Department of Immunology, Biogen Idec, Inc., Cambridge, Massachusetts 02142, USA.
J Biol Chem. 2010 Apr 23;285(17):12971-8. doi: 10.1074/jbc.M109.076091. Epub 2010 Feb 25.
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 NFkappaB. Here, we report that TRAF3 functions as a negative regulator of LTBR signaling via both canonical and non-canonical NFkappaB pathways by two distinct mechanisms. Analysis of NFkappaB signaling in cell lines with functionally intact NFkappaB pathway but lacking LTBR-mediated induction of NFkappaB target genes revealed an inverse association of cellular TRAF3 levels with LTBR-specific defect in canonical NFkappaB activation. Increased expression of TRAF3 correlated with its increased recruitment to LTBR-induced signaling complexes, decreased recruitment of TRAF2, and attenuated phosphorylation of IkappaB alpha and RelA. In contrast, activation of NFkappaB 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 NFkappaB signaling and NFkappaB target gene expression. TRAF3 knock-down also increased mRNA and protein expression of several non-canonical NFkappaB components, including NFkappaB2/p100, RelB, and NIK, accompanied by processing of NFkappaB2/p100 into p52. These effects of TRAF3 depletion did not require LTBR signaling and were consistent with autonomous activation of the non-canonical NFkappaB pathway. Our data illustrate the function of TRAF3 as a dual-mode repressor of LTBR signaling that controls activation of canonical NFkappaB, and de-repression of the intrinsic activity of non-canonical NFkappaB. Modulation of cellular TRAF3 levels may thus contribute to regulation of NFkappaB-dependent gene expression by LTBR by affecting the balance of LTBR-dependent activation of canonical and non-canonical NFkappaB pathways.
淋巴毒素β受体 (LTBR) 相关信号复合物的组成部分，包括 TRAF2、TRAF3、NIK、IKK1 和 IKK2，已被证明参与 LTBR 与 NFκB 的偶联。在这里，我们报告 TRAF3 通过两种不同的机制作为 LTBR 信号的负调节剂，通过两种不同的机制作用于经典和非经典 NFκB 途径。在具有完整 NFκB 途径但缺乏 LTBR 介导的 NFκB 靶基因诱导的细胞系中分析 NFκB 信号，发现细胞 TRAF3 水平与 LTBR 特异性经典 NFκB 激活缺陷呈负相关。TRAF3 的表达增加与 LTBR 诱导的信号复合物中 TRAF3 的募集增加、TRAF2 的募集减少以及 IκBα和 RelA 的磷酸化减少相关。相比之下，TNF 激活的 NFκB 不依赖于 TRAF3 水平。siRNA 介导的 TRAF3 耗竭促进了 TRAF2 和 IKK1 向激活的 LTBR 的募集，使 LTBR 诱导的经典 NFκB 信号和 NFκB 靶基因表达成为可能。TRAF3 敲低也增加了几种非经典 NFκB 成分的 mRNA 和蛋白表达，包括 NFκB2/p100、RelB 和 NIK，同时 NFκB2/p100 被加工成 p52。TRAF3 耗竭的这些影响不需要 LTBR 信号，并且与非经典 NFκB 途径的自主激活一致。我们的数据说明了 TRAF3 作为 LTBR 信号的双模式抑制剂的功能，该抑制剂控制经典 NFκB 的激活，并解除非经典 NFκB 的内在活性。因此，细胞 TRAF3 水平的调节可能通过影响 LTBR 依赖性经典和非经典 NFκB 途径的激活平衡，有助于 LTBR 依赖性 NFκB 依赖性基因表达的调节。
