Trans-well experiment was carried out manually. In brief, 1 × 105 cells were plated onto 24-well cell chambers with a non-coated membrane (pore size: 8 μm, BD Biosciences). The cells were plated in medium without serum, and medium with 20% serum was used as a chemoattractant in the lower chamber. The cells were incubated at 37°C for 16 h, and cells that did not migrate through the pores were removed with a cotton swab. Cells on the lower surface of the membrane were fixed with 100% methanol for 5 min and stained with 0.1% crystal violet for 2 min (Sigma). Cells in 10 random fields of view at 100× magnification were counted and expressed as the average number of cells per field of view. Images were acquired using an inverted phase-contrast microscope at 100× magnification (Olympus, CKX41).