Cytotoxic T lymphocytes express a beta3 integrin which can induce the phosphorylation of focal adhesion kinase and the related PYK-2

Inhibition of Integrin αvβ3-FAK-MAPK signaling constrains the invasion of T-ALL cells

The role of adhesion receptor integrin αvβ3 in T-ALL was unclear. Firstly, we performed quantitative real-time PCR to assess medullary expression of integrin β3(ITGB3) in T-ALL patients and high ITGB3 expression was relevant with the central nervous system leukemia(CNSL) incidence. Decreasing of cell invasion was observed in Jurkat and Molt4 treated with integrin αvβ3 specific antibody and inhibitor as well as cells with ITGB3 interference. Further, phosphorylation of FAK, cRAF, MEK and ERK decreased in cells with integrin αvβ3 inhibition or interference. Invasion decreased in T-ALL cells treated with FAK and ERK inhibitors. In conclusion, inhibition of integrin αvβ3 signals significantly limits the cell invasion of T-ALL cells.

Pyk2 Controls Integrin-Dependent CTL Migration through Regulation of De-Adhesion

Protein tyrosine kinase 2 (Pyk2) is required for T cell adhesion to ICAM-1; however, the mechanism by which it regulates adhesion remains unexplored. Pyk2 function in murine CTL clones and activated ex vivo CD8(+) T cells was disrupted by pharmacological inhibition, knockdown of expression with small interfering RNA, or expression of the dominant-negative C-terminal domain. We found that Pyk2 is not absolutely required for adhesion of CTL to ICAM-1, but rather delays the initial adhesion. Disruption of Pyk2 function caused cells to display an unusual elongated appearance after 1 h on ICAM-1, consistent with abnormally strong adhesion. Furthermore, the random mobility of CTL on ICAM-1 was severely compromised using all three methods of disrupting Pyk2 function. Live-cell imaging studies revealed that the decreased migration is the result of a defect in the detachment from ICAM-1 at the trailing edge when Pyk2 function is inhibited. Examination of Pyk2 tyrosine phosphorylation in normal polarized cells demonstrated that Pyk2 phosphorylated at Y579 and Y580 preferentially localizes to the leading edge, whereas Y881-phosphorylated Pyk2 is enriched at the trailing edge, suggesting that the tyrosine phosphorylation of Pyk2 is spatially regulated in migrating CTL. Additionally, inhibition of Pyk2 caused cells to form multiple LFA-1-rich tails at the trailing edge, most likely resulting from a defect in LFA-1 release required for forward movement. Our results show that Pyk2 contributes to CTL migration by regulating detachment of CTL at the trailing edge, which could explain why Pyk2 is important for chemotactic and migratory responses.

Inflammatory Th17 Cells Express Integrin αvβ3 for Pathogenic Function

Interleukin-23 (IL-23) is required for inflammatory Th17 cell function in experimental autoimmune encephalomyelitis (EAE), and IL-23 blockade reduces the number of effector Th17 cells in the CNS. We report that pro-inflammatory Th17 cells express high integrin β3 that is IL-23 dependent. Integrin β3 was not upregulated on all activated T cells; rather, integrin β3 was upregulated along with its functional partner integrin αv on effector Th17 cells and "ex-Th17" cells, and αvβ3(hi) RORγt(+) cells expanded during EAE. Integrin αvβ3 inhibitors ameliorated clinical signs of EAE, and integrin β3 deficiency on CD4(+) T cells alone was sufficient to block EAE induction. Furthermore, integrin-β3-deficient Th17 cells, but not Th1 cells, were impaired in their ability to induce EAE. Integrin β3(-/-) T cells induced smaller demyelinated lesions and showed reduced spread and accumulation within the CNS, corresponding with impaired extracellular-matrix-mediated migration. Hence, integrin β3 is required for Th17 cell-mediated autoimmune CNS inflammation.

Functions of the FAK family kinases in T cells: beyond actin cytoskeletal rearrangement

T cells control the focus and extent of adaptive immunity in infectious and pathological diseases. The activation of T cells occurs when the T cell antigen receptor (TCR) and costimulatory and/or adhesion receptors are engaged by their ligands. This process drives signaling that promotes cytoskeletal rearrangement and transcription factor activation, both of which regulate the quality and magnitude of the T cell response. However, it is not fully understood how different receptor-induced signals combine to alter T cell activation. The related non-receptor tyrosine kinases focal adhesion kinase (FAK) and proline-rich tyrosine kinase 2 (Pyk2) are phosphorylated downstream of the TCR and several costimulatory and adhesion receptors. FAK family proteins integrate receptor-mediated signals that influence actin cytoskeletal rearrangement and effector T cell responses. In this review, we summarize the receptor-specific roles that FAK and Pyk2 control to influence T cell development and activation.

The role of focal adhesion kinase in the regulation of cellular mechanical properties

The regulation of mechanical properties is necessary for cell invasion into connective tissue or intra- and extravasation through the endothelium of blood or lymph vessels. Cell invasion is important for the regulation of many healthy processes such as immune response reactions and wound healing. In addition, cell invasion plays a role in disease-related processes such as tumor metastasis and autoimmune responses. Until now the role of focal adhesion kinase (FAK) in regulating mechanical properties of cells and its impact on cell invasion efficiency is still not well known. Thus, this review focuses on mechanical properties regulated by FAK in comparison to the mechano-regulating protein vinculin. Moreover, it points out the connection between cancer cell invasion and metastasis and FAK by showing that FAK regulates cellular mechanical properties required for cellular motility. Furthermore, it sheds light on the indirect interaction of FAK with vinculin by binding to paxillin, which then impairs the binding of paxillin to vinculin. In addition, this review emphasizes whether FAK fulfills regulatory functions similar to vinculin. In particular, it discusses the differences and the similarities between FAK and vinculin in regulating the biomechanical properties of cells. Finally, this paper highlights that both focal adhesion proteins, vinculin and FAK, synergize their functions to regulate the mechanical properties of cells such as stiffness and contractile forces. Subsequently, these mechanical properties determine cellular invasiveness into tissues and provide a source sink for future drug developments to inhibit excessive cell invasion and hence, metastases formation.

Paxillin associates with the microtubule cytoskeleton and the immunological synapse of CTL through its leucine-aspartic acid domains and contributes to microtubule organizing center reorientation

The cytoskeletal adaptor protein paxillin localizes to the microtubule organizing center (MTOC) in T cells and, upon target cell binding, is recruited to the supramolecular activation complex (SMAC). We mapped the region of paxillin that associates with both the MTOC and SMAC to the leucine-aspartic acid (LD) domains and showed that a protein segment containing LD2-4 was sufficient for MTOC and SMAC recruitment. Examination of the localization of paxillin at the SMAC revealed that paxillin localizes to the peripheral area of the SMAC along with LFA-1, suggesting that LFA-1 may contribute to its recruitment. LFA-1 or CD3 engagement alone was insufficient for paxillin recruitment because there was no paxillin accumulation at the site of CTL contact with anti-LFA-1- or anti-CD3-coated beads. In contrast, paxillin accumulation was detected when beads coated with both anti-CD3 and anti-LFA-1 were bound to CTL, suggesting that signals from both the TCR and LFA-1 are required for paxillin accumulation. Paxillin was shown to be phosphorylated downstream of ERK, but when we generated a mutation (S83A/S130A) that abolished the mobility shift as a result of phosphorylation, we found that paxillin still bound to the MTOC and was recruited to the SMAC. Furthermore, ERK was not absolutely required for MTOC reorientation in CTL that require ERK for killing. Finally, expression of the LD2-4 region of paxillin substantially reduced MTOC reorientation. These studies demonstrated that paxillin is recruited, through its LD domains, to sites of integrin engagement and may contribute to MTOC reorientation required for directional degranulation.

