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

Integrin signaling in epithelial cells

Although most cells of adult mammals express multiple different integrins, particular types of cells have a characteristic repertoire of integrin expression. Benign and malignant epithelial cells use specific integrins to allow the epithelial microenvironment to modulate a wide variety of cell functions, including cell survival, proliferation, morphogenesis, differentiation, motility, invasion and metastasis. An important concept emerging from the data on integrin signal transduction is that integrin signaling impinges on pathways downstream of other receptors, creating elaborate intracellular signaling networks. This review highlights signal transduction functions of epithelial integrins, with particular emphasis on signaling pathways underlying some of the most important functions of epithelium.

The role of integrin-linked kinase in liver wound healing

Liver wound healing is an integrated process in which hepatic stellate cells play a major role. We hypothesized that the cellextracellular signaling protein integrin-linked kinase (ILK) is important in transducing signals from the extracellular matrix to stellate cells and thus plays a critical role in stellate cell activation and fibrogenesis during liver injury. Liver injury and subsequent stellate cell activation led to a 3-fold increase in ILK expression and increased kinase activity. Overexpression of ILK in isolated stellate cells led to enhanced motility and adhesion as well as increases in smooth muscle alpha-actin and type I collagen mRNA expression. The effects of ILK on stellate cell phenotypes were phosphatidylinositol 3-kinase-dependent. Forced expression of ILK in vivo led to increases in type I collagen, smooth muscle alpha-actin, transforming growth factor-beta, and extra domain A (EDA) fibronectin mRNAs (by 3.2-, 3.5-, 2.5-, and 2.2-fold, respectively; n = 8, p < 0.05 for each versus the control), whereas inhibition of ILK in vivo led to significant reductions in these mRNAs. Morphometric analysis revealed that ILK overexpression led to a 31.4% increase in liver collagen content (n = 8, p < 0.05 versus the control); in contrast ILK knockdown in vivo led to a significant reduction in fibrogenesis. We conclude that ILK plays an important pathophysiological role in vivo in liver wound healing.

Simultaneous inhibition of PDK1/AKT and Fms-like tyrosine kinase 3 signaling by a small-molecule KP372-1 induces mitochondrial dysfunction and apoptosis in acute myelogenous leukemia

Phosphoinositol-3-kinase (PI3K)/protein kinase B (AKT) and Fms-like tyrosine kinase 3 (FLT3) signaling are aberrantly activated in acute myelogenous leukemia (AML) cells. Constitutively activated AKT and FLT3 regulate leukemia cell survival and resistance to chemotherapy. In this study, we investigated the effects of the novel multiple kinase inhibitor KP372-1 on the survival of AML cell lines and primary AML samples. KP372-1 directly inhibited the kinase activity of AKT, PDK1, and FLT3 in a concentration-dependent manner. Western blot analysis indicated that KP372-1 decreased the phosphorylation of AKT on both Ser(473) and Thr(308); abrogated the phosphorylation of p70S6 kinase, BAD, and Foxo3a via PI3K/AKT signaling; and down-regulated expression of PIM-1 through direct inhibition of FLT3. Treatment of AML cell lines with KP372-1 resulted in rapid generation of reactive oxygen species and stimulation of oxygen consumption, followed by mitochondrial depolarization, caspase activation, and phosphatidylserine externalization. KP372-1 induced pronounced apoptosis in AML cell lines and primary samples irrespective of their FLT3 status, but not in normal CD34(+) cells. Moreover, KP372-1 markedly decreased the colony-forming ability of primary AML samples (IC(50) < 200 nmol/L) with minimal cytotoxic effects on normal progenitor cells. Taken together, our results show that the simultaneous inhibition of critical prosurvival kinases by KP372-1 leads to mitochondrial dysfunction and apoptosis of AML but not normal hematopoietic progenitor cells.

Role of IkappaB kinase in tumorigenesis

IkappaB kinase plays a central role in regulating NF-kappaB, is a key signaling molecule involved in controlling cell proliferation, survival, antiapoptosis, and tumorigenesis. Alternative pathways can also regulate NF-kappaB in an IkappaB kinase-independent manner. Emerging evidence indicates that IKK phosphorylates and inactivates forkhead box, class O (FOXO)-3a and promotes cell growth and tumorigenesis. Moreover, IKK and NF-kappaB play an important role in linking inflammation and tumorigenesis, and facilitate tumor maintenance and invasion. Thus, IKK and NF-kappaB are promising targets for drug discovery, and agents targeting the IKK/NF-kappaB and FOXO pathways may become therapeutic intervention for those patients with IKK/NF-kappaB-overexpressing cancers in the future.

Integrin-linked kinase and its partners: A modular platform regulating cell–matrix adhesion dynamics and cytoskeletal organization

Integrin-linked kinase (ILK) represents a key component of integrin signaling complexes that functions in concert with multiple binding partners to transmit cues from the extracellular matrix environment to the actin cytoskeleton. Both gain- and loss-of-function approaches to study ILK have confirmed the essential role of this protein in regulating cell-matrix adhesion dynamics and cytoskeletal organization.

ILK, PINCH and parvin: the tIPP of integrin signalling

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

Phosphoinositide 3-kinase: from viral oncoprotein to drug target

The catalytic subunit p110alpha of the phosphoinositide 3-kinase (PI3K) and the serine-threonine protein kinase Akt have been extensively studied as retroviral oncoproteins. The experimental tools developed with the retroviral vectors are now being applied to PI3K mutations in human cancer. The most frequently occurring mutants of p110alpha are oncogenic in vitro and in vivo, show gain of enzymatic function, activate Akt, and their oncogenic activity is sensitive to rapamycin. The related isoforms p110beta, gamma and delta induce oncogenic transformation as wild-type proteins. Mutated p110alpha proteins are ideal drug targets. Identification of small molecule inhibitors that specifically target these mutant proteins is a realistic and urgent goal.

PTEN and GSK3beta: key regulators of progression to androgen-independent prostate cancer

Prostate cancer (PrCa) is characterized by progression from an androgen-dependent phenotype to one that is inevitably androgen independent (AI) and lethal. Recent evidence strongly suggests that the phosphatidylinositol-3-kinase/Akt (PI3K/Akt) and androgen receptor (AR) signalling pathways provide prostatic epithelium with the necessary signalling events to escape the apoptotic response associated with androgen withdrawal therapy. Silencing of phosphatase and tensin homologue deleted on chromosome 10 (PTEN) and glycogen synthase kinase beta (GSK3beta) are frequently associated with advanced PrCa systems and likely serve critical roles in promoting AR and PI3K/Akt gain-of-function. That PTEN negatively regulates AR and is sufficient to promote metastatic PrCa in murine models strongly implies its role as a gatekeeper of progressive PrCa. In human PrCa, PTEN loss is correlated with substantial increases in Akt(Ser473) and integrin-linked kinase expression, both of which promote Ser(9) phospho-inhibition of GSK3beta and inactivation of apoptotic factors. Sufficient evidence also suggests that GSK3beta is not only a critical regulator of proproliferative signalling but also a promiscuous one as PI3K/Akt pools of GSK3beta are, at least in part, functionally interchangeable with those of the Wnt/beta-catenin pathway. Thus, GSK3beta may serve not only as a mediator of PI3K/Akt activation but may also regulate the potent transactivation and proproliferative effects that Wnt3a and beta-catenin confer upon AR. These data suggest that prostate-specific activation of GSK3beta may serve as a viable pharmacological option. Thus, in this review, we emphasize that temporal changes in GSK3beta and PTEN expression during progression to AI PrCa are important factors when considering the potential for therapies targeting the oncogenic contributions of PI3K/Akt and AR signalling pathways.

