilitate prostate cancer progression are changes in cell adhesion molecule expression along with the increased resistance to apoptosis (
      • Abate-Shen C.
      • Shen M.M.
      ). E-cadherin is essential for adhesion of glandular epithelial cells to each other, and homophilic cell-cell adhesion results in the formation of adheren junctions, which in turn regulates differentiation, growth, and survival within the tissue microenvironment (
      • Boudreau N.
      • Bissell M.J.
      ,
      • Vallorosi C.J.
      • Day K.C.
      • Zhao X.
      • Rashid M.G.
      • Rubin M.A.
      • Johnson K.R.
      • Wheelock M.J.
      • Day M.L.
      ). Decreased expression of E-cadherin is an important factor in the regulation of carcinoma invasiveness and metastasis (
      • Birchmeier W.
      • Behrens J.
      ). The dysregulation of cell adhesion in prostate cancer metastasis is, in part, facilitated by the gain in expression of nonepithelial cadherins and members of the integrin family (
      • Cress A.E.
      • Rabinovitz I.
      • Zhu W.
      • Nagle R.B.
      ). N-cadherin is up-regulated in dedifferentiated, invasive prostate carcinomas (
      • Tran N.L.
      • Nagle R.B.
      • Cress A.E.
      • Heimark R.L.
      ,
      • Bussemakers M.J.
      • Van Bokhoven A.
      • Tomita K.
      • Jansen C.F.
      • Schalken J.A.
      ) and N-cadherin expression is reported to induce a motile scattered phenotype in breast carcinoma cells (
      • Hazan R.B.
      • Phillips G.R.
      • Qiao R.F.
      • Norton L.
      • Aaronson S.A.
      ) and squamous cell carcinomas (
      • Islam S.
      • Carey T.E.
      • Wolf G.T.
      • Wheelock M.J.
      • Johnson K.R.
      ).
      Evidence indicates that disruption of cadherin adhesion can initiate apoptosis in both normal and cancer cells (
      • Hermiston M.L.
      • Gordon J.I.
      ,
      • Kantak S.S.
      • Kramer R.H.
      ) similar to the inhibition of integrin adhesion to extracellular matrix (
      • Frisch S.M.
      • Francis H.
      ). Several studies have partly delineated the molecular mechanisms that link integrin-mediated signal transduction pathways to the intracellular apoptotic machinery, but the mechanism whereby cadherin adhesion contributes to cell survival is poorly understood. To test the importance of cadherin subtypes in cell survival, we examined the homophilic ligation of cadherins on the phosphorylation and activation of Akt. Increased Akt activity has been implicated in prostate tumor progression androgen independence (
      • Graff J.R.
      • Konicek B.W.
      • McNulty A.M.
      • Wang Z.
      • Houck K.
      • Allen S.
      • Paul J.D.
      • Hbaiu A.
      • Goode R.G.
      • Sandusky G.E.
      • Vessella R.L.
      • Neubauer B.L.
      ). We demonstrate that the anti-apoptotic effects of N-cadherin in prostate carcinomas depend on its ability to recruit PI 3-kinase to the N-cadherin·catenin·actin complex and results in an increased Bcl-2/Bax protein level. Overexpression of Bcl-2 has been observed in androgen-independent prostate adenocarcinomas, which correlated with the presence of metastases (
      • Colombel M.
      • Symmans F.
      • Gil S.
      • O'Toole K.M.
      • Chopin D.
      • Benson M.
      • Olsson C.A.
      • Korsmeyer S.
      • Buttyan R.
      ,
      • Krajewska M.
      • Krajewski S.
      • Epstein J.I.
      • Shabaik A.
      • Sauvageot J.
      • Song K.
      • Kitada S.
      • Reed J.C.
      ). Our findings suggest that the gain of N-cadherin expression by prostate carcinomas in advanced stages of metastasis may be an inhibitor of apoptosis through stabilization of Bcl-2.

      DISCUSSION

      The studies presented here suggest new insights into the function and interaction of the cadherin subtypes, E-cadherin and N-cadherin. Our data indicates that N-cadherin has dual functional roles in the regulation of apoptosis and in homophilic cell-cell adhesion. Genetic studies have also demonstrated a dual role for N-cadherin in null mouse embryos where populations of neural cells in the homozygous null embryos were apoptotic and myocardium was disorganized but not apoptotic (
      • Luo Y.
