Initiation of cofilin activity in response to EGF is uncoupled from cofilin phosphorylation and dephosphorylation in carcinoma cells

ALDH1L1 inhibits cell motility via dephosphorylation of cofilin by PP1 and PP2A

Here we report that ALDH1L1 (FDH, a folate enzyme with tumor suppressor-like properties) inhibits cell motility. The underlying mechanism involves F-actin stabilization, re-distribution of cytoplasmic actin toward strong preponderance of filamentous actin and formation of actin stress fibers. A549 cells expressing FDH showed a much slower recovery of green fluorescent protein-actin fluorescence in a fluorescence recovery after photobleaching assay, as well as an increase in G-actin polymerization and a decrease in F-actin depolymerization rates in pyren-actin fluorescence assays indicating the inhibition of actin dynamics. These effects were associated with robust dephosphorylation of the actin depolymerizing factor cofilin by PP1 and PP2A serine/threonine protein phosphatases, but not the cofilin-specific phosphatases slingshot and chronophin. In fact, the PP1/PP2A inhibitor calyculin prevented cofilin dephosphorylation and restored motility. Inhibition of FDH-induced apoptosis by the Jun N-terminal kinase inhibitor SP600125 or the pan-caspase inhibitor zVAD-fmk did not restore motility or levels of phosphor-cofilin, indicating that the observed effects are independent of FDH function in apoptosis. Interestingly, cofilin small interfering RNA or expression of phosphorylation-deficient S3A cofilin mutant resulted in a decrease of G-actin and the actin stress fiber formation, the effects seen upon FDH expression. In contrast, the expression of S3D mutant, mimicking constitutive phosphorylation, prevented these effects further supporting the cofilin-dependent mechanism. Dephosphorylation of cofilin and inhibition of motility in response to FDH can also be prevented by the increased folate in media. Furthermore, folate depletion itself, in the absence of FDH, resulted in cofilin dephosphorylation and inhibition of motility in several cell lines. Our experiments showed that these effects were folate specific and not a general response to nutrient starvation. Overall, this study shows the presence of distinct intracellular signaling pathways regulating motility in response to folate status and points toward mechanisms involving folates in promoting a malignant phenotype.

beta-Arrestins scaffold cofilin with chronophin to direct localized actin filament severing and membrane protrusions downstream of protease-activated receptor-2

Protease-activated receptor-2 (PAR-2) mediates pro-inflammatory signals in a number of organs, including enhancing leukocyte recruitment to sites of injury and infection. At the cellular level, PAR-2 promotes activation of the actin filament-severing protein cofilin, which is crucial for the reorganization of the actin cytoskeleton and chemotaxis. These responses require the scaffolding functions of beta-arrestins; however, the mechanism by which beta-arrestins spatially regulate cofilin activity and the role of this pathway in primary cells has not been investigated. Here, using size-exclusion chromatography and co-immunoprecipitation, we demonstrate that PAR-2 promotes the formation of a complex containing beta-arrestins, cofilin, and chronophin (CIN) in primary leukocytes and cultured cells. Both association of cofilin with CIN and cell migration are inhibited in leukocytes from beta-arrestin-2(-/-) mice. We show that, in response to PAR-2 activation, beta-arrestins scaffold cofilin with its upstream activator CIN, to facilitate the localized generation of free actin barbed ends, leading to membrane protrusion. These studies suggest that a major role of beta-arrestins in chemotaxis is to spatially regulate cofilin activity to facilitate the formation of a leading edge, and that this pathway may be important for PAR-2-stimulated immune cell migration.

Phosphoinositide signalling in cancer: beyond PI3K and PTEN

There are numerous studies that suggest multiple links between the cellular phosphoinositide system and cancer. As key roles in cancer have been established for PI3K and PTEN - enzymes that regulate the levels of phosphatidylinositol-3,4,5-trisphosphate - compounds targeting this pathway are entering the clinic at a rapid pace. Several other phosphoinositide-modifying enzymes, including phosphoinositide kinases, phosphatases and phospholipase C enzymes, have been implicated in the generation and progression of tumours. Studies of these enzymes are providing new insights into the mechanisms and the extent of their involvement in cancer, highlighting new potential targets for therapeutic intervention.

