Photoaffinity labeling of the DDT1 MF-2 cell α1-adrenergic receptor

Summary In this study, we have used an ₁-adrenergic receptor photoaffinity ligand, 2-[4-(4-azido-3-iodo-benzoyl)-piperazin-1-yl]-4-amino-6, 7-dimethoxyquinazoline (¹²⁵I-APD), to label covalently the ₁-adrenergic receptor in a smooth muscle cell line. Our results indicate that in the absence of light, (¹²⁵I)APD binds reversibly to a site in the DDT₁ MF-2 cell membranes having pharmacological characteristics of an ₁-adrenergic receptor. Following incorporation of (¹²⁵I)ADP into partially purified membranes a single labeled band of protein with a M_r of 81 000 was visualized by autoradiography following sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Incorporation of (¹²⁵I)-APD into this band was affected by adrenergic agonists and antagonists in a manner consistent with an ₁-adrenergic interaction. Prazosin (₁-selective) blocked incorporation of the label into the Mr = 81 000 protein while yohimbine (₂-selective) did not. Of the adrenergic agonists, (–)-epinephrine and (–)-norepinephrine but not (–)-isoproterenol blocked labeling of the M_r – 81 000 protein. We conclude that the ligand binding site of the DDT₁ MF-2 cell ₁-adrenergic receptor resides in a M_r = 81 000 protein.     

Effect of the L- or D-aspartate on ecto-5′nucleotidase activity and on cellular viability in cultured neurons: participation of the adenosine A<Subscript>2A</Subscript> receptors

Summary. Glutamate increases the extracellular adenosine levels, an important endogenous neuromodulator. The neurotoxicity induced by glutamate increases the ecto-5′-nucleotidase activity in neurons, which produces adenosine from AMP. L- and D-aspartate (Asp) mimic most of the actions of glutamate in the N-methyl-D-aspartate (NMDA) receptors. In the present study, both amino acids stimulated the ecto-5′-nucleotidase activity in cerebellar granule cells. MK-801 and AP-5 prevented the L- and D-Asp-evoked activation of ecto-5′-nucleotidase. Both NMDA receptor antagonists prevented completely the damage induced by L-Asp, but partially the D-Asp-induced damage. The antagonist of adenosine A_2A receptors (ZM 241385) prevented totally the L- Asp-induced cellular death, but partially the neurotoxicity induced by D-Asp and the antagonist of adenosine A₁ receptors (CPT) had no effect. The results indicated a different involvement of NMDA receptors on the L- or D-Asp-evoked activation of ecto-5′-nucleotidase and on cellular damage. The adenosine formed from ecto-5′-nucleotidase stimulation preferentially acted on adenosine A_2A receptor which is probably co-operating with the neurotoxicity induced by amino acids.     

Regional localization of human ecto-5′nucleotidase to chromosome 6q14–q21

Summary Human and mouse hybrids that contain fragments of human chromosome 6 as translocations were analysed for expression of ecto-5nucleotidase enzymic activity measured by the conversion of AMP to adenosine and for antigenicity recognized by a monoclonal antibody specific for the human isozyme. Both methods allow a regional assignment of ecto-5nucleotidase to 6q14–q21.     

Effect of an epoxy derivative of 2′5′-trioligoadenylate on human neuroblastoma cells

We studied the effect of an epoxy derivative of dephosphorylated 2′,5′-trioligoadenylate (5′,5′ApApAepoxy) resistive to the action of cellular phosphodiesterase on cells of human neuroblastoma IMR 32 cultured in vitro. Twenty-two hours after the addition of 5·10⁻⁶ M 2′,5′ApApAepoxy to the culture medium, the number of cells decreased by 20% (P < 0.05), while the content of protein in these cells increased, on average, by 52% (P < 0.01), as compared with the control. The activities of Na⁺,K⁺-and Ca²⁺, Mg²⁺-ATPases in a microsomal fraction obtained from cells cultured in the presence of 2′, 5′ ApApAepoxy decreased by 50% (P < 0.001) as compared with those in the control cells. Our data indicate that 2′,5′ApApAepoxy possess antiproliferative activity. According to our findings, the antiproliferative effect of 2′,5′ ApApAepoxy can, to a great extent, be explained by the fact that this oligoadenylate derivative significantly modulates the activities of Na⁺,K⁺-and Ca²⁺,Mg²⁺-ATPases. Neirofiziologiya/Neurophysiology, Vol. 38, No. 2, pp. 97–102, March–April, 2006.     

