Sphingomyelinase stimulates p38 MAPK-dependent CREB phosphorylation. U937 cells were treated with bacterial sphingomyelinase (100 mU/mL) for the indicated times, lysed in sample buffer, and immunoblot analysis performed to detect phospho-CREB (top) and reprobed to detect CREB (middle). In a separate experiment, U937 cells were preincubated with SB203580 (1 μmol/L) for 20 minutes, which significantly blocked the ability of several concentrations of bacterial sphingomyelinase (50 to 200 mU/mL for 5 minutes) to stimulate CREB phosphorylation (bottom).