cGMP signalling in pre- and post-conditioning: the role of mitochondria

Much of cell death from ischaemia/reperfusion in heart and other tissues is generally thought to arise from mitochondrial permeability transition (MPT) in the first minutes of reperfusion. In ischaemic pre-conditioning, agonist binding to G(i) protein-coupled receptors prior to ischaemia triggers a signalling cascade that protects the heart from MPT. We believe that the cytosolic component of this trigger pathway terminates in activation of guanylyl cyclase resulting in increased production of cGMP and subsequent activation of protein kinase G (PKG). PKG phosphorylates a protein on the mitochondrial outer membrane (MOM), which then causes the mitochondrial K(ATP) channel (mitoK(ATP)) on the mitochondrial inner membrane to open, leading to increased production of reactive oxygen species (ROS) by the mitochondria. This implies that the protective signal is somehow transmitted from the MOM to its inner membrane. This is accomplished by a series of intermembrane signalling steps that includes protein kinase C (PKCepsilon) activation. The resulting ROS then activate a second PKC pool which, through another signal transduction pathway termed the mediator pathway, causes inhibition of MPT and reduction in cell death.

Association of PI3K-Akt signaling pathway with digitalis-induced hypertrophy of cardiac myocytes

Our previous studies on cardiac myocytes showed that positive inotropic concentrations of the digitalis drug ouabain activated signaling pathways linked to Na(+)-K(+)-ATPase through Src and epidermal growth factor receptor (EGFR) and led to myocyte hypertrophy. In view of the known involvement of phosphatidylinositol 3-kinase (PI3K)-Akt pathways in cardiac hypertrophy, the aim of the present study was to determine whether these pathways are also linked to cardiac Na(+)-K(+)-ATPase and, if so, to assess their role in ouabain-induced myocyte growth. In a dose- and time-dependent manner, ouabain activated Akt and phosphorylation of its substrates mammalian target of rapamycin and glycogen synthase kinase in neonatal rat cardiac myocytes. Akt activation by ouabain was sensitive to PI3K inhibitors and was also noted in adult myocytes and isolated hearts. Ouabain caused a transient increase of phosphatidylinositol 3,4,5-trisphosphate content of neonatal myocytes, activated class IA, but not class IB, PI3K, and increased coimmunoprecipitation of the alpha-subunit of Na(+)-K(+)-ATPase with the p85 subunit of class IA PI3K. Ouabain-induced activation of ERK1/2 was prevented by Src, EGFR, and MEK inhibitors, but not by PI3K inhibitors. Activation of Akt by ouabain, however, was sensitive to inhibitors of PI3K and Src, but not to inhibitors of EGFR and MEK. Similarly, ouabain-induced myocyte hypertrophy was prevented by PI3K and Src inhibitors, but not by an EGFR inhibitor. These findings 1) establish the linkage of the class IA PI3K-Akt pathway to Na(+)-K(+)-ATPase and the essential role of this linkage to ouabain-induced myocyte hypertrophy and 2) suggest cross talk between these PI3K-Akt pathways and the signaling cascades previously identified to be associated with cardiac Na(+)-K(+)-ATPase.

Protective effect of an acute oral loading dose of trimetazidine on myocardial injury following percutaneous coronary intervention

OBJECTIVE

To evaluate the effect of pre-procedural acute oral administration of trimetazidine (TMZ) on percutaneous coronary intervention (PCI)-induced myocardial injury.

DESIGN

Single-centre, prospective, randomised evaluation study.

SETTING

Patients with stable angina pectoris and single-vessel disease undergoing PCI.

PATIENTS

582 patients were prospectively randomised. Patients who underwent more than one inflation during PCI were excluded, resulting in 266 patients randomly assigned to 2 groups.

INTERVENTIONS

Patients were randomly assigned to receive or not an acute loading dose of 60 mg of TMZ prior to intervention.

MAIN OUTCOME

The frequency and the increase in the level of cardiac troponin Ic (cTnI) after successful PCI. cTnI levels were measured before and 6, 12, 18 and 24 h after PCI.

RESULTS

136 patients were assigned to the TMZ group and 130 to the control group. Although no statistically significant difference was observed in the frequency of cTnI increase between the two groups, post-procedural cTnI levels were significantly reduced in the TMZ group at all time points (6 h: mean (SD) 4.2 (0.8) vs 1.7 (0.2), p<0.001; 12 h: 5.5 (1.5) vs 2.3 (0.4), p<0.001; 18 h: 9 (2.3) vs 3 (0.5), p<0.001; and 24 h: 3.2 (1.2) vs 1 (0.5), p<0.001). Moreover, the total amount of cTnI released after PCI, as assessed by the area under the curve of serial measurement, was significantly reduced in the TMZ group (p<0.05).

