Angiotensin II stimulates c-Jun NH2-terminal kinase in cultured cardiac myocytes of neonatal rats

Reactive oxygen species modulate angiotensin II-induced beta-myosin heavy chain gene expression via Ras/Raf/extracellular signal-regulated kinase pathway in neonatal rat cardiomyocytes

Angiotensin II (Ang II) causes cardiomyocytes hypertrophy. Cardiac beta-myosin heavy chain (beta-MyHC) gene expression can be altered by Ang II. The molecular mechanisms are not completely known. Reactive oxygen species (ROS) are involved in signal transduction pathways of Ang II. However, the role of ROS on Ang II-induced beta-MyHC gene expression remains unclear. Here we found that Ang II increased beta-MyHC promoter activity and it was blocked by Ang II type 1 receptor antagonist losartan. Ang II dose-dependently increased the intracellular ROS. Cardiomyocytes cotransfected with a dominant negative mutant of Ras (RasN17), Raf-1 (Raf301), or a catalytically inactive mutant of extracellular signal regulated kinase (mERK2) inhibited Ang II-induced beta-MyHC promoter activity, indicating Ras/Raf/ERK pathway was involved. Antioxidants such as catalase or N-acetyl-cysteine decreased Ang II-activated ERK phosphorylation and inhibited Ang II-induced beta-MyHC promoter activity. These data indicate that Ang II increases beta-MyHC gene expression in part via the generation of ROS.

Stage-specific differential activation of mitogen-activated protein kinases in hypertrophied and failing rat hearts

Mitogen-activated protein kinases (MAPKs) are involved in the early development of cardiac hypertrophy, but their roles in chronic left ventricular hypertrophy (LVH) are unclear. We studied the angiotensin (Ang) II-induced cardiac MAPK activation of the hypertensive Dahl salt-sensitive (DS) rats in the subacute developing LVH stage, the chronic compensated LVH stage, and the congestive heart failure (CHF) stage. In the isolated, coronary-perfused heart preparation, Ang II infusion (1x10(-6)mol/l) activated extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK) and p38-MAPK in the LV myocardium. No substantial differences were observed in the Ang II-induced ERK activation between the normotensive control DS rats and the hypertensive DS rats in either stage. In contrast, the Ang II-induced activation of JNK and p38-MAPK was augmented in the subacute LVH stage of the hypertensive DS rats, but then progressively attenuated in the chronic LVH and CHF stages. Chronic treatment with an angiotensin converting enzyme inhibitor, temocapril (20 mg/kg/day), ameliorated the responsiveness of the JNK/p38-MAPK activation, suggesting that the decreased JNK/p38-MAPK activation is a consequence of negative feedback regulation for the activated cardiac renin-angiotensin system in chronic LVH and CHF. Thus, the Ang II-induced activation of multiple cardiac MAPK pathways are differentially regulated, depending on the stages of chronic hypertrophic process. The JNK and p38-MAPK activation may be involved in the early development of adaptive LVH. However, the responsiveness of the cardiac JNK/p38-MAPK pathways progressively decreased in chronic LVH and CHF under the chronic activation of tissue renin-angiotensin system.

Activation of calcineurin and stress activated protein kinase/p38-mitogen activated protein kinase in hearts of utrophin-dystrophin knockout mice

Dilated cardiomyopathy is a common complication of Duchenne and Becker muscular dystrophies, which are caused by mutations in the dystrophin gene. The mdx mouse is an animal model for Duchenne muscular dystrophy (DMD) and shows mildly dystrophic changes in the heart. By contrast, the utrophin-dystrophin knockout (dko) mouse shows severe dystrophic changes in cardiac muscle, that more closely resembles DMD cardiomyopathy than mdx mouse. However the pathogenesis of development has not been fully understood. Recently many reports have revealed that calcineurin and stress activated protein kinase (SAPK)/p38-mitogen activated protein kinase (MAPK) hypertrophic signalling pathways are associated with the development of some forms of hypertrophic and dilated cardiomyopathies. These signalling pathways may have some roles in the development of dystrophin-deficient cardiomyopathy. Here we report that calcineurin and SAPK/p38-MAPK signalling pathways were constantly activated in dko hearts, but the activation varied in mdx hearts. The pathogenesis of the development of dystrophin-deficient cardiomyopathy may be associated with the activation of these signalling pathways.

Apoptosis induction and inhibition of cellular proliferation by angiotensin II: possible implication and perspectives

The renin-angiotensin system plays a pivotal role in the regulation of fluid, electrolyte metabolism and blood pressure. Molecular cloning and pharmacological studies have defined two major classes of Angiotensin II (Ang II) receptors, designated AT1 and AT2. Recently, it has been well recognized that Ang II, beside its classical physiological actions, is a profibrogenic peptide and displays characteristics of a growth factor. The emerging picture suggests that angiotensin receptor subtypes exert opposing features in many aspects of their biological function, most importantly in cellular growth and proliferation. Accordingly, the proliferative and/or growth-promoting effects of Ang II are thought to be mediated by AT1 receptor, whereas the AT2 receptor subtype may have growth-inhibitory properties. The novel finding that Ang II is able to induce apoptosis by AT2 receptors in diverse cell types is of great scientific interest, as recent studies revealed a role for apoptosis as a deliberate form of cell death in the pathogenesis of various cardiovascular diseases such as heart failure and vascular remodeling. Furthermore apoptotic cell death might occur during the development of progressive glomerulosclerosis. It is tempting to speculate that autocrine-paracrine vasoactive substances such as Ang II might regulate these apoptotic processes during pathogenic conditions.

