STE20-like Protein Kinase Research Articles

RHO binding to FAM65A regulates Golgi reorientation during cell migration.

ABSTRACTDirectional cell migration involves reorientation of the secretory machinery. However, the molecular mechanisms that control this reorientation are not well characterised. Here, we identify a new Rho effector protein, named FAM65A, which binds to active RHOA, RHOB and RHOC. FAM65A links RHO proteins to Golgi-localising cerebral cavernous malformation-3 protein (CCM3; also known as PDCD10) and its interacting proteins mammalian STE20-like protein kinases 3 and 4 (MST3 and MST4; also known as STK24 and STK26, respectively). Binding of active RHO proteins to FAM65A does not affect the kinase activity of MSTs but results in their relocation from the Golgi in a CCM3-dependent manner. This relocation is crucial for reorientation of the Golgi towards the leading edge and subsequent directional cell migration. Our results reveal a previously unidentified pathway downstream of RHO that regulates the polarity of migrating cells through Golgi reorientation in a FAM65A-, CCM3- and MST3- and MST4-dependent manner.

Upregulation of miR-181c contributes to chemoresistance in pancreatic cancer by inactivating the Hippo signaling pathway.

The Hippo signaling pathway plays a crucial role in regulating tissue homeostasis, organ size, tumorigenesis and cancer chemoresistance when deregulated. Physiologically, the Hippo core kinase cassette that consists of mamma-lian STE20-like protein kinase 1/2 (MST1/2), and large tumour suppressor 1/2 (LATS1/2), together with the adaptor proteins Salvador homologue 1 (SAV1) and MOB kinase activator 1 (MOB1), tightly restricts the activities of homologous oncoproteins Yes-associated protein (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ) to low levels. However, how the Hippo kinase cassette core components are simultaneously inhibited, to exhibit constitutively inactivated Hippo signaling and activated YAP/TAZ in cancer remains puzzling. Herein, we reported that miR-181c directly repressed MST1, LATS2, MOB1 and SAV1 expression in human pancreatic cancer cells. Overexpression of miR-181c induced hyperactivation of the YAP/TAZ and enhanced expression of the Hippo signaling downstream genes CTGF, BIRC5 and BLC2L1, leading to pancreatic cancer cell survival and chemoresistance in vitro and in vivo. Importantly, high miR-181c levels were significantly correlated with Hippo signaling inactivation in pancreatic cancer samples, and predicted a poor patient overall survival. These findings provide a novel mechanism for Hippo signaling inactivation in cancer, indicating not only a potentially pivotal role for miR-181c in the progression of pancreatic cancer, but also may represent a new therapeutic target and prognostic marker.

MST2-RASSF protein-protein interactions through SARAH domains.

The detailed, atomistic-level understanding of molecular signaling along the tumor-suppressive Hippo signaling pathway that controls tissue homeostasis by balancing cell proliferation and death through apoptosis is a promising avenue for the discovery of novel anticancer drug targets. The activation of kinases such as Mammalian STE20-Like Protein Kinases 1 and 2 (MST1 and MST2)-modulated through both homo- and heterodimerization (e.g. interactions with Ras association domain family, RASSF, enzymes)-is a key upstream event in this pathway and remains poorly understood. On the other hand, RASSFs (such as RASSF1A or RASSF5) act as important apoptosis activators and tumor suppressors, although their exact regulatory roles are also unclear. We present recent molecular studies of signaling along the Ras-RASSF-MST pathway, which controls growth and apoptosis in eukaryotic cells, including a variety of modern molecular modeling and simulation techniques. Using recently available structural information, we discuss the complex regulatory scenario according to which RASSFs perform dual signaling functions, either preventing or promoting MST2 activation, and thus control cell apoptosis. Here, we focus on recent studies highlighting the special role being played by the specific interactions between the helical Salvador/RASSF/Hippo (SARAH) domains of MST2 and RASSF1a or RASSF5 enzymes. These studies are crucial for integrating atomistic-level mechanistic information about the structures and conformational dynamics of interacting proteins, with information available on their system-level functions in cellular signaling.

