Addition of a mannose-6-phosphate-containing oligosaccharide alters cellular processing of low density lipoprotein by parental and LDL-receptor-defective Chinese hamster ovary cells.

Low density lipoprotein (LDL) was chemically modified by the addition of omega-(6-phospho)-tetra(alpha 1-3)mannosyl-(alpha 1-2)mannose (M56P), a phosphorylated oligosaccharide containing a terminal mannose 6-phosphate residue. Uptake and degradation of this modified LDL (M56P-LDL) by Chinese hamster ovary (CHO) cells occurred via the lysosomal enzyme (mannose 6-phosphate) receptor pathway. Cellular processing of M56P-LDL was saturable, specific for the mannose 6-phosphate marker, and occurred with approximately threefold higher affinity than that of native LDL by the LDL receptor pathway. Mannose 6-phosphate receptor activity, as measured by degradation of M56P-LDL, was ninefold lower than the LDL receptor activity. Degradation of M56P-LDL was more sensitive to inhibition by the lysosomotropic agent chloroquine than was degradation of LDL, suggesting differences in the intracellular processing of mannose 6-phosphate-bearing ligands and LDL. Previously isolated CHO cell lines defective in LDL receptor activity resembled parental CHO cells in their ability to process M56P-LDL. The potential use of M56P-LDL in the isolation of cells with pleiotropic mutations affecting receptor-mediated endocytosis is discussed.