beta-Adrenergic pathway induces apoptosis through calcineurin activation in cardiac myocytes

Elucidating the Candida albicans calcineurin signaling cascade controlling stress response and virulence

The protein phosphatase calcineurin is a key mediator of virulence and antifungal susceptibility of multiple fungal pathogens including Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus, and has clinical potential as a therapeutic target to increase the efficacy of the current antifungal armamentarium. Despite the importance of this signaling pathway, few components of the calcineurin-signaling pathway are known in C. albicans. Here we identified and analyzed additional components of the C. albicans calcineurin cascade, including the RCN1 (regulator of calcineurin1), MID1, and CCH1 genes, which mediate calcineurin functions in other species. When heterologously expressed in Saccharomyces cerevisiae, C. albicans Rcn1 inhibited calcineurin function. Although rcn1/rcn1, mid1/mid1, and cch1/cch1 mutant strains share some phenotypes with calcineurin mutants, they do not completely recapitulate the phenotypes of a calcineurin mutant strain. These studies extend our understanding of the C. albicans calcineurin signaling cascade and its host-niche specific role in virulence.

The BH4 domain of Bcl-2 inhibits ER calcium release and apoptosis by binding the regulatory and coupling domain of the IP3 receptor

Although the presence of a BH4 domain distinguishes the antiapoptotic protein Bcl-2 from its proapoptotic relatives, little is known about its function. BH4 deletion converts Bcl-2 into a proapoptotic protein, whereas a TAT-BH4 fusion peptide inhibits apoptosis and improves survival in models of disease due to accelerated apoptosis. Thus, the BH4 domain has antiapoptotic activity independent of full-length Bcl-2. Here we report that the BH4 domain mediates interaction of Bcl-2 with the inositol 1,4,5-trisphosphate (IP3) receptor, an IP3-gated Ca(2+) channel on the endoplasmic reticulum (ER). BH4 peptide binds to the regulatory and coupling domain of the IP3 receptor and inhibits IP3-dependent channel opening, Ca(2+) release from the ER, and Ca(2+)-mediated apoptosis. A peptide inhibitor of Bcl-2-IP3 receptor interaction prevents these BH4-mediated effects. By inhibiting proapoptotic Ca(2+) signals at their point of origin, the Bcl-2 BH4 domain has the facility to block diverse pathways through which Ca(2+) induces apoptosis.

Novel cardiac apoptotic pathway: the dephosphorylation of apoptosis repressor with caspase recruitment domain by calcineurin

BACKGROUND

Apoptosis repressor with caspase recruitment domain (ARC) is abundantly expressed in cardiomyocytes. Protein kinase CK2 can phosphorylate ARC at threonine-149, thereby enabling ARC to antagonize apoptosis. ARC phosphorylation occurs in a constitutive manner. Nevertheless, cardiomyocytes still undergo apoptosis that is related to cardiac diseases such as myocardial infarction and heart failure. Whether the occurrence of apoptosis is related to the loss of protection by ARC under pathological conditions remains unknown.

METHODS AND RESULTS

ARC phosphorylation levels are decreased in cardiomyocytes treated with isoproterenol or aldosterone. We explored the molecular mechanism by which ARC phosphorylation levels are decreased. Our results reveal that either direct incubation or coexpression with calcineurin leads to a decrease in ARC phosphorylation levels. Inhibition of calcineurin can attenuate the reduction in ARC phosphorylation levels on treatment with isoproterenol or aldosterone. These data indicate that the reduction in ARC phosphorylation levels is related to its dephosphorylation by calcineurin. Our results further reveal that ARC can prevent isoproterenol- and aldosterone-induced apoptosis, but this function depends on its phosphorylation status. Isoproterenol and aldosterone upregulate Fas ligand expression, and Fas ligand and caspase-8 are required for isoproterenol and aldosterone to induce apoptosis. However, phosphorylated but not dephosphorylated ARC is able to inhibit caspase-8-mediated apoptosis. Phosphorylated ARC exerts its effects against caspase-8 by directly associating with procaspase-8 and inhibiting its interaction with Fas-associated protein with death domain.

CONCLUSIONS

Our study identifies a novel cardiac apoptotic pathway in which ARC is dephosphorylated by calcineurin. This pathway could be a component in the cardiac apoptotic machinery.

Interleukin-1beta-dependent signaling between astrocytes and neurons depends critically on astrocytic calcineurin/NFAT activity

Interleukin-1beta (IL-1beta) and the Ca(2+)/calmodulin-dependent protein phosphatase, calcineurin, have each been shown to play an important role in neuroinflammation. However, whether these signaling molecules interact to coordinate immune/inflammatory processes and neurodegeneration has not been investigated. Here, we show that exogenous application of IL-1beta (10 ng/ml) recruited calcineurin/NFAT (nuclear factor of activated T cells) activation in primary astrocyte-enriched cultures within minutes, through a pathway involving IL-1 receptors and L-type Ca(2+) channels. Adenovirus-mediated delivery of the NFAT inhibitor, VIVIT, suppressed the IL-1beta-dependent induction of several inflammatory mediators and/or markers of astrocyte activation, including tumor necrosis factor alpha, granulocyte/macrophage colony-stimulating factor, and vimentin. Expression of an activated form of calcineurin in one set of astrocyte cultures also triggered the release of factors that, in turn, stimulated NFAT activity in a second set of "naive" astrocytes. This effect was prevented when calcineurin-expressing cultures co-expressed VIVIT, suggesting that the calcineurin/NFAT pathway coordinates positive feedback signaling between astrocytes. In the presence of astrocytes and neurons, 48-h delivery of IL-1beta was associated with several excitotoxic effects, including NMDA receptor-dependent neuronal death, elevated extracellular glutamate, and hyperexcitable synaptic activity. Each of these effects were reversed or ameliorated by targeted delivery of VIVIT to astrocytes. IL-1beta also caused an NFAT-dependent reduction in excitatory amino acid transporter levels, indicating a possible mechanism for IL-1beta-mediated excitotoxicity. Taken together, the results have potentially important implications for the propagation and maintenance of neuroinflammatory signaling processes associated with many neurodegenerative conditions and diseases.

Isoproterenol-induced Cardiotoxicity in Sprague-Dawley Rats: Correlation of Reversible and Irreversible Myocardial Injury with Release of Cardiac Troponin T and Roles of iNOS in Myocardial Injury

The present study was undertaken to characterize myocardial lesions in the rat induced by low doses of isoproterenol (Iso) and to correlate lesion severity with release of cardiac troponin T (cTnT) and changes in myocyte iNOS expression. Two types of cardiac injury patterns were observed. A Type I response, noted 3 or 6 hours postdosing with 8, 16, 32, or 64 μg/kg Iso, included potential reversible myocardial alterations associated with slight increases in serum cTnT (< 0.3 ng/mL) and a slight reduction in myocyte cTnT immunoreactivity. The second type of response noted 3, 6, 12, 24 or 48 hours postdosing with 125, 250, or 500 μg/kg Iso consisted of irreversible myocyte alterations, together with significant increases in serum cTnT (3–14 ng/mL) and a marked reduction of cTnT immunoreactivity. By 48 hours the hearts of rats dosed with 125–500 μg/kg Iso had developed interstitial fibrosis, and serum cTnT had declined to near control levels (0.06–0.18 ng/mL). Increases in iNOS immunoreactivity correlated with the lesion severity. These findings suggest that low doses of Iso exert complex effects on the myocardium and that the generation of NO through increased expression of iNOS could be an important factor in the pathogenesis of myocyte injury.

Amlodipine inhibits granulation tissue cell apoptosis through reducing calcineurin activity to attenuate postinfarction cardiac remodeling

Although amlodipine, a long-acting L-type calcium channel blocker, reportedly prevents left ventricular remodeling and dysfunction after myocardial infarction, the mechanism responsible is not yet well understood. Myocardial infarction was induced in mice by ligating the left coronary artery. Treatment of mice with amlodipine (10 mg·kg−1·day−1), beginning on the third day postinfarction, significantly improved survival and attenuated left ventricular dilatation and dysfunction 4 wk postinfarction compared with treatment with saline or hydralazine. Although infarct sizes did not differ among the groups, the infarcted wall thickness was greater and the infarct segment length was smaller in the amlodipine-treated group, and cellular components, including vessels and myofibroblasts, were abundant within the infarcted area. Ten days postinfarction (the subacute stage), the proliferation of granulation tissue cells in the infarcted area was similar among the groups, but the incidence of apoptosis was significantly lower in the amlodipine-treated group, where Bad, a proapoptotic Bcl-2 family protein, was significantly phosphorylated (inactivated). Calcineurin, which dephosphorylates (activates) Bad, was upregulated in infarcted hearts, but its levels were significantly reduced by amlodipine treatment. In vitro, Fas stimulation augmented calcineurin activity and induced apoptosis among infarct tissue-derived myofibroblasts; both of those effects were strongly inhibited by amlodipine, two other calcium channel blockers (verapamil or nifedipine), and two calcineurin inhibitors (cyclosporin A or FK-506). Amlodipine inhibits Fas-mediated granulation tissue cell apoptosis in infarcted hearts, possibly by attenuating the activities of calcineurin and Bad. These findings may provide new insight into the mechanism by which calcium channel blockers attenuate postinfarction cardiac remodeling and dysfunction.

