Induction of Murine Macrophage Growth by Oxidized Low Density Lipoprotein Is Mediated by Granulocyte Macrophage Colony-stimulating Factor

Abstract

We have examined whether certain secreted factor(s) is involved in oxidized low density lipoprotein (Ox-LDL)-induced murine macrophage growth. An antibody against granulocyte-macrophage colony-stimulating factor (GM-CSF) effectively inhibited Ox-LDL-induced macrophage growth by >80%. Ox-LDL as well as phospholipase A 2 -treated acetylated LDL enhanced mRNA levels and protein release of GM-CSF from macrophages, while neither acetylated LDL nor lysophosphatidylcholine (lyso-PC) showed such effects. The maximal induction of GM-CSF by Ox-LDL was noted at 4 h, followed by a time-dependent decrease to a basal level within 24 h. Ox-LDL-induced macrophage growth was inhibited by 75% by replacement of the culture medium at 24 h by a fresh medium containing the same concentration of Ox-LDL, when GM-CSF had already returned to the basal level. Thus, a cytokine(s) other than GM-CSF is also expected to participate in Ox-LDL-induced macrophage growth in a later phase. The Ox-LDL-induced GM-CSF release was inhibited by calphostin C, a protein kinase C inhibitor, and was significantly reduced in macrophages from the knockout mice lacking class A, type I and type II macrophage scavenger receptors (MSR-AI/AII). These results taken together indicate that effective endocytosis of lyso-PC of Ox-LDL by macrophages through MSR-AI/AII and subsequent protein kinase C activation have led to GM-CSF release into the medium which may play a priming role in conjunction with other cytokines in Ox-LDL-induced macrophage growth.

Macrophage Scavenger Receptor Class A

“…67 Some studies have also reported a modifying effect of the activation response to LPS by oxLDL through SRA. 71,72 How these processes are exactly intertwined is not yet clear.…”

“…This GM-CSF secretion was strongly reduced in SRA-deficient macrophages. 67 In the atherosclerotic lesion, this GM-CSF production in response to oxLDL may play a very important role in priming macrophage growth, in conjunction with other cytokines. In addition to this proliferative role for the SRA, more recently, SRA expression was shown to confer resistance of macrophages to apoptosis.…”

Glucocorticoid Inhibits Oxidized LDL-Induced Macrophage Growth by Suppressing the Expression of Granulocyte/Macrophage Colony-Stimulating Factor

“…These results suggested that inhibition of GM-CSF production by glucocorticoids might cause inhibition of Ox-LDL-induced macrophage growth. Our previous report 15 and the present study demonstrated that Ox-LDL could induce an increase in GM-CSF mRNA level and protein release. Moreover, our subsequent preliminary experiments using luciferase assay in which we examined the promoter activity of GM-CSF gene showed that Ox-LDL activated reporter gene expression, suggesting that Ox-LDL could enhance GM-CSF mRNA expression at transcription level.…”

“…The concentration of GM-CSF was determined by a commercially available GM-CSFspecific ELISA kit (sensitivity, 5 pg/mL, Amersham) using recombinant murine GM-CSF as a standard. 15 …”

“…12 Moreover, our biochemical studies showed that scavenger receptor-mediated endocytic internalization of lysophosphatidylcholine (lyso-PC), a major modified lipid moiety of Ox-LDL, is important for Ox-LDL-induced macrophage growth 13,14 and that Ox-LDL-induced granulocyte/ macrophage colony-stimulating factor (GM-CSF) production plays an important role in macrophage growth. 15 Atherosclerosis is viewed as a chronic inflammatory process because one of its features is accumulation of inflammatory cells and cytokines. 1 Moreover, administration of an anti-inflammatory agent, dexamethasone, suppresses the development of atherosclerosis in various experimental animal models, including Watanabe heritable hyperlipidemic rabbits, 16 cholesterol-fed rabbits, 17 cuff-induced intimal thickening of the rabbit carotid artery, 18 and rat balloon angioplasty model.…”