Body composition of prolactin-, growth hormone, and thyrotropin-deficient Ames dwarf mice

Gene and transcript expression patterns, coupled with isoform switching and long non-coding RNA dynamics in adipose tissue, underlie the longevity of Ames dwarf mice

Our study investigates gene expression in adipose tissue of Ames dwarf (df/df) mice, whose deficiency in growth hormone is linked to health and extended lifespan. Recognizing adipose tissue influence on metabolism, aging, and related diseases, we aim to understand its contribution to the health and longevity of df/df mice. We have identified gene and transcript expression patterns associated with critical biological functions, including metabolism, stress response, and resistance to cancer. Intriguingly, we identified genes that, despite maintaining unchanged expression levels, switch between different isoforms, impacting essential cellular functions such as tumor suppression, oncogenic activity, ATP transport, and lipid biosynthesis and storage. The isoform switching is associated with changes in protein domains, retention of introns, initiation of nonsense-mediated decay, and emergence of intrinsically disordered regions. Moreover, we detected various alternative splicing events that may drive these structural alterations. We also found changes in the expression of long non-coding RNAs (lncRNAs) that may be involved in the aging process and disease resistance by regulating crucial genes in survival and metabolism. Through weighted gene co-expression network analysis, we have linked four lncRNAs with 29 genes, which contribute to protein complexes such as the Mili-Tdrd1-Tdrd12 complex. Beyond safeguarding DNA integrity, this complex also has a wider impact on gene regulation, chromatin structure, and metabolic control. Our detailed investigation provides insight into the molecular foundations of the remarkable health and longevity of df/df mice, emphasizing the significance of adipose tissue in aging and identifying new avenues for health-promoting therapeutic strategies.

Effects of growth hormone-releasing hormone deficiency in mice beyond growth

This paper provides a critical overview on GHRH and its deficiency, discussing its multiple roles in both central and peripheral tissues. Genetically engineered mice have been instrumental in elucidating the multifaceted roles of GHRH and GH, each offering unique insights into the physiological and pathological roles of these hormones, although in many of these models dissecting the direct effect of GHRH from the effect of GH is not possible. Key findings highlight the effects of GHRH deficiency on emotional behavior, including anxiety and depression, its impact on memory and learning capabilities, as well as on adipose tissue, immune system, inflammation and pain.

Differential Responses of White Adipose Tissue and Brown Adipose Tissue to Calorie Restriction During Aging

Age-related adipose tissue dysfunction is potentially important in the development of insulin resistance and metabolic disorder. Caloric restriction (CR) is a robust intervention to reduce adiposity, improve metabolic health, and extend healthy life span. Both white adipose tissue (WAT) and brown adipose tissue (BAT) are involved in energy homeostasis. CR triggers the beiging of WAT in young mice; however, the effects of CR on beiging of WAT and function of BAT during aging are unclear. This study aimed to investigate how age and CR impact the beiging of WAT, the function of BAT, and metabolic health in mice. C57BL/6 mice were fed CR diet (40% less than the ad libitum [AL] diet) for 3 months initiated in young (3 months), middle-aged (12 months), and old (19 months) stage. We found age-related changes in different types of adipose tissue, including adipocyte enlargement, declined beiging of WAT, and declined thermogenic and β-oxidational function of BAT. Moreover, CR attenuated age-associated adipocyte enlargement and prevented the age-related decline in beiging potential of WAT. These protective effects on the beiging potential were significant in inguinal WAT at all three ages, which were significant in epididymal WAT at young and old age. In contrast, thermogenic and β-oxidational function of BAT further declined after CR in the young age group. In conclusion, our findings reveal the contribution of WAT beiging decline to age-related metabolic disorder and suggest nutritional intervention, specifically targeting WAT beiging, as an effective approach to metabolic health during aging.

Physiological and metabolic characteristics of novel double-mutant female mice with targeted disruption of both growth hormone-releasing hormone and growth hormone receptor

Mice with disruptions of growth hormone-releasing hormone (GHRH) or growth hormone receptor (GHR) exhibit similar phenotypes of prolonged lifespan and delayed age-related diseases. However, these two models respond differently to calorie restriction indicating that they might carry different and/or independent mechanisms for improved longevity and healthspan. In order to elucidate these mechanisms, we generated GHRH and GHR double-knockout mice (D-KO). In the present study, we focused specifically on the characteristics of female D-KO mice. The D-KO mice have reduced body weight and enhanced insulin sensitivity compared to wild-type (WT) controls. Growth retardation in D-KO mice is accompanied by decreased GH expression in pituitary, decreased circulating IGF-1, increased high-molecular-weight (HMW) adiponectin, and leptin hormones compared to WT controls. Generalized linear model-based regression analysis, which controls for body weight differences between D-KO and WT groups, shows that D-KO mice have decreased lean mass, bone mineral density, and bone mineral content, but increased adiposity. Indirect calorimetry markers including oxygen consumption, carbon dioxide production, and energy expenditure were significantly lower in D-KO mice relative to the controls. In comparison with WT mice, the D-KO mice displayed reduced respiratory exchange ratio (RER) values only during the light cycle, suggesting a circadian-related metabolic shift toward fat utilization. Interestingly, to date survival data suggest extended lifespan in D-KO female mice.

The effects of early-life growth hormone intervention on tissue specific histone H3 modifications in long-lived Ames dwarf mice

Histone modifications, specifically in the lysine residues of histone H3, have been implicated in lifespan regulation in several model organisms. Our previous studies showed that growth hormone (GH) treatment during early life can dramatically influence lifespan in long-lived Ames dwarf mice. However, the effects of this hormonal intervention on epigenetic modifications have never been examined. In this study, we sought to compare tissue-specific histone H3 lysine methylation and acetylation markers in Ames dwarf and wild type (WT) mice and to determine how these markers are affected by early-life GH intervention. Ames dwarf mice exhibited suppressed H3K4me in both hepatic and brain tissues, while showing elevated H3K27me in the brain. Early-life GH intervention significantly altered the histone H3 markers in those tissues. Furthermore, early GH intervention increased expression of histone H3 acetylation at multiple lysine residues in a tissue-specific manner. This included changes in H3K14ac and H3K18ac in the liver and brain, H3K18ac in visceral adipose tissue and H3K9ac, H3K14ac and H3K27ac in subcutaneous adipose tissue. This study serves as an initial, but important step in elucidating the epigenetic mechanisms by which hormonal signals during early life can influence aging and longevity in mammals.

