The role of focal adhesion kinase binding in the regulation of tyrosine phosphorylation of paxillin

Mechanisms of CAS substrate domain tyrosine phosphorylation by FAK and Src

Tyrosine phosphorylation of CAS (Crk-associated substrate, p130(Cas)) has been implicated as a key signaling step in integrin control of normal cellular behaviors, including motility, proliferation, and survival. Aberrant CAS tyrosine phosphorylation may contribute to cell transformation by certain oncoproteins, including v-Crk and v-Src, and to tumor growth and metastasis. The CAS substrate domain (SD) contains 15 Tyr-X-X-Pro motifs, which are thought to represent the major tyrosine phosphorylation sites and to function by recruiting downstream signaling effectors, including c-Crk and Nck. CAS makes multiple interactions, direct and indirect, with the tyrosine kinases Src and focal adhesion kinase (FAK), and as a result of this complexity, several plausible models have been proposed for the mechanism of CAS-SD phosphorylation. The objective of this study was to provide experimental tests of these models in order to determine the most likely mechanism(s) of CAS-SD tyrosine phosphorylation by FAK and Src. In vitro kinase assays indicated that FAK has a very poor capacity to phosphorylate CAS-SD, relative to Src. However, FAK expression along with Src was found to be important for achieving high levels of CAS tyrosine phosphorylation in COS-7 cells, as well as recovery of CAS-associated Src activity toward the SD. Structure-functional studies for both FAK and CAS further indicated that FAK plays a major role in regulating CAS-SD phosphorylation by acting as a docking or scaffolding protein to recruit Src to phosphorylate CAS, while a secondary FAK-independent mechanism involves Src directly bound to the CAS Src-binding domain (SBD). Our results do not support models in which FAK either phosphorylates CAS-SD directly or phosphorylates CAS-SBD to promote Src binding to this site.

Paxillin: a focal adhesion-associated adaptor protein

Paxillin is a focal adhesion-associated, phosphotyrosine-containing protein that may play a role in several signaling pathways. Paxillin contains a number of motifs that mediate protein-protein interactions, including LD motifs, LIM domains, an SH3 domain-binding site and SH2 domain-binding sites. These motifs serve as docking sites for cytoskeletal proteins, tyrosine kinases, serine/threonine kinases, GTPase activating proteins and other adaptor proteins that recruit additional enzymes into complex with paxillin. Thus paxillin itself serves as a docking protein to recruit signaling molecules to a specific cellular compartment, the focal adhesions, and/or to recruit specific combinations of signaling molecules into a complex to coordinate downstream signaling. The biological function of paxillin coordinated signaling is likely to regulate cell spreading and motility.

Selected contribution: roles of focal adhesion kinase and paxillin in the mechanosensitive regulation of myosin phosphorylation in smooth muscle

The increase in intracellular Ca(2+) and myosin light chain (MLC) phosphorylation in response to the contractile activation of tracheal smooth muscle is greater at longer muscle lengths (21). However, MLC phosphorylation can also be stimulated by Ca(2+)-insensitive signaling pathways (19). The cytoskeletal proteins paxillin and focal adhesion kinase (FAK) mediate a Ca(2+)-independent length-sensitive signaling pathway in tracheal smooth muscle (30). We used alpha-toxin-permeabilized tracheal smooth muscle strips to determine whether the length sensitivity of MLC phosphorylation can be regulated by a Ca(2+)-insensitive signaling pathway and whether the length sensitivity of active tension depends on the length sensitivity of myosin activation. Although active tension remained length sensitive, ACh-induced MLC phosphorylation was the same at optimal muscle length (L(o)) and 0.5 L(o) when intracellular Ca(2+) was maintained at pCa 7. MLC phosphorylation was also the same at L(o) and 0.5 L(o) in strips stimulated with 10 microM Ca(2+). In contrast, the Ca(2+)-insensitive tyrosine phosphorylation of FAK and paxillin stimulated by ACh was higher at L(o) than at 0.5 L(o). We conclude that the length-sensitivity of MLC phosphorylation depends on length-dependent changes in intracellular Ca(2+) but that length-dependent changes in MLC phosphorylation are not the primary mechanism for the length sensitivity of active tension.

