Rapid regulation of the activity of the low density lipoprotein receptor of cultured human fibroblasts.

Regulation of low density lipoprotein (LDL) receptor activity of cultured human skin fibroblasts was studied by measuring 4 degrees C binding of 125I-LDL after cells were incubated at 37 degrees C with medium containing varying lipoprotein and serum compositions. When cells grown on medium containing 10% human whole serum were exposed to medium containing 10% human lipoprotein-deficient serum, LDL receptor activity increased within 4 h and then decreased between 12 and 24 h. This early, transient increase in binding was inhibited by cycloheximide, suggesting that new protein synthesis was involved. Changing the medium also produced a drop in cell total cholesterol content within the first 4 h, suggesting that cholesterol efflux initiated the rapid increase in LDL receptor activity. Cells incubated for 4 h with medium designed to promote cholesterol flux into the cell (plus whole serum or LDL) or minimize efflux (no serum or lipoprotein-"free" medium) had lower LDL receptor activities than cells incubated for 4 h with medium that appeared to promote cholesterol efflux (plus lipoprotein-deficient serum or high density lipoproteins3). The promotion of cholesterol efflux and acute activation of the LDL receptor by lipoprotein-deficient serum appear to be saturable processes. The direct addition of fresh lipoprotein-deficient serum to the medium partially reversed the secondary decrease in LDL receptor activity that followed the initial acute increase. Frequent medium changes enhanced the long term rate of activation of the receptor, depleted the cell of cholesterol, and increased the rate of sterol synthesis. These results suggest that regulation of the LDL receptor activity is a potentially rapid process than can respond acutely to changes in the rate of cholesterol flux into or out of the cell.