Adipocyte-Specific GH Receptor-Null (AdGHRKO) Mice Have Enhanced Insulin Sensitivity With Reduced Liver Triglycerides

Growth Hormone Action as a Target in Cancer: Significance, Mechanisms, and Possible Therapies

Growth hormone (GH) is a pituitary-derived endocrine hormone required for normal postnatal growth and development. Hypo- or hypersecretion of endocrine GH results in 2 pathologic conditions, namely GH deficiency (GHD) and acromegaly. Additionally, GH is also produced in nonpituitary and tumoral tissues, where it acts rather as a cellular growth factor with an autocrine/paracrine mode of action. An increasingly persuasive and large body of evidence over the last 70 years concurs that GH action is implicit in escalating several cancer-associated events, locally and systemically. This pleiotropy of GH's effects is puzzling, but the association with cancer risk automatically raises a concern for patients with acromegaly and for individuals treated with GH. By careful assessment of the available knowledge on the fundamental concepts of cancer, suggestions from epidemiological and clinical studies, and the evidence from specific reports, in this review we aimed to help clarify the distinction of endocrine vs autocrine/paracrine GH in promoting cancer and to reconcile the discrepancies between experimental and clinical data. Along this discourse, we critically weigh the targetability of GH action in cancer-first by detailing the molecular mechanisms which posit GH as a critical node in tumor circuitry; and second, by enumerating the currently available therapeutic options targeting GH action. On the basis of our discussion, we infer that a targeted intervention on GH action in the appropriate patient population can benefit a sizable subset of current cancer prognoses.

Growth hormone receptor in VGLUT2 or Sim1 cells regulates glycemia and insulin sensitivity

Growth hormone (GH) has several metabolic effects, including a profound impact on glucose homeostasis. For example, GH oversecretion induces insulin resistance and increases the risk of developing diabetes mellitus. Here, we show that GH receptor (GHR) ablation in vesicular glutamate transporter 2 (VGLUT2)-expressing cells, which comprise a subgroup of glutamatergic neurons, led to a slight decrease in lean body mass without inducing changes in body adiposity. VGLUT2∆GHR mice exhibited reduced glycemia and improved glucose tolerance and insulin sensitivity. Among different glutamatergic neuronal populations, we found that GHR inactivation in Sim1-expressing cells recapitulated the phenotype observed in VGLUT2∆GHR mice. Furthermore, Sim1∆GHR mice exhibited reduced endogenous glucose production and improved hepatic insulin sensitivity without alterations in whole-body or muscle glucose uptake. Sim1∆GHR mice were protected against acute but not chronic diabetogenic effects of exogenous GH administration. Pharmacological activation of ATP-sensitive potassium channels in the brain normalized blood glucose levels in Sim1∆GHR mice. In conclusion, the absence of GHR signaling in VGLUT2/Sim1-expressing cells causes a persistent reduction in glycemia and improves hepatic insulin sensitivity. Central glucose-sensing mechanisms are likely involved in the reduced glycemia exhibited by Sim1∆GHR mice. The current findings uncover a mechanism involved in the effects of GHR signaling in regulating glucose homeostasis.

Neuroendocrine control of brown adipocyte function by prolactin and growth hormone

Growth hormone (GH) is fundamental for growth and glucose homeostasis, and prolactin for optimal pregnancy and lactation outcome, but additionally, both hormones have multiple functions that include a strong impact on energetic metabolism. In this respect, prolactin and GH receptors have been found in brown, and white adipocytes, as well as in hypothalamic centers regulating thermogenesis. This review describes the neuroendocrine control of the function and plasticity of brown and beige adipocytes, with a special focus on prolactin and GH actions. Most evidence points to a negative association between high prolactin levels and the thermogenic capacity of BAT, except in early development. During lactation and pregnancy, prolactin may be a contributing factor that limits unneeded thermogenesis, downregulating BAT UCP1. Furthermore, animal models of high serum prolactin have low BAT UCP1 levels and whitening of the tissue, while lack of Prlr induces beiging in WAT depots. These actions may involve hypothalamic nuclei, particularly the DMN, POA and ARN, brain centers that participate in thermogenesis. Studies on GH regulation of BAT function present some controversies. Most mouse models with GH excess or deficiency point to an inhibitory role of GH on BAT function. Even so, a stimulatory role of GH on WAT beiging has also been described, in accordance with whole-genome microarrays that demonstrate divergent response signatures of BAT and WAT genes to the loss of GH signaling. Understanding the physiology of BAT and WAT beiging may contribute to the ongoing efforts to curtail obesity.

Growth hormone insensitivity and adipose tissue: tissue morphology and transcriptome analyses in pigs and humans

PURPOSE

Growth hormone receptor knockout (GHR-KO) pigs have recently been developed, which serve as a large animal model of Laron syndrome (LS). GHR-KO pigs, like individuals with LS, are obese but lack some comorbidities of obesity. The purpose of this study was to examine the histological and transcriptomic phenotype of adipose tissue (AT) in GHR-KO pigs and humans with LS.

