The actin cytoskeleton in cancer cell motility. Clin Exp Metastasis. 2009;26:273-287. [PMID: 18498004 DOI: 10.1007/s10585-008-9174-2]

Actin cortex mechanics and cellular morphogenesis

The cortex is a thin, crosslinked actin network lying immediately beneath the plasma membrane of animal cells. Myosin motors exert contractile forces in the meshwork. Because the cortex is attached to the cell membrane, it plays a central role in cell shape control. The proteic constituents of the cortex undergo rapid turnover, making the cortex both mechanically rigid and highly plastic, two properties essential to its function. The cortex has recently attracted increasing attention and its functions in cellular processes such as cytokinesis, cell migration, and embryogenesis are progressively being dissected. In this review, we summarize current knowledge on the structural organization, composition, and mechanics of the actin cortex, focusing on the link between molecular processes and macroscopic physical properties. We also highlight consequences of cortex dysfunction in disease.

RKI-1447 is a potent inhibitor of the Rho-associated ROCK kinases with anti-invasive and antitumor activities in breast cancer

The Rho-associated kinases ROCK1 and ROCK2 are critical for cancer cell migration and invasion, suggesting they may be useful therapeutic targets. In this study, we describe the discovery and development of RKI-1447, a potent small molecule inhibitor of ROCK1 and ROCK2. Crystal structures of the RKI-1447/ROCK1 complex revealed that RKI-1447 is a Type I kinase inhibitor that binds the ATP binding site through interactions with the hinge region and the DFG motif. RKI-1447 suppressed phosphorylation of the ROCK substrates MLC-2 and MYPT-1 in human cancer cells, but had no effect on the phosphorylation levels of the AKT, MEK, and S6 kinase at concentrations as high as 10 μmol/L. RKI-1447 was also highly selective at inhibiting ROCK-mediated cytoskeleton re-organization (actin stress fiber formation) following LPA stimulation, but does not affect PAK-meditated lamellipodia and filopodia formation following PDGF and Bradykinin stimulation, respectively. RKI-1447 inhibited migration, invasion and anchorage-independent tumor growth of breast cancer cells. In contrast, RKI-1313, a much weaker analog in vitro, had little effect on the phosphorylation levels of ROCK substrates, migration, invasion or anchorage-independent growth. Finally, RKI-1447 was highly effective at inhibiting the outgrowth of mammary tumors in a transgenic mouse model. In summary, our findings establish RKI-1447 as a potent and selective ROCK inhibitor with significant anti-invasive and antitumor activities and offer a preclinical proof-of-concept that justify further examination of RKI-1447 suitability as a potential clinical candidate.

ERK3 signals through SRC-3 coactivator to promote human lung cancer cell invasion

In contrast to the well-studied classic MAPKs, such as ERK1/2, little is known concerning the regulation and substrates of the atypical MAPK ERK3 signaling cascade and its function in cancer progression. Here, we report that ERK3 interacted with and phosphorylated steroid receptor coactivator 3 (SRC-3), an oncogenic protein overexpressed in multiple human cancers at serine 857 (S857). This ERK3-mediated phosphorylation at S857 was essential for interaction of SRC-3 with the ETS transcription factor PEA3, which promotes upregulation of MMP gene expression and proinvasive activity in lung cancer cells. Importantly, knockdown of ERK3 or SRC-3 inhibited the ability of lung cancer cells to invade and form tumors in the lung in a xenograft mouse model. In addition, ERK3 was found to be highly upregulated in human lung carcinomas. Our study identifies a previously unknown role for ERK3 in promoting lung cancer cell invasiveness by phosphorylating SRC-3 and regulating SRC-3 proinvasive activity by site-specific phosphorylation. As such, ERK3 protein kinase may be an attractive target for therapeutic treatment of invasive lung cancer.

Biophysical control of invasive tumor cell behavior by extracellular matrix microarchitecture

Fibrillar collagen gels, which are used extensively in vitro to study tumor-microenvironment interactions, are composed of a cell-instructive network of interconnected fibers and pores whose organization is sensitive to polymerization conditions such as bulk concentration, pH, and temperature. Using confocal reflectance microscopy and image autocorrelation analysis to quantitatively assess gel microarchitecture, we show that additional polymerization parameters including culture media formulation and gel thickness significantly affect the dimensions and organization of fibers and pores in collagen gels. These findings enabled the development of a three-dimensional culture system in which cell-scale gel microarchitecture was decoupled from bulk gel collagen concentration. Interestingly, morphology and migration characteristics of embedded MDA-MB-231 cells were sensitive to gel microarchitecture independently of collagen gel concentration. Cells adopted a polarized, motile phenotype in gels with larger fibers and pores and a rounded or stellate, less motile phenotype in gels with small fibers and pores regardless of bulk gel density. Conversely, cell proliferation was sensitive to gel concentration but not microarchitecture. These results indicate that cell-scale gel microarchitecture may trump bulk-scale gel density in controlling specific cell behaviors, underscoring the biophysical role of gel microarchitecture in influencing cell behavior.

Mena, a new available marker in tumors of salivary glands?

Mena (mammalian Ena) is an actin regulatory protein involved in cell motility and adhesion. Based on its potential role in malignant transformation revealed in other organs, we analyzed the Mena expression in normal salivary glands (SG) and salivary tumors. Mena expression was determined in normal SG (n=10) and also benign (n=20) and malignant (n=35) lesions of SG. For the immunohistochemical staining we used the anti-Mena antibody. All normal SG and the benign lesions (10 pleomorphic adenomas, 10 Warthin's tumors) were Mena negative. Salivary duct carcinomas (n=5), carcinomas in pleomorphic adenoma (n=5), acinic cell carcinomas (n=5), squamous cell carcinomas (n=10) and high-grade mucoepidermoid carcinomas (n=2) were positive. The lymphomas (n=5) and low-grade mucoepidermoid carcinomas (n=1) were Mena negative. In one case the lymphoblastic cells stained positive for Mena. Some of the endothelial cells, in the peritumoral vessels, were Mena positive. To the best of our knowledge, this is the first study in the literature about Mena expression in salivary tumors. Our study suggests that Mena protein seems to play a role in malignant transformation and its intensity is correlated with the type and grade of tumor and also with vascular invasion. Its positivity in endothelial cells may suggest its potential role in tumor angiogenesis.

