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

Molecular endocrinology and physiology of the aging central nervous system

Aging is associated with a progressive decline in physical and cognitive functions. The impact of age-dependent endocrine changes regulated by the central nervous system on the dynamics of neuronal behavior, neurodegeneration, cognition, biological rhythms, sexual behavior, and metabolism are reviewed. We also briefly review how functional deficits associated with increases in glucocorticoids and cytokines and declining production of sex steroids, GH, and IGF are likely exacerbated by age-dependent molecular misreading and alterations in components of signal transduction pathways and transcription factors.

Gene or size: metabolic rate and body temperature in obese growth hormone-deficient dwarf mice

OBJECTIVE

SMA1 mice carry a missense mutation in the growth hormone gene that leads to semidominant dwarfism and obesity. In this study, the basic thermal and metabolic properties of SMA1 mice were examined to detect metabolic alterations that can support the accretion of excess fat.

RESEARCH METHODS AND PROCEDURES

Basal and resting metabolic rates (RMRs) in wild-type and SMA1 (sma1/+ and sma1/sma1) mice were determined by indirect calorimetry. Body temperature (T(b)) was recorded using intraperitoneally implanted temperature-sensitive transmitters, and body composition was determined by DXA.

RESULTS

SMA1 mice have proportionally lower basal and resting metabolic rates, higher body mass (BM)-specific RMRs, and a higher lower critical temperature, and display a decrease in T(b) by 0.4 degrees C in sma1/+ and 0.9 degrees C in sma1/sma1.

DISCUSSION

The analysis of gene effects on BM and energy expenditure in mouse mutants must consider the appropriate allometric relationship between BM and metabolic rate. With the exception of T(b), all metabolic alterations observed in SMA1 reflect reduced size.

The incorporation of iodine in thyroid hormone may stem from its role as a prehistoric signal of ecologic opportunity: An evolutionary perspective and implications for modern diseases

To optimize fitness under conditions of varying Darwinian opportunity, organisms demonstrate tremendous plasticity in their life-history strategies based on their perception of available resources. Higher-energy environments generally promote more aggressive life-history strategies, such as faster growth, larger adult size, greater genetic variation, shorter lifespan, larger brood sizes, and offspring ratio skewed towards the larger-sized gender. While numerous mechanisms regulate life-history plasticity including genetic imprinting, methylation, and growth factors, evidence suggests that thyroid hormone plays a central role. Given the pivotal adaptive role of thyroid hormone, the teleology of its dependence on dietary iodine for production remains unexplained. We hypothesize that iodine may have emerged as a substrate for production of thyroid hormone in prehistoric ecosystems because the former represented a reliable proxy for ecologic potential that enabled the latter to modulate growth, reproduction, metabolic rate, and lifespan. Such a scenario may have existed in early marine ecosystems where ocean-surface vegetation, which concentrates iodine for its antimicrobial and antioxidant properties, formed the basis of the food chain. Teleologic parallels can be drawn to the food-chain accumulation of antimicrobials that also exhibit antioxidant properties and promote adult size, brood size, and offspring quality by modulating central hormonal axes. As each higher species in the food chain tunes its life-history strategy based on iodine intake, the coupling of this functional role of iodine with its value as a resource signal to the next member of the food-chain may promote runaway evolution. Whereas predators in prehistoric ecosystems successfully tuned their life-history strategy using iodine as a major input, the strategy may prove maladaptive in modern humans for whom the pattern of iodine intake is decoupled from resource availability. Iodine acquired through sodium iodide supplementation may independently contribute to some biologic dysfunctions currently attributed to sodium.

