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

Targeting of integrin-linked kinase with a small interfering RNA suppresses progression of experimental proliferative vitreoretinopathy

Integrin-linked kinase (ILK) is a serine/threonine kinase that interacts through its COOH terminus with beta1 and beta3 integrins, which mediates a diversity of cell functions by coupling integrins and growth factors to cascades of downstream signaling events. The purpose of this work was to investigate the effects of ILK on development of experimental proliferative vitreoretinopathy (PVR). Cultured human RPE cell line D407 was knocked down for ILK using a small interfering RNA (siRNA). For this, cellular ILK expression was quantified by real-time quantitative PCR, Western blot analysis and immunocytochemical assay, and cytotoxicity of transfection was determined by MTT assay. Moreover, cell attachment, spreading, migration, microfilament dynamics, and cell cycling assays were performed. Furthermore, the impact of the ILK-specific siRNA on PVR was tested using a rabbit model in which PVR was induced by the injection of human RPE cells. Prevalence of PVR and retinal detachment were determined by indirect ophthalmoscopy on days 1, 3, 7, 14, 21 and 28 post-injection. The results showed that blocking the expression of ILK by siRNA significantly inhibited human RPE cell attachment, spreading, migration and proliferation. The knockdown of ILK also disturbed F-actin assembly and induced a cellular arrest in the G1 phase of the cell cycle. Though the eyes injected with ILK-specific siRNA also developed features of PVR, the severities of day 28 post-injection were significantly lower than those in the control eyes (P<0.01). We conclude that targeting of ILK with a small interfering RNA not only inhibits human RPE cell attachment, spreading, migration and proliferation in vitro, but also effectively suppresses development of proliferative vitreoretinopathy in a rabbit model. This may be a potential therapeutic usefulness in treating PVR.

Lamin A Ser404 is a nuclear target of Akt phosphorylation in C2C12 cells

Akt/PKB is a central activator of multiple signaling pathways coupled with a large number of stimuli. Although both localization and activity of Akt in the nuclear compartment are well-documented, most Akt substrates identified so far are located in the cytoplasm, while nuclear substrates have remained elusive. A proteomic-based search for nuclear substrates of Akt was undertaken, exploiting 2D-electrophoresis/MS in combination with an anti-Akt phosphosubstrate antibody. This analysis indicated lamin A/C as a putative substrate of Akt in C2C12 cells. In vitro phosphorylation of endogenous lamin A/C by recombinant Akt further validated this result. Moreover, by phosphopeptide analysis and point mutation, we established that lamin A/C is phosphorylated by Akt at Ser404, in an evolutionary conserved Akt motif. To delve deeper into this, we raised an antibody against the lamin A Ser404 phosphopeptide which allowed us to determine that phosphorylation of lamin A Ser404 is triggered by the well-known Akt activator insulin, and is therefore to be regarded as a physiological response. Remarkably, expression of S404A lamin A in primary cells from healthy tissue caused the nuclear abnormalities that are a hallmark of Emery-Dreifuss muscular dystrophy (EDMD) cells. Indeed, it is known that mutations at several sites in lamin A/C cause autosomal dominant EDMD. Very importantly, we show here that Akt failed to phosphorylate lamin A/C in primary cells from an EDMD-2 patient with lamin A/C mutated in the Akt consensus motif. Together, our data demonstrate that lamin A/C is a novel signaling target of Akt, and implicate Akt phosphorylation of lamin A/C in the correct function of the nuclear lamina.

EPAC and PKA allow cAMP dual control over DNA-PK nuclear translocation

We identify a compartmentalized signaling system that identifies a functional role for the GTP exchange factor, exchange protein activated by cAMP (EPAC) coupled to Rap2 in the nucleus. In this system, cAMP regulates the nuclear/cytoplasmic trafficking of DNA-dependent protein kinase (DNA-PK), a critical kinase that acts to repair double-stranded breaks (DSBs) in damaged DNA and to phosphorylate the cell survival kinase, PKB/Akt. Intersecting regulatory inputs for cAMP employ EPAC to transduce positive effects, namely the Rap2-dependent nuclear exit and activation of DNA-PK, whereas protein kinase A (PKA) provides the negative input by antagonizing these actions. We identify this as a compartmentalized regulatory system where modulation of cAMP input into the stimulatory, EPAC and inhibitory, PKA intersecting arms is provided by spatially discrete, cAMP degradation systems. The distribution of DNA-PK between nuclear and cytoplasmic compartments can thus potentially be influenced by relative inputs of cAMP signaling through the EPAC and PKA pathways. Through this signaling system EPAC activation can thereby impact on the Ser-473 phosphorylation status of PKB/Akt and the repair of etoposide-induced DSBs.

Dp71f modulates GSK3-beta recruitment to the beta1-integrin adhesion complex

Previously, it was shown that Dp71f binds to the beta1-integrin adhesion complex to modulate PC12 cell adhesion. The absence of Dp71f led to a failure in the beta1-integrin adhesion complex formation. One of the structural proteins which links the beta1-integrin cytoplasmic domain to the actin cytoskeleton is ILK. GSK3-beta is an ILK substrate and the carboxi-terminal region of dystrophin 427 is a substrate for hierarchical phosphorylation by GSK3-beta. Dp71f contains the carboxi-terminal domain present in dystrophin 427. By using co-immunoprecipitation assays, in the present work it is demonstrated that in the neuronal PC12 cell line an interaction between Dp71f and GSK3-beta occurs. This interaction was corroborated by in vitro pulldown assays. We show that GSK3-beta is recruited to the beta1-integrin complex and that a reduced expression of Dp71f induces a reduced GSK3-beta recruitment to the beta1-integrin complex. In addition, the present work establishes that adhesion of PC12 cells to laminin does not influence the phosphorylation status of Dp71f.

Identification of phosphoproteins in kidney tissues from patients with autosomal dominant polycystic kidney disease

Protein phosphorylation is a very important PTM. Phosphorylation/dephosphorylation of a protein can alter its behavior in almost every conceivable way. Previous studies indicate that abnormal phosphorylation is involved in the pathogenesis of autosomal dominant polycystic kidney disease (ADPKD). However, large-scale proteomic analysis of altered phosphoproteins in ADPKD has not been reported. In this study, total proteins from ADPKD cystic kidney tissues (n = 5) and normal kidney tissues (n = 5) were extracted and phosphoproteins were enriched by phosphate metal affinity chromatography, then separated by 2-DE and identified by LC-MS/MS. Between the two groups, 48 protein spots showing more than a twofold difference were detected. Among them, 28 spots were up-regulated and 20 down-regulated in ADPKD kidney tissues. Of these, 38 different proteins were identified including cell signaling proteins, cytoskeleton proteins, mitochondria metabolic enzymes, antioxidant proteins, molecular chaperones, transcription factors and regulators. Two differential phosphoproteins, annexin II and tropomyosin, were further confirmed by immunoprecipitation and Western blot analysis. The results show that there are many kinds of abnormal phosphoproteins in ADPKD cystic kidney tissues. More studies on the functions of the differential phosphoproteins may provide us new clues for ADPKD pathogenesis and treatment.

Integrin-linked kinase in the vascular smooth muscle cell response to injury

Integrin-mediated interactions between smooth muscle cells (SMCs) and the extracellular matrix regulate cell migration and proliferation during neointimal hyperplasia. Integrin-linked kinase (ILK) is a serine-threonine kinase and scaffolding molecule that acts downstream of integrin receptors to modulate cell adhesion; therefore, we examined ILK function in SMCs during wound repair. Silencing of ILK expression with siRNA in vitro decreased cell adhesion to fibronectin and accelerated both cell proliferation and wound closure in the cell monolayer; it also resulted in the rearrangement of focal adhesions and diminished central actin stress fibers. Akt and GSK3beta are ILK substrates that are important in cell motility; however, ILK siRNA silencing did not attenuate injury-induced increases in Akt and GSK3beta phosphorylation. Following balloon catheter injury of the rat carotid artery in vivo, a dramatic decrease in ILK levels coincided with both the proliferation and migration of SMCs, which leads to the formation of a thickened neointima. Immunostaining revealed decreased ILK levels in the media and deep layers of the neointima, but increased ILK levels in the subluminal layers of the intima. Taken together, these results suggest that ILK functions to maintain SMC quiescence in the normal artery. A decrease in ILK levels after injury may permit SMC migration, proliferation, and neointimal thickening, and its re-expression at the luminal surface may attenuate this process during later stages of the injury response.

