Tyrosine phosphorylation of the focal adhesion kinase pp125FAK during development: relation to paxillin

Endosomal sorting and c-Cbl targeting of paxillin to autophagosomes regulate cell-matrix adhesion turnover in human breast cancer cells

Post-translational mechanisms regulating cell-matrix adhesion turnover during cell locomotion are not fully elucidated. In this study, we uncovered an essential role of Y118 site-specific tyrosine phosphorylation of paxillin, an adapter protein of focal adhesion complexes, in paxillin recruitment to autophagosomes to trigger turnover of peripheral focal adhesions in human breast cancer cells. We demonstrate that the Rab-7 GTPase is a key upstream regulator of late endosomal sorting of tyrosine118-phosphorylated paxillin, which is subsequently recruited to autophagosomes via the cargo receptor c-Cbl. Essentially, this recruitment involves a direct and selective interaction between Y118-phospho-paxillin, c-Cbl, and LC3 and is independent from c-Cbl E3 ubiquitin ligase activity. Interference with the Rab7-paxillin-autophagy regulatory network using genetic and pharmacological approaches greatly impacted focal adhesion stability, cell locomotion and progression to metastasis using a panel of human breast cancer cells. Together, these results provide novel insights into the requirement of phospho-site specific post-translational mechanism of paxillin for autophagy targeting to regulate cell-matrix adhesion turnover and cell locomotion in breast cancer cells.

Mechanoregulation of stem cell fate via micro-/nano-scale manipulation for regenerative medicine

Recent developments in the field of mechanobiology have renewed the call for a better understanding of the role of mechanical forces as potent regulators and indicators of stem cell fate. Although it is well established that mechanical forces play a crucial role in guiding tissue development, little is known about how submicroscopic biomechanical forces can influence key stem cell behaviors. This review will detail the use of micro-/nano-technologies that are advancing our current understanding of stem cell mechanobiology, and mechanoregulation of stem cell fate using engineered surface topographies and small-scale patterning techniques. The involvement of focal adhesions and the cytoskeleton systems as a common biophysical impetus through which these mechanical signals are transduced via distinct signaling pathways will also be discussed. These insights are envisioned to provide the basis for the rational design of future biocompatible materials and may inspire alternative drug-free therapeutic strategies to manage diseased sites via biomechanical management.

Paxillin and hydrogen peroxide-inducible clone 5 expression and distribution in control and Alzheimer disease hippocampi

Hydrogen peroxide-inducible clone 5 (Hic-5) and paxillin are members of the Group III LIM domain protein family that localize to both cell nuclei and focal adhesions and link integrin-mediated signaling to the actin cytoskeleton. Prior in vitro studies have implicated paxillin in beta-amyloid-induced cell death, but little is known about the expression and function of Hic-5 and paxillin in the brain. We performed a blinded retrospective cross-sectional study of Hic-5 and paxillin expression in the hippocampi of Alzheimer disease (AD) and control subjects using immunohistochemistry and laser scanning confocal microscopy. The analysis included assessment of the expression of phosphorylated isoforms of paxillin that reflect activation of distinct signaling pathways. We found changes in the subcellular distribution of Hic-5, paxillin, and specific phosphorylated isoforms of paxillin in the AD brains. The Hic-5 and phosphorylated isoforms of paxillin colocalized with neurofibrillary tangles. Paxillin was predominantly found in reactive astrocytes in the AD hippocampi, and activated paxillin was also detected in granulovacuolar degeneration bodies in AD. These data indicate that these important scaffolding proteins that link various intracellular signaling pathways to the extracellular matrix are modified and have altered subcellular distribution in AD.

Focal adhesions disassemble during early pregnancy in rat uterine epithelial cells

During early pregnancy in rodents, invasion of the blastocyst into the endometrial decidual cells is accompanied by the removal of uterine epithelial cells around the implantation sites. The present study investigated the distribution and expression of two focal adhesion proteins, namely talin and paxillin, in rat uterine epithelial cells during early pregnancy and their role in the loss of these cells at the time of implantation. A major distributional change of talin and paxillin was demonstrated in uterine epithelial cells during early pregnancy. From a highly concentrated expression along the basal cell surface on Day 1 of pregnancy, talin and paxillin were lost from the basal cell surface at the time of implantation. There was also a corresponding statistically significant decrease in paxillin seen through western blotting analysis. Together, these observations suggest that uterine epithelial cells are less adherent to the underlying basal lamina due to the disassembly of talin and paxillin from focal adhesions, facilitating removal of these cells at the time of implantation. This phenomenon was restricted to the period of receptivity because talin and paxillin reappeared along the basal cell surface soon after implantation.

Paxillin comes of age

Paxillin is a multi-domain scaffold protein that localizes to the intracellular surface of sites of cell adhesion to the extracellular matrix. Through the interactions of its multiple protein-binding modules, many of which are regulated by phosphorylation, paxillin serves as a platform for the recruitment of numerous regulatory and structural proteins that together control the dynamic changes in cell adhesion, cytoskeletal reorganization and gene expression that are necessary for cell migration and survival. In particular, paxillin plays a central role in coordinating the spatial and temporal action of the Rho family of small GTPases, which regulate the actin cytoskeleton, by recruiting an array of GTPase activator, suppressor and effector proteins to cell adhesions. When paxillin was first described 18 years ago, the amazing complexity of cell-adhesion organization, dynamics and signaling was yet to be realized. Herein we highlight our current understanding of how the multiple protein interactions of paxillin contribute to the coordination of cell-adhesion function.

Src-dependent phosphorylation of ASAP1 regulates podosomes

Invadopodia are Src-induced cellular structures that are thought to mediate tumor invasion. ASAP1, an Arf GTPase-activating protein (GAP) containing Src homology 3 (SH3) and Bin, amphiphysin, and RVS161/167 (BAR) domains, is a substrate of Src that controls invadopodia. We have examined the structural requirements for ASAP1-dependent formation of invadopodia and related structures in NIH 3T3 fibroblasts called podosomes. We found that both predominant splice variants of ASAP1 (ASAP1a and ASAP1b) associated with invadopodia and podosomes. Podosomes were highly dynamic, with rapid turnover of both ASAP1 and actin. Reduction of ASAP1 levels by small interfering RNA blocked formation of invadopodia and podosomes. Podosomes were formed in NIH 3T3 fibroblasts in which endogenous ASAP1 was replaced with either recombinant ASAP1a or ASAP1b. ASAP1 mutants that lacked the Src binding site or GAP activity functioned as well as wild-type ASAP1 in the formation of podosomes. Recombinant ASAP1 lacking the BAR domain, the SH3 domain, or the Src phosphorylation site did not support podosome formation. Based on these results, we conclude that ASAP1 is a critical target of tyrosine kinase signaling involved in the regulation of podosomes and invadopodia and speculate that ASAP1 may function as a coincidence detector of simultaneous protein association through the ASAP1 SH3 domain and phosphorylation by Src.