CONCLUSION

Pre-procedural acute oral TMZ administration significantly reduces PCI-induced myocardial infarction.

The Na,K-ATPase receptor complex: its organization and membership

A major difference between the Na,K-ATPase ion pump and other P-type ATPases is its ability to bind cardiotonic steroids such as ouabain. Na,K-ATPase also interacts with many membrane and cytosolic proteins. In addition to their role in Na,K-ATPase regulation, it became apparent that some of the newly identified interactions are capable of organizing the Na,K-ATPase into various signaling complexes. This new function confers a ligand-like effect to cardiotonic steroids on cellular signal transduction. This article reviews these new developments and provides a comparison of Na,K-ATPase-mediated signal transduction with other receptors and ion transporters.

Involvement of Na+/K+-ATPase in hydrogen peroxide-induced hypertrophy in cardiac myocytes

We have shown that increased production of reactive oxygen species (ROS) was required for ouabain-induced hypertrophy in cultured cardiac myocytes. In the present study we assessed whether long-term exposure of myocytes to nontoxic ROS stress alone is sufficient to induce hypertrophy. A moderate amount of H2O2 was continuously generated in culture media by glucose oxidase. This resulted in a steady increase in intracellular ROS in cultured cardiac myocytes for at least 12 h. Such sustained, but not transient, increase in intracellular ROS at a level comparable to that induced by ouabain was sufficient to stimulate protein synthesis, increase cell size, and change the expression of several hypertrophic marker genes. Like ouabain, glucose oxidase increased intracellular Ca2+ and activated extracellular signal-regulated kinases 1 and 2 (ERK1/2). These effects of glucose oxidase were additive to ouabain-induced cellular changes. Furthermore, glucose oxidase stimulated endocytosis of the plasma membrane Na+/K+-ATPase, resulting in significant inhibition of sodium pump activity. While inhibition of ERK1/2 abolished glucose oxidase-induced increases in protein synthesis, chelating intracellular Ca2+ by BAPTA-AM showed no effect. These results, taken together with our prior observations, suggest that ROS may cross talk with Na+/K+-ATPase, leading to the activation of hypertrophic pathways in cardiac myocytes.

Ouabain protects rat hearts against ischemia-reperfusion injury via pathway involving src kinase, mitoKATP, and ROS

We showed recently that mitochondrial ATP-dependent K(+) channel (mitoK(ATP)) opening is required for the inotropic response to ouabain. Because mitoK(ATP) opening is also required for most forms of cardioprotection, we investigated whether exposure to ouabain was cardioprotective. We also began to map the signaling pathways linking ouabain binding to Na(+)-K(+)-ATPase with the opening of mitoK(ATP). In Langendorff-perfused rat hearts, 10-80 microM ouabain given before the onset of ischemia resulted in cardioprotection against ischemia-reperfusion injury, as documented by an improved recovery of contractile function and a reduction of infarct size. In skinned cardiac fibers, a ouabain-induced protection of mitochondrial outer membrane integrity, adenine nucleotide compartmentation, and energy transfer efficiency was evidenced by a decreased release of cytochrome c and preserved half-saturation constant of respiration for ADP and adenine nucleotide translocase-mitochondrial creatine kinase coupling, respectively. Ouabain-induced positive inotropy was dose dependent over the range studied, whereas ouabain-induced cardioprotection was maximal at the lowest dose tested. Compared with bradykinin (BK)-induced preconditioning, ouabain was equally efficient. However, the two ligands clearly diverge in the intracellular steps leading to mitoK(ATP) opening from their respective receptors. Thus BK-induced cardioprotection was blocked by inhibitors of cGMP-dependent protein kinase (PKG) or guanylyl cyclase (GC), whereas ouabain-induced protection was not blocked by either agent. Interestingly, however, ouabain-induced inotropy appears to require PKG and GC. Thus 5-hydroxydecanoate (a selective mitoK(ATP) inhibitor), N-(2-mercaptopropionyl)glycine (MPG; a reactive oxygen species scavenger), ODQ (a GC inhibitor), PP2 (a src kinase inhibitor), and KT-5823 (a PKG inhibitor) abolished preconditioning by BK and blocked the inotropic response to ouabain. However, only PP2, 5-HD, and MPG blocked ouabain-induced cardioprotection.

Ouabain triggers preconditioning through activation of the Na+,K+-ATPase signaling cascade in rat hearts

OBJECTIVE

Because ouabain activates several pathways that are critical to cardioprotective mechanisms such as ischemic preconditioning, we tested if this digitalis compound could protect the heart against ischemia-reperfusion injury through activation of the Na+,K+-ATPase/c-Src receptor complex.