Hic-5-reduced cell spreading on fibronectin: competitive effects between paxillin and Hic-5 through interaction with focal adhesion kinase

Hic-5 is a paxillin homologue that is localized to focal adhesion complexes. Hic-5 and paxillin share structural homology and interacting factors such as focal adhesion kinase (FAK), Pyk2/CAKbeta/RAFTK, and PTP-PEST. Here, we showed that Hic-5 inhibits integrin-mediated cell spreading on fibronectin in a competitive manner with paxillin in NIH 3T3 cells. The overexpression of Hic-5 sequestered FAK from paxillin, reduced tyrosine phosphorylation of paxillin and FAK, and prevented paxillin-Crk complex formation. In addition, Hic-5-mediated inhibition of spreading was not observed in mouse embryo fibroblasts (MEFs) derived from FAK(-/-) mice. The activity of c-Src following fibronectin stimulation was decreased by about 30% in Hic-5-expressing cells, and the effect of Hic-5 was restored by the overexpression of FAK and the constitutively active forms of Rho-family GTPases, Rac1 V12 and Cdc42 V12, but not RhoA V14. These observations suggested that Hic-5 inhibits cell spreading through competition with paxillin for FAK and subsequent prevention of downstream signal transduction. Moreover, expression of antisense Hic-5 increased spreading in primary MEFs. These results suggested that the counterbalance of paxillin and Hic-5 expression may be a novel mechanism regulating integrin-mediated signal transduction.

Focal adhesion kinase signaling is decreased in polyamine-depleted IEC-6 cells

Polyamines are essential to the migration of epithelial cells in the intestinal mucosa. Cells depleted of polyamines do not attach as rapidly to the extracellular matrix and do not form the actin stress fibers essential for migration. Because both attachment and stress fiber formation depend on integrin signaling and the formation of focal adhesions, we examined these and related processes in polyamine-depleted IEC-6 cells. There was general decreased tyrosine phosphorylation of focal adhesion kinase (FAK), and, specifically, decreased phosphorylation of Tyr-925, the paxillin binding site. In control cells, FAK phosphorylation was rapid after attachment to the extracellular matrix, while attached cells depleted of polyamines had significantly delayed phosphorylation. FAK activity was also significantly inhibited in polyamine-depleted cells as was the phosphorylation of paxillin. Polyamine-depleted cells failed to spread normally after attachment, and immunocytochemistry showed little colocalization of FAK and actin compared with controls. Focal adhesion complex formation was greatly reduced in the absence of polyamines. These data suggest that defective integrin signaling may, at least in part, account for the decreased rates of attachment, actin stress fiber formation, spreading, and migration observed in polyamine-depleted cells.

Biochemical signals and biological responses elicited by the focal adhesion kinase

The focal adhesion kinase, FAK, is an important component of an integrin-dependent signaling pathway, which functions to transmit signals from the extracellular matrix into the cytoplasm. FAK is an essential gene product, since the fak-/- mouse exhibits embryonic lethality. A number of important biological processes, including cell motility and cell survival, are controlled by integrin-dependent signals and FAK has been implicated in regulating these processes. This review will focus upon recent findings providing insight into the mechanisms by which FAK transmits biochemical signals and elicits biological effects.

Differential phosphorylation of paxillin in response to surface-bound serum proteins during early osteoblast adhesion

An early signaling event during the adhesion and spreading of cells is integrin-mediated tyrosine phosphorylation of the cytoskeletal adaptor protein paxillin and the non-receptor tyrosine kinase pp125(FAK) at focal contacts. To determine the influence of surface-charge and -adsorbed adhesion proteins on this signaling pathway, paxillin phosphorylation was examined during attachment of MC3T3-E1 osteoblast-like cell onto charged and uncharged polystyrene, and on adsorbed layers of serum proteins, fibronectin (Fn), vitronectin (Vn), a mixture of Fn and Vn, and albumin. Paxillin phosphorylation was induced 2.4-fold (P < 0.05) on charged vs uncharged polystyrene only in the presence of serum proteins. Activation of paxillin via Fn or Vn alone, or in combination, resulted in significantly lower phosphorylation signals compared to whole serum (41 +/- 6.9%, P < 0.05, 45 +/- 5.9%, P < 0.05, and 76 +/- 9.8%, P < 0.075, respectively). Confocal laser microscopy confirmed increased co-localization of phosphotyrosine and paxillin at protruding lamellopodia of spreading osteoblasts on charged vs uncharged serum-pretreated polystyrene. Taken together, these data suggest that subtle differences in surface characteristics mediate effects on adhering cells via adsorbed serum proteins involving the cytoskeletal adaptor protein paxillin.

