The effects of subfractions of high density lipoprotein on cholesterol efflux from cultured fibroblasts. Regulation of low density lipoprotein receptor activity.

When cultured human fibroblasts were incubated with medium of varying composition, the activity of the low density lipoprotein (LDL) receptor was strongly correlated with the amount of cholesterol that egressed from the cell. Particles isolated from lipoprotein-deficient serum (d > 1.25) by anti-apoprotein (apo) A-I affinity chromatography were as effective as the d > 1.25 serum in activating the LDL receptor, suggesting that the acceptors for cellular cholesterol in human lipoprotein-deficient serum are apo A-I-containing particles, Incubations with high density lipoprotein (HDL) subfractions designated as “very high” density lipoprotein (VHDL, d = 1.21-1.25) and HDL3 (d = 1.100-1.21) enhanced LDL receptor activity while incubations with HDL, (d = 1.063-1.100) had a slight inhibitory effect. Based on apo A-I content, VHDL was a more potent activator than HDL,. Incubations with HDL3 also enhanced the rate of sterol synthesis, inhibited cholesterol esterification, and decreased cell cholesterol, while incubations with HDL had the opposite effect. When apo E was removed from HDL by heparin-Sepharose affinity chromatography, HDLz retained its ability to activate the LDL receptor while HDLz was still inhibitory. Addition of chloroquine and compactin to the medium, to block supply of cholesterol to the cell, resulted in cell death between 12 and 24 h of incubation with VHDL and HDL,, but not with HDL,. With blocking agents present, incubations with VHDL, but not HDLz, decreased the cellular free cholesterol content within 12 h. These results suggest that net efflux occurs only in the presence of HDL particles from d > 1.100 serum fractions. The greater the density of the apo AI-containing particles, the greater the ability to remove cholesterol from the cells.