Mitogen-activated protein kinases signal inhibition of apoptosis in lipopolysaccharide-stimulated neutrophils.
BACKGROUND: Neutrophil (PMN) apoptosis is critical to the resolution of infection and the limitation of inflammation. Bacterial endotoxin (lipopolysaccharide [LPS]) inhibits PMN apoptosis and activates the p38 mitogen-activated protein kinase (MAPK) signal cascade. The role of p38 and other MAPKs (ERK and SAPK/JNK) in regulating PMN apoptosis after LPS stimulation is unknown. We hypothesize that MAPK activation by LPS signals inhibition of PMN apoptosis. METHODS: PMNs were isolated from the blood of healthy human volunteers and incubated with PD98059 (ERK inhibitor), SB203580 (p38 inhibitor), or 0.1% dimethyl sulfoxide (vehicle) for 1 hour before treatment with LPS (0, 10, or 1000 ng/mL). Neutrophil MAPK activation was determined by Western blot analysis for phosphorylated p38, ERK, and SAPK/JNK. Apoptosis was quantified by flow cytometry with use of propidium iodide and annexin V. RESULTS: LPS inhibited PMN apoptosis and activated p38 and ERK in a dose- and time-dependent fashion. SAPK/JNK was not activated by LPS. Treatment of cells with ERK inhibitor before LPS stimulation abrogated LPS signaled inhibition of PMN apoptosis. Conversely, p38 inhibition with SB203580 augmented inhibition of apoptosis by LPS. CONCLUSIONS: These data demonstrate opposing roles of MAPKs in mediating PMN apoptosis after LPS stimulation. We conclude that LPS signal transduction by ERK inhibits PMN apoptosis while activation of p38 promotes apoptosis.