Ammonium chloride causes reversible inhibition of low density lipoprotein receptor recycling and accelerates receptor degradation

The effects of the acidotropic agent, NH₄Cl, on the recycling and turnover of low density lipoprotein (LDL) receptors were analyzed in human skin fibroblasts using ligand binding assays, [³⁵S]methionine pulse-chase experiments, and electron microscopy. NH₄Cl did not prevent receptor internalization but caused a marked redistribution of LDL receptors to intracellular sites (endosomes) that was completely dependent on the presence of apolipoprotein-B- or -E-containing ligands. Maximal inhibition of recycling was observed at LDL concentrations that only partially saturated receptors, suggesting that the receptors function as oligomers. In contrast, full receptor occupancy by the multivalent, apolipoprotein-E-containing β-very low density lipoprotein was required for the same effect. The intracellular accumulation was reversible and the majority of receptors returned to the cell surface when NH₄Cl was removed after short treatments. The rate of degradation of LDL receptors was greatly accelerated in the presence of NH₄Cl and ligand, with a t_1/2 of about 2 h (approximately 6 times faster than receptor degradation in the absence of NH₄Cl). Neither the redistribution nor the accelerated loss of immunoprecipitable LDL receptors was observed in an LDL receptor internalization-defective mutant cell line. We conclude that NH₄Cl inhibited the recycling specifically of occupied receptors, thereby accelerating their degradation, probably in endosomes.