Defective endocytosis of low-density lipoprotein in monensin-resistant mutants of the mouse Balb/3T3 cell line

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Abstract
Two monensin‐resistant clones show similar low‐density lipoprotein binding activity but less internalization or degradation of low‐density lipoerotein than the parental Balb/3T3 or other resistant clone. Sterol synthesis from radioactive acetate in the resistant mutant, MO‐5, is inhibited by more than 70% of control in the presence of tenfold higher amounts of low‐density lipoprotein than the dose that inhibits the parental Balb/3T3 to similar level. 3‐Hydroxy‐3‐methylglutaryl coenzyme A reductase activity of Balb/3T3 and MO‐5 is inhibited by 48% and 27% of control, respectively, in the presence of 10 μg/ml of low‐density lipoprotein. Colloidal silica gradient centrifugation shows that transport of low‐density lipoprotein from the surface membrane to the lysosome is much slower in MO‐5 cells than in Balb/3T3 cells. Down regulation of low‐density lipoprotein receptors on the cell surface in Balb/3T3 is observed by exposing the cells to 5–15 μg/ml low‐density lipoprotein, whereas only slight if any down regulation is observed when MO‐5 cells are treated with low‐density lipoprotein. The altered endocytosis of low‐density lipoprotein behaves as a dominant trait in hybrids of MO‐5 and THO2‐2, a derivative of Balb/3T3 resistant to both ouabain and 6‐thioguanine.

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