Expression of integrin-linked kinase is increased in differentiated cells

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

Regulation of cell-matrix contacts and beta-catenin signaling in VSMC by integrin-linked kinase: implications for intimal thickening

Vascular smooth muscle cell (VSMC) proliferation and migration is responsible for intimal thickening that occurs in restenosis and atherosclerosis. Integrin-linked kinase (ILK) is a serine/threonine protein kinase implicated in signaling pathways involved in cell proliferation and migration. We studied the involvement of ILK in intimal thickening. ILK expression was significantly increased in two models of intimal thickening: balloon-injured rat carotid arteries and human saphenous vein organ cultures. Over-expression of a dominant negative ILK (DN-ILK) significantly reduced intimal thickening by approximately 50% in human saphenous vein organ cultures, demonstrating an important role in intimal thickening. ILK protein and activity was reduced on laminin and up-regulated on fibronectin, indicating ILK protein expression is modulated by extracellular matrix composition. Inhibition of ILK by siRNA knockdown and DN-ILK significantly decreased VSMC proliferation and migration while wild type ILK significantly increased proliferation and migration on laminin, confirming an essential role of ILK in both processes. Localization of paxillin and vinculin and protein levels of FAK and phospho-FAK indicated that inhibition of ILK reduced focal adhesion formation. Additionally, inhibition of ILK significantly attenuated the presence of the cell-cell complex proteins N-cadherin and beta-catenin, and beta-catenin signaling. We therefore suggest ILK modulates VSMC proliferation and migration at least in part by acting as a molecular scaffold in focal adhesions as well as modulating the stability of cell-cell contact proteins and beta-catenin signaling. In summary, ILK plays an important role in intimal thickening by modulating VSMC proliferation and migration via regulation of cell-matrix and cell-cell contacts and beta-catenin signaling.

Rictor and integrin-linked kinase interact and regulate Akt phosphorylation and cancer cell survival

An unbiased proteomic screen to identify integrin-linked kinase (ILK) interactors revealed rictor as an ILK-binding protein. This finding was interesting because rictor, originally identified as a regulator of cytoskeletal dynamics, is also a component of mammalian target of rapamycin complex 2 (mTORC2), a complex implicated in Akt phosphorylation. These functions overlap with known ILK functions. Coimmunoprecipitation analyses confirmed this interaction, and ILK and rictor colocalized in membrane ruffles and leading edges of cancer cells. Yeast two-hybrid assays showed a direct interaction between the NH(2)- and COOH-terminal domains of rictor and the ILK kinase domain. Depletion of ILK and rictor in breast and prostate cancer cell lines resulted in inhibition of Akt Ser(473) phosphorylation and induction of apoptosis, whereas, in several cell lines, depletion of mTOR increased Akt phosphorylation. Akt and Ser(473)P-Akt were detected in ILK immunoprecipitates and small interfering RNA-mediated depletion of rictor, but not mTOR, inhibited the amount of Ser(473)P-Akt in the ILK complex. Expression of the NH(2)-terminal (1-398 amino acids) rictor domain also resulted in the inhibition of ILK-associated Akt Ser(473) phosphorylation. These data show that rictor regulates the ability of ILK to promote Akt phosphorylation and cancer cell survival.

Zebrafish integrin-linked kinase is required in skeletal muscles for strengthening the integrin-ECM adhesion complex

Mechanical instability of skeletal muscle cells is the major cause of congenital muscular dystrophy. Here we show that the zebrafish lost-contact mutant, that lacks a functional integrin-linked kinase (ilk) gene, suffers from mechanical instability of skeletal muscle fibres. With genetic and morpholino knock-down experiments we demonstrate that: 1) laminin, itgalpha7, Ilk and beta-parvin are all critical for mechanical stability in skeletal muscles. 2) Ilk acts redundantly with the dystrophin/dystroglycan adhesion complex in maintaining mechanical stability of skeletal muscles. 3) Ilk protein is recruited to the myotendinous junctions, which requires the ECM component laminin and the presence of itgalpha7 in the sarcolemma. 4) Ilk, unexpectedly, is dispensable for formation of the adhesion complex. Ilk, however, is required for strengthening the adhesion of the muscle fibre with the ECM and this activity requires the presence of a functional kinase domain in Ilk. 5) We identified a novel interaction between Ilk and the mechanical stretch sensor protein MLP. Thus, Ilk is an essential intracellular component downstream of laminin and itgalpha7, providing strengthening of skeletal muscle fibre adhesion with the ECM and therefore qualified as a novel candidate gene for congenital muscular dystrophy.

Mapping the integrin-linked kinase interactome using SILAC

Protein-protein interactions play an essential role in the regulation of vital biological functions. Through a network of interactions, integrin-linked kinase (ILK) functions downstream of integrin receptors to control cell spreading, migration, growth, survival, and cell cycle progression. Despite many reports on the role of ILK in the regulation of multiple signaling pathways, it is still not understood how ILK integrates and controls complex cellular signals. A more global analysis of ILK-protein complexes will give important insights in the complexity of ILK-dependent signal transduction. Here, we applied a SILAC (stable isotope labeling with amino acids in cell culture)-based proteomics approach to discover novel ILK-interacting proteins. Of 752 proteins identified in ILK immunoprecipitates, 24 proteins had SILAC ratios higher than PINCH, previously identified as direct ILK-binding partner. Some of the newly identified proteins specifically enriched in ILK immunoprecipitates, with potentially interesting roles in ILK biology, include rapamycin-insensitive companion of mTOR (Rictor), alpha- and beta-tubulin, RuvB-like 1 and 2, HS1-associating protein 1 (HAX-1), T-complex protein 1 subunits, and Ras-GTP-ase activating-like protein 1 (IQ-GAP1). Functional interactions between ILK and several of the new binding partners were confirmed by coimmunoprecipitation/Western blot and colocalization experiments. Detailed analysis showed that when ILK is found in a complex with alpha-tubulin and RuvB-like 1, alpha-parvin and PINCH are not present, suggesting that ILK has the ability to form distinct protein complexes throughout the cell. Inhibition of ILK activity with an ILK-kinase inhibitor QLT0267 or downregulation of its expression impaired the ability of RuvB-like 1 to bind to tubulin pointing toward a possible role of ILK in the regulation of RuvB-like 1/tubulin interaction. Using the power of quantitative proteomics to resolve specific from nonspecific protein interactions, we identified several novel ILK-binding proteins, which sheds light on the molecular mechanisms of regulation of ILK-dependent signal transduction.

PI(3)K-Akt-mTOR pathway as a potential therapeutic target in neuroendocrine tumors

Constitutive activation of PI(3)K-Akt-mTOR signaling is a frequently occurring event in human cancer and has also been detected in the majority of neuroendocrine tumors (NETs) of the gastroenteropancreatic system. Molecular analysis of NETs suggests, that in addition to mutations in certain tumor-suppressor genes (e.g., PTEN), multiple autocrine growth factor loops contribute to hyperactive PI(3)K-Akt-mTOR signaling, thus promoting unrestricted proliferation and resistance to apoptosis. These insights opened new perspectives for targeted therapy in NETs. In particular, several novel small-molecule inhibitors of tyrosine and serine/threonine kinases have demonstrated potent anti-tumor activity. This review will summarize current knowledge on PI(3)K-Akt-mTOR signaling, its role in proliferation and apoptosis, as well as novel therapeutic approaches targeting PI(3)K-Akt-mTOR pathway components in NET disease.

