PAK4 mediates morphological changes through the regulation of GEF-H1

Altered prefrontal cortical MARCKS and PPP1R9A mRNA expression in schizophrenia and bipolar disorder

BACKGROUND

We previously observed dendritic spine loss in the dorsolateral prefrontal cortex (DLPFC) from schizophrenia and bipolar disorder subjects. In the current study, we sought to determine if the mRNA expression of genes known to regulate the actin cytoskeleton and spines correlated with spine loss.

METHODS

Five candidate genes were identified using previously obtained microarray data from the DLPFC from schizophrenia and control subjects. The relative mRNA expression of the genes linked to dendritic spine growth and function, i.e. IGF1R, MARCKS, PPP1R9A, PTPRF, and ARHGEF2, was assessed using quantitative real-time PCR (qRT-PCR) in the DLPFC from a second cohort including schizophrenia, bipolar disorder, and control subjects. Functional pathway analysis was conducted to determine which actin cytoskeleton-regulatory pathways the genes of interest interact with.

RESULTS

MARCKS mRNA expression was increased in both schizophrenia and bipolar disorder subjects. PPP1R9A mRNA expression was increased in bipolar disorder subjects. For IGF1R, mRNA expression did not differ significantly among groups; however, it did show a significant, negative correlation with dendrite length. MARCKS and PPP1R9A mRNA expression did not correlate with spine loss, but they interact with NMDA receptor signaling pathways that regulate the actin cytoskeleton and spines.

CONCLUSIONS

MARCKS and PPP1R9A might contribute to spine loss in schizophrenia and bipolar disorder through their interactions, possibly indirect ones, with NMDA signaling pathways that regulate spine structure and function.

Signaling, Regulation, and Specificity of the Type II p21-activated Kinases

The p21-activated kinases (PAKs) are a family of six serine/threonine kinases that act as key effectors of RHO family GTPases in mammalian cells. PAKs are subdivided into two groups: type I PAKs (PAK1, PAK2, and PAK3) and type II PAKs (PAK4, PAK5, and PAK6). Although these groups are involved in common signaling pathways, recent work indicates that the two groups have distinct modes of regulation and have both unique and common substrates. Here, we review recent insights into the molecular level details that govern regulation of type II PAK signaling. We also consider mechanisms by which signal transduction is regulated at the level of substrate specificity. Finally, we discuss the implications of these studies for clinical targeting of these kinases.

Interaction between the type III effector VopO and GEF-H1 activates the RhoA-ROCK pathway

Vibrio parahaemolyticus is an important pathogen that causes food-borne gastroenteritis in humans. The type III secretion system encoded on chromosome 2 (T3SS2) plays a critical role in the enterotoxic activity of V. parahaemolyticus. Previous studies have demonstrated that T3SS2 induces actin stress fibers in various epithelial cell lines during infection. This stress fiber formation is strongly related to pathogenicity, but the mechanisms that underlie T3SS2-dependent actin stress fiber formation and the main effector have not been elucidated. In this study, we identified VopO as a critical T3SS2 effector protein that activates the RhoA-ROCK pathway, which is an essential pathway for the induction of the T3SS2-dependent stress fiber formation. We also determined that GEF-H1, a RhoA guanine nucleotide exchange factor (GEF), directly binds VopO and is necessary for T3SS2-dependent stress fiber formation. The GEF-H1-binding activity of VopO via an alpha helix region correlated well with its stress fiber-inducing capacity. Furthermore, we showed that VopO is involved in the T3SS2-dependent disruption of the epithelial barrier. Thus, VopO hijacks the RhoA-ROCK pathway in a different manner compared with previously reported bacterial toxins and effectors that modulate the Rho GTPase signaling pathway.

Many bacterial pathogens manipulate the actin cytoskeleton of mammalian cells to establish pathogenesis via invasion, to evade killing by phagocytes, to disrupt a barrier function, and to induce inflammation caused by translocation type III secretion (T3S) effector proteins. We demonstrated that the T3S effector protein (VopO) of the enteric pathogen Vibrio parahaemolyticus induced robust actin stress fiber formation in infected host cells. Furthermore, this actin rearrangement induced barrier disruption in a colon epithelial cell line. Although many types of effector proteins have been reported, VopO does not share homology with previously reported effector proteins, and no putative functional motifs could be identified. Finally, we determined that the direct binding of VopO to a RhoA guanine nucleotide exchange factor (GEF) is a key step in the induction of stress fiber formation. These findings indicate that VopO plays a unique role in the pathogenicity of V. parahaemolyticus.

Tumor cells chronically treated with a trastuzumab-maytansinoid antibody-drug conjugate develop varied resistance mechanisms but respond to alternate treatments

Antibody-drug conjugates (ADC) are emerging as clinically effective therapy. We hypothesized that cancers treated with ADCs would acquire resistance mechanisms unique to immunoconjugate therapy and that changing ADC components may overcome resistance. Breast cancer cell lines were exposed to multiple cycles of anti-Her2 trastuzumab-maytansinoid ADC (TM-ADC) at IC80 concentrations followed by recovery. The resistant cells, 361-TM and JIMT1-TM, were characterized by cytotoxicity, proteomic, transcriptional, and other profiling. Approximately 250-fold resistance to TM-ADC developed in 361-TM cells, and cross-resistance was observed to other non-cleavable-linked ADCs. Strikingly, these 361-TM cells retained sensitivity to ADCs containing cleavable mcValCitPABC-linked auristatins. In JIMT1-TM cells, 16-fold resistance to TM-ADC developed, with cross-resistance to other trastuzumab-ADCs. Both 361-TM and JIMT1-TM cells showed minimal resistance to unconjugated mertansine (DM1) and other chemotherapeutics. Proteomics and immunoblots detected increased ABCC1 (MRP1) drug efflux protein in 361-TM cells, and decreased Her2 (ErbB2) in JIMT1-TM cells. Proteomics also showed alterations in various pathways upon chronic exposure to the drug in both cell models. Tumors derived from 361-TM cells grew in mice and were refractory to TM-ADC compared with parental cells. Hence, acquired resistance to trastuzumab-maytansinoid ADC was generated in cultured cancer cells by chronic drug treatment, and either increased ABCC1 protein or reduced Her2 antigen were primary mediators of resistance. These ADC-resistant cell models retain sensitivity to other ADCs or standard-of-care chemotherapeutics, suggesting that alternate therapies may overcome acquired ADC resistance. Mol Cancer Ther; 14(4); 952-63. ©2015 AACR.

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.

PAK4-6 in cancer and neuronal development

PAKs 4, 5 and 6 are members of the group B family of p21-activated kinases. Among this group, PAK4 has been most extensively studied. While it has essential roles in embryonic development, in adults high levels of PAK4 are frequently associated with cancer. PAK4 is overexpressed in a variety of cancers, and the Pak4 gene is amplified in some cancers. PAK4 overexpression is sufficient to cause oncogenic transformation in cells and in mouse models. The tight connection between PAK4 and cancer make it a promising diagnostic tool as well as a potential drug target. The group B PAKs also have important developmental functions. PAK4 is important for many early developmental processes, while PAK5 and PAK6 play roles in learning and memory in mice. This chapter provides an overview of the roles of the group B PAKs in cancer as well as development, and includes a discussion of PAK mediated signaling pathways and cellular functions.

