Cardioprotective effects of adenosine A1 and A 3 receptor activation during hypoxia in isolated rat cardiac myocytes

Adenosine (ADO) is a well-known regulator of a variety of physiological functions in the heart. In stress conditions, like hypoxia or ischemia, the concentration of adenosine in the extracellular fluid rises dramatically, mainly through the breakdown of ATP. The degradation of adenosine in the ischemic myocytes induced damage in these cells, but it may simultaneously exert protective effects in the heart by activation of the adenosine receptors. The contribution of ADO to stimulation of protective effects was reported in human and animal hearts, but not in rat hearts. The aim of this study was to evaluate the role of adenosine A₁ and A₃ receptors (A₁R and A₃R), in protection of isolated cardiac myocytes of newborn rats from ischemic injury. The hypoxic conditions were simulated by exposure of cultured rat cardiomyocytes (4–5 days in vitro), to an atmosphere of a N₂ (95%) and CO₂ (5%) mixture, in glucose-free medium for 90 min. The cardiotoxic and cardioprotective effects of ADO ligands were measured by the release of lactate dehydrogenase (LDH) into the medium. Morphological investigation includes immunohistochemistry, image analysis of living and fixed cells and electron microscopy were executed. Pretreatment with the adenosine deaminase considerably increased the hypoxic damage in the cardiomyocytes indicating the importance of extracellular adenosine. Blocking adenosine receptors with selective A₁ and A₃ receptor antagonists abolished the protective effects of adenosine. A₁R and A₃R activation during the hypoxic insult delays onset of irreversible cell injury and collapse of mitochondrial membrane potential as assessed using DASPMI fluorochrom. Cardioprotection induced by the A₁R agonist, CCPA, was abolished by an A₁R antagonist, DPCPX, and was not affected by an A₃R antagonist, MRS1523. Cardioprotection caused by the A₃R agonist, Cl-IB-MECA, was antagonized completely by MRS1523 and only partially by DPCPX. Activation of both A₁R and A₃R together was more efficient in protection against hypoxia than by each one alone. Our study indicates that activation of either A₁ or A₃ adenosine receptors in the rat can attenuate myocyte injury during hypoxia. Highly selective A₁R and A₃R agonists may have potential as cardioprotective agents against ischemia or heart surgery.     

Activation of the p70 S6 kinase and phosphorylation of the 4E-BP1 repressor of mRNA translation by type I interferons

The Type I IFN receptor-generated signals required for initiation of mRNA translation and, ultimately, induction of protein products that mediate IFN responses, remain unknown. We have previously shown that IFNalpha and IFNbeta induce phosphorylation of insulin receptor substrate proteins and downstream engagement of the phosphatidylinositol (PI) 3'-kinase pathway. In the present study we provide evidence for the existence of a Type I IFN-dependent signaling cascade activated downstream of PI 3'-kinase, involving p70 S6 kinase. Our data demonstrate that p70 S6K is rapidly phosphorylated on threonine 421 and serine 424 and is activated during treatment of cells with IFNalpha or IFNbeta. Such activation of p70 S6K is blocked by pharmacological inhibitors of the PI 3'-kinase or the FKBP 12-rapamycin-associated protein/mammalian target of rapamycin (FRAP/mTOR). Consistent with this, the Type I IFN-dependent phosphorylation/activation of p70 S6K is defective in embryonic fibroblasts from mice with targeted disruption of the p85alpha and p85beta subunits of the PI 3'-kinase (p85alpha-/-beta-/-). Treatment of sensitive cell lines with IFNalpha or IFNbeta also results in phosphorylation/inactivation of the 4E-BP-1 repressor of mRNA translation. Such 4E-BP1 phosphorylation is also PI3'-kinase-dependent and rapamycin-sensitive, indicating that the Type I IFN-inducible activation of PI3'-kinase and FRAP/mTOR results in dissociation of 4E-BP1 from the eukaryotic initiation factor-4E (eIF4E) complex. Altogether, our data establish that the Type I IFN receptor-activated PI 3'-kinase pathway mediates activation of the p70 S6 kinase and inactivation of 4E-BP1, to regulate mRNA translation and induction of Type I IFN responses.     

