LncRNA Bigheart stimulates Regulator of calcineurin 1 (Rcan1) expression in an auto-amplification loop that stimulates calcineurin-NFAT signalling in heart failure

Background

Cardiac hypertrophy precedes many heart diseases and understanding its molecular basis remains one of the greatest challenges in cardiovascular medicine. Recent studies highlighted the sporadic involvement of long noncoding RNAs (lncRNAs) in cardiac development and disease but our understanding of lncRNAs in heart failure is still limited.

Results

Expression profiling of lncRNAs in failing mouse hearts revealed dysregulation of “Bigheart”, a novel lncRNA that is evolutionary conserved. Bigheart overexpressed using a serotype 9 adeno-associated virus (AAV) in neonatal rat cardiomyocytes (NRCMs) induced spontaneous hypertrophy, while silencing this lncRNA with specific siRNAs blunted the hypertrophic response in agonist-stimulated cardiomyocytes. GapmeR-mediated silencing of Bigheart prevented transverse aortic constriction (TAC)-induced pathological cardiac remodeling in the mouse in vivo. Mechanistically, analysis of the Bigheart genomic locus revealed several binding sites for the transcription factor nuclear factor of activated T-cells (NFAT), a downstream transcription factor of the pro-hypertrophic calcineurin-NFAT signaling cascade. The sensitivity of Bigheart transcriptional induction for calcineurin-NFAT signalling was further demonstrated by luciferase assays using a Bigheart promoter-luciferase construct. Finally, RNA-sequencing of Gapmer-silenced mouse hearts and chromatin isolation by RNA purification (ChIRP) coupled to mass spectrometry (MS), revealed that Bigheart interacts with heterogeneous nuclear ribonucleoproteins (hnRNPs) and High Mobility Group Box 1 (HMGB1) acts in trans to stimulate the transcription of Regulator of calcineurin 1 (Rcan1), a facilitator of calcineurin-NFAT signaling.

Conclusions

These results indicate that lncRNA Bigheart constitutes a positive feedforward loop in hypertrophic signaling and a promising target to attenuate maladaptive cardiac hypertrophy

Funding Acknowledgement

Type of funding sources: Public Institution(s). Main funding source(s): Maastricht university

P4996The adult heart requires baseline expression of the transcription factor Hand2 to withstand right ventricular pressure overload

Introduction

Heart and neural crest derivatives expressed-2 (Hand2) has been identified as an important embryonic basic helix-loop-helix-transcription factor, with different functions in the development of the first and second heart field, from which the left and right ventricle originate, respectively. Our previous work revealed that Hand2, under conditions of left ventricular (LV) pressure overload, is re-expressed in the adult heart and activates a “fetal gene” program contributing to pathological cardiac remodeling. Ablation of cardiac expression of Hand2 resulted in protection to cardiac stress and attenuated maladaptive remodeling.

Purpose

In this study, we aimed at unraveling the role of Hand2 during cardiac remodeling in response to right ventricular (RV) pressure overload induced by pulmonary artery banding (PAB).

Methods

Hand2F/F and MCM− Hand2F/F mice were treated with tamoxifen (control and knockout, respectively) and subjected to six weeks of RV pressure overload induced by PAB. Echocardiographic and MRI derived hemodynamic parameters, and molecular remodelling were assessed for experimental groups and compared to sham-operated controls (Fig. 1a). RNA sequencing and gene ontology enrichment analysis were performed to compare the dysregulated genes between the pressure overloaded RV of the control and Hand2 knockout mice.

Results

After six weeks of increased pressure load (Fig. 1b), levels of Hand2 increased in the control banded animals but, as expected, remained absent in the knockout hearts (Fig. 1c). In contrast to the what was previously observed for the pressure overloaded LV, in the pressure loaded RV, Hand2 depletion resulted in more severe remodelling and dysfunction as reflected by increased hypertrophic growth, increased RV end-diastolic and end-systolic volumes as well as decreased RV ejection fraction (Fig. 1d–g). In addition, RNA sequencing revealed a distinct set of genes that are dysregulated in the pressure-overloaded RV, compared to the previously described pressure-overloaded LV. These include components of the extracellular matrix structure, collagen assembly and organization and several types of collagens. Genes associated with inflammation response, adhesion and muscle organization were also affected in the RV of the Hand2 KO mice (Fig. 1h).

