Phytomitogen-induced stimulation of syntheses de novo of phosphatidylinositol, phosphatidic acid and diacylglycerol in rat and human lymphocytes.

The biosynthetic route of phosphatidylinositol in lymphocytes stimulated with either concanavalin A or Phaseolus vulgaris hemagglutinin is examined by the determination of the [2-3H]glycerol and 32PO4 incorporations into glycerolipids of rat and human lymphocytes. In rat lymph-node cells, the [2-3H]glycerol incorporation into phosphatidylinositol was accelerated 23-fold by 50 microgram concanavalin A/ml culture; the same cells exhibited a 37-fold concanavalin A-induced stimulation in 32PO4 incorporation into the phospholipid. Moreover, concanavalin A markedly stimulated [2-3H]glycerol incorporation into phosphatidic acid and diacylglycerol. [2-3H]Glycerol incorporation into the other glycerolipids was affected to a very small degree by concanavalin A. In human peripheral blood lymphocytes, concanavalin A and P. vulgaris hemagglutinin enhanced the rate of the [2-3H]glycerol incorporation into phosphatidylinositol to almost the same degree (4--8-fold) as the rate of the 32PO4 incorporation into the phospholipid. The [2-3H]glycerol incorporation into phosphatidic acid and phosphatidylserine was markedly stimulated after addition of these lectins; the enhancement was 3--4-times higher than the enhancement found in the 32PO4 incorporation into these lipids. The [2-3H]glycerol incorporation into diacylglycerol was enhanced 4--5-fold by these lectins. These mitogenic lectins stimulated the [2-3H]glycerol and 32PO4 incorporation into phosphatidylcholine and phosphatidylethanolamine to a smaller degree. Our results indicate that phosphatidylinositol in phytomitogen-stimulated lymphocytes is synthesized mainly via the de novo pathway, in contrast to the previously postulated pathway involving turnover of the inositol and phosphate moieties of phosphatidylinositol while conserving the diacylglycerol portion of this molecule.