Fromm C, Coso O A, Montaner S, Xu N, Gutkind J S
Oral and Pharyngeal Cancer Branch, National Institute of Dental Research, National Institutes of Health, 9000 Rockville Pike, Building 30, Room 212, Bethesda, MD 20892-4330, USA.
Proc Natl Acad Sci U S A. 1997 Sep 16;94(19):10098-103. doi: 10.1073/pnas.94.19.10098.
Receptors coupled to heterotrimeric G proteins can effectively stimulate growth promoting pathways in a large variety of cell types, and if persistently activated, these receptors can also behave as dominant-acting oncoproteins. Consistently, activating mutations for G proteins of the Galphas and Galphai2 families were found in human tumors; and members of the Galphaq and Galpha12 families are fully transforming when expressed in murine fibroblasts. In an effort aimed to elucidate the molecular events involved in proliferative signaling through heterotrimeric G proteins we have focused recently on gene expression regulation. Using NIH 3T3 fibroblasts expressing m1 muscarinic acetylcholine receptors as a model system, we have observed that activation of this transforming G protein-coupled receptors induces the rapid expression of a variety of early responsive genes, including the c-fos protooncogene. One of the c-fos promoter elements, the serum response element (SRE), plays a central regulatory role, and activation of SRE-dependent transcription has been found to be regulated by several proteins, including the serum response factor and the ternary complex factor. With the aid of reporter plasmids for gene expression, we observed here that stimulation of m1 muscarinic acetylcholine receptors potently induced SRE-driven reporter gene activity in NIH 3T3 cells. In these cells, only the Galpha12 family of heterotrimeric G protein alpha subunits strongly induced the SRE, while Gbeta1gamma2 dimers activated SRE to a more limited extent. Furthermore, our study provides strong evidence that m1, Galpha12 and the small GTP-binding protein RhoA are components of a novel signal transduction pathway that leads to the ternary complex factor-independent transcriptional activation of the SRE and to cellular transformation.
与异源三聚体G蛋白偶联的受体能够在多种细胞类型中有效刺激促进生长的信号通路，并且如果持续激活，这些受体还可表现为显性作用的癌蛋白。同样，在人类肿瘤中发现了Gαs和Gαi2家族G蛋白的激活突变；而Gαq和Gα12家族的成员在鼠成纤维细胞中表达时具有完全转化能力。为了阐明通过异源三聚体G蛋白参与增殖信号传导的分子事件，我们最近聚焦于基因表达调控。使用表达m1毒蕈碱型乙酰胆碱受体的NIH 3T3成纤维细胞作为模型系统，我们观察到这种转化型G蛋白偶联受体的激活诱导了多种早期反应基因的快速表达，包括原癌基因c-fos。c-fos启动子元件之一，即血清反应元件（SRE），发挥着核心调控作用，并且已发现SRE依赖的转录激活受多种蛋白质调控，包括血清反应因子和三元复合因子。借助用于基因表达报告质粒，我们在此观察到刺激m1毒蕈碱型乙酰胆碱受体可有效诱导NIH 3T3细胞中SRE驱动的报告基因活性。在这些细胞中，只有异源三聚体G蛋白α亚基的Gα12家族强烈诱导SRE，而Gβ1γ2二聚体对SRE的激活程度则较为有限。此外，我们的研究提供了有力证据，表明m1、Gα12和小GTP结合蛋白RhoA是一条新信号转导通路的组成部分，该通路导致SRE的三元复合因子非依赖性转录激活并引发细胞转化。
