A non-antibacterial oxazolidinone derivative that inhibits epithelial cell sheet migration

RKIP localizes to the nucleus through a bipartite nuclear localization signal and interaction with importin α to regulate mitotic progression

Raf kinase inhibitor protein (RKIP) is a multifunctional modulator of intracellular signal transduction. Although most of its functions have been considered cytosolic, we show here that the localization of RKIP is primarily nuclear in both growing and quiescent Madin-Darby canine kidney epithelial cells and in Cal-51 and BT-20 human breast cancer cells. We have identified a putative bipartite nuclear localization signal (NLS) in RKIP that maps to the surface of the protein surrounding a known regulatory region. Like classical NLS sequences, the putative NLS of RKIP is rich in arginine and lysine residues. Deletion of and point mutations in the putative NLS lead to decreased nuclear localization. Point mutation of all the basic residues in the putative NLS of RKIP particularly strongly reduces nuclear localization. We found consistent results in reexpression experiments with wildtype or mutant RKIP in RKIP-silenced cells. A fusion construct of the putative NLS of RKIP alone to a heterologous reporter protein leads to nuclear localization of the fusion protein, demonstrating that this sequence alone is sufficient for import into the nucleus. We found that RKIP interacts with the nuclear transport factor importin α in BT-20 and MDA-MB-231 human breast cancer cells, suggesting importin-mediated active nuclear translocation. Taken together, these findings suggest that a bipartite NLS in RKIP interacts with importin α for active transport of RKIP into the nucleus and that this process may be involved in the regulation of mitotic progression. Evaluating the biological function of nuclear localization of RKIP, we found that the presence of the putative NLS is important for the role of RKIP in mitotic checkpoint regulation in MCF-7 human breast cancer cells.

Investigation of Marine-Derived Natural Products as Raf Kinase Inhibitory Protein (RKIP)-Binding Ligands

Raf kinase inhibitory protein (RKIP) is an essential regulator of the Ras/Raf-1/MEK/ERK signaling cascade and functions by directly interacting with the Raf-1 kinase. The abnormal expression of RKIP is linked with numerous diseases including cancers, Alzheimer's and diabetic nephropathy. Interestingly, RKIP also plays an indispensable role as a tumor suppressor, thus making it an attractive therapeutic target. To date, only a few small molecules have been reported to modulate the activity of RKIP, and there is a need to explore additional scaffolds. In order to achieve this objective, a pharmacophore model was generated that explores the features of locostatin, the most potent RKIP modulator. Correspondingly, the developed model was subjected to screening, and the mapped compounds from Marine Natural Products (MNP) library were retrieved. The mapped MNPs after ensuing drug-likeness filtration were escalated for molecular docking, where locostatin was regarded as a reference. The MNPs exhibiting higher docking scores than locostatin were considered for molecular dynamics simulations, and their binding affinity towards RKIP was computed via MM/PBSA. A total of five molecules revealed significantly better binding free energy scores than compared to locostatin and, therefore, were reckoned as hits. The hits from the present in silico investigation could act as potent RKIP modulators and disrupt interactions of RKIP with its binding proteins. Furthermore, the identification of potent modulators from marine natural habitat can act as a future drug-discovery source.

Reviewing Hit Discovery Literature for Difficult Targets: Glutathione Transferase Omega-1 as an Example

Early stage drug discovery reporting on relatively new or difficult targets is often associated with insufficient hit triage. Literature reviews of such targets seldom delve into the detail required to critically analyze the associated screening hits reported. Here we take the enzyme glutathione transferase omega-1 (GSTO1-1) as an example of a relatively difficult target and review the associated literature involving small-molecule inhibitors. As part of this process we deliberately pay closer-than-usual attention to assay interference and hit quality aspects. We believe this Perspective will be a useful guide for future development of GSTO1-1 inhibitors, as well serving as a template for future review formats of new or difficult targets.

Locostatin, a disrupter of Raf kinase inhibitor protein, inhibits extracellular matrix production, proliferation, and migration in human uterine leiomyoma and myometrial cells

OBJECTIVE

To investigate the presence of Raf kinase inhibitor protein (RKIP) in human myometrium and leiomyoma as well as to determine the effect of locostatin (RKIP inhibitor) on extracellular matrix (ECM) production, proliferation, and migration in human myometrial and leiomyoma cells.

DESIGN

Laboratory study.

SETTING

Human myometrium and leiomyoma.

PATIENT(S)

Thirty premenopausal women who were admitted to the hospital for myomectomy or hysterectomy.

INTERVENTION(S)

Myometrial and leiomyoma tissues were used to investigate the localization and the expression level of RKIP through immunohistochemistry and Western blotting. Myometrial and leiomyoma cells were treated with locostatin (10 μM) to measure ECM expression by real-time polymerase chain reaction, GSK3β expression by Western blotting, cell migration by wound-healing assay, and cell proliferation by MTT assay and immunocytochemistry.

MAIN OUTCOME MEASURE(S)

The expression of RKIP in human myometrial and leiomyoma tissue; ECM components and GSK3β expression, migration, and proliferation in myometrial and leiomyoma cells.

