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Objective: Cancer stem cells (CSCs) are responsible for the drug resistance of breast cancers. Vitamin D deficiency promotes tumor resistance. The present study examined the effect of vitamin D and vitamin D receptor (VDR) expression on the tamoxifen resistance of CSCs. Methods: MCF-7 cells were treated with 1,25(OH) 2 D 3 and their levels of VDR expression, viability, and apoptosis were detected. CD133 + MCF-7 stem cells were identified and transfected with a VDR-overexpression plasmid. The tamoxifen concentration that reduced MCF-7 cell viability by 50% (IC 50 ) was determined. The activation of Wnt/β-catenin signaling was also investigated. Results: Vitamin D reduced the viability of MCF-7 cells and promoted their apoptosis. Vitamin D enhanced VDR expression and induced DNA damage. When CD133 + stem cells were separated from MCF-7 cells, the IC 50 of tamoxifen for stem cells was significantly higher than that of parental MCF-7 cells, suggesting a higher tamoxifen resistance in MCF-7 stem cells. Levels of VDR expression and Wnt/β-catenin signaling in CD133 + cells were markedly lower and higher than those in CD133 - cells, respectively. Stem cells transfected with VDR overexpression plasmids showed decreased tamoxifen IC 50 values, viability, spheroid formation, and expression of Wnt and β-catenin proteins when compared with control cells. Cell apoptosis was increased by transfection with a VDR overexpression plasmid. Finally, the inhibitory effects induced by VDR overexpression could be reversed by the VDR inhibitor, calcifediol. Conclusion: Stem cells contributed to the tamoxifen resistance of MCF-7 cells. Vitamin D-induced VDR expression increased the sensitivity of MCF-7 stem cells to tamoxifen by inhibiting Wnt/β-catenin signaling.

Vitamin D-induced vitamin D receptor expression induces tamoxifen sensitivity in MCF-7 stem cells via suppression of Wnt/β-catenin signaling