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

Inhibition of P21-activated kinases 1 and 4 synergistically suppresses the growth of pancreatic cancer by stimulating anti-tumour immunity

BACKGROUND

Pancreatic ductal adenocarcinoma (PDA) is one of the most lethal types of cancer, and KRAS oncogene occurs in over 90% of cases. P21-activated kinases (PAK), containing six members (PAK1 to 6), function downstream of KRAS. PAK1 and PAK4 play important roles in carcinogenesis, but their combinational effect remains unknown. In this study, we have determined the effect of dual inhibition of PAK1 and PAK4 in PDA progression using knockout (KO) cancer cell lines.

METHODS

Murine wild-type (WT) and PAK1KO pancreatic cancer cell lines were isolated from PAK1+/+ and PAK1-/- KPC (LSL-KrasG12D/+; LSL-Trp53 R172H/+; Pdx-1-Cre) mice. KPC PAK4KO and KPC PAK1&4 KO cell lines were generated from KPC WT and KPC PAK1KO cell lines respectively using the CRISPR-CAS9 gene knockout technique. PAK WT and KO cell lines were used in mouse models of pancreatic tumours. Cells and tumour tissue were also used in flow cytometry and proteomic studies. A human PDA tissue microarray was stained by immunohistochemistry.

RESULTS

Double knock out of PAK1 and PAK4 caused complete regression of tumour in a syngeneic mouse model. PAK4KO inhibited tumour growth by stimulating a rapid increase of cytotoxic CD8+ T cell infiltration. PAK1KO synergistically with PAK4KO increased cytotoxic CD8+ T cell infiltration and stimulated a sustained infiltration of CD8+ T cells at a later phase to overcome the immune evasion in the PAK4KO tumour. The human PDA tissue microarray study showed the important role of PAK1 and PAK4 in intra-tumoral T-cell function.

CONCLUSION

Our results demonstrated that dual inhibition of PAK1 and PAK4 synergistically suppressed PDA progression by stimulating cytotoxic CD8 + T cell response.

GLUT3 promotes macrophage signaling and function via RAS-mediated endocytosis in atopic dermatitis and wound healing

The facilitative GLUT1 and GLUT3 hexose transporters are expressed abundantly in macrophages, but whether they have distinct functions remains unclear. We confirmed that GLUT1 expression increased after M1 polarization stimuli and found that GLUT3 expression increased after M2 stimulation in macrophages. Conditional deletion of Glut3 (LysM-Cre Glut3fl/fl) impaired M2 polarization of bone marrow-derived macrophages. Alternatively activated macrophages from the skin of patients with atopic dermatitis showed increased GLUT3 expression, and a calcipotriol-induced model of atopic dermatitis was rescued in LysM-Cre Glut3fl/fl mice. M2-like macrophages expressed GLUT3 in human wound tissues as assessed by transcriptomics and costaining, and GLUT3 expression was significantly decreased in nonhealing, compared with healing, diabetic foot ulcers. In an excisional wound healing model, LysM-Cre Glut3fl/fl mice showed significantly impaired M2 macrophage polarization and delayed wound healing. GLUT3 promoted IL-4/STAT6 signaling, independently of its glucose transport activity. Unlike plasma membrane-localized GLUT1, GLUT3 was localized primarily to endosomes and was required for the efficient endocytosis of IL-4Rα subunits. GLUT3 interacted directly with GTP-bound RAS in vitro and in vivo through its intracytoplasmic loop domain, and this interaction was required for efficient STAT6 activation and M2 polarization. PAK activation and macropinocytosis were also impaired without GLUT3, suggesting broader roles for GLUT3 in the regulation of endocytosis. Thus, GLUT3 is required for efficient alternative macrophage polarization and function, through a glucose transport-independent, RAS-mediated role in the regulation of endocytosis and IL-4/STAT6 activation.

The trilogy of P21 activated kinase, autophagy and immune evasion in pancreatic ductal adenocarcinoma

Pancreatic Ductal Adenocarcinoma (PDA) is one of the most lethal types of cancer with a dismal prognosis. KRAS mutation is a commonly identified oncogene in PDA tumorigenesis and P21-activated kinases (PAKs) are its downstream mediator. While PAK1 is more well-studied, PAK4 also attracted increasing interest. In PDA, PAK inhibition not only reduces cancer cell viability but also sensitises it to chemotherapy. While PDA remains resistant to existing immunotherapies, PAK inhibition has been shown to increase cancer immunogenicity of melanoma, glioblastoma and PDA. Furthermore, autophagy plays an important role in PDA immune evasion, and accumulating evidence has pointed to a connection between PAK and cancer cell autophagy. In this literature review, we aim to summarize currently available studies that have assessed the potential connection between PAK, autophagy and immune evasion in PDA biology to guide future research.

Drug resistance in papillary RCC: from putative mechanisms to clinical practicalities

Papillary renal cell carcinoma (pRCC) is the second most common renal cell carcinoma (RCC) subtype and accounts for 10-15% of all RCCs. Despite clinical need, few pharmacogenomics studies in pRCC have been performed. Moreover, current research fails to adequately include pRCC laboratory models, such as the ACHN or Caki-2 pRCC cell lines. The molecular mechanisms involved in pRCC development and drug resistance are more diverse than in clear-cell RCC, in which inactivation of VHL occurs in the majority of tumours. Drug resistance to multiple therapies in pRCC occurs via genetic alteration (such as mutations resulting in abnormal receptor tyrosine kinase activation or RALBP1 inhibition), dysregulation of signalling pathways (such as GSK3β-EIF4EBP1, PI3K-AKT and the MAPK or interleukin signalling pathways), deregulation of cellular processes (such as resistance to apoptosis or epithelial-to-mesenchymal transition) and interactions between the cell and its environment (for example, through activation of matrix metalloproteinases). Improved understanding of resistance mechanisms will facilitate drug discovery and provide new effective therapies. Further studies on novel resistance biomarkers are needed to improve patient prognosis and stratification as well as drug development.

PAK1 Expression in Pancreatic Cancer: Clinicopathological Characteristics and Prognostic Significance

Background:

Improvement of the management of pancreatic cancer requires a better understanding of the genetic and molecular changes responsible for the development of the disease. The family of p21-activated kinases (PAKs) and especially PAK1 appears to mediate many cellular processes that contribute to the development and progression of pancreatic cancer, but the clinical relevance of PAK1 expression with the disease still remains unclear. Aim of the study was to assess the clinical value and the potential prognostic significance of PAK1 in pancreatic adenocarcinoma.

Methods:

We investigated the relationship between the PAK1 expression and the clinical and histopathologic characteristics of pancreatic cancer patients and the potential significance of PAK1 on survival. We examined tissue samples from 51 patients operated for pancreatic cancer. PAK1 expression was investigated with immunohistochemistry and correlated to clinicopathological parameters.

Results:

PAK1 was detected in all tumor samples and high expression was found in most patients. High PAK1 expression was also associated with younger age and well-differentiated tumors, but no association was found between PAK1 expression and Tumor-Node-Metastasis stage as well as deceased or alive status on follow-up. Moderate to high PAK1 expression favored higher 6-month and 1-year survival and low PAK1 expression 2-year survival but without statistical significance.

