Effect of the caspase inhibitors zVAD, DEVD, and YVAD in preventing RA-induced PML/RARα degradation. (A) Cells were treated with 1 μmol/L RA for 96 hours. U937/PR9 cells were induced to express PML/RARα by treatment with 100 μmol/L Zn for 16 hours. zVAD (100 μmol/L) was added for 1 hour before RA treatment. (B) Zn-induced U937/PR9 cells were treated with the indicated concentrations of DEVD or YVAD. Immunoblot analysis was performed on total cellular proteins (50 μg) using the anti-RARα RPα(F) antibody. Immunoreactivity to β-tubulin was used for loading control.