Vijayan Elimban

Subcellular remodelling may induce cardiac dysfunction in congestive heart failure

It is commonly held that cardiac remodelling, represented by changes in muscle mass, size, and shape of the heart, explains the progression of congestive heart failure (CHF). However, this concept does not provide any clear information regarding the development of cardiac dysfunction in CHF. Extensive research has revealed that various subcellular organelles such as the extracellular matrix, sarcolemma, sarcoplasmic reticulum, myofibrils, mitochondria, and nucleus undergo varying degrees of changes in their biochemical composition and molecular structure in CHF. This subcellular remodelling occurs due to alterations in cardiac gene expression as well as activation of different proteases and phospholipases in the failing hearts. Several mechanisms including increased ventricular wall stress, prolonged activation of the renin-angiotensin and sympathetic systems, and oxidative stress have been suggested to account for subcellular remodelling in CHF. Furthermore, subcellular remodelling is associated with changes in cardiomyocyte structure, cation homeostasis as well as functional activities of cation channels and transporters, receptor-mediated signal transduction, Ca(2+)-cycling proteins, contractile and regulatory proteins, and energy production during the development of heart failure. The existing evidence supports the view that subcellular remodelling may result in cardiac dysfunction and thus play a critical role in the transition of cardiac hypertrophy to heart failure.

Role of proteases in the pathophysiology of cardiac disease.

Cardiovascular disease is a major cause of death and thus a great deal of effort has been made in salvaging the diseased myocardium. Although various factors have been identified as possible causes of different cardiac diseases such as heart failure and ischemic heart disease, there is a real need to elucidate their role for the better understanding of the cardiac disease pathology and formulation of strategies for developing newer therapeutic interventions. In view of the intimate involvement of different types of proteases in maintaining cellular structure, the role of proteases in various cardiac diseases has become the focus of recent research. Proteases are present in the cytosol as well as are localized in a number of subcellular organelles in the cell. These are known to use extracellular matrix, cytoskeletal, sarcolemmal, sarcoplasmic reticular, mitochondrial and myofibrillar proteins as substrates. Work from different laboratories using a wide variety of techniques has shown that the activation of proteases causes alterations of a number of specific proteins leading to subcellular remodeling and cardiac dysfunction. Inhibition of protease action by different drugs and agents, therefore, has a clinical relevance and is expected to form a part of new treatment paradigm for improving heart function. This review examines the biochemistry and localization of some of the proteases in the cardiac tissue in addition to identification of the sites of action of some protease inhibitors. (Mol Cell Biochem 263: 241–256, 2004)

Paradoxical role of lipid metabolism in heart function and dysfunction

The heart utilizes fatty acids as a substrate in preference to glucose for the production of energy. The rate of fatty acid uptake and oxidation by heart muscle is controlled by the availability of exogenous fatty acids, the rate of acyl translocation across the mitochondrial membrane and the rate of acetyl-CoA oxidation by the citric acid cycle. Carnitine acyl-CoA tranferase appears to have an important function in coupling the fatty acid activation and acyl transfer to the oxidative phosphorylation. Activated fatty acids are also utilized for the synthesis of triglycerides and membrane phospholipids in the myocardium. The inhibition of long chain acyl-carnitine transferase I reduces the oxidation of fatty acids and promotes the synthesis of lipids in the myocardium. Accumulation of fatty acids and their metabolites such as long chain acyl-CoA and long chain acyl-carnitine has been associated with cardiac dysfunction and cell damage in both ischemic and diabetic hearts. Alterations in the composition of membrane phospholipids are also consiered to change the activities of various membrane bound enzymes and subsequently heart function under different pathophysiological conditions. Chronic diabetes was found to be associated with increased plasma lipids, subcellular defects and cardiac dysfunction. Lowering the plasma lipids or reducing the oxidation of fatty acids by agents such as etomoxir, an inhibitor of palmitoylcarnitine transferase I was found to promote glucose utilization and remodel the subcellular membranous organelles in the heart. The crucial role of fatty acids in membrane phospholipids for the maintenance of structural integrity and production of energy for cardiac contractile activity as well as the toxic effects of fatty acids and their long chain acyl-derivatives support the concept of ‘lipid paradox’ in the myocardium. (Mol Cell Biochem 116:3–9, 1992)

Cardiac remodeling and subcellular defects in heart failure due to myocardial infarction and aging

