Endothelin-1 and fibroblast growth factors stimulate the mitogen-activated protein kinase signaling cascade in cardiac myocytes. The potential role of the cascade in the integration of two signaling pathways leading to myocyte hypertrophy

Regulation of Ras.GTP loading and Ras-Raf association in neonatal rat ventricular myocytes by G protein-coupled receptor agonists and phorbol ester. Activation of the extracellular signal-regulated kinase cascade by phorbol ester is mediated by Ras

The small G protein Ras has been implicated in hypertrophy of cardiac myocytes. We therefore examined the activation (GTP loading) of Ras by the following hypertrophic agonists: phorbol 12-myristate 13-acetate (PMA), endothelin-1 (ET-1), and phenylephrine (PE). All three increased Ras.GTP loading by 10-15-fold (maximal in 1-2 min), as did bradykinin. Other G protein-coupled receptor agonists (e.g. angiotensin II, carbachol, isoproterenol) were less effective. Activation of Ras by PMA, ET-1, or PE was reduced by inhibition of protein kinase C (PKC), and that induced by ET-1 or PE was partly sensitive to pertussis toxin. 8-(4-Chlorophenylthio)-cAMP (CPT-cAMP) did not inhibit Ras.GTP loading by PMA, ET-1, or PE. The association of Ras with c-Raf protein was increased by PMA, ET-1, or PE, and this was inhibited by CPT-cAMP. However, only PMA and ET-1 increased Ras-associated mitogen-activated protein kinase kinase 1-activating activity, and this was decreased by PKC inhibition, pertussis toxin, and CPT-cAMP. PMA caused the rapid appearance of phosphorylated (activated) extracellular signal-regulated kinase in the nucleus, which was inhibited by a microinjected neutralizing anti-Ras antibody. We conclude that PKC- and Gi-dependent mechanisms mediate the activation of Ras in myocytes and that Ras activation is required for stimulation of extracellular signal-regulated kinase by PMA.

Cell density and contraction regulate p38 MAP kinase-dependent responses in neonatal rat cardiac myocytes

In vitro cardiac myocyte hypertrophy is characterized by increased cell size, sarcomere organization, and induction of several genes including atrial natriuretic factor (ANF). The hypertrophic growth program has been associated with activation of various mitogen-activated protein kinase (MAP) kinase family members, one of which is a stress kinase, p38. In this study, we found that the p38-specific inhibitor SB-203580 failed to inhibit phenylephrine-induced ANF-driven gene expression in low-density myocyte cultures but did inhibit gene expression in higher density cultures. Dense myocyte cultures also had a higher metabolic activity and contraction rate than cells plated at low density. We found that mimicking this effect by rapid electrical pacing activated ANF-driven gene expression and that this expression was inhibited by inactivation of p38. However, addition of SB-203580 at time points ranging between 1 and 72 h suggests that the effect of p38 on the ANF promoter may be both direct and indirect. Electrical pacing induced a small, but consistent, increase in p38 phosphorylation (phospho-p38) at time points ranging from 30 min to 4 h, but at later times phospho-p38 levels were reduced. When myocytes were treated with phenylephrine or electrically paced in the presence of the p38 inhibitor, there was little discernible change in morphology or rates of protein synthesis from DMSO-treated cells at 48 or 72 h. These data indicate that cell density and myocyte contraction may modulate p38-dependent pathways for ANF gene expression, but these pathways may not be direct and have limited effects on hypertrophic morphology.

Activation of protein kinase cascades in the heart by hypertrophic G protein-coupled receptor agonists

Cardiac myocyte hypertrophy involves changes in cell structure and alterations in protein expression regulated at both the transcriptional and translational levels. Hypertrophic G protein-coupled receptor (GPCR) agonists such as endothelin-(ET-1) and phenylephrine stimulate a number of protein kinase cascades in the heart. Mitogen-activated protein kinase (MAPK) cascades stimulated include the extracellularly regulated kinase cascade, the stress-activated protein kinase/c-Jun N-terminal kinase cascade, and the p38 MAPK cascade. All 3 pathways have been implicated in hypertrophy, but recent ex vivo evidence also suggests that there may be additional effects on cell survival. ET-1 and phenylephrine also stimulate the protein kinase B pathway, and this may be involved in the regulation of protein synthesis by these agonists. Thus, protein kinase-mediated signaling may be important in the regulation of the development of myocyte hypertrophy.

The effects of the selective ROCK inhibitor, Y27632, on ET-1-induced hypertrophic response in neonatal rat cardiac myocytes--possible involvement of Rho/ROCK pathway in cardiac muscle cell hypertrophy

A small GTPase, Rho, participates in agonist-induced cytoskeletal organization and gene expression in many cell types including cardiac myocytes. However, little is known about the functions of Rho's downstream targets in cardiac myocytes. We examined the role of ROCK, a downstream target of Rho, in ET-1-induced hypertrophic response. Y27632, a selective ROCK inhibitor, inhibited ET-1-induced increases in natriuretic peptide production, cell size, protein synthesis, and myofibrillar organization. In addition, a dominant-negative mutant of p160ROCK suppressed ET-1-induced transcription of the BNP gene. These findings suggest that the Rho/ROCK pathway is an important component of ET-1-induced hypertrophic signals in cardiac myocytes.

Low affinity Fc gamma receptors on murine macrophages: mitogen-activated protein kinase activation and AP-1 DNA binding activity

Mouse macrophage cell lines such as J774 express Fc receptors for IgG2a immune complexes, which upon binding of the proper ligand, trigger several signal transduction pathways. A surface to nucleus signaling through these receptors has been demonstrated. We describe here the activation of the mitogen-activated protein kinase (MAPK) and an increase in the binding of the activator protein 1 (AP-1) to DNA upon receptor stimulation. The described effects are only partially blocked by inhibitors of the Ca2+/diacylglycerol-dependent protein kinase (PKC), suggesting that differential signaling pathways are activated upon receptor cross-linking and that they converge at or above the MAPK level. These results pave the way to our understanding of Fc gammaR cross-linking induced gene expression regulation.

Arachidonic acid in platelet microparticles up-regulates cyclooxygenase-2-dependent prostaglandin formation via a protein kinase C/mitogen-activated protein kinase-dependent pathway

Activation of platelets results in shedding of membrane microparticles (MP) with potentially bioactive properties. Platelet MP modulate platelet, monocyte, and vascular endothelial cell function, both by direct effects of MP arachidonic acid (AA) and by its metabolism to bioactive prostanoids. We have previously reported that platelet MP induce expression of cyclooxygenase (COX)-2 and prostacyclin production in monocytes and endothelial cells. To elucidate further the molecular mechanisms that underlie MP-induced up-regulation of COX-2 expression, we investigated the response of a human monocytoid (U-937) cell line to platelet MP stimulation. In U-937 cells, MP-induced COX-2 expression and eicosanoid formation is prevented by pharmacological inhibitors of protein kinase C (PKC), PI 3-kinase, mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase, and p38 kinase. Treatment with the PI 3-kinase inhibitors wortmannin and LY294002 also blocked MP-induced p42/p44 MAPK, p38, and JNK1 phosphorylation. Conversely, platelet MP stimulation of U-937 cells results in direct activation of PKC, p42/p44 MAPK, p38 kinase, and c-Jun N-terminal kinase (JNK) as well as activation of the transcription factors c-Jun and Elk-1. However, MP failed to activate the cAMP response element. Activation of U-937 cells by MP induces translocation of classical (PKCbeta), novel (PKCdelta) and atypical (PKCzeta and PKClambda) isozymes of PKC from the cytosol to the membrane, with concomitant activation of downstream MAPK. While MP-induced activation of p42/p44 MAPK and p38 kinase is transient, a sustained activation of JNK1 was observed. Although PKC activation is required for MP-induced p42/p44 MAPK, activation of the stress kinases p38 and JNK1 was PKC-independent. The fatty acid fraction of the MP accounted for these effects, which were mimicked by MP AA. Rather than acting directly via nuclear receptors, MP AA activates COX-2-dependent prostaglandin production by a PKC/p42/p44 MAPK/p38 kinase-sensitive pathway in which PI 3-kinase plays a significant role. MP AA also stimulates transcriptional activation of COX-2 as well as c-Jun and Elk-1.

