Purinergic stimulation of rat cardiomyocytes induces tyrosine phosphorylation and membrane association of phospholipase C gamma: a major mechanism for InsP3 generation

Myocyte membrane and microdomain modifications in diabetes: determinants of ischemic tolerance and cardioprotection

Cardiovascular disease, predominantly ischemic heart disease (IHD), is the leading cause of death in diabetes mellitus (DM). In addition to eliciting cardiomyopathy, DM induces a ‘wicked triumvirate’: (i) increasing the risk and incidence of IHD and myocardial ischemia; (ii) decreasing myocardial tolerance to ischemia–reperfusion (I–R) injury; and (iii) inhibiting or eliminating responses to cardioprotective stimuli. Changes in ischemic tolerance and cardioprotective signaling may contribute to substantially higher mortality and morbidity following ischemic insult in DM patients. Among the diverse mechanisms implicated in diabetic impairment of ischemic tolerance and cardioprotection, changes in sarcolemmal makeup may play an overarching role and are considered in detail in the current review. Observations predominantly in animal models reveal DM-dependent changes in membrane lipid composition (cholesterol and triglyceride accumulation, fatty acid saturation vs. reduced desaturation, phospholipid remodeling) that contribute to modulation of caveolar domains, gap junctions and T-tubules. These modifications influence sarcolemmal biophysical properties, receptor and phospholipid signaling, ion channel and transporter functions, contributing to contractile and electrophysiological dysfunction, cardiomyopathy, ischemic intolerance and suppression of protective signaling. A better understanding of these sarcolemmal abnormalities in types I and II DM (T1DM, T2DM) can inform approaches to limiting cardiomyopathy, associated IHD and their consequences. Key knowledge gaps include details of sarcolemmal changes in models of T2DM, temporal patterns of lipid, microdomain and T-tubule changes during disease development, and the precise impacts of these diverse sarcolemmal modifications. Importantly, exercise, dietary, pharmacological and gene approaches have potential for improving sarcolemmal makeup, and thus myocyte function and stress-resistance in this ubiquitous metabolic disorder.

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.

An inositol 1,4,5-triphosphate (IP3)-IP3 receptor pathway is required for insulin-stimulated glucose transporter 4 translocation and glucose uptake in cardiomyocytes

Intracellular calcium levels ([Ca2+]i) and glucose uptake are central to cardiomyocyte physiology, yet connections between them have not been studied. We investigated whether insulin regulates [Ca2+]i in cultured cardiomyocytes, the participating mechanisms, and their influence on glucose uptake via SLC2 family of facilitative glucose transporter 4 (GLUT4). Primary neonatal rat cardiomyocytes were preloaded with the Ca2+ fluorescent dye fluo3-acetoxymethyl ester compound (AM) and visualized by confocal microscopy. Ca2+ transport pathways were selectively targeted by chemical and molecular inhibition. Glucose uptake was assessed using [3H]2-deoxyglucose, and surface GLUT4 levels were quantified in nonpermeabilized cardiomyocytes transfected with GLUT4-myc-enhanced green fluorescent protein. Insulin elicited a fast, two-component, transient increase in [Ca2+]i. Nifedipine and ryanodine prevented only the first component. The second one was reduced by inositol-1,4,5-trisphosphate (IP3)-receptor-selective inhibitors (xestospongin C, 2 amino-ethoxydiphenylborate), by type 2 IP3 receptor knockdown via small interfering RNA or by transfected Gβγ peptidic inhibitor βARKct. Insulin-stimulated glucose uptake was prevented by bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid-AM, 2-amino-ethoxydiphenylborate, and βARK-ct but not by nifedipine or ryanodine. Similarly, insulin-dependent exofacial exposure of GLUT4-myc-enhanced green fluorescent protein was inhibited by bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid-AM and xestospongin C but not by nifedipine. Phosphatidylinositol 3-kinase and Akt were also required for the second phase of Ca2+ release and GLUT4 translocation. Transfected dominant-negative phosphatidylinositol 3-kinase γ inhibited the latter. In conclusion, in primary neonatal cardiomyocytes, insulin induces an important component of Ca2+ release via IP3 receptor. This component signals to glucose uptake via GLUT4, revealing a so-far unrealized contribution of IP3-sensitive Ca2+ stores to insulin action. This pathway may influence cardiac metabolism in conditions yet to be explored in adult myocardium.

IP3-induced cytosolic and nuclear Ca2+ signals in HL-1 atrial myocytes: possible role of IP3 receptor subtypes

HL-1 cells are the adult cardiac cell lines available that continuously divide while maintaining an atrial phenotype. Here we examined the expression and localization of inositol 1,4,5-trisphosphate receptor (IP(3)R) subtypes, and investigated how pattern of IP(3)-induced subcellular local Ca(2+) signaling is encoded by multiple IP(3)R subtypes in HL-1 cells. The type 1 IP(3)R (IP(3)R1) was expressed in the perinucleus with a diffuse pattern and the type 2 IP(3)R (IP(3)R2) was expressed in the cytosol with a punctate distribution. Extracellular ATP (1 mM) elicited transient intracellular Ca(2+) releases accompanied by a Ca(2+) oscillation, which was eliminated by the blocker of IP(3)Rs, 2-APB, and attenuated by ryanodine. Direct introduction of IP(3) into the permeabilized cells induced Ca(2+) transients with Ca(2+) oscillations at [Symbol: see text] 20 muM of IP(3), which was removed by the inhibition of IP(3)Rs using 2-APB and heparin. IP(3)-induced local Ca(2+) transients contained two distinct time courses: a rapid oscillation and a monophasic Ca(2+) transient. The magnitude of Ca(2+) oscillation was significantly larger in the cytosol than in the nucleus, while the monophasic Ca(2+) transient was more pronounced in the nucleus. These results provide evidence for the molecular and functional expression of IP(3)R1 and IP(3)R2 in HL-1 cells, and suggest that such distinct local Ca(2+) signaling may be correlated with the punctate distribution of IP(3)R2s in the cytosol and the diffuse localization of IP(3)R1 in the peri-nucleus.

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.

