Spatial regulation of RhoC activity defines protrusion formation in migrating cells

RBFOX2 deregulation promotes pancreatic cancer progression and metastasis through alternative splicing

RNA splicing is an important biological process associated with cancer initiation and progression. However, the contribution of alternative splicing to pancreatic cancer (PDAC) development is not well understood. Here, we identify an enrichment of RNA binding proteins (RBPs) involved in splicing regulation linked to PDAC progression from a forward genetic screen using Sleeping Beauty insertional mutagenesis in a mouse model of pancreatic cancer. We demonstrate downregulation of RBFOX2, an RBP of the FOX family, promotes pancreatic cancer progression and liver metastasis. Specifically, we show RBFOX2 regulates exon splicing events in transcripts encoding proteins involved in cytoskeletal remodeling programs. These exons are differentially spliced in PDAC patients, with enhanced exon skipping in the classical subtype for several RBFOX2 targets. RBFOX2 mediated splicing of ABI1, encoding the Abelson-interactor 1 adapter protein, controls the abundance and localization of ABI1 protein isoforms in pancreatic cancer cells and promotes the relocalization of ABI1 from the cytoplasm to the periphery of migrating cells. Using splice-switching antisense oligonucleotides (AONs) we demonstrate the ABI1 ∆Ex9 isoform enhances cell migration. Together, our data identify a role for RBFOX2 in promoting PDAC progression through alternative splicing regulation.

Role of RhoC in cancer cell migration

Migration is one of the five major behaviors of cells. Although RhoC—a classic member of the Rho gene family—was first identified in 1985, functional RhoC data have only been widely reported in recent years. Cell migration involves highly complex signaling mechanisms, in which RhoC plays an essential role. Cell migration regulated by RhoC—of which the most well-known function is its role in cancer metastasis—has been widely reported in breast, gastric, colon, bladder, prostate, lung, pancreatic, liver, and other cancers. Our review describes the role of RhoC in various types of cell migration. The classic two-dimensional cell migration cycle constitutes cell polarization, adhesion regulation, cell contraction and tail retraction, most of which are modulated by RhoC. In the three-dimensional cell migration model, amoeboid migration is the most classic and well-studied model. Here, RhoC modulates the formation of membrane vesicles by regulating myosin II, thereby affecting the rate and persistence of amoeba-like migration. To the best of our knowledge, this review is the first to describe the role of RhoC in all cell migration processes. We believe that understanding the detail of RhoC-regulated migration processes will help us better comprehend the mechanism of cancer metastasis. This will contribute to the study of anti-metastatic treatment approaches, aiding in the identification of new intervention targets for therapeutic or genetic transformational purposes.

TC10 regulates breast cancer invasion and metastasis by controlling membrane type-1 matrix metalloproteinase at invadopodia

During breast cancer metastasis, cancer cell invasion is driven by actin-rich protrusions called invadopodia, which mediate the extracellular matrix degradation required for the success of the invasive cascade. In this study, we demonstrate that TC10, a member of a Cdc42 subfamily of p21 small GTPases, regulates the membrane type 1 matrix metalloproteinase (MT1-MMP)-driven extracellular matrix degradation at invadopodia. We show that TC10 is required for the plasma membrane surface exposure of MT1-MMP at these structures. By utilizing our Förster resonance energy transfer (FRET) biosensor, we demonstrate the p190RhoGAP-dependent regulation of spatiotemporal TC10 activity at invadopodia. We identified a pathway that regulates invadopodia-associated TC10 activity and function through the activation of p190RhoGAP and the downstream interacting effector Exo70. Our findings reveal the role of a previously unknown regulator of vesicular fusion at invadopodia, TC10 GTPase, in breast cancer invasion and metastasis.

Actin dynamics during tumor cell dissemination

The actin cytoskeleton is a dynamic network that regulates cellular behavior from development to disease. By rearranging the actin cytoskeleton, cells are capable of migrating and invading during developmental processes; however, many of these cellular properties are hijacked by cancer cells to escape primary tumors and disseminate to distant organs in the body. In this review article, we highlight recent work describing how cancer cells regulate the actin cytoskeleton to achieve efficient invasion and metastatic colonization. We also review new imaging technologies that are capable of revealing the complex architecture and regulation of the actin cytoskeleton during motility and invasion of tumor cells.

GPER1 Signaling Initiates Migration of Female V-SVZ-Derived Cells

In the rodent ventricular-subventricular zone (V-SVZ) neurons are generated throughout life. They migrate along the rostral migratory stream (RMS) into the olfactory bulb before their final differentiation into interneurons and integration into local circuits. Estrogen receptors (ERs) are steroid hormone receptors with important functions in neurogenesis and synaptic plasticity. In this study, we show that the ER GPER1 is expressed in subsets of cells within the V-SVZ of female animals and provide evidence for a potential local estrogen source from aromatase-positive astrocytes surrounding the RMS. Blocking of GPER1 in Matrigel cultures of female animals significantly impairs migration of V-SVZ-derived cells. This outgrowth is accompanied by regulation of phosphorylation of the actin-binding protein cofilin by GPER1 signaling including an involvement of the p21-Ras pathway. Our results point to a prominent role of GPER1 in the initiation of neuronal migration from the V-SVZ to the olfactory bulb.

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GPER1 is expressed within all cell types of the stem cell lineage in the V-SVZ

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Blocking of GPER1 leads to a decrease in migration of V-SVZ-derived neuroblasts

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GPER1 signaling in V-SVZ Matrigel cultures involves Ras-induced p21

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Blocking of GPER1 signaling leads to an increase in the ratio of p-cofilin/cofilin

The Role of Rho GTPases in VEGF Signaling in Cancer Cells

Vascular endothelial growth factors (VEGFs) consist of five molecules (VEGFA through D as well as placental growth factor) which are crucial for regulating key cellular and tissue functions. The role of VEGF and its intracellular signaling and downstream molecular pathways have been thoroughly studied. Activation of VEGF signal transduction can be initiated by the molecules' binding to two classes of transmembrane receptors: (1) the VEGF tyrosine kinase receptors (VEGF receptors 1 through 3) and (2) the neuropilins (NRP1 and 2). The involvement of Rho GTPases in modulating VEGFA signaling in both cancer cells and endothelial cells has also been well established. Additionally, different isoforms of Rho GTPases, namely, RhoA, RhoC, and RhoG, have been shown to regulate VEGF expression as well as blood vessel formation. This review article will explore how Rho GTPases modulate VEGF signaling and the consequences of such interaction on cancer progression.

