Targeting group II PAKs in cancer and metastasis

Targeting P21-Activated Kinase-1 for Metastatic Prostate Cancer

Metastatic prostate cancer (mPCa) has limited therapeutic options and a high mortality rate. The p21-activated kinase (PAK) family of proteins is important in cell survival, proliferation, and motility in physiology, and pathologies such as infectious, inflammatory, vascular, and neurological diseases as well as cancers. Group-I PAKs (PAK1, PAK2, and PAK3) are involved in the regulation of actin dynamics and thus are integral for cell morphology, adhesion to the extracellular matrix, and cell motility. They also play prominent roles in cell survival and proliferation. These properties make group-I PAKs a potentially important target for cancer therapy. In contrast to normal prostate and prostatic epithelial cells, group-I PAKs are highly expressed in mPCA and PCa tissue. Importantly, the expression of group-I PAKs is proportional to the Gleason score of the patients. While several compounds have been identified that target group-I PAKs and these are active in cells and mice, and while some inhibitors have entered human trials, as of yet, none have been FDA-approved. Probable reasons for this lack of translation include issues related to selectivity, specificity, stability, and efficacy resulting in side effects and/or lack of efficacy. In the current review, we describe the pathophysiology and current treatment guidelines of PCa, present group-I PAKs as a potential druggable target to treat mPCa patients, and discuss the various ATP-competitive and allosteric inhibitors of PAKs. We also discuss the development and testing of a nanotechnology-based therapeutic formulation of group-I PAK inhibitors and its significant potential advantages as a novel, selective, stable, and efficacious mPCa therapeutic over other PCa therapeutics in the pipeline.

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.

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.

Cell Motility and Cancer

Cell migration is an essential systemic behavior, tightly regulated, of all living cells endowed with directional motility that is involved in the major developmental stages of all complex organisms such as morphogenesis, embryogenesis, organogenesis, adult tissue remodeling, wound healing, immunological cell activities, angiogenesis, tissue repair, cell differentiation, tissue regeneration as well as in a myriad of pathological conditions. However, how cells efficiently regulate their locomotion movements is still unclear. Since migration is also a crucial issue in cancer development, the goal of this narrative is to show the connection between basic findings in cell locomotion of unicellular eukaryotic organisms and the regulatory mechanisms of cell migration necessary for tumor invasion and metastases. More specifically, the review focuses on three main issues, (i) the regulation of the locomotion system in unicellular eukaryotic organisms and human cells, (ii) how the nucleus does not significantly affect the migratory trajectories of cells in two-dimension (2D) surfaces and (iii) the conditioned behavior detected in single cells as a primitive form of learning and adaptation to different contexts during cell migration. New findings in the control of cell motility both in unicellular organisms and mammalian cells open up a new framework in the understanding of the complex processes involved in systemic cellular locomotion and adaptation of a wide spectrum of diseases with high impact in the society such as cancer.

Advances in covalent kinase inhibitors

This comprehensive review details recent advances, challenges and innovations in covalent kinase inhibition within a 10 year period (2007–2018).

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.

PAK5 promotes the migration and invasion of cervical cancer cells by phosphorylating SATB1

p21-activated kinase 5 (PAK5) is involved in several oncogenic signaling pathways and its amplification or overexpression has been found in various types of cancer; however, the pathophysiologic role of PAK5 in cervical cancer (CC) remains elusive. This study aims to elucidate the effects of PAK5 on CC metastasis and its specific regulation mechanism. We performed western blotting and immunohistochemistry (IHC) analysis and found that the expression levels of PAK5 were significantly upregulated in CC cells and tissues. In addition, statistical analysis via IHC showed that increased PAK5 significantly correlated with CC progression. Mn²⁺-Phos-tag SDS-PAGE, western blotting, immunofluorescence and dual luciferase reporter assays were utilized to determine the involvement of SATB1 in PAK5-mediated epithelial-mesenchymal transition (EMT). We found that PAK5-mediated special AT-rich binding protein-1 (SATB1) phosphorylation on Ser47 initiated EMT cascade and promoted migration and invasion of CC cells. Furthermore, overexpression of PAK5 induced lung metastasis of CC cells in xenograft modes. Taken together, we conclude that PAK5 is a novel prognostic indicator and plays an important role in the CC 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.

PAK4 crystal structures suggest unusual kinase conformational movements

In order for protein kinases to exchange nucleotide they must open and close their catalytic cleft. These motions are associated with rotations of the N-lobe, predominantly around the 'hinge region'. We conducted an analysis of 28 crystal structures of the serine-threonine kinase, p21-activated kinase 4 (PAK4), including three newly determined structures in complex with staurosporine, FRAX486, and fasudil (HA-1077). We find an unusual motion between the N-lobe and C-lobe of PAK4 that manifests as a partial unwinding of helix αC. Principal component analysis of the crystal structures rationalizes these movements into three major states, and analysis of the kinase hydrophobic spines indicates concerted movements that create an accessible back pocket cavity. The conformational changes that we observe for PAK4 differ from previous descriptions of kinase motions, and although we observe these differences in crystal structures there is the possibility that the movements observed may suggest a diversity of kinase conformational changes associated with regulation.

AUTHOR SUMMARY

Protein kinases are key signaling proteins, and are important drug targets, therefore understanding their regulation is important for both basic research and clinical points of view. In this study, we observe unusual conformational 'hinging' for protein kinases. Hinging, the opening and closing of the kinase sub-domains to allow nucleotide binding and release, is critical for proper kinase regulation and for targeted drug discovery. We determine new crystal structures of PAK4, an important Rho-effector kinase, and conduct analyses of these and previously determined structures. We find that PAK4 crystal structures can be classified into specific conformational groups, and that these groups are associated with previously unobserved hinging motions and an unusual conformation for the kinase hydrophobic core. Our findings therefore indicate that there may be a diversity of kinase hinging motions, and that these may indicate different mechanisms of regulation.

