Involvement of calcium and arachidonate metabolism in acetylated‐low‐density‐lipoprotein‐stimulated tumor‐necrosis‐factor‐α production by rat peritoneal macrophages

Abstract

We show that lipopolysaccharide‐free actetylated low‐density lipoprotein (LDL), but not native LDL, stimulates tumor‐necrosis factor‐α (TNF‐α) secretion by rat peritoneal macrophages and the signal‐transduction pathways involved. The role of the scavenger receptor (SR) in this response was suggested by the absence of an effect induced by native LDL, signal coupling involving pertussis‐toxin‐dependent guanine‐nucleotide‐binding regulatory (G) protein, and the complete inhibition of this response by SR ligands [poly(I) and dextran sulfate]. Acetylated LDL induces rapid Ca 2+ release from inositol‐phosphate‐sensitive Ca 2+ stores mediated by pertussis‐sensitive G proteins and a sustained Ca 2+ rise mediated by Ca 2+ influx and by Ca 2+ release from ryanodine‐sensitive Ca 2+ stores. Acetylated LDL‐induced Ca 2+ influx and TNF‐α production were abolished by inhibitors of phospholipase C (U73122) and phospholipase A 2 (bromophenacyl bromide), but were not affected by an inhibitor of protein kinase C (calphostine C). Therefore, Ca 2+ influx induced by acetylated LDL is dependent on Ca 2+ store depletion. Arachidonate released by acetylated LDL acts as a second messenger to activate TNF‐α secretion via Ca 2+ influx. While the Ca 2+ signal was not modified by an inhibitor of protein tyrosine kinases (PTK; herbimycin A), this inhibitor completely blocked TNF‐α production, suggesting the involvement of PTK downstream of the Ca 2+ signal. These results suggest that a sustained elevation of intracellular Ca 2+ , mediated through Ca 2+ influx via the phospholipase‐A 2 ‐dependent pathway, is essential for induction of TNF‐α secretion. The type of SR class involved in these pathways remains to be identified.