Roles of PAK family kinases

Mitogen-activated protein kinase (MAPK) cascades-A yeast perspective

Discovery of the class of protein kinase now dubbed a mitogen (or messenger)-activated protein kinase (MAPK) is an illustrative example of how disparate lines of investigation can converge and reveal an enzyme family universally conserved among eukaryotes, from single-celled microbes to humans. Moreover, elucidation of the circuitry controlling MAPK function defined a now overarching principle in enzyme regulation-the concept of an activation cascade mediated by sequential phosphorylation events. Particularly ground-breaking for this field of exploration were the contributions of genetic approaches conducted using several model organisms, but especially the budding yeast Saccharomyces cerevisiae. Notably, examination of how haploid yeast cells respond to their secreted peptide mating pheromones was crucial in pinpointing genes encoding MAPKs and their upstream activators. Fully contemporaneous biochemical analysis of the activities elicited upon stimulation of mammalian cells by insulin and other growth- and differentiation-inducing factors lead eventually to the demonstration that components homologous to those in yeast were involved. Continued studies of these pathways in yeast were integral to other foundational discoveries in MAPK signaling, including the roles of tethering, scaffolding and docking interactions.

Restored autophagy is protective against PAK3-induced cardiac dysfunction

Despite the development of clinical treatments, heart failure remains the leading cause of mortality. We observed that p21-activated kinase 3 (PAK3) was augmented in failing human and mouse hearts. Furthermore, mice with cardiac-specific PAK3 overexpression exhibited exacerbated pathological remodeling and deteriorated cardiac function. Myocardium with PAK3 overexpression displayed hypertrophic growth, excessive fibrosis, and aggravated apoptosis following isoprenaline stimulation as early as two days. Mechanistically, using cultured cardiomyocytes and human-relevant samples under distinct stimulations, we, for the first time, demonstrated that PAK3 acts as a suppressor of autophagy through hyper-activation of the mechanistic target of rapamycin complex 1 (mTORC1). Defective autophagy in the myocardium contributes to the progression of heart failure. More importantly, PAK3-provoked cardiac dysfunction was mitigated by administering an autophagic inducer. Our study illustrates a unique role of PAK3 in autophagy regulation and the therapeutic potential of targeting this axis for heart failure.

Targeting PAK4 Inhibits Ras-Mediated Signaling and Multiple Oncogenic Pathways in High-Risk Rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is the most prevalent pediatric soft-tissue sarcoma. Multimodal treatment, including surgery and traditional chemotherapy with radiotherapy, has contributed to improvements in overall survival rates. However, patients with recurrent or metastatic disease have 5-year survival rates of less than 30%. One reason for the lack of therapeutic advancement is identification and targeting of critical signaling nodes. p21-activated kinases (PAK) are a family of serine/threonine kinases downstream of multiple critical tumorigenic receptor tyrosine kinase receptors and oncogenic regulators, including IGFR and RAS signaling, that significantly contribute to aggressive malignant phenotypes. Here, we report that RMS cell lines and tumors exhibit enhanced PAK4 expression levels and activity, which are further activated by growth factors involved in RMS development. Molecular perturbation of PAK4 in multiple RMS models in vitro and in vivo resulted in inhibition of RMS development and progression. Fusion-positive and -negative RMS models were sensitive to two PAK4 small-molecule inhibitors, PF-3758309 and KPT-9274, which elicited significant antitumor and antimetastatic potential in several primary and metastatic in vivo models, including a relapsed RMS patient-derived xenograft model. Transcriptomic analysis of PAK4-targeted tumors revealed inhibition of the RAS-GTPase, Hedgehog, and Notch pathways, along with evidence of activation of antitumor immune response signatures. This PAK4-targeting gene signature showed prognostic significance for patients with sarcoma. Overall, our results show for the first time that PAK4 is a novel and viable therapeutic target for the treatment of high-risk RMS. SIGNIFICANCE: These data demonstrate a novel oncogenic role for PAK4 in rhabdomyosarcoma and show that targeting PAK4 activity is a promising viable therapeutic option for advanced rhabdomyosarcoma.

Methoxetamine Induces Cytotoxicity in H9c2 Cells: Possible Role of p21 Protein (Cdc42/Rac)-Activated Kinase 1

The abuse of new psychoactive substances (NPS) is an emerging social problem. Methoxetamine, one of the NPS, was designed as an alternative to ketamine and it was considered an NPS candidate owing to its high addictive potential. However, cardiotoxicity of the phencyclidine analogue, methoxetamine, has not been extensively evaluated. P21 protein (Cdc42/Rac)-activated kinase 1 (PAK-1) is associated with the drug-induced cardiotoxicity and hypertrophy of cardiomyocytes. In the present study, we investigated the effects of methoxetamine on rat cardiomyocytes and PAK-1. Methoxetamine (at 10 µM) reduced cell viability and PAK-1 mRNA levels in H9c2 cells. Methoxetamine treatment (100 µM) decreased the beating rate of primary cardiomyocytes. However, 100 µM methoxetamine-induced heart rate decline was less than 100 µM PCP- or ketamine-induced heart rate decline. Meanwhile, fingolimod hydrochloride (FTY720, 1 µM), a PAK-1 activator, increased cell viability and inhibited hypertrophy induced by methoxetamine in H9c2 cells. These results suggest that methoxetamine may have harmful effects on the cardiovascular system through the regulation of the expression and function of PAK-1.

PAK4 signaling in health and disease: defining the PAK4-CREB axis

p21-Activated kinase 4 (PAK4), a member of the PAK family, regulates a wide range of cellular functions, including cell adhesion, migration, proliferation, and survival. Dysregulation of its expression and activity thus contributes to the development of diverse pathological conditions. PAK4 plays a pivotal role in cancer progression by accelerating the epithelial–mesenchymal transition, invasion, and metastasis. Therefore, PAK4 is regarded as an attractive therapeutic target in diverse types of cancers, prompting the development of PAK4-specific inhibitors as anticancer drugs; however, these drugs have not yet been successful. PAK4 is essential for embryonic brain development and has a neuroprotective function. A long list of PAK4 effectors has been reported. Recently, the transcription factor CREB has emerged as a novel effector of PAK4. This finding has broad implications for the role of PAK4 in health and disease because CREB-mediated transcriptional reprogramming involves a wide range of genes. In this article, we review the PAK4 signaling pathways involved in prostate cancer, Parkinson’s disease, and melanogenesis, focusing in particular on the PAK4-CREB axis.

An enzyme that regulates an important controller of gene expression may offer a therapeutic target for cancer and other diseases. cAMP response element-binding protein (CREB) interacts with various other proteins to switch a myriad of target genes on and off in different cells. A review by Eung-Gook Kim, Eun-Young Shin and colleagues at Chungbuk National University, Cheongju, South Korea, explores the interplay between CREB and an enzyme called p21-activated kinase 4 (PAK4) in human health and disease. PAK4, for example, has been shown to promote CREB’s gene-activating function in prostate cancer, and PAK4 overexpression is a feature of numerous other tumor types. Disruptions in PAK4-mediated regulation of CREB activity have also been observed in neurons affected by Parkinson’s disease. The authors see strong clinical promise in further exploring the biology of the PAK4-CREB pathway.

Loss of ARHGEF6 Causes Hair Cell Stereocilia Deficits and Hearing Loss in Mice

ARHGEF6 belongs to the family of guanine nucleotide exchange factors (GEFs) for Rho GTPases, and it specifically activates Rho GTPases CDC42 and RAC1. Arhgef6 is the X-linked intellectual disability gene also known as XLID46, and clinical features of patients carrying Arhgef6 mutations include intellectual disability and, in some cases, sensorineural hearing loss. Rho GTPases act as molecular switches in many cellular processes. Their activities are regulated by binding or hydrolysis of GTP, which is facilitated by GEFs and GTPase-activating proteins, respectively. RAC1 and CDC42 have been shown to play important roles in hair cell (HC) stereocilia development. However, the role of ARHGEF6 in inner ear development and hearing function has not yet been investigated. Here, we found that ARHGEF6 is expressed in mouse cochlear HCs, including the HC stereocilia. We established Arhgef6 knockdown mice using the clustered regularly interspaced short palindromic repeat-associated Cas9 nuclease (CRISPR-Cas9) genome editing technique. We showed that ARHGEF6 was indispensable for the maintenance of outer hair cell (OHC) stereocilia, and loss of ARHGEF6 in mice caused HC stereocilia deficits that eventually led to progressive HC loss and hearing loss. However, the loss of ARHGEF6 did not affect the synapse density and did not affect the mechanoelectrical transduction currents in OHCs at postnatal day 3. At the molecular level, the levels of active CDC42 and RAC1 were dramatically decreased in the Arhgef6 knockdown mice, suggesting that ARHGEF6 regulates stereocilia maintenance through RAC1/CDC42.

