α5β1 integrin recycling promotes Arp2/3-independent cancer cell invasion via the formin FHOD3

Myosin X is required for efficient melanoblast migration and melanoma initiation and metastasis

Myosin X (Myo10), an actin-associated molecular motor, has a clear role in filopodia induction and cell migration in vitro, but its role in vivo in mammals is not well understood. Here, we investigate the role of Myo10 in melanocyte lineage and melanoma induction. We found that Myo10 knockout (Myo10KO) mice exhibit a white spot on their belly caused by reduced melanoblast migration. Myo10KO mice crossed with available mice that conditionally express in melanocytes the BRAFV600E mutation combined with Pten silencing exhibited reduced melanoma development and metastasis, which extended medial survival time. Knockdown of Myo10 (Myo10kd) in B16F1 mouse melanoma cell lines decreased lung colonization after tail-vein injection. Myo10kd also inhibited long protrusion (LP) formation by reducing the transportation of its cargo molecule vasodilator-stimulated phosphoprotein (VASP) to the leading edge of migrating cells. These findings provide the first genetic evidence for the involvement of Myo10 not only in melanoblast migration, but also in melanoma development and metastasis.

Actin-Based Cell Protrusion in a 3D Matrix

Cell migration controls developmental processes (gastrulation and tissue patterning), tissue homeostasis (wound repair and inflammatory responses), and the pathobiology of diseases (cancer metastasis and inflammation). Understanding how cells move in physiologically relevant environments is of major importance, and the molecular machinery behind cell movement has been well studied on 2D substrates, beginning over half a century ago. Studies over the past decade have begun to reveal the mechanisms that control cell motility within 3D microenvironments – some similar to, and some highly divergent from those found in 2D. In this review we focus on migration and invasion of cells powered by actin, including formation of actin-rich protrusions at the leading edge, and the mechanisms that control nuclear movement in cells moving in a 3D matrix.

Cell migration has been well studied in 2D, but how this relates to movement in physiological 3D tissues and matrix is not clear, particularly in vertebrate interstitial matrix.

In 3D matrix cells actin polymerisation directly contributes to the formation of lamellipodia to facilitate migration and invasion (mesenchymal movement), analogous to 2D migration; actomyosin contractility promotes bleb formation to indirectly promote protrusion (amoeboid movement).

Mesenchymal migration can be characterised by polymerisation of actin to form filopodial protrusions, in the absence of lamellipodia.

Translocation of the nucleus is emerging as a critical step due to the constrictive environment of 3D matrices, and the mechanisms that transmit force to the nucleus and allow movement are beginning to be uncovered.

Collagen abundance controls melanoma phenotypes through lineage-specific microenvironment sensing

Despite the general focus on an invasive and de-differentiated phenotype as main driver of cancer metastasis, in melanoma patients many metastatic lesions display a high degree of pigmentation, indicative for a differentiated phenotype. Indeed, studies in mice and fish show that melanoma cells switch to a differentiated phenotype at secondary sites, possibly because in melanoma differentiation is closely linked to proliferation through the lineage-specific transcriptional master regulator MITF. Importantly, while a lot of effort has gone into identifying factors that induce the de-differentiated/invasive phenotype, it is not well understood how the switch to the differentiated/proliferative phenotype is controlled. We identify collagen as a contributor to this switch. We demonstrate that collagen stiffness induces melanoma differentiation through a YAP/PAX3/MITF axis and show that in melanoma patients increased collagen abundance correlates with nuclear YAP localization. However, the interrogation of large patient datasets revealed that in the context of the tumour microenvironment, YAP function is more complex. In the absence of fibroblasts, YAP/PAX3-mediated transcription prevails, but in the presence of fibroblasts tumour growth factor-β suppresses YAP/PAX3-mediated MITF expression and induces YAP/TEAD/SMAD-driven transcription and a de-differentiated phenotype. Intriguingly, while high collagen expression is correlated with poorer patient survival, the worst prognosis is seen in patients with high collagen expression, who also express MITF target genes such as the differentiation markers TRPM1, TYR and TYRP1, as well as CDK4. In summary, we reveal a distinct lineage-specific route of YAP signalling that contributes to the regulation of melanoma pigmentation and uncovers a set of potential biomarkers predictive for poor survival.