Cell adhesion-dependent cofilin serine 3 phosphorylation by the integrin-linked kinase.c-Src complex

Integrin-linked kinase (ILK) is involved in signal transduction by integrin-mediated cell adhesion that leads to dynamic actin reorganization. Actin (de)polymerization is regulated by cofilin, the Ser(3) phosphorylation (pS(3)cofilin) of which inhibits its actin-severing activity. To determine how ILK regulates pS(3)cofilin, we examined the effects of ILK on pS(3)cofilin using normal RIE1 cells. Compared with suspended cells, fibronectin-adherent cells showed enhanced pS(3)cofilin, depending on ILK expression and c-Src activity. The ILK-mediated pS(3)cofilin in RIE1 cells did not involve Rho-associated kinase, LIM kinase, or testicular protein kinases, which are known to be upstream of cofilin. The kinase domain of ILK, including proline-rich regions, appeared to interact physically with the Src homology 3 domain of c-Src. In vitro kinase assay revealed that ILK immunoprecipitates phosphorylated the recombinant glutathione S-transferase-cofilin, which was abolished by c-Src inhibition. Interestingly, epidermal growth factor treatment abolished the ILK effects, indicating that the linkage from ILK to cofilin is biologically responsive to extracellular cues. Altogether, this study provides evidence for a new signaling connection from ILK to cofilin for dynamic actin polymerization during cell adhesion, depending on the activity of ILK-associated c-Src.

Tenascin-C induction by cyclic strain requires integrin-linked kinase

Induction of tenascin-C mRNA by cyclic strain in fibroblasts depends on RhoA and Rho dependent kinase (ROCK). Here we show that integrin-linked kinase (ILK) is required upstream of this pathway. In ILK-deficient fibroblasts, RhoA was not activated and tenascin-C mRNA remained low after cyclic strain; tenascin-C expression was unaffected by ROCK inhibition. In ILK wild-type but not ILK-/- fibroblasts, cyclic strain-induced reorganization of actin stress fibers and focal adhesions, as well as nuclear translocation of MAL, a transcriptional co-activator that links actin assembly to gene expression. These findings support a role for RhoA in ILK-mediated mechanotransduction. Rescue of ILK -/- fibroblasts by expression of wild-type ILK restored these responses to cyclic strain. Mechanosensation is not entirely abolished in ILK -/- fibroblasts, since cyclic strain activated Erk-1/2 and PKB/Akt, and induced c-fos mRNA in these cells. Conversely, lysophosphatidic acid stimulated RhoA and induced both c-fos and tenascin-C mRNA in ILK -/- cells. Thus, the signaling pathways controlling tenascin-C expression are functional in the absence of ILK, but are not triggered by cyclic strain. Our results indicate that ILK is selectively required for the induction of specific genes by mechanical stimulation via RhoA-mediated pathways.

17beta-estradiol attenuates glycogen synthase kinase-3beta activation and tau hyperphosphorylation in Akt-independent manner

Decline of estrogen is associated with high incidence of Alzheimer's disease (AD) characterized pathologically with tau hyperphosphorylation, and glycogen synthase kinase-3beta (GSK-3beta) is a major tau kinase. However, the role of estrogen on GSK3beta-induced tau hyperphosphorylation is elusive. Here, we treated N2a cells with wortmannin (Wort) and GF-109203X (GFX) or gene transfection to activate GSK-3beta and to induce tau hyperphosphorylation and then the effects of 17beta-estradiol (betaE2) on tau phosphorylation and GSK-3beta activity were studied. We found that betaE2 could attenuate tau hyperphosphorylation at multiple AD-related sites, including Ser396/404, Thr231, Thr205, and Ser199/202, induced by Wort/GFX or transient overexpression of GSK-3beta. Simultaneously, it increased the level of Ser9-phosphorylated (inactive) GSK-3beta. To study whether the protective effect of betaE2 on GSK-3beta and tau phosphorylation involves protein kinase B (Akt), an upstream effector of GSK-3, we transiently expressed the dominant negative Akt (dnAkt) in the cells. We found that betaE2 could attenuate Wort/GFX-induced GSK-3beta activation and tau hyperphosphorylation with Akt-independent manner. It suggests that betaE2 may arrest AD-like tau hyperphosphorylation by directly targeting GSK-3beta.

Ultrasound Induces Hypoxia-inducible Factor-1 Activation and Inducible Nitric-oxide Synthase Expression through the Integrin/Integrin-linked Kinase/Akt/Mammalian Target of Rapamycin Pathway in Osteoblasts

It has been shown that ultrasound (US) stimulation accelerates fracture healing in the animal models and clinical studies. Nitric oxide (NO) is a crucial early mediator in mechanically induced bone formation. Here we found that US stimulation increased NO formation and the protein level of inducible nitric-oxide synthase (iNOS). US-mediated iNOS expression was attenuated by anti-integrin alpha5beta1 or beta1 antibodies but not anti-integrin alphavbeta3 or beta3 antibodies or focal adhesion kinase mutant. Integrin-linked kinase (ILK) inhibitor (KP-392), Akt inhibitor (1L-6-hydroxymethyl-chiro-inositol-2-[(R)-2-O-methyl-3-O-octadecylcarbonate]) or mammalian target of rapamycin (mTOR) inhibitor (rapamycin) also inhibited the potentiating action of US. US stimulation increased the kinase activity of ILK and phosphorylation of Akt and mTOR. Furthermore, US stimulation also increased the stability and activity of HIF-1 protein. The binding of HIF-1alpha to the HRE elements on the iNOS promoter was enhanced by US stimulation. Moreover, the use of pharmacological inhibitors or genetic inhibition revealed that both ILK/Akt and mTOR signaling pathway were potentially required for US-induced HIF-1alpha activation and subsequent iNOS up-regulation. Taken together, our results provide evidence that US stimulation up-regulates iNOS expression in osteoblasts by an HIF-1alpha-dependent mechanism involving the activation of ILK/Akt and mTOR pathways via integrin receptor.

Ultrasound induces cyclooxygenase-2 expression through integrin, integrin-linked kinase, Akt, NF-kappaB and p300 pathway in human chondrocytes

It has been shown that ultrasound (US) stimulation accelerates fracture healing in the animal models and in clinical studies. However, the precise molecular events generated by US in chondrocytes have not been clarified well. Here we found that US stimulation transiently increased the surface expression of alpha2, alpha5, beta1 or beta3 but not alpha3 or alpha4 integrins in human chondrocytes, as shown by flow cytometric analysis. US stimulation increased prostaglandin E(2) formation as well as the protein and mRNA levels of cyclooxygenase-2 (COX-2). At the mechanistic level, anti-integrin beta1 and beta3 antibodies or beta1 and beta3 integrin small interference RNA attenuated the US-induced COX-2 expression. Integrin-linked kinase (ILK) inhibitor (KP-392), Akt inhibitor, NF-kappaB inhibitor (PDTC) or IkappaB protease inhibitor (TPCK) also inhibited the potentiating action of US. US stimulation promotes kinase activity of ILK, phosphorylation of Akt. In addition, US stimulation also induces IKKalpha/beta phosphorylation, IkappaBalpha phosphorylation, IkappaBalpha degradation, p65 phosphorylation at Ser(276), p65 and p50 translocation from the cytosol to the nucleus, and kappaB-luciferase activity. The binding of p65 to the NF-kappaB element, as well as the recruitment of p300 and the enhancement of p50 acetylation on the COX-2 promoter was enhanced by US. Taken together, our results provide evidence that US stimulation increases COX-2 expression in chondrocytes via the integrin/ILK/Akt/NF-kappaB and p300 signaling pathway.

