Extracellular ATP inhibits adrenergic agonist-induced hypertrophy of neonatal cardiac myocytes

P2 Receptors in Cardiac Myocyte Pathophysiology and Mechanotransduction

ATP is a major energy source in the mammalian cells, but it is an extracellular chemical messenger acting on P2 purinergic receptors. A line of evidence has shown that ATP is released from many different types of cells including neurons, endothelial cells, and muscle cells. In this review, we described the distribution of P2 receptor subtypes in the cardiac cells and their physiological and pathological roles in the heart. So far, the effects of external application of ATP or its analogues, and those of UTP on cardiac contractility and rhythm have been reported. In addition, specific genetic alterations and pharmacological agonists and antagonists have been adopted to discover specific roles of P2 receptor subtypes including P2X4-, P2X7-, P2Y2- and P2Y6-receptors in cardiac cells under physiological and pathological conditions. Accumulated data suggest that P2X4 receptors may play a beneficial role in cardiac muscle function, and that P2Y2- and P2Y6-receptors can induce cardiac fibrosis. Recent evidence further demonstrates P2Y1 receptor and P2X4 receptor as important mechanical signaling molecules to alter membrane potential and Ca²⁺ signaling in atrial myocytes and their uneven expression profile between right and left atrium.

Kinome and Transcriptome Profiling Reveal Broad and Distinct Activities of Erlotinib, Sunitinib, and Sorafenib in the Mouse Heart and Suggest Cardiotoxicity From Combined Signal Transducer and Activator of Transcription and Epidermal Growth Factor Receptor Inhibition

BACKGROUND

Most novel cancer therapeutics target kinases that are essential to tumor survival. Some of these kinase inhibitors are associated with cardiotoxicity, whereas others appear to be cardiosafe. The basis for this distinction is unclear, as are the molecular effects of kinase inhibitors in the heart.

METHODS AND RESULTS

We administered clinically relevant doses of sorafenib, sunitinib (cardiotoxic multitargeted kinase inhibitors), or erlotinib (a cardiosafe epidermal growth factor receptor inhibitor) to mice daily for 2 weeks. We then compared the effects of these 3 kinase inhibitors on the cardiac transcriptome using RNAseq and the cardiac kinome using multiplexed inhibitor beads coupled with mass spectrometry. We found unexpectedly broad molecular effects of all 3 kinase inhibitors, suggesting that target kinase selectivity does not define either the molecular response or the potential for cardiotoxicity. Using in vivo drug administration and primary cardiomyocyte culture, we also show that the cardiosafety of erlotinib treatment may result from upregulation of the cardioprotective signal transducer and activator of transcription 3 pathway, as co-treatment with erlotinib and a signal transducer and activator of transcription inhibitor decreases cardiac contractile function and cardiomyocyte fatty acid oxidation.

CONCLUSIONS

Collectively our findings indicate that preclinical kinome and transcriptome profiling may predict the cardiotoxicity of novel kinase inhibitors, and suggest caution for the proposed therapeutic strategy of combined signal transducer and activator of transcription/epidermal growth factor receptor inhibition for cancer treatment.

Effects of 4(1H)-quinolinone derivative, a novel non-nucleotide allosteric purinergic P2Y 2 agonist, on cardiomyocytes in neonatal rats

Purinergic P2Y₂ receptors, G-protein coupled receptors that primarily couple with Gα_q/11-proteins, are activated equipotently by adenosine-5′-triphosphate (ATP) and uridine-5′-triphosphate. Evidence suggests that P2Y₂ agonists make potential drug candidates for the treatment of cardiovascular diseases. However, selective non-nucleotide, small-molecule P2Y₂ agonists have yet to be developed. In this report, we discuss Compound 89, a novel non-nucleotide allosteric P2Y₂ agonist that was active in signal transduction and gene induction, and in our in vitro cardiac hypertrophy model. Compound 89 exhibited selective P2Y₂ agonistic activity and potentiated responses to the endogenous agonist ATP, while exhibiting no agonistic activities for four other Gα_q/11-coupled human P2Y (hP2Y) receptors and one representative Gα_i/o-coupled hP2Y₁₂ receptor. Its P2Y₂ agonistic effect on mouse P2Y₂ receptors suggested non-species-specific activity. Compound 89 acted as a pure positive allosteric modulator in a Ca²⁺ mobilization assay of neonatal rat cardiomyocytes; it potentiated ATP-induced expression of genes in the nuclear receptor 4A family (negative regulators of hypertrophic stimuli in cardiomyocytes). Additionally, Compound 89 attenuated isoproterenol-induced cardiac hypertrophy, presumably through dose-dependent interaction with pericellular ATP. These results indicate that Compound 89 is potentially efficacious against cardiomyocytes and therefore a good proof-of-concept tool for elucidating the therapeutic potential of P2Y₂ activation in various cardiovascular diseases.

Interplay between renal endothelin and purinergic signaling systems

Alterations in extracellular fluid volume regulation and sodium balance may result in the development and maintenance of salt-dependent hypertension, a major risk factor for cardiovascular disease. Numerous pathways contribute to the regulation of sodium excretion and blood pressure, including endothelin and purinergic signaling. Increasing evidence suggests a link between purinergic receptor activation and endothelin production within the renal collecting duct as a means of promoting natriuresis. A better understanding of the relationship between these two systems, especially in regard to sodium homeostasis, will fill a significant knowledge gap and may provide novel antihypertensive treatment options. Therefore, this review focuses on the cross talk between endothelin and purinergic signaling as it relates to the renal regulation of sodium and blood pressure homeostasis.

Purinergic signalling and development of the autonomic nervous system

Most early studies of the role of nucleotides in development have evidenced their crucial importance as carriers of energy in all organisms. However, an increasing number of studies are now available to suggest that purines and pyrimidines, acting as extracellular ligands specifically on receptors of the plasma membrane, may play a pivotal role throughout pre- and postnatal development in a wide variety of organisms including amphibians, birds, and mammals. Purinergic receptor expression and functions have been studied in the development of many organs, including the autonomic nervous system (ANS). Nucleotide receptors can induce a multiplicity of cellular signalling pathways via crosstalk with bioactive molecules acting on growth factors and neurotransmitter receptors which are fundamental for the development of a mature and functional ANS. Purines and pyrimidines may influence all the stages of neuronal development, including neural cell proliferation, migration, differentiation and phenotype determination of differentiated cells. Indeed, the normal development of the ANS is disturbed by dysfunction of purinergic signalling in animal models. To establish the primitive and fundamental nature of purinergic neurotransmission in the ontogeny of the ANS, in this review the roles of purines and pyrimidines as signalling molecules during embryological and postnatal development are considered.

Cardiac purinergic signalling in health and disease

This review is a historical account about purinergic signalling in the heart, for readers to see how ideas and understanding have changed as new experimental results were published. Initially, the focus is on the nervous control of the heart by ATP as a cotransmitter in sympathetic, parasympathetic, and sensory nerves, as well as in intracardiac neurons. Control of the heart by centers in the brain and vagal cardiovascular reflexes involving purines are also discussed. The actions of adenine nucleotides and nucleosides on cardiomyocytes, atrioventricular and sinoatrial nodes, cardiac fibroblasts, and coronary blood vessels are described. Cardiac release and degradation of ATP are also described. Finally, the involvement of purinergic signalling and its therapeutic potential in cardiac pathophysiology is reviewed, including acute and chronic heart failure, ischemia, infarction, arrhythmias, cardiomyopathy, syncope, hypertrophy, coronary artery disease, angina, diabetic cardiomyopathy, as well as heart transplantation and coronary bypass grafts.

