The responsiveness of low density lipoprotein (LDL) binding and uptake was measured in a human hepatoblastoma cell line, Hep G2. These cells exhibited both saturable, high-affinity and nonsaturable low-affinity components similar to that described for LDL binding in other mammalian cells. In addition, receptor-mediated uptake of ['2"I]LDL was dependent on the presence of Ca++ and required intact lysine residues in LDL as evidenced by abolition of binding following reductive methylation of LDL. The receptor activity was rapidly responsive to changes made in LDL concentrations in the medium, up-regulating by 200% in the absence of LDL, and downregulating by 56% in the presence of LDL, both changes occuring within 2 hr. Cholesterol synthesis was estimated by measuring 3-hydroxymethylglutaryl coenzyme A reductase activity. The activity of this enzyme was also found to increase rapidly in the absence of LDL and decrease rapidly in the presence of LDL in the media. The human cell line Hep G2 possesses an active and responsive receptor for LDL and provides a useful model for the study of liproprotein uptake and metabolism in intact human liver.
In human serum, low density lipoprotein (LDL) is the major transport vehicle for cholesterol, and its elevation in man is one of the principal risk factors for the development of atherosclerotic vascular disease. The protein component of LDL, apoprotein B, comprises a portion of the polar surface of the structure (1) and is responsible for recognition (2-4) of the particle by specific receptors, both extrahepatic (5) and hepatic (6). The activity of hepatic LDL receptors and the clearance of serum LDL in animals by hepatic LDL receptors (7,8) suggests that the liver plays an important role in the regulation of serum LDL levels by maintaining a balance between exogenous and endogenous pools of cholesterol.
In human fibroblasts, recognition and binding of LDL is followed by internalization within membrane-limited vesicles by receptor-mediated endocytosis (4). In these