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.

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.

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.

(-)-β-hydrastine suppresses the proliferation and invasion of human lung adenocarcinoma cells by inhibiting PAK4 kinase activity

(-)-β-hydrastine is one of the main active components of the medicinal plant, Hydrastis canadensis, which is used in many dietary supplements intended to enhance the immune system. However, whether (-)-β-hydrastine affects the tumor signaling pathway remains unexplored. In the present study, we found that (-)-β-hydrastine inhibited the kinase activity of p21-activated kinase 4 (PAK4), which is involved in the regulation of cytoskeletal reorganization, cell proliferation, gene transcription, oncogenic transformation and cell invasion. In the present study, (-)-β-hydrastine suppressed lung adenocarcinoma cell proliferation by inhibiting expression of cyclin D1/D3 and CDK2/4/6, leading to cell cycle arrest at the G1 phase, in a PAK4 kinase-dependent manner. Moreover, inhibition of PAK4 kinase activity by (-)-β-hydrastine also promoted the early apoptosis of lung adenocarcinoma cells through the mitochondrial apoptosis pathway. In addition, (-)-β-hydrastine significantly suppressed the migration and invasion of human lung adenocarcinoma cells in conjunction with concomitant blockage of the PAK4/LIMK1/cofilin, PAK4/SCG10 and PAK4/MMP2 pathways. All of these data indicate that (-)-β-hydrastine, as a novel PAK4 inhibitor, suppresses the proliferation and invasion of lung adenocarcinoma cells. Taken together, these results provide novel insight into the development of a PAK4 kinase inhibitor and a potential therapeutic strategy for lung cancer.

The potential role of miRNAs 21 and 199-a in early diagnosis of hepatocellular carcinoma

BACKGROUND

Hepatocellular carcinoma (HCC) is regarded as one of the most common malignancies and among the leading causes of cancer death among the whole world. The most urgent needs are to find sensitive markers for early diagnosis for HCC. MicroRNAs (miRNAs) are reported as a group of small non-coding RNAs that can function as endogenous RNA interference to regulate expression of the targeted genes. This study was conducted to detect the serum and tissue expression of miR 21 and miR 199-a to be applied as early detectors for HCC.

METHODS

A total of 40 serum and tissue samples (17 samples from chronic hepatitis and 23 samples from HCC patients) were collected. The levels of the two mature miRNAs (miR-21 and miR-199-a) were detected by real time quantitative reverse-transcriptase PCR (RT-qPCR) in sera and tissues of chronic hepatitis and HCC patients. Besides, miR-21 and miR-199-a levels in relation to clinical and pathological factors were explored.

RESULTS

We found that the expression of serum miR-21 was distinctly increased in HCC compared with chronic hepatitis (P<0.001). miR 199-a was distinctly decreased in HCC compared with chronic hepatitis (P<0.001). In addition, median of miR 21 was increased in malignant when compared to adjacent non-malignant tissues without significant differences (P=0.191) while miR 199-a was significantly decreased in malignant when compared to adjacent nonmalignant tissues (P<0.001). ROC analysis showed that miR-21 and miR-199-a might be potential biomarkers for HCC.

CONCLUSIONS

In conclusion, the expression of miR-21 was significantly up-regulated and miR-199-a was significantly down regulated in serum of patients with HCC. Due to their reasonable sensitivity and specificity for disease progression, miR-21 and miR-199-a could be used as potential circulating biomarkers for HCC.

p21-Activated Kinases 1, 2 and 4 in Endometrial Cancers: Effects on Clinical Outcomes and Cell Proliferation

p21-activated kinases (Paks) are serine/threonine protein kinases involved in biological events linked to malignant tumor progression. In this study, expression of Pak1, p-Pak2 Ser²⁰, Pak4, pPak4 Ser⁴⁷⁴ in 21 normal endometrium, 16 hyperplastic endometrium without atypia, 17 atypical complex hyperplasia and 67 endometrial cancers was assessed by immunohistochemistry and correlated with clinicopathological parameters. We also accessed the proliferative role and downstream targets of Pak1 in endometrial cancer. Pak1 was expressed in cytoplasm whereas Pak4 and p-Pak4 were expressed in both cytoplasm and nucleus of endometrial tissues. In normal endometrium, significantly higher Pak1 (P = 0.028) and cytoplasmic p-Pak2 (P = 0.048) expression was detected in proliferative endometrium than secretory endometrium. Pak1, cytoplasmic and nuclear Pak4 and nuclear p-Pak4 was significantly overexpressed in endometrial cancer when compared to atrophic endometrium (all P<0.05). Moreover, type I endometrioid carcinomas showed significantly higher Pak1 expression than type II non-endometrioid carcinomas (P<0.001). On the other hand, Pak1, Pak4 and p-Pak4 expression negatively correlated with histological grade (all P<0.05) while p-Pak2 and cytoplasmic Pak4 expression inversely correlated with myometrial invasion (all P<0.05). Furthermore, patients with endometrial cancers with lower cytoplasmic Pak4 expression showed poorer survival (P = 0.026). Multivariate analysis showed cytoplasmic Pak4 is an independent prognostic factor. Functionally, knockdown of Pak1, but not Pak4, in endometrial cancer cell line led to reduced cell proliferation along with reduced cyclin D1, estrogen receptor (ERα) and progestogen receptor (PR) expression. Significant correlation between Pak1 and PR expression was also detected in clinical samples. Our findings suggest that Pak1 and cytoplasmic p-Pak2 may promote cell proliferation in normal endometrium during menstral cycle. Pak1, cytoplasmic and nuclear Pak4 and nuclear p-Pak4 are involved in the pathogenesis of endometrial cancer especially in postmenopausal women. Pak1 promote endometrial cancer cell proliferation, particular in type I endometrioid carcinoma. Cytoplasmic Pak4 can be potential prognostic marker in endometrial cancer.

