Nuclear targeting of Akt enhances kinase activity and survival of cardiomyocytes

Atrial natriuretic peptide promotes cardiomyocyte survival by cGMP-dependent nuclear accumulation of zyxin and Akt

This study delineates a mechanism for antiapoptotic signaling initiated by atrial natriuretic peptide (ANP) stimulation leading to elevation of cGMP levels and subsequent nuclear accumulation of Akt kinase associated with zyxin, a cytoskeletal LIM-domain protein. Nuclear targeting of zyxin induces resistance to cell death coincident with nuclear accumulation of activated Akt. Nuclear translocation of zyxin triggered by cGMP also promotes nuclear Akt accumulation. Additional supportive evidence for nuclear accumulation of zyxin-enhancing cardiomyocyte survival includes the following: (a) promotion of zyxin nuclear localization by cardioprotective stimuli; (b) zyxin association with phospho-Akt473 induced by cardioprotective stimuli; and (c) recruitment of zyxin to the nucleus by activated nuclear-targeted Akt as well as recruitment of Akt by nuclear-targeted zyxin. Nuclear accumulation of zyxin requires both Akt activation and nuclear localization. Potentiation of cell survival is sensitive to stimulation intensity with high-level induction by ANP or cGMP signaling leading to apoptotic cell death rather than enhancing resistance to apoptotic stimuli. Myocardial nuclear accumulation of zyxin and Akt responds similarly in vivo following treatment of mice with ANP or cGMP. Thus, zyxin and activated Akt participate in a cGMP-dependent signaling cascade leading from ANP receptors to nuclear accumulation of both molecules. Nuclear accumulation of zyxin and activated Akt may represent a fundamental mechanism that facilitates nuclear-signal transduction and potentiates cell survival.

Differential regulation of cardiomyocyte survival and hypertrophy by MDM2, an E3 ubiquitin ligase

MDM2 is an E3 ubiquitin ligase that regulates the proteasomal degradation and activity of proteins involved in cell growth and apoptosis, including the tumor suppressors p53 and retinoblastoma and the transcription factor E2F1. Although the effect of several MDM2 targets on cardiomyocyte survival and hypertrophy has already been investigated, the role of MDM2 in these processes has not yet been established. We have, therefore, analyzed the effect of overexpression as well as inhibition of MDM2 on cardiac ischemia/reperfusion injury and hypertrophy. Here we show that isolated cardiac myocytes overexpressing MDM2 acquired resistance to hypoxia/reoxygenation-induced cell death. Conversely, inactivation of MDM2 by a peptide inhibitor resulted in elevated p53 levels and promoted hypoxia/reoxygenation-induced apoptosis. Consistent with this, decreased expression of MDM2 in a genetic mouse model was accompanied by reduced functional recovery of the left ventricles determined with the Langendorff ex vivo model of ischemia/reperfusion. In contrast to cell survival, cell hypertrophy induced by the alpha-agonists phenylephrine or endothelin-1 was inhibited by MDM2 overexpression. Collectively, our studies indicate that MDM2 promotes survival and attenuates hypertrophy of cardiac myocytes. This differential regulation of cell growth and cell survival is unique, because most other survival factors are prohypertrophic. MDM2, therefore, might be a potential therapeutic target to down-regulate both cell death and pathologic hypertrophy during remodeling upon cardiac infarction. In addition, our data also suggest that cancer treatments with MDM2 inhibitors to reactivate p53 may have adverse cardiac side effects by promoting cardiomyocyte death.