Mechanism of the cardioprotective effect of inhibition of the renin-angiotensin system on ischemia/reperfusion-induced myocardial injury

Inhibition of the renin-angiotensin system (RAS) has been shown to be beneficial in providing cardioprotective effects in humans, but the mechanism of these effects is not well understood. In this study, we examined the effects and mechanism of RAS inhibitors on ischemia/reperfusion (IR)-induced myocardial injury in rats. Rats were randomly divided into five groups and treated with vehicle (C), angiotensin converting enzyme inhibitor (ACE-I), angiotensin II type 1 receptor antagonist (AT1-A), angiotensin II type 2 receptor antagonist (AT2-A) or ACE-I plus bradykinin B2 antagonist. Ten minutes after administration, the left main coronary artery was ligated for 45 min, and then reperfused for 120 min. IR-induced cardiomyocyte apoptosis was assessed by terminal deoxyribonucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) assay and confirmed by typical DNA laddering. Mitogen-activated protein kinase, extracellular signal-regulated protein kinase (ERK) and c-Jun NH2-terminal protein kinase (JNK) activity in the ischemic zone were measured by an in vitro kinase assay. The duration of ventricular tachycardia (VT) during ischemia was reduced by AT2-A and ACE-I, and increased by AT1-A and ACE-I+icatibant. ACE-I and AT2-A reduced apoptosis (by 54% and 53%) and infarct size (by 42% and 41%), while AT1-A increased apoptosis (by 86%) and infarct size (by 45%). These changes were negatively correlated with the change in ERK activity. The effects of ACE-I on apoptosis and infarct size were abolished by the coadministration of icatibant. Apoptosis was correlated with the occurrence of VT (r=0.837, p<0.001). These results suggest that both the accumulation of bradykinin and inhibition of AT2 receptor are cardioprotective against IR injury through the activation of ERK, but not JNK.

Antioxidant therapy attenuates JNK activation and apoptosis in the remote noninfarcted myocardium after large myocardial infarction

We hypothesized that the concomitant occurrence of increased oxidative stress, JNK activation, and myocyte apoptosis in the remote myocardium (RM) following a large myocardial infarction (MI) are causally related. Three days following coronary ligation, rats were randomized to treatment with probucol and PDTC (MI-T) or vehicle (MI). Control rats (C) underwent sham operation. At 7 weeks, TBARS assay showed increased level of lipid-peroxidation within the RM in the MI group vs C, which was completely inhibited in the MI-T group. Similarly, Western blot analysis showed a twofold increase in p-JNK in the MI group, vs C, which was attenuated in MI-T, a result confirmed by a JNK-kinase activity. Furthermore, apoptosis was increased within the RM in MI vs C, while this was inhibited in MI-T. We conclude that long-term antioxidant therapy with probucol and PDTC attenuates oxidative stress, JNK activation, and myocyte apoptosis within the RM after large MI.

Calcineurin blockade prevents cardiac mitogen-activated protein kinase activation and hypertrophy in renovascular hypertension

Chronic stimulation of the renin-angiotensin system induces an elevation of blood pressure and the development of cardiac hypertrophy via the actions of its effector, angiotensin II. In cardiomyocytes, mitogen-activated protein kinases as well as protein kinase C isoforms have been shown to be important in the transduction of trophic signals. The Ca(2+)/calmodulin-dependent phosphatase calcineurin has also been suggested to play a role in cardiac growth. In the present report, we investigate possible cross-talks between calcineurin, protein kinase C, and mitogen-activated protein kinase pathways in controlling angiotensin II-induced hypertrophy. Angiotensin II-stimulated cardiomyocytes and mice with angiotensin II-dependent renovascular hypertension were treated with the calcineurin inhibitor cyclosporin A. Calcineurin, protein kinase C, and mitogen-activated protein kinase activations were determined. We show that cyclosporin A blocks angiotensin II-induced mitogen-activated protein kinase activation in cultured primary cardiomyocytes and in the heart of hypertensive mice. Cyclosporin A also inhibits specific protein kinase C isoforms. In vivo, cyclosporin A prevents the development of cardiac hypertrophy, and this effect appears to be independent of hemodynamic changes. These data suggest cross-talks between the calcineurin pathway, the protein kinase C, and the mitogen-activated protein kinase signaling cascades in transducing angiotensin II-mediated stimuli in cardiomyocytes and could provide the basis for an integrated model of cardiac hypertrophy.

Interaction between angiotensin II and Smad proteins in fibroblasts in failing heart and in vitro

Angiotensin II (angiotensin) and transforming growth factor (TGF)-beta(1) play an important role in cardiac fibrosis. We examined Smad proteins in 8-wk post-myocardial infarction (MI) rat hearts. AT(1) blockade (losartan) attenuated the activation of TGF-beta(1) in target tissues. Losartan administration (8 wk, 15 mg. kg(-1). day(-1)) normalized total Smad 2 overexpression in infarct scar and remnant heart tissue and normalized Smad 4 in infarct scar. Phosphorylated Smad 2 (P-Smad 2) staining decreased in cytosol from failing heart vs. the control, which was normalized by losartan, suggesting augmented P-Smad 2 movement into nuclei in untreated failing hearts. Using adult primary rat fibroblasts treated with angiotensin (10(-6) M), we noted rapid translocation (15 min) of P-Smad 2 into the nuclei from the cytosol. Nuclear P-Smad 2 protein level increased with angiotensin treatment, which was blocked by losartan. We conclude that angiotensin may influence total Smad 2 and 4 expression in post-MI heart failure and that angiotensin treatment is associated with rapid P-Smad 2 nuclear translocation in isolated fibroblasts. This study suggests that cross talk between angiotensin and Smad signaling is associated with fibrotic events in post-MI hearts.

Stretch activation of jun N-terminal kinase/stress-activated protein kinase in mesangial cells

BACKGROUND

Mesangial cells (MCs) grown on extracellular matrix (ECM)-coated plates and exposed to cyclic stretch/relaxation proliferate and produce ECM protein, suggesting that this may be a useful in vitro model for MC behavior in response to increased physical forces. The induction of c-fos in response to MC stretch has been shown. Stimuli that lead to c-fos induction pass through mitogen-activated protein (MAP) kinase pathways. We have seen early activation of jun N-terminal kinase/stress-activated protein kinase (SAPK/JNK) in MCs exposed to cyclic stretch. Accordingly, we studied SAPK/JNK activation in stretched MCs and the downstream consequences of this signaling.

