Mevalonate regulates polysome distribution and blocks translation-dependent suppression of 3-hydroxy-3-methylglutaryl...

We reported previously that 3-hydroxy-3-methylglutaryl coenzyme A reductase synthesis is regulated at 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 treated with lovastatin alone; lovastatin and 25-hydroxycholesterol; or lovastatin, 25-hydroxycholesterol, and mevalonate. In lovastatin-treated 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 all polysome fractions, with no accompanying redistribution of reductase mRNA into lighter polysome fractions. However, addition of both 25-hydroxycholesterol and mevalonate to lovastatin-treated cells shifted reductase mRNA from heavier to lighter polysome fractions. No change in the distribution of control beta-actin or ribosomal protein S17 mRNA occurred with any of the treatments. These results suggest that mevalonate suppresses reductase synthesis at the level of initiation. When the translation inhibitor cycloheximide was added to all three regimens, reductase mRNA shifted into heavy polysome fractions. Treatment with either lovastatin alone or lovastatin plus 25-hydroxycholesterol resulted in a 50% greater loss of reductase mRNA from the heavy polysome fractions compared to the same fractions from noncycloheximide-treated cells. No loss of reductase mRNA occurred when cycloheximide was added to cells treated with both 25-hydroxycholesterol and mevalonate. beta-Actin mRNA levels and polysome distribution were not significantly changed by cycloheximide under any of these conditions. Translationally mediated 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 rate of elongation.