Interaction of kindlin-2 with integrin β3 promotes outside-in signaling responses by the αVβ3 vitronectin receptor

Integrins as Drug Targets in Vascular and Related Diseases

Integrins are transmembrane receptors that, as critical participants in a vast range of pathological processes, are potential therapeutic targets. However, in only a few cases has the promise been realized by drug approval. In this review, we briefly review basic integrin biology and participation in disease, challenges in the development of safe, effective integrin-targeted therapies, and recent advances that may lead to progress.

Vitronectin promotes insulin resistance in trophoblast cells by activating JNK in gestational diabetes mellitus

Gestational diabetes mellitus (GDM) is a common disorder in the clinic, which may lead to severe detrimental outcomes both for mothers and infants. However, the underlying mechanisms for GDM are still not clear. In the present study, we performed label-free proteomics using placentas from GDM patients and normal controls. Vitronectin caused our attention among differentially expressed proteins due to its potential role in the pathological progression of GDM. Vitronectin was increased in the placentas of GDM patients, which was confirmed by Western blot analysis. Vitronectin represses insulin signal transduction in trophoblast cells, whereas the knockdown of vitronectin further potentiates insulin-evoked events. Neutralization of CD51/61 abolishes the repressed insulin signal transduction in vitronectin-treated trophoblast cells. Moreover, vitronectin activates JNK in a CD51/61-depedent manner. Inhibition of JNK rescues impaired insulin signal transduction induced by vitronectin. Overall, our data indicate that vitronectin binds CD51/61 in trophoblast cells to activate JNK, and thus induces insulin resistance. In this regard, increased expression of vitronectin is likely a risk factor for the pathological progression of GDM. Moreover, blockade of vitronectin production or its receptors (CD51/61) may have therapeutic potential for dealing with GDM.

Kindlin-2 controls angiogenesis through modulating Notch1 signaling

Kindlin-2 is critical for development and homeostasis of key organs, including skeleton, liver, islet, etc., yet its role in modulating angiogenesis is unknown. Here, we report that sufficient KINDLIN-2 is extremely important for NOTCH-mediated physiological angiogenesis. The expression of KINDLIN-2 in HUVECs is significantly modulated by angiogenic factors such as vascular endothelial growth factor A or tumor necrosis factor α. A strong co-localization of CD31 and Kindlin-2 in tissue sections is demonstrated by immunofluorescence staining. Endothelial-cell-specific Kindlin-2 deletion embryos die on E10.5 due to hemorrhage caused by the impaired physiological angiogenesis. Experiments in vitro show that vascular endothelial growth factor A-induced multiple functions of endothelial cells, including migration, matrix proteolysis, morphogenesis and sprouting, are all strengthened by KINDLIN-2 overexpression and severely impaired in the absence of KINDLIN-2. Mechanistically, we demonstrate that KINDLIN-2 inhibits the release of Notch intracellular domain through binding to and maintaining the integrity of NOTCH1. The impaired angiogenesis and avascular retinas caused by KINDLIN-2 deficiency can be rescued by DAPT, an inhibitor of γ-secretase which releases the intracellular domain from NOTCH1. Moreover, we demonstrate that high glucose stimulated hyperactive angiogenesis by increasing KINDLIN-2 expression could be prevented by KINDLIN-2 knockdown, indicating Kindlin-2 as a potential therapeutic target in treatment of diabetic retinopathy. Our study for the first time demonstrates the significance of Kindlin-2 in determining Notch-mediated angiogenesis during development and highlights Kindlin-2 as the potential therapeutic target in angiogenic diseases, such as diabetic retinopathy.

Integrin-based mechanosensing through conformational deformation

Conversion of integrins from low to high affinity states, termed activation, is important in biological processes, including immunity, hemostasis, angiogenesis, and embryonic development. Integrin activation is regulated by large-scale conformational transitions from closed, low affinity states to open, high affinity states. Although it has been suggested that substrate stiffness shifts the conformational equilibrium of integrin and governs its unbinding, here, we address the role of integrin conformational activation in cellular mechanosensing. Comparison of wild-type versus activating mutants of integrin αVβ3 show that activating mutants shift cell spreading, focal adhesion kinase activation, traction stress, and force on talin toward high stiffness values at lower stiffness. Although all activated integrin mutants showed equivalent binding affinity for soluble ligands, the β3 S243E mutant showed the strongest shift in mechanical responses. To understand this behavior, we used coarse-grained computational models derived from molecular level information. The models predicted that wild-type integrin αVβ3 displaces under force and that activating mutations shift the required force toward lower values, with S243E showing the strongest effect. Cellular stiffness sensing thus correlates with computed effects of force on integrin conformation. Together, these data identify a role for force-induced integrin conformational deformation in cellular mechanosensing.