Mechanobiology of tumor invasion: engineering meets oncology

The physical sciences and engineering have introduced novel perspectives into the study of cancer through model systems, tools, and metrics that enable integration of basic science observations with clinical data. These methods have contributed to the identification of several overarching mechanisms that drive processes during cancer progression including tumor growth, angiogenesis, and metastasis. During tumor cell invasion - the first clinically observable step of metastasis - cells demonstrate diverse and evolving physical phenotypes that cannot typically be defined by any single molecular mechanism, and mechanobiology has been used to study the physical cell behaviors that comprise the "invasive phenotype". In this review, we discuss the continually evolving pathological characterization and in vitro mechanobiological characterization of tumor invasion, with emphasis on emerging physical biology and mechanobiology strategies that have contributed to a more robust mechanistic understanding of tumor cell invasion. These physical approaches may ultimately help to better predict and identify tumor metastasis.

Modeling and imaging 3-dimensional collective cell invasion

A defining characteristic of cancer malignancy is invasion and metastasis. In some cancers (e.g. glioma), local invasion into surrounding healthy tissue is the root cause of disease and death. For other cancers (e.g. breast, lung, etc.), it is the process of metastasis, in which tumor cells move from a primary tumor mass, colonize distal sites and ultimately contribute to organ failure, that eventually leads to morbidity and mortality. It has been estimated that invasion and metastasis are responsible for 90% of cancer deaths. As a result, there has been intense interest in identifying the molecular processes and critical protein mediators of invasion and metastasis for the purposes of improving diagnosis and treatment. A challenge for cancer scientists is to develop invasion assays that sufficiently resemble the in vivo situation to enable accurate disease modeling. Two-dimensional cell motility assays are only informative about one aspect of invasion and do not take into account extracellular matrix (ECM) protein remodeling which is also a critical element. Recently, research has refined our understanding of tumor cell invasion and revealed that individual cells may move by elongated or rounded modes. In addition, there has been greater appreciation of the contribution of collective invasion, in which cells invade in strands, sheets and clusters, particularly in highly differentiated tumors that maintain epithelial characteristics, to the spread of cancer. We present a refined method for examining the contributions of candidate proteins to collective invasion. In particular, by engineering separate pools of cells to express different fluorescent proteins, it is possible to molecularly dissect the activities and proteins required in leading cells versus those required in following cells. The use of RNAi provides the molecular tool to experimentally disassemble the processes involved in individual cell invasion as well as in different positions of collective invasion. In this procedure, mixtures of fluorescently-labeled cells are plated on the bottom of a Transwell insert previously filled with Matrigel ECM protein, then allowed to invade "upwards" through the filter and into the Matrigel. Reconstruction of z-series image stacks, obtained by confocal imaging, into three-dimensional representations allows for visualization of collectively invading strands and analysis of the representation of fluorescently-labeled cells in leading versus following positions.

Reliable measurement of elastic modulus of cells by nanoindentation in an atomic force microscope

The elastic modulus of an oral cancer cell line UM1 is investigated by nanoindentation in an atomic force microscope with a flat-ended tip. The commonly used Hertzian method gives apparent elastic modulus which increases with the loading rate, indicating strong effects of viscoelasticity. On the contrary, a rate-jump method developed for viscoelastic materials gives elastic modulus values which are independent of the rate-jump magnitude. The results show that the rate-jump method can be used as a standard protocol for measuring elastic stiffness of living cells, since the measured values are intrinsic properties of the cells.

Integration of cell biology, pharmacological modeling and statistical analysis: part I: cell biology and PK/PD in the Oncology paradigm

The motivation of this two-part review article is to provide a comprehensive picture of cancer, cancer drugs and the detection and treatment of cancer. In order to do so, this article integrates the cell biology and biophysics of cancer as well as the modeling of preclinical Oncology drug data and statistical analysis of Oncology clinical trials data. It also discusses novel cancer diagnostic tools and standard and potential treatment options.

Correlative nanomechanical profiling with super-resolution F-actin imaging reveals novel insights into mechanisms of cisplatin resistance in ovarian cancer cells

The exact molecular mechanisms of ovarian cancer platinum resistance are not well understood, and biomarkers to reliably predict ovarian cancer resistance to platinum and other chemotherapeutic agents are lacking. Biomechanics of cisplatin-treated ovarian cancer cells were measured quantitatively at nanoscale level using atomic force microscopy. We demonstrate that cisplatin modulates the cellular nanomechanics of ovarian cancer cells; sensitive cells show dose-dependent increase in cell stiffness, which is effected by disrupting the F-actin polymerization. In contrast, resistant cells show no significant changes in cell stiffness upon cisplatin treatment. Further, stimulated emission depletion, an emerging super-resolution microscopy, shows that at the molecular level, F-actin is indeed remodeled considerably in cisplatin-sensitive and cisplatin-resistant cells. These findings reveal a direct role of the actin remodeling mechanism in cisplatin resistance of ovarian cancer cells, suggesting potential future applications of nanomechanical profiling as a marker for cancer drug sensitivity.

FROM THE CLINICAL EDITOR

In this paper, nanomechanical profiling and an emerging super-resolution microscopy method was utilized to decipher the mechanisms of cisplatin resistance in ovarian cancer cells, paving the way to future studies of this and similar other problems with drug resistance in cancer biology.

Co-crystal structures of inhibitors with MRCKβ, a key regulator of tumor cell invasion

MRCKα and MRCKβ (myotonic dystrophy kinase-related Cdc42-binding kinases) belong to a subfamily of Rho GTPase activated serine/threonine kinases within the AGC-family that regulate the actomyosin cytoskeleton. Reflecting their roles in myosin light chain (MLC) phosphorylation, MRCKα and MRCKβ influence cell shape and motility. We report further evidence for MRCKα and MRCKβ contributions to the invasion of cancer cells in 3-dimensional matrix invasion assays. In particular, our results indicate that the combined inhibition of MRCKα and MRCKβ together with inhibition of ROCK kinases results in significantly greater effects on reducing cancer cell invasion than blocking either MRCK or ROCK kinases alone. To probe the kinase ligand pocket, we screened 159 kinase inhibitors in an in vitro MRCKβ kinase assay and found 11 compounds that inhibited enzyme activity >80% at 3 µM. Further analysis of three hits, Y-27632, Fasudil and TPCA-1, revealed low micromolar IC(50) values for MRCKα and MRCKβ. We also describe the crystal structure of MRCKβ in complex with inhibitors Fasudil and TPCA-1 bound to the active site of the kinase. These high-resolution structures reveal a highly conserved AGC kinase fold in a typical dimeric arrangement. The kinase domain is in an active conformation with a fully-ordered and correctly positioned αC helix and catalytic residues in a conformation competent for catalysis. Together, these results provide further validation for MRCK involvement in regulation of cancer cell invasion and present a valuable starting point for future structure-based drug discovery efforts.