Protein kinase activity in membrane preparations from ox brain. Stimulation of intrinsic activity by adenosine 3′:5′-cyclic monophosphate

When Ac(2)-l-Lys-d-Ala-d-Ala and either meso-diaminopimelic acid or Gly-l-Ala are exposed to the exocellular dd-carboxypeptidase-transpeptidase of Streptomyces R61, transpeptidation reactions yielding Ac(2)-l-Lys-d-Ala-(d)-meso- diaminopimelic acid and Ac(2)-l-Lys-d-Ala-Gly-l-Ala occur concomitantly with the hydrolysis of the tripeptide into Ac(2)-l-Lys-d-Ala. The proportion of the enzyme activity which can be channelled in the transpeptidation and the hydrolysis pathways depends upon the pH and the polarity of the environment. Transpeptidation is favoured both by increasing the pH and by decreasing the water content of the reaction mixtures. Kinetics suggest that the reactions proceed through an ordered mechanism in which the acceptor molecule (meso-diaminopimelic acid or Gly-l-Ala) binds first to the enzyme. Both acceptors behave as non-competitive inhibitors of the hydrolysis pathway. Transpeptidation is inhibited by high concentrations of Gly-l-Ala but not by high concentrations of meso-diaminopimelic acid. The occurrence on the enzyme of an additional inhibitory binding site for Gly-l-Ala is suggested.     

Evaluation of molecular model-based discovery of ecto-5′-nucleotidase inhibitors on the basis of X-ray structures

The enzyme ecto-5′-nucleotidase (e5NT, CD73), a metallophosphoesterase, is a critical component of adenosine metabolism and signaling and implicated in different disease states. Therefore, attempts have been made to discover inhibitors of e5NT. For example, a virtual screening study using a molecular model of the enzyme has led to the identification of a new series of sulfonamide-containing e5NT inhibitors. The recent availability of several X-ray structures of human e5NT in complex with inhibitors has made it possible to re-evaluate this model building and virtual screening effort. We have assessed the quality of the model in detail and analyzed the question why it was possible to identify a new series of inhibitors on the basis of model-based docking calculations. The model utilized for virtual screening was found to be topologically correct and approach experimental accuracy in the active site region. Two key features within the active site were identified as major determinants for the successfully identification of inhibitors. Taken together, the results rationalize the computer-aided discovery of sulfonamide inhibitors of e5NT and provide further support for the use of carefully built protein models for virtual screening.     

Effect of deletions 5′ to the translation initiation sequence on the expression of an mRNA in animal cells

To learn if an mRNA·18S rRNA interaction or a special secondary structure in the mRNA start region is essential for translation in eukaryotic cells, we constructed recombinant plasmids with the SV40 early promoter 5 to part of the Escherichia coli tuf B-lacZ gene. Deletion of bases potentially complementary to the 18S rRNA highly increased the transient -galactosidase expressed in transfected CHO cells. Deletion of bases that fostered formation of potential hairpins with the mRNA 5-terminus or altered the structure of the coding region reduced -galactosidase activity suggesting that these features of the mRNA secondary structure may be essential for initiation of translation. Computer aided analysis of the potential structure of 290 mRNAs suggests these are conserved features of the initiation region.     

Studies of the orotidine 5′-monophosphate decarboxylase activity of crude extracts of human cells

The specific activity of orotidine 5-monophosphate (OMP) decarboxylase in cultured human fibroblasts is an exponential function of the concentration of cell protein in the extract. Low concentrations of uridylic or cytidylic acid augment the catalytic activity of dilute solutions of cell extract but not of concentrated ones. In the presence of urea, specific activity becomes independent of protein concentration, and uridylic or cytidylic acid augments activity over all concentrations of cell extract. These results, as well as other observations, suggest that the decarboxylase may be composed of subunits which are in dynamic equilibrium with an aggregate. At least one of the subunits is likely to have catalytic activity for the reaction, though less activity than the aggregate. The effect of the mutant gene for orotic aciduria on OMP decarboxylase is easily demonstrated when cell extracts are assayed in either the presence or the absence of urea.Supported by Program Project Grants 1-PO1-GM 15419 and GM 18153-1, National Institutes of Health, United States Public Health Service.     

