Caspase cleavage of MST1 promotes nuclear translocation and chromatin condensation

Protein kinases of the Hippo pathway: regulation and substrates

The "Hippo" signaling pathway has emerged as a major regulator of cell proliferation and survival in metazoans. The pathway, as delineated by genetic and biochemical studies in Drosophila, consists of a kinase cascade regulated by cell-cell contact and cell polarity that inhibits the transcriptional coactivator Yorkie and its proliferative, anti-differentiation, antiapoptotic transcriptional program. The core pathway components are the GC kinase Hippo, which phosphorylates the noncatalytic polypeptide Mats/Mob1 and, with the assistance of the scaffold protein Salvador, phosphorylates the ndr-family kinase Lats. In turn phospho-Lats, after binding to phospho-Mats, autoactivates and phosphorylates Yorkie, resulting in its nuclear exit. Hippo also uses the scaffold protein Furry and a different Mob protein to control another ndr-like kinase, the morphogenetic regulator Tricornered. Architecturally homologous kinase cascades consisting of a GC kinase, a Mob protein, a scaffolding polypeptide and an ndr-like kinase are well described in yeast; in Saccharomyces cerevisiae, e.g., the MEN pathway promotes mitotic exit whereas the RAM network, using a different GC kinase, Mob protein, scaffold and ndr-like kinase, regulates cell polarity and morphogenesis. In mammals, the Hippo orthologs Mst1 and Mst2 utilize the Salvador ortholog WW45/Sav1 and other scaffolds to regulate the kinases Lats1/Lats2 and ndr1/ndr2. As in Drosophila, murine Mst1/Mst2, in a redundant manner, negatively regulate the Yorkie ortholog YAP in the epithelial cells of the liver and gut; loss of both Mst1 and Mst2 results in hyperproliferation and tumorigenesis that can be largely negated by reduction or elimination of YAP. Despite this conservation, considerable diversification in pathway composition and regulation is already evident; in skin, e.g., YAP phosphorylation is independent of Mst1Mst2 and Lats1Lats2. Moreover, in lymphoid cells, Mst1/Mst2, under the control of the Rap1 GTPase and independent of YAP, promotes integrin clustering, actin remodeling and motility while restraining the proliferation of naïve T cells. This review will summarize current knowledge of the structure and regulation of the kinases Hippo/Mst1&2, their noncatalytic binding partners, Salvador and the Rassf polypeptides, and their major substrates Warts/Lats1&2, Trc/ndr1&2, Mats/Mob1 and FOXO.

Threonine-120 phosphorylation regulated by phosphoinositide-3-kinase/Akt and mammalian target of rapamycin pathway signaling limits the antitumor activity of mammalian sterile 20-like kinase 1

Mst1/Stk4, a hippo-like serine-threonine kinase, is implicated in many cancers, including prostate cancer. However, the mechanisms regulating Mst1 remain obscure. Here, we characterized the effects of phospho-Thr-120 on Mst1 in prostate cancer cells. We demonstrated that phospho-Thr-120 did not alter the nuclear localization or cleavage of Mst1 in a LNCaP or castration-resistant C4-2 prostate tumor cell model, as revealed by a mutagenesis approach. Phospho-Thr-120 appeared to be specific to cancer cells and predominantly localized in the nucleus. In contrast, phospho-Thr-183, a critical regulator of Mst1 cell death, was exclusively found in the cytoplasm. As assessed by immunohistochemistry, a similar distribution of phospho-Mst1-Thr-120/Thr-183 was also observed in a prostate cancer specimen. In addition, the blockade of PI3K signaling by a small molecule inhibitor, LY294002, increased cytoplasmic phospho-Mst1-Thr-183 without having a significant effect on nuclear phospho-Mst1-Thr-120. However, the attenuation of mammalian target of rapamycin (mTOR) activity by a selective pharmacologic inhibitor, Ku0063794 or CCI-779, caused the up-regulation of nuclear phospho-Mst1-Thr-120 without affecting cytoplasmic phospho-Mst1-Thr-183. This suggests that PI3K and mTOR pathway signaling differentially regulate phospho-Mst1-Thr-120/Thr-183. Moreover, mutagenesis and RNAi data revealed that phospho-Thr-120 resulted in C4-2 cell resistance to mTOR inhibition and reduced the Mst1 suppression of cell growth and androgen receptor-driven gene expression. Collectively, these findings indicate that phospho-Thr-120 leads to the loss of Mst1 functions, supporting cancer cell growth and survival.

The SARAH Domain of RASSF1A and Its Tumor Suppressor Function

The Ras association domain family 1A (RASSF1A) tumor suppressor encodes a Sav-RASSF-Hpo domain (SARAH), which is an interaction domain characterized by hWW45 (dSAV) and MST1/2 (dHpo). In our study, the interaction between RASSF1A and RASSF1C with MST1 and MST2 was demonstrated and it was shown that this interaction depends on the SARAH domain. SARAH domain-deleted RASSF1A had a similar growth-reducing effect as full-length RASSF1A and inhibited anchorage independent growth of the lung cancer cell lines A549 significantly. In cancer cells expressing the SARAH deleted form of RASSF1A, reduced mitotic rates (P = 0.001) with abnormal metaphases (P < 0.001) were observed and a significantly increased rate of apoptosis was found (P = 0.006) compared to full-length RASSF1A. Although the association with microtubules and their stabilization was unaffected, mitotic spindle formation was altered by deletion of the SARAH domain of RASSF1A. In summary, our results suggest that the SARAH domain plays an important role in regulating the function of RASSF1A.

Ras family small GTPase-mediated neuroprotective signaling in stroke

Selective neuronal cell death is one of the major causes of neuronal damage following stroke, and cerebral cells naturally mobilize diverse survival signaling pathways to protect against ischemia. Importantly, therapeutic strategies designed to improve endogenous anti-apoptotic signaling appear to hold great promise in stroke treatment. While a variety of complex mechanisms have been implicated in the pathogenesis of stroke, the overall mechanisms governing the balance between cell survival and death are not well-defined. Ras family small GTPases are activated following ischemic insults, and in turn, serve as intrinsic switches to regulate neuronal survival and regeneration. Their ability to integrate diverse intracellular signal transduction pathways makes them critical regulators and potential therapeutic targets for neuronal recovery after stroke. This article highlights the contribution of Ras family GTPases to neuroprotective signaling cascades, including mitogen-activated protein kinase (MAPK) family protein kinase- and AKT/PKB-dependent signaling pathways as well as the regulation of cAMP response element binding (CREB), Forkhead box O (FoxO) and hypoxiainducible factor 1(HIF1) transcription factors, in stroke.

