Hepatocyte toll-like receptor 2 expression in vivo and in vitro:: Role of cytokines in induction of rat TLR2 gene expression by lipopolysaccharide

We and others have demonstrated previously that cytokines, including interleukin-1 (IL-1) and tumor necrosis factor-alpha (TNF alpha), regulate LPS recognition proteins such as CD14 in the liver and on hepatocytes. Based on recent findings that the mammalian homologue of Drosophila Toll participates in LPS signaling, we examined the regulation of Toll-Like Receptor (TLR) gene expression by cytokines in vitro and its distribution in vivo with a focus on the liver as a site of host-microbe interaction. Our results show that IL-1 beta and/or TNF alpha participate in the upregulation of TLR2 mRNA levels in hepatocytes. Rats treated concurrently with LPS and antagonists of the IL-1 or TNF alpha receptor demonstrated significantly reduced LPS-induced hepatic expression of TLR2 compared to animals treated with LPS alone. The increase in hepatic TLR2 mRNA expression was associated with enhanced transcription as determined by nuclear run-on analysis. LPS treatment in vivo caused a marked TLR2 mRNA up-regulation in all of the tissues examined, with liver showing the highest expression. The high level of TLR2 expression in the liver may have important implications for pathogen-host interactions or microbial signaling.