The epithelial-mesenchymal transition: new insights in signaling, development, and disease

The conversion of an epithelial cell to a mesenchymal cell is critical to metazoan embryogenesis and a defining structural feature of organ development. Current interest in this process, which is described as an epithelial–mesenchymal transition (EMT), stems from its developmental importance and its involvement in several adult pathologies. Interest and research in EMT are currently at a high level, as seen by the attendance at the recent EMT meeting in Vancouver, Canada (October 1–3, 2005). The meeting, which was hosted by The EMT International Association, was the second international EMT meeting, the first being held in Port Douglas, Queensland, Australia in October 2003. The EMT International Association was formed in 2002 to provide an international body for those interested in EMT and the reverse process, mesenchymal–epithelial transition, and, most importantly, to bring together those working on EMT in development, cancer, fibrosis, and pathology. These themes continued during the recent meeting in Vancouver.

Discussion at the Vancouver meeting spanned several areas of research, including signaling pathway activation of EMT and the transcription factors and gene targets involved. Also covered in detail was the basic cell biology of EMT and its role in cancer and fibrosis, as well as the identification of new markers to facilitate the observation of EMT in vivo. This is particularly important because the potential contribution of EMT during neoplasia is the subject of vigorous scientific debate (Tarin, D., E.W. Thompson, and D.F. Newgreen. 2005. Cancer Res. 65:5996–6000; Thompson, E.W., D.F. Newgreen, and D. Tarin. 2005. Cancer Res. 65:5991–5995).

Guidelines and definitions for research on epithelial-mesenchymal transition

Epithelial–mesenchymal transition (EMT) encompasses dynamic changes in cellular organization from epithelial to mesenchymal phenotypes, which leads to functional changes in cell migration and invasion. EMT occurs in a diverse range of physiological and pathological conditions and is driven by a conserved set of inducing signals, transcriptional regulators and downstream effectors. With over 5,700 publications indexed by Web of Science in 2019 alone, research on EMT is expanding rapidly. This growing interest warrants the need for a consensus among researchers when referring to and undertaking research on EMT. This Consensus Statement, mediated by ‘the EMT International Association’ (TEMTIA), is the outcome of a 2-year-long discussion among EMT researchers and aims to both clarify the nomenclature and provide definitions and guidelines for EMT research in future publications. We trust that these guidelines will help to reduce misunderstanding and misinterpretation of research data generated in various experimental models and to promote cross-disciplinary collaboration to identify and address key open questions in this research field. While recognizing the importance of maintaining diversity in experimental approaches and conceptual frameworks, we emphasize that lasting contributions of EMT research to increasing our understanding of developmental processes and combatting cancer and other diseases depend on the adoption of a unified terminology to describe EMT.

In this Consensus Statement, the authors (on behalf of the EMT International Association) propose guidelines to define epithelial–mesenchymal transition, its phenotypic plasticity and the associated multiple intermediate epithelial–mesenchymal cell states. Clarification of nomenclature and definitions will help reduce misinterpretation of research data generated in different experimental model systems and promote cross-disciplinary collaboration.

Regulation of cell adhesion and anchorage-dependent growth by a new beta 1-integrin-linked protein kinase

The interaction of cells with the extracellular matrix regulates cell shape, motility, growth, survival, differentiation and gene expression, through integrin-mediated signal transduction. We used a two-hybrid screen to isolate genes encoding proteins that interact with the beta 1-integrin cytoplasmic domain. The most frequently isolated complementary DNA encoded a new, 59K serine/threonine protein kinase, containing four ankyrin-like repeats. We report here that this integrin-linked kinase (ILK) phosphorylated a beta 1-integrin cytoplasmic domain peptide in vitro and coimmunoprecipitated with beta 1 in lysates of mammalian cells. Endogenous ILK kinase activity was reduced in response to fibronectin. Overexpression of p59ILK disrupted epithelial cell architecture and inhibited adhesion to integrin substrates, while inducing anchorage-independent growth. We propose that ILK is a receptor-proximal protein kinase regulating integrin-mediated signal transduction.