Carboxy-terminal modulator protein induces Akt phosphorylation and activation, thereby enhancing antiapoptotic, glycogen synthetic, and glucose uptake pathways

Carboxy-terminal modulator protein (CTMP) was identified as binding to the carboxy terminus of Akt and inhibiting the phosphorylation and activation of Akt. In contrast to a previous study, we found CTMP overexpression to significantly enhance Akt phosphorylation at both Thr(308) and Ser(473) as well as the kinase activity of Akt, while phosphatidylinositol 3-kinase (PI3-kinase) activity was unaffected. Translocation of Akt to the membrane fraction was also markedly increased in response to overexpression of CTMP, with no change in the whole cellular content of Akt. Furthermore, the phosphorylations of GSK-3beta and Foxo1, well-known substrates of Akt, were increased by CTMP overexpression. On the other hand, suppression of CTMP with small interfering RNA partially but significantly attenuated this Akt phosphorylation. The cellular activities reportedly mediated by Akt activation were also enhanced by CTMP overexpression. UV-B-induced apoptosis of HeLa cells was significantly reversed not only by overexpression of the active mutant of Akt (myr-Akt) but also by that of CTMP. Increases in glucose transport activity and glycogen synthesis were also induced by overexpression of either myr-Akt or CTMP in 3T3-L1 adipocytes. Taking these results into consideration, it can be concluded that CTMP induces translocation of Akt to the membrane and thereby increases the level of Akt phosphorylation. As a result, CTMP enhances various cellular activities that are principally mediated by the PI3-kinase/Akt pathway.

A co-clustering model involving alpha5beta1 integrin for the biological effects of GPI-anchored human carcinoembryonic antigen (CEA)

CEA functions as an intercellular adhesion molecule and is up-regulated in a wide variety of human cancers, including colon, breast and lung. Its over-expression inhibits cellular differentiation, blocks cell polarization, distorts tissue architecture, and inhibits anoikis of many different cell types. Here we report results concerning the molecular mechanism involved in these biological effects, where relatively rapid molecular changes not requiring alterations in gene expression were emphasized. Confocal microscopy experiments showed that antibody-mediated clustering of a deletion mutant of CEA (DeltaNCEA), normally incapable of self binding and clustering, led to the co-localization of integrin alpha5beta1 with patches of DeltaNCEA on the cell surface. Activation of alpha5, as defined by an anti-alpha5 mAb-sensitive increase in cell adhesion to immobilized fibronectin, and an increased binding of soluble fibronectin to cells, was also observed. This was accompanied by the recruitment of integrin-linked kinase (ILK), protein kinase B (PKB/Akt), and the mitogen-activated protein kinase (MAPK) to membrane microdomains and the phosphorylation of Akt and MAPK. Inhibition of PI3-K and ILK, but not MAPK, prevented the alpha5beta1 integrin activation. Conversely, anti-alpha5 antibody inhibited the PI3-K-mediated activation of Akt, implying the involvement of outside-in and inside-out signaling in integrin activation. Therefore we propose that CEA-mediated signaling involves clustering of CEA and co-clustering and activation of the alpha5beta1 and associated specific signaling elements on the internal surfaces of membrane microdomains. These changes may represent a molecular mechanism for the biological effects of CEA.

Interaction of integrin-linked kinase with the kidney chloride/bicarbonate exchanger, kAE1

Kidney anion exchanger 1 (kAE1) mediates chloride/bicarbonate exchange at the basolateral membrane of kidney alpha-intercalated cells, thereby facilitating bicarbonate reabsorption into the blood. Human kAE1 lacks the N-terminal 65 residues of the erythroid form (AE1, band 3), which are essential for binding of cytoskeletal and cytosolic proteins. Yeast two-hybrid screening identified integrin-linked kinase (ILK), a serine/threonine kinase, and an actin-binding protein as an interacting partner with the N-terminal domain of kAE1. Interaction between kAE1 and ILK was confirmed in co-expression experiments in HEK 293 cells and is mediated by a previously unidentified calponin homology domain in the kAE1 N-terminal region. The calponin homology domain of kAE1 binds the C-terminal catalytic domain of ILK to enhance association of kAE1 with the actin cytoskeleton. Overexpression of ILK increased kAE1 levels at the cell surface as shown by flow cytometry, cell surface biotinylation, and anion transport activity assays. Pulse-chase experiments revealed that ILK associates with kAE1 early in biosynthesis, likely in the endoplasmic reticulum. ILK co-localized with kAE1 at the basolateral membrane of polarized Madin-Darby canine kidney cells and in alpha-intercalated cells of human kidneys. Taken together these results suggest that ILK and kAE1 traffic together from the endoplasmic reticulum to the basolateral membrane. ILK may provide a linkage between kAE1 and the underlying actin cytoskeleton to stabilize kAE1 at the basolateral membrane, resulting in higher levels of cell surface expression.

Role of the integrin-linked kinase (ILK) in determining neuronal polarity

The establishment of axon-dendrite polarity in mammalian neurons has recently been shown to involve the kinases Akt and GSK-3beta. Here we report the function of the integrin-linked kinase (ILK) in neuronal polarization. ILK distribution is differential: with more of it present in the axonal tips than that in the dendritic tips of a polarized neuron. Inactivation of ILK by chemical inhibitors, a kinase-inactive mutant or siRNAs inhibited axon formation, whereas a kinase hyperactive ILK mutant induced the formation of multiple axons. Biochemical studies indicate that ILK is upstream of Akt and GSK-3beta. Manipulations of multiple intracellular components indicate that ILK is functionally upstream of Akt and GSK-3beta but downstream of PI3K in neuronal polarity. These results reveal a key role of ILK in the formation of neuronal polarity and suggest a signaling pathway important for neuronal polarity.

Integrin-Linked Kinase at the Heart of Cardiac Contractility, Repair, and Disease

Recent advances in cardiac physiology identify the integrin-linked kinase (ILK) as an essential molecule regulating cardiac growth, contractility, and repair. A key transducer of biochemical signals initiated at the plasma membrane by cell–matrix interactions, ILK now emerges as a crucial player in mechanotransduction by integrins. Animal models have been particularly instructive in dissecting the cardiac functions of ILK and its associated proteins, such as parvins and PINCH, and have clearly established ILK as a major contributor to cardiac health. ILK gene knockouts in mice, flies, and worms result in early embryonic lethality because of cell adhesion defects and cytoskeletal disorganization. Although widely distributed in mammalian tissues, ILK expression is highest in the heart, and cardiac-specific ablation of ILK causes cardiomyopathy and sudden death in mice. ILK protein complexes are found in the sarcomere, which is the basic contractile unit of myocytes. A natural inactivating mutation in the kinase domain of ILK disrupts ILK protein interactions in the sarcomere, causing a contractile defect in the zebrafish heart. The relatively subtle phenotype of mutant ILK hearts, compared with ILK-ablated hearts, suggests multiple cardiac ILK functions. Cardiac-specific expression of ILK in transgenic mice induces a hypertrophic program, pointing to ILK as a proximal regulator of multiple hypertrophic signal transduction pathways. ILK protein interactions may also be important in mediating postinfarct cell migration and myocardial repair.