RESULT(S)

RKIP is expressed in human myometrial and leiomyoma tissue. Locostatin treatment resulted in the activation of the mitogen-activated protein kinase (MAPK) signal pathway (ERK phosphorylation), providing a powerful validation of our targeting protocol. Further, RKIP inhibition by locostatin reduces ECM components. Moreover, the inhibition of RKIP by locostatin impaired cell proliferation and migration in both leiomyoma and myometrial cells. Finally, locostatin treatment reduced GSK3β expression. Therefore, even if the activation of MAPK pathway should increase proliferation and migration, the destabilization of GSK3β leads to the reduction of proliferation and migration of myometrial and leiomyoma cells.

CONCLUSION(S)

Our results indicate that RKIP may be involved in leiomyoma pathophysiology.

SSPaQ: A Subtractive Segmentation Approach for the Exhaustive Parallel Quantification of the Extent of Protein Modification at Every Possible Site

Protein modifications, whether chemically induced or post-translational (PTMs), play an essential role for the biological activity of proteins. Understanding biological processes and alterations thereof will rely on the quantification of these modifications on individual residues. Here we present SSPaQ, a subtractive method for the parallel quantification of the extent of modification at each possible site of a protein. The method combines uniform isotopic labeling and proteolysis with MS, followed by a segmentation approach, a powerful tool to refine the quantification of the degree of modification of a peptide to a segment containing a single modifiable amino acid. The strength of this strategy resides in: (1) quantification of all modifiable sites in a protein without prior knowledge of the type(s) of modified residues; (2) insensitivity to changes in the solubility and ionization efficiency of peptides upon modification; and (3) detection of missed cleavages caused by the modification for mitigation. The SSPaQ method was applied to quantify modifications resulting from the interaction of human phosphatidyl ethanolamine binding protein 1 (hPEBP1), a metastasis suppressor gene product, with locostatin, a covalent ligand and antimigratory compound with demonstrated activity towards hPEBP1. Locostatin is shown to react with several residues of the protein. SSPaQ can more generally be applied to induced modification in the context of drugs that covalently bind their target protein. With an alternate front-end protocol, it could also be applied to the quantification of protein PTMs, provided a removal tool is available for that PTM. Graphical Abstract ᅟ.

Screening and target identification of bioactive compounds that modulate cell migration and autophagy

Cell migration is a fundamental step for embryonic development, wound repair, immune responses, and tumor cell invasion and metastasis. It is well known that protrusive structures, namely filopodia and lamellipodia, can be observed at the leading edge of migrating cells. The formation of these structures is necessary for cell migration; however, the molecular mechanisms behind the formation of these structures remain largely unclear. Therefore, bioactive compounds that modulate protrusive structures are extremely powerful tools for studying the mechanisms behind the formation of these structures and subsequent cell migration. Therefore, we have screened for bioactive compounds that inhibit the formation of filopodia, lamellipodia, or cell migration from natural products, and attempted to identify the target molecules of our isolated compounds. Additionally, autophagy is a bulk, non-specific protein degradation system that is involved in the pathogenesis of cancer and neurodegenerative disorders. Recent extensive studies have revealed the molecular mechanisms of autophagy, however, they also remain largely unclear. Thus, we also have screened for bioactive compounds that modulate autophagy, and identified the target molecules. In the present article, we introduce the phenotypic screening system and target identification of four bioactive compounds.

Chemistry and biology of the compounds that modulate cell migration

Cell migration is a fundamental step for embryonic development, wound repair, immune responses, and tumor cell invasion and metastasis. Extensive studies have attempted to reveal the molecular mechanisms behind cell migration; however, they remain largely unclear. Bioactive compounds that modulate cell migration show promise as not only extremely powerful tools for studying the mechanisms behind cell migration but also as drug seeds for chemotherapy against tumor metastasis. Therefore, we have screened cell migration inhibitors and analyzed their mechanisms for the inhibition of cell migration. In this mini-review, we introduce our chemical and biological studies of three cell migration inhibitors: moverastin, UTKO1, and BU-4664L.

Cardiac glycoside activities link Na(+)/K(+) ATPase ion-transport to breast cancer cell migration via correlative SAR

The cardiac glycosides ouabain and digitoxin, established Na⁺/K⁺ ATPase inhibitors, were found to inhibit MDA-MB-231 breast cancer cell migration through an unbiased chemical genetics screen for cell motility. The Na⁺/K⁺ ATPase acts both as an ion-transporter and as a receptor for cardiac glycosides. To delineate which function is related to breast cancer cell migration, structure–activity relationship (SAR) profiles of cardiac glycosides were established at the cellular (cell migration inhibition), molecular (Na⁺/K⁺ ATPase inhibition), and atomic (computational docking) levels. The SAR of cardiac glycosides and their analogs revealed a similar profile, a decrease in potency when the parent cardiac glycoside structure was modified, for each activity investigated. Since assays were done at the cellular, molecular, and atomic levels, correlation of SAR profiles across these multiple assays established links between cellular activity and specific protein–small molecule interactions. The observed antimigratory effects in breast cancer cells are directly related to the inhibition of Na⁺/K⁺ transport. Specifically, the orientation of cardiac glycosides at the putative cation permeation path formed by transmembrane helices αM1–M6 correlates with the Na⁺ pump activity and cell migration. Other Na⁺/K⁺ ATPase inhibitors that are structurally distinct from cardiac glycosides also exhibit antimigratory activity, corroborating the conclusion that the antiport function of Na⁺/K⁺ ATPase and not the receptor function is important for supporting the motility of MDA-MB-231 breast cancer cells. Correlative SAR can establish new relationships between specific biochemical functions and higher-level cellular processes, particularly for proteins with multiple functions and small molecules with unknown or various modes of action.

