Mammalian Sterile 20-like Kinase Research Articles

Hydrogen sulfide ameliorates subarachnoid hemorrhage-induced neuronal apoptosis via the ROS-MST1 pathway.

BackgroundHydrogen sulfide (H2S) has shown a neuroprotective role in several cerebrovascular diseases. This study aimed to explore the underlying mechanisms of H2S in early brain injury after subarachnoid hemorrhage (SAH).MethodsOne hundred seventy-seven male Sprague-Dawley rats were employed in this study. Sodium hydrosulfide (NaHS), a donor of H2S, was injected intraperitoneally at 60 min after SAH was induced by endovascular perforation. Western blot analysis determined the expression of several proteins of interest, and an immunofluorescence assay was used to examine neuronal apoptosis.ResultsExogenous NaHS markedly improved neurological scores, attenuated brain edema, and ameliorated neuronal apoptosis at 24 h after SAH induction. The underlying mechanisms of H2S in ameliorating neuronal apoptosis might be executed through inhibition of the activity of mammalian sterile 20-like kinase 1 (MST1) protein. Western blot analysis demonstrated that exogenous NaHS decreased cleaved MST1 (cl-MST1) while increasing full-length MST1 expression. This anti-apoptotic effect of H2S could be reversed by chelerythrine, which could activate MST1 via caspase-dependent cleavage.ConclusionsExogenous NaHS, as a donor of H2S, could ameliorate early brain injury after SAH by inhibiting neuronal apoptosis by reducing the activity of the MST1 protein.

Mammalian sterile 20-like kinase 1 expression and its prognostic significance in patients with breast cancer.

Mammalian sterile 20-like kinase 1 (Mst1) is a major inhibitor of cell proliferation, and is involved in apoptosis, oncogenesis and organ growth via its ubiquitously encoded serine threonine kinase. Previous studies have demonstrated that Mst1 has a tumor suppressor function in human breast cancer. Mst1 deletion or mutation is associated with tumorigenesis, whereas Mst1 overexpression leads to tumor cell apoptosis and decreases proliferation of tumor cells. Our previous study reported the tumor suppressive function of Mst1, and debated Mst1 as a prognostic factor in human breast cancer. In the present study, Mst1 levels were measured in the plasma of patients in order to elucidate their association with overall and disease-free survival. The results of the present study indicated that Mst1 is a strong prognostic and predictive factor in human breast cancer and a promising anticancer target.

PCMT1 Ameliorates Neuronal Apoptosis by Inhibiting the Activation of MST1 after Subarachnoid Hemorrhage in Rats.

Mammalian sterile 20-like kinase 1 (MST1) is found to promote neuronal apoptosis. Protein-L-isoaspartate (D-aspartate) O-methyltransferase (PCMT1), an anti-apoptosis factor, was recently identified as an MST1-interacting protein. This study aims to explore the potential role of PCMT1 in reducing MST1-induced neuronal apoptosis after subarachnoid hemorrhage (SAH) in rats. One hundred ninety-eight male Sprague-Dawley rats were used. An exogenous PCMT1 agonist, CGP 3466B, was injected subcutaneously 1h after the SAH induced by endovascular perforation. Chelerythrine or calyculin A was given immediately via intracerebroventricular administration after SAH. The SAH grade, Garcia score, and brain water content were measured at 24 and 72h after the SAH. Neuronal apoptosis was detected by an immunofluorescent assay. The expression levels of endogenous PCMT1, MST1, phospho-MST1 (p-MST1), cleaved MST1 (cl-MST1), and apoptosis-related proteins were studied by western blotting. The expression of PCMT1 and MST1 decreased, while the level of active caspase 3 increased in rats after SAH. CGP 3466B treatment improved neurobehavioral function, reduced brain water content, inhibited the activity of MST1, and relieved neuronal apoptosis. These neuroprotective effects were significantly weakened either through accelerating MST1 phosphorylation by calyculin A or increasing cl-MST1 by chelerythrine. PCMT1 inhibited neuronal apoptosis by reducing MST1 phosphorylation and the level of cl-MST1. PCMT1/MST1 pathway might be an alternative therapeutic target for alleviating early brain injury after SAH.

