Signaling Pathways Potentially Responsible for Foam Cell Formation: Cholesterol Accumulation or Inflammatory Response-What is First?

Alexander N Orekhov,Ichiro Manabe,Philip Stegmaier,Yumiko Oishi,Igor A Sobenin,Alexander Kel,Vsevolod Y Makeev,Sergey Pintus,Marina V Kubekina,Artem S Kasianov,Vasily N Sukhorukov,Daria Stelmashenko,Wei-Kai Wu,Kathy K Foxx,Nikita G Nikiforov,Anastasia V Poznyak

doi:10.3390/ijms21082716

Abstract

Accumulation of lipid-laden (foam) cells in the arterial wall is known to be the earliest step in the pathogenesis of atherosclerosis. There is almost no doubt that atherogenic modified low-density lipoproteins (LDL) are the main sources of accumulating lipids in foam cells. Atherogenic modified LDL are taken up by arterial cells, such as macrophages, pericytes, and smooth muscle cells in an unregulated manner bypassing the LDL receptor. The present study was conducted to reveal possible common mechanisms in the interaction of macrophages with associates of modified LDL and non-lipid latex particles of a similar size. To determine regulatory pathways that are potentially responsible for cholesterol accumulation in human macrophages after the exposure to naturally occurring atherogenic or artificially modified LDL, we used transcriptome analysis. Previous studies of our group demonstrated that any type of LDL modification facilitates the self-association of lipoprotein particles. The size of such self-associates hinders their interaction with a specific LDL receptor. As a result, self-associates are taken up by nonspecific phagocytosis bypassing the LDL receptor. That is why we used latex beads as a stimulator of macrophage phagocytotic activity. We revealed at least 12 signaling pathways that were regulated by the interaction of macrophages with the multiple-modified atherogenic naturally occurring LDL and with latex beads in a similar manner. Therefore, modified LDL was shown to stimulate phagocytosis through the upregulation of certain genes. We have identified at least three genes (F2RL1, EIF2AK3, and IL15) encoding inflammatory molecules and associated with signaling pathways that were upregulated in response to the interaction of modified LDL with macrophages. Knockdown of two of these genes, EIF2AK3 and IL15, completely suppressed cholesterol accumulation in macrophages. Correspondingly, the upregulation of EIF2AK3 and IL15 promoted cholesterol accumulation. These data confirmed our hypothesis of the following chain of events in atherosclerosis: LDL particles undergo atherogenic modification; this is accompanied by the formation of self-associates; large LDL associates stimulate phagocytosis; as a result of phagocytosis stimulation, pro-inflammatory molecules are secreted; these molecules cause or at least contribute to the accumulation of intracellular cholesterol. This chain of events may explain the relationship between cholesterol accumulation and inflammation. The primary sequence of events in this chain is related to inflammatory response rather than cholesterol accumulation.

Highlights

The accumulation of lipid-laden cells in the arterial wall is the earliest step in the pathogenesis of atherosclerosis [1,2,3]
High-throughput sequencing of mRNA obtained from the cells treated with naturally occurring, modified low-density lipoproteins (LDL), or latex beads was performed on 21 samples from 7 groups, as shown in Figure 1
We have shown that not less than 12 signaling pathways are likewise regulated by the interaction of macrophages with the multiply modified atherogenic naturally occurring LDL and with latex beads

Summary

Introduction

The accumulation of lipid-laden (foam) cells in the arterial wall is the earliest step in the pathogenesis of atherosclerosis [1,2,3]. It is generally accepted that the main sources of accumulating lipids (mostly cholesterol and cholesteryl esters) in foam cells are atherogenic modified low-density lipoproteins (LDL). The mechanism of interaction of native (unmodified) LDL with a specific cell receptor is currently well known [5,6,7,8]. This interaction does not lead to excessive deposition of intracellular lipids since the lipid components of LDL are utilized by cells, and the excess is removed. Native LDL is not atherogenic because it does not induce the formation of foam cells

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