Diurnal variation in growth hormone receptor messenger ribonucleic acid in liver and skeletal muscle of lit/+ and lit/lit mice

The present study investigated the diurnal variation in GH receptor (GHR) mRNA in liver and skeletal muscle of 3-month-old GH-deficient and -sufficient mice using quantitative real-time RT-PCR. lit/lit (GH deficient) or lit/+ (GH sufficient) mice were fed ad libitum and lights were on between 0600 and 2000. Tissues were collected at 0800-1000, 1200-1400 and 2000-2200. Hepatic GHR mRNA levels of lit/+ mice at 0800-1000 were significantly lower than those at 1200-1400 and 2000-2200. There was no significant variation in hepatic GHR mRNA of lit/lit mice. In skeletal muscle, GHR mRNA levels of both lit/+ and lit/lit mice at 1200-1400 were significantly higher than those at 0800-1000 and 2000-2200. There was also a diurnal change in hepatic IGF-I mRNA levels of lit/+ but not of lit/lit mice; the levels were lowest at 0800-1000 in lit/+ mice. On the other hand, there was no variation in IGF-I mRNA levels in skeletal muscle. These results suggest that 1) there is a diurnal variation in GHR expression in liver and skeletal muscle and the pattern of the variation is tissue specific; 2) GH deficiency blunted the diurnal variation in GHR mRNA in liver but not that in skeletal muscle; 3) IGF-I mRNA expression in liver is more closely related to GHR mRNA expression than that in skeletal muscle.

Lack of contribution of 11betaHSD1 and glucocorticoid action to reduced muscle mass associated with reduced growth hormone action

11Beta-hydroxysteroid dehydrogenase 1 (11beta-HSD1) is expressed in several tissues and converts inactive glucocorticoids (GC) to active GC. 11betaHSD1 activity, evaluated by urine cortisol metabolites, is increased in patients with hypopituitarism and decreased by GH replacement. Skeletal muscle wasting is one of the major characteristics of GH deficiency (GHD). We hypothesized that increased 11betaHSD1 activity and increased GC action in skeletal muscle may play a role in the development of muscle atrophy observed in GHD patients. Glutamine synthetase (GS) mRNA in muscle has been reported to be related to GC-induced muscle atrophy. In this study, we measured mRNA levels of 11betaHSD1 and GS in skeletal muscle of GH receptor gene disrupted (GHR-/-) mice and of their age-matched wild-type mice controls to elucidate the physiological significance of 11betaHSD1 and GC in the development of GHD-associated muscle atrophy in vivo. We also measured the expression of these genes in hypertrophied muscles of giant, bovine GH transgenic mice. In skeletal muscle, although IGF-I mRNA levels were decreased in GHR-/- mice, 11betaHSD1 mRNA levels were not significantly changed compared to wild-type mice. In addition, expression level of 11betaHSD1 in muscle was lower compared to that seen in liver. GS mRNA in skeletal muscle of GHR-/- mice was not significantly different from that of controls. In bGH mice, 11betaHSD1 and GS mRNA levels were not altered compared to control mice. These data do not support a significant role of 11betaHSD1 and GC action in skeletal muscle in the development of muscle atrophy associated with GHD.

The pancreas of the naked mole-rat (Heterocephalus glaber): an ultrastructural and immunocytochemical study of the endocrine component of thermoneutral and cold acclimated animals

Endocrine cell distribution within the islets of Langerhans may vary both between species and under different energetically demanding conditions such as cold acclimation. The naked mole-rat, Heterocephalus glaber, lacking an effective insulatory pelage, is effectively a poikilotherm, yet it shows a typical mammalian cold-acclimation response by substantially increasing food intake to meet higher energy requirements when housed at lower temperatures. The endocrine component of the pancreas of thermoneutral and cold-acclimated naked mole-rats was thus characterized using immunocytochemistry and ultrastructural analyses. Four distinct endocrine cells were identified: alpha (glucagon-producing), beta (insulin-producing), delta (somatostatin-producing), and PP (pancreatic polypeptide-producing) cells. Distribution of these cells differed from that of other rodents, in that beta cells formed the mantle while alpha cells formed the core of the islets. This distribution may contribute to the observed insulin insensitivity of this species, as indicated in abnormal responses to glucose tolerance tests. Insulin-producing cells, however, were more numerous than glucagon-producing cells. This ratio was unchanged with cold acclimation. Immunoreactivity of alpha and beta cells was more intense in cold-acclimated than in thermoneutral animals, possibly indicative of a change in hormonal production in animals housed at a lower temperature.

