3D structure analysis of PAKs: A clue to the rational design for affinity reagents and blockers

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

AIMS

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

MAIN METHODS

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

KEY FINDINGS

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

SIGNIFICANCE

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

Minor Kinases with Major Roles in Cytokinesis Regulation

Cytokinesis, the conclusive act of cell division, allows cytoplasmic organelles and chromosomes to be faithfully partitioned between two daughter cells. In animal organisms, its accurate regulation is a fundamental task for normal development and for preventing aneuploidy. Cytokinesis failures produce genetically unstable tetraploid cells and ultimately result in chromosome instability, a hallmark of cancer cells. In animal cells, the assembly and constriction of an actomyosin ring drive cleavage furrow ingression, resulting in the formation of a cytoplasmic intercellular bridge, which is severed during abscission, the final event of cytokinesis. Kinase-mediated phosphorylation is a crucial process to orchestrate the spatio-temporal regulation of the different stages of cytokinesis. Several kinases have been described in the literature, such as cyclin-dependent kinase, polo-like kinase 1, and Aurora B, regulating both furrow ingression and/or abscission. However, others exist, with well-established roles in cell-cycle progression but whose specific role in cytokinesis has been poorly investigated, leading to considering these kinases as "minor" actors in this process. Yet, they deserve additional attention, as they might disclose unexpected routes of cell division regulation. Here, we summarize the role of multifunctional kinases in cytokinesis with a special focus on those with a still scarcely defined function during cell cleavage. Moreover, we discuss their implication in cancer.

Targeting PKCι-PAK1 signaling pathways in EGFR and KRAS mutant adenocarcinoma and lung squamous cell carcinoma

Introduction

p21-activated kinase 1 (PAK1) stimulates growth and metastasis in non-small cell lung cancer (NSCLC). Protein kinase C iota (PKCι) is an enzyme highly expressed in NSCLC, regulating PAK1 signaling. In the present study we explored whether the PKCι-PAK1 signaling pathway approach can be an efficient target in different types of NSCLC cell and mouse models.

Methods

The effect of IPA-3 (PAK1 inhibitor) plus auranofin (PKCι inhibitor) combination was evaluated by cell viability assay, colony formation and western blotting assay, using three types of NSCLC cell lines: EGFR or KRAS mutant adenocarcinoma and squamous cell carcinoma with PAK1 amplification. In addition, for clinical availability, screening for new PAK1 inhibitors was carried out and the compound OTSSP167 was evaluated in combination with auranofin in cell and mice models.

Results

The combination of IPA-3 or OTSSP167 plus auranofin showed high synergism for inhibiting cell viability and colony formation in three cell lines. Mechanistic characterization revealed that this drug combination abrogated expression and activation of membrane receptors and downstream signaling proteins crucial in lung cancer: EGFR, MET, PAK1, PKCι, ERK1/2, AKT, YAP1 and mTOR. A nude mouse xenograft assay demonstrated that this drug combination strongly suppressed tumor volume compared with single drug treatment.

Conclusions

Combination of IPA-3 or OTSSP167 and auranofin was highly synergistic in EGFR or KRAS mutant adenocarcinoma and squamous cell carcinoma cell lines and decreased tumor volume in mice models. It is of interest to further test the targeting of PKCι-PAK1 signaling pathways in EGFR mutant, KRAS mutant and squamous NSCLC patients.

Targeting PAK1

p21-Activated kinase 1 (PAK1) has attracted much attention as a potential therapeutic target due to its central role in many oncogenic signaling pathways, its frequent dysregulation in cancers and neurological disorders, and its tractability as a target for small-molecule inhibition. To date, several PAK1-targeting compounds have been developed as preclinical agents, including one that has been evaluated in a clinical trial. A series of ATP-competitive inhibitors, allosteric inhibitors and peptide inhibitors with distinct biochemical and pharmacokinetic properties represent useful laboratory tools for studies on the role of PAK1 in biology and in disease contexts, and could lead to promising therapeutic agents. Given the central role of PAK1 in vital signaling pathways, future clinical development of PAK1 inhibitors will require careful investigation of their safety and efficacy.

Identification of NCK1 as a novel downstream effector of STAT3 in colorectal cancer metastasis and angiogenesis

Signal transducer and activator of transcription 3 (STAT3) is known to activate targets associated with invasion, proliferation, and angiogenesis in a wide variety of cancers. The adaptor protein NCK1 is involved in cytoskeletal movement and was identified as a STAT3-associated target in human tumors. However, the underlying molecular mechanism associated with colorectal cancer (CRC) metastasis is not yet completely understood. In this study, we report a novel STAT3 to NCK1 signaling pathway in colorectal cancer (CRC). We investigated the expression of NCK1 and its potential clinical and biological significance in CRC. NCK1 was noticeably up-regulated in human CRC tissues. NCK1 was also significantly associated with serosal invasion, lymph metastasis, and tumor-node-metastasis classification but was inversely correlated with differentiation. Gain-of-function and loss-of-function studies have shown that ectopic expression of NCK1 enhanced metastasis and angiogenesis in CRC cells. By gene expression analyses, we revealed a high co-overexpression of STAT3 and NCK1 in CRC tissues. Ectopic overexpression of STAT3 in CRC cells induced the expression of NCK1, whereas STAT3 knockdown decreased the expression of NCK1. Promoter activation and binding analyses demonstrated that STAT3 promoted the expression of NCK1 via direct action on the NCK1 promoter. The knock down of NCK1 partially reduced the CRC cell metastasis and angiogenesis promoted by STAT3. Additionally, by co-immunoprecipitation assays, we verified that NCK1 interacted with PAK1, which resulted in the activation of the PAK1/ERK pathway. STAT3 induced the transcription of NCK1 and triggered a PAK1/ERK cascade in CRC. These findings suggest a novel STAT3 to NCK1 to PAK1/ERK signaling mechanism that is potentially critical for CRC metastasis and angiogenesis.