Protective effect of astragalosides on myocardial injury by isoproterenol in SD rats

We have extracted and roughly purified astragalosides (AS) from Astragalus membranaceus, a natural herb used as a traditional Chinese medicine, regarded to have pharmacodynamic benefits of protecting injured myocardium. We hypothesized that the astragalosides might exert beneficial effect in myocardial lesion by preserving both energy metabolism and Ca(2+) homeostasis. Sprague-Dauley (SD) rats were injected with isoproterenol (ISO) subcutaneous (s.c.) at a dose of 5 mg/kg/day consecutively for two days as models and were treated with astragalosides and trimetazidine intraperitoneally (i.p.) respectively, at a dose of 5 mg/kg/day one day prior to isoproterenol for 8 days. The histological changes were alleviated in isoproterenol-injected SD rats treated with astragalosides. Compared with isoproterenol-injected rats, the concentration of myocardial intracellular [Ca(2+)]i was decreased, L-type Ca(2+) current density and sarcoplasmic reticulum (SR) Ca(2+) load were recovered, the concentration of myocardial ATP was increased and phosphocreatine (PCr) was decreased in rats treated with astragalosides. In conclusion, the efficacious treatment of astragalosides for myocardial injury might be through regulating intracellular Ca(2+) homeostasis and energy metabolism.

Loss of muscle-specific RING-finger 3 predisposes the heart to cardiac rupture after myocardial infarction

RING-finger proteins commonly function as ubiquitin ligases that mediate protein degradation by the ubiquitin-proteasome pathway. Muscle-specific RING-finger (MuRF) proteins are striated muscle-restricted components of the sarcomere that are thought to possess ubiquitin ligase activity. We show that mice lacking MuRF3 display normal cardiac function but are prone to cardiac rupture after acute myocardial infarction. Cardiac rupture is preceded by left ventricular dilation and a severe decrease in cardiac contractility accompanied by myocyte degeneration. Yeast two-hybrid assays revealed four-and-a-half LIM domain (FHL2) and gamma-filamin proteins as MuRF3 interaction partners, and biochemical analyses showed these proteins to be targets for degradation by MuRF3. Accordingly, FHL2 and gamma-filamin accumulated to abnormal levels in the hearts of mice lacking MuRF3. These findings reveal an important role of MuRF3 in maintaining cardiac integrity and function after acute myocardial infarction and suggest that turnover of FHL2 and gamma-filamin contributes to this cardioprotective function of MuRF3.

Induction of cardioplegic arrest immediately activates the myocardial apoptosis signal pathway

Myocardial ischemia-reperfusion, including cardioplegic arrest (CA), has been associated with cardiac apoptosis induction. However, the time course of apoptosis activation and the trigger mechanisms are still unclear. Because apoptosis inhibition may represent a novel therapeutic strategy for long-term myocardial preservation, we sought to investigate the time course of apoptosis signal-pathway induction during CA. As to method, Sprague-Dawley rats (300–350 g) were anesthetized, intubated, and mechanically ventilated. CA was initiated by infusion of ice-cold crystalloid solution (Custodiol, 10 ml/kg) into the aortic root, and hearts were rapidly excised and stored for 0, 30, 60, and 120 min in 0.9% sodium chloride solution (28°C). In controls, no CA was initiated before removal and storage at 28°C. In another group, calcium-rich cardioplegia was used, and an additional group received a caspase-8 inhibitor before CA induction. Left ventricular cytosolic extracts were isolated and investigated for the activity of caspase-3 and -6 (effector caspases) and caspase-8 and -9 (involved in extrinsic and intrinsic pathways of apoptosis induction). Fluorometric activity assays were performed by using specific substrates. As a result, activities of all tested caspases were significantly increased immediately after CA induction compared with controls. Administration of the caspase-8 inhibitor significantly reduced activities of all caspases. With calcium-rich cardioplegia, caspase activities were significantly lower compared with low-calcium CA. Control hearts also showed an increase of caspase activities during cold-storage ischemia without CA but had significantly different time courses compared with hearts with CA. In conclusion, our data show rapid apoptosis signal-pathway induction immediately following CA exposure. Thus apoptosis signal-pathway inhibition as a potential strategy for improved myocardial preservation would have the greatest effect when applied before CA exposure.

Silibinin protects against isoproterenol-induced rat cardiac myocyte injury through mitochondrial pathway after up-regulation of SIRT1

Terminally differentiated adult injured cardiac myocytes have been used for various animal models of heart failure. It has recently been shown that isoproterenol induces injury in rat neonatal cardiac myocytes via a beta-adrenergic pathway, suggesting that it might be one of the factors involved in myocardial cell injury in heart failure in vivo. In the study, silibinin, a plant flavanoid from milk thistle was first evaluated for its protective effect against beta-adrenergic agonist isoproterenol-induced injury in cultured rat neonatal cardiac myocytes. The viability, activation of lactate dehydrogenase (LDH), and content of maleic dialdehyde (MDA) were chosen for measuring the degree of cardiac myocytes injury. As a result, silibinin protected isoproterenol-treated rat cardiac myocytes from death and significantly decreased LDH release and MDA production. Silibinin increased superoxide dismutase activity, decreased [Ca(2+)](i), and increased mitochondrial membrane potential (DeltaPsi). Furthermore, the release of pro-apoptotic cytochrome c from mitochondria was reduced by silibinin. Silibinin increased the expression of anti-apoptotic Bcl-2 family protein Bcl-2, and up-regulation of SIRT1 inhibited the translocation of Bax from cytoplasm to mitochondria, which caused mitochondrial dysfunction and cell injury. These results demonstrate that silibinin protects against isoproterenol-induced cardiac myocytes injury through resuming mitochondrial function and regulating the expression of SIRT1 and Bcl-2 family members.

Genetic disruption of calcineurin improves skeletal muscle pathology and cardiac disease in a mouse model of limb-girdle muscular dystrophy

Calcineurin (Cn) is a Ca(2+)/calmodulin-dependent serine/threonine phosphatase that regulates differentiation-specific gene expression in diverse tissues, including the control of fiber-type switching in skeletal muscle. Recent studies have implicated Cn signaling in diminishing skeletal muscle pathogenesis associated with muscle injury or disease-related muscle degeneration. For example, use of the Cn inhibitor cyclosporine A has been shown to delay muscle regeneration following toxin-induced injury and inhibit regeneration in the dystrophin-deficient mdx mouse model of Duchenne muscular dystrophy. In contrast, transgenic expression of an activated mutant of Cn in skeletal muscle was shown to increase utrophin expression and reduce overall disease pathology in mdx mice. Here we examine the effect of altered Cn activation in the context of the delta-sarcoglycan-null (scgd(-/-)) mouse model of limb-girdle muscular dystrophy. In contrast to results discussed in mdx mice, genetic deletion of a loxP-targeted calcineurin B1 (CnB1) gene using a skeletal muscle-specific cre allele in the scgd(-/-) background substantially reduced skeletal muscle degeneration and histopathology compared with the scgd(-/-) genotype alone. A similar regression in scgd-dependent disease manifestation was also observed in calcineurin Abeta (CnAbeta) gene-targeted mice in both skeletal muscle and heart. Conversely, increased Cn expression using a muscle-specific transgene increased cardiac fibrosis, decreased cardiac ventricular shortening, and increased muscle fiber loss in the quadriceps. Our results suggest that inhibition of Cn may benefit select types of muscular dystrophy.

Lipopolysaccharide activates calcineurin in ventricular myocytes

OBJECTIVES

We investigated whether lipopolysaccharide (LPS), a proximate cause of inflammation, activates calcineurin in cardiac myocytes and if calcineurin regulates apoptosis in this setting.

BACKGROUND

Calcineurin regulates myocardial growth and hypertrophy, but its role in inflammation is unknown. Calcineurin has proapoptotic or antiapoptotic effects depending on the stimuli.