Decreased ezrin and paxillin expression in human urothelial bladder tumors correlate with tumor progression

OBJECTIVES

F-actin binding proteins ezrin and paxillin are involved in cell adhesion and cell migration/invasion. The aim of the study was to investigate their role in urothelial bladder carcinogenesis.

MATERIALS AND METHODS

Expression of ezrin and paxillin was studied by immunohistochemistry in 104 and 96 cases of urothelial bladder tumors, respectively. Correlations with clinicopathologic data and expression of p53, E-cadherin, and β-catenin were examined.

RESULTS

Positive ezrin and paxillin protein expression was found in 99% and 93.7% of cases, respectively. Membranous expression of ezrin was significantly lower in high grade tumors and correlated with invasion. Multivariate analysis showed that ezrin is an independent predictor of muscularis propria invasion. Paxillin expression was significantly decreased in urothelial carcinomas compared with tumors of low malignant potential and low paxillin levels also correlated with advancing tumor stage and invasion. A statistically significant correlation was found between membranous ezrin and E-cadherin as well as between ezrin and paxillin expression in urothelial tumors.

CONCLUSIONS

Down-regulation of ezrin and paxillin in urothelial bladder tumors is associated with aggressive tumor features and invasiveness.

Identification and characterization of a set of conserved and new regulators of cytoskeletal organization, cell morphology and migration

BACKGROUND

Cell migration is essential during development and in human disease progression including cancer. Most cell migration studies concentrate on known or predicted components of migration pathways.

RESULTS

Here we use data from a genome-wide RNAi morphology screen in Drosophila melanogaster cells together with bioinformatics to identify 26 new regulators of morphology and cytoskeletal organization in human cells. These include genes previously implicated in a wide range of functions, from mental retardation, Down syndrome and Huntington's disease to RNA and DNA-binding genes. We classify these genes into seven groups according to phenotype and identify those that affect cell migration. We further characterize a subset of seven genes, FAM40A, FAM40B, ARC, FMNL3, FNBP3/FBP11, LIMD1 and ZRANB1, each of which has a different effect on cell shape, actin filament distribution and cell migration. Interestingly, in several instances closely related isoforms with a single Drosophila homologue have distinct phenotypes. For example, FAM40B depletion induces cell elongation and tail retraction defects, whereas FAM40A depletion reduces cell spreading.

CONCLUSIONS

Our results identify multiple regulators of cell migration and cytoskeletal signalling that are highly conserved between Drosophila and humans, and show that closely related paralogues can have very different functions in these processes.

Actomyosin-mediated cellular tension drives increased tissue stiffness and β-catenin activation to induce epidermal hyperplasia and tumor growth

Tumors and associated stroma manifest mechanical properties that promote cancer. Mechanosensation of tissue stiffness activates the Rho/ROCK pathway to increase actomyosin-mediated cellular tension to re-establish force equilibrium. To determine how actomyosin tension affects tissue homeostasis and tumor development, we expressed conditionally active ROCK2 in mouse skin. ROCK activation elevated tissue stiffness via increased collagen. β-catenin, a key element of mechanotranscription pathways, was stabilized by ROCK activation leading to nuclear accumulation, transcriptional activation, and consequent hyperproliferation and skin thickening. Inhibiting actomyosin contractility by blocking LIMK or myosin ATPase attenuated these responses, as did FAK inhibition. Tumor number, growth, and progression were increased by ROCK activation, while ROCK blockade was inhibitory, implicating actomyosin-mediated cellular tension and consequent collagen deposition as significant tumor promoters.

Metastasis suppressor genes at the interface between the environment and tumor cell growth

The molecular mechanisms and genetic programs required for cancer metastasis are sometimes overlapping, but components are clearly distinct from those promoting growth of a primary tumor. Every sequential, rate-limiting step in the sequence of events leading to metastasis requires coordinated expression of multiple genes, necessary signaling events, and favorable environmental conditions or the ability to escape negative selection pressures. Metastasis suppressors are molecules that inhibit the process of metastasis without preventing growth of the primary tumor. The cellular processes regulated by metastasis suppressors are diverse and function at every step in the metastatic cascade. As we gain knowledge into the molecular mechanisms of metastasis suppressors and cofactors with which they interact, we learn more about the process, including appreciation that some are potential targets for therapy of metastasis, the most lethal aspect of cancer. Until now, metastasis suppressors have been described largely by their function. With greater appreciation of their biochemical mechanisms of action, the importance of context is increasingly recognized especially since tumor cells exist in myriad microenvironments. In this chapter, we assemble the evidence that selected molecules are indeed suppressors of metastasis, collate the data defining the biochemical mechanisms of action, and glean insights regarding how metastasis suppressors regulate tumor cell communication to-from microenvironments.

Transition to invasion in breast cancer: a microfluidic in vitro model enables examination of spatial and temporal effects

The transition of ductal carcinoma in situ (DCIS) to invasive ductal carcinoma (IDC) is a critical step in breast cancer progression. We introduce a simple microfluidic 3D compartmentalized system in which mammary epithelial cells (MCF-DCIS) are co-cultured with human mammary fibroblasts (HMFs), which promotes a transition from DCIS to IDC in vitro. The model enables control of both spatial (distance-dependence) and temporal (transition from larger clusters) aspects within the microenvironment, allowing recapitulation of the in vivo environment in ways not practical with existing experimental models. When HMFs were cultured some distance (0.5-1.5 mm) from the MCF-DCIS cells, we observed an initial morphological change, suggesting soluble factors can begin the transition. However, cell-cell contact with HMFs allowed the MCF-DCIS cells to complete the transition to invasion. Uniquely, the compartmentalized platform enables the analysis of the intrinsic second harmonic generation signal of collagen, providing a label-free quantitative analysis of DCIS-associated collagen remodeling. The arrayed microchannel-based model is compatible with existing infrastructure and, for the first time, provides a cost effective approach to test for inhibitors of pathways involved in DCIS progression to IDC allowing a screening approach to the identification of potential therapeutic targets. Importantly, the model can be easily adapted and generalized to a variety of cell-cell signaling studies.

