Abstract
Placenta expresses various lipase activities. However, a detailed characterization of the involved genes and proteins is lacking. In this study, we compared the expression of endothelial lipase (EL) and LPL in human term placenta. When placental protein extracts were separated by heparin-Sepharose affinity chromatography, the EL protein eluted as a single peak without detectable phospholipid or triglyceride (TG) lipase activity. The major portion of LPL protein eluted slightly after EL. This peak also had no lipase activity and most likely contained monomeric LPL. Fractions eluting at a higher NaCl concentration contained small amounts of LPL protein (most likely dimeric LPL) and had substantial TG lipase activity. In situ hybridization studies showed EL mRNA expression in syncytiotrophoblasts and endothelial cells and LPL mRNA in syncytiotrophoblasts. In contrast, immunohistochemistry showed EL and LPL protein associated with both cell types. In mouse placentas, lack of LPL expression resulted in increased EL mRNA expression. These results suggest that the cellular expression of EL and LPL in human placenta is different. Nevertheless, the two lipases might have overlapping functions in the mouse placenta. Our data also suggest that the major portions of both proteins are stored in an inactive form in human term placenta.
Highlights
IntroductionWe compared the expression of endothelial lipase (EL) and LPL in human term placenta
Endothelial cells and syncytiotrophoblasts were identified with immunohistochemistry using antibodies against CD34 and cytokeratin (Fig. 2C, D, H)
LPL mRNA was mainly associated with the trophoblast layer; we cannot exclude the possibility that there might be a low level of LPL expression in endothelial cells
Summary
We compared the expression of endothelial lipase (EL) and LPL in human term placenta. Albumin-bound FFAs probably can be transferred directly to the placental membrane-fatty acid binding protein, the release of FFAs from lipoprotein-TGs and phospholipids requires hydrolysis in the maternal circulation. Human placental tissue expresses TG lipase activities that are distinct from that of LPL, the cells and proteins involved have not been identified [4]. EL is expressed in vascular endothelial cells and mainly hydrolyzes phospholipids in HDL, but it has some TG lipase activity [11]. To improve the understanding of the putative roles of EL and LPL in placenta, we have characterized the expression of EL and LPL mRNA, protein, and lipase activities in human term placenta. By studying genetically modified mice, we tested the idea that placental EL expression is increased when LPL expression is eliminated
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