The PAKs come of age: Celebrating 18 years of discovery

PAK5 potentiates slug transactivation of N-cadherin to facilitate metastasis of renal cell carcinoma

Renal cell carcinoma (RCC) is an aggravating cancer with a poor prognosis and a high rate of metastasis. PAK5, a p21-activated kinases, has shown to be overexpressed in a variety of cancers, including RCC. In previous studies, we discovered that PAK5 regulates cell migration and invasion in RCC cell lines. However, the underlying mechanisms remain obscure. In this study, we consolidated that PAK5 confers a pro-metastatic phenotype RCC cells in vitro and exacerbates metastasis in vivo. High PAK5 expression was associated with an advanced TNM stage and a lower overall survival. Furthermore, PAK5 increases the expression level of N-cadherin. In terms of mechanism, PAK5 bound to Slug and phosphorylated it at serine 87. As a result, phosphorylated Slug transactivated N-cadherin, accelerating the epithelial-mesenchymal transition. Collectively, Slug is a novel PAK5 substrate, and PAK5-mediated phosphorylation of Slug-S87 increases N-cadherin and the pro-metastatic phenotype of RCC, implying that phosphorylated Slug-S87 could be a therapeutic target in progressive RCC.

Combined Analysis of DNA Methylome and Transcriptome Reveal Novel Candidate Genes Related to Porcine Escherichia coli F4ab/ac-Induced Diarrhea

Enterotoxigenic Escherichia coli (ETEC) that express F4 (K88) fimbriae are the principal microorganisms responsible for bacterial diarrhea in neonatal and pre-weaning piglets. To better understand the molecular effects of ETEC F4ab/ac infection, we performed a genome-wide comparison of the changes in DNA methylation and gene expression in ETEC F4ab/ac infected porcine intestinal epithelial cells. We characterized the pattern of changes in methylation and found 3297 and 1593 differentially methylated regions in cells infected with F4ab and F4ac, respectively. Moreover, 606 and 780 differentially expressed genes (DEGs) in ETEC F4ab and F4ac infected cells were detected and these genes were highly enriched in immune/defense response related pathways. Integrative analysis identified 27 and 10 genes showing inverse correlations between promoter methylation and expression with ETEC F4ab/ac infection. Altered DNA methylation and expression of various genes suggested their roles and potential functional interactions upon ETEC F4ab/ac infection. Further functional analyses revealed that three DEGs (S100A9, SGO1, and ESPL1) in F4ab infected cells and three DEGs (MAP3K21, PAK6, and MPZL1) in F4ac infected cells are likely involved in the host cells response to ETEC infection. Our data provides further insight into the epigenetic and transcriptomic alterations of ETEC F4ab/ac infected porcine intestinal epithelial cells, and may advance the identification of biomarkers and drug targets for predicting susceptibility to and controlling ETEC F4ab/ac induced diarrhea.

PAK1 expression determines poor prognosis and immune evasion in metastatic renal cell carcinoma patients

BACKGROUND

Previous studies have shown the prognostic value of PAK1 expression in different tumor patients, including nonmetastatic renal cell carcinoma. In this study, we explored the prognostic and drug predictive value of PAK1 expression in metastatic renal cell carcinoma (mRCC) patients treated with tyrosine kinase inhibitors (TKIs).

MATERIALS AND METHODS

We retrospectively enrolled 138 mRCC patients treated with TKIs from a single institution from 2005 to 2014. Analyses were based on 111 patients who met our inclusion criteria. The validation set enrolled 538 RCC patients from The Cancer Genome Atlas Kidney Renal Clear Cell Carcinoma cohort (TCGA KIRC) between 1998 and 2013 in North America. PAK1 expression was assessed by immunohistochemistry (IHC) on tissue microarrays.

RESULTS

High PAK1 expression was associated with short overall survival (OS) (P < 0.001) and progression-free survival (PFS) (P = 0.008). Multivariate analyses further indicated that PAK1 expression was an independent prognostic factor for OS (hazard ratio 3.301 [95% confidence interval 2.579-10.899], P < 0.001) and PFS (hazard ratio 3.108 [95% confidence interval 1.795-5.381], P < 0.001). Subgroup analyses suggested that PAK1 was more significant in patients with the intermediate risk group of Heng risk criteria (OS, P = 0.004). Of note, patients treated with Sunitinib showed improved outcome in the low PAK1 subgroup (OS, P = 0.002; PFS, P = 0.013). Finally, relationship was found between PAK1 expression and natural killer cell-mediated cytotoxicity according to gene profile investigation.

