Effects of bacterial lipopolysaccharide on the hydrolysis of phosphatidylinositol-4,5-bisphosphate in murine peritoneal macrophages

LPS and lipid A initiated enhanced hydrolysis of PIP₂ in macrophages. When murine peritoneal macrophages were labeled with [2-³H]myoinositol and stimulated with either LPS or lipid A, a rapid (within 10 sec) rise in Ins(1,4,5)P₃ was observed. The breakdown pattern of Ins(1,4,5)P₃ was complex; this included breakdown of Ins(1,4,5)P₃ and formation of Ins(1,3,4,5)P₄ (~10 to 30 sec), and ultimately formation of Ins(1,3,4)P₃ (~60 sec). Within 10 sec after treatment, LPS caused an average increase of about fourfold to fivefold in Ins(1,4,5)P₃, which declined over 5 min. When the total isomers of InsP₃ were measured, levels rose about twofold in response to LPS or to lipid A and remained elevated for as long as 5 min. Lipid A, in the concentration range of 0.1 to 10 μg/ml, induced elevated intracellular levels of Ca²⁺ as quantified by fluorescence with Quin 2 or with Fura 2. When single, adherent Fura 2-loaded macrophages were treated with lipid A, basal levels of calcium rose over 10 sec from ~55 nM to almost 600 nM. LPS, paradoxically, did not cause such substantial increases in intracellular calcium (i.e., increases of ~26 nM) when judged by Fura 2 fluorescence. LPS treatment led to enhanced phosphorylation of a characteristic set of proteins, similar to those induced by stimulating protein kinase C (PKC) with phorbol myristate acetate as previously reported. The enhanced phosphorylation of pp28, pp33, and pp67 in macrophages was evident by 15 min and optimal by 30 min. Taken together, these observations indicate that LPS and lipid A cause increased breakdown of phosphatidylinositol 4,5-biphosphate, which led to enhanced intracellular levels of calcium and also to enhanced protein phosphorylation, presumably mediated by PKC. The data thus suggest that one major intracellular signal transduction mechanism, initiated by LPS and lipid A in macrophages, is the rapid breakdown of PIP₂.