M1 muscarinic receptors heterologously expressed in cardiac myocytes mediate Ras-dependent changes in gene expression

Epithelium integrity is crucial for the relaxant activity of brain natriuretic peptide in human isolated bronchi

BACKGROUND AND PURPOSE Brain natriuretic peptide (BNP) plays an important role in several biological functions, including bronchial relaxation. Here, we have investigated the role of BNP and its cognate receptors in human bronchial tone. EXPERIMENTAL APPROACH Effects of BNP on responses to carbachol and histamine were evaluated in non-sensitized, passively sensitized, epithelium-intact or denuded isolated bronchi and in the presence of methoctramine, N(ω) -nitro-L-arginine methyl ester (L-NAME) and aminoguanidine. Natriuretic peptide receptors (NPRs) were investigated by immunohistochemistry, RT-PCR and real-time PCR. Release of NO and acetylcholine from bronchial tissues and cultured BEAS-2B bronchial epithelial cells was also investigated. KEY RESULTS BNP reduced contractions mediated by carbachol and histamine, with decreased E(max) (carbachol: 22.7 ± 4.7%; histamine: 59.3 ± 1.8%) and increased EC(50) (carbachol: control 3.33 ± 0.88 µM, BNP 100 ± 52.9 µM; histamine: control 16.7 ± 1.7 µM, BNP 90 ± 30.6 µM); BNP was ineffective in epithelium-denuded bronchi. Among NPRs, only atrial NPR (NPR1) transcripts were detected in bronchial tissue. Bronchial NPR1 immunoreactivity was detected in epithelium and inflammatory cells but faint or absent in airway smooth muscle cells. NPR1 transcripts in bronchi increased after incubation with BNP, but not after sensitization. Methoctramine and quinine abolished BNP-induced relaxant activity. The latter was associated with increased bronchial mRNA for NO synthase and NO release, inhibited by L-NAME and aminoguanidine. In vitro, BNP increased acetylcholine release from bronchial epithelial cells, whereas NO release was unchanged. CONCLUSIONS AND IMPLICATIONS Epithelial cells mediate the BNP-induced relaxant activity in human isolated bronchi.

CaMKIIdelta isoforms differentially affect calcium handling but similarly regulate HDAC/MEF2 transcriptional responses

The delta(B) and delta(C) splice variants of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII), which differ by the presence of a nuclear localization sequence, are both expressed in cardiomyocytes. We used transgenic (TG) mice and CaMKII expression in cardiomyocytes to test the hypothesis that the CaMKIIdelta(C) isoform regulates cytosolic Ca(2+) handling and the delta(B) isoform, which localizes to the nucleus, regulates gene transcription. Phosphorylation of CaMKII sites on the ryanodine receptor (RyR) and on phospholamban (PLB) were increased in CaMKIIdelta(C) TG. This was associated with markedly enhanced sarcoplasmic reticulum (SR) Ca(2+) spark frequency and decreased SR Ca(2+) content in cardiomyocytes. None of these parameters were altered in TG mice expressing the nuclear-targeted CaMKIIdelta(B). In contrast, cardiac expression of either CaMKIIdelta(B) or delta(C) induced transactivation of myocyte enhancer factor 2 (MEF2) gene expression and up-regulated hypertrophic marker genes. Studies using rat ventricular cardiomyocytes confirmed that CaMKIIdelta(B) and delta(C) both regulate MEF2-luciferase gene expression, increase histone deacetylase 4 (HDAC4) association with 14-3-3, and induce HDAC4 translocation from nucleus to cytoplasm, indicating that either isoform can stimulate HDAC4 phosphorylation. Finally, HDAC4 kinase activity was shown to be increased in cardiac homogenates from either CaMKIIdelta(B) or delta(C) TG mice. Thus CaMKIIdelta isoforms have similar effects on hypertrophic gene expression but disparate effects on Ca(2+) handling, suggesting distinct roles for CaMKIIdelta isoform activation in the pathogenesis of cardiac hypertrophy versus heart failure.