Phosphoinositide-3-OH kinase-dependent regulation of glycogen synthase kinase 3 and protein kinase B/AKT by the integrin-linked kinase

Integrin-linked kinase (ILK) is an ankyrin-repeat containing serine-threonine protein kinase capable of interacting with the cytoplasmic domains of integrin beta1, beta2, and beta3 subunits. Overexpression of ILK in epithelial cells disrupts cell-extracellular matrix as well as cell-cell interactions, suppresses suspension-induced apoptosis (also called Anoikis), and stimulates anchorage-independent cell cycle progression. In addition, ILK induces nuclear translocation of beta-catenin, where the latter associates with a T cell factor/lymphocyte enhancer-binding factor 1 (TCF/LEF-1) to form an activated transcription factor. We now demonstrate that ILK activity is rapidly, but transiently, stimulated upon attachment of cells to fibronectin, as well as by insulin, in a phosphoinositide-3-OH kinase [Pi(3)K]-dependent manner. Furthermore, phosphatidylinositol(3,4,5)trisphosphate specifically stimulates the activity of ILK in vitro, and in addition, membrane targetted constitutively active Pi(3)K activates ILK in vivo. We also demonstrate here that ILK is an upstream effector of the Pi(3)K-dependent regulation of both protein kinase B (PKB/AKT) and glycogen synthase kinase 3 (GSK-3). Specifically, ILK can directly phosphorylate GSK-3 in vitro and when stably, or transiently, overexpressed in cells can inhibit GSK-3 activity, whereas the overexpression of kinase-deficient ILK enhances GSK-3 activity. In addition, kinase-active ILK can phosphorylate PKB/AKT on serine-473, whereas kinase-deficient ILK severely inhibits endogenous phosphorylation of PKB/AKT on serine-473, demonstrating that ILK is involved in agonist stimulated, Pi(3)K-dependent, PKB/AKT activation. ILK is thus a receptor-proximal effector for the Pi(3)K-dependent, extracellular matrix and growth factor mediated, activation of PKB/AKT, and inhibition of GSK-3.

Targeting tumor hypoxia: suppression of breast tumor growth and metastasis by novel carbonic anhydrase IX inhibitors

Carbonic anhydrase IX (CAIX) is a hypoxia and HIF-1-inducible protein that regulates intra- and extracellular pH under hypoxic conditions and promotes tumor cell survival and invasion in hypoxic microenvironments. Interrogation of 3,630 human breast cancers provided definitive evidence of CAIX as an independent poor prognostic biomarker for distant metastases and survival. shRNA-mediated depletion of CAIX expression in 4T1 mouse metastatic breast cancer cells capable of inducing CAIX in hypoxia resulted in regression of orthotopic mammary tumors and inhibition of spontaneous lung metastasis formation. Stable depletion of CAIX in MDA-MB-231 human breast cancer xenografts also resulted in attenuation of primary tumor growth. CAIX depletion in the 4T1 cells led to caspase-independent cell death and reversal of extracellular acidosis under hypoxic conditions in vitro. Treatment of mice harboring CAIX-positive 4T1 mammary tumors with novel CAIX-specific small molecule inhibitors that mimicked the effects of CAIX depletion in vitro resulted in significant inhibition of tumor growth and metastasis formation in both spontaneous and experimental models of metastasis, without inhibitory effects on CAIX-negative tumors. Similar inhibitory effects on primary tumor growth were observed in mice harboring orthotopic tumors comprised of lung metatstatic MDA-MB-231 LM2-4(Luc+) cells. Our findings show that CAIX is vital for growth and metastasis of hypoxic breast tumors and is a specific, targetable biomarker for breast cancer metastasis.

Reciprocal interactions between beta1-integrin and epidermal growth factor receptor in three-dimensional basement membrane breast cultures: a different perspective in epithelial biology