Optogenetics-based localization of talin to the plasma membrane promotes activation of β3 integrins

Interaction of talin with the cytoplasmic tails of integrin β triggers integrin activation, leading to an increase of integrin affinity/avidity for extracellular ligands. In talin KO mice, loss of talin interaction with platelet integrin αIIbβ3 causes a severe hemostatic defect, and loss of talin interaction with endothelial cell integrin αVβ3 affects angiogenesis. In normal cells, talin is autoinhibited and localized in the cytoplasm. Here, we used an optogenetic platform to assess whether recruitment of full-length talin to the plasma membrane was sufficient to induce integrin activation. A dimerization module (Arabidopsis cryptochrome 2 fused to the N terminus of talin; N-terminal of cryptochrome-interacting basic helix-loop-helix domain ended with a CAAX box protein [C: cysteine; A: aliphatic amino acid; X: any C-terminal amino acid]) responsive to 450 nm (blue) light was inserted into Chinese hamster ovary cells and endothelial cells also expressing αIIbβ3 or αVβ3, respectively. Thus, exposure of the cells to blue light caused a rapid and reversible recruitment of Arabidopsis cryptochrome 2-talin to the N-terminal of cryptochrome-interacting basic helix-loop-helix domain ended with a CAAX box protein [C: cysteine; A: aliphatic amino acid; X: any C-terminal amino acid]-decorated plasma membrane. This resulted in β3 integrin activation in both cell types, as well as increasing migration of the endothelial cells. However, membrane recruitment of talin was not sufficient for integrin activation, as membrane-associated Ras-related protein 1 (Rap1)-GTP was also required. Moreover, talin mutations that interfered with its direct binding to Rap1 abrogated β3 integrin activation. Altogether, these results define a role for the plasma membrane recruitment of talin in β3 integrin activation, and they suggest a nuanced sequence of events thereafter involving Rap1-GTP.

miRNA-200c-3p promotes endothelial to mesenchymal transition and neointimal hyperplasia in artery bypass grafts

Increasing evidence has suggested a critical role for endothelial‐to‐mesenchymal transition (EndoMT) in a variety of pathological conditions. MicroRNA‐200c‐3p (miR‐200c‐3p) has been implicated in epithelial‐to‐mesenchymal transition. However, the functional role of miR‐200c‐3p in EndoMT and neointimal hyperplasia in artery bypass grafts remains largely unknown. Here we demonstrated a critical role for miR‐200c‐3p in EndoMT. Proteomics and luciferase activity assays revealed that fermitin family member 2 (FERM2) is the functional target of miR‐200c‐3p during EndoMT. FERMT2 gene inactivation recapitulates the effect of miR‐200c‐3p overexpression on EndoMT, and the inhibitory effect of miR‐200c‐3p inhibition on EndoMT was reversed by FERMT2 knockdown. Further mechanistic studies revealed that FERM2 suppresses smooth muscle gene expression by preventing serum response factor nuclear translocation and preventing endothelial mRNA decay by interacting with Y‐box binding protein 1. In a model of aortic grafting using endothelial lineage tracing, we observed that miR‐200c‐3p expression was dramatically up‐regulated, and that EndoMT contributed to neointimal hyperplasia in grafted arteries. MiR‐200c‐3p inhibition in grafted arteries significantly up‐regulated FERM2 gene expression, thereby preventing EndoMT and reducing neointimal formation. Importantly, we found a high level of EndoMT in human femoral arteries with atherosclerotic lesions, and that miR‐200c‐3p expression was significantly increased, while FERMT2 expression levels were dramatically decreased in diseased human arteries. Collectively, we have documented an unexpected role for miR‐200c‐3p in EndoMT and neointimal hyperplasia in grafted arteries. Our findings offer a novel therapeutic opportunity for treating vascular diseases by specifically targeting the miR‐200c‐3p/FERM2 regulatory axis. © 2020 The Authors. The Journal of Pathology published by John Wiley & Sons, Ltd. on behalf of The Pathological Society of Great Britain and Ireland.