The actin-bundling protein L-plastin is essential for marginal zone B cell development

B cell development is exquisitely sensitive to location within specialized niches in the bone marrow and spleen. Location within these niches is carefully orchestrated through chemotactic and adhesive cues. In this article, we demonstrate the requirement for the actin-bundling protein L-plastin (LPL) in B cell motility toward the chemokines CXCL12 and CXCL13 and the lipid chemoattractant sphingosine-1-phosphate, which guide normal B cell development. Impaired motility of B cells in LPL(-/-) mice correlated with diminished splenic maturation of B cells, with a moderate (40%) loss of follicular B cells and a profound (>80%) loss of marginal zone B cells. Entry of LPL(-/-) B cells into the lymph nodes and bone marrow of mice was also impaired. Furthermore, LPL was required for the integrin-mediated enhancement of Transwell migration but was dispensable for integrin-mediated lymphocyte adhesion. These results suggest that LPL may participate in signaling that enables lymphocyte transmigration. In support of this hypothesis, the phosphorylation of Pyk-2, a tyrosine kinase that integrates chemotactic and adhesive cues, is diminished in LPL(-/-) B cells stimulated with chemokine. Finally, a well-characterized role of marginal zone B cells is the generation of a rapid humoral response to polysaccharide Ags. LPL(-/-) mice exhibited a defective Ab response to Streptococcus pneumoniae, indicating a functional consequence of defective marginal zone B cell development in LPL(-/-) mice.

Compensatory function of Pyk2 protein in the promotion of focal adhesion kinase (FAK)-null mammary cancer stem cell tumorigenicity and metastatic activity

Mammary cancer stem cells (MaCSCs) have been identified as a rare population of cells capable of self-renewal to drive mammary tumorigenesis and metastasis. Nevertheless, relatively little is known about the intracellular signaling pathways regulating self-renewal and metastatic activities of MaCSCs in vivo. Using a recently developed breast cancer mouse model with focal adhesion kinase (FAK) deletion in mammary tumor cells (MFCKO-MT mice), here we present evidence suggesting a compensatory function of Pyk2, a FAK-related kinase, in the regulation of MaCSCs and metastasis in these mice. Increased expression of Pyk2 was found selectively in pulmonary metastatic nodules of MFCKO-MT mice, and its inhibition significantly reduced mammary tumor development and metastasis in these mice. Consistent with the idea of metastasis driven by MaCSCs, we detected selective up-regulation of Pyk2 in MaCSCs, but not bulk mammary tumor cells, of primary tumors developed in MFCKO-MT mice. We further showed that inhibition of Pyk2 in FAK-null MaCSCs significantly decreased their tumorsphere formation and migration in vitro as well as self-renewal, tumorigenicity, and metastatic activity in vivo. Last, we identified PI3K/Akt signaling as a major mediator of FAK regulation of MaCSCs as well as a target for the compensatory function of Pyk2 in FAK-null MaCSCs. Together, these results further advance our understanding of FAK and its related tyrosine kinase Pyk2 in regulation of MaCSCs in breast cancer and suggest that pharmaceutically targeting these kinases may hold promise as a novel treatment for the disease by targeting and eradicating MaCSCs.

Regulation of the tyrosine kinase Pyk2 by calcium is through production of reactive oxygen species in cytotoxic T lymphocytes

Pyk2 was identified as a Ca(2+)-dependent kinase, however, the regulation of Pyk2 by Ca(2+) in T cells remains controversial. We found that Ca(2+) mobilization preferentially induced Pyk2 phosphorylation in cytotoxic T lymphocytes (CTL). Furthermore, Pyk2 phosphorylation in CTL was not absolutely Ca(2+) dependent but relied on the strength of T cell receptor stimulation. Ionomycin-stimulated Pyk2 phosphorylation did not require calmodulin activity, because phosphorylation was not inhibited by the calmodulin inhibitor W7, and we detected no Ca(2+)-regulated association between Pyk2 and calmodulin. Ca(2+)-stimulated Pyk2 phosphorylation was dependent on Src-family kinase activity, even at the Pyk2 autophosphorylation site. We sought to identify a Ca(2+)-regulated pathway that could trigger Pyk2 phosphorylation in T cells and found that ionomycin stimulated the production of reactive oxygen species and an H(2)O(2) scavenger inhibited ionomycin-induced Pyk2 phosphorylation. Additionally, H(2)O(2) induced strong Erk activation and ionomycin-stimulated Pyk2 phosphorylation was Erk dependent. These data support the conclusion that Ca(2+) mobilization induces the production of reactive oxygen species, which in turn activate the Erk pathway, leading to Src-family kinase-dependent Pyk2 phosphorylation. Our data demonstrate that Pyk2 is not a Ca(2+)-dependent kinase in T cells but instead, increased intracellular Ca(2+) induces Pyk2 phosphorylation through production of reactive oxygen species. These findings are consistent with the possibility that Pyk2 acts as an early sensor of numerous extracellular signals that trigger a Ca(2+) flux and/or reactive oxygen species to amplify tyrosine phosphorylation signaling events.

Activation signal transduction by proline-rich tyrosine kinase 2 (PYK2) in peripheral blood mononuclear cells from patients with systemic lupus erythematosus

To explore the role of proline-rich tyrosine kinase 2 (PYK2) in systemic lupus erythematosus (SLE), we studied the expression, activation, and function of PYK2 in peripheral blood mononuclear cells (PBMC) from 36 SLE patients. As controls, samples from 19 patients with rheumatoid arthritis and 15 healthy individuals were studied simultaneously. We found a significant increase of both the total PYK2 protein and its activated/phosphorylated form in PBMCs from patients with SLE, particularly those with the complication of nephritis (WHO class IV). There is a clear correlation between the activation of PYK2 and the level of serum complements. In active SLE patients, activation of PYK2 in PBMCs accompanies the increased cell proliferation and the induced expression of co-stimulatory molecules CD40L and CTLA4. These results indicate that phosphorylated PYK2 may induce the expression of CD40L and CTLA4, and subsequently the cell proliferation. PYK2 signaling enhances the autoreactive lymphocyte activation and plays an important role in the pathogenesis of SLE.

Expression and activation of proline-rich tyrosine kinase 2 (PYK2) in peripheral blood mononuclear cells from patients with systemic lupus erythematosus

This study analysed the expression and activation of proline-rich tyrosine kinase 2 (PYK2) in peripheral blood mononuclear cells (PBMCs) from 36 systemic lupus erythematosus (SLE) patients and explored whether activation of PYK2 correlates with disease activity or organ damage in SLE. Samples from 19 patients with rheumatoid arthritis (RA) and 15 healthy individuals were included as controls. There was a significant increase in both total PYK2 protein and its activated/phosphorylated form in PBMCs from patients with SLE, particularly in those with the complication of World Health Organization class IV lupus nephritis. There was a clear correlation between the activation of PYK2 and the level of serum complement, but no relationship was found between the activation of PYK2 and SLE Disease Activity Index (SLEDAI). As previous studies have shown that PYK2 provides important signals during the activation of lymphocytes, PYK2 could be a major contributor to the complex autoimmune pathogenesis of SLE.

Functional activation of proline-rich tyrosine kinase2 (PYK2) in peripheral blood mononuclear cells from patients with systemic lupus erythematosus

Background

Systemic lupus erythematosus (SLE) is a representative systemic autoimmune disease characterized by activated T cells and polyclonally activated B cells that produce autoantibodies. Activation of autoreactive T and B cells plays a pivotal role in the pathogenesis of this disease. A role of focal adhesion kinase (FAK) in the pathogenesis has been suggested. Proline-rich tyrosine kinase2 (PYK2) is structurally related to FAK, however, the functional activation of PYK2 in SLE remains unclear. In the present study, we showed that PYK2 is significantly increased and activated in peripheral blood mononuclear cells (PBMCs) of patients with SLE. In addition, we showed the involvement of PYK2 proteins in the up-regulation of CD40L and CTLA4 expression and PBMC proliferation.

