Growth hormone acts on liver to stimulate autophagy, support glucose production, and preserve blood glucose in chronically starved mice

Fasting and prolonged food restriction differentially affect GH secretion independently of GH receptor signaling in AgRP neurons

Growth hormone (GH) receptor (GHR) is abundantly expressed in neurons that co-release the agouti-related protein (AgRP) and neuropeptide Y (NPY) in the arcuate nucleus of the hypothalamus (ARH). Since ARHAgRP/NPY neurons regulate several hypothalamic-pituitary-endocrine axes, this neuronal population possibly modulates GH secretion via a negative feedback loop, particularly during food restriction, when ARHAgRP/NPY neurons are highly active. The present study aims to determine the importance of GHR signaling in ARHAgRP/NPY neurons on the pattern of GH secretion in fed and food-deprived male mice. Additionally, we compared the effect of two distinct situations of food deprivation: 16 h of fasting or four days of food restriction (40% of usual food intake). Overnight fasting strongly suppressed both basal and pulsatile GH secretion. Animals lacking GHR in ARHAgRP/NPY neurons (AgRP∆GHR mice) did not exhibit differences in GH secretion either in the fed or fasted state, compared to control mice. In contrast, four days of food restriction increased GH pulse frequency, basal GH secretion, and pulse irregularity/complexity (measured by sample entropy), whereas pulsatile GH secretion was not affected in both control and AgRP∆GHR mice. Hypothalamic Ghrh mRNA levels were unaffected by fasting or food restriction, but Sst expression increased in acutely fasted mice, but decreased after prolonged food restriction in both control and AgRP∆GHR mice. Our findings indicate that short-term fasting and prolonged food restriction differentially affect the pattern of GH secretion, independently of GHR signaling in ARHAgRP/NPY neurons.

A long-acting LEAP2 analog reduces hepatic steatosis and inflammation and causes marked weight loss in mice

OBJECTIVE

The number of individuals affected by metabolic dysfunction associated fatty liver disease [1] is on the rise, yet hormonal contributors to the condition remain incompletely described and only a single FDA-approved treatment is available. Some studies suggest that the hormones ghrelin and LEAP2, which act as agonist and antagonist/inverse agonist, respectively, for the G protein coupled receptor GHSR, may influence the development of MAFLD. For instance, ghrelin increases hepatic fat whereas synthetic GHSR antagonists do the opposite. Also, hepatic steatosis is less prominent in standard chow-fed ghrelin-KO mice but more prominent in 42% high-fat diet-fed female LEAP2-KO mice.

METHODS

Here, we sought to determine the therapeutic potential of a long-acting LEAP2 analog (LA-LEAP2) to treat MAFLD in mice. LEAP2-KO and wild-type littermate mice were fed a Gubra-Amylin-NASH (GAN) diet for 10 or 40 wks, with some randomized to an additional 28 or 10 days of GAN diet, respectively, while treated with LA-LEAP2 vs Vehicle. Various metabolic parameters were followed and biochemical and histological assessments of MAFLD were made.

RESULTS

Among the most notable metabolic effects, daily LA-LEAP2 administration to both LEAP2-KO and wild-type littermates during the final 4 wks of a 14 wk-long GAN diet challenge markedly reduced liver weight, hepatic triglycerides, plasma ALT, hepatic microvesicular steatosis, hepatic lobular inflammation, NASH activity scores, and prevalence of higher-grade fibrosis. These changes were accompanied by prominent reductions in body weight, without effects on food intake, and reduced plasma total cholesterol. Daily LA-LEAP2 administration during the final 10 d of a 41.5 wk-long GAN diet challenge also reduced body weight, plasma ALT, and plasma total cholesterol in LEAP2-KO and wild-type littermates and prevalence of higher grade fibrosis in LEAP2-KO mice.

CONCLUSIONS

Administration of LA-LEAP2 to mice fed a MAFLD-prone diet markedly improves several facets of MAFLD, including hepatic steatosis, hepatic lobular inflammation, higher-grade hepatic fibrosis, and transaminitis. These changes are accompanied by prominent reductions in body weight and lowered plasma total cholesterol. Taken together, these data suggest that LEAP2 analogs such as LA-LEAP2 hold promise for the treatment of MAFLD and obesity.

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.

