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.

A cDNA encoding human nucleophosmin (protein B23) was obtained by screening a human placental cDNA library in lambda gtll first with monoclonal antibody to rat nucleophosmin and then with confirmed partial cDNA of human nucleophosmin as probes. The cDNA had 1311 bp with a coding sequence encoding a protein of 294 amino acids. The identity of the cDNA was confirmed by the presence of encoded amino acid sequences identical with those determined by sequencing pure rat nucleophosmin (a total of 138 amino acids). The most striking feature of the sequence is an acidic cluster located in the middle of the molecule. The cluster consists of 26 Asp/Glu and 1 Phe and Ala. Comparison of human nucleophosmin and Xenopus nucleolar protein NO38 shows 64.3% sequence identity. The N-terminal 130 amino acids of human nucleophosmin also bear 50% identity with that of Xenopus nucleoplasmin. Northern blot analysis of rat liver total RNA with a partial nucleophosmin cDNA as probe demonstrated a homogeneous mRNA band of about 1.6 kb. Similar observations were made in hypertrophic rat liver and Novikoff hepatoma. However, the quantity of nucleophosmin mRNA is 50- and 5-fold higher in Novikoff hepatoma and hypertrophic rat liver, respectively, when compared with normal rat liver. Dot blot analysis also showed a nucleophosmin mRNA ratio of 64:5:1 in the three types of rat liver. When the protein levels were compared with Western blot immunoassays, Novikoff hepatoma showed 20 times more nucleophosmin, while only about 5 times more nucleophosmin was observed in hypertrophic rat liver than in unstimulated normal liver.

HBV reactivation is a common cause of liver damage during CT in HBsAg-positive HCC patients. The only identifiable associated risk factor was elevated pre-treatment ALT. Further studies into the role of antiviral and novel anticancer therapies are required to improve the prognosis of these patients.

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