Bacterial lipopolysaccharide stimulates protein tyrosine phosphorylation in macrophages

Lipopolysaccharide (LPS), a membrane component of Gram-negative bacteria, stimulates immune responses by activating macrophages, B lymphocytes, and other cells of the immune system. The mechanisms by which LPS activates these cells are poorly characterized. Since protein tyrosine phosphorylation appears to be a major intracellular signaling event that mediates cellular responses, we examined whether LPS alters tyrosine phosphorylation in macrophages. We found that Escherichia coli K235 LPS increased tyrosine phosphorylation of several proteins in the RAW 264.7 murine macrophage cell line and in resident peritoneal macrophages from C3H/HeSNJ mice. Changes in tyrosine phosphorylation were detectable by 4-5 min, reached a maximum by 15 min, and declined after 30-60 min. Protein tyrosine phosphorylation increased following stimulation with LPS at 100 pg/ml and was maximal with 10 ng/ml. Similar changes in tyrosine phosphorylation were induced by Salmonella minnesota R595 LPS and by the biologically active domain of LPS, lipid A, but not by the inactive lipid A derivative N2-monoacylglucosamine 1-phosphate. Phorbol 12-myristate 13-acetate also stimulated protein tyrosine phosphorylation, but some of the modulated proteins were different than those phosphorylated by LPS. Treatment of RAW 264.7 cells with a tyrosine kinase inhibitor, herbimycin A, inhibited both LPS-stimulated tyrosine phosphorylation and LPS-stimulated release of arachidonic acid metabolites. Thus, increased protein tyrosine phosphorylation is a rapid LPS-activated signaling event that may mediate release of arachidonic acid metabolites in RAW 264.7 cells.