Signaling, Regulation, and Specificity of the Type II p21-activated Kinases

The p21-activated kinases (PAKs) are a family of six serine/threonine kinases that act as key effectors of RHO family GTPases in mammalian cells. PAKs are subdivided into two groups: type I PAKs (PAK1, PAK2, and PAK3) and type II PAKs (PAK4, PAK5, and PAK6). Although these groups are involved in common signaling pathways, recent work indicates that the two groups have distinct modes of regulation and have both unique and common substrates. Here, we review recent insights into the molecular level details that govern regulation of type II PAK signaling. We also consider mechanisms by which signal transduction is regulated at the level of substrate specificity. Finally, we discuss the implications of these studies for clinical targeting of these kinases.

PAK1 regulates RUFY3-mediated gastric cancer cell migration and invasion

Actin protrusion at the cell periphery is central to the formation of invadopodia during tumor cell migration and invasion. Although RUFY3 (RUN and FYVE domain containing 3)/SINGAR1 (single axon-related1)/RIPX (Rap2 interacting protein X) has an important role in neuronal development, its pathophysiologic role and relevance to cancer are still largely unknown. The purpose of this study was to elucidate the molecular mechanisms by which RUFY3 involves in gastric cancer cell migration and invasion. Here, our data show that overexpression of RUFY3 leads to the formation of F-actin-enriched protrusive structures at the cell periphery and induces gastric cancer cell migration. Furthermore, P21-activated kinase-1 (PAK1) interacts with RUFY3, and promotes RUFY3 expression and RUFY3-induced gastric cancer cell migration; inhibition of PAK1 attenuates RUFY3-induced SGC-7901 cell migration and invasion. Importantly, we found that the inhibitory effect of cell migration and invasion is significantly enhanced by knockdown of both PAK1 and RUFY3 compared with knockdown of RUFY3 alone or PAK1 alone. Strikingly, we found significant upregulation of RUFY3 in gastric cancer samples with invasive carcinoma at pathologic TNM III and TNM IV stages, compared with their non-tumor counterparts. Moreover, an obvious positive correlation was observed between the protein expression of RUFY3 and PAK1 in 40 pairs of gastric cancer samples. Therefore, these findings provide important evidence that PAK1 can positively regulate RUFY3 expression, which contribute to the metastatic potential of gastric cancer cells, maybe blocking PAK1-RUFY3 signaling would become a potential metastasis therapeutic strategy for gastric cancer.

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.

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

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

Protein kinase d isoforms differentially modulate cofilin-driven directed cell migration

Background

Protein kinase D (PKD) enzymes regulate cofilin-driven actin reorganization and directed cell migration through both p21-activated kinase 4 (PAK4) and the phosphatase slingshot 1L (SSH1L). The relative contributions of different endogenous PKD isoforms to both signaling pathways have not been elucidated, sufficiently.

Methodology/Principal Findings

We here analyzed two cell lines (HeLa and MDA-MB-468) that express the subtypes protein kinase D2 (PKD2) and protein kinase D3 (PKD3). We show that under normal growth conditions both isoforms can form a complex, in which PKD3 is basally-active and PKD2 is inactive. Basal activity of PKD3 mediates PAK4 activity and downstream signaling, but does not significantly inhibit SSH1L. This signaling constellation was required for facilitating directed cell migration. Activation of PKD2 and further increase of PKD3 activity leads to additional phosphorylation and inhibition of endogenous SSH1L. Net effect is a dramatic increase in phospho-cofilin and a decrease in cell migration, since now both PAK4 and SSH1L are regulated by the active PKD2/PKD3 complex.

Conclusions/Significance

Our data suggest that PKD complexes provide an interface for both cofilin regulatory pathways. Dependent on the activity of involved PKD enzymes signaling can be balanced to guarantee a functional cofilin activity cycle and increase cell migration, or imbalanced to decrease cell migration. Our data also provide an explanation of how PKD isoforms mediate different effects on directed cell migration.

P21 activated kinases: structure, regulation, and functions

The p21 activated kinases (Paks) are well known effector proteins for the Rho GTPases Cdc42 and Rac. The Paks contain 6 members, which fall into 2 families of proteins. The first family consists of Paks 1, 2, and 3, and the second consists of Paks 4, 5, and 6. While some of the Paks are ubiquitously expressed, others have more restrictive tissue specificity. All of them are found in the nervous system. Studies using cell culture, transgenic mice, and knockout mice, have revealed important roles for the Paks in cytoskeletal organization and in many aspects of cell growth and development. This review discusses the basic structures of the Paks, and their roles in cell growth, development, and in cancer.

P21-activated kinase 5 plays essential roles in the proliferation and tumorigenicity of human hepatocellular carcinoma

AIM

To investigate the roles of P21-activated kinase 5 (PAK5) in proliferation and tumorigenicity of human hepatocellular carcinoma (HCC).

METHODS

HCC and matched paraneoplastictis tissue samples were obtained from 30 patients. Human HCC cell lines SMMC7721, HepG2, Hep3B, SK-HEP-1, Huh-7, and liver cell line HL-7702 were examined. The expression of PAK5 gene was studied using real-time qPCR and Western blotting. Cell proliferation was quantified with the MTT assay. Cell cycle was analyzed with flow cytometry. The tumorigenicity of Lv-shRNA-transfected HepG2 cells was evaluated in BALB/cA nude mice.

RESULTS

The mRNA level of PAK5 was significantly higher in 25 out of 30 HCC samples compared to the matched paraneoplastic tissues. The HCC cell lines showed varying expression of PAK5 protein, and the highest level was found in the HepG2 cells. PAK5 gene silencing in HepG2 cells markedly reduced the cell proliferation and colony formation, and induced cell cycle arrest in the G1 phase. Furthermore, PAK5 gene silencing suppressed the tumor formation in nude mice, and significantly decreased the expression of HCC-related genes Cyclin D1 and beta-catenin.

