NDR kinase is activated by RASSF1A/MST1 in response to Fas receptor stimulation and promotes apoptosis

SENP2-NDR2-p21 axis modulates lung cancer cell growth

Sentrin/small ubiquitin-like modifier (SUMO)-specific proteases (SENPs) perform pivotal roles in SUMO maturation and recycling, which modulate the balance of SUMOylation/de-SUMOylation and spatiotemporal functions of SUMOylation targets. The malfunction of SENPs often results in cellular dysfunction and various diseases. However, studies rarely investigated the correlation between SENP2 and lung cancer. This study revealed that SENP2 is a required contributor to lung cancer-cell growth and targets nuclear Dbf2-related 2 (NDR2, also known as serine/threonine kinase 38L or STK38L) for de-SUMOylation, which improves NDR2 kinase activity. This condition leads to the instability of downstream target p21 in accelerating the G1/S cell cycle transition and suggests SENP2 as a promising therapeutic target for lung cancer in the future. Specifically, astragaloside IV, an active ingredient of Jinfukang Oral Liquid (JOL, a clinical combination antilung cancer drug approved by the National Food and Drug Administration (FDA) of China), can repress lung cancer-cell growth via the SENP2-NDR2-p21 axis, which provides new insights into the molecular mechanism of JOL for lung cancer treatment.

YAP in development and disease: Navigating the regulatory landscape from retina to brain

The distinctive role of Yes-associated protein (YAP) in the nervous system has attracted widespread attention. This comprehensive review strategically uses the retina as a vantage point, embarking on an extensive exploration of YAP's multifaceted impact from the retina to the brain in development and pathology. Initially, we explore the crucial roles of YAP in embryonic and cerebral development. Our focus then shifts to retinal development, examining in detail YAP's regulatory influence on the development of retinal pigment epithelium (RPE) and retinal progenitor cells (RPCs), and its significant effects on the hierarchical structure and functionality of the retina. We also investigate the essential contributions of YAP in maintaining retinal homeostasis, highlighting its precise regulation of retinal cell proliferation and survival. In terms of retinal-related diseases, we explore the epigenetic connections and pathophysiological regulation of YAP in diabetic retinopathy (DR), glaucoma, and proliferative vitreoretinopathy (PVR). Lastly, we broaden our exploration from the retina to the brain, emphasizing the research paradigm of "retina: a window to the brain." Special focus is given to the emerging studies on YAP in brain disorders such as Alzheimer's disease (AD) and Parkinson's disease (PD), underlining its potential therapeutic value in neurodegenerative disorders and neuroinflammation.

Hippo and PI5P4K signaling intersect to control the transcriptional activation of YAP

Phosphoinositides are essential signaling molecules. The PI5P4K family of phosphoinositide kinases and their substrates and products, PI5P and PI4,5P2, respectively, are emerging as intracellular metabolic and stress sensors. We performed an unbiased screen to investigate the signals that these kinases relay and the specific upstream regulators controlling this signaling node. We found that the core Hippo pathway kinases MST1/2 phosphorylated PI5P4Ks and inhibited their signaling in vitro and in cells. We further showed that PI5P4K activity regulated several Hippo- and YAP-related phenotypes, specifically decreasing the interaction between the key Hippo proteins MOB1 and LATS and stimulating the YAP-mediated genetic program governing epithelial-to-mesenchymal transition. Mechanistically, we showed that PI5P interacted with MOB1 and enhanced its interaction with LATS, thereby providing a signaling connection between the Hippo pathway and PI5P4Ks. These findings reveal how these two important evolutionarily conserved signaling pathways are integrated to regulate metazoan development and human disease.

The NDR family of kinases: essential regulators of aging

Aging is defined as a progressive decline of cognitive and physiological functions over lifetime. Since the definition of the nine hallmarks of aging in 2013 by López-Otin, numerous studies have attempted to identify the main regulators and contributors in the aging process. One interesting group of proteins whose participation has been implicated in several aging hallmarks are the nuclear DBF2-related (NDR) family of serine-threonine AGC kinases. They are one of the core components of the Hippo signaling pathway and include NDR1, NDR2, LATS1 and LATS2 in mammals, along with its highly conserved metazoan orthologs; Trc in Drosophila melanogaster, SAX-1 in Caenorhabditis elegans, CBK1, DBF20 in Saccharomyces cerevisiae and orb6 in Saccharomyces pombe. These kinases have been independently linked to the regulation of widely diverse cellular processes disrupted during aging such as the cell cycle progression, transcription, intercellular communication, nutrient homeostasis, autophagy, apoptosis, and stem cell differentiation. However, a comprehensive overview of the state-of-the-art knowledge regarding the post-translational modifications of and by NDR kinases in aging has not been conducted. In this review, we summarize the current understanding of the NDR family of kinases, focusing on their relevance to various aging hallmarks, and emphasize the growing body of evidence that suggests NDR kinases are essential regulators of aging across species.

Discovery of a small-molecule NDR1 agonist for prostate cancer therapy

Prostatic cancer (PCa) is a common malignant neoplasm in men worldwide. Most patients develop castration-resistant prostate cancer (CRPC) after treatment with androgen deprivation therapy (ADT), usually resulting in death. Therefore, investigating new therapeutic targets and drugs for PCa patients is urgently needed. Nuclear Dbf2-related kinase 1 (NDR1), also known as STK38, is a serine/threonine kinase in the NDR/LATS kinase family that plays a critical role in cellular processes, including immunity, inflammation, metastasis, and tumorigenesis. It was reported that NDR1 inhibited the metastasis of prostate cancer cells by suppressing epithelial-mesenchymal transition (EMT), and decreased NDR1 expression might lead to a poorer prognosis, suggesting the enormous potential of NDR1 in antitumorigenesis. In this study, we characterized a small-molecule agonist named aNDR1, which specifically bound to NDR1 and potently promoted NDR1 expression, enzymatic activity and phosphorylation. aNDR1 exhibited drug-like properties, such as favorable stability, plasma protein binding capacity, cell membrane permeability, and PCa cell-specific inhibition, while having no obvious effect on normal prostate cells. Meanwhile, aNDR1 exhibited good antitumor activity both in vitro and in vivo. aNDR1 inhibited proliferation and migration of PCa cells and promoted apoptosis of PCa cells in vitro. We further found that aNDR1 inhibited subcutaneous tumors and lung metastatic nodules in vivo, with no obvious toxicity to the body. In summary, our study presents a potential small-molecule lead compound that targets NDR1 for clinical therapy of PCa patients.

The Hippo Pathway Effectors YAP/TAZ-TEAD Oncoproteins as Emerging Therapeutic Targets in the Tumor Microenvironment

Various cancer cell-associated intrinsic and extrinsic inputs act on YAP/TAZ proteins to mediate the hyperactivation of the TEAD transcription factor-based transcriptome. This YAP/TAZ-TEAD activity can override the growth-limiting Hippo tumor-suppressor pathway that maintains normal tissue homeostasis. Herein, we provide an integrated summary of the contrasting roles of YAP/TAZ during normal tissue homeostasis versus tumor initiation and progression. In addition to upstream factors that regulate YAP/TAZ in the TME, critical insights on the emerging functions of YAP/TAZ in immune suppression and abnormal vasculature development during tumorigenesis are illustrated. Lastly, we discuss the current methods that intervene with the YAP/TAZ-TEAD oncogenic signaling pathway and the emerging applications of combination therapies, gut microbiota, and epigenetic plasticity that could potentiate the efficacy of chemo/immunotherapy as improved cancer therapeutic strategies.

