Inhibition of focal adhesion kinase (FAK) signaling in focal adhesions decreases cell motility and proliferation

Regulation of somitogenesis by Ena/VASP proteins and FAK during Xenopus development

The metameric organization of the vertebrate body plan is established during somitogenesis as somite pairs sequentially form along the anteroposterior axis. Coordinated regulation of cell shape, motility and adhesion are crucial for directing the morphological segmentation of somites. We show that members of the Ena/VASP family of actin regulatory proteins are required for somitogenesis in Xenopus. Xenopus Ena (Xena) localizes to the cell periphery in the presomitic mesoderm (PSM), and is enriched at intersomitic junctions and at myotendinous junctions in somites and the myotome, where it co-localizes with β1-integrin, vinculin and FAK. Inhibition of Ena/VASP function with dominant-negative mutants results in abnormal somite formation that correlates with later defects in intermyotomal junctions. Neutralization of Ena/VASP activity disrupts cell rearrangements during somite rotation and leads to defects in the fibronectin (FN) matrix surrounding somites. Furthermore, inhibition of Ena/VASP function impairs FN matrix assembly, spreading of somitic cells on FN and autophosphorylation of FAK, suggesting a role for Ena/VASP proteins in the modulation of integrin-mediated processes. We also show that inhibition of FAK results in defects in somite formation, blocks FN matrix deposition and alters Xena localization. Together, these results provide evidence that Ena/VASP proteins and FAK are required for somite formation in Xenopus and support the idea that Ena/VASP and FAK function in a common pathway to regulate integrin-dependent migration and adhesion during somitogenesis.

mda-9/Syntenin: a positive regulator of melanoma metastasis

Metastasis is a significant event in cancer progression and continues to pose the greatest challenge for a cancer cure. Defining genes that control metastasis in vivo may provide new targets for intervening in this process with profound therapeutic implications. Melanoma differentiation associated gene-9 (mda-9) was initially identified by subtraction hybridization as a novel gene displaying biphasic expression during terminal differentiation in human melanoma cells. Mda-9, also known as syntenin, is a PDZ-domain protein overexpressed in many types of human cancers, where it is believed to function in tumor progression. However, a functional role of mda-9/syntenin in tumor growth and metastasis and the signaling pathways involved in mediating these biological activities remain to be defined. Evidence is now provided, using weakly and highly metastatic isogenic melanoma variants, that mda-9/syntenin regulates metastasis. Expression of mda-9/syntenin correlates with advanced stages of melanoma progression. Regulating mda-9/syntenin expression using a replication-incompetent adenovirus expressing either sense or antisense mda-9/syntenin modifies the transformed phenotype and alters metastatic ability in immortal human melanocytes and metastatic melanoma cells in vitro and in vivo in newborn rats. A direct relationship is observed between mda-9/syntenin expression and increased phosphorylation of focal adhesion kinase, c-Jun-NH2-kinase, and p38. This study provides the first direct link between mda-9/syntenin expression and tumor cell dissemination in vivo and indicates that mda-9/syntenin expression activates specific signal transduction pathways, which may regulate melanoma tumor progression. Based on its ability to directly alter metastasis, mda-9/syntenin provides a promising new focus for melanoma cancer research with potential therapeutic applications for metastatic diseases.

Integrin-dependent signal transduction regulating cyclin D1 expression and G1 phase cell cycle progression

Integrins and growth factor receptors coordinately regulate proliferation in nontransformed cells. Coordinate signaling from these receptors controls the activation of the G1 phase cyclin-dependent kinases, largely by regulating levels of cyclin D1 and p27(kip1). Induction of cyclin D1 is one of the best understood examples of an integrin/growth factor receptor-regulated G1 phase target. This review focuses on the integrin-dependent signal transduction events that regulate the expression of cyclin D1 during G1 phase.

Adhesion and proliferation of fibroblasts on RF plasma-deposited nanostructured fluorocarbon coatings: Evidence of FAK activation

We used combined plasma-deposition process to deposit smooth and nanostructured fluorocarbon coatings on polyethylenethereftalate (PET) substrates, to obtain surfaces with identical chemical composition and different roughness, and investigate the effect of surface nanostructures on adhesion and proliferation of 3T3 Swiss Albino Mouse fibroblasts. Untreated PET and polystyrene (PS) were used as controls for cell culture. We have found that the statistically significant increase of cell proliferation rate and FAK (a nonreceptor tyrosine kinase) activation detected on ROUGH fluorocarbon surfaces is due to the presence of nanostructures. Changes in cytoskeletal organization and phospho FAK (tyr 397) localization were evident after 60 min on cells adhering to ROUGH surfaces. This change was characterized by the formation of actin stress fibers along lamellar membrane protrusion instead of usual focal contacts. Also the morphology of the adhering fibroblasts (60 min) adhering on ROUGH surfaces was found quite different compared to cells adhering on smooth ones.

Mechanotransduction in endothelial cell migration

The migration of endothelial cells (ECs) plays an important role in vascular remodeling and regeneration. EC migration can be regulated by different mechanisms such as chemotaxis, haptotaxis, and mechanotaxis. This review will focus on fluid shear stress-induced mechanotransduction during EC migration. EC migration and mechanotransduction can be modulated by cytoskeleton, cell surface receptors such as integrins and proteoglycans, the chemical and physical properties of extracellular matrix (ECM) and cell-cell adhesions. The shear stress applied on the luminal surface of ECs can be sensed by cell membrane and associated receptor and transmitted throughout the cell to cell-ECM adhesions and cell-cell adhesions. As a result, shear stress induces directional migration of ECs by promoting lamellipodial protrusion and the formation of focal adhesions (FAs) at the front in the flow direction and the disassembly of FAs at the rear. Persistent EC migration in the flow direction can be driven by polarized activation of signaling molecules and the positive feedback loops constituted by Rho GTPases, cytoskeleton, and FAs at the leading edge. Furthermore, shear stress-induced EC migration can overcome the haptotaxis of ECs. Given the hemodynamic environment of the vascular system, mechanotransduction during EC migration has a significant impact on vascular development, angiogenesis, and vascular wound healing.

Elevated Src activity promotes cellular invasion and motility in tamoxifen resistant breast cancer cells

Src kinase plays a central role in growth factor signalling, regulating a diverse array of cellular functions including proliferation, migration and invasion. Recent studies have demonstrated that Src activity is frequently elevated in human tumours and correlates with disease stage. We have previously demonstrated that, upon acquisition of tamoxifen resistance, MCF7 cells display increased epidermal growth factor receptor (EGFR) activation and a more aggressive phenotype in vitro. Since tumours exhibiting elevated EGFR signalling may possess elevated levels of Src activity, we wished to investigate the role of Src in our MCF7 model of endocrine resistance. Src kinase activity was significantly elevated in tamoxifen-resistant (TamR) cells in comparison to wild type MCF7 cells. This increase was not due to elevated Src protein or gene expression. Treatment of TamR cells with the novel Src inhibitor, AZD0530, significantly reduced the amount of activated Src detectable in both cell types whilst having no effect on total Src levels. AZD0530 significantly suppressed the motile and invasive nature of TamR cells in vitro, reduced basal levels of activated focal adhesion kinase (FAK) and paxillin and promoted elongation of focal adhesions. Furthermore, the use of this compound in conjunction with the EGFR inhibitor, gefitinib, was markedly additive towards inhibition of TamR cell motility and invasion. These observations suggest that Src plays a pivotal role in mediating the motile and invasive phenotype observed in endocrine-resistant breast cancer cells. The use of Src inhibitors in conjunction with EGFR inhibitors such as gefitinib may provide an effective method with which to prevent cancer progression and metastasis.

