Tie2 signalling through Erk1/2 regulates TLR4 driven inflammation

Area of Science:

• Immunology
• Cell Biology
• Molecular Biology

Background:

• The inflammatory response is critical for pathogen clearance but must be tightly regulated to prevent vascular damage.
• Endothelial cells initiate inflammation via Toll-like Receptor 4 (TLR4) sensing of lipopolysaccharide (LPS) and resolve it through receptor tyrosine kinase Tie2.
• The precise mechanisms by which Tie2 modulates LPS-TLR4-induced inflammation remain incompletely understood.

Purpose of the Study:

• To elucidate the role of Tie2 signaling in the attenuation of LPS-TLR4-mediated inflammatory responses.
• To identify the specific domains and signaling pathways within Tie2 essential for its anti-inflammatory function.
• To investigate the impact of Tie2 on key inflammatory mediators and signaling molecules downstream of TLR4.

Main Methods:

• Utilized Tie2 mutants with tyrosine-to-phenylalanine substitutions to assess the importance of tyrosine phosphorylation.
• Monitored nuclear factor-kappa B (NF-κB) activation as a key readout of TLR4 signaling.
• Examined the expression levels of TLR4 signaling proteins (TRAF6, IRAK1) and NF-κB inhibitor (IκBα).
• Assessed the regulation of microRNA-146b-5p (miRNA-146b-5p) by Tie2.
• Validated findings in an in vivo model of LPS-induced inflammation.

Main Results:

• Tie2 activation significantly attenuated LPS-induced NF-κB activation, dependent on Tie2 kinase activity and the Y1100 residue.
• The Y1100 residue was crucial for Tie2-mediated reduction in TRAF6 and IRAK1 expression and IκBα stabilization.
• Upregulation of the TLR4 antagonist miRNA-146b-5p required all three tyrosine phosphorylation sites in Tie2.
• In vivo studies confirmed that Tie2 activation reduces LPS-mediated inflammation.

Conclusions:

• Tie2 kinase activity, specifically through the Y1100 residue and subsequent Erk1/2 signaling, is essential for suppressing LPS-TLR4-induced inflammation.
• Tie2 influences multiple steps in the TLR4 signaling cascade, including protein expression and inhibitor stabilization.
• These findings reveal a critical mechanism for Tie2 in regulating vascular inflammation and offer potential therapeutic targets.