The role of conserved amino acid motifs within the integrin beta3 cytoplasmic domain in triggering focal adhesion kinase phosphorylation

ITGB3 and associated molecules as critical biomarkers in Cesarean Scar Pregnancy

BACKGROUND

Cesarean scar pregnancy (CSP) is a life-threatening condition with a rising incidence in China. The pathogenesis of CSP remains poorly understood, partly due to the limited availability of comprehensive datasets constrained by spatiotemporal factors.

OBJECTIVE

This study aimed to explore key regulatory molecules and mechanisms involved in CSP through a multi-omics approach.

METHODS

Proteomic analysis was performed on decidual and villous tissues from clinical patients (n = 6, including 3 CSP cases and 3 controls). Gene expression datasets (n = 9) were obtained from the GEO and SRA databases. Bioinformatics analyses were conducted using DAVID, Metascape, and STRING, with transcription factor prediction performed via the JASPAR database. Data analysis was conducted using SPSS 27, with a significance threshold set at P < 0.05.

RESULTS

CSP shared differentially expressed genes (DEGs) with cesarean delivery (CD) and embryo implantation (EI). Enrichment analysis revealed that biological processes and KEGG pathways related to adhesion, with Integrin Subunit Beta 3 (ITGB3), Integrin Subunit Alpha 2b (ITGA2B), and Vitronectin (VTN) playing significant roles. ITGB3 expression was significantly downregulated following CD compared to spontaneous delivery, followed by upregulation in subsequent pregnancies. The transcription factor GATA4 was identified as a key regulator of ITGB3, potentially contributing to CSP pathogenesis.

CONCLUSION

Our findings suggest that CSP development is closely associated with CD and EI, with ITGB3 and its regulatory network playing a crucial role. GATA4-mediated regulation of ITGB3 may represent an important molecular mechanism contributing to CSP formation.

c-Met-integrin cooperation: Mechanisms, tumorigenic effects, and therapeutic relevance

c-Met is a receptor tyrosine kinase which upon activation by its ligand, the hepatocyte growth factor, mediates many important signalling pathways that regulate cellular functions such as survival, proliferation, and migration. Its oncogenic and tumorigenic signalling mechanisms, greatly contributing to cancer development and progression, are well documented. Integrins, heterogeneous adhesion receptors which facilitate cell-extracellular matrix interactions, are important in biomechanically sensitive cell adhesion and motility but also modulate diverse cell behaviour. Here we review the studies which reported cooperation between c-Met and several integrins, particularly β1 and β4, in various cell models including many tumour cell types. From the various experimental models and results analysed, we propose that c-Met-integrin cooperation occurs via inside-out or outside-in signalling. Thus, either c-Met activation triggers integrin activation and cell adhesion or integrin adhesion to its extracellular ligand triggers c-Met activation. These two modes of cooperation require the adhesive function of integrins and mostly lead to cell migration and invasion. In a third, less conventional, mode of cooperation, the integrin plays the role of a signalling adaptor for c-Met, independently from its adhesive property, leading to anchorage independent survival. Recent studies have revealed the influence of endocytic trafficking in c-Met-integrin cooperation including the adaptor function of integrin occurring on endomembranes, triggering an inside-in signalling, believed to promote survival of metastatic cells. We present the evidence of the cooperation in vivo and in human tissues and highlight its therapeutic relevance. A better understanding of the mechanisms regulating c-Met-integrin cooperation in cancer progression could lead to the design of new therapies targeting this cooperation, providing more effective therapeutic approaches than c-Met or integrin inhibitors as monotherapies used in the clinic.

Regulation of the integrin αVβ3- actin filaments axis in early osteogenesis of human fibroblasts under cyclic tensile stress

Background

Integrins play a prominent role in osteogenic differentiation by transmitting both mechanical and chemical signals. Integrin expression is closely associated with tensile stress, which has a positive effect on osteogenic differentiation. We investigated the relationship between integrin αVβ3 and tensile stress.

Methods

Human fibroblasts were treated with c (RGDyk) and lentivirus transduction to inhibit function of integrin αVβ3. Y-15, cytochalasin D and verteporfin were used to inhibit phosphorylation of FAK, polymerization of microfilament and function of nuclear YAP, respectively. Fibroblasts were exposed to a cyclic tensile stress of 10% at 0.5 Hz, once a day for 2 h each application. Fibroblasts were harvested on day 4 and 7 post-treatment. The expression of ALP, RUNX2, integrin αVβ3, β-actin, talin-1, FAK, vinculin, and nuclear YAP was detected by Western blot or qRT-PCR. The expression and distribution of integrin αVβ3, vinculin, microfilament and nuclear YAP.

Results

Cyclic tensile stress was found to promote expression of ALP and RUNX2. Inhibition of integrin αVβ3 activation downregulated the rearrangement of microfilament and the expression of ALP, RUNX2 and nuclear YAP. When the polymerization of microfilament was inhibited the expression of ALP, RUNX2 and nuclear YAP were decreased. The phosphorylation of FAK induced by cyclic tensile stress reduced by the inhibition of integrin αVβ3. The expression of ALP and RUNX2 was decreased by inhibition of phosphorylation of FAK and inhibition of nuclear YAP.

Conclusions

Cyclic tensile stress promotes osteogenesis of human fibroblasts via integrin αVβ3-microfilament axis. Phosphorylation of FAK and nuclear YAP participates in this process.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02597-y.

β Integrins mediate FAK Y397 autophosphorylation of resistance arteries during eutrophic inward remodeling in hypertension

Human essential hypertension is characterized by eutrophic inward remodeling of the resistance arteries with little evidence of hypertrophy. Upregulation of αVβ3 integrin is crucial during this process. In order to investigate the role of focal adhesion kinase (FAK) activation in this process, the level of FAK Y397 autophosphorylation was studied in small blood vessels from young TGR(mRen2)27 animals as blood pressure rose and eutrophic inward remodeling took place. Between weeks 4 and 5, this process was completed and accompanied by a significant increase in FAK phosphorylation compared with normotensive control animals. Phosphorylated (p)FAK Y397 was coimmunoprecipitated with both β1- and β3-integrin-specific antibodies. In contrast, only a fraction (<10-fold) was coprecipitated with the β3 integrin subunit in control vessels. Inhibition of eutrophic remodeling by cRGDfV treatment of TGR(mRen2)27 rats resulted in the development of smooth-muscle-cell hypertrophy and a significant further enhancement of FAK Y397 phosphorylation, but this time with exclusive coassociation of pFAK Y397 with integrin β1. We established that phosphorylation of FAK Y397 with association with β1 and β3 integrins occurs with pressure-induced eutrophic remodeling. Inhibiting this process leads to an adaptive hypertrophic vascular response induced by a distinct β1-mediated FAK phosphorylation pattern.

Nov/CCN3 regulates long-term repopulating activity of murine hematopoietic stem cells via integrin αvβ3

Throughout life, hematopoietic stem cells (HSCs) sustain the blood cell supply through their capacities for self-renewal and multilineage differentiation. These processes are regulated within a specialized microenvironment termed the 'niche'. Here, we show a novel mechanism for regulating HSC function that is mediated by nephroblastoma overexpressed (Nov/CCN3), a matricellular protein member of the CCN family. We found that Nov contributes to the maintenance of long-term repopulating (LTR) activity through association with integrin αvβ3 on HSCs. The resultant β3 integrin outside-in signaling is dependent on thrombopoietin (TPO), a crucial cytokine involved in HSC maintenance. TPO was required for Nov binding to integrin αvβ3, and stimulated Nov expression in HSCs. However, in the presence of IFNγ, a cytokine known to impair HSC function, not only was TPO-induced expression of Nov suppressed, but the LTR activity was conversely impaired by TPO-mediated ligation of integrin αvβ3 with exogenous ligands, including Nov, as well. Thus, Nov/integrin αvβ3-mediated maintenance of HSCs appears to be modulated by simultaneous stimulation by other cytokines. Our finding suggests that this system contributes to the regulation of HSCs within the bone marrow niche.

