A distinct role of neutrophil lactoferrin in RelA/p65 phosphorylation on Ser⁵³⁶ by recruiting TNF receptor-associated factors to IκB kinase signaling complex

The activation of NF-κB by neutrophil lactoferrin (Lf) is regulated via the IκB kinase (IKK) signaling cascade, resulting in the sequential phosphorylation and degradation of IκB. In this study, we observed that Lf protein augmented p65 phosphorylation at the Ser⁵³⁶, but not the Ser²⁷⁶ residue, and stimulated the translocation of p65 into the nucleus. Lf was also shown to enhance the association between p65 and CREB-binding protein/p300 in vivo. To elucidate the mechanism by which Lf triggers these signaling pathways, we attempted to delineate the roles of the upstream components of the IKK complex, using their dominant-negative mutants and IKKα^-/- and IKKβ^-/- mouse embryonic cells. We demonstrated that both IKKα and IKKβ as well as NF-κB-inducing kinase are indispensable for Lf-induced p65 phosphorylation. However, MAPK kinase kinase 1 is not essentially required for this activation. We also observed that Lf-induced p65 phosphorylation was either partially or completely abrogated as the result of treatment with the mutant forms of TNFR-associated factor (TRAF) 2, TRAF5, or TRAF6. Moreover, we demonstrated that Lf directly interacted with TRAF5. Expression of the dominant-negative mutant of TRAF5 or its small interfering RNA almost completely abrogated the Lf-induced p65 phosphorylation. These results suggest that signaling pathways, including TRAFs/NF-κB-inducing kinase/IKKs, may be involved in the regulation of Lf-induced p65 activation, thereby resulting in the activation of members of the NF-κB family.