Hypoxia and EGF Stimulation Regulate VEGF Expression in Human Glioblastoma Multiforme (GBM) Cells by Differential Regulation of the PI3K/Rho-GTPase and MAPK Pathways

Glioblastoma multiforme (GBM) is one of the most common and deadly cancers of the central nervous system (CNS). It is characterized by the presence of hypoxic regions, especially in the core, leading to an increase in vascularity. This increased vascularization is driven by the expression of the major angiogenic inducer VEGF and the indirect angiogenic inducer Epidermal growth factor (EGF), which stimulates VEGF expression. In this study, we examine the regulation of VEGF by both hypoxia and the EGF signaling pathway. We also examine the involvement of pathways downstream from EGF signaling, including the mitogen-activated protein kinase/extracellular regulated kinase (MAPK/ERK) pathway and the Phosphatidylinositol-3-kinase/RhoA/C (PI3K/RhoA/C) pathway in this regulation. Our results show that VEGF expression and secretion levels increase following either hypoxia or EGF stimulation, with the two stimuli signaling in parallel. We also observed an increase in ERK and protein kinase B (Akt) phosphorylation, in response to EGF stimulation, with kinetics that correlated with the kinetics of the effect on VEGF. Using pharmacological inhibitors against ERK and PI3K and small interfering RNAs (siRNAs) against RhoA and RhoC, we found that both the ERK and the PI3K/RhoA/C pathways have to cooperate in order to lead to an increase in VEGF expression, downstream from EGF. In response to hypoxia, however, only ERK was involved in the regulation of VEGF. Hypoxia also led to a surprising decrease in the activation of PI3K and RhoA/C. Finally, the decrease in the activation of these Rho-GTPases was found to be mediated through a hypoxia-driven overexpression of the Rho-GTPase GTPase activating protein (GAP), StarD13. Therefore, while under normoxic conditions, EGF stimulates the activation of both the PI3K and the MAPK pathways and the induction of VEGF, in glioblastoma cells, hypoxic conditions lead to the suppression of the PI3K/RhoA/C pathway and an exclusive switch to the MAPK pathway.

Textures of the tumour microenvironment

In this review, we present recent findings on the dynamic nature of the tumour microenvironment (TME) and how intravital microscopy studies have defined TME components in a spatiotemporal manner. Intravital microscopy has shed light into the nature of the TME, revealing structural details of both tumour cells and other TME co-habitants in vivo, how these cells communicate with each other, and how they are organized in three-dimensional space to orchestrate tumour growth, invasion, dissemination and metastasis. We will review different imaging tools, imaging reporters and fate-mapping strategies that have begun to uncover the complexity of the TME in vivo.

The Effect of Ras Homolog C/Rho-Associated Coiled-Protein Kinase (Rho/ROCK) Signaling Pathways on Proliferation and Apoptosis of Human Myeloma Cells

BACKGROUND The aim of this study was to explore the impact of Ras homolog C/Rho-associated coiled-protein kinase (Rho/ROCK) signaling pathways intervention on biological characteristics of the human multiple myeloma cell lines RPMI-8226 and U266 cells, and to investigate the expression of RhoC, ROCK1, and ROCK2 in RPMI-8226 and U266 cells. MATERIAL AND METHODS RPMI8226 and U266 cell lines were treated by 5-aza-2-deoxycytidine (5-Aza-Dc), trichostatin A (TSA), RhoA inhibitor CCG-1423, Rac1 inhibitor NSC23766, and ROCK inhibitor fasudil. Cell proliferation was examined by Cell Counting Kit-8 (CCK-8) assay and clone formation. Cell apoptosis was examined by flow cytometry and TUNEL assay. The mRNA and protein expressions of RhoC, ROCK1, and ROCK2 were detected by quantitative reverse transcription-polymerase chain reaction (qRT-PCR) and western blot, respectively. RESULTS CCG-1423, NSC23766, and fasudil could significantly inhibit the proliferation of RPMI8226 and U266 cells. The inhibitory effect was dose- and time-dependent within a certain concentration range (P<0.05). After treatment with CCG-1423, NSC23766, and fasudil for 24 hours, the apoptosis rates of RPMI8226 and U266 cells were significantly higher than those of the control group, which were dose-dependent (P<0.05). Compared with the control group, the mRNA and protein expressions of RhoC, ROCK1, and ROCK2 in RPMI8226 and U266 cells were significantly decreased with single 5-Aza-Dc or TSA treatment. However, the effects were obviously stronger after combined treatment of 5-Aza-CdR and TSA (P<0.05). CONCLUSIONS We found that 5-Aza-Dc and TSA can effectively decrease the mRNA and protein expressions of RhoC, ROCK1, and ROCK2. Furthermore, Rho and ROCK inhibitors significantly inhibit cell growth and induce cell apoptosis in the human multiple myeloma cell lines RPMI-8226 and U266.

Ubiquitin-based modifications in endothelial cell-cell contact and inflammation

Endothelial cell-cell contacts are essential for vascular integrity and physiology, protecting tissues and organs from edema and uncontrolled invasion of inflammatory cells. The vascular endothelial barrier is dynamic, but its integrity is preserved through a tight control at different levels. Inflammatory cytokines and G-protein-coupled receptor agonists, such as histamine, reduce endothelial integrity and increase vascular leakage. This is due to elevated myosin-based contractility, in conjunction with phosphorylation of proteins at cell-cell contacts. Conversely, reducing contractility stabilizes or even increases endothelial junctional integrity. Rho GTPases are key regulators of such cytoskeletal dynamics and endothelial cell-cell contacts. In addition to signaling-induced regulation, the expression of junctional proteins, such as occludin, claudins and vascular endothelial cadherin, also controls endothelial barrier function. There is increasing evidence that, in addition to protein phosphorylation, ubiquitylation (also known as ubiquitination) is an important and dynamic post-translational modification that regulates Rho GTPases, junctional proteins and, consequently, endothelial barrier function. In this Review, we discuss the emerging role of ubiquitylation and deubiquitylation events in endothelial integrity and inflammation. The picture that emerges is one of increasing complexity, which is both fascinating and promising given the clinical relevance of vascular integrity in the control of inflammation, and of tissue and organ damage.

Endothelial RhoA GTPase is essential for in vitro endothelial functions but dispensable for physiological in vivo angiogenesis

Imbalanced angiogenesis is a characteristic of several diseases. Rho GTPases regulate multiple cellular processes, such as cytoskeletal rearrangement, cell movement, microtubule dynamics, signal transduction and gene expression. Among the Rho GTPases, RhoA, Rac1 and Cdc42 are best characterized. The role of endothelial Rac1 and Cdc42 in embryonic development and retinal angiogenesis has been studied, however the role of endothelial RhoA is yet to be explored. Here, we aimed to identify the role of endothelial RhoA in endothelial cell functions, in embryonic and retinal development and explored compensatory mechanisms. In vitro, RhoA is involved in cell proliferation, migration and tube formation, triggered by the angiogenesis inducers Vascular Endothelial Growth Factor (VEGF) and Sphingosine-1 Phosphate (S1P). In vivo, through constitutive and inducible endothelial RhoA deficiency we tested the role of endothelial RhoA in embryonic development and retinal angiogenesis. Constitutive endothelial RhoA deficiency, although decreased survival, was not detrimental for embryonic development, while inducible endothelial RhoA deficiency presented only mild deficiencies in the retina. The redundant role of RhoA in vivo can be attributed to potential differences in the signaling cues regulating angiogenesis in physiological versus pathological conditions and to the alternative compensatory mechanisms that may be present in the in vivo setting.