Conclusions

Our results indicate that PAK1 could potentially be used as a prognostic marker in pancreatic cancer. Further studies could clarify whether utilization of PAK1 in therapeutic protocols for the treatment of pancreatic cancer will render them more effective.

Periostin/β1integrin interaction regulates p21-activated kinases in valvular interstitial cell survival and in actin cytoskeleton reorganization

The matricellular protein periostin (PN) promotes postnatal valve remodeling and maturation. Incomplete remodeling of the valve can trigger degenerative processes that lead to a myxomatous phenotype that includes loss of PN. However, signaling pathways involved that link valvular-interstitial-fibroblast cells (VICs) to proliferation, migration and actin remodeling functions are unclear. The p21-activated kinases (Paks) have been shown to regulate cytoskeleton rearrangements and cell proliferation/adhesion/migration functions in a variety of cellular contexts, including normal cells and cancer cells. This study shows that Pak1, but not Pak2 and Pak4, is a critical mediator of VIC survival and actin organization, and that the molecular signaling regulating actin-remodeling is initiated upon PN/beta-integrin-induced phosphorylation of the focal-adhesion-kinase (Fak) (Y397). Molecular and pharmacological inhibition of key components of PN/Fak/Akt1 signaling abolished the PN-induced actin polymerization and the activation of mTOR, p70S6K and Pak1. Similarly, blocking mTOR inhibited p70S6K, Pak1 phosphorylation and consequently actin-polymerization. Accordingly, inhibiting p70S6K blocked Pak1 phosphorylation and actin polymerization, and subsequently inhibited adhesion and growth of VICs. Periostin-induced Akt1 activation of Pak1 is independent of Cdc42 and Rac1 GTPases, and Akt1 is both downstream and upstream of Pak1. Further, the PN-Pak1-induced Akt1 protects cells from apoptosis through suppression of transcriptional activation of Forkhead-Transcription-Factor (FKHR). In contrast, kinase deficient Pak1 increases apoptosis by increasing FKHR-mediated transcriptional activation. These studies define new functional significance of PN-Fak-Akt1-Pak1 signaling that at least partly regulates Akt1-induced actin polymerization and FKHR-mediated transcriptional activation, which may eventually regulate the mature-valve-leaflet remodeling function, and also FKHR-mediated transcriptional activation for pro-survival of VICs.

p-21 Activated Kinase as a Molecular Target for Chemoprevention in Diabetes

Hypothesis: Anti-diabetic drugs modulate p-21 activated kinase (PAK) signaling. Introduction: Type 2 diabetes mellitus (T2DM) is a chronic inflammatory disease associated with increased cancer risk. PAK signaling is implicated in cellular homeostasis when regulated, and cancer when unrestrained. Recent reports provided a role for PAK signaling in glucose homeostasis, but the role of PAKs in the pathogenesis of T2DM is unknown. Here, we performed a mini-meta-analysis to explore if anti-diabetic drugs modify PAK signaling pathways, and provide insight regarding modulation of these pathways, to potentially reduce diabetes-associated cancer risk. Methods: PAK interacting partners in T2DM were identified using the online STRING database. Correlation studies were performed via systematic literature review to understand the effect of anti-diabetic drugs on PAK signaling. A mini-meta-analysis correlated multiple clinical studies and revealed the overall clinical response rate and percentage of adverse events in piogliazone (n = 53) and metformin (n = 91) treated patients with PAK-associated diseases. Results: A total of 30 PAK interacting partners were identified (10: reduced beta-cell mass; 10: beta-cell dysfunction; 10: obesity-insulin resistance), which were highly associated with Wnt, and G-protein signaling. The anti-diabetic drug metformin activated signaling pathways upstream; whereas pioglitazone inhibited pathways downstream of PAK. Overall, clinical response upon pioglitazone treatment was 53%. Seventy-nine percent of pioglitazone and 75% of metformin treated patients had adverse events. Pioglitazone reduced molecular-PAK biomarkers of proliferation (Ki67 and CyclinD1), and metformin had the opposite effect. Conclusions: PAK signaling in T2DM likely involves Wnt and G-protein signaling, which may be altered by the anti-diabetic drugs metformin and pioglitazone. Apart from the therapeutic limitations of adverse events, pioglitazone may be promising in chemoprevention. However long-term multi-centered studies, which initiate pioglitazone treatment early will be required to fully assess the full potential of these drugs.

Cloning and functional characterization of human Pak1 promoter by steroid hormones

P21-activated kinase 1 (Pak1) is known to be involved in a plethora of functions including cell growth, survival and can lead to cell transformation and tumor progression especially in breast tissue. Multiple studies have shown Pak1 dysregulation as a change in DNA copy number as well as gene expression levels, suggesting many regulatory mechanisms at transcriptional and translational level. However, very little is known about the transcriptional regulation of the human Pak1 promoter. Here, we focus on Pak1 promoter regulation by steroid hormones along with their respective receptors that are also crucial players in breast tissue function and tumorigenesis. Our results show high Pak1 expression in breast cancer cell lines and in breast tumor tissue. It also suggests that Pak1 is hormone responsive, whose expression can be modulated by steroid hormones namely, estrogen in the form of 17β-estradiol (E2) and progesterone (P4). Sequence analysis of a 3.2kb Pak1 proximal promoter region shows the presence of PRE (progesterone response element) and ERE (estrogen response element) half sites, that were further cloned and characterized. Results from promoter analysis showed that Pak1 promoter activity is mediated by PR via its binding to PRE present on the Pak1 promoter that was further reaffirmed in vitro by electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation assay (ChIP). Our results together suggest that it is the PR isoform B regulates Pak1 promoter. To our knowledge, this is the first study to report the detailed characterization and transcriptional regulation of the human Pak1 promoter by steroid hormones.

PAKs in the brain: Function and dysfunction

p21-Activated kinases (PAKs) comprise a family of proteins covering a central role in signal transduction. They are downstream effectors of Rho GTPases and can affect a variety of processes in different cell types and tissues by remodeling the cytoskeleton and by promoting gene transcription and cell survival. Given the relevance of cytoskeletal organization in neuronal development as well as synaptic function and the importance of pro-survival signals in controlling neuronal cell fate, accumulating studies investigated the role of PAKs in the nervous system. In this review, we provide a critical overview of the role of PAKs in the nervous system, both in neuronal and non-neuronal cells, and discuss their potential link with neurodegenerative diseases.

The p21-activated kinase 1 (Pak1) signalling pathway in cardiac disease: from mechanistic study to therapeutic exploration

p21-activated kinase 1 (Pak1) is a member of the highly conserved family of serine/threonine protein kinases regulated by Ras-related small G-proteins, Cdc42/Rac1. It has been previously demonstrated to be involved in cardiac protection. Based on recent studies, this review provides an overview of the role of Pak1 in cardiac diseases including disrupted Ca²⁺ homoeostasis-related cardiac arrhythmias, adrenergic stress- and pressure overload-induced hypertrophy, and ischaemia/reperfusion injury. These findings demonstrate the important role of Pak1 mediated through the phosphorylation and transcriptional modification of hypertrophy and/or arrhythmia-related genes. This review also discusses the anti-arrhythmic and anti-hypertrophic, protective function of Pak1 and the beneficial effects of fingolimod (an FDA-approved sphingolipid drug), a Pak1 activator, and its ability to prevent arrhythmias and cardiac hypertrophy. These findings also highlight the therapeutic potential of Pak1 signalling in the treatment and prevention of cardiac diseases.

