Cytoskeleton remodeling Integrin outside-in signaling
Integrin outside-in signaling Integrins are heterodimeric adhesion receptors composed of alpha- andbeta-subunits. It is known that at least 18 distinct alpha subunits and 8 or more betasubunits lead to generation of 24 alpha/beta heterodimeric receptors. Most integrinsrecognize extracellular matrix (ECM) proteins, such as Laminin,Fibronectin, Vitronectin and Collagen ( types I, II and IV ). Unstimulated cells are non-adherent, and cell adhesion to ECM can alter integrinavidity through intracellular signaling (inside-out signaling). Talin binding toIntegrin beta subunit triggers conformational changes of Integrins, increasingtheir affinity to the ECM proteins . Cell adhesion to ECMs can activate diverse intracellular kinases, includingtyrosine-protein kinase c-Src, focal adhesion kinase FAK1 andintegrin-linked protein kinase ILK . FAK1 is autophosphorylated in response to integrin activation, and/or isphosphorylated by c-Src leading to enhancement of catalytic activity ofFAK1 . Integrin binding of FAK1 is not required forFAK1 localization to the focal adhesions. This may be due to the fact thatFAK1 binds to Talin and Paxillin, which interacts withIntegrins , . A major function of integrin signaling is to link ECM proteins to intracellular actinfilaments via interactions of integrins with actin-binding proteins such as FilaminA, Talin, Alpha-actinin and ILK/ Pinch/ Parvin( alpha - and/or beta -) complex . Filamin A links actin filaments in orthogonal networks or parallel bundles, andcan induce reorganization of the cytoskeleton by different pathways. Filamin Arecruits a guanine nucleotide exchange factor (GEF) TRIO, which catalyses thetransition of Filamin -bound Ras-like protein Rac1 from the GDP bound tothe the GTP form . Rac1 -GTP then activates p21-activated kinase 1(PAK1). Alternatively, Filamin A directly associates with and activatedPAK1, leading to PAK-induced phosphorylation of Filamin A . PAK1 also promotes activation of Actin polymerization byphosphorylating of Arp2/3 (complex of actin-related proteins) ,. Talin is also important for the linkage of integrins to the cytoskeleton. Uponintegrin clustering Talin is recruited to focal complexes where it binds andactivates phosphatidylinositol phosphate kinase type 1 gamma ( PIPKI gamma ), thatleads to the production of phosphatidylinositol-4,5-bisphosphate ( PtdIns(4,5)P2 ). PtdIns(4,5)P2 binds to Talin and increases itsinteraction with Integrin. PtdIns(4,5)P2 also binds to Vinculin,which then interacts with Talin. PtdIns(4,5)P2 -bound Vinculin isreplaced by Actin filaments. PtdIns(4,5)P2 -associated vinculin cantransiently bind to the Arp2/3 complex, which nucleates Actinpolymerization. This interaction also requires Rac1 activation leading toArp2/3 phosphorylation by PAK1 . Alpha-actinin connects actin fibrils to the cytoplasmic tail ofintegrins and crosslinks actin filaments to actin bundles and networks.PtdIns(4,5)P2 binding to Alpha-actinin increases its interaction withActin . Phosphorylation of Alpha-actinin by FAK1decreases its associating with Actin. Alpha-actinin can also connect toActin filaments via its interaction with Vinculin . ILK binds to beta Integrin subunits and phosphorylates AKT(PKB)kinase, leading to integrin-mediated cell proliferation and survival. Another ILKtarget is GSK-3 beta, whose phosphorylation leads to stabilization and increaseof Beta-catenin in nucleus where it associates with transcription factorsTcf/Lef-1 to activate gene expression including c-Myc and Cyclin D1. ILK was shown to bind adapter protein PINCH which interacts with adapterprotein NCK2(Grb4) , , .NCK2(Grb4) can recruit PAK1 and WASP (Wiskott-Aldrich Sydromeprotein) interacting protein WIRE, leading to Arp2/3 activation andActin polymerization , . Cell adhesion to extracellular matrix also regulates cell proliferation and survival.Integrin activation by Fibronectin induces phosphorylation of FAK1 andPaxillin and causes the formation of FAK1/ GRB-2/ SOScomplexes, leading to H-Ras/ c-Raf-1/ MEK1, MEK2/ Erk(MAPK1/3) activation . ILK can recruit a family of F-actin binding proteins includingAlpha-parvin and Beta-parvin. Formation of ILK/ PINCH/Parvin complex is essential but not sufficient for cell adhesion .Paxillin is an additional interactor, which binds Integrins, ILK,Alpha-parvin to focal adhesions via interaction with Vinculin .