A positive inotropic effect of UTP in the human cardiac atrium

In the cardio-vascular system extracellular UTP can induce receptor-mediated vasoconstriction via smooth muscle cells and vasodilatation via endothelial cells. We evaluated inotropic effects of UTP in preparations from human heart. Contractile effects were studied in atrial preparations from patients undergoing cardiac bypass surgery. For comparison, contractility in isolated spontaneously beating right atrial and paced left atrial preparations from mice was investigated. UTP and UTPγS concentration-dependently exerted a positive inotropic effect with a maximum at 100 µM UTP that amounted to 156% of pre-drug value (n=13) without changing time parameters of contraction. UTP was able to partially attenuate the positive inotropic effect of β-adrenoceptor stimulation. UTP did not change the beating rate in right atrial mouse preparations. The positive inotropic effect of UTP could not be blocked by the P2 purinoceptor antagonists suramin (100 µM and 500 µM), PPADS (50 µM) and reactive blue (100 µM). Likewise inhibitors of PLC activity (U73122) and of adenylyl cyclase activity (SQ22563; 10 µM each) failed to affect the effects of UTP. In summary, we describe a novel positive inotropic effect of UTP on force contraction in the isolated human atrium. We tentatively suggest that UTP might act via P2Y2- or P2Y4-like receptors.

K(ATP) channels mediate the antihypertrophic effects afforded by κ-opioid receptor stimulation in neonatal rat ventricular myocytes

Recent evidence suggests that κ-opioid receptor (OR) agonists and K(ATP) channel activation exert antihypertrophic effects on cardiac myocytes. We studied the role of K(ATP) channels in the antihypertrophic effects of ORs in primary cultures of neonatal rat ventricular myocytes exposed for 48 h to the α(1) adrenoceptor agonist phenylephrine and the relative contributions of mitochondrial K(ATP) (mitoK(ATP)) and sarcolemmal K(ATP) (sarcK(ATP)). Furthermore, we elucidated the pathway between ORs and K(ATP) channels and their impact on intracellular Ca(2+) ([Ca(2+)](i)) transients. Hypertrophy of cardiomyocytes was characterized by increases in i) total protein content; ii) cell size and iii) [(3)H]leucine incorporation. Phenylephrine (10 μM) increased the three parameters. Trans-(±)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]-benzeneacetamid methanesulfonate salt (U50,488H), a selective κ-opioid receptor agonist, prevented phenylephrine-induced hypertrophy and [Ca(2+)](i) transients. The effect of U50,488H was abolished by nor-binaltorphimine, a selective κ-OR antagonist, indicating that the effect was κ-OR-mediated. The protein kinase C inhibitor chelerythrine and the K(ATP) channel inhibitors glibenclamide (50 μM), a nonselective K(ATP) antagonist, and 5-hydroxydecanoic acid (100 μM), a mitochondrial selective K(ATP) antagonist, reversed the antihypertrophic effect of U50,488H, and there was no significant difference between the two K(ATP) channel blockers. Moreover, we also determined the expression of the Kir6.2 subunits of the K(ATP) channel, which increased in response to U50,488H in the presence of phenylephrine, but was suppressed by chelerythrine, glibenclamide and 5-hydroxydecanoic acid. U50,488H also attenuated the elevation of [Ca(2+)](i). This study suggests that K(ATP), and particularly the mitochondrial K(ATP,) mediates the antihypertrophic effects of κ-opioid receptor stimulation via the PKC signaling pathway.

P2Y receptors in health and disease

The purine- and pyrimidine-sensitive P2Y receptors belong to the large group of G-protein-coupled receptors that are the target of approximately one-third of the pharmaceutical drugs used in the clinic today. It is therefore not unexpected that the P2Y receptors could be useful targets for drug development. This chapter will discuss P2Y receptor-based therapies currently used, in development and possible future developments. The platelet inhibitors blocking the ADP-receptor P2Y(12) reduce myocardial infarction, stroke, and mortality in patients with cardiovascular disease. Clopidogrel (Plavix) was for many years the second most selling drug in the world. The improved P2Y(12) inhibitors prasugrel, ticagrelor, and elinogrel are now entering the clinic with even more pronounced protective effects. The UTP-activated P2Y(2) receptor stimulates ciliary movement and secretion from epithelial cells. Cystic fibrosis is a monogenetic disease where reduced chloride ion secretion results in a severe lung disease and early death. No specific treatment has been available, but the P2Y(2) agonist Denufosol has been shown to improve lung function and is expected to be introduced as treatment for cystic fibrosis soon. In preclinical studies, there are indications that P2Y receptors can be important for diabetes, osteoporosis, cardiovascular, and atherosclerotic disease. In conclusion, P2Y receptors are important for the health of humans for many diseases, and we can expect even more beneficial drugs targeting P2Y receptors in the future.

Signal transduction pathways associated with ATP-induced proliferation of colon adenocarcinoma cells

BACKGROUND

In previous work, we have demonstrated that extracellular adenosine 5'-triphosphate (ATP) acts on intestinal Caco-2 cell P2Y receptors promoting a rapid increase in the phosphorylation of ERK1/2, p46 JNK and p38 MAP kinases (MAPKs).

METHODS AND RESULTS

In this study, we investigated whether the extracellular ATP-P2Y receptor signalling pathways were required for the proliferation of Caco-2 cells. Confocal microscopy and immunobloting studies showed that ERK1/2 and JNK translocate into the nucleus of the cells stimulated by ATP, where they participate, together with p38 MAPK, in the phosphorylation of JunD, ATF-1 and ATF-2 transcription factors. In addition, ATP through the activation of MAPKs induces the expression of the immediate early genes products of the Jun family, c-Fos and MAP kinase phosphatase-1 (MKP-1). Moreover, ERK1/2 and p38 MAPK are involved in the phosphorylation of MKP-1 in Caco-2 cells. Of physiological significance, in agreement with the mitogenic role of the MAPK cascade, ATP increased Caco-2 cell proliferation, and this effect was blocked by UO126, SB203580 and SP600125, the specific inhibitors of ERK1/2, p38 MAPK and JNK1/2, respectively.

CONCLUSION

Extracellular ATP induces proliferation of Caco-2 human colonic cancer cells by activating MAPK cascades and modulation of transcription factors.

GENERAL SIGNIFICANCE

These findings and identification of the specific P2Y subtype receptors involved in the mitogenic effect of ATP on Caco-2 cells might be relevant for understanding tumor cell development, resistance to treatment regimens and the design of new therapeutic strategies.

