Late stage control of cholesterol synthesis in lens epithelial cells

Document Type

Article

Publication Title

Investigative Ophthalmology and Visual Science

Abstract

Purpose. Non-sterol isoprenoids (NSI) formed from intermediates between mevalonate and squalene in cholesterol biosynthesis function to regulate cholesterol synthesis by lowering the concentration of the rate limiting enzyme, 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR). NSI enhance proteolysis of this enzyme in other cells and, therefore, function to decrease sterol biosynthesis. In view of the importance of cholesterol synthesis to the lens, our aim was to examine the role of NSI in regulating the concentration of HMGR. Methods. Bovine lens epithelial cells (BLEC) and human skin fibroblasts were cultured for 24 hr in the presence or absence of U18666A or lovastatin at levels which produced 98-99% inhibition of cholesterol synthesis. Lovastatin would also block NSI formation, since it inhibits formation of mevalonate; whereas, U18666A should not lower NSI since it inhibits at a site after their formation. We measured relative changes in the concentration of HMGR protein in response to drug exposure by Western blotting. Changes in HMGR mRNA levels were estimated by a competitive reverse-transcriptase-PCR procedure using the RNA plasmid pAW109 as internal standard. If NSI are important regulators in lens cells, exposure to lovastatin but not U18666A should markedly elevate HMGR protein levels. Results. Exposure of BLEC to lovastatin resulted in a 100 to 200 fold increase in the relative concentration of HMGR and a 3 to 4 fold increase in HMGR mRNA levels. Treatment with U18666A produced only a 1-2 fold increase in HMGR protein and a comparable increase in HMGR mRNA. In contrast to BLEC, lovastatin resulted in only a 4 fold increase in HMGR protein levels in fibroblast and U18666A had no apparent effect. Neither drug appeared to increase HMGR mRNA levels in fibroblasts. Conclusions. The concentration of HMGR protein in BLEC appears to be largely controlled by posttranscriptional mechanisms. Non-sterol isoprenes appear important in regulating the concentration of HMGR protein in the lens cells.

Publication Date

2-15-1996

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