Xu Yong, McDonald Jessica, Perloff Emily, Butticè Giovanna, Schreiber Barbara M, Smith Barbara D
Department of Biochemistry, Boston University School of Medicine, Boston, MA 02118, USA.
Mol Immunol. 2007 Mar;44(7):1709-21. doi: 10.1016/j.molimm.2006.07.294. Epub 2006 Sep 18.
Major histocompatibility class II (MHC II) transactivator (CIITA) is critical for interferon-gamma (IFN-gamma)-induced repression of collagen [Xu, Y., Wang, L., Buttice, G., Sengupta, P.K., Smith, B.D., 2004. Major histocompatibility class II transactivator (CIITA) mediates repression of collagen (COL1A2) transcription by interferon gamma (IFN-gamma). J. Biol. Chem. 279, 41319-41332] and activation of MHC II transcription. To better understand the role of CIITA and IFN-gamma induced repression of collagen, mesenchymal cells (lung fibroblasts, adventitial fibroblasts, and smooth muscle cells) were isolated from a CIITA deficient mouse (C2ta(tm1Ccum)). IFN-gamma induced MHC II expression and repressed collagen type I expression in all three cell types isolated from the wild type background. As expected, IFN-gamma treatment of cells isolated from CIITA deficient mice did not induce MHC II production or activate the MHC II promoter. Interestingly, collagen gene expression and promoter activity was similar to that of wild type. Moreover, IFN-gamma induced CIITA mRNA and a truncated form of CIITA protein in all cells isolated from CIITA deficient mice. Most importantly, truncated CIITA occupied the collagen alpha 2(I) gene (col1a2) transcription start site during IFN-gamma treatment, but it did not occupy the MHC II promoter as judged by chromatin immunoprecipitation assays. Exogenous expression of a similar truncated form of CIITA maintained its ability to repress col1a2 transcription, but lost its ability to activate MHC II gene transcription suggesting a role for the CIITA C-terminal domain in activation, but not repression. IFN-gamma induced primarily types I and IV CIITA isoforms in the mouse cells. All three isoforms of CIITA were capable of repressing col1a2 and activating MHC II gene transcription. These data suggest that the previously described CIITA knockout mouse carries a hypomorphic mutation, rather than a null mutation. The removal of the leucine rich region in CIITA blocks activation of MHC II without altering repression of collagen transcription.
主要组织相容性复合体II类（MHC II）反式激活因子（CIITA）对于干扰素-γ（IFN-γ）诱导的胶原蛋白抑制作用至关重要[Xu, Y., Wang, L., Buttice, G., Sengupta, P.K., Smith, B.D., 2004. 主要组织相容性复合体II类反式激活因子（CIITA）介导干扰素γ（IFN-γ）对胶原蛋白（COL1A2）转录的抑制作用。《生物化学杂志》279, 41319 - 41332]以及MHC II转录的激活。为了更好地理解CIITA的作用以及IFN-γ诱导的胶原蛋白抑制作用，从CIITA缺陷小鼠（C2ta(tm1Ccum)）中分离出间充质细胞（肺成纤维细胞、外膜成纤维细胞和平滑肌细胞）。IFN-γ在从野生型背景分离出的所有三种细胞类型中诱导MHC II表达并抑制I型胶原蛋白表达。正如预期的那样，用IFN-γ处理从CIITA缺陷小鼠分离出的细胞不会诱导MHC II产生或激活MHC II启动子。有趣的是，胶原蛋白基因表达和启动子活性与野生型相似。此外，IFN-γ在从CIITA缺陷小鼠分离出的所有细胞中诱导CIITA mRNA和一种截短形式的CIITA蛋白。最重要的是，通过染色质免疫沉淀分析判断，截短的CIITA在IFN-γ处理期间占据胶原蛋白α2（I）基因（col1a2）转录起始位点，但不占据MHC II启动子。外源性表达类似截短形式的CIITA保持了其抑制col1a2转录的能力，但失去了激活MHC II基因转录的能力，这表明CIITA C末端结构域在激活而非抑制中起作用。IFN-γ在小鼠细胞中主要诱导I型和IV型CIITA亚型。CIITA的所有三种亚型都能够抑制col1a2并激活MHC II基因转录。这些数据表明，先前描述的CIITA基因敲除小鼠携带的是一个低表达突变，而非无效突变。去除CIITA中富含亮氨酸的区域会阻断MHC II的激活，而不会改变胶原蛋白转录的抑制作用。
