Effect of p110β inactivation on EC apoptosis. Active caspase-3 in (A) mouse or (B) human ECs was determined as in “Methods” with or without tumor necrosis factor (TNF)/cycloheximide (CHX) challenge. Where indicated, ECs were pretreated with TGX-221 (100 nM) for 60 minutes or treated with siRNA (Siβ) as in supplemental Figure 3 to decrease p110β expression (mean ± SEM; *P < .05 vs the respective unchallenged group; n ≥ 3 independent experiments). NS, nonspecific siRNA.