A low-affinity serum response element allows other transcription factors to activate inducible gene expression in cardiac myocytes

Hypertrophic growth of cardiac muscle cells is induced by a variety of physiological and pathological stimuli and is associated with a number of changes, including activation of genes such as atrial natriuretic factor. We found that two serum response element (SRE)-like DNA elements, one of which does not meet the consensus sequence and binds serum response factor (SRF) with low affinity, regulate the activity of this promoter. Surprisingly, the ability to induce the promoter by two different physiologic stimuli, as well as various activated transcription factors, including SRF-VP16, was primarily dependent upon the nonconsensus rather than the consensus SRE. This SRE controls the induction of gene expression via an unusual mechanism in that it is required to allow some, but not all, active transcription factors at unrelated sites on the promoter to stimulate gene expression. Thus, in addition to regulation of SRF activity by growth stimuli, regulation of a low-affinity SRE element controls inducible gene expression by modulating the ability of other transcription factors to stimulate the transcription machinery.

Distinct and common pathways in the regulation of insulin-like growth factor-1 receptor gene expression by angiotensin II and basic fibroblast growth factor

Angiotensin II (Ang II) and basic fibroblast growth factor (bFGF) are important modulators of cell growth under physiological and pathophysiological conditions. We and others have previously shown that these growth factors increase insulin-like growth factor-1 receptor (IGF-1R) number and mRNA in vascular smooth muscle cells and that this effect is transcriptionally regulated. To study the mechanisms and the signaling pathways involved, IGF-1R promoter reporter constructs were transiently transfected in CHO-AT1 cells that overexpress angiotensin AT1 receptors. Our findings indicate that Ang II and bFGF significantly increased IGF-1R promoter activity up to 7- and 3-fold, respectively. The effect induced by Ang II was mediated via a tyrosine kinase-dependent mechanism, since tyrphostin A25 largely inhibited the Ang II-induced increase in promoter activity. In addition, co-transfection of dominant negative Ras, Raf, and MEK1 or pretreatment with the MEK inhibitor PD 98059 dose-dependently decreased both the Ang II- and bFGF-induced increase in IGF-1R transcription and protein expression, suggesting that the Ras-Raf-mitogen-activated protein kinase kinase pathway is required for both growth factors. Reactive oxygen species have been shown to act as second messengers in Ang II-induced signaling, and activation of the transcription factor NF-kappaB is redox-sensitive. While co-transfection of dominant negative IkappaBalpha mutant completely inhibited the Ang II-induced increase in transcription, it had no effect on the bFGF signaling. In contrast, co-transfection studies indicated that the transcription factors STAT1, STAT3, and c-Jun and the Janus kinase 2 kinase are required in the signaling pathway of bFGF, whereas only dominant c-Jun inhibited the Ang II-induced effect. In summary, these data demonstrate that Ang II and bFGF increase IGF-1R gene transcription via distinct as well as shared pathways and have important implications for understanding growth-stimulatory effects of these growth factors on vascular cells.

c-Src activation plays a role in endothelin-dependent hypertrophy of the cardiac myocyte

Activation of the atrial natriuretic peptide (ANP) gene is regarded as one of the earliest and most reliable markers of hypertrophy in the ventricular cardiac myocyte. We have examined the role of the nonreceptor tyrosine kinases in the signaling mechanism(s) leading to hypertrophy using human ANP gene promoter activity as a marker. Endothelin (ET), a well known hypertrophic agonist, increased activity of c-Src, c-Yes, and Fyn within minutes and promoted a selective redistribution of each of these kinases within the cell. Overexpression of c-Src effected a significant increase in activity of a cotransfected human ANP promoter-driven chloramphenicol acetyl transferase reporter, while expression of either c-Yes or Fyn was considerably less effective in this regard. ET-dependent stimulation of the human ANP gene promoter was partially inhibited by co-transfection with dominant negative Ras or dominant negative Src or Csk or by treatment with the potent Src family-selective tyrosine kinase inhibitor PP1, suggesting that the Src family kinases are involved in signaling ET-dependent activation of this promoter. Both ET- and Src-dependent activation of the ANP promoter required the presence of a CArG motif in a serum response element-like structure between -422 and -413 but did not appear to require assembly of a ternary complex for full activity. These findings support a role for Src in the activation of ANP gene expression and suggest that this kinase may contribute in an important way to the signaling mechanisms that activate hypertrophy in the cardiac myocyte.

Modulation of Mdm2 expression and p53-induced apoptosis in immortalized human ovarian granulosa cells

The activity of the tumor suppressor gene p53 is implicated in arrest of the cell cycle and the induction of apoptosis. The mdm2 oncogene is transcriptionally activated by p53, and the protein products of this gene can down-modulate biochemical activities and biological effects of p53 in a cell context-dependent manner. We have established highly steroidogenic human granulosa cell lines expressing the Ha-ras oncogene and a temperature sensitive (ts) mutant of p53 (p53val135) to test the involvement of p53-downstream genes in the modulation of apoptosis in these cells. We find that ras-transformed granulosa cells expressing p53val135 undergo apoptosis following a shift from 37 C to 32 C, a temperature at which p53val135 exerts its wild-type activity. Elevating the cellular content of cAMP at 32 C markedly enhances apoptosis. Basic fibroblast growth factor (bFGF) effectively blocks the p53/cAMP-induced apoptosis, but suppresses steroidogenesis. A naturally produced basement membrane-like extracellular matrix (ECM) containing immobilized bFGF exerts a similar antiapoptotic effect, but unlike soluble bFGF, it enhances steroidogenesis in these cells. While cAMP markedly suppresses the p53-induced Mdm2 expression, bFGF and ECM elevate Mdm2 expression 3-5-fold. These effects on Mdm2 expression are most pronounced 2-4 h after the shift to 32 C, before nuclear fragmentation is detected. Cells grown at 32 C in contact with ECM have a more developed actin cytoskeleton both in the absence and presence of cAMP stimulation, compared with cells grown on plastic dishes. We conclude that bFGF and components of the ECM can cross-talk with p53/cAMP-generated signals for apoptosis. These signals may, at least in part, be coordinated by the modulation of Mdm2 expression, which precedes the biochemical events characteristic of apoptosis. The multicomponent ECM also induced differentiation in these ras-transformed cells, while soluble bFGF inhibited differentiation, suggesting that ECM components other than bFGF stimulate differentiation. Organization of the actin cytoskeleton is likely to play an important role in the cross-talk between p53/cAMP- and bFGF/ECM-generated signals. Because the tumor suppressor gene p53 is implicated with apoptosis of primary granulosa cells and the ECM is involved in the prevention of this process, the newly established cell lines can serve as a useful model for apoptosis in highly luteinized granulosa cells.

Calcium channel blockers in cardiac failure

Congestive cardiac failure is an increasingly prevalent syndrome associated with a high morbidity and mortality. The role of calcium channel blockers in the treatment of heart failure is unclear. The potential benefits of these agents derive not only from their vasodilator properties, but also from anti-ischemic effects, beneficial effects on endothelial function and the development of atherosclerosis, and favorable effects on calcium cycling at a molecular level. Pitted against this array of potential benefits are direct negative inotropic effects and the potential for neuroendocrine activation. Treatment with short-acting dihydropyridine agents has not resulted in long-term clinical benefits in patients with cardiac failure. Diltiazem may be beneficial in patients with nonischemic heart failure, and verapamil has a neutral effect in cardiac failure, although it may have a role in combination with ace inhibition. To date, amlodipine has been associated with the most promising results, with evidence of a mortality benefit in nonischemic heart failure. Mibefradil is of no benefit in the management of heart failure, although the trend toward increased mortality in the treatment arm of the Mortality Assessment in Congestive Heart Failure (MACH)-1 trial may have been due to drug interactions. The potential role of calcium blockers in diastolic dysfunction and in combination with ace-inhibition requires further study.

