Transcriptional Regulation of Selectin-ligand Synthetase (Fucosyltransferase VII) Related to Ability of Metastasis in Cancer Cells

岩藻糖基转移酶VII（Fucosyltransferase VII，Fuc-T VII）基因编码白细胞/内皮细胞粘附分子-配体唾液酸刘易斯X合成酶，其表达在炎症反应甚至某些癌细胞的转移中起重要作用。我们已经表明，成人T细胞白血病患者的白血病细胞强烈表达唾液酸刘易斯X，其表达水平与白血病细胞的血管外浸润程度显著相关，并且Fuc-T VII基因被HTLV-1 Tax反式激活。我们鉴定了Fuc-T VII基因启动子中的Tax应答元件（TaxRE），其含有半回文cAMP反应元件（CRE）。我们发现CREB/ATF家族的几个成员和未鉴定的因子与该TaxRE结合，该TaxRE与HTLV-1长末端重复启动子中的Tax应答性病毒CRE非常同源。除结构相似外，报告基因分析显示TaxRE与病毒CRE在功能上也相似，与细胞克雷斯如生长抑素基因启动子不同，为了阐明TaxRE中这种反式激活的机制，我们利用酵母单杂交系统克隆了编码这种反式激活的TaxRE结合因子的cDNA。除了CREB/ATF家族成员外，还鉴定出一个cDNA是转录因子T box家族T-bet的人类同源物，据报道T-bet在T淋巴细胞向Th 1分化中起关键作用。然而，T-bet的确切结合序列和反式激活机制仍有待研究。我们发现，T-bet实际上与TaxRE的5'部分结合，并且在体内也与Tax相关，并且T-bet参与与CREB/ATF家族和Tax相关的Fuc-T VII基因的调节。这些结果表明T-bet在HTLV-1 Tax介导的Fuc-T VII基因反式激活中起重要作用。

Fucosyltransferase VII (Fuc-T VII) gene encodes a leukocyte/endothelial cell adhesion molecule -ligand sialyl Lewis X synthetase, the expression of which plays important roles in inflammation and even in metastasis of some cancer cells. We have shown that leukemia cells in patients with adult T-cell leukemia strongly express the sialyl Lewis X, the expression level of which significantly correlates with the degree of extravascular infiltration of the leukemic cells, and that the Fuc-T VII gene is transactivated by the HTLV-1 Tax.We identified the Tax-responsive element (TaxRE) in the promoter of the Fuc-T VII gene, which contains a half-palindromic cAMP-responsive element (CRE). We found that several members of the CREB/ATF family and unidentified factors bind to this TaxRE being much homologous to the Tax-responsive viral CRE in the HTLV-1 long terminal repeat-promoter. In addition to the structural similarity, reporter analysis revealed that the TaxRE also bears functional resemblance to the viral CRE, differing from cellular CREs such as ** the somatostatin gene promoter.To clarify the mechanism of such fransactivation in the TaxRE, we utilized the yeast one-hybrid system to clone cDNAs encoding TaxRE-binding factors involved in this transactivation. Besides members of CREB/ATF family, one cDNA was identified as the human homologue of one of the transcription factor T box family, T-bet, T-bet reportedly plays a critical role in differentiation of T lymphocyte to Th1. However, the exact binding sequence of T-bet and the mechanism of transactivation remain to be studied. We showed that T-bet actually binds to the 5' part of the TaxRE and also associates with Tax in vivo, and that T-bet is involved in the regulation of the Fuc-T VII gene in association with CREB/ATF family and Tax. These results suggest an important role for T-bet in HTLV-1 Tax-mediated Fuc-T VII gene transactivation.