Galpha(12/13) mediates alpha(1)-adrenergic receptor-induced cardiac hypertrophy

In neonatal cardiomyocytes, activation of the G(q)-coupled alpha(1)-adrenergic receptor (alpha(1)AR) induces hypertrophy by activating mitogen-activated protein kinases, including c-Jun NH(2)-terminal kinase (JNK). Here, we show that JNK activation is essential for alpha(1)AR-induced hypertrophy, in that alpha(1)AR-induced hypertrophic responses, such as reorganization of the actin cytoskeleton and increased protein synthesis, could be blocked by expressing the JNK-binding domain of JNK-interacting protein-1, a specific inhibitor of JNK. We also identified the classes and subunits of G proteins that mediate alpha(1)AR-induced JNK activation and hypertrophic responses by generating several recombinant adenoviruses that express polypeptides capable of inhibiting the function of specific G-protein subunits. alpha(1)AR-induced JNK activation was inhibited by the expression of carboxyl terminal regions of Galpha(q), Galpha(12), and Galpha(13). JNK activation was also inhibited by the Galpha(q/11)- or Galpha(12/13)-specific regulator of G-protein signaling (RGS) domains and by C3 toxin but was not affected by treatment with pertussis toxin or by expression of the carboxyl terminal region of G protein-coupled receptor kinase 2, a polypeptide that sequesters Gbetagamma. alpha(1)AR-induced hypertrophic responses were inhibited by Galpha(q/11)- and Galpha(12/13)-specific RGS domains, C3 toxin, and the carboxyl terminal region of G protein-coupled receptor kinase 2 but not by pertussis toxin. Activation of Rho was inhibited by carboxyl terminal regions of Galpha(12) and Galpha(13) but not by Galpha(q). Our findings suggest that alpha(1)AR-induced hypertrophic responses are mediated in part by a Galpha(12/13)-Rho-JNK pathway, in part by a G(q/11)-JNK pathway that is Rho independent, and in part by a Gbetagamma pathway that is JNK independent.

Guanine nucleotide exchange factor-like factor (Rlf) induces gene expression and potentiates alpha 1-adrenergic receptor-induced transcriptional responses in neonatal rat ventricular myocytes

Expression of constitutively active Ras (V12Ras) in cultured neonatal rat ventricular myocytes or targeted cardiac expression of V12Ras in transgenic mice induces myocardial cell growth and expression of genes that are markers of cardiac hypertrophy including atrial natriuretic factor (ANF) and myosin light chain-2. However, the signaling pathways that modulate the effects of Ras on acquisition of the various features of cardiac hypertrophy are not known. We identified the Ral guanine nucleotide exchange factor-like factor (Rlf) in a yeast two-hybrid screen of human heart cDNA library using Ras as bait, suggesting that Ras signaling in the heart may involve Rlf. We demonstrate here that Rlf is expressed in human heart. Expression of wild type Rlf or Rlf-CAAX, a membrane-targeted mutant of Rlf, transactivated ANF and myosin light chain-2 promoters but did not activate canonical cAMP responsive elements or phorbol ester responsive elements, suggesting that Rlf expression does not lead to a generalized increase in transcription. Transfection of mutant ANF promoter-reporter gene constructs demonstrated that the proximal serum response element is both necessary and sufficient for Rlf-inducible ANF expression. Rlf-induced ANF promoter activation required Ral and Cdc42 but not RhoA, Rac1, ERK, or p38 kinase activation. In addition, Rlf potentiated alpha(1)-adrenergic receptor (alpha(1)-AR)-induced ANF expression. Prolonged activation of the alpha(1)-AR increases RalGTP levels in neonatal rat ventricular myocytes, further emphasizing a role for Ral guanine nucleotide exchange factors in alpha(1)-AR signaling. Overall, this study supports the concept that Rlf and Ral are important previously unrecognized signaling components that regulate transcriptional responses in myocardial cells.