Identification of p122RhoGAP (deleted in liver cancer-1) Serine 322 as a substrate for protein kinase B and ribosomal S6 kinase in insulin-stimulated cells

Protein kinase B (PKB or Akt) plays an essential role in the actions of insulin, cytokines, and growth factors, although the substrates for PKB that are relevant to many of its actions require identification. In this study, we have reported the identification of p122RhoGAP, a GTPase-activating protein selective for RhoA and rodent homologue of the tumor suppressor deleted in liver cancer (DLC1) as a novel insulin-stimulated phosphoprotein in primary rat adipocytes. We have demonstrated that Ser-322 is phosphorylated upon insulin stimulation of intact cells and that this site is directly phosphorylated in vitro by PKB and ribosomal S6 kinase, members of the AGC (protein kinases A, G, and C) family of insulin-stimulated protein kinases. Furthermore, expression of constitutively active mutants of PKB or mitogen-activated protein kinase/extracellular signal-regulated kinase kinase (MEK) stimulates Ser-322 phosphorylation in intact cells, demonstrating that activation of the PKB or MEK pathway is sufficient for Ser-322 phosphorylation in vivo. Indeed, in primary adipocytes, insulin-stimulated Ser-322 phosphorylation was almost exclusively regulated by the phosphatidylinositol 3-kinase/PKB pathway, whereas in immortalized cells, insulin-stimulated phosphorylation was predominantly regulated by the MEK/extracellular signal-regulated kinase/ribosomal S6 kinase pathway, with the phosphatidylinositol 3-kinase/PKB pathway playing a minor role. These results demonstrate that p122RhoGAP Ser-322 acts as an integrator of signal transduction in a manner dependent on the cellular context.

Preferential dependence of breast cancer cells versus normal cells on integrin-linked kinase for protein kinase B/Akt activation and cell survival

The emerging paradigm of "oncogene addiction" has been called an Achilles' heel of cancer that can be exploited therapeutically. Here, we show that integrin-linked kinase (ILK), which is either activated or overexpressed in many types of cancers, is a critical regulator of breast cancer cell survival through the protein kinase B (PKB)/Akt pathway but is largely dispensable for the survival of normal breast epithelial cells and mesenchymal cells. We show that inhibition of ILK activity with a pharmacologic ILK inhibitor, QLT-0267, results in the inhibition of PKB/Akt Ser473 phosphorylation, stimulation of apoptosis, and a decrease in mammalian target of rapamycin (mTOR) expression in human breast cancer cells. In contrast, QLT-0267 treatment has no effect on PKB/Akt Ser473 phosphorylation or apoptosis in normal human breast epithelial, mouse fibroblast, or vascular smooth muscle cells. The inhibition of PKB/Akt Ser473 phosphorylation by QLT-0267 in breast cancer cells was rescued by a kinase-active ILK mutant but not by a kinase-dead ILK mutant. Furthermore, a dominant-negative ILK mutant increased apoptosis in the MDA-MB-231 breast cancer cell line but not in normal human breast epithelial cells. The inhibitor was active against ILK isolated from all cell types but did not have any effect on cell attachment and spreading. Our data point to an "ILK addiction" of breast cancer cells whereby they become dependent on ILK for cell survival through the mTOR-PKB/Akt signaling pathway and show that ILK is a promising target for the treatment of breast cancer.

Integrin-linked kinase deletion from mouse cortex results in cortical lamination defects resembling cobblestone lissencephaly

Integrin-linked kinase (Ilk) is a scaffold and kinase that links integrin receptors to the actin cytoskeleton and to signaling pathways involved in cell adhesion, migration, and extracellular matrix deposition. Targeted deletion of Ilk from embryonic mouse dorsal forebrain neuroepithelium results in severe cortical lamination defects resembling cobblestone (type II) lissencephaly. Defects in adult mutants include neuronal invasion of the marginal zone, downward displacement of marginal zone components, fusion of the cerebral hemispheres, and scalloping of the dentate gyrus. These lesions are associated with abundant astrogliosis and widespread fragmentation of the basal lamina at the cortical surface. During cortical development, neuronal ectopias are associated with severe disorganization of radial glial processes and displacement of Cajal-Retzius cells. Lesions are not seen when Ilk is specifically deleted from embryonic neurons. Interestingly, targeted Ilk deletion has no effect on proliferation or survival of cortical cells or on phosphorylation of two Ilk substrates, Pkb/Akt and Gsk-3beta, suggesting that Ilk does not regulate cortical lamination via these enzymes. Instead, Ilk acts in vivo as a major intracellular mediator of integrin-dependent basal lamina formation. This study demonstrates a critical role for Ilk in cortical lamination and suggests that Ilk-associated pathways are involved in the pathogenesis of cobblestone lissencephalies.

Integrin signaling via RGD peptides and anti-beta1 antibodies confers resistance to apoptosis in islets of Langerhans

Islet transplantation is associated with a high rate of early graft failure caused by early immune attack and poor functionality of islets. Apoptosis of islet cells appears soon after islet isolation and primarily involves the beta-cell. The purpose of this study was to determine the effect of ligation to extracellular matrix (ECM) proteins on survival of the islets of Langerhans following islet isolation. Islets that had been cultured for 24 h on collagen type I showed an islet survival of 59.7 +/- 8.7%, while islets that had been cultured on collagen type IV and laminin showed an islet survival of 88.6 +/- 10.3 and 94.3 +/- 5.6%, respectively. Islets that had been pretreated with anti-beta1 antibodies and argenin-glycin-aspartic acid (RGD) peptides showed a decrease in the level of apoptosis by a factor of 2.5 and 3.1, respectively, and an increase of phospho-Akt Ser 473 activity by a factor of 3.1 and 2.9, respectively, compared with untreated islets. When detached from their natural ECM surrounding in the pancreas, islet cells undergo apoptosis, unless islets are cultured on collagen IV or laminin or treated with anti-beta1 integrin antibodies or RGD peptides to mimic ECM ligation. These results indicate that inhibition of anoikis may offer opportunities to improve function and viability of islet cells.

Integrin-linked kinase is an essential link between integrins and uptake of bacterial pathogens by epithelial cells

Entry of Streptococcus pyogenes or group A streptococcus (GAS) into host cells is mediated by fibronectin bound to surface proteins, M1 or PrtF1, forming a bridge to alpha5beta1 integrins. This interaction leads to cytoskeletal rearrangement and uptake of streptococci. We postulated that integrin-linked kinase (ILK), which directly associates with integrins, is the universal link between integrins and several bacterial pathogens. We showed that inhibition of ILK expression by siRNA silencing, or ILK kinase activity by chemical inhibitors or expression of a dominant negative form of ILK reduced M1-mediated invasion of epithelial cells up to 80%. To evaluate the ILK requirement for PrtF1-mediated GAS invasion, a M1-PrtF1+ recombinant strain within the M1 background was constructed. Inhibition of ILK kinase activity also significantly reduced invasion of epithelial cells by this recombinant and wild-type strain JRS4 that expresses PrtF1. In addition, impaired ILK kinase activity results in significant reduction of integrin-dependent invasion mediated by invasins of two other important pathogens, Staphylococcus aureus and Yersinia spp. This study suggests that bacterial pathogens evolved different molecules and strategies to exploit the host integrin signalling pathway for their survival.

