Integrin-mediated signal transduction linked to Ras pathway by GRB2 binding to focal adhesion kinase

Focal adhesion kinase pp125FAK is associated with both intercellular junctions and matrix adhesion sites in vivo

Previous studies have characterized pp125FAK as a focal adhesion (FA)-associated non-receptor tyrosine kinase. However, there are few data available on the expression and localization of this kinase in tissues. In this study we show that in human tissues the highest expression of pp125FAK is found in some developing epithelia, where pp125FAK is associated with either intercellular junctions or with sites of adhesion to the basement membrane, whereas the same adult tissues show only a faint reactivity. Connective tissue cells do not show any reactivity for pp125FAK in vivo, but developing arterial smooth muscle expresses pp125FAK at high levels. The expression pattern in malignant tissues is variable, but most carcinomas do not express this kinase. In primary cultures of human amnion epithelial cells pp125FAK first becomes associated with the polarized adhesion lamellae, but is subsequently translocated to the forming adherens junctions (AJs). Later upon culturing pp125FAK becomes associated with prominent FAs, as in cultured cell lines. Taken together, our results suggest that the association of pp125FAK with FAs in cultured cells is principally due to a process of adaptation, whereas in vivo pp125FAK mainly functions as a regulatory component of intercellular AJs and cell-matrix adhesions of developing epithelia and also in developing arterial smooth muscle.

Cytoskeletal integrity is required throughout the mitogen stimulation phase of the cell cycle and mediates the anchorage-dependent expression of cyclin D1

The proliferation of many nontransformed cells depends on cell adhesion. We report here that disrupting the cytoskeleton in normal human fibroblasts causes the same cell cycle phenotype that is observed after blocking cell adhesion: suspended cells and cytochalasin D-treated monolayers fail to progress through G1 despite normal mitogen-induced expression of c-myc mRNA. Midway between G0 and the beginning of S-phase, cell cycle progression becomes independent of adhesion and the cytoskeleton. At this stage, the cells are also mitogen independent. Molecular analyses showed that Rb hyperphosphorylation and the induction of cyclin D1 occur slightly earlier than the transition to cytoskeleton independence. Moreover, these molecular events are blocked by cytochalasin D. Overall, our data indicate the following: 1) anchorage and cytoskeletal integrity are required throughout the mitogen-dependent part of G1; 2) mitogens and the cytoskeleton jointly regulate the phosphorylation of Rb; and 3) this interdependence is manifest in the regulation of cyclin D1.

Beta 1 integrin-dependent and -independent polymerization of fibronectin

The mouse cell line GD25, which lacks expression of the beta 1 family of integrin heterodimers due to disruption of the beta 1 integrin subunit gene, was used for expression of full-length cDNA coding for splice variant A of the mouse beta 1 integrin subunit. In a stably transformed clone (GD25-beta 1A), the expressed protein was found to form functional heterodimeric receptors together with the subunits alpha 3, alpha 5, and alpha 6. Both GD25 and GD25-beta 1A attached to fibronectin and formed focal contacts which contained alpha v beta 3, but no detectable alpha 5 beta 1A. The presence of GRGDS peptide allowed alpha 5 beta 1A to locate to focal contacts of GD25-beta 1A cultured on fibronectin, while the beta 1-null GD25 cells were unable to attach under these conditions. Affinity chromatography revealed that alpha 5 beta 1A and alpha v beta 3 could bind to a large cell-binding fragment of fibronectin. alpha 5 beta 1A strongly promoted polymerization of fibronectin into a fibrillar network on top of the cells. Whereas little alpha v beta 3 was colocalized with the fibronectin fibrils in GD25-beta 1A cells, this integrin was able to support fibronectin fibril polymerization in GD25 cells. However, the alpha v beta 3-induced polymerization was less efficient and occurred mainly in dense cultures of the GD25 cells. Thus, while both alpha 5 beta 1A and alpha v beta 3 are able to support adhesion to fibronectin, alpha v beta 3 dominates in the formation of focal contacts, and alpha 5 beta 1A has a prime function in fibronectin matrix assembly. This is the first report on fibronectin matrix assembly in the absence of beta 1 integrins.

CRKL links p210BCR/ABL with paxillin in chronic myelogenous leukemia cells

The Philadelphia chromosome translocation generates a chimeric oncogene, BCR/ABL, which causes chronic myelogenous leukemia (CML). In primary neutrophils from patients with CML, the major novel tyrosine-phosphorylated protein is CRKL, an SH2-SH3-SH3 linker protein which has an overall homology of 60% to CRK, the human homologue of the v-crk oncogene product. Anti-CRKL immunoprecipitates from CML cells, but not normal cells, were found to contain p210BCR/ABL and c-ABL. Several other phosphoproteins were also detected in anti-CRKL immunoprecipitates, one of which has been identified as paxillin, a 68-kDa focal adhesion protein which we have previously shown to be phosphorylated by p210BCR/ABL. Using GST-CRKL fusion proteins, the SH3 domains of CRKL were found to bind c-ABL and p210BCR/ABL, while the SH2 domain of CRKL bound to paxillin, suggesting that CRKL could physically link p210BCR/ABL to paxillin. Paxillin contains three tyrosines in Tyr-X-X-Pro (Y-X-X-P) motifs consistent with amino acid sequences predicted to be optimal for binding to the CRKL-SH2 domain (at positions Tyr-31, Tyr-118, and Tyr-181). Each of these tyrosine residues was mutated to a phenylalanine residue, and in vitro binding assays indicated that paxillin tyrosines at positions 31 and 118, but not 181, are likely to be involved in CRKL-SH2 binding. These results suggest that the p210BCR/ABL oncogene may be physically linked to the focal adhesion-associated protein paxillin in hematopoietic cells by CRKL. This interaction could contribute to the known adhesive defects of CML cells.

The assembly of integrin adhesion complexes requires both extracellular matrix and intracellular rho/rac GTPases

Interaction of cells with extracellular matrix via integrin adhesion receptors plays an important role in a wide range of cellular: functions, for example cell growth, movement, and differentiation. Upon interaction with substrate, integrins cluster and associate with a variety of cytoplasmic proteins to form focal complexes and with the actin cytoskeleton. Although the intracellular signals induced by integrins are at present undefined, it is thought that they are mediated by proteins recruited to the focal complexes. It has been suggested, for example, that after recruitment to focal adhesions p125FAK can activate the ERK1/2 MAP kinase cascade. We have previously reported that members of the rho family of small GTPases can trigger the assembly of focal complexes when activated in cells. Using microinjection techniques, we have now examined the role of the extracellular matrix and of the two GTP-binding proteins, rac and rho, in the assembly of integrin complexes in both mouse and human fibroblasts. We find that the interaction of integrins with extracellular matrix alone is not sufficient to induce integrin clustering and focal complex formation. Similarly, activation of rho or rac by extracellular growth factors does not lead to focal complex formation in the absence of matrix. Focal complexes are only assembled in the presence of both matrix and functionally active members of the rho family. In agreement with this, the interaction of integrins with matrix in the absence of rho/rac activity is unable to activate the ERK1/2 kinases in Swiss 3T3 cells. In fact, ERK1/2 can be activated fully by growth factors in the absence of matrix and it seems unlikely, therefore, that the adhesion dependence of fibroblast growth is mediated through the ras/MAP kinase pathway. We conclude that extracellular matrix is not sufficient to trigger focal complex assembly and subsequent integrin-dependent signal transduction in the absence of functionally active members of the rho family of GTPases.