CHI3L1 (YKL-40) is expressed in human gliomas and regulates the invasion, growth and survival of glioma cells

Chitinase 3-like 1 (CHI3L1) is a secreted glycoprotein that has pleiotropic activity in aggressive cancers. In our study, we examined the expression and function of CHI3L1 in glioma cells. CHI3L1 was highly expressed in human glioma tissue, whereas its expression in normal brain tissue was very low. CHI3L1 suppression by shRNA reduced glioma cell invasion, anchorage-independent growth and increased cell death triggered by several anticancer drugs, including cisplatin, etoposide and doxorubicin, whereas CHI3L1 overexpression had the opposite effect in glioma cells. Because the invasive nature of glioma cells plays a critical role in the high morbidity of glioma, we have further defined the role of CHI3L1 in the process of glioma invasion. Downregulation of CHI3L1 results in decreased cell-matrix adhesion and causes a marked increase in stress fiber formation and cell size with fewer cellular processes. Furthermore, the expression and activity of matrix metalloproteinase-2 was also decreased in glioma cells in which CHI3L1 was knocked down. Taken together, these results suggest that CHI3L1 plays an important role in the regulation of malignant transformation and local invasiveness in gliomas. Thus, targeting the CHI3L1 molecule may be a potential therapeutic molecular target for gliomas.

Cellular uptake and subcellular distribution of ruthenium-based metallodrugs under clinical investigation versus cisplatin

The cellular uptake and subcellular distribution including adduct formation with genomic DNA and uptake into mitochondria of two ruthenium(iii)-based drugs in clinical trials, KP1019 and NAMI-A, and cisplatin, was investigated in cisplatin sensitive and resistant A2780 human ovarian carcinoma cells. These data indicate that reduced metal uptake into mitochondria in combination with increased binding towards low molecular weight components involved in detoxification mechanisms is essential for cisplatin resistance. The ruthenium drugs show distinct differences with respect to cisplatin, especially in the cisplatin resistant cells; in comparison to the sensitive cells, KP1019 exhibits higher cytotoxicity and an only slightly changed metabolism of the drug, whereas NAMI-A treatment results in increased intracellular ruthenium levels and a higher number of ruthenium-DNA adducts. In addition, size exclusion-inductively coupled mass spectrometry indicates that adduct formation with high molecular weight components in the particulate and nuclear fractions is crucial for the therapeutic effect of KP1019 in both cisplatin resistant and sensitive cell lines.

Changes in gene expression and cellular architecture in an ovarian cancer progression model

BACKGROUND

Ovarian cancer is the fifth leading cause of cancer deaths among women. Early stage disease often remains undetected due the lack of symptoms and reliable biomarkers. The identification of early genetic changes could provide insights into novel signaling pathways that may be exploited for early detection and treatment.

METHODOLOGY/PRINCIPAL FINDINGS

Mouse ovarian surface epithelial (MOSE) cells were used to identify stage-dependent changes in gene expression levels and signal transduction pathways by mouse whole genome microarray analyses and gene ontology. These cells have undergone spontaneous transformation in cell culture and transitioned from non-tumorigenic to intermediate and aggressive, malignant phenotypes. Significantly changed genes were overrepresented in a number of pathways, most notably the cytoskeleton functional category. Concurrent with gene expression changes, the cytoskeletal architecture became progressively disorganized, resulting in aberrant expression or subcellular distribution of key cytoskeletal regulatory proteins (focal adhesion kinase, α-actinin, and vinculin). The cytoskeletal disorganization was accompanied by altered patterns of serine and tyrosine phosphorylation as well as changed expression and subcellular localization of integral signaling intermediates APC and PKCβII.

CONCLUSIONS/SIGNIFICANCE

Our studies have identified genes that are aberrantly expressed during MOSE cell neoplastic progression. We show that early stage dysregulation of actin microfilaments is followed by progressive disorganization of microtubules and intermediate filaments at later stages. These stage-specific, step-wise changes provide further insights into the time and spatial sequence of events that lead to the fully transformed state since these changes are also observed in aggressive human ovarian cancer cell lines independent of their histological type. Moreover, our studies support a link between aberrant cytoskeleton organization and regulation of important downstream signaling events that may be involved in cancer progression. Thus, our MOSE-derived cell model represents a unique model for in depth mechanistic studies of ovarian cancer progression.

F-actin reorganization and inactivation of rho signaling pathway involved in the inhibitory effect of Coptidis Rhizoma on hepatoma cell migration

HYPOTHESIS

Hepatocellular carcinoma (HCC) is one of the most malignant human tumors and one of the risk factors is its highly metastatic property. Coptidis Rhizoma aqueous extract (CRAE) is able to suppress the migration and invasion of HCC cells, MHCC97-L, and F-actin reorganization and Rho signaling inhibition is involved.

MAIN METHODS

CRAE was prepared and analyzed by high-performance liquid chromatography combined with mass spectrometry. The cytotoxicity and antimigration action of CRAE on MHCC97-L cells were evaluated; Immunofluorescence and immunoblotting were used to investigate the proposed mechanism of CRAE action.

KEY FINDINGS

Chemical analysis reveals that the active components in CRAE are berberine and berberine-like alkaloids. CRAE exhibits a significant inhibitory effect on MHCC97-L cell migration as indicated by wound-healing and invasion-chamber assays. No significant alteration of matrix metalloproteinases and urokinase-type plasminogen activator (uPA) expression were observed in MHCC97-L cells exposed to CRAE. Reduction of F-actin polymerization and damage to cytoskeleton network in MHCC97-L cells were observed after CRAE treatment. Furthermore, it was found that CRAE significantly downregulated the Rho/ROCK signaling pathway.

SIGNIFICANCE

These results indicate that CRAE may act as a Rho/ROCK signaling inhibitor to suppress MHCC97-L cell migration in vitro and suggested that total alkaloids in Coptidis Rhizoma may be a potential agent for suppressing liver cancer invasion.

Effect of selenocystine on gene expression profiles in human keloid fibroblasts

In this study, selenocystine, a nutritionally available selenoamino acid, was identified for the first time as a novel agent with anti proliferative activity on human keloids. The 20 μM concentration after 48 h treatment used here was the most effective to reduce keloid fibroblast growth. We analyzed the gene expression profile of selenocystine treatment response in keloid fibroblasts by the microarray system to characterize the effects of selenocystine on human keloids. The major alterations in keloid fibroblasts following selenocystine exposure included up-regulation of the genes encoding cell death and transcription factors. Prominent down-regulation of genes involved in development, cell adhesion and cytoskeleton, as well as extra cellular matrix genes, usually strongly up-regulated in keloids, resulted following selenocystine exposure. The range of the down-regulated genes and the degree of the decreased expression appeared to be correlated with the degree of the morphological alterations in selenocystine treated keloids.

