- Scavenger receptor
Scavenger receptors are a group of receptors that recognize modified
low density lipoprotein(LDL) by oxidation or acetylation. This naming is based on a function of cleaning (scavenging): scavenger receptors widely recognize and uptake macromolecules having a negative charge as well as modified LDL.
It is thought that scavenger receptors participate in the removal of many foreign substances and waste materials in the living body by extensive
ligandspecificity and a variety of receptor molecules.
In atherosclerotic lesions,
macrophages that express scavenger receptors on their plasma membrane aggressively uptake the oxidized LDL deposited in the blood vessel wall inside and become foam cells, and they secrete various inflammatory cytokines and accelerate the development of atherosclerosis.
Scavenger receptors are categorized into classes A, B, and C according to their structural characteristics.
* Class A is mainly expressed in the
macrophage, and a protein whose molecular weight is about 80 kDa makes a trimer; it is composed of 1) cytosoldomain, 2) transmembrane domain, 3) spacer domain, 4) alpha-helical coiled-coil domain, 5) collagen-like domain, and 6) cysteine-rich domain.
* Class B has two transmembrane regions.
* Class C is a transmembrane protein whose N-terminus is located extracellularly.
cavenger receptor class A
Scavenger receptors type 1 (SR-A1) and 2 (SR-A2) are trimers with a molecular weight of about 220-250 kDa (the molecular weight of monomeric protein is about 80 kDa). They preferentially bind modified
LDLby acetic acid and oxidized LDL. They have a collagen-like domain, which is essential for ligandbinding. SR-A1 scavenger receptors have a cysteine-rich domain, which can be found in a series of cell surface receptors and soluble proteins, but SR-A2 do not. Another scavenger receptor class A, MARCO, has collagen-like and cysteine-rich domains.
cavenger receptor class B
CD36and scavenger receptor class B1(SR-B1) are identified as oxidized LDL receptors and classified into class B. Both proteins have two transmembrane domains, and they are concentrated in a specific plasma membranemicrodomain, the caveolae. CD36 has been thought to be implicated in cell adhesion, in the phagocytosisof apoptotic cells, and in the metabolismof long-chain fatty acids. SR-B1 can interact not only with oxidized LDL but also with normal LDLand high density lipoproteins ( HDL). Recent studies have indicated that SR-B1 are involved in HDLmetabolism.
Some receptors that can bind to oxidized LDL have been discovered.
CD68and its mouse homologue, macrosialin, has a unique N-terminal mucin-like domain.
Mucinis a viscous substance (found in " natto" or okra) that is composed of a protein and polysaccharides binding it. A " Drosophila" class C scavenger receptor (dSR-C1) also has a mucin-like structure.
Lectin-like oxidized LDL receptor-1 ( LOX-1) was isolated from an aortic endothelial cell, and recently it has been discovered in macrophages and vascular smooth muscle cells in artery vessels. The expression of LOX-1 is inducted by inflammatory stimuli, so LOX-1 is thought to be involved in the development of atherosclerotic lesions.
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