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

The Sall2 transcription factor promotes cell migration regulating focal adhesion turnover and integrin β1 expression

SALL2/Sall2 is a transcription factor associated with development, neuronal differentiation, and cancer. Interestingly, SALL2/Sall2 deficiency leads to failure of the optic fissure closure and neurite outgrowth, suggesting a positive role for SALL2/Sall2 in cell migration. However, in some cancer cells, SALL2 deficiency is associated with increased cell migration. To further investigate the role of Sall2 in the cell migration process, we used immortalized Sall2 knockout (Sall2−/−) and Sall2 wild-type (Sall2+/+) mouse embryonic fibroblasts (iMEFs). Our results indicated that Sall2 positively regulates cell migration, promoting cell detachment and focal adhesions turnover. Sall2 deficiency decreased cell motility and altered focal adhesion dynamics. Accordingly, restoring Sall2 expression in the Sall2−/− iMEFs by using a doxycycline-inducible Tet-On system recovered cell migratory capabilities and focal adhesion dynamics. In addition, Sall2 promoted the autophosphorylation of Focal Adhesion Kinase (FAK) at Y397 and increased integrin β1 mRNA and its protein expression at the cell surface. We demonstrated that SALL2 increases ITGB1 promoter activity and binds to conserved SALL2-binding sites at the proximal region of the ITGB1 promoter, validated by ChIP experiments. Furthermore, the overexpression of integrin β1 or its blockade generates a cell migration phenotype similar to that of Sall2+/+ or Sall2−/− cells, respectively. Altogether, our data showed that Sall2 promotes cell migration by modulating focal adhesion dynamics, and this phenotype is associated with SALL2/Sall2-transcriptional regulation of integrin β1 expression and FAK autophosphorylation. Since deregulation of cell migration promotes congenital abnormalities, tumor formation, and spread to other tissues, our findings suggest that the SALL2/Sall2-integrin β1 axis could be relevant for those processes.

KLF14 targets ITGB1 to inhibit the progression of cervical cancer via the PI3K/AKT signalling pathway

Our study aimed to determine whether Krüppel-like factor 14 (KLF14) inhibits the proliferation and promotes the apoptosis of cervical cancer cells through integrin β1 (ITGB1). Immunohistochemistry was performed to determine the expression of KLF14. The effect of KLF14 on the proliferation of cervical cancer cells was verified by Cell Counting Kit-8 (CCK-8) assays, colony formation assays and in vivo experiments. The effect of KLF14 on cervical cancer cell apoptosis was detected by flow cytometry. The targeting relationship between KLF14 and ITGB1 was evaluated by Western blotting and a dual-luciferase reporter assay. Moreover, Flow cytometry was performed to verify the relationship between KLF14 and ITGB1 on the apoptosis of cervical cancer cells. Additionally, Western blot analysis was performed to investigate the relationship between KLF14 and ITGB1 on the expression of downstream related molecules. As a result, the expression of KLF14 in cervical cancer tissues was lower than that in paracancerous tissues. KLF14 inhibited proliferation and promoted apoptosis in cervical cancer cells. Mechanistically, ITGB1 expression was significantly downregulated in KLF14-overexpressing cervical cancer cells. At the same time, we found that the effects of KLF14 and ITGB1 on apoptosis of cervical cancer cells could be mutually affected. KLF14 directly targeted ITGB1 to regulate its downstream PI3K/AKT signalling pathway. In summary, KLF14 inhibits the progression of cervical cancer by targeting ITGB1 via the PI3K/AKT signalling pathway.

BMPR2 acts as a gatekeeper to protect endothelial cells from increased TGFβ responses and altered cell mechanics

Balanced transforming growth factor-beta (TGFβ)/bone morphogenetic protein (BMP)-signaling is essential for tissue formation and homeostasis. While gain in TGFβ signaling is often found in diseases, the underlying cellular mechanisms remain poorly defined. Here we show that the receptor BMP type 2 (BMPR2) serves as a central gatekeeper of this balance, highlighted by its deregulation in diseases such as pulmonary arterial hypertension (PAH). We show that BMPR2 deficiency in endothelial cells (ECs) does not abolish pan-BMP-SMAD1/5 responses but instead favors the formation of mixed-heteromeric receptor complexes comprising BMPR1/TGFβR1/TGFβR2 that enable enhanced cellular responses toward TGFβ. These include canonical TGFβ-SMAD2/3 and lateral TGFβ-SMAD1/5 signaling as well as formation of mixed SMAD complexes. Moreover, BMPR2-deficient cells express genes indicative of altered biophysical properties, including up-regulation of extracellular matrix (ECM) proteins such as fibrillin-1 (FBN1) and of integrins. As such, we identified accumulation of ectopic FBN1 fibers remodeled with fibronectin (FN) in junctions of BMPR2-deficient ECs. Ectopic FBN1 deposits were also found in proximity to contractile intimal cells in pulmonary artery lesions of BMPR2-deficient heritable PAH (HPAH) patients. In BMPR2-deficient cells, we show that ectopic FBN1 is accompanied by active β1-integrin highly abundant in integrin-linked kinase (ILK) mechano-complexes at cell junctions. Increased integrin-dependent adhesion, spreading, and actomyosin-dependent contractility facilitates the retrieval of active TGFβ from its latent fibrillin-bound depots. We propose that loss of BMPR2 favors endothelial-to-mesenchymal transition (EndMT) allowing cells of myo-fibroblastic character to create a vicious feed-forward process leading to hyperactivated TGFβ signaling. In summary, our findings highlight a crucial role for BMPR2 as a gatekeeper of endothelial homeostasis protecting cells from increased TGFβ responses and integrin-mediated mechano-transduction.

NDRG4 promoter hypermethylation is a mechanistic biomarker associated with metastatic progression in breast cancer patients

The risk of developing metastatic disease in breast cancer patients is traditionally predictable based on the number of positive axillary lymph nodes, complemented with additional clinicopathological factors. However, since lymph node-negative patients have a 20-30% probability of developing metastatic disease, lymph node information alone is insufficient to accurately assess individual risk. Molecular approaches, such as multigene expression panels, analyze a set of cancer-related genes that more accurately predict the early risk of metastasis and the treatment response. Here, we present N-Myc downstream-regulated gene 4 (NDRG4) epigenetic silencing as a mechanistic biomarker of metastasis in ductal invasive breast tumors. While aberrant NDRG4 DNA hypermethylation is significantly associated with the development of metastatic disease, downregulation of NDRG4 transcription and protein expression is functionally associated with enhanced lymph node adhesion and cell mobility. Here, we show that epigenetic silencing of NDRG4 modulates integrin signaling by assembling β1-integrins into large punctate clusters at the leading edge of tumor cells to promote an "adhesive switch," decreasing cell adhesion to fibronectin and increasing cell adhesion and migration towards vitronectin, an important component of human lymph nodes. Taken together, our functional and clinical observations suggest that NDRG4 is a potential mechanistic biomarker in breast cancer that is functionally associated with metastatic disease.

