Integrin-linked protein kinase regulates fibronectin matrix assembly, E-cadherin expression, and tumorigenicity

Ganglioside loss promotes survival primarily by activating integrin-linked kinase/Akt without phosphoinositide 3-OH kinase signaling

Keratinocyte gangliosides influence cellular functions, including proliferation, adhesion, migration, and differentiation. The effects of endogenous depletion of membrane gangliosides by gene transfection of a human ganglioside-specific sialidase on cell survival were investigated. Ganglioside depletion promotes survival of the human keratinocyte-derived SCC12 cell line through upregulated phosphorylation of beta1 integrin, and increased phosphorylation and activity of integrin-linked kinase, protein kinase B/Akt, and Bad, with resultant inhibition of caspase-9 activation. Ganglioside deficiency also increases expression of cyclins D1 and E, promoting cell cycle progression from G1 phase to S phase. Inhibition of either protein kinase B/Akt or integrin-linked kinase activity renders the ganglioside-deficient cells susceptible to triggers of apoptosis. Both serine-473 and threonine-308 sites of protein kinase B/Akt show increased phosphorylation in ganglioside-deficient cells, but the cell survival correlates with increased phosphorylation of the serine-473 site of Akt, not with increased phosphorylation of the threonine-308 site. Consistently, blockade of ganglioside GT1b function activates integrin-linked kinase and only the serine-473 site of protein kinase B/Akt. In contrast, antibody-induced blockade of GM3 function increases only threonine-308 phosphorylation of ganglioside-deficient cells. Whereas blockade of phosphoinositide 3-OH kinase function suppresses threonine-308 phosphorylation, it neither inhibits serine-473 phosphorylation nor triggers apoptosis. These data suggest that ganglioside depletion modulates cell survival primarily through protein kinase B/Akt stimulation by a pathway that does not require phosphoinositide 3-OH kinase and epidermal growth factor receptor signaling.

Protection against anoikis and down-regulation of cadherin expression by a regulatable beta-catenin protein

beta-Catenin signaling plays a key role in a variety of cellular contexts during embryonic development and tissue differentiation. Aberrant beta-catenin signaling has also been implicated in promoting human colorectal carcinomas as well as a variety of other cancers. To study the molecular and cellular biological functions of beta-catenin in a controlled fashion, we created a regulatable form of activated beta-catenin by fusion to a modified estrogen receptor (ER) ligand binding domain (G525R). Transfection of tissue culture cells with expression vectors encoding this hybrid protein allows the signal transduction function of beta-catenin to be induced by the synthetic estrogen, 4-hydroxytamoxifen, leading to regulated activation of a beta-catenin-lymphocyte enhancer-binding factor-dependent reporter gene as well as induction of endogenous cyclin D1 expression. The activation of ER-beta-catenin signaling rescues RK3E cells from anoikis and correlates with an increased phosphorylation of mitogen-activated protein kinase. The inhibition of anoikis by ER-beta-catenin can be abolished by a mitogen-activated protein kinase pathway inhibitor, PD98059. Evidence is also provided to show that ER-beta-catenin down-regulates cadherin protein levels. These findings support a key role for activated beta-catenin signaling in processes that contribute to tumor formation and progression.

Regulation of adhesive interaction between podocytes and glomerular basement membrane

Glomerular filtration depends on well-orchestrated cell-cell and cell-matrix contacts of glomerular podocytes. Over the last years critical constituents of these contacts have been identified via molecular approaches. Podocyte cell-matrix interactions have been shown to be mediated in part by alpha(3)beta(1)-integrin heterodimers. Disturbances of integrin matrix interaction lead to detachment of podocytes in vitro, corresponding to the critical event of foot process retraction and glomerular basement membrane (GBM) denudation in vivo. Further, dystroglycan-mediated matrix attachment appears to play a critical role for podocyte foot process architecture. Downstream signaling events are currently elucidated concentrating mainly on integrin-dependent cascades and their consequences for podocyte adhesion and proliferation. An activation of the integrin-linked kinase in podocyte damage in vivo and in vitro makes this molecule a particularly interesting candidate for integrin-mediated inside-out and outside-in signaling in podocytes. Podocyte cell-cell interaction has been characterized in a few studies in vitro, indicating the slit diaphragm to be a modified adherens junction. The structural link between the cell-matrix and cell-cell contacts is maintained by the actin cytoskeleton, which may also enable cross-talk between these two cell contact sites. Examining podocyte function in tissue culture, animal models and human expression studies should allow further detailed dissection of the molecular pathways responsible for maintenance and failure of the glomerular filtration barrier.

C. elegans PAT-4/ILK functions as an adaptor protein within integrin adhesion complexes

BACKGROUND

Mammalian integrin-linked kinase (ILK) was identified in a yeast two-hybrid screen for proteins binding the integrin beta(1) subunit cytoplasmic domain. ILK has been implicated in integrin-mediated signaling and is also an adaptor within integrin-associated cytoskeletal complexes.

RESULTS

We identified the C. elegans pat-4 gene in previous genetic screens for mutants unable to assemble integrin-mediated muscle cell attachments. Here, we report that pat-4 encodes the sole C. elegans homolog of ILK. In pat-4 null mutants, embryonic muscle cells form integrin foci, but the subsequent recruitment of vinculin and UNC-89 as well as actin and myosin filaments to these in vivo focal adhesion analogs is blocked. Conversely, PAT-4/ILK requires the ECM component UNC-52/perlecan, the transmembrane protein integrin, and the novel cytoplasmic attachment protein UNC-112 to be properly recruited to nascent attachments. Transgenically expressed "kinase-dead" ILK fully rescues pat-4 loss-of-function mutants. We also identify UNC-112 as a new binding partner for ILK.

CONCLUSIONS

Our data strengthens the emerging view that ILK functions primarily as an adaptor protein within integrin adhesion complexes and identifies UNC-112 as a new ILK binding partner.

Promoter characterization and genomic organization of the gene encoding integrin-linked kinase 1

Integrin-linked kinase (ILK)-1 is a 59-kDa serine-threonine protein kinase, which associates with the cytoplasmic domain of beta 1, beta 2 and beta 3 integrins and acts as a receptor proximal kinase regulating integrin-mediated signal transduction. We have recently identified an isoform of ILK (ILK-2), which is expressed, in a TGF-beta 1-dependent manner, in a highly invasive tumor cell line but not in normal adult tissues. In contrast, ILK-1 is ubiquitously expressed in normal tissues and is up-regulated in various tumors independent of TGF-beta 1. Here, we report the structural organization and the promoter activity of the human ILK-1 gene, contained within a 8.8-kb genomic fragment cloned from a human BAC library. The mature protein is encoded by 13 exons. The last coding exon contains the entire 3' UTR of the ILK-1 gene, which overlaps with the complementary 3' UTR sequence of the TAF2H gene, a TATA box binding protein-associated factor. A major transcriptional initiation start site was found 138 bp upstream of exon 1 in close proximity to a consensus initiator element (Inr). The ILK gene is transcribed by a TATA-less and CAAT-less promoter with typical features of housekeeping genes. The promoter activity was characterized by a luciferase reporter assay and the minimal sequence conferring promoter activity was 349 bp in size and located immediately upstream of exon 1.

Decreased nitric oxide synthesis in human endothelial cells cultured on type I collagen

Endothelial dysfunction, considered as a defective vascular dilatation after certain stimuli, is characteristic of different pathological conditions, such as hypertension, atherosclerosis, or diabetes. A decreased synthesis or an increased degradation of nitric oxide (NO) has been postulated as the mechanism responsible for this alteration. The present experiments were designed to test the hypothesis that the presence of an abnormal extracellular matrix in vessel walls could be responsible for the decreased NO synthesis observed in these pathological conditions. Experiments were performed in cultured human umbilical vein endothelial cells (HUVECs) grown on type IV (Col. IV) or type I (Col. I) collagen. Cells seeded on Col. I showed decreased nitrite synthesis, nitric oxide synthase activity, eNOS protein content, and eNOS mRNA expression when compared with cells grown on Col. IV. Moreover, cells grown on Col. I failed to respond to glucose oxidase activation of the eNOS system. In both cases, the changes in the eNOS mRNA expression seemed to depend on the modulation of eNOS promoter activity. The downregulation of eNOS induced by Col. I was blocked by D6Y, a peptide that interferes with the Col. I-dependent signals through integrins, as well as by specific anti-integrin antibodies. Moreover, a decreased activation of integrin-linked kinase (ILK) may explain the effects observed in Col. I-cultured cells because the activity of this kinase was decreased in these cells and ILK modulation prevented the Col. I-induced changes in HUVECs. Taken together, these findings may contribute to explaining the basis of endothelial dysfunction in some vascular diseases.

