Glucose uptake and glycolysis reduce hypoxia-induced apoptosis in cultured neonatal rat cardiac myocytes

Apoptosis in myocardial ischemia, infarction, and altered myocardial states

The work ahead necessary to develop and refine clinically useful antiapoptotic therapy in ischemic-reperfusion injury is daunting. There are many unanswered questions. What is the best method of detecting apoptosis in the cardiac myocytes? What will be the most practical method to deliver this therapy to the cardiac myocyte? Will antiapoptotic agents act selectively on affected myocytes to provide clinical efficacy? Will antiapoptotic agents be effective, or will they be limited by dose heterogeneity? If antiapoptotic is proven to have long lasting efficacy, should it be used for all patients with myocardial infarction or confined only to patients with left ventricular dysfunction. Will antiapoptotic therapy be so effective that it replaces ACE inhibitors and betablockers, or will it always be used as an adjunct to an ACE inhibitor or a betablocker? These questions lay the foundation for investigation for the next decade.

New imaging techniques for cardiovascular autonomic neuropathy: a window on the heart

Cardiovascular autonomic neuropathy (CAN) is a common complication of diabetes, which results in disabling clinical manifestations and may predispose to sudden cardiac death. Recently, direct scintigraphic assessment of cardiac sympathetic integrity has become possible with the introduction of radiolabeled analogues of norepinephrine, which are actively taken up by the sympathetic nerve terminals of the heart. This article reviews how these techniques have been utilized to improve understanding of CAN complicating diabetes. Quantitative scintigraphic assessment of cardiac sympathetic innervation heart is possible with either [123I]-metaiodobenzylguanidine (MIBG) and single photon emission computed tomography (SPECT) or [11C]-hydroxyephedrine (HED) and positron emission tomography (PET). Studies in diabetic patients have explored the sensitivity of these techniques to detect CAN, characterize the effects of glycemic control on the progression of CAN and evaluate the effects of CAN on myocardial electrophysiology, blood flow regulation and function. Deficits of left ventricular (LV) [123I]-MIBG and [11C]-HED retention have been identified in diabetic subjects without abnormalities on cardiovascular reflex testing consistent with increased sensitivity to detect CAN. Poor glycemic control results in the progression of LV tracer deficits, which can be prevented or reversed by the institution of near-euglycemia. Deficits begin distally in the LV and may extend proximally. Paradoxically, however, absolute HED retention is increased in the proximal segments of the severe CAN subjects consistent with regional "hyperinnervation." These regions also exhibit abnormal blood flow regulation. Impaired myocardial MIBG uptake correlates with altered LV diastolic filling and myocardial electrophysiological deficits and is predictive of sudden death. Scintigraphic studies have provided unique insights into the effects of diabetes on cardiac sympathetic integrity and the pathophysiological consequences of LV sympathetic dysinnervation. Future studies using complementary neurotransmitter analogues will allow different aspects of regional dysfunction to be characterized with the aim of developing therapeutic strategies to prevent or reverse CAN.

Decreased glucose transporter expression triggers BAX-dependent apoptosis in the murine blastocyst

We report that a decrease in facilitative glucose transporter (GLUT1) expression and reduced glucose transport trigger apoptosis in the murine blastocyst. Inhibition of GLUT1 expression either by high glucose conditions or with antisense oligodeoxynucleotides significantly lowers protein expression and function of GLUT1 and as a result induces a high rate of apoptosis at the blastocyst stage. Similar to wild-type mice, embryos from streptozotocin-induced diabetic Bax -/- mice experienced a significant decrease in glucose transport compared with embryos from non-diabetic Bax -/- mice. However, despite this decrease, these blastocysts demonstrate significantly fewer apoptotic nuclei as compared with blastocysts from hyperglycemic wild-type mice. This decrease in preimplantation apoptosis correlates with a decrease in resorptions and malformations among the infants of the hyperglycemic Bax -/- mice versus the Bax +/+ and +/- mice. These findings suggest that hyperglycemia by decreasing glucose transport acts as a cell death signal to trigger a BAX-dependent apoptotic cascade in the murine blastocyst. This work also supports the hypothesis that increased apoptosis at a blastocyst stage because of maternal hyperglycemia may result in loss of key progenitor cells and manifest as a resorption or malformation, two adverse pregnancy outcomes more common in diabetic women.