Expression of α1-adrenergic receptor subtypes in heart cell culture

Three ₁AR subtypes have been cloned so far and are designated as _1a, _1b, and _1d. Organspecific distribution pattern and subtype-specific effects are known but not fully understood. To address a cell-type specific expression pattern in the heart we investigated expression pattern of ₁AR subtypes on RNA and proteinlevel in heart tissue, cultured cardiomyocytes and nonmyocytes of the rat. Each ₁ARsubtype mRNA was present in neonatal and adult rat heart culture but the relative distribution pattern was significantly different. While the _1aAR subtype is preferentially expressed in adult cardiomyocytes, the _1bAR subtype was preferentially expressed in the nonmyocyte cell fraction. The RTPCR results were confirmed by Westernblotting (_1b) and immunocytochemical studies. Incubation with an ₁agonist (phenylephrine) for 72 h led to a significant reduction of the _1bAR in neonatal heart cell culture on both mRNA and protein level. In contrast, incubation with an ₁antagonist (prazosin) induced a 1.6 fold upregulation of the _1aAR mRNA without significant effects on radioligand binding and functional assay. The results indicate a distribution pattern of the ₁AR subtype which is specific for cell type and ontogeny of the rat heart and may be regulated by adrenergic agents.     

Interaction of transmembrane-spanning segments of the α2-adrenergic receptor with model membranes

Adrenergic receptors are integral membrane proteins involved in cellular signalling that belong to the G protein-coupled receptors. Synthetic peptides resembling the putative transmembrane (TM) segments TM4, TM6 and TM7, of the human α2-adrenergic receptor subtype C10 (P08913) and defined lipid vesicles were used to assess protein-lipid interactions that might be relevant to receptor structure/function. P6 peptide contains the hydrophobic core of TM6 plus the N-terminal hydrophilic motif REKR, while peptides P4 and P7 contained just the hydrophobic stretches of TM4 and TM7, respectively. All the peptides increase their helical tendency at moderate concentrations of TFE (30–50%) and in presence of 1,2-dielaidoyl-sn-glycero-3-phosphatidylethanolamine (DEPE) lipids. However, only P6 displays up to 19% of α-helix in the presence of just the DEPE lipids, evidences a transmembrane orientation and stabilizes the Lα lipid phase. Conversely, P4 and P7 peptides form only stable β-sheet structures in DEPE and favour the non-lamellar, inverted hexagonal (H_II) phase of DEPE by lowering its phase transition temperature. This study highlights the potential of using synthetic peptides derived from the amino acid sequence in the native proteins as templates to understand the behaviour of the transmembrane segments and underline the importance of interfacial anchoring interactions to meet hydrophobic matching requirements and define membrane organization.     

Comparative studies on the carbohydrate-containing membrane components of normal and adenosine 3′:5′-cyclic monophosphate-treated Chinese-hamster ovary cells

1. We investigated some of the changes in plasma-membrane composition that accompany the alteration in cell growth and morphology induced by treating Chinese-hamster ovary cells with dibutyryladenosine 3':5'-cyclic monophosphate (dibutyryl cyclic AMP). 2. A double-labelling technique was employed in which normal cells were given (14)C-labelled precursor, and those treated with dibutyryl cyclic AMP were given (3)H label. l-Leucine, d-glucosamine, and l-fucose were used to label the membranes. 3. After 3 days growth, the two populations of cells were harvested by trypsin treatment, the cells were pooled, and plasma membranes isolated. Proteins and glycoproteins of the membranes were separated by electrophoresis on sodium dodecyl sulphate-polyacrylamide gels, and the radioactive profiles for (14)C and (3)H and the staining patterns with Amido Black were compared. 4. Although certain components of the membrane from treated cells showed marked quantitative changes, there was neither major addition nor major deletions of components. 5. Complete proteolysis of the mixed membranes, of the material released from the cell surface by trypsin, and of the glycoproteins released from the cells into the medium, gave a series of radioactive glycopeptides when either fucose or glucosamine was employed as precursor. 6. After such glycopeptides were fractionated on columns of Sephadex G-50, marked differences in the elution profiles of (3)H and (14)C were noted. Dibutyryl cyclic AMP evidently causes alterations in the overall composition of the carbohydrate components of the cell surface. It was not possible to decide whether this was solely the result of the same glycoproteins being formed but in different proportions, or the result of modifications of oligosaccharide side chains on some of the glycoproteins. 7. Some of the changes were not unlike the reverse of those that accompany the transformation of fibroblasts by oncogenic viruses, and our results lend credence to the idea that the lowered amount of cyclic AMP noted in transformed cells is responsible for their altered surface properties.     