METHODS

Calcineurin activity was measured in left ventricular myocytes from adult Sprague Dawley rats. Cardiac apoptosis was measured by terminal deoxy-nucleotidyl transferase-mediated dUTP nick end-labeling staining and caspase-3 activity after in vitro and in vivo exposure to LPS.

RESULTS

Lipopolysaccharide increased calcineurin activity in myocytes over 1 to 24 h (t 1/2 = 4.8 h) with an EC(50) of 0.80 ng/ml LPS (p < 0.05, n = 4). The LPS (10 ng/ml) effects were mimicked by angiotensin II (Ang II) (100 nmol/l); both increased calcineurin activity and induced apoptosis without additive effects (p < 0.05, n = 5 to 9). Lipopolysaccharide and/or Ang II effects were prevented by 1 h pre-treatment with an Ang II type 1 receptor blocker (losartan, 1 micromol/l), calcineurin inhibitor (cyclosporin A, 0.5 micromol/l), calcium chelator (1,2-Bis(2-amino-5-fluorophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl) ester, 0.1 micromol/l), or by inhibiting sarcoplasmic reticulum (SR) calcium (Ca)-ATPase (thapsigargin, 1 micromol/l) or SR calcium release channel (ryanodine, 1 micromol/l). Left ventricular apoptosis increased from 4 to 24 h after LPS (1 mg/kg intravenously) in vivo, but not in rats pre-treated with cyclosporin A (20 mg/kg/day subcutaneously) for 3 days (p < 0.05, n = 5).

CONCLUSIONS

In cardiac myocytes, LPS activates calcineurin in association with apoptosis by Ang II and SR calcium-dependent mechanisms. This expands the paradigm for cardiac calcineurin to be activated by low levels of LPS in inflammation and chronic conditions (e.g., infections, smoking, and heart failure).

Mimicry and Antibody-Mediated Cell Signaling in Autoimmune Myocarditis

The mechanisms by which autoantibodies against cardiac myosin (CM) may lead to heart dysfunction is unknown. We show that autoantibodies to CM in anti-CM sera and mAbs derived from experimental autoimmune myocarditis targeted the heart cell surface and induced Ab-mediated cAMP-dependent protein kinase A activity. Ab-mediated cell signaling of protein kinase A was blocked by CM, anti-IgG, or by specific inhibitors of the beta-adrenergic receptor (beta-AR) pathway. mAbs confirmed mimicry between CM and the beta-AR. Passive transfer of purified Ab (IgG) from CM-immunized rats resulted in IgG deposition and apoptosis in the heart, leading to a cardiomyopathic heart disease phenotype in recipients. Our novel findings link anti-CM Ab with the beta-AR and subsequent Ab-mediated cell signaling in the heart.

Direct interaction and reciprocal regulation between ASK1 and calcineurin-NFAT control cardiomyocyte death and growth

The calcium-calmodulin-activated protein phosphatase calcineurin functions as a key mediator of diverse biologic processes, including differentiation, apoptosis, growth, and adaptive responses, in part through dephosphorylation and activation of nuclear factor of activated T-cell (NFAT) transcription factors. Apoptosis signal-regulating kinase 1 (ASK1) is an upstream component of the mitogen-activated protein kinases that serves as a pivotal regulator of cytokine-, oxidative-, and stress-induced cell death. Here, we performed a yeast two-hybrid screen with calcineurin B as bait, which identified ASK1 as a direct physical interacting partner. The C-terminal 218 amino acids of ASK1 were sufficient to mediate interaction with calcineurin B in yeast, as well as in mammalian cell lysates. Importantly, endogenous calcium binding B subunit (CnB) protein interacted with endogenous ASK1 protein in cardiomyocytes at baseline, suggesting that the interaction observed in yeast was of potential biologic relevance. Indeed, calcineurin directly dephosphorylated ASK1 at serine 967 using purified proteins or mammalian cell lysates. Dephosphorylation of ASK1 serine 967 by calcineurin promoted its disassociation from 14-3-3 proteins, resulting in ASK1 activation. Calcineurin and ASK1 cooperatively enhanced cardiomyocyte apoptosis, while expression of a dominant negative ASK1 blocked calcineurin-induced apoptosis. Mouse embryonic fibroblasts deficient in ask1 were also partially resistant to calcineurin- or ionomycin-induced apoptosis. Finally, ASK1 negatively regulated calcineurin-NFAT signaling indirectly through c-Jun NH2-terminal kinase (JNK)- and p38-mediated phosphorylation of NFAT, which blocked calcineurin- and agonist-dependent hypertrophic growth of cardiomyocytes. Thus, ASK1 and calcineurin-NFAT constitute a feedback regulatory circuit in which calcineurin positively regulates ASK1 through direct dephosphorylation, while ASK1 negatively regulates calcineurin-NFAT signaling through p38- and JNK-mediated NFAT phosphorylation.

Interrelations between oxidative stress and calcineurin in the attenuation of cardiac apoptosis by eugenol

In view of the known involvement of oxidative stress and calcineurin (Ca(2+)-calmodulin dependent protein phosphatase) in beta-Adrenergic stimulated events, we examined the influence of eugenol (an antioxidant generally regarded as safe by the Food and Agricultural Organization of the United Nations) on isoproterenol-induced apoptosis in neonatal cardiomyocytes. In comparison to unstimulated controls, cardiomyocytes stimulated with 50 microM isoproterenol for 48 h demonstrated (a) increased intracellular Ca(2+) levels (b) oxidative stress involving enhanced reactive oxygen species, decreased GSH/GSSG ratio, enhanced lipid peroxidation, increased activities of superoxide dismutase and glutathione peroxidase (c) apoptosis, evidenced by increased number of annexin V/TUNEL positive cells, enhanced membrane fluidity, decreased mitochondrial membrane potential, increased activities of caspase 3 and 9 along with (d) increased calcineurin activity. Pre-incubation of cardiomyocytes with 100 microM eugenol for 1 h, followed by isoproterenol treatment for 48 h, led to reversal of enhanced intracellular Ca(2+) levels, oxidative stress, calcineurin activation and apoptosis caused by isoproterenol. In addition, similar treatment of cardiomyocytes with 10 nM FK506, a calcineurin inhibitor, could also attenuate isoproterenol-induced apoptosis. These results indicate the beneficial effects of eugenol in preventing cardiomyocyte apoptosis.

Glucosamine inhibits angiotensin II-induced cytoplasmic Ca2+elevation in neonatal cardiomyocytes via protein-associatedO-linkedN-acetylglucosamine

We previously reported that glucosamine and hyperglycemia attenuate the response of cardiomyocytes to inositol 1,4,5-trisphosphate-generating agonists such as ANG II. This appears to be related to an increase in flux through the hexosamine biosynthesis pathway (HBP) and decreased Ca2+entry into the cells; however, a direct link between HBP and intracellular Ca2+homeostasis has not been established. Therefore, using neonatal rat ventricular myocytes, we investigated the relationship between glucosamine treatment; the concentration of UDP- N-acetylglucosamine (UDP-GlcNAc), an end product of the HBP; and the level of protein O-linked N-acetylglucosamine ( O-GlcNAc) on ANG II-mediated changes in intracellular free Ca2+concentration ([Ca2+]i). We found that glucosamine blocked ANG II-induced [Ca2+]iincrease and that this phenomenon was associated with a significant increase in UDP-GlcNAc and O-GlcNAc levels. O-(2-acetamido-2-deoxy-d-glucopyranosylidene)-amino- N-phenylcarbamate, an inhibitor of O-GlcNAcase that increased O-GlcNAc levels without changing UDP-GlcNAc concentrations, mimicked the effect of glucosamine on the ANG II-induced increase in [Ca2+]i. An inhibitor of O-GlcNAc-transferase, alloxan, prevented the glucosamine-induced increase in O-GlcNAc but not the increase in UDP-GlcNAc; however, alloxan abrogated the inhibition of the ANG II-induced increase in [Ca2+]i. These data support the notion that changes in O-GlcNAc levels mediated via increased HBP flux may be involved in the regulation of [Ca2+]ihomeostasis in the heart.

Role of calcineurin in Porphyromonas gingivalis-induced myocardial cell hypertrophy and apoptosis

BACKGROUND AND OBJECTIVE

Periodontal pathogen Porphyromonas gingivalis (P. gingivalis) increased cardiomyocyte hypertrophy and apoptosis whereas Actinobaeillus actinomycetemcomitans and Prevotella intermedia had no effects. The purpose of this study is to clarify the role of calcineurin signaling pathway in P. gingivalis-induced H9c2 myocardial cell hypertrophy and apoptosis.