The mechanical microenvironment regulates ovarian cancer cell morphology, migration, and spheroid disaggregation

There is growing appreciation of the importance of the mechanical properties of the tumor microenvironment on disease progression. However, the role of extracellular matrix (ECM) stiffness and cellular mechanotransduction in epithelial ovarian cancer (EOC) is largely unknown. Here, we investigated the effect of substrate rigidity on various aspects of SKOV3 human EOC cell morphology and migration. Young’s modulus values of normal mouse peritoneum, a principal target tissue for EOC metastasis, were determined by atomic force microscopy (AFM) and hydrogels were fabricated to mimic these values. We find that cell spreading, focal adhesion formation, myosin light chain phosphorylation, and cellular traction forces all increase on stiffer matrices. Substrate rigidity also positively regulates random cell migration and, importantly, directional increases in matrix tension promote SKOV3 cell durotaxis. Matrix rigidity also promotes nuclear translocation of YAP1, an oncogenic transcription factor associated with aggressive metastatic EOC. Furthermore, disaggregation of multicellular EOC spheroids, a behavior associated with dissemination and metastasis, is enhanced by matrix stiffness through a mechanotransduction pathway involving ROCK, actomyosin contractility, and FAK. Finally, this pattern of mechanosensitivity is maintained in highly metastatic SKOV3ip.1 cells. These results establish that the mechanical properties of the tumor microenvironment may play a role in EOC metastasis.

Programmed death ligand 1 promotes lymph node metastasis and glucose metabolism in cervical cancer by activating integrin β4/SNAI1/SIRT3 signaling pathway

Although PD-L1 has been shown to play a well-characterized role in inhibiting antitumor immunity via engagement of its receptor PD-1 in T lymphocytes, little is known about the tumor cell-intrinsic function of PD-L1 and its association with prognosis. Here, we investigate this issue and dissect the molecular mechanisms underlying the role of PD-L1 in glucose metabolism, proliferation, migration, and invasion in human cervical cancer cells. As a result, we found that PD-L1 overexpression in cervical cancer cells increases glucose metabolism and metastasis-related behaviors. Mechanistically, PD-L1 bound directly to integrin β4 (ITGB4), activating the AKT/GSK3β signaling pathway and consequently inducing the expression of the transcriptional repressor SNAI1. SNAIL in turn influenced the expression of genes involved in the epithelial-to-mesenchymal transition and regulated glucose metabolism by inhibiting SIRT3 promoter activity. High expression of PD-L1 and ITGB4 in human cervical carcinomas was significantly associated with lymph node metastasis and poor prognosis. Finally, ¹⁸F-fluorodeoxyglucose microPET/CT and bioluminescence imaging analyses of cervical xenograft tumors in mice revealed that PD-L1 overexpression markedly increases tumor glucose uptake and promotes lymph node metastasis. Together, these results demonstrate that PD-L1 can promote the growth and metastasis of cervical cancer by activating the ITGB4/SNAI1/SIRT3 signaling pathway, and also suggest the possibility of targeting PD-L1 and its downstream effectors as a potential approach for interfering with cervical cancer growth and metastasis.

Cell matrix adhesions in cancer: The proteins that form the glue

The main purposes of Integrin-mediated cell contacts are to interpret bi-directional signals between the extracellular environment and intracellular proteins, as well as, anchor the cell to a matrix. Many cell adhesion molecules have been discovered with a wide spectrum of responsibilities, including recruiting, activating, elongating, and maintaining. This review will perlustrate some of the key incidences that precede focal adhesion formation. Tyrosine phosphorylation is a key signaling initiation event that leads to the recruitment of multiple proteins to focal adhesion sites. Recruitment and concentration of proteins such as Paxillin and Vinculin to Integrin clutches is necessary for focal adhesion development. The assembled networks are responsible for transmitting signals back and forth from the extracellular matrix (ECM) to Actin and its binding proteins. Cancer cells exhibit highly altered focal adhesion dynamics. This review will highlight some key discoveries in cancer cell adhesion, as well as, identify current gaps in knowledge.