The paxillin LD motifs

Adapter/scaffold proteins, through their multidomain structure, perform a fundamental role in facilitating signal transduction within cells. Paxillin is a focal adhesion adapter protein implicated in growth factor- as well as integrin-mediated signaling pathways. The amino-terminus of paxillin contains five leucine-rich sequences termed LD motifs. These paxillin LD motifs are highly conserved between species as well as within the paxillin superfamily. They mediate interactions with several structural and regulatory proteins important for coordinating changes in the actin cytoskeleton associated with cell motility and cell adhesion as well as in the regulation of gene expression.

Emerging targets in the AKT pathway for treatment of androgen-independent prostatic adenocarcinoma

Prostatic adenocarcinoma (CaP) is the most common, non-cutaneous malignancy and the second-leading cause of cancer death in men. The disease has two distinct phases: the androgen-dependent phase, which can be treated effectively with androgen ablation therapies, and the androgen-independent phase, for which there is no effective life-prolonging therapy. An estimated 32,000 men will die this year from androgen-independent, metastatic CaP. Efforts to understand the metastatic progression of CaP and the emergence of androgen-independent disease have begun to illuminate the molecular events involved. Recent work suggests that CaP progression to androgen-independent, metastatic disease involves a dampened apoptotic response, a release from the cell cycle block that initially follows androgen withdrawal and a shift from dependence on paracrine-derived growth and survival factors to autonomous production of these key proteins. Functional loss of the tumour suppressor phosphatase and tensin homologue deleted on chromosome ten (PTEN) and subsequent activation of the AKT pathway, have been prominently implicated in the progression of CaP to androgen-independence. Activation of the AKT pathway can suppress the apoptotic response, undermine cell cycle control and selectively enhance the production of key growth and survival factors. Though many proteins and intracellular signalling pathways can influence these biological processes, activation of the AKT pathway may be a particularly potent signal involved in CaP progression to androgen-independence and therefore presents a series of potential targets for therapy of advanced androgen-independent CaP.

Integrin-linked kinase, a promising cancer therapeutic target: biochemical and biological properties

Integrin-linked kinase (ILK) is an ankyrin repeat-containing Ser/Thr kinase that interacts with the cytoplasmic domains of beta(1) and beta(3) integrins. ILK is widely expressed in tissues throughout the body, and, as might be expected, appears to mediate a diversity of functions relating to its role in coupling integrins and growth factor receptors to downstream signaling pathways. Through its downstream targets protein kinase B/Akt and glycogen synthase kinase-3beta, ILK appears to be involved in several oncogenesis-related events, including suppression of apoptosis and promotion of cell survival, as well as cell migration and invasion. Over-expression of ILK in epithelial cells results in anchorage-independent cell growth with increased cell cycle progression. Inoculation of nude mice with ILK over-expressing cells leads to tumor formation. Furthermore, increased ILK expression and activity have been correlated with malignancy in several human tumor types, including breast, prostate, brain, and colon carcinomas. Based on these findings, ILK represents an excellent therapeutic target for the prevention of tumor progression. Here, we provide an overview of the physical and biochemical properties of ILK, and present data describing the impact of small-molecule ILK inhibitors on several ILK-mediated cellular functions.

Molecular dissection of actopaxin-integrin-linked kinase-Paxillin interactions and their role in subcellular localization

Paxillin is a focal adhesion adapter protein involved in integrin signaling. We have recently reported that the paxillin LD1 motif acts as a binding interface for both the actin-binding protein actopaxin and the serine/threonine integrin-linked kinase (ILK). In this report we demonstrate the direct association between actopaxin and ILK and dissect the role of the respective interactions in their subcellular localization. Co-immunoprecipitation experiments were employed to map the binding sites on ILK and actopaxin. ILK binds to the CH2 domain of actopaxin. However, an actopaxin CH2 domain mutant defective for paxillin binding (paxillin binding subdomain mutant) retains the capacity to bind ILK, indicating that paxillin and ILK binding sites on actopaxin are distinct. Actopaxin binds to the C terminus of ILK. Despite the direct binding between actopaxin and ILK, mutation analysis confirmed a primary role for paxillin in their localization to focal adhesions. Interestingly, an ILK mutant (E359K) that was previously reported to act as dominant negative for ILK function was unable to bind actopaxin or paxillin and failed to localize to focal adhesions. This mutant also exhibited in vitro kinase activity comparable with wild-type ILK. Taken together, these data suggest that normal ILK signaling is dependent on efficient localization involving multiple protein interactions.

Protection against oxidized low-density lipoprotein-induced vascular endothelial cell death by integrin-linked kinase

BACKGROUND

Integrin-linked kinase (ILK) is a protein that plays important roles in extracellular matrix-mediated signaling. It has been shown that ILK is expressed preferentially in cardiac and skeletal muscles. Evidence points to its role as an upstream regulator of protein kinase B, a critical player in apoptosis. Because oxidized LDL (oxLDL) is thought to promote atherogenesis by causing the apoptosis of endothelial cells, we investigated the potential roles that ILK may play in oxLDL-induced apoptosis in vascular endothelial cells.

METHODS AND RESULTS

Transcriptional and translational levels of ILK were investigated with reverse-transcriptase polymerase chain reaction and Western analysis. oxLDL treatment induced both the transcription and the translation of the ILK gene in endothelial cells. A recombinant adenovirus vector encoding the ILK gene was constructed to investigate its potential role in oxLDL-induced apoptosis in human umbilical vein endothelial cells and mouse lymphoid vein endothelial cells transformed by simian virus 40. In both types of cells, overexpression of the ILK gene significantly prevented oxLDL-induced apoptosis or cell death, as evaluated by 2 independent assay methods. Furthermore, we showed that ILK could inhibit oxLDL-induced upregulation of the kinase activity of p38 mitogen-activated protein kinase, which is often associated with stress-induced pro-apoptotic signal transduction. Finally, examination of other factors, such as bcl-2, bcl-xl, caspase 3, and caspase 9, demonstrated significant changes that were correlated with oxLDL treatment and ILK overexpression.

CONCLUSIONS

ILK may be an important factor involved in the regulation of oxLDL-induced apoptosis in vascular endothelial cells. Modifying its activity may be a useful approach for prevention of endothelial cell injury in oxLDL-induced atherosclerosis.

Inhibition of integrin-linked kinase/protein kinase B/Akt signaling: mechanism for ganglioside-induced apoptosis

Ganglioside GT1b inhibits keratinocyte attachment to and migration on a fibronectin matrix by binding to alpha(5)beta(1) and preventing alpha(5)beta(1) interaction with fibronectin. The role of gangliosides in triggering keratinocyte apoptosis, however, is unknown. Addition of GT1b to keratinocyte-derived SCC12 cells, grown in serum-free medium but exposed to fibronectin, suppressed Bad phosphorylation, activated caspase-9, and inhibited cyclin D and E expression, resulting in cell cycle arrest at G(1) phase and initiation of apoptosis. The mechanism of GT1b activation of caspase-9 involved inhibition of beta(1) integrin serine/threonine phosphorylation and decreased phosphorylation of both integrin-linked kinase and protein kinase B/Akt at its Ser-473 site, leading to cytochrome c release from mitochondria. Consistently, blockade of GT1b function with anti-GT1b antibody specifically activated the Ser-473 site of Akt, markedly suppressing apoptosis. The ganglioside-induced inhibition of Akt phosphorylation was GT1b-specific and was not observed when cells were treated with other keratinocyte gangliosides, including GD3. These studies suggest that the modulation of keratinocyte cell cycle and survival by GT1b is mediated by its direct interaction with alpha(5)beta(1) and resultant inhibition of the integrin/integrin-linked kinase/protein kinase B/Akt signaling pathway.

