Role Of Pak1 Research Articles

Abstract 754: Nuclear liaison between PAK4 and PPARγ regulates radiation-induced MOX-1-mediated epithelial-mesenchymal transition and cell migration via a ROS-dependent mechanism in glioblastoma.

Abstract Despite the use of post-surgical radiation and chemotherapy, the mean survival rate is less than two years in glioblastoma (GBM), the most lethal human brain tumor. Augmented cell invasion and migration and increased tumor recurrence after ionizing radiation (IR) treatment is, in part, attributed to the IR-induced elevation in reactive oxygen species (ROS) levels and ROS-dependent signaling in tumors. Several reports, including our own studies, have confirmed the PAK4 oncogenic role in the regulation of tumor cell migration and invasion in GBM and various other tumors. However, the potential role of PAK4 in post-irradiated tumor malignancy and the underlying molecular mechanisms still remain unknown. Here, we show the significant PAK4 nuclear translocation in correlation with elevated ROS-mediated epithelial-mesenchymal transition (EMT) in IR-treated human glioma xenograft cells 4910 and 5310. Surprisingly, protein-transcription factor (TF) interaction arrays revealed a significant binding of nuclear-PAK4 with several TFs involved in tumor progression including AhR/ARNT, GATA1/2, FOXI1, Pax4, PPARα, PPARγ and TCF/LEF. We also observed that the nuclear co-localization of PAK4/PPARγ was elevated in correlation with increasing GBM grades. Most importantly, IR treatment elevated the PAK4/PPARγ nuclear interaction and increased MOX-1 expression, a PPARγ-target gene. In addition, EMSA and ChIP assays confirmed the presence of PAK4 in the PPARγ-DNA binding complex and recruitment onto MOX-1 promoter, which is further elevated in IR-treated cells. Stable shRNA-mediated knockdown of PPARγ or inhibition with a specific inhibitor, GW9662, abrogated IR-induced ROS levels, EMT and cell migration in vitro. siRNA-mediated PAK4 downregulation significantly decreased PPARγ DNA-binding activity and MOX-1 expression and inhibited EMT. On the other hand, PAK4 overexpression reversed GW9662-inhibited ROS levels, EMT and migration. In vivo experiments in nude mice showed the IR-induced PAK4/PPARγ nuclear co-localization in tumors. Furthermore, PAK4.si inhibited tumor growth and ROS-induced MOX-1 levels in IR-treated tumors. In summary, our studies not only demonstrate a prominent PAK4/PPARγ nuclear interaction but also suggest a novel regulation of PPARγ-mediated EMT by PAK4, thereby indicating the future therapeutic potential of targeting PAK4/PPARγ binding in glioma treatment. Citation Format: Divya Kesanakurti, Dilip Maddirela, Meena Gujrati, Sanjeeva Mohanam, Jasti S. Rao. Nuclear liaison between PAK4 and PPARγ regulates radiation-induced MOX-1-mediated epithelial-mesenchymal transition and cell migration via a ROS-dependent mechanism in glioblastoma. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 754. doi:10.1158/1538-7445.AM2013-754

Depletion of PAK1 enhances Ubiquitin-mediated Survivin degradation in pancreatic β-cells

Functional β-cell mass deficiency in diabetes results from imbalanced β-cell death and replication, and decreased PAK1 protein levels in human islets from donors with type 2 diabetes implicates a possible role for PAK1 in maintaining β-cell mass. Here, we aim to address the linkage between PAK1 and Survivin, a protein essential for β-cell replication. PAK1 knockout (KO) mouse islets exhibited decreased expression of Survivin protein. MIN6 β-cells with siRNA-mediated suppression of PAK1 also had decreased Survivin protein and exhibited an increased level of ubiquitinated-Survivin. However, no significant changes in Survivin mRNA were found in islets from PAK1 KO mice and PAK1-depleted MIN6 β-cells. The decreased Survivin level in MIN6 cells subjected to hyperglycemic stress was prevented by expression of exogenous PAK1. Moreover, overexpressing Survivin restored proliferation of β-cells that was impaired by the loss of PAK1. These data implicate a role for PAK1 in regulating Survivin protein stability in the β-cell and suggest PAK1 as a potential molecular target for the restoration of β-cell mass.