A transcriptional cross-talk between RhoA and c-Myc inhibits the RhoA/Rock-dependent cytoskeleton

The GTPase RhoA and the transcriptional factor c-Myc are closely intertwined in cancer cells. Although this cross-talk results in potent synergistic effects that favor the transformed phenotype of cancer cells, recent results from our laboratory indicate that c-Myc also participates in a negative feed-back loop that blocks specific RhoA signaling branches connected to the induction of stress fibers, focal adhesions and actomyosin contractility. Using microarray analysis, we have unveiled a RhoA/c-Myc-dependent gene signature in charge of this negative cross-talk. This signature is composed of upregulated and repressed transcripts encoding cytoskeletal modulators located downstream of both RhoA and Rock. Our results also indicate that this negative feed-back loop modifies the invasion and adhesion properties of RhoA-transformed cells, suggesting that it may be important to ensure fluid cytoskeletal dynamics of cancer cells. Preliminary data indicate that c-Myc may also use a different transcriptional program to interfere with the RhoA/Rock-dependent cytoskeletal branch in non-transformed cells.

PAK family kinases: Physiological roles and regulation

The p21-activated kinases (PAKs) are a family of Ser/Thr protein kinases that are represented by six genes in humans (PAK 1-6), and are found in all eukaryotes sequenced to date. Genetic and knockdown experiments in frogs, fish and mice indicate group I PAKs are widely expressed, required for multiple tissue development, and particularly important for immune and nervous system function in the adult. The group II PAKs (human PAKs 4-6) are more enigmatic, but their restriction to metazoans and presence at cell-cell junctions suggests these kinases emerged to regulate junctional signaling. Studies of protozoa and fungal PAKs show that they regulate cell shape and polarity through phosphorylation of multiple cytoskeletal proteins, including microtubule binding proteins, myosins and septins. This chapter discusses what we know about the regulation of PAKs and their physiological role in different model organisms, based primarily on gene knockout studies.

Mechanistic insight into GPCR-mediated activation of the microtubule-associated RhoA exchange factor GEF-H1

The RhoGEF GEF-H1 can be sequestered in an inactive state on polymerized microtubules by the dynein motor light-chain Tctex-1. Phosphorylation of GEF-H1 Ser885 by PKA or PAK kinases creates an inhibitory 14-3-3-binding site. Here we show a new mode of GEF-H1 activation in response to the G-protein-coupled receptor (GPCR) ligands lysophosphatidic acid (LPA) or thrombin that is independent of microtubule depolymerization. LPA/thrombin stimulates disassembly of the GEF-H1:dynein multi-protein complex through the concerted action of Gα and Gβγ. Gα binds directly to GEF-H1 and displaces it from Tctex-1, while Gβγ binds to Tctex-1 and disrupts its interaction with the dynein intermediate chain, resulting in the release of GEF-H1. Full activation of GEF-H1 requires dephosphorylation of Ser885 by PP2A, which is induced by thrombin. The coordinated displacement of GEF-H1 from microtubules by G-proteins and its dephosphorylation by PP2A demonstrate a multistep GEF-H1 activation and present a unique mechanism coupling GPCR signalling to Rho activation.

The role of p21-activated kinases in hepatocellular carcinoma metastasis

The p21-activated kinases (PAKs) are downstream effectors of the Rho family small GTPases as well as a wide variety of mitogenic factors and have been implicated in cancer formation, development and metastasis. PAKs phosphorylate a wide spectrum of substrates to mediate extracellular signals and regulate cytoskeletal remodeling, cell motility and survival. In this review, we aim to summarize the findings regarding the oncogenic role and the underlying mechanisms of PAKs signaling in various cancers, and in particular highlight the prime importance of PAKs in hepatocellular carcinoma (HCC) progression and metastasis. Recent studies exploring the potential therapeutic application of PAK inhibitors will also be discussed.

Rho GTPases in collective cell migration

The family of Rho GTPases are intracellular signal transducers that link cell surface signals to multiple intracellular responses. They are best known for their role in regulating actin dynamics required for cell migration, but in addition control cell-cell adhesion, polarization, vesicle trafficking, and the cell cycle. The roles of Rho GTPases in single mesenchymal cell migration are well established and rely on Cdc42- and Rac-dependent cell protrusion of a leading edge, coupled to Rho-dependent contractility required to move the cell body forward. In cells migrating collectively, cell-cell junctions are maintained, and migrating leader cells are mechanically coupled to, and coordinate, migration with follower cells. Recent evidence suggests that Rho GTPases provide multifunctional input to collective cell polarization, cell-cell interaction, and migration. Here, we discuss the role of Rho GTPases in initiating and maintaining front-rear, apical-basal cell polarization, mechanotransduction, and cell-cell junction stability between leader and follower cells, and how these roles are integrated in collective migration. Thereby, spatiotemporal fine-tuning of Rho GTPases within the same cell and among cells in the cell group are crucial in controlling potentially conflicting, divergent cell adhesion and cytoskeletal functions to achieve supracellular coordination and mechanocoupling.

Discovery and the structural basis of a novel p21-activated kinase 4 inhibitor

Functional versatility and elevated expression in cancers have endowed p21-activated kinase 4 (PAK4) as one of the first-in-class anti-cancer drug target. In this study, a novel PAK4 inhibitor, KY-04031 (N(2)-(2-(1H-indol-3-yl)ethyl)-N(4)-(1H-indazol-5-yl)-6-methoxy-1,3,5-triazine-2,4-diamine), was discovered using a high-throughput screening. Analysis of the complex crystal structure illustrated that both indole and indazole of KY-04031 are responsible for PAK4 hinge interaction. Moreover, the molecule's triazine core was found to mimic the ribose of the natural ATP substrate. The cell-based anti-cancer potency of KY-04031 was less effective than the pyrroloaminopyrazoles; however, the unique molecular feature of KY-04031 can be exploited in designing new PAK4 inhibitors.

P21 activated kinases: structure, regulation, and functions

The p21 activated kinases (Paks) are well known effector proteins for the Rho GTPases Cdc42 and Rac. The Paks contain 6 members, which fall into 2 families of proteins. The first family consists of Paks 1, 2, and 3, and the second consists of Paks 4, 5, and 6. While some of the Paks are ubiquitously expressed, others have more restrictive tissue specificity. All of them are found in the nervous system. Studies using cell culture, transgenic mice, and knockout mice, have revealed important roles for the Paks in cytoskeletal organization and in many aspects of cell growth and development. This review discusses the basic structures of the Paks, and their roles in cell growth, development, and in cancer.