RhoC: a fascinating journey from a cytoskeletal organizer to a Cancer stem cell therapeutic target

Tumor heterogeneity results in differential response to therapy due to the existence of plastic tumor cells, called cancer stem cells (CSCs), which exhibit the property of resistance to therapy, invasion and metastasis. These cells have a distinct, signaling network active at every stage of progression. It is difficult to envisage that the CSCs will have a unique set of signaling pathways regulating every stage of disease progression. Rather, it would be easier to believe that a single pivotal pathway having significant contribution at every stage, which can further turn on a battery of signaling mechanisms specific to that stage, would be instrumental in regulating the signaling network, enabling easy transition from one state to another. In this context, we discuss the role of RhoC which has contributed to several phenotypes during tumor progression.

RhoC (Ras homolog gene family member C) has been widely reported to regulate actin organization. It has been shown to impact the motility of cancer cells, resultantly affecting invasion and metastasis, and has contributed to carcinoma progression of the breast, pancreas, lung, ovaries and cervix, among several others. The most interesting finding has been its indispensable role in metastasis. Also, it has the ability to modulate various other phenotypes like angiogenesis, motility, invasion, metastasis, and anoikis resistance. These observations suggest that RhoC imparts the plasticity required by tumor cells to exhibit such diverse functions based on microenvironmental cues. This was further confirmed by recent reports which show that it regulates cancer stem cells in breast, ovary and head and neck cancers. Studies also suggest that the inhibition of RhoC results in abolition of advanced tumor phenotypes.

Our review throws light on how RhoC, which is capable of modulating various phenotypes may be the apt core signaling candidate regulating disease progression. Additionally, mice studies show that RhoC is not essential for embryogenesis, giving scope for its development as a possible therapeutic target. This review thus stresses on the need to understand the protein and its functioning in greater detail to enable its development as a stem cell marker and a possible therapeutic target.

DLC3 suppresses MT1-MMP-dependent matrix degradation by controlling RhoB and actin remodeling at endosomal membranes

Cancer cells degrade the extracellular matrix through actin-rich protrusions termed invadopodia. The formation of functional invadopodia requires polarized membrane trafficking driven by Rho GTPase-mediated cytoskeletal remodeling. We identify the Rho GTPase-activating protein deleted in liver cancer 3 (DLC3; also known as STARD8) as an integral component of the endosomal transport and sorting machinery. We provide evidence for the direct regulation of RhoB by DLC3 at endosomal membranes to which DLC3 is recruited by interacting with the sorting nexin SNX27. In TGF-β-treated MCF10A breast epithelial cells, DLC3 knockdown enhanced metalloproteinase-dependent matrix degradation, which was partially rescued by RhoB co-depletion. This was recapitulated in MDA-MB-231 breast cancer cells in which early endosomes demonstrated aberrantly enriched F-actin and accumulated the metalloproteinase MT1-MMP (also known as MMP14) upon DLC3 knockdown. Remarkably, Rab4 (herein referring to Rab4A) downregulation fully rescued the enhanced matrix degradation of TGF-β-treated MCF10A and MDA-MB-231 cells. In summary, our findings establish a novel role for DLC3 in the suppression of MT1-MMP-dependent matrix degradation by inactivating RhoB signaling at endosomal membranes. We propose that DLC3 function is required to limit endosomal actin polymerization, Rab4-dependent recycling of MT1-MMP and, consequently, matrix degradation mediated by invadopodial activity.

p190RhoGAPs, the ARHGAP35- and ARHGAP5-Encoded Proteins, in Health and Disease

Small guanosine triphosphatases (GTPases) gathered in the Rat sarcoma (Ras) superfamily represent a large family of proteins involved in several key cellular mechanisms. Within the Ras superfamily, the Ras homolog (Rho) family is specialized in the regulation of actin cytoskeleton-based mechanisms. These proteins switch between an active and an inactive state, resulting in subsequent inhibiting or activating downstream signals, leading finally to regulation of actin-based processes. The On/Off status of Rho GTPases implicates two subsets of regulators: GEFs (guanine nucleotide exchange factors), which favor the active GTP (guanosine triphosphate) status of the GTPase and GAPs (GTPase activating proteins), which inhibit the GTPase by enhancing the GTP hydrolysis. In humans, the 20 identified Rho GTPases are regulated by over 70 GAP proteins suggesting a complex, but well-defined, spatio-temporal implication of these GAPs. Among the quite large number of RhoGAPs, we focus on p190RhoGAP, which is known as the main negative regulator of RhoA, but not exclusively. Two isoforms, p190A and p190B, are encoded by ARHGAP35 and ARHGAP5 genes, respectively. We describe here the function of each of these isoforms in physiological processes and sum up findings on their role in pathological conditions such as neurological disorders and cancers.

Connexin 43 Loss Triggers Cell Cycle Entry and Invasion in Non-Neoplastic Breast Epithelium: A Role for Noncanonical Wnt Signaling

(1) Background: The expression of connexin 43 (Cx43) is disrupted in breast cancer, and re-expression of this protein in human breast cancer cell lines leads to decreased proliferation and invasiveness, suggesting a tumor suppressive role. This study aims to investigate the role of Cx43 in proliferation and invasion starting from non-neoplastic breast epithelium. (2) Methods: Nontumorigenic human mammary epithelial HMT-3522 S1 cells and Cx43 shRNA-transfected counterparts were cultured under 2-dimensional (2-D) and 3-D conditions. (3) Results: Silencing Cx43 induced mislocalization of β-catenin and Scrib from apicolateral membrane domains in glandular structures or acini formed in 3-D culture, suggesting the loss of apical polarity. Cell cycle entry and proliferation were enhanced, concomitantly with c-Myc and cyclin D1 upregulation, while no detectable activation of Wnt/β-catenin signaling was observed. Motility and invasion were also triggered and were associated with altered acinar morphology and activation of ERK1/2 and Rho GTPase signaling, which acts downstream of the noncanonical Wnt pathway. The invasion of Cx43-shRNA S1 cells was observed only under permissive stiffness of the extracellular matrix (ECM). (4) Conclusion: Our results suggest that Cx43 controls proliferation and invasion in the normal mammary epithelium in part by regulating noncanonical Wnt signaling.

Gap Junctions and Wnt Signaling in the Mammary Gland: a Cross-Talk?