Recognition and reactivity in the binding between Raf kinase inhibitor protein and its small-molecule inhibitor locostatin

The present work is aimed to provide detail on the binding process between Raf kinase inhibitor protein (RKIP) and locostatin, the only exogenous compound known to alter the function of RKIP. Understanding the basis of RKIP inhibition for use in pharmacological applications is of considerable interest, as dysregulated RKIP expression has the potential to contribute to pathophysiological processes. Herein, we report a series of atomistic models to describe the protein-ligand recognition step and the subsequent reactivity steps. Modeling approaches include ligand docking, molecular dynamics, and quantum mechanics/molecular mechanics calculations. We expect that such a computational assay will serve to study similar complexes in which potency is associated with recognition and reactivity. Although previous data suggested a single amino acid residue (His86) to be involved in the binding of locostatin, the actual ligand conformation and the steps involved in the reactivity process remain elusive from a detailed atomistic description. We show that the first reaction step, consisting of a nucleophilic attack of the nitrogen (Nε) of His86 at the sp(2)-hybridized carbon (C2) of locostatin, presents a late transition state (almost identical to the product). The reaction is followed by a hydrogen abstraction and hydrolysis. The theoretically predicted overall rate constant (6 M(-1) s(-1)) is in a very good agreement with the experimentally determined rate constant (13 M(-1) s(-1)).

Semisynthetic analogues of the marine cembranoid sarcophine as prostate and breast cancer migration inhibitors

Sarcophine (1) is a bioactive cembranoid diterpene isolated from the Red Sea soft coral Sarcophyton glaucum. Previous semisynthesis attempts resulted in decreased or complete loss of 1's anticancer activity. Sarcophine and analogues showed antimigratory activity against breast and prostate cancer cell lines. This encouraged further semisynthestic optimizations to improve its activity and establish a preliminary structure-activity relationship. Eight new and five known semisynthetic analogues were generated. These compounds were evaluated for their ability to inhibit growth, proliferation, and migration of the prostate and breast metastatic cancer cell lines PC-3 and MDA-MB-231, respectively. Most analogues exhibited enhanced antimigratory activity.

Diversity through a branched reaction pathway: generation of multicyclic scaffolds and identification of antimigratory agents

A library of 91 heterocyclic compounds composed of 16 distinct scaffolds has been synthesized through a sequence of phosphine-catalyzed ring-forming reactions, Tebbe reactions, Diels-Alder reactions, and, in some cases, hydrolysis. This effort in diversity-oriented synthesis produced a collection of compounds that exhibited high levels of structural variation both in terms of stereochemistry and the range of scaffolds represented. A simple but powerful sequence of reactions thus led to a high-diversity library of relatively modest size with which to explore biologically relevant regions of chemical space. From this library, several molecules were identified that inhibit the migration and invasion of breast cancer cells and may serve as leads for the development of antimetastatic agents.

Locostatin Disrupts Association of Raf Kinase Inhibitor Protein With Binding Proteins by Modifying a Conserved Histidine Residue in the Ligand-Binding Pocket

Raf kinase inhibitor protein (RKIP) interacts with a number of different proteins and regulates multiple signaling pathways. Here, we show that locostatin, a small molecule that covalently binds RKIP, not only disrupts interactions of RKIP with Raf-1 kinase, but also with G protein-coupled receptor kinase 2. In contrast, we found that locostatin does not disrupt binding of RKIP to two other proteins: inhibitor of κB kinase α and transforming growth factor β-activated kinase 1. These results thus imply that different proteins interact with different regions of RKIP. Locostatin's mechanism of action involves modification of a nucleophilic residue on RKIP. We observed that after binding RKIP, part of locostatin is slowly hydrolyzed, leaving a smaller RKIP-butyrate adduct. We identified the residue alkylated by locostatin as His86, a highly conserved residue in RKIP's ligand-binding pocket. Computational modeling of the binding of locostatin to RKIP suggested that the recognition interaction between small molecule and protein ensures that locostatin's electrophilic site is poised to react with His86. Furthermore, binding of locostatin would sterically hinder binding of other ligands in the pocket. These data provide a basis for understanding how locostatin disrupts particular interactions of RKIP with RKIP-binding proteins and demonstrate its utility as a probe of specific RKIP interactions and functions.

Cucurbitacin I inhibits cell motility by indirectly interfering with actin dynamics

BACKGROUND

Cucurbitacins are plant natural products that inhibit activation of the Janus kinase 2 (JAK2)/signal transducer and activator of transcription 3 (STAT3) pathway by an unknown mechanism. They are also known to cause changes in the organization of the actin cytoskeleton.