The Pak4 Protein Kinase in Breast Cancer

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

Mesalamine modulates intercellular adhesion through inhibition of p-21 activated kinase-1

Mesalamine (5-ASA) is widely used for the treatment of ulcerative colitis, a remitting condition characterized by chronic inflammation of the colon. Knowledge about the molecular and cellular targets of 5-ASA is limited and a clear understanding of its activity in intestinal homeostasis and interference with neoplastic progression is lacking. We sought to identify molecular pathways interfered by 5-ASA, using CRC cell lines with different genetic background. Microarray was performed for gene expression profile of 5-ASA-treated and untreated cells (HCT116 and HT29). Filtering and analysis of data identified three oncogenic pathways interfered by 5-ASA: MAPK/ERK pathway, cell adhesion and β-catenin/Wnt signaling. PAK1 emerged as a consensus target of 5-ASA, orchestrating these pathways. We further investigated the effect of 5-ASA on cell adhesion. 5-ASA increased cell adhesion which was measured by cell adhesion assay and transcellular-resistance measurement. Moreover, 5-ASA treatment restored membranous expression of adhesion molecules E-cadherin and β-catenin. Role of PAK1 as a mediator of mesalamine activity was validated in vitro and in vivo. Inhibition of PAK1 by RNA interference also increased cell adhesion. PAK1 expression was elevated in APCmin polyps and 5-ASA treatment reduced its expression. Our data demonstrates novel pharmacological mechanism of mesalamine in modulation of cell adhesion and role of PAK1 in APCmin polyposis. We propose that inhibition of PAK1 expression by 5-ASA can impede with neoplastic progression in colorectal carcinogenesis. The mechanism of PAK1 inhibition and induction of membranous translocation of adhesion proteins by 5-ASA might be independent of its known anti-inflammatory action.

Abstract 4004: P21-activated kinases (PAKs) are novel therapeutic targets in human breast cancer

Abstract P21-activated kinases (PAKs) are important mediators of Rac and Cdc42 GTPase function as well as pathways required for Ras-driven tumorigenesis. PAKs have been implicated in signaling by growth factor receptors and morphogenetic processes that control cell polarity, invasion and actin cytoskeleton organization. To better understand the role of PAK1 and PAK4 in breast tumorigenesis, genomic copy number, mRNA and protein expression were determined for a panel of 291 human breast adenocarcinomas and loss-of function was assessed in cell lines using genetic and pharmacologic approaches. Interestingly, PAK1 genomic amplification at 11q13 was prevalent in luminal breast cancer and PAK1 levels in early ductal carcinoma in situ (DCIS) samples was similar to that of primary adenocarcinoma suggesting that PAK1 dysregulation could be an early event in some breast cancers. PAK1 protein expression in adenocarcinoma was associated with lymph node invasion and further increased expression was observed in metastatic tumors. In addition, primary and secondary (28 local recurrences, 56 metastases) adenocarcinomas from a total of 86 patients (81 with matching samples) were evaluated for PAK1 and PAK4 expression to determine modulation of these kinases following adjuvant hormone therapy. Breast cancer cell lines with PAK1 genomic amplification rapidly underwent caspase activation and apoptosis following inhibition of this kinase. Together, our results provide evidence for dysregulation of PAK1 in breast cancer and a role for PAK1 in cellular survival and proliferation in this indication. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4004. doi:1538-7445.AM2012-4004

Abstract 228: PAK1 kinase promotes cell motility through CRK serine phosphorylation in non-small cell lung cancer cells

