Inhibition of squalene synthase but not squalene cyclase prevents mevalonate-mediated suppression of 3-hydroxy-3-methylglutaryl coenzyme A reductase synthesis at a posttranscriptional level.
Previously, we found that mevalonate-derived products together with an oxysterol regulated reductase synthesis at a posttranscriptional level. To determine which products were responsible for this regulation, either the squalene synthase inhibitor zaragozic acid A or the squalene cyclase inhibitor 4,4,10-beta-trimethyl-trans-decal-3beta-ol (TMD) was added to lovastatin-treated Syrian hamster cells in conjunction with mevalonate. Mevalonate alone decreased reductase synthesis 50% compared with lovastatin-treated cells. In contrast, when both zaragozic acid A and mevalonate were added to lovastatin-treated cells, there was no change in reductase synthesis. With either treatment, reductase mRNA levels did not change compared with lovastatin-treated cells. When both 25-hydroxycholesterol and mevalonate were added to lovastatin-treated cells, reductase synthesis and mRNA levels were decreased 95 and 50%, respectively. The 10-fold difference between changes in reductase synthesis and mRNA levels under these conditions reflects a specific effect of mevalonate-derived isoprenoids on reductase synthesis at the translational level. In contrast, coincubation of cells with mevalonate plus 25-hydroxycholesterol in the presence of zaragozic acid decreased reductase synthesis and mRNA levels 60 and 50%, respectively, compared with lovastatin-treated cells. Moreover, degradation of reductase was increased approximately 7-fold in cells treated with mevalonate alone but only 3-fold in cells treated with mevalonate and zaragozic acid A. These results indicate that isoprenoid products between mevalonate and squalene affect reductase at a posttranslational level by increasing degradation but do not regulate reductase synthesis at a posttranscriptional level. In contrast, when both TMD and mevalonate were added to lovastatin-treated cells, reductase synthesis was decreased approximately 50% with no corresponding decrease in reductase mRNA levels, similar to mevalonate only. Reductase degradation was increased approximately 7-fold under these conditions. Cellular incubation in TMD, mevalonate, and 25-hydroxycholesterol decreased reductase synthesis and mRNA levels 95 and 50%, respectively. From these results we concluded that mevalonate-derived nonsterols synthesized between squalene and lanosterol decrease reductase synthesis at a translational level-either alone or in combination with 25-hydroxycholesterol-and also increase reductase degradation.
Peffley
D M
DM
Department of Pharmacology and Molecular Biology, Finch University of Health Sciences/The Chicago Medical School, Illinois 60064, USA. peffleyd@mis.finchcms.edu
Gayen
A K
AK
D000818:Animals; D019086:Bridged Bicyclo Compounds, Heterocyclic / Q000494:pharmacology; D002460:Cell Line; D006224:Cricetinae; D004791:Enzyme Inhibitors / Q000494:pharmacology; D013186:Farnesyl-Diphosphate Farnesyltransferase / Q000037:antagonists & inhibitors*; D006888:Hydroxycholesterols / Q000494:pharmacology; D006903:Hydroxymethylglutaryl CoA Reductases / Q000096:biosynthesis* / Q000235:genetics / Q000378:metabolism; D008148:Lovastatin / Q000494:pharmacology; D008190:Lyases / Q000037:antagonists & inhibitors*; D008647:Mesocricetus; D008798:Mevalonic Acid / Q000494:pharmacology*; D009284:Naphthols / Q000494:pharmacology; D014176:Protein Biosynthesis; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D014158:Transcription, Genetic; D014233:Tricarboxylic Acids / Q000494:pharmacology
D019086:Bridged Bicyclo Compounds, Heterocyclic; D004791:Enzyme Inhibitors; D006888:Hydroxycholesterols; D009284:Naphthols; D012333:RNA, Messenger; D014233:Tricarboxylic Acids; C075117:squalestatin 1; C022178:4,4,10-trimethyl-trans-decal-3-ol; C007997:25-hydroxycholesterol; D008148:Lovastatin; D006903:Hydroxymethylglutaryl CoA Reductases; D013186:Farnesyl-Diphosphate Farnesyltransferase; D008190:Lyases; C022651:squalene cyclase; D008798:Mevalonic Acid