Diadenosine polyphosphate-induced inhibition of cardiac KATP channels: operative state-dependent regulation by a nucleoside diphosphate

Moonlighting matrix metalloproteinase substrates: Enhancement of proinflammatory functions of extracellular tyrosyl-tRNA synthetase upon cleavage

Tyrosyl-tRNA synthetase ligates tyrosine to its cognate tRNA in the cytoplasm, but it can also be secreted through a noncanonical pathway. We found that extracellular tyrosyl-tRNA synthetase (YRS) exhibited proinflammatory activities. In addition to acting as a monocyte/macrophage chemoattractant, YRS initiated signaling through Toll-like receptor 2 (TLR2) resulting in NF-κB activation and release of tumor necrosis factor α (TNFα) and multiple chemokines, including MIP-1α/β, CXCL8 (IL8), and CXCL1 (KC) from THP1 monocyte and peripheral blood mononuclear cell–derived macrophages. Furthermore, YRS up-regulated matrix metalloproteinase (MMP) activity in a TNFα-dependent manner in M0 macrophages. Because MMPs process a variety of intracellular proteins that also exhibit extracellular moonlighting functions, we profiled 10 MMPs for YRS cleavage and identified 55 cleavage sites by amino-terminal oriented mass spectrometry of substrates (ATOMS) positional proteomics and Edman degradation. Stable proteoforms resulted from cleavages near the start of the YRS C-terminal EMAPII domain. All of the MMPs tested cleaved at ADS³⁸⁶↓³⁸⁷LYV and VSG⁴⁰⁵↓⁴⁰⁶LVQ, generating 43- and 45-kDa fragments. The highest catalytic efficiency for YRS was demonstrated by MMP7, which is highly expressed by monocytes and macrophages, and by neutrophil-specific MMP8. MMP-cleaved YRS enhanced TLR2 signaling, increased TNFα secretion from macrophages, and amplified monocyte/macrophage chemotaxis compared with unprocessed YRS. The cleavage of YRS by MMP8, but not MMP7, was inhibited by tyrosine, a substrate of the YRS aminoacylation reaction. Overall, the proinflammatory activity of YRS is enhanced by MMP cleavage, which we suggest forms a feed-forward mechanism to promote inflammation.

Diadenosine tetra- and pentaphosphates affect contractility and bioelectrical activity in the rat heart via P2 purinergic receptors

Diadenosine polyphosphates (Ap(n)As) are endogenously produced molecules which have been identified in various tissues of mammalian organism, including myocardium. Ap(n)As contribute to the blood clotting and are also widely accepted as regulators of blood vascular tone. Physiological role of Ap(n)As in cardiac muscle has not been completely elucidated. The present study aimed to investigate the effects of diadenosine tetra- (Ap4A) and penta- (Ap5A) polyphosphates on contractile function and action potential (AP) waveform in rat supraventricular and ventricular myocardium. We have also demonstrated the effects of A4pA and Ap5A in myocardial sleeves of pulmonary veins (PVs), which play a crucial role in genesis of atrial fibrillation. APs were recorded with glass microelectrodes in multicellular myocardial preparations. Contractile activity was measured in isolated Langendorff-perfused rat hearts. Both Ap4A and Ap5A significantly reduced contractility of isolated Langendorff-perfused heart and produced significant reduction of AP duration in left and right auricle, interatrial septum, and especially in right ventricular wall myocardium. Ap(n)As also shortened APs in rat pulmonary veins and therefore may be considered as potential proarrhythmic factors. Cardiotropic effects of Ap4A and Ap5A were strongly antagonized by selective blockers of P2 purine receptors suramin and pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), while P1 blocker DPCPX was not effective. We conclude that Ap(n)As may be considered as new class of endogenous cardioinhibitory compounds. P2 purine receptors play the central role in mediation of Ap4A and Ap5A inhibitory effects on electrical and contractile activity in different regions of the rat heart.

A dual mechanism of cytoprotection afforded by M-LDH in embryonic heart H9C2 cells

Muscle form of lactate dehydrogenase (M-LDH), a minor LDH form in cardiomyocytes, physically interacts with ATP-sensitive K+ (K ATP) channel-forming subunits. Here, we have shown that expression of 193gly-M-LDH, an inactive mutant of M-LDH, inhibit regulation of the K ATP channels activity by LDH substrates in embryonic rat heart H9C2 cells. In cells expressing 193gly-M-LDH chemical hypoxia has failed to activate K ATP channels. The similar results were obtained in H9C2 cells expressing Kir6.2AFA, a mutant form of Kir6.2 with largely decreased K+ conductance. Kir6.2AFA has slightly, but significantly, reduced cellular survival under chemical hypoxia while the deleterious effect of 193gly-M-LDH was significantly more pronounced. The levels of total and subsarcolemmal ATP in H9C2 cells were not affected by Kir6.2AFA, but the expression of 193gly-M-LDH led to lower levels of subsarcolemmal ATP during chemical hypoxia. We conclude that M-LDH regulates both the channel activity and the levels of subsarcolemmal ATP and that both mechanism contribute to the M-LDH-mediated cytoprotection.