Role of p-21-activated kinases in cancer progression

The p-21-activated kinases (PAKs) are downstream effectors of Rho GTPases Rac and Cdc42. The PAK family consists of six members which are segregated into two subgroups (Group I and Group II) based on sequence homology. Group I PAKs (PAK1-3) are the most extensively studied but there is increasing interest in the functionality of Group II PAKs (PAK4-6). The PAK family proteins are thought to play an important role in many different cellular processes, some of which have particular significance in the context of cancer progression. This review explores established and more recent data, linking the PAK family kinases to cancer progression including expression profiles, evasion of apoptosis, promotion of cell survival, and regulation of cell invasion. Finally, we discuss attempts to therapeutically target the PAK family and outline the major obstacles that still need to be overcome.

Control of vascular permeability by atrial natriuretic peptide via a GEF-H1-dependent mechanism

Microtubule (MT) dynamics is involved in a variety of cell functions, including control of the endothelial cell (EC) barrier. Release of Rho-specific nucleotide exchange factor GEF-H1 from microtubules activates the Rho pathway of EC permeability. In turn, pathologic vascular leak can be prevented by treatment with atrial natriuretic peptide (ANP). This study investigated a novel mechanism of vascular barrier protection by ANP via modulation of GEF-H1 function. In pulmonary ECs, ANP suppressed thrombin-induced disassembly of peripheral MT and attenuated Rho signaling and cell retraction. ANP effects were mediated by the Rac1 GTPase effector PAK1. Activation of Rac1-PAK1 promoted PAK1 interaction with the Rho activator GEF-H1, inducing phosphorylation of total and MT-bound GEF-H1 and leading to attenuation of Rho-dependent actin remodeling. In vivo, ANP attenuated lung injury caused by excessive mechanical ventilation and TRAP peptide (TRAP/HTV), which was further exacerbated in ANP(-/-) mice. The protective effects of ANP against TRAP/HTV-induced lung injury were linked to the increased pool of stabilized MT and inactivation of Rho signaling via ANP-induced, PAK1-dependent inhibitory phosphorylation of GEF-H1. This study demonstrates a novel protective mechanism of ANP against pathologic hyperpermeability and suggests a novel pharmacological intervention for the prevention of increased vascular leak via PAK1-dependent modulation of GEF-H1 activity.

Barrier enhancing signals in pulmonary edema

Increased endothelial permeability and reduction of alveolar liquid clearance capacity are two leading pathogenic mechanisms of pulmonary edema, which is a major complication of acute lung injury, severe pneumonia, and acute respiratory distress syndrome, the pathologies characterized by unacceptably high rates of morbidity and mortality. Besides the success in protective ventilation strategies, no efficient pharmacological approaches exist to treat this devastating condition. Understanding of fundamental mechanisms involved in regulation of endothelial permeability is essential for development of barrier protective therapeutic strategies. Ongoing studies characterized specific barrier protective mechanisms and identified intracellular targets directly involved in regulation of endothelial permeability. Growing evidence suggests that, although each protective agonist triggers a unique pattern of signaling pathways, selected common mechanisms contributing to endothelial barrier protection may be shared by different barrier protective agents. Therefore, understanding of basic barrier protective mechanisms in pulmonary endothelium is essential for selection of optimal treatment of pulmonary edema of different etiology. This article focuses on mechanisms of lung vascular permeability, reviews major intracellular signaling cascades involved in endothelial monolayer barrier preservation and summarizes a current knowledge regarding recently identified compounds which either reduce pulmonary endothelial barrier disruption and hyperpermeability, or reverse preexisting lung vascular barrier compromise induced by pathologic insults.

Extracellular signal-regulated kinase regulates RhoA activation and tumor cell plasticity by inhibiting guanine exchange factor H1 activity

In certain Ras mutant cell lines, the inhibition of extracellular signal-regulated kinase (ERK) signaling increases RhoA activity and inhibits cell motility, which was attributed to a decrease in Fra-1 levels. Here we report a Fra-1-independent augmentation of RhoA signaling during short-term inhibition of ERK signaling. Using mass spectrometry-based proteomics, we identified guanine exchange factor H1 (GEF-H1) as mediating this effect. ERK binds to the Rho exchange factor GEF-H1 and phosphorylates it on S959, causing inhibition of GEF-H1 activity and a consequent decrease in RhoA activity. Knockdown experiments and expression of a nonphosphorylatable S959A GEF-H1 mutant showed that this site is crucial in regulating cell motility and invasiveness. Thus, we identified GEF-H1 as a critical ERK effector that regulates motility, cell morphology, and invasiveness.

PAK4 kinase-mediated SCG10 phosphorylation involved in gastric cancer metastasis

Superior cervical ganglia 10 (SCG10), as a microtubule (MT) destabilizer, maintains MT homeostasis and has a critical role in neuronal development, but its function in tumorigenesis has not been characterized. In the present study, we demonstrated that p21-activated kinase 4 (PAK4)-mediated SCG10 phosphorylation regulates MT homeostasis in metastatic gastric cancer. Our results indicate that SCG10 is a physiological substrate of PAK4, which is phosphorylated on serine 50 (Ser50) in a PAK4-dependent manner. Phosphorylated SCG10 regulated MT dynamics to promote gastric cancer cell migration and invasion in vitro and metastasis in a xenograft mouse models. Inhibiting PAK4, either by LCH-7749944 or RNA interference, resulted in the inhibition of Ser50 phosphorylation and a blockade to cell invasion, suggesting that PAK4-SCG10 signaling occurs in gastric cancer cell invasion. Moreover, we demonstrated a strong positive correlation between PAK4 and phospho-Ser50 SCG10 expression in gastric cancer samples. We also showed that high expression of SCG10 phospho-Ser50 is highly correlated to an aggressive phenotype of clinical gastric cancer. These findings revealed a novel function of SCG10 in promoting invasive potential of gastric cancer cells, suggesting that blocking PAK4-mediated SCG10 phosphorylation might be a potential therapeutic strategy for metastasis of gastric cancer.

Systems-wide analysis of K-Ras, Cdc42, and PAK4 signaling by quantitative phosphoproteomics

Although K-Ras, Cdc42, and PAK4 signaling are commonly deregulated in cancer, only a few studies have sought to comprehensively examine the spectrum of phosphorylation-mediated signaling downstream of each of these key signaling nodes. In this study, we completed a label-free quantitative analysis of oncogenic K-Ras, activated Cdc42, and PAK4-mediated phosphorylation signaling, and report relative quantitation of 2152 phosphorylated peptides on 1062 proteins. We define the overlap in phosphopeptides regulated by K-Ras, Cdc42, and PAK4, and find that perturbation of these signaling components affects phosphoproteins associated with microtubule depolymerization, cytoskeletal organization, and the cell cycle. These findings provide a resource for future studies to characterize novel targets of oncogenic K-Ras signaling and validate biomarkers of PAK4 inhibition.

