TRAF3 Controls Activation of the Canonical and Alternative NFκB by the Lymphotoxin Beta Receptor

Components of lymphotoxin beta receptor (LTBR)-associated signaling complexes, including TRAF2, TRAF3, NIK, IKK1, and IKK2 have been shown to participate in the coupling of LTBR to NF kappa B. Here, we report that TRAF3 functions as a negative regulator of LTBR signaling via both canonical and non-canonical NF kappa B pathways by two distinct mechanisms. Analysis of NF kappa B signaling in cell lines with functionally intact NF kappa B pathway but lacking LTBR-mediated induction of NF kappa B target genes revealed an inverse association of cellular TRAF3 levels with LTBR-specific defect in canonical NF kappa B activation. Increased expression of TRAF3 correlated with its increased recruitment to LTBR-induced signaling complexes, decreased recruitment of TRAF2, and attenuated phosphorylation of I kappa B alpha and RelA. In contrast, activation of NF kappa B by TNF did not depend on TRAF3 levels. siRNA-mediated depletion of TRAF3 promoted recruitment of TRAF2 and IKK1 to activated LTBR, enabling LTBR-inducible canonical NF kappa B signaling and NF kappa B target gene expression. TRAF3 knock-down also increased mRNA and protein expression of several non-canonical NF kappa B components, including NF kappa B2/p100, RelB, and NIK, accompanied by processing of NF kappa B2/p100 into p52. These effects of TRAF3 depletion did not require LTBR signaling and were consistent with autonomous activation of the non-canonical NF kappa B pathway. Our data illustrate the function of TRAF3 as a dual-mode repressor of LTBR signaling that controls activation of canonical NF kappa B, and de-repression of the intrinsic activity of non-canonical NF kappa B. Modulation of cellular TRAF3 levels may thus contribute to regulation of NF kappa B-dependent gene expression by LTBR by affecting the balance of LTBR-dependent activation of canonical and non-canonical NF kappa B pathways.