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Abstractp38 mitogen-activated protein kinases (MAPKs) are serine/threonine specific protein kinases that respond to cellular stress and regulate a broad range of cellular activities. There are four major isoforms of p38MAPK:α, β, γ, and δ. To date, the prominent isoform in heart has been thought to be p38α. We examined the expression of each p38 isoform at both the mRNA and protein level in murine heart. mRNA for all four p38 isoforms was detected. p38γ and p38δ were expressed at protein levels comparable to p38α and 38β, respectively. In the early phase of pressure-overload hypertrophy (1-7 days after constriction of the transverse aorta), the abundance of p38β, p38γ and p38δ mRNA increased; however, no corresponding changes were detected at the protein level. Confocal immunofluorescence studies revealed p38α and p38γ in both the cytoplasm and nucleus. In the established phase of hypertrophy induced by chronic pressure overload (7-28 days after constriction of the transverse aorta), p38γ immunoreactivity accumulated in the nucleus whereas the distribution of p38α remained unaffected. Hence, both p38α and p38γ are prominent p38 isoforms in heart and p38γ may play a role in mediating the changes in gene expression associated with cardiac remodeling during pressure-overload hypertrophy.

BackgroundCardiac fibroblasts play important functional and pathophysiological roles. Intracellular (“intracrine”) angiotensin‐II (Ang‐II) signaling regulates intercellular communication, excitability, and gene expression in cardiomyocytes; however, the existence and role of intracrine Ang‐II signaling in cardiac fibroblasts is unstudied. Here, we evaluated the localization of Ang‐II receptors on atrial fibroblast nuclei and associated intracrine effects of potential functional significance.Methods and ResultsImmunoblots of subcellular protein‐fractions from isolated canine atrial fibroblasts indicated the presence of nuclear Ang‐II type 1 receptors (AT1Rs) and Ang‐II type 2 receptors (AT2Rs). Fluorescein isothiocyanate–Ang‐II binding displaceable by AT1R‐ and AT2R‐blockers was present on isolated fibroblast nuclei. G‐protein subunits, including Gαq/11, Gαi/3, and Gβ, were observed in purified fibroblast nuclear fractions by immunoblotting and intact‐fibroblast nuclei by confocal immunocytofluorescence microscopy. Nuclear AT1Rs and AT2Rs regulated de novo RNA synthesis ([α32P]UTP incorporation) via IP3R‐ and NO‐dependent pathways, respectively. In intact cultured fibroblasts, intracellular Ang‐II release by photolysis of a membrane‐permeable caged Ang‐II analog led to IP3R‐dependent nucleoplasmic Ca2+‐liberation, with IP3R3 being the predominant nuclear isoform. Intracellular Ang‐II regulated fibroblast proliferation ([3H]thymidine incorporation), collagen‐1A1 mRNA‐expression, and collagen secretion. Intracellular Ang‐II and nuclear AT1R protein levels were significantly increased in a heart failure model in which atrial fibrosis underlies atrial fibrillation.ConclusionsFibroblast nuclei possess AT1R and AT2R binding sites that are coupled to intranuclear Ca2+‐mobilization and NO liberation, respectively. Intracellular Ang‐II signaling regulates fibroblast proliferation, collagen gene expression, and collagen secretion. Heart failure upregulates Ang‐II intracrine signaling‐components in atrial fibroblasts. These results show for the first time that nuclear angiotensin‐II receptor activation and intracrine Ang‐II signaling control fibroblast function and may have pathophysiological significance.

