| Summary: | 博士 === 國立清華大學 === 生命科學系 === 87 === Ganodermic acid S (GAS) [lanosta-7,9(11),24-triene-3beta,15alpha-diacetoxy-26-oic acid], a triterpene isolated from the Chinese medicinal fungus Ganoderma lucidum (Fr.) Karst (Polyporaceae), exerts multiple effects on human platelet responses to various aggregating agonists. The thesis investigated how GAS less than 16 microM may affect the responses of human gel-filtered platelets (GFP) to (1) thromboxane (TX) A2 mimetic U46619, (2) collagen, and (3) prostaglandin (PG) E1.
In platelet response to U46619, GAS inhibited Ca2+ mobilization, phosphorylation of myosin light chain and pleckstrin, alpha-granule secretion, and cell aggregation. GAS at 7.5 microM also strongly inhibited U46619-induced diacylglycerol formation, arachidonic acid release, and TXB2 formation. Immunoblotting study of protein-tyrosine phosphorylation showed that GAS inhibited the phosphotyrosine proteins at the steps involving the engagement of integrin alphaIIbbeta3 and aggregation. However, GAS did not inhibit U46619-induced platelet shape change, nor the inhibitory effect of U46619 on PGE1-evoked cyclic AMP level in GFP. It is concluded that GAS inhibits platelet response to TXA2 on the receptor-Gq-phospholipase Cbeta1 pathway, but not on the receptor-Gi pathway.
In platelet response to collagen, GAS inhibited cell aggregation by prolonging lag and shape change periods and decreasing the initial cell aggregation rate. However, the inhibitory efficiency was less than its inhibition on GFP response to U46619. In the agent-effect on biochemical events, GAS effectively inhibited Ca2+ mobilization, phosphorylation of myosin light chain, dense granule secretion and TXB2 generation. The inhibition might be originated from blocking Ca2+ mobilization of the TXA2-dependent pathway. GAS partially decreased the phosphorylation of most phosphotyrosine proteins from early activation to the integrin alphaIIbbeta3-regulated steps. The agent did not affect the phosphorylation of four proteins at the steps regulated by integrin alphaIIbbeta3. These results suggest that GAS inhibits the collagen response predominantly on the TXA2-dependent signaling, and the tyrosine kinase-dependent pathway in collagen response plays a major role in cell aggregation.
In platelet response to PGE1, GAS at < 20 microM did not affect the basal cyclic AMP level. However, GAS potentiated the PGE1-evoked cyclic AMP level in a bell-shaped, concentration-dependent manner. The agent at 7.5 microM enhanced the level up to 1.8 fold of that evoked by PGE1 alone. Collagen did not inhibit the PGE1-induced cyclic AMP level in platelets pretreated with GAS of 6 to 7.5 microM. Besides, the agent at 7.5 microM enhanced the inhibition of PGE1 on platelet response to collagen in: phosphorylation of myosin light chain and pleckstrin, alpha-granule secretion, cell aggregation and protein-tyrosine phosphorylation. In addition, the agent along with PGE1 almost abolished the dense granule secretion and TXB2 formation. The results suggest that GAS potentiated the PGE1-induced cyclic AMP synthesis. The elevated cyclic AMP level might contribute to part of the enhancement of GAS in PGE1-inhibition of platelet response to collagen.
These studies indicate that the insertion of GAS less than 7.5 microM into human platelet plasma membrane exhibits multiple effects on platelet functions. It may be due that the lipid environments surrounding the receptors of various agonists are different. The thesis establishes the inhibitory mechanisms of GAS on platelet response to TXA2 mimetic U46619 and collagen, and distinguishes the effects of GAS on both two agonists. Besides, the study shows that GAS plays another role on platelets by potentiation of PGE1-induced cyclic AMP synthesis and PGE1-inhibition of collagen response.
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