Abstract The role of c-Crk (CRK) in promoting metastasis is well described but the role of CRK phosphorylation and the corresponding signaling events are not well explained. We have noticed that CRK serine 41 phosphorylation is inversely correlated with p120-catenin and E-cadherin expressions in non-small cell lung cancer (NSCLC) cells. Therefore, we investigated the role of CRK serine 41 phosphorylation in the down-regulation of p120-catenin and cell motility in NSCLC cells. We prepared A549 cell lines stably expressing phosphomimetic and phosphodeficient CRK serine 41 constructs. A549 cells expressing phosphomimetic CRK seine 41 demonstrated a more aggressive behavior in would healing assay and on the contrary, expression of phosphodeficient CRK serine 41 in A549 cells reduced cell motility. A549 cells expressing phosphomimetic CRK serine 41 had lower p120-catenin mRNA and promoter activity and vice versa. In addition, we provide evidence that PAK1 mediates CRK serine 41 phosphorylation. RNAi mediated silencing of PAK1 increased p120-catenin level and also decreased cell motility in A549 and H157 cells. These effects were abrogated in A549 cells expressing phosphomimetic CRK serine 41. In summary, our data provide evidence for the role of PAK1 in promotion of cell motility and down regulation of p120-catenin through CRK serine 41 phosphorylation in NSCLC cells. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 228. doi:1538-7445.AM2012-228

Novel roles of PAK1 in the heart

Our work and others’ over the past few years have led to the identification of new roles of PAK1 in cardiac physiology, such as the regulation of cardiac ion channel and actomyosin function. More recent studies have revealed that PAK1-deficient mice were vulnerable to cardiac hypertrophy and readily progress to failure under sustained pressure overload and susceptible to ischemia/reperfusion injury. Our further study indicated that the PAK1 activator FTY720 was able to prevent this pressure overload-induced hypertrophy in wild-type mice without compromising their cardiac functions. A cardiac protective effect against ischemia/reperfusion injury by FTY720 was also observed in both rat and mouse models by us and others. Thus, these studies suggest that PAK1 is more important in the heart than previously thought, in particular a therapeutic potential of PAK1 activators. In the future, in-depth investigations are required to further substantiate our hypotheses on mechanisms for PAK1 function in the heart and to explore a therapeutic potential of FTY720 and other PAK1 activators in heart disease conditions.

Abstract B8: Targeting p21-activated kinases in human lung cancer

Abstract P21-activated kinases (PAKs) are important mediators of Rac and Cdc42 GTPase function as well as pathways required for Ras-driven tumorigenesis. PAKs have been implicated in signaling by growth factor receptors and morphogenetic processes that control cell polarity, invasion and actin cytoskeleton organization. To better understand the role of PAK1 in lung tumorigenesis, PAK1 genomic copy number and expression were determined for a panel of adenocarcinoma and squamous non-small cell lung cancers (NSCLC) and small cell lung cancers (SCLC). 64% of squamous NSCLC samples were positive for PAK1 expression and 52% of all cases showed staining of moderate (2+) or strong (3+) immunohistochemical intensity in the malignant cells. Nuclear localization of PAK1 was also evident in a significant proportion of squamous NSCLCs. Adjacent normal lung tissue did not express appreciable levels of PAK1. Selective PAK1 inhibition was associated with delayed cell cycle progression in vitro and in vivo. NSCLC cells were profiled using a library of pathway-targeted small molecule inhibitors and several synergistic combination therapies, including combination with antagonists of inhibitor of apoptosis (IAP) proteins, were revealed for PAK1. Dual inhibition of PAK1 and X chromosome-linked IAP (XIAP) efficiently increased effector caspase activation and apoptosis of NSCLC cells. Together, our results provide evidence for dysregulation of PAK1 in NSCLC and a role for PAK1 in cellular survival and proliferation in this indication.

