Regulation of protein kinase B/Akt-serine 473 phosphorylation by integrin-linked kinase: critical roles for kinase activity and amino acids arginine 211 and serine 343

Implication of the MAGI‐1b/PTEN signalosome in stabilization of adherens junctions and suppression of invasiveness

We recently established the critical role of the lipid phosphatase activity of the PTEN tumor suppressor in stabilizing cell-cell contacts and suppressing invasiveness. To delineate the effector systems involved, we investigated the interaction of PTEN with E-cadherin junctional complexes in kidney and colonic epithelial cell lines. PTEN and the p85 regulatory subunit of phosphatidylinositol 3-OH kinase (PI3K) co-immunoprecipitated with E-cadherin and catenins. By using a yeast two-hybrid assay, we demonstrated that PTEN interacted indirectly with beta-catenin by binding the scaffolding protein MAGI-1b. This model was corroborated in various ways in mammalian cells. Ectopic expression of MAGI-1b potentiated the interaction of PTEN with junctional complexes, promoted E-cadherin-dependent cell-cell aggregation, and reverted the Src-induced invasiveness of kidney MDCKts-src cells. In this model, MAGI-1b slightly decreased the activity of AKT, a downstream effector of PI3K. By using dominant-negative and constitutively active AKT expression vectors, we demonstrated that this kinase was included in the pathways involved in Src-induced destabilization of junctional complexes and was necessary and sufficient to trigger invasiveness. We propose that the recruitment of PTEN at adherens junctions by MAGI-1b and the local down-regulation of phosphatidylinositol-3,4,5-trisphosphate pools and downstream effector systems at the site of cell-cell contacts are focal points for restraining both disruption of junctional complexes and induction of tumor cell 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.

Inhibition of Chk1 by the G2 DNA damage checkpoint inhibitor isogranulatimide

Inhibitors of the G2 DNA damage checkpoint can selectively sensitize cancer cells with mutated p53 to killing by DNA-damaging agents. Isogranulatimide is a G2 checkpoint inhibitor containing a unique indole/maleimide/imidazole skeleton identified in a phenotypic cell-based screen; however, the mechanism of action of isogranulatimide is unknown. Using natural and synthetic isogranulatimide analogues, we show that the imide nitrogen and a basic nitrogen at position 14 or 15 in the imidazole ring are important for checkpoint inhibition. Isogranulatimide shows structural resemblance to the aglycon of UCN-01, a potent bisindolemaleimide inhibitor of protein kinase Cβ (IC50, 0.001 μmol/L) and of the checkpoint kinase Chk1 (IC50, 0.007 μmol/L). In vitro kinase assays show that isogranulatimide inhibits Chk1 (IC50, 0.1 μmol/L) but not protein kinase Cβ. Of 13 additional protein kinases tested, isogranulatimide significantly inhibits only glycogen synthase kinase-3β (IC50, 0.5 μmol/L). We determined the crystal structure of the Chk1 catalytic domain complexed with isogranulatimide. Like UCN-01, isogranulatimide binds in the ATP-binding pocket of Chk1 and hydrogen bonds with the backbone carbonyl oxygen of Glu85 and the amide nitrogen of Cys87. Unlike UCN-01, the basic N15 of isogranulatimide interacts with Glu17, causing a conformation change in the kinase glycine-rich loop that may contribute importantly to inhibition. The mechanism by which isogranulatimide inhibits Chk1 and its favorable kinase selectivity profile make it a promising candidate for modulating checkpoint responses in tumors for therapeutic benefit.

Integrin-linked Kinase Regulates the Nuclear Entry of the c-Jun Coactivator α-NAC and Its Coactivation Potency

Overexpression of the integrin-linked kinase (ILK) was shown to increase c-Jun-dependent transcription. We now show that this effect of ILK involves the c-Jun transcriptional coactivator, nascent polypeptide-associated complex and coactivator alpha (alpha-NAC). ILK phosphorylated alpha-NAC on residue Ser-43 upon adhesion of cells to fibronectin. Co-expression of constitutively active ILK with alpha-NAC led to the nuclear accumulation of the coactivator. Conversely, alpha-NAC remained in the cytoplasm of cells transfected with a dominant-negative ILK mutant, and a mutated alpha-NAC at phosphoacceptor position Ser-43 (S43A) also localized outside of the nucleus. The S43A alpha-NAC mutant could not potentiate the effect of ILK on c-Jun-dependent transcription. We conclude that ILK-dependent phosphorylation of alpha-NAC induced the nuclear accumulation of the coactivator and that phosphorylation of alpha-NAC by ILK is required for the potentiation of c-Jun-mediated responses by the kinase. The results represent one of the rare examples of a transcriptional coactivator shuttling between the cytosol and the nucleus.

Signal transduction pathways relevant for neoplastic effects of fibrous and non-fibrous particles

Apart from their genotoxic effects, both fibrous and non-fibrous particles are known to induce signalling pathways involved in the development of malignant lung diseases. Different direct effects of particles as well as indirect cellular effects are believed to induce changes in apoptosis or proliferation in target cells. Signalling events, e.g. the induction of mitogen-activated protein kinase (MAPK) cascades resulting in the activation of the transcription factor AP-1, as well as the induction of the transcription factor NFkappaB which mainly mediates the expression of pro-inflammatory genes are discussed. There is some insight into the molecular mechanisms eliciting these pathways. Therefore, this review aims to give an overview on signalling pathways as well as initial events including effects of reactive oxygen and nitrogen species, membrane receptors and particle uptake.

Protein kinase C betaII regulates Akt phosphorylation on Ser-473 in a cell type- and stimulus-specific fashion

Akt (= protein kinase B), a subfamily of the AGC serine/threonine kinases, plays critical roles in survival, proliferation, glucose metabolism, and other cellular functions. Akt activation requires the recruitment of the enzyme to the plasma membrane by interacting with membrane-bound lipid products of phosphatidylinositol 3-kinase. Membrane-bound Akt is then phosphorylated at two sites for its full activation; Thr-308 in the activation loop of the kinase domain is phosphorylated by 3-phosphoinositide-dependent kinase-1 (PDK1) and Ser-473 in the C-terminal hydrophobic motif by a putative kinase PDK2. The identity of PDK2 has been elusive. Here we present evidence that conventional isoforms of protein kinase C (PKC), particularly PKCbetaII, can regulate Akt activity by directly phosphorylating Ser-473 in vitro and in IgE/antigen-stimulated mast cells. By contrast, PKCbeta is not required for Ser-473 phosphorylation in mast cells stimulated with stem cell factor or interleukin-3, in serum-stimulated fibroblasts, or in antigen receptor-stimulated T or B lymphocytes. Therefore, PKCbetaII appears to work as a cell type- and stimulus-specific PDK2.

