ABSTRACT
NF-κB activation depends on the IKK complex consisting of the catalytically active IKK1 and 2 subunits and the scaffold protein NEMO. Hitherto, IKK2 activation has always been associated with IκBα degradation, NF-κB activation, and cytokine production. In contrast, we found that in SCF-stimulated primary bone marrow-derived mast cells (BMMCs), IKK2 is alternatively activated. Mechanistically, activated TAK1 mediates the association between c-Kit and IKK2 and therefore facilitates the Lyn-dependent IKK2 activation which suffices to mediate mitogenic signaling but, surprisingly, does not result in NF-κB activation. Moreover, the c-Kit-mediated and Lyn-dependent IKK2 activation is targeted by MyD88-dependent pathways leading to enhanced IKK2 activation and therefore to potentiated effector functions. In neoplastic cells, expressing constitutively active c-Kit mutants, activated TAK1 and IKKs do also not induce NF-κB activation but mediate uncontrolled proliferation, resistance to apoptosis and enables IL-33 to mediate c-Kit-dependent signaling. Together, we identified the formation of the c-Kit-Lyn-TAK1 signalosome which mediates IKK2 activation. Unexpectedly, this IKK activation is uncoupled from the NF-κB-machinery but is critical to modulate functional cell responses in primary-, and mediates uncontrolled proliferation and survival of tumor-mast cells. Therefore, targeting TAK1 and IKKs might be a novel approach to treat c-Kit-driven diseases.
Subject(s)
I-kappa B Kinase/metabolism , MAP Kinase Kinase Kinases/metabolism , Mast Cells/drug effects , Proto-Oncogene Proteins c-kit/metabolism , Signal Transduction/drug effects , Stem Cell Factor/pharmacology , Animals , Apoptosis , Cell Differentiation , Cell Proliferation , Dose-Response Relationship, Drug , Enzyme Activation , Genotype , HEK293 Cells , Humans , I-kappa B Kinase/antagonists & inhibitors , I-kappa B Kinase/deficiency , I-kappa B Kinase/genetics , Interleukin-33/metabolism , MAP Kinase Kinase Kinases/genetics , Mast Cells/enzymology , Mast Cells/pathology , Mice, Knockout , Mutation , Myeloid Differentiation Factor 88/genetics , Myeloid Differentiation Factor 88/metabolism , NF-kappa B/metabolism , Neoplasms/enzymology , Neoplasms/genetics , Neoplasms/pathology , Phenotype , Primary Cell Culture , Protein Binding , Protein Kinase Inhibitors/pharmacology , Proto-Oncogene Proteins c-kit/genetics , Time Factors , Transfection , src-Family Kinases/genetics , src-Family Kinases/metabolismABSTRACT
Mast cell differentiation and proliferation depends on IL-3. IL-3 induces the activation of MAP-kinases and STATs and consequently induces proliferation and survival. Dysregulation of IL-3 signaling pathways also contribute to inflammation and tumorigenesis. We show here that IL-3 induces a SFK- and Ca²âº-dependent activation of the inhibitor of κB kinases 2 (IKK2) which results in mast cell proliferation and survival but does not induce IκBα-degradation and NFκB activation. Therefore we propose the term "subthreshold IKK activation".This subthreshold IKK activation also primes mast cells for enhanced responsiveness to IL-33R signaling. Consequently, co-stimulation with IL-3 and IL-33 increases IKK activation and massively enhances cytokine production induced by IL-33.We further reveal that in neoplastic mast cells expressing constitutively active Ras, subthreshold IKK activation is associated with uncontrolled proliferation. Consequently, pharmacological IKK inhibition reduces tumor growth selectively by inducing apoptosis in vivo.Together, subthreshold IKK activation is crucial to mediate the full IL-33-induced effector functions in primary mast cells and to mediate uncontrolled proliferation of neoplastic mast cells. Thus, IKK2 is a new molecularly defined target structure.