Anchorage and growth factor independence are cardinal features of the transformed phenotype. Although it is logical that the two pathways must be coregulated in normal tissues to maintain homeostasis, this has not been demonstrated directly. We showed previously that down-modulation of beta1-integrin signaling reverted the malignant behavior of a human breast tumor cell line (T4-2) derived from phenotypically normal cells (HMT-3522) and led to growth arrest in a three-dimensional (3D) basement membrane assay in which the cells formed tissue-like acini (14). Here, we show that there is a bidirectional cross-modulation of beta1-integrin and epidermal growth factor receptor (EGFR) signaling via the mitogen-activated protein kinase (MAPK) pathway. The reciprocal modulation does not occur in monolayer (2D) cultures. Antibody-mediated inhibition of either of these receptors in the tumor cells, or inhibition of MAPK kinase, induced a concomitant down-regulation of both receptors, followed by growth-arrest and restoration of normal breast tissue morphogenesis. Cross-modulation and tissue morphogenesis were associated with attenuation of EGF-induced transient MAPK activation. To specifically test EGFR and beta1-integrin interdependency, EGFR was overexpressed in nonmalignant cells, leading to disruption of morphogenesis and a compensatory up-regulation of beta1-integrin expression, again only in 3D. Our results indicate that when breast cells are spatially organized as a result of contact with basement membrane, the signaling pathways become coupled and bidirectional. They further explain why breast cells fail to differentiate in monolayer cultures in which these events are mostly uncoupled. Moreover, in a subset of tumor cells in which these pathways are misregulated but functional, the cells could be "normalized" by manipulating either pathway.

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.

NGF-induced axon growth is mediated by localized inactivation of GSK-3beta and functions of the microtubule plus end binding protein APC

Little is known about how nerve growth factor (NGF) signaling controls the regulated assembly of microtubules that underlies axon growth. Here we demonstrate that a tightly regulated and localized activation of phosphatidylinositol 3-kinase (PI3K) at the growth cone is essential for rapid axon growth induced by NGF. This spatially activated PI3K signaling is conveyed downstream through a localized inactivation of glycogen synthase kinase 3beta (GSK-3beta). These two spatially coupled kinases control axon growth via regulation of a microtubule plus end binding protein, adenomatous polyposis coli (APC). Our results demonstrate that NGF signals are transduced to the axon cytoskeleton via activation of a conserved cell polarity signaling pathway.

Ureido-substituted benzenesulfonamides potently inhibit carbonic anhydrase IX and show antimetastatic activity in a model of breast cancer metastasis

A series of ureido-substituted benzenesulfonamides was prepared that showed a very interesting profile for the inhibition of several human carbonic anhydrases (hCAs, EC 4.2.1.1), such as hCAs I and II (cytosolic isoforms) and hCAs IX and XII (transmembrane, tumor-associated enzymes). Excellent inhibition of all these isoforms has been observed with various members of the series, depending on the substitution pattern of the urea moiety. Several low nanomolar CA IX/XII inhibitors also showing good selectivity for the transmembrane over the cytosolic isoforms have been discovered. One of them, 4-{[(3'-nitrophenyl)carbamoyl]amino}benzenesulfonamide, significantly inhibited the formation of metastases by the highly aggressive 4T1 mammary tumor cells at pharmacologic concentrations of 45 mg/kg, constituting an interesting candidate for the development of conceptually novel antimetastatic drugs.

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

Protein kinase B (PKB/Akt) is a regulator of cell survival and apoptosis. To become fully activated, PKB/Akt requires phosphorylation at two sites, threonine 308 and serine 473, in a phosphatidylinositol (PI) 3-kinase-dependent manner. The kinase responsible for phosphorylation of threonine 308 is the PI 3-kinase-dependent kinase-1 (PDK-1), whereas phosphorylation of serine 473 has been suggested to be regulated by PKB/Akt autophosphorylation in a PDK-1-dependent manner. However, the integrin-linked kinase (ILK) has also been shown to regulate phosphorylation of serine 473 in a PI 3-kinase-dependent manner. Whether ILK phosphorylates this site directly or functions as an adapter molecule has been debated. We now show by in-gel kinase assay and matrix-assisted laser desorption-ionization time-of-flight mass spectrometry that biochemically purified ILK can phosphorylate PKB/Akt directly. Co-immunoprecipitation analysis of cell extracts demonstrates that ILK can complex with PKB/Akt as well as PDK-1 and that ILK can disrupt PDK-1/PKB association. The amino acid residue serine 343 of ILK within the activation loop is required for kinase activity as well as for its interaction with PKB/Akt. Mutational analysis of ILK further shows a crucial role for arginine 211 of ILK within the phosphoinositide phospholipid binding domain in the regulation of PKB- serine 473 phosphorylation. A highly selective small molecule inhibitor of ILK activity also inhibits the ability of ILK to phosphorylate PKB/Akt in vitro and in intact cells. These data demonstrate that ILK is an important upstream kinase for the regulation of PKB/Akt.

