Possible role of ILK-affixin complex in integrin-cytoskeleton linkage during platelet aggregation

Parvin: A hub of intracellular signalling pathways regulating cellular behaviour and disease progression

α-actinin superfamily houses the family of parvins, comprising α, β and γ isoforms in the vertebrates and a single orthologue in the invertebrates. Parvin as an adaptor protein is a member of the ternary IPP-complex including Integrin Linked Kinase (ILK) and particularly-interesting-Cys-His-rich protein (PINCH). Each of the complex proteins showed a conserved lineage and was principally used by the evolutionarily primitive integrin-adhesome machinery to regulate cellular behaviour and signalling pathways. Parvin facilitated integrin mediated integration of the extracellular matrix with cytoskeletal framework culminating in regulation of cellular adhesion and spreading, cytoskeleton reorganisation and cell survival. Studies have established role of parvin in pregnancy, lactation, matrix degradation, blood vessel formation and in several diseases such as cancer, NAFLD and cardiac diseases etc. This review narrates the history of parvin discovery, its elaborate gene structure and conservation across phyla including cellular expression, localisation and interacting partners in vertebrates as well as invertebrates. The review further discusses how parvin acts as an epicentre of signalling pathways, its associated mutants and diseased conditions.

Fak silencing impairs osteogenic differentiation of bone mesenchymal stem cells induced by uniaxial mechanical stretch

Background/purpose

Mechanical stretch plays a key role in promoting proliferation and differentiation of bone marrow mesenchymal stem cells (BMSCs) in distraction osteogenesis (DO). A better understanding of how the extracellular biomechanical stimulation is transferred to intracellular signal expression will benefit DO. Focal adhesion kinase (FAK) is a key factor in integrin signaling pathway. However, little is known about the effect of integrin-FAK signaling during the process of stretch induced osteogenic differentiation of BMSCs.

Materials and methods

A specific short hairpin RNAs (shRNAs) lentiviral expression vector was used to silence Fak gene and a well-established in vitro uniaxial dynamic stretching device was applied to stimulate DO. Fak silencing was confirmed by fluorescence microscopy and the detection of Fak mRNA and FAK, p-FAK protein expression. Alkaline phosphatase (ALP) activity, expression of osteogenic differentiation markers - runt-related transcription factor 2 (RUNX2/Runx2) and alkaline phosphatase (Alp) together with integrin upstream signal transduction molecules integrin beta-1 (ITGB1/Itgb1) and downstream signal transduction molecules integrin-linked kinase (ILK) were detected after the stretch.

Results

The results showed that mechanical stretch in control groups significantly induced the osteogenic differentiation of BMSCs with increased ALP activity, expression of RUNX2/Runx2 and Alp, together with upregulated ITGB1/Itgb1 and ILK, which all vanished in Fak silencing group.

Conclusion

Silencing of the Fak gene inhibited the osteogenic differentiation of rat BMSCs induced by in vitro mechanical stretch through integrin signaling pathway.

Differences in Whole Blood Gene Expression Associated with Infection Time-Course and Extent of Fetal Mortality in a Reproductive Model of Type 2 Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) Infection

Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) infection of pregnant females causes fetal death and increased piglet mortality, but there is substantial variation in the extent of reproductive pathology between individual dams. This study used RNA-sequencing to characterize the whole blood transcriptional response to type 2 PRRSV in pregnant gilts during the first week of infection (at 0, 2, and 6 days post-inoculation), and attempted to identify gene expression signatures associated with a low or high level of fetal mortality rates (LFM and HFM; n = 8/group) at necropsy, 21 days post-inoculation. The initial response to infection measured at 2 days post-inoculation saw an upregulation of genes involved in innate immunity, such as interferon-stimulated antiviral genes and inflammatory markers, and apoptosis. A concomitant decrease in expression of protein synthesis and T lymphocyte markers was observed. By day 6 the pattern had reversed, with a drop in innate immune signaling and an increase in the expression of genes involved in cell division and T cell signaling. Differentially expressed genes (DEGs) associated with extremes of litter mortality rate were identified at all three time-points. Among the 15 DEGs upregulated in LFM gilts on all three days were several genes involved in platelet function, including integrins ITGA2B and ITGB3, and the chemokine PF4 (CXCL4). LFM gilts exhibited a higher baseline expression of interferon-stimulated and pro-inflammatory genes prior to infection, and of T cell markers two days post-infection, indicative of a more rapid progression of the immune response to PRRSV. This study has increased our knowledge of the early response to PRRSV in the blood of pregnant gilts, and could ultimately lead to the development of a biomarker panel that can be used to predict PRRSV-associated reproductive pathology.