The phenotype of human STK4 deficiency

We describe a novel clinical phenotype associating T- and B-cell lymphopenia, intermittent neutropenia, and atrial septal defects in 3 members of a consanguineous kindred. Their clinical histories included recurrent bacterial infections, viral infections, mucocutaneous candidiasis, cutaneous warts, and skin abscesses. Homozygosity mapping and candidate gene sequencing revealed a homozygous premature termination mutation in the gene STK4 (serine threonine kinase 4, formerly having the symbol MST1). STK4 is the human ortholog of Drosophila Hippo, the central constituent of a highly conserved pathway controlling cell growth and apoptosis. STK4-deficient lymphocytes and neutrophils exhibit enhanced loss of mitochondrial membrane potential and increased susceptibility to apoptosis. STK4 deficiency is a novel human primary immunodeficiency syndrome.

Histone modifiers in cancer: friends or foes?

Covalent modifications of histones can regulate all DNA-dependent processes. In the last few years, it has become more and more evident that histone modifications are key players in the regulation of chromatin states and dynamics as well as in gene expression. Therefore, histone modifications and the enzymatic machineries that set them are crucial regulators that can control cellular proliferation, differentiation, plasticity, and malignancy processes. This review discusses the biology and biochemistry of covalent histone posttranslational modifications (PTMs) and evaluates the dual role of their modifiers in cancer: as oncogenes that can initiate and amplify tumorigenesis or as tumor suppressors.

Apoptosis signaling kinases: from stress response to health outcomes

Apoptosis is a highly regulated process essential for the development and homeostasis of multicellular organisms. Whereas caspases, a large family of intracellular cysteine proteases, play central roles in the execution of apoptosis, other proapoptotic and antiapoptotic regulators such as the members of the Bcl-2 family are also critically involved in the regulation of apoptosis. A large body of evidence has revealed that a number of protein kinases are among such regulators and regulate cellular sensitivity to various proapoptotic signals at multiple steps in apoptosis. However, recent progress in the analysis of these apoptosis signaling kinases demonstrates that they generally act as crucial regulators of diverse cellular responses to a wide variety of stressors, beyond their roles in apoptosis regulation. In this review, we have cataloged apoptosis signaling kinases involved in cellular stress responses on the basis of their ability to induce apoptosis and discuss their roles in stress responses with particular emphasis on health outcomes upon their dysregulation.

Daxx mediates activation-induced cell death in microglia by triggering MST1 signalling

Microglia, the resident macrophages of the mammalian central nervous system, migrate to sites of tissue damage or infection and become activated. Although the persistent secretion of inflammatory mediators by the activated cells contributes to the pathogenesis of various neurological disorders, most activated microglia eventually undergo apoptosis through the process of activation-induced cell death (AICD). The molecular mechanism of AICD, however, has remained unclear. Here, we show that Daxx and mammalian Ste20-like kinase-1 (MST1) mediate apoptosis elicited by interferon-γ (IFN-γ) in microglia. IFN-γ upregulated the expression of Daxx, which in turn mediated the homodimerization, activation, and nuclear translocation of MST1 and apoptosis in microglial cells. Depletion of Daxx or MST1 by RNA interference also attenuated IFN-γ-induced cell death in primary rat microglia. Furthermore, the extent of IFN-γ-induced death of microglia in the brain of MST1-null mice was significantly reduced compared with that apparent in wild-type mice. Our results thus highlight new functions of Daxx and MST1 that they are the key mediators of microglial cell death initiated by the proinflammatory cytokine IFN-γ.

MST1 is a multifunctional caspase-independent inhibitor of androgenic signaling

The MST1 serine-threonine kinase, a component of the RASSF1-LATS tumor suppressor network, is involved in cell proliferation and apoptosis and has been implicated in cancer. However, the physiologic role of MST1 in prostate cancer (PCa) is not well understood. Here, we investigated the possibility of a biochemical and functional link between androgen receptor (AR) and MST1 signaling. We showed that MST1 forms a protein complex with AR and antagonizes AR transcriptional activity as shown by coimmunoprecipitation (co-IP), promoter reporter analysis, and molecular genetic methods. In vitro kinase and site-specific mutagenesis approaches indicate that MST1 is a potent AR kinase; however, the kinase activity of MST1 and its proapoptotic functions were shown not to be involved in inhibition of AR. MST1 was also found in AR-chromatin complexes, and enforced expression of MST1 reduced the binding of AR to a well-characterized, androgen-responsive region within the prostate-specific antigen promoter. MST1 suppressed PCa cell growth in vitro and tumor growth in mice. Because MST1 is also involved in regulating the AKT1 pathway, this kinase may be an important new link between androgenic and growth factor signaling and a novel therapeutic target in PCa.

Functional PAK-2 knockout and replacement with a caspase cleavage-deficient mutant in mice reveals differential requirements of full-length PAK-2 and caspase-activated PAK-2p34

p21-Activated protein kinase 2 (PAK-2) has both anti- and pro-apoptotic functions depending on its mechanism of activation. Activation of full-length PAK-2 by the monomeric GTPases Cdc42 or Rac stimulates cell survival, whereas caspase activation of PAK-2 to the PAK-2p34 fragment is involved in the apoptotic response. In this study we use functional knockout of PAK-2 and gene replacement with the caspase cleavage-deficient PAK-2D212N mutant to differentiate the biological functions of full-length PAK-2 and caspase-activated PAK-2p34. Knockout of PAK-2 results in embryonic lethality at early stages before organ development, whereas replacement with the caspase cleavage-deficient PAK-2D212N results in viable and healthy mice, indicating that early embryonic lethality is caused by deficiency of full-length PAK-2 rather than lack of caspase activation to the PAK-2p34 fragment. However, deficiency of caspase activation of PAK-2 decreased spontaneous cell death of primary mouse embryonic fibroblasts and increased cell growth at high cell density. In contrast, stress-induced cell death by treatment with the anti-cancer drug cisplatin was not reduced by deficiency of caspase activation of PAK-2, but switched from an apoptotic to a nonapoptotic, caspase-independent mechanism. Homozygous PAK-2D212N primary mouse embryonic fibroblasts that lack the ability to generate the proapoptotic PAK-2p34 show less activation of the effector caspase 3, 6, and 7, indicating that caspase activation of PAK-2 amplifies the apoptotic response through a positive feedback loop resulting in more activation of effector caspases.

Mammalian Hippo pathway: from development to cancer and beyond

The Hippo pathway was discovered as a signal transduction pathway that regulates organ size in Drosophila melanogaster. It is composed of three components: cell surface upstream regulators including cell adhesion molecules and cell polarity complexes; a kinase cascade comprising two serine-threonine kinases with regulators and adaptors; and a downstream target, a transcription coactivator. The coactivator mediates the transcription of cell proliferation-promoting and anti-apoptotic genes. The pathway negatively regulates the coactivator to restrict cell proliferation and to promote cell death. Thus, the pathway prevents tissue overgrowth and tumourigenesis. The framework of the pathway is conserved in mammals. A dysfunction of the pathway is frequently detected in human cancers and correlates with a poor prognosis. Recent works indicated that the Hippo pathway plays an important role in tissue homoeostasis through the regulation of stem cells, cell differentiation and tissue regeneration.