Euglycemia is affected by stress defense factor hepatocyte NRF1, but not NRF2

Hepatocyte stress signaling has been established to alter glucose metabolism and impair systemic glucose homeostasis. In contrast, the role of stress defenses in the control of glucose homeostasis is less understood. Nuclear factor erythroid 2 related factor-1 (NRF1) and -2 (NRF2) are transcription factors that promote stress defense and can exert hepatocyte stress defense programming via complementary gene regulation. To identify whether there are independent or complementary roles of these factors in hepatocytes on glucose homeostasis, we investigated the effect of adult-onset, hepatocyte-specific deletion of NRF1, NRF2, or both on glycemia in mice fed 1-3 weeks with a mildly stressful diet enriched with fat, fructose, and cholesterol. Compared to respective control, NRF1 deficiency and combined deficiency reduced glycemia, in some cases resulting in hypoglycemia, whereas there was no effect of NRF2 deficiency. However, reduced glycemia in NRF1 deficiency did not occur in the leptin-deficient mouse model of obesity and diabetes, suggesting hepatocyte NRF1 support defenses that counteract hypoglycemia but does not promote hyperglycemia. Consistent with this, NRF1 deficiency was associated with reduced liver glycogen and glycogen synthase expression as well as marked alteration to circulating level of glycemia-influencing hormones, growth hormone and insulin-like growth factor-1 (IGF1). Overall, we identify a role for hepatocyte NRF1 in modulating glucose homeostasis, which may be linked to liver glycogen storage and the growth hormone/IGF1 axis.

Unexpected role for IGF-1 in starvation: Maintenance of blood glucose

Wild-type (WT) mice maintain viable levels of blood glucose even when adipose stores are depleted by 6 d of 60% calorie restriction followed by a 23-h fast (hereafter designated as "starved" mice). Survival depends on ghrelin, an octanoylated peptide hormone. Mice that lack ghrelin suffer lethal hypoglycemia when subjected to the same starvation regimen. Ghrelin is known to stimulate secretion of growth hormone (GH), which in turn stimulates secretion of IGF-1 (insulin-like growth factor-1). In the current study, we found that starved ghrelin-deficient mice had a 90% reduction in plasma IGF-1 when compared with starved WT mice. Injection of IGF-1 in starved ghrelin-deficient mice caused a twofold increase in glucose production and raised blood glucose to levels seen in starved WT mice. Increased glucose production was accompanied by increases in plasma glycerol, fatty acids and ketone bodies, and hepatic triglycerides. All of these increases were abolished when the mice were treated with atglistatin, an inhibitor of adipose tissue triglyceride lipase. We conclude that IGF-1 stimulates adipose tissue lipolysis in starved mice and that this lipolysis supplies energy and substrates that restore hepatic gluconeogenesis. This action of IGF-1 in starved mice is in contrast to its known action in inhibiting adipose tissue lipase in fed mice. Surprisingly, the ghrelin-dependent maintenance of plasma IGF-1 in starved mice was not mediated by GH. Direct injection of GH into starved ghrelin-deficient mice failed to increase plasma IGF-1. These data call attention to an unsuspected role of IGF-1 in the adaptation to starvation.

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.

Metabolic Effects of Recombinant Human Growth Hormone Replacement Therapy on Juvenile Patients after Craniopharyngioma Resection

