Seyfert V L, Allman D, He Y, Staudt L M
Metabolism Branch, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892, USA.
Oncogene. 1996 Jun 6;12(11):2331-42.
In up to 45% of reported cases of the non-Hodgkin's lymphoma, diffuse large cell lymphoma, there are translocations of the BCL-6 gene, which are presumed to deregulate its expression. The BCL-6 protein, which is unmutated in these lymphomas, contains six Krüppel-like zinc fingers at its carboxy terminus and a 121 amino acid domain at its amino terminus, termed the POZ domain, which bears homology with amino terminal domains in a subset zinc finger transcription factors. In this study, we tested whether BCL-6 regulates transcription and if the POZ domain has a role in this function. The BCL-6 POZ domain, when fused to the GAL4 DNA binding domain, strongly repressed transcriptional activation initiated from several different promoters including the SV40 enhancer/promoter. Repression was also observed when the fusion protein was bound at a distance of 200 bp 5' of the promoter. When the GAL4/BCL6 POZ domain fusion protein was expressed in yeast, it was able to homodimerize in the nucleus. Nevertheless, in contrast with mammalian cells, the fusion protein did not repress transcription. To test the ability of the full length BC1-6 protein to repress transcription when bound to DNA through its zinc finger DNA binding domain, high affinity BCL-6 binding sites were selected from a pool of random oligonucleotides. Full length BCL-6 was able to strongly repress transcription when bound to its cognate site cloned upstream of the thymidine kinase promoter. This repression was mediated, in large measure, by the POZ domain, although a variant of BCL-6 lacking the POZ domain was able to repress transcription modestly. The ability of BCL-6 to function as a transcriptional repressor may contribute to its ability to transform B lymphocytes in diffuse large cell lymphoma.
在高达45%的已报道非霍奇金淋巴瘤弥漫大B细胞淋巴瘤病例中，存在BCL-6基因易位，推测这会使其表达失调。在这些淋巴瘤中未发生突变的BCL-6蛋白，在其羧基末端含有六个Krüppel样锌指结构，在其氨基末端含有一个121个氨基酸的结构域，称为POZ结构域，该结构域与一部分锌指转录因子的氨基末端结构域具有同源性。在本研究中，我们测试了BCL-6是否调节转录以及POZ结构域在该功能中是否起作用。当BCL-6的POZ结构域与GAL4 DNA结合结构域融合时，它强烈抑制从包括SV40增强子/启动子在内的几种不同启动子起始的转录激活。当融合蛋白结合在启动子5'端200 bp处时，也观察到了抑制作用。当GAL4/BCL6 POZ结构域融合蛋白在酵母中表达时，它能够在细胞核中同源二聚化。然而，与哺乳动物细胞不同，该融合蛋白不抑制转录。为了测试全长BC1-6蛋白通过其锌指DNA结合结构域与DNA结合时抑制转录的能力，从随机寡核苷酸库中筛选出高亲和力的BCL-6结合位点。全长BCL-6与克隆在胸苷激酶启动子上游其同源位点结合时，能够强烈抑制转录。这种抑制在很大程度上是由POZ结构域介导的，尽管缺乏POZ结构域的BCL-6变体也能够适度抑制转录。BCL-6作为转录抑制因子发挥作用的能力可能有助于其在弥漫大B细胞淋巴瘤中转化B淋巴细胞的能力。
