Caspase-3-mediated cleavage of PICOT in apoptosis.

Mammalian protein kinase C-interacting cousin of thioredoxin (PICOT) is a multi-domain mono-thiol glutaredoxin that is involved in several signal transduction pathways and is necessary for cell growth and metastasis. Here, we demonstrate that PICOT is a cleavage substrate of the apoptosis-related protein caspase-3. In vitro cleavage assays indicated that PICOT was specifically cleaved by caspase-3. Similarly, endogenous PICOT was cleaved in cell death responses induced by staurosporine and etoposide. These phenomena were blocked in the presence of a pan-caspase inhibitor. Using site-directed mutagenesis, we identified two putative caspase-3 cleavage sequences in PICOT, DRLD(101)/G and EELD(226)/T. Interestingly, overexpression of either PICOT wild type or the D101A/D226A double point mutant accelerated etoposide-induced activation of caspase-3 whereas siRNA-mediated knockdown of PICOT blocked this phenomenon. Our data raise the possibility that the pro-apoptotic role of PICOT is actively regulated via caspase-3-mediated cleavage.

Targeted ablation of the histidine-rich Ca(2+)-binding protein (HRC) gene is associated with abnormal SR Ca(2+)-cycling and severe pathology under pressure-overload stress.

The histidine-rich Ca(2+)-binding protein (HRC) is located in the lumen of the sarcoplasmic reticulum (SR) and exhibits high-capacity Ca(2+)-binding properties. Overexpression of HRC in the heart resulted in impaired SR Ca(2+) uptake and depressed relaxation through its interaction with SERCA2a. However, the functional significance of HRC in overall regulation of calcium cycling and contractility is not currently well defined. To further elucidate the role of HRC in vivo under physiological and pathophysiological conditions, we generated and characterized HRC-knockout (KO) mice. The KO mice were morphologically and histologically normal compared to wild-type (WT) mice. At the cellular level, ablation of HRC resulted in significantly enhanced contractility, Ca(2+) transients, and maximal SR Ca(2+) uptake rates in the heart. However, after-contractions were developed in 50 % of HRC-KO cardiomyocytes, compared to 11 % in WT mice under stress conditions of high-frequency stimulation (5 Hz) and isoproterenol application. A parallel examination of the electrical activity revealed significant increases in the occurrence of Ca(2+) spontaneous SR Ca(2+) release and delayed afterdepolarizations with ISO in HRC-KO, compared to WT cells. The frequency of Ca(2+) sparks was also significantly higher in HRC-KO cells with ISO, consistent with the elevated SR Ca(2+) load in the KO cells. Furthermore, HRC-KO cardiomyocytes showed significantly deteriorated cell contractility and Ca(2+)-cycling caused possibly by depressed SERCA2a expression after transverse-aortic constriction (TAC). Also HRC-null mice exhibited severe cardiac hypertrophy, fibrosis, pulmonary edema and decreased survival after TAC. Our results indicate that ablation of HRC is associated with poorly regulated SR Ca(2+)-cycling, and severe pathology under pressure-overload stress, suggesting an essential role of HRC in maintaining the integrity of cardiac function.

PICOT increases cardiac contractility by inhibiting PKCζ activity.

Protein kinase C (PKC)-interacting cousin of thioredoxin (PICOT) has distinct anti-hypertrophic and inotropic functions. We have previously shown that PICOT exerts its anti-hypertrophic effect by inhibiting calcineurin-NFAT signaling through its C-terminal glutaredoxin domain. However, the mechanism underlying the inotropic effect of PICOT is unknown. The results of protein pull-down experiments showed that PICOT directly binds to the catalytic domain of PKCζ through its N-terminal thioredoxin-like domain. Purified PICOT protein inhibited the kinase activity of PKCζ in vitro, which indicated that PICOT is an endogenous inhibitor of PKCζ. The inhibition of PKCζ activity with a PKCζ-specific pseudosubstrate peptide inhibitor was sufficient to increase the cardiac contractility in vitro and ex vivo. Overexpression of PICOT or inhibition of PKCζ activity down-regulated PKCα activity, which led to the elevation of sarcoplasmic reticulum Ca(2+)-ATPase (SERCA) 2a activity, concomitant with the increased phosphorylation of phospholamban (PLB). Overexpression of PICOT or inhibition of PKCζ activity also down-regulated protein phosphatase (PP) 2A activity, which subsequently resulted in the increased phosphorylation of troponin (Tn) I and T, key myofilament proteins associated with the regulation of contractility. PICOT appeared to inhibit PP2A activity through the disruption of the functional PKCζ/PP2A complex. In contrast to the overexpression of PICOT or inhibition of PKCζ, reduced PICOT expression resulted in up-regulation of PKCα and PP2A activities, followed by decreased phosphorylation of PLB, and TnI and T, respectively, supporting the physiological relevance of these events. Transgene- or adeno-associated virus (AAV)-mediated overexpression of PICOT restored the impaired contractility and prevented further morphological and functional deterioration of the failing hearts. Taken together, the results of the present study suggest that PICOT exerts its inotropic effect by negatively regulating PKCα and PP2A activities through the inhibition of PKCζ activity. This finding provides a novel insight into the regulation of cardiac contractility.