β-Integrin mediates LPS-induced coelomocyte apoptosis in sea cucumber Apostichopus japonicus via the integrin/FAK/caspase-3 signaling pathway

Lipopolysaccharides (LPS) can induce the apoptosis of coelomocytes in Apostichopus japonicus (A. japonicus), and β-integrin serves as an apoptotic inhibitor during this process. However, the underlying mechanism in invertebrates is largely unknown. Integrin/focal adhesion kinase (FAK) signaling pathway modulates the apoptosis in vertebrates. In this study, a novel FAK was identified from A. japonicus (designated as AjFAK) by β-integrin (designated as AjITGB) -mediated GST-pull down assay. This interaction was further validated in the LPS-exposed coelomocytes through co-immunoprecipitation and immunofluorescence analyses. To investigate the functional role of AjFAK in AjITGB-mediated coelomocyte apoptosis, we cloned the full-length cDNA of AjFAK and characterized its relationship with AjITGB through real-time PCR. The mRNA expression levels of AjFAK exhibited consistent expression patterns with those of AjITGB in our previous work with 0.48- and 0.22-fold decreases at 12 and 96 h in LPS-exposed coelomocytes and in Vibrio splendidus challenged sea cucumber, respectively. Moreover, the expression level of AjFAK decreased to 0.35-fold in AjITGB knockdown treatment by specific small interference RNA (siRNA). We further performed an assay for the apoptotic rate of coelomocytes in AjITGB, AjFAK, and AjITGB/AjFAK silencing conditions and found that their apoptotic percentages increased to 26%, 25%, and 30%, respectively, compared with those of the control. Finally, the expression levels of four caspases from A. japonicus were also investigated to determine the apoptotic effector. After AjITGB or AjFAK was silenced, the mRNA levels of caspase-3 were 6.6-fold and 2.5-fold higher than those of the control, respectively. In addition, the enzymatic activity of caspase-3 was enhanced to 1.82- and 1.79-fold that of the control in the two groups. However, no significant changes were detected in caspase-2/6/8. All our results supported that β-integrin mediated the LPS-induced coelomocyte apoptosis in sea cucumber via the integrin/FAK/caspase-3 signaling pathway.

Novel sulphamoylated 2-methoxy estradiol derivatives inhibit breast cancer migration by disrupting microtubule turnover and organization

BACKGROUND

The estrogen metabolite 2-methoxyestradiol (2ME2) and a number of synthesised derivatives have been shown to bind to microtubules thereby arresting cancer cells in mitosis which leads to apoptosis. In interphase cells, microtubules play an important role in the delivery of proteins to subcellular locations including the focal adhesions. In fact, focal adhesion dynamics and cell migration are in part regulated by microtubules. We hypothesised that novel 2ME2 derivatives can alter cell migration by influencing microtubule dynamics in interphase cells. In this report we describe 2ME2 derivatives that display anti-migratory capabilities in a metastatic breast cancer cell line through their effects on the microtubule network resulting in altered focal adhesion signalling and RhoA activity.

METHODS

Cell migration was assayed using wound healing assays. To eliminate mitosis blockage and cell rounding as a confounding factor cell migration was also assessed in interphase blocked cells. Fluorescence confocal microscopy was used to visualise microtubule dynamics and actin cytoskeleton organisation while western blot analysis was performed to analyse focal adhesion signalling and RhoA activation.

RESULTS

2ME2 derivatives, ESE-one and ESE-15-one, inhibited cell migration in cycling cells as expected but equally diminished migration in cells blocked in interphase. While no significant effects were observed on the actin cytoskeleton, focal adhesion kinase activity was increased while RhoA GTPase activity was inhibited after exposure to either compound. Microtubule stability was increased as evidenced by the increased length and number of detyrosinated microtubules while at the same time clear disorganisation of the normal radial microtubule organisation was observed including multiple foci.

CONCLUSIONS

ESE-15-one and ESE-one are potent migration inhibitors of metastatic breast cancer cells. This ability is coupled to alterations in focal adhesion signalling but more importantly is associated with severe disorganisation of microtubule dynamics and polarity. Therefore, these compounds may offer potential as anti-metastatic therapies.

By modulating α2β1 integrin signalling, gastrin increases adhesion oF AGS-GR gastric cancer cells

Peritoneal metastasis is a major cause of recurrence of gastric cancer and integrins are key molecules involved in gastric cancer cells attachment to the peritoneum. The peptide hormone, gastrin, initially identified for its role in gastric acid secretion is also a growth factor for gastric mucosa. Gastrin has also been shown to contribute to gastric cancers progression. Here, we provide the first evidence that gastrin increases the adhesion of gastric cancer cells. Gastrin treatment induces the expression of α2 integrin subunit through a mechanism that involves the ERK pathway. We also observed in response to gastrin an increase in the amount of α2 integrin associated with β1subunit. In addition, gastrin-stimulated cell adhesion was blocked with an anti-α2β1 integrin neutralizing antibody. We also show that gastrin activates the integrin pathway via the phosphorylation of β1 integrin by a Src family kinase. This mechanism may contribute to the enhancement of cell adhesion observed in response to gastrin since we found an inhibition of gastrin-mediated cell adhesion when cells were treated with a Src inhibitor. By regulating one of the key step of the metastatic process gastrin might contribute to increase the aggressive behaviour of human gastric tumours.

Phagocytic Receptors Activate Syk and Src Signaling during Borrelia burgdorferi Phagocytosis

Phagocytosis of the Lyme disease-causing pathogen Borrelia burgdorferi has been shown to be important for generating an inflammatory response to the pathogen. As a result, understanding the mechanisms of phagocytosis has been an area of great interest in the field of Lyme disease. Several cell surface receptors that participate in B. burgdorferi phagocytosis have been reported, including the scavenger receptor MARCO and integrin α3β1. We sought to define the mechanisms by which these receptors mediate phagocytosis and to identify signaling pathways activated downstream of these receptors upon contact with B. burgdorferi We identified both Syk and Src signaling pathways as ones that participate in B. burgdorferi phagocytosis and the resulting cytokine activation. In our studies, we found that both MARCO and integrin β1 play a role in the activation of the Src kinase pathway. However, only integrin β1 participates in the activation of Syk. Interestingly, the integrin activates Syk without the help of the signaling adaptor Dap12 or FcRγ. Thus, we report that multiple pathways participate in B. burgdorferi internalization and that different cell surface receptors act simultaneously in cooperation and independently to mediate phagocytosis.

PRL-3/PTP4A3 phosphatase regulates integrin β1 in adhesion structures during migration of human ocular melanoma cells

In a previous transcriptomic analysis of 63 ocular melanomas of the uvea, we found that expression of the PRL-3/PTP4A3 gene, encoding a phosphatase that is anchored to the plasma membrane, was associated with the risk of metastasis, and a poor prognosis. We also showed that PRL-3 overexpression in OCM-1 ocular melanoma cells significantly increased cell migration in vitro and invasiveness in vivo, suggesting a direct role for PRL-3 in the metastatic spreading of uveal melanoma. Here, we aimed to identify PRL-3 substrates at the plasma membrane involved in adhesion to the extracellular matrix. We focused on integrin β1, which is the most highly expressed integrin in our cohort of uveal melanomas. We show that preventing PRL-3 anchorage to the plasma membrane i) abolishes PRL-3-induced migration in OCM-1 cells, ii) specifically enhances the spreading of OCM-1 cells overexpressing PRL-3, and iii) favors the maturation of large focal adhesions (FAs) containing integrin β1 on collagen I. Knockdown experiments confirmed integrin β1 involvement in PRL3-induced migration. We identified interactions between PRL-3 and integrin β1, as well as with FAK P-Y397, an auto-activated form of Focal Adhesion Kinase found in FAs. We also show that integrin β1 may be dephosphorylated by PRL-3 in its intracytoplasmic S/T region, an important motif for integrin-mediated cell adhesion. Finally, we observed that PRL-3 regulated the clustering of integrin β1 in FAs on collagen I but not on fibronectin. This work identifies PRL-3 as a new regulator of cell adhesion structures to the extracellular matrix, and further supports PRL-3 as a key actor of metastasis in uveal melanoma, of which molecular mechanisms are still poorly understood.