P21-activated protein kinase 1 is overexpressed in gastric cancer and induces cancer metastasis

P21-activated protein kinase (Pak1), a main downstream effector of small Rho GTPases, plays an important role in the regulation of cell morphogenesis, motility, mitosis and angiogenesis. However, the role of Pak1 in gastric cancer metastasis remains unclear. Here, we showed that Pak1 is overexpressed in gastric cancer tissues from 74 patients by immunohistochemistry. Overexpression of Pak1 was associated with metastasis and prognosis of gastric cancer. In addition, overexpression of Pak1 increased gastric cancer cell motility and invasion, whereas downregulation of Pak1 expression reduced gastric cancer cell migration and invasion. In further study, data showed that activated Pak1 inhibited stress fiber and focal adhesion complex formation in gastric cancer cells and led to the formation of motile phenotypes. Importantly, activated Pak1 elicited phosphorylation of the ERK and JNK-dependent pathway in gastric cancer cell lines. In conclusion, our results suggest that Pak1 is overexpressed in gastric cancer and plays an important role in the metastasis of gastric cancer. The mechanism by which Pak1 induces cancer metastasis may involve activation of ERK and JNK.

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.

PAK4 is required for regulation of the cell‐cycle regulatory protein p21, and for control of cell‐cycle progression

The serine/threonine kinase PAK4 regulates cytoskeletal architecture, and controls cell proliferation and survival. In most adult tissues PAK4 is expressed at low levels, but overexpression of PAK4 is associated with uncontrolled proliferation, inappropriate cell survival, and oncogenic transformation. Here we have studied for the first time, the role for PAK4 in the cell cycle. We found that PAK4 levels peak dramatically but transiently in the early part of G1 phase. Deletion of Pak4 was also associated with an increase in p21 levels, and PAK4 was required for normal p21 degradation. In serum-starved cells, the absence of PAK4 led to a reduction in the amount of cells in G1, and an increase in the amount of cells in G2/M phase. We propose that the transient increase in PAK4 levels at early G1 reduces p21 levels, thereby abrogating the activity of CDK4/CDK6 kinases, and allowing cells to proceed with the cell cycle in a precisely coordinated way.

Erk1-Mediated Development of Left Ventricular Cardiac Hypertrophy in a P21 Activated Kinase-1 (Pak1) Knockout Mouse Model

Pak1, a serine/threonine protein kinase, plays a critical role in cardiac excitation-contraction. We hypothesize that Pak1 prevents or attenuates the development of left ventricular (LV) cardiac hypertrophy. Six wild-type (WT) and six Pak1-knockout (KO) mice were randomized into four groups of three mice each to receive subcutaneously 25μg/g/day of isoproterenol (ISO) or saline (CTRL) for seven days. Transthoracic echocardiography showed in the Pak1-KO/ISO group vs. WT/ISO group, respectively: i) increased LV fractional shortening (%)(61.65 ± 2.72 vs. 48.02 ± 4.01; p = 0.048); ii) reduced LV chamber volume (μL) in diastole (36.77 ± 4.19 vs. 73.92 ± 3.95; p = 0.003) and systole (4.14 ±0.54 vs. 16.24 ± 2.19; p = 0.0058); iii) increased isoproterenol-induced LV cardiac hypertrophy in Pak1-KO mice (40% increase in LV mass, p = 0.049) vs. WT mice (15.2 % increase in LV mass, p = 0.07); iv) enhanced early filling deceleration time (mm/s2) (−80460 ± 13620 vs. −21550 ± 3560; p = 0.014). We also generated novel evidence that Erk1 and Pak1 establish protein-protein interaction in whole cardiac tissue, as assessed by co-immunoprecipitation. Western immunoblotting of Erk1 phosphorylation in whole cardiac tissue showed maximal Erk1 activation in Pak1-KO/ISO mice vs. all other groups (phosphoErk1/totalErk1 ratio in Pak1-KO/ISO 0.80 ± 0.005 vs.: i) WT/CTRL 0.042 ± 0.02171, p<0.0001; ii) WT/ISO 0.3086 ± 0.0924, p=0.006; iii) Pak1-KO/CTRL 0.6020 ± 0.046, p=0.013), whereas Erk1 phosphorylation was consistently reduced in lysates obtained from adenovirally-infected adult rat cardiomyocytes that express constitutively active Pak1 vs. dominant negative Pak1 (phosphoErk1/actin ratio: 7.06% vs. 73.73%, respectively). In conclusion, Pak1-null mice develop LV cardiac hypertrophy due to increased Erk1 activation, indicating a role for Pak1 as a natural inhibitor of Erk1 and a novel anti-hypertrophic signaling molecule.