Diadenosine tetraphosphate stimulates atrial ANP release via A(1) receptor: involvement of K(ATP) channel and PKC

Diadenosine polyphosphates (APnAs) are endogenous compounds and exert diverse cardiovascular functions. However, the effects of APnAs on atrial ANP release and contractility have not been studied. In this study, the effects of diadenosine tetraphosphate (AP4A) on atrial ANP release and contractility, and their mechanisms were studied using isolated perfused rat atria. Treatment of atria with AP4A resulted in decreases in atrial contractility and extracellular fluid (ECF) translocation whereas ANP secretion and cAMP levels in perfusate were increased in a dose-dependent manner. These effects of AP4A were attenuated by A(1) receptor antagonist but not by A(2A) or A(3) receptor antagonist. Other purinoceptor antagonists also did not show any effects on AP4A-induced ANF release and contractility. The increment of ANP release and negative inotropy induced by AP4A was similar to those induced by AP3A, AP5A, and AP6A. Protein kinase A inhibitors accentuated AP4A-induced ANP secretion. In contrast, an inhibitor of phospholipase C, protein kinase C or sarcolemma K(ATP) channel completely blocked AP4A-induced ANP secretion. However, an inhibitor of adenylyl cyclase or mitochondria K(ATP) channel had no significant modification of AP4A effects. These results suggest that AP4A regulates atrial inotropy and ANP release mainly through A(1) receptor signaling involving phospholipase C-protein kinase C and sarcolemmal K(ATP) channel and that protein kinase A negatively modulates the effects of AP4A.

The pnhA gene of Pasteurella multocida encodes a dinucleoside oligophosphate pyrophosphatase member of the Nudix hydrolase superfamily

The pnhA gene of Pasteurella multocida encodes PnhA, which is a member of the Nudix hydrolase subfamily of dinucleoside oligophosphate pyrophosphatases. PnhA hydrolyzes diadenosine tetra-, penta-, and hexaphosphates with a preference for diadenosine pentaphosphate, from which it forms ATP and ADP. PnhA requires a divalent metal cation, Mg(2+) or Mn(2+), and prefers an alkaline pH of 8 for optimal activity. A P. multocida strain that lacked a functional pnhA gene, ACP13, was constructed to further characterize the function of PnhA. The cellular size of ACP13 was found to be 60% less than that of wild-type P. multocida, but the growth rate of ACP13 and its sensitivity to heat shock conditions were similar to those of the wild type, and the wild-type cell size was restored in the presence of a functional pnhA gene. Wild-type and ACP13 strains were tested for virulence by using the chicken embryo lethality model, and ACP13 was found to be up to 1,000-fold less virulent than the wild-type strain. This is the first study to use an animal model in assessing the virulence of a bacterial strain that lacked a dinucleoside oligophosphate pyrophosphatase and suggests that the pyrophosphatase PnhA, catalyzing the hydrolysis of diadenosine pentaphosphates, may also play a role in facilitating P. multocida pathogenicity in the host.

