Intermittent ischemia reduces warm hypoxia-reoxygenation-induced JNK(1)/SAPK(1) activation and apoptosis in rat hepatocytes.

Prolonged liver ischemia followed by reperfusion (I/R) causes functional and structural damage to liver cells, resulting in necrosis and apoptosis. c-jun N-terminal kinase 1/stress-activated protein kinase 1 (JNK(1)/SAPK(1)) is activated during I/R and participates in the onset of the apoptosis program. Excessive blood loss during surgery can hinder postoperative recovery. Intermittent portal triad clamping (PTC) is better tolerated than prolonged continuous ischemia. This study was designed to demonstrate that intermittent ischemia could improve postischemic survival rates by a decrease of JNK(1)/SAPK(1) and caspase 3 activation, which were involved in the apoptosis process. Rats were subjected to intermittent 1-hour ischemia (15-minute ischemia/5-minute reperfusion, 4 times), followed by 220-minute reperfusion, or to continuous ischemia (1 hour), followed by 240-minute reperfusion. Mortality rates were assessed on day 7. Serum aspartate transaminase (AST), alanine transaminase (ALT), and lactate dehydrogenase levels (LDH) were measured 6 hours after ischemia. This study was completed in primary cultured isolated rat hepatocytes, subjected to the same continuous or intermittent hypoxic conditions. The activation status of JNK(1)/SAPK(1) was evaluated by immunoprecipitation or Western blotting experiments. Apoptosis was assessed by measuring caspase activation and by terminal deoxynucleotidyl transferase-mediated dUTP biotin nick end labeling (TUNEL) reaction. Eighty percent of the intermittent-ischemia group was alive 7 days after surgery and serum enzyme levels were significantly decreased. Intermittent hypoxia or ischemia did not lead to JNK(1)/SAPK(1) activation, but at least 3 hypoxia-reoxygenation (H/R) sets were necessary to inhibit kinase activation. Consequently, caspase 3 activation and apoptosis were dramatically reduced. Intermittent ischemia is a powerful, protective way to reduce I/R damage of the liver, by reduction of JNK(1)/SAPK(1) activation associated with a down-regulation of caspase 3 activity, which leads to inhibition of hepatocyte apoptosis.     

Preferential activation of the p46 isoform of JNK/SAPK in mouse macrophages by TNFα

A pleiotropic cytokine, tumor necrosis factor-α (TNFα), regulates the expression of multiple macrophage gene products and thus contributes a key role in host defense. In this study, we have investigated the specificity and mechanism of activation of members of the c-Jun-NH₂-terminal kinase/stress-activated protein kinase (JNK/SAPK) subfamily of mitogen-activated protein kinases (MAPKs) in mouse macrophages in response to stimulation with TNFα. Exposure of macrophages to TNFα stimulated a preferential increase in catalytic activity of the p46 JNK/SAPK isoform compared with the p54 JNK/SAPK isoform as determined by: (i) separation of p46 and p54 JNK/SAPKs by anion exchange liquid chromatography and (ii) selective immunodepletion of the p46 JNK/SAPK from macrophage lysates. To investigate the level of regulation of p46 JNK/SAPK activation, we determined the ability of MKK4/SEK1/JNKK, an upstream regulator of JNK/SAPKs, to phosphorylate recombinant kinase-inactive p46 and p54 JNK/SAPKs. Endogenous MKK4 was able to transphosphorylate both isoforms. In addition, both the p46 and p54 JNK/SAPK isoforms were phosphorylated on their TPY motif in response to TNFα stimulation as reflected by immunoblotting with a phospho-specific antibody that recognizes both kinases. Collectively, these results suggest that the level of control of p46 JNK/SAPK activation is distal not only to MKK4 but also to the p54 JNK/SAPK. Preferential isoform activation within the JNK/SAPK subfamily of MAPKs may be an important mechanism through which TNFα regulates macrophage phenotypic heterogeneity and differentiation.     

Requirement of activation of JNK and p38 for environmental stress-induced erythroid differentiation and apoptosis and of inhibition of ERK for apoptosis.

