The emerging importance of group II PAKs

miR-224-5p acts as a tumour suppressor and reverses the resistance to BRAF inhibitor in melanoma through directly targeting PAK4 to block the MAPK pathway

miR-224-5p has been shown to play both an oncogene and tumour suppressor role in many human tumours. However, the role and molecular mechanism of miR-224-5p in cutaneous melanoma remains unclear. miR-224-5p levels were downregulated in melanoma tissue, and low miR-224-5p expression was an independent risk factor for melanoma patients. miR-224-5p blocked proliferation, epithelial-to-mesenchymal transition (EMT), invasion, migration in BRAF wild-type melanoma cell, and overcome acquired BRAFi resistance in VMF-resistant melanoma cells. miR-224-5p exerted its role by directly repressing PAK4 to block the downstream CRAF/MEK/ERK pathways. We demonstrated that miR-224-5p inhibited melanoma growth and metastasis in vivo though xenograft tumor and pulmonary metastasis assay. Thus, miR-224-5p/PAK4-mediated CRAF/MEK/ERK pathways have therapeutic potential in melanoma treatment.

The role of the miR-4306/PAK6 axis in degenerative nucleus pulposus cells in human intervertebral disc degeneration

Intervertebral disc degeneration (IDD), characterized by degenerative changes that occur in intervertebral discs due to aging or structural injury, is thought to be the most common cause of lower back pain. Recent studies have shown that microRNAs (miRNAs) have a critical role in the etiopathogenesis of IDD. In the current study, we aimed to determine the role of miRNAs in mediating the underlying mechanisms associated with IDD. First, differentially expressed miRNAs (DEmiRNAs) were identified using the GEO database, and subsequently confirmed by RT-qPCR and in situ hybridization. We found that miR-4306 expression was significantly decreased in human nucleus pulposus (NP) tissues compared with healthy controls, and was negatively correlated with the patients' Pfirrmann grade. To determine the mechanism by which miR-4306 was involved in IDD pathogenesis, we examined the effects of overexpressing or silencing miR-4306 on extracellular matrix (ECM) synthesis/degradation, proliferation, autophagy and apoptosis of human degenerated NP cells isolated from IDD patients. Next, we used dual-luciferase reporter assays to demonstrate that miR-4306 interacted with the 3'-untranslated regions of p21-activated kinase 6 (PAK6) mRNA, resulting in significant suppression of PAK6 expression. This effect was abolished by miR-4306 binding site mutations. Using miR-4306/PAK6 gain-of-function and loss-of-function studies in human degenerated NP cells, we demonstrated that miR-4306 promoted NP cell proliferation, ECM synthesis and autophagy, while inhibiting apoptosis and ECM degradation via PAK6. Thus, our findings indicate that miR-4306, acting via PAK6, has an important role in IDD and can be used as a promising therapeutic target for the treatment of patients with IDD.

Molecular basis for integrin adhesion receptor binding to p21-activated kinase 4 (PAK4)

Integrin adhesion receptors provide links between extracellular ligands and cytoplasmic signaling. Multiple kinases have been found to directly engage with integrin β tails, but the molecular basis for these interactions remain unknown. Here, we assess the interaction between the kinase domain of p21-activated kinase 4 (PAK4) and the cytoplasmic tail of integrin β5. We determine three crystal structures of PAK4-β5 integrin complexes and identify the PAK-binding site. This is a region in the membrane-proximal half of the β5 tail and confirmed by site-directed mutagenesis. The β5 tail engages the kinase substrate-binding groove and positions the non-phosphorylatable integrin residue Glu767 at the phosphoacceptor site. Consistent with this, integrin β5 is poorly phosphorylated by PAK4, and in keeping with its ability to occlude the substrate-binding site, weakly inhibits kinase activity. These findings demonstrate the molecular basis for β5 integrin-PAK4 interactions but suggest modifications in understanding the potential cellular role of this interaction.

Microfluidic devices as model platforms of CNS injury-ischemia to study axonal regeneration by regulating mitochondrial transport and bioenergetic metabolism

Central nervous system (CNS) neurons typically fail to regenerate their axons after injury leading to neurological impairment. Axonal regeneration is a highly energy-demanding cellular program that requires local mitochondria to supply most energy within injured axons. Recent emerging lines of evidence have started to reveal that injury-triggered acute mitochondrial damage and local energy crisis contribute to the intrinsic energetic restriction that accounts for axon regeneration failure in the CNS. Characterizing and reprogramming bioenergetic signaling and mitochondrial maintenance after axon injury-ischemia is fundamental for developing therapeutic strategies that can restore local energy metabolism and thus facilitate axon regeneration. Therefore, establishing reliable and reproducible neuronal model platforms is critical for assessing axonal energetic metabolism and regeneration capacity after injury-ischemia. In this focused methodology article, we discuss recent advances in applying cutting-edge microfluidic chamber devices in combination with state-of-the-art live-neuron imaging tools to monitor axonal regeneration, mitochondrial transport, bioenergetic metabolism, and local protein synthesis in response to injury-ischemic stress in mature CNS neurons.

Differential roles and regulation of the protein kinases PAK4, PAK5 and PAK6 in melanoma cells

The protein kinases PAK4, PAK5 and PAK6 comprise a family of ohnologues. In multiple cancers including melanomas PAK5 most frequently carries non-synonymous mutations; PAK6 and PAK4 have fewer; and PAK4 is often amplified. To help interpret these genomic data, initially we compared the cellular regulation of the sister kinases and their roles in melanoma cells. In common with many ohnologue protein kinases, PAK4, PAK5 and PAK6 each have two 14-3-3-binding phosphosites of which phosphoSer99 is conserved. PAK4 localises to the leading edge of cells in response to phorbol ester-stimulated binding of 14-3-3 to phosphoSer99 and phosphoSer181, which are phosphorylated by two different PKCs or PKDs. These phosphorylations of PAK4 are essential for its phorbol ester-stimulated phosphorylation of downstream substrates. In contrast, 14-3-3 interacts with PAK5 in response to phorbol ester-stimulated phosphorylation of Ser99 and epidermal growth factor-stimulated phosphorylation of Ser288; whereas PAK6 docks onto 14-3-3 and is prevented from localising to cell–cell junctions when Ser133 is phosphorylated in response to cAMP-elevating agents via PKA and insulin-like growth factor 1 via PKB/Akt. Silencing of PAK4 impairs viability, migration and invasive behaviour of melanoma cells carrying BRAF^V600E or NRAS^Q61K mutations. These defects are rescued by ectopic expression of PAK4, more so by a 14-3-3-binding deficient PAK4, and barely by PAK5 or PAK6. Together these genomic, biochemical and cellular data suggest that the oncogenic properties of PAK4 are regulated by PKC–PKD signalling in melanoma, while PAK5 and PAK6 are dispensable in this cancer.