The mammalian STE20-like kinase 1 (MST1) is a substrate for the apoptosis inhibiting protein kinase CK2

Apoptosis and the response to cell stress are evolutionary highly conserved mechanisms. Both processes require strict regulation, which is often performed by protein kinases. The mammalian Sterile 20-like kinase 1 (MST1) is a pro-apoptotic protein kinase, which is activated and cleaved by caspases upon the induction of cell stress. Being a phosphoprotein itself, the activity of MST1 is regulated by phosphorylation. Protein kinase CK2 is an anti-apoptotic protein kinase which seems to be involved in the regulation of many different cellular processes including apoptosis. There is increasing evidence that the cleavage of many substrates by caspases is regulated by phosphorylation in the close vicinity of the caspase cleavage sites. One of these kinases, implicated in the phosphorylation of caspase substrates, is protein kinase CK2. Here, we report that serine 320 of the MST1 protein is a novel phosphorylation site for the anti-apoptotic protein kinase CK2. Although serine 320 is in close vicinity to the caspase 3 cleavage site, caspase 3 cleavage of MST1 is not affected by CK2 phosphorylation. Using biochemical approaches, we were able to show that MST1 co-localizes with the CK2 subunits in the pancreatic β-cell line INS-1 and that full-length MST1 and the activated N-terminal fragment of MST1 both interacted with the CK2 subunits in vitro and in vivo. MST1 is a basophilic kinase whereas CK2 is an acidophilic kinase. Thus, binding of these two kinases in the cytosol and in the nucleus opens the door to the phosphorylation of a variety of new substrates.

Pyruvate kinase M2 interacts with mammalian sterile 20-like kinase 1 and inhibits tamoxifen-induced apoptosis in human breast cancer cells.

Tamoxifen has been reported to be associated with antagonism of estrogen-mediated cell growth signaling and activation of estrogen receptor-independent apoptosis events. It has been demonstrated that mammalian sterile 20-like kinase 1 is a direct target of Caspases to amplify the apoptotic signaling pathway. Here, we presented that breast cancer MCF-7 and SKBR3 cells under treatment with 4-hydroxytamoxifen displayed decreased level of pyruvate kinase M2. Western blot results also showed that 4-hydroxytamoxifen induced the activity of pro-apoptotic protein Caspase-3 in MCF-7 and SKBR3 cells, as evidenced by the cleavage of mammalian sterile 20-like kinase 1 substrate in a dose-dependent manner. Co-immunoprecipitation and immunofluorescence experiments were performed to clarify the relationship between pyruvate kinase M2 and mammalian sterile 20-like kinase 1. The results indicated that mammalian sterile 20-like kinase 1 was associated with pyruvate kinase M2 in cultured mammalian cells, and the interaction between mammalian sterile 20-like kinase 1 and pyruvate kinase M2 was decreased in response to 4-hydroxytamoxifen treatment. In addition, knockdown of pyruvate kinase M2 upregulated the level of cleaved Caspase-3 and subsequently facilitated the nuclear translocation of mammalian sterile 20-like kinase 1. Our data further supplemented the extensive functions of pyruvate kinase M2 in mediating breast cancer cell viability by substantially abating the mammalian sterile 20-like kinase 1-mediated apoptosis. In summary, our results identified that mammalian sterile 20-like kinase 1 is a novel downstream target of pyruvate kinase M2, and knockdown of pyruvate kinase M2 contributes apoptosis via promoting nuclear translocation of mammalian sterile 20-like kinase 1 by enhancing Caspase-3-dependent cleavage.

Morin impedes Yap nuclear translocation and fosters apoptosis through suppression of Wnt/β-catenin and NF-κB signaling in Mst1 overexpressed HepG2 cells

Recent clinical and experimental evidences strongly acclaim Yes-associated protein (Yap), a key oncogenic driver in liver carcinogenesis, as a therapeutic target. Of the known multiple schemes to inhibit Yap activity, activation of Mammalian Sterile 20-like Kinase 1 (Mst1), an upstream regulator of Yap, appears to be a promising one. In this study, we hypothesize that morin, a bioflavonoid, mediates its anti-cancer effect through the activation of Mst1/hippo signaling in liver cancer cells. To test this hypothesis, both full length Mst1 (F-Mst1) and kinase active N-terminal Mst1 (N-Mst1)-overexpressed HepG2 cells were used. Exposure of F-Mst1 overexpressed HepG2 cells to morin activated Mst1 by caspase-3 cleavage and thereby inhibited Yap nuclear translocation and fostered apoptosis. Morin suppressed NF-κB p65 and Wnt/β-catenin signaling through Mst1 activation via cleavage and phosphorylation, leading to cell death. Annexin-V/PI staining further confirmed the induction of apoptosis in morin treated F-Mst1 overexpressed cells. The present study shows that morin targets cell survival molecules such as NF-κB p65 and β-catenin through activation of hippo signaling. Therefore, morin could be considered as a potential anti-cancer agent against liver cancer.