Mammalian MST2 kinase and human Salvador activate and reduce estrogen receptor alpha in the absence of ligand

Mammalian MST2 kinase plays an important role in cell proliferation, survival, and apoptosis. In search of interacting proteins of MST2, we found that estrogen receptor α (ERα) co-immunoprecipitates with MST2 and its adaptor protein human Salvador (hSAV). Using reporter assays, we observed that overexpression of MST2 and hSAV leads to ligand-independent activation of ERα in human breast cancer MCF-7 cells, which was attenuated by the knockdown of hSAV. Furthermore, using truncated mutants of hSAV, we observed that the C terminus of hSAV is necessary and sufficient for the induction of ERα transactivation. The expression of hSAV and MST2 results in the phosphorylation of ERα at serine residues 118 and 167 and represses ERα expression. We then investigated the incidence of MST2 and ERα expression with other tumor biomarkers using commercially available tissue microarrays. Among 40 breast cancer samples analyzed, 60% (24 out of 40) expressed MST2. Nineteen among the 40 cases were MST2-positive and ERα-negative, implying a correlation between expressions of MST2 with loss of ERα in breast tumor samples. This study suggests that MST and hSAV act as novel co-regulators of ERα and may play an important role in breast cancer pathogenesis.

Mst1/2 signalling to Yap: gatekeeper for liver size and tumour development

The mechanisms controlling mammalian organ size have long been a source of fascination for biologists. These controls are needed to both ensure the integrity of the body plan and to restrict inappropriate proliferation that could lead to cancer. Regulation of liver size is of particular interest inasmuch as this organ maintains the capacity for regeneration throughout life, and is able to regain precisely its original mass after partial surgical resection. Recent studies using genetically engineered mouse strains have shed new light on this problem; the Hippo signalling pathway, first elucidated as a regulator of organ size in Drosophila, has been identified as dominant determinant of liver growth. Defects in this pathway in mouse liver lead to sustained liver overgrowth and the eventual development of both major types of liver cancer, hepatocellular carcinoma and cholangiocarcinoma. In this review, we discuss the role of Hippo signalling in liver biology and the contribution of this pathway to liver cancer in humans.

MST1 promotes apoptosis through phosphorylation of histone H2AX

MST1 (mammalian STE20-like kinase 1) is a serine/threonine kinase that is cleaved and activated by caspases during apoptosis. Overexpression of MST1 induces apoptotic morphological changes such as chromatin condensation, but the mechanism is not clear. Here we show that MST1 induces apoptotic chromatin condensation through its phosphorylation of histone H2AX at Ser-139. During etoposide-induced apoptosis in Jurkat cells, the cleavage of MST1 directly corresponded with strong H2AX phosphorylation. In vitro kinase assay results showed that MST1 strongly phosphorylates histone H2AX. Western blot and kinase assay results with a mutant S139A H2AX confirmed that MST1 phosphorylates H2AX at Ser-139. Direct binding of MST1 and H2AX can be detected when co-expressed in HEK293 cells and was also confirmed by an endogenous immunoprecipitation study. When overexpressed in HeLa cells, both the MST1 full-length protein and the MST1 kinase domain (MST1-NT), but not the kinase-negative mutant (MST1-NT-KN), could induce obvious endogenous histone H2AX phosphorylation. The caspase-3 inhibitor benzyloxycarbonyl-DEVD-fluoromethyl ketone (Z-DEVD-fmk) attenuates phosphorylation of H2AX by MST1 but cannot inhibit MST1-NT-induced histone H2AX phosphorylation, indicating that cleaved MST1 is responsible for H2AX phosphorylation during apoptosis. Histone H2AX phosphorylation and DNA fragmentation were suppressed in MST1 knockdown Jurkat cells after etoposide treatment. Taken together, our data indicated that H2AX is a substrate of MST1, which functions to induce apoptotic chromatin condensation and DNA fragmentation.

Tumor suppressor ras association domain family 5 (RASSF5/NORE1) mediates death receptor ligand-induced apoptosis

Epigenetic silencing of RASSF (Ras association domain family) genes RASSF1 and RASSF5 (also called NORE1) by CpG hypermethylation is found frequently in many cancers. Although the physiological roles of RASSF1 have been studied in some detail, the exact functions of RASSF5 are not well understood. Here, we show that RASSF5 plays an important role in mediating apoptosis in response to death receptor ligands, TNF-α and TNF-related apoptosis-inducing ligand. Depletion of RASSF5 by siRNA significantly reduced TNF-α-mediated apoptosis, likely through its interaction with proapoptotic kinase MST1, a mammalian homolog of Hippo. Consistent with this, siRNA knockdown of MST1 also resulted in resistance to TNF-α-induced apoptosis. To further study the role of Rassf5 in vivo, we generated Rassf5-deficient mouse. Inactivation of Rassf5 in mouse embryonic fibroblasts (MEFs) resulted in resistance to TNF-α- and TNF-related apoptosis-inducing ligand-mediated apoptosis. Importantly, Rassf5-null mice were significantly more resistant to TNF-α-induced apoptosis and failed to activate Mst1. Loss of Rassf5 also resulted in spontaneous immortalization of MEFs at earlier passages than the control MEFs, and Rassf5-null immortalized MEFs, but not the immortalized wild type MEFs, were fully transformed by K-RasG12V. Together, our results demonstrate a direct role for RASSF5 in death receptor ligand-mediated apoptosis and provide further evidence for RASSF5 as a tumor suppressor.

Structural comparison of human mammalian ste20-like kinases

BACKGROUND

The serine/threonine mammalian Ste-20 like kinases (MSTs) are key regulators of apoptosis, cellular proliferation as well as polarization. Deregulation of MSTs has been associated with disease progression in prostate and colorectal cancer. The four human MSTs are regulated differently by C-terminal regions flanking the catalytic domains.

PRINCIPAL FINDINGS

We have determined the crystal structure of kinase domain of MST4 in complex with an ATP-mimetic inhibitor. This is the first structure of an inactive conformation of a member of the MST kinase family. Comparison with active structures of MST3 and MST1 revealed a dimeric association of MST4 suggesting an activation loop exchanged mechanism of MST4 auto-activation. Together with a homology model of MST2 we provide a comparative analysis of the kinase domains for all four members of the human MST family.