Involvement of UTP in protection of cardiomyocytes from hypoxic stress

Massive amounts of nucleotides are released during ischemia in the cardiovascular system. Although the effect of the purine nucleotide ATP has been intensively studied in myocardial infarction, the cardioprotective role of the pyrimidine nucleotide UTP is still unclear, especially in the cardiovascular system. The purpose of our study was to elucidate the protective effects of UTP receptor activation and describe the downstream cascade for the cardioprotective effect. Cultured cardiomyocytes and left anterior descending (LAD)-ligated rat hearts were pretreated with UTP and exposed to hypoxia-ischemia. In vitro experiments revealed that UTP reduced cardiomyocyte death induced by hypoxia, an effect that was diminished by suramin. UTP caused several effects that could trigger a cardioprotective response: a transient increase of [Ca2+]i, an effect that was abolished by PPADS or RB2; phosphorylation of the kinases ERK and Akt, which was abolished by U0126 and LY294002, respectively; and reduced mitochondrial calcium elevation after hypoxia. In vivo experiments revealed that UTP maintained ATP levels, improved mitochondrial activity, and reduced infarct size. In conclusion, UTP administrated before ischemia reduced infarct size and improved myocardial function. Reduction of mitochondrial calcium overload can partially explain the protective effect of UTP after hypoxic-ischemic injury.

Protein kinase C-Fyn kinase cascade mediates the oleic acid-induced disassembly of neonatal rat cardiomyocyte adherens junctions

Oleic acid (OA) affects assembly of gap junctions in neonatal cardiomyocytes. Adherens junction (AJ) regulates the stability of gap junction integrity; however, the effect of OA on AJ remains largely unexplored. The distribution of N-cadherin and catenins at cell-cell junction was decreased by OA. OA induced activation of protein kinase C(PKC)-alpha and -epsilon and Src family kinase, and all three kinases were involved in the oleic acid-induced disassembly of the adherens junction, since it was blocked by pretreatment with Gö6976 (a PKCalpha inhibitor), epsilonV1-2 (a PKCepsilon inhibitor), or PP2 (a Src family kinase inhibitor). Src family kinase appeared to be the downstream of PKC-alpha and -epsilon, as blockade of either PKC-alpha or -epsilon activity prevented the OA-induced activation of Src family kinase. Immunoprecipitation analyses showed that OA activated Fyn and Fer. OA promoted the association of p120 catenin/beta-catenin with Fyn and Fer and caused increased tyrosine phosphorylation of p120 catenin and beta-catenin, resulting in decreased binding of the former to N-cadherin and of the latter to alpha-catenin. Pretreatment with PP2 abrogated this OA-induced tyrosine phosphorylation of p120 catenin and beta-catenin and restored the association of N-cadherin with p120 catenin and that of beta-catenin with alpha-catenin. In conclusion, these results show that OA activates the PKC-Fyn signaling pathway, leading to the disassembly of the AJ. Therefore, inhibitors of PKC-alpha/-epsilon and Src family kinase are potential candidates as cardioprotection agents against OA-induced heart injury during ischemia-reperfusion.

A positive inotropic effect of ATP in the human cardiac atrium

We studied contractile effects in isolated electrically driven (1 Hz) atrial preparations from patients undergoing cardiac bypass surgery. ATP concentration dependently (10, 30, and 100 microM) and rapidly decreased force of contraction (negative inotropic effect, NIE) and thereafter more slowly increased force of contraction. The maximum positive inotropic effect (PIE) at 100 microM ATP amounted to 152% of the predrug value (n = 9) and was stable and could be washed out fast and completely. The PIE did not affect time parameters of contraction (time to peak tension and time of relaxation). Moreover, a similar NIE and PIE were noted with adenosine 5'-O-(2-thiotriphosphate) (100 microM). In contrast 2-methyl-thio-ATP did not exert a NIE but only a PIE. In a second set of experiments, preparations were first incubated for 30 min with purinoreceptor antagonists and, in their continuous presence, 100 microM ATP was applied. However, the PIE and NIE of ATP could neither be blocked with suramin (100 and 500 microM), pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (50 microM), nor reactive blue 2 (30, 100, and 500 microM), which are known blockers for subtypes of P(2) receptors, or 1,3-dipropyl-cyclopentvl-xanthine (1 and 10 microM), a subtype (A(1) adenosine) P(1) receptor blocker. Likewise, the inhibitor of phospholipase C (PLC) activity (U-73122) and the inhibitor of adenylate cyclase activity (SQ-022563) (10 microM each) failed to affect the NIE and the PIE of ATP. We tentatively suggest that the PIE of ATP might be mediated via P(2X4)-like receptors. In summary, we describe a novel biphasic effect of ATP on force contraction in the isolated human atrium. It is conceivable that ATP plays a physiological role in the human heart, for instance, after cardiac injury to sustain contractility.

P2 receptors in cardiovascular regulation and disease

The role of ATP as an extracellular signalling molecule is now well established and evidence is accumulating that ATP and other nucleotides (ADP, UTP and UDP) play important roles in cardiovascular physiology and pathophysiology, acting via P2X (ion channel) and P2Y (G protein-coupled) receptors. In this article we consider the dual role of ATP in regulation of vascular tone, released as a cotransmitter from sympathetic nerves or released in the vascular lumen in response to changes in blood flow and hypoxia. Further, purinergic long-term trophic and inflammatory signalling is described in cell proliferation, differentiation, migration and death in angiogenesis, vascular remodelling, restenosis and atherosclerosis. The effects on haemostasis and cardiac regulation is reviewed. The involvement of ATP in vascular diseases such as thrombosis, hypertension and diabetes will also be discussed, as well as various heart conditions. The purinergic system may be of similar importance as the sympathetic and renin-angiotensin-aldosterone systems in cardiovascular regulation and pathophysiology. The extracellular nucleotides and their cardiovascular P2 receptors are now entering the phase of clinical development.

Mechanism of oleic acid-induced myofibril disassembly in rat cardiomyocytes

This study investigated the mechanism of oleic acid (OA)-induced disassembly of myofibrils in cardiomyocytes. OA treatment disrupted myofibrils, as revealed by the disorganization of several sarcomeric proteins. Since focal adhesions (FAs) are implicated in myofibril assembly, we examined structural changes in FAs after OA treatment. Immunofluorescence studies with antibodies against FA proteins (vinculin, integrin beta1D, and paxillin) showed that FAs and costameres disintegrated or disappeared after OA treatment and that the changes in FA proteins occurred prior to myofibril disassembly. The effects of OA on FAs and myofibrils were reversed after removal of OA. OA decreased expression of integrin beta1D, paxillin, vinculin, and actin, and induced tyrosine dephosphorylation of FA kinase (FAK) and paxillin. These effects were blocked by pretreatment with sodium orthovanadate, a protein tyrosine phosphatase (PTP) inhibitor. This inhibitor also prevented OA-induced myofibril disassembly, indicating the involvement of PTP in myofibril disassembly. Furthermore, OA increased protein levels of PTP-PEST. The upregulation of this phosphatase correlated with the tyrosine dephosphorylation of paxillin and FAK, which are targets for PTP-PEST. In addition, OA decreased RhoA activity and the phosphorylation of cofilin, a downstream target of RhoA. Cofilin dephosphorylation increased its actin-severing activity and led to the depolymerization of F-actin, which might provide another potential mechanism for OA-induced myofibril disassembly.

