title: Regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity in avian myeloblasts. Mode of action of 25-hydroxycholesterol.
abstract: 25-Hydroxycholesterol inhibits cholesterol biosynthesis by inhibiting the activity of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase. Addition of 25-hydroxycholesterol to chicken myeloblasts caused a rapid inhibition of HMG-CoA reductase activity, producing approximately an 80% decrease in enzyme activity after 60 min. The mode of action of 25-hydroxycholesterol was determined by immunoprecipitating radiolabeled enzyme from 25-hydroxycholesterol-treated myeloblasts. The decline in enzyme activity due to addition of 25-hydroxycholesterol was not associated with increased levels of [32P]PO4 incorporation into the immunoprecipitated reductase polypeptide (Mr = 94,000). Hence, 25-hydroxycholesterol did not appear to regulate reductase activity by enzyme phosphorylation, as observed for other modulators of HMG-CoA reductase. However, 25-hydroxycholesterol was shown to inhibit reductase activity by causing a 350% increase in the relative rate of reductase degradation and a 72% decrease in the relative rate of reductase synthesis. These alterations in the rates of degradation and synthesis occurred rapidly (within 10-30 min after addition of 25-hydroxycholesterol) and can account completely for the 25-hydroxycholesterol-induced inhibition of enzyme activity. The rapid decline in the rate of synthesis of HMG-CoA reductase in 25-hydroxycholesterol-treated cells was not associated with concomitant changes in the levels of reductase mRNA; therefore, suggesting that 25-hydroxycholesterol must inhibit the rate of reductase synthesis by translational regulation. We also present evidence that mRNA purified from chicken myeloblasts codes for two reductase polypeptides of Mr = 94,000 and 102,000.
mesh_terms: D000818:Animals; D002460:Cell Line; D002645:Chickens; D006888:Hydroxycholesterols / Q000494:pharmacology*; D006903:Hydroxymethylglutaryl CoA Reductases / Q000235:genetics* / Q000302:isolation & purification / Q000378:metabolism; D007700:Kinetics; D008970:Molecular Weight; D009132:Muscles / Q000201:enzymology*; D010766:Phosphorylation; D011061:Poly A / Q000235:genetics; D014176:Protein Biosynthesis; D012313:RNA / Q000235:genetics; D012333:RNA, Messenger
chemical_list: D006888:Hydroxycholesterols; D012333:RNA, Messenger; D011061:Poly A; D012313:RNA; C007997:25-hydroxycholesterol; D006903:Hydroxymethylglutaryl CoA Reductases