SIGNIFICANCE

The comparative analysis identified new structural features in the MST ATP binding pocket and has also defined the mechanism for autophosphorylation. Both structural features may be further explored for inhibitors design.

ENHANCED VERSION

This article can also be viewed as an enhanced version in which the text of the article is integrated with interactive 3D representations and animated transitions. Please note that a web plugin is required to access this enhanced functionality. Instructions for the installation and use of the web plugin are available in Text S1.

The life span of short-lived plasma cells is partly determined by a block on activation of apoptotic caspases acting in combination with endoplasmic reticulum stress

Apoptosis of short-lived plasma cells after a few days of intense immunoglobulin secretion is critical for maintaining a controlled humoral immune response. The mechanisms that regulate this process are poorly understood. Here we report that the key apoptotic caspases, caspase-3 and caspase-9, become resistant to activation by apoptotic stimuli when B cells differentiate into short-lived plasma cells. As a consequence, apoptosis of most short-lived plasma cells in vitro and in vivo is effector caspase-independent. We also show that a triaspartic acid repeat that normally prevents activation of caspase-3 becomes stabilized in short-lived plasma cells and myeloma cell lines. The block on caspase activation occurs before the accumulation of intracellular immunoglobulins and a progressive rise in secretory stress in the endoplasmic reticulum (ER). Plasma cells show increased susceptibility to ER stress-induced apoptosis and activate the ER-associated caspase-12, which is required specifically for nuclear apoptotic events. In nonlymphoid cells that cannot activate effector caspases, programmed cell death is delayed in response to ER stress. These observations suggest that the block on activation of key apoptotic caspases has evolved in short-lived plasma cells to prolong survival under conditions of ER stress resulting from high-level immunoglobulin secretion.

Cracking the death code: apoptosis-related histone modifications

The degradation and compaction of chromatin are long-standing hallmark features of apoptosis. The histones, chief protein components of chromatin, are subjected to a wide range of post-translational modifications. An increasing body of evidence suggests that combinations of epigenetic histone modifications influence the overall chromatin structure and have clear functional consequences in cellular processes including apoptosis. This review describes the work to date on the post-translational modification of histones during apoptosis, their regulation by enzymatic complexes and discusses the existence of the apoptotic histone code.

The MST1 and hMOB1 tumor suppressors control human centrosome duplication by regulating NDR kinase phosphorylation

BACKGROUND

Human MST/hSAV/LATS/hMOB tumor suppressor cascades are regulators of cell death and proliferation; however, little is known about other functions of MST/hMOB signaling. Mob1p, one of two MOB proteins in yeast, appears to play a role in spindle pole body duplication (the equivalent of mammalian centrosome duplication). We therefore investigated the role of human MOB proteins in centrosome duplication. We also addressed the regulation of human centrosome duplication by mammalian serine/threonine Ste20-like (MST) kinases, considering that MOB proteins can function together with Ste20-like kinases in eukaryotes.

RESULTS

By studying the six human MOB proteins and five MST kinases, we found that MST1/hMOB1 signaling controls centrosome duplication. Overexpression of hMOB1 caused centrosome overduplication, whereas RNAi depletion of hMOB1 or MST1 impaired centriole duplication. Significantly, we delineated an hMOB1/MST1/NDR1 signaling pathway regulating centrosome duplication. More specifically, analysis of shRNA-resistant hMOB1 and NDR1 mutants revealed that a functional NDR/hMOB1 complex is critical for MST1 to phosphorylate NDR on the hydrophobic motif that in turn is required for human centrosome duplication. Furthermore, shRNA-resistant MST1 variants revealed that MST1 kinase activity is crucial for centrosome duplication whereas MST1 binding to the hSAV and RASSF1A tumor suppressor proteins is dispensable. Finally, by studying the PLK4/HsSAS-6/CP110 centriole assembly machinery, we also observed that normal daughter centriole formation depends on intact MST1/hMOB1/NDR signaling, although HsSAS-6 centriolar localization is not affected.

CONCLUSIONS

Our observations propose a novel pathway in control of human centriole duplication after recruitment of HsSAS-6 to centrioles.

Nuclear entry of active caspase-3 is facilitated by its p3-recognition-based specific cleavage activity

As a critical apoptosis executioner, caspase-3 becomes activated and then enters into the nucleus to exert its function. However, the molecular mechanism of this nuclear entry of active caspase-3 is still unknown. In this study, we revealed that caspase-3 harbors a crm-1-independent nuclear export signal (NES) in its small subunit. Using reverse-caspase-3 as the study model, we found that the function of the NES in caspase-3 was not disturbed by the conformational changes during induced caspase-3 activation. Mutations disrupting the cleavage activity or p3-recognition site resulted in a defect in the nuclear entry of active caspase-3. We provide evidence that the p3-mediated specific cleavage activity of active caspase-3 abrogated the function of the NES. In conclusion, our results demonstrate that during caspase-3 activation, NES is constitutively present. p3-mediated specific cleavage activity abrogates the NES function in caspase-3, thus facilitating the nuclear entry of active caspase-3.

Proapoptotic kinase MST2 coordinates signaling crosstalk between RASSF1A, Raf-1, and Akt

Mammalian MST kinases function in stress-induced apoptosis to limit tumor progression. However, there is limited understanding about MST2 control by key regulators of cell division and survival. Raf-1 binds and inhibits MST2 kinase, whereas dissociation from Raf-1 and binding to tumor suppressor protein RASSF1A activates MST2. Akt phosphorylates MST2 in response to mitogens, oncogenic Ras, or depletion of tumor suppressor phosphatase and tensin homologue deleted on chromosome 10. We identified T117 and T384 as Akt phosphorylation sites in MST2. Mutation of these sites inhibited MST2 binding to Raf-1 kinase but enhanced binding to tumor suppressor RASSF1A, accentuating downstream c-Jun NH(2)-terminal kinase and p38 mitogen-activated protein kinase signaling and promoting apoptosis. We determined that MST2 phosphorylation by Akt limits MST2 activity in two ways: first, by blocking its binding to RASSF1A and by promoting its association into the Raf-1 inhibitory complex, and second, by preventing homodimerization of MST2, which is needed for its activation. Dissociation of the Raf-1-MST2 complex promoted mitogenic signaling and coordinately licensed apoptotic risk. Using Ras effector domain mutants, we found that Akt is essential to prevent MST2 activation after mitogenic stimulation. Our findings elucidate how MST2 serves as a hub to integrate biological outputs of the Raf-1 and Akt pathways.

