Activation of genes involved in xenobiotic metabolism is a shared signature of mouse models with extended lifespan

Nuclear hormone receptor DHR96 mediates the resistance to xenobiotics but not the increased lifespan of insulin-mutant Drosophila

Lifespan of laboratory animals can be increased by genetic, pharmacological, and dietary interventions. Increased expression of genes involved in xenobiotic metabolism, together with resistance to xenobiotics, are frequent correlates of lifespan extension in the nematode worm Caenorhabditis elegans, the fruit fly Drosophila, and mice. The Green Theory of Aging suggests that this association is causal, with the ability of cells to rid themselves of lipophilic toxins limiting normal lifespan. To test this idea, we experimentally increased resistance of Drosophila to the xenobiotic dichlordiphenyltrichlorethan (DDT), by artificial selection or by transgenic expression of a gene encoding a cytochrome P450. Although both interventions increased DDT resistance, neither increased lifespan. Furthermore, dietary restriction increased lifespan without increasing xenobiotic resistance, confirming that the two traits can be uncoupled. Reduced activity of the insulin/Igf signaling (IIS) pathway increases resistance to xenobiotics and extends lifespan in Drosophila, and can also increase longevity in C. elegans, mice, and possibly humans. We identified a nuclear hormone receptor, DHR96, as an essential mediator of the increased xenobiotic resistance of IIS mutant flies. However, the IIS mutants remained long-lived in the absence of DHR96 and the xenobiotic resistance that it conferred. Thus, in Drosophila IIS mutants, increased xenobiotic resistance and enhanced longevity are not causally connected. The frequent co-occurrence of the two traits may instead have evolved because, in nature, lowered IIS can signal the presence of pathogens. It will be important to determine whether enhanced xenobiotic metabolism is also a correlated, rather than a causal, trait in long-lived mice.

Cell nonautonomous activation of flavin-containing monooxygenase promotes longevity and health span

Stabilization of the hypoxia-inducible factor 1 (HIF-1) increases life span and health span in nematodes through an unknown mechanism. We report that neuronal stabilization of HIF-1 mediates these effects in Caenorhabditis elegans through a cell nonautonomous signal to the intestine, which results in activation of the xenobiotic detoxification enzyme flavin-containing monooxygenase-2 (FMO-2). This prolongevity signal requires the serotonin biosynthetic enzyme TPH-1 in neurons and the serotonin receptor SER-7 in the intestine. Intestinal FMO-2 is also activated by dietary restriction (DR) and is necessary for DR-mediated life-span extension, which suggests that this enzyme represents a point of convergence for two distinct longevity pathways. FMOs are conserved in eukaryotes and induced by multiple life span-extending interventions in mice, which suggests that these enzymes may play a critical role in promoting health and longevity across phyla.

Biochemical Genetic Pathways that Modulate Aging in Multiple Species

The mechanisms underlying biological aging have been extensively studied in the past 20 years with the avail of mainly four model organisms: the budding yeast Saccharomyces cerevisiae, the nematode Caenorhabditis elegans, the fruitfly Drosophila melanogaster, and the domestic mouse Mus musculus. Extensive research in these four model organisms has identified a few conserved genetic pathways that affect longevity as well as metabolism and development. Here, we review how the mechanistic target of rapamycin (mTOR), sirtuins, adenosine monophosphate-activated protein kinase (AMPK), growth hormone/insulin-like growth factor 1 (IGF-1), and mitochondrial stress-signaling pathways influence aging and life span in the aforementioned models and their possible implications for delaying aging in humans. We also draw some connections between these biochemical pathways and comment on what new developments aging research will likely bring in the near future.

SKN-1/Nrf, stress responses, and aging in Caenorhabditis elegans

The mammalian Nrf/CNC proteins (Nrf1, Nrf2, Nrf3, p45 NF-E2) perform a wide range of cellular protective and maintenance functions. The most thoroughly described of these proteins, Nrf2, is best known as a regulator of antioxidant and xenobiotic defense, but more recently has been implicated in additional functions that include proteostasis and metabolic regulation. In the nematode Caenorhabditis elegans, which offers many advantages for genetic analyses, the Nrf/CNC proteins are represented by their ortholog SKN-1. Although SKN-1 has diverged in aspects of how it binds DNA, it exhibits remarkable functional conservation with Nrf/CNC proteins in other species and regulates many of the same target gene families. C. elegans may therefore have considerable predictive value as a discovery model for understanding how mammalian Nrf/CNC proteins function and are regulated in vivo. Work in C. elegans indicates that SKN-1 regulation is surprisingly complex and is influenced by numerous growth, nutrient, and metabolic signals. SKN-1 is also involved in a wide range of homeostatic functions that extend well beyond the canonical Nrf2 function in responses to acute stress. Importantly, SKN-1 plays a central role in diverse genetic and pharmacologic interventions that promote C. elegans longevity, suggesting that mechanisms regulated by SKN-1 may be of conserved importance in aging. These C. elegans studies predict that mammalian Nrf/CNC protein functions and regulation may be similarly complex and that the proteins and processes that they regulate are likely to have a major influence on mammalian life- and healthspan.