Cellular Targets of HIV-1 Protease: Just the Tip of the Iceberg?

Human immunodeficiency virus 1 (HIV-1) viral protease (PR) is one of the most studied viral enzymes and a crucial antiviral target. Despite its well-characterized role in virion maturation, an increasing body of research is starting to focus on its ability to cleave host cell proteins. Such findings are apparently in contrast with the dogma of HIV-1 PR activity being restricted to the interior of nascent virions and suggest catalytic activity within the host cell environment. Given the limited amount of PR present in the virion at the time of infection, such events mainly occur during late viral gene expression, mediated by newly synthesized Gag-Pol polyprotein precursors, rather than before proviral integration. HIV-1 PR mainly targets proteins involved in three different processes: those involved in translation, those controlling cell survival, and restriction factors responsible for innate/intrinsic antiviral responses. Indeed, by cleaving host cell translation initiation factors, HIV-1 PR can impair cap-dependent translation, thus promoting IRES-mediated translation of late viral transcripts and viral production. By targeting several apoptotic factors, it modulates cell survival, thus promoting immune evasion and viral dissemination. Additionally, HIV-1 PR counteracts restriction factors incorporated in the virion that would otherwise interfere with nascent virus vitality. Thus, HIV-1 PR appears to modulate host cell function at different times and locations during its life cycle, thereby ensuring efficient viral persistency and propagation. However, we are far from having a complete picture of PR-mediated host cell modulation, which is emerging as a field that needs further investigation.

Effect of STK3 on proliferation and apoptosis of pancreatic cancer cells via PI3K/AKT/mTOR pathway

Pancreatic cancer, as a malignant tumor with a very poor prognosis, has a high mortality. It is imperative to clarify the mechanism of pancreatic cancer development and find suitable targets for diagnosis and treatment. Serine/threonine kinase 3 (STK3) is one of the core kinases of the Hippo pathway and has the ability to inhibit tumor growth. But the biological function of STK3 in pancreatic cancer remains unknown. Here, we confirmed that STK3 has an impact on the growth, apoptosis, and metastasis of pancreatic cancer cells and investigated the related molecular mechanisms. In our research, we found that STK3 is reduced in pancreatic cancer by RT-qPCR, IHC and IF, its expression level is correlated with the clinicopathological features. CCK-8 assay, colony formation assay and flow cytometry were used to detect the effect of STK3 on the proliferation and apoptosis of pancreatic cancer cells. In addition, the Transwell assay was used to detect the ability of cell migration and invasion. The results showed that STK3 promoted apoptosis and inhibited cell migration, invasion and proliferation in pancreatic cancer. Gene set enrichment analysis (GSEA) and western blotting are used to predict and verify the pathways related to STK3. Subsequently, we found that the effect of STK3 on proliferation and apoptosis is closely related to the PI3K/AKT/mTOR pathway. Moreover, the assistance of RASSF1 plays a significant role in the regulation of PI3K/AKT/mTOR pathway by STK3. The nude mouse xenograft experiment demonstrated the tumor suppressive ability of STK3 in vivo. Collectively, this study found that STK3 regulates pancreatic cancer cell proliferation and apoptosis by suppressing the PI3K/AKT/mTOR pathway with the assistance of RASSF1.

The XPA Protein-Life under Precise Control

Nucleotide excision repair (NER) is a central DNA repair pathway responsible for removing a wide variety of DNA-distorting lesions from the genome. The highly choreographed cascade of core NER reactions requires more than 30 polypeptides. The xeroderma pigmentosum group A (XPA) protein plays an essential role in the NER process. XPA interacts with almost all NER participants and organizes the correct NER repair complex. In the absence of XPA's scaffolding function, no repair process occurs. In this review, we briefly summarize our current knowledge about the XPA protein structure and analyze the formation of contact with its protein partners during NER complex assembling. We focus on different ways of regulation of the XPA protein's activity and expression and pay special attention to the network of post-translational modifications. We also discuss the data that is not in line with the currently accepted hypothesis about the functioning of the XPA protein.

Hippo signaling pathway: A comprehensive gene expression profile analysis in breast cancer

Breast cancer (BC) is the most frequently diagnosed malignancy in women and a major public health concern. The Hippo pathway is an evolutionarily conserved signaling pathway that serves as a key regulator for a wide variety of biological processes. Hippo signaling has been shown to have both oncogenic and tumor-suppressive functions in various cancers. Core components of the Hippo pathway consist of various kinases and downstream effectors such as YAP/TAZ. In the current report, differential expression of Hippo pathway elements as well as the correlation of Hippo pathway mRNAs with various clinicopathologic characteristics, including molecular subtypes, receptor status, and methylation status, has been investigated in BC using METABRIC and TCGA datasets. In this review, we note deregulation of several Hippo signaling elements in BC patients. Moreover, we see examples of negative correlations between methylation of Hippo genes and mRNA expression. The expression of Hippo genes significantly varies between different receptor subgroups. Because of the clear associations between mRNA expression and methylation status, DNA methylation may be one of the mechanisms that regulate the Hippo pathway in BC cells. Differential expression of Hippo genes among various BC molecular subtypes suggests that Hippo signaling may function differently in different subtypes of BC. Our data also highlights an interesting link between Hippo components' transcription and ER negativity in BC. In conclusion, substantial deregulation of Hippo signaling components suggests an important role of these genes in breast cancer.

Molecular Alterations in Malignant Pleural Mesothelioma: A Hope for Effective Treatment by Targeting YAP

Malignant pleural mesothelioma is a rare and aggressive neoplasm, which has primarily been attributed to the exposure to asbestos fibers (83% of cases); yet, despite a ban of using asbestos in many countries, the incidence of malignant pleural mesothelioma failed to decline worldwide. While little progress has been made in malignant pleural mesothelioma diagnosis, bevacizumab at first, then followed by double immunotherapy (nivolumab plus ipilumumab), were all shown to improve survival in large phase III randomized trials. The morphological analysis of the histological subtyping remains the primary indicator for therapeutic decision making at an advanced disease stage, while a platinum-based chemotherapy regimen combined with pemetrexed, either with or without bevacizumab, is still the main treatment option. Consequently, malignant pleural mesothelioma still represents a significant health concern owing to poor median survival (12-18 months). Given this context, both diagnosis and therapy improvements require better knowledge of the molecular mechanisms underlying malignant pleural mesothelioma's carcinogenesis and progression. Hence, the Hippo pathway in malignant pleural mesothelioma initiation and progression has recently received increasing attention, as the aberrant expression of its core components may be closely related to patient prognosis. The purpose of this review was to provide a critical analysis of our current knowledge on these topics, the main focus being on the available evidence concerning the role of each Hippo pathway's member as a promising biomarker, enabling detection of the disease at earlier stages and thus improving prognosis.

tRNA overexpression rescues peripheral neuropathy caused by mutations in tRNA synthetase

[Figure: see text].