Objective: To investigate the effect of short-term recombinant human growth hormone (rhGH) replacement therapy on metabolic parameters in juvenile patients following craniopharyngioma (CP) resection. Methods. This retrospective study included 42 cases of juvenile patients that had undergone CP resection in the Department of Endocrinology at the Peking Union Medical College Hospital, from April 2013 to August 2020. According to whether they received growth hormone replacement therapy, the patients were divided into either the growth hormone replacement therapy (GHRT) group (30 cases) or the control group (12 cases). Changes in body mass index (BMI), BMI z-score, transaminase activity, fasting blood glucose (FBG) levels, blood lipid profile, and high-sensitivity C-reactive protein (hsCRP) levels were evaluated after one year of GHRT treatment. Results. The average age of the GHRT group was 13.00 (8.00-14.00) years old and these patients had undergone a CP operation an average of 2.00 (1.62-3.15) years earlier. Prior to receiving GHRT treatment, they received appropriate doses of adrenocortical hormone and thyroid hormone replacement therapy. After one year of GHRT treatment, the average BMI z-score decreased from 1.60 ± 1.76 to 1.13 ± 1.73 (P=0.005). Alanine aminotransferase (ALT) activity decreased from 26.50 (17.00∼98.00) U/L to 18.00 (13.00∼26.48) U/L (P ≤ 0.001), and similar changes were observed with regard to aspartate aminotransferase (AST) and glutamyl transferase (GGT) activity in the GHRT treatment group. The average total cholesterol (TC) decreased from 4.67 (4.10-6.14) mmol/L to 4.32 ± 0.85 mmol/L (P=0.002), and low-density lipoprotein (LDL) levels decreased from 3.05 ± 0.95 mmol/L to 2.56 ± 0.65 mmol/L (P=0.001) in the GHRT treatment group. The average blood urea nitrogen level decreased from 4.53 ± 1.09 mmol/L to 3.92 ± 0.82 mmol/L (P=0.016) and the average serum creatinine (SCr) level decreased from 55.59 ± 12.54 µmol/L to 51.15 ± 10.51 µmol/L (P=0.005) in the GHRT treatment group. The average hsCRP level decreased from 3.23 (1.79∼4.34) mg/L to 0.92 (0.42∼1.21) mg/L in the GHRT treatment group. In the control group, the average ALT activity increased from 26.58 ± 8.75 U/L to 42.58 ± 24.59 U/L (P=0.039), GGT activity increased from 19.0 (13.25-29.25) U/L to 25.00 (14.75-34.75) U/L (P=0.026), and LDL levels increased from 2.27 ± 0.76 mmol/L to 3.43 ± 1.28 mmol/L (P=0.04). Conclusion. GHRT treatment improves the metabolic parameters of juvenile patients that have undergone craniopharyngioma resection by reducing BMI z-scores, low-density lipoprotein, and hsCRP levels and improving liver function.

The Metabolism Reprogramming of microRNA Let-7-Mediated Glycolysis Contributes to Autophagy and Tumor Progression

Cancer is usually a result of abnormal glucose uptake and imbalanced nutrient metabolization. The dysregulation of glucose metabolism, which controls the processes of glycolysis, gives rise to various physiological defects. Autophagy is one of the metabolic-related cellular functions and involves not only energy regeneration but also tumorigenesis. The dysregulation of autophagy impacts on the imbalance of metabolic homeostasis and leads to a variety of disorders. In particular, the microRNA (miRNA) Let-7 has been identified as related to glycolysis procedures such as tissue repair, stem cell-derived cardiomyocytes, and tumoral metastasis. In many cancers, the expression of glycolysis-related enzymes is correlated with Let-7, in which multiple enzymes are related to the regulation of the autophagy process. However, much recent research has not comprehensively investigated how Let-7 participates in glycolytic reprogramming or its links to autophagic regulations, mainly in tumor progression. Through an integrated literature review and omics-related profiling correlation, this review provides the possible linkage of the Let-7 network between glycolysis and autophagy, and its role in tumor progression.

High-throughput mediation analysis of human proteome and metabolome identifies mediators of post-bariatric surgical diabetes control

To improve the power of mediation in high-throughput studies, here we introduce High-throughput mediation analysis (Hitman), which accounts for direction of mediation and applies empirical Bayesian linear modeling. We apply Hitman in a retrospective, exploratory analysis of the SLIMM-T2D clinical trial in which participants with type 2 diabetes were randomized to Roux-en-Y gastric bypass (RYGB) or nonsurgical diabetes/weight management, and fasting plasma proteome and metabolome were assayed up to 3 years. RYGB caused greater improvement in HbA1c, which was mediated by growth hormone receptor (GHR). GHR’s mediation is more significant than clinical mediators, including BMI. GHR decreases at 3 months postoperatively alongside increased insulin-like growth factor binding proteins IGFBP1/BP2; plasma GH increased at 1 year. Experimental validation indicates (1) hepatic GHR expression decreases in post-bariatric rats; (2) GHR knockdown in primary hepatocytes decreases gluconeogenic gene expression and glucose production. Thus, RYGB may induce resistance to diabetogenic effects of GH signaling.

Trial Registration: Clinicaltrials.gov NCT01073020.

Factors underlying the effects of gastric bypass surgery on glucose homeostasis are incompletely understood. Here the authors developed and applied high-throughput mediation analysis to identify proteome/metabolome mediators of improved glucose homeostasis after to gastric bypass surgery, and report that improved glycemia was mediated by the growth hormone receptor.

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.

