Glucocorticoid regulation of insulin receptor and substrate IRS-1 tyrosine phosphorylation in rat skeletal muscle in vivo

Treating the Side Effects of Exogenous Glucocorticoids; Can We Separate the Good From the Bad?

It is estimated that 2% to 3% of the population are currently prescribed systemic or topical glucocorticoid treatment. The potent anti-inflammatory action of glucocorticoids to deliver therapeutic benefit is not in doubt. However, the side effects associated with their use, including central weight gain, hypertension, insulin resistance, type 2 diabetes (T2D), and osteoporosis, often collectively termed iatrogenic Cushing's syndrome, are associated with a significant health and economic burden. The precise cellular mechanisms underpinning the differential action of glucocorticoids to drive the desirable and undesirable effects are still not completely understood. Faced with the unmet clinical need to limit glucocorticoid-induced adverse effects alongside ensuring the preservation of anti-inflammatory actions, several strategies have been pursued. The coprescription of existing licensed drugs to treat incident adverse effects can be effective, but data examining the prevention of adverse effects are limited. Novel selective glucocorticoid receptor agonists and selective glucocorticoid receptor modulators have been designed that aim to specifically and selectively activate anti-inflammatory responses based upon their interaction with the glucocorticoid receptor. Several of these compounds are currently in clinical trials to evaluate their efficacy. More recently, strategies exploiting tissue-specific glucocorticoid metabolism through the isoforms of 11β-hydroxysteroid dehydrogenase has shown early potential, although data from clinical trials are limited. The aim of any treatment is to maximize benefit while minimizing risk, and within this review we define the adverse effect profile associated with glucocorticoid use and evaluate current and developing strategies that aim to limit side effects but preserve desirable therapeutic efficacy.

Secondary diabetes mellitus due to primary aldosteronism

Primary aldosteronism (PA) and diabetes mellitus (DM) are clinical conditions that increase cardiovascular risk. Approximately one in five patients with PA have DM. Nevertheless, the pathophysiology linking these two entities is not entirely understood. In addition, the majority of patients with PA have glucocorticoid co-secretion, which is associated with increased risk of impaired glucose homeostasis. In the present review, we aim to comprehensively discuss all the available research data concerning the interplay between mineralocorticoid excess and glucose metabolism, with separate analysis of the sequalae in muscle, adipose tissue, liver and pancreas. Aldosterone binds both mineralocorticoid and glucocorticoid receptors and amplifies tissue glucocorticoid activity, via 11-β-hydroxysteroid dehydrogenase type 1 stimulation. A clear classification of the molecular events as per specific receptor in insulin-sensitive tissues is impossible, while their synergistic interaction is plausible. Furthermore, aldosterone induces oxidative stress and inflammation, perturbs adipokine expression, thermogenesis and lipogenesis in adipose tissue, and increases hepatic steatosis. In pancreas, enhanced oxidative stress and inflammation of beta cells, predominantly upon glucocorticoid receptor activation, impair insulin secretion. No causality between hypokalemia and impaired insulin response is yet proven; in contrast, hypokalemia appears to be implicated with insulin resistance and hepatic steatosis. The superior efficacy of adrenalectomy in ameliorating glucose metabolism vs. mineralocorticoid receptor antagonists in clinical studies highlights the contribution of non-mineralocorticoid receptor-mediated mechanisms in the pathophysiologic process. The exact role of hypokalemia, the mechanisms linking mineralocorticoid excess with hepatic steatosis, and possible disease-modifying role of pioglitazone warrant further studies.

Animal Models of Cushing's Syndrome

Endogenous Cushing's syndrome is characterized by unique clinical features and comorbidities, and progress in the analysis of its genetic pathogenesis has been achieved. Moreover, prescribed glucocorticoids are also associated with exogenous Cushing's syndrome. Several animal models have been established to explore the pathophysiology and develop treatments for Cushing's syndrome. Here, we review recent studies reporting animal models of Cushing's syndrome with different features and complications induced by glucocorticoid excess. Exogenous corticosterone (CORT) administration in drinking water is widely utilized, and we found that CORT pellet implantation in mice successfully leads to a Cushing's phenotype. Corticotropin-releasing hormone overexpression mice and adrenal-specific Prkar1a-deficient mice have been developed, and AtT20 transplantation methods have been designed to examine the medical treatments for adrenocorticotropic hormone-producing pituitary neuroendocrine tumors. We also review recent advances in the molecular pathogenesis of glucocorticoid-induced complications using animal models.

Investigating the Effects of Maternal Separation on Hypothalamic-Pituitary-Adrenal Axis and Glucose Homeostasis under Chronic Social Defeat Stress in Young Adult Male Rat Offspring

INTRODUCTION

Given the suggested metabolic regulatory effects of stress-responsive genes and based on the impacts of early-life stress on HPA axis development, this study aimed to characterize the maternal separation (MS) impact on the communication between glucose metabolism and HPA axis dysregulations under chronic social defeat stress (CSDS).

METHODS

During the first 2 weeks of life, male Wistar rats were either exposed to MS or left undisturbed with their mothers (Std). Starting on postnatal day 50, the animals of each group were either left undisturbed in the standard group housing (Con) or underwent CSDS for 3 weeks. There were four groups (n = 10/group): Std-Con, MS-Con, Std-CSDS, and MS-CSDS.

RESULTS

Early and/or adult life adversity reduced β-cell number, muscular FK506-binding protein 51 (FKBP51) content, and BMI in adulthood. The reduction of β-cell number and BMI in the MS-CSDS rats were more profound than MS-Con group. CSDS either alone or in combination with MS reduced locomotor activity and increased and decreased corticotropin-releasing factor type 1 receptor (CRFR1) content, respectively, in hypothalamus and pancreas. Although, under CSDS, MS intensified HPA axis overactivity and reduced isolated islets' insulin secretion, it could promote resilience to depression symptoms. No differences were observed in hypothalamic Fkbp5 gene DNA methylation and glucose tolerance among groups.

CONCLUSION

MS exacerbated HPA axis overactivity and the endocrine pancreas dysfunctions under CSDS. The intensified corticosterone secretion and the diminished content of pancreatic CRFR1 protein could be involved in the reduced β-cell number and islets' insulin secretion under CSDS. The decreased muscular FKBP51 content might be a homeostatic response to slow down insulin resistance development under chronic stress.

