A critical analysis of the role of growth hormone and IGF-1 in aging and lifespan

Reduced insulin/IGF-1 signalling and human longevity

Evidence is accumulating that aging is hormonally regulated by an evolutionarily conserved insulin/IGF-1 signalling (IIS) pathway. Mutations in IIS components affect lifespan in Caenorhabditis elegans, Drosophila melanogaster and mice. Most long-lived IIS mutants also show increased resistance to oxidative stress. In D. melanogaster and mice, the long-lived phenotype of several IIS mutants is restricted to females. Here, we analysed the relationship between IIS signalling, body height and longevity in humans in a prospective follow-up study. Based on the expected effects (increased or decreased signalling) of the selected variants in IIS pathway components (GHRHR, GH1, IGF1, INS, IRS1), we calculated composite IIS scores to estimate IIS pathway activity. In addition, we analysed the relative impact on lifespan and body size of the separate variants in multivariate models. In women, lower IIS scores are significantly associated with lower body height and improved old age survival. Multivariate analyses showed that these results were most pronounced for the GH1 SNP, IGF1 CA repeat and IRS1 SNP. In females, for variant allele carriers of the GH1 SNP, body height was 2 cm lower (P = 0.007) and mortality 0.80-fold reduced (P = 0.019) when compared with wild-type allele carriers. Thus, in females, genetic variation causing reduced IIS activation is beneficial for old age survival. This effect was stronger for the GH1 SNP than for variation in the conserved IIS genes that were found to affect longevity in model organisms.

Molecular aging in human prefrontal cortex is selective and continuous throughout adult life

BACKGROUND

Aging leads to morphologic and functional changes in the brain and is associated with increased risk for psychiatric and neurological disorders.

METHODS

To identify age-related transcriptional changes in the human brain, we profiled gene expression in two prefrontal cortex (PFC) areas in postmortem samples from 39 subjects, ranging in age from 13 to 79 years.

RESULTS

Robust transcriptional age-related changes were identified for at least 540 genes. Gene expression correlates of aging were highly specific, and the large majority of the 22,000 transcripts investigated were unaffected by age. Across subjects, changes were progressive throughout adult life and accurately predicted chronological age. Age-upregulated transcripts were mostly of glial origin and related to inflammation and cellular defenses, whereas downregulated genes displayed mostly neuron-enriched transcripts relating to cellular communication and signaling.

CONCLUSIONS

Continuous changes in gene expression with increasing age revealed a "molecular profile" of aging in human PFC. The restricted scope of the transcript changes suggests cellular populations or functions that are selectively vulnerable during aging. Because age-related gene expression changes begin early in adulthood and are continuous throughout life, our results suggest the possibility of identifying early cellular mechanisms that may be engaged in preventive or detrimental age-related brain functions.

Differential Effects of Aging and Insulin-like Growth Factor-1 on Synapses in CA1 of Rat Hippocampus

Aging-related impairments of learning and memory can be ameliorated by 28 days of intracerebroventricular (icv) infusion of insulin-like growth factor-1 (IGF-1) in old rats. The present study investigated whether there is an aging-related synaptic decline in the stratum radiatum of hippocampal CA1 and whether IGF-1 can ameliorate that decline. Five young (4 months), five middle-aged (18 months) and five old (29 months) Fischer 344xBrown Norway rats received saline infusion; five old (29 months) rats received IGF-1 infusion for 28 days preceding sacrifice. Pyramidal neurons, total synaptic profiles as well as synaptic profiles in multiple spine bouton (MSB) complexes in CA1 were quantified stereologically with the physical disector technique and the postsynaptic density (PSD) length was determined as well. The results indicated a decrease of total synapses between middle and old age but a maintenance of PSD length and MSB synapses throughout life. IGF-1 infusion in old rats did not reverse the aging-related decline in total synapses but did increase PSD length and the number of MSB synapses. These changes in synaptic configurations are morphological correlates of enhanced synaptic efficacy. Thus, aging and IGF-1 affect different, but complementary, aspects of synapses in hippocampal CA1.