Mechanotransduction pathways linking the extracellular matrix to the nucleus

Cells contain several mechanosensing components that transduce mechanical signals into biochemical cascades. During cell-ECM adhesion, a complex network of molecules mechanically couples the extracellular matrix (ECM), cytoskeleton, and nucleoskeleton. The network comprises transmembrane receptor proteins and focal adhesions, which link the ECM and cytoskeleton. Additionally, recently identified protein complexes extend this linkage to the nucleus by linking the cytoskeleton and the nucleoskeleton. Despite numerous studies in this field, due to the complexity of this network, our knowledge of the mechanisms of cell-ECM adhesion at the molecular level remains remarkably incomplete. Herein, we present a review of the structures of key molecules involved in cell-ECM adhesion, along with an evaluation of their predicted roles in mechanical sensing. Additionally, specific binding events prompted by force-induced conformational changes of each molecule are discussed. Finally, we propose a model for the biomechanical events prominent in cell-ECM adhesion.

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

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

Skelemin in integrin α(IIb)β(3) mediated cell spreading

Skelemin, a myosin-associated protein in skeletal muscle, has been demonstrated to interact with integrin α(IIb)β(3) in nonmuscle cells during initial stages of cell spreading. The significance of this interaction and the role of skelemin in integrin signaling and cytoskeletal reorganization were investigated in this study. We established a series of Chinese hamster ovary cell lines expressing wild-type or mutant α(IIb)β(3) receptors in which skelemin binding residues at the membrane proximal region of integrin tails were mutated to alanine. Most cells displayed unimpaired adhesive capacity and spreading on immobilized fibrinogen at the early stages of cell spreading. In addition, they formed normal focal adhesions and stress fibers with no indication of impaired cell spreading. R995A/R997A/L1000A, H722A, and K716A exhibited the greatest cell spreading, which was associated with enhanced p-Src activation but was independent of FAK activation. Transfection of the cells with GFP-skelemin, containing only the C2 integrin binding domain, caused wild-type cells to round up, but had no effect on R995A/R997A/L1000A, H722A, and K716A cell spreading. Furthermore, the protrusions of the leading edge of K716A cells showed strong colocalization of talin with α(IIb)β(3) which was associated with a loss in skelemin binding. Thus, we propose that during early stages of cell spreading, skelemin exerts contractile force on cell spreading and modulates the attachment of cytoskeletal proteins and Src to integrin clusters.

Calcium Integrin Binding Protein Associates with Integrins αVβ3 and αIIbβ3 Independent of β3 Activation Motifs

The Calcium Integrin Binding protein (CIB) has been identified as interacting specifically with the cytoplasmic tail of the integrin α_IIb domain to induce receptor activation and integrin α_IIbβ₃ mediated cell adhesion to extracellular proteins. In K562 cells stably expressing mutated integrin α_Vβ₃, or chimeric α_Vβ₃ carrying α_IIb cytoplasmic tail, we report that the interaction of CIB with β₃ integrins is not α_IIbβ₃ specific but binds α_IIb as well as α_V cytoplasmic tail domains. A double mutation of two proline residues to alanine residues in the α_IIb cytoplasmic domain, previously shown to disturb its conformation, inhibits chimeric α_V/α_IIbβ₃-CIB interaction. This demonstrates that α_IIb cytoplasmic domain loop-like conformation is required for interaction with CIB. Moreover, mutations of β₃ cytoplasmic domain residues Tyr-747 and/or Tyr-759 to phenylalanine residues (Y747F, Y759F, and Y747,759F) as well as residues Ser-752 to proline or alanine (S752P and S752A), do not affect the α_IIbβ₃ or α_Vβ₃ interaction with CIB. Since tyrosine residues Tyr-747 and/or Tyr-759 are the sites of tyrosine phosphorylation of β₃ subunit, these results suggest that the β₃ integrin-CIB interaction occurs through a β₃-phosphorylation independent mechanism. Likewise, ablation of conformation-dependent affinity change in β₃ Ser752Pro mutation had no affect on CIB-β₃ interaction. In summary, our results demonstrate that the α_IIb-subunit integrin and CIB interaction is non-exclusive and requires the loop-like α_IIb-cytoplasmic domain conformation. An interaction of CIB with α_V-containing integrins provides an additional role for this molecule in keeping with its expression outside of platelets.

Glanzmann thrombasthenia: a review of ITGA2B and ITGB3 defects with emphasis on variants, phenotypic variability, and mouse models

Characterized by mucocutaneous bleeding arising from a lack of platelet aggregation to physiologic stimuli, Glanzmann thrombasthenia (GT) is the archetype-inherited disorder of platelets. Transmitted by autosomal recessive inheritance, platelets in GT have quantitative or qualitative deficiencies of the fibrinogen receptor, αIIbβ3, an integrin coded by the ITGA2B and ITGB3 genes. Despite advances in our understanding of the disease, extensive phenotypic variability with respect to severity and intensity of bleeding remains poorly understood. Importantly, genetic defects of ITGB3 also potentially affect other tissues, for β3 has a wide tissue distribution when present as αvβ3 (the vitronectin receptor). We now look at the repertoire of ITGA2B and ITGB3 gene defects, reexamine the relationship between phenotype and genotype, and review integrin structure in the many variant forms. Evidence for modifications in platelet production is assessed, as is the multifactorial etiology of the clinical expression of the disease. Reports of cardiovascular disease and deep vein thrombosis, cancer, brain disease, bone disorders, and pregnancy defects in GT are discussed in the context of the results obtained for mouse models where nonhemostatic defects of β3-deficiency or nonfunction are being increasingly described.

Emerging roles of SUMO modification in arthritis

Dynamic modification involving small ubiquitin-like modifier (SUMO) has emerged as a new mechanism of protein regulation in mammalian biology. Sumoylation is an ATP-dependent, reversible post-translational modification which occurs under both basal and stressful cellular conditions. Sumoylation profoundly influences protein functions and pertinent biological processes. For example, sumoylation modulates multiple components in the NFkappaB pathway and exerts an anti-inflammatory effect. Likewise, sumoylation of peroxisome proliferator-activated receptor gamma (PPARgamma) augments its anti-inflammatory activity. Current evidence suggests a role of sumoylation for resistance to apoptosis in synovial fibroblasts. Dynamic SUMO regulation controls the biological outcomes initiated by various growth factors involved in cartilage homeostasis, including basic fibroblast growth factors (bFGF or FGF-2), transforming growth factor-beta (TGF-beta) and insulin-like growth factor-1 (IGF-1). The impact of these growth factors on cartilage are through sumoylation-dependent control of the transcription factors (e.g., Smad, Elk-1, HIF-1) that are key regulators of matrix components (e.g., aggrecan, collagen) or cartilage-degrading enzymes (e.g., MMPs, aggrecanases). Thus, SUMO modification appears to profoundly affect chondrocyte and synovial fibroblast biology, including cell survival, inflammatory responses, matrix metabolism and hypoxic responses. More recently, evidence suggests that, in addition to their nuclear roles, the SUMO pathways play crucial roles in mitochondrial activity, cellular senescence, and autophagy. With an increasing number of reports linking SUMO to human diseases like arthritis, it is probable that novel and equally important functions of the sumoylation pathway will be elucidated in the near future.

Ancient origin of the integrin-mediated adhesion and signaling machinery

The evolution of animals (metazoans) from their unicellular ancestors required the emergence of novel mechanisms for cell adhesion and cell-cell communication. One of the most important cell adhesion mechanisms for metazoan development is integrin-mediated adhesion and signaling. The integrin adhesion complex mediates critical interactions between cells and the extracellular matrix, modulating several aspects of cell physiology. To date this machinery has been considered strictly metazoan specific. Here we report the results of a comparative genomic analysis of the integrin adhesion machinery, using genomic data from several unicellular relatives of Metazoa and Fungi. Unexpectedly, we found that core components of the integrin adhesion complex are encoded in the genome of the apusozoan protist Amastigomonas sp., and therefore their origins predate the divergence of Opisthokonta, the clade that includes metazoans and fungi. Furthermore, our analyses suggest that key components of this apparatus have been lost independently in fungi and choanoflagellates. Our data highlight the fact that many of the key genes that had formerly been cited as crucial for metazoan origins have a much earlier origin. This underscores the importance of gene cooption in the unicellular-to-multicellular transition that led to the emergence of the Metazoa.

