LKB1 suppresses p21-activated kinase-1 (PAK1) by phosphorylation of Thr109 in the p21-binding domain

Rapamycin vs TORin-1 or Gleevec vs Nilotinib: Simple chemical evolution that converts PAK1-blockers to TOR-blockers or vice versa?

Both PAK1 (RAC/CDC42-activating kinase 1) and TOR (Target of Rapamycin) are among the major oncogenic/ageing kinases. However, they play the opposite role in our immune system, namely immune system is suppressed by PAK1, while it requires TOR. Thus, PAK1-blockers, would be more effective for therapy of cancers, than TOR-blockers. Since 2015 when we discovered genetically that PDGF-induced melanogenesis depends on "PAK1", we are able to screening a series of PAK1-blockers as melanogenesis-inhibitors which could eventually promote longevity. Interestingly, rapamycin, the first TOR-inhibitor, promotes melanogenesis, clearly indicating that TOR suppresses melanogenesis. However, a new TOR-inhibitor called TORin-1 no longer suppresses immune system, and blocks melanogenesis in cell culture. These observations strongly indicate that TORin-1 acts as PAK1-blockers, instead of TOR-blockers, in vivo. Thus, it is most likely that melanogenesis in cell culture could enable us to discriminate PAK1-blockers from TORblockers.

LKB1 loss promotes colorectal cancer cell metastasis through regulating TNIK expression and actin cytoskeleton remodeling

Colorectal cancer (CRC) is one of the most common malignant tumors. Approximately 5%-6% of CRC cases are associated with hereditary CRC syndromes, including the Peutz-Jeghers syndrome (PJS). Liver kinase B1 (LKB1), also known as STK11, is the major gene responsible for PJS. LKB1 heterozygotic deficiency is involved in intestinal polyps in mice, while the mechanism of LKB1 in CRC remains elusive. In this study, we generated LKB1 knockout (KO) CRC cell lines by using CRISPR-Cas9. LKB1 KO promoted CRC cell motility in vitro and tumor metastases in vivo. LKB1 attenuated expression of TRAF2 and NCK-interacting protein kinase (TNIK) as accessed by RNA-seq and western blots, and similar suppression was also detected in the tumor tissues of azoxymethane/dextran sodium sulfate-induced intestinal-specific LKB1-KO mice. LKB1 repressed TNIK expression through its kinase activity. Moreover, attenuating TNIK by shRNA inhibited cell migration and invasion of CRC cells. LKB1 loss-induced high metastatic potential of CRC cells was depended on TNIK upregulation. Furthermore, TNIK interacted with ARHGAP29 and further affected actin cytoskeleton remodeling. Taken together, LKB1 deficiency promoted CRC cell metastasis via TNIK upregulation and subsequently mediated cytoskeleton remodeling. These results suggest that LKB1-TNIK axis may play a crucial role in CRC progression.

Liver Kinase B1 Functions as a Regulator for Neural Development and a Therapeutic Target for Neural Repair

The liver kinase B1 (LKB1), also known as serine/threonine kinase 11 (STK11) and Par-4 in C. elegans, has been identified as a master kinase of AMPKs and AMPK-related kinases. LKB1 plays a crucial role in cell growth, metabolism, polarity, and tumor suppression. By interacting with the downstream signals of SAD, NUAK, MARK, and other kinases, LKB1 is critical to regulating neuronal polarization and axon branching during development. It also regulates Schwann cell function and the myelination of peripheral axons. Regulating LKB1 activity has become an attractive strategy for repairing an injured nervous system. LKB1 upregulation enhances the regenerative capacity of adult CNS neurons and the recovery of locomotor function in adult rodents with CNS axon injury. Here, we update the major cellular and molecular mechanisms of LKB1 in regulating neuronal polarization and neural development, and the implications thereof for promoting neural repair, axon regeneration, and functional recovery in adult mammals.

p21-Activated kinase 1 (PAK1) in aging and longevity: An overview

The p21-activated kinases (PAKs) belong to serine/threonine kinases family, regulated by ∼21 kDa small signaling G proteins RAC1 and CDC42. The mammalian PAK family comprises six members (PAK1-6) that are classified into two groups (I and II) based on their domain architecture and regulatory mechanisms. PAKs are implicated in a wide range of cellular functions. PAK1 has recently attracted increasing attention owing to its involvement in oncogenesis, tumor progression, and metastasis as well as several life-limiting diseases and pathological conditions. In Caenorhabditis elegans, PAK1 functions limit the lifespan under basal conditions by inhibiting forkhead transcription factor DAF-16. Interestingly, PAK depletion extended longevity and attenuated the onset of age-related phenotypes in a premature-aging mouse model and delayed senescence in mammalian fibroblasts. These observations implicate PAKs as not only oncogenic but also aging kinases. Therefore, PAK-targeting genetic and/or pharmacological interventions, particularly PAK1-targeting, could be a viable strategy for developing cancer therapies with relatively no side effects and promoting healthy longevity. This review describes PAK family proteins, their biological functions, and their role in regulating aging and longevity using C. elegans. Moreover, we discuss the effect of small-molecule PAK1 inhibitors on the lifespan and healthspan of C. elegans.