Disruption of growth hormone receptor gene causes diminished pancreatic islet size and increased insulin sensitivity in mice

Growth hormone, acting through its receptor (GHR), plays an important role in carbohydrate metabolism and in promoting postnatal growth. GHR gene-deficient (GHR(-/-)) mice exhibit severe growth retardation and proportionate dwarfism. To assess the physiological relevance of growth hormone actions, GHR(-/-) mice were used to investigate their phenotype in glucose metabolism and pancreatic islet function. Adult GHR(-/-) mice exhibited significant reductions in the levels of blood glucose and insulin, as well as insulin mRNA accumulation. Immunohistochemical analysis of pancreatic sections revealed normal distribution of the islets despite a significantly smaller size. The average size of the islets found in GHR(-/-) mice was only one-third of that in wild-type littermates. Total beta-cell mass was reduced 4.5-fold in GHR(-/-) mice, significantly more than their body size reduction. This reduction in pancreatic islet mass appears to be related to decreases in proliferation and cell growth. GHR(-/-) mice were different from the human Laron syndrome in serum insulin level, insulin responsiveness, and obesity. We conclude that growth hormone signaling is essential for maintaining pancreatic islet size, stimulating islet hormone production, and maintaining normal insulin sensitivity and glucose homeostasis.

Growth hormone secretagogues: prospects and potential pitfalls

The growth hormone secretagogues (GHSs) are the first well-characterised agents that rejuvenate the growth hormone (GH)/insulin-like growth factor (IGF-1) axis. This property was discovered during investigations of the underlying causative mechanisms of age-related endocrine changes. Chronic administration of the long acting GHS, MK-0677, reverses the age-related decline in pulse-amplitude of GH secretion and restores IGF-1 levels producing profiles typical of young adults. This restoration is accompanied by improvements in body composition in frail elderly subjects. When given acutely, the GHSs also increase appetite. Following cloning and characterisation of the GHS-receptor (GHS-R) an endogenous ligand, ghrelin, was isolated and identified. Ghrelin shares the GH releasing and orexigenic properties of the GHSs. Studies using Ghsr-null mice confirmed that the GHS-R was the ghrelin-receptor; hence, the GHSs should be considered to be 'ghrelin mimetics.' Ghrelin levels are reported to decline during ageing, therefore long-acting GHSs are ideal candidates for ghrelin replacement therapy.

Comparing adiposity profiles in three mouse models with altered GH signaling

Three mouse lines with altered growth hormone (GH) signaling were used to study GH's role in adiposity. Dwarf GH receptor knockout mice (GHR -/-) and bovine GH antagonist expressing mice (GHA) had an increased percent body fat with most of the excess fat mass accumulating in the subcutaneous region. Giant bovine GH expressing mice (bGH) had a reduced percent body fat. Only GHA mice consumed significantly more food per body weight. Serum leptin levels were significantly increased in GHA mice and decreased in bGH mice but unchanged in the GHR -/- mice. Interestingly, serum adiponectin levels were significantly increased in the GHR -/- and GHA lines but decreased in bGH mice. These data suggest that suppression or absence of GH action and enhanced GH action indeed have opposite metabolic effects in terms of adiposity. Interestingly, adiponectin levels were positively correlated with previously reported insulin sensitivity of these mice, but also positively correlated with adiposity, which is contrary to findings in other mouse models. Thus, adiponectin levels were negatively correlated with GH function suggesting a role for adiponectin in GH-induced insulin resistance.