Nrf2 Signaling and the Slowed Aging Phenotype: Evidence from Long-Lived Models

Studying long-lived animals provides novel insight into shared characteristics of aging and represents a unique model to elucidate approaches to prevent chronic disease. Oxidant stress underlies many chronic diseases and resistance to stress is a potential mechanism governing slowed aging. The transcription factor nuclear factor (erythroid-derived 2)-like 2 is the "master regulator" of cellular antioxidant defenses. Nrf2 is upregulated by some longevity promoting interventions and may play a role in regulating species longevity. However, Nrf2 expression and activity in long-lived models have not been well described. Here, we review evidence for altered Nrf2 signaling in a variety of slowed aging models that accomplish lifespan extension via pharmacological, nutritional, evolutionary, genetic, and presumably epigenetic means.

Improving healthspan via changes in gut microbiota and fermentation

Dietary resistant starch impact on intestinal microbiome and improving healthspan is the topic of this review. In the elderly population, dietary fiber intake is lower than recommended. Dietary resistant starch as a source of fiber produces a profound change in gut microbiota and fermentation in animal models of aging. Dietary resistant starch has the potential for improving healthspan in the elderly through multiple mechanisms as follows: (1) enhancing gut microbiota profile and production of short-chain fatty acids, (2) improving gut barrier function, (3) increasing gut peptides that are important in glucose homeostasis and lipid metabolism, and (4) mimicking many of the effects of caloric restriction including upregulation of genes involved in xenobiotic metabolism.

Nuclear hormone receptors: Roles of xenobiotic detoxification and sterol homeostasis in healthy aging

Health during aging can be improved by genetic, dietary and pharmacological interventions. Many of these increase resistance to various stressors, including xenobiotics. Up-regulation of xenobiotic detoxification genes is a transcriptomic signature shared by long-lived nematodes, flies and mice, suggesting that protection of cells from toxicity of xenobiotics may contribute to longevity. Expression of genes involved in xenobiotic detoxification is controlled by evolutionarily conserved transcriptional regulators. Three closely related subgroups of nuclear hormone receptors (NHRs) have a major role, and these include DAF-12 and NHR-8 in C. elegans, DHR96 in Drosophila and FXR, LXRs, PXR, CAR and VDR in mammals. In the invertebrates, these NHRs have been experimentally demonstrated to play a role in extension of lifespan by genetic and environmental interventions. NHRs represent critical hubs in that they regulate detoxification enzymes with broad substrate specificities, metabolizing both endo- and xeno-biotics. They also modulate homeostasis of steroid hormones and other endogenous cholesterol derivatives and lipid metabolism, and these roles, as well as xenobiotic detoxification, may contribute to the effects of NHRs on lifespan and health during aging, an issue that is being increasingly addressed in C. elegans and Drosophila. Disentangling the contribution of these processes to longevity will require more precise understanding of the molecular mechanisms by which each is effected, including identification of ligands and co-regulators of NHRs, patterns of tissue-specificity and mechanisms of interaction between tissues. The roles of vertebrate NHRs in determination of health during aging and lifespan have yet to be investigated.

Lipid-mediated regulation of SKN-1/Nrf in response to germ cell absence

In Caenorhabditis elegans, ablation of germline stem cells (GSCs) extends lifespan, but also increases fat accumulation and alters lipid metabolism, raising the intriguing question of how these effects might be related. Here, we show that a lack of GSCs results in a broad transcriptional reprogramming in which the conserved detoxification regulator SKN-1/Nrf increases stress resistance, proteasome activity, and longevity. SKN-1 also activates diverse lipid metabolism genes and reduces fat storage, thereby alleviating the increased fat accumulation caused by GSC absence. Surprisingly, SKN-1 is activated by signals from this fat, which appears to derive from unconsumed yolk that was produced for reproduction. We conclude that SKN-1 plays a direct role in maintaining lipid homeostasis in which it is activated by lipids. This SKN-1 function may explain the importance of mammalian Nrf proteins in fatty liver disease and suggest that particular endogenous or dietary lipids might promote health through SKN-1/Nrf.

