Abstract: Objective To explore the effect of Vitamin D on the activity of breast cancer stem cells and its possible mechanism. Methods The effects of different concentrations（1 μmol/L， 5 μmol/L， 10 μmol/L and 20 μmol/L） of Vitamin D on the viability of breast cancer SUM159 cells were detected by MTT assay. The breast cancer stem cells CSCSUM159 were enriched in suspension culture in serum-free medium to analyze the effect of Vitamin D on the activity of breast cancer stem cells.　Western blot and Real-time qPCR were used to detect the expression of the breast cancer stem cell markers CD133， CD44， Oct-4 and Nanog，and Wnt/β-catenin signaling pathway-related genes p-GSK-3β， β-catenin， c-Myc and cyclin-D1. Results MTT assay showed that 1 μmol/L， 5 μmol/L， 10 μmol/L and 20 μmol/L Vitamin D inhibited the viability of breast cancer SUM159 cells significantly（P＜0.01）. Serum-free medium culture could effectively enrich breast cancer stem cells CSCSUM159. Vitamin D reduced the number and size of SUM159 tumorsphere. Western blot and Real-time qPCR showed that Vitamin D down-regulated the protein expression of CD133， CD44 and Nanog and inhibited the protein expression levels of p-GSK-3β， β-catenin and c-Myc，and down-regulated the mRNA expression of β-catenin，c-Myc and cyclin-D1 in breast cancer stem cell CSCSUM159（P＜0.05）. Conclusion Vitamin D can inhibit the activity of breast cancer stem cells， which may be related to its regulation of Wnt/β-catenin signaling pathway.

Key words: Breast cancer, Vitamin D, Cancer stem cells, Wnt/β?鄄catenin signaling pathway