The serum-free spent medium of lipopolysaccharide-activated rabbit peritoneal macrophages contains a proteinaceous factor that stimulates the synthesis of PGE2 in rabbit articular chondrocytes. Synthesis of this factor by macrophages is inhibited by cycloheximide. Stimulation of PGE2 in chondrocytes is detected after a four-hour exposure to the macrophage factor and is completely abolished by the addition of either cycloheximide or indomethacin to the chondrocyte cultures. The macrophage derived factor has an apparent molecular weight of 30,000, is heat stable and not inactivated upon reductive alkylation or on treatment with phenylglyoxal. Activity is partially destroyed upon treatment with acid (pH 2.0) and upon trypsin treatment.
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