C-Jun amino terminal kinase/stress-activated protein kinases (JNK/SAPK) and p38 subgroups of mitogen-activated protein kinases have been suggested to play a critical role in apoptosis, cell growth, and/or differentiation. We found that a short exposure of SKT6 cells, which respond to erythropoietin (Epo) and induce erythroid differentiation, to osmotic or heat shock induced transient activation of JNK/SAPK and p38 and inactivation of ERK and resulted in erythroid differentiation without Epo, whereas long exposure of the cells to these stresses induced prolonged activation/inactivation of the same kinases and caused apoptosis. Inhibition of JNK/SAPK and p38 resulted in inhibition of stress-induced erythroid differentiation and apoptosis. Inhibition of ERK had no effect on stress-induced erythroid differentiation, but stimulated apoptosis. Activation of p38 and/or JNK/SAPK for a short time caused erythroid differentiation without Epo, although its prolonged activation induced apoptosis. Activation of ERK suppressed stress-induced apoptosis. These results indicate that short cellular stresses, inducing transient activation of JNK/SAPK and p38, lead to cell differentiation rather than apoptosis. Furthermore, activation of JNK/SAPK and p38 is required for both cell differentiation and apoptosis, and the duration of their activation may determine the cell fate, cell differentiation, and apoptosis. In contrast, inactivation of ERK is required for stress-induced apoptosis but not cell differentiation.     

Signal transduction involved in MCP-1-mediated monocytic transendothelial migration

Monocyte chemoattractant protein-1 (MCP-1) is a major chemoattractant for monocytes and T lymphocytes. The MonoMac6 cell line was used to examine MCP-1 receptor-mediated signal transduction events in relation to MCP-1-mediated monocytic transendothelial migration. MCP-1 stimulates, with distinct time courses, extracellular signal-related kinases (ERK1 and ERK2) and stress-activated protein kinases (SAPK1/JNK1 and SAPK2/p38). SAPK1/JNK1 activation was blocked by piceatannol, indicating that it is regulated by Syk kinase, whereas SAPK2/p38 activation was inhibited by PP2, revealing an upstream regulation by Src-like kinases. In contrast, ERK activation was insensitive to PP2 and piceatannol. Pertussis toxin, a blocker of Go/Gi proteins, abrogated MCP-1-induced ERK activation, but was without any effect on SAPK1/JNK1 and SAPK2/p38 activation. These results underscore the major implication of Go/Gi proteins and nonreceptor tyrosine kinases in the early MCP-1 signaling. Furthermore, MCP-1-mediated chemotaxis and transendothelial migration were significantly diminished by a high concentration of SB202190, a broad SAPK inhibitor, or by SB203580, a specific inhibitor of SAPK2/p38, and abolished by pertussis toxin treatment. Altogether, these data suggest that coordinated action of distinct signal pathways is required to produce a full response to MCP-1 in terms of monocytic locomotion.     

Mitogen-activated protein kinase-mediated Fas apoptotic signaling pathway

Ligation of the cell surface receptor Fas/APO-1 (CD95) by its specific ligand or by anti-Fas antibodies rapidly induces apoptosis in susceptible cells. To characterize the molecular events involved in Fas-induced apoptosis, we examined the contribution of two subgroups of the mitogen-activated protein (MAP) kinase family, the Jun kinases or stress-activated protein kinases (JNKs/SAPKs) and the extracellular signal-regulated kinases (ERKs), in a Fas-sensitive neuroblastoma cell line. Here we show that both JNK and ERK protein kinases were activated upon Fas crosslinking through a Ras-dependent mechanism. Interference with either the JNK or ERK pathway by ectopic expression of dominant-interfering mutant proteins blocked Fas-mediated apoptosis. ERK activation was transient and associated with induced expression of the Fas receptor. In contrast, JNK activation was sustained and correlated with the onset of apoptosis. These data indicate that the ERK and the JNK groups of MAP kinases cooperate in the induction of cell death by Fas. Inhibition of Fas killing by an interleukin 1β-converting enzyme (ICE)-like protease inhibitor peptide did not modify Fas-induced JNK activation upon Fas ligation. In contrast, changes in Bcl-2 level due to expression of sense and antisense vectors influenced the sensitivity to Fas killing and Fas-induced JNK activation.     