Oat consumption reduced intestinal fat deposition and improved health span in Caenorhabditis elegans model

In addition to their fermentable dietary fiber and the soluble β-glucan fiber, oats have unique avenanthramides that have anti-inflammatory and antioxidant properties that reduce coronary heart disease in human clinical trials. We hypothesized that oat consumption will increase insulin sensitivity, reduce body fat, and improve health span in Caenorhabditis elegans through a mechanism involving the daf-2 gene, which codes for the insulin/insulin-like growth factor-1–like receptor, and that hyperglycemia will attenuate these changes. Caenorhabditis elegans wild type (N2) and the null strains sir-2.1, daf-16, and daf-16/daf-2 were fed Escherichia coli (OP50) and oat flakes (0.5%, 1.0%, or 3%) with and without 2% glucose. Oat feeding decreased intestinal fat deposition in N2, daf-16, or daf-16/daf-2 strains (P < .05); and glucose did not affect intestinal fat deposition response. The N2, daf-16, or sir-2.1 mutant increased the pharyngeal pumping rate (P < .05), a surrogate marker of life span, following oat consumption. Oat consumption increased ckr-1, gcy-8, cpt-1, and cpt-2 mRNA expression in both the N2 and the sir-2.1 mutant, with significantly higher expression in sir-2.1 than in N2 (P < .01). Additional glucose further increased expression 1.5-fold of the 4 genes in N2 (P < .01), decreased the expression of all except cpt-1 in the daf-16 mutant, and reduced mRNA expression of the 4 genes in the daf-16/daf-2 mutant (P < .01). These data suggest that oat consumption reduced fat storage and increased ckr-1, gcy-8, cpt-1, or cpt-2 through the sir-2.1 genetic pathway. Oat consumption may be a beneficial dietary intervention for reducing fat accumulation, augmenting health span, and improving hyperglycemia-impaired lipid metabolism.

Transient early food restriction leads to hypothalamic changes in the long-lived crowded litter female mice

Transient nutrient restriction in the 3 weeks between birth and weaning (producing "crowded litter" or CL mice) leads to a significant increase in lifespan and is associated with permanent changes in energy homeostasis, leptin, and insulin sensitivity. Here, we show this brief period of early food restriction leads to permanent modulation of the arcuate nucleus of the hypothalamus (ARH), markedly increasing formation of both orexigenic agouti-related peptide (AgRP) and anorexigenic proopiomelanocortin (POMC) projections to the paraventricular nucleus of the hypothalamus (PVH). An additional 4 weeks of caloric restriction, after weaning, does not further intensify the formation of AgRP and POMC projections. Acute leptin stimulation of 12-month-old mice leads to a stronger increase in the levels of hypothalamic pStat3 and cFos activity in CL mice than in controls, suggesting that preweaning food restriction leads to long-lasting enhancement of leptin signaling. In contrast, FoxO1 nuclear exclusion in response to insulin is equivalent in young adult CL and control mice, suggesting that hypothalamic insulin signaling is not modulated by the crowded litter intervention. Markers of hypothalamic reactive gliosis associated with aging, such as Iba1-positive microglia and GFAP-positive astrocytes, are significantly reduced in CL mice as compared to controls at 12 and 22 months of age. Lastly, age-associated overproduction of TNF-α in microglial cells is reduced in CL mice than in age-matched controls. Together, these results suggest that transient early life nutrient deprivation leads to long-term hypothalamic changes which may contribute to the longevity of CL mice.

Reduced expression of MYC increases longevity and enhances healthspan

MYC is a highly pleiotropic transcription factor whose deregulation promotes cancer. In contrast, we find that Myc haploinsufficient (Myc(+/-)) mice exhibit increased lifespan. They show resistance to several age-associated pathologies, including osteoporosis, cardiac fibrosis, and immunosenescence. They also appear to be more active, with a higher metabolic rate and healthier lipid metabolism. Transcriptomic analysis reveals a gene expression signature enriched for metabolic and immune processes. The ancestral role of MYC as a regulator of ribosome biogenesis is reflected in reduced protein translation, which is inversely correlated with longevity. We also observe changes in nutrient and energy sensing pathways, including reduced serum IGF-1, increased AMPK activity, and decreased AKT, TOR, and S6K activities. In contrast to observations in other longevity models, Myc(+/-) mice do not show improvements in stress management pathways. Our findings indicate that MYC activity has a significant impact on longevity and multiple aspects of mammalian healthspan.

ATF4 activity: a common feature shared by many kinds of slow-aging mice

ATF4, a DNA-binding factor that modulates responses to amino acid availability and ribosomal function, has been shown to be altered in both liver and fibroblasts from two strains of long-lived mice, i.e. Snell dwarf and PAPP-A knockout mice. New data now show elevated ATF4 levels, and elevation of ATF4-dependent proteins and mRNAs, in liver of mice treated with acarbose or rapamycin, calorically restricted mice, methionine-restricted mice, and mice subjected to litter crowding. Elevation of ATF4, at least in liver, thus seems to be a shared feature of diets, drugs, genes, and developmental alterations that extend maximum lifespan in mice.

