U2-OS and U2-OS/MTX 300 cells were treated with quercetin and methotrexate. Cell viability was evaluated by MTT and colony formation assay. Cell-cycle status

apoptosis inducing effect

and the expression of apoptosis-related proteins were evaluated by flow cytometry

fluorescent staining

DNA fragmentation assay

and Western blotting

respectively. 【Results】 Quercetin inhibited cell growth in U2-OS/MTX300. The 72 h IC50 value of quercetin for U2-OS/MTX300 was (22 ± 4)μmol/L. 10

25

and 50 μmol/L quercetin inhibited the colony formation obviously. There was no cross resistance between methotrexate and quercetin. Induction of S phase arrest and apoptosis were confirmed by flow cytometry and fluorescent staining in U2-OS/MTX300. Quercetin induced Bak

Bax up-regulation

Bcl-2 down-regulation

cytosolic cytochrome C accumulation and cleavage of PARP in U2-OS/MTX300 cells.【Conclusion】Quercetin inhibits the growth and induces apoptosis in MTX resistant human osteosarcoma cell line U2-OS/MTX300. The mitochondrial apoptosis pathway probably plays an important role in this process.