Targeting integrin-linked kinase inhibits Akt signaling pathways and decreases tumor progression of human glioblastoma

The phosphatidylinositol 3-kinase pathway is an important regulator of a wide spectrum of tumor-related biological processes, including cell proliferation, survival, and motility, as well as neovascularization. Protein kinase B/Akt is activated in a complex manner through the phosphorylation of protein kinase B/Akt on Thr308 and Ser473. Although protein-dependent kinase-1 has been shown to phosphorylate Akt at Thr308, it is not clear whether there is a distinct kinase that exclusively phosphorylates Akt at Ser473. A possible candidate is integrin-linked kinase (ILK), which has been shown to phosphorylate Akt at Ser473 in vitro. ILK is a multidomain focal adhesion protein that is believed to be involved in signal transmission from integrin and growth factor receptors. Further, ILK is implicated in the regulation of anchorage-dependent cell growth/survival, cell cycle progression, invasion and migration, and tumor angiogenesis. In this study, we tested the hypothesis that ILK inhibition would inhibit these processes in gliomas in which it is constitutively expressed. We found that a newly developed small-molecule compound (QLT0267) effectively inhibited signaling through the ILK/Akt cascade in glioma cells by blocking the phosphorylation of Akt and downstream targets, including mammalian target of rapamycin and glycogen synthase kinase-3beta. Treatment of glioma cells with 12.5 micromol/L QLT0267 inhibited cell growth by 50% at 48 hours. An anchorage-dependent cell growth assay confirmed the cell growth-inhibitory effect of QLT0267. Further, the decrease in cell growth was associated with a dramatic accumulation of cells in the G2-M phase of the cell cycle. Although the cell growth-inhibitory effects of the ILK inhibitor were achieved only at a high concentration, the QLT0267 was able to reduce cellular invasion and angiogenesis at much lower concentrations as shown by in vitro invasion assays and vascular endothelial growth factor secretion. Thus, blocking the ILK/Akt pathway is a potential strategy for molecular targeted therapy for gliomas.

Elevated phosphorylation and activation of PDK-1/AKT pathway in human breast cancer

Activation of kinases signalling pathways contributes to various malignant phenotypes in human cancers, including breast tumour. To examine the possible activation of these signalling molecules, we examined the phosphorylation status in 12 protein kinases and transcription factors in normal primary human mammary epithelial cells, telomerase-immortalised human breast epithelial cell line, and two breast cancer lines, MDA-MB-468 and MCF-7, using Kinexus phosphorylated protein screening assays. The phosphorylation of FAK, mTOR, p70S6K, and PDK-1 were elevated in both breast cancer cell lines, whereas the phosphorylation of AKT, EGFR, ErbB2/Her2, PDGFR, Shc, and Stat3 were elevated in only one breast cancer line compared to normal primary mammary epithelial cells and telomerase-immortalised breast epithelial cells. The same findings were confirmed by Western blotting and by kinase assays. We further substantiated the phosphorylation status of these molecules in tissue microarray slides containing 89 invasive breast cancer tissues as well as six normal mammary tissues with immunohistochemistry staining using phospho-specific antibodies. Consistent findings were obtained as greater than 70% of invasive breast carcinomas expressed moderate to high levels of phosphorylated PDK-1, AKT, p70S6K, and EGFR. In sharp contrast, phosphorylation of the same proteins was nearly undetectable or was at low levels in normal mammary tissues under the same assay. Elevated phosphorylation of PDK-1, AKT, mTOR, p70S6K, S6, EGFR, and Stat3 were highly associated with invasive breast tumours (P<0.05). Taken together, our results suggest that activation of these kinase pathways by phosphorylation may in part account for molecular pathogenesis of human breast carcinoma. Particularly, moderate to high level of PDK-1 phosphorylation was found in 86% of high-grade metastasised breast tumours. This is the first report demonstrating phosphorylation of PDK-1 is frequently elevated in breast cancer with concomitantly increased phosphorylation of downstream kinases, including AKT, mTOR, p70S6K, S6, and Stat3. This finding thus suggested PDK-1 may promote oncogenesis in part through the activation of AKT and p70S6K and rationalised that PDK-1 as well as downstream components of PDK-1 signalling pathway may be promising therapeutic targets to treat breast cancer.

Integrin-linked kinase (ILK) is highly expressed in first trimester human chorionic villi and regulates migration of a human cytotrophoblast-derived cell line

The placenta represents a critically important fetal-maternal interaction. Trophoblast migration and invasion into the uterine wall is a precisely controlled process and aberrations in these processes are implicated in diseases such as preeclampsia. Integrin-linked kinase (ILK) is a multifunctional, cytoplasmic, serine/threonine kinase that has been implicated in regulating processes such as cell proliferation, survival, migration, and invasion; yet the temporal and spatial pattern of expression of ILK in human chorionic villi and its role in early human placental development are completely unknown. We hypothesized that ILK would be expressed in trophoblast subtypes of human chorionic villi during early placental development and that it would regulate trophoblast migration. Immunoblot analysis revealed that ILK protein was highly detectable in placental tissue samples throughout gestation. In floating branches of chorionic villi, from 6 to 15 wk of gestation immunofluorescence analysis of ILK expression in placental tissue sections demonstrated that ILK was highly detectable in the cytoplasm and membranes of villous cytotrophoblast cells and in stromal mesenchyme, whereas it was barely detectable in the syncytiotrophoblast layer. In anchoring branches of villi, ILK was highly localized to plasma membranes of extravillous trophoblast cells. Transient expression of dominant negative E359K-ILK in the villous explant-derived trophoblast cell line HTR8-SVneo dramatically reduced migration into wounds compared to cells expressing wild-type ILK or empty vector. Therefore, our work has demonstrated that ILK is highly expressed in trophoblast subtypes of human chorionic villi during the first trimester of pregnancy and is a likely mediator of trophoblast migration during this period of development.

Critical role of integrin-linked kinase in granule cell precursor proliferation and cerebellar development

Integrin-linked kinase (ILK) is a serine/threonine protein kinase that plays an important role in integrin signaling and cell proliferation. We used Cre recombinase (Cre)-loxP technology to study CNS restricted knock-out of the ilk gene by either Nestin-driven or gfap-driven Cre-mediated recombination. Developmental changes in ilk-excised brain regions are similar to those observed in mice lacking the integrin beta1 subunit in the CNS, including defective laminin deposition, abnormal glial morphology, and alterations in granule cell migration. Decreases in 6-bromodeoxyuridine (BrdU) pulse labeling and proliferating cell nuclear antigen expression in the external granule cell layer of the cerebellum demonstrated that proliferation is disrupted in granule cells lacking ILK. Previous studies have shown that laminin-sonic hedgehog (Shh)-induced granule cell precursor (GCP) proliferation is dependent on beta1 integrins, several of which bind laminin and interact with ILK through the beta1 cytoplasmic domain. Both ex vivo deletion of ilk and a small molecule inhibitor of ILK kinase activity decreased laminin-Shh-induced BrdU labeling in cultured GCPs. Together, these results implicate ILK as a critical effector in a signaling pathway necessary for granule cell proliferation and cerebellar development.

Costameres, focal adhesions, and cardiomyocyte mechanotransduction

Mechanotransduction refers to the cellular mechanisms by which load-bearing cells sense physical forces, transduce the forces into biochemical signals, and generate appropriate responses leading to alterations in cellular structure and function. This process affects the beat-to-beat regulation of cardiac performance but also affects the proliferation, differentiation, growth, and survival of the cellular components that comprise the human myocardium. This review focuses on the experimental evidence indicating that the costamere and its structurally related structure the focal adhesion complex are critical cytoskeletal elements involved in cardiomyocyte mechanotransduction. Biochemical signals originating from the extracellular matrix-integrin-costameric protein complex share many common features with those signals generated by growth factor receptors. The roles of key regulatory kinases and other muscle-specific proteins involved in mechanotransduction and growth factor signaling are discussed, and issues requiring further study in this field are outlined.

The parvins

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

Integrin-linked kinase in renal disease: connecting cell-matrix interaction to the cytoskeleton

PURPOSE OF REVIEW

Cellular functions like proliferation, differentiation, migration, morphogenesis and apoptosis are modulated by the extracellular matrix. Integrins are the prototypic heterodimeric transmembrane matrix receptors with competing affinities for individual extracellular matrix ligands. The intracellular integrin domain clusters cytoplasmic proteins into focal adhesion plaques for bidirectional (outside-in and inside-out) signalling. Integrin-linked kinase organizes the connections of the extracellular matrix via integrins to the cytoskeleton and is involved in adhesion plaque signalling. In this review, an introduction of integrin-linked kinase structure and function is followed by a summary of our current understanding of integrin-linked kinase in renal disease with special focus on glomerular cell-matrix interaction.