Integrin-linked kinase is required for radial sorting of axons and Schwann cell remyelination in the peripheral nervous system

During development, Schwann cells (SCs) interpret different extracellular cues to regulate their migration, proliferation, and the remarkable morphological changes associated with the sorting, ensheathment, and myelination of axons. Although interactions between extracellular matrix proteins and integrins are critical to some of these processes, the downstream signaling pathways they control are still poorly understood. Integrin-linked kinase (ILK) is a focal adhesion protein that associates with multiple binding partners to link integrins to the actin cytoskeleton and is thought to participate in integrin and growth factor–mediated signaling. Using SC-specific gene ablation, we report essential functions for ILK in radial sorting of axon bundles and in remyelination in the peripheral nervous system. Our in vivo and in vitro experiments show that ILK negatively regulates Rho/Rho kinase signaling to promote SC process extension and to initiate radial sorting. ILK also facilitates axon remyelination, likely by promoting the activation of downstream molecules such as AKT/protein kinase B.

The antithrombotic potential of selective blockade of talin-dependent integrin alpha IIb beta 3 (platelet GPIIb-IIIa) activation

In vitro studies indicate that binding of talin to the beta(3) integrin cytoplasmic domain (tail) results in integrin alpha(IIb)beta(3) (GPIIb-IIIa) activation. Here we tested the importance of talin binding for integrin activation in vivo and its biological significance by generating mice harboring point mutations in the beta(3) tail. We introduced a beta(3)(Y747A) substitution that disrupts the binding of talin, filamin, and other cytoplasmic proteins and a beta(3)(L746A) substitution that selectively disrupts interactions only with talin. Platelets from animals homozygous for each mutation showed impaired agonist-induced fibrinogen binding and platelet aggregation, providing proof that inside-out signals that activate alpha(IIb)beta(3) require binding of talin to the beta(3) tail. beta(3)(L746A) mice were resistant to both pulmonary thromboembolism and to ferric chloride-induced thrombosis of the carotid artery. Pathological bleeding, measured by the presence of fecal blood and development of anemia, occurred in 53% of beta(3)(Y747A) and virtually all beta(3)-null animals examined. Remarkably, less than 5% of beta(3)(L746A) animals exhibited this form of bleeding. These results establish that alpha(IIb)beta(3) activation in vivo is dependent on the interaction of talin with the beta(3) integrin cytoplasmic domain. Furthermore, they suggest that modulation of beta(3) integrin-talin interactions may provide an attractive target for antithrombotics and result in a reduced risk of pathological bleeding.

Reconciling the roles of FAK in osteoblast differentiation, osteoclast remodeling, and bone regeneration

Integrins link the inside of a cell with its outside environment and in doing so regulate a wide variety of cell behaviors. Integrins are well known for their roles in angiogenesis and cell migration but their functions in bone formation are less clear. The majority of integrin signaling proceeds through focal adhesion kinase (FAK), an essential component of the focal adhesion complex. We generated transgenic mice in which FAK was deleted in osteoblasts and uncovered a previously unknown role in osteoblast differentiation associated with bone healing. FAK mutant cells migrated to the site of skeletal injury and angiogenesis was unaffected yet the transgenic mice still exhibited numerous defects in reparative bone formation. Osteoblast differentiation itself was unperturbed by the loss of FAK, whereas the attachment of osteoclasts to the bone matrix was disrupted in vivo. We postulate that defective bi-directional integrin signaling affects the organization of the collagen matrix. Finally, we present a compensatory candidate molecule, Pyk2, which localized to the focal adhesions in osteoblasts that were lacking FAK.

Calcitonin receptor-stimulated migration of prostate cancer cells is mediated by urokinase receptor-integrin signaling

Abundance of calcitonin (CT) and calcitonin receptor (CTR) mRNA in primary prostate tumors positively correlates with tumor grade, and exogenously added CT increases the invasion of prostate cancer cell lines. We examined acute and chronic actions of CT on migration of highly metastatic PC-3M cells and poorly invasive LNCaP cells on several extracellular matrices in a spheroid disaggregation/migration assay. While PC-3M spheroids displayed maximum disaggregation/migration on vitronectin (VN), LNCaP spheroids preferred collagen but also migrated significantly on VN. Up-regulation of CT significantly enhanced disaggregation/migration of PC-3M spheroids on VN, but not on fibronectin. In contrast, down-regulation of CT, CTR, protein kinase A or urokinase-type plasminogen activator receptor (uPAR) led to amelioration of PC-3M spheroid disaggregation/migration. CT selectively increased surface activity of alpha v beta 3 or alpha 6 beta 5 integrins in PC-3M and LNCaP cell lines, respectively, and uPAR-integrin association. Finally, either CT or urokinase could completely restore migration of CT-knock-down PC-3M spheroids. But, only forced expression of urokinase receptor coupled with exogenous addition of urokinase restored migration of CTR-knock-down spheroids. These results support our hypothesis that up-regulation of CT biosynthesis and activation of CT-CTR axis in primary prostate tumors may have direct relevance in their progression to the metastatic phenotype.

Two distinct cytoplasmic regions of the beta2 integrin chain regulate RhoA function during phagocytosis

α_Mβ₂ integrins mediate phagocytosis of opsonized particles in a process controlled by RhoA, Rho kinase, myosin II, Arp2/3, and actin polymerization. α_Mβ₂, Rho, Arp2/3, and F-actin accumulate underneath bound particles; however, the mechanism regulating Rho function during α_Mβ₂-mediated phagocytosis is poorly understood. We report that the binding of C3bi-opsonized sheep red blood cells (RBCs) to α_Mβ₂ increases Rho-GTP, but not Rac-GTP, levels. Deletion of the cytoplasmic domain of β₂, but not of α_M, abolished Rho recruitment and activation, as well as phagocytic uptake. Interestingly, a 16–amino acid (aa) region in the membrane-proximal half of the β₂ cytoplasmic domain was necessary for activating Rho. Three COOH-terminal residues (aa 758–760) were essential for β₂-induced accumulation of Rho at complement receptor 3 (CR3) phagosomes. Activation of Rho was necessary, but not sufficient, for its stable recruitment underneath bound particles or for uptake. However, recruitment of active Rho was sufficient for phagocytosis. Our data shed light on the mechanism of outside-in signaling, from ligated integrins to the activation of Rho GTPase signaling.

Regulation of the catalytic activity of PTP1B: roles for cell adhesion, tyrosine residue 66, and proline residues 309 and 310

The reversible phosphorylation of proteins on tyrosine residues is fundamental to a variety of intracellular signaling pathways and is controlled by the actions of protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs). While much progress has been made in understanding the regulation of PTKs, there is still relatively little known concerning the regulation of PTPs. Using immune complex phosphatase assays, we demonstrated that the enzymatic activity of the nonreceptor type PTP, PTP1B, is regulated by cell adhesion. Placing primary human foreskin fibroblasts (HFFs) in suspension leads to a distinct increase in PTP1B activity, whereas the readhesion of suspended HFFs onto fibronectin or collagen I inhibited activity. To gain insight into the mechanisms involved, we analyzed recombinant forms of PTP1B mutated at potential regulatory sites. Our results indicated that tyrosine residue 66 is essential for maintaining activity at 37 degrees C. We also found that the C-terminal region of PTP1B and localization to the endoplasmic reticulum are not required for the inhibition of activity by cell adhesion. However, analysis of PA-PTP1B, in which alanines are substituted for prolines 309 and 310, revealed an important role for these residues as the catalytic activity of this mutant did not decrease following readhesion onto collagen I. Since the binding of p130cas and Src to PTP1B is dependent upon these proline residues, we assayed the regulation of PTP1B in mouse embryo fibroblasts deficient in these proteins. We found that neither p130cas nor Src is required for the inhibition of PTP1B activity by adhesion to extracellular matrix proteins. Additionally, pretreatment with cytochalasin D did not prevent the reduction of PTP1B activity when cells adhered to collagen I, indicating that cell spreading is not required for this regulation. The control of the catalytic activity of PTP1B by cell adhesion demonstrated in this study is likely to have important implications for growth factor and insulin signaling.

