Tumor-initiating function of nucleostemin-enriched mammary tumor cells

Nucleostemin (NS) is highly expressed in normal stem cells and tumors and is upregulated by estradiol in MCF7 breast cancer cells. To investigate the role of NS in mammary tumorigenesis, we established first that NS is expressed at higher levels in the basal cell type than in the luminal cell type in mouse mammary tumors and human breast cancer cells. NS expression was also increased during progression of mammary tumors in MMTV-Wnt1 and MMTV-PyMT transgenic mice and by the tumor sphere culture. To determine the function of NS-enriched tumor cells, we generated a bacterial artificial chromosome transgenic mouse line expressing green fluorescent protein (GFP) from the NS promoter and bred it to MMTV-Wnt1 mice, so that NS-expressing cells can be prospectively isolated based on their GFP levels. Notably, NS-enriched mammary tumor cells exhibited stronger in vitro and in vivo tumorigenic activities and expressed higher levels of K5, CD133, Oct4, telomerase reverse transcriptase, and C-X-C chemokine ligand 12 compared with NS-deficient mammary tumor cells. Furthermore, knockdown of NS dramatically reduced the sphere-forming activity of MDA-MB-231 and MCF7 human breast cancer cells. Our findings establish the tumor-initiating and molecular features of NS-enriched mammary tumor cells, suggesting that NS may offer a valuable therapeutic target.