Regulation of reactive oxygen species-induced endothelial cell-cell and cell-matrix contacts by focal adhesion kinase and adherens junction proteins

Oxidants, generated by activated neutrophils, have been implicated in the pathophysiology of vascular disorders and lung injury; however, mechanisms of oxidant-mediated endothelial barrier dysfunction are unclear. Here, we have investigated the role of focal adhesion kinase (FAK) in regulating hydrogen peroxide (H2O2)-mediated tyrosine phosphorylation of intercellular adhesion proteins and barrier function in endothelium. Treatment of bovine pulmonary artery endothelial cells (BPAECs) with H2O2increased tyrosine phosphorylation of FAK, paxillin, β-catenin, and vascular endothelial (VE)-cadherin and decreased transendothelial electrical resistance (TER), an index of cell-cell adhesion and/or cell-matrix adhesion. To study the role of FAK in H2O2-induced TER changes, BPAECs were transfected with vector or FAK wild-type or FAK-related non-kinase (FRNK) plasmids. Overexpression of FRNK reduced FAK expression and attenuated H2O2-mediated tyrosine phosphorylation of FAK, paxillin, β-catenin, and VE-cadherin and cell-cell adhesion. Additionally, FRNK prevented H2O2-induced distribution of FAK, paxillin, β-catenin, or VE-cadherin toward focal adhesions and cell-cell adhesions but not actin stress fiber formation. These results suggest that activation of FAK by H2O2is an important event in oxidant-mediated VE barrier function regulated by cell-cell and cell-matrix contacts.

Molecular basis of the effects of shear stress on vascular endothelial cells

Blood vessels are constantly exposed to hemodynamic forces in the form of cyclic stretch and shear stress due to the pulsatile nature of blood pressure and flow. Endothelial cells (ECs) are subjected to the shear stress resulting from blood flow and are able to convert mechanical stimuli into intracellular signals that affect cellular functions, e.g., proliferation, apoptosis, migration, permeability, and remodeling, as well as gene expression. The ECs use multiple sensing mechanisms to detect changes in mechanical forces, leading to the activation of signaling networks. The cytoskeleton provides a structural framework for the EC to transmit mechanical forces between its luminal, abluminal and junctional surfaces and its interior, including the cytoplasm, the nucleus, and focal adhesion sites. Endothelial cells also respond differently to different modes of shear forces, e.g., laminar, disturbed, or oscillatory flows. In vitro studies on cultured ECs in flow channels have been conducted to investigate the molecular mechanisms by which cells convert the mechanical input into biochemical events, which eventually lead to functional responses. The knowledge gained on mechano-transduction, with verifications under in vivo conditions, will advance our understanding of the physiological and pathological processes in vascular remodeling and adaptation in health and disease.

Activation of focal adhesion kinase enhances the adhesion and invasion of pancreatic cancer cells via extracellular signal-regulated kinase-1/2 signaling pathway activation

Background

Interaction with integrin and focal adhesion kinase (FAK) regulates the cancer cell adhesion and invasion into extracellular matrix (ECM). In addition, phosphorylation of FAK correlates with the increase of cell motility and invasion. Adhesion and spreading of cancer cells on a variety of ECM proteins, including collagen type IV (Coll IV), leads to an increase in tyrosine phosphorylation and activation of FAK. In this study, we investigated the mechanism of activation of FAK and its downstream extracellular signal-regulated kinase (ERK)-1/2 signaling following stimulation by interleukin (IL)-1α and adhesion to ECM with subsequent enhancement of pancreatic cancer cell adhesion and invasion.

Results

In immunoblotting analysis, all three pancreatic cancer cell lines (AsPC-1, BxPC-3, and Capan-2) expressed the protein of FAK and β₁ integrin. Enhancement of FAK protein association with β₁ integrin when cells were plated on Coll IV was more increased by stimulation with IL-1α. Preincubation with anti-β₁ integrin antibody and FAK siRNA transfection inhibited the association of FAK with β₁ integrin of pancreatic cancer cells. FAK phosphorylation was observed by adhesion to Coll IV, furthermore, stronger FAK phosphorylation was observed by stimulation with IL-1α of pancreatic cancer cells adhered to Coll IV in time-dependent manner. Genistein, a tyrosine kinase inhibitor, markedly inhibited the FAK phosphorylation. IL-1α stimulation and Coll IV adhesion enhanced the activation of Ras, as evidenced by the increased Ras-GTP levels in pancreatic cancer cells. Activation of Ras correlated with the phosphorylation of ERK. While not statistical affecting the apoptosis of pancreatic cancer cells, IL-1α-induced adhesion and invasion on Coll IV were inhibited with FAK gene silencing by siRNA, β₁ integrin blocking, and inhibition of FAK phosphorylation. PD98059, a MEK inhibitor, also inhibited IL-1α-induced enhancement of adhesion and invasion in pancreatic cancer cells.

Conclusion

Our results demonstrated that activation of FAK is involved with the aggressive capability in pancreatic cancer through Ras/ERK signaling pathway. Based on our results, we suggest that the modification of IL-1, FAK, and integrins functions might be a novel therapeutic approach to aggressive spread of pancreatic cancer.

Biochemistry and biomechanics of cell motility

Cell motility is an essential cellular process for a variety of biological events. The process of cell migration requires the integration and coordination of complex biochemical and biomechanical signals. The protrusion force at the leading edge of a cell is generated by the cytoskeleton, and this force generation is controlled by multiple signaling cascades. The formation of new adhesions at the front and the release of adhesions at the rear involve the outside-in and inside-out signaling mediated by integrins and other adhesion receptors. The traction force generated by the cell on the extracellular matrix (ECM) regulates cell-ECM adhesions, and the counter force exerted by ECM on the cell drives the migration. The polarity of cell migration can be amplified and maintained by the feedback loop between the cytoskeleton and cell-ECM adhesions. Cell migration in three-dimensional ECM has characteristics distinct from that on two-dimensional ECM. The migration of cells is initiated and modulated by external chemical and mechanical factors, such as chemoattractants and the mechanical forces acting on the cells and ECM, as well as the surface density, distribution, topography, and rigidity of the ECM.

Differentiation of C2C12myoblasts is critically regulated by FAK signaling

This study examined whether focal adhesion kinase (FAK) plays a role in the differentiation of C2C12myoblasts into myotubes. Differentiation of C2C12myoblasts induced by switch to differentiation culture medium was accompanied by a transient reduction of FAK phosphorylation at Tyr-397 (to ∼50%, at 1 and 2 h), followed by an increase thereafter (to 240% up to 5 days), although FAK protein expression remained unchanged. FAK and phosphorylated FAK were found at the edge of lamellipodia in proliferating cells, whereas the later increase in FAK phosphorylation in differentiating cells was accompanied by its preferential location at the tip of well-organized actin stress fibers. Hyperexpression of FAK autophosphorylation site (Tyr-397) mutant (MT-FAK) reduced FAK phosphorylation at Tyr-397 in proliferating cells and was accompanied by reduction of cyclin D1 and increase of myogenin expression. These cells failed to progress to myotubes in differentiation medium. In contrast, hyperexpression of a wild-type FAK construction (WT-FAK) increased baseline and abolished the transient reduction of FAK phosphorylation at Tyr-397 in serum-starved C2C12cells. Cells transfected with WT-FAK failed to reduce cyclin D1 and to increase myogenin expression, as well as to progress to terminal differentiation in differentiation medium. These data indicate that FAK signaling plays a critical role in the control of cell cycle as well as in the progression of C2C12cells to terminal differentiation. Transient inhibition of FAK phosphorylation at Tyr-397 contributes to trigger the myogenic genetic program, but its later activation is also central to terminal differentiation into myotubes.

Conditional knockout of focal adhesion kinase in endothelial cells reveals its role in angiogenesis and vascular development in late embryogenesis

Focal adhesion kinase (FAK) is a critical mediator of signal transduction by integrins and growth factor receptors in a variety of cells including endothelial cells (ECs). Here, we describe EC-specific knockout of FAK using a Cre-loxP approach. In contrast to the total FAK knockout, deletion of FAK specifically in ECs did not affect early embryonic development including normal vasculogenesis. However, in late embryogenesis, FAK deletion in the ECs led to defective angiogenesis in the embryos, yolk sac, and placenta, impaired vasculature and associated hemorrhage, edema, and developmental delay, and late embryonic lethal phenotype. Histologically, ECs and blood vessels in the mutant embryos present a disorganized, detached, and apoptotic appearance. Consistent with these phenotypes, deletion of FAK in ECs isolated from the floxed FAK mice led to reduced tubulogenesis, cell survival, proliferation, and migration in vitro. Together, these results strongly suggest a role of FAK in angiogenesis and vascular development due to its essential function in the regulation of multiple EC activities.

