Submitted to: Journal of Biological Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/1/2009
Publication Date: N/A
Interpretive Summary: Toll-like receptor 4 (TLR4) is a protein that activates the immune system in the presence of lipopolysaccharide (LPS), but saturated fatty acids can also cause TLR4 activation while unsaturated fatty acids prevent activation. In this paper, we report that the mechanism for TLR4 activation. TLR4 is recruited to lipid rafts, regions of the membrane that contain high cholesterol concentrations, in the presence of LPS and saturated fatty acid. In these lipid rafts, increased TLR4 dimerization and association with downstream signaling molecules were observed in the presence of LPS and saturated fatty acid, but TLR4 dimerization was not observed in non-lipid rafts portions of the cell. The unsaturated fatty acid, docosahexaenoic acid, inhibited the effects of LPS and the saturated fatty acid. These results show how fatty acids can regulate TLR4 signaling and their role with inflammatory responses that cause many chronic diseases.
Technical Abstract: The saturated fatty acids acylated on Lipid A of lipopolysaccharide (LPS) or bacterial lipoproteins play critical roles in ligand recognition and receptor activation for Toll-like Receptor 4 (TLR4) and TLR2. The results from our previous studies (J Biol Chem 2003, 2004) demonstrated that saturated and polyunsaturated fatty acids reciprocally modulate the activation of TLR4. However, the underlying mechanism has not been understood. Here, we report for the first time that the saturated fatty acid lauric acid induced dimerization and recruitment of TLR4 into lipid rafts. Dimerized TLR4 was detected only in lipid raft fractions, but not in non-lipid raft fractions, suggesting that dimerization is coupled with recruitment of TLR4 into lipid rafts. Consequently, lauric acid enhanced the association of TLR4 with MD-2 and downstream adaptor molecules, TRIF and MyD88 in lipid rafts leading to the activation of downstream signaling pathways and target gene expression. However, docosahexaenoic acid (DHA), an n-3 polyunsaturated fatty acid, inhibited LPS- or lauric acid-induced dimerization and recruitment of TLR4 into lipid raft fractions. Together, these results demonstrated that lauric acid and DHA reciprocally modulate TLR4 activation by regulation of the dimerization and recruitment of TLR4 into lipid rafts. These results provide a new insight in understanding the mechanism by which fatty acids differentially modulate TLR4-mediated signaling pathway and consequent inflammatory responses that are implicated in the development and progression of many chronic diseases.