Connexins (Cxs), the building blocks of gap junctions (GJs), exhibit spatiotemporal patterns of expression and regulate the development and differentiation of the mammary gland, acting via channel-dependent and channel-independent mechanisms. Impaired Cx expression and localization are reported in breast cancer, suggesting a tumor suppressive role for Cxs. The signaling events that mediate the role of GJs in the development and tumorigenesis of the mammary gland remain poorly identified. The Wnt pathways, encompassing the canonical or the Wnt/β-catenin pathway and the noncanonical β-catenin-independent pathway, also play important roles in those processes. Indeed, aberrant Wnt signaling is associated with breast cancer. Despite the coincident roles of Cxs and Wnt pathways, the cross-talk in the breast tissue is poorly defined, although this is reported in a number of other tissues. Our previous studies revealed a channel-independent role for Cx43 in inducing differentiation or suppressing tumorigenesis of mammary epithelial cells by acting as a negative regulator of the Wnt/β-catenin pathway. Here, we provide a brief overview of mammary gland development, with emphasis on the role of Cxs in development and tumorigenesis of this tissue. We also discuss the role of Wnt signaling in similar contexts, and review the literature illustrating interplay between Cxs and Wnt pathways.

RHOG Activates RAC1 through CDC42 Leading to Tube Formation in Vascular Endothelial Cells

Angiogenesis is a hallmark of cancer cell malignancy. The role of the RHO family GTPase RHOG in angiogenesis in vascular endothelial cells has recently been elucidated. However, the regulation of RHOG during this process, as well as its cross-talk with other RHO GTPases, have yet to be fully examined. In this study, we found that siRNA-mediated depletion of RHOG strongly inhibits tube formation in vascular endothelial cells (ECV cells), an effect reversed by transfecting dominant active constructs of CDC42 or RAC1 in the RHOG-depleted cells. We also found CDC42 to be upstream from RAC1 in these cells. Inhibiting either Phosphatidyl inositol (3) kinase (PI3K) with Wortmannin or the mitogen-activated protein kinase extracellular-regulated kinase (MAPK ERK) with U0126 leads to the inhibition of tube formation. While knocking down either RHO, GTPase did not affect p-AKT levels, and p-ERK decreased in response to the knocking down of RHOG, CDC42 or RAC1. Recovering active RHO GTPases in U0126-treated cells also did not reverse the inhibition of tube formation, placing ERK downstream from PI3K-RHOG-CDC42-RAC1 in vascular endothelial cells. Finally, RHOA and the Rho activated protein kinases ROCK1 and ROCK2 positively regulated tube formation independently of ERK, while RHOC seemed to inhibit the process. Collectively, our data confirmed the essential role of RHOG in angiogenesis, shedding light on a potential new therapeutic target for cancer malignancy and metastasis.

Proteomics and bioinformatics analyses identify novel cellular roles outside mitochondrial function for human miro GTPases

The human Miro GTPases (hMiros) have recently emerged as important mediators of mitochondrial transport and may significantly contribute to the development of disorders such as Alzheimer’s and schizophrenia. The hMiros represent two highly atypical members of the Ras superfamily, and exhibit several unique features: the presence of a GTPase domain at both the N-terminus and C-terminus, the presence of two calcium-binding EF-hand domains and localisation to the mitochondrial outer membrane. Here, elucidation of Miro GTPase signalling pathway components was achieved through the use of molecular biology, cell culture techniques and proteomics. An investigation of this kind has not been performed previously; we hoped, through these techniques, to enable the profiling and identification of pathways regulated by the human Miro GTPases. The results indicate several novel putative interaction partners for hMiro1 and hMiro2, including numerous proteins previously implicated in neurodegenerative pathways and the development of schizophrenia. Furthermore, we show that the N-terminal GTPase domain appears to fine-tune hMiro signalling, with GTP-bound versions of this domain associated with a diverse range of interaction partners in comparison to corresponding GDP-bound versions. Recent evidences suggest that human Miros participate in host–pathogen interactions with Vibrio Cholerae type III secretion proteins. We have undertaken a bioinformatics investigation to identify novel pathogenic effectors that might interact with Miros.

MS-based quantification of RhoA/B and RhoC ADP-ribosylation

Mono ADP-ribosylation is a common characteristic of bacterial toxins resulting to aberrant activation or inactivation of target proteins. The C3 exoenzyme of Clostridium botulinum (C3bot) ADP-ribosylates the small GTPases RhoA, RhoB and RhoC, leading to inactivation of these important regulators and impaired down-stream signaling. Quantification of ADP-ribosylation using gel migration assays, antibodies, and radioactivity-based methods are limited. Therefore a novel LC-MS-based method to specifically determine and quantify ADP-ribosylation of Rho GTPases was established. A heavy labeled protein standard that contained ADP-ribosylation specific peptides in similar amounts in ADP ribosylated and non ADP ribosylated form was used for relative quantification in vivo. In a proof of principle experiment HT22 cells were treated with C3bot and the kinetics of RhoA/B and RhoC ADP-ribosylation were determined in vivo.

Cofilin-2 Acts as a Marker for Predicting Radiotherapy Response and Is a Potential Therapeutic Target in Nasopharyngeal Carcinoma

Background

The purpose of this study was to determine whether cofilin-2 could serve as a protein marker for predicting radiotherapy response and as a potential therapeutic target in nasopharyngeal carcinoma (NPC).

Material/Methods

Cofilin-2 protein levels in serum and tissue samples from patients with NPC were assessed by sandwich ELISA and IHC. In vitro, cofilin-2 levels in CNE-2R cells were significantly higher than those of CNE-2 cells. Meanwhile, CNE-2R cells were silenced for cofilin-2 to obtain a stable cofilin-2-RNAi-LV3 cell line. Then, cell proliferation, radiosensitivity, invasion and migration abilities, cell cycle, and apoptosis were evaluated by Cell Counting Kit 8 assay (CCK-8), flow cytometry (FCM), clone formation assay, and in vitro.

Results

The secreted levels of the cofilin-2 protein in radioresistant NPC patients were significantly higher than those of radiosensitive cases. After cofilin-2 knockdown in nasopharyngeal carcinoma CNE-2R cells, proliferation was decreased, while apoptosis and radiosensitivity were enhanced; cell cycle distribution was altered, and the transplanted tumors in nude mice grew significantly less.

Conclusions

Overall, our findings suggest that cofilin-2 acts as a marker for predicting radiotherapy response and is a potential therapeutic target in nasopharyngeal carcinoma.

Rho GTPase signaling complexes in cell migration and invasion

Cell migration is dependent on the dynamic formation and disassembly of actin filament-based structures, including lamellipodia, filopodia, invadopodia, and membrane blebs, as well as on cell-cell and cell-extracellular matrix adhesions. These processes all involve Rho family small guanosine triphosphatases (GTPases), which are regulated by the opposing actions of guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). Rho GTPase activity needs to be precisely tuned at distinct cellular locations to enable cells to move in response to different environments and stimuli. In this review, we focus on the ability of RhoGEFs and RhoGAPs to form complexes with diverse binding partners, and describe how this influences their ability to control localized GTPase activity in the context of migration and invasion.