Anti-aging effects of anti-lipolytic drugs

Genetic disruption of insulin and insulin-like signaling pathways may extend lifespan. Hyperinsulinemia and insulin resistance may accelerate aging. The hypothesis was tested that a once-a-week life-long inhibition of insulin secretion by the administration of anti-lipolytic drugs might have anti-aging effects. Groups of 3-month-old male Sprague-Dawley rats were (a) given standard laboratory food ad libitum (AL); (b) fed AL 6 days and fasted 1 day every week (FW); (c) fed AL every other day (EOD), (d) fed like FW and given Acipimox (50 mg/kg b.w.) on the day of fasting (FWA) by the gastric tube. The AL, FW and EOD groups received saline intragastrically. Treatment with ACIPIMOX transiently decreased plasma free fatty acids, glucose and insulin and increased valine plasma levels, and had no long-term effect on food consumption and body weight. By age 6, 12 and 24 months subgroups were taken and the age-related changes in liver dolichol and autophagic proteolysis--which are correlated with life-expectancy--were measured. Liver dolichol levels increased and autophagic proteolysis decreased in mature and older AL rats; EOD and FWA fully counteracted these changes; FW rats had significant but smaller beneficial effects. It is concluded that life-long weekly-repeated transient inhibition of insulin secretion by antilipolytic drugs may have an anti-aging effect, additive to the anti-aging effect of a milder caloric restriction. Speculation is that transiently lower plasma insulin levels might stimulate the anti-aging cell-repair mechanism autophagy, which has longer lasting effects on cell housekeeping.

Mutations in chemosensory cilia cause resistance to paraquat in nematode Caenorhabditis elegans

The relationship between oxidative stress and longevity is a matter of concern in various organisms. We isolated mutants resistant to paraquat from nematode Caenorhabditis elegans. One mutant named mev-4 was long-lived and showed cross-resistance to heat and Dyf phenotype (defective in dye filling). Genetic and sequence analysis revealed that mev-4 had a nonsense mutation on the che-11 gene, homologues of which are involved in formation of cilia and flagella in other organisms. The paraquat resistance was commonly observed in various Dyf mutants and did not depend on the daf-16 gene, whereas the extension of life span did depend on it. Expression of antioxidant enzyme genes seemed normal. These results suggest that chemosensory neurons are a target of oxidative stress and influence longevity dependent on the daf-16 signaling in C. elegans.

Signalling pathways for insulin-like growth factor type 1-mediated expression of uncoupling protein 3

Uncoupling protein 3 (UCP3) is a mitochondrial protein with antioxidant properties and its regulation by factors promoting cell-survival may be important for protection of, for instance, neurons in states of oxidative stress. In the present study, we investigated regulatory pathways for UCP3 expression mediated by the neuroprotective hormone insulin-like growth factor type 1 (IGF-1) in human neuroblastoma SH-SY5Y cells. Northern blot analysis and RT-PCR showed that treatment with 10 nm IGF-1 increased the UCP3 mRNA levels 2.5-fold after 5 h. Co-incubation with the phosphatidylinositol 3 (PI3)-kinase inhibitor LY294002 prohibited IGF-1-mediated induction of both UCP3 mRNA and protein in a concentration-dependent manner, with a complete blockage at 1 microm, as shown by RT-PCR and western blot analyses. The mitogen-activated protein (MAP) kinase kinase 1 (MKK1 or MEK) inhibitor PD98059 also decreased the UCP3 mRNA expression at 10 microm, however, this concentration only partly inhibited the protein expression. We conclude that IGF-1 enhanced UCP3 expression at transcriptional level, primarily through the PI3-kinase-dependent pathway and partly through the MAP kinase pathway.