Redefining the roles of p38 and JNK signaling in cardiac hypertrophy: dichotomy between cultured myocytes and animal models

The mitogen-activated protein kinase (MAPK) signaling pathways serve as pivotal transducers of diverse biologic functions including cell growth, differentiation, proliferation, and apoptosis. The c-Jun N-terminal kinases (JNKs) and p38 kinases constitute two important branches of the greater MAPK signaling cascade that function as specialized transducers of stress or injury responses, hence they are subclassified as stress-activated protein kinases (SAPKs). In the myocardium, both p38 and JNK transduction cascades have been implicated in regulating the hypertrophic response, as well as cardiomyopathy and heart failure. Most reports proposing a pro-hypertrophic regulatory role for JNK and p38 signaling placed a heavy or exclusive reliance on culture-based models of cellular growth. More recently, a number of studies in genetically modified animal models have challenged the previously proposed role of JNK and p38 as pro-hypertrophic signaling effectors in the myocardium. This review will discuss an increasing body of evidence suggesting that the SAPKs (JNK and p38) do not positively regulate cardiac hypertrophy in vivo, but in fact may actually serve as negative regulators of this response in the adult heart. However, SAPK signaling is likely maladaptive, despite its putative anti-hypertrophic role in vivo, given the observation of dilated cardiomyopathy and heart failure in gain-of-function transgenic models.     

Signal transduction pathways involved in soluble fractalkine-induced monocytic cell adhesion

Fractalkine displays features that distinguishes it from the other chemokines. In particular, besides its chemoattractant action it promotes, under physiologic flow, the rapid capture and the firm adhesion of a subset of leukocytes or intervenes in the neuron/microglia interaction. This study verified that indeed the human monocytic MonoMac6 cell line adheres to fibronectin-coated filters in response to soluble fractalkine (s-FKN). s-FKN stimulates, with distinct time courses, extracellular signal-related kinases (ERK1 and ERK2) and stress-activated protein kinases (SAPK1/JNK1 and SAPK2/p38). Both p60 Src and p72 Syk were activated under s-FKN stimulation with a rapid kinetic profile compatible with a downstream regulation on the mitogen-activated protein kinase (MAPK) congeners. The use of specific tyrosine kinase inhibitors revealed that the ERK pathway is strictly controlled by Syk, whereas c-Src up-regulated the downstream SAPK2/p38. In contrast, the SAPK1/JNK1 pathway was not regulated by any of these nonreceptor tyrosine kinases. The s-FKN-mediated increased adherence of MonoMac6 cells was partially inhibited by SB202190, a broad SAPKs inhibitor, PD98059, an MEK inhibitor, LY294002, a phosphatidyl inositol 3-kinase inhibitor, and a pertussis toxin-sensitive G protein. These data highlight that the integration of a complex array of signal transduction pathways is necessary to complete the full s-FNK-dependent adherence of human monocytic cells to fibronectin. (Blood. 2001;97:2031-2037)     

CD22 cross-linking generates B-cell antigen receptor-independent signals that activate the JNK/SAPK signaling cascade.

CD22 is a B-cell-specific adhesion molecule that modulates BCR-mediated signal transduction. Ligation of human CD22 with monoclonal antibodies (MoAbs) that block the ligand binding site triggers rapid tyrosine phosphorylation of CD22 and primary B-cell proliferation. Because extracellular signal-regulated kinases (ERKs) couple upstream signaling pathways to gene activation and are activated by B-cell antigen receptor (BCR) signaling, we examined whether CD22 ligation also activated ERKs and/or modified BCR-induced ERK activation. Ligation of CD22 on either primary B cells or B-cell lines failed to significantly activate the mitogen activated protein kinase (MAPK) ERK-2, but did activate the stress-activated protein kinases (SAPKs; c-jun NH2-terminal kinases or JNKs). In contrast, BCR ligation resulted in ERK-2 activation without significant SAPK activation. Concurrent ligation of CD22 and BCR enhanced BCR-mediated ERK-2 activation without appreciably modulating CD22-induced SAPK activation. Consistent with its induction of SAPK activity, there was a marked increase in nuclear extracts of activator protein-1 (AP-1) and c-jun levels within 2 hours of exposure of primary B cells to the CD22 MoAb. Despite their differences in ERK activation, both CD22 and BCR ligation triggered several Burkitt lymphoma cell lines to undergo apoptosis, and the 2 stimuli together induced greater cell death than either signal alone. The pro-apoptotic effects were CD22-blocking MoAb-specific and dose-dependent. Examination of expression levels of Bcl-2 protoncogene family members (Bcl-2, Bcl-x(L), Mcl-1, and Bax) showed a downregulation of Bcl-x(L) and Mcl-1 after CD22 ligation. This study provides a plausible mechanism to explain how CD22 and BCR signaling can costimulate B-cell proliferation and induce apoptosis in Burkitt lymphoma cell lines.     