Figure 1

Conclusion

Cardiac-specific depletion of Hand2 is associated with severe cardiac dysfunction in conditions of RV pressure overload. While inhibiting Hand2 expression can prevent cardiac dysfunction in conditions of LV pressure overload, the same does not hold true for conditions of RV pressure overload. This study highlights the need to better understand the molecular mechanisms driving pathological remodelling of the RV, in contrast to the LV, in order to better diagnose and treat patients with RV or LV failure.

A novel miR-371a-5p-mediated pathway, leading to BAG3 upregulation in cardiomyocytes in response to epinephrine, is lost in Takotsubo cardiomyopathy

Molecular mechanisms protecting cardiomyocytes from stress-induced death, including tension stress, are essential for cardiac physiology and defects in these protective mechanisms can result in pathological alterations. Bcl2-associated athanogene 3 (BAG3) is expressed in cardiomyocytes and is a component of the chaperone-assisted autophagy pathway, essential for homeostasis of mechanically altered cells. BAG3 ablation in mice results in a lethal cardiomyopathy soon after birth and mutations of this gene have been associated with different cardiomyopathies including stress-induced Takotsubo cardiomyopathy (TTC). The pathogenic mechanism leading to TTC has not been defined, but it has been suggested that the heart can be damaged by excessive epinephrine (epi) spillover in the absence of a protective mechanism. The aim of this study was to provide more evidence for a role of BAG3 in the pathogenesis of TTC. Therefore, we sequenced BAG3 gene in 70 TTC patients and in 81 healthy donors with the absence of evaluable cardiovascular disease. Mutations and polymorphisms detected in the BAG3 gene included a frequent nucleotide change g2252c in the BAG3 3′-untranslated region (3′-UTR) of Takotsubo patients (P<0.05), resulting in loss of binding of microRNA-371a-5p (miR-371a-5p) as evidenced by dual-luciferase reporter assays and argonaute RNA-induced silencing complex catalytic component 2/pull-down assays. Moreover, we describe a novel signaling pathway in cardiomyocytes that leads to BAG3 upregulation on exposure to epi through an ERK-dependent upregulation of miR-371a-5p. In conclusion, the presence of a g2252c polymorphism in the BAG3 3′-UTR determines loss of miR-371a-5p binding and results in an altered response to epi, potentially representing a new molecular mechanism that contributes to TTC pathogenesis.

MicroRNA regulation in cardiovascular disease

The molecular biology dogma that DNA replicates its genetic information within nucleotide sequences and transcribes it to RNA where it codes for the generation of mRNA, failed to consider a significant part of the genetic code. Although it has been generally assumed that most genetic information is executed by proteins, recent evidence suggests that the majority of the genomes of mammals and other complex organisms is transcribed into non-coding RNA (ncRNA), many of which are alternatively spliced and/or processed into smaller functional RNA molecules. ncRNAs are predominantly involved in processes that require highly specific nucleic acid recognition, revealing a, so far hidden, layer of internal signals that control various levels of gene expression in developmental and (patho)physiological processes. MicroRNAs (miRNAs) are a large class of evolutionary conserved, small ncRNAs, typically 18 to 24 nucleotides in length, that primarily function at the posttranscriptional level by interacting with the 3' untranslated region (UTR) of specific target mRNAs in a sequence-specific manner. Despite the advances in miRNA discovery, the role of miRNAs in physiological and pathological processes is just rising, revealing their cellular functions in proliferation and differentiation, apoptosis, the stress response and tumorgenesis. MiRNA expression profiling and the manipulation of their expression in cardiac tissue has led to the discovery of regulatory roles for these small ncRNAs during cardiac development and disease, implicating them in regulation of cardiac gene expression. Here we review the basic mechanisms by which cardiovascular miRNAs are regulated in the larger context of cardiogenesis and in cardiovascular disease.

