Activated PAK4 regulates cell adhesion and anchorage-independent growth

Anticancer Efficacy of KRASG12C Inhibitors Is Potentiated by PAK4 Inhibitor KPT9274 in Preclinical Models of KRASG12C-Mutant Pancreatic and Lung Cancers

KRASG12C inhibitors, such as sotorasib and adagrasib, have revolutionized cancer treatment for patients with KRASG12C-mutant tumors. However, patients receiving these agents as monotherapy often develop drug resistance. To address this issue, we evaluated the combination of the PAK4 inhibitor KPT9274 and KRASG12C inhibitors in preclinical models of pancreatic ductal adenocarcinoma (PDAC) and non-small cell lung cancer (NSCLC). PAK4 is a hub molecule that links several major signaling pathways and is known for its tumorigenic role in mutant Ras-driven cancers. We found that cancer cells resistant to KRASG12C inhibitor were sensitive to KPT9274-induced growth inhibition. Furthermore, KPT9274 synergized with sotorasib and adagrasib to inhibit the growth of KRASG12C-mutant cancer cells and reduce their clonogenic potential. Mechanistically, this combination suppressed cell growth signaling and downregulated cell-cycle markers. In a PDAC cell line-derived xenograft (CDX) model, the combination of a suboptimal dose of KPT9274 with sotorasib significantly reduced the tumor burden (P= 0.002). Similarly, potent antitumor efficacy was observed in an NSCLC CDX model, in which KPT9274, given as maintenance therapy, prevented tumor relapse following the discontinuation of sotorasib treatment (P= 0.0001). Moreover, the combination of KPT9274 and sotorasib enhances survival. In conclusion, this is the first study to demonstrate that KRASG12C inhibitors can synergize with the PAK4 inhibitor KPT9274 and combining KRASG12C inhibitors with KPT9274 can lead to remarkably enhanced antitumor activity and survival benefits, providing a novel combination therapy for patients with cancer who do not respond or develop resistance to KRASG12C inhibitor treatment.

Elucidating Fibroblast Growth Factor-Induced Kinome Dynamics Using Targeted Mass Spectrometry and Dynamic Modeling

Fibroblast growth factors (FGFs) are paracrine or endocrine signaling proteins that, activated by their ligands, elicit a wide range of health and disease-related processes, such as cell proliferation and the epithelial-to-mesenchymal transition. The detailed molecular pathway dynamics that coordinate these responses have remained to be determined. To elucidate these, we stimulated MCF-7 breast cancer cells with either FGF2, FGF3, FGF4, FGF10, or FGF19. Following activation of the receptor, we quantified the kinase activity dynamics of 44 kinases using a targeted mass spectrometry assay. Our system-wide kinase activity data, supplemented with (phospho)proteomics data, reveal ligand-dependent distinct pathway dynamics, elucidate the involvement of not earlier reported kinases such as MARK, and revise some of the pathway effects on biological outcomes. In addition, logic-based dynamic modeling of the kinome dynamics further verifies the biological goodness-of-fit of the predicted models and reveals BRAF-driven activation upon FGF2 treatment and ARAF-driven activation upon FGF4 treatment.

Anticancer efficacy of KRASG12C inhibitors is potentiated by PAK4 inhibitor KPT9274 in preclinical models of KRASG12C mutant pancreatic and lung cancers

KRASG12C inhibitors have revolutionized the treatment landscape for cancer patients harboring the G12C mutant isoform of KRAS. With the recent FDA approval of sotorasib and adagrasib, patients now have access to more promising treatment options. However, patients who receive these agents as a monotherapy usually develop drug resistance. Thus, there is a need to develop logical combination strategies that can delay or prevent the onset of resistance and simultaneously enhance the antitumor effectiveness of the treatment regimen. In this study, we aimed at pharmacologically targeting PAK4 by KPT9274 in combination with KRASG12C inhibitors in KRASG12C mutant pancreatic ductal adenocarcinoma (PDAC) and nonâ€"small cell lung cancer (NSCLC) preclinical models. PAK4 is a hub molecule that links several major signaling pathways and is known for its tumorigenic role in mutant Ras-driven cancers. We assessed the cytotoxicity of PAK4 and KRASG12C inhibitors combination in KRASG12C mutant 2D and 3D cellular models. KPT9274 synergized with both sotorasib and adagrasib in inhibiting the growth of KRASG12C mutant cancer cells. The combination was able to reduce the clonogenic potential of KRASG12C mutant PDAC cells. We also evaluated the antitumor activity of the combination in a KRASG12C mutant PDAC cell line-derived xenograft (CDX) model. Oral administration of a sub-optimal dose of KPT9274 in combination with sotorasib (at one-fourth of MTD) demonstrated significant inhibition of the tumor burden ( p = 0.002). Similarly, potent antitumor efficacy was observed in an NSCLC CDX model where KPT9274, acting as an adjuvant, prevented tumor relapse following the discontinuation of sotorasib treatment ( p = 0.0001). KPT9274 and sotorasib combination also resulted in enhanced survival. This is the first study showing that KRASG12C inhibitors can synergize with PAK4 inhibitor KPT9274 both in vitro and in vivo resulting in remarkably enhanced antitumor activity and survival outcomes.

Significance

KRASG12C inhibitors demonstrate limited durable response in patients with KRASG12C mutations. In this study, combining PAK4 inhibitor KPT9274 with KRASG12C inhibitors has resulted in potent antitumor effects in preclinical cancer models of PDAC and NSCLC. Our results bring forward a novel combination therapy for cancer patients that do not respond or develop resistance to KRASG12C inhibitor treatment.

Pan-cancer analysis identifies the immunological and prognostic role of PAK4

AIMS

P21-activated kinase 4 (PAK4) belongs to the wider family of Serine/Threonine p21-activated kinases (PAKs) and functions as a hub for signaling pathways in cancer progression. Numerous studies have indicated the significance of PAK4 for tumorigenesis, but no systematic pan-cancer analysis has been performed.

MAIN METHODS

The current study aimed to investigate the prognostic and immunological functions of PAK4 through bioinformatic analysis of datasets from The Cancer Genome Atlas, UALCAN, GEPIA2, cBioPortal, TIMER2, and Human Protein Atlas. PAK4 expression was correlated with prognosis, DNA methylation, tumor mutational burden, microsatellite instability, and immune cell infiltration.

KEY FINDINGS

PAK4 was highly expressed in various cancers but showed decreased expression in colon adenocarcinoma, kidney renal clear cell carcinoma, kidney renal papillary cell carcinoma, and thyroid carcinoma. PAK4 was found to have a positive or negative correlation with prognosis of different cancers. PAK4 expression was related to tumor mutational burden in 11 tumor types, and associated with microsatellite instability in 10 tumor types and was correlated with immune infiltration and immune checkpoint genes.

SIGNIFICANCE

PAK4 could be considered as a prognostic and immunotherapeutic marker for some types of malignant tumor.

Rho family GTPase signaling through type II p21-activated kinases

Signaling from the Rho family small GTPases controls a wide range of signaling outcomes. Key among the downstream effectors for many of the Rho GTPases are the p21-activated kinases, or PAK group. The PAK family comprises two types, the type I PAKs (PAK1, 2 and 3) and the type II PAKs (PAK4, 5 and 6), which have distinct structures and mechanisms of regulation. In this review, we discuss signal transduction from Rho GTPases with a focus on the type II PAKs. We discuss the role of PAKs in signal transduction pathways and selectivity of Rho GTPases for PAK family members. We consider the less well studied of the Rho GTPases and their PAK-related signaling. We then discuss the molecular basis for kinase domain recognition of substrates and for regulation of signaling. We conclude with a discussion of the role of PAKs in cross talk between Rho family small GTPases and the roles of PAKs in disease.