Potential applications for rhIGF-I: Bone disease and IGFI

Growth hormone (GH) and insulin like growth factor-I (IGFI) are key bone trophic hormones, whose rising levels during puberty are critical for pubertal bone accrual. Conditions of GH deficiency and genetic resistance impact cortical and trabecular bone deleteriously with reduced estimates of bone strength. In humans, conditions of undernutrition (as in anorexia nervosa (AN), or subsequent to chronic illnesses) are associated with low IGF-I levels, which correlate with disease severity, and also with lower bone mineral density (BMD), impaired bone structure and lower strength estimates. In adolescents and adults with AN, studies have demonstrated a nutritionally acquired GH resistance with low IGF-I levels despite high concentrations of GH. IGF-I levels go up with increasing body weight, and are associated with rising levels of bone turnover markers. In short-term studies lasting 6-10 days, recombinant human IGF-I (rhIGF-I) administration in physiologic replacement doses normalized IGF-I levels and increased levels of bone formation markers in both adults and adolescents with AN. In a randomized controlled trial in adults with AN in which participants were randomized to one of four arms: (i) rhIGF-I with oral estrogen-progesterone (EP), (ii) rhIGF-I alone, (iii) EP alone, or (iv) neither for 9 months, a significant increase in bone formation markers was noted in the groups that received rhIGF-I, and a significant decrease in bone resorption markers in the groups that received EP. The group that received both rhIGF-I and EP had a significant increase in bone density at the spine and hip compared to the group that received neither. Side effects were minimal, with no documented fingerstick glucose of <50 mg/dl. These data thus suggest a potential role for rhIGF-I administration in optimizing bone accrual in states of undernutrition associated with low IGF-I.

Physiological and metabolic features of mice with CRISPR/Cas9-mediated loss-of-function in growth hormone-releasing hormone

Our previous study demonstrated that the loss of growth hormone releasing hormone (GHRH) results in increased lifespan and improved metabolic homeostasis in the mouse model generated by classical embryonic stem cell-based gene-targeting method. In this study, we targeted the GHRH gene using the CRISPR/Cas9 technology to avoid passenger alleles/mutations and performed in-depth physiological and metabolic characterization. In agreement with our previous observations, male and female GHRH-/- mice have significantly reduced body weight and enhanced insulin sensitivity when compared to wild type littermates. Dual-energy X-ray absorptiometry showed that there were significant decreases in lean mass, bone mineral content and density, and a dramatic increase in fat mass of GHRH-/- mice when compared to wild type littermates. Indirect calorimetry measurements showed dramatic reductions in oxygen consumption, carbon dioxide production and energy expenditure in GHRH-/- mice compared to wild type mice in both light and dark cycles. Respiratory exchange ratio was significantly lower in GHRH-/- mice during the light cycle, but not during the dark cycle, indicating a circadian related metabolic shift towards fat utilization in the growth hormone deficient mice. The novel CRISPR/Cas9 GHRH-/- mice are exhibiting the consistent and unique physiological and metabolic characteristics, which might mediate the longevity effects of growth hormone deficiency in mice.

The effects of growth hormone on adipose tissue: old observations, new mechanisms

The ability of growth hormone (GH) to induce adipose tissue lipolysis has been known for over five decades; however, the molecular mechanisms that mediate this effect and the ability of GH to inhibit insulin-stimulated glucose uptake have scarcely been documented. In this same time frame, our understanding of adipose tissue has evolved to reveal a complex structure with distinct types of adipocyte, depot-specific differences, a biologically significant extracellular matrix and important endocrine properties mediated by adipokines. All these aforementioned features, in turn, can influence lipolysis. In this Review, we provide a historical and current overview of the lipolytic effect of GH in humans, mice and cultured cells. More globally, we explain lipolysis in terms of GH-induced intracellular signalling and its effect on obesity, insulin resistance and lipotoxicity. In this regard, findings that define molecular mechanisms by which GH induces lipolysis are described. Finally, data are presented for the differential effect of GH on specific adipose tissue depots and on distinct classes of metabolically active adipocytes. Together, these cellular, animal and human studies reveal novel cellular phenotypes and molecular pathways regulating the metabolic effects of GH on adipose tissue.

Inadequate control of thyroid hormones sensitizes to hepatocarcinogenesis and unhealthy aging

An inverse correlation between thyroid hormone levels and longevity has been reported in several species and reduced thyroid hormone levels have been proposed as a biomarker for healthy aging and metabolic fitness. However, hypothyroidism is a medical condition associated with compromised health and reduced life expectancy. Herein, we show, using wild-type and the Pax8 ablated model of hypothyroidism in mice, that hyperthyroidism and severe hypothyroidism are associated with an overall unhealthy status and shorter lifespan. Mild hypothyroid Pax8 +/- mice were heavier and displayed insulin resistance, hepatic steatosis and increased prevalence of liver cancer yet had normal lifespan. These pathophysiological conditions were precipitated by hepatic mitochondrial dysfunction and oxidative damage accumulation. These findings indicate that individuals carrying mutations on PAX8 may be susceptible to develop liver cancer and/or diabetes and raise concerns regarding the development of interventions aiming to modulate thyroid hormones to promote healthy aging or lifespan in mammals.

GH Knockout Mice Have Increased Subcutaneous Adipose Tissue With Decreased Fibrosis and Enhanced Insulin Sensitivity

In 1997, our laboratory used targeted gene disruption of the GH receptor (GHR) to generate GHR knockout (GHR-/-) mice, which have been used in >127 published studies to help elucidate GH's numerous activities. However, because GH replacement studies cannot be performed using this line, a GH knockout mouse line via targeted disruption of the GH gene is needed. Therefore, we created and characterized GH gene-disrupted (GH-/-) mice. GH-/- mice have severely decreased IGF-1 levels, small body size, and altered body composition with increased adiposity. GH-/- mice are extremely insulin sensitive but glucose intolerant, with a dramatic reduction in pancreatic islet size. Importantly, disruption of the GH gene had profound and depot-specific effects on white adipose tissue (WAT). Subcutaneous WAT from male and female GH-/- mice have significantly larger adipocytes and reduced fibrosis, neither of which occurred in perigonadal WAT, suggesting that GH has a more pronounced effect on subcutaneous WAT. Comparisons of GH-/- mice to previously published data on GHR-/- mice show a remarkably similar phenotype. Finally, we demonstrate that GH-/- mice are responsive to GH treatment, as shown by changes to serum IGF-1 levels; body length, weight, and composition; and insulin sensitivity. This study not only provides characterization of the first mouse line with targeted mutation of the GH gene but also indicates that GH gene disruption dramatically influences fibrosis of subcutaneous WAT.

