Mannose 6-phosphate receptor-mediated uptake of modified low density lipoprotein results in down regulation of hydroxymethylglutaryl-CoA reductase in normal and familial hypercholesterolemic fibroblasts.

Monophosphotetramannosyl-1-deoxymannitol-1-yl-low density lipoprotein (Man-6-P-LDL) was prepared by covalent attachment of the pentasaccharide omega-(6-phospho-tetra(alpha 1-3)mannosyl(alpha 1-2)mannose to amino groups on low density lipoprotein. Normal human fibroblasts were shown to specifically bind, internalize, and degrade 125I-labeled Man-6-P-LDL. Specificity for the mannose 6-phosphate (Man-6-P) receptor was demonstrated by competitive displacement with cold Man-6-P-LDL, Man-6-P, or mannose. No displacement was seen with cold LDL. Kd is estimated to be less than or equal to 2 X 10(-9) M. Degradation of 125I-labeled Man-6-P-LDL in familial hypercholesterolemic fibroblasts showed the same time course and specificity as observed in normal fibroblasts. Man-y-P-LDL was also able to deliver cholesterol to the cytosol where down regulation of the enzyme 3-hydroxy-3-methylglutaryl CoA reductase was observed in both normal and familial hypercholesterolemic fibroblasts. Down regulation could be blocked by Man-6-P in both cell lines. The possible uses of agents such as Man-6-P-LDL as research probes and therapeutic tools directed to specific cell types are discussed.