Fresh insights into glucocorticoid-induced diabetes mellitus and new therapeutic directions

Glucocorticoid hormones were discovered to have use as potent anti-inflammatory and immunosuppressive therapeutics in the 1940s and their continued use and development have successfully revolutionized the management of acute and chronic inflammatory diseases. However, long-term use of glucocorticoids is severely hampered by undesirable metabolic complications, including the development of type 2 diabetes mellitus. These effects occur due to glucocorticoid receptor activation within multiple tissues, which results in inter-organ crosstalk that increases hepatic glucose production and inhibits peripheral glucose uptake. Despite the high prevalence of glucocorticoid-induced hyperglycaemia associated with their routine clinical use, treatment protocols for optimal management of the metabolic adverse effects are lacking or underutilized. The type, dose and potency of the glucocorticoid administered dictates the choice of hypoglycaemic intervention (non-insulin or insulin therapy) that should be provided to patients. The longstanding quest to identify dissociated glucocorticoid receptor agonists to separate the hyperglycaemic complications of glucocorticoids from their therapeutically beneficial anti-inflammatory effects is ongoing, with selective glucocorticoid receptor modulators in clinical testing. Promising areas of preclinical research include new mechanisms to disrupt glucocorticoid signalling in a tissue-selective manner and the identification of novel targets that can selectively dissociate the effects of glucocorticoids. These research arms share the ultimate goal of achieving the anti-inflammatory actions of glucocorticoids without the metabolic consequences.

Family history of diabetes moderates metabolic depression endophenotypes in overweight/obese adults

OBJECTIVE

Insulin resistance (IR) is linked to depressive disorders, and there is growing evidence that targeting IR may be beneficial in treating them. We examine the association between depressive symptoms and a direct measure of IR, and whether family history of type 2 diabetes (FHx-T2DM) or major depressive disorder (FHx-MDD) moderate this relationship.

METHODS

Cross-sectional data were collected from 96 primarily overweight/obese adults ages 25-50 without diabetes or clinical depression. Multiple regression and correlation analyses were used to assess the association between depressive symptoms and a direct measure of IR (steady-state plasma glucose) as well as moderating effects of FHx-T2DM or FHx-MDD.

RESULTS

In the total sample, elevated depressive symptoms were positively associated with IR (p = 0.005). IR was associated with depressive symptoms in subjects with FHx-T2DM (p = 0.002) or FHx-MDD (p = 0.009) whereas BMI was associated with depressive symptoms in subjects without FHx-T2DM (p = 0.049) or FHx-MDD (p = 0.029). The odds of being in the top tertile of IR increased with elevated depressive symptoms alone (OR, 4.22; 95%CI, 1.15 to 17.33), presence of FHx-T2DM alone (OR, 3.42; 95%CI, 1.26 to 10.00), and presence of both FHx-T2DM and elevated depressive symptoms (OR, 10.08; 95%CI, 1.94 to 96.96).

CONCLUSIONS

Our findings indicate that depressive symptoms are positively associated with a direct measure of IR in overweight/obese individuals without diabetes or clinical depression. This association is moderated by FHx-T2DM. Early identification of groups vulnerable to IR related to depressive symptomatology may be useful for determining personalized interventions that have the potential to reduce morbidity in later years.

Investigation of the effects of maternal separation on the pancreatic oxidative and inflammatory damages along with metabolic impairment in response to chronic social defeat stress in young adult male rats

Purpose

Chronic glucocorticoid release during the stress response has been proposed to initiate certain damages, which in turn produce metabolic disorders. The present study is the first work to test whether maternal separation (MS) would impact the metabolic alterations associated with pancreatic oxidative and inflammatory damages under chronic exposure to social defeat stress (CSDS) in adulthood.

Methods

During the first 2 weeks of life, male Wistar rats were exposed to MS or left undisturbed with their mothers (Std). Starting on postnatal day 50, the animals of each group were either left undisturbed in the standard group housing (Con) or underwent CSDS for 3 weeks. Thus, there were 4 groups (n = 7/group): Std-Con, Ms-Con, Std-CSDS, MS-CSDS. Each animal was weighed and then decapitated so that we could collect trunk blood for assessment of fasting plasma corticosterone, insulin, glucose, lipid profile, and insulin resistance. Plasma and pancreatic catalase activity, reduced glutathione (GSH), malondialdehyde levels and pancreatic interleukin-1 beta (IL-1β) content were also measured.

Results

MS-CSDS animals showed elevated plasma corticosterone and insulin levels (P < 0.01) along with insulin resistance (P < 0.05). According to one-way ANOVA results, chronic exposure to early or adult life adversity decreased body weight (P < 0.0001), Catalase activity and GSH levels (P < 0.0001) and increased malondialdehyde level (P = 0.0006) in plasma. Pancreatic MDA and IL-1β contents elevated just in MS-CSDS rats (P < 0.05).

Conclusion

Maternal separation shapes vulnerability to develop corticosterone hypersecretion, insulin resistance, pancreatic oxidative, and inflammatory damages associated with chronic exposure to later social challenges, which could potentially trigger metabolic disorders.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40200-021-00902-3.

Molecular Mechanisms of Glucocorticoid-Induced Insulin Resistance

Glucocorticoids (GCs) are steroids secreted by the adrenal cortex under the hypothalamic-pituitary-adrenal axis control, one of the major neuro-endocrine systems of the organism. These hormones are involved in tissue repair, immune stability, and metabolic processes, such as the regulation of carbohydrate, lipid, and protein metabolism. Globally, GCs are presented as ‘flight and fight’ hormones and, in that purpose, they are catabolic hormones required to mobilize storage to provide energy for the organism. If acute GC secretion allows fast metabolic adaptations to respond to danger, stress, or metabolic imbalance, long-term GC exposure arising from treatment or Cushing’s syndrome, progressively leads to insulin resistance and, in fine, cardiometabolic disorders. In this review, we briefly summarize the pharmacological actions of GC and metabolic dysregulations observed in patients exposed to an excess of GCs. Next, we describe in detail the molecular mechanisms underlying GC-induced insulin resistance in adipose tissue, liver, muscle, and to a lesser extent in gut, bone, and brain, mainly identified by numerous studies performed in animal models. Finally, we present the paradoxical effects of GCs on beta cell mass and insulin secretion by the pancreas with a specific focus on the direct and indirect (through insulin-sensitive organs) effects of GCs. Overall, a better knowledge of the specific action of GCs on several organs and their molecular targets may help foster the understanding of GCs’ side effects and design new drugs that possess therapeutic benefits without metabolic adverse effects.

The steroid hormone 20-hydroxyecdysone counteracts insulin signaling via insulin receptor dephosphorylation

The insulin receptor (INSR) binds insulin to promote body growth and maintain normal blood glucose levels. While it is known that steroid hormones such as estrogen and 20-hydroxyecdysone counteract insulin function, the molecular mechanisms responsible for this attenuation remain unclear. In the present study, using the agricultural pest lepidopteran Helicoverpa armigera as a model, we proposed that the steroid hormone 20-hydroxyecdysone (20E) induces dephosphorylation of INSR to counteract insulin function. We observed high expression and phosphorylation of INSR during larval feeding stages that decreased during metamorphosis. Insulin upregulated INSR expression and phosphorylation, whereas 20E repressed INSR expression and induced INSR dephosphorylation in vivo. Protein tyrosine phosphatase 1B (PTP1B, encoded by Ptpn1) dephosphorylated INSR in vivo. PTEN (phosphatase and tensin homolog deleted on chromosome 10) was critical for 20E-induced INSR dephosphorylation by maintaining the transcription factor Forkhead box O (FoxO) in the nucleus, where FoxO promoted Ptpn1 expression and repressed Insr expression. Knockdown of Ptpn1 using RNA interference maintained INSR phosphorylation, increased 20E production, and accelerated pupation. RNA interference of Insr in larvae repressed larval growth, decreased 20E production, delayed pupation, and accumulated hemolymph glucose levels. Taken together, these results suggest that a high 20E titer counteracts the insulin pathway by dephosphorylating INSR to stop larval growth and accumulate glucose in the hemolymph.

