Mildly Oxidized LDL Induces Expression of Group IIa Secretory Phospholipase A ₂ in Human Monocyte–Derived Macrophages

Abstract

Abstract-Phospholipase A 2 s (PLA 2 s) constitute a family of enzymes that hydrolyze fatty acids of membrane phospholipids, thus initiating the synthesis of proinflammatory mediators. Various PLA 2 s have been detected in human atherosclerotic arteries (advanced lesions); however, only the secretory group of PLA 2 has been shown to specifically hydrolyze low density lipoprotein (LDL)-associated phospholipids and, as such, may play a potential role in atherogenesis. In the present study, we investigated the expression pattern of group IIa, IV, and V PLA 2 s in human macrophages, which are the key cells involved in the onset and perpetuation of atherosclerosis. Immunohistochemical staining by double labeling showed that the secretory nonpancreatic PLA 2 (snpPLA 2 ) is detectable in macrophages in the intima of early atherosclerotic lesions. Reverse transcription-polymerase chain reaction analysis of RNA extracted from human monocytes clearly showed that expression of group IV PLA 2 was enhanced during differentiation into macrophages, with an onset of induction at days 2 to 3 of differentiation. Group V snpPLA 2 was constitutively expressed on differentiation, whereas the detection of group IIa snpPLA 2 was dependent on both differentiation and subsequent stimulation of macrophages. Indeed, the transcription of group IIa snpPLA 2 in macrophages was induced by treatment with minimally modified or mildly oxidized LDL, whereas native, extensively oxidized, or acetylated LDL had no effect. To our knowledge, this is the first report describing induction of group IIa snpPLA 2 expression in human monocyte-derived macrophages. The mRNA levels of cytosolic PLA 2 group IV and snpPLA 2 group V remained unchanged on LDL treatment. Thus, our results show that the expression of distinct PLA 2 enzymes is regulated not only during differentiation of monocytes into macrophages but also on exposure of macrophages to distinct LDL species. Consequently, our results indicate a potential role for both cytosolic and secretory PLA 2 enzymes in inflammation and in macrophage functions related to atherosclerosis, with a specific role for group IIa snpPLA2 in LDL scavenging.

Oxidized LDL activates phospholipase A2 to supply fatty acids required for cholesterol esterification

“…However, oxLDL fails to stimulate the expression of cPLA 2 mRNA in human monocyte-derived macrophages (44). This inability of oxLDL is consistent with the present result that oxLDL did not affect amounts of cPLA 2 proteins (Fig.…”

“…It has been reported that group IIA and group X secretory PLA 2 s are markedly expressed in the arterial intima or media of human and mouse atherosclerotic lesions (40)(41)(42), and contribute to the facilitation of uptake of LDL by macrophages to form foam cells through the modification (hydrolysis) of LDL (42,43). Furthermore, oxLDL has been shown to accelerate the expression of group IIA secretory PLA 2 in human monocytederived macrophages (44). In the present study, we showed that the oxLDL-induced formation of cholesteryl oleate was insensitive to manoalide, a secretory PLA 2 inhibitor, in RAW264.7 macrophages.…”

Functional Coupling between Secretory and Cytosolic Phospholipase A2 Modulates Tumor Necrosis Factor-α- and Interleukin-1β-induced NF-κB Activation

“…cDNAs encoding human PLA 2 enzymes were amplified from 3-5 l of the cDNA reaction mixture using specific gene primers. PCR reactions for group IIa, IV, V, ␤-actin, and 5-LO were performed as previously described (44,45). For group IId, VI, and X PLA 2 , the primers synthesized according to known human cDNA sequences were the following: 5Ј-GTTCCTCAG-CATGGAGCTC-3Ј (forward primer) and 5Ј-TCCGAGACTATATTG-GAGG-3Ј (reverse primer) for group IId; 5Ј-TGACAATTCTCAGGT-GCTGC-3Ј (forward primer) and 5Ј-TCTTTCCAGGGAGAAGGGAT-3Ј (reverse primer) for group VI; and 5Ј-GGTGCCTCCGGTGGCCCTG-CAG-3Ј (forward primer) and 5Ј-GTTCTCTGCCGGTCCGCACAG-GCTCTG-3Ј (reverse primer) for group X.…”

Biomarkers of Vascular Disease Linking Inflammation to Endothelial Activation

“…63 Secretion of sPLA 2 is induced by proinflammatory compounds within the atherosclerotic environment such as IL-1, IL-6, TNF-␣, interferon-␥, and oxLDL, enhancing plaque formation. 63,64 Plaque formation is further enhanced via the role of sPLA 2 in increasing intracellular lipid accumulation within macrophages leading to foam-cell formation. 63 sPLA 2 activity has been implicated in promoting monocyte chemoattractant protein-1 secretion by monocytes as well as inducing the surface display of functional CD40 on monocytic cells.…”