Netrin-1 exerts oncogenic activities through enhancing Yes-associated protein stability

Yes-associated protein (YAP), a transcription coactivator, is the major downstream effector of the Hippo pathway, which plays a critical role in organ size control and cancer development. However, how YAP is regulated by extracellular stimuli in tumorigenesis remains incompletely understood. Netrin-1, a laminin-related secreted protein, displays proto-oncogenic activity in cancers. Nonetheless, the downstream signaling mediating its oncogenic effects is not well defined. Here we show that netrin-1 via its transmembrane receptors, deleted in colorectal cancer and uncoordinated-5 homolog, up-regulates YAP expression, escalating YAP levels in the nucleus and promoting cancer cell proliferation and migration. Inactivating netrin-1, deleted in colorectal cancer, or uncoordinated-5 homolog B (UNC5B) decreases YAP protein levels, abrogating cancer cell progression by netrin-1, whereas knockdown of mammalian STE20-like protein kinase 1/2 (MST1/2) or large tumor suppressor kinase 1/2 (Lats1/2), two sets of upstream core kinases of the Hippo pathway, has no effect in blocking netrin-1-induced up-regulation of YAP. Netrin-1 stimulates phosphatase 1A to dephosphorylate YAP, which leads to decreased ubiquitination and degradation, enhancing YAP accumulation and signaling. Hence, our findings support that netrin-1 exerts oncogenic activity through YAP signaling, providing a mechanism coupling extracellular signals to the nuclear YAP oncogene.

An emerging role for Hippo-YAP signaling in cardiovascular development

The Hippo signaling pathway was originally discovered in Drosophila and shown to be critical for organ size control and tumorigenesis. This pathway consists of a cascade of kinases and several adaptors that lead to the phosphorylation and inhibition, through nuclear exclusion, of the transcriptional cofactor Yorkie in Drosophila or YAP (yes associated protein) in mammals. Recent studies demonstrate that cardiac-specific deletion of the Hippo pathway kinase Mst (STE20-like protein kinases) co-activator WW45 (WW domain-containing adaptor 45), Mst1, Mst2, or Lats2 (large tumor suppressor homologue 2) in mice result in over-grown hearts with elevated cardiomyocyte proliferation. Consistent with these observations, over-expression of YAP in the mouse embryonic heart increases heart size and promotes cardiac regeneration and contractility after myocardial infarction by inducing cardiomyocyte proliferation, whereas deletion of YAP in the mouse heart impedes cardiomyocyte proliferation, causing myocardial hypoplasia and embryonic or premature lethality. YAP has also been shown to play an important role in the vascular system. Specific-deletion of YAP from vascular smooth muscle cells in mice results in aberrant development of large arteries with a hypoplastic arterial wall phenotype. Hippo-YAP signaling cross-talks with other signaling pathways such as IGF (insulin-like growth factor) and Wnt signaling to promote heart growth by increasing expression of cell cycle genes. The purpose of this review is to summarize these recent findings and discuss potential diagnostic or therapeutic strategies in cardiovascular system based on manipulating the Hippo-YAP signaling.

The Hippo Pathway Is Activated and Is a Causal Mechanism for Adipogenesis in Arrhythmogenic Cardiomyopathy