Nongenomic Actions of Thyroid Hormone: The Integrin Component

The extracellular domain of plasma membrane integrin αvβ3 contains a cell surface receptor for thyroid hormone analogues. The receptor is largely expressed and activated in tumor cells and rapidly dividing endothelial cells. The principal ligand for this receptor is l-thyroxine (T₄), usually regarded only as a prohormone for 3,5,3'-triiodo-l-thyronine (T₃), the hormone analogue that expresses thyroid hormone in the cell nucleus via nuclear receptors that are unrelated structurally to integrin αvβ3. At the integrin receptor for thyroid hormone, T₄ regulates cancer and endothelial cell division, tumor cell defense pathways (such as anti-apoptosis), and angiogenesis and supports metastasis, radioresistance, and chemoresistance. The molecular mechanisms involve signal transduction via mitogen-activated protein kinase and phosphatidylinositol 3-kinase, differential expression of multiple genes related to the listed cell processes, and regulation of activities of other cell surface proteins, such as vascular growth factor receptors. Tetraiodothyroacetic acid (tetrac) is derived from T₄ and competes with binding of T₄ to the integrin. In the absence of T₄, tetrac and chemically modified tetrac also have anticancer effects that culminate in altered gene transcription. Tumor xenografts are arrested by unmodified and chemically modified tetrac. The receptor requires further characterization in terms of contributions to nonmalignant cells, such as platelets and phagocytes. The integrin αvβ3 receptor for thyroid hormone offers a large panel of cellular actions that are relevant to cancer biology and that may be regulated by tetrac derivatives.

Reciprocal integrin/integrin antagonism through kindlin-2 and Rho GTPases regulates cell cohesion and collective migration

Collective cell behaviour during embryogenesis and tissue repair requires the coordination of intercellular junctions, cytoskeleton-dependent shape changes controlled by Rho GTPases, and integrin-dependent cell-matrix adhesion. Many different integrins are simultaneously expressed during wound healing, embryonic development, and sprouting angiogenesis, suggesting that there is extensive integrin/integrin cross-talk to regulate cell behaviour. Here, we show that fibronectin-binding β1 and β3 integrins do not act synergistically, but rather antagonize each other during collective cell processes in neuro-epithelial cells, placental trophoblasts, and endothelial cells. Reciprocal β1/β3 antagonism controls RhoA activity in a kindlin-2-dependent manner, balancing cell spreading, contractility, and intercellular adhesion. In this way, reciprocal β1/β3 antagonism controls cell cohesion and cellular plasticity to switch between extreme and opposing states, including epithelial versus mesenchymal-like phenotypes and collective versus individual cell migration. We propose that integrin/integrin antagonism is a universal mechanism to effectuate social cellular interactions, important for tissue morphogenesis, endothelial barrier function, trophoblast invasion, and sprouting angiogenesis.

Site-specific phosphorylation regulates the functions of kindlin-3 in a variety of cells

Studies of isolated cells, mice, and humans have demonstrated the vital role of the FERM domain protein kindlin-3 in integrin activation in certain hematopoietic and non-hematopoietic cells, consequent to binding to integrin β-subunits. To explore regulatory mechanisms, we developed a monoclonal antibody that selectively recognizes the phosphorylated form of Ser⁴⁸⁴ (pS⁴⁸⁴) in kindlin-3. Activation of platelets, HEL megakaryocytic-like cells and BT549 breast cancer cells led to enhanced expression of pS⁴⁸⁴ as assessed by immunofluorescence or Western blotting. In platelets, pS⁴⁸⁴ rose rapidly and transiently upon stimulation. When a mutant form of kindlin-3, T⁴⁸²S⁴⁸⁴/AA kindlin-3, was transduced into mouse megakaryocytes, it failed to support activation of integrin α_IIbβ₃, whereas wild-type kindlin-3 did. In MDA-MB231 breast cancer cells, expression of T⁴⁸²S⁴⁸⁴/AA kindlin-3 suppressed cell spreading, migration, invasion, and VEGF production. Wild-type kindlin-3 expressing cells markedly increased tumor growth in vivo, whereas T⁴⁸²S⁴⁸⁴/AA kindlin-3 significantly blunted tumor progression. Thus, our data establish that a unique phosphorylation event in kindlin-3 regulates its cellular functions.

Elevated kindlin-2 promotes tumour progression and angiogenesis through the mTOR/VEGFA pathway in melanoma

Background: In our previous study, kindlin-2 promoted skin wound healing and decreased the permeability of neovascularization during angiogenesis. Herein, we explored the biological function and underlying mechanism of kindlin-2 in cutaneous melanoma.

