An oxysterol-derived positive signal for 3-hydroxy-3-methylglutaryl-CoA reductase degradation in yeast

Sterol synthesis by the mevalonate pathway is modulated, in part, through f eedback-regulated degradation of 3-hydroxy-3-methylglutaryl-CoA reductase ( HMGR). In mammals, both a non-sterol isoprenoid signal derived from farnesy l diphosphate (FPP) and a sterol-derived signal appear to act together to p ositively regulate the rate of HMGR degradation. Although the nature and nu mber of sterol-derived signals are not clear, there is growing evidence tha t oxysterols can serve in this capacity. In yeast, a similar non-sterol iso prenoid signal generated from FPP acts to positively regulate HMGR degradat ion, but the existence of any sterol-derived signal has thus far not been r evealed. We now demonstrate, through the use of genetic and pharmacological manipulation of oxidosqualene-lanosterol cyclase, that an oxysterol-derive d signal positively regulated HMGR degradation in yeast. The oxysterol-deri ved signal acted by specifically modulating HMGR stability, not endoplasmic reticulum-associated degradation in general. Direct biochemical labeling o f mevalonate pathway products confirmed that oxysterols were produced endog enously in yeast and that their levels varied appropriately in response to genetic or pharmacological manipulations that altered HMGR stability. Genet ic manipulation of oxidosqualene-lanosterol cyclase did result in the build up of detectable levels of 24,25-oxidolanosterol by gas chromatography, gas chromatography-mass spectroscopy, and MMR analyses, whereas no detectable amounts were observed in wild-type cells or cells with squalene epoxidase d own-regulated. In contrast to mammalian cells, the yeast oxysterol-derived signal was not required for HMGR degradation in yeast, Rather, the function of this second signal was to enhance the ability of the FPP-derived signal to promote HMGR degradation. Thus, although differences do exist, both yea st and mammalian cells employ a similar strategy of multiinput regulation o f HMGR degradation.