Highly bioavailable Berberine formulation improves Glucocorticoid Receptor-mediated Insulin Resistance via reduction in association of the Glucocorticoid Receptor with phosphatidylinositol-3-kinase

Excess glucocorticoid (GC) production is known to induce obesity and insulin resistance through increased activation of the glucocorticoid receptor (GR). The molecular mechanism for the non-genomic effects of excessive circulating GC on the insulin-signalling pathway in skeletal muscle is unknown. The plant alkaloid berberine has been shown to attenuate insulin resistance and inhibit gluconeogenesis in type 2 diabetic animals. A highly bioavailable berberine formulation termed Huang-Gui solid dispersion (HGSD), is a preparation of berberine coupled to sodium caprate and this markedly improving berberines bioavailability. Here we examined how HGSD treatment attenuated GR-mediated alteration in PI3K signalling and insulin resistance in diabetic rats, dexamethasone-treated mice and in insulin resistant C2C12 skeletal muscle cells. Blood glucose and skeletal muscle GC levels were increased and insulin signalling impaired in skeletal muscle of type 2 diabetic rats compared to controls. Treatment of these animals with HGSD restored blood glucose and skeletal muscle GC levels to that of controls. Insulin resistant C2C12 skeletal muscle cells exhibited impaired insulin signalling compared to controls and treatment of HGSD and RU486, an antagonist of GR, restored insulin signalling to that of control cells. Administration of dexamethasone to mice increased GR/GRα-associated PI3K and reduced IRS1-associated PI3K, phosphorylated-AKT, and membrane GLUT4 translocation resulting in a higher blood glucose concentration compared to controls. HGSD treatment of these mice improved insulin resistance by reducing the association of GR/GRα with PI3K. Excess GC-induced insulin resistance is mediated by increased association of GR with PI3K and treatment with HGSD attenuates these effects. We hypothesize that HGSD may be a promising candidate drug for the treatment of type 2 diabetes by reducing the association of GR with PI3K in skeletal muscle.

[Corticosteroid-induced diabetes: Novelties in pathophysiology and management]

Diabetes frequently occurs during corticosteroid treatment, sometimes necessitating urgent therapeutic management, with insulin for example. Corticosteroids induce insulin resistance in the liver, adipocytes and skeletal muscle, and have direct deleterious effects on insulin secretion. The development of insulin resistance during corticosteroid treatment, and the insufficient adaptation of insulin secretion, are key elements in the pathophysiology of corticosteroid-induced diabetes. The capacity of pancreatic β-cells to increase insulin secretion in response to insulin resistance is partly genetically determined. A familial history of type 2 diabetes is, therefore, a major risk factor for diabetes development on corticosteroid treatment. Corticosteroid treatments are usually initiated at a fairly high dose, which is subsequently decreased to the lowest level sufficient to achieve disease control. Pharmacological management of diabetes is needed in patients with blood glucose levels exceeding 2.16 g/l (12 mmol/l) and insulin therapy can be started when blood glucose levels are higher than 3.6 g/l (20 mmol/l) with clinical symptoms of diabetes. Insulin can then be replaced with oral hypoglycemic compounds when both blood glucose levels and corticosteroid dose have decreased. Patient education is essential, particularly for the management of hypoglycemia when corticosteroids are withdrawn or their dose tapered.

Poor Diet, Stress, and Inactivity Converge to Form a "Perfect Storm" That Drives Alzheimer's Disease Pathogenesis

North American incidence of Alzheimer's disease (AD) is expected to more than double over the coming generation. Although genetic factors surrounding the production and clearance of amyloid-β and phosphorylated tau proteins are known to be responsible for a subset of early-onset AD cases, they do not explain the pathogenesis of the far more prevalent sporadic late-onset variant of the disease. It is thus likely that lifestyle and environmental factors contribute to neurodegenerative processes implicated in the pathogenesis of AD. Herein, we review evidence that (1) excess sucrose consumption induces AD-associated liver pathologies and brain insulin resistance, (2) chronic stress overdrives activity of locus coeruleus neurons, leading to loss of function (a common event in neurodegeneration), (3) high-sugar diets and stress promote the loss of neuroprotective sex hormones in men and women, and (4) Western dietary trends set the stage for a lithium-deficient state. We propose that these factors may intersect as part of a "perfect storm" to contribute to the widespread prevalence of neurodegeneration and AD. In addition, we put forth the argument that exercise and supplementation with trace lithium can counteract many of the deleterious consequences associated with excessive caloric intake and perpetual stress. We conclude that lifestyle and environmental factors likely contribute to AD pathogenesis and that simple lifestyle and dietary changes can help counteract their effects.

Maternal Omega-3 Nutrition, Placental Transfer and Fetal Brain Development in Gestational Diabetes and Preeclampsia

Omega-3 fatty acids, particularly docosahexaenoic fatty acid (DHA), are widely recognized to impact fetal and infant neurodevelopment. The impact of DHA on brain development, and its inefficient synthesis from the essential alpha-linolenic acid (ALA), has led to recommended DHA intakes of 250-375 mg eicosapentaenoic acid + DHA/day for pregnant and lactating women by the Dietary Guidelines for Americans. Despite these recommendations, the intake of omega-3s in women of child-bearing age in the US remains very low. The low maternal status of DHA prior to pregnancy could impair fetal neurodevelopment. This review focuses on maternal omega-3 status in conditions of gestational diabetes mellitus (GDM) and preeclampsia, and the subsequent impact on placental transfer and cord blood concentration of omega-3s. Both GDM and preeclampsia are associated with altered maternal omega-3 status, altered placental omega-3 metabolism, reduced cord blood omega-3 levels and have an impact on neurodevelopment in the infant and on brain health later in life. These findings indicate lower DHA exposure of the developing baby may be driven by lower placental transfer in both conditions. Thus, determining approaches which facilitate increased delivery of DHA during pregnancy and early development might positively impact brain development in infants born to mothers with these diseases.

Whole Egg Consumption Impairs Insulin Sensitivity in a Rat Model of Obesity and Type 2 Diabetes

BACKGROUND

The literature regarding the relation between egg consumption and type 2 diabetes (T2D) is inconsistent and there is limited evidence pertaining to the impact of egg consumption on measures of insulin sensitivity.

OBJECTIVES

The aim of this study was to investigate the effect of dietary whole egg on metabolic biomarkers of insulin resistance in T2D rats.