      • Ferreira-Cornwell M.
      • Baldwin H.
      • Kostetskii I.
      • Lenox J.
      • Lieberman M.
      • Radice G.
      ). We found that N-cadherin homophilic adhesion in prostate carcinoma cells is linked to Akt signaling and inhibition of the mitochondrial apoptotic pathway. Members of the Bcl-2 family of proteins affect mitochondrial function and regulate the release of apoptosis-activating factors (
      • Adams J.M.
      • Cory S.
      ). Anti-apoptotic Bcl-2 family members act to preserve mitochondrial integrity by suppressing the release of cytochrome c. The clustering of N-cadherin extracellular domains between adjacent PC3N cells leads to organization of actin cytoskeletal structure and provides specific outside-in signals that regulate the steady state level of Bcl-2. Inhibition of homophilic binding of the N-cadherin extracellular domain correlates with decreased cellular levels of activated Akt and Bcl-2 protein expression. Cadherin engagement between adjacent cells has an essential role in the recruitment of the actin cytoskeleton to the adherens junction complex and regulation of actin polymerization (
      • Kovacs E.M.
      • Ali R.G.
      • McCormack A.J.
      • Yap A.S.
      ,
      • Adams C.L.
      • Chen Y.T.
      • Smith S.J.
      • Nelson W.J.
      ,
      • Vasioukhin V.
      • Bauer C.
      • Yin M.
      • Fuchs E.
      ). We identified that formation of the adherens junction complex leads to the recruitment and activation of PI 3-kinase and phosphorylation of Akt. Subsequent activation of the PI 3-kinase/Akt pathway leads to phosphorylation of Bad at Ser-136 and stabilization of Bcl-2. The cytoprotective effects of Bcl-2 are in part through the inhibition of formation of Bax homodimers in the mitochondrial outer membrane (
      • Mikhailov V.
      • Mikhailova M.
      • Pulkrabek D.J.
      • Dong Z.
      • Venkatachalam M.A.
      • Saikumar P.
      ).
      In previous studies (
      • Pece S.
      • Chiariello M.
      • Murga C.
      • Gutkind J.S.
      ,
      • Kovacs E.M.
      • Ali R.G.
      • McCormack A.J.
      • Yap A.S.
      ) E-cadherin ligation has been implicated in the regulation of PI 3-kinase and the localized production of phosphatidylinositol 3,4,5-triphosphate that initiates phosphorylation of Akt. Thus, both E-cadherin and N-cadherin have the potential to activate Akt signaling pathways. In a prostate carcinoma cell line, which expresses both E-cadherin and N-cadherin, our results using an antibody immobilization assay to ligate cadherin subtypes with specific antibodies show that only N-cadherin leads to phosphorylation of Akt. Ligation of E-cadherin in PC-3 cells under the same conditions failed to activate Akt (Ser-473 phosphorylation). In prostate carcinomas, E-cadherin adhesion can prevent apoptosis in suspension, however, this is through a survival mechanism controlling retinoblastoma activation and G1 arrest (
      • Day M.L.
      • Zhao X.
      • Vallorosi C.J.
      • Putzi M.
      • Powell C.T.
      • Lin C.
      • Day K.C.
      ). Results from studies by Laure et al. (
      • Larue L.
      • Antos C.
      • Butz S.
      • Huber O.
      • Delmas V.
      • Dominis M.
      • Kemler R.
      ) indicate that cadherin subtypes are directly involved in the differentiation of different tissue types suggesting that cadherins influence specific signal tranduction pathways regulating gene expression activity. Taken together these results suggest that the outside-in signaling of cadherin subtypes depends on their expression in a cellular context.
      Evidence suggests that following cellular attachment the suppression of apoptosis is not mediated solely by integrin-mediated adhesion to extracellular matrix but, rather, by the ability of the cells to spread and adopt an optimal cell shape (
      • Chen C.S.
      • Mrksich M.
      • Huang S.
      • Whitesides G.M.
      • Ingber D.E.
      ). Cadherin-mediated cell-cell adhesion is an additional mechanism to control cell shape and organization of the cytoskeleton. Formation of intercellular adherens junctions is a dynamic process that involves actin reorganization and polymerization initiated by cadherin adhesion (
      • Vasioukhin V.
      • Bauer C.
      • Yin M.