Targeting Integrin-Linked Kinase Suppresses Invasion and Metastasis through Downregulation of Epithelial-to-Mesenchymal Transition in Renal Cell Carcinoma

Renal cell carcinoma (RCC) is the most common malignancy in the kidney. Antiangiogenic targeted therapies inhibit the progression of RCC, but have limited impacts on invasion or metastasis of tumor cells. Integrin-linked kinase (ILK) is a serine/threonine kinase implicated in the regulation of cell growth/survival, cell-cycle progression, epithelial-mesenchymal transition (EMT), invasion/migration, and angiogenesis. However, the role of ILK in RCC has not been evaluated. We investigated the role of ILK on cancer progression and metastasis and the therapeutic potential of ILK inhibition in RCC. Our investigation reveals that ILK is expressed at a low level in normal cells and low-stage RCC cells and is highly expressed in advanced and metastatic cells. Caki-1, a metastatic RCC cell line, showed higher expression of molecular EMT markers, including Snail and Zeb1, but decreased activity of GSK3β. Knockdown of ILK using small interference (si)-ILK minimally inhibited tumor proliferation and cell-cycle progression was not significantly affected. However, ILK knockdown suppressed the formation of stress fibers and focal adhesions and impeded phenotypic EMT markers, including cell migration and invasion, in Caki-1 and UMRC-3 cells. Finally, in vivo knockdown of ILK suppressed the progression, invasion, and metastasis of primary RCC in nude mice by downregulation of EMT markers (Snail, Zeb1, vimentin, and E-cadherin). Our results show that ILK may be essential for invasion and metastasis in RCC and regulates vimentin and E-cadherin expression by regulating the EMT-related transcription factors Snail and Zeb1. These results suggest that ILK may be a potential target in RCC.

The integrin-linked kinase-PINCH-parvin complex supports integrin αIIbβ3 activation

Integrin-linked kinase (ILK) is an important signaling regulator that assembles into the heteroternary complex with adaptor proteins PINCH and parvin (termed the IPP complex). We recently reported that ILK is important for integrin activation in a Chinese hamster ovary (CHO) cell system. We previously established parental CHO cells expressing a constitutively active chimeric integrin (αIIbα6Bβ3) and mutant CHO cells expressing inactive αIIbα6Bβ3 due to ILK deficiency. In this study, we further investigated the underlying mechanisms for ILK-dependent integrin activation. ILK-deficient mutant cells had trace levels of PINCH and α-parvin, and transfection of ILK cDNA into the mutant cells increased not only ILK but also PINCH and α-parvin, resulting in the restoration of αIIbα6Bβ3 activation. In the parental cells expressing active αIIbα6Bβ3, ILK, PINCH, and α-parvin were co-immunoprecipitated, indicating the formation of the IPP complex. Moreover, short interfering RNA (siRNA) experiments targeting PINCH-1 or both α- and β-parvin mRNA in the parent cells impaired the αIIbα6Bβ3 activation as well as the expression of the other components of the IPP complex. In addition, ILK mutants possessing defects in either PINCH or parvin binding failed to restore αIIbα6Bβ3 activation in the mutant cells. Kindlin-2 siRNA in the parental cells impaired αIIbα6Bβ3 activation without disturbing the expression of ILK. For CHO cells stably expressing wild-type αIIbβ3 that is an inactive form, overexpression of a talin head domain (THD) induced αIIbβ3 activation and the THD-induced αIIbβ3 activation was impaired by ILK siRNA through a significant reduction in the expression of the IPP complex. In contrast, overexpression of all IPP components in the αIIbβ3-expressing CHO cells further augmented THD-induced αIIbβ3 activation, whereas they did not induce αIIbβ3 activation without THD. These data suggest that the IPP complex rather than ILK plays an important role and supports integrin activation probably through stabilization of the active conformation.