High expression and activity of ecto-5′-nucleotidase/CD73 in the male murine reproductive tract

Extracellular ATP and its hydrolysis product adenosine modulate various reproductive functions such as those requiring contraction, steroidogenesis, and maintenance of fluid composition. Interestingly, adenosine might act as a key capacitative effector for mammalian spermatozoa to acquire the capacity for fertilisation. Extracellular nucleotide levels are affected by cell surface ectonucleotidases, amongst which the ectonucleoside triphosphate diphosphohydrolase (E-NTPDase) family regroups the most abundant and effective enzymes to hydrolyse ATP and ADP to AMP in physiological conditions. In the male reproductive tract three members of this family have been indentified: NTPDase1, NTPDase2 and NTPDase3 (Martín-Satué et al. in Histochem Cell Biol 131:615–628, 2009). The purpose of the present study was to characterize in the male reproductive tract the expression profile of the main enzyme responsible for the generation of adenosine from AMP, namely the ecto-5′-nucleotidase (CD73). The enzyme was identified by immunological techniques and by in situ enzymatic assays, including inhibition experiments with α,β-methylene-ADP, a specific CD73 inhibitor. High levels of ecto-5′-nucleotidase were detected in testes in association with both germinal and somatic cells, in smooth muscle cells throughout the tract, in secretory epithelia from exocrine glands, and remarkably, in principal cells of epididymis, where co-localization with NTPDase3 was found. The relevance of this co-expression on nucleotide hydrolysis in these cells directly involved in the control of sperm fluid composition was addressed biochemically. This study suggests close regulation of extracellular nucleoside and nucleotide levels in the genital tract by ecto-5′-nucleotidase that, in concurrence with NTPDases, may impact male fertility.     

Presence of an α-galactolipid on the cell surfaces of endothelial cells of human kidney

Summary The presence of an -galactolipid was investigated with a peroxidase-labelled lectin fromGriffonia simplicifolia (GSA-I) with specific binding for terminal -d-galactose residues. Normal kidney tissue was obtained from patients undergoing nephrectomy for renal neoplasms. For light microscopy, tissue was snap-frozen; 4 µm-thick sections were briefly fixed in paraformaldehyde and incubated with GSA (0.025 mg ml^–1). The peroxidase activity was developed with 3-amino-9-ethylcarbazole. Adjacent sections were stained at the same time after lipid extraction with 3:1 (v/v) chloroform/methanol. For electron microscopy, 0.2–0.5 mm-thick paraformaldehyde-fixed blocks, with or without lipid extraction, were stained with peroxidase-labelled GSA. The label was developed with diaminobenzidine and osmium tetroxide. Some structures, such as tubular epithelia, stained both in lipid-extracted and non-extracted tissues, suggesting that glycoproteins were most likely involved. In addition, tissue stained immediately after fixation showed GSA reactivity on endothelial cell surfaces of intertubular capillaries and larger vessels. In lipid-extracted tissues, however, tubular epithelium was still positive for GSA but endothelial cells failed to stain. These findings suggest that a glycolipid, bearing a terminal -galactose residue, is present on the endothelial cells in human kidney and possibly on tubular epithelia. Our data may explain the preferential storage of -galactolipid in endothelial cells of patients with Fabry's disease and other biological phenomena such asEscherichia coli adhesion.     

Molecular cloning, stable expression and cellular localization of human α1-adrenergic receptor subtypes: effect of charcoal/dextran treated serum on expression and localization of α1D -adrenergic receptor

The cDNAs encoding for three subtypes of adrenergic receptors, α_1A-, α_1B- and α_1D-ARs, were cloned and expressed in HEK 293 cells. Expression of α_1A- and α_1B-AR subtypes in HEK 293 cells was stable even with increased passages but that of α_1D-AR was not. Cellular localization studies using immunofluorescence and flow cytometry revealed that expression of α_1A- and α_1B-ARs was primarily localized on the cell membrane whereas expression of α_1D-AR was␣predominantly intracellular. Our studies clearly demonstrated that the culturing of the recombinant cell lines expressing α_1D-AR in charcoal/dextran treated fetal bovine serum (FBS) resulted in targeting of α_1D-AR to the cell membrane and thus, significantly improving its stability and availability for ligand binding studies.Sunil M. Khattar, Roop Singh Bora and Priyanka Priyadarsiny contributed equally to this work.     