METHODS

Male Zucker diabetic fatty (ZDF) rats (n = 12; 6 wk of age) and age-matched lean controls (n = 12) were randomly assigned to be fed a casein- or whole egg-based diet. At week 5 of dietary treatment, an insulin tolerance test (ITT) was performed on all rats and blood glucose was measured by glucometer. After 7 wk of dietary treatment, rats were anesthetized and whole blood was collected via a tail vein bleed. Following sedation, the extensor digitorum longus muscle was removed before and after an intraperitoneal insulin injection, and insulin signaling in skeletal muscle was analyzed by Western blot. Serum glucose and insulin were analyzed by ELISA for calculation of the homeostatic model assessment of insulin resistance (HOMA-IR).

RESULTS

Mean ITT blood glucose over the course of 60 min was 32% higher in ZDF rats fed the whole egg-based diet than in ZDF rats fed the casein-based diet. Furthermore, whole egg consumption increased fasting blood glucose by 35% in ZDF rats. Insulin-stimulated phosphorylation of key proteins in the insulin signaling pathway did not differ in skeletal muscle of ZDF rats fed casein- and whole egg-based diets. In lean rats, no differences were observed in insulin tolerance, HOMA-IR and skeletal muscle insulin signaling, regardless of experimental dietary treatment.

CONCLUSIONS

These data suggest that whole body insulin sensitivity may be impaired by whole egg consumption in T2D rats, although no changes were observed in skeletal muscle insulin signaling that could explain this finding.

Insulin receptors in the kidneys in health and disease

Insulin is an important hormone that affects various metabolic processes, including kidney function. Impairment in insulin’s action leads to insulin resistance in the target tissue. Besides defects in post-receptor insulin signaling, impairment at the receptor level could significantly affect insulin sensitivity of the target tissue. The kidney is a known target of insulin; however, whether the kidney develops “insulin resistance” is debatable. Regulation of the insulin receptor (IR) expression and its function is very well studied in major metabolic tissues like liver, skeletal muscles, and adipose tissue. The physiological relevance of IRs in the kidney has recently begun to be clarified. The credit goes to studies that showed a wide distribution of IR throughout the nephron segments and their reduced expression in the insulin resistance state. Moreover, altered renal and systemic metabolism observed in mice with targeted deletion of the IR from various epithelial cells of the kidney has strengthened this proposition. In this review, we recapitulate the crucial findings from literature that have expanded our knowledge regarding the significance of the renal IR in normal- and insulin-resistance states.

Deletion of insulin receptor in the proximal tubule and fasting augment albumin excretion

The contribution of proximal tubules (PT) to albumin uptake is now well recognized, however, its regulation is understudied area. There are reports suggesting that insulin resistance is associated with the development of albuminuria in nondiabetic individuals. We have previously reported reduced insulin receptor (IR) expression in renal-tubular-epithelial cells, including PT in various models of insulin resistance. However, the effect of a physiological fall in insulin levels and the role for IR in PT in tubular albumin uptake is not clear. To address these gaps in our understanding, we estimated urine excretion and renal uptake of albumin in fasted and fed C57Bl/6 mice injected with fluorescein isothiocyanate (FITC)-albumin (5 µg/mL/kg body weight, intraperitoneal, n = 6 per group). In addition, we compared spot urine analysis from 33 clinically healthy humans after overnight fasting (when insulin levels are lower than in the fed state) and then at 2 hours after 75 g oral glucose challenge (postprandial). Fasted mice had attenuated renal uptake of FITC-albumin and higher excretion in urine, relative to fed mice ( P = 0.04). Moreover, a significant drop in urine albumin-to-creatinine ratio (ACR) and urine albumin concentration (UAC) was observed in the postprandial state in these subjects ( P = 0.001 and P = 0.017, for ACR and UAC, respectively). The drop was negatively associated with postprandial blood glucose levels (ρ = -0.36, P = 0.03 for ΔUAC and ρ = -0.34, P = 0.05 for ΔACR). To test the role of IR in PT, we analyzed 24-hour urine albumin excretion in male mice with targeted deletion of IR from PT (insulin receptor knockout [IRKO]) and their wild-type (WT) littermates ( n = 7 per group). IRKO mice had significantly higher 24-hour urine albumin excretion relative to WT. Moreover, kidneys from KO mice revealed reduced expression of megalin and cubulin proteins in the PT relative to the WT. We also demonstrated insulin (100 nM) induced albumin internalization in human proximal tubule cells (hPT) and this effect of insulin was attenuated in hydroxy-2-naphthalenylmethylphosphonic acid (100 µM), a tyrosine kinase inhibitor, pretreated hPT. Our findings revealed albumin excretion was attenuated by glucose administration to fasting individuals implying a regulatory role for insulin in PT albumin reabsorption. Thus albuminuria associated with insulin resistance/diabetes may relate not only to glomerular dysfunction but also to impairment in insulin-mediated reabsorption.

Mifepristone enhances insulin-stimulated Akt phosphorylation and glucose uptake in skeletal muscle cells

Mifepristone is the only FDA-approved drug for glycaemia control in patients with Cushing's syndrome and type 2 diabetes. Mifepristone also has beneficial effects in animal models of diabetes and patients with antipsychotic treatment-induced obesity. However, the mechanisms through which Mifepristone produces its beneficial effects are not completely elucidated.

PURPOSE

To determine the effects of mifepristone on insulin-stimulated glucose uptake on a model of L6 rat-derived skeletal muscle cells.

RESULTS

Mifepristone enhanced insulin-dependent glucose uptake, GLUT4 translocation to the plasma membrane and Akt Ser⁴⁷³ phosphorylation in L6 myotubes. In addition, mifepristone reduced oxygen consumption and ATP levels and increased AMPK Thr¹⁷² phosphorylation. The knockdown of AMPK prevented the effects of mifepristone on insulin response.

CONCLUSIONS

Mifepristone enhanced insulin-stimulated glucose uptake through a mechanism that involves a decrease in mitochondrial function and AMPK activation in skeletal muscle cells.

Sulforaphane ameliorates glucose intolerance in obese mice via the upregulation of the insulin signaling pathway

Sulforaphane (SFN) is a dietary component with multiple bioactivities; however, its role in obesity-related metabolic derangement remains unclear.

The Association of COPD Exacerbations with New Onset Type 2 Diabetes among Medicare Patients

Objective: Chronic obstructive pulmonary disease (COPD) is highly prevalent in the elderly population and typically reduces overall quality of life. Exacerbations of COPD are commonly treated with corticosteroids, a class of drug known to cause insulin resistance. The objective of this study was to assess the rate of exacerbations requiring emergency room visits, hospitalizations or any medical encounter (a combination of emergency room and hospitalizations) between COPD patients who did and did not develop type 2 diabetes. Research Design and Methods: A case-control study of COPD patients from the 2011-2012 Medicare 5% sample Limited Data Set (LDS) was conducted. Beneficiaries with at least 1 year of continuous enrollment and evidence of > 2 COPD-related claims (>1 primary diagnosis) were included in the study. Cases were defined as a beneficiary with a new claim for type 2 diabetes, whereas controls lacked evidence of type 2 diabetes (beneficiaries with evidence of non-incident type 2 diabetes were excluded). Results: Of 27 456 COPD beneficiaries, 1274 developed incident type 2 diabetes (4.6%). After matching, 2536 beneficiaries were assigned as cases (n = 1268) and controls (n = 1268). Cases in the emergency room (1.97 claims per person) (p = <0.001) and hospitalizations (2.02 claims per person) (p = <0.001) had a higher rate of exacerbations. Conclusion: Our findings suggest that patients that were hospitalized and visited the emergency room for COPD exacerbations had a greater likelihood of type 2 diabetes. Type 2 diabetes may be associated with exposure to corticosteroids as a result of the treatment for exacerbations. Future work should investigate the risk for type 2 diabetes in COPD patients treated with corticosteroids.