Ischemic-reperfused isolated working mouse hearts: membrane damage and type IIA phospholipase A2

For the murine heart the relationships between ischemia-reperfusion-induced loss of cardiac function, enzyme release, high-energy phosphate (HEP), and membrane phospholipid metabolism are ill-defined. Accordingly, isolated ejecting murine hearts were subjected to varying periods of ischemia, whether or not followed by reperfusion. On reperfusion, hemodynamic function was almost completely restored after 10 min of ischemia [83 +/- 14% recovery of cardiac output (CO)], but was severely depressed after 15 and 20 min of ischemia (40 +/- 24 and 31 +/- 24% recovery of CO, respectively). Reperfusion was associated with partial recovery of HEP stores and enhanced degradation of phospholipids as indicated by the accumulation of fatty acids (FA). Myocardial FA content and enzyme release during reperfusion were correlated (r = 0.70), suggesting that membrane phospholipid degradation and cellular damage are closely related phenomena. To investigate the role of type IIA secretory phospholipase A2 (sPLA2) in this process, hearts from wild-type and sPLA2-deficient mice were subjected to ischemia-reperfusion. Postischemic functional recovery, ATP depletion, enzyme release, and FA accumulation were not significantly different between wild-type and sPLA2- deficient hearts. These findings argue against a prominent role of type IIA sPLA2 in the development of irreversible cell damage in the ischemic-reperfused murine myocardium.

The transcription factors GATA4 and GATA6 regulate cardiomyocyte hypertrophy in vitro and in vivo

The zinc finger-containing transcription factors GATA4 and GATA6 are important regulators of basal and inducible gene expression in cardiac and smooth muscle cell types. Here we demonstrate a direct functional role for GATA4 and GATA6 as regulators of cardiomyocyte hypertrophic growth and gene expression. To model the increase in endogenous GATA4 and GATA6 transcriptional activity that occurs in response to hypertrophic stimulation, each factor was overexpressed in cardiomyocytes using recombinant adenovirus. Overexpression of either GATA4 or GATA6 was sufficient to induce cardiomyocyte hypertrophy characterized by enhanced sarcomeric organization, a greater than 2-fold increase in cell surface area, and a significant increase in total protein accumulation. In vivo, transgenic mice with 2.5-fold overexpression of GATA4 within the adult heart demonstrated a slowly progressing increase in heart to body weight ratio, histological features of cardiomyopathy, and activation of hypertrophy-associated genes, suggesting that GATA factors are sufficient regulators of cardiomyocyte hypertrophy in vitro and in vivo. To evaluate the requirement of GATA factors as downstream transcriptional mediators of hypertrophy, a dominant negative GATA4-engrailed repressor fusion-encoding adenovirus was generated. Expression of GATA4-engrailed blocked GATA4- and GATA6-directed transcriptional responses and agonist-induced cardiomyocyte hypertrophy, demonstrating that cardiac-expressed GATA factors are necessary mediators of this process.

Targeted inhibition of calcineurin attenuates cardiac hypertrophy in vivo

The Ca(2+)-calmodulin-activated Ser/Thr protein phosphatase calcineurin and the downstream transcriptional effectors of calcineurin, nuclear factor of activated T cells, have been implicated in the hypertrophic response of the myocardium. Recently, the calcineurin inhibitory agents cyclosporine A and FK506 have been extensively used to evaluate the importance of this signaling pathway in rodent models of cardiac hypertrophy. However, pharmacologic approaches have rendered equivocal results necessitating more specific or genetic-based inhibitory strategies. In this regard, we have generated Tg mice expressing the calcineurin inhibitory domains of Cain/Cabin-1 and A-kinase anchoring protein 79 specifically in the heart. DeltaCain and DeltaA-kinase-anchoring protein Tg mice demonstrated reduced cardiac calcineurin activity and reduced hypertrophy in response to catecholamine infusion or pressure overload. In a second approach, adenoviral-mediated gene transfer of DeltaCain was performed in the adult rat myocardium to evaluate the effectiveness of an acute intervention and any potential species dependency. DeltaCain adenoviral gene transfer inhibited cardiac calcineurin activity and reduced hypertrophy in response to pressure overload without reducing aortic pressure. These results provide genetic evidence implicating calcineurin as an important mediator of the cardiac hypertrophic response in vivo.