Nuclear Dbf2-related (NDR1) functions as tumor suppressor in glioblastoma by phosphorylation of Yes-associated protein (YAP)

Background:

The Nuclear Dbf2-related (NDR1) kinase is a member of the NDR/LATS family, which was a supplementary of Hippo pathway. However, whether NDR1 could inhibit glioblastoma (GBM) growth by phosphorylating Yes-associated protein (YAP) remains unknown. Meanwhile, the role of NDR1 in GBM was not clear. This study aimed to investigate the role of NDR1-YAP pathway in GBM.

Methods:

Bioinformation analysis and immunohistochemistry (IHC) were performed to identify the expression of NDR1 in GBM. The effect of NDR1 on cell proliferation and cell cycle was analyzed utilizing CCK-8, clone formation, immunofluorescence and flow cytometry, respectively. In addition, the xenograft tumor model was established as well. Protein interaction was examined by Co-immunoprecipitation and immunofluorescence to observe co-localization.

Results:

Bioinformation analysis and IHC of our patients’ tumor tissues showed that expression of NDR1 in tumor tissue was relatively lower than that in normal tissues and was positively related to a lower survival rate. NDR1 could markedly reduce the proliferation and colony formation of U87 and U251. Furthermore, the results of flow cytometry showed that NDR1 led to cell cycle arrest at the G1 phase. Tumor growth was also inhibited in xenograft nude mouse models in NDR1-overexpression group. Western blotting and immunofluorescence showed that NDR1 could integrate with and phosphorylate YAP at S127 site. Meanwhile, NDR1 could mediate apoptosis process.

Conclusion:

In summary, our findings point out that NDR1 functions as a tumor suppressor in GBM. NDR1 is identified as a novel regulator of YAP, which gives us an in-depth comprehension of the Hippo signaling pathway.

Furry is required for cell movements during gastrulation and functionally interacts with NDR1

Gastrulation is a key event in animal embryogenesis during which germ layer precursors are rearranged and the embryonic axes are established. Cell polarization is essential during gastrulation, driving asymmetric cell division, cell movements, and cell shape changes. The furry (fry) gene encodes an evolutionarily conserved protein with a wide variety of cellular functions, including cell polarization and morphogenesis in invertebrates. However, little is known about its function in vertebrate development. Here, we show that in Xenopus, Fry plays a role in morphogenetic processes during gastrulation, in addition to its previously described function in the regulation of dorsal mesoderm gene expression. Using morpholino knock-down, we demonstrate a distinct role for Fry in blastopore closure and dorsal axis elongation. Loss of Fry function drastically affects the movement and morphological polarization of cells during gastrulation and disrupts dorsal mesoderm convergent extension, responsible for head-to-tail elongation. Finally, we evaluate a functional interaction between Fry and NDR1 kinase, providing evidence of an evolutionarily conserved complex required for morphogenesis.

The STK38-XPO1 axis, a new actor in physiology and cancer

The Hippo signal transduction pathway is an essential regulator of organ size during developmental growth by controlling multiple cellular processes such as cell proliferation, cell death, differentiation, and stemness. Dysfunctional Hippo signaling pathway leads to dramatic tissue overgrowth. Here, we will briefly introduce the Hippo tumor suppressor pathway before focusing on one of its members and the unexpected twists that followed our quest of its functions in its multifarious actions beside the Hippo pathway: the STK38 kinase. In this review, we will precisely discuss the newly identified role of STK38 on regulating the nuclear export machinery by phosphorylating and activating, the major nuclear export receptor XPO1. Finally, we will phrase STK38's role on regulating the subcellular distribution of crucial cellular regulators such as Beclin1 and YAP1 with its implication in cancer.

The role of the Hippo pathway in the pathogenesis of inflammatory bowel disease

Inflammatory bowel disease (IBD) is a chronic and recurrent inflammatory disorder that primarily comprises Crohn's disease (CD) and ulcerative colitis (UC). Owing to its increasing prevalence in Eastern countries and the intractable challenges faced during IBD treatment, extensive research on IBD has been carried out over the last few years. Although the precise aetiology of IBD is undefined, the currently accepted hypothesis for IBD pathogenesis considers it to be a combination of environment, genetic predisposition, gut microbiota, and abnormal immunity. A recently emerged signalling pathway, the Hippo pathway, acts as a key regulator of cell growth, tissue homoeostasis, organ size, and has been implicated in several human cancers. In the past few years, studies have revealed the importance of the Hippo pathway in gastrointestinal tract physiology and gastrointestinal diseases, such as colorectal cancer and IBD. However, the role of the Hippo pathway and its exact impact in IBD remains to be elucidated. This review summarises the latest scientific literature on the involvement of this pathway in IBD from the following perspectives that account for the IBD pathogenesis: intestinal epithelial cell regeneration, immune regulation, gut microbiota, and angiogenesis. A comprehensive understanding of the specific role of the Hippo pathway in IBD will provide novel insights into future research directions and clinical implications of the Hippo pathway.

Neuronal Hippo signaling: From development to diseases

Hippo signaling pathway is a highly conserved and familiar tissue growth regulator, primarily dealing with cell survival, cell proliferation, and apoptosis. The Yes-associated protein (YAP) is the key transcriptional effector molecule, which is under negative regulation of the Hippo pathway. Wealth of studies have identified crucial roles of Hippo/YAP signaling pathway during the process of development, including the development of neuronal system. We provide here, an overview of the contributions of this signaling pathway at multiple stages of neuronal development including, proliferation of neural stem cells (NSCs), migration of NSCs toward their destined niche, maintaining NSCs in the quiescent state, differentiation of NSCs into neurons, neuritogenesis, synaptogenesis, brain development, and in neuronal apoptosis. Hyperactivation of the neuronal Hippo pathway can also lead to a variety of devastating neurodegenerative diseases. Instances of aberrant Hippo pathway leading to neurodegenerative diseases along with the approaches utilizing this pathway as molecular targets for therapeutics has been highlighted in this review. Recent evidences suggesting neuronal repair and regenerative potential of this pathway has also been pointed out, that will shed light on a novel aspect of Hippo pathway in regenerative medicine. Our review provides a better understanding of the significance of Hippo pathway in the journey of neuronal system from development to diseases as a whole.

MOB: Pivotal Conserved Proteins in Cytokinesis, Cell Architecture and Tissue Homeostasis

The MOB family proteins are constituted by highly conserved eukaryote kinase signal adaptors that are often essential both for cell and organism survival. Historically, MOB family proteins have been described as kinase activators participating in Hippo and Mitotic Exit Network/ Septation Initiation Network (MEN/SIN) signaling pathways that have central roles in regulating cytokinesis, cell polarity, cell proliferation and cell fate to control organ growth and regeneration. In metazoans, MOB proteins act as central signal adaptors of the core kinase module MST1/2, LATS1/2, and NDR1/2 kinases that phosphorylate the YAP/TAZ transcriptional co-activators, effectors of the Hippo signaling pathway. More recently, MOBs have been shown to also have non-kinase partners and to be involved in cilia biology, indicating that its activity and regulation is more diverse than expected. In this review, we explore the possible ancestral role of MEN/SIN pathways on the built-in nature of a more complex and functionally expanded Hippo pathway, by focusing on the most conserved components of these pathways, the MOB proteins. We discuss the current knowledge of MOBs-regulated signaling, with emphasis on its evolutionary history and role in morphogenesis, cytokinesis, and cell polarity from unicellular to multicellular organisms.

