NGF Withdrawal Induces Apoptosis in CESS B Cell Line through p38 MAPK Activation and Bcl-2 Phosphorylation

The sIgG+ lymphoblastoid B cell line CESS spontaneously produces a high amount of NGF and expresses both high affinity (p140Trk-A) and low affinity (p75NTR) NGF receptors. Blocking NGF signals with neutralizing antibodies or specific Trk-A inhibitors induces a rapid phosphorylation of antiapoptotic Bcl-2 protein, followed by caspase activation, and apoptotic death of CESS cells. Bcl-2 phosphorylation in several sites within a ?60 aa "loop" domain of protein is known to regulate its antiapoptotic function. Accordingly, CESS cells expressing the loop deletional mutant cDNA constructs Bcl-2 ?40-91 were completely resistant to apoptosis induced by NGF withdrawal, indicating that Bcl-2 phosphorylation is a critical event. NGF withdrawal induces p38 MAPK, but not JNK, activation in CESS cells, and SB203580, a specific inhibitor of p38 MAPK, is able to prevent both Bcl-2 phosphorylation and apoptosis, indicating that p38 MAPK is the enzyme responsible for these events.