Phosphorylation-dependent regulation of nuclear localization and functions of integrin-linked kinase

Integrin-linked kinase (ILK) is a phosphorylated protein that regulates physiological processes that overlap with those regulated by p21-activated kinase 1 (PAK1). Here we report the possible role of ILK phosphorylation by PAK1 in ILK-mediated signaling and intracellular translocation. We found that PAK1 phosphorylates ILK at threonine-173 and serine-246 in vitro and in vivo. Depletion of PAK1 decreased the levels of endogenous ILK phosphorylation in vivo. Mutation of PAK1 phosphorylation sites on ILK to alanine reduced cell motility and cell proliferation. Biochemical fractionation, confocal microscopy, and chromatin-interaction analyses of human cells revealed that ILK localizes predominantly in the cytoplasm but also resides in the nucleus. Transfection of MCF-7 cells with point mutants ILK-T173A, ILK-S246A, or ILK-T173A; S246A (ILK-DM) altered ILK localization. Selective depletion of PAK1 dramatically increased the nuclear and focal point accumulation of ILK, further demonstrating a role for PAK1 in ILK translocation. We also identified functional nuclear localization sequence and nuclear export sequence motifs in ILK, delineated an apparently integral role for ILK in maintaining normal nuclear integrity, and established that ILK interacts with the regulatory region of the CNKSR3 gene chromatin to negatively modulate its expression. Together, these results suggest that ILK is a PAK1 substrate, undergoes phosphorylation-dependent shuttling between the cell nucleus and cytoplasm, and interacts with gene-regulatory chromatin.

The phosphatidyl inositol 3-kinase signaling network: implications for human breast cancer

The phosphatidyl inositol 3-kinase (PI3K)/Akt pathway is activated downstream of a variety of extracellular signals and activation of this signaling pathway impacts a number of cellular processes including cell growth, proliferation and survival. The alteration of components of this pathway, through either activation of oncogenes or inactivation of tumor suppressors, disrupts a signaling equilibrium and can thus lead to cellular transformation. The frequent dysregulation of the PI3K/Akt pathway in human cancer has made components of this pathway attractive for therapeutic targeting; however, a more comprehensive understanding of the signaling intricacies is necessary to develop pharmacological agents to target not only specific molecules, but also specific functions. Here, we review a series of experiments examining the contribution of molecules of this signaling network including PI3K, phosphatase and tensin homolog deleted on chromosome 10, integrin-linked kinase and Akt and address the significance to human breast cancer.

Akt inhibitor A-443654 induces rapid Akt Ser-473 phosphorylation independent of mTORC1 inhibition

Rapamycin, a natural product inhibitor of the Raptor-mammalian target of rapamycin complex (mTORC1), is known to induce Protein kinase B (Akt/PKB) Ser-473 phosphorylation in a subset of human cancer cell lines through inactivation of S6K1, stabilization of insulin receptor substrate (IRS)-1, and increased signaling through the insulin/insulin-like growth factor-I/phosphatidylinositol 3-kinase (PI3K) axis. We report that A-443654, a potent small-molecule inhibitor of Akt serine/threonine kinases, induces Akt Ser-473 phosphorylation in all human cancer cell lines tested, including PTEN- and TSC2-deficient lines. This phenomenon is dose-dependent, manifests coincident with Akt inhibition and likely represents an alternative, rapid-feedback pathway that can be functionally dissociated from mTORC1 inhibition. Experiments performed in TSC2-/- cells indicate that TSC2 and IRS-1 cooperate with, but are dispensable for, A-443654-mediated Akt phosphorylation. This feedback event does require PI3K activity, however, as it can be inhibited by LY294002 or wortmannin. Small interfering RNA-mediated knockdown of mTOR or Rictor, components of the rapamycin-insensitive mTORC2 complex, but not the mTORC1 component Raptor, also inhibited Akt Ser-473 phosphorylation induced by A-443654. Our data thus indicate that Akt phosphorylation and activity are coupled in a manner not previously appreciated and provide a novel mode of Akt regulation that is distinct from the previously described rapamycin-induced IRS-1 stabilization mechanism.

Quantitative analysis of Akt phosphorylation and activity in response to EGF and insulin treatment

The protein kinase Akt is a critical regulator of cell function and its overexpression and activation have been functionally linked to numerous pathologies such as cancer. Previous reports regarding the mechanism-regulating Akt's activation have revealed two phosphorylation events, at threonine 308 (T308) and serine 473 (S473), as necessary for the full activation of the kinase in response to insulin. For this reason and because of the availability of phospho-specific antibodies to both T308 and S473, many studies that focus on Akt's role in governing cell function rely on the measurement of these two sites to understand changes in kinase activity. Recent evidence, however, suggests the involvement of other phosphorylation sites; for example, in Src-transformed and epidermal growth factor (EGF)-treated cells, tyrosine phosphorylation has been found important for full kinase activation. In this study, we probed the quantitative reliability of using S473 and/or T308 phosphorylation as surrogates for Akt kinase activity across diverse treatment conditions. We performed quantitative Western blots and kinase activity assays on lysates generated during a 2h time course from two cell lines treated with either EGF or insulin. From the resulting approximately 250 quantitative measurements of phosphorylation and activity, we found that both T308 and S473 phosphorylation accurately captured quantitative changes in EGF-stimulated cells, but not in insulin-stimulated cells. Moreover, in all but one condition studied, we found a tight correlation between the onset of phosphorylation and dephosphorylation for both sites, despite the fact that they do not share common kinase- or phosphatase-mediated regulation. In sum, using a quantitative approach to study Akt activation identified ligand-dependent limits for the use of T308 or S473 as proxies for kinase activity and suggests the coregulation of Akt phosphorylation and dephosphorylation.

Activation of Integrin-Linked Kinase Is a Critical Prosurvival Pathway Induced in Leukemic Cells by Bone Marrow–Derived Stromal Cells

Integrin-linked kinase (ILK) directly interacts with beta integrins and phosphorylates Akt in a phosphatidylinositol 3-kinase (PI3K)-dependent manner. In this study, we examined the functional role of ILK activation in leukemic and bone marrow stromal cells on their direct contact. Coculture of leukemic NB4 cells with bone marrow-derived stromal mesenchymal stem cells (MSC) resulted in robust activation of multiple signaling pathways, including ILK/Akt, extracellular signal-regulated kinase 1/2 (ERK1/2), signal transducers and activators of transcription 3 (STAT3), and Notch1/Hes. Blockade of PI3K or ILK signaling with pharmacologic inhibitors LY294002 or QLT0267 specifically inhibited stroma-induced phosphorylation of Akt and glycogen synthase kinase 3beta, suppressed STAT3 and ERK1/2 activation, and decreased Notch1 and Hes1 expression in leukemic cells. This resulted in induction of apoptosis in both leukemic cell lines and in primary acute myelogenous leukemia samples that was not abrogated by MSC coculture. In turn, leukemic cells growing in direct contact with bone marrow stromal elements induce activation of Akt, ERK1/2, and STAT3 signaling in MSC, accompanied by significant increase in Hes1 and Bcl-2 proteins, which were all suppressed by QLT0267 and LY294002. In summary, our results indicate reciprocal activation of ILK/Akt in both leukemic and bone marrow stromal cells. We propose that ILK/Akt is a proximal signaling pathway critical for survival of leukemic cells within the bone marrow microenvironment. Hence, disruption of these interactions by ILK inhibitors represents a potential novel therapeutic strategy to eradicate leukemia in the bone marrow microenvironment by simultaneous targeting of both leukemic cells and activated bone marrow stromal cells.

Role of activator protein-1 on the effect of arginine-glycine-aspartic acid containing peptides on transforming growth factor-β1 promoter activity

While arginine-glycine-aspartic acid-based peptidomimetics have been employed for the treatment of cardiovascular disorders and cancer, their use in other contexts remains to be explored. Arginine-glycine-aspartic acid-serine induces Transforming growth factor-beta1 transcription in human mesangial cells, but the molecular mechanisms involved have not been studied extensively. We explored whether this effect could be due to Activator protein-1 activation and studied the potential pathways involved. Addition of arginine-glycine-aspartic acid-serine promoted Activator protein-1 binding to its cognate sequence within the Transforming growth factor-beta1 promoter as well as c-jun and c-fos protein abundance. Moreover, this effect was suppressed by curcumin, a c-Jun N terminal kinase inhibitor, and was absent when the Activator protein-1 cis-regulatory element was deleted. Activator protein-1 binding was dependent on the activity of integrin linked kinase, as transfection with a dominant negative mutant suppressed both Activator protein-1 binding and c-jun and c-fos protein increment. Integrin linked kinase was, in turn, dependent on Phosphoinositol-3 kinase activity. Arginine-glycine-aspartic acid-serine stimulated Phosphoinositol-3 kinase activity, and Transforming growth factor-beta1 promoter activation was abrogated by the use of Phosphoinositol-3 kinase specific inhibitors. In summary, we propose that arginine-glycine-aspartic acid-serine activates Integrin linked kinase via the Phosphoinositol-3 kinase pathway and this leads to activation of c-jun and c-fos and increased Activator protein-1 binding and Transforming growth factor-beta1 promoter activity. These data may contribute to understand the molecular mechanisms involved in the cellular actions of arginine-glycine-aspartic acid-related peptides and enhance their relevance as these products evolve into clinical therapeutic use.