Starvation-induced transcription factor CREBH negatively governs body growth by controlling GH signaling

cAMP responsive element-binding protein H (CREBH) is a hepatic transcription factor to be activated during fasting. We generated CREBH knock-in flox mice, and then generated liver-specific CREBH transgenic (CREBH L-Tg) mice in an active form. CREBH L-Tg mice showed a delay in growth in the postnatal stage. Plasma growth hormone (GH) levels were significantly increased in CREBH L-Tg mice, but plasma insulin-like growth factor 1 (IGF1) levels were significantly decreased, indicating GH resistance. In addition, CREBH overexpression significantly increased hepatic mRNA and plasma levels of FGF21, which is thought to be as one of the causes of growth delay. However, the additional ablation of FGF21 in CREBH L-Tg mice could not correct GH resistance at all. CREBH L-Tg mice sustained GH receptor (GHR) reduction and the increase of IGF binding protein 1 (IGFBP1) in the liver regardless of FGF21. As GHR is a first step in GH signaling, the reduction of GHR leads to impairment of GH signaling. These data suggest that CREBH negatively regulates growth in the postnatal growth stage via various pathways as an abundant energy response by antagonizing GH signaling.

Growth hormone receptor disrupts glucose homeostasis via promoting and stabilizing retinol binding protein 4

Rationale: The molecular mechanisms underlying the pathogenesis of systemic insulin resistance in type 2 diabetes remain elusive. Growth hormone receptor (GHR) deficiency has long been known to improved insulin sensitivity. However, whether hepatic GHR overexpression or activation is a cause of insulin resistance is still unknown. The aim of this study was to identify the new role of GHR in systemic insulin resistance and explore the underlying mechanism.

Method: Different samples obtained from obese humans, ob/ob mice, db/db mice, high-fat diet (HFD)-fed mice and primary mouse hepatocytes were used to evaluate the correlations between GHR and metabolic disorders. Recombinant adeno-associated viruses encoding GHR and STAT5 and GHR knockout mice were used to investigate the roles of hepatic GHR in glucose homeostasis. Tissue H&E, Oil Red O and PAS staining were performed for histomorphological analysis. Gel filtration chromatography was employed for the separation of serum RBP4-TTR complexes. Plasmids (related to GHR, STAT5 and HIF1α), siRNA oligos (siGHR and siSTAT5), luciferase activity and ChIP assays were used to explore the potential mechanism of hepatic GHR.

Results: Here, we found that hepatic GHR expression was elevated during metabolic disorder. Accordingly, hepatic GHR overexpression disrupted systemic glucose homeostasis by promoting gluconeogenesis and disturbing insulin responsiveness in the liver. Meanwhile, hepatic GHR overexpression promoted lipolysis in white adipose tissue and repressed glucose utilization in skeletal muscle by promoting the circulating level of RBP4, which contributed to impaired systemic insulin action. A mechanistic study revealed that hepatic GHR disrupted systemic insulin sensitivity by increasing RBP4 transcription by activating STAT5. Additionally, overexpression of hepatic GHR promoted TTR transcriptional levels by enhancing the expression of HIF1α, which not only increased the protein stability of RBP4 but also inhibited renal clearance of RBP4 in serum.

Conclusions: Hepatic GHR overexpression and activation accelerated systemic insulin resistance by increasing hepatic RBP4 production and maintaining circulating RBP4 homeostasis. Our current study provides novel insights into the pathogenesis of type 2 diabetes and its associated metabolic complications.

Missense variant in insulin receptor (Y1355H) segregates in family with fatty liver disease

A missense variant in the cytoplasmic domain of the insulin receptor (INSR) was identified by exome sequencing in affected members of a four-generation family with fatty liver disease (FLD). The variant (rs766457461, c.4063T>C, p.Y1355H) results in the substitution of histidine for a tyrosine that undergoes autophosphorylation in response to insulin stimulation in vitro. Because insulin promotes lipogenesis in hepatocytes, we hypothesized that the variant was causally linked to FLD in the family. To test this hypothesis, we used CRISPR/Cas9 technology to replace the corresponding tyrosine in mouse INSR with histidine (Y1345H). No significant differences were found in hepatic insulin signaling, as assessed by phosphorylation of INSR or AKT levels or in activation of the insulin-responsive transcription factor SREBP-1c. Glucose tolerance and hepatic triglyceride (TG) content in Insr^1345H/H mice fed a chow diet or diets rich in fat, sucrose or fructose did not differ significantly from WT littermates. Thus, our studies in mice failed to support the notion that INSR (Y1355H) is causally related to FLD in the family or that phosphorylation of this residue alters hepatic TG metabolism.