The Effects of Graded Levels of Calorie Restriction: X. Transcriptomic Responses of Epididymal Adipose Tissue

Calorie restriction (CR) leads to a remarkable decrease in adipose tissue mass and increases longevity in many taxa. Since the discovery of leptin, the secretory abilities of adipose tissue have gained prominence in the responses to CR. We quantified transcripts of epididymal white adipose tissue of male C57BL/6 mice exposed to graded levels of CR (0–40% CR) for 3 months. The numbers of differentially expressed genes (DEGs) involved in NF-κB, HIF1-α, and p53 signaling increased with increasing levels of CR. These pathways were all significantly downregulated at 40% CR relative to 12 h ad libitum feeding. In addition, graded CR had a substantial impact on DEGs associated with pathways involved in angiogenesis. Of the 497 genes differentially expressed with graded CR, 155 of these genes included a signal peptide motif. These putative signaling proteins were involved in the response to ketones, TGF-β signaling, negative regulation of insulin secretion, and inflammation. This accords with the previously established effects of graded CR on glucose homeostasis in the same mice. Overall these data suggest reduced levels of adipose tissue under CR may contribute to the protective impact of CR in multiple ways linked to changes in a large population of secreted proteins.

From White to Brown - Adipose Tissue Is Critical to the Extended Lifespan and Healthspan of Growth Hormone Mutant Mice

Growth hormone (GH) is a metabolic hormone that has major functions in the liver, muscle, and adipose tissue (AT). In the past 20 years, numerous studies have demonstrated that decreased growth hormone (GH) action is clearly linked to alterations in longevity. Therefore, it is not surprising that mechanisms underlying the extended longevity of GH-mutant animals include alterations in AT function. This Review aims to describe the basics of AT biology, GH secretion and action, and the effects of altered GH signaling in mice and humans. Lastly, this Review discusses the intersection of GH and AT, and how the influence of GH on AT may play a critical role in determining lifespan and healthspan.

Increased environmental temperature normalizes energy metabolism outputs between normal and Ames dwarf mice

Ames dwarf (Prop1^df) mice possess a loss-of-function mutation that results in deficiency of growth hormone, prolactin, and thyroid-stimulating hormone, as well as exceptional longevity. Work in other laboratories suggests that increased respiration and lipid utilization are important for maximizing mammalian longevity. Interestingly, these phenotypes are observed in Ames dwarf mice. We recently demonstrated that Ames dwarf mice have hyperactive brown adipose tissue (BAT), and hypothesized that this may in part be due to their increased surface to mass ratio leading to increased heat loss and an increased demand for thermogenesis. Here, we used increased environmental temperature (eT) to interrogate this hypothesis. We found that increased eT diminished BAT activity in Ames dwarf mice, and led to the normalization of both VO₂ and respiratory quotient between dwarf and normal mice, as well as partial normalization (i.e. impairment) of glucose homeostasis in Ames dwarf mice housed at an increased eT. Together, these data suggest that an increased demand for thermogenesis is partially responsible for the improved energy metabolism and glucose homeostasis which are observed in Ames dwarf mice.

VCAM-1 upregulation accompanies muscle remodeling following resistance-type exercise in Snell dwarf (Pit1dw/dw ) mice

Snell dwarf mice (Pit1^dw/dw ) exhibit deficiencies in growth hormone, prolactin, and thyroid stimulating hormone. Besides being an experimental model of hypopituitarism, these mice are long-lived (>40% lifespan extension) and utilized as a model of slowed/delayed aging. Whether this longevity is accompanied by a compromised quality of life in terms of muscular performance has not yet been characterized. In this study, we investigated nontrained and trained muscles 1 month following a general validated resistance-type exercise protocol in 3-month-old Snell dwarf mice and control littermates. Nontrained Snell dwarf gastrocnemius muscles exhibited a 1.3-fold greater muscle mass to body weight ratio than control values although muscle quality, maximum isometric torque normalized to muscle mass, and fatigue recovery were compromised. For control mice, training increased isometric torque (17%) without altering muscle mass. For Snell dwarf mice, isometric torque was unaltered by training despite decreased muscle mass that rendered muscle mass to body weight ratio comparable to control values. Muscle quality and fatigue recovery improved twofold and threefold, respectively, for Snell dwarf mice. This accompanied a fourfold increase in levels of vascular cell adhesion molecule-1 (VCAM-1), a mediator of progenitor cell recruitment, and muscle remodeling in the form of increased number of central nuclei, additional muscle fibers per unit area, and altered fiber type distribution. These results reveal a trade-off between muscle quality and longevity in the context of anterior pituitary hormone deficiency and that resistance-type training can diminish this trade-off by improving muscle quality concomitant with VCAM-1 upregulation and muscle remodeling.

FSH, Bone Mass, Body Fat, and Biological Aging

The Study of Women's Health Across the Nation has taught us that impending ovarian failure during late perimenopause is associated with a sharp rise in serum FSH, which coincides with the most rapid rate of bone loss and the onset of visceral adiposity. At this time in a woman's life, serum estrogen levels are largely unaltered, so the hypothesis that hypoestrogenemia is the sole cause of bone loss and visceral obesity does not offer a full explanation. An alternative explanation, arising from animal models and human data, is that both physiologic aberrations, obesity and osteoporosis, arise at least in part from rising FSH levels. Here, we discuss recent findings on the mechanism through which FSH exerts biological actions on bone and fat and review clinical data that support a role for FSH in causing osteoporosis and obesity. We will also provide a conceptual framework for using a single anti-FSH agent to prevent and treat both osteoporosis and obesity in women across the menopausal transition.

Growth Hormone's Effect on Adipose Tissue: Quality versus Quantity

Obesity is an excessive accumulation or expansion of adipose tissue (AT) due to an increase in either the size and/or number of its characteristic cell type, the adipocyte. As one of the most significant public health problems of our time, obesity and its associated metabolic complications have demanded that attention be given to finding effective therapeutic options aimed at reducing adiposity or the metabolic dysfunction associated with its accumulation. Growth hormone (GH) has therapeutic potential due to its potent lipolytic effect and resultant ability to reduce AT mass while preserving lean body mass. However, AT and its resident adipocytes are significantly more dynamic and elaborate than once thought and require one not to use the reduction in absolute mass as a readout of efficacy alone. Paradoxically, therapies that reduce GH action may ultimately prove to be healthier, in part because GH also possesses potent anti-insulin activities along with concerns that GH may promote the growth of certain cancers. This review will briefly summarize some of the newer complexities of AT relevant to GH action and describe the current understanding of how GH influences this tissue using data from both humans and mice. We will conclude by considering the therapeutic use of GH or GH antagonists in obesity, as well as important gaps in knowledge regarding GH and AT.