Overexpression of Hyperactive Integrin-Linked Kinase Leads to Increased Cellular Radiosensitivity

Integrin-linked kinase (ILK), bound to the cytoplasmic tails of integrin beta1, beta2, and beta3, is thought to signal through AKT and glycogen synthase kinase-3beta (GSK-3beta) for survival and proliferation regulation. To determine the role of ILK in the cellular radiation response, stably transfected A549 lung cancer cells overexpressing either wild-type (ILK-wk) or hyperactive ILK (ILK-hk) were studied for survival, signaling, proliferation, and examined in immunofluorescence and adhesion assays. Strong radiosensitization was observed in ILK-hk in contrast to ILK-wk mutants and empty vector controls. ILK small interfering RNA transfections showed radioresistance similar to irradiation on fibronectin. AKT, GSK-3beta-cyclin D1, mitogen-activated protein kinase kinase 1/2-mitogen-activated protein kinase, and c-Jun NH2-terminal kinase signaling was dysregulated in irradiated ILK-hk mutants. Immunofluorescence stainings of ILK-hk cells indicated disturbed ILK and paxillin membrane localization with concomitant decrease in focal adhesions. Profound ILK-hk-dependent changes in morphology were characterized by spindle-like cell shape, cell size reduction, increased cell protrusions, strong formation of membranous f-actin rings, and significantly reduced adhesion to matrix proteins. Additionally, ILK-wk and ILK-hk overexpression impaired beta1-integrin clustering and protein Tyr-phosphorylation. Taken together, the data provide evidence that ILK signaling modulates the cellular radiation response involving diverse signaling pathways and through changes in f-actin-based processes such as focal adhesion formation, cell adhesion, and spreading. Identification of ILK and its signaling partners as potential targets for tumor radiosensitization might promote innovative anticancer strategies by providing insight into the mechanism of cell adhesion-mediated radioresistance, oncogenic transformation, and tumor growth and spread.

The regulation of integrin-linked kinase in human platelets: evidence for involvement in the regulation of integrin alpha 2 beta 1

BACKGROUND

Activation of the platelet integrin alpha 2 beta 1 is closely regulated due to the high thrombogenicity of its ligand. As a beta 1 interacting kinase, ILK represents a candidate intracellular regulator of alpha 2 beta 1 in human platelets.

OBJECTIVES

We investigated the regulation of ILK in human platelets and the role of ILK in regulating alpha 2 beta 1 activation in HEL cells, a megakaryocytic cell line.

METHODS

An in-vitro kinase assay was used to determine the effect of platelet agonists on ILK kinase activity together with the contribution of PI3K and PKC on ILK activation. Interaction of ILK with beta 1-integrin subunits was investigated by coimmunoprecipitation and the role of ILK in regulating alpha 2 beta 1 function assessed by overexpression studies in HEL cells.

RESULTS

We report that collagen and thrombin modulate ILK kinase activity in human platelets in an aggregation-independent manner. Furthermore, ILK activity is dually regulated by PI3K and PKC in thrombin-stimulated platelets and regulated by PI3K in collagen-stimulated cells. ILK associates with the beta 1-integrin subunits immunoprecipitated from platelet cell lysates, an association which increased upon collagen stimulation. Overexpression of ILK in HEL cells enhanced alpha 2 beta 1-mediated adhesion whereas overexpression of kinase-dead ILK reduced adhesion, indicating a role for this kinase in the positive regulation of alpha 2 beta 1.

CONCLUSIONS

Our findings that ILK regulates alpha 2 beta 1 in HEL cells, is activated in platelets and associates with beta 1-integrins, raise the possibility that it may play a key role in adhesion events upon agonist stimulation of platelets.

Regulation of Bcl-2 proteins during anoikis and amorphosis

Adhesion to extracellular matrix regulates cell survival through both integrin engagement and appropriate cell spreading. Numerous signaling pathways converge to affect the levels and posttranslational modifications of Bcl-2 family proteins. Recent work has defined specific roles for different Bcl-2 proteins in the disruption of mitochondrial function that leads to cell death. Using this understanding of Bcl-2 protein function as a framework, we will consider the molecular mechanisms of apoptosis induced by integrin detachment (anoikis) and cell death stimulated by the loss of cytoskeletal architecture (amorphosis).

Akt2, a novel functional link between p38 mitogen-activated protein kinase and phosphatidylinositol 3-kinase pathways in myogenesis

Activation of either the phosphatidylinositol 3-kinase (PI 3-kinase)/Akt or the p38 mitogen-activated protein kinase (MAPK) signaling pathways accelerates myogenesis but only when the reciprocal pathway is functional. We therefore examined the hypothesis that cross-activation between these signaling cascades occurs to orchestrate myogenesis. We reveal a novel and reciprocal cross-talk and activation between the PI 3-kinase/Akt and p38 MAPK pathways that is essential for efficient myoblast differentiation. During myoblast differentiation, Akt kinase activity correlated with S473 but not T308 phosphorylation and occurred 24 h after p38 activation. Inhibition or activation of p38 with SB203580, dominant-negative p38, or MKK6EE regulated Akt kinase activity. Analysis of Akt isoforms revealed a specific increase in Akt2 protein levels that coincided with AktS473 phosphorylation during myogenesis and an enrichment of S473-phosphorylated Akt2. Akt2 promoter activity and protein levels were regulated by p38 activation, thus providing a mechanism for communication. Subsequent Akt activation by S473 phosphorylation was PI 3-kinase dependent and specific for Akt2 rather than Akt1. Complementary to p38-mediated transactivation of Akt, activation or inhibition of PI 3-kinase regulated p38 activity upstream of MKK6, demonstrating reciprocal communication and positive feedback characteristic of myogenic regulation. Our findings have identified novel communication between p38 MAPK and PI 3-kinase/Akt via Akt2.

New prognostic factors in uveal melanomas: potential molecular targets for therapy

Even with advances in the diagnosis and local treatment of uveal melanoma, there has been no significant change in the survival rates of these patients in the last decades. Metastatic disease still occurs at the same frequency, and no systemic therapy is currently offered to patients after local eye treatment. Therefore, experimental and clinical research has been focused on the metastatic cascade in order to elucidate its underlying mechanisms at the molecular level. As a result, new prognostic factors in uveal melanoma have been described that also serve as molecular targets for the development of novel treatments. These prognostic factors/molecular targets, such as membrane receptors, enzymes, cytokines, cytoskeleton components, oncogenes, tumour suppressor genes, cell-cycle proteins and nuclear antigens, are reviewed in this article.

Regulation of E-cadherin expression and beta-catenin/Tcf transcriptional activity by the integrin-linked kinase

Integrin-linked kinase (ILK) is a serine/threonine protein kinase which interacts with the cytoplasmic domains of beta1 and beta3 integrins. ILK structure and its localization at the focal adhesion allows it not only to interact with different structural proteins, but also to mediate many different signalling pathways. Extracellular matrices (ECM) and growth factors each stimulate ILK signalling. Constitutive activation of ILK in epithelial cells results in oncogenic phenotypes such as disruption of cell extracellular matrix and cell to cell interactions, suppression of suspension-induced apoptosis, and induction of anchorage independent cell growth and cell cycle progression. More specifically, pathological overexpression of ILK results in down-regulation of E-cadherin expression, and nuclear accumulation of beta-catenin, leading to the subsequent activation of the beta-catenin/Tcf transcription complex, the downstream components of the Wnt signalling pathway. Here we review the data implicating ILK in the regulation of these two signalling pathways, and discuss recent novel insights into the molecular basis and requirement of ILK in the process of epithelial to mesenchymal transformation (EMT).

