Objective   This study was designed to investigate the effects of ubenimex, a CD13 inhibitor, (ubenimex alone and ubenimex with cisplatin), on A549 cell proliferation and apoptosis. This study also aimed to determine the possible underlying mechanisms.

  Methods   A549 cells were cultured in RPMI 1640 medium supplemented with 10% fetal bovine serum. The cultured cells were then treated with ubenimex at varying concentrations or a combination of ubenimex (100 µg·mL^–1) and cisplatin at varying concentrations. Methyl thiazolyl tetrazolium assay was used to determine the cell proliferation ratio. Flow cytometer was then used to analyze the cell apoptotic rates (AnnexinV-FITC/PI double-staining). The protein expressions of p53, Bcl-2, and Bcl2-xL, as well as the cleavage of PARP-1 and caspase were determined through Western blot. Spectrophotometrical assay was performed using alanine-p-nitroanilido as a substrate to detect CD13 activity. Hochest 33342 staining combined with flow cytometry analysis was performed to detect the side population (SP) cells in A549 cells.

  Results   Ubenimex alone did not significantly inhibit A549 cell proliferation (P>0.05). By contrast, ubenimex significantly increased the chemosensitivity of A549 cells to cisplatin at 8 μM or 16 μM in a time-dependent manner. After the A549 cells were treated with different concentrations of cisplatin (i.e., 8, 16, 32, and 64 μM) and ubenimex (100 µg·mL^–1) for 24, 48, and 72 h, the cell proliferation ratios were obtained. In particular, at a cisplatin concentration of 8 μM, the cell proliferation ratios of the cisplatin group against the combined treatment of cisplatin with ubenimex (100 µg·mL^–1) were 89.44±15.84% versus 62.18±5.35% (24 h), 64.81±5.86% versus 33.09±3.14% (48 h), and 62.06±7.6% versus 27.92±5.84% (72 h). At 16 μM of cisplatin, the cell proliferation ratios (cisplatin against the combined treatment) were 84.61±3.73% versus 54.50±4.22% (24 h), 22.09±5.74% versus 3.62±3.28% (48 h), and 19.22±1.57% versus 0.67±0.42% (72 h). Ubenimex did not significantly increase the chemosensitivity of A549 cells to cisplatin (64 μM). Cellular apoptotic rates were also significantly increased in the combined treatment group cisplatin (16 μM) and ubenimex compared with that in the cisplatin group alone. Ubenimex alone cannot change the protein expression of p53, Bcl-2, and Bcl2-xL, as well as the cleavage of PARP-1 and caspase. By contrast, ubenimex can significantly inhibit the CD13 activity of A549 cells and decrease the percentage of SP cells of A549 cells (P < 0.05).

  Conclusion   Ubenimex can significantly enhance the chemosensitivity of low-dose cisplatin on A549 cells in vitro. Therefore, ubenimex may be used as a sensitizer of cisplatin for human non-small cell lung cancer with CD13 expression.