Structure, biochemistry, and biology of PAK kinases

PAKs, p21-activated kinases, play central roles and act as converging junctions for discrete signals elicited on the cell surface and for a number of intracellular signaling cascades. PAKs phosphorylate a vast number of substrates and act by remodeling cytoskeleton, employing scaffolding, and relocating to distinct subcellular compartments. PAKs affect wide range of processes that are crucial to the cell from regulation of cell motility, survival, redox, metabolism, cell cycle, proliferation, transformation, stress, inflammation, to gene expression. Understandably, their dysregulation disrupts cellular homeostasis and severely impacts key cell functions, and many of those are implicated in a number of human diseases including cancers, neurological disorders, and cardiac disorders. Here we provide an overview of the members of the PAK family and their current status. We give special emphasis to PAK1 and PAK4, the prototypes of groups I and II, for their profound roles in cancer, the nervous system, and the heart. We also highlight other family members. We provide our perspective on the current advancements, their growing importance as strategic therapeutic targets, and our vision on the future of PAKs.

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

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

Inhibitors of p21-activated kinases (PAKs)

The p21-activated kinase (PAK) family of serine/threonine protein kinases plays important roles in cytoskeletal organization, cellular morphogenesis, and survival, and members of this family have been implicated in many diseases including cancer, infectious diseases, and neurological disorders. Owing to their large and flexible ATP binding cleft, PAKs, particularly group I PAKs (PAK1, -2, and -3), are difficult to drug; hence, few PAK inhibitors with satisfactory kinase selectivity and druglike properties have been reported to date. Examples are a recently discovered group II PAK (PAK4, -5, -6) selective inhibitor series based on a benzimidazole core, a group I PAK selective series based on a pyrido[2,3-d]pyrimidine-7-one core, and an allosteric dibenzodiazepine PAK1 inhibitor series. Only one compound, an aminopyrazole based pan-PAK inhibitor, entered clinical trials but did not progress beyond phase I trials. Clinical proof of concept for pan-group I, pan-group II, or PAK isoform selective inhibition has yet to be demonstrated.

The PAKs come of age: Celebrating 18 years of discovery

Protein kinases are versatile signaling molecules that are involved in the regulation most physiological responses. The p21-activated kinases (PAKs) can be activated directly by the small GTPases Rac and Cdc42 and are among the best characterized downstream effectors of these Rho proteins. The structure, substrate specificity and functional role of PAKS are evolutionarily conserved from protozoa to mammals. Vertebrate PAKs are particularly important for cytoskeletal remodeling and focal adhesion assembly, thereby contributing to dynamic processes such as cell migration and synaptic plasticity. This issue of Cellular Logistics focuses on the PAK family of kinases, with ten reviews written by researchers currently working in the field. Here in this introductory overview we highlight some of the most interesting recent discoveries regarding PAK biochemistry and biology. The reviews in this issue cover a range of topics including the atomic structures of PAK1 and PAK4, their role in animals as assessed by knockout studies, and how PAKs are likely to contribute to cancer and neurodegenerative diseases. The promise remains that PAK inhibitors will emerge that validate current pre-clinical studies suggesting that blocking PAK activity will positively contribute to human health.

Untangling the complexity of PAK1 dynamics: The future challenge

PAK1 kinase is a crucial regulator of a variety of cellular processes, such as motility, cell division, gene transcription and apoptosis. Its deregulation is involved in several pathologies, including cancer, viral infection and neurodegenerative diseases. Due to this strong implication in human health, the complex network of signaling pathways centered on PAK1 is a subject of intensive investigations. This review summarizes the present knowledge on the multiple PAK1 intracellular localizations and on its shuttling between different compartments. The dynamics of PAK1 localization and activation are finely tuned by the cell and it is this tight control that underlies the capacity of PAK1 to participate in the regulation of many fundamental cell functions. Recently, PAK1 biosensors have been developed to visualize PAK1 activation in live cells. These new imaging tools should be of great help to better understand PAK1 biology and to conceive strategies for efficient and specific PAK1 inhibitors.

Signaling networks of Rho GTPases in cell motility

The last decades have witnessed an exponential increase in our knowledge of Rho GTPase signaling network which further highlighted the cross talk between these proteins and the complexity of their signaling pathways. In this review, we summarize the upstream and downstream players from Rho GTPases that are mainly involved in actin polymerization leading to cell motility and potentially playing a role in cancer cell metastasis.