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Integrin Signaling Revisited 466 Review TRENDS in Cell Biology Vol.11 No.12 December 2001 Integrin signaling revisited Martin A. Schwartz Adhesion to the extracellular matrix (ECM) is a crucial regulator of cell function, appear to be downstream of FAK and to contribute to and it is now well established that signaling by integrins mediates many of cell migration12,13. The adaptor protein p130cas also these effects. Ten years of research has seen integrin signaling advance on binds to FAK and has been linked to activation of the many fronts towards a molecular understanding of the control mechanisms. small GTPase Rac to promote motility. This diversity of Most striking is the merger with studies of other receptors, the cytoskeleton downstream pathways that converge on cell migration and mechanical forces within the general field of signaling networks. suggests that FAK is a central coordinator of this process. FAK has also been implicated in cell-cycle When I wrote a Trends in Cell Biology review on regulation through activation of both the Erk and integrin signaling in 19921, a 3000-word article could JNK pathways. And FAK plays an important role in cover the entire subject without omitting any key mediating integrin-dependent cell survival, possibly references. In the year 2000 alone, a literature search through phosphoinositide (PI) 3-kinase or JNK7,8,14,15. on ‘integrin’ plus ‘signal transduction’ yielded 480 Multiple physical associations of FAK with references. Given the impossibility of covering more other signaling molecules appear to mediate these than a sliver of what’s been written, I have chosen to multiple effector pathways. Proteins that bind to revisit the topics discussed in 1992, several of which autophosphorylated FAK through their Src-homology seem to have developed in interesting ways. 2 (SH2) domains include c-Src family kinases, GRB7 A good deal of this expansion has been lateral. Even (Ref. 16), phospholipase Cγ1 (Ref. 17) and PI 3-kinase. beyond my fairly wild dreams, integrins and integrin p130cas and a GTPase-activating protein (GAP) for Rho signaling have found their way into nearly every named GRAF bind to proline-rich sequences in the C- biological process. Integrin signals evidently play terminus of FAK through their Src-homology 3 (SH3) important roles in transplantation, angiogenesis, viral domains. Paxillin, talin and STAT1 bind through and bacterial infections, immune recognition, sequences near the C-terminus. The Etk tyrosine development, atherogenesis and nearly every other kinase directly binds to sequences in the N-terminus, complex physiological or pathological process in and the PDGF receptor also associates (although vertebrate organisms2–4. Luckily, I feel no obligation to perhaps indirectly) with the N-terminal region. Many review this vast literature. However, a second kind of of the proteins phosphorylated downstream of FAK expansion still complicates the task. Integrin signaling might actually be substrates of associated Src-family has undergone a remarkable merger with other areas kinases. Indeed, it has been suggested that FAK is best of signaling, particularly those involving the regarded as an adaptor protein with kinase activity7. cytoskeleton and growth factor/cytokine receptors5,6. Clearly, FAK promotes assembly of signaling This trend in cell biology reflects the general paradigm complexes downstream of integrins that lead to a wide shift towards understanding signal transduction in range of events; however, the detailed mechanisms by terms of spatially organized complex networks. which particular partners induce specific downstream events are not well understood. An overview of what is Tyrosine kinases known is presented in Fig. 1. In 1992, focal adhesion kinase (FAK) had just been The Src tyrosine kinases, in addition to their role identified as a protein tyrosine kinase activated by as cofactors for FAK-dependent responses, can be integrin-mediated adhesion and localized to sites of activated independently of FAK and contribute to a adhesion. FAK has since emerged as a remarkably distinct set of responses. Src tyrosine kinases mediate complex and interesting molecule. The reader is also phosphorylation of the adaptor Shc, which provides a directed to earlier reviews that cover FAK biochemistry separate link to the Ras/Erk pathway (reviewed in in greater detail7,8. FAK is essential for multicellular Ref. 18). They also phosphorylate Jab-1, which is a life, as, in its absence, mice die early during component of the nuclear signalosome that regulates embryogenesis. Even cells isolated from FAK–/– the proteasome-dependent degradation of a number embryos show severe defects in cytoskeletal of nuclear proteins19; this pathway also contributes to organization and