Renal ischemia/reperfusion injury: functional tissue preservation by anti-activated {beta}1 integrin therapy

Renal ischemia/reperfusion injury (IRI) is an important cause of acute renal failure. Cellular and molecular responses of the kidney to IRI are complex and not fully understood. beta1 integrins localize to the basal surface of tubular epithelium interacting with extracellular matrix components of the basal membrane, including collagen IV. Whether preservation of tubular epithelium integrity could be a therapeutic approach for IRI was assessed. The effects of HUTS-21 mAb administration, which recognizes an activation-dependent epitope of beta1 integrins, in a rat model of IRI were investigated. Preischemic HUTS-21 administration resulted in the preservation of renal functional and histopathologic parameters. Analyses of activated beta1 integrins expression and focal adhesion kinase phosphorylation suggest that its deactivation after IRI was prevented by HUTS-21 treatment. Moreover, HUTS-21 impaired the inflammatory response in vivo, as indicated by inhibition of proinflammatory mediators and the absence of infiltrating cells. Ex vivo adhesion assays using reperfused kidneys revealed that HUTS-21 induced a significant increase of epithelial cell attachment to collagen IV. In conclusion, the data provide evidence that HUTS-21 has a protective effect in renal IRI, preventing tubular epithelial cell detachment by preserving activated beta1 integrins functions.

The integrin-linked kinase regulates cell morphology and motility in a rho-associated kinase-dependent manner

The integrin-linked kinase (ILK) is a multidomain focal adhesion protein implicated in signal transmission from integrin and growth factor receptors. We have determined that ILK regulates U2OS osteosarcoma cell spreading and motility in a manner requiring both kinase activity and localization. Overexpression of wild-type (WT) ILK resulted in suppression of cell spreading, polarization, and motility to fibronectin. Cell lines overexpressing kinase-dead (S343A) or paxillin binding site mutant ILK proteins display inhibited haptotaxis to fibronectin. Conversely, spreading and motility was potentiated in cells expressing the "dominant negative," non-targeting, kinase-deficient E359K ILK protein. Suppression of cell spreading and motility of WT ILK U2OS cells could be rescued by treatment with the Rho-associated kinase (ROCK) inhibitor Y-27632 or introduction of dominant negative ROCK or RhoA, suggesting these cells have increased RhoA signaling. Activation of focal adhesion kinase (FAK), a negative regulator of RhoA, was reduced in WT ILK cells, whereas overexpression of FAK rescued the observed defects in spreading and cell polarity. Thus, ILK-dependent effects on ROCK and/or RhoA signaling may be mediated through FAK.

Competition for talin results in trans-dominant inhibition of integrin activation

The ability of integrin adhesion receptors to undergo rapid changes in affinity for their extracellular ligands (integrin activation) is essential for the development and function of multicellular animals and is dependent on interactions between the integrin beta subunit-cytoplasmic tail and the cytoskeletal protein talin. Cross-talk among different integrins and between integrins and other receptors impacts many cellular processes including adhesion, spreading, migration, clot retraction, proliferation, and differentiation. One form of integrin cross-talk, transdominant inhibition of integrin activation, occurs when ligand binding to one integrin inhibits the activation of a second integrin. This may be relevant clinically in a number of settings such as during platelet adhesion, leukocyte trans-migration, and angiogenesis. Here we report that competition for talin underlies the trans-dominant inhibition of integrin activation. This conclusion is based on our observations that (i). beta tails selectively defective in talin binding are unable to mediate trans-dominant inhibition, (ii). trans-dominant inhibition can be reversed by overexpression of integrin binding and activating fragments of talin, and (iii). expression of another non-integrin talin-binding protein, phosphatidylinositol phosphate kinase type Igamma-90, also inhibits integrin activation. Thus, the sequestration of talin by the suppressive species is both necessary and sufficient for trans-dominant inhibition of integrin activation.

Calcium rises locally trigger focal adhesion disassembly and enhance residency of focal adhesion kinase at focal adhesions

Focal adhesion kinase (FAK) activity and Ca(2+) signaling led to a turnover of focal adhesions (FAs) required for cell spreading and migration. We used yellow Cameleon-2 (Ycam), a fluorescent protein-based Ca(2+) sensor fused to FAK or to a FAK-related non-kinase domain, to measure simultaneously local Ca(2+) variations at FA sites and FA dynamics. Discrete subcellular Ca(2+) oscillators initiate both propagating and abortive Ca(2+) waves in migrating U87 astrocytoma cells. Ca(2+)-dependent FA disassembly occurs when the Ca(2+) wave reaches individual FAs, indicating that local but not global Ca(2+) increases trigger FA disassembly. An unexpectedly rapid flux of FAK between cytosolic and FA compartments was revealed by fluorescence recovery after photobleaching studies. The FAK-Ycam recovery half-time (17 s) at FAs was slowed (to 29 s) by Ca(2+) elevation. FAK-related non-kinase domain-Ycam had a faster, Ca(2+)-insensitive recovery half-time (11 s), which is consistent with the effect of Ca(2+) on FAK-Ycam dynamics not being due to a general modification of the dynamics of FA components. Because FAK association at FAs was prolonged by Ca(2+) and FAK autophosphorylation was correlated to intracellular Ca(2+) levels, we propose that local Ca(2+) elevations increase the residency of FAK at FAs, possibly by means of tyrosine phosphorylation of FAK, thereby leading to increased activation of its effectors involved in FA disassembly.

Regulation of focal adhesion kinase by its amino-terminal domain through an autoinhibitory interaction

We have investigated a role for the amino-terminal FERM-like domain of the focal adhesion kinase (FAK) as a negative regulator of its own activity and phosphorylation state. Deletion of the first 375 amino acids from the amino terminus of FAK increases its catalytic activity in vitro, its phosphorylation when expressed in mammalian cells, and the phosphorylation of a FAK substrate, paxillin. Deletion mutants are phosphorylated in suspension, suggesting that they are no longer regulated by adhesion. The amino terminus of FAK can interact with the kinase domain of FAK in vitro and in vivo, suggesting that it might act as an autoinhibitor of FAK activity. The amino terminus of FAK can act in trans to inhibit FAK phosphorylation when expressed in mammalian cells or to directly inhibit FAK activity in vitro. Expression of the amino terminus of FAK inhibits cell cycle progression in CHO cells, consistent with its inhibition of FAK phosphorylation and function in trans. A glutathione S-transferase fusion protein containing the cytoplasmic tail of the beta1 integrin stimulates FAK activity in vitro, suggesting that FAK could be regulated by molecular interactions with the amino terminus. Based on these and previous data, we propose a working model for activation of FAK in cell adhesion.

