Institutes of Biology and Medical Sciences, Soochow University, Suzhou, China.
Jiangsu Key Laboratory of Infection and Immunity, Soochow University, Suzhou, China.
Front Immunol. 2018 Dec 13;9:2930. doi: 10.3389/fimmu.2018.02930. eCollection 2018.
β transducin repeat-containing protein (β-TrCP) is a Skp1-Cul1-F-box ubiquitin ligase, which plays important roles in controlling numerous signaling pathways. Notably, β-TrCP induces ubiquitination and degradation of inhibitor of NF-κB (IκBα), thus triggering activation of NF-κB signaling. Here, we unexpectedly find that β-TrCP restricts TRAF6-IKK signaling upstream of IκBα induced by lipopolysaccharide (LPS). In LPS-Toll-like receptor 4 (TLR4) pathway, protein kinase D1 (PKD1) is essential for activation of TRAF6-IKK-IκBα signaling including TRAF6 ubiquitination, IKK phosphorylation and subsequent IκBα degradation. We found that LPS promotes binding of β-TrCP to PKD1, and results in downregulation of PKD1 and recovery of IκBα protein level. Knockdown of β-TrCP blocks LPS-induced downregulation of PKD1. Supplement of enough PKD1 in cells inhibits recovery of IκBα protein levels during LPS stimulation. Furthermore, we demonstrate that β-TrCP inhibits LPS-induced TRAF6 ubiquitination and IKK phosphorylation. Taken together, our findings identify β-TrCP as an important negative regulator for upstream signaling of IκBα in LPS pathway, and therefore renew the understanding of the roles of β-TrCP in regulating TLRs inflammatory signaling.
β 衔接蛋白重复含蛋白（β-TrCP）是一种 Skp1-Cul1-F-box 泛素连接酶，在控制众多信号通路中发挥重要作用。值得注意的是，β-TrCP 诱导 NF-κB 抑制剂（IκBα）的泛素化和降解，从而触发 NF-κB 信号的激活。在这里，我们意外地发现β-TrCP 限制了脂多糖（LPS）诱导的 IκBα 上游的 TRAF6-IKK 信号。在 LPS-Toll 样受体 4（TLR4）途径中，蛋白激酶 D1（PKD1）对于 TRAF6-IKK-IκBα 信号的激活是必不可少的，包括 TRAF6 泛素化、IKK 磷酸化以及随后的 IκBα 降解。我们发现 LPS 促进了β-TrCP 与 PKD1 的结合，导致 PKD1 下调和 IκBα 蛋白水平的恢复。β-TrCP 的敲低阻止了 LPS 诱导的 PKD1 下调。在细胞中补充足够的 PKD1 可抑制 LPS 刺激期间 IκBα 蛋白水平的恢复。此外，我们证明β-TrCP 抑制 LPS 诱导的 TRAF6 泛素化和 IKK 磷酸化。总之，我们的研究结果确定β-TrCP 是 LPS 通路中 IκBα 上游信号的重要负调控因子，从而更新了对β-TrCP 调节 TLRs 炎症信号的作用的认识。