METHODOLOGY/PRINCIPAL FINDINGS

We show that cucurbitacin I potently inhibits the migration of Madin-Darby canine kidney (MDCK) cell sheets during wound closure, as well as the random motility of B16-F1 mouse melanoma cells, but has no effect on movement of Dictyostelium discoideum amoebae. Upon treatment of MDCK or B16-F1 cells with cucurbitacin I, there is a very rapid cessation of motility and gradual accumulation of filamentous actin aggregates. The cellular effect of the compound is similar to that observed when cells are treated with the actin filament-stabilizing agent jasplakinolide. However, we found that, unlike jasplakinolide or phallacidin, cucurbitacin I does not directly stabilize actin filaments. In in vitro actin depolymerization experiments, cucurbitacin I had no effect on the rate of actin filament disassembly at the nanomolar concentrations that inhibit cell migration. At elevated concentrations, the depolymerization rate was also unaffected, although there was a delay in the initiation of depolymerization. Therefore, cucurbitacin I targets some factor involved in cellular actin dynamics other than actin itself. Two candidate proteins that play roles in actin depolymerization are the actin-severing proteins cofilin and gelsolin. Cucurbitacin I possesses electrophilic reactivity that may lead to chemical modification of its target protein, as suggested by structure-activity relationship data. However, mass spectrometry revealed no evidence for modification of purified cofilin or gelsolin by cucurbitacin I.

CONCLUSIONS/SIGNIFICANCE

Cucurbitacin I results in accumulation of actin filaments in cells by a unique indirect mechanism. Furthermore, the proximal target of cucurbitacin I relevant to cell migration is unlikely to be the same one involved in activation of the JAK2/STAT3 pathway.

Raf kinase inhibitor protein suppresses nuclear factor-κB-dependent cancer cell invasion through negative regulation of matrix metalloproteinase expression

Accumulating evidence suggests that Raf kinase inhibitor protein (RKIP), which negatively regulates multiple signaling cascades including the Raf and nuclear factor-κB (NF-κB) pathways, functions as a metastasis suppressor. However, the basis for this activity is not clear. We investigated this question in a panel of breast cancer, colon cancer and melanoma cell lines. We found that RKIP negatively regulated the invasion of the different cancer cells through three-dimensional extracellular matrix barriers by controlling the expression of matrix metalloproteinases (MMPs), particularly, MMP-1 and MMP-2. Silencing of RKIP expression resulted in a highly invasive phenotype and dramatically increased levels of MMP-1 and MMP-2 expression, while overexpression of RKIP decreased cancer cell invasion in vitro and metastasis in vivo of murine tumor allografts. Knockdown of MMP-1 or MMP-2 in RKIP-knockdown cells reverted their invasiveness to normal. In contrast, when examining migration of the different cancer cells in a two-dimensional, barrier-less environment, we found that RKIP had either a positive regulatory activity or no activity, but in no case a negative one (as would be expected if RKIP suppressed metastasis at the level of cell migration itself). Therefore, RKIP's function as a metastasis suppressor appears to arise from its ability to negatively regulate expression of specific MMPs, and thus invasion through barriers, and not from a direct effect on the raw capacity of cells to move. The NF-κB pathway, but not the Raf pathway, appeared to positively control the invasion of breast cancer cells. A regulatory loop involving an opposing relationship between RKIP and the NF-κB pathway may control the level of MMP expression and cell invasion.

Velocity fields in a collectively migrating epithelium

We report quantitative measurements of the velocity field of collectively migrating cells in a motile epithelium. The migration is triggered by presenting free surface to an initially confluent monolayer by using a microstencil technique that does not damage the cells. To avoid the technical difficulties inherent in the tracking of single cells, the field is mapped using the technique of particle image velocimetry. The main relevant parameters, such as the velocity module, the order parameter, and the velocity correlation function, are then extracted from this cartography. These quantities are dynamically measured on two types of cells (collectively migrating Madin-Darby canine kidney (MDCK) cells and fibroblastlike normal rat kidney (NRK) cells), first as they approach confluence, and then when the geometrical constraints are released. In particular, for MDCK cells filling up the patterns, we observe a sharp decrease in the average velocity after the point of confluence, whereas the densification of the monolayer is much more regular. After the peeling off of the stencil, a velocity correlation length of approximately 200 microm is measured for MDCK cells versus only approximately 40 microm for the more independent NRK cells. Our conclusions are supported by parallel single-cell tracking experiments. By using the biorthogonal decomposition of the velocity field, we conclude that the velocity field of MDCK cells is very coherent in contrast with the NRK cells. The displacements in the fingers arising from the border of MDCK epithelia are very oriented along their main direction. They influence the velocity field in the epithelium over a distance of approximately 200 microm.

