Direct and indirect effects of growth hormone receptor ablation on liver expression of xenobiotic metabolizing genes

Proteomic changes induced by longevity-promoting interventions in mice

Using mouse models and high-throughput proteomics, we conducted an in-depth analysis of the proteome changes induced in response to seven interventions known to increase mouse lifespan. This included two genetic mutations, a growth hormone receptor knockout (GHRKO mice) and a mutation in the Pit-1 locus (Snell dwarf mice), four drug treatments (rapamycin, acarbose, canagliflozin, and 17α-estradiol), and caloric restriction. Each of the interventions studied induced variable changes in the concentrations of proteins across liver, kidney, and gastrocnemius muscle tissue samples, with the strongest responses in the liver and limited concordance in protein responses across tissues. To the extent that these interventions promote longevity through common biological mechanisms, we anticipated that proteins associated with longevity could be identified by characterizing shared responses across all or multiple interventions. Many of the proteome alterations induced by each intervention were distinct, potentially implicating a variety of biological pathways as being related to lifespan extension. While we found no protein that was affected similarly by every intervention, we identified a set of proteins that responded to multiple interventions. These proteins were functionally diverse but tended to be involved in peroxisomal oxidation and metabolism of fatty acids. These results provide candidate proteins and biological mechanisms related to enhancing longevity that can inform research on therapeutic approaches to promote healthy aging.

Transgenerational effects of extracts containing Microcystin-LR exposure on reproductive toxicity and offspring growth inhibition in a model organism zebrafish

Cyanobacteria cell lysates release numerous toxic substances (e.g., cyanotoxins) into the water, posing a serious threat to human health and aquatic ecosystems. Microcystins (MCs) are among the most abundant cyanotoxins in the cell lysates, with microcystin-LR (MC-LR) being one of the most common and highly toxic congeners. In this study, zebrafish (Danio rerio) were exposed to different levels MC-LR that from extracts of Microcystis aeruginosa. Changes in the MC-LR accumulations, organ coefficients, and antioxidant enzyme activities in the zebrafish were analyzed. Transgenerational reproductive toxicity of MC-LR in the maternal and paternal generations was further investigated, as well as the influences of extracts containing MC-LR exposures of the F1 on the growth of zebrafish. The study found that high levels of MC-LR could be detected in the major organs of adult zebrafish, particularly in spleen. Notably, concentration of MC-LR in the spermary was significantly higher than that in the ovarium. MC-LR could induce oxidative damage by affecting the activities of catalase and superoxide dismutase. Inherited from F0, MC-LR led to impaired development in the F1 generation. Difference in offspring survival rates could be observed in the groups with different MC-LR levels of maternal and paternal exposures. This study reveals transgenerational effects of MC-LR on the reproductive toxicity and offspring growth inhibition to the aquatic organisms, which should be emphasized in the future ecological risk assessment.

GHR disruption in mature adult mice alters xenobiotic metabolism gene expression in the liver

BACKGROUND

Lifelong reduction of growth hormone (GH) action extends lifespan and improves healthspan in mice. Moreover, congenital inactivating mutations of GH receptor (GHR) in mice and humans impart resistance to age-associated cancer, diabetes, and cognitive decline. To investigate the consequences of GHR disruption at an adult age, we recently ablated the GHR at 6-months of age in mature adult (6mGHRKO) mice. We found that both, male and female 6mGHRKO mice have reduced oxidative damage, with males 6mGHRKO showing improved insulin sensitivity and cancer resistance. Importantly, 6mGHRKO females have an extended lifespan compared to controls.

OBJECTIVE AND METHODS

To investigate the possible mechanisms leading to health improvements, we performed RNA sequencing using livers from male and female 6mGHRKO mice and controls.

RESULTS

We found that disrupting GH action at an adult age reduced the gap in liver gene expression between males and females, making gene expression between sexes more similar. However, there was still a 6-fold increase in the number of differentially expressed genes when comparing male 6mGHRKO mice vs controls than in 6mGHRKO female vs controls, suggesting that GHR ablation affects liver gene expression more in males than in females. Finally, we found that lipid metabolism and xenobiotic metabolism pathways are activated in the liver of 6mGHRKO mice.