CONCLUSION

PAK5 may play essential roles in the initiation and progression of human HCC. Thus, it may be an effective therapeutic target or perhaps serve as a clinical diagnostic or prognostic marker in human HCC.

Targeting nuclear kinases in cancer: development of cell cycle kinase inhibitors

Cellular proliferation is a tightly controlled set of events that is regulated by numerous nuclear protein kinases. The proteins involved include checkpoint kinases (CHK), cyclin-dependent kinases (CDK), which regulate the cell cycle and aurora kinases (AURK) and polo-like kinases (PLK), which regulate mitosis. In cancer, these nuclear kinases are often dysregulated and cause uncontrolled cell proliferation and growth. Much work has gone into developing novel therapeutics that target each of these protein kinases in cancer but none have been approved in patients. In this review we provide an overview of the current compounds being developed clinically to target these nuclear kinases involved in regulating the cell cycle and mitosis.

Protein kinase D-mediated phosphorylation at Ser99 regulates localization of p21-activated kinase 4

PAKs (p21-activated kinases) are effectors of RhoGTPases. PAK4 contributes to regulation of cofilin at the leading edge of migrating cells through activation of LIMK (Lin-11/Isl-1/Mec-3 kinase). PAK4 activity is regulated by an autoinhibitory domain that is released upon RhoGTPase binding as well as phosphorylation at Ser474 in the activation loop of the kinase domain. In the present study, we add another level of complexity to PAK4 regulation by showing that phosphorylation at Ser99 is required for its targeting to the leading edge. This phosphorylation is mediated by PKD1 (protein kinase D1). Phosphorylation of PAK4 at Ser99 also mediates binding to 14-3-3 protein, and is required for the formation of a PAK4-LIMK-PKD1 complex that regulates cofilin activity and directed cell migration.

Substrate and inhibitor specificity of the type II p21-activated kinase, PAK6

The p21-activated kinases (PAKs) are important effectors of Rho-family small GTPases. The PAK family consists of two groups, type I and type II, which have different modes of regulation and signaling. PAK6, a type II PAK, influences behavior and locomotor function in mice and has an ascribed role in androgen receptor signaling. Here we show that PAK6 has a peptide substrate specificity very similar to the other type II PAKs, PAK4 and PAK5 (PAK7). We find that PAK6 catalytic activity is inhibited by a peptide corresponding to its N-terminal pseudosubstrate. Introduction of a melanoma-associated mutation, P52L, into this peptide reduces pseudosubstrate autoinhibition of PAK6, and increases phosphorylation of its substrate PACSIN1 (Syndapin I) in cells. Finally we determine two co-crystal structures of PAK6 catalytic domain in complex with ATP-competitive inhibitors. We determined the 1.4 Å co-crystal structure of PAK6 with the type II PAK inhibitor PF-3758309, and the 1.95 Å co-crystal structure of PAK6 with sunitinib. These findings provide new insights into the structure-function relationships of PAK6 and may facilitate development of PAK6 targeted therapies.

SH3RF2 functions as an oncogene by mediating PAK4 protein stability

SH3RF (SH3-domain-containing RING finger protein) family members, SH3RF1-3, are multidomain scaffold proteins involved in promoting cell survival and apoptosis. In this report, we show that SH3RF2 is an oncogene product that is overexpressed in human cancers and regulates p21-activated kinase 4 (PAK4) protein stability. Immunohistochemical analysis of 159 colon cancer tissues showed that SH3RF2 expression levels are frequently elevated in cancer tissues and significantly correlate with poor prognostic indicators, including increased invasion, early recurrence and poor survival rates. We also demonstrated that PAK4 protein is degraded by the ubiquitin-proteasome system and that SH3RF2 inhibits PAK4 ubiquitination via physical interaction-mediated steric hindrance, which results in the upregulation of PAK4 protein. Moreover, ablation of SH3RF2 expression attenuates TRADD (TNFR-associated death domain) recruitment to tumor necrosis factor-α (TNF-α) receptor 1 and hinders downstream signals, thereby inhibiting NF-κB (nuclear factor-kappaB) activity and enhancing caspase-8 activity, in the context of TNF-α treatment. Notably, ectopic expression of SH3RF2 effectively prevents apoptosis in cancer cells and enhances cell migration, colony formation and tumor growth in vivo. Taken together, our results suggest that SH3RF2 is an oncogene that may be a definitive regulator of PAK4. Therefore, SH3RF2 may represent an effective therapeutic target for cancer treatment.

Pak and Rac GTPases promote oncogenic KIT-induced neoplasms

An acquired somatic mutation at codon 816 in the KIT receptor tyrosine kinase is associated with poor prognosis in patients with systemic mastocytosis and acute myeloid leukemia (AML). Treatment of leukemic cells bearing this mutation with an allosteric inhibitor of p21-activated kinase (Pak) or its genetic inactivation results in growth repression due to enhanced apoptosis. Inhibition of the upstream effector Rac abrogates the oncogene-induced growth and activity of Pak. Although both Rac1 and Rac2 are constitutively activated via the guanine nucleotide exchange factor (GEF) Vav1, loss of Rac1 or Rac2 alone moderately corrected the growth of KIT-bearing leukemic cells, whereas the combined loss resulted in 75% growth repression. In vivo, the inhibition of Vav or Rac or Pak delayed the onset of myeloproliferative neoplasms (MPNs) and corrected the associated pathology in mice. To assess the role of Rac GEFs in oncogene-induced transformation, we used an inhibitor of Rac, EHop-016, which specifically targets Vav1 and found that EHop-016 was a potent inhibitor of human and murine leukemic cell growth. These studies identify Pak and Rac GTPases, including Vav1, as potential therapeutic targets in MPN and AML involving an oncogenic form of KIT.

