The influence of cellular and lipoprotein cholesterol contents on the flux of cholesterol between fibroblasts and high density lipoprotein.

Abstract

Previous studies indicate that free cholesterol moves passively between high density lipoprotein (HDL) and cell plasma membranes by uncatalyzed diffusion of cholesterol molecules in the extracellular aqueous phase. By this mechanism, the rate constants for free cholesterol influx (Cli) and efflux (ke) should not be very sensitive to the free cholesterol content of cells or HDL. Thus, at a given HDL concentration, the unidirectional influx and efflux of cholesterol mass (Fi, Fe) should be proportional to the cholesterol content of HDL and cells, respectively, and net efflux of cholesterol mass (Fe-Fi greater than 0) should occur when either cells are enriched with cholesterol or HDL is depleted of cholesterol. We have examined the influence of cell and HDL free cholesterol contents on the bidirectional flux of free cholesterol between HDL and human fibroblasts and also attempted to detect some dependence of flux on the binding of HDL to the cells. In the range of HDL concentrations from 1 to 1000 micrograms of protein/ml, ke for cell free cholesterol approximately doubled for every 10-fold increase in HDL concentration, reaching 0.04 h-1 at 1000 micrograms of HDL/ml. ke and Cli were not influenced by the doubling of fibroblast free cholesterol content (from 31 +/- 5 to 62 +/- 13 micrograms of cholesterol/mg of protein). There was an approximate exchange of cholesterol between HDL and the unenriched fibroblasts (e.g. at [HDL] = 100 micrograms/ml, Fe and Fi = 3.2 and 3.0 micrograms of cholesterol/[4 h.mg of cell protein], respectively). In contrast, there was substantial net efflux from the enriched cells (at [HDL] = 100 micrograms/ml, Fe and Fi = 5.5 and 3.1 micrograms of cholesterol/[4 h.mg of cell protein], respectively). The rate constants for cholesterol flux were not influenced by changing the free cholesterol content of HDL, so that there was net efflux of cell cholesterol in the presence of cholesterol-depleted HDL and net influx from cholesterol-rich HDL. The Kd of HDL binding to fibroblasts was reduced from 1.7 to 0.9 micrograms/ml by the enrichment of the cells with free cholesterol; this increase in affinity for HDL was not reflected in enhanced rate constants for cholesterol flux. The inhibition of specific HDL binding by treatment of the lipoprotein with dimethyl suberimidate did not affect cholesterol flux using either control or cholesterol-rich cells at any HDL concentration in the range 1-1000 micrograms/ml. The above results are consistent with the concept that net movement of free cholesterol between cells and HDL occurs by passive, mass-action effects.(ABSTRACT TRUNCATED AT 400 WORDS)