Caspase inhibitors and overexpression of Bcl-2 protect against apoptosis induced by SK1-I. U937 cells were treated without or with 10 μM SK1-I (A) or 20 μM SK1-I (B) for the indicated times. Cell lysates were prepared and equal amounts of proteins (20 μg) analyzed by Western blotting with the indicated antibodies. Blots were stripped and reprobed with anti–Mcl-1 to ensure equivalent loading and transfer. (C) U937 cells were pretreated for 30 minutes without or with 20 μM Boc-D-FMK (BOC) or Z-VAD-FMK (ZVAD) and then treated with 20 μM SK1-I or 10 μM etoposide. After 4 hours, apoptosis was determined by flow cytometric analysis of annexin V/PI-stained cells. *P ≤ .01. (D) U937 cells stably expressing vector, Bcl-2, or Bcl-xL were treated with the indicated concentrations of SK1-I for 8 hours. Apoptosis was determined by flow cytometric analysis of annexin V/PI-stained cells. Insets show overexpression of Bcl-2 and Bcl-xL by immunoblotting.