Cytoplasmic signaling pathways that regulate cardiac hypertrophy

This review discusses the rapidly progressing field of cardiomyocyte signal transduction and the regulation of the hypertrophic response. When stimulated by a wide array of neurohumoral factors or when faced with an increase in ventricular-wall tension, individual cardiomyocytes undergo hypertrophic growth as an adaptive response. However, sustained cardiac hypertrophy is a leading predictor of future heart failure. A growing number of intracellular signaling pathways have been characterized as important transducers of the hypertrophic response, including specific G protein isoforms, low-molecular-weight GTPases (Ras, RhoA, and Rac), mitogen-activated protein kinase cascades, protein kinase C, calcineurin, gp130-signal transducer and activator of transcription, insulin-like growth factor I receptor pathway, fibroblast growth factor and transforming growth factor beta receptor pathways, and many others. Each of these signaling pathways has been implicated as a hypertrophic transducer, which collectively suggests an emerging paradigm whereby multiple pathways operate in concert to orchestrate a hypertrophic response

G-proteins in growth and apoptosis: lessons from the heart

The acute contractile function of the heart is controlled by the effects of released nonepinephrine (NE) on cardiac adrenergic receptors. NE can also act in a more chronic fashion to induce cardiomyocyte growth, characterized by cell enlargement (hypertrophy), increased protein synthesis, alterations in gene expression and addition of sarcomeres. These responses enhance cardiomyocyte contractile function and thus allow the heart to compensate for increased stress. The hypertrophic effects of NE are mediated through Gq-coupled alpha(1)-adrenergic receptors and are mimicked by the actions of other neurohormones (endothelin, prostaglandin F(2alpha) angiotensin II) that also act on Gq-coupled receptors. Activation of phospholipase C by Gq is necessary for these responses, and protein kinase C and MAP kinases have also been implicated. Gq stimulated cardiac hypertrophy is also evident in transgenic mouse models. In contrast, stimulation of G(s)-coupled beta-adrenergic receptors or G(i)-coupled receptors do not directly effect cardiomyocyte hypertrophy. Apoptosis is also induced by G-protein-coupled receptor stimulation in cardiomyocytes. Sustained or excessive activation of either Gq- or Gs-signaling pathways results in apoptotic loss of cardiomyocytes both in vitro and in vivo. Apoptosis is associated with decreased ventricular function in the failing heart. Cardiomyocytes provide an ideal model system for understanding the basis for G-protein mediated hypertrophy and apoptosis, and the mechanisms responsible for the transition from compensatory to deleterious levels of signaling. This information may prove critical for designing interventions that prevent the pathophysiological consequences of heart failure.

Receptors coupling to G proteins: is there a signal behind the sequence?

Upon the binding of their ligands, G protein-coupled receptors couple to the heterotrimeric G proteins to transduce a signal. One receptor family may couple to a single G protein subtype and another family to several ones. Is there a signal in the receptor sequence that can give an indication of the G protein subtype selectivity? We used a sequence analysis method on biogenic amine and adenosine receptors and concluded that a weak signal can be detected in receptor families where specialization for coupling to a given G protein occurred during a recent divergent evolutionary process. Proteins 2000;41:448-459.

The Rho effector, PKN, regulates ANF gene transcription in cardiomyocytes through a serum response element

