Tocotrienols regulate cholesterol production in mammalian cells by post-transcriptional suppression of 3-hydroxy-3-methylglutaryl-coenzyme A reductase.
Tocotrienols are natural farnesylated analogues of tocopherols which decrease hepatic cholesterol production and reduce plasma cholesterol levels in animals. For several cultured cell types, incubation with gamma-tocotrienol inhibited the rate of [14C]acetate but not [3H] mevalonate incorporation into cholesterol in a concentration- and time-dependent manner, with 50% inhibition at approximately 2 microM and maximum approximately 80% inhibition observed within 6 h in HepG2 cells. 3-Hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase total activity and protein levels assayed by Western blot were reduced concomitantly with the decrease in cholesterol synthesis. In HepG2 cells, gamma-tocotrienol suppressed reductase despite strong blockade by inhibitors at several steps in the pathway, suggesting that isoprenoid flux is not required for the regulatory effect. HMG-CoA reductase protein synthesis rate was moderately diminished (57% of control), while the degradation rate was increased 2.4-fold versus control (t1/2 declined from 3.73 to 1.59 h) as judged by [35S]methionine pulse-chase/immunoprecipitation analysis of HepG2 cells treated with 10 microM gamma-tocotrienol. Under these conditions, the decrease in reductase protein levels greatly exceeded the minor decrease in mRNA (23 versus 76% of control, respectively), and the low density lipoprotein receptor protein was augmented. In contrast, 25-hydroxycholesterol strongly cosuppressed HMG-CoA reductase protein and mRNA levels and the low density lipoprotein receptor protein. Thus, tocotrienols influence the mevalonate pathway in mammalian cells by post-transcriptional suppression of HMG-CoA reductase, and appear to specifically modulate the intracellular mechanism for controlled degradation of the reductase protein, an activity that mirrors the actions of the putative non-sterol isoprenoid regulators derived from mevalonate.
Parker
R A
RA
Department of Metabolic Diseases, Bristol-Myers Squibb Pharmaceutical Research Institute, Princeton, New Jersey 08543.
Pearce
B C
BC
Clark
R W
RW
Gordon
D A
DA
Wright
J J
JJ
D000085:Acetates / Q000378:metabolism; D019342:Acetic Acid; D000736:Androstenes / Q000494:pharmacology; D000818:Animals; D016466:CHO Cells; D002250:Carbon Radioisotopes; D006528:Carcinoma, Hepatocellular; D002460:Cell Line; D002645:Chickens; D002784:Cholesterol / Q000096:biosynthesis*; D002839:Chromans; D006224:Cricetinae; D015971:Gene Expression Regulation, Enzymologic / Q000187:drug effects; D006801:Humans; D006888:Hydroxycholesterols / Q000494:pharmacology; D006903:Hydroxymethylglutaryl CoA Reductases / Q000235:genetics / Q000378:metabolism*; D007654:Ketoconazole / Q000494:pharmacology; D007700:Kinetics; D008113:Liver Neoplasms; D008114:Liver Neoplasms, Experimental; D008148:Lovastatin / Q000494:pharmacology*; D008798:Mevalonic Acid / Q000378:metabolism; D012323:RNA Processing, Post-Transcriptional; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D013489:Suppression, Genetic / Q000187:drug effects*; D014316:Tritium; D014407:Tumor Cells, Cultured; D014810:Vitamin E / Q000031:analogs & derivatives* / Q000494:pharmacology
D000085:Acetates; D000736:Androstenes; D002250:Carbon Radioisotopes; D002839:Chromans; D006888:Hydroxycholesterols; D012333:RNA, Messenger; D014316:Tritium; D014810:Vitamin E; C006261:3-beta-(2-(diethylamino)ethoxy)androst-5-en-17-one; C013649:plastochromanol 8; C007997:25-hydroxycholesterol; D002784:Cholesterol; D008148:Lovastatin; D006903:Hydroxymethylglutaryl CoA Reductases; D019342:Acetic Acid; D007654:Ketoconazole; D008798:Mevalonic Acid