Expression of mitogen-activated protein kinase pathways during postnatal development of rat heart

The loss of ability to proliferate (terminal differentiation) and reduction in capability to resist ischemia are key phenomena observed during postnatal development of the heart. Mitogen-activated protein kinases (MAPKs) mediate signaling pathways for cell proliferation/differentiation and stress responses such as ischemia. In this study, the expression of these kinases and their associated kinases were investigated in rat heart ventricle. Extracts of 1-, 10-, 20-, 50-, and 365-day-old rat heart ventricles were probed with specific antibodies and their immunoreactivities were quantified by densitometry. Most of the mitogenic protein kinases including Raf1, RafB, Mek1, Erk2, and Rsk1 were significantly down-regulated, whereas the stress signaling kinases, such as Mlk3, Mekkl, Sekl, Mkk3, and Mapkapk2 were up-regulated in expression during postnatal development. Most MAP kinases including Erk1, JNKs, p38 Hog, as well as Rsk2, however, did not exhibit postnatal changes in expression. The proto-oncogene-encoded kinases Mos and Cot/Tpl 2 were up-regulated up to two- and four-fold, respectively, during development. Pakl, which may be involved in the regulation of cytoskeleton as well as in stress signaling, was downregulated with age, but the Pak2 isoform increased only after 50 days. All of these proteins, except RafB, were also detected in the isolated adult ventricular myocytes at comparable levels to those found in adult ventricle. Tissue distribution studies revealed that most of the protein kinases that were up-regulated during heart development tended to be preferentially expressed in heart, whereas the downregulated protein kinases were generally expressed in heart at relatively lesser amounts than in most of other tissues.

Differential regulation of parallel mitogen-activated protein kinases in cardiac myocytes revealed by phosphatase inhibition

Previous studies have suggested that the contribution of inducible phosphatases to ERK MAPK deactivation is both cell-type- and agonist-specific. The aim of this study was to define the role of inducible phosphatases in ERK MAPK regulation in cardiac myocytes. We examined the kinetics of activation/deactivation of ERK MAPKs following the exposure of cardiac myocytes to endothelin-1 or phorbol ester. Deactivation was prevented by inhibition of protein synthesis indicating a contribution of inducible phosphatases. In contrast, okadaic acid failed to prolong ERK MAPK activation, but activated three myelin basic protein kinases (MBPKs, 55, 62, and 87 kDa) and two c-Jun kinases (46 and 55 kDa). Although the identity of the MBPKs is unknown, the c-Jun kinases corresponded to JNK MAPKs. Simultaneous exposure of cardiac myocytes to okadaic acid and osmotic shock potentiated JNK MAPK activation. Thus, inducible phosphatases regulate ERK MAPK deactivation, whereas okadaic acid-sensitive phosphatases regulate JNK MAPKs and three novel MBPKs.

Regulation of mitogen-activated protein kinase cascades in the heart

Using primary cultures of neonatal rat ventricular myocytes and isolated adult rat hearts as models, we have characterized extensively the regulation of MAPKs in the heart. The ERKs are activated primarily by GPCR agonists acting through PKC. These agonists can also activate the JNKs although the mechanism is unclear. Cellular stresses stimulate strong activation of the JNKs, but also cause some stimulation of ERKs. Activation of p38-MAPK has so far only been demonstrated in intact adult hearts subjected to stresses and probably leads to activation of MAPKAPK2. Both cellular stresses and GPCR agonists induce phosphorylation of c-Jun, but only the latter causes upregulation of c-Jun protein.

Regulation of myogenesis by fibroblast growth factors requires beta-gamma subunits of pertussis toxin-sensitive G proteins

Terminal differentiation of skeletal muscle cells in culture is inhibited by a number of different growth factors whose subsequent intracellular signaling events are poorly understood. In this study, we have investigated the role of heterotrimeric G proteins in mediating fibroblast growth factor (FGF)-dependent signals that regulate myogenic differentiation. Pertussis toxin, which ADP-ribosylates and inactivates susceptible G proteins, promotes terminal differentiation in the presence of FGF-2, suggesting that Galpha or Gbeta gamma subunits or both are involved in transducing the FGF-dependent signal(s) that inhibits myogenesis. We found that Gbetagamma subunits are likely to be involved since the expression of the C terminus of beta-adrenergic receptor kinase 1, a Gbetagamma subunit-sequestering agent, promotes differentiation in the presence of FGF-2, and expression of the free Gbeta gamma dimer can replace FGF-2, rescuing cells from pertussis toxin-induced differentiation. Addition of pertussis toxin also blocked FGF-2-mediated activation of mitogen-activated protein kinases (MAPKs). Ectopic expression of dominant active mutants in the Ras/MAPK pathway rescued cells from pertussis toxin-induced terminal differentiation, suggesting that the Gbeta gamma subunits act upstream of the Ras/MAPK pathway. It is unlikely that the pertussis toxin-sensitive pathway is activated by other, as yet unidentified FGF receptors since PDGF (platelet-derived growth factor)-stimulated MM14 cells expressing a chimeric receptor containing the FGF receptor-1 intracellular domain and the PDGF receptor extracellular domain were sensitive to pertussis toxin. Our data suggest that FGF-mediated signals involved in repression of myogenic differentiation are transduced by a pertussis toxin-sensitive G-protein-coupled mechanism. This signaling pathway requires the action of Gbeta gamma subunits and activation of MAPKs to repress skeletal muscle differentiation.

Activation of mitogen-activated protein kinases (p38-MAPKs, SAPKs/JNKs and ERKs) by adenosine in the perfused rat heart

Adenosine and mitogen-activated protein kinases (MAPKs) have been separately implicated in cardiac ischaemic preconditioning. We investigated the activation of MAPK subfamilies by adenosine in perfused rat hearts. p38-MAPK was rapidly phosphorylated and activated (10-fold activation, maximal at 5 min) by 10 mM adenosine, as was the p38-MAPK substrate, MAPKAPK2 (4.5-fold). SAPKs/JNKs were activated (5-fold) and ERKs were phosphorylated (both maximal at 5 min). The concentration dependences of activation of p38-MAPK and ERKs were biphasic with a 'high affinity' component (maximal at 10-100 microM adenosine) and a 'low affinity' component that had not saturated at 10 mM. SAPKs/JNKs were activated only by 10 mM adenosine. These results are consistent with MAPK involvement in adenosine-mediated ischaemic preconditioning.

Role of AT1 and AT2 receptors in regulation of MAPKs and MKP-1 by ANG II in adult cardiac myocytes

ANG II has been implicated in the hypertrophic response in ventricular myocytes by acting at the angiotensin type 1 (AT1) receptor. However, the role of the angiotensin type 2 (AT2) receptor in the adult heart is not as clearly understood. In adult rat ventricular myocytes (ARVM) and cardiac microvascular endothelial cells (CMEC), we examined the role of ANG II signaling, via AT1 and AT2 receptors, on the activation of the extracellular signal-regulated protein kinases (ERKs) and on the expression of the mitogen-activated protein kinase (MAPK) phosphatase MKP-1. ANG II caused no detectable increase in ERK activity or in c-fos mRNA abundance in ARVM but increased ERK activity within 5 min in CMEC and increased c-fos mRNA levels. However, in the presence of the selective phosphoprotein phosphatase (PP-2A/PP-1) inhibitor okadaic acid (OA), a sustained increase in ERK activity, as well as in c-jun NH2-terminal protein kinase activity, in ARVM was observed. ANG II increased MKP-1 mRNA levels within 15 min in ARVM and CMEC. In contrast to the response in endothelial cells, however, ANG II activation of MKP-1 in ARVM was mediated by AT2-receptor activation. Thus there is constitutive as well as inducible suppression of ERKs and c-jun NH2-terminal protein kinases by MKP and PP-2A/PP-1 in the adult cardiac myocyte phenotype.

Activation of gp130 transduces hypertrophic signals via STAT3 in cardiac myocytes

BACKGROUND

gp130, a signal transducer of the IL-6-related cytokines, is expressed ubiquitously, including in the heart. The activation of gp130 in cardiac myocytes was reported to induce myocardial hypertrophy. The downstream side of gp130 consists of two distinct pathways in cardiac myocytes, one a Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway, the other a mitogen-activated protein kinase (MAPK) pathway. In the present study, we examined whether the JAK/STAT pathway, especially the STAT3-mediated pathway, plays a critical role in gp130-dependent myocardial hypertrophy by transfecting wild-type and mutated-type STAT3 cDNA to cardiac myocytes.