RECENT FINDINGS

Differential regulation of integrin-linked kinase has been observed during the pathogenesis of glomerular disease and tubulo-interstitial fibrosis. In outside-in signalling integrin-linked kinase mediates the response of renal cells to alterations in matrix and growth factor environments. Inside-out signalling transduces inflammatory and oxidative stress responses into decreased matrix attachment. Downstream signalling of integrin-linked kinase activates the Wnt pathway with a switch towards a proliferative, mesenchymal phenotype. In concert with interacting molecules integrin-linked kinase influences the actin cytoskeleton, resulting in shape change and focal adhesion dysfunction observed in podocyte failure and tubulo-interstitial fibrosis.

SUMMARY

Integrin-linked kinase has emerged as a key player at the interface between extracellular matrix, integrins, actin-based cytoskeleton and cellular phenotype in kidney diseases. Future studies focusing on interacting molecules and modification of integrin-linked kinase function in vivo will better define the role of cell matrix signalling in progressive renal failure.

The PI3' kinase pathway in interferon signaling

Interferons (IFNs) are pleiotropic cytokines with important immunomodulatory, antitumor, antiviral, and growth inhibitory properties. Consistent with the multiplicity of their effects, the IFNs activate several cellular cascades after they bind to their receptors. Although the Jak-Stat pathway is the most studied pathway, it appears that additional signaling cascades play roles in IFN signaling. The phosphatidylinositol 3'-kinase (PI3'K) pathway has emerged as one of the critical players in IFN signal transduction and is the focus of this review. This pathway is activated distinctively from the Stat pathway but plays important roles in the regulation of IFN-dependent gene transcription and initiation of mRNA translation.

Oncogenic PI3K deregulates transcription and translation

There have long been indications of a role for PI3K (phosphatidylinositol 3-kinase) in cancer pathogenesis. Experimental data document a requirement for deregulation of both transcription and translation in PI3K-mediated oncogenic transformation. The recent discoveries of cancer-specific mutations in PIK3CA, the gene that encodes the catalytic subunit p110alpha of PI3K, have heightened the interest in the oncogenic potential of this lipid kinase and have made p110alpha an ideal drug target.

HIV protease inhibitors block Akt signaling and radiosensitize tumor cells both in vitro and in vivo

In tumor cells with mutations in epidermal growth factor receptor (SQ20B), H-Ras (T24), or K-Ras (MIAPACA2 and A549), the inhibition of Akt phosphorylation increases radiation sensitivity in clonogenic assays, suggesting that Akt is a potential molecular target when combined with therapeutic radiation. Insulin resistance and diabetes are recognized side effects of HIV protease inhibitors (HPIs), suggesting that these agents may inhibit Akt signaling. Because activation of the phosphatidylinositol 3-kinase (PI3K)-Akt signaling pathway is common in human cancers, we hypothesized that HPIs can inhibit Akt activity resulting in increased tumor cell sensitivity to ionizing radiation-induced cell death. Five first-generation HPIs were subsequently tested and three of the five (amprenavir, nelfinavir, and saquinavir but not ritonavir or indinavir) inhibited Akt phosphorylation at Ser473 at serum concentrations routinely achieved in HIV patients. In both tumor cell colony formation assays and tumor regrowth delay experiments, combinations of drug and radiation exerted synergistic effects compared with either modality alone. In addition, in vivo, doses of amprenavir or nelfinavir comparable with the therapeutic levels achieved in HIV patients were sufficient to down-regulate phosphorylation of Akt in SQ20B and T24 xenografts. Finally, overexpression of active PI3K in cells without activation of Akt resulted in radiation resistance that could be inhibited with HPIs. Because there is abundant safety data on HPIs accumulated in thousands of HIV patients over the last 5 years, these agents are excellent candidates to be tested as radiation sensitizers in clinical trials.

The Akt/PKB pathway: molecular target for cancer drug discovery

The serine/threonine kinase Akt/PKB pathway presents an exciting new target for molecular therapeutics, as it functions as a cardinal nodal point for transducing extracellular (growth factor and insulin) and intracellular (receptor tyrosine kinases, Ras and Src) oncogenic signals. In addition, alterations of the Akt pathway have been detected in a number of human malignancies. Ectopic expression of Akt, especially constitutively activated Akt, is sufficient to induce oncogenic transformation of cells and tumor formation in transgenic mice as well as chemoresistance. Akt has a wide range of downstream targets that regulate tumor-associated cell processes such as cell growth, cell cycle progression, survival, migration, epithelial-mesenchymal transition and angiogenesis. Blockage of Akt signaling results in apoptosis and growth inhibition of tumor cells with elevated Akt. The observed dependence of certain tumors on Akt signaling for survival and growth has wide implications for cancer therapy, offering the potential for preferential tumor cell killing. In the last several years, through combinatorial chemistry, high-throughput and virtual screening, and traditional medicinal chemistry, a number of inhibitors of the Akt pathway have been identified. This review focuses on ongoing translational efforts to therapeutically target the Akt pathway.

An expanding role for mTOR in cancer

Rapamycin, a valuable drug with diverse clinical applications, inhibits mTOR (mammalian target of rapamycin), which is a protein kinase that controls cell growth by regulating many cellular processes, including protein synthesis and autophagy. The sensitivity of select tumor cells to rapamycin has ignited considerable excitement over its potential as an anti-cancer therapeutic. Recent findings identified a rapamycin-insensitive function of mTOR in regulating a cell-survival pathway that is hyperactive in many cancers, particularly those with elevated PtdIns3K signaling or harboring mutations in the tumor suppressor PTEN (phosphatase and tensin homolog deleted on chromosome 10). These new findings suggest that targeting this function of mTOR might have broader applications in cancer therapy. In this article, we re-evaluate mTOR signaling, suggesting a more central role for mTOR in cancers with defective PtdIns3K-PTEN signaling and conceptually discuss these implications in the context of drug discovery.

mTOR.RICTOR is the Ser473 kinase for Akt/protein kinase B in 3T3-L1 adipocytes

The insulin-signaling pathway leading to the activation of Akt/protein kinase B has been well characterized except for a single step, the phosphorylation of Akt at Ser-473. Double-stranded DNA-dependent protein kinase (DNA-PK), ataxia telangiectasia mutated (ATM) gene product, integrin-linked kinase (ILK), protein kinase Calpha (PKCalpha), and mammalian target of rapamycin (mTOR), when complexed to rapamycin-insensitive companion of mTOR (RICTOR), have all been identified as playing a critical role in Akt Ser-473 phosphorylation. However, the apparently disparate results reported in these studies are difficult to evaluate, given that different stimuli and cell types were examined and that all of the candidate proteins have never been systematically studied in a single system. Additionally, none of these studies were performed in a classical insulin-responsive cell type or tissue such as muscle or fat. We therefore examined each of these candidates in 3T3-L1 adipocytes. In vitro kinase assays, using different subcellular fractions of 3T3-L1 adipocytes, revealed that phosphatidylinositol 3,4,5-trisphosphate-stimulated Ser-473 phosphorylation correlated well with the amount of DNA-PK, mTOR, and RICTOR but did not correlate with levels of ATM, ILK, and PKCalpha. PKCalpha was completely absent from compartments with Ser-473 phosphorylation activity. Although purified DNA-PK could phosphorylate a peptide derived from Akt that contains amino acid Ser-473, it could not phosphorylate full-length Akt2. Vesicles immunoprecipitated from low density microsomes using antibodies directed against mTOR or RICTOR had phosphatidylinositol 3,4,5-trisphosphate-stimulated Ser-473 activity that was sensitive to wortmannin but not staurosporine. In contrast, immunopurified low density microsome vesicles containing ILK could not phosphorylate Akt on Ser-473 in vitro. Small interference RNA knockdown of RICTOR, but not DNA-PK, ATM, or ILK, suppressed insulin-activated Ser-473 phosphorylation and, to a lesser extent, Thr-308 phosphorylation in 3T3-L1 adipocytes. Based on our cell-free kinase and small interference RNA results, we conclude that mTOR complexed to RICTOR is the Ser-473 kinase in 3T3-L1 adipocytes.

