INVOLVEMENT OF CALCIUM AND ARACHIDONATE METABOLISM IN ACETYLATED-LOW-DENSITY-LIPOPROTEIN-STIMULATED TUMOR-NECROSIS-FACTOR-ALPHA PRODUCTION BY RAT PERITONEAL-MACROPHAGES

We show that lipopolysaccharide-free actetylated low-density lipoprote in (LDL), but not native LDL, stimulates tumor-necrosis factor-alpha ( TNF-alpha) secretion by rat peritoneal macrophages and the signal-tran sduction pathways involved. The role of the scavenger receptor (SR) in this response was suggested by the absence of an effect induced by na tive 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]. Acetyla ted LDL induces rapid Ca2+ release from inositol-phosphate sensitive C a2+ stores mediated by pertussis-sensitive G proteins and a sustained Ca2+ rise mediated by Ca2+ influx and by Ca2+ release from ryanodine-s ensitive Ca2+ stores. Acetylated LDL-induced Ca2+ influx and TNF-alpha production were abolished by inhibitors of phospholipase C (U73122) a nd phospholipase A(2) (bromophenacyl bromide), but were not affected b y an inhibitor of protein kinase C (calphostine C). Therefore, Ca2+ in flux induced by acetylated LDL is dependent on Ca2+ store depletion. A rachidonate released by acetylated LDL acts as a second messenger to a ctivate TNF-alpha secretion via Ca2+ influx. While the Ca2+ signal was not modified by an inhibitor of protein tyrosine kinases (PTK; herbim ycin A): this inhibitor completely blocked TNF-alpha production, sugge sting the involvement of PTK downstream of the Ca2+ signal. These resu lts suggest that a sustained elevation of intracellular Ca2+, mediated through Ca2+ influx via the phospholipase-A(2)-dependent pathway, is essential for induction of TNF-alpha secretion. The type of SR class i nvolved in these pathways remains to be identified.