Integrin signaling: roles for the cytoplasmic tails of alpha IIb beta 3 in the tyrosine phosphorylation of pp125FAK

pp125FAK (focal adhesion kinase) a protein tyrosine kinase that may mediate cellular responses to adhesion, is activated and tyrosine-phosphorylated when platelets adhere to fibrinogen via the integrin, alpha IIb beta 3. To determine whether either of the cytoplasmic tails of alpha IIb beta 3 regulates FAK phosphorylation, CHO cells were stably transfected with alpha IIb beta 3 or various cytoplasmic tail truncation mutants. Cells expressing wild-type alpha IIb beta 3 or alpha IIb beta 3 that lacked the COOH-terminal 13 or 18 residues of the 20 residue alpha IIb tail adhered to and spread on fibrinogen or on an anti-alpha IIb antibody, and FAK became tyrosine-phosphorylated. FAK also became phosphorylated in adherent cells lacking the COOH-terminal 35 or 39 residues of the 47 residue beta 3 tail, although the extent of phosphorylation was reduced by about 50% in the latter mutant. Little or no FAK phosphorylation was observed if 46 residues were deleted from the beta 3 tail. None of these beta 3 truncation mutants spread on the anti-alpha IIb antibody. When cells with wild-type alpha IIb beta 3 or truncated beta 3 were detached from a surface, FAK became rapidly dephosphorylated. In contrast, FAK remained phosphorylated in the two alpha IIb truncation mutants for up to 90 minutes in suspension. This persistent phosphorylation was not due to occupancy of alpha IIb beta 3 by adhesive ligands because it was also observed with an alpha IIb tail truncation mutant that contained an additional mutation in the extracellular portion of the receptor that prevents ligand binding. These studies demonstrate that: (1) the beta 3 cytoplasmic tail, including the membrane-proximal portion, is involved in initiation of FAK phosphorylation; (2) FAK phosphorylation can be initiated by cell adhesion in the absence of cell spreading; and (3) the membrane-distal portion of the alpha IIb cytoplasmic tail may normally function to dampen FAK phosphorylation in non-anchored cells.

Evidence that farnesyltransferase inhibitors suppress Ras transformation by interfering with Rho activity

Small-molecule inhibitors of the housekeeping enzyme farnesyltransferase (FT) suppress the malignant growth of Ras-transformed cells. Previous work suggested that the activity of these compounds reflected effects on actin stress fiber regulation rather than Ras inhibition. Rho proteins regulate stress fiber formation, and one member of this family, RhoB, is farnesylated in vivo. Therefore, we tested the hypothesis that interference with RhoB was the principal basis by which the peptidomimetic FT inhibitor L-739,749 suppressed Ras transformation. The half-life of RhoB was found to be approximately 2 h, supporting the possibility that it could be functionally depleted within the 18-h period required by L-739,749 to induce reversion. Cell treatment with L-739,749 disrupted the vesicular localization of RhoB but did not effect the localization of the closely related RhoA protein. Ras-transformed Rat1 cells ectopically expressing N-myristylated forms of RhoB (Myr-rhoB), whose vesicular localization was unaffected by L-739,749, were resistant to drug treatment. The protective effect of Myr-rhoB required the integrity of the RhoB effector domain and was not due to a gain-of-function effect of myristylation on cell growth. In contrast, Rat1 cells transformed by a myristylated Ras construct remained susceptible to growth inhibition by L-739,749. We concluded that Rho is necessary for Ras transformation and that FT inhibitors suppress the transformed phenotype at least in part by direct or indirect interference with Rho, possibly with RhoB itself.

T cell receptor- and beta 1 integrin-mediated signals synergize to induce tyrosine phosphorylation of focal adhesion kinase (pp125FAK) in human T cells

The beta 1 subfamily of integrins is thought to play an important role in both the adhesion/migration and proliferation/differentiation of T cells. beta 1 integrins can provide T cell costimulation through interaction of very late antigen (VLA) 4 (VLA-4) (alpha 4 beta 1) and VLA-5 (alpha 5 beta 1) with the extracellular matrix protein fibronectin (FN), or by VLA-4 binding to its cell surface ligand, vascular cell adhesion molecule (VCAM) 1. The mechanism by which beta 1 integrin members transduce T cell-costimulatory signals is poorly understood. Studies in non-T cells have demonstrated regulation of the tyrosine focal adhesion kinase pp125FAK by beta 1 integrin engagement and, most recently, indicate a role for pp125FAK in linking integrin-mediated signal transduction to the Ras pathway (Schaller, M. D., and J. T. Parsons, 1994, Curr. Opin. Cell. Biol. 6: 705-710; Schlaepfer, D. D., S. K. Hanks, T. Hunter, and P. Van der Geer. 1994. Nature (Lond.), 372:786-790). Although pp125FAK kinase occurs in T cells, there are no reports on its regulation in this cell type. The studies described in this article characterize novel regulation of pp125FAK by the T cell receptor (TCR)-CD3 antigen complex and beta 1 integrins, and provide the first account, in any cell type, of integrin alpha 4 beta 1-mediated pp125FAK tyrosine phosphorylation. We demonstrate a rapid and sustained synergistic increase in tyrosine phosphorylation of human pp125FAK in Jurkat T cells after simultaneous (a) triggering of the TCR-CD3 complex, and (b) alpha 4 beta 1 and alpha 5 beta 1 integrin-mediated binding of these cells to immobilized FN or alpha 4 beta 1 integrin-mediated binding to immobilized VCAM-1. Studies with normal peripheral blood-derived CD4+ human T blasts confirm the synergistic action of a TCR-CD3 complex-mediated costimulus with a FN- or VCAM-1-dependent signal in the induction of T cell pp125FAK tyrosine phosphorylation. In vitro kinase assays performed on pp125FAK immunoprecipitates isolated from Jurkat cells and normal CD4+ T cells identified a coprecipitating 57-kD tyrosine-phosphorylated protein (pp57), distinct from pp59fyn or pp56lck. These results indicate, for the first time, the involvement of a specific kinase, pp125FAK, in alpha 4 beta 1- and alpha 5 beta 1-mediated T cell-costimulatory signaling pathways. In addition, the data demonstrate novel regulation of pp125FAK tyrosine phosphorylation by the TCR-CD3 complex.

c-Cbl is inducibly tyrosine-phosphorylated by epidermal growth factor stimulation in fibroblasts, and constitutively tyrosine-phosphorylated and associated with v-Src in v-src-transformed fibroblasts

The c-cbl gene was cloned as the cellular homolog of the v-cbl oncogene that is the transforming component of a murine tumorigenic retrovirus, CAS NS-1, though the biological roles of c-Cbl remain to be elucidated. We have previously reported that c-Cbl is implicated in the signal transduction triggered by granulocyte-macrophage colony-stimulating factor or erythropoietin in hematopoietic cells. Here, we observed tyrosine phosphorylation of C-cbl in cells expressing epidermal growth factor receptor depending on EGF stimulation and in v-src transformed cells. Furthermore, c-Cbl was revealed to associate with v-Src in vivo. By means of binding experiments using glutathione S-transferase fusion proteins, we have found that the SH2 and SH3 domains of many proteins bind to c-Cbl. These findings strongly suggest that c-Cbl is implicated in a wide variety of signal transduction pathways, including those of EGF receptor and Src protein, as well as in the signaling pathways of hematopoietic cells.

Calcium influx induces neurite growth through a Src-Ras signaling cassette

We find that calcium influx through voltage-dependent calcium channels causes extensive neurite outgrowth in PC12 cells. The calcium signal transduction pathway promoting neurite outgrowth causes the rapid activation of protein tyrosine kinases, which include Src. Protein tyrosine phosphorylation results in the formation of an Shc/Grb2 complex, leading to Ras activation, MAP kinase activation, and the subsequent induction of the immediate early gene NGFI-A. Protein tyrosine phosphorylation, gene induction, and neurite outgrowth are inhibited by the expression of dominant negative forms of both Src and Ras, indicating a requirement for both proto-oncoproteins in calcium signaling. Our results suggest that a signaling cassette which includes Src and Ras is likely to underlie a broad range of calcium of actions in the nervous system.