Mastic oil inhibits the metastatic phenotype of mouse lung adenocarcinoma cells

Mastic oil from Pistacia lentiscus variation chia, a natural combination of bioactive terpenes, has been shown to exert anti-tumor growth effects against a broad spectrum of cancers including mouse Lewis lung adenocarcinomas (LLC). However, no studies have addressed its anti-metastatic actions. In this study, we showed that treatment of LLC cells with mastic oil within a range of non-toxic concentrations (0.01–0.04% v/v): (a) abrogated their Matrigel invasion and migration capabilities in transwell assays; (b) reduced the levels of secreted MMP-2; (c) restricted phorbol ester-induced actin remodeling and (d) limited the length of neo-vessel networks in tumor microenvironment in the model of chick embryo chorioallantoic membrane. Moreover, exposure of LLC and endothelial cells to mastic oil impaired their adhesive interactions in a co-culture assay and reduced the expression of key adhesion molecules by endothelial cells upon their stimulation with tumor necrosis factor-alpha. Overall, this study provides novel evidence supporting a multipotent role for mastic oil in prevention of crucial processes related to cancer metastasis.

Cell migration: an overview

Cell migration is a fundamental process that controls morphogenesis and inflammation. Its deregulation causes or is part of many diseases, including autoimmune syndromes, chronic inflammation, mental retardation, and cancer. Cell migration is an integral part of the cell biology, embryology, immunology, and neuroscience fields; as such, it has benefited from quantum leaps in molecular biology, biochemistry, and imaging techniques, and the emergence of the genomic and proteomic era. Combinations of these techniques have revealed new and exciting insights that explain how cells adhere and move, how the migration of multiple cells are coordinated and regulated, and how the cells interact with neighboring cells and/or react to changes in their microenvironment. This introduction provides a primer of the molecular and cellular insights, particularly the signaling networks, which control the migration of individual cells as well as collective migrations. The rest of the chapters are devoted to describe in detail some of the most salient technical advances that have illuminated the field of cell migration in recent years.

Actin cytoskeleton participation in the onset of IL-1beta induction of an invasive mesenchymal-like phenotype in epithelial MCF-7 cells

BACKGROUND

Interleukin 1 beta (IL-1beta) and other inflammatory cytokines are reported to induce phenotypic changes in epithelial breast cancer tumor cells related to increased invasiveness. Mechanisms involved in the process are not well understood.

METHODS

The noninvasive breast cancer epithelial cell line MCF-7 was used to investigate the IL-1beta-induced phenotype. Live cells expressing EGFP-actin were monitored for cell morphology changes and actin cytoskeleton dynamics by time-lapse video microscopy in the presence of IL-1beta and specific inhibitors of actin signaling pathways. Chemotaxis, invasion of Matrigel, MMP activity and expression of S100A4 in cells treated with IL-1beta were assessed by migration assays, zymograms and immunoblots.

RESULTS

Exposure to IL-1beta specifically induced a change in MCF-7 cells from a typical epithelial morphology into elongated cells, showing numerous dynamic actin-rich lamellae and peripheral ruffles characteristic of fibroblasts. These cells could scatter from compact cell colonies and respond to chemoattractants such as the homing-associated chemokine CXCL-12. Pharmacological blockage of actin signaling pathways and negative mutants of RhoGTPases revealed that actin reorganization and enhanced motility are regulated via PI3K/Rac 1 activation. IL-1beta-stimulated cells expressed the metastasis promoter S100A4, increased secretion of active MMP-9 and MMP-2 and invasion of extracellular matrix proteins.

CONCLUSIONS

IL-1beta induces a PI3K/Rac 1-regulated reorganization of the actin cytoskeleton of MCF-7 cells that is required for cell scattering, elongation and migration. The enhanced motility is accompanied by expression of protein markers correlated with invasive behavior.

Biological functions of the genes in the mammaprint breast cancer profile reflect the hallmarks of cancer

Background:

MammaPrint was developed as a diagnostic tool to predict risk of breast cancer metastasis using the expression of 70 genes. To better understand the tumor biology assessed by MammaPrint, we interpreted the biological functions of the 70-genes and showed how the genes reflect the six hallmarks of cancer as defined by Hanahan and Weinberg.

Results:

We used a bottom-up system biology approach to elucidate how the cellular processes reflected by the 70-genes work together to regulate tumor activities and progression. The biological functions of the genes were analyzed using literature research and several bioinformatics tools. Protein-protein interaction network analyses indicated that the 70-genes form highly interconnected networks and that their expression levels are regulated by key tumorigenesis related genes such as TP53, RB1, MYC, JUN and CDKN2A. The biological functions of the genes could be associated with the essential steps necessary for tumor progression and metastasis, and cover the six well-defined hallmarks of cancer, reflecting the acquired malignant characteristics of a cancer cell along with tumor progression and metastasis-related biological activities.

Conclusion:

Genes in the MammaPrint gene signature comprehensively measure the six hallmarks of cancer-related biology. This finding establishes a link between a molecular signature and the underlying molecular mechanisms of tumor cell progression and metastasis.

A p27(kip1)-binding protein, p27RF-Rho, promotes cancer metastasis via activation of RhoA and RhoC

Rho family proteins regulate multiple cellular functions including motility and invasion through regulation of the actin cytoskeleton and gene expression. Activation of Rho proteins is controlled precisely by multiple regulators in a spatiotemporal manner. RhoA and/or RhoC are key players that regulate the metastatic activity of malignant tumor cells, and it is therefore of particular interest to understand how activation of these Rho proteins is controlled. We recently identified an upstream regulator of RhoA activation, p27RF-Rho (p27(kip1) releasing factor from RhoA) that acts by freeing RhoA from inhibition by p27(kip1). p27(kip1) is a cell cycle regulator when it is localized to the nucleus, but it binds RhoA and inhibits activation of the latter when it is localized to the cytoplasm. Here, we show that a metastatic variant of mouse melanoma B16 cells (F10) exhibits greater expression of p27RF-Rho, RhoA, and RhoC than the nonmetastatic parental cells (F0). Injection of F10 cells into mouse tail vein resulted in the formation of metastatic lung colonies, whereas prior knockdown of expression of either one of the three proteins using specific shRNA sequences decreased metastasis markedly. p27RF-Rho regulated the activation of RhoA and RhoC and thereby modulated cellular adhesion and motility, in addition to pericellular proteolysis. The Rho activities enhanced by p27RF-Rho had a marked effect upon efficiency of lodging of F10 cells in the lung, which represents an early step of metastasis. p27RF-Rho also regulated metastasis of human melanoma and fibrosarcoma cells. Thus, p27RF-Rho is a key upstream regulator of RhoA and RhoC that controls spreading of tumor cells.