P21-Activated Kinase 1: Emerging biological functions and potential therapeutic targets in Cancer

The p21-Activated kinase 1 (PAK1), a member of serine-threonine kinases family, was initially identified as an interactor of the Rho GTPases RAC1 and CDC42, which affect a wide range of processes associated with cell motility, survival, metabolism, cell cycle, proliferation, transformation, stress, inflammation, and gene expression. Recently, the PAK1 has emerged as a potential therapeutic target in cancer due to its role in many oncogenic signaling pathways. Many PAK1 inhibitors have been developed as potential preclinical agents for cancer therapy. Here, we provide an overview of essential roles that PAK1 plays in cancer, including its structure and autoactivation mechanism, its crucial function from onset to progression to metastasis, metabolism, immune escape and even drug resistance in cancer; endogenous regulators; and cancer-related pathways. We also summarize the reported PAK1 small-molecule inhibitors based on their structure types and their potential application in cancer. In addition, we provide overviews on current progress and future challenges of PAK1 in cancer, hoping to provide new ideas for the diagnosis and treatment of cancer.

Current trends and opportunities in targeting p21 activated kinase-1(PAK1) for therapeutic management of breast cancers

Breast cancer is the most frequently diagnosed cancer in women worldwide. Identifying reliable biomarkers and druggable molecular targets pose to be a significant quest in breast cancer research. p21-activated kinase 1 (PAK1) is a serine/threonine kinase that direct cell motility, cytoskeletal remodelling, and has been shown to function as a downstream regulator for various cancer signalling cascades that promote cell proliferation, apoptosis deregulation and hasten mitotic abnormalities, resulting in tumor formation and progression. The heterogeneity and acquired drug resistance are important factors that challenge the treatment of breast cancer. p21-activated kinase 1 signalling is crucial for activation of the Ras/RAF/MEK/ERK, PI3K/Akt/mTOR and Wnt signalling cascades which regulate cell survival, cell cycle progression, differentiation, and proliferation. A study involving proteogenomics analysis on breast cancer tissues showed the PAK1 as outlier kinase. In addition to this, few outlier molecules were identified specific to subtypes of breast cancer. A few substrates of PAK1 in breast cancer are already known. In this paper, we have discussed a similar approach called Kinase Interacting Substrate Screening (KISS) for the identification of novel oncogenic substrates of p21-activated kinase specific to subtypes of breast cancer. Such high throughput approaches are expected to accelerate the process of identifying novel drug targets and biomarkers.

Crosstalk of LKB1- and PTEN-regulated signals in liver morphogenesis and tumor development

Liver kinase B 1 (LKB1 or STK11) and phosphatase and tensin homologue deleted on chromosome 10 (PTEN) are two tumor suppressors that regulate the mammalian target of rapamycin signaling pathway. Deletion studies show that loss of either Lkb1 (Lkb^+/–) or Pten (Pten^loxP/loxP; Alb‐Cre⁺) leads to liver injury and development of hepatocarcinoma. In this study, we investigated the crosstalk of LKB1 and PTEN loss during tumorigenesis and liver development. We show that haplo‐insufficiency of Lkb1 in the liver leads to advanced tumor development in Pten‐null mice (Pten^loxP/loxP; Lkb^loxP/+; Alb‐Cre⁺). Our analysis shows that LKB1 and PTEN interact with each other in their regulation of fatty acid synthase as well as p21 expression. The combined loss of LKB1 and PTEN (Pten^loxP/loxP; Lkb^loxP/loxP; Alb‐Cre⁺) also leads to the inability to form zonal structures in the liver. The lack of metabolic zonal structures is consistent with the inability of the livers to store glycogen as well as elevated plasma bilirubin and alanine aminotransferase, indicative of liver dysfunction. These structural and functional defects are associated with cytoplasm distribution of a canalicular membrane protein multidrug resistant protein 2, which is responsible for clearing bilirubin. This observed regulation of multidrug resistant protein 2 by LKB1 likely contributes to the lack of cellular polarity and the early lethality phenotype associated with the homozygous loss of Lkb1 alone or in combination with Pten. Finally, Pten deletion does not rescue the precocious ductal plate formation reported for Lkb1‐deleted livers. Conclusion: Our study dissected the functional and molecular crosstalk of PTEN and LKB1 and elucidated key molecular targets for such interactions. (Hepatology Communications 2017;1:153‐167)

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.