κ-opioid receptor stimulation inhibits cardiac hypertrophy induced by β1-adrenoceptor stimulation in the rat

To test the hypothesis that kappa-opioid receptor stimulation inhibits cardiac hypertrophy induced by beta1-adrenoceptor stimulation, we determined the effects of trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]-benzeneacetamide methanesulfonate salt (U50,488H), a selective kappa-opioid receptor agonist, on cardiac hypertrophy induced by isoprenaline, a selective beta-adrenoceptor agonist, in neonatal ventricular myocytes upon blockade of beta2-adrenoceptor. Hypertrophy of cardiomyocytes was determined by increases in (i) total protein content; (ii) [3H]leucine incorporation; and iii) cell size. 10 micromol/l isoprenaline increased all three parameters. The effects were abolished by 2 micromol/l propranolol, a beta-adrenergic receptor antagonist, or 300 nmol/l CGP20712A, a beta1-adrenoceptor antagonist, but not by 100 nmol/l ICI118,551, a beta2-adrenoceptor antagonist. The effects were also abolished by Rp-cAMPs 100 micromol/l, a protein kinase A inhibitor and not by pertussis toxin 5 mg/l. The effects of isoprenaline in the presence or absence of ICI118,551 were also abolished by 1 micromol/l U50,488H. The inhibitory effects of U50,488H were abolished by 1 micromol/l nor-binaltorphimine, a selective kappa-opioid receptor antagonist. U50,488H also abolished the increases in the amplitude and frequency of the spontaneous intracellular Ca2+ transient induced by 10 micromol/l isoprenaline in the presence or absence of ICI118,551, an effect also abolished by nor-binaltorphimine. In conclusion the results show that kappa-opioid receptor stimulation abolished both the cardiac hypertrophy and enhanced amplitude and frequency of the spontaneous intracellular Ca2+ transient induced by beta1-adrenoceptor stimulation.

Phospholipase C and cAMP-dependent positive inotropic effects of ATP in mouse cardiomyocytes via P2Y11-like receptors

ATP is released as a cotransmitter together with catecholamines from sympathetic nerves. In the heart ATP has been shown to cause a pronounced positive inotropic effect and may also act in synergy with beta-adrenergic agonists to augment cardiomyocyte contractility. The aim of the present study was to investigate the inotropic effects mediated by purinergic P2 receptors using isolated mouse cardiomyocytes. Stable adenine nucleotide analogs were used and the agonist rank order for adenine nucleotide stimulation of the mouse cardiomyocytes was AR-C67085>ATPgammaS>2-MeSATP>>>2-MeSADP=0, that fits the agonist profile of the P2Y11 receptor. ATPgammaS induced a positive inotropic response in single mouse cardiomyocytes. The response was similar to that for the beta1 receptor agonist isoproterenol. The most potent response was obtained using AR-C67085, a P2Y11 receptor agonist. This agonist also potentiated contractions in isolated trabecular preparations. The adenylyl cyclase blocker (SQ22563) and phospholipase C (PLC) blocker (U73122) demonstrated that both pathways were required for the inotropic response of AR-C67085. A cAMP enzyme immunoassay confirmed that AR-C67085 increased cAMP in the cardiomyocytes. These findings are in agreement with the P2Y11 receptor, coupled both to activation of IP3 and cAMP, being a major receptor for ATP induced inotropy. Analyzing cardiomyocytes from desmin deficient mice, Des-/-, with a congenital cardiomyopathy, we found a lower sensitivity to AR-C67085, suggesting a down-regulation of P2Y11 receptor function in heart failure. The prominent action of the P2Y11 receptor in controling cardiomyocyte contractility and possible alterations in its function during cardiomyopathy may suggest this receptor as a potential therapeutic target. It is possible that agonists for the P2Y11 receptor could be used to improve cardiac output in patients with circulatory shock and that P2Y11 receptor antagonist could be beneficial in patients with congestive heart failure (CHF).

Role of ATP in DNA synthesis of renal proximal tubule cells: involvement of calcium, MAPKs, and CDKs

Although ATP has been shown to act as a modulator in various kidney functions, its effect on renal proximal tubule cell (PTC) proliferation has not been elucidated. This study investigated the effect of ATP on cell proliferation and the effect of its related signal pathways on primary cultured PTCs. Treatment with >10(-5) M ATP for 1 h stimulated incorporation of thymidine and bromodeoxyuridine. ATP (10(-4) M)-induced stimulation of thymidine incorporation was blocked by suramin (a P2X and P2Y receptor antagonist), reactive blue 2 (a P2Y receptor antagonist), MRS-2159 (a P2X1 receptor antagonist), and MRS-2179 (a P2Y1 receptor antagonist). ATP increased intracellular Ca2+ concentration, which was blocked by suramin, methoxyverapamil, and EGTA. ATP-induced stimulation of cell proliferation was also blocked by EGTA (an extracellular Ca2+ chelator), methoxyverapamil (a Ca2+ antagonist), and nifedipine (an L-type Ca2+ channel blocker), suggesting a role for Ca2+ influx. ATP-induced phosphorylation of p38 and p44/42 MAPKs was blocked by nifedipine. ATP increased expression levels of cyclin-dependent kinase (CDK)-2, CDK-4, and cyclin E, which were blocked by suramin, reactive blue 2, MRS-2179, MRS-2159, and nifedipine. However, ATP decreased expression levels of p21WAF1/Cip1 and p27kip1. ATP-induced stimulation of thymidine incorporation and increase of CDK-2 and CDK-4 expression were blocked by SB-203580 (a p38 MAPK inhibitor) and PD-98059 (an MEK inhibitor), but not by SP-600125 (a JNK inhibitor). In conclusion, ATP stimulates proliferation by increasing intracellular Ca2+ concentration and activating p38, p44/42 MAPKs, and CDKs in PTCs.

Involvement of uracil nucleotides in protection of cardiomyocytes from hypoxic stress

Cardiomyocytes express one or more subtypes of P2 receptors for extracellular nucleotides. P2 purinoceptors, which are activated by nucleotides, are classified as P2X or P2Y: P2X receptors are ligand-gated intrinsic ion channels, and P2Y receptors are G protein-coupled receptors. Extracellular pyrimidine and purine nucleotides are released from the heart during hypoxia. Although the cardioprotective effects of purines acting via purinoceptors were studied intensively, the physiological role of uracil nucleotide-responsive P2Y2, P2Y4, P2Y6, and P2Y14 receptors is still unclear, especially in the cardiovascular system. This study revealed that uridine-5'-triphosphate (UTP) protected cultured rat cardiomyocytes during hypoxia and explored the UTP signaling pathway leading to this cardioprotection. We found that UTP, but not UDP or uridine, significantly reduced cardiomyocyte death induced by hypoxia. Incubation with UTP for 1 h, before exposure to hypoxic conditions, protected the cells 24 h later. The cardioprotective effect of UTP was reduced in the presence of the P2 antagonist suramin. In addition, UTP caused a transient increase of [Ca2+]i in cardiomyocytes. Pyridoxal-5'-phosphate-6-azophenyl-2,4-disulfonate (PPADS) or Reactive blue 2 (RB-2), other antagonists of P2 receptors, abolished the [Ca2+]i elevation caused by UTP. We used various inhibitors of the Ca2+ signaling pathway to show that UTP elevated levels of [Ca2+]i, originating from intracellular sources, via activation of phospholipase C and the IP3 receptor. Interestingly, these inhibitors of the Ca2+ signaling pathway did not prevent the immediate protective effect caused by UTP. Although mitochondrial KATP channels are involved in other preconditioning mediator pathways, the involvement of these channels in the cardioprotective effect induced by UTP was ruled out, because 5-hydroxydecanoic acid (5-HD), a specific inhibitor of these channels, did not prevent the protection.