Knockout of p21-activated kinase-1 attenuates exercise-induced cardiac remodelling through altered calcineurin signalling

AIMS

Despite its known cardiovascular benefits, the intracellular signalling mechanisms underlying physiological cardiac growth remain poorly understood. Therefore, the purpose of this study was to investigate a novel role of p21-activated kinase-1 (Pak1) in the regulation of exercise-induced cardiac hypertrophy.

METHODS AND RESULTS

Wild-type (WT) and Pak1 KO mice were subjected to 6 weeks of treadmill endurance exercise training (ex-training). Cardiac function was assessed via echocardiography, in situ haemodynamics, and the pCa-force relations in skinned fibre preparations at baseline and at the end of the training regimen. Post-translational modifications to the sarcomeric proteins and expression levels of calcium-regulating proteins were also assessed following ex-training. Heart weight/tibia length and echocardiography data revealed that there was marked hypertrophy following ex-training in the WT mice, which was not evident in the KO mice. Additionally, following ex-training, WT mice demonstrated an increase in cardiac contractility, myofilament calcium sensitivity, and phosphorylation of cardiac myosin-binding protein C, cardiac TnT, and tropomyosin compared with KO mice. With ex-training in WT mice, there were also increased protein levels of calcineurin and increased phosphorylation of phospholamban.

CONCLUSIONS

Our data suggest that Pak1 is essential for adaptive physiological cardiac remodelling and support previous evidence that demonstrates Pak1 signalling is important for cardiac growth and survival.

Novel insights into mechanisms for Pak1-mediated regulation of cardiac Ca(2+) homeostasis

A series of recent studies report novel roles for Pak1, a key member of the highly conserved family of serine-threonine protein kinases regulated by Ras-related small G-proteins, Cdc42/Rac1, in cardiac physiology and cardioprotection. Previous studies had identified Pak1 in the regulation of hypertrophic remodeling that could potentially lead to heart failure. This article provides a review of more recent findings on the roles of Pak1 in cardiac Ca(2+) homeostasis. These findings identified crucial roles for Pak1 in cardiomyocyte Ca(2+) handling and demonstrated that it functions through unique mechanisms involving regulation of the post-transcriptional activity of key Ca(2+)-handling proteins, including the expression of Ca(2+)-ATPase SERCA2a, along with the speculative possibility of an involvement in the maintenance of transverse (T)-tubular structure. They highlight important regulatory functions of Pak1 in Ca(2+) homeostasis in cardiac cells, and identify novel potential therapeutic strategies directed at manipulation of Pak1 signaling for the management of cardiac disease, particularly heart failure.

PAK1 is a novel cardiac protective signaling molecule

We review here the novel cardiac protective effects of the multifunctional enzyme, p21-activated kinase 1 (PAK1), a member of a serine/threonine protein kinase family. Despite the large body of evidence from studies in noncardiac tissue indicating that PAK1 activity is key in the regulation of a number of cellular functions, the role of PAK1 in the heart has only been revealed over the past few years. In this review, we assemble an overview of the recent findings on PAK1 signaling in the heart, particularly its cardiac protective effects. We present a model for PAK1 signaling that provides a mechanism for specifically affecting cardiac cellular processes in which regulation of protein phosphorylation states by protein phosphatase 2A (PP2A) predominates.We discuss the anti-adrenergic and antihypertrophic cardiac protective effects of PAK1, as well as its role in maintaining ventricular Ca(2+) homeostasis and electrophysiological stability under physiological, β-adrenergic and hypertrophic stress conditions.

Cdc42 regulates Schwann cell radial sorting and myelin sheath folding through NF2/merlin-dependent and independent signaling

The Rho family GTPase Cdc42 has been implicated in developmental Schwann cell (SC) proliferation, providing sufficient SCs for radial sorting of axons preceding SC differentiation in the peripheral nervous system. We generated Cdc42 conditional knockout (Cdc42-CKO) mice and confirmed aberrant axon sorting in Cdc42-CKO nerves. In adult Cdc42-CKO nerves, blood vessels were enlarged, and mature Remak bundles containing small axons were absent. Abnormal infoldings and outfoldings of myelin sheaths developed in Cdc42-CKO nerves, mimicking pathological features of Charcot-Marie-Tooth (CMT) disease. The NF2/merlin tumor suppressor has been implicated up- and down-stream of Cdc42. In Cdc42-CKO;NF2-del double mutant mice, radial sorting defects seen in Cdc42-CKO nerves were rescued, while changes in myelin sheaths in Cdc42-CKO nerves were not. Phosphorylation of Focal adhesion kinase (FAK) and P-GSK3β, as well as expression of β-catenin were decreased in Cdc42-CKO nerves, and these changes were rescued by NF2/merlin mutation in Cdc42-CKO;NF2-del double mutant mice. Thus, Cdc42 regulates SC radial sorting in vivo through NF2/merlin dependent signaling pathways, while Cdc42 modulation of myelin sheath folding is NF2/merlin independent.

Glial ensheathment of peripheral axons in Drosophila

The ensheathment of neurons and their axons creates an ion-sensitive microenvironment that allows rapid conduction of nerve impulses. One of the fundamental questions about axonal ensheathment is how insulating glial cells wrap around axons. The mechanisms that underlie insulation of axons in invertebrates and vertebrates are not fully understood. In the present article we address cellular aspects of axonal ensheathment in Drosophila by taking advantage of glial mutants that illustrate a range of phenotypic defects including ensheathment of axons. From the findings of these mutant studies, we summarize that loss of glial cells, defects in glial membrane wrapping, failure of glial migration, and loss of specialized ladderlike septate junctions between ensheathing glial membranes result in axon-glial functional defects. These studies provide a broad perspective on glial ensheathment of axons in Drosophila and key insights into the anatomical and cellular aspects of axonal insulation. Given the powerful genetic approaches available in Drosophila, the axonal ensheathment process can be dissected in great detail to reveal the fundamental principles of ensheathment. These observations will be relevant to understanding the very similar processes in vertebrates, where defects in glial cell functions lead to devastating neurological diseases.

PAK is regulated by PI3K, PIX, CDC42, and PP2Calpha and mediates focal adhesion turnover in the hyperosmotic stress-induced p38 pathway

Fractionation of brain extracts and functional biochemical assays identified PP2Calpha, a serine/threonine phosphatase, as the major biochemical activity inhibiting PAK1. PP2Calpha dephosphorylated PAK1 and p38, both of which were activated upon hyperosmotic shock with the same kinetics. In comparison to growth factors, hyperosmolality was a more potent activator of PAK1. Therefore we characterize the PAK signaling pathway in the hyperosmotic shock response. Endogenous PAKs were recruited to the p38 kinase complex in a phosphorylation-dependent manner. Overexpression of a PAK inhibitory peptide or dominant negative Cdc42 revealed that p38 activation was dependent on PAK and Cdc42 activities. PAK mutants deficient in binding to Cdc42 or PAK-interacting exchange factor were not activated. Using a panel of kinase inhibitors, we identified PI3K acting upstream of PAK, which correlated with PAK repression by pTEN overexpression. RNA interference knockdown of PAK expression reduced stress-induced p38 activation and conversely, PP2Calpha knockdown increased its activation. Hyperosmotic stress-induced PAK translocation away from focal adhesions to the perinuclear compartment and resulted in disassembly of focal adhesions, which are hallmarks of PAK activation. Inhibition of PAK by overexpression of PP2Calpha or the kinase inhibitory domain prevented sorbitol-induced focal adhesion dissolution. Inhibition of MAPK pathways showed that MEK-ERK signaling but not p38 is required for full PAK activation and focal adhesion turnover. We conclude that 1) PAK plays a required role in hyperosmotic signaling through the PI3K/pTEN/Cdc42/PP2Calpha/p38 pathway, and 2) PAK and PP2Calpha modulate the effects of this pathway on focal adhesion dynamics.