Methods

Freshly isolated PBMCs from 48 SLE patients, 32 patients with rheumatoid arthritis(RA) and 24 healthy individuals were analyzed for the expression and activation of PYK2 by western-blotting and immunocytochemistry. The other isolated PBMCs from patients with this condition were cultured and stimulated with PMA or TyrA9, and then the expression of costimulatory molecules CD40L and CTLA4 was evaluated using flow cytometry, PBMCs proliferation was determined with [³H]-thymidine incorporation (CPM).

Results

Compared with RA patients and healthy donors, PBMCs from SLE patients expressed more of both the total PYK2 protein and its activated/phosphorylated form. The increase of activated PYK2 protein in SLE PBMCs was correlated with the complication of nephritis and inversly associated the level of serum complements. In active SLE patients, activation of PYK2 in PBMCs is accompanying the increased cell proliferation and the induced expression of costimulatory molecules CD40L and CTLA4.

Conclusion

Our findings indicate that phosphorylated PYK2 in SLE PBMCs may induce the expression of CD40L and CTLA4, and subsequently the cell proliferation. PYK2 signaling enhances the autoreactive lymphocyte activation and plays an important role in the pathogenesis of SLE.

Role of beta3 integrin in acute renal allograft rejection in humans

BACKGROUND AND OBJECTIVES

Beta3 Integrin may play a role in the process of acute rejection by increasing leukocyte adhesion to the endothelium, cytotoxic T lymphocyte activation, and platelet aggregation.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

For investigation of the role of beta3 integrin in the pathogenesis of acute rejection, this study examined the surface expression of beta3 integrin on leukocyte subsets and analyzed a common single-nucleotide polymorphism in exon 2 of the gene encoding the beta3 subunit that generates two beta3 integrin isoforms, termed Pl(A1) and Pl(A2). Pl(A) genotype was determined in blood samples from 445 renal allograft recipients at two centers. Patients were then grouped by Pl(A) genotype, and clinical outcomes as recorded in a preexisting database were analyzed.

RESULTS

Although almost all monocytes express beta3 integrin, its expression was also found on all leukocyte subsets, including T, B, and NK cells. The percentage of patients who experienced acute rejection was noted to be significantly higher in those with Pl(A1)/Pl(A1) (TT) genotype versus patients with the Pl(A1)/Pl(A2) or Pl(A2)/Pl(A2) (CT or CC) genotypes (33% for TT versus 20% for CT or CC). In a multivariate analysis, the Pl(A1)/Pl(A1) (TT) genotype remained significantly associated with acute rejection. Patients with Pl(A1)/Pl(A1) (TT) genotype also exhibited a higher number of acute rejection episodes per patient.

CONCLUSIONS

The Pl(A1)/Pl(A1) (TT) genotype is associated with an increased incidence of acute renal allograft rejection in humans, supporting a role for beta3 integrin in the pathophysiology of acute rejection.

Focal adhesion kinase-related protein tyrosine kinase Pyk2 in T-cell activation and function

Pyk2 is a protein tyrosine kinase expressed primarily in brain and hematopoietic cells. It becomes activated in response to stimulation through numerous receptors, including integrins, chemokine receptors, and antigen receptors, and is found in association with src-family kinases. Although this enzyme associates with many proteins known to be important for activation and has many characteristics of a scaffolding protein, its function remains elusive. A number of studies in non-T-cells suggest that Pyk2 is important for cell spreading, cell migration, and integrin function; however, a defined role in T-cells has not been established. Here, we discuss evidence that implicates Pyk2 in directionality of signaling, which is essential to establishment of the directional killing mediated by cytotoxic lymphocytes.

The beta1 and beta3 integrins promote T cell receptor-mediated cytotoxic T lymphocyte activation

Recognition by CD8+ cytotoxic T lymphocytes (CTLs) of antigenic peptides bound to major histocompatibility class (MHC) I molecules on target cells leads to sustained calcium mobilization and CTL degranulation resulting in perforin-dependent killing. We report that beta1 and beta3 integrin-mediated adhesion to extracellular matrix proteins on target cells and/or surfaces dramatically promotes CTL degranulation. CTLs, when adhered to fibronectin but not CTL in suspension, efficiently degranulate upon exposure to soluble MHC.peptide complexes, even monomeric ones. This adhesion induces recruitment and activation of the focal adhesion kinase Pyk2, the cytoskeleton linker paxillin, and the Src kinases Lck and Fyn in the contact site. The T cell receptor, by association with Pyk2, becomes part of this adhesion-induced activation cluster, which greatly increases its signaling.

Integrin receptors are crucial for the restimulation of activated T lymphocytes

Stimulation via the T-cell receptor results in proliferation of naive T cells and activation-induced death of activated T cells. The expression of Fas ligand and activation-induced cell death are major mechanisms by which immune responses are modulated in the lung. Although it is known that the binding of integrin receptors to extracellular matrix proteins provides co-stimulatory signals to naive T cells, it is not clear whether these signals are critical for activated T cells. The activation and differentiation of T cells is marked by significant changes in integrin expression and affinity. To determine the role of integrin signaling in restimulation of activated T cells, we blocked integrin receptors with RGD peptides. Using murine activated CD4+ T cells and the T-cell hybridoma DO11.10, we found that RGD peptides inhibit tyrosine phosphorylation of CD3 epsilon-chain and ZAP-70, clustering of T-cell receptors, extracellular signal-regulated kinase mitogen-activated protein-kinase activation, and Fas ligand expression and prevent activation-induced cell death. We demonstrate that activated T cells are sensitive to integrin co-stimulation and that integrin receptors are required for the successful restimulation of activated T cells. This indicates that matrix proteins may play a major role in regulating T-cell-mediated immune responses in the lung.

Beta 1/beta 3 integrin ligation is uncoupled from ERK1/ERK2 activation in cytotoxic T lymphocytes

beta 3 integrins mediate fibronectin binding and enhanced activation of cytotoxic T lymphocytes (CTL). The intracellular signals initiated by beta 3 integrins in lymphocytes are not well characterized, but in many cell types, beta 1 integrin ligation activates mitogen-activated protein (MAP) kinases. In the present study, we find that fibronectin can synergize with very low levels of CD3 stimulation to activate the extracellular signal-regulated kinase (ERK)1 and ERK2 MAP kinases but that fibronectin alone induces no detectable MAP kinase activation in CTL. Surprisingly, antibodies to beta1 or beta 3 integrins were also unable to stimulate MAP kinase activation, suggesting that although beta 1 integrins are capable of stimulating MAP kinase activation in other cells, they cannot do so in CTL. In CTL, phosphorylation of proline-rich tyrosine kinase 2 downstream of integrin stimulation did not result in recruitment of the adaptor protein Grb2. Additionally, we examined the role of MAP kinases in regulating integrin-mediated adhesion. Anti-CD3-triggered adhesion to fibronectin was largely insensitive to the MAP kinase kinase inhibitor PD98059. Triggered cell-spreading on fibronectin was inhibited by PD98059 but not by U0126. In summary, ligation of beta 3 integrin by antibodies or fibronectin or of beta1 integrin by monoclonal antibodies fails to activate ERK MAP kinases, but integrin ligation synergizes with T cell receptor stimulation upstream of MAP kinases.

Cas, Fak and Pyk2 function in diverse signaling cascades to promote Yersinia uptake

The interplay between pathogen-encoded virulence factors and host cell signaling networks is critical for both the establishment and clearance of microbial infections. Yersinia uptake into host cells serves as an in vitro model for exploring how host cells respond to Yersinia adherence. In this study, we provide insight into the molecular nature and regulation of signaling networks that contribute to the uptake process. Using a reconstitution approach in Fak-/- fibroblasts, we have been able to specifically address the interplay between Fak, Cas and Pyk2 in this process. We show that both Fak and Cas play roles in the Yersinia uptake process and that Cas can function in a novel pathway that is independent of Fak. Fak-dependent Yersinia uptake does not appear to involve Cas-Crk signaling. By contrast, Cas-mediated uptake in the absence of Fak requires Crk as well as the protein tyrosine kinases Pyk2 and Src. In spite of these differences, the requirement for Rac1 activity is a common feature of both pathways. Furthermore, blocking the function of either Fak or Cas induces similar morphological defects in Yersinia internalization, which are manifested by incomplete membrane protrusive activity that is consistent with an inhibition of Rac1 activity. Pyk2 also functions in Yersinia uptake by macrophages, which are physiologically important for clearing Yersinia infections. Taken together, these data provide new insight into the host cellular signaling networks that are initiated upon infection with Y. pseudotuberculosis. Importantly, these findings also contribute to a better understanding of other cellular processes that involve actin remodeling, including the host response to other microbial pathogens, cell adhesion and migration.

