Inhibition of IRAK1 Ubiquitination Determines Glucocorticoid Sensitivity for TLR9-Induced Inflammation in Macrophages

Abstract

Inflammatory responses are controlled by signaling mediators that are regulated by various posttranslational modifications. Recently, transcription-independent functions for glucocorticoids (GC) in restraining inflammation have emerged, but the underlying mechanisms are unknown. In this study, we report that GC receptor (GR)–mediated actions of GC acutely suppress TLR9-induced inflammation via inhibition of IL-1R–associated kinase 1 (IRAK1) ubiquitination. β-TrCP–IRAK1 interaction is required for K48-linked ubiquitination of IRAK1 at Lys134 and subsequent membrane-to-cytoplasm trafficking of IRAK1 interacting partners TNFR-associated factor 6 and TAK1 that facilitates NF-κB and MAPK activation. Upon costimulation of macrophages with GC and TLR9-engaging ligand, GR physically interacts with IRAK1 and interferes with protein–protein interactions between β-TrCP and IRAK1. Ablation of GR in macrophages prevents GC-dependent suppression of β-TrCP–IRAK1 interactions. This GC-mediated suppression of IRAK1 activation is unique to TLR9, as GC treatment impairs TLR9 but not TLR4 ligand–induced K48-linked IRAK1 ubiquitination and trafficking of IRAK1 interacting partners. Furthermore, mutations in IRAK1 at Lys134 prevent TLR9 ligand–induced activation of inflammatory signaling mediators and synthesis of proinflammatory cytokines to an extent comparable to GC-mediated inhibition. Collectively, these findings identify a transcription-independent, rapid, and nongenomic GC suppression of TLR9 ligand–mediated IRAK1 ubiquitination as a novel mechanism for restraining acute inflammatory reactions.