Mitogen-activated protein kinases: a new therapeutic target in cardiac pathology

Eukaryotic cells respond to different external stimuli by activation of mechanisms of cell signaling. One of the major systems participating in the transduction of signal from the cell membrane to nuclear and other intracellular targets is the highly conserved mitogen-activated protein kinase (MAPK) superfamily. The members of MAPK family are involved in the regulation of a large variety of cellular processes such as cell growth, differentiation, development, cell cycle, death and survival. Several MAPK subfamilies, each with apparently unique signaling pathway, have been identified in the mammalian myocardium. These cascades differ in their upstream activation sequence and in downstream substrate specifity. Each pathway follows the same conserved three-kinase module consisting of MAPK, MAPK kinase (MAPKK, MKK or MEK), and MAPK kinase kinase (MAPKKK, MEKK). The major groups of MAPKs found in cardiac tissue include the extracellular signal-regulated kinases (ERKs), the stress-activated/c-Jun NH2-terminal kinases (SAPK/JNKs), p38-MAPK, and ERK5/big MAPK 1 (BMK1). The ERKs are strongly activated by mitogenic and growth factors and by physical stress, whereas SAPK/JNKs and p38-MAPK can be activated by various cell stresses, such as hyperosmotic shock, metabolic stress or protein synthesis inhibitors, UV radiation, heat shock, cytokines, and ischemia. Activation of MAPKs family plays a key role in the pathogenesis of various processes in the heart, e.g. myocardial hypertrophy and its transition to heart failure, in ischemic and reperfusion injury, as well in the cardioprotection conferred by ischemia- or pharmacologically-induced preconditioning. The following approaches are currently utilized to elucidate the role of MAPKs in the myocardium: (i) studies of the effects of myocardial processes on the activity of these kinases; (ii) pharmacological modulations of MAPKs activity and evaluation of their impact on the (patho)physiological processes in the heart; (iii) gene targeting or expression of constitutively active and dominant-negative forms of enzymes (adenovirus-mediated gene transfer). This review is focused on the regulatory role of MAPKs in the myocardium, with particular regard to their involvement in pathophysiological processes, such as myocardial hypertrophy and heart failure, ischemia/reperfusion injury, as well as in the mechanisms of cardioprotection. In addition, it summarizes current information on pharmacological modulations of MAPKs activity and their impact on the cardiac response to pathophysiological processes.

Changes in the expression and/or activation of regulatory proteins in rat hearts adapted to chronic hypoxia

Chronic intermittent high altitude (IHA) hypoxia results in long-term adaptation protecting the heart against acute ischemia/reperfusion injury; however, molecular mechanisms of this phenomenon are not completely elucidated so far. The present study was aimed at investigation of a modulating effect of IHA hypoxia on the expression and/or activation of selected regulatory proteins, with particular emphasis on differential responses in the right ventricle (RV) and left ventricle (LV). Adult male Wistar rats were exposed to IHA hypoxia of 7000 m simulated in a hypobaric chamber (8 h/day, 25 exposures), and protein contents and activities in myocardial fractions were determined by Western blot analysis. In markedly hypertrophic RV of hypoxic rats, gelatinolytic activity of MMP-2 and protein levels of carbonic anhydrase IX (a marker of hypoxia) were significantly enhanced. Study of mitogen-activated protein kinases (MAPKs) revealed no differences in the contents of total p38-MAPK in both ventricles between the IHA and normoxic control rats, whereas activation of p38-MAPK was decreased in the RV and moderately increased in the LV of IHA rats as compared to controls. Extracellular signal regulated kinase-2 (ERK-2) was partially up-regulated in the RV of IHA rats, and, in addition, expression of acidic fibroblast growth factor (aFGF), a potential activator of ERK cascade, was also significantly increased. In contrast, expression of ERKs in the LV as well as their activities in both ventricles, were not affected by IHA hypoxia. Differential effects of IHA hypoxia on c-Jun-N-terminal protein kinases (JNKs) in the RV and LV were also observed. As compared with the controls, total content of JNKs was increased in the RV of the IHA rats, while expression of JNKs in the LV was down-regulated. IHA hypoxia changed neither total levels of Akt kinase in both RV and LV, nor Akt kinase activity in the RV. However, increased levels of activated phospho-Akt kinase were found in the LV of IHA rats. The results demonstrate that adaptation of rat hearts to chronic IHA hypoxia is associated with disctinct changes in the levels and/or activation of several regulatory proteins in two ventricles. The latter could be attributed to both myocardial remodeling and cardioprotection induced by chronic hypoxia.

