LET-60 RAS modulates effects of insulin/IGF-1 signaling on development and aging in Caenorhabditis elegans

The DAF-2 insulin/insulin-like growth factor 1 (IGF-1) receptor signals via a phosphatidylinositol 3-kinase (PI3K) pathway to control dauer larva formation and adult longevity in Caenorhabditis elegans. Yet epistasis analysis suggests signal bifurcation downstream of DAF-2. We have used epistasis analysis to test whether the Ras pathway (which plays a role in signaling from mammalian insulin receptors) acts downstream of DAF-2. We find that an activated Ras mutation, let-60(n1046gf), weakly suppresses constitutive dauer diapause in daf-2 and age-1 (PI3K) mutants. Moreover, increased Ras pathway signaling partially suppresses the daf-2 mutant feeding defect, while reduced Ras pathway signaling enhances it. By contrast, activated Ras extends the longevity induced by mutation of daf-2, while reduced Ras pathway signaling partially suppresses it. Thus, Ras pathway signaling appears to act with insulin/IGF-1 signaling during larval development, but against it during aging.     

Insulin and aging

In invertebrates, signaling pathways homologous to mammalian insulin and insulin-like growth factor (IGF-1) signal transduction have a major role in the control of longevity. There are numerous indications that these pathways also influence aging in mammals, but separating the role of insulin from the effects of IGF-1 and growth hormone (GH) is difficult. In mice, selective disruption of the insulin receptor in the adipose tissue extends longevity. Increases in lifespan were also reported in mice with deletion of insulin receptor substrate 1 (IRS1) in whole body or IRS2 only in the brain. GH deficiency or resistance in mutant mice leads to hypoinsulinemia and enhanced insulin sensitivity along with remarkably extended longevity. These characteristics resemble animals subjected to calorie restriction. Studies of physiological characteristics and polymorphisms of insulin-related genes in exceptionally long-lived people suggest a role of insulin signaling in the control of human aging.     

Longevity effect of IGF‐1R+/− mutation depends on genetic background‐specific receptor activation

Growth hormone (GH) and insulin-like growth factor (IGF) signaling regulates lifespan in mice. The modulating effects of genetic background gained much attention because it was shown that life-prolonging effects in Snell dwarf and GH receptor knockout vary between mouse strains. We previously reported that heterozygous IGF-1R inactivation (IGF-1R^+/−) extends lifespan in female mice on 129/SvPas background, but it remained unclear whether this mutation produces a similar effect in other genetic backgrounds and which molecules possibly modify this effect. Here, we measured the life-prolonging effect of IGF-1R^+/− mutation in C57BL/6J background and investigated the role of insulin/IGF signaling molecules in strain-dependent differences. We found significant lifespan extension in female IGF-1R^+/− mutants on C57BL/6J background, but the effect was smaller than in 129/SvPas, suggesting strain-specific penetrance of longevity phenotypes. Comparing GH/IGF pathways between wild-type 129/SvPas and C57BL/6J mice, we found that circulating IGF-I and activation of IGF-1R, IRS-1, and IRS-2 were markedly elevated in 129/SvPas, while activation of IGF pathways was constitutively low in spontaneously long-lived C57BL/6J mice. Importantly, we demonstrated that loss of one IGF-1R allele diminished the level of activated IGF-1R and IRS more profoundly and triggered stronger endocrine feedback in 129/SvPas background than in C57BL/6J. We also revealed that acute oxidative stress entails robust IGF-1R pathway activation, which could account for the fact that IGF-1R^+/− stress resistance phenotypes are fully penetrant in both backgrounds. Together, these results provide a possible explanation why IGF-1R^+/− was less efficient in extending lifespan in C57BL/6J compared with 129/SvPas.     