Differential regulation of phosphoinositide metabolism by alphaVbeta3 and alphaVbeta5 integrins upon smooth muscle cell migration

Smooth muscle cell migration is a key step of atherosclerosis and angiogenesis. We demonstrate that alpha(V)beta(3) and alpha(V)beta(5) integrins synergistically regulate smooth muscle cell migration onto vitronectin. Using an original haptotactic cell migration assay, we measured a strong stimulation of phosphoinositide metabolism in migrating vascular smooth muscle cells. Phosphatidic acid production and phosphoinositide 3-kinase IA activation were triggered only upon alpha(V)beta(3) engagement. Blockade of alpha(V)beta(3) engagement or phospholipase C activity resulted in a strong inhibition of smooth muscle cell spreading on vitronectin. By contrast, blockade of alpha(V)beta(5) reinforced elongation and polarization of cell shape. Moreover, Pyk2-associated tyrosine kinase and phosphoinositide 4-kinase activities measured in Pyk2 immunoprecipitates were stimulated upon cell migration. Blockade of either alpha(V)beta(3) or alpha(V)beta(5) function, as well as inhibition of phospholipase C activity, decreased both Pyk2-associated activities. We demonstrated that the Pyk2-associated phosphoinositide 4-kinase corresponded to the beta isoform. Our data point to the metabolism of phosphoinositides as a regulatory pathway for the differential roles played by alpha(V)beta(3) and alpha(V)beta(5) upon cell migration and identify the Pyk2-associated phosphoinositide 4-kinase beta as a common target for both integrins.

B cell receptor signaling involves physical and functional association of FAK with Lyn and IgM

B cell receptor (BCR) stimulation induces phosphorylation of a number of proteins, leading to functional activation of B lymphocytes. Focal adhesion kinase (FAK) is a non-receptor protein tyrosine kinase, involved in a variety of signaling pathways. In this study, we show that FAK is tyrosine-phosphorylated and activated following BCR stimulation. We also demonstrate constitutive association of FAK with the Src-family kinase Lyn and with components of the BCR. Association of Lyn with FAK which was not correlated with BCR-induced activation of both kinases, appeared to be mediated via the binding of Lyn to the COOH-terminal part of the FAK molecule. Our results indicate that FAK is a component of the BCR complex and that it participates in BCR signaling.

Tumor necrosis factor-alpha activation of the c-Jun N-terminal kinase pathway in human neutrophils. Integrin involvement in a pathway leading from cytoplasmic tyrosine kinases apoptosis

The intensity and duration of an inflammatory response depends on the balance of factors that favor perpetuation versus resolution. At sites of inflammation, neutrophils adherent to other cells or matrix components are exposed to tumor necrosis factor-alpha (TNFalpha). Although TNFalpha has been implicated in induction of pro-inflammatory responses, it may also inhibit the intensity of neutrophilic inflammation by promoting apoptosis. Since TNFalpha is not only an important activator of the stress-induced pathways leading to p38 MAPk and c-Jun N-terminal kinase (JNK) but also a potent effector of apoptosis, we investigated the effects of TNFalpha on the JNK pathway in adherent human neutrophils and the potential involvement of this pathway in neutrophil apoptosis. Stimulation with TNFalpha was found to result in beta2 integrin-mediated activation of the cytoplasmic tyrosine kinases Pyk2 and Syk, and activation of a three-part MAPk module composed of MEKK1, MKK7, and/or MKK4 and JNK1. JNK activation was attenuated by blocking antibodies to beta2 integrins, the tyrosine kinase inhibitors, genistein, and tyrphostin A9, a Pyk2-specific inhibitor, and piceatannol, a Syk-specific inhibitor. Exposure of adherent neutrophils to TNFalpha led to the rapid onset of apoptosis that was demonstrated by augmented annexin V binding and caspase-3 cleavage. TNFalpha-induced increases in annexin V binding to neutrophils were attenuated by blocking antibodies to beta2 integrins, and the caspase-3 cleavage was attenuated by tyrphostin A9. Hence, exposure of adherent neutrophils to TNFalpha leads to utilization of the JNK-signaling pathways that may contribute to diverse functional responses including induction of apoptosis and subsequent resolution of the inflammatory response.

Targeting of PYK2 to focal adhesions as a cellular mechanism for convergence between integrins and G protein-coupled receptor signaling cascades

The non-receptor tyrosine kinase PYK2 appears to function at a point of convergence of integrins and certain G protein-coupled receptor (GPCR) signaling cascades. In this study, we provide evidence that translocation of PYK2 to focal adhesions is triggered both by cell adhesion to extracellular matrix proteins and by activation of the histamine GPCR. By using different mutants of PYK2 as green fluorescent fusion proteins, we show that the translocation of PYK2 to focal adhesions is not dependent on its catalytic activity but rather is mediated by its carboxyl-terminal domain. Translocation of PYK2 to focal adhesions was attributed to enhanced tyrosine phosphorylation of PYK2 and its association with the focal adhesion proteins paxillin and p130(Cas). Translocation of PYK2 to focal adhesions, as well as its tyrosine phosphorylation in response to histamine treatment, was abolished in the presence of protein kinase C inhibitors or cytochalasin D treatment, whereas activation of protein kinase C by phorbol ester resulted in focal adhesion targeting of PYK2 and its tyrosine phosphorylation in an integrin-clustering dependent manner. Overexpression of a wild-type PYK2 enhanced ERK activation in response to histamine, whereas a kinase-deficient mutant substantially inhibited this response. Furthermore, inhibition of PYK2 translocation to focal adhesions abolished ERK activation in response to histamine treatment. These results suggest that PYK2 apparently links between GPCRs and focal adhesion-dependent ERK activation and can provide the molecular basis underlying PYK2 function at a point of convergence between signaling pathways triggered by extracellular matrix proteins and certain GPCR agonists.

Focal adhesion kinase functions as a receptor-proximal signaling component required for directed cell migration

In performing host-defense functions, cells of the immune system become activated by soluble chemokine signals and must migrate through endothelial cell or solid tissue barriers to reach sites of inflammation or infection. Regulated adhesive interactions of immune cells with endothelium, extracellular matrix components, and cells of solid organs are critical control points of the overall immune response. Both the soluble chemokine and cell adhesion receptor-mediated migration signals must converge on common intracellular targets to engage the cell migration machinery. In this article, we focus on the role of focal adhesion kinase (FAK) and its homolog Pyk2 as cytoplasmic mediators of motility events in multiple cell types. We introduce the overall domain structure of the FAK and Pyk2 nonreceptor protein tyrosine kinases (PTKs), highlight some of the signals that activate these PTKs, and detail the molecules that functionally interact and signal transduction pathways that may mediate cell migration responses. Emphasis is placed on the knowledge gained from studies using FAK-null cells as a model system to decipher the role of this PTK in promoting cell motility.