The significance of PAK4 in signaling and clinicopathology: A review

P21-activated protein kinases (PAKs) are thought to be at the center of tumor signaling pathways. As a representative member of the group II PAK family, P21-activated protein kinase 4 (PAK4) plays an important role in the development of tumors, with several biological functions such as participating in oncogenic transformation, promoting cell division, resisting aging and apoptosis, regulating cytoskeleton and adhesion, as well as suppressing antitumor immune responses. PAK4 is also crucial in biological processes, including the occurrence, proliferation, survival, migration, invasion, drug resistance, and immune escape of tumor cells. It is closely related to poor prognosis and tumor-related pathological indicators, which have significant clinical and pathological significance. Therefore, this article offers a review of the structure, activation, and biological functions of PAK4 and its clinical and pathological importance. This overview should be of assistance for future research on PAK4 and tumors and provide new ideas for tumor treatment and prognostic evaluation of patients.

Small-Molecule RAS Inhibitors as Anticancer Agents: Discovery, Development, and Mechanistic Studies

Mutations of RAS oncogenes are responsible for about 30% of all human cancer types, including pancreatic, lung, and colorectal cancers. While KRAS1 is a pseudogene, mutation of KRAS2 (commonly known as KRAS oncogene) is directly or indirectly associated with human cancers. Among the RAS family, KRAS is the most abundant oncogene related to uncontrolled cellular proliferation to generate solid tumors in many types of cancer such as pancreatic carcinoma (over 80%), colon carcinoma (40-50%), lung carcinoma (30-50%), and other types of cancer. Once described as 'undruggable', RAS proteins have become 'druggable', at least to a certain extent, due to the continuous efforts made during the past four decades. In this account, we discuss the chemistry and biology (wherever available) of the small-molecule inhibitors (synthetic, semi-synthetic, and natural) of KRAS proteins that were published in the past decades. Commercial drugs, as well as investigational molecules from preliminary stages to clinical trials, are categorized and discussed in this study. In summary, this study presents an in-depth discussion of RAS proteins, classifies the RAS superfamily, and describes the molecular mechanism of small-molecule RAS inhibitors.

The Use of Nanomedicine to Target Signaling by the PAK Kinases for Disease Treatment

P21-activated kinases (PAKs) are serine/threonine kinases involved in the regulation of cell survival, proliferation, inhibition of apoptosis, and the regulation of cell morphology. Some members of the PAK family are highly expressed in several types of cancer, and they have also been implicated in several other medical disorders. They are thus considered to be good targets for treatment of cancer and other diseases. Although there are several inhibitors of the PAKs, the utility of some of these inhibitors is reduced for several reasons, including limited metabolic stability. One way to overcome this problem is the use of nanoparticles, which have the potential to increase drug delivery. The overall goals of this review are to describe the roles for PAK kinases in cell signaling and disease, and to describe how the use of nanomedicine is a promising new method for administering PAK inhibitors for the purpose of disease treatment and research. We discuss some of the basic mechanisms behind nanomedicine technology, and we then describe how these techniques are being used to package and deliver PAK inhibitors.

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.

Reprogramming an energetic AKT-PAK5 axis boosts axon energy supply and facilitates neuron survival and regeneration after injury and ischemia

Mitochondria supply adenosine triphosphate (ATP) essential for neuronal survival and regeneration. Brain injury and ischemia trigger acute mitochondrial damage and a local energy crisis, leading to degeneration. Boosting local ATP supply in injured axons is thus critical to meet increased energy demand during nerve repair and regeneration in adult brains, where mitochondria remain largely stationary. Here, we elucidate an intrinsic energetic repair signaling axis that boosts axonal energy supply by reprogramming mitochondrial trafficking and anchoring in response to acute injury-ischemic stress in mature neurons and adult brains. P21-activated kinase 5 (PAK5) is a brain mitochondrial kinase with declined expression in mature neurons. PAK5 synthesis and signaling is spatiotemporally activated within axons in response to ischemic stress and axonal injury. PAK5 signaling remobilizes and replaces damaged mitochondria via the phosphorylation switch that turns off the axonal mitochondrial anchor syntaphilin. Injury-ischemic insults trigger AKT growth signaling that activates PAK5 and boosts local energy supply, thus protecting axon survival and facilitating regeneration in in vitro and in vivo models. Our study reveals an axonal mitochondrial signaling axis that responds to injury and ischemia by remobilizing damaged mitochondria for replacement, thereby maintaining local energy supply to support central nervous system (CNS) survival and regeneration.

Human CPPED1 belongs to calcineurin-like metallophosphoesterase superfamily and dephosphorylates PI3K-AKT pathway component PAK4

Protein kinases and phosphatases regulate cellular processes by reversible phosphorylation and dephosphorylation events. CPPED1 is a recently identified serine/threonine protein phosphatase that dephosphorylates AKT1 of the PI3K-AKT signalling pathway. We previously showed that CPPED1 levels are down-regulated in the human placenta during spontaneous term birth. In this study, based on sequence comparisons, we propose that CPPED1 is a member of the class III phosphodiesterase (PDE) subfamily within the calcineurin-like metallophosphoesterase (MPE) superfamily rather than a member of the phosphoprotein phosphatase (PPP) or metal-dependent protein phosphatase (PPM) protein families. We used a human proteome microarray to identify 36 proteins that putatively interact with CPPED1. Of these, GRB2, PAK4 and PIK3R2 are known to regulate the PI3K-AKT pathway. We further confirmed CPPED1 interactions with PAK4 and PIK3R2 by coimmunoprecipitation analyses. We characterized the effect of CPPED1 on phosphorylation of PAK4 and PIK3R2 in vitro by mass spectrometry. CPPED1 dephosphorylated specific serine residues in PAK4, while phosphorylation levels in PIK3R2 remained unchanged. Our findings indicate that CPPED1 may regulate PI3K-AKT pathway activity at multiple levels. Higher CPPED1 levels may inhibit PI3K-AKT pathway maintaining pregnancy. Consequences of decreased CPPED1 expression during labour remain to be elucidated.

Inhibiting p21-activated kinase (PAK7) enhances radiosensitivity in hepatocellular carcinoma

OBJECTIVE

In light of the upregulation of p21-activated kinase (PAK7) in a variety of cancers, including hepatocellular carcinoma (HCC), we aimed to investigate the effect of PAK7 on the sensitivity of HCC cells to radiotherapy.

METHODS

PAK7 expression was determined in normal adult liver epithelial THLE-2 and human HCC cell lines. The effect of ionizing radiation (IR) on the HCC cell viability was evaluated by Sulforhodamine B (SRB) assay. HCC cell lines Mahlavu and Huh7 were chosen to assess the effect of PAK7 shRNAs on the viability, clone formation, apoptosis, cycle distribution and γ-H2AX expression after exposure to IR.

RESULTS

As compared to THLE-2 cells, PAK7 was upregulated in poorly differentiated Mahlavu and SK-Hep-1 cells, but moderately or lowly expressed in well-differentiated Huh7 and HepG2 cells. HCC cells with moderate or low expression of PAK7 presented a decreased viability at 2 Gy IR, which had no significant effect on PAK7_high HCC cells. Mahlavu and Huh7 cells transfected with PAK7 shRNAs showed increased inhibitory effect of IR on viability. In addition, PAK7 shRNAs reduced clone formation, facilitated the cell apoptosis, arrested cells at G2/M phase, and increased γ-H2AX expression. Moreover, changes above were more evident in the HCC cells co-treated with IR and PAK7 shRNAs.