Impact of Growth Hormone on Regulation of Adipose Tissue

Increasing prevalence of obesity and obesity-related conditions worldwide has necessitated a more thorough understanding of adipose tissue (AT) and expanded the scope of research in this field. AT is now understood to be far more complex and dynamic than previously thought, which has also fueled research to reevaluate how hormones, such as growth hormone (GH), alter the tissue. In this review, we will introduce properties of AT important for understanding how GH alters the tissue, such as anatomical location of depots and adipokine output. We will provide an overview of GH structure and function and define several human conditions and cognate mouse lines with extremes in GH action that have helped shape our understanding of GH and AT. A detailed discussion of the GH/AT relationship will be included that addresses adipokine production, immune cell populations, lipid metabolism, senescence, differentiation, and fibrosis, as well as brown AT and beiging of white AT. A brief overview of how GH levels are altered in an obese state, and the efficacy of GH as a therapeutic option to manage obesity will be given. As we will reveal, the effects of GH on AT are numerous, dynamic and depot-dependent. © 2017 American Physiological Society. Compr Physiol 7:819-840, 2017.

Altered structure and function of adipose tissue in long-lived mice with growth hormone-related mutations

A major focus of biogerontology is elucidating the role(s) of the endocrine system in aging and the accumulation of age-related diseases. Endocrine control of mammalian longevity was first reported in Ames dwarf (Prop1^df) mice, which are long-lived due to a recessive Prop1 loss-of-function mutation resulting in deficiency of growth hormone (GH), thyroid-stimulating hormone, and prolactin. Following this report, several other GH-related mutants with altered longevity have been described including long-lived Snell dwarf and growth hormone receptor knockout mice, and short-lived GH overexpressing transgenic mice. One of the emerging areas of interest in these mutant mice is the role of adipose tissue in their altered healthspan and lifespan. Here, we provide an overview of the alterations in body composition of GH-related mutants, as well as the altered thermogenic potential of their brown adipose tissue and the altered cellular senescence and adipokine production of their white adipose tissue.

Neuropeptide Y resists excess loss of fat by lipolysis in calorie-restricted mice: a trait potential for the life-extending effect of calorie restriction

Neuropeptide Y (NPY) is an orexigenic peptide that plays an essential role in caloric restriction (CR)‐mediated lifespan extension. However, the mechanisms underlying the NPY‐mediated effects in CR are poorly defined. Here, we report that NPY deficiency in male mice during CR increases mortality in association with lipodystrophy. NPY^−/− mice displayed a rapid decrease in body weight and fat mass, as well as increased lipolysis during CR. These alterations in fat regulation were inhibited by the lipolysis inhibitor, acipimox, a treatment associated with reduced mortality. The lipolytic/thermogenic signaling, β3‐adrenergic receptor/hormone sensitive lipase, was markedly activated in white adipose tissue of NPY^−/− mice compared with that of NPY^+/+ mice, and thermogenesis was controlled by NPY under negative energy balance. These results demonstrate the critical role of NPY in the regulation of lipid metabolic homeostasis and survival via control of lipolysis and thermogenesis in a state of negative energy balance.

GH and ageing: Pitfalls and new insights

The interrelationships of growth hormone (GH) actions and aging are complex and incompletely understood. The very pronounced age-related decline in GH secretion together with benefits of GH therapy in individuals with congenital or adult GH deficiency (GHD) prompted interest in GH as an anti-aging agent. However, the benefits of treatment of normal elderly subjects with GH appear to be marginal and counterbalanced by worrisome side effects. In laboratory mice, genetic GH deficiency or resistance leads to a remarkable extension of longevity accompanied by signs of delayed and/or slower aging. Mechanisms believed to contribute to extended longevity of GH-related mutants include improved anti-oxidant defenses, enhanced insulin sensitivity and reduced insulin levels, reduced inflammation and cell senescence, major shifts in mitochondrial function and energy metabolism, and greater stress resistance. Negative association of the somatotropic signaling and GH/insulin-like growth factor 1 (IGF-1)-dependent traits with longevity has also been shown in other mammalian species. In humans, syndromes of GH resistance or deficiency have no consistent effect on longevity, but can provide striking protection from cancer, diabetes and atherosclerosis. More subtle alterations in various steps of GH and IGF-1 signaling are associated with reduced old-age mortality, particularly in women and with improved chances of attaining extremes of lifespan. Epidemiological studies raise a possibility that the relationship of IGF-1 and perhaps also GH levels with human healthy aging and longevity may be biphasic. However, the impact of somatotropic signaling on neoplastic disease is difficult to separate from its impact on aging, and IGF-1 levels exhibit opposite associations with different chronic, age-related diseases.

Reduced growth hormone signaling and methionine restriction: interventions that improve metabolic health and extend life span

Interventions that improve health are often associated with longevity. Reduced growth hormone signaling has been shown to increase life span in mice by over 50%. Similarly, reductions in dietary intake of methionine, in rats and mice, result in life-span extension. Many factors affect metabolic health, mitochondrial function, and resistance to stressors, each of which influence aging and life span. This paper presents a comparison of these two interventions, as well as the results of a study combining these interventions, to understand potential mechanisms underlying their effectiveness in enhancing healthy aging.

Inhibition of adenylyl cyclase type 5 increases longevity and healthful aging through oxidative stress protection

Mice with disruption of adenylyl cyclase type 5 (AC5 knockout, KO) live a third longer than littermates. The mechanism, in part, involves the MEK/ERK pathway, which in turn is related to protection against oxidative stress. The AC5 KO model also protects against diabetes, obesity, and the cardiomyopathy induced by aging, diabetes, and cardiac stress and also demonstrates improved exercise capacity. All of these salutary features are also mediated, in part, by oxidative stress protection. For example, chronic beta adrenergic receptor stimulation induced cardiomyopathy was rescued by AC5 KO. Conversely, in AC5 transgenic (Tg) mice, where AC5 is overexpressed in the heart, the cardiomyopathy was exacerbated and was rescued by enhancing oxidative stress resistance. Thus, the AC5 KO model, which resists oxidative stress, is uniquely designed for clinical translation, since it not only increases longevity and exercise, but also protects against diabetes, obesity, and cardiomyopathy. Importantly, inhibition of AC5's action to prolong longevity and enhance healthful aging, as well as its mechanism through resistance to oxidative stress, is unique among all of the nine AC isoforms.