Focal adhesion kinase: a potential target in cancer therapy

Focal adhesion kinase (FAK) is a non-receptor tyrosine kinase that plays an important role in signal transduction pathways that are initiated at sites of integrin-mediated cell adhesions and by growth factor receptors. FAK is a key regulator of survival, proliferation, migration and invasion: processes that are all involved in the development and progression of cancer. FAK is also linked to oncogenes at both a biochemical and functional level. Moreover, overexpression and/or increased activity of FAK is common in a wide variety of human cancers, implicating a role for FAK in carcinogenesis. Given the important role of FAK in a large number of processes involved in tumorigenesis, metastasis and survival signalling FAK should be regarded as a potential target in the development of anti-cancer drugs. Therefore, selective inhibitors of FAK need to be developed. Combination of these selective FAK inhibitors with cytotoxic agents could be a very promising anti-cancer therapy.

The stimulus-induced tyrosine phosphorylation of Munc18c facilitates vesicle exocytosis

Stimulus-induced tyrosine phosphorylation of Munc18c was investigated as a potential regulatory mechanism by which the Munc18c-Syntaxin 4 complex can be dissociated in response to divergent stimuli in multiple cell types. Use of [(32)P]orthophosphate incorporation, pervanadate treatment, and phosphotyrosine-specific antibodies demonstrated that Munc18c underwent tyrosine phosphorylation. Phosphorylation was apparent under basal conditions, but levels were significantly increased within 5 min of glucose stimulation in MIN6 beta cells. Tyrosine phosphorylation of Munc18c was also detected in 3T3L1 adipocytes and increased with insulin stimulation, suggesting that this may be a conserved mechanism. Syntaxin 4 binding to Munc18c decreased as Munc18c phosphorylation levels increased in pervanadate-treated cells, suggesting that phosphorylation dissociates the Munc18c-Syntaxin 4 complex. Munc18c phosphorylation was localized to the N-terminal 255 residues. Mutagenesis of one residue in this region, Y219F, significantly increased the affinity of Munc18c for Syntaxin 4, whereas mutation of three other candidate sites was without effect. Moreover, Munc18c-Y219F expression in MIN6 cells functionally inhibited glucose-stimulated SNARE complex formation and insulin granule exocytosis. These data support a novel and conserved mechanism for the dissociation of Munc18c-Syntaxin 4 complexes in a stimulus-dependent manner to facilitate the increase in Syntaxin 4-VAMP2 association and to promote vesicle/granule fusion.

High expression of focal adhesion kinase (p125FAK) in node-negative breast cancer is related to overexpression of HER-2/neu and activated Akt kinase but does not predict outcome

INTRODUCTION

Focal adhesion kinase (FAK) regulates multiple cellular processes including growth, differentiation, adhesion, motility and apoptosis. In breast carcinoma, FAK overexpression has been linked to cancer progression but the prognostic relevance remains unknown. In particular, with regard to lymph node-negative breast cancer it is important to identify high-risk patients who would benefit from further adjuvant therapy.

METHODS

We analyzed 162 node-negative breast cancer cases to determine the prognostic relevance of FAK expression, and we investigated the relationship of FAK with major associated signaling pathways (HER2, Src, Akt and extracellular regulated kinases) by immunohistochemistry and western blot analysis.

RESULTS

Elevated FAK expression did not predict patient outcome, in contrast to tumor grading (P = 0.005), Akt activation (P = 0.0383) and estrogen receptor status (P = 0.0033). Significant positive correlations were observed between elevated FAK expression and HER2 overexpression (P = 0.001), as well as phospho-Src Tyr-215 (P = 0.021) and phospho-Akt (P < 0.001), but not with phospho-ERK1/2 (P = 0.108). Western blot analysis showed a significant correlation of FAK Tyr-861 activation and HER2 overexpression (P = 0.01).

CONCLUSIONS

Immunohistochemical detection of FAK expression is of no prognostic significance in node-negative breast cancer but provides evidence that HER2 is involved in tumor malignancy and metastatic ability of breast cancer through a novel signaling pathway participating FAK and Src.

Cytoskeletal remodeling in vascular smooth muscle cells in response to angiotensin II-induced activation of the SHP-2 tyrosine phosphatase

Angiotensin II is an octapeptide that regulates diverse cellular responses including the actin cytoskeletal organization. In this study, stable cell lines overexpressing wild-type or catalytically inactive SHP-2 were employed to elucidate the signaling pathway utilized by the SHP-2 tyrosine phosphatase that mediates an angiotensin II-induced reorganization of the actin cytoskeleton in vascular smooth muscle cells (VSMC). The expression of wild-type SHP-2 prevented an angiotensin II dependent increase in stress fiber formation. In contrast, the catalytically inactive mutant SHP-2 increased stress fiber formation. Additional observations further established that SHP-2 regulates the reorganization of the actin cytoskeleton through RhoA- and Vav2-dependent signaling pathways. The expression of wild-type SHP-2 caused a dephosphorylation of several focal adhesion associated proteins including paxillin, p130Cas, and tensin in VSMC. This dephosphorylation of focal adhesion associated proteins was accompanied by significantly decreased numbers of focal adhesions within cells. These results demonstrate a unique role for SHP-2 in the regulation of the cellular architecture of VSMC, suggesting the possibility that this phosphatase might be instrumental in vascular remodeling.

Nuclear translocation of Xenopus laevis paxillin

Paxillin has been recognized as a focal adhesion adapter protein that participates in the integrin-mediated signaling. An earlier study [Ogawa et al. Biochim. Biophys. Acta 1519 (2001) 235] found that frog paxillin was expressed in the kidney epithelial cell line A6 and localized in the nucleus. Here, in this study, we have found that the expression of frog paxillin is up-regulated in the S phase of cell cycle. The protein became phosphorylated on tyrosine when the cells were grown on vitronectin; the tyrosine phosphorylation was not detectable when the cells were cultured on fibronectin, laminin or poly-D-lysine. On the other hand, MAP kinase was revealed to phosphorylate frog paxillin on serine. Both phosphorylation events, namely on tyrosine and serine, were essential for the nuclear translocation of this protein. Our results suggest that the integrin-mediated signaling pathway and the MAP kinase pathway meet at paxillin.