Long-lived crowded-litter mice have an age-dependent increase in protein synthesis to DNA synthesis ratio and mTORC1 substrate phosphorylation

Increasing mouse litter size [crowded litter (CL)] presumably imposes a transient nutrient stress during suckling and extends lifespan through unknown mechanisms. Chronic calorically restricted and rapamycin-treated mice have decreased DNA synthesis and mTOR complex 1 (mTORC1) signaling but maintained protein synthesis, suggesting maintenance of existing cellular structures. We hypothesized that CL would exhibit similar synthetic and signaling responses to other long-lived models and, by comparing synthesis of new protein to new DNA, that insight may be gained into the potential preservation of existing cellular structures in the CL model. Protein and DNA synthesis was assessed in gastroc complex, heart, and liver of 4- and 7-mo CL mice. We also examined mTORC1 signaling in 3- and 7-mo aged animals. Compared with controls, 4-mo CL had greater DNA synthesis in gastroc complex with no differences in protein synthesis or mTORC1 substrate phosphorylation across tissues. Seven-month CL had less DNA synthesis than controls in heart and greater protein synthesis and mTORC1 substrate phosphorylation across tissues. The increased new protein-to-new DNA synthesis ratio suggests that new proteins are synthesized more so in existing cells at 7 mo, differing from 4 mo, in CL vs. controls. We propose that, in CL, protein synthesis shifts from being directed toward new cells (4 mo) to maintenance of existing cellular structures (7 mo), independently of decreased mTORC1.

The measurement of protein synthesis for assessing proteostasis in studies of slowed aging

Slowing the aging process can reduce the risk for multiple chronic diseases simultaneously. It is increasingly recognized that maintaining protein homeostasis (or proteostasis) is important for slowing the aging process. Since proteostasis is a dynamic process, monitoring it is not a simple task and requires use of appropriate methods. This review will introduce methods to assess protein and DNA synthesis using deuterium oxide (D2O), and how protein and DNA synthesis outcomes provide insight into proteostatic mechanisms. Finally, we provide a discussion on how these assessments of protein and DNA synthesis are "mechanistic" investigations and provide an appropriate framework for the further development of slowed aging treatments.

A novel kinase regulates dietary restriction-mediated longevity in Caenorhabditis elegans

Although dietary restriction (DR) is known to extend lifespan across species, from yeast to mammals, the signalling events downstream of food/nutrient perception are not well understood. In Caenorhabditis elegans, DR is typically attained either by using the eat-2 mutants that have reduced pharyngeal pumping leading to lower food intake or by feeding diluted bacterial food to the worms. In this study, we show that knocking down a mammalian MEKK3-like kinase gene, mekk-3 in C. elegans, initiates a process similar to DR without compromising food intake. This DR-like state results in upregulation of beta-oxidation genes through the nuclear hormone receptor NHR-49, a HNF-4 homolog, resulting in depletion of stored fat. This metabolic shift leads to low levels of reactive oxygen species (ROS), potent oxidizing agents that damage macromolecules. Increased beta-oxidation, in turn, induces the phase I and II xenobiotic detoxification genes, through PHA-4/FOXA, NHR-8 and aryl hydrocarbon receptor AHR-1, possibly to purge lipophilic endotoxins generated during fatty acid catabolism. The coupling of a metabolic shift with endotoxin detoxification results in extreme longevity following mekk-3 knock-down. Thus, MEKK-3 may function as an important nutrient sensor and signalling component within the organism that controls metabolism. Knocking down mekk-3 may signal an imminent nutrient crisis that results in initiation of a DR-like state, even when food is plentiful.

P62/SQSTM1 at the interface of aging, autophagy, and disease

Advanced age is characterized by increased incidence of many chronic, noninfectious diseases that impair the quality of living of the elderly and pose a major burden on the healthcare systems of developed countries. These diseases are characterized by impaired or altered function at the tissue and cellular level, which is a hallmark of the aging process. Age-related impairments are likely due to loss of homeostasis at the cellular level, which leads to the accumulation of dysfunctional organelles and damaged macromolecules, such as proteins, lipids, and nucleic acids. Intriguingly, aging and age-related diseases can be delayed by modulating nutrient signaling pathways converging on the target of rapamycin (TOR) kinase, either by genetic or dietary intervention. TOR signaling influences aging through several potential mechanisms, such as autophagy, a degradation pathway that clears the dysfunctional organelles and damaged macromolecules that accumulate with aging. Autophagy substrates are targeted for degradation by associating with p62/SQSTM1, a multidomain protein that interacts with the autophagy machinery. p62/SQSTM1 is involved in several cellular processes, and its loss has been linked to accelerated aging and to age-related pathologies. In this review, we describe p62/SQSTM1, its role in autophagy and in signaling pathways, and its emerging role in aging and age-associated pathologies. Finally, we propose p62/SQSTM1 as a novel target for aging studies and age-extending interventions.