CONCLUSIONS

High PAK1 expression predicted dismal prognosis in mRCC patients treated with TKIs. Besides, PAK1 was a potential predictor for TKIs treatments.

The subcellular localization of type I p21-activated kinases is controlled by the disordered variable region and polybasic sequences

p21-activated kinases (PAKs) are serine/threonine kinase effectors of the small GTPases Rac and Cdc42 and major participants in cell adhesion, motility, and survival. Type II PAKs (PAK4, -5, and -6) are recruited to cell-cell boundaries, where they regulate adhesion dynamics and colony escape. In contrast, the type I PAK, PAK1, does not localize to cell-cell contacts. We have now found that the other type I PAKs (PAK2 and PAK3) also fail to target to cell-cell junctions. PAKs contain extensive similarities in sequence and domain organization; therefore, focusing on PAK1 and PAK6, we used chimeras and truncation mutants to investigate their differences in localization. We observed that a weakly conserved sequence region (the variable region), located between the Cdc42-binding CRIB domain and the kinase domain, inhibits PAK1 targeting to cell-cell junctions. Accordingly, substitution of the PAK1 variable region with that from PAK6 or removal of this region of PAK1 resulted in its localization to cell-cell contacts. We further show that Cdc42 binding is required, but not sufficient, to direct PAKs to cell-cell contacts and that an N-terminal polybasic sequence is necessary for PAK1 recruitment to cell-cell contacts, but only if the variable region-mediated inhibition is released. We propose that all PAKs contain cell-cell boundary-targeting motifs but that the variable region prevents type I PAK accumulation at junctions. This highlights the importance of this poorly conserved, largely disordered region in PAK regulation and raises the possibility that variable region inhibition may be released by cellular signals.

PAK5 Induces EMT and Promotes Cell Migration and Invasion by Activating the PI3K/AKT Pathway in Ovarian Cancer

Ovarian cancer is the most lethal gynecologic cancer and currently ranks fifth in causing cancer-related deaths among women. P21^cdc42/rac1-activated kinase 5 (PAK5) is a newly identified protein that has been indicated to have oncogenic potential. The present study investigated the expression level of PAK5 in clinical ovarian cancer and the functional roles of PAK5 in ovarian cancer progression. It was initially found that PAK5 was highly expressed in ovarian cancer tissues, particularly in patients with distant metastasis. Higher expression of PAK5 predicted poor survival fates in patients with ovarian cancer (p = 0.008). Knockdown of PAK5 in SKOV3 cells caused epithelial cell phenotypes, whereas overexpression of PAK5 led to remarkable mesenchymal cell phenotypes in A2780 cells. When PAK5 was depleted from SKOV3 cells, cells exhibited impaired wound recovery abilities. Cell migration and invasion abilities were also significantly inhibited. On the contrary, when PAK5 was overexpressed in A2780 cells, the wound recovery ability was enhanced by 68%. Cell migration and invasion abilities were consistently increased to approximately 2-fold. After knockdown of PAK5, the phosphorylation levels of PI3K p85 at Tyr458 and its downstream AKT at Ser473 were both decreased. The total protein of PI3K and AKT as well as the phosphorylation level of AKT at Thr308 remained unaffected. These data suggested that PI3K induced epithelial-to-mesenchymal transition and promoted cell migration and invasion by activating the PI3K/AKT pathway in ovarian cancer. The oncogenic potential of PAK5 in ovarian cancer might suggest that any therapeutic strategies targeting PAK5 had the promising value for ovarian cancer treatment.

RIT1 controls actin dynamics via complex formation with RAC1/CDC42 and PAK1

RIT1 belongs to the RAS family of small GTPases. Germline and somatic RIT1 mutations have been identified in Noonan syndrome (NS) and cancer, respectively. By using heterologous expression systems and purified recombinant proteins, we identified the p21-activated kinase 1 (PAK1) as novel direct effector of RIT1. We found RIT1 also to directly interact with the RHO GTPases CDC42 and RAC1, both of which are crucial regulators of actin dynamics upstream of PAK1. These interactions are independent of the guanine nucleotide bound to RIT1. Disease-causing RIT1 mutations enhance protein-protein interaction between RIT1 and PAK1, CDC42 or RAC1 and uncouple complex formation from serum and growth factors. We show that the RIT1-PAK1 complex regulates cytoskeletal rearrangements as expression of wild-type RIT1 and its mutant forms resulted in dissolution of stress fibers and reduction of mature paxillin-containing focal adhesions in COS7 cells. This effect was prevented by co-expression of RIT1 with dominant-negative CDC42 or RAC1 and kinase-dead PAK1. By using a transwell migration assay, we show that RIT1 wildtype and the disease-associated variants enhance cell motility. Our work demonstrates a new function for RIT1 in controlling actin dynamics via acting in a signaling module containing PAK1 and RAC1/CDC42, and highlights defects in cell adhesion and migration as possible disease mechanism underlying NS.