The cofilin activity cycle in lamellipodia and invadopodia

The actin severing protein cofilin is essential for directed cell migration and chemotaxis, in many cell types and is also important for tumor cell invasion during metastasis. Through its severing activity, cofilin increases the number of free barbed ends to initiate actin polymerization for actin-based protrusion in two distinct subcellular compartments in invasive tumor cells: lamellipodia and invadopodia. Cofilin severing activity is tightly regulated and multiple mechanisms are utilized to regulate cofilin activity. In this prospect, we have grouped the primary on/off regulation into two broad categories, both of which are important for inhibiting cofilin from binding to F-actin or G-actin: (1) Blocking cofilin activity by the binding of cofilin to either PI(4,5)P(2) at lamellipodia, or cortactin at invadopodia. (2) Blocking cofilin's ability to bind to actin via serine phosphorylation. Although the literature suggests that these cofilin regulatory mechanisms may be cell-type dependent, we propose the existence of a common cofilin activity cycle in which both operate. In this common cycle, the mechanism used to initiate cofilin activity is determined by the starting point in the cycle in a given subcellular compartment.

Amiloride inhibits macropinocytosis by lowering submembranous pH and preventing Rac1 and Cdc42 signaling

Inhibitors of Na⁺/H⁺ exchange proteins block macropinocytosis by lowering the pH near the plasma membrane, which in turn inhibits actin remodeling by Rho family GTPases.

Macropinocytosis is differentiated from other types of endocytosis by its unique susceptibility to inhibitors of Na⁺/H⁺ exchange. Yet, the functional relationship between Na⁺/H⁺ exchange and macropinosome formation remains obscure. In A431 cells, stimulation by EGF simultaneously activated macropinocytosis and Na⁺/H⁺ exchange, elevating cytosolic pH and stimulating Na⁺ influx. Remarkably, although inhibition of Na⁺/H⁺ exchange by amiloride or HOE-694 obliterated macropinocytosis, neither cytosolic alkalinization nor Na⁺ influx were required. Instead, using novel probes of submembranous pH, we detected the accumulation of metabolically generated acid at sites of macropinocytosis, an effect counteracted by Na⁺/H⁺ exchange and greatly magnified when amiloride or HOE-694 were present. The acidification observed in the presence of the inhibitors did not alter receptor engagement or phosphorylation, nor did it significantly depress phosphatidylinositol-3-kinase stimulation. However, activation of the GTPases that promote actin remodelling was found to be exquisitely sensitive to the submembranous pH. This sensitivity confers to macropinocytosis its unique susceptibility to inhibitors of Na⁺/H⁺ exchange.

DGCR6L, a novel PAK4 interaction protein, regulates PAK4-mediated migration of human gastric cancer cell via LIMK1

Overexpression, genetic amplification and mutations of p21-activated kinase 4 (PAK4) were found in a variety of human cancers. PAK4 regulated actin cytoskeleton reorganization by phosphorylating LIMK1 and promoted cancer cells migration. Using yeast two-hybrid screen, we identified a novel PAK4 binding protein, DGCR6L, which was associated with cancer cell metastasis. We confirmed PAK4 binding to the DGCR6L specifically by GST pull-down assay, and found an association between endogenous PAK4 and DGCR6L by immunoprecipitation in mammalian cells. Furthermore, L115 of DGCR6L was the critical amino acid to bind 466-572aa in the very C-terminus of PAK4. Importantly, DGCR6L was required for the formation of PAK4-DGCR6L-beta-actin complex. Overexpressed DGCR6L promoted migration of AGS cells mediated by PAK4, whereas knock-down of DGCR6L markedly inhibited the migration of those cells. Moreover, DGCR6L (L115V), which did not bind to PAK4, lost the ability to promote AGS cells migration. DGCR6L colocalized with PAK4 or F-actin and enhanced the phosphorylation level of LIMK1 and cofilin in a dose dependent manner. Taken together, our results demonstrated that DGCR6L, a novel PAK4 interacting protein, regulated PAK4-mediated migration of human gastric cancer cells via LIMK1.