Differing contributions of LIMK and ROCK to TGFβ-induced transcription, motility and invasion

The ability of transforming growth factor β (TGFβ) to induce epithelial-mesenchymal transition (EMT) is mediated by SMAD-dependent and SMAD-independent pathways such as the activation of Rho GTPase signalling. Upon activation, GTP-bound Rho stimulates the ROCK kinases, which in turn phosphorylate numerous substrates including the LIM kinases (LIMK). The net result of ROCK activation is increased actin-myosin contractile force generation, with a contribution from LIMK-induced actin filament stabilisation. In this study, we made use of siRNA-mediated knockdown and selective inhibitors to determine the contributions of ROCK and LIMK to TGFβ-induced responses. We find that both ROCK and LIMK are required for TGFβ stimulation of serum-response factor (SRF) transcriptional activity and actin stress fibre formation during EMT. In contrast, although LIMK inhibition had little effect on cell motility in scratch wound and Transwell migration assays, ROCK inhibition actually promoted TGFβ-induced cell motility by helping individual cells to break free from the epithelial sheet. Furthermore, we demonstrate that selective inhibition of LIMK, but not ROCK, effectively blocked TGFβ driven invasion through a layer of matrigel extracellular matrix protein. These results indicate that the roles of LIMK and ROCK in the Rho signalling pathway downstream of TGFβ are not identical and suggest that LIMK represents an attractive therapeutic target in TGFβ driven organ fibrosis and metastatic cancer spread.

Cortactin phosphorylated by ERK1/2 localizes to sites of dynamic actin regulation and is required for carcinoma lamellipodia persistence

Background

Tumor cell motility and invasion is governed by dynamic regulation of the cortical actin cytoskeleton. The actin-binding protein cortactin is commonly upregulated in multiple cancer types and is associated with increased cell migration. Cortactin regulates actin nucleation through the actin related protein (Arp)2/3 complex and stabilizes the cortical actin cytoskeleton. Cortactin is regulated by multiple phosphorylation events, including phosphorylation of S405 and S418 by extracellular regulated kinases (ERK)1/2. ERK1/2 phosphorylation of cortactin has emerged as an important positive regulatory modification, enabling cortactin to bind and activate the Arp2/3 regulator neuronal Wiskott-Aldrich syndrome protein (N-WASp), promoting actin polymerization and enhancing tumor cell movement.

Methodology/Principal Findings

In this report we have developed phosphorylation-specific antibodies against phosphorylated cortactin S405 and S418 to analyze the subcellular localization of this cortactin form in tumor cells and patient samples by microscopy. We evaluated the interplay between cortactin S405 and S418 phosphorylation with cortactin tyrosine phosphorylation in regulating cortactin conformational forms by Western blotting. Cortactin is simultaneously phosphorylated at S405/418 and Y421 in tumor cells, and through the use of point mutant constructs we determined that serine and tyrosine phosphorylation events lack any co-dependency. Expression of S405/418 phosphorylation-null constructs impaired carcinoma motility and adhesion, and also inhibited lamellipodia persistence monitored by live cell imaging.

Conclusions/Significance

Cortactin phosphorylated at S405/418 is localized to sites of dynamic actin assembly in tumor cells. Concurrent phosphorylation of cortactin by ERK1/2 and tyrosine kinases enables cells with the ability to regulate actin dynamics through N-WASp and other effector proteins by synchronizing upstream regulatory pathways, confirming cortactin as an important integration point in actin-based signal transduction. Reduced lamellipodia persistence in cells with S405/418A expression identifies an essential motility-based process reliant on ERK1/2 signaling, providing additional understanding as to how this pathway impacts tumor cell migration.

Myosin light chain mono- and di-phosphorylation differentially regulate adhesion and polarity in migrating cells

Myosin II is a critical regulator of cell migration that generates polarity, controls protrusion, and promotes adhesion maturation and retraction of the rear. Myosin II has an ATPase motor domain that is regulated by phosphorylation of the regulatory light chain (RLC) on Thr18 and Ser19. Here, we address the activation and specific function of the two phosphorylation states of the RLC, mono- (S19) and/or di-phosphorylation (T18+S19), in cell polarity and adhesion. Specific phospho-antibodies reveal that adhesion to fibronectin via the α5β1 integrin promotes localized mono- and di-phosphorylation of the RLC that follow different kinetics. Using phospho-mimetic mutants, we show that mono-phosphorylation promotes adhesion maturation in protrusions resulting in focal adhesions throughout the cell. In contrast, di-phosphorylation produces large, stable actomyosin bundles and large, non-dynamic adhesions that define the rear. Finally, RLC phosphorylation regulates the assembly and stability of MIIB, but not MIIA. Our data reveal a novel mechanism by which the degree of phosphorylation of the RLC differentially controls cell adhesion and polarity.

WASP and WAVE family proteins: friends or foes in cancer invasion?

Wiskott-Aldrich syndrome protein (WASP) and WASP family verprolin-homologous protein (WAVE) family proteins activate cells' major actin nucleating machinery, the actin-related protein 2/3 (Arp2/3) complex, leading to the formation and remodeling of cortical actin filament networks. Cortical actin regulation is critical in many aspects of cell physiology including cell-cell adhesion and cell motility, whose dysregulation is directly associated with cancer invasion and metastasis. In line with this association, the WASP and WAVE family proteins have been reported to be involved in cancer malignancies. What is puzzling, however, is that they can act as either enhancers or suppressors of cancer malignancies depending on the type of cancer and its pathological stage. We are still far from understanding the roles of the WASP and WAVE family proteins in cancer progression. Here, we summarize the recent advances of studies of the WASP and WAVE family proteins with respect to cancer invasion and we offer a model that can account for the diverse outcomes originating from dysregulated WASP and WAVE family proteins in cancer development.

The transcription factor EGR1 regulates metastatic potential of v-src transformed sarcoma cells

Metastatic spreading of cancer cells is a highly complex process directed primarily by the interplay between tumor microenvironment, cell surface receptors, and actin cytoskeleton dynamics. To advance our understanding of metastatic cancer dissemination, we have developed a model system that is based on two v-src transformed chicken sarcoma cell lines-the highly metastatic parental PR9692 and a non-metastasizing but fully tumorigenic clonal derivative PR9692-E9. Oligonucleotide microarray analysis of both cell lines revealed that the gene encoding the transcription factor EGR1 was downregulated in the non-metastatic PR9692-E9 cells. Further investigation demonstrated that the introduction of exogenous EGR1 into PR9692-E9 cells restored their metastatic potential to a level indistinguishable from parental PR9692 cells. Microarray analysis of EGR1 reconstituted cells revealed the activation of genes that are crucial for actin cytoskeleton contractility (MYL9), filopodia formation (MYO10), the production of specific extracellular matrix components (HAS2, COL6A1-3) and other essential pro-metastatic abilities.

