LYSOPHOSPHATIDIC ACID MIMICS SERUM-INDUCED SENSITIZATION OF CYCLIC-AMP ACCUMULATION

Pretreatment of 1321N1 human astrocytoma cells with serum induces a pr onounced increase in subsequent stimulation by forskolin and other age nts of intracellular cyclic AMP accumulation, a phenomenon referred to as sensitization (Mol. Pharmacol. 39, 399-406, 1991). Pretreatment of these cells with lysophosphatidic acid induced sensitization to a sim ilar extent as that with serum (approximately fivefold for forskolin s timulation and twofold for isoproterenol and prostaglandin E1 stimulat ion), with half-maximal effects at approximately 30 nm lysophosphatidi c acid. Phosphatidic acid was effective but less potent whereas other lipids were ineffective. Sensitization by serum and by lysophosphatidi c acid were almost completely inhibited by pertussis toxin pretreatmen t and partially inhibited by prolonged phorbol ester exposure to induc e protein kinase C down-regulation. Among nine cell lines tested, thos e that exhibited sensitization with serum showed comparable sensitizat ion with lysophosphatidic acid. The effects of both lysophosphatidic a cid and serum were markedly inhibited by treatment with phospholipase B but only minimally altered with phospholipases A2, D, and C. Exposur e of cells to phospholipase C alone induced approximately threefold se nsitization, but both serum and lysophosphatidic acid were able to ind uce further three- to fourfold sensitization above that induced by pho spholipase C alone. In contrast, the effects of serum and lysophosphat idic acid were not additive with each other. Together these results su ggest that lysophosphatidic acid or a closely related compound present in serum is the factor responsible for sensitization of the cyclic AM P pathway.