Chinese hamster ovary cell mutant with defective down-regulation of low density lipoprotein receptors

A monensin-resistant clone (Mon(r)-31) shows a related series of differences from its parental Chinese hamster ovary (CHO) cell line in the cellular response to several ligands. The uptake and metabolism of low density lipoprotein (LDL) in the mutant cells are defective. Accumulation of fluorescent-labeled LDL as well as internalization and degradation of ¹²⁵I-LDL are greatly reduced in Mon(r)-31 cells. The receptor number for LDL on the cell surface of Mon(r)-31 is about one-third that for CHO cells, but affinity constants for both cell lines are similar. Electrophoretic analysis shows a slightly reduced molecular weight of LDL receptor in Mon(r)-31 cells in comparison to that in CHO cells. The internalization index (internalization plus degradation per binding) of LDL of the mutant is about one-half that of CHO cells, suggesting a failure of internalization of LDL as well as LDL binding. Hybrids (hyb-1, -2, and -3) between CHO and Mon(r)-31 cells show LDL binding and LDL internalization activities comparable to that of CHO cells, suggesting that the altered LDL response in Mon(r)-31 cells is recessive. Addition of exogenous LDL to culture medium down-regulates the LDL receptor activity of CHO, hyb-2, and hyb-3 cells, whereas no such down-regulation is seen in Mon(r)-31 cells. Probably as a result of the failure of down-regulation, the prominent inhibition of sterol synthesis from acetate and 3-hydroxy-3-methylglutaryl-coenzyme A reductase observed in CHO cells is scarcely detectable in Mon(r)-31 cells. As a correlated result, sterol synthesis from acetate is 6-fold higher in the mutant. The failure of down-regulation of LDL receptors in Mon(r)-31 cells is discussed in relation to the altered binding and internalization of LDL.