Effects of diadenosine polyphosphates on glomerular volume

1. Diadenosine polyphosphates (P(1),P(3)-diadenosine triphosphate, Ap(3)A; P(1),P(4)-diadenosine tetraphosphate, Ap(4)A; and P(1),P(5)-diadenosine pentaphosphate, Ap(5)A) are vasoactive molecules. The experimental model of isolated rat renal glomeruli was used to investigate their effects on glomerular vasculature. We measured the changes of glomerular inulin space (GIS) as a marker of glomeruli contractility. 2. Ap(4)A and Ap(5)A induced concentration- and time-dependent reduction of GIS whereas Ap(3)A had no effect. The effects of Ap(4)A and Ap(5)A (both at 1 microM) were prevented by a nonselective P2 receptor antagonist, that is, suramin (10 microM) and P2Y receptor antagonist - reactive blue 2 (50 microM). However, the antagonist of P1 receptor, that is, theophylline (1 microM) and A(1) receptor 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 10 microM) did not affect the responses of glomeruli to Ap(4)A or Ap(5)A. 3. Ap(3)A, in contrast to Ap(4)A and Ap(5)A, prevented angiotensin II-induced reduction of GIS in a concentration- and time-dependent manner. This effect was partially prevented by suramin and markedly reduced by reactive blue 2 and the specific antagonist of P2Y(1) receptor - MRS 2179 (10 microM). However, theophylline and the specific antagonist of A(2) receptor - 3,7-dimethyl-1-propargylxanthine (DMPX; 10 microM) - did not affect Ap(3)A action. 4. We indicate that diadenosine polyphosphates changed the glomerular volume via activation of P2 receptors. We suggest that extracellular Ap(4)A and Ap(5)A via P2X and P2Y receptors may decrease and Ap(3)A via, at least in part, P2Y(1) receptors may increase filtration surface, which in turn may modify glomerular filtration rate.

Long-chain acyl-CoA esters and phosphatidylinositol phosphates modulate ATP inhibition of KATP channels by the same mechanism

Phosphatidylinositol phosphates (PIPs, e.g. PIP2) and long-chain acyl-CoA esters (e.g. oleoyl-CoA) are potent activators of KATP channels that are thought to link KATP channel activity to the cellular metabolism of PIPs and fatty acids. Here we show that the two types of lipid act by the same mechanism: oleoyl-CoA potently reduced the ATP sensitivity of cardiac (Kir6.2/SUR2A) and pancreatic (Kir6.2/SUR1) KATP channels in a way very similar to PIP2. Mutations (R54Q, R176A) in the C- and N-terminus of Kir6.2 that greatly reduced the PIP2 modulation of ATP sensitivity likewise reduced the modulation by oleoyl-CoA, indicating that the two lipids interact with the same site. Polyvalent cations reduced the effect of oleoyl-CoA and PIP2 on the ATP sensitivity with similar potency suggesting that electrostatic interactions are of similar importance. However, experiments with differently charged inhibitory adenosine phosphates (ATP4-, ADP3- and 2'(3')-O-(2,4,6-trinitrophenyl)adenosine 5'-monophosphate (TNP-AMP2-)) and diadenosine tetraphosphate (Ap4A5-) ruled out a mechanism where oleoyl-CoA or PIP2 attenuate ATP inhibition by reducing ATP binding through electrostatic repulsion. Surprisingly, CoA (the head group of oleoyl-CoA) did not activate but inhibited KATP channels (IC50 = 265 +/- 33 muM). We provide evidence that CoA and diadenosine polyphosphates (e.g. Ap4A) are ligands of the inhibitory ATP-binding site on Kir6.2.

Diadenosine-5-phosphate exerts A1-receptor-mediated proarrhythmic effects in rabbit atrial myocardium

(1) Diadenosine polyphosphates have been described to be present in the myocardium and exert purinergic- and nonreceptor-mediated effects. Since the electrophysiological properties of atrial myocardium are effectively regulated by A(1) receptors, we investigated the effect of diadenosine pentaphosphate (Ap(5)A) in rabbit myocardium. (2) Parameters of supraventricular electrophysiology and atrial vulnerability were measured in Langendorff-perfused rabbit hearts. Muscarinic potassium current (I(K(ACh/Ado))) and ATP-sensitive potassium current (I(K(ATP))) were measured by using the whole-cell voltage clamp method. (3) Ap(5)A prolonged the cycle length of spontaneously beating Langendorff perfused hearts from 225+/-14 (control) to 1823+/-400 ms (Ap(5)A 50 micro M; n=6; P<0.05). This effect was paralleled by higher degree of atrio-ventricular block. Atrial effective refractory period (AERP) in control hearts was 84+/-14 ms (n=6). Ap(5)A>/=1 micro M reduced AERP (100 micro M, 58+/-11 ms; n=6). (4) Extrastimuli delivered to hearts perfused with Ap(5)A- or adenosine (>/= micro M)-induced atrial fibrillation, the incidence of which correlated to the concentration added to the perfusate. The selective A(1)-receptor antagonist CPX (20 micro M) inhibited the Ap(5)A- and adenosine-induced decrease of AERP. Atrial fibrillation was no longer observed in the presence of CPX. (5) The described Ap(5)A-induced effects in the multicellular preparation were enhanced by dipyridamole (10 micro M), which is a cellular adenosine uptake inhibitor. Dipyridamole-induced enhancement was inhibited by CPX. (6) Ap(5)A (</=1 mM) did neither induce I(K(Ado)) nor I(K(ATP)). No effect on activated I(K(Ado/ATP)) was observed in myocytes superfused with Ap(5)A. However, effluents from Langendorff hearts perfused with Ap(5)A 100 micro M activated I(K(Ado)) by using A(1) receptors. (7) Ap(5)A did not activate A(1) receptors in rabbit atrial myocytes. The Ap(5)A induced A(1)-receptor-mediated effects on supraventricular electrophysiology and vulnerability suggest that in the multicellular preparation Ap(5)A is hydrolyzed to yield adenosine, which acts via A(1) receptors. An influence on atrial electrophysiology or a facilitation of atrial fibrillation under conditions resulting in increased interstitial Ap(5)A concentrations might be of physiological/pathophysiological relevance.

