Submitted on March 10, 2009
Revised on April 24, 2009
Accepted on April 28, 2009
Regulation of fibroblast mitochondrial 27-hydroxycholesterol production by active plasma membrane cholesterol
Yvonne Lange, Theodore L. Steck, Jin Ye, M. Hunter Lanier, Vasumathi Molugu, and Daniel S. Ory
Pathology, Rush University Medical Center, Chicago, IL 60612
Corresponding Author: ylange{at}rush.edu
Side chain oxysterols are cholesterol derivatives thought to signal the abundance of cell cholesterol to homeostatic effector proteins. Here, we investigated how plasma membrane (PM) cholesterol might regulate 27-hydroxycholesterol (27-HC) biosynthesis in cultured fibroblasts. We showed that PM cholesterol was a major substrate for 27-HC production. Biosynthesis commenced within minutes of loading depleted cells with cholesterol, concurrent with the rapid inactivation of hydroxy-3-methylglutaryl coenzyme A reductase (HMGR). 27-HC production rose ~30-fold in normal and Niemann-Pick C1 fibroblasts when PM cholesterol was increased by ~60%. 27-HC production was also stimulated by 1-octanol which displaces PM cholesterol from its phospholipid complexes and thereby increases its activity (escape tendency) and elevates its intracellular abundance. Conversely, lysophosphatidylserine and U18666A inhibited 27-HC biosynthesis and the inactivation of HMGR, presumably by reducing the activity of PM cholesterol and, therefore, its circulation to mitochondria. We conclude that, in this in vitro system, excess (active) PM cholesterol rapidly reaches mitochondria where, as the rate-limiting substrate, it stimulates 27-HC biosynthesis. The oxysterol product then promotes the rapid degradation of HMGR, along with other homeostatic effects. The regulation of 27-HC production by the active excess of PM cholesterol can thus provide a feedback mechanism in the homeostasis of PM cholesterol.
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