Qin B, McClarty G
Department of Medical Microbiology, University of Manitoba, Winnipeg, Canada.
J Bacteriol. 1992 May;174(9):2865-73. doi: 10.1128/jb.174.9.2865-2873.1992.
Chlamydiae have evolved a biphasic life cycle to facilitate their survival in two discontinuous habitats. The unique growth cycle is represented by two alternating forms of the organism, the elementary body and the reticulate body. Chlamydiae have an absolute nutritional dependency on the host cell to provide ribonucleoside triphosphates and other essential intermediates of metabolism. This report describes the pleiotropic effects of the purine antimetabolite 6-thioguanine on chlamydial replication. In order to display cytotoxicity, 6-thioguanine must first be converted to the nucleotide level by the host cell enzyme hypoxanthine-guanine phosphoribosyltransferase. Our results show that 6-thioguanine is an effective inhibitor of chlamydial growth with either wild-type or hypoxanthine-guanine phosphoribosyltransferase-deficient cell lines as the host. Interestingly, the mechanism of 6-thioguanine-induced inhibition of chlamydial growth is different depending on which cell line is used. With wild-type cells as the host, the cytotoxic effects of 6-thioguanine on chlamydial growth are relatively fast and irreversible. Under these circumstances, cytotoxicity likely results from the combined effect of starving chlamydiae for purine ribonucleotides and incorporation of host-derived 6-thioguanine-containing nucleotides into chlamydial nucleic acids. With hypoxanthine-guanine phosphoribosyltransferase-deficient cells as the host, 6-thioguanine must be present at the start of the chlamydial infection cycle to be effective and the growth inhibition is reversible upon removal of the antimetabolite. These findings suggest that in hypoxanthine-guanine phosphoribosyltransferase-deficient cells, the free base 6-thioguanine may inhibit the differentiation of elementary bodies to reticulate bodies. With hypoxanthine-guanine phosphoribosyltransferase-deficient cells as the host, 6-thioguanine was used as a selective agent in culture to isolate a Chlamydia trachomatis isolate resistant to the effects of the drug. This drug resistant C. trachomatis isolate was completely resistant to 6-thioguanine in hypoxanthine-guanine phosphoribosyltransferase-deficient cells; however, it displayed wildtype sensitivity to 6-thioguanine when cultured in wild-type host cells.
衣原体进化出了双相生命周期，以利于其在两个不连续的栖息地中生存。独特的生长周期由该生物体的两种交替形式代表，即原体和网状体。衣原体在营养方面绝对依赖宿主细胞来提供三磷酸核糖核苷和其他必需的代谢中间产物。本报告描述了嘌呤抗代谢物6-硫鸟嘌呤对衣原体复制的多效性作用。为了表现出细胞毒性，6-硫鸟嘌呤必须首先被宿主细胞酶次黄嘌呤-鸟嘌呤磷酸核糖转移酶转化为核苷酸水平。我们的结果表明，以野生型或次黄嘌呤-鸟嘌呤磷酸核糖转移酶缺陷型细胞系作为宿主时，6-硫鸟嘌呤是衣原体生长的有效抑制剂。有趣的是，6-硫鸟嘌呤诱导的衣原体生长抑制机制因所使用的细胞系而异。以野生型细胞作为宿主时，6-硫鸟嘌呤对衣原体生长的细胞毒性作用相对较快且不可逆。在这种情况下，细胞毒性可能是由于衣原体缺乏嘌呤核糖核苷酸以及宿主来源的含6-硫鸟嘌呤的核苷酸掺入衣原体核酸的综合作用所致。以次黄嘌呤-鸟嘌呤磷酸核糖转移酶缺陷型细胞作为宿主时，6-硫鸟嘌呤必须在衣原体感染周期开始时存在才有效，并且在去除抗代谢物后生长抑制是可逆的。这些发现表明，在次黄嘌呤-鸟嘌呤磷酸核糖转移酶缺陷型细胞中，游离碱6-硫鸟嘌呤可能会抑制原体向网状体的分化。以次黄嘌呤-鸟嘌呤磷酸核糖转移酶缺陷型细胞作为宿主时，6-硫鸟嘌呤在培养中用作选择剂，以分离出对该药物作用具有抗性的沙眼衣原体分离株。这种耐药物的沙眼衣原体分离株在次黄嘌呤-鸟嘌呤磷酸核糖转移酶缺陷型细胞中对6-硫鸟嘌呤完全耐药；然而，当在野生型宿主细胞中培养时，它对6-硫鸟嘌呤表现出野生型敏感性。
