Direct and systemic actions of growth hormone receptor (GHR)-signaling on hepatic glycolysis, de novo lipogenesis and insulin sensitivity, associated with steatosis.

BACKGROUND: Evidence is accumulating that growth hormone (GH) protects against the development of steatosis and progression of non-alcoholic fatty liver disease (NAFLD). GH may control steatosis indirectly by altering systemic insulin sensitivity and substrate delivery to the liver and/or by the direct actions of GH on hepatocyte function. APPROACH: To better define the hepatocyte-specific role of GH receptor (GHR) signaling on regulating steatosis, we used a mouse model with adult-onset, hepatocyte-specific GHR knockdown (aHepGHRkd). To prevent the reduction in circulating insulin-like growth factor 1 (IGF1) and the subsequent increase in GH observed after aHepGHRkd, subsets of aHepGHRkd mice were treated with adeno-associated viral vectors (AAV) driving hepatocyte-specific expression of IGF1 or a constitutively active form of STAT5b (STAT5bCA). The impact of hepatocyte-specific modulation of GHR, IGF1 and STAT5b on carbohydrate and lipid metabolism was studied across multiple nutritional states and in the context of hyperinsulinemic:euglycemic clamps. RESULTS: Chow-fed male aHepGHRkd mice developed steatosis associated with an increase in hepatic glucokinase (GCK) and ketohexokinase (KHK) expression and de novo lipogenesis (DNL) rate, in the post-absorptive state and in response to refeeding after an overnight fast. The aHepGHRkd-associated increase in hepatic KHK, but not GCK and steatosis, was dependent on hepatocyte expression of carbohydrate response element binding protein (ChREBP), in re-fed mice. Interestingly, under clamp conditions, aHepGHRkd also increased the rate of DNL and expression of GCK and KHK, but impaired insulin-mediated suppression of hepatic glucose production, without altering plasma NEFA levels. These effects were normalized with AAV-mediated hepatocyte expression of IGF1 or STAT5bCA. Comparison of the impact of AAV-mediated hepatocyte IGF1 versus STAT5bCA in aHepGHRkd mice across multiple nutritional states, indicated the restorative actions of IGF1 are indirect, by improving systemic insulin sensitivity, independent of changes in the liver transcriptome. In contrast, the actions of STAT5b are due to the combined effects of raising IGF1 and direct alterations in the hepatocyte gene program that may involve suppression of BCL6 and FOXO1 activity. However, the direct and IGF1-dependent actions of STAT5b cannot fully account for enhanced GCK activity and lipogenic gene expression observed after aHepGHRkd, suggesting other GHR-mediated signals are involved. CONCLUSION: These studies demonstrate hepatocyte GHR-signaling controls hepatic glycolysis, DNL, steatosis and hepatic insulin sensitivity indirectly (via IGF1) and directly (via STAT5b). The relative contribution of these indirect and direct actions of GH on hepatocytes is modified by insulin and nutrient availability. These results improve our understanding of the physiologic actions of GH on regulating adult metabolism to protect against NAFLD progression.

Liver insulin-like growth factor-1 mediates effects of low-intensity vibration on wound healing in diabetic mice.

Chronic wounds in diabetic patients are associated with significant morbidity and mortality; however, few therapies are available to improve healing of diabetic wounds. Our group previously reported that low-intensity vibration (LIV) could improve angiogenesis and wound healing in diabetic mice. The purpose of this study was to begin to elucidate the mechanisms underlying LIV-enhanced healing. We first demonstrate that LIV-enhanced wound healing in db/db mice is associated with increased IGF1 protein levels in liver, blood, and wounds. The increase in insulin-like growth factor (IGF) 1 protein in wounds is associated with increased Igf1 mRNA expression both in liver and wounds, but the increase in protein levels preceded the increase in mRNA expression in wounds. Since our previous study demonstrated that liver was a primary source of IGF1 in skin wounds, we used inducible ablation of IGF1 in the liver of high-fat diet (HFD)-fed mice to determine whether liver IGF1 mediated the effects of LIV on wound healing. We demonstrate that knockdown of IGF1 in liver blunts LIV-induced improvements in wound healing in HFD-fed mice, particularly increased angiogenesis and granulation tissue formation, and inhibits the resolution of inflammation. This and our previous studies indicate that LIV may promote skin wound healing at least in part via crosstalk between the liver and wound. © 2023 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

Sex matters in liver fat regulation.

