Effects of caloric restriction on insulin pathway gene expression in the skeletal muscle and liver of normal and long-lived GHR-KO mice

Current Update on Categorization of Migraine Subtypes on the Basis of Genetic Variation: a Systematic Review

Migraine is a complex neurovascular disorder that is characterized by severe behavioral, sensory, visual, and/or auditory symptoms. It has been labeled as one of the ten most disabling medical illnesses in the world by the World Health Organization (Aagaard et al Sci Transl Med 6(237):237ra65, 2014). According to a recent report by the American Migraine Foundation (Shoulson et al Ann Neurol 25(3):252-9, 1989), around 148 million people in the world currently suffer from migraine. On the basis of presence of aura, migraine is classified into two major subtypes: migraine with aura (Aagaard et al Sci Transl Med 6(237):237ra65, 2014) and migraine without aura. (Aagaard K et al Sci Transl Med 6(237):237ra65, 2014) Many complex genetic mechanisms have been proposed in the pathophysiology of migraine but specific pathways associated with the different subtypes of migraine have not yet been explored. Various approaches including candidate gene association studies (CGAS) and genome-wide association studies (Fan et al Headache: J Head Face Pain 54(4):709-715, 2014). have identified the genetic markers associated with migraine and its subtypes. Several single nucleotide polymorphisms (Kaur et al Egyp J Neurol, Psychiatry Neurosurg 55(1):1-7, 2019) within genes involved in ion homeostasis, solute transport, synaptic transmission, cortical excitability, and vascular function have been associated with the disorder. Currently, the diagnosis of migraine is majorly behavioral with no focus on the genetic markers and thereby the therapeutic intervention specific to subtypes. Therefore, there is a need to explore genetic variants significantly associated with MA and MO as susceptibility markers in the diagnosis and targets for therapeutic interventions in the specific subtypes of migraine. Although the proper characterization of pathways based on different subtypes is yet to be studied, this review aims to make a first attempt to compile the information available on various genetic variants and the molecular mechanisms involved with the development of MA and MO. An attempt has also been made to suggest novel candidate genes based on their function to be explored by future research.

Sex-specific effects of dietary restriction on physiological variables in Japanese quails

Nutritional limitation is a common phenomenon in nature that leads to trade-offs among processes competing for limited resources. These trade-offs are mediated by changes in physiological traits such as growth factors and circulating lipids. However, studies addressing the sex-specific effect of nutritional deficiency on these physiological variables are limited in birds. We used dietary restriction to mimic the depletion of resources to various degrees and investigated sex-specific effects on circulating levels of insulin-like growth factor 1 (IGF-1) and triglycerides in Japanese quails (Coturnix japonica) subjected to ad libitum, 20%, 30% or 40% restriction of their daily requirement, for 2 weeks. We also explored the association of both physiological variables with body mass and egg production. While dietary restriction showed no effects on circulating IGF-1, this hormone exhibited a marked sexual difference, with females having 64.7% higher IGF-1 levels than males. Dietary restriction significantly reduced plasma triglyceride levels in both sexes. Females showed more than six-fold higher triglyceride levels than males. Triglyceride levels were positively associated with body mass in females while showed not association in males. Overall, our findings revealed sex-specific expression of physiological variables under dietary restriction conditions, which coincide with body size.

Calorie restriction modulates the transcription of genes related to stress response and longevity in human muscle: The CALERIE study

The lifespan extension induced by 40% caloric restriction (CR) in rodents is accompanied by postponement of disease, preservation of function, and increased stress resistance. Whether CR elicits the same physiological and molecular responses in humans remains mostly unexplored. In the CALERIE study, 12% CR for 2 years in healthy humans induced minor losses of muscle mass (leg lean mass) without changes of muscle strength, but mechanisms for muscle quality preservation remained unclear. We performed high-depth RNA-Seq (387-618 million paired reads) on human vastus lateralis muscle biopsies collected from the CALERIE participants at baseline, 12- and 24-month follow-up from the 90 CALERIE participants randomized to CR and "ad libitum" control. Using linear mixed effect model, we identified protein-coding genes and splicing variants whose expression was significantly changed in the CR group compared to controls, including genes related to proteostasis, circadian rhythm regulation, DNA repair, mitochondrial biogenesis, mRNA processing/splicing, FOXO3 metabolism, apoptosis, and inflammation. Changes in some of these biological pathways mediated part of the positive effect of CR on muscle quality. Differentially expressed splicing variants were associated with change in pathways shown to be affected by CR in model organisms. Two years of sustained CR in humans positively affected skeletal muscle quality, and impacted gene expression and splicing profiles of biological pathways affected by CR in model organisms, suggesting that attainable levels of CR in a lifestyle intervention can benefit muscle health in humans.

A cross-sectional analysis of association between visceral adiposity index and serum anti-aging protein Klotho in adults

BACKGROUND

The visceral adiposity index (VAI) is regarded as a reliable indicator to assess body fat distribution and dysfunction. Klotho protein is a hormone with anti-aging biological functions. However, the relationship between them has not been researched.

OBJECTS

This study aimed to evaluate the association between VAI and serum anti-aging protein klotho in American adults.

METHODS

A cross-sectional study of participants was conducted based on the National Health and Nutrition Examination Surveys (NHANES) 2007-2016. Visceral adiposity was determined using the VAI score, while the klotho protein concentration was measured by ELISA kit. After adjusting some possible confounding variables, multivariate regression model was conducted to estimate the relationship between VAI and klotho protein. Furthermore, the smooth curve fitting and the segmented regression model were applied to examine the threshold effect and to calculate the inflection point.

RESULT

In total, 6 252 adults were eligible, with a mean VAI of 2.04 ± 0.03 and a mean klotho protein concentration of 848.79 ± 6.98 pg/ml. Multivariate regression analysis indicated that serum klotho protein concentration was lower in participants with high VAI score. When VAI was divided into quartiles, participants in the fourth quartiles of higher VAI had lower klotho protein levels (Q4: -32.25 pg/ml) than participants in the lowest quartile (Q1) after full adjustment (P < 0.05). Segmented regression suggested that the turning point value of VAI was 3.21. A 1-unit increase in VAI was significantly associated with lower klotho protein levels by -18.61 pg/ml (95% CI: -28.87, -8.35; P < 0.05) when VAI ranged from 0.29 to 3.21(accounting for 83.7% of the participants), however, the association was not significant when VAI ranged from 3.21 to 11.81 (P = 0.77).

CONCLUSION

There was a nonlinear correlation between VAI score and the serum anti-aging protein klotho concentrations, showing a saturation effect. When VAI was less than 3.21, they were negatively correlated, and when VAI was greater than 3.21, they had no obvious correlation.