Transcription controls growth, cell kinetics and cholesterol supply to sustain ACTH responses

Chronic ACTH exposure is associated with adrenal hypertrophy and steroidogenesis. The underlying molecular processes in mice have been analysed by microarray, histological and immunohistochemical techniques. Synacthen infused for 2 weeks markedly increased adrenal mass and plasma corticosterone levels. Microarray analysis found greater than 2-fold changes in expression of 928 genes (P < 0.001; 397 up, 531 down). These clustered in pathways involved in signalling, sterol/lipid metabolism, cell proliferation/hypertrophy and apoptosis. Signalling genes included some implicated in adrenal adenomas but also upregulated genes associated with cyclic AMP and downregulated genes associated with aldosterone synthesis. Sterol metabolism genes were those promoting cholesterol supply (Scarb1, Sqle, Apoa1) and disposal (Cyp27a1, Cyp7b1). Oil red O staining showed lipid depletion consistent with reduced expression of genes involved in lipid synthesis. Genes involved in steroidogenesis (Star, Cyp11a1, Cyp11b1) were modestly affected (P < 0.05; <1.3-fold). Increased Ki67, Ccna2, Ccnb2 and Tk1 expression complemented immunohistochemical evidence of a 3-fold change in cell proliferation. Growth arrest genes, Cdkn1a and Cdkn1c, which are known to be active in hypertrophied cells, were increased >4-fold and cross-sectional area of fasciculata cells was 2-fold greater. In contrast, genes associated with apoptosis (eg Casp12, Clu,) were downregulated and apoptotic cells (Tunel staining) were fewer (P < 0.001) and more widely distributed throughout the cortex. In summary, long-term steroidogenesis with ACTH excess is sustained by genes controlling cholesterol supply and adrenal mass. ACTH effects on adrenal morphology and genes controlling cell hypertrophy, proliferation and apoptosis suggest the involvement of different cell types and separate molecular pathways.

Glucose Metabolism Abnormalities in Cushing Syndrome: From Molecular Basis to Clinical Management

An impaired glucose metabolism, which often leads to the onset of diabetes mellitus (DM), is a common complication of chronic exposure to exogenous and endogenous glucocorticoid (GC) excess and plays an important part in contributing to morbidity and mortality in patients with Cushing syndrome (CS). This article reviews the pathogenesis, epidemiology, diagnosis, and management of changes in glucose metabolism associated with hypercortisolism, addressing both the pathophysiological aspects and the clinical and therapeutic implications. Chronic hypercortisolism may have pleiotropic effects on all major peripheral tissues governing glucose homeostasis. Adding further complexity, both genomic and nongenomic mechanisms are directly induced by GCs in a context-specific and cell-/organ-dependent manner. In this paper, the discussion focuses on established and potential pathologic molecular mechanisms that are induced by chronically excessive circulating levels of GCs and affect glucose homeostasis in various tissues. The management of patients with CS and DM includes treating their hyperglycemia and correcting their GC excess. The effects on glycemic control of various medical therapies for CS are reviewed in this paper. The association between DM and subclinical CS and the role of screening for CS in diabetic patients are also discussed.

11β-Hydroxysteroid Dehydrogenase Type 1 in Obese Subjects With Type 2 Diabetes Mellitus

Obesity is one of the most significant contributors to the development of type 2 diabetes mellitus. Tissue-specific glucocorticoids regulated by 11β-hydroxysteroid dehydrogenase enzyme (11β-HSD) type 1 are involved in central obesity and obesity-related comorbidities. Moderate downregulation of 11β-HSD1 can attenuate insulin insensitivity and the impairment of glucose-stimulated insulin secretion. Some of the beneficial effects of 11β-HSD1 inhibition may be mediated, at least in part, through inactivation of tissue-specific glucocorticoid action related to insulin signaling mechanisms, alleviation of abnormal cytokine profile and the improvement of β-cell function. Thus, 11β-HSD1 is a promising target for the treatment and prevention of type 2 diabetes mellitus with obesity.

The Metabolic Implications of Glucocorticoids in a High-Fat Diet Setting and the Counter-Effects of Exercise

Glucocorticoids (GCs) are steroid hormones, naturally produced by activation of the hypothalamic-pituitary-adrenal (HPA) axis, that mediate the immune and metabolic systems. Synthetic GCs are used to treat a number of inflammatory conditions and diseases including lupus and rheumatoid arthritis. Generally, chronic or high dose GC administration is associated with side effects such as steroid-induced skeletal muscle loss, visceral adiposity, and diabetes development. Patients who are taking exogenous GCs could also be more susceptible to poor food choices, but the effect that increasing fat consumption in combination with elevated exogenous GCs has only recently been investigated. Overall, these studies show that the damaging metabolic effects initiated through exogenous GC treatment are significantly amplified when combined with a high fat diet (HFD). Rodent studies of a HFD and elevated GCs demonstrate more glucose intolerance, hyperinsulinemia, visceral adiposity, and skeletal muscle lipid deposition when compared to rodents subjected to either treatment on its own. Exercise has recently been shown to be a viable therapeutic option for GC-treated, high-fat fed rodents, with the potential mechanisms still being examined. Clinically, these mechanistic studies underscore the importance of a low fat diet and increased physical activity levels when individuals are given a course of GC treatment.

Glucocorticoids and Diabetes

Glucocorticoids (GCs), stress hormones produced by the adrenal gland, are involved in many pathways in physiology and metabolism including glucose homeostasis and inflammation. Excess GC signaling results in Cushing's syndrome and possibly metabolic syndrome. Diabetes, central adiposity, and hyperlipidemia are components of both syndromes. Here, we discuss the mechanisms of GC action, clinical syndromes of GC excess, modulation of glucose homeostasis by GCs, and future treatments for diabetes based on GC signaling.