Phospholipase Cgamma1 signalling regulates lipopolysaccharide-induced cyclooxygenase-2 expression in cardiomyocytes

Lipopolysaccharide (LPS) induces cyclooxygenase-2 (COX-2) expression in cardiomyocytes, which plays a role in myocardial depression during endotoxemia. The purpose of this study was to investigate the role of phosphatidylinositol (PI)-phospholipase Cgamma1 (PLCgamma1) in cardiac COX-2 expression in vitro and in vivo. In cultured mouse neonatal cardiomyocytes, LPS increased PLCgamma1 phosphorylation and COX-2 expression. Knockdown of PLCgamma1 with specific siRNA or inhibition of PI-PLC with U73122 attenuated COX-2 mRNA and protein expression induced by LPS (1 microg/ml). PLCgamma1 activation by LPS also increased ERK1/2 MAPK phosphorylation, and inhibition of ERK1/2 MAPK blocked the effect of PLCgamma1 on COX-2 expression. Furthermore, activation of PLCgamma1 is a consequence of the Src family activation since inhibition of Src abrogated whereas over-expression of Src enhanced PLCgamma1 phosphorylation and COX-2 expression in LPS-stimulated cardiomyocytes. To investigate the role of PLCgamma1 in endotoxemia, wild-type and PLCgamma1(+/-) adult mice were pre-treated with U73122, or its inactive analog, U73343 (9 mg/kg, i.p.), or vehicle for 15 min followed by LPS (4 mg/kg, i.p.) for 4 h. U73122 or heterozygous deletion of PLCgamma1 decreased cardiac COX-2 expression. The phosphorylation of ERK1/2 MAPK induced by LPS was also attenuated in U73122- or PLCgamma1(+/-) compared to U73343-treated or wild-type littermate hearts, respectively. In conclusion, our study suggests that PLCgamma1 signalling represents a novel pathway regulating cardiac COX-2 expression during LPS stimulation. The Src family is responsible for PLCgamma1 activation, which signals the ERK1/2 MAPK pathway, resulting in COX-2 production in LPS-stimulated cardiomyocytes.

Activation of Src/kinase/phospholipase C/mitogen-activated protein kinase and induction of neurite expression by ATP, independent of nerve growth factor

Extracellular ATP has been reported to potentiate the neurite outgrowth induced by nerve growth factor. In the present study the neurotrophic effect of ATP and other nucleotides was examined in mouse neuroblastoma neuro2a cells which lack nerve growth factor receptor. Exposure of neuro2a cells to ATP resulted in a dramatic increase in neurite bearing cells as compared with untreated control cells. Experiments performed with purinergic receptor agonists and antagonists suggest that the ATP stimulates neurite outgrowth via P2 receptors. Neurite outgrowth was completely blocked by P2 receptor antagonist suramin whereas the P1 receptor antagonist CGS15943 was ineffective. P1 receptor agonist 5'-(N-ethylcarboxamido)adenosine failed to induce neurite outgrowth. The potency order of different P2 receptor agonists was ATP=ATPgammaS>ADP>>2Me-S-ATP. It was insensitive to UTP and antagonist pyridoxal phosphate-6-azo (benzene-2,4-disulfonic acid) suggesting the involvement of P2Y11 receptor in the observed neuritogenic effect. The signaling pathway leading to ATP-induced neuritogenesis was investigated. The neuritogenic effect of ATP is independent of rise in intracellular Ca(2+) as pharmacological profile of neuritogenic P2Y receptor does not match with that of P2Y2 receptor associated with [Ca(2+)](i) signaling cascade. Exposure of cells to ATP caused activation of Src kinase, phospholipase Cgamma and extracellular signal-regulated kinases ERK1/2. Mitogen-activated protein kinase (MAPK) inhibitor U0126 drastically reduced the number of neurite bearing cells in ATP-treated cultures implying that the neurotrophic effect of ATP is mediated by MAPK. Our results demonstrate that ATP can stimulate neurite outgrowth independent of other neurotrophic factors and can be an effective trophic agent.

Do rat cardiac myocytes release ATP on contraction?

ATP is released by numerous cell types in response to mechanical strain. It then acts as a paracrine or autocrine signaling molecule, inducing a variety of biological responses. In this work, we addressed the question whether mechanical force acting on the membranes of contracting cardiomyocytes during periodic longitudinal shortening can stimulate the release of ATP. Electrically stimulated isolated adult rat cardiomyocytes as well as spontaneously contracting mouse cardiomyocytes derived from embryonic stem (ES) cells were assayed for ATP release with the use of luciferase and a sensitive charge-coupled device camera. Sensitivity of soluble luciferase in the supernatant of cardiomyocytes was 100 nM ATP, which is approximately 10-fold below the EC(50) values for most purinergic receptors expressed in the heart (1.5-20 microM). Light intensities were not different between resting or contracting adult rat cardiomyocytes. Similar results were obtained with ES-cell-derived contracting mouse cardiomyocytes. ATP release was measurable only from obviously damaged or permeabilized cells. To increase selectivity and sensitivity of ATP detection we have targeted a recombinant luciferase to the sarcolemmal membrane using a wheat germ agglutinin-IgG linker. Contraction of labeled adult rat cardiomyocytes was not associated with measurable bioluminescence. However, when human umbilical vein endothelial cells were targeted with membrane-bound luciferase, shear stress-induced ATP release could be clearly detected, demonstrating the sensitivity of the detection method. In the present study, we did not detect ATP release from contracting cardiomyocytes on the single cell level, despite adequate sensitivity of the detection system. Thus deformation of the contracting cardiomyocyte is not a key stimulus for the release of cellular ATP.

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.

Attenuation of extracellular ATP response in cardiomyocytes isolated from hearts subjected to ischemia-reperfusion

Extracellular ATP is known to augment cardiac contractility by increasing intracellular Ca2+ concentration ([Ca2+]i) in cardiomyocytes; however, the status of ATP-mediated Ca2+ mobilization in hearts undergoing ischemia-reperfusion (I/R) has not been examined previously. In this study, therefore, isolated rat hearts were subjected to 10-30 min of global ischemia and 30 min of reperfusion, and the effect of extracellular ATP on [Ca2+]i was measured in purified cardiomyocytes by fura-2 microfluorometry. Reperfusion for 30 min of 20-min ischemic hearts, unlike 10-min ischemic hearts, revealed a partial depression in cardiac function and ATP-induced increase in [Ca2+]i; no changes in basal [Ca2+]i were evident in 10- or 20-min I/R preparations. On the other hand, reperfusion of 30-min ischemic hearts for 5, 15, or 30 min showed a marked depression in both cardiac function and ATP-induced increase in [Ca2+]i and a dramatic increase in basal [Ca2+]i. The positive inotropic effect of extracellular ATP was attenuated, and the maximal binding characteristics of 35S-labeled adenosine 5'-[gamma-thio]triphosphate with crude membranes from hearts undergoing I/R was decreased. ATP-induced increase in [Ca2+]i in cardiomyocytes was depressed by verapamil and Cibacron Blue in both control and I/R hearts; however, this response in I/R hearts, unlike control hearts, was not affected by ryanodine. I/R-induced alterations in cardiac function and ATP-induced increase in [Ca2+]i were attenuated by treatment with an antioxidant mixture and by ischemic preconditioning. The observed changes due to I/R were simulated in hearts perfused with H2O2. The results suggest an impairment of extracellular ATP-induced Ca2+ mobilization in I/R hearts, and this defect appears to be mediated through oxidative stress.

