Biomedical subjects

G Bessou

Publications and source records attributed to G Bessou.

21 records · Page 2Linked to original sources

PubMed1987

Biosynthesis of paf-acether. IX. Role for a phosphorylation-dependent activation of acetyltransferase in antigen-stimulated mouse mast cells.

Mouse bone marrow-derived mast cells passively sensitized with monoclonal IgE released paf-acether (platelet-activating factor) and beta-hexosaminidase when challenged with the specific antigen. The formation and the release of paf-acether followed an early increase in the activity of the acetyltransferase, the main enzyme in paf-acether biosynthesis. The antigen-induced activation of the acetyltransferase was dependent on physiologic temperature and on the presence of Ca2+. By using microsomal fractions from unchallenged and challenged mast cells, the Vmax values were 3.5 and 12.0 nmol/min/mg of protein, respectively, whereas in both cases a Km value for acetyl-coenzyme A of 172 microM was measured. The stimulation of acetyltransferase could be mimicked in vitro under experimental conditions which favor phosphorylation, i.e. adding ATP and Mg2+ to lysates from unchallenged mast cells. In contrast, ATP and Mg2+ were uneffective on lysates from challenged cells that exhibited high level of acetyltransferase activity, suggesting that phosphorylation of the enzyme already took place at the time of cell stimulation. Moreover, addition of alkaline phosphatase to microsomal fraction obtained from either antigen-challenged mouse bone marrow-derived mast cells or unchallenged cells, resulted in 52% and 43% loss of acetyltransferase activity, respectively. Phorbol myristate acetate treatment of cells doubled the enzyme activity supporting the phosphorylation hypothesis. Thus, we report on the immunologic activation of a key enzyme for paf-acether synthesis and on the mechanism of this activation in a pure mast cell population. A link between bridging of IgE receptors and the activation of an enzyme critical to the formation of a lipid mediator is thereby evidenced.

Acetyltransferases↗

PubMed1986

Decrease in IgE Fc receptor expression on mouse bone marrow-derived mast cells and inhibition of paf-acether formation and of beta-hexosaminidase release by dexamethasone.

The effect of dexamethasone (DM) on the immunologic and nonimmunologic release of paf-acether and of the granule marker beta-hexosaminidase (BHEX) from mouse bone marrow-derived mast cells (BMMC) was studied. BMMC (1 X 10(6] in a modified Tyrode's solution containing 0.25% bovine serum albumin (BSA) were sensitized with an optimal dose of dinitrophenyl (DNP)-specific monoclonal IgE, and were washed before challenge with 40 ng/ml of DNP coupled to BSA. Preincubation of BMMC for 24 hr with 1 nM to 1 microM DM inhibited in a dose-dependent fashion the immunologic release of paf-acether and of BHEX as compared with control cells, with a half-maximal effect at 20 nM and 4 nM respectively. By contrast, the ionophore A23187 (1 microM)-induced release of paf-acether and of BHEX was unaffected by DM pretreatment. Finally, the antigen-induced increase in acetyltransferase activity, used as an index of cellular activation, was inhibited by 37 +/- 16% in 1 microM DM-treated BMMC as compared with untreated cells. Preincubation of BMMC with DM for 24 hr caused a dose-dependent inhibition of 125I-IgE binding to the cells, with a half-maximal effect at 14 nM. As determined by Scatchard analysis, the number of IgE Fc receptors was decreased by 55% in 1 microM DM-treated BMMC as compared with untreated cells, although the dissociation constants were comparable (control: 12.6 +/- 4.1 nM; DM-treated cells: 14.1 +/- 6.7 nM; mean +/- 1 SD; n = 3). Cytofluorometer analysis of BMMC sensitized with a saturating amount of purified monoclonal IgE, followed by addition of a fluoresceinated anti-mouse IgG (heavy and light chains), revealed a single cellular population for both DM-treated and untreated BMMC. This demonstrates that the DM-induced decrease in IgE Fc receptor expression was exhibited by every BMMC. The possible link between the decreased sensitization of the cells consequent to the reduction in IgE Fc receptor expression and the alteration of the secretory response and acetyltransferase activity was investigated. BMMC were incubated with IgE under experimental conditions giving half-sensitization of the cells. Upon antigen challenge, a 10.5 +/- 3.7% decrease in acetyltransferase activity and a 29.2 +/- 3.5% decrease in paf-acether release were observed with half-sensitized cells as compared with cells sensitized with a saturating amount of IgE.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1985

Ether-phospholipid composition in neutrophils and platelets.

It is well documented that ether-lipids, especially the 1-0-alkyl-2-0-acyl-sn-glycero-3-phosphocholine, can serve as precursors in paf-acether (platelet-activating factor) biosynthesis. This study was undertaken to determine the amount of these compounds in two cell types which are good producers of paf-acether: human neutrophils and rabbit platelets. The method of phospholipid analysis was based on selective destruction of diacyl molecules by lipase from guinea pig pancreas and of plasmalogens by acidolysis; phospholipids were then separated by bidimensional thin-layer chromatography. In platelets as in neutrophils, the major phospholipids were choline (37%) and ethanolamine (30 and 32%, respectively) phosphoglycerides and sphingomyelin (18 and 16%, respectively). In rabbit platelets the 1-alkyl molecules represented 15% of the choline class (0.7 nmol/10(7) cells). Neutrophils differed strikingly from platelets by the high level of these molecules which constituted 50% of the choline class (16.3 nmol/10(7) cells). It therefore appears that in the two cell types, the amount of 1-0-alkyl-2-0-acyl-sn-glycero-3-phosphocholine largely exceeded that would be necessary for paf-acether biosynthesis.

Animals↗