Vasoactive intestinal peptide suppresses toll-like receptor 4 expression in macrophages via Akt1 reducing their responsiveness to lipopolysaccharide

Toll-like receptor 4 (TLR4) recognizes and initiates signals from Gram-negative bacterial lipopolysaccharide (LPS) triggering the inflammatory response. Expression levels of TLR4 on macrophages partly regulate the magnitude of the response to LPS. Vasoactive Intestinal Peptide (VIP) is known to block inflammatory responses by inhibiting pro-inflammatory cytokine production from activated macrophages. In the present report we demonstrate that VIP directly suppressed TLR4 expression on naïve primary mouse macrophages utilizing signalling cascades that control TLR4 transcription. VIP-induced suppression of TLR4 occurred at the transcriptional level by decreasing PU.1 DNA binding. Mutation of the proximal PU.1 but not the AP-1-binding site on the TLR4 promoter abrogated VIP-induced suppression of TLR4 transcription. Moreover, inhibition of PI3K by wortmannin or homologous deletion of the Akt1 isoform, a pathway known to act as a negative regulator of macrophage activation, alleviated the suppressive action of VIP on TLR4 expression. To evaluate the biological significance of VIP effect on TLR4 expression, Raw264.7 macrophages were pre-treated with VIP for 24 h and then exposed to LPS. Pre-treatment with VIP rendered macrophages hypo-responsive to LPS resulting in reduced pro-inflammatory cytokine production. Moreover, in vivo administration of VIP in C57BL/6 mice resulted in lower IL-6 production upon treatment with LPS. Overall, the results indicate that VIP promotes endotoxin tolerance by downregulating TLR4 expression via Akt1.