3-Hydroxy-3-methylglutaryl coenzyme A reductase activity in cultured fibroblasts from patients with mevalonate kinase deficiency: Differential response to lipid supplied by fetal bovine serum in tissue culture medium

3-Hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity was measured in extracts of cultured fibroblasts derived from patients with mevalonate kinase deficiency (MKD). For six patients studied, the mean activity of 63.3 ± 41.1 pmol/min-mg protein (± 1 SD, range 37.7-146.2) was significantly higher than the mean value in three control fibroblast lines of 11.1 ± 3.5 (± 1 SD, rang 8.0-14.9). These values were obtained using cells subcultured in medium supplemented with 10% fetal bovine serum (FBS) 21 h prior to assay. When cells were deprived of cholesterol by subculturing for 21 h in delipidated FBS, the mean value for patient cells were deprived of cholesterol by subculturing for 21 h in delipidated FBS, he mean value for patient cells was increased to 230.8 ± 78.5 pmol/min-mg protein (range 13.9-333.8) as compared to 109.5 ± 47.1 (range 78.0-163.6) for controls. The activity of HMG-CoA synthase in extracts of fibroblasts derived from the patients was not elevated. The mevalonic acid concentration in the surrounding culture medium was assessed by stable isotope dilution assay. For five patients, the mean concentration in medium containing FBS was 0.92 ± 0.37 μm (± 1 SD, range 0.46-1.48) in contrast to 1.24 ± 0.83 μM (range 0.46-2.54) for cells subcultured in delipidated FBS. The mean value for three control fibroblast lines was 0.22 ± 0.12 μM (± 1 SD, range 0.11-0.35) for cells subcultured in FBS as compared to 0.01 ± 0.01 μM (range 0.0-0.01 μM) for cells subcultured in delipadated FBS. The activity of the cholisterol biosynthetic pathway, assessed by monitoring [¹⁴C]acetate incorporation into cholesterol in fibroblast monolayers for cells subcultured in both FBS and delipidated FBS, was comparable between patient and control cell lines despite the finding of virtually undetectable mevalonate kinase activity in fibroblast extracts of the patients. The absolute levels of control mevalonate kinase activity were comparable for cells subcultured in FBS or delipidated FBS. These data suggest that MKD cells compensate for diminished mevalonate kinase actitivity by regulating HMG-CoA reductase upward to increase intracellular concentration of mevalonic acid and permit the normal process of sterol synthesis.