Glucose-Related Traits and Risk of Migraine—A Potential Mechanism and Treatment Consideration

Migraine and glucose-related (glycaemic) traits (fasting glucose, fasting insulin, and type 2 diabetes) are common and complex comorbid disorders that cause major economic and social burdens on patients and their families. Studies on the relationship between migraine and glucose-related traits have yielded inconsistent results. The purpose of this review is to synthesise and discuss the information from the available literature on the relationship between fasting glucose, fasting insulin, and type 2 diabetes (T2D) with migraine. Publications on migraine and fasting glucose, migraine and fasting insulin, and migraine and T2D were identified from a PubMed and Google Scholar database search and reviewed for this article. Multiple publications have suggested that the comorbidity of migraine and glucose-related traits may have a similar complex pathogenic mechanism, including impaired glucose homeostasis, insulin resistance, reduced cerebrovascular reactivity, abnormal brain metabolism, shared genetic factors, neurotransmitters, and sex hormones. Furthermore, several studies have found a bi-directional link between migraine with insulin resistance and T2D. There is strong evidence for a biological association between migraine headache and glucose-related traits, and burgeoning evidence for shared genetic influences. Therefore, genetic research into these comorbid traits has the potential to identify new biomarkers and therapeutic targets and provide biological insight into their relationships. We encourage healthcare professionals to consider the co-occurrence of migraine with glucose-related traits in the evaluation and treatment of their patients.

Growth hormone and aging

Growth hormone (GH) actions impact growth, metabolism, and body composition and have been associated with aging and longevity. Lack of GH results in slower growth, delayed maturation, and reduced body size and can lead to delayed aging, increased healthspan, and a remarkable extension of longevity. Adult body size, which is a GH-dependent trait, has a negative association with longevity in several mammalian species. Mechanistic links between GH and aging include evolutionarily conserved insulin/insulin-like growth factors and mechanistic target of rapamycin signaling pathways in accordance with long-suspected trade-offs between anabolic/growth processes and longevity. Height and the rate and regulation of GH secretion have been related to human aging, but longevity is not extended in humans with syndromes of GH deficiency or resistance. However, the risk of age-related chronic disease is reduced in individuals affected by these syndromes and various indices of increased healthspan have been reported.

The role of liver metabolism in compensatory-growth piglets induced by protein restriction and subsequent protein realimentation

The aim of this work was to study the role of hepatic metabolism of compensatory growth in piglets induced by protein restriction and subsequent protein realimentation. Thirty-six weaned piglets were randomly distributed in a control group and a treatment group. The control group piglets were fed with a normal protein level diet (18.83% CP) for the entire experimental period (day 1-28). The treatment group piglets were fed with a protein-restriction diet (13.05% CP) for day 1 to day 14, and the diet was restored to normal protein level diet for day 15 to day 28. RNA-seq is used to analyze samples of liver metabolism on day 14 and day 28, respectively. Hepatic RNA-sequencing analysis revealed that some KEGG signaling pathways involved in glycolipid metabolism (eg, "AMPK signaling pathway," "insulin signaling pathway," and "glycolysis or gluconeogenesis") were significantly enriched on day 14 and day 28. On day 14, protein restriction promoted hepatic lipogenesis by increasing the genes expression level of ACACA, FASN, GAPM, and SREBP1C, decreasing protein phosphorylation levels of AMPKɑ and ACC in AMPK signaling pathway. In contrast, on day 28, protein realimentation promoted hepatic gluconeogenesis by increasing the concentration of G6Pase and PEPCK, decreasing protein phosphorylation levels of IRS1, Akt, and FoXO1 in insulin signaling pathway. In addition, protein realimentation activated the GH-IGF1 axis between the liver and skeletal muscle. Overall, these findings revealed the importance of liver metabolism in achieving compensatory growth.

Analysis of Bone Mineral Profile After Prolonged Every-Other-Day Feeding in C57BL/6J Male and Female Mice

Intermitted fasting or every-other-day feeding (EOD) has many positive effects in rodents and humans. Our goal was to describe how EOD influences bone mineral composition in female and male mice under prolonged EOD feeding. Male and female adult mice were fed EOD for 9 months. After this time, we used a direct method of measurement of mineral components in ashes of long bones (humerus and radius) to estimate the content of calcium (Ca), phosphorus (P), potassium (K), magnesium (Mg), and sodium (Na). We also performed histological analysis of sections of long bones. We found no significant changes in mineral composition between ad libitum and EOD fed males and females. We noted higher Ca and P contents in control males vs. females and lower content of Mg in control males vs. females. We observed the presence of marrow adipose tissue (MAT) in sections of EOD-fed females. EOD without supplementation during feeding days did not increase loss of mineral content of bones in C57BL/6J mice, but the presence of MAT only in EOD females indicates a gender-dependent response to EOD treatment in C57BL/6J mice.

The endocrine function of adipose tissues in health and cardiometabolic disease

In addition to their role in glucose and lipid metabolism, adipocytes respond differentially to physiological cues or metabolic stress by releasing endocrine factors that regulate diverse processes, such as energy expenditure, appetite control, glucose homeostasis, insulin sensitivity, inflammation and tissue repair. Both energy-storing white adipocytes and thermogenic brown and beige adipocytes secrete hormones, which can be peptides (adipokines), lipids (lipokines) and exosomal microRNAs. Some of these factors have defined targets; for example, adiponectin and leptin signal through their respective receptors that are expressed in multiple organs. For other adipocyte hormones, receptors are more promiscuous or remain to be identified. Furthermore, many of these hormones are also produced by other organs and tissues, which makes defining the endocrine contribution of adipose tissues a challenge. In this Review, we discuss the functional role of adipose tissue-derived endocrine hormones for metabolic adaptations to the environment and we highlight how these factors contribute to the development of cardiometabolic diseases. We also cover how this knowledge can be translated into human therapies. In addition, we discuss recent findings that emphasize the endocrine role of white versus thermogenic adipocytes in conditions of health and disease.

Depot-specific and GH-dependent regulation of IGF binding protein-4, pregnancy-associated plasma protein-A, and stanniocalcin-2 in murine adipose tissue

INTRODUCTION

Pregnancy-associated plasma protein-A (PAPP-A) stimulates insulin-like growth factor (IGF)-I action through proteolytic cleavage of IGF binding protein-4 (IGFBP-4). Recently, stanniocalcin-2 (STC2) was discovered as an inhibitor of PAPP-A. Most members of the IGF system are expressed in adipose tissue (AT), but there is a relative paucity of information on the distribution of IGFBP-4, PAPP-A, and STC2 in different AT depots. Since IGF-I expression in AT is highly GH-dependent, we used bovine GH transgenic (bGH) and GH receptor knockout (GHR-/-) mice to investigate AT depot-specific expression patterns of IGFBP-4, PAPP-A, and STC2, and whether the regulation is GH-dependent.

METHODS

Seven-month-old male bGH, GHR-/- and wild type (WT) control mice were used. Body composition was determined, and subcutaneous, epididymal, retroperitoneal, mesenteric and brown adipose tissue (BAT) depots were collected. RNA expression of Igfbp4, Pappa, and Stc2 was assessed by reverse transcription quantitative PCR and IGFBP-4 protein by Western blotting.

RESULTS

Igfbp4, Pappa, and Stc2 RNA levels were differentially expressed in an AT depot-dependent manner in WT mice. Igfbp4 RNA levels were significantly higher in all white AT depots than in BAT. Pappa was most highly expressed in the mesenteric depot: levels were 7.5-fold higher in mesenteric than in subcutaneous AT (p < .001). Although intraabdominal in origin, epididymal and retroperitoneal Pappa expression levels were 69% and 68% lower, respectively, as compared to mesenteric levels (p < .001). Stc2 RNA expression was significantly higher in all intraabdominal white AT as compared to subcutaneous AT and BAT; levels in epididymal, retroperitoneal, and mesenteric were all more than three-fold higher than in subcutaneous AT (p < .001) and 12-fold higher than in BAT (p < .001). Gene expression patterns in bGH and GHR-/- mice mimicked those in WT mice, suggesting that GH does not affect the transcription of the STC2-PAPP-A-IGFBP-4-axis in AT. However, proteins levels of intact IGFBP-4 were significantly increased in bGH mice and decreased in GHR-/- mice, whereas the PAPP-A-generated IGFBP-4 fragment level was unaltered.