LINKED ARTICLES

This article is part of a themed section on Spotlight on Small Molecules in Cardiovascular Diseases. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.8/issuetoc.

The p21-activated kinase, PAK2, is important in the activation of numerous pancreatic acinar cell signaling cascades and in the onset of early pancreatitis events

In a recent study we explored Group-1-p21-activated kinases (GP.1-PAKs) in rat pancreatic acini. Only PAK2 was present; it was activated by gastrointestinal-hormones/neurotransmitters and growth factors in a PKC-, Src- and small-GTPase-mediated manner. PAK2 was required for enzyme-secretion and ERK/1-2-activation. In the present study we examined PAK2's role in CCK and TPA-activation of important distal signaling cascades mediating their physiological/pathophysiological effects and analyzed its role in pathophysiological processes important in early pancreatitis. In rat pancreatic acini, PAK2-inhibition by the specific, GP.1.PAK-inhibitor, IPA-3-suppressed cholecystokinin (CCK)/TPA-stimulated activation of focal-adhesion kinases and mitogen-activated protein-kinases. PAK2-inhibition reversed the dual stimulatory/inhibitory effect of CCK/TPA on the PI3K/Akt/GSK-3β pathway. However, its inhibition did not affect PKC activation. PAK2-inhibition protected acini from CCK-induced ROS-generation; caspase/trypsin-activation, important in early pancreatitis; as well as from cell-necrosis. Furthermore, PAK2-inhibition reduced proteolytic-activation of PAK-2p34, which is involved in programmed-cell-death. To ensure that the study did not only rely in the specificity of IPA-3 as a PAK inhibitor, we used two other approaches for PAK inhibition, FRAX597 a ATP-competitive-GP.1-PAKs-inhibitor and infection with a PAK2-dominant negative(DN)-Advirus. Those two approaches confirmed the results obtained with IPA-3. This study demonstrates that PAK2 is important in mediating CCK's effect on the activation of signaling-pathways known to mediate its physiological/pathophysiological responses including several cellular processes linked to the onset of pancreatitis. Our results suggest that PAK2 could be a new, important therapeutic target to consider for the treatment of diseases involving deregulation of pancreatic acinar cells.

Gastrointestinal hormones/neurotransmitters and growth factors can activate P21 activated kinase 2 in pancreatic acinar cells by novel mechanisms

P-21-activated kinases (PAKs) are serine/threonine kinases comprising six isoforms divided in two groups, group-I (PAK1-3)/group-II (PAK4-6) which play important roles in cell cytoskeletal dynamics, survival, secretion and proliferation and are activated by diverse stimuli. However, little is known about PAKs ability to be activated by gastrointestinal (GI) hormones/neurotransmitters/growth-factors. We used rat pancreatic acini to explore the ability of GI-hormones/neurotransmitters/growth-factors to activate Group-I-PAKs and the signaling cascades involved. Only PAK2 was present in acini. PAK2 was activated by some pancreatic growth-factors [EGF, PDGF, bFGF], by secretagogues activating phospholipase-C (PLC) [CCK, carbachol, bombesin] and by post-receptor stimulants activating PKC [TPA], but not agents only mobilizing cellular calcium or increasing cyclic AMP. CCK-activation of PAK2 required both high- and low-affinity-CCK1-receptor-state activation. It was partially reduced by PKC- or Src-inhibition, but not with PI3K-inhibitors (wortmannin, LY294002) or thapsigargin. IPA-3, which prevents PAK2 binding to small-GTPases partially inhibited PAK2-activation, as well as reduced CCK-induced ERK1/2 activation and amylase release induced by CCK or bombesin. This study demonstrates pancreatic acini, possess only one Group-I-PAK, PAK2. CCK and other GI-hormones/neurotransmitters/growth-factors activate PAK2 via small GTPases (CDC42/Rac1), PKC and SFK but not cytosolic calcium or PI3K. CCK-activation of PAK2 showed several novel features being dependent on both receptor-activation states, having PLC- and PKC-dependent/independent components and small-GTPase-dependent/independent components. These results show that PAK2 is important in signaling cascades activated by numerous pancreatic stimuli which mediate their various physiological/pathophysiological responses and thus could be a promising target for the development of therapies in some pancreatic disorders such as pancreatitis.

p21-activated kinase 1 determines stem-like phenotype and sunitinib resistance via NF-κB/IL-6 activation in renal cell carcinoma

The p21-activated kinase 1 (PAK1), a serine/threonine kinase that orchestrates cytoskeletal remodeling and cell motility, has been shown to function as downstream node for various oncogenic signaling pathways to promote cell proliferation, regulate apoptosis and accelerate mitotic abnormalities, resulting in tumor formation and invasiveness. Although alterations in PAK1 expression and activity have been detected in various human malignancies, its potential biological and clinical significance in renal cell carcinoma (RCC) remains obscure. In this study, we found increased PAK1 and phosphorylated PAK1 levels in tumor tissues according to TNM stage progression. Elevated phosphorylated PAK1 levels associated with progressive features and indicated unfavorable overall survival (OS) as an independent adverse prognosticator for patients with RCC. Moreover, PAK1 kinase activation with constitutive active PAK1 mutant T423E promoted growth, colony formation, migration, invasion and stem-like phenotype of RCC cells, and vice versa, in PAK1 inhibition by PAK1 kinase inactivation with specific PAK1 shRNA, dead kinase PAK1 mutant K299R or allosteric inhibitor IPA3. Stem-like phenotype due to sunitinib administration via increased PAK1 kinase activation could be ameliorated by PAK1 shRNA, PAK1 mutant K299R and IPA3. Furthermore, nuclear factor-κB (NF-κB)/interleukin-6 (IL-6) activation was found to be responsible for PAK1-mediated stem-like phenotype following sunitinib treatment. Both IL-6 neutralizing antibody and IPA3 administration enhanced tumor growth inhibition effect of sunitinib treatment on RCC cells in vitro and in vivo. Our results unraveled that oncogenic activation of PAK1 defines an important mechanism for maintaining stem-like phenotype and sunitinib resistance through NF-κB/IL-6 activation in RCC, lending PAK1-mediated NF-κB/IL-6 activation considerable appeal as novel pharmacological therapeutic targets against sunitinib resistance.

p21-activated kinases in Erbb2-positive breast cancer: A new therapeutic target?