Synthesis and evaluation of antimigratory and antiproliferative activities of lipid-linked [13]-macro-dilactones

The biological activities of a family of novel, lipid-linked 13-membered-ring macro-dilactones are reported. These [13]-macro-dilactones were synthesized by diacylation of functionalized diols, followed by ring-closing metathesis under conditions we had previously reported. Antimigratory, cytostatic and cytotoxic activities of the compounds against cancer cells were evaluated. Compound 13 was the most potent in the series, while compound 10 had the broadest concentration range of subtoxic antiproliferative activity. These compounds share common structural components, namely the [13]-macro-dilactone templated by an octyl alpha-glucoside 4,6-diol.

The oxazolidinone derivative locostatin induces cytokine appeasement

Damaging inflammation arising from autoimmune pathology and septic responses results in severe cases of disease. In both instances, anti-inflammatory compounds are used to limit the excessive or deregulated cytokine responses. We used a model of robust T cell stimulation to identify new proteins involved in triggering a cytokine storm. A comparative proteomic mining approach revealed the differential mapping of Raf kinase inhibitory protein after T cell recall in vivo. Treatment with locostatin, an Raf kinase inhibitory protein inhibitor, induced T cell anergy by blocking cytokine production after Ag recall. This was associated with a reduction in Erk phosphorylation. Importantly, in vivo treatment with locostatin profoundly inhibited TNF-alpha production upon triggering the Ag-specific T cells. This effect was not limited to a murine model because locostatin efficiently inhibited cytokine secretion by human lymphocytes. Therefore, locostatin should be a useful therapeutic to control inflammation, sepsis, and autoimmune diseases.

G protein-coupled receptor kinase 2 activates radixin, regulating membrane protrusion and motility in epithelial cells

Ezrin/radixin/moesin (ERM) proteins are membrane-cytoskeleton linkers that also have roles in signal transduction. Here we show that G protein-coupled receptor kinase 2 (GRK2) regulates membrane protrusion and cell migration during wound closure in Madin-Darby canine kidney (MDCK) epithelial cell monolayers at least partly through activating phosphorylation of radixin on a conserved, regulatory C-terminal Thr residue. GRK2 phosphorylated radixin exclusively on Thr 564 in vitro. Expression of a phosphomimetic (Thr-564-to-Asp) mutant of radixin resulted in increased Rac1 activity, membrane protrusion and cell motility in MDCK cells, suggesting that radixin functions "upstream" of Rac1, presumably as a scaffolding protein. Phosphorylation of ERM proteins was highest during the most active phase of epithelial cell sheet migration over the course of wound closure. In view of these results, we explored the mode of action of quinocarmycin/quinocarcin analog DX-52-1, an inhibitor of cell migration and radixin function with considerable selectivity for radixin over the other ERM proteins, finding that its mechanism of inhibition of radixin does not appear to involve binding and antagonism at the site of regulatory phosphorylation.

Raf Kinase Inhibitory Protein protects cells against locostatin-mediated inhibition of migration

BACKGROUND

Raf Kinase Inhibitory Protein (RKIP, also PEBP1), a member of the Phosphatidylethanolamine Binding Protein family, negatively regulates growth factor signaling by the Raf/MAP kinase pathway. Since an organic compound, locostatin, was reported to bind RKIP and inhibit cell migration by a Raf-dependent mechanism, we addressed the role of RKIP in locostatin function.

METHODS/FINDINGS

We analyzed locostatin interaction with RKIP and examined the biological consequences of locostatin binding on RKIP function. NMR studies show that a locostatin precursor binds to the conserved phosphatidylethanolamine binding pocket of RKIP. However, drug binding to the pocket does not prevent RKIP association with its inhibitory target, Raf-1, nor affect RKIP phosphorylation by Protein Kinase C at a regulatory site. Similarly, exposure of wild type, RKIP-depleted HeLa cells or RKIP-deficient (RKIP(-/-)) mouse embryonic fibroblasts (MEFs) to locostatin has no effect on MAP kinase activation. Locostatin treatment of wild type MEFs causes inhibition of cell migration following wounding. RKIP deficiency impairs migration further, indicating that RKIP protects cells against locostatin-mediated inhibition of migration. Locostatin treatment of depleted or RKIP(-/-) MEFs reveals cytoskeletal disruption and microtubule abnormalities in the spindle.

CONCLUSIONS/SIGNIFICANCE

These results suggest that locostatin's effects on cytoskeletal structure and migration are caused through mechanisms independent of its binding to RKIP and Raf/MAP kinase signaling. The protective effect of RKIP against drug inhibition of migration suggests a new role for RKIP in potentially sequestering toxic compounds that may have deleterious effects on cells.