P21-activated protein kinase 1 induces colorectal cancer metastasis involving ERK activation and phosphorylation of FAK at Ser-910

Pak1 has been reported to be overexpressed in colorectal cancer, but the role of Pak1 in colorectal cancer remains unclear. In this study, Pak1 expression and activity were associated with aggressive behavior of colorectal cancer. Overexpression of Pak1 increased colorectal cancer cell motility and invasion, whereas down-regulation of Pak1 expression or activity reduced colorectal cancer cell migration and invasion. In addition, activated Pak1 inhibited stress fiber and focal adhesion complex formation in colorectal cancer cells and led to formation of motile phenotypes. Importantly, activated Pak1 elicited phosphorylation of FAK at Ser-910 via an ERK-dependent pathway in colorectal cancer cell lines and clinical samples. In conclusion, our results suggest that activated Pak1 regulates colorectal cancer metastasis requiring an ERK-dependent phosphorylation of FAK at Ser-910.

Increased expression of p21-activated kinase in adenomyosis

The present study was designed to evaluate whether the expression of p21-activated kinase 1 (Pak1), which recently has been shown to be increased in the eutopic endometrium of women with endometriosis, is also increased in adenomyotic nodules as well as in the eutopic endometrium in women with adenomyosis. Comparative immunohistochemical study using a monoclonal antihuman Pak1 antibody revealed that the expression of Pak1 is increased in the eutopic endometrium of women with adenomyosis during the secretory phase, along with its increased expression in adenomyotic nodules, suggesting a possible role of Pak1 in the establishment of adenomyosis.

Downregulation of p21‐activated kinase‐1 inhibits the growth of gastric cancer cells involving cyclin B1

p21-Activated kinase 1 (Pak1), a serine/threonine kinase, has been implicated in cytoskeletal remodelling, cell motility, apoptosis and transformation. However, the role of Pak1 in gastric cancer remains unclear. In this study, we detected Pak1 expression in gastric cancer tissues from 40 patients by western blot. Overexpression of Pak1 was associated with progression, metastasis and prognosis of gastric cancer. In addition, we found that knockdown of Pak1 expression significantly inhibited anchorage-dependent and anchorage-independent growth in gastric cancer cells, and markedly inhibited gastric cancer cell xenograft tumor growth. In further study, data showed that Pak1 could regulate the expression of cyclin B1 at the mRNA and protein levels, and impact the subcellular distribution and the promoter activity of cyclin B1. Results from deletion and mutant analysis supplied a new NF-kappaB binding sites at position -321 of cyclin B1 promoter, and indicated that Pak1 regulated the transcription of cyclin B1 in gastric cancer through NF-kappaB. In conclusion, Pak1 may be a potential prognostic marker and therapeutic target in gastric cancer.

The role of PAK-1 in activation of MAP kinase cascade and oncogenic transformation by Akt

The activity of protein kinase B, also known as Akt, is commonly elevated in human malignancies and plays a crucial role in oncogenic transformation. The relationship between Akt and the mitogen-activated protein kinase cascade, which is also frequently associated with oncogenesis, remains controversial. We report here examples of cooperation between Akt and cRaf in oncogenic transformation, which was accompanied by elevated activity of extracellular signal-regulated mitogen-activated protein kinases. The effect of Akt on extracellular signal-regulated kinases depended on the status of p21-activated kinase (PAK). Importantly, disruption of the function of PAK not only uncoupled the activation of Akt from that of extracellular signal-regulated kinases, but also greatly reduced the capacity of Akt to act as a transforming oncogene. For the malignancies with hyperactive Akt, our observations support the role for PAK-1 as a potential target for therapeutic intervention.