Drosophila and human FHOD family formin proteins nucleate actin filaments

Formins are a conserved group of proteins that nucleate and processively elongate actin filaments. Among them, the formin homology domain-containing protein (FHOD) family of formins contributes to contractility of striated muscle and cell motility in several contexts. However, the mechanisms by which they carry out these functions remain poorly understood. Mammalian FHOD proteins were reported not to accelerate actin assembly in vitro; instead, they were proposed to act as barbed end cappers or filament bundlers. Here, we show that purified Drosophila Fhod and human FHOD1 both accelerate actin assembly by nucleation. The nucleation activity of FHOD1 is restricted to cytoplasmic actin, whereas Drosophila Fhod potently nucleates both cytoplasmic and sarcomeric actin isoforms. Drosophila Fhod binds tightly to barbed ends, where it slows elongation in the absence of profilin and allows, but does not accelerate, elongation in the presence of profilin. Fhod antagonizes capping protein but dissociates from barbed ends relatively quickly. Finally, we determined that Fhod binds the sides of and bundles actin filaments. This work establishes that Fhod shares the capacity of other formins to nucleate and bundle actin filaments but is notably less effective at processively elongating barbed ends than most well studied formins.

Cell-derived matrices for studying cell proliferation and directional migration in a complex 3D microenvironment

2D surfaces offer simple analysis of cells in culture, yet these often yield different cell morphologies and responses from those observed in vivo. Considerable effort has therefore been expended on the generation of more tissue-like environments for the study of cell behavior in vitro. Purified matrix proteins provide a 3D scaffold that better mimics the in vivo situation; however, these are far removed from the complex tissue composition seen in vivo. Cell-derived matrices (CDMs) offer a more physiologically relevant alternative for studying in vivo-like cell behavior in vitro. In the protocol described here, fibroblasts cultured on gelatin-coated surfaces are maintained in the presence of ascorbic acid to strengthen matrix deposition over 1-3 weeks. The resulting fibrillar CDMs are denuded of cells, and other cells are subsequently cultured on them, after which their behavior is monitored. We also demonstrate how to use CDMs as an in vivo-relevant reductionist model for studying tumor-stroma-induced changes in carcinoma cell proliferation and migration.

FiloQuant reveals increased filopodia density during breast cancer progression

Defective filopodia formation is linked to pathologies such as cancer, wherein actively protruding filopodia, at the invasive front, accompany cancer cell dissemination. Despite wide biological significance, delineating filopodia function in complex systems remains challenging and is particularly hindered by lack of compatible methods to quantify filopodia properties. Here, we present FiloQuant, a freely available ImageJ plugin, to detect filopodia-like protrusions in both fixed- and live-cell microscopy data. We demonstrate that FiloQuant can extract quantifiable information, including protrusion dynamics, density, and length, from multiple cell types and in a range of microenvironments. In cellular models of breast ductal carcinoma in situ, we reveal a link between filopodia formation at the cell-matrix interface, in collectively invading cells and 3D tumor spheroids, and the in vitro invasive capacity of the carcinoma. Finally, using intravital microscopy, we observe that tumor spheroids display filopodia in vivo, supporting a potential role for these protrusions during tumorigenesis.

Exploring the Role of RGD-Recognizing Integrins in Cancer

Integrins are key regulators of communication between cells and with their microenvironment. Eight members of the integrin superfamily recognize the tripeptide motif Arg-Gly-Asp (RGD) within extracelluar matrix (ECM) proteins. These integrins constitute an important subfamily and play a major role in cancer progression and metastasis via their tumor biological functions. Such transmembrane adhesion and signaling receptors are thus recognized as promising and well accessible targets for novel diagnostic and therapeutic applications for directly attacking cancer cells and their fatal microenvironment. Recently, specific small peptidic and peptidomimetic ligands as well as antibodies binding to distinct integrin subtypes have been developed and synthesized as new drug candidates for cancer treatment. Understanding the distinct functions and interplay of integrin subtypes is a prerequisite for selective intervention in integrin-mediated diseases. Integrin subtype-specific ligands labelled with radioisotopes or fluorescent molecules allows the characterization of the integrin patterns in vivo and later the medical intervention via subtype specific drugs. The coating of nanoparticles, larger proteins, or encapsulating agents by integrin ligands are being explored to guide cytotoxic reagents directly to the cancer cell surface. These ligands are currently under investigation in clinical studies for their efficacy in interference with tumor cell adhesion, migration/invasion, proliferation, signaling, and survival, opening new treatment approaches in personalized medicine.