CONCLUSION

PAK7 downregulation could inhibit the viability, promote the apoptosis, arrest cells in G2/M phase, and induce the DNA damage in HCC cells, thereby enhancing the radiosensitivity in HCC.

Escherichia coli Rho GTPase-activating toxin CNF1 mediates NLRP3 inflammasome activation via p21-activated kinases-1/2 during bacteraemia in mice

Inflammasomes are signalling platforms that are assembled in response to infection or sterile inflammation by cytosolic pattern recognition receptors. The consequent inflammasome-triggered caspase-1 activation is critical for the host defence against pathogens. During infection, NLRP3, which is a pattern recognition receptor that is also known as cryopyrin, triggers the assembly of the inflammasome-activating caspase-1 through the recruitment of ASC and Nek7. The activation of the NLRP3 inflammasome is tightly controlled both transcriptionally and post-translationally. Despite the importance of the NLRP3 inflammasome regulation in autoinflammatory and infectious diseases, little is known about the mechanism controlling the activation of NLRP3 and the upstream signalling that regulates the NLRP3 inflammasome assembly. We have previously shown that the Rho-GTPase-activating toxin from Escherichia coli cytotoxic necrotizing factor-1 (CNF1) activates caspase-1, but the upstream mechanism is unclear. Here, we provide evidence of the role of the NLRP3 inflammasome in sensing the activity of bacterial toxins and virulence factors that activate host Rho GTPases. We demonstrate that this activation relies on the monitoring of the toxin's activity on the Rho GTPase Rac2. We also show that the NLRP3 inflammasome is activated by a signalling cascade that involves the p21-activated kinases 1 and 2 (Pak1/2) and the Pak1-mediated phosphorylation of Thr 659 of NLRP3, which is necessary for the NLRP3-Nek7 interaction, inflammasome activation and IL-1β cytokine maturation. Furthermore, inhibition of the Pak-NLRP3 axis decreases the bacterial clearance of CNF1-expressing UTI89 E. coli during bacteraemia in mice. Taken together, our results establish that Pak1 and Pak2 are critical regulators of the NLRP3 inflammasome and reveal the role of the Pak-NLRP3 signalling axis in vivo during bacteraemia in mice.

GTPases, genome, actin: A hidden story in DNA damage response and repair mechanisms

The classical small Rho GTPase (Rho, Rac, and Cdc42) protein family is mainly responsible for regulating cell motility and polarity, membrane trafficking, cell cycle control, and gene transcription. Cumulative recent evidence supports important roles for these proteins in the maintenance of genomic stability. Indeed, DNA damage response (DDR) and repair mechanisms are some of the prime biological processes that underlie several disease phenotypes, including genetic disorders, cancer, senescence, and premature aging. Many reports guided by different experimental approaches and molecular hypotheses have demonstrated that, to some extent, direct modulation of Rho GTPase activity, their downstream effectors, or actin cytoskeleton regulation contribute to these cellular events. Although much attention has been paid to this family in the context of canonical actin cytoskeleton remodeling, here we provide a contextualized review of the interplay between Rho GTPase signaling pathways and the DDR and DNA repair signaling components. Interesting questions yet to be addressed relate to the spatiotemporal dynamics of this collective response and whether it correlates with different subcellular pools of Rho GTPases. We highlight the direct and indirect targets, some of which still lack experimental validation data, likely associated with Rho GTPase activation that provides compelling evidence for further investigation in DNA damage-associated events and with potential therapeutic applications in translational medicine.

p21-activated kinase 6 controls mitosis and hepatocellular carcinoma progression by regulating Eg5

P21-activated kinases 6 (PAK6) associated with many fundamental cellular processes in cancer including cell-cell adhesion, migration and apoptosis. Here, we report a novel function of PAK6 in mitosis. Expression of PAK6 peaks in the M phase. Knockdown of PAK6 increases cell number in G2/M and promotes cell proliferation. PAK6 specifically colocalizes with Eg5 in the centrosome. Depletion of PAK6 results in multipolar spindle and a simultaneous upregulation of Eg5. Further, the PAK6 depletion-induced multiple spindle and cell cycle progression is reversed by knockdown of Eg5. These data suggest that PAK6 regulates spindle formation and cell cycle by regulating Eg5 expression. Additionally, expression of PAK6 is upregulated when Eg5 is downregulated or inhibited. Thus, PAK6 and Eg5 negatively inter-regulate each other. Significantly, the effect of PAK6 expression on the outcome of the HCC patients is controlled by Eg5 expression. Inhibition of Eg5 reverses PAK6 depletion-promoted cell invasion. Collectively, our data indicate that the inter-regulation between PAK6 and Eg5 might promote the progression of HCC.

PAK4 Kinase Activity Plays a Crucial Role in the Podosome Ring of Myeloid Cells

p21-Activated kinase 4 (PAK4), a serine/threonine kinase, is purported to localize to podosomes: transient adhesive structures that degrade the extracellular matrix to facilitate rapid myeloid cell migration. We find that treatment of transforming growth factor β (TGF-β)-differentiated monocytic (THP-1) cells with a PAK4-targeted inhibitor significantly reduces podosome formation and induces the formation of focal adhesions. This switch in adhesions confers a diminution of matrix degradation and reduced cell migration. Furthermore, reduced PAK4 expression causes a significant reduction in podosome number that cannot be rescued by kinase-dead PAK4, supporting a kinase-dependent role. Concomitant with PAK4 depletion, phosphorylation of Akt is perturbed, whereas a specific phospho-Akt signal is detected within the podosomes. Using superresolution analysis, we find that PAK4 specifically localizes in the podosome ring, nearer to the actin core than other ring proteins. We propose PAK4 kinase activity intersects with the Akt pathway at the podosome ring:core interface to drive regulation of macrophage podosome turnover.

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.

PAK5 facilitates the proliferation, invasion and migration in colorectal cancer cells

Colorectal cancer (CRC) is the third-most common cancer around the world, accounting for approximately 10% of cancer-related mortality. Deeper molecular understanding of colorectal carcinogenesis will provide evidences for identification of early diagnostic indicators and novel therapeutic strategies for CRC treatment. The p21cdc42/rac1 -activated kinase 5 (PAK5) has been reported to be involved in a variety of tumor-promoting behaviors, whereas the underlying mechanisms of PAK5 in CRC progression are still obscure. Our current study revealed an upregulated expression of PAK5 in human CRC tissues as compared with normal adjacent biopsies, which was associated with tumor progression and metastasis. We further unraveled that inhibition of PAK5 was correlated with restrained proliferation, migration, and invasion of CRC cells in vitro and in vivo. Moreover, we showed an indispensable role of PAK5 in interacting with Cdc42 and Integrin β1, β3, thus, to facilitate the migration and invasion of CRC cells. Collectively, we pointed out a potential of PAK5 to serve as a novel therapeutic target in restricting CRC proliferation and metastasis. The uncovered mechanisms will deepen the comprehension with regard to the mechanisms of CRC progression, as well as providing new insights for therapeutic intervention in colorectal cancer.

