Elevation of cyclic AMP decreases phosphoinositide turnover and inhibits thrombin-induced secretion in human platelets

Anita Ryningen, Baard Olav Jensen, Holm Holmsen
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引用次数: 35

Abstract

Elevation of cyclic AMP (cAMP) in platelets inhibits agonist-induced, G protein-mediated responses and activation of polyphosphoinositide-specific phospholipase C (PLC) by ill-defined mechanism(s). Signal transduction steps downstream of PLC are inhibited by elevated cAMP, suggesting an inhibitory effect of cAMP, via protein kinase A, on PLC. In [32P]i-prelabeled platelets, forskolin increased intracellular cAMP (104 nmol/1011 cells at 10−5 M forskolin) and [32P]phosphatidylinositol 4-phosphate (Δ[32P]PIP) (30% at 10−7–10−6 M forskolin). The thrombin-induced (0.1 U/ml) increase in production of [32P]PA, ‘overshoots’ in [32P]PIP and [32P]PIP2 ([32P]phosphatidylinositol 4,5-bisphosphate), and the increase in [32P]PI and secretion of ADP+ATP were abolished by forskolin (10−7 M). Forskolin stimulated total [32P]Pi uptake in resting platelets (48%), increased 32P incorporation into PIP (110%), and inhibited 32P incorporation into PI (50%). The latter inhibition was most likely considerably greater due to the forskolin-induced stimulation of [32P]Pi uptake. The changes in radioactive PA, PIP and PIP2 are regarded as being proportional with their masses in the prelabeled platelets, while the increase in PI (phosphatidylinositol) is regarded as a change in specific radioactivity, and hence in its synthesis. The results suggest that cAMP elevation inhibits the flux in the polyphosphoinositide cycle through both inhibition of PIP 5-kinase and PI synthesis. The inverse relation between forskolin-produced ΔPIP and [32P]PA production suggests that the PLC reaction is inhibited by elevated cAMP through reduction of substrate (PIP2) resynthesis, and not by inhibition of the PLC enzyme.

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升高的环AMP减少磷酸肌肽的周转和抑制人血小板中凝血酶诱导的分泌
血小板中环AMP (cAMP)的升高抑制激动剂诱导的、G蛋白介导的反应和多磷肌醇特异性磷脂酶C (PLC)的激活,机制尚不明确。PLC下游的信号转导步骤被升高的cAMP抑制,这表明cAMP通过蛋白激酶A对PLC有抑制作用。在[32P]i预标记的血小板中,forskolin增加了细胞内cAMP(10−5 M forskolin时为104 nmol/1011个细胞)和[32P]磷脂酰肌醇4-磷酸(Δ[32P]PIP)(10−7-10−6 M forskolin时为30%)。凝血酶诱导的[32P]PA的生成增加(0.1 U/ml), [32P]PIP和[32P]PIP2 ([32P]磷脂酰肌醇4,5-二磷酸)的“过量”,[32P]PI的增加和ADP+ATP的分泌被forskolin(10−7 M)所消除。forskolin刺激静息血小板中[32P]PI的总摄取(48%),增加32P并入PIP(110%),抑制32P并入PI(50%)。由于福斯克林诱导的[32P]Pi摄取刺激,后一种抑制很可能要大得多。在预标记的血小板中,放射性PA、PIP和PIP2的变化被认为与它们的质量成正比,而PI(磷脂酰肌醇)的增加被认为是比放射性的变化,从而是其合成的变化。结果表明,cAMP升高通过抑制PIP 5-激酶和PI合成来抑制多磷酸肌醇循环的通量。福斯克林生成ΔPIP与[32P]PA生成之间的反比关系表明,通过减少底物(PIP2)的再合成,升高的cAMP可以抑制PLC反应,而不是通过抑制PLC酶。
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