Noonan syndrome (NS) belongs to the RASopathies, a group of developmental diseases caused by mutations in genes encoding RAS-MAPK pathway components. Germline mutations in RIT1 have been identified in NS. RIT1 belongs to the RAS superfamily, however, the cellular function of RIT1 remains elusive. We show that RIT1 binds p21-activated kinase 1 (PAK1), an effector of the RHO GTPases RAC1 and CDC42, which are important regulators of cytoskeletal dynamics. NS-associated RIT1 mutants enhance complex formation between RIT1, RAC1/CDC42 and PAK1. Expression of wild-type or mutant forms of RIT1 caused loss of stress fibers and mature focal adhesions and enhanced cell motility. Our data suggest that dysfunction in actin dynamics is a novel aspect in the pathophysiology of RASopathies.

Mouse Genetic Models of Human Brain Disorders

Over the past three decades, genetic manipulations in mice have been used in neuroscience as a major approach to investigate the in vivo function of genes and their alterations. In particular, gene targeting techniques using embryonic stem cells have revolutionized the field of mammalian genetics and have been at the forefront in the generation of numerous mouse models of human brain disorders. In this review, we will first examine childhood developmental disorders such as autism, intellectual disability, Fragile X syndrome, and Williams-Beuren syndrome. We will then explore psychiatric disorders such as schizophrenia and lastly, neurodegenerative disorders including Alzheimer’s disease and Parkinson’s disease. We will outline the creation of these mouse models that range from single gene deletions, subtle point mutations to multi-gene manipulations, and discuss the key behavioral phenotypes of these mice. Ultimately, the analysis of the models outlined in this review will enhance our understanding of the in vivo role and underlying mechanisms of disease-related genes in both normal brain function and brain disorders, and provide potential therapeutic targets and strategies to prevent and treat these diseases.

PAK6 targets to cell-cell adhesions through its N-terminus in a Cdc42-dependent manner to drive epithelial colony escape

The six serine/threonine kinases in the p21-activated kinase (PAK) family are important regulators of cell adhesion, motility and survival. PAK6, which is overexpressed in prostate cancer, was recently reported to localize to cell-cell adhesions and to drive epithelial cell colony escape. Here we report that PAK6 targeting to cell-cell adhesions occurs through its N-terminus, requiring both its Cdc42/Rac interactive binding (CRIB) domain and an adjacent polybasic region for maximal targeting efficiency. We find PAK6 localization to cell-cell adhesions is Cdc42-dependent, as Cdc42 knockdown inhibits PAK6 targeting to cell-cell adhesions. We further find the ability of PAK6 to drive epithelial cell colony escape requires kinase activity and is disrupted by mutations that perturb PAK6 cell-cell adhesion targeting. Finally, we demonstrate that all type II PAKs (PAK4, PAK5 and PAK6) target to cell-cell adhesions, albeit to differing extents, but PAK1 (a type I PAK) does not. Notably, the ability of a PAK isoform to drive epithelial colony escape correlates with its targeting to cell-cell adhesions. We conclude that PAKs have a broader role in the regulation of cell-cell adhesions than previously appreciated.

Dysregulation of PAK1 Is Associated with DNA Damage and Is of Prognostic Importance in Primary Esophageal Small Cell Carcinoma

Primary esophageal small cell carcinoma (PESCC) is a rare, but fatal subtype of esophageal carcinoma. No effective therapeutic regimen for it. P21-activated kinase 1 (PAK1) is known to function as an integrator and an indispensable node of major growth factor signaling and the molecular therapy targeting PAK1 has been clinical in pipeline. We thus set to examine the expression and clinical impact of PAK1 in PESCC. The expression of PAK1 was detected in a semi-quantitative manner by performing immunohistochemistry. PAK1 was overexpressed in 22 of 34 PESCC tumors, but in only 2 of 18 adjacent non-cancerous tissues. Overexpression of PAK1 was significantly associated with tumor location (p = 0.011), lymph node metastasis (p = 0.026) and patient survival (p = 0.032). We also investigated the association of PAK1 with DNA damage, a driven cause for malignancy progression. γH2AX, a DNA damage marker, was detectable in 18 of 24 (75.0%) cases, and PAK1 expression was associated with γH2AX (p = 0.027). Together, PAK1 is important in metastasis and progression of PESCC. The contribution of PAK1 to clinical outcomes may be involved in its regulating DNA damage pathway. Further studies are worth determining the potentials of PAK1 as prognostic indicator and therapeutic target for PESCC.