The dual-specificity phosphatase MKP-1 limits the cardiac hypertrophic response in vitro and in vivo

Mitogen-activated protein kinase (MAPK) signaling pathways are important regulators of cell growth, proliferation, and stress responsiveness. A family of dual-specificity MAP kinase phosphatases (MKPs) act as critical counteracting factors that directly regulate the magnitude and duration of p38, c-Jun N-terminal kinase (JNK), and extracellular signal-regulated kinase (ERK) activation. Here we show that constitutive expression of MKP-1 in cultured primary cardiomyocytes using adenovirus-mediated gene transfer blocked the activation of p38, JNK1/2, and ERK1/2 and prevented agonist-induced hypertrophy. Transgenic mice expressing physiological levels of MKP-1 in the heart showed (1) no activation of p38, JNK1/2, or ERK1/2; (2) diminished developmental myocardial growth; and (3) attenuated hypertrophy in response to aortic banding and catecholamine infusion. These results provide further evidence implicating MAPK signaling factors as obligate regulators of cardiac growth and hypertrophy and demonstrate the importance of dual-specificity phosphatases as counterbalancing regulatory factors in the heart.

The MEK1-ERK1/2 signaling pathway promotes compensated cardiac hypertrophy in transgenic mice

Members of the mitogen-activated protein kinase (MAPK) cascade such as extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK) and p38 are implicated as important regulators of cardiomyocyte hypertrophic growth in culture. However, the role that individual MAPK pathways play in vivo has not been extensively evaluated. Here we generated nine transgenic mouse lines with cardiac-restricted expression of an activated MEK1 cDNA in the heart. MEK1 transgenic mice demonstrated concentric hypertrophy without signs of cardiomyopathy or lethality up to 12 months of age. MEK1 transgenic mice showed a dramatic increase in cardiac function, as measured by echocardiography and isolated working heart preparation, without signs of decompensation over time. MEK1 transgenic mice and MEK1 adenovirus-infected neonatal cardiomyocytes each demonstrated ERK1/2, but not p38 or JNK, activation. MEK1 transgenic mice and MEK1 adenovirus-infected cultured cardiomyocytes were also partially resistant to apoptotic stimuli. The results of the present study indicate that the MEK1-ERK1/2 signaling pathway stimulates a physiologic hypertrophy response associated with augmented cardiac function and partial resistance to apoptotsis.

A calcineurin-NFATc3-dependent pathway regulates skeletal muscle differentiation and slow myosin heavy-chain expression

The differentiation and maturation of skeletal muscle cells into functional fibers is coordinated largely by inductive signals which act through discrete intracellular signal transduction pathways. Recently, the calcium-activated phosphatase calcineurin (PP2B) and the family of transcription factors known as NFAT have been implicated in the regulation of myocyte hypertrophy and fiber type specificity. Here we present an analysis of the intracellular mechanisms which underlie myocyte differentiation and fiber type specificity due to an insulinlike growth factor 1 (IGF-1)-calcineurin-NFAT signal transduction pathway. We demonstrate that calcineurin enzymatic activity is transiently increased during the initiation of myogenic differentiation in cultured C2C12 cells and that this increase is associated with NFATc3 nuclear translocation. Adenovirus-mediated gene transfer of an activated calcineurin protein (AdCnA) potentiates C2C12 and Sol8 myocyte differentiation, while adenovirus-mediated gene transfer of noncompetitive calcineurin-inhibitory peptides (cain or DeltaAKAP79) attenuates differentiation. AdCnA infection was also sufficient to rescue myocyte differentiation in an IGF-depleted myoblast cell line. Using 10T1/2 cells, we demonstrate that MyoD-directed myogenesis is dramatically enhanced by either calcineurin or NFATc3 cotransfection, while a calcineurin inhibitory peptide (cain) blocks differentiation. Enhanced myogenic differentiation directed by calcineurin, but not NFATc3, preferentially specifies slow myosin heavy-chain expression, while enhanced differentiation through mitogen-activated protein kinase kinase 6 (MKK6) promotes fast myosin heavy-chain expression. These data indicate that a signaling pathway involving IGF-calcineurin-NFATc3 enhances myogenic differentiation whereas calcineurin acts through other factors to promote the slow fiber type program.

