Mechanism of activation of interferon-stimulated genes by HTLV-1 Tax

用RT-PCR差异显示技术研究了HTLV-1基因Tax转化大鼠1成纤维细胞后的基因表达。分析显示，在Tax转化的细胞中，8个基因上调，1个基因被抑制。六例上调依赖于NF-κ B通路，有趣的是，其中至少有四个是干扰素刺激基因（ISG）。在Tax转化的Rat-1细胞和原代ATL细胞中均被激活的2 ′-5 ′寡腺苷酸合成酶（2-5 OAS）基因的启动子分析表明Tax和NF-κ B均激活其干扰素应答增强子元件。此外，占主导地位的活性形式的NF-κ B分子抑制激活。虽然在Tax转化的大鼠1细胞和原代ATL细胞中观察到干扰素B基因的表达，但添加针对干扰素a和B的抗体并不能消除Tax对ISRE报告基因的激活。结果表明Tax可能通过诱导干扰素应答因子（interferon responsive factors，IRF）等ISRE结合蛋白的表达而间接激活ISRE，但在Tax转化细胞中未观察到IRF-1、IRF-3和IRF-7的表达增强，而IRF-2和IRF-4的表达增强。由于已报道IRF-4在HTLV-1转化的T细胞中高度表达，并参与rel癌基因对鸡成纤维细胞的细胞转化，因此建议IRF-4参与Tax介导的Rat-1细胞转化。

Gene expression in Rat-1 fibroblast cells transformed by Tax from HTLV-1 was studied using the RT-PCR differential display technique. The analysis revealed that eight genes were upregulated and one gene was suppressed in Tax-transformed cells. Six cases of upregulation were dependent on the NF-kB pathway and, interestingly, at least four of them were interferon-stimulated genes (ISGs). Promoter analysis of the 2'-5' oligoadenylate synthetase (2-5 OAS) gene, which was activated in both Tax-transformed Rat-1 cells and primary ATL cells, demonstrated that both Tax and NF-kB activate its interferon-responsive enhancer element. Furthermore, dominant active form of the NF-kB molecule suppressed the activation. Although the expression of interferon b gene was observed in Tax-transformed rat1 cells and primary ATL cells, addition of antibodies against interferon a and b did not abrogate the activation of ISRE reporter by Tax. It was thus indicated that Tax activates the ISRE indirectly, probably through induction of some ISRE binding proteins such as members of interferon responsive factors, IRFs.Enhanced expression of IRF-1, -3 and -7 was not observed in Tax-transformed cells but overexpression of IRF-2 and IRF-4, which were originally identified as repressors, with expression vectors activated the ISRE reporter plasmid. Since IRF-4 has been reported to be highly expressed in HTLV-1 transformed T-cells and to be involved in cellular transformation of chicken fibroblasts by rel oncogene, involvement of IRF-4 in Tax-mediated transformation of Rat-1 cells was suggested.