Integrin-linked kinase controls neurite outgrowth in N1E-115 neuroblastoma cells

Mouse N1E-115 cells grown on a laminin matrix exhibit neurite outgrowth in response to serum deprivation. Treatment of cells with an antibody against beta(1) integrin inhibits neurite outgrowth. Thus, beta(1) integrin is involved in the neuritogenesis of N1E-115 cells on a laminin matrix. Integrin-linked kinase (ILK), a recently identified cytoplasmic serine/threonine protein kinase that binds to the cytoplasmic domain of beta(1) integrin, has an important role in transmembrane signal transduction via integrins. We report that ILK is expressed in N1E-115 cells, the expression levels of which are constant under both normal and differentiating conditions. A stable transfection of a kinase-deficient mutant of ILK (DN-ILK) results in inhibition of neurite outgrowth in serum-starved N1E-115 cells grown on laminin. On the other hand, a transient expression of wild type ILK stimulated neurite outgrowth. The ILK activity in the parental cells was transiently activated after seeding on the laminin matrix, whereas that in the DN-ILK-transfected cells was not. These results suggest that transient activation of ILK is required for neurite outgrowth in serum-starved N1E-115 cells on laminin. Under the same conditions, p38 mitogen-activated protein (MAP) kinase, but neither MAP kinase/extracellular signal-regulated kinase kinase (MEK) nor extracellular signal-regulated kinases (ERK), was transiently activated after N1E-115 cell attachment to laminin, but not in the DN-ILK-expressed cells. The time course of p38 MAP kinase activation was very similar to that of ILK activation. Furthermore, a p38 MAP kinase inhibitor, SB203580, significantly blocked neurite outgrowth. Thus, activation of p38 MAP kinase is involved in ILK-mediated signal transduction leading to integrin-dependent neurite outgrowth in N1E-115 cells.

Integrin-linked kinase (ILK) and its interactors: a new paradigm for the coupling of extracellular matrix to actin cytoskeleton and signaling complexes

How intracellular cytoskeletal and signaling proteins connect and communicate with the extracellular matrix (ECM) is a fundamental question in cell biology. Recent biochemical, cell biological, and genetic studies have revealed important roles of cytoplasmic integrin-linked kinase (ILK) and its interactive proteins in these processes. Cell adhesion to ECM is an important process that controls cell shape change, migration, proliferation, survival, and differentiation. Upon adhesion to ECM, integrins and a selective group of cytoskeletal and signaling proteins are recruited to cell matrix contact sites where they link the actin cytoskeleton to the ECM and mediate signal transduction between the intracellular and extracellular compartments. In this review, we discuss the molecular activities and cellular functions of ILK, a protein that is emerging as a key component of the cell-ECM adhesion structures.

The distribution and regulation of integrin-linked kinase in normal and diabetic kidneys

Alteration in cell adhesion and extracellular matrix deposition is a hallmark of diabetic glomerulosclerosis. Integrin-linked kinase (ILK) is a recently identified integrin cytoplasmic-binding protein that has been implicated in the regulation of cell adhesion and extracellular matrix deposition. To begin to investigate whether ILK is involved in the pathogenesis of diabetic glomerulosclerosis, we have analyzed the distribution and regulation of ILK in normal and diabetic kidneys as well as in isolated mesangial cells. We have found that ILK is normally expressed at high concentration in visceral epithelial cells. In diabetic glomeruli, ILK expression in the mesangium is dramatically increased. The increase in ILK level is associated with diffuse mesangial expansion. In glomeruli where advanced nodular sclerosis and global sclerosis were dominant, ILK level was reduced, suggesting that the increase in ILK expression likely associates with relatively early glomerulosclerosis. Additionally, we have found that exposure of mesangial cells to high concentrations of glucose significantly increased the ILK level. Finally, we show that ILK localizes to regions of cell membranes that are in close contact with mesangial fibronectin matrix. These results suggest that ILK is likely involved in mesangial matrix expansion in response to hyperglycemia in the pathogenesis of diabetic glomerulosclerosis.

Brain metastases: biology and the role of the brain microenvironment

Metastatic lesions constitute the most frequently occurring malignancy in the brain, and their detection portends a grim prognosis. Efforts to treat these lesions have failed partly because the biologic processes that govern their development are poorly understood. In recent years, it has become evident that metastases occur as a result of a multistep process involving a rigorous natural selection of cells in the primary tumor that bear molecular and biologic characteristics permitting brain metastasis. In addition, recent studies have uncovered the importance of the brain microenvironment and its contribution to the metastatic process. The development of targeted therapies against brain metastases demands a better understanding of these molecular processes and the factors that influence them. This review examines the interplay between tumor cells and host brain tissue in the context of our current understanding of the role of various molecules involved in the metastatic process.

Integrin up-regulation in chronic liver disease: relationship with inflammation and fibrosis in chronic hepatitis C

In 94 patients with chronic hepatitis C, the pattern of integrin expression was correlated with firstly, the histological activity index, necro-inflammatory grade, and stage of fibrosis; secondly, the expression of inflammatory markers including ICAM-1; and thirdly, the extent and intensity of laminin deposition in the perisinusoidal matrix. Immunohistochemical results were evaluated according to a semi-quantitative scoring system or by image analysis. Increased beta1 expression was observed in 88.2% of cases. The expression of alpha1 and alpha5 was increased in 55% and 58.5% of cases, respectively. alpha6 chain was detected in 78.7% of cases. There were no statistically significant differences in integrin expression level according to Knodell's score, inflammatory grade, or stage of fibrosis. ICAM-1 expression was higher in patients with high scores for beta1 expression, but the differences were not statistically significant. There were significantly more patients with high scores for beta1 expression among those with continuous perisinusoidal deposition of laminin. Moreover, a close statistical correlation was observed between alpha6 induction and perisinusoidal laminin deposition (p<0.001). The results suggest that integrin up-regulation in chronic hepatitis C is more closely related to the fibrotic process than to the inflammatory lesions. This reinforces the idea that integrin induction in chronic liver disease is part of a coordinated process involved in the progression of liver fibrosis.

Mammary epithelial-specific expression of the integrin-linked kinase (ILK) results in the induction of mammary gland hyperplasias and tumors in transgenic mice

The integrin linked kinase (ILK) is a cytoplasmic effector of integrin receptors, involved in the regulation of integrin binding properties as well as the activation of cell survival and proliferative pathways, including those involving MAP kinase, PKB/Akt and GSK-3beta. Overexpression of ILK in cultured intestinal and mammary epithelial cells has been previously shown to induce changes characteristic of oncogenic transformation, including anchorage-independent growth, invasiveness, suppression of anoikis and tumorigenicity in nude mice. In order to determine if ILK overexpression can result in the formation of mammary tumors in vivo, we generated transgenic mice expressing ILK in the mammary epithelium, under the transcriptional control of the mouse mammary tumor virus (MMTV) long terminal repeat (LTR). By the age of 6 months, female MMTV/ILK mice developed a hyperplastic mammary phenotype, which was accompanied by the constitutive phosphorylation of PKB/Akt, GSK-3beta and MAP kinase. Focal mammary tumors subsequently appeared in 34% of the animals at an average age of 18 months. Given the focal nature and long latency of the tumors, however, additional genetic events are likely required for tumor induction in the MMTV/ILK mice. These results provide the first direct demonstration of a potential oncogenic role for ILK, which is upregulated in human tumors and tumor cell lines.

Dysregulation of integrin-linked kinase (ILK) signaling in colonic polyposis

Mutation of the adenomatous polyposis coli (APC) gene and the subsequent dysregulation of beta-catenin are well-documented abnormalities in familial adenomatous polyposis (FAP), as well as sporadic polyposis. Intriguingly, overexpression of the integrin-linked kinase (ILK) has been shown to modulate beta-catenin subcellular localization and function. However, the significance of this finding for human carcinogenesis remains unclear. Here, we report the increased biochemical activity and expression of ILK protein in polyps from FAP patients. Furthermore, dramatic increases in ILK immunoreactivity were observed in all abnormal crypts from sporadic polyps, when compared with the normal appearing crypts within the same resected specimens. As sulindac and aspirin are the two most important therapeutic/chemopreventative agents demonstrated in colorectal carcinogenesis, in both humans and animals, further investigation revealed that these non-steroidal anti-inflammatory drugs (NSAIDs) target ILK and ILK-mediated events in vivo. These include inhibition of, both the biochemical activation of ILK, inhibition of serine 9 GSK3beta phosphorylation and the enhancement of TCF-4 transcriptional activity. In conclusion, ILK protein hyperexpression appears to be an early event in colonic polyposis. Additionally, ILK signaling is shown to undergo modulation by sulindac (and aspirin) for the first time, indicating that it is likely to be one of the targets affected by these agents in vivo.