Platelet signaling-a primer

OBJECTIVE

To review the receptors and signal transduction pathways involved in platelet plug formation and to highlight links between platelets, leukocytes, endothelium, and the coagulation system.

DATA SOURCES

Original studies, review articles, and book chapters in the human and veterinary medical fields.

DATA SYNTHESIS

Platelets express numerous surface receptors. Critical among these are glycoprotein VI, the glycoprotein Ib-IX-V complex, integrin α(IIb) β(3) , and the G-protein-coupled receptors for thrombin, ADP, and thromboxane. Activation of these receptors leads to various important functional events, in particular activation of the principal adhesion receptor α(IIb) β(3) . Integrin activation allows binding of ligands such as fibrinogen, mediating platelet-platelet interaction in the process of aggregation. Signals activated by these receptors also couple to 3 other important functional events, secretion of granule contents, change in cell shape through cytoskeletal rearrangement, and procoagulant membrane expression. These processes generate a stable thrombus to limit blood loss and promote restoration of endothelial integrity.

CONCLUSIONS

Improvements in our understanding of how platelets operate through their signaling networks are critical for diagnosis of unusual primary hemostatic disorders and for rational antithrombotic drug design.

There's something about ILK

PURPOSE

Integrin-Linked Kinase (ILK) is associated with integrin and growth factor receptor signalling. As both signalling pathways contribute to cancer cell resistance, ILK seems well suited as a promising tumour target.

MATERIAL AND METHODS

Data were obtained by performing a PubMed database search and summarised with a focus on the function of ILK in cancer biology.

RESULTS

The findings on the catalytic function of ILK, on the putative substrates of ILK and on the expression of ILK in tumour and normal tissues are heterogeneous. In the context of cancer, two of these issues might be of importance. First, a variety of reports indicate a lack of ILK overexpression in tumours. Second, wild-type or overexpression of ILK has been found to considerably sensitise tumour cells to ionising irradiation as compared to ILK knockout or ILK knockdown conditions. In contrast, wild-type or overexpression of ILK has been shown to protect tumour cells from chemotherapy-induced cell death.

CONCLUSIONS

Due to these conflicting data, it is difficult to evaluate if therapeutic targeting of ILK is a reasonable strategy in cancer therapy. A more comprehensive understanding of the molecular mechanisms controlled by ILK may help to answer this question.

Proteomic analysis of integrin alphaIIbbeta3 outside-in signaling reveals Src-kinase-independent phosphorylation of Dok-1 and Dok-3 leading to SHIP-1 interactions

BACKGROUND AND OBJECTIVES

Outside-in integrin alphaIIbbeta3 signaling involves a series of tyrosine kinase reactions that culminate in platelet spreading on fibrinogen. The aim of this study was to identify novel tyrosine phosphorylated signaling proteins downstream of alphaIIbbeta3, and explore their role in platelet signaling.