METHODS

Intraabdominal (IA) and subcutaneous (SubQ) AT was collected from GHR-KO pigs and examined histologically for adipocyte size and collagen content. RNA was isolated and cDNA sequenced, and the results were analyzed to determine differentially expressed genes that were used for enrichment and pathway analysis in pig samples. For comparison, we also performed limited analyses on human AT collected from a single individual with and without LS.

RESULTS

GHR-KO pigs have increased adipocyte size, while the LS AT had a trend towards an increase. Transcriptome analysis revealed 55 differentially expressed genes present in both depots of pig GHR-KO AT. Many significant terms in the enrichment analysis of the SubQ depot were associated with metabolism, while in the IA depot, IGF and longevity pathways were negatively enriched. In pathway analysis, multiple expected and novel pathways were significantly affected by genotype, i.e. KO vs. controls. When GH related gene expression was analyzed, SOCS3 and CISH showed species-specific changes.

CONCLUSION

AT of GHR-KO pigs has several similarities to that of humans with LS in terms of adipocyte size and gene expression profile that help describe the depot-specific adipose phenotype of both groups.

Growth hormone receptor gene disruption

Much of our understanding of growth hormone's (GH)'s numerous activities stems from studies utilizing GH receptor (GHR) knockout mice. More recently, the role of GH action has been examined by creating mice with tissue-specific or temporal GHR disruption. To date, 37 distinct GHR knockout mouse lines have been created. Targeted tissues include fat, liver, muscle, heart, bone, brain, macrophage, intestine, hematopoietic stem cells, pancreatic β cells, and inducible multi-tissue "global" disruption at various ages. In this chapter, a summary of each mouse line is provided with background information on the generation of the mouse line as well as important physiological outcomes resulting from GHR gene disruption. Collectively, these mouse lines provide unique insights into GH action and have resulted in the development of new hypotheses about the functions ascribed to GH action in particular tissues.

Increased Fibrosis in White Adipose Tissue of Male and Female bGH Transgenic Mice Appears Independent of TGF-β Action

Fibrosis is a pathological state caused by excess deposition of extracellular matrix proteins in a tissue. Male bovine growth hormone (bGH) transgenic mice experience metabolic dysfunction with a marked decrease in lifespan and with increased fibrosis in several tissues including white adipose tissue (WAT), which is more pronounced in the subcutaneous (Sc) depot. The current study expanded on these initial findings to evaluate WAT fibrosis in female bGH mice and the role of transforming growth factor (TGF)-β in the development of WAT fibrosis. Our findings established that female bGH mice, like males, experience a depot-dependent increase in WAT fibrosis, and bGH mice of both sexes have elevated circulating levels of several markers of collagen turnover. Using various methods, TGF-β signaling was found unchanged or decreased-as opposed to an expected increase-despite the marked fibrosis in WAT of bGH mice. However, acute GH treatments in vivo, in vitro, or ex vivo did elicit a modest increase in TGF-β signaling in some experimental systems. Finally, single nucleus RNA sequencing confirmed no perturbation in TGF-β or its receptor gene expression in any WAT cell subpopulations of Sc bGH WAT; however, a striking increase in B lymphocyte infiltration in bGH WAT was observed. Overall, these data suggest that bGH WAT fibrosis is independent of the action of TGF-β and reveals an intriguing shift in immune cells in bGH WAT that should be further explored considering the increasing importance of B cell-mediated WAT fibrosis and pathology.

Understanding the role of growth hormone in situations of metabolic stress

Growth hormone (GH) is secreted by the anterior pituitary gland and plays a key role in controlling tissue and body growth. While basal GH secretion is considerably reduced along adulthood and aging, several situations of metabolic stress can lead to robust increases in circulating GH levels. The objective of the present review is to summarize and discuss the importance of GH regulating different physiological functions in situations of metabolic stress, including prolonged food restriction, hypoglycemia, exercise, pregnancy, and obesity. The presented data indicate that GH increases hunger perception/food intake, fat mobilization, blood glucose levels, and insulin resistance and produces changes in energy expenditure and neuroendocrine responses during metabolic challenges. When all these effects are considered in the context of situations of metabolic stress, they contribute to restore homeostasis by (1) helping the organism to use appropriate energy substrates, (2) preventing hypoglycemia or increasing the availability of glucose, (3) stimulating feeding to provide nutrients in response to energy-demanding activities or to accelerate the recovery of energy stores, and (4) affecting the activity of neuronal populations involved in the control of metabolism and stress response. Thus, the central and peripheral effects of GH coordinate multiple adaptations during situations of metabolic stress that ultimately help the organism restore homeostasis, increasing the chances of survival.