Integrin-linked kinase (ILK) binding to paxillin LD1 motif regulates ILK localization to focal adhesions

Paxillin is a focal adhesion adapter protein involved in integrin signaling. Paxillin LD motifs bind several focal adhesion proteins including the focal adhesion kinase, vinculin, the Arf-GTPase-activating protein paxillin-kinase linker, and the newly identified actin-binding protein actopaxin. Microsequencing of peptides derived from a 50-kDa paxillin LD1 motif-binding protein revealed 100% identity with integrin-linked kinase (ILK)-1, a serine/threonine kinase that has been implicated in integrin, growth factor, and Wnt signaling pathways. Cloning of ILK from rat smooth muscle cells generated a cDNA that exhibited 99.6% identity at the amino acid level with human ILK-1. A monoclonal antibody raised against a region of the carboxyl terminus of ILK, which is identical in rat and human ILK-1 protein, recognized a 50-kDa protein in all cultured cells and tissues examined. Binding experiments showed that ILK binds directly to the paxillin LD1 motif in vitro. Co-immunoprecipitation from fibroblasts confirmed that the association between paxillin and ILK occurs in vivo in both adherent cells and cells in suspension. Immunofluorescence microscopy of fibroblasts demonstrated that endogenous ILK as well as transfected green fluorescent protein-ILK co-localizes with paxillin in focal adhesions. Analysis of the deduced amino acid sequence of ILK identified a paxillin-binding subdomain in the carboxyl terminus of ILK. In contrast to wild-type ILK, paxillin-binding subdomain mutants of ILK were unable to bind to the paxillin LD1 motif in vitro and failed to localize to focal adhesions. Thus, paxillin binding is necessary for efficient focal adhesion targeting of ILK and may therefore impact the role of ILK in integrin-mediated signal transduction events.

A novel integrin-linked kinase-binding protein, affixin, is involved in the early stage of cell-substrate interaction

Focal adhesions (FAs) are essential structures for cell adhesion, migration, and morphogenesis. Integrin-linked kinase (ILK), which is capable of interacting with the cytoplasmic domain of beta1 integrin, seems to be a key component of FAs, but its exact role in cell-substrate interaction remains to be clarified. Here, we identified a novel ILK-binding protein, affixin, that consists of two tandem calponin homology domains. In CHOcells, affixin and ILK colocalize at FAs and at the tip of the leading edge, whereas in skeletal muscle cells they colocalize at the sarcolemma where cells attach to the basal lamina, showing a striped pattern corresponding to cytoplasmic Z-band striation. When CHO cells are replated on fibronectin, affixin and ILK but not FA kinase and vinculin concentrate at the cell surface in blebs during the early stages of cell spreading, which will grow into membrane ruffles on lamellipodia. Overexpression of the COOH-terminal region of affixin, which is phosphorylated by ILK in vitro, blocks cell spreading at the initial stage, presumably by interfering with the formation of FAs and stress fibers. The coexpression of ILK enhances this effect. These results provide evidence suggesting that affixin is involved in integrin-ILK signaling required for the establishment of cell-substrate adhesion.

Modulation of integrin signal transduction by ILKAP, a protein phosphatase 2C associating with the integrin-linked kinase, ILK1

ILKAP, a protein serine/threonine (S/T) phosphatase of the PP2C family, was isolated in a yeast two-hybrid screen baited with integrin-linked kinase, ILK1. Association of ILK1 and ILKAP was independent of the catalytic activity of either partner, as assayed in co-precipitation and two-hybrid experiments. Condi tional expression of ILKAP in HEK 293 cells resulted in selective inhibition of ECM- and growth factor-stimulated ILK1 activity, but did not inhibit Raf-1 kinase activity. A catalytic mutant of ILKAP, H154D, did not inhibit ILK1 kinase activity. Two cellular targets of ILK1, glycogen synthase kinase 3 beta (GSK3beta) and protein kinase B (PKB)/AKT, were differentially affected by ILKAP-mediated inhibition of ILK1. Catalytically active, but not mutant ILKAP, strongly inhibited insulin-like growth factor-1-stimulated GSK3beta phosphorylation on Ser9, but did not affect phosphorylation of PKB on Ser473, suggesting that ILKAP selectively affects ILK-mediated GSK3beta signalling. Consistent with this, active, but not H154D mutant or the related PP2Calpha, selectively inhibited transactivation of a Tcf/Lef reporter gene, TOPFlash, in 293 cells. We propose that ILKAP regulates ILK1 activity, targeting ILK1 signalling of Wnt pathway components via modulation of GSK3beta phosphorylation.

A new focal adhesion protein that interacts with integrin-linked kinase and regulates cell adhesion and spreading

Integrin-linked kinase (ILK) is a multidomain focal adhesion (FA) protein that functions as an important regulator of integrin-mediated processes. We report here the identification and characterization of a new calponin homology (CH) domain-containing ILK-binding protein (CH-ILKBP). CH-ILKBP is widely expressed and highly conserved among different organisms from nematodes to human. CH-ILKBP interacts with ILK in vitro and in vivo, and the ILK COOH-terminal domain and the CH-ILKBP CH2 domain mediate the interaction. CH-ILKBP, ILK, and PINCH, a FA protein that binds the NH(2)-terminal domain of ILK, form a complex in cells. Using multiple approaches (epitope-tagged CH-ILKBP, monoclonal anti-CH-ILKBP antibodies, and green fluorescent protein-CH-ILKBP), we demonstrate that CH-ILKBP localizes to FAs and associates with the cytoskeleton. Deletion of the ILK-binding CH2 domain abolished the ability of CH-ILKBP to localize to FAs. Furthermore, the CH2 domain alone is sufficient for FA targeting, and a point mutation that inhibits the ILK-binding impaired the FA localization of CH-ILKBP. Thus, the CH2 domain, through its interaction with ILK, mediates the FA localization of CH-ILKBP. Finally, we show that overexpression of the ILK-binding CH2 fragment or the ILK-binding defective point mutant inhibited cell adhesion and spreading. These findings reveal a novel CH-ILKBP-ILK-PINCH complex and provide important evidence for a crucial role of this complex in the regulation of cell adhesion and cytoskeleton organization.

alpha 2 integrin subunit cytoplasmic domain-dependent cellular migration requires p38 MAPK

The alpha(2) integrin subunit cytoplasmic domain uniquely supported epidermal growth factor (EGF)-stimulated migration on type I collagen. p38 MAP kinase- and phosphatidylinositol 3-kinase-specific inhibitors, but not a MEK-specific inhibitor, eliminated EGF-stimulated and unstimulated alpha(2)-cytoplasmic domain-dependent migration. Following adhesion to collagenous matrices, cells expressing the full-length alpha(2) integrin subunit, but not cells expressing a chimeric alpha(2) integrin subunit in which the alpha(2)-cytoplasmic domain was replaced by the cytoplasmic domain of the alpha(1)-subunit, exhibited sustained and robust phosphorylation of p38 MAP kinase. Expression of dominant negative p38 MAP kinase inhibited alpha(2)-cytoplasmic domain-dependent, EGF-stimulated migration as well as unstimulated migration on collagen. Expression of constitutively active Rac1(Val-12) augmented p38 MAP kinase activation and alpha(2)-cytoplasmic domain-dependent migration. It also rescued the ability of cells expressing the alpha(1)-cytoplasmic domain to activate p38 MAPK and to migrate. These results suggest that the alpha(2) integrin cytoplasmic domain uniquely stimulates the p38 MAP kinase pathway that is required for unstimulated and EGF-stimulated migration on type I collagen.

Overexpression of the integrin-linked kinase mesenchymally transforms mammary epithelial cells

Signals generated by the interaction of (β)1 integrins with laminin in the basement membrane contribute to mammary epithelial cell morphogenesis and differentiation. The integrin-linked kinase (ILK) is one of the signaling moieties that associates with the cytoplasmic domain of (β)1 integrin subunits with some specificity. Forced expression of a dominant negative, kinase-dead form of ILK subtly altered mouse mammary epithelial cell morphogenesis but it did not prevent differentiative milk protein expression. In contrast, forced overexpression of wild-type ILK strongly inhibited both morphogenesis and differentiation. Overexpression of wild-type ILK also caused the cells to lose the cell-cell adhesion molecule E-cadherin, become invasive, reorganize cortical actin into cytoplasmic stress fibers, and switch from an epithelial cytokeratin to a mesenchymal vimentin intermediate filament phenotype. Forced expression of E-cadherin in the latter mesenchymal cells rescued epithelial cytokeratin expression and it partially restored the ability of the cells to differentiate and undergo morphogenesis. These data demonstrate that ILK, which responds to interactions between cells and the extracellular matrix, induces a mesenchymal transformation in mammary epithelial cells, at least in part, by disrupting cell-cell junctions.

