Search PubMed⌕ Search

Biomedical subjects

J Axelrod

Publications and source records attributed to J Axelrod.

At least 109 records · Page 6Linked to original sources
PubMed1980

Phospholipid methylation and biological signal transmission.

Many types of cells methylate phospholipids using two methyltransferase enzymes that are asymmetrically distributed in membranes. As the phospholipids are successively methylated, they are translocated from the inside to the outside of the membrane. When catecholamine neurotransmitters, lectins, immunoglobulins or chemotaxic peptides bind to the cell surface, they stimulate the methyltransferase enzymes and reduce membrane viscosity. The methylation of phospholipids is coupled to Ca2+ influx and the release of arachidonic acid, lysophosphatidylcholine, and prostaglandins. These closely associated biochemical changes facilitate the transmission of many signals through membranes, resulting in the generation of adenosine 3',5'-monophophate in many cell types, release of histamine in mast cells and basophils, mitogenesis in lymphocytes, and chemotaxis in neutrophils.

Adrenal Medulla↗
PubMed1980

Phospholipid methyltransferase asymmetry in synaptosomal membranes.

The sequential methylation of phosphatidylethanolamine to form phosphatidylcholine is carried out by two methyltransferases in rat brain synaptosomes. The first enzyme methylates phosphatidylethanolamine to form phosphatidylmonomethylethanolamine. The second enzyme methylates the monomethylated phospholipid two additional times, forming phosphatidylcholine. Experiments comparing the rate of methylation between intact and lysed synaptosomes indicate that synaptosomes accumulate S-adenosyl-L-methionine and that the first methylation takes place on the cytoplasmic side of the membrane. Studies comparing trypsin digestion of proteins in intact and lysed synaptosomes indicate that the first enzyme is localized on the cytoplasmic side of the membrane and the second enzyme faces the external surface. Phospholipase C hydrolyzed phosphatidylcholine formed by methylation, suggesting its localization in the external layer of the phospholipid bilayer. A mechanism for an enzyme-mediated flip-flop of phospholipids from the cytoplasmic side to the outer surface of the synaptosomal plasma membrane is presented.

Animals↗
PubMed1980

Phospholipid methylation and the transmission of biological signals through membranes.

Enzymatic methylation of phosphotidylethanolamine (PE) to form phosphatidylcholine (PC) is associated with translocation of the lipid from the inner cell membrane (PE) to the outer membrane (PC), a concomitant decrease in membrane viscosity, and in some cases, activation of phospholipase A and release of arachidonic acid. Changes in phospholipid methylation are induced by a variety of ligands upon interaction with their specific receptors. In each case stimulation of phospholipid methylation appears to contribute to the propagation of the particular physiological response (e.g., activation of adenylate cyclase in rat reticulocytes; release of histamine by mast cells; chemotactic movement of neutrophils; mitogenesis of lymphocytes). Thus, receptor-mediated changes in phospholipid methylation and membrane fluidity may represent a general mechanism leading to a specific cellular response.

Animals↗
PubMed1980

Phospholipid methylation: a biochemical signal modulating lymphocyte mitogenesis.

Phospholipid methylation in murine T lymphocytes but not B cells was stimulated by mitogenic lectins such as concanavalin A and phytohemagglutinin, and the methylation was then returned to the control level by the concomitant activation of phospholipase A2. A parallelism between dose-response curves of concanavalin A for phospholipid methylation and thymidine incorporation was found. Inhibition of either synthesis or degradation of methylated phospholipids resulted in a decrease in the thymidine incorporation. Although prostaglandins such as the E and F series were the main products of arachidonic acid released by phospholipase A2 activation, inhibition of synthesis of these compounds by indomethacin did not reduce the thymidine incorporation significantly. These results suggest that the mitogenesis of murine T lymphocytes is triggered by the activation of both phospholipid methyltransferase(s) and phospholipase A2.

Animals↗
PubMed1980

Mepacrine blocks beta-adrenergic agonist-induced desensitization in astrocytoma cells.

C6 astrocytoma cells contain beta-adrenergic receptors coupled to adenylate cyclase. A 2-hr exposure to l-isoproterenol results in an 80% decrease in cyclic AMP production in response to a subsequent challenge by l-isoproterenol (desensitization). This loss in responsiveness is paralleled by a 20-30% decrease in the apparent number of beta-adrenergic receptors and by increased release of arachidonic aciid into the medium. The increased release of arachidonic acid is caused by the action of phospholipase A2 (phosphatide 2-acylhydrolase, EC 3.1.1.4) and corresponds to increased turnover of methylated phospholipids. Mepacrine and tetracaine, both inhibitors of this phospholipase A2, are able to block l-isoproterenol-induced desensitization of cyclic AMP production and the decrease in beta-adrenergic receptors. Mellitin and phorbol ester, two activators of phospholipase A2, when preincubated with the cells cause a decreased cyclic AMP response of the cells to l-isoproterenol. These results suggest that the activation of phospholipase A2 in the local domain of the beta-adrenergic receptor may be involved in desensitization.

Adenylyl Cyclases↗
PubMed1980

Stimulation of phospholipid methylation, Ca2+ influx, and histamine release by bridging of IgE receptors on rat mast cells.