Although several risk factors including hypertension, cardiac hypertrophy, coronary artery disease, and diabetes are known to result in heart failure, elderly subjects are more susceptible to myocardial infarction and more likely to develop heart failure. This article is intended to discuss that cardiac dysfunction in hearts failing due to myocardial infarction and aging is associated with cardiac remodeling and defects in the subcellular organelles such as sarcolemma (SL), sarcoplasmic reticulum (SR), and myofibrils. Despite some differences in the pattern of heart failure due to myocardial infarction and aging with respect to their etiology and sequence of events, evidence has been presented to show that subcellular remodeling plays a critical role in the occurrence of intracellular Ca2+-overload and development of cardiac dysfunction in both types of failing heart. In particular, alterations in gene expression for SL and SR proteins induce Ca2+-handling abnormalities in cardiomyocytes, whereas those for myofibrillar proteins impair the interaction of Ca2+ with myofibrils in hearts failing due to myocardial infarction and aging. In addition, different phosphorylation mechanisms, which regulate the activities of Ca2+-cycling proteins in SL and SR membranes as well as Ca2+-binding proteins in myofibrils, become defective in the failing heart. Accordingly, it is suggested that subcellular remodeling involving defects in Ca2+-handling and Ca2+-binding proteins as well as their regulatory mechanisms is intimately associated with cardiac remodeling and heart failure due to myocardial infarction and aging.

Mechanisms of subcellular remodeling in heart failure due to diabetes.

Diabetic cardiomyopathy is not only associated with heart failure but there also occurs a loss of the positive inotropic effect of different agents. It is now becoming clear that cardiac dysfunction in chronic diabetes is intimately involved with Ca2+-handling abnormalities, metabolic defects and impaired sensitivity of myofibrils to Ca2+ in cardiomyocytes. On the other hand, loss of the inotropic effect in diabetic myocardium is elicited by changes in signal transduction mechanisms involving hormone receptors and depressions in phosphorylation of various membrane proteins. Ca2+-handling abnormalities in the diabetic heart occur mainly due to defects in sarcolemmal Na+–K+ ATPase, Na+–Ca2+ exchange, Na+–H+ exchange, Ca2+-channels and Ca2+-pump activities as well as changes in sarcoplasmic reticular Ca2+-uptake and Ca2+-release processes; these alterations may lead to the occurrence of intracellular Ca2+ overload. Metabolic defects due to insulin deficiency or ineffectiveness as well as hormone imbalance in diabetes are primarily associated with a shift in substrate utilization and changes in the oxidation of fatty acids in cardiomyocytes. Mitochondria initially seem to play an adaptive role in serving as a Ca2+ sink, but the excessive utilization of long-chain fatty acids for a prolonged period results in the generation of oxidative stress and impairment of their function in the diabetic heart. In view of the activation of sympathetic nervous system and renin-angiotensin system as well as platelet aggregation, endothelial dysfunction and generation of oxidative stress in diabetes and blockade of their effects have been shown to attenuate subcellular remodeling, metabolic derangements and signal transduction abnormalities in the diabetic heart. On the basis of these observations, it is suggested that oxidative stress and subcellular remodeling due to hormonal imbalance and metabolic defects play a critical role in the genesis of heart failure during the development of diabetic cardiomyopathy.

Mechanism of depression in cardiac sarcolemmal Na+-K+-ATPase by hypochlorous acid

Oxidative stress during pathological conditions such as ischemia-reperfusion is known to promote the formation of hypochlorous acid (HOCl) in the heart and to result in depression of cardiac sarcolemmal (SL) Na+-K+-ATPase activity. In this study, we examined the direct effects of HOCl on SL Na+-K+-ATPase from porcine heart. HOCl decreased SL Na+-K+-ATPase activity in a concentration- and time-dependent manner. Characterization of Na+-K+-ATPase activity in the presence of different concentrations of MgATP revealed a decrease in the maximal velocity ( V max) value, without a change in affinity for MgATP on treatment of SL membranes with 0.1 mM HOCl. The V max value of Na+-K+-ATPase, when determined in the presence of different concentrations of Na+, was also decreased, but affinity for Na+ was increased when treated with HOCl. Formation of acylphosphate by SL Na+-K+-ATPase was not affected by HOCl. Scatchard plot analysis of [3H]ouabain binding data indicated no significant change in the affinity or maximum binding capacity value for ouabain binding following treatment of SL membranes with HOCl. Western blot analysis of Na+-K+-ATPase subunits in HOCl-treated SL membranes showed a decrease (34 ± 9% of control) in the β1-subunit without any change in the α1- or α2-subunits. These data suggest that the HOCl-induced decrease in SL Na+-K+-ATPase activity may be due to a depression in the β1-subunit of the enzyme.