Inka2, a novel Pak4 inhibitor, regulates actin dynamics in neuronal development

The actin filament is a fundamental part of the cytoskeleton defining cell morphology and regulating various physiological processes, including filopodia formation and dendritic spinogenesis of neurons. Serine/threonine-protein kinase Pak4, an essential effector, links Rho GTPases to control actin polymerization. Previously, we identified the Inka2 gene, a novel mammalian protein exhibiting sequence similarity to Inka1, which serves as a possible inhibitor for Pak4. Although Inka2 is dominantly expressed in the nervous system and involved in focal-adhesion dynamics, its molecular role remains unclear. Here, we found that Inka2-iBox directly binds to Pak4 catalytic domain to suppress actin polymerization. Inka2 promoted actin depolymerization and inhibited the formation of cellular protrusion caused by Pak4 activation. We further generated the conditional knockout mice of the Inka2 gene. The beta-galactosidase reporter indicated the preferential Inka2 expression in the dorsal forebrain neurons. Cortical pyramidal neurons of Inka2^-/- mice exhibited decreased density and aberrant morphology of dendritic spines with marked activation/phosphorylation of downstream molecules of Pak4 signal cascade, including LIMK and Cofilin. These results uncovered the unexpected function of endogenous Pak4 inhibitor in neurons. Unlike Inka1, Inka2 is a critical mediator for actin reorganization required for dendritic spine development.

Actin filaments are an essential part of the cytoskeleton defining cell morphology and regulating various cellular processes, such as cell migration and synapse formation in the brain. Actin polymerization is controlled by the kinase activity of the Pak4 signaling cascade, including LIMK and Cofilin. Previously, we identified the Inka2 gene, which is strongly expressed in the mammalian central nervous system and a similar sequence as Inka1. Inka1 was reported to serve as a Pak4 inhibitor in cancer cell lines; however, the physiological function of Inka2 is unclear. In this study, we found that (i) Inka2 overexpression inhibits the formation of cell-protrusion caused by Pak4 activation; (ii) Inka2 directly binds to the catalytic domain of Pak4 to inhibit intracellular actin polymerization; (iii) Inka2 is specifically expressed in neurons in the forebrain region, including the cerebral cortex and hippocampus that are known to be essential for brain plasticity, such as learning and memory; and (iv) cortical neurons of Inka2-deficient mice showed decreased synapse formation and abnormal spine morphology, probably due to the marked phosphorylation of LIMK and Cofilin. These results indicate that Inka2 is an endogenous Pak4 inhibitor in neurons required for normal synapse formation through the modulation of actin reorganization.

Remodeling of the tumor microenvironment through PAK4 inhibition sensitizes tumors to immune checkpoint blockade

PAK4 inhibition can sensitize tumors to immune checkpoint blockade (ICB) therapy, however, the underlying mechanisms remain unclear. We report that PAK4 inhibition reverses immune cell exclusion by increasing the infiltration of CD8 T cells and CD103+ dendritic cells (DCs), a specific type of DCs that excel at cross-presenting tumor antigens and constitute a source of CXCL10. Interestingly, in melanoma clinical datasets, PAK4 expression levels negatively correlate with the presence of CCL21, the ligand for CCR7 expressed in CD103+ DCs. Furthermore, we extensively characterized the transcriptome of PAK4 knock out (KO) tumors, in vitro and in vivo, and established the importance of PAK4 expression in the regulation of the extracellular matrix, which can facilitate immune cell infiltration. Comparison between PAK4 wild type (WT) and KO anti-PD-1 treated tumors revealed how PAK4 deletion sensitizes tumors to ICB from a transcriptomic perspective. In addition, we validated genetically and pharmacologically that inhibition of PAK4 kinase activity is sufficient to improve anti-tumor efficacy of anti-PD-1 blockade in multiple melanoma mouse models. Therefore, this study provides novel insights into the mechanism of action of PAK4 inhibition and provides the foundation for a new treatment strategy that aims to overcome resistance to PD-1 blockade by combining anti-PD-1 with a small molecule PAK4 kinase inhibitor.

CORO1C, a novel PAK4 binding protein, recruits phospho-PAK4 at serine 99 to the leading edge and promotes the migration of gastric cancer cells

Gastric cancer is one of the malignant tumors in the world. PAK4 plays an important role in the occurrence and development of gastric cancer, especially in the process of invasion and metastasis. Here we discover that CORO1C, a member of coronin family that regulates microfilament and lamellipodia formation, recruits cytoplasmic PAK4 to the leading edge of gastric cancer cells by C-terminal extension (CE) domain of CORO1C (353-457 aa). The localization of PAK4 on the leading edge of the cell depends on two necessary conditions: the phosphorylation of PAK4 on serine 99 and the binding to the CE domain of CORO1C. Unphosphorylated PAK4 on serine 99 is closely associated with microtubules by PAK4/GEF-H1/Tctex-1 complex. Once phosphorylated, PAK4 is released from microtubule, and then is recruited by CORO1C to the leading edge and regulates the CORO1C/RCC2 (regulator of chromosome condensation 2) complex, leading to the migration of gastric cancer cells. Our results reveal a new mechanism by which PAK4 regulates the migration potential of gastric cancer cells through microtubule-microfilament cross talk.

STE20 phosphorylation of AMPK related kinases revealed by biochemical purifications combined with genetics

We have recently purified mammalian sterile 20 (STE20)–like kinase 3 (MST3) as a kinase for the multifunctional kinases, AMP-activated protein kinase–related kinases (ARKs). However, unresolved questions from this study, such as remaining phosphorylation activities following deletion of the Mst3 gene from human embryonic kidney cells and mice, led us to conclude that there were additional kinases for ARKs. Further purification recovered Ca²⁺/calmodulin-dependent protein kinase kinases 1 and 2 (CaMKK1 and 2), and a third round of purification revealed mitogen-activated protein kinase kinase kinase kinase 5 (MAP4K5) as potential kinases of ARKs. We then demonstrated that MST3 and MAP4K5, both belonging to the STE20-like kinase family, could phosphorylate all 14 ARKs both in vivo and in vitro. Further examination of all 28 STE20 kinases detected variable phosphorylation activity on AMP-activated protein kinase (AMPK) and the salt-inducible kinase 3 (SIK3). Taken together, our results have revealed novel relationships between STE20 kinases and ARKs, with potential physiological and pathological implications.

CDK15 promotes colorectal cancer progression via phosphorylating PAK4 and regulating β-catenin/ MEK-ERK signaling pathway

Colorectal cancer (CRC) is the third most diagnosed cancer and the second leading cause of cancer-related deaths. However, there are few effective therapeutic targets for CRC patients. Here, we found that CDK15 was highly expressed in human CRC and negatively correlated with patient prognosis and overall survival in tissue microarray. Knockdown of CDK15 suppressed cell proliferation and anchorage-independent growth of CRC cells and inhibited tumor growth in cell line-derived xenograft (CDX) model. Importantly, knockout of CDK15 in mice retarded AOM/DSS-induced tumorigenesis and CDK15 silencing by lentivirus significantly suppressed tumor progression in patient-derived xenograft (PDX) model. Mechanistically, CDK15 could bind PAK4 and phosphorylate PAK4 at S291 site. Phosphorylation of PAK4 at the S291 residue promoted cell proliferation and anchorage-independent growth through β-catenin/c-Myc, MEK/ERK signaling pathway in CRC. Moreover, inhibition of PAK4 reversed the tumorigenic function of CDK15 in CRC cells and pharmacological targeting PAK4 suppressed tumor growth in PDX models. Thus, our data reveal the pivotal role of CDK15 in CRC progression and demonstrate CDK15 promotes CRC tumorigenesis by phosphorylating PAK4. Hence, the CDK15-PAK4 axis may serve as a novel therapeutic target for CRC.