Cell death in the nervous system: lessons from insulin and insulin-like growth factors

Programmed cell death is an essential process for proper neural development. Cell death, with its similar regulatory and executory mechanisms, also contributes to the origin or progression of many or even all neurodegenerative diseases. An understanding of the mechanisms that regulate cell death during neural development may provide new targets and tools to prevent neurodegeneration. Many studies that have focused mainly on insulin-like growth factor-I (IGF-I), have shown that insulin-related growth factors are widely expressed in the developing and adult nervous system, and positively modulate a number of processes during neural development, as well as in adult neuronal and glial physiology. These factors also show neuroprotective effects following neural damage. Although some specific actions have been demonstrated to be anti-apoptotic, we propose that a broad neuroprotective role is the foundation for many of the observed functions of the insulin-related growth factors, whose therapeutical potential for nervous system disorders may be greater than currently accepted.

Mammalian hibernation: cellular and molecular responses to depressed metabolism and low temperature

Mammalian hibernators undergo a remarkable phenotypic switch that involves profound changes in physiology, morphology, and behavior in response to periods of unfavorable environmental conditions. The ability to hibernate is found throughout the class Mammalia and appears to involve differential expression of genes common to all mammals, rather than the induction of novel gene products unique to the hibernating state. The hibernation season is characterized by extended bouts of torpor, during which minimal body temperature (Tb) can fall as low as -2.9 degrees C and metabolism can be reduced to 1% of euthermic rates. Many global biochemical and physiological processes exploit low temperatures to lower reaction rates but retain the ability to resume full activity upon rewarming. Other critical functions must continue at physiologically relevant levels during torpor and be precisely regulated even at Tb values near 0 degrees C. Research using new tools of molecular and cellular biology is beginning to reveal how hibernators survive repeated cycles of torpor and arousal during the hibernation season. Comprehensive approaches that exploit advances in genomic and proteomic technologies are needed to further define the differentially expressed genes that distinguish the summer euthermic from winter hibernating states. Detailed understanding of hibernation from the molecular to organismal levels should enable the translation of this information to the development of a variety of hypothermic and hypometabolic strategies to improve outcomes for human and animal health.

The retardation of aging by caloric restriction: its significance in the transgenic era

The retardation of aging and diseases by caloric restriction (CR) is a widely-studied and robust phenomenon. Recent publications describe transgenic and other mutant rodents displaying lifespan extension, and the rapid pace at which these animals are being generated raises the possibility that the importance of the CR paradigm is declining. Here we discuss these models and evaluate the evidence whether or not the aging process is retarded based on longevity, disease patterns and age-associated biological changes. A comparison to rodents on CR is made. Because CR has been investigated for approximately 70 years with increasing intensity, there exists extensive data to document aging retardation. In contrast, for nearly all of the genetically abnormal models of lifespan extension, such data are minimal and often unconvincing; additional studies will be required to validate these strains as suitable models for aging research.

Ceramide, stress, and a "LAG" in aging

Recent studies have implicated the longevity assurance gene LAG1 in ceramide synthesis. In light of a role for ceramide in yeast and mammalian stress responses and mammalian cellular senescence, important connections are emerging between ceramide and organismal aging. In this Perspective, we examine the evidence for these connections in yeast, Drosophila, and mammals, and speculate on their implications.

Age-related changes in the metabolism and body composition of three dog breeds and their relationship to life expectancy