Single chain antibodies specific for fatty acids derived from a semi-synthetic phage display library

The biological activities of many acylated molecules are lipid dependent. Lipids, however, are poorly immunogenic or non-immunogenic. We employed a phage display semi-synthetic human antibody library to isolate anti-lipid antibodies. Selection was done against methyl palmitate, a 16 carbon aliphatic chain, and a major component of bacterial glycolipids and lipoproteins in animal cells. The selected single chain variable fragment (scFv) bound specifically to a 16 carbon aliphatic chain and to a lesser extent to a 14 or 18 carbon aliphatic chain and poorly to either 12, 22 or 8 carbon aliphatic chains. Furthermore, the scFv prevented micelle formation of lipoteichoic acid from Gram-positive bacteria; inhibited lipopolysaccharide-induced tumor necrosis factor alpha release in mononuclear cells; bound to hydrophobic bacterial surfaces, especially those of Gram-positive bacteria, and bound to Lck, a mammalian palmitated lipoprotein. Our data suggest that the phage antibody library can be successfully employed to obtain human anti-aliphatic scFv human antibody fragment with potential therapeutic applications in neutralizing the deleterious effects of bacterial toxins as well as in structure--function analysis of lipoproteins in animal cells.     

Metabolism of 15NH3 in Organotypic Cerebellar Explants and Cultured Astrocytes: Studies with Gas Chromatography-Mass Spectrometry

Gas chromatography-mass spectrometry was used to study the metabolism of 15NH3 in organotypic cerebellar explants and cultured astrocyte monolayers. A steady-state level of 15NH3 was present by 1 min in both systems. Steady-state labeling in L-[amide-15N] glutamine, L-[15N]alanine, L-[15N]glutamate, and L-[15N]aspartate was attained by 1 min after 15NH3 addition in the organotypic cerebellar explants and by approximately 5 min in the cultured astrocytes. No measurable 15N labeling was noted in either glycine or serine in either system.     

Simultaneous determination of [2-15N]- and [5-15N]glutamine with gas chromatography-mass spectroscopy: applications to nitrogen metabolic studies

A gas chromatographic-mass spectrometric method for analysis of L-[2-15N]- and L-[5-15N]glutamine is described. The method is based on direct acylation of glutamine with trifluoroacetic anhydride and the formation of the N,N-bis-trifluoroacetyl-L-glutamine derivative. This simple and sensitive method is capable of detecting approximately 0.5 atom% excess 15N in as little as 10 microliter of plasma with a mean coefficient of variance of 11.6%. The method was applied to determine the appearance of 15N enrichment in plasma amino-N and amide-N of glutamine in a healthy adult volunteer during a constant infusion of 15NH4Cl. A plateau level of 3.7 and 2.6 atom% excess was observed in amide-N and amino-N, respectively, at 1 and 2 h after 15NH4Cl infusion was started.     

Neuronal Glutamine Utilization: Pathways of Nitrogen Transfer tudied with [15N]Glutamine

Gas chromatography-mass spectrometry was used to evaluate the metabolism of [15N]glutamine in isolated rat brain synaptosomes. In the presence of 0.5 mM glutamine, synaptosomes accumulated this amino acid to a level of 25-35 nmol/mg protein at an initial rate greater than 9 nmol/min/mg of protein. The metabolism of [15N]glutamine generated 15N-labelled glutamate, aspartate, and gamma-aminobutyric acid (GABA). An efflux of both [15N]glutamate and [15N]aspartate from synaptosomes to the medium was observed. Enrichment of 15N in alanine could not be detected because of a limited pool size. Elimination of glucose from the incubation medium substantially increased the rate and amount of [15N]aspartate formed. It is concluded that: (1) With 0.5 mM external glutamine, the glutaminase reaction, and not glutamine transport, determines the rate of metabolism of this amino acid. (2) The primary route of glutamine catabolism involves aspartate aminotransferase which generates 2-oxoglutarate, a substrate for the tricarboxylic acid cycle. This reaction is greatly accelerated by the omission of glucose. (3) Glutamine has preferred access to a population of synaptosomes or to a synaptosomal compartment that generates GABA. (4) Synaptosomes maintain a constant internal level of glutamate plus aspartate of about 70-80 nmol/mg protein. As these amino acids are produced from glutamine in excess of this value, they are released into the medium. Hence synaptosomal glutamine and glutamate metabolism are tightly regulated in an interrelated manner.     