METHODS

MCs (passages 5 to 10) cultured on type 1 collagen-coated, flexible-bottom plates were exposed to 2 to 60 minutes of cyclic strain (60 cycles per minute) by generation of vacuums of -10 to -27 kPa, inducing approximately 16 to 28% maximum elongation in the diameter of the surfaces. Control MCs were grown on coated rigid bottom plates. Protein levels (by Western blot) and activity assays for SAPK/JNK were performed under these conditions. We observed marked activation at -18 kPa and above and at two minutes, and then we studied activation mechanisms under these conditions. Nuclear protein binding to activator protein-1 (AP-1) consensus sequences was also examined. The role of calcium was studied with EGTA and BAPTA-AM to chelate extra- and intracellular calcium, respectively. Protein kinase C (PKC) was down-regulated by incubation with phorbol ester (PMA) for 24 hours prior to stretch. In unstretched MCs, A23187 was used as a calcium ionophore, and PKC was up-regulated with PMA application for 30 minutes to determine the effects on SAPK/JNK. Nuclear protein binding to AP-1 was also determined under these conditions. The effects of stretch, acute PMA, and A23187 on fibronectin mRNA levels were studied using reverse transcriptase-polymerase chain reaction (RT-PCR).

RESULTS

Cyclic strain/relaxation led to increased SAPK/JNK activity only at two minutes and -18 kPa and above. The activation of SAPK/JNK was dependent on intracellular calcium, with BAPTA-AM almost completely abrogating the response to stretch. EGTA was without effect. Down-regulation of PKC also led to a diminution of activity. In static cells, the calcium ionophore A23187 increased SAPK/JNK activity, and this was potentiated by acute PMA. Stretch, acute PMA, and A23187 all increased nuclear protein binding to AP-1 consensus sequences. mRNA levels for fibronectin were increased by stretch in MCs and by PMA and A23187 in static MCs. No change was observed in the amount of SAPK/JNK protein present in stretched MCs by Western blot.

CONCLUSIONS

Stretch leads to early activation of SAPK/JNK in MCs. This is dependent on intracellular calcium and PKC and can be replicated by activation of these stimuli in static MCs. A downstream induction of nuclear protein binding to AP-1 consensus sequences was seen in a pattern that was completely concordant with the SAPK/JNK induction.

Taurine attenuates hypertrophy induced by angiotensin II in cultured neonatal rat cardiac myocytes

The effect of taurine on angiotensin II-induced changes in cell morphology and biochemistry of the cultured neonatal cardiomyocyte was examined. Angiotensin II (1-100 nM) alone caused a slow increase in the surface area of the myocyte accompanied by an induction of the expression of atrial natriuretic peptide (ANP) and an upregulation of transforming growth factor beta(1) gene (TGF-beta(1)). The signaling pathway of angiotensin II (1-100 nM) was found to proceed through protein kinase C and the rapid activation of mitogen-activated protein (MAP) kinases. Pretreatment of the myocyte with taurine (20 mM) in the absence of angiotensin II had no visible effect on cell size or growth rate. However, the cells that were pretreated with taurine (20 mM) for 24 h exhibited reduced responsiveness to angiotensin II (100 nM) relative to surface cell area enlargement and the upregulation of the late and growth factor genes(ANP, TGF-beta(1)). Angiotensin II-mediated activation of the MAP kinases (extracellular signal-regulated protein kinase 1/2: ERK1/2) was not blocked by taurine. Taurine reduced the phosphorylation of a 29-kDa protein, a reaction which was enhanced by angiotensin II and appears to involve protein kinase C step. The results indicate that taurine is an effective inhibitor of certain aspects of angiotensin II action.

Angiotensin II-induced cardiac hypertrophy is associated with different mitogen-activated protein kinase activation in normotensive and hypertensive mice

OBJECTIVE

In addition to its haemodynamic effects, angiotensin II (AngII) is thought to contribute to the development of cardiac hypertrophy via its growth factor properties. The activation of mitogen-activated protein kinases (MAPK) is crucial for stimulating cardiac growth. Therefore, the present study aimed to determine whether the trophic effects of AngII and the AngII-induced haemodynamic load were associated with specific cardiac MAPK pathways during the development of hypertrophy. Methods The activation of the extracellular-signal-regulated kinase (ERK), the c-jun N-terminal kinase (JNK) and the p38 kinase was followed in the heart of normotensive and hypertensive transgenic mice with AngII-mediated cardiac hypertrophy. Secondly, we used physiological models of AngII-dependent and AngII-independent renovascular hypertension to study the activation of cardiac MAPK pathways during the development of hypertrophy.

RESULTS

In normotensive transgenic animals with AngII-induced cardiac hypertrophy, p38 activation is associated with the development of hypertrophy while ERK and JNK are modestly stimulated. In hypertensive transgenic mice, further activation of ERK and JNK is observed. Moreover, in the AngII-independent model of renovascular hypertension and cardiac hypertrophy, p38 is not activated while ERK and JNK are strongly stimulated. In contrast, in the AngII-dependent model, all three kinases are stimulated.

CONCLUSIONS

These data suggest that p38 activation is preferentially associated with the direct effects of AngII on cardiac cells, whereas stimulation of ERK and JNK occurs in association with AngII-induced mechanical stress.

Evidence for angiotensin II type 2 receptor-mediated cardiac myocyte enlargement during in vivo pressure overload

The pathophysiological roles of the angiotensin II type 2 receptor (AT(2)) in cardiac hypertrophy remain unclear. By the targeted deletion of mouse AT(2) we were able to prevent the left ventricular hypertrophy resulting from pressure overload, while cardiac contractile functions remained normal. This implies that AT(2) is a mediator of cardiac hypertrophy in response to increased blood pressure. The effects of AT(2) deletion were independent of activation of embryonic genes for cardiac hypertrophy. However, p70(S6k), one of the key factors in cardiac hypertrophy, was markedly and specifically reduced in the ventricles of Agtr2(-)/Y mice. We propose that p70(S6k) plays a major role in AT(2)-mediated ventricular hypertrophy. This article may have been published online in advance of the print edition. The date of publication is available from the JCI website, http://www.jci.org.