Proximity proteomics identifies PAK4 as a component of Afadin-Nectin junctions

Human PAK4 is an ubiquitously expressed p21-activated kinase which acts downstream of Cdc42. Since PAK4 is enriched in cell-cell junctions, we probed the local protein environment around the kinase with a view to understanding its location and substrates. We report that U2OS cells expressing PAK4-BirA-GFP identify a subset of 27 PAK4-proximal proteins that are primarily cell-cell junction components. Afadin/AF6 showed the highest relative biotin labelling and links to the nectin family of homophilic junctional proteins. Reciprocally >50% of the PAK4-proximal proteins were identified by Afadin BioID. Co-precipitation experiments failed to identify junctional proteins, emphasizing the advantage of the BioID method. Mechanistically PAK4 depended on Afadin for its junctional localization, which is similar to the situation in Drosophila. A highly ranked PAK4-proximal protein LZTS2 was immuno-localized with Afadin at cell-cell junctions. Though PAK4 and Cdc42 are junctional, BioID analysis did not yield conventional cadherins, indicating their spatial segregation. To identify cellular PAK4 substrates we then assessed rapid changes (12') in phospho-proteome after treatment with two PAK inhibitors. Among the PAK4-proximal junctional proteins seventeen PAK4 sites were identified. We anticipate mammalian group II PAKs are selective for the Afadin/nectin sub-compartment, with a demonstrably distinct localization from tight and cadherin junctions.

The role of PAK4 in the immune system and its potential implication in cancer immunotherapy

The p21 activated kinases (PAKs) are known to play a role in the regulation of cell morphology and functions. Among the various members of PAKs family, only the PAK4 protein has been shown to be overexpressed in cancer cells and its upregulation was associated with tumor development. Indeed, several studies have shown that PAK4 overexpression is implicated in carcinogenesis by different mechanisms including promotion of cell proliferation, invasion and migration, protection of cells from apoptosis, stimulation of the tumor-specific anchorage-independent cell growth and regulation of the cytoskeletal organisation and adhesion. Moreover, high PAK4 protein levels have been observed in several solid tumors and have been shown able to enhance cancer cell resistance to many treatments especially chemotherapy. Interestingly, it has been recently demonstrated that PAK4 downregulation can inhibit the PD-1/PD-L1 immune regulatory pathway. Taken together, these findings not only implicate PAK4 in oncogenic transformation and in prediction of tumor response to treatment but also suggest its role as an attractive target for immunotherapy. In the current review we will summarize the different mechanisms of PAK4 implication in tumor development, describe its role as a regulator of the immune response and as a potential novel target for cancer immunotherapy.

p21-activated kinases as viable therapeutic targets for the treatment of high-risk Ewing sarcoma

Ewing sarcoma (ES) is the second most common bone tumor in children and young adults. Unfortunately, there have been minimal recent advancements in improving patient outcomes, especially in metastatic and recurrent diseases. In this study, we investigated the biological role of p21-activated kinases (PAKs) in ES, and the ability to therapeutically target them in high-risk disease. Via informatics analysis, we established the inverse association of PAK1 and PAK4 expression with clinical stage and outcome in ES patients. Through expression knockdown and small-molecule inhibition of PAKs, utilizing FRAX-597, KPT-9274, and PF-3758309 in multiple ES cell lines and patient-derived xenograft models, we further explored the role of PAKs in ES tumor growth and metastatic capabilities. In vitro studies in several ES cell lines indicated that diminishing PAK1 and PAK4 expression reduces tumor cell viability, migratory, and invasive properties. In vivo studies using PAK4 inhibitors, KPT-9274 and PF-3758309 demonstrated significant inhibition of primary and metastatic tumor formation, while transcriptomic analysis of PAK4-inhibitor-treated tumors identified concomitant suppression of Notch, β-catenin, and hypoxia-mediated signatures. In addition, the analysis showed enrichment of anti-tumor immune regulatory mechanisms, including interferon (IFN)-ɣ and IFN-α responses. Altogether, our molecular and pre-clinical studies are the first to establish a critical role for PAKs in ES development and progression, and consequently as viable therapeutic targets for the treatment of high-risk ES in the near future.

PAK1 and PAK4 as therapeutic targets for Ewing sarcoma: a commentary

Ewing sarcoma (ES) is an aggressive pediatric bone tumor that is prone to metastasis. Due to low five-year survival rates and limited therapeutic options for metastatic disease, there is a dire clinical need for improved ES treatments. Targeting p21-activated kinases (PAKs) may be key. PAK1 and PAK4 are associated with aggressive ES and poor patient outcomes, although their molecular mechanisms remain largely uncharacterized in this disease. This commentary aims to highlight the recent advancements made to the understanding of PAK1 and PAK4 in ES in the paper "p21-activated kinases as viable therapeutic targets for the treatment of high-risk Ewing sarcoma" by Qasim et al.

PAK4 methylation by the methyltransferase SETD6 attenuates cell adhesion

P21-activated kinase 4 (PAK4), a member of serine/threonine kinases family is over-expressed in numerous cancer tumors and is associated with oncogenic cell proliferation, migration and invasion. Our recent work demonstrated that the SET-domain containing protein 6 (SETD6) interacts with and methylates PAK4 at chromatin in mammalian cells, leading to activation of the Wnt/β-catenin signaling pathway. In our current work, we identified lysine 473 (K473) on PAK4 as the primary methylation site by SETD6. Methylation of PAK4 at K473 activates β-catenin transcriptional activity and inhibits cell adhesion. Specific methylation of PAK4 at K473 also attenuates paxillin localization to focal adhesions leading to overall reduction in adhesion-related features, such as filopodia and actin structures. The altered adhesion of the PAK4 wild-type cells is accompanied with a decrease in the migrative and invasive characteristics of the cells. Taken together, our results suggest that methylation of PAK4 at K473 plays a vital role in the regulation of cell adhesion and migration.

Recognition of physiological phosphorylation sites by p21-activated kinase 4

Many serine/threonine protein kinases discriminate between serine and threonine substrates as a filter to control signaling output. Among these, the p21-activated kinase (PAK) group strongly favors phosphorylation of Ser over Thr residues. PAK4, a group II PAK, almost exclusively phosphorylates its substrates on serine residues. The only well documented exception is LIM domain kinase 1 (LIMK1), which is phosphorylated on an activation loop threonine (Thr508) to promote its catalytic activity. To understand the molecular and kinetic basis for PAK4 substrate selectivity we compared its mode of recognition of LIMK1 (Thr508) with that of a known serine substrate, β-catenin (Ser675). We determined X-ray crystal structures of PAK4 in complex with synthetic peptides corresponding to its phosphorylation sites in LIMK1 and β-catenin to 1.9 Å and 2.2 Å resolution, respectively. We found that the PAK4 DFG + 1 residue, a key determinant of phosphoacceptor preference, adopts a sub-optimal orientation when bound to LIMK1 compared to β-catenin. In peptide kinase activity assays, we find that phosphoacceptor identity impacts catalytic efficiency but does not affect the K_m value for both phosphorylation sites. Although catalytic efficiency of wild-type LIMK1 and β-catenin are equivalent, T508S mutation of LIMK1 creates a highly efficient substrate. These results suggest suboptimal phosphorylation of LIMK1 as a mechanism for controlling the dynamics of substrate phosphorylation by PAK4.