Stressing diabetes? The hidden links between insulinotropic peptides and the HPA axis

Diabetes mellitus exerts metabolic stress on cells and it provokes a chronic increase in the long-term activity of the hypothalamus-pituitary-adrenocortical (HPA) axis, perhaps thereby contributing to insulin resistance. GLP-1 receptor (GLP-1R) agonists are pleiotropic hormones that not only affect glycaemic and metabolic control, but they also produce many other effects including activation of the HPA axis. In fact, several of the most relevant effects of GLP-1 might involve, at least in part, the modulation of the HPA axis. Thus, the anorectic activity of GLP-1 could be mediated by increasing CRF at the hypothalamic level, while its lipolytic effects could imply a local increase in glucocorticoids and glucocorticoid receptor (GC-R) expression in adipose tissue. Indeed, the potent activation of the HPA axis by GLP-1R agonists occurs within the range of therapeutic doses and with a short latency. Interestingly, the interactions of GLP-1 with the HPA axis may underlie most of the effects of GLP-1 on food intake control, glycaemic metabolism, adipose tissue biology and the responses to stress. Moreover, such activity has been observed in animal models (mice and rats), as well as in normal humans and in type I or type II diabetic patients. Accordingly, better understanding of how GLP-1R agonists modulate the activity of the HPA axis in diabetic subjects, especially obese individuals, will be crucial to design new and more efficient therapies for these patients.

Overexpression of Glucocorticoid Receptor β Enhances Myogenesis and Reduces Catabolic Gene Expression

Unlike the glucocorticoid receptor α (GRα), GR β (GRβ) has a truncated ligand-binding domain that prevents glucocorticoid binding, implicating GRα as the mediator of glucocorticoid-induced skeletal muscle loss. Because GRβ causes glucocorticoid resistance, targeting GRβ may be beneficial in impairing muscle loss as a result of GRα activity. The purpose of this study was to determine how the overexpression of GRβ affects myotube formation and dexamethasone (Dex) responsiveness. We measured GR isoform expression in C₂C12 muscle cells in response to Dex and insulin, and through four days of myotube formation. Next, lentiviral-mediated overexpression of GRβ in C₂C12 was performed, and these cells were characterized for cell fusion and myotube formation, as well as sensitivity to Dex via the expression of ubiquitin ligases. GRβ overexpression increased mRNA levels of muscle regulatory factors and enhanced proliferation in myoblasts. GRβ overexpressing myotubes had an increased fusion index. Myotubes overexpressing GRβ had lower forkhead box O3 (Foxo3a) mRNA levels and a blunted muscle atrophy F-box/Atrogen-1 (MAFbx) and muscle ring finger 1 (MuRF1) response to Dex. We showed that GRβ may serve as a pharmacological target for skeletal muscle growth and protection from glucocorticoid-induced catabolic signaling. Increasing GRβ levels in skeletal muscle may cause a state of glucocorticoid resistance, stabilizing muscle mass during exposure to high doses of glucocorticoids.

Italian Society for the Study of Diabetes (SID)/Italian Endocrinological Society (SIE) guidelines on the treatment of hyperglycemia in Cushing's syndrome and acromegaly

Hyperglycemia is a common feature associated with states of increased growth hormone secretion and glucocorticoid levels. The purpose of these guidelines is to assist clinicians and other health care providers to take evidence-based therapeutic decisions for the treatment of hyperglycemia in patients with growth hormone and corticosteroid excess. Both the SID and SIE appointed members to represent each society and to collaborate in Guidelines writing. Members were chosen for their specific knowledge in the field. Each member agreed to produce-and regularly update-conflicts of interest. The authors of these guidelines prepared their contributions following the recommendations for the development of Guidelines, using the standard classes of recommendation shown below. All members of the writing committee provided editing and systematic review of each part of the manuscript, and discussed the grading of evidence. Consensus was guided by a systematic review of all available trials and by interactive discussions.

Italian Society for the Study of Diabetes (SID)/Italian Endocrinological Society (SIE) guidelines on the treatment of hyperglycemia in Cushing's syndrome and acromegaly

BACKGROUND

Hyperglycemia is a common feature associated with states of increased growth hormone secretion and glucocorticoid levels.

AIMS

The purpose of these guidelines is to assist clinicians and other health care providers to take evidence-based therapeutic decisions for the treatment of hyperglycemia in patients with growth hormone and corticosteroid excess.

METHODOLOGY

Both the SID and SIE appointed members to represent each society and to collaborate in Guidelines writing. Members were chosen for their specific knowledge in the field. Each member agreed to produce--and regularly update--conflicts of interest. The Authors of these guidelines prepared their contributions following the recommendations for the development of Guidelines, using the standard classes of recommendation shown below. All members of the writing committee provided editing and systematic review of each part of the manuscript, and discussed the grading of evidence. Consensus was guided by a systematic review of all available trials and by interactive discussions.

Glucocorticoids and Metabolic Control

In response to stress, the central nervous system initiates a signaling cascade, which leads to the production of glucocorticoids (GCs). GCs act through the glucocorticoid receptor (GR) to coordinate the appropriate cellular response with the primary goal of mobilizing the storage forms of carbon precursors to generate a continuous glucose supply for the brain. Although GCs are critical for maintaining energy homeostasis, excessive GC stimulation leads to a number of undesirable side effects, including hyperglycemia, insulin resistance, fatty liver, obesity, and muscle wasting leading to severe metabolic dysfunction. Summarized below are the diverse metabolic roles of glucocorticoids in energy homeostasis and dysregulation, focusing specifically on glucose, lipid, and protein metabolism.

Regulation of Glucose Homeostasis by Glucocorticoids

Glucocorticoids are steroid hormones that regulate multiple aspects of glucose homeostasis. Glucocorticoids promote gluconeogenesis in liver, whereas in skeletal muscle and white adipose tissue they decrease glucose uptake and utilization by antagonizing insulin response. Therefore, excess glucocorticoid exposure causes hyperglycemia and insulin resistance. Glucocorticoids also regulate glycogen metabolism. In liver, glucocorticoids increase glycogen storage, whereas in skeletal muscle they play a permissive role for catecholamine-induced glycogenolysis and/or inhibit insulin-stimulated glycogen synthesis. Moreover, glucocorticoids modulate the function of pancreatic α and β cells to regulate the secretion of glucagon and insulin, two hormones that play a pivotal role in the regulation of blood glucose levels. Overall, the major glucocorticoid effect on glucose homeostasis is to preserve plasma glucose for brain during stress, as transiently raising blood glucose is important to promote maximal brain function. In this chapter we will discuss the current understanding of the mechanisms underlying different aspects of glucocorticoid-regulated mammalian glucose homeostasis.

Glucose uptake through translocation and activation of GLUT4 in PI3K/Akt signaling pathway by asiatic acid in diabetic rats

In this study, we examined the in vivo effect and the mechanism of asiatic acid (AA) on glucose uptake in an insulin target skeletal muscle. Diabetic rats showed significantly increased levels of plasma glucose, thiobarbituric acid reactive substances, and lipid hydroperoxides, decreased levels of insulin and antioxidants, and impairment in insulin-signaling proteins such as insulin receptor (IR), insulin receptor substrate (IRS)-1/2, phosphoinositide 3-kinase (PI3K), Akt, and glucose transporter 4 (GLUT4) proteins. Oral treatment with AA (20 mg/kg body weight) showed near-normalized levels of plasma glucose, lipid peroxidation products, and antioxidants and improved insulin, IR, IRS-1/2, PI3K, Akt, and GLUT4 proteins. These findings suggest that AA improves glucose response by increasing GLUT4 in skeletal muscle through Akt and antioxidant defense in plasma and it also improves glucose homeostasis.

