Chen Han, Gilbert Carolyn A, Hudson John A, Bolick Sophia C, Wright Kenneth L, Piskurich Janet F
Division of Basic Medical Sciences, Mercer University School of Medicine, 1550 College St., Macon, GA 31207, USA.
Mol Immunol. 2007 Feb;44(6):1461-70. doi: 10.1016/j.molimm.2006.04.026. Epub 2006 Jun 12.
MHC class II transactivator (CIITA), a co-activator that controls MHC class II (MHC II) transcription, functions as the master regulator of MHC II expression. Persistent activity of the CIITA type III promoter (pIII), one of the four potential promoters of this gene, is responsible for constitutive expression of MHC II by B lymphocytes. In addition, IFN-gamma induces expression of CIITA in these cells through the type IV promoter (pIV). Positive regulatory domain 1-binding factor 1 (PRDI-BF1), called B lymphocyte-induced maturation protein 1 (Blimp-1) in mice, represses the expression of CIITA pIII in plasma and multiple myeloma cells. To investigate regulation of CIITA pIV expression by PRDI-BF1 in the B lymphocyte lineage, protein/DNA-binding studies, and functional promoter analyses were performed. PRDI-BF1 bound to the IFN regulatory factor-element (IRF-E) site in CIITA pIV. Ectopic expression of either PRDI-BF1 or Blimp-1 repressed this promoter in B lymphocytes. In vitro binding and functional analyses of CIITA pIV demonstrated that the IRF-E is the target of this repression. In vivo genomic footprint analysis demonstrated protein binding at the IRF-E site of CIITA pIV in U266 myeloma cells, which express PRDI-BF1. PRDI-BF1beta, a truncated form of PRDI-BF1 that is co-expressed in myeloma cells, also bound to the IRF-E site and repressed CIITA pIV. These findings demonstrate for the first time that, in addition to silencing expression of CIITA pIII in B lymphocytes, PRDI-BF1 is capable of binding and suppressing CIITA pIV.
主要组织相容性复合体II类反式激活因子（CIITA）是一种控制主要组织相容性复合体II类（MHC II）转录的共激活因子，作为MHC II表达的主要调节因子发挥作用。CIITA基因的四个潜在启动子之一的III型启动子（pIII）的持续活性，负责B淋巴细胞对MHC II的组成型表达。此外，γ干扰素通过IV型启动子（pIV）诱导这些细胞中CIITA的表达。阳性调节域1结合因子1（PRDI-BF1），在小鼠中称为B淋巴细胞诱导成熟蛋白1（Blimp-1），可抑制浆细胞和多发性骨髓瘤细胞中CIITA pIII的表达。为了研究PRDI-BF1对B淋巴细胞系中CIITA pIV表达的调控，进行了蛋白质/DNA结合研究和功能性启动子分析。PRDI-BF1与CIITA pIV中的干扰素调节因子元件（IRF-E）位点结合。PRDI-BF1或Blimp-1的异位表达在B淋巴细胞中抑制了该启动子。CIITA pIV的体外结合和功能分析表明，IRF-E是这种抑制作用的靶点。体内基因组足迹分析表明，在表达PRDI-BF1的U266骨髓瘤细胞中，蛋白质与CIITA pIV的IRF-E位点结合。PRDI-BF1β是PRDI-BF1的一种截短形式，在骨髓瘤细胞中共同表达，也与IRF-E位点结合并抑制CIITA pIV。这些发现首次证明，除了使B淋巴细胞中CIITA pIII的表达沉默外，PRDI-BF1还能够结合并抑制CIITA pIV。