Angiotensin II activates at least two tyrosine kinases in rat liver epithelial cells. Separation of the major calcium-regulated tyrosine kinase from p125FAK

In rat liver epithelial cell lines (WB or GN4), angiotensin II (Ang II) stimulates cytosolic tyrosine kinase activity, in part, through a calcium-dependent mechanism. In other cell types, selected hormones that activate Gi- or Gq-coupled receptors stimulate the soluble tyrosine kinase, p125FAK. Immunoprecipitation of p125FAK from Ang II-activated GN4 cells demonstrated a doubling of p125FAK kinase activity. However, an additional Ang II-activated tyrosine kinase (or kinases) representing the majority of the total activity was detected when the remaining cell lysate, immunodepleted of p125FAK, was reimmunoprecipitated with an anti-phosphotyrosine antibody. Cytochalasin D pretreatment blocks G-protein receptor-dependent tyrosine phosphorylation in Swiss 3T3 cells. While cytochalasin D decreased the Tyr(P) content of 65-75-kDa substrates in Ang II-treated GN4 cells, it did not diminish tyrosine phosphorylation of 115-130-kDa substrates, again suggesting activation of at least two tyrosine kinase pathways in GN4 cells. To search for additional Ang II-activated enzymes, we used molecular techniques to identify 20 tyrosine kinase sequences in these cell lines. None was the major cytosolic enzyme activated by Ang II. Specifically, JAK2, which had been shown by others to be stimulated by Ang II in smooth muscle cells, was not activated by Ang II in GN4 cells. Finally, we purified Tyr(P)-containing tyrosine kinases from Ang II-treated cells, using anti-Tyr(P) and ATP affinity resins; 80% of the tyrosine kinase activity migrated as a single 115-120-kDa tyrosine-phosphorylated protein immunologically distinct from p125FAK. In summary, Ang II activates at least two separate tyrosine kinases in rat liver epithelial cells; p125FAK and a presumably novel, cytosolic 115-120-kDa protein referred to as the calcium-dependent tyrosine kinase.

Interaction between focal adhesion kinase and Crk-associated tyrosine kinase substrate p130Cas

The focal adhesion kinase (FAK) has been implicated in integrin-mediated signaling events and in the mechanism of cell transformation by the v-Src and v-Crk oncoproteins. To gain further insight into FAK signaling pathways, we used a two-hybrid screen to identify proteins that interact with mouse FAK. The screen identified two proteins that interact with FAK via their Src homology 3 (SH3) domains: a v-Crk-associated tyrosine kinase substrate (Cas), p130Cas, and a still uncharacterized protein, FIPSH3-2, which contains an SH3 domain closely related to that of p130Cas. These SH3 domains bind to the same proline-rich region of FAK (APPKPSR) encompassing residues 711-717. The mouse p130Cas amino acid sequence was deduced from cDNA clones, revealing an overall high degree of similarity to the recently reported rat sequence. Coimmunoprecipitation experiments confirmed that p130Cas and FAK are associated in mouse fibroblasts. The stable interaction between p130Cas and FAK emerges as a likely key element in integrin-mediated signal transduction and further represents a direct molecular link between the v-Src and v-Crk oncoproteins. The Src family kinase Fyn, whose Src homology 2 (SH2) domain binds to the major FAK autophosphorylation site (tyrosine 397), was also identified in the two-hybrid screen.

Beta 3-endonexin, a novel polypeptide that interacts specifically with the cytoplasmic tail of the integrin beta 3 subunit

The adhesive and signaling functions of integrins are regulated through their cytoplasmic domains. We identified a novel 111 residue polypeptide, designated beta 3-endonexin, that interacted with the cytoplasmic tail of the beta 3 integrin subunit in a yeast two-hybrid system. This interaction is structurally specific, since it was reduced by 64% by a point mutation in the beta 3 cytoplasmic tail (S752-->P) that disrupts integrin signaling. Moreover, this interaction is integrin subunit specific since it was not observed with the cytoplasmic tails of the alpha IIb, beta 1, or beta 2 subunits. beta 3-Endonexin fusion proteins bound selectively to detergent-solubilized beta 3 from platelets and human umbilical vein endothelial cells, and beta 3-endonexin mRNA and protein were detected in platelets and other tissues. A related mRNA encoded a larger polypeptide that failed to bind to beta integrin tails. The apparent specificity of beta 3-endonexin for the beta 3 integrin subunit suggests potential mechanisms for selective modulation of integrin functions.

Integrin function: molecular hierarchies of cytoskeletal and signaling molecules

Integrin receptors play important roles in organizing the actin-containing cytoskeleton and in signal transduction from the extracellular matrix. The initial steps in integrin function can be analyzed experimentally using beads coated with ligands or anti-integrin antibodies to trigger rapid focal transmembrane responses. A hierarchy of transmembrane actions was identified in this study. Simple integrin aggregation triggered localized transmembrane accumulation of 20 signal transduction molecules, including RhoA, Rac1, Ras, Raf, MEK, ERK, and JNK. In contrast, out of eight cytoskeletal molecules tested, only tensin coaccumulated. Integrin aggregation alone was also sufficient to induce rapid activation of the JNK pathway, with kinetics of activation different from those of ERK. The tyrosine kinase inhibitors herbimycin A or genistein blocked both the accumulation of 19 out of 20 signal transduction molecules and JNK- and ERK-mediated signaling. Cytochalasin D had identical effects, whereas three other tyrosine kinase inhibitors did not. The sole exception among signaling molecules was the kinase pp125FAK which continued to coaggregate with alpha 5 beta 1 integrins even in the presence of these inhibitors. Tyrosine kinase inhibition also failed to block the ability of ligand occupancy plus integrin aggregation to trigger transmembrane accumulation of the three cytoskeletal molecules talin, alpha-actinin, and vinculin; these molecules accumulated even in the presence of cytochalasin D. However, it was necessary to fulfill all four conditions, i.e., integrin aggregation, integrin occupancy, tyrosine kinase activity, and actin cytoskeletal integrity, to achieve integrin-mediated focal accumulation of other cytoskeletal molecules including F-actin and paxillin. Integrins therefore mediate a transmembrane hierarchy of molecular responses.

Sphingosylphosphorylcholine activation of mitogen-activated protein kinase in Swiss 3T3 cells requires protein kinase C and a pertussis toxin-sensitive G protein

Sphingosylphosphorylcholine (SPC) is a potent mitogen for Swiss 3T3 cells, but the signaling mechanisms involved are poorly characterized. Here, we report that addition of SPC induces a rapid and transient activation of p42 mitogen-activated protein kinase (p42MAPK) in these cells. SPC-induced p42MAPK activation peaked at 5 min and was undetectable after 30 min of incubation with SPC. The effect of SPC on p42MAPK activation was comparable to that induced by bombesin and platelet-derived growth factor. As SPC strongly induced phosphorylation of the major protein kinase C (PKC) substrate 80K/MARCKS in either intact or permeabilized cells, we examined whether PKC could be involved in SPC-induced p42MAPK activation. Here, we demonstrate that p42MAPK activation by SPC was dependent on PKC activity as shown by inhibition of PKC with the bisindolymaleimide GF 109203X or down-regulation of PKC by prolonged treatment of Swiss 3T3 cells with phorbol esters. Activation of both PKC and p42MAPK by SPC was markedly inhibited by treatment with pertussis toxin, implicating a G proteins(s) of the Gi/G(o) subfamily in the action of SPC. SPC-induced rapid activation of a downstream target of p42MAPK, p90 ribosomal S6 kinase (p90rsk), also required PKC and a pertussis toxin-sensitive G protein. In addition, SPC-induced mitogenesis was dependent on a Gi protein in Swiss 3T3 cells. SPC also induced p42MAPK activation and DNA synthesis in secondary cultures of mouse embryo fibroblasts through a pertussis toxin-sensitive pathway. As G proteins link many cell surface receptors to effector proteins, we hypothesize, therefore, that SPC could bind to a receptor that mediates at least some of its biological effects in Swiss 3T3 cells and mouse embryo fibroblasts.