Ca(2+)-dependent activation of c-jun NH(2)-terminal kinase in primary rabbit proximal tubule epithelial cells

Previous work from this laboratory demonstrated that arachidonic acid activates c-jun NH(2)-terminal kinase (JNK) through oxidative intermediates in a Ca(2+)-independent manner (Cui X and Douglas JG. Arachidonic acid activates c-jun N-terminal kinase through NADPH oxidase in rabbit proximal tubular epithelial cells. Proc Natl Acad Sci USA 94: 3771-3776, 1997.). We now report that JNK can also be activated via a Ca(2+)-dependent mechanism by agents that increase the cytosolic Ca(2+) concentration (Ca(2+) ionophore A(23187), Ca(2+)-ATPase inhibitor thapsigargin) or deplete intracellular Ca(2+) stores [intracellular Ca(2+) chelator 1, 2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA)-AM]. The activation of JNK by BAPTA-AM occurs despite a decrease in cytosolic Ca(2+) concentration as detected by the indicator dye fura 2, but appears to be related to Ca(2+) metabolism, because modification of BAPTA with two methyl groups increases not only the chelation affinity for Ca(2+), but also the potency for JNK activation. BAPTA-AM stimulates Ca(2+) influx across the plasma membrane, and the resulting local Ca(2+) increases are probably involved in activation of JNK because Ca(2+) influx inhibitors (SKF-96365, nifedipine) and lowering of the free extracellular Ca(2+) concentration with EGTA reduce the BAPTA-induced JNK activation.

Intact mitochondrial electron transport function is essential for signalling by hydrogen peroxide in cardiac myocytes

Oxidative stress has been proposed as a mediator of cardiac injury during ischemia and reperfusion. We examined the signalling events initiated by short-term exposure of cardiac myocytes to oxidative stress elicited by hydrogen peroxide. A potent stimulation of tyrosine phosphorylation was observed within 1 to 2 min exposure to 1 m m hydrogen peroxide. Within 5 min, the ERK mitogen-activated protein kinases (ERK MAPKs) were activated. This activation of ERK MAPKs was blocked by N-acetylcysteine (NAC), implicating a role for free radicals in the signalling events. NAC failed to inhibit ERK MAPK activation by the hypertrophic agent, phenylephrine, or hyperosmotic shock. Myxothiazol, an inhibitor of complex III of the mitochondrial electron transport chain, also inhibited ERK MAPK activation by hydrogen peroxide, but not by 12- O -tetradecanoylphorbol-13-acetate (TPA) or hyperosmotic shock. Myxothiazol completely inhibited the increase in tyrosine phosphorylated proteins observed with hydrogen peroxide treatment. A variety of inhibitors which act at different levels of the mitochondrial electron transport chain (rotenone, theonyltrifluoroacetone, antimycin A, cyanide) also inhibited activation of the ERK MAPKs by hydrogen peroxide but not TPA or hyperosmotic shock. These studies suggest a novel mechanism of regulation of the ERK MAPK pathway and oxidative stress signalling by hydrogen peroxide.

Isoenzyme-specific protein kinase C and c-Jun N-terminal kinase activation by electrically stimulated contraction of neonatal rat ventricular myocytes

Previous studies from our laboratory and others indicate that contraction-induced mechanical loading of cultured neonatal rat ventricular myocytes produces many of the phenotypic changes associated with cardiomyocyte hypertrophy in vivo, and that these changes occur via the activation of serine-threonine protein kinases. These may include the extracellular regulated protein kinases (ERK1 and ERK2), the c-Jun N-terminal kinases (JNK1, JNK2, and JNK3), and one or more isoenzymes of protein kinase C. In this study, we assessed whether one or more of these kinases are activated by stimulated contraction, and whether activation was isoenzyme-specific. Low-density, quiescent cultures of neonatal rat ventricular myocytes were maintained in serum-free medium, or electrically stimulated to contract (3 Hz) for up to 48 h. ERK and JNK activation was assessed by Western blotting with polyclonal antibodies specific for the phosphorylated forms of both kinases. PKC activation was analysed by subcellular fractionation, detergent extraction, and Western blotting using isoenzyme-specific monoclonal antibodies. Stimulated contractile activity produced myocyte hypertrophy, as indicated by increased cell size, a 15+/-5% increase in total protein/DNA ratio, and induction of ANF and beta MHC gene transcription. Electrical pacing did not cause ERK1/2 or JNK1 activation, but increased JNK2 and JNK3 phosphorylation by;two-fold. Subcellular fractionation revealed a time-dependent increase in PKC delta, and to a much lesser extent PKC xi, in a Triton X-100-soluble membrane fraction within 5 min of the onset of stimulated contraction. PKC alpha was not activated by electrical pacing. These results indicate that contraction-induced mechanical loading acutely activates some but not all of the specific isoenzymes of JNKs and PKCs in cardiomyocytes.

PKC-dependent delayed metabolic preconditioning is independent of transient MAPK activation

In this study we used an in vitro model of delayed preconditioning to investigate activation of mitogen-activated protein kinases (MAPKs) and their potential role in protection. Neonatal rat cardiomyocytes were preconditioned using a buffer containing glycolytic inhibitors and low pH (minimal metabolic preconditioning; MMPC) consisting of modified Krebs buffer, 10 mM 2-deoxyglucose, and 20 mM lactate, pH 6.8, for 2 h followed by 24 h of simulated reperfusion before lethal simulated ischemia (LSI). MAPK activation during the MMPC protocol was determined using phospho-specific antisera and the effect on protection determined following LSI. Rapid, transient phosphorylation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) and p38 MAPK was observed during each of the MMPC, reperfusion, and LSI phases; an effect blocked by MAPK inhibitors PD-98059 and SB-203580, respectively, but not by the protein kinase C (PKC) inhibitor Ro31-8220. However, although MMPC was blocked by Ro31-8220, treatment with the MAPK inhibitors during the preconditioning protocol did not block delayed protection conferred by MMPC. Thus the data suggest that, in this model of delayed preconditioning, protection appears to be PKC dependent but independent of ERK1/2 or p38 MAPK activation.