The low-molecular-weight GTP-binding protein RhoA mediates hypertrophic growth and atrial natriuretic factor (ANF) gene expression in neonatal rat ventricular myocytes. Neither the effector nor the promoter elements through which Rho exerts its regulatory effects on ANF gene expression have been elucidated. When constitutively activated forms of Rho kinase and two protein kinase C-related kinases, PKN (PRK1) and PRK2, were compared, only PKN generated a robust stimulation of a luciferase reporter gene driven by a 638-bp fragment on the ANF promoter. This ANF promoter fragment contains a proximal serum response element (SRE) and an Sp-1-like element required for the transcriptional response to phenylephrine (PE). This response was inhibited by dominant negative Rho. The ability of dominant negative Rho to inhibit the response to PE and the ability of PKN to stimulate ANF reporter gene expression were both lost when the SRE was mutated. Mutation of the Sp-1-like element also attenuated the response to PKN. A minimal promoter driven by ANF SRE sequences was sufficient to confer Rho- and PKN-mediated gene expression. Interestingly, PKN preferentially stimulated the ANF versus the c-fos SRE reporter gene. Thus PKN and Rho are able to regulate transcriptional activation of the ANF SRE by a common element that could implicate PKN as a downstream effector of Rho in transcriptional responses associated with hypertrophy.

Differential and selective inhibition of protein kinase A and protein kinase C in intact cells by balanol congeners

The fungal metabolite balanol is a potent inhibitor of protein kinase A (PKA) and protein kinase C (PKC) in vitro that acts by competing with ATP for binding (K(i) approximately 4 nM); congeners of balanol show specificity for PKA over PKC. We have characterized the effects of balanol and 10"-deoxybalanol in intact cells to determine whether these compounds cross the cell membrane and whether the potency and specificity noted in vitro are preserved in vivo. In neonatal rat myocytes and cultured A431 cells transiently transfected with a cyclic AMP response element-luciferase reporter construct, balanol inhibits the induction of luciferase activity by isoproterenol, indicating inhibition of PKA. Western analysis shows that both balanol and 10"-deoxybalanol reduce phosphorylation of cAMP response element-binding protein in isoproterenol-stimulated A431 cells; inhibition is concentration dependent with an IC(50) value of approximately 3 microM. Balanol, but not 10"-deoxybalanol, inhibits phosphorylation of the myristoylated alanine-rich C kinase substrate protein, a PKC substrate, in phorbol ester-stimulated A431 cells (IC(50) approximately 7 microM). Our data demonstrate that balanol is a potent inhibitor of PKA and PKC in several whole-cell systems and causes no obvious toxicity. In addition, balanol congeners inhibit PKA and PKC with the specificity and potency predicted by in vitro experiments.

A Ras-dependent pathway regulates RNA polymerase II phosphorylation in cardiac myocytes: implications for cardiac hypertrophy

Despite extensive evidence implicating Ras in cardiac muscle hypertrophy, the mechanisms involved are unclear. We previously reported that Ras, through an effector-like function of Ras GTPase-activating protein (GAP) in neonatal cardiac myocytes (M. Abdellatif et al., J. Biol. Chem. 269:15423-15426, 1994; M. Abdellatif and M. D. Schneider, J. Biol. Chem. 272:527-533, 1997), can up-regulate expression from a comprehensive set of promoters, including both cardiac cell-specific and constitutive ones. To investigate the mechanism(s) underlying these earlier findings, we have used recombinant adenoviruses harboring a dominant negative Ras (17N Ras) allele or the N-terminal domain of GAP (nGAP), responsible for the Ras-like effector function. Inhibition of endogenous Ras reduced basal levels of [3H]uridine and [3H]phenylalanine incorporation into total RNA, mRNA, and protein, with parallel changes in apparent cell size. In addition, 17N Ras markedly inhibited phosphorylation of the C-terminal domain (CTD) of RNA polymerase II (pol II), known to regulate transcript elongation, accompanied by down-regulation of its principal kinase, cyclin-dependent kinase 7 (Cdk7). In contrast, nGAP elicited the opposite effects on each of these parameters. Furthermore, cotransfection of constitutively active Ras (12R Ras) with wild-type pol II, rather than a truncated mutant lacking the CTD, demonstrated that Ras activation of transcription was dependent on the pol II CTD. Consistent with a potential role for this pathway in the development of cardiac myocyte hypertrophy, alpha1-adrenergic stimulation similarly enhanced pol II phosphorylation and Cdk7 expression, where both effects were inhibited by dominant negative Ras, while pressure overload hypertrophy led to an increase in both hyperphosphorylated and hypophosphorylated pol II in addition to Cdk7.