Mutations in the intercalated disc proteins, such as plakophilin 2 (PKP2), cause arrhythmogenic cardiomyopathy (AC). AC is characterized by the replacement of cardiac myocytes by fibro-adipocytes, cardiac dysfunction, arrhythmias, and sudden death. To delineate the molecular pathogenesis of AC. Localization and levels of selected intercalated disc proteins, including signaling molecules, were markedly reduced in human hearts with AC. Altered protein constituents of intercalated discs were associated with activation of the upstream Hippo molecules in the human hearts, in Nkx2.5-Cre:Dsp(W/F) and Myh6:Jup mouse models of AC, and in the PKP2 knockdown HL-1 myocytes (HL-1(PKP2:shRNA)). Level of active protein kinase C-α isoform, which requires PKP2 for activity, was reduced. In contrast, neurofibromin 2 (or Merlin), a molecule upstream of the Hippo pathway and that is inactivated by protein kinase C-α isoform, was activated. Consequently, the downstream Hippo molecules mammalian STE20-like protein kinases 1/2 (MST1/2), large tumor suppressor kinases 1/2 (LATS1/2), and Yes-associated protein (YAP) (the latter is the effector of the pathway) were phosphorylated. Coimmunoprecipitation detected binding of phosphorylated YAP, phosphorylated β-catenin, and junction protein plakoglobin (the latter translocated from the junction). RNA sequencing, transcript quantitative polymerase chain reaction, and reporter assays showed suppressed activity of SV40 transcriptional enhancer factor domain (TEAD) and transcription factor 7-like 2 (TCF7L2), which are transcription factors of the Hippo and the canonical Wnt signaling, respectively. In contrast, adipogenesis was enhanced. Simultaneous knockdown of Lats1/2, molecules upstream to YAP, rescued inactivation of YAP and β-catenin and adipogenesis in the HL-1(PKP2:shRNA) myocytes. Molecular remodeling of the intercalated discs leads to pathogenic activation of the Hippo pathway, suppression of the canonical Wnt signaling, and enhanced adipogenesis in AC. The findings offer novel mechanisms for the pathogenesis of AC.

Abstract IA34: Hippo signaling in development and cancer

Abstract The Hippo signaling pathway regulates organ size and tissue homeostasis in diverse species from Drosophila to mammals. Central to the Hippo pathway is a functionally conserved kinase cascade leading from the Ste20-like protein kinase Hippo (Hpo) (Mst1/2 in mammals) and the NDR family kinase Warts (Lats1/2 in mammals) to the transcription factor complex formed by the coactivator Yorkie (Yki) (YAP/TAZ in mammals) and its major DNA-binding partner Scalloped (Sd) (TEAD1/2/3/4 in mammals). Inactivation of the Hippo pathway tumor suppressors or overexpression/activation of the YAP oncoprotein results in uncontrolled growth and tumorigenesis in transgenic mouse models and has been implicated in diverse human cancers. Compared to the core kinase cascade leading from Hpo to Yki, signaling events upstream of the Hippo kinase cascade are less well understood. My presentation will focus on Merlin/NF2, which encodes a member of the ezrin, radixin, moesin (ERM) family of membrane-cytoskeleton adapter proteins. Merlin/NF2 was discovered twenty years ago as a tumor suppressor underlying Neurofibromatosis type II, an autosomal dominant disorder characterized by the development of benign schwannomas, meningiomas and ependymomas. Although it is generally believed that Merlin/NF2 impacts cell signaling as a membrane-cytoskeleton scaffold, its precise molecular mechanism remains poorly understood. Recent studies in Drosophila revealed a potential link between Merlin and the Hippo pathway by placing Merlin genetically upstream of the kinase Hpo. These genetic findings have led to the prevailing view in the field that Merlin functions upstream of Hpo in a linear biochemical pathway, even though the genetic epistasis is equally compatible with Merlin and Hpo acting in parallel to promote Hippo signaling output. Using a combination of genetics, cell biology and biochemistry, we investigated the molecular relationship between Merlin/NF2 and the Hippo pathway in both Drosophila and mammals. In contrast to the commonly assumed linear pathway of Merlin/NF2 functioning through Hpo, we show that in both Drosophila and mammals, Merlin/NF2 promotes downstream Hippo signaling without activating the intrinsic kinase activity of Hpo. Instead, Merlin/NF2 directly binds and recruits the effector kinase Wts to plasma membrane. Membrane recruitment, in turn, promotes Wts phosphorylation and activation by the membrane-associated Hpo-Sav kinase complex. Accordingly, disease-causing mutations in Merlin/NF2 impair its ability to bind/recruit Wts to plasma membrane. Conversely, gain-of-function mutations in Merlin/NF2 potentiate its ability to bind/recruit Wts to plasma membrane. Our studies therefore uncover a critical function of Merlin in spatial organization of Hippo signaling at the plasma membrane. Citation Format: Duojia Pan. Hippo signaling in development and cancer. [abstract]. In: Proceedings of the Third AACR International Conference on Frontiers in Basic Cancer Research; Sep 18-22, 2013; National Harbor, MD. Philadelphia (PA): AACR; Cancer Res 2013;73(19 Suppl):Abstract nr IA34.

