The TNF Family Member 4-1BBL Sustains Inflammation by Interacting with TLR Signaling Components During Late-Phase Activation

Abstract

Activation of Toll-like receptor (TLR)-dependent signaling leads to the expression of genes that encode pro-inflammatory factors, such as tumor necrosis factor α (TNF-α), and this process is sustained for the duration of the inflammatory response. TLR-mediated inflammation, which occurs in two phases, depends on the TNF family member 4-1BB ligand (4-1BBL) to sustain TNF-α production during late-phase signaling. Here, we showed that Toll–interleukin-1 receptor (TIR) domain–containing adaptor protein (TIRAP) and IL-1R–associated kinase 2 (IRAK2) were required to mediate the late phase of the TLR4 response through their interaction with 4-1BBL. Expression of 4-1bbl was dependent on early TLR signaling that also induced the expression of Tnf, and the resulting 4-1BBL protein translocated to the plasma membrane, where it physically interacted with TLRs to mediate late-phase, secondary TLR signaling. TLR4–4-1BBL–mediated signaling depended on TIRAP and IRAK2, as well as a downstream complex consisting of the E3 ubiquitin ligase TRAF6, the kinase TAK1, and the adaptor protein TAB1, leading to the stimulation of mitogen-activated protein kinases. Whereas early TLR4 signaling involved the formation of a signaling complex consisting of TLR4, the adaptor proteins MyD88 and TIRAP, and IRAK2, formation of a secondary complex consisting of TLR4, 4-1BBL, TIRAP, and IRAK2 enabled late-phase signaling. Inhibition of this late-phase pathway reduced the extent of TNF-α production by mouse macrophages exposed to the TLR4 ligand lipopolysaccharide (LPS), and ameliorated LPS-induced sepsis in mice. Together, these data suggest that TIRAP and IRAK2 are critical for the sustained inflammatory response that is mediated by late-phase signaling by the TLR–4-1BBL complex.

Ablation of Akt2 protects against lipopolysaccharide-induced cardiac dysfunction: Role of Akt ubiquitination E3 ligase TRAF6

“…Findings from our study revealed that TRAF6 peptide inhibitor or siRNA knockdown (but not non-target RNA) alleviated or ablated LPS-induced contractile anomalies, apoptosis and Akt activation, indicating a role of TRAF6 in LPS-induced cell injury. Inhibition of TRAF6 using RNA silence or decoy peptides decreases tumor cell proliferation and promotes apoptosis [26, 27, 43] although insufficient information is available with regards to the role of TRAF6 in heart failure. Ubiquitination, an essential component of the ubiquitin-proteasome system (UPS), is a cell degradation machinery through which mammalian cells degrade and recycle macromolecules and organelles [44].…”

SA-4-1BBL and Monophosphoryl Lipid A Constitute an Efficacious Combination Adjuvant for Cancer Vaccines

“…It has been shown that 4-1BB/4-1BBL signaling is critical for the activation and survival of dendritic cells (29) and the conversion of monocytes into dendritic cells (30). Interesting in this context, it was shown that TLR signaling induces 4-1BBL expression on the surface of macrophages and the physical interaction of 4-1BBL with TLR-4 is necessary for sustained TNF production (31, 32). We did not observe synergy between MPL and SA-4-1BBL on bone marrow derived DCs for the induction of costimulatory molecules or elaboration of various proinflammatory cytokines (Table S2).…”

Inhibition of 4‐1BBL‐regulated TLR response in macrophages ameliorates endotoxin‐induced sepsis in mice

“…Unlike the early phase of TLR responses, this later phase controlled by 4‐1BBL does not induce the activation of NF‐κB pathway, while MAPKs, protein kinases, and transcription factors, such as CREB and C/EBP that play an essential role in sustaining expression of TNF. Specifically, we found that inhibition of the 4‐1BBL‐TIRAP interaction significantly reduced TNF production in macrophages in vitro and improved survival of mice in a model of endotoxin‐induced sepsis , suggesting that the 4‐1BBL‐mediated signaling may be a target of anti‐inflammation treatments.…”

“…Explaining this phenomenon, we showed that the TLR response induces the expression of 4‐1BBL during the early phase of macrophage activation and subsequently the newly expressed 4‐1BBL interacts with the TLRs to initiate a secondary late‐phase signaling response to sustain TNF expression . The TLR‐4‐1BBL complex interacts with an adaptor protein TIRAP and a kinase IRAK2, but not with MyD88 and TRIF, to activate a downstream signaling cascade that includes TRAF6‐TAK1‐TAB1 . Unlike the early phase of TLR responses, this later phase controlled by 4‐1BBL does not induce the activation of NF‐κB pathway, while MAPKs, protein kinases, and transcription factors, such as CREB and C/EBP that play an essential role in sustaining expression of TNF.…”