ET-1 pretreatment augments the chemotactic activity of SDF-1 α in 6-10B and 5-8F nasopharyngeal carcinoma cells. A-B. The migration of 6-10B cells in response to SDF-1α was measured using chemotaxis chambers. Initially, 6-10B cells were serum-starved and then stimulated with increasing concentrations (0, 0.1, 1, 10, and 50 nM) of ET-1 for 12 hours, along with SDF-1α (100 nM), in the lower chamber. ET-1 stimulation together with SDF-1α in the lower chamber had an additive effect and produced a level of migration that was significantly higher than the level induced by SDF-1α alone. C-D. After transfection with siETAR and siCXCR4, ET-1 and/or SDF-1α no longer augmented chemotaxis in 5-8F cells. All panels show the mean ± SE of the migration index from at least three separate experiments, which were each performed in triplicate (Student’s unpaired t test; **P < 0.001 compared to SDF-1α alone).