Reciprocal epigenetic modification of histone H2B occurs in chromatin during apoptosis in vitro and in vivo

Histone H2B phosphorylation at Serine 14 (phosS14) has been proposed as an epigenetic marker of apoptotic cells whereas acetylation at the adjacent Lysine 15 (acK15) is a property of non-dying cells. We investigated the relationship and the potential regulatory mechanisms between these two epigenetic histone modifications and internucleosomal DNA degradation during apoptosis. Using rat primary thymocytes induced to undergo apoptosis with glucocorticoids we found that H2B phosphorylated at Ser14 was associated with soluble, cleaved DNA in apoptotic nuclei. In contrast acK15 was prevalent in non-apoptotic nuclei and scarce in apoptotic nuclei. This switch between K15 acetylation and S14 phosphorylation on H2B was also observed in apoptotic thymocytes from animals treated in vivo with glucocorticoids and in a rat hepatoma cell line (HTC) induced to die by UV-C or Fas ligand. Interestingly the combined use of a histone deacetylase inhibitor and glucocorticoid suppressed both S14 phosphorylation and internucleosomal DNA degradation without inhibiting apoptosis in thymocytes. Using synthetic peptides and a PKC phosphorylation assay system, we show that the deacetylation of K15 was necessary to allow the S14 phosphorylation. These findings suggest that selective chromatin post-translational modifications are associated with DNA degradation during apoptosis.

Mst1 and Mst2 maintain hepatocyte quiescence and suppress hepatocellular carcinoma development through inactivation of the Yap1 oncogene

Hippo-Lats-Yorkie signaling regulates tissue overgrowth and tumorigenesis in Drosophila. We show that the Mst1 and Mst2 protein kinases, the mammalian Hippo orthologs, are cleaved and constitutively activated in the mouse liver. Combined Mst1/2 deficiency in the liver results in loss of inhibitory Ser127 phosphorylation of the Yorkie ortholog, Yap1, massive overgrowth, and hepatocellular carcinoma (HCC). Reexpression of Mst1 in HCC-derived cell lines promotes Yap1 Ser127 phosphorylation and inactivation and abrogates their tumorigenicity. Notably, Mst1/2 inactivates Yap1 in liver through an intermediary kinase distinct from Lats1/2. Approximately 30% of human HCCs show low Yap1(Ser127) phosphorylation and a majority exhibit loss of cleaved, activated Mst1. Mst1/2 inhibition of Yap1 is an important pathway for tumor suppression in liver relevant to human HCC.

Caspases and kinases in a death grip

The complex process of apoptosis is orchestrated by caspases, a family of cysteine proteases with unique substrate specificities. Accumulating evidence suggests that cell death pathways are finely tuned by multiple signaling events, including direct phosphorylation of caspases, whereas kinases are often substrates of active caspases. Importantly, caspase-mediated cleavage of kinases can terminate prosurvival signaling or generate proapoptotic peptide fragments that help to execute the death program and facilitate packaging of the dying cells. Here, we review caspases as kinase substrates and kinases as caspase substrates and discuss how the balance between cell survival and cell death can be shifted through crosstalk between these two enzyme families.

Crucial role for Mst1 and Mst2 kinases in early embryonic development of the mouse

Mammalian sterile 20-like kinases 1 and 2 (Mst1 and Mst2, respectively) are potent serine/threonine kinases that are involved in cell proliferation and cell death. To investigate the physiological functions of Mst1 and Mst2, we generated Mst1 and Mst2 mutant mice. Mst1(-/-) and Mst2(-/-) mice were viable and fertile and developed normally, suggesting possible functional overlaps between the two genes. A characterization of heterozygous and homozygous combinations of Mst1 and Mst2 mutant mice showed that mice containing a single copy of either gene underwent normal organ development; however, Mst1(-/-); Mst2(-/-) mice lacking both Mst1 and Mst2 genes started dying in utero at approximately embryonic day 8.5. Mst1(-/-); Mst2(-/-) mice exhibited severe growth retardation, failed placental development, impaired yolk sac/embryo vascular patterning and primitive hematopoiesis, increased apoptosis in placentas and embryos, and disorganized proliferating cells in the embryo proper. These findings indicate that both Mst1 and Mst2 kinases play essential roles in early mouse development, regulating placental development, vascular patterning, primitive hematopoiesis, and cell proliferation and survival.

The DeMSTification of mammalian Ste20 kinases

When first reported in 1995, the mammalian Ste20-like kinases (Mst) 1 and 2 were so named both for their similarity to the yeast kinase Ste20 and for the fact that their function was, to us, a deep mystery. While much remains to be explained about the regulation and role of these kinases, the veil has been at least partly raised on the Msts, revealing unexpected modes of activation and function. Work in model organisms suggests a central growth-suppressive role for Mst orthologs, with intriguing possible links to other established tumor suppressors. This minireview underlines our current understanding of how Mst1 and Mst2 are regulated, and how activation of these proteins influences cell survival and proliferation.

RASSF2 associates with and stabilizes the proapoptotic kinase MST2

RASSF2 is a tumour suppressor that in common with the rest of the RASSF family contains Ras association and SARAH domains. We identified the proapoptotic kinases, MST1 and MST2, as the most significant binding partners of RASSF2, confirmed the interactions at endogenous levels and showed that RASSF2 immunoprecipitates active MST1/2. We then showed that RASSF2 can be phosphorylated by a co-immunoprecipitating kinase that is likely to be MST1/2. Furthermore, we showed that RASSF2 and MST2 do indeed colocalize, but whereas RASSF2 alone is nuclear, the presence of MST1 or MST2 results in colocalization in the cytoplasm. Expression of RASSF2 (stably in MCF7 or transiently in HEK-293) increases MST2 levels and knockdown of RASSF2 in HEK-293 cells reduces MST2 levels, in addition colorectal tumour cell lines and primary tumours with low RASSF2 levels show decreased MST2 protein levels. This is likely to be mediated by RASSF2-dependent protection of MST2 against proteolytic degradation. Our findings suggest that MST2 and RASSF2 form an active complex in vivo, in which RASSF2 is maintained in a phosphorylated state and protects MST2 from degradation and turnover. Thus, we propose that the frequent loss of RASSF2 in tumours results in the destablization of MST2 and thus decreased apoptotic potential.

The human WW45 protein enhances MST1-mediated apoptosis in vivo

Mammalian sterile 20-like kinase 1 (MST1) is a serine/threonine protein kinase that is activated in response to a variety of apoptotic stimuli and causes apoptosis when over-expressed in mammalian cells. The physiological regulation and cellular targets of MST1 are not well understood. Using a yeast two-hybrid system, we identified human WW45 (hWW45, also called hSav1) as an MST1-binding protein. The association between the two proteins was confirmed by immunofluorescence and co-immunoprecipitation, and hWW45 was present in both the cytoplasm and nucleus. When hWW45 alone was over-expressed, it weakly induced apoptosis. However, hWW45 augmented MST1-induced apoptosis when the two were co-expressed. Conversely, RNA interference-mediated depletion of endogenous hWW45 suppressed MST1-induced apoptosis. These results indicate that hWW45 is required to enhance MST1-mediated apoptosis in vivo and thus is a critical player in an MST1-driven cell death signaling pathway.