Targeting of alpha(v) integrins interferes with FAK activation and smooth muscle cell migration and invasion

Aberrant migration of smooth muscle cells (SMCs) is a key feature of restenosis. Since extracellular matrix proteins and their receptors of the integrin family play a critical role in this process, it is instrumental to understand their contribution to cell migration and invasive motility of SMC on the molecular level. Therefore, we investigated the role of alpha(v)-containing integrins expressed by primary human coronary artery smooth muscle cells (hCASMCs) in vitronectin (VN)-initiated signaling events and cell migration. In hCASMC plated on VN, alpha(v)-containing integrins were localized at focal adhesion sites. Haptotactic stimulation through VN led to a dose-dependent increase in cell migration and concomitantly to enhanced tyrosine phosphorylation of focal adhesion kinase. Both events were completely blocked by a specific inhibitor of integrin alpha(v). Additionally, the integrin alpha(v) inhibitor abolished PDGF-BB-stimulated chemotactic migration. Confocal microscopy confirmed the increased tyrosine phosphorylation at VN-initiated focal contact sites in hCASMC, that was abolished upon alpha(v) inhibition. In vitro invasion of hCASMC was severely compromised in the presence of the integrin alpha(v) inhibitor paralleled by decreased levels of secreted matrix metalloprotease 2 (MMP-2). Together, integrin alpha(v) inhibition abrogates tyrosine phosphorylation at focal adhesion sites and diminishes MMP-2 secretion leading to reduced migration and invasion of hCASMCs.

Focal adhesion kinase is required for bombesin-induced prostate cancer cell motility

Clinical evidence links neuroendocrine differentiation (NED) to prostate cancer progression. In the prostate carcinoma PC-3 cell model, the action of the gastrin releasing peptide (GRP) analog, bombesin (BN), on the activation of focal adhesion kinase (FAK) and invasiveness suggests that this kinase might favor metastasis. Given that components of the FAK signalling pathway are also up regulated in prostate cancer, the aim of the present investigation was to test if FAK function is required for BN-induced motility in PC-3 cells. In wound assays designed to investigate the fate of FAK in cells undergoing BN-induced motility, it was observed that BN treatment resulted in relocalization of FAK in focal contacts concomitantly with its tyrosine phosphorylation on residue 397 (FAK [pY(397)]) and with the formation of actin lamellipodia. Moreover, BN-induced cell motility was significantly reduced in the presence of FAK inhibitors (either anti-FAK [pY(397)] antibody or FRNK, the FAK-related non-kinase). Altogether, these observations point towards a critical role for FAK in the action of BN on PC-3 cell motility.

Genetic alterations in prostate cancer

Prostate cancer is the most common nondermatologic malignancy in men. Prostate cancer is characterized by clinical and biologic heterogeneity that has complicated molecular and epidemiologic studies. Like other epithelial malignancies, prostate tumors exhibit complex karyotypic abnormalities and harbor many specific genetic alterations. Although recent work has begun to elucidate many of the specific mutations associated with prostate cancer, we still lack a clear understanding of the complement of genetic changes that suffice to program the malignant state. Here, we review our current understanding of the genetic changes found in prostate cancer and explore the connections between specific genetic alterations and malignant phenotypes including cell growth, survival, invasion, and metastasis.

G protein-coupled receptor activation rapidly stimulates focal adhesion kinase phosphorylation at Ser-843. Mediation by Ca2+, calmodulin, and Ca2+/calmodulin-dependent kinase II

A rapid increase in the tyrosine phosphorylation of focal adhesion kinase (FAK) has been extensively documented in cells stimulated by multiple signaling molecules, but little is known about the regulation of FAK phosphorylation at serine residues. Stimulation of Swiss 3T3 cells with the G protein-coupled receptor agonists bombesin, vasopressin, or bradykinin induced an extremely rapid (within 5 s) increase in FAK phosphorylation at Ser-843. The phosphorylation of this residue preceded FAK phosphorylation at Tyr-397, the major autophosphorylation site, and FAK phosphorylation at Ser-910. Treatment of intact cells with ionomycin stimulated a rapid increase in FAK phosphorylation at Ser-843, indicating that an increase in intracellular Ca2+ concentration ([Ca2+]i) is a potential pathway leading to FAK-Ser-843 phosphorylation. Indeed, treatment with agents that prevent an agonist-induced increase in [Ca2+]i (e.g. thapsigargin or BAPTA (1,2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid)), interfere with calmodulin function (e.g. trifluoperazine, W13, and W7), or block Ca2+/calmodulin-dependent protein kinase II (CaMKII) activation (KN93) or expression (small interfering RNA) abrogated the rapid FAK phosphorylation at Ser-843 induced by bombesin, bradykinin, or vasopressin. Furthermore, activated CaMKII directly phosphorylated the recombinant COOH-terminal region of FAK at a residue equivalent to Ser-843. Thus, our results demonstrate that G protein-coupled receptor activation induces rapid FAK phosphorylation at Ser-843 through Ca2+, calmodulin, and CaMKII.

Activation of focal adhesion kinase in human lung cancer cells involves multiple and potentially parallel signaling events

Integrins are adhesion receptors that transmit signals bidirectionally across the plasma membrane. In our previous report we have shown that the squamous lung cancer cell line, Calu-1, binds to collagen type IV (Coll IV) through beta1-integrin and results in phosphorylation of focal adhesion kinase (FAK) (Ann Thorac Surg 2004; 78:450-457). Considering the critical role of FAK in cell migration, proliferation, and survival, here we investigated potential mechanisms of its activation and regulation in Calu-1 cells. We observed the phosphorylation of Tyr397 of FAK (the autophosphorylation site of FAK) and paxillin, the immediate downstream substrate of FAK following the adhesion of Calu-1 cells to Coll IV. FAK remains phosphorylated during proliferation either on Coll IV or on uncoated plates for 72 h, as determined by peroxivanadate treatment. Exposure of Calu-1 cells with 60 microM genistein, reduces FAK phosphorylation (7.6 fold) and cell proliferation. Extracellular signal regulated kinases (ERKs) were also phosphorylated after Coll IV attachment. Disruption of Calu-1 cell cytoskeleton integrity by 1-5 muM Cytochalasin D resulted in the inhibition of cell adhesion (50% to 75%, p<0.19 - 6.6 x 10(7)) and ERKs phosphorylation (2 fold) without any effect on FAK phosphorylation. Protein Kinase C inhibitor, Calphostin C at 100 and 250 nM concentrations did not block Coll IV induced FAK phosphorylation but activated the ERKs in a dose dependent manner. beta1-integrin is essential for Coll IV induced FAK activation, but it is not physically associated with FAK as determined by immunodetection assay. Collectively, this report defines the existence of multiple and potentially parallel Coll IV/beta1-integrin mediated signaling events in Calu-1 cells, which involve FAK, ERKs, and PKC.

Growth factors stimulate kidney proximal tubule cell migration independent of augmented tyrosine phosphorylation of focal adhesion kinase

Migration of human proximal tubule cells (HKC-5) was stimulated by epidermal growth factor (EGF), hepatocyte growth factor (HGF), and insulin-like growth factor-1 (IGF-1). Integrin signaling via phosphorylation of focal adhesion kinase (FAK) appears to play a central role in cell migration. Once stimulated, FAK undergoes autophosphorylation at tyrosine (Y) 397, followed by phosphorylation of several sites including Y576/Y577 which increases FAK's kinase activity, as well as at Y407, Y861, and Y925. EGF, HGF, and IGF-1 stimulate FAK phosphorylation in various cells. We showed that endothelin stimulated phosphorylation of Y397 in fibroblasts but not HKC-5 cells. After EGF stimulation, HKC-5 cells showed no change in tyrosine phosphorylation at FAK Y397, 407, 576, 861, or 925. Similarly, HGF and IGF-1 did not stimulate the phosphorylation of FAK Y397 in HKC-5 cells. Further, after inhibition of FAK expression by siRNA, cell migration was similar to cells treated with non-target siRNA and responded to EGF with increased migration. Thus, in proximal tubule cells, stimulation of cell migration by growth factors was independent of augmented FAK tyrosine phosphorylation.