Growth hormone orchestrates a complex, sex-dependent balancing act.

Constitutively Active STAT5b Feminizes Mouse Liver Gene Expression.

STAT5 is an essential transcriptional regulator of the sex-biased actions of GH in the liver. Delivery of constitutively active STAT5 (STAT5CA) to male mouse liver using an engineered adeno-associated virus with high tropism for the liver is shown to induce widespread feminization of the liver, with extensive induction of female-biased genes and repression of male-biased genes, largely mimicking results obtained when male mice are given GH as a continuous infusion. Many of the STAT5CA-responding genes were associated with nearby (< 50 kb) sites of STAT5 binding to liver chromatin, supporting the proposed direct role of persistently active STAT5 in continuous GH-induced liver feminization. The feminizing effects of STAT5CA were dose-dependent; moreover, at higher levels, STAT5CA overexpression resulted in some histopathology, including hepatocyte hyperplasia, and increased karyomegaly and multinuclear hepatocytes. These findings establish that the persistent activation of STAT5 by GH that characterizes female liver is by itself sufficient to account for the sex-dependent expression of a majority of hepatic sex-biased genes. Moreover, histological changes seen when STAT5CA is overexpressed highlight the importance of carefully evaluating such effects before considering STAT5 derivatives for therapeutic use in treating liver disease.

Growth Hormone and Insulin-Like Growth Factor 1 Regulation of Nonalcoholic Fatty Liver Disease.

Patients with obesity have a high prevalence of nonalcoholic fatty liver disease (NAFLD), representing a spectrum of simple steatosis to nonalcoholic steatohepatitis (NASH), without and with fibrosis. Understanding the etiology of NAFLD is clinically relevant since NAFLD is an independent risk factor for diabetes and cardiovascular disease. In addition, NASH predisposes patients to the development of cirrhosis and hepatocellular carcinoma, and NASH cirrhosis represents the fastest growing indication for liver transplantation in the United States. It is appreciated that multiple factors are involved in the development and progression of NAFLD. Growth hormone (GH) and insulin-like growth factor 1 (IGF1) regulate metabolic, immune, and hepatic stellate cell function, and alterations in the production and function of GH is associated with obesity and NAFLD/NASH. Therefore, this review will focus on the potential role of GH and IGF1 in the regulation of hepatic steatosis, inflammation, and fibrosis.

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.

Liver is a primary source of insulin-like growth factor-1 in skin wound healing.

Insulin-like growth factor (IGF)-1 plays important role in tissue repair through its ability to stimulate wound cell activity. While IGF-1 is expressed locally by wound cells, liver-derived IGF-1 is also present at high levels in the circulation, and the contributions of local vs circulating IGF-1 to wound levels remain undefined. The hypothesis of this study was that liver is a primary source of IGF-1 during skin wound healing. To test this hypothesis, we utilized a model that allows inducible ablation of IGF-1 specifically in liver of adult mice. We demonstrate that ablation of liver IGF-1 leads to >85% loss of circulating IGF-1 and ~60% decrease in wound IGF-1 during the proliferative phase of healing in both male and female mice. This reduction of liver-derived IGF-1 did not alter local mRNA expression of Igf1 in wounds. Knockdown of liver IGF-1 significantly delayed wound re-epithelialization and reduced granulation tissue formation and collagen deposition. Knockdown of liver IGF-1 also significantly reduced angiogenesis and resulted in persistent macrophage accumulation. In summary, liver is a primary source of IGF-1 in skin wounds and contributes to many aspects of both epithelial and dermal healing.

Rosiglitazone Requires Hepatocyte PPARγ Expression to Promote Steatosis in Male Mice With Diet-Induced Obesity.