Pyk2 and Syk participate in functional activation of granulocytic HL-60 cells in a different manner

The roles of the protein tyrosine kinases Pyk2 (also called RAFTK or CAK β) and Syk in the process of functional activation of human myeloid cells were examined. During granulocytic differentiation of HL-60 cells with dimethyl sulfoxide (DMSO), the amounts of Pyk2 and β2 integrin increased, whereas the amount of Syk was abundant before differentiation and did not change during differentiation. When the granulocytic cells were stimulated withN-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP), tyrosine phosphorylation of Pyk2 occurred promptly and subsequent association of Pyk2 with β2 integrin was detected. In contrast, Syk was not tyrosine phosphorylated by fMLP stimulation but constitutively associated with β2 integrin. Stimulation with fMLP also caused the alteration of β2 integrin to an activated form, a finding that was confirmed by the observation of fMLP-induced cell attachment on fibrinogen-coated dishes and inhibition of this attachment by pretreatment with anti-β2 integrin antibody. Cell attachment to fibrinogen caused the enhanced tyrosine phosphorylation of Pyk2 and the initial tyrosine phosphorylation of Syk, which was also inhibited by pretreatment with anti-β2 integrin antibody. In vitro kinase assays revealed that Pyk2 and Syk represented kinase activities to induce tyrosine phosphorylation of several molecules in the anti-β2 integrin immunoprecipitates of the attached cells. These results showed that Pyk2 is involved in the functional activation of granulocytic cells in 2 signaling pathways: an fMLP receptor–mediated “inside-out” signaling pathway that might cause β2 integrin activation and a subsequent β2 integrin–mediated “outside-in” signaling pathway. Syk was activated in relation to cell attachment to fibrinogen as a result of “outside-in” signaling, although it was already associated with β2 integrin before fMLP stimulation.

Pyk2 and Syk participate in functional activation of granulocytic HL-60 cells in a different manner

The roles of the protein tyrosine kinases Pyk2 (also called RAFTK or CAK β) and Syk in the process of functional activation of human myeloid cells were examined. During granulocytic differentiation of HL-60 cells with dimethyl sulfoxide (DMSO), the amounts of Pyk2 and β2 integrin increased, whereas the amount of Syk was abundant before differentiation and did not change during differentiation. When the granulocytic cells were stimulated withN-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP), tyrosine phosphorylation of Pyk2 occurred promptly and subsequent association of Pyk2 with β2 integrin was detected. In contrast, Syk was not tyrosine phosphorylated by fMLP stimulation but constitutively associated with β2 integrin. Stimulation with fMLP also caused the alteration of β2 integrin to an activated form, a finding that was confirmed by the observation of fMLP-induced cell attachment on fibrinogen-coated dishes and inhibition of this attachment by pretreatment with anti-β2 integrin antibody. Cell attachment to fibrinogen caused the enhanced tyrosine phosphorylation of Pyk2 and the initial tyrosine phosphorylation of Syk, which was also inhibited by pretreatment with anti-β2 integrin antibody. In vitro kinase assays revealed that Pyk2 and Syk represented kinase activities to induce tyrosine phosphorylation of several molecules in the anti-β2 integrin immunoprecipitates of the attached cells. These results showed that Pyk2 is involved in the functional activation of granulocytic cells in 2 signaling pathways: an fMLP receptor–mediated “inside-out” signaling pathway that might cause β2 integrin activation and a subsequent β2 integrin–mediated “outside-in” signaling pathway. Syk was activated in relation to cell attachment to fibrinogen as a result of “outside-in” signaling, although it was already associated with β2 integrin before fMLP stimulation.

Absence of marginal zone B cells in Pyk-2-deficient mice defines their role in the humoral response

The lymphoid organs contain specialized microanatomic structures composed of lymphoid, myeloid and stromal cells that are vital to the generation of an effective adaptive immune response. Although the existence of these specialized structures has been known for over a century, the developmental signals that generate them and the specific roles of these structures in the immune response have remained largely elusive. Because of their position adjacent to the marginal sinuses, marginal zone B (MZB) cells are amongst the first population of cells seen by blood born antigens and are presumed to have a critical role in host defense against bacterial pathogens. Here we demonstrate that a deficiency of the tyrosine kinase (Pyk-2) results in a cell autonomous defect of MZB cell production. In response to repetitive polysaccharide antigens (T-independent type II (TI-II)) Pyk-2-deficient mice displayed marked suppression of IgM, IgG3 and IgG2a production. Furthermore, complement receptor engagement proved necessary for the specific targeting of polysaccharide antigens to MZB cells. These results suggest how innate immune responses mediated through complement coupling are translated into an adaptive response by MZB cells, and provide a potential mechanism for the T cell independence of humoral responses to polysaccharide antigens.

Protein-tyrosine kinase Pyk2 is involved in interleukin-2 production by Jurkat T cells via its tyrosine 402

We established Jurkat transfectants that overexpress Pyk2 or its mutants, K457A (lysine 457 was mutated to alanine), Pyk2-Y402F (tyrosine 402 to phenylalanine), and Pyk2-Y881F to investigate the role of Pyk2 in T cell activation. Pyk2 as well as kinase-inactive Pyk2-K457A, was phosphorylated at tyrosine residues 402, 580, and 881 upon T cell antigen receptor cross-linking, indicating that these residues are phosphorylated by other tyrosine kinase(s). However, no tyrosine phosphorylation of Pyk2-Y402F was detected while more than 60% of the tyrosine phosphorylation was observed in Pyk2-Y881F. Pyk2-Y402F inhibited the activation of endogenous Pyk2. The degree of activation of both c-Jun NH(2)-terminal kinase and p38 mitogen-activated protein kinase but not extracellular signal-regulated protein kinase after concurrent ligation of T cell antigen receptor and CD28 was reduced by more than 50% in the clones expressing Pyk2-Y402F. Consistent with this inhibition, IL-2 production was significantly diminished in the Pyk2-Y402F-expressing clones. Furthermore, we found that Pyk2, when overexpressed, associates with Zap70 and Vav. Taken together, these findings suggest that Pyk2 is involved in the activation of T cells through its tyrosine 402.

The tyrosine kinase PYK-2/RAFTK regulates natural killer (NK) cell cytotoxic response, and is translocated and activated upon specific target cell recognition and killing

The compartmentalization of plasma membrane proteins has a key role in regulation of lymphocyte activation and development of immunity. We found that the proline-rich tyrosine kinase-2 (PYK-2/RAFTK) colocalized with the microtubule-organizing center (MTOC) at the trailing edge of migrating natural killer (NK) cells. When polyclonal NK cells bound to K562 targets, PYK-2 translocated to the area of NK-target cell interaction. The specificity of this process was assessed with NK cell clones bearing activatory or inhibitory forms of CD94/NKG2. The translocation of PYK-2, MTOC, and paxillin to the area of NK-target cell contact was regulated upon specific recognition of target cells through NK cell receptors, controlling target cell killing. Furthermore, parallel in vitro kinase assays showed that PYK-2 was activated in response to signals that specifically triggered its translocation and NK cell mediated cytotoxicity. The overexpression of both the wt and a dominant-negative mutant of PYK-2, but not ZAP-70 wt, prevented the specific translocation of the MTOC and paxillin, and blocked the cytotoxic response of NK cells. Our data indicate that subcellular compartmentalization of PYK-2 correlates with effective signal transduction. Furthermore, they also suggest an important role for PYK-2 on the assembly of the signaling complexes that regulate the cytotoxic response.

DFak56 is a novel Drosophila melanogaster focal adhesion kinase

The mammalian focal adhesion kinase (FAK) family of nonreceptor protein-tyrosine kinases have been implicated in controlling a multitude of cellular responses to the engagement of cell surface integrins and G protein-coupled receptors. We describe here a Drosophila melanogaster FAK homologue, DFak56, which maps to band 56D on the right arm of the second chromosome. Full-length DFak56 cDNA encodes a phosphoprotein of 140 kDa, which shares strong sequence similarity not only with mammalian p125(FAK) but also with the more recently described mammalian Pyk2 (also known as CAKbeta, RAFTK, FAK2, and CADTK) FAK family member. DFak56 has intrinsic tyrosine kinase activity and is phosphorylated on tyrosine in vivo. As is the case for FAK, tyrosine phosphorylation of DFak56 is increased upon plating Drosophila embryo cells on extracellular matrix proteins. In situ hybridization and immunofluorescence staining analysis showed that DFak56 is ubiquitously expressed with particularly high levels within the developing central nervous system. We utilized the UAS-GAL4 expression system to express DFak56 and analyze its function in vivo. Overexpression of DFak56 in the wing imaginal disc results in wing blistering in adults, a phenotype also observed with both position-specific integrin loss of function and position-specific integrin overexpression. Our results imply a role for DFak56 in adhesion-dependent signaling pathways in vivo during D. melanogaster development.

