Schwan W R, Kopecko D J
Laboratory of Enteric and Sexually Transmitted Diseases, Food and Drug Administration-Center for Biologics Evaluation and Research, Bethesda, Maryland 20892, USA.
Infect Immun. 1997 Nov;65(11):4754-60. doi: 10.1128/iai.65.11.4754-4760.1997.
A differential PCR technique detected the transcriptional downregulation of the mss1 (mammalian suppressor of svg1) gene in murine J774A.1 macrophages following uptake of Salmonella typhimurium. This downregulation was also noted after entry of virulent strains of Listeria monocytogenes and Shigella flexneri, two other facultative intracellular bacterial species. In contrast, uptake of nonpathogenic Escherichia coli HB101, an aroA mutant of S. typhimurium, an invasion plasmid antigen B (ipaB) mutant of S. flexneri, hemolysin (hly) and positive-regulatory factor (prfA) mutants of L. monocytogenes, or latex beads produced mss1 expression levels similar to that of uninfected macrophages. Transcriptional downregulation of mss1 was also shown to occur in S. typhimurium-infected human U937 cells, albeit to an extent less than that in murine J774A.1 cells. In addition to a lower abundance of mss1 transcripts, we also demonstrate for the first time that less MSS1 protein was detected in intracellular-bacterium-infected cells (beginning about 1 h after entry of the pathogenic intracellular bacteria) than in noninfected cells. Some strains with specific mutations in characterized genes, such as an ipaB mutant strain of S. flexneri and an hly mutant strain of L. monocytogenes, did not elicit this lower level of expression of MSS1 protein. The decrease in MSS1 within infected macrophages resulted in an accumulation of ubiquitinated proteins, substrates for MSS1. Since MSS1 comprises the ATPase part of the 26S protease that degrades ubiquitinated proteins, we hypothesize that downregulation of the mss1 gene by intracellular bacterial entry may help subvert the host cell's normal defensive response to internalized bacteria, allowing the intracellular bacteria to survive.
一种差异PCR技术检测到，鼠伤寒沙门氏菌被小鼠J774A.1巨噬细胞摄取后，mss1（哺乳动物svg1抑制因子）基因的转录下调。在单核细胞增生李斯特菌和福氏志贺氏菌这两种其他兼性胞内细菌的有毒菌株进入细胞后，也观察到了这种下调现象。相比之下，摄取非致病性大肠杆菌HB101、鼠伤寒沙门氏菌的aroA突变体、福氏志贺氏菌的侵袭质粒抗原B（ipaB）突变体、单核细胞增生李斯特菌的溶血素（hly）和正调控因子（prfA）突变体，或乳胶珠后，mss1的表达水平与未感染的巨噬细胞相似。mss1的转录下调在鼠伤寒沙门氏菌感染的人U937细胞中也有发生，尽管程度低于小鼠J774A.1细胞。除了mss1转录本丰度较低外，我们还首次证明，与未感染细胞相比，在细胞内细菌感染的细胞中（在致病性细胞内细菌进入后约1小时开始）检测到的MSS1蛋白更少。一些在特定基因中有特定突变的菌株，如福氏志贺氏菌的ipaB突变菌株和单核细胞增生李斯特菌的hly突变菌株，不会引发这种较低水平的MSS1蛋白表达。感染的巨噬细胞内MSS1的减少导致泛素化蛋白（MSS1的底物）积累。由于MSS1构成了降解泛素化蛋白的26S蛋白酶的ATP酶部分，我们推测细胞内细菌进入导致mss1基因下调可能有助于颠覆宿主细胞对内化细菌的正常防御反应，使细胞内细菌得以存活。
