Structure-Based Screen Identification of a Mammalian Ste20-like Kinase 4 (MST4) Inhibitor with Therapeutic Potential for Pituitary Tumors

Mst4, a novel cardiac STRIPAK complex-associated kinase, regulates cardiomyocyte growth and survival and is upregulated in human cardiomyopathy

Myocardial failure is associated with adverse remodeling, including loss of cardiomyocytes, hypertrophy, and alterations in cell-cell contacts. Striatin-interacting phosphatase and kinase (STRIPAK) complexes and their mammalian STE20-like kinase 4 (Mst4) have been linked to development of different diseases. The role and targets of Mst4 in cardiomyocytes have not been investigated yet. Multitissue immunoblot experiments show highly enriched Mst4 expression in rodent hearts. Analyses of human biopsy samples from patients suffering from dilated cardiomyopathy revealed that Mst4 is upregulated (5- to 8-fold p < 0.001) compared with nonfailing controls. Increased abundance of Mst4 could also be detected in mouse models of cardiomyopathy. We confirmed that Mst4 interacts with STRIPAK components in neonatal rat ventricular cardiomyocytes, indicating that STRIPAK is present in the heart. Immunofluorescence stainings and molecular interaction studies revealed that Mst4 is localized to the intercalated disc and interacts with several intercalated disc proteins. Overexpression of Mst4 in cardiomyocytes results in hypertrophy compared with controls. In adult rat cardiomyocytes, Mst4 overexpression increases cellular and sarcomeric fractional shortening (p < 0.05), indicating enhanced contractility. Overexpression of Mst4 also inhibits apoptosis shown by reduction of cleaved caspase3 (-69%, p < 0.0001), caspase7 (-80%, p < 0.0001), and cleaved Parp1 (-27%, p < 0.001). To elucidate potential Mst4 targets, we performed phosphoproteomics analyses in neonatal rat cardiomyocytes after Mst4 overexpression and inhibition. The results revealed target candidates of Mst4 at the intercalated disc. We identified Mst4 as a novel cardiac kinase that is upregulated in cardiomyopathy-regulating cardiomyocyte growth and survival.

The mammalian Sterile 20-like kinase 4 (MST4) signaling in tumor progression: Implications for therapy

Cancer is a leading cause of death in humans, with a complex and dynamic nature that makes it challenging to fully comprehend and treat. The Mammalian Sterile 20-Like Kinase 4 (MST4 or STK26) is a serine/threonine-protein kinase that plays a crucial role in cell migration and polarity in both normal and tumor cells via activation of intracellular signaling molecules and pathways. MST4 is involved in tumor cell proliferation, migration and invasion, epithelial-mesenchymal transition (EMT), survival, and cancer metastasis through modulation of downstream signaling pathways including the extracellular signal-regulated kinase (ERK) and protein kinase B (AKT) pathways. Additionally, MST4 interacts with programmed cell death 10 (PDCD10) to promote tumor proliferation and migration. MST4 phosphorylates autophagy related 4B cysteine peptidase (ATG4B) to mediate autophagy signaling, promote tumor cell survival and proliferation, and contribute to treatment resistance. Taken together, MST4 functions as an oncogene and is a promising therapeutic target which deserves further exploration.

MST4: A Potential Oncogene and Therapeutic Target in Breast Cancer

The mammalian STE 20-like protein kinase 4 (MST4) gene is highly expressed in several cancer types, but little is known about the role of MST4 in breast cancer, and the function of MST4 during epithelial-mesenchymal transition (EMT) has not been fully elucidated. Here we report that overexpression of MST4 in breast cancer results in enhanced cell growth, migration, and invasion, whereas inhibition of MST4 expression significantly attenuates these properties. Further study shows that MST4 promotes EMT by activating Akt and its downstream signaling molecules such as E-cadherin/N-cadherin, Snail, and Slug. MST4 also activates AKT and its downstream pro-survival pathway. Furthermore, by analyzing breast cancer patient tissue microarray and silicon datasets, we found that MST4 expression is much higher in breast tumor tissue compared to normal tissue, and significantly correlates with cancer stage, lymph node metastasis and a poor overall survival rate (p < 0.05). Taken together, our findings demonstrate the oncogenic potential of MST4 in breast cancer, highlighting its role in cancer cell proliferation, migration/invasion, survival, and EMT, suggesting a possibility that MST4 may serve as a novel therapeutic target for breast cancer.