Thiazolidinediones (TZD) are peroxisome proliferator-activated receptor γ (PPARγ) agonists that may reduce hepatic steatosis through their effects in adipose tissue and therefore have been assessed as potential therapies to treat nonalcoholic fatty liver disease (NAFLD) in humans. However, some studies suggest that expression and activation of hepatocyte PPARγ promotes steatosis and that would limit the benefits of TZD as a NAFLD therapy. To further explore this possibility, we examined the impact of short-term rosiglitazone maleate treatment after the development of moderate or severe diet-induced obesity, in both control and adult-onset hepatocyte-specific PPARγ knockout (PpargΔHep) mice. Independent of the level of obesity and hepatic PPARγ expression, the TZD treatment enhanced insulin sensitivity, associated with an increase in white adipose tissue (WAT) fat accumulation, consistent with clinical observations. However, TZD treatment increased hepatic triglyceride content only in control mice with severe obesity. Under these conditions, PpargΔHep reduced diet-induced steatosis and prevented the steatogenic effects of short-term TZD treatment. In these mice, subcutaneous WAT was enlarged and associated with increased levels of adiponectin, while hepatic levels of phosphorylated adenosine 5'-monophosphate-activated protein kinase were also increased. In addition, in mice with severe obesity, the expression of hepatic Cd36, Cidea, Cidec, Fabp4, Fasn, and Scd-1 was increased by TZD in a PPARγ-dependent manner. Taken together, these results demonstrate that hepatocyte PPARγ expression offsets the antisteatogenic actions of TZD in mice with severe obesity. Therefore, in obese and insulin resistant humans, TZD-mediated activation of hepatocyte PPARγ may limit the therapeutic potential of TZD to treat NAFLD.

Sexual dimorphic impact of adult-onset somatopause on life span and age-induced osteoarthritis.

Osteoarthritis (OA), the most prevalent joint disease, is a major cause of disability worldwide. Growth hormone (GH) has been suggested to play significant roles in maintaining articular chondrocyte function and ultimately articular cartilage (AC) homeostasis. In humans, the age-associated decline in GH levels was hypothesized to play a role in the etiology of OA. We studied the impact of adult-onset isolated GH deficiency (AOiGHD) on the life span and skeletal integrity including the AC, in 23- to 30-month-old male and female mice on C57/BL6 genetic background. Reductions in GH during adulthood were associated with extended life span and reductions in body temperature in female mice only. However, end-of-life pathology revealed high levels of lymphomas in both sexes, independent of GH status. Skeletal characterization revealed increases in OA severity in AOiGHD mice, evidenced by AC degradation in both femur and tibia, and significantly increased osteophyte formation in AOiGHD females. AOiGHD males showed significant increases in the thickness of the synovial lining cell layer that was associated with increased markers of inflammation (IL-6, iNOS). Furthermore, male AOiGHD showed significant increases in matrix metalloproteinase-13 (MMP-13), p16, and ß-galactosidase immunoreactivity in the AC as compared to controls, indicating increased cell senescence. In conclusion, while the life span of AOiGHD females increased, their health span was compromised by high-grade lymphomas and the development of severe OA. In contrast, AOiGHD males, which did not show extended life span, showed an overall low grade of lymphomas but exhibited significantly decreased health span, evidenced by increased OA severity.

Parameter-Dependency of Low-Intensity Vibration for Wound Healing in Diabetic Mice.

Chronic wounds in diabetic patients represent an escalating health problem, leading to significant morbidity and mortality. Our group previously reported that whole body low-intensity vibration (LIV) can improve angiogenesis and wound healing in diabetic mice. The purpose of the current study was to determine whether effects of LIV on wound healing are frequency and/or amplitude dependent. Wound healing was assessed in diabetic (db/db) mice exposed to one of four LIV protocols with different combinations of two acceleration magnitudes (0.3 and 0.6 g) and two frequencies (45 and 90 Hz) or in non-vibration sham controls. The low acceleration, low frequency protocol (0.3 g and 45 Hz) was the only one that improved wound healing, increasing angiogenesis and granulation tissue formation, leading to accelerated re-epithelialization and wound closure. Other protocols had little to no impact on healing with some evidence that 0.6 g accelerations negatively affected wound closure. The 0.3 g, 45 Hz protocol also increased levels of insulin-like growth factor-1 and tended to increase levels of vascular endothelial growth factor in wounds, but had no effect on levels of basic fibroblast growth factor or platelet derived growth factor-bb, indicating that this LIV protocol induces specific growth factors during wound healing. Our findings demonstrate parameter-dependent effects of LIV for improving wound healing that can be exploited for future mechanistic and therapeutic studies.