Extracellular lumican inhibits pancreatic cancer cell growth and is associated with prolonged survival after surgery

PURPOSE

To evaluate the relevance between lumican expression patterns and the clinical course of patients with pancreatic ductal adenocarcinoma (PDAC), and to investigate the role of lumican in PDAC progression.

EXPERIMENTAL DESIGN

One hundred thirty-one patient tumors were chosen for tissue microarray staining, and Cox regression analysis was used to test the associations between lumican expression and clinical, pathologic, and oncologic outcomes in all patients. Primary PDAC cells and recombinant human lumican protein were used to establish a working model to mimic the in vivo interactions between stromal lumican and PDAC cells. Using this model, we tested the effects of lumican on EGFR signaling via Akt and hypoxia-inducible factor-1α (HIF1α) and its subsequent influence on glucose consumption, lactate production, intracellular ATP, and apoptotic cell death.

RESULTS

Lumican was present in the stroma surrounding PDAC cells in roughly one-half of primary tumors and the direct xenografts. Patients with stromal lumican were associated with a profound reduction in metastatic recurrence after surgery and 3-fold longer survival than patients without stromal lumican. In PDAC cells, extracellular lumican reduced EGFR expression and phosphorylation through enhanced dimerization and internalization of EGFR and the resultant inhibition of Akt kinase activity. Lumican also reduced HIF1α expression and activity via Akt. PDAC cells with enhanced HIF1α activity were resistant to lumican-induced inhibition of glucose consumption, lactate production, intracellular ATP, and apoptosis.

CONCLUSIONS

There is a positive association between stromal lumican in primary PDAC tumors and prolonged survival after tumor resection. Lumican plays a restrictive role in EGFR-expressing pancreatic cancer progression.

Republished: tracing PAKs from GI inflammation to cancer

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

Tracing PAKs from GI inflammation to cancer

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

p21-Activated protein kinases and their emerging roles in glucose homeostasis

p21-Activated protein kinases (PAKs) are centrally involved in a plethora of cellular processes and functions. Their function as effectors of small GTPases Rac1 and Cdc42 has been extensively studied during the past two decades, particularly in the realms of cell proliferation, apoptosis, and hence tumorigenesis, as well as cytoskeletal remodeling and related cellular events in health and disease. In recent years, a large number of studies have shed light onto the fundamental role of group I PAKs, most notably PAK1, in metabolic homeostasis. In skeletal muscle, PAK1 was shown to mediate the function of insulin on stimulating GLUT4 translocation and glucose uptake, while in pancreatic β-cells, PAK1 participates in insulin granule localization and vesicle release. Furthermore, we demonstrated that PAK1 mediates the cross talk between insulin and Wnt/β-catenin signaling pathways and hence regulates gut proglucagon gene expression and the production of the incretin hormone glucagon-like peptide-1 (GLP-1). The utilization of chemical inhibitors of PAK and the characterization of Pak1(-/-) mice enabled us to gain mechanistic insights as well as to assess the overall contribution of PAKs in metabolic homeostasis. This review summarizes our current understanding of PAKs, with an emphasis on the emerging roles of PAK1 in glucose homeostasis.

Herbal therapeutics that block the oncogenic kinase PAK1: a practical approach towards PAK1-dependent diseases and longevity

Over 35 years research on PAKs, RAC/CDC42(p21)-activated kinases, comes of age, and in particular PAK1 has been well known to be responsible for a variety of diseases such as cancer (mainly solid tumors), Alzheimer's disease, acquired immune deficiency syndrome and other viral/bacterial infections, inflammatory diseases (asthma and arthritis), diabetes (type 2), neurofibromatosis, tuberous sclerosis, epilepsy, depression, schizophrenia, learning disability, autism, etc. Although several distinct synthetic PAK1-blockers have been recently developed, no FDA-approved PAK1 blockers are available on the market as yet. Thus, patients suffering from these PAK1-dependent diseases have to rely on solely a variety of herbal therapeutics such as propolis and curcumin that block PAK1 without affecting normal cell growth. Furthermore, several recent studies revealed that some of these herbal therapeutics significantly extend the lifespan of nematodes (C. elegans) and fruit flies (Drosophila), and PAK1-deficient worm lives longer than the wild type. Here, I outline mainly pathological phenotypes of hyper-activated PAK1 and a list of herbal therapeutics that block PAK1, but cause no side (harmful) effect on healthy people or animals.