Sphingosylphosphorylcholine rapidly induces tyrosine phosphorylation of p125FAK and paxillin, rearrangement of the actin cytoskeleton and focal contact assembly. Requirement of p21rho in the signaling pathway

Sphingosylphosphorylcholine (SPC), a potent mitogen for Swiss 3T3 cells, rapidly induced tyrosine phosphorylation of multiple substrates including bands of M(r) 110,000-130,000 and M(r) 70,000-80,000 in Swiss 3T3 cells. Focal adhesion kinase (p125FAK) and paxillin were identified as prominent substrates for SPC-stimulated tyrosine phosphorylation. An increase in tyrosine phosphorylation of p125FAK was detected as soon as 30 s after SPC stimulation, reaching a maximum after 2.5 min. SPC induced tyrosine phosphorylation of p125FAK in a concentration-dependent fashion; a half-maximum effect occurred at 250 nM. Tyrosine phosphorylation of p125FAK induced by SPC could be dissociated from both protein kinase C activation and Ca2+ mobilization from intracellular stores. SPC induced a unique pattern of reorganization of the actin cytoskeleton with a rapid appearance of actin microspikes at the plasma membrane that was followed by the formation of actin stress fibers. This pattern of cytoskeletal changes was clearly distinguishable from that induced by bombesin and 1-oleoyl-lysophosphatidic acid. Formation of microspikes and actin stress fibers were accompanied by striking assembly of focal adhesion plaques. Cytochalasin D, which disrupts the network of actin microfilaments, completely prevented SPC-induced tyrosine phosphorylation of p125FAK. In addition, tyrosine phosphorylation of p125FAK was markedly inhibited in the presence of platelet-derived growth factor at a concentration (30 ng/ml) that disrupts actin stress fibers. Finally, microinjection of Clostridium botulinum C3 exoenzyme, which inactivates p21rho, prevented SPC-induced formation of actin stress fibers, focal adhesion assembly, and tyrosine phosphorylation. Thus, p21rho is upstream of both cytoskeletal reorganization and tyrosine phosphorylation in SPC-treated cells.

Reduced cell motility and enhanced focal adhesion contact formation in cells from FAK-deficient mice

The intracellular protein tyrosine kinase FAK (focal adhesion kinase) was originally identified gy its high level of tyrosine phosphorylation in v-src-transformed cells. FAK is also highly phosphorylated during early development. In cultured cells it is localized to focal adhesion contacts and becomes phosphorylated and activated in response to integrin-mediated binding of cells to the extracellular matrix, suggesting an important role in cell adhesion and/or migration. We have generated FAK-deficient mice by gene targeting to examine the role of FAK during development. Mutant embryos displayed a general defect of mesoderm development, and cells from these embryos had reduced mobility in vitro. Surprisingly, the number of focal adhesions was increased in FAK-deficient cells, suggesting that FAK may be involved in the turnover of focal adhesion contacts during cell migration.

Possible role of protein kinase C in the regulation of intracellular stability of focal adhesion kinase in mouse 3T3 cells

Effects of various types of protein kinase inhibitor on the adhesion and spreading of BALB/c mouse 3T3 cells and on the phosphorylation and stability of focal adhesion kinase (FAK) in the cells were studied. Inhibitors of protein tyrosine kinases, methyl 2,5-dihydroxycinnamate and herbimycin A, inhibited tyrosine-phosphorylation of FAK and the adhesion of 3T3 cells to fibronectin. Among inhibitors of serine/threonine kinases tested, calphostin C, a specific inhibitor of protein kinase C, inhibited cell spreading rather than cell adhesion, and it induced the decrease of intracellular FAK within 30 min. Inhibitors of tyrosine kinase, A kinase, G kinase, and myosin light chain kinase did not induce such a rapid and specific decrease of FAK. When calphostin C (20 microM) was added to sub-confluent monolayer cultures, serine-phosphorylation of FAK was inhibited by 67% within 2 h, and decrease in the amount of FAK and rounding up of the cells began after 4 h. Label-chase experiments indicated that about 60% of 35S-labeled FAK degraded within 1-2 h after addition of calphostin C to monolayer cultures. These results indicated that serine-phosphorylation of FAK induced by protein kinase C was important in the regulation of metabolic stability of FAK.

Shc and a novel 89-kDa component couple to the Grb2-Sos complex in fibroblast growth factor-2-stimulated cells

A major pathway for mitogenicity is gated via the small GTP-binding protein Ras. Receptor tyrosine kinases couple to Ras through the Src homology 2 (SH2) domain protein Grb2. The activated fibroblast growth factor receptor-1 (FGFR-1) expressed in L6 myoblasts did not bind Grb2 directly, but indirectly, through the small adaptor protein Shc, which was tyrosine-phosphorylated in response to fibroblast growth factor-2 (FGF-2) stimulation. A FGFR-1 mutant in which Tyr766, a known autophosphorylation site, was changed to Phe, mediated less efficient tyrosine phosphorylation of Shc. FGF-2 stimulation of mutant FGFR-1-expressing cells still allowed formation of complexes containing Shc, Grb2, and the nucleotide exchange factor Sos and mediation of a mitogenic signal. Another pool of Grb2 was found in complex with a tyrosine-phosphorylated 89-kDa component after FGF-2 stimulation. Stimulation with other growth factors did not lead to tyrosine phosphorylation of p89. As shown by "far-Western" analysis, p89 bound directly to the Grb2 SH2 domain, and this interaction was inhibited by a peptide containing the Y(P)-X-N motif. Tyrosine-phosphorylated p89 was found exclusively in the membrane fraction, indicating its role in bringing Grb2, as well as Sos, to the plasma membrane. These data support the concept of growth factor-specific coupling of Grb2 to the Ras pathway.

Beta 1 integrin expression in malignant melanoma predicts occult lymph node metastases

BACKGROUND

Elective lymph node dissection for malignant melanoma is still controversial. Experimental studies suggest that differential expression, activation, or both of beta1 integrins facilitate melanoma metastases. However, the clinical significance of beta1 integrin expression in human melanoma is unclear.

METHODS

We examined primary cutaneous melanomas from 76 patients undergoing elective lymph mode dissection. We quantified the percentage of tumor area stained by beta1 integrin antibody with an image analyzer.

RESULTS

beta1 integrin was expressed in all 23 primary tumors from patients with pathologically positive lymph nodes (LNs) but in only 14 (26%) of 53 cases with pathologically negative nodes (p < 0.001). No patients with beta1 integrin-negative tumors had LN involvement, whereas 23 (62%) of 37 patients with beta1 integrin-positive tumors had LN metastases (p < 0.001). Furthermore, 21 (91%) of 23 cases with LN metastases but only 4 (8%) of 53 cases without had beta1 integrin staining of 10% or more of tumor area (p < 0.001).

CONCLUSIONS

Our study is the first to show a correlation between expression of a molecular marker in the primary cutaneous melanoma and likelihood of regional LN metastases. beta1 immunostaining of 10% or more of tumor area reliably predicts patients most likely to harbor occult LN metastases and likely to benefit from ELND.