PIKE/nuclear PI 3-kinase signaling in preventing programmed cell death

PI 3-kinase enhancer (PIKE) is a nuclear GTPase that enhances PI 3-kinase (PI3K) activity. Nerve growth factor (NGF) treatment leads to PIKE activation by triggering the nuclear translocation of PLC-gamma1, which acts as a physiological guanine nucleotide exchange factor (GEF) for PIKE. PI3K occurs in the nuclei of a broad range of cell types, and various stimuli elicit PI3K nuclear translocation. While cytoplasmic PI3K has been well characterized, little is known about the biological function of nuclear PI3K. Surprisingly, nuclei from 30 min NGF-treated PC12 cells are resistant to DNA fragmentation initiated by the activated cell-free apoptosome, and both PIKE and nuclear PI3K are sufficient and necessary for this effect. Moreover, pretreatment of the control nucleus with PI(3,4,5)P3 alone mimics the anti-apoptotic activity of NGF by selectively preventing apoptosis, for which nuclear Akt is required but not sufficient. Recently, a nuclear PI(3,4,5)P3 receptor, nucleophosmin/B23, has been identified from NGF-treated PC12 nuclear extract. PI(3,4,5)P3/B23 complex mediates the anti-apoptotic effects of NGF by inhibiting DNA fragmentation activity of caspase-activated DNase (CAD). Thus, PI(3,4,5)P3/B23 complex and nuclear Akt effectors might coordinately mediate PIKE/nuclear PI3K signaling in promoting cell survival by NGF.

Nuclear targeting of Akt enhances ventricular function and myocyte contractility

Cytoplasmic overexpression of Akt in the heart results in a myopathy characterized by organ and myocyte hypertrophy. Conversely, nuclear-targeted Akt does not lead to cardiac hypertrophy, but the cellular basis of this distinct heart phenotype remains to be determined. Similarly, whether nuclear-targeted Akt affects ventricular performance and mechanics, calcium metabolism, and electrical properties of myocytes is unknown. Moreover, whether the expression and state of phosphorylation of regulatory proteins implicated in calcium cycling and myocyte contractility are altered in nuclear-targeted Akt has not been established. We report that nuclear overexpression of Akt does not modify cardiac size and shape but results in an increased number of cardiomyocytes, which are smaller in volume. Additionally, the heart possesses enhanced systolic and diastolic function, which is paralleled by increased myocyte performance. Myocyte shortening and velocity of shortening and relengthening are increased in transgenic mice and are coupled with a more efficient reuptake of calcium by the sarcoplasmic reticulum (SR). This process increases calcium loading of the SR during relengthening. The enhanced SR function appears to be mediated by an increase in SR Ca2+-ATPase2a activity sustained by a higher degree of phosphorylation of phospholamban. This posttranslational modification was associated with an increase in phospho-protein kinase A and a decrease in protein phosphatase-1. Together, these observations provide a plausible biochemical mechanism for the potentiation of myocyte and ventricular function in Akt transgenic mice. Therefore, nuclear-targeted Akt in myocytes may have important implications for the diseased heart.

Class reunion: PTEN joins the nuclear crew

Several recent reports have brought conclusive evidence that the tumor suppressor PTEN, once considered a strictly cytoplasmic protein, shuttles to the nuclear compartment, where it joins a variety of components of the same pathway it regulates in the cytoplasm, among which PI3K, PDK1 and AKT. In this review, we focus on the growing supporting evidence for an important physiological role of this nuclear pathway and on the role that alteration of this novel regulatory circuit may play during cell transformation.

Ca2+ Dysregulation Induces Mitochondrial Depolarization and Apoptosis

We previously reported that constitutively activated Galpha(q) (Q209L) expression in cardiomyocytes induces apoptosis through opening of the mitochondrial permeability transition pore. We assessed the hypothesis that disturbances in Ca(2+) handling linked Galpha(q) activity to apoptosis because resting Ca(2+) levels were significantly increased prior to development of apoptosis. Treating cells with EGTA lowered Ca(2+) and blocked both loss of mitochondrial membrane potential (an indicator of permeability transition pore opening) and apoptosis (assessed by DNA fragmentation). When cytosolic Ca(2+) and mitochondrial membrane potential were simultaneously measured by confocal microscopy, sarcoplasmic reticulum (SR)-driven slow Ca(2+) oscillations (time-to-peak approximately 4 s) were observed in Q209L-expressing cells. These oscillations were seen to transition into sustained increases in cytosolic Ca(2+), directly paralleled by loss of mitochondrial membrane potential. Ca(2+) transients generated by caffeine-induced release of SR Ca(2+) were greatly prolonged in Q209L-expressing cells, suggesting a decreased ability to extrude Ca(2+). Indeed, the Na(+)/Ca(2+) exchanger (NCX), which removes Ca(2+) from the cell, was markedly down-regulated at the mRNA and protein levels. Adenoviral NCX expression normalized cytosolic Ca(2+) levels and prevented DNA fragmentation in cells expressing Q209L. Interestingly, constitutively activated Akt, which rescues cells from Q209L-induced apoptosis, prevented the decrease in NCX expression, normalized cytosolic Ca(2+) levels and spontaneous Ca(2+) oscillations, shortened caffeine-induced Ca(2+) transients, and prevented loss of the mitochondrial membrane potential. Our findings demonstrate that NCX down-regulation and consequent increases in cytosolic and SR Ca(2+) can lead to Ca(2+) overloading-induced loss of mitochondrial membrane potential and suggest that recovery of Ca(2+) dysregulation is a target of Akt-mediated protection.