Reduced expression of MYC increases longevity and enhances healthspan

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

Diminished mTOR signaling: a common mode of action for endocrine longevity factors

Since the initial observation that a calorie-restricted (CR) diet can extend rodent lifespan, many genetic and pharmaceutical interventions that also extend lifespan in mammals have been discovered. The mechanism by which CR and these other interventions extend lifespan is the subject of significant debate and research. One proposed mechanism is that CR promotes longevity by increasing insulin sensitivity, but recent findings that dissociate longevity and insulin sensitivity cast doubt on this hypothesis. These findings can be reconciled if longevity is promoted not via increased insulin sensitivity, but instead via decreased PI3K/Akt/mTOR pathway signaling. This review presents a unifying hypothesis that explains the lifespan-extending effects of a variety of genetic mutations and pharmaceutical interventions and points towards new molecular pathways which may also be leveraged to promote healthy aging.

The GH/IGF-1 axis in obesity: pathophysiology and therapeutic considerations

Obesity has become one of the most common medical problems in developed countries, and this disorder is associated with high incidences of hypertension, dyslipidaemia, cardiovascular disease, type 2 diabetes mellitus and specific cancers. Growth hormone (GH) stimulates the production of insulin-like growth factor 1 in most tissues, and together GH and insulin-like growth factor 1 exert powerful collective actions on fat, protein and glucose metabolism. Clinical trials assessing the effects of GH treatment in patients with obesity have shown consistent reductions in total adipose tissue mass, in particular abdominal and visceral adipose tissue depots. Moreover, studies in patients with abdominal obesity demonstrate a marked effect of GH therapy on body composition and on lipid and glucose homeostasis. Therefore, administration of recombinant human GH or activation of endogenous GH production has great potential to influence the onset and metabolic consequences of obesity. However, the clinical use of GH is not without controversy, given conflicting results regarding its effects on glucose metabolism. This Review provides an introduction to the role of GH in obesity and summarizes clinical and preclinical data that describe how GH can influence the obese state.

A dwarf mouse model with decreased GH/IGF-1 activity that does not experience life-span extension: potential impact of increased adiposity, leptin, and insulin with advancing age

Reduced growth hormone (GH) action is associated with extended longevity in many vertebrate species. GH receptor (GHR) null (GHR(-)(/-)) mice, which have a disruption in the GHR gene, are a well-studied example of mice that are insulin sensitive and long lived yet obese. However, unlike other mouse lines with reduced GH action, GH receptor antagonist (GHA) transgenic mice have reduced GH action yet exhibit a normal, not extended, life span. Understanding why GHA mice do not have extended life span though they share many physiological attributes with GHR(-)(/-) mice will help provide clues about how GH influences aging. For this study, we examined age- and sex-related changes in body composition, glucose homeostasis, circulating adipokines, and tissue weights in GHA mice and littermate controls. Compared with previous studies with GHR(-)(/-) mice, GHA mice had more significant increases in fat mass with advancing age. The increased obesity resulted in significant adipokine changes. Euglycemia was maintained in GHA mice; however, hyperinsulinemia developed in older male GHA mice. Overall, GHA mice experience a more substantial, generalized obesity accompanied by altered adipokine levels and glucose homeostasis than GHR(-)(/-) mice, which becomes more exaggerated with advancing age and which likely contributes to the lack of life-span extension in these mice.

Treatment of Obesity Using GH

The obesity epidemic calls for complementary treatment possibilities in addition to lifestyle changes. One of the important regulators of lipid homeostasis is growth hormone (GH). Clinical trials have tested if GH can reduce obesity in humans. The mechanisms underlying the response to GH administration have also been investigated in animal models of human obesity. A literature search yielded 19 randomized placebo-controlled clinical studies and several animal studies investigating chronic GH treatment of obesity. Significant effects were found in some of the larger trials. One clinical trial showed significantly increased weight loss due to GH treatment, and in seven trials, a significant reduction of fat mass was found. The improvements observed were modest, but even minor improvements have been shown to be beneficial, especially if the reduction in fat mass includes visceral adipose tissue, as was reported in three of six trials. In principle, animal data support the clinical observations although the reduction of fat mass was more dramatic than observed in humans. The mechanisms resulting in lipid mobilization most likely include adipose tissue lipo-protein lipase (LPL) inhibition and antagonization of the anti-lipolytic activity of insulin. By feeding a restricted amount of a high fat diet to GH exposed rats hyper-insulinemia was avoided, loss of body fat was accelerated and metabolic markers were improved. Provision of a diet suitable for the metabolic conditions during GH treatment shows promise for improving metabolic control and can perhaps increase the efficacy and/or widen the therapeutic window of GH.

GH and IGF1: roles in energy metabolism of long-living GH mutant mice

Of the multiple theories to explain exceptional longevity, the most robust of these has centered on the reduction of three anabolic protein hormones, growth hormone (GH), insulin-like growth factor, and insulin. GH mutant mice live 50% longer and exhibit significant differences in several aspects of energy metabolism as compared with wild-type mice. Mitochondrial metabolism is upregulated in the absence of GH, whereas in GH transgenic mice and dwarf mice treated with GH, multiple aspects of these pathways are suppressed. Core body temperature is markedly lower in dwarf mice, yet whole-body metabolism, as measured by indirect calorimetry, is surprisingly higher in Ames dwarf and Ghr-/- mice compared with normal controls. Elevated adiponectin, a key antiinflammatory cytokine, is also very likely to contribute to longevity in these mice. Thus, several important components related to energy metabolism are altered in GH mutant mice, and these differences are likely critical in aging processes and life-span extension.

Insulin-like growth factor 1 physiology: lessons from mouse models

Insulin-like growth factor 1 (IGF-1) is a pleiotropic polypeptide. Its expression is tightly regulated and it plays significant roles during early development, maturation, and adulthood. This article discusses the roles of IGF-1 in determination of body size, skeletal acquisition, muscle growth, carbohydrate metabolism, and longevity, as learned from mouse models.