Methods and Results: Through a series of in vitro assays, we found that high levels of kindlin-2 promoted migration and invasion of melanoma cells without influencing cell proliferation. Quantitative real-time polymerase chain reaction (qRT-PCR) and western blot analyses showed that upregulated kindlin-2 promoted the cellular epithelial-mesenchymal transition (EMT). Importantly, we found that melanoma cells overexpressing kindlin-2 promoted angiogenesis and VEGFA secretion in vitro and facilitated tumour growth and lung metastasis in vivo. To unveil the underlying mechanism, we conducted Next-generation sequencing (NGS) and differential expression analyses. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis showed that overlapping differentially expressed genes (DEGs) were primarily enriched in the TGF-β, mTOR and VEGF signalling pathways. Then, we confirmed that the mTOR/VEGFA pathway was activated during the process of kindlin-2-induced melanoma progression and angiogenesis. Moreover, we demonstrated that kindlin-2 was significantly overexpressed in clinical melanoma samples and that a high level of kindlin-2 predicted a poor prognosis.

Conclusions: Taken together, these findings showed that kindlin-2 promotes angiogenesis and tumour progression via the mTOR/VEGFA pathway.

Are Integrins Still Practicable Targets for Anti-Cancer Therapy?

Correlative clinical evidence and experimental observations indicate that integrin adhesion receptors, in particular those of the αV family, are relevant to cancer cell features, including proliferation, survival, migration, invasion, and metastasis. In addition, integrins promote events in the tumor microenvironment that are critical for tumor progression and metastasis, including tumor angiogenesis, matrix remodeling, and the recruitment of immune and inflammatory cells. In spite of compelling preclinical results demonstrating that the inhibition of integrin αVβ3/αVβ5 and α5β1 has therapeutic potential, clinical trials with integrin inhibitors targeting those integrins have repeatedly failed to demonstrate therapeutic benefits in cancer patients. Here, we review emerging integrin functions and their proposed contribution to tumor progression, discuss preclinical evidence of therapeutic significance, revisit clinical trial results, and consider alternative approaches for their therapeutic targeting in oncology, including targeting integrins in the other cells of the tumor microenvironment, e.g., cancer-associated fibroblasts and immune/inflammatory cells. We conclude that integrins remain a valid target for cancer therapy; however, agents with better pharmacological properties, alternative models for their preclinical evaluation, and innovative combination strategies for clinical testing (e.g., together with immuno-oncology agents) are needed.

uPAR isoform 2 forms a dimer and induces severe kidney disease in mice

Soluble urokinase plasminogen activator receptor (suPAR) is an immune-derived circulating signaling molecule that has been implicated in chronic kidney disease, such as focal segmental glomerulosclerosis (FSGS). Typically, native uPAR (isoform 1) translates to a 3-domain protein capable of binding and activating integrins, yet the function of additional isoforms generated by alternative splicing is unknown. Here, we characterized mouse uPAR isoform 2 (msuPAR2), encoding domain I and nearly one-half of domain II, as a dimer in solution, as revealed by 3D electron microscopy structural analysis. In vivo, msuPAR2 transgenic mice exhibited signs of severe renal disease characteristic of FSGS with proteinuria, loss of kidney function, and glomerulosclerosis. Sequencing of the glomerular RNAs from msuPAR2-Tg mice revealed a differentially expressed gene signature that includes upregulation of the suPAR receptor Itgb3, encoding β3 integrin. Crossing msuPAR2-transgenic mice with 3 different integrin β3 deficiency models rescued msuPAR2-mediated kidney function. Further analyses indicated a central role for β3 integrin and c-Src in msuPAR2 signaling and in human FSGS kidney biopsies. Administration of Src inhibitors reduced proteinuria in msuPAR2-transgenic mice. In conclusion, msuPAR2 may play an important role in certain forms of scarring kidney disease.

Cellular tension encodes local Src-dependent differential β1 and β3 integrin mobility

Integrins are transmembrane receptors that have a pivotal role in mechanotransduction processes by connecting the extracellular matrix to the cytoskeleton. Although it is well established that integrin activation/inhibition cycles are due to highly dynamic interactions, whether integrin mobility depends on local tension and cytoskeletal organization remains surprisingly unclear. Using an original approach combining micropatterning on glass substrates to induce standardized local mechanical constraints within a single cell with temporal image correlation spectroscopy, we measured the mechanosensitive response of integrin mobility at the whole cell level and in adhesion sites under different mechanical constraints. Contrary to β1 integrins, high tension increases β3 integrin residence time in adhesive regions. Chimeric integrins and structure–function studies revealed that the ability of β3 integrins to specifically sense local tensional organization is mostly encoded by its cytoplasmic domain and is regulated by tuning the affinity of its NPXY domains through phosphorylation by Src family kinases.

