Oxygen-Mediated Regulation of Cholesterol Synthesis through Accelerated Degradation of HMG COA Reductase

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Title: Oxygen-Mediated Regulation of Cholesterol Synthesis through Accelerated Degradation of HMG COA Reductase
Author: Nguyen, Andrew Tuan Duc
Abstract: Endoplasmic reticulum -associated degradation of the enzyme 3 -hydroxy -3 -methylglutaryl CoA reductase represents one mechanism by which cholesterol synthesis is controlled in mammalian cells . The key reaction in this degradation is binding of reductase to Insig proteins in the endoplasmic reticulum , which is stimulated by the methylated cholesterol precursors lanosterol and 24 ,25 -dihydrolanosterol . Conversion of these sterols to cholesterol requires the removal of three methyl groups , which consumes nine molecules of oxygen . Here , we report that oxygen deprivation (hypoxia ) slows the rate of demethylation of lanosterol and its reduced metabolite 24 ,25 -dihydrolanosterol , causing both sterols to accumulate in cells . These methylated sterols serve as one signal to stimulate rapid Insig -mediated degradation of reductase . In addition , hypoxia increases the expression of Insig -2 in a response mediated by hypoxia -inducible factor . Our analysis of the mouse Insig -2 gene revealed the presence of a functional hypoxia response element in the first intron . Importantly , hepatic Insig -2a expression is upregulated in three independent mouse models of hypoxia . These studies establish that Insig -2 is a target gene of hypoxia -inducible factor . The hypoxia -dependent increase in Insig levels confers cells with enhanced sensitivity to sterol -induced degradation of reductase . In this way , hypoxia -inducible factor -mediated induction of Insig -2 provides a second signal for stimulating reductase degradation . To address the specificity of methylated sterols in promoting reductase degradation , we reconstituted Insig -dependent , sterol -accelerated degradation of the membrane domain of mammalian reductase in Drosophila S2 cells . Studies in this system revealed that 24 ,25 -dihydrolanosterol , and lanosterol , is active in accelerating degradation of reductase . These results were confirmed by examining ubiquitination of reductase in vitro using permeabilized mammalian cells . Collectively , these studies show that under hypoxic conditions reductase undergoes accelerated Insig -dependent degradation as the combined result of two events : 1 ) accumulation of 24 ,25 -dihydrolanosterol and 2 ) hypoxia -inducible factor -mediated upregulation of Insig -2 . Degradation of reductase ultimately slows a rate -determining step in cholesterol synthesis . These results highlight the importance of 24 ,25 -dihydrolanosterol as a physiologic regulator of reductase degradation and define a novel oxygen -sensing mechanism in the mammalian cholesterol biosynthetic pathway .
URI: http : / /hdl .handle .net /2152 .5 /514
Date: 2009-09-04

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Oxygen-Mediated Regulation of Cholesterol Synthesis through Accelerated Degradation of HMG COA Reductase. Graduate School of Biomedical Sciences. Available electronically from http : / /hdl .handle .net /2152 .5 /514 .

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