The chemical chaperone 4-phenylbutyric acid attenuates pressure-overload cardiac hypertrophy by alleviating endoplasmic reticulum stress.

Evidence has shown that endoplasmic reticulum stress (ERS) is associated with the pathogenesis of cardiac hypertrophy. The aim of this study was to investigate whether direct alleviation of ER stress by 4-phenylbutyric acid (PBA), a known chemical chaperone drug, could attenuate pressure-overload cardiac hypertrophy in mice. The effects of orally administered PBA (100mg/kg body weight daily for a week) were examined using mice undergoing transverse aortic constriction (TAC-mice), an animal model to produce pressure overload. TAC application for 1 week led to a 1.8-fold increase in the ratio of the heart weight over body weight (HW/BW) and up-regulation of the hypertrophy markers ANF and BNF accompanied by up-regulation of ERS markers (GRP78, p-PERK, and p-elF2α). The oral administration of PBA to the TAC-mice reduced hypertrophy (19%) and severely downregulated the fibrosis-related genes (transforming growth factor-ß1, phospho-smad2, and pro-collagen isoforms). We conclude that ERS is induced as a consequence of remodeling during pathological hypertrophy and that PBA may help to relieve ERS and play a protective role against cardiac hypertrophy and possibly heart failure. We suggest PBA as a novel therapeutic agent for cardiac hypertrophy and fibrosis.

The opposing effects of CCN2 and CCN5 on the development of cardiac hypertrophy and fibrosis.

CCN family members are matricellular proteins with diverse roles in cell function. The differential expression of CCN2 and CCN5 during cardiac remodeling suggests that these two members of the CCN family play opposing roles during the development of cardiac hypertrophy and fibrosis. We aimed to evaluate the role of CCN2 and CCN5 in the development of cardiac hypertrophy and fibrosis. In isolated cardiomyocytes, overexpression of CCN2 induced hypertrophic growth, whereas the overexpression of CCN5 inhibited both phenylephrine (PE)- and CCN2-induced hypertrophic responses. Deletion of the C-terminal (CT) domain of CCN2 transformed CCN2 into a CCN5-like dominant negative molecule. Fusion of the CT domain to the Carboxy-terminus of CCN5 transformed CCN5 into a CCN2-like pro-hypertrophic molecule. CCN2 transgenic (TG) mice did not develop cardiac hypertrophy at baseline but showed significantly increased fibrosis in response to pressure overload. In contrast, hypertrophy and fibrosis were both significantly inhibited in CCN5 TG mice. CCN2 TG mice showed an accelerated deterioration of cardiac function in response to pressure overload, whereas CCN5 TG mice showed conserved cardiac function. TGF-beta-SMAD signaling was elevated in CCN2 TG mice, but was inhibited in CCN5 TG mice. CCN2 is pro-hypertrophic and -fibrotic, whereas CCN5 is anti-hypertrophic and -fibrotic. CCN5 lacking the CT domain acts as a dominant negative molecule. CCN5 may provide a novel therapeutic target for the treatment of cardiac hypertrophy and heart failure.

PICOT attenuates cardiac hypertrophy by disrupting calcineurin-NFAT signaling.

