Casalino Laura, De Cesare Dario, Verde Pasquale
Institute of Genetics and Biophysics "A Buzzati-Traverso," Consiglio Nazionale delle Ricerche, 80125 Naples, Italy.
Mol Cell Biol. 2003 Jun;23(12):4401-15. doi: 10.1128/MCB.23.12.4401-4415.2003.
The AP-1 transcription factor plays an essential role in cell proliferation and tumorigenesis. It was previously shown that the fra-1 gene product is upregulated by various oncogenes and is involved in the in vitro and in vivo transformation of thyroid cells. Here we show that the ras oncogene-dependent accumulation of Fra-1 is mediated by a positive feedback mechanism which requires both transcriptional autoregulation and posttranslational stabilization of the protein. The oncogene-dependent transcriptional activation involves the cooperation between both Raf-dependent and Raf-independent pathways and is mediated by an AP-1 site within the fra-1 first intron, which becomes stably occupied by a transcriptionally active Fra-1-containing complex in ras-transformed cells. The posttranslational stabilization results in a drastic increase in the Fra-1 half-life in ras-transformed cells and is totally dependent on the activity of the MEK/ERK phosphorylation pathway. The analysis of the Fra-1 transactivation potential shows that the protein is able to stimulate a heterologous promoter in a ras-dependent manner, but the transactivating activity requires the recruitment of a heterodimeric partner. These data show that the alteration of multiple regulatory mechanisms is required for the constitutive activation of Fra-1 as a nuclear target of ras transformation.
AP-1转录因子在细胞增殖和肿瘤发生中起关键作用。先前研究表明，fra-1基因产物受多种癌基因上调，且参与甲状腺细胞的体外和体内转化。在此我们表明，Fra-1的ras癌基因依赖性积累由一种正反馈机制介导，该机制既需要蛋白质的转录自调控，也需要翻译后稳定化。癌基因依赖性转录激活涉及Raf依赖性和Raf非依赖性途径之间的协同作用，并由fra-1第一个内含子内的一个AP-1位点介导，在ras转化细胞中，该位点被一个含转录活性Fra-1的复合物稳定占据。翻译后稳定化导致ras转化细胞中Fra-1半衰期急剧增加，且完全依赖于MEK/ERK磷酸化途径的活性。对Fra-1反式激活潜能的分析表明，该蛋白能够以ras依赖性方式刺激异源启动子，但反式激活活性需要募集一个异二聚体伴侣。这些数据表明，Fra-1作为ras转化的核靶点的组成型激活需要多种调控机制的改变。