Brain IGF-1 receptors control mammalian growth and lifespan through a neuroendocrine mechanism

Mutations that decrease insulin-like growth factor (IGF) and growth hormone signaling limit body size and prolong lifespan in mice. In vertebrates, these somatotropic hormones are controlled by the neuroendocrine brain. Hormone-like regulations discovered in nematodes and flies suggest that IGF signals in the nervous system can determine lifespan, but it is unknown whether this applies to higher organisms. Using conditional mutagenesis in the mouse, we show that brain IGF receptors (IGF-1R) efficiently regulate somatotropic development. Partial inactivation of IGF-1R in the embryonic brain selectively inhibited GH and IGF-I pathways after birth. This caused growth retardation, smaller adult size, and metabolic alterations, and led to delayed mortality and longer mean lifespan. Thus, early changes in neuroendocrine development can durably modify the life trajectory in mammals. The underlying mechanism appears to be an adaptive plasticity of somatotropic functions allowing individuals to decelerate growth and preserve resources, and thereby improve fitness in challenging environments. Our results also suggest that tonic somatotropic signaling entails the risk of shortened lifespan.     

Lifespan extension in hypomorphic daf-2 mutants of Caenorhabditis elegans is partially mediated by glutathione transferase CeGSTP2-2

Electrophilic stress caused by lipid peroxidation products such as 4-hydroxynonenal (4-HNE) and/or related compounds may contribute to aging. The major mode of 4-HNE metabolism involves glutathione conjugation catalyzed by specialized glutathione transferases. We have previously shown that glutathione transferase CeGSTP2-2, the product of the Caenorhabditis elegans gst-10 gene, has the ability to conjugate 4-HNE, and that its overexpression extends lifespan of C. elegans. We now demonstrate that the expression level of CeGSTP2-2 correlates highly with lifespan in a series of hypomorphic daf-2 mutants of C. elegans. The overexpression of CeGSTP2-2 in daf-2 is abrogated in daf-16; daf-2 mutants, indicating that expression of the gst-10 gene is modulated by insulin-like growth factor signaling. To determine whether the relationship between CeGSTP2-2 and lifespan is causal, we used RNAi to knock down CeGSTP2-2. Treatment with gst-10-specific dsRNA decreased CeGSTP2-2 protein in wild-type N2 and in daf-2 strains to an approximately equal level. The ability to conjugate 4-HNE was similarly decreased by RNAi, suggesting that the increment of that activity in daf-2 over N2 is due largely to the overexpression of CeGSTP2-2. RNAi-mediated knock-down of CeGSTP2-2 led to an increased susceptibility to 4-HNE, paraquat, and heat shock, and to a shortening of lifespan by 13% in both N2 and daf-2 strains. These results indicate that CeGSTP2-2 significantly contributes to the maintenance of the soma, and that this function is augmented in daf-2 mutants concordantly with other longevity assurance genes, probably via insulin-like growth factor signaling.     

The DAF-2 insulin-like signaling pathway independently regulates aging and immunity in C. elegans

The Caenorhabditis elegans DAF-2 insulin-like signaling pathway, which regulates lifespan and stress resistance, has also been implicated in resistance to bacterial pathogens. Loss-of-function daf-2 and age-1 mutants have increased lifespans and are resistant to a variety of bacterial pathogens. This raises the possibility that the increased longevity and the pathogen resistance of insulin-like signaling pathway mutants are reflections of the same underlying mechanism. Here we report that regulation of lifespan and resistance to the bacterial pathogen Pseudomonas aeruginosa is mediated by both shared and genetically distinguishable mechanisms. We find that loss of germline proliferation enhances pathogen resistance and this effect requires daf-16, similar to the regulation of lifespan. In contrast, the regulation of pathogen resistance and lifespan is decoupled within the DAF-2 pathway. Long-lived mutants of genes downstream of daf-2, such as pdk-1 and sgk-1, show wildtype resistance to pathogens. However, mutants of akt-1 and akt-2, which we find to individually have modest effects on lifespan, show enhanced resistance to pathogens. We also demonstrate that pathogen resistance of daf-2, akt-1, and akt-2 mutants is associated with restricted bacterial colonization, and that daf-2 mutants are better able to clear an infection after challenge with P. aeruginosa. Moreover, we find that pathogen resistance among insulin-like signaling mutants is associated with increased expression of immunity genes during infection. Other processes that affect organismal longevity, including Jun kinase signaling and caloric restriction, do not affect resistance to bacterial pathogens, further establishing that aging and innate immunity are regulated by genetically distinct mechanisms.     