Activation of the luteinizing hormone beta promoter by gonadotropin-releasing hormone requires c-Jun NH2-terminal protein kinase

Regulation of the mitogen-activated protein kinase (MAPK) family by gonadotropin-releasing hormone (GnRH) in the gonadotrope cell line LbetaT2 was investigated. Treatment with gonadotropin-releasing hormone agonist (GnRHa) activates extracellular signal-regulated kinase (ERK) and c-Jun NH(2)-terminal kinase (JNK). Activation of ERK by GnRHa occurred within 5 min, and declined thereafter, whereas activation of JNK by GnRHa occurred with a different time frame, i.e. it was detectable at 5 min, reached a plateau at 30 min, and declined thereafter. GnRHa-induced ERK activation was dependent on protein kinase C or extracellular and intracellular Ca(2+), whereas GnRHa-induced JNK activation was not dependent on protein kinase C or on extracellular or intracellular Ca(2+). To determine whether a mitogen-activated protein kinase family cascade regulates rat luteinizing hormone beta (LHbeta) promoter activity, we transfected the rat LHbeta (-156 to +7)-luciferase construct into LbetaT2 cells. GnRH activated the rat LHbeta promoter activity in a time-dependent manner. Neither treatment with a mitogen-activated protein kinase/ERK kinase (MEK) inhibitor, PD98059, nor cotransfection with a catalytically inactive form of a mitogen-activated protein kinase construct inhibited the induction of the rat LHbeta promoter by GnRH. Furthermore, cotransfection with a dominant negative Ets had no effect on the response of the rat LHbeta promoter to GnRH. On the other hand, cotransfection with either dominant negative JNK or dominant negative c-Jun significantly inhibited the induction of the rat LHbeta promoter by GnRH. In addition, GnRH did not induce either the rat LHbeta promoter activity in LbetaT2 cells transfected stably with dominant negative c-Jun. These results suggest that GnRHa differentially activates ERK and JNK, and a JNK cascade is necessary to elicit the rat LHbeta promoter activity in a c-Jun-dependent mechanism in LbetaT2 cells.

Divergent signaling pathways requiring discrete calcium signals mediate concurrent activation of two mitogen-activated protein kinases by gonadotropin-releasing hormone

Receptors coupled to heterotrimeric G proteins are linked to activation of mitogen-activated protein kinases (MAPKs) via receptor- and cell-specific mechanisms. We have demonstrated recently that gonadotropin-releasing hormone (GnRH) receptor occupancy results in activation of extracellular signal-regulated kinase (ERK) through a mechanism requiring calcium influx through L-type calcium channels in alphaT3-1 cells and primary rat gonadotropes. Further studies were undertaken to explore the signaling mechanisms by which the GnRH receptor is coupled to activation of another member of the MAPK family, c-Jun N-terminal kinase (JNK). GnRH induces activation of the JNK cascade in a dose-, time-, and receptor-dependent manner in clonal alphaT3-1 cells and primary rat pituitary gonadotrophs. Coexpression of dominant negative Cdc42 and kinase-defective p21-activated kinase 1 and MAPK kinase 7 with JNK and ERK indicated that specific activation of JNK by GnRH appears to involve these signaling molecules. Unlike ERK activation, GnRH-stimulated JNK activity does not require activation of protein kinase C and is not blocked after chelation of extracellular calcium with EGTA. GnRH-induced JNK activity was reduced after treatment with the intracellular calcium chelator BAPTA-AM (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester), whereas activation of ERK was not affected. Chelation of intracellular calcium also reduced GnRH-induced activation of JNK in rat pituitary cells in primary culture. GnRH-induced induction and activation of the JNK target c-Jun was inhibited after chelation of intracellular calcium, whereas induction of c-Fos, a known target of ERK, was unaffected. Therefore, although activation of ERK by GnRH requires a specific influx of calcium through L-type calcium channels, JNK activation is independent of extracellular calcium but sensitive to chelation of intracellular calcium. Our results provide novel evidence that GnRH activates two MAPK superfamily members via strikingly divergent signaling pathways with differential sensitivity to activation of protein kinase C and mobilization of discrete pools of calcium.

Angiotensin II and basic fibroblast growth factor mitogenic pathways in human fetal mesangial cells

Angiotensin II (Ang II) and basic fibroblast growth factor (bFGF/FGF-2) play relevant roles in renal development. Since the signaling pathways modulating the mitogenic effects of Ang II and bFGF in human fetal mesangial cells (HFMc) are not clearly defined, we carried out experiments to determine whether they would exert their mitogenic effects by modulating the activity of the mitogen-activated protein kinases (MAPK) [extracellular signal-regulated kinase-2 (ERK-2)] and cAMP signaling pathways. In confluent HFMc, bFGF (20 ng/mL) induced a significant 4-fold increase in ERK-2 activity and [3H]-thymidine incorporation (6-fold). In contrast, under similar tissue culture conditions, Ang II (10(-6) M) induced a more modest increase in ERK-2 activity (2-fold) and [3H]-thymidine incorporation (35 +/- 4%). The mitogen-activated protein kinase kinase-1 (MEK-1) inhibitor PD098059 (25 microM) almost completely abolished the bFGF-induced proliferation in HFMc but did not significantly affect Ang II proliferative effects. In the presence of the cAMP elevating agent isoproterenol, Ang II and bFGF induced opposite changes in cAMP accumulation and cell growth. Isoproterenol inhibited the basal and bFGF-induced proliferation of HFMc through a MEK-1/2-independent pathway that included the accumulation of cAMP. In contrast, isoproterenol increased Ang II mitogenic effects in correlation with a reduction in cAMP accumulation. We conclude that Ang II and bFGF modulate the proliferation of HFMc through the stimulation of different MEK-1/2-dependent and independent signaling pathways. Activation of MEK-1/2 is required but not sufficient for mitogenesis in HFMc. The accumulation of cAMP in HFMc counteracts the mitogenic effects of bFGF by a MEK-1/2-independent pathway.