Involvement of the beta3 E749ATSTFTN756 region in stabilizing integrin alphaIIbbeta3-ligand interaction

Platelet integrin alphaIIbbeta3 must be activated via intracellular mechanisms before it binds soluble ligands, and it is thought to be activated at its extracellular site by surface-bound ligands. Integrin activation is associated with rearrangement of the cytoskeleton and phosphorylation of proteins that become localized in focal contacts. In these processes, the cytoplasmic tail of the beta-subunit plays a central role. We introduced peptides homologous to the E749ATSTFTN756 domain (E-N peptide) and the T755NITYRGT762 domain (T-T peptide) of beta3 in streptolysin O-permeabilized platelets and analyzed the initial interaction with soluble fibronectin, fibrinogen and PAC-1 after stimulation with thrombin. E-N peptide left the initial binding of fibronectin intact but interfered with stable receptor occupancy. E-N peptide also inhibited fibrinogen binding, thereby reducing the formation of large aggregates. Strikingly, E-N peptide did not disturb the binding of PAC-1, which is known to reflect activation of the integrin. E-N peptide also inhibited tyrosine phosphorylation of focal adhesion kinase, a response known to be dependent on alphaIIbbeta3. T-T peptide did not affect these processes. In a model for outside-in integrin activation, E-N peptide disrupted the binding of CHO cells expressing alphaIIbbeta3 to surface-bound ligand. Again, T-T peptide had no effect. We conclude that the E749ATSTFTN756 region of the beta3-tail stabilizes the binding of soluble and surface-bound ligand to integrin alphaIIbbeta3 via a mechanism that involves the phosphorylation of FAK.

The integrin-actin connection, an eternal love affair

Integrin receptors connect the extracellular matrix to the actin cytoskeleton. This interaction can be viewed as a cyclical liaison, which develops again and again at new adhesion sites only to cease at sites of de-adhesion. Recent work has demonstrated that multidomain proteins play crucial roles in the integrin-actin connection by providing a high degree of regulation adjusted to the needs of the cell. In this review we present several examples of this paradigm and with special emphasis on the ILK-PINCH-parvin complex, which amply demonstrates how structural and signalling functions are linked together.

The integrin beta tail is required and sufficient to regulate adhesion signaling to Rac1

Rac1 is a small Rho family GTPase that regulates changes in cell morphology associated with cell spreading and migration. Integrin-mediated adhesion is known to activate Rac1 and to regulate the interaction of Rac1 with downstream effectors. Currently, it is not clear how integrins signal Rac1 activation following cell adhesion. Integrin beta cytoplasmic domains (beta-tails) are known to be required for integrin-mediated cell spreading, and isolated beta tails expressed as tac-beta tail chimeras can inhibit cell spreading indicating that protein interactions with beta tails can regulate this process. Our recent studies demonstrated that the expression of constitutively activated Rac1 can restore cell spreading inhibited by tac beta tail chimeras, suggesting a role for Rac1 in the regulation of cell spreading by beta tails. Hence, we examined the role of beta tails in integrin activation of Rac1. By using recombinant wild-type and mutant integrin heterodimers, we demonstrate that integrin beta tails are required for adhesion to increase Rac1-GTP loading. We demonstrate that clustering tac-beta tail chimeras, on the surface of cells in suspension, activates Rac1. Thus, beta tails are not only required, but also sufficient for integrin-triggered Rac1 activation. Our findings indicate that integrin beta-tails are an important link between integrin engagement and Rac1 signaling, and that protein interactions initiated at beta tails are sufficient for integrins to regulate Rac1 activity.

Mechanisms of magnesium-stimulated adhesion of osteoblastic cells to commonly used orthopaedic implants

Poor cell adhesion to orthopaedic and dental implants may result in implant failure. Cellular adhesion to biomaterial surfaces primarily is mediated by integrins, which act as signal transduction and adhesion proteins. Because integrin function depends on divalent cations, we investigated the effect of magnesium ions modified bioceramic substrata (Al(2)O(3)-Mg(2+)) on human bone-derived cell (HBDC) adhesion, integrin expression, and activation of intracellular signalling molecules. Immunohistochemistry, flow cytometry, cell adhesion, cell adhesion blocking, and Western blotting assays were used. Our findings demonstrated that adhesion of HBDC to Al(2)O(3)-Mg(2+) was increased compared to on the Mg(2+)-free Al(2)O(3). Furthermore, HBDC adhesion decreased significantly when the fibronectin receptor alpha5beta1- and beta1-integrins were blocked by functional blocking antibodies. HBDC grown on the Mg(2+)-modified bioceramic expressed significantly enhanced levels of beta1-, alpha5beta1-, and alpha3beta1-integrins receptors compared to those grown on the native unmodified Al(2)O(3). Tyrosine phosphorylation of intracellular integrin-dependent signalling proteins as well as the expression of key signalling protein Shc isoforms (p46, p52, p66), focal adhesion kinase, and extracellular matrix protein collagen type I were significantly enhanced when HBDC were grown on Al(2)O(3)-Mg(2+) compared to the native Al(2)O(3). We conclude that cell adhesion to biomaterial surfaces is probably mediated by alpha5beta1- and beta1-integrin. Cation-promoted cell adhesion depends on 5beta1- and beta1-integrins associated signal transduction pathways involving the key signalling protein Shc and results also in enhanced gene expression of extracellular matrix proteins. Therefore, Mg(2+) supplementation of bioceramic substrata may be a promising way to improve integration of implants in orthopaedic and dental surgery.

A Ser752-->Pro substitution in the cytoplasmic domain of beta3 in a Glanzmann thrombasthenia variant fails to prevent interactions between the alphaIIbbeta3 integrin and the platelet granule pool of fibrinogen

A Glanzmann thrombasthenia variant with a beta3 Ser752-->Pro cytoplasmic domain substitution has platelets that fail to aggregate or bind soluble fibrinogen (Fg) after activation. Despite this, Fg is normally present in the alpha-granules. We have used immunoelectron microscopy to examine the reactivity of Fg with the different pools of alphaIIbbeta3 in the patient's platelets. Immunogold labelling was performed on cryosections using an anti-ligand-induced binding site (LIBS) monoclonal antibody (mAb), which binds to alphaIIbbeta3 only when Fg is bound, or a mixture of two anti-receptor-induced binding site (RIBS) mAbs that specifically recognize receptor-bound Fg. Labelling of the alpha-granule membrane and channels of the surface-connected canalicular system in unstimulated platelets confirmed that the mutated alphaIIbbeta3 retains the capacity to transport Fg. When the patient's platelets were stimulated with ADP in the presence of Fg, as expected there was a much-decreased activation of surface-exposed alphaIIbbeta3. However, thrombin-induced activation was associated with both secretion and a rapid increase in the labelling of internal membrane systems by anti-RIBS and anti-LIBS mAbs, with mobilization of the internal Fg pool. Yet labelling on the surface of the patient's platelets was transient. Our studies implied that alphaIIbbeta3 in platelets may bind fibrinogen in different activation states and that this patient specifically lacked high-affinity binding.

Identification of the phosphotyrosine proteome from thrombin activated platelets

Signalling cascades are regulated both positively and negatively by tyrosine phosphorylation. Integrin mediated platelet adhesion triggers signal transduction cascades involving translocation of proteins and tyrosine phosphorylation events, ultimately causing large signalling complexes to be assembled. In resting platelets, a small number of phosphorylated proteins are evident with molecular mass of 50-62 kDa and 120-130 kDa. In thrombin activated human platelets, however, there is a large increase in the number of tyrosine phosphorylated signalling proteins detected. These proteins include pCas (130 kDa), FAK (125 kDa), PI(3)k (85 kDa) and src (85 kDa). However, it is unlikely that this list of proteins represents all the dynamic changes that occur after platelet activation and it is understood that more proteins remain unidentified. In this study, we propose a method for the isolation of the phosphotyrosine proteome from both resting and thrombin activated human platelets. All the dynamic phosphotyrosine events that occur in the platelet after thrombin activation were isolated by immunoprecipitation, using the monoclonal antibody 4G10, allowing us to obtain higher concentrations of relatively low abundant proteins. The resulting phosphotyrosine proteomes were separated by two-dimensional gel electrophoresis. Sixty-seven proteins were reproducibly found to be unique in the thrombin activated platelet proteome when compared to resting platelets. We have positively identified ten of these proteins by Western blotting and matrix-assisted laser desorption/ionisation-time of flight mass spectrometry and these include FAK, Syk, ALK-4, P2X6 and MAPK kinase kinase. This proteomics approach to understanding the signalling events following platelet activation may elucidate potential drug targets for the treatment of coronary thrombosis.

