Kinetic regulation of beta 3 integrin tyrosine phosphorylation

An RGDKGE-Containing Cryptic Collagen Fragment Regulates Phosphorylation of Large Tumor Suppressor Kinase-1 and Controls Ovarian Tumor Growth by a Yes-Associated Protein-Dependent Mechanism

The growth and spread of malignant tumors, such as ovarian carcinomas, are governed in part by complex interconnected signaling cascades occurring between stromal and tumor cells. These reciprocal cross-talk signaling networks operating within the local tissue microenvironment may enhance malignant tumor progression. Understanding how novel bioactive molecules generated within the tumor microenvironment regulate signaling pathways in distinct cellular compartments is critical for the development of more effective treatment paradigms. Herein, we provide evidence that blocking cellular interactions with an RGDKGE-containing collagen peptide that selectively binds integrin β3 on ovarian tumor cells enhances the phosphorylation of the hippo effector kinase large tumor suppressor kinase-1 and reduces nuclear accumulation of yes-associated protein and its target gene c-Myc. Selectively targeting this RGDKGE-containing collagen fragment inhibited ovarian tumor growth and the development of ascites fluid in vivo. These findings suggest that this bioactive collagen fragment may represent a previously unknown regulator of the hippo effector kinase large tumor suppressor kinase-1 and regulate ovarian tumor growth by a yes-associated protein-dependent mechanism. Taken together, these data not only provide new mechanistic insight into how a unique collagen fragment may regulate ovarian cancer, but in addition may help provide a useful new alternative strategy to control ovarian tumor progression based on selectively disrupting a previously unappreciated signaling cascade.

Effects of Xuefu Zhuyu Decoction on Cell Migration and Ocular Tumor Invasion inDrosophila

Xuefu Zhuyu Decoction (XFZYD), a Traditional Chinese Medicine (TCM) decoction mainly for treating blood stasis syndrome, has been widely investigated and applied in clinic and in laboratory. XFZYD contains 11 herbs and has been identified to promoting blood circulation to remove blood stasis for cardiovascular disease. Meanwhile, blood stasis is directly related to malignant tumor according to TCM basic theory. However, the effects of XFZYD on tumor metastasis and the underlying mechanisms are still largely unknown. Here, we employed well-establishedDrosophilacell migration and tumor invasion models to explore whether XFZYD has the anticancer activity on tumor metastasisin vivo. Our work has demonstrated that XFZYD could suppress cell migration and tumor invasion at the moderate concentrations. In addition, XFZYD altered the expression of MMP1,β-integrin, and E-cadherin to impede cell migration. Moreover, XFZYD inhibited ocular tumor invasion presumably by reducing the activity of Notch signaling. Together, these evidences reveal a positive role of XFZYD in suppressing cell migration and tumor metastasis, providing the potential drug targets and key clues for cancer clinical treatment strategies.

The thyroid hormone-αvβ3 integrin axis in ovarian cancer: regulation of gene transcription and MAPK-dependent proliferation

Ovarian carcinoma is the fifth common cause of cancer death in women, despite advanced therapeutic approaches. αvβ3 integrin, a plasma membrane receptor, binds thyroid hormones (L-thyroxine, T4; 3,5,3'-triiodo-L-thyronine, T3) and is overexpressed in ovarian cancer. We have demonstrated selective binding of fluorescently labeled hormones to αvβ3-positive ovarian cancer cells but not to integrin-negative cells. Physiologically relevant T3 (1 nM) and T4 (100 nM) concentrations in OVCAR-3 (high αvβ3) and A2780 (low αvβ3) cells promoted αv and β3 transcription in association with basal integrin levels. This transcription was effectively blocked by RGD (Arg-Gly-Asp) peptide and neutralizing αvβ3 antibodies, excluding T3-induced β3 messenger RNA, suggesting subspecialization of T3 and T4 binding to the integrin receptor pocket. We have provided support for extracellular regulated kinase (ERK)-mediated transcriptional regulation of the αv monomer by T3 and of β3 monomer by both hormones and documented a rapid (30-120 min) and dose-dependent (0.1-1000 nM) ERK activation. OVCAR-3 cells and αvβ3-deficient HEK293 cells treated with αvβ3 blockers confirmed the requirement for an intact thyroid hormone-integrin interaction in ERK activation. In addition, novel data indicated that T4, but not T3, controls integrin's outside-in signaling by phosphorylating tyrosine 759 in the β3 subunit. Both hormones induced cell proliferation (cell counts), survival (Annexin-PI), viability (WST-1) and significantly reduced the expression of genes that inhibit cell cycle (p21, p16), promote mitochondrial apoptosis (Nix, PUMA) and tumor suppression (GDF-15, IGFBP-6), particularly in cells with high integrin expression. At last, we have confirmed that hypothyroid environment attenuated ovarian cancer growth using a novel experimental platform that exploited paired euthyroid and severe hypothyroid serum samples from human subjects. To conclude, our data define a critical role for thyroid hormones as potent αvβ3-ligands, driving ovarian cancer cell proliferation and suggest that disruption of this axis may present a novel treatment strategy in this aggressive disease.

