Statins are cholesterol-lowering drugs that inhibit 3-hydroxy-3-methyl-glutaryl-CoA (HMG-CoA) reductase, the rate-limiting enzyme in the synthesis of cholesterol via the mevalonate pathway. This pathway also produces coenzyme Q (a component of the respiratory chain), dolichols (important for protein glycosylation), and isoprenoids (lipid moieties responsible for the membrane association of small GTPases). We previously showed that the nematode Caenorhabditis elegans is useful to study the noncholesterol effects of statins because its mevalonate pathway lacks the sterol synthesis branch but retains all other branches. Here, from a screen of 150,000 mutagenized genomes, we isolated four C. elegans mutants resistant to statins by virtue of gain-of-function mutations within the first six amino acids of the protein ATFS-1, the key regulator of the mitochondrial unfolded protein response that includes activation of the chaperones HSP-6 and HSP-60. The atfs-1 gain-of-function mutants are also resistant to ibandronate, an inhibitor of an enzyme downstream of HMG-CoA reductase, and to gliotoxin, an inhibitor acting on a subbranch of the pathway important for protein prenylation, and showed improved mitochondrial function and protein prenylation in the presence of statins. Additionally, preinduction of the mitochondrial unfolded protein response in wild-type worms using ethidium bromide or paraquat triggered statin resistance, and similar observations were made in Schizosaccharomyces pombe and in a mammalian cell line. We conclude that statin resistance through maintenance of mitochondrial homeostasis is conserved across species, and that the cell-lethal effects of statins are caused primarily through impaired protein prenylation that results in mitochondria dysfunction.

In spite of the worldwide impact of diabetes on human health, the mechanisms behind glucose toxicity remain elusive. Here we show that C. elegans mutants lacking paqr-2, the worm homolog of the adiponectin receptors AdipoR1/2, or its newly identified functional partner iglr-2, are glucose intolerant and die in the presence of as little as 20 mM glucose. Using FRAP (Fluorescence Recovery After Photobleaching) on living worms, we found that cultivation in the presence of glucose causes a decrease in membrane fluidity in paqr-2 and iglr-2 mutants and that genetic suppressors of this sensitivity act to restore membrane fluidity by promoting fatty acid desaturation. The essential roles of paqr-2 and iglr-2 in the presence of glucose are completely independent from daf-2 and daf-16, the C. elegans homologs of the insulin receptor and its downstream target FoxO, respectively. Using bimolecular fluorescence complementation, we also show that PAQR-2 and IGLR-2 interact on plasma membranes and thus may act together as a fluidity sensor that controls membrane lipid composition.

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The mevalonate pathway is responsible for the synthesis of cholesterol, coenzyme Q, and prenyl groups essential for small GTPase modification and function, and for the production of dolichols important for protein glycosylation. Statins, i.e., cholesterol-lowering drugs that inhibit the rate-limiting enzyme in the mevalonate pathway, HMG-CoA reductase, are lethal to Caenorhabditis elegans even though this animal lacks the branch of the mevalonate pathway that leads to cholesterol synthesis. To better understand the effects of statins that are not related to cholesterol, we have adopted the strategy of isolating statin-resistant C. elegans mutants. Previously, we showed that such mutants often have gain-of-function mutations in ATFS-1, a protein that activates the mitochondrial unfolded protein response. Here, we describe the isolation of a statin-resistant mutant allele of the NDUF-7 protein, which is a component of complex I in the mitochondrial electron transport chain. The novel nduf-7(et19) mutant also exhibits constitutive and ATFS-1-dependent activation of the mitochondrial unfolded protein response (UPRmt) and prolonged life span, both of which are mediated through production of ROS. Additionally, lifespan extension, but not activation, of the mitochondrial unfolded protein response was dependent on the pro-apoptotic gene ced-4. We conclude that the nduf-7(et19) mutant allele causes an increase in reactive oxygen species that activate ATFS-1, hence UPRmt-mediated statin resistance, and extends life span via CED-4.

HMG-CoA reductase is the rate-limiting enzyme in the mevalonate pathway and the target of cholesterol-lowering statins. We characterized the C. elegans hmgr-1(tm4368) mutant, which lacks HMG-CoA reductase, and show that its phenotypes recapitulate that of statin treatment, though in a more severe form. Specifically, the hmgr-1(tm4368) mutant has defects in growth, reproduction and protein prenylation, is rescued by exogenous mevalonate, exhibits constitutive activation of the UPRer and requires less mevalonate to be healthy when the UPRmt is activated by a constitutively active form of ATFS-1. We also show that different amounts of mevalonate are required for different physiological processes, with reproduction requiring the highest levels. Finally, we provide evidence that the mevalonate pathway is required for the activation of the UPRmt.

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Some ORF names, lengths and mutations in Table 1 are incorrect. Specifically, the ORF names and ORF lengths listed for the paqr-2(et36) and paqr-2(et35) alleles, and the ORF names and mutations listed for the iglr-2(et34), iglr-2(et37) and iglr-2(et38) alleles. The shift by one amino acid position in the mutation column also applies to the iglr-2 mutations indicated in Fig 2. Please see the corrected Table 1 and Fig 2 here.