Association of Mps one binder kinase activator 1 (MOB1) expression with poor disease-free survival in individuals with non-small cell lung cancer

Background

Mps one binder kinase activator 1 (MOB1) is a core component of the Hippo signaling pathway and has been implicated as a tumor suppressor. Here, we evaluated the possible relationship of MOB1 expression in non‐small cell lung cancer (NSCLC) to prognosis.

Methods

We retrospectively analyzed 205 lung adenocarcinoma patients treated at Kyushu University Hospital between November 2007 and October 2012. MOB1 expression in tumor cells of surgical specimens was evaluated by immunohistochemistry. Invasive activity of NSCLC cell lines in vitro was measured with a transwell assay.

Results

Expression of MOB1 was classified as high in 105 of the 205 (51.2%) tumor specimens, and such high expression was significantly associated with poor disease‐free survival (P = 0.0161). Among the various clinicopathologic parameters examined, high MOB1 expression was significantly associated only with intratumoral vascular invasion (P = 0.0005). Multivariate analysis also identified high MOB1 expression as a significant independent risk factor for disease‐free survival (P = 0.0319). The invasiveness of H1299 cells in vitro was increased or attenuated by overexpression or knockdown of MOB1, respectively.

Conclusions

Our results suggest that MOB1 might promote early recurrence of NSCLC by increasing vascular invasion by tumor cells.

Key points

Significant findings of the study

We found that high MOB1 expression in surgical specimens of lung adenocarcinoma was associated with poor disease‐free survival and with intratumoral vascular invasion. MOB1 expression also promoted the invasiveness of NSCLC cells in vitro.

What this study adds

Our results thus suggest that high MOB1 expression is a risk factor for early postoperative recurrence in lung adenocarcinoma.

New insights into the core Hippo signaling and biological macromolecules interactions in the biology of solid tumors

As an evolutionarily conserved pathway, Hippo signaling pathway impacts different pathology and physiology processes such as wound healing, tissue repair/size and regeneration. When some components of Hippo signaling dysregulated, it affects cancer cells proliferation. Moreover, the relation Hippo pathway with other signaling including Wnt, TGFβ, Notch, and EGFR signaling leaves effect on the proliferation of cancer cells. Utilizing a number of therapeutic approaches, such as siRNAs and long noncoding RNA (lncRNA) to prevent cancer cells through the targeting of Hippo pathways, can provide new insights into cancer target therapy. The purpose of present review, first of all, is to demonstrate the importance of Hippo signaling and its relation with other signaling pathways in cancer. It also tries to demonstrate targeting Hippo signaling progress in cancer therapy.

STK38 promotes ATM activation by acting as a reader of histone H4 ufmylation

The ATM (ataxia-telangiectasia mutated) kinase is rapidly activated following DNA damage and phosphorylates its downstream targets to launch DDR signaling. Recently, we and others showed that UFM1 signaling promotes ATM activation. We further discovered that monoufmylation of histone H4 at Lys³¹ by UFM1-specific ligase 1 (UFL1) is an important step in the amplification of ATM activation. However, how monoufmylated H4 enhances ATM activation is still unknown. Here, we report STK38, a kinase in the Hippo pathway, serves as a reader for histone H4 ufmylation to promote ATM activation in a kinase-independent manner. STK38 contains a potential UFM1 binding motif which recognizes ufmylated H4 and recruits the SUV39H1 to the double-strand breaks, resulting in H3K9 trimethylation and Tip60 activation to promote ATM activation. Together, STK38 is a previously unknown player in DNA damage signaling and functions as a reader of monoufmylated H4 at Lys³¹ to promote ATM activation.

MST1/2 Balance Immune Activation and Tolerance by Orchestrating Adhesion, Transcription, and Organelle Dynamics in Lymphocytes

The STE20-like serine/threonine kinases MST1 and MST2 (MST1/2) are mammalian homologs of Hippo in flies. MST1/2 regulate organ size by suppressing the transcription factor YAP, which promotes proliferation. MST1 is predominantly expressed in immune cells, where it plays distinct roles. Here, we review the functions of MST1/2 in immune cells, uncovered by a series of recent studies, and discuss the connection between MST1/2 function and immune responses. MST1/2 regulate lymphocyte development, trafficking, survival, and antigen recognition by naive T cells. MST1/2 also regulate the function of regulatory T cells and effector T cell differentiation, thus acting to balance immune activation and tolerance. Interestingly, MST1/2 elicit these functions not by the “canonical” Hippo pathway, but by the non-canonical Hippo pathway or alternative pathways. In these pathways, MST1/2 regulates cellular processes relating to immune response, such as chemotaxis, cell adhesion, immunological synapse, gene transcriptions. Recent advances in our understanding of the molecular mechanisms of these processes have revealed important roles of MST1/2 in regulating cytoskeleton remodeling, integrin activation, and vesicular transport in lymphocytes. We discuss the significance of the MST1/2 signaling in lymphocytes in the regulation of organelle dynamics.

PINK1-Dependent Mitophagy Regulates the Migration and Homing of Multiple Myeloma Cells via the MOB1B-Mediated Hippo-YAP/TAZ Pathway

The roles of mitochondrial dysfunction in carcinogenesis remain largely unknown. The effects of PTEN-induced putative kinase 1 (PINK1)-dependent mitophagy on the pathogenesis of multiple myeloma (MM) are determined. The levels of the PINK1-dependent mitophagy markers PINK1 and parkin RBR E3 ubiquitin protein ligase (PARK2) in CD138+ plasma cells are reduced in patients with MM and correlate with clinical outcomes in myeloma patients. Moreover, the induction of PINK1-dependent mitophagy with carbonylcyanide-m-chlorophenylhydrazone (CCCP) or salinomycin, or overexpression of PINK1 leads to inhibition of transwell migration, suppression of myeloma cell homing to calvarium, and decreased osteolytic bone lesions. Furthermore, genetic deletion of pink1 accelerates myeloma development in a spontaneous X-box binding protein-1 spliced isoform (XBP-1s) transgenic myeloma mouse model and in VK*MYC transplantable myeloma recipient mice. Additionally, treatment with salinomycin shows significant antimyeloma activities in vivo in murine myeloma xenograft models. Finally, the effects of PINK1-dependent mitophagy on myeloma pathogenesis are driven by the activation of the Mps one binder kinase activator (MOB1B)-mediated Hippo pathway and the subsequent downregulation of Yes-associated protein (YAP)/transcriptional co-activator with PDZ-binding motif (TAZ) expression. These data provide direct evidence that PINK1-dependent mitophagy plays a critical role in the pathogenesis of MM and is a potential therapeutic target.