METHODS AND RESULTS

Utilizing proteomics to search for novel platelet proteins that contribute to outside-in signaling by the integrin alphaIIbbeta3, we identified 27 proteins, 17 of which were not previously shown to be part of a tyrosine phosphorylation-based signaling complex downstream of alphaIIbbeta3. The proteins identified include the novel immunoreceptors G6f and G6b-B, and two members of the Dok family of adapters, Dok-1 and Dok-3, which underwent increased tyrosine phosphorylation following platelet spreading on fibrinogen. Dok-3 was also inducibly phosphorylated in response to the GPVI-specific agonist collagen-related peptide (CRP) and the PAR-1 and -4 agonist thrombin, independently of the integrin alphaIIbbeta3. Tyrosine phosphorylation of Dok-1 and Dok-3 was primarily Src kinase-independent downstream of the integrin, whereas it was Src kinase-dependent downstream of GPVI. Moreover, both proteins inducibly interacted with Grb-2 and SHIP-1 in fibrinogen-spread platelets.

CONCLUSIONS

This study provides new insights into the molecular mechanism regulating alphaIIbbeta3-mediated platelet spreading on fibrinogen. The novel platelet adapter Dok-3 and the structurally related Dok-1 are tyrosine phosphorylated in an Src kinase-independent manner downstream of alphaIIbbeta3 in human platelets, leading to an interaction with Grb2 and SHIP-1.

Integrin-linked kinase associated with integrin activation

Platelet integrin alphaIIbbeta3 activation is tightly controlled by intracellular signaling pathways, and several molecules, including talin, have been identified as critical for alphaIIbbeta3 activation. However, the whole pathway associated with alphaIIbbeta3 activation remains to be determined. To address this issue, we established a Chinese hamster ovary cell line (parental cells) that expresses constitutively activated chimeric integrin alphaIIbalpha6Bbeta3, and then obtained mutant cells expressing inactivated alphaIIbalpha6Bbeta3 by genome-wide mutagenesis. We have performed expression cloning to isolate signaling molecules responsible for integrin activation in the mutant cells. We show that integrin-linked kinase (ILK) complements defective integrin activation in the mutant cells. ILK mRNAs in the mutant cells contained 2 nonsense mutations, R317X and W383X, in a compound heterozygous state, resulting in a complete loss of ILK expression. Moreover, the mutant cells showed partially impaired activation of endogenous beta1 integrins. Knockdown of ILK in parental cells significantly suppressed the activated state of alphaIIbalpha6Bbeta3. However, ILK overexpression did not rescue the impaired integrin activation in talin knocked-down parental cells, whereas overexpression of talin-F3, a subdomain of the talin head domain, restored the function. Our present data suggest that ILK contributes to inside-out integrin activation.

Insulin resistance in striated muscle-specific integrin receptor beta1-deficient mice

Integrin receptor plays key roles in mediating both inside-out and outside-in signaling between cells and the extracellular matrix. We have observed that the tissue-specific loss of the integrin beta1 subunit in striated muscle results in a near complete loss of integrin beta1 subunit protein expression concomitant with a loss of talin and to a lesser extent, a reduction in F-actin content. Muscle-specific integrin beta1-deficient mice had no significant difference in food intake, weight gain, fasting glucose, and insulin levels with their littermate controls. However, dynamic analysis of glucose homeostasis using euglycemichyperinsulinemic clamps demonstrated a 44 and 48% reduction of insulin-stimulated glucose infusion rate and glucose clearance, respectively. The whole body insulin resistance resulted from a specific inhibition of skeletal muscle glucose uptake and glycogen synthesis without any significant effect on the insulin suppression of hepatic glucose output or insulin-stimulated glucose uptake in adipose tissue. The reduction in skeletal muscle insulin responsiveness occurred without any change in GLUT4 protein expression levels but was associated with an impairment of the insulin-stimulated protein kinase B/Akt serine 473 phosphorylation but not threonine 308. The inhibition of insulin-stimulated serine 473 phosphorylation occurred concomitantly with a decrease in integrin-linked kinase expression but with no change in the mTOR.Rictor.LST8 complex (mTORC2). These data demonstrate an in vivo crucial role of integrin beta1 signaling events in mediating cross-talk to that of insulin action.

