Cytoskeleton remodeling - Integrin outside-in signaling

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) [1].

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 [2].

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 [1].

FAK1 is autophosphorylated in response to integrin activation, and/or is phosphorylated by c-Src leading to enhancement of catalytic activity of FAK1 [3]. 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 [4], [5].

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 [6].

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 [7]. 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 [8]. PAK1 also promotes activation of Actin polymerization by phosphorylating of Arp2/3 (complex of actin-related proteins) [9], [10].

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) [11]. 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 [12].

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 [13]. Phosphorylation of Alpha-actinin by FAK1 decreases its associating with Actin. Alpha-actinin can also connect to Actin filaments via its interaction with Vinculin [14].

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 [15].

ILK was shown to bind adapter protein PINCH which interacts with adapter protein NCK2(Grb4) [16], [17], [18]. NCK2(Grb4) can recruit PAK1 and WASP (Wiskott-Aldrich Sydrome protein) interacting protein WIRE, leading to Arp2/3 activation and Actin polymerization [6], [19].

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 [20].

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 [6]. Paxillin is an additional interactor, which binds Integrins, ILK, Alpha-parvin to focal adhesions via interaction with Vinculin [21].

References:

1. Lee JW, Juliano R
   Mitogenic signal transduction by integrin- and growth factor receptor-mediated pathways. Molecules and cells 2004 Apr 30;17(2):188-202
2. Tadokoro S, Shattil SJ, Eto K, Tai V, Liddington RC, de Pereda JM, Ginsberg MH, Calderwood DA
   Talin binding to integrin beta tails: a final common step in integrin activation. Science (New York, N.Y.) 2003 Oct 3;302(5642):103-6
3. Schaller MD
   Biochemical signals and biological responses elicited by the focal adhesion kinase. Biochimica et biophysica acta 2001 Jul 25;1540(1):1-21
4. Hildebrand JD, Schaller MD, Parsons JT
   Paxillin, a tyrosine phosphorylated focal adhesion-associated protein binds to the carboxyl terminal domain of focal adhesion kinase. Molecular biology of the cell 1995 Jun;6(6):637-47
5. Shen Y, Schaller MD
   Focal adhesion targeting: the critical determinant of FAK regulation and substrate phosphorylation. Molecular biology of the cell 1999 Aug;10(8):2507-18
6. Brakebusch C, Fassler R
   The integrin-actin connection, an eternal love affair. The EMBO journal 2003 May 15;22(10):2324-33
7. Bellanger JM, Astier C, Sardet C, Ohta Y, Stossel TP, Debant A
   The Rac1- and RhoG-specific GEF domain of Trio targets filamin to remodel cytoskeletal actin. Nature cell biology 2000 Dec;2(12):888-92
8. Vadlamudi RK, Li F, Adam L, Nguyen D, Ohta Y, Stossel TP, Kumar R
   Filamin is essential in actin cytoskeletal assembly mediated by p21-activated kinase 1. Nature cell biology 2002 Sep;4(9):681-90
9. dos Remedios CG, Chhabra D, Kekic M, Dedova IV, Tsubakihara M, Berry DA, Nosworthy NJ
   Actin binding proteins: regulation of cytoskeletal microfilaments. Physiological reviews 2003 Apr;83(2):433-73
10. Vadlamudi RK, Li F, Barnes CJ, Bagheri-Yarmand R, Kumar R
    p41-Arc subunit of human Arp2/3 complex is a p21-activated kinase-1-interacting substrate. EMBO reports 2004 Feb;5(2):154-60
11. Di Paolo G, Pellegrini L, Letinic K, Cestra G, Zoncu R, Voronov S, Chang S, Guo J, Wenk MR, De Camilli P
    Recruitment and regulation of phosphatidylinositol phosphate kinase type 1 gamma by the FERM domain of talin. Nature 2002 Nov 7;420(6911):85-9
12. DeMali KA, Barlow CA, Burridge K
    Recruitment of the Arp2/3 complex to vinculin: coupling membrane protrusion to matrix adhesion. The Journal of cell biology 2002 Dec 9;159(5):881-91
13. Greenwood JA, Theibert AB, Prestwich GD, Murphy-Ullrich JE
    Restructuring of focal adhesion plaques by PI 3-kinase. Regulation by PtdIns (3,4,5)-p(3) binding to alpha-actinin. The Journal of cell biology 2000 Aug 7;150(3):627-42
14. Wachsstock DH, Wilkins JA, Lin S
    Specific interaction of vinculin with alpha-actinin. Biochemical and biophysical research communications 1987 Jul 31;146(2):554-60
15. Novak A, Hsu SC, Leung-Hagesteijn C, Radeva G, Papkoff J, Montesano R, Roskelley C, Grosschedl R, Dedhar S
    Cell adhesion and the integrin-linked kinase regulate the LEF-1 and beta-catenin signaling pathways. Proceedings of the National Academy of Sciences of the United States of America 1998 Apr 14;95(8):4374-9
16. Wu C
    Integrin-linked kinase and PINCH: partners in regulation of cell-extracellular matrix interaction and signal transduction. Journal of cell science 1999 Dec;112 ( Pt 24):4485-9
17. Tu Y, Li F, Goicoechea S, Wu C
    The LIM-only protein PINCH directly interacts with integrin-linked kinase and is recruited to integrin-rich sites in spreading cells. Molecular and cellular biology 1999 Mar;19(3):2425-34
18. Velyvis A, Vaynberg J, Yang Y, Vinogradova O, Zhang Y, Wu C, Qin J
    Structural and functional insights into PINCH LIM4 domain-mediated integrin signaling. Nature structural biology 2003 Jul;10(7):558-64
19. Aspenstrom P
    The WASP-binding protein WIRE has a role in the regulation of the actin filament system downstream of the platelet-derived growth factor receptor. Experimental cell research 2002 Sep 10;279(1):21-33
20. Illario M, Amideo V, Casamassima A, Andreucci M, di Matola T, Miele C, Rossi G, Fenzi G, Vitale M
    Integrin-dependent cell growth and survival are mediated by different signals in thyroid cells. The Journal of clinical endocrinology and metabolism 2003 Jan;88(1):260-9
21. Nikolopoulos SN, Turner CE
    Integrin-linked kinase (ILK) binding to paxillin LD1 motif regulates ILK localization to focal adhesions. The Journal of biological chemistry 2001 Jun 29;276(26):23499-505