Drosophila integrin-linked kinase is required at sites of integrin adhesion to link the cytoskeleton to the plasma membrane

Integrin-linked kinase (ILK) was identified by its interaction with the cytoplasmic tail of human beta1 integrin and previous data suggest that ILK is a component of diverse signaling pathways, including integrin, Wnt, and protein kinase B. Here we show that the absence of ILK function in Drosophila causes defects similar to loss of integrin adhesion, but not similar to loss of these signaling pathways. ILK mutations cause embryonic lethality and defects in muscle attachment, and clones of cells lacking ILK in the adult wing fail to adhere, forming wing blisters. Consistent with this, an ILK-green fluorescent protein fusion protein colocalizes with the position-specific integrins at sites of integrin function: muscle attachment sites and the basal junctions of the wing epithelium. Surprisingly, mutations in the kinase domain shown to inactivate the kinase activity of human ILK do not show any phenotype in Drosophila, suggesting a kinase-independent function for ILK. The muscle detachment in ILK mutants is associated with detachment of the actin filaments from the muscle ends, unlike integrin mutants, in which the primary defect is detachment of the plasma membrane from the extracellular matrix. Our data suggest that ILK is a component of the structure linking the cytoskeleton and the plasma membrane at sites of integrin-mediated adhesion.

Inhibition of integrin linked kinase (ILK) suppresses beta-catenin-Lef/Tcf-dependent transcription and expression of the E-cadherin repressor, snail, in APC-/- human colon carcinoma cells

Loss of functional adenomatous polyposis coli (APC) protein results in the stabilization of cytosolic beta-catenin and activation of genes that are responsive to Lef/Tcf family transcription factors. We have recently shown that an independent cell adhesion and integrin linked kinase (ILK)-dependent pathway can also activate beta-catenin/LEF mediated gene transcription and downregulate E-cadherin expression. In addition, ILK activity and expression are elevated in adenomatous polyposis and colon carcinomas. To examine the role of this pathway in the background of APC mutations we inhibited ILK activity in APC-/- human colon carcinoma cell lines. In all cases, inhibition of ILK resulted in substantial inhibition of TCF mediated gene transcription and inhibition of transcription and expression of the TCF regulated gene, cyclin D1. Inhibition of ILK resulted in decreased nuclear beta-catenin expression, and in the inhibition of phosphorylation of GSK-3 and stimulation of its activity, leading to accelerated degradation of beta-catenin. In addition, inhibition of ILK suppressed cell growth in culture as well as growth of human colon carcinoma cells in SCID mice. Strikingly, inhibition of ILK also resulted in the transcriptional stimulation of E-cadherin expression and correlated with the inhibition of gene transcription of snail, a repressor of E-cadherin gene expression. Overexpression of ILK caused a stimulation of expression of snail, but snail expression was found not to be regulated by beta-catenin/Tcf. These data demonstrate that ILK can regulate beta-catenin/TCF and snail transcription factors by distinct pathways. We propose that inhibition of ILK may be a useful strategy in the control of progression of colon as well as other carcinomas. Oncogene (2001) 20, 133 - 140.

Architectural transcription factor HMGI(Y) promotes tumor progression and mesenchymal transition of human epithelial cells

Numerous studies have demonstrated that overexpression or aberrant expression of the HMGI(Y) family of architectural transcription factors is frequently associated with both neoplastic transformation of cells and metastatic tumor progression. Little is known, however, about the molecular roles played by the HMGI(Y) proteins in these events. Here we report that human breast epithelial cells harboring tetracycline-regulated HMGI(Y) transgenes acquire the ability to form both primary and metastatic tumors in nude mice only when the transgenes are actively expressed. Unexpectedly, the HMG-Y, rather than the HMG-I, isoform of these proteins is the most effective elicitor of both neoplastic transformation and metastatic progression in vivo. Furthermore, expression of either antisense or dominant-negative HMGI(Y) constructs inhibits both the rate of proliferation of tumor cells and their ability to grow anchorage independently in soft agar. Array analysis of transcription profiles demonstrates that the HMG-I and HMG-Y isoform proteins each modulate the expression of distinctive constellations of genes known to be involved in signal transduction, cell proliferation, tumor initiation, invasion, migration, induction of angiogenesis, and colonization. Immunohistochemical analyses of tumors formed in nude mice indicate that many have undergone an epithelial-mesenchymal transition in vivo. Together, these findings demonstrate that overexpression of the HMGI(Y) proteins, more specifically, the HMG-Y isoform protein, is causally associated with both neoplastic transformation and metastatic progression and suggest that induction of integrins and their signaling pathways may play significant molecular roles in these biological events.

Fibronectin polymerization stimulates cell growth by RGD-dependent and -independent mechanisms

Many aspects of cell behavior are regulated by cell-extracellular matrix interactions, including cell migration and cell growth. We previously showed that the addition of soluble fibronectin to collagen-adherent fibronectin-null cells enhances cell growth. This growth-promoting effect of fibronectin depended upon the deposition of fibronectin into the extracellular matrix; occupancy and clustering of fibronectin-binding integrins was not sufficient to trigger enhanced cell growth. To determine whether the binding of integrins to fibronectin's RGD site is required for fibronectin-enhanced cell growth, the ability of fibronectin lacking the integrin-binding RGD site (FN(Delta)RGD) to promote cell growth was tested. FN(Delta)RGD promoted cell growth when used as an adhesive substrate or when added in solution to collagen-adherent fibronectin-null cells. Addition of FN(Delta)RGD to collagen-adherent fibronectin-null cells resulted in a 1.6-1.8x increase in cell growth in comparison with cells grown in the absence of fibronectin. The growth-promoting effects of FN(Delta)RGD and wild-type fibronectin were blocked by inhibitors of fibronectin polymerization, including the anti-fibronectin antibody, L8. In addition, FN(Delta)RGD-induced cell growth was completely inhibited by the addition of heparin, and was partially blocked by either heparitinase-treatment or by addition of recombinant fibronectin heparin-binding domain. Heparin and heparitinase-treatment also partially blocked the growth-promoting effects of wild-type fibronectin, as well as the deposition of wild-type fibronectin into the extracellular matrix. These data suggest that cell surface heparan-sulfate proteoglycans contribute to the growth-promoting effects of FN(Delta)RGD and wild-type fibronectin. Addition of heparin, treatment with heparitinase, or incubation with monoclonal antibody L8 all inhibited the formation of short linear FN(Delta)RGD fibrils on the cell surface. Inhibitory (beta)1 integrin antibodies had no effect on FN(Delta)RGD fibril formation, FN(Delta)RGD-induced cell growth, or cell adhesion on FN(Delta)RGD-coated substrates. These data suggest that fibronectin fibril formation can promote cell growth by a novel mechanism that is independent of RGD-integrin binding, and that involves cell surface proteoglycans.

The integrin linked kinase (ILK) induces an invasive phenotype via AP-1 transcription factor-dependent upregulation of matrix metalloproteinase 9 (MMP-9)

Overexpression of Integrin Linked Kinase (ILK) in intestinal and mammary epithelial cells results in a highly invasive phenotype, associated with increased levels of expression of the matrix metalloproteinase MMP-9. This increase was at the transcriptional level as determined by MMP-9 promoter-CAT reporter assays. Mutations in the two AP-1 binding sites within the MMP-9 promoter completely inhibited the reporter activity. We have previously shown that ILK inhibits glycogen synthase kinase-3 (GSK-3) activity. Transient transfection of wild-type GSK-3beta in ILK-overexpressing cells decreased MMP-9 promoter activity and AP-1 activity, indicating that ILK can stimulate MMP-9 expression via GSK-3beta and AP-1 transcription factor. A small molecule inhibitor of the ILK kinase reduced the in vitro invasiveness of ILK-overexpressing cells as well as the invasiveness of several human brain tumor cell lines. Furthermore, both MMP-9 promoter and AP-1 activities were inhibited by the ILK inhibitor. Invasiveness of ILK-overexpressing cells was also reduced by inhibition of MMP-9. These data demonstrate that ILK can induce an invasive phenotype via AP-1-dependent upregulation of MMP-9.