Regulation of integrin activity by phosphorylation

Integrins are heterodimeric complex type I membrane proteins involved in cellular adhesion and signaling. They exist as inactive molecules in resting cells, and need activation to become adhesive. Although much is known about their structure, and a large number of interacting molecules have been described, we still only partially understand how their activities are regulated. In this review we focus on the leukocyte-specific β2-integrins and, specifically, on the role of integrin phosphorylation in the regulation of activity. Phosphorylation reactions can be fast and reversible, thus enabling strictly directed regulatory activities both time-wise and locally in specific regions of the plasma membrane in different leukocytes.

Insulin-like growth factor (IGF) signaling requires αvβ3-IGF1-IGF type 1 receptor (IGF1R) ternary complex formation in anchorage independence, and the complex formation does not require IGF1R and Src activation

Integrin αvβ3 plays a role in insulin-like growth factor 1 (IGF1) signaling (integrin-IGF1 receptor (IGF1R) cross-talk) in non-transformed cells in anchorage-dependent conditions. We reported previously that IGF1 directly binds to αvβ3 and induces αvβ3-IGF1-IGF1R ternary complex formation in these conditions. The integrin-binding defective IGF1 mutant (R36E/R37E) is defective in inducing ternary complex formation and IGF signaling, whereas it still binds to IGF1R. We studied if IGF1 can induce signaling in anchorage-independent conditions in transformed Chinese hamster ovary cells that express αvβ3 (β3-CHO) cells. Here we describe that IGF1 signals were more clearly detectable in anchorage-independent conditions (polyHEMA-coated plates) than in anchorage-dependent conditions. This suggests that IGF signaling is masked by signals from cell-matrix interaction in anchorage-dependent conditions. IGF signaling required αvβ3 expression, and R36E/R37E was defective in inducing signals in polyHEMA-coated plates. These results suggest that αvβ3-IGF1 interaction, not αvβ3-extracellular matrix interaction, is essential for IGF signaling. Inhibitors of IGF1R, Src, AKT, and ERK1/2 did not suppress αvβ3-IGF-IGF1R ternary complex formation, suggesting that activation of these kinases are not required for ternary complex formation. Also, mutations of the β3 cytoplasmic tail (Y747F and Y759F) that block β3 tyrosine phosphorylation did not affect IGF1R phosphorylation or AKT activation. We propose a model in which IGF1 binding to IGF1R induces recruitment of integrin αvβ3 to the IGF-IGF1R complex and then β3 and IGF1R are phosphorylated. It is likely that αvβ3 should be together with the IGF1-IGF1R complex for triggering IGF signaling.

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.

Heparin modulates the conformation and signaling of platelet integrin αIIbβ3

INTRODUCTION

The glycosaminoglycan heparin has been shown to bind to platelet integrin αIIbβ3 and induce platelet activation and aggregation, although the relationship between binding and activation is unclear. We analyzed the interaction of heparin and αIIbβ3 in detail, to obtain a better understanding of the mechanism by which heparin acts on platelets.

