Surfactant protein D gene regulation. Interactions among the conserved CCAAT/enhancer-binding protein elements

Surfactant protein D (SP-D) plays roles in pulmonary host defense and surfactant homeostasis and is increased following acute lung injury. Given the importance of CCAAT/enhancer-binding protein (C/EBP)-binding elements in the systemic acute-phase response and lung development and the expression of C/EBP isoforms by lung epithelial cells, we hypothesized that conserved C/EBP motifs in the near-distal and proximal promoters contribute to the regulation of SP-D expression by C/EBPs. Five SP-D motifs (-432, -340, -319, -140, and -90) homologous to the C/EBP consensus sequence specifically bound to C/EBPs in gel shift assays, and four of the five sites (-432, -340, -319, and -90) efficiently competed for the binding of C/EBPalpha, C/EBPbeta, or C/EBPdelta to consensus oligomers. Cotransfection of C/EBPalpha, C/EBPbeta, or C/EBPdelta cDNA in H441 lung adenocarcinoma cells significantly increased the luciferase activity of a wild-type SP-D promoter construct containing 698 bp of upstream sequence (SS698). Transfection of C/EBP also increased the level of endogenous SP-D mRNA in H441 cells. Transactivation of the reporter construct was abrogated by deletion of sequences upstream of -205. Independent site-directed mutagenesis of the sites at -432, -340, and -319 reduced C/EBP-mediated activation by approximately 50%, and mutagenesis of the site at -432 in combination with either of the tandem sites at -340 and -319 blocked activation. The conserved AP-1 element at -109 was required for maximal promoter activity, but not for the transactivation of SS698 by C/EBPs. Thus, interactions among C/EBP elements in the near-distal promoter can modulate the promoter activity of SP-D.     

C／EBPβ作为LIF的中介维持小鼠胚干细胞于未分化状态

C／EBPβ（CCAAT／增强子结合蛋白β,又称NF—IL6）是一个多功能的转录因子,其主要功能之一是促进细胞分化。白血病抑制因子（LIF）是一个细胞因子．其在不同类型的细胞中具有不同的效应：它诱导前脂肪细胞分化,却抑制小鼠胚多能干细胞（mES）的分化。在mES中,C／EBPβ究竟起什么作用．尚未有报道。本文首次报告在mES中。在LIF蛋白存在下,C／EBPβ的作用是维持mES的未分化状态．即C／EBPβ是LIF的调控对象。主要事实如下。在mES细胞中．内源C／EBPβ蛋白的表达量与加到培养基中的LIF蛋白的量呈正相关。而在未分化的mES细胞中人工高表达的外源C／EBPβ蛋白和其截短形式LIP蛋白。在LIF存在下．也不但不促进反而抑制mES细胞分化,C／EBPβ的大分子异型蛋白还显著促进mES细胞的增殖：而且,在LIF去除后,这种促进mES细胞增殖的效应还能持续一段短时间。当LIF不存在时。C／EBPβ和LIP才如所预期的那样．诱导分化相关基因表达并促进细胞分化。C／EBPβ和LIP所调控的某些分化相关的基因的表达水平．当LIF存在时．也比没有LIF时显著降低。因此,在mES细胞中C／EBPβ是受LIF的调控而作为LIF的中介．维持mES于未分化状态。