Personalizing risk stratification by addition of PAK1 expression to TNM staging: improving the accuracy of clinical decision for gastroesophageal junction adenocarcinoma

Gastroesophageal junction adenocarcinoma (GEJA) is an aggressive malignancy with an alarmingly rising incidence. TNM staging is widely used by oncologists to stratify prognosis as well as direct therapeutic strategies. However, inadequate lymphadenectomy is frequently encountered for GEJA and largely confounds prognosis resulting from TNM staging. Thus, a molecular biomarker, which can accurately forecast the risk of nodal metastasis in patients with inadequate lymphadenectomy, is required to guide precisely clinical decision. In this study, bioinformatics and pathological analysis identified that p21 protein-activated kinase 1 (PAK1) is associated with lymph nodal metastasis of GEJA. The PAK1 H-score was lower in the patients with negative lymph nodes than that in patients with positive (metastatic) lymph nodes (6.865 ± 3.376, 9.370 ± 2.530, respectively; p < 0.001). The PAK1 H-score in lymph nodes was positively correlated with that in primary tumors (PTs; p < 0.001; r = 0.475). PAK1 H-scores in PTs had the best performance based on its area under the receiver-operating characteristic (ROC) curve compared with PAK1 H-scores in lymph nodes, histological grade, lymph nodal metastasis status, tumor size, depth of tumor, TNM stage and number of resected lymph nodes. Multivariate Cox proportional hazard and Fine and Gray models showed that histological grade 3, Charlson comorbidity index > 7 and high PAK1 expression in PTs were associated with significantly increased risk of recurrence and cancer-related death. In conclusion, high PAK1 expression in PTs is predictive of node metastasis and can be easily integrated in the clinical decision process for personalized therapeutics of GEJA.

Participation of group I p21-activated kinases in neuroplasticity

PAKs are a family of serine/threonine protein kinases activated by small GTPases of the Rho family, including Rac and Cdc42, and are categorized into group I (isoforms 1, 2 and 3) and group II (isoforms 4, 5 and 6). PAK1 and PAK3 are critically involved in biological mechanisms associated with neurodevelopment, neuroplasticity and maturation of the nervous system, and changes in their activity have been detected in pathological disorders, such as Alzheimer's disease, Huntington's disease and mental retardation. The group I PAKs have been associated with neurological processes due to their involvement in intracellular mechanisms that result in molecular and cellular morphological alterations that promote cytoskeletal outgrowth, increasing the efficiency of synaptic transmission. Their substrates in these processes include other intracellular signaling molecules, such as Raf, Mek and LIMK, as well as other components of the cytoskeleton, such as MLC and FLNa. In this review, we describe the characteristics of group I PAKs, such as their molecular structure, mechanisms of activation and importance in the neurobiological processes involved in synaptic plasticity.

Remarkable reductions of PAKs in the brain tissues of scrapie-infected rodent possibly linked closely with neuron loss

Prion diseases are irreversible progressive neurodegenerative diseases characterized in the brain by PrP(Sc) deposits, neuronal degeneration, gliosis and by cognitive, behavioral and physical impairments, leading to severe incapacity and inevitable death. Proteins of the p21-activated kinase (PAK) family are noted for roles in gene transcription, cytoskeletal dynamics, cell cycle progression and survival signaling. In the present study, we aimed to identify the potential roles of PAKs during prion infection, utilizing the brains of scrapie agent-infected hamsters. Western blots and immunohistochemical assays showed that brain levels of PAK3 and PAK1, as well as their upstream activator Rac/cdc42 and downstream substrate Raf1, were remarkably reduced at terminal stage. Double-stained immunofluorescent assay demonstrated that PAK3 was expressed mainly in neurons. Dynamic analyses of the brain samples collected at the different time points during the incubation period illustrated successive decreases of PAK3, PAK1 and Raf1, especially phosphor Raf1, which correlated well with neuron loss. Rac/cdc42 in the brain tissues increased at early stage and reached to the top at mid-late stage, but diminished at final stage. Unlike the alteration of PAKs in vivo, PAK3 and PAK1, as well as Rac/cdc42 and Raf1 in the prion-infected cell line SMB-S15 remained unchanged compared with those of its normal cell line SMB-PS. Our data here indicate that the functions of PAKs and their associated signaling pathways are seriously affected in the brains of prion disease, which appear to associate closely with the extensive neuron loss.