Arp2/3 Complex Is Required for Macrophage Integrin Functions but Is Dispensable for FcR Phagocytosis and In Vivo Motility

The Arp2/3 complex nucleates branched actin, forming networks involved in lamellipodial protrusion, phagocytosis, and cell adhesion. We derived primary bone marrow macrophages lacking Arp2/3 complex (Arpc2^-/-) and directly tested its role in macrophage functions. Despite protrusion and actin assembly defects, Arpc2^-/- macrophages competently phagocytose via FcR and chemotax toward CSF and CX3CL1. However, CR3 phagocytosis and fibronectin haptotaxis, both integrin-dependent processes, are disrupted. Integrin-responsive actin assembly and αM/β2 integrin localization are compromised in Arpc2^-/- cells. Using an in vivo system to observe endogenous monocytes migrating toward full-thickness ear wounds we found that Arpc2^-/- monocytes maintain cell speeds and directionality similar to control. Our work reveals that the Arp2/3 complex is not a general requirement for phagocytosis or chemotaxis but is a critical driver of integrin-dependent processes. We demonstrate further that cells lacking Arp2/3 complex function in vivo remain capable of executing important physiological responses that require rapid directional motility.

Modelling GTPase dynamics to understand RhoA-driven cancer cell invasion

Metastasis, initially driven by cells migrating and invading through the local environment, leads to most cancer-associated deaths. Cells can use a variety of modes to move in vitro, all of which depend on Rho GTPases at some level. While traditionally it was thought that Rac1 activity drives protrusive lamellipodia at the leading edge of a polarised cell while RhoA drives rear retraction, more recent work in 3D microenvironments has revealed a much more complicated picture of GTPase dynamics. In particular, RhoA activity can dominate the leading edge polymerisation of actin to form filopodial actin-spike protrusions that drive more invasive cell migration. We recently described a potential mechanism to abrogate this pro-invasive localised leading edge Rac1 to RhoA switch via manipulation of a negative feedback loop that was revealed by adopting a logical modelling approach. Both challenging dogma and taking a formal, mathematical approach to understanding signalling involved in motility may be vital to harnessing harmful cell migration and preventing metastasis in future research.

Actin cytoskeleton-dependent regulation of corticotropin-releasing factor receptor heteromers

A physical interaction is shown between CRF1R and CRF2R, two class B G protein–coupled receptors that mediate stress and immune responses. Trafficking of CRF2R but not CRF1R is actin dependent, and coexpression of the two receptors alters actin-independent trafficking. Receptor cross-talk alters agonist binding and signaling.

Stress responses are highly nuanced and variable, but how this diversity is achieved by modulating receptor function is largely unknown. Corticotropin-releasing factor receptors (CRFRs), class B G protein–coupled receptors, are pivotal in mediating stress responses. Here we show that the two known CRFRs interact to form heteromeric complexes in HEK293 cells coexpressing both CRFRs and in vivo in mouse pancreas. Coimmunoprecipitation and mass spectrometry confirmed the presence of both CRF₁R and CRF₂βR, along with actin in these heteromeric complexes. Inhibition of actin filament polymerization prevented the transport of CRF₂βR to the cell surface but had no effect on CRF₁R. Transport of CRF₁R when coexpressed with CRF_2βR became actin dependent. Simultaneous stimulation of cells coexpressing CRF₁R+CRF₂βR with their respective high-affinity agonists, CRF+urocortin2, resulted in approximately twofold increases in peak Ca²⁺ responses, whereas stimulation with urocortin1 that binds both receptors with 10-fold higher affinity did not. The ability of CRFRs to form heteromeric complexes in association with regulatory proteins is one mechanism to achieve diverse and nuanced function.

Mechanisms of Cell Polarity-Controlled Epithelial Homeostasis and Immunity in the Intestine

Intestinal epithelial cell polarity is instrumental to maintain epithelial homeostasis and balance communications between the gut lumen and bodily tissue, thereby controlling the defense against gastrointestinal pathogens and maintenance of immune tolerance to commensal bacteria. In this review, we highlight recent advances with regard to the molecular mechanisms of cell polarity-controlled epithelial homeostasis and immunity in the human intestine.