Cardiomyocyte apoptosis induced by Galphaq signaling is mediated by permeability transition pore formation and activation of the mitochondrial death pathway

Expression of the wild-type alpha subunit of Gq stimulates phospholipase C and induces hypertrophy in cardiomyocytes. Addition of Gq-coupled receptor agonists additionally activates phospholipase C, as does expression of a constitutively active mutant form of Galphaq. Under these conditions, hypertrophy is rapidly succeeded by apoptotic cellular and molecular changes, including myofilament disorganization, loss of mitochondrial membrane potential, alterations in Bcl-2 family protein levels, DNA fragmentation, increased caspase activity ( approximately 4-fold), cytochrome c redistribution, and nuclear chromatin condensation in approximately 12% of the cells. We used various interventions to define the molecular relationships between these events and identify potential sites at which these features of apoptosis could be rescued. Treatment with caspase inhibitors prevented DNA fragmentation and promoted myocyte survival; however, cytochrome c release and loss of mitochondrial membrane potential still occurred. In contrast, treatment with bongkrekic acid, an inhibitor of the mitochondrial permeability transition pore, not only prevented DNA fragmentation and reduced nuclear chromatin condensation but also preserved mitochondrial membrane potential and limited cytochrome c redistribution to only approximately 2% of cells. These data demonstrate the central role of mitochondrial membrane potential in initiation of caspase activation and downstream apoptotic events and suggest that preservation of mitochondrial integrity is crucial for prolonging the life and function of cardiomyocytes exposed to pathological levels of stress.

Morphological and molecular characterization of adult cardiomyocyte apoptosis during hypoxia and reoxygenation

Apoptosis has been implicated in ischemic heart disease, but its mechanism in cardiomyocytes has not been elucidated. In this study, we investigate the effects of hypoxia and reoxygenation in adult cardiomyocytes and the molecular mechanism involved in cardiomyocyte apoptosis. Morphologically, reoxygenation induced rounding up of the cells, appearance of membrane blebs that were filled with marginated mitochondria, and ultrastructural findings characteristic of apoptosis. Reoxygenation (18 hours of reoxygenation after 6 hours of hypoxia) and prolonged hypoxia (24 hours of hypoxia) resulted in a 59% and 51% decrease in cellular viability, respectively. During reoxygenation, cell death occurred predominantly via apoptosis associated with appearance of cytosolic cytochrome c and activation of caspase-3 and -9. However, nonapoptotic cell death predominated during prolonged hypoxia. Both caspase inhibition and Bcl-2 overexpression during reoxygenation significantly improved cellular viability through inhibition of apoptosis but had minimal effect on hypoxia-induced cell death. Bcl-2 overexpression blocked reoxygenation-induced cytochrome c release and activation of caspase -3 and -9, but caspase inhibition alone did not block cytochrome c release. These results suggest that apoptosis predominates in cardiomyocytes after reoxygenation through a mitochondrion-dependent apoptotic pathway, and Bcl-2 prevents reoxygenation-induced apoptosis by inhibiting cytochrome c release from the mitochondria and prevents activation of caspase-3 and -9.

Beneficial effects of MET-88, a gamma-butyrobetaine hydroxylase inhibitor in rats with heart failure following myocardial infarction

Myocardial ischemia can cause myocardial infarction and as a consequence, heart failure. 3-(2,2,2-trimethylhydrazinium) propionate (MET-88) inhibits gamma-butyrobetaine hydroxylase and has cardioprotective effects on the ischemic heart. We now examined the effects of MET-88 in rats with congestive heart failure following myocardial infarction. Congestive heart failure was produced by left coronary artery ligation in rats. MET-88 at 100 mg/kg/day was orally administered from the 2nd day after surgery. We performed a survival study for 181 days, and measured ventricular remodeling, cardiac function, and myocardial high-energy phosphate levels after treatment for 20 days. MET-88 prolonged survival with a median 50% survival of 103 days compared to 79 days for the heart-failure control rats. The expansion of the left ventricular cavity (ventricular remodeling) in heart-failure rats was prevented by treatment with MET-88, and the effect of MET-88 was similar to that of captopril at 20 mg/kg. MET-88 attenuated the rise in right atrial pressure in heart-failure rats and augmented cardiac functional adaptability against an increased load. Also, MET-88 improved the myocardial energy state in heart-failure rats. The present results indicate that MET-88 improves the pathosis in rats with heart failure induced by myocardial infarction.