Integrin-linked kinase activity regulates Rac- and Cdc42-mediated actin cytoskeleton reorganization via alpha-PIX

Cell spreading and migration are regulated in a Rho family GTPase-dependent manner by growth factors and integrin-mediated cell-extracellular matrix (ECM) interactions. The molecular mechanisms involved in the ECM- and growth factor-mediated activation of these small GTPases remain unclear. In the present study, we demonstrate that integrin-linked kinase (ILK), which is a focal adhesion protein activated by both ECM and growth factors, is required for the activation of Rac and Cdc42 in epithelial cells. Ectopic expression of active ILK in mammary epithelial cells induces dramatic reorganization of the actin cytoskeleton and promotes rapid cell spreading on fibronectin. These effects are associated with constitutive activation of both Rac and Cdc42, but not Rho. The use of ILK siRNA or small molecule inhibitors to inhibit ILK expression and kinase activity, respectively, results in diminished cell spreading and actin cytoskeleton reorganization, concomitant with a reduction in Rac and Cdc42 activation. Studies into the mechanism of ILK-mediated Rac activation suggest an important role for the ILK-beta-parvin interaction and the activity of the Rac/Cdc42-specific guanine nucleotide exchange factor alpha-PIX downstream of ILK. Taken together, these data demonstrate an essential role of ILK kinase activity in Rac- and Cdc42-mediated actin cytoskeleton reorganization in epithelial cells, further solidifying a role for ILK in the regulation of cancer cell motility and invasiveness.

SPARC Regulates Extracellular Matrix Organization through Its Modulation of Integrin-linked Kinase Activity

SPARC, a 32-kDa matricellular glycoprotein, mediates interactions between cells and their extracellular matrix, and targeted deletion of Sparc results in compromised extracellular matrix in mice. Fibronectin matrix provides provisional tissue scaffolding during development and wound healing and is essential for the stabilization of mature extracellular matrix. Herein, we report that SPARC expression does not significantly affect fibronectin-induced cell spreading but enhances fibronectin-induced stress fiber formation and cell-mediated partial unfolding of fibronectin molecules, an essential process in fibronectin matrix assembly. By phage display, we identify integrin-linked kinase as a potential binding partner of SPARC and verify the interaction by co-immunoprecipitation and colocalization in vitro. Cells lacking SPARC exhibit diminished fibronectin-induced integrin-linked kinase activation and integrin-linked kinase-dependent cell-contractile signaling. Furthermore, induced expression of SPARC in SPARC-null fibroblasts restores fibronectin-induced integrin-linked kinase activation, downstream signaling, and fibronectin unfolding. These data further confirm the function of SPARC in extracellular matrix organization and identify a novel mechanism by which SPARC regulates extracellular matrix assembly.

High glucose and angiotensin II increase β1 integrin and integrin-linked kinase synthesis in cultured mouse podocytes

Alterations of integrin alpha3beta1 may play a role in the development of diabetic nephropathy. We have investigated the effects of high glucose and angiotensin II on the expression of integrin alpha3 and beta1, and whether these changes are associated with integrin-linked kinase (ILK) in cultured mouse podocytes. Integrin beta1 and ILK mRNA expression and protein production were rapidly up-regulated in a dose-dependent manner by high glucose and angiotensin II stimulation. ILK mRNA levels in the mouse podocytes exposed to 30 mmol/l glucose were 1.66, 1.89, and 1.28 times higher than those in control cells at 6, 24, and 72 h exposure, respectively. ILK mRNA levels in mouse podocytes exposed to 1 nM, 10 nM, and 100 nM angiotensin II for 6 h were 1.38, 1.55, and 1.93 times higher, respectively, than those in control cells. Angiotensin-II-induced integrin beta1 and ILK mRNA expression was significantly inhibited by treatment with losartan (100 muM). In addition, the up-regulation of ILK synthesis induced by these stimuli was related to beta1 integrin synthesis and increased ILK kinase activity. Cell adhesion assay displayed inhibitory effects when podocytes were exposed to high concentrations of angiotensin II. Interestingly, glucose and angiotensin II stimulation induced shrinkage of the cell body and elongation of the podocyte processes, a phenotype similar to that of immature podocytes. In addition, beta1 integrin showed higher levels of staining on both the cell membranes and the cell-cell contact areas. Thus, high glucose and angiotensin II may affect the regulation of the integrin-ILK system in podocytes; this system may therefore play a role in the pathogenesis of diabetic nephropathy and other renal diseases affecting podocytes.

Therapeutic potential of target of rapamycin inhibitors

Target of rapamycin (TOR) functions within the cell as a transducer of information from various sources, including growth factors, energy sensors, and hypoxia sensors, as well as components of the cell regulating growth and division. Blocking TOR function mimics amino acid, and to some extent, growth factor deprivation and has a cytostatic effect on proliferating cells in vivo. Inhibition of TOR in vivo, utilising its namesake rapamycin, leads to immunosuppression. This property has been exploited successfully with the use of rapamycin and its derivatives as a therapeutic agent in the prevention of organ rejection after transplantation with relatively mild side effects when compared to other immunosuppressive agents. The cytostatic effect of TOR on vascular smooth muscle cell proliferation has also recently been exploited in the therapeutic application of rapamycin to drug eluting stents for angioplasty. These stents significantly reduce the amount of arterial reblockage that results from proliferating vascular smooth muscle cells. In cancer, the effect of blocking TOR function on tumour growth and disease progression is currently of major interest and is the basis for a number of ongoing clinical trials. However, different cell types and tumours respond differently to TOR inhibition, and TOR is clearly not cytostatic for all types of cancer cells in vitro or in vivo. As the molecular details of how TOR functions and the targets of TOR activity are further elucidated, tumour and tissue specific functions are being identified that implicate TOR in angiogenesis, apoptosis, and the reversal of some forms of cellular transformation. This review will describe our current understanding of TOR function, describe the current strategies for employing TOR inhibitors in clinical and preclinical development, and outline future strategies for appropriate targets of TOR inhibitors in the treatment of disease.

The TCL1 family of oncoproteins: co-activators of transformation

The T-cell leukaemia/lymphoma 1 (TCL1)-family oncoproteins augment AKT signal transduction and enhance cell proliferation and survival. Chromosome rearrangements, faulty developmental silencing and Epstein-Barr virus infection appear to dysregulate the expression of TCL1-family genes, provoking several important types of lymphocyte cancer. A key role for TCL1 proteins in cell transformation has been established in studies of transgenic mouse models, which develop a unique spectrum of T- and B-cell malignancies. How TCL1 proteins are regulated and dysregulated, how they promote transformation and the potential for therapies modelled on TCL1 interactions have important implications for understanding and treating lymphocyte cancers.

Akt1 contains a functional leucine-rich nuclear export sequence

Nuclear Akt1 expression and Akt activation are common in cancer invasion. However, the mechanisms for this association and its causal role in invasion are uncertain. In an effort to identify potential mechanisms for regulating Akt subcellular localization, we analyzed the Akt gene sequences and identified a highly conserved leucine-rich potential nuclear export sequence (NES). Initial experiments demonstrated that leptomycin B induced nuclear Akt1 localization. Transient expression experiments demonstrated that, in comparison to wild-type Akt1, NES-mutated (AKT/NES) Akt1 has reduced interactions with CRM-1 and persistent nuclear localization. Subsequent stable transfection experiments in Akt1-/- fibroblasts confirmed that expression of AKT/NES resulted in persistent nuclear localization and activation1. Finally, stable expression of AKT/NES in Akt1-/- fibroblasts was sufficient to enhance cell migration in vitro. Thus, Akt1 contains a functional NES and mutation of the NES results in nuclear-predominant Akt1 activation that is sufficient to induce migration.

