Reduced levels of thyroid hormones, insulin, and glucose, and lower body core temperature in the growth hormone receptor/binding protein knockout mouse

The critical role of metabolic pathways in aging

Aging is characterized by a deterioration in the maintenance of homeostatic processes over time, leading to functional decline and increased risk for disease and death. The aging process is characterized metabolically by insulin resistance, changes in body composition, and physiological declines in growth hormone (GH), insulin-like growth factor-1 (IGF-1), and sex steroids. Some interventions designed to address features of aging, such as caloric restriction or visceral fat depletion, have succeeded in improving insulin action and life span in rodents. Meanwhile, pharmacologic interventions and hormonal perturbations have increased the life span of several mammalian species without necessarily addressing features of age-related metabolic decline. These interventions include inhibition of the mammalian target of rapamycin and lifetime deficiency in GH/IGF-1 signaling. However, strategies to treat aging in humans, such as hormone replacement, have mostly failed to achieve their desired response. We will briefly discuss recent advances in our understanding of the complex role of metabolic pathways in the aging process and highlight important paradoxes that have emerged from these discoveries. Although life span has been the major outcome of interest in the laboratory, a special focus is made in this study on healthspan, as improved quality of life is the goal when translated to humans.

Healthy aging: is smaller better? - a mini-review

A recent report of virtually complete protection from diabetes and cancer in a population of people with hereditary dwarfism revived interest in elucidating the relationships between growth, adult body size, age-related disease and longevity. In many species, smaller individuals outlive those that are larger and a similar relationship was shown in studies of various human populations. Adult body size is strongly dependent on the actions of growth hormone (GH) and the absence of GH or GH receptor in mice leads to a remarkable extension of longevity. Many mechanisms that may account for, or contribute to, this association have been identified. It is suggested that modest modifications of the diet at different ages may extend human healthspan and lifespan by reducing levels of hormones that stimulate growth.

Biosensor technology in aging research and age-related diseases

Cell- and tissue-based biosensors comprise genetically engineered proteins that are incorporated into cells ex vivo or into cells of tissues in vivo. They enable the investigator to sense levels of hormones, drugs, or toxins, continuously and noninvasively, using biophotonics or other physical principles, and could potentially be used over the entire lifespan of an experimental animal. The present work reviews the state of the art of cell- and tissue-based biosensors and discusses how they could be of value in aging research. Examples of recently developed biosensors are given, including those that detect levels of a cytokine (TNFα) and drugs (activators of the mTOR pathway). Finally, we discuss the hurdles that would have to be overcome for biosensor technology to be used in humans in monitoring health status and disease treatment in late life.

Long-term calorie restriction, but not endurance exercise, lowers core body temperature in humans

Reduction of body temperature has been proposed to contribute to the increased lifespan in calorie restricted animals and mice overexpressing the uncoupling protein-2 in hypocretin neurons. However, nothing is known regarding the long-term effects of calorie restriction (CR) with adequate nutrition on body temperature in humans. In this study, 24-hour core body temperature was measured every minute by using ingested telemetric capsules in 24 men and women (mean age 53.7±9.4 yrs) consuming a CR diet for an average of 6 years, 24 age- and sex-matched sedentary (WD) and 24 body fat-matched exercise-trained (EX) volunteers, who were eating Western diets. The CR and EX groups were significantly leaner than the WD group. Energy intake was lower in the CR group (1769±348 kcal/d) than in the WD (2302±668 kcal/d) and EX (2798±760 kcal/d) groups (P<0.0001). Mean 24-hour, day-time and night-time core body temperatures were all significantly lower in the CR group than in the WD and EX groups (P≤0.01). Long-term CR with adequate nutrition in lean and weight-stable healthy humans is associated with a sustained reduction in core body temperature, similar to that found in CR rodents and monkeys. This adaptation is likely due to CR itself, rather than to leanness, and may be involved in slowing the rate of aging.

Endocrine parameters and phenotypes of the growth hormone receptor gene disrupted (GHR-/-) mouse

Disruption of the GH receptor (GHR) gene eliminates GH-induced intracellular signaling and, thus, its biological actions. Therefore, the GHR gene disrupted mouse (GHR-/-) has been and is a valuable tool for helping to define various parameters of GH physiology. Since its creation in 1995, this mouse strain has been used by our laboratory and others for numerous studies ranging from growth to aging. Some of the most notable discoveries are their extreme insulin sensitivity in the presence of obesity. Also, the animals have an extended lifespan, which has generated a large number of investigations into the roles of GH and IGF-I in the aging process. This review summarizes the many results derived from the GHR-/- mice. We have attempted to present the findings in the context of current knowledge regarding GH action and, where applicable, to discuss how these mice compare to GH insensitivity syndrome in humans.

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.

Insulin-like growth factor 1-mediated hyperthermia involves anterior hypothalamic insulin receptors

The objective is to investigate the role of insulin-like growth factor 1 (IGF-1) in the regulation of core body temperature. Sequencing cDNA libraries from individual warm-sensitive neurons from the preoptic area (POA) of the hypothalamus, a region involved in the central control of thermoregulation, identified neurons that express both IGF-1 receptor (IGF-1R) and insulin receptor transcripts. The effects of administration of IGF-1 into the POA was measured by radiotelemetry monitoring of core temperature, brown adipose tissue (BAT) temperature, metabolic assessment, and imaging of BAT by positron emission tomography of 2-[(18)F]fluoro-2-deoxyglucose uptake combined with computed tomography. IGF-1 injection into the POA caused dose-dependent hyperthermia that could be blocked by pretreatment with the IGF-1R tyrosine kinase inhibitor, PQ401. The IGF-1-evoked hyperthermia involved activation of brown adipose tissue and was accompanied by a switch from glycolysis to fatty acid oxidation as a source of energy as shown by lowered respiratory exchange ratio. Transgenic mice that lack neuronal insulin receptor expression in the brain (NIRKO mice) were unable to mount the full hyperthermic response to IGF-1, suggesting that the IGF-1 mediated hyperthermia is partly dependent on expression of functional neuronal insulin receptors. These data indicate a novel thermoregulatory role for both IGF-1R and neuronal insulin receptors in IGF-1 activation of BAT and hyperthermia. These central effects of IGF-1 signaling may play a role in regulation of metabolic rate, aging, and the risk of developing type 2 diabetes.