Phosphorylation of cardiac troponin I by mammalian sterile 20-like kinase 1

Mst1 (mammalian sterile 20-like kinase 1) is a ubiquitously expressed serine/threonine kinase and its activation in the heart causes cardiomyocyte apoptosis and dilated cardiomyopathy. Its myocardial substrates, however, remain unknown. In a yeast two-hybrid screen of a human heart cDNA library with a dominant-negative Mst1 (K59R) mutant used as bait, cTn [cardiac Tn (troponin)] I was identified as an Mst1-interacting protein. The interaction of cTnI with Mst1 was confirmed by co-immunoprecipitation in both co-transfected HEK-293 cells (human embryonic kidney cells) and native cardiomyocytes, in which cTnI interacted with full-length Mst1, but not with its N-terminal kinase fragment. in vitro phosphorylation assays demonstrated that cTnI is a sensitive substrate for Mst1. In contrast, cTnT was phosphorylated by Mst1 only when it was incorporated into the Tn complex. MS analysis indicated that Mst1 phosphorylates cTnI at Thr(31), Thr(51), Thr(129) and Thr(143). Substitution of Thr(31) with an alanine residue reduced Mst1-mediated cTnI phosphorylation by 90%, whereas replacement of Thr(51), Thr(129) or Thr(143) with alanine residues reduced Mst1-catalysed cTnI phosphorylation by approx. 60%, suggesting that Thr(31) is a preferential phosphorylation site for Mst1. Furthermore, treatment of cardiomyocytes with hydrogen peroxide rapidly induced Mst1-dependent phosphorylation of cTnI at Thr(31). Protein epitope analysis and binding assays showed that Mst1-mediated phosphorylation modulates the molecular conformation of cTnI and its binding affinity to TnT and TnC, thus indicating functional significances. The results of the present study suggest that Mst1 is a novel mediator of cTnI phosphorylation in the heart and may contribute to the modulation of myofilament function under a variety of physiological and pathophysiological conditions.

Apoptotic histone modification inhibits nuclear transport by regulating RCC1

A number of signalling pathways have been identified that regulate apoptosis, but the mechanism that initiates apoptosis remains incompletely understood. We have found that the nuclear RanGTP level is diminished during the early stages of apoptosis, which correlates with immobilization of RCC1 on the chromosomes. Furthermore, the expression of phosphomimetic histone H2B or caspase-activated Mst1 immobilizes RCC1 and causes reduction of nuclear RanGTP levels, which leads to inactivation of the nuclear transport machinery. As a consequence, nuclear localization signal (NLS)-containing proteins, including NF-kappaB-p65, remain bound to importins alpha and beta in the cytoplasm. Knocking down Mst1 allows resumption of nuclear transport and the nuclear entry of NF-kappaB-p65, which have important roles in rescuing cells from apoptosis. Therefore, we propose that RCC1 reads the histone code created by caspase-activated Mst1 to initiate apoptosis by reducing the level of RanGTP in the nucleus.

Characterization of two Mst1-deficient mouse models

Mammalian sterile 20-like kinase 1 (Mst1) is a ubiquitously expressed serine/threonine kinase belonging to the family of Sterile 20-like kinases. MST1 has been inferred to play important roles in apoptosis and in the inhibition of proliferation in mammalian cells. Here, we describe the genetic characterization of Mst1-deficient mice produced by two distinct gene-trap insertions. Animals generated from clone RRT293 exhibit transmission ratio distortion favoring the mutated allele which is amplified with each generation. Inexplicably, while the mutated allele is favored for transmission, its homozygosity is embryonic lethal. By contrast, animals generated from the second Mst1 gene-trap clone, AJ0315, do not show any gross abnormalities. We find that the discrepancy in phenotype is most likely attributable to a second insertion in the RRT293 clone. Thus, a mutation in Mst1 alone does not affect survival. Our results set the stage for identification of the lethal second-site mutation that is paradoxically favored for transmission.

SOK1 translocates from the Golgi to the nucleus upon chemical anoxia and induces apoptotic cell death

SOK1 is a Ste20 protein kinase of the germinal center kinase (GCK) family that has been shown to be activated by oxidant stress and chemical anoxia, a cell culture model of ischemia. More recently, it has been shown to be localized to the Golgi apparatus, where it functions in a signaling pathway required for cell migration and polarization. Herein, we demonstrate that SOK1 regulates cell death after chemical anoxia, as its down-regulation by RNA interference enhances cell survival. Furthermore, expression of SOK1 elicits apoptotic cell death by activating the intrinsic pathway. We also find that a cleaved form of SOK1 translocates from the Golgi to the nucleus after chemical anoxia and that this translocation is dependent on both caspase activity and on amino acids 275-292, located immediately C-terminal to the SOK1 kinase domain. Furthermore, SOK1 entry into the nucleus is important for the cell death response since SOK1 mutants unable to enter the nucleus do not induce cell death. In summary, SOK1 is necessary to induce cell death and can induce death when overexpressed. Furthermore, SOK1 appears to play distinctly different roles in stressed versus non-stressed cells, regulating cell death in the former.

Apoptosis: controlled demolition at the cellular level

Apoptosis is characterized by a series of dramatic perturbations to the cellular architecture that contribute not only to cell death, but also prepare cells for removal by phagocytes and prevent unwanted immune responses. Much of what happens during the demolition phase of apoptosis is orchestrated by members of the caspase family of cysteine proteases. These proteases target several hundred proteins for restricted proteolysis in a controlled manner that minimizes damage and disruption to neighbouring cells and avoids the release of immunostimulatory molecules.

The pro-apoptotic kinase Mst1 and its caspase cleavage products are direct inhibitors of Akt1

Akt kinases mediate cell growth and survival. Here, we report that a pro-apoptotic kinase, Mst1/STK4, is a physiological Akt1 interaction partner. Mst1 was identified as a component of an Akt1 multiprotein complex isolated from lipid raft-enriched fractions of LNCaP human prostate cancer cells. Endogenous Mst1, along with its paralog, Mst2, acted as inhibitors of endogenous Akt1. Surprisingly, mature Mst1 as well as both of its caspase cleavage products, which localize to distinct subcellular compartments and are not structurally homologous, complexed with and inhibited Akt1. cRNAs encoding full-length Mst1, and N- and C-terminal caspase Mst1 cleavage products, reverted an early lethal phenotype in zebrafish development induced by expression of membrane-targeted Akt1. Mst1 and Akt1 localized to identical subcellular sites in human prostate tumors. Mst1 levels declined with progression from clinically localized to hormone refractory disease, coinciding with an increase in Akt activation with transition from hormone naïve to hormone-resistant metastases. These results position Mst1/2 within a novel branch of the phosphoinositide 3-kinase/Akt pathway and suggest an important role in cancer progression.