Critical role for classical PKC in activating Akt by phospholipase A2-modified LDL in monocytic cells

OBJECTIVE

Modification of low density lipoprotein (LDL) by phospholipases confers pro-atherogenic properties, although signalling pathways of phospholipase-modified LDL (PLA-LDL) remain obscure. We questioned whether members of the protein kinase C (PKC) family are involved in PLA-LDL-induced Akt phosphorylation and survival of THP-1 monocytic cells.

METHODS

Akt phosphorylation in THP-1 cells was monitored by Western analysis. To modulate PKC expression cells were transfected with dominant-negative enhanced green fluorescent protein linked PKCalpha (PKCalpha-EGFP K368R) and PKCbeta (PKCbeta-EGFP K371M) constructs or with siRNA specific for PKCalpha/PKCbeta using nucleofection technology. Cell survival was assessed by Annexin V/propidium iodide staining or mitochondrial membrane potential measurement with 3,3'-dihexyloxacarbocyanine iodide (DiOC(6)) using flow cytometry.

RESULTS

Inhibitors of phospholipase C (PLC) or classical PKCs as well as PKC depletion following phorbol ester treatments, blocked Akt phosphorylation in response to PLA-LDL. In contrast, phosphatidylinositol 3-kinase (PI3K) activation by PLA-LDL was insensitive to PKC inhibition. Using RNA interference to knockdown PKCalpha and overexpression of dominant-negative PKCalpha as well as PKCbeta drastically lowered Akt phosphorylation after PLA-LDL. Moreover, inhibition of PKC attenuated a PLA-LDL-induced survival response towards oxidative stress in THP-1 cells.

CONCLUSION

We show that PKCalpha and PKCbeta are critical for PLA-LDL-induced Akt phosphorylation and survival in THP-1 monocytic cells.

Combinatorial activation of FAK and AKT by transforming growth factor-beta1 confers an anoikis-resistant phenotype to myofibroblasts

Transforming growth factor-beta (TGF-beta) is a prototypical tumour-suppressor cytokine with cytostatic and pro-apoptotic effects on most target cells; however, mechanisms of its pro-survival/anti-apoptotic signalling in certain cell types and contexts remain unclear. In human lung fibroblasts, TGF-beta1 is known to induce myofibroblast differentiation in association with the delayed activation of focal adhesion kinase (FAK) and protein kinase B (PKB/AKT). Here, we demonstrate that FAK and AKT are independently regulated by early activation of SMAD3 and p38 MAPK, respectively. Pharmacologic or genetic approaches that disrupt SMAD3 signalling block TGF-beta1-induced activation of FAK, but not AKT; in contrast, disruption of early p38 MAPK signalling abrogates AKT activation, but does not alter FAK activation. TGF-beta1 is able to activate AKT in cells expressing mutant FAK or in cells treated with an RGD-containing peptide that interferes with integrin signalling, inhibits FAK activation and induces anoikis (apoptosis induced by loss of adhesion signalling). TGF-beta1 protects myofibroblasts from anoikis, in part, by activation of the PI3K-AKT pathway. Thus, TGF-beta1 co-ordinately and independently activates the FAK and AKT protein kinase pathways to confer an anoikis-resistant phenotype to myofibroblasts. Activation of these pro-survival/anti-anoikis pathways in myofibroblasts likely contributes to essential roles of TGF-beta1 in tissue fibrosis and tumour-promotion.

Identification of Sin1 as an essential TORC2 component required for complex formation and kinase activity

Target of rapamycin (TOR) is an evolutionally conserved protein kinase in eukaryotes and a central cell growth controller. TOR exists in two distinct complexes, termed TORC1 and TORC2. Mammalian TORC2 has recently been shown to possess kinase activity toward the C-terminal hydrophobic site of Akt/PKB. Here, we report that Sin1 is an essential component of TORC2 but not of TORC1, and functions similarly to Rictor, the defining member of TORC2, in complex formation and kinase activity. Knockdown of Sin1decreases Akt phosphorylation in both Drosophila and mammalian cells and diminishes Akt function in vivo. It also disrupts the interaction between Rictor and mTOR. Furthermore, Sin1 is required for TORC2 kinase activity in vitro. Disruption of the Rictor gene in mice results in embryonic lethality and ablates Akt phosphorylation. These data demonstrate that Sin1 together with Rictor are key components of mTORC2 and play an essential role in Akt phosphorylation and signaling.

Targeted ablation of ILK from the murine heart results in dilated cardiomyopathy and spontaneous heart failure

A requirement for integrin-mediated adhesion in cardiac physiology is revealed through targeted deletion of integrin-associated genes in the murine heart. Here we show that targeted ablation of the integrin-linked kinase (ILK) expression results in spontaneous cardiomyopathy and heart failure by 6 wk of age. Deletion of ILK results in disaggregation of cardiomyocytes, associated with disruption of adhesion signaling through the beta1-integrin/FAK (focal adhesion kinase) complex. Importantly, the loss of ILK is accompanied by a reduction in cardiac Akt phosphorylation, which normally provides a protective response against stress. Together, these results suggest that ILK plays a central role in protecting the mammalian heart against cardiomyopathy and failure.

Hsp90 functions to balance the phosphorylation state of Akt during C2C12 myoblast differentiation

The function of the 90-kDa heat shock protein (Hsp90) is essential for the regulation of a myriad of signal transduction cascades that control all facets of a cell's physiology. Akt (PKB) is an Hsp90-dependent serine-threonine kinase that plays critical roles in the regulation of muscle cell physiology, including roles in the regulation of muscle differentiation and anti-apoptotic responses that modulate cell survival. In this report, we have examined the role of Hsp90 in regulating the activity of Akt in differentiating C2C12 myoblasts. While long-term treatment of differentiating C2C12 cells with the Hsp90 inhibitor geldanamycin led to the depletion of cellular Akt levels, pulse-chase analysis indicated that geldanamycin primarily enhanced the turnover rate of newly synthesized Akt. Hsp90 maintained an interaction with mature Akt, while Cdc37, Hsp90's kinase-specific co-chaperone, was lost from the chaperone complex upon Akt maturation. Geldanamycin partially disrupted the interaction of Cdc37 with Akt, but had a much less significant effect on the interaction of Hsp90 with Akt. Surprisingly, short-term treatment of differentiating C2C12 with geldanamycin increased the phosphorylation of Akt on Ser473, an effect mimicked by treatment of C2C12 cells with okadaic acid or the Hsp90 inhibitor novobiocin. Furthermore, Akt was found to interact directly with catalytic subunit of protein phosphatase 2A (PP2Ac) in C2C12 cells, and this interaction was not disrupted by geldanamycin. Thus, our findings indicate that Hsp90 functions to balance the phosphorylation state of Akt by modulating the ability of Akt to be dephosphorylated by PP2Ac during C2C12 myoblast differentiation.

