On day 0, CHO-7 cells were set up for experiments in medium A with 5% lipoprotein-deficient serum (LPDS) at a density of 2.5 x 105 cells/60-mm dish. On day 2, the medium was switched to fresh medium containing 5 µM sodium compactin and 50 µM sodium mevalonate. On day 3, the medium was switched to medium B containing either 10% LPDS or 10% fetal calf serum (FCS), 50 µM compactin, 50 µM mevalonate, and various concentrations of the indicated compound and then incubated for either 7 hr (A) or 6 hr (B) as described below. (A) Cholesterol esterification. After a 5 hr incubation with the above medium with 10% FCS and the indicated compounds, each cell monolayer was labeled for 2 hr with 0.2 mM sodium [14C]oleate (8133 dpm/nmol). The cells were then harvested for measurement of their content of cholesteryl [14C]oleate and [14C]triglycerides. Each value is the average of duplicate incubations. Values for [14C]triglyceride content in cells treated with 1 µM of U18666A, compound A, and U-X were 117, 116, and 106 nmol/hr/ mg protein, respectively. (B) SREBP-2 processing. After a 5 hr incubation with the above medium containing either 10% LPDS (lane 1) or 10% FCS (lanes 2–11), each monolayer received a direct addition of 20 µg/ml of N-acetyl-leu-leu-norleucinal (A.G. Scientific, San Diego, CA). After 1 hr, cells were harvested and fractionated into a nuclear extract and 105g membrane fraction (Sakai et al., 1996). Aliquots (30 and 10 μg protein for SREBP-2 and Niemann-Pick C1 [NPC1], respectively) were subjected to sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). Immunoblot analysis and image scanning were carried out with monoclonal antibodies directed against SREBP-2 or NPC1 as described in Materials and methods. (C) 125I-LDL degradation. On day 3, the medium was switched to medium B containing 5% human LPDS, 20 µg protein/ml of either 125I-LDL (for 125I-LDL degradation) or unlabeled LDL (for cholesterol esterification), 10 µM compactin, and 50 µM mevalonate in the presence of one of the following compounds: none, 0.3 µM U18666A, 0.3 µM U-X, and 50 µM chloroquine. For the 125I-LDL degradation assay, cells were incubated for 6 hr with 125I-LDL (48 cpm/ng protein), after which the medium from each monolayer was removed and its content of 125I-monoiodotyrosine was measured as previously described (Goldstein, 1983). The 100% control value for 125I-LDL degradation in the absence of any compound (none) was 4.1 µg/6 hr/mg of protein. The cholesterol esterification assay was carried out as in A except that the cells were pulse-labeled with 0.1 mM [14C]oleate (6515 dpm/nmol). The 100% control value for cholesteryl [14C]oleate formed was 5.9 nmol/hr/mg protein. The content of [14C]triglycerides in cells receiving the various compounds were not significantly different: 50, 57, 55, and 81 nmol/hr/mg protein, respectively, for no addition, U18666A, U-X, and chloroquine. All values are the mean of triplicate incubations with individual values shown.