METHODS

We assessed conformational changes in αIIbβ3 by flow cytometry of platelets exposed to unfractionated heparin. In human platelets and K562 cells engineered to express αIIbβ3, we assayed the effect of heparin on key steps in integrin signaling: phosphorylation of the β3 chain cytoplasmic tail, and activation of src kinase. We measured the heparin binding affinity of purified αIIbβ3, and of recombinant fragments of αIIb and β3, by surface plasmon resonance.

RESULTS AND CONCLUSIONS

Heparin binding results in conformational changes in αIIbβ3, similar to those observed upon ligand binding. Heparin binding alone is not sufficient to induce tyrosine phosphorylation of the integrin β3 cytoplasmic domain, but the presence of heparin increased both β3 phosphorylation and src kinase activation in response to ligand binding. Specific recombinant fragments derived from αIIb bound heparin, while recombinant β3 did not bind. This pattern of heparin binding, compared to the crystal structure of αIIbβ3, suggests that heparin-binding sites are located in clusters of basic amino acids in the headpiece and/or leg domains of αIIb. Binding of heparin to these clusters may stabilize the transition of αIIbβ3 to an open conformation with enhanced affinity for ligand, facilitating outside-in signaling and platelet activation.

Lnk regulates integrin alphaIIbbeta3 outside-in signaling in mouse platelets, leading to stabilization of thrombus development in vivo

The nature of the in vivo cellular events underlying thrombus formation mediated by platelet activation remains unclear because of the absence of a modality for analysis. Lymphocyte adaptor protein (Lnk; also known as Sh2b3) is an adaptor protein that inhibits thrombopoietin-mediated signaling, and as a result, megakaryocyte and platelet counts are elevated in Lnk-/- mice. Here we describe an unanticipated role for Lnk in stabilizing thrombus formation and clarify the activities of Lnk in platelets transduced through integrin alphaIIbbeta3-mediated outside-in signaling. We equalized platelet counts in wild-type and Lnk-/- mice by using genetic depletion of Lnk and BM transplantation. Using FeCl3- or laser-induced injury and in vivo imaging that enabled observation of single platelet behavior and the multiple steps in thrombus formation, we determined that Lnk is an essential contributor to the stabilization of developing thrombi within vessels. Lnk-/- platelets exhibited a reduced ability to fully spread on fibrinogen and mediate clot retraction, reduced tyrosine phosphorylation of the beta3 integrin subunit, and reduced binding of Fyn to integrin alphaIIbbeta3. These results provide new insight into the mechanism of alphaIIbbeta3-based outside-in signaling, which appears to be coordinated in platelets by Lnk, Fyn, and integrins. Outside-in signaling modulators could represent new therapeutic targets for the prevention of cardiovascular events.

FAK-independent alphavbeta3 integrin-EGFR complexes rescue from anoikis matrix-defective fibroblasts

Extracellular matrix (ECM) binding to integrin receptors regulates cell cycle progression and survival. In adherent cells, ECM disassembly induces anoikis, the apoptotic pathway switched on by loss of adhesion. ECM-deficient Ehlers-Danlos syndrome (EDS) fibroblasts, to adhere to rare fibronectin (FN) fibrils, and to proliferate, only organize, as FN receptor, the alphavbeta3 integrin. We report that in EDS cells the alphavbeta3 integrin is bound to talin and vinculin, but not to tensin, and that actin cytoskeleton is disorganized. Furthermore, in EDS cells Bcl-2 is down-regulated and caspases are active. We provide evidence that the antibody-mediated alphavbeta3 integrin or the FN inhibition induces anoikis in EDS cells. The alphavbeta3 integrin transduces survival signals to pp60src-mediated tyrosine phosphorylated paxillin, instead than to FAK, and interacts with EGF receptor (EGFR). This complex, when activated by EGF and FN, signals for the rescue of EDS cells from anoikis. Therefore, EDS cells, through the alphavbeta3 integrin-EGFR complexes, engage a paxillin- but not FAK-mediated pathway of cell survival.