Calcineurin expression, activation, and function in cardiac pressure-overload hypertrophy

BACKGROUND

Vascular hypertension resulting in increased cardiac load is associated with left ventricular hypertrophy and is a leading predicator for progressive heart disease. The molecular signaling pathways that respond to increases in cardiac load are poorly understood. One potential regulator of the hypertrophic response is the calcium-sensitive phosphatase calcineurin.

METHODS AND RESULTS

We showed that calcineurin enzymatic activity is increased 3. 2-fold in the heart in response to pressure-overload hypertrophy induced by abdominal aortic banding in the rat. Western blot analysis further demonstrates that calcineurin A (catalytic subunit) protein content and association with calmodulin are increased in response to pressure-overload hypertrophy. This increase in calcineurin protein content was prevented by administration of the calcineurin inhibitor cyclosporine A (CsA). CsA administration attenuated load-induced cardiac hypertrophy in a dose-dependent manner over a 14-day treatment protocol. CsA administration also partially reversed pressure-overload hypertrophy in aortic-banded rats after 14 days. CsA also attenuated the histological and molecular indexes of pressure-overload hypertrophy.

CONCLUSIONS

These data suggest that calcineurin is an important upstream regulator of load-induced hypertrophy.

Calcineurin promotes protein kinase C and c-Jun NH2-terminal kinase activation in the heart. Cross-talk between cardiac hypertrophic signaling pathways

Multiple intracellular signaling pathways have been shown to regulate the hypertrophic growth of cardiomyocytes. Both necessary and sufficient roles have been described for the mitogen activated protein kinase(1) (MAPK) signaling pathway, specific protein kinase C (PKC) isoforms, and calcineurin. Here we investigate the interdependence between calcineurin, MAPK, and PKC isoforms in regulating cardiomyocyte hypertrophy using three separate approaches. Hearts from hypertrophic calcineurin transgenic mice were characterized for PKC and MAPK activation. Transgenic hearts demonstrated activation of c-Jun NH(2)-terminal kinase (JNK) and extracellular signal-regulated kinase (ERK1/2), but not p38 MAPK factors. Calcineurin transgenic hearts demonstrated increased activation of PKCalpha, beta(1), and theta, but not of epsilon, beta(2), or lambda. In a second approach, cultured cardiomyocytes were infected with a calcineurin adenovirus to induce hypertrophy and the effects of pharmacologic inhibitors or co-infection with a dominant negative adenovirus were examined. Calcineurin-mediated hypertrophy was prevented with PKC inhibitors, Ca(2+) chelation, and attenuated with a dominant negative SEK-1 (MKK4) adenovirus, but inhibitors of ERK or p38 activation had no effect. In a third approach, we examined the activation of MAPK factors and PKC isoforms during the progression of load-induced hypertrophy in aortic banded rats with or without cyclosporine. We determined that inhibition of calcineurin activity with cyclosporine prevented PKCalpha, theta, and JNK activation, but did not affect PKCepsilon, beta, lambda, ERK1/2, or p38 activation. Collectively, these data indicate that calcineurin hypertrophic signaling is interconnected with PKCalpha, theta, and JNK in the heart, while PKCepsilon, beta, lambda, p38, and ERK1/2 are not involved in calcineurin-mediated hypertrophy.

Reversal of cardiac hypertrophy in transgenic disease models by calcineurin inhibition

Heart disease remains one of the leading causes of morbidity and mortality in the industrialized nations of the world. Intense investigation has centered around identifying and manipulating intracellular signaling pathways that direct hypertrophic and myopathic responses in an attempt to intervene in the progression or reverse certain forms of heart disease. We show here that cyclosporin A-mediated inhibition of the calcium-regulated phosphatase, calcineurin (PP2B), reverses cardiac hypertrophy and myopathic dilation in two transgenic mouse models of cardiomyopathy. Reversal was demonstrated by gravimetric analysis, echocardiography, histological analysis, and molecular analysis of hypertrophy-associated gene expression. In contrast, a third mouse model of hypertrophic cardiomyopathy due to activated NFAT3 cardiac-specific expression was not affected by cyclosporin A. These results suggest that calcineurin may function in the long-term maintenance of cardiac hypertrophy or myopathic disease states.