Rapamycin-mediated lifespan increase in mice is dose and sex dependent and metabolically distinct from dietary restriction

Rapamycin, an inhibitor of mTOR kinase, increased median lifespan of genetically heterogeneous mice by 23% (males) to 26% (females) when tested at a dose threefold higher than that used in our previous studies; maximal longevity was also increased in both sexes. Rapamycin increased lifespan more in females than in males at each dose evaluated, perhaps reflecting sexual dimorphism in blood levels of this drug. Some of the endocrine and metabolic changes seen in diet-restricted mice are not seen in mice exposed to rapamycin, and the pattern of expression of hepatic genes involved in xenobiotic metabolism is also quite distinct in rapamycin-treated and diet-restricted mice, suggesting that these two interventions for extending mouse lifespan differ in many respects.

Long-lived crowded-litter mice exhibit lasting effects on insulin sensitivity and energy homeostasis

The action of nutrients on early postnatal growth can influence mammalian aging and longevity. Recent work has demonstrated that limiting nutrient availability in the first 3 wk of life [by increasing the number of pups in the crowded-litter (CL) model] leads to extension of mean and maximal lifespan in genetically normal mice. In this study, we aimed to characterize the impact of early-life nutrient intervention on glucose metabolism and energy homeostasis in CL mice. In our study, we used mice from litters supplemented to 12 or 15 pups and compared those to control litters limited to eight pups. At weaning and then throughout adult life, CL mice are significantly leaner and consume more oxygen relative to control mice. At 6 mo of age, CL mice had low fasting leptin concentrations, and low-dose leptin injections reduced body weight and food intake more in CL female mice than in controls. At 22 mo, CL female mice also have smaller adipocytes compared with controls. Glucose and insulin tolerance tests show an increase in insulin sensitivity in 6 mo old CL male mice, and females become more insulin sensitive later in life. Furthermore, β-cell mass was significantly reduced in the CL male mice and was associated with reduction in β-cell proliferation rate in these mice. Together, these data show that early-life nutrient intervention has a significant lifelong effect on metabolic characteristics that may contribute to the increased lifespan of CL mice.

Glycerol extends lifespan of Brachionus manjavacas (Rotifera) and protects against stressors

Diet has profound effects on animal longevity and manipulation of nutrient sensing pathways is one of the primary interventions capable of lifespan extension. This often is done through caloric restriction (CR) and a variety of CR mimics have been identified that produce life extending effects without adhering to the rigorous CR dietary regimen. Glycerol is a dietary supplement capable mimicking CR by shifting metabolism away from glycolysis and towards oxidative phosphorylation. Glycerol supplementation has a number of beneficial effects, including lifespan extension, improved stress resistance, and enhanced locomotory and mitochondria activity in older age classes. Using rotifers as a model, we show that supplements of 150-300mM glycerol produced 40-50% extension of mean lifespan. This effect was produced by raising glycerol concentration only three times higher than its baseline concentration in rotifer tissues. Glycerol supplementation decreased rotifer reliance on glycolysis and reduced the pro-aging effects of glucose. Glycerol also acted as a chemical chaperone, mitigating damage by protein aggregation. Glycerol treatment improved rotifer swimming performance in older age classes and maintained more mitochondrial activity. Glycerol treatment provided increased resistance to starvation, heat, oxidation, and osmotic stress, but not UV stress. When glycerol was co-administered with the hexokinase inhibitor 2-deoxyglucose, the lifespan extending effect of glycerol was enhanced. Co-administration of glycerol with inhibitors like 2-deoxyglucose can lower their efficacious doses, thereby reducing their toxic side effects.