Signalling via integrins: implications for cell survival and anticancer strategies

Integrin-associated signalling renders cells more resistant to genotoxic anti-cancer agents like ionizing radiation and chemotherapeutic substances, a phenomenon termed cell adhesion-mediated radioresistance/drug resistance (CAM-RR, CAM-DR). Integrins are heterodimeric cell-surface molecules that on one side link the actin cytoskeleton to the cell membrane and on the other side mediate cell-matrix interactions. In addition to their structural functions, integrins mediate signalling from the extracellular space into the cell through integrin-associated signalling and adaptor molecules such as FAK (focal adhesion kinase), ILK (integrin-linked kinase), PINCH (particularly interesting new cysteine-histidine rich protein) and Nck2 (non-catalytic (region of) tyrosine kinase adaptor protein 2). Via these molecules, integrin signalling tightly and cooperatively interacts with receptor tyrosine kinase signalling to regulate survival, proliferation and cell shape as well as polarity, adhesion, migration and differentiation. In tumour cells of diverse origin like breast, colon or skin, the function and regulation of these molecules is partly disturbed and thus might contribute to the malignant phenotype and pre-existent and acquired multidrug resistance. These issues as well as a variety of therapeutic options envisioned to influence tumour cell growth, metastasis and resistance, including kinase inhibitors, anti-integrin antibodies or RNA interference, will be summarized and discussed in this review.

Integrin-linked kinase interacts with caspase-9 and -8 in an adhesion-dependent manner for promoting radiation-induced apoptosis in human leukemia cells

Integrin-mediated adhesion of leukemia cells to extracellular matrix proteins reduces apoptosis following radiation-induced genotoxic injury. To evaluate the role of integrin-linked kinase (ILK) in this process, HL60 human acute promyelocytic leukemia cells were stably transfected with ILK wild-type or kinase-hyperactive overexpression vectors. Suspension or fibronectin (FN) adhesion cultures were irradiated with X-rays and processed for measurement of apoptosis, mitochondrial transmembrane potential and caspase activation. Adhesion to FN pronouncedly reduced radiation-induced apoptosis of HL60 cells and vector controls. Intriguingly, overexpressed ILK enhanced apoptosis after irradiation by combined activation of caspase-3 through caspase-8 and -9 in irradiated FN cultures. Irradiation of ILK suspension cultures lacked caspase-8 activation, but showed serial cleavage of caspase-9, -3 and poly (ADP-ribose) polymerase. These findings further characterize the cell death-promoting function of ILK in DNA-damaged cells. Moreover, ILK might represent a potential therapeutic target for innovative chemo- and radiooncological approaches in hematological malignancies.

Quantitative analysis of anti-apoptotic function of Akt in Akt1 and Akt2 double knock-out mouse embryonic fibroblast cells under normal and stressed conditions

The serine/threonine kinases Akt1/PKBalpha, Akt2/PKBbeta, and Akt3/PKBgamma have been implicated in preventing cells from undergoing apoptosis. Although several small molecule inhibitors of Akt have been reported to induce apoptosis in cancer cells, these inhibitors may have additional targets. In the current study, we used an Akt3 small interfering RNA (Akt3 siRNA) to analyze apoptosis induction in Akt1 and Akt2 double knock-out mouse embryonic fibroblast cells (MEF-Akt1,2-DKO). Our data indicated that Akt3 siRNA inhibited Akt3 protein expression in a dose-dependent manner. As a result, phosphorylation of Akt and its downstream targets, including FKHRL1 and GSK3alpha/beta, were reduced accordingly. The treatment also induced apoptosis in MEF-Akt1,2-DKO cells. However, apoptosis induction is significant only when more than 80% of Akt3 protein was depleted. Reintroducing Akt3 totally rescued Akt3-siRNA-induced apoptosis in MEF-Akt1,2-DKO cells. In addition, reintroducing Akt1 also inhibited apoptosis induced by Akt3 siRNA. Moreover, Akt3 siRNA potentiated different stress-induced apoptosis in MEF-Akt1,2-DKO cells at a lower dose when compared with what is required for apoptosis induction by itself. Our study suggests that only a small portion of Akt is active in wild-type MEF cells and a threshold of Akt inhibition is required to induce apoptosis by pure Akt inhibitors. In addition, our data indicate that cells under stress require more Akt for its survival.

Integrin-linked kinase, a novel component of the cardiac mechanical stretch sensor, controls contractility in the zebrafish heart

The vertebrate heart possesses autoregulatory mechanisms enabling it first to sense and then to adapt its force of contraction to continually changing demands. The molecular components of the cardiac mechanical stretch sensor are mostly unknown but of immense medical importance, since dysfunction of this sensing machinery is suspected to be responsible for a significant proportion of human heart failure. In the hearts of the ethylnitros-urea (ENU)-induced, recessive embryonic lethal zebrafish heart failure mutant main squeeze (msq), we find stretch-responsive genes such as atrial natriuretic factor (anf) and vascular endothelial growth factor (vegf) severely down-regulated. We demonstrate through positional cloning that heart failure in msq mutants is due to a mutation in the integrin-linked kinase (ilk) gene. ILK specifically localizes to costameres and sarcomeric Z-discs. The msq mutation (L308P) reduces ILK kinase activity and disrupts binding of ILK to the Z-disc adaptor protein beta-parvin (Affixin). Accordingly, in msq mutant embryos, heart failure can be suppressed by expression of ILK, and also of a constitutively active form of Protein Kinase B (PKB), and VEGF. Furthermore, antisense-mediated abrogation of zebrafish beta-parvin phenocopies the msq phenotype. Thus, we provide evidence that the heart uses the Integrin-ILK-beta-parvin network to sense mechanical stretch and respond with increased expression of ANF and VEGF, the latter of which was recently shown to augment cardiac force by increasing the heart's calcium transients.

TcR and TcR-CD28 engagement of protein kinase B (PKB/AKT) and glycogen synthase kinase-3 (GSK-3) operates independently of guanine nucleotide exchange factor VAV-1

TcRzeta/CD3 and TcRzeta/CD3-CD28 signaling requires the guanine nucleotide exchange factor (GEF) Vav-1 as well as the activation of phosphatidylinositol 3-kinase, protein kinase B (PKB/AKT), and its inactivation of glycogen synthase kinase-3 (GSK-3). Whether these two pathways are connected or operate independently of each other in T-cells has been unclear. Here, we report that anti-CD3 and anti-CD3/CD28 can induce PKB and GSK-3alpha phosphorylation in the Vav-1(-/-) Jurkat cell line J. Vav.1 and in primary CD4-positive Vav-1(-/-) T-cells. Reduced GSK-3alpha phosphorylation was observed in Vav-1,2,3(-/-) T-cells together with a complete loss of FOXO1 phosphorylation. Furthermore, PKB and GSK-3 phosphorylation was unperturbed in the presence of GEF-inactive Vav-1 that inhibited interleukin-2 gene activation and a form of Src homology 2 domain-containing lymphocytic protein of 76-kDa (SLP-76) that is defective in binding to Vav-1. The pathway also was intact under conditions of c-Jun N-terminal kinase (JNK) inhibition and disruption of the actin cytoskeleton by cytochalasin D. Both events are down-stream targets of Vav-1. Overall, our findings indicate that the TcR and TcR-CD28 driven PKB-GSK-3 pathway can operate independently of Vav-1 in T-cells.