p21-Activated kinase 1 coordinates aberrant cell survival and pericellular proteolysis in a three-dimensional culture model for premalignant progression of human breast cancer

Overexpression of p21-activated kinase 1 (PAK1) occurs during the progression of human breast cancer. We have investigated the role of PAK1 in the premalignant progression of the MCF10 series of human breast epithelial cell lines. Levels of PAK1 expression and activation increased with premalignant progression, and expression of dominant-negative (DN) PAK1 reduced both cell proliferation and migration/invasion. In three-dimensional (3D) overlay cultures in reconstituted basement membrane, the MCF10 series produced an in vitro model for premalignant progression. MCF10AneoT cells formed a hyperplastic morphology in which some spheroids developed abnormal lumens. The MCF10.AT1 line exhibited an atypical hyperplastic morphology of abnormal spheroid clusters that did not form lumens. The MCF10.DCIS cells exhibited dysplastic growth. Expression of DN-PAK1 promoted lumen formation in 3D-cultured MCF10A, NeoT, and AT1 structures, suggesting partial reversion of the premalignant phenotype, but did not affect the atypical budding of AT1 structures or the dysplastic growth of ductal carcinoma in situ structures. Aberrant proteolysis is another important characteristic of breast cancer progression and invasion. DN-PAK1 or knock-down of PAK1 reduced pericellular proteolysis of DQ-collagen IV in the 3D cultures. Treatment of cells with an inhibitor of Rac1 also reduced pericellular proteolysis, and this reduction was reversed by the expression of activated PAK1. Our conclusion is that overexpressed and activated PAK1 may be a key coordinator of aberrant cell survival and proteolysis in breast cancer progression.

p21-Activated kinase-1 and its role in integrated regulation of cardiac contractility

We review here a novel concept in the regulation of cardiac contractility involving variations in the activity of the multifunctional enzyme, p21-activated kinase 1 (Pak1), a member of a family of proteins in the small G protein-signaling pathway that is activated by Cdc42 and Rac1. There is a large body of evidence from studies in noncardiac tissue that Pak1 activity is key in regulation of a number of cellular functions, including cytoskeletal dynamics, cell motility, growth, and proliferation. Although of significant potential impact, the role of Pak1 in regulation of the heart has been investigated in only a few laboratories. In this review, we discuss the structure of Pak1 and its sites of posttranslational modification and molecular interactions. We assemble an overview of the current data on Pak1 signaling in noncardiac tissues relative to similar signaling pathways in the heart, and we identify potential roles of Pak1 in cardiac regulation. Finally, we discuss the current state of Pak1 research in the heart in regard to regulation of contractility through functional myofilament and Ca(2+)-flux modification. An important aspect of this regulation is the modulation of kinase and phosphatase activity. We have focused on Pak1 regulation of protein phosphatase 2A (PP2A), which is abundant in cardiac muscle, thereby mediating dephosphorylation of sarcomeric proteins and sensitizing the myofilaments to Ca(2+). We present a model for Pak1 signaling that provides a mechanism for specifically affecting cardiac cellular processes in which regulation of protein phosphorylation states by PP2A dephosphorylation predominates.

Activation of myosin in HeLa cells causes redistribution of focal adhesions and F-actin from cell center to cell periphery

Activation of actomyosin II by phosphorylation of its regulatory light chain is one of the main factors involved in the regulation of cytoskeletal dynamics. Phosphorylation of myosin regulatory light chain may be mediated directly and indirectly by several kinases including myosin light chain kinase (MLCK) and kinases activated by small GTP-binding proteins. Most of the myosin kinases, including PAK, can also interact with other proteins through binding sites located outside of their catalytic domains. In an attempt to study the effects due only to phosphorylation of myosin light chain, we expressed the constitutively active catalytic domain of ameba PAK in HeLa cells. The catalytic domain phosphorylates myosin light chain in vitro with high specific activity but has none of the sequences that target mammalian PAK to other proteins and membranes. Expression of the catalytic domain caused disassembly of focal adhesions and stress fibers in the cell center and accumulation of focal adhesions and F-actin at the cell periphery. There was a twofold increase in the phosphorylation level of endogenous myosin light chain and changes in cell shape consistent with enhanced cell contractility. The phenotype was independent of MLCK, ROCK, MEK, Rac, and Rho activities but was abolished by blebbistatin, a specific inhibitor of myosin II activity. Our data are consistent with myosin being directly phosphorylated by the expressed catalytic domain of ameba PAK with the induced phenotype resulting from cell retraction driven by contraction of peripheral actomyosin. The phenotype induced by expression of the catalytic domain is reminiscent of that caused by expression of active mammalian PAK, suggesting that myosin phosphorylation may play an important role in PAK-induced cytoskeletal changes. The catalytic domain of ameba PAK may be a useful tool for studying the effects of myosin light chain phosphorylation in other cells.

The GIT-associated kinase PAK targets to the centrosome and regulates Aurora-A

Previously, we showed PAK-PIX-GIT targets and regulates focal adhesions; here, we uncover a different function for the complex at the centrosome. Active PAK1 is particularly evident in mitosis and phosphorylates the centrosomal adaptor GIT1 on serine 517. Interestingly, direct centrosome targeting activates the kinase via a process not requiring Rho GTPases; excision of the centrosome prevents this activation. Once activated, PAK1 dissociates from PIX/GIT but can bind to and phosphorylate the important centrosomal kinase Aurora-A. PAK1 promotes phosphorylation of Aurora-A on Thr288 and Ser342, which are key sites for kinase activation in mitosis. In vivo PAK activation causes an accumulation of activated Aurora-A; conversely, when betaPIX is depleted or PAK is inhibited, there is a delay in centrosome maturation. These observations may underlie reported effects of active PAK on cells, including histone H3 phosphorylation, alterations in centrosome number, and progression through mitosis.

Src and FAK kinases cooperate to phosphorylate paxillin kinase linker, stimulate its focal adhesion localization, and regulate cell spreading and protrusiveness

The ArfGAP paxillin kinase linker (PKL)/G protein-coupled receptor kinase-interacting protein (GIT)2 has been implicated in regulating cell spreading and motility through its transient recruitment of the p21-activated kinase (PAK) to focal adhesions. The Nck-PAK-PIX-PKL protein complex is recruited to focal adhesions by paxillin upon integrin engagement and Rac activation. In this report, we identify tyrosine-phosphorylated PKL as a protein that associates with the SH3-SH2 adaptor Nck, in a Src-dependent manner, after cell adhesion to fibronectin. Both cell adhesion and Rac activation stimulated PKL tyrosine phosphorylation. PKL is phosphorylated on tyrosine residues 286/392/592 by Src and/or FAK and these sites are required for PKL localization to focal adhesions and for paxillin binding. The absence of either FAK or Src-family kinases prevents PKL phosphorylation and suppresses localization of PKL but not GIT1 to focal adhesions after Rac activation. Expression of an activated FAK mutant in the absence of Src-family kinases partially restores PKL localization, suggesting that Src activation of FAK is required for PKL phosphorylation and localization. Overexpression of the nonphosphorylated GFP-PKL Triple YF mutant stimulates cell spreading and protrusiveness, similar to overexpression of a paxillin mutant that does not bind PKL, suggesting that failure to recruit PKL to focal adhesions interferes with normal cell spreading and motility.

Tumor cell invasion and survival in head and neck cancer

Squamous cell carcinoma (SCC) is the primary tumor type in head and neck cancer. Typically, these tumor cells show persistent invasion that frequently leads to local recurrence and distant lymphatic metastasis. The process of invasion involves concurrent infiltration and destruction of adjacent tissues. As with normal mucosal epithelium, SCC cells express receptors that mediate cell-extracellular matrix (ECM) adhesion (integrins) and cell-cell adhesion (cadherins). Both receptor families represent important signaling devices that are capable of promoting survival and proliferation. Recent results indicate that integrins and cadherins cooperate to regulate invasive behavior. During SCC invasion, cells actively migrate through the surrounding ECM with the simultaneous remodeling of their intercellular adhesions. During invasion, integrin receptor engagement with specific ECM ligands along with concurrent remodeling of cadherin adhesions induces changes in the cytoskeleton though modulation of the activities of Rho family members. Tumor development and progression of SCC proceeds with the generation of variant cells with potential alterations in expression of adhesion receptors, and their associated signaling pathways lead to a highly invasive and metastatic phenotype. Understanding the molecular events that define this subset of invasive cells will facilitate the development of new treatment strategies.