CONCLUSION

Expression of Igfbp4, Pappa, and Stc2 differ between AT depots and is generally higher in white AT than in BAT. The transcription appears to occur in a GH-independent manner, whereas IGFBP-4 protein levels are highly influenced by altered GH activity.

MicroRNAs and the metabolic hallmarks of aging

Aging, the natural process of growing older, is characterized by a progressive deterioration of physiological homeostasis at the cellular, tissue, and organismal level. Metabolically, the aging process is characterized by extensive changes in body composition, multi-tissue/multi-organ insulin resistance, and physiological declines in multiple signaling pathways including growth hormone, insulin/insulin-like growth factor 1, and sex steroids regulation. With this review, we intend to consolidate published information about microRNAs that regulate critical metabolic processes relevant to aging. In certain occasions we uncover relationships likely relevant to aging, which has not been directly described before, such as the miR-451/AMPK axis. We have also included a provocative section highlighting the potential role in aging of a new designation of miRNAs, namely fecal miRNAs, recently discovered to regulate intestinal microbiota in mammals.

A Long-lived Mouse Lacking Both Growth Hormone and Growth Hormone Receptor: A New Animal Model for Aging Studies

Disruption of the growth hormone (GH) signaling pathway promotes insulin sensitivity and is associated with both delayed aging and extended longevity. Two kinds of long-lived mice-Ames dwarfs (df/df) and GH receptor gene-disrupted knockouts (GHRKO) are characterized by a suppressed GH axis with a significant reduction of body size and decreased plasma insulin-like growth factor-1 (IGF-1) and insulin levels. Ames dwarf mice are deficient in GH, prolactin, and thyrotropin, whereas GHRKOs are GH resistant and are dwarf with decreased circulating IGF-1 and increased GH. Crossing Ames dwarfs and GHRKOs produced a new mouse line (df/KO), lacking both GH and GH receptor. These mice are characterized by improved glucose tolerance and increased adiponectin level, which could imply that these mice should be also characterized by additional life-span extension when comparing with GHRKOs and Ames dwarfs. Importantly, our longevity experiments showed that df/KO mice maintain extended longevity when comparing with N control mice; however, they do not live longer than GHRKO and Ames df/df mice. These important findings indicate that silencing GH signal is important to extend the life span; however, further decrease of body size in mice with already inhibited GH signal does not extend the life span regardless of improved some health-span markers.

Insulin, IGF-1, and GH Receptors Are Altered in an Adipose Tissue Depot-Specific Manner in Male Mice With Modified GH Action

Growth hormone (GH) is a determinant of glucose homeostasis and adipose tissue (AT) function. Using 7-month-old transgenic mice expressing the bovine growth hormone (bGH) gene and growth hormone receptor knockout (GHR-/-) mice, we examined whether changes in GH action affect glucose, insulin, and pyruvate tolerance and AT expression of proteins involved in the interrelated signaling pathways of GH, insulinlike growth factor 1 (IGF-1), and insulin. Furthermore, we searched for AT depot-specific differences in control mice. Glycated hemoglobin levels were reduced in bGH and GHR-/- mice, and bGH mice displayed impaired gluconeogenesis as judged by pyruvate tolerance testing. Serum IGF-1 was elevated by 90% in bGH mice, whereas IGF-1 and insulin were reduced by 97% and 61% in GHR-/- mice, respectively. Igf1 RNA was increased in subcutaneous, epididymal, retroperitoneal, and brown adipose tissue (BAT) depots in bGH mice (mean increase ± standard error of the mean in all five depots, 153% ± 27%) and decreased in all depots in GHR-/- mice (mean decrease, 62% ± 4%). IGF-1 receptor expression was decreased in all AT depots of bGH mice (mean decrease, 49% ± 6%) and increased in all AT depots of GHR-/- mice (mean increase, 94% ± 8%). Insulin receptor expression was reduced in retroperitoneal, mesenteric, and BAT depots in bGH mice (mean decrease in all depots, 56% ± 4%) and augmented in subcutaneous, retroperitoneal, mesenteric, and BAT depots in GHR-/- mice (mean increase: 51% ± 1%). Collectively, our findings indicate a role for GH in influencing hormone signaling in AT in a depot-dependent manner.

Changes in adipose tissue cellular composition during obesity and aging as a cause of metabolic dysregulation

Adipose tissue represents complex endocrine organ containing several different cellular populations including adipocytes, pre-adipocytes, mesenchymal stem cells, macrophages and lymphocytes. It is well establishing that these populations are not static but alter during obesity and aging. Changes in cellular populations alter inflammatory status and other common metabolic complications arise, therefore adipose tissue cellular composition helps dictate its endocrine and regulatory function. During excessive weight gain in obese individuals and as we age there is shift towards increase populations of inflammatory macrophages with a decrease of regulatory T cell. This altered cellular composition promote chronic low grade inflammation negatively affecting mesenchymal stem cell progenitor self-renewal, which result in deterioration of adipogenesis and increased cellular stress in adipocytes. All these changes promote metabolic disorders including age- or obese-related insulin resistance leading to type 2 diabetes.

Mild Calorie Restriction Does Not Affect Testosterone Levels and Testicular Gene Expression in Mutant Mice

The hypothalamic-pituitary-gonadal (HPG) axis and the somatotropic axis are influenced by nutritional factors. Calorie restriction (CR) extends lifespan but suppresses both the HPG and the somatotropic axes. Since most CR studies use a fairly severe (40%–60%) reduction of calorie intake, we hypothesized that a milder CR (20%) might not be deleterious to reproduction in male mice. To test this hypothesis, we evaluated the effects of 20% CR on testicular testosterone content and on testicular expression of genes that are relevant to testicular function and reproductive competence, including insulin-like growth factor-I, cytochrome P450 aromatase (Cyp19a1), androgen receptor, luteinizing hormone receptor, follicle-stimulating hormone receptor, cytochrome P450c17 and 3-β-hydroxysteroid dehydrogenase/isomerase. To relate CR effects to the activity of the somatotropic axis, we have used growth hormone–resistant GHR knockout mice as well as transgenic mice overexpressing GH. Mild CR did not affect testosterone levels in testis homogenates and had little effect on expression of the examined genes in the reproductive organs. Altered activity of the GH/insulin-like growth factor–1 axis had a major impact on the parameters analyzed. The results also suggest that expression of several key genes involved in the control of testicular function is preserved under conditions of mild CR and encourage speculation that mild regimens of CR can produce longevity benefits without impairing reproduction.