CONCLUSION

The present study shows for the first time the specific hepatic gene expression profile, cellular pathways, biological processes and molecular mechanisms that are driven by ablating GH action at a mature adult age in males and females. Importantly, these results and future studies on xenobiotic metabolism may help explain the lifespan extension seen in 6mGHRKO mice.

Lysosomal targetomics of ghr KO mice shows chaperone-mediated autophagy degrades nucleocytosolic acetyl-coA enzymes

Mice deficient in GHR (growth hormone receptor; ghr KO) have a dramatic lifespan extension and elevated levels of hepatic chaperone-mediated autophagy (CMA). Using quantitative proteomics to identify protein changes in purified liver lysosomes and whole liver lysates, we provide evidence that elevated CMA in ghr KO mice downregulates proteins involved in ribosomal structure, translation initiation and elongation, and nucleocytosolic acetyl-coA production. Following up on these initial proteomics findings, we used a cell culture approach to show that CMA is necessary and sufficient to regulate the abundance of ACLY and ACSS2, the two enzymes that produce nucleocytosolic (but not mitochondrial) acetyl-coA. Inhibition of CMA in NIH3T3 cells has been shown to lead to aberrant accumulation of lipid droplets. We show that this lipid droplet phenotype is rescued by knocking down ACLY or ACSS2, suggesting that CMA regulates lipid droplet formation by controlling ACLY and ACSS2. This evidence leads to a model of how constitutive activation of CMA can shape specific metabolic pathways in long-lived endocrine mutant mice.Abbreviations: CMA: chaperone-mediated autophagy; DIA: data-independent acquisition; ghr KO: growth hormone receptor knockout; GO: gene ontology; I-WAT: inguinal white adipose tissue; KFERQ: a consensus sequence resembling Lys-Phe-Glu-Arg-Gln; LAMP2A: lysosomal-associated membrane protein 2A; LC3-I: non-lipidated MAP1LC3; LC3-II: lipidated MAP1LC3; PBS: phosphate-buffered saline; PI3K: phosphoinositide 3-kinase.

Mice with gene alterations in the GH and IGF family

Much of our understanding of GH's action stems from animal models and the generation and characterization of genetically altered or modified mice. Manipulation of genes in the GH/IGF1 family in animals started in 1982 when the first GH transgenic mice were produced. Since then, multiple laboratories have altered mouse DNA to globally disrupt Gh, Ghr, and other genes upstream or downstream of GH or its receptor. The ability to stay current with the various genetically manipulated mouse lines within the realm of GH/IGF1 research has been daunting. As such, this review attempts to consolidate and summarize the literature related to the initial characterization of many of the known gene-manipulated mice relating to the actions of GH, PRL and IGF1. We have organized the mouse lines by modifications made to constituents of the GH/IGF1 family either upstream or downstream of GHR or to the GHR itself. Available data on the effect of altered gene expression on growth, GH/IGF1 levels, body composition, reproduction, diabetes, metabolism, cancer, and aging are summarized. For the ease of finding this information, key words are highlighted in bold throughout the main text for each mouse line and this information is summarized in Tables 1, 2, 3 and 4. Most importantly, the collective data derived from and reported for these mice have enhanced our understanding of GH action.

Elevated metallothionein expression in long-lived species mediates the influence of cadmium accumulation on aging

Cadmium (Cd) accumulates with aging and is elevated in long-lived species. Metallothioneins (MTs), small cysteine-rich proteins involved in metal homeostasis and Cd detoxification, are known to be related to longevity. However, the relationship between Cd accumulation, the role of MTs, and aging is currently unclear. Specifically, we do not know if long-lived species evolved an efficient metal stress response by upregulating their MT levels to reduce the toxic effects of environmental pollutants, such as Cd, that accumulate over their longer life span. It is also unknown if the number of MT genes, their expression, or both protect the organisms from potentially damaging effects during aging. To address these questions, we reanalyzed several cross-species studies and obtained data on MT expression and Cd accumulation in long-lived mouse models. We confirmed a relationship between species maximum life span in captive mammals and their Cd content in liver and kidney. We found that although the number of MT genes does not affect longevity, gene expression and protein amount of specific MT paralogs are strongly related to life span in mammals. MT expression rather than gene number may influence the high Cd levels and longevity of some species. In support of this, we found that overexpression of MT-1 accelerated Cd accumulation in mice and that tissue Cd was higher in long-lived mouse strains with high MT expression. We conclude that long-lived species have evolved a more efficient stress response by upregulating the expression of MT genes in presence of Cd, which contributes to elevated tissue Cd levels.

