Induction of the fibrinogen receptor on human platelets by intracellular mediators

We have used platelets permeabilized with saponin to examine the mechanism by which platelet activation causes the exposure of surface receptors for fibrinogen. Receptor exposure was detected using 125I-fibrinogen and 125I-PAC1, a monoclonal antibody specific for the activated form of the fibrinogen receptor. The potential mediators that were studied included guanyl-5'-yl imidodiphosphate (Gpp(NH)p) and guanosine 5'O-(thiotriphosphate) (GTP gamma S), which cause G protein-dependent phospholipase C activation in platelets; inositol 1,4,5-triphosphate (IP3), which causes Ca2+ release from the platelet dense tubular system; and diacylglycerol and phorbol ester, which activate protein kinase C. Each of these molecules caused fibrinogen and PAC1 binding. The effect of IP3 was mimicked by raising the cytosolic free Ca2+ concentration in the permeabilized platelets. However, IP3 and Ca2+-induced PAC1 binding were abolished by indomethacin or aspirin, which had no effect on PAC1 binding caused by Gpp(NH)p, phorbol ester, or diacylglycerol. This suggests that the response to IP3 and Ca2+ is due to the formation of metabolites of arachidonic acid. One such metabolite, TxA2, is believed to activate platelets by stimulating G protein-dependent phosphoinositide hydrolysis. Indeed, we found that the G protein inhibitor guanyl-5'-yl thiophosphate (GDP beta S) inhibited PAC1 binding caused by a thromboxane A2 analog (U46619), IP3, and Ca2+, but had no effect on diacylglycerol or phorbol ester-induced PAC1 binding. Thrombin-induced PAC1 binding and phosphoinositide hydrolysis were also inhibited by GDP beta S and by pertussis toxin. Increasing the thrombin concentration overcame the inhibition of PAC1 binding caused by GDP beta S but did not overcome the inhibition of phosphoinositide hydrolysis. These observations demonstrate that fibrinogen receptor exposure occurs by at least two routes. One of these, in response to agonists such as thrombin and U46619, is initiated by G protein-dependent phosphoinositide hydrolysis and involves the formation of IP3 and diacylglycerol. IP3 appears to act by stimulating Ca2+-dependent arachidonic acid metabolism which, in turn, triggers further phosphoinositide hydrolysis. Diacylglycerol acts by stimulating protein kinase C. A second route is activated by high concentrations of thrombin and is independent of phosphoinositide hydrolysis.