Associations of GHR, IGF-1 and IGFBP-3 expression in adipose tissue cells with obesity-related alterations in corresponding circulating levels and adipose tissue function in children

Components of the growth hormone (GH) axis, such as insulin-like growth factor-1 (IGF-1), IGF-1 binding protein-3 (IGFBP-3), GH receptor (GHR) and GH-binding protein (GHBP), regulate growth and metabolic pathways. Here, we asked if serum levels of these factors are altered with overweight/obesity and if this is related to adipose tissue (AT) expression and/or increased fat mass. Furthermore, we hypothesized that expression of GHR, IGF-1 and IGFBP-3 is associated with AT function. Serum GHBP levels were increased in children with overweight/obesity throughout childhood, while for IGF-1 levels and the IGF-1/IGFBP-3 molar ratio obesity-related elevations were detectable until early puberty. Circulating levels did not correlate with AT expression of these factors, which was decreased with overweight/obesity. Independent from obesity, expression of GHR, IGF-1 and IGFBP-3 was related to AT dysfunction,and increased insulin levels. Serum GHBP was associated with liver fat percentage and transaminase levels. We conclude that obesity-related elevations in serum GHBP and IGF-1 are unlikely to be caused by increased AT mass and elevations in GHBP are more closely related to liver status in children. The diminished AT expression of these factors with childhood obesity may contribute to early AT dysfunction and a deterioration of the metabolic state.

Disruption of Growth Hormone Receptor in Adipocytes Improves Insulin Sensitivity and Lifespan in Mice

Growth hormone receptor knockout (GHRKO) mice have been used for 25 years to uncover some of the many actions of growth hormone (GH). Since they are extremely long-lived with enhanced insulin sensitivity and protected from multiple age-related diseases, they are often used to study healthy aging. To determine the effect that adipose tissue has on the GHRKO phenotype, our laboratory recently created and characterized adipocyte-specific GHRKO (AdGHRKO) mice, which have increased adiposity but appear healthy with enhanced insulin sensitivity. To test the hypothesis that removal of GH action in adipocytes might partially replicate the increased lifespan and healthspan observed in global GHRKO mice, we assessed adiposity, cytokines/adipokines, glucose homeostasis, frailty, and lifespan in aging AdGHRKO mice of both sexes. Our results show that disrupting the GH receptor gene in adipocytes improved insulin sensitivity at advanced age and increased lifespan in male AdGHRKO mice. AdGHRKO mice also exhibited increased fat mass, reduced circulating levels of insulin, c-peptide, adiponectin, resistin, and improved frailty scores with increased grip strength at advanced ages. Comparison of published mean lifespan data from GHRKO mice to that from AdGHRKO and muscle-specific GHRKO mice suggests that approximately 23% of lifespan extension in male GHRKO is due to GHR disruption in adipocytes vs approximately 19% in muscle. Females benefited less from GHR disruption in these 2 tissues with approximately 19% and approximately 0%, respectively. These data indicate that removal of GH's action, even in a single tissue, is sufficient for observable health benefits that promote long-term health, reduce frailty, and increase longevity.

Growth Hormone and Counterregulation in the Pathogenesis of Diabetes

PURPOSE OF REVIEW

Canonical growth hormone (GH)-dependent signaling is essential for growth and counterregulatory responses to hypoglycemia, but also may contribute to glucose homeostasis (even in the absence of hypoglycemia) via its impact on metabolism of carbohydrates, lipids and proteins, body composition, and cardiovascular risk profile. The aim of this review is to summarize recent data implicating GH action in metabolic control, including both IGF-1-dependent and -independent pathways, and its potential role as target for T2D therapy.

RECENT FINDINGS

Experimental blockade of the GHR can modulate glucose metabolism. Moreover, the soluble form of the GH receptor (GHR, or GHBP) was recently identified as a mediator of improvement in glycemic control in patients with T2D randomized to bariatric surgery vs. medical therapy. Reductions in GHR were accompanied by increases in plasma GH, but unchanged levels of both total and free IGF-1. Likewise, hepatic GHR expression is reduced following both RYGB and VSG in rodents. Emerging data indicate that GH signaling is important for regulation of long-term glucose metabolism in T2D. Future studies will be required to dissect tissue-specific GH signaling and sensitivity and their contributions to systemic glucose metabolism.

Covert actions of growth hormone: fibrosis, cardiovascular diseases and cancer

Since its discovery nearly a century ago, over 100,000 studies of growth hormone (GH) have investigated its structure, how it interacts with the GH receptor and its multiple actions. These include effects on growth, substrate metabolism, body composition, bone mineral density, the cardiovascular system and brain function, among many others. Recombinant human GH is approved for use to promote growth in children with GH deficiency (GHD), along with several additional clinical indications. Studies of humans and animals with altered levels of GH, from complete or partial GHD to GH excess, have revealed several covert or hidden actions of GH, such as effects on fibrosis, cardiovascular function and cancer. In this Review, we do not concentrate on the classic and controversial indications for GH therapy, nor do we cover all covert actions of GH. Instead, we stress the importance of the relationship between GH and fibrosis, and how fibrosis (or lack thereof) might be an emerging factor in both cardiovascular and cancer pathologies. We highlight clinical data from patients with acromegaly or GHD, alongside data from cellular and animal studies, to reveal novel phenotypes and molecular pathways responsible for these actions of GH in fibrosis, cardiovascular function and cancer.