Perturbation of biological processes with small molecule kinase inhibitors

The reversible phosphorylation of substrates mediated by kinases and phosphatases affects their subcellular localization, catalytic activity, and/or interaction with other molecules. It is essential for signal transduction and the regulation of nearly all cellular processes, such as proliferation, apoptosis, metabolism, motility, and differentiation. Small molecule kinase inhibitors (SMKIs) have served as critical chemical probes to reveal the biological functions and mechanisms of kinases and their potential as therapeutic targets. In this review, we focused on a few novel SMKIs and their recent application in biological and preclinical studies to showcase how highly selective and potent SMKIs can be developed and utilized to propel the investigations on kinases and the biology behind.

Hesperadin suppresses pancreatic cancer through ATF4/GADD45A axis at nanomolar concentrations

Pancreatic cancer (PC) is a fatal disease with poor survival and limited therapeutic strategies. In this study, we identified Hesperadin as a potent anti-cancer compound against PC, from a high-throughput screening of a commercial chemical library associated with cell death. Hesperadin induced potent growth inhibition in PC cell lines and patient-derived tumor organoids in a dose- and time-dependent manner, with IC50 values in the nanomolar range. Cellular studies showed that Hesperadin caused mitochondria damage in PC cells, resulting in reactive oxygen species production, ER stress and apoptotic cell death. Transcriptomic analysis using RNA-sequencing data identified GADD45A as a potential target of Hesperadin. Mechanistic studies showed that Hesperadin could increase GADD45A expression in PC cells via ATF4, leading to apoptosis. Moreover, immunohistochemical staining of 92 PC patient samples demonstrated the correlation between ATF4 and GADD45A expression. PC xenograft studies demonstrated that Hesperadin could effectively inhibit the growth of PC cells in vivo. Together, these findings suggest that Hesperadin is a novel drug candidate for PC.

Novel CaMKII-δ Inhibitor Hesperadin Exerts Dual Functions to Ameliorate Cardiac Ischemia/Reperfusion Injury and Inhibit Tumor Growth

BACKGROUND

Cardiac ischemia/reperfusion (I/R) injury has emerged as an important therapeutic target for ischemic heart disease, the leading cause of morbidity and mortality worldwide. At present, there is no effective therapy for reducing cardiac I/R injury. CaMKII (Ca²⁺/calmodulin-dependent kinase II) plays a pivotal role in the pathogenesis of severe heart conditions, including I/R injury. Pharmacological inhibition of CaMKII is an important strategy in the protection against myocardial damage and cardiac diseases. To date, there is no drug targeting CaMKII for the clinical therapy of heart disease. Furthermore, at present, there is no selective inhibitor of CaMKII-δ, the major CaMKII isoform in the heart.

METHODS

A small-molecule kinase inhibitor library and a high-throughput screening system for the kinase activity assay of CaMKII-δ9 (the most abundant CaMKII-δ splice variant in human heart) were used to screen for CaMKII-δ inhibitors. Using cultured neonatal rat ventricular myocytes, human embryonic stem cell-derived cardiomyocytes, and in vivo mouse models, in conjunction with myocardial injury induced by I/R (or hypoxia/reoxygenation) and CaMKII-δ9 overexpression, we sought to investigate the protection of hesperadin against cardiomyocyte death and cardiac diseases. BALB/c nude mice with xenografted tumors of human cancer cells were used to evaluate the in vivo antitumor effect of hesperadin.

RESULTS

Based on the small-molecule kinase inhibitor library and screening system, we found that hesperadin, an Aurora B kinase inhibitor with antitumor activity in vitro, directly bound to CaMKII-δ and specifically blocked its activation in an ATP-competitive manner. Hesperadin functionally ameliorated both I/R- and overexpressed CaMKII-δ9-induced cardiomyocyte death, myocardial damage, and heart failure in both rodents and human embryonic stem cell-derived cardiomyocytes. In addition, in an in vivo BALB/c nude mouse model with xenografted tumors of human cancer cells, hesperadin delayed tumor growth without inducing cardiomyocyte death or cardiac injury.