Integrins in the Spotlight of Cancer

Integrins are heterodimeric cell surface receptors that bind to different extracellular ligands depending on their composition and regulate all processes which enable multicellular life. In cancer, integrins trigger and play key roles in all the features that were once described as the Hallmarks of Cancer. In this review, we will discuss the contribution of integrins to these hallmarks, including uncontrolled and limitless proliferation, invasion of tumor cells, promotion of tumor angiogenesis and evasion of apoptosis and resistance to growth suppressors, by highlighting the latest findings. Further on, given the paramount role of integrins in cancer, we will present novel strategies for integrin inhibition that are starting to emerge, promising a hopeful future regarding cancer treatment.

The kindlin family: functions, signaling properties and implications for human disease

The kindlin (or fermitin) family of proteins comprises three members (kindlin-1,-2 and -3) of evolutionarily conserved focal adhesion (FA) proteins, whose best-known task is to increase integrin affinity for a ligand (also referred as integrin activation) through binding of β-integrin tails. The consequence of kindlin-mediated integrin activation and integrin-ligand binding is cell adhesion, spreading and migration, assembly of the extracellular matrix (ECM), cell survival, proliferation and differentiation. Another hallmark of kindlins is their involvement in disease. Mutations in the KINDLIN-1 (also known as FERMT1) gene cause Kindler syndrome (KS)--in which mainly skin and intestine are affected, whereas mutations in the KINDLIN-3 (also known as FERMT3) gene cause leukocyte adhesion deficiency type III (LAD III), which is characterized by impaired extravasation of blood effector cells and severe, spontaneous bleedings. Also, aberrant expression of kindlins in various forms of cancer and in tissue fibrosis has been reported. Although the malfunctioning of integrins represent a major cause leading to kindlin-associated diseases, increasing evidence also point to integrin-independent functions of kindlins that play an important role in the pathogenesis of certain disease aspects. Furthermore, isoform-specific kindlin functions have been discovered, explaining, for example, why loss of kindlins differentially affects tissue stem cell homeostasis or tumor development. This Commentary focuses on new and isoform-specific kindlin functions in different tissues and discusses their potential role in disease development and progression.

Integrin endosomal signalling suppresses anoikis

Integrin-containing focal adhesions transmit extracellular signals across the plasma membrane to modulate cell adhesion, signalling and survival. Although integrins are known to undergo continuous endo/exocytic traffic, the potential impact of endocytic traffic on integrin-induced signals is unknown. Here, we demonstrate that integrin signalling is not restricted to cell-ECM adhesions and identify an endosomal signalling platform that supports integrin signalling away from the plasma membrane. We show that active focal adhesion kinase (FAK), an established marker of integrin-ECM downstream signalling, localizes with active integrins on endosomes. Integrin endocytosis positively regulates adhesion-induced FAK activation, which is early endosome antigen-1 and small GTPase Rab21 dependent. FAK binds directly to purified endosomes and becomes activated on them, suggesting a role for endocytosis in enhancing distinct integrin downstream signalling events. Finally, endosomal integrin signalling contributes to cancer-related processes such as anoikis resistance, anchorage independence and metastasis.

Contributions of the integrin β1 tail to cell adhesive forces

Integrin receptors connect the extracellular matrix to the cell cytoskeleton to provide essential forces and signals. To examine the contributions of the β1 integrin cytoplasmic tail to adhesive forces, we generated cell lines expressing wild-type and tail mutant β1 integrins in β1-null fibroblasts. Deletion of β1 significantly reduced cell spreading, focal adhesion assembly, and adhesive forces, and expression of human β1 (hβ1) integrin in these cells restored adhesive functions. Cells expressing a truncated tail mutant had impaired spreading, fewer and smaller focal adhesions, reduced integrin binding to fibronectin, and lower adhesion strength and traction forces compared to hβ1-expressing cells. All these metrics were equivalent to those for β1-null cells, demonstrating that the β1 tail is essential to these adhesive functions. Expression of the constitutively-active D759A hβ1 mutant restored many of these adhesive functions in β1-null cells, although with important differences when compared to wild-type β1. Even though there were no differences in integrin-fibronectin binding and adhesion strength between hβ1- and hβ1-D759A-expressing cells, hβ1-D759A-expressing cells assembled more but smaller adhesions than hβ1-expressing cells. Importantly, hβ1-D759A-expressing cells generated lower traction forces compared to hβ1-expressing cells. These differences between hβ1- and hβ1-D759A-expressing cells suggest that regulation of integrin activation is important for fine-tuning cell spreading, focal adhesion assembly, and traction force generation.

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.

SNARE-dependent interaction of Src, EGFR and β1 integrin regulates invadopodia formation and tumor cell invasion

Acquisition of an invasive phenotype is prerequisite for tumor metastasis. Degradation of the extracellular matrix (ECM), and subsequent invasion by tumor cells, is mediated, in part, through subcellular structures called invadopodia. Src-dependent cytoskeletal rearrangements are required to form invadopodia, and here we identify an association between Src, epidermal growth factor receptor (EGFR), and β1 integrin that facilitates invadopodia formation. The association of Src, EGFR and β1 integrin is dependent upon membrane traffic that is mediated by syntaxin13 (officially known as STX12) and SNAP23; a similar dependence on these two SNARE proteins was observed for invadopodium-based matrix degradation and cell invasion. Inhibition of SNARE function impaired the delivery of Src and EGFR to developing invadopodia, as well as the β1-integrin-dependent activation of Src and phosphorylation of EGFR on Tyr residue 845. We also identified an association between SNAP23 and β1 integrin, and inhibition of β1 integrin increased this association, whereas the interaction between syntaxin13 and SNAP23 was reduced. The results suggest that SNARE-dependent trafficking is regulated, in part, by β1 integrin and is required for the delivery of Src and EGFR to sites of invadopodia formation in order to support tumor cell invasion.

MicroRNA- 130b suppresses migration and invasion of colorectal cancer cells through downregulation of integrin β1 [corrected]

MicroRNA 130b (miR-130b) is significantly dysregulated in various human tumor types. In this study, using a microarray assay, we characterized the upregulation of miR-130b expression in colorectal cancer (CRC) specimens. However, there is limited knowledge about the roles of aberrant miR-130b expression in CRC. Our studies in CRC cells demonstrated that miR-130b significantly decreases cell migration and invasion, but it has no evidently effects on cell proliferation and apoptosis. In the overexpression miR-130b CRC cells and the CRC specimens, we observed a decreased level of integrin β1 protein, which is considered as a key molecule involved in cell motility. The targeting of the 3'-UTR region of integrin β1 gene by miR-130b was revealed using a luciferase reporter assay. The regulation of integrin β1 by miR-130b was further shown using the miR-130b mimics and the inhibitor of miR-130b. The impaired motility of the miR-130b overexpression cells is recovered partly by the expression of integrin β1 lacking the 3'-UTR. Additionally, the knockdown of integrin β1 also gives rise to a decrease in cell migration and invasion, which is similar to the impeded motility due to overexpression of miR-130b in CRC cells. Furthermore, the inverse expressions of miR-130b and integrin β1 were observed in CRC specimens. In summary, these data demonstrate that miR-130b downregulates its target-integrin β1, leading to the impaired migration and invasion of CRC cells.

Focal adhesion complex proteins in epidermis and squamous cell carcinoma

Focal adhesions (FAs) are large, integrin-containing, multi-protein assemblies spanning the plasma membrane that link the cellular cytoskeleton to surrounding extracellular matrix. They play critical roles in adhesion and cell signaling and are major regulators of epithelial homeostasis, tissue response to injury, and tumorigenesis. Most integrin subunits and their associated FA proteins are expressed in skin, and murine genetic models have provided insight into the functional roles of FAs in normal and neoplastic epidermis. Here, we discuss the roles of these proteins in normal epidermal proliferation, adhesion, wound healing, and cancer. While many downstream signaling mechanisms remain unclear, the critically important roles of FAs are highlighted by the development of therapeutics targeting FAs for human cancer.