Oncogenic PAK4 regulates Smad2/3 axis involving gastric tumorigenesis

The alteration of p21-activated kinase 4 (PAK4) and transforming growth factor-beta (TGF-β) signaling effector Smad2/3 was detected in several types of tumors, which acts as oncogenic factor and tumor suppressor, but the relationship between these events has not been explored. Here, we demonstrate that PAK4 interacts with and modulates phosphorylation of Smad2/3 via both kinase-dependent and kinase-independent mechanisms, which attenuate Smad2/3 axis transactivation and TGF-β-mediated growth inhibition in gastric cancer cells. First, PAK4 interaction with Smad2/3, which is independent of PAK4 kinase activity, blocks TGF-β1-induced phosphorylation of Smad2 Ser465/467 or Smad3 Ser423/425 and the consequent activation. In addition, PAK4 phosphorylates Smad2 on Ser465, leading to the degradation of Smad2 through ubiquitin-proteasome-dependent pathway under hepatocyte growth factor (HGF) stimulation. Interestingly, PAK4 expression correlates negatively with phospho-Ser465/467 Smad2 but positively with phospho-Ser465 Smad2 in gastric cancer tissues. Furthermore, the expressions of HGF, phospho-Ser474 PAK4 and phospho-Ser465 Smad2 are markedly increased in gastric cancer tissues, and the expression of Smad2 is decreased in gastric cancer tissues. Our results document an oncogenic role of PAK4 in repression of Smad2/3 transactivation that involved in tumorigenesis, and suggest PAK4 as a potential therapeutic target for gastric cancer.

CHL1 is involved in human breast tumorigenesis and progression

Neural cell adhesion molecules (CAM) play important roles in the development and regeneration of the nervous system. The L1 family of CAMs is comprised of L1, Close Homolog of L1 (CHL1, L1CAM2), NrCAM, and Neurofascin, which are structurally related trans-membrane proteins in vertebrates. Although the L1CAM has been demonstrated play important role in carcinogenesis and progression, the function of CHL1 in human breast cancer is limited. Here, we found that CHL1 is down-regulated in human breast cancer and related to lower grade. Furthermore, overexpression of CHL1 suppresses proliferation and invasion in MDA-MB-231 cells and knockdown of CHL1 expression results in increased proliferation and invasion in MCF7 cells in vitro. Finally, CHL1 deficiency promotes tumor formation in vivo. Our results may provide a strategy for blocking breast carcinogenesis and progression.

The Pak4 protein kinase is required for oncogenic transformation of MDA-MB-231 breast cancer cells

The Pak4 protein kinase, normally expressed at low level in the mammary gland, is commonly overexpressed in breast cancer. Overexpression of Pak4 transforms mouse mammary epithelial cells in vitro and renders these cells tumorigenic in athymic mice in vivo. Here we show that Pak4 is also required for oncogenic transformation of the human breast cancer cell line MDA-MB-231. These high Pak4-expressing human breast cancer cells form highly disorganized three-dimensional (3D) structures in vitro and readily give rise to orthotopic xenograft tumors in nude mice. We have found that when Pak4 levels are reduced, MDA-MB-231 cells exhibit decreased proliferation and migration in vitro, as well as gross restoration of normal 3D mammary acinar organization, the latter in association with a strong induction of apoptosis. Similarly, Pak4 knockdown suppresses MDA-MB-231 breast xenograft tumor formation in nude mice in vivo. These results indicate that Pak4 has a key role in the oncogenic transformation of breast cells.

Association of the epithelial-to-mesenchymal transition phenotype with responsiveness to the p21-activated kinase inhibitor, PF-3758309, in colon cancer models

The p21-activated kinase (PAK) family of serine/threonine kinases, which are overexpressed in several cancer types, are critical mediators of cell survival, motility, mitosis, transcription, and translation. In the study presented here, we utilized a panel of colorectal cancer (CRC) cell lines to identify potential biomarkers of sensitivity or resistance that may be used to individualize therapy to the PAK inhibitor PF-03758309. We observed a wide range of proliferative responses in the CRC cell lines exposed to PF-03758309, this response was recapitulated in other phenotypic assays such as anchorage-independent growth, three-dimensional (3D) tumor spheroid formation, and migration. Interestingly, we observed that cells most sensitive to PF-03758309 exhibited up-regulation of genes associated with a mesenchymal phenotype (CALD1, VIM, ZEB1) and cells more resistant had an up-regulation of genes associated with an epithelial phenotype (CLDN2, CDH1, CLDN3, CDH17) allowing us to derive an epithelial-to-mesenchymal transition (EMT) gene signature for this agent. We assessed the functional role of EMT-associated genes in mediating responsiveness to PF-3758309, by targeting known genes and transcriptional regulators of EMT. We observed that suppression of genes associated with the mesenchymal phenotype conferred resistance to PF-3758309, in vitro and in vivo. These results indicate that PAK inhibition is associated with a unique response phenotype in CRC and that further studies should be conducted to facilitate both patient selection and rational combination strategies with these agents.

