L-γ-Glutamyl-p-nitroanilide (GPNA) is widely used to inhibit the glutamine (Gln) transporter ASCT2, but recent studies have demonstrated that it is also able to inhibit other sodium-dependent and independent amino acid transporters. Moreover, GPNA is a well known substrate of the enzyme γ-glutamyltransferase (GGT). Our aim was to evaluate the effect of GGT-mediated GPNA catabolism on cell viability and Gln transport. The GGT-catalyzed hydrolysis of GPNA produced cytotoxic effects in lung cancer A549 cells, resulting from the release of metabolite p-nitroaniline (PNA) rather than from the inhibition of Gln uptake. Interestingly, compounds like valproic acid, verapamil and reversan were able to increase the cytotoxicity of GPNA and PNA, suggesting a key role of intracellular detoxification mechanisms. Our data indicate that the mechanism of action of GPNA is more complex than believed, and further confirm the poor specificity of GPNA as an inhibitor of Gln transport. Different factors may modulate the final effects of GPNA, ranging from GGT and ASCT2 expression to intracellular defenses against xenobiotics. Thus, other strategies - such as a genetic suppression of ASCT2 or the identification of new specific inhibitors - should be preferred when inhibition of ASCT2 function is required.

L-γ-谷氨酰对硝基苯胺（GPNA）被广泛用于抑制谷氨酰胺（Gln）转运蛋白ASCT2，但最近的研究表明，它也能够抑制其他钠依赖性和非依赖性氨基酸转运蛋白。此外，GPNA是酶γ-谷氨酰转移酶（GGT）的众所周知的底物。我们的目的是评估GGT介导的GPNA分解代谢对细胞活力和Gln转运的影响。GGT催化的GPNA水解在肺癌A549细胞中产生细胞毒性作用，这是由于代谢产物对硝基苯胺（PNA）的释放而不是抑制Gln的摄取所致。有趣的是，诸如丙戊酸，维拉帕米和白藜芦醇之类的化合物能够增加GPNA和PNA的细胞毒性，提示细胞内排毒机制的关键作用。我们的数据表明，GPNA的作用机理比人们认为的更为复杂，并且进一步证实了GPNA作为Gln转运抑制剂的特异性很差。不同的因素可能调节GPNA的最终作用，范围从GGT和ASCT2表达到细胞内对异源生物的防御作用。因此，当需要抑制ASCT2功能时，应首选其他策略-例如ASCT2的遗传抑制或鉴定新的特异性抑制剂。