Caloric restriction and longevity: effects of reduced body temperature

Caloric restriction (CR) causes a reduction in body temperature (T(b)) which is suggested to contribute to changes that increase lifespan. Moreover, low T(b) has been shown to improve health and longevity independent of CR. In this review we examine the connections between CR, T(b) and mechanisms that influence longevity and ageing. Recent findings regarding the overlapping mechanisms of CR and T(b) that benefit longevity are discussed, including changes in body composition, hormone regulation, and gene expression, as well as reductions in low-level inflammation and reactive oxygen species-induced molecular damage. This information is summarized in a model describing how CR and low T(b), both synergistically and independently, increase lifespan. Moreover, the nascent notion that the rate of ageing may be pre-programmed in response to environmental influences at critical periods of early development is also considered. Based on current evidence, it is concluded that low T(b) plays an integral role in mediating the effects of CR on health and longevity, and that low T(b) may exert independent biological changes that increase lifespan. Our understanding of the overlap between CR- and T(b)-mediated longevity remains incomplete and should be explored in future research.

Calorie restriction reduces oxidative stress by SIRT3-mediated SOD2 activation

A major cause of aging and numerous diseases is thought to be cumulative oxidative stress, resulting from the production of reactive oxygen species (ROS) during respiration. Calorie restriction (CR), the most robust intervention to extend life span and ameliorate various diseases in mammals, reduces oxidative stress and damage. However, the underlying mechanism is unknown. Here, we show that the protective effects of CR on oxidative stress and damage are diminished in mice lacking SIRT3, a mitochondrial deacetylase. SIRT3 reduces cellular ROS levels dependent on superoxide dismutase 2 (SOD2), a major mitochondrial antioxidant enzyme. SIRT3 deacetylates two critical lysine residues on SOD2 and promotes its antioxidative activity. Importantly, the ability of SOD2 to reduce cellular ROS and promote oxidative stress resistance is greatly enhanced by SIRT3. Our studies identify a defense program that CR provokes to reduce oxidative stress and suggest approaches to combat aging and oxidative stress-related diseases.

Caloric restriction: from soup to nuts

Caloric restriction (CR), reduced protein, methionine, or tryptophan diets; and reduced insulin and/or IGFI intracellular signaling can extend mean and/or maximum lifespan and delay deleterious age-related physiological changes in animals. Mice and flies can shift readily between the control and CR physiological states, even at older ages. Many health benefits are induced by even brief periods of CR in flies, rodents, monkeys, and humans. In humans and nonhuman primates, CR produces most of the physiologic, hematologic, hormonal, and biochemical changes it produces in other animals. In primates, CR provides protection from type 2 diabetes, cardiovascular and cerebral vascular diseases, immunological decline, malignancy, hepatotoxicity, liver fibrosis and failure, sarcopenia, inflammation, and DNA damage. It also enhances muscle mitochondrial biogenesis, affords neuroprotection; and extends mean and maximum lifespan. CR rapidly induces antineoplastic effects in mice. Most claims of lifespan extension in rodents by drugs or nutrients are confounded by CR effects. Transcription factors and co-activators involved in the regulation of mitochondrial biogenesis and energy metabolism, including SirT1, PGC-1alpha, AMPK and TOR may be involved in the lifespan effects of CR. Paradoxically, low body weight in middle aged and elderly humans is associated with increased mortality. Thus, enhancement of human longevity may require pharmaceutical interventions.

Daily energy balance in growth hormone receptor/binding protein (GHR -/-) gene-disrupted mice is achieved through an increase in dark-phase energy efficiency

The goal of this study was to examine factors that contribute to energy balance in female GHR -/- mice. We measured energy intake, energy expenditure (EE), fuel utilization, body mass (M(b)) changes and physical activity in 17month-old female GHR -/- mice and their age-matched wild type littermates. The GHR -/- mice were smaller, consumed more food per unit M(b), had greater EE per unit M(b) and had an increase in 24-h EE/M(b) that was similar to the increase in their surface-area-to-volume ratio. Locomotor activity (LMA) was reduced in the GHR -/- mice, but the energetic cost associated with their LMA was greater than in wild type controls. Furthermore, M(b) and LMA were independent explanatory covariates of most of the variance in EE, and when adjusted for M(b) and LMA, the GHR -/- mice had higher EE during both the light and dark phases of the daily cycle. Respiratory quotient was lower in GHR -/- mice during the light phase, which indicated a greater utilization of lipid relative to carbohydrate in these mice. Additionally, GHR -/- mice had higher ratios of caloric intake to EE at several intervals during the dark phase, and this effect was greater and more sustained in the final 3h of the dark phase. Therefore, we conclude that GHR -/- mice are able to overcome the substantial energetic challenges of dwarfism through several mechanisms that promote stable M(b). Relative to wild type mice, the GHR -/- mice consumed more calories per unit M(b), which offset the disproportionate increase in their daily energy expenditure. While GHR -/- mice oxidized a greater proportion of lipid during the light phase in order to meet their energy requirements, they achieved greater energy efficiency and storage during the dark phase through a combination of higher energy consumption and lower LMA.

Hypothalamic and dietary control of temperature-mediated longevity

Temperature is an important modulator of longevity and aging in both poikilotherms and homeotherm animals. In homeotherms, temperature homeostasis is regulated primarily in the preoptic area (POA) of the hypothalamus. This region receives and integrates peripheral, central and environmental signals and maintains a nearly constant core body temperature (T(core)) by regulating the autonomic and hormonal control of heat production and heat dissipation. Temperature sensitive neurons found in the POA are considered key elements of the neuronal circuitry modulating these effects. Nutrient homeostasis is also a hypothalamically regulated modulator of aging as well as one of the signals that can influence T(core) in homeotherms. Investigating the mechanisms of the regulation of nutrient and temperature homeostasis in the hypothalamus is important to understanding how these two elements of energy homeostasis influence longevity and aging as well as how aging can affect hypothalamic homeostatic mechanisms.

Low serum free triiodothyronine levels mark familial longevity: the Leiden Longevity Study

BACKGROUND

The hypothalamo-pituitary-thyroid axis has been widely implicated in modulating the aging process. Life extension effects associated with low thyroid hormone levels have been reported in multiple animal models. In human populations, an association was observed between low thyroid function and longevity at old age, but the beneficial effects of low thyroid hormone metabolism at middle age remain elusive.