METHODS

DNA fragmentation, nuclear condensation, cellular morphology, calcineurin protein, Bcl2-associated death promoter (Bad) and nuclear factor of activated T cell (NFAT)-3 protein products in cultured H9c2 myocardial cell were measured by agarose gel electrophoresis, DAPI, immunofluorescence, and Western blotting following P. gingivalis and/or pre-administration of CsA (calcineurin inhibitors cyclosporin A).

RESULTS

P. gingivalis not only increased calcineurin protein, NFAT-3 protein products and cellular hypertrophy, but also increased DNA fragmentation, nuclear condensation and Bad protein products in H9c2 cells. The increased cellular sizes, DNA fragmentation, nuclear condensation, and Bad of H9c2 cells treated with P. gingivalis were all significantly reduced after pre-administration of CsA.

CONCLUSION

Our findings suggest that the activity of calcineurin signal pathway may be initiated by P. gingivalis and further lead to cell hypertrophy and death in culture H9c2 myocardial cells.

Synchronous progression of calcium transient-dependent beating and sarcomere destruction in apoptotic adult cardiomyocytes

During early apoptosis, adult cardiomyocytes show unusual beating, suggesting possible participation of abnormal Ca(2+) transients in initiation of apoptotic processes in this cell type. Simultaneously with the beating, these cells show dynamic structural alteration resulting from cytoskeletal disintegration that is quite rapid. Because of the specialized structure and extensive cytoskeleton of cardiomyocytes, we hypothesized that its degradation in so short a time would require a particularly efficient mechanism. To better understand this mechanism, we used serial video microscopy to observe beta-adrenergic stimulation-induced apoptosis in isolated adult rat cardiomyocytes while simultaneously recording intracellular Ca(2+) concentration and cell length. Trains of Ca(2+) transients and corresponding rhythmic contractions and relaxations (beating) were observed in apoptotic cells. Frequencies of Ca(2+) transients and beating gradually increased with time and were accompanied by cellular shrinkage. As the cells shrank, amplitudes of Ca(2+) transients declined and diastolic intracellular Ca(2+) concentration increased until the transients were lost. Beating and progression of apoptosis were significantly inhibited by antagonists against the L-type Ca(2+) channel (nifedipine), ryanodine receptor (ryanodine), inositol 1,4,5-trisphosphate receptor (heparin), sarco(endo)plasmic Ca(2+)-ATPase (thapsigargin), and Na(+)/Ca(2+) exchanger (KB-R7943). Electron-microscopic examination of beating cardiomyocytes revealed progressive breakdown of Z disks. Immunohistochemical analysis and Western blot confirmed that disappearance of Z disk constituent proteins (alpha-actinin, desmin, and tropomyosin) preceded degradation of other cytoskeletal proteins. It thus appears that, in adult cardiomyocyte apoptosis, Ca(2+) transients mediate apoptotic beating and efficient sarcomere destruction initiated by Z disk breakdown.

Ca 2+ Influx–Induced Sarcoplasmic Reticulum Ca 2+ Overload Causes Mitochondrial-Dependent Apoptosis in Ventricular Myocytes

Increases in Ca 2+ influx through the L-type Ca 2+ channel (LTCC, Cav1.2) augment sarcoplasmic reticulum (SR) Ca 2+ loading and the amplitude of the cytosolic Ca 2+ transient to enhance cardiac myocyte contractility. Our hypothesis is that persistent increases in Ca 2+ influx through the LTCC cause apoptosis if the excessive influx results in SR Ca 2+ overload. Feline ventricular myocytes (VMs) in primary culture were infected with either an adenovirus (Ad) containing a rat Cav1.2 β 2a subunit-green fluorescent protein (GFP) fusion gene (Adβ 2a ) to increase Ca 2+ influx or with AdGFP as a control. Significantly fewer β 2a -VMs (21.4±5.6%) than GFP-VMs (99.6±1.7%) were viable at 96 hours. A fraction of β 2a -VMs (20.8±1.8%) contracted spontaneously (SC-β 2a -VMs), and viability was significantly correlated with the percentage of SC-β 2a -VMs. Higher percentages of apoptotic nuclei, DNA laddering, and cytochrome C release were detected in β 2a -VMs. This apoptosis was prevented with pancaspase or caspase-3 or caspase-9 inhibitors. L-type calcium current (I Ca-L ) density was greater in β 2a -VMs (23.4±2.8 pA/pF) than in GFP-VMs (7.6±1.6 pA/pF). SC-β 2a -VMs had higher diastolic intracellular Ca 2+ (Indo-1 ratio: 1.1±0.1 versus 0.7±0.03, P <0.05) and systolic Ca 2+ transients (1.89±0.27 versus 0.80±0.08) than GFP-VMs. Inhibitors of Ca 2+ influx, SR Ca 2+ uptake and release, mitochondrial Ca 2+ uptake, mitochondrial permeation transition pore, calpain, and Bcl-2-associated X protein protected β 2a -VMs from apoptosis. These results show that persistent increases in Ca 2+ influx through the I Ca-L enhance contractility but lead to apoptosis through a mitochondrial death pathway if SR Ca 2+ overload is induced.

Angiotensin II induces apoptosisin vivoin skeletal, as well as cardiac, muscle of the rat

Our previous work has established that angiotensin II is cardiotoxic. Here we sought to investigate whether skeletal muscle is similarly susceptible to damage. Male Wistar rats were either given a single subcutaneous injection of angiotensin II (range 1 microg kg-1 to 10 mg kg-1) or only the vehicle and killed 7 h later, or implanted with preconditioned osmotic pumps dispensing 1 mg kg-1 day-1 angiotensin II and killed 9 or 18 h later. Apoptotic (caspase 3 positive) myocytes were counted on cryosections of the heart, soleus, tibialis anterior and diaphragm muscle. Single injections of 100 microg kg-1 to 10 mg kg-1 angiotensin II induced significant (P<0.05) myocyte apoptosis (per 10(4) viable myocytes) in the heart and this was heterogeneously distributed, peaking (5.7+/-0.6; P<0.05) at a point 6 mm from the apex, i.e. approximately three-quarters of the way towards the base. The slow-twitch soleus muscle was also damaged significantly (peak=2.6+/-0.4; P<0.05), while only the administration of 1 mg kg-1 induced significant (P<0.05) apoptosis in the fast-twitch tibialis anterior muscle (peak=1.2+/-0.3). Infusion of 1 mg kg-1 day-1 angiotensin II induced more myocyte apoptosis than a single bolus administration of the same dose, and in general there was a higher incidence of apoptosis in muscles harvested after 18 than after 9 h. Infusion of 1 mg kg-1 day-1 angiotensin II over 18 h induced significant (P<0.05) myocyte apoptosis in the heart (3.3+/-0.4), soleus (3.9+/-1), tibialis anterior (5.9+/-0.4) and diaphragm (19.8+/-5.6) muscle. Depending on the muscle type, angiotensin II induces myocyte apoptosis in skeletal muscle to a similar or greater extent as in cardiac muscle, supporting the hypothesis that angiotensin II is generally toxic to all striated muscles.

Alcohol and mitochondria in cardiac apoptosis: mechanisms and visualization

Apoptosis of myocytes is likely to contribute to a variety of heart conditions and could also be important in the development of alcoholic heart disease. A fundamental pathway to apoptosis is through mitochondrial membrane permeabilization and release of proapoptotic factors from the mitochondrial intermembrane space to the cytosol. The authors' results show that prolonged exposure of cultured cardiac cells to ethanol (35 mM for 48 hr) promotes Ca2+-induced activation of the mitochondrial permeability transition pore (PTP). PTP-dependent mitochondrial membrane permeabilization is followed by release of cytochrome c and execution of apoptosis. The authors propose that chronic ethanol exposure, in combination with other stress signals, may allow for activation of the PTP by physiological calcium oscillations, providing a trigger for cardiac apoptosis during chronic alcohol abuse. Coincidence of apoptosis promoting factors occurs in only a small fraction of myocytes, but because of the absence of regeneration, even a modest increase in the rate of cell death may contribute to a decrease in cardiac contractility. Detection of apoptotic changes that are present in only a few myocytes at a certain time in the heart is not feasible with most of the apoptotic assays. Fluorescence imaging is a powerful technology to visualize changes that are confined to a minor fraction of cells in a tissue, and the use of multiphoton excitation permits imaging in situ deep in the wall of the intact heart. This article discusses potential mechanisms of the effect of alcohol on mitochondrial membrane permeabilization and visualization of mitochondria-dependent apoptosis in cardiac muscle.