An appetite for growth: The role of the hypothalamic - pituitary - growth hormone axis in energy balance

Links between the regulation of growth and energy balance are clear; to fuel growth, there must be consumption of energy. Therefore, it is perhaps intuitive that interactions between the hypothalamic - pituitary - growth hormone axis (growth axis) and pathways that drive metabolic processes exist. Overproduction of growth hormone has been associated with diabetes and metabolic disease for decades and the opposing effects of growth hormone and insulin have been studied since early experiments almost a century ago. The relationship between neuroendocrine axes can be complex and the growth axis is no exception, interacting with energy balance in several organ systems, both in the periphery and centrally in hypothalamic nuclei. Much is known about peripheral interactions between growth axis hormones and processes such as glucose homeostasis and adipogenesis. More is still being learned about the molecular actions of growth axis hormones in adipose and other metabolically active tissues, and recent findings are discussed in this perspective. However, less is known about interactions with central energy balance pathways in the hypothalamus. This perspective aims to summarise what is known about these interactions, taking lessons from human studies and animal genetic and seasonal models, and discusses what this may mean in an evolving landscape of personalised medicine.

Deficiency of C1QL1 Reduced Murine Ovarian Follicle Reserve Through Intraovarian and Endocrine Control

Ovarian aging is associated with depletion of the ovarian follicle reserve, which is the key determinant of fertility potential in females. In this study, we found that the small, secreted protein complement 1Q-like (C1QL1) is involved in the regulation of follicle depletion through intraovarian and endocrine control in a multidimensional collaborative manner. C1ql1 was detected to be conserved in the ovary and showed high transcript levels during folliculogenesis. Blockade of C1QL1 by IP and ovarian intrabursal injection of C1QL1 antiserum into prepubertal mice impaired folliculogenesis accompanied by reductions in body weight, fat mass, and intraovarian lipid accumulation. An elevation of circulating estradiol levels, reduction of hypothalamic KISS1 and GnRH expression, and a decrease in serum FSH levels were found in C1QL1-deficient mice. In C1QL1-deficient ovaries, many primordial follicles were recruited and developed into medium follicles but underwent atresia at the large follicle stages, which resulted in depletion of follicle reserve. Depletion of C1QL1 alleviated the inhibitory effect of C1QL1 on granulosa cell apoptosis and the stimulatory effect of C1QL1 on granulosa cell autophagy, which resulted in accumulation in the preantral and early antral follicles and an increase in the atretic follicles. The abnormal profile of endocrine hormones accelerated the intraovarian effect of C1QL1 deficiency and further led to depletion of ovarian reserve. Altogether, this study revealed the expression patterns and the mechanism of action of C1QL1 during folliculogenesis and demonstrated that deficiency of C1QL1 caused ovarian follicular depletion.

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.

First use of gene therapy to treat growth hormone resistant dwarfism in a mouse model

The only treatment tested for growth hormone receptor (GHR) defective Laron Syndrome (LS) is injections of recombinant insulin-like-growth factor 1 (rhIGF1). The response is suboptimal and associated with progressive obesity. In this study, we treated 4–5-week-old Laron dwarf mice (GHR−/−) with an adeno-associated virus expressing murine GHR (AAV-GHR) injection at a dose of 4 × 10¹⁰ vector genome per mouse. Serum growth hormone (GH) levels decreased, and GH-responsive IGF1, IGF binding protein 3 (IGFBP3) and acid labile subunit (ALS) increased. There was a significant but limited increase in body weight and length, similar to the response to rhIGF1 treatment in LS patients. All the major organs increased in weight except the brain. Our study is the first to use gene therapy to treat GH-receptor deficiency. We propose that gene therapy with AAV-GHR may eventually be useful for the treatment of human LS.

Loss of Adipocyte STAT5 Confers Increased Depot-Specific Adiposity in Male and Female Mice That Is Not Associated With Altered Adipose Tissue Lipolysis

STATs (Signal Transducers and Activators of Transcription) 5A and 5B are induced during adipocyte differentiation and are primarily activated by growth hormone (GH) and prolactin in fat cells. Previous studies in mice lacking adipocyte GH receptor or STAT5 support their roles in lipolysis-mediated reduction of adipose tissue mass. Male and female mice harboring adipocyte-specific deletion of both STAT5 genes (STAT5^AKO) exhibit increased subcutaneous or inguinal adipose tissue mass, but no changes in visceral or gonadal fat mass. Both depots display substantial increases in adipocyte size with no changes in lipolysis in adipose tissue explants. RNA sequencing analysis of subcutaneous adipose tissue and indirect calorimetry experiments reveal sex-dependent differences in adipose gene expression and whole-body energy expenditure, respectively, resulting from the loss of adipocyte STAT5.