Short-term calorie restriction feminizes the mRNA profiles of drug metabolizing enzymes and transporters in livers of mice

Calorie restriction (CR) is one of the most effective anti-aging interventions in mammals. A modern theory suggests that aging results from a decline in detoxification capabilities and thus accumulation of damaged macromolecules. The present study aimed to determine how short-term CR alters mRNA profiles of genes that encode metabolism and detoxification machinery in the liver. Male C57BL/6 mice were fed CR (0, 15, 30, or 40%) diets for one month, followed by mRNA quantification of 98 xenobiotic processing genes (XPGs) in the liver, including 7 uptake transporters, 39 phase-I enzymes, 37 phase-II enzymes, 10 efflux transporters, and 5 transcription factors. In general, 15% CR did not alter mRNAs of most XPGs, whereas 30 and 40% CR altered over half of the XPGs (32 increased and 29 decreased). CR up-regulated some phase-I enzymes (fold increase), such as Cyp4a14 (12), Por (2.3), Nqo1 (1.4), Fmo2 (5.4), and Fmo3 (346), and numerous number of phase-II enzymes, such as Sult1a1 (1.2), Sult1d1 (2.0), Sult1e1 (33), Sult3a1 (2.2), Gsta4 (1.3), Gstm2 (1.3), Gstm3 (1.7), and Mgst3 (2.2). CR feminized the mRNA profiles of 32 XPGs in livers of male mice. For instance, CR decreased the male-predominantly expressed Oatp1a1 (97%) and increased the female-predominantly expressed Oatp1a4 (11). In conclusion, short-term CR alters the mRNA levels of over half of the 98 XPGs quantified in livers of male mice, and over half of these alterations appear to be due to feminization of the liver.

Growth hormone-releasing hormone disruption extends lifespan and regulates response to caloric restriction in mice

We examine the impact of targeted disruption of growth hormone-releasing hormone (GHRH) in mice on longevity and the putative mechanisms of delayed aging. GHRH knockout mice are remarkably long-lived, exhibiting major shifts in the expression of genes related to xenobiotic detoxification, stress resistance, and insulin signaling. These mutant mice also have increased adiponectin levels and alterations in glucose homeostasis consistent with the removal of the counter-insulin effects of growth hormone. While these effects overlap with those of caloric restriction, we show that the effects of caloric restriction (CR) and the GHRH mutation are additive, with lifespan of GHRH-KO mutants further increased by CR. We conclude that GHRH-KO mice feature perturbations in a network of signaling pathways related to stress resistance, metabolic control and inflammation, and therefore provide a new model that can be used to explore links between GHRH repression, downregulation of the somatotropic axis, and extended longevity.

DOI:http://dx.doi.org/10.7554/eLife.01098.001

There is increasing evidence that the hormonal systems involved in growth, the metabolism of glucose, and the processes that balance energy intake and expenditure might also be involved in the aging process. In rodents, mutations in genes involved in these hormone-signaling pathways can substantially increase lifespan, as can a diet that is low in calories but which avoids malnutrition. As well as living longer, such mice also show reductions in age-related conditions such as diabetes, memory loss and cancer.

Many of these effects appear to involve the actions of growth hormone. Mice with mutations that disrupt the development of the pituitary gland, which produces growth hormone, show increased longevity, as do mice that lack the receptor for growth hormone. However, these animals also show changes in a number of other hormones, making it difficult to be sure that the reduction in growth hormone signaling is responsible for their increased lifespan.

Now, Sun et al. have studied mutant mice that lack a gene called GHRH, which promotes the release of growth hormone. These mice, which have normal levels of all other pituitary hormones, lived for up to 50% longer than their wild-type littermates. They were more active than normal mice and had more body fat, and showed greatly increased sensitivity to insulin.

Some of the changes in these mutant mice resembled those seen in animals with a restricted calorie intake, suggesting that the same mechanisms may be implicated in both. However, Sun et al. found that caloric restriction further increased the lifespans of their GHRH knockout mice, indicating that at least some of the effects of caloric restriction are independent of disrupted growth hormone signaling.

The results of this study are an important step forward for understanding how growth hormone signaling and caloric restriction regulate aging, both individually and in combination. The GHRH knockout mice are likely to become an important model system for studying these processes and for understanding the complex interactions between diet and hormonal pathways.

DOI:http://dx.doi.org/10.7554/eLife.01098.002

Direct and indirect effects of growth hormone receptor ablation on liver expression of xenobiotic metabolizing genes

Detoxification of ingested xenobiotic chemicals, and of potentially toxic endogenous metabolites, is carried out largely through a series of enzymes synthesized in the liver, sometimes called "xenobiotic metabolizing enzymes" (XME). Expression of these XME is sexually dimorphic in rodents and humans, with many of the XME expressed at higher levels in females. This expression pattern is thought to be regulated, in part, by the sex differences in circadian growth hormone (GH) pulsatility. We have evaluated mRNA, in the liver, for 52 XME genes in male and female mice of four mutant stocks, with diminished levels of GH receptor (GHR) either globally (GKO), or in liver (LKO), fat (FKO), or muscle (MKO) tissue specifically. The data show complex, sex-specific changes. For some XME, the expression pattern is consistent with direct control of hepatic mRNA by GHR in the liver. In contrast, other XME show evidence for indirect pathways in which hepatic XME expression is altered by GH signals in fat or skeletal muscle. The effects of GHR-null mutations on glucose control, responses to dietary interventions, steroid metabolism, detoxification pathways, and lifespan may depend on a mixture of direct hepatic effects and cross talk between different GH-responsive tissues.