Ste20-like protein kinase SLK (LOSK) regulates microtubule organization by targeting dynactin to the centrosome

The microtubule- and centrosome-associated Ste20-like kinase (SLK; long Ste20-like kinase [LOSK]) regulates cytoskeleton organization and cell polarization and spreading. Its inhibition causes microtubule disorganization and release of centrosomal dynactin. The major function of dynactin is minus end-directed transport along microtubules in a complex with dynein motor. In addition, dynactin is required for maintenance of the microtubule radial array in interphase cells, and depletion of its centrosomal pool entails microtubule disorganization. Here we demonstrate that SLK (LOSK) phosphorylates the p150(Glued) subunit of dynactin and thus targets it to the centrosome, where it maintains microtubule radial organization. We show that phosphorylation is required only for centrosomal localization of p150(Glued) and does not affect its microtubule-organizing properties: artificial targeting of nonphosphorylatable p150(Glued) to the centrosome restores microtubule radial array in cells with inhibited SLK (LOSK). The phosphorylation site is located in a microtubule-binding region that is variable for two isoforms (1A and 1B) of p150(Glued) expressed in cultured fibroblast-like cells (isoform 1B lacks 20 amino acids in the basic microtubule-binding domain). The fact that SLK (LOSK) phosphorylates only a minor isoform 1A of p150(Glued) suggests that transport and microtubule-organizing functions of dynactin are distinctly divided between the two isoforms. We also show that dynactin phosphorylation is involved in Golgi reorientation in polarized cells.

Molecular cloning, characterization and differential expression of a Sporothrix schenckii STE20-like protein kinase SsSte20

Dimorphic switching requires fungal cells to undergo changes in polarized growth in response to environmental stimuli. The Ste20-related kinases are involved in signaling through mitogen-activated protein kinase pathways and in morphogenesis through the regulation of cytokinesis and actin-dependent polarized growth. In this report, we isolated and characterized an Ste20 homologue gene, designated SsSte20, from yeast-form Sporothrix schenckii (S. schenckii). The full length SsSte20 cDNA sequence is 2846 bp in size, and contains an open reading frame of 2505 bp encoding 835 amino acids. The predicted molecular mass of SsSte20 is 91.31 kDa with an estimated theoretical isoelectric point of 5.76. SsSte20 kinase domain shows 63% identity with that of Don3, a germinal centre kinase (GCK) from Ustilago maydis. Two exons and one intron are identified within the 2578 bp SsSte20 genomic DNA sequence of S. schenckii. Differential expression of the SsSte20 was demonstrated by real-time RT-PCR. The expression of SsSte20 was much higher in the yeast stage compared with that in the mycelial stage, which indicated that the SsSte20 may be involved in the pathogenesis of the yeast phase of S. schenckii.

Mammalian Ste20-like protein kinase 3 plays a role in hypoxia-induced apoptosis of trophoblast cell line 3A-sub-E

Mammalian Ste20-like protein kinase 3 (Mst3) is a key player in inducing apoptosis in a variety of cell types and has recently been shown to participate in the signaling pathway of hypoxia-induced apoptosis of human trophoblast cell line 3A-sub-E (3A). It is believed that oxidative stress may occur during hypoxia and induce the expression of Mst3 in 3A cells via the activation of c-Jun N-terminal protein kinase 1 (JNK1). This hypothesis was demonstrated by the suppressive effect of dl-α-lipoic acid, a reactive oxygen species scavenger, in hypoxia-induced responses of 3A cells such as Mst3 expression, nitrotyrosine formation, JNK1 activation and apoptosis. Similar results were also observed in trophoblasts of human placental explants in both immunohistochemical studies and immunoblot analyses. These suggested that the activation of Mst3 might trigger the apoptotic process in trophoblasts by activating caspase 3 and possibly other apoptotic pathways. The role of nitric oxide synthase (NOS) and NADPH oxidase (NOX) in hypoxia-induced Mst3 up-regulation was also demonstrated by the inhibitory effect of NG-nitro-l-arginine and apocynin, which inhibits NOS and NOX, respectively. Oxidative stress was postulated to be induced by NOS and NOX in 3A cells during hypoxia. In conclusion, hypoxia induces oxidative stress in human trophoblasts by activating NOS and NOX. Subsequently, Mst3 is up-regulated and plays an important role in hypoxia-induced apoptosis of human trophoblasts.

