Crespo P, Cachero T G, Xu N, Gutkind J S
Molecular Signaling Unit, NIDR, National Institutes of Health, Bethesda, Maryland 20892, USA.
J Biol Chem. 1995 Oct 20;270(42):25259-65. doi: 10.1074/jbc.270.42.25259.
The enzymatic activity of mitogen-activated protein kinases (MAP kinases) increases in response to agents acting on a variety of cell surface receptors, including receptors linked to heterotrimeric G proteins of the Gi and Gq family. Recently, it has been shown that stimulation of beta-adrenergic receptors, which are typical of those that act through Gs to activate adenylyl cyclases, potently activates MAP kinases in the heart, resulting in the hypertrophy of the cardiac muscle (Lazou, A., Bogoyevitch, M.A., Clerk, A., Fuller, S.J., Marshall, C.J., and Sudgen, P.H. (1994) Circ. Res. 75, 938-941). We have observed that exposure of COS-7 cells to a beta-adrenergic agonist, isoproterenol, raises intracellular levels of cAMP and effectively activates protein kinase A (PKA) and an epitope-tagged MAP kinase. However, MAP kinase stimulation by isoproterenol was neither mimicked by expression of an activated mutant of G alpha s, nor by treatment with PKA-stimulating agents. Moreover, pretreatment of COS-7 with a permeable cAMP analog, 8-Br-cAMP, markedly decreased MAP kinase activation by either isoproterenol or epidermal growth factor. Thus, in COS-7 cells cAMP and PKA do not appear to mediate MAP kinase activation by beta-adrenergic receptors. Signaling from beta-adrenergic receptors to MAP kinase was inhibited by transfection of a chimeric molecule consisting of the CD8 receptor and the carboxyl terminus of the beta-adrenergic receptor kinase, which includes the beta gamma-binding domain. MAP kinase activation by isoproterenol was not affected by depletion of protein kinase C, but it was completely abolished by expression of Ras-inhibiting molecules. We conclude that signaling from beta-adrenergic receptors to MAP kinase involves an activating signal mediated by beta gamma subunits acting on a Ras-dependent pathway and a G alpha s-induced inhibitory signal mediated by cAMP and PKA. The balance between these two opposing mechanisms of regulation would be expected to control the MAP kinase response to beta-adrenergic agonists as well as to other biologically active agents known to act on Gs coupled receptors, including a number of hormones, neurotransmitters, and lipid mediators.
丝裂原活化蛋白激酶（MAP激酶）的酶活性会因作用于多种细胞表面受体的因子而增加，这些受体包括与Gi和Gq家族的异源三聚体G蛋白相连的受体。最近有研究表明，通过Gs激活腺苷酸环化酶的典型β-肾上腺素能受体受到刺激后，能有效激活心脏中的MAP激酶，导致心肌肥大（Lazou, A., Bogoyevitch, M.A., Clerk, A., Fuller, S.J., Marshall, C.J., and Sudgen, P.H. (1994) Circ. Res. 75, 938 - 941）。我们观察到，将COS - 7细胞暴露于β-肾上腺素能激动剂异丙肾上腺素后，细胞内cAMP水平升高，并有效激活蛋白激酶A（PKA）和一个表位标记的MAP激酶。然而，异丙肾上腺素对MAP激酶的刺激既不能被激活的Gαs突变体的表达所模拟，也不能被PKA刺激剂处理所模拟。此外，用可渗透的cAMP类似物8 - Br - cAMP预处理COS - 7细胞，可显著降低异丙肾上腺素或表皮生长因子对MAP激酶的激活作用。因此，在COS - 7细胞中，cAMP和PKA似乎并不介导β-肾上腺素能受体对MAP激酶的激活。由CD8受体和β-肾上腺素能受体激酶的羧基末端（包括βγ结合结构域）组成的嵌合分子转染可抑制从β-肾上腺素能受体到MAP激酶的信号传导。异丙肾上腺素对MAP激酶的激活不受蛋白激酶C耗竭的影响，但通过表达Ras抑制分子可使其完全消除。我们得出结论，从β-肾上腺素能受体到MAP激酶的信号传导涉及由βγ亚基作用于Ras依赖性途径介导的激活信号，以及由cAMP和PKA介导的Gαs诱导的抑制信号。预计这两种相反调节机制之间的平衡将控制MAP激酶对β-肾上腺素能激动剂以及其他已知作用于Gs偶联受体的生物活性物质（包括多种激素、神经递质和脂质介质）的反应。