P21-activated kinase 4--not just one of the PAK

P21-activated kinase 4 (PAK4) is a member of the p21-activated kinase (PAK) family. Historically much of the attention has been directed towards founding family member PAK1 but the focus is now shifting towards PAK4. It is a pluripotent serine/threonine kinase traditionally recognised as a downstream effector of the Rho-family GTPases. However, emerging research over the last few years has revealed that this kinase is much more than that. New findings have shed light on the molecular mechanism of PAK4 activation and how this kinase is critical for early development. Moreover, the number of PAK4 substrates and binding partners is rapidly expanding highlighting the increasing amount of cellular functions controlled by PAK4. We propose that PAK4 should be considered a signalling integrator regulating numerous fundamental cellular processes, including actin cytoskeletal dynamics, cell morphology and motility, cell survival, embryonic development, immune defence and oncogenic transformation. This review will outline our current understanding of PAK4 biology.

EB1, p150Glued, and Clasp1 control endothelial tubulogenesis through microtubule assembly, acetylation, and apical polarization

Vascular tube morphogenesis requires the establishment of endothelial cell (EC) apical-basal polarity in three-dimensional (3D) extracellular matrices. To date, there is little understanding of how EC polarity is controlled during these highly dynamic and rapid morphogenic events. We show that the microtubule tip complex proteins, end binding 1 (EB1), p150(Glued), and Clasp1, control human EC tube formation by (1) inducing microtubule assembly and asymmetric cytoskeletal polarization, whereby acetylated and detyrosinated tubulins distribute in a subapical membrane location and filamentous actin distributes basally; (2) increasing tubulin posttranslational modifications, including required acetylation events; and (3) regulating an EC lumen signaling cascade that involves membrane type 1 matrix metallopatrinase (MT1-MMP)-dependent proteolysis as well as Pak, Raf, and Erk kinases. Another regulator of this process is the microtubule stabilizing protein, tau, which binds p150(Glued) and similarly affects EC lumen formation by controlling the levels of acetylated and detyrosinated tubulins. Increased expression of the tubulin deacetylases, sirtuin 2, and histone deacetylase 6 (HDAC6), blocks EC tube formation and cytoskeletal polarization, while siRNA suppression of these deacetylases stimulates these events. Overall, this work reveals a fundamental role for microtubule tip complex proteins in coordinating microtubule assembly, posttranslational modifications including acetylation, and apical-basal cytoskeletal polarization to control the developing apical membrane surface during blood vessel tubulogenesis in 3D matrix environments.

Central role of the exchange factor GEF-H1 in TNF-α-induced sequential activation of Rac, ADAM17/TACE, and RhoA in tubular epithelial cells

Tumor necrosis factor-α activates the enzyme TACE/ADAM17 through the guanine nucleotide exchange factor GEF-H1, Rac, and p38, leading to activation of the epidermal growth factor. GEF-H1 mediates hierarchical activation of Rac and RhoA through differential phosphorylation.

Transactivation of the epidermal growth factor receptor (EGFR) by tumor necrosis factor-α (TNF-α) is a key step in mediating RhoA activation and cytoskeleton and junction remodeling in the tubular epithelium. In this study we explore the mechanisms underlying TNF-α–induced EGFR activation. We show that TNF-α stimulates the TNF-α convertase enzyme (TACE/a disintegrin and metalloproteinase-17), leading to activation of the EGFR/ERK pathway. TACE activation requires the mitogen-activated protein kinase p38, which is activated through the small GTPase Rac. TNF-α stimulates both Rac and RhoA through the guanine nucleotide exchange factor (GEF)-H1 but by different mechanisms. EGFR- and ERK-dependent phosphorylation at the T678 site of GEF-H1 is a prerequisite for RhoA activation only, whereas both Rac and RhoA activation require GEF-H1 phosphorylation on S885. Of interest, GEF-H1-mediated Rac activation is upstream from the TACE/EGFR/ERK pathway and regulates T678 phosphorylation. We also show that TNF-α enhances epithelial wound healing through TACE, ERK, and GEF-H1. Taken together, our findings can explain the mechanisms leading to hierarchical activation of Rac and RhoA by TNF-α through a single GEF. This mechanism could coordinate GEF functions and fine-tune Rac and RhoA activation in epithelial cells, thereby promoting complex functions such as sheet migration.

Increased expression of microRNA-221 inhibits PAK1 in endothelial progenitor cells and impairs its function via c-Raf/MEK/ERK pathway

Coronary artery disease (CAD) is associated with high mortality and occurs via endothelial injury. Endothelial progenitor cells (EPCs) restore the integrity of the endothelium and protect it from atherosclerosis. In this study, we compared the expression of microRNAs (miRNAs) in EPCs in atherosclerosis patients and normal controls. We found that miR-221 expression was significantly up-regulated in patients compared with controls. We predicted and identified p21/Cdc42/Rac1-activated kinase 1 (PAK1) as a novel target of miR-221 in EPCs. We also demonstrated that miR-221 targeted a putative binding site in the 3'UTR of PAK1, and absence of this site was inversely associated with miR-221 expression in EPCs. We confirmed this relationship using a luciferase reporter assay. Furthermore, overexpression of miR-221 in EPCs significantly decreased EPC proliferation, in accordance with the inhibitory effects induced by decreased PAK1. Overall, these findings demonstrate that miR-221 affects the MEK/ERK pathway by targeting PAK1 to inhibit the proliferation of EPCs.

The pak4 protein kinase in breast cancer

Paks4, along with Paks5, and 6 are members of the group B family of p21-activated kinases (Paks). The Paks play multiple different roles in controlling cell morphology, cell growth, proliferation, and signaling. Pak4 has essential roles in embryonic development (Qu et al., 2003), but in adults high levels of Pak4 are frequently associated with cancer. Pak4 has been implicated in several types of cancer (Wells and Jones, 2010; Eswaran et al., 2009; Liu et al., 2008; and Liu et al., 2010) and it is strongly linked to breast cancer (Liu et al., 2008; Liu et al. 2010; Yu et al., 2009; Rafn et al., 2012; and So et al., 2012). Breast tumors and breast cancer cell lines frequently have high levels of Pak4 (Liu et al., 2008), and overexpression of Pak4 in mammary epithelial cells leads to tumorigenesis in mice (Liu et al., 2010). This paper summarizes the current work on the role of Pak4 in breast cancer.