PICOT (protein kinase C-interacting cousin of thioredoxin) was previously shown to inhibit pressure overload-induced cardiac hypertrophy, concomitant with an increase in ventricular function and cardiomyocyte contractility. The combined analyses of glutathione S-transferase pull-down experiments and mass spectrometry enabled us to determine that PICOT directly interacts with muscle LIM protein (MLP) via its carboxyl-terminal half (PICOT-C). It was also shown that PICOT colocalizes with MLP in the Z-disc. MLP is known to play a role in anchoring calcineurin to the Z-disc in the sarcomere, which is critical for calcineurin-NFAT (nuclear factor of activated T cells) signaling. We, therefore, suggested that PICOT may affect calcineurin-NFAT signaling through its interaction with MLP. Consistent with this hypothesis, PICOT, or more specifically PICOT-C, abrogated phenylephrine-induced increases in calcineurin phosphatase activity, NFAT dephosphorylation/nuclear translocation, and NFAT-dependent transcriptional activation in neonatal cardiomyocytes. In addition, pressure overload-induced upregulation of NFAT target genes was significantly diminished in the hearts of PICOT-overexpressing transgenic mice. PICOT interfered with MLP-calcineurin interactions in a dose-dependent manner. Moreover, calcineurin was displaced from the Z-disc, concomitant with an abrogated interaction between calcineurin and MLP, in the hearts of PICOT transgenic mice. Replenishment of MLP restored the hypertrophic responses and the increase in calcineurin phosphatase activity that was inhibited by PICOT in phenylephrine-treated cardiomyocytes. Finally, PICOT-C inhibited cardiac hypertrophy to an extent that was comparable to that of full-length PICOT. Taken together, these data suggest that PICOT inhibits cardiac hypertrophy largely by negatively regulating calcineurin-NFAT signaling via disruption of the MLP-calcineurin interaction.

PICOT is a critical regulator of cardiac hypertrophy and cardiomyocyte contractility.

PICOT (PKC-interacting cousin of thioredoxin) was previously shown to inhibit the development of cardiac hypertrophy, concomitant with an increase in cardiomyocyte contractility. To explore the physiological function of PICOT in the hearts, we generated a PICOT-deficient mouse line by using a gene trap approach. PICOT(-/-) mice were embryonic lethal indicating that PICOT plays an essential role during embryogenesis, whereas PICOT(+/-) mice were viable with no apparent morphological defects. The PICOT protein levels were reduced by about 50% in the hearts of PICOT(+/-) mice. Significantly exacerbated cardiac hypertrophy was induced by pressure overload in PICOT(+/-) mice relative to that seen in wild type littermates. In line with this observation, calcineurin-NFAT signaling was greatly enhanced by pressure overload in the hearts of PICOT(+/-) mice. Cardiomyocytes from PICOT(+/-) mice exhibited significantly reduced contractility, which may be due in part to hypophosphorylation of phospholamban and reduced SERCA activity. These data indicate that the precise PICOT protein level significantly affects the process of cardiac hypertrophy and cardiomyocyte contractility. We suggest that PICOT plays as a critical negative regulator of cardiac hypertrophy and a positive inotropic regulator.

PICOT inhibits cardiac hypertrophy and enhances ventricular function and cardiomyocyte contractility.

Multiple signaling pathways involving protein kinase C (PKC) have been implicated in the development of cardiac hypertrophy. We observed that a putative PKC inhibitor, PICOT (PKC-Interacting Cousin Of Thioredoxin) was upregulated in response to hypertrophic stimuli both in vitro and in vivo. This suggested that PICOT may act as an endogenous negative feedback regulator of cardiac hypertrophy through its ability to inhibit PKC activity, which is elevated during cardiac hypertrophy. Adenovirus-mediated gene transfer of PICOT completely blocked the hypertrophic response of neonatal rat cardiomyocytes to enthothelin-1 and phenylephrine, as demonstrated by cell size, sarcomere rearrangement, atrial natriuretic factor expression, and rates of protein synthesis. Transgenic mice with cardiac-specific overexpression of PICOT showed that PICOT is a potent inhibitor of cardiac hypertrophy induced by pressure overload. In addition, PICOT overexpression dramatically increased the ventricular function and cardiomyocyte contractility as measured by ejection fraction and end-systolic pressure of transgenic hearts and peak shortening of isolated cardiomyocytes, respectively. Intracellular Ca(2+) handing analysis revealed that increases in myofilament Ca(2+) responsiveness, together with increased rate of sarcoplasmic reticulum Ca(2+) reuptake, are associated with the enhanced contractility in PICOT-overexpressing cardiomyocytes. The inhibition of cardiac remodeling by of PICOT with a concomitant increase in ventricular function and cardiomyocyte contractility suggests that PICOT may provide an efficient modality for treatment of cardiac hypertrophy and heart failure.

AAV-mediated knock-down of HRC exacerbates transverse aorta constriction-induced heart failure.