Akt phosphorylates MstI and prevents its proteolytic activation, blocking FOXO3 phosphorylation and nuclear translocation

Oxidative stress can induce apoptosis through activation of MstI, subsequent phosphorylation of FOXO and nuclear translocation. MstI is a common component of apoptosis initiated by various stresses. MstI kinase activation requires autophosphorylation and proteolytic degradation by caspases. The role of Akt in regulating MstI activity has not been previously examined. Here, we show that MstI is a physiological substrate of Akt. Akt phosphorylation of MstI diminishes its apoptotic cleavage by caspases and prevents its kinase activity on FOXO3. MstI directly binds to Akt, which is regulated Akt kinase activity. Akt phosphorylates MstI on the Thr(387) residue and protects MstI from apoptotic cleavage in vitro and in apoptotic cells. Interestingly, Akt phosphorylation of MstI strongly inhibits its kinase activity on FOXO3. The phosphorylation mimetic mutant MST1 T387E blocks H2O2-triggered FOXO3 nuclear translocation and apoptosis. Thus, our findings support that Akt blocks MstI-triggered FOXO3 nuclear translocation by phosphorylating MstI, promoting cell survival.

Acinus-provoked protein kinase C δ isoform activation is essential for apoptotic chromatin condensation

Histone H2B phosphorylation tightly correlates with chromatin condensation during apoptosis. The caspase-cleaved acinus (apoptotic chromatin condensation inducer in the nucleus) provokes chromatin condensation in the nucleus, but the molecular mechanism accounting for this effect remains elusive. Here, we report that the active acinus p17 fragment initiates H2B phosphorylation and chromatin condensation by activating protein kinase C delta isoform (PKC-delta). We show that p17 binds to both Mst1 and PKC-delta, which is upregulated by apoptotic stimuli, enhancing their kinase activities. Acinus mutant susceptible to degradation elicits stronger chromatin condensation and higher H2B phosphorylation than wild-type acinus. Dominant-negative PKC-delta but not Mst1 robustly blocks acinus-initiated H2B phosphorylation. Surprisingly, depletion of Mst1 triggers caspase-3 activation, provoking H2B phosphorylation through activating PKC-delta. Further, acinus-elicited H2B phosphorylation and chromatin condensation are abrogated in PKC-delta-deficient mouse embryonic fibroblast cells and siRNA-knocked down PC12 cells. Thus, PKC-delta but not Mst1 acts as a physiological downstream kinase of acinus in promoting H2B phosphorylation and chromatin condensation.

Activation of the c-Jun N-terminal kinase pathway by MST1 is essential and sufficient for the induction of chromatin condensation during apoptosis

Chromatin condensation is the most recognizable nuclear hallmark of apoptosis. Cleavage and activation of MST1 by caspases induce chromatin condensation. It was previously reported that, during apoptosis, activated MST1 induced c-Jun N-terminal kinase (JNK) activation and also phosphorylated histone H2B. However, which of these mechanisms underlies MST1's induction of chromatin condensation has yet to be clarified. Here, we report that MST1-mediated activation of JNK is both essential and sufficient for chromatin condensation. MST1 activation did not result in chromatin condensation in mitogen-activate protein kinase kinase 4 (MKK4)/MKK7 double knockout (MKK4/7 DKO) embryonic stem (ES) cells, which genetically lack the ability to activate JNK. On the other hand, constitutively active JNK was able to induce chromatin condensation in MKK4/7 DKO ES cells. In contrast, histone H2B phosphorylation did not correlate with chromatin condensation in wild-type ES cells. Finally, inhibition of JNK as well as inhibitor of caspase-activated DNase blocked chromatin condensation during Fas-mediated apoptosis of Jurkat cells. Taken together, our results indicate that caspase-mediated cleavage of MST1, followed by MST1-mediated activation of the JNK pathway, is the mechanism responsible for inducing chromatin condensation during apoptosis.

Structural insight into dimeric interaction of the SARAH domains from Mst1 and RASSF family proteins in the apoptosis pathway

In eukaryotic cells, apoptosis and cell cycle arrest by the Ras --> RASSF --> MST pathway are controlled by the interaction of SARAH (for Salvador/Rassf/Hippo) domains in the C-terminal part of tumor suppressor proteins. The Mst1 SARAH domain interacts with its homologous domain of Rassf1 and Rassf5 (also known as Nore1) by forming a heterodimer that mediates the apoptosis process. Here, we describe the homodimeric structure of the human Mst1 SARAH domain and its heterotypic interaction with the Rassf5 and Salvador (Sav) SARAH domain. The Mst1 SARAH structure forms a homodimer containing two helices per monomer. An antiparallel arrangement of the long alpha-helices (h2/h2') provides an elongated binding interface between the two monomers, and the short 3(10) helices (h1/h1') are folded toward that of the other monomer. Chemical shift perturbation experiments identified an elongated, tight-binding interface with the Rassf5 SARAH domain and a 1:1 heterodimer formation. The linker region between the kinase and the SARAH domain is shown to be disordered in the free protein. These results imply a novel mode of interaction with RASSF family proteins and provide insight into the mechanism of apoptosis control by the SARAH domain.

Commuting (to) suicide: an update on nucleocytoplasmic transport in apoptosis

Commuting is the process of travelling between a place of residence and a place of work. In the context of biology, this expression evokes the continuous movement of macromolecules between different compartments of a eukaryotic cell. Transport in and out of the nucleus is a major example of intracellular commuting. This article discusses recent findings that substantiate the emerging link between nucleocytoplasmic transport and the signalling and execution of cell death.

Activity-dependent AIDA-1 nuclear signaling regulates nucleolar numbers and protein synthesis in neurons

Neuronal development, plasticity and survival require activity-dependent synapse-to-nucleus signaling. Most studies implicate an activity-dependent regulation of gene expression in this phenomenon. However, little is known about other nuclear functions that are regulated by synaptic activity. Here we show that a newly identified component of rat postsynaptic densities (PSDs), AIDA-1d, can regulate global protein synthesis by altering nucleolar numbers. AIDA-1d binds to the first two postsynaptic density-95/Discs large/zona occludens-1 (PDZ) domains of the scaffolding protein PSD-95 via its C-terminal three amino acids. Stimulation of NMDA receptors (NMDARs), which are also bound to PSD-95, results in a Ca2+-independent translocation of AIDA-1d to the nucleus, where it couples to Cajal bodies and induces Cajal body-nucleolar association. Long-term neuronal stimulation results in an AIDA-1-dependent increase in nucleolar numbers and protein synthesis. We propose that AIDA-1d mediates a link between synaptic activity and control of protein biosynthetic capacity by regulating nucleolar assembly.