Interaction between Mas1 and AT1RA contributes to enhancement of skeletal muscle angiogenesis by angiotensin-(1-7) in Dahl salt-sensitive rats

The heptapeptide angiotensin-(1-7) (Ang-(1-7)) is protective in the cardiovascular system through its induction of vasodilator production and angiogenesis. Despite acting antagonistically to the effects of elevated, pathophysiological levels of angiotensin II (AngII), recent evidence has identified convergent and beneficial effects of low levels of both Ang-(1-7) and AngII. Previous work identified the AngII receptor type I (AT1R) as a component of the protein complex formed when Ang-(1-7) binds its receptor, Mas1. Importantly, pharmacological blockade of AT1R did not alter the effects of Ang-(1-7). Here, we use a novel mutation of AT1RA in the Dahl salt-sensitive (SS) rat to test the hypothesis that interaction between Mas1 and AT1R contributes to proangiogenic Ang-(1-7) signaling. In a model of hind limb angiogenesis induced by electrical stimulation, we find that the restoration of skeletal muscle angiogenesis in SS rats by Ang-(1-7) infusion is impaired in AT1RA knockout rats. Enhancement of endothelial cell (EC) tube formation capacity by Ang-(1-7) is similarly blunted in AT1RA mutant ECs. Transcriptional changes elicited by Ang-(1-7) in SS rat ECs are altered in AT1RA mutant ECs, and tandem mass spectrometry-based proteomics demonstrate that the protein complex formed upon binding of Ang-(1-7) to Mas1 is altered in AT1RA mutant ECs. Together, these data support the hypothesis that interaction between AT1R and Mas1 contributes to proangiogenic Ang-(1-7) signaling.

Group II p21-activated kinase, PAK4, is needed for activation of focal adhesion kinases, MAPK, GSK3, and β-catenin in rat pancreatic acinar cells

PAK4 is the only member of the Group II p21-activated kinases (PAKs) present in rat pancreatic acinar cells and is activated by gastrointestinal hormones/neurotransmitters stimulating PLC/cAMP and by various pancreatic growth factors. However, little is known of the role of PAK4 activation in cellular signaling cascades in pancreatic acinar cells. In the present study, we examined the role of PAK4's participation in five different cholecystokinin-8 (CCK-8)-stimulated signaling pathways (PI3K/Akt, MAPK, focal adhesion kinase, GSK3, and β-catenin), which mediate many of its physiological acinar-cell effects, as well as effects in pathophysiological conditions. To define PAK4's role, the effect of two different PAK4 inhibitors, PF-3758309 and LCH-7749944, was examined under experimental conditions that only inhibited PAK4 activation and not activation of the other pancreatic PAK, Group I PAK2. The inhibitors' effects on activation of these five signaling cascades by both physiological and pathophysiological concentrations of CCK, as well as by 12-O-tetradecanoylphobol-13-acetate (TPA), a PKC-activator, were examined. CCK/TPA activation of focal adhesion kinases(PYK2/p125^FAK) and the accompanying adapter proteins (paxillin/p130^CAS), Mek1/2, and p44/42, but not c-Raf or other MAPKs (JNK/p38), were mediated by PAK4. Activation of PI3K/Akt/p70s6K was independent of PAK4, whereas GSK3 and β-catenin stimulation was PAK4-dependent. These results, coupled with recent studies showing PAK4 is important in pancreatic fluid/electrolyte/enzyme secretion and acinar cell growth, show that PAK4 plays an important role in different cellular signaling cascades, which have been shown to mediate numerous physiological and pathophysiological processes in pancreatic acinar cells.NEW & NOTEWORTHY In pancreatic acinar cells, cholecystokinin (CCK) or 12-O-tetradecanoylphobol-13-acetate (TPA) activation of focal adhesion kinases (p125^FAK,PYK2) and its accompanying adapter proteins, p130CAS/paxillin; Mek1/2, p44/42, GSK3, and β-catenin are mediated by PAK4. PI3K/Akt/p70s6K, c-Raf, JNK, or p38 pathways are independent of PAK4 activation.

Serine phosphorylation of the small phosphoprotein ICAP1 inhibits its nuclear accumulation

Nuclear accumulation of the small phosphoprotein integrin cytoplasmic domain-associated protein-1 (ICAP1) results in recruitment of its binding partner, Krev/Rap1 interaction trapped-1 (KRIT1), to the nucleus. KRIT1 loss is the most common cause of cerebral cavernous malformation, a neurovascular dysplasia resulting in dilated, thin-walled vessels that tend to rupture, increasing the risk for hemorrhagic stroke. KRIT1's nuclear roles are unknown, but it is known to function as a scaffolding or adaptor protein at cell-cell junctions and in the cytosol, supporting normal blood vessel integrity and development. As ICAP1 controls KRIT1 subcellular localization, presumably influencing KRIT1 function, in this work, we investigated the signals that regulate ICAP1 and, hence, KRIT1 nuclear localization. ICAP1 contains a nuclear localization signal within an unstructured, N-terminal region that is rich in serine and threonine residues, several of which are reportedly phosphorylated. Using quantitative microscopy, we revealed that phosphorylation-mimicking substitutions at Ser-10, or to a lesser extent at Ser-25, within this N-terminal region inhibit ICAP1 nuclear accumulation. Conversely, phosphorylation-blocking substitutions at these sites enhanced ICAP1 nuclear accumulation. We further demonstrate that p21-activated kinase 4 (PAK4) can phosphorylate ICAP1 at Ser-10 both in vitro and in cultured cells and that active PAK4 inhibits ICAP1 nuclear accumulation in a Ser-10-dependent manner. Finally, we show that ICAP1 phosphorylation controls nuclear localization of the ICAP1-KRIT1 complex. We conclude that serine phosphorylation within the ICAP1 N-terminal region can prevent nuclear ICAP1 accumulation, providing a mechanism that regulates KRIT1 localization and signaling, potentially influencing vascular development.

PAK4 Regulates Actin and Microtubule Dynamics during Meiotic Maturation in Mouse Oocyte

Meiotic maturation of oocyte is an important process for successful fertilization, in which cytoskeletal integrality takes a significant role. The p-21 activated kinases (PAKs) belong to serine/threonine kinases that affect wide range of processes that are crucial for cell motility, survival, cell cycle, and proliferation. In this study, we used a highly selective inhibitor of PAK4, PF-3758309, to investigate the functions of PAK4 during meiotic maturation of mouse oocytes. We found that PAK4 inhibition resulted in meiotic arrest by inducing abnormal microfilament and microtubule dynamics. PAK4 inhibition impaired the microtubule stability and led to the defective kinetochore-microtubule (K-M) attachment which inevitably resulted in aneuploidy. Also, PAK4 inhibition induced abnormal acentriolar centrosome assembly during meiotic maturation. In conclusion, all these combined results suggest that PAK4 is necessary for the oocyte meiosis maturation as a regulator of cytoskeleton.

Comprehensive profiling of the STE20 kinase family defines features essential for selective substrate targeting and signaling output

Specificity within protein kinase signaling cascades is determined by direct and indirect interactions between kinases and their substrates. While the impact of localization and recruitment on kinase-substrate targeting can be readily assessed, evaluating the relative importance of direct phosphorylation site interactions remains challenging. In this study, we examine the STE20 family of protein serine-threonine kinases to investigate basic mechanisms of substrate targeting. We used peptide arrays to define the phosphorylation site specificity for the majority of STE20 kinases and categorized them into four distinct groups. Using structure-guided mutagenesis, we identified key specificity-determining residues within the kinase catalytic cleft, including an unappreciated role for the kinase β3-αC loop region in controlling specificity. Exchanging key residues between the STE20 kinases p21-activated kinase 4 (PAK4) and Mammalian sterile 20 kinase 4 (MST4) largely interconverted their phosphorylation site preferences. In cells, a reprogrammed PAK4 mutant, engineered to recognize MST substrates, failed to phosphorylate PAK4 substrates or to mediate remodeling of the actin cytoskeleton. In contrast, this mutant could rescue signaling through the Hippo pathway in cells lacking multiple MST kinases. These observations formally demonstrate the importance of catalytic site specificity for directing protein kinase signal transduction pathways. Our findings further suggest that phosphorylation site specificity is both necessary and sufficient to mediate distinct signaling outputs of STE20 kinases and imply broad applicability to other kinase signaling systems.