Monitoring the cellular dynamic distribution of mammalian Ste20‐like protein kinase 3 by peptide aptamer from phage display

Mammalian Ste20‐like protein kinase 3 (Mst3), a member of the GC‐III subfamily protein kinases, is found to regulate various cellular affairs in response to environmental cues. The localization of endogenous Mst3 is in cytoplasm and may translocate to nucleus upon stimulation. We are interested in screening Mst3‐binding peptide for future drug development. Two phage display libraries, hepta‐peptide and dodeca‐peptide phage libraries had been subjected to an in vitro selection process for Mst3‐binding ligand identification. Following six biopanning process of Mst3‐binding selection and DNA sequencing of selected phages, five Mst3‐binding phages were selected. The five types of Mst3‐binding phages had been confirmed that all could bind with Mst3 but not Mst4 by ELISA assay and Western blotting. The peptides were constructed into EGFP vector to form EGFP fusion proteins in vivo. The peptide‐EGFP fusion proteins will be used to real time monitor the subcellular redistribution of endogenous Mst3 in response to various environmental stimuli.

Bioinformatics search for plant homologues of Ste20-like serine/threonine protein kinases

Eleven plant homologs of animal and yeast Ste20-like protein kinases were identified. It was shown that the nearest plant homologs of the Ste20-like protein kinases are the unknown proteins A9RVK0 from Physcomitrella patens ssp. patens and A7P2E2 from Vitis vinifera. Cladistic analysis showed a protein kinase dstl from Dictyostelium discoideum as the closest protein to the newly found plant homologs. A predicted spatial structure of the A9RVK0 from P. patens ssp. patens catalytic domain is presented.

Mst1-FoxO Signaling Protects Naïve T Lymphocytes from Cellular Oxidative Stress in Mice

BackgroundThe Ste-20 family kinase Hippo restricts cell proliferation and promotes apoptosis for proper organ development in Drosophila. In C. elegans, Hippo homolog also regulates longevity. The mammalian Ste20-like protein kinase, Mst1, plays a role in apoptosis induced by various types of apoptotic stress. Mst1 also regulates peripheral naïve T cell trafficking and proliferation in mice. However, its functions in mammals are not fully understood.Methodology/Principal FindingsHere, we report that the Mst1-FoxO signaling pathway plays a crucial role in survival, but not apoptosis, of naïve T cells. In Mst1−/− mice, peripheral T cells showed impaired FoxO1/3 activation and decreased FoxO protein levels. Consistently, the FoxO targets, Sod2 and catalase, were significantly down-regulated in Mst1−/− T cells, thereby resulting in elevated levels of intracellular reactive oxygen species (ROS) and induction of apoptosis. Expression of constitutively active FoxO3a restored Mst1−/− T cell survival. Crossing Mst1 transgenic mice (Mst1 Tg) with Mst1−/− mice reduced ROS levels and restored normal numbers of peripheral naïve T cells in Mst1 Tg;Mst1−/− progeny. Interestingly, peripheral T cells from Mst1−/− mice were hypersensitive to γ-irradiation and paraquat-induced oxidative stresses, whereas those from Mst1 Tg mice were resistant.Conclusions/SignificanceThese data support the hypothesis that tolerance to increased levels of intracellular ROS provided by the Mst1-FoxOs signaling pathway is crucial for the maintenance of naïve T cell homeostasis in the periphery.

