Phosphorylation Of Cardiac Proteins Research Articles

Receptor-independent modulation of cAMP-dependent protein kinase and protein phosphatase signaling in cardiac myocytes by oxidizing agents

The contraction and relaxation of the heart is controlled by stimulation of the β1-adrenoreceptor (AR) signaling cascade, which leads to activation of cAMP-dependent protein kinase (PKA) and subsequent cardiac protein phosphorylation. Phosphorylation is counteracted by the main cardiac protein phosphatases, PP2A and PP1. Both kinase and phosphatases are sensitive to intramolecular disulfide formation in their catalytic subunits that inhibits their activity. Additionally, intermolecular disulfide formation between PKA type I regulatory subunits (PKA-RI) has been described to enhance PKA's affinity for protein kinase A anchoring proteins, which alters its subcellular distribution. Nitroxyl donors have been shown to affect contractility and relaxation, but the mechanistic basis for this effect is unclear. The present study investigates the impact of several nitroxyl donors and the thiol-oxidizing agent diamide on cardiac myocyte protein phosphorylation and oxidation. Although all tested compounds equally induced intermolecular disulfide formation in PKA-RI, only 1-nitrosocyclohexalycetate (NCA) and diamide induced reproducible protein phosphorylation. Phosphorylation occurred independently of β1-AR activation, but was abolished after pharmacological PKA inhibition and thus potentially attributable to increased PKA activity. NCA treatment of cardiac myocytes induced translocation of PKA and phosphatases to the myofilament compartment as shown by fractionation, immunofluorescence, and proximity ligation assays. Assessment of kinase and phosphatase activity within the myofilament fraction of cardiac myocytes after exposure to NCA revealed activation of PKA and inhibition of phosphatase activity thus explaining the increase in phosphorylation. The data suggest that the NCA-mediated effect on cardiac myocyte protein phosphorylation orchestrates alterations in the kinase/phosphatase balance.

Divergent off-target effects of RSK N-terminal and C-terminal kinase inhibitors in cardiac myocytes

P90 ribosomal S6 kinases (RSK) are ubiquitously expressed and regulate responses to neurohumoral stimulation. To study the role of RSK signalling on cardiac myocyte function and protein phosphorylation, pharmacological RSK inhibitors were tested. Here, the ATP competitive N-terminal kinase domain-targeting compounds D1870 and SL0101 and the allosteric C-terminal kinase domain-targeting FMK were evaluated regarding their ability to modulate cardiac myocyte protein phosphorylation. Exposure to D1870 and SL0101 significantly enhanced phospholamban (PLN) Ser16 and cardiac troponin I (cTnI) Ser22/23 phosphorylation in response to D1870 and SL0101 upon exposure to phenylephrine (PE) that activates RSK. In contrast, FMK pretreatment significantly reduced phosphorylation of both proteins in response to PE. D1870-mediated enhancement of PLN Ser16 phosphorylation was also observed after exposure to isoprenaline or noradrenaline (NA) stimuli that do not activate RSK. Inhibition of β-adrenoceptors by atenolol or cAMP-dependent protein kinase (PKA) by H89 prevented the D1870-mediated increase in PLN phosphorylation, suggesting that PKA is the kinase responsible for the observed phosphorylation. Assessment of changes in cAMP formation by FRET measurements revealed increased cAMP formation in vicinity to PLN after exposure to D1870 and SL0101. D1870 inhibited phosphodiesterase activity similarly as established PDE inhibitors rolipram or 3-isobutyl-1-methylxanthine. Assessment of catecholamine-mediated force development in rat ventricular muscle strips revealed significantly reduced EC50 for NA after D1870 pretreatment (DMSO/NA: 2.33 μmol/L vs. D1870/NA: 1.30 μmol/L). The data reveal enhanced cardiac protein phosphorylation by D1870 and SL0101 that was not detectable in response to FMK. This disparate effect might be attributed to off-target inhibition of PDEs with impact on muscle function as demonstrated for D1870.