The signaling pathway of Campylobacter jejuni-induced Cdc42 activation: Role of fibronectin, integrin beta1, tyrosine kinases and guanine exchange factor Vav2

Background

Host cell invasion by the foodborne pathogen Campylobacter jejuni is considered as one of the primary reasons of gut tissue damage, however, mechanisms and key factors involved in this process are widely unclear. It was reported that small Rho GTPases, including Cdc42, are activated and play a role during invasion, but the involved signaling cascades remained unknown. Here we utilised knockout cell lines derived from fibronectin^-/-, integrin-beta1^-/-, focal adhesion kinase (FAK)^-/- and Src/Yes/Fyn^-/- deficient mice, and wild-type control cells, to investigate C. jejuni-induced mechanisms leading to Cdc42 activation and bacterial uptake.

Results

Using high-resolution scanning electron microscopy, GTPase pulldowns, G-Lisa and gentamicin protection assays we found that each studied host factor is necessary for induction of Cdc42-GTP and efficient invasion. Interestingly, filopodia formation and associated membrane dynamics linked to invasion were only seen during infection of wild-type but not in knockout cells. Infection of cells stably expressing integrin-beta1 variants with well-known defects in fibronectin fibril formation or FAK signaling also exhibited severe deficiencies in Cdc42 activation and bacterial invasion. We further demonstrated that infection of wild-type cells induces increasing amounts of phosphorylated FAK and growth factor receptors (EGFR and PDGFR) during the course of infection, correlating with accumulating Cdc42-GTP levels and C. jejuni invasion over time. In studies using pharmacological inhibitors, silencing RNA (siRNA) and dominant-negative expression constructs, EGFR, PDGFR and PI3-kinase appeared to represent other crucial components upstream of Cdc42 and invasion. siRNA and the use of Vav1/2^-/- knockout cells further showed that the guanine exchange factor Vav2 is required for Cdc42 activation and maximal bacterial invasion. Overexpression of certain mutant constructs indicated that Vav2 is a linker molecule between Cdc42 and activated EGFR/PDGFR/PI3-kinase. Using C. jejuni mutant strains we further demonstrated that the fibronectin-binding protein CadF and intact flagella are involved in Cdc42-GTP induction, indicating that the bacteria may directly target the fibronectin/integrin complex for inducing signaling leading to its host cell entry.

Conclusion

Collectively, our findings led us propose that C. jejuni infection triggers a novel fibronectin→integrin-beta1→FAK/Src→EGFR/PDGFR→PI3-kinase→Vav2 signaling cascade, which plays a crucial role for Cdc42 GTPase activity associated with filopodia formation and enhances bacterial invasion.

β1-integrins signaling and mammary tumor progression in transgenic mouse models: implications for human breast cancer

Consistent with their essential role in cell adhesion to the extracellular matrix, integrins and their associated signaling pathways have been shown to be involved in cell proliferation, migration, invasion and survival, processes required in both tumorigenesis and metastasis. β1-integrins represent the predominantly expressed integrins in mammary epithelial cells and have been proven crucial for mammary gland development and differentiation. Here we provide an overview of the studies that have used transgenic mouse models of mammary tumorigenesis to establish β1-integrin as a critical mediator of breast cancer progression and thereby as a potential therapeutic target for the development of new anticancer strategies.

β1-integrin: a potential therapeutic target in the battle against cancer recurrence

Primary cancer treatment, involving both local and often systemic adjuvant therapy, is often successful, especially if the cancer is detected at an early stage of progression. However, for some patients, the cancer may recur either locally or as distant metastases, in some cases many years after apparently successful primary treatment. Significant tumor dormancy has been documented in several cancers, such as breast, melanoma, and renal cancer. Tumor dormancy has long been recognized as an important problem in management of cancer patients. Recent work has clarified biologic aspects of tumor dormancy and has shown that dormant tumor cells may be resistant to cytotoxic chemotherapy and radiation. This work has led to recognition of a key role for β1-integrin in regulating the switch from a dormant state to active proliferation and metastasis. Here we discuss the role of β1-integrin and its signaling partners in regulating the dormant phenotype. We also consider possible therapeutic approaches, such as small molecules or antibodies (ATN-161, volociximab, and JSM6427), directed against β1-integrin signaling to target dormant cancer cells and to prevent metastatic recurrence.

Cooperation between integrins and growth factor receptors in signaling and endocytosis

All multicellular animals express receptors for growth factors (GFs) and extracellular matrix (ECM) molecules. Integrin-type ECM receptors anchor cells to their surroundings and concomitantly activate intracellular signal transduction pathways. The same signaling mechanisms are regulated by GF receptors (GFRs). Recently, intensive research efforts have revealed novel mechanisms describing how the two receptor systems collaborate at many different levels. Integrins can directly bind to GFs and promote their activation. Adhesion receptors also organize signaling platforms and assist GFRs or even activate them via ligand-independent mechanisms. Furthermore, integrins can orchestrate endocytosis and recycling of GFRs. Here, we review the present knowledge about the interplay between integrins and GFRs and discuss recent ideas of how this collaboration may explain some previous controversies in integrin research.

Downregulation of cPLA2γ expression inhibits EGF-induced chemotaxis of human breast cancer cells through Akt pathway

Phospholipids play an important role in mediating cell migration. In the present study, we investigated the role of cPLA(2)γ in chemotaxis of human breast cancer cells. Inhibition of cPLA(2)γ expression by small interference RNA severely inhibits EGF-induced chemotaxis in a dose-dependent manner in MDA-MB-231, MCF-7, T47D and ZR-75-30 cells. Furthermore, silencing cPLA(2)γ expression also impaired directional migration, adhesion and invasion in MDA-MB-231 cells. In addition, we investigated the molecular mechanism by which cPLA(2)γ regulated migration. Knockdown of cPLA(2)γ suppressed the phosphorylation of Akt at both Thr308 and Ser473. Phosphorylation of PKCζ, downstream of Akt, was also dampened. Knockdown of cPLA(2)γ also impaired the phosphorylation of integrin β1 and cofilin, key regulators of cell adhesion and actin polymerization, respectively. Taken together, our results suggest that cPLA(2)γ plays an important role in cancer cell chemotaxis.

Significance of talin in cancer progression and metastasis

Upon detachment from the extracellular matrix, tumor epithelial cells and tumor-associated endothelial cells are capable of overcoming anoikis, gain survival benefits, and hence contribute to the process of metastasis. The focal-adhesion complex formation recruits the association of key adaptor proteins such as FAK (focal-adhesion kinase). Vimentin, paxillin, and talin are responsible for mediating the interaction between the actin cytoskeleton and integrins. Talin is an early-recruited focal-adhesion player that is of structural and functional significance in mediating interactions with integrin cytoplasmic tails leading to destabilization of the transmembrane complex and resulting in rearrangements in the extracellular integrin compartments that mediate integrin activation. Talin-mediated integrin activation plays a definitive role in integrin-mediated signaling and induction of downstream survival pathways leading to protection from anoikis and consequently resulting in cancer progression to metastasis. We recently reported that talin expression is significantly increased in prostate cancer compared with benign and normal prostate tissue and that this overexpression correlates with progression to metastatic disease implicating a prognostic value for talin during tumor progression. At the molecular level, talin is functionally associated with enhanced survival and proliferation pathways and confers anoikis resistance and metastatic spread of primary tumor cells via activation of the Akt survival pathway. In this review, we discuss the growing evidence surrounding the value of talin as a prognostic marker of cancer progression to metastasis and as therapeutic target in advanced prostate cancer, as well as the current understanding of mechanisms regulating its signaling activity in cancer.