METHODS AND RESULTS

In Langendorff-perfused rat hearts, a short (4 min) administration of ouabain 10 muM followed by an 8-minute washout before 30 min of global ischemia and reperfusion improved cardiac function, decreased lactate dehydrogenase release and reduced infarct size by 40%. Western blot analysis revealed that ouabain activated the cardioprotective phospholipase Cgamma1/protein kinase Cepsilon (PLC-gamma1/PKCepsilon) pathway. Pre-treatment of the hearts with the Src kinase family inhibitor 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolol[3,4-d]pyrimidine (PP2) blocked not only ouabain-induced activation of PLC-gamma1/PKCepsilon pathway, but also cardiac protection. This protection was also blocked by a PKCepsilon translocation inhibitor peptide (PKCepsilon TIP).

CONCLUSION

Short exposure to a low concentration of ouabain protects the heart against ischemia/reperfusion injury. This effect of ouabain on the heart is most likely due to the activation of the Na+,K+-ATPase/c-Src receptor complex and subsequent stimulation of key mediators of preconditioning, namely PLC-gamma1 and PKCepsilon.

Regions conferring isoform-specific function in the catalytic subunit of the Na,K-pump

The Na,K-pump (i.e., Na,K-ATPase) is critical for maintaining the ionic gradients across the plasma membranes of animal cells. Its component subunits are expressed in multiple forms, but the physiological relevance of this subunit diversity remains unknown. The primary contributor to overall catalysis, the alpha subunit, exists in four isoforms. There are observed kinetic differences among these isoforms, but their subtlety makes them an unlikely basis for physiological significance. Instead, recent work suggests that the major functional distinction among the isoforms is their interaction with regulatory proteins. Moreover, the isoform-specific effects of modulatory agents such as protein kinase C seem to originate within two regions of structural divergence: the amino terminus and eleven residues near the center of the alpha subunit, the isoform-specific region.

The Isoform-Specific Region of the Na,K-ATPase Catalytic Subunit: Role in Enzyme Kinetics and Regulation by Protein Kinase C

Comparisons of the primary structures of the Na,K-ATPase alpha-isoforms reveal the existence of regions of structural divergence, suggesting that they are involved in unique functions. One of these regions is the isoform-specific region (ISR), located near the ATP binding site in the major cytoplasmic loop. To evaluate its importance, we constructed mutants of the rodent wild-type alpha1 and alpha3 isoforms in which the ISR was replaced with irrelevant sequences, i.e., the analogous region from the rat gastric H,K-ATPase catalytic subunit or a region from the human c-myc oncogene. Opossum kidney (OK) cells were transfected with wild-type rat alpha1, alpha3, or their corresponding chimeras and selected in ouabain. Introduction of either mutant produced ouabain-resistant colonies, consistent with functional expression of the chimeric protein and indicating that the ISR is not essential for overall Na,K-ATPase function. The introduced chimeras were then characterized enzymatically by measuring the relative rate of K(+) and Li(+) deocclusions. Results showed that exchanges of both alpha1 and alpha3 ISRs significantly modified the sensitivity for the enzyme to either K(+) or Li(+). Subsequent treatment of the cells with phorbol esters revealed an altered Na,K-ATPase transport in response to protein kinase C activation for the alpha1 chimeras. No changes were observed for the alpha3 isoform, suggesting that it is not sensitive to PKC regulation. These results demonstrated that the ISR plays an important role in ion deocclusion and in the response to PKC (only for the alpha1 isoform).

Structure/function analysis of Na(+)-K(+)-ATPase central isoform-specific region: involvement in PKC regulation

Specific functions served by the various Na(+)-K(+)-ATPase alpha-isoforms are likely to originate in regions of structural divergence within their primary structures. The isoforms are nearly identical, with the exception of the NH(2) terminus and a 10-residue region near the center of each molecule (isoform-specific region; ISR). Although the NH(2) terminus has been clearly identified as a source of isoform functional diversity, other regions seem to be involved. We investigated whether the central ISR could also contribute to isoform variability. We constructed chimeric molecules in which the central ISRs of rat alpha(1)- and alpha(2)-isoforms were exchanged. After stable transfection into opossum kidney cells, the chimeras were characterized for two properties known to differ dramatically among the isoforms: their K(+) deocclusion pattern and their response to PKC activation. Comparisons with rat full-length alpha(1)- and alpha(2)-isoforms expressed under the same conditions suggest an involvement of the central ISR in the response to PKC but not in K(+) deocclusion.