Phosphorylation of Nucleophosmin at Threonine 234/237 is associated with HCC metastasis

Abstract

Hepatocellular carcinoma (HCC) is frequently complicated by the occurrence of intrahepatic and extrahepatic metastases, leading to poor prognosis. To improve the prognosis for HCC patients, there is an urgent need to understand the molecular mechanisms of metastasis in HCC. Since protein Serine/Threonine phosphorylation emerges to be an important posttranslational modification critical in signaling process associated with cell proliferation, survival and metastasis, we employed a pair of primary tumor-derived and corresponding lung-metastatic counterparts (PLC/PRF/5-PT and PLC/PRF/5-LM) and aimed to identify these changes using CelluSpot™ Serine/Threonine kinase peptide array. Upon analysis, we found phosphorylated level of nucleophosmin (NPM) at Threonine 234/237 (p-NPM-Thr^234/237) had remarkably high level in metastatic HCC cells (PLC-LM) than the corresponding primary HCC cell line (PLC-PT). Similar observation was observed in another match primary and their metastatic counterparts (MHCC-97L and MHCC-97H). By immunohistochemical staining, p-NPM-Thr^234/237 was consistently found to be preferentially expressed in metastatic HCCs when compared with primary HCC in 28 HCC cases (p < 0.0001). By overexpressing Flag-tagged NPM and its phosphorylation site mutant (Thr234/237A) into low p-NPM-Thr^234/237 expressing cells (Hep3B and Huh7) using a lentiviral based approach, we demonstrated that p-NPM-Thr^234/237 is critical in invasion and migration of HCC cells, and this effect was mediated by cyclin-dependent kinase 1 (CDK1). Wild-type NPM was found to physically interact with a metastatic gene, ROCK2, and defective in Thr234/237 phosphorylation decreased its binding affinity, resulting in decrease in ROCK2 mediated signaling pathway. Identification of CDK1/p-NPM/ROCK2 signaling pathway provides a novel target for molecular therapy against HCC metastasis.