IRAK-2 participates in multiple Toll-like receptor signaling pathways to NFκB via activation of TRAF6 ubiquitination

Abstract

Toll-like receptor (TLR) signaling is known to involve interleukin-1 receptor-associated kinases (IRAKs), however the particular role of IRAK-2 has remained unclear. Further, although IRAK-1 was originally thought to be central for the TLR-NFκB signaling axis, recent data have shown that it is dispensable for NFκB activation for some TLRs and demonstrated an alternative role for it in interferon regulatory factor activation. Here we show that IRAK-2 is critical for the TLR-mediated NFκB activation pathway. The poxviral TLR antagonist A52 inhibited NFκB activation by TLR2, -3, -4, -5, -7, and -9 ligands, via its interaction with IRAK-2, while not affecting interferon regulatory factor activation. Knockdown of IRAK-2 expression by small interfering RNA suppressed TLR3, TLR4, and TLR8 signaling to NFκB in human cell lines, and importantly, TLR4-mediated chemokine production in primary human cells. IRAK-2 usage by different TLRs was distinct, because it acted downstream of the TLR adaptors MyD88 and Mal but upstream of TRIF. Expression of IRAK-2, but not IRAK-1, led to TRAF6 ubiquitination, an event critical for NFκB activation. Further, IRAK-2 loss-of-function mutants, which could not activate NFκB, were incapable of promoting TRAF6 ubiquitination. Thus we propose that IRAK-2 plays a more central role than IRAK-1 in TLR signaling to NFκB. © 2007 by The American Society for Biochemistry and Molecular Biology, Inc.