CONCLUSIONS

Here, we identified hesperadin as a specific small-molecule inhibitor of CaMKII-δ with dual functions of cardioprotective and antitumor effects. These findings not only suggest that hesperadin is a promising leading compound for clinical therapy of cardiac I/R injury and heart failure, but also provide a strategy for the joint therapy of cancer and cardiovascular disease caused by anticancer treatment.

Identification of a dual TAOK1 and MAP4K5 inhibitor using a structure-based virtual screening approach

The STE20 kinase family is a complex signalling cascade that regulates cytoskeletal organisation and modulates the stress response. This signalling cascade includes various kinase mediators, such as TAOK1 and MAP4K5. The dysregulation of the STE20 kinase pathway is linked with cancer malignancy. A small-molecule inhibitor targeting the STE20 kinase pathway has therapeutic potential. In this study, a structure-based virtual screening (SBVS) approach was used to identify potential dual TAOK1 and MAP4K5 inhibitors. Enzymatic assays confirmed three potential dual inhibitors (>50% inhibition) from our virtual screening, and analysis of the TAOK1 and MAP4K5 binding sites indicated common interactions for dual inhibition. Compound 1 revealed potent inhibition of colorectal and lung cancer cell lines. Furthermore, compound 1 arrested cancer cells in the G0/G1 phase, which suggests the induction of apoptosis. Altogether, we show that the STE20 signalling mediators TAOK1 and MAP4K5 are promising targets for drug research.

MST4 attenuates NLRP3 inflammasome-mediated neuroinflammation and affects the prognosis after intracerebral hemorrhage in mice

BACKGROUND

The kinase MST4 limits inflammatory responses through direct phosphorylation of the adaptor TRAF6. TRAF6 interacts with NLRP3 to promote the activation of NLRP3 inflammasome. However, the role of MST4 in neuroinflammation after intracerebral hemorrhage (ICH) and how it interacts with NLRP3 inflammasome remain unclear.

METHODS

Mice were administered MST4 AAV four weeks before collagenase-induced ICH. ICH mice received either hesperadin (MST4 selective inhibitor), or MCC950 (NLRP3 inflammasome selective inhibitor). Neurological deficits and brain water content were assessed. Western blot and immunofluorescence were performed to evaluate the proteins content and localization in MST4/NLRP3 signaling pathway.

RESULTS

The expression of endogenous MST4 and NLRP3 was increased after ICH compared to sham group. MST4 and NLRP3 were respectively colocalized in microglia. Upregulation of MST4 gene inhibited the activation of NLRP3 inflammasome, the release of IL-1β and TNF-α, and significantly improved brain edema and neurological deficits. Hesperadin pretreatment inhibited the expression of MST4 and increased the expression of NLRP3 inflammasome-mediated proteins, which aggravated neurological deficits and cerebral edema. MCC950 markedly alleviated neurological deficits and brain edema but had no effect on the expression of MST4 protein.

CONCLUSIONS

MST4 alleviates inflammatory progression and brain injury in ICH mice possibly by inhibiting NLRP3 inflammasome activation.

Downregulation of MST4 Underlies a Novel Inhibitory Role of MicroRNA Let-7a in the Progression of Retinoblastoma

Purpose

Retinoblastoma (RB) is the most common intraocular malignancy in children. Deregulation of several microRNAs (miRNAs) has been identified in RB. However, the specific effect of let-7a on RB remains unclear. The present study aims to explore the effect of let-7a on malignant biological behaviors of RB cells and angiogenesis in RB.

Methods

The expressions of let-7a and mammalian sterile-20 like kinase 4 (MST4) in RB were determined with the use of real-time quantitative polymerase chain reaction (RT-qPCR) and Western blot analysis. Next, in order to explore effects of let-7a and MST4 on RB cellular functions, RB cells were transfected with let-7a-mimic, let-7a inhibitor, si-MST4, or co-transfected with let-7a-mimic and oe-MST4 plasmids. Subsequently, the interaction among let-7a, MST4, and the MAPK signaling pathway was evaluated by RT-qPCR, dual-luciferase reporter gene assay, and Western blot analysis. Finally, the effects of let-7a and MST4 were further confirmed in vivo by injecting nude mice with RB cells stably expressing let-7a agomir or sh-MST4.