Inhibitory effect and mechanism of exogenous mammalian sterile 20-like kinase 1 on the growth of human colorectal cancer.

The present study aimed to observe the inhibitory effect and preliminary mechanism of exogenous mammalian sterile 20-like kinase 1 (MST1) on the growth of colorectal cancer SW480 cells. The SW480 cells were randomly divided into the following groups: Control, empty enhanced green fluorescent protein (EGFP) plasmid (pEGFP-N1), MST1 EGFP plasmid (pEGFP-MST1), 20 µmol/l fluorouracil (5-FU) and pEGFP-MST1 + 5-FU. An MTS colorimetric assay was used to detect cell viability, Hoechst 33342 staining was used to observe cell apoptosis, and western blotting and immunohistochemistry were used to detect the levels of the proteins MST1, yes-associated protein (YAP), phospho-YAP1 (Ser127), p53 and p53 upregulated modulator of apoptosis (PUMA). In addition, nude mice were injected with SW480 cells to assess the tumor inhibition rates. Compared with the control group, the growth inhibition and apoptosis rates, the levels of MST1, p53 and PUMA, and the ratios of phospho-YAP1/YAP in the pEGFP-MST1 and pEGFP-MST1 + 5-FU groups were increased significantly (P<0.01). Additionally, relative to the control group, the tumor inhibition rates in the nude mice transplanted with SW480 cells of the pEGFP-MST1 and pEGFP-MST1 + 5-FU groups were 48.52±1.63 and 87.28±2.58%, respectively, and the positive rates of phospho-YAP1 (Ser127) protein in nuclei increased significantly (P<0.01). Overall, exogenous MST1 effectively inhibited the proliferation and growth of transplanted human colorectal cancer cells and promoted cancer cell apoptosis. The mechanism involved may be associated with the increase of intracellular phospho-YAP1 (Ser127) protein.

Deletion of mammalian sterile 20-like kinase 1 attenuates neuronal loss and improves locomotor function in a mouse model of spinal cord trauma.

Neuronal cell death following spinal cord injury (SCI) is an important contributor to neurological deficits. The purpose of our work was to delineate the function of mammalian sterile 20-like kinase 1 (Mst1), a pro-apoptotic kinase and key mediator of apoptotic signaling, in the pathogenesis of an experimental mouse model of SCI. Male mice received a mid-thoracic spinal contusion injury, and it was found that phosphorylation of Mst1 at the injured site was enhanced significantly following SCI. Furthermore, when compared to the wild-type controls, Mst1-deficient mice displayed improved locomotor function by increased Basso mouse scale score. Deletion of Mst1 in mice attenuated loss of motor neurons and suppressed microglial and glial activation following SCI. Deletion of Mst1 in mice reduced apoptosis via suppressing cytochrome c release and caspase-3 activation following SCI. Deletion of Mst1 attenuated mitochondrial dysfunction and increased ATP formation following SCI. Deletion of Mst1 in mice inhibited local inflammation following SCI, evidenced by reduced activities of myeloperoxidase and protein levels of TNF-α, IL-1β, and IL-6. In conclusion, the present study demonstrated that deletion of Mst1 attenuated neuronal loss and improved locomotor function in a mouse model of SCI, via preserving mitochondrial function, attenuating mitochondria-mediated apoptotic pathway, and suppressing inflammation, at least in part.

Dendritic cell MST1 inhibits Th17 differentiation

Although the differentiation of CD4+T cells is widely studied, the mechanisms of antigen-presenting cell-dependent T-cell modulation are unclear. Here, we investigate the role of dendritic cell (DC)-dependent T-cell differentiation in autoimmune and antifungal inflammation and find that mammalian sterile 20-like kinase 1 (MST1) signalling from DCs negatively regulates IL-17 producing-CD4+T helper cell (Th17) differentiation. MST1 deficiency in DCs increases IL-17 production by CD4+T cells, whereas ectopic MST1 expression in DCs inhibits it. Notably, MST1-mediated DC-dependent Th17 differentiation regulates experimental autoimmune encephalomyelitis and antifungal immunity. Mechanistically, MST1-deficient DCs promote IL-6 secretion and regulate the activation of IL-6 receptor α/β and STAT3 in CD4+T cells in the course of inducing Th17 differentiation. Activation of the p38 MAPK signal is responsible for IL-6 production in MST1-deficient DCs. Thus, our results define the DC MST1–p38MAPK signalling pathway in directing Th17 differentiation.