The integrin-linked kinase regulates the cyclin D1 gene through glycogen synthase kinase 3beta and cAMP-responsive element-binding protein-dependent pathways

The cyclin D1 gene encodes the regulatory subunit of a holoenzyme that phosphorylates and inactivates the pRB tumor suppressor protein. Cyclin D1 is overexpressed in 20-30% of human breast tumors and is induced both by oncogenes including those for Ras, Neu, and Src, and by the beta-catenin/lymphoid enhancer factor (LEF)/T cell factor (TCF) pathway. The ankyrin repeat containing serine-threonine protein kinase, integrin-linked kinase (ILK), binds to the cytoplasmic domain of beta(1) and beta(3) integrin subunits and promotes anchorage-independent growth. We show here that ILK overexpression elevates cyclin D1 protein levels and directly induces the cyclin D1 gene in mammary epithelial cells. ILK activation of the cyclin D1 promoter was abolished by point mutation of a cAMP-responsive element-binding protein (CREB)/ATF-2 binding site at nucleotide -54 in the cyclin D1 promoter, and by overexpression of either glycogen synthase kinase-3beta (GSK-3beta) or dominant negative mutants of CREB or ATF-2. Inhibition of the PI 3-kinase and AKT/protein kinase B, but not of the p38, ERK, or JNK signaling pathways, reduced ILK induction of cyclin D1 expression. ILK induced CREB transactivation and CREB binding to the cyclin D1 promoter CRE. Wnt-1 overexpression in mammary epithelial cells induced cyclin D1 mRNA and targeted overexpression of Wnt-1 in the mammary gland of transgenic mice increased both ILK activity and cyclin D1 levels. We conclude that the cyclin D1 gene is regulated by the Wnt-1 and ILK signaling pathways and that ILK induction of cyclin D1 involves the CREB signaling pathway in mammary epithelial cells.

Integrin-linked kinase (ILK): a "hot" therapeutic target

Integrin-mediated cell adhesion is known to regulate gene expression through the activation of transcription factors. We have recently revealed that these activations are mediated through integrin-linked kinase (ILK). ILK is an ankyrin repeat-containing serine-threonine protein kinase that can interact directly with the cytoplasmic domain of the beta1 and beta3 integrin subunits and whose kinase activity is modulated by cell-extracellular matrix interactions. We have shown that ILK overexpression results in the translocation of beta-catenin to the nucleus, which then forms a complex formation with the lymphoid enhancer binding factor 1 (LEF-1) transcription factor, subsequently activating the transcriptional activity of promoters containing LEF-1 response elements. ILK phosphorylates the glycogen synthase kinase-3 (GSK-3), which inhibits GSK-3 activity. We have demonstrated that ILK stimulates activator protein-1 transcriptional activity through GSK-3 and the subsequent regulation of the c-Jun-DNA interaction. ILK also phosphorylates protein kinase B (PKB/Akt) and stimulates its activity. We have shown that ILK is an upstream effector of the phosphatidylinositol 3-kinase-dependent regulation of PKB/Akt. ILK has been shown to phosphorylate PKB/Akt on Ser-473 in vitro and in vivo. Our results clearly indicate that ILK is a key element in the regulation of integrin signaling as well as growth factor and Wnt signaling pathways. PTEN (phosphatase and tensin homolog detected on chromosome 10) is a tumor suppressor gene located on chromosome 10q23 that encodes a protein and phospholipid phosphatase. It is now estimated that inactivation mutants of PTEN exist in 60% of all forms of solid tumors. Loss of expression or mutational inactivation of PTEN leads to the constitutive activation of PKB/Akt via enhanced phosphorylation of Thr-308 and Ser-473. We have demonstrated that the activity of ILK is constitutively elevated in PTEN mutant cells. A small molecule ILK inhibitor suppresses the phosphorylation of PKB at the Ser-473 but not the Thr-308 site in the PTEN mutant cells. These results indicate that inhibition of ILK may be of significant value in solid tumor therapy.

Cellular interactions and signaling in cartilage development

The long bones of the developing skeleton, such as those of the limb, arise from the process of endochondral ossification, where cartilage serves as the initial anlage element and is later replaced by bone. One of the earliest events of embryonic limb development is cellular condensation, whereby pre-cartilage mesenchymal cells aggregate as a result of specific cell-cell interactions, a requisite step in the chondrogenic pathway. In this review an extensive examination of historical and recent literature pertaining to limb development and mesenchymal condensation has been undertaken. Topics reviewed include limb initiation and axial induction, mesenchymal condensation and its regulation by various adhesion molecules, and regulation of chondrocyte differentiation and limb patterning. The complexity of limb development is exemplified by the involvement of multiple growth factors and morphogens such as Wnts, transforming growth factor-beta and fibroblast growth factors, as well as condensation events mediated by both cell-cell (neural cadherin and neural cell adhesion molecule) and cell-matrix adhesion (fibronectin, proteoglycans and collagens), as well as numerous intracellular signaling pathways transduced by integrins, mitogen activated protein kinases, protein kinase C, lipid metabolites and cyclic adenosine monophosphate. Furthermore, information pertaining to limb patterning and the functional importance of Hox genes and various other signaling molecules such as radical fringe, engrailed, Sox-9, and the Hedgehog family is reviewed. The exquisite three-dimensional structure of the vertebrate limb represents the culmination of these highly orchestrated and strictly regulated events. Understanding the development of cartilage should provide insights into mechanisms underlying the biology of both normal and pathologic (e.g. osteoarthritis) adult cartilage.

The roles of integrin-linked kinase in the regulation of myogenic differentiation

Myogenic differentiation is a highly orchestrated, multistep process that is coordinately regulated by growth factors and cell adhesion. We show here that integrin-linked kinase (ILK), an intracellular integrin- and PINCH-binding serine/threonine protein kinase, is an important regulator of myogenic differentiation. ILK is abundantly expressed in C2C12 myoblasts, both before and after induction of terminal myogenic differentiation. However, a noticeable amount of ILK in the Triton X-100-soluble cellular fractions is significantly reduced during terminal myogenic differentiation, suggesting that ILK is involved in cellular control of myogenic differentiation. To further investigate this, we have overexpressed the wild-type and mutant forms of ILK in C2C12 myoblasts. Overexpression of ILK in the myoblasts inhibited the expression of myogenic proteins (myogenin, MyoD, and myosin heavy chain) and the subsequent formation of multinucleated myotubes. Furthermore, mutations that eliminate either the PINCH-binding or the kinase activity of ILK abolished its ability to inhibit myogenic protein expression and allowed myotube formation. Although overexpression of the ILK mutants is permissive for the initiation of terminal myogenic differentiation, the myotubes derived from myoblasts overexpressing the ILK mutants frequently exhibited an abnormal morphology (giant myotubes containing clustered nuclei), suggesting that ILK functions not only in the initial decision making process, but also in later stages (fusion or maintaining myotube integrity) of myogenic differentiation. Additionally, we show that overexpression of ILK, but not that of the PINCH-binding defective or the kinase-deficient ILK mutants, prevents inactivation of MAP kinase, which is obligatory for the initiation of myogenic differentiation. Finally, inhibition of MAP kinase activation reversed the ILK-induced suppression of myogenic protein expression. Thus, ILK likely influences the initial decision making process of myogenic differentiation by regulation of MAP kinase activation.

Phosphorylation of the beta1 integrin cytoplasmic domain: toward an understanding of function and mechanism

As F9 stem cells differentiate into parietal endoderm they form focal adhesion sites. There is a concomitant decrease in the level of phosphorylation of S785 in the cytoplasmic domain of the beta1 integrin subunit. Previous transfection studies demonstrate that site-specific mutations at this residue, mimicking different phosphorylation states, can alter the subcellular localization of the subunit in differentiating F9 cells. We now extend these observations in an attempt to substantiate the function of beta1 phosphorylation and determine how the phosphorylation levels are regulated. We show that treatment of parietal endoderm with okadaic acid induces an increase in beta1 phosphorylation and selective loss of beta1 from focal adhesion sites. Using a PCR approach, we identify two phosphatases expressed in parietal endoderm, including PP2A. Using a crosslinking approach, where antibodies are added to live cells, we show that the catalytic subunit of PP2A co-immunoprecipitates with beta1. Immunocytochemistry shows PP2A colocalizing to focal adhesion sites with beta1. In addition integrin-linked kinase (ILK) co-immunoprecipitates with beta1 in parietal endoderm and localizes to focal adhesion sites. Okadaic acid treatment significantly decreases the level of ILK associated with beta1. A possible role for regulated beta1 phosphorylation in cell migration is discussed.