Molecular mechanism of mechanical stress-induced cardiac hypertrophy

Mechanical stress is a major cause of cardiac hypertrophy. Although the mechanisms by which mechanical load induces cardiomyocyte hypertrophy have long been a subject of great interest for cardiologists, the lack of a good in vitro system has hampered the understanding of the biochemical mechanisms. For these past several years, however, an in vitro neonatal cardiocyte culture system has made it possible to examine the biochemical basis for the signal transduction of mechanical stress. Passive stretch of cardiac myocytes cultured on silicone membranes activates phosphorylation cascades of many protein kinases including protein kinase C, Raf-1 kinase and extracellular signal regulated kinases, and induces the expression of specific genes as well as an increase in protein synthesis. During that process, the secretion and production of vasoactive peptides such as angiotensin II and endothelin, are increased and they play critical roles in the induction of these hypertrophic responses. Although the involvement of vasoactive peptides in the development of cardiac hypertrophy is clinically important, the "mechanoreceptor" which receives the mechanical stress and converts it into intracellular biochemical signals remained unknown. We have recently obtained evidence suggesting that ion channels and integrins may be the "mechanoreceptor", the activation of which leads to cardiac hypertrophy.

Hydrogen peroxide dose dependent induction of cell death or hypertrophy in cardiomyocytes

Cardiomyocyte hypertrophy and cell death are often observed in the end stages of heart failure. The triggers of these two cellular processes are not known under most pathological conditions. Oxidants are by-products of aerobic metabolism. The level of oxidants increases as a result of ischemic reperfusion. Using H9C2 and primary cultured neonatal rat cardiomyocytes, we found that a 2-h pulse treatment with H(2)O(2) at 250 microM or lower caused activation of DEVD sequence specific caspases. The activity of DEVD-ase peaked with 200 microM H(2)O(2) at 24 h. While a fraction of the cells detached and showed nuclear condensation, the majority of the cells (>55%) survived the treatment and appeared to enlarge when cultured for 5 days. These cells showed increases in cell surface area, cell volume, and protein content. With 200 microM H(2)O(2), treated cells appeared to be six times bigger in volume and contained three times more protein per cell than untreated cells. The enlarged cells showed enhanced actin stress fibers and disrupted myofibrils. Our data indicate that while H(2)O(2) can cause cell death, the surviving cardiomyocytes undergo hypertrophy.

Inhibition of extracellular signal-regulated protein kinase or c-Jun N-terminal protein kinase cascade, differentially activated by cisplatin, sensitizes human ovarian cancer cell line

We have studied the roles of c-Jun N-terminal protein kinase (JNK) and extracellular signal-regulated protein kinase (ERK) cascade in both the cisplatin-resistant Caov-3 and the cisplatin-sensitive A2780 human ovarian cancer cell lines. Treatment of both cells with cisplatin but not transplatin isomer activates JNK and ERK. Activation of JNK by cisplatin occurred at 30 min, reached a plateau at 3 h, and declined thereafter, whereas activation of ERK by cisplatin showed a biphasic pattern, indicating the different time frame. Activation of JNK by cisplatin was maximal at 1000 microM, whereas activation of ERK was maximal at 100 microM and was less at higher concentrations, indicating the different dose dependence. Cisplatin-induced JNK activation was neither extracellular and intracellular Ca(2+)- nor protein kinase C-dependent, whereas cisplatin-induced ERK activation was extracellular and intracellular Ca(2+)- dependent and protein kinase C-dependent. A mitogen-activated protein kinase/extracellular signal-regulated kinase kinase inhibitor, PD98059, had no effect on the cisplatin-induced JNK activity, suggesting an absence of cross-talk between the ERK and JNK cascades. We further examined the effect of each cascade on the viability following cisplatin treatment. Either exogenous expression of dominant negative c-Jun or the treatment by PD98059 induced sensitivity to cisplatin in both cells. Our findings suggest that cisplatin-induced DNA damage differentially activates JNK and ERK cascades and that inhibition of either of these cascades sensitizes ovarian cancer cells to cisplatin.

Role of angiotensin AT1, and AT2 receptors in cardiac hypertrophy and disease

Angiotensin II modulates beat-to-beat cardiac performance as a potent vasocontrictor, inotrope, and regulator of water and electrolyte balance. It is also a growth factor that can stimulate the early molecular growth responses of proto-oncogene activation and new protein synthesis, and the later event of cardiocyte hypertrophy independent from load. Its effects are mediated through the angiotensin II type 1 (AT1) receptor, which exists as the AT1a and AT1b isoforms, and the angiotensin II type 2 (AT2) receptor. There is still controversy regarding the role of activation of the AT1 receptor subtype(s) as a mandatory signal versus modulatory regulator of the transduction of mechanical load in pressure-overload hypertrophy due to hypertension or aortic stenosis. The role of the AT2 receptor subtype in the heart is even less well understood, although this receptor appears to serve as an antigrowth signal in proliferating cells. Here we review current data on these controversies, including new data that support the notion that angiotensin II activation of the cardiac AT2 receptor subtype inhibits the effects of angiotensin II on the immediate growth response in the adult heart.

The renin-angiotensin system and its receptors

The renin-angiotensin system (RAS) plays an important role in blood pressure control and in water and salt homeostasis. It is involved in the pathophysiology of hypertension and structural alterations of the vasculature, kidney, and heart, including neointima formation, nephrosclerosis, postinfarction remodeling, and cardiac left ventricular hypertrophy (LVH). Recently, an increased knowledge of the effector peptides of the RAS, their receptors, and their respective functions has led to a new principle of treatment for hypertension: the inhibition of angiotensin (Ang) II via angiotensin-converting enzyme inhibitors or Ang II-receptor antagonists. In this review, the Ang receptors AT1 and AT2 and the potential roles of shorter angiotensin fragments, including Ang III(2-8), Ang IV(3-8), and Ang(1-7), are discussed.

Modulation of angiotensin II signaling for GATA4 activation by homocysteine

Homocysteine (Hcy) is a redox active thiol-containing compound with pro-oxidant and pathogenic properties in the cardiovascular system. Angiotensin II (Ang II) also plays important roles in age-associated cardiovascular disease. Recently, the GATA4 transcription factor was recognized as a mediator of heart failure. We investigated the interrelationship of these elements in NIH/3T3 fibroblasts and found that Ang II induces GATA4 activity and Hcy alters Ang II signaling. Electrophoretic mobility shift assays determined that treatment of cells with Ang II induced DNA binding activity to the GATA consensus sequence. This activation was transient with a peak occurring at 30 min. Supershift analysis revealed the GATA binding protein as GATA4. Ang II also induced NFAT activity with similar kinetics. Pretreatment of cells with Hcy (100 microM) delayed the peak of Ang II-induced NFAT and GATA activation to 60 min. Ang II-mediated activation of c-fos serum response factor (SRF) was similarly delayed by Hcy. These results suggest the pathogenic mechanism of Hcy action may be mediated in part via modulation of Ang II-signaling for gene transcription.

