IL-21–induced apoptosis is caspase dependent and is associated with changes in expression of Bcl-2 family members. (A) RC-K8 and OCI-LY-3 cells were treated with IL-21 (100 ng/mL) for 24 hours, lysed, and assayed for caspase activation as described in “Proliferation and apoptosis studies.” Relative luciferase units (RLUs) readings were normalized to total protein content. Results are shown as the means ± SD and are representative of 3 independent experiments. (B) RC-K-8 and MC116 cells were pretreated with either dimethyl sulfoxide (DMSO) or the pan-caspase inhibitor Z-VAD-FMK, the selective caspase-9 inhibitor Z-LEHD-FMK, or caspase-8 inhibitor Z-IETD-FMK at 50 μM for 30 minutes and then treated with IL-21 (100 ng/mL). Cell viability was assayed after 48 hours by YO-PRO/PI staining. (C) Cells were treated with IL-21 (100 ng/mL) for the specified time period and protein expression was assayed by immunoblotting with specific antibodies. Immunoblotting for GAPDH served as a loading control. The results in panels B-C are representative of 3 independent experiments. (D) Densitometry analysis of Western blots from 3 independent experiments. The values in specimens at time point 0 were arbitrarily defined as 1. Error bars represent SE. *A statistically significant difference (P < .05) between experimental conditions marked by arrowheads.