The Consequences of Altered Somatotropic System on Reproduction1

Although the primary control of gonadotropin secretion is by the hypothalamic GnRH and the gonadal function is controlled by the pituitary gonadotropins and prolactin, the emerging evidence suggests a vital role of the somatotropic axis, growth hormone (GH), and insulin-like growth factor-I (IGF-I) in the control of the pituitary and gonadal functions. It has been shown that GH deficiency, GH resistance, and experimental alterations in IGF-I secretion modify folliculogenesis, ovarian maturation, ovulation, and pregnancy, and in the male, GH/IGF-I plays an important role in spermatogenesis and the Leydig cell function. The primary focus of this review is to examine the role of GH/ IGF-I on the onset of puberty, fertility, pituitary, and gonadal endocrine functions. A number of studies have revealed that fertility is affected in GH-deficient dwarf and in IGF-I gene-ablated mice, possibly due to subnormal function of either the pituitary gland or the gonads. In the female GH receptor gene knockout (GHR-KO) mice, there was impairment in follicular development, ovulation rate, sexual maturation, production of and responsiveness to pheromonal signals, and the corpus luteum function. In IGF-I-deficient male GHR-KO mice, puberty is delayed, spermatogenesis is affected, and neuroendocrine-gonadal function is attenuated. Similarly, in some of the human Laron syndrome patients, puberty is delayed due to GH resistance. These data suggest that, in addition to GnRH and gonadotropins, GH/IGF-I influences the pituitary and gonadal functions in animals and humans.

Transgenic growth hormone mice exposed to lifetime constant illumination: gender-specific effects

Photoperiod affects most of the features altered in transgenic growth hormone (TG) mice, and laboratory rats and mice retain some sensitivity to photoperiod. We examined growth, feeding, longevity, and reproduction of TG mice and normal control mice (Mus musculus L., 1758) in 12 h light : 12 h dark (LD) and 24 h light (LL) photoperiods. Sexual dichotomy in growth and hepatic gene expression are considered to require gender-specific patterns of growth hormone secretion that are absent in TG mice. Regardless, in the LD photoperiod mature TG females were 82.8% (46.8 g) of the mass of TG males (56.5 g, p < 0.05), whereas control mice showed no size dichotomy (≈33 g). Mature masses of TG males and of control mice of either gender were unaffected by the LL photoperiod. TG females, however, reached a mature mass 92% (50.9 g) of that of mature TG males in the LL photoperiod, attenuating the sexual size dichotomy expressed in the LD photoperiod. Growth of females was slower than that of males, even in the control group. TG females in the LL photoperiod expressed faster growth, higher reproduction, and greater mean longevity than TG females in the LD photoperiod. Differences in age-related feeding associated with gender and photoperiod reflected differential growth rates. Females grew more slowly and ate more than males of similar age because they were smaller (i.e., had lower growth efficiencies). The LL photoperiod improved the energy balance of TG females. Possible mechanisms mediating such gender-specific effects are explored.

Temporal linkage between the phenotypic and genomic responses to caloric restriction

Caloric restriction (CR), the consumption of fewer calories while avoiding malnutrition, decelerates the rate of aging and the development of age-related diseases. CR has been viewed as less effective in older animals and as acting incrementally to slow or prevent age-related changes in gene expression. Here we demonstrate that CR initiated in 19-month-old mice begins within 2 months to increase the mean time to death by 42% and increase mean and maximum lifespans by 4.7 (P = 0.000017) and 6.0 months (P = 0.000056), respectively. The rate of age-associated mortality was decreased 3.1-fold. Between the first and second breakpoints in the CR survival curve (between 21 and 31 months of age), tumors as a cause of death decreased from 80% to 67% (P = 0.012). Genome-wide microarray analysis of hepatic RNA from old control mice switched to CR for 2, 4, and 8 weeks showed a rapid and progressive shift toward the gene expression profile produced by long-term CR. This shift took place in the time frame required to induce the health and longevity effects of CR. Shifting from long-term CR to a control diet, which returns animals to the control rate of aging, reversed 90% of the gene expression effects of long-term CR within 8 weeks. These results suggest a cause-and-effect relationship between the rate of aging and the CR-associated gene expression biomarkers. Therefore, therapeutics mimicking the gene-expression biomarkers of CR may reproduce its physiological effects.