Role of Integrin-linked Kinase in Regulating Phosphorylation of Akt and Fibroblast Survival in Type I Collagen Matrices through a β1 Integrin Viability Signaling Pathway

A beta1 integrin phosphatidylinositol 3-kinase/Akt pathway regulates fibroblast survival in collagen matrices. When fibroblasts attach to collagen, Akt becomes phosphorylated, providing a survival signal. In contrast, in response to mechanical forces generated during collagen contraction, Akt is dephosphorylated and fibroblasts undergo apoptosis. The kinase(s) responsible for regulating Akt phosphorylation in response to matrix-derived mechanical signals are unclear. Integrin-linked kinase (ILK) is associated with the beta1 integrin in the focal adhesion complex and as such is a candidate kinase that may regulate Akt phosphorylation and fibroblast viability. Nevertheless, there is no direct evidence that matrix-derived mechanical forces regulate cell viability by modulating ILK activity. Here, we show that ILK activity decreased in response to collagen matrix contraction, which correlated with Akt dephosphorylation and induction of fibroblast apoptosis. In contrast, enforced activation of beta1 integrin by activating antibody preserved ILK and Akt activity during collagen matrix contraction, and this is associated with protection from collagen contraction-induced apoptosis. Knock-down of ILK by small, interfering RNA (siRNA) attenuated Akt phosphorylation in response to ligation of beta1 integrin by collagen or activating antibody and enhanced fibroblast apoptosis in response to collagen contraction. Kinase dead ILK attenuated Akt phosphorylation and enhanced fibroblast apoptosis, whereas hyperactive and wild type ILK augmented Akt phosphorylation and protected fibroblasts from apoptosis. Constitutively active Akt preserved Akt activity and rescued ILK siRNA-treated fibroblasts from collagen contraction-induced apoptosis. These data establish that matrix-derived mechanical forces sensed by beta1 integrin are capable of modulating ILK activity which regulates fibroblast viability via an Akt-dependent mechanism.

Inhibition of ILK in PTEN-mutant human glioblastomas inhibits PKB/Akt activation, induces apoptosis, and delays tumor growth

The tumor suppressor gene phosphatase and tensin homologue (PTEN) regulates the phosphatidylinositol-3'-kinase (PI3K) signaling pathway and has been shown to correlate with poor prognosis in high-grade astrocytomas when mutational inactivation or loss of the PTEN gene occurs. PTEN mutation leads to constitutive activation of protein kinase B (PKB)/Akt with phosphorylation at the PKB/Akt sites Thr-308 and Ser-473. Integrin-linked kinase (ILK) has been shown to regulate PKB/Akt activity with the loss of PTEN in prostate cancer. We now demonstrate that ILK activity regulates PKB/Akt activity in glioblastoma cells. The activity of ILK is constitutively elevated in a serum-independent manner in PTEN mutant cells, and transfection of wild-type PTEN under the control of an inducible promoter into mutant PTEN cells inhibits ILK activity. Transfection of ILK antisense (ILKAS) or exposure to a small-molecule ILK inhibitor suppresses the constitutive phosphorylation of PKB/Akt on Ser-473 in PTEN-mutant glioblastoma cell lines. In addition, the delivery of ILKAS to PTEN-negative glioblastoma cells resulted in apoptosis. Rag-2M mice bearing established ( approximately 100 mg) human U87MG glioblastoma tumors, treated QD x 5 for 3 consecutive weeks with ILKAS (i.p. 5 mg/kg), exhibited stable disease with < or =7% increase in tumor volume over the 3-week course of treatment. In contrast, animals treated with an oligonucleotide control or saline exhibited a >100% increase in tumor volume over the same time period. Our initial results indicate that therapeutic strategies targeting ILK may be beneficial in the treatment of glioblastomas.

Akt phosphorylates the Y-box binding protein 1 at Ser102 located in the cold shock domain and affects the anchorage-independent growth of breast cancer cells

Akt/PKB is a serine/threonine kinase that promotes tumor cell growth by phosphorylating transcription factors and cell cycle proteins. There is particular interest in finding tumor-specific substrates for Akt to understand how this protein functions in cancer and to provide new avenues for therapeutic targeting. Our laboratory sought to identify novel Akt substrates that are expressed in breast cancer. In this study, we determined that activated Akt is positively correlated with the protein expression of the transcription/translation factor Y-box binding protein-1 (YB-1) in primary breast cancer by screening tumor tissue microarrays. We therefore questioned whether Akt and YB-1 might be functionally linked. Herein, we illustrate that activated Akt binds to and phosphorylates the YB-1 cold shock domain at Ser102. We then addressed the functional significance of disrupting Ser102 by mutating it to Ala102. Following the stable expression of Flag:YB-1 and Flag:YB-1 (Ala102) in MCF-7 cells, we observed that disruption of the Akt phosphorylation site on YB-1 suppressed tumor cell growth in soft agar and in monolayer. This correlated with an inhibition of nuclear translocation by the YB-1(Ala102) mutant. In conclusion, YB-1 is a new Akt substrate and disruption of this specific site inhibits tumor cell growth.

ILK mediates actin filament rearrangements and cell migration and invasion through PI3K/Akt/Rac1 signaling

One of the hallmarks of integrin signaling is an increase in cell migration and invasion, both of which are associated with actin filament rearrangements. Integrin-linked kinase (ILK) is a cytoplasmic effector of integrin receptors. ILK is known to be involved in multiple cellular functions. However, the signaling pathways involved in ILK-mediated cellular structure and motility remain to be elucidated. Here, we have demonstrated that overexpression of ILK was sufficient to induce actin filament rearrangements, to form cell motility structures, and to increase cell migration and invasion in a phosphatidylinositol 3-kinase (PI3K)-dependent manner. This corresponds with the activation of both Akt and p70 ribosomal protein S6 kinase (p70S6K1). Overexpression of dominant-negative mutants of Akt inhibited ILK-dependent activation of p70S6K1, indicating that Akt is upstream of p70S6K1 in response to ILK signaling. Overexpression of ILK was sufficient to induce Rac1 activation, which was abolish by a PI3K inhibitor, indicating that Rac1 activity is involved in ILK signaling in a PI3K dependent manner. Inhibition of Akt, Rac1, or p70S6K1 inhibited the effects of ILK on actin filaments and cell migration, suggesting a regulatory role of the PI3K/Akt/p70S6K1/Rac1 signaling pathway in response to ILK signaling. We have shown that overexpression of a dominant-negative ILK was sufficient to abolish fibronectin peptide (PHSRN)-induced rearrangements of actin filaments and cell migration and invasion. Taken together, our results identify a mechanism through which ILK can regulate both integrin-associated rearrangements of actin filaments and cell migration and invasion at the integrin receptor-proximal region.