PI3-kinase p110α mediates β1 integrin-induced Akt activation and membrane protrusion during cell attachment and initial spreading

Integrin-mediated cell adhesion activates several signaling effectors, including phosphatidylinositol 3-kinase (PI3K), a central mediator of cell motility and survival. To elucidate the molecular mechanisms of this important pathway the specific members of the PI3K family activated by different integrins have to be identified. Here, we studied the role of PI3K catalytic isoforms in β1 integrin-induced lamellipodium protrusion and activation of Akt in fibroblasts. Real-time total internal reflection fluorescence imaging of the membrane-substrate interface demonstrated that β1 integrin-mediated attachment induced rapid membrane spreading reaching essentially maximal contact area within 5-10 min. This process required actin polymerization and involved activation of PI3K. Isoform-selective pharmacological inhibition identified p110α as the PI3K catalytic isoform mediating both β1 integrin-induced cell spreading and Akt phosphorylation. A K756L mutation in the membrane-proximal part of the β1 integrin subunit, known to cause impaired Akt phosphorylation after integrin stimulation, induced slower cell spreading. The initial β1 integrin-regulated cell spreading as well as Akt phosphorylation were sensitive to the tyrosine kinase inhibitor PP2, but were not dependent on Src family kinases, FAK or EGF/PDGF receptor transactivation. Notably, cells expressing a Ras binding-deficient p110α mutant were severely defective in integrin-induced Akt phosphorylation, but exhibited identical membrane spreading kinetics as wild-type p110α cells. We conclude that p110α mediates β1 integrin-regulated activation of Akt and actin polymerization important for survival and lamellipodia dynamics. This could contribute to the tumorigenic properties of cells expressing constitutively active p110α.

The glycoprotein B disintegrin-like domain binds beta 1 integrin to mediate cytomegalovirus entry

Cellular integrins were identified as human cytomegalovirus (HCMV) entry receptors and signaling mediators in both fibroblasts and endothelial cells. The goal of these studies was to determine the mechanism by which HCMV binds to cellular integrins to mediate virus entry. HCMV envelope glycoprotein B (gB) has sequence similarity to the integrin-binding disintegrin-like domain found in the ADAM (a disintegrin and metalloprotease) family of proteins. To test the ability of this region to bind to cellular integrins, we generated a recombinant soluble version of the gB disintegrin-like domain (gB-DLD). The gB-DLD protein bound to human fibroblasts in a specific, dose-dependent and saturable manner that required the expression of an intact beta1 integrin ectodomain. Furthermore, a physical association between gB-DLD and beta1 integrin was demonstrated through in vitro pull-down assays. The function of this interaction was shown by the ability of cell-bound gB-DLD to efficiently block HCMV entry and the infectivity of multiple in vivo target cells. Additionally, rabbit polyclonal antibodies raised against gB-DLD neutralized HCMV infection. Mimicry of the ADAM family disintegrin-like domain by HCMV gB represents a novel mechanism for integrin engagement by a virus and reveals a unique therapeutic target for HCMV neutralization. The strong conservation of the DLD across beta- and gammaherpesviruses suggests that integrin recognition and utilization may be a more broadly conserved feature throughout the Herpesviridae.

Beta integrin tyrosine phosphorylation is a conserved mechanism for regulating talin-induced integrin activation

Integrins are large membrane-spanning receptors fundamental to cell adhesion and migration. Integrin adhesiveness for the extracellular matrix is activated by the cytoskeletal protein talin via direct binding of its phosphotyrosine-binding-like F3 domain to the cytoplasmic tail of the beta integrin subunit. The phosphotyrosine-binding domain of the signaling protein Dok1, on the other hand, has an inactivating effect on integrins, a phenomenon that is modulated by integrin tyrosine phosphorylation. Using full-length tyrosine-phosphorylated (15)N-labeled beta3, beta1A, and beta7 integrin tails and an NMR-based protein-protein interaction assay, we show that talin1 binds to the NPXY motif and the membrane-proximal portion of beta3, beta1A, and beta7 tails, and that the affinity of this interaction is decreased by integrin tyrosine phosphorylation. Dok1 only interacts weakly with unphosphorylated tails, but its affinity is greatly increased by integrin tyrosine phosphorylation. The Dok1 interaction remains restricted to the integrin NPXY region, thus phosphorylation inhibits integrin activation by increasing the affinity of beta integrin tails for a talin competitor that does not form activating membrane-proximal interactions with the integrin. Key residues governing these specificities were identified by detailed structural analysis, and talin1 was engineered to bind preferentially to phosphorylated integrins by introducing the mutation D372R. As predicted, this mutation affects talin1 localization in live cells in an integrin phosphorylation-specific manner. Together, these results indicate that tyrosine phosphorylation is a common mechanism for regulating integrin activation, despite subtle differences in how these integrins interact with their binding proteins.

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.

Integrin trafficking regulated by Rab21 is necessary for cytokinesis

Adherent cells undergo remarkable changes in shape during cell division. However, the functional interplay between cell adhesion turnover and the mitotic machinery is poorly understood. The endo/exocytic trafficking of integrins is regulated by the small GTPase Rab21, which associates with several integrin alpha subunits. Here, we show that targeted trafficking of integrins to and from the cleavage furrow is required for successful cytokinesis, and that this is regulated by Rab21. Rab21 activity, integrin-Rab21 association, and integrin endocytosis are all necessary for normal cytokinesis, which becomes impaired when integrin-mediated adhesion at the cleavage furrow fails. We also describe a chromosomal deletion and loss of Rab21 gene expression in human cancer, which leads to the accumulation of multinucleate cells. Importantly, reintroduction of Rab21 rescued this phenotype. In conclusion, Rab21-regulated integrin trafficking is essential for normal cell division, and its defects may contribute to multinucleation and genomic instability, which are hallmarks of cancer.

Extended interaction of beta1 integrin subunit-deficient cells (GD25) with surfaces modified with fibronectin-derived peptides: Culture optimization, adhesion and cytokine panel studies

The modification of biomaterials with extracellular matrix-mimicking factors is a common technique used to influence the cellular response through integrin-mediated signaling. The inherent limitations of antibody-inhibition studies necessitate the use of complementary methods to block integrin function to confirm cell-surface interaction. In this study, we employed a beta1 integrin-deficient cell line, GD25, to investigate the role of beta1 subunit in cell adhesion and subsequent cytokine (granulocyte macrophage colony stimulating factor; interleukin (IL)-1alpha; IL-1beta; IL-6; monocyte chemoattractant protein-1; regulated upon activation, normal T-cell expressed, and secreted; tumor necrosis factor-alpha) release kinetics in the presence of tissue culture polystyrene (TCPS) and semi-interpenetrating polymer networks (sIPN) modified with fibronectin (FN)-mimic peptides (RGD, PHSRN). Culture conditions (i.e. seeding density, medium, serum supplementation) were optimized for long-term observation. Differences in cell adhesion, cell viability and cytokine release behavior were dependent on the presence of the beta1 integrin subunit, FN, sIPN cast method and peptide identity. By comparing two complementary techniques for assaying integrin function, we observed both similarities (i.e. decreased adhesion to FN-absorbed TCPS and increased IL-1beta release at 96h) and differences (i.e. no difference in adhesion or IL-1beta release in the presence of different sIPN surfaces) when the function of the beta1 subunit was blocked in cell adhesion and signaling in the presence of biomaterials.