GH directly inhibits steatosis and liver injury in a sex-dependent and IGF1-independent manner.

A reduction in hepatocyte growth hormone (GH)-signaling promotes non-alcoholic fatty liver disease (NAFLD). However, debate remains as to the relative contribution of the direct effects of GH on hepatocyte function vs indirect effects, via alterations in insulin-like growth factor 1 (IGF1). To isolate the role of hepatocyte GH receptor (GHR) signaling, independent of changes in IGF1, mice with adult-onset, hepatocyte-specific GHR knockdown (aHepGHRkd) were treated with a vector expressing rat IGF1 targeted specifically to hepatocytes. Compared to GHR-intact mice, aHepGHRkd reduced circulating IGF1 and elevated GH. In male aHepGHRkd, the shift in IGF1/GH did not alter plasma glucose or non-esterified fatty acids (NEFA), but was associated with increased insulin, enhanced systemic lipid oxidation and reduced white adipose tissue (WAT) mass. Livers of male aHepGHRkd exhibited steatosis associated with increased de novo lipogenesis, hepatocyte ballooning and inflammation. In female aHepGHRkd, hepatic GHR protein levels were not detectable, but moderate levels of IGF1 were maintained, with minimal alterations in systemic metabolism and no evidence of steatosis. Reconstitution of hepatocyte IGF1 in male aHepGHRkd lowered GH and normalized insulin, whole body lipid utilization and WAT mass. However, IGF1 reconstitution did not reduce steatosis or eliminate liver injury. RNAseq analysis showed IGF1 reconstitution did not impact aHepGHRkd-induced changes in liver gene expression, despite changes in systemic metabolism. These results demonstrate the impact of aHepGHRkd is sexually dimorphic and the steatosis and liver injury observed in male aHepGHRkd mice is autonomous of IGF1, suggesting GH acts directly on the adult hepatocyte to control NAFLD progression.

Statins Directly Regulate Pituitary Cell Function and Exert Antitumor Effects in Pituitary Tumors.

INTRODUCTION: Pituitary neuroendocrine tumors (PitNETs), the most abundant of all intracranial tumors, entail severe comorbidities. First-line therapy is transsphenoidal surgery, but subsequent pharmacological therapy is often required. Unfortunately, many patients are/become unresponsive to available drugs (somatostatin analogues [SSAs]/dopamine agonists), underscoring the need for new therapies. Statins are well-known drugs commonly prescribed to treat hyperlipidemia/cardiovascular diseases, but can convey additional beneficial effects, including antitumor actions. The direct effects of statins on normal human pituitary or PitNETs are poorly known. Thus, we aimed to explore the direct effects of statins, especially simvastatin, on key functional parameters in normal and tumoral pituitary cells, and to evaluate the combined effects of simvastatin with metformin (MF) or SSAs. METHODS: Effects of statins in cell proliferation/viability, hormone secretion, and signaling pathways were evaluated in normal pituitary cells from a primate model (Papio anubis), tumor cells from corticotropinomas, somatotropinomas, nonfunctioning pituitary tumors, and PitNET cell-lines (AtT20/GH3-cells). RESULTS: All statins decreased AtT20-cell proliferation, simvastatin showing stronger effects. Indeed, simvastatin reduced cell viability and/or hormone secretion in all PitNETs subtypes and cell-lines, and ACTH/GH/PRL/FSH/LH secretion (but not expression), in primate cell cultures, by modulating MAPK/PI3K/mTOR pathways and expression of key receptors (GH-releasing hormone-receptor/ghrelin-R/Kiss1-R) regulating pituitary function. Addition of MF or SSAs did not enhance simvastatin antitumor effects. CONCLUSION: Our data reveal direct antitumor effects of simvastatin on PitNET-cells, paving the way to explore these compounds as a possible tool to treat PitNETs.

Imaging and Manipulating Pituitary Function in the Awake Mouse.