METHODS

We have compared serum thyroid hormone function parameters in a group of middle-aged offspring of long-living nonagenarian siblings and a control group of their partners, all participants of the Leiden Longevity Study.

RESULTS

When compared with their partners, the group of offspring of nonagenarian siblings showed a trend toward higher serum thyrotropin levels (1.65 vs157 mU/L, p = .11) in conjunction with lower free thyroxine levels (15.0 vs 15.2 pmol/L, p = .045) and lower free triiodothyronine levels (4.08 vs 4.14 pmol/L, p = .024).

CONCLUSIONS

Compared with their partners, the group of offspring of nonagenarian siblings show a lower thyroidal sensitivity to thyrotropin. These findings suggest that the favorable role of low thyroid hormone metabolism on health and longevity in model organism is applicable to humans as well.

Genetic predisposition to elevated serum thyrotropin is associated with exceptional longevity

CONTEXT

Exceptional longevity is associated with raised serum TSH.

OBJECTIVE

The aim of this study was to examine whether offspring of people with exceptional longevity have elevated serum TSH and whether specific single nucleotide polymorphisms (SNPs) in the TSH-B gene and TSH receptor (TSHR) gene are associated with this phenotype.

DESIGN/SETTING/PATIENTS

We measured serum TSH and free T(4) in Ashkenazi Jewish centenarians (n = 232; median age, 97 yr), their offspring (n = 366; median age, 69 yr), and age-matched controls without familial longevity (n = 163; median age, 70 yr). We determined TSH heritability, its distribution, and association with SNPs in the TSH-B and TSHR genes.

RESULTS

Offspring had higher median serum TSH [1.68 mIU/liter (97.5% confidence interval, 0.65 to 4.79 mIU/liter)], compared to controls [1.50 mIU/liter (97.5% confidence interval, 0.63 to 3.93 mIU/liter); P = 0.02], with estimated heritability of 0.33 (P = 0.004). Allele frequency of two SNPs in the promoter/enhancer region of TSHR gene, associated with increased serum TSH, was higher in centenarians and their offspring compared to controls (rs10149689 G allele frequency, 0.57 and 0.53 vs. 0.48; P = 0.001 and P = 0.08; odds ratio, 1.56 and 1.22, respectively; and rs12050077 A allele frequency, 0.57 and 0.53 vs. 0.46; P = 0.0001 and P = 0.01; odds ratio, 1.68 and 1.32, respectively). Linkage disequilibrium between the two SNPs was high (r(2) = 0.95), suggesting interaction between them. Furthermore, GA haplotype frequency was significantly higher among centenarians and offspring compared to controls (0.57 and 0.53 vs. 0.46; P = 0.0001 and P = 0.01, respectively).

CONCLUSIONS

A heritable phenotype characterized by raised serum TSH is associated with human longevity. Carriers of rs12050077 and rs10149689 SNPs in the TSHR have higher serum TSH, possibly contributing to decreased thyroid function and longevity.

Two-year body composition analyses of long-lived GHR null mice

Growth hormone receptor gene-disrupted (GHR-/-) mice exhibit increased life span and adipose tissue mass. Although this obese phenotype has been reported extensively for young adult male GHR-/- mice, data for females and for other ages in either gender are lacking. Thus, the purpose of this study was to evaluate body composition longitudinally in both male and female GHR-/- mice. Results show that GHR-/- mice have a greater percent fat mass with no significant difference in absolute fat mass throughout life. Lean mass shows an opposite trend with percent lean mass not significantly different between genotypes but absolute mass reduced in GHR-/- mice. Differences in body composition are more pronounced in male than in female mice, and both genders of GHR-/- mice show specific enlargement of the subcutaneous adipose depot. Along with previously published data, these results suggest a consistent and intriguing protective effect of excess fat mass in the subcutaneous region.

Central ghrelin regulates peripheral lipid metabolism in a growth hormone-independent fashion

GH plays a major role in the regulation of lipid metabolism and alterations in GH axis elicit major changes in fat distribution and mobilization. For example, in patients with GH deficiency (GHD) or in mice lacking the GH receptor, the percentage of fat is increased. In addition to the direct actions of GH on lipid metabolism, current evidence indicates that ghrelin, a stomach-derived peptide hormone with potent GH secretagogue action, increases lipogenesis in white adipose tissue (WAT) through a hypothalamic-mediated mechanism. Still, the mechanism by which GH tone modulates ghrelin actions on WAT remains unclear. Here we investigated the effect of central ghrelin administration on lipid metabolism in lipogenic tissues (liver and WAT) in the absence of GH, by using a model for the study of GHD, namely the spontaneous dwarf rat, which shows increased body fat. Our data demonstrate that central chronic ghrelin administration regulates adipose lipid metabolism, mainly in a GH-independent fashion, as a result of increased mRNA, protein expression, and activity levels of fatty acid metabolism enzymes. On the contrary, central ghrelin regulates hepatic lipogenesis de novo in a GH-independent fashion but lipid mobilization in a GH-dependent fashion because carnitine palmitoyltransferase 1 was decreased only in wild-type Lewis rats. These findings suggest the existence of a new central nervous system-based neuroendocrine circuit, regulating metabolic homeostasis of adipose tissue. Understanding the molecular mechanism underlying the interplay between GH and ghrelin and their effects on lipid metabolism will provide new strategies for the design and development of suitable drugs for the treatment of GHD, obesity, and its comorbidities.

Overexpression of Mn superoxide dismutase does not increase life span in mice

Genetic manipulations of Mn superoxide dismutase (MnSOD), SOD2 expression have demonstrated that altering the level of MnSOD activity is critical for cellular function and life span in invertebrates. In mammals, Sod2 homozygous knockout mice die shortly after birth, and alterations of MnSOD levels are correlated with changes in oxidative damage and in the generation of mitochondrial reactive oxygen species. In this study, we directly tested the effects of overexpressing MnSOD in young (4-6 months) and old (26-28 months) mice on mitochondrial function, levels of oxidative damage or stress, life span, and end-of-life pathology. Our data show that an approximately twofold overexpression of MnSOD throughout life in mice resulted in decreased lipid peroxidation, increased resistance against paraquat-induced oxidative stress, and decreased age-related decline in mitochondrial ATP production. However, this change in MnSOD expression did not alter either life span or age-related pathology.