An endothelin-transforming growth factor beta pathway in the nephrotoxicity of immunosuppressive drugs

An endothelin-transforming growth factor beta type 1 pathway is proposed to account for cyclosporin nephrotoxicity. Cyclosporin amplifies the production of endothelin. Enhanced endothelin production accentuates acute vascular events and promotes the synthesis and activation of transforming growth factor beta type 1, contributing to acute and chronic pathology. This scheme integrates many observations, including the involvement of the renin-angiotensin pathway and other activators of endothelin production, and provides a rationale for the amelioration of cyclosporin nephrotoxicity.

Regulation of c-Jun NH2-terminal kinases in isolated canine gastric parietal cells

c-Jun NH2-terminal kinases (JNKs) are protein kinases that are activated by a wide variety of extracellular signals. This study investigated the expression and regulation of JNKs in isolated gastric canine parietal cells. Western blot analysis of cell lysates from highly purified (>95%) parietal cells with an antibody recognizing JNK1 and to a lesser degree JNK2 revealed the presence of two bands of 46 and 54 kDa, respectively. JNK1 activity was quantitated by immunoprecipitation and in-gel kinase assays. Of the different agents tested, carbachol was the most potent inducer of JNK1 activity, whereas histamine and epidermal growth factor induced weaker responses. The proinflammatory cytokine tumor necrosis factor-alpha stimulated JNK1 but had no effect on extracellular signal-regulated kinase (ERK2) induction, suggesting that activation of JNK1 might represent an important event in mediation of the inflammatory response in the stomach. The action of carbachol was dose (0.1-100 microM) and time dependent, with a maximal stimulatory effect (fourfold) detected after 30 min of incubation and sustained for 2 h. Addition of the specific protein kinase C (PKC) inhibitor GF109203X did not affect the stimulatory action of carbachol. The intracellular Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N', N'-tetraacetic acid-AM inhibited carbachol induction of JNK1 activity by 60%. Thapsigargin (1 microM), an intracellular Ca2+-rising agent, induced JNK1 activity more than threefold. Carbachol activation of JNK1 resulted in induction of c-Jun (protein) transcriptional activity and in stimulation of parietal cell mRNA content of c-jun. In conclusion, our data indicate that carbachol induces JNK activity in gastric parietal cells via intracellular Ca2+-dependent, PKC-independent pathways, leading to induction of c-jun gene expression via phosphorylation and transcriptional activation of c-Jun.

Role of the stress-activated protein kinases in endothelin-induced cardiomyocyte hypertrophy

The signal transduction pathways governing the hypertrophic response of cardiomyocytes are not well defined. Constitutive activation of the stress-activated protein kinase (SAPK) family of mitogen-activated protein (MAP) kinases or another stress-response MAP kinase, p38, by overexpression of activated mutants of various components of the pathways is sufficient to induce a hypertrophic response in cardiomyocytes, but it is not clear what role these pathways play in the response to physiologically relevant hypertrophic stimuli. To determine the role of the SAPKs in the hypertrophic response, we used adenovirus-mediated gene transfer of SAPK/ERK kinase-1 (KR) [SEK-1(KR)], a dominant inhibitory mutant of SEK-1, the immediate upstream activator of the SAPKs, to block signal transmission down the SAPK pathway in response to the potent hypertrophic agent, endothelin-1 (ET-1). SEK-1(KR) completely inhibited ET-1-induced SAPK activation without affecting activation of the other MAP kinases implicated in the hypertrophic response, p38 and extracellular signal-regulated protein kinases (ERK)-1/ERK-2. Expression of SEK-1(KR) markedly inhibited the ET-1-induced increase in protein synthesis. In contrast, the MAPK/ERK kinase inhibitor, PD98059, which blocks ERK activation, and the p38 inhibitor, SB203580, had no effect on ET-1-induced protein synthesis. ET-1 also induced a significant increase in atrial natriuretic factor mRNA expression as well as in the percentage of cells with highly organized sarcomeres, responses which were also blocked by expression of SEK-1(KR). In summary, inhibiting activation of the SAPK pathway abrogated the hypertrophic response to ET-1. These data are the first demonstration that the SAPKs are necessary for the development of agonist-induced cardiomyocyte hypertrophy, and suggest that in response to ET-1, they transduce critical signals governing the hypertrophic response.

Mechanical stretch induces hypertrophic responses in cardiac myocytes of angiotensin II type 1a receptor knockout mice

Many lines of evidence have suggested that angiotensin II (AngII) plays an important role in the development of cardiac hypertrophy through AngII type 1 receptor (AT1). To determine whether AngII is indispensable for the development of mechanical stress-induced cardiac hypertrophy, we examined the activity of mitogen-activated protein kinase (MAPK) family and the expression of the c-fos gene as hypertrophic responses after stretching cultured cardiac myocytes of AT1a knockout (KO) mice. When cardiac myocytes were stretched by 20% for 10 min, extracellular signal-regulated protein kinases (ERKs) were strongly activated in KO cardiomyocytes as well as wild type (WT) myocytes. Both basal and stimulated levels of ERKs were higher in cardiomyocytes of KO mice than in those of WT mice. Activation of another member of the MAPK family, p38(MAPK), and expression of the c-fos gene were also induced by stretching cardiac myocytes of both types of mice. An AT1 antagonist attenuated stretch-induced activation of ERKs in WT cardiomyocytes but not in KO cardiomyocytes. Down-regulation of protein kinase C inhibited stretch-induced ERK activation in WT cardiomyocytes, whereas a broad spectrum tyrosine kinase inhibitor (genistein) and selective inhibitors of epidermal growth factor receptor (tyrphostin, AG1478, and B42) suppressed stretch-induced activation of ERKs in KO cardiac myocytes. Epidermal growth factor receptor was phosphorylated at tyrosine residues by stretching cardiac myocytes of KO mice. These results suggest that mechanical stretch could evoke hypertrophic responses in cardiac myocytes that lack the AT1 signaling pathway possibly through tyrosine kinase activation.