Cardiac stem cells and mechanisms of myocardial regeneration

This review discusses current understanding of the role that endogenous and exogenous progenitor cells may have in the treatment of the diseased heart. In the last several years, a major effort has been made in an attempt to identify immature cells capable of differentiating into cell lineages different from the organ of origin to be employed for the regeneration of the damaged heart. Embryonic stem cells (ESCs) and bone marrow-derived cells (BMCs) have been extensively studied and characterized, and dramatic advances have been made in the clinical application of BMCs in heart failure of ischemic and nonischemic origin. However, a controversy exists concerning the ability of BMCs to acquire cardiac cell lineages and reconstitute the myocardium lost after infarction. The recognition that the adult heart possesses a stem cell compartment that can regenerate myocytes and coronary vessels has raised the unique possibility to rebuild dead myocardium after infarction, to repopulate the hypertrophic decompensated heart with new better functioning myocytes and vascular structures, and, perhaps, to reverse ventricular dilation and wall thinning. Cardiac stem cells may become the most important cell for cardiac repair.

Disruption of coordinated cardiac hypertrophy and angiogenesis contributes to the transition to heart failure

Although increased external load initially induces cardiac hypertrophy with preserved contractility, sustained overload eventually leads to heart failure through poorly understood mechanisms. Here we describe a conditional transgenic system in mice characterized by the sequential development of adaptive cardiac hypertrophy with preserved contractility in the acute phase and dilated cardiomyopathy in the chronic phase following the induction of an activated Akt1 gene in the heart. Coronary angiogenesis was enhanced during the acute phase of adaptive cardiac growth but reduced as hearts underwent pathological remodeling. Enhanced angiogenesis in the acute phase was associated with mammalian target of rapamycin-dependent induction of myocardial VEGF and angiopoietin-2 expression. Inhibition of angiogenesis by a decoy VEGF receptor in the acute phase led to decreased capillary density, contractile dysfunction, and impaired cardiac growth. Thus, both heart size and cardiac function are angiogenesis dependent, and disruption of coordinated tissue growth and angiogenesis in the heart contributes to the progression from adaptive cardiac hypertrophy to heart failure.

Cardioprotection afforded by NF-kappaB ablation is associated with activation of Akt in mice overexpressing TNF-alpha

When selectively overexpressed in mouse heart, TNF-alpha effects the development of a cardiomyopathy that closely mimics that seen in human failing hearts. It has been suggested that two intracellular signaling pathways, the Akt protein kinase and the NF-kappaB transcription factor, mediated TNF-alpha signaling. The present experiments assessed the effects of TNF-alpha overexpression on these two target proteins in vivo. We measured cardiac Akt kinase phosphorylation and NF-kappaB activity in mice overexpressing TNF-alpha (TNF1.6). Both basal and insulin-stimulated Akt phosphorylation were reduced by almost 70% by TNF-alpha overexpression. By contrast, NF-kappaB was robustly activated. These effects were absent when TNF-alpha receptor 1 (TNFR1) was selectively ablated. Cardiomyocyte-specific overexpression of the dominant-negative inhibitory kappaB protein transgene and subsequent inhibition of NF-kappaB activity attenuated the effects of TNF-alpha on Akt phosphorylation. NF-kappaB inhibition also significantly improved fractional shortening and diminished ventricular hypertrophy and survival without affecting infiltrative inflammation or cytokine expression. Thus, while overexpression of TNF-alpha effected a marked Akt inhibition and NF-kappaB activation in mouse hearts, inhibition of NF-kappaB offered salutary benefits mediated at least in part through activation of Akt.