Structural features of the focal adhesion kinase-paxillin complex give insight into the dynamics of focal adhesion assembly

The C-terminal region of focal adhesion kinase (FAK) consists of a right-turn, elongated, four-helix bundle termed the focal adhesion targeting (FAT) domain. The structure of this domain is maintained by hydrophobic interactions, and this domain is also the proposed binding site for the focal adhesion protein paxillin. Paxillin contains five well-conserved LD motifs, which have been implicated in the binding of many focal adhesion proteins. In this study we determined that LD4 binds specifically to only a single site between the H2 and H3 helices of the FAT domain and that the C-terminal end of LD4 is oriented toward the H2-H3 loop. Comparisons of chemical-shift perturbations in NMR spectra of the FAT domain in complex with the binding region of paxillin and the FAT domain bound to both the LD2 and LD4 motifs allowed us to construct a model of FAK-paxillin binding and suggest a possible mechanism of focal adhesion disassembly.

Thrombospondin Induces RhoA Inactivation through FAK-dependent Signaling to Stimulate Focal Adhesion Disassembly

Cells utilize dynamic interactions with the extracellular matrix to adapt to changing environmental conditions. Thrombospondin 1 (TSP1) induces focal adhesion disassembly and cell migration through a sequence (hep I) in its heparin-binding domain signaling through the calreticulin-low density lipoprotein receptor-related protein receptor complex. This involves the Galphai-dependent activation of ERK and phosphoinositide (PI) 3-kinase, both of which are required for focal adhesion disassembly. Focal adhesion kinase (FAK) regulates adhesion dynamics, acting in part by modulating RhoA activity, and FAK is implicated in ERK and PI 3-kinase activation. In this work, we sought to determine the role of FAK in TSP1-induced focal adhesion disassembly. TSP1/hep I does not stimulate focal adhesion disassembly in FAK knockout fibroblasts, whereas re-expressing FAK rescues responsiveness. Inhibiting FAK signaling through FRNK or FAK Y397F expression in endothelial cells also abrogates this response. TSP1/hep I stimulates a transient increase in FAK phosphorylation that requires calreticulin and Galphai, but not ERK or PI 3-kinase. Hep I does not activate ERK or PI 3-kinase in FAK knockout fibroblasts, suggesting activation occurs downstream of FAK. TSP1/hep I stimulates RhoA inactivation with kinetics corresponding to focal adhesion disassembly in a FAK, ERK, and PI 3-kinase-dependent manner. Furthermore, hep I does not stimulate focal adhesion disassembly in cells expressing constitutively active RhoA, suggesting that RhoA inactivation is required for this response. This is the first work to illustrate a connection between FAK phosphorylation in response to a soluble factor and RhoA inactivation, as well as the first report of PI 3-kinase and ERK in FAK regulation of RhoA activity.

The interplay between Src and integrins in normal and tumor biology

Src family nonreceptor protein tyrosine kinases transduce signals that control normal cellular processes such as cell proliferation, adhesion and motility. Normally, cellular Src is held in an inactive state, but in several cancer types, abnormal events lead to elevated kinase activity of the protein and cause pleiotropic cellular responses inducing transformation and metastasis. A prerequisite of the ability of a cancer cell to undergo metastasis into distant tissues is to penetrate surrounding extracellular matrices. These processes are facilitated by the integrin family of cell adhesion molecules. As is the case with Src, altered integrin activity or substrate affinity can contribute to the neoplastic phenotype. Therefore, understanding the interplay between Src and integrin function has been of intense interest over the past few years. This review focuses on the role of Src and integrin signaling in normal cells and how this is deregulated in human cancer. We will identify the key players in the integrin-mediated signaling pathways involved in cell motility and apoptosis, such as FAK, paxillin and p130(CAS), and discuss how Src signaling affects the formation of focal adhesions and the extracellular matrix.

Focal adhesion kinase (FAK) expression and phosphorylation in sea urchin embryos

We have cloned three cDNA isoforms of focal adhesion kinase (FAK) from the sea urchin, Lytechinus variegatus. The sea urchin FAK is more closely related to FAK from other deuterostomes than from invertebrate protostomes or to cell adhesion kinase beta (CAKbeta/Pyk2/FAK2). FAK is expressed in all cells of sea urchin embryos by the 120-cell stage and strongly in blastulae. Phospho-FAK concentrates on basal surfaces of epithelial cells in early blastulae and occurs in syncytial cables of primary mesenchyme cells (PMC). Inhibition of FAK by constructs of FAK-related non-kinase delays blastocoel expansion and early PMC ingression. These results suggest that FAK has roles in cell adhesion and in the shape and integrity of the epithelial cells in sea urchin embryos.

Focal adhesion kinase suppresses apoptosis by binding to the death domain of receptor-interacting protein

Tumor cells resist the apoptotic stimuli associated with invasion and metastasis by activating survival signals that suppress apoptosis. Focal adhesion kinase (FAK), a tyrosine kinase that is overexpressed in a variety of human tumors, mediates one of these survival signals. Attenuation of FAK expression in tumor cells results in apoptosis that is mediated by caspase 8- and FADD-dependent pathways, suggesting that death receptor pathways are involved in the process. Here, we report a functional link between FAK and death receptors. We have demonstrated that FAK binds to the death domain kinase receptor-interacting protein (RIP). RIP is a major component of the death receptor complex and has been shown to interact with Fas and tumor necrosis factor receptor 1 through its binding to adapter proteins. We have shown that RIP provides proapoptotic signals that are suppressed by its binding to FAK. We thus propose that FAK overexpression in human tumors provides a survival signal function by binding to RIP and inhibiting its interaction with the death receptor complex.

The compositional structure of Gene Ontology terms

An analysis of the term names in the Gene Ontology reveals the prevalence of substring relations between terms: 65.3% of all GO terms contain another GO term as a proper substring. This substring relation often coincides with a derivational relationship between the terms. For example, the term regulation of cell proliferation (GO:0042127) is derived from the term cell proliferation (GO:0008283) by addition of the phrase regulation of. Further, we note that particular substrings which are not themselves GO terms (e.g. regulation of in the preceding example) recur frequently and in consistent subtrees of the ontology, and that these frequently occurring substrings often indicate interesting semantic relationships between the related terms. We describe the extent of these phenomena--substring relations between terms, and the recurrence of derivational phrases such as regulation of--and propose that these phenomena can be exploited in various ways to make the information in GO more computationally accessible, to construct a conceptually richer representation of the data encoded in the ontology, and to assist in the analysis of natural language texts.

FERM domain interaction promotes FAK signaling

From the results of deletion analyses, the FERM domain of FAK has been proposed to inhibit enzymatic activity and repress FAK signaling. We have identified a sequence in the FERM domain that is important for FAK signaling in vivo. Point mutations in this sequence had little effect upon catalytic activity in vitro. However, the mutant exhibits reduced tyrosine phosphorylation and dramatically reduced Src family kinase binding. Further, the abilities of the mutant to transduce biochemical signals and to promote cell migration were severely impaired. The results implicate a FERM domain interaction in cell adhesion-dependent activation of FAK and downstream signaling. We also show that the purified FERM domain of FAK interacts with full-length FAK in vitro, and mutation of this sequence disrupts the interaction. These findings are discussed in the context of models of FAK regulation by its FERM domain.