Metabotyping of long-lived mice using 1H NMR spectroscopy

Significant advances in understanding aging have been achieved through studying model organisms with extended healthy lifespans. Employing 1H NMR spectroscopy, we characterized the plasma metabolic phenotype (metabotype) of three long-lived murine models: 30% dietary restricted (DR), insulin receptor substrate 1 null (Irs1-/-), and Ames dwarf (Prop1df/df). A panel of metabolic differences were generated for each model relative to their controls, and subsequently, the three long-lived models were compared to one another. Concentrations of mobile very low density lipoproteins, trimethylamine, and choline were significantly decreased in the plasma of all three models. Metabolites including glucose, choline, glycerophosphocholine, and various lipids were significantly reduced, while acetoacetate, d-3-hydroxybutyrate and trimethylamine-N-oxide levels were increased in DR compared to ad libitum fed controls. Plasma lipids and glycerophosphocholine were also decreased in Irs1-/- mice compared to controls, as were methionine and citrate. In contrast, high density lipoproteins and glycerophosphocholine were increased in Ames dwarf mice, as were methionine and citrate. Pairwise comparisons indicated that differences existed between the metabotypes of the different long-lived mice models. Irs1-/- mice, for example, had elevated glucose, acetate, acetone, and creatine but lower methionine relative to DR mice and Ames dwarfs. Our study identified several potential candidate biomarkers directionally altered across all three models that may be predictive of longevity but also identified differences in the metabolic signatures. This comparative approach suggests that the metabolic networks underlying lifespan extension may not be exactly the same for each model of longevity and is consistent with multifactorial control of the aging process.

Growth hormone and adipose tissue: beyond the adipocyte

The last two decades have seen resurgence in research focused on adipose tissue. In part, the enhanced interest stems from an alarming increase in obesity rates worldwide. However, an understanding that this once simple tissue is significantly more intricate and interactive than previously realized has fostered additional attention. While few would argue that growth hormone (GH) radically alters fat mass, newer findings revealing the complexity of adipose tissue requires that GH's influence on this tissue be reexamined. Therefore, the objective of this review is to describe the more recent understanding of adipose tissue and to summarize our current knowledge of how GH may influence and contribute to these newer complexities of this tissue with special focus on the available data from mice with altered GH action.

The hippocampus of Ames dwarf mice exhibits enhanced antioxidative defenses following kainic acid-induced oxidative stress

INTRODUCTION

The vulnerability of the hippocampus to the effects of aging has been found to be associated with a decline in growth hormone/insulin like growth factor-1 (GH/IGF-1), and an increase in oxidative stress. We have evidence that long-living GH-deficient Ames dwarf mice have enhanced antioxidant protection in the periphery but the protection in the central nervous system is less clear.

MATERIAL AND METHODS

In the present study, we evaluated the antioxidative defense enzyme status in the hippocampus of Ames dwarf and wild type mice at 3, 12 and 24 months of age and examined the ability of each genotype to resist kainic acid-induced (KA) oxidative stress. An equiseizure concentration of KA was administered such that both genotypes responded with similar seizure scores and lipid peroxidation.

RESULTS

We found that GH-sufficient wild type mice showed an increase in oxidative stress as indicated by the reduced ratio of glutathione: glutathione disulfide following KA injection while this ratio was maintained in GH-deficient Ames dwarf mice. In addition, glutathione peroxidase activity (GPx) as well as GPx1 mRNA expression was enhanced in KA-injected Ames dwarf mice but decreased in wild type mice. There was no induction of Nrf-2 (an oxidative stress-induced transcription factor) gene expression in Ames dwarf mice following KA further suggesting maintenance of antioxidant defense in GH-deficiency under oxidative stress conditions.

DISCUSSION

Therefore, based on equiseizure administration of KA, Ames dwarf mice have an enhanced antioxidant defense capacity in the hippocampus similar to that observed in the periphery. This improved defense capability in the brain is likely due to increased GPx availability in Ames mice and may contribute to their enhanced longevity.

IGF-1 and bone: New discoveries from mouse models

Insulin-like growth factor-1 (IGF-1) plays a central role in cellular growth, differentiation, survival, and cell cycle progression. It is expressed early during development and its effects are mediated through binding to a tyrosine kinase receptor, the insulin-like growth factor-1 receptor (IGF-1R). In the circulation, the IGFs bind to IGF-binding proteins (IGFBPs), which determine their bioavailability and regulate the interaction between the IGFs and IGF-1R. Studies in animal models and in humans have established critical roles for IGFs in skeletal growth and development. In this review we present new and old findings from mouse models of the IGF system and discuss their clinical relevance to normal and pathological skeletal physiology.

Spatial memory is enhanced in long-living Ames dwarf mice and maintained following kainic acid induced neurodegeneration

INTRODUCTION

Age associated cognitive impairment is associated with low levels of IGF-1, oxidative stress, and neuronal loss in the hippocampus. Ames dwarf mice are long-lived animals that exhibit peripheral IGF-1 deficiency. Hippocampal-based spatial memory (a homolog of cognitive function) has not been evaluated in these long-living mice.

MATERIALS AND METHODS

We evaluated the hippocampal-based spatial memory in 3-, 12- and 24-month-old Ames dwarf and wild type mice using the Barnes maze and the T-maze. We also examined the effect of a hippocampal-specific toxin, kainic acid (KA), on spatial memory to determine whether Ames mice were resistant to the cognitive impairment induced by this compound.

RESULTS

We found that Ames dwarf mice exhibit enhanced learning, making fewer errors and using less time to solve both the Barnes and T-mazes. Dwarf mice also have significantly better short-term memory as compared to wild type mice. Both genotypes exhibited neuronal loss in the CA1 and CA3 areas of the hippocampus following KA, but Ames dwarf mice retained their spatial memory.

DISCUSSION

Our results show that Ames dwarf mice retained their spatial memory despite neurodegeneration when compared to wild type mice at an "equiseizure" dose of KA.

Ames dwarf (Prop1(df)/Prop1(df)) mice display increased sensitivity of the major GH-signaling pathways in liver and skeletal muscle

CONTEXT

Growth hormone (GH) is an anabolic hormone that regulates growth and metabolism. Ames dwarf mice are natural mutants for Prop1, with impaired development of anterior pituitary and undetectable levels of circulating GH, prolactin and TSH. They constitute an endocrine model of life-long GH-deficiency. The main signaling cascades activated by GH binding to its receptor are the JAK2/STATs, PI-3K/Akt and the MAPK Erk1/2 pathways.