Phosphorylation Sites in Protein Kinases and Phosphatases Regulated by Formyl Peptide Receptor 2 Signaling

FPR1, FPR2, and FPR3 are members of Formyl Peptides Receptors (FPRs) family belonging to the GPCR superfamily. FPR2 is a low affinity receptor for formyl peptides and it is considered the most promiscuous member of this family. Intracellular signaling cascades triggered by FPRs include the activation of different protein kinases and phosphatase, as well as tyrosine kinase receptors transactivation. Protein kinases and phosphatases act coordinately and any impairment of their activation or regulation represents one of the most common causes of several human diseases. Several phospho-sites has been identified in protein kinases and phosphatases, whose role may be to expand the repertoire of molecular mechanisms of regulation or may be necessary for fine-tuning of switch properties. We previously performed a phospho-proteomic analysis in FPR2-stimulated cells that revealed, among other things, not yet identified phospho-sites on six protein kinases and one protein phosphatase. Herein, we discuss on the selective phosphorylation of Serine/Threonine-protein kinase N2, Serine/Threonine-protein kinase PRP4 homolog, Serine/Threonine-protein kinase MARK2, Serine/Threonine-protein kinase PAK4, Serine/Threonine-protein kinase 10, Dual specificity mitogen-activated protein kinase kinase 2, and Protein phosphatase 1 regulatory subunit 14A, triggered by FPR2 stimulation. We also describe the putative FPR2-dependent signaling cascades upstream to these specific phospho-sites.

Phosphoproteomic profiling reveals a defined genetic program for osteoblastic lineage commitment of human bone marrow-derived stromal stem cells

Bone marrow-derived mesenchymal stem cells (MSCs) differentiate into osteoblasts upon stimulation by signals present in their niche. Because the global signaling cascades involved in the early phases of MSCs osteoblast (OB) differentiation are not well-defined, we used quantitative mass spectrometry to delineate changes in human MSCs proteome and phosphoproteome during the first 24 h of their OB lineage commitment. The temporal profiles of 6252 proteins and 15,059 phosphorylation sites suggested at least two distinct signaling waves: one peaking within 30 to 60 min after stimulation and a second upsurge after 24 h. In addition to providing a comprehensive view of the proteome and phosphoproteome dynamics during early MSCs differentiation, our analyses identified a key role of serine/threonine protein kinase D1 (PRKD1) in OB commitment. At the onset of OB differentiation, PRKD1 initiates activation of the pro-osteogenic transcription factor RUNX2 by triggering phosphorylation and nuclear exclusion of the histone deacetylase HDAC7.

Phosphorylation-dependent activity-based conformational changes in P21-activated kinase family members and screening of novel ATP competitive inhibitors

P21-activated kinases (PAKs) are serine/threonine protein kinases that are subdivided into two groups on the basis of their domain architecture: group-I (PAK1–3) and group-II (PAK4–6). PAKs are considered as attractive drug targets that play vital role in cell proliferation, survival, motility, angiogenesis and cytoskeletal dynamics. In current study, molecular dynamics simulation-based comparative residual contributions and differential transitions were monitored in both active and inactive states of human PAK homologs for therapeutic intervention. Due to their involvement in cancer, infectious diseases, and neurological disorders, it is inevitable to develop novel therapeutic strategies that specifically target PAKs on the basis of their activity pattern. In order to isolate novel inhibitors that are able to bind at the active sites of PAK1 and PAK4, high throughput structure-based virtual screening was performed. Multiple lead compounds were proposed on the basis of their binding potential and targeting region either phosphorylated (active) or unphosphorylated PAK isoform (inactive). Thus, ATP-competitive inhibitors may prove ideal therapeutic choice against PAK family members. The detailed conformational readjustements occurring in the PAKs upon phosphorylation-dephosphorylation events may serve as starting point for devising novel drug molecules that are able to target on activity basis. Overall, the observations of current study may add valuable contribution in the inventory of novel inhibitors that may serve as attractive lead compounds for targeting PAK family members on the basis of activity-based conformational changes.

Molecular dynamics simulation and QM/MM calculation reveal the selectivity mechanism of type I 1/2 kinase inhibitors: the effect of intramolecular H-bonds and conformational restriction for improved selectivity

Understanding the selectivity mechanisms of inhibitors towards highly similar proteins is extremely important work on the way to a new drug. Here, we aim to reveal the selectivity mechanisms of type I 1/2 kinase inhibitors towards p21-activated kinase (PAK4) and mitogen-activated protein kinase kinase kinase 14 (MAP3K14, NIK). PAK4, belonging to the serine/threonine protein kinases, is involved in cell signaling pathways and controls cellular functions and has received attention as an attractive drug target. The high sequence identity between PAK4 and NIK makes it challenging to design selective PAK4 inhibitors. In this work, computational methods including protein comparison, molecular docking, QM/MM, molecular dynamics simulations, and density functional theory (DFT) calculation were employed to explore the binding mechanisms of selective inhibitors against NIK and PAK4. The simulation results revealed the crucial factors accounting for selective inhibition of PAK4 over NIK, including different protein-ligand interactions, the positions and conformations of key residues, and the ligands flexibilities. This study will shed light on understanding the selectivity mechanisms of PAK4 and NIK inhibitors.

Identification of the Apoptosis and Autophagy Bi-functional Proteins

Although autophagy and apoptosis, two main kinds of programmed cell death, constitute distinct cellular processes with often opposing outcomes, there have been shown a complex interplay between them in recent years. This interplay is surely critical to the overall fate of the cell. However, a full-scale identification of those bi-functional proteins involved in both functions is currently beyond reach by existing databases or traditional biological experiments. And that makes the interplay impossible to be well understood. Here we built a large, comprehensiveness apoptosis and autophagy related PPI (protein-protein interaction) network and then used topology clustering to undergo a network analysis workflow. Finally, we concluded a list of 151 apoptosis and autophagy bi-functional proteins from this network. By this way we showed a global view on these bi-functional proteins about their unique characteristics and provided clues of new functions of some proteins which are not focused in present researches.

Human Testis Phosphoproteome Reveals Kinases as Potential Targets in Spermatogenesis and Testicular Cancer

Spermatogenesis is a complex cell differentiation process that includes marked genetic, cellular, functional and structural changes. It requires tight regulation, because disturbances in any of the spermatogenic processes would lead to fertility deficiencies as well as disorders in offspring. To increase our knowledge of signal transduction during sperm development, we carried out a large-scale identification of the phosphorylation events that occur in the human male gonad. Metal oxide affinity chromatography using TiO2 combined with LC-MS/MS was conducted to profile the phosphoproteome of adult human testes with full spermatogenesis. A total of 8187 phosphopeptides derived from 2661 proteins were identified, resulting in the most complete report of human testicular phosphoproteins to date. Phosphorylation events were enriched in proteins functionally related to spermatogenesis, as well as to highly active processes in the male gonad, such as transcriptional and translational regulation, cytoskeleton organization, DNA packaging, cell cycle and apoptosis. Moreover, 174 phosphorylated kinases were identified. The most active human protein kinases in the testis were predicted both by the number of phosphopeptide spectra identified and the phosphorylation status of the kinase activation loop. The potential function of cyclin-dependent kinase 12 (CDK12) and p21-activated kinase 4 (PAK4) has been explored by in silico, protein-protein interaction analysis, immunodetection in testicular tissue, and a functional assay in a human embryonal carcinoma cell line. The colocalization of CDK12 with Golgi markers suggests a potential crucial role of this protein kinase during sperm formation. PAK4 has been found expressed in human spermatogonia, and a role in embryonal carcinoma cell response to apoptosis has been observed. Together, our protein discovery analysis confirms that phosphoregulation by protein kinases is highly active in sperm differentiation and opens a window to detailed characterization and validation of potential targets for the development of drugs modulating male fertility and tumor behavior.