Migration of isogenic cell lines quantified by dynamic multivariate analysis of single-cell motility

Cell migration is essential in many physiological and pathological processes. To understand this complex behavior, researchers have turned to quantitative, in vitro, image-based measurements to dissect the steps of cellular motility. With the rise of automated microscopy, the bottleneck in these approaches is no longer data acquisition, but data analysis. Using time-lapse microscopy and computer-assisted image analysis, we have developed a novel, quantitative assay that extracts a multivariate profile for cellular motility. This technique measures three dynamic parameters per single cell: speed, surface area, and an in-dex of cell expansion/contraction activity (DECCA). Our assay can be used in combination with a variety of extracellular matrix components, or other soluble agents, to analyze the effects of the microenvironment on cellular migration dynamics in vitro. Our application was developed and tested using A431 and HT-1080 cell lines plated on laminin-332 or fibronectin substrates. Our results indicate that HT-1080 cells migrate faster, have a greater surface area, and have a higher DECCA index than A431 cells on both matrices (for all parameters, p < 0.05). Spearman's correlation coefficients suggest that for these cell lines and matrices, various combinations of the three measurements display low to medium-high levels of correlation. These findings compare well with previous literature. Our approach provides new tools to measure cellular migration dynamics and address questions on the relationship between cell motility and the microenvironment, using only common microscopy techniques, accessible image analysis applications, and a basic desktop computer for image processing.

Raf kinase inhibitor protein inhibits cell proliferation but promotes cell migration in rat hepatic stellate cells

AIM

Hepatic stellate cells (HSCs) play an important role in the pathogenesis of liver fibrosis and cirrhosis. Raf kinase inhibitor protein (RKIP), an inhibitor of extracellular signal-regulated kinases (ERK)/mitogen-activated protein kinase (MAPK) signalling pathway, has been proved to suppress tumor metastasis. Interestingly, RKIP promotes cell migration in Madin-Darby canine kidney epithelial cells. However, the effects of RKIP on HSC behaviours are unknown. The purpose of the present study is to investigate the role of RKIP in HSC proliferation, apoptosis and migration.

METHODS

Two types of cells, freshly isolated HSC and HSC-T6 cell line, were used in this study. The amount of RKIP, the phosphorylation of RKIP, Raf and ERK (pRKIP, pRaf and pERK) were analysed in quiescent and activated HSCs by Western blots. HSC-T6 cells were transfected with RKIP-expressing plasmid or treated with locostatin, a RKIP inhibitor. HSC proliferation, apoptosis and migration were evaluated with 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labelling (TUNEL) staining and Transwell cell migration assay respectively.

RESULTS

In activated HSCs, RKIP protein expression was downregulated whereas pRKIP, pRaf and pERK were upregulated. RKIP overexpression significantly mitigated the phosphorylation of RKIP, Raf and ERK. This in turn inhibited HSC proliferation. Locostatin not only inhibited RKIP protein expression but also, to some extent, reversed the RKIP-inhibited phosphorylation of RKIP, Raf and ERK. RKIP augmented HSC migration and enhanced wound closure. Locostatin reversed the effects of RKIP.

CONCLUSION

Raf kinase inhibitor protein inhibits ERK/MAPK signalling and this inhibition impedes HSC proliferation. RKIP promotes HSC migration and wound closure.

Raf kinase inhibitor protein positively regulates cell-substratum adhesion while negatively regulating cell-cell adhesion

Raf kinase inhibitor protein (RKIP) regulates a number of cellular processes, including cell migration. Exploring the role of RKIP in cell adhesion, we found that overexpression of RKIP in Madin-Darby canine kidney (MDCK) epithelial cells increases adhesion to the substratum, while decreasing adhesion of the cells to one another. The level of the adherens junction protein E-cadherin declines profoundly, and there is loss of normal localization of the tight junction protein ZO-1, while expression of the cell-substratum adhesion protein beta1 integrin dramatically increases. The cells also display increased adhesion and spreading on multiple substrata, including collagen, gelatin, fibronectin and laminin. In three-dimensional culture, RKIP overexpression leads to marked cell elongation and extension of long membrane protrusions into the surrounding matrix, and the cells do not form hollow cysts. RKIP-overexpressing cells generate considerably more contractile traction force than do control cells. In contrast, RNA interference-based silencing of RKIP expression results in decreased cell-substratum adhesion in both MDCK and MCF7 human breast adenocarcinoma cells. Treatment of MDCK and MCF7 cells with locostatin, a direct inhibitor of RKIP and cell migration, also reduces cell-substratum adhesion. Silencing of RKIP expression in MCF7 cells leads to a reduction in the rate of wound closure in a scratch-wound assay, although not as pronounced as that previously reported for RKIP-knockdown MDCK cells. These results suggest that RKIP has important roles in the regulation of cell adhesion, positively controlling cell-substratum adhesion while negatively controlling cell-cell adhesion, and underscore the complex functions of RKIP in cell physiology.

Analogs of tetrahydroisoquinoline natural products that inhibit cell migration and target galectin-3 outside of its carbohydrate-binding site

