SHIP Negatively Regulates IgE + Antigen-Induced IL-6 Production in Mast Cells by Inhibiting NF-κB Activity¹

We demonstrate in this study that IgE + Ag-induced proinflammatory cytokine production is substantially higher in Src homology-2-containing inositol 5'-phosphatase (SHIP)^-/- than in SHIP^+/+ bone marrow-derived mast cells (BMMCs). Focusing on IL-6, we found that the repression of IL-6 mRNA and protein production in SHIP^+/+ BMMCs requires the enzymatic activity of SHIP, because SHIP^-/- BMMCs expressing wild-type, but not phosphatase-deficient (D675G), SHIP revert the IgE + Ag-induced increase in IL-6 mRNA and protein down to levels seen in SHIP^+/+ BMMCs. Comparing the activation of various signaling pathways to determine which ones might be responsible for the elevated IL-6 production in SHIP^-/- BMMCs, we found the phosphatidylinositol 3-kinase/protein kinase B (PKB), extracellular signal-related kinase (Erk), p38, c-Jun N-terminal kinase, and protein kinase C (PKC) pathways are all elevated in IgE + Ag-induced SHIP^-/- cells. Moreover, inhibitor studies suggested that all these pathways play an essential role in IL-6 production. Looking downstream, we found that IgE + Ag-induced IL-6 production is dependent on the activity of NF-κB and that IκB phosphorylation/degradation and NF-κB translocation, DNA binding and transactivation are much higher in SHIP^-/- BMMCs. Interestingly, using various pathway inhibitors, it appears that the phosphatidylinositol 3-kinase/PKB and PKC pathways elevate IL-6 mRNA synthesis, at least in part, by enhancing the phosphorylation of IκB and NF-κB DNA binding while the Erk and p38 pathways enhance IL-6 mRNA synthesis by increasing the transactivation potential of NF-κB. Taken together, our data are consistent with a model in which SHIP negatively regulates NF-κB activity and IL-6 synthesis by reducing IgE + Ag-induced phosphatidylinositol-3,4,5-trisphosphate levels and thus PKB, PKC, Erk, and p38 activation