Cardiac phosphoproteomics during remote ischemic preconditioning: a role for the sarcomeric Z-disk proteins.

Remote ischemic preconditioning (RIPC) induced by brief ischemia/reperfusion cycles of remote organ (e.g., limb) is cardioprotective. The myocardial cellular changes during RIPC responsible for this phenomenon are not currently known. The aim of this work was to identify the activation by phosphorylation of cardiac proteins following RIPC. To achieve our aim we used isobaric tandem mass tagging (TMT) and reverse phase nanoliquid chromatography tandem spectrometry using a Linear Trap Quadropole (LTQ) Orbitrap Velos mass spectrometer. Male C57/Bl6 mice were anesthetized by an intraperitoneal injection of Tribromoethanol. A cuff was placed around the hind limb and inflated at 200 mmHg to prevent blood flow as confirmed by Laser Doppler Flowmetry. RIPC was induced by 4 cycles of 5 min of limb ischemia followed by 5 min of reperfusion. Hearts were extracted for phosphoproteomics. We identified approximately 30 phosphoproteins that were differentially expressed in response to RIPC protocol. The levels of several phosphoproteins in the Z-disk of the sarcomere including phospho-myozenin-2 were significantly higher than control. This study describes and validates a novel approach to monitor the changes in the cardiac phosphoproteome following the cardioprotective intervention of RIPC and prior to index ischemia. The increased level of phosphorylated sarcomeric proteins suggests they may have a role in cardiac signaling during RIPC.

Pulmonary exposure of rats to ultrafine titanium dioxide enhances cardiac protein phosphorylation and substance P synthesis in nodose ganglia

The inhalation of engineered nanoparticles stimulates the development of atherosclerosis and impairs vascular function. However, the cardiac effects of inhaled engineered nanoparticles are unknown. Here, we investigate the effects of ultrafine titanium dioxide (UFTiO2) on the heart, and we define the possible mechanisms underlying the measured effects. Pulmonary exposure of rats to UFTiO2 increased the phosphorylation levels of p38 mitogen-activated protein kinase and cardiac troponin I, but not Akt, in the heart and substance P synthesis in nodose ganglia. Circulatory levels of pro-inflammatory cytokines, and blood cell counts and differentials were not significantly changed after pulmonary exposure. Separately, the incubation of cardiac myocytes isolated from naïve adult rat hearts in vitro with UFTiO2 did not alter the phosphorylation status of the same cardiac proteins. In conclusion, the inhalation of UFTiO2 enhanced the phosphorylation levels of cardiac proteins. Such responses are likely independent of systemic inflammation, but may involve a lung-neuron-regulated pathway.

Calmodulin Dependent Protein Kinase II (CaMKII) Interacts with and Phosphorylates Cardiac Troponin and Tropomyosin

Protein phosphorylation is a major regulatory mechanism for contractility and rate of relaxation in the heart. The cAMP-dependent protein kinase A (PKA) and the calmodulin-dependent protein kinase II (CaMKII) are well established modulators of cardiac protein phosphorylation. PKA and CaMKII phosphorylate a set of key regulatory proteins that control the excitation contraction coupling cycle. CaMKII phosphorylation of cardiac MyBP-C at Ser282 increases the number of myosin heads simultaneously attaching to actin, enhances the maximum force development and decreases myofibrillar calcium sensitivity accelerating relaxation. Immunoprecipitation of CaMKII delta (the predominant CaMKII form in heart) from murine cardiac tissues showed troponin subunits as potential interacting partners. Using mammalian two-hybrid system CaMKII delta was found to weakly interact with cardiac troponin I and troponin T and moderately with cardiac troponin C. CaMKII binds to and is activated by calmodulin (CaM). The tertiary structure of troponin C is similar to that of CaM and it may be possible that troponin C is binding CaMKII in a similar manner to how CaM binds CaMKII. CaMKII delta was also found to interact with alpha tropomyosin suggesting that CaMKII could readily bind several proteins on the thin filament. In vitro phosphorylation of recombinant cardiac troponin complexes showed that CaMKII readily phosphorylates cardiac troponin T with little phosphorylation of troponin I or C detected. Phosphorylation studies also showed that tropomyosin can also be phosphorylated by CaMKII. These results suggest that CaMKII can interact with several thin filament proteins but phosphorylates only a few of these proteins. Proteins which interact with but are not phosphorylated by CaMKII may serve as anchoring proteins allowing the CaMKII to phosphorylate other proteins nearby.

