MEVALONATE REGULATES POLYSOME DISTRIBUTION AND BLOCKS TRANSLATION-DEPENDENT SUPPRESSION OF 3-HYDROXY-3-METHYLGLUTARYL COENZYME-A REDUCTASE MESSENGER-RNA - RELATIONSHIP TO TRANSLATIONAL CONTROL

We reported previously that 3-hydroxy-3-methylglutaryl coenzyme A redu ctase synthesis is regulated al the translational level by mevalonate. To determine at what stage mevalonate affects reductase synthesis, we examined the distribution of reductase mRNA in polysomes from cells t reated with lovastatin alone; lovastatin and 25-hydroxycholesterol; or lovastatin, 25-hydroxycholesterol, and mevalonate. In lovastatin-trea ted cells, reductase mRNA was primarily associated with heavy polysome fractions. When 25-hydroxycholesterol was added to lovastatin-treated cells, reductase mRNA levels were reduced approximately fourfold in a ll polysome fractions, with no accompanying redistribution of reductas e mRNA into lighter polysome fractions. However, addition of both 25-h ydroxycholesterol and mevalonate to lovastatin-treated cells shifted r eductase mRNA from heavier to lighter polysome fractions. No change in the distribution of control p-actin or ribosomal protein S17 mRNA occ urred with any of the treatments. These results suggest that mevalonat e suppresses reductase synthesis at the level of initiation. When the translation inhibitor cycloheximide was added to all three regimens, r eductase mRNA shifted into heavy polysome fi actions. Treatment with e ither lovastatin alone ol lovastatin plus 25-hydroxycholesterol result ed in a 50% greater loss of reductase mRNA from the heavy polysome fra ctions compared to the same factions from noncycloheximide-treated cel ls. No loss of reductase mRNA occurred when cycloheximide was added to cells treated with both 25-hydroxycholesterol and mevalonate. P-Actin mRNA levels and polysome distribution were not significantly changed by cycloheximide under any of these conditions. Translationally mediat ed suppression of reductase mRNA did not occur when protein synthesis was inhibited with puromycin. Our results indicate that regulation of reductase mRNA levels is translation-dependent and is linked to the ra te of elongation.