Electroacupuncture plus metformin lowers glucose levels and facilitates insulin sensitivity by activating MAPK in steroid-induced insulin-resistant rats

BACKGROUND

Type 2 diabetes mellitus is the predominant form of diabetes. Although metformin is the preferred first-line drug for treatment of the disease, it is associated with a risk of secondary failure. Electroacupuncture (EA) can enhance insulin sensitivity and reduce blood glucose levels.

OBJECTIVES

To examine, in an animal study, whether EA combined with metformin (EA-metformin) results in a better glucose-lowering effect and greater insulin sensitivity than metformin alone in steroid-induced insulin-resistant rats.

METHODS

Adult Wistar rats were injected with dexamethasone to induce diabetes and subsequently treated with EA plus metformin or metformin alone. Variations in plasma glucose, plasma insulin, and plasma free fatty acid levels were studied at the midpoint and end of the experimental course. Insulin receptor substrate 1 (IRS-1) and peroxisome proliferator-activated receptor γ (PPAR-γ), which are associated with glucose transporter type 4 (GLUT4) translocation, and mitogen-activated protein kinase (MAPK), which is related to GLUT4 activation, were measured after EA treatment.

RESULTS

We found that EA-metformin resulted in a better glucose-lowering effect, greater insulin sensitivity, lower plasma free fatty acid levels and higher levels of MAPK than metformin alone (p<0.05). There were no significant differences between treatment groups in expression of IRS-1 or PPAR-γ.

CONCLUSIONS

The glucose-lowering effect and increased insulin sensitivity associated with EA-metformin administration is governed, at least in part, by its ability to stimulate the activation of GLUT4 via upregulation of MAPK expression.

11β-Hydroxysteroid dehydrogenase type 1 shRNA ameliorates glucocorticoid-induced insulin resistance and lipolysis in mouse abdominal adipose tissue

Long-term glucocorticoid exposure increases the risk for developing type 2 diabetes. Prereceptor activation of glucocorticoid availability in target tissue by 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) coupled with hexose-6-phosphate dehydrogenase (H6PDH) is an important mediator of the metabolic syndrome. We explored whether the tissue-specific modulation of 11β-HSD1 and H6PDH in adipose tissue mediates glucocorticoid-induced insulin resistance and lipolysis and analyzed the effects of 11β-HSD1 inhibition on the key lipid metabolism genes and insulin-signaling cascade. We observed that corticosterone (CORT) treatment increased expression of 11β-HSD1 and H6PDH and induced lipase HSL and ATGL with suppression of p-Thr(172) AMPK in adipose tissue of C57BL/6J mice. In contrast, CORT induced adipose insulin resistance, as reflected by a marked decrease in IR and IRS-1 gene expression with a reduction in p-Thr(308) Akt/PKB. Furthermore, 11β-HSD1 shRNA attenuated CORT-induced 11β-HSD1 and lipase expression and improved insulin sensitivity with a concomitant stimulation of pThr(308) Akt/PKB and p-Thr(172) AMPK within adipose tissue. Addition of CORT to 3T3-L1 adipocytes enhanced 11β-HSD1 and H6PDH and impaired p-Thr(308) Akt/PKB, leading to lipolysis. Knockdown of 11β-HSD1 by shRNA attenuated CORT-induced lipolysis and reversed CORT-mediated inhibition of pThr(172) AMPK, which was accompanied by a parallel improvement of insulin signaling response in these cells. These findings suggest that elevated adipose 11β-HSD1 expression may contribute to glucocorticoid-induced insulin resistance and adipolysis.

Insulin regulates nitric oxide production in the kidney collecting duct cells

The kidney is an important organ for arterial blood pressure (BP) maintenance. Reduced NO generation in the kidney is associated with hypertension in insulin resistance. NO is a critical regulator of vascular tone; however, whether insulin regulates NO production in the renal inner medullary collecting duct (IMCD), the segment with the greatest enzymatic activity for NO production in kidney, is not clear. Using an NO-sensitive 4-amino-5-methylamino-2',7'-difluorofluorescein (DAF-FM) fluorescent dye, we found that insulin increased NO production in mouse IMCD cells (mIMCD) in a time- and dose-dependent manner. A concomitant dose-dependent increase in the NO metabolite (NOx) was also observed in the medium from insulin-stimulated cells. NO production peaked in mIMCD cells at a dose of 100 nm insulin with simultaneously increased NOx levels in the medium. At this dose, insulin significantly increased p-eNOS(Ser1177) levels in mIMCD cells. Pretreatment of cells with a PI 3-kinase inhibitor or insulin receptor silencing with RNA interference abolished these effects of insulin, whereas insulin-like growth factor-1 receptor (IGF-1R) silencing had no effect. We also showed that chronic insulin infusion to normal C57BL/6J mice resulted in increased endothelial NOS (eNOS) protein levels and NO production in the inner medulla. However, insulin-infused IRKO mice, with targeted deletion of insulin receptor from tubule epithelial cells of the kidney, had ∼50% reduced eNOS protein levels in their inner medulla along with a significant rise in BP relative to WT littermates. We have previously reported increased baseline BP and reduced urine NOx in IRKO mice. Thus, reduced insulin receptor signaling in IMCD could contribute to hypertension in the insulin-resistant state.

Decreased gene expressions of insulin signal molecules in canine hyperadrenocorticism

Hyperadrenocorticism (HAC) is a common endocrine disorder in dogs, in which excess glucocorticoid causes insulin resistance. Disturbance of insulin action may be caused by multiple factors, including transcriptional modulation of insulin signal molecules which lie downstream of insulin binding to insulin receptors. In this study, gene expressions of insulin signal molecules were examined using neutrophils of the HAC dogs (the untreated dogs and the dogs which had been treated with trilostane). Insulin receptor substrate (IRS)-1, IRS-2, phosphatidylinositol 3-kinase (PI3-K), protein kinase B/Akt kinase (Akt)-2 and protein kinase C (PKC)-lambda were analyzed in the HAC dogs and compared with those from normal dogs. The IRS-1 gene expressions decreased by 37% and 35% of the control dogs in the untreated and treated groups, respectively. The IRS-2 gene expressions decreased by 61% and 72%, the PI3-K gene expressions decreased by 47% and 55%, and the Akt-2 gene expressions decreased by 45% and 56% of the control dogs, similarly. Collectively, gene expressions of insulin signal molecules are suppressed in the HAC dogs, which may partially contribute to the induction of insulin resistance.