Adhesion of B cell lines to endothelial cells from human lymphoid tissue modulates tyrosine phosphorylation and endothelial cell activation

Through the production of cytokines and growth factors the endothelium of secondary lymphoid organs plays a crucial role in controlling lymphocyte migration to the lymphoid microenvironment, an essential step in the initiation of the immune response. Here we demonstrate that direct contact of B cell lines with tonsil-derived human endothelial cells resulted in changes in the phosphorylation state of endothelial cells, causing their functional activation. We found a rapid (<15-s) and transient dephosphorylation, followed by a rapid rephosphorylation of tyrosine residues of the focal adhesion kinase, paxillin, and ERK2. Maximal rephosphorylation occurred after 15-30 min of B cell contact. Preincubation of lymphoid B cells with an adhesion-blocking Ab directed against alpha(4)beta(1) integrin abrogated adhesion-mediated changes of endothelial cell tyrosine phosphorylation, suggesting that cell contact was essential. Similar patterns of tyrosine phosphorylation, but with slightly different kinetics were induced after cross-linking of beta(1) integrin or CD40 on endothelial cells. Functional activation of endothelial cells by B cell adhesion was confirmed by the production of IL-6, IL-8, monocyte chemoattractant protein-1, M-CSF, and macrophage inflammatory protein-1beta mRNA. However, direct cross-linking of beta(1) integrin and CD40 failed to accomplish the same functional activation. These data indicate that direct contact of lymphoid B cells with the endothelium from lymphoid tissue induce endothelial cell signaling, resulting in chemokine and cytokine production. This phenomenon may provide a mechanism for the remodeling of the endothelium from lymphoid tissues, thus contributing to the free migration of lymphocytes and other cells into the lymphoid organs.

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.

mRNA sequence of the Xenopus laevis paxillin gene and its expression

Paxillin is a cytoskeletal protein found in structures of focal adhesions where cells adhere to the extracellular matrix. We isolated paxillin cDNA from the Xenopus laevis ovary. The cDNA sequence encodes a protein of 539 amino acids with four LIM and five LD motifs. 80% of the amino acids of frog paxillin are shared by human and chicken paxillins. Northern analysis showed that the frog gene is expressed in the spleen, kidney, testis and ovary. Immunocytochemistry showed that paxillin protein is accumulated in the nucleus as well as in the periphery of the cytoplasm of the A6 cell. This intriguing result shows that paxillin, which has been characterized as a cytoskeletal protein, is capable of translocating to the nucleus.

Protein kinase C-mediated tyrosine phosphorylation of paxillin and focal adhesion kinase requires cytoskeletal integrity and is uncoupled to mitogen-activated protein kinase activation in human hepatoma cells

Treatment of cultured human hepatoma HepG2 cells with the protein kinase C (PKC) activator, 12-O-tetradecanoylphorbol-13-acetate (TPA), results in an increase in tyrosine phosphorylation of several proteins, including the focal adhesion kinase (FAK) and paxillin using anti-phosphotyrosine Western blotting and immunoprecipitation. However, when cells are in suspension or in the presence of cytochalasin D which disrupts the intracellular network of actin microfilaments, TPA loses its ability to stimulate tyrosine phosphorylation of FAK and paxillin but it still activates mitogen-activated protein kinase (MAPK) and induces PKC translocation from cytosol to the membrane in HepG2 cells. On the other hand, PD98059, a specific inhibitor of mitogen-activated protein kinase kinase, blocks TPA-induced MAPK activation but has no effect on TPA-induced tyrosine phosphorylation. Our findings suggest that TPA-induced tyrosine phosphorylation of FAK and paxillin in human hepatoma cells is PKC dependent and requires the integrity of the cell cytoskeleton but is uncoupled to the signal transduction pathway of PKC leading to the translocation of PKC and MAPK activation.

Diperoxovanadate alters endothelial cell focal contacts and barrier function: role of tyrosine phosphorylation

Diperoxovanadate (DPV), a potent tyrosine kinase activator and protein tyrosine phosphatase inhibitor, was utilized to explore bovine pulmonary artery endothelial cell barrier regulation. DPV produced dose-dependent decreases in transendothelial electrical resistance (TER) and increases in permeability to albumin, which were preceded by brief increases in TER (peak TER effect at 10-15 min). The significant and sustained DPV-mediated TER reductions were primarily the result of decreased intercellular resistance, rather than decreased resistance between the cell and the extracellular matrix, and were reduced by pretreatment with the tyrosine kinase inhibitor genistein but not by inhibition of p42/p44 mitogen-activating protein kinases. Immunofluorescent analysis after DPV challenge revealed dramatic F-actin polymerization and stress-fiber assembly and increased colocalization of tyrosine phosphoproteins with F-actin in a circumferential pattern at the cell periphery, changes that were abolished by genistein. The phosphorylation of focal adhesion and adherens junction proteins on tyrosine residues was confirmed in immunoprecipitates of focal adhesion kinase and cadherin-associated proteins in which dramatic dose-dependent tyrosine phosphorylation was observed after DPV stimulation. We speculate that DPV enhances endothelial cell monolayer integrity via focal adhesion plaque phosphorylation and produces subsequent monolayer destabilization of adherens junctions initiated by adherens junction protein tyrosine phosphorylation catalyzed by p60(src) or Src-related tyrosine kinases.

Association of focal adhesion kinase with fibronectin and paxillin is required for precartilage condensation of chick mesenchymal cells

We show that tyrosine phosphorylation of FAK was increased as precartilage condensation occurred, followed by a subsequent decrease in proliferation of in vitro micromass culture of wing bud mesenchymal cells. FAK was associated with fibronectin and paxillin, which were maximal at day 3 of culture. FAK was also associated with signaling molecules such as PLC-gamma and PI3-kinase through c-Src. The beta1 integrin antibody and several inhibitors of signaling molecules such as herbimycin A, U73122, LY294002, as well as cytochalasin D, an actin depolymerizing agent, remarkably decreased tyrosine phosphorylation of FAK and its association with fibronectin and paxillin during condensation. resulting in a marked inhibition of condensation and chondrogenesis. Taken together, our findings suggest that beta1 integrin-mediated interaction of mesenchymal cells and fibronectin signals to accelerate the precartilage condensation through tyrosine phosphorylation of FAK and its association with paxillin. This signaling pathway is required for precartilage condensation and subsequent cartilage nodule formation in chondrogenesis.

