Regulatory role of nucleophosmin during the differentiation of human liver cancer cells.

Nucleophosmin (NPM, also known as B23), mainly localized in the nucleolus, has been reported to be overexpressed in many types of human cancer, including colon, ovarian, prostate and gastric cancer. NPM was identified while screening the differential nuclear matrix proteins during HMBA-induced differentiation of human liver cancer cells. We investigated the aberrant expression and subcellular localization of NPM in clinical liver cancer tissues and a cell line with the aim of providing more evidence for revealing the roles of NPM on regulating liver cancer cell proliferation and differentiation. In addition, we studied the potential interaction between NPM and several important proteins. Our results revealed that NPM protein was overexpressed in cancer cells, which was in accordance with the overexpressed mRNA in cancer tissues compared to the corresponding non-cancer tissues. We also found a decrease of NPM in protein and mRNA levels upon treatment with the differentiation reagent HMBA. We focused on the aberrant localization of NPM. Immunochemistry and immunofluorescence revealed aberrant cytoplasmic and nucleoplasm localization of NPM in liver cancer tissues and its colocalization with c-Myc, c-Fos, P53 and Rb in the SMMC-7721 cell line. The interactions between NPM and the above proteins were confirmed by GST pull-down assay and co-immunoprecipitation assay. These findings indicate that NPM plays a regulatory role in liver cancer, which deserves in-depth investigation.