Akt phosphorylates acinus and inhibits its proteolytic cleavage, preventing chromatin condensation

Akt promotes cell survival by phosphorylating and inhibiting components of the intrinsic cell death machinery. Akt translocates into the nucleus upon exposure of cells to survival factors, but little is known about its functions in the nucleus. Here, we show that acinus, a nuclear factor required for apoptotic chromatin condensation, is a direct target of Akt. We demonstrate that Akt phosphorylation of acinus on serine 422 and 573 results in its resistance to caspase cleavage in the nucleus and the inhibition of acinus-dependent chromatin condensation. Abolishing acinus phosphorylation by Akt through mutagenesis accelerates its proteolytic degradation and chromatin condensation. Acinus S422, 573D, a mutant mimicking phosphorylation, resists against apoptotic cleavage and prevents chromatin condensation. Knocking down of acinus substantially decreases chromatin condensation, and depletion of Akt provokes the apoptotic cleavage of acinus. Thus, Akt inhibits chromatin condensation during apoptosis by phosphorylating acinus in the nucleus, revealing a specific mechanism by which nuclear Akt promotes cell survival.

Characterization of the protective effects of cardiotrophin-1 against non-ischemic death stimuli in adult cardiomyocytes

The aim of this study was to investigate the cytoprotective effects of CT-1 against non-ischemic death stimuli in adult cardiomyocytes. Primary cultures of cardiomyocytes isolated from adult rats were stimulated with either angiotensin II (Ang II) or H(2)O(2) in the presence or absence of CT-1. Cell death was determined by trypan blue exclusion, cell viability by MTT assay and apoptosis by TUNEL-Annexin-V staining. Intracellular pathways were analyzed by the employment of chemical inhibitors and by the assessment of signalling intermediates phosphorylation by Western blot analysis. CT-1 reduced (p<0.01) total cell death and apoptosis induced by either Ang II or H(2)O(2), and increased (p<0.01) cell viability in cardiomyocytes exposed to these stimuli. These effects of CT-1 were abolished in the presence of antibodies specific for gp130 or LIFR and did not require RNA or protein synthesis. Both Wortmannin and PD98059 abolished protective effects of CT-1 against H(2)O(2), whereas only Wortmannin inhibited protection against Ang II. In both cases, Akt kinase activation and Bad phosphorylation were observed. These findings suggest that CT-1 protects adult cardiomyocytes against Ang II- and oxidative stress-induced cell death, via gp130/LIFR and by means of the PI3K/Akt and the p42/44 MAPK intracellular cascades.

Serum and Glucocorticoid-Responsive Kinase-1 Regulates Cardiomyocyte Survival and Hypertrophic Response

Background— Serum- and glucocorticoid-responsive kinase-1 (SGK1), a serine-threonine kinase that is highly expressed in the heart, has been previously reported to regulate sodium channels. Because SGK1 is a PI 3-kinase–dependent kinase with structural homology to Akt, we examined its regulation in the heart and its effects on cardiomyocyte (CM) apoptosis and hypertrophy in vitro.

Methods and Results— Rats were subjected to aortic banding, and expression of total and phosphorylated SGK1 was examined. Both phospho- and total SGK1 increased 2 to 7 days after banding. Phospho-SGK1 was also upregulated in CMs stimulated in vitro with IGF-I or phenylephrine. Infection of CMs with an adenoviral vector encoding constitutively active SGK1 (Ad.SGK1.CA) inhibited apoptosis after serum-deprivation or hypoxia ( P <0.05), whereas expression of kinase-dead SGK1 (Ad.SGK1.KD) increased it and partially mitigated the protective effects of IGF-I ( P <0.05). SGK1 activation was also sufficient to increase cell size, protein synthesis, sarcomere organization, and ANF expression both at baseline and in response to phenylephrine but was not necessary for the hypertrophic response to phenylephrine. Evaluation of potential downstream signaling pathways demonstrated that SGK1 induces phosphorylation of tuberin, p70s6kinase, and GSK3β in CMs, which may contribute to its effects.