Serine phosphorylation of paxillin by heregulin-beta1: role of p38 mitogen activated protein kinase

The mechanisms through which heregulin (HRG) regulates the progression of breast cancer cells to a more invasive phenotype are currently unknown. Recently we have shown that HRG treatment of breast cancer cells leads to the formation of lamellipodia/filopodia, and increased cell migration and invasiveness through the phosphatidylinositol 3-kinase (PI-3 kinase). Since the process of cell migration must involve changes in adhesion, we explored the potential HRG regulation of paxillin, a major cytoskeletal phosphoprotein of focal adhesion. We report that HRG stimulation of non-invasive breast cancer cells resulted in stimulation of p38 mitogen-activated protein kinase (p38MAPK), extracellular signal-regulated kinases (ERK) and PI-3K, and a concurrent unexpected increase in the level of paxillin phosphorylation on serine residue which was sensitive to protein-phosphatase 2b but not to protein tyrosine phosphatase 1. In addition, HRG triggered a rapid redistribution of paxillin to the perinuclear regions from the tyrosine-phosphorylated focal adhesions, and increased cell scattering. There was no effect of HRG on the state of phosphorylation and localization of focal adhesion kinase. The HRG-induced increase in serine phosphorylation of paxillin and cell scattering were selectively inhibited by a specific inhibitor of p38MAPK or a dominant-negative p38MAPK mutant, but not by inhibitors of p42/44MAPK or PI-3 kinase pathways. For the first time our results have shown that HRG, a potent migratory growth factor stimulates serine phosphorylation of paxillin. These findings suggest a role of p38MAPK-dependent signal transduction pathway(s) in serine phosphorylation and disassembly of the paxillin from the focal complexes during HRG-induced cell shape alterations and motility.

Focal adhesion kinase, paxillin, and bcl-2: analysis of expression, phosphorylation, and association during morphogenesis

Cell adhesive mechanisms which determine tissue architecture during morphogenesis are tightly regulated and have an impact on apoptosis, cell migration, proliferation, and differentiation. Bcl-2 is a death repressor that protects cells from apoptosis initiated by a variety of stimuli including loss of cell adhesion. Utilizing the kidney as a model of an organ that undergoes three-dimensional development we demonstrate that bcl-2 directly associates with paxillin. Focal adhesion kinase (FAK)(p125) and paxillin(p68) were highly expressed and tyrosine phosphorylated during development but declined to low levels following renal maturation (postnatal day 20) in normal mice. The decline in the expression of p125 FAK and p68 paxillin occurred together with an increase in specific cleavage products of lower molecular weights. Mice deficient in bcl-2 are born with renal hypoplasia and succumb to renal failure as a result of renal multicystic disease. In kidneys from postnatal day 20 bcl-2 -/- mice, tyrosine phosphorylation of p125 FAK and p68 paxillin was not down-regulated. However, the level of expression was similar to that of normal mice. These results demonstrate that the developmentally regulated expression and phosphorylation of FAK and paxillin, in the presence of bcl-2, is necessary for normal morphogenesis. The interaction of paxillin with bcl-2 during nephrogenesis may provide an alternative to integrin(s) signaling through paxillin/FAK thus bypassing the need for adhesion-mediated survival during three dimensional morphogenesis. Dev Dyn 1999;215:371-382.

Regulated expression of focal adhesion kinase-related nonkinase, the autonomously expressed C-terminal domain of focal adhesion kinase

Focal adhesion kinase (FAK) has been implicated in cellular processes that control cell adhesion, migration, cell cycle progression, and apoptosis. FRNK (FAK-related nonkinase) is the autonomously expressed, noncatalytic C-terminal portion of FAK. When ectopically expressed in cells, FRNK has been shown to act as a negative regulator of FAK activity, inhibiting cell spreading, migration, and cell cycle progression. The mechanisms that regulate FRNK expression during embryonic development and the functional role of FRNK in normal cell homeostasis remain poorly understood. Herein we show that FRNK expression in chicken cells is directed by an alternative promoter residing within an intron of FAK, positioned 3' of the exon encoding sequences for the catalytic domain and 5' of the exon encoding sequences for the C-terminal domain of FAK (e.g., FRNK). Using probes specific for FRNK, we show that FRNK expression occurs early in chicken embryogenesis, being readily detected at day 3, 6, or 9. Late in embryogenesis, at day 18, FRNK is expressed in a tissue-specific manner, predominately in lung and intestine cells. Western blot analysis of mouse tissues with a FAK-specific antibody revealed the expression of FRNK in the mouse lung. Reverse transcriptase PCR analysis of mouse lung RNA revealed the presence of spliced FRNK mRNAs containing 5' untranslated sequences derived from a positionally conserved exon present in the mouse genome. FAK is the first example of a tyrosine kinase regulated by a domain under the control of an alternative intronic promoter. It is also the first example of a focal adhesion-associated protein regulated by such a mechanism and thus represents a novel means for the modulation of cell adhesion signaling.

Focal adhesion kinase, paxillin, and bcl-2: analysis of expression, phosphorylation, and association during morphogenesis

Cell adhesive mechanisms which determine tissue architecture during morphogenesis are tightly regulated and have an impact on apoptosis, cell migration, proliferation, and differentiation. Bcl-2 is a death repressor that protects cells from apoptosis initiated by a variety of stimuli including loss of cell adhesion. Utilizing the kidney as a model of an organ that undergoes three-dimensional development we demonstrate that bcl-2 directly associates with paxillin. Focal adhesion kinase (FAK)(p125) and paxillin(p68) were highly expressed and tyrosine phosphorylated during development but declined to low levels following renal maturation (postnatal day 20) in normal mice. The decline in the expression of p125 FAK and p68 paxillin occurred together with an increase in specific cleavage products of lower molecular weights. Mice deficient in bcl-2 are born with renal hypoplasia and succumb to renal failure as a result of renal multicystic disease. In kidneys from postnatal day 20 bcl-2 -/- mice, tyrosine phosphorylation of p125 FAK and p68 paxillin was not down-regulated. However, the level of expression was similar to that of normal mice. These results demonstrate that the developmentally regulated expression and phosphorylation of FAK and paxillin, in the presence of bcl-2, is necessary for normal morphogenesis. The interaction of paxillin with bcl-2 during nephrogenesis may provide an alternative to integrin(s) signaling through paxillin/FAK thus bypassing the need for adhesion-mediated survival during three dimensional morphogenesis. Dev Dyn 1999;215:371-382.

Role of PTP-1B in aortic smooth muscle cell motility and tyrosine phosphorylation of focal adhesion proteins

Recent studies have focused attention on the role of protein tyrosine kinases in vascular smooth muscle cell biology, but similar information regarding protein tyrosine phosphatases (PTP) is sparse. PTP-1B is a ubiquitous nonreceptor phosphatase with uncertain function and substrates that are mostly unidentified. We used antisense oligodeoxynucleotides (ODN) against PTP-1B to investigate the role of endogenous PTP-1B in motility of primary cultures of rat aortic smooth muscle cells (RASMC). Antisense ODN decreased PTP-1B protein levels and activity in a concentration-dependent fashion, whereas sense, scrambled, or three-base mismatch antisense ODN had little or no effect. Treatment of cells with antisense ODN, but not sense, scrambled, or three-base mismatch antisense ODN, enhanced cell motility and increased tyrosine phosphorylation levels of focal adhesion proteins paxillin, p130(cas), and focal adhesion kinase. Our findings indicate that PTP-1B is a negative regulator of RASMC motility via modulation of phosphotyrosine levels in several focal adhesion proteins and suggest the involvement of PTP-1B in events such as atherosclerosis and restenosis, which are associated with increased vascular smooth muscle cell motility.