Circulating microRNA signature of genotype-by-age interactions in the long-lived Ames dwarf mouse

Recent evidence demonstrates that serum levels of specific miRNAs significantly change with age. The ability of circulating sncRNAs to act as signaling molecules and regulate a broad spectrum of cellular functions implicates them as key players in the aging process. To discover circulating sncRNAs that impact aging in the long‐lived Ames dwarf mice, we conducted deep sequencing of small RNAs extracted from serum of young and old mice. Our analysis showed genotype‐specific changes in the circulating levels of 21 miRNAs during aging [genotype‐by‐age interaction (GbA)]. Genotype‐by‐age miRNAs showed four distinct expression patterns and significant overtargeting of transcripts involved in age‐related processes. Functional enrichment analysis of putative and validated miRNA targets highlighted cellular processes such as tumor suppression, anti‐inflammatory response, and modulation of Wnt, insulin, mTOR, and MAPK signaling pathways, among others. The comparative analysis of circulating GbA miRNAs in Ames mice with circulating miRNAs modulated by calorie restriction (CR) in another long‐lived mouse suggests CR‐like and CR‐independent mechanisms contributing to longevity in the Ames mouse. In conclusion, we showed for the first time a signature of circulating miRNAs modulated by age in the long‐lived Ames mouse.

Genetic Variation and Insulin Resistance in Middle-Aged Chinese Men

We investigated the effect of variants in the first three genes in the insulin signaling pathway and genes identified from genome wide association studies (GWAS) of T2D quantitative traits with IR (fasting insulin and the homeostasis model assessment of IR, HOMA-IR) and evaluated gene-environment interactions with IR traits among 1879 nondiabetic middle-aged men from a population-based study conducted in Shanghai, China. One candidate gene, IGF1, was associated with fasting insulin and HOMA-IR. We observed four BMI-gene interactions (P < 0.05) with HOMA-IR (INRS rs7254060, INRS rs7254358, GLU4 rs2113050, and GLU4 rs7713127) and seven BMI-gene interactions with fasting insulin (INRS rs7254060, INRS rs7254358, INRS rs10417205, INRS rs1799817, GLU4 rs12054720 GLU4 rs2113050, and GLU4 rs7713127). There were four WHR-gene interactions with HOMA-IR (INRS rs10417205, INRS rs12971499, INRS rs7254060, and INRS rs7254358), five WHR-gene interactions with fasting insulin (INRS rs10417205, INRS rs7254060, INRS rs7254358, GLU4 rs2113050, and GLU4 rs7713127), eight physical activity-gene interactions with HOMA-IR (INRS rs10411676, INRS rs11671297, INRS rs2229431, INRS rs12461909, INRS rs6510950, INRS rs10420382, IRS2 rs913949, and IRS2 rs2241745) and five physical activity-gene interactions with fasting insulin (INRS rs2229431, INRS rs12461909, INRS rs10420382, IRS2 rs913949, and IRS2 rs2241745). Our results suggest that BMI, WHR and physical activity may modify IR-associated variants.

The effect of calorie restriction on insulin signaling in skeletal muscle and adipose tissue of Ames dwarf mice

Long-living Ames dwarf (df/df) mice are homozygous for a mutation of the Prop1(df) gene. As a result, mice are deficient in growth hormone (GH), prolactin (PRL) and thyrotropin (TSH). In spite of the hormonal deficiencies, df/df mice live significantly longer and healthier lives compared to their wild type siblings. We studied the effects of calorie restriction (CR) on the expression of insulin signaling genes in skeletal muscle and adipose tissue of normal and df/df mice. The analysis of genes expression showed that CR differentially affects the insulin signaling pathway in these insulin target organs. Moreover, results obtained in both normal and Ames dwarf mice indicate more direct effects of CR on insulin signaling genes in adipose tissue than in skeletal muscle. Interestingly, CR reduced the protein levels of adiponectin in the epididymal adipose tissue of normal and Ames dwarf mice, while elevating adiponectin levels in skeletal muscle and plasma of normal mice only. In conclusion, our findings suggest that both skeletal muscle and adipose tissue are important mediators of insulin effects on longevity. Additionally, the results revealed divergent effects of CR on expression of genes in the insulin signaling pathway of normal and Ames dwarf mice.

Gene expression of key regulators of mitochondrial biogenesis is sex dependent in mice with growth hormone receptor deletion in liver

Mitochondrial biogenesis is an essential process for cell viability. Mice with disruption of the growth hormone receptor (GHR) gene (Ghr gene) in the liver (LiGHRKO), in contrast to long-lived mice with global deletion of the Ghr gene (GHRKO), are characterized by lack of improved insulin sensitivity and severe hepatic steatosis. Tissue-specific disruption of the GHR in liver results in a mouse model with dramatically altered GH/IGF1 axis. We have previously shown increased levels of key regulators of mitochondrial biogenesis in insulin-sensitive GHRKO mice. The aim of the present study is to assess, using real-time PCR, the gene expression of key regulators of mitochondrial biogenesis (Pgc1α, Ampk, Sirt1, Nrf2 and Mfn2) and a marker of mitochondrial activity (CoxIV) in brains, kidneys and livers of male and female LiGHRKO and wild-type (WT) mice. There were significant differences between males and females. In the brain, expression of Pgc1α, Ampk, Sirt1, Nrf2 and Mfn2 was lower in pooled females compared to pooled males. In the kidneys, expression of Ampk and Sirt1 was also lower in female mice. In the liver, no differences between males and females were observed. Sexual dimorphism may play an important role in regulating the biogenesis of mitochondria.

Growth hormone abolishes beneficial effects of calorie restriction in long-lived Ames dwarf mice

Disruption of the growth hormone (GH) axis promotes longevity and delays aging. In contrast, GH over-expression may lead to accelerated aging and shorter life. Calorie restriction (CR) improves insulin sensitivity and may extend lifespan. Long-lived Ames dwarf (df/df) mice have additional extension of longevity when subjected to 30% CR. The aim of the study was to assess effects of CR or GH replacement therapy separately and as a combined (CR+GH) treatment in GH-deficient df/df and normal mice, on selected metabolic parameters (e.g., insulin, glucose, cholesterol), insulin signaling components (e.g., insulin receptor [IR] β-subunit, phosphorylated form of IR [IR pY1158], protein kinase C ζ/λ [p-PKCζ/λ] and mTOR [p-mTOR]), transcription factor p-CREB, and components of the mitogen-activated protein kinase (MAPK) signaling (p-ERK1/2, p-p38), responsible for cell proliferation, differentiation and survival. CR decreased plasma levels of insulin, glucose, cholesterol and leptin, and increased hepatic IR β-subunit and IR pY1158 levels as well as IR, IRS-1 and GLUT-2 gene expression compared to ad libitum feeding, showing a significant beneficial diet intervention effect. Moreover, hepatic protein levels of p-PKCζ/λ, p-mTOR and p-p38 decreased, and p-CREB increased in CR mice. On the contrary, GH increased levels of glucose, cholesterol and leptin in plasma, and p-mTOR or p-p38 in livers, and decreased plasma adiponectin and hepatic IR β-subunit compared to saline treatment. There were no GH effects on adiponectin in N mice. Moreover, GH replacement therapy did not affect IR, IRS-1 and GLUT-2 gene expression. GH treatment abolishes the beneficial effects of CR; it may suggest an important role of GH-IGF1 axis in mediating the CR action. Suppressed somatotrophic signaling seems to predominate over GH replacement therapy in the context of the examined parameters and signaling pathways.