Knockdown of the KINDLIN-2 Gene and Reduced Expression of Kindlin-2 Affects Vascular Permeability in Angiogenesis in a Mouse Model of Wound Healing

Background

Angiogenesis is an important component of wound healing and tissue repair. Kindlin-2 is an integrin-associated protein, encoded by the KINDLIN-2 gene, which has been shown to affect cell adhesion and migration of cells, including endothelial cells. The aim of this study was to use a mouse model of wound healing to evaluate the effects of expression of KINDLIN-2 on angiogenesis in wound healing in vivo.

Material/Methods

Thirty-six male C57BL/6 mice were studied in an established model that used a wound created on the back. Mice were divided randomly into three groups: the normal group (n=12) received injections of normal (0.9%) saline; the KINDLIN-2(−) group (n=12) received injections of adeno-associated virus with small interfering (si)RNA targeting the KINDLIN-2 gene (AAV-KINDLIN-2-siRNA); and the control (group (n=12) received injections of adeno-associated virus containing a scrambled RNA sequence (AAV-control-RNA). Wound healing was analyzed by biochemical examination of the exudates and histology. Evans blue dye was injected into the caudal vein of each mouse, two weeks after wound healing to assess neovascular permeability.

Results

Wound healing was significantly delayed in the KINDLIN-2 gene knockdown mice (AAV-KINDLIN-2-siRNA) compared with that of the normal group and the control group, and neovascular permeability was increased. In the AAV-KINDLIN-2-siRNA group, blood vessels were shorter and thinner compared with the normal group and the control group.

Conclusions

In a mouse model of wound healing, KINDLIN-2 gene knockdown inhibited wound healing, and increased neovascular permeability in vivo.

Peptides derived from the integrin β cytoplasmic tails inhibit angiogenesis

BACKGROUND

Integrins are essential regulators of angiogenesis. However, the antiangiogenic potential of peptides derived from the integrin cytoplasmic tails (CT) remains mostly undetermined.

METHODS

Here we designed a panel of membrane-penetrating peptides (termed as mβCTPs), each comprising a C-terminal NxxY motif from one of the conserved integrin β CTs, and evaluated their antiangiogenic ability using both in vitro and in vivo approaches.

RESULTS

We found that mβ3CTP, mβ5CTP and mβ6CTP, derived respectively from the integrin β3, β5 and β6 CTs, but not others, exhibit antiangiogenic ability. Interestingly, we observed that the integrin β3, β5 and β6 CTs but not others are able to interact with β₃-endonexin. In addition, the antiangiogenic core in mβ3CTP is identical to a previously identified β₃-endonexin binding region in the integrin β3 CT, indicating that the antiangiogenic mβCTPs may function via their binding to β₃-endonexin. Consistently, knockdown of endogenous β₃-endonexin in HUVECs significantly suppresses tube formation, suggesting that β₃-endonexin is proangiogenic. However, neither treatment with the antiangiogenic mβCTPs nor knockdown of endogenous β₃-endonexin affects integrin-mediated HUVEC adhesion and migration, indicating that their antiangiogenic effect may not rely on directly regulating integrin activity. Importantly, both treatment with the antiangiogenic mβCTPs and knockdown of endogenous β₃-endonexin in HUVECs inhibit VEGF expression and cell proliferation, thereby providing mechanistic explanations for the functional consequences.

CONCLUSION

Our results suggest that the antiangiogenic mβCTPs can interact with β₃-endonexin in vascular endothelial cells and suppress its function in regulating VEGF expression and cell proliferation, thus disclosing a unique pathway that may be useful for developing novel antiangiogenic strategies.

Collagen structure regulates MSCs behavior by MMPs involved cell–matrix interactions

Various scaffolds have been studied in the formation of cell niches and regulation of mesenchymal stem cells (MSCs) behaviors.

Optogenetic interrogation of integrin αVβ3 function in endothelial cells

The integrin αVβ3 is reported to promote angiogenesis in some model systems but not in others. Here, we used optogenetics to study the effects of αVβ3 interaction with the intracellular adapter kindlin-2 (Fermt2) on endothelial cell functions potentially relevant to angiogenesis. Because interaction of kindlin-2 with αVβ3 requires the C-terminal three residues of the β3 cytoplasmic tail (Arg-Gly-Thr; RGT), optogenetic probes LOVpep and ePDZ1 were fused to β3ΔRGT-GFP and mCherry-kindlin-2, respectively, and expressed in β3 integrin-null microvascular endothelial cells. Exposure of the cells to 450 nm (blue) light caused rapid and specific interaction of kindlin-2 with αVβ3 as assessed by immunofluorescence and total internal reflection fluorescence (TIRF) microscopy, and it led to increased endothelial cell migration, podosome formation and angiogenic sprouting. Analyses of kindlin-2 mutants indicated that interaction of kindlin-2 with other kindlin-2 binding partners, including c-Src, actin, integrin-linked kinase and phosphoinositides, were also likely necessary for these endothelial cell responses. Thus, kindlin-2 promotes αVβ3-dependent angiogenic functions of endothelial cells through its simultaneous interactions with β3 integrin and several other binding partners. Optogenetic approaches should find further use in clarifying spatiotemporal aspects of vascular cell biology.