FAK and PYK2/CAKbeta in the nervous system: a link between neuronal activity, plasticity and survival?

A major aim of neurobiology today is to improve understanding of the signaling pathways that couple rapid events, such as the action potential and neurotransmitter release, to long-lasting changes in synaptic strength and increased neuronal survival. These adaptations involve interactions of neurons with other cells and with the extracellular matrix. They use, in part, the same molecular machinery that controls adhesion, motility or survival in non-neuronal cells. This machinery includes two homologous non-receptor tyrosine kinases, FAK and PYK2/CAKbeta, and the associated SRC-family tyrosine kinases. Specific brain isoforms of FAK with distinct properties are regulated by neurotransmitters, whereas PYK2/CAKbeta is highly sensitive to depolarization. The multiplicity of the pathways that can be activated by these tyrosine kinases indicates their importance in signal transduction in the adult brain.

Activation of three classes of nonreceptor tyrosine kinases following Fc gamma receptor crosslinking in human monocytes

Fc gamma receptors on monocytes/macrophages play an important role in both host defense and autoimmune disorders. Fc gamma receptor signaling can lead to such downstream events as phagocytosis and the release of intracellular cytokines and reactive oxygen species. Freshly isolated human monocytes express two major classes of Fc gamma receptor proteins, Fc gamma RI (CD64) and Fc gamma RII (CD32). Crosslinking of Fc gamma RI and Fc gamma RII gives rise to rapid and transient phosphorylation of multiple monocyte intracellular proteins including proteins of 40, 68-72, 75-85, 95, and 115-165 kDa. A 72-kDa protein was earlier identified as the tyrosine kinase Syk. Here we identify one of the proteins in the 115- to 165-kDa cluster as FAK, a protein tyrosine kinase localized to focal adhesions. A 68-kDa phosphoprotein was identified as paxillin, a cytoskeleton associated substrate for tyrosine kinases, and a 95-kDa protein was found to be the proto-oncogene product Vav. The Src family protein tyrosine kinase Fgr (p58) also displayed enhanced tyrosine phosphorylation after Fc gamma RI and Fc gamma RII crosslinking. Although Fc gamma RIIA utilizes tyrosines within its own cytoplasmic domain for signaling while Fc gamma RI utilizes the cytoplasmic tyrosines of its associated gamma subunit, our results indicate sharing of several proteins for signaling in monocytes by these Fc receptors. These molecules include three distinct classes of tyrosine kinases, Syk, FAK, and Fgr, and the functionally diverse proteins Vav and paxillin.

Focal adhesion kinase and its potential involvement in tumor invasion and metastasis

BACKGROUND

Integrins are cell surface receptors which, in part, mediate the adhesion of cells to the extracellular matrix. In addition to providing a molecular "glue" essential for tissue organization and survival, integrins are dynamic signaling molecules. Integrins allow normal, nontransformed cells to sense that they are adhered to the extracellular matrix, thus providing a cell survival signal. This signal allows cells to proliferate in the presence of growth factors and in some instances prevents apoptosis. Integrins also mediate cell migration as it occurs in normal processes such as angiogenesis, wound healing, immune system function, and development. Aberrances in the expression and function of integrins contribute to many disease states including cancer.

RESULTS

Focal adhesion kinase (FAK) becomes phosphorylated and activated during integrin-mediated cell adhesion. Focal adhesion kinase is a signal transducer of integrins (and certain soluble growth factors). Cells derived from FAK -/- mouse embryos exhibit reduced migration relative to wild-type cells. Cells which overexpress FAK show increased migration relative to wild-type cells. Focal adhesion kinase promotes cell survival under certain in vitro conditions. Focal adhesion kinase is overexpressed in invasive and metastatic colon, breast, thyroid, and prostate cancers. Enhanced FAK immunostaining is detected in small populations of preinvasive (carcinoma in situ) oral cancers and in large populations of cells in invasive oral cancers.

CONCLUSIONS

Focal adhesion kinase is probably not a classical oncogene but may be involved in the progression of cancer to invasion and metastasis. It is hypothesized that overexpression of FAK in subpopulations of tumor cells leads to populations of cells with a high propensity toward invasion and metastasis. Focal adhesion kinase would have a dual role in this regard: Overexpression of FAK leads to (1) increased cell migration and (2) increased cell survival under anchorage-independent conditions. Further work is needed to test this model and to determine whether FAK represents a viable target for anticancer therapy.

Decreased tyrosine phosphorylation of focal adhesion kinase after estradiol treatment of MCF-7 human breast carcinoma cells

MCF-7 human breast carcinoma cultures grown in the presence of 17-beta estradiol form solid, multicellular nodules, a process that reflects changes in cell-substrate adhesions and loss of growth inhibition. We examined the effects of estradiol on the status of tyrosine phosphorylation in focal adhesion kinase (FAK) and the association of FAK with paxillin using immunoprecipitations and then probing western blots for FAK, phosphotyrosine, and paxillin. Culture of MCF-7 cells for seven days in the presence of 1 nM E2 resulted in decreased tyrosine phosphorylation of FAK compared to controls. The estradiol-induced effect was blocked by 100 nM of the estrogen receptor antagonist 4-hydroxytamoxifen, indicating dephosphorylation of FAK is an estrogen receptor-mediated event. FAK immunoprecipitated from either estradiol or DMSO-treated cells phosphorylated the exogenous substrate poly(Glu,Tyr), suggesting that the potential kinase activity of FAK was not changed by estradiol. Estradiol treatment also resulted in a reduced association between FAK and paxillin. The decreased phosphorylation levels and reduced association between FAK and paxillin may be important steps leading to the loss of stable focal contacts and loss of growth inhibition during MCF-7 nodulation.

Activation of human endothelial cells via S-endo-1 antigen (CD146) stimulates the tyrosine phosphorylation of focal adhesion kinase p125(FAK)

S-Endo-1 antigen (CD146), a transmembrane receptor also known as MUC18/MCAM, is a member of the immunoglobulin superfamily and belongs to a group of cell adhesion molecules. CD146 is highly expressed on the whole vascular tree. We demonstrate here that engagement of CD146 on human endothelial cells isolated from cord blood results in tyrosine phosphorylation of a large panel of cellular proteins, although no tyrosine phosphorylation of CD146 was detected. In particular, CD146 cross-linking induces the tyrosine phosphorylation of the protein tyrosine kinase p125(FAK) as well as p125(FAK) association with paxillin, both events being inhibited by cytochalasin D. No direct association of CD146 with p125(FAK) was observed. Consistent with these data, CD146 associates with p59(fyn), a Src family kinase known to phosphorylate p125(FAK). The identification of a signaling pathway initiated by CD146 engagement and which includes p59(fyn), p125(FAK), and paxillin indicates that CD146 participates in outside-in signaling in endothelial cells.