Disruption of C-terminal cytoplasmic domain of betaPS integrin subunit has dominant negative properties in developing Drosophila

We have analyzed a set of new and existing strong mutations in the myospheroid gene, which encodes the betaPS integrin subunit of Drosophila. In addition to missense and other null mutations, three mutants behave as antimorphic alleles, indicative of dominant negative properties. Unlike null alleles, the three antimorphic mutants are synthetically lethal in double heterozygotes with an inflated (alphaPS2) null allele, and they fail to complement very weak, otherwise viable alleles of myospheroid. Two of the antimorphs result from identical splice site lesions, which create a frameshift in the C-terminal half of the cytoplasmic domain of betaPS. The third antimorphic mutation is caused by a stop codon just before the cytoplasmic splice site. These mutant betaPS proteins can support cell spreading in culture, especially under conditions that appear to promote integrin activation. Analyses of developing animals indicate that the dominant negative properties are not a result of inefficient surface expression, or simple competition between functional and nonfunctional proteins. These data indicate that mutations disrupting the C-terminal cytoplasmic domain of integrin beta subunits can have dominant negative effects in situ, at normal levels of expression, and that this property does not necessarily depend on a specific new protein sequence or structure. The results are discussed with respect to similar vertebrate beta subunit cytoplasmic mutations.

Dancing with multiple partners

Transmembrane proteins, such as G protein-coupled receptors (GPCRs) and integrins, activate intracellular signaling pathways through interactions with downstream binding partners. Woodside discusses two examples in which GPCRs and integrins interact in a noncompeting manner with more than one partner. The specific GPCR described is the thrombin receptor, in experiments where G protein peptides selectively block signaling through a particular G protein that does not appear to inhibit coupling of the receptor to other G proteins. The second system described is the alphaIIbbeta3 integrin and its activation of the nonreceptor tyrosine kinase Syk. Syk appeared capable of interacting with both the integrin and intracellular domains of immune response receptors, because binding of Syk to the integrin was not inhibited by peptides based on the Syk binding site in immune response receptors. Thus, multiple, noncompeting binding partners add to the complexity of signal transduction outputs from a single receptor complex.

The involvement of beta1 integrin in the modulation by collagen of chondrocyte-response to transforming growth factor-beta1

The physiologic response of chondrocytes to maintenance of the matrix and response to injury likely involves signaling from multiple sources including soluble cytokines, mechanical stimulation, and signaling from the extracellular matrix. The signaling from the extracellular matrix may serve to effect cell differentiation and to modulate the response to cytokines. We have previously reported that type II collagen modulates the response of bovine articular chondrocytes to TGF-beta1. The molecular nature of the signaling mechanism has not been elucidated but presumably involves a similar mechanism by which the cell attaches to the surrounding matrix. An alginate bead culture system is utilized to which exogenous type II collagen is added. The inclusion of type II collagen results in an alteration of integrin expression with a down regulation of alpha2. The response of the chondrocyte to TGF-beta1 can be modulated by the inclusion of exogenous type II collagen. The modulation of DNA and proteoglycan synthesis was blocked by the treatment of anti-beta1 integrin antibody (4B4) or by cyclic RGD containing peptides. These events occur at concentrations that block cell adhesion to type II collagen. Linear RGD containing peptides and anti-anchorin antibodies had no effect on the modulation by type II collagen. These results suggest that type II collagen binding by chondrocytes at least in part occurs through the beta1 integrin. This binding results in modulation of the cell response to TGF-beta1. This modulation may serve to provide physiologic specificity to the cytokine-signaling cascade. An understanding of the regulatory milieu of the chondrocyte may permit the stimulation of an intrinsic repair of articular cartilage in the future. A near term application of this understanding can be made to tissue engineering attempts at articular cartilage repair.

Apoptosis of adherent cells by recruitment of caspase-8 to unligated integrins

Integrin-mediated adhesion promotes cell survival in vitro, whereas integrin antagonists induce apoptosis of adherent cells in vivo. Here, we demonstrate that cells adherent within a three-dimensional extracellular matrix undergo apoptosis due to expression of unligated integrins, the beta subunit cytoplasmic domain, or its membrane proximal sequence KLLITIHDRKEF. Integrin-mediated death requires initiator, but not stress, caspase activity and is distinct from anoikis, which is caused by the loss of adhesion per se. Surprisingly, unligated integrin or beta integrin tails recruit caspase-8 to the membrane, where it becomes activated in a death receptor-independent manner. Integrin ligation disrupts this integrin-caspase containing complex and increases survival, revealing an unexpected role for integrins in the regulation of apoptosis and tissue remodeling.

Roles for beta(pat-3) integrins in development and function of Caenorhabditis elegans muscles and gonads

Heterodimeric integrin receptors for extracellular matrix (ECM) play vital roles in bidirectional signaling during tissue development, organization, remodeling, and repair. The beta integrin subunit cytoplasmic domain is essential for transmission of many of these signals and overexpression of an unpaired beta tail in cultured cells inhibits endogenous integrins. Unlike vertebrates, which have at least nine beta subunit genes, the nematode Caenorhabditis elegans expresses only one beta subunit (betapat-3), and a null mutation in this gene causes embryonic lethality. To determine the functions of integrins during larval development and in adult tissues, we have taken a dominant negative approach by expression of an HA-betatail transgene composed of a hemagglutinin (HA) epitope tag extracellular domain connected to the betapat-3 transmembrane and cytoplasmic domains. Expression of this transgene in muscle and gonad, major sites of integrin expression, caused a variety of phenotypes dependent on the level of transgene expression. Abnormalities in body wall and sex muscles led to uncoordinated movement and egg-laying defects. Significant anomalies in migration and pathfinding were caused by tissue-specific expression of HA-betatail in the distal tip cells (DTC), the cells that direct gonad morphogenesis. A pat-3 gene with Tyr to Phe mutations in the cytoplasmic domain was able to rescue pat-3 null animals but also showed DTC migration defects. These results show that betapat-3 plays important roles in post-embryonic organogenesis and tissue function.

A functional comparison of mutations in integrin β cytoplasmic domains

Cell adhesion is a multistep process that requires the interaction of integrins with their ligands in cell attachment, the activation of integrin-triggered signals, and cell spreading. Integrin β subunit cytoplasmic domains (β tails) participate in regulating each of these steps; however, it is not known whether the same or different regions within β tails are required. We generated a panel of amino acid substitutions within the β1 and β3 cytoplasmic domains to determine whether distinct regions within β tails regulate different steps in adhesion. We expressed these β cytoplasmic domains in the context of interleukin 2 (IL-2) receptor (tac) chimeras and tested their ability to activate tyrosine phosphorylation, to regulate β1 integrin conformation and to inhibit β1 integrin function in cell attachment and spreading. We found that many of the mutant β1 and β3 chimeras either had no effect on these parameters or dramatically inhibited the function of the β tail in most assays. However, one set of analogous Ala substitutions in the β1 and β3 tails differentially affected the ability of the tac-β1 and tac-β3 chimeras to activate tyrosine phosphorylation. The tac-β1 mutant containing Ala substitutions for the VTT motif did not signal, whereas the analogous tac-β3 mutant was able to activate tyrosine phosphorylation, albeit not to wild-type levels. We also identified a few mutations that inhibited β tail function in only a subset of assays. Ala substitutions for the Val residue in the VTT motif of the β1 tail or for the conserved Asp and Glu residues in the membrane-proximal region of the β3 tail greatly diminished the ability of tac-β1 and tac-β3 to inhibit cell spreading, but had minimal effects in other assays. Ala substitutions for the Trp and Asp residues in the conserved WDT motif in the β1 tail had dramatic effects on the ability of tac-β1 to regulate integrin conformation and function in cell spreading, but had no or intermediate effects in other assays. The identification of mutations in the β1 and β3 tails that specifically abrogated the ability of these β tails to regulate β1 integrin conformation and function in cell spreading suggests that distinct protein interactions with β tails regulate β cytoplasmic domain function in these processes.

