High density lipoprotein binding by rat Fu5AH hepatoma cells is not related to cholesterol content

The binding of 125I-HDL obtained from nephrotic rats (HDLne) containing only apo A-I and apo C, to rat hepatoma cells (Fu5AH) grown to confluency was studied under conditions which increased the free cholesterol or the cholesteryl ester content. The high affinity binding (Kd = 5 nM) measured at 4 degrees C was unchanged. This transformed cell line also exhibited greater specificity for rat HDL compared to human HDL than has been reported for other types of cultured cells. When the cells were allowed to internalize and degrade HDLne at 37 degrees C, the acid-soluble products were derived almost entirely from the breakdown of apo A-I. Competition experiments with an LDL fraction from nephrotic rat plasma (LDLne) which contained 20% of apo A-I indicated that it was as effective as other rat HDL preparations in competing for the binding of HDLne at 4 degrees C, based on its content of apo A-I. Control experiments indicated that labeled apo A-I in HDLne exchanged less than 1% when incubated with a 50-fold excess of unlabeled LDLne for 2 h at 4 degrees C. These results point to a critical role of cell type in HDL binding. They support the view that apo A-I is a ligand. The up-regulation of high affinity HDL binding by cholesterol which has been reported with cultured human fibroblasts and Hep G2 cells does not occur in the Fu5AH rat hepatoma cell line.