Snail induction of epithelial to mesenchymal transition in tumor cells is accompanied by MUC1 repression and ZEB1 expression

E-cadherin protein plays a key role in the establishment and maintenance of adherent junctions. Recent evidence implicates the transcription factor Snail in the blockage of E-cadherin expression in fibroblasts and some epithelial tumor cells through direct binding to three E-boxes in the E-cadherin promoter. Transfection of Snail into epithelial cells leads to a more fibroblastic phenotype. Cells expressing Snail presented a scattered flattened phenotype with low intercellular contacts. Other epithelial markers like Cytokeratin 18 or MUC1 were also repressed. The effects of Snail on MUC1 transcription were mediated by two E-boxes present in the proximal promoter. Snail also induced expression of the mesenchymal markers fibronectin and LEF1 and the transcription repressor ZEB1. ZEB1 and Snail had a similar pattern of expression in epithelial cell lines, and both were induced by overexpression of ILK1, a kinase that causes the loss of E-cadherin and the acquisition of a fibroblastic phenotype. Snail overexpression in several cell lines raised ZEB1 RNA levels and increased the activity of ZEB1 promoter. ZEB1 could also repress E-cadherin and MUC1 promoters but less strongly than Snail. However, since ZEB1 expression persisted after Snail was down-regulated, ZEB1 may regulate epithelial genes in several tumor cell lines.

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.

Cell adhesion and the integrin-linked kinase regulate the LEF-1 and beta-catenin signaling pathways

The integrin-linked kinase (ILK) is an ankyrin repeat containing serine-threonine protein kinase that can interact directly with the cytoplasmic domains of the beta1 and beta3 integrin subunits and whose kinase activity is modulated by cell-extracellular matrix interactions. Overexpression of constitutively active ILK results in loss of cell-cell adhesion, anchorage-independent growth, and tumorigenicity in nude mice. We now show that modest overexpression of ILK in intestinal epithelial cells as well as in mammary epithelial cells results in an invasive phenotype concomitant with a down-regulation of E-cadherin expression, translocation of beta-catenin to the nucleus, formation of a complex between beta-catenin and the high mobility group transcription factor, LEF-1, and transcriptional activation by this LEF-1/beta-catenin complex. We also find that LEF-1 protein expression is rapidly modulated by cell detachment from the extracellular matrix, and that LEF-1 protein levels are constitutively up-regulated at ILK overexpression. These effects are specific for ILK, because transformation by activated H-ras or v-src oncogenes do not result in the activation of LEF-1/beta-catenin. The results demonstrate that the oncogenic properties of ILK involve activation of the LEF-1/beta-catenin signaling pathway, and also suggest ILK-mediated cross-talk between cell-matrix interactions and cell-cell adhesion as well as components of the Wnt signaling pathway.

Recent developments in targeting carbonic anhydrase IX for cancer therapeutics

Carbonic anhydrase IX (CAIX) is a hypoxia-inducible enzyme that is overexpressed by cancer cells from many tumor types, and is a component of the pH regulatory system invoked by these cells to combat the deleterious effects of a high rate of glycolytic metabolism. CAIX functions to help produce and maintain an intracellular pH (pHi) favorable for tumor cell growth and survival, while at the same time participating in the generation of an increasingly acidic extracellular space, facilitating tumor cell invasiveness. Pharmacologic interference of CAIX catalytic activity using monoclonal antibodies or CAIX-specific small molecule inhibitors, consequently disrupting pH regulation by cancer cells, has been shown recently to impair primary tumor growth and metastasis. Many of these agents are in preclinical or clinical development and constitute a novel, targeted strategy for cancer therapy.

Interaction of nuclear receptors with the Wnt/beta-catenin/Tcf signaling axis: Wnt you like to know?