Convergence of integrin and growth factor receptor signaling pathways within the focal adhesion complex

Extracellular matrix controls capillary endothelial cell sensitivity to soluble mitogens by binding integrin receptors and thereby activating a chemical signaling response that rapidly integrates with growth factor-induced signaling mechanisms. Here we report that in addition to integrins, growth factor receptors and multiple molecules that transduce signals conveyed by both types of receptors are immobilized on the cytoskeleton (CSK) and spatially integrated within the focal adhesion complex (FAC) at the site of integrin binding. FACs were rapidly induced in round cells and physically isolated from the remainder of the CSK after detergent-extraction using magnetic microbeads coated with fibronectin or a synthetic RGD-containing peptide. Immunofluorescence microscopy revealed that multiple signaling molecules (e.g., pp60c-src, pp125FAK, phosphatidylinositol-3-kinase, phospholipase C-gamma, and Na+/H+ antiporter) involved in both integrin and growth factor receptor signaling pathways became associated with the CSK framework of the FAC within 15 min after binding to beads coated with integrin ligands. Recruitment of tyrosine kinases to the FAC was also accompanied by a local increase in tyrosine phosphorylation, as indicated by enhanced phosphotyrosine staining at the site of integrin binding. In contrast, neither recruitment of signaling molecules nor increased phosphotyrosine staining was observed when cells bound to beads coated with a control ligand (acetylated low density lipoprotein) that ligates transmembrane scavenger receptors, but does not induce FAC formation. Western blot analysis confirmed that FACs isolated using RGD-beads were enriched for pp60c-src, pp125FAK, phospholipase C-gamma, and the Na+/H+ antiporter when compared with intact CSK or basal cell surface preparations that retained lipid bilayer. Isolated FACs were also greatly enriched for the high affinity fibroblast growth factor receptor flg. Most importantly, isolated FACs continued to exhibit multiple chemical signaling activities in vitro, including protein tyrosine kinase activities (pp60c-src and pp125FAK) as well as the ability to undergo multiple sequential steps in the inositol lipid synthesis cascade. These data suggest that many of the chemical signaling events that are induced by integrins and growth factor receptors in capillary cells may effectively function in a "solid-state" on insoluble CSK scaffolds within the FAC and that the FAC may represent a major site for signal integration between these two regulatory pathways. Future investigations into the biochemical and biophysical basis of signal transduction may be facilitated by this method, which results in isolation of FACs that retain the CSK framework as well as multiple associated chemical signaling activities.

Direct association of pp125FAK with paxillin, the focal adhesion-targeting mechanism of pp125FAK

Focal adhesion kinase (pp125FAK) is localized to focal adhesions and tyrosine phosphorylated by the engagement of beta 1 integrins. However, it is unclear how pp125FAK is linked to integrin molecules. We demonstrate that pp125FAK is directly associated with paxillin, a 68-kD cytoskeleton protein. The COOH-terminal domain of pp125FAK spanning FAK residues 919-1042 is sufficient for paxillin binding and has vinculin-homologous amino acids, which are essential for paxillin binding. Microinjection and subsequent immunohistochemical analysis reveal that glutathione S-transferase-FAK fusion proteins, which bind to paxillin, localize to focal adhesions, whereas fusion proteins with no paxillin-binding activity do not localize to focal adhesions. These findings strongly suggest that pp125FAK is localized to focal adhesions by the direct association with paxillin.

Beta 1 integrins signal lipid second messengers required during cell adhesion

Clustering of integrin receptors during cell adhesion stimulates signal transduction across the cell membrane. Second messengers are generated, activating cytosolic proteins and causing cytoskeletal assembly and rearrangement. HeLa cell adhesion to a collagen substrate has been shown to initiate an arachidonic acid-mediated signaling pathway, leading to the activation of protein kinase C (PKC) and cell spreading. To determine the role of integrin receptors in triggering this signaling pathway, monoclonal antibodies to beta 1 integrins were used to either cluster integrins on the cell surface or to provide an integrin-dependent substrate for cell adhesion. Using this approach, we have defined a pathway required for cell spreading that can be initiated by the ligation of integrins and leads to the activation of PKC. Specifically, our results indicate that clustering beta 1 integrins results in the activation of phospholipase A2 leading to the production of arachidonic acid and the activation of PKC.

Ins and outs of LFA-1

Leukocyte function-associated molecule 1 (LFA-1) is an integrin that plays a major role in the immune system. Recent findings demonstrate that LFA-1 has a two-way signaling function, mediating cell adhesion and stimulating intracellular processes at the same time. Here, Marijke Lub, Yvette van Kooyk and Carl Figdor discuss the 'inside-out' and 'outside-in' signaling properties of LFA-1, as a prototype leukocyte integrin, in normal and malignant T cells. They integrate data into a model that highlights the role of the cytoskeleton in the regulation of LFA-1.

Increased phosphorylation of focal adhesion kinase in diabetic rat kidney glomeruli

Altered extracellular matrix production by the glomerular mesangium is a feature of diabetes mellitus. Matrix proteins, including fibronectin, via interaction with cell-surface receptors (the integrins) may activate intracellular pathways such as prostaglandin production, shown previously to be stimulated by addition of fibronectin to glomerular cores. However, the signalling pathways involved are unclear. An intracellular tyrosine kinase (focal adhesion kinase), associated with focal adhesions, is known to be phosphorylated after interaction with matrix proteins. We now show for the first time, in glomeruli from diabetic rats, that focal adhesion kinase has increased phosphorylation on tyrosine, when compared with non-diabetic control rats. This phosphorylation was labile and disappeared with extended time of sample preparation or digestion of glomeruli to glomerular cores. Cultured mesangial cells, from non-diabetic rats, plated onto fibronectin also showed increased tyrosine phosphorylation of focal adhesion kinase accompanied by a twofold increase in prostaglandin production. However, it may not be possible to replicate fully the diabetic ¿state¿ in vitro merely by use of raised glucose concentrations, as these conditions (for 3 weeks) resulted in decreased focal adhesion kinase phosphorylation, despite increased fibronectin and prostaglandin levels. A role for increased focal adhesion kinase phosphorylation in kidney glomeruli isolated from diabetic rats, and any linkage to intracellular signalling pathways remains to be determined.

A hierarchy of ECM-mediated signalling regulates tissue-specific gene expression

A dynamic and reciprocal flow of information between cells and the extracellular matrix contributes significantly to the regulation of form and function in developing systems. Signals generated by the extracellular matrix do not act in isolation. Instead, they are processed within the context of global signalling hierarchies whose constituent inputs and outputs are constantly modulated by all the factors present in the cell's surrounding microenvironment. This is particularly evident in the mammary gland, where the construction and subsequent destruction of such a hierarchy regulates changes in tissue-specific gene expression, morphogenesis and apoptosis during each developmental cycle of pregnancy, lactation and involution.

Activation of the p21 pathway of growth arrest and apoptosis by the beta 4 integrin cytoplasmic domain

The integrin alpha 6 beta 4, a receptor for members of the laminin family of basement membrane components, contributes to the function of epithelial cells and their oncogenically transformed derivatives. In our efforts to study alpha 6 beta 4-mediated functions in more detail and to assess the contribution of the beta 4 cytoplasmic domain in such functions, we identified a rectal carcinoma cell line that lacks expression of the beta 4 integrin subunit. This cell line, termed RKO, expresses alpha 6 beta 1 but not alpha 6 beta 4, and it interacts with laminin-1 less avidly than similar cell lines that express alpha 6 beta 4. We expressed a full-length beta 4 cDNA, as well as a mutant cDNA that lacks the beta 4 cytoplasmic domain, in RKO cells and isolated stable subclones of these transfectants. In this study, we report that subclones that expressed the full-length beta 4 cDNA in association with endogenous alpha 6 exhibited partial G1 arrest and apoptosis, properties that were not evident in RKO cells transfected with either the cytoplasmic domain mutant or the expression vector alone. In an effort to define a mechanism for these observed changes in growth, we observed that expression of the alpha 6 beta 4 integrin induced expression of the p21 (WAF1; CiP1) protein, an inhibitor of cyclin-dependent kinases. These data suggest that the beta 4 integrin cytoplasmic domain is linked to a signaling pathway involved in cell cycle regulation in the beta 4 transfected RKO cells.