Tumor P-Glycoprotein Correlates with Efficacy of PF-3758309 in in vitro and in vivo Models of Colorectal Cancer

P-glycoprotein (P-gp), a member of the ATP-binding cassette transporter family, is overexpressed in a number of different cancers and some studies show that P-gp overexpression can be correlated to poor prognosis or therapeutic resistance. Here we sought to elucidate if PF-3758309 (PF-309), a novel p-21 activated kinase inhibitor, efficacy was influenced by tumor P-gp. Based on in vitro proliferation data, a panel of colorectal cancer cell lines were ranked as sensitive or resistant and ABCB1 (P-gp) expression was evaluated by microarray for these cell lines. P-gp expression was determined by western blot and activity determined by rhodamine efflux assay. Knock down of P-gp and pharmacologic inhibition of P-gp to restore PF-309 activity was performed in vitro. PF-309 activity was evaluated in vivo in cell line xenograft models and in primary patient derived tumor xenografts (PDTX). Mice were treated with 25 mg/kg PF-309 orally, twice daily. On the last day of treatment, tumor and plasma were collected for PF-309 analysis. Here we show that ABCB1 gene expression correlates with resistance to PF-309 treatment in vitro and the expression and activity of P-gp was verified in a panel of resistant cells. Furthermore, inhibition of P-gp increased the sensitivity of resistant cells, resulting in a 4–100-fold decrease in the IC50s. Eleven cell line xenografts and 12 PDTX models were treated with PF-309. From the cell line xenografts, we found a significant correlation between ABCB1 gene expression profiles and tumor response. We evaluated tumor and plasma concentrations for eight tumor models (three cell line xenografts and five PDTX models) and a significant correlation was found between tumor concentration and response. Additionally, we show that tumor concentration is approximately fourfold lower in tumors that express P-gp, verified by western blot. Our in vitro and in vivo data strongly suggests that PF-309 efficacy is influenced by the expression of tumor P-gp.

The pak4 protein kinase in breast cancer

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

Knockdown of PAK4 or PAK1 inhibits the proliferation of mutant KRAS colon cancer cells independently of RAF/MEK/ERK and PI3K/AKT signaling

The p21-activated kinase (PAK) serine/threonine kinases are important effectors of the small GTPases Rac and Cdc42, and play significant roles in controlling cell growth, motility, and transformation. Knockdown of PAK4 or PAK1 inhibited the proliferation of mutant KRAS or BRAF colon cancer cells in vitro. Dependence on PAK4 or PAK1 protein for colon cancer cell proliferation was independent of PAK4 or PAK1 protein expression levels. Mutant KRAS HCT116 colorectal cells were the most sensitive to PAK4 or PAK1 knockdown resulting in the potent inhibition of anchorage-dependent and -independent proliferation as well as the formation and proliferation of HCT116 colon cancer spheroids. This inhibition of proliferation did not correlate with inhibition of RAF/MEK/ERK or PI3K/AKT signaling. In HCT116 cells, knockdown of PAK4 or PAK1 caused changes to the actin cytoskeleton resulting in reduced basal spread and cell elongation and increased cell rounding. These cytoskeletal rearrangements seemed to be independent of LIMK/cofilin/paxillin phosphorylation. PAK4 or PAK1 knockdown initially induced growth arrest in HCT116 cells followed by cell death at later time points. Inhibition of the antiapoptotic proteins Bcl-2 and Bcl-X(L) with the pharmacologic inhibitor ABT-737 increased effector caspase activation and apoptosis, and reduced cell survival with PAK4 or PAK1 knockdown. These results support a role for the PAKs in the proliferation of mutant KRAS-driven colorectal carcinoma cells via pathways not involving RAF/MEK/ERK and PI3K/AKT signaling.

Signaling-dependent phosphorylation of mitotic centromere-associated kinesin regulates microtubule depolymerization and its centrosomal localization

BACKGROUND

Although PAK1 regulates cytoskeleton and microtubule dynamics, its role in controlling the functions of MCAK remains unknown.

RESULTS

PAK1 phosphorylates MCAK and thereby regulates both its localization and function.

CONCLUSION

MCAK is a cognate substrate of PAK1.

SIGNIFICANCE

This study provides a novel mechanistic insight into PAK1 regulation of MCAK functions. Although p21-activated kinase 1 (PAK1) and microtubule (MT) dynamics regulate numerous fundamental processes including cytoskeleton remodeling, directional motility, and mitotic functions, the significance of PAK1 signaling in regulating the functions of MT-destabilizing protein mitotic centromere-associated kinesin (MCAK) remains unknown. Here we found that MCAK is a cognate substrate of PAK1 wherein PAK1 phosphorylates MCAK on serines 192 and 111 both in vivo and in vitro. Furthermore, we found that PAK1 phosphorylation of MCAK on serines 192 and 111 preferentially regulates its microtubule depolymerization activity and localization to centrosomes, respectively, in the mammalian cells.

Approaches to Ras signaling modulation and treatment of Ras-dependent disorders: a patent review (2007--present)

INTRODUCTION

Ras proteins are small GTPases molecular switches that cycle through two alternative conformational states, a GDP-bound inactive state and a GTP-bound active state. In the active state, Ras proteins interact with and modulate the activity of several downstream effectors regulating key cellular processes including proliferation, differentiation, survival, senescence, migration and metabolism. Activating mutations of RAS genes and of genes encoding Ras signaling members have a great incidence in proliferative disorders, such as cancer, immune and inflammatory diseases and developmental syndromes. Therefore, Ras and Ras signaling represent important clinical targets for the design and development of pharmaceutically active agents, including anticancer agents.

AREAS COVERED

The authors summarize methods available to down-regulate the Ras pathway and review recent patents covering Ras signaling modulators, as well as methods designed to kill specifically cancer cells bearing activated RAS oncogene.

EXPERT OPINION

Targeted therapy approach based on direct targeting of molecules specifically altered in Ras-dependent diseases is pursued with molecules that down-regulate expression or inhibit the biological function of mutant Ras or Ras signaling members. The low success rate in a clinical setting of molecules targeting activated members of the Ras pathway may require development of novel approaches, including combined and synthetic lethal therapies.