Cell surface receptors activate p21-activated kinase 1 via multiple Ras and PI3-kinase-dependent pathways

p21-activated kinases (PAKs) were the first identified mammalian members of a growing family of Ste20-like serine-threonine protein kinases. In this study, we show that PAK1 can be stimulated by carbachol, lysophosphatidic acid (LPA), epidermal growth factor (EGF), and phorbol 12-myristate 13-acetate (PMA) by multiple independent and overlapping pathways. Dominant-negative Ras, Rac, and Cdc42 inhibited PAK1 activation by all of these agonists, while active Rac1 and Cdc42 were sufficient to maximally activate PAK1 in the absence of any treatment. Active Ras induced only a weak activation of PAK1 that could be potentiated by muscarinic receptor stimulation. Studies using inhibitors of the EGF receptor tyrosine kinase, phosphatidylinositol 3-kinase (PI3-kinase) and protein kinase C (PKC) revealed that all of the cell surface agonists could activate PAK1 through pathways independent of PKC, that EGF stimulated a PI3-kinase dependent pathway to stimulate PAK1, and that muscarinic receptor stimulation of PAK1 was predominantly mediated through this EGF-R-dependent mechanism. Activation of PAK1 by LPA was independent of PI3-kinase and the EGF receptor, but was inhibited by dominant-negative RhoA. These results identify multiple Ras-dependent pathways to activation of PAK1.

Phosphorylation of Mnk1 by caspase-activated Pak2/gamma-PAK inhibits phosphorylation and interaction of eIF4G with Mnk

The mitogen-activated protein kinase-interacting kinase 1 (Mnk1) is phosphorylated by caspase-cleaved protein kinase Pak2/gamma-PAK but not by Cdc42-activated Pak2. Phosphorylation of Mnk1 is rapid, reaching 1 mol/mol within 15 min of incubation with Pak2. A kinetic analysis of the phosphorylation of Mnk1 by Pak2 yields a K(m) of 0.6 microm and a V(max) of 14.9 pmol of (32)P/min/microg of Pak2. Two-dimensional tryptic phosphopeptide mapping of Mnk1 phosphorylated by Pak2 yields two distinct phosphopeptides. Analysis of the phosphopeptides by automated microsequencing and manual Edman degradation identified the sites in Mnk1 as Thr(22) and Ser(27). Mnk1, activated by phosphorylation with Erk2, phosphorylates the eukaryotic initiation factor (eIF) 4E and the eIF4G components of eIF4F. Phosphorylation of Mnk1 by Pak2 does not activate Mnk1, as measured with either eIF4E or eIF4F as substrate. Phosphorylation of Erk2-activated Mnk1 by Pak2 has no effect on phosphorylation of eIF4E but reduces phosphorylation of eIF4G by Mnk1 by up to 50%. Phosphorylation of Mnk1 by Pak2 inhibits binding of eIF4G peptides containing the Mnk1 binding site by up to 80%. When 293T cells are subjected to apoptotic induction by hydrogen peroxide, Mnk1 is phosphorylated at both Thr(22) and Ser(27). These results indicate a role for Pak2 in the down-regulation of translation initiation in apoptosis by phosphorylation of Mnk1.

The Down syndrome cell adhesion molecule (DSCAM) interacts with and activates Pak

The Down syndrome cell adhesion molecule (DSCAM) is a member of the immunoglobulin superfamily that maps to a Down syndrome region of chromosome 21q22.2-22.3. In Drosophila, Dscam functions as an axon guidance receptor regulating targeting and branching. Genetic and biochemical studies have shown that in Drosophila, Dscam activates Pak1 via the Dock adaptor molecule. The extracellular domain of human DSCAM is highly homologous to the Drosophila protein; however, the intracellular domains of both human and Drosophila DSCAM share no obvious sequence identity. To study the signaling mechanisms of human DSCAM, we investigated the interaction between DSCAM and potential downstream molecules. We found that DSCAM directly binds to Pak1 and stimulates Pak1 phosphorylation and activity, unlike Drosophila where an adaptor protein Dock mediates the interaction between Dscam and Pak1. We also observed that DSCAM activates both JNK and p38 MAP kinases. Furthermore, expression of the cytoplasmic domain of DSCAM induces a morphological change in cultured cells that is JNK-dependent. These observations suggest that human DSCAM also signals through Pak1 and may function in axon guidance similar to the Drosophila Dscam.

Intracellular localization and functional effects of P21-activated kinase-1 (Pak1) in cardiac myocytes

We investigated intracellular localization and substrate specificity of P21-activated kinase-1 (Pak1) in rat cardiac myocytes. Pak1 is a serine/threonine protein kinase that is activated by Rac1/Cdc42 and important in signaling of stress responses. Yet the localization and in vivo function of Pak1 in heart cells is poorly understood. Studies reported here indicate that Pak1 physically interacts with protein phosphatase 2a and localizes to the Z-disk, cell membrane, intercalated disc, and nuclear membrane of adult rat heart myocytes. We compared levels of phosphorylation of cardiac troponin I (cTnI) in control myocytes with phosphorylation of cTnI and myosin binding protein C (C-protein) in myocytes with increased Pak1 activity. The increase in activity was induced by infection of myocytes with a recombinant adenovirus (AdPak1) containing cDNA for a constitutively active Pak1. Control cells were infected with a virus (AdLacZ) containing LacZ. Basal levels of phosphorylation of cTnI and C-protein were relatively high in the myocytes infected with AdLacZ. However, phosphorylation of cTnI and C-protein in cells expressing constitutively active Pak1 was significantly reduced compared with those expressing LacZ. Measurement of Ca2+ tension relations in single myocytes demonstrated that this reduction in phosphorylation of cTnI and C-protein was associated with the predicted increase in sensitivity to Ca2+. Our data provide evidence for a novel pathway of phosphatase regulation in cardiac myocytes.

Gβγ subunits stimulate p21-activated kinase 1 (PAK1) through activation of PI3-kinase and Akt but act independently of Rac1/Cdc42

The p21-activated kinase (PAK) family is homologous to the yeast sterile 20 (Ste20) and regulates a wide variety of cellular responses, including cell morphology, proliferation, and survival. In this study we examined the activation of PAK1 by Gbetagamma subunits. Co-transfection of COS7 cells with Gbeta1gamma2 or Gbeta1gamma5 was sufficient to induce agonist-independent activation of PAK1. Expression of dominant/negative Rac, Cdc42, or Ras did not inhibit this Gbetagamma-dependent activation. Wortmannin, which inhibits phosphoinositide 3-kinase (PI3-kinase) activity, and expression of a dominant/negative form of Akt were sufficient to abrogate the activation of PAK1 that was induced by Gbetagamma. These results reveal that stimulation of PAK1 by Gbetagamma can occur via a PI3-kinase and Akt pathway that does not require Rac1 or Cdc42.

Mechanisms in LPA-induced tumor cell migration: critical role of phosphorylated ERK

Lysophosphatidic acid (LPA) is a serum-borne phospholipid with hormone and growth factor-like properties. LPA has been shown to modulate tumor cell invasion and malignant cell growth. Here, we report that two human pancreatic carcinoma cell lines, PANC-1 and BxPC-3, express functionally active LPA receptors coupled to pertussis toxin-sensitive Gi/o-proteins. In contrast to other cell types, LPA does not act as a mitogen, but is an efficacious stimulator of cell migration of these tumor cells. LPA-induced chemotaxis is markedly dependent on activation of PTX-sensitive heterotrimeric G-proteins, on activation of the small GTPases Ras, Rac and RhoA, and on GTPase-dependent activation of ERK. LPA-induced ERK activation results in a transient translocation of the phosphorylated ERK to newly forming focal contact sites at the leading edge of the migrating cells. Inhibition of ERK activation and its subsequent translocation impaired LPA-induced chemotaxis and LPA-induced actin reorganization. Thus, pancreatic tumor cell migration in response to LPA is essentially controlled by activation of a Gi/o-ERK pathway and requires the LPA-induced activation of Ras, Rac1 and RhoA.