Functional cooperativity by direct interaction between PAK4 and MMP-2 in the regulation of anoikis resistance, migration and invasion in glioma

Gliomas display anoikis resistance, enhanced invasion in to the adjacent brain parenchyma and eventually recur despite using the standard therapies. Our studies on increased anoikis sensitization in matrix metalloproteinase-2 (MMP-2)-knockdown 4910 and 5310 human glioma xenograft cells were interestingly correlated with p21-activated kinase 4 (PAK4) inhibition, prompting us to further investigate the role of PAK4 in glioma. Here, we report the PAK4 upregulation in positive correlation with increasing glioma pathological grades. The siRNA-mediated PAK4 knockdown elevated anoikis, and inhibited invasion and migration by downregulating MMP-2, αvβ3-integrin and phospho-epidermal growth factor receptor (phospho-EGFR). The cDNA-PCR arrays revealed a transcriptional suppression of essential proteins involved in cell proliferation and adhesion in PAK4-knockdown cells. Most importantly, glutathione S-transferase pull-down assays demonstrated the MMP-2 as a new PAK4-interacting protein which binds to PAK4 kinase domain. Individual EGFR/ErbB2 inhibitor and αvβ3 antibody treatments in PAK4si-treated cells indicated the regulation of αvβ3/EGFR survival signaling by PAK4. Overexpression of PAK4 significantly reversed the MMP2si-induced cell death in both cell lines. Codepletion of PAK4 and MMP-2 resulted in robust anoikis-mediated cell death, and severely inhibited invasive and migratory properties in these cells. PAK4si inhibited in vivo tumor growth in nude mice by inhibiting MMP-2, β3-integrin and phospho-EGFR levels in tumors. Our findings indicate a physical association between PAK4 and MMP-2, and suggest the future therapeutic potential of PAK4/MMP-2 dual targeting in glioma treatment.

Pak2 kinase restrains mast cell FcεRI receptor signaling through modulation of Rho protein guanine nucleotide exchange factor (GEF) activity

p21-activated kinase-1 (Pak1) is a serine/threonine kinase that plays a key role in mediating antigen-stimulated extracellular calcium influx and degranulation in mast cells. Another isoform in this kinase family, Pak2, is expressed at very high levels in mast cells, but its function is unknown. Here we show that Pak2 loss in murine bone marrow-derived mast cells, unlike loss of Pak1, induces increased antigen-mediated adhesion, degranulation, and cytokine secretion without changes to extracellular calcium influx. This phenotype is associated with an increase in RhoA-GTPase signaling activity to downstream effectors, including myosin light chain and p38(MAPK), and is reversed upon treatment with a Rho-specific inhibitor. Pak2, but not Pak1, negatively regulates RhoA via phosphorylation of the guanine nucleotide exchange factor GEF-H1 at an inhibitory site, leading to increased GEF-H1 microtubule binding and loss of RhoA stimulation. These data suggest that Pak2 plays a unique inhibitory role in mast cell degranulation by down-regulating RhoA via GEF-H1.

Targeting and transport: how microtubules control focal adhesion dynamics

Directional cell migration requires force generation that relies on the coordinated remodeling of interactions with the extracellular matrix (ECM), which is mediated by integrin-based focal adhesions (FAs). Normal FA turnover requires dynamic microtubules, and three members of the diverse group of microtubule plus-end-tracking proteins are principally involved in mediating microtubule interactions with FAs. Microtubules also alter the assembly state of FAs by modulating Rho GTPase signaling, and recent evidence suggests that microtubule-mediated clathrin-dependent and -independent endocytosis regulates FA dynamics. In addition, FA-associated microtubules may provide a polarized microtubule track for localized secretion of matrix metalloproteases (MMPs). Thus, different aspects of the molecular mechanisms by which microtubules control FA turnover in migrating cells are beginning to emerge.

Hyperosmotic stress regulates the distribution and stability of myocardin-related transcription factor, a key modulator of the cytoskeleton

Hyperosmotic stress initiates several adaptive responses, including the remodeling of the cytoskeleton. Besides maintaining structural integrity, the cytoskeleton has emerged as an important regulator of gene transcription. Myocardin-related transcription factor (MRTF), an actin-regulated coactivator of serum response factor, is a major link between the actin skeleton and transcriptional control. We therefore investigated whether MRTF is regulated by hyperosmotic stress. Here we show that hypertonicity induces robust, rapid, and transient translocation of MRTF from the cytosol to the nucleus in kidney tubular cells. We found that the hyperosmolarity-triggered MRTF translocation is mediated by the RhoA/Rho kinase (ROK) pathway. Moreover, the Rho guanine nucleotide exchange factor GEF-H1 is activated by hyperosmotic stress, and it is a key contributor to the ensuing RhoA activation and MRTF translocation, since siRNA-mediated GEF-H1 downregulation suppresses these responses. While the osmotically induced RhoA activation promotes nuclear MRTF accumulation, the concomitant activation of p38 MAP kinase mitigates this effect. Moderate hyperosmotic stress (600 mosM) drives MRTF-dependent transcription through the cis-element CArG box. Silencing or pharmacological inhibition of MRTF prevents the osmotic stimulation of CArG-dependent transcription and renders the cells susceptible to osmotic shock-induced structural damage. Interestingly, strong hyperosmolarity promotes proteasomal degradation of MRTF, concomitant with apoptosis. Thus, MRTF is an osmosensitive and osmoprotective transcription factor, whose intracellular distribution is regulated by the GEF-H1/RhoA/ROK and p38 pathways. However, strong osmotic stress destabilizes MRTF, concomitant with apoptosis, implying that hyperosmotically induced cell death takes precedence over epithelial-myofibroblast transition, a potential consequence of MRTF-mediated phenotypic reprogramming.

Maternal pak4 expression is required for primitive myelopoiesis in zebrafish

Transcripts of pak4, the zebrafish ortholog of p21-activated kinase 4 (PAK4), are most abundant in the egg and fall to low levels by the end of gastrulation, after which expression is essentially ubiquitous. Translation of maternal mRNA into pak4 protein is first detectable at high stage (3.3hpf). Splice-blocking morpholino oligonucleotides (MOs) were used to prevent zygotic pak4 expression. This had no discernable effect on development through larval stages. In contrast, a translation-blocking MO, alone or in combination with the splice MOs, resulted in a complex lethal phenotype. In addition to disrupted somite development and other morphogenetic abnormalities, the knockdown of maternal pak4 expression led to alterations in regulatory gene expression in the primitive hematopoietic domains, leading to deficiencies in granulocyte and leukocyte lineages. At least some of the effects of pak4 knockdown on gene expression could be mimicked by treatment with actin depolymerization agents, suggesting a mechanistic link between regulation of microfilament dynamics by pak4 and regulation of gene expression in primitive myeloid cell differentiation.

Type II p21-activated kinases (PAKs) are regulated by an autoinhibitory pseudosubstrate

The type II p21-activated kinases (PAKs) are key effectors of RHO-family GTPases involved in cell motility, survival, and proliferation. Using a structure-guided approach, we discovered that type II PAKs are regulated by an N-terminal autoinhibitory pseudosubstrate motif centered on a critical proline residue, and that this regulation occurs independently of activation loop phosphorylation. We determined six X-ray crystal structures of either full-length PAK4 or its catalytic domain, that demonstrate the molecular basis for pseudosubstrate binding to the active state with phosphorylated activation loop. We show that full-length PAK4 is constitutively autoinhibited, but mutation of the pseudosubstrate releases this inhibition and causes increased phosphorylation of the apoptotic regulation protein Bcl-2/Bcl-X(L) antagonist causing cell death and cellular morphological changes. We also find that PAK6 is regulated by the pseudosubstrate region, indicating a common type II PAK autoregulatory mechanism. Finally, we find Src SH3, but not β-PIX SH3, can activate PAK4. We provide a unique understanding for type II PAK regulation.

