INHIBITION OF PROTEIN-SYNTHESIS IN BABY-HAMSTER KIDNEY-CELLS BLOCKS OXYSTEROL-MEDIATED SUPPRESSION OF 3-HYDROXY-3-METHYLGLUTARYL-COA REDUCTASE MESSENGER-RNA AT A POSTTRANSCRIPTIONAL LEVEL

The effects of the protein-synthesis inhibitor cycloheximide on 25-hyd roxycholesterol-mediated suppression of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase mRNA levels were evaluated in the baby-hamster ki dney cell line C100. Cells cultured in medium supplemented with delipi dized fetal bovine serum and 25 muM lovastatin for 12-24- h had a 5-fo ld higher level of HMG-CoA reductase mRNA than cells grown in medium s upplemented with non-delipidized fetal bovine serum (FBS). The higher level was due to increased transcription, as determined by run-on assa ys with isolated nuclei. Addition of 25-hydroxycholesterol to lovastat in-treated cells lowered HMGCoA reductase mRNA levels within 4 h of tr eatment to those of cells grown in FBS-supplemented medium. This decre ase was due in part to a decrease in gene transcription. Cycloheximide added in conjunction with 25-hydroxycholesterol to lovastatin-treated cells blocked the suppression of mRNA levels, but did not block oxyst erol-mediated suppression of transcription. In addition, cycloheximide added to cells grown in FBS-supplemented medium rapidly increased mRN A levels by 10-fold relative to untreated cells, with no comparable in crease in transcription. No comparable increase in either the mRNA lev el or rate of transcription for beta-actin was observed under such con ditions. These results indicate that cycloheximide specifically stabil izes HMG-CoA reductase mRNA in the presence of oxysterols and suggests that continuous synthesis of a short lived protein regulator is requi red for oxysterol-mediated suppression of HMG-CoA reductase mRNA at a post-transcriptional level.