Nucleophosmin/B23 Is a Target of CDK2/Cyclin E in Centrosome Duplication

Abstract

In animal cells, duplication of centrosomes and DNA is coordinated. Since CDK2/cyclin E triggers initiation of both events, activation of CDK2/cyclin E is thought to link these two events. We identified nucleophosmin (NPM/B23) as a substrate of CDK2/cyclin E in centrosome duplication. NPM/B23 associates specifically with unduplicated centrosomes, and NPM/B23 dissociates from centrosomes by CDK2/cyclin E-mediated phosphorylation. An anti-NPM/B23 antibody, which blocks this phosphorylation, suppresses the initiation of centrosome duplication in vivo. Moreover, expression of a nonphosphorylatable mutant NPM/ B23 in cells effectively blocks centrosome duplication. Thus, NPM/B23 is a target of CDK2/cyclin E in the initiation of centrosome duplication.

Increased expression of nucleophosmin/B23 in hepatocellular carcinoma and correlation with clinicopathological parameters

“…Previous reports have shown that NPM is involved in the assembly and/or intranuclear transport of pre-ribosomal particles and in cytoplasmic/nuclear trafficking (Spector et al, 1984;Yung et al, 1985;Borer et al, 1989). In mitotic cells, most nuclear proteins were dispersed throughout the cytoplasm, including NPM, which then re-localised to the centrosomes, and later to each daughter cell that received one centrosome bound by NPM after mitosis (Okuda et al, 2000). Interestingly, we also found that some of the non-mitotic cells, such as adult cerebral neuronal cells, epithelial cells of the adult kidney proximal tubule, and embryo osteoblast and thymic cells contain cytoplasmic NPM.…”

“…Earlier studies have shown that the level of NPM is markedly and promptly increased in association with cell commitment to mitogenesis (Feuerstein and Mond, 1987;Feuerstein et al, 1988). It has also been shown that NPM is a direct target of the Myc transcription factor during ribosomal biogenesis (Zeller et al, 2001) and cyclin E/CDK2 kinase activity during centrosome duplication (Okuda et al, 2000). Nucleophosmin physically interacts with several other cellular factors such as the retinoblastoma susceptibility gene product pRB (Takemura et al, 1994(Takemura et al, , 1999, the cell cycle G2 -M checkpoint factor Gadd45a (Gao et al, 2005), and the tumour suppressor p14 ARF (Itahana et al, 2003).…”

“…Overexpression and tumorigenic activation of the Smoothened (SMO) protooncogene mediates c-myc overexpression, suggesting that SMO may also be a prognostic factor in HCC tumorigenesis (Sicklick et al, 2006). Nucleophosmin (NPM) is a major nucleolar phosphoprotein implicated in multiple cellular functions, including ribosomal protein assembly and transport (Verheggen et al, 2000;Huang et al, 2005), centrosome duplication (Okuda et al, 2000;Okuda, 2002;Grisendi et al, 2005), molecular chaperone activity in preventing protein aggregation (Hingorani et al, 2000;Szebeni et al, 2003), and regulating the activity of the tumour suppressors p53 (Colombo et al, 2002;Li et al, 2004;Maiguel et al, 2004) and p14 ARF (Itahana et al, 2003;Bertwistle et al, 2004;Brady et al, 2004). Earlier studies have shown that the level of NPM is markedly and promptly increased in association with cell commitment to mitogenesis (Feuerstein and Mond, 1987;Feuerstein et al, 1988).…”

Proteome analysis of proliferative response of bystander cells adjacent to cells exposed to ionizing radiation

“…This protein is mainly localized in granular regions of the nucleolus and is associated with ribosome biogenesis. It also plays a substantial role in cell-cycle progression and cell division [26][27][28]. NPM/B23 binds to retinoblastoma protein (pRb) and synergistically stimulates DNA polymerase-α [26].…”

“…Down-regulation of NPM/B23 mRNA delays the entry of cells into mitosis [27]. Unduplicated centrosomes have been found to bear NPM/B23, phosphorylation of which by CDK2-cyclin E appears to be required for the initiation of centrosome duplication during mitosis [28,29]. The fact that NPM/B23 shuttles between nucleoli and cytoplasm suggests that its function is not restricted within the nucleolus [30].…”