Extensive efforts have been made to explore how the activities of multiple brain cells combine to alter physiology through imaging and cell-specific manipulation in different animal models. However, the temporal regulation of peripheral organs by the neuroendocrine factors released by the brain is poorly understood. We have established a suite of adaptable methodologies to interrogate in vivo the relationship of hypothalamic regulation with the secretory output of the pituitary gland, which has complex functional networks of multiple cell types intermingled with the vasculature. These allow imaging and optogenetic manipulation of cell activities in the pituitary gland in awake mouse models, in which both neuronal regulatory activity and hormonal output are preserved. These methodologies are now readily applicable for longitudinal studies of short-lived events (e.g., calcium signals controlling hormone exocytosis) and slowly evolving processes such as tissue remodeling in health and disease over a period of days to weeks.

Tissue-dependent effects of cis-9,trans-11- and trans-10,cis-12-CLA isomers on glucose and lipid metabolism in adult male mice.

Mixtures of the two major conjugated linoleic acid (CLA) isomers trans-10,cis-12-CLA and cis-9,trans-11-CLA are used as over the counter supplements for weight loss. Because of the reported adverse effects of CLA on insulin sensitivity in some mouse studies, we sought to compare the impact of dietary t10c12-CLA and c9t11-CLA on liver, adipose tissue, and systemic metabolism of adult lean mice. We fed 8 week-old C57Bl/6J male mice with low fat diets (10.5% Kcal from fat) containing 0.8% t10c12-CLA or c9t11-CLA for 9 or 38 days. Diets containing c9t11-CLA had minimal impact on the endpoints studied. However, 7 days after starting the t10c12-CLA diet, we observed a dramatic reduction in fat mass measured by NMR spectroscopy, which interestingly rebounded by 38 days. This rebound was apparently due to a massive accumulation of lipids in the liver, because adipose tissue depots were visually undetectable. Hepatic steatosis and the disappearance of adipose tissue after t10c12-CLA feeding was associated with elevated plasma insulin levels and insulin resistance, compared to mice fed a control diet or c9t11-CLA diet. Unexpectedly, despite being insulin resistant, mice fed t10c12-CLA had normal levels of blood glucose, without signs of impaired glucose clearance. Hepatic gene expression and fatty acid composition suggested enhanced hepatic de novo lipogenesis without an increase in expression of gluconeogenic genes. These data indicate that dietary t10c12-CLA may alter hepatic glucose and lipid metabolism indirectly, in response to the loss of adipose tissue in mice fed a low fat diet.

Dysregulation of the Splicing Machinery Is Associated to the Development of Nonalcoholic Fatty Liver Disease.

CONTEXT: Nonalcoholic fatty liver disease (NAFLD) is a common obesity-associated pathology characterized by hepatic fat accumulation, which can progress to fibrosis, cirrhosis, and hepatocellular carcinoma. Obesity is associated with profound changes in gene-expression patterns of the liver, which could contribute to the onset of comorbidities. OBJECTIVE: As these alterations might be linked to a dysregulation of the splicing process, we aimed to determine whether the dysregulation in the expression of splicing machinery components could be associated with NAFLD. PARTICIPANTS: We collected 41 liver biopsies from nonalcoholic individuals with obesity, with or without hepatic steatosis, who underwent bariatric surgery. INTERVENTIONS: The expression pattern of splicing machinery components was determined using a microfluidic quantitative PCR-based array. An in vitro approximation to determine lipid accumulation using HepG2 cells was also implemented. RESULTS: The liver of patients with obesity and steatosis exhibited a severe dysregulation of certain splicing machinery components compared with patients with obesity without steatosis. Nonsupervised clustering analysis allowed the identification of three molecular phenotypes of NAFLD with a unique fingerprint of alterations in splicing machinery components, which also presented distinctive hepatic and clinical-metabolic alterations and a differential response to bariatric surgery after 1 year. In addition, in vitro silencing of certain splicing machinery components (i.e., PTBP1, RBM45, SND1) reduced fat accumulation and modulated the expression of key de novo lipogenesis enzymes, whereas conversely, fat accumulation did not alter spliceosome components expression. CONCLUSION: There is a close relationship between splicing machinery dysregulation and NAFLD development, which should be further investigated to identify alternative therapeutic targets.