The activation of receptor tyrosine kinases, particularly ErbB2, has been linked to the genesis and progression of breast cancer. Two of the central signaling pathways activated by ErbB2 are the Ras/Raf-1/Mek/Erk pathway, which plays an important role in tumor cell growth and migration, and the PI3K/Akt pathway, which plays an important role in cell survival. Recently, we and others have shown that signaling through the Ras-Erk pathway can be influenced by p21-activated kinase 1 (Pak1), an effector of the Rho family GTP ases Rac and Cdc42. Expression of activated forms of Rac promotes activation of Erk through mechanisms involving Pak1 phosphorylation of Raf-1 and Mek1. In addition, Pak1 has also been implicated in the activation of Akt. However, our understanding regarding the degree to which Rho GTPases, and their effectors such as Pak1, contribute to ErbB2-mediated signaling is very limited.Recent results from our laboratory indicate that ErbB2 expression correlates with Pak activation in estrogen receptor negative human breast tumor samples. Using a three-dimensional (3D) culture of human MCF-10A mammary epithelial cells, we found that activation of Rac-Pak pathway by ErbB2 induces growth factor independent proliferation and promotes disruption of acini-like structures through the activation of the Erk and Akt pathways. We also observed that blocking Pak1 activity by small molecule inhibitors impeded the ability of activated ErbB2 to transform these cells and to activate its associated downstream signaling targets. In addition, we found that suppressing Pak activity in ErbB2-amplified breast cancer cells delayed tumor formation and downregulated Erk and Akt signaling in vivo. These results support a model in which Pak, by activating Erk and Akt, cooperates with ErbB2 in transforming mammary epithelial cells.

Transcriptional regulation of fibronectin by p21-activated kinase-1 modulates pancreatic tumorigenesis

Pancreatic ductal adenocarcinoma (PDAC) is the eighth largest cause of cancer-related mortality across the world, with a median 5-year survival rate of less than 3.5%. This is partly because the molecules and the molecular mechanisms that contribute to PDAC are not well understood. Our goal is to understand the role of p21-activated kinase 1 (Pak1) signaling axis in the progression of PDAC. Pak1, a serine/threonine kinase, is a well-known regulator of cytoskeletal remodeling, cell motility, cell proliferation and cell survival. Recent reports suggest that Pak1 by itself can have an oncogenic role in a wide variety of cancers. In this study, we analyzed the expression of Pak1 in human pancreatic cancer tissues and found that Pak1 levels are significantly upregulated in PDAC samples as compared with adjacent normals. Further, to study the functional role of Pak1 in pancreatic cancer model systems, we developed stable overexpression and lentiviral short hairpin RNA-mediated knockdown (KD) clones of Pak1 and studied the changes in transforming properties of the cells. We also observed that Pak1 KD clones failed to form tumors in nude mice. By adopting a quantitative PCR array-based approach, we identified fibronectin, a component of the extracellular matrix and a mesenchymal marker, as a transcriptional target of Pak1 signaling. The underlying molecular mechanism of Pak1-mediated transformation includes its nuclear import and recruitment to the fibronectin promoter via interaction with nuclear factor-κB (NF-κB)-p65 complex. To our knowledge, this is the first study illustrating Pak1-NF-κB-p65-mediated fibronectin regulation as a potent tumor-promoting mechanism in KRAS intact model.

Activated Ras as a Therapeutic Target: Constraints on Directly Targeting Ras Isoforms and Wild-Type versus Mutated Proteins

The ability to selectively and directly target activated Ras would provide immense utility for treatment of the numerous cancers that are driven by oncogenic Ras mutations. Patients with disorders driven by overactivated wild-type Ras proteins, such as type 1 neurofibromatosis, might also benefit from progress made in that context. Activated Ras is an extremely challenging direct drug target due to the inherent difficulties in disrupting the protein:protein interactions that underlie its activation and function. Major investments have been made to target Ras through indirect routes. Inhibition of farnesyl transferase to block Ras maturation has failed in large clinical trials. Likely reasons for this disappointing outcome include the significant and underappreciated differences in the isoforms of Ras. It is still plausible that inhibition of farnesyl transferase will prove effective for disease that is driven by activated H-Ras. The principal current focus of drugs entering clinic trial is inhibition of pathways downstream of activated Ras, for example, trametinib, a first-in-class MEK inhibitor. The complexity of signaling that is driven by activated Ras indicates that effective inhibition of oncogenic transduction through this approach will be difficult, with resistance being likely to emerge through switch to parallel pathways. Durable disease responses will probably require combinatorial block of several downstream targets.

miR-23b regulates cytoskeletal remodeling, motility and metastasis by directly targeting multiple transcripts

Uncontrolled cell proliferation and cytoskeletal remodeling are responsible for tumor development and ultimately metastasis. A number of studies have implicated microRNAs in the regulation of cancer cell invasion and migration. Here, we show that miR-23b regulates focal adhesion, cell spreading, cell-cell junctions and the formation of lamellipodia in breast cancer (BC), implicating a central role for it in cytoskeletal dynamics. Inhibition of miR-23b, using a specific sponge construct, leads to an increase of cell migration and metastatic spread in vivo, indicating it as a metastatic suppressor microRNA. Clinically, low miR-23b expression correlates with the development of metastases in BC patients. Mechanistically, miR-23b is able to directly inhibit a number of genes implicated in cytoskeletal remodeling in BC cells. Through intracellular signal transduction, growth factors activate the transcription factor AP-1, and we show that this in turn reduces miR-23b levels by direct binding to its promoter, releasing the pro-invasive genes from translational inhibition. In aggregate, miR-23b expression invokes a sophisticated interaction network that co-ordinates a wide range of cellular responses required to alter the cytoskeleton during cancer cell motility.

Tyrosyl phosphorylated PAK1 regulates breast cancer cell motility in response to prolactin through filamin A

The p21-activated serine-threonine kinase (PAK1) is activated by small GTPase-dependent and -independent mechanisms and regulates cell motility. Both PAK1 and the hormone prolactin (PRL) have been implicated in breast cancer by numerous studies. We have previously shown that the PRL-activated tyrosine kinase JAK2 (Janus tyrosine kinase 2) phosphorylates PAK1 in vivo and identified tyrosines (Tyr) 153, 201, and 285 in the PAK1 molecule as sites of JAK2 tyrosyl phosphorylation. Here, we have used human breast cancer T47D cells stably overexpressing PAK1 wild type or PAK1 Y3F mutant in which Tyr(s) 153, 201, and 285 were mutated to phenylalanines to demonstrate that phosphorylation of these three tyrosines are required for maximal PRL-dependent ruffling. In addition, phosphorylation of these three tyrosines is required for increased migration of T47D cells in response to PRL as assessed by two independent motility assays. Finally, we show that PAK1 phosphorylates serine (Ser) 2152 of the actin-binding protein filamin A to a greater extent when PAK1 is tyrosyl phosphorylated by JAK2. Down-regulation of PAK1 or filamin A abolishes the effect of PRL on cell migration. Thus, our data presented here bring some insight into the mechanism of PRL-stimulated motility of breast cancer cells.

Governing epidermal homeostasis by coupling cell-cell adhesion to integrin and growth factor signaling, proliferation, and apoptosis

Cadherin/catenin-based adhesions coordinate cellular growth, survival, migration, and differentiation within a tissue by mechanically anchoring cells to their neighbors. They also intersect with diverse signaling pathways in development and cancer. Although the adhesive functions of adherens junction proteins are well characterized, their contribution to other signaling pathways is less well understood. Here, we show that ablation of α-catenin in the epidermis selectively induces apoptosis in suprabasal differentiating keratinocytes while sparing basal cell progenitors. This protection from death is coupled to elevated focal adhesion signaling, faster migration, and an altered distribution of growth factor receptors. We show that simultaneous depletion of α-catenin and focal adhesion kinase or p21-activated kinase eliminates basal cell protection as well as the elevated migration and proliferation of cells. The increased dependency of cells upon matrix interactions for their survival when cell-cell adhesions are destabilized has important implications for cancer progression and metastasis.