An oncogenic kinase: putting PAK5 forward

INTRODUCTION

Overexpression of p21-activated kinase 5 (PAK5) is discovered in many tumors, probably due to its regulation in cytoskeleton, antiapoptosis and proliferation. A better understanding of the modulation mechanisms of PAK5 is needed for the development of tumor treatment where current therapeutics is inadequate.

AREAS COVERED

This review discusses the current understanding of PAK5 functions as an oncogenic kinase in tumor cellular regulation. Mechanisms of action and molecular pathways involved in cytoskeleton regulation, antiapoptosis and proliferation of tumors are discussed.

EXPERT OPINION

PAKs are serine/threonine kinases and downstream effectors for Cdc42 and Rac, the subfamilies of Rho small GTPases. PAK5 shares sequence identities in p21-GTPase-binding domain and kinase domain and is completely different in other regions compared with other PAKs. Overexpression of PAK5 has been found in several tumors, probably due to its contribution to proliferation, cytoskeleton and anti-apoptosis. Additional regulation mechanisms which are independent of Rho GTPases also indicate that PAK5 functions as a special signal molecule in cellular signaling pathways of tumor progression.

Protein kinase CK2 phosphorylates and activates p21-activated kinase 1

Activation of the p21-activated kinase 1 (PAK1) is achieved through a conformational change that converts an inactive PAK1 dimer to an active monomer. In this paper, we show that this change is necessary but not sufficient to activate PAK1 and that it is, rather, required for CK2-dependent PAK1(S223) phosphorylation that converts a monomeric PAK1 into a catalytically active form. This phosphorylation appears to be essential for autophosphorylation at specific residues and overall activity of PAK1. A phosphomimetic mutation (S223E) bypasses the requirement for GTPases in PAK1 activation, whereas the constitutive activity of the PAK1 mutant (PAK1(H83,86L)), postulated to mimic GTPase-induced structural changes, is abolished by inhibition of S223 phosphorylation. Thus, S223 is likely accessible to CK2 upon conformational changes of PAK1 induced by GTPase-dependent and GTPase-independent stimuli, suggesting that S223 phosphorylation may play a key role in the final step of the PAK1 activation process. The physiological significance of this phosphorylation is reinforced by the observations that CK2 is responsible for epidermal growth factor-induced PAK1 activation and that inhibition of S223 phosphorylation abrogates PAK1-mediated malignant transformation of prostate epithelial cells. Taken together, these findings identify CK2 as an upstream activating kinase of PAK1, providing a novel mechanism for PAK1 activation.

Prognostic importance and therapeutic implications of PAK1, a drugable protein kinase, in gastroesophageal junction adenocarcinoma

Gastroesophageal junction (GEJ) adenocarcinoma is a lethal cancer with rising incidence, yet the molecular biomarkers that have strong prognostic impact and also hold great therapeutic promise remain elusive. We used a data mining approach and identified the p21 protein-activated kinase 1 (PAK1), an oncogene and drugable protein kinase, to be among the most promising targets for GEJ adenocarcinoma. Immunoblot analysis and data mining demonstrated that PAK1 protein and mRNA were upregulated in cancer tissues compared to the noncancerous tissues. Immunohistochemistry revealed PAK1 overexpression in 72.6% of primary GEJ adenocarcinomas (n = 113). A step-wise increase in PAK1 levels was noted from paired normal epithelium, to atypical hyperplasia and adenocarcinoma. PAK1 overexpression in tumor was associated with lymph node (LN) metastasis (P<0.001), advanced tumor stage (P<0.001), large tumor size (P = 0.006), residual surgical margin (P = 0.033), and unfavorable overall survival (P<0.001). Multivariate analysis showed PAK1 overexpression is an independent high-risk prognostic predictor (P<0.001). Collectively, PAK1 is overexpressed during tumorigenic progression and its upregulation correlates with malignant properties mainly relevant to invasion and metastasis. PAK1 expression could serve as a prognostic predictor that holds therapeutic promise for GEJ adenocarcinoma.