A dual role for integrin-linked kinase in platelets: regulating integrin function and alpha-granule secretion

Integrin-linked kinase (ILK) has been implicated in the regulation of a range of fundamental biological processes such as cell survival, growth, differentiation, and adhesion. In platelets ILK associates with beta1- and beta3-containing integrins, which are of paramount importance for the function of platelets. Upon stimulation of platelets this association with the integrins is increased and ILK kinase activity is up-regulated, suggesting that ILK may be important for the coordination of platelet responses. In this study a conditional knockout mouse model was developed to examine the role of ILK in platelets. The ILK-deficient mice showed an increased bleeding time and volume, and despite normal ultrastructure the function of ILK-deficient platelets was decreased significantly. This included reduced aggregation, fibrinogen binding, and thrombus formation under arterial flow conditions. Furthermore, although early collagen stimulated signaling such as PLCgamma2 phosphorylation and calcium mobilization were unaffected in ILK-deficient platelets, a selective defect in alpha-granule, but not dense-granule, secretion was observed. These results indicate that as well as involvement in the control of integrin affinity, ILK is required for alpha-granule secretion and therefore may play a central role in the regulation of platelet function.

Platelet protein interactions: map, signaling components, and phosphorylation groundstate

OBJECTIVE

Assembly of a comprehensive proteome and transcriptome database of human platelets, derivation of a model of the platelet-specific interactome, and generation of a functional interaction map of platelet phosphorylations and kinases.

METHODS AND RESULTS

Interactions are derived from literature-curated data from HPRD and yeast two hybrid (Y2H) and mapped to platelet-specific expression data (SAGE or proteome). From this a cell-type specific model of platelet proteins and protein-protein interactions is derived. The obtained inventory of platelet-specific proteins includes key domains, protein GO annotations, and receptors. Collected interactions point to new platelet signaling components, actin remodeling processes, and pharmacological targets and offer incentives for further studies (eg, on the IPP complex). Integration of platelet-specific phosphoproteins and the characterization of the platelet kinase repertoire sketch a first outline of kinase signaling in human platelets.

CONCLUSIONS

A first view of the platelet interactome, platelet phosphorylation, and platelet kinome is available from the in silico data.

Variations in platelet protein associated with arterial thrombosis

INTRODUCTION

Hyperactivity of platelets has been associated with thrombotic episodes by molecular mechanisms not yet elucidated. The present work aimed at identifying whether the platelet protein content from patients who had suffered an arterial thrombosis episode differed from that of platelets obtained from normal healthy donors.

METHODS

Differential platelet protein profiles were determined by 2-dimensional (2-D) gel electrophoresis and Western blot analysis of total platelet lysates. Identification of differentially expressed proteins was carried out by mass spectrometry (MALDI-TOF).

RESULTS

We found a decreased platelet content of three protein spots in patients of arterial thrombosis: integrin linked kinase (ILK), fructose bisphosphate aldolase (aldolase) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) whereas the content of four other protein spots was increased: actin binding protein, coronine like (p57), non-muscle myosin heavy chain (NMMHC-A), pyruvate kinase M2 isoenzyme (PK) and phosphoglycerate kinase (PGK). The variations in ILK, GAPDH and PK were validated by Western blot analysis. The proteins showing a decreased platelet content in arterial thrombosis patients are associated with the cytoskeletal insoluble fraction and the detected increase in some proteins seems to be due to the generation of peptides caused by a limited proteolysis. Differences in the protein profiles of circulating platelets from arterial thrombosis were maintained months after the acute thrombotic event and disappear in the long term.

CONCLUSIONS

The observed variations in some platelet proteins suggest the existence of a perturbation in the cytoskeletal organization and increased proteolysis, both indicative of a platelet pro-active state, persistent after the thrombotic event.