We measured body composition and resting metabolic rates (RMR) of three dog breeds (Papillons, mean body mass 3.0 kg (n = 35), Labrador retrievers, mean body mass 29.8 kg (n = 35) and Great Danes, mean body mass 62.8 kg (n = 35)) that varied between 0.6 and 14.3 years of age. In Papillons, lean body mass (LBM) increased with age but fat mass (FBM) was constant; in Labradors, both LBM and FBM were constant with age, and in Great Danes, FBM increased with age but LBM was constant. FBM averaged 14.8% and 15.7% of body mass in Papillons and Labradors, respectively. Great Danes were leaner and averaged only 10.5% FBM. Pooling the data for all individuals, the RMR was significantly and positively associated with LBM and FBM and negatively associated with age. Once these factors had been taken into account there was still a significant breed effect on RMR, which was significantly lower in Labradors than in the other two breeds. Using the predictive multiple regression equation for RMR and the temporal trends in body composition, we modelled the expenditure of energy (at rest) over the first 8 years of life, and over the entire lifespan for each breed. Over the first 8 years of life the average expenditure of energy per kg LBM were 0.985, 0.675 and 0.662 GJ for Papillons, Labradors and Great Danes, respectively. This energy expenditure was almost 60% greater for the smallest compared with the largest breed. On average, however, the life expectancy for the smallest breed was a further 6 years (i.e. 14 years in total), whereas for the largest breed it was only another 6 months (i.e. 8.5 years in total). Total lifetime expenditure of energy at rest per kg LBM averaged 1.584, 0.918 and 0.691 GJ for Papillons, Labradors and Great Danes, respectively. In Labradors, total daily energy expenditure, measured by the doubly labelled water method in eight animals, was only 16% greater than the observed RMR. High energy expenditure in dogs appears positively linked to increased life expectancy, contrary to the finding across mammal species and within exotherms, yet resembling observations in other intra-specific studies. These contrasting correlations suggest that metabolism is affecting life expectancy in different ways at these different levels of enquiry.

Deletion, but not antagonism, of the mouse growth hormone receptor results in severely decreased body weights, insulin, and insulin-like growth factor I levels and increased life span

GH participates in growth, metabolism, and cellular differentiation. To study these roles, we previously generated two different dwarf mouse lines, one expressing a GH antagonist (GHA) and the other having a disrupted GH receptor and binding protein gene (GHR -/-). In this study we compared the two dwarf lines in the same genetic background (C57BL/6J). One of the most striking differences between the mouse lines was their weight gain profile after weaning. The weights of the GHA dwarfs gradually approached controls over time, but the weights of the GHR -/- dwarfs remained low throughout the analysis period. Additionally, fasting insulin and glucose levels were reduced in the GHR -/- mice but normal in the GHA mice. IGF-I and IGF binding protein 3 (IGFBP-3) levels were significantly reduced, but by different degrees, in both mouse lines, but IGFBP-1 and -4 levels were reduced and IGFBP-2 levels increased in GHR -/- mice but unaltered in GHA mice. Finally, life span was significantly extended for the GHR -/- mice but remained unchanged for GHA dwarfs. These results suggest that the degree of blockade of GH signaling can lead to dramatically different phenotypes.

The anti-ageing effects of caloric restriction may involve stimulation of macroautophagy and lysosomal degradation, and can be intensified pharmacologically

Caloric restriction (CR) and a reduced growth hormone (GH)-insulin-like growth factor (IGF-1) axis are associated with an extension of lifespan across taxa. Evidence is reviewed showing that CR and reduced insulin of GH-IGF-1 axis may exhibit their effects at least partly by their common stimulatory action on autophagy, the cell repair mechanism responsible for the housekeeping of cell membranes and organelles including the free radical generators peroxisomes and mitochondria. It is shown that the life-long weekly administration of an anti-lipolytic drug may decrease glucose and insulin levels and stimulate autophagy and intensify anti-ageing effects of submaximal CR.

Hormone levels and cataract scores as sex-specific, mid-life predictors of longevity in genetically heterogeneous mice

Serum levels of thyroxine (T4), leptin, and insulin-like growth factor-I (IGF-I), as well as cataract severity, were evaluated as predictors of life span in a population of genetically heterogeneous mice (UM-HET3). Long life span was predicted by low levels of leptin at age 4 months in females, and by low levels of IGF-I at age 15 months and high levels of T4 at age 4 months, in males. Cataract severity at either 18 or 24 months was also a significant predictor of life span in females only, but in contrast to what has been reported in human studies, relatively severe cataract was correlated with longer life span. Additional work is needed to evaluate the role of these hormones as potential modulators of the aging process, and to resolve the conflicting data obtained for cataract severity as a predictor of life span.