Tumor necrosis factor-alpha enhances hypoxia-reoxygenation-mediated apoptosis in cultured human coronary artery endothelial cells: critical role of protein kinase C.

BACKGROUND: Ischemia and tumor necrosis factor-alpha (TNF alpha) released during ischemia both cause apoptosis and necrosis of myocardial tissues. Since endothelium may be critically important in determination of cardiac function, we examined the interaction between TNF alpha and hypoxia-reoxygenation with regard to induction of apoptosis and underlying signaling pathway in cultured human coronary artery endothelial cells (HCAECs). METHODS AND RESULTS: HCAECs were cultured and exposed to hypoxia alone, hypoxia-reoxygenation, TNF alpha alone, TNF alpha plus hypoxia-reoxygenation, or TNF alpha only during the period of reoxygenation. Apoptosis was evaluated by transmission electron microscopy, DNA nick-end labeling and DNA laddering. Hypoxia alone caused modest time-dependent apoptosis of cultured HCAECs, and reoxygenation increased the number of apoptotic cells (P < 0.01 vs. hypoxia alone). TNF alpha induced concentration-dependent apoptosis, and enhanced reoxygenation-mediated apoptosis in cultured HCAECs (P < 0.01 vs. hypoxia-reoxygenation alone). As expected, monoclonal antibody to TNF alpha significantly blocked the pro-apoptotic effect of TNF alpha-induced apoptosis (P < 0.01). TNF alpha-induced apoptosis was found to be associated with marked activation of protein kinase C (PKC), and pretreatment of cells with a specific PKC inhibitor markedly reduced TNF alpha-mediated PKC activity and apoptosis. CONCLUSION: These observations indicate that hypoxia alone causes modest apoptosis, reoxygenation increases apoptosis beyond that caused by hypoxia in cultured HCAECs. TNF alpha alone causes apoptosis, and further enhances apoptosis caused by hypoxia-reoxygenation. The activation of PKC plays a critical role in TNF alpha-induced apoptosis of cultured HCAECs.     

Taurolithocholic acid-3 sulfate induces CD95 trafficking and apoptosis in a c-Jun N-terminal kinase-dependent manner

BACKGROUND & AIMS: Prevention of bile acid-induced apoptosis is of therapeutic interest and requires the understanding of underlying mechanisms. METHODS: The effect of tauroursodeoxycholate (TUDC) on taurolithocholic acid-3 sulfate (TLCS)-induced apoptosis was studied in cultured rat hepatocytes. RESULTS: TLCS induced activation of caspases 8, 9, and 3 and hepatocyte apoptosis. These effects were abolished by TUDC in a PI 3-kinase-/protein kinase B (PKB)-, p38(MAPK)-, and extracellular signal-regulated kinase-2 (Erk-2)-independent manner. These protein kinases were activated by both TLCS and TUDC, however, with different kinetics. TLCS, but not TUDC, led to a sustained activation of c-Jun N-terminal kinase (JNK) and CD95 trafficking to the plasma membrane; both TLCS effects were prevented by TUDC. Inhibition of JNK1 or protein kinase C prevented TLCS-induced CD95 membrane trafficking and blunted the apoptotic response. The apoptotic potency of other bile acids paralleled their ability to induce sustained JNK activation. CONCLUSIONS: Protection by TUDC against TLCS-induced apoptosis starts upstream of caspase 8 activation and is independent of a PI 3-kinase-dependent survival pathway. JNK activation may be important for bile acid-induced apoptosis by triggering ligand-independent CD95 surface trafficking and activation of apoptosis.     

Attenuation of acetaldehyde-induced cell injury by overexpression of aldehyde dehydrogenase-2 (ALDH2) transgene in human cardiac myocytes: role of MAP kinase signaling