RSK2 drives cell motility by serine phosphorylation of LARG and activation of Rho GTPases

Cell motility is a dynamic process that requires the directed application of force and continuous coordinated changes in cell adhesion and cytoskeletal architecture often in response to extracellular stimuli. Here we have defined a mechanism by which RSK2 can promote cell migration and invasion in response to promotility stimuli. We show that in response to these signals RSK2 directly binds the RhoGEF LARG and phosphorylates it, thereby promoting LARG activation of RhoA GTPases. Moreover, we find that RSK2 is important for epidermal growth factor activation of Rho GTPases. These results advance our understanding of cell motility, RSK kinase function, and LARG/RhoA activation by revealing that these pathways are integrated and the precise mechanism by which that is accomplished.

Directed migration is essential for cell motility in many processes, including development and cancer cell invasion. RSKs (p90 ribosomal S6 kinases) have emerged as central regulators of cell migration; however, the mechanisms mediating RSK-dependent motility remain incompletely understood. We have identified a unique signaling mechanism by which RSK2 promotes cell motility through leukemia-associated RhoGEF (LARG)-dependent Rho GTPase activation. RSK2 directly interacts with LARG and nucleotide-bound Rho isoforms, but not Rac1 or Cdc42. We further show that epidermal growth factor or FBS stimulation induces association of endogenous RSK2 with LARG and LARG with RhoA. In response to these stimuli, RSK2 phosphorylates LARG at Ser1288 and thereby activates RhoA. Phosphorylation of RSK2 at threonine 577 is essential for activation of LARG-RhoA. Moreover, RSK2-mediated motility signaling depends on RhoA and -B, but not RhoC. These results establish a unique RSK2-dependent LARG-RhoA signaling module as a central organizer of directed cell migration and invasion.

Rac3 regulates breast cancer invasion and metastasis by controlling adhesion and matrix degradation

The initial step of metastasis is the local invasion of tumor cells into the surrounding tissue. Invadopodia are actin-based protrusions that mediate the matrix degradation necessary for invasion and metastasis of tumor cells. We demonstrate that Rac3 GTPase is critical for integrating the adhesion of invadopodia to the extracellular matrix (ECM) with their ability to degrade the ECM in breast tumor cells. We identify two pathways at invadopodia important for integrin activation and delivery of matrix metalloproteinases: through the upstream recruiter CIB1 as well as the downstream effector GIT1. Rac3 activity, at and surrounding invadopodia, is controlled by Vav2 and βPIX. These guanine nucleotide exchange factors regulate the spatiotemporal dynamics of Rac3 activity, impacting GIT1 localization. Moreover, the GTPase-activating function of GIT1 toward the vesicular trafficking regulator Arf6 GTPase is required for matrix degradation. Importantly, Rac3 regulates the ability of tumor cells to metastasize in vivo. The Rac3-dependent mechanisms we show in this study are critical for balancing proteolytic activity and adhesive activity to achieve a maximally invasive phenotype.

RhoD Inhibits RhoC-ROCK-Dependent Cell Contraction via PAK6

RhoA-mediated regulation of myosin-II activity in the actin cortex controls the ability of cells to contract and bleb during a variety of cellular processes, including cell migration and division. Cell contraction and blebbing also frequently occur as part of the cytopathic effect seen during many different viral infections. We now demonstrate that the vaccinia virus protein F11, which localizes to the plasma membrane, is required for ROCK-mediated cell contraction from 2 hr post infection. Curiously, F11-induced cell contraction is dependent on RhoC and not RhoA signaling to ROCK. Moreover, RhoC-driven cell contraction depends on the upstream inhibition of RhoD signaling by F11. This inhibition prevents RhoD from regulating its downstream effector Pak6, alleviating the suppression of RhoC by the kinase. Our observations with vaccinia have now demonstrated that RhoD recruits Pak6 to the plasma membrane to antagonize RhoC signaling during cell contraction and blebbing.

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Vaccinia F11 protein is required for virus-induced cell contraction and blebbing

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F11-induced cell contraction depends on RhoC, but not RhoA, signaling to ROCK

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RhoD recruits Pak6 to the plasma membrane to antagonize RhoC signaling

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F11 inhibits RhoD signaling to its downstream effector Pak6

Paving the Rho in cancer metastasis: Rho GTPases and beyond

Malignant carcinomas are often characterized by metastasis, the movement of carcinoma cells from a primary site to colonize distant organs. For metastasis to occur, carcinoma cells first must adopt a pro-migratory phenotype and move through the surrounding stroma towards a blood or lymphatic vessel. Currently, there are very limited possibilities to target these processes therapeutically. The family of Rho GTPases is an ubiquitously expressed division of GTP-binding proteins involved in the regulation of cytoskeletal dynamics and intracellular signaling. The best characterized members of the Rho family GTPases are RhoA, Rac1 and Cdc42. Abnormalities in Rho GTPase function have major consequences for cancer progression. Rho GTPase activation is driven by cell surface receptors that activate GTP exchange factors (GEFs) and GTPase-activating proteins (GAPs). In this review, we summarize our current knowledge on Rho GTPase function in the regulation of metastasis. We will focus on key discoveries in the regulation of epithelial-mesenchymal-transition (EMT), cell-cell junctions, formation of membrane protrusions, plasticity of cell migration and adaptation to a hypoxic environment. In addition, we will emphasize on crosstalk between Rho GTPase family members and other important oncogenic pathways, such as cyclic AMP-mediated signaling, canonical Wnt/β-catenin, Yes-associated protein (YAP) and hypoxia inducible factor 1α (Hif1α) and provide an overview of the advancements and challenges in developing pharmacological tools to target Rho GTPase and the aforementioned crosstalk in the context of cancer therapeutics.

Diverse roles of guanine nucleotide exchange factors in regulating collective cell migration

Effective collective cell migration depends on delicate tuning of cell motility forces and cell–cell communication. Zaritsky, Tseng, and coworkers identified differential roles for RhoA, RhoC, and their upstream activators in mediating long-range transmission of guidance cues.

Efficient collective migration depends on a balance between contractility and cytoskeletal rearrangements, adhesion, and mechanical cell–cell communication, all controlled by GTPases of the RHO family. By comprehensive screening of guanine nucleotide exchange factors (GEFs) in human bronchial epithelial cell monolayers, we identified GEFs that are required for collective migration at large, such as SOS1 and β-PIX, and RHOA GEFs that are implicated in intercellular communication. Down-regulation of the latter GEFs differentially enhanced front-to-back propagation of guidance cues through the monolayer and was mirrored by down-regulation of RHOA expression and myosin II activity. Phenotype-based clustering of knockdown behaviors identified RHOA-ARHGEF18 and ARHGEF3-ARHGEF28-ARHGEF11 clusters, indicating that the latter may signal through other RHO-family GTPases. Indeed, knockdown of RHOC produced an intermediate between the two phenotypes. We conclude that for effective collective migration, the RHOA-GEFs → RHOA/C → actomyosin pathways must be optimally tuned to compromise between generation of motility forces and restriction of intercellular communication.

