The UPR Protects from Mitochondrial Dysfunction by Upregulating Specific Enzymes of the Mevalonate Pathway.

The mevalonate pathway is the primary target of the cholesterol-lowering drugs statins, some of the most widely prescribed medicines of all time. The pathway's enzymes not only catalyze the synthesis of cholesterol but also of diverse metabolites such as mitochondrial electron carriers and isoprenyls. Recently, it has been shown that one type of mitochondrial stress response, the UPR, can protect yeast, , and cultured human cells from the deleterious effects of mevalonate pathway inhibition by statins. The mechanistic basis for this protection, however, remains unknown. Using , we found that the UPR does not directly affect the levels of the statin target HMG-CoA reductase, the rate-controlling enzyme of the mevalonate pathway in mammals. Instead, in the UPR upregulates the first dedicated enzyme of the pathway, HMG-CoA synthase (HMGS-1). A targeted RNA interference (RNAi) screen identified two UPR transcription factors, ATFS-1 and DVE-1, as regulators of HMGS-1 A comprehensive analysis of the pathway's enzymes found that, in addition to HMGS-1, the UPR upregulates enzymes involved with the biosynthesis of electron carriers and geranylgeranylation intermediates. Geranylgeranylation, in turn, is requisite for the full execution of the UPR 3response. Thus, the UPR acts in at least three coordinated, compensatory arms to upregulate specific branches of the mevalonate pathway, thereby alleviating mitochondrial stress. We propose that statin-mediated inhibition of the mevalonate pathway blocks this compensatory system of the UPR and consequentially impedes mitochondrial homeostasis. This effect is likely one of the principal bases for the adverse side effects of statins.