Adult-Onset Hepatocyte GH Resistance Promotes NASH in Male Mice, Without Severe Systemic Metabolic Dysfunction.

Nonalcoholic fatty liver disease (NAFLD), which includes nonalcoholic steatohepatitis (NASH), is associated with reduced GH input/signaling, and GH therapy is effective in the reduction/resolution of NAFLD/NASH in selected patient populations. Our laboratory has focused on isolating the direct vs indirect effects of GH in preventing NAFLD/NASH. We reported that chow-fed, adult-onset, hepatocyte-specific, GH receptor knockdown (aHepGHRkd) mice rapidly (within 7 days) develop steatosis associated with increased hepatic de novo lipogenesis (DNL), independent of changes in systemic metabolic function. In this study, we report that 6 months after induction of aHepGHRkd early signs of NASH develop, which include hepatocyte ballooning, inflammation, signs of mild fibrosis, and elevated plasma alanine aminotransferase. These changes occur in the presence of enhanced systemic lipid utilization, without evidence of white adipose tissue lipolysis, indicating that the liver injury that develops after aHepGHRkd is due to hepatocyte-specific loss of GH signaling and not due to secondary defects in systemic metabolic function. Specifically, enhanced hepatic DNL is sustained with age in aHepGHRkd mice, associated with increased hepatic markers of lipid uptake/re-esterification. Because hepatic DNL is a hallmark of NAFLD/NASH, these studies suggest that enhancing hepatocyte GH signaling could represent an effective therapeutic target to reduce DNL and treat NASH.

The Pituitary Gland is a Novel Major Site of Action of Metformin in Non-Human Primates: a Potential Path to Expand and Integrate Its Metabolic Actions.

BACKGROUND/AIMS: Biguanides are anti-hyperglycaemic agents used to treat diabetes by acting primarily on the liver, inhibiting hepatic gluconeogenesis. However, biguanides may target other key metabolic tissues to exert beneficial actions. As the "master endocrine gland", the pituitary is a true homeostatic sensor that controls whole body homeostasis and metabolism by integrating central and peripheral signals. However, whether the pituitary is a primary site of biguanides action in normal adult humans/primates remains unknown. Therefore, we aimed to elucidate the direct effects of two biguanides (metformin/phenformin) on the expression and secretion of all anterior pituitary hormones in two non-human primate species (Papio anubis and Macaca fascicularis), and the molecular/signalling-mechanisms behind these actions. METHODS: Primary pituitary cell cultures from baboons and macaques were used to determine the direct impact of metformin/phenformin (alone and combined with primary regulators) on the functioning of all pituitary cell-types (i.e. expression/secretion/signaling-pathways, etc). RESULTS: Metformin/phenformin inhibited basal, but not GHRH/ghrelin-stimulated GH/ACTH/ FSH-secretion and GH/POMC-expression, without altering secretion or expression of other pituitary hormones (PRL/LH/TSH), FSH-expression or cell viability in both primate models. These biguanide actions are likely mediated through modulation of: 1) common (mTOR/PI3K/intracellular-Ca2+mobilization) and distinct (MAPK) signaling pathways; and 2) gene expression of key receptors regulating somatotrope/corticotrope/gonadotrope function (i.e. upregulation of SSTR2/SSTR5/INSR/IGF1R/LEPR). CONCLUSION: The pituitary gland is a primary target of biguanide actions wherein they modulate somatotrope/corticotrope/gonadotrope-function through multiple molecular/signaling pathways in non-human primate-models. This suggests that the well-known metabolic effects of biguanides might be, at least in part, influenced by their actions at the pituitary level.

40 YEARS of IGF1: Understanding the tissue-specific roles of IGF1/IGF1R in regulating metabolism using the Cre/loxP system.

