ABSTRACT
Although the cellular function of group IVC phospholipase A(2) (IVC-PLA(2)) remains to be understood, the expression of IVC-PLA(2) in human monocytic THP-1 cells was increased during phorbol ester-induced macrophage differentiation. We therefore examined the role of IVC-PLA(2) in macrophage differentiation using THP-1 cells. Two THP-1 cell lines stably expressing IVC-PLA(2)-specific shRNA were established. Differentiation and maturation into macrophages on treatment with phorbol ester were facilitated by knockdown of IVC-PLA(2) without the compensatory induction of mRNA expression for other group IV and VI PLA(2)s. Furthermore, the enhancement of macrophage differentiation by IVC-PLA(2)-knockdown were abolished by treatment with lysophosphatidylcholine, a metabolite of phospholipids generated by PLA(2)-mediated hydrolysis, indicating that PLA(2) activity is necessary for the inhibition of macrophage differentiation by IVC-PLA(2). Additionally, we found that the differentiation into classically activated M1 macrophage was superior in IVC-PLA(2)-knockdown cells, whereas the differentiation into alternatively activated M2 macrophage was suppressed by IVC-PLA(2)-knockdown. These findings suggest that IVC-PLA(2) is involved in regulations of macrophage differentiation and macrophage polarization.
Subject(s)
Cell Differentiation , Cell Polarity , Group IV Phospholipases A2/metabolism , Macrophages/physiology , Cell Adhesion , Cells, Cultured , Fatty Acids, Nonesterified/metabolism , Gene Knockdown Techniques , Group IV Phospholipases A2/genetics , Human Umbilical Vein Endothelial Cells/physiology , Humans , Lysophosphatidylcholines/metabolism , Macrophages/enzymology , Monocytes/physiology , RNA, Messenger/biosynthesis , RNA, Messenger/genetics , RNA, Small Interfering/geneticsABSTRACT
The accumulation of hepatocellular triacylglycerol (TG), a major symptom of fatty liver, is associated with the excessive incorporation of exogenous free fatty acids into hepatocytes, the free fatty acids inducing an increase in TG bearing acyl chains derived from not only themselves but also endogenous fatty acids. However, the mechanisms responsible for the supply of endogenous fatty acids, which are mainly esterified into phospholipids, remain unclear. In the present study, we examined the possible involvement of intracellular phospholipase A(2) (PLA(2))s including group IVA, IVC, VIA, and VIB PLA(2)s, which catalyze the release of endogenous fatty acids, in the deposition of TG in hepatocytes. Stimulation of human hepatoma Huh-7 cells with oleate or linoleate for 48 h increased TG contents time-dependently. Under the conditions, increased expression of group IVC PLA(2) mRNA and protein was observed at 6-12 h and 24-48 h after the stimulation, respectively. However, mRNA levels of group IVA, VIA, or VIB PLA(2) did not change. When cells were treated with methyl arachidonyl fluorophosphonate used as an inhibitor of group IVC PLA(2), the fatty acid-induced deposition of TG was partially but significantly suppressed at 48 h, although no significant inhibition was observed at 24 h. Overexpression of wild-type group IVC PLA(2) but not a catalytically inactive mutant of group IVC PLA(2) tended to increase cellular TG levels. The present findings suggest that stimulation of Huh-7 hepatocytes with free fatty acids induces the expression of group IVC PLA(2), which is involved in the fatty acid-induced deposition of TG.