The Crosstalk Between Hippo-YAP Pathway and Innate Immunity

Recognition of pathogen-associated molecular patterns (PAMPs) triggers expression of antiviral interferons and proinflammatory cytokines, which functions as the frontier of host defense against microbial pathogen invasion. Hippo-YAP pathway regulates cell proliferation, survival, differentiation and is involved in diverse life processes, including tissue homeostasis and tumor suppression. Emerging discoveries elucidated that the components of Hippo-YAP pathway, such as MST1/2, NDR1/2, and YAP/TAZ played crucial regulatory roles in innate immunity. Meanwhile the innate immune signaling also exhibited regulatory effect on Hippo-YAP pathway. As for the importance of these two pathways, it would be interesting to figure out the deeper biological implications of their interplays. This review focuses on the regulation between Hippo-YAP pathway and innate immune signaling. We also propose the possible contribution of these interplays to tumor development.

RASSF1A Tumour Suppressor: Target the Network for Effective Cancer Therapy

The RASSF1A tumour suppressor is a scaffold protein that is involved in cell signalling. Increasing evidence shows that this protein sits at the crossroad of a complex signalling network, which includes key regulators of cellular homeostasis, such as Ras, MST2/Hippo, p53, and death receptor pathways. The loss of expression of RASSF1A is one of the most common events in solid tumours and is usually caused by gene silencing through DNA methylation. Thus, re-expression of RASSF1A or therapeutic targeting of effector modules of its complex signalling network, is a promising avenue for treating several tumour types. Here, we review the main modules of the RASSF1A signalling network and the evidence for the effects of network deregulation in different cancer types. In particular, we summarise the epigenetic mechanism that mediates RASSF1A promoter methylation and the Hippo and RAF1 signalling modules. Finally, we discuss different strategies that are described for re-establishing RASSF1A function and how a multitargeting pathway approach selecting druggable nodes in this network could lead to new cancer treatments.

The Hippo Pathway, YAP/TAZ, and the Plasma Membrane

The plasma membrane allows the cell to sense and adapt to changes in the extracellular environment by relaying external inputs via intracellular signaling networks. One central cellular signaling pathway is the Hippo pathway, which regulates homeostasis and plays chief roles in carcinogenesis and regenerative processes. Recent studies have found that mechanical stimuli and diffusible chemical components can regulate the Hippo pathway primarily through receptors embedded in the plasma membrane. Morphologically defined structures within the plasma membrane, such as cellular junctions, focal adhesions, primary cilia, caveolae, clathrin-coated pits, and plaques play additional key roles. Here, we discuss recent evidence highlighting the importance of these specialized plasma membrane domains in cellular feedback via the Hippo pathway.

STK38 kinase acts as XPO1 gatekeeper regulating the nuclear export of autophagy proteins and other cargoes

STK38 (also known as NDR1) is a Hippo pathway serine/threonine protein kinase with multifarious functions in normal and cancer cells. Using a context-dependent proximity-labeling assay, we identify more than 250 partners of STK38 and find that STK38 modulates its partnership depending on the cellular context by increasing its association with cytoplasmic proteins upon nutrient starvation-induced autophagy and with nuclear ones during ECM detachment. We show that STK38 shuttles between the nucleus and the cytoplasm and that its nuclear exit depends on both XPO1 (aka exportin-1, CRM1) and STK38 kinase activity. We further uncover that STK38 modulates XPO1 export activity by phosphorylating XPO1 on serine 1055, thus regulating its own nuclear exit. We expand our model to other cellular contexts by discovering that XPO1 phosphorylation by STK38 regulates also the nuclear exit of Beclin1 and YAP1, key regulator of autophagy and transcriptional effector, respectively. Collectively, our results reveal STK38 as an activator of XPO1, behaving as a gatekeeper of nuclear export. These observations establish a novel mechanism of XPO1-dependent cargo export regulation by phosphorylation of XPO1's C-terminal auto-inhibitory domain.

SIRT1 and p300/CBP regulate the reversible acetylation of serine-threonine kinase NDR2

Nuclear Dbf2-related kinase 2 (NDR2) is a highly conserved kinase that belongs to the NDR/LATS serine-threonine kinase family. NDR2 is involved in many cellular processes as a kinase or a scaffolding protein. As a known kinase, NDR2 requires self-phosphorylation and trans-phosphorylation to become fully active. However, beside phosphorylation, little is known about other posttranslational modifications of NDR2. In this study, we found that NDR2 can be specially acetylated at K463 in cells. In addition, SIRT1 acts as the major deacetylase for NDR2, while p300 and CBP function as specific acetyltransferases for NDR2. Interestingly, in SIRT1 deficient cells HDAC6 and HDAC1/2 can deacetylate NDR2, which provides a novel insight in deacetylation regulation. Our results demonstrate that NDR2 is a reversible acetylated kinase regulated by SIRT1 and p300/CBP.

The Hippo network kinase STK38 contributes to protein homeostasis by inhibiting BAG3-mediated autophagy

Chaperone-assisted selective autophagy (CASA) initiated by the cochaperone Bcl2-associated athanogene 3 (BAG3) represents an important mechanism for the disposal of misfolded and damaged proteins in mammalian cells. Under mechanical stress, the cochaperone cooperates with the small heat shock protein HSPB8 and the cytoskeleton-associated protein SYNPO2 to degrade force-unfolded forms of the actin-crosslinking protein filamin. This is essential for muscle maintenance in flies, fish, mice and men. Here, we identify the serine/threonine protein kinase 38 (STK38), which is part of the Hippo signaling network, as a novel interactor of BAG3. STK38 was previously shown to facilitate cytoskeleton assembly and to promote mitophagy as well as starvation and detachment induced autophagy. Significantly, our study reveals that STK38 exerts an inhibitory activity on BAG3-mediated autophagy. Inhibition relies on a disruption of the functional interplay of BAG3 with HSPB8 and SYNPO2 upon binding of STK38 to the cochaperone. Of note, STK38 attenuates CASA independently of its kinase activity, whereas previously established regulatory functions of STK38 involve target phosphorylation. The ability to exert different modes of regulation on central protein homeostasis (proteostasis) machineries apparently allows STK38 to coordinate the execution of diverse macroautophagy pathways and to balance cytoskeleton assembly and degradation.

MOB (Mps one Binder) Proteins in the Hippo Pathway and Cancer

The family of MOBs (monopolar spindle-one-binder proteins) is highly conserved in the eukaryotic kingdom. MOBs represent globular scaffold proteins without any known enzymatic activities. They can act as signal transducers in essential intracellular pathways. MOBs have diverse cancer-associated cellular functions through regulatory interactions with members of the NDR/LATS kinase family. By forming additional complexes with serine/threonine protein kinases of the germinal centre kinase families, other enzymes and scaffolding factors, MOBs appear to be linked to an even broader disease spectrum. Here, we review our current understanding of this emerging protein family, with emphases on post-translational modifications, protein-protein interactions, and cellular processes that are possibly linked to cancer and other diseases. In particular, we summarise the roles of MOBs as core components of the Hippo tissue growth and regeneration pathway.