Pharmacological plasticity of cardiac ATP-sensitive potassium channels toward diazoxide revealed by ADP

The pharmacological phenotype of ATP-sensitive potassium (K(ATP)) channels is defined by their tissue-specific regulatory subunit, the sulfonylurea receptor (SUR), which associates with the pore-forming channel core, Kir6.2. The potassium channel opener diazoxide has hyperglycemic and hypotensive properties that stem from its ability to open K(ATP) channels in pancreas and smooth muscle. Diazoxide is believed not to have any significant action on cardiac sarcolemmal K(ATP) channels. Yet, diazoxide can be cardioprotective in ischemia and has been found to bind to the presumed cardiac sarcolemmal K(ATP) channel-regulatory subunit, SUR2A. Here, in excised patches, diazoxide (300 microM) activated pancreatic SUR1/Kir6.2 currents and had little effect on native or recombinant cardiac SUR2A/Kir6.2 currents. However, in the presence of cytoplasmic ADP (100 microM), SUR2A/Kir6.2 channels became as sensitive to diazoxide as SUR1/Kir6. 2 channels. This effect involved specific interactions between MgADP and SUR, as it required Mg(2+), but not ATP, and was abolished by point mutations in the second nucleotide-binding domain of SUR, which impaired channel activation by MgADP. At the whole-cell level, in cardiomyocytes treated with oligomycin to block mitochondrial function, diazoxide could also activate K(ATP) currents only after cytosolic ADP had been raised by a creatine kinase inhibitor. Thus, ADP serves as a cofactor to define the responsiveness of cardiac K(ATP) channels toward diazoxide. The present demonstration of a pharmacological plasticity of K(ATP) channels identifies a mechanism for the control of channel activity in cardiac cells depending on the cellular ADP levels, which are elevated under ischemia.

Identification and characterization of diadenosine 5',5"'-P1,P2 -diphosphate and diadenosine 5',5"'-P1,P3-triphosphate in human myocardial tissue

We examined whether human cardiac tissue contains diadenosine polyphosphates and investigated their physiological role. Extracts from human cardiac tissue from transplant recipients were fractionated by size exclusion-, affinity-, anion exchange- and reversed-phase chromatography. MALDI-MS analysis of two absorbing fractions revealed molecular masses of 676.2 Da and 756.0 Da. The UV spectra of both fractions were identical to that of adenosine. Postsource decay MALDI mass spectrometry indicated that the molecules with a mass of 676.2 Da and 757.0 Da contained AMP and ATP, respectively. As shown by enzymatic cleavage, both molecules consist of two adenosines interconnected by either two or three phosphates in 5'-positions of the riboses. Two substances can be identified as 5',5"'-P1,P2-diphosphate (Ap2A) and 5',5"'-P1, P3-triphosphate (Ap3A). Ap2A and Ap3A, together with ATP and ADP, are stored in myocardial-specific granules in biologically active concentrations. In the isolated perfused rat heart, Ap2A and Ap3A caused dose-dependent coronary vasodilations. In myocardial preparations, Ap2A and Ap3A attenuated the effect of isoproterenol, exerting a negative inotropic effect. The calcium current of guinea pig ventricular myocytes, stimulated by isoproterenol, was also attenuated by Ap2A and Ap3A. The presence of Ap2A and Ap3A in cardiac-specific granules and the actions of these substances on the myocardium and coronary vessels indicate a role for these substances as endogenous modulators of myocardial functions and coronary perfusion.