Expression of apoptosis-related genes in liver-specific growth hormone receptor gene-disrupted mice is sex dependent

Apoptosis is a process that affects life span and health. Mice with liver-specific disruption of the growth hormone receptor (GHR) gene (ie, Ghr gene) liver-specific growth hormone receptor knockout [LiGHRKO] mice), as opposed to mice with global deletion of the Ghr gene (GHRKO; Ghr-/-), are characterized by severe hepatic steatosis and lack of improved insulin sensitivity. We have previously shown that levels of proapoptotic factors are decreased in long-lived and insulin-sensitive GHRKO mice. In the current study, expression of specific apoptosis-related genes was assessed in brains, kidneys, and livers of male and female LiGHRKO and wild-type mice using real-time PCR. In the brain, expression of Caspase 3, Caspase 9, Smac/DIABLO, and p53 was decreased in females compared with males. Renal expression of Caspase 3 and Noxa also decreased in female mice. In the liver, no differences were seen between males and females. Also, no significant genotype effects were detected in the examined organs. Lack of significant genotype effect in kidneys contrasts with previous observations in GHRKO mice. Apparently, global GHR deletion induces beneficial changes in apoptotic factors, whereas liver-specific GHR disruption does not. Furthermore, sexual dimorphism may play an important role in regulating apoptosis during liver-specific suppression of the somatotrophic signaling.

Age-related and depot-specific changes in white adipose tissue of growth hormone receptor-null mice

Growth hormone receptor-null (GHR(-/-)) mice are dwarf, insulin sensitive, and long-lived in spite of increased adiposity. However, their adiposity is not uniform, with select white adipose tissue (WAT) depots enlarged. To study WAT depot-specific effects on insulin sensitivity and life span, we analyzed individual WAT depots of 12- and 24-month-old GHR(-) (/-) and wild-type (WT) mice, as well as their plasma levels of selected hormones. Adipocyte sizes and plasma insulin, leptin, and adiponectin levels decreased with age in both GHR(-) (/-) and WT mice. Two-dimensional gel electrophoresis proteomes of WAT depots were similar among groups, but several proteins involved in endocytosis and/or cytoskeletal organization (Ehd2, S100A10, actin), anticoagulation (S100A10, annexin A5), and age-related conditions (alpha2-macroglobulin, apolipoprotein A-I, transthyretin) showed significant differences between genotypes. Because Ehd2 may regulate endocytosis of Glut4, we measured Glut4 levels in the WAT depots of GHR(-) (/-) and WT mice. Inguinal WAT of 12-month-old GHR(-) (/-) mice displayed lower levels of Glut4 than WT. Overall, the protein changes detected in this study offer new insights into possible mechanisms contributing to enhanced insulin sensitivity and extended life span in GHR(-) (/-) mice.

Mouse models of growth hormone action and aging: a proteomic perspective

Growth hormone (GH) is a protein secreted by the anterior pituitary and circulates throughout the body to exert important actions on growth and metabolism. GH stimulates the secretion of insulin-like growth factor-I (IGF-I) that mediates some of the growth promoting actions of GH. The GH/IGF-I axis has recently been recognized as important in terms of longevity in organisms ranging from Caenorhabditis elegans to mice. For example, GH transgenic mice possess short lifespans while GH receptor null (GHR-/-) mice have extended longevity. Thus, the actions of GH (or IGF-I) or lack thereof impact the aging process. In this review, we summarize the proteomic analyses of plasma and white adipose tissue in these two mouse models of GH action, i.e. GH transgenic and GHR-/- mice. At the protein level, we wanted to establish novel plasma biomarkers of GH action as a function of age and to determine differences in adipose tissue depots. We have shown that these proteomic approaches have not only confirmed several known physiological actions of GH, but also resulted in novel protein biomarkers and targets that may be indicative of the aging process and/or new functions of GH. These results may generate new directions for GH and/or aging research.

Ghrelin improves body weight loss and skeletal muscle catabolism associated with angiotensin II-induced cachexia in mice

Ghrelin is a gastric peptide that regulates energy homeostasis. Angiotensin II (Ang II) is known to induce body weight loss and skeletal muscle catabolism through the ubiquitin-proteasome pathway. In this study, we investigated the effects of ghrelin on body weight and muscle catabolism in mice treated with Ang II. The continuous subcutaneous administration of Ang II to mice for 6 days resulted in cardiac hypertrophy and significant decreases in body weight gain, food intake, food efficiency, lean mass, and fat mass. In the gastrocnemius muscles of Ang II-treated mice, the levels of insulin-like growth factor 1 (IGF-1) were decreased, and the levels of mRNA expression of catabolic factors were increased. Although the repeated subcutaneous injections of ghrelin (1.0mg/kg, twice daily for 5 days) did not affect cardiac hypertrophy, they resulted in significant body weight gains and improved food efficiencies and tended to increase both lean and fat mass in Ang II-treated mice. Ghrelin also ameliorated the decreased IGF-1 levels and the increased mRNA expression levels of catabolic factors in the skeletal muscle. IGF-1 mRNA levels in the skeletal muscle significantly decreased 24h after Ang II infusion, and this was reversed by two subcutaneous injections of ghrelin. In C2C12-derived myocytes, the dexamethasone-induced mRNA expression of atrogin-1 was decreased by IGF-1 but not by ghrelin. In conclusion, we demonstrated that ghrelin improved body weight loss and skeletal muscle catabolism in mice treated with Ang II, possibly through the early restoration of IGF-1 mRNA in the skeletal muscle and the amelioration of nutritional status.

Insulin concentration modulates hepatic lipid accumulation in mice in part via transcriptional regulation of fatty acid transport proteins

Background

Fatty liver disease (FLD) is commonly associated with insulin resistance and obesity, but interestingly it is also observed at low insulin states, such as prolonged fasting. Thus, we asked whether insulin is an independent modulator of hepatic lipid accumulation.

Methods/Principal Findings

In mice we induced, hypo- and hyperinsulinemia associated FLD by diet induced obesity and streptozotocin treatment, respectively. The mechanism of free fatty acid induced steatosis was studied in cell culture with mouse liver cells under different insulin concentrations, pharmacological phosphoinositol-3-kinase (PI3K) inhibition and siRNA targeted gene knock-down. We found with in vivo and in vitro models that lipid storage is increased, as expected, in both hypo- and hyperinsulinemic states, and that it is mediated by signaling through either insulin receptor substrate (IRS) 1 or 2. As previously reported, IRS-1 was up-regulated at high insulin concentrations, while IRS-2 was increased at low levels of insulin concentration. Relative increase in either of these insulin substrates, was associated with an increase in liver-specific fatty acid transport proteins (FATP) 2&5, and increased lipid storage. Furthermore, utilizing pharmacological PI3K inhibition we found that the IRS-PI3K pathway was necessary for lipogenesis, while FATP responses were mediated via IRS signaling. Data from additional siRNA experiments showed that knock-down of IRSs impacted FATP levels.

Conclusions/Significance

States of perturbed insulin signaling (low-insulin or high-insulin) both lead to increased hepatic lipid storage via FATP and IRS signaling. These novel findings offer a common mechanism of FLD pathogenesis in states of both inadequate (prolonged fasting) and ineffective (obesity) insulin signaling.