Proapoptotic activity of cell-permeable anti-Akt single-chain antibodies

We developed anti-Akt1 single-chain antibodies (scFv) by panning a mouse phage-displayed scFv recombinant antibody library. Recombinant scFv that bound glutathione S-transferase (GST)-Akt1 were screened for their ability to inhibit Akt activity in vitro in a kinase reaction containing human recombinant Akt1 and an Akt/serum glucocorticoid-inducible kinase (SGK) substrate. Michaelis-Menten analysis of kinase inhibition by a selected scFv was consistent with scFv-mediated competition with enzyme's substrate for the catalytic site of Akt. To generate a membrane-permeable version of the anti-Akt1 scFv, the scFv gene was subcloned into a GST expression vector carrying a membrane-translocating sequence (MTS) from Kaposi fibroblast growth factor. A purified GST-anti-Akt1-MTS fusion protein accumulated intracellularly in 293T, BT-474, and PyVmT cells in a dose- and time-dependent fashion. Intracellular accumulation correlated temporally with inhibition of p-Ser(473) Akt and GSK-3alpha/beta phosphorylation, suggesting that Ser(473) is an Akt autophosphorylation site. Phosphorylated (activated) phosphoinositide-dependent kinase 1, mitogen-activated protein kinase, p38, and HER2 (erbB2) were not affected, supporting Akt kinase specificity for the inhibitory scFv. Exogenously expressed constitutively active Akt2 and Akt3 were also inhibited in vitro by the anti-Akt1 fusion protein. Furthermore, GST-anti-Akt1-MTS induced apoptosis in three cancer cell lines that express constitutively active Akt. Finally, systemic treatment with the anti-Akt scFv reduced tumor volume and neovascularization and increased apoptosis in PyVmT-expressing transgenic tumors implanted in mouse dorsal window chambers. Thus, GST-anti-Akt1-MTS is a novel cell-permeable inhibitor of Akt, which selectively inhibits Akt-mediated survival in intact cells both in vitro and in vivo.

Involvement of integrin-linked kinase in capillary/tube-like network formation of human vascular endothelial cells

Angiogenesis is a complex process involving an ECM and vascular endothelial cells (EC), and is regulated by various angiogenic factors including VEGF. The ability to form a capillary/tube-like network is a specialized function of EC. Therefore, in vitro angiogenesis was assessed by a capillary/tube-like network formation assay. There are three angiogenic parameters: capillary length, number of capillaries, and relative capillary area per field. We evaluated capillary length per field in the assay. VEGF promoted capillary/tube-like network formation of EC in a type I collagen gel matrix in vitro. Moreover, we demonstrated the involvement of ILK in a VEGF signaling pathway mediating capillary/tube-like network formation of EC using dominant-negative, kinase deficient ILK. This is a straightforward assay to monitor responses of human vascular endothelial cells.

The activation of Akt/PKB signaling pathway and cell survival

Akt/PKB is a serine/threonine protein kinase that functions as a critical regulator of cell survival and proliferation. Akt/PKB family comprises three highly homologous members known as PKBalpha/Akt1, PKBbeta/Akt2 and PKBgamma/Akt3 in mammalian cells. Similar to many other protein kinases, Akt/PKB contains a conserved domain structure including a specific PH domain, a central kinase domain and a carboxyl-terminal regulatory domain that mediates the interaction between signaling molecules. Akt/PKB plays important roles in the signaling pathways in response to growth factors and other extracellular stimuli to regulate several cellular functions including nutrient metabolism, cell growth, apoptosis and survival. This review surveys recent developments in understanding the molecular mechanisms of Akt/PKB activation and its roles in cell survival in normal and cancer cells.

Activation of a GST-tagged AKT2/PKBbeta

The protein kinase AKT is a key regulator for cell growth, cell survival and metabolic insulin action. However, the mechanism of activation of AKT in vivo, which presumably involves membrane recruitment of the kinase, oligomerization, and multiple phosphorylation events, is not fully understood. In the present study, we have expressed and purified dimeric GST-fusion proteins of human protein kinase AKT2 (DeltaPH-AKT2) in milligram quantities via the baculovirus expression system. Treatment of virus-infected insect cells with the phosphatase inhibitor okadaic acid (OA) led to phosphorylation of the two regulatory phosphorylation sites, Thr309 and Ser474, and to activation of the kinase. Likewise, phosphorylation of Thr309 in vitro by recombinant PDK1 or mutation of Thr309 and Ser474 to acidic residues rendered the kinase constitutively active. However, even though the specific activity of our AKT2 was increased 15-fold compared to previous reports, GST-mediated dimerization alone did not lead to an activation of the kinase. Whereas both mutagenesis and phosphorylation led to an increase in the turnover number of the enzyme, only the latter resulted in a marked reduction (20-fold) of the apparent Km value for the exogenous substrate Crosstide, indicating that this widely used mutagenesis only partially mimics phosphorylation. Kinetic analysis of GST-AKT2 demonstrates that phosphorylation of Thr309 in the activation loop of the kinase is largely responsible for the observed reduction in Km and for a subsequent 150-fold increase in the catalytic efficiency (k(cat)/Km) of the enzyme. Highly active AKT2 constructs were used in autophosphorylation reactions in vitro, where inactive AKT2 kinases served as substrates. As a matter of fact, we found evidence for a minor autophosphorylation activity of AKT2 but no significant autophosphorylation of any of the two regulatory sites, Thr309 or Ser474.

Differential effects of Hsp90 inhibition on protein kinases regulating signal transduction pathways required for myoblast differentiation

As derivatives of the Hsp90-inhibitor and tumoricidal agent geldanamycin move into phase II clinical trials, its potential for triggering adverse effects in non-tumor cell populations requires closer examination. In this report, the effect of geldanamycin on the differentiation and survival of C2C12 myoblasts was investigated. Treatment of differentiating C2C12 myoblasts with geldanamycin blocked myogenin expression, inhibited myotubule formation, and led to the depletion of three Hsp90-dependent protein kinases, ErbB2, Fyn, and Akt, and induction of apoptosis. ErbB2 levels declined rapidly, while Fyn and Akt levels decreased at a slower rate. Geldanamycin blocked the interaction of Hsp90 and its "kinase-specific" co-chaperone Cdc37 with Fyn, indicating that Fyn is an Hsp90-dependent kinase. Pulse-chase experiments indicated that geldanamycin caused newly synthesized Akt and Fyn to be degraded rapidly, but geldanamycin had little effect on the turnover rate of mature Fyn and Akt. Curiously, total cellular Src (c-Src) protein levels and the turnover rate of newly synthesized c-Src were unaffected by geldanamycin. While, geldanamycin had no effect on the levels of the putative Hsp90 client protein MyoD expressed in C2C12 cells, geldanamycin disrupted the interaction of Cdc37 with MyoD. Thus, inhibition of Hsp90 caused C2C12 cells to become depleted of multiple signal transduction proteins whose functions are essential for myoblast differentiation, and muscle cell survival, suggesting that geldanamycin derivatives may have the prospective of adversely affecting the physiology of certain sensitive muscle cell populations in vivo.

Integrin-linked kinase: integrin's mysterious partner

Integrin-mediated cell adhesion regulates a vast number of biological processes including migration, survival and proliferation of cells. It is therefore not surprising that defects in integrin function are often rate-limiting for development and profoundly affect the progression of several diseases. The functions of integrins are mediated through the recruitment of cytoplasmic plaque proteins. One of these is integrin-linked kinase, which connects integrins to the actin cytoskeleton and transduces signals through integrins to the extracellular matrix and from integrins to various subcellular compartments.

Novel small molecule inhibitors of 3-phosphoinositide-dependent kinase-1

The phosphoinositide 3-kinase/3-phosphoinositide-dependent kinase 1 (PDK1)/Akt signaling pathway plays a key role in cancer cell growth, survival, and tumor angiogenesis and represents a promising target for anticancer drugs. Here, we describe three potent PDK1 inhibitors, BX-795, BX-912, and BX-320 (IC(50) = 11-30 nm) and their initial biological characterization. The inhibitors blocked PDK1/Akt signaling in tumor cells and inhibited the anchorage-dependent growth of a variety of tumor cell lines in culture or induced apoptosis. A number of cancer cell lines with elevated Akt activity were >30-fold more sensitive to growth inhibition by PDK1 inhibitors in soft agar than on tissue culture plastic, consistent with the cell survival function of the PDK1/Akt signaling pathway, which is particularly important for unattached cells. BX-320 inhibited the growth of LOX melanoma tumors in the lungs of nude mice after injection of tumor cells into the tail vein. The effect of BX-320 on cancer cell growth in vitro and in vivo indicates that PDK1 inhibitors may have clinical utility as anticancer agents.