Unbalancing the phosphatidylinositol-4,5-bisphosphate-cofilin interaction impairs cell steering

Cofilin is a key player in actin dynamics during cell migration. Its activity is regulated by (de)phosphorylation, pH, and binding to phosphatidylinositol-4,5-bisphosphate [PI(4,5)P(2)]. Here, we here use a human cofilin-1 (D122K) mutant with increased binding affinity for PI(4,5)P(2) and slower release from the plasma membrane to study the role of the PI(4,5)P(2)-cofilin interaction in migrating cells. In fibroblasts in a background of endogenous cofilin, D122K cofilin expression negatively affects cell turning frequency. In carcinoma cells with down-regulated endogenous cofilin, D122K cofilin neither rescues the drastic morphological defects nor restores the effects in cell turning capacity, unlike what has been reported for wild-type cofilin. In cofilin knockdown cells, D122K cofilin expression promotes outgrowth of an existing lamellipod in response to epidermal growth factor (EGF) but does not result in initiation of new lamellipodia. This indicates that, next to phospho- and pH regulation, the normal release kinetics of cofilin from PI(4,5)P(2) is crucial as a local activation switch for lamellipodia initiation and as a signal for migrating cells to change direction in response to external stimuli. Our results demonstrate that the PI(4,5)P(2) regulatory mechanism, that is governed by EGF-dependent phospholipase C activation, is a determinant for the spatial and temporal control of cofilin activation required for lamellipodia initiation.

Interleukin 6 secreted from adipose stromal cells promotes migration and invasion of breast cancer cells

Excessive adiposity has long been associated with increased incidence of breast cancer in post-menopausal women, and with increased mortality from breast cancer, regardless of the menopausal status. Although adipose tissue-derived estrogen contributes to obesity-associated risk for estrogen receptor (ER)-positive breast cancer, the estrogen-independent impact of adipose tissue on tumor invasion and progression needs to be elucidated. Here, we show that adipose stromal cells (ASCs) significantly stimulate migration and invasion of ER-negative breast cancer cells in vitro and tumor invasion in a co-transplant xenograft mouse model. Our study also identifies cofilin-1, a known regulator of actin dynamics, as a determinant of the tumor-promoting activity of ASCs. The cofilin-1-dependent pathway affects the production of interleukin 6 (IL-6) in ASCs. Depletion of IL-6 from the ASC-conditioned medium abrogated the stimulatory effect of ASCs on the migration and invasion of breast tumor cells. Thus, our study uncovers a link between a cytoskeleton-based pathway in ASCs and the stromal impact on breast cancer cells.

A common cofilin activity cycle in invasive tumor cells and inflammatory cells

In many cell types, the formation of membrane protrusions and directional migration depend on the spatial and temporal regulation of the actin-binding protein cofilin. Cofilin, which is important for the regulation of actin-polymerization initiation, increases the number of actin free barbed ends through three mechanisms: its intrinsic actin-nucleation activity; binding and severing of existing actin filaments; and recycling actin monomers from old filaments to new ones through its actin-depolymerization activity. The increase in free barbed ends that is caused by cofilin initiates new actin polymerization, which can be amplified by the actin-nucleating ARP2/3 complex. Interestingly, different cell systems seem to have different mechanisms of activating cofilin. The initial activation of cofilin in mammary breast tumors is dependent on PLCgamma, whereas cofilin activation in neutrophils is additionally dependent on dephosphorylation, which is promoted through Rac2 signaling. Although the literature seems to be confusing and inconsistent, we propose that all of the data can be explained by a single activity-cycle model. In this Opinion, we give an overview of cofilin activation in both tumor cells and inflammatory cells, and demonstrate how the differences in cofilin activation that are observed in various cell types can be explained by different starting points in this single common activity cycle.