Exploration of MST1-Mediated Secondary Brain Injury Induced by Intracerebral Hemorrhage in Rats via Hippo Signaling Pathway

Intracerebral hemorrhage (ICH) is a serious public health problem which causes high rates of disability and mortality in adults. Cell apoptosis is a sign of secondary brain injury (SBI) following ICH. Mammalian sterile 20-like kinase-1 (MST1), an apoptosis-promoting kinase, is a part of the Hippo signaling pathway and involved in cell death, oxidative stress, and inflammation. However, the role and underlying mechanism of MST1 in SBI induced by ICH have not yet been fully explained. The main purpose of present research was to explore the role of MST1 and its potential mechanism in SBI after ICH. An ICH model was established by injecting autologous blood into the right basal ganglia in male SD rats. We found that MST1 phosphorylation was significantly increased in brain tissues of rats after ICH. Additionally, inhibition of MST1 phosphorylation by a chemical inhibitor (Xmu-mp-1) and genetic knockdown could effectively reduce the activation of P-LATS1 and P-YAP which are downstream proteins of MST1 and decrease neuronal cell death and inflammation reaction in ICH rats. Furthermore, the decreased of MST1 phosphorylation reduced brain edema, blood-brain barrier (BBB) damage, and neurobehavioral impairment during ICH. Over-expression of MST1 resulted in opposite effects. Finally, deletion of MST1 significantly reduced neuronal apoptosis in vitro. In summary, our study revealed that MST1 played an important role in the SBI following ICH, and inhibition of MST1 could alleviate ICH-induced SBI. Therefore, MST1 may be considered as a potential therapeutic target for SBI following ICH.

NDR2 promotes the antiviral immune response via facilitating TRIM25-mediated RIG-I activation in macrophages

Retinoic acid-inducible gene I (RIG-I), a pivotal cytosolic sensor, recognizes viral RNAs to initiate antiviral innate immunity. However, posttranslational regulation of RIG-I signaling is not well understood. We report here that nuclear Dbf2-related kinase 2 (NDR2) functions as a crucial positive regulator of the RIG-I-mediated antiviral immune response. Overexpression of NDR2 or its kinase-inactive mutants potentiates RNA virus-induced production of type I interferons and proinflammatory cytokines and dampens viral replication. NDR2 conditional knockout mice (Lysm+NDR2f/f) show an impaired antiviral immune response. Mechanistically, NDR2 directly associates with RIG-I and TRIM25, thus facilitating the RIG-I/TRIM25 complex and enhancing the TRIM25-mediated K63-linked polyubiquitination of RIG-I, which is required for the RIG-I-mediated antiviral immune response. Furthermore, NDR2 expression is notably down-regulated in peripheral blood from respiratory syncytial virus-infected patients and in virus-infected macrophages. Collectively, these findings provide insights into the function of NDR2 in antiviral immunity and its related clinical significance.

Measuring the Kinase Activities of the LATS/NDR Protein Kinases

The Hippo tissue growth control and regeneration pathway is a main regulator of the YAP/TAZ effectors. In this regard, the LATS/NDR serine/threonine protein kinases can function as central components of the Hippo core module. More specifically, LATS/NDR-mediated phosphorylation of YAP/TAZ on different residues can regulate the subcellular localization and/or stability of YAP/TAZ. Therefore, the assessment of LATS/NDR activities can serve as readout for the activity status of the Hippo pathway. Here, we describe our preferred methodology regarding the measurement of the activities of LATS/NDR kinases.

RASSF1A promoter methylation correlates development, progression, and poor cancer-specific survival of renal cell carcinoma: trial sequential analysis

Background

This meta-analysis evaluated the clinicopathologic and prognostic significance of RASSF1A promoter methylation in renal cell carcinoma (RCC).

Materials and methods

The ORs or HRs and their 95% CIs were calculated. Trial sequential analysis was conducted.

Results

Twenty-two articles that included 1,421 patients with RCC and 724 controls were identified. RASSF1A promoter methylation correlated with RCC in tissue, blood, and urine samples. On multivariate analysis, RASSF1A promoter methylation was associated with tumor grade (grade 3–4 vs 1–2: OR=3.59), clinical stage (stage 3–4 vs 1–2: OR=2.15), T classification (pT2–4 vs pT1: OR=2.66), histologic subtypes (papillary vs clear cell: OR=2.91), and cancer-specific survival (HR=1.78), but it was not linked to age, gender, lymph node status, distant metastasis, or overall survival. The Cancer Genome Atlas data also showed that RASSF1A methylation was significantly more likely to be seen in papillary vs clear-cell RCC (OR=23.19).

Conclusion

RASSF1A promoter methylation may be associated with the development and progression of RCC, as well as poor cancer-specific survival. Methylation was more frequent in papillary vs clear-cell RCC. More studies are needed to confirm these findings in blood or urine samples.

Targeting the Hippo Pathway for Breast Cancer Therapy

Breast cancer (BC) is one of the most prominent diseases in the world, and the treatments for BC have many limitations, such as resistance and a lack of reliable biomarkers. Currently the Hippo pathway is emerging as a tumor suppressor pathway with its four core components that regulate downstream transcriptional targets. In this review, we introduce the present targeted therapies of BC, and then discuss the roles of the Hippo pathway in BC. Finally, we summarize the evidence of the small molecule inhibitors that target the Hippo pathway, and then discuss the possibilities and future direction of the Hippo-targeted drugs for BC therapy.

Reduction of Thermotolerance by Heat Shock Protein 90 Inhibitors in Murine Erythroleukemia Cells

Cells induce heat shock proteins (HSPs) against various stress. However, murine erythroleukemia (MEL) cells do not express HSP72, a heat-inducible member of HSP70 family. So, it is of interest to examine how MEL cells respond to heat stress (44°C, 30 min). Heat stress-induced apoptosis was suppressed by pretreatment of heat shock (44°C, 10 min). Such suppressive effects were maximal at 6 h after heat shock and remained up to 12 h. Interestingly, such effects of heat shock were abrogated by specific inhibitors of HSP90 such as 17-allylamino-17-demethoxygeldanamycin (17-AAG) and novobiocin. From flow cytometric analysis, it was found that MEL cells arrest in G₂ phase at 6 h after heat shock, but restore original cell cycle at 12 h. High expression level of HSP90 was maintained before and after heat shock. Phosphorylation of HSP90α was observed in apoptotic cells induced by heat stress, but inhibited by pretreatment of heat shock. Such inhibition was abrogated by 17-AAG. Moreover, c-Jun NH₂-terminal kinase (JNK) was activated in heat stress-induced apoptotic cells. Taken together, these results suggest that HSP90α in MEL cells plays an important role in the thermotolerance, i.e., suppression of heat stress-induced apoptosis by heat shock.

Tumor suppressor C-RASSF proteins

Human genome has ten genes that are collectedly called Ras association domain family (RASSF). RASSF is composed of two subclasses, C-RASSF and N-RASSF. Both N-RASSF and C-RASSF encode Ras association domain-containing proteins and are frequently suppressed by DNA hypermethylation in human cancers. However, C-RASSF and N-RASSF are quite different. Six C-RASSF proteins (RASSF1-6) are characterized by a C-terminal coiled-coil motif named Salvador/RASSF/Hippo domain, while four N-RASSF proteins (RASSF7-10) lack it. C-RASSF proteins interact with mammalian Ste20-like kinases-the core kinases of the tumor suppressor Hippo pathway-and cross-talk with this pathway. Some of them share the same interacting molecules such as MDM2 and exert the tumor suppressor role in similar manners. Nevertheless, each C-RASSF protein has distinct characters. In this review, we summarize our current knowledge of how C-RASSF proteins play tumor suppressor roles and discuss the similarities and differences among C-RASSF proteins.