Nerve growth factor (NGF) induces a rapid and sustained downregulation of the focal adhesion kinase (FAK)

1. Exposure of PC12 cells to nerve growth factor (NGF) induces an early tyrosine phosphorylation of many proteins, a number of which is still unidentified. Although NGF is known to bind to and activate the receptor tyrosine kinase TrkA, many downstream targets of NGF signaling may be possibly phosphorylated by nonreceptor tyrosine kinases such as c-Src and focal adhesion kinase (FAK). 2. In the present study, exposure of TrkA-overexpressing PC12 cells to NGF is found to cause a rapid and sustained loss in the recovery of a subpopulation of nominally active FAK (i.e., being autophosphorylated on the positive site of regulation). 3. Consistent with the possibility that NGF induces the proteolysis of FAK via recruitment of Src family kinases, the use of various phosphorylation site-specific anti-FAK antibodies revealed an NGF-inducible and PP1-sensitive accumulation of a putative fragment (i.e., p62) of FAK. Significantly, the mitogenic epidermal growth factor (EGF) failed to induce the downregulation of FAK and the accumulation of tyrosine phosphorylated p62. Such differential response of FAK to NGF and EGF may shape the specificity by which these growth factors control the status of cell-matrix adhesion and the adhesion-driven signaling.

Activation of p21-activated kinase 1 is required for lysophosphatidic acid-induced focal adhesion kinase phosphorylation and cell motility in human melanoma A2058 cells

Lysophosphatidic acid (LPA), one of the naturally occurring phospholipids, stimulates cell motility through the activation of Rho family members, but the signaling mechanisms remain to be elucidated. In the present study, we investigated the roles of p21-activated kinase 1 (PAK1) on LPA-induced focal adhesion kinase (FAK) phosphorylation and cell motility. Treatment of human melanoma cells A2058 with LPA increased phosphorylation and activation of PAK1, which was blocked by treatment with pertussis toxin and by inhibition of phosphoinositide 3-kinase (PI3K) with an inhibitor LY294002 or by overexpression of catalytically inactive mutant of PI3Kgamma, indicating that LPA-induced PAK1 activation was mediated via a Gi protein and the PI3Kgamma signaling pathway. In addition, we demonstrated that Rac1/Cdc42 signals acted as upstream effector molecules of LPA-induced PAK activation. However, Rho-associated kinase, MAP kinase kinase 1/2 or phospholipase C might not be involved in LPA-induced PAK1 activation or cell motility stimulation. Furthermore, PAK1 was necessary for FAK phosphorylation by LPA, which might cause cell migration, as transfection of the kinase deficient mutant of PAK1 or PAK auto-inhibitory domain significantly abrogated LPA-induced FAK phosphorylation. Taken together, these findings strongly indicated that PAK1 activation was necessary for LPA-induced cell motility and FAK phosphorylation that might be mediated by sequential activation of Gi protein, PI3Kgamma and Rac1/Cdc42.

Recruitment of focal adhesion kinase and paxillin to beta1 integrin promotes cancer cell migration via mitogen activated protein kinase activation

Background

Integrin-extracellular matrix interactions activate signaling cascades such as mitogen activated protein kinases (MAPK). Integrin binding to extracellular matrix increases tyrosine phosphorylation of focal adhesion kinase (FAK). Inhibition of FAK activity by expression of its carboxyl terminus decreases cell motility, and cells from FAK deficient mice also show reduced migration. Paxillin is a focal adhesion protein which is also phosphorylated on tyrosine. FAK recruitment of paxillin to the cell membrane correlates with Shc phosphorylation and activation of MAPK. Decreased FAK expression inhibits papilloma formation in a mouse skin carcinogenesis model. We previously demonstrated that MAPK activation was required for growth factor induced in vitro migration and invasion by human squamous cell carcinoma (SCC) lines.

Methods

Adapter protein recruitment to integrin subunits was examined by co-immunoprecipitation in SCC cells attached to type IV collagen or plastic. Stable clones overexpressing FAK or paxillin were created using the lipofection technique. Modified Boyden chambers were used for invasion assays.

Results

In the present study, we showed that FAK and paxillin but not Shc are recruited to the β1 integrin cytoplasmic domain following attachment of SCC cells to type IV collagen. Overexpression of either FAK or paxillin stimulated cancer cell migration on type IV collagen and invasion through reconstituted basement membrane which was dependent on MAPK activity.

Conclusions

We concluded that recruitment of focal adhesion kinase and paxillin to β1 integrin promoted cancer cell migration via the mitogen activated protein kinase pathway.

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Arginine stimulates intestinal cell migration through a focal adhesion kinase dependent mechanism

BACKGROUND

L-Arginine is a nutritional supplement that may be useful for promoting intestinal repair. Arginine is metabolised by the oxidative deiminase pathway to form nitric oxide (NO) and by the arginase pathway to yield ornithine and polyamines.

AIMS

To determine if arginine stimulates restitution via activation of NO synthesis and/or polyamine synthesis.

METHODS

We determined the effects of arginine on cultured intestinal cell migration, NO production, polyamine levels, and activation of focal adhesion kinase, a key mediator of cell migration.

RESULTS

Arginine increased the rate of cell migration in a dose dependent biphasic manner, and was additive with bovine serum concentrate (BSC). Arginine and an NO donor activated focal adhesion kinase (a tyrosine kinase which localises to cell matrix contacts and mediates beta1 integrin signalling) after wounding. Arginine stimulated cell migration was dependent on focal adhesion kinase (FAK) signalling, as demonstrated using adenovirus mediated transfection with a kinase negative mutant of FAK. Arginine stimulated migration was dependent on NO production and was blocked by NO synthase inhibitors. Arginine dependent migration required synthesis of polyamines but elevating extracellular arginine concentration above 0.4 mM did not enhance cellular polyamine levels.

CONCLUSIONS

These results showed that L-arginine stimulates cell migration through NO and FAK dependent pathways and that combination therapy with arginine and BSC may enhance intestinal restitution via separate and convergent pathways.

Disruption of Ca2+-dependent cell-matrix adhesion enhances c-Src kinase activity, but causes dissociation of the c-Src/FAK complex and dephosphorylation of tyrosine-577 of FAK in carcinoma cells

The nonreceptor tyrosine kinase c-Src is activated in most invasive cancers. Activated c-Src binds to FAK in the focal adhesion complex, resulting in the activation of the c-Src/FAK signaling cascade, which regulates cytoskeletal functions. However, the mechanisms by which c-Src/FAK signaling is regulated during conditions of anchorage-independent growth, a hallmark of tumor progression, are not clearly known. Here, an in vivo approach to measure c-Src activity was studied using phospho-specific antibodies against phosphorylated Y418 of c-Src (Src[pY418]), an autophosphorylation site of c-Src, and phosphorylated Y577 of FAK (FAK[pY577]), a known substrate of c-Src. Using genetic and pharmacological approaches to modulate c-Src activity, we showed that the levels of Src[pY418] and FAK[pY577], and the formation of a c-Src/FAK[pY577] complex correlated with the activation state of c-Src in adherent cells. Interestingly, both the in vivo level of Src[pY418] and in vitro c-Src kinase activity were increased in carcinoma cells following disruption of Ca(2+)-dependent cell-matrix adhesion. In contrast, the level of FAK[pY577] and its association with c-Src were reduced in suspended cells. The amount of FAK[pY577] in suspended cells was recovered following attachment of rounded cells to fibronectin-coated polystyrene beads, indicating that cell spreading was not required for phosphorylation of FAK. Moreover, cells expressing activated c-Src showed sustained Src[Y418] phosphorylation, but required Ca(2+)-dependent cell adhesion for phosphorylation of FAK[Y577] and association of c-Src with FAK[pY577]. These findings indicate an important role of integrin-based cell-matrix adhesion in regulating c-Src/FAK signaling under decreased anchorage conditions.

Focal Adhesion Kinase Regulation of N-WASP Subcellular Localization and Function

N-WASP is a member of the WASP family of proteins that regulate actin cytoskeleton remodeling. FAK is a cytoplasmic tyrosine kinase implicated in integrin signaling during cell migration. Here we identify a direct interaction between N-WASP and FAK and show that N-WASP is phosphorylated by FAK at a conserved tyrosine residue, Tyr(256). We found that phosphorylation of Tyr(256) affected N-WASP nuclear localization, suggesting that phosphorylation of N-WASP by FAK may regulate its activity in vivo by altering its subcellular localization. We also showed that the nuclear localization of N-WASP is dependent on its being in the open conformation either after its activation by Cdc42 or the truncation of the C-terminal VCA domain. Phosphorylation of Tyr(256) of N-WASP could reduce its interaction with nuclear importin NPI-1, which might be responsible for its decreased nuclear localization. Lastly, we show that phosphorylation of Tyr(256) plays an important role in promoting cell migration. Together, these results suggest a novel regulatory mechanism of N-WASP by tyrosine phosphorylation and subcellular localization and its potential role in the regulation of cell migration.