Pak1 and its T212 phosphorylated form accumulate in neurones and epithelial cells of the developing rodent

The serine/threonine kinase Pak1 is a target of the RhoGTPases Rac and Cdc42 and an important regulator of cell morphology and migration. Recent work from several laboratories has indicated that Pak1 controls microtubule dynamics as well as the organisation of F-actin microfilaments. Pak1 is phosphorylated on T212 by the p35/Cdk5 or cyclin B1/Cdc2 kinase in postmitotic neurones and mitotic cells, respectively. To understand its function during development, we have carried out a detailed temporal and spatial analysis of Pak1 expression and phosphorylation on T212. In the embryonic forebrain, Pak1 and Pak1T212(PO4) were seen to accumulate in the corpus callosum, intermediate zone, lateral olfactory tracts, and anterior commissures. Epithelial cells of the mouse embryo lung, kidney, intestine, and skin also exhibited high levels of Pak1 and Pak1T212(PO4), suggesting a previously unsuspected role in epithelial differentiation. Pak1T212(PO4) was undetectable in all adult tissues. Together, these data indicate a specific, developmentally regulated role of the Pak1 kinase.

Localization of p21-activated protein kinase gamma-PAK/Pak2 in the endoplasmic reticulum is required for induction of cytostasis

The intracellular localization and physiological functions of the p21-activated protein kinase gamma-PAK have been examined in human embryonic kidney 293T and COS-7 cells. At 1-4 days post-transfection, cell division is inhibited by the expression of wild type (WT) gamma-PAK and the mutant S490A, whereas cells expressing S490D and the inactive mutants K278R and T402A grow exponentially, indicating a role for gamma-PAK in the induction of cytostasis. WT gamma-PAK and S490A are localized in a region surrounding the nucleus identified as the endoplasmic reticulum (ER), as determined by immunofluorescence, whereas K278R, T402A, and S490D lack localization. As shown by sucrose density gradient centrifugation, WT gamma-PAK, S490A, and endogenous gamma-PAK are distributed among the high density (ER-associated), intermediate density, and low density fractions, whereas the mutants that do not inhibit cell division are present only as soluble enzyme. The amount of endogenous gamma-PAK associated with the particulate fractions is increased 4-fold when cell division is inhibited by ionizing radiation. gamma-PAK in the ER and intermediate density fractions has high specific activity and is active, whereas the soluble form of gamma-PAK has low activity and is activable. The importance of localization of gamma-PAK is supported by data with the C-terminal mutants S490D and Delta 488; these mutants have high levels of protein kinase activity but do not induce cytostasis and are not bound to the ER. A model for the induction of cytostasis by gamma-PAK through targeting of gamma-PAK to the ER is presented in which gamma-PAK activity and Ser-490 are implicated in the regulation of cytostasis.

Biological aspects of signal transduction by cell adhesion receptors

Cell adhesion receptors such as integrins, cadherins, selectins, and immunoglobulin family receptors profoundly modulate many signal transduction cascades. In this review we examine aspects of adhesion receptor signaling and how this impinges on key biological processes. We have chosen to focus on cell migration and on programmed cell death. We examine many of the cytoplasmic signaling molecules that interface with adhesion receptors, including focal adhesion kinase (FAK), phosphatidylinositol-3-kinase (PI3K), and elements of the Erk/MAP kinase pathway. In many cases these molecules impinge on both the regulation of cell movement and on control of apoptosis.

The TRE17 oncogene encodes a component of a novel effector pathway for Rho GTPases Cdc42 and Rac1 and stimulates actin remodeling

The Rho family GTPases Cdc42 and Rac1 play fundamental roles in transformation and actin remodeling. Here, we demonstrate that the TRE17 oncogene encodes a component of a novel effector pathway for these GTPases. TRE17 coprecipitated specifically with the active forms of Cdc42 and Rac1 in vivo. Furthermore, the subcellular localization of TRE17 was dramatically regulated by these GTPases and mitogens. Under serum-starved conditions, TRE17 localized predominantly to filamentous structures within the cell. Epidermal growth factor (EGF) induced relocalization of TRE17 to the plasma membrane in a Cdc42-/Rac1-dependent manner. Coexpression of activated alleles of Cdc42 or Rac1 also caused complete redistribution of TRE17 to the plasma membrane, where it partially colocalized with the GTPases in filopodia and ruffles, respectively. Membrane recruitment of TRE17 by EGF or the GTPases was dependent on actin polymerization. Finally, we found that a C-terminal truncation mutant of TRE17 induced the accumulation of cortical actin, mimicking the effects of activated Cdc42. Together, these results identify TRE17 as part of a novel effector complex for Cdc42 and Rac1, potentially contributing to their effects on actin remodeling. The present study provides insights into the regulation and cellular function of this previously uncharacterized oncogene.

A new constitutively active brain PAK3 isoform displays modified specificities toward Rac and Cdc42 GTPases

p21-activated kinases (PAK) are involved in the control of cytoskeleton dynamics and cell cycle progression. Here we report the characterization of a new mammalian PAK3 mRNA that contains a 45-bp alternatively spliced exon. This exon encodes for 15 amino acids that are inserted in the regulatory domain, inside the autoinhibitory domain but outside the Cdc42 and Rac interactive binding domain. The transcript of the 68-kDa new isoform named PAK3b is expressed in various areas of the adult mouse brain. In contrast to PAK3 without the exon b (PAK3a), whose basal kinase activity is weak in resting cells, PAK3b displays a high kinase activity in starved cells that is not further stimulated by active GTPases. Indeed, we demonstrate that the autoinhibitory domain of PAK3b no longer inhibits the kinase activity of PAK3. Moreover, we show that the 15-amino acid insertion within the autoinhibitory domain impedes the ability of PAK3b to bind to the GTPases Rac and Cdc42 and changes its specificity toward the GTPases. Altogether, our results show that the new PAK3b isoform has unique properties and would signal differently from PAK3a in neurons.

Pak1 phosphorylation on t212 affects microtubules in cells undergoing mitosis

The Pak kinases are targets of the Rho GTPases Rac and Cdc42, which regulate cell shape and motility. It is increasingly apparent that part of this function is due to the effect Pak kinases have on microtubule organization and dynamics. Recently, overexpression of Xenopus Pak5 was shown to enhance microtubule stabilization, and it was shown that mammalian Pak1 may inhibit a microtubule-destabilizing protein, Op18/Stathmin. We have identified a specific phosphorylation site on mammalian Pak1, T212, which is targeted by the neuronal p35/Cdk5 kinase. Pak1 phosphorylated on T212, Pak1T212(PO(4)), is enriched in axonal growth cones and colocalizes with small peripheral bundles of microtubules. Cortical neurons overexpressing a Pak1A212 mutant display a tangled neurite morphology, which suggests that the microtubule cytoskeleton is affected. Here, we show that cyclin B1/Cdc2 phosphorylates Pak1 in cells undergoing mitosis. In the developing cortex and in cultured fibroblasts, Pak1T212(PO(4)) is enriched in microtubule-organizing centers and along parts of the spindles. In living cells, a peptide mimicking phosphorylated T212 accumulates at the centrosomes and spindles and causes an increased length of astral microtubules during metaphase or following nocodazole washout. Together these results suggest that similar signaling pathways regulate microtubule dynamics in a remodeling axonal growth cone and during cell division.

The role of Rho GTPases and associated kinases in regulating neurite outgrowth

Neurones are highly specialised cells that can extend over great distances, enabling the complex networking of the nervous system. We are beginning to understand in detail the molecular mechanisms that control the shape of neurones during development. One family of proteins that are clearly essential are the Rho GTPases which have a pivotal role in regulating the actin cytoskeleton in all cell types. The Rho GTPases are responsible for the activation and downregulation of many downstream kinases. This review discusses individual kinases that are regulated by three members of the Rho GTPases, Rac, Rho and Cdc42 and their function during neurite outgrowth and remodelling.

