HEK293 cells were grown in Dulbecco’s modified Eagle’s medium (DMEM) supplemented with 10% fetal bovine serum (FBS), 50 U/ml penicillin and 50 μg/ml streptomycin, and were transfected using polyethylenimine as previously described . Briefly, cells were cultured in six-well plates to 80% confluence and incubated for 6 h with precipitates formed by 2 μg of plasmid DNA and 2 μl of 0.5% (wt/vol, pH 7.0) polyethylenimine (Sigma-Aldrich, catalog no. 40 872-7). After replacing with fresh medium, cells were cultured in DMEM containing 10% FBS for 24 h before harvesting.