APOBR

APOBR Full Name

apolipoprotein B receptor

APOBR Introduction

APOBR (apolipoprotein B receptor, also known as APOB48R or APOB100R) is a macrophage receptor that specifically binds to apolipoprotein B48 (APOB), the structural protein of dietary triglyceride-rich lipoproteins (TRL) including chylomicrons. The gene is located on chromosome 16p12.1 and encodes a protein of 1,088 amino acids with a molecular weight of approximately 120 kDa. APOBR is primarily expressed by reticuloendothelial cells including monocytes, macrophages, and endothelial cells, with highest expression detected in peripheral blood leukocytes, bone marrow, spleen, lung, and placenta. The receptor plays a critical role in the clearance of dietary triglycerides and the delivery of essential lipids and lipid-soluble vitamins to reticuloendothelial cells. However, when plasma triglyceride levels are elevated, APOBR-mediated uptake of TRL can become pathological, leading to foam cell formation, endothelial dysfunction, and atherothrombogenesis, thereby contributing to the development of atherosclerosis and cardiovascular disease

Figure 1. Schematic structure of APOBR.

Gene Structure and Protein Architecture
The human APOBR gene spans approximately 4.3 kb and contains 4 exons. The full-length protein consists of 1,088 amino acids and shares no significant sequence homology with other known proteins, making it a unique member of the receptor family. APOBR is anchored to the cell membrane through an interaction with an integral membrane protein, rather than possessing its own transmembrane domain. Two major forms of the protein have been identified in macrophages: a 200 kDa membrane-binding protein (MBP 200) and a 235 kDa form (MBP 235), both of which can be reduced to a single active ligand-binding species with intermediate mobility (MBP 200R). The receptor functions as a homodimer, and this dimerization is essential for its ligand-binding activity. APOBR specifically recognizes apolipoprotein B48 on dietary triglyceride-rich lipoproteins and also binds to a similar domain of APOB present on hypertriglyceridemic very low density lipoprotein (HTG-VLDL). Importantly, APOBR mediates the rapid high-affinity uptake of chylomicrons, HTG-VLDL, and trypsinized VLDL devoid of apolipoprotein E (APOE) in macrophages, distinguishing it from other lipoprotein receptors that typically require APOE for ligand recognition.

Regulation and Tissue-Specific Expression
APOBR expression is tightly regulated at the transcriptional level by several factors. The receptor is expressed primarily by reticuloendothelial cells including monocytes, macrophages, and endothelial cells, with lower expression detected in peripheral blood leukocytes, bone marrow, spleen, lymph node, lung, placenta, kidney, liver, pancreas, and heart. APOBR expression is significantly suppressed by PPARA and PPARG, which are activated by fatty acids and synthetic ligands including fibrates and thiazolidinediones, respectively. This negative regulation provides a feedback mechanism linking lipid sensing to APOBR expression and foam cell formation. Conversely, inflammatory cytokines and oxidized lipids may upregulate APOBR expression, contributing to the pro-atherogenic environment. Tissue expression analysis shows that APOBR is also expressed in the colon, with lower levels in the appendix and thymus. The receptor is conserved across mammals, with orthologs identified in chimpanzee, rhesus monkey, dog, cow, mouse, rat, gorilla, and other species, indicating an evolutionarily conserved role in lipid metabolism. However, the precise signaling pathways downstream of APOBR activation and the full spectrum of its biological functions remain areas of active investigation.

Alternate Names for APOBR

APOBR; apolipoprotein B receptor; Apob48r; Apob-48r; apolipoprotein B48 receptor; apolipoprotein B-48 receptor; apolipoprotein B-100 receptor;