Posttranscriptional regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase in lens epithelial cells by mevalonate-derived nonsterols.

The ocular lens must continuously synthesize the cholesterol required to support membrane formation for its life-long growth. The roles of transcriptional and posttranscriptional mechanisms in controlling 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) protein levels in cultured lens epithelial cells were examined by measuring the effect of restricting exogenous cholesterol, endogenous cholesterol synthesis and mevalonate derived nonsterols upon HMGR protein and mRNA levels and upon the synthesis and degradation of HMGR protein. Sterols were restricted by culturing in lipoprotein deficient media and blocking 2,3-oxidosqualene cyclase with U18666A. Mevalonate derived nonsterols were additionally restricted by inhibition of HMGR activity with lovastatin. A 4-fold increase in HMGR protein levels due to restricting sterols with U18666A could be explained by comparably increased mRNA levels and enzyme protein synthesis. The very rapid turnover of HMGR protein (T(1/2) approximately 45 min) was unaffected. The additional restriction of mevalonate derived nonsterols increased HMGR protein levels to about 400-fold. A 10-fold slowing in the rate of enzyme degradation coupled with at least a 5-fold increase in mRNA levels likely accounted for this accumulated protein mass. The capacity of the nonsterol regulators to promote enzyme degradation appeared independent of sterols, since mevalonate restored rapid degradation of HMGR protein when 2,3-oxidosqualene cyclase activity was simultaneously blocked. Thus, in cultured lens epithelial cells, sterols appear to exert a modest influence on HMGR protein levels solely by suppressing transcription; whereas, mevalonate derived nonsterols exert major influence mainly by accelerating enzyme protein degradation. We speculate that nonsterol isoprenes might be important for preventing overexpression of cholesterol biosynthesis in the intact lens.