Mechanism of oleic acid-induced gap junctional disassembly in rat cardiomyocytes

This study investigated the mechanism of oleic acid (OA) on gap junctions and identified the protein kinase C (PKC) isoforms involved in OA-mediated gap junction disassembly in cardiomyocytes. Control cardiomyocytes showed continuous staining of the plasma membrane at cell-cell contact areas using antibodies reacting with connexin 43 (Cx43). The spontaneous contraction rate of cultured cardiomyocytes was reduced in a time-dependent manner by OA. In addition, Cx43 expression at cell-cell junction decreased, suggesting the disassembly of gap junction. Staining for PKC and PKCalpha, which were shown to colocalize with Cx43, also decreased with increased duration of OA treatment. The effects of OA on these distributional changes at cell junctions were reversed by 24 h incubation in fresh culture medium devoid of OA. Immunoprecipitation assays confirmed the biochemical binding between Cx43 and PKC/PKCalpha, and this protein interaction was not affected by OA. This may provide the basis for simultaneous detachment of Cx and PKC/PKCalpha from the cell-cell junction to the cytosol upon OA stimulation. Western blot analysis showed that OA-induced Cx43 Ser368 phosphorylation, and that this effect could be blocked by cotreatment with the general PKC inhibitor, calphostin C, the PKC inhibitor, eV1-2, or the Src kinase inhibitor, PP1, but not by the PKCalpha inhibitor, Gö6976. eV1-2 also prevented the OA-induced disassembly of gap junctions. Taken together, these data suggest that OA-induced Cx43 Ser368 phosphorylation is mediated by activation of PKC and Src kinase and might be responsible for OA-induced gap junctional disassembly.

κ-Opioid receptor stimulation inhibits growth of neonatal rat ventricular myocytes

The effects of trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]-benzeneacetamide methanesulfonate salt (U50,488H), a selective kappa-opioid receptor agonist, on growth in neonatal ventricular myocytes were determined. In 15% serum culture medium, U50,488H at 0.1-1 microM significantly reduced the protein content, [3H]leucine uptake and cell size of the myocytes. The effect of U50,488H on protein content was abolished in the presence of 1 microM nor-binaltorphimine (nor-BNI), a selective kappa-opioid receptor antagonist. In a 0.4% serum medium, U50,488H at 0.1-1 microM had no effect on myocyte growth. Interestingly, 1 microM U50,488H abolished the stimulatory effects of 1 microM norepinephrine on protein content, [3H]leucine uptake and cell size of the myocytes in the low serum medium. The effect of U50,488H was abolished by 1 microM nor-BNI. With the exception of cell size, the effects of norepinephrine were completely abolished by blockade of both alpha- and beta-adrenoceptors, but only partially blocked by blockade of either adrenoceptors. These results provide first evidence that kappa-opioid receptor stimulation inhibits growth of the neonatal ventricular myocyte as a result of direct action as well as by inhibiting sympathetic stimulation of the heart. The stimulatory effects of sympathetic activity on growth occurs via both alpha- and beta-adrenoceptors.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Purine nucleotides modulate proliferation of brown fat preadipocytes

The hypothesis that purine nucleotides and nucleosides affect brown fat preadipocyte proliferation was tested using isolated rat interscapular brown fat preadipocytes in culture. Daily addition of 100 microM adenosine triphosphate (ATP) (n = 4) to cultures enhanced the relative DNA content by 1.5-fold compared to control cultures (P < 0.05) measured using CyQUANT-GR fluorescence. Higher concentrations of ATP inhibited growth and 500 (n = 2) or 1000 microM ATP (n = 3) almost completely inhibited growth. ATP (100 microM) did not affect while 250-1000 microM ATP decreased protein content relative to control cultures. Adenosine (100 microM; n = 3) did not affect DNA or protein content, but 500 microM and 1000 microM adenosine suppressed brown adipocyte proliferation and inhibited protein synthesis. Cultured brown adipocytes quickly removed or degraded ATP in the culture media as determined by luciferin-luciferase bioluminescence, suggesting that the inhibitory effects of high ATP concentrations may result from its breakdown to adenosine. The results support the conclusion that ATP promotes and adenosine inhibits brown adipocyte proliferation.

Transforming growth factor beta-1 attenuates endothelin-1-induced functions in neonatal cardiac myocytes

In the present study we characterized a "crosstalk" mechanism between transforming growth factor beta-1 (TGF beta-1) and endothelin-1 (ET1) signaling pathways in neonatal cardiac myocytes. A 5 minute pretreatment with 1 ng/ml concentrations of TGF beta-1 attenuated ET1-induced negative chronotropic effects and translocation of the alpha, delta and varepsilonPKC isozymes to the particulate cell fraction. We found no effect of TGF beta-1 on responses induced by the P(2) purinergic agonist ATP or phorbol ester. Treatment of cardiac myocytes with acidic fibroblast growth factor (aFGF) did not alter ET1- or ATP-mediated effects on contraction rate or translocation of PKC isozymes to the particulate fraction. Our studies suggest that TGF beta-1 may act as a negative modulator of ET1- but not ATP- or phorbol ester-induced PKC isozyme signaling events in neonatal cardiac myocytes. A better understanding of the complex ET1 and TGF beta-1 signaling mechanisms in neonatal heart cells should enhance our knowledge regarding the interplay between these pathways.

Effect of a regulatory mutation on the rat atrial natriuretic peptide gene transcription

To investigate the functional relevance of a regulatory mutation affecting the enhancer element PEA2 of the rat ANP gene we transfected rat cardiomyocytes and aortic endothelial cells with either the mutant or the wild-type ANP promoter construct (-683 +54) and performed CAT assays both at baseline and in response to Phenylephrine and Angiotensin II. In the myocardial cells we also determined the DNA/nuclear protein interaction through electrophoretic mobility shift assay. These studies showed a significantly lower degree of ANP transcription in the presence of the mutant PEA2 site, thus demonstrating its functional significance and the biological relevance of ANP gene structural alterations.

Adenosine 5'-triphosphate: a P2-purinergic agonist in the myocardium

ATP, besides an intracellular energy source, is an agonist when applied to a variety of different cells including cardiomyocytes. Sources of ATP in the extracellular milieu are multiple. Extracellular ATP is rapidly degraded by ectonucleotidases. Today ionotropic P2X(1--7) receptors and metabotropic P2Y(1,2,4,6,11) receptors have been cloned and their mRNA found in cardiomyocytes. On a single cardiomyocyte, micromolar ATP induces nonspecific cationic and Cl(-) currents that depolarize the cells. ATP both increases directly via a G(s) protein and decreases Ca(2+) current. ATP activates the inward-rectifying currents (ACh- and ATP-activated K(+) currents) and outward K(+) currents. P2-purinergic stimulation increases cAMP by activating adenylyl cyclase isoform V. It also involves tyrosine kinases to activate phospholipase C-gamma to produce inositol 1,4,5-trisphosphate and Cl(-)/HCO(3)(-) exchange to induce a large transient acidosis. No clear correlation is presently possible between an effect and the activation of a given P2-receptor subtype in cardiomyocytes. ATP itself is generally a positive inotropic agent. Upon rapid application to cells, ATP induces various forms of arrhythmia. At the tissue level, arrhythmia could be due to slowing of electrical spread after both Na(+) current decrease and cell-to-cell uncoupling as well as cell depolarization and Ca(2+) current increase. In as much as the information is available, this review also reports analog effects of UTP and diadenosine polyphosphates.