It is clear that insulin-like growth factor-1 (IGF1) is important in supporting growth and regulating metabolism. The IGF1 found in the circulation is primarily produced by the liver hepatocytes, but healthy mature hepatocytes do not express appreciable levels of the IGF1 receptor (IGF1R). Therefore, the metabolic actions of IGF1 are thought to be mediated via extra-hepatocyte actions. Given the structural and functional homology between IGF1/IGF1R and insulin receptor (INSR) signaling, and the fact that IGF1, IGF1R and INSR are expressed in most tissues of the body, it is difficult to separate out the tissue-specific contributions of IGF1/IGF1R in maintaining whole body metabolic function. To circumvent this problem, over the last 20 years, investigators have taken advantage of the Cre/loxP system to manipulate IGF1/IGF1R in a tissue-dependent, and more recently, an age-dependent fashion. These studies have revealed that IGF1/IGF1R can alter extra-hepatocyte function to regulate hormonal inputs to the liver and/or alter tissue-specific carbohydrate and lipid metabolism to alter nutrient flux to liver, where these actions are not mutually exclusive, but serve to integrate the function of all tissues to support the metabolic needs of the organism.

Neuronostatin exerts actions on pituitary that are unique from its sibling peptide somatostatin.

Neuronostatin, a somatostatin gene-encoded peptide, exerts important physiological and metabolic actions in diverse tissues. However, the direct biological effects of neuronostatin on pituitary function of humans and primates are still unknown. This study used baboon (Papio anubis) primary pituitary cell cultures, a species that closely models human physiology, to demonstrate that neuronostatin inhibits basal, but not ghrelin-/GnRH-stimulated, growth hormone (GH) and luteinizing hormone (LH) secretion in a dose- and time-dependent fashion, without affecting the secretion of other pituitary hormones (prolactin, ACTH, FSH, thyroid-stimulating hormone (TSH)) or changing mRNA levels. Actions of neuronostatin differs from somatostatin which in this study reduced GH/PRL/ACTH/LH/TSH secretion and GH/PRL/POMC/LH gene expression. Remarkably, we found that inhibitory actions of neuronostatin are likely mediated through: (1) the orphan receptor GPCR107 (found to be highly expressed in pituitary compared to somatostatin-receptors), (2) common (i.e. adenylyl cyclase/protein kinase A/MAPK/extra-/intracellular Ca2+ mobilization, but not phospholipase C/protein kinase C/mTOR) and distinct (i.e. PI3K) signaling pathways than somatostatin and; (3) dissimilar molecular mechanisms than somatostatin (i.e. upregulation of GPCR107 and downregulation of GHS-R/Kiss1-R expression by neuronostatin and, upregulation of sst1-5 expression by somatostatin). Altogether, the results of this study provide the first evidence that there is a functional neuronostatin signaling circuit, unique from somatostatin, which may work in concert with somatostatin to fine-tune hormone release from somatostropes and gonadotropes.

Somatotroph-Specific Aip-Deficient Mice Display Pretumorigenic Alterations in Cell-Cycle Signaling.

Patients with familial isolated pituitary adenoma are predisposed to pituitary adenomas, which in a subset of cases is due to germline inactivating mutations of the aryl hydrocarbon receptor-interacting protein (AIP) gene. Using Cre/lox and Flp/Frt technology, a conditional mouse model was generated to examine the loss of the mouse homolog, Aip, in pituitary somatotrophs. By 40 weeks of age, >80% of somatotroph specific Aip knockout mice develop growth hormone (GH) secreting adenomas. The formation of adenomas results in physiologic effects recapitulating the human syndrome of acromegaly, including increased body size, elevated serum GH and insulin-like growth factor 1 levels, and glucose intolerance. The pretumorigenic Aip-deficient somatotrophs secrete excess GH and exhibit pathologic hyperplasia associated with cytosolic compartmentalization of the cyclin-dependent kinase (CDK) inhibitor p27kip1 and perinuclear accentuation of CDK-4. Following tumor formation, the Aip-deficient somatotrophs display reduced expression of somatostatin receptor subtype 5 with impaired response to octreotide. The delayed tumor emergence, even with loss of both copies of Aip, implies that additional somatic events are required for adenoma formation. These findings suggest that pituitary hyperplasia precedes adenomatous transformation in somatotroph-specific Aip-deficient mice and reveal potential mechanisms involved in the pretumorigenic state that ultimately contribute to transformation.