      • Fuchs E.
      ,
      • Drubin D.G.
      • Nelson W.J.
      ). Cadherin homophilic binding initiates clustering of the extracellular domains, and interaction of the cytoplasmic domain with the actin cytoskeleton is an important mechanism to strengthen the weak forces provided by homophilic binding (
      • Yap A.S.
      • Brieher W.M.
      • Pruschy M.
      • Gumbiner B.M.
      ). Association of the N-cadherin·catenin·vinculin complexes in PC3 cells with the peripheral actin cytoskeleton provides sites for attachment of the cytoskeleton at the membrane, which are important for the development of mechanical stress during epithelial polarization and control of cell shape (
      • Drubin D.G.
      • Nelson W.J.
      ). This cadherin-mediated interaction also transmits signals from the plasma membrane to the actin cytoskeleton, ranging from activation of the Rho family of small GTPases (
      • Braga V.M.
      • Machesky L.M.
      • Hall A.
      • Hotchin N.A.
      ,
      • Noren N.K.
      • Niessen C.M.
      • Gumbiner B.M.
      • Burridge K.
      ,
      • Anastasiadis P.Z.
      • Moon S.Y.
      • Thoreson M.A.
      • Mariner D.J.
      • Crawford H.C.
      • Zheng Y.
      • Reynolds A.B.
      ) to production of phosphoinositol 3,4,5-triphosphate (
      • Sechi A.S.
      • Wehland J.
      ), which can regulate actin cytoskeleton organization. Homophilic E-cadherin ligation activates the GTPases Cdc42 and Rac, which regulates actin cytoskeletal rearrangement and can modulate cadherin function (
      • Braga V.M.
      • Machesky L.M.
      • Hall A.
      • Hotchin N.A.
      ,
      • Kim S.H., Li, Z.
      • Sacks D.B.
      ). The PI 3-kinase product phosphoinositol 3,4,5-triphosphate is required for the activation and recruitment of exchange factors for the GTPases such as Tiam-1, which is required for epithelial adherens junction formation (
      • Hordijk P.L.
      • ten Klooster J.P.
      • van der Kammen R.A.
      • Michiels F.
      • Oomen L.C.
      • Collard J.G.
      ). The survival signals initiated by integrin adhesion activate specific pathways that converge on the activation of Akt (
      • Giancotti F.G.
      ), in addition to the activation of other molecular mechanisms that inhibit apoptosis (
      • Stromblad S.
      • Becker J.C.
      • Yebra M.
      • Brooks P.C.
      • Cheresh D.A.
      ). Thus, our results suggest an overlap in the cell survival signal transduction cascades that are stimulated by integrin adhesion to extracellular matrix, and intercellular adhesion by N-cadherin.
      Following homophilic ligation of N-cadherin, our observations show that an intact actin cytoskeleton with the N-cadherin·catenin complex recruits PI 3-kinase. In addition to the remodeling of the actin cytoskeleton at the membrane by N-cadherin engagement, Lambert et al. (
      • Lambert M.
      • Padilla F.
      • Mege R.M.
      ) demonstrated that local accumulation of N-cadherin signaling complexes result in tyrosine phosphorylation of proteins in the adhesion complex. These tyrosine-phosphorylated proteins apparently serve as binding sites for the two Src homology 2 domains of the p85 regulatory subunit of PI 3-kinase to the cadherin·catenin complex. The major role of the p85 Src homology 2 domain is to mediate tyrosine kinase-dependent regulation of PI 3-kinase by increasing the catalytic activity of the p110 and recruitment of PI 3-kinase to the signaling complex (
      • Carpenter C.L.
      • Cantley L.C.
      ). Binding of the p85 subunit of PI 3-kinase has been identified to tyrosine-phosphorylated cytosolic and cadherin-associated β-catenin (
      • Woodfield R.J.
      • Hodgkin M.N.
      • Akhtar N.
      • Morse M.A.
      • Fuller K.J.
      • Saqib K.
      • Thompson N.T.
      • Wakelam M.J.
      ). The activation and autophosphorylation of EGF receptor by EGF elicits remodeling of actin (
      • Chan A.Y.
      • Raft S.
      • Bailly M.
      • Wyckoff J.B.
      • Segall J.E.
      • Condeelis J.S.
      ), which initiates the formation of lamellipodia (
      • Hill K.
