Macrophage colony stimulating factor activates phosphatidylcholine hydrolysis by cytoplasmic phospholipase A2.

Abstract

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 Ca(2+)‐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 PGF2 alpha 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.

Lysophosphatidylcholine Stimulates Activator Protein 1 and the c-Jun N-terminal Kinase Activity

“…Another possibility is that lyso-PC may enter cells and serve as an intracellular second messenger with JNK as a downstream target. This hypothesis has been strengthened by a series of studies demonstrating that lyso-PC was generated intracellularly through activation of phospholipase A 2 following stimulation of tyrosine kinase receptors by macrophage colony-stimulating factor (23), and stimulation of G-proteincoupled receptors by thrombin (24,25), bombesin (26), interferon-␥ (27), or angiotensin (28,29). The identification of potential intracellular effectors of lyso-PC should help in elucidating the mechanism(s) for the multiple biological functions of this phospholipid.…”

“…It has also been reported that lyso-PC significantly potentiated protein kinase C (PKC)-mediated cellular responses such as primary T-lymphocyte activation (20,21) and HL-60 cell differentiation into macrophages (22). Since actions of many extracellular agonists are associated with the activation of membrane phospholipase A 2 and the subsequent accumulation of lyso-PC (23)(24)(25)(26)(27)(28)(29), it is conceivable that lyso-PC may act as a second messenger, transducing signals elicited from membrane receptors. This is consistent with experiments showing that lyso-PC and phospholipase A 2 under certain circumstances have similar effects when incubated with cultured cells (21,22).…”

Characteristics of lysophospholipase activity expressed by cytosolic phospholipase A2

“…sion was also documented in certain cytokine-treated cells (Schalkwijk et al, 1992, Nakamura et al, 1992Lin et al, 1992b). It was reported previously that cPLA, prepared from the mouse macrophage-like cell line, RAW264.7, apparently exhibited both phospholipase A, and lysophospholipase activities (Leslie, 1991), although the activity of phospholipase A, was about 10% that of phospholipase A,.…”

Identification of Phosphorylation Sites of Human 85-kDa Cytosolic Phospholipase A2 Expressed in Insect Cells and Present in Human Monocytes

“…Phosphorylation of cPLA 2 in Human Monocytes-cPLA 2 plays a key role in releasing arachidonic acid for prostaglandin production in human monocytes, and previous studies revealed that monocyte cPLA 2 is regulated by phosphorylation (2,38). Therefore, monocytes were used to determine if similar sites were phosphorylated on cPLA 2 in a mammalian system, as identified on the cPLA 2 expressed in Sf9 cells.…”