Can Integrin Agonists Have Cards to Play against Cancer? A Literature Survey of Small Molecules Integrin Activators

The ability of integrins to activate and integrate intracellular communication illustrates the potential of these receptors to serve as functional distribution hubs in a bi-directional signal transfer outside-in and inside-out of the cells. Tight regulation of the integrin signaling is paramount for normal physiological functions such as migration, proliferation, and differentiation, and misregulated integrin activity could be associated with several pathological conditions. Because of the important roles of integrins and their ligands in biological development, immune responses, leukocyte traffic, haemostasis, and cancer, their potential as therapeutic tools is now widely recognized. Nowadays extensive efforts have been made to discover and develop small molecule ligands as integrin antagonists, whereas less attention has been payed to agonists. In recent years, it has been recognized that integrin agonists could open up novel opportunities for therapeutics, which gain benefits to increase rather than decrease integrin-dependent adhesion and transductional events. For instance, a significant factor in chemo-resistance in melanoma is a loss of integrin-mediated adhesion; in this case, stimulation of integrin signaling by agonists significantly improved the response to chemotherapy. In this review, we overview results about small molecules which revealed an activating action on some integrins, especially those involved in cancer, and examine from a medicinal chemistry point of view, their structure and behavior.

Hypoxia Selectively Enhances Integrin α5β1 Receptor Expression in Breast Cancer to Promote Metastasis

Metastasis is the leading cause of breast cancer mortality. Previous studies have implicated hypoxia-induced changes in the composition and stiffness of the extracellular matrix (ECM) in the metastatic process. Therefore, the contribution of potential ECM-binding receptors in this process was explored. Using a bioinformatics approach, the expression of all integrin receptor subunits, in two independent breast cancer patient datasets, were analyzed to determine whether integrin status correlates with a validated hypoxia-inducible gene signature. Subsequently, a large panel of breast cancer cell lines was used to validate that hypoxia induces the expression of integrins that bind to collagen (ITGA1, ITGA11, ITGB1) and fibronectin (ITGA5, ITGB1). Hypoxia-inducible factors (HIF-1 and HIF-2) are directly required for ITGA5 induction under hypoxic conditions, which leads to enhanced migration and invasion of single cells within a multicellular 3D tumor spheroid but did not affect migration in a 2D microenvironment. ITGB1 expression requires HIF-1α, but not HIF-2α, for hypoxic induction in breast cancer cells. ITGA5 (α₅ subunit) is required for metastasis to lymph nodes and lungs in breast cancer models, and high ITGA5 expression in clinical biopsies is associated with an increased risk of mortality.Implications: These results reveal that targeting ITGA5 using inhibitors that are currently under consideration in clinical trials may be beneficial for patients with hypoxic tumors. Mol Cancer Res; 15(6); 723-34. ©2017 AACR.

Downregulation of ARID1A, a component of the SWI/SNF chromatin remodeling complex, in breast cancer

Recent studies unraveled that AT-rich interactive domain-containing protein 1A (ARID1A), a subunit of the mammary SWI/SNF chromatin remodeling complex, acts as a tumor suppressor in various cancers. In this study, we first evaluated ARID1A expression by immunohistochemistry in invasive breast cancer tissue specimens and assessed the correlation with the prognosis of patients with breast cancer. Non-tumorous mammary duct epithelial cells exhibited strong nuclear ARID1A staining, whereas different degrees of loss in ARID1A immunoreactivity were observed in many invasive breast cancer cells. We scored ARID1A immunoreactivity based on the sum of the percentage score in invasive cancer cells (on a scale of 0 to 5) and the intensity score (on a scale of 0 to 3), for a possible total score of 0 to 8. Interestingly, partial loss of ARID1A expression, score 2 to 3, was significantly correlated with poor disease free survival of the patients. Subsequently, we performed siRNA-mediated ARID1A knockdown in cultured breast cancer cells followed by comprehensive gene profiling and quantitative RT-PCR. Interestingly, many genes were downregulated by partial loss of ARID1A, whereas RAB11FIP1 gene expression was significantly upregulated by partial loss of ARID1A expression in breast cancer cells. In contrast, a more than 50% reduction in ARID1A mRNA decreased RAB11FIP1gene expression. Immunoblotting also demonstrated that partial downregulation of ARID1A mRNA at approximately 20% reduction significantly increased the expression of RAB11FIP1 protein in MCF-7 cells, whereas, over 50% reduction of ARID1A mRNA resulted in reduction of RAB11FIP1 protein in cultured breast cancer cells. Recent studies reveal that RAB11FIP1 overexpression leads to breast cancer progression. Altogether, the present findings indicated that partial loss of ARID1A expression is linked to unfavorable outcome for patients with breast cancer, possibly due to increased RAB11FIP1 expression.