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

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

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

Cyclic AMP-dependent protein kinase A and EPAC mediate VIP and secretin stimulation of PAK4 and activation of Na+,K+-ATPase in pancreatic acinar cells

Rat pancreatic acinar cells possess only the p21-activated kinase (PAKs), PAK4 of the group II PAK, and it is activated by gastrointestinal hormones/neurotransmitters stimulating PLC and by a number of growth factors. However, little is known generally of cAMP agents causing PAK4 activation, and there are no studies with gastrointestinal hormones/neurotransmitters activating cAMP cascades. In the present study, we examined the ability of VIP and secretin, which stimulate cAMP generation in pancreatic acini, to stimulate PAK4 activation, the signaling cascades involved, and their possible role in activating sodium-potassium adenosine triphosphatase (Na⁺,K⁺-ATPase). PAK4 activation was compared with activation of the well-established cAMP target, cyclic AMP response element binding protein (CREB). Secretin-stimulated PAK4 activation was inhibited by KT-5720 and PKA Type II inhibitor (PKI), protein kinase A (PKA) inhibitors, whereas VIP activation was inhibited by ESI-09 and HJC0197, exchange protein directly activated by cAMP (EPAC) inhibitors. In contrast, both VIP/secretin-stimulated phosphorylation of CREB (pCREB) via EPAC activation; however, it was inhibited by the p44/42 inhibitor PD98059 and the p38 inhibitor SB202190. The specific EPAC agonist 8-CPT-2- O-Me-cAMP as well 8-Br-cAMP and forskolin stimulated PAK4 activation. Secretin/VIP activation of Na⁺,K⁺-ATPase, was inhibited by PAK4 inhibitors (PF-3758309, LCH-7749944). These results demonstrate PAK4 is activated in pancreatic acini by stimulation of both VIP-/secretin-preferring receptors, as is CREB. However, they differ in their signaling cascades. Furthermore, PAK4 activation is needed for Na⁺,K⁺ATPase activation, which mediates pancreatic fluid secretion. These results, coupled with recent studies reporting PAKs are involved in both pancreatitis/pancreatic cancer growth/enzyme secretion, show that PAK4, similar to PAK2, likely plays an important role in both pancreatic physiological/pathological responses. NEW & NOTEWORTHY Pancreatic acini possess only the group II p21-activated kinase, PAK4, which is activated by PLC-stimulating agents/growth factors and is important in enzyme-secretion/growth/pancreatitis. Little information exists on cAMP-activating agents stimulating group II PAKs. We studied ability/effect of cyclic AMP-stimulating agents [vasoactive intestinal polypeptide (VIP), secretin] on PAK4 activity in rat pancreatic-acini. Both VIP/secretin activated PAK4/CREB, but the cAMP signaling cascades differed for EPAC, MAPK, and PKA pathways. Both hormones require PAK4 activation to stimulate sodium-potassium adenosine triphosphatase activity. This study shows PAK4 plays an important role in VIP-/secretin-stimulated pancreatic fluid secretion and suggests it plays important roles in pancreatic acinar physiological/pathophysiological responses mediated by cAMP-activating agents.

P21-activated kinase 4 involves TSH induced papillary thyroid cancer cell proliferation

Papillary thyroid cancer is a common endocrine malignancy. Although p21-activated kinase 4 (PAK4) is involved in the development of different types of tumor, its function has not been investigated in papillary thyroid cancer. Here, we identified a role for PAK4 in papillary thyroid cancer progression. Levels of PAK4 and PAK4 phosphorylated at serine 474 correlated significantly with tumor size and TNM stage. Furthermore, stable knockdown of PAK4 retarded cellular proliferation, migration, and invasion. Moreover, thyroid stimulating hormone-induced cellular proliferation in papillary thyroid cancer was found to be dependent on TSHR/cAMP/PKA/PAK4 signaling, with levels of phosphorylated PAK4 correlating positively with serum thyroid stimulating hormone and PKA Cα levels in patients with papillary thyroid cancer. These findings revealed a novel function of PAK4 in thyroid stimulating hormone-induced papillary thyroid cancer progression and suggest that PAK4 may become a promising diagnostic and therapeutic target for this disease.

Nigral dopaminergic PAK4 prevents neurodegeneration in rat models of Parkinson's disease

Parkinson's disease (PD) is characterized by progressive loss of dopaminergic (DA) neurons in the substantia nigra. No neuroprotective treatments have successfully prevented the progression of this disease. We report that p21-activated kinase 4 (PAK4) is a key survival factor for DA neurons. We observed PAK4 immunoreactivity in rat and human DA neurons in brain tissue, but not in microglia or astrocytes. PAK4 activity was markedly decreased in postmortem brain tissue from PD patients and in rodent models of PD. Expression of constitutively active PAK4^S445N/S474E (caPAK4) protected DA neurons in both the 6-hydroxydopamine and α-synuclein rat models of PD and preserved motor function. This neuroprotective effect of caPAK4 was mediated by phosphorylation of CRTC1 [CREB (adenosine 3',5'-monophosphate response element-binding protein)-regulated transcription coactivator] at S215. The nonphosphorylated form of CRTC1^S215A compromised the ability of caPAK4 to induce the expression of the CREB target proteins Bcl-2, BDNF, and PGC-1α. Our results support a neuroprotective role for the PAK4-CRTC1^S215-CREB signaling pathway and suggest that this pathway may be a useful therapeutic target in PD.

Decrypting the PAK4 transcriptome profile in mammary tumor forming cells using Next Generation Sequencing

The p-21 Activated Kinase 4 (PAK4) protein kinase is implicated in many cancers, including breast cancer. Overexpression of PAK4 is sufficient to cause mouse mammary epithelial cells (iMMECs) to become tumorigenic. To gain insight into the long-term gene expression changes that occur downstream to PAK4, we performed Next Generation Sequencing of RNA collected from PAK4 overexpressing iMMECs and wild-type iMMECs. We identified a list of genes whose expression levels were altered in response to PAK4 overexpression in iMMECs. Some of these genes, including FoxC2 and ParvB, are consistent with a role for PAK4 in cancer. In addition, PAK4 regulates many genes that are frequently associated with the inflammatory response, raising the possibility that there is a connection between PAK4, inflammation, and the tumor microenvironment. This study delineates the PAK4 transcriptome profile in transformed mammary cells and can provide translational utility in other types of cancers as well.

Deciphering the link between PI3K and PAK: An opportunity to target key pathways in pancreatic cancer?

The development of personalised therapies has ushered in a new and exciting era of cancer treatment for a variety of solid malignancies. Yet pancreatic ductal adenocarcinoma (PDAC) has failed to benefit from this paradigm shift, remaining notoriously refractory to targeted therapies. Chemotherapy is the cornerstone of management but can offer only modest survival benefits of a few months with 5-year survival rates rarely exceeding 3%. Despite these disappointing statistics, significant strides have been made towards understanding the complex biology of pancreatic cancer, with deep genomic sequencing identifying novel genetic aberrations and key signalling pathways. The PI3K-PDK1-AKT pathway has received great attention due to its prominence in carcinogenesis. However, efforts to target several components of this network have resulted in only a handful of drugs demonstrating any survival benefit in solid tumors; despite promising pre-clinical results. p-21 activated kinase 4 (PAK4) is a gene that is recurrently amplified or overexpressed in PDAC and both PAK4 and related family member PAK1, have been linked to aberrant RAS activity, a common feature in pancreatic cancer. As regulators of PI3K, PAKs have been highlighted as a potential prognostic marker and therapeutic target. In this review, we discuss the biology of pancreatic cancer and the close interaction between PAKs and the PI3K pathway. We also suggest proposals for future research that may see the development of effective targeted therapies that could finally improve outcomes for this disease.

