Celia M. Pombo

Stress-Activated Protein Kinases in Cardiovascular Disease

Cells respond to extracellular stimuli by activating signal transduction pathways, which culminate in changes in gene expression. The particular genetic program activated determines, in large part, the response of the cell (eg, growth versus growth arrest versus apoptosis; differentiation versus dedifferentiation). A critical component of eukaryotic signal transduction is the activation of protein kinases, which phosphorylate a host of cellular substrates, including transcription factors controlling the induction of various genes. For example, the Ras/ERK-1 and ERK-2 (or MAP kinase) pathway transduces critical components of the growth factor–induced mitogenic response to the nucleus. Expression of inactive or interfering mutants of components of the pathway disrupts, and expression of constitutively active mutants activates, mitogenesis.1 Recently, protein serine/threonine kinases related to ERK-1 and -2 have been identified; these kinases transduce signals to the nucleus not in response to growth factors and other mitogens but in response to cellular stresses such as inflammatory cytokines (IL-1β and TNF-α), ischemia, reversible ATP depletion, heat shock, endotoxin, and genotoxic stress. These kinases, called the SAPKs2 or, alternatively, c-Jun N-terminal (amino-terminal) kinases (JNKs, named after one of their physiological substrates),3 and p38,4 likely play critical roles in the genetic response of many components of the cardiovascular system to disease processes (Table⇓). In this review, we will discuss these stress-activated kinases, how they are regulated, and the evidence suggesting roles they may play in cardiovascular disease. View this table: Table 1. Postulated Effects of SAPKs and p38 in Cardiovascular Disease States The SAPKs were first described in 1990 as the dominant microtubule-associated protein 2 kinase activated in rat liver in response to systemic administration of the protein synthesis inhibitor cycloheximide.5 The kinase shared two major characteristics with the mitogen-activated p42 and p44 MAP kinases (later renamed ERK-2 and ERK-1, respectively). The first was proline-directed substrate specificity. Serine or …

Activation of a human Ste20-like kinase by oxidant stress defines a novel stress response pathway.

Mammalian homologs of the yeast protein kinase, Sterile 20 (Ste20), can be divided into two groups based on their regulation and structure. The first group, which includes PAK1, is regulated by Rac and Cdc42Hs, and activators have been identified. In contrast, very little is known about activators, regulatory mechanisms or physiological roles of the other group, which consists of GC kinase and MST1. We have identified a human Ste20‐like kinase from the GC kinase group, SOK‐1 (Ste20/oxidant stress response kinase‐1), which is activated by oxidant stress. The kinase is activated by autophosphorylation and is markedly inhibited by its non‐catalytic C‐terminal region. SOK‐1 is activated 3‐ to 7‐fold by reactive oxygen intermediates, but is not activated by growth factors, alkylating agents, cytokines or environmental stresses including heat shock and osmolar stress. Although these data place SOK‐1 on a stress response pathway, SOK‐1, unlike GC kinase and PAK1, does not activate either of the stress‐activated MAP kinase cascades (p38 and SAPKs). SOK‐1 is the first mammalian Ste20‐like kinase which is activated by cellular stress, and the activation is relatively specific for oxidant stress. Since SOK‐1 does not activate any of the known MAP kinase cascades, its activation defines a novel stress response pathway which is likely to include a unique stress‐activated MAP kinase cascade.

CCM3/PDCD10 stabilizes GCKIII proteins to promote Golgi assembly and cell orientation.

Mutations in CCM3/PDCD10 result in cerebral cavernous malformations (CCMs), a major cause of cerebral hemorrhage. Despite intense interest in CCMs, very little is known about the function of CCM3. Here, we report that CCM3 is located on the Golgi apparatus, forming a complex with proteins of the germinal center kinase III (GCKIII) family and GM130, a Golgi-resident protein. Cells depleted of CCM3 show a disassembled Golgi apparatus. Furthermore, in wound-healing assays, CCM3-depleted cells cannot reorient the Golgi and centrosome properly, and demonstrate impaired migration. Golgi disassembly after either depletion of CCM3 or dissociation of CCM3 from the GM130-GCKIII complex is the result of destabilization of GCKIII proteins and dephosphorylation of their substrate, 14-3-3ζ. Significantly, the phenotype induced by CCM3 depletion can be reverted by expression of wild-type CCM3, but not by disease-associated mutants. Our findings suggest that Golgi dysfunction and the ensuing abnormalities of cell orientation and migration resulting from CCM3 mutations contribute to CCM pathogenesis.