PAK1 is a novel MEK‐independent raf target controlling expression of the IAP survivin in M‐CSF‐mediated osteoclast survival

As activation of the Ras/Raf/MEK/ERK pathway is a critical component of M-CSF-promoted osteoclast survival, determining specific mechanism by which M-CSF activates this signal transduction pathway is paramount towards advancing treatment of pathological conditions resulting in increased bone turnover. The p21 activated kinase PAK1 modulates activation of the Raf/MEK/ERK pathway by either directly activating Raf or priming MEK for activation by Raf. Therefore a role for PAK1 in M-CSF-mediated activation of the MEK/ERK pathway controlling osteoclast survival was assessed. Here we show that PAK1 is activated by M-CSF in a Ras-dependent mechanism that promotes osteoclast survival. Surprisingly, PAK1 did not modulate Raf activation or Raf-mediated MEK activation. M-CSF mediated activation of Raf was required for PAK1 activation and osteoclast survival promoted by PAK1. This survival response was MEK-independent as expression of constitutively active MEK did not rescue osteoclasts from apoptosis induced by blocking PAK1 function. Functionally, PAK1 promoted osteoclast survival by modulating expression of the IAP family member Survivin. M-CSF therefore functions to promote PAK1 activation as a novel MEK-independent Raf target to control Survivin-mediated osteoclast survival.

PAK4 Functions in Tumor Necrosis Factor (TNF) α-induced Survival Pathways by Facilitating TRADD Binding to the TNF Receptor

PAK4 is a member of the group B family of p21-activated kinases. Its expression is elevated in many cancer cell lines, and activated PAK4 is highly transforming, suggesting that it plays an important role in tumorigenesis. Although most previous work was carried out with overexpressed PAK4, here we used RNA interference to knock down endogenous PAK4 in cancer cells. By studying PAK4 knockdown HeLa cells, we demonstrated that endogenous PAK4 is required for anchorage-independent growth. Because cell survival is a key part of tumorigenesis and anchorage-independent growth, we studied whether PAK4 has a role in protecting cells from cell death. To address this, we studied the role for PAK4 downstream to the tumor necrosis factor (TNF) α receptor. Although overexpressed PAK4 was previously shown to abrogate proapoptotic pathways, here we demonstrate that endogenous PAK4 is required for the full activation of prosurvival pathways induced by TNFα. Our results indicate that PAK4 is required for optimal binding of the scaffold protein TRADD to the activated TNFα receptor through both kinase-dependent and kinase-independent mechanisms. Consequently, activation of several prosurvival pathways, including the NFκB and ERK pathways, is reduced in the absence of PAK4. Interestingly, constitutive activation of the NFκB and ERK pathways could compensate for the lack of PAK4, indicating that these pathways function downstream to PAK4. The role for PAK4 in regulating prosurvival pathways is a completely new function for this protein, and the connection between PAK4 and cell survival under stress helps explain its role in tumorigenesis and development.

PAK interacts with NCK and MLK2 to regulate the activation of jun N-terminal kinase

The p21-GTPase activated kinase, PAK1, and the mixed lineage kinase, MLK2, have been implicated in the activation of jun N-terminal kinase (JNK). However, the role of PAK1 in JNK activation is still not understood. Here we show that over-expression of the SH3-SH2 adapter Nck ‘squelches’ JNK activation but this squelching is relieved by over-expression of PAK1. In turn, PAK1 squelches activation of JNK by MLK2 and these kinases interact via their catalytic domains. The data suggest that PAK1 recruits MLK2 to an activated receptor via the adapter Nck, but cannot itself induce activation of the JNK cascade.