Conditional expression of anti-apoptotic protein p35 by Cre-mediated DNA recombination in cardiomyocytes from loxP-p35-transgenic mice

p35, a viral inhibitor of caspase, prevents cell death induced by various stimuli. We established an experimental system to study the involvement of caspases in cell death, using primary cultured cells from p35 transgenic mice in which the p35 open reading frame (ORF) had been disrupted by the insertion of a DNA segment flanked by loxP sites, the Cre recognition sites. In this system, p35 expression can be initiated by Cre recombinase. Cardiomyocytes, which are highly sensitive to hypoxic stress, were infected with an adenovirus carrying the cre gene (AxCANCre). Expression of p35 by infection with AxCANCre resulted in inhibition of caspase-3 activation and resistance to hypoxia-induced cell death. Hypoxia-induced cytochrome c release was also attenuated in p35-expressing cardiomyocytes. Our transgenic mice can be used as an experimental model for studying the involvement of caspases in various degenerative diseases as well as programmed cell death both in vitro and in vivo.

GLUT-1 reduces hypoxia-induced apoptosis and JNK pathway activation

Many studies have suggested that enhanced glucose uptake protects cells from hypoxic injury. More recently, it has become clear that hypoxia induces apoptosis as well as necrotic cell death. We have previously shown that hypoxia-induced apoptosis can be prevented by glucose uptake and glycolytic metabolism in cardiac myocytes. To test whether increasing the number of glucose transporters on the plasma membrane of cells could elicit a similar protective response, independent of the levels of extracellular glucose, we overexpressed the facilitative glucose transporter GLUT-1 in a vascular smooth muscle cell line. After 4 h of hypoxia, the percentage of cells that showed morphological changes of apoptosis was 30.5 +/- 2.6% in control cells and only 6.0 +/- 1.1 and 3.9 +/- 0.3% in GLUT-1-overexpressing cells. Similar protection against cell death and apoptosis was seen in GLUT-1-overexpressing cells treated for 6 h with the electron transport inhibitor rotenone. In addition, hypoxia and rotenone stimulated c-Jun-NH(2)-terminal kinase (JNK) activity >10-fold in control cell lines, and this activation was markedly reduced in GLUT-1-overexpressing cell lines. A catalytically inactive mutant of MEKK1, an upstream kinase in the JNK pathway, reduced hypoxia-induced apoptosis by 39%. These findings show that GLUT-1 overexpression prevents hypoxia-induced apoptosis possibly via inhibition of stress-activated protein kinase pathway activation.

Unresolved issues regarding the role of apoptosis in the pathogenesis of ischemic injury and heart failure

Apoptosis is "suicidal" programmed cell death followed by necrosis, i.e. cellular degradation. This review presents a critical evaluation of the methods used for detection of apoptosis and on data regarding the role of apoptosis in ischemia and heart failure.

METHODS

DNA laddering by electrophoresis and the TUNEL method in histology for the final stage of apoptosis, Annexin V labeling, evidence of caspase activation, cleavage of substrates, measurements of mitochondrial pro-apoptotic and anti-apoptotic factors (Bcl-2, Bax and others) and determination of the mitochondrial transitional pore potential. Much work has been carried out regarding the mechanism and the importance of apoptosis in ischemia and heart failure but many issues still remain unsolved: (1)Time needed for completion of apoptosis from stimulus to DNA fragmentation? (2)Importance of mitochondrial pathway considering the fact that cardiomyocytes contain the highest volume density of mitochondria of all mammalian cells (25% in humans, 37% in mice)? (3)Means of removal of dead cells, disconnection at the intercalated disc from neighbouring myocytes, time frame of this process? (4)Reversibility of apoptosis? (5)Differences between physiological (postnatal differentiation of the conduction system) v pathological apoptotic cell death? (6)Why do cells, under ischemic conditions, die by either apoptosis or oncosis? (7)Is apoptosis an epiphenomenon or a true cause of heart failure? (8)Quantification of the rate of apoptosis in different pathophysiological situations? Clarification of these unresolved issues will then allow an estimation of the importance of apoptosis in cardiac pathophysiology and, if necessary because the role of apoptosis has been established, the development of new therapeutic concepts.

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.

