LPS induces altered phosphate labeling of proteins in murine peritoneal macrophages.

Covalent modification of proteins via phosphorylation is a well-documented mechanism whereby intracellular events are controlled by external stimuli. Treatment of thioglycollate-elicited, C57Bl/6 murine peritoneal macrophages with nanogram quantities of bacterial lipopolysaccharide (LPS) consistently results in altered 32Pi labeling of a specific set of proteins (e.g., proteins of 67, 37, 33, and 28 kD), as measured by autoradiography after SDS-polyacrylamide gel electrophoresis. Induction of this pattern of phosphorylation is duplicated by the lipid A moiety of LPS. The LPS-stimulated changes in phosphate labeling are both dose- and time-dependent. Of various pharmacologic agents tested, the phosphorylation pattern induced in macrophages by the tumor promoter phorbol myristic acetate shows similarity to the pattern induced by LPS. Analysis of pp 28 and pp 37 from both LPS- and PMA-treated macrophages by limited proteolysis demonstrates that these phosphoproteins are structurally related and that the sites of phosphorylation are similar for both treatment conditions. Macrophages from the genetically LPS-unresponsive C3H/HeJ strain show no alteration in their pattern of phosphorylation after treatment with LPS. Control macrophages, from C3H/HeN mice, respond to LPS in a fashion identical to that seen in C57Bl/6 macrophages. Pretreatment of macrophages with IFN-gamma potentiates the effect of LPS (i.e., yields a level of altered phosphate labeling greater than that observed with LPS or PMA alone). Together, the data indicate that LPS causes altered phosphate labeling of a defined set of proteins, and that the circumstances of this response are consistent with a possible role in coupling LPS-initiated signals to the induction of functional competence in macrophages.