Imidapril treatment improves the attenuated inotropic and intracellular calcium responses to ATP in heart failure due to myocardial infarction

1. Adenosine 5'-triphosphate (ATP) is known to augment cardiac contractile activity and cause an increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) in isolated cardiomyocytes. However, no information regarding the ATP-mediated signal transduction in the myocardium in congestive heart failure (CHF) is available. 2. CHF due to myocardial infarction (MI) in rats was induced by the occlusion of the left coronary artery for 8 weeks. The positive inotropy due to ATP was depressed in failing hearts. Treatment of 3 weeks infarcted animals with imidapril (1 mg kg(-1) day(-1)) for a period of 5 weeks improved the left ventricle function and decreased the attenuation of inotropic response to ATP. 3. ATP-induced increase in [Ca(2+)](i) was significantly depressed in cardiomyocytes isolated from the failing heart and this change was partially attenuated by imidapril treatment. However, the binding characteristics of (35)S-labeled adenosine 5'-(gamma-thio) triphosphate in sarcolemma isolated from the failing heart remained unaltered. 4. ATP-induced increase in [Ca(2+)](i) was depressed by verapamil and cibacron blue in both control and failing heart cardiomyocytes; however, the ATP response in the failing hearts, unlike the control preparations, was not decreased by ryanodine. This insensitivity to ryanodine was attenuated by imidapril treatment. 5. Treatment of infarcted rats with enalapril and losartan produced effects similar to imidapril. 6. These findings indicate that the positive inotropic response to ATP and ATP-induced increase in [Ca(2+)](i) in cardiomyocytes are impaired in heart failure. Furthermore, blockade of renin angiotensin system prevented the impairment of the ATP-mediated inotropic and [Ca(2+)](i) responses in the failing heart.

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.

Insulin-like growth factor-1 induces an inositol 1,4,5-trisphosphate-dependent increase in nuclear and cytosolic calcium in cultured rat cardiac myocytes

In the heart, insulin-like growth factor-1 (IGF-1) is a pro-hypertrophic and anti-apoptotic peptide. In cultured rat cardiomyocytes, IGF-1 induced a fast and transient increase in Ca(2+)(i) levels apparent both in the nucleus and cytosol, releasing this ion from intracellular stores through an inositol 1,4,5-trisphosphate (IP(3))-dependent signaling pathway. Intracellular IP(3) levels increased after IGF-1 stimulation in both the presence and absence of extracellular Ca(2+). A different spatial distribution of IP(3) receptor isoforms in cardiomyocytes was found. Ryanodine did not prevent the IGF-1-induced increase of Ca(2+)(i) levels but inhibited the basal and spontaneous Ca(2+)(i) oscillations observed when cardiac myocytes were incubated in Ca(2+)-containing resting media. Spatial analysis of fluorescence images of IGF-1-stimulated cardiomyocytes incubated in Ca(2+)-containing resting media showed an early increase in Ca(2+)(i), initially localized in the nucleus. Calcium imaging suggested that part of the Ca(2+) released by stimulation with IGF-1 was initially contained in the perinuclear region. The IGF-1-induced increase on Ca(2+)(i) levels was prevented by 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-AM, thapsigargin, xestospongin C, 2-aminoethoxy diphenyl borate, U-73122, pertussis toxin, and betaARKct (a peptide inhibitor of Gbetagamma signaling). Pertussis toxin also prevented the IGF-1-dependent IP(3) mass increase. Genistein treatment largely decreased the IGF-1-induced changes in both Ca(2+)(i) and IP(3). LY29402 (but not PD98059) also prevented the IGF-1-dependent Ca(2+)(i) increase. Both pertussis toxin and U73122 prevented the IGF-1-dependent induction of both ERKs and protein kinase B. We conclude that IGF-1 increases Ca(2+)(i) levels in cultured cardiac myocytes through a Gbetagamma subunit of a pertussis toxin-sensitive G protein-PI3K-phospholipase C signaling pathway that involves participation of IP(3).

C-terminal fragment of tetanus toxin heavy chain activates Akt and MEK/ERK signalling pathways in a Trk receptor-dependent manner in cultured cortical neurons

Previous publications from our group [Gil, Chaib, Pelliccioni and Aguilera (2000) FEBS Lett. 481, 177-182; Gil, Chaib, Blasi and Aguilera (2001) Biochem. J. 356, 97-103] have reported the activation, in rat brain synaptosomes, of several phosphoproteins, such as neurotrophin tyrosine kinase (Trk) A receptor, phospholipase Cgamma-1, protein kinase C (PKC) isoforms and extracellular-signal-regulated kinases 1 and 2 (ERK-1/2). In the present study, we examined, by means of phospho-specific antibodies, the activation of the signalling cascades involving neurotrophin Trk receptor, Akt kinase and ERK pathway, in cultured cortical neurons from foetal rat brain, by tetanus toxin (TeTx) as well as by the C-terminal part of its heavy chain (H(C)-TeTx). TeTx and H(C)-TeTx induce fast and transient phosphorylation of Trk receptor at Tyr(674) and Tyr(675), but not at Tyr(490), although the potency of TeTx in this action was higher when compared with H(C)-TeTx action. Moreover, H(C)-TeTx and TeTx also induced phosphorylation of Akt (at Ser(473) and Thr(308)) and of ERK-1/2 (Thr(202)/Tyr(204)), in a time- and concentration-dependent manner. The detection of TeTx- and H(C)-TeTx-induced phosphorylation at Ser(9) of glycogen synthase kinase 3beta confirms Akt activation. In the extended analysis of the ERK pathway, phosphorylation of the Raf, mitogen-activated protein kinase kinase (MEK)-1/2 and p90Rsk kinases and phosphorylation of the transcription factor cAMP-response-element-binding protein were detected. The use of tyrphostin AG879, an inhibitor of Trk receptors, demonstrates their necessary participation in the H(C)-TeTx-induced activation of Akt and ERK pathways, as well as in the phosphorylation of phospholipase Cgamma-1. Furthermore, both pathways are totally dependent on phosphatidylinositol 3-kinase action, and they are independent of PKC action, as assessed using wortmannin and Ro-31-8220 as inhibitors. The activation of PKC isoforms was determined by their translocation from the cytosolic compartment to the membranous compartment, showing a clear H(C)-TeTx-induced translocation of PKC-alpha and -beta, but not of PKC- epsilon.

Phospholipase C-gamma is required for agonist-induced Ca2+ entry

We report here that PLC-gamma isoforms are required for agonist-induced Ca2+ entry (ACE). Overexpressed wild-type PLC-gamma1 or a lipase-inactive mutant PLC-gamma1 each augmented ACE in PC12 cells, while a deletion mutant lacking the region containing the SH3 domain of PLC-gamma1 was ineffective. RNA interference to deplete either PLC-gamma1 or PLC-gamma2 in PC12 and A7r5 cells inhibited ACE. In DT40 B lymphocytes expressing only PLC-gamma2, overexpressed muscarinic M5 receptors (M5R) activated ACE. Using DT40 PLC-gamma2 knockout cells, M5R stimulation of ER Ca2+ store release was unaffected, but ACE was abolished. Normal ACE was restored by transient expression of PLC-gamma2 or a lipase-inactive PLC-gamma2 mutant. The results indicate a lipase-independent role of PLC-gamma in the physiological agonist-induced activation of Ca2+ entry.