Adhesion to fibronectin promotes the activation of the p125(FAK)/Zap-70complex in human T cells

The beta1 integrins are a family of heterodimeric adhesion receptors involved in cell-to-cell contacts and cell-to-extracellular matrix interactions. Through their adhesive role, integrins participate in transduction of outside/inside signals and contribute to trigger a multitude of cellular events such as differentiation, cell activation, and motility. The fibronectin integrin receptors, alpha4beta1 and alpha5beta1, can function as costimulatory molecules in T-cell receptor (TCR)-dependent T-cell activation. In the current study the Jurkat T-cell line was used as a model system to investigate the TCR-independent role of cell adhesion to fibronectin in the activation of Zap-70, a central molecule in the signalling events in T cells. Upon adhesion to plastic immobilized fibronectin but not to bovine serum albumin (BSA) the phosphorylation of p125FAK, a protein kinase that localizes to focal adhesion sites, was induced. Moreover, clustering of fibronectin receptors led to the detection of a p125FAK/Zap-70 complex. Finally, while the complex between fak-B, another protein kinase localized to focal adhesion sites, and Zap-70 was detected in cells plated either on BSA or on fibronectin, the formation of the p125FAK/Zap-70 complex appeared specifically induced following fibronectin-mediated integrin clustering. These data suggest the existence of a high degree of specificity when the members of the beta1 integrin family mediate signalling pathways in T cells.

Signaling Through CD43 Induces Natural Killer Cell Activation, Chemokine Release, and PYK-2 Activation

Natural killer (NK) cell activation is the result of a balance between positive and negative signals triggered by specific membrane receptors. We report here the activation of NK cells induced through the transmembrane glycoprotein CD43 (leukosialin, sialophorin). Engagement of CD43 by specific antibodies stimulated the secretion of the chemokines RANTES, macrophage inflammatory protein (MIP)-1, and MIP-1β, which was prevented by treatment of cells with the specific tyrosine kinase inhibitor genistein. Furthermore, signaling through CD43 increased the cytotoxic activity of NK cells and stimulated an increase in the tyrosine kinase activity in antiphosphotyrosine immune complexes of NK cell lysates. PYK-2 was identified among the tyrosine kinase proteins that become activated. Hence, PYK-2 activation was observed after 20 minutes of CD43 stimulation, reached a maximum after 45 to 60 minutes, and decreased to almost basal levels after 120 minutes of treatment. Together, these results demonstrate the role of CD43 as an activation molecule able to transduce positive activation signals in NK cells, including the regulation of chemokine synthesis, killing activity, and tyrosine kinase activation.

Cell volume-dependent regulation of L-selectin shedding in neutrophils. A role for p38 mitogen-activated protein kinase

Neutrophil-mediated organ damage is a common feature of many disease states. We previously demonstrated that resuscitation with hypertonic salt solutions prevented the endotoxin-induced leukosequestration and consequent lung injury, and this effect was partially attributed to an altered surface expression of adhesion molecules, CD11b and L-selectin. In this study we investigated the mechanisms whereby osmotic stress evokes L-selectin shedding. The metalloprotease inhibitor RO 31-9790 prevented the osmotic down-regulation of L-selectin, indicating that this process was catalyzed by the same "sheddase" responsible for L-selectin cleavage induced by diverse inflammatory stimuli. The trigger for hypertonic shedding was cell shrinkage and not increased osmolarity, ionic strength, or intracellular pH. Volume reduction caused robust tyrosine phosphorylation and its inhibition by genistein and erbstatin abrogated shedding. Shrinkage stimulated tyrosine kinases Hck, Syk, and Pyk2, but prevention of their activation by the Src-family inhibitor PP1 failed to affect the L-selectin response. Hypertonicity elicited the Src family-independent activation of p38, and the inhibition of this kinase by SB203580 strongly reduced shedding. p38 was also essential for the N-formyl-methionyl-leucyl-phenylalanine- and lipopolysaccharide-induced shedding but not the phorbol ester-induced shedding. Thus, cell volume regulates L-selectin surface expression in a p38-mediated, metalloprotease-dependent manner. Moreover, p38 has a central role in shedding induced by many inflammatory mediators.

Signaling through focal adhesion kinase

Integrin receptor binding to extracellular matrix proteins generates intracellular signals via enhanced tyrosine phosphorylation events that are important for cell growth, survival, and migration. This review will focus on the functions of the focal adhesion kinase (FAK) protein-tyrosine kinase (PTK) and its role in linking integrin receptors to intracellular signaling pathways. FAK associates with several different signaling proteins such as Src-family PTKs, p130Cas, Shc, Grb2, PI 3-kinase, and paxillin. This enables FAK to function within a network of integrin-stimulated signaling pathways leading to the activation of targets such as the ERK and JNK/mitogen-activated protein kinase pathways. Focus will be placed on the structural domains and sites of FAK tyrosine phosphorylation important for FAK-mediated signaling events and how these sites are conserved in the FAK-related PTK, Pyk2. We will review what is known about FAK activation by integrin receptor-mediated events and also non-integrin stimuli. In addition, we discuss the emergence of a consensus FAK substrate phosphorylation sequence. Emphasis will also be placed on the role of FAK in generating cell survival signals and the cleavage of FAK during caspase-mediated apoptosis. An in-depth discussion will be presented of integrin-stimulated signaling events occurring in the FAK knockout fibroblasts (FAK-) and how these cells exhibit deficits in cell migration. FAK re-expression in the FAK- cells confirms the role of this PTK in the regulation of cell morphology and in promoting cell migration events. In addition, these results reinforce the potential role for FAK in promoting an invasive phenotype in human tumors.

The interaction of activated integrin lymphocyte function-associated antigen 1 with ligand intercellular adhesion molecule 1 induces activation and redistribution of focal adhesion kinase and proline-rich tyrosine kinase 2 in T lymphocytes

Integrin receptors play a central role in the biology of lymphocytes, mediating crucial functional aspects of these cells, including adhesion, activation, polarization, migration, and signaling. Here we report that induction of activation of the beta2-integrin lymphocyte function-associated antigen 1 (LFA-1) in T lymphocytes with divalent cations, phorbol esters, or stimulatory antibodies is followed by a dramatic polarization, resulting in a characteristic elongated morphology of the cells and the arrest of migrating lymphoblasts. This cellular polarization was prevented by treatment of cells with the specific tyrosine kinase inhibitor genistein. Furthermore, the interaction of the activated integrin LFA-1 with its ligand intercellular adhesion molecule 1 induced the activation of the cytoplasmic tyrosine kinases focal adhesion kinase (FAK) and proline-rich tyrosine kinase 2 (PYK-2). FAK activation reached a maximum after 45 min of stimulation; in contrast, PYK-2 activation peaked at 30 min, declining after 60 min. Upon polarization of lymphoblasts, FAK and PYK-2 redistributed from a diffuse localization in the cytoplasm to a region close to the microtubule-organizing center in these cells. FAK and PYK-2 activation was blocked when lymphoblasts were pretreated with actin and tubulin cytoskeleton-interfering agents, indicating its cytoskeletal dependence. Our results demonstrate that interaction of the beta2-integrin LFA-1 with its ligand intercellular adhesion molecule 1 induces remodeling of T lymphocyte morphology and activation and redistribution of the cytoplasmic tyrosine kinases FAK and PYK-2.