Mitogen-activated protein kinases and their role in regulation of cellular processes

Mitogen-activated protein kinases (MAPKs) are evolutionary conserved enzymes connecting cell-surface receptors to critical regulatory targets within cells. The three major MAPK cascades are known, the extracellular signal-regulated protein kinase (ERK) cascade, c-Jun amino-terminal protein kinase/stress-activated protein kinase (JNK/SAPK) cascade and p38-MAPK cascade. This paper is focused on characterization of these MAPK cascades in terms of their distribution and biological role in some pathological processes (apoptosis, hypertrophy) with a special orientation on the role of MAPKs in cardiovascular system during ischemia/reperfusion.

Short-term NO synthase inhibition and the Na+-binding properties of cardiac Na,K-ATPase

It is known that hypertension is accompanied by increased [Na+]i. The functional properties of Na,K-ATPase, which transports the Na+ out and K+ into myocardial cells during the relaxation phase, were investigated in the left ventricle (LV), septum (SV) and the right ventricle (RV) of anesthetized dogs with moderate acute blood pressure elevation elicited by short-term (4-hour) NO synthase inhibition. The NO-insufficiency was induced by administration of an L-arginine analogue, the N(G)-nitro-L-arginine methyl ester (L-NAME). Concerning the function of Na,K-ATPase under the conditions of lowered NO synthesis, we focused our attention to the binding of Na+ to the enzyme molecule. Activation of the enzyme by increasing Na+ concentrations revealed significant changes in both the maximal velocity (Vmax) and the affinity for Na+ (K(Na)) in all investigated heart sections. The Vmax increased by 27% in LV, by 87% in SV and by 58% in RV. The K(Na) value increased by 86% in LV, by 105% in SV and by 93% in RV, indicating an apparent decrease in the sensitivity of the Na+-binding site in the Na,K-ATPase molecule. This apparently decreased pump affinity for Na+ together with the increase of Vmax suggest that, during the short-term inhibition of NO synthesis, the Na,K-ATPase is capable of extruding the excessive Na+ from the myocardial cells more effectively at higher [Na+]i, as compared to the Na,K-ATPase of control animals.

Short-term NO synthase inhibition and the ATP affinity of cardiac Na,K-ATPase

It was previously shown that 4 hours lasting inhibition of nitric oxide synthesis by administration of an L-arginine analogue, the A(G)-nitro-L-arginine methyl ester (L-NAME) changed the affinity of the Na-binding site of Na,K-ATPase thus resulting in elevation of enzyme activity especially at higher concentrations of sodium. Using the same experimental model, we focused our attention in the present study to the question of binding of ATP to the enzyme molecule in the left ventricle (LV), ventricular septum (S) and the right ventricle (RV) of the dog heart. Activation of the enzyme by increasing concentrations of ATP revealed a significant increase of the Vmax only in septum (by 38 %). The K(M) increased significantly in septum (by 40 %) and in left ventricle (by 56 %) indicating an altered sensitivity of the ATP-binding site of Na,K-ATPase in the hearts of NO-deficient animals. The alterations of Na,K-ATPase in its ability to bind and hydrolyze ATP are localized to the tissue surrounding the cavity of the left ventricle.