Disrupting the ghrelin-growth hormone axis limits ghrelin's orexigenic but not glucoregulatory actions

Objective

Acyl-ghrelin regulates eating, body weight, blood glucose, and GH secretion upon binding to its receptor GHSR (growth hormone secretagogue receptor; ghrelin receptor). GHSR is distributed in several brain regions and some peripheral cell-types including pituitary somatotrophs. The objective of the current study was to determine the functional significance of acyl-ghrelin's action on GHSR-expressing somatotrophs in mediating GH secretion and several of acyl-ghrelin's metabolic actions.

Methods

GH-IRES-Cre mice and loxP-flanked (floxed) GHSR mice were newly developed and then crossed to one another to generate mice that lacked GHSR selectively from somatotrophs. Following validation of mice with somatotroph-selective GHSR deletion, metabolic responses of these mice and control littermates were assessed following both acute and chronic acyl-ghrelin administration, a 24-h fast, and a prolonged 60% chronic caloric restriction protocol modeling starvation.

Results

In mice with somatotroph-selective GHSR deletion, a single peripheral injection of acyl-ghrelin failed to induce GH secretion or increase food intake, unlike wild-type and other littermate control groups. However, the usual acute blood glucose increase in response to the acyl-ghrelin bolus was preserved. Similarly, chronic s.c. acyl-ghrelin administration to mice with somatotroph-selective GHSR deletion failed to increase plasma GH, food intake, or body weight. Physiologically elevating plasma acyl-ghrelin via a 24-h fast also failed to raise plasma GH and resulted in a limited hyperphagic response upon food reintroduction in mice with somatotroph-selective GHSR deletion, although those mice nonetheless did not exhibit an exaggerated reduction in blood glucose. Physiologically elevating plasma acyl-ghrelin via a 15-day caloric restriction protocol which provided only 40% of usual daily calories failed to raise plasma GH in mice with somatotroph-selective GHSR deletion, although those mice did not exhibit life-threatening hypoglycemia.

Conclusions

These results reveal that direct engagement of GHSR-expressing somatotrophs is required for a peripheral ghrelin bolus to acutely stimulate GH secretion and the actions of chronic acyl-ghrelin delivery and physiological plasma acyl-ghrelin elevations to increase plasma GH. These results also suggest that actions of acyl-ghrelin to increase food intake and body weight are reliant on direct activation of GHSRs expressed on somatotrophs. Furthermore, these results suggest that the glucoregulatory actions of acyl-ghrelin – in particular, its actions to raise blood glucose when acutely administered, prevent small blood glucose drops following a 24-h fast, and avert life-threatening hypoglycemia during an acute-on-chronic caloric restriction protocol – do not depend on GHSR expression by somatotrophs.

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Mice with pituitary somatotroph-selective GHSR deletion were generated.

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Somatotroph-expressed GHSRs mediate GH secretion and food intake after acute ghrelin.

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Body weight effects of chronic ghrelin infusion require somatotroph-expressed GHSRs.

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Somatotroph-expressed GHSRs enable GH to increase upon chronic caloric restriction.

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Mice lacking somatotroph GHSRs maintain euglycemia upon chronic caloric restriction.

A Balanced Act: The Effects of GH-GHR-IGF1 Axis on Mitochondrial Function

Mitochondrial function is multifaceted in response to cellular energy homeostasis and metabolism, with the generation of adenosine triphosphate (ATP) through the oxidative phosphorylation (OXPHOS) being one of their main functions. Selective elimination of mitochondria by mitophagy, in conjunction with mitochondrial biogenesis, regulates mitochondrial function that is required to meet metabolic demand or stress response. Growth hormone (GH) binds to the GH receptor (GHR) and induces the JAK2/STAT5 pathway to activate the synthesis of insulin-like growth factor 1 (IGF1). The GH–GHR–IGF1 axis has been recognized to play significant roles in somatic growth, including cell proliferation, differentiation, division, and survival. In this review, we describe recent discoveries providing evidence for the contribution of the GH–GHR–IGF1 axis on mitochondrial biogenesis, mitophagy (or autophagy), and mitochondrial function under multiple physiological conditions. This may further improve our understanding of the effects of the GH–GHR–IGF1 axis on mitochondrial function, which may be controlled by the delicate balance between mitochondrial biogenesis and mitophagy. Specifically, we also highlight the challenges that remain in this field.

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.

"A LEAP 2 conclusions? Targeting the ghrelin system to treat obesity and diabetes"

BACKGROUND

The hormone ghrelin stimulates food intake, promotes adiposity, increases body weight, and elevates blood glucose. Consequently, alterations in plasma ghrelin levels and the functioning of other components of the broader ghrelin system have been proposed as potential contributors to obesity and diabetes. Furthermore, targeting the ghrelin system has been proposed as a novel therapeutic strategy for obesity and diabetes.

