Cullen Sean P, Afonina Inna S, Donadini Roberta, Lüthi Alexander U, Medema Jan Paul, Bird Phillip I, Martin Seamus J
Molecular Cell Biology Laboratory, Department of Genetics, The Smurfit Institute, Trinity College, Dublin 2, Ireland.
J Biol Chem. 2009 Feb 20;284(8):5137-47. doi: 10.1074/jbc.M807913200. Epub 2008 Dec 22.
Natural killer (NK) cells kill virus-infected or transformed target cells by delivering cytotoxic proteases called granzymes to the target cell cytosol. One of these proteases, granzyme M, is specifically expressed in NK cells and is thought to instigate a form of cell death distinct from that mediated by granzyme A or granzyme B. However, the mechanism of granzyme M-induced cell death is unclear at present, and few substrates for this granzyme have been reported to date. Here we show that the abundant nucleolar phosphoprotein, nucleophosmin (NPM), is cleaved and inactivated by granzyme M. NPM is essential for cell viability as RNA interference-mediated ablation of NPM expression in human cells resulted in spontaneous apoptosis. Significantly, overexpression of wild-type NPM rescued cells treated with NPM small interference RNA, whereas overexpression of the granzyme M-cleaved form of NPM did not. Because NPM is essential for cell viability, these data suggest that targeting of NPM by granzyme M may contribute to tumor cell eradication by abolishing NPM function.
自然杀伤（NK）细胞通过将称为颗粒酶的细胞毒性蛋白酶递送至靶细胞胞质溶胶来杀死病毒感染或转化的靶细胞。这些蛋白酶之一，颗粒酶M，在NK细胞中特异性表达，并被认为可引发一种不同于颗粒酶A或颗粒酶B介导的细胞死亡形式。然而，目前颗粒酶M诱导的细胞死亡机制尚不清楚，迄今为止报道的该颗粒酶的底物很少。在这里，我们表明丰富的核仁磷蛋白，核仁素（NPM），被颗粒酶M切割并失活。NPM对细胞活力至关重要，因为RNA干扰介导的人细胞中NPM表达的缺失导致自发凋亡。值得注意的是，野生型NPM的过表达挽救了用NPM小干扰RNA处理的细胞，而颗粒酶M切割形式的NPM的过表达则没有。由于NPM对细胞活力至关重要，这些数据表明颗粒酶M对NPM的靶向作用可能通过消除NPM功能而有助于根除肿瘤细胞。