Sustained Hemodynamic Stress Disrupts Normal Circadian Rhythms in Calcineurin-Dependent Signaling and Protein Phosphorylation in the Heart

Despite overwhelming evidence of the importance of circadian rhythms in cardiovascular health and disease, little is known regarding the circadian regulation of intracellular signaling pathways controlling cardiac function and remodeling. To assess circadian changes in processes dependent on the protein phosphatase calcineurin, relative to changes in phosphorylation of cardiac proteins, in normal, hypertrophic, and failing hearts. We found evidence of large circadian oscillations in calcineurin-dependent activities in the left ventricle of healthy C57BL/6 mice. Calcineurin-dependent transcript levels and nuclear occupancy of the NFAT (nuclear factor of activated T cells) regularly fluctuated as much as 20-fold over the course of a day, peaking in the morning when mice enter a period of rest. Phosphorylation of the protein phosphatase 1 inhibitor 1 (I-1), a direct calcineurin substrate, and phospholamban, an indirect target, oscillated directly out of phase with calcineurin-dependent signaling. Using a surgical model of cardiac pressure overload, we found that although calcineurin-dependent activities were markedly elevated, the circadian pattern of activation was maintained, whereas, oscillations in phospholamban and I-1 phosphorylation were lost. Changes in the expression of fetal gene markers of heart failure did not mirror the rhythm in calcineurin/NFAT activation, suggesting that these may not be direct transcriptional target genes. Cardiac function in mice subjected to pressure overload was significantly lower in the morning than in the evening when assessed by echocardiography. Normal, opposing circadian oscillations in calcineurin-dependent activities and phosphorylation of proteins that regulate contractility are disrupted in heart failure.

Agonistic antibody to the α1-adrenergic receptor mobilizes intracellular calcium and induces phosphorylation of a cardiac 15-kDa protein

Hypertension is a major cause for hypertrophic remodelling of the myocardium. Agonistic autoantibodies to extracellular loops of the alpha(1)-adrenergic receptor (alpha(1)-AR) have been identified in patients with arterial hypertension. However, intracellular reactions elicited by these agonistic antibodies remain elusive. An anti-peptide antibody (anti-alpha(1)) was generated against the second extracellular loop of the alpha(1)-AR that bound to its peptide epitope with high affinity (K (D) approximately 50 nM). We studied anti-alpha(1) effects on intracellular calcium (Ca(i)), a key factor in cellular remodelling, and receptor-mediated cardiac protein phosphorylation. Anti-alpha(1) induced pronounced but transient increases in Ca(i) in CHO cells expressing the human alpha(1)-AR (CHO-alpha(1)) and in neonatal cardiomyocytes. Preincubation experiments failed to demonstrate a tonic effect of anti-alpha(1) on Ca(i). However, preincubation with the antibody attenuated the effect of the alpha(1)-AR antagonist prazosin. In neonatal cardiomyocytes anti-alpha(1) induced a robust phosphorylation of a 15-kDa protein that is involved in alpha(1)-AR signalling. Our data support the notion that elevation of Ca(i) is a general feature of agonistic antibodies' action and constitute an important pathogenic component of hypertension-associated autoantibodies. Furthermore, we suggest that agonistic antibodies to the alpha(1)-AR contribute to hypertrophic remodelling of cardiac myocytes, and that the cardiac 15-kDa protein is a relevant downstream target of their action.