Integrin-mediated signaling events in human endothelial cells

Vascular endothelial cells are important in a variety of physiological and pathophysiological processes. The growth and functions of vascular endothelial cells are regulated both by soluble mitogenic and differentiation factors and by interactions with the extracellular matrix; however, relatively little is known about the role of the matrix. In the present study, we investigate whether integrin-mediated anchorage to a substratum coated with the extracellular matrix protein fibronectin regulates growth factor signaling events in human endothelial cells. We show that cell adhesion to fibronectin and growth factor stimulation trigger distinct initial tyrosine phosphorylation events in endothelial cells. Thus, integrin-dependent adhesion of endothelial cells leads to tyrosine phosphorylation of both focal adhesion kinase and paxillin, but not of several growth factor receptors. Conversely, EGF stimulation causes receptor autophosphorylation, with no effect on focal adhesion kinase or paxillin tyrosine phosphorylation. Adhesion to fibronectin, in the absence of growth factors, leads to activation of MAPK. In addition, adhesion to fibronectin also potentiates growth factor signaling to MAPK. Thus, polypeptide growth factor activation of MAPK in anchored cells is far more effective than in cells maintained in suspension. Other agonists known to activate MAPK were also examined for their ability to activate MAPK in an anchorage-dependent manner. The neuropeptide bombesin, the bioactive lipid lysophosphatidic acid (LPA), and the cytokine tumor necrosis factor alpha, which signal through diverse mechanisms, were all able to activate MAPK to a much greater degree in fibronectin-adherent cells than in suspended cells. In addition, tumor necrosis factor alpha activation of c-Jun kinase (JNK) was also much more robust in anchored cells. Together, these data suggest a cooperation between integrins and soluble mitogens in efficient propagation of signals to downstream kinases. This cooperation may contribute to anchorage dependence of mitogenic cell cycle progression.

The urokinase-type plasminogen activator receptor mediates tyrosine phosphorylation of focal adhesion proteins and activation of mitogen-activated protein kinase in cultured endothelial cells

Urokinase-type plasminogen activator (uPA) binds to cells via a specific glycosylphosphatidylinositol-anchored receptor. Although occupancy of the uPA receptor (uPAR) has been shown to alter cellular function and to induce gene expression, the signaling mechanism has not been characterized. Urokinase induced an increase in the tyrosine phosphorylation of multiple proteins in bovine aortic endothelial cells. In contrast, low molecular weight uPA did not induce this response. Analysis by immunoblotting demonstrated tyrosine phosphorylation of focal adhesion kinase (FAK), the focal adhesion-associated proteins paxillin and p130(cas), and mitogen-activated protein kinase (MAPK) following the occupancy of the uPAR by uPA. Treatment of cells with phosphatidylinositol-specific phospholipase C, which cleaves glycosylphosphatidylinositol-linked proteins from the cell surface, blocked the uPA-induced tyrosine phosphorylation of FAK, indicating the requirement of an intact uPAR on the cell surface. The uPA-induced activation of MAPK was completely inhibited by genistein, but not by 4-amino-5-(4-methylphenyl)-7-(t-butyl)pyrazolo[3, 4-d]pyrimidine, a specific inhibitor of Src family kinases. Thus, this study demonstrates a novel role for the uPAR in endothelial cell signal transduction that involves the activation of FAK and MAPK, which are mediated by the receptor-binding domain of uPA. This may have important implications for the mechanism through which uPA influences cell migration and differentiation.

Serine and threonine phosphorylation of the paxillin LIM domains regulates paxillin focal adhesion localization and cell adhesion to fibronectin

We have previously shown that the LIM domains of paxillin operate as the focal adhesion (FA)-targeting motif of this protein. In the current study, we have identified the capacity of paxillin LIM2 and LIM3 to serve as binding sites for, and substrates of serine/threonine kinases. The activities of the LIM2- and LIM3-associated kinases were stimulated after adhesion of CHO.K1 cells to fibronectin; consequently, a role for LIM domain phosphorylation in regulating the subcellular localization of paxillin after adhesion to fibronectin was investigated. An avian paxillin-CHO.K1 model system was used to explore the role of paxillin phosphorylation in paxillin localization to FAs. We found that mutations of paxillin that mimicked LIM domain phosphorylation accelerated fibronectin-induced localization of paxillin to focal contacts. Further, blocking phosphorylation of the LIM domains reduced cell adhesion to fibronectin, whereas constitutive LIM domain phosphorylation significantly increased the capacity of cells to adhere to fibronectin. The potentiation of FA targeting and cell adhesion to fibronectin was specific to LIM domain phosphorylation as mutation of the amino-terminal tyrosine and serine residues of paxillin that are phosphorylated in response to fibronectin adhesion had no effect on the rate of FA localization or cell adhesion. This represents the first demonstration of the regulation of protein localization through LIM domain phosphorylation and suggests a novel mechanism of regulating LIM domain function. Additionally, these results provide the first evidence that paxillin contributes to "inside-out" integrin-mediated signal transduction.

Syk Activation Is Required for Spreading and H2O2 Release in Adherent Human Neutrophils

Chemoattractant-stimulated polymorphonuclear leukocytes (PMNs) that are adherent to extracellular matrix proteins exhibit a massive, sustained respiratory burst that requires cell spreading. However, the signaling pathways culminating in PMN spreading are not well characterized. Studies showing that protein tyrosine phosphorylation increases with PMN spreading suggest that phosphorylation is critical for this process. In the present study, we observed increased tyrosine phosphorylation of both focal adhesion kinase and Syk in FMLP-activated PMNs that had been plated onto fibrinogen; an increase in Syk activity, but not focal adhesion kinase activity, was apparent. The time course of Syk phosphorylation correlated with the initiation of cell spreading and H2O2 release. Pretreatment of PMNs with piceatannol, a Syk-selective inhibitor, blocked Syk activity, cell spreading, and H2O2 release, indicating that Syk activity was required for the activation of adherent PMNs. Paxillin is a cytoskeletally associated protein that is also tyrosine phosphorylated during PMN spreading and H2O2 release. Paxillin phosphorylation is kinetically slower than Syk phosphorylation and is inhibited with piceatannol, suggesting that paxillin is a substrate for Syk. An analysis of Syk immunoprecipitates indicated that Syk and paxillin associate during PMN spreading. This interaction is not mediated by the src kinases Lyn and Fgr, since neither kinase coprecipitated with Syk. Syk from FMLP-activated, adherent PMNs phosphorylated paxillin-glutathione S-transferase, suggesting that paxillin is a substrate for Syk in vivo. These results indicate that PMN spreading and H2O2 release require a Syk-dependent signaling pathway leading to paxillin phosphorylation.