Paxillin-dependent paxillin kinase linker and p21-activated kinase localization to focal adhesions involves a multistep activation pathway

The precise temporal-spatial regulation of the p21-activated serine-threonine kinase PAK at the plasma membrane is required for proper cytoskeletal reorganization and cell motility. However, the mechanism by which PAK localizes to focal adhesions has not yet been elucidated. Indirect binding of PAK to the focal adhesion protein paxillin via the Arf-GAP protein paxillin kinase linker (PKL) and PIX/Cool suggested a mechanism. In this report, we demonstrate an essential role for a paxillin-PKL interaction in the recruitment of activated PAK to focal adhesions. Similar to PAK, expression of activated Cdc42 and Rac1, but not RhoA, stimulated the translocation of PKL from a generally diffuse localization to focal adhesions. Expression of the PAK regulatory domain (PAK1-329) or the autoinhibitory domain (AID 83-149) induced PKL, PIX, and PAK localization to focal adhesions, indicating a role for PAK scaffold activation. We show PIX, but not NCK, binding to PAK is necessary for efficient focal adhesion localization of PAK and PKL, consistent with a PAK-PIX-PKL linkage. Although PAK activation is required, it is not sufficient for localization. The PKL amino terminus, containing the PIX-binding site, but lacking paxillin-binding subdomain 2 (PBS2), was unable to localize to focal adhesions and also abrogated PAK localization. An identical result was obtained after PKLDeltaPBS2 expression. Finally, neither PAK nor PKL was capable of localizing to focal adhesions in cells overexpressing paxillinDeltaLD4, confirming a requirement for this motif in recruitment of the PAK-PIX-PKL complex to focal adhesions. These results suggest a GTP-Cdc42/GTP-Rac triggered multistep activation cascade leading to the stimulation of the adaptor function of PAK, which through interaction with PIX provokes a functional PKL PBS2-paxillin LD4 association and consequent recruitment to focal adhesions. This mechanism is probably critical for the correct subcellular positioning of PAK, thereby influencing the ability of PAK to coordinate cytoskeletal reorganization associated with changes in cell shape and motility.

The p21-activated kinase, Shk1, is required for proper regulation of microtubule dynamics in the fission yeast, Schizosaccharomyces pombe

The p21-activated kinase, Shk1, is required for the proper establishment of cell polarity in the fission yeast, Schizosaccharomyces pombe. We showed recently that loss of the essential Shk1 inhibitor, Skb15, causes significant spindle defects in fission yeast, thus implicating Shk1 as a potential regulator of microtubule dynamics. Here, we show that cells deficient in Shk1 function have malformed interphase microtubules and mitotic microtubule spindles, are hypersensitive to the microtubule-destabilizing drug thiabendazole (TBZ) and cold sensitive for growth. TBZ treatment causes a downregulation of Shk1 kinase activity, which increases rapidly after release of cells from the drug, thus providing a correlation between Shk1 kinase function and active microtubule polymerization. Consistent with a role for Shk1 as a regulator of microtubule dynamics, green fluorescent protein (GFP)-Shk1 fusion proteins localize to interphase microtubules and mitotic microtubule spindles, as well as to cell ends and septum-forming regions of fission yeast cells. We show that loss of Tea1, a cell end- and microtubule-localized protein previously implicated as a regulator of microtubule dynamics in fission yeast, exacerbates the growth and microtubule defects resulting from partial loss of Shk1 and that Shk1 localizes to illicit growth tips produced by tea1 mutant cells. Our results demonstrate that Shk1 is required for the proper regulation of microtubule dynamics in fission yeast and implicate Tea1 as a potential Shk1 regulator.

p21-activated protein kinase gamma-PAK in pituitary secretory granules phosphorylates prolactin

p21-activated protein kinase gamma-PAK phosphorylates prolactin (PRL) in rat pituitary secretory granules on Ser-177 and on the equivalent site, Ser-179, in recombinant human PRL. This is shown by comparison of phosphopeptide maps with the human PRL mutant S179D. gamma-PAK is present in rat and bovine granules as identified by in-gel phosphorylation of histone H4, and by immunoblotting. Thus, phosphorylation of PRL by gamma-PAK in granules produces the PRL molecule that has been shown to antagonize the growth-promoting activity of unmodified PRL, and is consistent with the identified role of gamma-PAK in the induction and maintenance of cytostasis.

The hematopoiesis-specific GTP-binding protein RhoH is GTPase deficient and modulates activities of other Rho GTPases by an inhibitory function

The Rho subfamily of small GTP-binding proteins mediates many fundamental cellular functions. The commonly studied members (Rho, Rac, and CDC42) regulate actin reorganization, affecting diverse cellular responses, including adhesion, cytokinesis, and motility. Another major function of the Rho GTPases is their role in regulating transcriptional factors and nuclear signaling. RhoH is encoded by a hematopoiesis-specific Rho-related gene recently identified in a fusion transcript with bcl6 in lymphoma cell lines. Significantly, translocations and a high frequency of RhoH mutation have been detected in primary lymphoma cells. We show here that RhoH functions differently from other Rho GTPases. RhoH exerts no significant effect on actin reorganization. However, RhoH is a potent inhibitor of the activation of NFkappaB and p38 by other Rho GTPases. This property, together with the differential expression of RhoH in the Th1 subset of T cells, suggests a role for RhoH in the functional differentiation of T cells. RhoH has different amino acids in two highly conserved residues critical for GTPase activity. Consequently, RhoH is GTPase deficient, remaining in a GTP-bound activated state without cycling. Reduction of RhoH levels in T cells augments the response to Rac activation. Furthermore, RhoH is dramatically down regulated after phorbol myristate acetate treatment and in Th1 cells after activation by anti-CD3. Hence, a mechanism for regulation of RhoH function is likely to exist at the transcriptional level. The inhibitory function of RhoH supports a model in which Rho GTPases with opposing functions may compete to modulate the final outcome of a particular GTPase-activated pathway.

Calmodulin-binding and autoinhibitory domains of Acanthamoeba myosin I heavy chain kinase, a p21-activated kinase (PAK)

The sequence homology between Acanthamoeba myosin I heavy chain kinase (MIHCK) and other p21-activated kinases (PAKs) is relatively low, including only the catalytic domain and a short PAK N-terminal motif (PAN), and even these regions are not highly homologous. In this paper, we report the expression in insect cells of full-length, fully regulated Acanthamoeba MIHCK and further characterize the regulation of this PAK by Rac, calmodulin, and autoinhibition. We map the autoinhibitory region of MIHCK to its PAN region and show that the PAN region inhibits autophosphorylation and kinase activity of unphosphorylated full-length MIHCK and its expressed catalytic domain but has very little effect on either when they are phosphorylated. These properties are similar to those reported for mammalian PAK1. Unlike PAK1, MIHCK is activated by Rac only in the presence of phospholipid. However, peptides containing the PAN region of MIHCK bind Rac in the absence of lipid, and Rac binding reverses the inhibition of the MIHCK catalytic domain by PAN peptides. Our data suggest that a region N-terminal to PAN is required for optimal binding of Rac. Also unlike mammalian PAK, phospholipid stimulation of Acanthamoeba MIHCK and Dictyostelium MIHCK) (which is also a PAK) is inhibited by Ca(2+)-calmodulin. In contrast to Dictyostelium MIHCK, however, Ca(2+)-calmodulin also inhibits Rac-induced activity of Acanthamoeba MIHCK. The basic region N-terminal to PAN is essential for calmodulin binding.

A novel p21-activated kinase binds the actin and microtubule networks and induces microtubule stabilization

Coordination of the different cytoskeleton networks in the cell is of central importance for morphogenesis, organelle transport, and motility. The Rho family proteins are well characterized for their effects on the actin cytoskeleton, but increasing evidence indicates that they may also control microtubule (MT) dynamics. Here, we demonstrate that a novel Cdc42/Rac effector, X-p21-activated kinase (PAK)5, colocalizes and binds to both the actin and MT networks and that its subcellular localization is regulated during cell cycle progression. In transfected cells, X-PAK5 promotes the formation of stabilized MTs that are associated in bundles and interferes with MTs dynamics, slowing both the elongation and shrinkage rates and inducing long paused periods. X-PAK5 subcellular localization is regulated tightly, since coexpression with active Rac or Cdc42 induces its shuttling to actin-rich structures. Thus, X-PAK5 is a novel MT-associated protein that may communicate between the actin and MT networks during cellular responses to environmental conditions.