Growth hormone receptor gene disruption in mature-adult mice improves male insulin sensitivity and extends female lifespan

Studies in multiple species indicate that reducing growth hormone (GH) action enhances healthy lifespan. In fact, GH receptor knockout (GHRKO) mice hold the Methuselah prize for the world's longest-lived laboratory mouse. We previously demonstrated that GHR ablation starting at puberty (1.5 months), improved insulin sensitivity and female lifespan but results in markedly reduced body size. In this study, we investigated the effects of GHR disruption in mature-adult mice at 6 months old (6mGHRKO). These mice exhibited GH resistance (reduced IGF-1 and elevated GH serum levels), increased body adiposity, reduced lean mass, and minimal effects on body length. Importantly, 6mGHRKO males have enhanced insulin sensitivity and reduced neoplasms while females exhibited increased median and maximal lifespan. Furthermore, fasting glucose and oxidative damage was reduced in females compared to males irrespective of Ghr deletion. Overall, disrupted GH action in adult mice resulted in sexual dimorphic effects suggesting that GH reduction at older ages may have gerotherapeutic effects.

Adipocyte STAT5 deficiency does not affect blood glucose homeostasis in obese mice

The aim of this study was to investigate whether the lack of signal transducer and activator of transcription 5 (STAT5) in mature adipocytes of obese mice (Stat5Adipoq mice) improves glucose and lipid metabolism as previously observed in lean mice. Male Stat5Adipoq mice and their wild type (WT) littermates were fed high-fat diet (HFD). Effects of adipocyte STAT5 deficiency on adiposity as well as on glucose and lipid metabolism were determined under ad libitum feeding and after weight loss induced by calorie restriction. Compared to WT mice, obese Stat5Adipoq mice showed modestly accelerated weight gain and blunted depletion of fat stores under calorie restriction (reduction in % body fat after 3 weeks: WT, -9.3±1.1, vs Stat5Adipoq, -5.9±0.8, p = 0.04). No differences were observed between Stat5Adipoq and WT mice with regard to parameters of glucose and lipid metabolism including basal glycaemia, glucose tolerance, and plasma triglycerides. In conclusion, STAT5 deficiency in the adipocyte of HFD-fed obese mice was associated with increased fat accumulation. In contrast to previous findings in lean mice, however, lipid accumulation was not associated with any improvement in glucose and lipid metabolism. Our results do not support adipocyte STAT5 as a promising target for the treatment of obesity-associated metabolic derangements.

Systemic Deficiency of GHR in Pigs leads to Hepatic Steatosis via Negative Regulation of AHR Signaling

Laron syndrome (LS) is an autosomal recessive genetic disease mainly caused by mutations in the human growth hormone receptor (GHR) gene. Previous studies have focused on Ghr mutant mice, but compared with LS patients, Ghr knockout (KO) mice exhibit differential lipid metabolism. To elucidate the relationship between GHR mutation and lipid metabolism, the role of GHR in lipid metabolism was examined in GHR KO pigs and hepatocytes transfected with siGHR. We observed high levels of free fatty acids and hepatic steatosis in GHR KO pigs, which recapitulates the abnormal lipid metabolism in LS patients. RNAseq analysis revealed that genes related to the fatty acid oxidation pathway were significantly altered in GHR KO pigs. AHR, a transcription factor related to lipid metabolism, was significantly downregulated in GHR KO pigs and siGHR-treated human hepatocytes. We found that AHR directly regulated fatty acid oxidation by directly binding to the promoters of ACOX1 and CPT1A and activating their expression. These data indicate that loss of GHR disturbs the ERK-AHR-ACOX1/CPT1A pathway and consequently leads to hepatic steatosis. Our results established AHR as a modulator of hepatic steatosis, thereby providing a therapeutic target for lipid metabolism disorder.

Towards Understanding the Direct and Indirect Actions of Growth Hormone in Controlling Hepatocyte Carbohydrate and Lipid Metabolism

Growth hormone (GH) is critical for achieving normal structural growth. In addition, GH plays an important role in regulating metabolic function. GH acts through its GH receptor (GHR) to modulate the production and function of insulin-like growth factor 1 (IGF1) and insulin. GH, IGF1, and insulin act on multiple tissues to coordinate metabolic control in a context-specific manner. This review will specifically focus on our current understanding of the direct and indirect actions of GH to control liver (hepatocyte) carbohydrate and lipid metabolism in the context of normal fasting (sleep) and feeding (wake) cycles and in response to prolonged nutrient deprivation and excess. Caveats and challenges related to the model systems used and areas that require further investigation towards a clearer understanding of the role GH plays in metabolic health and disease are discussed.

Growth Hormone and Aging: New Findings

Complex relationships between growth hormone (GH) signaling and mammalian aging continue to attract attention of many investigators. Recent results include evidence that the impact of GH on genome maintenance (DNA damage and repair) is drastically different in normal as compared to cancer cells, consistent with GH promoting aging and cancer progression. Impact of GH on DNA methylation was studied as a possible mechanism linking actions of GH during early life to the trajectory of aging. Animals with reduced or enhanced GH signaling and novel animals with adipocyte-specific deletion of GH receptors were used to elucidate the effects of GH on white and brown adipose tissue, including the impact of this hormone on lipolysis, fibrosis, and thermogenesis. Effects of GH on adipose tissue related to lipid and energy metabolism emerge as mechanistic links between GH, healthspan, and lifespan. Treatment of healthy men with a combination of GH, dehydroepiandrosterone, and metformin was reported to restore thymus function and reduce epigenetic age. Studies of human subjects with deficiency of GH or GH receptors and studies of mice with the same endocrine syndromes identified several phenotypic changes related (positively or negatively) to the previously reported predisposition to healthy aging. Results of these and other recent studies advance present understanding of the mechanisms by which GH influences aging and longevity and of the trade-offs involved.