Integrin-binding protein nischarin interacts with tumor suppressor liver kinase B1 (LKB1) to regulate cell migration of breast epithelial cells

Biallelic inactivation of LKB1, a serine/threonine kinase, has been detected in 30% of lung adenocarcinomas, and inhibition of breast tumor growth has been demonstrated. We have identified the tumor suppressor, Nischarin, as a novel binding partner of LKB1. Our mapping analysis shows that the N terminus of Nischarin interacts with amino acids 44-436 of LKB1. Time lapse microscopy and Transwell migration data show that the absence of both Nischarin and LKB1 from an invasive breast cancer cell line (MDA-MB-231) enhances migration as measured by increased distance and speed of migrating cells. Our data suggest that this is a result of elevated PAK1 and LIMK1 phosphorylation. Moreover, the absence of Nischarin and LKB1 increased tumor growth in vivo. Consistent with this, the percentage of S phase cells was increased, as demonstrated by flow cytometry and enhanced cyclin D1. The absence of Nischarin and LKB1 also led to a dramatic increase in the formation of lung metastases. Our studies, for the first time, demonstrate functional interaction between LKB1 and Nischarin to inhibit cell migration and breast tumor progression. Mechanistically, we show that these two proteins together regulate PAK-LIMK-Cofilin and cyclin D1/CDK4 pathways.

Altered LKB1/AMPK/TSC1/TSC2/mTOR signaling causes disruption of Sertoli cell polarity and spermatogenesis

Male patients with Peutz-Jeghers syndrome (PJS) have defective spermatogenesis and are at increased risk of developing Sertoli cell tumors. Mutations in the Liver Kinase B1 (LKB1/STK11) gene are associated with the pathogenesis of PJS and have been identified in non-PJS patients with sporadic testicular cancers. The mechanisms controlled by LKB1 signaling in Sertoli cell functions and testicular biology have not been described. We have conditionally deleted the Lkb1 gene (Lkb1(cko)) in somatic testicular cells to define the molecular mechanisms involved in the development of the testicular phenotype observed in PJS patients. Focal vacuolization in some of the seminiferous tubules was observed in 4-week-old mutant testes but germ cell development appeared to be normal. However, similar to PJS patients, we observed progressive germ cell loss and Sertoli cell only tubules in Lkb1(cko) testes from mice older than 10 weeks, accompanied by defects in Sertoli cell polarity and testicular junctional complexes and decreased activation of the MAP/microtubule affinity regulating and focal adhesion kinases. Suppression of AMP kinase and activation of mammalian target of rapamycin (mTOR) signaling were also observed in Lkb1(cko) testes. Loss of Tsc1 or Tsc2 copies the progressive Lkb1(cko) phenotype, suggesting that dysregulated activation of mTOR contributes to the pathogenesis of the Lkb1(cko) testicular phenotype. Pten(cko) mice had a normal testicular phenotype, which could be explained by the comparative lack of mTOR activation detected. These studies describe the importance of LKB1 signaling in testicular biology and the possible molecular mechanisms driving the pathogenesis of the testicular defects observed in PJS patients.

Molecular pathways: targeting p21-activated kinase 1 signaling in cancer--opportunities, challenges, and limitations

The evolution of cancer cells involves deregulation of highly regulated fundamental pathways that are central to normal cellular architecture and functions. p21-activated kinase 1 (PAK1) was initially identified as a downstream effector of the GTPases Rac and Cdc42. Subsequent studies uncovered a variety of new functions for this kinase in growth factor and steroid receptor signaling, cytoskeleton remodeling, cell survival, oncogenic transformation, and gene transcription, largely through systematic discovery of its direct, physiologically relevant substrates. PAK1 is widely upregulated in several human cancers, such as hormone-dependent cancer, and is intimately linked to tumor progression and therapeutic resistance. These exciting developments combined with the kinase-independent role of PAK1-centered phenotypic signaling in cancer cells elevated PAK1 as an attractive drug target. Structural and biochemical studies revealed the precise mechanism of PAK1 activation, offering the possibility to develop PAK1-targeted cancer therapeutic approaches. In addition, emerging reports suggest the potential of PAK1 and its specific phosphorylated substrates as cancer prognostic markers. Here, we summarize recent findings about the PAK1 molecular pathways in human cancer and discuss the current status of PAK1-targeted anticancer therapies.