Acetaldehyde, the major metabolite of ethanol, which is far more toxic and reactive than ethanol, may be responsible for alcohol-induced cardiac damage. This study was designed to examine the impact of facilitated acetaldehyde metabolism using transfection of human aldehyde dehydrogenase-2 (ALDH2) transgene on acetaldehyde- and ethanol-induced cell injury. Fetal human cardiac myocytes were transfected with ALDH2, the efficacy of which was verified by flow cytometry, Western blot and ALDH2 activity assays. Generation of reactive oxygen species (ROS) was detected using 5-(6)-chloromethyl-2',7'-dichlorodihydrofluorescein diacetate (CM-H2DCFDA). Apoptosis was evaluated by 4',6'-diamidino-2'-phenylindoladihydrochloride (DAPI) fluorescence microscopy, quantitative DNA fragmentation ELISA and caspase 3 activity. Acetaldehyde and ethanol elicited overt ROS generation and apoptosis in human cardiac myocytes following 24-48 h of incubation. Immunostaining revealed activation of the MAP kinase cascades ERK1/2, SAPK/JNK and p38 MAP kinase in acetaldehyde-treated myocytes. Interestingly, ALDH2 transgene significantly attenuated acetaldehyde-induced ROS generation, apoptosis and phosphorylation of ERK1/2 and SAPK/JNK. Time-dependent response (0-12 h) revealed ROS accumulation and activation of MAP kinases prior to acetaldehyde-induced apoptosis. In addition, acetaldehyde-induced ROS generation and apoptosis were antagonized by non-enzymatic antioxidants. Our results suggested that ALDH2 transgene overexpression may effectively alleviate acetaldehyde-elicited cell injury through an ERK1/2 and SPAK/JNK-dependent mechanism. Our data are consistent with the notion of acetaldehyde as a contributor to alcoholic cardiomyopathy and implicate the therapeutic potential of ALDH2 enzyme in alcoholic complications.     

Activation, differential localization, and regulation of the stress-activated protein kinases, extracellular signal-regulated kinase, c-JUN N-terminal kinase, and p38 mitogen-activated protein kinase, in synovial tissue and cells in rheumatoid arthritis

OBJECTIVE: To investigate whether stress- and mitogen-activated protein kinases (SAPK/MAPK), such as extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38 MAPK, are significantly activated in rheumatoid arthritis (RA) synovial tissue compared with their activation in degenerative joint disease; to assess the localization of SAPK/MAPK activation in rheumatoid synovial tissue; and to search for the factors leading to stress kinase activation in human synovial cells. METHODS: Immunoblotting and immunohistology by antibodies specific for the activated forms of SAPK/MAPK were performed on synovial tissue samples from patients with RA and osteoarthritis (OA). In addition, untreated and cytokine-treated human synovial cells were assessed for SAPK/MAPK activation and downstream signaling by various techniques. RESULTS: ERK, JNK, and p38 MAPK activation were almost exclusively found in synovial tissue from RA, but not OA, patients. ERK activation was localized around synovial microvessels, JNK activation was localized around and within mononuclear cell infiltrates, and p38 MAPK activation was observed in the synovial lining layer and in synovial endothelial cells. Tumor necrosis factor alpha, interleukin-1 (IL-1), and IL-6 were the major inducers of ERK, JNK, and p38 MAPK activation in cultured human synovial cells. CONCLUSION: Signaling through SAPK/MAPK pathways is a typical feature of chronic synovitis in RA, but not in degenerative joint disease. SAPK/MAPK signaling is found at distinct sites in the synovial tissue, is induced by proinflammatory cytokines, and could lead to the design of highly targeted therapies.     

Role of MAP kinases and their cross-talk in TGF-beta1-induced apoptosis in FaO rat hepatoma cell line

Transforming growth factor (TGF) beta1 is a potent inducer of apoptosis in the liver. During TGF-beta1-induced apoptosis, 3 mitogen-activated protein (MAP) kinases (extracellular signal-regulated kinase [ERK], c-Jun N-terminal kinase [JNK], and p38 kinase) showed simultaneously sustained activation in FaO rat hepatoma cells. TGF-beta1-induced apoptosis was markedly enhanced when ERK activation was selectively inhibited by the mitogen-activated protein kinase/extracellular signal-regulated kinase kinase inhibitor PD98059. In contrast, both interfering with p38 activity by overexpression of the dominant negative (DN) MKK6 mutant and inhibition of the JNK pathway by overexpression of the DN SEK1 mutant resulted in suppression of mitochondrial cytochrome c release, abrogating TGF-beta1-induced apoptosis. In addition, antiapoptotic Bcl-2 blocked mitochondrial cytochrome c release, suppressing TGF-beta1-induced activation of JNK and p38. Inhibition of ERK activity enhanced TGF-beta1-induced p38 and JNK activation. However, inhibition of the JNK pathway suppressed p38 but induced transient ERK activation. Similarly, interfering with the p38 pathway also attenuated JNK activation but generated transient ERK activation in response to TGF-beta1. These results indicate that disrupting one MAP kinase pathway affects the TGF-beta1-induced activation of other MAP kinases, suggesting cross-talk among MAP kinase pathways. In conclusion, we propose that the balance and integration of MAP kinase signaling may regulate commitment to TGF-beta1-induced apoptosis modulating the release of cytochrome c from mitochondria.     

