Figure 4. CD13⁺ cells resist chemotherapy, and inhibition of CD13 elicits cellular apoptosis.

(A) The HuH7 and PLC/PRF/5 cells were treated with 0.1 μg/ml of DXR or 1 μg/ml of 5-FU for 72 hours. The changes in cell-surface markers were compared with controls. The percentages of CD13⁺CD133⁺ in HuH7 and CD13⁺CD90^– populations in PLC/PRF/5 are shown in figure. (B) Effect of CD13 inhibition on cell proliferation. HuH7 cells were treated with various concentrations of anti-human mouse IgG₁ CD13-neutralizing antibody. As a negative control, 10 μg/ml of anti-human mouse IgG₁ antibody was used. (C) Inhibition of CD13 induces cell apoptosis. Cells were treated with 1–20 μg/ml of CD13-neutralizing antibody or 50–500 μg/ml of ubenimex for 24 hours. Data show each case of 5 μg/ml of CD13-neutralizing antibody and 100 μg/ml of ubenimex treatment. (D) The effect of CD13-neutralizing antibody on DXR-R HuH7. The DXR-R clone was established with continuous treatment in 1 μg/ml of DXR and a selection of viable colonies. In 0.5 μg/ml of DXR, most control HuH7 cells die after 72 hours, whereas over 90% of DXR-R cells survive. The DXR-R HuH7 cells were cultured with 1–20 μg/ml of CD13-neutralizing antibody for 72 hours. Control, treated with 10 μg/ml of anti-human mouse IgG₁ antibody.