The lipopolysaccharide-induced stimulation of peritoneal macrophages involves at least two signal pathways. Partial stimulation by lipid A precursors.

Bacterial lipopolysaccharides (LPS) stimulate the ability of macrophages to convert arginine into ornithine. This effect is inhibited by the cycloxygenase inhibitor indomethacin and reconstituted by application of exogenous prostaglandin E2 (PGE2) or cholera toxin, i.e. two substances which are known to raise the intracellular concentration of cyclic AMP. Moreover, the LPS-mediated effect is augmented in a synergistic manner by PGE2, cholera toxin and the dibutyryl-derivative of cyclic AMP. Lipid A precursor IA, which lacks lauric, myristic and palmitic acids, and lipid A precursor IB, which is devoid of lauric and myristic acids of the complete lipid A structure, are capable of augmenting PGE2 synthesis but do not stimulate the conversion of arginine into ornithine. Taken together, our experiments suggest that the LPS-induced production of PGE2 and the PGE2-induced increase of the intracellular cAMP concentrations are essential elements of an autoregulatory loop that controls the LPS-mediated stimulation of the ornithine production by macrophages. The stimulation of the autoregulatory loop is necessary but not sufficient for this effect, indicating that an additional signal is required which is provided by the complete lipid A structure but not by the incomplete lipid A structures IA and IB. This additional signal can be provided by a lipid A structure containing the 3-acyloxyacyl residues with lauric and myristic acids.