Pak4, a novel Gab1 binding partner, modulates cell migration and invasion by the Met receptor

Hepatocyte growth factor (HGF), the ligand for the Met receptor tyrosine kinase, induces epithelial cell dispersal, invasion, and morphogenesis, events that require remodeling of the actin cytoskeleton. The scaffold protein Gab1 is essential for these biological responses downstream from Met. We have identified p21-activated kinase 4 (Pak4) as a novel Gab1-interacting protein. We show that in response to HGF, Gab1 and Pak4 associate and colocalize at the cell periphery within lamellipodia. The association between Pak4 and Gab1 is dependent on Gab1 phosphorylation but independent of Pak4 kinase activity. The interaction is mediated through a region in Gab1, which displays no homology to known Gab1 interaction motifs and through the guanine exchange factor-interacting domain of Pak4. In response to HGF, Gab1 and Pak4 synergize to enhance epithelial cell dispersal, migration, and invasion, whereas knockdown of Pak4 attenuates these responses. A Gab1 mutant unable to recruit Pak4 fails to promote epithelial cell dispersal and an invasive morphogenic program in response to HGF, demonstrating a physiological requirement for Gab1-Pak4 association. These data demonstrate a novel association between Gab1 and Pak4 and identify Pak4 as a key integrator of cell migration and invasive growth downstream from the Met receptor.

Memo is a cofilin-interacting protein that influences PLCγ1 and cofilin activities, and is essential for maintaining directionality during ErbB2-induced tumor-cell migration

Heregulin (HRG) activates ErbB2-ErbB3 heterodimers thereby stimulating many cellular responses, including motility. Memo and PLCγ1 interact with ErbB2 autophosphorylation sites and are essential for HRG-induced chemotaxis. By tracing HRG-stimulated cell migration in Dunn chambers, we found that Memo- or PLCγ1 knockdown (KD) strongly impairs cell directionality. Memo has no obvious enzymatic activity and was discovered via its ability to complex with ErbB2. Using the yeast two-hybrid approach to gain insight into Memo function, an interaction between Memo and cofilin, a regulator of actin dynamics, was uncovered. The interaction was confirmed in vitro using recombinant proteins and in vivo in co-immunoprecipitation experiments where Memo was detected in complexes with cofilin, ErbB2 and PLCγ1. Interestingly, in Memo KD cells, HRG-induced PLCγ1 phosphorylation was decreased, suggesting that Memo regulates PLCγ1 activation. Furthermore, HRG-induced recruitment of GFP-cofilin to lamellipodia is impaired in Memo and in PLCγ1 KD cells, suggesting that both proteins lie upstream of cofilin in models of ErbB2-driven tumor-cell migration. Finally, in vitro F-actin binding and depolymerization assays showed that Memo enhances cofilin depolymerizing and severing activity. In summary, these data indicate that Memo also regulates actin dynamics by interacting with cofilin and enhancing its function.

Akt2 is required for macrophage chemotaxis

Tumor-associated macrophages play an important role in tumorigenesis and metastasis. Trafficking of macrophages to the proximity of tumors is mediated by CSF-1, a growth factor. In this study, we investigated the role of PKB/Akt in CSF-1-induced macrophage migration. Disruption of Akt2 expression by small interference RNA impaired chemotaxis of both THP-1 cells and mouse peritoneal macrophages. Phosphorylation of PKCzeta, an essential component in chemotaxis signaling pathway, was reduced. LIMK/Cofilin, downstream of PKCzeta, regulated cytoskeleton rearrangement during cell migration. Disruption of Akt2 expression inhibited CSF-1-induced LIMK/Cofilin phosphorylation, which contributed to defects in actin polymerization and chemotaxis. Furthermore, MCP-1, a chemokine, -induced macrophage chemotaxis was also impaired. Taken together, our results demonstrated that Akt2 plays an essential role in both CSF-1- and chemokine-induced chemotaxis of macrophages.

Cofilin is a pH sensor for actin free barbed end formation: role of phosphoinositide binding

Newly generated actin free barbed ends at the front of motile cells provide sites for actin filament assembly driving membrane protrusion. Growth factors induce a rapid biphasic increase in actin free barbed ends, and we found both phases absent in fibroblasts lacking H(+) efflux by the Na-H exchanger NHE1. The first phase is restored by expression of mutant cofilin-H133A but not unphosphorylated cofilin-S3A. Constant pH molecular dynamics simulations and nuclear magnetic resonance (NMR) reveal pH-sensitive structural changes in the cofilin C-terminal filamentous actin binding site dependent on His133. However, cofilin-H133A retains pH-sensitive changes in NMR spectra and severing activity in vitro, which suggests that it has a more complex behavior in cells. Cofilin activity is inhibited by phosphoinositide binding, and we found that phosphoinositide binding is pH-dependent for wild-type cofilin, with decreased binding at a higher pH. In contrast, phosphoinositide binding by cofilin-H133A is attenuated and pH insensitive. These data suggest a molecular mechanism whereby cofilin acts as a pH sensor to mediate a pH-dependent actin filament dynamics.