SCOPE OF REVIEW

The current review focuses on the potential for targeting ghrelin and other proteins comprising the ghrelin system as a treatment for obesity and diabetes. The main components of the ghrelin system are introduced. Data supporting a role for the endogenous ghrelin system in the development of obesity and diabetes along with data that seemingly refute such a role are outlined. An argument for further research into the development of ghrelin system-targeted therapeutic agents is delineated. Also, an evidence-based discussion of potential factors and contexts that might influence the efficacy of this class of therapeutics is provided.

MAJOR CONCLUSIONS

It would not be a "leap to" conclusions to suggest that agents which target the ghrelin system - including those that lower acyl-ghrelin levels, raise LEAP2 levels, block GHSR activity, and/or raise desacyl-ghrelin signaling - could represent efficacious novel treatments for obesity and diabetes.

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.

Effects of gastric sleeve surgery on the serum levels of GH, IGF-1 and IGF-binding protein 2 in healthy obese patients

Background

Bariatric surgery is an effective treatment for severe obesity. It also ameliorates diabetes independently of weight loss through mechanisms that are not fully understood. In this study, we investigated the levels of GH, IGF-1 and IGF-binding protein 2 (IGFBP-2) after gastric sleeve surgery in healthy obese individuals.

Method

This study was conducted in 33 obese (BMI > 38.3) healthy male subjects aged 25 to 50 years undergoing sleeve gastrectomy. GH, IGF-1 and IGFBP-2 levels were evaluated by ELISA at baseline and 6–12 months after surgery. Other parameters, such as glucose, BMI, insulin, HOMA-IR and lipid profile, were also investigated.

Results

Systemic GH (12.32 vs. 50.97 pg/mL, p < 0.001) and IGFBP-2 levels (51.86 vs. 68.81 pg/mL, p < 0.001) were elevated after bariatric surgery. There was no change in IGF-1 level from before to after surgery. BMI (52.18 vs. 40.11, p = 0.001), insulin (19.35 vs. 8.80 mIU/L, p < 0.001) and HOMA-IR index (6.48 to 2.52, p < 0.001) were reduced after surgery. Lipid profile analysis revealed that total cholesterol (4.26 vs. 5.12 mmol/L, p < 0.001) and high-density lipoprotein (HDL) (0.90 to 1.55 mmol/L, p < 0.001) were increased, while triglycerides were decreased, after surgery (1.62 vs. 1.05 mmol/L p < 0.001). GH, IGF-1, and IGFBP-2 were not correlated with insulin or lipid parameters.

Conclusions

Our study suggests that improved circulating GH and IGFBP-2 levels may mediate the beneficial effects of gastric sleeve surgery in improving insulin sensitivity and reducing insulin demand.

Ghrelin-insulin-like growth factor-1 axis is activated via autonomic neural circuits in the non-alcoholic fatty liver disease

BACKGROUND

The correlation of the growth hormone (GH) and insulin-like growth factor-1 (IGF-1) with non-alcoholic fatty liver disease (NAFLD) has been reported in epidemiological studies. However, the mechanisms of molecular and inter-organ systems that render these factors to influence on NAFLD have not been elucidated. In this study, we examined the induction of ghrelin which is the GH-releasing hormone and IGF-1, and involvement of autonomic neural circuits, in the pathogenesis of NAFLD.

METHODS

The expression of gastric and hypothalamic ghrelin, neural activation in the brain, and serum IGF-1 were examined in NAFLD models of choline-deficient defined l-amino-acid diet-fed, melanocortin 4 receptor knockout mice, and partial hepatectomy mice with or without the blockades of autonomic nerves to test the contribution of neural circuits connecting the brain, liver, and stomach.

KEY RESULTS

The fatty changes in the liver increased the expression of gastric ghrelin through the autonomic pathways which sends the neural signals to the arcuate nucleus in the hypothalamus through the afferent vagal nerve which reached the pituitary gland to release GH and then stimulate the IGF-1 release from the liver. In addition, high levels of ghrelin expression in the arcuate nucleus were correlated with NAFLD progression regardless of the circuits.

CONCLUSIONS

Our study demonstrated that the fatty liver stimulates the autonomic nervous signal circuits which suppress the progression of the disease by activating the gastric ghrelin expression, the neural signal transduction in the brain, and the release of IGF-1 from the liver.