Life extension in the dwarf mouse

Ames dwarf mice and Snell dwarf mice lack growth hormone (GH), prolactin (PRL), and thyroid-stimulating hormone (TSH), live much longer than their normal siblings, and exhibit many symptoms of delayed aging. "Laron dwarf mice," produced by targeted disruption of the GH receptor/GH-binding protein gene (GHR-KO mice), are GH resistant and also live much longer than normal animals from the same line. Isolated GH deficiency in "little" mice is similarly associated with increased life span, provided that obesity is prevented by reducing fat content in the diet. Long-lived dwarf mice share many phenotypic characteristics with genetically normal (wild-type) animals subjected to prolonged caloric restriction (CR) but are not CR mimetics. We propose that mechanisms linking GH deficiency and GH resistance with delayed aging include reduced hepatic synthesis of insulin-like growth factor 1 (IGF-1), reduced secretion of insulin, increased hepatic sensitivity to insulin actions, reduced plasma glucose, reduced generation of reactive oxygen species, improved antioxidant defenses, increased resistance to oxidative stress, and reduced oxidative damage. The possible role of hypothyroidism, reduced body temperature, reduced adult body size, delayed puberty, and reduced fecundity in producing the long-lived phenotype of dwarf mice remains to be evaluated. An important role of IGF-1 and insulin in the control of mammalian longevity is consistent with the well-documented actions of homologous signaling pathways in invertebrates.

Constitutively active signal transducer and activator of transcription 5 can replace the requirement for growth hormone in adipogenesis of 3T3-F442A preadipocytes

Although it is the best characterized in vitro model of GH action, the mechanisms used by GH to induce differentiation of murine 3T3-F442A preadipocytes remain unclear. Here we have examined the role of three transcriptional regulators in adipogenesis. These regulators are either rapidly induced in response to GH [Stra13, signal transducer and activator of transcription (Stat)3] or of central importance to GH signaling (Stat5). Retroviral transfection of 3T3-F442A preadipocytes was used to increase expression of Stra13, Stat3, and Stat5a. Only Stat5a transfection increased the expression of adipogenic markers peroxisome proliferator-activated receptor gamma, CCAAT enhancer binding protein (C/EBP)alpha, and adipose protein 2/fatty acid-binding protein in response to GH, as determined by quantitative RT-PCR. Transfection with constitutively active Stat3 and Stat5a revealed that constitutively active Stat5a but not Stat3 was able to replace the GH requirement for adipogenesis. Constitutively active Stat5a but not Stat3 was able to increase the formation of lipid droplets and expression of alpha-glycerol phosphate dehydrogenase toward levels seen in mature adipocytes. Constitutively active Stat5a was also able to increase the expression of transcripts for C/EBPalpha to similar levels as GH, and of C/EBPbeta, peroxisome proliferator-activated receptor gamma, and adipose protein 2/fatty acid-binding protein transcripts to a lesser extent. An in vivo role for GH in murine adipogenesis is supported by significantly decreased epididymal fat depot size in young GH receptor-deleted mice, before manifestation of the lipolytic actions of GH. We conclude that Stat5 is a critical factor in GH-induced, and potentially prolactin-induced, murine adipogenesis.

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.

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.

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.

A dietary supplement abolishes age-related cognitive decline in transgenic mice expressing elevated free radical processes

We previously found that transgenic mice overexpressing growth hormone (TGM) have elevated and progressively increasing free radical processes in brain that strongly correlates with reduced survivorship. Young mature TGM, however, displayed vastly enhanced learning of an eight-choice cued maze and qualitatively different learning curves than normal controls. Here we document the age-related patterns in learning ability of TGM and normal mice. Learning appeared inferior in both genotypes of very young mice but TGM were confirmed to be superior to normal mice upon maturity. Older TGM, however, showed rapid age-related loss of their exceptional learning, whereas normal mice at 1 year of age showed little change. The cognitive decline of TGM was abolished by a complex "anti-aging" dietary supplement formulated to promote membrane and mitochondrial integrity, increase insulin sensitivity, reduce reactive oxygen and nitrogen species, and ameliorate inflammation. Results are discussed in the context of reactive oxygen and nitrogen species, long-term potentiation, learning, aging and neuropathology, based on known impacts of the growth hormone axis on the brain, and characteristics of TGM.