Regulation of Rho GTPase activity at the leading edge of migrating cells by p190RhoGAP

Cell migration, a key feature of embryonic development, immunity, angiogenesis, and tumor metastasis, is based on the coordinated regulation of actin dynamics and integrin-mediated adhesion. Rho GTPases play a major role in this phenomenon by regulating the onset and maintenance of actin-based protruding structures at cell leading edges (i.e. lamellipodia and filopodia) and contractile structures (i.e., stress fibers) at their trailing edge. While spatio-temporal analysis demonstrated the tight regulation of Rho GTPases at the migration front during cell locomotion, little is known about how the main regulators of Rho GTPase activity, such as GAPs, GEFs and GDIs, play a role in this process. In this review, we focus on a major negative regulator of RhoA, p190RhoGAP-A and its close isoform p190RhoGAP-B, which are necessary for efficient cell migration. Recent studies, including our, demonstrated that p190RhoGAP-A localization and activity undergo a complex regulatory mechanism, accounting for the tight regulation of RhoA, but also other members of the Rho GTPase family, at the cell periphery.

Macrophage-dependent tumor cell transendothelial migration is mediated by Notch1/MenaINV-initiated invadopodium formation

The process of intravasation involving transendothelial migration is a key step in metastatic spread. How the triple cell complex composed of a macrophage, Mena over-expressing tumor cell and endothelial cell, called the tumor microenvironment of metastasis (TMEM), facilitates tumor cell transendothelial migration is not completely understood. Previous work has shown that the physical contact between a macrophage and tumor cell results in the formation of invadopodia, actin-rich matrix degrading protrusions, important for tumor cell invasion and transendothelial migration and tumor cell dissemination. Herein, we show that the macrophage-induced invadopodium is formed through a Notch1/Mena^INV signaling pathway in the tumor cell upon macrophage contact. This heterotypic tumor cell – macrophage interaction results in the upregulation of Mena^INV through the activation of MENA transcription. Notch1 and Mena^INV expression are required for tumor cell transendothelial migration, a necessary step during intravasation. Inhibition of the Notch signaling pathway blocked macrophage-induced invadopodium formation in vitro and the dissemination of tumor cells from the primary tumor in vivo. Our findings indicate a novel role for Notch1 signaling in the regulation of Mena^INV expression and transendothelial migration and provide mechanistic information essential to the use of therapeutic inhibitors of metastasis.

The dynamics of spatio-temporal Rho GTPase signaling: formation of signaling patterns

Rho GTPases are crucial signaling molecules that regulate a plethora of biological functions. Traditional biochemical, cell biological, and genetic approaches have founded the basis of Rho GTPase biology. The development of biosensors then allowed measuring Rho GTPase activity with unprecedented spatio-temporal resolution. This revealed that Rho GTPase activity fluctuates on time and length scales of tens of seconds and micrometers, respectively. In this review, we describe Rho GTPase activity patterns observed in different cell systems. We then discuss the growing body of evidence that upstream regulators such as guanine nucleotide exchange factors and GTPase-activating proteins shape these patterns by precisely controlling the spatio-temporal flux of Rho GTPase activity. Finally, we comment on additional mechanisms that might feed into the regulation of these signaling patterns and on novel technologies required to dissect this spatio-temporal complexity.

Optical Tools To Study the Isoform-Specific Roles of Small GTPases in Immune Cells

Despite the 92% homology of the hematopoietic cell-specific Rac2 to the canonical isoform Rac1, these isoforms have been shown to play nonredundant roles in immune cells. To study isoform-specific dynamics of Rac in live cells, we developed a genetically encoded, single-chain FRET-based biosensor for Rac2. We also made significant improvements to our existing single-chain Rac1 biosensor. We optimized the biosensor constructs for facile expression in hematopoietic cells and performed functional validations in murine macrophage sublines of RAW264.7 cells. Rac2, Rac1, and Cdc42 have been implicated in the formation of actin-rich protrusions by macrophages, but their individual activation dynamics have not been previously characterized. We found that both Rac1 and Rac2 had similar activation kinetics, yet they had distinct spatial distributions in response to the exogenous stimulus, fMLF. Active Rac1 was mainly localized to the cell periphery, whereas active Rac2 was distributed throughout the cell, with an apparent higher concentration in the perinuclear region. We also performed an extensive morphodynamic analysis of Rac1, Rac2, and Cdc42 activities during the extension of random protrusions. We found that Rac2 appears to play a leading role in the generation of random protrusions, as we observed an initial strong activation of Rac2 in regions distal from the leading edge, followed by the activation of Rac1, a second burst of Rac2 and then Cdc42 immediately behind the leading edge. Overall, isoform-specific biosensors that have been optimized for expression should be valuable for interrogating the coordination of Rho family GTPase activities in living cells.

Toward understanding RhoGTPase specificity: structure, function and local activation

Cell adhesion and migration are regulated through the concerted action of cytoskeletal dynamics and adhesion proteins, the activity of which is governed by RhoGTPases. Specific RhoGTPase signaling requires spatio-temporal activation and coordination of subsequent protein-protein and protein-lipid interactions. The nature, location and duration of these interactions are dependent on polarized extracellular triggers, such as cell-cell contact, and intracellular modifying events, such as phosphorylation. RhoA, RhoB, and RhoC are highly homologous GTPases that, however, succeed in generating specific intracellular responses. Here, we discuss the key features that contribute to this specificity. These not only include the well-studied switch regions, the conformation of which is nucleotide-dependent, but also additional regions and seemingly small differences in primary sequence that also contribute to specific interactions. These differences translate into differential surface charge distribution, local exposure of amino acid side-chains and isoform-specific post-translational modifications. The available evidence supports the notion that multiple regions in RhoA/B/C cooperate to provide specificity in binding to regulators and effectors. These specific interactions are highly regulated in time and space. We therefore subsequently discuss current approaches means to visualize and analyze localized GTPase activation using biosensors that allow imaging of isoform-specific, localized regulation.

Spatial regulation of tumor cell protrusions by RhoC

Systemic metastasis is the dissemination of cancer cells from the primary tumor to distant organs and is the primary cause of death in cancer patients. How do cancer cells leave the primary tumor mass? The ability of the tumor cells to form different types of actin-rich protrusions including invasive protrusions (invadopodia) and locomotory protrusions (lamellipodia [2D] or pseudopodia [3D]), facilitate the invasion and dissemination of the tumor cells. Rho-family of p21 small GTPases plays a direct role in regulating the actin dynamics in these intracellular compartments. Recent studies have shown that the signaling molecules including RhoC/p190RhoGEF/p190RhoGAP acts as a “molecular compass” in order to direct the spatial and temporal dynamics of the formation of these invasive and locomotory protrusions leading to efficient invasion.