FAM123A binds to microtubules and inhibits the guanine nucleotide exchange factor ARHGEF2 to decrease actomyosin contractility

The FAM123 gene family comprises three members: FAM123A, the tumor suppressor WTX (also known as FAM123B), and FAM123C. WTX is required for normal development and causally contributes to human disease, in part through its regulation of β-catenin-dependent WNT signaling. The roles of FAM123A and FAM123C in signaling, cell behavior, and human disease remain less understood. We defined and compared the protein-protein interaction networks for each member of the FAM123 family by affinity purification and mass spectrometry. Protein localization and functional studies suggest that the FAM123 family members have conserved and divergent cellular roles. In contrast to WTX and FAM123C, we found that microtubule-associated proteins were enriched in the FAM123A protein interaction network. FAM123A interacted with and tracked with the plus end of dynamic microtubules. Domain interaction experiments revealed a "SKIP" amino acid motif in FAM123A that mediated interaction with the microtubule tip tracking proteins end-binding protein 1 (EB1) and EB3--and therefore with microtubules. Cells depleted of FAM123A showed compartment-specific effects on microtubule dynamics, increased actomyosin contractility, larger focal adhesions, and decreased cell migration. These effects required binding of FAM123A to and inhibition of the guanine nucleotide exchange factor ARHGEF2, a microtubule-associated activator of RhoA. Together, these data suggest that the SKIP motif enables FAM123A, but not the other FAM123 family members, to bind to EB proteins, localize to microtubules, and coordinate microtubule dynamics and actomyosin contractility.

MiR-145 regulates PAK4 via the MAPK pathway and exhibits an antitumor effect in human colon cells

MicroRNAs (miRNAs) are regulators of numerous cellular events; accumulating evidence indicates that miRNAs play a key role in a wide range of biological functions, such as cellular proliferation, differentiation, and apoptosis in cancer. Down-regulated expression of miR-145 has been reported in colon cancer tissues and cell lines. The molecular mechanisms underlying miR-145 and the regulation of colon carcinogenesis remain unclear. In this study, we investigated the levels of miR-145 in human colon cancer cells using qRT-PCR and found markedly decreased levels compared to normal epithelial cells. We identified PAK4 as a novel target of miR-145 using informatics screening. Additionally, we demonstrated that miR-145 targets a putative binding site in the 3'UTR of PAK4 and that its abundance is inversely associated with miR-145 expression in colon cancer cells; we confirmed this relationship using the luciferase reporter assay. Furthermore, restoration of miR-145 by mimics in SW620 cells significantly attenuated cell growth in vitro, in accordance with the inhibitory effects induced by siRNA mediated knockdown of PAK4. Taken together, these findings demonstrate that miR-145 downregulates P-ERK expression by targeting PAK4 and leads to inhibition of tumor growth.

Group I and II mammalian PAKs have different modes of activation by Cdc42

p21-activated kinases (PAKs) are Cdc42 effectors found in metazoans, fungi and protozoa. They are subdivided into PAK1-like (group I) or PAK4-like (group II) kinases. Human PAK4 is widely expressed and its regulatory mechanism is unknown. We show that PAK4 is strongly inhibited by a newly identified auto-inhibitory domain (AID) formed by amino acids 20 to 68, which is evolutionarily related to that of other PAKs. In contrast to group I kinases, PAK4 is constitutively phosphorylated on Ser 474 in the activation loop, but held in an inactive state until Cdc42 binding. Thus, group II PAKs are regulated through conformational changes in the AID rather than A-loop phosphorylation.

PAK4 kinase activity and somatic mutation promote carcinoma cell motility and influence inhibitor sensitivity

Hepatocyte growth factor (HGF) and its receptor (c-Met) are associated with cancer cell motility and invasiveness. p21-activated kinase 4 (PAK4), a potential therapeutic target, is recruited to and activated by c-Met. In response, PAK4 phosphorylates LIM kinase 1 (LIMK1) in an HGF-dependent manner in metastatic prostate carcinoma cells. PAK4 overexpression is known to induce increased cell migration speed but the requirement for kinase activity has not been established. We have used a panel of PAK4 truncations and mutations in a combination of over-expression and RNAi rescue experiments to determine the requirement for PAK4 kinase activity during carcinoma cell motility downstream of HGF. We find that neither the kinase domain alone nor a PAK4 mutant unable to bind Cdc42 is able to fully rescue cell motility in a PAK4-deficient background. Nevertheless, we find that PAK4 kinase activity and associated LIMK1 activity are essential for carcinoma cell motility, highlighting PAK4 as a potential anti-metastatic therapeutic target. We also show here that overexpression of PAK4 harboring a somatic mutation, E329K, increased the HGF-driven motility of metastatic prostate carcinoma cells. E329 lies within the G-loop region of the kinase. Our data suggest E329K mutation leads to a modest increase in kinase activity conferring resistance to competitive ATP inhibitors in addition to promoting cell migration. The existence of such a mutation may have implications for the development of PAK4-specific competitive ATP inhibitors should PAK4 be further explored for clinical inhibition.

Microtubules regulate GEF-H1 in response to extracellular matrix stiffness

Breast epithelial cells sense the stiffness of the extracellular matrix through Rho-mediated contractility. In turn, matrix stiffness regulates RhoA activity. However, the upstream signaling mechanisms are poorly defined. Here we demonstrate that the Rho exchange factor GEF-H1 mediates RhoA activation in response to extracellular matrix stiffness. We demonstrate the novel finding that microtubule stability is diminished by a stiff three-dimensional (3D) extracellular matrix, which leads to the activation of GEF-H1. Surprisingly, activation of the mitogen-activated protein kinase kinase/extracellular signal-regulated kinase pathway did not contribute to stiffness-induced GEF-H1 activation. Loss of GEF-H1 decreases cell contraction of and invasion through 3D matrices. These data support a model in which matrix stiffness regulates RhoA through microtubule destabilization and the subsequent release and activation of GEF-H1.

