TOCOTRIENOLS REGULATE CHOLESTEROL PRODUCTION IN MAMMALIAN-CELLS BY POSTTRANSCRIPTIONAL SUPPRESSION OF 3-HYDROXY-3-METHYLGLUTARYL-COENZYME-AREDUCTASE

Tocotrienols are natural farnesylated analogues of tocopherols which d ecrease hepatic cholesterol production and reduce plasma cholesterol l evels in animals. For several cultured cell types, incubation with gam ma-to-cotrienol inhibited the rate of [C-14]acetate but not [H-3] meva lonate incorporation into cholesterol in a concentration- and time-dep endent manner, with 50% inhibition at approximately 2 muM and maximum approximately 80% inhibition observed within 6 h in HepG2 cells. 3-Hyd roxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase total activity an d prot-in levels assayed by Western blot were reduced concomitantly wi th the decrease in cholesterol synthesis. In HepG2 cells, gamma-tocotr ienol suppressed reductase despite strong blockade by inhibitors at se veral steps in the pathway, suggesting that isoprenoid flux is not req uired for the regulatory effect. HMG-CoA reductase protein synthesis r ate 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 [S-35]methionine pulse-chase/immunoprecipitation analysis of HepG2 cells treated with 10 muM gamma-tocotrienol. Under these conditions, the decrease in reductase protein levels greatly exc eeded the minor decrease in mRNA (23 versus 76% of control, respective ly), and the low density lipoprotein receptor protein was augmented. I n contrast, 25-hydroxycholesterol strongly cosuppressed HMG-CoA reduct ase protein and mRNA levels and the low density lipoprotein receptor p rotein. Thus, tocotrienols influence the mevalonate pathway in mammali an cells by post-transcriptional suppression of HMG-CoA reductase, and appear to specifically modulate the intracellular mechanism for contr olled degradation of the reductase protein, an activity that mirrors t he actions of the putative non-sterol isoprenoid regulators derived fr om mevalonate.