The tissue engineeting puzzle: a molecular perspective

The inability of biomaterial scaffolds to functionally integrate into surrounding tissue is one of the major roadblocks to developing new biomaterials and tissue-engineering scaffolds. Despite considerable advances, current approaches to engineering cell-surface interactions fall short in mimicking the complexity of signals through which surrounding tissue regulates cell behavior. Cells adhere and interact with their extracellular environment via integrins, and their ability to activate associated downstream signaling pathways depends on the character of adhesion complexes formed between cells and their extracellular matrix. In particular, alpha5beta1 and alphavbeta3 integrins are central to regulating downstream events, including cell survival and cell-cycle progression. In contrast to previous findings that alphavbeta3 integrins promote angiogenesis, recent evidence argues that alphavbeta3 integrins may act as negative regulators of proangiogenic integrins such as alpha5beta1. This suggests that fibronectin is critical for scaffold vascularization because it is the only mammalian adhesion protein that binds and activates alpha5beta1 integrins. Cells are furthermore capable of stretching fibronectin matrices such that the protein partially unfolds, and recent computational simulations provide structural models of how mechanical stretching affects fibronectin function. We propose a model whereby excessive tension generated by cells in contact to biomaterials may in fact render fibronectin fibrils nonangiogenic and potentially inhibit vascularization. The model could explain why current biomaterials independent of their surface chemistries and textures fail to vascularize.

PDGF and FGF induce focal adhesion kinase (FAK) phosphorylation at Ser-910: Dissociation from Tyr-397 phosphorylation and requirement for ERK activation

A rapid increase in the tyrosine phosphorylation of focal adhesion kinase (FAK) has been extensively documented in cells stimulated by multiple signaling molecules, but very little is known about the regulation of FAK phosphorylation at serine residues. Stimulation of Swiss 3T3 cells with platelet-derived growth factor (PDGF) promoted a striking increase in the phosphorylation of FAK at Ser-910, as revealed by site-specific antibodies that recognized the phosphorylated state of this residue. FAK phosphorylation at Ser-910 could be distinguished from that at Tyr-397 in terms of dose-response relationships and kinetics. Furthermore, the selective phosphoinositide 3-kinase (PI 3-kinase) inhibitors wortmannin and LY 294002 abrogated FAK phosphorylation at Tyr-397 but did not interfere with PDGF-induced FAK phosphorylation at Ser-910. Conversely, treatment with U0126, a potent inhibitor of MEK-mediated ERK activation, prevented FAK phosphorylation at Ser-910 induced by PDGF but did not interfere with PDGF-induced FAK phosphorylation at Tyr-397. These results were extended using growth factors that either stimulate, fibroblast growth factor (FGF), or do not stimulate (insulin) the ERK pathway activation in Swiss 3T3 cells. FGF but not insulin promoted a striking ERK-dependent phosphorylation of FAK at Ser-910. Our results indicate that FAK phosphorylation at Tyr-397 and FAK phosphorylation at Ser-910 are induced in response to PDGF stimulation through different signaling pathways, namely PI 3-kinase and ERK, respectively.

Sezary Syndrome Cells Unlike Normal Circulating T Lymphocytes Fail to Migrate Following Engagement of NT1 Receptor

Circulating malignant Sezary cells are a clonal proliferation of CD4+CD45RO+ T lymphocytes primarily involving the skin. To study the biology of these malignant T lymphocytes, we tested their ability to migrate in chemotaxis assays. Previously, we had shown that the neuropeptide neurotensin (NT) binds to freshly isolated Sezary malignant cells and induces through NT1 receptors the cell migration of the cutaneous T cell lymphoma cell line Cou-L. Here, we report that peripheral blood Sezary cells as well as the Sezary cell line Pno fail to migrate in response to neurotensin although they are capable of migrating to the chemokine stromal-cell-derived factor 1 alpha. This is in contrast with normal circulating CD4+ or CD8+ lymphocytes, which respond to both types of chemoattractants except after ex vivo short-time anti-CD3 monoclonal antibody activation, which abrogates the neurotensin-induced lymphocyte migration. Furthermore, we demonstrate that neurotensin-responsive T lymphocytes express the functional NT1 receptor responsible for chemotaxis. In these cells, but not in Sezary cells, neurotensin induces recruitment of phosphatidylinositol-3 kinase, and redistribution of phosphorylated cytoplasmic tyrosine kinase focal adhesion kinase and filamentous actin. Taken together, these results, which show functional distinctions between normal circulating lymphocytes and Sezary syndrome cells, contribute to further understanding of the physiopathology of these atypical cells.

FAK promotes organization of fibronectin matrix and fibrillar adhesions

Targeted disruption of the focal adhesion kinase (FAK) gene in mice is lethal at embryonic day 8.5 (E8.5). Vascular defects in FAK-/- mice result from the inability of FAK-deficient endothelial cells to organize themselves into vascular network. We found that, although fibronectin (FN) levels were similar, its organization was less fibrillar in both FAK-/- endothelial cells and mesoderm of E8.5 FAK-/- embryos, as well as in mouse embryonic fibroblasts isolated from mutant embryos. FAK catalytic activity, proline-rich domains, and location in focal contacts were all required for proper allocation and patterning of FN matrix. Cells lacking FAK in focal adhesions fail to translocate supramolecular complexes of integrin-bound FN and focal adhesion proteins along actin filaments to form mature fibrillar adhesions. Taken together, our data suggest that proper FN allocation and organization are dependent on FAK-mediated remodeling of focal adhesions.

Alterations in Granule Matrix and Cell Surface of Focal Adhesion Kinase-Deficient Mast Cells

Focal adhesion kinase (FAK) is a nonreceptor protein tyrosine kinase that plays an important role in many cellular processes and is tyrosine phosphorylated after FcepsilonRI aggregation in mast cells. In mice, null mutation of the fak gene results in a lethal phenotype in which the embryos fail to develop past day 8.5 of gestation. To study the role of FAK in these mast cells, 8.5-day embryos were isolated and placed in culture with IL-3 and stem cell factor (SCF). Although FAK was not required for the development of mast cells in culture, the FAK(-/-) embryo-derived mast cells had several distinct characteristics. Compared with the controls, the mast cells that lack FAK were less metachromatic and by electron microscopy had granules that appeared largely electron lucid, although their histamine content was unchanged. The FAK-deficient mast cells had a reduction in the content of chondroitin/dermatan sulfate, the major glycosaminoglycan component of the granular matrix. The FAK-deficient cells had fewer microvilli that were fused with each other, giving the cell surface a ruffled appearance. There was also a 3-fold increase in the number of cells highly expressing beta(7) integrin. However, signal transduction from the high affinity IgE receptor for the secretion of histamine was similar in the wild-type, heterozygote, and the FAK-deficient cells. The FcepsilonRI-induced tyrosine phosphorylation of paxillin, Crk-associated tyrosine kinase substrate (CAS), and mitogen-activated protein kinase proteins was independent of FAK. These results indicate that FAK plays a role in regulating the glycosaminoglycan content of the secretory granules and influences the cell surface morphology of mast cells.