Calcineurin-mediated hypertrophy protects cardiomyocytes from apoptosis in vitro and in vivo: An apoptosis-independent model of dilated heart failure

We have previously shown that the calcium-calmodulin-regulated phosphatase calcineurin (PP2B) is sufficient to induce cardiac hypertrophy that transitions to heart failure in transgenic mice. Given the rapid onset of heart failure in these mice, we hypothesized that calcineurin signaling would stimulate myocardial cell apoptosis. However, utilizing multiple approaches, we determined that calcineurin-mediated hypertrophy protected cardiac myocytes from apoptosis, suggesting a model of heart failure that is independent of apoptosis. Adenovirally mediated gene transfer of a constitutively active calcineurin cDNA (AdCnA) was performed in cultured neonatal rat cardiomyocytes to elucidate the mechanism whereby calcineurin affected myocardial cell viability. AdCnA infection, which induced myocyte hypertrophy and atrial natriuretic factor expression, protected against apoptosis induced by 2-deoxyglucose or staurosporine, as assessed by terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling (TUNEL) labeling, caspase-3 activation, DNA laddering, and cellular morphology. The level of protection conferred by AdCnA was similar to that of adenoviral Bcl-x(L) gene transfer or hypertrophy induced by phenylephrine. In vivo, failing hearts from calcineurin-transgenic mice did not demonstrate increased TUNEL labeling and, in fact, demonstrated a resistance to ischemia/reperfusion-induced apoptosis. We determined that the mechanism whereby calcineurin afforded protection from apoptosis was partially mediated by nuclear factor of activated T cells (NFAT3) signaling and partially by Akt/protein kinase B (PKB) signaling. Although calcineurin activation protected myocytes from apoptosis, inhibition of calcineurin with cyclosporine was not sufficient to induce TUNEL labeling in Gqalpha-transgenic mice or in cultured cardiomyocytes. Collectively, these data identify a calcineurin-dependent mouse model of dilated heart failure that is independent of apoptosis.

Targeted inhibition of calcineurin prevents agonist-induced cardiomyocyte hypertrophy

Cardiac hypertrophy is a major predictor of future morbidity and mortality. Recent investigation has centered around identifying the molecular signaling pathways that regulate cardiac myocyte reactivity with the goal of modulating pathologic hypertrophic programs. One potential regulator of cardiomyocyte hypertrophy is the calcium-sensitive phosphatase calcineurin. We show here that calcineurin enzymatic activity, mRNA, and protein levels are increased in cultured neonatal rat cardiomyocytes by hypertrophic agonists such as angiotensin II, phenylephrine, and 1% fetal bovine serum. This induction of calcineurin activity was associated with an increase in calcineurin Abeta (CnAbeta) mRNA and protein, but not in CnAalpha or CnAgamma. Agonist-dependent increases in calcineurin enzymatic activity were specifically inhibited with an adenovirus expressing a noncompetitive peptide inhibitor of calcineurin known as cain [Lai, M. M., Burnett, P. E., Wolosker, H., Blackshaw, S. & Snyder, S. H. (1998) J. Biol. Chem. 273, 18325-18331]. Targeted inhibition of calcineurin with cain or an adenovirus expressing only the calcineurin inhibitory domain of AKAP79 attenuated cardiomyocyte hypertrophy and atrial natriuretic factor expression in response to angiotensin II, phenylephrine, and 1% fetal bovine serum. These data demonstrate that calcineurin is an important regulator of cardiomyocyte hypertrophy in response to certain agonists and suggest that cyclosporin A and FK506 function to attenuate cardiac hypertrophy by specifically inhibiting calcineurin.

