Azzi Mounia, Charest Pascale G, Angers Stéphane, Rousseau Guy, Kohout Trudy, Bouvier Michel, Piñeyro Graciela
Department of Biochemistry, Université de Montréal, Montréal, QC, Canada H3C 3J7.
Proc Natl Acad Sci U S A. 2003 Sep 30;100(20):11406-11. doi: 10.1073/pnas.1936664100. Epub 2003 Sep 17.
It is becoming increasingly clear that signaling via G protein-coupled receptors is a diverse phenomenon involving receptor interaction with a variety of signaling partners. Despite this diversity, receptor ligands are commonly classified only according to their ability to modify G protein-dependent signaling. Here we show that beta2AR ligands like ICI118551 and propranolol, which are inverse agonists for Gs-stimulated adenylyl cyclase, induce partial agonist responses for the mitogen-activated protein kinases extracellular signal-regulated kinase (ERK) 1/2 thus behaving as dual efficacy ligands. ERK1/2 activation by dual efficacy ligands was not affected by ADP-ribosylation of Galphai and could be observed in S49-cyc- cells lacking Galphas indicating that, unlike the conventional agonist isoproterenol, these drugs induce ERK1/2 activation in a Gs/i-independent manner. In contrast, this activation was inhibited by a dominant negative mutant of beta-arrestin and was abolished in mouse embryonic fibroblasts lacking beta-arrestin 1 and 2. The role of beta-arrestin was further confirmed by showing that transfection of beta-arrestin 2 in these knockout cells restored ICI118551 promoted ERK1/2 activation. ICI118551 and propranolol also promoted beta-arrestin recruitment to the receptor. Taken together, these observations suggest that beta-arrestin recruitment is not an exclusive property of agonists, and that ligands classically classified as inverse agonists rely exclusively on beta-arrestin for their positive signaling activity. This phenomenon is not unique to beta2-adrenergic ligands because SR121463B, an inverse agonist on the V2 vasopressin receptor-stimulated adenylyl cyclase, recruited beta-arrestin and stimulated ERK1/2. These results point to a multistate model of receptor activation in which ligand-specific conformations are capable of differentially activating distinct signaling partners.
越来越明显的是，通过G蛋白偶联受体的信号传导是一种多样的现象，涉及受体与多种信号传导伙伴的相互作用。尽管存在这种多样性，但受体配体通常仅根据其修饰G蛋白依赖性信号传导的能力进行分类。在这里，我们表明，像ICI118551和普萘洛尔这样的β2肾上腺素能受体配体，它们是Gs刺激的腺苷酸环化酶的反向激动剂，却能诱导丝裂原活化蛋白激酶细胞外信号调节激酶（ERK）1/2产生部分激动剂反应，因此表现为双重效能配体。双重效能配体对ERK1/2的激活不受Gαi的ADP核糖基化影响，并且在缺乏Gαs 的S49-cyc-细胞中也能观察到，这表明与传统激动剂异丙肾上腺素不同，这些药物以不依赖Gs/i的方式诱导ERK1/2激活。相反，这种激活被β-抑制蛋白的显性负性突变体所抑制，并且在缺乏β-抑制蛋白1和2的小鼠胚胎成纤维细胞中被消除。通过在这些基因敲除细胞中转染β-抑制蛋白2可恢复ICI118551促进ERK1/2激活，进一步证实了β-抑制蛋白的作用。ICI118551和普萘洛尔还促进β-抑制蛋白募集到受体上。综上所述，这些观察结果表明，β-抑制蛋白募集并非激动剂的独有特性，并且传统上归类为反向激动剂的配体完全依赖β-抑制蛋白来实现其正向信号传导活性。这种现象并非β2-肾上腺素能配体所特有，因为V2血管加压素受体刺激的腺苷酸环化酶的反向激动剂SR121463B也能募集β-抑制蛋白并刺激ERK1/2激活。这些结果指向一种受体激活的多状态模型,其中配体特异性构象能够差异性地激活不同的信号传导伙伴。