Endothelin receptors are present on the nuclear membranes in adult cardiac ventricular myocytes. The objectives of the present study were to determine 1) which endothelin receptor subtype is in cardiac nuclear membranes, 2) if the receptor and ligand traffic from the cell surface to the nucleus, and 3) the effect of increased intracellular ET-1 on nuclear Ca 2+ signalling. Confocal microscopy using fluorescently-labeled endothelin analogs confirmed the presence of ETB at the nuclear membrane of rat cardiomyocytes in skinned-cells and isolated nuclei. Furthermore, in both cardiac myocytes and aortic endothelial cells, endocytosed ET:ETB complexes translocated to lysosomes and not the nuclear envelope. Although ETA and ETB can form heterodimers, the presence or absence of ETA did not alter ETB trafficking. Treatment of isolated nuclei with * Abbreviations: The abbreviations used are: 2-APB, 2-aminoethoxydiphenyl borate; A, Angiotensin II; αAR, α-adrenergic receptor;[Ca 2+ ] n , nucleoplasmic free calcium concentration; CaMKII, Ca 2+ /calmodulin-dependent protein kinase II; DMNB, 4,5-dimethoxynitrobenzyl group; DNA, deoxyribonucleic acid; DCC, 1,3-dicyclohexylcarbodiimide; DCM, dichloromethane; DTT, dithiothreitol; ECE1-a, endothelin converting enzyme 1a; cET-1, caged ET-1; caged ET-1, [Trp-ODMNB 21 ]ET-1; ET-1, endothelin 1; ET-2, endothelin 2; ET-3, endothelin 3; ETA, endothelin type A receptor; ETB, endothelin type B receptor; ETR, endothelin receptor; GPCR, G protein-coupled receptor; HDAC5; histone deacetylase 5; IP3, inositol 1,4,5-trisphosphate; IP3R, inositol 1,4,5-trisphosphate receptor; ISO, isoproterenol; MEF2, myocyte enhancing factor-2; PBS, phosphate buffered saline; PMSF, phenylmethanesulphonylfluoride; PNGase F, peptide N-glycosidase F; qPCR, quantitative real-time polymerase chain reaction; RNA, ribonucleic acid; RyR, ryanodine receptor; TCA, trichloroacetic acid; TFA, trifluoroacetic acid; TX-100, Triton X-100. Address correspondence to: Bruce G. Allen, Montreal Heart Institute, 5000 Belanger St,. Montréal, Québec, Canada, H1T 1C8., Telephone: (514) ] n whereas extracellular application of ETA and ETB receptor antagonists did not. These data suggest that 1) the endothelin receptor in the cardiac nuclear membranes is ETB, 2) ETB traffic directly to the nuclear membrane after biosynthesis, 3) exogenous endothelins are not ligands for ETB on nuclear membranes, and 4) ETB associated with the nuclear membranes regulate nuclear Ca 2+ signalling.

Cholera toxin (CT) produced by Vibrio cholerae is the major virulence factor responsible for the massive secretory diarrhea of infected humans [1]. CT interacts with intestinal epithelial cells and induces chloride secretion due to toxin-mediated activation of adenylate cyclase and elevation of intracellular cAMP. Activation of adenylate cyclase results from ADP-ribosylation of Arg201 of the a-subunit of the stimulatory GTP-binding regulatory protein, Gsa, catalyzed by the toxin [2].CT ( 84 kDa) is an oligomeric protein of the A-B type composed of one activating A subunit (CT-A, r.m.m. 27 400 Da) and five identical B subunits (CT-B, r.m.m. 11 600 Da) arranged in a ring-like configuration that bind ganglioside GM1 at the cell surface [3]. The CT-A subunit is comprised of two functional domains termed the A1 and A2 peptides linked by a single disulfide bond. The A1 peptide exhibits the toxin's ADP-ribosyltransferase activity which is necessary for CT cytotoxic action. The A2 peptide contains the endoplasmic reticulum-targeting motif KDEL at its C-terminus. For full ADP-ribosylation of the stimulatory heterotrimeric GTPase Gsa, enzymatic production of a degradative fragment generated from native CT and structurally related to the A1 peptide must occur followed by its targeting to the Gsa substrate. This process, which takes 30-40 min in most cell types, We have defined the in vivo and in vitro metabolic fate of internalized cholera toxin (CT) in the endosomal apparatus of rat liver. In vivo, CT was internalized and accumulated in endosomes where it underwent degradation in a pH-dependent manner. In vitro proteolysis of CT using an endosomal lysate required an acidic pH and was sensitive to pepstatin A, an inhibitor of aspartic acid proteases. By nondenaturating immunoprecipitation, the acidic CT-degrading activity was attributed to the luminal form of endosomal cathepsin D. The rate of toxin hydrolysis using an endosomal lysate or pure cathepsin D was found to be high for native CT and free CT-B subunit, and low for free CT-A subunit. On the basis of IC 50 values, competition studies revealed that CT-A and CT-B subunits share a common binding site on the cathepsin D enzyme, with native CT and free CT-B subunit displaying the highest affinity for the protease. By immunofluorescence, partial colocalization of internalized CT with cathepsin D was confirmed at early times of endocytosis in both hepatoma HepG2 and intestinal Caco-2 cells. Hydrolysates of CT generated at low pH by bovine cathepsin D displayed ADP-ribosyltransferase activity towards exogenous Gsa protein suggesting that CT cytotoxicity, at least in part, may be related to proteolytic events within endocytic vesicles. Together, these data identify the endocytic apparatus as a critical subcellular site for the accumulation and proteolytic degradation of endocytosed CT, and define endosomal cathepsin D an enzyme potentially responsible for CT cytotoxic activation.Abbreviations CT, cholera toxin; EN, endosomes; HI, human insulin; PA, pepstatin-A; PDI, protein...

