Wensi Tao, Robert Moore, Yue Meng, Elizabeth R Smith, Xiang-Xi Xu

High serum cholesterol (hypercholesterolemia) strongly associates with cardiovascular diseases as the atherogenic low density lipoproteins (LDL) promote atheroma development in arteries (atherosclerosis). LDL clearance from the circulation by LDL receptor (LDLR)-mediated endocytosis by hepatic and peripheral tissues and subsequent feed back regulation of endogenous synthesis of cholesterol is a key determinant of serum LDL level. Human mutation analysis revealed that ARH, an LDLR endocytic adaptor, perturbs LDLR function and thus impacts serum cholesterol levels. In our genetic analysis of mutant mice, we found that deletion of another LDLR endocytic adaptor, Disabled-2 (Dab2), only slightly affected serum cholesterol levels. However, elimination of both arh and dab2 genes in mice resulted in profound hypercholesterolemia similar to that resulting from ldlr homozygous deletion. In the liver, Dab2 is expressed in sinusoid endothelial cells but not in hepatocytes. In the absence of both Dab2 and Arh, HMG-CoA reductase level increased to the level similar to that of ldlr knockout. Thus, in the absence of Arh, Dab2 in liver endothelial cells regulates cholesterol synthesis in hepatocytes. We conclude that the combination of Arh and Dab2 is responsible for the majority of adaptor function for LDLR endocytosis and LDLR-mediated cholesterol homeostasis.