Growth Hormone-Releasing Hormone Stimulates Mitogen-Activated Protein Kinase

GH-releasing hormone (GHRH) can induce proliferation of somatotroph cells. The pathway involving adenylyl cyclase/cAMP/protein kinase A pathway in its target cells seems to be important for this action, or at least it is deregulated in some somatotroph pituitary adenomas. We studied in this work whether GHRH can also stimulate mitogen-activated protein (MAP) kinase. GHRH can activate MAP kinase both in pituitary cells and in a cell line overexpressing the GHRH receptor. Although both protein kinase A and protein kinase C could activate MAP kinase in the CHO cell line studied, neither protein kinase A nor protein kinase C appears to be required for GHRH activation of MAP kinase in this system. However, sequestration of the βγ-subunits of the G protein coupled to the receptor inhibits MAP kinase activation mediated by GHRH. This pathway also involves p21ras and a phosphatidylinositol 3-kinase, probably phosphatidylinositol 3-kinase-γ. Despite the involvement of p21ras, the protein kinase Raf-1 is not hyperp...

Serine 58 of 14-3-3ζ Is a Molecular Switch Regulating ASK1 and Oxidant Stress-Induced Cell Death

ABSTRACT Oxidant stress is a ubiquitous stressor with negative impacts on multiple cell types. ASK1 is a central mediator of oxidant injury, but while mechanisms of its inhibition, such as sequestration by 14-3-3 proteins and thioredoxin, have been identified, mechanisms of activation have remained obscure and the signaling pathways regulating this are not clear. Here, we report that phosphorylation of 14-3-3ζ at serine 58 (S58) is dynamically regulated in the cell and that the phosphorylation status of S58 is a critical factor regulating oxidant stress-induced cell death. Phosphorylation of S58 releases ASK1 from 14-3-3ζ, and ASK1 then activates stress-activated protein kinases, leading to cell death. While several members of the mammalian sterile 20 (Mst) family of kinases can phosphorylate S58 when overexpressed, we identify Ste20/oxidant stress response kinase 1 (SOK-1), an Mst family member known to be activated by oxidant stress, as a central endogenous regulator of S58 phosphorylation and thereby of ASK1-mediated cell death. Our findings identify a novel pathway that regulates ASK1 activation and oxidant stress-induced cell death.

Hormonal Control of Growth Hormone Secretion

Growth hormone secretion by the somatotroph cells depends upon the interaction between hypothalamic regulatory peptides, target gland hormones and a variety of growth factors acting in a paracrine or autocrine fashion. This review will be focused on recent data regarding the mechanism by which growth hormone-releasing hormone (GHRH) influences somatotroph cell function and the physiological role played by Ghrelin and leptin in the regulation of growth hormone (GH) secretion. It is well established that binding of GHRH to its receptor leads to activation of protein kinase A (PKA). More recently, it was found that GHRH can also activate mitogen-activated protein (MAP) kinase both in pituitary cells and in a cell line overexpressing the GHRH receptor. Whether somatotroph adenomas, either with or without a GS-alpha mutation, have alterations in some of the components of the activation of the MAP kinase pathway remains to be known. The recent isolation of Ghrelin, the endogenous ligand of the growth hormone secretagogue receptor, can be considered a landmark in the GH field, which opens up the possibility of gaining greater insight into our understanding of the mechanisms involved in the regulation of GH secretion and somatic growth. Indeed, preliminary evidences indicate that this peptide exerts a marked stimulatory effect on plasma GH levels in both rats and humans. Finally, it is well known that GH secretion is markedly influenced by nutritional status. Leptin has emerged as an important adipose tissue-generated signal that is involved in the regulation of GH secretion, thus providing an integrated regulatory system of growth and metabolism. Although the effects of leptin on GH secretion in humans remain to be clarified, indirect evidences indicate that it may play an inhibitory role.