Expression of integrin-linked kinase is increased in differentiated cells

Integrin-linked kinase (ILK), a mediator of beta integrin signals, has emerged as a therapeutic target in malignant tumors. Because malignant transformation is accompanied by dedifferentiation, ILK expression was evaluated in diverse normal and tumor tissue samples with regard to tissue differentiation. In single sections and in a tissue microarray (323 tumor tissues, 181 normal tissues), immunohistochemistry was performed [ILK, Akt, phospho-Akt-S473, loricrin, transforming growth factor beta2 (TGFbeta2)], and staining intensities were semiquantitatively scored. Increased ILK expression was clearly associated with increased differentiation in normal gastrointestinal, neural, bone marrow, renal tissue, and in more differentiated areas of malignant tumors. ILK colocalized with its putative downstream target Akt and with loricrin or TGFbeta2. Our findings clearly show that elevated levels of ILK are associated with cellular differentiation in high turnover tissues but not generally with a malignant phenotype. Our study indicates that ILK is not a general molecular target for cancer therapy but rather an indicator of differentiation. This manuscript contains online supplemental material at http://www.jhc.org. Please visit this article online to view these materials.

Integrin-mediated host cell invasion by type 1-piliated uropathogenic Escherichia coli

Uropathogenic Escherichia coli (UPEC), the primary causative agent of urinary tract infections, typically express filamentous adhesive organelles called type 1 pili that mediate both bacterial attachment to and invasion of bladder urothelial cells. Several host proteins have previously been identified as receptors for type 1 pili, but none have been conclusively shown to promote UPEC entry into host bladder cells. Using overlay assays with FimH, the purified type 1 pilus adhesin, and mass spectroscopy, we have identified β1 and α3 integrins as key host receptors for UPEC. FimH recognizes N-linked oligosaccharides on these receptors, which are expressed throughout the urothelium. In a bladder cell culture system, β1 and α3 integrin receptors co-localize with invading type 1–piliated bacteria and F-actin. FimH-mediated bacterial invasion of host bladder cells is inhibited by β1 and α3 integrin–specific antibodies and by disruption of the β1 integrin gene in the GD25 fibroblast cell line. Phosphorylation site mutations within the cytoplasmic tail of β1 integrin that alter integrin signaling also variably affect UPEC entry into host cells, by either attenuating or boosting invasion frequencies. Furthermore, focal adhesion and Src family kinases, which propagate integrin-linked signaling and downstream cytoskeletal rearrangements, are shown to be required for FimH-dependent bacterial invasion of target host cells. Cumulatively, these results indicate that β1 and α3 integrins are functionally important receptors for type 1 pili–expressing bacteria within the urinary tract and possibly at other sites within the host.

Strains of bacteria called uropathogenic Escherichia coli (UPEC) are the primary cause of urinary tract infections (UTIs), which by some estimates are the second most common type of infectious disease in the world today. UPEC strains typically express hair-like fibers called type 1 pili on their surface that allow them to bind and invade the host cells that line the urinary tract. The ability of UPEC to enter these host cells likely promotes the establishment and persistence of UTIs. The invasion process requires that the incoming pathogens first bind specific receptor molecules on the target cell surface. Here, we identify two host proteins known as β1 and α3 integrin as key receptors for type 1 pili–expressing UPEC. The adhesive tips of type 1 pili recognize sugars that decorate these integrin receptors, thereby activating a signaling cascade that stimulates the host plasma membrane to zipper around and envelop bound bacteria. β1 and α3 integrins have important roles in a number of host cell functions and are widely distributed within the urinary tract and in other tissues. Interestingly, type 1 pili are expressed by many different types of bacteria in addition to UPEC, raising the possibility that interactions between type 1 pili and integrin receptors can facilitate bacterial colonization within diverse niches throughout the host.

NPXY motifs in the beta1 integrin cytoplasmic tail are required for functional reovirus entry

Reovirus cell entry is mediated by attachment to cell surface carbohydrate and junctional adhesion molecule A (JAM-A) and internalization by beta1 integrin. The beta1 integrin cytoplasmic tail contains two NPXY motifs, which function in recruitment of adaptor proteins and clathrin for endocytosis and serve as sorting signals for internalized cargo. As reovirus infection requires disassembly in the endocytic compartment, we investigated the role of the beta1 integrin NPXY motifs in reovirus internalization. In comparison to wild-type cells (beta1+/+ cells), reovirus infectivity was significantly reduced in cells expressing mutant beta1 integrin in which the NPXY motifs were altered to NPXF (beta1+/+Y783F/Y795F cells). However, reovirus displayed equivalent binding and internalization levels following adsorption to beta1+/+ cells and beta1+/+Y783F/Y795F cells, suggesting that the NPXY motifs are essential for transport of reovirus within the endocytic pathway. Reovirus entry into beta1+/+ cells was blocked by chlorpromazine, an inhibitor of clathrin-mediated endocytosis, while entry into beta1+/+Y783F/Y795F cells was unaffected. Furthermore, virus was distributed to morphologically distinct endocytic organelles in beta1+/+ and beta1+/+Y783F/Y795F cells, providing further evidence that the beta1 integrin NPXY motifs mediate sorting of reovirus in the endocytic pathway. Thus, NPXY motifs in the beta1 integrin cytoplasmic tail are required for functional reovirus entry, which indicates a key role for these sequences in endocytosis of a pathogenic virus.

Quantitative time-resolved phosphoproteomic analysis of mast cell signaling

Mast cells play a central role in type I hypersensitivity reactions and allergic disorders such as anaphylaxis and asthma. Activation of mast cells, through a cascade of phosphorylation events, leads to the release of mediators of the early phase allergic response. Understanding the molecular architecture underlying mast cell signaling may provide possibilities for therapeutic intervention in asthma and other allergic diseases. Although many details of mast cell signaling have been described previously, a systematic, quantitative analysis of the global tyrosine phosphorylation events that are triggered by activation of the mast cell receptor is lacking. In many cases, the involvement of particular proteins in mast cell signaling has been established generally, but the precise molecular mechanism of the interaction between known signaling proteins often mediated through phosphorylation is still obscure. Using recently advanced methodologies in mass spectrometry, including automation of phosphopeptide enrichments and detection, we have now substantially characterized, with temporal resolution as short as 10 s, the sites and levels of tyrosine phosphorylation across 10 min of FcepsilonRI-induced mast cell activation. These results reveal a far more extensive array of tyrosine phosphorylation events than previously known, including novel phosphorylation sites on canonical mast cell signaling molecules, as well as unexpected pathway components downstream of FcepsilonRI activation. Furthermore, our results, for the first time in mast cells, reveal the sequence of phosphorylation events for 171 modification sites across 121 proteins in the MCP5 mouse mast cell line and 179 modification sites on 117 proteins in mouse bone marrow-derived mast cells.

Postischemic gene transfer of soluble Flt-1 protects against brain ischemia with marked attenuation of blood-brain barrier permeability

Brain edema is a major and often mortal complication of brain ischemia. Vascular endothelial growth factor (VEGF) is also known as a potent vascular permeability factor and may play detrimental roles at the acute stage of brain infarction. Our goal in this study was to explore protective effects of gene transfer of soluble flt-1 (sFlt-1), a natural inhibitor of VEGF, on focal brain ischemia. Adenoviral vector encoding sFlt-1 or beta-galactosidase as control was injected into the lateral ventricle 90 mins after photochemical distal middle cerebral artery occlusion in male spontaneously hypertensive rats. The transduced sFlt-1 was released to the cerebrospinal fluid from the ventricular wall and significantly increased 6 h, 1 and 7 days after sFlt-1 transfection. One day after brain ischemia, sFlt-1 gene transfer significantly reduced infarct volume (by 35%), brain edema (by 35%), and blood-brain barrier permeability (Evans blue extravasation; by 69%) with diminished phosphorylation of focal adhesion kinase (FAKtyr397 and FAKtyr861) in the ischemic vessels. Seven days after ischemia, sFlt-1 gene transfer also significantly attenuated infarct volume (by 29%) and monocyte/macrophage infiltration (by 27%), although there were no reductions in angiogenesis by sFlt-1 overexpression. These results suggest that sFlt-1 gene therapy targeting brain edema in acute stage of brain ischemia may be useful for brain infarction.