Why do sexes differ in lifespan extension? Sex-specific pathways of aging and underlying mechanisms for dimorphic responses

Males and females typically have different lifespans and frequently differ in their responses to anti-aging interventions. These sex-specific responses are documented in mice and Drosophila species, in addition to other organisms where interventions have been tested. While the prevalence of sex-specific responses to anti-aging interventions is now recognised, the underlying causes remain poorly understood. This review first summarises the main pathways and interventions that lead to sex-specific lifespan responses, including the growth-hormone/insulin-like growth factor 1 (GH-IGF1) axis, mechanistic target of rapamycin (mTOR) signalling, and nutritional and pharmacological interventions. After summarising current evidence, several different potential causes for sex-specific responses are discussed. These include sex-differences in xenobiotic metabolism, differing disease susceptibility, sex-specific hormone production and chromosomes, and the relative importance of different signalling pathways in the control of male and female life-history. Understanding why sex-differences in lifespan-extension occur should provide a greater understanding of the mechanisms that regulate the aging process in each sex, and will be crucial for understanding the full implications of these treatments if they are translated to humans.

Effects of tissue-specific GH receptor knockouts in mice

Growth hormone (GH) is pituitary derived hormone which acts on most tissues of the body either directly or indirectly and affects many metabolic processes throughout life. Genetically engineered mouse lines have become vital tools for uncovering the various in vivo activities of a GH. A particularly useful mouse line has been the GH receptor (GHR) gene disrupted or knockout (KO) mouse which has been used world-wide in many studies. Recent advances in biotechnology have allowed the development of tissue-specific knockout mouse lines which allows for more direct enquiries on the activities of a given protein in specific tissues or cell types. Accordingly, twenty-two novel tissue-specific GHRKO mouse lines have been developed in the last eleven years. In this paper we provide a detailed list and review the phenotypic changes that occur in each of these tissue-specific GHRKO mouse lines.

Muscle-dependent regulation of adipose tissue function in long-lived growth hormone-mutant mice

Altered adipose tissue may contribute to the longevity of Snell dwarf and growth hormone receptor (GHR) knock-out mice. We report here that white (WAT) and brown (BAT) fat have elevated UCP1 in both kinds of mice, and that adipocytes in WAT depots turn beige/brown. These imply increased thermogenesis and are expected to lead to improved glucose control. Both kinds of long-lived mice show lower levels of inflammatory M1 macrophages and higher levels of anti-inflammatory M2 macrophages in BAT and WAT, with correspondingly lower levels of TNFα, IL-6, and MCP1. Experiments with mice with tissue-specific disruption of GHR showed that these adipocyte and macrophage changes were not due to hepatic IGF1 production nor to direct GH effects on adipocytes, but instead reflect GH effects on muscle. Muscles deprived of GH signals, either globally (GKO) or in muscle only (MKO), produce higher levels of circulating irisin and its precursor FNDC5. The data thus suggest that the changes in adipose tissue differentiation and inflammatory status seen in long-lived mutant mice reflect interruption of GH-dependent irisin inhibition, with consequential effects on metabolism and thermogenesis.

New insights of growth hormone (GH) actions from tissue-specific GH receptor knockouts in mice

In order to provide new insights into the various activities of GH in specific tissues, recent advances have allowed for the generation of tissue-specific GHR knockout mice. To date, 21 distinct tissue-specific mouse lines have been created and reported in 28 publications. Targeted tissues include liver, muscle, fat, brain, bone, heart, intestine, macrophage, pancreatic beta cells, hematopoietic stem cells, and multi-tissue "global". In this review, we provide a brief history and description of the 21 tissue-specific GHR knockout mouse lines. Arch Endocrinol Metab. 2019;63(6):557-67.

