MACROPHAGE COLONY STIMULATING FACTOR ACTIVATES PHOSPHATIDYLCHOLINE HYDROLYSIS BY CYTOPLASMIC PHOSPHOLIPASE-A(2)

The macrophage colony stimulating factor (M-CSF) is required for the proliferation and differentiation of monocytes. Previous studies have demonstrated that M-CSF stimulation is associated with phosphatidylcholine (PC) hydrolysis and increased formation of both diacylglycerol (DAG) and phosphorylcholine. The present work extends those results by demonstrating that treatment of human monocytes with M-CSF is associated with increases in a cytoplasmic Ca2+-dependent activity which hydrolyzes 1-palmitoyl,2-arachidonoyl PC to arachidonic acid. The finding that this hydrolysis of PC is associated with increases in production of lysophosphatidylcholine indicates that M-CSF stimulates a cytoplasmic phospholipase A2 (cPLA2) activity. These results are supported by the demonstration that M-CSF induces cPLA2 gene expression. M-CSF-induced increases in cPLA2 mRNA levels were biphasic and corresponded with rapid (30-60 min) and delayed (24-72 h) increases in cPLA2 activity. The results demonstrate that this effect of M-CSF on cPLA2 expression is controlled at least in part by post-transcriptional stabilization of cPLA2 transcripts. The finding that M-CSF treatment is also associated with phosphorylation of the cPLA2 protein further suggests that expression of this enzyme is regulated at multiple levels. Finally, the stimulation of cPLA2 activity and arachidonate release is supported by increases in prostaglandin (PG) synthesis. In this regard, levels of both PGE2 and PGF2alpha were increased in response to M-CSF. Taken together, these results indicate that M-CSF stimulates PC hydrolysis in human monocytes by inducing cPLA2 activity and thereby formation of eicosanoids.