A comparison of the effects of phytohaemagglutinin and of calcium ionophore A23187 on the metabolism of glycerolipids in small lymphocytes

Abstract

1. The effects of phytohaemagglutinin and of a Ca2+ ionophore (A23 187) on glycerolipid metabolism in lymphocytes from pig lymph nodes were compared (a) by studying the incorporation of [32P] Phytohaemagglutinin only stimulated 32p incorporation into phosphatidylinositol and, to a slight extent, phosphatidate. Removal of most of the extracellular Ca2+ somewhat decreased this response. 3. lonophore A23187 stimulated the labellingofphosphatidateand phosphatidylinositol with 32p to a much greater extent than did phytohaemagglutinin: the increase in phosphatidate labelling, but not that of phosphatidylinositol, was almost abolished by the removal of extracellular Ca2+. 4. The combined effects of phytohaemagglutinin and ionophore appeared to be additive, rather than synergistic. 5. Treatment with ionophore A23187 somewhat decreased the total incorporation of [3H]glycerol into glycerolipids, possibly because it lowered cell ATP content. In these experiments di-and tri-acylglycerol behaved anomalously, triacylglycerol labelling being suppressed completely, whereas that of diacylglycerol was enhanced. The pulse-chase results revealed that triacylglycerol was converted into diacylglycerol in the ionophore-treated cells, and the availability of this diacylglycerol probably led to the enhanced labelling of phosphatidate and phosphatidylinositol in these cells. 6. Thus an increase in intracellular Ca2+ concentration appeared to have three effects on glycerolipid metabolism: (a) slight inhibition of some metabolic step preceding phosphatidate synthesis, (b) inhibition of diacylglycerol acyltransferase and (c) activation of a triacylglycerol lipase. 7. In contrast, it seems likely that the only effect of phytohaemagglutinin is to stimulate phosphatidylinositol breakdown. 8. Pig polymorphonuclear leucocytes treated with ionophore A23187 showed metabolic changes that were similar to those demonstrated with lymphocytes. 9. A possible similarity is suggested between Ca2+-stimulated triacylglycerol lipase in lymphocytes and polymorphonuclear leucocytes and previous observations of enhanced triacylglyerol metabolism in stimulated cells whose metabolic functions involve membrane fusion.

The dependence on Ca2+ of phosphatidylinositol breakdown and enzyme secretion in rabbit neutrophils stimulated by formylmethionyl-leucylphenylalanine or ionomycin

“…Diacylglycerol from this source could be a substrate for phosphatidic acid formation. A Ca2+-sensitive triacylglycerol lipase has previously been demonstrated in human neutrophils and lymphocytes (Allan & Michell, 1977).…”

A transient increase in diacylglycerols is associated with the action of vasopressin on hepatocytes

“…Triacylglycerols, phosphatidic acid or other phospholipids could also be precursors of diacylglycerol. Evidence that vasopressin or Ca2 + affect some of the reactions involved in the conversion of these lipids into diacylglycerol has been obtained (Allan & Michell, 1977;Williamson et al, 1980;Sugden & Williamson, 1981;Alemany et al, 1982;Pollard & Brindley, 1984). Comparison of the fatty acid composition of the diacylglycerols formed in the presence of vasopressin with that of triacylglycerols (Akesson, 1969), the observations that vasopressin increases the rate of fatty acid esterification (Williamson et al, 1980) and the amount of [32P]phosphatidic acid in cells prelabelled with [32P]P; (Thomas et al, 1983;Litosch et al, 1983), and the failure of glucagonwhichlhas been shown to alter hepatic triacylglycerol metab-olism (reviewed by Saggerson & Bates, 1981;Pelech et al, 1983)] to induce a transient change in diacylglycerols suggest that stimulation by vasopressin of the net conversion of triacylglycerol or phosphatidic acid into diacylglycerol is unlikely.…”

Studies of receptor-stimulated inositol lipid metabolism should focus upon measurements of inositol lipid breakdown

“…Enhanced phosphatidylinositol labelling can also occur: (a) when any other lipid breaks down to yield diacylglycerol which can then be converted to phosphatidate and phosphatidylinositol (e.g. triacylglycerol breakdown in response to cytosolic Ca2+; Allan & Michell, 1977); or (b) when synthesis of phosphatidylinositol de novo is enhanced (e.g. during the redirection of lipid synthetic pathways in cells exposed to amphiphilic cations; Brindley et al, 1975).…”