Identification and Application of Gene Expression Signatures Associated with Lifespan Extension

Several pharmacological, dietary, and genetic interventions that increase mammalian lifespan are known, but general principles of lifespan extension remain unclear. Here, we performed RNA sequencing (RNA-seq) analyses of mice subjected to 8 longevity interventions. We discovered a feminizing effect associated with growth hormone regulation and diminution of sex-related differences. Expanding this analysis to 17 interventions with public data, we observed that many interventions induced similar gene expression changes. We identified hepatic gene signatures associated with lifespan extension across interventions, including upregulation of oxidative phosphorylation and drug metabolism, and showed that perturbed pathways may be shared across tissues. We further applied the discovered longevity signatures to identify new lifespan-extending candidates, such as chronic hypoxia, KU-0063794, and ascorbyl-palmitate. Finally, we developed GENtervention, an app that visualizes associations between gene expression changes and longevity. Overall, this study describes general and specific transcriptomic programs of lifespan extension in mice and provides tools to discover new interventions.

The Growth Hormone Receptor: Mechanism of Receptor Activation, Cell Signaling, and Physiological Aspects

The growth hormone receptor (GHR), although most well known for regulating growth, has many other important biological functions including regulating metabolism and controlling physiological processes related to the hepatobiliary, cardiovascular, renal, gastrointestinal, and reproductive systems. In addition, growth hormone signaling is an important regulator of aging and plays a significant role in cancer development. Growth hormone activates the Janus kinase (JAK)-signal transducer and activator of transcription (STAT) signaling pathway, and recent studies have provided a new understanding of the mechanism of JAK2 activation by growth hormone binding to its receptor. JAK2 activation is required for growth hormone-mediated activation of STAT1, STAT3, and STAT5, and the negative regulation of JAK-STAT signaling comprises an important step in the control of this signaling pathway. The GHR also activates the Src family kinase signaling pathway independent of JAK2. This review covers the molecular mechanisms of GHR activation and signal transduction as well as the physiological consequences of growth hormone signaling.

mTOR regulates the expression of DNA damage response enzymes in long-lived Snell dwarf, GHRKO, and PAPPA-KO mice

Studies of the mTOR pathway have prompted speculation that diminished mTOR complex‐1 (mTORC1) function may be involved in controlling the aging process. Our previous studies have shown diminished mTORC1 activity in tissues of three long‐lived mutant mice: Snell dwarf mice, growth hormone receptor gene disrupted mice (GHRKO), and in this article, mice deficient in the pregnancy‐associated protein‐A (PAPPA‐KO). The ways in which lower mTOR signals slow aging and age‐related diseases are, however, not well characterized. Here, we show that Snell, GHKRO, and PAPPA‐KO mice express high levels of two proteins involved in DNA repair, O‐6‐methylguanine‐DNA methyltransferase (MGMT) and N‐myc downstream‐regulated gene 1 (NDRG1). Furthermore, we report that lowering mTOR enhances MGMT and NDRG1 protein expression via post‐transcriptional mechanisms. We show that the CCR4‐NOT complex, a post‐transcriptional regulator of gene expression, is downstream of the mTORC1 pathway and may be responsible for the upregulation of MGMT and NDRG1 in all three varieties of long‐lived mice. Our data thus suggest a novel link between DNA repair and mTOR signaling via post‐transcriptional regulation involving specific alteration in the CCR4‐NOT complex, whose modulation could control multiple aspects of the aging process.

Deconstructing the Growth Hormone Receptor(GHR): Physical and Metabolic Phenotypes of Tissue-Specific GHR Gene-Disrupted Mice

Growth hormone (GH)-induced signaling results in numerous effects in multiple tissues throughout the body. Elucidation of several of these effects has come from studies observing the various phenotypes of the GH receptor (GHR) gene-disrupted (GHR-/-) mouse. These mice are dwarf and obese with increased insulin sensitivity, are resistant to cancer and diabetes, and have an extended lifespan. While these mice have proven to be a valuable tool for understanding the pleiotropic effects of GH, we and others have generated novel tissue-specific GHR gene-disrupted mouse lines that are now helping to further dissect the actions of GH on specific cells/tissues. In this chapter, we summarize the various phenotypes observed in these mice.

