The signal transduction pathway of CD23 (Fc epsilon RIIb) targets I kappa B kinase

Alveolar macrophages play a crucial role in initiating the inflammatory response in allergic asthma through the cross-linking of the low affinity IgE receptors (Fc epsilon RIIb or CD23) by IgE-allergen immunocomplexes. We have previously shown that CD23 cross-linking in monocytes and U937 cells targets I kappa B alpha, leading to the activation of the transcription factor NF-kappa B. We demonstrate in this paper that CD23-initiated signaling in U937 cells leads to hyperphosphorylation of I kappa B alpha at Ser32/Ser36 residues. Overexpression of a dominant-negative I kappa B alpha transgene containing mutations at Ser32/Ser36 completely inhibits degradation of I kappa B alpha, NF-kappa B activation, and gene transcription that follows CD23 cross-linking. Investigation of the second messengers mediating the CD23-dependent activation of NF kappa B demonstrates that I kappa B kinases (IKKs) but not p90rsk are selectively activated following CD23 cross-linking and mediates the phosphorylation of I kappa B alpha. Cotransfection experiments with an IKK beta negative dominant completely inhibit CD23 induced NF kappa B activation. Furthermore, the activation of tyrosine kinase(s) by CD23 is required for the induction of IKK activity, I kappa B alpha degradation, and NF-kappa B nuclear translocation. Taken together, our results show that CD23 cross-linking in the monocytic lineage induces tyrosine kinase activation followed by activation of IKK, which phosphorylates I kappa B alpha at the N-terminal domain (Ser32/Ser36), inducing its degradation, NF-kappa B activation and gene transcription.