Calcium mobilization evoked by hepatocellular swelling is linked to activation of phospholipase Cgamma

Recovery from swelling of hepatocytes and selected other epithelia is triggered by intracellular Ca(2+) release from the endoplasmic reticulum, which leads to fluid and electrolyte efflux through volume-sensitive K(+) and Cl(-) channels. The aim of this study was to determine the mechanisms responsible for swelling-mediated hepatocellular Ca(2+) mobilization. Swelling of HTC rat hepatoma cells, evoked by exposure to hypotonic medium, elicited transient increases in intracellular levels of inositol 1,4,5-trisphosphate (IP(3)) and cytosolic [Ca(2+)]. The latter was attenuated by inhibition of phospholipase C (PLC) with and by IP(3) receptor blockade with 2-aminoethoxydiphenyl borate, but it was unaffected by ryanodine, an inhibitor of intracellular Ca(2+)-induced Ca(2+) release channels. Hypotonic swelling was associated with a transient increase in tyrosine phosphorylation of PLCgamma, with kinetics that paralleled the increases in intracellular IP(3) levels and cytosolic [Ca(2+)]. Confocal imaging of HTC cells exposed to hypotonic medium revealed a swelling-induced association of tyrosine-phosphorylated PLCgamma with the plasma membrane. These findings suggest that activation of PLCgamma by hepatocellular swelling leads to the generation of IP(3) and stimulates discharge of Ca(2+) from the endoplasmic reticulum via activation of IP(3) receptors. By extension, these data support the concept that tyrosine phosphorylation of PLCgamma represents a critical step in adaptive responses to hepatocellular swelling.

Calcium cycling, historic overview and perspectives. Role for autonomic nervous system regulation

The human heart proceeds from a relaxed state (diastole) to a fully contracted state (systole) and recovery in 600ms. During this period, Ca(2+) inside the myocardial cell rises from about 10nM to about 100nM and returns to the former. The contractile-relaxation cycle is tightly coupled to the Ca(2+)transient. In the normal physiological state, the autonomic nervous system (ANS) plays a major role in the regulation of cardiac function and important changes occur in diseases of the heart. Sympathetic overdrive is a major determinant of the critical transition from initial compensatory hypertrophy to decompensated failure. Cardiac myocytes from failing hearts are characterized by a number of abnormalities in excitation-contraction coupling, that are a direct consequence of beta-adrenergic signaling defects. Although desensitized in cardiac hypertrophy and failure, the beta-adrenergic signaling pathway retains receptor capacity, a characteristic that is used in therapeutic approaches. There are several putative Ca(2+)-dependent pathways that exert counterbalancing negative regulation over cAMP-dependent positive inotropic effect and may represent potential targets for contractile stimulation. This review is focused on the interactions between sympathetic drive and aspects of calcium signaling in the heart.

Evidence for selective coupling of alpha 1-adrenergic receptors to phospholipase C-beta 1 in rat neonatal cardiomyocytes

Activation of phospholipase C (PLC) in neonatal rat cardiomyocytes (NCM) generates primarily inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3)) in response to rises in intracellular Ca(2+), or inositol 1,4-bisphosphate (Ins(1,4)P(2)) in response to norepinephrine (NE) (Matkovich, S. J. and Woodcock, E. A. (2000) J. Biol. Chem. 275, 10845-10850). To examine the PLC subtype mediating the alpha(1)-adrenergic receptor response, PLC-beta(1) and PLC-beta(3) were overexpressed in NCM using adenoviral infection (Ad-PLC-beta(1) NCM and Ad-PLC-beta(3) NCM, respectively) and PLC responses assessed from [(3)H]inositol phosphate (InsP) generation in the presence of 10 mm LiCl. The [(3)H]InsP response to NE (100 microm) was enhanced in Ad-PLC-beta(1) NCM relative to cells infected with blank virus (Ad-MX NCM), but was reduced in Ad-PLC-beta(3) NCM. In contrast, the [(3)H]InsP response to ATP (100 microm) was not elevated in Ad-PLC-beta(1) NCM, and was enhanced rather than diminished in Ad-PLC-beta(3) NCM, showing that effects of the two PLC-beta isoforms were specific for particular receptor types. PLC-delta(1) overexpression selectively reduced NE-induced [(3)H]InsP responses, without affecting the ATP stimulation. The reduced NE response was associated with a selective loss of PLC-beta(1) expression in Ad-PLC-delta(1) NCM. alpha(1)-Adrenergic receptor activation caused phosphorylation of PLC-beta(1) but not PLC-beta(3), whereas stimulation by ATP induced phosphorylation of PLC-beta(3) but not PLC-beta(1.) Taken together, these studies provide evidence that NE-stimulated InsP generation in NCM is primarily mediated by PLC-beta(1), despite the presence of both PLC-beta(1) and PLC-beta(3) isoforms.

Directed inhibition of nuclear import in cellular hypertrophy

Each nuclear pore is responsible for both nuclear import and export with a finite capacity for bidirectional transport across the nuclear envelope. It remains poorly understood how the nuclear transport pathway responds to increased demands for nucleocytoplasmic communication. A case in point is cellular hypertrophy in which increased amounts of genetic material need to be transported from the nucleus to the cytosol. Here, we report an adaptive down-regulation of nuclear import supporting such an increased demand for nuclear export. The induction of cardiac cell hypertrophy by phenylephrine or angiotensin II inhibited the nuclear translocation of H1 histones. The removal of hypertrophic stimuli reversed the hypertrophic phenotype and restored nuclear import. Moreover, the inhibition of nuclear export by leptomycin B rescued import. Hypertrophic reprogramming increased the intracellular GTP/GDP ratio and promoted the nuclear redistribution of the GTP-binding transport factor Ran, favoring export over import. Further, in hypertrophy, the reduced creatine kinase and adenylate kinase activities limited energy delivery to the nuclear pore. The reduction of activities was associated with the closure of the cytoplasmic phase of the nuclear pore preventing import at the translocation step. Thus, to overcome the limited capacity for nucleocytoplasmic transport, cells requiring increased nuclear export regulate the nuclear transport pathway by undergoing a metabolic and structural restriction of nuclear import.

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.

A specific role of phosphatidylinositol 3-kinase gamma. A regulation of autonomic Ca(2)+ oscillations in cardiac cells