Two synergistic activation mechanisms of alpha2beta1 integrin-mediated collagen binding

Activation of protein kinase C by 12-O-tetradecanoylphorbol-13-acetate (TPA) induces ligand-independent aggregation of a cell surface collagen receptor, alpha2beta1 integrin. Concomitantly, TPA increases the avidity of alpha2beta1 for collagen and the number of conformationally activated alpha2beta1 integrins. The structural change was shown using a monoclonal antibody 12F1 that recognizes the "open" (active) conformation of the inserted domain in the alpha2 subunit (alpha2I). Amino acid residue Glu-336 in alpha2 subunit is proposed to mediate the interaction between alpha2I domain and beta1 subunit. Glu-336 seems to regulate a switch between open and "closed" conformations, since the mutation alpha2E336A inhibited the TPA-related increase in the number of 12F1 positive integrins. E336A also reduced cell adhesion to collagen. However, E336A did not prevent the TPA-related increase in adhesion to collagen or alpha2beta1 aggregation. Thus, alpha2beta1 integrin avidity is regulated by two synergistic mechanisms, first an alpha2E336-dependent switch to the open alpha2I conformation, and second an alpha2E336-independent mechanism temporally associated with receptor aggregation.

Coculture with endothelial cells enhances vascular smooth muscle cell adhesion and spreading via activation of β1-integrin and phosphatidylinositol 3-kinase/Akt

The interactions between endothelial cells (ECs) and vascular smooth muscle cells (VSMCs) play significant roles in the homeostasis of the blood vessel during vascular remodeling. Cell adhesion and spreading are an essential process for VSMC migration, survival and proliferation in the events of vascular physiology and pathophysiology. However, effects of ECs on adhesion and spreading of VSMCs have not been characterized yet. Here, the interaction of ECs and VSMCs on adhesion and spreading of VSMCs were investigated by using a coculture system. The results showed that VSMCs cocultured with ECs exhibited a significant increase in the number of adherent and spreading cells, and much more mRNA (twofold, P<0.01) and protein (threefold, P<0.05) expression of beta(1)-integrin comparing to the control, i.e., VSMCs cultured alone. Furthermore, the enhanced functional activity of beta(1)-integrin expression was confirmed by FACS. A beta(1)-integrin blocking antibody (P5D2) could inhibit the EC-induced VSMC adhesion and spreading. It was demonstrated that in correspondence with enhanced cell adhesion, ECs also prompted focal adhesion complex assembly and stress fiber formation of VSMCs. The phosphatidylinositol 3-kinase (PI3K)/Akt pathway was more pronouncedly activated in response to VSMC attachment. Our results for the first time show that coculture with ECs enhances VSMC adhesion and spreading by up-regulating beta(1)-integrin expression and activating the PI3K/Akt pathway, suggesting that the interaction between ECs and VSMCs serves an important role in vascular homeostasis and remodeling.

α-Actinin-4 Is Required for Normal Podocyte Adhesion

Mutations in the alpha-actinin-4 gene ACTN4 cause an autosomal dominant human kidney disease. Mice deficient in alpha-actinin-4 develop a recessive phenotype characterized by kidney failure, proteinuria, glomerulosclerosis, and retraction of glomerular podocyte foot processes. However, the mechanism by which alpha-actinin-4 deficiency leads to glomerular disease has not been defined. Here, we examined the effect of alpha-actinin-4 deficiency on the adhesive properties of podocytes in vivo and in a cell culture system. In alpha-actinin-4-deficient mice, we observed a decrease in the number of podocytes per glomerulus compared with wild-type mice as well as the presence of podocyte markers in the urine. Podocyte cell lines generated from alpha-actinin-4-deficient mice were less adherent than wild-type cells to glomerular basement membrane (GBM) components collagen IV and laminin 10 and 11. We also observed markedly reduced adhesion of alpha-actinin-4-deficient podocytes under increasing shear stresses. This adhesion deficit was restored by transfecting cells with alpha-actinin-4-GFP. We tested the strength of the integrin receptor-mediated linkages to the cytoskeleton by applying force to microbeads bound to integrin using magnetic pulling cytometry. Beads bound to alpha-actinin-4-deficient podocytes showed greater displacement in response to an applied force than those bound to wild-type cells. Consistent with integrin-dependent alpha-actinin-4-mediated adhesion, phosphorylation of beta1-integrins on alpha-actinin-4-deficient podocytes is reduced. We rescued the phosphorylation deficit by transfecting alpha-actinin-4 into alpha-actinin-4-deficient podocytes. These results suggest that alpha-actinin-4 interacts with integrins and strengthens the podocyte-GBM interaction thereby stabilizing glomerular architecture and preventing disease.

Genetic analysis of beta1 integrin "activation motifs" in mice

Akey feature of integrins is their ability to regulate the affinity for ligands, a process termed integrin activation. The final step in integrin activation is talin binding to the NPXY motif of the integrin beta cytoplasmic domains. Talin binding disrupts the salt bridge between the alpha/beta tails, leading to tail separation and integrin activation. We analyzed mice in which we mutated the tyrosines of the beta1 tail and the membrane-proximal aspartic acid required for the salt bridge. Tyrosine-to-alanine substitutions abolished beta1 integrin functions and led to a beta1 integrin-null phenotype in vivo. Surprisingly, neither the substitution of the tyrosines with phenylalanine nor the aspartic acid with alanine resulted in an obvious defect. These data suggest that the NPXY motifs of the beta1 integrin tail are essential for beta1 integrin function, whereas tyrosine phosphorylation and the membrane-proximal salt bridge between alpha and beta1 tails have no apparent function under physiological conditions in vivo.

beta1-integrin-mediated signaling essentially contributes to cell survival after radiation-induced genotoxic injury

Integrin-mediated adhesion to extracellular matrix proteins confers resistance to radiation- or drug-induced genotoxic injury. To analyse the underlying mechanisms specific for beta1-integrins, wild-type beta1A-integrin-expressing GD25beta1A cells were compared to GD25beta1B cells, which express signaling-incompetent beta1B variants. Cells grown on fibronectin, collagen-III, beta1-integrin-IgG or poly-l-lysine were exposed to 0-6 Gy X-rays in presence or depletion of growth factors and phosphatidylinositol-3 kinase (PI3K) inhibitors (LY294002, wortmannin). In order to test the relevance of these findings in tumor cells, human A-172 glioma cells were examined under the same conditions after siRNA-mediated silencing of beta1-integrins. We found that beta1A-integrin-mediated adhesion to fibronectin, collagen-III or beta1-IgG was essential for cell survival after radiation-induced genotoxic injury. Mediated by PI3K, pro-survival beta1A-integrin/Akt signaling was critically involved in this process. Additionally, the beta1-integrin downstream targets p130Cas and paxillin-impaired survival-regulating PI3K-dependent JNK. In A-172 glioma cells, beta1-integrin knockdown and PI3K inhibition confirmed the central role of beta1-integrins in Akt- and p130Cas/paxillin-mediated prosurvival signaling. These findings suggest beta1-integrins as critical regulators of cell survival after radiation-induced genotoxic injury. Elucidation of the molecular circuitry of prosurvival beta1-integrin-mediated signaling in tumor cells may promote the development of innovative molecular-targeted therapeutic antitumor strategies.

Interactions of the integrin subunit beta1A with protein kinase B/Akt, p130Cas and paxillin contribute to regulation of radiation survival

BACKGROUND AND PURPOSE

Cell adhesion-mediated radioresistance is a common phenomenon particularly relevant in tumor cells, which might hamper anticancer therapies. To analyze the role of adhesion-mediating beta1-integrins, stably transfected functional beta1A-integrin-expressing GD25beta1A and GD25beta1B cells, which express mutant beta1B-integrins, were compared in terms of radiation survival and beta1-integrin signaling.