A PAK4-LIMK1 pathway drives prostate cancer cell migration downstream of HGF

Hepatocyte growth factor (HGF) is associated with tumour progression and increases the invasiveness of prostate carcinoma cells. Cell migration and invasion requires reorganisation of the actin cytoskeleton; processes mediated by the Rho family GTPases. p21 activated kinase 4 (PAK4), an effector of the Rho family protein Cdc42, is activated downstream of HGF. We report here the novel finding that in prostate cancer cells PAK4 binds to and phosphorylates LIM kinase 1 (LIMK1) in an HGF-dependent manner. We show for the first time that variations in the level of PAK4 expression change the level of cofilin phosphorylation in cells, a change we correlate with LIMK1 activity, cell morphology and migratory behaviour. We identify for the first time a direct and localised interaction between PAK4 and LIMK1 within cells using FRET: FLIM. Moreover we show here that HGF mediates this interaction which is concentrated in small foci at the cell periphery. PAK4 and LIMK1 act synergistically to increase cell migration speed, whilst a reduction in PAK4 expression decreases cell speed. It is well established that unphosphorylated (active) cofilin is a required to drive cell migration. Our results support a model whereby HGF-stimulated cell migration also requires a cofilin phosphorylation step that is mediated by PAK4.

Ins and outs of ADF/cofilin activity and regulation

The actin-binding proteins of the actin-depolymerisation factor (ADF)/cofilin family were first described more than three decades ago, but research on these proteins still occupies a front role in the actin and cell migration field. Moreover, cofilin activity is implicated in the malignant, invasive properties of cancer cells. The effects of ADF/cofilins on actin dynamics are diverse and their regulation is complex. In stimulated cells, multiple signalling pathways can be initiated resulting in different activation/deactivation switches that control ADF/cofilin activity. The output of this entire regulatory system, in combination with spatial and temporal segregation of the activation mechanisms, underlies the contribution of ADF/cofilins to various cell migration/invasion phenotypes. In this framework, we describe current views on how ADF/cofilins function in migrating and invading cells.

RhoA/ROCK-mediated switching between Cdc42- and Rac1-dependent protrusion in MTLn3 carcinoma cells

Rho GTPases are versatile regulators of cell shape that act on the actin cytoskeleton. Studies using Rho GTPase mutants have shown that, in some cells, Rac1 and Cdc42 regulate the formation of lamellipodia and filopodia, respectively at the leading edge, whereas RhoA mediates contraction at the rear of moving cells. However, recent reports have described a zone of RhoA/ROCK activation at the front of cells undergoing motility. In this study, we use a FRET-based RhoA biosensor to show that RhoA activation localizes to the leading edge of EGF-stimulated cells. Inhibition of Rho or ROCK enhanced protrusion, yet markedly inhibited cell motility; these changes correlated with a marked activation of Rac-1 at the cell edge. Surprisingly, whereas EGF-stimulated protrusion in control MTLn3 cells is Rac-independent and Cdc42-dependent, the opposite pattern is observed in MTLn3 cells after inhibition of ROCK. Thus, Rho and ROCK suppress Rac-1 activation at the leading edge, and inhibition of ROCK causes a switch between Cdc42 and Rac-1 as the dominant Rho GTPase driving protrusion in carcinoma cells. These data describe a novel role for Rho in coordinating signaling by Rac and Cdc42.