The cross-regulation of Wnt/beta-catenin/Tcf ligands, kinases, and transcription factors with members of the nuclear receptor (NR) family has emerged as a clinically and developmentally important area of endocrine cell biology. Interactions between these signaling pathways result in a diverse array of cellular effects including altered cellular adhesion, tissue morphogenesis, and oncogenesis. Analyses of NR interactions with canonical Wnt signaling reveal two broad themes: Wnt/beta-catenin modulation of NRs (theme I), and ligand-dependent NR inhibition of the Wnt/beta-catenin/Tcf cascade (theme II). Beta-catenin, a promiscuous Wnt signaling member, has been studied intensively in relation to the androgen receptor (AR). Beta-catenin acts as a coactivator of AR transcription and is also involved in co-trafficking, increasing cell proliferation, and prostate pathogenesis. T cell factor, a transcriptional mediator of beta-catenin and AR, engages in a dynamic reciprocity of nuclear beta-catenin, p300/CREB binding protein, and transcriptional initiation factor 2/GC receptor-interaction protein, thereby facilitating hormone-dependent coactivation and transrepression. Beta-catenin responds in an equally dynamic manner with other NRs, including the retinoic acid (RA) receptor (RAR), vitamin D receptor (VDR), glucocorticoid receptor (GR), progesterone receptor, thyroid receptor (TR), estrogen receptor (ER), and peroxisome proliferator-activated receptor (PPAR). The NR ligands, vitamin D(3), trans/cis RA, glucocorticoids, and thiazolidines, induce dramatic changes in the physiology of cells harboring high Wnt/beta-catenin/Tcf activity. Wnt signaling regulates, directly or indirectly, developmental processes such as ductal branching and adipogenesis, two processes dependent on NR function. Beta-catenin has been intensively studied in colorectal cancer; however, it is now evident that beta-catenin may be important in cancers of the breast, prostate, and thyroid. This review will focus on the cross-regulation of AR and Wnt/beta-catenin/Tcf but will also consider the dynamic manner in which RAR/RXR, GR, TR, VDR, ER, and PPAR modulate canonical Wnt signaling. Although many commonalities exist by which NRs interact with the Wnt/beta-catenin signaling pathway, striking cell line and tissue-specific differences require deciphering and application to endocrine pathology.

Calreticulin is essential for integrin-mediated calcium signalling and cell adhesion

Integrins are important mediators of cell adhesion to extracellular ligands and can transduce biochemical signals both into and out of cells. The cytoplasmic domains of integrins interact with several structural and signalling proteins and consequently participate in the regulation of cell shape, motility, growth and differentiation. It has been shown that calreticulin associates with the cytoplasmic domains of integrin alpha-subunits and that this interaction can influence integrin-mediated cell adhesion to extracellular matrix. We have now developed calreticulin-deficient embryonic stem (ES) cells and isolated embryonic fibroblasts from calreticulin mutant mice. We find that in both cell types integrin-mediated adhesion is severely impaired, although integrin expression is unaltered. Expression of recombinant calreticulin in double knockout ES cells by complementary DNA transfection rescued integrin-mediated adhesion. In wild-type cells, engagement of surface integrins induced a transient elevation in cytosolic calcium concentration owing to influx of extracellular calcium. This calcium transient was absent in calreticulin-deficient cells. In contrast, the amount of calcium in endomembrane stores, which is sensitive to both inositol 1,4,5-trisphosphate and thapsigargin, was indistinguishable in the two cell types. Our results indicate that calreticulin is an essential modulator both of integrin adhesive functions and integrin-initiated signalling, but that it may not play a significant role in the storage of luminal calcium.

Integrin cytoplasmic interactions and bidirectional transmembrane signalling

Integrins are heterodimeric integral plasma membrane proteins containing extracellular, transmembrane, and cytoplasmic domains. These highly versatile receptors mediate not only cell adhesion and migration, but also the bidirectional transfer of information across the plasma membrane. The cytoplasmic domains of integrins are required for the transduction of this bidirectional information, and have recently been shown to participate in direct interactions with some novel cytoplasmic proteins, such as an ankyrin repeat containing serine/threonine protein kinase (integrin-linked kinase) and beta3 endonexin. New evidence also suggests that, via interactions with focal adhesion kinase, the integrin cytoplasmic domains can coordinate actin cytoskeletal organization and responses to growth factors. The elucidation of the signal transduction pathways activated by integrins is an intense area of investigation that has shown that integrins have some unique properties as signal transducing receptors.