Quantitative proteomics of extracellular vesicles derived from human primary and metastatic colorectal cancer cells

Cancer cells actively release extracellular vesicles (EVs), including exosomes and microvesicles, into surrounding tissues. These EVs play pleiotropic roles in cancer progression and metastasis, including invasion, angiogenesis, and immune modulation. However, the proteomic differences between primary and metastatic cancer cell-derived EVs remain unclear. Here, we conducted comparative proteomic analysis between EVs derived from human primary colorectal cancer cells (SW480) and their metastatic derivatives (SW620). Using label-free quantitation, we identified 803 and 787 proteins in SW480 EVs and SW620 EVs, respectively. Based on comparison between the estimated abundance of EV proteins, we identified 368 SW480 EV-enriched and 359 SW620 EV-enriched proteins. SW480 EV-enriched proteins played a role in cell adhesion, but SW620 EV-enriched proteins were associated with cancer progression and functioned as diagnostic indicators of metastatic cancer; they were overexpressed in metastatic colorectal cancer and played roles in multidrug resistance. As the first proteomic analysis comparing primary and metastatic cancer-derived EVs, this study increases our understanding of the pathological function of EVs in the metastatic process and provides useful biomarkers for cancer metastasis.

Group I p21-activated kinases (PAKs) promote tumor cell proliferation and survival through the AKT1 and Raf-MAPK pathways

Group I p21-activated kinases (PAK) are important effectors of the small GTPases Rac and Cdc42, which regulate cell motility/migration, survival, proliferation, and gene transcription. Hyperactivation of these kinases have been reported in many tumor types, making PAKs attractive targets for therapeutic intervention. PAKs are activated by growth factor-mediated signaling and are negatively regulated by the tumor suppressor neurofibromatosis type 2 (NF2)/Merlin. Thus, tumors characterized by NF2 inactivation would be expected to show hyperactivated PAK signaling. On the basis of this rationale, we evaluated the status of PAK signaling in malignant mesothelioma, an aggressive neoplasm that is resistant to current therapies and shows frequent inactivation of NF2. We show that group I PAKs are activated in most mesotheliomas and mesothelioma cell lines and that genetic or pharmacologic inhibition of PAKs is sufficient to inhibit mesothelioma cell proliferation and survival. We also identify downstream effectors and signaling pathways that may contribute mechanistically to PAK-related tumorigenesis. Specifically, we show that inhibition of PAK results in attenuation of AKT and Raf-MAPK signaling and decreased tumor cell viability. Collectively, these data suggest that pharmacologic inhibition of group I PAKs may have therapeutic efficacy in tumors characterized by PAK activation.

Pharmacological inhibition of LIM kinase stabilizes microtubules and inhibits neoplastic growth

The emergence of tumor resistance to conventional microtubule-targeting drugs restricts their clinical use. Using a cell-based assay that recognizes microtubule polymerization status to screen for chemicals that interact with regulators of microtubule dynamics, we identified Pyr1, a cell permeable inhibitor of LIM kinase, which is the enzyme that phosphorylates and inactivates the actin-depolymerizing factor cofilin. Pyr1 reversibly stabilized microtubules, blocked actin microfilament dynamics, inhibited cell motility in vitro and showed anticancer properties in vivo, in the absence of major side effects. Pyr1 inhibition of LIM kinase caused a microtubule-stabilizing effect, which was independent of any direct effects on the actin cytoskeleton. In addition, Pyr1 retained its activity in multidrug-resistant cancer cells that were resistant to conventional microtubule-targeting agents. Our findings suggest that LIM kinase functions as a signaling node that controls both actin and microtubule dynamics. LIM kinase may therefore represent a targetable enzyme for cancer treatment.

p21-Activated kinase 4 promotes prostate cancer progression through CREB

Prostate cancer is initially androgen-dependent but, over time, usually develops hormone- and chemo-resistance. The present study investigated a role for p21-activated kinase 4 (PAK4) in prostate cancer progression. PAK4 activation was markedly inhibited by H89, a specific protein kinase A (PKA) inhibitor, and PAK4 was activated by the elevation of cAMP. The catalytic subunit of PKA interacted with the regulatory domain of PAK4, and directly phosphorylated PAK4 at serine 474 (S474). Catalytically active PAK4 enhanced the transcriptional activity of CREB independent of S133 phosphorylation. Stable knockdown of PAK4 in PC-3 and DU145 prostate cancer cells inhibited tumor formation in nude mice. Decreased tumorigenicity correlated with decreased expression of CREB and its targets, including Bcl-2 and cyclin A1. Additionally, in androgen-dependent LNCap-FGC cells, PAK4 regulated cAMP-induced neuroendocrine differentiation, which is known to promote tumor progression. Finally, PAK4 enhanced survival and decreased apoptosis following chemotherapy. These results suggested that PAK4 regulates progression toward hormone- and chemo-resistance in prostate cancer, and this study identified both a novel activation mechanism and potential downstream effector pathways. Therefore, PAK4 may be a promising therapeutic target in prostate cancer.

PAK4 kinase activity and somatic mutation promote carcinoma cell motility and influence inhibitor sensitivity

Hepatocyte growth factor (HGF) and its receptor (c-Met) are associated with cancer cell motility and invasiveness. p21-activated kinase 4 (PAK4), a potential therapeutic target, is recruited to and activated by c-Met. In response, PAK4 phosphorylates LIM kinase 1 (LIMK1) in an HGF-dependent manner in metastatic prostate carcinoma cells. PAK4 overexpression is known to induce increased cell migration speed but the requirement for kinase activity has not been established. We have used a panel of PAK4 truncations and mutations in a combination of over-expression and RNAi rescue experiments to determine the requirement for PAK4 kinase activity during carcinoma cell motility downstream of HGF. We find that neither the kinase domain alone nor a PAK4 mutant unable to bind Cdc42 is able to fully rescue cell motility in a PAK4-deficient background. Nevertheless, we find that PAK4 kinase activity and associated LIMK1 activity are essential for carcinoma cell motility, highlighting PAK4 as a potential anti-metastatic therapeutic target. We also show here that overexpression of PAK4 harboring a somatic mutation, E329K, increased the HGF-driven motility of metastatic prostate carcinoma cells. E329 lies within the G-loop region of the kinase. Our data suggest E329K mutation leads to a modest increase in kinase activity conferring resistance to competitive ATP inhibitors in addition to promoting cell migration. The existence of such a mutation may have implications for the development of PAK4-specific competitive ATP inhibitors should PAK4 be further explored for clinical inhibition.

