Atherosclerosis (AS) is an inflammatory disease characterized by the infiltration of immune cells into the arterial wall and lipid deposition, which is the main pathological basis of cardiovascular and cerebrovascular diseases [22]. Macrophages are the main component of AS plaque, and the formation of macrophage foam cells rich in cholesterol esters is an important sign of early AS [27]. There are mainly two types of polarized macrophage phenotypes in AS plaques of humans and mice, i.e. classically activated M1 macrophages and alternatively activated M2 macrophages. M1 macrophages secrete inflammatory factors to promote the formation of foam cells and the growth of arterial plaque [1]. M2 macrophages inhibit inflammation, enhance cholesterol trafficking and plaque stability to promote plaque regression [36]. Thus, polarizing macrophage towards M2 phenotype and simultaneously reducing the proportion of M1 macrophages in plaques are essential to the effective prevention and therapeutic treatment of atherosclerosis.

The macrophages in AS plaques mainly come from circulating monocytes produced in bone marrow [46]. Murine monocytes can be divided into two phenotypes: Ly6Chi inflammatory monocytes (Gr1+CCR2+CX3CR1lo) and Ly6Clo resident monocytes (Gr-1-CCR2-CX3CR1hi) based on different chemokine receptors, corresponding to human CD14hiCD16- and CD14+CD16+ monocytes, respectively [16]. These subsets are characterized by distinct migratory and inflammatory properties. The classical Ly6Chi monocytes can efficiently infiltrate inflammatory sites, dominate monocytosis and be recruited into progressing plaques in ApoE–/– mice [40]. During atherosclerosis progression, Ly6Chi inflammatory monocytes constantly migrate from bone marrow into the circulating blood and then move into arterial plaques. A high-fat diet can induce a 14-fold increase in Ly6Chi inflammatory monocytes in the circulating blood of ApoE-/- mice. These cells in the atherosclerotic lesions are thought to become M1 macrophages in inflammatory sites [40]. In contrast, the nonclassical Ly6Clo monocytes patrol blood vessels and also accumulate at inflammatory sites, where they are thought to give rise to M2 macrophages [21]. Therefore, exploring key molecules that regulate the differentiation of monocytes into M2 macrophages may have great significance for the prevention and control of AS.

Cell metabolism plays a crucial role in macrophage polarization [48]. Metabolic reprogramming of macrophages is not only crucial for energy homeostasis, but also directly affects the polarization fate of M1/M2 macrophages (Y [4,19,24,28]; S [44]). During the process of cell differentiation, metabolic changes are required to meet the needs of cell differentiation (Z [15,34,43]). Therefore, identifying the key molecules involved in metabolic regulation during the differentiation of monocytes into macrophages is also informative for elucidating the immune metabolic pathway of monocyte differentiation into macrophages.

AMP-activated protein kinase (AMPK) is a heterotrimeric protein kinase, which is composed of catalytic subunit α (α 1 or α 2), regulatory subunits β (β 1 or β 2) and γ (γ 1, γ 2, or γ 3) [13]. AMPK is a central regulator of multiple metabolic pathways and may have therapeutic implication for obesity [11], insulin resistance [10], type 2 diabetes (T2D) [39], non-alcoholic fatty liver disease (NAFLD)(Q [20]), and cardiovascular disease (CVD) [8]. AMPKα1 can regulate macrophage polarization toward M2 phenotype in the skeletal muscle regeneration mouse model [30], tissue damage mouse model after unilateral ureteral obstruction [29], and obesity related insulin resistance mouse model [2] with unknown mechanisms. In addition, AMPKα1 can promote M2 polarization of obesity-related adipose tissue macrophages by regulating Sirtuin 1 (SIRT1) [9]. Consistently, AMPK activation induces the polarization of macrophages to M2 state and inhibits inflammation in ApoE-/- mice [25]. However, whether AMPKα1 can regulate monocyte differentiation phenotype is still unknown.

In this study, we explored the role of AMPKα1 in monocyte-to-macrophage differentiation during atherosclerosis and found that AMPKα1 can enhance the decomposition of cholesterol esters by lysosomal acid lipase, which may represent a key mechanism to promote FAO and inflammatory monocytes differentiation towards M2 phenotype to prevent atherosclerosis.