Caspase activation and mitochondrial cytochrome C release during hypoxia-mediated apoptosis of adult ventricular myocytes

Oxygen deprivation for prolonged periods leads to cardiac cell death and ventricular dysfunction. The ability to prevent myocardial cell death would be of significant therapeutic value in maintaining cardiac function after injury. While caspases have been suggested to play a critical role in apoptosis, their involvement during hypoxic injury has not been formally determined. In this report, we show that adult ventricular myocytes subjected to hypoxia for 1 h undergo a three-fold increase (P<0.05) in the incidence of apoptosis as determined by TUNEL analysis and Hoechst 33258 nuclear staining. Western blot analysis of hypoxic myocytes revealed a 10-fold increase in the proteolytic processing of caspase 3 to p17 with a concomitant cleavage of the caspase 3 substrate PARP from 116 kd to p85 kd compared to normoxic controls. Defects in mitochondrial membrane integrity were also observed as evidenced by the translocation of cytochrome c from the mitochondrial to cytosolic compartment of hypoxic cells. Pretreatment of ventricular myocytes with the peptide-caspase inhibitor known to block caspases related to caspase 1 (Ac-YVAD-CHO) attenuated cytochrome c release, processing of caspase 3, and apoptosis. While the caspase inhibitor (Ac-DEVD-CHO) which blocks caspases related to caspase 3, suppressed the cleavage of PARP and apoptosis, it had no effect on cytochrome c release by mitochondria. The data provide direct evidence for the proteolytic activation of caspases during hypoxia-mediated apoptosis of adult ventricular myocytes. Furthermore, the data suggest a hierarchical scheme for caspase activation with mitochondrial cytochrome c release occurring proximally to DEVD-CHO-inhibitable caspases.

ARC inhibits cytochrome c release from mitochondria and protects against hypoxia-induced apoptosis in heart-derived H9c2 cells

Ischemia induces apoptosis as well as necrosis of cardiac myocytes. We recently reported the cloning of a cDNA that encodes an apoptotic inhibitor, ARC, that is expressed predominantly in cardiac and skeletal muscle. In the present study, we examined the ability of ARC to protect rat embryonic heart-derived H9c2 cells from apoptosis induced by hypoxia, a component of ischemia. We found that H9c2 cells express ARC and that exposure to hypoxia substantially reduces ARC expression while inducing apoptosis. Transfected H9c2 cells in which cytosolic ARC protein levels remain elevated during hypoxia were significantly more resistant to hypoxia-induced apoptosis than parental H9c2 cells or H9c2 cells transfected with a control vector. Loss of endogenous ARC in the cytosol of H9c2 cells was associated with translocation of ARC from the cytosol to intracellular membranes, release of cytochrome c from the mitochondria, activation of caspase-3, poly(ADP-ribose)polymerase (PARP) cleavage, and DNA fragmentation. All of these events were inhibited in H9c2 cells overexpressing ARC when compared with control cells. In contrast, caspase inhibitors prevented PARP cleavage but not cytochrome c release, suggesting that exogenously expressed ARC acts upstream of caspase activation in this model of apoptosis. These results demonstrate that ARC can protect heart myogenic H9c2 cells from hypoxia-induced apoptosis, and that ARC prevents cytochrome c release by acting upstream of caspase activation, perhaps at the mitochondrial level.

The mitochondrial apoptotic pathway is activated by serum and glucose deprivation in cardiac myocytes

Many cell types undergo apoptosis under conditions of ischemia. Little is known, however, about the molecular pathways that mediate this response. A cellular and biochemical approach to elucidate such signaling pathways was undertaken in primary cultures of cardiac myocytes, a cell type that is especially sensitive to ischemia-induced apoptosis. Deprivation of serum and glucose, components of ischemia in vivo, resulted in myocyte apoptosis, as determined by nuclear fragmentation, internucleosomal cleavage of DNA, and processing of caspase substrates. These manifestations of apoptosis were blocked by zVAD-fmk, a peptide caspase inhibitor, indicating that caspase activity is necessary for the progression of apoptosis in this model. In contrast to control cells, apoptotic myocytes exhibited cytoplasmic accumulation of cytochrome c, indicating release from the mitochondria. Furthermore, both caspase-9 and caspase-3 were processed to their active forms in serum-/glucose-deprived myocytes. Caspase processing, but not cytochrome c release, was inhibited by zVAD-fmk, placing the latter event upstream of caspase activation. This evidence demonstrates that components of ischemia activate the mitochondrial death pathway in cardiac myocytes.