The gamma-parvin-integrin-linked kinase complex is critically involved in leukocyte-substrate interaction

Leukocyte extravasation is an important step of inflammation, in which integrins have been demonstrated to play an essential role by mediating the interaction of leukocytes with the vascular endothelium and the subendothelial extracellular matrix. Previously, we identified an integrin-linked kinase (ILK)-binding protein affixin (beta-parvin), which links initial integrin signals to rapid actin reorganization, and thus plays critical roles in fibroblast migration. In this study, we demonstrate that gamma-parvin, one of three mammalian parvin family members, is specifically expressed in several lymphoid and monocytic cell lines in a complementary manner to affixin. Like affixin, gamma-parvin directly associates with ILK through its CH2 domain and colocalizes with ILK at focal adhesions as well as the leading edge of PMA-stimulated U937 cells plated on fibronectin. The overexpression of the C-terminal fragment containing CH2 domain or the depletion of gamma-parvin by RNA interference inhibits the substrate adhesion of MCP-1-stimulated U937 cells and the spreading of PMA-stimulated U937 cells on fibronectin. Interestingly, the overexpression of the CH2 fragment or the gamma-parvin RNA interference also disrupts the asymmetric distribution of PTEN and F-actin observed at the very early stage of cell spreading, suggesting that the ILK-gamma-parvin complex is essential for the establishment of cell polarity required for leukocyte migration. Taken together with the results that gamma-parvin could form a complex with some important cytoskeletal proteins, such as alphaPIX, alpha-actinin, and paxillin as demonstrated for affixin and actopaxin (alpha-parvin), the results in this study suggest that the ILK-gamma-parvin complex is critically involved in the initial integrin signaling for leukocyte migration.

ILK, PINCH and parvin: the tIPP of integrin signalling

The ternary complex of integrin-linked kinase (ILK), PINCH and parvin functions as a signalling platform for integrins by interfacing with the actin cytoskeleton and many diverse signalling pathways. All these proteins have synergistic functions at focal adhesions, but recent work has indicated that these proteins might also have separate roles within a cell. They function as regulators of gene transcription or cell-cell adhesion.

A role for PKCtheta in outside-in alpha(IIb)beta3 signaling

Fibrinogen binding to platelets triggers alpha(IIb)beta3-dependent outside-in signals that promote actin rearrangements and cell spreading. Studies with chemical inhibitors or activators have implicated protein kinase C (PKC) in alpha(IIb)beta3 function. However, the role of individual PKC isoforms is poorly understood. Biochemical and genetic approaches were used to determine whether PKCtheta is involved in alpha(IIb)beta3 signaling. PKCtheta was constitutively associated with alpha(IIb)beta3 in human and murine platelets. Fibrinogen binding to alpha(IIb)beta3 stimulated the association of PKCtheta with tyrosine kinases Btk and Syk, and tyrosine phosphorylation of PKCtheta, Btk and the actin regulator, Wiskott-Aldrich syndrome protein (WASP). Mouse platelets deficient in PKCtheta or Btk failed to spread on fibrinogen. Furthermore, PKCtheta was required for phosphorylation of WASP-interacting protein on Ser-488, an event that has been linked to WASP activation of the Arp2/3 complex and actin polymerization in lymphocytes. Neither PKCtheta nor Btk were required for agonist-induced inside-out signaling and fibrinogen binding to alpha(IIb)beta3. Thus, PKCtheta is a newly identified, essential member of a dynamic outside-in signaling complex that includes Btk and that couples alpha(IIb)beta3 to the actin cytoskeleton.