Structural basis of kindlin-mediated integrin recognition and activation

Kindlins and talins are integrin-binding proteins that are critically involved in integrin activation, an essential process for many fundamental cellular activities including cell-matrix adhesion, migration, and proliferation. As FERM-domain-containing proteins, talins and kindlins, respectively, bind different regions of β-integrin cytoplasmic tails. However, compared with the extensively studied talin, little is known about how kindlins specifically interact with integrins and synergistically enhance their activation by talins. Here, we determined crystal structures of kindlin2 in the apo-form and the β1- and β3-integrin bound forms. The apo-structure shows an overall architecture distinct from talins. The complex structures reveal a unique integrin recognition mode of kindlins, which combines two binding motifs to provide specificity that is essential for integrin activation and signaling. Strikingly, our structures uncover an unexpected dimer formation of kindlins. Interrupting dimer formation impairs kindlin-mediated integrin activation. Collectively, the structural, biochemical, and cellular results provide mechanistic explanations that account for the effects of kindlins on integrin activation as well as for how kindlin mutations found in patients with Kindler syndrome and leukocyte-adhesion deficiency may impact integrin-mediated processes.

Biological characterization of three immortalized esophageal epithelial cell lines

The key molecular events that contribute to tumorigenesis are incompletely understood. The aim of the present study was to characterize and compare the biological phenotypes of three human telomerase reverse transcriptase (hTERT) and/or human papillomavirus 16 E6 and E7‑immortalized esophageal epithelial cell lines, NE2‑hTERT (NE2), NE3‑E6E7‑hTERT (NE3) and NEcA6‑E6E7‑hTERT (NEcA6). The present study used soft‑agar colony formation assays, tumorigenicity assays in nude mice, and cell proliferation, adhesion and migration assays to identify the biological characteristics of NE2, NE3 and NEcA6 cells. NE2 and NE3 cells exhibited characteristics of benign cells, such as the inability to grow in soft agar or form tumors in nude mice. By contrast, NEcA6 cells had undergone transformation, as demonstrated by the ability to grow in soft agar and form tumors in nude mice. In addition, NEcA6 cells exhibited increased migration and adhesion capabilities when compared with NE2 and NE3 cells. In order to identify mechanism(s) that may contribute to the altered biological phenotypes exhibited by these cells, the expression of three proteins involved in modulating cell migration [fascin, ezrin/radixin/moesin family proteins and phosphorylated‑focal adhesion kinase (Tyr 397)], as well as the expression status and subcellular localization of three key focal adhesions components (paxillin, talin and kindlin‑2) were examined. Paxillin, talin and kindlin‑2 were localized to adhesive sites that connect F‑actin with the extracellular matrix in transformed NEcA6 cells, but were distributed in a diffuse manner in NE2 and NE3 cells. Knockdown of kindlin‑2 in NE3 and NEcA6 cells decreased cell adhesion, however, NEcA6 cells demonstrated a greater sensitivity to knockdown of kindlin‑2. No significant differences were observed in the protein expression levels of fascin, exrin/radixin/moesin and p‑FAK in the three cell lines. In conclusion, these results demonstrate that these three focal adhesion components, particularly kindlin‑2, may contribute to the carcinogenesis of esophageal squamous cells.

αvβ3 Integrin and tumour blood vessels-learning from the past to shape the future

Angiogenesis, the formation of new blood vessels from pre-existing ones, is thought to enhance tumour growth and these blood vessels can act as conduits of tumour cell metastasis. Integrins, the family of cell surface extracellular matrix receptors, can promote endothelial cell migration and survival, both essential features of angiogenesis, and were thus considered good targets for anti-angiogenic therapy. This sparked the development of agents to block integrin function as new cancer therapies. Here, we review the current status of αvβ3-integrin in tumour angiogenesis. Learning from what we now know about integrin conformational changes and endocytosis, we discuss the possible future of targeting blood vessel αvβ3-integrin in the control of cancer.

