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Biomedical subjects

J Axelrod

Publications and source records attributed to J Axelrod.

At least 91 records · Page 5Linked to original sources
PubMed1982

Somatostatin inhibits multireceptor stimulation of cyclic AMP formation and corticotropin secretion in mouse pituitary tumor cells.

The AtT-20/D16-16 mouse pituitary tumor cell secretes corticotropin (ACTH) in response to corticotropin-releasing factor (CRF), (-)-isoproterenol, and vasoactive intestinal peptide (VIP). These responses are associated with a rapid increase in cyclic AMP formation. Somatostatin (SRIF) markedly decreases the stimulatory effect of CRF, (-)-isoproterenol, and VIP on both cyclic AMP formation and immunoreactive ACTH secretion. Forskolin and cholera toxin, adenylate cyclase activators, also stimulate cyclic AMP formation and ACTH secretion in AtT-20 cells and these responses are all inhibited by SRIF. The ACTH secretory responses to melittin and to the calcium ionophore A23187, neither of which increases cyclic AMP in AtT-20 cells, were not inhibited by SRIF. SRIF did not affect the binding of a tritiated beta-adrenergic receptor antagonist to AtT-20 membranes nor did it decrease basal cyclic AMP formation even in the presence of excess phosphodiesterase inhibitor, indicating that the reduction of cyclic AMP levels by SRIF did not involve either an interference with beta-adrenergic agonist binding to receptors or stimulation of cyclic AMP degradation. These results indicate that the inhibition of CRF-, (-)-isoproterenol-, and VIP-stimulated ACTH secretion by SRIF may be regulated by its inhibitory action on adenylate cyclase.

Adrenocorticotropic Hormone↗
PubMed1982

Phospholipid metabolism, calcium flux, and the receptor-mediated induction of chemotaxis in rabbit neutrophils.

Rabbit neutrophils were stimulated with the chemotactic peptide fMet-Leu-Phe in the presence of the methyltransferase inhibitors homocysteine (HCYS) and 3-deazaadenosine (3-DZA). HCYS and 3-DZA inhibited chemotaxis, phospholipid methylation, and protein carboxymethylation in a dose-dependent manner. The chemotactic peptide-stimulated release of [14C]arachidonic acid previously incorporated into phospholipid was also partially blocked by the methyltransferase inhibitors. Stimulation by fMet-Leu-Phe or the calcium ionophore A23187 caused release of arachidonic acid but not of previously incorporated [14C]-labeled linoleic, oleic, or stearic acids. Unlike the arachidonic acid release caused by fMet-Leu-Phe, release stimulated by the ionophore could not be inhibited by HCYS and 3-DZA, suggesting that the release was caused by a different mechanism or by stimulating a step after methylation in the pathway from receptor activation to arachidonic acid release. Extracellular calcium was required for arachidonic acid release, and methyltransferase inhibitors were found to partially inhibit chemotactic peptide-stimulated calcium influx. These results suggest that methylation pathways may be associated with the chemotactic peptide receptor stimulation of calcium influx and activation of a phospholipase A2 specific for cleaving arachidonic acid from phospholipids.

Animals↗
PubMed1982

Variants of the rat basophilic leukemia cell line for the study of histamine release.

Cloning of the rat basophilic leukemia (RBL) cell lines demonstrates variability in cell chromosome number (approximately 44-70) and in their capacity to release histamine following an IgE- or Ca2+-ionophore stimulus. After IgE activation there is increased phospholipid methylation, Ca2+ influx, arachidonic acid, and histamine release. On Ca2+ ionophore A23187 stimulation, phospholipid methylation is not increased, but Ca2+ influx, arachidonic acid, and histamine release occur. Variants of the RBL-cloned sublines defective at different stages in the release process were obtained and used to sequence the different events in the release process: IgE activation is followed by methylation, Ca2+ influx, arachidonic acid, and histamine release. However, there are other variants with defects in intermediate steps in the pathway, e.g., increased phospholipid methylation that is not followed by Ca2+ influx or arachidonic acid release not followed by histamine release. Isolating variants carrying drug-resistance markers made hybridization (reconstitution) experiments possible. Two variants were recognized, each of which was deficient in one of the two phospholipid methyltransferase enzymes. Neither of these two variants released histamine; hybrids formed by fusion of these two cell lines have both phospholipid methyltransferase enzymes and release histamine. By other complementation experiments, groups of variants with defects at different steps in the histamine release sequence were recognized. Clearly, these basophilic leukemia cell lines provide a unique system for the study of the mechanism of histamine release.