Low insulin‐like growth factor‐1 level predicts survival in humans with exceptional longevity

Attenuated growth hormone and insulin-like growth factor-1 (GH/IGF-1) signaling is associated with extended lifespan in several animal models. However, the effect of diminished GH/IGF-1 activity on survival in humans has not been confirmed. We tested the hypothesis that IGF-1 levels in nonagenarians (n = 184), measured at study enrollment, predict the duration of their incremental survival. In the Kaplan–Meier analysis, females with IGF-1 levels below the median (≤ 96 ng mL⁻¹) had significantly longer survival compared with females with levels above the median, P < 0.01. However, this survival advantage was not observed in males (P = 0.83). On the other hand, in both males and females with a history of cancer, lower IGF-1 levels predicted longer survival (P < 0.01). IGF-1 level remained a significant predictor of survival duration in linear regression models after multivariable adjustment in females (P = 0.01) and individuals with a history of cancer (P < 0.01). We show for the first time that low IGF-1 levels predict life expectancy in exceptionally long-lived individuals.     

Single-gene mutations and healthy ageing in mammals

Studies of the effects of single-gene mutations on longevity in Caenorhabditis elegans, Drosophila melanogaster and Mus musculus identified homologous, highly conserved signalling pathways that influence ageing. In each of these very distantly related species, single mutations which lead-directly or indirectly-to reduced insulin, insulin-like growth factor (IGF) or insulin/IGF-like signalling (IIS) can produce significant increases in both average and maximal lifespan. In mice, most of the life-extending mutations described to date reduce somatotropic (growth hormone (GH) and IGF-1) signalling. The reported extensions of longevity are most robust in GH-deficient and GH-resistant mice, while suppression of somatotropic signalling 'downstream' of the GH receptor produces effects that are generally smaller and often limited to female animals. This could be due to GH influencing ageing by both IGF-1-mediated and IGF-1-independent mechanisms. In mutants that have been examined in some detail, increased longevity is associated with various indices of delayed ageing and extended 'healthspan'. The mechanisms that probably underlie the extension of both lifespan and healthspan of these animals include increased stress resistance, improved antioxidant defences, alterations in insulin signalling (e.g. hypoinsulinaemia combined with improved insulin sensitivity in some mutants and insulin resistance in others), a shift from pro- to anti-inflammatory profile of circulating adipokines, reduced mammalian target of rapamycin-mediated translation and altered mitochondrial function including greater utilization of lipids when compared with carbohydrates.     

Role of Insulin and Growth Hormone/Insulin-Like Growth Factor-I Signaling in Lifespan Extension: Rodent Longevity Models for Studying Aging and Calorie Restriction

Insulin/insulin-like growth factor-I (IGF-I) pathways are recognized as critical signaling pathways involved in the control of lifespans in lower organisms to mammals. Caloric restriction (CR) reduces plasma concentration of insulin, growth hormone (GH), and IGF-I. CR retards various age-dependent disorders such as nuerodegenerative diseases and extends lifespan in laboratory rodents. These beneficial effects of CR are partly mimicked in spontaneous or genetically engineered rodent models of reduced insulin and GH/IGF-I axis. Most of these long-living rodents show increased insulin sensitivity; however, recent study has revealed that some other rodents show normal or reduced insulin sensitivity. Thus, increased insulin sensitivity might be not prerequisite for lifespan extension in insulin/GH/IGF-I altered longevity rodent models. These results highlighted that, for lifespan extension, the intracellular signaling molecules of insulin/GH/IGF-I pathways might be more important than actual peripheral or systemic insulin action.     

