Distribution of exogenous 25-hydroxycholesterol in Hep G2 cells between two different pools.

Binding of [26,27-(3)H]25-hydroxycholesterol (25HC) to human hepatoma Hep G2 cells was saturated within 120 min. Two intracellular pools of 25HC were identified in a pulse-chase experiment: (i) an exchangeable pool which was in dynamic equilibrium with 25HC in the medium (t(1/2) of reversible exchange 15 min) and (ii) an unexchangeable pool which remained in cells during incubation in medium containing LPDS. 25HC from the exchangeable pool inhibits cholesterol biosynthesis, decreases the HMG CoA reductase mRNA level and stimulates cholesterol acylation. 25HC from the unexchangeable pool was partially bound to cytosolic proteins and apparently utilized for metabolic transformation. Incubation of Hep G2 cells with [26,27-(3)H]25HC in the presence of a 30-fold molar excess of 3beta-hydroxy-5alpha-cholest-8(14)-en-15-one was found to cause (i) 2-fold decrease in the binding of [26,27-(3)H]25HC to cytosolic proteins (sedimentation constant of radioactive complex was 4-5 S) and (ii) the 35% inhibition of 25HC transformation to polar metabolites.