Animals↗
PubMed1981

Mepacrine treatment prevents immobilization-induced desensitization of beta-adrenergic receptors in rat hypothalamus and brain stem.

Forced immobilization is a severe stress in rats which diminishes levels of epinephrine in specific nuclei in the hypothalamus and brain stem, suggesting that release of epinephrine is stimulated to a rate which exceeds the rate of its replacement. In the pineal gland, frog erythrocytes, C6 astrocytoma cells and rat brain, beta-adrenoceptor agonists appear to regulate the number of their receptors. Exposure to high concentrations of an agonist leads to apparent decrease in receptors reflected by a decrease in maximal specific binding of antagonists. The apparent decreases in receptors have been shown to be attended by decreases in physiologic responsiveness. In C6 astrocytoma cells, beta-agonists stimulate methylation of phosphatidylethanolamine to increase formation of membrane phosphatidylcholine which in turn appears to enhance activation of adenyl cyclase. Interference with the metabolism of phospholipids by exposure to phospholipase A2 inhibitor, mepacrine (quinacrine), prevents agonist-induced desensitization of beta-adrenoceptors in astrocytoma cells. In the present study repeated immobilization stress has been found to decrease significantly the number of beta-adrenoceptors in hypothalamus and brain stem while increasing the number of alpha 2-adrenoceptors. The desensitization of beta-adrenoceptors was prevented by treatment with mepacrine.

Animals↗
PubMed1981

Rat basophilic leukemia cell lines defective in phospholipid methyltransferase enzymes, Ca2+ influx, and histamine release: reconstitution by hybridization.

Variants of the rat basophilic leukemia (RBL) cell line were isolated and screened for phospholipid methyltransferase I and II activities, enzymes that convert phosphatidylethanolamine to phosphatidylcholine. Two variants were found that had decreased phospholipid methyltransferase enzyme levels and were unable to cause an influx of Ca2+ or release histamine in an IgE-mediated reaction. However, these cells were able to release histamine through an ionophore-induced reaction, indicating that the releasing mechanism distal to the Ca2+ channel was intact. One cell line, 1C1.B1, had low specific activity for phospholipid methyltransferase I. A second variant, 2H3.B6, had reduced phospholipid methyltransferase II activity. Although both variants were unable to incorporate label from [methyl-3H]methionine or [3H]serine into phosphatidylcholine, they were able to incorporate [methyl-3H]choline and myo-[2-3H(N)]inositol into phospholipids. Fusion of the two cell lines and isolation on selective media resulted in the growth of eight independent hybrids. All eight had an increased number of chromosomes and normal phospholipid methyltransferase activities. Stimulation of the hybrids with IgE resulted in CA2+ influx and histamine release. These results indicate that phospholipid methylation precedes and is necessary for Ca2+ influx, and they further support the hypothesis that methylation is a necessary early step in the IgE-mediated histamine release reaction in RBL cells.

Animals↗
PubMed1981

Presence of autoantibody for phospholipase inhibitory protein, lipomodulin, in patients with rheumatic diseases.