Activation of platelet-activating factor receptor-coupled G alpha q leads to stimulation of Src and focal adhesion kinase via two separate pathways in human umbilical vein endothelial cells

Platelet-activating factor (PAF), a phospholipid second messenger, has diverse physiological functions, including responses in differentiated endothelial cells to external stimuli. We used human umbilical vein endothelial cells (HUVECs) as a model system. We show that PAF activated pertussis toxin-insensitive G alpha(q) protein upon binding to its seven transmembrane receptor. Elevated cAMP levels were observed via activation of adenylate cyclase, which activated protein kinase A (PKA) and was attenuated by a PAF receptor antagonist, blocking downstream activity. Phosphorylation of Src by PAF required G alpha(q) protein and adenylate cyclase activation; there was an absolute requirement of PKA for PAF-induced Src phosphorylation. Immediate (1 min) PAF-induced STAT-3 phosphorylation required the activation of G alpha(q) protein, adenylate cyclase, and PKA, and was independent of these intermediates at delayed (30 min) and prolonged (60 min) PAF exposure. PAF activated PLC beta 3 through its G alpha(q) protein-coupled receptor, whereas activation of phospholipase C gamma 1 (PLC gamma 1) by PAF was independent of G proteins but required the involvement of Src at prolonged PAF exposure (60 min). We demonstrate for the first time in vascular endothelial cells: (i) the involvement of signaling intermediates in the PAF-PAF receptor system in the induction of TIMP2 and MT1-MMP expression, resulting in the coordinated proteolytic activation of MMP2, and (ii) a receptor-mediated signal transduction cascade for the tyrosine phosphorylation of FAK by PAF. PAF exposure induced binding of p130(Cas), Src, SHC, and paxillin to FAK. Clearly, PAF-mediated signaling in differentiated endothelial cells is critical to endothelial cell functions, including cell migration and proteolytic activation of MMP2.

Src mediates prolactin-dependent proliferation of T47D and MCF7 cells via the activation of focal adhesion kinase/Erk1/2 and phosphatidylinositol 3-kinase pathways

Prolactin (PRL) stimulates breast cancer cell proliferation; however, the involvement of PRL-activated signaling molecules in cell proliferation is not fully established. Here we studied the role of c-Src on PRL-stimulated proliferation of T47D and MCF7 breast cancer cells. We initially observed that PRL-dependent activation of focal adhesion kinase (Fak), Erk1/2, and cell proliferation was mediated by c-Src in T47D cells, because expression of a dominant-negative form of c-Src (SrcDM, K295A/Y527F) blocked the PRL-dependent effects. The Src inhibitor PP1 abrogated PRL-dependent in vivo activation of Fak, Erk1/2, p70S6K, and Akt and the proliferation of T47D and MCF7 cells; Janus kinase 2 (Jak2) activation was not affected. However, in vitro, Fak and Jak2 kinases were not directly inhibited by PP1, demonstrating the effect of PP1 on c-Src kinase as an upstream activator of Fak. Expression of Fak mutant Y397F abrogated PRL-dependent activation of Fak, Erk1/2, and thymidine incorporation, but had no effect on p70S6K and Akt kinases. MAPK kinase 1/2 (Mek1/2) inhibitor PD184352 blocked PRL-induced stimulation of Erk1/2 and cell proliferation; however, p70S6K and Akt activation were unaffected. The phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002 abolished cell proliferation and activation of p70S6K and Akt; however, PRL-dependent activation of Erk1/2 was not modified. Moreover, we show that both c-Src/PI3K and c-Src/Fak/Erk1/2 pathways are involved in the up-regulation of c-myc and cyclin d1 expression mediated by PRL. The previous findings suggest the existence of two PRL-dependent signaling cascades, initiated by the c-Src-mediated activation of Fak/Erk1/2 and PI3K pathways that, subsequently, control the expression of c-Myc and cyclin D1 and the proliferation of T47D and MCF7 breast cancer cells.

Regulation of the expression and activity of beta1,4-galactosyltransferase I by focal adhesion kinase

beta1,4-Galactosyltransferase I (GalT) participates in both glycoconjugates and cellular interactions. GalT's role in lamellipodia formation and cell migration on laminin is associated with a transient phosphorylation of focal adhesion kinase (FAK) and a consequent reorganization of the actin cytoskeleton and focal adhesions. We transfected wild type FAK and different FAK mutants into NIH3T3 cell line, measured GalT gene expression by Northern blot hybridization, and evaluated its activity. It was found that wtFAK and FAKY576F up-regulated GalT gene expression and its surface activity, while FAKY397F down-regulated them. At the same time, we used ricinus communis agglutinin (RCA)-I lectin staining to demonstrate its binding reactions. We found that wtFAK and FAKY576F bound stronger, while FAKY397F bound weaker than the control. By flow cytometry analysis, it was found that FAK promoted G1/S transition and enhanced the expression of cyclin D1 while FAKY397F inhibited these steps compared with the control NIH3T3 cells. G1/S checkpoint regulation proteins control GalT mRNA transcription. The results indicate that FAK regulated the expression of GalT and its activity in NIH3T3 cells may contribute to the effect of FAK on the cell-cycle.

Integrin signaling and mammary cell function

The mammary gland is a highly organized tissue, containing ductal structures, secretory alveolar units, and a supporting stroma. The organization of the epithelial cells within the tissue depends upon cell-cell adhesion as well as cell interactions with the extracellular matrix that underlies the epithelial units and makes up most of the organization of the stroma. Adhesion to the extracellular matrix is mediated by a class of heterodimeric transmembrane receptors called integrins, which cluster at focal adhesions. Integrins link the matrix with an intracellular structural scaffold, the cytoskeleton, as well as with signaling enzymes that direct cell survival, proliferation, differentiation, and migration. Two key enzymes that are recruited to sites of integrin clustering are focal adhesion kinase and integrin-linked kinase. Both enzymes are involved with communication downstream of integrins and have key roles in regulating cell behavior. This review will focus on what is known about focal adhesion kinase and integrin-linked kinase signaling and will discuss current evidence about their role in mammary gland biology and neoplasia.

Negative regulation of FAK signaling by SOCS proteins

Focal adhesion kinase (FAK) becomes activated upon integrin-mediated cell adhesion and controls cellular responses to the engagement of integrins, including cell migration and survival. We show here that a coordinated signaling by integrins and growth factor receptors induces expression of suppressor of cytokine signaling-3 (SOCS-3) and subsequent interaction between endogenous FAK and SOCS-3 proteins in 3T3 fibroblasts. Cotransfection studies demonstrated that SOCS-3, and also SOCS-1, interact with FAK in a FAK-Y397-dependent manner, and that both the Src homology 2 (SH2) and the kinase inhibitory region (KIR) domains of the SOCS proteins contribute to FAK binding. SOCS-1 and SOCS-3 were found to inhibit FAK-associated kinase activity in vitro and tyrosine phosphorylation of FAK in cells. The SOCS proteins also promoted polyubiquitination and degradation of FAK in a SOCS box-dependent manner and inhibited FAK-dependent signaling events, such as cell motility on fibronectin. These studies suggest a negative role of SOCS proteins in FAK signaling, and for a previously unidentified regulatory mechanism for FAK function.

Cytoskeletal remodeling of the airway smooth muscle cell: a mechanism for adaptation to mechanical forces in the lung

Airway smooth muscle is continuously subjected to mechanical forces caused by changes in lung volume during breathing. These mechanical oscillations have profound effects on airway smooth muscle contractility both in vivo and in vitro. Alterations in airway smooth muscle properties in response to mechanical forces may result from adaptive changes in the organization of the actin cytoskeleton. Recent advances suggest that in airway smooth muscle, two cytosolic signaling proteins that associate with focal adhesion complexes, focal adhesion kinase (FAK) and paxillin, are involved in transducing external mechanical signals. FAK and paxillin regulate changes in the organization of the actin cytoskeleton and the activation of contractile proteins. Actin is in a dynamic state in airway smooth muscle and undergoes polymerization and depolymerization during the contraction-relaxation cycle. The organization of the cytoskeletal proteins, vinculin, talin, and alpha-actinin, which mediate linkages between actin filaments and transmembrane integrins, is also regulated by contractile stimulation in airway smooth muscle. The fluidity of the cytoskeletal structure of the airway smooth muscle cell may be fundamental to its ability to adapt and respond to the mechanical forces imposed on it in the lung during breathing.