An improved isolated, left ventricular ejecting, murine heart model. Functional and metabolic evaluation

An improved, isolated, left ventricular-ejecting, murine heart model is described and evaluated. Special attention was paid to the design and impedance characteristics of the artificial aortic outflow tract and perfusate composition, which contained glucose (10 mM plus insulin) and pyruvate (1.5 mM) as substrates. Temperature of the isolated perfused hearts was maintained at 38.5 degrees C. During antegrade perfusion (preload 10 mm Hg, afterload 50 mm Hg, 2.5 mM Ca2+) proper design of the aortic outflow tract provided baseline values for cardiac output (CO), left ventricular developed pressure (LVDP) and the maximum first derivative of left ventricular pressure (LV dP/dtmax) of 11.1+/-1.7 ml min-1, 83+/-5 mm Hg and 6283+/-552 mm Hg s-1, respectively, resembling findings in the intact mouse. During 100 min normoxic antegrade perfusion CO declined non-significantly by less than 10%. Varying pre- and afterloads resulted in typical Frank-Starling relationships with maximal CO values of 18.6+/-1.8 ml min-1 at pre- and afterload pressures of 25 and 50 mm Hg, respectively. Left ventricular function curves were constructed at free [Ca2+] of 1.5 and 2.5 mM in the perfusion medium. Significantly higher values for CO, LVDP and LV dP/dtmax and LV dP/dtmin were obtained at 2.5 mM Ca2+ at all loading conditions investigated. Phosphocreatine and creatine levels remained stable throughout the perfusion period. Despite a small but significant decline in tissue ATP content, the sum of adenine nucleotides did not change during the normoxic perfusion period. The tissue content of glycogen increased significantly.

Phospholipase A2-mediated hydrolysis of cardiac phospholipids: the use of molecular and transgenic techniques

Under pathophysiological conditions, like myocardial ischemia and reperfusion, cardiac phospholipid homeostasis is severely disturbed, resulting in a net degradation of phospholipids and the accumulation of degradation products, such as lysophospholipids and (non-esterified) fatty acids. The derangements in phospholipid metabolism are thought to be involved in the sequence of events leading to irreversible myocardial injury. The net degradation of phospholipids as observed during myocardial ischemia may result from increased hydrolysis and/or reduced resynthesis, while during reperfusion hydrolysis is likely to prevail in this net degradation. Several studies indicate that the activation of phospholipases A2 plays an important role in the hydrolysis of phospholipids. In this review current knowledge regarding the potential role of the different types of phospholipases A2 in ischemia and reperfusion-induced damage is being evaluated. Furthermore, it is indicated how recent advances in molecular biological techniques could be helpful in determining whether disturbances in phospholipid metabolism indeed play a crucial role in the transition from reversible to irreversible myocardial ischemia and reperfusion-induced injury, the knowledge of which could be of great therapeutic relevance.

Cloning and cellular distribution of a group II phospholipase A2 expressed in the heart

Phospholipase A2 has been considered to play a role in physiological membrane turnover in cardiac tissue and in the degradation of membrane lipids under pathophysiological conditions, such as ischemia and reperfusion. We report the cloning of a cDNA encoding a member of the Ca2+-dependent, low molecular mass phospholipase A2 (PLA2) present in rat heart. The cDNA predicts a mature protein of 146 amino acid residues including a 21 amino acid sequence at the N-terminal end, which has the features characteristic of eukaryotic secretory signal peptides. The deduced amino acid sequence constitutes an enzyme of the group II class of PLA2s, and resembles PLA2s from other mammalian sources. A Northern blot analysis performed to determine the tissue distribution showed that rat ileum contains the largest amount of the PLA2 transcript among the tissues examined, a weaker signal was present in heart, spleen and soleus muscle, and no signal could be detected in EDL muscle, stomach, liver, kidney, brain and lung. Northern blot analysis and reverse transcriptase-polymerase chain reaction (RT-PCR) techniques indicate the presence of this enzyme in neonatal and adult rat cardiomyocytes and in a cultured rat cardiac fibroblast-like cell line, but not in rat cardiac-derived endothelial cell lines. Transcription levels of rat heart group II PLA2 in isolated neonatal rat cardiomyocytes were found to increase after stimulating the cells with tumor necrosis factor-alpha (TNF-alpha) or the alpha1-adrenergic agonist phenylephrine.