METHODS AND RESULTS

We constructed three kinds of replication-defective adenovirus vectors carrying wild-type (AD/WT) or mutated-type (AD/DN) STAT3 cDNA or adenovirus vector itself (AD). Cultured murine cardiac myocytes infected with adenovirus were stimulated with leukemia inhibitory factor (LIF), and the expression of c-fos and atrial natriuretic factor (ANF) mRNAs and [3H]leucine incorporation were examined. There were no significant differences in MAPK activity among the three groups. Compared with AD-transfected cardiac myocytes, induction of c-fos and ANF mRNAs and protein synthesis after LIF stimulation were significantly augmented in AD/WT-transfected cells. In contrast, induction of c-fos and ANF mRNA expression and protein synthesis were attenuated after LIF stimulation in cardiac myocytes transfected with AD/DN.

CONCLUSIONS

These results suggest that the STAT3-dependent signaling pathway downstream of gp 130 promotes cardiac myocyte hypertrophy under stimulation with LIF.

Stimulation of the p38 mitogen-activated protein kinase pathway in neonatal rat ventricular myocytes by the G protein-coupled receptor agonists, endothelin-1 and phenylephrine: a role in cardiac myocyte hypertrophy?

We examined the activation of the p38 mitogen-activated protein kinase (p38-MAPK) pathway by the G protein-coupled receptor agonists, endothelin-1 and phenylephrine in primary cultures of cardiac myocytes from neonatal rat hearts. Both agonists increased the phosphorylation (activation) of p38-MAPK by approximately 12-fold. A p38-MAPK substrate, MAPK-activated protein kinase 2 (MAPKAPK2), was activated approximately fourfold and 10 microM SB203580, a p38-MAPK inhibitor, abolished this activation. Phosphorylation of the MAPKAPK2 substrate, heat shock protein 25/27, was also increased. Using selective inhibitors, activation of the p38-MAPK pathway by endothelin-1 was shown to involve protein kinase C but not Gi/Go nor the extracellularly responsive kinase (ERK) pathway. SB203580 failed to inhibit the morphological changes associated with cardiac myocyte hypertrophy induced by endothelin-1 or phenylephrine between 4 and 24 h. However, it decreased the myofibrillar organization and cell profile at 48 h. In contrast, inhibition of the ERK cascade with PD98059 prevented the increase in myofibrillar organization but not cell profile. These data are not consistent with a role for the p38-MAPK pathway in the immediate induction of the morphological changes of hypertrophy but suggest that it may be necessary over a longer period to maintain the response.

Oncogenic src, raf, and ras stimulate a hypertrophic pattern of gene expression and increase cell size in neonatal rat ventricular myocytes

In response to hormones and growth factors, cultured neonatal ventricular myocytes increase in profile, exhibit myofibrillogenesis, and re-express genes whose expression is normally restricted to the fetal stage of ventricular development. These include atrial natriuretic factor (ANF), beta-myosin heavy chain (beta-MHC), and skeletal muscle (SkM)-alpha-actin. By using luciferase reporter plasmids, we examined whether oncogenes that activate the extracellular signal-regulated kinase cascade (srcF527, Ha-rasV12, and v-raf) increased expression of "fetal" genes. Transfection of myocytes with srcF527 stimulated expression of ANF, SkM-alpha-actin, and beta-MHC by 62-, 6.7-, and 50-fold, respectively, but did not induce DNA synthesis. Stimulation of ANF expression by srcF527 was greater than by Ha-rasV12, which in turn was greater than by v-raf. General gene expression was also increased but to a lesser extent. The response to srcF527 was inhibited by dominant-negative Ha-rasN17. Myocyte area was increased by srcF527, Ha-rasV12, and v-raf, and although it altered myocyte morphology by causing a pseudopodial appearance, srcF527 did not detectably increase myofibrillogenesis either alone or in combination with Ha-rasV12. A kinase-dead src mutant increased myocyte size to a much lesser extent than srcF527 and also did not inhibit ANF-luciferase expression in response to phenylephrine. We conclude that members of the Src family of tyrosine kinases may be important in mediating the transcriptional changes occurring during cardiac myocyte hypertrophy and that Ras and Raf may be downstream effectors.

Beta1 integrins participate in the hypertrophic response of rat ventricular myocytes

Multiple signaling pathways have been implicated in the hypertrophic response of ventricular myocytes, yet the importance of cell-matrix interactions has not been extensively examined. Integrins are cell-surface molecules that link the extracellular matrix to the cellular cytoskeleton. They can function as cell signaling molecules and transducers of mechanical information in noncardiac cells. Given these properties and their abundance in cardiac cells, we evaluated the hypothesis that beta1 integrin function is involved in the alpha1-adrenergic mediated hypertrophic response of neonatal rat ventricular myocytes. The hypertrophic response of this model required interaction with extracellular matrix proteins. Specificity of these results was confirmed by demonstrating that ventricular myocytes plated onto an anti-beta1 integrin antibody supported the hypertrophic gene response. Adenovirus-mediated overexpression of beta1 integrin augmented the myocyte hypertrophic response when assessed by protein synthesis and atrial natriuretic factor production, a marker gene of hypertrophic induction. DNA synthesis was not altered by integrin overexpression. Transfection of cultured cardiac myocytes with either the ubiquitously expressed beta1A integrin or the cardiac/skeletal muscle-specific beta1 isoform (beta1D) activated reporter expression from both the atrial natriuretic factor and myosin light chain-2 ventricular promoters, genetic markers of ventricular cell hypertrophy. Finally, suppression of integrin signaling by overexpression of free beta1 integrin cytoplasmic domains inhibited the adrenergically mediated atrial natriuretic factor response. These findings show that integrin ligation and signaling are involved in the cardiac hypertrophic response pathway.

Multiple signal transduction pathways link Na+/K+-ATPase to growth-related genes in cardiac myocytes. The roles of Ras and mitogen-activated protein kinases

We showed before that in neonatal rat cardiac myocytes partial inhibition of Na+/K+-ATPase by nontoxic concentrations of ouabain causes hypertrophic growth and transcriptional regulations of genes that are markers of cardiac hypertrophy. In view of the suggested roles of Ras and p42/44 mitogen-activated protein kinases (MAPKs) as key mediators of cardiac hypertrophy, the aim of this work was to explore their roles in ouabain-initiated signal pathways regulating four growth-related genes of these myocytes, i.e. those for c-Fos, skeletal alpha-actin, atrial natriuretic factor, and the alpha3-subunit of Na+/K+-ATPase. Ouabain caused rapid activations of Ras and p42/44 MAPKs; the latter was sustained longer than 90 min. Using high efficiency adenoviral-mediated expression of a dominant-negative Ras mutant, and a specific inhibitor of MAPK kinase (MEK), activation of Ras-Raf-MEK-p42/44 MAPK cascade by ouabain was shown. The effects of the mutant Ras, an inhibitor of Ras farnesylation, and the MEK inhibitor on ouabain-induced changes in mRNAs of the four genes indicated that (a) skeletal alpha-actin induction was dependent on Ras but not on p42/44 MAPKs, (b) alpha3 repression was dependent on the Ras-p42/44 MAPK cascade, and (c) induction of c-fos or atrial natriuretic factor gene occurred partly through the Ras-p42/44 MAPK cascade, and partly through pathways independent of Ras and p42/44 MAPKs. All ouabain effects required extracellular Ca2+, and were attenuated by a Ca2+/calmodulin antagonist or a protein kinase C inhibitor. The findings show that (a) signal pathways linked to sarcolemmal Na+/K+-ATPase share early segments involving Ca2+ and protein kinase C, but diverge into multiple branches only some of which involve Ras, or p42/44 MAPKs, or both; and (b) there are significant differences between this network and the related gene regulatory pathways activated by other hypertrophic stimuli, including those whose responses involve increases in intracellular free Ca2+ through different mechanisms.