β-adrenergic blockade attenuates cardiac dysfunction and myofibrillar remodelling in congestive heart failure

Although β‐adrenoceptor (β‐AR) blockade is an important mode of therapy for congestive heart failure (CHF), subcellular mechanisms associated with its beneficial effects are not clear. Three weeks after inducing myocardial infarction (MI), rats were treated daily with or without 20 and 75 mg/kg atenolol, a selective β1‐AR antagonist, or propranolol, a non‐selective β‐AR antagonist, for 5 weeks. Sham operated rats served as controls. All animals were assessed haemodynamically and echocardiographically and the left ventricle (LV) was processed for the determination of myofibrillar ATPase activity, α‐ and β‐myosin heavy chain (MHC) isoforms and gene expression as well as cardiac troponin I (cTnI) phosphorylation. Both atenolol and propranolol at 20 and 75 mg/kg doses attenuated cardiac hypertrophy and lung congestion in addition to increasing LV ejection fraction and LV systolic pressure as well as decreasing heart rate, LV end‐diastolic pressure and LV diameters in the infarcted animals. Treatment of infarcted animals with these agents also attenuated the MI‐induced depression in myofibrillar Ca2+‐stimulated ATPase activity and phosphorylated cTnI protein content. The MI‐induced decrease in α‐MHC and increase in β‐MHC protein content were attenuated by both atenolol and propranolol at low and high doses; however, only high dose of propranolol was effective in mitigating changes in the gene expression for α‐MHC and β‐MHC. Our results suggest that improvement of cardiac function by β‐AR blockade in CHF may be associated with attenuation of myofibrillar remodelling.

Subcellular Remodeling and Heart Dysfunction in Cardiac Hypertrophy due to Pressure Overloada

Rats were treated with etomoxir, an inhibitor of palmitoyltransferase‐1, to examine the role of a shift in myocardial metabolism in cardiac hypertrophy. Pressure overload was induced by abdominal aorta banding for 8 weeks. Sham‐operated animals served as control. Left ventricular dysfunction, as reflected by decreased LVDP, +dP/dt, −dP/dt, and elevated LVEDP in the pressure overloaded animals, was improved by treatment with etomoxir. Cardiac hypertrophy in pressure‐overload rats decreased the sarcoplasmic reticular (SR) Ca2+ uptake and Ca2+ release as well as myofibrillar Ca2+‐stimulated ATPase and myosin Ca2+‐ATPase activities; these changes were attenuated by treatment with etomoxir. Steady‐state mRNA levels for α‐ and β‐myosin heavy chains, SR Ca2+‐pump, and protein content of SR Ca2+‐pump were reduced in hypertrophied hearts; these alterations were prevented by etomoxir treatment. The results indicate that modification of changes in myocardial metabolism by etomoxir may prevent remodeling of myofibrils and SR membrane and thereby improve cardiac function in hypertrophied heart.

Diabetes-like action of intermittent fasting on sarcoplasmic reticulum Ca2+-pump ATPase and myosin isoenzymes can be prevented by sucrose

Experimental diabetes results in a reduction of the sarcoplasmic reticulum (SR) Ca2+-stimulated ATPase activity and a redirection of myosin isoenzymes from V1 to V3. Similar, but less pronounced, changes were induced by subjecting rats to intermittent fasting for 6 weeks. Low amounts of sucrose (0.8%) in the drinking water prevented the subcellular changes in fasted rats; however, sucrose neither affected the levels of plasma thyroid hormones nor normalized the reduced body weight. Plasma glucose was lowered without any changes in plasma insulin in the fasted rats receiving sucrose; this suggested an enhanced peripheral glucose utilization. Thus, the signals in the diabetic heart leading to changes in SR and myosin can be mimicked by intermittent fasting and seem to be linked to a shift in fuel utilization by the myocytes.

Sucrose feeding prevents changes in myosin isoenzymes and sarcoplasmic reticulum Ca2+-pump ATPase in pressure-loaded rat heart

Pressure-overload due to banding of the abdominal aorta in rats for 10 weeks resulted in cardiac hypertrophy, redistribution of myosin isoenzymes and reduction in the sarcoplasmic reticulum (SR) Ca2+-stimulated ATPase activity. Administration of sucrose in the drinking water (0.8%, w/v) to rats prevented changes in myosin isoenzymes and SR Ca2+-stimulated ATPase in hypertrophied hearts. This beneficial effect of sucrose feeding with respect to remodeling of the subcellular organelles in the myocardium was not associated with any significant changes in plasma glucose or thyroid hormone levels. It is suggested that the prevention of subcellular changes in the hypertrophied hearts due to sucrose feeding may be due to a shift in fuel utilization by the myocardium.