Force regulated conformational change of integrin αVβ3

Integrins mediate cell adhesion to extracellular matrix and transduce signals bidirectionally across the membrane. Integrin α_Vβ₃ has been shown to play an essential role in tumor metastasis, angiogenesis, hemostasis and phagocytosis. Integrins can take several conformations, including the bent and extended conformations of the ectodomain, which regulate integrin functions. Using a biomembrane force probe, we characterized the bending and unbending conformational changes of single α_Vβ₃ integrins on living cell surfaces in real-time. We measured the probabilities of conformational changes, rates and speeds of conformational transitions, and the dynamic equilibrium between the two conformations, which were regulated by tensile force, dependent on the ligand, and altered by point mutations. These findings provide insights into how α_Vβ₃ acts as a molecular machine and how its physiological function and molecular structure are coupled at the single-molecule level.

Phosphatidylinositol 3-Kinase/Akt Mediates Integrin Signaling To Control RNA Polymerase I Transcriptional Activity

RNA polymerase I-mediated rRNA production is a key determinant of cell growth. Despite extensive studies, the signaling pathways that control RNA polymerase I-mediated rRNA production are not well understood. Here we provide original evidence showing that RNA polymerase I transcriptional activity is tightly controlled by integrin signaling. Furthermore, we show that a signaling axis consisting of focal adhesion kinase (FAK), Src, phosphatidylinositol 3-kinase (PI3K), Akt, and mTOR mediates the effect of integrin signaling on rRNA transcription. Additionally, we show that in kindlin-2 knockout mouse embryonic fibroblasts, overactivation of Ras, Akt, and Src can successfully rescue the defective RNA polymerase I activity induced by the loss of kindlin-2. Finally, through experiments with inhibitors of FAK, Src, and PI3K and rescue experiments in MEFs, we found that the FAK/Src/PI3K/Akt signaling pathway to control rRNA transcription is linear. Collectively, these studies reveal, for the first time, a pivotal role of integrin signaling in regulation of RNA polymerase I transcriptional activity and shed light on the downstream signaling axis that participates in regulation of this key aspect of cell growth.

Kindlin-2 directly binds actin and regulates integrin outside-in signaling

Bledzka et al. show that kindlin-2 binds actin via its F0 domain, and mutation of this site diminishes cell spreading, revealing a new mechanism by which kindlin-2 regulates cellular responses.

Reduced levels of kindlin-2 (K2) in endothelial cells derived from K2^+/− mice or C2C12 myoblastoid cells treated with K2 siRNA showed disorganization of their actin cytoskeleton and decreased spreading. These marked changes led us to examine direct binding between K2 and actin. Purified K2 interacts with F-actin in cosedimentation and surface plasmon resonance analyses and induces actin aggregation. We further find that the F0 domain of K2 binds actin. A mutation, LK⁴⁷/AA, within a predicted actin binding site (ABS) of F0 diminishes its interaction with actin by approximately fivefold. Wild-type K2 and K2 bearing the LK⁴⁷/AA mutation were equivalent in their ability to coactivate integrin αIIbβ3 in a CHO cell system when coexpressed with talin. However, K2-LK⁴⁷/AA exhibited a diminished ability to support cell spreading and actin organization compared with wild-type K2. The presence of an ABS in F0 of K2 that influences outside-in signaling across integrins establishes a new foundation for considering how kindlins might regulate cellular responses.

Kindlin-3 Is Essential for the Resting α4β1 Integrin-mediated Firm Cell Adhesion under Shear Flow Conditions

Integrin-mediated rolling and firm cell adhesion are two critical steps in leukocyte trafficking. Integrin α4β1 mediates a mixture of rolling and firm cell adhesion on vascular cell adhesion molecule-1 (VCAM-1) when in its resting state but only supports firm cell adhesion upon activation. The transition from rolling to firm cell adhesion is controlled by integrin activation. Kindlin-3 has been shown to bind to integrin β tails and trigger integrin activation via inside-out signaling. However, the role of kindlin-3 in regulating resting α4β1-mediated cell adhesion is not well characterized. Herein we demonstrate that kindlin-3 was required for the resting α4β1-mediated firm cell adhesion but not rolling adhesion. Knockdown of kindlin-3 significantly decreased the binding of kindlin-3 to β1 and down-regulated the binding affinity of the resting α4β1 to soluble VCAM-1. Notably, it converted the resting α4β1-mediated firm cell adhesion to rolling adhesion on VCAM-1 substrates, increased cell rolling velocity, and impaired the stability of cell adhesion. By contrast, firm cell adhesion mediated by Mn(2+)-activated α4β1 was barely affected by knockdown of kindlin-3. Structurally, lack of kindlin-3 led to a more bent conformation of the resting α4β1. Thus, kindlin-3 plays an important role in maintaining a proper conformation of the resting α4β1 to mediate both rolling and firm cell adhesion. Defective kindlin-3 binding to the resting α4β1 leads to a transition from firm to rolling cell adhesion on VCAM-1, implying its potential role in regulating the transition between integrin-mediated rolling and firm cell adhesion.