Mouse models of growth hormone insensitivity

Growth hormone (GH) induces pleiotropic effects on growth and metabolism via binding and subsequent activation of the growth hormone receptor (GHR) and its downstream signaling pathways. Growth hormone insensitivity (GHI) describes a group of disorders in which there is resistance to the action of GH and resultant insulin-like growth factor I (IGF-I) deficiency. GHI is commonly due to genetic disorders of the GH receptor causing GH receptor deficiency (e.g. Laron Syndrome (LS)), decreased activation of GHR, or defects in post-receptor signaling molecules. Genetically altered mouse lines have been invaluable to better understand the physiological impact of GHI due to the ability to do invasive and longitudinal measures of metabolism, growth, and health on a whole animal or in individual tissues/cells. In the current review, the phenotype of mouse lines with GHI will be reviewed. Mouse lines to be discussed include: 1) GHR-/- mice with a gene disruption in the GHR that results in no functional GHR throughout life, also referred to as the Laron mouse, 2) mice with temporal loss of GHR (aGHRKO) starting at 6 weeks of age, 3) mice transgenic for a GHR antagonist (GHA mice), 4) mice with GHI in select tissues or cells generated via Cre-lox or related technology, and 5) assorted mice with defects in post-receptor signaling molecules. Collectively, these mouse lines have revealed an intriguing role of GH action in health, disease, and aging.

Integrative Analyses of mRNA Expression Profile Reveal SOCS2 and CISH Play Important Roles in GHR Mutation-Induced Excessive Abdominal Fat Deposition in the Sex-Linked Dwarf Chicken

Sex-linked dwarf (SLD) chicken, which is caused by a recessive mutation of the growth hormone receptor (GHR), has been widely used in the Chinese broiler industry. However, it has been found that the SLD chicken has more abdominal fat deposition than normal chicken. Excessive fat deposition not only reduced the carcass quality of the broilers but also reduced the immunity of broilers to diseases. To find out the key genes and the precise regulatory pathways that were involved in the GHR mutation-induced excessive fat deposition, we used high-fat diet (HFD) and normal diet to feed the SLD chicken and normal chicken and analyzed the differentially expressed genes (DEGs) among the four groups. Results showed that the SLD chicken had more abdominal fat deposition and larger adipocytes size than normal chicken and HFD can promote abdominal fat deposition and induce adipocyte hypertrophy. RNA sequencing results of the livers and abdominal fats from the above chickens revealed that many DEGs between the SLD and normal chickens were enriched in fat metabolic pathways, such as peroxisome proliferator-activated receptor (PPAR) signaling, extracellular matrix (ECM)-receptor pathway, and fatty acid metabolism. Importantly, by constructing and analyzing the GHR-downstream regulatory network, we found that suppressor of cytokine signaling 2 (SOCS2) and cytokine-inducible SH2-containing protein (CISH) may involve in the GHR mutation-induced abdominal fat deposition in chicken. The ectopic expression of SOCS2 and CISH in liver-related cell line leghorn strain M chicken hepatoma (LMH) cell and immortalized chicken preadipocytes (ICP) revealed that these two genes can regulate fatty acid metabolism, adipocyte differentiation, and lipid droplet accumulation. Notably, overexpression of SOCS2 and CISH can rescue the hyperactive lipid metabolism and excessive lipid droplet accumulation of primary liver cell and preadipocytes that were isolated from the SLD chicken. This study found some genes and pathways involved in abdominal fat deposition of the SLD chicken and reveals that SOCS2 and CISH are two key genes involved in the GHR mutation-induced excessive fat deposition of the SLD chicken.

Both proliferation and lipogenesis of brown adipocytes contribute to postnatal brown adipose tissue growth in mice

Brown adipose tissue (BAT) is the primary non-shivering thermogenesis organ in mammals, which plays essential roles in maintaining the body temperature of infants. Although the development of BAT during embryogenesis has been well addressed in rodents, how BAT grows after birth remains unknown. Using mouse interscapular BAT (iBAT) as an example, we studied the cellular and molecular mechanisms that regulate postnatal BAT growth. By analyzing the developmental dynamics of brown adipocytes (BAs), we found that BAs size enlargement partially accounts for iBAT growth. By investigating the BAs cell cycle activities, we confirmed the presence of proliferative BAs in the neonatal mice. Two weeks after birth, most of the BAs exit cell cycle, and the further expansion of the BAT was mainly due to lipogenesis-mediated BAs volume increase. Microscopy and fluorescence-activated cell sorting analyses suggest that most BAs are mononuclear and diploid. Based on the developmental dynamics of brown adipocytes, we propose that the murine iBAT has two different growth phases between birth and weaning: increase of BAs size and number in the first two weeks, and BAs size enlargement thereafter. In summary, our data demonstrate that both lipogenesis and proliferation of BAs contribute to postnatal iBAT growth in mice.