Insulin sensitivity as a key mediator of growth hormone actions on longevity

Reduced insulin sensitivity and glucose intolerance have been long suspected of having important involvement in aging. Here we report that in studies of calorie restriction (CR) effects in mutant (Prop1(df) and growth hormone receptor knockout [GHRKO]) and normal mice, insulin sensitivity was strongly associated with longevity. Of particular interest was enhancement of the already increased insulin sensitivity in CR df/df mice in which longevity was also further extended and the lack of changes in insulin sensitivity in calorically restricted GHRKO mice in which there was no further increase in average life span. We suggest that enhanced insulin sensitivity, in conjunction with reduced insulin levels, may represent an important (although almost certainly not exclusive) mechanism of increased longevity in hypopituitary, growth hormone (GH)-resistant, and calorie-restricted animals. We also report that the effects of GH treatment on insulin sensitivity may be limited to the period of GH administration.

Alterations in oxygen consumption, respiratory quotient, and heat production in long-lived GHRKO and Ames dwarf mice, and short-lived bGH transgenic mice

Growth hormone (GH) signaling influences longevity in mice, with decreased GH signaling associated with longer life span and increased GH signaling with shortened life span. A proposed mechanism through which GH signaling influences life span postulates that decreased GH signaling lowers metabolic rate, thus slowing aging by decreasing production of damaging free radicals. The influence of altered GH signaling on metabolism was tested by monitoring oxygen consumption (VO(2)), respiratory quotient (RQ), and heat production in long-lived GH receptor knockout (GHRKO) and Ames dwarf mice, and short-lived bovine GH-overexpressing transgenic (bGH TG) mice. Intriguingly, both GHRKO and Ames dwarf mice have increased VO(2) and heat per gram body weight, and decreased RQ, whereas bGH TG mice have decreased VO(2) and heat per gram body weight and increased RQ. In conclusion, decreased GH signaling associates with increased metabolism per body weight and may beneficially affect mitochondrial flexibility by increasing the capacity for fat oxidation; generally, GH excess produces opposite metabolic effects.

Reduced incidence and delayed occurrence of fatal neoplastic diseases in growth hormone receptor/binding protein knockout mice

Although studies of Ames and Snell dwarf mice have suggested possible important roles of the growth hormone (GH)/insulin-like growth factor-1 (IGF-1) axis in aging and age-related diseases, the results cannot rule out the possibility of other hormonal changes playing an important role in the life extension exhibited by these dwarf mice. Therefore, growth hormone receptor/binding protein (GHR/BP) knockout (KO) mice would be valuable animals to directly assess the roles of somatotropic axis in aging and age-related diseases because the primary hormonal change is due to GH/IGF-1 deficiency. Our pathological findings showed GHR/BP KO mice to have a lower incidence and delayed occurrence of fatal neoplastic lesions compared with their wild-type littermates. These changes of fatal neoplasms are similar to the effects observed with calorie restriction and therefore could possibly be a major contributing factor to the extended life span observed in the GHR/BP KO mice.

Endocrine function in naturally long-living small mammals

The complex, highly integrative endocrine system regulates all aspects of somatic maintenance and reproduction and has been widely implicated as an important determinant of longevity in short-lived traditional model organisms of aging research. Genetic or experimental manipulation of hormone profiles in mice has been proven to definitively alter longevity. These hormonally induced lifespan extension mechanisms may not necessarily be relevant to humans and other long-lived organisms that naturally show successful slow aging. Long-lived species may have evolved novel anti-aging defenses germane to naturally retarding the aging process. Here, we examine the available endocrine data associated with the vitamin D, insulin, glucocorticoid and thyroid endocrine systems of naturally long-living small mammals. Generally, long-living rodents and bats maintain tightly regulated lower basal levels of these key pleiotropic hormones than shorter lived rodents. Similarities with genetically manipulated long-lived rodent models of aging suggest that evolutionary well-conserved hormonal mechanisms are integrally involved in lifespan determination.

Hormonal control of aging in rodents: the somatotropic axis

There is a growing body of literature focusing on the somatotropic axis and regulation of aging and longevity. Many of these reports derive data from multiple endocrine mutants, those that exhibit both elevated growth hormone (GH) and insulin-like growth factor I (IGF-1) or deficiencies in one or both of these hormones. In general, both spontaneous and genetically engineered GH and IGF-1 deficiencies have lead to small body size, delayed development of sexual maturation and age-related pathology, and life span extension. In contrast, characteristics of high circulating GH included larger body sizes, early puberty and reproductive senescence, increased cancer incidence and reduced life span when compared to wild-type animals with normal plasma hormone concentrations. This information, along with that found in multiple other species, implicates this anabolic pathway as the major regulator of longevity in animals.

Neuroendocrine inhibition of glucose production and resistance to cancer in dwarf mice

Pit1 null (Snell dwarf) and Proph1 null (Ames dwarf) mutant mice lack GH, PRL and TSH. Snell and Ames dwarf mice also exhibit reduced IGF-I, resistance to cancer and a longer lifespan than control mice. Endogenous glucose production during fasting is reduced in Snell dwarf mice compared to fasting control mice. In view of cancer cell dependence on glucose for energy, low endogenous glucose production may provide Snell dwarf mice with resistance to cancer. We investigated whether endogenous glucose production is lower in Snell dwarf mice during feeding. Inhibition of endogenous glucose production by glucose injection was enhanced in 12 to 14 month-old female Snell dwarf mice. Thus, we hypothesize that lower endogenous glucose production during feeding and fasting reduces cancer cell glucose utilization providing Snell dwarf mice with resistance to cancer. The elevation of circulating adiponectin, a hormone produced by adipose tissue, may contribute to the suppression of endogenous glucose production in 12 to 14 month-old Snell dwarf mice. We compared the incidence of cancer at time of death between old Snell dwarf and control mice. Only 18% of old Snell dwarf mice had malignant lesions at the time of death compared to 82% of control mice. The median ages at death for old Snell dwarf and control mice were 33 and 26 months, respectively. By contrast, previous studies showed a high incidence of cancer in old Ames dwarf mice at the time of death. Hence, resistance to cancer in old Snell dwarf mice may be mediated by neuroendocrine factors that reduce glucose utilization besides elevated adiponectin, reduced IGF-I and a lack of GH, PRL and TSH, seen in both Snell and Ames dwarf mice. Proteomics analysis of pituitary secretions from Snell dwarf mice confirmed the absence of GH and PRL, the secretion of ACTH and elevated secretion of Chromogranin B and Secretogranin II. Radioimmune assays confirmed that circulating Chromogranin B and Secretogranin II were elevated in 12 to 14 month-old Snell dwarf mice. In summary, our results in Snell dwarf mice suggest that the pituitary gland and adipose tissue are part of a neuroendocrine loop that lowers the risk of cancer during aging by reducing the availability of glucose.