Integrins control dendritic spine plasticity in hippocampal neurons through NMDA receptor and Ca2+/calmodulin-dependent protein kinase II-mediated actin reorganization

The formation of dendritic spines during development and their structural plasticity in the adult brain are critical aspects of synaptogenesis and synaptic plasticity. Many different factors and proteins have been shown to control dendritic spine development and remodeling (Ethell and Pasquale, 2005). The extracellular matrix (ECM) components and their cell surface receptors, integrins, have been found in the vicinity of synapses and shown to regulate synaptic efficacy and play an important role in long-term potentiation (Bahr et al., 1997; Chavis and Westbrook, 2001; Chan et al., 2003; Lin et al., 2003; Bernard-Trifilo et al., 2005). Although molecular mechanisms by which integrins affect synaptic efficacy have begun to emerge, their role in structural plasticity is poorly understood. Here, we show that integrins are involved in spine remodeling in cultured hippocampal neurons. The treatment of 14 d in vitro hippocampal neurons with arginine-glycine-aspartate (RGD)-containing peptide, an established integrin ligand, induced elongation of existing dendritic spines and promoted formation of new filopodia. These effects were also accompanied by integrin-dependent actin reorganization and synapse remodeling, which were partially inhibited by function-blocking antibodies against beta1 and beta3 integrins. This actin reorganization was blocked with the NMDA receptor (NMDAR) antagonist MK801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate]. The Ca2+/calmodulin-dependent protein kinase II (CaMKII) inhibitor KN93 (N-[2-[N-(4-chlorocinnamyl)-N-methylaminomethyl]phenyl]-N-(2-hydroxyethyl)-4-methoxybenzenesulfonamide) also suppressed RGD-induced actin reorganization and synapse remodeling. Our findings show that integrins control ECM-mediated spine remodeling in hippocampal neurons through NMDAR/CaMKII-dependent actin reorganization.

Purified Integrin Adhesion Complexes Exhibit Actin-Polymerization Activity

BACKGROUND

Cell adhesion and motility are accomplished through a functional linkage of the extracellular matrix with the actin cytoskeleton via adhesion complexes composed of integrin receptors and associated proteins. To determine whether this linkage is attained actively or passively, we isolated integrin complexes from nonadherent hematopoietic cells and determined their influence on the polymerization of actin.

RESULTS

We observed that alpha(V)beta3 complexes are capable of dramatically accelerating the rate of actin assembly, resulting in actin fibers tethered at their growing ends by clustered integrins. The ability to enhance actin polymerization was dependent upon Arg-Gly-Asp-ligand-induced beta3 tyrosine phosphorylation, agonist-induced cellular activation, sequestration of Diaphanous formins, and clustering of the receptor.

CONCLUSIONS

These results suggest that adhesion complexes actively promote actin assembly from their cytosolic face in order to establish a mechanical linkage with the extracellular matrix.

Evidence for a differential functional regulation of the two beta(3)-integrins alpha(V)beta(3) and alpha(IIb)beta(3)

The functional regulation of integrins is a major determinant of cell adhesion, migration and tissue maintenance. The binding of cytoskeletal proteins to various sites of integrin cytoplasmic domains is a key mechanism of this functional regulation. Expression of recombinant integrin alpha(IIb)beta(3) and alpha(M)beta(2) lacking the GFFKR-region in CHO cells results in constitutively activated integrins. In contrast, CHO cells stably expressing either a GFFKR-deleted alpha(V(del))beta(3) or a FF to AA-substituted alpha(V(AA))beta(3) do not reveal a constitutively activated integrin. Adhesion to immobilized fibrinogen is strongly impaired in alpha(V(del))beta(3) or alpha(V(AA))beta(3)-expressing cells, whereas it is not impaired in alpha(IIb)beta(3) and alpha(M)beta(2), both lacking the GFFKR-region. In a parallel plate flow chamber assay, alpha(V)beta(3)-expressing cells adhere firmly to fibrinogen and spread even at shear rates of 15 to 20 dyn/cm(2), whereas alpha(V(del))beta(3) or alpha(V(AA))beta(3) cells are detached at 15 dyn/cm(2). Actin stress fiber formation and focal adhesion plaques containing alpha(V)beta(3) are observed in alpha(V)beta(3) cells but not in alpha(V(del))beta(3) or alpha(V(AA))beta(3)-expressing cells. As an additional manifestation of impaired outside-in signaling, phosphorylation of pp125(FAK) was reduced in these cells. In summary, we report that the GFFKR-region of the alpha(V)-cytoplasmic domain and in particular two phenylalanines are essential for integrin alpha(V)beta(3) function, especially for outside-in signaling. Our results suggest that the two beta(3)-integrins alpha(IIb)beta(3) and alpha(V)beta(3) are differentially regulated via their GFFKR-region.