Novel mutations of the growth hormone 1 (GH1) gene disclosed by modulation of the clinical selection criteria for individuals with short stature

Subtle mutations in the growth hormone 1 (GH1) gene have been regarded as a comparatively rare cause of short stature. Such lesions were sought in a group of 41 individuals selected for short stature, reduced height velocity, and bone age delay; a group of 11 individuals with short stature and idiopathic growth hormone deficiency (IGHD); and a group of 154 controls. Heterozygous mutations were identified in all three groups but disproportionately in the individuals with short stature, both with (odds ratio 25.2; 95% CI, 5.1-132.2) and without (odds ratio 3.6; 95% CI, 1.0-12.9) IGHD. Twenty-four novel GH1 gene lesions were found. Thirteen novel missense mutations were characterized by assaying the signal transduction activity of in vitro expressed variants; six (T27I, K41R, N47D, S71F, S108R, and T175A) exhibited a reduced ability to activate the JAK/STAT pathway. Molecular modeling suggested that both K41R and T175A might compromise GH receptor binding. Seven GH variants (R16C, K41R, S71F, E74K, Q91L, S108C, and a functional polymorphism, V110I) manifested reduced secretion in rat pituitary cells after allowance had been made for the level of expression attributable to the associated GH1 proximal promoter haplotype. A further leader peptide variant (L-11P) was not secreted. Eleven novel mutations in the GH1 gene promoter were assessed by reporter gene assay but only two, including a GH2 gene-templated gene conversion, were found to be associated with a significantly reduced level of expression. Finally, a novel intron 2 acceptor splice-site mutation, detected in a family with autosomal dominant type II IGHD, was shown to lead to the skipping of exon 3 from the GH1 transcript. A total of 15 novel GH1 gene mutations were thus considered to be of probable phenotypic significance. Such lesions are more prevalent than previously recognized and although most may be insufficient on their own to account for the observed clinical phenotype, they are nevertheless likely to play a contributory role in the etiology of short stature.

IGF-1 activates p21 to inhibit UV-induced cell death

The insulin-like growth factor-1 (IGF-1) and its downstream effector Akt have been documented as survival factors in response to a variety of stress signals. In this study, we show that IGF-1 activates p21 protein expression in a p53-dependent manner. Inhibition of PI-3 kinase or ectopic expression of a dominant-negative Akt blocks the effect of IGF-1 on the upregulation of p21 expression. In addition, IGF-1 prevents the UV irradiation-mediated suppression of p21 and MDM2 expression. Furthermore, p21 is important for IGF-1-mediated cell survival upon UV irradiation. Taken together, these data indicate that IGF-1 may activate p21 in executing its survival function upon genotoxic insults.

Evolutionary medicine: from dwarf model systems to healthy centenarians?

Restriction of the number of calories consumed extends longevity in many organisms. In rodents, caloric restriction decreases the levels of plasma glucose and insulin-like growth factor I (IGF-1) and postpones or attenuates cancer, immunosenescence, and inflammation without irreversible side effects. In organisms ranging from yeast to mice, mutations in glucose or IGF-I-like signaling pathways extend life-span but also cause glycogen or fat accumulation and dwarfism. This information suggests a new category of drugs that could prevent or postpone diseases of aging with few adverse effects.

Head rush

For energetic and good-humored William Sonntag, attacking the dullness of everyday routine--and just plain having fun--is one key to happiness in life as well as science. As a psychologist who branched out into endocrinology, Sonntag also epitomizes the notion that it takes all kinds of minds to decipher the science of aging. The Wake Forest University neuroendocrinologist has pioneered insights into the hormonal networks that shape aging-associated behavioral and cognitive changes.

Of worms, flies, dwarfs, and things that go bump in the night

Studies over the past several years have found that antagonism of the insulin/insulin-like growth factor 1 (IGF-1) signaling pathway increases life-span in Caenorhabditis elegans and Drosophila. However, a persistent problem in these studies is the fact that the genetic mutation has effects on the development of the organism as well as on reproductive function. These effects act as potential confounding variables that complicate the interpretation of results. Kenyon and colleagues circumvent these issues by suppressing the insulin/IGF-1-like daf-2 signaling pathway at specific stages in the life-span of C. elegans. The results of their investigation challenge our understanding of the evolution of aging and provide opportunities for future studies in mammalian models.