Rho GTPase isoforms in cell motility: Don't fret, we have FRET

The Rho-family of p21 small GTPases are directly linked to the regulation of actin-based motile machinery and play a key role in the control of cell migration. Aside from the original and most well-characterized canonical Rho GTPases RhoA, Rac1, and Cdc42, numerous isoforms of these key proteins have been identified and shown to have specific roles in regulating various cellular motility processes. The major difficulty in addressing these isoform-specific effects is that isoforms typically contain highly similar primary amino acid sequences and thus are able to interact with the same upstream regulators and the downstream effector targets. Here, we will introduce the major members of each GTPase subfamily and discuss recent advances in the design and application of fluorescent resonance energy transfer-based probes, which are at the forefront of the technologies available to directly probe the differential, spatiotemporal activation dynamics of these proteins in live single cells. Currently, it is possible to specifically detect the activation status of RhoA vs. RhoC isoforms, as well as Cdc42 vs. TC-10 isoforms in living cells. Clearly, additional efforts are still required to produce biosensor systems capable of detecting other isoforms of Rho GTPases including RhoB, Rac2/3, RhoG, etc. Through such efforts, we will uncover the isoform-specific roles of these near-identical proteins in living cells, clearly an important area of the Rho GTPase biology that is not yet fully appreciated.

Rho GAPs and GEFs: controling switches in endothelial cell adhesion

Within blood vessels, endothelial cell–cell and cell–matrix adhesions are crucial to preserve barrier function, and these adhesions are tightly controlled during vascular development, angiogenesis, and transendothelial migration of inflammatory cells. Endothelial cellular signaling that occurs via the family of Rho GTPases coordinates these cell adhesion structures through cytoskeletal remodelling. In turn, Rho GTPases are regulated by GTPase-activating proteins (GAPs) and guanine nucleotide exchange factors (GEFs). To understand how endothelial cells initiate changes in the activity of Rho GTPases, and thereby regulate cell adhesion, we will discuss the role of Rho GAPs and GEFs in vascular biology. Many potentially important Rho regulators have not been studied in detail in endothelial cells. We therefore will first overview which GAPs and GEFs are highly expressed in endothelium, based on comparative gene expression analysis of human endothelial cells compared with other tissue cell types. Subsequently, we discuss the relevance of Rho GAPs and GEFs for endothelial cell adhesion in vascular homeostasis and disease.

Localization of cofilin mRNA to the leading edge of migrating cells promotes directed cell migration

mRNA trafficking, which enables the localization of mRNAs to particular intracellular targets, occurs in a wide variety of cells. The importance of the resulting RNA distribution for cellular functions, however, has been difficult to assess. We have found that cofilin-1 mRNA is rapidly localized to the leading edge of human lung carcinoma cells and that VICKZ family RNA-binding proteins help mediate this localization through specific interactions with the 3′UTR of cofilin mRNA. Using a phagokinetic assay for cell motility, we have been able to quantify the effect of mRNA localization on the rescue of lung carcinoma cells in which cofilin was knocked down by using short hairpin RNA (shRNA). Although restoring cofilin protein to normal endogenous levels rescues general lamellipodia formation around the periphery of the cell, only when the rescuing cofilin mRNA can localize to the leading edge is it capable of also fully rescuing directed cell movement. These results demonstrate that localization of an mRNA can provide an additional level of regulation for the function of its protein product.

Rac1 nucleocytoplasmic shuttling drives nuclear shape changes and tumor invasion

Nuclear membrane microdomains are proposed to act as platforms for regulation of nuclear function, but little is known about the mechanisms controlling their formation. Organization of the plasma membrane is regulated by actin polymerization, and the existence of an actin pool in the nucleus suggests that a similar mechanism might operate here. We show that nuclear membrane organization and morphology are regulated by the nuclear level of active Rac1 through actin polymerization-dependent mechanisms. Rac1 nuclear export is mediated by two internal nuclear export signals and through its interaction with nucleophosmin-1 (B23), which acts as a Rac1 chaperone inside the nucleus. Rac1 nuclear accumulation alters the balance between cytosolic Rac1 and Rho, increasing RhoA signaling in the cytoplasm and promoting a highly invasive phenotype. Nuclear Rac1 shuttling is a finely tuned mechanism for controlling nuclear shape and organization and cell invasiveness.

I'm coming to GEF you: Regulation of RhoGEFs during cell migration

Cell migration is a highly regulated multistep process that requires the coordinated regulation of cell adhesion, protrusion, and contraction. These processes require numerous protein–protein interactions and the activation of specific signaling pathways. The Rho family of GTPases plays a key role in virtually every aspect of the cell migration cycle. The activation of Rho GTPases is mediated by a large and diverse family of proteins; the guanine nucleotide exchange factors (RhoGEFs). GEFs work immediately upstream of Rho proteins to provide a direct link between Rho activation and cell–surface receptors for various cytokines, growth factors, adhesion molecules, and G protein-coupled receptors. The regulated targeting and activation of RhoGEFs is essential to coordinate the migratory process. In this review, we summarize the recent advances in our understanding of the role of RhoGEFs in the regulation of cell migration.

Intramolecular interactions between the Dbl homology (DH) domain and the carboxyl-terminal region of myosin II-interacting guanine nucleotide exchange factor (MyoGEF) act as an autoinhibitory mechanism for the regulation of MyoGEF functions

We have reported previously that nonmuscle myosin II-interacting guanine nucleotide exchange factor (MyoGEF) plays an important role in the regulation of cell migration and cytokinesis. Like many other guanine nucleotide exchange factors (GEFs), MyoGEF contains a Dbl homology (DH) domain and a pleckstrin homology domain. In this study, we provide evidence demonstrating that intramolecular interactions between the DH domain (residues 162-351) and the carboxyl-terminal region (501-790) of MyoGEF can inhibit MyoGEF functions. In vitro and in vivo pulldown assays showed that the carboxyl-terminal region (residues 501-790) of MyoGEF could interact with the DH domain but not with the pleckstrin homology domain. Expression of a MyoGEF carboxyl-terminal fragment (residues 501-790) decreased RhoA activation and suppressed actin filament formation in MDA-MB-231 breast cancer cells. Additionally, Matrigel invasion assays showed that exogenous expression of the MyoGEF carboxyl-terminal region decreased the invasion activity of MDA-MB-231 cells. Moreover, coimmunoprecipitation assays showed that phosphorylation of the MyoGEF carboxyl-terminal region by aurora B kinase interfered with the intramolecular interactions of MyoGEF. Furthermore, expression of the MyoGEF carboxyl-terminal region interfered with RhoA localization during cytokinesis and led to an increase in multinucleation. Together, our findings suggest that binding of the carboxyl-terminal region of MyoGEF to its DH domain acts as an autoinhibitory mechanism for the regulation of MyoGEF activation.

