Barbieri J T
Department of Microbiology and Molecular Genetics, Medical College of Wisconsin, Milwaukee 53226, USA.
Int J Med Microbiol. 2000 Oct;290(4-5):381-7. doi: 10.1016/S1438-4221(00)80047-8.
Our recent studies have shown ExoS to be a bifunctional type-III secreted cytotoxin. Intracellular expression of the amino terminus of ExoS (C234) in eukaryotic cells stimulates actin reorganization without cytotoxicity, which involves small-molecular-weight GTPases of the Rho subfamily. Expression of the carboxyl terminus of ExoS comprises an ADP-ribosyltransferase domain, which is cytotoxic when expressed in cultured cells (Pederson and Barbieri, 1998). Rho and Ras are molecular switches, which control numerous cellular processes. Recent signaling studies suggest that there is crosstalk between Rho and Ras (Keely et al, 1997). Ras and Rho also contribute to wound healing processes and tissue regeneration. Recent studies have shown that microinjection of endothelial cells with activated Ras stimulated their motility, while microinjection of Ras-blocking antibodies inhibited cellular motility that is a component of the wound healing process (Fox et al., 1994). In addition, hepatocyte growth factor/scatter factor (HGF/ SF) and epidermal growth factor stimulate cellular motility through the Ras signal transduction pathway (Ridley et al., 1995). Rac and Rho are also involved in motility and tissue regeneration, since dominant negative Rac inhibits the cellular motility stimulated by HGF/SF (Santos et al., 1997) and inhibition of Rho by either C. difficile ToxA and ToxB or the C. botulinum C3 transferase inhibits wound healing (Santos et al., 1997). Inhibition of tissue regeneration and wound healing appear to play a role in the pathogenesis of C. difficile, since treatment of gastrointestinal mucosa with C. difficile ToxA and ToxB alone inhibits regeneration of the gastric mucosa. Thus, ExoS may contribute to the establishment of P. aeruginosa infections by inhibiting wound healing and tissue regeneration by two mechanisms. The amino terminus of ExoS could inhibit Rho function and wound healing in a manner similar to C. difficile. Alternatively, ExoS could inhibit the cellular motility and angiogenesis required for wound healing by ADP-ribosylating Ras. Through the inhibition of tissue regeneration and wound healing, ExoS may play a pivotal role in chronic disease by maintaining sites of colonization. Inhibition of Ras or Rho signaling may also interfere with both innate and acquired immunity. Small-molecular-weight GTP-binding proteins of the Ras superfamily are required for cellular processes, such as phagocytosis, as Rho proteins contribute to phagocytosis (Caron and Hall, 1998). Since Ras functions upstream of Rho in cellular signaling processes (Ridley et al., 1995), ADP-ribosylation of Ras by ExoS or the inhibition of Rho function by C234 may inhibit phagocytosis of P. aeruginosa by macrophages. Other studies indicate that Ras plays a role in T cell activation (Cantrell, 1994). Thus, ExoS may inhibit acquired immunity by inhibiting T-cell activation.
我们最近的研究表明，ExoS是一种双功能III型分泌细胞毒素。ExoS氨基末端（C234）在真核细胞中的胞内表达可刺激肌动蛋白重组且无细胞毒性，这涉及Rho亚家族的小分子量GTP酶。ExoS羧基末端的表达包含一个ADP核糖基转移酶结构域，当在培养细胞中表达时具有细胞毒性（佩德森和巴比里，1998年）。Rho和Ras是分子开关，控制着众多细胞过程。最近的信号转导研究表明，Rho和Ras之间存在相互作用（基利等人，1997年）。Ras和Rho也有助于伤口愈合过程和组织再生。最近的研究表明，向内皮细胞显微注射活化的Ras可刺激其运动性，而显微注射Ras阻断抗体则抑制了作为伤口愈合过程一部分的细胞运动性（福克斯等人，1994年）。此外，肝细胞生长因子/分散因子（HGF/SF）和表皮生长因子通过Ras信号转导途径刺激细胞运动性（里德利等人，1995年）。Rac和Rho也参与运动性和组织再生，因为显性负性Rac抑制HGF/SF刺激的细胞运动性（桑托斯等人，1997年），并且艰难梭菌ToxA和ToxB或肉毒杆菌C3转移酶对Rho的抑制作用会抑制伤口愈合（桑托斯等人，1997年）。组织再生和伤口愈合的抑制似乎在艰难梭菌的发病机制中起作用，因为仅用艰难梭菌ToxA和ToxB处理胃肠道黏膜会抑制胃黏膜的再生。因此，ExoS可能通过两种机制抑制伤口愈合和组织再生，从而有助于铜绿假单胞菌感染的建立。ExoS的氨基末端可能以类似于艰难梭菌的方式抑制Rho功能和伤口愈合。或者，ExoS可能通过对Ras进行ADP核糖基化来抑制伤口愈合所需的细胞运动性和血管生成。通过抑制组织再生和伤口愈合，ExoS可能通过维持定植部位在慢性疾病中起关键作用。对Ras或Rho信号转导的抑制也可能干扰先天性和获得性免疫。Ras超家族的小分子量GTP结合蛋白是细胞过程所必需的，例如吞噬作用，因为Rho蛋白有助于吞噬作用（卡龙和霍尔，1998年）。由于Ras在细胞信号转导过程中位于Rho的上游（里德利等人，1995年），ExoS对Ras的ADP核糖基化或C234对Rho功能的抑制可能会抑制巨噬细胞对铜绿假单胞菌的吞噬作用。其他研究表明，Ras在T细胞活化中起作用（坎特雷尔，1994年）。因此，ExoS可能通过抑制T细胞活化来抑制获得性免疫。