Cdc42-independent activation and translocation of the cytostatic p21-activated protein kinase gamma-PAK by sphingosine

Autophosphorylation of p21-activated protein kinase gamma-PAK is stimulated at 10 microM sphingosine in vitro and is maximal at 100 microM. Sites autophosphorylated on gamma-PAK in response to sphingosine are identical to those obtained with Cdc42(GTP). Autophosphorylation is paralleled by stimulation of gamma-PAK activity as measured with peptide and protein substrates. In 3T3-L1 cells, sphingosine stimulates the autophosphorylation and activity of gamma-PAK associated with the membrane-containing particulate fraction by 2.8-fold, but does not stimulate the activity of the soluble enzyme. Thus, gamma-PAK is activatable via a Cdc42-independent mechanism, suggesting sphingosine has a role in gamma-PAK activation under conditions of cell stress.

Leukemia-associated Rho guanine nucleotide exchange factor, a Dbl family protein found mutated in leukemia, causes transformation by activation of RhoA

Leukemia-associated Rho guanine nucleotide exchange factor (LARG) was originally identified as a fusion partner with mixed-lineage leukemia in a patient with acute myeloid leukemia. LARG possesses a tandem Dbl homology and pleckstrin homology domain structure and, consequently, may function as an activator of Rho GTPases. In this study, we demonstrate that LARG is a functional Dbl protein. Expression of LARG in cells caused activation of the serum response factor, a known downstream target of Rho-mediated signaling pathways. Transient overexpression of LARG did not activate the extracellular signal-regulated kinase or c-Jun NH(2)-terminal kinase mitogen-activated protein kinase cascade, suggesting LARG is not an activator of Ras, Rac, or Cdc42. We performed in vitro exchange assays where the isolated Dbl homology (DH) or DH/pleckstrin homology domains of LARG functioned as a strong activator of RhoA, but exhibited no activity toward Rac1 or Cdc42. We found that LARG could complex with RhoA, but not Rac or Cdc42, in vitro, and that expression of LARG caused an increase in the levels of the activated GTP-bound form of RhoA, but not Rac1 or Cdc42, in vivo. Thus, we conclude that LARG is a RhoA-specific guanine nucleotide exchange factor. Finally, like activated RhoA, we determined that LARG cooperated with activated Raf-1 to transform NIH3T3 cells. These data demonstrate that LARG is the first functional Dbl protein mutated in cancer and indicate LARG-mediated activation of RhoA may play a role in the development of human leukemias.

p21-activated protein kinase gamma-PAK suppresses programmed cell death of BALB3T3 fibroblasts

In response to stress stimulants, cells activate opposing signaling pathways for cell survival and programmed cell death. p21-activated protein kinase gamma-PAK is involved in both cell survival and cell death pathways. Many stress stimulants activate gamma-PAK as a full-length enzyme and as a proteolytic fragment. Caspase-mediated proteolytic activation parallels cell death and appears to be a pro-apoptotic factor in stress-induced cell death. Here, we show that activation of full-length gamma-PAK promotes cell survival and suppresses stress-induced cell death. Expression of constitutively active gamma-PAK-T402E, which mimics activated full-length gamma-PAK, stimulates cell survival of BALB3T3 fibroblasts in response to tumor necrosis factor alpha, growth factor withdrawal, and UVC light. This stimulation of cell survival is mainly due to protection of cells from cell death rather than by stimulation of proliferation. Expression of gamma-PAK-T402E increases phosphorylation of the pro-apoptotic Bcl-2 family protein Bad and protects from cell death induced by ectopic expression of Bad. In response to tumor necrosis factor alpha, expression of gamma-PAK-T402E increases the early but reduces the late activation of ERK, JNK, and p38. Our results indicate that the ubiquitous gamma-PAK may have a crucial function in cell survival by regulating the pro-apoptotic activity of Bad and the stress-induced activation of ERK, JNK, and p38 pathways.

Constitutive ERK1/2 activation in esophagogastric rib bone marrow micrometastatic cells is MEK-independent

In this study, we examined the mitogen-activated protein kinase (MAPK) cascade in micrometastatic cell lines generated from rib bone marrow (RBM) of patients undergoing resection of esophagogastric malignancies. The molecular mechanism(s) involved in esophagogastric MAPK activation have not previously been investigated. Constitutive activation of both ERK1 and -2 isoforms was evident in each of the five RBM cell lines. Elk-1, a transcription factor activated by the ERK1/2 pathway was also found to be constitutively activated. Cell lines generated from metastases of involved lymph nodes (OC2) and ascites (OC1) of patients with esophageal cancer do not display, however, hyperphosphorylation of ERK1/2. Constitutive RBM ERK1/2 activation is protein kinase C and phosphatidylinositol 3-kinase dependent. Surprisingly, constitutive ERK1/2 activation is MEK-independent. Pharmacological inhibition of MEK with two specific inhibitors, PD 98059 and U0126, were both ineffective in blocking ERK activation. Similarly, the use of a dominant negative MEK mutant was without effect. Interestingly, experiments overexpressing two different dominant negative Pak1 mutants significantly reduced RBM ERK1/2 activation, albeit not to the same extent for all cell lines. We also examined the role of three different phosphatases, PAC1, MKP-1, and -2. While RBM ERK1/2 activation was found to be PAC1- and MKP-2-independent, surprisingly, MKP-1 was down-regulated in all five RBM cell lines. In conclusion, we provide evidence for the first time for a MEK-independent constitutive ERK1/2 activation pathway in esophagogastric RBM cell lines. These findings have important implications for drug treatment strategies which currently target MEK in other forms of cancer.

Mesodermal patterning defect in mice lacking the Ste20 NCK interacting kinase (NIK)

We have previously shown that the Drosophila Ste20 kinase encoded by misshapen (msn) is an essential gene in Drosophila development. msn function is required to activate the Drosophila c-Jun N-terminal kinase (JNK), basket (Bsk), to promote dorsal closure of the Drosophila embryo. Later in development, msn expression is required in photoreceptors in order for their axons to project normally. A mammalian homolog of msn, the NCK-interacting kinase (NIK) (recently renamed to mitogen-activated protein kinase kinase kinase kinase 4; Map4k4), has been shown to activate JNK and to bind the SH3 domains of the SH2/SH3 adapter NCK. To determine whether NIK also plays an essential role in mammalian development, we created mice deficient in NIK by homologous recombination at the Nik gene. Nik(−/−) mice die postgastrulation between embryonic day (E) 9.5 and E10.5. The most striking phenotype in Nik(−/−) embryos is the failure of mesodermal and endodermal cells that arise from the anterior end of the primitive streak (PS) to migrate to their correct location. As a result Nik(−/−)embryos fail to develop somites or a hindgut and are truncated posteriorly. Interestingly, chimeric analysis demonstrated that NIK has a cell nonautonomous function in stimulating migration of presomitic mesodermal cells away from the PS and a second cell autonomous function in stimulating the differentiation of presomitic mesoderm into dermomyotome. These findings indicate that despite the large number of Ste20 kinases in mammalian cells, members of this family play essential nonredundant function in regulating specific signaling pathways. In addition, these studies provide evidence that the signaling pathways regulated by these kinases are diverse and not limited to the activation of JNK because mesodermal and somite development are not perturbed in JNK1-, and JNK2-deficient mice.

Control of cell polarity and chemotaxis by Akt/PKB and PI3 kinase through the regulation of PAKa

We demonstrate that PI3 kinase and protein kinase B (PKB or Akt) control cell polarity and chemotaxis, in part, through the regulation of PAKa, which is required for myosin II assembly. We demonstrate that PI3K and PKB mediate PAKa's subcellular localization, PAKa's activation in response to chemoattractant stimulation, and chemoattractant-mediated myosin II assembly. Mutation of the PKB phosphorylation site in PAKa to Ala blocks PAKa's activation and inhibits PAKa redistribution in response to chemoattractant stimulation, whereas an Asp substitution leads to an activated protein. Addition of the PI3K inhibitor LY294002 results in a rapid loss of cell polarity and the axial distribution of actin, myosin, and PAKa. These results provide a mechanism by which PI3K regulates chemotaxis.