Opposing effects of Jun kinase and p38 mitogen-activated protein kinases on cardiomyocyte hypertrophy

c-Jun N-terminal protein kinase (JNK) and p38, two distinct members of the mitogen-activated protein (MAP) kinase family, regulate gene expression in response to various extracellular stimuli, yet their physiological functions are not completely understood. In this report we show that JNK and p38 exerted opposing effects on the development of myocyte hypertrophy, which is an adaptive physiological process characterized by expression of embryonic genes and unique morphological changes. In rat neonatal ventricular myocytes, both JNK and p38 were stimulated by hypertrophic agonists like endothelin-1, phenylephrine, and leukemia inhibitory factor. Expression of MAP kinase kinase 6b (EE), a constitutive activator of p38, stimulated the expression of atrial natriuretic factor (ANF), which is a genetic marker of in vivo cardiac hypertrophy. Activation of p38 was required for ANF expression induced by the hypertrophic agonists. Furthermore, a specific p38 inhibitor, SB202190, significantly changed hypertrophic morphology induced by the agonists. Surprisingly, activation of JNK led to inhibition of ANF expression induced by MEK kinase 1 (MEKK1) and the hypertrophic agonists. MEKK1-induced ANF expression was also negatively regulated by expression of c-Jun. Our results demonstrate that p38 mediates, but JNK suppresses, the development of 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.

Role of protein kinase C alpha in endothelin-1 stimulation of cytosolic phospholipase A2 and arachidonic acid release in cultured cat iris sphincter smooth muscle cells

We have investigated the role and mechanism of protein kinase C (PKC) isoforms in endothelin-1 (ET-1)-induced arachidonic acid (AA) release in cat iris sphincter smooth muscle (CISM) cells. ET-1 increased AA release in a concentration (EC50=8 nM) and time-dependent (t1/2=1.2 min) manner. Cytosolic phospholipase A2 (cPLA2), but not phospholipase C (PLC), is involved in the liberation of AA in the stimulated cells. This conclusion is supported by the findings that ET-1-induced AA release is inhibited by AACOCF3, quinacrine and manoalide, PLA2 inhibitors, but not by U-73122, a PLC inhibitor, or by RHC-80267, a diacylglycerol lipase inhibitor. A role for PKC in ET-1-induced AA release is supported by the findings that the phorbol ester, PDBu, increased AA release by 96%, that prolonged treatment of the cells with PDBu resulted in the selective down regulation of PKCalpha and the complete inhibition of ET-1-induced AA release, and that pretreatment of the cells with staurosporine or RO 31-8220, PKC inhibitors, blocked the ET-1-induced AA release. Gö-6976, a compound that inhibits PKCalpha and beta specifically, blocked ET-1-induced AA release in a concentration-dependent manner with an IC50 value of 8 nM. Thymeatoxin (0.1 microM), a specific activator of PKCalpha, beta, and gamma induced a 150% increase in AA release. Treatment of the cells with ET-1 caused significant translocation of PKCalpha, but not PKCbeta, from cytosol to the particulate fraction. These results suggest that PKCalpha plays a critical role in ET-1-induced AA release in these cells. Immunochemical analysis revealed the presence of cPLA2, p42mapk and p44mapk in the CISM cells. The data presented are consistent with a role for PKCalpha, but not for p42/p44 mitogen-activated protein kinase (MAPK), in cPLA2 activation and AA release in ET-1-stimulated CISM cells since: (i) the PKC inhibitor, RO 31-8220, inhibited ET-1-induced AA release, cPLA2 phosphorylation and cPLA2 activity, but had no inhibitory effect on p42/p44 MAPK activation, (ii) genistein, a tyrosine kinase inhibitor, inhibited ET-1-stimulated MAPK activity but had no inhibitory effect on AA release in the ET-1-stimulated cells. We conclude that in CISM cells, ET-1 activates PKCalpha, which activates cPLA2, which liberates AA for prostaglandin synthesis.

Neuregulins promote survival and growth of cardiac myocytes. Persistence of ErbB2 and ErbB4 expression in neonatal and adult ventricular myocytes

Neuregulins (i.e. neuregulin-1 (NRG1), also called neu differentiation factor, heregulin, glial growth factor, and acetylcholine receptor-inducing activity) are known to induce growth and differentiation of epithelial, glial, neuronal, and skeletal muscle cells. Unexpectedly, mice with loss of function mutations of NRG1 or of either of two of their cognate receptors, ErbB2 and ErbB4, die during midembryogenesis due to the aborted development of myocardial trabeculae in ventricular muscle. To examine the role of NRG and their receptors in developing and postnatal myocardium, we studied the ability of a soluble NRG1 (recombinant human glial growth factor 2) to promote proliferation, survival, and growth of isolated neonatal and adult rat cardiac myocytes. Both ErbB2 and ErbB4 receptors were found to be expressed by neonatal and adult ventricular myocytes and activated by rhGGF2. rhGGF2 (30 ng/ml) provoked an approximate 2-fold increase in embryonic cardiac myocyte proliferation. rhGGF2 also promoted survival and inhibited apoptosis of subconfluent, serum-deprived myocyte primary cultures and also induced hypertrophic growth in both neonatal and adult ventricular myocytes, which was accompanied by enhanced expression of prepro-atrial natriuretic factor and skeletal alpha-actin. Moreover, NRG1 mRNA could be detected in coronary microvascular endothelial cell primary cultures prepared from adult rat ventricular muscle. NRG1 expression in these cells was increased by endothelin-1, another locally acting cardiotropic peptide within the heart. The persistent expression of both a neuregulin and its cognate receptors in the postnatal and adult heart suggests a continuing role for neuregulins in the myocardial adaption to physiologic stress or injury.

Tumor necrosis factor-alpha confers resistance to hypoxic injury in the adult mammalian cardiac myocyte

BACKGROUND

Previous studies in isolated cardiac myocytes have shown that tumor necrosis factor (TNF)-alpha provokes increased expression of 27- and 70-kD stress proteins as well as manganese superoxide dismutase, suggesting that TNF-alpha might play a role in mediating stress responses in the heart.

METHODS AND RESULTS

To determine whether TNF-alpha stimulation would protect isolated cardiac myocytes against environmental stress, myocyte cultures were pretreated with TNF-alpha for 12 hours and then subjected to continuous hypoxic injury (O2 content, 3 to 5 ppm) for 12 hours, followed by reoxygenation. Cell injury was assessed in terms of lactic dehydrogenase (LDH) release, 45Ca2+ uptake, and MTT metabolism. Pretreatment with TNF-alpha concentrations > or = 50 U/mL significantly attenuated LDH release by hypoxic cells compared with diluent-treated hypoxic cells. Similar findings were observed with respect to 45Ca2+ uptake and MTT metabolism in TNF-alpha-pretreated cells that were subjected to prolonged hypoxia. To determine the mechanism for the TNF-alpha-induced protective effect, the cells were pretreated with heat shock protein (HSP) 72 antisense oligonucleotides. These studies showed that the protective effect of TNF-alpha was not inhibited by antisense oligonucleotides, despite use of a concentration of antisense that was sufficient to attenuate the TNF-alpha-induced increase in HSP 72 expression. Subsequent studies using mutated TNF ligands showed that activation of both types 1 and 2 TNF receptors was sufficient to confer a protective response in isolated cardiac myocytes through an as yet unknown pathway(s).

CONCLUSIONS

Taken together, the above observations demonstrate that TNF-alpha pretreatment confers resistance to hypoxic stress in the adult cardiac myocyte through a novel mechanism that appears to be different from but not necessarily exclusive of the protective response conferred by HSP 72 expression.