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.

Tyrosine kinase and c-Jun NH2-terminal kinase mediate hypertrophic responses to prostaglandin F2alpha in cultured neonatal rat ventricular myocytes

Myocardial infarction results in focal areas of ischemia, hypoxia, necrosis, and decreased contractile function. To compensate for loss of contractile function, remaining viable myocytes undergo hypertrophic growth. Prostaglandin F2alpha (PGF2alpha), which is released from cells of the myocardium during periods of stress such as hypoxia or ischemia/reperfusion, has recently been shown to stimulate hypertrophic growth in neonatal rat ventricular myocytes. In the present study, we determine which growth-related intracellular pathways are required for PGF2alpha to induce morphological and genetic features characteristic of the hypertrophic phenotype. In cardiomyocytes, PGF2alpha increases the hydrolysis of inositol phosphates and induces the translocation of protein kinase C epsilon to the myocyte membrane, consistent with PGF2alpha receptor coupling to Gq. PGF2alpha also activates the extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinase pathways. Surprisingly, studies using pharmacological inhibitors and transfection of dominant-interfering proteins demonstrate that PGF2alpha-induced myocyte hypertrophy occurs independent of either PKC, p38, or ERK pathways. Additional studies demonstrate that PGF2alpha stimulates protein tyrosine phosphorylation and activates c-Jun NH2-terminal kinase and suggest that these pathways mediate hypertrophic growth in response to PGF2alpha.

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.

The nuclear deltaB isoform of Ca2+/calmodulin-dependent protein kinase II regulates atrial natriuretic factor gene expression in ventricular myocytes

Cultured neonatal ventricular myocytes display features of myocardial hypertrophy including increased cell size, myofilament organization, and reexpression of the embryonic gene for atrial natriuretic factor (ANF). KN-93, an inhibitor of multifunctional Ca2+/calmodulin-dependent protein kinase (CaM kinase II), blocked the induction of these responses by the alpha1-adrenergic receptor agonist phenylephrine, whereas its inactive analog KN-92 did not. To directly determine whether CaM kinase II could regulate ANF gene expression, we transiently expressed each of three isoforms of CaM kinase II (alpha, deltaB, and deltaC) along with an ANF promoter/luciferase reporter gene. The deltaB isoform markedly increased luciferase gene expression, whereas comparable levels of the deltaC and alpha isoforms were ineffective. Expression of deltaB-CaM kinase II also potentiated phenylephrine-mediated ANF gene expression, and this effect was blocked by KN-93 but not by KN-92. The ability of deltaB-CaM kinase II to transactivate a truncated ANF promoter, containing a serum response element (SRE) required for phenylephrine-inducible gene expression, was lost when this SRE was mutated. The deltaB isoform of CaM kinase II has been shown to exhibit nuclear localization. Coexpression of the non-nuclear deltaC or alpha isoforms, which can form multimers with the deltaB isoform, prevented the nuclear localization of deltaB-CaM kinase II and also blocked its effects on ANF reporter gene and protein expression. In addition, a chimeric alpha-CaM kinase II which contains the nuclear localization signal of the deltaB isoform was able to induce ANF reporter gene expression, albeit to a lesser extent than deltaB-CaM kinase II. These data are the first to assign a function to the deltaB isoform of CaM kinase II and to link its nuclear localization to subsequent activation of cardiac gene expression.

