Yan Z, Yang D C, Jett M
Chemistry Department, Georgetown University, Washington, DC 20056, USA.
Mol Cell Biol Res Commun. 1999 Aug;2(2):124-30. doi: 10.1006/mcbr.1999.0162.
Cholera toxin covalently ADP-ribosylates the a subunit of Gs proteins. The modified Gsalpha activates adenylate cyclase and leads to a dramatic increase in intracellular cAMP. The effect of cholera toxin on the production of tumor necrosis factor (TNF-alpha), a critical mediator of toxicity for a number of bacterial and viral infections, has not been examined. Here we show that cholera toxin stimulated human monocytes to secrete TNF-alpha. The subunit A of cholera toxin alone also induced TNF-alpha production, suggesting that TNF-alpha production is mediated through ADP-ribosylation activity of the toxin. Inhibitors of ADP-ribosylation such as 3-aminobenzamide and niacinamide blocked TNF-alpha induction. However, cyclic AMP analogs and adenylate cyclase activator forskolin did not induce TNF-alpha production in monocytes, suggesting that TNF-alpha induction is independent of cAMP. Furthermore, cholera toxin-induced TNF-alpha production was suppressed by protein kinase C inhibitors H7 and sphingosine and by phospholipase C inhibitors U73122 and ET-18-OCH3, suggesting that PLC and PKC mediate TNF-alpha induction. Cholera toxin-mediated induction of TNF-alpha occurs at the transcription level as demonstrated by the time-dependent expression of TNF-alpha mRNA. These results raise the possibility that TNF-alpha may play an important role in cholera toxin-mediated toxicity and demonstrate that cholera toxin activates TNF-alpha production through PLC-dependent and cAMP-independent pathways. The probable mechanisms of signal transduction from cholera toxin to PLC in monocytes will be discussed.
霍乱毒素可使Gs蛋白的α亚基发生共价ADP核糖基化。修饰后的Gsα激活腺苷酸环化酶，导致细胞内cAMP急剧增加。霍乱毒素对肿瘤坏死因子（TNF-α）产生的影响尚未得到研究，而TNF-α是多种细菌和病毒感染毒性的关键介质。在此我们表明，霍乱毒素刺激人单核细胞分泌TNF-α。单独的霍乱毒素A亚基也可诱导TNF-α的产生，这表明TNF-α的产生是通过毒素的ADP核糖基化活性介导的。ADP核糖基化抑制剂如3-氨基苯甲酰胺和烟酰胺可阻断TNF-α的诱导。然而，环AMP类似物和腺苷酸环化酶激活剂福斯可林并未在单核细胞中诱导TNF-α的产生，这表明TNF-α的诱导与cAMP无关。此外，霍乱毒素诱导的TNF-α产生受到蛋白激酶C抑制剂H7和鞘氨醇以及磷脂酶C抑制剂U73122和ET-18-OCH3的抑制，这表明PLC和PKC介导TNF-α的诱导。如TNF-α mRNA的时间依赖性表达所示，霍乱毒素介导的TNF-α诱导发生在转录水平。这些结果增加了TNF-α可能在霍乱毒素介导的毒性中起重要作用的可能性，并证明霍乱毒素通过PLC依赖性和cAMP非依赖性途径激活TNF-α的产生。将讨论霍乱毒素在单核细胞中向PLC信号转导的可能机制。