Beta2 integrin-dependent phosphorylation of protein-tyrosine kinase Pyk2 stimulated by tumor necrosis factor alpha and fMLP in human neutrophils adherent to fibrinogen

Tumor necrosis factor alpha and fMLP can activate a broad range of cellular functions in neutrophils adherent to biological surfaces. These functions are mediated by integrins and involve the activation of tyrosine kinases. Here, we report that Pyk2, a member of the focal adhesion kinase family, was present in human neutrophils and was rapidly phosphorylated and activated following tumor necrosis factor alpha and fMLP stimulation in an adhesion-dependent manner. Tyrosine phosphorylation of Pyk2 was attenuated by beta2 integrin blocking with specific antibodies. The tyrosine phosphorylation of Pyk2 was downstream of protein kinases Lyn, Syk and protein kinase C and cytoskeletal organization. The activation of Pyk2 may play a role in adhesion/cytoskeleton-associated neutrophils function.

Interactions between two cytoskeleton-associated tyrosine kinases: calcium-dependent tyrosine kinase and focal adhesion tyrosine kinase

The calcium-dependent tyrosine kinase (CADTK), also known as Pyk2/RAFTK/CAKbeta/FAK2, is a cytoskeleton-associated tyrosine kinase. We compared CADTK regulation with that of the highly homologous focal adhesion tyrosine kinase (FAK). First, we generated site-specific CADTK mutants. Mutation of Tyr402 eliminated autophosphorylation and significantly decreased kinase activity. Mutation of Tyr881, a putative Src kinase phosphorylation site predicted to bind Grb2, had little effect on CADTK regulation. Src family tyrosine kinases resulted in CADTK tyrosine phosphorylation even when co-expressed with the Tyr402/Tyr881 double mutant, suggesting that Src/Fyn etc. phosphorylate additional tyrosine residues. Interestingly, CADTK tyrosine-phosphorylated FAK when both were transiently expressed, but FAK did not phosphorylate CADTK. Biochemical experiments confirmed direct CADTK phosphorylation of FAK. This phosphorylation utilized tyrosine residues other than Tyr397, Tyr925, or Tyr576/Tyr577, suggesting that new SH2-binding sites might be created by CADTK-dependent FAK phosphorylation. Last, expression of the CADTK carboxyl terminus (CRNK) abolished CADTK but not FAK autophosphorylation. In contrast, FAK carboxyl terminus overexpression inhibited both FAK and CADTK autophosphorylation, suggesting that a FAK-dependent cytoskeletal function may be necessary for CADTK activation. Thus, CADTK and FAK, which both bind to some, but not necessarily the same, cytoskeletal elements, may be involved in coordinate regulation of cytoskeletal structure and signaling.

Matrix valency regulates integrin-mediated lymphoid adhesion via Syk kinase

Lymphocytes accumulate within the extracellular matrix (ECM) of tumor, wound, or inflammatory tissues. These tissues are largely comprised of polymerized adhesion proteins such as fibrin and fibronectin or their fragments. Nonactivated lymphoid cells attach preferentially to polymerized ECM proteins yet are unable to attach to monomeric forms or fragments of these proteins without previous activation. This adhesion event depends on the appropriate spacing of integrin adhesion sites. Adhesion of nonactivated lymphoid cells to polymeric ECM components results in activation of the antigen receptor-associated Syk kinase that accumulates in adhesion-promoting podosomes. In fact, activation of Syk by antigen or agonists, as well as expression of an activated Syk mutant in lymphoid cells, facilitates their adhesion to monomeric ECM proteins or their fragments. These results reveal a cooperative interaction between signals emanating from integrins and antigen receptors that can serve to regulate stable lymphoid cell adhesion and retention within a remodeling ECM.

Pyk2 is differentially regulated by beta1 integrin- and CD28-mediated co-stimulation in human CD4+ T lymphocytes

Beta1 integrins can provide T cell co-stimulation, but little is known concerning their downstream signaling pathways. We found that Pyk2, a focal adhesion kinase-related tyrosine kinase, is regulated by beta1 integrin signaling in human T cells. Stimulation of Jurkat T cells with the alpha4beta1 integrin ligand VCAM-1 results in Pyk2 tyrosine phosphorylation, and combined stimulation with VCAM-1 and anti-CD3 mAb induces rapid and sustained synergistic Pyk2 phosphorylation. Studies with mAb suggest that in synergistic CD3- and alpha4beta1 integrin-mediated Pyk2 tyrosine phosphorylation, a major contribution of CD3-derived signals is independent of their effects on regulating integrin adhesion. Analysis of resting human CD4+ T cells confirmed the ability of CD3-derived signals to synergize with beta1 integrin-dependent signals in the induction of Pyk2 tyrosine phosphorylation. In addition, although CD28-mediated co-stimulatory signals were able to synergize with CD3-mediated signals in inducing ERK and JNK activation and secretion of IL-2 in the primary T cells, they did not contribute to the induction of Pyk2 phosphorylation. Taken together, these results indicate a potential role for Pyk2 in T cell co-stimulation mediated specifically by beta1 integrins.

Pyk2 and Src-family protein-tyrosine kinases compensate for the loss of FAK in fibronectin-stimulated signaling events but Pyk2 does not fully function to enhance FAK- cell migration

The focal adhesion kinase (FAK) protein-tyrosine kinase (PTK) links transmembrane integrin receptors to intracellular signaling pathways. We show that expression of the FAK-related PTK, Pyk2, is elevated in fibroblasts isolated from murine fak-/- embryos (FAK-) compared with cells from fak+/+ embryos (FAK+). Pyk2 was localized to perinuclear regions in both FAK+ and FAK- cells. Pyk2 tyrosine phosphorylation was enhanced by fibronectin (FN) stimulation of FAK- but not FAK+ cells. Increased Pyk2 tyrosine phosphorylation paralleled the time-course of Grb2 binding to Shc and activation of ERK2 in FAK- cells. Pyk2 in vitro autophosphorylation activity was not enhanced by FN plating of FAK- cells. However, Pyk2 associated with active Src-family PTKs after FN but not poly-L-lysine replating of the FAK- cells. Overexpression of both wild-type (WT) and kinase-inactive (Ala457), but not the autophosphorylation site mutant (Phe402) Pyk2, enhanced endogenous FN-stimulated c-Src in vitro kinase activity in FAK- cells, but only WT Pyk2 overexpression enhanced FN-stimulated activation of co-transfected ERK2. Interestingly, Pyk2 overexpression only weakly augmented FAK- cell migration to FN whereas transient FAK expression promoted FAK- cell migration to FN efficiently compared with FAK+ cells. Significantly, repression of endogenous Src-family PTK activity by p50(csk) overexpression inhibited FN-stimulated cell spreading, Pyk2 tyrosine phosphorylation, Grb2 binding to Shc, and ERK2 activation in the FAK- but not in FAK+ cells. These studies show that Pyk2 and Src-family PTKs combine to promote FN-stimulated signaling events to ERK2 in the absence of FAK, but that these signaling events are not sufficient to overcome the FAK- cell migration defects.

PYK2 in osteoclasts is an adhesion kinase, localized in the sealing zone, activated by ligation of alpha(v)beta3 integrin, and phosphorylated by src kinase

Osteoclast activation is initiated by adhesion to the bone surface, followed by cytoskeletal rearrangement, the formation of the sealing zone, and a polarized ruffled membrane. This study shows that PYK2/CAKbeta/RAFTK, a cytoplasmic kinase related to the focal adhesion kinase, is highly expressed in rat osteoclasts in vivo. Using murine osteoclast-like cells (OCLs) or their mononuclear precursors (pOCs), generated in a coculture of bone marrow and osteoblastic MB1.8 cells, we show: (a) tyrosine phosphorylation of PYK2 upon ligation of beta3 integrins or adhesion of pOCs to serum, vitronectin, osteopontin, or fibronectin but not to laminin or collagen; (b) coimmunoprecipitation of PYK2 and c-Src from OCLs; (c) PYK2 binding to the SH2 domains of Src; (d) marked reduction in tyrosine phosphorylation and kinase activity of PYK2 in OCLs derived from Src (-/-) mice, which do not form actin rings and do not resorb bone; (e) PYK2 phosphorylation by exogeneous c-Src; (f) translocation of PYK2 to the Triton X-100 insoluble cytoskeletal fraction upon adhesion; (g) localization of PYK2 in podosomes and the ring-like structures in OCLs plated on glass and in the sealing zone in OCLs plated on bone; and (h) activation of PYK2, in the presence of MB1.8 cells, parallels the formation of sealing zones and pit resorption in vitro and is reduced by echistatin or calcitonin and cytochalasin D. Taken together, these findings suggest that Src-dependent tyrosine phosphorylation of PYK2 is involved in the adhesion-induced formation of the sealing zone, required for osteoclastic bone resorption.