Evidence for calcineurin-mediated regulation of SERCA 2a activity in human myocardium.

Compromised SERCA 2a activity is a key malfunction leading to the Ca(2+) cycling alterations in failing human myocardium. SERCA 2a activity is regulated by the Ca(2+)/calmodulin-dependent protein kinase (CaM-kinase) but alterations of the CaM-kinase pathway regarding SERCA 2a in heart failure are unresolved. Therefore we investigated the CaM-kinase and phosphatase calcineurin mediated regulation of SERCA 2a in failing and non-failing human myocardium. We studied human myocardial preparations from explanted hearts from non-failing organ donors (NF, n=8) and from patients with terminal heart failure undergoing cardiac transplantation (dilated cardiomyopathy, DCM, n=8). SERCA 2a activity was determined using a NADH-coupled enzyme assay [expressed in nmol ATP/(mg protein x min)] and by(45)Ca(2+) uptake. Protein expression of SERCA 2a, phospholamban, calsequestrin and calcineurin was assessed by Western blotting (expressed as densitometric units/microg protein); phosphorylation of cardiac proteins was detected with specific phospho-antibodies for phospholamban at threonine-17 (PT17) or by incorporation of [gamma -(32)P] (expressed as pmol(32)P/mg). Maximal(45)Ca(2+) uptake (in pmol/mg/min) (NF: 3402+/-174; DCM: 2488+/-189) and maximal SERCA 2a activity were reduced in DCM compared to NF (V(max): NF: 125+/-9; DCM: 98+/-5). The V(max) reduction could be mimicked by calcineurin in vitro in NF (NF(control): 72.1+/-3.7; NF(+calcineurin): 49.8+/-2.9) and restored in DCM by CaM-kinase in vitro (DCM(control): 98+/-5; DCM(+CaM-kinase): 120+/-6). Protein expression of SERCA 2a, phospholamban and calsequestrin remained similar, but calcineurin expression was significantly increased in failing human hearts (NF: 11.6+/-1.5 v DCM: 17.1+/-1.6). Although the capacity of endogenous CaM-kinase to phosphorylate PT17 was significantly higher in DCM (DCM(control): 128+/-36; DCM(+endogenous CaM-kinase): 205+/-20) compared to NF myocardium (NF(control): 273+/-37; NF(+endogenous CaM-kinase): 254+/-31), net phosphorylation at threonine-17 phospholamban was significantly lower in DCM (DCM 130+/-11 v NF 170+/-11). A calcineurin-dependent dephosphorylation of phospholamban could be mimicked in vitro by incubation of NF preparations with calcineurin (NF(control) 80.7+/-4.4 v NF(+calcineurin) 30.7+/-4.1, P<0.05). In human myocardium, the V(max) of SERCA 2a and the phosphorylation of phospholamban is modulated by CaM-kinase and calcineurin, at least in vitro. In failing human myocardium, despite increased CaM-kinase activity, calcineurin dephosphorylation leads to decreased net phosphorylation of threonine-17 phospholamban in vivo. Increased calcineurin activity contributes to the impaired V(max) of SERCA 2a in failing human myocardium and the disorder in Ca(2+)-handling in heart failure.

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.

Protein phosphorylation and cardiac function: cholinergic-adrenergic interaction.