Genetic Inhibition of Galectin-3 Suppressed Cardiac Fibrosis and Improved Function in a Mouse Model of Dilated Cardiomyopathy

Background: Pre-clinical studies have consistently reported a 2∼5 fold increase in expression of Galectin-3 (Gal-3) in different heart disease models, and beneficial effects of anti-Gal-3 inhibitors. It remains unknown whether Gal-3 expression levels differ in dilated cardiomyopathy (DCM), and the effects of anti-Gal-3 interventions. We determined Gal-3 expression in transgenic (TG) mice with cardiac-restricted overexpression of mammalian sterile 20-like kinase 1 (Mst1), and studied the effects of Gal-3 gene deletion. Methods: Gal-3 deletion in Mst1-TG mice was achieved by cross-breeding Mst1-TG mice with Gal-3 knockout (KO) mice to produce TG, TG/Gal-3 KO and non-TG (NTG) genotypes. Male mice were studied (n = 6-12/group). Echocardiography (Vevo2100 system), gene expression (RT-PCR), Gal-3 protein levels (ELISA), and collagen content (hydroxyproline assay) were all measured. Results: Relative to NTG controls, TG mice had atrial dilatation (by 3-fold), suppressed left ventricular (LV) ejection fraction (EF, 27 ± 2 vs 55 ± 4%), higher myocardial collagen content (by 3-fold), and upregulated fibrotic genes (4-8 fold). Gal-3 expression was increased in TG hearts by 35-40-fold at mRNA and protein levels compared with NTG. Gal-3 gene deletion in TG mice was associated with lower cardiac fibrosis (by 17%) and suppressed expression of fibrotic genes (by 40-70%). Echocardiography revealed reduced LV volume at end-diastole (51 ± 2 vs 70 ± 7 μL) and improved EF (37 ± 2 vs 27 ± 2%, all p < 0.01). Conclusion: Gal-3 expression is markedly increased in Mst1-TG hearts exhibiting DCM. Gal-3 gene deletion effectively suppressed cardiac fibrotic signalling and LV dilatation, and improved function. Thus, suppression of Gal-3 expression may form a new therapeutic approach in DCM.

Hippo signaling interactions with Wnt/β-catenin and Notch signaling repress liver tumorigenesis.

Malignant tumors develop through multiple steps of initiation and progression, and tumor initiation is of singular importance in tumor prevention, diagnosis, and treatment. However, the molecular mechanism whereby a signaling network of interacting pathways restrains proliferation in normal cells and prevents tumor initiation is still poorly understood. Here, we have reported that the Hippo, Wnt/β-catenin, and Notch pathways form an interacting network to maintain liver size and suppress hepatocellular carcinoma (HCC). Ablation of the mammalian Hippo kinases Mst1 and Mst2 in liver led to rapid HCC formation and activated Yes-associated protein/WW domain containing transcription regulator 1 (YAP/TAZ), STAT3, Wnt/β-catenin, and Notch signaling. Previous work has shown that abnormal activation of these downstream pathways can lead to HCC. Rigorous genetic experiments revealed that Notch signaling forms a positive feedback loop with the Hippo signaling effector YAP/TAZ to promote severe hepatomegaly and rapid HCC initiation and progression. Surprisingly, we found that Wnt/β-catenin signaling activation suppressed HCC formation by inhibiting the positive feedback loop between YAP/TAZ and Notch signaling. Furthermore, we found that STAT3 in hepatocytes is dispensable for HCC formation when mammalian sterile 20-like kinase 1 and 2 (Mst1 and Mst2) were removed. The molecular network we have identified provides insights into HCC molecular classifications and therapeutic developments for the treatment of liver tumors caused by distinct genetic mutations.

Integrin α2β1 inhibits MST1 kinase phosphorylation and activates Yes-associated protein oncogenic signaling in hepatocellular carcinoma.