Considerations on temperature, longevity and aging

A modest reduction in body temperature prolongs longevity and may retard aging in both poikilotherm and homeotherm animals. Some of the possible mechanisms mediating these effects are considered here with respect to major aging models and theories.

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.

Low utilization of circulating glucose after food withdrawal in Snell dwarf mice

Glucose metabolism is altered in long-lived people and mice. Although it is clear that there is an association between altered glucose metabolism and longevity, it is not known whether this link is causal or not. Our current hypothesis is that decreased fasting glucose utilization may increase longevity by reducing oxygen radical production, a potential cause of aging. We observed that whole body fasting glucose utilization was lower in the Snell dwarf, a long-lived mutant mouse. Whole body fasting glucose utilization may be reduced by a decrease in the production of circulating glucose. Our isotope labeling analysis indicated both gluconeogenesis and glycogenolysis were suppressed in Snell dwarfs. Elevated circulating adiponectin may contribute to the reduction of glucose production in Snell dwarfs. Adiponectin lowered the appearance of glucose in the media over hepatoma cells by suppressing gluconeogenesis and glycogenolysis. The suppression of glucose production by adiponectin in vitro depended on AMP-activated protein kinase, a cell mediator of fatty acid oxidation. Elevated fatty acid oxidation was indicated in Snell dwarfs by increased utilization of circulating oleic acid, reduced intracellular triglyceride content, and increased phosphorylation of acetyl-CoA carboxylase. Finally, protein carbonyl content, a marker of oxygen radical damage, was decreased in Snell dwarfs. The correlation between high glucose utilization and elevated oxygen radical production was also observed in vitro by altering the concentrations of glucose and fatty acids in the media or pharmacologic inhibition of glucose and fatty acid oxidation with 4-hydroxycyanocinnamic acid and etomoxir, respectively.

Endocrine regulation of ageing

Over the past 15 years it has become clear that mutations in genes that regulate endocrine signalling pathways can prolong lifespan. Lifespan can be increased by altered endocrine signalling in a group of cells or a single tissue, which indicates that crosstalk between tissues functions to coordinate ageing of the organism. These endocrine pathways might serve as targets for the manipulation of the ageing process and prevention of age-related diseases.

Hormonal regulation of longevity in mammals

Multiple biological and environmental factors impact the life span of an organism. The endocrine system is a highly integrated physiological system in mammals that regulates metabolism, growth, reproduction, and response to stress, among other functions. As such, this pervasive entity has a major influence on aging and longevity. The growth hormone, insulin-like growth factor-1 and insulin pathways have been at the forefront of hormonal control of aging research in the last few years. Other hormones, including those from the thyroid and reproductive system have also been studied in terms of life span regulation. The relevance of these hormones to human longevity remains to be established, however the evidence from other species including yeast, nematodes, and flies suggest that evolutionarily well-conserved mechanisms are at play and the endocrine system is a key determinant.

Loss of the Par-1b/MARK2 polarity kinase leads to increased metabolic rate, decreased adiposity, and insulin hypersensitivity in vivo

Obesity is a major factor central to the development of insulin resistance and type 2 diabetes. The identification and characterization of genes involved in regulation of adiposity, insulin sensitivity, and glucose uptake are key to the design and development of new drug therapies for this disease. In this study, we show that the polarity kinase Par-1b/MARK2 is required for regulating glucose metabolism in vivo. Mice null for Par-1b were lean, insulin hypersensitive, resistant to high-fat diet-induced weight gain, and hypermetabolic. (18)F-FDG microPET and hyperinsulinemic-euglycemic clamp analyses demonstrated increased glucose uptake into white and brown adipose tissue, but not into skeletal muscle of Par-1b null mice relative to wild-type controls. Taken together, these data indicate that Par-1b is a regulator of glucose metabolism and adiposity in the whole animal and may be a valuable drug target for the treatment of both type 2 diabetes and obesity.

Interaction of growth hormone and calorie restriction

Sustaining health and extending longevity have been perpetual goals of all human societies. For almost as long, there has been an ongoing effort to develop treatments that could prevent aging and, more importantly, make us live longer and more healthily. At present, there is one known intervention that delays aging, increases lifespan and prevents diseases in many animal species: calorie restriction. There are other physiological factors that are believed to have corresponding impacts on longevity and aging, including growth hormone and the insulin/insulin-like growth factor 1 signaling pathway. However, there is still much debate regarding the complex action of growth hormone on lifespan and aging.

Adaptive stress response in segmental progeria resembles long-lived dwarfism and calorie restriction in mice

How congenital defects causing genome instability can result in the pleiotropic symptoms reminiscent of aging but in a segmental and accelerated fashion remains largely unknown. Most segmental progerias are associated with accelerated fibroblast senescence, suggesting that cellular senescence is a likely contributing mechanism. Contrary to expectations, neither accelerated senescence nor acute oxidative stress hypersensitivity was detected in primary fibroblast or erythroblast cultures from multiple progeroid mouse models for defects in the nucleotide excision DNA repair pathway, which share premature aging features including postnatal growth retardation, cerebellar ataxia, and death before weaning. Instead, we report a prominent phenotypic overlap with long-lived dwarfism and calorie restriction during postnatal development (2 wk of age), including reduced size, reduced body temperature, hypoglycemia, and perturbation of the growth hormone/insulin-like growth factor 1 neuroendocrine axis. These symptoms were also present at 2 wk of age in a novel progeroid nucleotide excision repair-deficient mouse model (XPD^G602D/R722W/XPA^−/−) that survived weaning with high penetrance. However, despite persistent cachectic dwarfism, blood glucose and serum insulin-like growth factor 1 levels returned to normal by 10 wk, with hypoglycemia reappearing near premature death at 5 mo of age. These data strongly suggest changes in energy metabolism as part of an adaptive response during the stressful period of postnatal growth. Interestingly, a similar perturbation of the postnatal growth axis was not detected in another progeroid mouse model, the double-strand DNA break repair deficient Ku80^−/− mouse. Specific (but not all) types of genome instability may thus engage a conserved response to stress that evolved to cope with environmental pressures such as food shortage.