OBJECTIVES

We have previously reported that GH-induced STAT5 activation was higher in Ames dwarf mice liver compared to non-dwarf controls. The aim of this study was to evaluate the principal components of the main GH-signaling pathways under GH-deficiency in liver and skeletal muscle, another GH-target tissue.

METHODS

Ames dwarf mice and their non-dwarf siblings were assessed. Animals were injected i.p. with GH or saline 15min before tissue removal. Protein content and phosphorylation of signaling mediators were determined by immunoblotting of tissue solubilizates.

RESULTS

GH was able to induce STAT5 and STAT3 tyrosine phosphorylation in both liver and muscle, but the response was higher for Ames dwarf mice than for non-dwarf controls. When Erk1/2 activation was assessed in liver, only dwarf mice showed GH-induced phosphorylation, while in muscle no response to the hormone was found in either genotype. GH-induced Akt phosphorylation at Ser473 in liver was only detected in dwarf mice. In skeletal muscle, both normal and dwarf mice responded to a GH stimulus, although dwarf mice presented higher GH activation levels. The phosphorylation of GSK-3, a substrate of Akt, increased upon hormone stimulation only in dwarf mice in both tissues. In contrast, no differences in the phosphorylation of mTOR, another substrate of Akt, were observed after GH stimulus, either in normal or dwarf mice in liver, while we were unable to determine mTOR in muscle. Protein content of GH-receptor and of the signaling mediators studied did not vary between normal and dwarf animals in the assessed tissues.

CONCLUSION

These results show that several components of the main GH-signaling pathways exhibit enhanced sensitivity to the hormone in liver and muscle of Ames dwarf mice.

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.

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.

Role of the GH/IGF-1 axis in lifespan and healthspan: lessons from animal models

Animal models are fundamentally important in our quest to understand the genetic, epigenetic, and environmental factors that contribute to human aging. In comparison to humans, relatively short-lived mammals are useful models as they allow for rapid assessment of both genetic manipulation and environmental intervention as related to longevity. These models also allow for the study of clinically relevant pathologies as a function of aging. Data associated with more distant species offers additional insight and critical consideration of the basic physiological processes and molecular mechanisms that influence lifespan. Consistently, two interventions, caloric restriction and repression of the growth hormone (GH)/insulin-like growth factor-1/insulin axis, have been shown to increase lifespan in both invertebrates and vertebrate animal model systems. Caloric restriction (CR) is a nutrition intervention that robustly extends lifespan whether it is started early or later in life. Likewise, genes involved in the GH/IGF-1 signaling pathways can lengthen lifespan in vertebrates and invertebrates, implying evolutionary conservation of the molecular mechanisms. Specifically, insulin and insulin-like growth factor-1 (IGF-1)-like signaling and its downstream intracellular signaling molecules have been shown to be associated with lifespan in fruit flies and nematodes. More recently, mammalian models with reduced growth hormone (GH) and/or IGF-1 signaling have also been shown to have extended lifespans as compared to control siblings. Importantly, this research has also shown that these genetic alterations can keep the animals healthy and disease-free for longer periods and can alleviate specific age-related pathologies similar to what is observed for CR individuals. Thus, these mutations may not only extend lifespan but may also improve healthspan, the general health and quality of life of an organism as it ages. In this review, we will provide an overview of how the manipulation of the GH/IGF axis influences lifespan, highlight the invertebrate and vertebrate animal models with altered lifespan due to modifications to the GH/IGF-1 signaling cascade or homologous pathways, and discuss the basic phenotypic characteristics and healthspan of these models.

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.

Age to survive: DNA damage and aging

Aging represents the progressive functional decline and increased mortality risk common to nearly all metazoans. Recent findings experimentally link DNA damage and organismal aging: longevity-regulating genetic pathways respond to the accumulation of DNA damage and other stress conditions and conversely influence the rate of damage accumulation and its impact for cancer and aging. This novel insight has emerged from studies on human progeroid diseases and mouse models that have deficient DNA repair pathways. Here we discuss a unified concept of an evolutionarily conserved 'survival' response that shifts the organism's resources from growth to maintenance as an adaptation to stresses, such as starvation and DNA damage. This shift protects the organism from cancer and promotes healthy aging.

Effects of lactogen resistance and GH deficiency on mouse metabolism: pancreatic hormones, adipocytokines, and expression of adiponectin and insulin receptors

We recently described a novel mouse model that combines resistance to lactogenic hormones with GH deficiency (GHD). The GHD/lactogen-resistant males develop obesity and insulin resistance with age. We hypothesized that altered production of pancreatic hormones and dysregulation of adipocytokine secretion and action contribute to the pathogenesis of their insulin resistance. Double-mutant males (age 12-16 months) had fasting hyperinsulinemia, hyperamylinemia, hyperleptinemia, and a decreased ratio of adiponectin to leptin. Adiponectin receptor 1 and 2 (AdipoR1 and R2) mRNA levels in liver and skeletal muscle were normal but hepatic insulin receptor mRNA was increased. Relative to double-mutant males, GHD males had lower levels of insulin, amylin, and leptin, higher levels of adiponectin, and higher expression of hepatic AdipoR1 and insulin receptor mRNAs. Lactogen-resistant mice had reduced hepatic adipoR2 mRNA. In response to stress the plasma concentrations of MCP-1 and IL-6 increased in double-mutant males but not GHD or lactogen-resistant males. Our findings suggest that the insulin resistance of GHD/lactogen-resistant males is accompanied by dysregulation of pancreatic hormone and adipocytokine secretion and receptor expression. Phenotypic differences between double-mutant and GHD males suggest that lactogens and GH exert differential but overlapping effects on fat deposition and adipocytokine secretion and action.