The Hippo pathway regulates the down-stream target Yes-associated protein (YAP) to maintain organ homeostasis, which is commonly inactivated in many types of cancers. However, how cell adhesion dysregulates the Hippo pathway activating YAP oncogene in hepatocellular carcinoma (HCC) remains unclear. Our findings demonstrate that α2β1 integrin (but not other β1 integrins) expressed in HCC cells, after binding to collagen extracellular matrix, could inhibit MST1 kinase phosphorylation and activate YAP pro-oncogenic activities. Knockdown of integrin α2 gene (ITGA2) suppressed YAP targeted gene expression in vitro. α2β1 and collagen binding resulted in suppressing Hippo signaling of mammalian sterile 20-like kinase 1 (MST1) and Large tumor suppressor homolog 1 (LATS1) with concomitant activation of YAP-mediated connective tissue growth factor (CTGF) gene expression. In vitro kinase assay showed that MST1 is an immediate downstream target of integrin α2 with S1180 residue as the critical phosphorylation site. Clinical correlational analysis using a gene expression dataset of 228 HCC tumors revealed that ITGA2 expression was significantly associated with tumor progression, and co-expression with YAP targeted genes (AXL receptor tyrosine kinase, CTGF, cyclin D1, glypican 3, insulin like growth factor 1 receptor, and SRY-box 4) correlated with survivals of HCC patients. In conclusion, α2β1 integrin activation through cellular adhesion impacts the Hippo pathway in solid tumors and modulates MST1-YAP signaling cascade. Targeting integrin α2 holds promises for treating YAP-positive HCC.

MST1-dependent vesicle trafficking regulates neutrophil transmigration through the vascular basement membrane.

Neutrophils need to penetrate the perivascular basement membrane for successful extravasation into inflamed tissue, but this process is incompletely understood. Recent findings have associated mammalian sterile 20-like kinase 1 (MST1) loss of function with a human primary immunodeficiency disorder, suggesting that MST1 may be involved in immune cell migration. Here, we have shown that MST1 is a critical regulator of neutrophil extravasation during inflammation. Mst1-deficient (Mst1-/-) neutrophils were unable to migrate into inflamed murine cremaster muscle venules, instead persisting between the endothelium and the basement membrane. Mst1-/- neutrophils also failed to extravasate from gastric submucosal vessels in a murine model of Helicobacter pylori infection. Mechanistically, we observed defective translocation of VLA-3, VLA-6, and neutrophil elastase from intracellular vesicles to the surface of Mst1-/- neutrophils, indicating that MST1 is required for this crucial step in neutrophil transmigration. Furthermore, we found that MST1 associates with the Rab27 effector protein synaptotagmin-like protein 1 (JFC1, encoded by Sytl1 in mice), but not Munc13-4, thereby regulating the trafficking of Rab27-positive vesicles to the cellular membrane. Together, these findings highlight a role for MST1 in vesicle trafficking and extravasation in neutrophils, providing an additional mechanistic explanation for the severe immune defect observed in patients with MST1 deficiency.

Phosphoinositide 3-kinase/Akt inhibits MST1-mediated pro-apoptotic signaling through phosphorylation of threonine 120.

The protein kinase mammalian sterile 20-like kinase 1 (MST1) is a mammalian homologue of the Drosophila hippo and plays a critical role in regulation of programmed cell death. MST1 exerts pro-apoptotic function through cleavage, autophosphorylation-Thr183 and subsequent translocation to the nucleus where it phosphorylates a number of molecules, including LATS1/2, FOXO, JNK, and histone H2B. Here, we show that the cleavage of MST1 is inhibited by the phosphatidylinositol 3-kinase/Akt pathway. Akt interacts with MST1 and phosphorylates a highly conserved residue threonine 120 of MST1, which leads to inhibition of its kinase activity and nuclear translocation as well as the autophosphorylation of Thr183. Phospho-MST1-Thr120 failed to activate downstream targets FOXO3a and JNK. Further, inverse correlation between pMST1-Thr120 and pMST1-Thr183 was observed in human ovarian tumors. These findings indicate that the phosphorylation of MST1-Thr120 by Akt could be a major mechanism of regulation of the Hippo/MST1 pathway by cell survival signaling.