MicroRNA-10a targets CHL1 and promotes cell growth, migration and invasion in human cervical cancer cells

MicroRNAs (miRNAs) play an important role in cancer initiation, progression and metastasis by regulating their target genes. Here, we found microRNA-10a (miR-10a) is upregulated in human cervical cancer and promotes the colony formation activity, migration and invasion of HeLa and C33A cells. Subsequently, CHL1 is confirmed as a target of miR-10a and is negatively regulated by miR-10a at mRNA and protein levels. Furthermore, knockdown of CHL1 expression results in increased colony formation activity, migration and invasion. Finally, overexpression of CHL1 lacked the 3'UTR abolished the effects of miR-10a. Our results may provide a strategy for blocking tumor metastasis.

Role of p21-activated kinase 1 in regulating the migration and invasion of fibroblast-like synoviocytes from rheumatoid arthritis patients

OBJECTIVE

To investigate the role of p21-activated kinase 1 (PAK1) in regulating migration, invasion and MMP expression in RA fibroblast-like synoviocytes (FLS).

METHODS

RA FLS migration and invasion in vitro were measured by the Boyden chamber method. Invasion of RA FLS into cartilage was detected in the severe combined immunodeficiency (SCID) mouse co-implantation model of RA in vivo. PAK1 and MT1-MMP expression were examined by western blotting. ELISA was used to measure the production and activity of MMPs.

RESULTS

Phosphorylated PAK1 (p-PAK1) protein expression was increased in ex vivo synovial membrane cells from RA patients. Stimulation with IL-1β or TNF-α up-regulated p-PAK1 expression. Inhibition of PAK1 by transfection with dominant negative PAK1 mutant (dnPAK1) reduced in vitro migration and invasion of RA FLS. In the SCID mouse model, RA FLS invasion into cartilage was attenuated by transfection with dnPAK1 in vivo. PAK1 regulated IL-1β-induced production and activity of MMP-13 and MT1-MMP. Inhibition of MMP-13 or MT1-MMP activity also reduced RA FLS invasion. Furthermore, dnPAK1 transfection inhibited c-Jun N-terminal kinase (JNK) activation, but did not affect the activities of extracellular signal-regulated kinases and p38. Inhibition of the JNK activity by chemical inhibitor significantly reduced the migration, invasion and production of MMP-13 and MT1-MMP.

CONCLUSION

PAK1 plays an important role in regulating the migration, invasion and production and activity of MMPs in RA FLS, which is mediated by the JNK pathway. This suggests a novel strategy targeting PAK1 to prevent joint destruction of RA.

Identification of allosteric inhibitors of p21-activated kinase

Protein kinases are among the most important drug targets; however the structural conservation of the ATP-binding pocket of kinases can lead to promiscuous inhibition of additional unintended kinase targets. Allosteric inhibitors that target less conserved regions of protein kinases represent an alternative approach that may provide more selective kinase inhibition. In this report, protocols are provided for the screening and identification of Pak1 inhibitors acting via an allosteric mechanism.

p21-activated kinase 1: PAK'ed with potential

The p21-activated kinases (PAKs) are central players in growth factor signaling networks and morphogenetic processes that control proliferation, cell polarity, invasion and actin cytoskeleton organization. This raises the possibility that interfering with PAK activity may produce significant anti-tumor activity. In this perspective, we summarize recent data concerning the contribution of the PAK family member, PAK1, in growth factor signaling and tumorigenesis. We further discuss mechanisms by which inhibition of PAK1 can arrest tumor growth and promote cell apoptosis, and the types of cancers in which PAK1 inhibition may hold promise.

miRNA signature of schwannomas: possible role(s) of "tumor suppressor" miRNAs in benign tumors

miRNAs have been recently implicated as drivers in several carcinogenic processes, where they can act either as oncogenes or as tumor suppressors. Schwannomas arise from Schwann cells, the myelinating cells of the peripheral nervous system. These benign tumors typically result from loss of the neurofibromatosis type 2 (NF2) tumor suppressor gene. We have recently carried out high-throughput miRNA expression profiling of human vestibular schwannomas using an array representing 407 known miRNAs in order to explore the role of miRNAs in the tumorigenesis of schwannomas. We found that miR-7 functions as a "tumor suppressor" by targeting proteins in three major oncogenic pathways - EGFR, Pak1, and Ack1. Interestingly, in this study, we also observed that several previously described potential tumor suppressor miRNAs that are down-regulated in malignant tumors were up-regulated in schwannomas. Here we discuss the possibility that "tumor suppressor" miRNAs may play a role in the transition stage(s) of cancer from benign to malignant forms.

Protein kinase D regulates cofilin activity through p21-activated kinase 4

Dynamic reorganization of the actin cytoskeleton at the leading edge is required for directed cell migration. Cofilin, a small actin-binding protein with F-actin severing activities, is a key enzyme initiating such actin remodeling processes. Cofilin activity is tightly regulated by phosphorylation and dephosphorylation events that are mediated by LIM kinase (LIMK) and the phosphatase slingshot (SSH), respectively. Protein kinase D (PKD) is a serine/threonine kinase that inhibits actin-driven directed cell migration by phosphorylation and inactivation of SSH. Here, we show that PKD can also regulate LIMK through direct phosphorylation and activation of its upstream kinase p21-activated kinase 4 (PAK4). Therefore, active PKD increases the net amount of phosphorylated inactive cofilin in cells through both pathways. The regulation of cofilin activity at multiple levels may explain the inhibitory effects of PKD on barbed end formation as well as on directed cell migration.