Oxidative damage to cellular components, including fats, proteins, and DNA, is an inevitable consequence of cellular energy use and may underlie both normal and pathological aging. Calorie restriction delays the aging process and extends lifespan in a number of lower organisms including rodents. Inborn defects in the postnatal growth axis resulting in dwarfism can also extend lifespan. Both may function via overlapping pathways impacting on energy metabolism. Here, we report a novel DNA repair-deficient mouse model with symptoms of the related premature aging disorders Cockayne syndrome and trichothiodystrophy, namely reduced fat deposits, neurological dysfunction, failure to thrive, and reduced lifespan. Surprisingly, we also observed traits usually associated with extended longevity as found in calorie restriction and dwarfism, including reduced blood sugar and reduced insulin-like growth factor-1. These characteristics were present at 2 wk of age, that is, during the period of rapid postnatal development, but returned to normal by sexual maturation at 10 wk. Furthermore, they were absent altogether in another premature aging mouse model with a distinct DNA repair defect. Specific types of unrepaired DNA damage may thus elicit a preservative organismal response affecting energy metabolism that is similar to the one that evolved to cope with the stress of food shortage.

Reduced recruitment and survival of primordial and growing follicles in GH receptor-deficient mice

GH influences female fertility. The goal of the present study was to obtain more insight into the effect of loss of GH signalling, as observed in humans suffering from Laron syndrome, on ovarian function. Therefore, serial paraffin sections of ovaries of untreated and IGF-I-treated female GH receptor knock-out (GHR/GHBP-KO) mice were examined to determine the follicular reserve and the percentage of follicular atresia in each ovary. Our observations demonstrate that the amount of primordial follicles was significantly elevated in GHR/GHBP-KO mice, while the numbers of primary, preantral and antral follicles were lower compared with wild-type values. The reduced number of healthy growing follicles in GHR/GHBP-KO mice was accompanied by a significant increase in the percentage of atretic follicles. IGF-I treatment of GHR/GHBP-KO mice for 14 days resulted in a reduced number of primordial follicles, an increased number of healthy antral follicles, and a decreased percentage of atretic follicles. The results of the present study suggest that GH may play a role, either directly or indirectly, via for instance IGF-I, in the recruitment of primordial follicles into the growing pool. Furthermore, GH seems to protect antral follicles, directly or indirectly from undergoing atresia.

Aging-related characteristics of growth hormone receptor/binding protein gene-disrupted mice

Since generation of the growth hormone receptor/binding protein (GHR/BP) gene-disrupted mouse nearly 10 years ago, use of this mouse model has become widespread in the elucidation of the physiological roles of GH and insulin-like growth factor-1 (IGF-1). In particular, it serves as a useful model to study mechanisms of aging. This review highlights the evidence demonstrating that the loss of GH signaling leads to lifespan extension in mice, and presents the multiple characteristics of this mouse line that suggest the life extension is due to alteration of the aging process.

Do long-lived mutant and calorie-restricted mice share common anti-aging mechanisms?--a pathological point of view

Rodent models are an invaluable resource for studying the mechanism of mammalian aging. In recent years, the availability of transgenic and knockout mouse models has facilitated the study of potential mechanisms of aging. Since 1996, aging studies with several long-lived mutant mice have been conducted. Studies with the long-lived mutant mice, Ames and Snell dwarf, and growth hormone receptor/binding protein knockout mice, are currently providing important clues regarding the role of the growth hormone/insulin like growth factor-1 axis in the aging process. Interestingly, these studies demonstrate that these long-lived mutant mice have physiological characteristics that are similar to the effects of calorie restriction, which has been the most effective experimental manipulation capable of extending lifespan in various species. However, a question remains to be answered: do these long-lived mutant and calorie-restricted mice extend their lifespan through a common underlying mechanism?

Effect of growth hormone on susceptibility to diet-induced obesity

Mice with a deficiency in GH function due to disruption of the GH receptor/binding protein gene (GHR(-/-)) are long lived, insulin sensitive, and obese, whereas mice with excess GH function due to expression of a bovine GH transgene (bGH) are short lived, glucose intolerant, and lean. When challenged with a high-fat (HF) diet, we hypothesized that these mice would be differentially susceptible to diet-induced obesity. To test this hypothesis, GHR(-/-), bGH, and littermate control (WT) mice were fed a HF diet (40% kcal) or a nutrient-matched low-fat diet (9% kcal) for 12 wk. On the HF diet, all mice, regardless of genotype, showed a similar percent weight gain and exhibited a significant increase in percent body fat and the mass of epididymal, retroperitoneal, and sc fat pads. For bGH mice, the increase in adipose tissue was relatively small, compared with the WT or GHR(-/-) mice, suggesting some resiliency, although not immunity, to diet-induced obesity. GHR(-/-) mice, which are relatively obese on a low-fat diet, responded to the dietary challenge in a manner similar to WT controls. With HF feeding, all genotypes experienced an increase in insulin levels and depot-dependent effect of adipose tissue. Together, these results further support a role for GH in energy balance regulation and nutrient partitioning. More importantly, because there were genotype-specific effects of diet, these data stress the importance of diet selection and sampling multiple adipose depots in studies with these mouse models.

Methionine flux to transsulfuration is enhanced in the long living Ames dwarf mouse

