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Published on: January 7, 2019
MLK4 has negative effect on TLR4 signaling
Alim Seit-Nebi1, Wei Cheng, Hong Xu
1Key Laboratory of the Ministry of Education for Cell Biology and Tumor Cell Engineering, School of Life Sciences, Xiamen University, Xiamen, China.
Abstract:
The stimulation of Toll-like receptors (TLRs) on macrophages triggers production of proinflammatory cytokines such as tumor-necrosis factor-α (TNF-α). The TNF production is mediated by a series of signaling events and subsequent transcriptional and post-transcriptional activation of the TNF gene. Termination of TLR-mediated cellular signaling is also important for a proper immunoresponse, since sustained cytokine expression can result in immune disorders. Here we identified that mixed-lineage kinase (MLK) 4 is a TLR4-interacting protein. Unlike previously characterized MLK group members, MLK4 cannot act as a mitogen-activated protein kinase kinase kinase (MAP3K) to mediate c-Jun N-terminal kinase (JNK), p38 or extracellular signal-regulated kinase (ERK) activation. Rather, MLK4 appears to be able to inhibit lipopolysaccharide (LPS)-induced activation of the JNK or ERK pathways, but does not have effect on LPS-induced p38 or NF-κB activation. The LPS-induced TNF production in MLK4 knockdown and overexpression cells were also increased and reduced, respectively. These data demonstrate that MLK4 is a negative regulator of TLR4 signaling.
Insights
Mixed-lineage kinase 4 (MLK4) negatively regulates Toll-like receptor 4 (TLR4) signaling. MLK4 inhibits lipopolysaccharide-induced activation of JNK and ERK pathways, reducing pro-inflammatory cytokine production.
Area of Science:
- Immunology
- Cell Signaling
- Molecular Biology
Background:
- Toll-like receptors (TLRs) on macrophages initiate inflammatory responses by producing cytokines like tumor-necrosis factor-α (TNF-α).
- Proper termination of TLR signaling is crucial to prevent immune disorders caused by sustained cytokine expression.
Purpose of the Study:
- To identify novel proteins interacting with TLR4.
- To elucidate the role of mixed-lineage kinase (MLK) 4 in TLR4-mediated signaling pathways.
Main Methods:
- Identified MLK4 as a TLR4-interacting protein.
- Investigated MLK4's effect on mitogen-activated protein kinase (MAPK) pathways (JNK, p38, ERK) and NF-κB activation following lipopolysaccharide (LPS) stimulation.
- Analyzed TNF-α production in cells with altered MLK4 expression (knockdown and overexpression).
Main Results:
- MLK4 interacts with TLR4 but does not function as a MAP3K.
- MLK4 inhibits LPS-induced JNK and ERK activation, but not p38 or NF-κB activation.
- MLK4 knockdown increased, while overexpression reduced, LPS-induced TNF-α production.
Conclusions:
- MLK4 acts as a negative regulator of TLR4 signaling.
- MLK4 plays a critical role in controlling the magnitude and duration of the inflammatory response initiated by TLR4 stimulation.