A low-fat, whole-food vegan diet, as well as other strategies that down-regulate IGF-I activity, may slow the human aging process

A considerable amount of evidence is consistent with the proposition that systemic IGF-I activity acts as pacesetter in the aging process. A reduction in IGF-I activity is the common characteristic of rodents whose maximal lifespan has been increased by a wide range of genetic or dietary measures, including caloric restriction. The lifespans of breeds of dogs and strains of rats tend to be inversely proportional to their mature weight and IGF-I levels. The link between IGF-I and aging appears to be evolutionarily conserved; in worms and flies, lifespan is increased by reduction-of-function mutations in signaling intermediates homologous to those which mediate insulin/IGF-I activity in mammals. The fact that an increase in IGF-I activity plays a key role in the induction of sexual maturity, is consistent with a broader role for-IGF-I in aging regulation. If down-regulation of IGF-I activity could indeed slow aging in humans, a range of practical measures for achieving this may be at hand. These include a low-fat, whole-food, vegan diet, exercise training, soluble fiber, insulin sensitizers, appetite suppressants, and agents such as flax lignans, oral estrogen, or tamoxifen that decrease hepatic synthesis of IGF-I. Many of these measures would also be expected to decrease risk for common age-related diseases. Regimens combining several of these approaches might have a sufficient impact on IGF-I activity to achieve a useful retardation of the aging process. However, in light of the fact that IGF-I promotes endothelial production of nitric oxide and may be of especial importance to cerebrovascular health, additional measures for stroke prevention-most notably salt restriction-may be advisable when attempting to down-regulate IGF-I activity as a pro-longevity strategy.

The endocrine regulation of aging by insulin-like signals

Reduced signaling of insulin-like peptides increases the life-span of nematodes, flies, and rodents. In the nematode and the fly, secondary hormones downstream of insulin-like signaling appear to regulate aging. In mammals, the order in which the hormones act is unresolved because insulin, insulin-like growth factor-1, growth hormone, and thyroid hormones are interdependent. In all species examined to date, endocrine manipulations can slow aging without concurrent costs in reproduction, but with inevitable increases in stress resistance. Despite the similarities among mammals and invertebrates in insulin-like peptides and their signal cascade, more research is needed to determine whether these signals control aging in the same way in all the species by the same mechanism.

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

Studies in Caenorhabditis elegans demonstrate that disruption of the daf-2 signaling pathways extends lifespan. Similarities among the daf-2 pathway, insulin-like signaling in flies and yeast, and the mammalian insulin-like growth factor 1 (IGF-1) signaling cascade raise the possibility that modifications to IGF-1 signaling could also extend lifespan in mammals. In fact, growth hormone (GH)/IGF-1-deficient dwarf mice do live significantly longer than their wild-type counterparts. However, multiple endocrine deficiencies and developmental anomalies inherent in these models confound this interpretation. Here, we critique the current mammalian models of GH/IGF-1 deficiency and discuss the actions of GH/IGF-1 on biological aging and lifespan.

Models of growth hormone and IGF-1 deficiency: applications to studies of aging processes and life-span determination

The remarkable progress in understanding the genetic basis of life-span determination in invertebrates indicates that impairments in the insulin-insulin-like growth factor 1 (IGF-1) signaling cascade increase longevity. Similarities among insulin and IGF-1-like signaling pathways in invertebrates and mammals raise the possibility that modifications of these pathways may extend life span in mammals. Investigators using Ames, Snell, and growth hormone receptor knockout models have concluded that decreased growth hormone and IGF-1 are responsible for increased life span. In this review, we critique the dwarf models and, based on multiple endocrine deficiencies and developmental anomalies, conclude that these models may not be sufficient to assess the consequences of growth hormone or IGF-1 deficiency on either biological aging or life span. We attempt to resolve some of these issues by presenting an alternative animal model of growth hormone-IGF-1 deficiency. Finally, we propose an integrated explanation of growth hormone and IGF-1's contribution to the aging phenotype and life-span determination.