Activation of the Ste20-like Oxidant Stress Response Kinase-1 during the Initial Stages of Chemical Anoxia-induced Necrotic Cell Death REQUIREMENT FOR DUAL INPUTS OF OXIDANT STRESS AND INCREASED CYTOSOLIC [Ca2+]

Signal transduction mechanisms activated during the early stages of necrotic cell death are poorly characterized. We have recently identified the Sterile 20 (Ste20)-like oxidant stress response kinase-1, SOK-1, which is a member of the Ste20 kinase family. We report that SOK-1 is markedly activated as early as 20 min after chemical anoxia induced by exposure of Madin-Darby canine kidney or LLC-PK1 renal tubular epithelial cells to 2-deoxyglucose (2-DG) and any one of three inhibitors of the electron transport chain, cyanide (CN), rotenone, or antimycin A. Since oxidant stress activates SOK-1, we postulated that reactive oxygen species (ROS), which are produced by the electron transport chain during chemical anoxia, might be responsible for SOK-1 activation. The time course of CN/2-DG-induced SOK-1 activation and of production of ROS, measured in cells loaded with dichlorofluorescein, were compatible with a role for ROS in SOK-1 activation. Furthermore, preincubation of LLC-PK1 cells with three unrelated scavengers of ROS, pyrrolidine dithiocarbamate, pyruvate, or nordihydroguaiaretic acid, reduced both cellular oxidant stress and activation of SOK-1 by CN/2-DG. An increase in cytosolic free [Ca2+] ([Ca2+] i ) was necessary but not sufficient for CN/2-DG-induced activation of SOK-1. Preincubation of cells with BAPTA-AM prevented activation of SOK-1. Incubation of cells with thapsigargin or the calcium ionophore, A23187, had no effect on SOK-1 activity, but preincubation of cells with either of these agents markedly enhanced CN/2-DG-induced activation of SOK-1 (20-foldversus 7-fold). In summary, chemical anoxia activates SOK-1 via an oxidant stress-dependent mechanism that is both critically dependent upon and markedly amplified by an increase in [Ca2+] i . This requirement for dual inputs of oxidant stress and an increase in [Ca2+] i may prevent inappropriate activation of the kinase by milder degrees of oxidant stress, which are insufficient to generate an increase in [Ca2+] i . The activation of SOK-1 may be one of the cell’s earliest responses to inducers of necrotic cell death.

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.

A novel cardioprotective p38-MAPK/mTOR pathway

Despite intensive study, the mechanisms regulating activation of mTOR and the consequences of that activation in the ischemic heart remain unclear. This is particularly true for the setting of ischemia/reperfusion (I/R) injury. In a mouse model of I/R injury, we observed robust mTOR activation, and its inhibition by rapamycin increased injury. Consistent with the in-vivo findings, mTOR activation was also protective in isolated cardiomyocytes exposed to two models of I/R. Moreover, we identify a novel oxidant stress-activated pathway regulating mTOR that is critically dependent on p38-MAPK and Akt. This novel p38-regulated pathway signals downstream through REDD1, Tsc2, and 14-3-3 proteins to activate mTOR and is independent of AMPK. The protective role of p38/Akt and mTOR following oxidant stress is a general phenomenon since we observed it in a wide variety of cell types. Thus we have identified a novel protective pathway in the cardiomyocyte involving p38-mediated mTOR activation. Furthermore, the p38-dependent protective pathway might be able to be selectively modulated to enhance cardio-protection while not interfering with the inhibition of the better-known detrimental p38-dependent pathways.

Adaptor Protein Cerebral Cavernous Malformation 3 (CCM3) Mediates Phosphorylation of the Cytoskeletal Proteins Ezrin/Radixin/Moesin by Mammalian Ste20-4 to Protect Cells from Oxidative Stress

Background: The adaptor protein cerebral cavernous malformation 3 (CCM3) is involved in cell death. Results: Ezrin/radixin/moesin (ERM) proteins are phosphorylated after oxidative stress, and this requires CCM3 and the ERM kinase Mst4. Conclusion: CCM3 is necessary for ERM protein phosphorylation after stress, which enhances survival. Significance: This is a novel, functionally significant pathway that protects cells from death. While studying the functions of CCM3/PDCD10, a gene encoding an adaptor protein whose mutation results in vascular malformations, we have found that it is involved in a novel response to oxidative stress that results in phosphorylation and activation of the ezrin/radixin/moesin (ERM) family of proteins. This phosphorylation protects cells from accidental cell death induced by oxidative stress. We also present evidence that ERM phosphorylation is performed by the GCKIII kinase Mst4, which is activated and relocated to the cell periphery after oxidative stress. The cellular levels of Mst4 and its activation after oxidative stress depend on the presence of CCM3, as absence of the latter impairs the phosphorylation of ERM proteins and enhances death of cells exposed to reactive oxygen species. These findings shed new light on the response of cells to oxidative stress and identify an important pathophysiological situation in which ERM proteins and their phosphorylation play a significant role.