Differential effects of bestatin on cell proliferation of ES-2 and HRA cells. A; Decrease of APN/CD13 enzyme activity by bestatin in a concentration-dependent manner in ES-2 cells, which expressed APN/CD13. More than 100 μg/ml of bestatin significantly inhibited APN/CD13 enzyme activity in ES-2 cells (p < 0.01). B; Cytotoxicity of bestatin was evaluated by trypan blue dye-exclusion test for APN/CD13-positive ES-2 cells and APN/CD13-negative HRA cells. C; Bestatin dose-dependently suppressed cell proliferation of ES-2 cells, which strongly expressed APN/CD13. More than 100 μg/ml of bestatin significantly inhibited the proliferation of ES-2 cells (p < 0.01). D; Bestatin exerted no significant effect on the proliferation of HRA cells, which expressed a low level of APN/CD13.