The phrase “scavenger receptor’’ was coined to describe the activity of macrophages that mediates the uptake of modified LDL in cell culture. These receptors recognize a broad range of ligands, hence the term “scavenger receptor (SR)”. However, the overlapping binding behavior between the receptor classes can make it challenging to characterize their respective behaviors in terms of ligand uptake. For example, most scavenger receptors bind an array of polyanionic ligands.
A number of different scavenger receptors for oxLDL have been identified and the receptors are grouped into classes A through G, according to their structure. These include SR-AI/II/III, macrophage receptor with collagenous structure (MARCO), SR-BI, CD36, SR-C, CD68, LOX-1, and SR-PSOX. The significance of some of these oxLDL receptors has not been fully quantified in vivo; however SR-A and CD36 have been established to be central to atherogenesis. Kunjathoor et al. demonstrated, through the use of transgenic mice lacking both SR-A and CD36 scavenger receptors that together, the receptors accounted for nearly 90% of total oxLDL uptake by macrophages. In addition, Nakagawa-Toyama and colleagues completed an investigation of CD36 and SR-A ex
SR-A is principally expressed on macrophages, although the receptors have also been identified on endothelial and smooth muscle cells. Class A receptors can bind modified LDL (both acLDL and oxLDL), polynucleic acids, bacteria and bacteria fragments, and certain carbohydrate-based ligands. Early assessments on the ability of SR-A deficient macrophages to take in oxLDL implied that the receptor was responsible for at least 50% of OxLDL uptake. Despite this partial prevention of oxLDL uptake by macrophages, SR-A knockout mice show a considerable decrease in atherosclerotic lesions in the apoE–/– and LDLR–/– mouse models.
The SR-A structure consists five distinct segments totaling 451 amino acid residues: a cysteine-rich C-terminal region, a collagen-like region, a segment of α-helical coiled-coils, a single transmembrane domain, and an N-terminal cytoplasmic domain. It has been reported that a cluster of four lysines within the collagen-like region is responsible for ligand recognition, as changing these basic amino acids to neutral residues eliminated ligand binding. This is especially noteworthy as SR-A is known to principally bind to anionic ligands. The cycle of SR-A begins when the receptor binds to oxLDL at the cell surface and the complexes (SR-A + oxLDL) cluster in vesicles and clathrin-coated pits before becoming internalized and delivered to the endosomes. Once in the endosomes, the acidic pH causes the receptor to release the ligand and the receptor is then likely recycled to the cell surface.
Scavenger receptor class B type I
Scavenger receptor class B type I (SR-BI) is a 509-amino acid transmembrane protein belonging to the class B scavenger receptor family. As with CD36 and lysosome membrane protein 2 (LIMP2), the other members of this family, SR-BI has a horseshoe-like structure with short N- and C-terminal cytoplasmic domains, two transmembrane domains and a large heavily N-glycosylated extracellular loop. The extracellular domain of SR-BI is predicted to contain a helical bundle which binds with ligands and a tunnel used for substrates delivery. SR-BI also has a splice variant, SR-BII, which differs from SR-BI in its carboxyl terminal and serves as another member of class B scavenger receptor family.
Like many other scavenger receptors, SR-BI can recognize a wide range of self- and foreign ligands. The self-originated ligands of SR-BI include modified low density lipoprotein (LDL), naïve LDL, high density lipoprotein (HDL), maleylated bovine serum albumin (BSA), anionic phospholipids and apoptotic cells. The non-self-ligands include lipopolysaccharide (LPS), lipoteichoic acid (LTA), gram-negative bacteria, gram-positive bacteria and soluble hepatitis c virus (HCV) E2 glycoprotein. SR-BI is most abundantly expressed in liver, adrenal gland and ovaries, which is closely related with its major functions. SR-BI is also expressed many other cell types, such as endothelial cells, epithelial cells, macrophages and lymphocytes. The wide range of ligands SR-BI can bind and the diverse tissues SR-BI is expressed underlie the multiple functions of SR-BI within the body.