The regulation of cell motility and chemotaxis by phospholipid signaling

Phosphoinositide 3-kinase (PI3K), PTEN and localized phosphatidylinositol (3,4,5)-trisphosphate [PtdIns(3,4,5)P3] play key roles in chemotaxis, regulating cell motility by controlling the actin cytoskeleton in Dictyostelium and mammalian cells. PtdIns(3,4,5)P3, produced by PI3K, acts via diverse downstream signaling components, including the GTPase Rac, Arf-GTPases and the kinase Akt (PKB). It has become increasingly apparent, however, that chemotaxis results from an interplay between the PI3K-PTEN pathway and other parallel pathways in Dictyostelium and mammalian cells. In Dictyostelium, the phospholipase PLA2 acts in concert with PI3K to regulate chemotaxis, whereas phospholipase C (PLC) plays a supporting role in modulating PI3K activity. In adenocarcinoma cells, PLC and the actin regulator cofilin seem to provide the direction-sensing machinery, whereas PI3K might regulate motility.

EGF-induced PIP2 hydrolysis releases and activates cofilin locally in carcinoma cells

Lamellipodial protrusion and directional migration of carcinoma cells towards chemoattractants, such as epidermal growth factor (EGF), depend upon the spatial and temporal regulation of actin cytoskeleton by actin-binding proteins (ABPs). It is generally hypothesized that the activity of many ABPs are temporally and spatially regulated by PIP₂; however, this is mainly based on in vitro–binding and structural studies, and generally in vivo evidence is lacking. Here, we provide the first in vivo data that directly visualize the spatial and temporal regulation of cofilin by PIP₂ in living cells. We show that EGF induces a rapid loss of PIP₂ through PLC activity, resulting in a release and activation of a membrane-bound pool of cofilin. Upon release, we find that cofilin binds to and severs F-actin, which is coincident with actin polymerization and lamellipod formation. Moreover, our data provide evidence for how PLC is involved in the formation of protrusions in breast carcinoma cells during chemotaxis and metastasis towards EGF.

The slingshot family of phosphatases mediates Rac1 regulation of cofilin phosphorylation, laminin-332 organization, and motility behavior of keratinocytes

The motility of keratinocytes is an essential component of wound closure and the development of epidermal tumors. In vitro, the specific motile behavior of keratinocytes is dictated by the assembly of laminin-332 tracks, a process that is dependent upon alpha6beta4 integrin signaling to Rac1 and the actin-severing protein cofilin. Here we have analyzed how cofilin phosphorylation is regulated by phosphatases (slingshot (SSH) or chronophin (CIN)) downstream of signaling by alpha6beta4 integrin/Rac1 in human keratinocytes. Keratinocytes express all members of the SSH family (SSH1, SSH2, and SSH3) and CIN. However, expression of phosphatase-dead versions of all three SSH proteins, but not dominant inactive CIN, results in phosphorylation/inactivation of cofilin, changes in actin cytoskeleton organization, loss of cell polarity, and assembly of aberrant arrays of laminin-332 in human keratinocytes. SSH activity is regulated by 14-3-3 protein binding, and intriguingly, 14-3-3/alpha6beta4 integrin protein interaction is required for keratinocyte migration. We wondered whether 14-3-3 proteins function as regulators of Rac1-mediated keratinocyte migration patterns. In support of this hypothesis, inhibition of Rac1 results in an increase in 14-3-3 protein association with SSH. Thus, we propose a novel mechanism in which alpha6beta4 integrin signaling via Rac1, 14-3-3 proteins, and SSH family members regulates cofilin activation, cell polarity, and matrix assembly, leading to specific epidermal cell migration behavior.

Activity of cofilin can be regulated by a mechanism other than phosphorylation/dephosphorylation in muscle cells in culture

