Cell-released substances interfere with low density lipoprotein receptor determination in fibroblasts

Low density lipoprotein (LDL) receptor activity in fibroblasts decreases with increasing cell concentration. This was found for radioligand assays at 4 degrees C and 37 degrees C. Thus, it seems that it is the binding of 125I-LDL to the LDL receptor which is the step in the LDL receptor pathway that is influenced by differences in cell concentration. The reduced binding of 125I-LDL with increasing cell concentration seems to result from a modification of 125I-LDL by fibroblasts. Binding of 125I-LDL to cell-free plastic tubes was used to study this modification of 125I-LDL. Such studies indicated that direct cell contact was not required for the modification to occur. The rapid modification of 125I-LDL (within 10 min) took place even at 4 degrees C. These findings suggest that substances released from the cell membrane interact with LDL and decrease its binding to the LDL receptor. The higher the cell concentration is, the more modification of LDL takes place. Fibroblast modified LDL has reduced electrophoretic mobility. We conclude that differences in cell concentration influence LDL receptor activity by interfering with the binding of 125I-LDL to the LDL receptors, rather than affecting the receptor number on the cells' surfaces.