alpha-8/beta-1 integrin, Filamin A, Beta-catenin, WASP, Fibronectin, alpha-V/beta-3 integrin, MEK2, alpha-2/beta-1 integrin, Collagen I, H-Ras, Alpha-actinin, c-Src, Alpha-parvin, Beta- parvin, PtdIns(4,5)P2, Collagen IV, Vitronectin, GSK3 beta, Laminin 1, Collagen II, Erk (MAPK1/3), Cyclin D1, Paxillin, AKT, alpha-5/beta-1 integrin, GRB2, PINCH, c-Jun, Rac1, c-Raf-1, Actin cytoskeletal, PIPKI gamma, alpha-V/beta-3 integrin, Vinculin, PtdIns(4)P, TRIO, alpha-10/beta-1 integrin, FAK1, WIRE, Talin, alpha-3/beta-1 integrin, ILK, 188.8.131.52, PAK1, SOS, Tcf(Lef), MEK1, NCK2 (Grb4), alpha-11/beta-1 integrin, Arp2/3
Integrin outside-in signaling
Integrins are heterodimeric adhesion receptors composed of alpha- and beta-subunits. It is known that at least 18 distinct alpha subunits and 8 or more beta subunits lead to generation of 24 alpha/beta heterodimeric receptors. Most integrins recognize 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 integrin avidity through intracellular signaling (inside-out signaling). Talin binding to Integrin beta subunit triggers conformational changes of Integrins, increasing their affinity to the ECM proteins .
Cell adhesion to ECMs can activate diverse intracellular kinases, including tyrosine-protein kinase c-Src, focal adhesion kinase FAK1 and integrin-linked protein kinase ILK .
FAK1 is autophosphorylated in response to integrin activation, and/or is phosphorylated by c-Src leading to enhancement of catalytic activity of FAK1 . Integrin binding of FAK1 is not required for FAK1 localization to the focal adhesions. This may be due to the fact that FAK1 binds to Talin and Paxillin, which interacts with Integrins , .
A major function of integrin signaling is to link ECM proteins to intracellular actin filaments via interactions of integrins with actin-binding proteins such as Filamin A, Talin, Alpha-actinin and ILK/ Pinch/ Parvin ( alpha - and/or beta -) complex .
Filamin A links actin filaments in orthogonal networks or parallel bundles, and can induce reorganization of the cytoskeleton by different pathways. Filamin A recruits a guanine nucleotide exchange factor (GEF) TRIO, which catalyses the transition of Filamin -bound Ras-like protein Rac1 from the GDP bound to the the GTP form . Rac1 -GTP then activates p21-activated kinase 1 (PAK1). Alternatively, Filamin A directly associates with and activated PAK1, leading to PAK-induced phosphorylation of Filamin A . PAK1 also promotes activation of Actin polymerization by phosphorylating of Arp2/3 (complex of actin-related proteins) , .
Talin is also important for the linkage of integrins to the cytoskeleton. Upon integrin clustering Talin is recruited to focal complexes where it binds and activates phosphatidylinositol phosphate kinase type 1 gamma ( PIPKI gamma ), that leads to the production of phosphatidylinositol-4,5-bisphosphate ( PtdIns(4,5)P2 ) . PtdIns(4,5)P2 binds to Talin and increases its interaction with Integrin. PtdIns(4,5)P2 also binds to Vinculin, which then interacts with Talin. PtdIns(4,5)P2 -bound Vinculin is replaced by Actin filaments. PtdIns(4,5)P2 -associated vinculin can transiently bind to the Arp2/3 complex, which nucleates Actin polymerization. This interaction also requires Rac1 activation leading to Arp2/3 phosphorylation by PAK1 .
Alpha-actinin connects actin fibrils to the cytoplasmic tail of integrins and crosslinks actin filaments to actin bundles and networks. PtdIns(4,5)P2 binding to Alpha-actinin increases its interaction with Actin . Phosphorylation of Alpha-actinin by FAK1 decreases its associating with Actin. Alpha-actinin can also connect to Actin 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 ILK target is GSK-3 beta, whose phosphorylation leads to stabilization and increase of Beta-catenin in nucleus where it associates with transcription factors Tcf/Lef-1 to activate gene expression including c-Myc and Cyclin D1 .
ILK was shown to bind adapter protein PINCH which interacts with adapter protein NCK2(Grb4) , , . NCK2(Grb4) can recruit PAK1 and WASP (Wiskott-Aldrich Sydrome protein) interacting protein WIRE, leading to Arp2/3 activation and Actin polymerization , .
Cell adhesion to extracellular matrix also regulates cell proliferation and survival. Integrin activation by Fibronectin induces phosphorylation of FAK1 and Paxillin and causes the formation of FAK1/ GRB-2/ SOS complexes, leading to H-Ras/ c-Raf-1/ MEK1, MEK2/ Erk (MAPK1/3) activation .
ILK can recruit a family of F-actin binding proteins including Alpha-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 .