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.

Cholinergic neurons in the hypothalamus and dorsal motor nucleus of the vagus are directly responsive to growth hormone

AIMS

Cholinergic neurons are distributed in brain areas containing growth hormone (GH)-responsive cells. We determined if cholinergic neurons are directly responsive to GH and the metabolic consequences of deleting the GH receptor (GHR) specifically in choline acetyltransferase (ChAT)-expressing cells.

MAIN METHODS

Mice received an acute injection of GH to detect neurons co-expressing ChAT and phosphorylated STAT5 (pSTAT5), a well-established marker of GH-responsive cells. For the physiological studies, mice carrying ablation of GHR exclusively in ChAT-expressing cells were produced and possible changes in energy and glucose homeostasis were determined when consuming regular chow or high-fat diet (HFD).

KEY FINDINGS

The majority of cholinergic neurons in the arcuate nucleus (60%) and dorsomedial nucleus (84%) of the hypothalamus are directly responsive to GH. Approximately 34% of pre-ganglionic parasympathetic neurons in the dorsal motor nucleus of the vagus also exhibited GH-induced pSTAT5. GH-induced pSTAT5 in these ChAT neurons was absent in GHR ChAT knockout mice. Mice carrying ChAT-specific GHR deletion, either in chow or HFD, did not exhibit significant changes in body weight, body adiposity, lean body mass, food intake, energy expenditure, respiratory quotient, ambulatory activity, serum leptin levels, glucose tolerance, insulin sensitivity and metabolic responses to 2-deoxy-d-glucose. However, GHR deletion in ChAT neurons caused decreased hypothalamic Pomc mRNA levels in HFD mice.

SIGNIFICANCE

Cholinergic neurons that regulate the metabolism are directly responsive to GH, although GHR signaling in these cells is not required for energy and glucose homeostasis. Thus, the physiological importance of GH action on cholinergic neurons still needs to be identified.

Insulin sensitivity in long-lived growth hormone-releasing hormone knockout mice

Our previous studies showed that loss-of-function mutation of growth hormone releasing hormone (GHRH) results in increased longevity and enhanced insulin sensitivity in mice. However, the details of improved insulin action and tissue-specific insulin signaling are largely unknown in this healthy-aging mouse model. We conducted hyperinsulinemic-euglycemic clamp to investigate mechanisms underlying enhanced insulin sensitivity in growth hormone (GH) deficient mice. Further, we assessed in vivo tissue-specific insulin activity via activation of PI3K-AKT and MAPK-ERK1/2 cascades using western blot. Clamp results showed that the glucose infusion rate required for maintaining euglycemia was much higher in GHRH^-/- mice compared to WT controls. Insulin-mediated glucose production was largely suppressed, whereas glucose uptake in skeletal muscle and brown adipose tissue were significant enhanced in GHRH^-/- mice compared to WT controls. Enhanced capacity of insulin-induced activation of the PI3K-AKT and MAPK-ERK1/2 signaling were observed in a tissue-specific manner in GHRH^-/- mice. Enhanced systemic insulin sensitivity in long-lived GHRH^-/- mice is associated with differential activation of insulin signaling cascades among various organs. Improved action of insulin in the insulin sensitive tissues is likely to mediate the prolonged longevity and healthy-aging effects of GH deficiency in mice.

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.

Tissue-specific disruption of the growth hormone receptor (GHR) in mice: An update

The Growth hormone receptor (GHR) is expressed in many cells/tissues in the body. To investigate the specific metabolic effects of GH action in distinct tissues, several tissue-specific GHR gene disrupted or knockout (KO) mouse lines have been generated. Previously, we have described the effects of GHRKO in several known insulin sensitive tissues, namely liver, muscle and adipose tissue. In this review, we further explore and summarize the main findings of recently published GHRKO results in liver, adipocytes, intestine, bone, brain and heart.

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

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

Tissue-Specific GHR Knockout Mice: An Updated Review

Growth hormone (GH) signaling plays a key role in mediating growth, development, metabolism, and lifespan regulation. However, the mechanisms of longevity regulation at the cellular and molecular level are still not well-understood. An important area in the field of GH research is in the development of advanced transgenic systems for conditional expression of GH signaling in a cell type- or tissue-specific manner. There have been many recent studies conducted to examine the effects of tissue-specific GHR disruption. This review updates our previous discussions on this topic and summarizes recent data on the newly-made tissue-specific GHR-KO mice including intestinal epithelial cells, bone, hematopoietic stem cells, cardiac myocytes, and specific brain regions. The data from these new genetically-engineered mice have a significant impact on our understanding of the local GH signaling function.