Rationale and Goal Glutamine, the most abundant amino acid in plasma, has attracted considerable interest for its cardioprotective properties. The primary effect of glutamine in the heart is commonly believed to be mediated via its anaplerotic metabolism to citric acid cycle (CAC) intermediates; however, there is little direct evidence to support this concept. Another potential candidate is the hexosamine biosynthetic pathway (HBP), which has recently been shown to modulate cardiomyocyte function and metabolism. Therefore, the goal of this study was to evaluate the contribution of anaplerosis and the HBP to the acute metabolic effects of glutamine in the heart. Methods Normoxic ex vivo working rat hearts were perfused with 13C-labeled substrates to assess relevant metabolic fluxes perfused with a physiological mixture of carbohydrates and a fatty acid (control) or under conditions of restricted pyruvate anaplerosis. Results Addition of a physiological concentration of glutamine (0.5 mM) had no effect on contractile function of hearts perfused under the control condition, but improved that of hearts perfused under restricted pyruvate anaplerosis. Changes in CAC intermediate concentrations as well as 13C-enrichment from [U-13C]glutamine did not support a major role of glutamine anaplerosis under any conditions. Under the control condition, however, glutamine significantly increased the contribution of exogenous oleate to β-oxidation, 1.6-fold, and triglyceride formation, 2.8-fold. Glutamine had no effect on malonyl-CoA or AMPK kinase activity levels; however, it resulted in a higher plasma membrane level of the fatty acid transporter CD36. These metabolic effects of glutamine were reversed by azaserine, which inhibits glucose entry into the HPB. Conclusion Our results reveal a metabolic role of physiological concentration of glutamine in the healthy working heart beyond anaplerosis. This role appears to involve the HBP and regulation of fatty acid entry and metabolism via CD36.

At the cell surface, βARs and endothelin receptors can regulate nitric oxide (NO) production. β-adrenergic receptors (βARs) and type B endothelin receptors (ETB) are present in cardiac nuclear membranes and regulate transcription. The present study investigated the role of the NO pathway in the regulation of gene transcription by these nuclear G protein-coupled receptors. Nitric oxide production and transcription initiation were measured in nuclei isolated from the adult rat heart. The cell-permeable fluorescent dye 4,5-diaminofluorescein diacetate (DAF2 DA) was used to provide a direct assessment of nitric oxide release. Both isoproterenol and endothelin increased NO production in isolated nuclei. Furthermore, a β3AR-selective agonist, BRL 37344, increased NO synthesis whereas the β1AR-selective agonist xamoterol did not. Isoproterenol increased, whereas ET-1 reduced, de novo transcription. The NO synthase inhibitor l-NAME prevented isoproterenol from increasing either NO production or de novo transcription. l-NAME also blocked ET-1-induced NO-production but did not alter the suppression of transcription initiation by ET-1. Inhibition of the cGMP-dependent protein kinase (PKG) using KT5823 also blocked the ability of isoproterenol to increase transcription initiation. Furthermore, immunoblotting revealed eNOS, but not nNOS, in isolated nuclei. Finally, caged, cell-permeable isoproterenol and endothelin-1 analogs were used to selectively activate intracellular β-adrenergic and endothelin receptors in intact adult cardiomyocytes. Intracellular release of caged ET-1 or isoproterenol analogs increased NO production in intact adult cardiomyocytes. Hence, activation of the NO synthase/guanylyl cyclase/PKG pathway is necessary for nuclear β3ARs to increase de novo transcription. Furthermore, we have demonstrated the potential utility of caged receptor ligands in selectively modulating signaling via endogenous intracellular G protein-coupled receptors.

To assess glucagon receptor compartmentalization and signal transduction in liver parenchyma, we have studied the functional relationship between glucagon receptor endocytosis, phosphorylation and coupling to the adenylate cyclase system. Following administration of a saturating dose of glucagon to rats, a rapid internalization of glucagon receptor was observed coincident with its serine phosphorylation both at the plasma membrane and within endosomes. Co-incident with glucagon receptor endocytosis, a massive internalization of both the 45- and 47-kDa Gsalpha proteins was also observed. In contrast, no change in the subcellular distribution of adenylate cyclase or beta-arrestin 1 and 2 was observed. In response to des-His(1)-[Glu(9)]glucagon amide, a glucagon receptor antagonist, the extent and rate of glucagon receptor endocytosis and Gsalpha shift were markedly reduced compared with wild-type glucagon. However, while the glucagon analog exhibited a wild-type affinity for endosomal acidic glucagonase activity and was processed at low pH with similar kinetics and rates, its proteolysis at neutral pH was 3-fold lower. In response to tetraiodoglucagon, a glucagon receptor agonist of enhanced biological potency, glucagon receptor endocytosis and Gsalpha shift were of higher magnitude and of longer duration, and a marked and prolonged activation of adenylate cyclase both at the plasma membrane and in endosomes was observed. The subsequent post-endosomal fate of internalized Gsalpha was evaluated in a cell-free rat liver endosome-lysosome fusion system following glucagon injection. A sustained endo-lysosomal transfer of the two 45- and 47-kDa Gsalpha isoforms was observed. Therefore, these results reveal that within hepatic target cells and consequent to glucagon-mediated internalization of the serine-phosphorylated glucagon receptor and the Gsalpha protein, extended signal transduction may occur in vivo at the locus of the endo-lysosomal apparatus.