Activation of NF-κB in B cell receptor signaling through Bruton's tyrosine kinase-dependent phosphorylation of IκB-α

The antigen-mediated triggering of B cell receptor (BCR) activates the transcription factor NF-kappa B that regulates the expression of genes involved in B cell differentiation, proliferation, and survival. The tyrosine kinase Btk is essentially required for the activation of NF-kappa B in BCR signaling through the canonical pathway of IKK-dependent phosphorylation and proteasomal degradation of I kappa B-alpha, the main repressor of NF-kappa B. Here, we provide the evidence of an additional mechanism of NF-kappa B activation in BCR signaling that is Btk-dependent and IKK-independent. In DeFew B lymphoma cells, the anti-IgM stimulation of BCR activated Btk and NF-kappa B p50/p65 within 0.5 min in absence of IKK activation and I kappa B-alpha degradation. IKK silencing did not affect the rapid activation of NF-kappa B. Within this short time, Btk associated and phosphorylated I kappa B-alpha at Y289 and Y305, and, concomitantly, p65 translocated from cytosol to nucleus. The mutant I kappa B-alpha Y289/305A inhibited the NF-kappa B activation after BCR triggering, suggesting that the phosphorylation of I kappa B-alpha at tyrosines 289 and 305 was required for NF-kappa B activation. In primary chronic lymphocytic leukemia cells, Btk was constitutively active and associated with I kappa B-alpha, which correlated with Y305-phosphorylation of I kappa B-alpha and increased NF-kappa B activity compared with healthy B cells. Altogether, these results describe a novel mechanism of NF-kappa B activation in BCR signaling that could be relevant for Btk-targeted therapy in B-lymphoproliferative disorders. Key messages Anti-IgM stimulation of BCR activates NF-kappa B p50/p65 within 30 s by a Btk-dependent and IKK-independent mechanism. Btk associates and phosphorylates I kappa B-alpha at Y289 and Y305, promoting NF-kappa B activation. In primary CLLs, the binding of Btk to I kappa B-alpha correlates with tyrosine phosphorylation of I kappa B-alpha and increased NF-kappa B activity.