A mandatory role of nuclear PAK4-LIFR axis in breast-to-bone metastasis of ERα-positive breast cancer cells

The mechanism of estrogen receptor alpha (ERα)-positive breast cancer-associated bone metastasis is poorly understood. In this article, we report that nuclear p21-activated kinase 4 (nPAK4) is a novel repressor of ERα-mediated transactivation in a 17β-estradiol (E2)-dependent manner and promotes PAK4–ERα axis-mediated bone metastasis by targeting leukemia inhibitory factor receptor (LIFR) in ERα-positive breast cancer. An evaluation of clinical breast cancer samples revealed that nPAK4 is linked to ERα expression and appears to be associated with a poor prognosis in bone metastatic breast cancer. PAK4 bound and co-translocated with ERα from the cytoplasm to the nucleus upon stimulation with E2. nPAK4 enhanced the invasive potential of ERα-positive breast cancer cells in vitro and promoted breast cancer metastasis in vivo. Mechanistically, nPAK4 promoted the metastasis of ERα-positive breast cancer cells by targeting LIFR, a bone metastasis suppressor. Strikingly, the nuclear accumulation of PAK4 might promote aggressive phenotypes, highlighting nPAK4 as a novel predictive biomarker for ERα-positive breast cancer bone metastasis.

Inhibition of Cdc42 is essential for Mig-6 suppression of cell migration induced by EGF

The adaptor protein Mig-6 is a negative regulator of EGF signaling. It is shown that Mig-6 inhibits cell migration via direct interaction with the ErbB receptors, thereby inhibiting cross-phosphorylation or targeting the receptors for degradation. Mig-6 has also been shown to bind to and inhibit the Rho GTPase Cdc42 to suppress cytoskeletal rearrangement. However, the molecular mechanism(s) by which Mig-6 inhibits cell migration via Cdc42 is still not entirely clear. Here, we show that Mig-6 binding to Cdc42 is necessary and sufficient to inhibit EGF-induced filopodia formation and migration. This binding, mediated by four specific residues (I11, R12, M26, R30) in the Mig-6 CRIB domain, is essential for Mig-6 function. In addition, ectopic expression of Cdc42 reverses Mig-6 inhibition of cell migration. Mig-6 CRIB domain, alone, is sufficient to inhibit cell migration. Conversely, Mig-6 binding to EGFR is dispensable for Mig-6-mediated inhibition of cell migration. Moreover, we found that decreased Mig-6 expression correlates with cancer progression in breast and prostate cancers. Together, our results demonstrate that Mig-6 inhibition of Cdc42 signaling is critical in Mig-6 function to suppress cell migration and that dysregulation of this pathway may play a critical role in cancer development.

Discovery of 2-(4-Substituted-piperidin/piperazine-1-yl)-N-(5-cyclopropyl-1H-pyrazol-3-yl)-quinazoline-2,4-diamines as PAK4 Inhibitors with Potent A549 Cell Proliferation, Migration, and Invasion Inhibition Activity

A series of novel 2,4-diaminoquinazoline derivatives were designed, synthesized, and evaluated as p21-activated kinase 4 (PAK4) inhibitors. All compounds showed significant inhibitory activity against PAK4 (half-maximal inhibitory concentration IC₅₀ < 1 μM). Among them, compounds 8d and 9c demonstrated the most potent inhibitory activity against PAK4 (IC₅₀ = 0.060 μM and 0.068 μM, respectively). Furthermore, we observed that compounds 8d and 9c displayed potent antiproliferative activity against the A549 cell line and inhibited cell cycle distribution, migration, and invasion of this cell line. In addition, molecular docking analysis was performed to predict the possible binding mode of compound 8d. This series of compounds has the potential for further development as PAK4 inhibitors for anticancer activity.

LINC01088 inhibits tumorigenesis of ovarian epithelial cells by targeting miR-24-1-5p

The roles of long non-coding RNAs (lncRNAs), a class of long non-protein-coding RNAs, in the tumorigenesis of ovarian epithelial cells remain unknown. In this study, we discovered that the expression of long intergenic non-coding RNA 1088 (LINC01088) was clearly reduced in benign epithelial ovarian tumor tissues compared to matched normal ovarian tissues. This was shown by global cDNA gene chip scanning and real-time qPCR, and validated in 42 clinical specimens. Furthermore, we found that LINC01088 inhibited the growth of ovarian cancer xenografts in nude mice. Correlation analysis between LINC01088 and mircoRNAs (miRNAs) conducted using primary clinical samples and RNA co-precipitation experiments revealed that miR-24-1-5p was one of the targets of LINC01088. Overexpression of miR-24-1-5p facilitated cell proliferation both in vitro and in vivo, however, LINC01088 could partially reverse the cell proliferation induced by miR-24-1-5p. Finally, we demonstrated that p21 activated kinase 4 (PAK4) was one of the downstream key targets of miR-24-1-5p by luciferase reporter assay and Western blotting; and our results showed a remarkable decrease in cell proliferation after overexpression of PAK4. We conclude that LINC01088 might function as a tumor suppressor, inhibiting the tumorigenesis of ovarian epithelial cells through LINC01088/ miR-24-1-5p/ PAK4 axis.

A key role for Rac and Pak signaling in neutrophil extracellular traps (NETs) formation defines a new potential therapeutic target

NET formation in mice (NETosis) is supported by reactive oxygen species (ROS) production by NADPH oxidase and histone hypercitrullination by peptidylarginine deiminase 4 (PAD4). Rac1 and Rac2, expressed in polymorphonuclear neutrophils (PMNs), regulate the cytoskeleton, cell shape, adhesion, and migration and are also essential components of the NADPH oxidase complex. We aimed to explore the role of the Rac signaling pathway including the upstream guanosine exchange factor (GEF) activator, Vav, and a downstream effector, the p21-activated kinase, Pak, on NETosis in PMNs using a previously described flow-cytometry-based assay. Rac2^-/- PMNs showed reduced levels of citrullinated histone H3 (H3Cit)-positive cells and defective NETosis. Rac1^Δ/Δ ; Rac2^-/- PMNs demonstrated a further reduction in PMA-induced H3Cit levels and a more profound impairment of NETosis than deletion of Rac2 alone, suggesting an overlapping role of these two highly related proteins. Genetic knockouts of Vav1, or Vav2, did not impair H3Cit response to phorbol myristate ester (PMA) or NETosis. Combined, Vav1 and Vav3 deletions decreased H3Cit response and caused a modest but significant impairment of NETosis. Pharmacologic inhibition of Pak by two inhibitors with distinct mechanisms of action, led to reduced H3Cit levels after PMA stimulation, as well as significant inhibition of NETosis. We validated the importance of Pak using Pak2^Δ/Δ PMNs, which demonstrated significantly impaired histone H3 citrullination and NETosis. These data confirm and more comprehensively define the key role of the Rac signaling pathway in PMN NETosis. The Rac signaling cascade may represent a valuable target for inhibition of NETosis and related pathological processes.