Ubiquitin-Dependent Regulation of the Mammalian Hippo Pathway: Therapeutic Implications for Cancer

The Hippo pathway serves as a key barrier for oncogenic transformation. It acts by limiting the activity of the proto-oncogenes YAP and TAZ. Reduced Hippo signaling and elevated YAP/TAZ activities are frequently observed in various types of tumors. Emerging evidence suggests that the ubiquitin system plays an important role in regulating Hippo pathway activity. Deregulation of ubiquitin ligases and of deubiquitinating enzymes has been implicated in increased YAP/TAZ activity in cancer. In this article, we review recent insights into the ubiquitin-mediated regulation of the mammalian Hippo pathway, its deregulation in cancer, and possibilities for targeting the Hippo pathway through the ubiquitin system.

Altering Neurospora crassa MOB2A exposes its functions in development and affects its interaction with the NDR kinase COT1

The Neurospora crassa Mps One Binder (MOB) proteins MOB2A and MOB2B physically interact with the Nuclear Dbf2 Related (NDR) kinase COT1 and have been shown to have overlapping functions in various aspects of asexual development. Here, we identified two N. crassa MOB2A residues, Tyr117 and Tyr119, which are potentially phosphorylated. Using phosphomimetic mob-2a mutants we have been able to establish that apart from their previously described roles, MOB2A/B are involved in additional developmental processes. Enhanced conidial germination, accompanied by conidial agglutination, in the phosphomimetic mutants indicated that MOB2A is a negative regulator of germination. Thick-section imaging of perithecia revealed slow maturation and a lack of asci alignment in the mutant strains demonstrating a role for MOB2A in sexual development. We demonstrate that even though MOB2A and MOB2B have some overlapping functions, MOB2B cannot compensate for the roles MOB2A has in conidiation and germination. Altering Tyr residues 117 and 119 impaired the physical interactions between MOB2A and COT1, most likely contributing to some of the observed effects. As cot-1 and the phosphomimetic mutants share an extragenic suppressor (gul-1), we concluded that at least some of the effects imposed by altering Tyr117 and Tyr119 are mediated by the NDR kinase.

Fryl deficiency is associated with defective kidney development and function in mice

FRY like transcription coactivator ( Fryl) gene located on chromosome 5 is a paralog of FRY microtubule binding protein ( Fry) in vertebrates. It encodes a protein with unknown functions. Fryl gene is conserved in various species ranging from eukaryotes to human. Although there are several reports on functions of Fry gene, functions of Fryl gene remain unclear. A mouse line containing null mutation in Fryl gene by gene trapping was produced in this study for the first time. The survival and growth of Fryl^-/- mice were observed. Fryl gene expression levels in mouse tissues were determined and histopathologic analyses were conducted. Most Fryl^-/- mice died soon after birth. Rare Fryl^-/- survivors showed growth retardation with significantly lower body weight compared to their littermate controls. Although they could breed, more than half of Fryl^-/- survivors died of hydronephrosis before age 1. No abnormal histopathologic lesion was apparent in full-term embryo or adult tissues except the kidney. Abnormal lining cell layer detachments from walls of collecting and convoluted tubules in kidneys were apparent in Fryl^-/- neonates and full-term embryos. Fryl gene was expressed in renal tubular tissues including the glomeruli and convoluted and collecting tubules. This indicates that defects in tubular systems are associated with Fryl functions and death of Fryl^-/- neonates. Fryl protein is required for normal development and functional maintenance of kidney in mice. This is the first report of in vivo Fryl gene functions. Impact statement FRY like transcription coactivator ( Fryl) gene is conserved in various species ranging from eukaryotes to human. It expresses a protein with unknown function. We generated a Fryl gene mutant mouse line and found that most homozygous mice died soon after their birth. Rare Fryl^-/- survivors showed growth retardation with significantly lower body weight compared to their littermate controls. Although they could breed, more than half of Fryl^-/- survivors died of hydronephrosis before age 1. Full-term mutant embryos showed abnormal collecting and convoluted tubules in kidneys where Fryl gene was expressed. Collectively, these results indicate that Fryl protein is required for normal development and functional maintenance of kidney in mice. To the best of our knowledge, this is the first report on in vivo Fryl gene functions.

Monopolar spindle-one-binder protein 2 regulates the activity of large tumor suppressor/yes-associated protein to inhibit the motility of SMMC-7721 hepatocellular carcinoma cells

Accumulating evidence implicates monopolar spindle-one-binder protein (MOB)2 as an inhibitor of nuclear-Dbf2-related kinase (NDR) by competing with MOB1 for interaction with NDR1/2. NDR/large tumor suppressor (LATS) kinases may function similarly to yes-associated protein (YAP) kinases and be considered as members of the Hippo core cassette. MOB2 appears to serve roles in cell survival, cell cycle progression, responses to DNA damage and cell motility. However, the underlying mechanisms involved remain unclarified. In the present study, it was demonstrated that the knockout of MOB2 by clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR associated protein 9 promoted migration and invasion, induced phosphorylation of NDR1/2 and decreased phosphorylation of YAP in SMMC-7721 cells when compared with the blank vector-transduced cells. By contrast, the overexpression of MOB2 resulted in the opposite results. Mechanistically, MOB2 regulated the alternative interaction of MOB1 with NDR1/2 and LATS1, which resulted in increased phosphorylation of LATS1 and MOB1 and thereby led to the inactivation of YAP and consequently inhibition of cell motility. The results of the present study provide evidence of MOB2 serving a positive role in LATS/YAP activation by activating the Hippo signaling pathway.

Mitochondrial clearance by the STK38 kinase supports oncogenic Ras-induced cell transformation

Oncogenic Ras signalling occurs frequently in many human cancers. However, no effective targeted therapies are currently available to treat patients suffering from Ras-driven tumours. Therefore, it is imperative to identify downstream effectors of Ras signalling that potentially represent promising new therapeutic options. Particularly, considering that autophagy inhibition can impair the survival of Ras-transformed cells in tissue culture and mouse models, an understanding of factors regulating the balance between autophagy and apoptosis in Ras-transformed human cells is needed. Here, we report critical roles of the STK38 protein kinase in oncogenic Ras transformation. STK38 knockdown impaired anoikis resistance, anchorage-independent soft agar growth, and in vivo xenograft growth of Ras-transformed human cells. Mechanistically, STK38 supports Ras-driven transformation through promoting detachment-induced autophagy. Even more importantly, upon cell detachment STK38 is required to sustain the removal of damaged mitochondria by mitophagy, a selective autophagic process, to prevent excessive mitochondrial reactive oxygen species production that can negatively affect cancer cell survival. Significantly, knockdown of PINK1 or Parkin, two positive regulators of mitophagy, also impaired anoikis resistance and anchorage-independent growth of Ras-transformed human cells, while knockdown of USP30, a negative regulator of PINK1/Parkin-mediated mitophagy, restored anchorage-independent growth of STK38-depleted Ras-transformed human cells. Therefore, our findings collectively reveal novel molecular players that determine whether Ras-transformed human cells die or survive upon cell detachment, which potentially could be exploited for the development of novel strategies to target Ras-transformed cells.