Identification of a novel stretch-responsive skeletal muscle gene (Smpx)

Skeletal muscle is able to respond to a range of stimuli, including stretch and increased load, by increasing in diameter and length in the absence of myofiber division. This type of cellular growth (hypertrophy) is a highly complex process involving division of muscle precursor cells (myoblasts) and their fusion to existing muscle fibers as well as increased protein synthesis and decreased protein degradation. Underlying the alterations in protein levels are increases in a range of specific mRNAs including those coding for structural proteins and proteins that regulate the hypertrophic process. Seven days of passive stretch in vivo of tibialis anterior (TA) muscle has been shown to elicit muscle hypertrophy. We have identified a cDNA corresponding to an mRNA that exhibits increased expression in response to 7 days of passive stretch imposed on TA muscles in vivo. This 944-bp novel murine transcript is expressed primarily in cardiac and skeletal muscle and to a lesser extent in brain. Translation of the transcript revealed an open reading frame of 85 amino acids encoding a nuclear localization signal and two overlapping casein kinase II phosphorylation sites. This gene has been called "small muscle protein (X chromosome)" (Smpx; HGMW-approved human gene symbol SMPX) and we hypothesize that it plays a role in skeletal muscle hypertrophy.

Modulation of MLC-2v gene expression by AP-1: complex regulatory role of Jun in cardiac myocytes

Hypertrophic stimulation of cardiac myocytes results in rapid induction of a number of transcription factors, including members of the AP-1 family, which is followed by a programmed alteration in the pattern of gene expression. In the ventricular cardiocytes there is re-expression of the fetal atrial natriuretic factor (ANF) gene and upregulation of its myosin light chain-2 (MLC-2v). The mechanism(s) by which the induction ofAP-1 is coupled to the promoters of these target genes is largely unknown. In this report, we demonstrate that in transient co-transfection assay, c-Jun inhibited while Jun B stimulated the MLC-2v promoter activity. Mutant c-Jun recombinants, in which the activation domains were deleted, still remained inhibitory, but a specific mutation in the leucine zipper, which changes the alignment of Jun with its dimerization partner, caused a reversal of its effect on the target MLC-2v promoter. Based on these findings, we propose that in chicken cardiac myocytes, the regulation of MLC-2v promoter by Jun may occur via its interaction with other proteins, possibly of the leucine zipper family.

Increased sensitivity of neonate atrial myocytes to adenosine A1 receptor stimulation in regulation of the L-type Ca2+ current

Adenosine has cardioprotective effects against ischemia, and newborn hearts show high resistance to ischemia. The effects of purinoceptor stimulation by adenosine and ATP on the L-type Ca2+ current (ICa) were examined in atrial cells from neonate and adult rabbits. ICa was measured by the membrane-perforated patch method. Adenosine inhibited the isoproterenol-stimulated ICa more potently in neonate cells than in adult cells. The high sensitivity of neonate myocytes to adenosine was accompanied not only by an increased maximum response but also by a lower IC50 concentration. ATP also inhibited isoproterenol-stimulated ICa. The effect of ATP on neonate cells was stronger than that on adult cells at high concentrations (greater than or = 100 microM). The effect of adenosine was antagonized by an A1 adenosine receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX). DPCPX or an ecto-5'-nucleosidase inhibitor (alpha,beta-methylene-ADP) blocked most (approximately 60%) of the effect of ATP (30 microM), and co-addition of DPCPX and suramin (P2 receptor blocker) abolished the effect of ATP. Suramin alone did not reduce the effect of ATP significantly in neonate cells. Both the effects of adenosine and ATP were eliminated by pre-treatment with pertussis toxin or by superfusion with forskolin plus 3-isobutyl-1-methylxanthine (IBMX). Inhibitors of the nitric oxide-cyclic GMP pathway did not affect the adenosine inhibition of ICa. In summary, neonatal myocardial cells are highly sensitive to adenosine A1 receptor stimulation. ATP stimulates both the adenosine A1 and P2 receptors. Adenosine A1 receptor stimulation, as a result of hydrolysis of ATP, predominantly mediates the effect of ATP, and the role of P2 receptors in the ATP inhibition of ICa is relatively small in neonate cells. The high sensitivity to adenosine may contribute to the ischemic tolerance of newborn hearts.

cAMP activates an ATP-permeable pathway in neonatal rat cardiac myocytes

The molecular mechanisms associated with intracellular ATP release by the heart are largely unknown. In this study the luciferin-luciferase assay and patch-clamp techniques were used to characterize the pathways responsible for ATP release in neonatal rat cardiac myocytes (NRCM). Spontaneous ATP release by NRCM was significantly increased after cAMP stimulation under physiological conditions. cAMP stimulation also induced an anion-selective electrodiffusional pathway that elicited linear, diphenylamine-2-carboxylate (DPC)-inhibitable Cl(-) currents in either symmetrical MgCl(2) or NaCl. ATP, adenosine 5'-O-(3-thiotriphosphate), and the ATP derivatives ADP and AMP, permeated this pathway; however, GTP did not. The cAMP-induced ATP currents were inhibited by DPC and glibenclamide and by a monoclonal antibody raised against the R domain of the cystic fibrosis transmembrane conductance regulator (CFTR). The channel-like nature of the cAMP-induced ATP-permeable pathway was also determined by assessing protein kinase A-activated single channel Cl(-) and ATP currents in excised inside-out patches of NRCM. Single channel currents were inhibited by DPC and the anti-CFTR R domain antibody. Thus the data in this report demonstrate the presence of a cAMP-inducible electrodiffusional ATP transport mechanism in NRCM. Based on the pharmacology, patch-clamping data, and luminometry studies, the data are most consistent with the role of a functional CFTR as the anion channel implicated in cAMP-activated ATP transport in NRCM.