Integrin alphaIIb-subunit cytoplasmic domain mutations demonstrate a requirement for tyrosine phosphorylation of beta3-subunits in actin cytoskeletal organization

Using truncated or mutated alphaIIb integrin cytoplasmic domains fused to the alphaV extracellular domain and expressed with the beta3 integrin subunit, we demonstrate that the double mutation of proline residues 998 and 999 to alanine (PP998/999AA), previously shown to disturb the C-terminal conformation of the alphaIIb integrin cytoplasmic domain, prevents tyrosine phosphorylation of beta3 integrin induced by Arg-Gly-Asp peptide ligation. This mutation also inhibits integrin mediated actin assembly and cell adhesion to vitronectin. In contrast, progressive truncation of the alphaIIb-subunit cytoplasmic domain did not reproduce these effects. Interestingly, the PP998/999AA mutations of alphaIIb did not affect beta3 tyrosine phosphorylation, cell adhesion, or actin polymerization induced by manganese. Exogenous addition of manganese was sufficient to rescue beta3 phosphorylation, cell adhesion, and actin assembly in cells expressing the PP998/999AA mutation when presented with a vitronectin substrate. Further, induction of the high affinity conformation of this mutant beta3 integrin by incubation with either Arg-Gly-Asp peptide or exogenous manganese was equivalent. These results suggest that the extracellular structure of beta3 integrins in the high affinity conformation is not directly related to the structure of the cytoplasmic face of the integrin. Moreover, the requirement for beta3 phosphorylation is demonstrated without mutation of the beta3 subunit. In support of our previous hypothesis of a role for beta3 phosphorylation in adhesion, these studies demonstrate a strong correlation between beta3 tyrosine phosphorylation and assembly of a cytoskeleton competent to support firm cell adhesion.

β3 Tyrosine Phosphorylation and αvβ3-mediated Adhesion Are Required for Vav1 Association and Rho Activation in Leukocytes

Integrin alpha(v)beta(3)-mediated adhesion of hematopoietic cells to vitronectin results in activation of the Rho GTPases. Mutation of beta(3) tyrosine residue 747, previously shown to disrupt cell adhesion, results in sustained activation of Cdc42 and diminished Rac and Rho activity. We investigated the role of the hematopoietically restricted guanine nucleotide exchange factor Vav1 in alpha(v)beta(3)-mediated adhesion. We find that Vav1, a guanine nucleotide exchange factor for Rac and Rho, associates with alpha(v)beta(3) upon cell adhesion to vitronectin and that this association requires beta(3) tyrosine phosphorylation. Expression of exogenous Vav1 demonstrates that Y160F, but not wild type or the Vav1Y174F mutant, inhibits Rac and Rho activation during alpha(v)beta(3)-mediated cell adhesion to vitronectin. Cells expressing Vav1Y160F exhibit a sustained Cdc42 activation similar to nonphosphorylatable beta(3) mutants. In addition, cytoskeletal reorganization and cell adhesion are severely suppressed in Vav1Y160F-transfected cells, and Vav1Y160F fails to associate with beta(3) integrins. Furthermore, Vav1 itself is selectively phosphorylated upon tyrosine 160 after alpha(v)beta(3)-mediated adhesion, and the association between Vav1 and beta(3) occurs in specific response to adhesion to substrate. These studies describe a phosphorylation-dependent association between beta(3) integrin and Vav1 which is essential for cell progression to a Rho-dominant phenotype during cell adhesion.