Identification of the PAK4 interactome reveals PAK4 phosphorylation of N-WASP and promotion of Arp2/3-dependent actin polymerization

p21-activated kinase 4 (PAK4) regulates cell proliferation, apoptosis, cell motility and F-actin remodeling, but the PAK4 interactome has not been systematically analyzed. Here, we comprehensively characterized the human PAK4 interactome by iTRAQ quantitative mass spectrometry of PAK4-immunoprecipitations. Consistent with its multiple reported functions, the PAK4 interactome was enriched in diverse protein networks, including the 14-3-3, proteasome, replication fork, CCT and Arp2/3 complexes. Because PAK4 co-immunoprecipitated most subunits of the Arp2/3 complex, we hypothesized that PAK4 may play a role in Arp2/3 dependent actin regulation. Indeed, we found that PAK4 interacts with and phosphorylates the nucleation promoting factor N-WASP at Ser484/Ser485 and promotes Arp2/3-dependent actin polymerization in vitro. Also, PAK4 ablation in vivo reduced N-WASP Ser484/Ser485 phosphorylation and altered the cellular balance between G- and F-actin as well as the actin organization. By presenting the PAK4 interactome, we here provide a powerful resource for further investigations and as proof of principle, we also indicate a novel mechanism by which PAK4 regulates actin cytoskeleton remodeling.

LC-0882 targets PAK4 and inhibits PAK4-related signaling pathways to suppress the proliferation and invasion of gastric cancer cells

Gastric cancer is the most common malignant tumor and globally the third leading cause of cancer-related deaths. Therefore, there exists an urgent need to identify new effective gastric cancer treatments. Given the important roles in tumorigenesis and progression, p21-activated kinase 4 (PAK4) has been regarded as an attractive high-value druggable target. In this study, we examined the effects and molecular mechanisms of action of the small molecular compound LC-0882 on gastric cancer cells in vitro. LC-0882 was found to significantly inhibit the proliferation of human gastric cancer cells by repressing phospho-PAK4/cyclin D1 and CDK4/6 expression. In addition, LC-0882 was found to attenuate cell invasion by blocking the PAK4/LIMK1/cofilin signaling pathway. Finally, analysis of immunofluorescence revealed that LC-0882 exposure decreased filopodia formation and induced cell elongation in BGC823 and SGC7901 gastric cancer cells. These findings suggest that targeting PAK4 with the novel compound LC-0882 may provide a new chemotherapeutic approach in gastric cancer treatment.

A novel interaction of PAK4 with PPARγ to regulate Nox1 and radiation-induced epithelial-to-mesenchymal transition in glioma

Tumor recurrence in glioblastoma (GBM) is, in part, attributed to increased epithelial-to-mesenchymal transition (EMT) and enhanced tumor cell dissemination in adjacent brain parenchyma after ionizing radiation (IR). EMT is associated with aggressive behavior, increased stem-like characteristics and treatment resistance in malignancies; however, the underlying signaling mechanisms that regulate EMT are poorly understood. We identified grade-dependent PAK4 upregulation in gliomas and further determined its role in mesenchymal transition and radioresistance. IR treatment significantly elevated expression and nuclear localization of PAK4 in correlation with induction of reactive oxygen species (ROS) and mesenchymal transition in GBM cells. Stable PAK4 overexpression promoted mesenchymal transition by elevating EMT marker expression in these cells. Of note, transcription factor-DNA binding arrays and chromatin immunoprecipitation experiments identified the formation of a novel nuclear PAK4/PPARγ complex which was recruited to the promoter of Nox1, a PPARγ target gene. In addition, IR further elevated PAK4/PPARγ complex co-recruitment to Nox1 promoter, and increased Nox1 expression and ROS levels associated with mesenchymal transition in these cells. Conversely, specific PAK4 downregulation decreased PPARγ-mediated Nox1 expression and suppressed EMT in IR-treated cells. In vivo orthotopic tumor experiments showed inhibition of growth and suppression of IR-induced PPARγ and Nox1 expression by PAK4 downregulation in tumors. Our results provide the first evidence of a novel role for PAK4 in IR-induced EMT and suggest potential therapeutic efficacy of targeting PAK4 to overcome radioresistance in gliomas.

Activated-PAK4 predicts worse prognosis in breast cancer and promotes tumorigenesis through activation of PI3K/AKT signaling

The p21-activated kinase 4 (PAK4) is sufficient to transform noncancerous mammary epithelial cells and to form tumors in the mammary glands of mice. The accumulated information suggests that PAK4 might be an oncogenic protein in breast cancer. In this study, we sought to identify the role for PAK4 in breast cancer progression. Immunohistochemical study revealed that high PAK4 expression is associated with larger tumor size, lymph node metastasis, and advanced stage cancer in 93 invasive breast carcinoma patients. Moreover, high PAK4 expression was significantly associated with poor overall and disease-free survival. PAK4 remained an independent adverse prognosticator after univariate and multivariate analysis. Ectopic expression of wild-type PAK4 in MDA-MB-231 cells activated PI3K/AKT signaling and resulted in the enhancement of the cell proliferation, migration, and invasion, whereas PAK4-induced effects were blocked by the PAK4 kinase inhibitor PF- 3758309, PAK4 siRNAs or the PI3K inhibitor LY294002. Furthermore, a kinase-active PAK4 (S474E) strongly induced PI3K/AKT activation, and promoted proliferation, migration and invasion in breast cancer cells. A kinase-inactive PAK4 KD (K350A/K351A) did partially upregulate PI3K/AKT, and promoted invasive phenotype. Taken together, these findings suggest that PAK4-activated PI3K/AKT signaling is both kinase-dependent and -independent, which contributes to breast cancer progression. Thus, our results imply that dual inhibition of PAK4 and PI3K/AKT signaling might be a potential therapeutic approach for breast cancer therapy.

A Systems Biology Comparison of Ovarian Cancers Implicates Putative Somatic Driver Mutations through Protein-Protein Interaction Models

Ovarian carcinomas can be aggressive with a high mortality rate (e.g., high-grade serous ovarian carcinomas, or HGSOCs), or indolent with much better long-term outcomes (e.g., low-malignant-potential, or LMP, serous ovarian carcinomas). By comparing LMP and HGSOC tumors, we can gain insight into the mechanisms underlying malignant progression in ovarian cancer. However, previous studies of the two subtypes have been focused on gene expression analysis. Here, we applied a systems biology approach, integrating gene expression profiles derived from two independent data sets containing both LMP and HGSOC tumors with protein-protein interaction data. Genes and related networks implicated by both data sets involved both known and novel disease mechanisms and highlighted the different roles of BRCA1 and CREBBP in the two tumor types. In addition, the incorporation of somatic mutation data revealed that amplification of PAK4 is associated with poor survival in patients with HGSOC. Thus, perturbations in protein interaction networks demonstrate differential trafficking of network information between malignant and benign ovarian cancers. The novel network-based molecular signatures identified here may be used to identify new targets for intervention and to improve the treatment of invasive ovarian cancer as well as early diagnosis.

p21-activated kinase group II small compound inhibitor GNE-2861 perturbs estrogen receptor alpha signaling and restores tamoxifen-sensitivity in breast cancer cells

Estrogen receptor alpha (ERα) is highly expressed in most breast cancers. Consequently, ERα modulators, such as tamoxifen, are successful in breast cancer treatment, although tamoxifen resistance is commonly observed. While tamoxifen resistance may be caused by altered ERα signaling, the molecular mechanisms regulating ERα signaling and tamoxifen resistance are not entirely clear. Here, we found that PAK4 expression was consistently correlated to poor patient outcome in endocrine treated and tamoxifen-only treated breast cancer patients. Importantly, while PAK4 overexpression promoted tamoxifen resistance in MCF-7 human breast cancer cells, pharmacological treatment with a group II PAK (PAK4, 5, 6) inhibitor, GNE-2861, sensitized tamoxifen resistant MCF-7/LCC2 breast cancer cells to tamoxifen. Mechanistically, we identified a regulatory positive feedback loop, where ERα bound to the PAK4 gene, thereby promoting PAK4 expression, while PAK4 in turn stabilized the ERα protein, activated ERα transcriptional activity and ERα target gene expression. Further, PAK4 phosphorylated ERα-Ser305, a phosphorylation event needed for the PAK4 activation of ERα-dependent transcription. In conclusion, PAK4 may be a suitable target for perturbing ERα signaling and tamoxifen resistance in breast cancer patients.