Biochemical signals and biological responses elicited by the focal adhesion kinase

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

NMR analysis of structure and dynamics of the cytosolic tails of integrin alpha IIb beta 3 in aqueous solution

The structural and dynamic properties of the cytosolic tails of the adhesion receptor integrin alphaIIbbeta3, fused to a coiled-coil construct via (Gly)(3) linkers, were studied in aqueous solution by nuclear magnetic resonance (NMR) spectroscopy. Both tails were largely flexible and unstructured, although, in the beta3 tail, residues Arg(724)-Ala(735) have a propensity to form a helical structure and residues Asn(744)-Tyr(747) (NPLY) have a propensity to adopt reverse-turn conformations. The mutation beta3(Y747A) disrupted this reverse-turn tendency and markedly reduced the affinity of the head domain of the cytoskeletal protein, talin for the beta3 tail. Omission of the (Gly)(3) linker connecting the coiled-coiled helices and the integrin tails lead to helix propagation into the beta3 tail extending up to eight residues. A variety of different tail constructs were made and studied to reveal tail-tail interactions, but surprisingly no significant interactions between both tails could be detected within the context of our constructs. These results provide structural insight into a highly conserved beta tail motif (NPXY/F) required for integrin signaling and highlight a second transiently structured region (residues Arg(724)-Ala(735)), which might also be of functional significance.

A membrane-distal segment of the integrin alpha IIb cytoplasmic domain regulates integrin activation

Previous evidence suggests that interactions between integrin cytoplasmic domains regulate integrin activation. We have constructed and validated recombinant structural mimics of the heterodimeric alpha(IIb)beta(3) cytoplasmic domain. The mimics elicited polyclonal antibodies that recognize a combinatorial epitope(s) formed in mixtures of the alpha(IIb) and beta(3) cytoplasmic domains but not present in either isolated tail. This epitope(s) is present within intact alpha(IIb)beta(3), indicating that interaction between the tails can occur in the native integrin. Furthermore, the combinatorial epitope(s) is also formed by introducing the activation-blocking beta(3)(Y747A) mutation into the beta(3) tail. A membrane-distal heptapeptide sequence in the alpha(IIb) tail ((997)RPPLEED) is responsible for this effect on beta(3). Membrane-permeant palmitoylated peptides, containing this alpha(IIb) sequence, specifically blocked alpha(IIb)beta(3) activation in platelets. Thus, this region of the alpha(IIb) tail causes the beta(3) tail to resemble that of beta(3)(Y747A) and suppresses activation of the integrin.

The NITY motif of the beta-chain cytoplasmic domain is involved in stimulated internalization of the beta3 integrin A isoform

Beta3 integrin adhesion molecules play important roles in wound repair and the regulation of vascular development and three beta3 integrin isoforms (beta3-A, -B, -C) have been described so far. Surface expression of beta3 integrins is dynamically regulated through internalization of beta3 integrins, however, the molecular mechanisms are understood incompletely. To evaluate the role of the cytoplasmic domain of beta3 integrins for internalization, we have generated single chain chimeras with variant and mutated forms of beta3 cytoplasmic domains. Upon transient transfection into chinese hamster ovary cells, it was found that the beta3-A chimera had strongly reduced cell surface expression compared with the corresponding beta3-B, or beta3-C fusion proteins, or the tail-less constructs, whereas steady state levels of all chimeras were near identical. Studies employing cytoplasmic domain mutants showed that the NITY motif at beta3-A 756–759 is critical for plasma membrane expression of beta3-A. Furthermore, delivery of beta3-A to the cell surface was specifically modulated by the cytoplasmic protein beta3-endonexin, a previously described intracellular protein. Coexpression of the native, long form of beta3-endonexin, which does not interact with the beta3 tail, acted as a dominant negative inhibitor of beta3-A-internalization and enhanced steady-state surface expression of the beta3-A-chimera. Furthermore, anti-beta3 antibody-induced internalization of the native beta3 integrin (alpha(IIb)beta3 was dramatically reduced for the Tyr(759)-Ala substitution mutant (alpha(IIb)beta3) (Y759A) and expression of the long isoform of beta3-endonexin substantially decreased the internalization of wild-type alpha(IIb)beta3. Thus, the NITY motif of the beta-chain cytoplasmic domain is involved in stimulated internalization of the beta3 integrin A isoform and beta3-endonexin appears to couple the beta3-A isoform to a specific receptor-recycling pathway.

Identification of a novel interaction between integrin beta1 and 14-3-3beta

Integrins are cell surface receptors for extracellular matrix, which play important roles in a variety of biological processes. 14-3-3 proteins are a highly conserved family of cytoplasmic proteins that associate with several intracellular signaling molecules in regulation of various cellular functions. Here, we report identification of an interaction between the integrin beta1 cytoplasmic domain and 14-3-3beta by using the yeast two-hybrid screen. Like several other proteins, the integrin beta1 cytoplasmic domain associated with 14-3-3beta by a non-phosphoserine mechanism. The 14-3-3beta/integrin beta1 interaction was confirmed by in vitro binding assays as well as co-precipitation in vivo. Furthermore, we found that 14-3-3beta co-localized with integrin beta1 during the early stage of cell spreading on fibronectin, suggesting a potential role of the 14-3-3beta/integrin beta1 interaction in the regulation of cell adhesion. Using tetracycline-regulated expression system, we showed that overexpression of 14-3-3beta stimulated cell spreading and migration on fibronectin but not on poly-L-lysine. However, the induced expression of 14-3-3beta did not affect tyrosine phosphorylation of FAK or its substrates, p130(cas) and paxillin, suggesting that 14-3-3beta regulated integrin-mediated cell spreading and migration by FAK-independent mechanisms. Taken together, these results identify an interaction between integrin and 14-3-3 proteins and suggest a potentially novel cellular function for 14-3-3 proteins in the regulation of integrin-mediated cell adhesion and signaling events.

The ability of integrin alpha(v)beta(3) To function as a receptor for foot-and-mouth disease virus is not dependent on the presence of complete subunit cytoplasmic domains

The integrin alpha(v)beta(3) has been shown to function as one of the integrin receptors on cultured cells for foot-and-mouth disease virus (FMDV), and high-efficiency utilization of the bovine homolog of this integrin is dependent on the cysteine-rich repeat region of the bovine beta(3) subunit. In this study we have examined the role of the cytoplasmic domains of the alpha(v) and beta(3) subunits in FMDV infection. We have found that truncations or extensions of these domains of either subunit, including deletions removing almost all of the cytoplasmic domains, had little or no effect on the ability of the integrin to function as a receptor for FMDV. The lysosomotropic agent monensin inhibited viral replication in cells transfected with either intact or cytoplasmic domain-truncated alpha(v)beta(3). In addition, viral replication in transfected cells was inhibited by an alpha(v)beta(3) function-blocking antibody but not by function-blocking antibodies to three other RGD-directed integrins, suggesting that these integrins are not involved in the infectious process. These results indicate that alterations to the cytoplasmic domains of either subunit, which lead to the inability of the integrin receptor to function normally, do not abolish the ability of the integrin to bind and internalize this viral ligand.

Integrin cytoplasmic domain-binding proteins

Integrins are a large family of cell surface receptors that mediate cell adhesion and influence migration, signal transduction, and gene expression. The cytoplasmic domains of integrins play a pivotal role in these integrin-mediated cellular functions. Through interaction with the cytoskeleton, signaling molecules, and other cellular proteins, integrin cytoplasmic domains transduce signals from both the outside and inside of the cell and regulate integrin-mediated biological functions. Identification and functional analyses of integrin cytoplasmic domain-binding proteins have been pursued intensively. In recent years, more cellular proteins have been reported to directly interact with integrin cytoplasmic domains and some of these interactions may play important roles in integrin-mediated biological responses. Integrin (β) chains, for example, interact with actin-binding proteins (e.g. talin and filamin), which form mechanical links to the cytoskeleton. These and other proteins (e.g. FAK, ILK and novel proteins such as TAP20) might also link integrins to signaling mechanisms and, in some cases (e.g. JAB1) mediate integrin-dependent gene regulation.