Organization of the Mammalian Metabolome according to Organ Function, Lineage Specialization, and Longevity

Biological diversity among mammals is remarkable. Mammalian body weights range seven orders of magnitude and lifespans differ more than 100-fold among species. While genetic, dietary, and pharmacological interventions can be used to modulate these traits in model organisms, it is unknown how they are determined by natural selection. By profiling metabolites in brain, heart, kidney, and liver tissues of 26 mammalian species representing ten taxonomical orders, we report metabolite patterns characteristic of organs, lineages, and species longevity. Our data suggest different rates of metabolite divergence across organs and reveal patterns representing organ-specific functions and lineage-specific physiologies. We identified metabolites that correlated with species lifespan, some of which were previously implicated in longevity control. We also compared the results with metabolite changes in five long-lived mouse models and observed some similar patterns. Overall, this study describes adjustments of the mammalian metabolome according to lifespan, phylogeny, and organ and lineage specialization.

Removal of growth hormone receptor (GHR) in muscle of male mice replicates some of the health benefits seen in global GHR-/- mice

Global disruption of the GH receptor in mice (GHR-/-) produces a large and reproducible extension in lifespan. Since lack of GH action in muscle resulting in improved glucose homeostasis is potentially a mechanism by which GHR-/- mice are long-lived, and since no information on muscle-specific GHR disruption in females is available, we generated and characterized a line of muscle-specific GHR disrupted (MuGHRKO) mice. As expected, male MuGHRKO mice had improved fasting blood glucose, insulin, c-peptide, and glucose tolerance. In contrast, female MuGHRKO mice exhibited normal glucose, insulin, and glucose tolerance. Body weight was mildly but significantly altered in opposite directions in males (decreased) and females (increased) compared to controls. Grip strength and treadmill endurance were unchanged with advanced age in both sexes, suggesting that the direct action of GH on muscle has minimal effect on age-related musculoskeletal frailty. Longevity was unchanged in both sexes at Ohio University and significantly increased for males at University of Michigan. These data suggest that removal of GHR in muscle of male MuGHRKO mice replicates some of the health benefits seen in global GHR-/- mice including improvements to glucose homeostasis and smaller body weight in males, which may explain the trends observed in lifespan.

Regulation of mTOR activity in Snell dwarf and GH receptor gene-disrupted mice

The involvement of mammalian target of rapamycin (mTOR) in lifespan control in invertebrates, calorie-restricted rodents, and extension of mouse lifespan by rapamycin have prompted speculation that diminished mTOR function may contribute to mammalian longevity in several settings. We show here that mTOR complex-1 (mTORC1) activity is indeed lower in liver, muscle, heart, and kidney tissue of Snell dwarf and global GH receptor (GHR) gene-disrupted mice (GHR-/-), consistent with previous studies. Surprisingly, activity of mTORC2 is higher in fasted Snell and GHR-/- than in littermate controls in all 4 tissues tested. Resupply of food enhanced mTORC1 activity in both controls and long-lived mutant mice but diminished mTORC2 activity only in the long-lived mice. Mice in which GHR has been disrupted only in the liver do not show extended lifespan and also fail to show the decline in mTORC1 and increase in mTORC2 seen in mice with global loss of GHR. The data suggest that the antiaging effects in the Snell dwarf and GHR-/- mice are accompanied by both a decline in mTORC1 in multiple organs and an increase in fasting levels of mTORC2. Neither the lifespan nor mTOR effects appear to be mediated by direct GH effects on liver or by the decline in plasma IGF-I, a shared trait in both global and liver-specific GHR-/- mice. Our data suggest that a more complex pattern of hormonal effects and intertissue interactions may be responsible for regulating both lifespan and mTORC2 function in these mouse models of delayed aging.

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.

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.