A requirement for the rac1 GTPase in the signal transduction pathway leading to cardiac myocyte hypertrophy

We have used adenoviral-mediated gene transfer of a constitutively active (V12rac1) and dominant negative (N17rac1) isoform of rac1 to assess the role of this small GTPase in cardiac myocyte hypertrophy. Expression of V12rac1 in neonatal cardiac myocytes results in sarcomeric reorganization and an increase in cell size that is indistinguishable from ligand-stimulated hypertrophy. In addition, V12rac1 expression leads to an increase in atrial natriuretic peptide secretion. In contrast, expression of N17rac1, but not a truncated form of Raf-1, attenuated the morphological hypertrophy associated with phenylephrine stimulation. Consistent with the observed effects on morphology, expression of V12rac1 resulted in an increase in new protein synthesis, while N17rac1 expression inhibited phenylephrine-induced leucine incorporation. These results suggest rac1 is an essential element of the signaling pathway leading to cardiac myocyte hypertrophy.

Cardiac protein phosphorylation: functional and pathophysiological correlates

Protein phosphorylation acts a pivotal mechanism in regulating the contractile state of the heart by modulating particular levels of autonomic control on cardiac force/length relationships. Early studies of changes in cardiac protein phosphorylation focused on key components of the excitation-coupling process, namely phospholamban of the sarcoplasmic reticulum and myofibrillar troponin I. In more recent years the emphasis has shifted towards the identification of other phosphoproteins, and more importantly, the delineation of the mechanistic and signaling pathways regulating the various known phosphoproteins. In addition to cAMP- and Ca(2+)-calmodulin-dependent kinase processes, these have included regulation by protein kinase C and the ever-emerging family of growth factor-related kinases such as the tyrosine-, mitogen- and stress-activated protein kinases. Similarly, the role of protein dephosphorylation by protein phosphatases has been recognized as integral in modulating normal cardiac cellular function. Recent studies involving a variety of cardiovascular pathologies have demonstrated that changes in the phosphorylation states of key cardiac regulatory proteins may underlie cardiac dysfunction in disease states. The emphasis of this comprehensive review will be on discussing the role of cardiac phosphoproteins in regulating myocardial function and pathophysiology based not only on in vitro data, but more importantly, from ex vivo experiments with corroborative physiological and biochemical evidence.

Activation of JNK in the remote myocardium after large myocardial infarction in rats

A large myocardial infarction (MI) causes a chronic hemodynamic load on the uninjured remote myocardium (RM). This may lead to oxidative stress, activation of stress-induced cell signaling and increase in myocyte apoptosis. MI was produced in 6 rats (INF) while 4 rats underwent sham operation (CON). At four weeks, there was 128% increase in right ventricular hypertrophy in the hearts from INF vs. CON. Western blot analysis showed 3.8 fold increase in JNK phosphorylation within the RM from INF vs. CON, confirmed by a 4.2 fold increase in JNK kinase activity. There was a 52% increase in TBARS within the RM from INF vs. CON, suggesting increased lipid peroxidation. Furthermore, there was a twofold increase in myocyte apoptosis within the RM in INF vs. CON. We conclude that the RM from INF is associated with activation of JNK, increased oxidative stress and enhanced myocyte apoptosis.

A calcineurin-dependent transcriptional pathway for cardiac hypertrophy

In response to numerous pathologic stimuli, the myocardium undergoes a hypertrophic response characterized by increased myocardial cell size and activation of fetal cardiac genes. We show that cardiac hypertrophy is induced by the calcium-dependent phosphatase calcineurin, which dephosphorylates the transcription factor NF-AT3, enabling it to translocate to the nucleus. NF-AT3 interacts with the cardiac zinc finger transcription factor GATA4, resulting in synergistic activation of cardiac transcription. Transgenic mice that express activated forms of calcineurin or NF-AT3 in the heart develop cardiac hypertrophy and heart failure that mimic human heart disease. Pharmacologic inhibition of calcineurin activity blocks hypertrophy in vivo and in vitro. These results define a novel hypertrophic signaling pathway and suggest pharmacologic approaches to prevent cardiac hypertrophy and heart failure.

Oxidative stress activates extracellular signal-regulated kinases through Src and Ras in cultured cardiac myocytes of neonatal rats

A growing body of evidence has suggested that oxidative stress causes cardiac injuries during ischemia/reperfusion. Extracellular signal-regulated kinases (ERKs) have been reported to play pivotal roles in many aspects of cell functions and to be activated by oxidative stress in some types of cells. In this study, we examined oxidative stress-evoked signal transduction pathways leading to activation of ERKs in cultured cardiomyocytes of neonatal rats, and determined their role in oxidative stress-induced cardiomyocyte injuries. ERKs were transiently and concentration-dependently activated by hydrogen peroxide (H2O2) in cardiac myocytes. A specific tyrosine kinase inhibitor, genistein, suppressed H2O2-induced ERK activation, while inhibitors of protein kinase A and C or Ca2+ chelators had no effects on the activation. When CSK, a negative regulator of Src family tyrosine kinases, or dominant-negative mutant of Ras or of Raf-1 kinase was overexpressed, activation of transfected ERK2 by H2O2 was abolished. The treatment with H2O2 increased the number of cells stained positive by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling, and induced formation of DNA ladder and activation of CPP32, suggesting that H2O2 induced apoptosis of cardiac myocytes. When H2O2-induced activation of ERKs was selectively inhibited by PD98059, the number of cardiac myocytes which showed apoptotic death was increased. These results suggest that Src family tyrosine kinases, Ras and Raf-1 are critical for ERK activation by hydroxyl radicals and that activation of ERKs may play an important role in protecting cardiac myocytes from apoptotic death following oxidative stress.