Integrin-linked kinase--essential roles in physiology and cancer biology

Integrin-linked kinase (ILK) is a multifunctional intracellular effector of cell-matrix interactions and regulates many cellular processes, including growth, proliferation, survival, differentiation, migration, invasion and angiogenesis. The use of recently developed Cre-lox-driven recombination and RNA-interference technologies has enabled the evaluation of the physiological roles of ILK in several major organ systems. Significant developmental and tissue-homeostasis defects occur when the gene that encodes ILK is deleted, whereas the expression of ILK is often elevated in human malignancies. Although the cause(s) of ILK overexpression remain to be fully elucidated, accumulating evidence suggests that its oncogenic capacity derives from its regulation of several downstream targets that provide cells with signals that promote proliferation, survival and migration, supporting the concept that ILK is a relevant therapeutic target in human cancer. Furthermore, a global analysis of the ILK 'interactome' has yielded several novel interactions, and has revealed exciting and unexpected cellular functions of ILK that might have important implications for the development of effective therapeutic agents.

Regulation of Snail transcription during epithelial to mesenchymal transition of tumor cells

Expression of Snail transcriptional factor is a determinant in the acquisition of a mesenchymal phenotype by epithelial tumor cells. However, the regulation of the transcription of this gene is still unknown. We describe here the characterization of a human SNAIL promoter that contains the initiation of transcription and regulates the expression of this gene in tumor cells. This promoter was activated in cell lines in response to agents that induce Snail transcription and the mesenchymal phenotype, as addition of the phorbol ester PMA or overexpression of integrin-linked kinase (ILK) or oncogenes such as Ha-ras or v-Akt. Although other regions of the promoter were required for a complete stimulation by Akt or ILK, a minimal fragment (-78/+59) was sufficient to maintain the mesenchymal specificity. Activity of this minimal promoter and SNAIL RNA levels were dependent on ERK signaling pathway. NFkappaB/p65 also stimulated SNAIL transcription through a region located immediately upstream the minimal promoter, between -194 and -78. These results indicate that Snail transcription is driven by signaling pathways known to induce epithelial to mesenchymal transition, reinforcing the role of Snail in this process.

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 nuclear hormone receptor activity by calreticulin

We have shown that a polypeptide of M(r) 60,000 (60K) that shares N-terminal homology with a calcium-binding protein, calreticulin, can bind to an amino-acid sequence motif, KXGFFKR, found in the cytoplasmic domains of all integrin alpha-subunits. The homologous amino-acid sequence, KXFFKR (where X is either G, A or V), is also present in the DNA-binding domain of all known members of the steroid hormone receptor family; amino acids in this sequence make direct contact with nucleotides in their DNA-responsive elements and are crucial for DNA binding. Here we show that both the 60K protein (p60), purified on a KLGFFKR-Sepharose affinity matrix, and recombinant calreticulin can inhibit the binding of androgen receptor to its hormone-responsive DNA element in a KXFFKR-sequence-specific manner. Calreticulin can also inhibit androgen receptor and retinoic acid receptor transcriptional activities in vivo, as well as retinoic acid-induced neuronal differentiation. Our results indicate that calreticulin can act as an important modulator of the regulation of gene transcription by nuclear hormone receptors.

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.

Cytoskeletal rearrangements accompanying salmonella entry into epithelial cells

Salmonella bacteria can enter (invade) eukaryotic cells, and exist as intracellular parasites. Confocal, light immunofluorescence and electron microscopy were used to examine various cytoskeletal components of cultured Madin Darby canine kidney (MDCK) and HeLa epithelial cells after infection with Salmonella typhimurium. These bacteria entered and remained within membrane-bound vacuoles and were surrounded by large (5–10 microns) dense structures composed of various cytoskeletal components. These structures consisted of extensive aggregations of polymerized actin, alpha-actinin and tropomyosin above and beside the invading bacterium in both epithelial cell lines. These structures were evident soon after bacterial addition (maximal at 20 min for HeLa cells, 60 min for MDCK cells), and disappeared later in the infection as the cytoskeletal components returned to a more normal distribution after bacterial internalization. Surprisingly, tubulin also aggregated above internalized Salmonella although bacterial entry or penetration through polarized monolayers was not disrupted by the microtubule-inhibiting agent nocadazole (this treatment actually enhanced tubulin accumulation around these organisms). There were little if any rearrangements in intermediate filaments composed of keratin or vimentin. Large amounts of talin also accumulated above and around invading Salmonella, but there was only a minor accumulation of vinculin around a few organisms. Pretreatment of epithelial cells with the microfilament inhibitor cytochalasin D blocked bacterial internalization but did not prevent accumulation of polymerized actin and alpha-actinin directly beneath uninternalized bacteria, yet prevented accumulation of the other cytoskeletal components. These results suggest that Salmonella bind to the surface and trigger a signal in epithelial cells that causes marked rearrangements in various cytoskeletal components, including recruitment of actin filaments and alpha-actinin, which then generates the force necessary for bacterial uptake.