Cadherin 6 promotes neural crest cell detachment via F-actin regulation and influences active Rho distribution during epithelial-to-mesenchymal transition

The epithelial-to-mesenchymal transition (EMT) is a complex change in cell phenotype that is important for cell migration, morphogenesis and carcinoma metastasis. Loss of epithelial cell adhesion and tight regulation of cadherin adhesion proteins are crucial for EMT. Cells undergoing EMT often display cadherin switching, where they downregulate one cadherin and induce expression of another. However, the functions of the upregulated cadherins and their effects on cell motility are poorly understood. Neural crest cells (NCCs), which undergo EMT during development, lose N-cadherin and upregulate Cadherin 6 (Cdh6) prior to EMT. Cdh6 has been suggested to suppress EMT via cell adhesion, but also to promote EMT by mediating pro-EMT signals. Here, we determine novel roles for Cdh6 in generating cell motility during EMT. We use live imaging of NCC behavior in vivo to show that Cdh6 promotes detachment of apical NCC tails, an important early step of EMT. Furthermore, we show that Cdh6 affects spatiotemporal dynamics of F-actin and active Rho GTPase, and that Cdh6 is required for accumulation of F-actin in apical NCC tails during detachment. Moreover, Cdh6 knockdown alters the subcellular distribution of active Rho, which is known to promote localized actomyosin contraction that is crucial for apical NCC detachment. Together, these data suggest that Cdh6 is an important determinant of where subcellular actomyosin forces are generated during EMT. Our results also identify mechanisms by which an upregulated cadherin can generate cell motility during EMT.

Modeling the synergy of cofilin and Arp2/3 in lamellipodial protrusive activity

Rapid polymerization of actin filament barbed ends generates protrusive forces at the cell edge, leading to cell migration. Two important regulators of free barbed ends, cofilin and Arp2/3, have been shown to work in synergy (net effect greater than additive). To explore this synergy, we model the dynamics of F-actin at the leading edge, motivated by data from EGF-stimulated mammary carcinoma cells. We study how synergy depends on the localized rates and relative timing of cofilin and Arp2/3 activation at the cell edge. The model incorporates diffusion of cofilin, membrane protrusion, F-actin capping, aging, and severing by cofilin and branch nucleation by Arp2/3 (but not G-actin recycling). In a well-mixed system, cofilin and Arp2/3 can each generate a large pulse of barbed ends on their own, but have little synergy; high synergy occurs only at low activation rates, when few barbed ends are produced. In the full spatially distributed model, both synergy and barbed-end production are significant over a range of activation rates. Furthermore, barbed-end production is greatest when Arp2/3 activation is delayed relative to cofilin. Our model supports a direct role for cofilin-mediated actin polymerization in stimulated cell migration, including chemotaxis and cancer invasion.

Talin regulates moesin-NHE-1 recruitment to invadopodia and promotes mammary tumor metastasis

Invadopodia are actin-rich protrusions that degrade the extracellular matrix and are required for stromal invasion, intravasation, and metastasis. The role of the focal adhesion protein talin in regulating these structures is not known. Here, we demonstrate that talin is required for invadopodial matrix degradation and three-dimensional extracellular matrix invasion in metastatic breast cancer cells. The sodium/hydrogen exchanger 1 (NHE-1) is linked to the cytoskeleton by ezrin/radixin/moesin family proteins and is known to regulate invadopodium-mediated matrix degradation. We show that the talin C terminus binds directly to the moesin band 4.1 ERM (FERM) domain to recruit a moesin-NHE-1 complex to invadopodia. Silencing talin resulted in a decrease in cytosolic pH at invadopodia and blocked cofilin-dependent actin polymerization, leading to impaired invadopodium stability and matrix degradation. Furthermore, talin is required for mammary tumor cell motility, intravasation, and spontaneous lung metastasis in vivo. Thus, our findings provide a novel understanding of how intracellular pH is regulated and a molecular mechanism by which talin enhances tumor cell invasion and metastasis.

The on-off relationship of Rho and Rac during integrin-mediated adhesion and cell migration

Rho GTPases play an essential role in regulating cell spreading, adhesion, and migration downstream of integrin engagement with the extracellular matrix. In this review, we focus on RhoA and Rac1--2 Rho GTPases that are required for efficient adhesion and migration--and describe how specific guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs) regulate the extensive crosstalk that exists between them. In particular, we assess the role of GEFs and GAPs in light of recent, unexpected evidence concerning the spatiotemporal relationship between RhoA and Rac1 at the leading edge of migrating cells. Force is increasingly recognized as a key regulator of cell adhesion and we highlight the role of GEFs and GAPs in mechanotransduction, before debating the controversial role of tension in focal adhesion maturation.

A RhoC biosensor reveals differences in the activation kinetics of RhoA and RhoC in migrating cells

RhoA and RhoC GTPases share 92% amino acid sequence identity, yet play different roles in regulating cell motility and morphology. To understand these differences, we developed and validated a biosensor of RhoC activation (RhoC FLARE). This was used together with a RhoA biosensor to compare the spatio-temporal dynamics of RhoA and RhoC activity during cell protrusion/retraction and macropinocytosis. Both GTPases were activated similarly at the cell edge, but in regions more distal from the edge RhoC showed higher activation during protrusion. The two isoforms differed markedly in the kinetics of activation. RhoC was activated concomitantly with RhoA at the cell edge, but distally, RhoC activation preceded RhoA activation, occurring before edge protrusion. During macropinocytosis, differences were observed during vesicle closure and in the area surrounding vesicle formation.

Functions of cofilin in cell locomotion and invasion

Recently, a consensus has emerged that cofilin severing activity can generate free actin filament ends that are accessible for F-actin polymerization and depolymerization without changing the rate of G-actin association and dissociation at either filament end. The structural basis of actin filament severing by cofilin is now better understood. These results have been integrated with recently discovered mechanisms for cofilin activation in migrating cells, which led to new models for cofilin function that provide insights into how cofilin regulation determines the temporal and spatial control of cell behaviour.

Phosphorylation-mediated regulation of GEFs for RhoA

Spatio-temporal control of RhoA GTPase is critical for regulation of cell migration, attachment to extracellular matrix, and cell-cell adhesions. Activation of RhoA is mediated by guanine nucleotide exchange factors (GEFs), a diverse family of enzymes that are controlled by multiple signaling pathways regulating actin cytoskeleton and cell migration. GEFs can be regulated by different mechanisms. Growing evidence demonstrates that phosphorylation serves as one of the predominant signals controlling activity, interactions, and localization of RhoGEFs. It acts as a positive and a negative regulator, and allows for regulation of RhoGEFs by multiple signaling cascades. Although there are common trends in phosphorylation-mediated regulation of some RhoGEF homologs, the majority of GEFs utilize distinct mechanisms that are dictated by their unique structure and interaction networks. This diversity enables multiple signaling pathways to use different RhoGEFs for regulation of a single central-RhoA. Here, we review current examples of phosphorylation-mediated regulation of GEFs for RhoA and its role in cell migration, discuss mechanisms, and provide insights into potential future directions.