STE20-related kinase adaptor protein α (STRADα) regulates cell polarity and invasion through PAK1 signaling in LKB1-null cells

LKB1 is a Ser/Thr kinase, and its activity is regulated by the pseudokinase, STE20-related adaptor α (STRADα). The STRADα-LKB1 pathway plays critical roles in epithelial cell polarity, neuronal polarity, and cancer metastasis. Though much attention is given to the STRADα-LKB1 pathway, the function of STRADα itself, including a role outside of the LKB1 pathway, has not been well-studied. Data in Caenorhabditis elegans suggest that STRADα has an LKB1-independent role in regulating cell polarity, and therefore we tested the hypothesis that STRADα regulates cancer cell polarity and motility when wild-type LKB1 is absent. These results show that STRADα protein is reduced in LKB1-null cell lines (mutation or homozygous deletion) and this partial degradation occurs through the Hsp90-dependent proteasome pathway. The remaining STRADα participates in cell polarity and invasion, such that STRADα depletion results in misaligned lamellipodia, improper Golgi positioning, and reduced invasion. To probe the molecular basis of this defect, we show that STRADα associates in a complex with PAK1, and STRADα loss disrupts PAK1 activity via Thr(423) PAK1 phosphorylation. When STRADα is depleted, PAK1-induced invasion could not occur, suggesting that STRADα is necessary for PAK1 to drive motility. Furthermore, STRADα overexpression caused increased activity of the PAK1-activating protein, rac1, and a constitutively active rac1 mutant (Q61L) rescued pPAK(Thr423) and STRADα invasion defects. Taken together, these results show that a STRADα-rac1-PAK1 pathway regulates cell polarity and invasion in LKB1-null cells. It also suggests that while the function of LKB1 and STRADα undoubtedly overlap, they may also have mutually exclusive roles.

LKB1 signaling in advancing cell differentiation

The Peutz-Jeghers syndrome (PJS) culprit kinase LKB1 phosphorylates and activates multiple intracellular kinases regulating cell metabolism and polarity. The relevance of each of these pathways is highly variable depending on the tissue type, but typically represents functions of differentiated cells. These include formation and maintenance of specialized cell compartments in nerve axons, swift refunneling of metabolites and restructuring of cell architecture in response to environmental cues in committed lymphocytes, and ensuring energy-efficient oxygen-based energy expenditure. Such features are often lost or reduced in cancer cells, and indeed LKB1 defects in PJS-associated and sporadic cancers and even the benign PJS polyps lead to differentiation defects, including expansion of partially differentiated epithelial cells in PJS polyps and epithelial-to-mesenchymal transition in carcinomas. This review focuses on the involvement of LKB1 in the differentiation of epithelial, mesenchymal, hematopoietic and germinal lineages.

Liver kinase B1 (LKB1) in the pathogenesis of epithelial cancers

LKB1 acts as a master kinase, with its major protein targets being the family of AMPKs. Through activation of multiple signaling pathways, LKB1's main physiologic functions involve regulating cellular growth, metabolism, and polarity. Germline mutations in LKB1 result in Peutz-Jeghers Syndrome, a rare cancer susceptibility syndrome. In addition, multiple LKB1 mutations have been identified in sporadic cancers, especially those of the lung. Recent studies from a variety of murine models have helped characterize LKB1's role in the pathogenesis of epithelial cancers. In some tumor types, LKB1 might function chiefly to suppress cell growth or invasion, while in other cases, it may serve to prevent metastasis. Moreover, molecular signatures of individual tumors likely influence LKB1's operational role, as multiple studies have shown that LKB1 can synergize with other tumor suppressors and/or oncogenes to accelerate tumorigenesis. To date, LKB1 has been considered mainly a tumor suppressor; however, some studies have suggested its potential oncogenic role, mainly through the suppression of apoptosis. In short, LKB1 is a tissue and context-specific kinase. This review aims to summarize our current understanding of its role in the pathogenesis of epithelial cancers.

LKB1 regulated pathways in lung cancer invasion and metastasis

Metastasis is characterized by the ability of cancer cells to invade into adjacent tissue, intravasate into blood or lymphatic vessels, and extravasate into a distant tissue. Metastatic disease is primarily responsible for the low 5-year survival rate of non-small cell lung cancer (NSCLC), and therefore, an understanding of the molecular mechanisms that regulate NSCLC metastasis is clearly warranted. The serine/threonine kinase and tumor suppressor LKB1 is mutated in 30% of NSCLC tumors, and recent evidence points to a prominent role in NSCLC metastasis. This review summarizes LKB1-dependent invasion pathways where compromised LKB1 function could promote NSCLC metastasis.