Chan Narisa, Lim Tit Meng
Department of Biological Sciences, National University of Singapore, 14 Science Drive 4, Singapore 117543.
Sci Rep. 2015 Jun 30;5:11777. doi: 10.1038/srep11777.
The cytoplasmic mutant of nucleophosmin (NPMc) is found approximately in one-third of acute myeloid leukemia (AML) cases and is highly associated with normal karyotype. Whereas previous studies have focused on wtNPM in centrosome duplication, we further elucidate the role of NPM in the cell cycle by utilizing the increased cytoplasmic load of NPMc. Overexpression of NPMc causes increased phosphorylation of NPM on T199 and, to a lesser degree, S4. T199 phosphorylation is dependent on cdk2 but activators of cdk2 were not elevated. Upon inhibition of cdk2, NPMc-overexpressing cells demonstrate a greater G2/M phase arrest than wtNPM or GFP counterparts. However, the number of cells with 2 centrosomes did not increase concordantly. This suggests that the arrest was caused by a delay in centrosome duplication, most likely due to the inhibition of centrosome duplication caused by unphosphorylated NPMc. Overall, these results suggest that the phosphorylation of T199 is important in the mitotic progression of NPMc-expressing cells. This further supports the hypothesis that NPMc is associated with normal karyotypes in AML because the higher cytoplasmic load of NPM can better suppress centrosome overduplication which would otherwise result in unequal segregation of chromosomes during mitosis, leading to aneuploidy and other genomic instabilities.
核磷蛋白的细胞质突变体（NPMc）约在三分之一的急性髓系白血病（AML）病例中被发现，且与正常核型高度相关。尽管先前的研究集中在野生型NPM在中心体复制中的作用，但我们通过利用NPMc增加的细胞质负荷进一步阐明了NPM在细胞周期中的作用。NPMc的过表达导致NPM在T199位点的磷酸化增加，在较小程度上也导致S4位点的磷酸化增加。T199位点的磷酸化依赖于cdk2，但cdk2的激活剂并未升高。在抑制cdk2后，过表达NPMc的细胞比野生型NPM或绿色荧光蛋白对照细胞表现出更大程度的G2/M期阻滞。然而，具有两个中心体的细胞数量并没有相应增加。这表明这种阻滞是由中心体复制延迟引起的，很可能是由于未磷酸化的NPMc对中心体复制的抑制作用。总体而言，这些结果表明T199位点的磷酸化在表达NPMc的细胞的有丝分裂进程中很重要。这进一步支持了NPMc与AML中的正常核型相关的假说，因为较高的NPM细胞质负荷可以更好地抑制中心体过度复制，否则这会导致有丝分裂期间染色体的不等分离，从而导致非整倍体和其他基因组不稳定。