Results

Rb tissues and cells presented with downregulated Let-7a and upregulated MST4. Let-7a negatively targeted MST4 to block the activation of the MAPK signaling pathway. Upregulation of let-7a promoted apoptosis, and facilitated proliferation, angiogenesis, migration, and invasion of RB cells by decreasing MST4. Elevation of let-7a or silencing MST4 restricted angiogenesis and tumorigenesis in RB mice.

Conclusions

Taken together, let-7a inhibits angiogenesis in RB by silencing MST4 and inhibiting the MAPK signaling pathway.

MST4 Kinase Inhibitor Hesperadin Attenuates Autophagy and Behavioral Disorder via the MST4/AKT Pathway in Intracerebral Hemorrhage Mice

Background

The aim of this study was to explore the role of hesperadin in intracerebral hemorrhage (ICH) mice, with the involvement of the mammalian ste20-like kinase 4 (MST4)/AKT signaling pathway.

Methods

All mice were divided into four groups: sham group, sham+hesperidin group, ICH group, and ICH+hesperadin group. The effects of hesperadin were assessed on the basis of brain edema and neurobehavioral function. Furthermore, we observed MST4, AKT, phosphorylation of AKT (pAKT), and microtubule-associated protein light chain 3 (LC3) by western blotting. Protein localization of MST4 and LC3 was determined by immunofluorescence.

Results

The expression of MST4 was upregulated at 12 h and 24 h after ICH. Brain edema was significantly decreased and neurological function was improved in the hesperadin treatment group compared to the ICH group (P < 0.05). Hesperadin decreases the expressions of MST and increases pAKT after ICH. Autophagy significantly increased in the ICH group, while hesperadin reduced this increase.

Conclusion

Hesperadin provides neuroprotection against ICH by inhibiting the MST4/AKT signaling pathway.

Regulation of cell proliferation and metastasis by microRNA-593-5p in human gastric cancer

Background

MicroRNA (miRNA) array analysis has reported that the expression of miR-593-5p is associated with lymph node metastasis in gastric cancer (GC); however, the function and mechanism of miR-593-5p in GC have not been described yet. miR-593-5p has also not been elucidated widely in other cancers.

Methods

miR-593-5p expression was detected by quantitative RT-PCR (qRT-PCR) in human GC tissues and cell lines. Cell proliferation was investigated using CCK-8 assays, cell cycle was detected by flow cytometric method, and cell migration and invasion abilities were evaluated by wound-healing and transwell assays. miR-593-5p-influenced gene expression profiles were detected by total gene expression chip method in MGC-803 cells, and miR-593-5p candidate target genes were predicted using bioinformatics methods. The candidate target gene and downstream of miR-593-5p were determined by qRT-PCR, Western blot, and dual-luciferase reporter assays. The effects of miR-593-5p on the growth and metastasis of GC were evaluated by tumor xenograft experiment in vivo.

Results

miR-593-5p was frequently downregulated in GC patients and GC cell lines. miR-593-5p was significantly correlated with tumor size and distant metastasis in GC patients. miR-593-5p inhibited cell proliferation, migration, and invasion and also arrested cell cycle at the G0/G1 phase in SGC-7901 and MGC-803 cells in vitro. miR-593-5p also suppressed tumor growth and metastasis in vivo. miR-593-5p influenced gene expression profile in MGC-803 cells. MST4 was indirectly targeted by miR-593-5p. miR-593-5p also downregulated FAK, MMP12, and JUN protein expression.

Conclusion

Our study suggests that miR-593-5p may function as a tumor suppressor in GC through a mechanism that regulates JUN pathway via indirectly targeting the MST4 gene.