The p38-MAPK inhibitor, SB203580, inhibits cardiac stress-activated protein kinases/c-Jun N-terminal kinases (SAPKs/JNKs)

SB203580 is a recognised inhibitor of p38-MAPKs. Here, we investigated the effects of SB203580 on cardiac SAPKs/JNKs. The IC50 for inhibition of p38-MAPK stimulation of MAPKAPK2 was approximately 0.07 microM, whereas that for total SAPK/JNK activity was 3-10 microM. SB203580 did not inhibit immunoprecipitated JNK1 isoforms. Three peaks of SAPK/JNK activity were separated by anion exchange chromatography, eluting in the isocratic wash (44 kDa), and at 0.08 M (46 and 52 kDa) and 0.15 M NaCl (54 kDa). SB203580 (10 microM) completely inhibited the 0.15 M NaCl activity and partially inhibited the 0.08 M NaCl activity. Since JNK1 antibodies immunoprecipitate the 46 kDa activity, this indicates that SB203580 selectively inhibits 52 and 54 kDa SAPKs/JNKs.

Stimulation of "stress-regulated" mitogen-activated protein kinases (stress-activated protein kinases/c-Jun N-terminal kinases and p38-mitogen-activated protein kinases) in perfused rat hearts by oxidative and other stresses

"Stress-regulated" mitogen-activated protein kinases (SR-MAPKs) comprise the stress-activated protein kinases (SAPKs)/c-Jun N-terminal kinases (JNKs) and the p38-MAPKs. In the perfused heart, ischemia/reperfusion activates SR-MAPKs. Although the agent(s) directly responsible is unclear, reactive oxygen species are generated during ischemia/reperfusion. We have assessed the ability of oxidative stress (as exemplified by H2O2) to activate SR-MAPKs in the perfused heart and compared it with the effect of ischemia/reperfusion. H2O2 activated both SAPKs/JNKs and p38-MAPK. Maximal activation by H2O2 in both cases was observed at 0.5 mM. Whereas activation of p38-MAPK by H2O2 was comparable to that of ischemia and ischemia/reperfusion, activation of the SAPKs/JNKs was less than that of ischemia/reperfusion. As with ischemia/reperfusion, there was minimal activation of the ERK MAPK subfamily by H2O2. MAPK-activated protein kinase 2 (MAPKAPK2), a downstream substrate of p38-MAPKs, was activated by H2O2 to a similar extent as with ischemia or ischemia/reperfusion. In all instances, activation of MAPKAPK2 in perfused hearts was inhibited by SB203580, an inhibitor of p38-MAPKs. Perfusion of hearts at high aortic pressure (20 kilopascals) also activated the SR-MAPKs and MAPKAPK2. Free radical trapping agents (dimethyl sulfoxide and N-t-butyl-alpha-phenyl nitrone) inhibited the activation of SR-MAPKs and MAPKAPK2 by ischemia/reperfusion. These data are consistent with a role for reactive oxygen species in the activation of SR-MAPKs during ischemia/reperfusion.

Continuous administration of insulin-like growth factor-I and basic fibroblast growth factor does not affect left ventricular geometry after acute myocardial infarction in rats

We examined the long-term effect of exogenous administration of bFGF and IGF-I on myocardial geometry in 72 Sprague-Dawley male rats subjected to AMI. A preloaded miniature osmotic pump subsequently implanted in the peritoneum for continuous infusion (1 week) of IGF-I, bFGF, IGF-I+bFGF or rat albumin. Six weeks following AMI the rats were killed and cross-section slices were analyzed for left ventricular geometry. No differences were observed between IGF-I-treated, bFGF-treated, IGF-I+bFGF-treated and control groups in all parameters of the left ventricle.

Fibroblast variants nonresponsive to fibroblast growth factor 1 are defective in its nuclear translocation

Fibroblast growth factors (FGF) elicit biological effects by binding to high affinity cell-surface receptors and activation of receptor tyrosine kinase. We previously reported that two NIH/3T3 derivatives, NR31 and NR33 (NR cells), express high levels of full-length FGF-1 and exhibit a complete spectrum of transformed phenotype. In the present study, we report that NR cells respond to the mitogenic stimulation of truncated FGF-1 but not to the full-length FGF-1. Incubation of the NR cells with either form of FGF-1 resulted in its binding to cell-surface FGF receptors, activation of mitogen-activated protein (MAP) kinase, and induction of c-fos and c-myc. These data demonstrate that the FGF receptor-mediated, MAP kinase-dependent signaling pathway is not defective in the NR cells. Our data further suggest that the activation of MAP kinase in response to full-length FGF-1 is not sufficient for mitogenesis. Subcellular distribution of exogenously added FGF-1 demonstrated that full-length FGF-1 fails to translocate to the nuclei of NR31 cells. Although the full-length FGF-1 was detected in the nuclear fractions of both NIH/3T3 and NR33 cells, its half-life is much shortened in NR33 than in NIH/3T3 cells. These observations suggest that non-responsiveness of the two NR cell lines may be due to defectiveness at different steps of nuclear translocation mechanism of FGF-1.

Heparin inhibits phorbol ester-induced ornithine decarboxylase gene expression in endothelial cells

Glycosaminoglycans regulate angiogenesis by affecting the availability of different growth factors for the endothelial cell (EC). However, little is known about the molecular and functional consequences resulting from direct interaction of these polyelectrolytes with the EC. Here we show that heparin markedly inhibited serum-stimulated DNA synthesis and ornithine decarboxylase (ODC) mRNA expression in human endothelial cells (HEC). About 50% of the serum effect on DNA synthesis and ODC gene expression was prevented by the selective protein kinase C (PKC) inhibitor chelerythrine or by PKC down-regulation. Heparin was ineffective in counteracting that part of the effect of serum that was resistant to PKC inhibition or down-regulation. In serum-free cultured HEC, heparin completely abolished the increase in DNA synthesis and ODC mRNA expression elicited by a number of PKC activators. Cell exposure to difluoromethylornithine, an irreversible inhibitor of ODC enzyme, dramatically antagonised both serum- and phorbol 12-myristate 13-acetate (PMA)-stimulated DNA synthesis. These results suggest that inhibition of PKC-mediated ODC gene expression by glycosaminoglycans may represent an important mechanism in the regulation of HEC proliferation.

Endothelin-1 is a potent survival factor for c-Myc-dependent apoptosis

Many vertebrate cells are resistant to apoptotic stimuli, whose variety and the mechanisms involved are not fully understood. Endothelin-1 is an endothelium-derived vasoactive peptide that mediates many physiological functions, such as vasoconstriction and cell proliferation. Deregulated expression of c-Myc induces apoptosis in serum-deprived fibroblasts. Using a panel of isogenic fibroblast cell lines with differential c-myc expression levels, we demonstrate that low doses of endothelin-1 protect fibroblasts against serum deprivation-induced apoptosis, which occurs through a c-Myc-dependent process. The endothelin-1-induced cell survival was mediated by the ET(A) receptor and was not linked to the ability of endothelin-1 to induce cell proliferation. The survival function of endothelin-1 was abrogated by inhibiting the mitogen-activated protein kinase pathway. These results demonstrate a hitherto unappreciated role of endothelin-1 as a potent survival factor for c-Myc-dependent apoptosis, a process mediated by the ET(A) receptor and the mitogen-activated protein kinase pathway.

Effect of inhibitors of signal transduction on IGF-1-induced protein synthesis associated with hypertrophy in cultured neonatal rat ventricular myocytes

IGF-1 increased 2-fold protein synthesis in cardiac myocytes. Genistein, whether added during preincubation or with IGF-1 at the start of incubation, significantly inhibited the IGF-1-induced stimulation of protein synthesis, autophosphorylation of the beta-subunit of IGF-1 receptor and inhibition of ERK. When added 1 or 6 h after IGF-1, however, genistein was without effect. IGF-1-stimulated protein synthesis was also significantly inhibited by PD-098059, staurosporine, and rapamycin, but not by wortmannin, in cardiac myocytes. Some inhibitors produced a reduction in cell size. Activation of the ERK cascade by IGF-1 may be responsible for some of the features associated with cardiac myocyte hypertrophy.