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

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

Phosphorylation and regulation of Akt/PKB by the rictor-mTOR complex

Deregulation of Akt/protein kinase B (PKB) is implicated in the pathogenesis of cancer and diabetes. Akt/PKB activation requires the phosphorylation of Thr308 in the activation loop by the phosphoinositide-dependent kinase 1 (PDK1) and Ser473 within the carboxyl-terminal hydrophobic motif by an unknown kinase. We show that in Drosophila and human cells the target of rapamycin (TOR) kinase and its associated protein rictor are necessary for Ser473 phosphorylation and that a reduction in rictor or mammalian TOR (mTOR) expression inhibited an Akt/PKB effector. The rictor-mTOR complex directly phosphorylated Akt/PKB on Ser473 in vitro and facilitated Thr308 phosphorylation by PDK1. Rictor-mTOR may serve as a drug target in tumors that have lost the expression of PTEN, a tumor suppressor that opposes Akt/PKB activation.

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

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

RGD peptides confer survival to hepatocytes via the beta1-integrin-ILK-pAkt pathway

BACKGROUND/AIMS

Allogeneic cell transplantation is characterized by a lack of sustained survival of the transplanted cells in the recipient. Activation of the appropriate integrin-linked signaling pathways in cells can promote cell survival. The purpose of this study was to determine how presence or absence of anti-beta1 integrin chain antibodies or RGD peptides affects the survival of hepatocytes.

METHODS

Hepatocytes of BN rats were isolated. Hepatocyte survival was tested after the hepatocytes had been cultured in the presence of anti-beta1 integrin antibodies or RGD peptides. Hepatocytes that had been given a different treatment were stained for caspase 3 (apoptosis marker) and phospho-Akt Ser 473 (survival marker) and were measured for their integrin-linked kinase (ILK) activity.

RESULTS

Ligation of integrins using antibodies against the beta1 integrin chain or RGD peptides protected isolated hepatocytes from apoptosis and resulted in an increased ILK activity and persistent phosphorylation of protein kinase B/Akt at serine 473.

CONCLUSIONS

Integrin activation in isolated hepatocytes contributes to the activation of ILK, phosphorylation of Akt on serine residue 473, and inhibition of apoptosis. Integrin signaling through the ILK-phospho Akt pathway protects isolated hepatocytes from apoptosis. This notion may potentially be applied to render the transplantation of hepatocytes more effective.

Cell-free 59 kDa immunoreactive integrin-linked kinase: a novel marker for ovarian carcinoma

PURPOSE

We reported that the expression of integrin-linked kinase (ILK) is up-regulated in ovarian carcinomas and that ovarian cancer cells have high expression of ILK. In this study, we have examined the expression of cell-free 59 kDa immunoreactive (ir)ILK in the serum and peritoneal fluid (PTF) of patients with ovarian cancer and evaluated its potential as a serum biomarker for early-stage screening and for monitoring clinical status of patients after chemotherapy treatment.

EXPERIMENTAL DESIGN

Thirty-six serum specimens, including normal (n = 6), benign (n = 6), borderline (n = 4), grade 1 (n = 5), grade 2 (n = 5), and grade 3 (n = 10), were evaluated for the expression of irILK by Western blotting. The expression of irILK was evaluated in PTF (n = 10) and peritoneal washings from women with benign ovarian cysts (n = 4). In addition, tissue-conditioned medium obtained from the cultures of primary ovarian tumors (n = 9) was examined for the presence of irILK. Finally, the potential of serum irILK as a biomarker for ovarian cancer screening was evaluated by comparison with cancer antigen 125 (CA 125) concentrations in cancer patients before and after chemotherapy.

RESULTS

irILK expression was present in normal serum and in serum of patients with benign ovarian tumors. irILK expression was 6-9-fold higher in the serum of patients with grade 1, grade 2, and grade 3 ovarian cancer than in the serum of healthy volunteers and patients with benign ovarian tumors (P < 0.01). Enhanced expression of irILK in the serum of ovarian cancer patients correlated with the concentration of CA 125. High expression of irILK was present in all 10 PTF tested. Tissue-conditioned medium prepared from malignant ovarian tumors had 4-fold more irILK expression than conditioned medium obtained from borderline and benign tumors (P < 0.01). irILK expression in serum of cancer patients was reduced to basal normal levels after six cycles of Taxol/carboplatin and was consistent with the change of CA 125 levels before and after chemotherapy.

CONCLUSIONS

These data suggest that irILK is an ovarian tumor-associated antigen and implicates its potential not only as a biomarker for early-stage screening but also as a marker for monitoring the clinical condition of patients after treatment.

The PINCH-ILK-parvin complexes: assembly, functions and regulation

Cell-extracellular matrix (ECM) adhesion is mediated by transmembrane cell adhesion receptors (e.g., integrins) and receptor proximal cytoplasmic proteins. Over the past several years, studies using biochemical, structural, cell biological and genetic approaches have provided important evidence suggesting crucial roles of integrin-linked kinase (ILK), PINCH and CH-ILKBP/actopaxin/affixin/parvin (abbreviated as parvin herein) in ECM control of cell behavior. One general theme emerging from these studies is that the formation of ternary protein complexes consisting of ILK, PINCH and parvin is pivotal to the functions of PINCH, ILK and parvin proteins. In addition, recent studies have begun to uncover the molecular mechanisms underlying the assembly, functions and regulation of the PINCH-ILK-parvin (PIP) complexes. The PIP complexes provide crucial physical linkages between integrins and the actin cytoskeleton and transduce diverse signals from ECM to intracellular effectors. Among the challenges of future studies are to define the functions of different PIP complexes in various cellular processes, identify additional partners of the PIP complexes that regulate and/or mediate the functions of the PIP complexes, and determine the roles of the PIP complexes in the pathogenesis of human diseases involving abnormal cell-ECM adhesion and signaling.

The Serine/Threonine Kinase Akt Promotes Fcγ Receptor-mediated Phagocytosis in Murine Macrophages through the Activation of p70S6 Kinase

Fc gamma receptor (Fc gamma R) clustering by immune complexes activates multiple signaling pathways leading to phagocytosis. We and others have previously reported that Akt is phosphorylated in response to Fc gamma R clustering. However, the functional consequence of Akt activation by Fc gamma R is not known. Using Raw 264.7 macrophage cells transfected to overexpress either constitutively active myristoylated (Myr)-Akt or a dominant-negative CAAX-Akt and bone marrow macrophages (BMMs) from wild-type and transgenic mice expressing macrophage-specific Myr-Akt, we analyzed the function of Akt in phagocytosis. We report that overexpression of Myr-Akt resulted in significant increase in phagocytic efficiency, whereas CAAX-Akt down-regulated phagocytosis in Raw 264.7 cells. Likewise BMMs expressing Myr-Akt displayed enhanced phagocytic ability. Analyzing the downstream effectors of Akt, we demonstrate that p70S6 kinase is constitutively phosphorylated in Myr-Akt-expressing BMMs. p70S6 kinase is reported to influence actin cytoskeleton and cell migration, suggesting that Akt may influence phagocytosis through the activation of p70S6 kinase. Consistent with this, overexpression of either wild-type or constitutively active but not a kinase-inactive p70S6 kinase in Raw 264.7 cells significantly enhanced phagocytosis. Likewise suppression of p70S6 kinase with rapamycin down-regulated phagocytic efficiency conferred by the expression of constitutively active Akt. These findings demonstrate a novel role for Akt in phagocytosis through the activation of p70S6 kinase.