      • Welti S., Yu, J.
      • Murray J.T.
      • Yip S.C.
      • Condeelis J.S.
      • Segall J.E.
      • Backer J.M.
      ) and requires the activity of PI 3-kinase. Distinct signaling properties for the class I isoforms of the p110 catalytic subunit of PI 3-kinase have been demonstrated (
      • Vanhaesebroeck B.
      • Jones G.E.
      • Allen W.E.
      • Zicha D.
      • Hooshmand-Rad R.
      • Sawyer C.
      • Wells C.
      • Waterfield M.D.
      • Ridley A.J.
      ). EGF-stimulated lamellipodia extension was shown to be a function of the α isoform and not the β isoform of the catalytic subunit (
      • Hill K.
      • Welti S., Yu, J.
      • Murray J.T.
      • Yip S.C.
      • Condeelis J.S.
      • Segall J.E.
      • Backer J.M.
      ), although both p110 catalytic subunits are capable of leading to downstream Akt activation (
      • Kurosu H.K.
      ). The differential utilization of the p110 isoforms could reflect the specific signaling properties that distinguish between the N-cadherin and EGF receptor signal transduction complexes.
      Because the protein and lipid phosphatase PTEN is a negative regulator of the PI 3-kinase pathway and is lost in advanced prostate cancer (
      • Stambolic V.
      • Suzuki A.
      • de la Pompa J.L.
      • Brothers G.M.
      • Mirtsos C.
      • Sasaki T.
      • Ruland J.
      • Penninger J.M.
      • Siderovski D.P.
      • Mak T.W.
      ), tumor progression is likely to be associated with increased responsiveness to extracellular signals, such as from cadherin adhesion that activates the PI 3-kinase pathway. Our studies show that the level of Bcl-2 protein expression is enhanced inPTEN negative prostate carcinoma cell lines that form N-cadherin-mediated adherens junctions. The association of PI 3-kinase with the N-cadherin·catenin complex was transient even though PC3 cells are deficient in PTEN.Tyrosine phosphatases, such as PTP1B (
      • Arregui C.
      • Pathre P.
      • Lilien J.
      • Balsamo J.
      ), are also associated with the N-cadherin·catenin adhesion complex and potentially function to reduce PI 3-kinase binding. We observed that following N-cadherin ligation, Akt translocated from the plasma membrane to the cytoplasm and then to the nucleus (data not shown), and returned to a basal level of phosphorylation by 24 h suggesting action of a phosphatase, such as PP2A.
      Altogether our results suggest a potential mechanism where N-cadherin adhesion can contribute to the regulation of apoptosis in carcinoma cells. Several studies have shown that regulation of apoptosis by the Bcl-2 family of proteins involves regulation of the mitochondrial outer membrane integrity (
      • Adams J.M.
      • Cory S.
      ). In the unphosphorylated state Bad can form heterodimers with anti-apoptotic Bcl-2 homologs and promotes cell death by allowing Bax-Bax homo-oligomers in the mitochondrial membrane. Monomeric Bax is predominantly cytoplasmic in cells under normal conditions, but translocates to the outer mitochondria membrane in response to apoptotic signals (
      • Narita M.
      • Shimizu S.
      • Ito T.
      • Chittenden T.
      • Lutz R.J.
      • Matsuda H.
      • Tsujimoto Y.
      ). During apoptosis the activation of caspases is a critical step and may contribute to the loss of anti-apoptotic signaling from cadherins because many of the substrates of caspases have been shown to be components of adherens junctions including plakoglobin, β-catenin, and E-cadherin (
      • Vallorosi C.J.
      • Day K.C.
      • Zhao X.
      • Rashid M.G.
      • Rubin M.A.
      • Johnson K.R.
      • Wheelock M.J.
      • Day M.L.
      ,
      • Steinhusen U.
      • Weiske J.
      • Badock V.
      • Tauber R.
      • Bommert K.
      • Huber O.
      ). Thus these new insights into the function of the cadherin survival signaling pathway may provide new approaches for the therapeutic treatment of uncontrolled growth and metastasis of prostate cancer cells.

      Acknowledgments

      We thank Nelson R. Alexander, Dr. Anne Cress, and Dr. G. Tim Bowden for helpful discussions, and Dr. Gabriel Nunez for providing the Akt wild type and kinase-dead expression constructs.

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