Novel role for integrin-linked kinase in modulation of coxsackievirus B3 replication and virus-induced cardiomyocyte injury

Viral myocarditis is a major cause of sudden cardiac death in children and young adults. Among viruses, coxsackievirus B3 (CVB3) is the most common agent for myocarditis. Recently, more consideration has been given to the role of signaling pathways in pathogenesis of enteroviral myocarditis, providing new platform for identifying a new potential therapeutic target for this, so far, incurable disease. Previously, we reported on the role of the protein kinase-B/Akt in CVB3 replication and virus-induced cell injury. Here, we report on regulation of virus-induced Akt activation by the integrin-linked kinase in infected mouse cardiomyocytes and HeLa cells. This study also presents the first observation that inhibition of ILK in CVB3-infected cells significantly improves the viability of infected cells, while blocking viral replication and virus release. Complementary experiments using a constitutively active form of Akt1 revealed that the observed protective effect of ILK inhibition is dependent on the associated downregulation of virus-induced Akt activation. To our knowledge, this is the first report of such beneficial effects of ILK inhibition in a viral infection model and conveys new insights in our efforts to characterize a novel therapeutic target for treatment of enteroviral myocarditis.

p38 MAPK downregulates phosphorylation of Bad in doxorubicin-induced endothelial apoptosis

Doxorubicin is the anthracycline with the widest spectrum of antitumor activity, and it has been shown that the antitumor activity is mediated in vivo by selective triggering of apoptosis in proliferating endothelial cells. We studied cultured human endothelial cells and observed that doxorubicin-induced apoptosis was mediated by p38 mitogen-activated protein kinase (MAPK). Doxorubicin-provoked apoptosis was significantly inhibited by expression of dominant negative p38 MAPK or pharmacological inhibition with SB203580. Furthermore, blocking phosphatidylinositol-3-kinase/Akt signaling significantly increased doxorubicin-induced caspase-3 activity and cell death, indicating that Akt is a survival factor in this system. Notably, we also found that doxorubicin-provoked apoptosis included p38 MAPK-mediated inhibition of Akt and Bad phosphorylation. Furthermore, doxorubicin-stimulated phosphorylation of Bad in cells expressing dominant negative p38 MAPK was impeded by the inhibition of PI3-K. In addition to the impact on Bad phosphorylation, doxorubicin-treatment caused p38 MAPK-dependent downregulation of Bcl-xL protein.

Integrin-linked kinase, a hypoxia-responsive molecule, controls postnatal vasculogenesis by recruitment of endothelial progenitor cells to ischemic tissue

BACKGROUND

Recruitment and adhesion of endothelial progenitor cells (EPCs) to hypoxic endothelial cells (ECs) is essential for vasculogenesis in ischemic tissue; little is known, however, about the key signals or intracellular signaling pathways involved in orchestrating the expression of adhesion molecules by ECs in response to hypoxia and how this is related to the recruitment of EPCs to the ischemic tissue. Here, we report that endogenous integrin-linked kinase (ILK) is a novel molecule that responds to hypoxia in ECs that regulates the expression of stromal cell-derived factor-1 (SDF-1) and intercellular adhesion molecule-1 (ICAM-1) through nuclear factor-kappaB and hypoxia-inducible factor-1alpha and induces recruitment of EPCs to ischemic areas.

METHODS AND RESULTS

Under hypoxia, both the endogenous amount and kinase activity of ILK were time-dependently upregulated in ECs, which was associated with increased ICAM-1 and SDF-1. This upregulation of ILK was mediated by stabilization of ILK by heat shock protein 90. ILK overexpression in normoxic ECs resulted in ICAM-1 and SDF-1 upregulation through dual control by nuclear factor-kappaB and hypoxia-inducible factor-1alpha. Blockade of ILK in hypoxic ECs significantly abrogated the expression of both molecules, which led to decreased EPC incorporation into ECs. A hindlimb ischemia model showed that ILK blockade significantly reduced EPC homing to ischemic limb and consequently led to poor neovascularization. Overexpression of ILK in the Matrigel plug significantly improved neovascularization in vivo, whereas the blockade of ILK resulted in the opposite effect.

CONCLUSIONS

Endogenous ILK is a novel and physiological upstream responder of numerous intracellular molecules involved in hypoxic stress in ECs and may control the recruitment of EPCs to ischemic tissue.

Modulation of Wnt3a-mediated nuclear beta-catenin accumulation and activation by integrin-linked kinase in mammalian cells

The Wnt gene family encodes secreted signaling molecules that play important roles in tumorgenesis and embryogenesis. The canonical Wnt signaling pathway regulates target gene expression via the stabilization and nuclear translocation of the cytoplasmic pool of beta-catenin. The activation of integrin-linked kinase (ILK) is also known to regulate the stabilization and subsequent nuclear translocation of beta-catenin in several epithelial cell models. We now report that molecular and pharmacological inhibition of ILK activity in mammalian cells directly modulates Wnt signaling by suppressing the stabilization and nuclear translocation of beta-catenin, as well as beta-catenin/Lef-mediated transcription. Inhibition of ILK activity, but not phosphatidylinositol-3 kinase (PI3K) or MEK activities suppresses nuclear beta-catenin stabilization in cells stably expressing Wnt3a as well as in cells exposed to either Wnt3a conditioned media or purified Wnt3a. Furthermore, ILK inhibition reverses the Wnt3a-induced suppression of beta-catenin phosphorylation that accompanies beta-catenin stabilization. In addition, we show that ILK can be identified in a complex with Wnt pathway components such as adenomatous polyposis coli and GSK-3. Upon treatment of L cells with Wnt3a-CM, glycogen synthase kinase-3 (GSK-3beta) becomes highly phosphorylated on Ser 9, which is completely abolished upon inhibition of ILK activity. However, acute exposure of L cells to purified Wnt3a does not result in the stimulation of GSK-3beta Ser 9 phosphorylation, despite beta-catenin stabilization. Together our data demonstrate that ILK activity can modulate acute Wnt3a mediated beta-catenin phosphorylation, stabilization and nuclear activation in a PI3K-independent manner, as well as the more prolonged PI3K-dependent secondary effects of Wnt signaling on GSK-3 phosphorylation. Finally, we suggest that a novel small molecule inhibitor of ILK, QLT-0267, may be a useful tool in the regulation of pathological Wnt signaling.

Integrin-linked kinase activity is associated with interleukin-1 alpha-induced progressive behavior of pancreatic cancer and poor patient survival

Cancer cell adhesion and invasion into extracellular matrix are regulated by integrin-linked kinase (ILK) activity in a phosphatidylinositol 3-kinase (PI3-K)-dependent manner. In this study, we demonstrated that ILK and beta(1)-integrin play important roles in interleukin (IL)-1alpha-induced enhancement of adhesion and invasion of pancreatic cancer cells through p38 mitogen-activated protein kinase (MAPK) signaling pathway and activator protein-1 (AP-1) activation. Alteration of ILK kinase activity controlled IL-1alpha-induced p38 MAPK phosphorylation and its downstream AP-1 activation with subsequent regulation of pancreatic cancer cell adhesion and invasion. Overexpressed ILK enhances the IL-1alpha-induced p38 MAPK phosphorylation more strongly through glycogen synthase kinase 3 (GSK-3) activation, and subsequently induces AP-1 activation, which promotes aggressive capabilities of pancreatic cancer cells. In contrast, knockdown of ILK kinase activity inhibits the IL-1alpha-induced activation of MAPK/AP-1 pathway via inhibition of GSK-3 phosphorylation. In immunohistochemical analysis, statistically significant association between strong expression of ILK and poor prognosis of pancreatic cancer patients were observed, and strong expression of ILK in cancerous tissues can be a significant prognostic indicator of pancreatic cancer patients. Our results suggest that ILK is involved with aggressive capability in pancreatic cancer and that these regulations can be helpful to understand biological processes for a better translational treatment for pancreatic cancer patients.

Modulation of response to tumor therapies by the extracellular matrix

The composition of the extracellular matrix in tumors is vastly different from that found in the normal tissue counterparts. As the extracellular matrix can signal to cells via integrin binding and activation, which is known to modulate cell proliferation, survival and migration, it may influence the response of both tumor and endothelial cells to anticancer therapies. Certain tumor-associated extracellular matrix proteins have been shown to confer resistance to chemotherapeutic drugs, radiation and anti-angiogenic factors. The current literature regarding this phenomenon and the potential therapeutic modalities to overcome extracellular matrix-induced resistance will be discussed.