Macrophage death in atherosclerosis: potential role in calcification

Abstract

Cell death is an important aspect of atherosclerotic plaque development. Insufficient efferocytosis of death cells by phagocytic macrophages leads to the buildup of a necrotic core that impacts stability of the plaque. Furthermore, in the presence of calcium and phosphate, apoptotic bodies resulting from death cells can act as nucleation sites for the formation of calcium phosphate crystals, mostly in the form of hydroxyapatite, which leads to calcification of the atherosclerotic plaque, further impacting plaque stability. Excessive uptake of cholesterol-loaded oxidized LDL particles by macrophages present in atherosclerotic plaques leads to foam cell formation, which not only reduces their efferocytosis capacity, but also can induce apoptosis in these cells. The resulting apoptotic bodies can contribute to calcification of the atherosclerotic plaque. Moreover, other forms of macrophage cell death, such as pyroptosis, necroptosis, parthanatos, and ferroptosis can also contribute by similar mechanisms to plaque calcification. This review focuses on macrophage death in atherosclerosis, and its potential role in calcification. Reducing macrophage cell death and/or increasing their efferocytosis capacity could be a novel therapeutic strategy to reduce the formation of a necrotic core and calcification and thereby improving atherosclerotic plaque stability.

Keywords: atherosclerosis; calcification; cardiovascular disease; cell death; macrophages; plaque stability.

Figure 1

Schematic summary of the role of macrophage death in calcification. Macrophage cholesterol loading by oxidized LDL (Ox-LDL) (foam cell formation) leads to ER stress, followed by activation of caspases leading to apoptosis and release of apoptotic bodies. Another pathway that can induce macrophage apoptosis, followed by release of apoptotic bodies, is the ferrylhemoglobin internalization by CD163. Furthermore, high mobility group box 1 (HMGB1), released by dying cells, can induce exosome secretion by macrophages through activation of neutral sphingomyelinase-2 (nSMase2) by the receptor for advanced glycation end products (RAGE)/p38 mitogen-activated protein kinase (p38MAPK) pathway. The resulting apoptotic bodies and exosomes, or other secreted extra-cellular vesicles (ECVs), such as microvesicles and microparticles, bind to collagen in the extracellular matrix and act as calcium binding sites for hydroxyapatite (HA) crystals. More specifically, an annexin V - phosphatidylserine (PS) - S100A9/MRP14 membrane complex on the surface of ECVs acts as a nucleation site for HA crystal formation. More specifically, the anionic PS serves as a binding site for cationic Ca²⁺. HA crystals can also bind to collagen directly through electrostatic interactions; cationic moieties on collagen interact with negatively charged groups on HA crystal surface. Inorganic phosphate (Pi), derived from pyrophosphate (PPi) through alkaline phosphatase (ALP), can be transported into the ECV by the Pi transporter 1 (PiT1), and calcium can be transported by Annexin V. This accumulation of calcium and Pi also leads to HA crystal formation inside the ECV. An increase in the presence of calcium and inorganic phosphate (Pi) decreases the content of calcification inhibitors, such as Fetuin A and MGP, in these ECVs, and increases the presence of S100A9 on their surface, leading to further stimulation of HA crystal formation. Together, these mechanisms allow macrophages to contribute directly to VC. Macrophage foam cells also have reduced efferocytosis, and these lipid-loaded stressed macrophages secrete osteogenic factors, such as TNF-α, IL-6, and oncostatin, that stimulate trans-differentiation of smooth muscle cells (SMC) to osteoblast-like cells, thereby also indirectly contributing to VC. Lastly, other forms of cell death, such as pyroptosis, necroptosis, parthanatos, and ferroptosis can also lead to similar induction of calcification.