Induction of early gene expression in murine macrophages by synthetic lipid A analogs with differing endotoxic potentials

Abstract

Numerous lipid A analogs have been synthesized in an attempt to dissociate endotoxic activities from beneficial immunomodulatory activities. In the present study, we have evaluated select lipid A analogs in macrophages for their ability to induce a panel of lipopolysaccharide (LPS)-inducible genes to gain insights into the molecular mechanisms which underlie endotoxicity. We evaluated three monosaccharide lipid A analogs: SDZ MRL 953, an agonist with an improved therapeutic margin over endotoxin; SDZ 281.288, a more toxic analog; and SDZ 880.431, an analog with proven LPS-inhibitory activity. In addition, three disaccharide lipid A analogs (i.e., lipid IV(A), SDZ 880.611, and SDZ 880.924) that differ in acylation and phosphorylation patterns were also examined and compared with synthetic lipid A. With the exception of SDZ 880.431, each of these structurally diverse analogs was able to induce the complete panel of LPS-inducible genes, specifically genes which encode tumor necrosis factor alpha (TNF-α), interleukin-1β, 75-kDa type 2 TNF receptor (D7), IP-10, D3, and D8. These results underscore that macrophage stimulation by lipid A analogs is permissive to considerable structural diversity. Structures with favorable therapeutic indices (SDZ MRL 953, SDZ 880.611, and SDZ 880.924) were not different from structures with poor therapeutic indices (lipid A, lipid IV(A), and SDZ 281.288) with regard to gene induction. Nonetheless, the nontoxic SDZ MRL 953 was ~1,000-fold less potent than synthetic lipid A at inducing TNF-α secretion, and perhaps this contributes to the lack of toxicity exhibited by this compound. The ability of compound SDZ 880.431 to inhibit TNF-α secretion induced by both SDZ MRL 953 and smooth LPS suggests that the monosaccharide and smooth LPS share a receptor or a portion thereof. A pattern of protein tyrosine phosphorylation similar to that induced by LPS was stimulated by the monosaccharides SDZ MRL 953 and SDZ 281.288 and disaccharides lipid IV(A), SDZ 880.924, and SDZ 880.611, providing evidence for a common signalling pathway.