Cofilin plays a critical role in actin filament dynamics in a variety of eukaryotic cells. Its activity is regulated by phosphorylation/dephosphorylation of a Ser3 residue on the N-terminal side and/or its binding to a phosphoinositide, PIP(2). To clarify how cofilin activity is regulated in muscle cells, we generated analogues of the unphosphorylated form (A3-cofilin) and phosphorylated form (D3-cofilin) by converting the phosphorylation site (Ser3) of cofilin to Ala and Asp, respectively. These mutated proteins, as well as the cofilin having Ser3 residue (S3-cofilin), were produced in an E. coli expression system and conjugated with fluorescent dyes. In an in vitro functional assay, A3-cofilin retained the ability to bind to F-actin. Upon injection into cultured muscle cells, A3-cofilin and S3-cofilin promptly disrupted actin filaments in the cytoplasm, and many cytoplasmic rods containing both the exogenous cofilin and actin were generated, while D3-cofilin was simply diffused in the cytoplasm without affecting actin filaments. Several hours after the injection, however, the activity of A3-cofilin and S3-cofilin was suppressed: the actin-A3-cofilin (or S3-cofilin) rods disappeared, the cofilin diffused in the cytoplasm like D3-cofilin, and actin filaments reformed. Both GFP-fused A3-cofilin and S3-cofilin that were produced by cDNA transfection were also suppressed in the cytoplasm of muscle cells in culture. Thus, some mechanism(s) other than phosphorylation can suppress A3-cofilin activity. We observed that PIP(2) can bind to A3-cofilin just as to S3-cofilin and inhibits the interaction of A3-cofilin with actin. Our results suggest that the activity of A3-cofilin and also S3-cofilin can be regulated by PIP(2) in the cytoplasm of muscle cells.

Anticancer therapeutics: A surge of new developments increasingly target tumor and stroma

The Annual Meeting of the American Association for Cancer Research (AACR) brings together research in fundamental biology, translational science, drug development and clinical testing of emerging anticancer therapies. Among the highlights of the 2007 Annual Meeting were major research themes on drug action, drug resistance and new drug development. Instead of striving for a comprehensive overview, we showcase several trends, concepts and research areas that exemplify the complexity of drug resistance and its reversal as we currently understand it. Many of the studies discussed here deal with the interaction of tumor cells with their stromal microenvironment; structural proteins as well as cellular components, fibroblasts as well as inflammatory cells. Target identification, target validation and dealing with the challenge of resistance are recurring themes. Specific classes of molecules discussed are the taxanes, tyrosine kinase inhibitors, anti-angiogenic, anti-stromal and anti-metastatic agents. In the latter three categories, targets reviewed are delta-like ligand 4 (DLL4), integrins, nodal, galectins, lysyl oxidases and thrombospondins, several of which belong to the p53-tumor suppressor repertoire of secreted proteins. Finally, developments in other inhibitor classes such as PI3K/Akt and Rho GTPase inhibitors and thoughts on possible novel combination therapies are briefly summarized. The report also includes relevant publications to July 2007.

EphA4 signaling regulates phospholipase Cgamma1 activation, cofilin membrane association, and dendritic spine morphology

Specialized postsynaptic structures known as dendritic spines are the primary sites of glutamatergic innervation at synapses of the CNS. Previous studies have shown that spines rapidly remodel their actin cytoskeleton to modify their shape and this has been associated with changes in synaptic physiology. However, the receptors and signaling intermediates that restructure the actin network in spines are only beginning to be identified. We reported previously that the EphA4 receptor tyrosine kinase regulates spine morphology. However, the signaling pathways downstream of EphA4 that induce spine retraction on ephrin ligand binding remain poorly understood. Here, we demonstrate that ephrin stimulation of EphA4 leads to the recruitment and activation of phospholipase Cgamma1 (PLCgamma1) in heterologous cells and in hippocampal slices. This interaction occurs through an Src homology 2 domain of PLCgamma1 and requires the EphA4 juxtamembrane tyrosines. In the brain, PLCgamma1 is found in multiple compartments of synaptosomes and is readily found in postsynaptic density fractions. Consistent with this, PLC activity is required for the maintenance of spine morphology and ephrin-induced spine retraction. Remarkably, EphA4 and PLC activity modulate the association of the actin depolymerizing/severing factor cofilin with the plasma membrane. Because cofilin has been implicated previously in the structural plasticity of spines, this signaling may enable cofilin to depolymerize actin filaments and restructure spines at sites of ephrin-EphA4 contact.