Modes of invasion during tumour dissemination

Cancer cell migration and invasion underlie metastatic dissemination, one of the major problems in cancer. Tumour cells exhibit a striking variety of invasion strategies. Importantly, cancer cells can switch between invasion modes in order to cope with challenging environments. This ability to switch migratory modes or plasticity highlights the challenges behind antimetastasis therapy design. In this Review, we present current knowledge on different tumour invasion strategies, the determinants controlling plasticity and arising therapeutic opportunities. We propose that targeting master regulators controlling plasticity is needed to hinder tumour dissemination and metastasis.

L-type calcium channels regulate filopodia stability and cancer cell invasion downstream of integrin signalling

Mounting in vitro, in vivo and clinical evidence suggest an important role for filopodia in driving cancer cell invasion. Using a high-throughput microscopic-based drug screen, we identify FDA-approved calcium channel blockers (CCBs) as potent inhibitors of filopodia formation in cancer cells. Unexpectedly, we discover that L-type calcium channels are functional and frequently expressed in cancer cells suggesting a previously unappreciated role for these channels during tumorigenesis. We further demonstrate that, at filopodia, L-type calcium channels are activated by integrin inside-out signalling, integrin activation and Src. Moreover, L-type calcium channels promote filopodia stability and maturation into talin-rich adhesions through the spatially restricted regulation of calcium entry and subsequent activation of the protease calpain-1. Altogether we uncover a novel and clinically relevant signalling pathway that regulates filopodia formation in cancer cells and propose that cycles of filopodia stabilization, followed by maturation into focal adhesions, directs cancer cell migration and invasion.

Filopodia have a prominent role in driving cancer cell invasion. Here, the authors show that L-type calcium channels are a druggable target regulating filopodia stability and maturation into focal adhesions in metastatic breast cancer cells.

Ankyrin-B is a PI3P effector that promotes polarized α5β1-integrin recycling via recruiting RabGAP1L to early endosomes

Endosomal membrane trafficking requires coordination between phosphoinositide lipids, Rab GTPases, and microtubule-based motors to dynamically determine endosome identity and promote long-range organelle transport. Here we report that ankyrin-B (AnkB), through integrating all three systems, functions as a critical node in the protein circuitry underlying polarized recycling of α5β1-integrin in mouse embryonic fibroblasts, which enables persistent fibroblast migration along fibronectin gradients. AnkB associates with phosphatidylinositol 3-phosphate (PI3P)-positive organelles in fibroblasts and binds dynactin to promote their long-range motility. We demonstrate that AnkB binds to Rab GTPase Activating Protein 1-Like (RabGAP1L) and recruits it to PI3P-positive organelles, where RabGAP1L inactivates Rab22A, and promotes polarized trafficking to the leading edge of migrating fibroblasts. We further determine that α5β1-integrin depends on an AnkB/RabGAP1L complex for polarized recycling. Our results reveal AnkB as an unexpected key element in coordinating polarized transport of α5β1-integrin and likely of other specialized endocytic cargos.

Endocytic Trafficking of Integrins in Cell Migration

Integrins are a family of heterodimeric receptors that bind to components of the extracellular matrix and influence cellular processes as varied as proliferation and migration. These effects are achieved by tight spatiotemporal control over intracellular signalling pathways, including those that mediate cytoskeletal reorganisation. The ability of integrins to bind to ligands is governed by integrin conformation, or activity, and this is widely acknowledged to be an important route to the regulation of integrin function. Over the last 15 years, however, the pathways that regulate endocytosis and recycling of integrins have emerged as major players in controlling integrin action, and studying integrin trafficking has revealed fresh insight into the function of this fascinating class of extracellular matrix receptors, in particular in the context of cell migration and invasion. Here, we review our current understanding of the contribution of integrin trafficking to cell motility.

A two-stage approach for combining gene expression and mutation with clinical data improves survival prediction in myelodysplastic syndromes and ovarian cancer

Motivation

Many traditional clinical prognostic factors have been known for cancer for years, but usually provide poor survival prediction. Genomic information is more easily available now which offers opportunities to build more accurate prognostic models. The challenge is how to integrate them to improve survival prediction. The common approach of jointly analyzing all type of covariates directly in one single model may not improve the prediction due to increased model complexity and cannot be easily applied to different datasets.