Integrin α6β4 cooperates with LPA signaling to stimulate Rac through AKAP-Lbc-mediated RhoA activation

The α(6)β(4) integrin promotes carcinoma invasion through its ability to promote directed migration and polarization of carcinoma cells. In this study, we explore how the α(6)β(4) integrin cooperates with lysophosphatidic acid (LPA) to activate Rho and Rac small GTPases. Through the use of dominant negative Rho constructs, C3 exotransferase, and Rho kinase inhibitor, we find that Rho is critical for LPA-dependent chemotaxis and lamellae formation. However, utilization of specific Rho isoforms depends on integrin α(6)β(4) expression status. Integrin α(6)β(4)-negative MDA-MB-435 cells utilize only RhoC for motility, whereas integrin α(6)β(4)-expressing cells utilize RhoC but additionally activate and utilize RhoA for LPA-dependent cell motility and lamellae formation. Notably, the activation of RhoA by cooperative LPA and integrin α(6)β(4) signaling requires the Rho guanine nucleotide exchange factor AKAP-Lbc. We also determine that integrin α(6)β(4) cannot activate Rac1 directly but promotes LPA-mediated Rac1 activation that is dependent on RhoA activity and de novo β(1) integrin ligation. Finally, we find that the regulation of Rac1 and RhoA in response to LPA is differentially regulated by phosphodiesterases, PKA, and phosphatidylinositol 3-kinase, thus supporting their spatially distinct compartmentalization. In summary, signaling from integrin α(6)β(4) facilitates LPA-stimulated chemotaxis through preferential activation of RhoA, which, in turn, facilitates activation of Rac1.

p21-activated kinase improves cardiac contractility during ischemia-reperfusion concomitant with changes in troponin-T and myosin light chain 2 phosphorylation

p21-activated kinase 1 (Pak1) is a serine/threonine kinase that activates protein phosphatase 2a, resulting in the dephosphorylation of cardiac proteins and increased myofilament Ca(2+) sensitivity. Emerging evidence indirectly indicates a role for Pak1 in ischemia-reperfusion (I/R), but direct evidence is lacking. We hypothesize that activation of the Pak1 signaling pathway is a cardioprotective mechanism that prevents or reverses the detrimental effects of ischemic injury by inducing posttranslational modifications in myofilament proteins that ultimately improve cardiac contractility following ischemic insult. In the present study, we subjected ex vivo hearts from wild-type (WT) and Pak1-knockout (KO) mice to 20 min of global cardiac ischemia followed by 30 min of reperfusion. In the absence of Pak1, there was an exacerbation of the increased end-diastolic pressure and reduced left ventricular developed pressure occurring after I/R injury. ProQ analysis revealed an increase in troponin-T phosphorylation at baseline in Pak1-KO hearts compared with WT. Significantly decreased myosin light chain 2 (MLC2) phosphorylation in Pak1-KO hearts compared with WT after I/R injury was confirmed by Western immunoblotting. These data indicate that Pak1-KO hearts have reduced recovery of myocardial performance after global I/R injury concomitant with changes in troponin-T and MLC2 phosphorylation. Finally, a protein-protein association between Pak1 and MLC2, and Pak1 and troponin-T, was determined by coimmunoprecipitation. Thus, results of our study provide a basis for targeting a novel pathway, including Pak1, in the therapies for patients with ischemic events.

A Rac-Pak signaling pathway is essential for ErbB2-mediated transformation of human breast epithelial cancer cells

The activation of receptor tyrosine kinases, particularly ErbB2, plays an important role in the genesis of breast cancer. ErbB2 kinase activity promotes Ras-mediated stimulation of downstream protein kinase cascades, including the Ras/Raf-1/Mek/extracellular-signal regulated kinase (Erk) pathway, leading to tumor cell growth and migration. Signaling through the Ras-Erk pathway can be influenced by p21-activated kinase-1 (Pak1), an effector of the Rho family GTPases Rac and Cdc42. In this study, we asked if ErbB2 expression correlates with Pak1 and Erk activity in human breast cancer specimens, and if Pak1 signaling is required for ErbB2 transformation in a 3D in vitro setting and in xenografts. We found a correlation between ErbB2 expression and activation of Pak in estrogen-receptor positive human breast tumor samples and observed that in 3D cultures, activation of Rac-Pak1 pathway by ErbB2 homodimers induced growth factor independent proliferation and promoted disruption of three-dimensional mammary acinar-like structures through activation of the Erk and Akt pathways. Further, we found that inhibition of Pak1 by small molecules compromised activation of Erk and Akt, resulting in reversion of the malignant phenotype and restoration of normal acinar architecture. Finally, ErbB2-amplified breast cancer cells expressing a specific Pak inhibitor showed delayed tumor formation and down-regulation of Erk and Akt signaling in vivo. These data imply that the Rac-Pak pathway is vital to ErbB2-mediated transformation and that Pak inhibitors represent plausible drug targets in breast cancers in which ErbB2 signaling is activated.

Arpc1b, a centrosomal protein, is both an activator and substrate of Aurora A

In addition to its function as an Arp2/3 complex subunit, Arp1cb interacts with and stimulates Aurora A at centrosomes, functioning in cell cycle progression.

Here we provide evidence in support of an inherent role for Arpc1b, a component of the Arp2/3 complex, in regulation of mitosis and demonstrate that its depletion inhibits Aurora A activation at the centrosome and impairs the ability of mammalian cells to enter mitosis. We discovered that Arpc1b colocalizes with γ-tubulin at centrosomes and stimulates Aurora A activity. Aurora A phosphorylates Arpc1b on threonine 21, and expression of Arpc1b but not a nonphosphorylatable Arpc1b mutant in mammalian cells leads to Aurora A kinase activation and abnormal centrosome amplification in a Pak1-independent manner. Together, these findings reveal a new function for Arpc1b in centrosomal homeostasis. Arpc1b is both a physiological activator and substrate of Aurora A kinase and these interactions help to maintain mitotic integrity in mammalian cells.

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.

Pak protein kinases and their role in cancer

Some of the characteristics of cancer cells are high rates of cell proliferation, cell survival, and the ability to invade surrounding tissue. The cytoskeleton has an essential role in these processes. Dynamic changes in the cytoskeleton are necessary for cell motility and cancer cells are dependent on motility for invasion and metastasis. The signaling pathways behind the reshaping and migrating properties of the cytoskeleton in cancer cells involve a group of Ras-related small GTPases and their effectors, including the p21-activated kinases (Paks). Paks are a family of serine/threonine protein kinases comprised of six isoforms (Pak 1-6), all of which are direct targets of the small GTPases Rac and Cdc42. Besides their role in cytoskeletal dynamics, Paks have recently been shown to regulate various other cellular activities, including cell survival, mitosis, and transcription. Paks are overexpressed and/or hyperactivated in several human tumors and their role in cell transformation makes them attractive therapeutic targets. Pak-targeted therapeutics may efficiently inhibit certain types of tumors and efforts to identify selective Pak-inhibitors are underway.