Mechanistic insight into the microtubule and actin cytoskeleton coupling through dynein-dependent RhoGEF inhibition

Actin-based stress fiber formation is coupled to microtubule depolymerization through the local activation of RhoA. While the RhoGEF Lfc has been implicated in this cytoskeleton coupling process, it has remained elusive how Lfc is recruited to microtubules and how microtubule recruitment moderates Lfc activity. Here, we demonstrate that the dynein light chain protein Tctex-1 is required for localization of Lfc to microtubules. Lfc residues 139-161 interact with Tctex-1 at a site distinct from the cleft that binds dynein intermediate chain. An NMR-based GEF assay revealed that interaction with Tctex-1 represses Lfc nucleotide exchange activity in an indirect manner that requires both polymerized microtubules and phosphorylation of S885 by PKA. We show that inhibition of Lfc by Tctex-1 is dynein dependent. These studies demonstrate a pivotal role of Tctex-1 as a negative regulator of actin filament organization through its control of Lfc in the crosstalk between microtubule and actin cytoskeletons.

Rac signaling in breast cancer: a tale of GEFs and GAPs

Rac GTPases, small G-proteins widely implicated in tumorigenesis and metastasis, transduce signals from tyrosine-kinase, G-protein-coupled receptors (GPCRs), and integrins, and control a number of essential cellular functions including motility, adhesion, and proliferation. Deregulation of Rac signaling in cancer is generally a consequence of enhanced upstream inputs from tyrosine-kinase receptors, PI3K or Guanine nucleotide Exchange Factors (GEFs), or reduced Rac inactivation by GTPase Activating Proteins (GAPs). In breast cancer cells Rac1 is a downstream effector of ErbB receptors and mediates migratory responses by ErbB1/EGFR ligands such as EGF or TGFα and ErbB3 ligands such as heregulins. Recent advances in the field led to the identification of the Rac-GEF P-Rex1 as an essential mediator of Rac1 responses in breast cancer cells. P-Rex1 is activated by the PI3K product PIP3 and Gβγ subunits, and integrates signals from ErbB receptors and GPCRs. Most notably, P-Rex1 is highly overexpressed in human luminal breast tumors, particularly those expressing ErbB2 and estrogen receptor (ER). The P-Rex1/Rac signaling pathway may represent an attractive target for breast cancer therapy.

Role for p21-activated kinase PAK4 in development of the mammalian heart

The serine-threonine kinase PAK4 plays a pivotal role in cell proliferation, survival, and control of the cytoskeleton. Mice that lack Pak4 die in midgestation prior to embryonic day E11 from unidentified causes. Analysis of PAK4 protein levels demonstrated that it was highly expressed in the whole embryo and in the developing heart but became low in the hearts of adult mice. In this study we analyzed development of the heart in conventional and conditional Pak4 knockout mice and embryos. We found that in conventional Pak4 knockout mice cardiogenesis is strongly affected from early developmental stages and by E9.5, hearts of Pak4⁻/⁻ embryos developed multiple profound deficits. Conditional deletion of Pak4 in the progenitors of the secondary heart field led to abnormal development of the outflow tract, in which the pulmonary artery had a smaller diameter, and the aortal wall was thinner than in wildtype mice. The conditional knockout mice also displayed the characteristic enlargement of the right ventricles and right atria. Pak4⁻/⁻ embryos and cardiomyocytes in which PAK4 was depleted exhibited low levels of LIMK1, a protein that plays key roles in cytoskeletal organization. Knock down of PAK4 in cultured cardiomyocytes led to severely compromised sarcomeric structure and deficits in contraction. These results indicate that PAK4 functions, including control of actin dynamics, are necessary for normal development of the heart.

P21-activated kinase 4 overexpression in metastatic gastric cancer patients

INTRODUCTION

P21-activated kinase 4 (PAK), a subfamily of serine/threonine kinases originally known as a regulator of cytoskeletal dynamics and cell motility, has recently been revealed to play a key role in oncogenic signaling pathways. We studied the frequency and clinical features of PAK4-overexpressed metastatic gastric cancer.

PATIENTS AND METHODS

PAK4 overexpression was screened by Western blot in 18 human gastric cancer cell lines. Immunohistochemical staining of PAK4 protein was performed in tumor specimens of 49 metastatic gastric cancer patients who received palliative capecitabine/cisplatin as first-line treatment.

RESULTS

PAK4 protein overexpression was detected strongly in five gastric cell lines (AGS, MGK-28, MKN-74, SNU-216, SNU-601) and weakly in four cell lines (KATOIII, MKN-1, SNU-620, and SNU-719). PAK4 knockdown by small interfering RNA induced apoptosis in PAK4-overexpressed AGS gastric cancer cells. Immunohistochemical staining revealed PAK4 overexpressions in 4 (8.1%) of 49 metastatic gastric cancer specimens. None of the four patients with PAK4(+) responded to capecitabine/cisplatin chemotherapy, and PAK4(+) gastric cancer patients had a trend of poorer survival compared with PAK(-)(P = .876).

CONCLUSIONS

We demonstrated PAK4 overexpression in a subset of gastric cancer patients, implicating a role in gastric cancer tumorigenesis. Its prognostic significance and efficacy as a drug target should be further studied.

Molecular mechanisms controlling vascular lumen formation in three-dimensional extracellular matrices

Considerable progress has been made toward a molecular understanding of how cells form lumen and tube structures in three-dimensional (3D) extracellular matrices (ECM). This progress has occurred through work performed with endothelial and epithelial cell models using both in vitro and in vivo approaches. Despite the apparent similarities between endothelial and epithelial cell lumen and tube formation mechanisms, there are clear distinctions that directly relate to their functional differences. This review will focus on endothelial cell (EC) lumen formation mechanisms which control blood vessel formation during development and postnatal life. Of great interest is that an EC lumen signaling complex has been identified which controls human EC lumen and tube formation in 3D matrices and which coordinates integrin-ECM contacts, cell surface proteolysis, cytoskeletal rearrangements, and cell polarity. This complex consists of the collagen-binding integrin α2β1, the collagen-degrading membrane-type 1 matrix metalloproteinase (MT1-MMP), junction adhesion molecule (Jam)C, JamB, polarity proteins Par3 and Par6b, and the Rho GTPase Cdc42-GTP. These interacting proteins are necessary to stimulate 3D matrix-specific signaling events (including activation of protein kinase cascades that regulate the actin and microtubule cytoskeletons) to control the formation of EC lumens and tube networks. Also, EC lumen formation is directly coupled to the generation of vascular guidance tunnels, enzymatically generated ECM conduits that facilitate EC tube remodeling and maturation. Mural cells such as pericytes are recruited along EC tubes within these tunnel spaces to control ECM remodeling events resulting in vascular basement membrane matrix assembly, a key step in tube maturation and stabilization.

Rho protein crosstalk: another social network?

Many fundamental processes in cell biology are regulated by Rho GTPases, including cell adhesion, migration and differentiation. While regulating cellular functions, members of the Rho protein family cooperate or antagonize each other. The resulting molecular network exhibits many levels of interaction dynamically regulated in time and space. In the first part of this review we describe the main mechanisms of this crosstalk, which can occur at three different levels of the pathway: (i) through regulation of activity, (ii) through regulation of protein expression and stability, and (iii) through regulation of downstream signaling pathways. In the second part we illustrate the importance of Rho protein crosstalk with two examples: integrin-based adhesion and cell migration.