MST kinases in innate immune signaling

The mammalian STE20-like (MST) protein kinases are composed of MST1, MST2, MST3, MST4 and YSK1. They play crucial roles in cell growth, migration, polarity and apoptosis. Dysfunction of these kinases often leads to diseases. MST kinases are extensively involved in development and function of immune system. Here, we review recent progresses on the regulatory function of MST kinases in innate immune signaling.

Disease implication of hyper-Hippo signalling

The Hippo signalling pathway regulates cellular proliferation, apoptosis and differentiation, thus exerting profound effects on cellular homeostasis. Inhibition of Hippo signalling has been frequently implicated in human cancers, indicating a well-known tumour suppressor function of the Hippo pathway. However, it is less certain whether and how hyperactivation of the Hippo pathway affects biological outcome in living cells. This review describes current knowledge of the regulatory mechanisms of the Hippo pathway, mainly focusing on hyperactivation of the Hippo signalling nexus. The disease implications of hyperactivated Hippo signalling have also been discussed, including arrhythmogenic cardiomyopathy, Sveinsson's chorioretinal atrophy, Alzheimer's disease, amyotrophic lateral sclerosis and diabetes. By highlighting the significance of disease-relevant Hippo signalling activation, this review can offer exciting prospects to address the onset and potential reversal of Hippo-related disorders.

Deletion of Mst1 attenuates neuronal loss and improves neurological impairment in a rat model of traumatic brain injury

Neuronal cell death following traumatic brain injury (TBI) is a considerable contributor to neurological deficits. In our work, we explored the functions of Mammalian STE20-like kinase-1 (Mst1), a apoptosis-promoting kinase and also a pivotal bridgebuilder of apoptotic signaling, in the etiopathogenesis of an experimental rat model of TBI. We found that the phosphorylation level of Mst1 in injured area was significantly increased after TBI. Furthermore, we discovered that inhibition of Mst1 phosphorylation can effectively reduce neuronal cell death by inhibiting the activation of caspase 3 and suppressing the damage of DNA during TBI. In addition, the decreased of Mst1 phosphorylation level, not only reduced brain edema and blood-brain barrier (BBB) damage in injured region but also weakened the impairment of neurologic behavior during TBI. In conclusion, our work demonstrates that Mst1 plays an important role in TBI-induced neuronal cell death, suggesting that Mst1 is expected to be a potential therapeutic target for TBI.

Stable MOB1 interaction with Hippo/MST is not essential for development and tissue growth control

The Hippo tumor suppressor pathway is essential for development and tissue growth control, encompassing a core cassette consisting of the Hippo (MST1/2), Warts (LATS1/2), and Tricornered (NDR1/2) kinases together with MOB1 as an important signaling adaptor. However, it remains unclear which regulatory interactions between MOB1 and the different Hippo core kinases coordinate development, tissue growth, and tumor suppression. Here, we report the crystal structure of the MOB1/NDR2 complex and define key MOB1 residues mediating MOB1's differential binding to Hippo core kinases, thereby establishing MOB1 variants with selective loss-of-interaction. By studying these variants in human cancer cells and Drosophila, we uncovered that MOB1/Warts binding is essential for tumor suppression, tissue growth control, and development, while stable MOB1/Hippo binding is dispensable and MOB1/Trc binding alone is insufficient. Collectively, we decrypt molecularly, cell biologically, and genetically the importance of the diverse interactions of Hippo core kinases with the pivotal MOB1 signal transducer.The Hippo tumor suppressor pathway is essential for development and tissue growth control. Here the authors employ a multi-disciplinary approach to characterize the interactions of the three Hippo kinases with the signaling adaptor MOB1 and show how they differently affect development, tissue growth and tumor suppression.

STK38L kinase ablation promotes loss of cell viability in a subset of KRAS-dependent pancreatic cancer cell lines

Pancreatic ductal adenocarcinomas (PDACs) are highly aggressive malignancies, associated with poor clinical prognosis and limited therapeutic options. Oncogenic KRAS mutations are found in over 90% of PDACs, playing a central role in tumor progression. Global gene expression profiling of PDAC reveals 3-4 major molecular subtypes with distinct phenotypic traits and pharmacological vulnerabilities, including variations in oncogenic KRAS pathway dependencies. PDAC cell lines of the aberrantly differentiated endocrine exocrine (ADEX) subtype are robustly KRAS-dependent for survival. The KRAS gene is located on chromosome 12p11-12p12, a region amplified in 5-10% of primary PDACs. Within this amplicon, we identified co-amplification of KRAS with the STK38L gene in a subset of primary human PDACs and PDAC cell lines. Therefore, we determined whether PDAC cell lines are dependent on STK38L expression for proliferation and viability. STK38L encodes a serine/threonine kinase, which shares homology with Hippo pathway kinases LATS1/2. We show that STK38L expression is elevated in a subset of primary PDACs and PDAC cell lines displaying ADEX subtype characteristics, including overexpression of mutant KRAS. RNAi-mediated depletion of STK38L in a subset of ADEX subtype cell lines inhibits cellular proliferation and induces apoptosis. Concomitant with these effects, STK38L depletion causes increased expression of the LATS2 kinase and the cell cycle regulator p21. LATS2 depletion partially rescues the cytostatic and cytotoxic effects of STK38L depletion. Lastly, high STK38L mRNA expression is associated with decreased overall patient survival in PDACs. Collectively, our findings implicate STK38L as a candidate targetable vulnerability in a subset of molecularly-defined PDACs.

Stk38 Modulates Rbm24 Protein Stability to Regulate Sarcomere Assembly in Cardiomyocytes

RNA-binding protein Rbm24 is a key regulator of heart development and required for sarcomere assembly and heart contractility. Yet, its underlying mechanism remains unclear. Here, we link serine/threonine kinase 38 (Stk38) signaling to the regulation of Rbm24 by showing that Rbm24 phosphorylation and its function could be modulated by Stk38. Using co-immunoprecipitation coupled with mass spectrometry technique, we identified Stk38 as an endogenous binding partner of Rbm24. Stk38 knockdown resulted in decreased Rbm24 protein level in cardiomyocytes. Further studies using Stk38 kinase inhibitor or activator showed that Rbm24 protein stability was regulated in a kinase activity-dependent manner. Deficiency of Stk38 caused reduction of sarcomere proteins and disarrangement of sarcomere, suggesting that Stk38 is essential for Rbm24 to regulate sarcomere assembly. Our results revealed that Stk38 kinase catalyzes the phosphorylation of Rbm24 during sarcomerogensis and this orchestrates accurate sarcomere alignment. This furthers our understanding of the regulatory mechanism of cardiac sarcomere assembly in both physiologic and pathologic contexts, and uncovers a potential novel pathway to cardiomyopathy through modulating the Stk38/Rbm24 protein activity.