Objective      To determine the effect of testosterone on the expression of flap endonuclease 1 (FEN1) in breast cancer.       Methods      The expression of FEN1 in MCF-7 cells was observed by RT-PCR after the cells were treated with 17β-estradiol (E2)

E2 combined with ICI182

780

an estrogen receptor antagonist

and E2 combined with U0126

a MEK inhibitor

respectively. In MCF-7 cells over-expressing aromatase (MCF-7/Aro)

the expression of FEN1 was observed by RT-PCR when cells were treated with testerone or testerone combined with letrozole

one of aromatase inhibitors. The expression of FEN1 protein

p-ERK and p-Elk was observed by Western blotting when the cells were treated with testosterone

testosterone combined with letrozole

or combined with U0126.       Results      In MCF-7 cells

E2 resulted in an increase in the expression of FEN1 mRNA by 2.04 folds (P0.01)

which was inhibited by ICI182

780 or U0126 （10.63 and 2.17 folds

P0.01）. In MCF-7aro cells

testosterone resulted in an increase in the expression of FEN1 at mRNA and protein levels by 1.66 and 1.80 folds respectively（P0.01）

which was inhibited by letrozole (2.38 and 1.84 folds

P0.01) or U0126（2.28 folds for protein only

P0.01）. Testosterone increased the phosphorylation of p-ERK and p-Elk by 2.28 and 2.60 folds

which was inhibited by letrozole by 10.38 folds or U0126 by 119.50 folds（P0.01）.       Conclusion      Testosterone up-regulates FEN1 in MCF-7aro cells through MAPK signaling pathway.