Inhibiting JNK Dephosphorylation and Induction of Apoptosis by Novel Anticancer Agent NSC-741909 in Cancer Cells

NSC-741909 is a recently identified novel anticancer agent that suppresses the growth of several NCI-60 cancer cell lines with a unique anticancer spectrum. However, its molecular mechanisms remain unknown. To determine the molecular mechanisms of NSC-741909-induced antitumor activity, we analyzed the changes of 77 protein biomarkers in a sensitive lung cancer cell line after treatment with this compound by using reverse-phase protein microarray. The results showed that phosphorylation of mitogen-activated protein (MAP) kinases (P38 MAPK, ERK, and JNK) were persistently elevated by the treatment with NSC-741909. However, only the JNK-specific inhibitor SP600125 effectively blocked the apoptosis induced by NSC-741909. Moreover, NSC-741909-mediated apoptosis was also blocked by a dominant-negative JNK construct, suggesting that sustained activation of JNK is critical for the apoptosis induction. Further studies revealed that treatment with NSC-741909 suppressed dephosphorylation of JNK and the expression of MAPK phosphatase-1. Thus, NSC-741909-mediated inhibition of JNK dephosphorylation results in sustained JNK activation, which leads to apoptosis in cancer cells.

Mst3 Plays an Important Role in Hypoxia‐induced placental Apoptosis in Preeclampsia

Preeclampsia, a disorder unique to humans, affects 5‐10% of all pregnancies, and remains a leading cause of fetal and maternal morbidity and mortality. A key histopathologic correlation is shallow invasion and aberrant remodeling of maternal spiral arteries, leading to decreased uteroplacental perfusion and hypoxia. In preeclampic human placenta, hypoxia is thought to be the main cause to elevate oxidative stress and induced trophoblast cell death. Recently, mammalian Ste20‐like protein kinase 3 (Mst3) was found to be elevated in human preeclampic placenta that exhibit extensive sign of apoptosis, suggesting the participation of Mst3 in preeclampsia. Hypoxia is postulated to induce the expression of Mst3 and the apoptosis of human placenta during preeclampsia. The role of Mst3 in hypoxia‐induced apoptosis was demonstrated in both human placental explant culture and the human trophoblast cell line 3A‐sub‐E. Further studies show that, in human trophoblast, hypoxia may induce the expression of Mst3 via oxidative stress and the activation of JNK. The activated Mst3 then triggers apoptosis of human trophoblasts by both caspase‐dependent and independent pathways. (The work was supported by National Science Council, ROC (Taiwan) under Contract No. NSC‐97‐2311‐B‐009‐001‐MY3)

Ste20-related Protein Kinase LOSK (SLK) Controls Microtubule Radial Array in Interphase

Interphase microtubules are organized into a radial array with centrosome in the center. This organization is a subject of cellular regulation that can be driven by protein phosphorylation. Only few protein kinases that regulate microtubule array in interphase cells have been described. Ste20-like protein kinase LOSK (SLK) was identified as a microtubule and centrosome-associated protein. In this study we have shown that the inhibition of LOSK activity by dominant-negative mutant K63R-DeltaT or by LOSK depletion with RNAi leads to unfocused microtubule arrangement. Microtubule disorganization is prominent in Vero, CV-1, and CHO-K1 cells but less distinct in HeLa cells. The effect is a result neither of microtubule stabilization nor of centrosome disruption. In cells with suppressed LOSK activity centrosomes are unable to anchor or to cap microtubules, though they keep nucleating microtubules. These centrosomes are depleted of dynactin. Vero cells overexpressing K63R-DeltaT have normal dynactin "comets" at microtubule ends and unaltered morphology of Golgi complex but are unable to polarize it at the wound edge. We conclude that protein kinase LOSK is required for radial microtubule organization and for the proper localization of Golgi complex in various cell types.

