Russell M, Winitz S, Johnson G L
Division of Basic Sciences, National Jewish Center for Immunology and Respiratory Medicine, Denver, Colorado 80206.
Mol Cell Biol. 1994 Apr;14(4):2343-51. doi: 10.1128/mcb.14.4.2343-2351.1994.
Acetylcholine muscarinic m2 receptors (m2R) couple to heterotrimeric Gi proteins and activate the Ras/Raf/mitogen-activated protein kinase pathway and phosphatidylinositol 3-kinase in Rat 1a cells. In contrast to the m2R, stimulation of the acetylcholine muscarinic m1 receptor (m1R) does not activate the Ras/Raf/mitogen-activated protein kinase regulatory pathway in Rat 1a cells but rather causes a pronounced inhibition of epidermal growth factor and platelet-derived growth factor receptor activation of Raf. In Rat 1a cells, m1R stimulation of phospholipase C beta and the marked rise in intracellular calcium stimulated cyclic AMP (cAMP) synthesis, resulting in the activation of protein kinase A. Stimulation of protein kinase A inhibited Raf activation in response to growth factors. Platelet-derived growth factor receptor stimulation of phosphatidylinositol 3-kinase activity was not affected by either m1R stimulation or protein kinase A activation in response to forskolin-stimulated cAMP synthesis. GTP loading of Ras in response to growth factors was unaffected by protein kinase A activation but was partially inhibited by carbachol stimulation of the m1R. Therefore, protein kinase A action at the Ras/Raf activation interface selectively inhibited only one branch of the signal transduction network initiated by tyrosine kinases. Specific adenylyl cyclases responding to different signals, including calcium, with enhanced cAMP synthesis will regulate Raf activation in response to Ras.GTP. Taken together, the data indicate that G protein-coupled receptors can positively and negatively regulate the responsiveness of tyrosine kinase-stimulated mitogenic response pathways.
乙酰胆碱毒蕈碱型m2受体（m2R）与异源三聚体Gi蛋白偶联，并激活大鼠1a细胞中的Ras/Raf/丝裂原活化蛋白激酶途径和磷脂酰肌醇3激酶。与m2R不同，乙酰胆碱毒蕈碱型m1受体（m1R）的刺激不会激活大鼠1a细胞中的Ras/Raf/丝裂原活化蛋白激酶调节途径，反而会显著抑制表皮生长因子和血小板衍生生长因子受体对Raf的激活。在大鼠1a细胞中，m1R对磷脂酶Cβ的刺激以及细胞内钙的显著升高刺激了环磷酸腺苷（cAMP）的合成，从而导致蛋白激酶A的激活。蛋白激酶A的刺激抑制了对生长因子的Raf激活。血小板衍生生长因子受体对磷脂酰肌醇3激酶活性的刺激不受m1R刺激或响应福斯可林刺激的cAMP合成的蛋白激酶A激活的影响。响应生长因子的Ras的GTP负载不受蛋白激酶A激活的影响，但受到m1R的卡巴胆碱刺激的部分抑制。因此，蛋白激酶A在Ras/Raf激活界面的作用仅选择性地抑制了由酪氨酸激酶启动的信号转导网络的一个分支。对包括钙在内的不同信号作出反应并增强cAMP合成的特定腺苷酸环化酶将调节对Ras.GTP的Raf激活。综上所述，数据表明G蛋白偶联受体可以正向和负向调节酪氨酸激酶刺激的有丝分裂反应途径的反应性。