Stimulation of 42/44 kDa mitogen-activated protein kinases by arginine vasopressin in rat cardiomyocytes

Vasoconstrictors, such as angiotensin II (Ang II), are involved in the regulatory mechanisms of post myocardial infarction (MI) hypertrophy. Arginine vasopressin (AVP), may be another vasoconstrictor that influences the mechanisms that lead to post MI hypertrophy. In these studies we investigated the possible activation of the 42/44 kDa mitogen-activated protein kinases (MAPKs), also referred as extracellular signal regulated kinases (ERKs), in cultured cardiomyocytes. Treatment of rat cardiomyocytes with AVP, Ang II and phorbol 12-myristate 13-acetate (PMA) increases the activation of ERKs. The activity of the 42/44 kDa MAPKs was tested using the phosphorylation of: (1) EGF receptor peptide (EGFR-P); (2) myelin basic protein (MBP) immobilized in poly acrylamide gels; and (3) T183 and Y185 residues of these proteins. The activity of the MAPKs, induced by AVP or PMA was inhibited by downregulation of protein kinase C (PKC), by the tyrosine kinase inhibitor genistein and by MAPK kinase (MEK) inhibitor, PD98059. In addition, the AVP-induced stimulation of MAPKs was shown to be mediated through a V1 receptor. We suggest that AVP activates the 42/44kDa MAPKs through a signal transduction pathway that involves stimulation of AVP-V1 receptor, tyrosine kinase, PKC and MEK. These results suggest that AVP may be involved in ERKs dependent regulatory functions of cardiomyocytes growth.

Differential coupling of alpha1-adrenoreceptor subtypes to phospholipase C and mitogen activated protein kinase in neonatal rat cardiac myocytes

Activation of cardiac alpha1-adrenoreceptors has a number of physiological effects. Ascribing these effects to a specific alpha1-adrenoreceptor subtype first requires the elucidation of the subtypes that are present in the tissue of interest. In the present study, mRNA transcripts for the alpha1A, alpha1B and alpha1D-adrenoreceptor subtypes were detected in cultured neonatal rat cardiac myocytes, using reverse transcriptase-polymerase chain reaction analysis. However, binding sites for only the alpha1A and alpha1B-adrenoreceptor subtypes were detected in cultured neonatal rat cardiac myocytes, using competition binding analysis with a variety of alpha1 selective receptor antagonists. Phenylephrine-stimulated phosphatidylinositol hydrolysis was inhibited by alpha1 selective receptor antagonists with affinities consistent with the alpha1A-adrenoreceptor subtype, whereas phenylephrine-induced activation of the mitogen activated protein kinase cascade was inhibited by these same antagonists with affinities more closely resembling the alpha1B-adrenoreceptor subtype. In the case of both signaling pathways, the alpha1D selective receptor antagonist, BMY 7378, exhibited affinities suggestive of the relative absence of a alpha1D-adrenoreceptor subtype. Thus, despite the presence of mRNA transcripts for all three alpha1-adrenoreceptor subtypes, only the alpha1A and alpha1B-adrenoreceptor subtypes were expressed and functionally coupled at detectable levels in neonatal rat cardiac myocytes. Of particular interest, phenylephrine-induced activation of the mitogen activated protein kinase cascade appears to be mediated by a subtype resembling most closely the pharmacological profile of the alpha1B-adrenoreceptor subtype.

Synergistic activation of guinea-pig cardiac cystic fibrosis transmembrane conductance regulator by the tyrosine kinase inhibitor genistein and cAMP

1. The regulation of cardiac Cl- current (ICl) by tyrosine and serine/threonine phosphorylation was examined in guinea-pig and rat ventricular myocytes. The protein tyrosine kinase (PTK) inhibitor genistein (GST) and phosphotyrosine phosphatase (PTP) inhibitor sodium orthovanadate (VO4) were used to modify tyrosine phosphorylation, whereas forskolin (FSK), cAMP, and other agents were used to modify cytoplasmic cAMP concentration and protein kinase A (PKA) phosphorylation. 2. Low concentrations (0.1 microM) of FSK did not activate the PKA-regulated cystic fibrosis transmembrane regulator (CFTR) ICl in guinea-pig ventricular myocytes, but strongly potentiated activation of an ICl by 20-100 microM GST. The potentiation did not occur when GST was replaced by PTK-inactive daidzein, and it was strongly inhibited by 1 mM VO4. 3. Potentiation by 0.1 microM FSK was linked to a small stimulation of the adenylate cyclase-cAMP-PKA pathway. The potentiation was not mimicked by inactive 1,9-dideoxyforskolin, and was inhibited by muscarinic stimulation (ACh) and by a PKA inhibitor. Internal application of a cAMP solution that alone was too weak to activate CFTR ICl strongly potentiated the activation of ICl by 50 microM GST and occluded potentiation by 0.1 microM FSK. 4. The foregoing suggests that potentiated ICl flows through cAMP-dependent CFTR channels. In agreement with this interpretation, GST did not increase ICl when CFTR was maximally activated by a high concentration (5 microM) of FSK and okadaic acid, and neither GST nor GST plus FSK activated an ICl in CFTR-deficient rat myocytes. The lack of effect in rat myocytes was not due to the absence of functional, channel-relevant PKA and PTK-PTP systems, because (as in guinea-pig myocytes) L-type Ca2+ current (ICa,L) was stimulated by FSK and inhibited in a VO4-reversible manner by GST. 5. The synergistic activation of CFTR by low concentrations of FSK and GST cannot be explained by either a GST-induced elevation of cAMP concentration or inhibition of serine/threonine phosphatase. Rather, it appears to be due to tyrosine dephosphorylation that facilitates PKA-mediated phosphorylation of the channels.

Trophic effect of human pericardial fluid on adult cardiac myocytes. Differential role of fibroblast growth factor-2 and factors related to ventricular hypertrophy

Pericardial fluid (PF) may contain myocardial growth factors that exert paracrine actions on cardiac myocytes. The aims of this study were (1) to investigate the effects of human PF and serum, collected from patients undergoing cardiac surgery, on the growth of cultured adult rat cardiac myocytes and (2) to relate the growth activity of both fluids to the adaptive changes in overloaded human hearts. Both PF and serum increased the rate of protein synthesis, measured by [14C]phenylalanine incorporation in adult rat cardiomyocytes (PF, +71.9 +/- 8.2% [n = 17]; serum, +14.9 +/- 6.5% [n = 13]; both P < .01 versus control medium). The effects of both PF and serum on cardiomyocyte growth correlated positively with the respective left ventricular (LV) mass. However, the magnitude of change with PF was 3-fold greater than with serum (P < .01). These trophic effects of PF were mimicked by exogenous basic fibroblast growth factor (FGF2) and inhibited by anti-FGF2 antibodies and transforming growth factor-beta (TGF-beta), suggesting a relationship to FGF2. In addition, FGF2 concentration in PF was 20 times greater than in serum. On the other hand, the LV mass-dependent trophic effect, present in both fluids, was independent of FGF2 concentration or other factors, such as angiotensin II, atrial natriuretic factor, and TGF-beta. These data suggest that FGF2 in human PF is a major determining factor in normal myocyte growth, whereas unidentified LV mass-dependent factor(s), present in both PF and serum, participates in the development of ventricular hypertrophy.

A role for the p38 mitogen-activated protein kinase pathway in myocardial cell growth, sarcomeric organization, and cardiac-specific gene expression

Three hallmark features of the cardiac hypertrophic growth program are increases in cell size, sarcomeric organization, and the induction of certain cardiac-specific genes. All three features of hypertrophy are induced in cultured myocardial cells by alpha1- adrenergic receptor agonists, such as phenylephrine (PE) and other growth factors that activate mitogen- activated protein kinases (MAPKs). In this study the MAPK family members extracellular signal-regulated kinase (ERK), c-jun NH2-terminal kinase (JNK), and p38 were activated by transfecting cultured cardiac myocytes with constructs encoding the appropriate kinases possessing gain-of-function mutations. Transfected cells were then analyzed for changes in cell size, sarcomeric organization, and induction of the genes for the A- and B-type natriuretic peptides (NPs), as well as the alpha-skeletal actin (alpha-SkA) gene. While activation of JNK and/or ERK with MEKK1COOH or Raf-1 BXB, respectively, augmented cell size and effected relatively modest increases in NP and alpha-SkA promoter activities, neither upstream kinase conferred sarcomeric organization. However, transfection with MKK6 (Glu), which specifically activated p38, augmented cell size, induced NP and alpha-Ska promoter activities by up to 130-fold, and elicited sarcomeric organization in a manner similar to PE. Moreover, all three growth features induced by MKK6 (Glu) or PE were blocked with the p38-specific inhibitor, SB 203580. These results demonstrate novel and potentially central roles for MKK6 and p38 in the regulation of myocardial cell hypertrophy.