The parvins

The parvins are a family of proteins involved in linking integrins and associated proteins with intracellular pathways that regulate actin cytoskeletal dynamics and cell survival. Both alpha-parvin (PARVA) and beta-parvin (PARVB) localize to focal adhesions and function in cell adhesion, spreading, motility and survival through interactions with partners, such as integrin-linked kinase (ILK), paxillin, alpha-actinin and testicular kinase 1. A complex of PARVA with ILK and the LIM protein PINCH-1 is critical for cell survival in a variety of cells, including certain cancer cells, kidney podocytes and cardiac myocytes. While PARVA inhibits the activities of Rac1 and testicular kinase 1 and cell spreading, PARVB binds alphaPIX and alpha-actinin, and can promote cell spreading. In contrast to PARVA, PARVB inhibits ILK activity and reverses some of its oncogenic effects in cancer cells. This review focuses on the structure and function of the parvins and some possible roles in human diseases.

Platelet adhesion signalling and the regulation of thrombus formation

Platelets perform a central role in haemostasis and thrombosis. They adhere to subendothelial collagens exposed at sites of blood vessel injury via the glycoprotein (GP) Ib-V-IX receptor complex, GPVI and integrin alpha(2)beta(1). These receptors perform distinct functions in the regulation of cell signalling involving non-receptor tyrosine kinases (e.g. Src, Fyn, Lyn, Syk and Btk), adaptor proteins, phospholipase C and lipid kinases such as phosphoinositide 3-kinase. They are also coupled to an increase in cytosolic calcium levels and protein kinase C activation, leading to the secretion of paracrine/autocrine platelet factors and an increase in integrin receptor affinities. Through the binding of plasma fibrinogen and von Willebrand Factor to integrin alpha(IIb)beta(3), a platelet thrombus is formed. Although increasing evidence indicates that each of the adhesion receptors GPIb-V-IX and GPVI and integrins alpha(2)beta(1) and alpha(IIb)beta(3) contribute to the signalling that regulates this process, the individual roles of each are only beginning to be dissected. By contrast, adhesion receptor signalling through platelet endothelial cell adhesion molecule 1 (PECAM-1) is implicated in the inhibition of platelet function and thrombus formation in the healthy circulation. Recent studies indicate that understanding of platelet adhesion signalling mechanisms might enable the development of new strategies to treat and prevent thrombosis.

Assembly and Signaling of Adhesion Complexes

Cell-extracellular matrix (ECM) adhesion is crucial for control of cell behavior. It connects the ECM to the intracellular cytoskeleton and transduces bidirectional signals between the extracellular and intracellular compartments. The subcellular machinery that mediates cell-ECM adhesion and signaling is complex. It consists of transmembrane proteins (e.g., integrins) and at least several dozens of membrane-proximal proteins that assemble into a network through multiple protein interactions. Furthermore, despite sharing certain common components, cell-ECM adhesions exhibit considerable heterogeneity in different types of cells (e.g., the cell-ECM adhesions in cardiac myocytes are considerably different from those in fibroblasts). Here, we will first briefly describe the general properties of the integrin-mediated cell-ECM adhesion and signal transduction. Next, we will focus on one of the recently discovered cell-ECM adhesion protein complexes consisting of PINCH, integrin-linked kinase (ILK), and Parvin and use it as an example to illustrate the molecular basis underlying the assembly and functions of cell-ECM adhesions. Finally, we will discuss in detail the structure and regulation of cell-ECM adhesion complexes in cardiac myocytes, which illustrate the importance and complexity of the cell-ECM adhesion structures in organogenesis and diseases.

Integrin-linked kinase: a cancer therapeutic target unique among its ILK

Cancer development requires the acquisition of several capabilities that include increased replicative potential, anchorage and growth-factor independence, evasion of apoptosis, angiogenesis, invasion of surrounding tissues and metastasis. One protein that has emerged as promoting many of these phenotypes when dysregulated is integrin-linked kinase (ILK), a unique intracellular adaptor and kinase that links the cell-adhesion receptors, integrins and growth factors to the actin cytoskeleton and to a range of signalling pathways. The recent findings of increased levels of ILK in various cancers, and that inhibition of ILK expression and activity is antitumorigenic, makes ILK an attractive target for cancer therapeutics.