Inflammation and mortality in a frail mouse model

Mice homozygous for targeted deletion of the interleukin 10 gene (Il-10) have been partially characterized as a model for human frailty. These mice have increased serum interleukin (IL)-6 in midlife, skeletal muscle weakness, and an altered skeletal muscle gene expression profile compared to age and sex-matched C57BL/6 (B6) control mice. In order to further characterize for use as a frailty model, we evaluated the evolution of inflammatory pathway activation, endocrine change, and mortality in these mice. Serum was collected in groups of age- and sex-matched B6.129P2-Il10(tm1Cgn)/J (IL-10(tm/tm)) mice and B6 control mice at age 12, 24, 48, 72, and 90 weeks. Cytokines including IL-6, interleukin 1 beta (IL-1β), tumor necrosis factor-alpha (TNF-α), interferon-gamma (IFN-γ), chemokine (C-X-C motif) ligand 1 (KC), IL-12, and IL-10 were measured using electro-chemiluminescent multiplex immunoassay and insulin-like growth factor 1 (IGF-1) was measured using solid-phase enzyme-linked immunosorbent assay. A separate longitudinal cohort was monitored from age 35 weeks to approximately 100 weeks. Survival was evaluated by Kaplan-Meier survival estimates and detailed necropsy information was gathered in a subset of mice that died or were sacrificed. In IL-10(tm/tm) mice compared to B6 controls, serum IL-6, IL-1β, TNF-α, IFN-γ, KC levels were significantly elevated across the age groups, serum mean IGF-1 levels were higher in the 48-week-old groups, and overall mortality rate was significantly higher. The quadratic relationship between IGF-1 and age was significantly different between the two strains of mice. Serum IL-6 was positively associated with IGF-1 but the effect was significantly larger in IL-10(tm/tm) mice. These findings provide additional rationale for the use of the IL-10(tm/tm) mouse as a model for frailty and for low-grade inflammatory pathway activation.

Insulin in central nervous system: more than just a peripheral hormone

Insulin signaling in central nervous system (CNS) has emerged as a novel field of research since decreased brain insulin levels and/or signaling were associated to impaired learning, memory, and age-related neurodegenerative diseases. Thus, besides its well-known role in longevity, insulin may constitute a promising therapy against diabetes- and age-related neurodegenerative disorders. More interestingly, insulin has been also faced as the potential missing link between diabetes and aging in CNS, with Alzheimer's disease (AD) considered as the "brain-type diabetes." In fact, brain insulin has been shown to regulate both peripheral and central glucose metabolism, neurotransmission, learning, and memory and to be neuroprotective. And a future challenge will be to unravel the complex interactions between aging and diabetes, which, we believe, will allow the development of efficient preventive and therapeutic strategies to overcome age-related diseases and to prolong human "healthy" longevity. Herewith, we aim to integrate the metabolic, neuromodulatory, and neuroprotective roles of insulin in two age-related pathologies: diabetes and AD, both in terms of intracellular signaling and potential therapeutic approach.

Metabolic effects of intra-abdominal fat in GHRKO mice

Mice with targeted deletion of the growth hormone receptor (GHRKO mice) are growth hormone (GH) resistant, small, obese, hypoinsulinemic, highly insulin sensitive and remarkably long-lived. To elucidate the unexpected coexistence of adiposity with improved insulin sensitivity and extended longevity, we examined effects of surgical removal of visceral (epididymal and perinephric) fat on metabolic traits related to insulin signaling and longevity. Comparison of results obtained in GHRKO mice and in normal animals from the same strain revealed disparate effects of visceral fat removal (VFR) on insulin and glucose tolerance, adiponectin levels, accumulation of ectopic fat, phosphorylation of insulin signaling intermediates, body temperature, and respiratory quotient (RQ). Overall, VFR produced the expected improvements in insulin sensitivity and reduced body temperature and RQ in normal mice and had opposite effects in GHRKO mice. Some of the examined parameters were altered by VFR in opposite directions in GHRKO and normal mice, and others were affected in only one genotype or exhibited significant genotype × treatment interactions. Functional differences between visceral fat of GHRKO and normal mice were confirmed by measurements of adipokine secretion, lipolysis, and expression of genes related to fat metabolism. We conclude that in the absence of GH signaling, the secretory activity of visceral fat is profoundly altered and unexpectedly promotes enhanced insulin sensitivity. The apparent beneficial effects of visceral fat in GHRKO mice may also explain why reducing adiposity by calorie restriction fails to improve insulin signaling or further extend longevity in these animals.

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

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

Decreased expression level of apoptosis-related genes and/or proteins in skeletal muscles, but not in hearts, of growth hormone receptor knockout mice

The long-lived growth hormone (GH) receptor knockout (GHRKO; KO) mice are GH-resistant due to targeted disruption of the GH receptor (Ghr) gene. Apoptosis is a physiological process in which cells play an active role in their own death and is a normal component of the development and health of multicellular organisms. Aging is associated with the progressive loss of strength of skeletal and heart muscles. Calorie restriction (CR) is a well-known experimental model to delay aging and increase lifespan. The aim of the study was to examine the expression of the following apoptosis-related genes: caspase-3, caspase-9, caspase-8, bax, bcl-2, Smac/DIABLO, p53 and cytochrome c1 (cyc1) in the skeletal muscles and hearts of female normal and GHRKO mice, fed ad libitum or subjected to 40% CR for six months, starting at two months of age. Moreover, skeletal muscle caspase-3, caspase-9, caspase-8, bax, bcl-2, Smac/DIABLO, Apaf-1, bad, phospho-bad (pbad), phospho-p53 and cytochrome c (cyc) protein expression levels were assessed. Expression of caspase-3, caspase-9, bax and Smac/DIABLO genes and proteins was decreased in GHRKO's skeletal muscles. The Apaf-1 protein expression also was diminished in this tissue. In contrast, bcl-2 and pbad protein levels were increased in skeletal muscles in knockouts. No changes were demonstrated for the examined genes' expression in GHRKO's hearts except for the increased level of cyc1 mRNA. CR did not alter the expression of the examined genes and proteins in skeletal muscles of knockouts versus normal (N) mice. In heart homogenates, CR increased caspase-3 mRNA level as compared with ad libitum mice. Decreased expression of certain proapoptotic genes and/or proteins may constitute the potential mechanism of prolonged longevity in GHRKO mice, protecting these animals from aging; this potential beneficial mechanism is not affected by CR.