GL-1196 Suppresses the Proliferation and Invasion of Gastric Cancer Cells via Targeting PAK4 and Inhibiting PAK4-Mediated Signaling Pathways

Gastric cancer, which is the most common malignant gastrointestinal tumor, has jumped to the third leading cause of cancer-related mortality worldwide. It is of great importance to identify novel and potent drugs for gastric cancer treatment. P21-activated kinase 4 (PAK4) has emerged as an attractive target for the development of anticancer drugs in consideration of its vital functions in tumorigenesis and progression. In this paper, we reported that GL-1196, as a small molecular compound, effectively suppressed the proliferation of human gastric cancer cells through downregulation of PAK4/c-Src/EGFR/cyclinD1 pathway and CDK4/6 expression. Moreover, GL-1196 prominently inhibited the invasion of human gastric cancer cells in parallel with blockage of the PAK4/LIMK1/cofilin pathway. Interestingly, GL-1196 also inhibited the formation of filopodia and induced cell elongation in SGC7901 and BGC823 cells. Taken together, these results provided novel insights into the potential therapeutic strategy for gastric cancer.

PAKs in Human Cancer Progression: From Inception to Cancer Therapeutic to Future Oncobiology

Since the initial recognition of a mechanistic role of p21-activated kinase 1 (PAK1) in breast cancer invasion, PAK1 has emerged as one of the widely overexpressed or hyperactivated kinases in human cancer at-large, allowing the PAK family to make in-roads in cancer biology, tumorigenesis, and cancer therapeutics. Much of our current understanding of the PAK family in cancer progression relates to a central role of the PAK family in the integration of cancer-promoting signals from cell membrane receptors as well as function as a key nexus-modifier of complex, cytoplasmic signaling network. Another core aspect of PAK signaling that highlights its importance in cancer progression is through PAK's central role in the cross talk with signaling and interacting proteins, as well as PAK's position as a key player in the phosphorylation of effector substrates to engage downstream components that ultimately leads to the development cancerous phenotypes. Here we provide a comprehensive review of the recent advances in PAK cancer research and its downstream substrates in the context of invasion, nuclear signaling and localization, gene expression, and DNA damage response. We discuss how a deeper understanding of PAK1's pathobiology over the years has widened research interest to the PAK family and human cancer, and positioning the PAK family as a promising cancer therapeutic target either alone or in combination with other therapies. With many landmark findings and leaps in the progress of PAK cancer research since the infancy of this field nearly 20 years ago, we also discuss postulated advances in the coming decade as the PAK family continues to shape the future of oncobiology.

Leucine-rich repeat kinase 2 interacts with p21-activated kinase 6 to control neurite complexity in mammalian brain

Leucine‐rich repeat kinase 2 (LRRK2) is a causative gene for Parkinson's disease, but the physiological function and the mechanism(s) by which the cellular activity of LRRK2 is regulated are poorly understood. Here, we identified p21‐activated kinase 6 (PAK6) as a novel interactor of the GTPase/ROC domain of LRRK2. p21‐activated kinases are serine‐threonine kinases that serve as targets for the small GTP binding proteins Cdc42 and Rac1 and have been implicated in different morphogenetic processes through remodeling of the actin cytoskeleton such as synapse formation and neuritogenesis. Using an in vivo neuromorphology assay, we show that PAK6 is a positive regulator of neurite outgrowth and that LRRK2 is required for this function. Analyses of post‐mortem brain tissue from idiopathic and LRRK2 G2019S carriers reveal an increase in PAK6 activation state, whereas knock‐out LRRK2 mice display reduced PAK6 activation and phosphorylation of PAK6 substrates. Taken together, these results support a critical role of LRRK2 GTPase domain in cytoskeletal dynamics in vivo through the novel interactor PAK6, and provide a valuable platform to unravel the mechanism underlying LRRK2‐mediated pathophysiology.

We propose p21‐activated kinase 6 (PAK6) as a novel interactor of leucine‐rich repeat kinase 2 (LRRK2), a kinase involved in Parkinson's disease (PD). In health, PAK6 regulates neurite complexity in the brain and LRRK2 is required for its function, (a) whereas PAK6 is aberrantly activated in LRRK2‐linked PD brain (b) suggesting that LRRK2 toxicity is mediated by PAK6.

p120-catenin participates in the progress of gastric cancer through regulating the Rac1 and Pak1 signaling pathway

p120-catenin (p120), an E-cadherin regulator, has been implicated as central to a series of genetic and epigenetic changes that ultimately lead to tumor progression and metastasis. Ras-related C3 botulinum toxin substrate 1 (Rac1)and p21-activated kinases (PAKs) are effectors of p120. In the present study, we examined the expression of p120, Rac1 and Pak1 using immunohistochemistry in human gastric cancer tissues. Then, we used the gastric cancer SGC7901 and AGS cell lines to explore the possible mechanism of p120, Rac1 and Pak1 in the progress of gastric cancer. Western blotting was used to detect the expression of p120, Rac1 and Pak1 in the two cell lines. Next, p120 was silenced using p120 siRNA or overexpression of p120 by transfection of the plasmid p120 1A into the two cell types, western blotting was used to investigate the expression changes of Rac1 and Pak1. Furthermore, the effects of p120 siRNA-mediated knockdown or overexpression on the proliferation and invasive ability of gastric cancer cells were investigated using wound healing test and Matrigel invasion assays. The results showed that p120 was downregulated in both poorly differentiated group and well differentiated human gastric cancer. However, Rac1 and Pak1 were upregulated in poorly differentiated tissues and remain low in well differentiated gastric cancer tissues. In the two gastric cancer cell lines, although the expression of Rac1 and Pak1 remained unchanged after the p120 knockdown, the expressions of Rac1 and Pak1 protein were decreased after p120 overexpression in both SGC7901 and AGS cells. Furthermore, knockdown of p120 promoted gastric cancer cell proliferation and invasion; overexpression of p120 reduced the proliferation and invasion of gastric cancer cells. In conclusion, based on our results, we speculate that p120 participates in the progress of gastric cancer through regulating Rac1 and Pak1, which provides a potential prevention and a promising therapeutical approach for the patients with gastric cancer.