Prognostic significance of mammalian sterile20-like kinase 1 in colorectal cancer

Mammalian Sterile20-like kinase 1 (Mst1) is a member of the yeast Ste20-related kinase family known to be involved in the process of apoptosis initiated by a variety of stimuli. The aim of this study was to determine the prognostic significance of Mst1 expression in colorectal cancer. A series of 1197 mismatch-repair-proficient colorectal cancers retrieved from a tissue microarray were randomized into two study groups. On the first group (n=599) receiver operating characteristic curves were used to determine the most clinically useful cutoffs to describe Mst1 expression with respect to T stage, N stage, tumor grade, vascular invasion and overall survival. The association of Mst1 expression and each outcome was immunohistochemically evaluated on the second study group (n=598) as well as on a third study group comprising 141 mismatch-repair-deficient colorectal cancers. A univariate analysis in the second study group showed that loss of cytoplasmic Mst1 was associated with higher T stage (P=0.001), higher N stage (P=0.029), vascular invasion (P=0.017) and overall survival (P=0.014). Nuclear Mst1 expression was not significantly associated with N stage, T stage or vascular invasion but was associated with tumor grade. In mismatch-repair-deficient colorectal cancers, loss of cytoplasmic Mst1 was associated with higher N stage (P=0.019) and shortened survival (P=0.0001). In a multivariate analysis, loss of cytoplasmic Mst1 was an independent adverse prognostic factor in this group of patients. Methylation analysis on 32 cases showed that the loss of cytoplasmic Mst1 expression is not likely due to promoter methylation. In summary, loss of cytoplasmic Mst1 expression is a marker of tumor progression in mismatch-repair-proficient as well as mismatch-repair-deficient colorectal cancers. These results are suggestive of a tumor suppressor role for Mst1 in human colorectal cancer.

Association of mammalian sterile twenty kinases, Mst1 and Mst2, with hSalvador via C-terminal coiled-coil domains, leads to its stabilization and phosphorylation

Genetic screens in Drosophila have revealed that the serine/threonine kinase Hippo (Hpo) and the scaffold protein Salvador participate in a pathway that controls cell proliferation and apoptosis. Hpo most closely resembles the pro-apoptotic mammalian sterile20 kinases 1 and 2 (Mst1 and 2), and Salvador (Sav) has a human orthologue hSav (also called hWW45). Here we show that Mst and hSav heterodimerize in an interaction requiring the conserved C-terminal coiled-coil domains of both proteins. hSav was also able to homodimerize, but this did not require its coiled-coil domain. Coexpression of Mst and hSav led to phosphorylation of hSav and also increased its abundance. In vitro phosphorylation experiments indicate that the phosphorylation of Sav by Mst is direct. The stabilizing effect of Mst was much greater on N-terminally truncated hSav mutants, as long as they retained the ability to bind Mst. Mst mutants that lacked the C-terminal coiled-coil domain and were unable to bind to hSav, also failed to stabilize or phosphorylate hSav, whereas catalytically inactive Mst mutants that retained the ability to bind to hSav were still able to increase its abundance, although they were no longer able to phosphorylate hSav. Together these results show that hSav can bind to, and be phosphorylated by, Mst, and that the stabilizing effect of Mst on hSav requires its interaction with hSav but is probably not due to phosphorylation of hSav by Mst.

Activation of sterile20-like kinase 1 in proteasome inhibitor bortezomib-induced apoptosis in oncogenic K-ras-transformed cells

Bortezomib (PS-341), a specific proteasome inhibitor, exhibits antitumor activity against a wide range of malignancies. However, the molecular mechanisms by which bortezomib causes apoptosis selectively in cancer cells still remain unclear. Ras signaling is involved in multiple cellular processes, including cell cycle progression, differentiation, and apoptosis, and can either promote or inhibit apoptosis depending on the type of apoptotic stimuli and the cell model. Here, we investigated the role of K-ras signaling in bortezomib-induced apoptosis. We found that K-ras-transformed cells were more susceptible to bortezomib-induced apoptosis than were nontransformed cells and that bortezomib-induced apoptosis was mainly caspase dependent in K-ras-transformed cells. We also found that mammalian sterile20-like kinase 1 (MST1) was activated by bortezomib in K-ras-transformed cells and K-ras-mutated cancer cells. Treatment of K-ras-transformed cells with bortezomib resulted in translocation of MST1 from cytoplasm into the nucleus and an increase of phosphorylated histone H2B and histone H2AX. Moreover, pretreatment with leptomycin B, an inhibitor of the nuclear export signal receptor, dramatically enhanced bortezomib-mediated MST1 activation, phosphorylation of histones H2B and H2AX, and apoptosis induction in K-ras-transformed cells. Knockdown of MST1 expression by small interfering RNA diminished bortezomib-induced apoptosis or caspase-3 activation. Our data suggested that bortezomib may be useful for treatment of K-ras-mutated cancer cells, and MST1 is one of the mediators for bortezomib-induced apoptosis in K-ras-transformed cells.

ERK promotes hydrogen peroxide-induced apoptosis through caspase-3 activation and inhibition of Akt in renal epithelial cells

Reactive oxygen species, including hydrogen peroxide (H(2)O(2)), are generated during ischemia-reperfusion and are critically involved in acute renal failure. The present studies examined the role of the extracellular signal-regulated kinase (ERK) pathway in H(2)O(2)-induced renal proximal tubular cells (RPTC) apoptosis. Exposure of RPTC to 1 mM H(2)O(2) resulted in apoptosis and activation of ERK1/2 and Akt. Pretreatment with the specific MEK inhibitors, U0126 and PD98059, or adenoviral infection with a construct that encodes a negative mutant of MEK1, protected cells against H(2)O(2)-induced apoptosis. In contrast, expression of constitutively active MEK1 enhanced H(2)O(2)-induced apoptosis. H(2)O(2) induced activation of caspase-3 and phosphorylation of histone H2B at serine 14, a posttranslational modification required for nuclear condensation, which also were blocked by ERK1/2 inhibition. Furthermore, blockade of ERK1/2 resulted in an increase in Akt phosphorylation and blockade of Akt potentiated apoptosis and diminished the protective effect conferred by ERK inhibition in H(2)O(2)-treated cells. Although Z-DEVD-FMK, a caspase-3 inhibitor, was able to inhibit histone H2B phosphorylation and apoptosis, it did not affect ERK1/2 phosphorylation. We suggest that ERK elicits apoptosis in epithelial cells by activating caspase-3 and inhibiting Akt pathways and elicits nuclear condensation through caspase-3 and histone H2B phosophorylation during oxidant injury.