Injury of stromal fibroblasts induces phosphorylation of focal adhesion proteins

PURPOSE

The extracellular matrix serves as a structural support for the corneal stroma and mediates signaling events that regulate the intracellular environment of stromal keratocytes. We hypothesize that adhesion and injury mediate signal transduction events causing the phosphorylation of tyrosine residues of specific adhesion proteins and that phosphorylation is required for cellular adhesion and migration.

METHODS

For the adhesion experiments; primary rabbit stromal fibroblasts were seeded and phosphorylation of tyrosine residues was followed from 1 min to 24 h. For the injury experiments, confluent primary cultures were rendered quiescent, wounded, and tyrosine phosphorylation was followed from 30 s to 6 h. The antibody (py-20) was used to detect proteins phosphorylated on tyrosine residues. We examined changes in the phosphorylation of focal adhesion kinase (FAK), paxillin and cortactin, using immunoprecipitation and Western blot analysis.

RESULTS

In the adhesion experiments, the phosphorylation of a 68-kDa protein was detected after 1 min, and the phosphorylation of a 125-kDa protein was not detected until 15 min. These proteins were identified in re-probed blots as paxillin and FAK. In the injury experiments, FAK phosphorylation was detected within 30 s and remained elevated for 6 h when cells were cultured on fibronectin. Both FAK and paxillin phosphorylation were prominent after injury, but unlike FAK phosphorylation, paxillin phosphorylation decreased over time. Phosphorylation was prominent at the wound margin. After wound closure, it returned to background levels. Tyrosine kinase inhibitors, genistein and herbimycin, decreased the number of adherent cells and altered the rate of cell migration after injury, compared to control (DMSO alone).

CONCLUSION

The results indicate that injury and cell-matrix interaction mediate the phosphorylation of specific adhesion proteins and that phosphorylation is required for wound repair.

Integrins and development: how might these receptors regulate differentiation of the lens

Integrins transduce both internal signals and signals from the matrix. These interactions between integrins, their extracellular matrix ligands, and their cytoskeletal partners play an important role in the regulation of cellular differentiation. We have shown them to be important in lens cell differentiation. In the lens capsule there is a compartmentalization of matrix components with fibronectin, primarily localized to the anterior capsule, and tenascin in the posterior capsule. Integrins are developmentally regulated in the lens. alpha 5 beta 1 integrin, like fibronectin, is primarily associated with the lens epithelial cells, where together they are likely to be important in regulation of adhesion and proliferation. alpha 6A beta 1, the integrin laminin receptor, is expressed at its highest levels in the equatorial epithelium and the peripheral fiber cells, both migratory populations. Because laminin is uniformly distributed in the lens capsule, such changes in alpha 6A integrin expression are likely critical to the cell's ability to regulate its response to laminin in the matrix. The organization of cytoskeletal molecules associated with the integrin cytoplasmic face also changes with development. In the epithelial regions of the lens, where the initiation of lens cell differentiation occurs, expression of the cytoskeletal proteins involved in cell-substrate interactions, talin, alpha-actinin, and the signaling proteins, are high. In the fiber cell region of the lens, where the cells establish stable cell-cell contacts, vinculin predominates and becomes highly associated with the cytoskeletal fraction. The role of integrins in lens development is not only regulated by changes in the expression of different integrin receptors but is also closely correlated with the expression and organization of the molecules with which they associate.

Relationship between paxillin and myosin phosphorylation during muscarinic stimulation of smooth muscle

The tyrosine phosphorylation of paxillin increases in association with force development during tracheal smooth muscle contraction, suggesting that paxillin plays a role in the contractile activation of smooth muscle [Z. L. Wang, F. M. Pavalko, and S. J. Gunst. Am. J. Physiol. 271 (Cell Physiol. 40): C1594-C1602, 1996]. We compared the Ca2+ sensitivity of the tyrosine phosphorylation of paxillin and myosin light chain (MLC) phosphorylation in tracheal muscle and evaluated whether MLC phosphorylation is necessary to induce paxillin phosphorylation. Ca(2+)-depleted muscle strips were stimulated with 10(-7)-10(-4) M acetylcholine (ACh) in 0,0.05, 0.1, or 0.5 mM extracellular Ca2+. In the absence of extracellular Ca2+, 10(-4) M ACh induced a maximal increase in paxillin phosphorylation without increasing MLC phosphorylation or force. Increases in extracellular Ca2+ concentration did not further increase paxillin phosphorylation. However, during stimulation with 10(-6) M ACh, paxillin phosphorylation increased with increases in extracellular Ca2+ concentration. We conclude that the tyrosine phosphorylation of paxillin can be stimulated by signaling pathways that do not depend on Ca2+ mobilization and that the activation of contractile proteins is not required to elicit paxillin phosphorylation.

Ascaris suum: protein phosphotyrosine phosphatases in oocytes and developing stages

Protein tyrosine phosphatases were analyzed in oocytes of Ascaris suum. Phosphatases dephosphorylating modified acidic lysozyme were present in high-molecular-weight form (M(r) > 600,000) and as a 50- to 55-kDa protein in the soluble fraction. The low-molecular-weight form of the phosphatase cross-reacted with an antiserum raised against human T-cell protein tyrosine phosphatase and was not distinguishable from the 50- to 55-kDa protein tyrosine phosphatase previously described in the muscular layer of the adult worms (B. Schmid et al. 1996, Molecular and Biochemical Parasitology 77, 183-192). The low-molecular-weight form was also present on immunoblots of high-molecular-weight protein tyrosine phosphatase preparations after denaturing electrophoresis. The same or a similar form of the tyrosine phosphatase was also found in detergent extracts from the pelletal fraction. In addition, another tyrosine phosphatase of 180 kDa molecular mass that dephosphorylated myelin basic protein was also found in extracts from the soluble compartment as well as in detergent extracts from the pelletal fraction. It showed no cross-reactivity with antisera raised against soluble mammalian phosphatases and was resistant to inhibition by vanadate. While the activities of the myelin basic protein-dephosphorylating protein phosphatase remained fairly constant during early development of the oocytes, the activity of the enzyme dephosphorylating modified lysozyme in the pelletal fraction decreased to less than 10% of the initial activity between days 3 and 28 of incubation. Immunocytochemical studies of unfertilized and developing Ascaris eggs revealed association of protein tyrosine kinase and protein tyrosine phosphatase with the egg shell, in addition to their presence in the neighborhood of mitochondria. The amount of enzyme changed with the stage of development. In the larval stage (21 days) protein tyrosine kinase had increased in the chitin layer of the shell and in the nuclei while the relative amount of tyrosine phosphatase decreased in accordance with the biochemical data.