The activity of phospholipase inhibitory protein, lipomodulin, partially purified from rabbit neutrophils, was markedly decreased after treatment with sera from patients with rheumatic diseases such as systemic lupus erythematosus, rheumatoid arthritis, and dermatomyositis. The decrease of the protein's inhibitory activity on phospholipase A2 paralleled the amount of [35S]methionine-labeled lipomodulin precipitated by the sera. Absorption of patients' sera with anti-human IgM (mu chain) or protein A-agarose, but not with anti-human IgG (gamma chain), decreased their ability to decrease the activity of lipomodulin on phospholipase A2 or to precipitate the radioactive lipomodulin. The IgM fraction of patients' sera could precipitate [35S]methionine-labeled lipomodulin (40,000 daltons) which comigrated with highly purified lipomodulin on gel electrophoresis with sodium dodecyl sulfate. All of these observations suggest that the sera of many patients with rheumatic diseases contain autoantibody against lipomodulin. A monoclonal antibody against lipomodulin was also obtained. Stimulating human fibroblasts with bradykinin in the presence of monoclonal antilipomodulin antibody markedly enhanced arachidonic acid release due to the activation of phospholipase(s) in the intact cells, and this stimulatory effect was blocked by adding purified lipomodulin. These findings suggest that lipomodulin regulates the activity of phospholipase(s) on the cell surface and that autoantibodies against lipomodulin may play a role in certain symptoms of rheumatic diseases, especially by the formation of prostaglandins and other metabolites of arachidonic acid.

Animals↗
PubMed1981

Phospholipid methylation and phospholipase A2 activation in cytotoxicity by human natural killer cells.

The role of phospholipid methylation and phospholipase A2 (phosphatide 2-acylhydrolase, EC 3.1.1.4) in natural killer (NK) function by human peripheral blood mononuclear cells was studied. Pretreatment of effector cells with a methyltransferase inhibitor, 3-deazaadenosine, in the presence of homocysteine thiolactone, reduced cytotoxicity in a dose-dependent fashion. This effect was closely associated with inhibition of methylation of lipids but not of nucleic acids or proteins. The suggestion for a role of phospholipid methylation was supported by the observation that the interaction between NK-susceptible tumor targets and peripheral blood mononuclear cells caused increased phospholipid methylation only when susceptible target cells were used. Phospholipase A2 was also implicated in human NK activity. Inhibitors of the enzyme such as tetracaine, mepacrine, Rosenthal's inhibitor, and corticosteroids impaired NK function. Rosenthal's inhibitor was also shown to exert an inhibitory effect on a purified NK-cell population obtained by the isolation of large granular lymphocytes on Percoll gradients. Peripheral blood mononuclear cells were also directly shown to display phospholipase A2-like activity, as measured by the decrease in radioactive arachidonate from prelabeled phospholipids, specifically phosphatidylcholine, in effector cells. These data suggest that enhanced phospholipid methylation occurs during the recognition function of NK cells. Consequent activation of phospholipase A2 might be involved in the mechanisms leading to lytic events within the target cell.

Adrenal Cortex Hormones↗
PubMed1981

Changes in phospholipid methyltransferases and membrane microviscosity during induction of rat liver microsomal cytochrome P-450 by phenobarbital and 3-methylcholanthrene.

Rat liver microsomes contained two methyltransferases which converted phosphatidylethanolamine (PE) to phosphatidylcholine (PC). The first methyltransferase converted PE to phosphatidyl-N-methylethanolamine (PME) and the second methyltransferase converted PME to PC. Previous work has shown that increased PME synthesis decreases membrane microviscosity. Therefore, changes in the rat liver microsomal cytochrome P-450, phospholipid methyltransferases and membrane microviscosity after induction by phenobarbital and 3-methylcholanthrene were studied. Phenobarbital and 3-methylcholanthrene increased cytochrome P-450 levels 2- to 3-fold. At low SAM concentration, the proportion of PME among the total phospholipids formed increased significantly, and at a high SAM concentration, the proportion of PC among the total phospholipids formed decreased significantly in microsomes of treated rats. Treatment of rats with phenobarbital and 3-methylcholanthrene also decreased microviscosities of the microsomal membranes and liposomes which were prepared from phospholipids extracted from the microsomes. In synthetic liposomes containing PE, PME and PC, microviscosity decreased when the proportion of PME was increased or the proportion of PC was decreased. These results suggest that the membrane fluidity increases with phenobarbital and 3-methylcholanthrene treatment, and changes in phospholipid methyltransferases may contribute to the process of enzyme induction. During induction with phenobarbital, all three factors known to increase membrane fluidity (linoleic acid content, the formation of phosphatidyl-N-methylethanolamine, and decreases in the cholesterol/phospholipid ratio) contribute to the decrease in microviscosity. During induction with 3-methylcholanthrene, alterations in phospholipid methylation is possibly the primary cause of the decrease in membrane microviscosity.