Calpain-6, a microtubule-stabilizing protein, regulates Rac1 activity and cell motility through interaction with GEF-H1

Crosstalk between microtubules and actin filaments is crucial for various cellular functions, including cell migration, spreading and cytokinesis. The Rac1 GTPase plays a key role in such crosstalk at the leading edge of migrating cells in order to promote lamellipodial formation. However, the mechanism underlying the link between microtubules and Rac1 activation remains unclear. Here, we show that calpain-6 (CAPN6), a non-proteolytic calpain with microtubule-binding and -stabilizing activity, might participate in this crosstalk. Small interfering RNA (siRNA)-induced knockdown of Capn6 in NIH 3T3 cells resulted in Rac1 activation, which promoted cell migration, spreading and lamellipodial protrusion. This increase in Rac1 activity was abolished by knockdown of the Rho guanine nucleotide exchange factor GEF-H1 (officially known as Arhgef2). CAPN6 and GEF-H1 colocalized with microtubules and also interacted with each other through specific domains. Upon knockdown of Capn6, GEF-H1 was shown to translocate from microtubules to the lamellipodial region and to interact with Rac1. By contrast, RhoA activity was decreased upon knockdown of Capn6, although low levels of active RhoA or the presence of RhoA molecules appeared to be required for the Capn6-knockdown-induced Rac1 activation. We suggest that CAPN6 acts as a potential regulator of Rac1 activity, through a mechanism involving interaction with GEF-H1, to control lamellipodial formation and cell motility.

Dynamic regulation of GEF-H1 localization at microtubules by Par1b/MARK2

Par1b/MARK2 is a serine/threonine kinase that plays key roles in the development of cell polarity, but its precise mechanism of action remains unknown. Here we report that GEF-H1, a guanine nucleotide exchange factor for Rho-family small GTPases, is a novel substrate for Par1b. GEF-H1 directly associates with microtubules via its N-terminal C1 domain, which is known to regulate the activity of GEF-H1. Ectopically expressed GEF-H1 markedly promotes stabilization of microtubules, resulting in acetylation of microtubules. We find that Par1b phosphorylates GEF-H1 at three serine residues conserved in vertebrates and releases GEF-H1 from microtubules, which abrogates stabilization and acetylation of microtubules induced by GEF-H1 overexpression. The alanine mutant for the three phosphorylation sites (3SA) of GEF-H1 strongly induces stabilization and acetylation of microtubules, which was resistant to Par1b. Time-lapse imaging analyses reveal that GFP-fused GEF-H1 dynamically moved on microtubules from one protrusion to another, whereas the 3SA mutant was static. These data suggest that Par1b-phosphorylation regulates turnover of GEF-H1 localization by regulating its interaction with microtubules, which may contribute to cell polarization.

Integrin-mediated cell attachment induces a PAK4-dependent feedback loop regulating cell adhesion through modified integrin alpha v beta 5 clustering and turnover

Cell-to-extracellular matrix adhesion is regulated by a multitude of pathways initiated distally to the core cell-matrix adhesion machinery, such as via growth factor signaling. In contrast to these extrinsically sourced pathways, we now identify a regulatory pathway that is intrinsic to the core adhesion machinery, providing an internal regulatory feedback loop to fine tune adhesion levels. This autoinhibitory negative feedback loop is initiated by cell adhesion to vitronectin, leading to PAK4 activation, which in turn limits total cell-vitronectin adhesion strength. Specifically, we show that PAK4 is activated by cell attachment to vitronectin as mediated by PAK4 binding partner integrin αvβ5, and that active PAK4 induces accelerated integrin αvβ5 turnover within adhesion complexes. Accelerated integrin turnover is associated with additional PAK4-mediated effects, including inhibited integrin αvβ5 clustering, reduced integrin to F-actin connectivity and perturbed adhesion complex maturation. These specific outcomes are ultimately associated with reduced cell adhesion strength and increased cell motility. We thus demonstrate a novel mechanism deployed by cells to tune cell adhesion levels through the autoinhibitory regulation of integrin adhesion.

Heparin inhibits pulmonary artery smooth muscle cell proliferation through guanine nucleotide exchange factor-H1/RhoA/Rho kinase/p27

Ras homolog gene family member A (RhoA) through Rho kinase kinase (ROCK), one of its downstream effectors, regulates a wide range of cell physiological functions, including vascular smooth muscle cell (SMC) proliferation, by degrading cyclin-dependent kinase inhibitor, p27. Our previous studies found that heparin inhibition of pulmonary artery SMC (PASMC) proliferation and pulmonary hypertension was dependent on p27 up-regulation. To investigate whether ROCK, a regulator of p27, is involved in regulation of heparin inhibition of PASMC proliferation, we analyzed ROCK expression in the lungs from mice and from human PASMCs exposed to hypoxia, and investigated the effect of ROCK expression in vitro by RhoA cDNA transfection. We also investigated the effect of guanine nucleotide exchange factor (GEF)-H1, an upstream regulator of RhoA, on heparin inhibition of PASMC proliferation by GEF-H1 cDNA transfection. We found that: (1) hypoxia increased ROCK expression in mice and PASMCs; (2) overexpression of RhoA diminished the inhibitory effect of heparin on PASMC proliferation and down-regulated p27 expression; and (3) overexpression of GEF-H1 negated heparin inhibition of PASMC proliferation, which was accompanied by increased GTP-RhoA and decreased p27. This study demonstrates that the RhoA/ROCK pathway plays an important role in heparin inhibition on PASMC proliferation, and reveals that heparin inhibits PASMC proliferation through GEF-H1/RhoA/ROCK/p27 signaling pathway, by down-regulating GEF-H1, RhoA, and ROCK, and then up-regulating p27.

PAK as a therapeutic target in gastric cancer

IMPORTANCE OF THE FIELD

Gastric cancer is one of the most common causes of cancer death worldwide. P21-activated kinases (PAKs), regulators of cancer-cell signalling networks, play fundamental roles in a range of cellular processes through their binding partners or kinase substrates.

AREAS COVERED IN THIS REVIEW

The complex regulation of PAKs through their upstream or downstream effectors in human cancers, especially in gastric cancer, are described and the identified inhibitors of PAKs are summarized.

WHAT THE READERS WILL GAIN

The structural differences and activation mechanisms between two subgroups of PAK are described. Both groups of PAKs play complicated and important roles in human gastric cancer, which indicated a possible way for us to identify the specific inhibitors targeting PAKs for gastric cancer.

TAKE HOME MESSAGE

PAKs play important roles in progression of many cancer types, the full mechanisms of PAKs in gastric cancer are still unclear. It seems there are different roles for two groups of PAKs in cancers. Group I PAKs play their functions mostly through their specific substrates, however, many binding partners that are independent of phosphorylation by group II PAKs were identified. Finding specific inhibitors of PAKs will help us discover the roles of PAKs and target these kinases in human gastric cancer.