|Detection of LDL R in HepG2 Human Cell Line by Flow Cytometry. HepG2 human hepatocellular carcinoma cell line was stained with Mouse Anti-Human LDL R PerCP‑conjugated Monoclonal Antibody (Catalog # FAB2148C, filled histogram) or isotype control antibody (Catalog # IC002C, open histogram). View our protocol for Staining Membrane-associated Proteins.|
The Low Density Lipoprotein Receptor (LDL R) is the founding member of the LDL R family of receptors (1, 2). This family contains transmembrane molecules that are characterized by the presence of EGF repeats, complement-like repeats, and YWTD motifs that form beta -propellers. Human LDL R is synthesized as an 860 amino acid (aa) type I transmembrane precursor that contains a 21 aa signal sequence, a 767 aa extracellular region, a 22 aa transmembrane segment and a 50 aa cytoplasmic tail (3). The extracellular region is complex. It consists of seven N-terminal complement-like cysteine-rich repeats that bind ligand. Cysteine residues in this region participate in intrachain disulfide bonds. This region is followed by three EGF-like repeats with a beta -propeller YWTD containing motif. The EGF-like repeats are responsible for ligand binding and dissociation. Finally, there is a 50 aa membrane proximal Ser/Thr-rich region that serves as a carbohydrate attachment point (1, 3, 4). There is extensive O-linked and modest N-linked glycosylation. Thus the receptor’s predicted molecular weight of 93 kDa is increased to a native molecular weight of 120-160 kDa (3, 4). Within the 50 aa cytoplasmic tail, there is an NPXY motif that links the receptor to clathrin pits (1, 2). The extracellular region of human LDL R shows 51% aa sequence identity to the extracellular region of human VLDL R, and 79% aa sequence identity to the extracellular region of mouse LDL R. LDL R is ubiquitously expressed, binds ApoB of LDL and ApoE of VLDL, and is responsible for clearing 70% of plasma LDL in liver (1, 5). It also is reported to bind to Factor VIII, PCSK9, and the human rhinovirus serotype 2 (1, 2, 7). Upon LDL particle binding to the hepatic LDL R, HMGCR expression is suppressed, reducing cholesterol biosynthesis; ACAT activity is increased, reducing toxic free cholesterol; and LDL R synthesis is curtailed, thereby reducing LDL uptake (1).