The role of β-adrenergic receptor signaling in the proliferation of hemangioma-derived endothelial cells

Background

Infantile hemangioma (IH) is a benign vascular neoplasm that arises from the abnormal proliferation of endothelial cells and enhanced angiogenesis. Recently, propranolol has been found to be effective in the management of IH, suggesting that β-adrenergic receptors (β-ARs) may play an important role in the pathogenesis of IH.

Results

In the present study, we investigated the β-adrenergic signaling that is associated with hemangioma-derived endothelial cell (HemEC) proliferation. The results showed that both β₁- and β₂-ARs were expressed in HemECs. Stimulation of the β-ARs by isoprenaline induced cell proliferation and elevation of second messenger cAMP levels. The proliferation-promoting action of isoprenaline was abolished by a β₁-selective antagonist and was more effectively abolished by a β₂-selective antagonist; the mechanism for the action of the antagonists was a G₀/G₁ phase cell cycle arrest which was associated with decreased cyclin D1, CDK-4, CDK-6 and phospho-Rb expression. Pre-treatment of the cells with VEGFR-2 or ERK inhibitors also prevented the isoprenaline-mediated proliferation of cells. In agreement with the involvement of β-ARs and VEGFR-2 in the HemEC response, β-AR antagonists and the VEGFR-2 inhibitor significantly attenuated isoprenaline-induced ERK phosphorylation. Moreover, treating the cells with isoprenaline markedly increased VEGF-A expression and VEGFR-2 activity in a β₂-AR-dependent manner.

Conclusions

We have demonstrated that the activation of the β-ARs in the ERK pathway may be important mechanisms in promoting HemEC growth. Furthermore, stimulation of the β-AR may transactivate VEGFR-2 signaling and further increase HemEC proliferation.     

Vesicle-associated membrane protein 2 mediates trafficking of α5β1 integrin to the plasma membrane

Integrins are major receptors for cell adhesion to the extracellular matrix (ECM). As transmembrane proteins, the levels of integrins at the plasma membrane or the cell surface are ultimately determined by the balance between two vesicle trafficking events: endocytosis of integrins at the plasma membrane and exocytosis of the vesicles that transport integrins. Here, we report that vesicle-associated membrane protein 2 (VAMP2), a SNARE protein that mediates vesicle fusion with the plasma membrane, is involved in the trafficking of α5β1 integrin. VAMP2 was present on vesicles containing endocytosed β1 integrin. Small interfering RNA (siRNA) silencing of VAMP2 markedly reduced cell surface α5β1 and inhibited cell adhesion and chemotactic migration to fibronectin, the ECM ligand of α5β1, without altering cell surface expression of α2β1 integrin or α3β1 integrin. By contrast, silencing of VAMP8, another SNARE protein, had no effect on cell surface expression of the integrins or cell adhesion to fibronectin. In addition, VAMP2-mediated trafficking is involved in cell adhesion to collagen but not to laminin. Consistent with disruption of integrin functions in cell proliferation and survival, VAMP2 silencing diminished proliferation and triggered apoptosis. Collectively, these data indicate that VAMP2 mediates the trafficking of α5β1 integrin to the plasma membrane and VAMP2-dependent integrin trafficking is critical in cell adhesion, migration and survival.     

Effect of ConA—receptor interaction on the structure of cell membrane

Recently,the effect of ligand receptor interaction on the membrane structure of liposomes has been studied extensively,However,little is known about how it exists on biological membranes,In this paper,the effect of Concanavalin A(ConA) receptorinteratcion on the structure of cell membranes was studied by Circular DIchrosim(CD) and 31P Nuclear Magnetic Resonance(NMR).CD results of both the purified macrophage membranes and human erythrocyte hgosts(EG) showed that the conformation of membrane proteins changed after ConA binding.For further research,31P-NMR was used to detect the orgainzation of phosp[holipid molecules on macrophage membranes.After ConA binding,the tendercy to form non bilayer structure increased with the amount of ConA.The changes of 31P-NMR spectra of living macrophages might be partly due to the above stated reason too.In addition,ConA-receptor interaction also induced similar results of 31P-NMR spectra in EG.In contrast,wheat germ agglutinin (WGA),another kind of lectin,rarely showed the same influence.