Epigenetic oxidative redox shift (EORS) theory of aging unifies the free radical and insulin signaling theories

Harman's free radical theory of aging posits that oxidized macromolecules accumulate with age to decrease function and shorten life-span. However, nutritional and genetic interventions to boost anti-oxidants have generally failed to increase life-span. Furthermore, the free radical theory fails to explain why exercise causes higher levels of oxyradical damage, but generally promotes healthy aging. The separate anti-aging paradigms of genetic or caloric reductions in the insulin signaling pathway is thought to slow the rate of living to reduce metabolism, but recent evidence from Westbrook and Bartke suggests metabolism actually increases in long-lived mice. To unify these disparate theories and data, here, we propose the epigenetic oxidative redox shift (EORS) theory of aging. According to EORS, sedentary behavior associated with age triggers an oxidized redox shift and impaired mitochondrial function. In order to maintain resting energy levels, aerobic glycolysis is upregulated by redox-sensitive transcription factors. As emphasized by DeGrey, the need to supply NAD(+) for glucose oxidation and maintain redox balance with impaired mitochondrial NADH oxidoreductase requires the upregulation of other oxidoreductases. In contrast to the 2% inefficiency of mitochondrial reduction of oxygen to the oxyradical, these other oxidoreductases enable glycolytic energy production with a deleterious 100% efficiency in generating oxyradicals. To avoid this catastrophic cycle, lactate dehydrogenase is upregulated at the expense of lactic acid acidosis. This metabolic shift is epigenetically enforced, as is insulin resistance to reduce mitochondrial turnover. The low mitochondrial capacity for efficient production of energy reinforces a downward spiral of more sedentary behavior leading to accelerated aging, increased organ failure with stress, impaired immune and vascular functions and brain aging. Several steps in the pathway are amenable to reversal for exit from the vicious cycle of EORS. Examples from our work in the aging rodent brain as well as other aging models are provided.

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

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

Is altered expression of hepatic insulin-related genes in growth hormone receptor knockout mice due to GH resistance or a difference in biological life spans?

Growth hormone receptor knockout (GHRKO) mice live about 40%-55% longer than their normal (N) littermates. Previous studies of 21-month-old GHRKO and N mice showed major alterations of the hepatic expression of genes involved in insulin signaling. Differences detected at this age may have been caused by the knockout of the growth hormone receptor (GHR) or by differences in biological age between GHRKO and N mice. To address this question, we compared GHRKO and N mice at ages corresponding to the same percentage of median life span to see if the differences of gene expression persisted. Comparison of GHRKO and N mice at approximately 50% of biological life span showed significant differences in hepatic expression of all 14 analyzed genes. We conclude that these changes are due to disruption of GHR gene and the consequent suppression of growth hormone signaling rather than to differences in "biological age" between mutant and normal animals sampled at the same chronological age.

Persistent transcription-blocking DNA lesions trigger somatic growth attenuation associated with longevity

The accumulation of stochastic DNA damage throughout an organism's lifespan is thought to contribute to ageing. Conversely, ageing seems to be phenotypically reproducible and regulated through genetic pathways such as the insulin-like growth factor-1 (IGF-1) and growth hormone (GH) receptors, which are central mediators of the somatic growth axis. Here we report that persistent DNA damage in primary cells from mice elicits changes in global gene expression similar to those occurring in various organs of naturally aged animals. We show that, as in ageing animals, the expression of IGF-1 receptor and GH receptor is attenuated, resulting in cellular resistance to IGF-1. This cell-autonomous attenuation is specifically induced by persistent lesions leading to stalling of RNA polymerase II in proliferating, quiescent and terminally differentiated cells; it is exacerbated and prolonged in cells from progeroid mice and confers resistance to oxidative stress. Our findings suggest that the accumulation of DNA damage in transcribed genes in most if not all tissues contributes to the ageing-associated shift from growth to somatic maintenance that triggers stress resistance and is thought to promote longevity.

Maternal protein restriction affects postnatal growth and the expression of key proteins involved in lifespan regulation in mice

We previously reported that maternal protein restriction in rodents influenced the rate of growth in early life and ultimately affected longevity. Low birth weight caused by maternal protein restriction followed by catch-up growth (recuperated animals) was associated with shortened lifespan whereas protein restriction and slow growth during lactation (postnatal low protein: PLP animals) increased lifespan. We aim to explore the mechanistic basis by which these differences arise. Here we investigated effects of maternal diet on organ growth, metabolic parameters and the expression of insulin/IGF1 signalling proteins and Sirt1 in muscle of male mice at weaning. PLP mice which experienced protein restriction during lactation had lower fasting glucose (P = 0.038) and insulin levels (P = 0.046) suggesting improved insulin sensitivity. PLP mice had higher relative weights (adjusted by body weight) of brain (P = 0.0002) and thymus (P = 0.031) compared to controls suggesting that enhanced functional capacity of these two tissues is beneficial to longevity. They also had increased expression of insulin receptor substrate 1 (P = 0.021) and protein kinase C zeta (P = 0.046). Recuperated animals expressed decreased levels of many insulin signalling proteins including PI3 kinase subunits p85alpha (P = 0.018), p110beta (P = 0.048) and protein kinase C zeta (P = 0.006) which may predispose these animals to insulin resistance. Sirt1 protein expression was reduced in recuperated offspring. These observations suggest that maternal protein restriction can affect major metabolic pathways implicated in regulation of lifespan at a young age which may explain the impact of maternal diet on longevity.

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.

Metabolic consequences of pregnancy-associated plasma protein-A deficiency in mice: exploring possible relationship to the longevity phenotype

Mice born with the deletion of the gene for pregnancy-associated plasma protein-A (PAPP-A), a model of reduced local IGF activity, live approximately 30% longer than their wild-type (WT) littermates. In this study, we investigated metabolic consequences of PAPP-A gene deletion and possible relationship to lifespan extension. Specifically, we determined whether 18-month-old PAPP-A knockout (KO) mice when compared with their WT littermates have reduced energy expenditure and/or altered glucose-insulin sensitivity. Food intake, and total energy expenditure and resting energy expenditure as measured by calorimetry were not different between PAPP-A KO and WT mice when subjected to the analysis of covariance with body weight as the covariate. However, there was an increase in spontaneous physical activity in PAPP-A KO mice. Both WT and PAPP-A KO mice exhibited mild insulin resistance with age, as assessed by fasting glucose/insulin ratios. Oral glucose tolerance and insulin sensitivity were not significantly different between the two groups of mice, although there appeared to be a decrease in the average size of the pancreatic islets in PAPP-A KO mice. Thus, neither reduced 'rate of living' nor altered glucose-insulin homeostasis can be considered key determinants of the enhanced longevity of PAPP-A KO mice. These findings are discussed in the context of those from other long-lived mouse models.

Age-related alterations in pituitary and testicular functions in long-lived growth hormone receptor gene-disrupted mice

The somatotropic axis, GH, and IGF-I interact with the hypothalamic-pituitary-gonadal axis in health and disease. GH-resistant GH receptor-disrupted knockout (GHRKO) male mice are fertile but exhibit delayed puberty and decreases in plasma FSH levels, testicular content of LH, and prolactin (PRL) receptors, whereas PRL levels are elevated. Because the lifespan of GHRKO mice is much greater than the lifespan of their normal siblings, it was of interest to compare age-related changes in the hypothalamic-pituitary-gonadal axis in GHRKO and normal animals. Plasma IGF-I, insulin, PRL, LH, FSH, androstenedione and testosterone levels, and acute responses to GnRH and LH were measured in young (2-4 and 5-6 months of age) and old (18-19 and 23-26 months of age) male GHRKO mice and their normal siblings. Plasma IGF-I was not detectable in GHRKO mice. Plasma PRL levels increased with age in normal mice but declined in GHRKO males, and did not differ in old GHRKO and normal animals. Plasma LH responses to acute GnRH stimulation were attenuated in GHRKO mice but increased with age only in normal mice. Plasma FSH levels were decreased in GHRKO mice regardless of age. Plasma testosterone responses to LH stimulation were attenuated in old mice regardless of genotype, whereas plasma androstenedione responses were reduced with age only in GHRKO mice. Testicular IGF-I mRNA levels were normal in young and increased in old GHRKO mice, whereas testicular concentrations and total IGF-I levels were decreased in these animals. These findings indicate that GH resistance due to targeted disruption of the GH receptor gene in mice leads to suppression of testicular IGF-I levels, and modifies the effects of aging on plasma PRL levels and responses of the pituitary and testes to GnRH and LH stimulation. Plasma testosterone levels declined during aging in normal but not in GHRKO mice, and the age-related increase in the LH responses to exogenous GnRH was absent in GHRKO mice, perhaps reflecting a delay of aging in these remarkably long-lived animals.