The impact of stress on tumor growth: peripheral CRF mediates tumor-promoting effects of stress

Introduction

Stress has been shown to be a tumor promoting factor. Both clinical and laboratory studies have shown that chronic stress is associated with tumor growth in several types of cancer. Corticotropin Releasing Factor (CRF) is the major hypothalamic mediator of stress, but is also expressed in peripheral tissues. Earlier studies have shown that peripheral CRF affects breast cancer cell proliferation and motility. The aim of the present study was to assess the significance of peripheral CRF on tumor growth as a mediator of the response to stress in vivo.

Methods

For this purpose we used the 4T1 breast cancer cell line in cell culture and in vivo. Cells were treated with CRF in culture and gene specific arrays were performed to identify genes directly affected by CRF and involved in breast cancer cell growth. To assess the impact of peripheral CRF as a stress mediator in tumor growth, Balb/c mice were orthotopically injected with 4T1 cells in the mammary fat pad to induce breast tumors. Mice were subjected to repetitive immobilization stress as a model of chronic stress. To inhibit the action of CRF, the CRF antagonist antalarmin was injected intraperitoneally. Breast tissue samples were histologically analyzed and assessed for neoangiogenesis.

Results

Array analysis revealed among other genes that CRF induced the expression of SMAD2 and β-catenin, genes involved in breast cancer cell proliferation and cytoskeletal changes associated with metastasis. Cell transfection and luciferase assays confirmed the role of CRF in WNT- β-catenin signaling. CRF induced 4T1 cell proliferation and augmented the TGF-β action on proliferation confirming its impact on TGFβ/SMAD2 signaling. In addition, CRF promoted actin reorganization and cell migration, suggesting a direct tumor-promoting action. Chronic stress augmented tumor growth in 4T1 breast tumor bearing mice and peripheral administration of the CRF antagonist antalarmin suppressed this effect. Moreover, antalarmin suppressed neoangiogenesis in 4T1 tumors in vivo.

Conclusion

This is the first report demonstrating that peripheral CRF, at least in part, mediates the tumor-promoting effects of stress and implicates CRF in SMAD2 and β-catenin expression.

LIM kinases are required for invasive path generation by tumor and tumor-associated stromal cells

Leading cells require LIMK for matrix degradation and invadopodia formation during collective cell migration.

LIM kinases 1 and 2 (LIMK1/2) are centrally positioned regulators of actin cytoskeleton dynamics. Using siRNA-mediated knockdown or a novel small molecule inhibitor, we show LIMK is required for path generation by leading tumor cells and nontumor stromal cells during collective tumor cell invasion. LIMK inhibition lowers cofilin phosphorylation, F-actin levels, serum response factor transcriptional activity and collagen contraction, and reduces invasion in three-dimensional invasion assays. Although motility was unaffected, LIMK inhibition impairs matrix protein degradation and invadopodia formation associated with significantly faster recovery times in FRAP assays indicative of reduced F-actin stability. When LIMK is knocked down in MDA-MB-231 cells, they lose the ability to lead strands of collectively invading cells. Similarly, when LIMK activity is blocked in cancer-associated fibroblasts, they are unable to lead the collective invasion of squamous carcinoma cells in an organotypic skin model. These results show that LIMK is required for matrix remodeling activities for path generation by leading cells in collective invasion.

Claudin-6, 7, or 9 overexpression in the human gastric adenocarcinoma cell line AGS increases its invasiveness, migration, and proliferation rate

Altered claudin expression is related to metastatic potential, poor prognosis, or tumor recurrence. We analyzed if the overexpression of claudin-6, claudin-7, or claudin-9 in AGS cells altered cell motility, invasiveness, or proliferation rate. Claudin-7, claudin-9, and claudin-6 enhanced their invasive potential by 3.4-fold, 1.6-fold, and 2.0-fold, respectively. Claudin-6 and claudin-9 enhanced cell migration, while the proliferation rate of claudin-6-, claudin-7-, and claudin-9-transfected cells increased by 12.7%, 9.0%, and 13.3%, respectively. Claudin-7 and claudin-9 overexpression increased claudin-1 and zonula occludens-1 levels. In summary, individual increased expression of claudin-6, claudin-7, or claudin-9 is sufficient to enhance tumorigenic properties of a gastric adenocarcinoma cell line.

Cancer dissemination--lessons from leukocytes

Cancer cells can move through tissues in a variety of different ways. In some cases, an epithelial-to-mesenchymal transition enables cancer cells to acquire fibroblast-like migratory properties. However, it is also becoming apparent that some cancer cells move in an amoeboid way similar to leukocytes. This theme will be the focus of the review, where we will discuss the similarities and differences between the mechanisms used by cancer cells and leukocytes to cross parenchymal basement membranes, move through interstitial tissue, and enter and exit the vasculature. Further, we propose that the ability to switch between different migratory mechanisms is critical for cells to relocate from one tissue to another.

Prolyl hydroxylase domain (PHD) 2 affects cell migration and F-actin formation via RhoA/rho-associated kinase-dependent cofilin phosphorylation

Cells are responding to hypoxia via prolyl-4-hydroxylase domain (PHD) enzymes, which are responsible for oxygen-dependent hydroxylation of the hypoxia-inducible factor (HIF)-1α subunit. To gain further insight into PHD function, we generated knockdown cell models for the PHD2 isoform, which is the main isoform regulating HIF-1α hydroxylation and thus stability in normoxia. Induction of a PHD2 knockdown in tetracycline-inducible HeLa PHD2 knockdown cells resulted in increased F-actin formation as detected by phalloidin staining. A similar effect could be observed in the stably transfected PHD2 knockdown cell clones 1B6 and 3B7. F-actin is at least in part responsible for shaping cell morphology as well as regulating cell migration. Cell migration was impaired significantly as a consequence of PHD2 knockdown in a scratch assay. Mechanistically, PHD2 knockdown resulted in activation of the RhoA (Ras homolog gene family member A)/Rho-associated kinase pathway with subsequent phosphorylation of cofilin. Because cofilin phosphorylation impairs its actin-severing function, this may explain the F-actin phenotype, thereby providing a functional link between PHD2-dependent signaling and cell motility.

The ROCK signaling and breast cancer metastasis

Metastasis is the predominant cause of death in most breast cancer patients. The molecular mechanisms underlying metastasis from primary tumors to distant organs are not clearly characterized. In this review, we depict the role of ROCK signaling in regulating cell motility and growth, and discuss the contribution of this signaling to breast cancer metastasis.