CIDE-A gene expression is decreased in white adipose tissue of growth hormone receptor/binding protein gene disrupted mice and with high-fat feeding of normal mice

Growth hormone's (GH) lipolytic activity in white adipose tissue (WAT) results in decreased body fat in giant GH transgenic mice and increased subcutaneous fat in dwarf growth hormone receptor/binding protein gene-disrupted mice (GHR -/-). We therefore hypothesized that GH action would affect expression of CIDE-A (cell-death-inducing DFF45-like effector-A), a protein found in white adipose tissue (WAT) and involved in lipid metabolism. CIDE-A RNA levels were determined in subcutaneous, retroperitoneal and epididymal adipose tissue isolated from wild-type and GHR -/- mice. The adipose tissue was also analyzed for adipocyte size. We determined that the lack of GH action has depot-specific effects on the levels of CIDE-A RNA and affected adipocyte cell size. CIDE-A expression is significantly reduced in GHR -/- subcutaneous fat compared to wild-type but is not altered in retroperitoneal or epididymal fat. Likewise, adipocytes are significantly enlarged in GHR -/- subcutaneous adipose tissue relative wild-type mice. A high-fat diet also influenced the level of CIDE-A RNA in mouse adipose tissue. The high-fat diet significantly reduced CIDE-A expression in wild-type subcutaneous fat but did not alter CIDE-A expression in subcutaneous fat of GHR -/- mice. The diet also reduced CIDE-A expression in wild-type retroperitoneal fat but the levels of CIDE-A in epididymal fat were unchanged. In contrast, the high-fat diet reduced CIDE-A expression in both retroperitoneal and epididymal fat of GHR -/- mice. These data demonstrate that CIDE-A levels are reduced in two different mouse models of obesity and this reduction may contribute to altered lipid metabolism.

Adipocytokines and lipid levels in Ames dwarf and calorie-restricted mice

Ames dwarf mice are long-lived and insulin sensitive, and have a normal or reduced percentage of body fat. Calorie restriction (CR) is known to improve insulin sensitivity and reduce body fat. The purpose of this study was to evaluate the mechanism of improved insulin sensitivity in the Ames dwarfs and the effects of CR on adipose signaling and metabolism in normal and dwarf mice. Enhanced insulin sensitivity in dwarf mice may be partly due to increased release of adiponectin and the reduced release of tumor necrosis factor-alpha (TNF-alpha) and interleukin-6 (IL-6). Altered levels of adipocytokines might be consequent to the decreased lipid synthesis, plasma triglycerides, and free fatty acid levels. In normal mice, CR improves insulin sensitivity by affecting the release of adipocytokines, and decreasing circulating fatty acid and triglycerides concentrations as well as liver triglyceride accumulation. However, CR may reduce rather than enhance some of the insulin effects in the highly insulin-sensitive dwarf mice.

Long-Lived Growth Hormone Receptor Knockout Mice Show a Delay in Age-Related Changes of Body Composition and Bone Characteristics

There is conflicting information on the physiological role of growth hormone (GH) in the control of aging. This study reports dual-energy x-ray absorptiometry (DXA) measurements of body composition and bone characteristics in young, adult, and aged long-lived GH receptor knockout (GHR-KO) and normal mice to determine the effects of GH resistance during aging. Compared to controls, GHR-KO mice showed an increased percentage of body fat. GHR-KO mice have reduced total-body bone mineral density (BMD), bone mineral content, and bone area, but these parameters increased with age. In addition, GHR-KO mice have decreased femur length, femur BMD, and lower lumbar BMD compared to controls in all age groups. These parameters also continued to increase with age. Our results indicate that GH resistance alters body composition, bone growth, and bone maintenance during aging in GHR-KO mice.

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.

Body size, energy metabolism and lifespan

Bigger animals live longer. The scaling exponent for the relationship between lifespan and body mass is between 0.15 and 0.3. Bigger animals also expend more energy, and the scaling exponent for the relationship of resting metabolic rate (RMR) to body mass lies somewhere between 0.66 and 0.8. Mass-specific RMR therefore scales with a corresponding exponent between -0.2 and -0.33. Because the exponents for mass-specific RMR are close to the exponents for lifespan, but have opposite signs, their product (the mass-specific expenditure of energy per lifespan) is independent of body mass (exponent between -0.08 and 0.08). This means that across species a gram of tissue on average expends about the same amount of energy before it dies regardless of whether that tissue is located in a shrew, a cow, an elephant or a whale. This fact led to the notion that ageing and lifespan are processes regulated by energy metabolism rates and that elevating metabolism will be associated with premature mortality--the rate of living theory. The free-radical theory of ageing provides a potential mechanism that links metabolism to ageing phenomena, since oxygen free radicals are formed as a by-product of oxidative phosphorylation. Despite this potential synergy in these theoretical approaches, the free-radical theory has grown in stature while the rate of living theory has fallen into disrepute. This is primarily because comparisons made across classes (for example, between birds and mammals) do not conform to the expectations, and even within classes there is substantial interspecific variability in the mass-specific expenditure of energy per lifespan. Using interspecific data to test the rate of living hypothesis is, however, confused by several major problems. For example, appeals that the resultant lifetime expenditure of energy per gram of tissue is 'too variable' depend on the biological significance rather than the statistical significance of the variation observed. Moreover, maximum lifespan is not a good marker of ageing and RMR is not a good measure of total energy metabolism. Analysis of residual lifespan against residual RMR reveals no significant relationship. However, this is still based on RMR. A novel comparison using daily energy expenditure (DEE), rather than BMR, suggests that lifetime expenditure of energy per gram of tissue is NOT independent of body mass, and that tissue in smaller animals expends more energy before expiring than tissue in larger animals. Some of the residual variation in this relationship in mammals is explained by ambient temperature. In addition there is a significant negative relationship between residual lifespan and residual daily energy expenditure in mammals. A potentially much better model to explore the links of body size, metabolism and ageing is to examine the intraspecific links. These studies have generated some data that support the original rate of living theory and other data that conflict. In particular several studies have shown that manipulating animals to expend more or less energy generate the expected effects on lifespan (particularly when the subjects are ectotherms). However, smaller individuals with higher rates of metabolism live longer than their slower, larger conspecifics. An addition to these confused observations has been the recent suggestion that under some circumstances we might expect mitochondria to produce fewer free radicals when metabolism is higher--particularly when they are uncoupled. These new ideas concerning the manner in which mitochondria generate free radicals as a function of metabolism shed some light on the complexity of observations linking body size, metabolism and lifespan.

Murine Models of Life Span Extension

Mice are excellent experimental models for genetic research and are being used to investigate the genetic component of organismal aging. Several mutant mice are known to possess defects in the growth hormone/insulin-like growth factor 1 (GH/IGF-1) neurohormonal pathway and exhibit dwarfism together with extended life span. Their phenotypes resemble those of mice subjected to caloric restriction. Targeted mutations that affect components of this pathway, including the GH receptor, p66Shc, and the IGF-1 receptor (IGF-1R), also extend life span; mutations that affect IGF-1R or downstream components of the pathway decouple longevity effects from dwarfism. These effects on life span may result from an increased capacity to resist oxidative damage.