Identification of Ankrd2, a novel skeletal muscle gene coding for a stretch-responsive ankyrin-repeat protein

Mechanically induced hypertrophy of skeletal muscles involves shifts in gene expression leading to increases in the synthesis of specific proteins. Full characterization of the regulation of muscle hypertrophy is a prerequisite for the development of novel therapies aimed at treating muscle wasting (atrophy) in human aging and disease. Using suppression subtractive hybridization, cDNAs corresponding to mRNAs that increase in relative abundance in response to mechanical stretch of mouse skeletal muscles in vivo were identified. A novel 1100-bp transcript was detected exclusively in skeletal muscle. This exhibited a fourfold increase in expression after 7 days of stretch. The transcript had an open reading frame of 328 amino acids encoding an ATP/GTP binding domain, a nuclear localization signal, two PEST protein-destabilization motifs, and a 132-amino-acid ankyrin-repeat region. We have named this gene ankyrin-repeat domain 2 (stretch-responsive muscle) (Ankrd2). We hypothesize that Ankrd2 plays an important role in skeletal muscle hypertrophy.

c-fos and c-jun mRNA expression in a pig liver model of ischemia/reperfusion: effect of extended cold storage and the antioxidant idebenone

OBJECTIVES

Expression of immediate early genes has been reported during reperfusion after ischemia in rat livers due to oxygen radical formation. This study investigates in perfused pig livers the effect of the antioxidant idebenone and of cold ischemia time on the gene expression of c-fos and c-jun.

DESIGN AND METHODS

Livers were perfused for 210 min after 0.5 h or 20 h ischemic storage (4 degrees C). One group of pigs was fed idebenone (280 mg/day/7days) prior to organ harvesting. C-fos and c-jun mRNA were determined by RT-PCR at 3, 30, 60, 120 180, 210 min during reperfusion.

RESULTS

Lipid peroxidation increased in liver tissue form 0.54 +/- 0.21 to 1. 09 +/- 0.54 nmol MDA/mg protein during reperfusion after 20 h compared to 0.5 h cold storage. This was antagonized by idebenone (0. 68 +/- 0.20 nmol/MDA/mg protein). C-fos and c-jun were strongly induced in livers stored for 20 h, which was attenuated by idebenone (p < 0.05).

CONCLUSIONS

These findings suggest that cold ischemia time and oxygen radicals are critical for immediate early gene expression and that application of an effective antioxidant can attenuate this early stress reaction of the pig liver.

Characterization of MAP kinase and PKC isoform and effect of ACE inhibition in hypertrophy in vivo

Protein kinase C (PKC) and mitogen-activated protein (MAP) kinase activation appear important in conferring hypertrophy in vitro. However, the response of PKC and MAP kinase to stimuli known to induce hypertrophy in vivo has not been determined. We recently demonstrated that pressure-overload hypertrophy induced a transiently transfected gene driven by an hypertrophy responsive enhancer (HRE) through a marked increase in binding activity of its interacting nuclear factor (HRF). These data suggested that the HRE/HRF could serve as a target for evaluating the signal transduction events responsible for hypertrophy in vivo. Accordingly, we characterized MAP kinase and PKC isoform activation, injected HRE driven reporter gene expression, and HRF binding activity in rat hearts subjected to ascending aortic clipping or sham operation in the presence of the angiotensin-converting enzyme (ACE) inhibitor fosinopril, hydralazine, or no treatment. Analyses showed that PKC-epsilon and MAP kinase were acutely activated following ascending aortic ligature and that fosinopril significantly inhibited but did not completely abrogate PKC-epsilon and MAP kinase activation. However, fosinopril completely prevented pressure overload-mediated induction of HRE containing constructs and obviated increased HRF binding activity. These results suggest a direct relationship between ACE activity and HRE/HRF-mediated gene activation and imply that PKC-epsilon and MAP kinase may be involved in transducing this signal.

Increase in cardiac P2X1-and P2Y2-receptor mRNA levels in congestive heart failure

We wanted to study the expression of P2-receptors at the mRNA-level in the heart and if it is affected by congestive heart failure (CHF). To quantify the P2 receptor mRNA-expression we used a competitive RT-PCR protocol which is based on an internal RNA standard. The P2 receptor mRNA-expression was quantified in hearts from CHF rats and compared to sham-operated rats. Furthermore, the presence of receptor mRNA was studied in the myocardium from patients with heart failure. In the sham operated rats the G-protein coupled P2Y-receptors were expressed at a higher level than the ligand gated ion-channel receptor (P2X1). Among the P2Y-receptors the P2Y6-receptor was most abundantly expressed (P2Y6 > P2Y1 > P2Y2 = P2Y4 > P2X1). A prominent change was seen for the P2X1- and P2Y2-receptor mRNA levels which were increased 2.7-fold and 4.7-fold respectively in the myocardium from the left ventricle of CHF-rats. In contrast, the P2Y1-, P2Y4- and P2Y6-receptor mRNA levels were not significantly altered in CHF rats. In human myocard the P2X1-, P2Y1-, P2Y2-, P2Y6- and P2Y11-receptors were detected by RT-PCR in both right and left atria and ventricles, while the P2Y4-receptor band was weak or absent. In conclusion, most of the studied P2-receptors were expressed in both rat and human hearts. Furthermore, the P2X1- and P2Y2-receptor mRNA were upregulated in CHF, suggesting a pathophysiological role for these receptors in the development of heart failure.

Extracellular signal-regulated protein kinase activation is required for the anti-hypertrophic effect of atrial natriuretic factor in neonatal rat ventricular myocytes

Atrial natriuretic factor (ANF) inhibits proliferation in non-myocardial cells and is thought to be anti-hypertrophic in cardiomyocytes. We investigated the possibility that the anti-hypertrophic actions of ANF involved the mitogen-activated protein kinase signal transduction cascade. Cultured neonatal rat ventricular myocytes treated for 48 h with the alpha(1)-adrenergic agonist phenylephrine (PE) had an 80% increase in cross-sectional area (CSA). ANF alone had no effect but inhibited PE-induced increases in CSA by approximately 50%. The mitogen-activated protein kinase/ERK kinase (MEK) inhibitor PD098059 minimally inhibited PE-induced increases in CSA, but it completely abolished ANF-induced inhibition of PE-induced increases. ANF-induced extracellular signal-regulated protein kinase (ERK) nuclear translocation was also eliminated by PD098059. ANF treatment caused MEK phosphorylation and activation but failed to activate any of the Raf isoforms. ANF induced a rapid increase in ERK phosphorylation and in vitro kinase activity. PE also increased ERK activity, and the combined effect of ANF and PE appeared to be additive. ANF-induced ERK phosphorylation was eliminated by PD098059. ANF induced minimal phosphorylation of JNK or p38, indicating that its effect on ERK was specific. ANF-induced activation of ERK was mimicked by cGMP analogs, suggesting that ANF-induced ERK activation involves the guanylyl cyclase activity of the ANF receptor. These data suggest that there is an important linkage between cGMP signaling and the mitogen-activated protein kinase cascade and that selective ANF activation of ERK is required for the anti-hypertrophic action of ANF. Thus, ANF expression might function as the natural defense of the heart against maladaptive hypertrophy through its ability to activate ERK.

Early response kinase and PI 3-kinase activation in adult cardiomyocytes and their role in hypertrophy

The present study investigated the role of early response kinase (ERK) and phosphatidylinositol 3 (PI 3)-kinase in ventricular cardiomyocytes from adult rat for the hypertrophic response to alpha-adrenoceptor stimulation. Parameters of the hypertrophic response were stimulation of protein synthesis and induction of creatine kinase BB. The alpha-adrenoceptor agonist phenylephrine (10 micromol/l) activated ERK2 and PI 3-kinase. The protein kinase C inhibitor bisindolylmaleimide (5 micromol/l) and the mitogen-activated protein kinase kinase inhibitor PD-98059 (10 micromol/l) but not the tyrosine kinase inhibitor genistein (100 micromol/l) blocked ERK2 activation. Inhibition of ERK2 activation abolished induction of creatine kinase BB by phenylephrine but not the increase in protein synthesis. The PI 3-kinase inhibitor wortmannin (100 nmol/l) blocked protein synthesis under alpha-adrenoceptor stimulation but did not interfere with ERK2 activation. Inhibition of the ERK2 pathway with PD-98059 did not affect PI 3-kinase activation. We conclude that ERK2- and PI 3-kinase-dependent pathways represent two mutually exclusive ways of signaling that lead to different aspects of the hypertrophic response to alpha-adrenoceptor stimulation.