Metabolic insights from a GHSR-A203E mutant mouse model

Objective

Binding of ghrelin to its receptor, growth hormone secretagogue receptor (GHSR), stimulates GH release, induces eating, and increases blood glucose. These processes may also be influenced by constitutive (ghrelin-independent) GHSR activity, as suggested by findings in short people with naturally occurring GHSR-A204E mutations and reduced food intake and blood glucose in rodents administered GHSR inverse agonists, both of which impair constitutive GHSR activity. In this study, we aimed to more fully determine the physiologic relevance of constitutive GHSR activity.

Methods

We generated mice with a GHSR mutation that replaces alanine at position 203 with glutamate (GHSR-A203E), which corresponds to the previously described human GHSR-A204E mutation, and used them to conduct ex vivo neuronal electrophysiology and in vivo metabolic assessments. We also measured signaling within COS-7 and HEK293T cells transfected with wild-type GHSR (GHSR-WT) or GHSR-A203E constructs.

Results

In COS-7 cells, GHSR-A203E resulted in lower baseline IP₃ accumulation than GHSR-WT; ghrelin-induced IP₃ accumulation was observed in both constructs. In HEK293T cells co-transfected with voltage-gated Ca_V2.2 calcium channel complex, GHSR-A203E had no effect on basal Ca_V2.2 current density while GHSR-WT did; both GHSR-A203E and GHSR-WT inhibited Ca_V2.2 current in the presence of ghrelin. In cultured hypothalamic neurons from GHSR-A203E and GHSR-deficient mice, native calcium currents were greater than those in neurons from wild-type mice; ghrelin inhibited calcium currents in cultured hypothalamic neurons from both GHSR-A203E and wild-type mice. In brain slices, resting membrane potentials of arcuate NPY neurons from GHSR-A203E mice were hyperpolarized compared to those from wild-type mice; the same percentage of arcuate NPY neurons from GHSR-A203E and wild-type mice depolarized upon ghrelin exposure. The GHSR-A203E mutation did not significantly affect body weight, body length, or femur length in the first ∼6 months of life, yet these parameters were lower in GHSR-A203E mice after 1 year of age. During a 7-d 60% caloric restriction regimen, GHSR-A203E mice lacked the usual marked rise in plasma GH and demonstrated an exaggerated drop in blood glucose. Administered ghrelin also exhibited reduced orexigenic and GH secretagogue efficacies in GHSR-A203E mice.

Conclusions

Our data suggest that the A203E mutation ablates constitutive GHSR activity and that constitutive GHSR activity contributes to the native depolarizing conductance of GHSR-expressing arcuate NPY neurons. Although the A203E mutation does not block ghrelin-evoked signaling as assessed using in vitro and ex vivo models, GHSR-A203E mice lack the usual acute food intake response to administered ghrelin in vivo. The GHSR-A203E mutation also blunts GH release, and in aged mice leads to reduced body length and femur length, which are consistent with the short stature of human carriers of the GHSR-A204E mutation.

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We generated mice with a GHSR mutation replacing Ala at position 203 with Glu.

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The A203E mutation ablates constitutive GHSR activity & hyperpolarizes NPY neurons.

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GHSR-A203E mice lack the usual orexigenic response to administered ghrelin.

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The GHSR-A203E mutation blunts GH release and causes reduced body length.

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This finding is consistent with short stature in human carriers of the GHSR-A204E mutation.

The new mechanism of Ghrelin/GHSR-1a on autophagy regulation

Autophagy is associated with several diseases. In recent years, accumulating evidence has suggested that ghrelin pathway exerts a protective effect by regulating autophagy. This review aims to assess the potential role and use of ghrelin as a new treatment for obesity, cardiovascular diseases, nonalcoholic fatty liver disease (NFALD), neurodegenerative diseases, and tissue damage associated with autophagy. Ghrelin reduces the basal expression of autophagy-related genes in obesity-associated type 2 diabetes and ghrelin level changes in obesity, heart failure, and NFALD as well as altered autophagy. Ghrelin and its receptor GHSR-1 activation induce the phosphorylation of ERK1/2 and the induction of PI-3 kinase (PI3 K) and phosphorylation of Akt. In the myocardium and hypothalamic NPY/AgRP neurons, ghrelin increases levels of the intracellular energy sensor AMPK and enhances autophagy, protecting cardiac ischemia and inducing neural stem cells. Nonetheless, ghrelin activates the PI3 K/Akt/Bcl-2 pathway and inhibits the activation of autophagy, such as tissues injured by sepsis or doxorubicin. In conclusion, endogenous ghrelin system could be considered as a new target or treatment for metabolism disorders, cardiac diseases, neurodegenerative diseases, and tissue injuries.