Effects of Ketone Bodies on Astrocyte Amino Acid Metabolism

Abstract: The effects of acetoacetate and 3-hydroxybutyrate on glial amino acid metabolism were studied in primary cultures of astrocytes. The exchange of nitrogen among amino acids was measured with ¹⁵N as a metabolic probe and gas chromatography-mass spectrometry as a tool with which to quantify isotope abundance. Addition of either acetoacetate or 3-hydroxybutyrate (5 m M ) to the incubation medium did not alter the initial rate of appearance of [¹⁵N]glutamate in the glia, but it did inhibit transamination of glutamate to [¹⁵N]aspartate. Addition of acetoacetate also inhibited formation of [2-¹⁵N]glutamine, but 3-hydroxybutyrate had a stimulatory effect. The presence in the medium of sodium acetate (5 m M ) was also associated with diminished production of [¹⁵N]aspartate and [2-¹⁵N]glutamine with [¹⁵N]glutamate as precursor. Studies with [2-¹⁵N]glutamine as precursor indicated that treatment of the astrocytes with ketone bodies did not alter flux through the glutaminase pathway. Nor did the presence of the ketone bodies reduce significantly the flux of nitrogen from [¹⁵N]GABA to [2-¹⁵N]glutamine when the former species served as a metabolic tracer. The concentration of internal citrate increased in the presence of acetoacetate, 3-hydroxybutyrate, and acetate. Studies with purified sheep brain glutamine synthetase showed that citrate inhibited this enzyme. These findings are considered in terms of the known anticonvulsant effect of a ketogenic diet.     

Generation and epitope mapping of high-affinity scFv to eukaryotic elongation factor 1A by dual application of phage display.

To generate specific tools for, in particular, localization studies of the eukaryotic elongation factor 1A (eEF1A), we have applied phage display in various formats to affinity-improve and map epitopes of two previously isolated, low-affinity single-chain Fv (scFv) G3 and D1. The scFv differ in their reactivity toward the eEF1A isoforms, eEF1A-1 and eEF1A-2. By PCR-based randomization of six residues within the variable light chain CDR3 (LCDR3), and subsequent phage-based affinity-selection, two 'families' of affinity-improved scFv were obtained. The scFv of highest affinity, A8, has a Kd of 9 nM to eEF1A-1. Interestingly, two affinity-improved scFvs have abnormally short LCDR3 consisting of two and four residues compared to 11 in the parental scFv. Hence, the LCDR3 of the parental clones may play a modulating rather than a direct role in antigen-binding. Despite different preferences for the eEF1A isoforms, both families of scFv recognize antigenic determinant(s), which was mapped to residues 413-450 of eEF1A-1/2 by Western blot analysis of recombinant human eEF1A (hEF1A) fragments. Prior to the Western blotting analysis, the epitope location had been suggested using a novel approach where phage-antibody repertoire derived scFv were used to select phage-displayed peptides. Hereby, peptides containing a SFXD motif, matching the SFSD(414-418) sequence found in hEF1A-1 were isolated. The structure of eukaryotic EF1A from yeast indicates a discontinuous nature of the epitope with distal functional elements juxtaposed by the protein fold. Finally, the scFv A8 was applied for immunofluorescence studies of transformed human amnion cells and MCF-7 fibroblasts. In both cases a perinuclear localization of hEF1A was observed. No evidence for the reported nuclear localization of hEF1A was obtained.