Vitamin D inhibits the activation of stress-activated protein kinases by physiological and environmental stresses in keratinocytes

In addition to its known effects on keratinocyte proliferation and differentiation, the hormonal form of vitamin D, 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)), has been shown to protect keratinocytes from UV- and chemotherapy-induced damage. Epidermal keratinocytes contain both the machinery needed to produce 1,25(OH)(2)D(3) and vitamin D receptors. The activation of the stress-activated protein kinases (SAPKs), such as c-Jun N-terminal kinase (JNK) and p38, is an early cellular response to stress signals and an important determinant of cell fate. This study examines whether modulation of these SAPKs is associated with the effects of 1,25(OH)(2)D(3) on keratinocytes under stress. HaCaT keratinocytes were exposed to heat shock, hyperosmotic concentrations of sorbitol, the epidermal growth factor receptor tyrosine kinase inhibitor AG1487, the pro-inflammatory cytokine tumor necrosis factor alpha, and H(2)O(2). These stresses activated both SAPKs. Pretreatment with 1,25(OH)(2)D(3) inhibited the activation of JNK by all stresses and the activation of p38 by heat shock, AG1478 and tumor necrosis factor alpha. Under the same conditions, treatment with 1,25(OH)(2)D(3) protected HaCaT keratinocytes from cytotoxicity induced by exposure to H(2)O(2) and hyperosmotic shock. The effect of 1,25(OH)(2)D(3) was dose-dependent, already apparent at nanomolar concentrations, and time-dependent, maximal after a 24-h pre-incubation. We suggest that inhibition of SAPK activation may account for some of the well-documented protective effects of 1,25(OH)(2)D(3) on epidermal cells during exposure to UV or chemotherapy and may also be related to the anti-inflammatory actions of the hormone in skin.     

Reactive oxygen species-mediated pancreatic beta-cell death is regulated by interactions between stress-activated protein kinases, p38 and c-Jun N-terminal kinase, and mitogen-activated protein kinase phosphatases

Pancreatic beta-cells are susceptible to reactive oxygen species (ROS), which are known to be generated by high or low glucose (LG), hypoxic, or cytokine-producing conditions. When we cultured mouse beta-cell-derived MIN6 cells in a LG condition, we detected a significant generation of ROS, including hydrogen peroxide, which was comparable to the ROS production in hypoxic or cytokine-treated conditions. ROS accumulation induced by the LG culture led to cell death, which was prevented by the ROS scavengers N-acetylcysteine and manganese(III)tetrakis(4-benzoic acid) porphyrin. We next investigated the mechanism of stress-activated protein kinases (SAPKs), c-jun N-terminal kinase (JNK) and p38, in ROS-induced MIN6 cell death. Activation of p38 occurred immediately after the LG culture, whereas JNK activation increased slowly 8 h later. Adenoviral p38 expression decreased MIN6 cell death, whereas the JNK expression increased it. Consistently, blocking p38 activation by inhibitors increased beta-cell death, whereas JNK inhibitors decreased it. We then examined the role of MAPK phosphatases (MKPs) specific for stress-activated protein kinases in beta-cell death. We found that MKP-1 presented an increase in its oxidized product after the LG culture. ROS scavengers prevented the appearance of this oxidized product and JNK activation. Thus, ROS-induced MKP inactivation causes sustained activation of JNK, which contributes to beta-cell death. Adenoviral overexpression of MKP-1 and MKP-7 prevented the phosphorylation of JNK at 36 h after the LG culture, and decreased MIN6 beta-cell death. We suggest that beta-cell death is regulated by interactions between JNK and its specific MKPs.     