Cell migration is central to a number of normal and disease processes. Small organic molecules that inhibit cell migration have potential as both research probes and therapeutic agents. We have identified two tetrahydroisoquinoline natural product analogs with antimigratory activities on Madin-Darby canine kidney epithelial cells: a semisynthetic derivative of quinocarmycin (also known as quinocarcin), DX-52-1, and a more complex synthetic molecule, HUK-921, related to the naphthyridinomycin family. It has been assumed that the cellular effects of reactive tetrahydroisoquinolines result from the alkylation of DNA. We have reported previously that the primary target of DX-52-1 relevant to cell migration appears to be the membrane-cytoskeleton linker protein radixin. Here we extend the analysis of the protein targets of DX-52-1, reporting that the multifunctional carbohydrate-binding protein galectin-3 is a secondary target of DX-52-1 that may also be relevant to the antimigratory effects of both DX-52-1 and HUK-921. All known inhibitors of galectin-3 target its beta-galactoside-binding site in the carbohydrate recognition domain. However, we found that DX-52-1 and HUK-921 bind galectin-3 outside of its beta-galactoside-binding site. Intriguingly HUK-921, although a less potent inhibitor of cell migration than DX-52-1, had far greater selectivity for galectin-3 over radixin, exhibiting little binding to radixin, both in vitro and in cells. Overexpression of galectin-3 in cells led to a dramatic increase in cell adhesion on different extracellular matrix substrata as well as changes in cell-cell adhesion and cell motility. Galectin-3-overexpressing cells had greatly reduced sensitivity to DX-52-1 and HUK-921, and these compounds caused a change in localization of the overexpressed galectin-3 and reversion of the cells to a more normal morphology. The converse manipulation, RNA interference-based silencing of galectin-3 expression, resulted in reduced cell-matrix adhesion and cell migration. In aggregate, the data suggest that DX-52-1 and HUK-921 inhibit a carbohydrate binding-independent function of galectin-3 that is involved in cell migration.

Quinocarmycin Analog DX-52-1 Inhibits Cell Migration and Targets Radixin, Disrupting Interactions of Radixin with Actin and CD44

In the course of screening for new small-molecule modulators of cell motility, we discovered that quinocarmycin (also known as quinocarcin) analog DX-52-1 is an inhibitor of epithelial cell migration. While it has been assumed that the main target of DX-52-1 is DNA, we identified and confirmed radixin as the relevant molecular target of DX-52-1 in the cell. Radixin is a member of the ezrin/radixin/moesin family of membrane-actin cytoskeleton linker proteins that also participate in signal transduction pathways. DX-52-1 binds specifically and covalently to the C-terminal region of radixin, which contains the domain that interacts with actin filaments. Overexpression of radixin in cells abrogates their sensitivity to DX-52-1's antimigratory activity. Small interfering RNA-mediated silencing of radixin expression reduces the rate of cell migration. Finally, we found that DX-52-1 disrupts radixin's ability to interact with both actin and the cell adhesion molecule CD44.

Glycogen synthase kinase-3 acts upstream of ADP-ribosylation factor 6 and Rac1 to regulate epithelial cell migration

Cell sheet movement during epithelial wound closure is a complex process involving collective cell migration. We have found that glycogen synthase kinase-3 (GSK-3) activity is required for membrane protrusion and crawling of cells at the wound edge and those behind it in wounded Madin-Darby canine kidney (MDCK) epithelial cell monolayers. RNA interference-based silencing of GSK-3alpha and GSK-3beta expression also results in slowed cell sheet migration, with the effect being more pronounced with knockdown of GSK-3beta. Both GSK-3alpha and GSK-3beta are in activated states during the most active phase of cell migration. In addition to having a positive control or permissive, rather than negative, function in MDCK cell migration, GSK-3 appears to act upstream of the small GTPases ADP-ribosylation factor 6 (ARF6) and Rac1. Expression of constitutively active ARF6 restores a protrusive, migratory phenotype in cells treated with GSK-3 inhibitors. It does not, however, restore to normal levels the directional polarization of cells behind the wound edge toward the wound area, implying the existence of a separate ARF6-independent branch of the GSK-3 pathway that regulates proper wound-directed polarization of these cells. Finally, inhibition of GSK-3 also strongly reduces activation of Rac1 and cell scatter in response to hepatocyte growth factor/scatter factor, which triggers dispersal and migration of cells in monolayer culture as fibroblast-like individual cells, a mode of epithelial cell motility distinct from the collective migration of wound closure.

A chemical inhibitor reveals the role of Raf kinase inhibitor protein in cell migration

Raf kinase inhibitor protein (RKIP) is a modulator of cell signaling that functions as an endogenous inhibitor of multiple kinases. We demonstrate here a positive role for RKIP in the regulation of cell locomotion. We discovered that RKIP is the relevant cellular target of locostatin, a cell migration inhibitor. Locostatin abrogates RKIP's ability to bind and inhibit Raf-1 kinase, and it acts by disrupting a protein-protein interaction, an uncommon mode of action for a small molecule. Small interfering RNA-mediated silencing of RKIP expression also reduces cell migration rate. Overexpression of RKIP converts epithelial cells to a highly migratory fibroblast-like phenotype, with dramatic reduction in the sensitivity of cells to locostatin. RKIP is therefore the compound's valid target and a key regulator of cell motility.

A role for RKIP in cell motility

The target of locostatin, a small-molecule inhibitor of cell movement, has been identified as RKIP, a Raf-1 kinase modulator [1]. In addition to advancing our understanding of cell locomotion, this work represents a major landmark in the development of chemical genetics.