Role of the growth hormone-insulin-like growth factor-1-insulin system signaling in aging and longevity: evolutional aspect

Growth hormone (GH)/ insulin-like growth factor 1 (IGF-1)/ insulin signaling molecules linked to longevity include DAF-2 and insulin-receptor and their homologues in mammals, and to inactivation of corresponding genes followed by increased life span in nematodes, fruit flies, and mice. It is possible that the life-prolonging effect of calorie restriction is due to decreasing IGF-1 levels. A search of pharmacological modulators of life-span-extending mutations in the GH/IGF-1/insulin signaling pathway and mimetic effects of caloric restriction is a priority directions in the regulation of longevity. Some literature and our own observations suggest that antidiabetic drugs could be promising candidates for both life span extension and prevention of cancer.     

A heady message for lifespan regulation

Mutant Caenorhabditis elegans in which the age-1 and daf-2 genes (involved in insulin-receptor-like signalling) are expressed at low levels exhibit extended lifespan. Wolkow and colleagues recently showed that restricted re-expression of age-1 and daf-2 genes in neurons of these mutants rescues wild-type lifespan as effectively as ubiquitous re-expression. Low levels of insulin-like signalling in neurons might control longevity by enhancing protection against free radical damage. However, in mammalian cells (including neurons) reduced insulin-like signalling is generally thought to be deleterious to antioxidant defence and to neuron survival. Here we discuss the new work and several hypotheses to explain this apparent contradiction.     

Impact of reduced insulin-like growth factor-1/insulin signaling on aging in mammals: novel findings

Growth hormone deficiency or resistance resulting from spontaneous or experimentally produced mutations in laboratory mice delay aging and increase lifespan. Alterations in insulin-like growth factor-1 (IGF-1) and insulin signaling emerged as likely mechanisms linking growth hormone and aging, and increased longevity was reported in mice with selective deletion of IGF-1 receptor in all tissues or insulin receptor in fat. Recent studies in mice with reduced IGF-1 levels or deletion of pregnancy-associated plasma protein-A, a protease that cleaves one of the IGF-1 binding proteins, strongly support the role of IGF-1 in the control of longevity. Reports of increased lifespan in mice with deletion of insulin receptor substrate (IRS) 1, reduced expression of IRS2, or selective deletion of IRS2 in the brain specifically implicate the IRS-PI3K-Akt-Foxo signaling pathway (which is shared by IGF-1 and insulin) in the control of aging. These important novel findings also strengthen the evidence for evolutionary conservation of mechanisms regulating lifespan in worms, insects and mammals.     

Caenorhabditis elegans SDF-9 enhances insulin/insulin-like signaling through interaction with DAF-2

SDF-9 is a modulator of Caenorhabditis elegans insulin/IGF-1 signaling that may interact directly with the DAF-2 receptor. SDF-9 is a tyrosine phosphatase-like protein that, when mutated, enhances many partial loss-of-function mutants in the dauer pathway except for the temperature-sensitive mutant daf-2(m41). We propose that SDF-9 stabilizes the active phosphorylated state of DAF-2 or acts as an adaptor protein to enhance insulin-like signaling.     

Metabolism, physiology and stress defense in three aging Ins/IGF-1 mutants of the nematode Caenorhabditis elegans

The insulin/insulin-like growth factor-1 (Ins/IGF-1) pathway regulates the aging rate of the nematode Caenorhabditis elegans. We describe other features of the three Ins/IGF-1 mutants daf-2, age-1 and aap-1. We show that the investigated Ins/IGF-1 mutants all have a reduced body volume, reduced reproductive capacity, increased ATP concentrations and an elevated stress resistance. We also observed that heat production is lower in these mutants, although the respiration rate was similar or higher compared with wild-type individuals, suggesting a metabolic shift in these mutants.