The muscarinic inhibition of cyclic AMP (cAMP) mediated contractile effects may not only be the result of a reduction in cAMP levels, but may also involve cAMP linked protein phosphorylation in the heart. To show that a cholinergic agonist may antagonise the effects of agents that induce the cAMP signalling sequence at the level of protein phosphorylation, the effect of carbachol was investigated on the in vivo phosphorylation of phospholamban, troponin I, and a 15 kDa protein during positive inotropic intervention with isoprenaline, forskolin, and a phosphodiesterase inhibitor in the isolated rat heart. The contractile activity of the heart was monitored during drug administration, and the hearts were freeze clamped for assay of protein phosphorylation and metabolites [cAMP, cyclic guanosine monophosphase (cGMP), cAMP dependent protein kinase (cA-PK), and phosphorylase a]. For estimation of the amount of in vivo phosphorylation, a "back phosphorylation" method was used. Carbachol attenuated in vivo phosphorylation, induced by cAMP increasing drugs, of all the phosphoproteins studied, whereas the accumulation of cAMP was not affected or was decreased to 61% after exposure to isoprenaline alone. In addition, the cAMP mediated phosphorylase a activation was completely reversed by carbachol. similarly, sodium nitroprusside reduced phosphorylation as well as the contractile force of the investigated phosphoproteins. These data confirm the reduction of cAMP mediated cardiac protein phosphorylation by carbachol. The cholinergic antagonism may primarily involve cellular target systems distal to sites of cAMP generation. The observed similarity in the cardiac response to carbachol and sodium nitroprusside indicates a role of cGMP in the antiadrenergic action of carbachol in the myocardium.

Changes in cyclic AMP dependent protein kinase and active stiffness in the rat volume overload model of heart hypertrophy

The aim was to clarify the role of cyclic AMP dependent protein kinase (PKA) and changes in mechanical heart function during development of cardiac hypertrophy induced by volume overload. Protein and DNA contents, PKA activity, and peak systolic stress-strain relationships in hearts from animals submitted to aortocaval shunt were assessed as a function of time. Sham operated (control) rats were used as controls. Heart weight to body weight ratio and cardiac protein content per heart increased from d 7 (p < 0.005 and p < 0.01, respectively) reaching their highest values by d 56; the same occurred with cardiac DNA content. PKA activity.g-1 tissue in soluble extracts of hearts from rats with aortocaval shunt increased by 2.7-fold on d 2 (p < 0.005), reached a ninefold peak increase by d 7 (p < 0.0001) and declined to fourfold by d 56 with respect to control values. The end peak systolic stress-strain relation slopes were: control, 368(SEM 14) g.cm-2 (n = 16); aortocaval shunt values: 2 d, 514(28) g.cm-2 (n = 6); 7 d, 579(10) g.cm-2 (n = 7); and 56 d, 554(28) g.cm-2 (n = 7). The force generating capacity at 0% strain was also significantly higher in the shunt groups as compared to sham operated controls (p < 0.01). Early activation of PKA was also confirmed through endogenous cardiac protein phosphorylation. SDS-PAGE gel electrophoretogram and autoradiography showed more heavily phosphorylated bands in aortocaval shunt hearts than in the control group. PKA activity and the slope of systolic stress-strain regression line followed a similar trend throughout the study, with an early increase in both variables by d 2 in the shunt group, reaching a peak at d 7, and decreasing thereafter but remaining higher than in controls. PKA activity appears to be related to increased force generating capacity rather than to hypertrophy or increased cardiac protein content. Thus PKA activation is an early biochemical event after aortocaval shunt, followed later by cardiac hypertrophy. Changes in PKA activity showed a similar trend to mechanical heart function over time. These findings help to explain the changes in the mechanical properties of the heart preceding the development of cardiac hypertrophy in the rat model of volume overload.

Evidence for physiological functions of protein phosphatases in the heart: evaluation with okadaic acid.

Okadaic acid exerts a positive inotropic effect in cardiac preparations. We studied whether the positive inotropic effect of okadaic acid in cardiac preparations could be due to phosphatase inhibition and whether this inhibition affects the phosphorylation of cardiac proteins. In papillary muscles from guinea pigs, 30 microM okadaic acid increased force of contraction to 175% of predrug value. In isolated guinea pig ventricular cardiomyocytes, okadaic acid augmented single Ca(2+)-channel currents by enhancing channel availability. In homogenates from ventricles, 1 microM okadaic acid completely inhibited phosphorylase a phosphatase activity. In isolated 32P-labeled ventricular cardiomyocytes, 30 microM okadaic acid increased phosphorylation of phospholamban (PLB) and troponin inhibitor (TnI) to 325 and 284% of control, respectively. Furthermore, 30 microM okadaic acid increased phosphorylation of a hitherto unknown 23-kDa protein to 352% of control. It is concluded that the effects of okadaic acid could be mediated by increasing the phosphorylation state of several proteins including PLB, a 23-kDa protein, and TnI.