MST2 phosphorylation at serine 385 in mitosis inhibits its tumor suppressing activity

Mammalian sterile 20-like kinase 1/2 (MST1/2) are core tumor suppressors in the Hippo signaling pathway. MST1/2 have been shown to regulate mitotic progression. Here, we report a novel mechanism for phospho-regulation of MST2 in mitosis and its biological significance in cancer. We found that the mitotic kinase cyclin-dependent kinase 1 (CDK1) phosphorylates MST2 in vitro and in vivo at serine 385 during antimitotic drug-induced G2/M phase arrest. This phosphorylation occurs transiently during unperturbed mitosis. Mitotic phosphorylation of MST2 does not affect its kinase activity or Hippo-YAP signaling. We further showed that mitotic phosphorylation-deficient mutant MST2-S385A possesses higher activity in suppressing cell proliferation and anchorage-independent growth in vitro and tumorigenesis in vivo. Together, our findings reveal a novel layer of regulation for MST2 in mitosis and its role in tumorigenesis.

MST1 coordinately regulates autophagy and apoptosis in diabetic cardiomyopathy in mice.

Diabetic cardiomyopathy (DCM) is associated with suppressed autophagy and augmented apoptosis in the heart although the interplay between the two remains elusive. The ability of mammalian sterile 20-like kinase 1 to regulate both autophagy and apoptosis prompted us to investigate it as a possible candidate in the progression of DCM. Wild-type, Mst1 (also known as Stk4) transgenic and Mst1-knockout mice were challenged with streptozotocin to induce experimental diabetes. In addition, cultured neonatal mouse cardiomyocytes were subjected to simulated diabetes to probe mechanisms. Mst1 knockout alleviated while Mst1 overexpression aggravated cardiac dysfunction in diabetes. Diabetic Mst1 transgenic mice exhibited decreased LC3 expression and enhanced protein aggregation. In contrast, typical autophagosomes were observed in diabetic Mst1-knockout mice with increased LC3 expression and reduced protein aggregation. Mst1 downregulation promoted autophagic flux as demonstrated by increased LC3-II and decreased p62 expression in the presence of bafilomycin A1. Furthermore, Mst1 overexpression increased, while Mst1 knockout decreased, cardiomyocyte apoptosis both in vivo and in vitro. Co-immunoprecipitation assays showed that Mst1 overexpression promoted Beclin1 binding to B cell lymphoma 2 (Bcl-2) and induced dissociation of Bcl-2 from Bax in diabetic mice. Conversely, Mst1 knockout disrupted the Beclin1-Bcl-2 complex and enhanced the interaction between Bcl-2 and Bax. Mst1 knockout restores autophagy and protects against apoptosis in cardiomyocytes, en route to the rescue against DCM.

Hippo/YAP signaling pathway is involved in osteosarcoma chemoresistance.

BackgroundOsteosarcoma is the most common bone malignancy in children and adolescents, and 20%–30% of the patients suffer from poor prognosis because of individual chemoresistance. The Hippo/yes-associated protein (YAP) signaling pathway has been shown to play a role in tumor chemoresistance, but no previous report has focused on its involvement in osteosarcoma chemoresistance. This study aimed to investigate the role of the Hippo/YAP signaling pathway in osteosarcoma chemoresistance and to determine potential treatment targets.MethodsUsing the Cell Titer-Glo Luminescent cell viability assay and flow cytometry analysis, we determined the proliferation and chemosensitivity of YAP-overexpressing and YAP-knockdown osteosarcoma cells. In addition, using western blotting and the real-time polymerase chain reaction technique, we investigated the alteration of the Hippo/YAP signaling pathway in osteosarcoma cells treated with chemotherapeutic agents.ResultsMammalian sterile 20-like kinase 1 (MST1) degradation was increased, and large tumor suppressor kinase 1/2 (LATS1/2) total protein levels were decreased by methotrexate and doxorubicin, which increased activation and nuclear translocation of YAP. Moreover, YAP increased the proliferation and chemoresistance of MG63 cells.ConclusionsThe Hippo/YAP signaling pathway plays a role in osteosarcoma chemoresistance, and YAP is a potential target for reducing chemoresistance.

Mst1 shuts off cytosolic antiviral defense through IRF3 phosphorylation.