Conclusions— SGK1 is dynamically regulated during acute biomechanical stress in the heart and inhibits CM apoptosis while enhancing the hypertrophic response.

The FOXO3a transcription factor regulates cardiac myocyte size downstream of AKT signaling

Although signaling mechanisms inducing cardiac hypertrophy have been extensively studied, little is known about the mechanisms that reverse cardiac hypertrophy. Here, we describe the existence of a similar Akt/forkhead signaling axis in cardiac myocytes in vitro and in vivo, which is regulated by insulin, insulin-like growth factor (IGF), stretch, pressure overload, and angiotensin II stimulation. FOXO3a gene transfer prevented both IGF and stretch-induced hypertrophy in rat neonatal cardiac myocyte cultures in vitro. Transduction with FOXO3a also caused a significant reduction in cardiomyocyte size in mouse hearts in vivo. Akt/FOXO signaling regulated the expression of multiple atrophy-related genes "atrogenes," including the ubiquitin ligase atrogin-1 (MAFbx). In cardiac myocyte cultures, transduction with constitutively active Akt or treatment with IGF suppressed atrogin-1 mRNA expression, whereas transduction with FOXO3a stimulated its expression. FOXO3a transduction activated the atrogin-1 promoter in both cultured myocytes and mouse heart. Thus, in cardiomyocytes, as in skeletal muscle, FOXO3a activates an atrogene transcriptional program, which retards or prevents hypertrophy and is down-regulated by multiple physiological and pathological stimuli of myocyte growth.

Molecular and genetic studies imply Akt-mediated signaling promotes protein kinase CbetaII alternative splicing via phosphorylation of serine/arginine-rich splicing factor SRp40

Insulin regulates alternative splicing of PKCbetaII mRNA by phosphorylation of SRp40 via a phosphatidylinositol 3-kinase pathway (Patel, N. A., Chalfant, C. E., Watson, J. E., Wyatt, J. R., Dean, N. M., Eichler, D. C., and Cooper, D. C. (2001) J. Biol. Chem. 276, 22648-22654). Transient transfection of constitutively active Akt2 kinase promotes PKCbetaII exon inclusion. Serine/arginine-rich (SR) RNA-binding proteins regulating the selection of alternatively spliced exons are potential substrates of Akt kinase because many of them contain RXRXX(S/T) motifs. Here we show that Akt2 kinase phosphorylated SRp40 in vivo and in vitro. Mutation of Ser86 on SRp40 blocked in vitro phosphorylation. In control Akt2(+/+) fibroblasts, insulin treatment increased the phosphorylation of endogenous SR proteins, but their phosphorylation state remained unaltered by insulin in fibroblasts from Akt2(-/-) mice. Levels of PKCbetaII protein were up-regulated by insulin in Akt2(+/+) cells; however, only very low levels of PKCbetaII were detected in Akt2(-/-) cells and did not change following insulin treatment. Endogenous PKCbetaI and -betaII mRNA levels in Akt2(+/+) and Akt2(-/-) gastrocnemius muscle tissues were compared using quantitative real time PCR. The results indicated a 54% decrease in the expression of PKCbetaII levels in Akt(-/-), whereas PKCbetaI levels remained unchanged in both samples. Further, transfection of Akt2(-/-) cells with a PKCbetaII splicing minigene revealed defective betaII exon inclusion. Co-transfection of the mutated SRp40 attenuated betaII exon inclusion. This study provides in vitro and in vivo evidence showing Akt2 kinase directly phosphorylated SRp40, thereby connecting the insulin, PI 3-kinase/Akt pathway with phosphorylation of a site on a nuclear splicing protein promoting exon inclusion. This model is upheld in Akt2-deficient mice with insulin resistance leading to diabetes mellitus.