Fibroblast elongation and dendritic extensions in constrained versus unconstrained microtissues

Cytoskeletal tension is fundamental to many biological processes, including germ layer sorting during embryogenesis [Krieg et al., 2008]. In vitro, such tension influences cell sorting in self-assembled, 3D microtissues and can be of sufficient magnitude to cause complex-shaped microtissue failure [Dean et al., 2007]. To examine the process of failure under cell-derived tension, we subjected normal human fibroblasts (NHFs) to directed self-assembly [Dean et al., 2007] in micro-molds designed to yield self-constraining microtissues. As cells contracted in this assay, the constrained microtissues narrowed, thinned and ultimately failed at their midpoints. By adding small numbers of GFP+ cells, changes in cell movement and morphology were assessed and compared to those of unconstrained microtissues. We found that cells formed numerous dendritic extensions within an hour of self-assembly and retracted these extensions as they elongated up to 30 times their initial diameter ( approximately 600 microm) just prior to failure. Surprisingly, significant coordination in cell motility was observed over large distances within microtissues. Pharmacologic interventions showed that failure was myosin II and Rho kinase dependent and inhibition of failure resulted in shorter cells with greater numbers of extensions. These findings further our understanding of cellular self-assembly and introduce the use of GFP+ cells with directed self-assembly as a scaffold-free analogue to fibroblast-populated collagen gels (FPCGs).

Diacylglycerol kinase zeta regulates actin cytoskeleton reorganization through dissociation of Rac1 from RhoGDI

Activation of Rac1 GTPase signaling is stimulated by phosphorylation and release of RhoGDI by the effector p21-activated kinase 1 (PAK1), but it is unclear what initiates this potential feed-forward mechanism for regulation of Rac activity. Phosphatidic acid (PA), which is produced from the lipid second messenger diacylglycerol (DAG) by the action of DAG kinases (DGKs), is known to activate PAK1. Here, we investigated whether PA produced by DGKzeta initiates RhoGDI release and Rac1 activation. In DGKzeta-deficient fibroblasts PAK1 phosphorylation and Rac1-RhoGDI dissociation were attenuated, leading to reduced Rac1 activation after platelet-derived growth factor stimulation. The cells were defective in Rac1-regulated behaviors, including lamellipodia formation, membrane ruffling, migration, and spreading. Wild-type DGKzeta, but not a kinase-dead mutant, or addition of exogenous PA rescued Rac activation. DGKzeta stably associated with PAK1 and RhoGDI, suggesting these proteins form a complex that functions as a Rac1-selective RhoGDI dissociation factor. These results define a pathway that links diacylglycerol, DGKzeta, and PA to the activation of Rac1: the PA generated by DGKzeta activates PAK1, which dissociates RhoGDI from Rac1 leading to changes in actin dynamics that facilitate the changes necessary for cell motility.

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.

Diesel exhaust increases EGFR and phosphorylated C-terminal Tyr 1173 in the bronchial epithelium

Background

Epidemiological studies have demonstrated adverse health effects of environmental pollution. Diesel exhaust (DE) is a major contributor to particulate matter pollution. DE exposure has been shown to induce a pronounced inflammatory response in the airways, together with an enhanced epithelial expression of cytokines such as IL-8, Gro-α, IL-13 and activation of redox sensitive transcription factors (NFκB, AP-1), and MAP kinases (p38, JNK). The aim of the present investigation was to elucidate the involvement of the epidermal growth factor receptor (EGFR) signalling pathway in the epithelial response to DE in-vivo.

Results

Immunohistochemical staining was used to quantify the expression of the EGFR, phosphorylated Tyrosine residues, MEK and ERK in the bronchial epithelium of archived biopsies from 15 healthy subjects following exposure to DE (PM₁₀, 300 μg/m³) and air. DE induced a significant increases in the expression of EGFR (p = 0.004) and phosphorylated C-terminal Tyr 1173 (p = 0.02). Other investigated EGFR tyrosine residues, Src related tyrosine (Tyr 416), MEK and ERK pathway were not changed significantly by DE.

Conclusion

Exposure to DE (PM₁₀, 300 μg/m³) caused enhanced EGFR expression and phosphorylation of the tyrosine residue (Tyr 1173) which is in accordance with the previously demonstrated activation of the JNK, AP-1, p38 MAPK and NFkB pathways and associated downstream signalling and cytokine production. No effects were seen on the MEK and ERK pathway suggesting that at the investigated time point (6 hours post exposure) there was no proliferative/differentiation signalling in the bronchial epithelium. The present findings suggest a key role for EGFR in the bronchial response to diesel exhaust.

Modulation of p21-activated kinase 1 alters the behavior of renal cell carcinoma

The p21-activated kinase 1 (Pak1) is a serine/threonine kinase whose activity is regulated by both Rho GTPases and AGC kinase family members. It plays a role in cytoskeletal remodeling and cell motility as well as cell proliferation, angiogenesis, tumorigenesis and metastasis. An involvement of Pak1 in renal cell carcinoma (RCC), which remains highly refractory to chemotherapy and radiotherapy, remains to be investigated. Pak1 expression, phosphorylation and kinase activity were examined in RCC cell lines and human tissue from normal and renal carcinoma. We report increased Pak1 expression and constitutive activity in the membrane and nucleus but not the cytoplasm of resected human RCC. To study a role for Pak1 in RCC, we developed 786-0 clones that expressed either a kinase-active Pak1L83,L86 2 different Pak1 dominant negative mutants, Pak1R299 and Pak1L83,L86,R299 or Pak1 siRNA. The expression of Pak1L83,L86 increased 786-0 proliferation, motility and anchorage independent growth, while the dominant negative mutants and Pak1 siRNA abrogated these effects. In addition, Pak1L83,L86 conferred resistance to 5-fluorouracil with a 40%+/-10% increase in cell viability. Conversely, Pak1L83,L86,R299, Pak1R299 and Pak1 siRNA conferred sensitivity with a 65.2%+/-5.5%, 69.2%+/-3.3% and 73.0%+/-8.4% loss in viability, respectively. Finally, Pak1 plays a role in renal tumor growth in vivo. Only 33% of mice developed tumors in the Pak1L83,L86,R299 group and no tumors developed from Pak1R299 cell challenge. Together these findings point to Pak1 as an exciting target for therapy of renal cancer, which remains highly refractory to existing treatments.

p21-activated kinase 1 is activated through the mammalian target of rapamycin/p70 S6 kinase pathway and regulates the replication of hepatitis C virus in human hepatoma cells

