A novel synthetic acyclic lipid A-like agonist activates cells via the lipopolysaccharide/toll-like receptor 4 signaling pathway

ER-112022 is a novel acyclic synthetic lipid A analog that contains six symmetrically organized fatty acids on a noncarbohydrate backbone. Chinese hamster ovary (CHO)-K1 fibroblasts and U373 human astrocytoma cells do not respond to lipopolysaccharide (LPS) in the absence of CD14. In contrast, exposure to ER-112022 effectively induced activation of CHO and U373 cells under serum-free conditions. Expression of CD14 was not necessary for cells to respond to ER-112022, although the presence of soluble CD14 enhanced the sensitivity of the response. Several lines of evidence suggested that ER-112022 stimulates cells via the LPS signal transduction pathway. First, the diglucosamine based LPS antagonists E5564 and E5531 blocked ER-112022-induced stimulation of CHO-RI, U373, and RAW264.7 cells. Second, ER-112022 was unable to activate C3H/HeJ mouse peritoneal macrophages, containing a mutation in Toll-like receptor (TLR) 4, as well as HEK293 cells, an epithelial cell line that does not express TLR4, Third, ER-112022 activated NF-KB in HEK293 cells transfected with TLR4/ MD-2. Finally, tumor necrosis factor release from primary human monocytes exposed to ER-112022 was blocked by TLR4 antibodies but not by TLR2 antibodies. Our results suggest that ER-112022 and the family of lipid A-like LPS antagonists can functionally associate with TLR4 in the absence of CD14. Synthetic molecules like ER-112022 may prove to be valuable tools to characterize elements in the LPS receptor complex, as well as to activate or inhibit the TLR4 signaling pathway for therapeutic purposes.