The Role of Semaphorin 4D in Bone Remodeling and Cancer Metastasis

Semaphorin 4D (Sema4D; CD100) is a transmembrane homodimer 150-kDa glycoprotein member of the Semaphorin family. Semaphorins were first identified as chemorepellants that guide neural axon growth. Sema4D also possesses immune regulatory activity. Recent data suggest other Sema4D functions: inactivation of platelets, stimulation of angiogenesis, and regulation of bone formation. Sema4D is a coupling factor expressed on osteoclasts that inhibits osteoblast differentiation. Blocking Sema4D may, therefore, be anabolic for bone. Sema4D and its receptor Plexin-B1 are commonly dysregulated in cancers, suggesting roles in cancer progression, invasion, tumor angiogenesis, and skeletal metastasis. This review focuses on Sema4D in bone and cancer biology and the molecular pathways involved, particularly Sema4D-Plexin-B1 signaling crosstalk between cancer cells and the bone marrow microenvironment-pertinent areas since a humanized Sema4D-neutralizing antibody is now in early phase clinical trials in cancers and neurological disorders.

Structure-Based Design of 6-Chloro-4-aminoquinazoline-2-carboxamide Derivatives as Potent and Selective p21-Activated Kinase 4 (PAK4) Inhibitors

Herein, we report the discovery and characterization of a novel class of PAK4 inhibitors with a quinazoline scaffold. Based on the shape and chemical composition of the ATP-binding pocket of PAKs, we chose a 2,4-diaminoquinazoline series of inhibitors as a starting point. Guided by X-ray crystallography and a structure-based drug design (SBDD) approach, a series of novel 4-aminoquinazoline-2-carboxamide PAK4 inhibitors were designed and synthesized. The inhibitors' selectivity, therapeutic potency, and pharmaceutical properties were optimized. One of the best compounds, 31 (CZh226), showed remarkable PAK4 selectivity (346-fold vs PAK1) and favorable kinase selectivity profile. Moreover, this compound potently inhibited the migration and invasion of A549 tumor cells by regulating the PAK4-directed downstream signaling pathways in vitro. Taken together, these data support the further development of 31 as a lead compound for PAK4-targeted anticancer drug discovery and as a valuable research probe for the further biological investigation of group II PAKs.

Mechanisms of RhoA inactivation and CDC42 and Rac1 activation during zebrafish optic nerve regeneration

When axons of the mammalian central nervous system (CNS) are injured, they fail to regenerate, while those of lower vertebrates undergo regeneration after injury. Wingless-type MMTV integration site family (Wnt) proteins play important roles in the CNS, and are reported to be activated after mammalian spinal cord or brain injury. Moreover, for axon growth to proceed, it is thought that small G-proteins, such as CDC42 and Rac1, need to be activated, whereas RhoA must be inactivated. However, the cell and molecular mechanisms involved in optic nerve regeneration remain unclear. In this study, we investigated axonal regeneration after injury using the zebrafish optic nerve as a model system. We sought to clarify the role of Wnt proteins and the mechanisms involved in the activation and inactivation of small G-proteins in nerve regeneration. After optic nerve injury, mRNA levels of Wnt5b, TAX1BP3 and ICAT increased in the retina, while those of Wnt10a decreased. These changes were associated with a reduction in β-catenin in nuclei. We found that Wnt5b activated CDC42 and Rac1, leading to the inactivation of RhoA, which appeared to be dependent on increased TAX1BP3 mRNA levels. Furthermore, we found that mRNA levels of Daam1a and ARHGEF16 decreased. We speculate that the decrease in β-catenin levels, which also further reduces levels of active RhoA, might contribute to regeneration in the zebrafish. Collectively, our novel results suggest that Wnt5b, Wnt10a, ICAT and TAX1BP3 participate in the activation and inactivation of small G-proteins, such as CDC42, Rac1 and RhoA, during the early stage of optic nerve regeneration in the zebrafish.

The role of Tec kinase signaling pathways in the development of Mallory Denk Bodies in balloon cells in alcoholic hepatitis

Several research strategies have been used to study the pathogenesis of alcoholic hepatitis (AH). These strategies have shown that various signaling pathways are the target of alcohol in liver cells. However, few have provided specific mechanisms associated with Mallory-Denk Bodies (MDBs) formed in Balloon cells in AH. The formation of MDBs in these hepatocytes is an indication that the mechanisms of protein quality control have failed. The MDB is the result of aggregation and accumulation of proteins in the cytoplasm of balloon degenerated liver cells. To understand the mechanisms that failed to degrade and remove proteins in the hepatocyte from patients suffering from alcoholic hepatitis, we investigated the pathways that showed significant up regulation in the AH liver biopsies compared to normal control livers (Liu et al., 2015). Analysis of genomic profiles of AH liver biopsies and control livers by RNA-seq revealed different pathways that were up regulated significantly. In this study, the focus was on Tec kinase signaling pathways and the genes that significantly interrupt this pathway. Quantitative PCR and immunofluorescence staining results, indicated that several genes and proteins are significantly over expressed in the livers of AH patients that affect the Tec kinase signaling to PI3K which leads to activation of Akt and its downstream effectors.

A CDC42-centered signaling unit is a dominant positive regulator of endothelial integrity

Endothelial barrier function is carefully controlled to protect tissues from edema and damage inflicted by extravasated leukocytes. RhoGTPases, in conjunction with myriad regulatory proteins, exert both positive and negative effects on the endothelial barrier integrity. Precise knowledge about the relevant mechanisms is currently fragmented and we therefore performed a comprehensive analysis of endothelial barrier regulation by RhoGTPases and their regulators. Combining RNAi with electrical impedance measurements we quantified the relevance of 270 Rho-associated genes for endothelial barrier function. Statistical analysis identified 10 targets of which six promoted- and four reduced endothelial barrier function upon downregulation. We analyzed in more detail two of these which were not previously identified as regulators of endothelial integrity. We found that the Rac1-GEF (Guanine nucleotide Exchange Factor) TIAM2 is a positive regulator and the Cdc42(Rac1)-GAP (GTPase-Activating Protein) SYDE1 is a negative regulator of the endothelial barrier function. Finally, we found that the GAP SYDE1 is part of a Cdc42-centered signaling unit, also comprising the Cdc42-GEF FARP1 and the Cdc42 effector PAK7 which controls the integrity of the endothelial barrier. In conclusion, using a siRNA-based screen, we identified new regulators of barrier function and found that Cdc42 is a dominant positive regulator of endothelial integrity.