Role of environmental temperature in aging and longevity: insights from neurolipofuscin

The available evidence for thermal modulation of neurolipofuscin deposition in poikilotherms is reviewed here and additional data are contributed. Mainly decapod crustacean models are employed and neurolipofuscin is treated as an index of physiological aging. In all cases, neurolipofuscin accumulation rate is positively correlated with environmental temperature but there appears to be lowered sensitivity in the thermal mid-range, an 'optimum' temperature for neurolipofuscin accumulation and possibly age-associated variation. The geographical position of the population within the species' thermal range may determine sensitivity of the response. There is seasonal oscillation of neurolipofuscin accumulation rate, providing preliminary evidence for neurolipofuscin turnover with net loss in winter. Spatial and temporal thermal variations of similar magnitude appear to have comparable effects on neurolipofuscin accumulation rate. Such effects may be extreme, suggesting important implications for physiological aging even in homeotherms. Inter-specific comparisons indicate that species-specific neurolipofuscin accumulation rates are positively correlated with habitat temperature and inversely correlated with maximum lifespan and age at maturity. These findings help explain some well-known bioclimatic trends in maturation- and maximum body size, such as Bergmann's rule. They also highlight the fact that global warming is likely to cause significant changes in life history parameters, population dynamics and responses to exploitation for many species.

Consequences of growth hormone (GH) overexpression and GH resistance

Development of transgenic mice overexpressing GH and GHR-KO mice with GH resistance provided novel animal models for study of the somatotropic axis and for identifying GH actions that may be relevant to its current and contemplated use in medicine and agriculture. Studies of phenotypic characteristics of these animals revealed previously unsuspected actions of GH and IGF-I on neuroendocrine functions related to reproduction and to the release of "stress hormones" (glucocorticoids and prolactin). These studies also provided novel and still-disputed evidence for involvement of somatotropic axis in the control of aging and life span and in mediating the actions of longevity genes.

Growth hormone-induced alterations in the insulin-signaling system

Growth hormone (GH) counteracts insulin action on lipid and glucose metabolism. However, the sequence of molecular events leading to these changes is poorly understood. Insulin action is initiated by binding of the hormone to its cell surface receptor (IR). This event activates the intrinsic tyrosine kinase activity residing in the beta-subunit of the IR and leads to autophosphorylation of the cytoplasmic portion of the beta-subunit and further activation of its tyrosine kinase towards several intermediate proteins, including the family of IR substrates (IRS) and the Shc proteins. When tyrosine phosphorylated, these cellular substrates connect the IR with several downstream signaling molecules. One of them is the enzyme phosphatidylinositol (PI) 3-kinase. The insulin antagonistic action of GH is not a consequence of a direct interaction with the IR. Instead, long-term exposure to GH is, in general, associated with hyperinsulinemia, which leads to a reduction of IR levels and an impairment of its tyrosine kinase activity. The signals of GH and insulin may converge at post-receptor levels. The signaling pathway leading to activation of PI 3-kinase appears to be an important site of convergence between the signals of these two hormones and seems to be mediated principally by IRS-1. Rodent models of chronic GH excess have been useful tools to investigate the mechanism by which GH induces insulin resistance. Decreased IR, IRS-1, and IRS-2 tyrosyl phosphorylation in response to insulin was found in skeletal muscle, whereas a chronic activation of the IRS-PI 3-kinase pathway was found in liver. The induction of the expression of proteins that inhibit IR signaling such as suppressors of cytokine signaling (SOCS)-1 and -6 may also be involved in this alteration. Interestingly, the modulation of insulin signaling and action observed in states of GH excess, deficiency, or resistance seems to be relevant to the changes in longevity associated with those states.