Tyrosine phosphorylation of p130Cas and cortactin accompanies integrin-mediated cell adhesion to extracellular matrix

We show in this report that two v-src substrate proteins, p130Cas and cortactin, become tyrosine-phosphorylated during integrin-mediated cell adhesion to extracellular matrix substrata and upon cell attachment onto immobilized anti-integrin antibodies. This tyrosine phosphorylation does not occur when cells attach to polylysine or through antibodies against major histocompatibility complex. It also does not take place when adhesion-mediated reorganization of the actin cytoskeleton is inhibited with cytochalasin D. Tyrosine phosphorylation of p130Cas and cortactin coincides with tyrosine phosphorylation of focal adhesion kinase during integrin-mediated cell adhesion but is independent of cell adhesion in v-src-transformed cells. The tyrosine-phosphorylated sites in p130Cas and cortactin may serve as binding sites for proteins containing Src homology 2 domains, as is the case with two other integrin-regulated docking proteins, focal adhesion kinase and paxillin. Thus, these results suggest that ligand binding of integrins regulates the tyrosine phosphorylation state of multiple docking proteins. These proteins may mediate anchorage dependence of growth; their misregulation in v-src-transformed and other tumorigenic cells may be responsible for the anchorage independence of such cells.

Stat5 as a target for regulation by extracellular matrix

Transcription of tissue-specific genes in mammary gland requires signals from both prolactin and basement membrane. Here we address the mechanism by which this specialized extracellular matrix regulates transcription. Using mammary cell cultures derived from transgenic mice harboring the ovine beta-lactoglobulin gene, we show that either a basement membrane extract, or purified laminin-1, induced high levels of beta-lactoglobulin synthesis. It is known that prolactin signals through Stat5 (signal transducer and activator of transcription). This transcription factor interacts with gamma-interferon activation site-related motifs within the beta-lactoglobulin promoter, which we show are required for matrix dependence of beta-lactoglobulin expression. The DNA binding activity of Stat5 was present only in extracts of mammary cells cultured on basement membrane, indicating that the activation state of Stat5 is regulated by the type of substratum the cell encounters. Thus, basement membrane controls transcription of milk protein genes through the Stat5-mediated prolactin signaling pathway, providing a molecular explanation for previous studies implicating extracellular matrix in the control of mammary differentiation.

Association of mitogen-activated protein kinase with the microtubule cytoskeleton

Using indirect immunofluorescence microscopy and biochemical techniques, we have determined that approximately one-third of the total mitogen-activated protein kinase (MAPK) is associated with the microtubule cytoskeleton in NIH 3T3 mouse fibroblasts. This population of enzyme can be separated from the soluble form that is found distributed throughout the cytosol and is also present in the nucleus after mitogen stimulation. The microtubule-associated enzyme pool constitutes half of all detectable MAPK activity after mitogenic stimulation. These findings extend the known in vivo associations of MAPK with microtubules to include the entire microtubule cytoskeleton of proliferating cells, and they suggest that a direct association of MAPK with microtubules may be in part responsible for the observed correlations between MAPK activities and cytoskeletal alteration.

Cloning and characterization of cell adhesion kinase beta, a novel protein-tyrosine kinase of the focal adhesion kinase subfamily

A second protein-tyrosine kinase (PTK) of the focal adhesion kinase (FAK) subfamily, cell adhesion kinase beta (CAK beta), was identified by cDNA cloning. The rat CAK beta is a 115.7-kDa PTK that contains N- and C-terminal domains of 418 and 330 amino acid residues besides the central kinase domain. The rat CAK beta has a homology with mouse FAK over their entire lengths except for the extreme N-terminal 88 residues and shares 45% overall sequence identity (60% identical in the catalytic domain), which indicates that CAK beta is a protein structurally related to but different from FAK. The CAK beta gene is less evenly expressed in a variety of rat organs than the FAK gene. Anti-CAK beta antibody immunoprecipitated a 113-kDa protein from rat brain, 3Y1 fibroblasts, and COS-7 cells transfected with CAK beta cDNA. The tyrosine-phosphorylated state of CAK beta was not reduced on trypsinization, nor enhanced in response to plating 3Y1 cells onto fibronectin. CAK beta localized to sites of cell-to-cell contact in COS-7 transfected with CAK beta cDNA, in which FAK was found at the bottom of the cells. Thus, CAK beta is a PTK possibly participating in the signal transduction regulated by cell-to-cell contacts.

Exogenous human immunodeficiency virus type-1 Tat protein selectively stimulates a phosphatidylinositol-specific phospholipase C nuclear pathway in the Jurkat T cell line

We investigated the effect of extracellular Tat protein of human immunodeficiency virus-type 1 (HIV-1) on the phosphatidylinositol (PI) cycle, which represents a major signal transduction pathway in lymphoid cells. Recombinant Tat, recombinant HIV-1 p24 and cross-linked anti-CD3 monoclonal antibody (mAb) were added in culture for 1-60 min to Jurkat lymphoblastoid CD4+ T cells. The stimulation of T cell receptor by cross-linked anti-CD3 mAb resulted in a rapid increase of the phosphatidylinositol-specific phospholipase C (PI-PLC) activity in whole cell lysates. On the other hand, Tat protein, either alone or in combination with anti-CD3 mAb, showed little effect on the PI turnover of whole cell extracts. Tat, however, selectively stimulated a nuclear-specific PI-PLC with a peak of activity after 30 min from the addition in culture to Jurkat cells. Interestingly, this time corresponded to that required for the uptake and nuclear localization of recombinant Tat protein, as demonstrated by electron microscope immunocytochemistry experiments with anti-Tat mAb. Moreover, exogenous Tat reached the nucleus of Jurkat cells in a bioactive form, as shown in a HIV-1 long terminal repeat-chloramphenicol acetyl transferase transactivation assay. The specific increase of a nuclear PI-PLC activity was further demonstrated by the ability of Tat to stimulate PI turnover also when added directly to isolated nuclei. As a whole, these data demonstrate that Tat selectively stimulates a nuclear polyphosphoinositide hydrolysis, which appears to be independent of the cellular PI turnover. The relevance of these findings for a better understanding of the biological functions of extracellular Tat is discussed.

Macrophage response to microbial pathogens: modulation of the expression of adhesion, CD14, and MHC class II molecules by viruses, bacteria, protozoa and fungi

The ability of inactivated viruses, bacteria, protozoa and fungi to modulate the expression of CD14, CD49d, CD49f, CD11a (LFA-1), and CD54 (ICAM-1) molecules in unprimed bone marrow-derived mononuclear phagocytes (BMM phi) was investigated by means of flow cytometry. Incubation with bacterial agents resulted in the large majority of experimental situations in enhanced expression of these macrophage surface molecules. In contrast, viruses and fungi down-regulated the expression of several adhesion receptors, especially integrins. Amplification of MHC class II expression triggered in macrophages by interferon gamma was clearly inhibited by viruses, bacteria, protozoa and fungi. The findings explain earlier results showing that, under the same experimental conditions, bacterial agents are, for the most part, potent stimulators of secretory and cell-mediated macrophage activities while viruses, protozoa and fungi are poor in this respect.