Affixin interacts with alpha-actinin and mediates integrin signaling for reorganization of F-actin induced by initial cell-substrate interaction

The linking of integrin to cytoskeleton is a critical event for an effective cell migration. Previously, we have reported that a novel integrin-linked kinase (ILK)-binding protein, affixin, is closely involved in the linkage between integrin and cytoskeleton in combination with ILK. In the present work, we demonstrated that the second calponin homology domain of affixin directly interacts with alpha-actinin in an ILK kinase activity-dependent manner, suggesting that integrin-ILK signaling evoked by substrate adhesion induces affixin-alpha-actinin interaction. The overexpression of a peptide corresponding to the alpha-actinin-binding site of affixin as well as the knockdown of endogenous affixin by small interference RNA resulted in the blockade of cell spreading. Time-lapse observation revealed that in both experiments cells were round with small peripheral blebs and failed to develop lamellipodia, suggesting that the ILK-affixin complex serves as an integrin-anchoring site for alpha-actinin and thereby mediates integrin signaling to alpha-actinin, which has been shown to play a critical role in actin polymerization at focal adhesions.

Phosphorylation of actopaxin regulates cell spreading and migration

Actopaxin is an actin and paxillin binding protein that localizes to focal adhesions. It regulates cell spreading and is phosphorylated during mitosis. Herein, we identify a role for actopaxin phosphorylation in cell spreading and migration. Stable clones of U2OS cells expressing actopaxin wild-type (WT), nonphosphorylatable, and phosphomimetic mutants were developed to evaluate actopaxin function. All proteins targeted to focal adhesions, however the nonphosphorylatable mutant inhibited spreading whereas the phosphomimetic mutant cells spread more efficiently than WT cells. Endogenous and WT actopaxin, but not the nonphosphorylatable mutant, were phosphorylated in vivo during cell adhesion/spreading. Expression of the nonphosphorylatable actopaxin mutant significantly reduced cell migration, whereas expression of the phosphomimetic increased cell migration in scrape wound and Boyden chamber migration assays. In vitro kinase assays demonstrate that extracellular signal-regulated protein kinase phosphorylates actopaxin, and treatment of U2OS cells with the MEK1 inhibitor UO126 inhibited adhesion-induced phosphorylation of actopaxin and also inhibited cell migration.

Integration of cell attachment, cytoskeletal localization, and signaling by integrin-linked kinase (ILK), CH-ILKBP, and the tumor suppressor PTEN

Cell attachment and the assembly of cytoskeletal and signaling complexes downstream of integrins are intimately linked and coordinated. Although many intracellular proteins have been implicated in these processes, a new paradigm is emerging from biochemical and genetic studies that implicates integrin-linked kinase (ILK) and its interacting proteins, such as CH-ILKBP (alpha-parvin), paxillin, and PINCH in coupling integrins to the actin cytoskeleton and signaling complexes. Genetic studies in Drosophila, Caenorhabditis elegans, and mice point to an essential role of ILK as an adaptor protein in mediating integrin-dependent cell attachment and cytoskeletal organization. Here we demonstrate, using several different approaches, that inhibiting ILK kinase activity, or expression, results in the inhibition of cell attachment, cell migration, F-actin organization, and the specific cytoskeletal localization of CH-ILKBP and paxillin in human cells. We also demonstrate that the kinase activity of ILK is elevated in the cytoskeletal fraction and that the interaction of CH-ILKBP with ILK within the cytoskeleton stimulates ILK activity and downstream signaling to PKB/Akt and GSK-3. Interestingly, the interaction of CH-ILKBP with ILK is regulated by the Pi3 kinase pathway, because inhibition of Pi3 kinase activity by pharmacological inhibitors, or by the tumor suppressor PTEN, inhibits this interaction as well as cell attachment and signaling. These data demonstrate that the kinase and adaptor properties of ILK function together, in a Pi3 kinase-dependent manner, to regulate integrin-mediated cell attachment and signal transduction.