Phosphotransfer reactions in the regulation of ATP-sensitive K+ channels

ATP-sensitive K+ (K(ATP)) channels are nucleotide-gated channels that couple the metabolic status of a cell with membrane excitability and regulate a number of cellular functions, including hormone secretion and cardioprotection. Although intracellular ATP is the endogenous inhibitor of K(ATP) channels and ADP serves as the channel activator, it is still a matter of debate whether changes in the intracellular concentrations of ATP, ADP, and/or in the ATP/ADP ratio could account for the transition from the ATP-liganded to the ADP-liganded channel state. Here, we overview evidence for the role of cellular phosphotransfer cascades in the regulation of K(ATP) channels. The microenvironment of the K(ATP) channel harbors several phosphotransfer enzymes, including adenylate, creatine, and pyruvate kinases, as well as other glycolytic enzymes that are able to transfer phosphoryls between ATP and ADP in the absence of major changes in cytosolic levels of adenine nucleotides. These phosphotransfer reactions are governed by the metabolic status of a cell, and their phosphotransfer rate closely correlates with K(ATP) channel activity. Adenylate kinase catalysis accelerates the transition from ATP to ADP, leading to K(ATP) channel opening, while phosphotransfers driven by creatine and pyruvate kinases promote ADP to ATP transition and channel closure. Thus, through delivery and removal of adenine nucleotides at the channel site, phosphotransfer reactions could regulate ATP/ADP balance in the immediate vicinity of the channel and thereby the probability of K(ATP) channel opening. In this way, phosphotransfer reactions could provide a transduction mechanism coupling cellular metabolic signals with K(ATP) channel-associated functions.

Characterization of oxytocin actions in guinea-pig isolated uterine artery: the effect of pregnancy

While the contractile effect of oxytocin on uterine artery has been reported, little is known about whether pregnancy affects the responsiveness of this artery to oxytocin. If it does, is it a consequence of changed endothelial function, as has been proposed for some other vasoconstrictors. Furthermore, the receptor subtypes involved in oxytocin action on uterine artery has not been yet determined. Therefore the purposes of this study were to (1) determine the receptor subtypes involved in oxytocin action in non-pregnant and pregnant guinea-pig uterine artery and to (2) determine whether possible changes in uterine artery sensitivity to oxytocin during pregnancy are due to altered endothelial function. Therefore, the effect of oxytocin on non-pregnant and pregnant guinea-pig uterine arterial rings with and without endothelium was investigated. In non-pregnant guinea-pig uterine artery oxytocin induced contraction (pEC50 = 7.63) with greater potency than in pregnant guinea-pig uterine artery (pEC50 = 7.17). Removal of the endothelium did not affect oxytocin-induced contractions, regardless of the pregnancy status. The uterine arteries did not respond to [Thr4, Gly7]oxytocin. In the preparations studied, [d(CH2)5Tyr(Me)2]vasopressin and [d(CH2)5, D-Ile2, Ile4]vasopressin antagonized oxytocin action with the following pKB values ([d(CH2)5Tyr(Me)2]vasopressin versus [d(CH2)5, D-Ile2, Ile4]vasopressin): 8.24 versus 7.29 and 8.11 versus 7.17 for non-pregnant guinea-pig uterine artery with and without endothelium, respectively; 8.39 versus 7.25 and 8.35 versus 7.25 for pregnant guinea-pig uterine artery with and without endothelium, respectively. We suggest that, in uterine arteries, oxytocin induces contraction by activation of vasopressin V1A receptors. The potency of oxytocin in uterine artery is decreased during pregnancy and this is not associated with altered endothelial function.

Ligand-insensitive state of cardiac ATP-sensitive K+ channels. Basis for channel opening

The mechanism by which ATP-sensitive K+ (KATP) channels open in the presence of inhibitory concentrations of ATP remains unknown. Herein, using a four-state kinetic model, we found that the nucleotide diphosphate UDP directed cardiac KATP channels to operate within intraburst transitions. These transitions are not targeted by ATP, nor the structurally unrelated sulfonylurea glyburide, which inhibit channel opening by acting on interburst transitions. Therefore, the channel remained insensitive to ATP and glyburide in the presence of UDP. "Rundown" of channel activity decreased the efficacy with which UDP could direct and maintain the channel to operate within intraburst transitions. Under this condition, the channel was sensitive to inhibition by ATP and glyburide despite the presence of UDP. This behavior of the KATP channel could be accounted for by an allosteric model of ligand-channel interaction. Thus, the response of cardiac KATP channels towards inhibitory ligands is determined by the relative lifetime the channel spends in a ligand-sensitive versus -insensitive state. Interconversion between these two conformational states represents a novel basis for KATP channel opening in the presence of inhibitory concentrations of ATP in a cardiac cell.