Detrimental effects after dobutamine infusion on rat left ventricular function: mechanical work and energetics

We have previously reported that continuous infusion of dobutamine into the coronary artery induces positive inotropic effects but induces no detrimental effects in cross-circulated, excised normal rat hearts and even in Ca2+ overload-induced contractile failing rat hearts. However, we hypothesized that some detrimental effects on left ventricular (LV) function are induced after continuous dobutamine infusion and the following clearance of blood dobutamine, as is the case after beta-adrenergic receptor stimulation. To test this hypothesis, we investigated LV mechanical work and energetics in the same type of preparations that underwent continuous dobutamine infusion and clearance of blood dobutamine. We found that both mean end-systolic pressure and systolic pressure-volume area (PVA; a measure of total mechanical energy per beat) at midrange LV volume were significantly (P < 0.01) decreased. The mean myocardial oxygen consumption per beat intercept, which is composed of for the total Ca2+ handling in excitation-contraction coupling and basal metabolism, of the and PVA linear relation was also significantly (P < 0.05) decreased (n=8). The mean slope of the linear relation was unchanged in such hearts. Post-dobutamine basal metabolism was unchanged (n = 5 of the 8 hearts). The moderate proteolysis of a cytoskeleton protein, alpha-fodrin was identified (n = 7 of the 8 hearts with the decreased intercept), after clearance of blood dobutamine. In agreement with our hypothesis, the detrimental effect of the post-beta-adrenergic receptor stimulation was induced by a moderate concentration of dobutamine; we found systolic dysfunction due to the impairment of Ca2+ handling in excitation-contraction coupling in the rat LV and proteolysis of a cytoskeleton protein, alpha-fodrin.

Species- and tissue-dependent effects of NO and cyclic GMP on cardiac ion channels

Biochemical studies have established the presence of a NO pathway in the heart, including sources of NO and various effectors. Several cardiac ion channels have been shown to be modified by NO, such as L-type Ca(2+), ATP-sensitive K(+), and pacemaker f-channels. Some of these effects are mediated by cGMP, through the activity of three main proteins: the cGMP-dependent protein kinase (PKG), the cGMP-stimulated phosphodiesterase (PDE2) and the cGMP-inhibited PDE (PDE3). Other effects appear independent of cGMP, as for instance the NO modulation of the ryanodine receptor-Ca(2+) channel. In the case of the cardiac L-type Ca(2+) channel current (I(Ca,L)), both cGMP-dependent and cGMP-independent effects have been reported, with important tissue and species specificity. For instance, in rabbit sinoatrial myocytes, NO inhibits the beta-adrenergic stimulation of I(Ca,L) through activation of PDE2. In cat and human atrial myocytes, NO potentiates the cAMP-dependent stimulation of I(Ca,L) through inhibition of PDE3. In rabbit atrial myocytes, NO enhances I(Ca,L) in a cAMP-independent manner through the activation of PKG. In ventricular myocytes, NO exerts opposite effects on I(Ca,L): an inhibition mediated by PKG in mammalian myocytes but by PDE2 in frog myocytes; a stimulation attributed to PDE3 inhibition in frog ventricular myocytes but to a direct effect of NO in ferret ventricular myocytes. Finally, NO can also regulate cardiac ion channels by a direct action on G-proteins and adenylyl cyclase.

Cyclosporin A But Not FK-506 Protects Against Dopamine-Induced Apoptosis in the Stunned Heart

BACKGROUND

Dopamine given at moderate doses for inotropy to postischemic hearts has been shown to augment myocyte apoptosis in association with elevated cytosolic calcium. We hypothesize that dopamine-mediated apoptosis occurs through calcium-induced opening of the mitochondrial permeability transition (mPT) pore. We also hypothesize that cyclosporin A (CSA), a calcineurin inhibitor known to block mPT pore opening, would prevent dopamine-induced apoptosis primarily by inhibiting pore opening (cyclophilin D binding).

METHODS

Isolated perfused rabbit hearts (n = 6/group) were subjected to 30 minutes of 37 degrees C cardioplegic arrest followed by 120 minutes reperfusion (ischemic injury that produces < 3% infarct by triphenyl-tetrazolium chloride [TTC] staining). Four groups were studied: (1) control; (2) dopamine (10 micromol/L) postischemia (dopa); (3) dopamine+CSA (0.2 micromol/L) (CSA+D) group; (4) dopamine+FK-506 (0.2 micromol/L) (FK+D) group. Left ventricular developed pressure and oxygen consumption were measured preischemia and postischemia. Bax, caspase-3 and caspase-9, and poly-ADP-ribose polymerase (PARP) activation were measured by Western blotting. Apoptotic nuclei were quantified by terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) staining.

RESULTS

Dopamine postischemia improved contractile function and heart rate and this was not affected by CSA or FK. However, TUNEL positive nuclei, Bax, caspase-3 and caspase-9 activation, and PARP cleavage were all increased in dopa and FK+D groups, but not in CSA+D.

CONCLUSIONS

Cyclosporin is effective in preventing dopamine-induced apoptosis in the postischemic heart. The mechanism is likely due to inhibition of mPT pore opening since FK-506, a potent calcineurin inhibitor that does not bind to cyclophilin, did not prevent this. Low dose cyclosporin may prove useful to prevent dopamine-induced apoptosis resulting in long-term preservation of cardiac function.

Intracellular signaling pathways for norepinephrine- and endothelin-1-mediated regulation of myocardial cell apoptosis

Accumulating data support the idea that apoptosis in cardiac myocytes, in part, contributes to the development of heart failure. Since a number of neurohormonal factors are activated in this state, these factors may be involved in the positive and negative regulation of apoptosis in cardiac myocytes. Norepinephrine is one such factor and induces apoptosis in cardiac myocytes via a beta-adrenergic receptor pathway. beta-adrenergic agonist-induced apoptosis in cardiac myocytes is dependent on the activation of the cAMP/protein kinase A pathway. Interestingly, the activation of this pathway protects PC12 cells from apoptosis, suggesting that cAMP/protein kinase A regulates apoptosis in a cell type-specific manner. Another neurohormonal factor activated in heart failure is endothelin-1, which acts as a potent survival factor against myocardial cell apoptosis. Intracellular signaling pathways for endothelin-1-mediated protection include activation of MEK-1 /ERK1/2 and PI3 kinase. In addition to these protective pathways common among cell types, endothelin- activates the calcium-activated phosphatase calcineurin, which is necessary for the nuclear import of NFAT transcription factors. These factors interact with the cardiac-restricted zinc finger protein GATA-4 and induce transcription and expression of anti-apoptotic molecule bcl-2. Thus, myocardial cell apoptosis is regulated by pathways unique to cardiac myocytes as well as by those common among cell types. It should be further determined whether agents that specifically block myocardial cell apoptosis will attenuate the progression of heart failure.

Swimming exercise in infancy has beneficial effect on the hearts in cardiomyopathic Syrian hamsters

The phenotypic expression of cardiomyopathy is greatly influenced by extrinsic factors other than intrinsic genetic defects, such as environmental stress. Exercise is assumed to be an important extrinsic factor, since sudden death is sometimes seen during exercise in young patients with hypertrophic cardiomyopathy (HCM). However, the long-term effects of mild exercise on phenotypic expression in cardiomyopathy remain unclear. To evaluate the effects of exercise performed during infancy or adolescence in cardiomyopathic patients, cardiomyopathic Syrian hamsters (BIO14.6) were subjected to swimming. BIO14.6 and age-matched congenic normal hamsters (CN) as controls were divided into three groups: sedentary (Sed), and trained during infancy (Inf) and during adolescence (Ado). Histological and biochemical analysis of 41-week-old hamsters revealed that (1) the relative level of beta-myosin heavy chain mRNA was significantly lower in the Inf group than in the Sed and Ado groups of BIO14.6. The level in the Inf group of BIO14.6 was compatible with that in the age-matched Sed group of the CN strain; (2) in BIO14.6, degenerative mitochondrial change in the cardiomyocytes was not seen in the Inf group while it was common in the Sed and Ado groups; (3) calcineurin phosphatase activity in the swimming group in 10-week-old CN was significantly higher than that of the age-matched sedentary group, and was as much as that of the swimming and sedentary groups in 10- and 41-week-old BIO14.6.