Interaction of paxillin with p21-activated Kinase (PAK). Association of paxillin alpha with the kinase-inactive and the Cdc42-activated forms of PAK3

p21-activated kinases (PAKs) are implicated in integrin signalings, and have been proposed to associate with paxillin indirectly. We show here that paxillin can bind directly to PAK3. We examined several representative focal adhesion proteins, and found that paxillin is the sole protein that associates with PAK3. PAK3 associated with the alpha and beta isoforms of paxillin, but not with gamma. We also show that paxillin alpha associated with both the kinase-inactive and the Cdc42-activated forms of PAK3 in vivo, without affecting the activation states of the kinase. A number of different functions have been ascribed to PAKs; and PAKs can bind directly to growth factor signaling-adaptor molecule, Nck, and a guanine nucleotide exchanger, betaPIX. Our results revealed that paxillin alpha can compete with Nck and betaPIX in the binding of PAK3. Moreover, paxillin alpha can be phosphorylated by PAK3 at serine. Therefore, paxillin alpha, but not gamma, appears to be capable of linking both the kinase-inactive and activated forms of PAK3 to integrins independent of Nck and betaPIX, as Nck links PAK1 to growth factor receptors. Our results also revealed that paxillin is involved in highly complexed protein-protein interactions in integrin signaling.

Stimulation of M3 muscarinic receptors induces phosphorylation of the Cdc42 effector activated Cdc42Hs-associated kinase-1 via a Fyn tyrosine kinase signaling pathway

The tyrosine kinase, activated Cdc42Hs-associated kinase-1 (ACK-1), is a specific effector of the Rho family GTPase Cdc42. GTP-bound Cdc42 has been shown to facilitate neurite outgrowth elicited by activation of muscarinic cholinergic receptors (mAChRs). Because tyrosine kinase activity is a requirement for neuritogenesis in several cell systems, we investigated whether endogenous mAChRs (principally of the M3 subtype) expressed in human SH-SY5Y neuroblastoma cells would signal to ACK-1. Incubation of cells with the cholinergic agonist oxotremorine-M (Oxo-M) induced an approximately 6-fold increase in the tyrosine phosphorylation of ACK-1 which was inhibited by atropine. ACK-1 phosphorylation was blocked by Clostridium difficile toxin B, an inhibitor of Rho family GTPases. In contrast, disruption of the actin cytoskeleton with cytochalasin D stimulated ACK-1 phosphorylation, and moreover, addition of Oxo-M to cells preincubated with this agent elicited a further increase in phosphorylation, indicating that an intact cytoskeleton is not required for mAChR signaling to ACK-1. Although stimulation of M3 mAChRs induces both an increase in intracellular Ca2+ and activation of protein kinase C (PKC), neither of these second messenger pathways was required for receptor-stimulated ACK-1 phosphorylation. Instead, inhibition of PKC resulted in a 2-fold increase in Oxo-M-stimulated ACK-1 phosphorylation, whereas acute activation of PKC with phorbol ester decreased ACK-1 phosphorylation. The agonist-induced tyrosine phosphorylation of ACK-1 was blocked by inhibitors of Src family kinases, and ACK-1 was coprecipitated with Fyn (but not Src) in an agonist-dependent manner. Finally, scrape loading cells with glutathione S-transferase fusion proteins of either the Fyn-SH2 or Fyn-SH3 domain significantly attenuated mAChR-stimulated ACK-1 tyrosine phosphorylation. The data are the first to show phosphorylation of ACK-1 after stimulation of a receptor coupled to neurite outgrowth and indicate that a Rho family GTPase (i.e. Cdc42) and Fyn are essential upstream elements of this signaling pathway.

Functional interaction between c-Abl and the p21-activated protein kinase gamma-PAK

A member of the p21-activated protein kinase (PAK) family, gamma-PAK has cytostatic properties and is activated by cellular stresses such as hyperosmolarity or DNA damage. We report herein that gamma-PAK is associated in vivo with the nonreceptor protein tyrosine kinase c-Abl. gamma-PAK phosphorylates c-Abl on sites located in the kinase domain, in a region that is implicated in protein-protein interactions and in subcellular localization. Activation of gamma-PAK in human embryonic kidney 293T cells by cotransfection with constitutively active Cdc42 induces activation of c-Abl, resulting in increased phosphotyrosine levels. Cotransfection of c-Abl and gamma-PAK elicits phosphorylation of gamma-PAK on tyrosine and down-regulation of gamma-PAK activity, promoting accumulation of inactive gamma-PAK. gamma-PAK is also phosphorylated in vitro by c-Abl. gamma-PAK activity is regulated by ubiquitination and proteolysis in vivo, as shown by immunoblotting with an anti-ubiquitin antibody in the presence of proteasome inhibitors. In summary, we describe a functional interaction between gamma-PAK and c-Abl in which gamma-PAK stimulates c-Abl tyrosine kinase activity and c-Abl phosphorylates and down-regulates gamma-PAK, suggesting the existence of a negative feedback loop between c-Abl and gamma-PAK.

Fray, a Drosophila serine/threonine kinase homologous to mammalian PASK, is required for axonal ensheathment

Fray is a serine/threonine kinase expressed by the peripheral glia of Drosophila, whose function is required for normal axonal ensheathment. Null fray mutants die early in larval development and have nerves with severe swelling and axonal defasciculation. The phenotype is associated with a failure of the ensheathing glia to correctly wrap peripheral axons. When the fray cDNA is expressed in the ensheathing glia of fray mutants, normal nerve morphology is restored. Fray belongs to a novel family of Ser/Thr kinases, the PF kinases, whose closest relatives are the PAK kinases. Rescue of the Drosophila mutant phenotype with PASK, the rat homolog of Fray, demonstrates a functional homology among these proteins and suggests that the Fray signaling pathway is widely conserved.

Mutational analysis suggests that activation of the yeast pheromone response mitogen-activated protein kinase pathway involves conformational changes in the Ste5 scaffold protein

Ste5 is essential for pheromone response and binds components of a mitogen-activated protein kinase (MAPK) cascade: Ste11 (MEKK), Ste7 (MEK), and Fus3 (MAPK). Pheromone stimulation releases Gbetagamma (Ste4-Ste18), which recruits Ste5 and Ste20 (p21-activated kinase) to the plasma membrane, activating the MAPK cascade. A RING-H2 domain in Ste5 (residues 177-229) negatively regulates Ste5 function and mediates its interaction with Gbetagamma. Ste5(C177A C180A), carrying a mutated RING-H2 domain, cannot complement a ste5Delta mutation, yet supports mating even in ste4Delta ste5Delta cells when artificially dimerized by fusion to glutathione S-transferase (GST). In contrast, wild-type Ste5 fused to GST permits mating of ste5Delta cells, but does not allow mating of ste4Delta ste5Delta cells. This differential behavior provided the basis of a genetic selection for STE5 gain-of-function mutations. MATa ste4Delta ste5Delta cells expressing Ste5-GST were mutagenized chemically and plasmids conferring the capacity to mate were selected. Three independent single-substitution mutations were isolated. These constitutive STE5 alleles induce cell cycle arrest, transcriptional activation, and morphological changes normally triggered by pheromone, even when Gbetagamma is absent. The first, Ste5(C226Y), alters the seventh conserved position in the RING-H2 motif, confirming that perturbation of this domain constitutively activates Ste5 function. The second, Ste5(P44L), lies upstream of a basic segment, whereas the third, Ste5(S770K), is situated within an acidic segment in a region that contacts Ste7. None of the mutations increased the affinity of Ste5 for Ste11, Ste7, or Fus3. However, the positions of these novel-activating mutations suggested that, in normal Ste5, the N terminus may interact with the C terminus. Indeed, in vitro, GST-Ste5(1-518) was able to associate specifically with radiolabeled Ste5(520-917). Furthermore, both the P44L and S770K mutations enhanced binding of full-length Ste5 to GST-Ste5(1-518), whereas they did not affect Ste5 dimerization. Thus, binding of Gbetagamma to the RING-H2 domain may induce a conformational change that promotes association of the N- and C-terminal ends of Ste5, stimulating activation of the MAPK cascade by optimizing orientation of the bound kinases and/or by increasing their accessibility to Ste20-dependent phosphorylation (or both). In accord with this model, the novel Ste5 mutants copurified with Ste7 and Fus3 in their activated state and their activation required Ste20.

Adhesion signaling: PAK meets Rac on solid ground

Interaction of cells with the extracellular matrix influences various aspects of cellular behavior. A recent study shows that cell-substrate adhesion is necessary for effective coupling of the small GTPase Rac to its effector PAK.