Evidence on the participation of the 3',5'-cyclic AMP pathway in the non-genomic action of 1,25-dihydroxy-vitamin D 3 in cardiac muscle

Several studies have suggested that vitamin D plays a role in cardiovascular function. It has been recently shown that in vitro treatment of vitamin D-deficient chick cardiac muscle with physiological concentrations of 1,25-dihydroxy-vitamin D 3 (1,25(OH) 2D 3) induces a rapid (1–10 min) increase of tissue 45Ca uptake which can be suppressed by Ca channel blockers. The hormone simultaneously stimulated heart microsomal membrane protein phosphorylation. Experiments were performed to investigate the existence of a relationship between these changes and to obtain information about the mechanism involved in 1,25(OH) 2D 3-induced modifications in cardiac protein phosphorylation. Dibutyryl cyclic AMP (10 μM) and forskolin (10 μM), known activators of the cAMP pathway, produced time courses of changes in 45Ca uptake by chick heart tissue similar to 1,25(OH) 2D 3 (10 −10 M). Analogously to the hormone, the effects of both compounds were abolished by nifedipine (30 μM) and verapamil (10 μM). In agreement with these observations, 1,25(OH) 2D 3 significantly increased (34–70%) heart muscle cAMP levels within 1–10 min of treatment. In addition, 1,25(OH) 2D 3 and forskolin caused similar changes in cardiac microsomal membrane protein phosphorylation (e.g. stimulation in 43 kDa and 55 kDa proteins). These changes were also evidenced by direct exposure of isolated heart microsomes to 1,25(OH) 2D 3, suggesting a direct membrane action of the hormone. The fast effects of 1,25(OH) 2D 3 on dihydropyridine-sensitive cardiac muscle Ca uptake could be reproduced in primary-cultured myocytes isolated from chick embryonic heart. Furthermore, the effects of the hormone could be suppressed by a specific protein kinase A inhibitor. These results suggest that 1,25(OH) 2D 3 affects heart cell calcium metabolism through regulation of Ca channel activity mediated by the cAMP pathway.

Substrate proteins for calmodulin-sensitive and phospholipid-sensitive Ca2+-dependent protein kinases in heart, and inhibition of their phosphorylation by palmitoylcarnitine.

At least two substrate proteins for phospholipid-sensitive Ca2+-dependent protein kinase and at least six substrates for calmodulin-sensitive Ca2+-dependent protein kinase were identified in the cytosol of the guinea pig heart. In the particulate subfractions enriched in nuclei, mitochondria, microsome, or plasma membrane, no substrates for the phospholipid-sensitive enzyme were demonstrated but at least four substrates for the calmodulin-sensitive enzyme were identified. The present studies suggest that phospholipid, acting independently of calmodulin, is likely to be involved in the regulation of Ca2+-dependent protein phosphorylation in the heart. Phosphorylation of endogenous substrates for the two enzyme systems was effectively inhibited by palmitoylcarnitine. When histone was used as exogenous substrate, the carnitine ester inhibited the cardiac phospholipid-sensitive Ca2+-dependent protein kinase but not the cardiac cyclic AMP-dependent and cyclic GMP-dependent protein kinases. It is suggested that inhibition of the Ca2+-dependent phosphorylation of cardiac proteins, regulated by either phospholipid or calmodulin, is probably related in part to the great increase in this fatty acid metabolic intermediate in the ischemic heart.