Effects of caloric restriction and growth hormone resistance on insulin-related intermediates in the skeletal muscle

Growth hormone receptor-deficient (GHRKO) mice are long-lived and have reduced insulin-like growth factor (IGF)-1 and insulin levels and enhanced insulin sensitivity thus resembling the phenotype of animals subjected to calorie restriction (CR). In contrast to its effects in normal mice, CR does not improve insulin sensitivity or increase longevity in GHRKO males. In an attempt to identify mechanisms underlying this differential response to CR, effects of CR on the expression of insulin-related genes were compared in GHRKO and normal mice. In addition to changes detected in both genotypes, and responses unique to GHRKO mice, the levels of Akt2 and peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC1alpha) were increased and levels of phosphorylated c-Jun N-terminal kinase (JNK)1 were reduced in response to CR only in normal mice. These changes may be related to mechanisms of improving insulin sensitivity and life expectancy.

Intrahepatic insulin resistance in a murine model of steatohepatitis: effect of PPARgamma agonist pioglitazone

Hepatic insulin resistance is associated with hepatic steatosis and is thought to play an important role in the pathogenesis of steatohepatitis. Using a murine model of steatohepatitis (mice fed a diet deficient in methionine and choline-MCD diet), we tested the effects of the insulin-sensitising, PPARgamma agonist drug pioglitazone (PGZ) on systemic and intrahepatic insulin sensitivity and on liver pathology. Compared to controls, mice fed the MCD diet develop a significant steatohepatitis, have enhanced glucose tolerance and enhanced systemic response to insulin. PGZ did not affect the systemic insulin sensitivity in control or MCD-fed mice as assessed in vivo by intraperitoneal glucose or insulin dynamic tests. However, PGZ prevented hepatic fat accumulation and steatohepatitis induced by the MCD diet. This effect was associated with an increased mass of adipose tissue and increased expression and release of adiponectin, while hepatic acyl co-enzyme A oxidase and acyl-co-enzyme A carboxylase, regulating hepatic beta-oxidation of fatty acid, remained unchanged. Steatohepatitis in MCD-diet-fed mice was associated with intrahepatic insulin resistance as shown by a reduced phosphorylation of hepatic insulin receptor, and Akt in response to an insulin stimulus. PGZ to MCD-fed mice restored the activation of the insulin receptor and of the Akt pathway in response to insulin. In conclusion, PGZ alleviates steatosis and steatohepatitis induced by the MCD diet, an effect associated with correction of intrahepatic insulin resistance.

Interaction of growth hormone and calorie restriction

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

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

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

Targeted disruption of growth hormone receptor interferes with the beneficial actions of calorie restriction

Reduced intake of nutrients [calorie restriction (CR)] extends longevity in organisms ranging from yeast to mammals. Mutations affecting somatotropic, insulin, or homologous signaling pathways can increase life span in worms, flies, and mice, and there is considerable evidence that reduced secretion of insulin-like growth factor I and insulin are among the mechanisms that mediate the effects of CR on aging and longevity in mammals. In the present study, mice with targeted disruption of the growth hormone (GH) receptor [GH receptor/GH-binding protein knockout (GHRKO) mice] and their normal siblings were fed ad libitum (AL) or subjected to 30% CR starting at 2 months of age. In normal females and males, CR produced the expected increases in overall, average, median, and maximal life span. Longevity of normal mice subjected to CR resembles that of GHRKO animals fed AL. In sharp contrast to its effects in normal mice, CR failed to increase overall, median, or average life span in GHRKO mice and increased maximal life span only in females. In a separate group of animals, CR for 1 year improved insulin sensitivity in normal mice but failed to further enhance the remarkable insulin sensitivity in GHRKO mutants. These data imply that somatotropic signaling is critically important not only in the control of aging and longevity under conditions of unlimited food supply but also in mediating the effects of CR on life span. The present findings also support the notion that enhanced sensitivity to insulin plays a prominent role in the actions of CR and GH resistance on longevity.

Calorie restriction mimetics: an emerging research field

When considering all possible aging interventions evaluated to date, it is clear that calorie restriction (CR) remains the most robust. Studies in numerous species have demonstrated that reduction of calories 30-50% below ad libitum levels of a nutritious diet can increase lifespan, reduce the incidence and delay the onset of age-related diseases, improve stress resistance, and decelerate functional decline. A current major focus of this research area is whether this nutritional intervention is relevant to human aging. Evidence emerging from studies in rhesus monkeys suggests that their response to CR parallels that observed in rodents. To assess CR effects in humans, clinical trials have been initiated. However, even if results from these studies could eventually substantiate CR as an effective pro-longevity strategy for humans, the utility of this intervention would be hampered because of the degree and length of restriction required. As an alternative strategy, new research has focused on the development of 'CR mimetics'. The objective of this strategy is to identify compounds that mimic CR effects by targeting metabolic and stress response pathways affected by CR, but without actually restricting caloric intake. For example, drugs that inhibit glycolysis (2-deoxyglucose), enhance insulin action (metformin), or affect stress signaling pathways (resveratrol), are being assessed as CR mimetics (CRM). Promising results have emerged from initial studies regarding physiological responses which resemble those observed in CR (e.g. reduced body temperature and plasma insulin) as well as protection against neurotoxicity (e.g. enhanced dopamine action and up-regulated neurotrophic factors). Ultimately, lifespan analyses in addition to expanded toxicity studies must be accomplished to fully assess the potential of any CRM. Nonetheless, this strategy clearly offers a very promising and expanding research endeavor.

Caloric restriction and growth hormone receptor knockout: effects on expression of genes involved in insulin action in the heart

Blockade of growth hormone (GH), decreased insulin-like growth factor-1 (IGF1) action and increased insulin sensitivity are associated with life extension and an apparent slowing of the aging process. We examined expression of genes involved in insulin action, IR, IRS1, IRS2, IGF1, IGF1R, GLUT4, PPARs and RXRs in the hearts of normal and GHR-/- (KO) mice fed ad libitum or subjected to 30% caloric restriction (CR). CR increased the cardiac expression of IR, IRS1, IGF1, IGF1R and GLUT4 in normal mice and IRS1, GLUT4, PPARalpha and PPARbeta/delta in GHR-KO animals. Expression of IR, IRS1, IRS2, IGF1, GLUT4, PPARgamma and PPARalpha did not differ between GHR-KO and normal mice. These unexpected results suggest that CR may lead to major modifications of insulin action in the heart, but high insulin sensitivity of GHR-KO mice is not associated with alterations in the levels of most of the examined molecules related to intracellular insulin signaling.