Fig. 1.

Acute effect of plasma membrane cholesterol concentration on HMGCoAR activity. To induce the expression of the enzyme, human fibroblasts were grown overnight in replicate flasks containing medium plus lipoprotein-deficient serum. Plasma membrane cholesterol was then reduced or increased by incubating the monolayers of cells (containing about 15 μgof cholesterol and 0.4 mg of protein) for 12 min at 37°C with 1 ml of hydroxypropyl-β-cyclodextrin (1.5–5.0 mg/ml) or complexes of hydroxypropyl-β-cyclodextrin (0.2–5 mg/ml) plus cholesterol (0–32 μg/ml). The modified cells were then incubated for 2 h (two experiments, filled symbols) or 4 h (three experiments, open symbols) and disrupted, and enzyme activity, protein, and cholesterol were assayed in duplicate. HMGCoAR activities, which are scaled to untreated controls (≈25 pmol of product per min per mg of protein), are plotted against cell cholesterol relative to untreated controls and fit to a sigmoid function.