Hepatic signaling by the mechanistic target of rapamycin complex 2 (mTORC2)

The mechanistic target of rapamycin (mTOR) exists in two complexes that regulate diverse cellular processes. mTOR complex 1 (mTORC1), the canonical target of rapamycin, has been well studied, whereas the physiological role of mTORC2 remains relatively uncharacterized. In mice in which the mTORC2 component Rictor is deleted in liver [Rictor-knockout (RKO) mice], we used genomic and phosphoproteomic analyses to characterize the role of hepatic mTORC2 in vivo. Overnight food withdrawal followed by refeeding was used to activate mTOR signaling. Rapamycin was administered before refeeding to specify mTORC2-mediated events. Hepatic mTORC2 regulated a complex gene expression and post-translational network that affects intermediary metabolism, ribosomal biogenesis, and proteasomal biogenesis. Nearly all changes in genes related to intermediary metabolic regulation were replicated in cultured fetal hepatocytes, indicating a cell-autonomous effect of mTORC2 signaling. Phosphoproteomic profiling identified mTORC2-related signaling to 144 proteins, among which were metabolic enzymes and regulators. A reduction of p38 MAPK signaling in the RKO mice represents a link between our phosphoproteomic and gene expression results. We conclude that hepatic mTORC2 exerts a broad spectrum of biological effects under physiological conditions. Our findings provide a context for the development of targeted therapies to modulate mTORC2 signaling.

Integration of the unfolded protein and oxidative stress responses through SKN-1/Nrf

The Unfolded Protein Response (UPR) maintains homeostasis in the endoplasmic reticulum (ER) and defends against ER stress, an underlying factor in various human diseases. During the UPR, numerous genes are activated that sustain and protect the ER. These responses are known to involve the canonical UPR transcription factors XBP1, ATF4, and ATF6. Here, we show in C. elegans that the conserved stress defense factor SKN-1/Nrf plays a central and essential role in the transcriptional UPR. While SKN-1/Nrf has a well-established function in protection against oxidative and xenobiotic stress, we find that it also mobilizes an overlapping but distinct response to ER stress. SKN-1/Nrf is regulated by the UPR, directly controls UPR signaling and transcription factor genes, binds to common downstream targets with XBP-1 and ATF-6, and is present at the ER. SKN-1/Nrf is also essential for resistance to ER stress, including reductive stress. Remarkably, SKN-1/Nrf-mediated responses to oxidative stress depend upon signaling from the ER. We conclude that SKN-1/Nrf plays a critical role in the UPR, but orchestrates a distinct oxidative stress response that is licensed by ER signaling. Regulatory integration through SKN-1/Nrf may coordinate ER and cytoplasmic homeostasis.

Proteins that are placed in membranes or secreted are produced in a cellular structure called the endoplasmic reticulum (ER). An accumulation of misfolded proteins in the ER contributes to many disease states, including diabetes and neurodegeneration. The ER protects against a toxic buildup of misfolded proteins by activating the unfolded protein response (UPR), which maintains ER homeostasis by slowing protein synthesis and enhancing ER functions such as protein folding and degradation. Many of these processes are controlled by three canonical ER/UPR gene regulatory factors. Here we identify the gene regulator SKN-1/Nrf as also playing a critical role in the UPR. SKN-1/Nrf is well known for its functions in oxidative stress defense and longevity. We now report that SKN-1/Nrf mobilizes an ER stress gene network that is distinct from its oxidative stress response, and includes regulation of other central UPR factors. Surprisingly, we also find that ER- and UPR-associated mechanisms are needed to “license” SKN-1/Nrf to defend against oxidative stresses. Our findings show that UPR and oxidative stress defense mechanisms are integrated through SKN-1/Nrf, and suggest that this integration may help maintain a healthy balance between ER and cytoplasmic functions, and stress defenses.

Farnesoid X receptor directly regulates xenobiotic detoxification genes in the long-lived Little mice

Activation of xenobiotic metabolism pathways has been linked to lifespan extension in different models of aging. However, the mechanisms underlying activation of xenobiotic genes remain largely unknown. Here we showed that although farnesoid X receptor (FXR, Nr1h4) mRNA levels do not change significantly, FXR protein levels are elevated in the livers of the long-lived Little mice, leading to increased DNA binding activity of FXR. Hepatic FXR expression is sex-dependent in wild-type mice but not in Little mice, implying that up-regulation of FXR might be dependent on the reduction of growth hormone in Little mice. Growth hormone treatment decreased hepatic expression of FXR and xenobiotic genes Abcb1a, Fmo3 and Gsta2 in both wild-type and Little mice, suggesting an association between FXR and xenobiotic gene expression. We found that Abcb1a is transactivated by FXR via direct binding of FXR/retinoid X receptor α (RXRα) heterodimer to a response element at the proximal promoter. FXR also positively controls Fmo3 and Gsta2 expression through direct interaction with the response elements in these genes. Our study demonstrates that xenobiotic genes are direct transcriptional targets of FXR and suggests that FXR signaling may play a critical role in the lifespan extension observed in Little mice.

