Low-density lipoprotein (LDL) as a transporter for 2,2',4,4',5,5'-hexabromobiphenyl (HBB) into the cell.

Abstract

This study investigated whether HBB could associate with LDL and, thereby, be carried into the cells. Wild type (K1) and a mutant strain (ldlA-7) of CHO cells which lacked functional receptors for LDL, were used to conduct kinetic studies in vitro designed to determine whether ($\sp{14}$C) HBB associated with LDL reconstituted with ($\sp3$H) cholesteryl linoleate, could be transported into the cell and, if so, to investigate the mechanism involved. LDL, reconstituted with a fluorescent probe, labeled K1 cells but not ldlA-7 cells indicating that K1 cells had functional receptors but ldlA-7 did not. Kinetic data confirmed this conclusion. The shapes of the kinetic curves for the entry of radiolabeled LDL showed that the rate of entry into the mutant cell was linear and slow. On the other hand, the entry of the LDL into K1 cells was initially rapid, ceased for a short period--presumably while the receptors were recycled from inside to the surface of the cell--and then resumed at a rapid rate. While free ($\sp{14}$C) HBB appeared to enter both cell types by diffusion, the rate of entry was reduced by about 75% when HBB was associated with LDL. This reduction in rate of entry and the observation that the ratio of cellular $\sp3$H and $\sp{14}$C remained constant as the amount of each isotope increased supported the view that ($\sp{14}$C) HBB and ($\sp3$H) LDL, when associated, entered both cell types at the same rate--a rate that was 4 times greater than the rate of entry of LDL alone into K1 cells and 15 times greater than the entry into the mutant cells. It was postulated that the increase in the rate of entry of LDL when associated with HBB was a function of an increase in hydrophobicity or charge. The findings that LDL associated with HBB entered both cell types at the same rate and that monensin, which blocks the recycling of LDL receptors in the cell, did not inhibit the entry of the LDL-HBB complex in K1 cells, suggested that the major route of entry for the complex occurred via a non-receptor pathway. (Abstract shortened with permission of author.).