Integrin-linked kinase function is required for transforming growth factor beta-mediated epithelial to mesenchymal transition

The role of integrin-linked kinase (ILK) in transforming growth factor beta (TGFbeta)-mediated epithelial to mesenchymal transition was investigated. A stable transfection of dominant-negative ILK results in the prevention of TGFbeta-mediated E-cadherin delocalization. TGFbeta-mediated phosphorylation of Akt at Ser-473 was inhibited by dominant-negative ILK and PI3K inhibitors, LY294002 and wortmannin. Treatment with TGFbeta stimulated induction of Akt and ILK kinase activity in HaCat control cells. This increased ILK activity by TGFbeta was lowered by PI3K inhibitor, LY294002. In addition, PI3K inhibitor, dominant-negative Akt, and dominant-negative ILK could not block TGFbeta-mediated C-terminal phosphorylation of Smad2. Taken together, these data suggest that PI3K-ILK-Akt pathway that is independent of the TGFbeta-induced Smad pathway is required for TGFbeta-mediated epithelial to mesenchymal transition.

Integrin-linked kinase (ILK) regulation of the cell viability in PTEN mutant glioblastoma and in vitro inhibition by the specific COX-2 inhibitor NS-398

We report the increased activity and expression of the ILK protein in human glioblastomas and demonstrate that ILK activity is regulated by PTEN. The transfection of wild type-PTEN into the glioblastoma cell line U-251 MG altered the localization of ILK in the cell membrane; transfection with PTEN down-regulated PKB/Akt-Ser-473 phosphorylation via the inhibition of ILK-signaling. Our results suggest that ILK is critical for the PTEN-sensitive regulation of PKB/Akt-dependent cell survival. The selective COX-2 inhibitor NS-398 was found capable of down-regulating ILK and PKB/Akt phosphorylation. Our data indicate that inhibition of ILK signaling may be beneficial in the treatment of PTEN-deficient glioblastoma.

Structure, regulation and function of PKB/AKT--a major therapeutic target

Protein phosphorylation and dephosphorylation play a major role in intracellular signal transduction activated by extracellular stimuli. Protein kinase B (PKB/Akt) is a central player in the signal transduction pathways activated in response to growth factors or insulin and is thought to contribute to several cellular functions including nutrient metabolism, cell growth and apoptosis. Recently, several significant publications have described novel mechanisms used to regulate PKB. Since the alteration of PKB activity is associated with several human diseases, including cancer and diabetes, understanding PKB regulation is an important task if we are to develop successful therapeutics.

PI3K/Akt signalling pathway and cancer

Phosphatidylinositol-3 kinases, PI3Ks, constitute a lipid kinase family characterized by their ability to phosphorylate inositol ring 3'-OH group in inositol phospholipids to generate the second messenger phosphatidylinositol-3,4,5-trisphosphate (PI-3,4,5-P(3)). RPTK activation results in PI(3,4,5)P(3) and PI(3,4)P(2) production by PI3K at the inner side of the plasma membrane. Akt interacts with these phospholipids, causing its translocation to the inner membrane, where it is phosphorylated and activated by PDK1 and PDK2. Activated Akt modulates the function of numerous substrates involved in the regulation of cell survival, cell cycle progression and cellular growth. In recent years, it has been shown that PI3K/Akt signalling pathway components are frequently altered in human cancers. Cancer treatment by chemotherapy and gamma-irradiation kills target cells primarily by the induction of apoptosis. However, the development of resistance to therapy is an important clinical problem. Failure to activate the apoptotic programme represents an important mode of drug resistance in tumor cells. Survival signals induced by several receptors are mediated mainly by PI3K/Akt, hence this pathway may decisively contribute to the resistant phenotype. Many of the signalling pathways involved in cellular transformation have been elucidated and efforts are underway to develop treatment strategies that target these specific signalling molecules or their downstream effectors. The PI3K/Akt pathway is involved in many of the mechanisms targeted by these new drugs, thus a better understanding of this crossroad can help to fully exploit the potential benefits of these new agents.

Ras GTPase-activating Protein Binds to Akt and Is Required for Its Activation

RasGAP (Ras GTPase-activating protein) is a negative regulator as well as a downstream effector of Ras. To identify partners of RasGAP we used it as the bait in a yeast two-hybrid screen. This resulted in discovering its interaction with Akt. Overexpression of RasGAP or a mutant lacking the GTPase-activating domain (nGAP) enhanced phosphorylation and activity of Akt, which was dependent on the upstream integrin-linked kinase. Also, nGAP protected the cells against staurosporin-induced apoptosis through an Akt-dependent pathway. To determine the role of RasGAP in receptor-mediated activation of Akt, we used short hairpin RNA interference to knock out endogenous RasGAP expression. Although this procedure resulted in enhanced Ras activity, it inhibited Akt phosphorylation. Thus, we propose that Ras-GAP interacts with Akt and is necessary for its activation, possibly via integrin-linked kinase-mediated phosphorylation of Ser-473. The data suggest that this effect is independent of Ras activity.

ILKAP regulates ILK signaling and inhibits anchorage-independent growth

ILKAP is a protein phosphatase 2C that selectively associates with integrin linked kinase, ILK, to modulate cell adhesion and growth factor signaling. We investigated the role of endogenous cellular ILKAP in antagonizing ILK signaling of two key targets, PKB and GSK3beta. Silencing of endogenous ILKAP by short interfering RNA (siRNA) stimulated GSK3beta phosphorylation at S9, with no effect on PKB S473 phosphorylation. In LNCaP prostate carcinoma cells, transient or stable expression of ILKAP suppressed ILK immune complex kinase activity, demonstrating an interaction between ILKAP and ILK. Consistent with the silencing data, ILKAP inhibition of ILK selectively inhibited S9 phosphorylation of GSK3beta without affecting S473 phosphorylation of PKB. The ILKAP-mediated inhibition of S9 phosphorylation was rescued by overexpression of ILK, but not by a dominant-negative ILK mutant. The expression level of cyclin D1, a target of ILK-GSK3beta signaling, was inversely correlated with ILKAP protein levels, suggesting that antagonism of ILK modulates cell cycle progression. ILKAP expression increased the proportion of LNCaP cells in G1, relative to vector control cells, and siRNA suppression of ILKAP increased entry of cells into the S phase, consistent with ILK antagonism. Anchorage-independent growth of LNCaP cells was inhibited by ILKAP, suggesting a critical role in the suppression of cellular transformation. Taken together, our results indicate that endogenous ILKAP activity inhibits the ILK-GSK3beta signaling axis, and suggest that ILKAP activity plays an important role in inhibiting oncogenic transformation.Oncogene (2004) 23, 3454-3461. doi:10.1038/sj.onc.1207473 Published online 1 March 2004

JAK/STAT, Raf/MEK/ERK, PI3K/Akt and BCR-ABL in cell cycle progression and leukemogenesis

The roles of the JAK/STAT, Raf/MEK/ERK and PI3K/Akt signal transduction pathways and the BCR-ABL oncoprotein in leukemogenesis and their importance in the regulation of cell cycle progression and apoptosis are discussed in this review. These pathways have evolved regulatory proteins, which serve to limit their proliferative and antiapoptotic effects. Small molecular weight cell membrane-permeable drugs that target these pathways have been developed for leukemia therapy. One such example is imatinib mesylate, which targets the BCR-ABL kinase as well as a few structurally related kinases. This drug has proven to be effective in the treatment of CML patients. However, leukemic cells have evolved mechanisms to become resistant to this drug. A means to combat drug resistance is to target other prominent signaling components involved in the pathway or to inhibit BCR-ABL by other mechanisms. Treatment of imatinib-resistant leukemia cells with drugs that target Ras (farnysyl transferase inhibitors) or with the protein destabilizer geldanamycin has proven to be a means to inhibit the growth of resistant cells. This review will tie together three important signal transduction pathways involved in the regulation of hematopoietic cell growth and indicate how their expression is dysregulated by the BCR-ABL oncoprotein.

