THE BIOACTIVE PHOSPHOLIPID, LYSOPHOSPHATIDYLCHOLINE, INDUCES CELLULAREFFECTS VIA G-PROTEIN-DEPENDENT ACTIVATION OF ADENYLYL-CYCLASE

The naturally occurring phospholipid, lysophosphatidylcholine (lyso-PC ), regulates a broad range of cell processes, including gene transcrip tion, mitogenesis, monocyte chemotaxis, smooth muscle relaxation, and platelet activation, Despite the growing list of cellular effects attr ibutable to lyso-PC, the mechanism(s) by which it alters cell function have not been elucidated, In this report, we have examined the effect s of exogenous lyso-PC on signal transduction processes within a varie ty of lyso-PC-responsive cells, including: human platelets, monocyte-l ike THP-1 cells, and the megakaryoblastic cell line, MEG-01. Pretreatm ent. of each of these cells with increasing concentrations of lyso-PC (25-150 mu g/ml) was associated with a progressive increase in the cyt osolic concentration of cAMP, The accumulation of cAMP in platelets co rrelated closely with the ability of lyso-PC to inhibit multiple plate let processes, including platelet aggregation, agonist-induced protein kinase C activation, thromboxane A(2) generation, and the tyrosine ph osphorylation of platelet proteins. In each of the cell types examined , the ability of lyso-PC to increase the cellular levels of cAMP was s ynergistically enhanced by pretreating the cells with the cAMP phospho diesterase inhibitor, theophylline (5 mM), and was specifically inhibi ted by the P-site inhibitor of adenylyl cyclase, 2,5-dideoxyadenosine. A role for the stimulatory G-protein, Gs, in the lyso-PC-induced acti vation of adenylyl cyclase was suggested by the ability of the GTPase inhibitor, guanylyl 5'-thiophosphate (0.2 mM), to inhibit the lyso-PC- stimulated increase in cAMP, and also by the ability of cholera toxin to inhibit increases in membrane GTPase activity in response to lyso-P C. The functional significance of lyso-PC-induced activation of adenyl yl cyclase was investigated in MEG-01 cells, Treatment of these cells with either lyso-PC or dibutyryl cAMP for 36-40 h resulted in a 3-5-fo ld increase in the surface expression of the natural anticoagulant pro tein, thrombomodulin (TM). The ability of lyso-PC to increase TM expre ssion was abolished by pretreating these cells with the adenylyl cycla se inhibitor, 2,5-dideoxyadenosine, whereas the dibutyryl cAMP-induced increase in TM remained insensitive to adenylyl cyclase inhibition. T hese studies define an important role for the adenylyl cyclase signali ng system in mediating cellular effects induced by lyso-PC.