Results

We proposed a two-stage procedure to better combine different sources of information for survival prediction, and applied the two-stage procedure in two cancer datasets: myelodysplastic syndromes (MDS) and ovarian cancer. Our analysis suggests that the prediction performance of different data types are very different, and combining clinical, gene expression and mutation data using the two-stage procedure improves survival prediction in terms of improved concordance index and reduced prediction error.

Availability and implementation

The two-stage procedure can be implemented in BhGLM package which is freely available at http://www.ssg.uab.edu/bhglm/.

Contact

nyi@uab.edu.

Rationalizing Rac1 and RhoA GTPase signaling: A mathematical approach

Precise spatiotemporal dynamics of Rho GTPases are essential for efficient cell migration. Manipulating Rac1 and RhoA signaling is thus a potential intervention strategy to abrogate harmful cell invasion and subsequent metastasis; however GTPase signaling can be extremely complicated due to crosstalk and the multitude of upstream regulators and downstream effectors. Studying Rho GTPase networks in a formal mathematical setting can therefore be of great use. We recently built a predictive model based on Boolean logic which identified a negative feedback loop critical for RhoA and Rac1 activity. Here, we discuss the value and potential pitfalls of different mathematical approaches which have been used to study Rho GTPase dynamics, and highlight the importance of choosing the correct approach given the data available and outputs desired. Overall, a mathematical approach, particularly when combined iteratively with in vitro experiments, can be of great use in deriving new biological insight to further harness the activity of Rho GTPases.

Function and regulation of the Arp2/3 complex during cell migration in diverse environments

As the first de novo actin nucleator discovered, the Arp2/3 complex has been a central player in models of protrusive force production via the dynamic actin network. Here, we review recent studies on the functional role of the Arp2/3 complex in the migration of diverse cell types in different migratory environments. These findings have revealed an unexpected level of plasticity, both in how cells rely on the Arp2/3 complex for migration and other physiological functions and in the intricate modulation of the Arp2/3 complex by other actin regulators and upstream signaling cascades.

A MAPK-Driven Feedback Loop Suppresses Rac Activity to Promote RhoA-Driven Cancer Cell Invasion

Cell migration in 3D microenvironments is fundamental to development, homeostasis and the pathobiology of diseases such as cancer. Rab-coupling protein (RCP) dependent co-trafficking of α5β1 and EGFR1 promotes cancer cell invasion into fibronectin (FN) containing extracellular matrix (ECM), by potentiating EGFR1 signalling at the front of invasive cells. This promotes a switch in RhoGTPase signalling to inhibit Rac1 and activate a RhoA-ROCK-Formin homology domain-containing 3 (FHOD3) pathway and generate filopodial actin-spike protrusions which drive invasion. To further understand the signalling network that drives RCP-driven invasive migration, we generated a Boolean logical model based on existing network pathways/models, where each node can be interrogated by computational simulation. The model predicted an unanticipated feedback loop, whereby Raf/MEK/ERK signalling maintains suppression of Rac1 by inhibiting the Rac-activating Sos1-Eps8-Abi1 complex, allowing RhoA activity to predominate in invasive protrusions. MEK inhibition was sufficient to promote lamellipodia formation and oppose filopodial actin-spike formation, and led to activation of Rac and inactivation of RhoA at the leading edge of cells moving in 3D matrix. Furthermore, MEK inhibition abrogated RCP/α5β1/EGFR1-driven invasive migration. However, upon knockdown of Eps8 (to suppress the Sos1-Abi1-Eps8 complex), MEK inhibition had no effect on RhoGTPase activity and did not oppose invasive migration, suggesting that MEK-ERK signalling suppresses the Rac-activating Sos1-Abi1-Eps8 complex to maintain RhoA activity and promote filopodial actin-spike formation and invasive migration. Our study highlights the predictive potential of mathematical modelling approaches, and demonstrates that a simple intervention (MEK-inhibition) could be of therapeutic benefit in preventing invasive migration and metastasis.