Regulation of gene and protein expression in cardiac myocyte hypertrophy and apoptosis

Considerable efforts have been expended in elucidating the inter-cellular and intra-cellular signaling pathways which elicit cardiac myocyte hypertrophy or apoptosis, and in identifying the changes which are associated with the end-stage of the response. The challenge now is to link the two. Although some of the signaling effects will be the acute modulation of existing protein function, long-term effects which bring about and maintain the hypertrophic state or which culminate in cell death are mediated at the level of gene and protein expression. With the advances in micro-array technology and genome sequencing, it is now possible to obtain a picture of the global gene expression profile in myocytes or in whole heart which dictates the proteins which could be made. This is not the final picture since additional regulation at the level of translation modulates the relative proportions of each protein that can be made from the transcriptome. Even here, further regulation of protein stability and turnover means that ultimately it is still necessary to examine the proteome to determine what may cause the functional changes in a cell. Thus, in order to gain a full picture of events which regulate the response and gain some insight into possible points of intervention for therapy, it is necessary to examine gene expression, mRNA translation and protein expression in concert.

Functional proteomic analysis of a three-tier PKCepsilon-Akt-eNOS signaling module in cardiac protection

Cardiac protective signaling networks have been shown to involve PKCepsilon. However, the molecular mechanisms by which PKCepsilon interacts with other members of these networks to form task-specific modules remain unknown. Among 93 different PKCepsilon-associated proteins that have been identified, Akt and endothelial nitric oxide (NO) synthase (eNOS) are of importance because of their independent abilities to promote cell survival and prevent cell death. The simultaneous association of PKCepsilon, Akt, and eNOS has not been examined, and, in particular, the formation of a module containing these three proteins and the role of such a module in the regulation of NO production and cardiac protection are unknown. The present study was undertaken to determine whether these molecules form a signaling module and, thereby, play a collective role in cardiac signaling. Using recombinant proteins in vitro and PKCepsilon transgenic mouse hearts, we demonstrate the following: 1) PKCepsilon, Akt, and eNOS interact and form signaling modules in vitro and in the mouse heart. Activation of either PKCepsilon or Akt enhances the formation of PKCepsilon-Akt-eNOS signaling modules. 2) PKCepsilon directly phosphorylates and enhances activation of Akt in vitro, and PKCepsilon activation increases phosphorylation and activation of Akt in PKCepsilon transgenic mouse hearts. 3) PKCepsilon directly phosphorylates eNOS in vitro, and this phosphorylation enhances eNOS activity. Activation of PKCepsilon in vivo increased phosphorylation of eNOS at Ser(1177), indicating eNOS activation. This study characterizes, for the first time, the physical, as well as functional, coupling of PKCepsilon, Akt, and eNOS in the heart and implicates these PKCepsilon-Akt-eNOS signaling modules as critical signaling elements during PKCepsilon-induced cardiac protection.

Molecular determinants of the physiological adaptation to stress in the cardiomyocyte: a focus on AKT

Cardiomyocytes (CMCs) adapt to physiological or pathological stimuli by undergoing molecular changes which differentiate according to the specificity of the stimulus and eventually generate a phenotype with peculiar molecular characteristics. Here, we review the literature on the molecular mechanisms activated in the CMC during physiologic adaptation to stress, as opposed to maladaptation. The critical role of the IGF-1 receptor/PI3K/Akt signaling pathway during this process is described, including effector targets regulating inotropism and cell size.

The biochemical response of the heart to hypertension and exercise

Mechanical stress on the heart can lead to crucially different outcomes. Exercise is beneficial because it causes heart muscle cells to enlarge (hypertrophy). Chronic hypertension also causes hypertrophy, but in addition it causes an excessive increase in fibroblasts and extracellular matrix (fibrosis), death of cardiomyocytes and ultimately heart failure. Recent research shows that stimulation of physiological (beneficial) hypertrophy involves several signaling pathways, including those mediated by protein kinase B (also known as Akt) and the extracellular-signal-regulated kinases 1 and 2 (ERK1/2). Hypertension, beta-adrenergic stimulation and agonists such as angiotensin II (Ang II) activate not only ERK1/2 but also p38 and the Jun N-terminal kinase (JNK), leading to pathological heart remodeling. Despite this progress, the mechanisms that activate fibroblasts to cause fibrosis and those that differentiate between exercise and hypertension to produce physiological and pathological responses, respectively, remain to be established.