Analysis methods of human cell migration

The autonomous migration of specialized cells is an essential characteristic in both physiological and pathological functions in the adult human organism. Leukocytes, fibroblasts, and stem cells, but also tumor cells, are thus the subject of intense investigation in a broad range of research fields. A wide spectrum of methods have therefore been established to analyze chemokinetic and chemotactic cell migration, ranging from easy-to-handle two-dimensional surface migration assays to highly specialized three-dimensional and intravital analysis methods. It is now manifest that the results obtained with these various migration assays substantially differ. This review therefore gives an overview of the migration assays which are currently in use, describes the methods, and critically enlightens the particular advantages and disadvantages of each method.

Scratch n’ Screen for Inhibitors of Cell Migration

Yarrow et al. have identified a small molecule inhibitor of cell migration, 3-(4-pyridyl)indole ("Rockout"), that targets Rho-kinase via a novel screening method using a scratch wound healing assay adapted to a high-throughput format and automated microscopy.

Multiple rows of cells behind an epithelial wound edge extend cryptic lamellipodia to collectively drive cell-sheet movement

The mechanism by which epithelial, endothelial and other strongly cell-cell adhesive cells migrate collectively as continuous sheets is not clear, even though this process is crucial for embryonic development and tissue repair in virtually all multicellular animals. Wound closure in Madin-Darby canine kidney (MDCK) epithelial cell monolayers involves Rac GTPase-dependent migration of cells both at and behind the wound edge. We report here for the first time that cells behind the margin of wounded MDCK cell monolayers, even hundreds of microns from the edge, extend 'cryptic' lamellipodia against the substratum beneath cells in front of them, toward the wound, as determined by confocal, two-photon and transmission electron microscopy. These so-called submarginal cells nevertheless strictly maintain their more apical cell-cell contacts when they migrate as part of a coherent cell sheet, hiding their basal protrusions from conventional microscopy. The submarginal protrusions display the hallmarks of traditional lamellipodia based on morphology and dynamics. Cells behind the margin therefore actively crawl, instead of just moving passively when cells at the margin pull on them. The rate of migration is inversely proportional to the distance from the margin, and cells move co-ordinately, yet still in part autonomously, toward the wound area. We also clarify the ancillary role played by nonprotrusive contractile actin bundles that assemble in a Rho GTPase-dependent manner at the margin after wounding. In addition, some cell proliferation occurs at a delay after wounding but does not contribute to closure. Instead, it apparently serves to replace damaged cells so that intact spread cells can revert to their normal cuboidal morphology and the original cell density of the unbroken sheet can be restored.

A high-throughput cell migration assay using scratch wound healing, a comparison of image-based readout methods

BACKGROUND

Cell migration is a complex phenomenon that requires the coordination of numerous cellular processes. Investigation of cell migration and its underlying biology is of interest to basic scientists and those in search of therapeutics. Current migration assays for screening small molecules, siRNAs, or other perturbations are difficult to perform in parallel at the scale required to screen large libraries.

RESULTS

We have adapted the commonly used scratch wound healing assay of tissue-culture cell monolayers to a 384 well plate format. By mechanically scratching the cell substrate with a pin array, we are able to create characteristically sized wounds in all wells of a 384 well plate. Imaging of the healing wounds with an automated fluorescence microscope allows us to distinguish perturbations that affect cell migration, morphology, and division. Readout requires ~1 hr per plate but is high in information content i.e. high content. We compare readouts using different imaging technologies, automated microscopy, scanners and a fluorescence macroscope, and evaluate the trade-off between information content and data acquisition rate.

CONCLUSIONS

The adaptation of a wound healing assay to a 384 well format facilitates the study of aspects of cell migration, tissue reorganization, cell division, and other processes that underlie wound healing. This assay allows greater than 10,000 perturbations to be screened per day with a quantitative, high-content readout, and can also be used to characterize small numbers of perturbations in detail.

c-Jun N-terminal kinase regulates lamellipodial protrusion and cell sheet migration during epithelial wound closure by a gene expression-independent mechanism

c-Jun N-terminal kinase (JNK) is emerging as an important regulator of cell migration. Perturbing the JNK signaling pathway with three structurally and mechanistically distinct inhibitors that selectively target either JNKs themselves or the upstream mixed-lineage kinases, we found dramatic inhibition of membrane protrusion and cell sheet migration during wound closure in Madin-Darby canine kidney (MDCK) epithelial cell monolayers. Extension of lamellipodia is blocked from the earliest times after wounding in the presence of JNK pathway inhibitors, whereas assembly of non-protrusive actin bundles at the wound margin is unaffected. Inhibitors of the other mitogen-activated protein kinase (MAPK) pathways, the extracellular signal-regulated kinase and p38 MAPK pathways, only have comparatively weak or marginal inhibitory effects on wound closure. Multiple splice variants of both JNK1 and JNK2 are expressed in MDCK cells, and JNK1 and JNK2 are rapidly and transiently activated upon wounding. Phosphorylation of c-Jun does not appear relevant to MDCK wound closure, and membrane protrusion directly after wounding is not affected by inhibitors of RNA or protein synthesis. While most known substrates of JNK are transcription factors or proteins regulating apoptosis, our data indicate that JNK regulates protrusion and migration in a gene expression-independent manner and suggest an important cytoplasmic role for JNK in the control of cell motility.