Calcineurin inhibits Na+/Ca2+ exchange in phenylephrine-treated hypertrophic cardiomyocytes

The cardiac Na(+)/Ca(2+) exchanger (NCX1) is the predominant mechanism for the extrusion of Ca(2+) from beating cardiomyocytes. The role of protein phosphorylation in the regulation of NCX1 function in normal and diseased hearts remains unclear. In our search for proteins that interact with NCX1 using a yeast two-hybrid screen, we found that the C terminus of calcineurin Abeta, containing the autoinhibitory domain, binds to the beta1 repeat of the central cytoplasmic loop of NCX1 that presumably constitutes part of the allosteric Ca(2+) regulatory site. The association of NCX1 with calcineurin was significantly increased in the BIO14.6 cardiomyopathic hamster heart compared with that in the normal control. In hypertrophic neonatal rat cardiomyocytes subjected to chronic phenylephrine treatment, we observed a marked depression of NCX activity measured as the rate of Na(+)(i)-dependent (45)Ca(2+) uptake or the rate of Na(+)(o)-dependent (45)Ca(2+) efflux. Depressed NCX activity was partially and independently reversed by the acute inhibition of calcineurin and protein kinase C activities with little effect on myocyte hypertrophic phenotypes. Studies of NCX1 deletion mutants expressed in CCL39 cells were consistent with the view that the beta1 repeat is required for the action of endogenous calcineurin and that the large cytoplasmic loop may be required to maintain the interaction of the enzyme with its substrate. Our data suggest that NCX1 is a novel regulatory target for calcineurin and that depressed NCX activity might contribute to the etiology of in vivo cardiac hypertrophy and dysfunction occurring under conditions in which both calcineurin and protein kinase C are chronically activated.

Mice lacking calsarcin-1 are sensitized to calcineurin signaling and show accelerated cardiomyopathy in response to pathological biomechanical stress

Signaling by the calcium-dependent phosphatase calcineurin profoundly influences the growth and gene expression of cardiac and skeletal muscle. Calcineurin binds to calsarcins, a family of muscle-specific proteins of the sarcomeric Z-disc, a focal point in the pathogenesis of human cardiomyopathies. We show that calsarcin-1 negatively modulates the functions of calcineurin, such that calcineurin signaling was enhanced in striated muscles of mice that do not express calsarcin-1. As a consequence of inappropriate calcineurin activation, mice with a null mutation in calsarcin-1 showed an excess of slow skeletal muscle fibers. The absence of calsarcin-1 also activated a hypertrophic gene program, despite the absence of hypertrophy, and enhanced the cardiac growth response to pressure overload. In contrast, cardiac adaptation to other hypertrophic stimuli, such as chronic catecholamine stimulation or exercise, was not affected. These findings show important roles for calsarcins as modulators of calcineurin signaling and the transmission of a specific subset of stress signals leading to cardiac remodeling in vivo.

Mechanisms of apoptosis in the heart

Apoptosis is a complex and highly regulated form of cell death, and believed to contribute to the continuous decline of ventricular function in heart failure. Apoptotic cell death is observed in a variety of cardiovascular diseases, including myocardial infarction, ischemia-reperfusion injury, end-stage heart failure, arrhythmias, and adriamycin cardiomyopathy. There are several pathways leading to programmed cell death. Apoptosis can be initiated by extracellular or intracellular stimuli, leading to the activation of caspases and subsequent cell death. A better understanding of the process of apoptosis in the heart is clearly important as it may lead to the identification of novel therapies for cardiovascular disease. This review is focused on the basic cellular mechanisms of apoptosis, as well as our current understanding of this process in the heart.

Effect of overexpressed adenylyl cyclase VI on beta 1- and beta 2-adrenoceptor responses in adult rat ventricular myocytes

1. Adenylyl cyclase VI (ACVI) is one of the most abundantly expressed beta adrenergic receptor (betaAR)-coupled cyclases responsible for cyclic AMP (cAMP) production within the mammalian myocardium. We investigated the role of ACVI in the regulation of cardiomyocyte contractility and whether it is functionally coupled with beta(1) adrenergic receptor (beta(1)AR). 2. Recombinant adenoviruses were generated for ACVI and for antisense to ACVI (AS). Adult rat ventricular myocytes were transfected with ACVI virus, AS or both (SAS). Adenovirus for green fluorescent protein (GFP) served as control. Myocyte contraction amplitudes (% shortening) and relaxation times (R50) were analysed. ACVI function was determined using cAMP assays. 3. ACVI-transfected cells demonstrated a strong 139 kDa ACVI protein band compared to controls. ACVI myocytes had higher steady-state intracellular cAMP levels than GFP myocytes when unstimulated (GFP vs ACVI=6.60+/-0.98 vs 14.2+/-2.1 fmol cAMP/viable cell, n=4, P<0.05) and in the presence of 1 microm isoprenaline or 10 microm forskolin. 4. ACVI myocytes had increased basal contraction (% shortening: GFP vs ACVI: 1.90+/-1.36 vs 3.91+/-2.29, P<0.0001) and decreased basal R50 (GFP vs ACVI: 62.6+/-24.2 ms (n=50) vs 45.0+/-17.2 ms (n=248), P<0.0001). ACVI myocyte responses were increased for forskolin (E(max): GFP=6.70+/-1.59 (n=6); ACVI=9.06+/-0.69 (n=14), P<0.01) but not isoprenaline. 5. ACVI myocyte responses were increased (E(max): GFP vs ACVI=3.16+/-0.77 vs 5.10+/-0.60, P<0.0001) to xamoterol (a partial beta(1)AR-selective agonist) under beta(2)AR blockade (+50 nm ICI 118, 551). AS decreased both control and ACVI-stimulated xamoterol responses (E(max): AS=2.59+/-1.42, SAS=1.38+/-0.5). ACVI response was not mimicked by IBMX. Conversely, response through beta(2) adrenergic receptor (beta(2)AR) was decreased in ACVI myocytes. 6. In conclusion, ACVI overexpression constitutively increases myocyte contraction amplitudes by raising cAMP levels. Native ACVI did not contribute to basal cAMP production or contraction amplitude and only to a minor extent to the forskolin response. beta(1)AR but not beta(2)AR coupling was dependent on ACVI.

Role of calcium in pancreatic islet cell death by IFN-gamma/TNF-alpha

We studied the intracellular events associated with pancreatic beta cell apoptosis by IFN-gamma/TNF-alpha synergism. IFN-gamma/TNF-alpha treatment of MIN6N8 insulinoma cells increased the amplitude of high voltage-activated Ca(2+) currents, while treatment with IFN-gamma or TNF-alpha alone did not. Cytosolic Ca(2+) concentration ([Ca(2+)](c)) was also increased by IFN-gamma/TNF-alpha treatment. Blockade of L-type Ca(2+) channel by nifedipine abrogated death of insulinoma cells by IFN-gamma/TNF-alpha. Diazoxide that attenuates voltage-activated Ca(2+) currents inhibited MIN6N8 cell death by IFN-gamma/TNF-alpha, while glibenclamide that accentuates voltage-activated Ca(2+) currents augmented insulinoma cell death. A protein kinase C inhibitor attenuated MIN6N8 cell death and the increase in [Ca(2+)](c) by IFN-gamma/TNF-alpha. Following the increase in [Ca(2+)](c), calpain was activated, and calpain inhibitors decreased insulinoma cell death by IFN-gamma/TNF-alpha. As a downstream of calpain, calcineurin was activated and the inhibition of calcineurin activation by FK506 diminished insulinoma cell death by IFN-gamma/TNF-alpha. BAD phosphorylation was decreased by IFN-gamma/TNF-alpha because of the increased calcineurin activity, which was reversed by FK506. IFN-gamma/TNF-alpha induced cytochrome c translocation from mitochondria to cytoplasm and activation of caspase-9. Effector caspases such as caspase-3 or -7 were also activated by IFN-gamma/TNF-alpha treatment. These results indicate that IFN-gamma/TNF-alpha synergism induces pancreatic beta cell apoptosis by Ca(2+) channel activation followed by downstream intracellular events such as mitochondrial events and caspase activation and also suggest the therapeutic potential of Ca(2+) modulation in type 1 diabetes.

Diphtheria toxin-induced autophagic cardiomyocyte death plays a pathogenic role in mouse model of heart failure

It is still not clear whether loss of cardiomyocytes through programmed cell death causes heart failure. To clarify the role of cell death in heart failure, we generated transgenic mice (TG) that express human diphtheria toxin receptor in the hearts. A mosaic expression pattern of the transgene was observed, and the transgene-expressing cardiomyocytes (17.3% of the total cardiomyocytes) were diffusely scattered throughout the ventricles. Intramuscular injection of diphtheria toxin induced complete elimination of the transgene-expressing cardiomyocytes within 7 days, and approximately 80% of TG showed pathophysiological features characteristic of heart failure and were dead within 14 days. Degenerated cardiomyocytes of the TG heart showed characteristic features indicative of autophagic cell death such as up-regulated lysosomal markers and abundant autophagosomes containing cytosolic organelles like cardiomyocytes of human dilated cardiomyopathy. The heart failure-inducible TG are a useful model for dilated cardiomyopathy, and provided evidence indicating that myocardial cell loss through autophagic cell death plays of a causal role in the pathogenesis heart failure.