BACKGROUND: Histidine-rich calcium binding protein (HRC) is located in the lumen of sarcoplasmic reticulum (SR) that binds to both triadin (TRN) and SERCA affecting Ca(2+) cycling in the SR. Chronic overexpression of HRC that may disrupt intracellular Ca(2+) homeostasis is implicated in pathogenesis of cardiac hypertrophy. Ablation of HRC showed relatively normal phenotypes under basal condition, but exhibited a significantly increased susceptibility to isoproterenol-induced cardiac hypertrophy. In the present study, we characterized the functions of HRC related to Ca(2+) cycling and pathogenesis of cardiac hypertrophy using the in vitro siRNA- and the in vivo adeno-associated virus (AAV)-mediated HRC knock-down (KD) systems, respectively. METHODOLOGY/PRINCIPAL FINDINGS: AAV-mediated HRC-KD system was used with or without C57BL/6 mouse model of transverse aortic constriction-induced failing heart (TAC-FH) to examine whether HRC-KD could enhance cardiac function in failing heart (FH). Initially we expected that HRC-KD could elicit cardiac functional recovery in failing heart (FH), since predesigned siRNA-mediated HRC-KD enhanced Ca(2+) cycling and increased activities of RyR2 and SERCA2 without change in SR Ca(2+) load in neonatal rat ventricular cells (NRVCs) and HL-1 cells. However, AAV9-mediated HRC-KD in TAC-FH was associated with decreased fractional shortening and increased cardiac fibrosis compared with control. We found that phospho-RyR2, phospho-CaMKII, phospho-p38 MAPK, and phospho-PLB were significantly upregulated by HRC-KD in TAC-FH. A significantly increased level of cleaved caspase-3, a cardiac cell death marker was also found, consistent with the result of TUNEL assay. CONCLUSIONS/SIGNIFICANCE: Increased Ca(2+) leak and cytosolic Ca(2+) concentration due to a partial KD of HRC could enhance activity of CaMKII and phosphorylation of p38 MAPK, causing the mitochondrial death pathway observed in TAC-FH. Our results present evidence that down-regulation of HRC could deteriorate cardiac function in TAC-FH through perturbed SR-mediated Ca(2+) cycling.

Parathyroid hormone accelerates decompensation following left ventricular hypertrophy.

Parathyroid hormone (PTH) treatment was previously shown to improve cardiac function after myocardial infarction by enhancing neovascularization and cell survival. In this study, pressure overload-induced left ventricular hypertrophy (LVH) was induced in mice by transverse aortic banding (TAB) for 2 weeks. We subsequently evaluated the effects of a 2-week treatment with PTH or saline on compensated LVH. After another 4 weeks, the hearts of the mice were analyzed by echocardiography, histology, and molecular biology. Echocardiography showed that hearts of the PTH-treated mice have more severe failing phenotypes than the saline-treated mice following TAB with a greater reduction in fractional shortening and left ventricular posterior wall thickness and with a greater increase in left ventricular internal dimension. Increases in the heart weight to body weight ratio and lung weight to body weight ratio following TAB were significantly exacerbated in PTH-treated mice compared to saline-treated mice. Molecular markers for heart failure, fibrosis, and angiogenesis were also altered in accordance with more severe heart failure in the PTH-treated mice compared to the saline-treated mice following TAB. In addition, the PTH-treated hearts were manifested with increased fibrosis accompanied by an enhanced SMAD2 phosphorylation. These data suggest that the PTH treatment may accelerate the process of decompensation of LV, leading to heart failure.

Identification of mouse heart transcriptomic network sensitive to various heart diseases.

Exploring biological systems from highly complex datasets is an important task for systems biology. The present study examined co-expression dynamics of mouse heart transcriptome by spectral graph clustering (SGC) to identify a heart transcriptomic network. SGC of microarray data produced 17 classified biological conditions (called condition spectrum, CS) and co-expression patterns by generating bi-clusters. The results showed dynamic co-expression patterns with a modular structure enriched in heart-related CS (CS-1 and -13) containing abundant heart-related microarray data. Consequently, a mouse heart transcriptomic network was constructed by clique analysis from the gene clusters exclusively present in the heart-related CS; 31 cliques were used for constructing the network. The participating genes in the network were closely associated with important cardiac functions (e. g., development, lipid and glycogen metabolisms). Online Mendelian Inheritance in Man (OMIM) database indicates that mutations of the genes in the network induced serious heart diseases. Many of the tested genes in the network showed significantly altered gene expression in an animal model of hypertrophy. The results suggest that the present approach is critical for constructing a heart-related transcriptomic network and for deducing important genes involved in the pathogenesis of various heart diseases.