Inhibition of the catalytic activity of cell adhesion kinase beta by protein-tyrosine phosphatase-PEST-mediated dephosphorylation

Protein-tyrosine phosphatase (PTP)-PEST is a cytoplasmic tyrosine phosphatase that can bind and dephosphorylate the focal adhesion-associated proteins p130(CAS) and paxillin. Focal adhesion kinase (FAK) and cell adhesion kinase beta (CAKbeta)/PYK2/CADTK/RAFTK are protein-tyrosine kinases that can colocalize with, bind to, and induce tyrosine phosphorylation of p130(CAS) and paxillin. Thus, we considered the possibility that these kinases might be substrates for PTP-PEST. Using a combination of substrate-trapping assays and overexpression of PTP-PEST in mammalian cells, CAKbeta was found to be a substrate for PTP-PEST. Both the major autophosphorylation site of CAKbeta (Tyr(402)) and activation loop tyrosine residues, Tyr(579) and Tyr(580), were targeted for dephosphorylation by PTP-PEST. Dephosphorylation of CAKbeta by PTP-PEST dramatically inhibited CAKbeta kinase activity. In contrast, FAK was a poor substrate for PTP-PEST, and treatment with PTP-PEST had no effect on FAK kinase activity. Tyrosine phosphorylation of paxillin, which is greatly enhanced by CAKbeta overexpression, was dramatically reduced upon coexpression of PTP-PEST. Finally, endogenous PTP-PEST and endogenous CAKbeta were found to localize to similar cellular compartments in epithelial and smooth muscle cells. These results suggest that CAKbeta is a substrate of PTP-PEST and that FAK is a poor PTP-PEST substrate. Further, PTP-PEST can negatively regulate CAKbeta signaling by inhibiting the catalytic activity of the kinase.

Y-27632, an inhibitor of Rho-associated kinases, prevents tyrosine phosphorylation of focal adhesion kinase and paxillin induced by bombesin: dissociation from tyrosine phosphorylation of p130(CAS)

A rapid increase in tyrosine phosphorylation of focal adhesion kinase (FAK), paxillin, and Crk-associated substrate (CAS) are prominent early events triggered by many G protein-coupled receptors (GPCRs), but the mechanisms involved remain unclear. Here, we examined whether the Rho-associated protein serine/threonine kinase family (ROCK) is a critical Rho effector in the pathway that links GPCR activation to the tyrosine phosphorylation of FAK, CAS, and paxillin. Treatment of Swiss 3T3 cells with Y-27632, a preferential inhibitor of ROCK, dramatically inhibited the formation of actin stress fibers, the assembly of focal contacts, and the increase in tyrosine phosphorylation of FAK and paxillin induced by bombesin in these cells. Surprisingly, we found that treatment with Y-27632 did not produce any detectable effect on bombesin-elicited CAS tyrosine phosphorylation even at the highest concentrations of Y-27632 tested. HA-1077, a preferential inhibitor of ROCK activity structurally unrelated to Y-27632, also attenuated the increase in the tyrosine phosphorylation of FAK and paxillin but did not affect the tyrosine phosphorylation of CAS induced by bombesin in Swiss 3T3 cells. The results demonstrate that ROCK-dependent tyrosine phosphorylation of FAK and paxillin can be dissociated from a ROCK-independent pathway leading to tyrosine phosphorylation of CAS.

