Mast cell tumor necrosis factor α production is regulated by MEK kinases

Mast cells synthesize and secrete specific cytokines and chemokines which play an important role in allergic inflammation. Aggregation of the high-affinity Fc receptor (FcɛRI) for immunoglobulin E (IgE) in MC/9 mouse mast cells stimulates the synthesis and secretion of tumor necrosis factor α (TNF-α). FcɛRI aggregation activates several sequential protein kinase pathways, leading to increased activity of extracellular signal-regulated kinases (ERKs), c-Jun amino-terminal kinases (JNKs), and the p38 mitogen-activated protein (MAP) kinase. Inhibition of ERKs with the compound PD 098059 had little effect on FcɛRI-stimulated TNF-α production. Aggregation of FcɛRI stimulated MEK kinase 1 (MEKK1) activity, which activates JNK kinase (JNKK), the kinase that phosphorylates and activates JNKs. Expression of activated MEKK1 (ΔMEKK1) in MC/9 cells strongly stimulated JNK activity but only weakly stimulated p38 activity, and it induced a large activation of TNF-α promoter-regulated luciferase gene expression. Inhibitory mutant JNK2 expressed in MC/9 cells significantly blunted FcɛRI stimulation of TNF-α promoter-driven luciferase expression. Wortmannin, an inhibitor of phosphatidylinositol 3-kinase, diminished FcɛRI-mediated TNF-α synthesis, significantly blunted JNK activation and TNF-α promoter-driven luciferase expression, and only weakly inhibited p38 kinase activation. Inhibition of NFκB activation resulting from ΔMEKK1 expression or FcɛRI stimulation did not affect TNF-α promoter-driven luciferase expression. Our findings define a MEKK-regulated JNK pathway activated by FcɛRI that regulates TNF-α production in mast cells.