Stereochemistry- Independent Sterols Coats for Fat Flies Permuted Proteins Report on Binding

Oxygenated derivatives of cholesterol, or oxysterols, regulate cholesterol homeostasis by interacting directly with Insig proteins and promoting both endoplasmic reticulum (ER) retention of transcription factors that control expression of cholesterol synthesis genes and degradation of the cholesterol biosynthetic enzyme HMG CoA reductase (HMGR). Gale et al. used the side chain oxy­ sterol 25­hydroxycholesterol (HC) and its enantiomer, ent­25­HC, to demonstrate that sterol regulatory element (SRE)­dependent transcription and HMGR degradation occur independently of HC stereochemistry. Because steroid­protein interactions are ex­ pected to be enantiospecific, these data are consistent with the oxysterols acting through a protein­independent pathway. Further in vitro experiments indicate that both oxy­ sterol enantiomers disorder membranes in phospholipid monolayers and bilayers. Interestingly, this disordering effect is coun­ teracted by higher concentrations of phos­ pholipids with saturated acyl chains. Indeed, incubation of cultured mammalian cells with palmitate abrogates the effects of oxysterols on SRE­dependent transcription and HMGR degradation. Evaluation of cell homogenates and ER membrane fractions indicates that palmitate does not affect the partitioning of cholesterol between the plasma membrane and the ER membrane, but it does promote ER phospholipid remodeling. Based on these data, the authors propose that oxysterols interact directly with phospholipids in the ER membrane and that fatty acids affect sterol metabolism by modulating the activ­ ity of oxysterols. (J. Biol. Chem., published online 6 November 2008, doi:10.1074/jbc. M807210200) AD