Ste20-related proline/alanine-rich kinase: A novel regulator of intestinal inflammation

Recently, inflammatory bowel disease (IBD) has been the subject of considerable research, with increasing attention being paid to the loss of intestinal epithelial cell barrier function as a mechanism of pathogenesis. Ste20-related proline/alanine-rich kinase (SPAK) is involved in regulating barrier function. SPAK is known to interact with inflammation-related kinases (such as p38, JNK, NKCC1, PKCtheta, WNK and MLCK), and with transcription factor AP-1, resulting in diverse biological phenomena, including cell differentiation, cell transformation and proliferation, cytoskeleton rearrangement, and regulation of chloride transport. This review examines the involvement of Ste20-like kinases and downstream mitogen-activated protein kinases (MAPKs) pathways in the pathogenesis and control of intestinal inflammation. The primary focus will be on the molecular features of intestinal inflammation, with an emphasis on the interaction between SPAK and other molecules, and the effect of these interactions on homeostatic maintenance, cell volume regulation and increased cell permeability in intestinal inflammation.

Mammalian Ste20-like protein kinase 3 mediates trophoblast apoptosis in spontaneous delivery

The placenta is essential in transferring gases and nutrients from the mother to the developing fetus. Trophoblast apoptosis may cause labor or other pregnancy-related disorders. This study demonstrated the essential role of Mst3, a human Ste20-like protein kinase, in the oxidative stress-induced apoptosis of trophoblasts of term placenta in normal spontaneous delivery. Oxidative stress, but not hormones released during labor such as prostaglandin E1, oxytocin or angiotensin II, induces the expression of Mst3 and apoptosis of human term placenta after elective Cesarean section without labor pain. The role of Mst3 in oxidative stress-induced apoptosis was further demonstrated in the 3A-sub-E, a human trophoblast cell line. The H2O2-induced apoptosis of 3A-sub-E cells was largely suppressed by overexpressed Mst3KR, the kinase-dead mutant or by selective knockdown of endogenous Mst3. Further studies showed that Jun N-terminal kinase (JNK) may participate in the signaling pathway of H2O2-induced apoptosis by mediating the level of Mst3. Subsequently, caspase 3 and other downstream apoptotic components may be activated by Mst3 and trigger the apoptotic process in human trophoblasts.

Mst3b, a purine-sensitive Ste20-like protein kinase, regulates axon outgrowth

The growth of axons is fundamental to the development and repair of brain circuitry. We show here that Mst3b, a neuron-specific homolog of the yeast kinase Ste20, is critical for axon outgrowth. Mst3b is activated in response to trophic factors, and suppressing its expression (via siRNAs) or its function (by a dominant-negative mutant) blocks axon outgrowth. Inosine, a purine nucleoside that stimulates axon outgrowth, activates Mst3b kinase activity, whereas 6-thioguanine, a purine analog that blocks outgrowth, inhibits the activity of this kinase. These findings place Mst3b as a key regulator of axon outgrowth and help explain the purine sensitivity of this process.

Dmp53 Activates the Hippo Pathway to Promote Cell Death in Response to DNA Damage

Developmental and environmental signals control a precise program of growth, proliferation, and cell death. This program ensures that animals reach, but do not exceed, their typical size . Understanding how cells sense the limits of tissue size and respond accordingly by exiting the cell cycle or undergoing apoptosis has important implications for both developmental and cancer biology. The Hippo (Hpo) pathway comprises the kinases Hpo and Warts/Lats (Wts), the adaptors Salvador (Sav) and Mob1 as a tumor suppressor (Mats), the cytoskeletal proteins Expanded and Merlin, and the transcriptional cofactor Yorkie (Yki) . This pathway has been shown to restrict cell division and promote apoptosis. The caspase repressor DIAP1 appears to be a primary target of the Hpo pathway in cell-death control. Firstly, Hpo promotes DIAP1 phosphorylation, likely decreasing its stability. Secondly, Wts phosphorylates and inactivates Yki, decreasing DIAP1 transcription. Although we understand some of the events downstream of the Hpo kinase, its mode of activation remains mysterious. Here, we show that Hpo can be activated by Ionizing Radiations (IR) in a Dmp53 (Drosophila melanogaster p53)-dependent manner and that Hpo is required (though not absolutely) for the cell death response elicited by IR or Dmp53 ectopic expression.