Abstract: Abstract: Objective To study the effect of mesenchymal stem cell derived exosomes (MSC-Exo) on the proliferation of breast cancer cells. Methods The human breast cancer cell line MDA-MB-231 (231-Fluc-GFP) was constructed by lentiviral transfection using a double-reporter gene (fluorescein-green fluorescent protein, fluc-GFP) vector based on bioluminescence imaging technology, so that the cells were monitored in vitro in real time. Cells were treated with MSCs-Exo (MSCs-Exo group) and PBS (control group) respectively. The morphological observation, proliferation, survival rate and results of real-time PCR in vitro cells were performed in two groups of cells. In vivo, a nude mouse breast cancer model was established, and the tumor formation under different treatment conditions was analyzed. Results The proliferation of breast cancer MDA-MB-231 cells was inhibited after treatment with mesenchymal stem cell-derived exosomes. There were significant differences in cell numbers on the 3rd day of treatment between the two groups (P＜0.05). The survival rate of breast cancer cells decreased after treatment (P＜0.01). The expression of STAT3 and its downstream genes C-MYC， BCL- XL， NANOG， OCT4 and SOX2 decreased in exosome treatment group. In vivo, the growth of tumor cells treated with MSCs- Exo was inhibited. Conclusion Mesenchymal stem cell-derived exosomes inhibit breast cancer cell growth.

Key words: mesenchymal stromal cells, exosomes, breast neoplasms, luminescent measurements, luciferases