Activated Pak4 expression correlates with poor prognosis in human gastric cancer patients

Despite considerable advances in gastrectomy and chemotherapy, the prognosis of gastric cancer (GC) has not noticeably improved due to lymph node or distant metastases. P21-activated serine/threonine kinase 4 (Pak4) plays an important role in cell morphology and cytoskeletal reorganization-both prerequisite steps for cell migration. However, it is still unclear if activated Pak4 (p-Pak4) is related to prognosis in GC patients. In our study, the level of p-Pak4 in 95 GC tissue specimens was examined by immunohistochemistry (IHC). We observed significant correlation between the level of p-Pak4 and grosstype (advanced stage GC vs. early stage GC, P = 0.04). Moreover, GC patients with higher p-Pak4 levels had a poorer prognosis than those with lower p-Pak4 levels (17 vs. 38 months, P = 0.001). Multivariate analysis showed that high phosphorylation level of Pak4, advanced stage GC, and lymph node metastasis were independent prognostic factors for GC patients (p-Pak4, P = 0.026; advanced stage GC, P = 0.030; lymph node metastasis, P = 0.016). In addition, in vitro assays indicated that knockdown of Pak4 accompanied with decreased p-Pak4, inhibited cell migration via downregulation of the traditional downstream signaling pathways of Pak4, LIMK1, and cofilin. In conclusion, this report reveals that high level of p-Pak4 correlates with poor prognosis in GC, thereby suggesting that p-Pak4 might be a potential prognostic marker for GC.

p-21 activated kinase 4 promotes proliferation and survival of pancreatic cancer cells through AKT- and ERK-dependent activation of NF-κB pathway

Identification of novel molecular targets and understanding the mechanisms underlying the aggressive nature of pancreatic cancer (PC) remain prime focus areas of research. Here, we investigated the expression and pathobiological significance of p21-activated kinase 4 (PAK4), a gene that was earlier shown to be amplified in a sub-set of PC. Our data demonstrate PAK4 overexpression in PC tissues and cell lines with little or no expression in the normal pancreas. PAK4 silencing in two PC cell lines, MiaPaCa and T3M4, by RNA interference causes suppression of growth and clonogenic ability due to decreased cell cycle progression and apoptosis-resistance. PAK4-silenced PC cells exhibit altered expression of proliferation- and survival-associated proteins. Moreover, we observe decreased nuclear accumulation and transcriptional activity of NF-κB in PAK4-silenced PC cells associated with stabilization of its inhibitory protein, IκBα. Transfection of PAK4-silenced PC cells with constitutively-active mutant of IKKβ, an upstream kinase of IκBα, leads to restoration of NF-κB activity and PC cell growth. Furthermore, we show that PAK4-induced NF-κB activity is mediated through activation and concerted action of ERK and Akt kinases. Together, these findings suggest that PAK4 is a regulator of NF-κB pathway in PC cells and can serve as a novel target for therapy.

Approaches of targeting Rho GTPases in cancer drug discovery

INTRODUCTION

Rho GTPases are master regulators of actomyosin structure and dynamics and play pivotal roles in a variety of cellular processes including cell morphology, gene transcription, cell cycle progression, and cell adhesion. Because aberrant Rho GTPase signaling activities are widely associated with human cancer, key components of Rho GTPase signaling pathways have attracted increasing interest as potential therapeutic targets. Similar to Ras, Rho GTPases themselves were, until recently, deemed "undruggable" because of structure-function considerations. Several approaches to interfere with Rho GTPase signaling have been explored and show promise as new ways for tackling cancer cells.

AREAS COVERED

This review focuses on the recent progress in targeting the signaling activities of three prototypical Rho GTPases, that is, RhoA, Rac1, and Cdc42. The authors describe the involvement of these Rho GTPases, their key regulators and effectors in cancer. Furthermore, the authors discuss the current approaches for rationally targeting aberrant Rho GTPases along their signaling cascades, upstream and downstream of Rho GTPases, and posttranslational modifications at a molecular level.

EXPERT OPINION

To date, while no clinically effective drugs targeting Rho GTPase signaling for cancer treatment are available, tool compounds and lead drugs that pharmacologically inhibit Rho GTPase pathways have shown promise. Small-molecule inhibitors targeting Rho GTPase signaling may add new treatment options for future precision cancer therapy, particularly in combination with other anti-cancer agents.

p21-Activated Kinase 4 (PAK4) as a Predictive Marker of Gemcitabine Sensitivity in Pancreatic Cancer Cell Lines

PURPOSE

p21-activated kinases (PAKs) are involved in cytoskeletal reorganization, gene transcription, cell proliferation and survival, and oncogenic transformation. Therefore, we hypothesized that PAK expression levels could predict the sensitivity of pancreatic cancer cells to gemcitabine treatment, and PAKs could be therapeutic targets.

MATERIALS AND METHODS

Cell viability inhibition by gemcitabine was evaluated in human pancreatic cancer cell lines (Capan-1, Capan-2, MIA PaCa-2, PANC-1, Aspc-1, SNU-213, and SNU-410). Protein expression and mRNA of molecules was detected by immunoblot analysis and reverse transcription polymerase chain reaction. To define the function of PAK4, PAK4 was controlled using PAK4 siRNA.

RESULTS

Capan-2, PANC-1, and SNU-410 cells were resistant to gemcitabine treatment. Immunoblot analysis of signaling molecules reported to indicate gemcitabine sensitivity showed higher expression of PAK4 and lower expression of human equilibrative nucleoside transporter 1 (hENT1), a well-known predictive marker for gemcitabine activity, in the resistant cell lines. Knockdown of PAK4 using siRNA induced the upregulation of hENT1. In resistant cell lines (Capan-2, PANC-1, and SNU-410), knockdown of PAK4 by siRNA resulted in restoration of sensitivity to gemcitabine.

CONCLUSION

PAK4 could be a predictive marker of gemcitabine sensitivity and a potential therapeutic target to increase gemcitabine sensitivity in pancreatic cancer.

Republished: tracing PAKs from GI inflammation to cancer

P-21 activated kinases (PAKs) are effectors of Rac1/Cdc42 which coordinate signals from the cell membrane to the nucleus. Activation of PAKs drive important signalling pathways including mitogen activated protein kinase, phospoinositide 3-kinase (PI3K/AKT), NF-κB and Wnt/β-catenin. Intestinal PAK1 expression increases with inflammation and malignant transformation, although the biological relevance of PAKs in the development and progression of GI disease is only incompletely understood. This review highlights the importance of altered PAK activation within GI inflammation, emphasises its effect on oncogenic signalling and discusses PAKs as therapeutic targets of chemoprevention.

PAK signaling in cancer

Transformation of a normal cell to a cancer cell is caused by mutations in genes that regulate proliferation, apoptosis, and invasion. Small GTPases such as Ras, Rho, Rac and Cdc42 orchestrate many of the signals that are required for malignant transformation. The p21-activated kinases (PAKs) are effectors of Rac and Cdc42. PAKs are a family of serine/threonine protein kinases comprised of six isoforms (PAK1–6), and they play important roles in cytoskeletal dynamics, cell survival and proliferation. They act as key signal transducers in several cancer signaling pathways, including Ras, Raf, NFκB, Akt, Bad and p53. Although PAKs are not mutated in cancers, they are overexpressed, hyperactivated or amplified in several human tumors and their role in cell transformation make them attractive therapeutic targets. This review discusses the evidence that PAK is important for cell transformation and some key signaling pathways it regulates. This review primarily discusses Group I PAKs (PAK1, PAK2 and PAK3) as Group II PAKs (PAK4, PAK5 and PAK6) are discussed elsewhere in this issue (by Minden).

PAK4-6 in cancer and neuronal development

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

Tracing PAKs from GI inflammation to cancer

P-21 activated kinases (PAKs) are effectors of Rac1/Cdc42 which coordinate signals from the cell membrane to the nucleus. Activation of PAKs drive important signalling pathways including mitogen activated protein kinase, phospoinositide 3-kinase (PI3K/AKT), NF-κB and Wnt/β-catenin. Intestinal PAK1 expression increases with inflammation and malignant transformation, although the biological relevance of PAKs in the development and progression of GI disease is only incompletely understood. This review highlights the importance of altered PAK activation within GI inflammation, emphasises its effect on oncogenic signalling and discusses PAKs as therapeutic targets of chemoprevention.

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

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