Global impact of Salmonella type III secretion effector SteA on host cells

Salmonella enterica is a Gram-negative bacterium that causes gastroenteritis, bacteremia and typhoid fever in several animal species including humans. Its virulence is greatly dependent on two type III secretion systems (T3SSs), encoded in pathogenicity islands 1 (SPI1) and 2 (SPI2), respectively. These systems translocate proteins called effectors into eukaryotic host cell. Effectors interfere with certain host signal transduction pathways to allow the internalization of pathogens and their survival and proliferation inside vacuoles. SteA is one of the few Salmonella effectors that are substrates of both T3SSs. Nothing is known about the function of this protein inside the host cells. Here, we used gene arrays and bioinformatics analysis to study the genetic response of human epithelial cells to SteA. We found that constitutive synthesis of SteA in epithelial cells leads to induction of genes related to extracellular matrix organization and regulation of cell proliferation and serine/threonine kinase signaling pathways. SteA also represses genes related to immune processes and regulation of purine nucleotide synthesis and pathway-restricted SMAD protein phosphorylation. Consisted with this analysis a cell biology approach revealed that epithelial cells expressing steA show altered cell morphology, reduction of cytotoxicity, cell-cell adhesion and migration capability, and increase in endocytosis. Three experiments. Each experiment replicated three times. Control: HeLa cell transfected with plasmid pBABE without steA, Experiment 1:HeLa line 2 transfected with plasmid pBABE with steA. Experiment 2: HeLa line 4 transfected with plasmid pBABE with steA

肠炎沙门氏菌（Salmonella enterica）是一类革兰氏阴性菌，可在包括人类在内的多种动物宿主中引发胃肠炎、菌血症与伤寒。其致病力主要依赖两套分别编码于致病岛1（pathogenicity islands 1, SPI1）与致病岛2（pathogenicity islands 2, SPI2）的Ⅲ型分泌系统（type III secretion systems, T3SSs）。上述系统可将名为效应蛋白的底物转运至真核宿主细胞内，效应蛋白会干扰宿主特定信号转导通路，以实现病原菌的内化以及其在宿主液泡内的存活与增殖。SteA是少数可同时作为两套T3SS底物的沙门氏菌效应蛋白之一，目前学界对其在宿主细胞内的功能尚一无所知。
本研究通过基因芯片（gene arrays）与生物信息学分析，探究了人类上皮细胞对SteA的遗传应答反应。我们发现，在上皮细胞中组成型表达SteA，会诱导与细胞外基质组织、细胞增殖调控及丝氨酸/苏氨酸激酶信号通路相关的基因；同时还会抑制与免疫过程、嘌呤核苷酸合成调控以及通路限制性SMAD蛋白磷酸化相关的基因。
与上述分析结果一致的是，细胞生物学实验显示，表达steA的上皮细胞会出现细胞形态改变、细胞毒性降低、细胞间黏附与迁移能力受损，以及胞吞作用增强的表型。本研究共设置3组独立实验，每组实验均重复3次。对照组：转染不含steA的pBABE质粒（plasmid pBABE）的海拉细胞（HeLa cell）；实验1：转染携带steA的pBABE质粒的HeLa细胞系2；实验2：转染携带steA的pBABE质粒的HeLa细胞系4。