Low Density Lipoprotein Receptor-Binding Activity in Human Tissues: Quantitative Importance of Hepatic Receptors and Evidence for Regulation of Their Expression in vivo

The heparin-sensitive binding of125I-labeled low-density lipoprotein (LDL) to homogenates from 18 different normal human tissues and some solid tumors was determined. The binding to adrenal and liver homogenates fulfilled criteria established for the binding of LDL to its receptor--namely, (i) saturability, (ii) sensitivity to proteolytic destruction, (iii) inhibition by EDTA, and (iv) heat sensitivity. When the binding of125I-labeled LDL was assayed at a constant concentration (50μg/ml), the adrenal gland and the ovary had the highest binding per g of tissue overall was obtained in homogenates of a gastric carcinoma and a parotid adenoma. When the weights of the parenchymatous organs were considered, the major amount of LDL receptors was contained in the liver. To study the possible regulation of hepatic LDL-receptor expression. 11 patients were pretreated with cholestyramine (8 g twice a day for 3 weeks). Increased binding activity (+105%,$P < 0.001$) was obtained in homogenates from liver biopsies from the cholestyramine-treated patients as compared with 12 untreated controls. It is concluded that the liver is the most important organ for LDL catabolism in humans and that the receptor activity in this organ can be regulated upon pharmacologic intervention. Further studies are needed to confirm the possibility that certain solid tumors can exhibit high numbers of LDL receptors.