Purinergic stimulation of cardiomyocytes turns on a Src family tyrosine kinase-dependent pathway that stimulates PLCgamma and generates IP(3), a breakdown product of phosphatidylinositol 4,5-bisphosphate (PIP2). This signaling pathway closely regulates cardiac cell autonomic activity (i.e., spontaneous cell Ca(2+) spiking). PIP2 is phosphorylated on 3' by phosphoinositide 3-kinases (PI3Ks) that belong to a broad family of kinase isoforms. The product of PI3K, phosphatidylinositol 3,4,5-trisphosphate, regulates activity of PLCgamma. PI3Ks have emerged as crucial regulators of many cell functions including cell division, cell migration, cell secretion, and, via PLCgamma, Ca(2+) homeostasis. However, although PI3Kalpha and -beta have been shown to mediate specific cell functions in nonhematopoietic cells, such a role has not been found yet for PI3Kgamma. We report that neonatal rat cardiac cells in culture express PI3Kalpha, -beta, and -gamma. The purinergic agonist predominantly activates PI3Kgamma. Both wortmannin and LY294002 prevent tyrosine phosphorylation, and membrane translocation of PLCgamma as well as IP(3) generation in ATP-stimulated cells. Furthermore, an anti-PI3Kgamma, but not an anti-PI3Kbeta, injected in the cells prevents the effect of ATP on cell Ca(2+) spiking. A dominant negative mutant of PI3Kgamma transfected in the cells also exerts the same action. The effect of ATP was observed on spontaneous Ca(2+) spiking of wild-type but not of PI3Kgamma(2/2) embryonic stem cell-derived cardiomyocytes. ATP activates the Btk tyrosine kinase, Tec, and induces its association with PLCgamma. A dominant negative mutant of Tec blocks the purinergic effect on cell Ca(2+) spiking. Tec is translocated to the T-tubes upon ATP stimulation of cardiac cells. Both an anti-PI3Kgamma antibody and a dominant negative mutant of PI3Kgamma injected or transfected into cells prevent the latter event. We conclude that PI3Kgamma activation is a crucial step in the purinergic regulation of cardiac cell spontaneous Ca(2+) spiking. Our data further suggest that Tec works in concert with a Src family kinase and PI3Kgamma to fully activate PLCgamma in ATP-stimulated cardiac cells. This cluster of kinases provides the cardiomyocyte with a tight regulation of IP(3) generation and thus cardiac autonomic activity.

ATP induces c-fos expression in C6 glioma cells by activation of P(2Y) receptors

BACKGROUND

Extracellular ATP functions in the enteric nervous system as a neurotransmitter, and recent evidence suggests ATP may regulate development through effects on cellular proliferation.

METHODS

The action of ATP at purinoceptors and the role of second messenger pathways in c-fos mRNA expression in C6 glioma cells were investigated using the techniques of Northern and Western blotting.

RESULTS

Treatment of C6 cells with ATP caused a time- and dose-dependent increase in c-fos expression. The rank order of agonist potency was ATP = ADP > gammasATP > alphabetaATP > betagammaATP > AMP = UTP. The ATP-induced c-fos increment was inhibited by three P(2Y) receptor antagonists-suramin, reactive blue, and DIDS-by 99+/-3, 89+/-7, and 61+/-14%, respectively. The ATP-stimulated c-fos expression was attenuated by phospholipase C inhibitor (U73122), protein kinase C (PKC) down-regulation (4alpha-phorbol 12-myristate 13-acetate and chelerythrine), mitogen-activated protein (MAP) kinase inhibition (apigenin), an inhibitor of MAP kinase kinase (PD98059), down-regulation of adenylate cyclase (SQ22536), and inhibition of type II protein kinase A (8-(4-chlorophenylthio)adenosine-3',5'-cyclic monophosphorothioate), but was not affected by inhibition of type I protein kinase A (8-bromoadenosine-3',5'-cyclic monophosphorothioate) and inhibitors of calmodulin kinase (KN93 and KN62). Phosphorylated MAP kinase was increased in cells exposed to ATP. This effect was suppressed by chelerythrine.

CONCLUSIONS

These studies demonstrate that ATP-induced c-fos mRNA expression is under multifactorial regulation.

Effects of angiotensin-converting enzyme inhibitors on glucose uptake

We investigated the effect of angiotensin-converting enzyme inhibitors on glucose uptake regulation as well as the effect of bradykinin (BK) on glucose uptake and its regulation by using inhibitors of phospholipase C, BK B2 receptor, protein kinase C, phosphatidylinositol 3-kinase, tyrosine kinase, and intracellular Ca(2+). We measured 2-deoxyglucose uptake by using L(6) skeletal muscle cells. In the presence of 1 nmol/L of insulin, 1 micromol/L of enalaprilat enhanced insulin-induced glucose uptake from 89.2+/-8. 1 to 138.0+/-13.6 pmol/h per mg protein. The stimulation of glucose uptake with enalaprilat was blocked to 92.7+/-7.8 pmol/h per mg protein by 10 micromol/L HOE 140 (a BK B2 receptor antagonist). In the presence of 1 nmol/L of insulin, exposure to 10 micromol/L BK stimulated glucose uptake from 89.2+/-8.1 to 171.6+/-10.1 pmol/h per mg protein. However, in the absence of insulin, BK could not enhance glucose uptake. One hundred nanomoles per liter of tyrphostin A-23 and genistein, which are tyrosine kinase inhibitors, significantly decreased the BK-induced glucose uptake from 142.0+/-8.4 to 87.6+/-6. 4 and 85.2+/-7.3 pmol/h per mg protein, respectively. BK-induced glucose uptake was inhibited significantly by 10 micromol/L U73122 (a phospholipase C antagonist) from 142.0+/-8.4 to 95.7+/-9.5 pmol/h per mg protein. One and 20 micromol/L of TMB-8 (an intracellular calcium antagonist) significantly decreased BK-induced glucose uptake from 142.0+/-8.4 to 108.0+/-9.6 and 100.8+/-11.4 pmol/h per mg protein. Angiotensin-converting enzyme inhibitors enhanced insulin-induced glucose uptake via the BK B2 receptor. BK-stimulated glucose uptake is related to phospholipase C, tyrosine kinase, and an increase in intracellular calcium.

Extracellular regulated kinase, but not protein kinase C, is an antiapoptotic signal of insulin-like growth factor-1 on cultured cardiac myocytes

This study aims to elucidate the signaling pathway for insulin-like growth factor-1 (IGF-1) in cultured neonatal rat cardiomyocytes and particularly the role of IGF-1 in cardiac apoptosis. IGF-1 stimulated polyphosphoinositide turnover, translocation of protein kinase C (PKC) isoforms (alpha, epsilon, and delta) from the soluble to the particulate fraction, activation of phospholipid-dependent and Ca(2+)-, phospholipid-dependent PKC, and activation of the extracellular-regulated kinase (ERK). IGF-1 attenuated sorbitol-induced cardiomyocyte viability and nuclear DNA fragmentation. These antiapoptotic effects of IGF-1 were blocked by PD-098059 (an MEK inhibitor) but not by bisindolylmaleimide I (BIM, a specific PKC inhibitor). The ERK pathway may therefore be an important component in the mechanism whereby IGF-1 exerts its antiapoptotic effect on the cardiomyocyte.

Advances in signalling by extracellular nucleotides. the role and transduction mechanisms of P2Y receptors

Nucleotides are ubiquitous intercellular messengers whose actions are mediated by specific receptors. Since the first clonings in 1993, it is known that nucleotide receptors belong to two families: the ionotropic P2X receptors and the metabotropic P2Y receptors. Five human P2Y receptor subtypes have been cloned so far and a sixth one must still be isolated. In this review we will show that they differ by their preference for adenine versus uracil nucleotides and triphospho versus diphospho nucleotides, as well as by their transduction mechanisms and cell expression.

