Activity and Regulation of Low Density Lipoprotein Receptors in a Human Hepatoblastoma Cell Line†

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Abstract
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 [125I]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 down-regulating 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.