Kindlin-2 cooperates with talin to activate integrins and induces cell spreading by directly binding paxillin

Integrins require an activation step prior to ligand binding and signaling. How talin and kindlin contribute to these events in non-hematopoietic cells is poorly understood. Here we report that fibroblasts lacking either talin or kindlin failed to activate β1 integrins, adhere to fibronectin (FN) or maintain their integrins in a high affinity conformation induced by Mn(2+). Despite compromised integrin activation and adhesion, Mn(2+) enabled talin- but not kindlin-deficient cells to initiate spreading on FN. This isotropic spreading was induced by the ability of kindlin to directly bind paxillin, which in turn bound focal adhesion kinase (FAK) resulting in FAK activation and the formation of lamellipodia. Our findings show that talin and kindlin cooperatively activate integrins leading to FN binding and adhesion, and that kindlin subsequently assembles an essential signaling node at newly formed adhesion sites in a talin-independent manner.

Identification of an Endogenously Generated Cryptic Collagen Epitope (XL313) That May Selectively Regulate Angiogenesis by an Integrin Yes-associated Protein (YAP) Mechano-transduction Pathway

Extracellular matrix (ECM) remodeling regulates angiogenesis. However, the precise mechanisms by which structural changes in ECM proteins contribute to angiogenesis are not fully understood. Integrins are molecules with the ability to detect compositional and structural changes within the ECM and integrate this information into a network of signaling circuits that coordinate context-dependent cell behavior. The role of integrin αvβ3 in angiogenesis is complex, as evidence exists for both positive and negative functions. The precise downstream signaling events initiated by αvβ3 may depend on the molecular characteristics of its ligands. Here, we identified an RGD-containing cryptic collagen epitope that is generated in vivo. Surprisingly, rather than inhibiting αvβ3 signaling, this collagen epitope promoted αvβ3 activation and stimulated angiogenesis and inflammation. An antibody directed to this RGDKGE epitope but not other RGD collagen epitopes inhibited angiogenesis and inflammation in vivo. The selective ability of this RGD epitope to promote angiogenesis and inflammation depends in part on its flanking KGE motif. Interestingly, a subset of macrophages may represent a physiologically relevant source of this collagen epitope. Here, we define an endothelial cell mechano-signaling pathway in which a cryptic collagen epitope activates αvβ3 leading to an Src and p38 MAPK-dependent cascade that leads to nuclear accumulation of Yes-associated protein (YAP) and stimulation of endothelial cell growth. Collectively, our findings not only provide evidence for a novel mechano-signaling pathway, but also define a possible therapeutic strategy to control αvβ3 signaling by targeting a pro-angiogenic and inflammatory ligand of αvβ3 rather than the receptor itself.

The Tyrosine Kinase c-Src Specifically Binds to the Active Integrin αIIbβ3 to Initiate Outside-in Signaling in Platelets

It is currently believed that inactive tyrosine kinase c-Src in platelets binds to the cytoplasmic tail of the β3 integrin subunit via its SH3 domain. Although a recent NMR study supports this contention, it is likely that such binding would be precluded in inactive c-Src because an auto-inhibitory linker physically occludes the β3 tail binding site. Accordingly, we have re-examined c-Src binding to β3 by immunoprecipitation as well as NMR spectroscopy. In unstimulated platelets, we detected little to no interaction between c-Src and β3. Following platelet activation, however, c-Src was co-immunoprecipitated with β3 in a time-dependent manner and underwent progressive activation as well. We then measured chemical shift perturbations in the (15)N-labeled SH3 domain induced by the C-terminal β3 tail peptide NITYRGT and found that the peptide interacted with the SH3 domain RT-loop and surrounding residues. A control peptide whose last three residues where replaced with those of the β1 cytoplasmic tail induced only small chemical shift perturbations on the opposite face of the SH3 domain. Next, to mimic inactive c-Src, we found that the canonical polyproline peptide RPLPPLP prevented binding of the β3 peptide to the RT- loop. Under these conditions, the β3 peptide induced chemical shift perturbations similar to the negative control. We conclude that the primary interaction of c-Src with the β3 tail occurs in its activated state and at a site that overlaps with PPII binding site in its SH3 domain. Interactions of inactive c-Src with β3 are weak and insensitive to β3 tail mutations.