(alpha)v(beta)3 integrins and Pyk2 mediate insulin-like growth factor I activation of Src and mitogen-activated protein kinase in 3T3-L1 cells

IGF-I stimulates cell growth through interaction of the IGF receptor with multiprotein signaling complexes. However, the mechanisms of IGF-I receptor-mediated signaling are not completely understood. We have previously shown that IGF-I-stimulated 3T3-L1 cell proliferation is dependent on Src activation of the ERK-1/2 MAPK pathway. We hypothesized that IGF-I activation of the MAPK pathway is mediated through integrin activation of Src-containing signaling complexes. The disintegrin echistatin decreased IGF-I phosphorylation of Src and MAPK, and blocking antibodies to (alpha)v and beta3 integrin subunits inhibited IGF-I activation of MAPK, suggesting that (alpha)v(beta)3 integrins mediate IGF-I mitogenic signaling. IGF-I increased ligand binding to (alpha)v(beta)3 as detected by immunofluorescent staining of ligand-induced binding site antibody and stimulated phosphorylation of the beta3 subunit, consistent with inside-out activation of (alpha)v(beta)3 integrins. IGF-I increased tyrosine phosphorylation of the focal adhesion kinase (FAK) Pyk2 (calcium-dependent proline-rich tyrosine kinase-2) to a much greater extent than FAK, and increased association of Src with Pyk2 but not FAK. The intracellular calcium chelator BAPTA prevented IGF-I phosphorylation of Pyk2, Src, and MAPK, suggesting that IGF-I activation of Pyk2 is calcium dependent. Transient transfection with a dominant-negative Pyk2, which lacks the autophosphorylation and Src binding site, decreased IGF-I activation of MAPK, but no inhibition was seen with transfected wild-type Pyk2. These results indicate that IGF-I signaling to MAPK is dependent on inside-out activation of (alpha)v(beta)3 integrins and integrin-facilitated multiprotein complex formation involving Pyk2 activation and association with Src.

Integrating an integrin: a direct route to actin

Integrins were so named for their ability to link the extracellular and intracellular skeletons. Now almost 20 years into integrin research, numerous questions remain as to how this interaction is accomplished and how it is modified to achieve a desired phenotype. As the cell adhesion and actin assembly fields are merging in combined approaches, novel actin assembly mechanisms are being uncovered. Some of the earliest identified cytoplasmic linker molecules, believed to mediate integrin-actin binding, are once again the subject of scrutiny as potential dynamic mediators of cell anchorage. It seems plausible that each unique cellular morphology occurs as the result of activation of distinct actin assembly systems that are either stabilized by unique bundling and linker proteins or modified for progression to a new phenotype. While this research initiative is likely to continue rapidly in a forward fashion, it remains to be clarified how integrins assemble the most stable and basic cytoskeletal phenotype, the adherent cell with prominent stress fibers. Recent investigations point towards a shift in the current model of anchoring at the cell periphery by providing both mechanisms and evidence for de novo actin assembly orchestrated by the adhesion site. Lacking a complete pathway from integrin ligation to an integrated extracellular-intracellular skeleton in any single system, this review proposes a simple model of integrin-mediated stress fiber integration by drawing from work in multiple systems.

A Computational Model of Transmembrane Integrin Clustering

The presented work describes a structural model for integrin homooligomerization, focusing on the transmembrane domains. The two noncovalently linked integrin subunits, alpha and beta, were previously shown to homodimerize or homotrimerize, respectively. Our work is based on published mutational work that induced homotrimerization of beta3 integrins. The mutations provided structural restraints for the creation of a structural model of the beta3 homotrimer by a computational search of the conformational space of homomeric interactions of the beta3 integrin. Additionally, we explored possible conformations of the alphaIIb integrin homodimer, for which no unique solution was found. Two possible models of signal transduction, involving two different alphaIIb conformations, are discussed. One of the possible homodimeric alphaIIb conformations is GpA like, which is in line with experimental evidence. Based on our here-presented structural models and on recent experiments, we will argue that most probably the heteromeric alpha/beta transmembrane complex separates in the course of clustering.