Insulin resistance affects the cytoprotective effect of insulin in cardiomyocytes through an impairment of MAPK phosphatase-1 expression

OBJECTIVE: Insulin protects cardiomyocytes from apoptosis. Insulin resistance usually refers to a defect in the ability of insulin to stimulate glucose uptake. It is unknown, however, whether or not insulin resistance compromises the cell-protective effect of the hormone. Caspases are a family of cysteine proteases that regulate apoptosis. We explored the effects of insulin resistance on hypoxia-induced caspase-3 activation in cardiomyocytes. METHODS: Experiments were performed in cultured neonatal rat cardiomyocytes. Insulin resistance was induced by treating cardiac myocytes with isoproterenol, a beta-adrenergic receptor agonist. RESULTS: Twelve hours of hypoxia-induced caspase-3 cleavage, which was inhibited by treatment with insulin, while pre-treatment with isoproterenol abolished the insulin effect. Hypoxia-induced cleavage of caspase-3 was mediated by p38 mitogen-activated protein kinase (MAPK). Insulin inhibited hypoxia-induced phosphorylation of p38 through MAPK phosphatase-1 (MKP-1). Insulin-induced MKP-1 expression was mediated by extracellular signal-regulated protein kinases (ERK) 1/2, c-Jun NH2-terminal kinases (JNK) MAPK, and phosphatidylinositol 3-kinase (PI3K)/Akt pathways. Isoproterenol stimulation failed to induce expression of MKP-1; moreover, insulin resistance induced by long-term beta-adrenergic stimulation inhibited insulin-evoked expression of MKP-1 by impairing insulin-induced phosphorylation of both ERK1/2 and JNK without affecting Akt kinase activity. Furthermore, concomitant activation of Akt, ERK 1/2, and JNK was required for insulin to exert its protective effect against the hypoxia-induced cleavage of caspase-3. CONCLUSIONS: The results of this study lead to the conclusions that, in cardiac myocytes, antiapoptotic signals induced by insulin are mediated by more than one signaling pathway, and that long-term beta-adrenergic receptor stimulation impairing some of these pathways affects the cytoprotective action of insulin.     

Hyperosmolarity triggers CD95 membrane trafficking and sensitizes rat hepatocytes toward CD95L-induced apoptosis

The effect of hyperosmolarity on CD95 membrane targeting and CD95 ligand (CD95L)-induced apoptosis was studied in rat hepatocytes. CD95 showed a predominant intracellular localization in normoosmotically exposed rat hepatocytes, whereas hyperosmotic exposure induced, within 1 hour, CD95 trafficking to the plasma membrane followed by activation of caspase-3 and -8. Hyperosmotic CD95 membrane targeting was sensitive to inhibition of c-Jun-N-terminal kinase (JNK), protein kinase C (PKC), and cyclic adenosine monophosphate, but not to inhibition of extracellular regulated kinases (Erks) or p38 mitogen activated protein kinase (p38(MAPK)). Hyperosmotic CD95 targeting to the plasma membrane was dose-dependently diminished by glutamine or taurine, probably caused by an augmentation of volume regulatory increase. Despite CD95 trafficking to the plasma membrane and caspase activation, hyperosmolarity per se did not induce apoptosis. Hyperosmolarity, however, sensitized hepatocytes toward CD95L-induced apoptosis, as assessed by annexin V staining and terminal deoxynucleotidyl transferase-mediated X-dUTP nick-end labeling (TUNEL) assay. This sensitization was abolished when hyperosmotic CD95 membrane trafficking was prevented by cyclic adenosine monophosphate, PKC, or JNK inhibition, whereas these effectors had no effect on CD95L-induced apoptosis in normoosmotically exposed hepatocytes. CD95L addition under normoosmotic conditions caused CD95 membrane trafficking, which was sensitive to JNK inhibition, but not to cyclic adenosine monophosphate or inhibition of PKC, Erks, and p38(MAPK). In conclusion, multiple signaling pathways are involved in CD95 membrane trafficking. Hyperosmotic hepatocyte shrinkage induces CD95 trafficking to the plasma membrane, which involves JNK-, PKA-, and PKC-dependent mechanisms and sensitizes hepatocytes toward CD95L-mediated apoptosis.     

Stress-activated MAP kinases in cardiac remodeling and heart failure; new insights from transgenic studies

Activation of stress-activated mitogen-activated protein kinases (SAPKs), mainly c-Jun N-terminal kinase (JNK) and p38, have long been associated with different forms of cardiac pathology across a wide spectrum of species. However, their specific roles in the development of heart failure are still unclear. Previous studies in neonatal myocytes in culture suggest a critical role for both JNK and p38 in hypertrophy and apoptosis. A far more complex picture has been provided by recent observations from both cellular and transgenic models that have not only challenged their role in hypertrophy and cell death but have also pointed out novel functions of SAPKs in different aspects of cardiac pathology, including contractile function, extracellular matrix remodeling, intercellular communication, and metabolic regulation.