Inositol 1,4,5-trisphosphate directs Ca(2+) flow between mitochondria and the Endoplasmic/Sarcoplasmic reticulum: a role in regulating cardiac autonomic Ca(2+) spiking

The signaling role of the Ca(2+) releaser inositol 1,4, 5-trisphosphate (IP(3)) has been associated with diverse cell functions. Yet, the physiological significance of IP(3) in tissues that feature a ryanodine-sensitive sarcoplasmic reticulum has remained elusive. IP(3) generated by photolysis of caged IP(3) or by purinergic activation of phospholipase Cgamma slowed down or abolished autonomic Ca(2+) spiking in neonatal rat cardiomyocytes. Microinjection of heparin, blocking dominant-negative fusion protein, or anti-phospholipase Cgamma antibody prevented the IP(3)-mediated purinergic effect. IP(3) triggered a ryanodine- and caffeine-insensitive Ca(2+) release restricted to the perinuclear region. In cells loaded with Rhod2 or expressing a mitochondria-targeted cameleon and TMRM to monitor mitochondrial Ca(2+) and potential, IP(3) induced transient Ca(2+) loading and depolarization of the organelles. These mitochondrial changes were associated with Ca(2+) depletion of the sarcoplasmic reticulum and preceded the arrest of cellular Ca(2+) spiking. Thus, IP(3) acting within a restricted cellular region regulates the dynamic of calcium flow between mitochondria and the endoplasmic/sarcoplasmic reticulum. We have thus uncovered a novel role for IP(3) in excitable cells, the regulation of cardiac autonomic activity.

Purinergic inhibition of glucose transport in cardiomyocytes

ATP is known to act as an extracellular signal in many organs. In the heart, extracellular ATP modulates ionic processes and contractile function. This study describes a novel, metabolic effect of exogenous ATP in isolated rat cardiomyocytes. In these quiescent (i.e. noncontracting) cells, micromolar concentrations of ATP depressed the rate of basal, catecholamine-stimulated, or insulin-stimulated glucose transport by up to 60% (IC50 for inhibition of insulin-dependent glucose transport, 4 microM). ATP decreased the amount of glucose transporters (GLUT1 and GLUT4) in the plasma membrane, with a concomitant increase in intracellular microsomal membranes. A similar glucose transport inhibition was produced by P2 purinergic agonists with the following rank of potencies: ATP approximately ATPgammaS approximately 2-methylthio-ATP (P2Y-selective) > ADP > alpha,betameATP (P2X-selective), whereas the P1 purinoceptor agonist adenosine was ineffective. The effect of ATP was suppressed by the poorly subtype-selective P2 antagonist pyridoxal-phosphate-6-azophenyl-2', 4'-disulfonic acid but, surprisingly, not by the nonselective antagonist suramin nor by the P2Y-specific Reactive Blue 2. Glucose transport inhibition by ATP was not affected by a drastic reduction of the extracellular concentrations of calcium (down to 10(-9) M) or sodium (down to 0 mM), and it was not mimicked by a potassium-induced depolarization, indicating that purinoceptors of the P2X family (which are nonselective cation channels whose activation leads to a depolarizing sodium and calcium influx) are not involved. Inhibition was specific for the transmembrane transport of glucose because ATP did not inhibit (i) the rate of glycolysis under conditions where the transport step is no longer rate-limiting nor (ii) the rate of [1-14C]pyruvate decarboxylation. In conclusion, extracellular ATP markedly inhibits glucose transport in rat cardiomyocytes by promoting a redistribution of glucose transporters from the cell surface to an intracellular compartment. This effect of ATP is mediated by P2 purinoceptors, possibly by a yet unknown subtype of the P2Y purinoceptor family.

Potentiation of adenosine A1 receptor-mediated inositol phospholipid hydrolysis by tyrosine kinase inhibitors in CHO cells

1. The effect of protein tyrosine kinase inhibitors on human adenosine A1 receptor-mediated [3H]-inositol phosphate ([3H]-IP) accumulation has been studied in transfected Chinese hamster ovary cells (CHO-A1) cells. 2. In agreement with our previous studies the selective adenosine A1 receptor agonist N6-cyclopentyladenosine (CPA) stimulated the accumulation of [3H]-IPs in CHO-A1 cells. Pre-treatment with the broad spectrum tyrosine kinase inhibitor genistein (100 microM; 30 min) potentiated the responses elicited by 1 microM (199+/-17% of control CPA response) and 10 microM CPA (234+/-15%). Similarly, tyrphostin A47 (100 microM) potentiated the accumulation of [3H]-IPs elicited by 1 microM CPA (280+/-32%). 3. Genistein (EC50 = 13.7+/-1.2 microM) and tyrphostin A47 (EC50 = 10.4+/-3.9 microM) potentiated the [3H]-IP response to 1 microM CPA in a concentration-dependent manner. 4. Pre-incubation with the inactive analogues of genistein and tyrphostin A47, daidzein (100 microM; 30 min) and tyrphostin A1 (100 microM; 30 min), respectively, had no significant effect on the accumulation of [3H]-IPs elicited by 1 microM CPA. 5. Genistein (100 microM) had no significant effect on the accumulation of [3H]-IPs produced by the endogenous thrombin receptor (1 u ml(-1); 100+/-10% of control response). In contrast, tyrphostin A47 produced a small augmentation of the thrombin [3H]-IP response (148+/-13%). 6. Genistein (100 microM) had no effect on the [3H]-IP response produced by activation of the endogenous Gq-protein coupled CCK(A) receptor with the sulphated C-terminal octapeptide of cholecystokinin (1 microM CCK-8; 96+/-6% of control). In contrast, tyrphostin A47 (100 microM) caused a small but significant increase in the response to 1 microM CCK-8 (113+/-3% of control). 7. The phosphatidylinositol 3-kinase inhibitor LY 294002 (30 microM) and the MAP kinase kinase inhibitor PD 98059 (50 microM) had no significant effect on the [3H]-IP responses produced by 1 microM CPA and 1 microM CCK-8. 8. These observations suggest that a tyrosine kinase-dependent pathway may be involved in the regulation of human adenosine A1 receptor mediated [3H]-IP responses in CHO-A1 cells.

Specific activation of adenylyl cyclase V by a purinergic agonist

The present study was designed to investigate whether and how the purinergic stimulation of rat ventricular myocytes modulates the cAMP-dependent pathway. Stimulation of cardiomyocytes with ATPgammaS in the presence of the phosphodiesterase inhibitor IBMX increases by 3-fold intracellular cAMP content. In contrast to beta-adrenergic stimulation, the purinergic stimulation of adenylyl cyclase was not inhibited by activation or enhanced by inhibition of a Gi protein. Forskolin did not potentiate the effect of extracellular ATPgammaS on intracellular cAMP content but the effect of isoproterenol did. Like isoproterenol, the purinergic agonist decreased subsequent ADP-ribosylation of a 45 kDa G(alpha s) by cholera toxin. ATPgammaS also increased cAMP content in neonatal rat cardiomyocytes, a cell type that expresses a long form of Gs protein (alpha(s), 52 kDa) in contrast to adult rat cardiomyocytes that express mostly a short form of Gs protein (alpha(s), 45 kDa). Both purinergic and beta-adrenergic agonists increased cAMP in HEK 293 cells expressing type V adenylyl cyclase while cAMP was only increased by beta-adrenergic stimulation of HEK expressing type IV or VI adenylyl cyclases. Thus, we propose that the purinergic and beta-adrenergic stimulations differentially activate adenylyl cyclase isoforms in rat cardiomyocytes and that adenylyl cyclase V is the specific target of the purinergic stimulation.