Effects of thrombin on interactions between beta3-integrins and extracellular matrix in platelets and vascular cells

The beta3-integrin family consists of alphaIIbbeta3 (also known as glycoprotein IIb/IIIa) and alpha(v)beta3. alphaIIbbeta3 is found on platelets and megakaryocytes and has an essential role in hemostasis. alpha(v)beta3 has a broader distribution, and it functions in angiogenesis, neointimal formation after vascular injury, and leukocyte trafficking. There are important interactions between thrombin and beta3-integrins relative to both "inside-out" (integrin activation) and "outside-in" (modification of cellular events by ligand binding to integrins) signaling. Thrombin, by binding to G protein-coupled, protease-activated receptors, is a potent activator of alphaIIbbeta3. Conversely, outside-in signaling through alphaIIbbeta3 amplifies events initiated by thrombin and is necessary for full platelet spreading, platelet aggregation, granule secretion, and the formation of a stable platelet thrombus. In smooth muscle cells, alpha(v)beta3-integrins influence various responses to thrombin, including proliferation, c-Jun NH2-terminal kinase-1 activation, and focal adhesion formation. Other interactions between beta3-integrins and thrombin include beta3-integrin promotion of the generation of thrombin by localizing prothrombin to cellular surfaces and/or enhancing the formation of procoagulant microparticles and the requirement of beta3-integrin function for platelet-dependent clot retraction. In summary, there is increasing evidence that interactions between beta3-integrins and thrombin play important roles in the regulation of hemostatic and vascular functions.

RGD-containing peptides activate S6K1 through beta3 integrin in adult cardiac muscle cells

The enzyme p70S6 kinase (S6K1) is critical for cell growth, and we have reported its activation during cardiac hypertrophy. Because cardiac hypertrophy also involves integrin activation, we analyzed whether integrins could contribute to S6K1 activation. Using adult feline cardiomyocytes, here we report that integrin-interacting Arg-Gly-Asp (RGD) peptides activate S6K1 as observed by band shifting, kinase activity and phosphorylation at Thr-389 and Thr-421/Ser-424 of S6K1, and S6 protein phosphorylation. Perturbation of specific integrin function with blocking antibodies and by overexpressing the beta1A cytoplasmic tail revealed that beta3 but not beta1 integrin mediates the RGD-induced S6K1 activation. This activation is focal adhesion complex-independent and is accompanied by the activation of extracellular signal-regulated kinases 1/2 (ERK) and mammalian target of rapamycin (mTOR). Studies using specific inhibitors and dominant negative c-Raf expression in cardiomyocytes indicate that the S6K1 activation involves mTOR, MEK/ERK, and phosphatidylinositol 3-kinase pathways and is independent of protein kinase C and c-Raf. Finally, addition of fluorescent-labeled RGD peptide to cardiomyocytes exhibits its internalization and localization to the endocytic vesicles, and pretreatment of cardiomyocytes with endocytic inhibitors reduced the S6K1 activation. These data suggest that RGD interaction with beta3 integrin and its subsequent endocytosis trigger specific signaling pathway(s) for S6K1 activation in cardiomyocytes and that this process may contribute to hypertrophic growth and remodeling of myocardium.

Ligand-dependent activation of integrin alpha vbeta 3

The ability of leukocytes to self-regulate adhesion during transendothelial and extravascular migration is fundamental to the performance of immune surveillance in complex extracellular matrices. Leukocyte adhesion is regulated through the modulation of integrin receptors such as alpha(v)beta(3). In this study, we examined the activation of alpha(v)beta(3) resulting from attachment to vitronectin or fibronectin. In K562 cells stably expressing transfected alpha(v)beta(3), adhesion to vitronectin required tyrosine phosphorylation of the beta(3) subunit and activation of phosphoinositide 3-kinase and protein kinase C. In contrast, adhesion to fibronectin proceeded without beta(3)-tyrosine phosphorylation or the activities of phosphoinositide 3-kinase or protein kinase C. Firm adhesion to both ligands and actin stress fiber formation required both Syk and Rho activity, suggesting that each ligand employs unique signaling pathways to achieve an active integrin complex, likely merging at a common RhoGEF such as Vav. Distinct signaling by a single integrin species interacting with different ligands permits initiation of additional cellular processes specific to the current task and provides an explanation for what has been described as promiscuous ligand specificity among integrins.