Focal adhesion kinase is required for endothelin-induced cell cycle progression of cultured astrocytes

When the brain is damaged, astrocytes often cause hyperplasia resulting in glial scar formation at the injured sites. Endothelins (ETs) have been shown to be involved in the pathophysiologic responses of astrocytes, including proliferation. In this study, we examined the mechanisms underlying the ET-induced astrocytic G1/S-phase cell cycle transition by focusing on focal adhesion kinase (FAK). A transient transfection with wild-type FAK was followed by an increase in bromodeoxyuridine (BrdU) incorporation into cultured rat astrocytes. The increases in BrdU incorporation induced by 100 nM ET-1 were not found in astrocytes transfected with dominant-negative FAK mutants (FRNK and dC14-FAK). The increases in BrdU incorporation induced by 10 nM phorbol 12-myristate 13-acetate (PMA) were not affected by the FAK mutants. Wild-type FAK did not induce stress fiber formation in cultured astrocytes. The dominant negative FAK mutant dC14-FAK did not prevent ET-induced astrocytic stress fiber formation. These results suggest that FAK mediated the astrocytic G1/S cell cycle transition induced by ET-1 downstream of the cytoskeletal actin reorganization.

The first three minutes: smooth muscle contraction, cytoskeletal events, and soft glasses

Smooth muscle exhibits biophysical characteristics and physiological behaviors that are not readily explained by present paradigms of cytoskeletal and cross-bridge mechanics. There is increasing evidence that contractile activation of the smooth muscle cell involves an array of cytoskeletal processes that extend beyond cross-bridge cycling and the sliding of thick and thin filaments. We review here the evidence suggesting that the biophysical and mechanical properties of the smooth muscle cell reflect the integrated interactions of an array of highly dynamic cytoskeletal processes that both react to and transform the dynamics of cross-bridge interactions over the course of the contraction cycle. The activation of the smooth muscle cell is proposed to trigger dynamic remodeling of the actin filament lattice within cellular microdomains in response to local mechanical and pharmacological events, enabling the cell to adapt to its external environment. As the contraction progresses, the cytoskeletal lattice stabilizes, solidifies, and forms a rigid structure well suited for transmission of tension generated by the interaction of myosin and actin. The integrated molecular transitions that occur within the contractile cycle are interpreted in the context of microscale agitation mechanisms and resulting remodeling events within the intracellular microenvironment. Such an interpretation suggests that the cytoskeleton may behave as a glassy substance whose mechanical function is governed by an effective temperature.

Focal adhesion kinase N-terminus in breast carcinoma cells induces rounding, detachment and apoptosis

Focal adhesion kinase (FAK) has a central role in adhesion-mediated cell signalling. The N-terminus of FAK is thought to function as a docking site for a number of proteins, including the Src-family tyrosine kinases. In the present study, we disrupted FAK signalling by expressing the N-terminal domain of FAK (FAK-NT) in human breast carcinoma cells, BT474 and MCF-7 lines, and non-malignant epithelial cells, MCF-10A line. Expression of FAK-NT led to rounding, detachment and apoptosis in human breast cancer cells. Apoptosis was accompanied by dephosphorylation of FAK Tyr(397), degradation of the endogenous FAK protein and activation of caspase-3. Over-expression of FAK rescued FAK-NT-mediated cellular rounding. Expression of FAK-NT in non-malignant breast epithelial cells did not lead to rounding, loss of FAK phosphorylation or apoptosis. Thus FAK-NT contributes to cellular adhesion and survival pathways in breast cancer cells which are not required for survival in non-malignant breast epithelial cells.

Bombesin, lysophosphatidic acid, and epidermal growth factor rapidly stimulate focal adhesion kinase phosphorylation at Ser-910: requirement for ERK activation

A rapid increase in the tyrosine phosphorylation of focal adhesion kinase (FAK) has been extensively documented in cells stimulated by multiple signaling molecules, but virtually nothing is known about the regulation of FAK phosphorylation at serine residues. Stimulation of Swiss 3T3 cells with bombesin promoted a striking increase ( approximately 13-fold) in the phosphorylation of FAK at Ser-910, as revealed by site-specific antibodies that recognized the phosphorylated state of this residue. Lysophosphatidic acid and epidermal growth factor (EGF) also stimulated FAK phosphorylation at Ser-910. Direct activation of protein kinase C isoforms with phorbol-12,13-dibutyrate (PDB) also promoted striking phosphorylation of FAK at Ser-910. Treatment with the protein kinase C inhibitor GF I or Ro 31-8220 or chronic exposure to PDB prevented the increase in FAK phosphorylation at Ser-910 induced by bombesin or PDB but not by EGF. Treatment with the ERK inhibitors U0126 and PD98059 prevented FAK phosphorylation at Ser-910 in response to all of the stimuli tested. Furthermore, incubation of activated ERK2 with FAK immunocomplexes leads to FAK phosphorylation at Ser-910 in vitro. Our results demonstrate, for the first time, that stimulation with bombesin, lysophosphatidic acid, PDB, or EGF induces phosphorylation of endogenous FAK at Ser-910 via an ERK-dependent pathway in Swiss 3T3 cells.

Paxillin binding to the alpha 4 integrin subunit stimulates LFA-1 (integrin alpha L beta 2)-dependent T cell migration by augmenting the activation of focal adhesion kinase/proline-rich tyrosine kinase-2

Engagement of very late Ag-4 (integrin alpha(4)beta(1)) by ligands such as VCAM-1 markedly stimulates leukocyte migration mediated by LFA-1 (integrin alpha(L)beta(2)). This form of integrin trans-regulation in T cells requires the binding of paxillin to the alpha(4) integrin cytoplasmic domain. This conclusion is based on the abolition of trans-regulation in Jurkat T cells by an alpha(4) mutation (alpha(4)(Y991A)) that disrupts paxillin binding. Furthermore, cellular expression of an alpha(4)-binding fragment of paxillin that blocks the alpha(4)-paxillin interaction, selectively blocked VCAM-1 stimulation of alpha(L)beta(2)-dependent cell migration. The alpha(4)-paxillin association mediates trans-regulation by enhancing the activation of tyrosine kinases, focal adhesion kinase (FAK) and/or proline-rich tyrosine kinase-2 (Pyk2), based on two lines of evidence. First, disruption of the paxillin-binding site in the alpha(4) tail resulted in much less alpha(4)beta(1)-mediated phosphorylation of Pyk2 and FAK. Second, transfection with cDNAs encoding C-terminal fragments of Pyk2 and FAK, which block the function of the intact kinases, blocked alpha(4)beta(1) stimulation of alpha(L)beta(2)-dependent migration. These results define a proximal protein-protein interaction of an integrin cytoplasmic domain required for trans-regulation between integrins, and establish that augmented activation of Pyk2 and/or FAK is an immediate signaling event required for the trans-regulation of integrin alpha(L)beta(2) by alpha(4)beta(1).

Mechanism and its regulation of tumor-induced angiogenesis

Tumor angiogenesis is the proliferation of a network of blood vessels that penetrates into cancerous growths, supplying nutrients and oxygen and removing waste products. The process of angiogenesis plays an important role in many physiological and pathological conditions. Solid tumors depend on angiogenesis for growth and metastasis in a hostile environment. In the prevascular phase, the tumor is rarely larger than 2 to 3 mm³ and may contain a million or more cells. Up to this size, tumor cells can obtain the necessary oxygen and nutrient supplies required for growth and survival by simple passive diffusion. The properties of tumors to release and induce several angiogenic and anti-angiogenic factors which play crucial roles in regulating endothelial cell (EC) proliferation, migration, apoptosis or survival, cell-cell and cell-matrix adhesion through different intracellular signaling are thought to be the essential mechanisms during tumor-induced angiogenesis. Tumor angiogenesis actually starts with tumor cells releasing molecules that send signals to surrounding normal host tissue. This signaling activates certain genes in the host tissue that, in turn, make proteins to encourage growth of new blood vessels. In this review, we focus the mechanisms of tumor-induced angiogenesis, with an emphasis on the regulatory role of several angiogenic and anti-angiogenic agents during the angiogenic process in tumors. Advances in understanding the mechanisms of tumor angiogenesis have led to the development of several most effective anti-angiogenic and anti-metastatic therapeutic agents and also have provided several techniques for the regulation of cancer's angiogenic switch. The suggestion is made that standard cytotoxic chemotherapy and angiogenesis inhibitors used in combination may produce complementary therapeutic benefits in the treatment of cancer.