Integrin and mechanosensitive ion channel-dependent tyrosine phosphorylation of focal adhesion proteins and beta-catenin in human articular chondrocytes after mechanical stimulation

Mechanical forces influence chondrocyte metabolism and function. We have previously shown that 0.33 Hz cyclical pressure-induced strain (PIS) results in membrane hyperpolarization of normal human articular chondrocytes (HAC) by activation of Ca(2+)-dependent K+ small conductance potassium activated calcium (SK) channels. The mechanotransduction pathway involves alpha 5 beta 1-integrin, stretch-activated ion channels (SAC) actin cytoskeleton and tyrosine protein kinases, with subsequent release of the chondroprotective cytokine interleukin-4 (IL-4). The objective of this study was to examine in detail tyrosine phosphorylation events in the mechanotransduction pathway. The results show tyrosine phosphorylation of three major proteins, p125, p90, and p70 within 1 minute of onset of mechanical stimulation. Immunoblotting and immunoprecipitation show these to be focal adhesion kinase (pp125FAK), beta-catenin, and paxillin, respectively. Tyrosine phosphorylation of all three proteins is inhibited by RGD containing oligopeptides and gadolinium, which is known to block SAC. beta-catenin coimmunoprecipitates with FAK and is colocalized with alpha 5-integrin and pp125FAK. These results indicate a previously unrecognized role for an integrin-beta-catenin signaling pathway in human articular chondrocyte (HAC) responses to mechanical stimulation.

Colocalization and redistribution of dishevelled and actin during Wnt-induced mesenchymal morphogenesis

Activation of the Wnt signaling pathway is important for induction of gene expression and cell morphogenesis throughout embryonic development. We examined the subcellular localization of dishevelled, the immediate downstream component from the Wnt receptor, in the embryonic mouse kidney. Using immunofluorescence staining, confocal microscopy, and coimmunoprecipitation experiments, we show that dishevelled associates with actin fibers and focal adhesion plaques in metanephric mesenchymal cells. Stimulation of Wnt signaling leads to profound changes in metanephric mesenchymal cell morphology, including disruption of the actin cytoskeleton, increased cell spreading, and increased karyokinesis. Upon activation of Wnt signaling, dishevelled also accumulates in and around the nucleus. Casein kinase Iepsilon colocalizes with dishevelled along actin fibers and in the perinuclear region, whereas axin and GSK-3 are only present around the nucleus. These data indicate a branched Wnt signaling pathway comprising a canonical signal that targets the nucleus and gene expression, and another signal that targets the cytoskeleton and regulates cell morphogenesis.

Cloning of an isoform of integrin-linked kinase (ILK) that is upregulated in HT-144 melanoma cells following TGF-beta1 stimulation

We have shown previously that integrin-linked kinase (ILK) is upregulated in human HT-144 melanoma cells following TGF-beta1 stimulation. Using mRNA from TGF-beta1 stimulated HT-144 cells and reverse transcriptase polymerase chain reaction, we have isolated a cDNA encoding a protein highly homologous to ILK. Sequencing of the full-length 1359 base pair cDNA and polypeptide translation revealed that this protein, designated ILK-2, differs from the known ILK (hereafter called ILK-1) by only four amino acids, while the cDNA sequence diverges by 102 nucleotides, thus excluding that ILK-2 is an allelic variant of ILK-1. Expression of ILK-2 mRNA was observed in metastatic human HT-144 melanoma and HT-1080 fibrosarcoma cell lines, but not in normal human tissues. Moreover, stimulation of HT-144 cells with TGF-beta1, but not with EGF, PDGF-AB or insulin, induced a selective overexpression of ILK-2 mRNA as compared to ILK-1 mRNA. Bacterially-expressed GST/ILK-2 autophosphorylated and labeled myelin basic protein as well as a recombinant GST/beta3 integrin cytoplasmic tail peptide. Transfection of either ILK-2 or ILK-1 cDNA into the non-metastatic melanoma cell line SK-Mel-2, expressing exclusively ILK-1, induced anchorage independent cell growth and cell proliferation, as demonstrated by growth in soft agar. Our data provide evidence that ILK-2 is a new isoform of ILK-1 that is expressed in some highly invasive tumor cell lines but not in normal adult human tissues and whose expression is regulated by TGF-beta1.

The nonreceptor tyrosine kinase fer mediates cross-talk between N-cadherin and beta1-integrins

Cadherins and integrins must function in a coordinated manner to effectively mediate the cellular interactions essential for development. We hypothesized that exchange of proteins associated with their cytoplasmic domains may play a role in coordinating function. To test this idea, we used Trojan peptides to introduce into cells and tissues peptide sequences designed to compete for the interaction of specific effectors with the cytoplasmic domain of N-cadherin, and assayed their effect on cadherin- and integrin-mediated adhesion and neurite outgrowth. We show that a peptide mimicking the juxtamembrane (JMP) region of the cytoplasmic domain of N-cadherin results in inhibition of N-cadherin and beta1-integrin function. The effect of JMP on beta1-integrin function depends on the expression of N-cadherin and is independent of transcription or translation. Treatment of cells with JMP results in the release of the nonreceptor tyrosine kinase Fer from the cadherin complex and its accumulation in the integrin complex. A peptide that mimics the first coiled-coil domain of Fer prevents Fer accumulation in the integrin complex and reverses the inhibitory effect of JMP. These findings suggest a new mechanism through which N-cadherin and beta1-integrins are coordinately regulated: loss of an effector from the cytoplasmic domain of N-cadherin and gain of that effector by the beta1-integrin complex.

Distinct role of protein phosphatase 2A subunit Calpha in the regulation of E-cadherin and beta-catenin during development

Protein phosphatase 2A (PP2A) plays central roles in development, cell growth and transformation. Inactivation of the gene encoding the PP2A catalytic subunit Calpha by gene targeting generates a lethal embryonic phenotype. No mesoderm is formed in Calpha(-/-) embryos. Here, we found that during normal early embryonic development Calpha was predominantly present at the plasma membrane whereas the highly homologous isoform Cbeta was localized to the cytoplasm and nuclei, suggesting the inability of Cbeta to compensate for vital functions of Calpha in Calpha(-/-) embryos. In addition, PP2A was found in a complex containing the PP2A substrates E-cadherin and beta-catenin. In Calpha(-/-) embryos, E-cadherin and beta-catenin were redistributed from the plasma membrane to the cytosol. Cytosolic concentrations of beta-catenin were low. Our results suggest that Calpha is required for stabilization of E-cadherin/beta-catenin complexes at the plasma membrane.

Cell-substrate interactions and signaling through ILK

Interactions between cells and the extracellular matrix (ECM) result in the regulation of cell growth, cell differentiation and cell migration. These interactions are mediated by integrins and growth factor receptors and intracellular effectors that couple these receptors to downstream components are key to the transduction of ECM signals. This review summarizes recent advances in our understanding of signal transduction via integrins, focusing on the role of integrin-linked kinase in some of these pathways. Research into this interesting protein is uncovering novel aspects of coordinated signaling by the ECM and growth factors.

Inhibition of integrin-linked kinase (ILK) suppresses activation of protein kinase B/Akt and induces cell cycle arrest and apoptosis of PTEN-mutant prostate cancer cells

PTEN is a tumor suppressor gene located on chromosome 10q23 that encodes a protein and phospholipid phosphatase. Somatic mutations of PTEN are found in a number of human malignancies, and loss of expression, or mutational inactivation of PTEN, leads to the constitutive activation of protein kinase B (PKB)/Akt via enhanced phosphorylation of Thr-308 and Ser-473. We recently have demonstrated that the integrin-linked kinase (ILK) can phosphorylate PKB/Akt on Ser-473 in a phosphoinositide phospholipid-dependent manner. We now demonstrate that the activity of ILK is constitutively elevated in a serum- and anchorage-independent manner in PTEN-mutant cells, and transfection of wild-type (WT) PTEN into these cells inhibits ILK activity. Transfection of a kinase-deficient, dominant-negative form of ILK or exposure to a small molecule ILK inhibitor suppresses the constitutive phosphorylation of PKB/Akt on Ser-473, but not on Thr-308, in the PTEN-mutant prostate carcinoma cell lines PC-3 and LNCaP. Transfection of dominant-negative ILK and WT PTEN into these cells also results in the inhibition of PKB/Akt kinase activity. Furthermore, dominant-negative ILK or WT PTEN induces G(1) phase cycle arrest and enhanced apoptosis. Together, these data demonstrate a critical role for ILK in PTEN-dependent cell cycle regulation and survival and indicate that inhibition of ILK may be of significant value in PTEN-mutant tumor therapy.