Positional control of cell fate through joint integrin/receptor protein kinase signaling

Cells adhere to the extracellular matrix throughout most of their lifetime. This close, intimate contact with the matrix exerts an extraordinary control on the behavior of cells, determining whether they move or stay put, proliferate or remain quiescent, and even live or die. Attachment to the matrix not only enables cells to respond to soluble growth factors and cytokines but also determines the nature of the response. The integrins are a large family of receptors that attach cells to the matrix, organize their cytoskeleton, and cooperate with receptor protein tyrosine kinases to regulate cell fate. Research on integrin signaling is beginning to explain the complex and specific effects that the extracellular matrix exerts on cells.

Application of a customized pathway-focused microarray for gene expression profiling of cellular homeostasis upon exposure to nicotine in PC12 cells

Maintenance of cellular homeostasis is integral to appropriate regulation of cellular signaling and cell growth and division. In this study, we report the development and quality assessment of a pathway-focused microarray comprising genes involved in cellular homeostasis. Since nicotine is known to have highly modulatory effects on the intracellular calcium homeostasis, we therefore tested the applicability of the homeostatic pathway-focused microarray on the gene expression in PC-12 cells treated with 1 mM nicotine for 48 h relative to the untreated control cells. We first provided a detailed description of the focused array with respect to its gene and pathway content and then assessed the array quality using a robust regression procedure that allows for the exclusion of unreliable measurements while decreasing the number of false positives. As a result, the mean correlation coefficient between duplicate measurements of the arrays used in this study (control vs. nicotine treatment, three samples each) has increased from 0.974+/-0.017 to 0.995+/-0.002. Furthermore, we found that nicotine affected various structural and signaling components of the AKT/PKB signaling pathway and protein synthesis and degradation processes in PC-12 cells. Since modulation of intracellular calcium concentrations ([Ca(2+)](i)) and phosphatidylinositol signaling are important in various biological processes such as neurotransmitter release and tissue pathogenesis including tumor formation, we expect that the homeostatic pathway-focused microarray potentially can be used for the identification of unique gene expression profiles in comparative studies of drugs of abuse and diverse environmental stimuli, such as starvation and oxidative stress.

Regulation of tumor angiogenesis by integrin-linked kinase (ILK)

We show that integrin-linked kinase (ILK) stimulates the expression of VEGF by stimulating HIF-1alpha protein expression in a PKB/Akt- and mTOR/FRAP-dependent manner. In human prostate cancer cells, knockdown of ILK expression with siRNA, or inhibition of ILK activity, results in significant inhibition of HIF-1alpha and VEGF expression. In endothelial cells, VEGF stimulates ILK activity, and inhibition of ILK expression or activity results in the inhibition of VEGF-mediated endothelial cell migration, capillary formation in vitro, and angiogenesis in vivo. Inhibition of ILK activity also inhibits prostate tumor angiogenesis and suppresses tumor growth. These data demonstrate an important and essential role of ILK in two key aspects of tumor angiogenesis: VEGF expression by tumor cells and VEGF-stimulated blood vessel formation.

beta1-integrin mediates asbestos-induced phosphorylation of AKT and ERK1/2 in a rat pleural mesothelial cell line

Integrin-mediated signalling has been implicated in asbestos-induced carcinogenesis. In studies here, we examined signal transduction events associated with integrin-directed cell reactions triggered by crocidolite asbestos in the pleural mesothelial cell line 4/4 RM-4. Crocidolite fibres induced a significant time- and dose-dependent activation of the extracellular-signal-regulated kinases ERK1 and ERK2. ERK activation was specifically inhibited by integrin-blocking agents, that are integrin-binding peptides containing the sequence arginine-glycine-aspartic acid (RGD), and monoclonal antibodies against the integrin beta1-chain. Integrin-dependent activation of ERK1/2 in response to asbestos appeared to be independent of focal adhesion kinase pp125FAK (FAK) since FAK autophosphorylation remained unaffected in crocidolite-exposed mesothelial cells. Instead, we observed striking similarities in the kinetics of asbestos-induced ERK1/2 responses and phosphorylation of protein kinase B (AKT) at serine 473, a possible target residue for integrin-linked kinase. As with ERK activation, asbestos-induced AKT stimulation was significantly blocked by both the RGD-peptide and the beta1-integrin antibodies. These studies are the first to establish that in mesothelial cells ERK1/2 and AKT are simultaneously phosphorylated upon asbestos exposure in a beta1-integrin-dependent manner.

Kinase-dead PKB gene therapy combined with hyperthermia for human breast cancer

We have previously demonstrated that protein kinase B (PKB) is a mediator of heat-induced apoptosis for human breast cancer cells. To investigate the therapeutic potential of abrogating the function of this important survival protein, a novel replication-deficient adenovirus was constructed, wherein a mutant, kinase-inactive PKB gene (AAA) was inserted downstream of the CMV promoter. Two human breast cancer cell lines, MCF-7 and MDA-468, were treated, along with the MCF-10 serving as a "normal" mammary epithelial comparator. Apoptosis was increased with adv.AAA (25 PFU/cell) infection alone, but was significantly enhanced with the addition of heat exposure. Differential survival was observed with the MDA-468 cancer cells being more sensitive than the MCF-7 cells. The MCF-10 cells, in contrast, were most resistant to these treatments. Results from the clonogenic assay reflected the apoptosis data, with an apparent additive interaction between adv.AAA and hyperthermia treatments, again with greater differential sensitivity of the malignant, compared to the "normal" mammary epithelial cells. Heat or adv.beta-gal treatments led to phosphorylation of PKB and Forkhead, but this phosphorylation was reduced with adv.AAA therapy. In parallel, the combination of adv.AAA and heat treatment reduced PKB kinase activity more so than with either heat or adv.beta-gal alone. In conclusion, our results demonstrate that inhibition of the PKB-dependent survival pathway will promote apoptosis and thermosensitization in malignant breast cancer cells, with relative sparing of their normal counterpart, suggesting that a therapeutic gain could be achievable using this therapeutic strategy.

Integrin-linked kinase regulates vascular morphogenesis induced by vascular endothelial growth factor

Integrin-linked kinase (ILK) is one of the signaling moieties that interact with the cytoplasmic domains of integrin beta1 and beta3 subunits. Integrin-mediated outside-in signals cooperate with vascular endothelial growth factor (VEGF) receptor to promote morphological changes, cell proliferation and motility in endothelial cells. In this report we demonstrate that VEGF-induced vessel morphogenesis of human umbilical vein endothelial cells (HUVEC) was inhibited by the transfection of a dominant negative, kinase-deficient ILK (ILK-KD), as well as by treatment with the phosphatidylinositol 3-kinase inhibitor LY294002. VEGF induced phosphorylation of protein kinase B (PKB/Akt), a regulator of cell survival and apoptosis, on serine 473, but not on threonine 308, in an ILK-dependent manner. Furthermore, transfection of antisense ILK (ILK-AS) blocked the survival effect of VEGF in annexin-V binding assays, and a VEGF-mediated decrease in caspase activity was reversed by both ILK-KD and ILK-AS as measured by a homogeneous caspase-3/7 assay. We also demonstrate that both chemotactic migration and cell proliferation of HUVEC induced by VEGF were suppressed by the inhibition of ILK. We conclude that ILK plays an important role in vascular morphogenesis mediated by VEGF.