MATERIALS AND METHODS

Cells grown on fibronectin, collagen-III, laminin, vitronectin, anti-beta1-integrin-IgG (beta1-IgG) or poly-l-lysine were irradiated with 0-6Gy in presence or absence of growth factors or inhibitors for phosphatidylinositol-3 kinase (PI3K), i.e. Ly294002 and wortmannin. In addition to colony formation, protein kinase B/Akt (PKB/Akt) kinase activity, focal adhesion kinase (FAK), p130Cas, paxillin and c-Jun N2-terminal kinase (JNK) expression and phosphorylation were analyzed by Western blot technique.

RESULTS

Adhesion of GD25beta1A cells to extracellular matrix proteins or beta1-IgG resulted in growth factor-independent radiation survival. In contrast, serum starved GD25beta1B cells showed a significant (P<0.01) reduction in radiation survival on all substrates. PI3K inhibition moderately or strongly radiosensitized GD25beta1A or GD25beta1B cells, respectively. The pro-survival effects detected in serum starved GD25beta1A cells were due to direct, PI3K-mediated stimulation of PKB/Akt activity by beta1-integrins and induced p130Cas and paxillin phosphorylation. Phosphorylated p130Cas and paxillin subsequently prevented activation of cell death-regulating JNK.

CONCLUSIONS

The data show that beta1-integrin-mediated signaling through the cytoplasmic integrin domains is critical for efficient pro-survival regulation after irradiation. Profound knowledge of the underlying mechanisms of integrin-mediated cellular radioresistance could foster the design of new molecular-targeted anticancer therapies.

Threonine 788 in integrin subunit beta1 regulates integrin activation

In the present study, the functional role of suggested phosphorylation of the conserved threonines in the cytoplasmic domain of integrin subunit beta1 was investigated. Mutants mimicking phosphorylated and unphosphorylated forms of beta1 were expressed in beta1 deficient GD25 cells. T788 in beta1 was identified as a site with major influence on integrin function. The mutation to A788 strongly reduced beta1-dependent cell attachment and exposure of the extracellular 9EG7 epitope, whereas replacement of T789 with alanine did not interfere with the ligand-binding ability. Talin has been shown to mediate integrin activation, but the talin head domain bound equally well to the wild-type beta1 and the mutants indicating that the T788A mutation caused defect integrin activation by another mechanism. The phosphorylation-mimicking mutation T788D was fully active in promoting cell adhesion. GD25 cells expressing beta1T788D accumulated increased number of focal contacts and migrated slowly compared to GD25 beta1 wild-type. An analogous phenotype is seen when focal adhesion kinase activation is abrogated. However, neither the beta1T788D nor the beta1T788A mutation failed to induce tyrosine phosphorylation of focal adhesion kinase. The results suggest that phosphorylation of T788 in integrin beta1 promotes inside-out receptor activation, as well as focal contact accumulation.

Integrin activation by talin

The development and integrity of the cardiovascular system depends on integrins, a family of adhesion receptors, vitally important for homeostasis of animal species from fruit fly to man. Integrins are critical players in cell migration, cell adhesion, cell cycle progression, differentiation, and apoptosis. Consequently, integrins have a major impact on the patterning and functions of the blood and cardiovascular system. Integrins undergo conformational changes, which alter their affinity for ligands through a process operationally defined as integrin activation. Integrin activation is important for platelet aggregation, leukocyte extravasation, and cell adhesion and migration, thus influencing such processes as hemostasis, inflammation and angiogenesis. Recently, a series of studies have begun to define the mechanism of integrin activation by demonstrating that binding of a cytoskeletal protein, talin, to integrin beta subunit cytoplasmic tail is a last common step in integrin activation. These findings indicate that talin is likely to be at the center of converging signaling pathways regulating integrin activation.

beta1-Integrins induce phosphorylation of Akt on serine 473 independently of focal adhesion kinase and Src family kinases

Adhesion by means of beta1-integrins induces the phosphorylation of Akt, an event strictly dependent on the activity of the phosphatidylinositol 3-kinase (PI3K). Binding of the p85 regulatory subunit of PI3K to phosphorylated tyrosine 397 in focal adhesion kinase (FAK) is considered to be the mechanism of cell adhesion-induced activation of class Ia PI3K. Here we show that PI3K-dependent phosphorylation of Akt in response to ligation of beta1-integrins occurs efficiently in the absence of FAK tyrosine phosphorylation. Akt S473 phosphorylation was strongly promoted both in cells expressing the integrin subunit splice variant beta1B, which is unable to activate FAK, and in FAK knockout cells. In addition, we found this phosphorylation to be independent of the Src family kinases Src, Fyn and Yes. These results indicate that a major pathway for adhesion-dependent activation of PI3K/Akt is triggered by the membrane proximal part of the beta1 subunit in a FAK and Src-independent manner.

Amplification and oscillations in the FAK/Src kinase system during integrin signaling

Integrin signaling is a major pathway of cell adhesion to extracellular matrices that regulates many physiological cell behaviors such as cell proliferation, migration or differentiation and is implied in pathologies such as tumor invasion. In this paper, we focused on the molecular system formed by the two kinases FAK (focal adhesion kinase) and Src, which undergo auto- and co-activation during early steps of integrin signaling. The system is modelled using classical kinetic equations and yields a set of three nonlinear ordinary differential equations describing the dynamics of the different phosphorylation forms of FAK. Analytical and numerical analysis of these equations show that this system may in certain cases amplify incoming signals from the integrins. A quantitative condition is obtained, which indicates that the total FAK charge in the system acts as a critical mass that must be exceeded for amplification to be effective. Furthermore, we show that when FAK activity is lower than Src activity, spontaneous oscillations of FAK phosphorylation forms may appear. The oscillatory behavior is studied using bifurcation and stability diagrams. We finally discuss the significance of this behavior with respect to recent experimental results evidencing FAK dynamics.

Focal adhesion regulation of cell behavior

Focal adhesions lie at the convergence of integrin adhesion, signaling and the actin cytoskeleton. Cells modify focal adhesions in response to changes in the molecular composition, two-dimensional (2D) vs. three-dimensional (3D) structure, and physical forces present in their extracellular matrix environment. We consider here how cells use focal adhesions to regulate signaling complexes and integrin function. Furthermore, we examine how this regulation controls complex cellular behaviors in response to matrices of diverse physical and biochemical properties. One event regulated by the physical structure of the ECM is phosphorylation of focal adhesion kinase (FAK) at Y397, which couples FAK to several signaling pathways that regulate cell proliferation, survival, migration, and invasion.

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.

CD98hc (SLC3A2) interaction with beta 1 integrins is required for transformation

CD98hc (SLC3A2) constitutively and specifically associates with beta(1) integrins and is highly expressed on the surface of human tumor cells irrespective of the tissue of origin. We have found here that expression of CD98hc promotes both anchorage- and serum-independent growth. This oncogenic activity is dependent on beta(1) integrin-mediated phosphoinositol 3-hydroxykinase stimulation and the level of surface expression of CD98hc. Using chimeras of CD98hc and the type II membrane protein CD69, we show that the transmembrane domain of CD98hc is necessary and sufficient for integrin association in cells. Furthermore, CD98hc/beta(1) integrin association is required for focal adhesion kinase-dependent phosphoinositol 3-hydroxykinase activation and cellular transformation. Amino acids 82-87 in the putative cytoplasmic/transmembrane region appear to be critical for the oncogenic potential of CD98hc and provide a novel mechanism for tumor promotion by integrins. These results explain how high expression of CD98hc in human cancers contributes to transformation; furthermore, the transmembrane association of CD98hc and beta(1) integrins may provide a new target for cancer therapy.