Long-lived Ames dwarf mice lack growth hormone, prolactin, and thyroid stimulating hormone. Additionally the dwarf mice have enzyme activities and levels that combat oxidative stress more efficiently than those of normal mice. We have shown that methionine metabolism in Ames mice is markedly different than in their wild type littermates. In our previous work we hypothesized that the flux of methionine to the transsulfuration pathway is enhanced in the dwarf mice. The current study was designed to determine whether the flux of methionine to the transsulfuration pathway is increased. We did this by injecting either l-[methyl-(3)H]-methionine or l-[(35)S]-methionine into dwarf or normal mice and then determined retained label (in form of S-adenosylmethionine) 45 min later. The amount of retained hepatic (3)H and (35)S label was significantly reduced in the dwarf mice; at 45 min the specific radioactivity of SAM (pCi/nmol SAM) was 56% lower (p < 0.05) for (3)H-label and 64% lower (p < 0.005) for (35)S-label in dwarf than wild type mice. Retention of (35)S was significantly lower in the brain (37%, p < .04) and kidney (47%, p < 0.02) of the dwarf compared to wild type mice; there was no statistical difference in retained (3)H-label in either brain or kidney. This suggests that both the methyl-moiety and the carbon chain of methionine are lost much faster in the dwarf compared to the wild type mouse, implying that both transmethylation in the liver and transsulfuration in the liver, brain, and kidney are increased in the dwarf mice. As further support, we determined by real-time RT PCR the expression of methionine metabolism genes in livers of mice. Compared to wild type, the Ames dwarf had increased expression of methionine adenosyltransferase 1a (2.3-fold, p = 0.013), glycine N-methyltransferase (3.8-fold, p = 0.023), betaine homocysteine methyltransferase (5.5-fold, p = 0.0006), S-adenosylhomocysteine hydrolase (3.8-fold, p = 0.0005), and cystathionase (2.6-fold; tended to be increased, p = 0.055). Methionine synthase expression was significantly decreased in dwarf compared to wild type (0.48-fold, p = 0.023). These results confirm that the flux of methionine to transsulfuration is enhanced in the Ames dwarf. This, along with data from previous studies support the hypothesis that altered methionine metabolism plays a significant role in the oxidative defense of the dwarf mouse and that the mechanism for the enhanced oxidative defense may be through altered GSH metabolism as a result of the distinctive methionine metabolism.

Growth hormone receptor deficiency results in blunted ghrelin feeding response, obesity, and hypolipidemia in mice

We have previously shown that growth hormone (GH) overexpression in the brain increased food intake, accompanied with increased hypothalamic agouti-related protein (AgRP) expression. Ghrelin, which stimulates both appetite and GH secretion, was injected intracerebroventricularly to GHR-/- and littermate control (+/+) mice to determine whether ghrelin's acute effects on appetite are dependent on GHR signaling. GHR-/- mice were also analyzed with respect to serum levels of lipoproteins, apolipoprotein (apo)B, leptin, glucose, and insulin as well as body composition. Central injection of ghrelin into the third dorsal ventricle increased food consumption in +/+ mice, whereas no change was observed in GHR-/- mice. After ghrelin injection, AgRP mRNA expression in the hypothalamus was higher in +/+ littermates than in GHR-/- mice, indicating a possible importance of AgRP in the GHR-mediated effect of ghrelin. Compared with controls, GHR-/- mice had increased food intake, leptin levels, and total and intra-abdominal fat mass per body weight and deceased lean mass. Moreover, serum levels of triglycerides, LDL and HDL cholesterol, and apoB, as well as glucose and insulin levels were lower in the GHR-/- mice. In summary, ghrelin's acute central action to increase food intake requires functionally intact GHR signaling. Long-term GHR deficiency in mice is associated with high plasma leptin levels, obesity, and increased food intake but a marked decrease in all lipoprotein fractions.

Medullary thyroid carcinoma arises in the absence of prolactin signaling

Prolactin, a pituitary hormone, exerts pleiotropic effects in various cells. These effects are mediated by a membrane receptor highly expressed in many tissues. To analyze prolactin effects on the thyroid gland, we first identified prolactin receptor (PRLR) mRNAs by in situ hybridization. To further evaluate the physiologic relevance of PRLR actions in the thyroid in vivo, we used PRLR knockout mice. Whereas the histologic structure of thyroid of PRLR-null mice was not disturbed, we show that T4 levels are lower in null animals (13.63 +/- 2.98 versus 10.78 +/- 2.25 pmol/L in null mice), confirming that prolactin participates in the control of thyroid metabolism. To further investigate thyroid effects in mice, we measured body temperature and thyroid-stimulating hormone in young and adult male and/or female PRLR-null mice and their normal siblings. Surprisingly, in null animals, we saw medullary thyroid carcinoma (MTC) arising from parafollicular C cells producing calcitonin. The incidence of these carcinomas attained 41% in PRLR-null mice, whereas this malignant tumor occurs sporadically or as a component of the familial cancer syndrome in humans. This finding suggests that PRLR-null mice could represent a valuable animal model for MTC, which could be compared with existing MTC models. These observations suggest a possible link between the appearance of this carcinoma and the absence of prolactin signaling.

Effect of rapid-eye-movement sleep deprivation on rat hypothalamic prostaglandins

In this study, we investigated thyroid hormones, thyroid stimulating hormone (TSH), prostaglandin D(2) (PGD(2)) and prostaglandin E(2) (PGE(2)) levels in rapid-eye-movement (REM) sleep-deprived rats compared with controls. The aim of the present study was to detect the effect of REM sleep deprivation (RSD) especially on hypothalamic prostaglandin levels. Twenty-seven male rats were randomly assigned in three groups as dry cage control, yoked control, and RSD. RSD rats were sleep deprived for 10 consecutive days. At the end of 10th day all rats were sacrificed for measurement. Our results indicated that total triiodothyronine (T(3)) and thyroxine (T(4)) decreased in the RSD group while there was no change in TSH. We also measured hypothalamic PGD(2) and PGE(2) levels, but we could not find any significant change between groups.

Age-related cataract progression in five mouse models for anti-oxidant protection or hormonal influence

Five mouse models with known alterations of resistance to oxidative damage were compared by slit lamp examination for the presence and degree of advancement of age-related cataract in young adult and old animals along with wild type controls. A group of young and old normal C57BL/6Jax mice were examined first to constitute a standard, and they were found to exhibit age-related cataract development. Following this, four models on the C57BL/6 background with imposed genetic alterations affecting anti-oxidant enzyme presence or activity, and one outbred model in which a deletion blocked the growth hormone/IGF-1 axis, were similarly examined. There was no evidence of foetal or juvenile cataract development in any of these models, and an age-related severity for lens opacities was shown between young adult and old mice in all groups. Model 1, mice null for the anti-oxidant gene glutathione peroxidase-1 (GPX1) had significantly advanced cataracts in older mice vs. same age controls. In mouse model 2 hemizygous knockout of SOD2 (MnSOD) did not affect age-related cataract development. In model 3 combining the GPX1 and SOD2 deficiencies in the same animal did not advance cataract development beyond that of the GPX1 null alone. In model 4 the addition of anti-oxidant protection in the lens by transfection of human catalase targeted only to the mitochondria resulted in a significant delay in cataract development. The 5th model, growth hormone receptor knockout (GHR-/-) mice, also demonstrated a significant reduction in age-related cataract development, as well as dwarfism. These findings, in general, support the oxidative theory of age-related cataract development. The exception, the partial deletion of SOD2 in the hemizygous KO model, probably did not represent a sufficiently severe deprivation of anti-oxidant protection to produce pathologic changes in the lens.