Cellular mechanisms that regulate the replication of hepatitis C virus (HCV) RNA are poorly understood. p21-activated kinase 1 (PAK1) is a serine/threonine kinase that has been suggested to participate in antiviral signaling. We studied its role in the cellular control of HCV replication. Transfection of PAK1-specific small interfering RNA enhanced viral RNA and protein abundance in established replicon cell lines as well as cells infected with chimeric genotype 1a/2a HCV, despite reducing cellular proliferation, suggesting specific regulation of HCV replication. PAK1 knockdown did not reduce interferon regulatory factor 3-dependent gene expression, indicating that this regulation is independent of the retinoic acid-inducible gene I/interferon regulatory factor 3 pathway. On the other hand, LY294002 and rapamycin abolished PAK1 phosphorylation and enhanced HCV abundance, suggesting that the mammalian target of rapamycin (mTOR) is involved in PAK1 regulation of HCV. Small interfering RNA knockdown of the mTOR substrate p70 S6 kinase abrogated PAK1 phosphorylation and enhanced HCV RNA abundance, whereas overexpression of a constitutively active alternate substrate, eukaryotic translation initiation factor 4E-binding protein 1, increased cap-independent viral translation and viral RNA abundance without influencing PAK1 phosphorylation. Similar data indicated that mTOR is regulated by both phosphatidylinositol 3-kinase/Akt and ERK. Taken together, the data indicate that p70 S6 kinase activates PAK1 and contributes to phosphatidylinositol 3-kinase- and ERK-mediated regulation of HCV RNA replication.

Molecular targets for emerging anti-tumor therapies for neurofibromatosis type 1

Neurofibromatosis type 1 (NF1) is the most common cancer predisposition syndrome. NF1 patients present with a constellation of clinical manifestations and have an increased risk of developing certain benign and malignant tumors. This disease results from mutation within the gene encoding neurofibromin, a GTPase activating protein (GAP) for Ras. Functional loss of this protein compromises Ras inactivation, which leads to the aberrant growth and proliferation of neural crest-derived cells and, ultimately, tumor formation. Current management of NF1-associated malignancy involves radiation, surgical excision, and cytotoxic drugs. The limited success of these strategies has fueled researchers to further elucidate the molecular changes that drive tumor formation and progression. This discussion will highlight how intracellular signaling molecules, cell-surface receptors, and the tumor microenvironment constitute potential therapeutic targets, which may be relevant not only to NF1-related malignancy but also to other human cancers.

Protein kinase C β and δ isoenzymes mediate cholesterol accumulation in PMA-activated macrophages

Previously, we showed that PMA activation of human monocyte-derived macrophages stimulates macropinocytosis (i.e., fluid-phase endocytosis) of LDL and transforms these macrophages into foam cells. The current study aimed to learn which PKC isoenzymes mediate cholesterol accumulation in PMA-activated human macrophages incubated with LDL. Cholesterol accumulation by PMA-activated macrophages incubated with LDL was nearly completely inhibited (>85%) by the pan PKC inhibitors Go6850, Go6983, and RO 32-0432, but only was inhibited about 50% by the classical group PKC inhibitor, Go6976. This indicated that cholesterol accumulation was mediated by both a classical group and some other PKC isoenzyme. PKC beta was determined to be the classical group isoenzyme that mediated PMA-stimulated cholesterol accumulation. A pseudosubstrate myristoylated peptide inhibitor of PKC alpha and beta showed partial inhibition (congruent with 50%) of cholesterol accumulation. However, a small molecule inhibitor of PKC alpha, HBDDE, show minimal inhibition of cholesterol accumulation while a small molecule inhibitor of PKC beta, LY333513, could completely account for the inhibition of cholesterol accumulation by the classical group PKC isoenzyme. Thus, our findings show that beta and some other PKC isoenzyme, most likely delta, mediate cholesterol accumulation when macropinocytosis of LDL is stimulated in PMA-activated human monocyte-derived macrophages.

An active form of Vav1 induces migration of mammary epithelial cells by stimulating secretion of an epidermal growth factor receptor ligand

Background

Vav proteins are guanine nucleotide exchange factors (GEF) for Rho family GTPases and are activated following engagement of membrane receptors. Overexpression of Vav proteins enhances lamellipodium and ruffle formation, migration, and cell spreading, and augments activation of many downstream signaling proteins like Rac, ERK and Akt. Vav proteins are composed of multiple structural domains that mediate their GEF function and binding interactions with many cellular proteins. In this report we examine the mechanisms responsible for stimulation of cell migration by an activated variant of Vav1 and identify the domains of Vav1 required for this activity.

Results

We found that expression of an active form of Vav1, Vav1Y3F, in MCF-10A mammary epithelial cells increases cell migration in the absence or presence of EGF. Vav1Y3F was also able to drive Rac1 activation and PAK and ERK phosphorylation in MCF-10A cells in the absence of EGF stimulation. Mutations in the Dbl homology, pleckstrin homology, or cysteine-rich domains of Vav1Y3F abolished Rac1 or ERK activation in the absence of EGF and blocked the migration-promoting activity of Vav1Y3F. In contrast, mutations in the SH2 and C-SH3 domains did not affect Rac activation by Vav1Y3F, but reduced the ability of Vav1Y3F to induce EGF-independent migration and constitutive ERK phosphorylation. EGF-independent migration of MCF-10A cells expressing Vav1Y3F was abolished by treatment of cells with an antibody that prevents ligand binding to the EGF receptor. In addition, conditioned media collected from Vav1Y3F expressing cells stimulated migration of parental MCF-10A cells. Lastly, treatment of cells with the EGF receptor inhibitory antibody blocked the Vav1Y3F-induced, EGF-independent stimulation of ERK phosphorylation, but had no effect on Rac1 activation or PAK phosphorylation.

Conclusion

Our results indicate that increased migration of active Vav1 expressing cells is dependent on Vav1 GEF activity and secretion of an EGF receptor ligand. In addition, activation of ERK downstream of Vav1 is dependent on autocrine EGF receptor stimulation while active Vav1 can stimulate Rac1 and PAK activation independent of ligand binding to the EGF receptor. Thus, stimulation of migration by activated Vav1 involves both EGF receptor-dependent and independent activities induced through the Rho GEF domain of Vav1.

Phospholipase Cgamma/diacylglycerol-dependent activation of beta2-chimaerin restricts EGF-induced Rac signaling

Although receptor-mediated regulation of small G-proteins and the cytoskeleton is intensively studied, the mechanisms for attenuation of these signals are poorly understood. In this study, we have identified the Rac-GAP beta2-chimaerin as an effector of the epidermal growth factor receptor (EGFR) via coupling to phospholipase Cgamma (PLCgamma) and generation of the lipid second messenger diacylglycerol (DAG). EGF redistributes beta2-chimaerin to promote its association with the small GTPase Rac1 at the plasma membrane, as determined by FRET. This relocalization and association with Rac1 were impaired by disruption of the beta2-chimaerin C1 domain as well as by PLCgamma1 RNAi, thus defining beta2-chimaerin as a novel DAG effector. On the other hand, GAP-deficient beta2-chimaerin mutants show enhanced translocation and sustained Rac1 association in the FRET assays. Remarkably, RNAi depletion of beta2-chimaerin significantly extended the duration of Rac activation by EGF, suggesting that beta2-chimaerin serves as a mechanism that self-limits Rac activity in response to EGFR activation. Our results represent the first direct evidence of divergence in DAG signaling downstream of a tyrosine-kinase receptor via a PKC-independent mechanism.