Levels of PTEN protein modulate Akt phosphorylation on serine 473, but not on threonine 308, in IGF-II-overexpressing rhabdomyosarcomas cells

Constitutive activation of Akt has been found in many types of human cancer, and is believed to promote proliferation and increased cell survival thereby contributing to cancer progression. In this study, we examined Akt phosphorylation on Ser473 and Thr308 in seven IGF-II-overexpressing rhabdomyosarcomas (RMS) cells. All the RMS cell lines tested had high levels of Akt phosphorylation on Thr308, whereas three cell lines (Rh5, Rh18, and CTR) had a much lower level of Akt phosphorylation on Ser473. To determine whether the difference in Akt phosphorylation on Ser473, but not on Thr308, observed among cell lines is a cell-specific phenomenon or due to other factors, which possibly downregulate Akt phosphorylation, we examined expression of PTEN protein, which acts as a negative regulator of the PI3K/Akt signaling pathway through its ability to dephosphorylate phosphatidylinositol 3,4,5-triphosphate (PIP3). The levels of PTEN expression inversely correlate with Akt phosphorylation on Ser473, but not on Thr308. Consistent with this finding, transfection of wild-type PTEN into RMS and mouse myoblast C2C12 cells resulted in reduced Akt phosphorylation on Ser473, but not on Thr308. Our data suggest that Ser473 may be a key target residue for PTEN to modulate the effects of IGF-II on activating the PI3K/Akt pathway in RMS cells. A better understanding of the pathway in RMS will likely contribute to insights into the biology of the RMS tumorigenesis and hopefully lead to novel therapeutic options.

Transforming growth factor-β1 stimulated protein kinase B serine-473 and focal adhesion kinase tyrosine phosphorylation dependent on cell adhesion in human hepatocellular carcinoma SMMC-7721 cells

Transforming growth factor-beta1 (TGF-beta1) is a potent growth inhibitor and apoptosis inducer for most normal cells. However, tumor cells are commonly nevertheless sensitive to the tumor-suppressing effects of TGF-beta1. In this paper, we focus on the effects of TGF-beta1 on two important anti-apoptotic protein kinases, protein kinase B (PKB), and focal adhesion kinase (FAK), in SMMC-7721 cells. We found that PKB-Ser-473 phosphorylation was apparently up-regulated by TGF-beta1. In the meantime, PKB-Thr-308 phosphorylation was slightly up-regulated by TGF-beta1. TGF-beta1 could also enhance FAK-Tyr phosphorylation. We observed that integrin-linked kinase (ILK) was also up-regulated by TGF-beta1 in good accordance with PKB-Ser-473 phosphorylation. We first found that TGF-beta1 could stimulate PKB-Ser-473 phosphorylation possibly via up-regulating ILK expression. Furthermore, we also failed to detect PKB-Ser-473 and FAK-Tyr phosphorylation with various concentrations of TGF-beta1 treatment when cells were kept in suspension. The above results indicate that PKB-Ser-473 and FAK-Tyr phosphorylation stimulated by TGF-beta1 are both dependent on cell adhesion.

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.

Epidermal growth factor receptor-dependent, NF-kappaB-independent activation of the phosphatidylinositol 3-kinase/Akt pathway inhibits ultraviolet irradiation-induced caspases-3, -8, and -9 in human keratinocytes

Both phosphatidylinositol 3-kinase (PI3K)/Akt and NF-kappaB pathways function to promote cellular survival following stress. Recent evidence indicates that the anti-apoptotic activity of these two pathways may be functionally dependent. Ultraviolet (UV) irradiation causes oxidative stress, which can lead to apoptotic cell death. Human skin cells (keratinocytes) are commonly exposed to UV irradiation from the sun. We have investigated activation of the PI3K/Akt and NF-kappaB pathways and their roles in protecting human keratinocytes (KCs) from UV irradiation-induced apoptosis. This activation of PI3K preceded increased levels (3-fold) of active/phosphorylated Akt. UV (50 mJ/cm2 from UVB source) irradiation caused rapid recruitment of PI3K to the epidermal growth factor receptor (EGFR). Pretreatment of KCs with EGFR inhibitor PD169540 abolished UV-induced Akt activation/phosphorylation, as did the PI3K inhibitors LY294002 or wortmannin. This inhibition of Akt activation was associated with a 3-4-fold increase of UV-induced apoptosis, as measured by flow cytometry and DNA fragmentation ELISA. In contrast to Akt, UV irradiation did not detectably increase nuclear localization of NF-kappaB, indicating that it was not strongly activated. Consistent with this observation, interference with NF-kappaB activation by adenovirus-mediated overexpression of dominant negative IKK-beta or IkappaB-alpha did not increase UV-induced apoptosis. However, adenovirus-mediated overexpression of constitutively active Akt completely blocked UV-induced apoptosis observed with PI3K inhibition by LY294002, whereas adenovirus mediated overexpression of dominant negative Akt increased UV-induced apoptosis by 2-fold. Inhibition of UV-induced activation of Akt increased release of mitochondrial cytochrome c 3.5-fold, and caused appearance of active forms of caspase-9, caspase-8, and caspase-3. Constitutively active Akt abolished UV-induced cytochrome c release and activation of caspases-9, -8, and -3. These data demonstrate that PI3K/Akt is essential for protecting human KCs against UV-induced apoptosis, whereas NF-kappaB pathway provides little, if any, protective role.

Akt signaling in skeletal muscle: regulation by exercise and passive stretch

Akt/protein kinase B is a serine/threonine kinase that has emerged as a critical signaling component for mediating numerous cellular responses. Contractile activity has recently been demonstrated to stimulate Akt signaling in skeletal muscle. Whether physiological exercise in vivo activates Akt is controversial, and the initiating factors that result in the stimulation of Akt during contractile activity are unknown. In the current study, we demonstrate that treadmill running exercise of rats using two different protocols (intermediate high or high-intensity exhaustive exercise) significantly increases Akt activity and phosphorylation in skeletal muscle composed of various fiber types. To determine if Akt activation during contractile activity is triggered by mechanical forces applied to the skeletal muscle, isolated skeletal muscles were incubated and passively stretched. Passive stretch for 10 min significantly increased Akt activity (2-fold) in the fast-twitch extensor digitorum longus (EDL) muscle. However, stretch had no effect on Akt in the slow-twitch soleus muscle, although there was a robust phosphorylation of the stress-activated protein kinase p38. Similar to contraction, stretch-induced Akt activation in the EDL was fully inhibited in the presence of the phosphatidylinositol 3-kinase inhibitor wortmannin, whereas glycogen synthase kinase-3 (GSK3) phosphorylation was only partially inhibited. Stretch did not cause dephosphorylation of glycogen synthase on GSK3-targeted sites in the absence or presence of wortmannin. We conclude that physiological exercise in vivo activates Akt in multiple skeletal muscle fiber types and that mechanical tension may be a part of the mechanism by which contraction activates Akt in fast-twitch muscles.