Effect of concomitant beta-adrenoceptor stimulation on alpha 1-adrenoceptor-mediated increase of inositol-1,4,5-trisphosphate mass in adult rat cardiomyocytes

The aim of the present study was to investigate the accumulation of inositol-1,4,5-trisphosphate (IP3) in isolated adult rat ventricular cardiomyocytes after alpha 1- and beta-adrenoceptor stimulation, separate and in combination, in order to elucidate a possible influence of concomitant beta-adrenoceptor stimulation on the alpha 1-adrenoceptor stimulated response. IP3 was measured by a radioligand binding assay based on an (1,4,5)IP3-specific binding protein from bovine adrenal cortex. The basal IP3 content was 4.06 +/- 0.31 pmol/mg protein (N = 56). alpha 1-Adrenoceptor stimulation resulted in a rapid increase in the IP3 level, which reached a plateau, 50-80% above basal level, at 10-30 sec. The plateau lasted at least up to 120 sec., while at 300 sec. there was no significant difference between control values and values after alpha 1-adrenoceptor stimulation. Li+ did not affect either the basal IP3 level, or the magnitude or time course of alpha 1-adrenoceptor-stimulated IP3 accumulation. Combined adrenoceptor stimulation gave a similar response as separate alpha 1-adrenoceptor stimulation, whereas there was no significant change in the IP3 level after beta-adrenoceptor stimulation. No inhibitory influence of simultaneous beta-adrenoceptor stimulation on the alpha 1-adrenoceptor-stimulated increase of IP3 mass was revealed.

Src family tyrosine kinase regulates intracellular pH in cardiomyocytes

The Anion Cl-/HCO3- Exchangers AE1, AE2, and AE3 are membrane pH regulatory ion transporters ubiquitously expressed in vertebrate tissues. Besides relieving intracellular alkaline and CO2 loads, the AEs have an important function during development and cell death and play a central role in such cellular properties as cell shape, metabolism, and contractility. The activity of AE(s) are regulated by neurohormones. However, little is known as to the intracellular signal transduction pathways that underlie this modulation. We show here that, in cardiomyocytes that express both AE1 and AE3, the purinergic agonist, ATP, triggers activation of anion exchange. The AE activation is observed in cells in which AE3 expression was blocked but not in cells microinjected with neutralizing anti-AE1 antibodies. ATP induces tyrosine phosphorylation of AE1, activation of the tyrosine kinase Fyn, and association of both Fyn and FAK with AE1. Inhibition of Src family kinases in vivo by genistein, herbimycin A, or ST638 prevents purinergic activation of AE1. Microinjection of either anti-Cst.1 antibody or recombinant CSK, both of which prevent activation of Src family kinase, significantly decreases ATP-induced activation of AE. Microinjection of an anti-FAK antibody as well as expression in cardiomyocytes of Phe397 FAK dominant negative mutant, also prevents purinergic activation of AE. Therefore, tyrosine kinases play a key role in acute regulation of intracellular pH and thus in cell function including excitation-contraction coupling of the myocardium.

Cardiac protein phosphorylation: functional and pathophysiological correlates

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

Hypotonic Ca2+ signaling and volume regulation in proliferating and quiescent cells from multicellular spheroids

Hypotonicity-induced Ca2+ signals and volume regulation were studied in proliferating and quiescent subpopulations of multicellular prostate cancer spheroids. Enzymatic dissociation of multicellular spheroids 100+/-19 microm in diameter, which are entirely proliferative, yielded a population of cells with a mean cell diameter of 17.5+/-1.4 microm. After dissociation of spheroids in a size class of 200+/-30, 300+/-60, and 400+/-65 microm in diameter, two subpopulations of cells with mean cell diameters corresponding to 12.9+/-1.9 microm and 16.7+/-2 microm were discriminated. The subpopulation of large cells was shown to be proliferative by positive Ki-67 antibody staining; the subpopulation of small cells was Ki-67 negative, indicating cell quiescence. In a spheroid size class of 100+/-19 microm, a distinct subpopulation of quiescent cells was absent. Superfusion by hypotonic solutions revealed that only the proliferating cell fraction showed a regulatory volume decrease (RVD) and a [Ca2+]i transient. Both effects were absent in the quiescent cell population. The [Ca2+]i transient persisted in low (10 nM) Ca2+ solution and in the presence of 4 mM extracellular Ni2+ but was abolished in the presence of the endoplasmic reticulum Ca2+-ATPase blocker 2,5-di-tert-butyl-hydrochinone (t-BHQ). The t-BHQ likewise inhibited RVD, indicating that Ca2+ release from intracellular stores was necessary for RVD. Moreover, [Ca2+]i and RVD were dependent on an intact microfilament cytoskeleton because after 30 min of preincubation with cytochalasin B the [Ca2+]i transient was significantly reduced and RVD was abolished. The absence of RVD and [Ca2+]i transient in quiescent cells may be due to differences in the amount and the cytosolic arrangement of F-actin observed in quiescent cells.

Angiotensin II-induced association of phospholipase Cgamma1 with the G-protein-coupled AT1 receptor

An early event in signaling by the G-protein-coupled angiotensin II (Ang II) AT1 receptor in vascular smooth muscle cells is the tyrosine phosphorylation and activation of phospholipase Cgamma1 (PLCgamma1). In the present study, we show that stimulation of this event by Ang II in vascular smooth muscle cells is accompanied by binding of PLCgamma1 to the AT1 receptor in an Ang II- and tyrosine phophorylation-dependent manner. The PLCgamma1-AT1 receptor interaction appears to depend on phosphorylation of tyrosine 319 in a YIPP motif in the C-terminal intracellular domain of the AT1 receptor and binding of the phosphorylated receptor by the most C-terminal of two Src homology 2 domains in PLCgamma1. PLCgamma1 thus binds to the same site in the receptor previously identified for binding by the SHP-2 phosphotyrosine phosphatase.JAK2 tyrosine kinase complex. A single site in the C-terminal tail of the AT1 receptor can, therefore, be bound in a ligand-dependent manner by two different downstream effector proteins. These data demonstrate that G-protein-coupled receptors can physically associate with intracellular proteins other than G proteins, creating membrane-delimited signal transduction complexes similar to those observed for classic growth factor receptors.