Stimulation of P2Y receptors activates c-fos gene expression and inhibits DNA synthesis in cultured cardiac fibroblasts

OBJECTIVES

The aims of this study were to determine (1) whether neonatal rat cardiac fibroblasts (CAFB) express P2Y receptors; (2) whether CAFB respond to extracellular ATP by inducing expression of c-fos mRNA; and (3) whether extracellular ATP modulates norepinephrine (NE)-stimulated cell growth in CAFB.

METHODS

Expression of P2Y1 and P2Y2 receptors and induction of c-fos were examined by Northern blot analysis. CAFB growth was assessed by measuring [3H]thymidine incorporation and DNA content. P2Y receptor pharmacology was studied using various ATP analogues.

RESULTS

Northern blot analysis of polyA enriched RNA confirmed that at least 2 subtypes of P2Y receptors (P2Y1 and P2Y2) are expressed in cultured CAFB. Extracellular ATP induced the expression of c-fos mRNA through a pathway that was sensitive to inhibitors of protein kinase C (PKC), but not to inhibitors of intracellular Ca2+ signaling. Extracellular ATP inhibited the NE-stimulated increases in DNA content and in [3H]thymidine incorporation into DNA. Whereas the potency order for stimulation of c-fos expression was ATP = UTP > ADP > adenosine, the potency order to inhibit the NE-induced increase of [3H]thymidine incorporation into DNA was ATP > ADP > UTP > adenosine.

CONCLUSIONS

These data demonstrate that CAFB express both P2Y1 and P2Y2 receptor mRNA and that CAFB respond to P2Y receptor stimulation by induction of c-fos and inhibition of DNA synthesis. These findings suggest that the effects of ATP on [3H]thymidine incorporation into DNA and on expression of c-fos mRNA are exerted via distinct P2Y receptor subtypes.

Nitric oxide, atrial natriuretic peptide, and cyclic GMP inhibit the growth-promoting effects of norepinephrine in cardiac myocytes and fibroblasts

This study tested the hypothesis that nitric oxide (NO) and atrial natriuretic peptide (ANP) can attenuate the effects of adrenergic agonists on the growth of cardiac myocytes and fibroblasts. In ventricular cells cultured from neonatal rat heart, ANP and the NO donor S-nitroso-N-acetyl-D,L-penicillamine (SNAP) caused concentration-dependent decreases in the norepinephrine (NE)-stimulated incorporation of [3H]leucine in myocytes and [3H]thymidine in fibroblasts. In myocytes, the NO synthase inhibitor NG-monomethyl-L-arginine potentiated NE-stimulated [3H]leucine incorporation. In both cell types, ANP and SNAP increased intracellular cGMP levels, and their growth-suppressing effects were mimicked by the cGMP analogue 8-bromo-cGMP. Furthermore, in myocytes, 8-bromo-cGMP attenuated the alpha1-adrenergic receptor-stimulated increases in c-fos. Likewise, ANP and 8-bromo-cGMP attenuated the alpha1-adrenergic receptor- stimulated increase in prepro-ANP mRNA and the alpha1-adrenergic receptor-stimulated decrease in sarcoplasmic reticulum calcium ATPase mRNA. The L-type Ca2+ channel blockers verapamil and nifedipine inhibited NE-stimulated incorporation of [3H]leucine in myocytes and [3H]thymidine in fibroblasts, and these effects were not additive with those of ANP, SNAP, or 8-bromo-cGMP. In myocytes, the Ca2+ channel agonist BAY K8644 caused an increase in [3H]leucine incorporation which was inhibited by ANP. These findings indicate that NO and ANP can attenuate the effects of NE on the growth of cardiac myocytes and fibroblasts, most likely by a cGMP-mediated inhibition of NE-stimulated Ca2+ influx.

Collaborative roles for c-Jun N-terminal kinase, c-Jun, serum response factor, and Sp1 in calcium-regulated myocardial gene expression

Electrical stimulation of contractions (pacing) of primary neonatal rat ventricular myocytes increases intracellular calcium and activates a hypertrophic growth program that includes expression of the cardiac-specific gene, atrial natriuretic factor (ANF). To investigate the mechanism whereby pacing increases ANF, pacing was tested for its ability to regulate mitogen-activated protein kinase family members, ANF promoter activity, and the trans-activation domain of the transcription factor, Sp1. Pacing and the calcium channel agonist BAYK 8644 activated c-Jun N-terminal kinase (JNK) but not extracellular signal-regulated kinase. Pacing stimulated ANF-promoter activity approximately 10-fold. Furthermore, transfection with an expression vector for c-Jun, a substrate for JNK, also activated the ANF promoter, and the combination of pacing and c-Jun was synergystic, consistent with roles for JNK and c-Jun in calcium-activated ANF expression. Proximal serum response factor and Sp1 binding sites were required for the effects of pacing or c-Jun on the ANF promoter. Pacing and c-Jun activated a GAL4-Sp1 fusion protein by 3- and 12-fold, respectively, whereas the two stimuli together activated GAL4-Sp1 synergistically, similar to their effect on the ANF promoter. Transfection with an expression vector for c-Fos inhibited the effects of c-Jun, suggesting that c-Jun acts independently of AP-1. These results demonstrate an interaction between c-Jun and Sp1 and are consistent with a novel mechanism of calcium-mediated transcriptional activation involving the collaborative actions of JNK, c-Jun, serum response factor, and Sp1.

Molecular biology of P2Y purinoceptors: expression in rat heart

1. Application of molecular biology to the study of P2Y purinoceptors has led to the identification of seven such receptors. Here we briefly review their properties and investigate qualitatively the expression of four rat receptor transcripts in heart. 2. The reverse transcriptase-polymerase chain reaction was used to ascertain whether the rat P2Y1, P2Y2, P2Y4 and P2Y6 receptor transcripts were expressed in whole heart, neonatal cardiac fibroblasts, neonatal cardiac myocytes and adult cardiac myocytes. 3. All receptor sequences could be amplified from neonatal rat whole heart, with P2Y6 appearing the most abundant transcript of the four. P2Y1 is expressed at higher levels in comparison to P2Y2, P2Y4 and P2Y6 in the neonatal myocyte. In the adult myocyte P2Y1, P2Y2 and P2Y6 could be amplified but P2Y4 could not be detected. In the neonatal fibroblast, P2Y1 and P2Y6 appear to be expressed at higher levels than P2Y2 and P2Y4. 4. In summary, it is concluded that multiple P2Y receptor subtypes are expressed in heart and that the expression in myocytes changes from the neonate to the adult.