Increased accumulation of acidic fibroblast growth factor in left ventricular myocytes of patients with idiopathic cardiomyopathy

To clarify the pathophysiologic role of fibroblast growth factors in idiopathic cardiomyopathy, we evaluated endomyocardial biopsy specimens obtained from 24 patients (nine with hypertrophic cardiomyopathy [HCM], 12 with dilated cardiomyopathy [DCM], and three with hypertensive hypertrophy) and six controls. All the specimens were stained for acidic fibroblast growth factor (aFGF) and basic FGF (bFGF) with immunohistochemistry. In situ hybridization was carried out for detection of aFGF mRNA. The average diameter of the myocytes, the percent area of interstitial fibrosis, and capillary vessel density were assessed in each biopsy specimen with morphometric methods. Positive staining of aFGF was observed in the myocytes of the biopsy specimens taken from 15 of 21 (71%) patients with cardiomyopathy (six of nine HCM, nine of 12 DCM) and all hypertensive hypertrophy patients but in none of the controls (p < 0.01). The average diameter of the myocytes was significantly larger in the patients with positive aFGF staining than in those with negative staining (23.1 +/- 1.5 versus 18.3 +/- 1.2 microm, p < 0.05). The percent area of fibrosis and the density of capillaries did not differ between the two groups. Intense expression of aFGF mRNA was observed in the myocytes from the patients with positive aFGF protein. In conclusion, the expression of FGF was significantly increased in myocytes obtained from the left ventricle of patients with cardiomyopathy. Acidic FGF may contribute to the hypertrophy of myocytes as the repair response to myocardial injury in patients with idiopathic cardiomyopathy.

Insulin-like growth factor-I rapidly activates multiple signal transduction pathways in cultured rat cardiac myocytes

In response to insulin-like growth factor-I (IGF-I), neonatal rat cardiac myocytes exhibit a hypertrophic response. The elucidation of the IGF-I signal transduction system in these cells remains unknown. We show here that cardiac myocytes present a single class of high affinity receptors (12,446 +/- 3,669 binding sites/cell) with a dissociation constant of 0.36 +/- 0.10 nM. Two different beta-subunits of IGF-I receptor were detected, and their autophosphorylation was followed by increases in the phosphotyrosine content of extracellular signal-regulated kinases (ERKs), insulin receptor substrate 1, phospholipase C-gamma1, and phosphatidylinositol 3-kinase. IGF-I transiently activates c-Raf in cultured neonatal cardiac myocytes, whereas A-raf is activated much less than c-Raf. Two peaks of ERK activity (ERK1 and ERK2) were resolved in cardiac myocytes treated with IGF-I by fast protein liquid chromatography, both being stimulated by IGF-I (with EC50 values for the stimulation of ERK1 and ERK2 by IGF-I of 0.10 and 0. 12 nM, respectively). Maximal activation of ERK2 (12-fold) and ERK1 (8.3-fold) activities was attained after a 5-min exposure to IGF-I. Maximal activation of p90 S6 kinase by IGF-I was achieved after 10 min, and then the activity decreased slowly. Interestingly, IGF-I stimulates incorporation of [3H]phenylalanine (1.6-fold) without any effect on [3H]thymidine incorporation. These data suggest that IGF-I activates multiple signal transduction pathways in cardiac myocytes some of which may be relevant to the hypertrophic response of the heart.

Regulation of the ERK subgroup of MAP kinase cascades through G protein-coupled receptors

The extracellularly-responsive kinase (ERK) subfamily of mitogen-activated protein kinases (MAPKs) has been implicated in the regulation of cell growth and differentiation. Activation of ERKs involves a two-step protein kinase cascade lying upstream from ERK, in which the Raf family are the MAPK kinase kinases and the MEK1/MEK2 isoforms are the MAPK kinases. The linear sequence of Raf --> MEK --> ERK constitutes the ERK cascade. Although the ERK cascade is activated through growth factor-regulated receptor protein tyrosine kinases, they are also modulated through G protein-coupled receptors (GPCRs). All four G protein subfamilies (Gq/11 Gi/o, Gs and G12/13) influence the activation state of ERKs. In this review, we describe the ERK cascade and characteristics of its activation through GPCRs. We also discuss the identity of the intervening steps that may couple agonist binding at GPCRs to activation of the ERK cascade.

Transgenic Galphaq overexpression induces cardiac contractile failure in mice

The critical cell signals that trigger cardiac hypertrophy and regulate the transition to heart failure are not known. To determine the role of Galphaq-mediated signaling pathways in these events, transgenic mice were constructed that overexpressed wild-type Galphaq in the heart using the alpha-myosin heavy chain promoter. Two-fold overexpression of Galphaq showed no detectable effects, whereas 4-fold overexpression resulted in increased heart weight and myocyte size along with marked increases in atrial naturietic factor ( approximately 55-fold), beta-myosin heavy chain ( approximately 8-fold), and alpha-skeletal actin ( approximately 8-fold) expression, and decreased ( approximately 3-fold) beta-adrenergic receptor-stimulated adenylyl cyclase activity. All of these signals have been considered markers of hypertrophy or failure in other experimental systems or human heart failure. Echocardiography and in vivo cardiac hemodynamic studies indeed revealed impaired intrinsic contractility manifested as decreased fractional shortening (19 +/- 2% vs. 41 +/- 3%), dP/dt max, a negative force-frequency response, an altered Starling relationship, and blunted contractile responses to the beta-adrenergic agonist dobutamine. At higher levels of Galphaq overexpression, frank cardiac decompensation occurred in 3 of 6 animals with development of biventricular failure, pulmonary congestion, and death. The element within the pathway that appeared to be critical for these events was activation of protein kinase Cepsilon. Interestingly, mitogen-activated protein kinase, which is postulated by some to be important in the hypertrophy program, was not activated. The Galphaq overexpressor exhibits a biochemical and physiologic phenotype resembling both the compensated and decompensated phases of human cardiac hypertrophy and suggests a common mechanism for their pathogenesis.

Signal transduction pathway of human fibroblast growth factor receptor 3. Identification of a novel 66-kDa phosphoprotein

Stimulation of fibroblast growth factor receptor 3 (FGFR3) results in a variety of functional effects, including regulation of epithelial cell growth and differentiation. In order to characterize the signaling pathway through which FGFR3 regulates cell growth, L6 cells lacking any endogenous FGFR were stably transfected with the two different human isoforms, FGFR3 IIIb and FGFR3 IIIc, that result from alternative splicing of exon III of the FGFR3 gene encoding the ligand binding domain. Expression of FGFR3 IIIc in stably transfected L6 cells conferred growth responses to several members of the FGF family including FGF-1, -2, -4, and -6, while FGFR3 IIIb-expressing cells responded only to FGF-1. Activation of FGFR3 upon ligand binding resulted in activation of mitogen-activated protein kinase pathway. FGFR3 utilizes two different pools of adapter protein GRB2 to link to Ras. Activated FGFR3 predominantly interacts with GRB2.Sos in complex with a previously identified 90-kDa protein and designated protein 80K-H. In addition, 80K-H.GRB2. Sos complex was found to contain a novel 66-kDa protein. Tyrosine phophorylation of the 66-kDa protein was dependent on ligand activation of FGFR3, suggesting that the 66-kDa protein may play an important role in FGFR3-specific signaling. In addition to this unique pathway, FGFR3 also links to GRB2.Sos complex via the adapter protein Shc. Furthermore, activated FGFR3 was not able to induce dissociation of GRB2.Sos complex following Sos phosphorylation. In summary, FGFR3 signaling pathway utilizes two GRB2-containing complexes; Shc.GRB2.Sos and 80K-H.pp66.GRB2.Sos; these two complexes may alternatively link FGFG3 to mitogen-activated protein kinase. Finally, activated FGFR3 was also found to result in phosphorylation of phospholipase C-gamma but reduced phosphorylation of c-Src.