Identification of BAG3 target proteins in anaplastic thyroid cancer cells by proteomic analysis

BAG3 protein is an apoptosis inhibitor and is highly expressed in Anaplastic Thyroid Cancer. We investigated the entire set of proteins modulated by BAG3 silencing in the human anaplastic thyroid 8505C cancer cells by using the Stable-Isotope Labeling by Amino acids in Cell culture strategy combined with mass spectrometry analysis. By this approach we identified 37 up-regulated and 54 down-regulated proteins in BAG3-silenced cells. Many of these proteins are reportedly involved in tumor progression, invasiveness and resistance to therapies. We focused our attention on an oncogenic protein, CAV1, and a tumor suppressor protein, SERPINB2, that had not previously been reported to be modulated by BAG3. Their expression levels in BAG3-silenced cells were confirmed by qRT-PCR and western blot analyses, disclosing two novel targets of BAG3 pro-tumor activity. We also examined the dataset of proteins obtained by the quantitative proteomics analysis using two tools, Downstream Effect Analysis and Upstream Regulator Analysis of the Ingenuity Pathways Analysis software. Our analyses confirm the association of the proteome profile observed in BAG3-silenced cells with an increase in cell survival and a decrease in cell proliferation and invasion, and highlight the possible involvement of four tumor suppressor miRNAs and TP53/63 proteins in BAG3 activity.

MST4 Phosphorylation of ATG4B Regulates Autophagic Activity, Tumorigenicity, and Radioresistance in Glioblastoma

ATG4B stimulates autophagy by promoting autophagosome formation through reversible modification of ATG8. We identify ATG4B as a substrate of mammalian sterile20-like kinase (STK) 26/MST4. MST4 phosphorylates ATG4B at serine residue 383, which stimulates ATG4B activity and increases autophagic flux. Inhibition of MST4 or ATG4B activities using genetic approaches or an inhibitor of ATG4B suppresses autophagy and the tumorigenicity of glioblastoma (GBM) cells. Furthermore, radiation induces MST4 expression, ATG4B phosphorylation, and autophagy. Inhibiting ATG4B in combination with radiotherapy in treating mice with intracranial GBM xenograft markedly slows tumor growth and provides a significant survival benefit. Our work describes an MST4-ATG4B signaling axis that influences GBM autophagy and malignancy, and whose therapeutic targeting enhances the anti-tumor effects of radiotherapy.

MST4 kinase phosphorylates ACAP4 protein to orchestrate apical membrane remodeling during gastric acid secretion

Digestion in the stomach depends on acidification of the lumen. Histamine-elicited acid secretion is triggered by activation of the PKA cascade, which ultimately results in the insertion of gastric H,K-ATPases into the apical plasma membranes of parietal cells. Our recent study revealed the functional role of PKA-MST4-ezrin signaling axis in histamine-elicited acid secretion. However, it remains uncharacterized how the PKA-MST4-ezrin signaling axis operates the insertion of H,K-ATPases into the apical plasma membranes of gastric parietal cells. Here we show that MST4 phosphorylates ACAP4, an ARF6 GTPase-activating protein, at Thr⁵⁴⁵ Histamine stimulation activates MST4 and promotes MST4 interaction with ACAP4. ACAP4 physically interacts with MST4 and is a cognate substrate of MST4 during parietal cell activation. The phosphorylation site of ACAP4 by MST4 was mapped to Thr⁵⁴⁵ by mass spectrometric analyses. Importantly, phosphorylation of Thr⁵⁴⁵ is essential for acid secretion in parietal cells because either suppression of ACAP4 or overexpression of non-phosphorylatable ACAP4 prevents the apical membrane reorganization and proton pump translocation elicited by histamine stimulation. In addition, persistent overexpression of MST4 phosphorylation-deficient ACAP4 results in inhibition of gastric acid secretion and blockage of tubulovesicle fusion to the apical membranes. Significantly, phosphorylation of Thr⁵⁴⁵ enables ACAP4 to interact with ezrin. Given the location of Thr⁵⁴⁵ between the GTPase-activating protein domain and the first ankyrin repeat, we reason that MST4 phosphorylation elicits a conformational change that enables ezrin-ACAP4 interaction. Taken together, these results define a novel molecular mechanism linking the PKA-MST4-ACAP4 signaling cascade to polarized acid secretion in gastric parietal cells.