A selective epsilon-protein kinase C antagonist inhibits protection of cardiac myocytes from hypoxia-induced cell death

Protein kinase C activation is thought to protect cardiac tissue from subsequent ischemic injury by a process termed preconditioning. The protein kinase C isozyme that mediates preconditioning has not yet been identified. Using a cell culture model of hypoxic preconditioning, we found that cardiac myocyte viability after 9 h of hypoxia was increased by more than 50% over control. Preconditioning activated protein kinase C isozymes as evidenced by translocation from one cell compartment to another as follows: there was a 2.1-fold increase in epsilon-protein kinase C activation, a 2. 8-fold increase in delta-protein kinase C activation, and no increase in betaI-protein kinase C activation. 4beta-Phorbol 12-myristate 13-acetate mimicked hypoxic preconditioning, increasing myocyte survival after prolonged hypoxia by 34% compared with control. We previously identified an epsilon-protein kinase C-selective antagonist, epsilonV1-2 peptide, that inhibits epsilon-protein kinase C translocation and function in cardiac myocytes (Johnson, J. A., Gray, M. O., Chen, C.-H., and Mochly-Rosen, D. (1996) J. Biol. Chem. 271, 24962-24966). epsilonV1-2 peptide abolished hypoxic preconditioning and phorbol ester-mediated cardiac protection. Therefore, preconditioning can be induced in this culture model, and activation of epsilon-protein kinase C is critical for cardiac myocyte protection.

Rho is required for Galphaq and alpha1-adrenergic receptor signaling in cardiomyocytes. Dissociation of Ras and Rho pathways

G protein-coupled receptor agonists initiate a cascade of signaling events in neonatal rat ventricular myocytes that culminates in changes in gene expression and cell growth characteristic of hypertrophy. These responses have been previously shown to be dependent on Gq and Ras. Rho, a member of the Ras superfamily of GTPases, regulates cytoskeletal rearrangement and transcriptional activation of the c-fos serum response element. Immunofluorescence staining of cardiomyocytes shows that Rho is present and predominantly cytosolic. We used two inhibitors of Rho function, dominant negative N19RhoA and Clostridium botulinum C3 transferase, to examine the possible requirement for Rho in alpha1-adrenergic receptor-mediated hypertrophy. Both inhibitors markedly attenuated atrial natriuretic factor (ANF) reporter gene expression induced by alpha1-adrenergic receptor stimulation with phenylephrine, and virtually abolished the increase in ANF reporter gene expression induced by GTPase-deficient Galphaq. These effects were reproduced with the myosin light chain-2 reporter gene. Notably, N19RhoA did not block the ability of activated Ras to induce ANF and myosin light chain-2 reporter gene expression. Furthermore, activation of the extracellular signal-regulated kinase by phenylephrine was not blocked by N19RhoA, nor was it stimulated by an activated mutant of RhoA. Since activated RhoA and Ras produce a large synergistic effect on ANF-luciferase gene expression, we conclude that Rho functions in a pathway separate from but complementary to Ras. Our results provide direct evidence that Rho is an effector of Galphaq signaling and suggest for the first time that a low molecular weight GTPase other than Ras is involved in regulating myocardial cell growth and gene expression in response to heterotrimeric G protein-linked receptor activation.

Phosphorylation-dependent activation of the Ras-GRF/CDC25Mm exchange factor by muscarinic receptors and G-protein beta gamma subunits

Muscarinic receptors activate Ras through a pathway distinct from that mediated through translocation of the exchange factor mSos1 by receptor tyrosine kinases. Here we report that muscarinic receptors can activate another Ras exchange factor, CDC25Mm, or p140Ras-GRF (refs 5,6). In NIH-3T3 cells expressing subtype 1 human muscarinic receptors (hm1), the agonist carbachol selectively increased the specific activity and phosphorylation state of epitope-tagged Ras-GRF. This stimulation was reversed by protein phosphatase 1 (PP1), and prevented by transducin alpha-subunits. Carbachol treatment of neonatal rat brain explants increasd Ras exchange factor activity and the phosphorylation state of endogenous Ras-GRF. In COS-7 cells, cotransfection of hm1 or hm2 receptors with Ras-GRF conferred carbachol-dependent increases in exchange-factor activity, whereas cotransfection with G-protein beta gamma subunits caused a constitutive activation that was sensitive to PP1. These results demonstrate a G-protein-coupled mechanism for Ras activation, mediated by p140 Ras-GRF.