Inhibition of integrin-linked kinase (ILK) suppresses activation of protein kinase B/Akt and induces cell cycle arrest and apoptosis of PTEN-mutant prostate cancer cells

PTEN is a tumor suppressor gene located on chromosome 10q23 that encodes a protein and phospholipid phosphatase. Somatic mutations of PTEN are found in a number of human malignancies, and loss of expression, or mutational inactivation of PTEN, leads to the constitutive activation of protein kinase B (PKB)/Akt via enhanced phosphorylation of Thr-308 and Ser-473. We recently have demonstrated that the integrin-linked kinase (ILK) can phosphorylate PKB/Akt on Ser-473 in a phosphoinositide phospholipid-dependent manner. We now demonstrate that the activity of ILK is constitutively elevated in a serum- and anchorage-independent manner in PTEN-mutant cells, and transfection of wild-type (WT) PTEN into these cells inhibits ILK activity. Transfection of a kinase-deficient, dominant-negative form of ILK or exposure to a small molecule ILK inhibitor suppresses the constitutive phosphorylation of PKB/Akt on Ser-473, but not on Thr-308, in the PTEN-mutant prostate carcinoma cell lines PC-3 and LNCaP. Transfection of dominant-negative ILK and WT PTEN into these cells also results in the inhibition of PKB/Akt kinase activity. Furthermore, dominant-negative ILK or WT PTEN induces G(1) phase cycle arrest and enhanced apoptosis. Together, these data demonstrate a critical role for ILK in PTEN-dependent cell cycle regulation and survival and indicate that inhibition of ILK may be of significant value in PTEN-mutant tumor therapy.

Phosphorylation of tyrosine residues 31 and 118 on paxillin regulates cell migration through an association with CRK in NBT-II cells

Identification of signaling molecules that regulate cell migration is important for understanding fundamental processes in development and the origin of various pathological conditions. The migration of Nara Bladder Tumor II (NBT-II) cells was used to determine which signaling molecules are specifically involved in the collagen-mediated locomotion. We show here that paxillin is tyrosine phosphorylated after induction of motility on collagen. Overexpression of paxillin mutants in which tyrosine 31 and/or tyrosine 118 were replaced by phenylalanine effectively impaired cell motility. Moreover, stimulation of motility by collagen preferentially enhanced the association of paxillin with the SH2 domain of the adaptor protein CrkII. Mutations in both tyrosine 31 and 118 diminished the phosphotyrosine content of paxillin and prevented the formation of the paxillin-Crk complex, suggesting that this association is necessary for collagen-mediated NBT-II cell migration. Other responses to collagen, such as cell adhesion and spreading, were not affected by these mutations. Overexpression of wild-type paxillin or Crk could bypass the migration-deficient phenotype. Both the SH2 and the SH3 domains of CrkII are shown to play a critical role in this collagen-mediated migration. These results demonstrate the important role of the paxillin-Crk complex in the collagen-induced cell motility.

Bi-directional signal transduction by integrin receptors

The integrin family of cell surface glycoproteins functions primarily as receptors for extracellular matrix ligands. There are now many well characterized integrin-ligand interactions which are known to influence many aspects of cell behaviour including cell morphology, cell adhesion, cell migration as well as cellular proliferation and differentiation. However, in fulfilling these functions, integrins are not simple adhesion receptors that physically mediate connections across the plasma membrane. Rather, integrin function itself is highly regulated, largely through the formation of specific associations with both structural and regulatory components within cells. It is these intracellular interactions which allow integrin function to effect many biochemical signalling pathways and therefore to impinge upon complex cellular activities. Recently, much research has focused on elucidating the molecular mechanisms which control integrin function and the molecular processes which transduce integrin-mediated signalling events. In this review, we discuss progress in the field of integrin signal transduction including, where applicable, potential therapeutic applications arising from the research.

The MMAC1 tumor suppressor phosphatase inhibits phospholipase C and integrin-linked kinase activity

Loss of the tumor suppressor MMAC1 has been shown to be involved in breast, prostate and brain cancer. Consistent with its identification as a tumor suppressor, expression of MMAC1 has been demonstrated to reduce cell proliferation, tumorigenicity, and motility as well as affect cell-cell and cell-matrix interactions of malignant human glioma cells. Subsequently, MMAC1 was shown to have lipid phosphatase activity towards PIP3 and protein phosphatase activity against focal adhesion kinase (FAK). The lipid phosphatase activity of MMAC1 results in decreased activation of the PIP3-dependent, anti-apoptotic kinase, AKT. It is thought that this inhibition of AKT culminates with reduced glioma cell proliferation. In contrast, MMAC1's effects on cell motility, cell - cell and cell - matrix interactions are thought to be due to its protein phosphatase activity towards FAK. However, recent studies suggest that the lipid phosphatase activity of MMAC1 correlates with its ability to be a tumor suppressor. The high rate of mutation of MMAC1 in late stage metastatic tumors suggests that effects of MMAC1 on motility, cell - cell and cell - matrix interactions are due to its tumor suppressor activity. Therefore the lipid phosphatase activity of MMAC1 may affect PIP3 dependent signaling pathways and result in reduced motility and altered cell - cell and cell - matrix interactions. We demonstrate here that expression of MMAC1 in human glioma cells reduced intracellular levels of inositol trisphosphate and inhibited extracellular Ca2+ influx, suggesting that MMAC1 affects the phospholipase C signaling pathway. In addition, we show that MMAC1 expression inhibits integrin-linked kinase activity. Furthermore, we show that these effects require the catalytic activity of MMAC1. Our data thus provide a link of MMAC1 to PIP3 dependent signaling pathways that regulate cell - matrix and cell - cell interactions as well as motility. Lastly, we demonstrate that AKT3, an isoform of AKT highly expressed in the brain, is also a target for MMAC1 repression. These data suggest an important role for AKT3 in glioblastoma multiforme. We therefore propose that repression of multiple PIP3 dependent signaling pathways may be required for MMAC1 to act as a tumor suppressor.

Integrin-linked kinase and PINCH: partners in regulation of cell-extracellular matrix interaction and signal transduction

Integrin-linked kinase (ILK) is a focal adhesion serine/threonine protein kinase that is emerging as a key signaling protein functioning at one of the early convergence points of integrin- and growth factor-signaling pathways. ILK binds to PINCH through the N-terminal ankyrin (ANK) repeat domain and the PINCH binding is crucial for focal adhesion localization of ILK. The ILK-PINCH interaction also connects ILK to Nck-2, an SH2-SH3-containing adaptor protein that interacts with components of growth factor and small GTPase signaling pathways. The kinase activity of ILK is regulated by both cell adhesion and growth factors in a phosphoinositide 3-kinase (PI3K)-dependent manner. ILK phosphorylates downstream targets such as protein kinase B (PKB, also known as Akt) and glycogen synthase kinase 3 (GSK-3) and regulates their activities. Overexpression of ILK in epithelial cells leads to striking morphological changes mimicking epithelial-mesenchymal transition, including upregulation of integrin-mediated fibronectin matrix assembly and downregulation of cell-cell adhesions. Furthermore, ILK regulates nuclear translocation of (beta)-catenin and gene expression, and promotes cell cycle progression and tumor formation. Recent genetic studies in Drosophila melanogaster and Caenorhabditis elegans have shown that lack of expression of ILK or PINCH results in phenotypes resembling those of integrin-null mutants, which demonstrates that ILK and PINCH are indispensable for integrin function during embryonic development.

Integrin-linked kinase is localized to cell-matrix focal adhesions but not cell-cell adhesion sites and the focal adhesion localization of integrin-linked kinase is regulated by the PINCH-binding ANK repeats

Integrin-linked kinase (ILK) is a ubiquitously expressed protein serine/threonine kinase that has been implicated in integrin-, growth factor- and Wnt-signaling pathways. In this study, we show that ILK is a constituent of cell-matrix focal adhesions. ILK was recruited to focal adhesions in all types of cells examined upon adhesion to a variety of extracellular matrix proteins. By contrast, ILK was absent in E-cadherin-mediated cell-cell adherens junctions. In previous studies, we have identified PINCH, a protein consisting of five LIM domains, as an ILK binding protein. We demonstrate in this study that the ILK-PINCH interaction requires the N-terminal-most ANK repeat (ANK1) of ILK and one (the C-terminal) of the two zinc-binding modules within the LIM1 domain of PINCH. The ILK ANK repeats domain, which is capable of interacting with PINCH in vitro, could also form a complex with PINCH in vivo. However, the efficiency of the complex formation or the stability of the complex was markedly reduced in the absence of the C-terminal domain of ILK. The PINCH binding defective ANK1 deletion ILK mutant, unlike the wild-type ILK, was unable to localize and cluster in focal adhesions, suggesting that the interaction with PINCH is necessary for focal adhesion localization and clustering of ILK. The N-terminal ANK repeats domain, however, is not sufficient for mediating focal adhesion localization of ILK, as an ILK mutant containing the ANK repeats domain but lacking the C-terminal integrin binding site failed to localize in focal adhesions. These results suggest that focal adhesions are a major subcellular compartment where ILK functions in intracellular signal transduction, and provide important evidence for a critical role of PINCH and integrins in regulating ILK cellular function.