Na+/Ca2+ exchanger plays a key role in inducing apoptosis after hypoxia in cultured guinea pig ventricular myocytes

Altered Na(+)/Ca(2+) exchanger (NCX) protein expression or activity is thought to contribute to various aspects of cardiac pathology. In guinea pig ventricular myocytes, NCX-mediated Ca(2+) entry is almost entirely responsible for Ca(2+) overload during hypoxia-reoxygenation. Because Ca(2+) overload is a common initiator of apoptosis, the purpose of this study was to test the hypotheses that NCX activity is critically involved in initiating apoptosis after hypoxia-reoxygenation and that hypoxia-reoxygenation-induced apoptosis can be modulated by changes in NCX protein expression or activity. An NCX antisense oligonucleotide was used to reduce NCX protein expression in cultured adult guinea pig ventricular myocytes. Caspase-3 activation and cytochrome c release were used as markers of apoptosis. Hypoxia-reoxygenation-induced apoptosis was significantly decreased in antisense-treated myocytes compared with untreated control or nonsense-treated myocytes. Pretreatment of cultured myocytes for 24 h with either endothelin-1 or phenylephrine was found to increase both NCX protein expression and evoked NCX activity as well as enhance hypoxia-reoxygenation-induced apoptosis. Control experiments demonstrated that endothelin-1 and phenylephrine did not induce apoptosis on their own nor did they enhance the apoptotic response in a model of Ca(2+)-dependent, NCX-independent apoptosis. Additional control experiments demonstrated that the NCX antisense oligonucleotide did not alter the apoptotic response of myocytes to either H(2)O(2) or isoproterenol. Taken together, these data suggest that the NCX has a critical and specific role in the initiation of apoptosis after hypoxia-reoxygenation in guinea pig myocytes and that hypoxia-reoxygenation-induced apoptosis is quite sensitive to changes in NCX activity.

Role of nitric oxide in myotoxic activity induced by crotoxin in vivo

This study was aimed to determine the role of nitric oxide on the skeletal myotoxic activity induced by crotoxin, the major component of the venom of Crotalus durissus terrificus. Rats were treated with N(G)-nitro-L-arginine methyl ester (L-NAME), a non-selective inhibitor of nitric oxide synthase or vehicle for 4 days, and on the 5th day received an intramuscular injection of crotoxin into the tibialis anterior muscle. Rats were also treated with aminoguanidine bicarbonate salt or 7-nitroindazole, inhibitors of the inducible and neuronal isoforms of nitric oxide synthase, respectively, for 4 days and on the 5th day injected with crotoxin. All treated groups were sacrificed 24 h after injection of crotoxin. Tibialis anterior and soleus muscles were removed, frozen and stored in liquid nitrogen. Histological sections were stained with toluidine blue and assayed for acid phosphatase. The results show that L-NAME significantly minimizes myonecrosis induced by crotoxin and both aminoguanidine and 7-nitroindazole partially prevented myonecrosis induced by crotoxin. Based on the present results we conclude that nitric oxide is a very important intracellular signaling molecule that mediates crotoxin myotoxic activity.

Myocyte contractile activity modulates norepinephrine cytotoxicity and survival effects of neuregulin-1β

The purpose of this study is to test the hypothesis that mechanical and electrical activity in adult rat ventricular myocytes (ARVM) alters responses to proapoptotic and prosurvival ligands. The effects of electrical stimulation on myocyte survival, stress signaling, response to β-adrenergic receptor (β-AR)-stimulated apoptosis, and neuregulin-1β (NRG) were examined. Electrical stimulation (6.6 V/cm; 0, 2, and 5 Hz; 2-ms duration; alternating polarity) of ARVM resulted in more than 70% capture. Although ARVM paced for 48 h showed higher mitochondrial uptake of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide ( P < 0.05, 0 vs. 2 and 5 Hz), electrical stimulation had little effect on cell survival assessed by trypan blue uptake, CPK release, and terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling staining. Electrical stimulation for 24 h did not induce stress response (heat shock protein 70, 90) nor stress kinase (Erk, JNK, p38) activation. NRG stimulation of Erk and Akt was similar between paced and quiescent cells. Pacing sensitized myocytes to β-AR-stimulated JNK phosphorylation and cell death with 0.1 μM norepinephrine (NE) in paced myocytes causing equivalent cytotoxicity to 10 μM NE in quiescent cells. NRG suppressed β-AR-induced apoptosis through a phosphatidylinositol-3-kinase-dependent pathway in both paced and quiescent cells, although it is overwhelmed by high-NE concentration in paced cells. Thus myocyte contractility modulates both NE cytotoxicity as well as the cytoprotective effect of NRG. These results demonstrate the feasibility and importance of using electrically paced cardiomyocytes in primary culture when examining the signaling pathways of cell survival.

Mechanical stretch induces mitochondria-dependent apoptosis in neonatal rat cardiomyocytes and G2/M accumulation in cardiac fibroblasts

Heart remodeling is associated with the loss of cardiomyocytes and increase of fibrous tissue owing to abnormal mechanical load in a number of heart disease conditions. In present study, a well-described in vitro sustained stretch model was employed to study mechanical stretch-induced responses in both neonatal cardiomyocytes and cardiac fibroblasts. Cardiomyocytes, but not cardiac fibroblasts, underwent mitochondria-dependent apoptosis as evidenced by cytochrome c (cyto c) and Smac/DIABLO release from mitochondria into cytosol accompanied by mitochondrial membrane potential (Deltapsi(m)) reduction, indicative of mitochondrial permeability transition pore (PTP) opening. Cyclosporin A, an inhibitor of PTP, inhibited stretch-induced cyto c release, Deltapsi(m) reduction and apoptosis, suggesting an important role of mitochondrial PTP in stretch-induced apoptosis. The stretch also resulted in increased expression of the pro-apoptotic Bcl-2 family proteins, including Bax and Bad, in cardiomyocytes, but not in fibroblasts. Bax was accumulated in mitochondria following stretch. Cell permeable Bid-BH3 peptide could induce and facilitate stretch-induced apoptosis and Deltapsi(m) reduction in cardiomyocytes. These results suggest that Bcl-2 family proteins play an important role in coupling stretch signaling to mitochondrial death machinery, probably by targeting to PTP. Interestingly, the levels of p53 were increased at 12 h after stretch although we observed that Bax upregulation and apoptosis occurred as early as 1 h. Adenovirus delivered dominant negative p53 blocked Bax upregulation in cardiomyocytes but showed partial effect on preventing stretch-induced apoptosis, suggesting that p53 was only partially involved in mediating stretch-induced apoptosis. Furthermore, we showed that p21 was upregulated and cyclin B1 was downregulated only in cardiac fibroblasts, which may be associated with G2/M accumulation in response to mechanical stretch.

Bcl-2 on the endoplasmic reticulum: protecting the mitochondria from a distance

Bcl-2 has been described both as an inhibitor of programmed cell death and as an inhibitor of mitochondrial dysfunction during apoptosis. It is still not clear what biochemical activity of Bcl-2 is responsible for its function, but increasing evidence indicates that a functional activity of Bcl-2 on the endoplasmic reticulum (ER) protects mitochondria under diverse circumstances. Indeed, an emerging hypothesis is that, during apoptosis, the Bcl-2 family regulates ER-to-mitochondrion communication by BH3-only proteins and calcium ions and thereby triggers mitochondrial dysfunction and cell death.

Calcineurin Abeta gene targeting predisposes the myocardium to acute ischemia-induced apoptosis and dysfunction

Cardiovascular disease is the leading cause of mortality and morbidity within the industrialized nations of the world, with coronary heart disease (CHD) accounting for as much as 66% of these deaths. Acute myocardial infarction is a typical sequelae associated with long-standing coronary heart disease resulting in large scale loss of ventricular myocardium through both apoptotic and necrotic cell death. In this study, we investigated the role that the calcium calmodulin-activated protein phosphatase calcineurin (PP2B) plays in modulating cardiac apoptosis after acute ischemia-reperfusion injury to the heart. Calcineurin Abeta gene-targeted mice showed a greater loss of viable myocardium, enhanced DNA laddering and TUNEL, and a greater loss in functional performance compared with strain-matched wild-type control mice after ischemia-reperfusion injury. RNA expression profiling was performed to uncover potential mechanisms associated with this loss of cardioprotection. Interestingly, calcineurin Abeta-/- hearts were characterized by a generalized downregulation in gene expression representing approximately 6% of all genes surveyed. Consistent with this observation, nuclear factor of activated T cells (NFAT)-luciferase reporter transgenic mice showed reduced expression in calcineurin Abeta-/- hearts at baseline and after ischemia-reperfusion injury. Finally, expression of an activated NFAT mutant protected cardiac myocytes from apoptotic stimuli, whereas directed inhibition of NFAT augmented cell death. These results represent the first genetic loss-of-function data showing a prosurvival role for calcineurin-NFAT signaling in the heart.