Signaling through beta-catenin and Lef/Tcf

Beta-catenin plays a structural role in cell adhesion by binding to cadherins at the intracellular surface of the plasma membrane and a signaling role in the cytoplasm as the penultimate downstream mediator of the wnt signaling pathway. The ultimate mediator of this pathway is a nuclear complex of beta-catenin acting as a coactivtor with lymphoid enhancer factor/T cell factor (Lef/Tcf) transcription factors to stimulate transcription of a variety of target genes. Signaling through beta-catenin is regulated by modulating its degradation and nuclear translocation. In the absence of an activating signal, phosphorylation of beta-catenin by glycogen synthase kinase 3 (GSK3) acting in conjunction with adenomatous polyposis coli and axin/conductin causes beta-catenin to interact with the beta-transducin repeat-containing protein which results in its ubiquitination and degradation. Signaling from the wnt pathway activates dishevelled which, in an as yet undefined manner, inhibits the activity of GSK3 resulting in an increase in the cytoplasmic free pool of beta-catenin, and translocation into the nucleus. The integrin-linked kinase (ILK) pathway also activates beta-catenin-Lef/Tcf signaling. ILK phosphorylates GSK3 to inhibit its activity and translocates beta-catenin into the nucleus. In addition, ILK downregulates the expression of E-cadherin and upregulates Lef-1 expression. In the final step of the beta-catenin-Lef/Tcf signaling pathway, nuclear beta-catenin binds pontin52-TATA binding protein and displaces Groucho-related gene or CREB-binding protein corepressors from Lef/Tcf resulting in stimulation of transcription. During development, beta-catenin-Lef/Tcf signaling is involved in the formation of dorsal mesoderm and dorsal axis. Furthermore, defects in the beta-catenin-Lef/Tcf pathway are involved in the development of several types of cancers.

A cell surface receptor complex for collagen type I recognizes the Arg-Gly-Asp sequence

To isolate collagen-binding cell surface proteins, detergent extracts of surface-iodinated MG-63 human osteosarcoma cells were chromatographed on affinity matrices of either type I collagen-Sepharose or Sepharose carrying a collagen-like triple-helical peptide. The peptide was designed to be triple helical and to contain the sequence Arg-Gly-Asp, which has been implicated as the cell attachment site of fibronectin, vitronectin, fibrinogen, and von Willebrand factor, and is also present in type I collagen. Three radioactive polypeptides having apparent molecular masses of 250 kD, 70 kD, and 30 kD were distinguishable in that they showed affinity toward the collagen and collagen-like peptide affinity columns, and could be specifically eluted from these columns with a solution of an Arg-Gly-Asp-containing peptide, Gly-Arg-Gly-Asp-Thr-Pro. These collagen-binding polypeptides associated with phosphatidylcholine liposomes, and the resulting liposomes bound specifically to type I collagen or the collagen-like peptide but not to fibronectin or vitronectin or heat-denatured collagen. The binding of these liposomes to type I collagen could be inhibited with the peptide Gly-Arg-Gly-Asp-Thr-Pro and with EDTA, but not with a variant peptide Gly-Arg-Gly-Glu-Ser-Pro. We conclude from these data that these three polypeptides are membrane molecules that behave as a cell surface receptor (or receptor complex) for type I collagen by interacting with it through the Arg-Gly-Asp tripeptide adhesion signal. The lack of binding to denatured collagen suggests that the conformation of the Arg-Gly-Asp sequence is important in the recognition of collagen by the receptor complex.