Growth factor regulation of integrin-mediated cell motility

Cell motility, a primary component of tumor cell invasion, is a continuum of sequential events in which the cell extends pseudopodia, forms nascent attachments, assembles and contracts the cytoskeleton, and finally, as it translocates forward, disengages distal adhesions. What triggers cells to move? Substratum contact mediated by integrin adhesion receptors is important, but other signals such as chemokinetic factors appear to be required for continued crawling. It is now apparent that integrins do not simply bind cells to matrix in a Velcro-like fashion, but also are potent signaling molecules. Initial engagement of integrins induces their condensation into focal contacts, forming anchors to the extracellular matrix and discrete signal-transducing complexes on the cytoplasmic surface. A number of growth factors, through either autocrine or paracrine pathways, can activate the cellular machinery that mobilizes the cell. Thus, these two classes of receptors--the integrin receptors that bind specific extracellular adhesion molecules, and growth factor receptors that bind their respective ligands--can regulate cell locomotion. Not surprisingly, there is 'cross-talk' between integrin and growth factor receptors that occurs through their common intracellular signaling pathways. In this way, each receptor type can either amplify or attenuate the other's signal and downstream response. An example of growth factor-induced motility is the epithelial-mesenchymal transition induced by hepatocyte growth factor/scatter factor (HGF/SF). When bound to its receptor, the c-met proto-oncogene product, HGF/SF induces a phenotypic conversion that appears to be an important aspect of tumor progression in malignant carcinomas. The motogenic response produced by HGF/SF in carcinoma cells occurs in discrete steps in which integrins and focal adhesion kinase (p125FAK) are first recruited to focal contacts. This is rapidly followed by cell spreading, disruption of focal adhesions and cell-cell contacts, and, finally, cell crawling. The precise mechanism by which growth factors such as HGF/SF and its receptor induce this motogenic response and modulate integrin function has not been clearly defined but appears to involve several signaling pathways. Understanding the process by which growth factor and integrin receptors interact and regulate motility may suggest novel targets for therapeutic intervention.

The nonreceptor protein-tyrosine kinase CSK complexes directly with the GTPase-activating protein-associated p62 protein in cells expressing v-Src or activated c-Src

CSK is a predominantly cytosolic protein-tyrosine kinase (PTK) that negatively regulates Src family PTKs by phosphorylation of a conserved tyrosine near their C termini. Little is known about how CSK itself is regulated. On the basis of immunofluorescence studies, a model has been proposed that when c-Src is activated, it is redistributed to podosomes, in which substrates become phosphorylated, creating binding sites for CSK. CSK is recruited to these sites of c-Src activation via its SH2 and SH3 domains and is then in a position to downregulate c-Src activity (B. W. Howell and J. A. Cooper, Mol. Cell. Biol. 14:5402-5411, 1994). To identify phosphotyrosine (P.Tyr)-containing proteins that may mediate translocation of CSK due to c-Src activation, we have examined the whole spectrum of P.Tyr-containing proteins that associate with CSK in v-Src NIH 3T3 cells by anti-P.Tyr immunoblotting. Nine P.Tyr-containing proteins coimmunoprecipitated with CSK from v-Src NIH 3T3 cells. One of these, an approximately 62-kDa protein, also associated with CSK in NIH 3T3 cells treated with vanadate prior to lysis and in NIH 3T3 cells expressing an activated c-Src mutant. This 62-kDa protein was shown to be identical to the GTPase-activating protein (GAP)-associated p62 (GAP-A.p62) protein. The interaction between CSK and GAP-A.p62 could be reconstituted in vitro with glutathione S-transferase fusion proteins containing full-length CSK or the CSK SH2 domain. Furthermore, our data show that CSK interacts directly with GAP.A-p62 and that the complex between the two proteins is localized in subcellular membrane or cytoskeletal fractions. Our results suggest that GAP-A.p62 may function as a docking protein and may mediate translocation of proteins, including GAP and CSK, to membrane or cytoskeletal regions upon c-Src activation.

Fibronectin and integrins in invasion and metastasis

The adhesive glycoprotein fibronectin and integrin receptors appear to play important roles in the progression of metastatic disease. Fibronectin is a multifunctional extracellular glycoprotein that has at lest two independent cell adhesion regions with different receptor specificities. The cell adhesive region in the central portion of fibronectin is comprised of at least two minimal amino acid sequences--an Arg-Gly-Asp (RGD) sequence and a Pro-His-Ser-Arg-Asn (PHSRN) sequence--which function in synergy. Another cell adhesive region is located near the carboxy-terminus in the alternatively spliced IIICS module. The critical minimal sequences for this region Leu-Asp-Val (LDV) and Arg-Glu-Asp-Val (REDV) which function in an additive rather than synergistic fashion. Integrins are heterodimeric, transmembrane cell adhesion receptors for fibronectin and other extracellular matrix molecules. Several different integrins bind to fibronectin. The alpha 5 beta 1 fibronectin-specific integrin binds to the central RGD/PHSRN site. The alpha 4 beta 1 integrin binds to the IIICS site. Fibronectin-integrin interactions are important in tumor cell migration, invasion, and metastasis. In addition to promoting cell adhesion to the extracellular matrix, these proteins may also function in chemotaxis and control of proliferation. Peptide and antibody inhibitors of fibronectin and integrin functions have been shown to be effective inhibitors of metastasis, and are potentially important reagents for the study and control of cancer.

Protein tyrosine kinase PYK2 involved in Ca(2+)-induced regulation of ion channel and MAP kinase functions

The protein tyrosine kinase PYK2, which is highly expressed in the central nervous system, is rapidly phosphorylated on tyrosine residues in response to various stimuli that elevate the intracellular calcium concentration, as well as by protein kinase C activation. Activation of PYK2 leads to modulation of ion channel function and activation of the MAP kinase signalling pathway. PYK2 activation may provide a mechanism for a variety of short- and long-term calcium-dependent signalling events in the nervous system.

Focal adhesion kinase in the brain: novel subcellular localization and specific regulation by Fyn tyrosine kinase in mutant mice

Signaling by tyrosine kinases is required for the induction of synaptic plasticity in the central nervous system. Comparison of fyn, src, yes, and abl nonreceptor tyrosine kinase mutant mice shows a specific requirement for Fyn in the induction of long-term potentiation at CA1 synapses in the hippocampus. To identify components of a Fyn-dependent pathway that may be involved with hippocampus function we examined tyrosine-phosphorylated proteins in kinase mutant mice. We found that nine proteins were hypophosphorylated specifically in fyn mutants. One of the hypophosphorylated proteins was focal adhesion tyrosine kinase (FAK). FAK also showed reduced activity in immunocomplex kinase assays only in fyn mutants. FAK is expressed at very high levels in the brain but in contrast to non-neural cells, FAK was not restricted to focal adhesion contacts. FAK was found in axons, dendrites, and the intermediate filament cytoskeleton of astrocytes. Brain extracts from the mutants also show specific patterns of compensatory changes in the activity of the remaining Src family kinases. Tyrosine phosphorylation is a critical regulator of FAK, and impairments in FAK signal transduction in fyn mutants may contribute to the mutant neural phenotype.