Phosphoproteome analysis of cardiomyocytes subjected to beta-adrenergic stimulation: identification and characterization of a cardiac heat shock protein p20

Posttranslational modification of target substrates underlies biological processes through activation/inactivation of signaling cascades. To concurrently identify the phosphoprotein substrates associated with cardiac beta-adrenergic signaling, the mouse myocyte phosphoproteome was analyzed using 2-D gel electrophoresis in combination with 32P autoradiography. Phosphoprotein spots, detected by silver staining, were identified using MALDI-TOF mass spectrometry in conjunction with computer-assisted protein spot matching. Stimulation with isoproterenol (1 micromol/L for 5 minutes) was associated with maximal increases in myocyte contractile parameters, and significant stimulation of the phosphorylation of troponin I (190+/-23%) and succinyl CoA synthetase (160+/-16%), whereas the phosphorylation of pyruvate dehydrogenase (48+/-10%), NADH-ubiquinone oxidoreductase (46+/-6%), heat shock protein 27 (18+/-3%), alphaB-crystallin (20+/-3%), and an unidentified 26-kDa protein (29+/-7%) was significantly decreased, compared with unstimulated cells (100%). After sustained (30 minutes) stimulation with isoproterenol, only the alterations in the phosphorylation levels of troponin I and NADH-ubiquinone oxidoreductase were maintained and de novo phosphorylation of a phosphoprotein (approximately 20 kDa and pI 5.5) was observed. The tryptic peptide fragments of this phosphoprotein were sequenced using postsource decay mass spectrometry, and the protein was subsequently cloned and designated as p20, based on its high sequence homology with rat and human skeletal p20. The mouse cardiac p20 contains the conserved domain sequences for heat shock proteins, and the RRAS consensus sequence for cAMP-PKA substrates. LC-MS/MS phosphorylation mapping confirmed phosphorylation of Ser16 in p20 on beta-agonist stimulation. Adenoviral gene transfer of p20 was associated with significant increases in contractility and Ca transient peak in adult rat cardiomyocytes, suggesting an important role of p20 in cardiac function. These findings suggest that cardiomyocytes undergo significant posttranslational modification via phosphorylation in a multitude of proteins to dynamically fine-tune cardiac responses to beta-adrenergic signaling.

Activation of calcineurin expression in ischemia-reperfused rat heart and in human ischemic myocardium

Calcineurin (CaN) has been reported as a critical mediator for cardiac hypertrophy and cardiac myocyte apoptosis. In the present study, we investigated the activity and expression of CaN and the effect of calpain in rat heart after ischemia and reperfusion. Rat ischemic heart showed significant increase in CaN activity. Western blot analysis of normal rat heart extract with a polyclonal antibody raised against bovine CaN indicated a prominent immunoreactive band of 60 kDa (CaN A). In ischemic-reperfused hearts, the expression of CaN A was significantly low and immunoreactivity was observed in proteolytic bands of 46 kDa. This may be due to the proteolytic degradation of CaN A in ischemic tissues by m-calpain. We also noticed in vitro proteolysis of bovine cardiac CaN A by m-calpain. Immunohistochemical studies showed strong staining of immunoreactivity in rat hearts that had gone under 30 min ischemia followed by 30 min reperfusion similar to that found in human ischemic heart. Ischemia is associated with multiple alterations in the extracellular and intracellular signaling of cardiomyocytes and may act as an inducer of apoptosis. The increase in CaN activity and strong immunostaining observed in ischemic/perfused rat heart may be due to the calpain-mediated proteolysis of this phosphatase.

Ins and outs of apoptosis in cardiovascular diseases

AIM

Cardiovascular disease (CVD) is the term used to define a group of disorders of the heart and blood vessels. Apoptosis, also known as programmed cell death (PCD), is genetically programmed "cell suicide" that plays an essential role in physiological processes such as embryo development, synaptogenesis, tissue turnover and the negative selection of T-cells, as well as in many diseases, such as cancer, and autoimmune and neurodegenerative diseases. The aim of this paper is to review the most recent data concerning the role of apoptosis in CVD, concentrating on the key apoptotic pathways in cardiomyocytes that may represent potential targets for therapeutic interventions.

DATA SUMMARY

The function of apoptosis in regulating CVD has recently been extensively investigated as a possible mechanism explaining the pathophysiological significance of various forms of CVD. Despite the difficulties of studying apoptosis in cardiomyocytes, a large number of studies of cellular and animal models suggest that they have the main apoptotic pathways that are also active in other cell types. However, the role of apoptosis in human pathologies, such as heart failure, ischemic heart disease and cardiac hypertrophy is still controversial. We revised classical (TUNEL) and novel experimental approaches (knock-out and transgenic mice; high-throughput genomics and proteomics) to address the role of apoptosis in CVD, concentrating on potential targets for therapeutic intervention.

CONCLUSION

Knowledge of the basic mechanisms regulating apoptosis activation and inhibition in cardiomyocytes may have important clinical and therapeutic implications.

Serotonin is a novel survival factor of cardiomyocytes: mitochondria as a target of 5-HT2B receptor signaling

Identification of factors regulating cardiomyocyte survival and growth is important to understand the pathogenesis of congenital heart diseases. Little is known about the molecular mechanism of cardiac functions triggered by serotonin. The link between signaling circuitry of external stimuli and the mitochondrial apoptotic machinery is of wide interest in cardiac diseases. Using cultured cardiomyocytes and 5-hydroxytryptamine (5-HT)2B-receptor knockout mice as an animal model of dilated cardiomyopathy, for the first time we show that serotonin via the Gq-coupled 5-HT2B-receptor protect cardiomyocytes against serum deprivation-induced apoptosis as manifested by DNA fragmentation, nuclear chromatin condensation, and TUNEL labeling. Serotonin prevents cytochrome c release and caspase-9 and -3 activation after serum deprivation via cross-talks between phosphatidylinositol-3 kinase/Akt and extracellular signal-regulated kinase (ERK) 1/2 signaling pathways. Serotonin binding to 5-HT2B-receptor activates ERK kinases to inhibit Bax expression induced by serum deprivation. Serotonin via phosphatidylinositol-3 kinase/Akt can activate NF-kappaB that is required for the regulation of the mitochondrial adenine nucleotide translocator (ANT-1). Parallel to these observations, ultrastructural analysis in the 5-HT2B-receptor knockout mice heart revealed pronounced mitochondrial defects in addition to altered mitochondrial enzyme activities (cytochrome oxidase and succinate dehydrogenase) and ANT-1 and Bax expressions. These findings identify 5-HT as a novel survival factor targeting mitochondria in cardiomyocytes.

A novel mechanism of myocyte degeneration involving the Ca2+-permeable growth factor-regulated channel

Disruption of the dystrophin-glycoprotein complex caused by genetic defects of dystrophin or sarcoglycans results in muscular dystrophy and/or cardiomyopathy in humans and animal models. However, the key early molecular events leading to myocyte degeneration remain elusive. Here, we observed that the growth factor-regulated channel (GRC), which belongs to the transient receptor potential channel family, is elevated in the sarcolemma of skeletal and/or cardiac muscle in dystrophic human patients and animal models deficient in dystrophin or delta-sarcoglycan. However, total cell GRC does not differ markedly between normal and dystrophic muscles. Analysis of the properties of myotubes prepared from delta-sarcoglycan-deficient BIO14.6 hamsters revealed that GRC is activated in response to myocyte stretch and is responsible for enhanced Ca2+ influx and resultant cell damage as measured by creatine phosphokinase efflux. We found that cell stretch increases GRC translocation to the sarcolemma, which requires entry of external Ca2+. Consistent with these findings, cardiac-specific expression of GRC in a transgenic mouse model produced cardiomyopathy due to Ca2+ overloading, with disease expression roughly parallel to sarcolemmal GRC levels. The results suggest that GRC is a key player in the pathogenesis of myocyte degeneration caused by dystrophin-glycoprotein complex disruption.