Influence of the crosstalk between growth hormone and insulin signalling on the modulation of insulin sensitivity

Growth hormone (GH) is an important modulator of insulin sensitivity. Multiple mechanisms appear to be involved in this modulatory effect. GH does not interact directly with the insulin receptor (IR), but conditions of GH excess are associated in general with hyperinsulinemia that induces a reduction of IR levels and impairment of its kinase activity. Several post-receptor events are shared between GH and insulin. This signaling crosstalk could be involved in the diabetogenic effects of GH. The utilization of animal models of GH excess, deficiency or resistance provided evidence that the signaling pathway leading to stimulation of the phosphatidylinositol 3-kinase (PI3K)/Akt cascade is an important site of regulation, and pointed to the liver as the major site of GH-induced insulin resistance. In skeletal muscle, GH-induced insulin resistance might involve an increase in the amount of the p85 subunit of PI3K that plays a negative role in insulin signalling. GH also reduces insulin sensitivity by enhancing events that negatively modulate insulin signaling such as stimulation of serine phosphorylation of IRS-1, which prevents its recruitment to the IR and induction of the suppressor of cytokine signalling (SOCS)-1 and SOCS-3 which modulate the signalling potential of the IRS proteins. In addition, GH has been shown to decrease the expression of the insulin-sensitizing adipo-cytokines adiponectin and visfatin. Finally, genetic manipulation of mice indicated that whereas GH plays a major role in reducing insulin sensitivity, circulating IGF-I also participates in the control of insulin sensitivity and plays an important role in the hormonal balance between GH and insulin.

Minireview: role of the growth hormone/insulin-like growth factor system in mammalian aging

The important role of IGF and insulin-related signaling pathways in the control of longevity of worms and insects is very well documented. In the mouse, several spontaneous or experimentally induced mutations that interfere with GH biosynthesis, GH actions, or sensitivity to IGF-I lead to extended longevity. Increases in the average life span in these mutants range from approximately 20-70% depending on the nature of the endocrine defect, gender, diet, and/or genetic background. Extended longevity of hypopituitary and GH-resistant mice appears to be due to multiple mechanisms including reduced insulin levels, enhanced insulin sensitivity, alterations in carbohydrate and lipid metabolism, reduced generation of reactive oxygen species, enhanced resistance to stress, reduced oxidative damage, and delayed onset of age-related disease. There is considerable evidence to suggest that the genetic and endocrine mechanisms that influence aging and longevity in mice may play a similar role in other mammalian species, including the human.

Adult-onset growth hormone and insulin-like growth factor I deficiency reduces neoplastic disease, modifies age-related pathology, and increases life span

Disruption of the insulin/IGF-I pathway increases life span in invertebrates. However, effects of decreased IGF-I signaling in mammalian models remain controversial. Using a rodent model with a specific and limited deficiency of GH and IGF-I, we report that GH and IGF-I deficiency throughout life [GH deficiency (GHD)] has no effect on life span compared with normal, heterozygous animals. However, treatment of GHD animals with GH from 4-14 wk of age [adult-onset (AO) GHD] increased median and maximal life span by 14% and 12%, respectively. Analysis of end-of-life pathology indicated that deficiency of these hormones decreased tumor incidence in GHD and AO-GHD animals (18 and 30%, respectively) compared with heterozygous animals and decreased the severity of, and eliminated deaths from, chronic nephropathy. Total disease burden was reduced by 24% in GHD and 16% in AO-GHD animals. Interestingly, the incidence of intracranial hemorrhage increased by 154 and 198% in GHD and AO-GHD animals, respectively, compared with heterozygous animals. Deaths from intracranial hemorrhage in AO-GHD animals were delayed by 14 wk accounting for the increased life span compared with GHD animals. The presence of GH and IGF-I was necessary to maximize reproductive fitness and growth of offspring early in life and to maintain cognitive function and prevent cartilage degeneration later in life. The diverse effects of GH and IGF-I are consistent with a model of antagonistic pleiotropy and suggest that, in response to a deficiency of these hormones, increased life span is derived at the risk of functional impairments and tissue degeneration.

Ageing and metabolism: drug discovery opportunities

There has recently been significant progress in our understanding of the mechanisms that regulate ageing, and it has been shown that changes in single genes can dramatically extend lifespan and increase resistance to many diseases. Furthermore, many of these genes belong to evolutionarily conserved pathways that also control energy metabolism. In this review, we describe the shared molecular machinery that regulates ageing and energy metabolism. Although drugs to slow ageing face severe regulatory hurdles, it is likely that an understanding of ageing pathways will help to identify novel drug targets to treat metabolic disorders and other age-related diseases.

Hyperglycemia, impaired glucose tolerance and elevated glycated hemoglobin levels in a long-lived mouse stock

We have previously demonstrated that two wild-derived stocks of mice, Idaho and Majuro, are significantly longer-lived than mice of a control stock (DC) generated as a four-way cross of commonly used laboratory strains of mice. This study provides independent confirmation of this earlier finding, as well as examining serum glucose, insulin, leptin, glycated hemoglobin (GHb), cataract severity, and glucose tolerance levels in each of the stocks. Both the mean (+20%) and maximum (+13%) life span of the Idaho mice were significantly increased relative to the DC stock, while in the Majuro mice only maximum (+15%) life span was significantly increased. In addition, Majuro mice were hyperglycemic in both the fed and fasted states compared both to laboratory-derived and Idaho stocks, had significantly elevated GHb levels and cataract scores, and were glucose intolerant although serum insulin levels did not differ between stocks. Body weight and body mass index (BMI)-corrected leptin levels were also dramatically (1.5-3-fold) higher in the Majuro mice. The longevity of Id mice was not accompanied by changes in serum glucose and insulin levels, or glucose tolerance compared to DC controls, although GHb levels were significantly lower in the Idaho mice. Taken together, these findings suggest that neither a reduction of blood glucose levels nor an increase in glucose tolerance is necessary for life span extension in mice.