Mutations at residues Tyr771 and Tyr783 of phospholipase C-gamma1 have different effects on cell actin-cytoskeleton organization and cell proliferation in CCL-39 cells

Currently a central question that remains unresolved is the potential importance of phospholipase C (PLC)-gamma1 in cell mitogenesis. In this study, we introduced wild-type PLC-gamma1 plasmid and mutants Y771F and Y783F into CCL-39 fibroblasts and investigated their effect on host cell functions. To our surprise, Y771F and Y783F plasmids appeared to have opposite effects on CCL-39 cell actin-cytoskeleton organization and cell proliferation. Y771F transfectants increased cell proliferation by two-fold. Y783F transfectants showed much thicker actin filaments and decreased cell growth rate by 50%. These results suggest that PLC-gamma1 mutations have an essential impact on cell mitogenesis.

Enhanced bradykinin-stimulated phospholipase C activity in murine embryonic stem cells lacking the G-protein alphaq-subunit

The gene coding for the G-protein alphaq subunit was interrupted by homologous recombination in murine embryonic stem cells (alphaq-null ES cells) as detected by Southern analysis and reverse-transcriptase PCR. The bradykinin (BK) B2 receptor was stably transfected into wild-type (WT) alphai-2-null and alphaq-null ES cells. The B2 receptor bound BK with high affinity and mobilized Ca2+. BK also activated phospholipase C (PLC), as determined by total inositol phosphate (IP) accumulation in a Bordetella pertussis toxin- and genistein-insensitive manner. In WT and alphai-2-null ES cells, BK increased IP levels approx. 4-fold above baseline. Most interestingly, in alphaq-null ES cells, BK increased IP accumulation approx. 9-fold above baseline. Re-expression of alphaq in alphaq-null ES cells resulted in normalization of the BK-stimulated IP accumulation (4-fold above baseline). These results suggest that the B2 receptor activates PLC through more than one member of the Gq family. Additionally, the absence of alphaq alters the kinetics of IP generation, which may reflect intrinsic characteristics of individual members of the Gq family or a decreased susceptibility to heterologous regulation in the alphaq-null ES cells, thus allowing for a more sustained generation of IP.

A new function for phospholipase C-gamma1: coupling to the adaptor protein GRB2

Epidermal growth factor (EGF)-induced autophosphorylation of the EGF receptor results in high-affinity binding of the adaptor protein GRB2, which serves as a convergence point for multiple signaling pathways. Present studies demonstrate that EGF induces the co-immunoprecipitation of phospholipase C (PLC)-gamma1 with the adaptor protein GRB2 and the guanine nucleotide exchange factor Sos, but not with the adaptor protein SHC, in WB cells. Inhibition of PLC-gamma1 tyrosine phosphorylation by phenylarsine oxide reduces the co-immunoprecipitation of PLC-gamma1 with GRB2. Furthermore, angiotensin II, a G protein-coupled receptor agonist, also induces the tyrosine phosphorylation of PLC-gamma1 and its co-immunoprecipitation with GRB2 in WB cells. Interestingly, angiotensin II stimulation also causes tyrosine phosphorylation of the EGF receptor, suggesting that angiotensin II-induced PLC-gamma1 tyrosine phosphorylation in WB cells may be via EGF receptor tyrosine kinase activation. In addition, there is some level of association between PLC-gamma1 and GRB2 that is independent of the tyrosine phosphorylation of PLC-gamma1 in both in vivo and in vitro studies. In vitro studies further demonstrate that the Tyr771 and Tyr783 region of PLC-gamma1 and the SH2 domain of GRB2 are potentially involved in the tyrosine phosphorylation-dependent association between PLC-gamma1 and GRB2. The association of PLC-gamma1 with GRB2 and Sos suggests that PLC-gamma1 may be directly involved in the Ras signaling pathway and that GRB2 may be involved in the translocation of PLC-gamma1 from cytosol to the plasma membrane as a necessary step for its effect on inositol lipid hydrolysis.

Enhancement of the ATP-sensitive K+ current by extracellular ATP in rat ventricular myocytes. Involvement of adenylyl cyclase-induced subsarcolemmal ATP depletion

ATP-sensitive K+ (KATP) channels are present at high density in membranes of cardiac cells, where they regulate cardiac function during metabolic impairment. The present study analyzes the effects of extracellular ATP (ATPc), a P2-purinergic agonist that can be released under various conditions in the myocardial cell bed, on KATP current (IK-ATP) in rat ventricular myocytes. Under the whole-cell patch-clamp configuration at a physiological level of intracellular ATP, applying ATPc in the micromolar range did not activate IK-ATP. However, dialyzing the cell with a low-ATP (100 mumol/L) pipette solution elicited a slowly, quasilinearly increasing IK-ATP that was markedly enhanced by applying ATPe in the presence of a Purinergic antagonist. The effect was reversible on washing out the agonist. The IK-ATP enhancement was inhibited by cholera toxin treatment of the myocytes, suggesting that a Gs protein was involved to mediate the effect. Experiments on excised patches allowed us to exclude a membrane-delimited G protein-dependent pathway. Rather, the results suggested that ATPe activates the adenylyl cyclase, since its inhibition by 2'-deoxyadenosine 3'-monophosphate and SQ-22536, which respectively interact with the purine and catalytic site of the cyclase, strongly reduced the ATPe-induced IK-ATP enhancement, whereas neither compound affected IK-ATP in inside-out patches. Inhibition of cAMP-dependent protein kinase by protein kinase inhibitor peptide 5-24 did not alter the purinergic effect. The findings suggests that ATPe triggers the activation of adenylyl cyclase, which causes a subsarcolemmal ATP depletion sufficient to enhance IK-ATP as it develops during low-ATP dialysis of rat ventricular myocytes.

Purinergic facilitation of ATP-sensitive potassium current in rat ventricular myocytes

1. The effects of different purinergic agonists on the cardiac adenosine 5'-triphosphate (ATP)-sensitive potassium current (IK(ATP)), appearing during dialysis of rat isolated ventricular myocytes with a low-ATP (100 microM) internal solution under whole-cell patch-clamp conditions, were examined in the presence of a P1 purinoceptor antagonist. 2. The extracellular application of ATP in the micromolar range induced, besides known inward currents through cationic and chloride channels, the facilitation of IK(ATP) once IK(ATP) had already been partially activated during the low-ATP dialysis. 3. Analogues of ATP, alpha, beta-methyleneadenosine 5'-triphosphate (alpha, beta meATP), 2-methylthioadenosine triphosphate (2MeSATP), adenosine 5'-O-3-thiotriphosphate (ATP gamma S) similarly facilitated IK(ATP). UTP and ADP were very weak agonists while AMP and adenosine had no detectable effect. 4. The half-maximal stimulating concentration (C50) of alpha, beta meATP, an analogue that did not elicite the interfering inward cationic current was 1.5 microM. Similar apparent C50 (1-2 microM) were observed for ATP and analogues tested with somewhat less maximal effect of ATP gamma S. 5. Suramin, a nonselective P2-purinoceptor antagonist, altered IK(ATP) at the relatively high concentration required to inhibit purinoceptors. Pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), a supposedly predominantly P2x-purinoceptor antagonist, at micromolar concentration inhibited the transient inward current but did not block the facilitation of IK(ATP). 6. Our results demonstrate that ATP and its analogues facilitate IK(ATP) in rat ventricular myocytes by stimulation of non-P1-, non-P2x-purinoceptors.