Normal rat mast cells were stimulated by antibodies against IgE receptors (anti-RBL) or by anti-IgE, and [3H]methyl group incorporation into phospholipids, 45Ca uptake, and histamine release were examined. Anti-RBL or its F(ab')2 fragments and anti-IgE induced an increase in the incorporation of [3H]methyl into phospholipids, in 45Ca influx, and in histamine release. By contrast, Fab' monomer fragments of anti-RBL induced none of these reactions. The transient increase of [3H]methyl incorporation in lipids peaked within 15 sec after the addition of either anti-RBL or anti-IgE and fell to basal level in 30 sec. This was then followed by an influx of 45Ca that increased to a maximum in 2 min and by histamine release that reached a maximum in 3 min. Inhibition of phospholipid methylation resulted in an inhibition of 45Ca influx and histamine release. These findings demonstrate that phospholipid methylation in rat mast cells is induced by bridging of IgE receptors on the cell surface and that increased methylation of phospholipids sets the stage for an influx of Ca2+ and subsequent release of histamine.

Animals↗
PubMed1980

A phospholipase A2 inhibitory protein in rabbit neutrophils induced by glucocorticoids.

When rabbit peritoneal neutrophils were treated with glucocorticoids, their chemotactic response to stimulation by the chemoattractant fMet-Leu-Phe was markedly reduced. Preincubation of cells with glucocorticoids also decreased phospholipase A2 (phosphatide 2-acylhydrolase, EC 3.1.1.4) activity in situ as measured by the release of [1-14C]arachidonic acid previously incorporated into phospholipids. The inhibitory potencies of glucocorticoids on phospholipase A2 activity correlated well with their anti-inflammatory activities and their abilities to bind to glucocorticoid receptors. Inhibitors of RNA and protein synthesis suppressed the inhibitory effect of glucocorticoids on phospholipase A2 activity. Digestion of the glucocorticoid-treated cells by Pronase overcame the inhibitory activity. Phospholipase A2 activity induced by Ca2+ ionophore A23187 was not affected by Pronase treatment. Gel filtration of proteins from neutrophil membranes labeled with [3H]lysine showed an induction of protein(s) (about 40,000 daltons) after glucocorticoid treatment. This protein inhibited a partially purified pancreatic phospholipase A2 and reduced the peptide-initiated chemotactic response of neutrophils.

Animals↗
PubMed1980

The enzymatic formation of catecholestrogens from 2-methoxyestrogens by rat liver microsomes.

Catecholestrogens are biologically active metabolites of estrogen which are synthesized by estrogen-2-hydroxylase in the liver, brain, and other organs. Although catecholestrogens are inactivated by enzymatic O-methylation, demethylation of the 2-methoxyestrogens to reform catecholestrogens also occurs. A sensitive and specific radioenzymatic assay for rat liver 2-methoxyestrogen demethylase has been developed which makes it possible to characterize its substrate requirements and to study the influence of various hormones on catecholestrogen formation. 2-Methoxyestrogen demethylase activity is localized in rat liver microsomes. The apparent Km for 2-methoxyestrone (2MeOE1) is 12 muM and that for 2-methoxyestradiol (2MeOE2) is 3 umM. The two most prevalent 2-methoxyestrogens, 2MeOE1 and 2 MeOE2, appear to be demethylated by different enzymes. The enzymes have absolute requirements for NADPH, an their activities are inhibited by CO and SKF-525A, indicating that they are cytochrome P450 dependent. 2MeOE2 demethylation but not 2MeOE1 demethylation exhibits substrate inhibition. 2MeOE1 demethylase activity in the female rat liver is only one third that in the male rat liver, but sexual dimorphism was not found in 2MeOE2 demethylation. Thyroidectomy and estradiol treatment of the male rat resulted in diminished 2MeOE1, but not 2MeOE2, demethylation.

Animals↗
PubMed1979

Estrogen-induced efflux of endogenous catecholamines from the hypothalamus in vitro.

Short-term organ cultures of the intact hypothalamus were used to study the effects of various estrogenic compounds on catecholamine release. Estradiol-17 beta (0.1--20 microM) produced a concentration-dependent efflux of norepinephrine and dopamine while its biologically inactive enantiomer, estradiol-17 alpha, was ineffective at concentrations up to 20 microM. Diethylstilbestrol, a potent non-steroidal estrogen, was as effective as estradiol-17 beta in inducing catecholamine efflux. In contrast, weakly or non-estrogenic steroids such as estrone, estriol, and corticosterone were without effect. The time course of the estrogen-induced efflux of hypothalamic catecholamines was similar to that previously reported for the estrogen-induced accumulation of hypothalamic cAMP, providing further evidence for the involvement of catecholamines in this effect. Theses results suggest that estrogen may facilitate the release of catecholamines within the hypothalamus.

Animals↗
PubMed1979

Phospholipid methylation unmasks cryptic beta-adrenergic receptors in rat reticulocytes.

The effect of phospholipid methylation on the number of beta-adrenergic receptor binding sites was examined in rat reticulocyte membranes. Stimulation of phosphatidylcholine synthesis by the introduction of the methyl donor S-adenosyl-L-methionine into reticulocyte ghosts increased the number of beta-adrenergic receptor sites. The appearance of beta-adrenergic binding sites was dependent on the formation of phosphatidylcholine by the enzyme that converts phosphatidyl-N-monomethylethanolamine from phosphatidylethanolamine. Both the synthesis of phosphatidylcholine and the unmasking of cryptic receptors were time and temperature dependent and did not occur in the presence of the methyl transferase inhibitor, S-adenosyl-L-homocysteine.

Animals↗