Cytosolic RNA/DNA sensing elicits primary defense against viral pathogens. Interferon regulatory factor 3 (IRF3), a key signal mediator/transcriptional factor of the antiviral-sensing pathway, is indispensible for interferon production and antiviral defense. However, how the status of IRF3 activation is controlled remains elusive. Through a functional screen of the human kinome, we found that mammalian sterile 20-like kinase 1 (Mst1), but not Mst2, profoundly inhibited cytosolic nucleic acid sensing. Mst1 associated with IRF3 and directly phosphorylated IRF3 at Thr75 and Thr253. This Mst1-mediated phosphorylation abolished activated IRF3 homodimerization, its occupancy on chromatin, and subsequent IRF3-mediated transcriptional responses. In addition, Mst1 also impeded virus-induced activation of TANK-binding kinase 1 (TBK1), further attenuating IRF3 activation. As a result, Mst1 depletion or ablation enabled an enhanced antiviral response and defense in cells and mice. Therefore, the identification of Mst1 as a novel physiological negative regulator of IRF3 activation provides mechanistic insights into innate antiviral defense and potential antiviral prevention strategies.

MST1: a promising therapeutic target to restore functional beta cell mass in diabetes.

The loss of insulin-producing beta cells by apoptosis is a hallmark of all forms of diabetes mellitus. Strategies to prevent beta cell apoptosis and dysfunction are urgently needed to restore the insulin-producing cells and to prevent severe diabetes progression. We recently identified the serine/threonine kinase known as mammalian sterile 20-like kinase 1 (MST1) as a critical regulator of apoptotic beta cell death and dysfunction. MST1 activates several apoptotic signalling pathways, which further stimulate its own cleavage, leading to a vicious cycle of cell death. This led us to hypothesise that MST1 signalling is central to the initiation of beta cell death in diabetes. We found that MST1 is strongly activated in a diabetic beta cell and induces not only its death but also directly impairs insulin secretion through promoting proteasomal degradation of key beta cell transcription factor, pancreatic and duodenal homeobox 1 (PDX1), which is critical for insulin production.Pre-clinical studies in various animal models of diabetes have reported that MST1 deficiency remarkably restores normoglycaemia and beta cell function and prevents the development of diabetes. Importantly, MST1 deficiency can revert fully diabetic beta cells to a non-diabetic state. MST1 may serve as a target for the development of novel therapies for diabetes that trigger the cause of the disease, namely, the destruction of the beta cells. The major current focus of our investigation is to identify and test the efficacy of potent inhibitors of this death signalling pathway to protect beta cells against the effects of autoimmune attack in type 1 diabetes and to preserve beta cell mass and function in type 2 diabetes. This review summarises a presentation given at the 'Can we make a better beta cell?' symposium at the 2015 annual meeting of the EASD. It is accompanied by two other reviews on topics from this symposium (by Heiko Lickert and colleagues, DOI: 10.1007/s00125-016-3949-9 , and by Harry Heimberg and colleagues, DOI: 10.1007/s00125-016-3879-6 ) and a commentary by the Session Chair, Shanta Persaud (DOI: 10.1007/s00125-016-3870-2 ).

N-acetylcysteine attenuates the development of cardiac fibrosis and remodeling in a mouse model of heart failure.

Oxidative stress plays a central role in the pathogenesis of heart failure. We aimed to determine whether the antioxidant N‐acetylcysteine can attenuate cardiac fibrosis and remodeling in a mouse model of heart failure. Minipumps were implanted subcutaneously in wild‐type mice (n = 20) and mice with cardiomyopathy secondary to cardiac specific overexpression of mammalian sterile 20‐like kinase 1 (MST‐1; n = 18) to administer N‐acetylcysteine (40 mg/kg per day) or saline for a period of 8 weeks. At the end of this period, cardiac remodeling and function was assessed via echocardiography. Fibrosis, oxidative stress, and expression of collagen types I and III were quantified in heart tissues. Cardiac perivascular and interstitial fibrosis were greater by 114% and 209%, respectively, in MST‐1 compared to wild type (P ≤ 0.001). In MST‐1 mice administered N‐acetylcysteine, perivascular and interstitial fibrosis were 40% and 57% less, respectively, compared to those treated with saline (P ≤ 0. 03). Cardiac oxidative stress was 119% greater in MST‐1 than in wild type (P < 0.001) and N‐acetylcysteine attenuated oxidative stress in MST‐1 by 42% (P = 0.005). These data indicate that N‐acetylcysteine can blunt cardiac fibrosis and related remodeling in the setting of heart failure potentially by reducing oxidative stress. This study provides the basis to investigate the role of N‐acetylcysteine in chronic heart failure.