Assembly and function of integrin receptors is dependent on opposing alpha and beta cytoplasmic domains

The membrane proximal regions of integrin alpha and beta subunits are highly conserved in evolution. In particular, all integrin alpha subunits share the KXGFFKR sequence at the beginning of their cytoplasmic domains. Previous work has shown that this domain is important in integrin receptor assembly. Using chimeric integrin alpha and beta subunits, we show that the native cytoplasmic domains of both subunits must be present for efficient assembly. Most strikingly, chimeric alpha 1 and beta 1 subunits with reciprocally swapped intracellular domains dimerize selectively into collagen IV receptors expressed at high levels on the surface. However, these receptors, which bind ligand efficiently, are deficient in a variety of post-ligand binding events, including cytoskeletal association and induction of tyrosine phosphorylation. Furthermore, deletion of the distal alpha cytoplasmic domain in the swapped heterodimers leads to ligand-independent focal contact localization, which also occurs in wild-type subunits when the distal cytoplasmic domain is deleted. These results show that proper integrin assembly requires opposed alpha and beta cytoplasmic domains, and this opposition prevents ligand-independent focal contact localization. Our working hypothesis is that these two domains may associate during receptor assembly and provide the mechanism for integrin receptor latency.

Transformation and pp60v-src autophosphorylation correlate with SHC-GRB2 complex formation in rat and chicken cells expressing host-range and kinase-active, transformation-defective alleles of v-src

The biochemical properties of several pp60v-src substrates believed to participate in src-mediated transformation were examined in cells expressing a kinase-active, transformation-defective v-src allele (v-src-F172 delta/Y416F) and its parental allele, v-src-F172 delta, a host-range--dependent allele that transforms chicken cells to a fusiform morphology, but does not transform rat cells. Because pp60v-src-F172 delta is dependent on autophosphorylation for transforming ability, these alleles provide a unique opportunity to examine the role of pp60v-src autophosphorylation in regulating substrate interactions. Increased pp125FAK tyrosine phosphorylation and high levels of pp60v-src-associated phosphotidylinositol-3' kinase activity were detected specifically in chicken cells exhibiting round, refractile transformation but not in cells transformed to a fusiform morphology. Increased pp125FAK kinase activity, but not increased pp125FAK tyrosine-phosphorylation correlated with pp60v-src autophosphorylation and increased anchorage-independent growth. Thus, pp125FAK and PI3'K may participate in morphological transformation by v-src. Furthermore, association of phosphorylated SHC with the adapter GRB2 correlated with increased anchorage-independent growth (and autophosphorylation) in both rat and chicken cells independent of the morphological phenotype induced. Therefore, host-range dependence for transformation may be regulated through association of SHC with GRB2, thus implicating SHC as a crucial substrate for src-dependent transformation.

The Grb2 adaptor

Grb2 is an 'adaptor' protein made of one SH2 and two SH3 domains. The SH3 domains bind to prolinerich motifs in the C-terminal part of the ras exchange factor Sos. Binding of the Grb2 SH2 domain to phosphotyrosine motifs on receptors, or other adaptor proteins such as Shc, recruits this Grb2/Sos complex at the plasma membrane where Sos stimulates nucleotide exchange on ras, then ras activates raf and leads to MAP kinase activation. The structure of Grb2, the precise motifs recognised by its SH2 and SH3 domains, the way Grb2 performs its function, a possible regulation of its association with Sos, and its ability to complex with other proteins in vivo, are discussed.

Focal adhesion kinase in rat central nervous system

Focal adhesion kinase (pp125FAK, FAK) is a 125 kDa non-receptor tyrosine kinase enriched in focal adhesions of various cell types, where it is thought to transduce signals triggered by contact with the extracellular matrix. We have studied the expression and localization of FAK in rat CNS. Immunoblotting, immunohistochemistry and in situ hybridization revealed the presence of FAK in all regions of the adult brain and demonstrated its enrichment in specific neuronal populations of the cerebral and cerebellar cortex, as well as in the hippocampus. During development, FAK protein levels were highest around birth in cerebral cortex and caudate putamen and decreased in the adult. In situ hybridization revealed enrichment of FAK mRNA in the ventricular germinative and external layers during the last period of embryonic growth. In primary cultures FAK immunoreactivity was localized in focal adhesions in astrocytes, whereas in developing neurons the highest levels were found in growth cones and perikarya. In the growth cone, FAK immunoreactivity colocalized with actin filaments. In mature neurons FAK appeared to be distributed in the whole cytoplasm, with no enrichment in any cellular compartment. Our results demonstrate the presence of high levels of FAK in rat CNS, maximal during development but persistent in the adult. Its enrichment in growth cones suggests that it may play a role in neurite outgrowth, as well as in plasticity in the adult.

The cytoplasmic tyrosine kinase FER is associated with the catenin-like substrate pp120 and is activated by growth factors

The FER gene encodes a cytoplasmic tyrosine kinase with a single SH2 domain and an extensive amino terminus. In order to understand the cellular function of the FER kinase, we analyzed the effect of growth factor stimulation on the phosphorylation and activity of FER. Stimulation of A431 cells and 3T3 fibroblasts with epidermal growth factor or platelet-derived growth factor results in the phosphorylation of FER and two associated polypeptides. The associated polypeptides were shown to be the epidermal growth factor receptor or the platelet-derived growth factor receptor and a previously identified target, pp120. Since pp120 had previously been shown to interact with components of the cadherin-catenin complex, these results implicate FER in the regulation of cell-cell interactions. The physical association of FER with pp120 was found to be constitutive and was mediated by a 400-amino-acid sequence in the amino terminus of FER. Analyses of that sequence revealed that it has the ability to form coiled coils and that it oligomerizes in vitro. The identification of a coiled coil sequence in the FER kinase and the demonstration that the sequence mediates association with a potential substrate suggest a novel mechanism for signal transduction by cytoplasmic tyrosine kinases.

Signal transduction by cell adhesion receptors

Over the last few years, it has become clear that cell adhesion receptors function in signal transduction processes leading to the regulation of cell growth and differentiation. Signal transduction by both integrins and CAMs has been shown to involve activation of tyrosine kinases, while CAM signaling in neural cells involves G proteins as well. In the case of integrins, some of the downstream signaling events intersect with the Ras pathway, particularly the activation of MAP kinases. In fibroblasts, integrin mediated anchorage to the substratum regulates cell cycle traverse, while in epithelial cells, loss of anchorage can trigger programmed cell death. In many cell types, but particularly monocytic cells, integrin ligation has a profound impact on gene expression. Preliminary evidence also implicates CAMs and selectins in gene regulation. A consistent theme in signal transduction mediated by adhesion receptors concerns the role of the cytoskeleton. Integrin mediated signaling processes are interrupted by cytoskeletal disassembly. Identification of the APC and neurofibromatosis type 2 tumor suppressors suggest that cytoskeletal complexes also play a key role in signaling by cadherins and CD44, respectively. Thus, signaling by cell adhesion receptors may involve aspects that impinge on previously known signaling pathways including the RTK/Ras pathway and serpentine receptor/G protein pathways. However, novel aspects of signal transduction involving cytoskeletal assemblies may also be critical.

The regulation of tyrosine kinase signalling pathways by growth factor and G-protein-coupled receptors

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Overexpression of the hyaluronan receptor RHAMM is transforming and is also required for H-ras transformation

Overexpression of the RHAMM gene by transfection into fibroblasts is transforming and causes spontaneous metastases in the lung. H-ras-transformed fibrosarcomas transfected with a dominant suppressor mutant of RHAMM exhibit a so-called revertant phenotype and are completely nontumorigenic and nonmetastatic. Conversely, fibroblasts stably expressing low levels of RHAMM as a result of antisense transfection are resistant to ras transformation. Collectively, these results indicate that RHAMM acts downstream of ras. The loss of functional RHAMM ablates signaling within focal adhesions, in particular changes in focal adhesion kinase phosphorylation, and as a result these focal adhesions are unable to turn over in response to hyaluronan. These results provide evidence of the oncogenic potential of a novel extracellular matrix receptor and establish a functional link between transformation by ras and signaling within focal adhesions that are required for transformation by this oncogene.