Foam cell

Foam cells are a type of cells that contain cholesterol. These can form a plaque that can lead to atherosclerosis and trigger heart attacks and stroke. Foam cells are a type of cells that contain cholesterol. These can form a plaque that can lead to atherosclerosis and trigger heart attacks and stroke. Foam cells are fat-laden M2 macrophages containing low density lipoproteins (LDL). They can only be truly detected by examining a fatty plaque under a microscope after it is removed from the body. They are named because the lipoproteins give the cell a foamy appearance. Despite the connection with cardiovascular diseases they are not inherently dangerous. Foam cell formation is triggered by a number of factors including the uncontrolled uptake of modified low density lipoproteins (LDL), the upregulation of cholesterol esterification and the impairment of mechanisms associated with cholesterol release. Foam cells are formed when circulating monocyte-derived cells are recruited to the atherosclerotic lesion site or fat deposits in the blood vessel walls. Recruitment is facilitated by the molecules P-selectin and E-selectin, intercellular adhesion molecule 1 (ICAM-1) and vascular cell adhesion molecule 1 (VCAM-1). Monocytes are then able to penetrate the arterial wall as a result of impaired endothelial integrity which increases permeability. Once in the sub endothelium space, inflammation processes induce the differentiation of monocytes into mature macrophages. Macrophages are then able to internalize modified lipoproteins like βVLDL (beta very low density lipoprotein), AcLDL (acetylated low density lipoprotein) and OxLDL (oxidized low density lipoprotein) through their binding to the scavenger receptors (SRs) such as CD36 and SR-A on the macrophage surface. These scavenger receptors act as 'Pattern recognition receptors' (PRR's) on macrophages and are responsible for recognizing and binding to oxLDL, which in turn promotes the formation of foam cells through internalization of these lipoproteins. Coated-pit endocytosis, phagocytosis and pinocytosis are also responsible for lipoprotein internalization.Once internalized, scavenged lipoproteins are transported to endosomes or lysosomes for degradation, whereby the cholesteryl esters (CE) are hydrolyzed to unesterified free cholesterol (FC) by lysosomal acid lipase (LPL). Free cholesterol is transported to the endoplasmic reticulum where it is re-esterified by ACAT1 (acyl-CoA: cholesterol acyltransferase 1) and subsequently stored as cytoplasmic liquid droplets. These droplets are responsible for the foamy appearance of the macrophage and thus the name of foam cells. At this point, foam cells can either be degraded though the de-esterification and secretion of cholesterol, or can further promote foam cell development and plaque formation – a process that is dependent on the balance of free cholesterol and esterified cholesterol. Low-density lipoprotein (LDL) cholesterol (LDL-C — also known as “bad” cholesterol) and particularly modified forms of LDL cholesterol such as oxidized, glycated, or acetylated LDL, is contained by a foam cell - a marker of atherosclerosis. The uptake of LDL-C alone does not cause foam cell formation; however, the co-internalization of LDL-C with modified LDL in macrophages can result in foam cell development. Modified LDL affects the intracellular trafficking and metabolism of native LDL, such that not all LDL need to be modified for foam cell formation when LDL levels are high. The maintenance of foam cells and the subsequent progression of plaque build-up is caused by the secretion of chemokines and cytokines from macrophages and foam cells. Foam cells secrete pro-inflammatory cytokines such as interleukins: IL-1, IL-6; tumour necrosis factor (TNF); chemokines: chemokines ligand 2, CCL5, CXC-chemokine ligand 1 (CXCL1); as well as macrophage retention factors. Macrophages within the atherosclerotic legion area have a decreased ability to migrate, which further promotes plaque formation as they are able to secrete cytokines, chemokines, reactive oxygen species (ROS) and growth factors that stimulate modified lipoprotein uptake and vascular smooth muscle cell (VSMC) proliferation. VSMC can also accumulate cholesteryl esters. In chronic hyperlipidemia, lipoproteins aggregate within the intima of blood vessels and become oxidized by the action of oxygen free radicals generated either by macrophages or endothelial cells. The macrophages engulf oxidized low-density lipoproteins (LDLs) by endocytosis via scavenger receptors, which are distinct from LDL receptors. The oxidized LDL accumulates in the macrophages and other phagocytes, which are then known as foam cells. Foam cells form the fatty streaks of the plaques of atheroma in the tunica intima of arteries. Foam cells are not dangerous as such, but can become a problem when they accumulate at particular foci thus creating a necrotic centre of atherosclerosis. If the fibrous cap that prevents the necrotic centre from spilling into the lumen of a vessel ruptures, a thrombus can form which can lead to emboli occluding smaller vessels. The occlusion of small vessels results in ischemia, and contributes to stroke and myocardial infarction, two of the leading causes of cardiovascular-related death.