Endogenous Enhancers of Toll-Like Receptor 4

Abstract

Lipopolysaccharide (LPS) is a bacterial component that triggers an inflammatory response that can lead to sepsis and death. Myeloid-related protein 8 (Mrp8) and Mrp14 interact with each other and are secreted by activated phagocytes, with Mrp8 as the active subunit of the complex. Vogl et al . investigated the role of these proteins in sepsis. Using DNA microarrays, they found that a number of genes encoding proinflammatory molecules were more highly expressed in LPS-treated bone marrow cells (BMCs) from wild-type mice than in those of Mrp14 –/– mice. ( Mrp8 –/– mice die as embryos.) Enzyme-linked immunosorbant assays revealed that LPS-treated Mrp14 –/– BMCs secreted lower quantities of tumor necrosis factor-α (TNF-α) than did LPS-treated wild-type BMCs. Addition of recombinant Mrp8-Mrp14 increased the responsiveness of Mrp14 –/– BMCs to LPS. Immunofluorescence and Western blotting analyses in human epithelial cells showed that Mrp8 stimulated the recruitment of the adaptor protein myeloid differentiation marker 88 to the plasma membrane and the phosphorylation of interleukin-1 receptor (IL-1R)-associated kinase-1 (IRAK-1), both of which are necessary for Toll-like receptor 4 (TLR4) signaling. Luciferase reporter assays performed in 293 cells transfected with TLR4 and its co-receptors MD2 and CD14 revealed that Mrp8 stimulated the activity of the transcription factor nuclear factor κB. Surface plasmon resonance studies showed that Mrp8 bound to the TLR4-MD2 complex. Administration of LPS and galactosamine, which sensitizes mice to LPS-induced TNF-α, to Mrp14 –/– mice resulted in less severe effects and better survival than did similar treatment of wild-type mice. The addition of Mrp8 to the Mrp14 –/– mice treated with LPS and galactosamine resulted in an outcome similar to that of LPS-treated wild-type mice. The role of Mrp8-Mrp14 as an endogenous enhancer of TLR4-induced TNF-α production in response to LPS thus reveals a new possible therapeutic target in the treatment of sepsis. T. Vogl, K. Tenbrock, S. Ludwig, N. Leukert, C. Ehrhardt, M. A. D. van Zoelen, W. Nacken, D. Foell, T. van der Poll, C. Sorg, J. Roth, Mrp8 and Mrp14 are endogenous activators of Toll-like receptor 4, promoting lethal, endotoxin-induced shock. Nat. Med . 13 , 1042-1049 (2007). [PubMed]