motility. These defects appear to be integrin regulation of c-Fos. due to a surprising number of molecular mechanisms. Other tyrosine kinases regulated by integrins Martin A. Schwartz –/– Dept of Vascular Biology, FAK cells show abnormal regulation of the small include syk, c-Abl and receptor tyrosine kinases such 9 Scripps Research GTPase Rho , of the mitogen-activated protein kinase as the epidermal growth factor receptor (EGFR) and Institute, 10550 North (MAPK) Erk10 and abnormal signaling through the c-Met. Activation of syk is a very early event following Torrey Pines Road, platelet-derived growth factor (PDGF) receptor11. stimulation of the integrin αIIbβ3 in platelets or β2 La Jolla, CA 92037, USA. e-mail: Additional mechanisms involve the Etk tyrosine kinase integrins on leukocytes (reviewed in Ref. 20). Unlike [email protected] and the transcription factor Stat1, both of which FAK, syk activation does not depend on the actin http://tcb.trends.com 0962-8924/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S0962-8924(01)02152-3 Review TRENDS in Cell Biology Vol.11 No.12 December 2001 467 Fig. 1. Focal adhesion stimulation do not show a tight correlation between kinase (FAK). FAK has FERM Kinase FAT their GTP loading and PIP 5-kinase activity29,30. This multiple binding partners, Y397 discrepancy might be resolved by recent work many of which appear to PR PR Y925 function as downstream showing that integrins regulate GTPase function at a PDGFR Etk effectors. A large number PI 3-K Src Grb7 Talin second step – that of membrane translocation. Rac in of these proteins, CAS GRAF Grb2 Paxillin non-adherent cells could undergo GTP loading but including the platelet- Stat1 derived growth factor failed to interact with effectors because it failed to receptor (PDGFR), Etk, Rac Rho Ras associate with membranes30. Rho and Cdc42 phosphoinositide (PI) evidently behave similarly (M. del Pozo and 3-kinase, Grb7, p130cas (CAS), GRAF, Stat1 and M. Schwartz, unpublished). This effect appears to Survival Migration Growth Erk appear to contribute account for nearly complete shut-off of Rac pathways to cell migration. TRENDS in Cell Biology in non-adherent cells. As membrane translocation Additionally, FAK can should be crucial for PIP 5-kinase to interact with its contribute to cell growth and survival, most likely substrate, this effect might account for a significant through pathways that cytoskeleton. The Syk kinase contributes to integrin- portion of the regulation of PIP2 synthesis by regulate Erk, JNK and mediated gene expression in monocytes and to the integrins, which could involve both Rho and Rac. As PI 3-kinase. FAK domains include the N-terminal spreading of platelets. This last effect is most likely PIP2 is also a key regulator of the actin cytoskeleton, a FERM (band 4.1, ezrin, due to phosphorylation by syk of the Rac nucleotide substrate for phosphoinositide 3-kinase and a binding radixin and moesin) exchange factor Vav1 (Ref. 21). c-Abl contributes to the site for some pleckstrin-homology domains31, domain and a FAT (focal integrin activation of Erk (Ref. 22) and serves to inhibit integrin-dependent changes in PIP levels or adhesion targeting) 2 sequence. cell migration through an inhibitory phosphorylation synthesis could have pleiotropic effects on cell of the SH2/SH3 domain adaptor protein c-Crk, leading signaling and membrane–protein interactions. to decreased assembly of a complex between Crk and p130cas and decreased activation of Rac (Ref. 23). Proximal signals Integrins also induce transactivation of c-met and The proximal mechanisms by which integrins signal EGFR tyrosine kinase growth factor receptors in the constitute crucially important unknowns to which we absence of their growth factor ligands24,25. This process have little insight. Lacking enzymatic activity, appears to be crucially dependent on the level of integrins must associate with other proteins, essentially growth factor receptor, being apparent only at high adaptors, to trigger signals. The major area of levels of expression. Transactivation of these receptors progress has been the identification of such adaptors. contributes to integrin activation of the Ras–Erk Integrin cytoplasmic domains bind directly to pathway24 and to tumorigenesis25. several cytoskeletal proteins that might associate with signaling molecules (reviewed in Ref. 32). For Phosphoinositides example, the β1A, β3 and β1D cytoplasmic domains The 1992 review described work from my laboratory bind to talin; the β1A tail binds to α-actinin; β1A, β2 that revealed how integrins could modulate the and β7 tails bind to filamin; and the α4 tail binds to ability of growth factors to stimulate paxillin. Filamin and paxillin are excellent phosphoinositide turnover. We found that
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