CD14 is a key mediator of both lysophosphatidic acid and lipopolysaccharide induction of foam cell formation

Dong An,Feng Hao,Fuqiang Zhang,Wei Kong,Jerold Chun,Xuemin Xu,Mei-Zhen Cui

doi:10.1074/jbc.m117.781807

Abstract

Macrophage uptake of oxidized low-density lipoprotein (oxLDL) plays an important role in foam cell formation and the pathogenesis of atherosclerosis. We report here that lysophosphatidic acid (LPA) enhances lipopolysaccharide (LPS)-induced oxLDL uptake in macrophages. Our data revealed that both LPA and LPS highly induce the CD14 expression at messenger RNA and protein levels in macrophages. The role of CD14, one component of the LPS receptor cluster, in LPA-induced biological functions has been unknown. We took several steps to examine the role of CD14 in LPA signaling pathways. Knockdown of CD14 expression nearly completely blocked LPA/LPS-induced oxLDL uptake in macrophages, demonstrating for the first time that CD14 is a key mediator responsible for both LPA- and LPS-induced oxLDL uptake/foam cell formation. To determine the molecular mechanism mediating CD14 function, we demonstrated that both LPA and LPS significantly induce the expression of scavenger receptor class A type I (SR-AI), which has been implicated in lipid uptake process, and depletion of CD14 levels blocked LPA/LPS-induced SR-AI expression. We further showed that the SR-AI–specific antibody, which quenches SR-AI function, blocked LPA- and LPS-induced foam cell formation. Thus, SR-AI is the downstream mediator of CD14 in regulating LPA-, LPS-, and LPA/LPS-induced foam cell formation. Taken together, our results provide the first experimental evidence that CD14 is a novel connecting molecule linking both LPA and LPS pathways and is a key mediator responsible for LPA/LPS-induced foam cell formation. The LPA/LPS–CD14–SR-AI nexus might be the new convergent pathway, contributing to the worsening of atherosclerosis.

Highlights

Macrophage uptake of oxidized low-density lipoprotein plays an important role in foam cell formation and the pathogenesis of atherosclerosis
To determine the molecular mechanism mediating CD14 function, we demonstrated that both Lysophosphatidic acid (LPA) and LPS significantly induce the expression of scavenger receptor class A type I (SR-AI), which has been implicated in lipid uptake process, and depletion of CD14 levels blocked LPA/LPS-induced SR-AI expression
Our results demonstrated that the depletion of CD14 largely abolished LPA, LPS, or LPA plus LPS–induced Dil-oxidized low-density lipoprotein (oxLDL) uptake in bone marrow– derived primary macrophages (BMMs) (Fig. 4A)

Summary

To whom correspondence should be addressed

Wide variety of living cells [1,2,3]. The biological effects of LPA are mediated by G protein-coupled receptors: LPA1–3, encoded by endothelial differentiation genes (Edg), and LPA4–6, encoded by a non-Edg family and classified as orphan G protein-coupled receptors belonging to the purinergic receptor family (4 –7). In early fatty streak lesions, macrophages gather lipoproteins and become lipid-loaded foam cells, playing a crucial role in the development of atherosclerosis. LPA was shown to induce oxidized low-density lipoprotein (oxLDL) uptake in the J774 macrophage cell line [19]; the underlying molecular mechanism is not well-understood. It has been shown that LPS induces macrophage-derived foam cell formation [26, 27]. In this study, using mouse bone marrow– derived primary macrophages (BMMs), we observed that LPA induces a specific and. CD14 –SR-AI mediates LPA- and LPS-induced oxLDL uptake in BMMs prominent induction of CD14 mRNA and protein expression and that LPA enhances LPS-induced CD14 expression. Our data revealed that the specific LPA receptor–CD14 – scavenger receptor axis mediates LPA- and LPS-induced oxLDL uptake in macrophages. The discovered new mechanism in foam cell formation in macrophages may contribute to atherogenesis

Results

Discussion

Experimental procedures