The Ras-GRF1 exchange factor coordinates activation of H-Ras and Rac1 to control neuronal morphology

The Ras-GRF1 exchange factor has regulated guanine nucleotide exchange factor (GEF) activity for H-Ras and Rac1 through separate domains. Both H-Ras and Rac1 activation have been linked to synaptic plasticity and thus could contribute to the function of Ras-GRF1 in neuronal signal transduction pathways that underlie learning and memory. We defined the effects of Ras-GRF1 and truncation mutants that include only one of its GEF activities on the morphology of PC12 phaeochromocytoma cells. Ras-GRF1 required coexpression of H-Ras to induce morphological effects. Ras-GRF1 plus H-Ras induced a novel, expanded morphology in PC12 cells, which was characterized by a 10-fold increase in soma size and by neurite extension. A truncation mutant of Ras-GRF1 that included the Ras GEF domain, GRFdeltaN, plus H-Ras produced neurite extensions, but did not expand the soma. This neurite extension was blocked by inhibition of MAP kinase activation, but was independent of dominant-negative Rac1 or RhoA. A truncation mutant of Ras-GRF1 that included the Rac GEF domains, GRFdeltaC, produced the expanded phenotype in cotransfections with H-Ras. Cell expansion was inhibited by wortmannin or dominant-negative forms of Rac1 or Akt. GRFdeltaC binds H-Ras.GTP in both pulldown assays from bacterial lysates and by coimmunoprecipitation from HEK293 cells. These results suggest that coordinated activation of H-Ras and Rac1 by Ras-GRF1 may be a significant controller of neuronal cell size.

P21-activated kinase 1: convergence point in PDGF- and LPA-stimulated collagen matrix contraction by human fibroblasts

Fibroblast three-dimensional collagen matrix culture provides a tissue-like model that can be used to analyze cell form and function. The physiological agonists platelet-derived growth factor (PDGF) and lysophosphatidic acid (LPA) both stimulate human fibroblasts to contract floating collagen matrices. In this study, we show that the PDGF and LPA signaling pathways required for matrix contraction converge on p21-activated kinase 1 (PAK1) and its downstream effector cofilin1 and that contraction depends on cellular ruffling activity, rather than on the protrusion and retraction of cellular dendritic extensions. We also show that, depending on the agonist, different Rho effectors cooperate with PAK1 to regulate matrix contraction, Rho kinase in the case of PDGF and mDia1 in the case of LPA. These findings establish a unified framework for understanding the cell signaling pathways involved in fibroblast contraction of floating collagen matrices.

Ligation of cell surface-associated glucose-regulated protein 78 by receptor-recognized forms of alpha 2-macroglobulin: activation of p21-activated protein kinase-2-dependent signaling in murine peritoneal macrophages

Previous studies of the plasma proteinase inhibitor alpha2-macroglobulin (alpha2M) demonstrated that alpha2M-proteinase complexes (alpha2M*) modulate immune responses and promotes macrophage locomotion and chemotaxis. Alpha2M* binds to cell surface-associated glucose-regulated protein 78 (GRP78), which activates downstream signaling events. The role of p21-activated protein kinase-1 and -2 (PAK-1 and -2) in promoting cellular motility is well documented. In the current study, we examined the ability of alpha2M* to activate PAK-1 and PAK-2. Upon macrophage stimulation with alpha2M*, PAK-2 is autophosphorylated, resulting in increased kinase activity; however, PAK-1 is negligibly affected. Alpha2M*-stimulated macrophages showed a marked elevation in the levels of Rac x GTP. Receptor tyrosine phosphorylation upon binding of alpha2M* to GRP78, recruits PAK-2 to the plasma membrane via the adaptor protein NCK. Consistent with this hypothesis, silencing of GRP78 gene expression greatly attenuated the levels of membrane-associated PAK-2 and NCK. PAK-2 activity was markedly decreased by inhibition of tyrosine kinases and PI3K before alpha2M* stimulation. We further demonstrate that phosphorylation of Lin-11, Isl-1, Mec-3 (LIM) kinase and cofilin is promoted by treating macrophages with alpha2M*. Thus, alpha2M* regulates activation of the PAK-2-dependent motility mechanism in these cells.

Active p21-activated kinase 1 rescues MCF10A breast epithelial cells from undergoing anoikis

The protein kinase, PAK1, is overexpressed in human breast cancer and may contribute to malignancy through induction of proliferation and invasiveness. In this study, we examined the role of PAK1 in the survival of detached MCF10A breast epithelial cells to test whether it may also regulate the early stages of neoplasia. MCF10A cells undergo anoikis, as measured by the cleavage of caspase 3 and poly(ADP-ribose) polymerase (PARP), after more than 8 hours of detachment. Endogenous Akt, PAK1, and BAD are phosphorylated in attached MCF10A cells, but these phosphorylation events are all lost during the first 8 hours of detachment. Expression of constitutively active PAK1 or Akt suppresses the cleavage of caspase 3 and PARP in detached MCF10A cells. Co-overexpression of active PAK1 with dominant-negative Akt, or of active Akt with dominant-negative PAK1, still suppresses anoikis. Thus, Akt and PAK1 enhance survival through pathways that are at least partially independent. PAK1-dependent regulation of anoikis is likely to occur early in the apoptotic cascade as expression of dominant-negative PAK1 increased the cleavage of the upstream caspase 9, while constitutively active PAK1 inhibited caspase 9 activation. These results support a role for activated PAK1 in the suppression of anoikis in MCF10A epithelial cells.

Nuclear localization and chromatin targets of p21-activated kinase 1

Pak1 (p21-activated kinase 1), a conserved, mammalian signaling kinase, is a downstream effector of small GTPases Rac1 and Cdc42 and of growth factor signaling. Until now, a major focus of study has been on the cytosolic functions of Pak1, where it is an important modulator of cytoskeletal reorganization, consequently playing a major role in cell survival, migration, and invasion. In this report, we demonstrate the nuclear localization of Pak1 upon stimulation by epidermal growth factor. Three nuclear localization signals (NLSs) were identified in the N-terminal domain of Pak1. With mutational analysis, the importance of each NLS was elucidated. Mutation of all three NLSs eliminated the nuclear localization of Pak1. Expression of Pak1 as a fusion protein with Gal4-DNA binding domain and Gal4-luciferase activity showed that Pak1 might increase transcription. To identify the potential targets of nuclear Pak1, we used a Pak1-specific chromatin immunoprecipitation-based screening assay and identified a series of Pak1-interacting target chromatins, including phosphofructokinase-muscle isoform (PFK-M) and nuclear factor of activated T-cell (NFAT1) genes. Pak1 associated with the upstream enhancer sequence and promoter of PFK-M and was involved in the stimulation of the PFK-M expression. It also associated with a portion of the NFAT1 gene and its upstream region, leading to the repression of NFAT1 expression. These investigations provide proof-of-principle evidence that Pak1 could influence the expression of its putative chromatin targets in both a positive and a negative manner. Together, for the first time, these findings defined the NLSs of the Pak1, its association with chromatin, and the resulting modulation of transcription, thus opening new avenues to further the search for nuclear Pak1 functions and identify putative Pak1-interacting nuclear proteins.

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.