Dissociation of p44 and p42 mitogen-activated protein kinase activation from receptor-induced hypertrophy in neonatal rat ventricular myocytes

In response to hormones and mechanical stretch, neonatal rat ventricular myocytes exhibit a hypertrophic response that is characterized by induction of cardiac-specific genes and increased myocardial cell size. Hypertrophic stimuli also activate mitogen-activated protein kinase (MAPK), an enzyme thought to play a central role in the regulation of cell growth and differentiation. To determine if MAPK activation is sufficient for acquisition of the molecular and morphological features of cardiac hypertrophy we compared four agonists that stimulate G protein-coupled receptors. Whereas phenylephrine and endothelin transactivate cardiac-specific promoter/luciferase reporter genes, increase atrial natriuretic factor (ANF) expression, and promote myofilament organization, neither carbachol nor ATP induces these responses. Interestingly, all four agonists activate both the p42 and the p44 isoforms of MAPK. Furthermore, the kinetics of MAPK activation are not different for the hypertrophic agonist phenylephrine and the nonhypertrophic agonist carbachol. Transient transfection of myocytes with dominant-interfering mutants of p42 and p44 MAPK failed to block phenylephrine-induced ANF expression, although Ras-induced gene expression was inhibited by expression of the mutant MAPK constructs. Moreover, PD 098059, an inhibitor of MAPK kinase, blocked phenylephrine-stimulated MAPK activity but not ANF reporter gene expression. Thus, MAPK activation is not sufficient for G protein receptor-mediated induction of cardiac cell growth and gene expression and is apparently not required for transcriptional activation of the ANF gene.

The role of protein kinases in adaptational growth of the heart

The ventricular myocyte is a terminally-differentiated cell that can no longer undergo cell division. In response to a variety of stimuli, including exposure to endothelin-1, phenylephrine or mechanical stretch, the myocyte increases its size and its complement of organized myofibrils. These adaptational changes during myocyte hypertrophy are accompanied by distinct changes in gene expression. The signalling cascades that initiate these changes are currently under intensive investigation. Many hypertrophic agonists activate protein kinase C (PKC). Transfection of ventricular myocytes with constitutively-active PKC isoforms initiates the changes in gene expression typical of the hypertrophic response. Similarly, the Ras/Raf/mitogen-activated protein kinase (MAPK) pathway can be activated by a variety of hypertrophic agents. Transfection of ventricular myocytes with components of this pathway has demonstrated that MAPK is essential for the changes in gene expression associated with the development of hypertrophy. However a Ras-dependent, but Raf-independent, pathway may regulate the organization of the contractile apparatus. Other protein kinases, such as ribosomal S6 kinases, p90RSK or p70/p85S6K, which are poorly characterized in the ventricular myocyte, may also regulate changes in gene expression. Further research is required to investigate cross-talk between these signal transduction pathways so that the spatial and temporal relationships that integrate the multiple signaling events leading to the adaptational growth of the ventricular myocyte may be understood.

Identification of a specific Ins(1,3,4,5)P4-binding protein as a member of the GAP1 family

Inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P4) is produced rapidly from inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) in stimulated cells. Despite extensive experimentation, no clearly defined cellular function has yet been described for this inositol phosphate. Binding sites specific for Ins(1,3,4,5)P4 have been identified in several tissues, and we have purified one such protein to homogeneity. Its high affinity for Ins(1,3,4,5)P4, and its exquisite specificity for this isomeric configuration, suggest it may be an Ins(1,3,4,5)P4 receptor. Here we report the cloning and characterization of this protein as a GTPase-activating protein, specifically a member of the GAP1 family. In vitro it shows GAP activity against both Rap and Ras, but only the Ras GAP activity is inhibited by phospholipids and is specifically stimulated by Ins(1,3,4,5)P4.