Identification of a new Pyk2 isoform implicated in chemokine and antigen receptor signaling

Pyk2 is a protein tyrosine kinase that links G-protein-coupled receptors, inflammatory cytokines, and extracellular stimuli that elevate intracellular calcium concentration with activation of the mitogen-activated protein kinase pathways and regulation of ion channel functions. Here we describe the identification, cloning, and characterization of a new isoform of Pyk2 (Pyk2-H) that is generated by alternative RNA splicing. Pyk2-H is mainly expressed in hematopoietic cells including T-cells, B-cells, and natural killer cells. Engagement of T-cell or B-cell antigen receptors leads to rapid tyrosine phosphorylation of Pyk2-H. Pyk2-H is also activated in response to the chemokines RANTES and macrophage inflammatory protein-1beta in T cells. In addition, we show that glutathione S-transferase fusion proteins containing the carboxyl termini of Pyk2 and Pyk2-H bind to a different set of tyrosine-phosphorylated proteins in thymus lysates. Specific expression of Pyk2-H and its activation by antigens or chemokines in hematopoietic cells may contribute toward the generation of cell type-specific signals involved in host immune responses.

Fluoroaluminate induces activation and association of Src and Pyk2 tyrosine kinases in osteoblastic MC3T3-E1 cells

Fluoride is known to increase bone mass in vivo, probably through stimulation of osteoblast proliferation; however, the mechanisms of fluoroaluminate action in osteoblasts have not yet been elucidated. We have previously shown that in osteoblastic MC3T3-E1 cells, fluoroaluminate stimulates G protein-mediated protein tyrosine phosphorylation (Scaronuscarona, M., Standke, G. J. R., Jeschke, M., and Rohner, D. (1997) Biochem. Biophys. Res. Commun. 235, 680-684). Although the Ser/Thr kinases Erk1, Erk2, and p70(S6K) were activated in response to fluoroaluminate, the identity of fluoroaluminate-activated tyrosine kinase(s) remained elusive. In this study, we show that in MC3T3-E1 cells, fluoroaluminate induces a 110-kDa tyrosine-phosphorylated protein that we identify as Pyk2, a cytoplasmic tyrosine kinase related to Fak (focal adhesion kinase). The tyrosine phosphorylation of Pyk2 increased in a dose- and time-dependent manner. The autophosphorylation activity of Pyk2 increased 3-fold and reached its maximum within 10 min of fluoroaluminate treatment. Fluoroaluminate also induced activation of Src and the association of Pyk2 with Src. The phosphorylation of Src-associated Pyk2 increased >20-fold in in vitro kinase assays, suggesting that Pyk2 is phosphorylated by Src. Although MC3T3-E1 cells express much more Fak than Pyk2, Src preferentially associated with Pyk2. In vitro, Pyk2 bound to the Src SH2 domain, suggesting that this interaction mediates the Src-Pyk2 association in cells. These data indicate that osteoblastic cells express Pyk2, which is tyrosine-phosphorylated and activated in response to G protein activation by fluoroaluminate, and that the mechanism of Pyk2 activation most likely involves Src. Thus, Src and Pyk2 are tyrosine kinases involved in G protein-mediated tyrosine phosphorylation in osteoblastic cells and may be important for the osteogenic action of fluoroaluminate.

Paxillin phosphorylation and association with Lck and Pyk2 in anti-CD3- or anti-CD45-stimulated T cells

Antibodies to either CD3 or CD45 have been shown to induce dramatic changes in cell morphology, increased tyrosine phosphorylation of cellular proteins, and the association of a subset of these proteins with the tyrosine kinase Lck. The current study was initiated to determine the identity of the tyrosine-phosphorylated 70-80 kDa protein that becomes Lck-associated after stimulation with anti-CD45 or anti-CD3. We demonstrate that the cytoskeletal protein paxillin becomes tyrosine-phosphorylated when cells are plated on immobilized antibodies specific for CD45 or CD3. Only tyrosine-phosphorylated paxillin is associated with Lck, suggesting that the association is through the SH2 domain of Lck. Consistent with this we demonstrate that the SH2 domain of Lck binds tyrosine-phosphorylated paxillin. In contrast, the association of paxillin with the FAK-related kinase Pyk2 was found to be constitutive and not altered by the phosphorylation of either protein. Finally, we establish that the phosphorylation of paxillin is dependent on the expression of Lck. Taken together, these results demonstrate that paxillin is physically associated with kinases from two different families in T cells and suggest that paxillin may function as an adaptor protein linking cellular signals with cytoskeletal changes during T cell activation.

Cell adhesion kinase beta forms a complex with a new member, Hic-5, of proteins localized at focal adhesions

Cell adhesion kinase beta (CAKbeta/PYK2) is the second protein-tyrosine kinase of the focal adhesion kinase subfamily. We identified a cDNA that encodes a CAKbeta-binding protein. This cDNA clone encodes the human homologue of Hic-5, the cDNA of which was cloned in 1994 as transforming growth factor beta1- and hydrogen peroxide-inducible mRNA. We found that Hic-5 exclusively localized at focal adhesions in a rat fibroblast line, WFB. This localization of Hic-5 was confirmed in WFB cells expressing Myc-tagged Hic-5. The amino acid sequence of Hic-5 is highly similar to that of paxillin in the four LD motifs as well as in the four contiguous LIM domains. The Hic-5 N-terminal domain directly associated in vitro with the extreme C-terminal region (residue 801 to the end) of CAKbeta. CAKbeta was coimmunoprecipitated with Hic-5 from the WFB cell lysate. The coimmunoprecipitation of CAKbeta with Hic-5 was markedly inhibited by the addition of the extreme C-terminal region of CAKbeta. Coimmunoprecipitation of Hic-5 with CAKbeta, which was shown in COS-7 cells doubly transfected with cDNA constructs of CAKbeta and Myc-tagged Hic-5, was lost when the CAKbeta amino acid residues 741-903 were deleted. Hic-5 was tyrosine-phosphorylated in Src-transformed 3Y1 cells and in cells treated with pervanadate. Hic-5 associated with CAKbeta was selectively tyrosine-phosphorylated in WFB cells exposed to hypertonic osmotic stress. These results indicate that Hic-5 is a paxillin-related component of focal adhesions and binds to CAKbeta, implying possible involvement of Hic-5 in the downstream signaling of CAKbeta.

Activation of protein-tyrosine kinase Pyk2 is downstream of Syk in FcepsilonRI signaling

Aggregation of the FcepsilonRI, a member of the immune receptor family, induces the activation of proteintyrosine kinases and results in tyrosine phosphorylation of proteins that are involved in downstream signaling pathways. Here we report that Pyk2, another member of the focal adhesion kinase family, was present in the RBL-2H3 mast cell line and was rapidly tyrosine-phosphorylated and activated after FcepsilonRI aggregation. Tyrosine phosphorylation of Pyk2 was also induced by the calcium ionophore A23187, by phorbol myristate acetate, or by stimulation of G-protein-coupled receptors. Adherence of cells to fibronectin dramatically enhanced the induced tyrosine phosphorylation of Pyk2. Although Src family kinases are activated by FcepsilonRI stimulation and tyrosine-phosphorylate the receptor subunits, the activation and tyrosine phosphorylation of Pyk2 were downstream of Syk. In contrast, tyrosine phosphorylation of Pyk2 by stimulation of G-protein-coupled receptors was independent of Syk. Therefore, the FcepsilonRI-induced tyrosine phosphorylation of Pyk2 is downstream of Syk and may play a role in cell secretion.