A cytoprotective perspective on longevity regulation

There are many mechanisms of lifespan extension, including the disruption of insulin/insulin-like growth factor 1 (IGF-1) signaling, metabolism, translation, and feeding. Despite the disparate functions of these pathways, inhibition of each induces responses that buffer stress and damage. Here, emphasizing data from genetic analyses in Caenorhabditis elegans, we explore the effectors and upstream regulatory components of numerous cytoprotective mechanisms activated as major elements of longevity programs, including detoxification, innate immunity, proteostasis, and oxidative stress response. We show that their induction underpins longevity extension across functionally diverse triggers and across species. Intertwined with the evolution of longevity, cytoprotective pathways are coupled to the surveillance of core cellular components, with important implications in normal and aberrant responses to drugs, chemicals, and pathogens.

mTOR is a key modulator of ageing and age-related disease

Many experts in the biology of ageing believe that pharmacological interventions to slow ageing are a matter of 'when' rather than 'if'. A leading target for such interventions is the nutrient response pathway defined by the mechanistic target of rapamycin (mTOR). Inhibition of this pathway extends lifespan in model organisms and confers protection against a growing list of age-related pathologies. Characterized inhibitors of this pathway are already clinically approved, and others are under development. Although adverse side effects currently preclude use in otherwise healthy individuals, drugs that target the mTOR pathway could one day become widely used to slow ageing and reduce age-related pathologies in humans.

Lifespan extension in the spontaneous dwarf rat and enhanced resistance to hyperoxia-induced mortality

Lifespan extension has been demonstrated in dwarfism mouse models relative to their wild-type. The spontaneous dwarf rat (SDR) was isolated from a closed colony of Sprague-Dawley (SD) rats. Growth hormone deficiencies have been indicated to be responsible for dwarfism in SDR. Survival time, the markers of oxidative stress, antioxidant enzymes, and resistance to hyperoxia were compared between SDR and SD rats, to investigate whether SDR, a dwarfism rat model, also extends lifespan and has an enhanced resistance to oxidative stress. SDRs lived 38% longer than SD rats on average. This is the first report to show that dwarf rats exhibit lifespan extensions similar to Ames and Snell mice. Decreased 8-oxo-2'-deoxyguanosine (8-oxodG) content, a marker of oxidative DNA damage, indicated suppressed oxidative stress in the liver, kidney, and lung of SDRs. Increased glutathione peroxidase enzyme activity was consistent with decreased 8-oxodG content in the same tissues. The heart and brain showed a similar tendency, but this was not significant. However, the catalase and superoxide dismutase enzyme activities of SDRs were not different from those of SD rats in any tissue. This was not what the original null hypothesis predicted. SDRs had potent resistance to the toxicity associated with high O2 (85%) exposure. The mean survival time in SDRs was more than 147% that of SD rats with 168h O2 exposure. These results suggest that the enhanced resistance to oxidative stress of SDRs associated with enhanced hydrogen peroxide elimination may support its potential role in lifespan extension.

Genetics of longevity in model organisms: debates and paradigm shifts

Discovering the biological basis of aging is one of the greatest remaining challenges for science. Work on the biology of aging has discovered a range of interventions and pathways that control aging rate. A picture is emerging of a signaling network that is sensitive to nutritional status and that controls growth, stress resistance, and aging. This network includes the insulin/IGF-1 and target of rapamycin (TOR) pathways and likely mediates the effects of dietary restriction on aging. Yet the biological processes upon which these pathways act to control life span remain unclear. A long-standing guiding assumption about aging is that it is caused by wear and tear, particularly damage at the molecular level. One view is that reactive oxygen species (ROS), including free radicals, generated as by-products of cellular metabolism, are a major contributor to this damage. Yet many recent tests of the oxidative damage theory have come up negative. Such tests have opened an exciting new phase in biogerontology in which fundamental assumptions about aging are being reexamined and revolutionary concepts are emerging. Among these concepts is the hyperfunction theory, which postulates that processes contributing to growth and reproduction run on in later life, leading to hypertrophic and hyperplastic pathologies. Here we reexamine central concepts about the nature of aging.