Quantitative proteomics reveals myosin and actin as promising saliva biomarkers for distinguishing pre-malignant and malignant oral lesions

Background

Oral cancer survival rates increase significantly when it is detected and treated early. Unfortunately, clinicians now lack tests which easily and reliably distinguish pre-malignant oral lesions from those already transitioned to malignancy. A test for proteins, ones found in non-invasively-collected whole saliva and whose abundances distinguish these lesion types, would meet this critical need.

Methodology/Principal Findings

To discover such proteins, in a first-of-its-kind study we used advanced mass spectrometry-based quantitative proteomics analysis of the pooled soluble fraction of whole saliva from four subjects with pre-malignant lesions and four with malignant lesions. We prioritized candidate biomarkers via bioinformatics and validated selected proteins by western blotting. Bioinformatic analysis of differentially abundant proteins and initial western blotting revealed increased abundance of myosin and actin in patients with malignant lesions. We validated those results by additional western blotting of individual whole saliva samples from twelve other subjects with pre-malignant oral lesions and twelve with malignant oral lesions. Sensitivity/specificity values for distinguishing between different lesion types were 100%/75% (p = 0.002) for actin, and 67%/83% (p<0.00001) for myosin in soluble saliva. Exfoliated epithelial cells from subjects' saliva also showed increased myosin and actin abundance in those with malignant lesions, linking our observations in soluble saliva to abundance differences between pre-malignant and malignant cells.

Conclusions/Significance

Salivary actin and myosin abundances distinguish oral lesion types with sensitivity and specificity rivaling other non-invasive oral cancer tests. Our findings provide a promising starting point for the development of non-invasive and inexpensive salivary tests to reliably detect oral cancer early.

Transcription profiling provides insights into gene pathways involved in horn and scurs development in cattle

BACKGROUND

Two types of horns are evident in cattle - fixed horns attached to the skull and a variation called scurs, which refers to small loosely attached horns. Cattle lacking horns are referred to as polled. Although both the Poll and Scurs loci have been mapped to BTA1 and 19 respectively, the underlying genetic basis of these phenotypes is unknown, and so far, no candidate genes regulating these developmental processes have been described. This study is the first reported attempt at transcript profiling to identify genes and pathways contributing to horn and scurs development in Brahman cattle, relative to polled counterparts.

RESULTS

Expression patterns in polled, horned and scurs tissues were obtained using the Agilent 44 k bovine array. The most notable feature when comparing transcriptional profiles of developing horn tissues against polled was the down regulation of genes coding for elements of the cadherin junction as well as those involved in epidermal development. We hypothesize this as a key event involved in keratinocyte migration and subsequent horn development. In the polled-scurs comparison, the most prevalent differentially expressed transcripts code for genes involved in extracellular matrix remodelling, which were up regulated in scurs tissues relative to polled.

CONCLUSION

For this first time we describe networks of genes involved in horn and scurs development. Interestingly, we did not observe differential expression in any of the genes present on the fine mapped region of BTA1 known to contain the Poll locus.

Preventing the activation or cycling of the Rap1 GTPase alters adhesion and cytoskeletal dynamics and blocks metastatic melanoma cell extravasation into the lungs

The Rap1 GTPase is a master regulator of cell adhesion, polarity, and migration. We show that both blocking Rap1 activation and expressing a constitutively active form of Rap1 reduced the ability of B16F1 melanoma cells to extravasate from the microvasculature and form metastatic lesions in the lungs. This correlated with a decreased ability of the tumor cells to undergo transendothelial migration (TEM) in vitro and form dynamic, F-actin-rich pseudopodia that penetrate capillary endothelial walls in vivo. Using multiple tumor cell lines, we show that the inability to form these membrane protrusions, which likely promote TEM and extravasation, can be explained by altered adhesion dynamics and impaired cell polarization that result when Rap1 activation or cycling is perturbed. Thus, targeting Rap1 could be a useful approach for reducing the metastatic dissemination of tumor cells that undergo active TEM.

From branched networks of actin filaments to bundles

Cross-linking proteins can mediate the emergence of rigid bundles from a dense branched network of actin filaments. To enable their binding, the filaments must first bend towards each other. We derive an explicit criterion for the onset of bundling, in terms of the initial length of filaments L, their spacing b, and cross-linker concentration f, reflecting the balance between bending and binding energies. Our model system contains actin, the branching complex Arp2/3 and the bundling protein fascin. In the first distinct stage, during which only actin and Arp2/3 are active, an entangled aster-like mesh of actin filaments is formed. Tens of seconds later, when filaments at the aster periphery are long and barely branched, a sharp transition takes place into a star-like structure, marking the onset of bundling. Now fascin and actin govern bundle growth; Arp2/3 plays no role. Using kinetic Monte Carlo simulations we calculate the temporal evolution of b and L, and predict the onset of bundling as a function of f. Our predictions are in good qualitative agreement with several new experiments that are reported herein and demonstrate how f controls the aster-star transition and bundle length. We also present two models for aster growth corresponding to different experimental realizations. The first treats filament and bundle association as an irreversible sequence of elongation-association steps. The second, applicable for low f, treats bundling as a reversible self-assembly process, where the optimal bundle size is dictated by the balance between surface and bending energies. Finally, we discuss the relevance of our conclusions for the lamellipodium to filopodia transition in living cells, noting that bundles are more likely nucleated by "tip complex" cross-linkers (e.g. mDia2 or Ena/VASP), whereas fascin is mainly involved in bundle maintenance.

Activating ROCK1 somatic mutations in human cancer

Cancer cells acquire characteristics of deregulated growth, survival and increased metastatic potential. Genetic mutations that provide a selective advantage by promoting these characteristics have been termed 'drivers,' whereas mutations that do not contribute to disease initiation/progression are termed 'passengers.' The advent of high-throughput methodologies has facilitated large-scale screening of cancer genomes and the subsequent identification of novel somatic mutations. Although this approach has generated valuable results, the data remain incomplete until the functional consequences of these mutations are determined to differentiate potential drivers from passengers. ROCK1 is an essential effector kinase downstream of Rho GTPases, an important pathway involved in cell migration. The Cancer Genome Project identified three nonsynonymous mutations in the ROCK1 gene. We now show that these somatic ROCK1 mutations lead to elevated kinase activity and drive actin cytoskeleton rearrangements that promote increased motility and decreased adhesion, characteristics of cancer progression. Mapping of the kinase-interacting regions of the carboxy terminus combined with structural modeling provides an insight into how these mutations likely affect the regulation of ROCK1. Consistent with the frequency of ROCK1 mutations in human cancer, these results support the conclusion that there is selective pressure for the ROCK1 gene to acquire 'driver' mutations that result in kinase activation.