Focal adhesion kinase: a regulator of focal adhesion dynamics and cell movement

Engagement of integrin receptors with extracellular ligands gives rise to the formation of complex multiprotein structures that link the ECM to the cytoplasmic actin cytoskeleton. These adhesive complexes are dynamic, often heterogeneous structures, varying in size and organization. In motile cells, sites of adhesion within filopodia and lamellipodia are relatively small and transient and are referred to as 'focal complexes,' whereas adhesions underlying the body of the cell and localized to the ends of actin stress fibers are referred to as 'focal adhesions'. Signal transduction through focal complexes and focal adhesions has been implicated in the regulation of a number of key cellular processes, including growth factor induced mitogenic signals, cell survival and cell locomotion. The formation and remodeling of focal contacts is a dynamic process under the regulation of protein tyrosine kinases and small GTPases of the Rho family. In this review, we consider the role of the focal complex associated protein tyrosine kinase, Focal Adhesion Kinase (FAK), in the regulation of cell movement with the emphasis on how FAK regulates the flow of signals from the ECM to the actin cytoskeleton.

Survey of the 1999 surface plasmon resonance biosensor literature

The application of surface plasmon resonance biosensors in life sciences and pharmaceutical research continues to increase. This review provides a comprehensive list of the commercial 1999 SPR biosensor literature and highlights emerging applications that are of general interest to users of the technology. Given the variability in the quality of published biosensor data, we present some general guidelines to help increase confidence in the results reported from biosensor analyses.

Differences in hyaluronic acid-mediated functions and signaling in arterial, microvessel, and vein-derived human endothelial cells

Hyaluronic acid (HA), a nonsulfated glycosaminoglycan, regulates cell adhesion and migration. Small HA fragments (3-25 disaccharide units) induce neovascularization. We investigated the effect of HA and a HA fragment (10-15 disaccharide units, F1) on primary human endothelial cells (ECs). Human pulmonary ECs (HPAEC) and lung microvessel ECs (HMVEC-L) bound HA (K(d) approximately 1 and 2.3 nm, respectively) and expressed 17,780 and 16,690 HA binding sites, respectively. Both ECs showed HA-mediated cell adhesion; however, HMVEC-L was 1.5-fold better. Human umbilical vein ECs neither bound HA nor showed HA-mediated adhesion. All three ECs expressed CD44 ( approximately 110 kDa). The expression of receptor for HA-mediated motility (RHAMM) (approximately 80 kDa) was the highest in HMVEC-L, followed by HPAEC and human umbilical vein ECs. RHAMM, not CD44, bound HA in all three ECs. F1 was better than HA and stimulated a 2. 5- and 1.8-fold mitogenic response in HMVEC-L and HPAEC, respectively. Both HA and F1 induced tyrosine phosphorylation of p125(FAK), paxillin, and p42/44 ERK in HMVEC-L and HPAEC, which was blocked by an anti-RHAMM antibody. These results demonstrate that RHAMM is the functional HA receptor in primary human ECs. Heterogeneity exists among primary human ECs of different vascular origins, with respect to functional HA receptor expression and function.

Paxillin binding is not the sole determinant of focal adhesion localization or dominant-negative activity of focal adhesion kinase/focal adhesion kinase-related nonkinase

The carboxy-terminal 150 residues of the focal adhesion kinase (FAK) comprise the focal adhesion-targeting sequence, which is responsible for its subcellular localization. The mechanism of focal adhesion targeting has not been fully elucidated. We describe a mutational analysis of the focal adhesion-targeting sequence of FAK to further examine the mechanism of focal adhesion targeting and explore additional functions encoded by the carboxy-terminus of FAK. The results demonstrate that paxillin binding is dispensable for focal adhesion targeting of FAK. Cell adhesion-dependent tyrosine phosphorylation strictly correlated with the ability of mutants to target to focal adhesions. Focal adhesion targeting was also a requirement for maximal FAK-dependent tyrosine phosphorylation of paxillin and FAK-related nonkinase (FRNK)-dependent inhibition of endogenous FAK function. However, there were additional requirements for these latter functions because we identified mutants that target to focal adhesions, yet are defective for the induction of paxillin phosphorylation or the dominant-negative function of FRNK. Furthermore, the paxillin-binding activity of FRNK mutants did not correlate with their ability to inhibit FAK, suggesting that FRNK has other targets in addition to paxillin.