The cofilin pathway in breast cancer invasion and metastasis

Recent evidence indicates that metastatic capacity is an inherent feature of breast tumours and not a rare, late acquired event. This has led to new models of metastasis. The interpretation of expression-profiling data in the context of these new models has identified the cofilin pathway as a major determinant of metastasis. Recent studies indicate that the overall activity of the cofilin pathway, and not that of any single gene within the pathway, determines the invasive and metastatic phenotype of tumour cells. These results predict that inhibitors directed at the output of the cofilin pathway will have therapeutic benefit in combating metastasis.

Dynamic cofilin phosphorylation in the control of lamellipodial actin homeostasis

During animal cell chemotaxis, signalling at the plasma membrane induces actin polymerisation to drive forward cell movement. Since the cellular pool of actin is limited, efficient protrusion formation also requires the coordinated disassembly of pre-existing actin filaments. To search for proteins that can monitor filamentous and globular actin levels to maintain the balance of polymerisation and disassembly, we followed changes in the proteome induced by RNA interference (RNAi)-mediated alterations in actin signalling. This unbiased approach revealed an increase in the levels of an inactive, phosphorylated form of the actin-severing protein cofilin in cells unable to generate actin-based lamellipodia. Conversely, an increase in F-actin levels induced the dephosphorylation and activation of cofilin via activation of the Ssh phosphatase. Similarly, in the context of acute phosphoinositide 3-kinase (PI3K) signalling, dynamic changes in cofilin phosphorylation were found to depend on the Ssh phosphatase and on changes in lamellipodial F-actin. These results indicate that changes in the extent of cofilin phosphorylation are regulated by Ssh in response to changes in the levels and/or organisation of F-actin. Together with the recent finding that Ssh phosphatase activity is augmented by F-actin binding, these results identify Ssh-dependent regulation of phosphorylated cofilin levels as an important feedback control mechanism that maintains actin filament homeostasis during actin signalling.

Chemotaxis: Cofilin in the driver's seat

A cell responds to a chemotactic signal by activating actin polymerization and forming a protrusion oriented towards the source. Recent work shows that the activity of cofilin, a protein that creates new barbed ends for actin filament elongation, amplifies and specifies the direction of the response in carcinoma cells.

Spatial and temporal control of cofilin activity is required for directional sensing during chemotaxis

BACKGROUND

Previous work has led to the hypothesis that cofilin severing, as regulated by PLC, is involved in chemotactic sensing. We have tested this hypothesis by investigating whether activation of endogenous cofilin is spatially and temporally linked to sensing an EGF point source in carcinoma cells.

RESULTS

We demonstrate that inhibition of endogenous cofilin activity with either siRNA or overexpression of LIMK suppresses directional sensing in carcinoma cells. LIMK siRNA knockdown, which suppresses cofilin phosphorylation, and microinjection of S3C cofilin, a cofilin mutant that is constitutively active and not phosphorylated by LIMK, also inhibits directional sensing and chemotaxis. These results indicate that phosphorylation of cofilin by LIMK, in addition to cofilin activity, is required for chemotaxis. Cofilin activity concentrates rapidly at the newly formed leading edge facing the gradient, whereas cofilin phosphorylation increases throughout the cell. Quantification of these results indicates that the amplification of asymmetric actin polymerization required for protrusion toward the EGF gradient occurs at the level of cofilin but not at the level of PLC activation by EGFR.

CONCLUSIONS

These results indicate that local activation of cofilin by PLC and its global inactivation by LIMK phosphorylation combine to generate the local asymmetry of actin polymerization required for chemotaxis.

Regulation of the actin cytoskeleton in cancer cell migration and invasion

Malignant cancer cells utilize their intrinsic migratory ability to invade adjacent tissues and the vasculature, and ultimately to metastasize. Cell migration is the sum of multi-step processes initiated by the formation of membrane protrusions in response to migratory and chemotactic stimuli. The driving force for membrane protrusion is localized polymerization of submembrane actin filaments. Recently, several studies revealed that molecules that link migratory signals to the actin cytoskeleton are upregulated in invasive and metastatic cancer cells. In this review, we summarize recent progress on molecular mechanisms of formation of invasive protrusions used by tumor cells, such as lamellipodia and invadopodia, with regard to the functions of key regulatory proteins of the actin cytoskeleton; WASP family proteins, Arp2/3 complex, LIM-kinase, cofilin, and cortactin.