Activation of the Rap GTPases in B lymphocytes modulates B cell antigen receptor-induced activation of Akt but has no effect on MAPK activation

Signaling by the B cell antigen receptor (BCR) activates the Rap1 and Rap2 GTPases, putative antagonists of Ras-mediated signaling. Because Ras can activate the Raf-1/ERK pathway and the phosphatidylinositol 3-kinase (PI3K)/Akt pathway, we asked whether Rap activation limits the ability of the BCR to signal via these pathways. To do this, we blocked the activation of endogenous Rap1 and Rap2 by expressing the Rap-specific GTPase-activating protein RapGAPII. Preventing Rap activation had no effect on BCR-induced activation of ERK. In contrast, BCR-induced phosphorylation of Akt on critical activating sites was increased 2- to 3-fold when Rap activation was blocked. Preventing Rap activation also increased the ability of the BCR to stimulate Akt-dependent phosphorylation of the FKHR transcription factor on negative regulatory sites and decreased the levels of p27Kip1, a pro-apoptotic factor whose transcription is enhanced by FKHR. Moreover, preventing Rap activation reduced BCR-induced cell death in the WEHI-231 B cell line. Thus activation of endogenous Rap by the BCR limits BCR-induced activation of the PI3K/Akt pathway, opposes the subsequent inhibition of the FKHR/p27Kip1 pro-apoptotic module, and enhances BCR-induced cell death. Consistent with the idea that Rap-GTP is a negative regulator of the PI3K/Akt pathway, expressing constitutively active Rap2 (Rap2V12) reduced BCR-induced phosphorylation of Akt and FKHR. Finally, our finding that Rap2V12 can bind PI3K and inhibit its activity in a manner that depends upon BCR engagement provides a potential mechanism by which Rap-GTP limits activation of the PI3K/Akt pathway, a central regulator of B cell growth and survival.

Integrin signaling and mammary cell function

The mammary gland is a highly organized tissue, containing ductal structures, secretory alveolar units, and a supporting stroma. The organization of the epithelial cells within the tissue depends upon cell-cell adhesion as well as cell interactions with the extracellular matrix that underlies the epithelial units and makes up most of the organization of the stroma. Adhesion to the extracellular matrix is mediated by a class of heterodimeric transmembrane receptors called integrins, which cluster at focal adhesions. Integrins link the matrix with an intracellular structural scaffold, the cytoskeleton, as well as with signaling enzymes that direct cell survival, proliferation, differentiation, and migration. Two key enzymes that are recruited to sites of integrin clustering are focal adhesion kinase and integrin-linked kinase. Both enzymes are involved with communication downstream of integrins and have key roles in regulating cell behavior. This review will focus on what is known about focal adhesion kinase and integrin-linked kinase signaling and will discuss current evidence about their role in mammary gland biology and neoplasia.

S-phase delay in human hepatocellular carcinoma cells induced by overexpression of integrin β1

AIM: To clarify the mechanisms of integrin overexpression in negatively regulating the cell cycle control of hepatocellular carcinoma cells SMMC-7721.

METHODS: The cell cycle pattern was determined by flow cytometry. The mRNA and protein expression levels were assayed by RT-PCR and Western blot, respectively. Stable transfection was performed by Lipofectamine 2000 reagent, and cells were screened by G418.

RESULTS: Overexpression of α5β1 or β1 integrin induced S-phase delay in SMMC-7721 cells, and this delay was possibly due to the accumulation of cyclin-dependent kinase inhibitors (CKIs) p21^cip1 and p27^kip1. The decrease of protein kinase B (PKB) phosphorylation was present in this signaling pathway, but focal adhesion kinase (FAK) was not involved. When phosphorylation of PKB was solely blocked by wortmannin, p27^kip1 protein level was increased. Moreover, S-phase delay was recurred when attachment of the parental SMMC-7721 cells was inhibited by the preparation of poly-HEME, and this cell cycle pattern was similar to that of β1-7721 or α5β1-7721 cells.

CONCLUSION: S-phase delay induced by overexpression of integrin β1 subunit is attributed to the decrease of PKB phosphorylation and subsequent increases of p21^cip1 and p27^kip1 proteins, and may be involved in the unoccupied α5β1 because of lack of its ligands.

Arg-Gly-Asp-Ser (RGDS) peptide stimulates transforming growth factor beta1 transcription and secretion through integrin activation

Extracellular matrix (ECM) components, through specific peptide motifs such as Arg-Gly-Asp (RGD), interact with integrins and can modify the behavior of cells. Transforming growth factor-beta1 (TGF-beta1) is the main cytokine involved in the synthesis of ECM proteins. We analyzed the effect of a RGD-containing peptide, as Arg-Gly-Asp-Ser (RGDS), on the regulation of TGF-beta1 secretion in cultured human mesangial cells. We found that RGDS increased mRNA expression and secretion of TGF-beta1 by stimulating the TGF-beta1 gene promoter. This effect was dependent on the interaction of RGDS with integrins. We evaluated the signaling pathways implicated in TGF-beta1 production by analyzing the effect of RGDS on kinase-related integrins. RGDS stimulated tyrosine phosphorylation as well as integrin-linked kinase (ILK) activity. However, tyrosine kinase inhibitors did not prevent the RGDS effect. In contrast, the inhibition of ILK by cell transfection with a kinase dead-ILK completely abolished the increased TGF-beta1 secretion and promoter activity in the presence of RGDS. Thus RGDS modulates the secretion of TGF-beta1, probably through increased synthesis by interacting with integrins and activating ILK. This supports a role for ECM components in the regulation of their own secretion.

Inactivation of integrin-linked kinase induces aberrant tau phosphorylation via sustained activation of glycogen synthase kinase 3beta in N1E-115 neuroblastoma cells

Integrin-linked kinase (ILK) is a focal adhesion serine/threonine protein kinase with an important role in integrin and growth factor signaling pathways. Recently, we demonstrated that ILK is expressed in N1E-115 neuroblastoma cells and controls integrin-dependent neurite outgrowth in serum-starved cells grown on laminin (Ishii, T., Satoh, E., and Nishimura, M. (2001) J. Biol. Chem. 276, 42994-43003). Here we report that ILK controls tau phosphorylation via regulation of glycogen synthase kinase-3beta (GSK-3beta) activity in N1E-115 cells. Stable transfection of a kinase-deficient ILK mutant (DN-ILK) resulted in aberrant tau phosphorylation in N1E-115 cells at sites recognized by the Tau-1 antibody that are identical to some of the phosphorylation sites in paired helical filaments, PHF-tau, in brains of patients with Alzheimer's disease. The tau phosphorylation levels in the DN-ILK-expressing cells are constant under normal and differentiating conditions. On the other hand, aberrant tau phosphorylation was not observed in the parental control cells. ILK inactivation resulted in an increase in the active form but a decrease in the inactive form of GSK-3beta, which is a candidate kinase involved in PHF-tau formation. Moreover, inhibition of GSK-3beta with lithium prevented aberrant tau phosphorylation in the DN-ILK-expressing cells. These results suggest that ILK inactivation results in aberrant tau phosphorylation via sustained activation of GSK-3beta in N1E-115 Cells. ILK directly phosphorylates GSK-3beta and inhibits its activity. Therefore, endogenous ILK protects against GSK-3beta-induced aberrant tau phosphorylation via inhibition of GSK-3beta activity in N1E-115 cells.