MicroRNA-526a targets p21-activated kinase 7 to inhibit tumorigenesis in hepatocellular carcinoma

MicroRNAs belong to a series of noncoding RNAs and have diverse roles in several biological processes. The association between aberrant microRNA expression and tumorigenesis is complex and remains to be fully elucidated. The present study investigated whether microRNA (miR) ‑526a can suppress the progression of hepatocellular carcinoma (HCC) in vitro and in vivo. Reverse transcription‑quantitative polymerase chain reaction, luciferase reporter assay, invasion assay, western blotting and in vivo implantation were used to investigate the potential function of miR‑526a. The present study observed that the level of miR‑526a was downregulated in HCC tissues and well‑established cell lines. In addition, the ectopic introduction of miR‑526a into Huh7 and HepG2 cells significantly attenuated HCC tumorigenesis, including proliferation, migration and invasion. The growth of tumor xenografts was also inhibited following transfection with miR‑526a. Using overlapping strategies, p21‑activated kinase 7 (PAK7) was predicted to be a target for miR‑526a, and this was verified experimentally. An inverse correlation was found between miR‑526a and PAK7 in HCC tissues. The results of the present study revealed a novel function of miR‑526a and may provide crucial insight into therapeutic interventions targeting microRNAs.

High expression of P21-activated kinase 5 protein is associated with poor survival in gastric cancer

P21-activated kinase 5 (PAK5), also termed PAK7, is one of the six members of the PAK family of serine/threonine kinases, which are downstream effectors in several cancer signaling pathways. PAK5 promotes neural outgrowth, contributes to microtubule stability and induces resistance to apoptosis. However, the clinical importance of PAK5 in gastric cancer has not been comprehensively investigated. In the present study, PAK5 expression was evaluated in gastric cancer tissue samples. Furthermore, the associations between high expression of PAK5, and clinicopathological features and prognosis were examined. PAK5 expression in primary gastric cancer specimens resected from 279 patients who underwent gastrectomy at the Tokyo Medical and Dental University Hospital was evaluated using immunohistochemistry. Of the 279 patients, 44 (15.8%) exhibited high PAK5 expression, which was significantly associated with the differentiated pathological type (differentiated vs. undifferentiated; P<0.001), depth of tumor invasion (T1 vs. T2-T4; P<0.001), lymph node metastasis (N0 vs. N1-N3; P<0.001), presence of distant metastasis or recurrence (present vs. absent; P=0.038), advanced tumor stage (I vs. II-IV; P=0.001) and worse disease-specific survival (P=0.013). In stage I-III disease, 38/254 (15.0%) patients exhibited high PAK5 expression, and high expression of PAK5 was significantly associated with relapse-free interval (P=0.044). PAK5 may serve an important role in tumor progression and influence the outcome of patients with gastric cancer.

p21-activated kinase 4 regulates HIF-1α translation in cancer cells

The p21-activated kinases (Paks) interact with Rac/Cdc42 GTPases to regulate the actin cytoskeleton as well as various signaling pathways. Although activation of Paks in many human cancers is known to mediate cancer progression, the role of Pak proteins in hypoxia is poorly understood. In this study, we found that both Pak1 and Pak4 are highly expressed in HeLa cervical cancer cells, but only Pak4 knockdown attenuates expression of hypoxia-inducible factor-1α (HIF-1α) in hypoxia. We further discovered that Pak4 regulates HIF-1α translation via the Akt-mTOR-4E-BP1 pathway under hypoxic conditions. These results support a novel connection between HIF-1α and Pak4 in hypoxic cancer cells, and provide insights into mechanisms whereby tumors respond to and thrive under oxygen-deficient conditions.

A novel orally bioavailable compound KPT-9274 inhibits PAK4, and blocks triple negative breast cancer tumor growth

Breast cancer is a heterogeneous disease consisting of several subtypes. Among these subtypes, triple negative breast cancer is particularly difficult to treat. This is due to a lack of understanding of the mechanisms behind the disease, and consequently a lack of druggable targets. PAK4 plays critical roles in cell survival, proliferation, and morphology. PAK4 protein levels are high in breast cancer cells and breast tumors, and the gene is often amplified in basal like breast cancers, which are frequently triple negative. PAK4 is also overexpressed in other types of cancer, making it a promising drug target. However, its inhibition is complicated by the fact that PAK4 has both kinase-dependent and -independent functions. Here we investigate a new clinical compound KPT-9274, which has been shown to inhibit PAK4 and NAMPT. We find that KPT-9274 (and its analog, KPT-8752) can reduce the steady state level of PAK4 protein in triple negative breast cancer cells. These compounds also block the growth of the breast cancer cells in vitro, and stimulate apoptosis. Most importantly, oral administration of KPT-9274 reduces tumorigenesis in mouse models of human triple negative breast cancer. Our results indicate that KPT-9274 is a novel therapeutic option for triple negative breast cancer therapy.

PAK4 suppresses PDZ-RhoGEF activity to drive invadopodia maturation in melanoma cells

Cancer cells are thought to use actin rich invadopodia to facilitate matrix degradation. Formation and maturation of invadopodia requires the co-ordained activity of Rho-GTPases, however the molecular mechanisms that underlie the invadopodia lifecycle are not fully elucidated. Previous work has suggested a formation and disassembly role for Rho family effector p-21 activated kinase 1 (PAK1) however, related family member PAK4 has not been explored. Systematic analysis of isoform specific depletion using in vitro and in vivo invasion assays revealed there are differential invadopodia-associated functions. We consolidated a role for PAK1 in the invadopodia formation phase and identified PAK4 as a novel invadopodia protein that is required for successful maturation. Furthermore, we find that PAK4 (but not PAK1) mediates invadopodia maturation likely via inhibition of PDZ-RhoGEF. Our work points to an essential role for both PAKs during melanoma invasion but provides a significant advance in our understanding of differential PAK function.

Significance of kinase activity in the dynamic invadosome

Invadosomes are actin rich protrusive structures that facilitate invasive migration in multiple cell types. Comprised of invadopodia and podosomes, these highly dynamic structures adhere to and degrade the extracellular matrix, and are also thought to play a role in mechanosensing. Many extracellular signals have been implicated in invadosome stimulation, activating complex signalling cascades to drive the formation, activity and turnover of invadosomes. While the structural components of invadosomes have been well studied, the regulation of invadosome dynamics is still poorly understood. Protein kinases are essential to this regulation, affecting all stages of invadosome dynamics and allowing tight spatiotemporal control of their activity. Invadosome organisation and function have been linked to pathophysiological states such as cancer invasion and metastasis; therapeutic targeting of invadosome regulatory components is thus warranted. In this review, we discuss the involvement of kinase signalling in every stage of the invadosome life cycle and evaluate its significance.

PAK5 is auto-activated by a central domain that promotes kinase oligomerization

The present study shows for the first time that self-association of PAK5 in vivo underlies its high basal activity, which contrasts with the inactive state of cellular PAK4. Such PAK5 self-association interferes with the engagement of the auto-inhibitory (AID) with the catalytic domain.