Threonine phosphorylation of the beta 3 integrin cytoplasmic tail, at a site recognized by PDK1 and Akt/PKB in vitro, regulates Shc binding

The mechanism of outside-in signaling by integrins parallels that for growth factor receptors. In both pathways, phosphorylation of a cytoplasmic segment on tyrosine generates a docking site for proteins containing Src homology 2 (SH2) and phosphotyrosine binding domains. We recently observed that phosphorylation of a threonine (Thr-753), six amino acids proximal to tyrosine 759 in beta(3) of the platelet specific integrin alpha(IIb)beta(3), inhibits outside-in signaling through this receptor. We hypothesized that the presence of phosphothreonine 753 either renders beta(3) a poor substrate for tyrosine kinases or inhibits the docking capabilities of the tyrosyl-phosphorylated form of beta(3.) The first alternative was tested by comparing the phosphorylation of beta(3) model peptides by the tyrosine kinase pp60(c-src) and we found that the presence of a phosphate group on a residue corresponding to Thr-753 did not detectably alter the kinetics of tyrosine phosphorylation. However, the presence of phosphate on this threonine inhibited the binding of Shc to tyrosyl-phosphorylated beta(3) peptide. The inhibitory effect of the phosphate group could be mimicked by substituting an aspartic acid for Thr-753, suggesting that a negative charge at this position modulates the binding of Shc and possibly other phosphotyrosine binding domain- and SH2-containing proteins. A survey of several protein kinases revealed that Thr-753 was avidly phosphorylated by PDK1 and Akt/PKB in vitro. These observations suggest that activation of PDK1 and/or Akt/PKB in platelets may modulate the binding activity and/or specificity of beta(3) for signaling molecules.

Restoration of positioning control following Disabled-2 expression in ovarian and breast tumor cells

The physical interaction of epithelial cells with the basement membrane ensures correct positioning and acts as a survival factor for epithelial cells. Cells that detach from the basement membrane often undergo apoptosis; however, in carcinomas, this positional control is absent, permitting disorganized cell proliferation. In the majority of breast and ovarian carcinomas (85-90%), the expression of a candidate tumor suppressor, Disabled-2 (Dab2), is frequently lost. The Dab2-negative tumor cells are no longer in contact with an intact basement membrane, as indicated by the absence of collagen IV (in about 90% of cases). However, in the subset (10-15%) of ovarian tumors in which Dab2 expression is positive, the presence of a basement membrane-like structure around tumor cells was observed. Recombinant adenovirus-mediated expression of Dab2 was used in Dab2-negative ovarian and breast cancer cells, and re-expression of Dab2 was found to lead to cell death or growth arrest. Dab2 expression suppressed MAPK activation and c-fos expression. Plating the infected cells on a basement membrane matrigel rescued the cells from death and growth arrest. Thus, Dab2 exhibits a negative activity for cell growth and survival, which can be countered by attachment of the cells to basement membrane matrix. We conclude that Dab2 functions in cell positioning control and mediates the exigency for basement membrane attachment of epithelial cells. Loss of Dab2 may contribute to the basement membrane-independent, disorganized proliferation of tumor cells in ovarian and breast carcinomas.

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

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

Differential expression of alphav integrins in K1735 melanoma cells

Tumor cell adherence to and migration on the extracellular matrix is an important aspect of cancer progression. This interaction with the extracellular matrix is mediated primarily through the integrin class of cell adhesion molecules. We identified a restricted expression of alphavbeta3 in highly metastatic K1735M2 and of alphavbeta5 in poorly metastatic K1735C23 murine melanoma cells. The highly metastatic cells were ten times more motile on vitronectin and fibronectin and approximately three times more invasive through a reconstituted basement membrane than the poorly metastatic cells. This motility was inhibited by addition of anti-beta3 antibodies. Injection of the alphavbeta3-negative K1735C23 cells into syngeneic mice resulted in the generation of a metastatic variant (K1735C23PM) that neo expressed the alphavbeta3 complex, indicating that expression of alphavbeta3 is required for K1735 melanoma metastasis. Injection of highly metastatic K1735M2 cells in the presence of blocking antibody to beta3 reduced tumor size by approximately 80%. Treatment of the K1735M2 cells with a retroviral antisense beta3 construct significantly reduced their expression of alphavbeta3 and also reduced their motility on extracellular matrix ligands and their invasion through a reconstituted basement membrane. In contrast, when the K1735C23 cells were treated with a construct containing the full-length beta3 cDNA, their motility on extracellular matrix proteins and invasion of a reconstituted basement membrane were significantly increased. These results indicate that alphavbeta3 is required for migration and invasion of K1735 melanoma cells in vitro and primary tumor growth and metastasis in vivo.

Disruption of integrin-dependent adhesion and survival of endothelial cells by recombinant adenovirus expressing isolated beta integrin cytoplasmic domains

We explored the possibility of using a genetic approach to inhibit integrin-mediated endothelial cell adhesion and survival. We constructed recombinant adenoviruses (Ads) expressing chimeric proteins consisting of the cytoplasmic and transmembrane domains of integrin beta1 (CH1), beta3 (CH3) or the beta1 transmembrane domain alone (CH2) connected to the extracellular domain of L3T4 placed under the control of the CMV promoter (AdCMV) or the endothelial cell specific Tie-1 promoter (AdTie). All constructs were expressed in a dose- and time-dependent manner with over 90% of cells expressing the constructs within 24 h (AdCMVs) or 72 h (AdTies) after infection. Confluent monolayers of HUVEC infected with AdCMVCH1 or AdCMVCH3 detached from the substrate in a time- and dose-dependent manner with over 95% of the cells being detached 2 days (AdCMVs) or 3 to 4 days (AdTies) after infection. Cell detachment was preceded by the disruption of focal adhesions and reorganization of the actin cytoskeleton and was associated with a reduced ligand-binding activity of beta1, while cell surface density of beta1 integrins remained unchanged. Detached cells failed to re-adhere to different matrix proteins, without, however, any specificity toward beta1 or beta3 integrin-mediated adhesion. Upon detachment, HUVEC rapidly died by apoptosis. These results demonstrate that dominant negative inhibition of integrin function is an effective approach to disrupt endothelial cell adhesion and survival in vitro.

The cytoplasmic tyrosines of integrin subunit beta1 are involved in focal adhesion kinase activation

We have previously shown that mutation of the two tyrosines in the cytoplasmic domain of integrin subunit beta1 (Y783 and Y795) to phenylalanines markedly reduces the capability of beta1A integrins to mediate directed cell migration. In this study, beta1-dependent cell spreading was found to be delayed in GD25 cells expressing beta1A(Y783/795F) compared to that in wild-type GD25-beta1A. Focal adhesion kinase (FAK) tyrosine phosphorylation and activation were severely impaired in response to beta1-dependent adhesion in GD25-beta1A(Y783/795F) cells compared to that in wild-type GD25-beta1A or mutants in which only a single tyrosine was altered (beta1A(Y783F) or beta1A(Y795F)). Phosphorylation site-specific antibodies selective for FAK phosphotyrosine 397 indicated that the defect in FAK phosphorylation via beta1A(Y783/795F) lies at the level of the initial autophosphorylation step. Indeed, beta1A-dependent tyrosine phosphorylation of tensin and paxillin was lost in the beta1A(Y783/795F) cells, consistent with the impairment in FAK activation. In contrast, p130(CAS) overall tyrosine phosphorylation was unaffected by the beta1 mutations. Despite the defect in beta1-mediated FAK activation, FAK was still localized to focal adhesions. Taken together, the phenotype of the GD25-beta1A(Y783/795F) cells resembles, but is distinct from, the phenotype observed in FAK-null cells. These observations argue that tyrosines 783 and 795 within the cytoplasmic tail of integrin subunit beta1A are critical mediators of FAK activation and cell spreading in GD25 cells.