Reduced chondrocyte proliferation and chondrodysplasia in mice lacking the integrin-linked kinase in chondrocytes

Chondrocyte proliferation and differentiation requires their attachment to the collagen type II-rich matrix of developing bone. This interaction is mediated by integrins and their cytoplasmic effectors, such as the integrin-linked kinase (ILK). To elucidate the molecular mechanisms whereby integrins control these processes, we have specifically inactivated the ILK gene in growth plate chondrocytes using the Cre-lox methodology. Mice carrying an ILK allele flanked by loxP sites (ILK-fl) were crossed to transgenic mice expressing the Cre recombinase under the control of the collagen type II promoter. Inactivation of both copies of the ILK-fl allele lead to a chondrodysplasia characterized by a disorganized growth plate and to dwarfism. Expression of chondrocyte differentiation markers such as collagen type II, collagen type X, Indian hedgehog and the PTH-PTHrP receptor was normal in ILK-deficient growth plates. In contrast, chondrocyte proliferation, assessed by BrdU or proliferating cell nuclear antigen labeling, was markedly reduced in the mutant growth plates. Cell-based assays showed that integrin-mediated adhesion of primary cultures of chondrocytes from mutant animals to collagen type II was impaired. ILK inactivation in chondrocytes resulted in reduced cyclin D1 expression, and this most likely explains the defect in chondrocyte proliferation observed when ILK is inactivated in growth plate cells.

Unravelling the activation mechanisms of protein kinase B/Akt

Over the past decade, protein kinase B (PKB, also termed Akt) has emerged as an important signaling mediator between extracellular cues and modulation of gene expression, metabolism, and cell survival. The enzyme is tightly controlled and consequences of its deregulation include loss of growth control and oncogenesis. Recent work has better characterized the mechanism of PKB activation, including upstream regulators and secondary binding partners. This minireview refreshes some old concepts with new twists and highlights current outstanding questions.

Conditional knock-out of integrin-linked kinase demonstrates an essential role in protein kinase B/Akt activation

Protein kinase B (PKB/Akt) plays a pivotal role in signaling pathways downstream of phosphatidylinositol 3-kinase, regulating fundamental processes such as cell survival, cell proliferation, differentiation, and metabolism. PKB/Akt activation is regulated by phosphoinositide phospholipid-mediated plasma membrane anchoring and by phosphorylation on Thr-308 and Ser-473. Whereas the Thr-308 site is phosphorylated by PDK-1, the identity of the Ser-473 kinase has remained unclear and controversial. The integrin-linked kinase (ILK) is a potential regulator of phosphorylation of PKB/Akt on Ser-473. Utilizing double-stranded RNA interference (siRNA) as well as conditional knock-out of ILK using the Cre-Lox system, we now demonstrate that ILK is essential for the regulation of PKB/Akt activity. ILK knock-out had no effect on phosphorylation of PKB/Akt on Thr-308 but resulted in almost complete inhibition of phosphorylation on Ser-473 and significant inhibition of PKB/Akt activity, accompanied by significant stimulation of apoptosis. The inhibition of PKB/Akt Ser-473 phosphorylation was rescued by kinase-active ILK but not by a kinase-deficient mutant of ILK, suggesting a role for the kinase activity of ILK in the stimulation of PKB/Akt phosphorylation. ILK knock-out also resulted in the suppression of phosphorylation of GSK-3beta on Ser-9 and cyclin D1 expression. These data establish ILK as an essential upstream regulator of PKB/Akt activation.

Anoikis: roadblock to cell transplantation?

Cell therapy, in particular liver cell transplantation, holds great therapeutic potential and is partially hindered by the high rate of apoptosis during cell isolation, cryopreservation, and engraftment. Apoptosis occurring due to cell detachment from the extracellular matrix is a phenomenon termed "anoikis." The purpose of this review is to describe signaling mechanisms pertinent to anoikis in both immortalized cell lines, but particularly in primary normal epithelial cells. The mechanisms described include integrin signaling and survival molecules, caspase activation, and the role of mitochondrial proteins in anoikis. Strategies to prevent anoikis during isolation and cryopreservation of hepatocytes are discussed.

Phosphoinositide-dependent kinase-2 is a distinct protein kinase enriched in a novel cytoskeletal fraction associated with adipocyte plasma membranes

By recombining subcellular components of 3T3-L1 adipocytes in a test tube, early insulin signaling events dependent on phosphatidylinositol 3-kinase (PI 3-kinase) were successfully reconstituted, up to and including the phosphorylation of glycogen synthase kinase-3 by the serine/threonine kinase, Akt (Murata, H., Hresko, R.C., and Mueckler, M. (2003) J. Biol. Chem. 278, 21607-21614). Utilizing the advantages provided by a cell-free methodology, we characterized phosphoinositide-dependent kinase 2 (PDK2), the putative kinase responsible for phosphorylating Akt on Ser-473. Immunodepleting cytosolic PDK1 from an in vitro reaction containing plasma membrane and cytosol markedly inhibited insulin-stimulated phosphorylation of Akt at the PDK1 site (Thr-308) but had no effect on phosphorylation at the PDK2 site (Ser-473). In contrast, PDK2 activity was found to be highly enriched in a novel cytoskeletal subcellular fraction associated with plasma membranes. Akt isoforms 1-3 and a kinase-dead Akt1 (K179A) mutant were phosphorylated in a phosphatidylinositol 3,4,5-trisphosphate-dependent manner at Ser-473 in an in vitro reaction containing this novel adipocyte subcellular fraction. Our data indicate that this PDK2 activity is the result of a kinase distinct from PDK1 and is not due to autophosphorylation or transphosphorylation of Akt.

The integrin-actin connection, an eternal love affair

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

Integrin-linked kinase regulates chondrocyte shape and proliferation

The interaction of chondrocytes with the extracellular-matrix environment is mediated mainly by integrins. Ligated integrins are recruited to focal adhesions (FAs) together with scaffolding proteins and kinases, such as integrin-linked kinase (Ilk). Ilk binds the cytoplasmic domain of beta1-, beta2- and beta3-integrins and recruits adaptors and kinases, and is thought to stimulate downstream signalling events through phosphorylation of protein kinase B/Akt (Pkb/Akt) and glycogen synthase kinase 3-beta (GSK3-beta). Here, we show that mice with a chondrocyte-specific disruption of the gene encoding Ilk develop chondrodysplasia, and die at birth due to respiratory distress. The chondrodysplasia was characterized by abnormal chondrocyte shape and decreased chondrocyte proliferation. In addition, Ilk-deficient chondrocytes showed adhesion defects, failed to spread and formed fewer FAs and actin stress fibres. Surprisingly, phosphorylation of Pkb/Akt and GSK3-beta is unaffected in Ilk-deficient chondrocytes. These findings suggest that Ilk regulates actin reorganization in chondrocytes and modulates chondrocyte growth independently of phosphorylation of Pkb/Akt and GSK3-beta.