P21-Activated Kinase Inhibitors FRAX486 and IPA3: Inhibition of Prostate Stromal Cell Growth and Effects on Smooth Muscle Contraction in the Human Prostate

Prostate smooth muscle tone and hyperplastic growth are involved in the pathophysiology and treatment of male lower urinary tract symptoms (LUTS). Available drugs are characterized by limited efficacy. Patients' adherence is particularly low to combination therapies of 5α-reductase inhibitors and α1-adrenoceptor antagonists, which are supposed to target contraction and growth simultaneously. Consequently, molecular etiology of benign prostatic hyperplasia (BPH) and new compounds interfering with smooth muscle contraction or growth in the prostate are of high interest. Here, we studied effects of p21-activated kinase (PAK) inhibitors (FRAX486, IPA3) in hyperplastic human prostate tissues, and in stromal cells (WPMY-1). In hyperplastic prostate tissues, PAK1, -2, -4, and -6 may be constitutively expressed in catecholaminergic neurons, while PAK1 was detected in smooth muscle and WPMY-1 cells. Neurogenic contractions of prostate strips by electric field stimulation were significantly inhibited by high concentrations of FRAX486 (30 μM) or IPA3 (300 μM), while noradrenaline- and phenylephrine-induced contractions were not affected. FRAX486 (30 μM) inhibited endothelin-1- and -2-induced contractions. In WPMY-1 cells, FRAX486 or IPA3 (24 h) induced concentration-dependent (1-10 μM) degeneration of actin filaments. This was paralleled by attenuation of proliferation rate, being observed from 1 to 10 μM FRAX486 or IPA3. Cytotoxicity of FRAX486 and IPA3 in WPMY-1 cells was time- and concentration-dependent. Stimulation of WPMY-1 cells with endothelin-1 or dihydrotestosterone, but not noradrenaline induced PAK phosphorylation, indicating PAK activation by endothelin-1. Thus, PAK inhibitors may inhibit neurogenic and endothelin-induced smooth muscle contractions in the hyperplastic human prostate, and growth of stromal cells. Targeting prostate smooth muscle contraction and stromal growth at once by a single compound is principally possible, at least under experimental conditions.

Proteomic Characterization of Inhibitory Synapses Using a Novel pHluorin-tagged γ-Aminobutyric Acid Receptor, Type A (GABAA), α2 Subunit Knock-in Mouse

The accumulation of γ-aminobutyric acid receptors (GABAARs) at the appropriate postsynaptic sites is critical for determining the efficacy of fast inhibitory neurotransmission. Although we know that the majority of synaptic GABAAR subtypes are assembled from α1-3, β, and γ2 subunits, our understanding of how neurons facilitate their targeting to and stabilization at inhibitory synapses is rudimentary. To address these issues, we have created knock-in mice in which the pH-sensitive green fluorescent protein (GFP) and the Myc epitope were introduced to the extracellular domain of the mature receptor α2 subunit (pHα2). Using immunoaffinity purification and mass spectroscopy, we identified a stable complex of 174 proteins that were associated with pHα2, including other GABAAR subunits, and previously identified receptor-associated proteins such as gephyrin and collybistin. 149 of these proteins were novel GABAAR binding partners and included G-protein-coupled receptors and ion channel subunits, proteins that regulate trafficking and degradation, regulators of protein phosphorylation, GTPases, and a number of proteins that regulate their activity. Notably, members of the postsynaptic density family of proteins that are critical components of excitatory synapses were not associated with GABAARs. Crucially, we demonstrated for a subset of these novel proteins (including cullin1, ephexin, potassium channel tetramerization domain containing protein 12, mitofusin2, metabotropic glutamate receptor 5, p21-activated kinase 7, and Ras-related protein 5A) bind directly to the intracellular domains of GABAARs, validating our proteomic analysis. Thus, our experiments illustrate the complexity of the GABAAR proteome and enhance our understanding of the mechanisms neurons use to construct inhibitory synapses.

PAK6 increase chemoresistance and is a prognostic marker for stage II and III colon cancer patients undergoing 5-FU based chemotherapy

p21-Activated kinase 6 (PAK6) has been implicated in radiotherapy and docetaxel resistance. We have further evaluated PAK6 as a predictor of 5-fluorouracil (5-FU) treatment response in colon cancer. Here we report that in colon cancer PAK6 promotes tumor progression and chemoresistance both in vitro and in vivo. In the clinical analysis, PAK6 was overexpressed in 104 of 147 (70.75%) stage II and III patients who received 5-FU based chemotherapy after surgery. Multivariate Cox regression analysis indicated that PAK6 was an independent prognostic factor for overall survival (P < 0.001) and disease-free survival (P < 0.001). Colon cancer cell lines showed increased PAK6 expression upon 5-FU treatment. In PAK6-knockdown cells treated with 5-FU, cell viability and phosphorylation of BAD decreased, and the number of apoptotic cells, levels of cleaved caspase 3 and PARP increased compared to control cells. The opposite was observed in PAK6 overexpressing cells. Short hairpin RNA knockdown of PAK6 blocked cells in G2-M phase. Furthermore, Animal experiments results in vivo are consistent with outcomes in vitro. This study demonstrates that PAK6 is an independent prognostic factor for adjuvant 5-FU-based chemotherapy in patients with stage II and stage III colon cancer.

Design, synthesis and biological evaluation of 1-phenanthryl-tetrahydroisoquinoline derivatives as novel p21-activated kinase 4 (PAK4) inhibitors

Functional versatility and elevated expression in cancers have promoted p21-activated kinase 4 (PAK4) as one of the first-in-class anti-cancer drug targets. In this study, a series of novel 1-phenanthryl-tetrahydroisoquinoline analogues have been designed and synthesized as a novel class of small-molecule PAK4 inhibitors to fit into the cavity of PAK4. All of the target compounds were evaluated for their in vitro PAK4 inhibitory activities and antiproliferative activities. Lead optimization identified all the derivatives with more potency than the lead compound, especially compound 21a. Moreover, compound 21a significantly induced the cell cycle in the G1/S phase, and inhibited migration and invasion of MCF-7 cells via the regulation of the PAK4-LIMK1-cofilin signaling pathway. A molecular modeling study showed possible novel binding modes between 21a and PAK4 and provided a structural basis for further structure-guided design of PAK4 inhibitors.

An in cellulo-derived structure of PAK4 in complex with its inhibitor Inka1

PAK4 is a metazoan-specific kinase acting downstream of Cdc42. Here we describe the structure of human PAK4 in complex with Inka1, a potent endogenous kinase inhibitor. Using single mammalian cells containing crystals 50 μm in length, we have determined the in cellulo crystal structure at 2.95 Å resolution, which reveals the details of how the PAK4 catalytic domain binds cellular ATP and the Inka1 inhibitor. The crystal lattice consists only of PAK4–PAK4 contacts, which form a hexagonal array with channels of 80 Å in diameter that run the length of the crystal. The crystal accommodates a variety of other proteins when fused to the kinase inhibitor. Inka1–GFP was used to monitor the process crystal formation in living cells. Similar derivatives of Inka1 will allow us to study the effects of PAK4 inhibition in cells and model organisms, to allow better validation of therapeutic agents targeting PAK4.

PAK4 is a metazoan-specific kinase, which acts downstream of the cell polarity regulator Cdc42. Here, Baskaran et al. determine the structure of PAK4 bound to the endogenous inhibitor Inka1 from crystals that form spontaneously in mammalian cells overexpressing both proteins.