Effects of bacterial lipopolysaccharide on protein synthesis in murine peritoneal macrophages: relationship to activation for macrophage tumoricidal function.

Early biochemical events in the response of murine peritoneal macrophages to bacterial lipopolysaccharide (LPS) have been examined (i.e., 0-4 hr after initiation of treatment). At concentrations of 10 ng/ml or less, LPS stimulated the new or enhanced synthesis of a series of at least six polypeptides of 85, 80, 75, 65, 57, and 38 kD. This effect was dependent upon the lipid A moiety of LPS as lipid A itself could induce the changes and the effect of LPS could be blocked by inclusion of polymixin B sulfate in the culture medium. The effect was specific for LPS in that other endotoxin-free agents known to alter macrophage physiology could not produce the same changes. The time course of LPS stimulation of macrophage protein synthesis was remarkable in that the synthesis of all six proteins was transient even in the continued presence of LPS, being first detected approximately 1 hr after exposure and no longer apparent by 8-10 hr after treatment was initiated. Furthermore, both pulse-chase and cumulative radiolabeling studies indicated that at least two of the proteins (85 and 38 kD) were short-lived and did not accumulate in LPS-treated cells, suggesting the possibility that they participate in a regulatory rather than a functional role. Macrophage tumoricidal activation involves cooperation in response to two independent signals; interferon gamma and LPS. Pretreatment of macrophages with interferon gamma increased the sensitivity of macrophages to LPS-stimulated protein synthesis by one to two orders of magnitude documenting such cooperativity in molecular terms. The LPS-induced stimulation of specific protein synthesis could be reproduced by treatment of macrophages with heat killed Listeria monocytogenes, a gram-positive, endotoxin-negative bacterial stain which has been shown to substitute effectively for LPS in macrophage tumoricidal activation. Furthermore, reversible inhibition (i.e., treatment with cycloheximide) of protein synthesis during LPS treatment abrogated the acquisition of tumoricidal function. These results identify an early biochemical response to LPS which may be a necessary component of the intracellular transduction of signals which regulate macrophage functional development.