HMG CoA reductase activity of lens epithelial cells: compared with true rates of sterol synthesis

Sphingolipids in human lens membranes: an update on their composition and possible biological implications

The unique nature of the most abundant phospholipids in human lens membranes remained overlooked until the 1990s when it was possible to discern dihydrosphingomyelins (DHSMs) from the more common sphingomyelins (SMs). Unlike in other mammalian membranes, DHSMs comprise nearly half of the phospholipids in adult human lenses. Compared to SMs with a trans double bond between carbons 4 and 5 of the sphingoid backbone, the absence of this unsaturation site in DHSMs allows the participation of the OH group on C3 in intermolecular H-bonds and leads to stronger interlipid interactions with both neighboring DHSMs and cholesterol. Phospholipid compositional changes with age and lens region observed in mammals with various life spans and lens growth rates, suggest that the highest levels of DHSMs along with the lowest amounts of phosphatidylcholines and SMs are found in lenses with the lowest growth rate, namely human lenses. The participation of phospholipid metabolites in the control of mitosis and elongation of lens cells is plausible and deserves investigation.

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.

Cholesterol and cataracts

Inherited defects in enzymes of cholesterol metabolism and use of drugs which inhibit lens cholesterol biosynthesis can be associated with cataracts in animals and man. The basis of this relationship apparently lies in the need of the lens to satisfy its sustained requirement for cholesterol by on-site synthesis, and impairing this synthesis can lead to alteration of lens membrane structure. Lens membrane contains the highest cholesterol content of any known membrane. The Smith-Lemli-Opitz syndrome, mevalonic aciduria, and cerebrotendinous xanthomatosis all involve mutations in enzymes of cholesterol metabolism, and affected patients can develop cataracts. Two established models of rodent cataracts are based on treatment with inhibitors of cholesterol biosynthesis. The long-term ocular safety of the very widely used vastatin class of hypocholesterolemic drugs is controversial. Some vastatins are potent inhibitors of cholesterol biosynthesis by animal lenses, can block cholesterol accumulation by these lenses and can produce cataracts in dogs. Whether these drugs inhibit cholesterol biosynthesis in human lenses at therapeutic doses is unknown. Results of clinical trials of 1-5 years duration in older patient populations indicate high ocular safety. However, considering the slow life-long growth of the lens and its continuing need for cholesterol, longterm safety of the vastatins should perhaps be viewed in units of 10 or 20 years, particularly with younger patients.

Tissue selectivity of hydroxymethylglutaryl coenzyme A (HMG CoA) reductase inhibitors

Hydroxymethylglutaryl coenzyme A (HMG CoA) reductase inhibitors are a class of lipid-lowering medications, with a major activity on plasma cholesterol levels, now enjoying a vast popularity among physicians and patients. These drugs, affecting a very early and key step of sterol biosynthesis, differ to a large extent in their physicochemical properties, tissue distribution and side effects in animals, possibly in humans. Some of these agents (namely lovastatin and simvastatin) are strikingly lipophilic and require enzymatic conversion from the lactone to the open-ring forms, whereas pravastatin, active per se, is hydrophilic. Liver uptake of pravastatin is regulated by a carrier-mediated mechanism. Other HMG CoA reductase inhibitors have been designed, with the objective of obtaining high levels of hepato-selectivity. Evaluation of available data in terms of potential advantages in tissue, namely liver selectivity, of HMG CoA reductase inhibitors, suggests, that, indeed, altered sterol biosynthesis in a number of tissues may potentially result in the appearance of significant side effects. While there is no clear-cut relationship between tissue selectivity and lipophilicity, the presence of this latter feature seems, in general, to dictate a lesser absorption to peripheral tissues vs the liver. At present, the toxicological profile of major HMG CoA reductase inhibitors appears safe; it is, however, possible that in selected patient groups liver selectivity may offer a considerable therapeutic advantage.

On the etiology of subcapsular lenticular opacities produced in dogs receiving HMG-CoA reductase inhibitors

The administration of high dosages of various hydroxymethylglutaryl-CoA (HMG-CoA) reductase inhibitors has resulted in the development of subcapsular lenticular opacities in dogs. While dogs receiving cataractogenic doses of HMG-CoA reductase inhibitors experienced profound decreases in circulating serum cholesterol concentrations (40-60% reductions in total serum cholesterol), a causal relationship between serum cholesterol lowering and cataractogenesis was not established. A strong relationship was demonstrated, however, between the systemic exposure to inhibitor (plasma drug levels) and the cataractogenic potential of the various compounds studied. Analysis of lenses from dogs chronically dosed with various HMG-CoA reductase inhibitors revealed the presence of low drug levels in the lens (less than 500 ng equivalents g-1), but no correlation was observed between the amount of drug associated with the lens after chronic treatment and cataract development. In addition, no abnormalities in cholesterol content or sterol composition were observed in clear and/or cataract containing lenses from dogs chronically dosed with HMG-CoA reductase inhibitors. The kinetics of drug appearance in the aqueous and lens cortex was assessed after doses of various HMG-CoA reductase inhibitors, and suggested somewhat higher but not statistically significant peak concentrations of inhibitor were achieved by compounds which produced a higher incidence of cataracts. These data have suggested that high doses of HMG-CoA reductase inhibitors may increase lenticular exposure to drug via the aqueous humor by producing a substantial systemic exposure to drug substance. This may result in an increased concentration of inhibitor in the outer cortical region of the lens where cholesterol synthesis is critical, thereby resulting in the development of opacities. The production of lenticular changes by a HMG-CoA reductase inhibitor of diverse chemical structure establishes, with reasonable assurance, that these lens changes are mechanism based (i.e. a product of the biochemical mechanism of action of this class of compounds). An extrapolation of these findings to patients receiving therapeutic dosages enables a favorable risk evaluation since the doses to be employed clinically are much lower and result in a far lower systemic exposure to drug substance.