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7-Dehydrocholesterol down-regulates cholesterol biosynthesis in cultured Smith-Lemli-Opitz syndrome skin fibroblasts
1Department of Medicine, The Liver Center, University of Medicine and Dentistry of New Jersey-New Jersey Medical School, Newark 07103, USA.
Abstract:
The Smith-Lemli-Opitz syndrome (SLOS) is a common birth defect-mental retardation syndrome caused by a defect in the enzyme that reduces 7-dehydrocholesterol to cholesterol. Because of this block, patients' plasma cholesterol levels are generally low while 7-dehydrocholesterol concentrations are markedly elevated. In addition, plasma total sterols are abnormally low and correlate negatively with the percent of 7-dehydrocholesterol (r = -0.65, P < 0.0001) suggesting that 7-dehydrocholesterol might inhibit the activity of HMG-CoA reductase. Cultured skin fibroblasts from SLOS patients grown in fetal bovine serum or for 1 day in delipidated medium contain little 7-dehydrocholesterol (3 +/- 1% of total sterols) and HMG-CoA reductase activities are indistinguishable from that measured in control cells. However, raising the 7-dehydrocholesterol concentration to 20 +/- 3% of total sterols, equal to the mean proportion in plasma of SLOS patients, by either growing cells for 1 week in delipidated medium or adding 20 microg/ml 7-dehydrocholesterol directly to the cells reduced HMG-CoA reductase activities from 74 +/- 7 to 9 +/- 2 pmol/min per mg protein, or from 92 +/- 22 to 16 +/- 4 pmol/min per mg protein, respectively (P < 0.01). In contrast, adding 20 microg/ml cholesterol evoked a 2- to 4-fold lesser suppression of activity (39 +/- 8 pmol/min per mg protein, P < 0.05, vs. 7-dehydrocholesterol). HMG-CoA synthase and LDL binding were inhibited equally by 7-dehydrocholesterol and cholesterol. Ketaconazole prevented the down-regulation of HMG-CoA reductase by 7-dehydrocholesterol, suggesting that an hydroxylated derivative of 7-dehydrocholesterol may be especially important in suppressing cholesterol synthesis. These results demonstrate that 7-dehydrocholesterol, perhaps as an hydroxylated derivative(s), is a very effective feedback inhibitor of HMG-CoA reductase.
Insights
Smith-Lemli-Opitz syndrome (SLOS) is linked to elevated 7-dehydrocholesterol, which acts as a potent inhibitor of HMG-CoA reductase, impacting cholesterol synthesis. This study reveals 7-dehydrocholesterol
Area of Science:
- Biochemistry
- Genetics
- Metabolic Disorders
Background:
- Smith-Lemli-Opitz syndrome (SLOS) is a genetic disorder characterized by impaired cholesterol synthesis due to a defect in 7-dehydrocholesterol reductase.
- SLOS patients exhibit low plasma cholesterol and high levels of 7-dehydrocholesterol, a precursor sterol.
- The precise mechanism by which elevated 7-dehydrocholesterol affects cholesterol biosynthesis regulation in SLOS is not fully understood.
Purpose of the Study:
- To investigate the inhibitory effect of 7-dehydrocholesterol on HMG-CoA reductase activity, a key enzyme in cholesterol synthesis.
- To determine if 7-dehydrocholesterol or its derivatives are responsible for feedback inhibition of cholesterol synthesis in SLOS.
- To compare the inhibitory potency of 7-dehydrocholesterol with cholesterol on HMG-CoA reductase.
Main Methods:
- Cultured skin fibroblasts from SLOS patients were used to assess sterol levels and HMG-CoA reductase activity.
- Cells were cultured under conditions to manipulate 7-dehydrocholesterol concentrations, including delipidated medium and direct addition of 7-dehydrocholesterol.
- HMG-CoA reductase activity was measured, and the effects of cholesterol and ketoconazole were evaluated.
Main Results:
- Elevated 7-dehydrocholesterol concentrations in SLOS fibroblasts significantly reduced HMG-CoA reductase activity.
- 7-dehydrocholesterol demonstrated a more potent inhibition of HMG-CoA reductase compared to cholesterol.
- Ketoconazole treatment blocked the down-regulation of HMG-CoA reductase by 7-dehydrocholesterol, suggesting a role for hydroxylated derivatives.
Conclusions:
- 7-dehydrocholesterol, potentially through its hydroxylated derivatives, acts as a potent feedback inhibitor of HMG-CoA reductase in SLOS.
- These findings elucidate a key mechanism contributing to the altered cholesterol metabolism observed in Smith-Lemli-Opitz syndrome.
- Understanding this inhibitory pathway may offer insights for therapeutic strategies targeting cholesterol synthesis defects.