The Complex Signaling Pathways of the Ghrelin Receptor

The ghrelin receptor (GhrR) is known for its strong orexigenic effects in pharmacological doses and has long been considered as a promising target for the treatment of obesity. Several antagonists have been developed to decrease the orexigenic signaling, but none of these have been approved for the treatment of obesity because of adverse effects and lack of efficacy. Heterodimerization and biased signaling are important concepts for G-protein coupled receptor (GPCR) signaling, and the influence of these aspects on the GhrR may be important for feeding behavior and obesity. GhrR has been described to heterodimerize with other GPCRs, such as the dopamine receptors 1 and 2, leading to a modulation of the signaling properties of both dimerization partners. Another complicating factor of GhrR-mediated signaling is its ability to activate several different signaling pathways on ligand stimulation. Importantly, some ligands have shown to be "biased" or "functionally selective," implying that the ligand favors a particular signaling pathway. These unique signaling properties could have a sizeable impact on the physiological functions of the GhrR system. Importantly, heterodimerization may explain why the GhrR is expressed in areas of the brain that are difficult for peptide ligands to access. One possibility is that the purpose of GhrR expression is to modulate the function of other receptors in addition to merely being independently activated. We suggest that a deeper understanding of the signaling properties of the GhrR will facilitate future drug discovery in the areas of obesity and weight management.

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.

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.

A new action of peptide hormones for survival in a low-nutrient environment

Malnutrition occurs when nutrient intake is too low for any reason and occurs regardless of gender or age. Therefore, besides loss of eating or digestive functionality due to illness, malnutrition can occur when a healthy individual undergoes an extreme diet and biases their nutrition, or when athletes exerts more energy than they can replenish through food. It has recently been reported that in Japan, the mortality rate of leaner individuals is equal to or higher than that of obese people. It is important to understand what homeostatic maintenance mechanism is behind this when the body is under hypotrophic conditions. Such mechanisms are generally endocranially controlled. We address this fundamental concern in this paper by focusing on peptide hormones. We introduce a mechanism for survival in a malnourished state via the regulation of food intake and temperature. Additionally, we will discuss the latest findings and future prospects for research on changes in the endocrine environment associated with malnutrition associated with exercise. We also review changes in next-generation endocrine environments when caused by malnutrition brought on by dieting.

Autophagy mediates hepatic GRK2 degradation to facilitate glucagon-induced metabolic adaptation to fasting

The liver plays a key role during fasting to maintain energy homeostasis and euglycemia via metabolic processes mainly orchestrated by the insulin/glucagon ratio. We report here that fasting or calorie restriction protocols in C57BL6 mice promote a marked decrease in the hepatic protein levels of G protein-coupled receptor kinase 2 (GRK2), an important negative modulator of both G protein-coupled receptors (GPCRs) and insulin signaling. Such downregulation of GRK2 levels is liver-specific and can be rapidly reversed by refeeding. We find that autophagy, and not the proteasome, represents the main mechanism implicated in fasting-induced GRK2 degradation in the liver in vivo. Reducing GRK2 levels in murine primary hepatocytes facilitates glucagon-induced glucose production and enhances the expression of the key gluconeogenic enzyme Pck1. Conversely, preventing full downregulation of hepatic GRK2 during fasting using adenovirus-driven overexpression of this kinase in the liver leads to glycogen accumulation, decreased glycemia, and hampered glucagon-induced gluconeogenesis, thus preventing a proper and complete adaptation to nutrient deprivation. Overall, our data indicate that physiological fasting-induced downregulation of GRK2 in the liver is key for allowing complete glucagon-mediated responses and efficient metabolic adaptation to fasting in vivo.

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.

Using CRISPR/Cas9 to model human liver disease

CRISPR/Cas9 gene editing has revolutionised biomedical research. The ease of design has allowed many groups to apply this technology for disease modelling in animals. While the mouse remains the most commonly used organism for embryonic editing, CRISPR is now increasingly performed with high efficiency in other species. The liver is also amenable to somatic genome editing, and some delivery methods already allow for efficient editing in the whole liver. In this review, we describe CRISPR-edited animals developed for modelling a broad range of human liver disorders, such as acquired and inherited hepatic metabolic diseases and liver cancers. CRISPR has greatly expanded the repertoire of animal models available for the study of human liver disease, advancing our understanding of their pathophysiology and providing new opportunities to develop novel therapeutic approaches.