Cellular phenotypic transformation during early embryogenesis: a role for focal adhesion kinase?

We have used the gastrulating chick embryo as a model for studying the potential role of focal adhesion kinase (FAK) in phenotypic transformation. In the gastrulating embryo, there is a well-defined epithelial to mesenchymal transformation as the upper epithelial epiblast layer of cells ingresses at the primitive streak to form the invasive mesenchymal mesoderm layer and the epithelioid endoderm layer. Immunolocalization showed that FAK was expressed primarily in the apical cytoplasm of the epiblast layer, together with some regions of the mesoderm and endoderm. Hensen's node and the primitive streak, where the transformation occurs, showed very low immunoreactivity. Levels of FAK in these individual tissues were quantified by densitometric analysis of Western blots, and FAK activation was quantified by stripping these blots and reprobing for phosphotyrosine. Immunoprecipitation indicated that the phosphotyrosine bands corresponded with the FAK bands on the blots. Although the blots confirmed that FAK was highly expressed in the epiblast, the level of FAK activation was highest in the endoderm, despite relatively low expression of the protein. Similar quantitative blotting was carried out using cells from each of the three layers cultured on different substrata. The results indicated that cells cultured on fibronectin, laminin, and Matrigel expressed differing levels of FAK, with differing levels of tyrosine phosphorylation, depending on the cell type and the substratum. We conclude that FAK is developmentally regulated during gastrulation, and that this regulation could be influenced by the changing substratum encountered by the differentiating cells during this process. However, the apical localization of FAK in much of the epiblast appears to preclude a consistent focal contact-like association of this molecule with integrins in vivo, and we therefore suggest that in the embryo, FAK may be involved in integrin-mediated signalling pathways without physical association with cell-substratum contacts.Key words: chick, embryo, gastrulation, phenotypic transformation, FAK.

Rapid impact of beta-amyloid on paxillin in a neural cell line

Beta-amyloid1-42 (Abeta) is a naturally occuring peptide whose accumulation in the brain is putatively coupled to Alzheimer's disease pathogenesis. Deleterious effects of Abeta on neurons have been linked to the inappropriate activation of signaling pathways within the cell (reviewed in Yankner, 1996), including tyrosine phosphorylation of focal adhesion kinase (FAK) (Zhang et al., 1994, 1996a,b). Here we have investigated the effects of Abeta on paxillin in a neural cell line. Paxillin, a substrate for FAK, is thought to act as a signal "integrator," functioning to link other proteins into multi-molecular signaling complexes (reviewed in Turner, 1994). Treatment of the rat central nervous system B103 cell line with aggregates of Abeta was found to induce the tyrosine phosphorylation of paxillin within 30 min, nearly 24 hr prior to significant cell death. Particularly striking was a subsequent "mobilization" of paxillin to the cytoskeleton in Abeta-treated cells. The amount of paxillin associated with the cytoskeleton in Abeta-treated cells was increased 10-fold over controls. The Abeta-induced paxillin accumulation could be visualized immunocytochemically, with an increase in number and size of paxillin-labeled focal contacts upon treatment with Abeta. This effect was specific, in that vinculin, another focal contact protein, was unaffected by Abeta. Disruption of f-actin, which inhibits both Abeta-induced neurotoxicity (Furukawa and Mattson, 1995) and focal contact signaling in B103 cells (Zhang et al., 1996b) was found to block the cytoskeletal paxillin accumulation. The rapid tyrosine phosphorylation and cytoskeletal mobilization of paxillin links Abeta to the activation of focal contact signaling events that may influence neuronal cytoskeletal architecture, gene expression, synaptic plasticity and cell viability.

mu-Opioids activate tyrosine kinase focal adhesion kinase and regulate cortical cytoskeleton proteins cortactin and vinculin in chick embryonic neurons

We have investigated the signal transduction pathway of the G-protein mu-opioid receptor upstream of phospholipase D (PLD) and protein kinase C-epsilon (PKC-epsilon) activation in postmitotic E6CH chick embryo cortical neurons. The mu-opioid receptor and PLD-PKC-epsilon functional coupling depends on upstream tyrosine kinase activation. We now report that the mu-opioid agonists specifically stimulated tyrosine phosphorylation and activation of the focal adhesion kinase (FAK) in a time-dependent manner. We also demonstrate that met-enkephalin, a mu-opioid agonist in E6CH cultures, significantly increases tyrosine phosphorylation of another Src kinase substrate, the cytoskeletal protein cortactin. Tyrosine phosphorylation of cortactin led to drastic changes in subcellular localization, an estimated 2-fold enrichment in the cytosol. Similarly, opioids stimulated a sustained tyrosine phosphorylation of vinculin, a protein enriched in focal adhesion sites. These data provide novel evidence that opioid receptor intracellular signaling engages the specific activation of tyrosine kinase FAK and regulates the neuronal cytoskeleton during central nervous system morphogenesis.

Role of focal adhesion kinase in integrin signaling

Integrins are the major cell surface receptors for extracellular matrix molecules, which play critical roles in a variety of biological processes. Focal adhesion kinase has recently been established as a key component of the signal transduction pathways triggered by integrins. Aggregation of FAK with integrins and cytoskeletal proteins in focal contacts has been proposed to be responsible for FAK activation and autophosphorylation by integrins in cell adhesion. This may be achieved by FAK interaction with talin or other cytoskeletal proteins that in turn associate with the cytoplasmic domain of integrin beta subunits. Autophosphorylation of FAK at Y397 leads to its association with Src, resulting in activation of both kinases. The activated FAK/Src complex acts on potential substrates tensin, paxillin and p130cas. Besides cytoskeletal regulation, FAK phosphorylation and/or binding to paxillin and p130cas may trigger downstream activation of MAP kinase by the adoptor protein Crk. Src association with FAK may also lead to its phosphorylation of other sites on FAK, including a binding site for Grb2. Cell adhesion-dependent association of FAK and Grb2 may provide a mechanism by which MAP kinase is activated in cell adhesion. PI 3-kinase has also been shown to bind FAK in a cell adhesion-dependent manner at the major autophosphorylation site Y397. This association could lead to activation of PI 3-kinase and its downstream effectors. Recent results from a number of different approaches have shown that integrin signaling through FAK leads to increased cell migration on fibronectin as well as potentially regulating cell proliferation and survival.