The majority of cancer-related fatalities are caused by the movement of cancer cells away from the primary site to form metastases, making understanding the signalling mechanisms which underpin cell migration and invasion through their local environment of paramount importance. Much has been discovered about key events leading to invasive cell migration. Here, we have taken this prior knowledge to build a powerful predictive model based on simple ON/OFF relations and logic to determine potential intervention targets to reduce harmful invasive migration. Interrogating our model, we have identified a negative feedback loop important to the signalling that determines invasive migration, the breaking of which reverts cells to a slower, less invasive phenotype. We have supported this feedback loop prediction using an array of in vitro experiments performed in cells within 2-D and physiologically relevant 3-D environments. Our findings demonstrate the predictive power of such modelling techniques, and could form the basis for clinical intervention to prevent metastasis in certain cancers.

Multiple mechanisms of 3D migration: the origins of plasticity

Cells migrate through 3D environments using a surprisingly wide variety of molecular mechanisms. These distinct modes of migration often rely on the same intracellular components, which are used in different ways to achieve cell motility. Recent work reveals that how a cell moves can be dictated by the relative amounts of cell-matrix adhesion and actomyosin contractility. A current concept is that the level of difficulty in squeezing the nucleus through a confining 3D environment determines the amounts of adhesion and contractility required for cell motility. Ultimately, determining how the nucleus controls the mode of cell migration will be essential for understanding both physiological and pathological processes dependent on cell migration in the body.

Mechanical confinement triggers glioma linear migration dependent on formin FHOD3

Glioblastomas are extremely aggressive brain tumors with highly invasive properties. Brain linear tracks such as blood vessel walls constitute their main invasive routes. Here we analyze rat C6 and patient-derived glioma cell motility in vitro using micropatterned linear tracks to mimic blood vessels. On laminin-coated tracks (3-10 μm), these cells used an efficient saltatory mode of migration similar to their in vivo migration. This saltatory migration was also observed on larger tracks (50-400 μm in width) at high cell densities. In these cases, the mechanical constraints imposed by neighboring cells triggered this efficient mode of migration, resulting in the formation of remarkable antiparallel streams of cells along the tracks. This motility involved microtubule-dependent polarization, contractile actin bundles and dynamic paxillin-containing adhesions in the leading process and in the tail. Glioma linear migration was dramatically reduced by inhibiting formins but, surprisingly, accelerated by inhibiting Arp2/3. Protein expression and phenotypic analysis indicated that the formin FHOD3 played a role in this motility but not mDia1 or mDia2. We propose that glioma migration under confinement on laminin relies on formins, including FHOD3, but not Arp2/3 and that the low level of adhesion allows rapid antiparallel migration.

Matrix rigidity differentially regulates invadopodia activity through ROCK1 and ROCK2

ROCK activity increases due to ECM rigidity in the tumor microenvironment and promotes a malignant phenotype via actomyosin contractility. Invasive migration is facilitated by actin-rich adhesive protrusions known as invadopodia that degrade the ECM. Invadopodia activity is dependent on matrix rigidity and contractile forces suggesting that mechanical factors may regulate these subcellular structures through ROCK-dependent actomyosin contractility. However, emerging evidence indicates that the ROCK1 and ROCK2 isoforms perform different functions in cells suggesting that alternative mechanisms may potentially regulate rigidity-dependent invadopodia activity. In this study, we found that matrix rigidity drives ROCK signaling in cancer cells but that ROCK1 and ROCK2 differentially regulate invadopodia activity through separate signaling pathways via contractile (NM II) and non-contractile (LIMK) mechanisms. These data suggest that the mechanical rigidity of the tumor microenvironment may drive ROCK signaling through distinct pathways to enhance the invasive migration required for cancer progression and metastasis.

β1 Integrins as Therapeutic Targets to Disrupt Hallmarks of Cancer

Integrins belong to a large family of αβ heterodimeric transmembrane proteins first recognized as adhesion molecules that bind to dedicated elements of the extracellular matrix and also to other surrounding cells. As important sensors of the cell microenvironment, they regulate numerous signaling pathways in response to structural variations of the extracellular matrix. Biochemical and biomechanical cues provided by this matrix and transmitted to cells via integrins are critically modified in tumoral settings. Integrins repertoire are subjected to expression level modifications, in tumor cells, and in surrounding cancer-associated cells, implicated in tumor initiation and progression as well. As critical players in numerous cancer hallmarks, defined by Hanahan and Weinberg (2011), integrins represent pertinent therapeutic targets. We will briefly summarize here our current knowledge about integrin implications in those different hallmarks focusing primarily on β1 integrins.