Functional Toll-like receptor 4 mutations modulate the response to fibrinogen

Fibrinogen has been implicated in atherosclerosis; in part by activating the lipopolysaccharide (LPS) receptor Toll-like receptor 4 (TLR4). The fibrinogen-TLR4 signalling pathway remains uncharacterised. In human macrophages fibrinogen stimulated interleukin (IL)6 expression and ERK (extracellular signal-related kinase) phosphorylation. In HEK293-CD14-MD2 cells expressing TLR4, fibrinogen induced robust phosphorylation of ERK 1, p38 alpha and JNK and activated transcription factors NF kappa B, EIK-I and AP-I (activator protein-1). The net effect of this signalling pathway was a pro-inflammatory response characterised by IL6 and TNF alpha synthesis and increased IL8, matrix metalloproteinase (MMP)1, MMP9, and MCP-I promoter activity. Two common TLR4 mutations, D299G and T3991, render the receptor LPS hyporesponsive. The effect of fibrinogen on polymorphic variant TLR4s was markedly different; enhancing activation of kinases, transcription factors, cytokine synthesis and promoter activity. This study indicates that fibrinogen activates TLR4, explaining how fibrinogen promotes inflammatory protein expression.