Rhythmic growth hormone secretion in physiological and pathological conditions: Lessons from rodent studies

Evolutionally conserved in all mammalians, the release of GH occurs in a rhythmic pattern, characterized by several dominant surges (pulsatile GH) with tonic low inter-pulse levels (tonic GH). Such pulsatile secretion pattern is essential for many physiological actions of GH on different tissues with defined gender dimorphism. Rhythmic release of pulsatile GH is tightly controlled by hypothalamic neurons as well as peripheral metabolic factors. Changes of GH pattern occur within a range of sophisticated physiological and pathological settings and significantly contribute to growth, ageing, survival and disease predispositions. Precise analysis of GH secretion pattern is vitally important for a comprehensive understanding of the function of GH and the components that regulate GH secretion pattern.

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.

Central growth hormone action regulates metabolism during pregnancy

The maternal organism undergoes numerous metabolic adaptations to become prepared for the demands associated with the coming offspring. These metabolic adaptations involve changes induced by several hormones that act at multiple levels, ultimately influencing energy and glucose homeostasis during pregnancy and lactation. Previous studies have shown that central growth hormone (GH) action modulates glucose and energy homeostasis. However, whether central GH action regulates metabolism during pregnancy and lactation is still unknown. In the present study, we generated mice carrying ablation of GH receptor (GHR) in agouti-related protein (AgRP)-expressing neurons, in leptin receptor (LepR)-expressing cells or in the entire brain to investigate the role played by central GH action during pregnancy and lactation. AgRP-specific GHR ablation led to minor metabolic changes during pregnancy and lactation. However, while brain-specific GHR ablation reduced food intake and body adiposity during gestation, LepR GHR knockout (KO) mice exhibited increased leptin responsiveness in the ventromedial nucleus of the hypothalamus during late pregnancy, although their offspring showed reduced growth rate. Additionally, both Brain GHR KO and LepR GHR KO mice had lower glucose tolerance and glucose-stimulated insulin secretion during pregnancy, despite presenting increased insulin sensitivity, compared with control pregnant animals. Our findings revealed that during pregnancy central GH action regulates food intake, fat retention, as well as the sensitivity to insulin and leptin in a cell-specific manner. Together, the results suggest that GH acts in concert with other "gestational hormones" to prepare the maternal organism for the metabolic demands of the offspring.

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

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

Loss of Adipose Growth Hormone Receptor in Mice Enhances Local Fatty Acid Trapping and Impairs Brown Adipose Tissue Thermogenesis

Growth hormone (GH) binds to its receptor (growth hormone receptor [GHR]) to exert its pleiotropic effects on growth and metabolism. Disrupted GH/GHR actions not only fail growth but also are involved in many metabolic disorders, as shown in murine models with global or tissue-specific Ghr deficiency and clinical observations. Here we constructed an adipose-specific Ghr knockout mouse model Ad-GHRKO and studied the metabolic adaptability of the mice when stressed by high-fat diet (HFD) or cold. We found that disruption of adipose Ghr accelerated dietary obesity but protected the liver from ectopic adiposity through free fatty acid trapping. The heat-producing brown adipose tissue burning and white adipose tissue browning induced by cold were slowed in the absence of adipose Ghr but were recovered after prolonged cold acclimation. We conclude that at the expense of excessive subcutaneous fat accumulation and lower emergent cold tolerance, down-tuning adipose GHR signaling emulates a healthy obesity situation which has metabolic advantages against HFD.

Characterization of an intestine-specific GH receptor knockout (IntGHRKO) mouse

OBJECTIVE

Growth hormone (GH) has been reported to enhance the intestinal barrier; as such, recombinant GH has been administered for several intestinal diseases. However, excess GH action has been implicated in increasing the risk of intestinal dysfunction. The goal of this study was to examine the direct effects of GH on the small and large intestines to clarify the role GH plays in intestinal function through the use of a mouse model.

DESIGN

An intestinal epithelial-specific GH receptor (GHR) knockout (IntGHRKO) mouse line was generated using Cre-lox with the villin promoter driving Cre expression. The generated mice were characterized with respect to growth and intestinal phenotypes.

RESULTS

IntGHRKO mice showed no significant changes in body length, weight, or composition compared to floxed controls. Male IntGHRKO mice had significantly shorter large intestines at 4 and 12 months of age. Intestinal barrier function was assessed by measuring the expression of tight junction related genes, as well as levels of serum endotoxin and fecal albumin. Results showed sex differences as males had an increase in occludin levels but normal serum endotoxin and fecal albumin; while, females had changes in fecal albumin levels with normal occludin and serum endotoxin. Evaluation of glucose tolerance and fat absorption also showed sex differences as females were glucose intolerant, while males had impaired fat absorption. Histopathology revealed a trend towards decreased villus height in males, which could explain the sex difference in glucose homeostasis.

CONCLUSIONS

Overall, the data demonstrate that disruption of GH on the intestinal epithelial cells modestly affects the intestinal gross anatomy, morphology, and function in a sex-specific manner.

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

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