Integrin-linked kinase (ILK) is required for polarizing the epiblast, cell adhesion, and controlling actin accumulation

Integrin-mediated cell-matrix interactions are essential for development, tissue homeostasis, and repair. Upon ligand binding, integrins are recruited into focal adhesions (FAs). Integrin-linked kinase (ILK) is an FA component that interacts with the cytoplasmic domains of integrins, recruits adaptor proteins that link integrins to the actin cytoskeleton, and phosphorylates the serine/threonine kinases PKB/Akt and GSK-3beta. Here we show that mice lacking ILK expression die at the peri-implantation stage because they fail to polarize their epiblast and to cavitate. The impaired epiblast polarization is associated with abnormal F-actin accumulation at sites of integrin attachments to the basement membrane (BM) zone. Likewise, ILK-deficient fibroblasts showed abnormal F-actin aggregates associated with impaired cell spreading and delayed formation of stress fibers and FAs. Finally, ILK-deficient fibroblasts have diminished proliferation rates. However, insulin or PDGF treatment did not impair phosphorylation of PKB/Akt and GSK-3beta, indicating that the proliferation defect is not due to absent or reduced ILK-mediated phosphorylation of these substrates in vivo. Furthermore, expression of a mutant ILK lacking kinase activity and/or paxillin binding in ILK-deficient fibroblasts can rescue cell spreading, F-actin organization, FA formation, and proliferation. Altogether these data show that mammalian ILK modulates actin rearrangements at integrin-adhesion sites.

Ca2+-dependent localization of integrin-linked kinase to cell junctions in differentiating keratinocytes

Integrin complexes are necessary for proper proliferation and differentiation of epidermal keratinocytes. Differentiation of these cells is accompanied by down-regulation of integrins and focal adhesions as well as formation of intercellular adherens junctions through E-cadherin homodimerization. A central component of integrin adhesion complexes is integrin-linked kinase (ILK), which can induce loss of E-cadherin expression and epithelial-mesenchymal transformation when ectopically expressed in intestinal and mammary epithelia. In cultured primary mouse keratinocytes, we find that ILK protein levels are independent of integrin expression and signaling, since they remain constant during Ca(2+)-induced differentiation. In contrast, keratinocyte differentiation is accompanied by marked reduction in kinase activity in ILK immunoprecipitates and altered ILK subcellular distribution. Specifically, ILK distributes in close apposition to actin fibers along intercellular junctions in differentiated but not in undifferentiated keratinocytes. ILK localization to cell-cell borders occurs independently of integrin signaling and requires Ca(2+) as well as an intact actin cytoskeleton. Further, and in contrast to what is observed in other epithelial cells, ILK overexpression in differentiated keratinocytes does not promote E-cadherin down-regulation and epithelial-mesenchymal transition. Thus, novel tissue-specific mechanisms control the formation of ILK complexes associated with cell-cell junctions in differentiating murine epidermal keratinocytes.

Role of integrin-linked kinase in nerve growth factor-stimulated neurite outgrowth

The role of integrin-linked kinase (ILK), a kinase that is involved in various cellular processes, including adhesion and migration, has not been studied in primary neurons. Using mRNA dot blot and Western blot analysis of ILK in rat and human brain tissue, we found that ILK is expressed in various regions of the CNS. Immunohistochemical and immunocytochemical techniques revealed granular ILK staining that is enriched in neurons and colocalizes with the beta1 integrin subunit. The role of ILK in neurite growth promotion by NGF was studied in rat pheochromocytoma cells and dorsal root ganglion neurons using a pharmacological inhibitor of ILK (KP-392) or after overexpression of dominant-negative ILK (ILK-DN). Both molecular and pharmacological inhibition of ILK activity significantly reduced NGF-induced neurite outgrowth. Survival assays indicate that KP-392-induced suppression of neurite outgrowth occurred in the absence of cell death. ILK kinase activity was stimulated by NGF. NGF-mediated stimulation of phosphorylation of both AKT and the Tau kinase glycogen synthase kinase-3 (GSK-3) was inhibited in the presence of KP-392 and after overexpression of ILK-DN. Consequently, ILK inhibition resulted in an increase in the hyperphosphorylation of Tau, a substrate of GSK-3. Together these findings indicate that ILK is an important effector in NGF-mediated neurite outgrowth.

alpha(v) integrins regulate cell proliferation through integrin-linked kinase (ILK) in ovarian cancer cells

Integrins regulate both adhesion and signaling processes involved in proliferation and survival. alpha(v)beta(3) and alpha(v)beta(5) integrins have been shown to mediate cell adhesion and migration. Here we used human ovarian cancer cell lines (IGROV1, SKOV-3) that express alpha(v)beta(3) and alpha(v)beta(5) to study their role in cell proliferation and the signaling pathways involved. We found that alpha(v) integrins regulate cell proliferation through activation of integrin-linked kinase (ILK). An anti-alpha(v)-blocking antibody specifically inhibits the growth of IGROV1 and SKOV-3. The inhibition of cell proliferation involves alpha(v)beta(3) in IGROV1 cells, and both alpha(v)beta(3) and alpha(v)beta(5) in SKOV-3 cells. The reduced growth rate induced by alpha(v) integrin blockade is linked in both cell lines to G1/S cell cycle arrest. alpha(v) integrin blockade by neutralizing antibody as well as cyclic-RGD peptide caused an inhibition of ILK activity and phosphorylation of PKB/Akt on serine-473 but not on threonine-308, and was accompanied by an increase in p27(Kip1) expression. Overexpression of wild-type ILK rescued the phosphorylation of PKB/Akt on serine-473 in cells treated with anti-alpha(v) antibody. Inhibition of ILK by a pharmacological inhibitor results in inhibition of cell proliferation, PKB/Akt phosphorylation and increase of p27(Kip1). These results demonstrate that alpha(v) integrins regulate ovarian cancer cell proliferation through ILK.

Mechanism of inhibition of insulin-stimulated glucose transport by 4-bromocrotonic acid in 3T3-L1 adipocytes

We have demonstrated previously that 4-bromocrotonic acid (Br-C4) inhibited insulin-stimulated glucose transport by interfering with GLUT4 translocation. In the present study, we further examined the underlying mechanism involved. Since insulin-induced insulin receptor substrate-1-associated phosphatidylinositol (PI) 3-kinase activity was not altered by Br-C4, we determined and found insulin activation of protein kinase B (PKB) and protein kinase Clambda (PKClambda) were both inhibited. However, time-course studies showed that only the inhibition of PKB activation correlated with the inhibition of insulin-stimulated glucose transport. In concert, insulin-stimulated Ser(473/474) phosphorylation on PKB(alpha/beta) were similarly decreased by Br-C4. The finding that okadaic acid-stimulated glucose transport and PKClambda activity were both inhibited by Br-C4 suggested that the effect of Br-C4 on Ser(473/474) phosphorylation was not mediated by protein phosphatase 2A. Moreover, whereas Br-C4 nearly abolished insulin-stimulated integrin-linked kinase (ILK) activity, it only inhibited insulin-stimulated PKB activity by 20%, implying that ILK was not the major kinase for Ser(473/474) phosphorylation. Taken together, these results support the notion that PKB is involved in insulin-stimulated glucose transport. In addition, Br-C4 seems to inhibit insulin-stimulated glucose transport via inhibiting insulin activation of PKB, probably by interfering with insulin activation of an upstream kinase responsible for the phosphorylation of Ser(473/474) residue.