Anthrax Toxin Entry into Polarized Epithelial Cells

We examined the entry of anthrax edema toxin (EdTx) into polarized human T84 epithelial cells using cyclic AMP-regulated Cl(−) secretion as an index of toxin entry. EdTx is a binary A/B toxin which self assembles at the cell surface from anthrax edema factor and protective antigen (PA). PA binds to cell surface receptors and delivers EF, an adenylate cyclase, to the cytosol. EdTx elicited a strong Cl(−) secretory response when it was applied to the basolateral surface of T84 cells but no response when it was applied to the apical surface. PA alone had no effect when it was applied to either surface. T84 cells exposed basolaterally bound at least 30-fold-more PA than did T84 cells exposed apically, indicating that the PA receptor is largely or completely restricted to the basolateral membrane of these cells. The PA receptor did not fractionate with detergent-insoluble caveola-like membranes as cholera toxin receptors do. These findings have implications regarding the nature of the PA receptor and confirm the view that EdTx and CT coopt fundamentally different subcellular systems to enter the cell and cause disease.

本研究以环腺苷酸（cyclic AMP）调控的氯离子（Cl⁻）分泌作为毒素入侵的检测指标，探究了炭疽水肿毒素（anthrax edema toxin，EdTx）入侵极化人T84上皮细胞的过程。该毒素属于二元A/B型毒素，可由炭疽水肿因子（anthrax edema factor，EF）与保护性抗原（protective antigen，PA）在细胞表面自发组装而成。PA可结合细胞表面受体，并将腺苷酸环化酶（adenylate cyclase）类的EF转运至胞质溶胶中。当将EdTx施加于T84细胞的基底外侧膜表面时，可引发强烈的Cl⁻分泌反应；而施加于顶膜表面时则无任何响应。仅单独施加PA时，无论作用于哪一侧膜表面均无任何效果。经基底外侧膜暴露的T84细胞结合的PA量至少是经顶膜暴露细胞的30倍，这表明PA受体大多或完全局限于该类细胞的基底外侧膜。PA受体并不会像霍乱毒素（cholera toxin，CT）受体那样，与去垢剂不溶性的窖蛋白样膜结构共分级分离。上述研究结果可为PA受体的本质提供相关启示，并证实了EdTx与CT利用完全不同的亚细胞系统入侵细胞并致病这一观点。