Autophagy Is Differentially Regulated in Leukocyte and Nonleukocyte Foam Cells During Atherosclerosis

Abstract

Rationale: Atherosclerosis is characterized by an accumulation of foam cells within the arterial wall, resulting from excess cholesterol uptake and buildup of cytosolic lipid droplets (LDs). Autophagy promotes LD clearance by freeing stored cholesterol for efflux, a process that has been shown to be atheroprotective. While the role of autophagy in LD catabolism has been studied in macrophage-derived foam cells, this has remained unexplored in vascular smooth muscle cell (VSMC)-derived foam cells that constitute a large fraction of foam cells within atherosclerotic lesions. Objective: We performed a comparative analysis of autophagy flux in lipid-rich aortic intimal populations to determine whether VSMC-derived foam cells metabolize LDs similarly to their macrophage counterparts. Methods and Results: Atherosclerosis was induced in GFP-LC3 transgenic mice by PCSK9 (proprotein convertase subtilisin/kexin type 9)-adeno-associated viral injection and Western diet feeding. Using flow cytometry of aortic digests, we observed a significant increase in dysfunctional autophagy of VSMC-derived foam cells during atherogenesis relative to macrophage-derived foam cells. Using cell culture models of lipid-loaded VSMC and macrophage, we show that autophagy-mediated cholesterol efflux from VSMC foam cells was poor relative to macrophage foam cells, and largely occurs when HDL (high-density lipoprotein) is used as a cholesterol acceptor, as opposed to apoA-1 (apolipoproteinA-1). This was associated with the predominant expression of ABCG1 in VSMC foam cells. Using metformin, an autophagy activator, cholesterol efflux to HDL was significantly increased in VSMC, but not in macrophage, foam cells. Conclusions: These data demonstrate that VSMC and macrophage foam cells perform cholesterol efflux by distinct mechanisms, and that autophagy flux is highly impaired in VSMC foam cells, but can be induced by pharmacological means. Further investigation is warranted into targeting autophagy specifically in VSMC foam cells, the predominant foam cell subtype of advanced atherosclerotic plaques, to promote reverse cholesterol transport and resolution of the atherosclerotic plaque.

Smooth Muscle Cell—Macrophage Interactions Leading to Foam Cell Formation in Atherosclerosis: Location, Location, Location

“…In general, staining for macrophages including macrophage foam cells indicates the majority of these cells reside primarily in the immediate subendothelial region throughout human atherosclerosis but at later stages can migrate somewhat deeper into the upper-mid intima region where they are in immediate proximity with additional SMCs in the intima. Accurate identification of macrophages requires use of markers that are not known to be expressed by SMCs, such as CD45 ( Allahverdian et al, 2014 ; Wang et al, 2019 ; Robichaud et al, 2022 ). Recent studies indicate that markers previously thought to be macrophage-specific, such as CD68, are expressed by a high percentage of intimal SMCs in both human and mouse lesions ( Allahverdian et al, 2014 ; Feil et al, 2014 ), and should no longer be used to identify macrophages specifically in lesions.…”

Autophagy in Atherosclerosis: Not All Foam Cells Are Created Equal

“…10 In this regard, attempts at parsing the differences in autophagy/lipophagy dysfunction between plaque SMCs and macrophages are important given SMCs appear to contribute the majority of foam cells in humans 6 and now in more than 1 mouse model of atherosclerosis. 3,8 To discriminate SMC and macrophage populations in the plaque with regard to both autophagosome formation and lysosome acidity/function, Robichaud et al 8 took advantage of the often-used GFP-LC3 mouse model together with LysoTracker staining. They observed clear differences between these populations isolated from lesions, which they further characterized in vitro using cultured cells of each type.…”

“…10 In this regard, attempts at parsing the differences in autophagy/lipophagy dysfunction between plaque SMCs and macrophages are important given SMCs appear to contribute the majority of foam cells in humans 6 and now in more than 1 mouse model of atherosclerosis. 3,8…”

“…Understanding differences and similarities in lipid uptake, storage, and efflux between plaque SMCs and macrophages will be critical in determining how cholesterol metabolic dysfunction occurs in these cells and how novel therapeutics can be leveraged to mitigate atherosclerosis. In the current issue of Circulation Research , Robichaud et al 8 contribute to this phenotyping by assessing the relative impairment in autophagy of SMCs and macrophages and whether this might provide further clues into mobilizing cholesterol from foam cells in atherosclerotic plaque. The importance of autophagy dysfunction in foam cell formation has been increasingly recognized in the last decade, first described in plaque macrophages.…”

Editorial: The Roles of Lipids in Immunometabolism: The Crosstalk Between Lipid Metabolisms and Inflammation

“…Chronic HDL treatment protected cardiac myocytes by reducing autophagy and inflammation. Autophagy is critical for lipid metabolism in both immune and non-immune cells during atherosclerosis ( 20 ) and it is a process coupling extracellular stress signals, cellular lipid handling and sensing, and immune cell activation ( 21 ). Therefore, the study by Al-Jarallah and Babiker on chronic HDL effects on autophagy deserves further exploration.…”