Animals↗
PubMed1981

Catecholamine neurotransmitters, psychoactive drugs, and biological clocks. The 1981 Harvey Cushing oration.

In his Cushing oration, the 1970 Nobel Laureate reviews the experimental history of the vital role which chemical agents play in the transmission of nerve impulses and the important functions of the brain. He reveals the intriguing steps in his own early involvement in the field of neurotransmitters. A beacon for neuroscientists of the future is his unique talent for not only looking, but seeing potentially significant clues.

Animals↗
PubMed1981

Quinacrine-blocked desensitization of adrenoceptors after immobilization stress or repeated injection of isoproterenol in rats.

Repeated forced immobilization or repeated administration of isoproterenol reduces the number of beta adrenoceptors in the heart and spleen of rats. Isoproterenol, but not immobilization, reduced the number of beta receptors in the lung. These changes in beta adrenoceptors were prevented by administration of quinacrine, a phospholipase A2 inhibitor, although the drug had no effect on beta adrenoceptors when given alone. Immobilization, but not isoproterenol, reduced the number of alpha-1 adrenoceptors in the heart but had no effect on the lung. Quinacrine treatment decreased the number of alpha-1 receptors in heart and lung but increased alpha-2 receptors in the spleen. The changes in receptor number attending exposure to agonists are usually consistent with the expected changes. The effects of quinacrine on such changes suggest that phospholipids are involved in modulating the changes in number of receptors or their availability to interact with ligands.

Animals↗
PubMed1981

Regulation of the beta-adrenergic receptor by methylation of membrane phospholipids.

Stimulation of the beta-adrenergic receptor increases the enzymatic methylation of membrane phospholipids. Increased synthesis of phosphatidyl-N-monomethylethanolamine by methyltransferase I increases fluidity and enhances the ability of the beta-adrenergic receptor to couple with adenylate cyclase. The number of beta-adrenergic receptors can be regulated by the rate of synthesis and of degradation of phosphatidylcholine formed by transmethylation.

Animals↗
PubMed1981

Phospholipid methylation: a possible mechanism of signal transduction across biomembranes.

The conversion of phosphatidylethanolamine (PE) to phosphatidylcholine (PC) is catalyzed by two methyltransferases with S-adenosylmethionine as the methyl donor. PC formed by transmethylation is further metabolized by phospholipase A2. The synthesis and degradation of methylated phospholipids are involved in regulating the number of the beta-adrenergic receptors and their coupling to adenylate cyclase in rat reticulocytes, HeLa cells, and rat astrocytoma cells. Methylation of the phospholipids in these cells is stimulated by binding of agonists to the beta-adrenergic receptors. Accumulation of phosphatidyl-N-monomethylethanolamine causes an increase in membrane fluidity and enhances the coupling of the receptors to adenylate cyclase. Agents that inhibit phospholipid methylation decrease the number of receptors in intact HeLa cells, while increased phospholipid methylation unmasks cryptic receptors. Conversely, the degradation of methylated phospholipids appears to be closely associated with the desensitization of the beta-adrenergic receptors following prolonged stimulation with isoproterenol. Inhibition and stimulation of phospholipase A2 causes inhibition and stimulation of this desensitization process.

Adenylyl Cyclases↗