Meta-modeling of methylprednisolone effects on glucose regulation in rats

A retrospective meta-modeling analysis was performed to integrate previously reported data of glucocorticoid (GC) effects on glucose regulation following a single intramuscular dose (50 mg/kg), single intravenous doses (10, 50 mg/kg), and intravenous infusions (0.1, 0.2, 0.3 and 0.4 mg/kg/h) of methylprednisolone (MPL) in normal and adrenalectomized (ADX) male Wistar rats. A mechanistic pharmacodynamic (PD) model was developed based on the receptor/gene/protein-mediated GC effects on glucose regulation. Three major target organs (liver, white adipose tissue and skeletal muscle) together with some selected intermediate controlling factors were designated as important regulators involved in the pathogenesis of GC-induced glucose dysregulation. Assessed were dynamic changes of food intake and systemic factors (plasma glucose, insulin, free fatty acids (FFA) and leptin) and tissue-specific biomarkers (cAMP, phosphoenolpyruvate carboxykinase (PEPCK) mRNA and enzyme activity, leptin mRNA, interleukin 6 receptor type 1 (IL6R1) mRNA and Insulin receptor substrate-1 (IRS-1) mRNA) after acute and chronic dosing with MPL along with the GC receptor (GR) dynamics in each target organ. Upon binding to GR in liver, MPL dosing caused increased glucose production by stimulating hepatic cAMP and PEPCK activity. In adipose tissue, the rise in leptin mRNA and plasma leptin caused reduction of food intake, the exogenous source of glucose input. Down-regulation of IRS-1 mRNA expression in skeletal muscle inhibited the stimulatory effect of insulin on glucose utilization further contributing to hyperglycemia. The nuclear drug-receptor complex served as the driving force for stimulation or inhibition of downstream target gene expression within different tissues. Incorporating information such as receptor dynamics, as well as the gene and protein induction, allowed us to describe the receptor-mediated effects of MPL on glucose regulation in each important tissue. This advanced mechanistic model provides unique insights into the contributions of major tissues and quantitative hypotheses for the multi-factor control of a complex metabolic system.

Metabolic control through glucocorticoid hormones: an update

In the past decades, glucocorticoid (GC) hormones and their cognate, intracellular receptor, the glucocorticoid receptor (GR), have been well established as critical checkpoints in mammalian energy homeostasis. Whereas many aspects in healthy nutrient metabolism require physiological levels and/or action of GC, aberrant GC/GR signalling has been linked to severe metabolic dysfunction, including obesity, insulin resistance and type 2 diabetes. Consequently, studies of the molecular mechanisms within the GC signalling axis have become a major focus in biomedical research, up-to-date particularly focusing on systemic glucose and lipid handling. However, with the availability of novel high throughput technologies and more sophisticated metabolic phenotyping capabilities, as-yet non-appreciated, metabolic functions of GC have been recently discovered, including regulatory roles of the GC/GR axis in protein and bile acid homeostasis as well as metabolic inter-organ communication. Therefore, this review summarises recent advances in GC/GR biology, and summarises findings relevant for basic and translational metabolic research.

Metabolic functions of glucocorticoid receptor in skeletal muscle

Glucocorticoids (GCs) exert key metabolic influences on skeletal muscle. GCs increase protein degradation and decrease protein synthesis. The released amino acids are mobilized from skeletal muscle to liver, where they serve as substrates for hepatic gluconeogenesis. This metabolic response is critical for mammals' survival under stressful conditions, such as fasting and starvation. GCs suppress insulin-stimulated glucose uptake and utilization and glycogen synthesis, and play a permissive role for catecholamine-induced glycogenolysis, thus preserving the level of circulating glucose, the major energy source for the brain. However, chronic or excess exposure of GCs can induce muscle atrophy and insulin resistance. GCs convey their signal mainly through the intracellular glucocorticoid receptor (GR). While GR can act through different mechanisms, one of its major actions is to regulate the transcription of its primary target genes through genomic glucocorticoid response elements (GREs) by directly binding to DNA or tethering onto other DNA-binding transcription factors. These GR primary targets trigger physiological and pathological responses of GCs. Much progress has been made to understand how GCs regulate protein and glucose metabolism. In this review, we will discuss how GR primary target genes confer metabolic functions of GCs, and the mechanisms governing the transcriptional regulation of these targets. Comprehending these processes not only contributes to the fundamental understanding of mammalian physiology, but also will provide invaluable insight for improved GC therapeutics.

C1-Ten is a protein tyrosine phosphatase of insulin receptor substrate 1 (IRS-1), regulating IRS-1 stability and muscle atrophy

Muscle atrophy occurs under various catabolic conditions, including insulin deficiency, insulin resistance, or increased levels of glucocorticoids. This results from reduced levels of insulin receptor substrate 1 (IRS-1), leading to decreased phosphatidylinositol 3-kinase activity and thereby activation of FoxO transcription factors. However, the precise mechanism of reduced IRS-1 under a catabolic condition is unknown. Here, we report that C1-Ten is a novel protein tyrosine phosphatase (PTPase) of IRS-1 that acts as a mediator to reduce IRS-1 under a catabolic condition, resulting in muscle atrophy. C1-Ten preferentially dephosphorylated Y612 of IRS-1, which accelerated IRS-1 degradation. These findings suggest a novel type of IRS-1 degradation mechanism which is dependent on C1-Ten and extends our understanding of the molecular mechanism of muscle atrophy under catabolic conditions. C1-Ten expression is increased by catabolic glucocorticoid and decreased by anabolic insulin. Reflecting these hormonal regulations, the muscle C1-Ten is upregulated in atrophy but downregulated in hypertrophy. This reveals a previously unidentified role of C1-Ten as a relevant PTPase contributing to skeletal muscle atrophy.

Insulin resistance and muscle metabolism in chronic kidney disease

Insulin resistance is a common finding in chronic kidney disease (CKD) and is manifested by mild fasting hyperglycemia and abnormal glucose tolerance testing. Circulating levels of glucocorticoids are high. In muscle, changes in the insulin signaling pathway occur. An increase in the regulatory p85 subunit of Class I phosphatidylinositol 3-Kinase enzyme leads to decreased activation of the downstream effector protein kinase B (Akt). Mechanisms promoting muscle proteolysis and atrophy are unleashed. The link of Akt to the ubiquitin proteasome pathway, a major degradation pathway in muscle, is discussed. Another factor associated with insulin resistance in CKD is angiotensin II (Ang II) which appears to induce its intracellular effects through inflammatory cytokines or reactive oxygen species. Skeletal muscle ATP is depleted and the ability of AMP-activated protein kinase (AMPK) to replenish energy stores is blocked. How this can be reversed is discussed. Interleukin-6 (IL-6) levels are elevated in CKD and impair insulin signaling at the level of IRS-1. With exercise, IL-6 levels are reduced; glucose uptake and utilization are increased. For patients with CKD, exercise may improve insulin signaling and build up muscle. Treatment strategies for preventing muscle atrophy are discussed.

Acute peripheral but not central administration of olanzapine induces hyperglycemia associated with hepatic and extra-hepatic insulin resistance

Atypical antipsychotic drugs such as Olanzapine induce weight gain and metabolic changes associated with the development of type 2 diabetes. The mechanisms underlying the metabolic side-effects of these centrally acting drugs are still unknown to a large extent. We compared the effects of peripheral (intragastric; 3 mg/kg/h) versus central (intracerebroventricular; 30 µg/kg/h) administration of Olanzapine on glucose metabolism using the stable isotope dilution technique (Experiment 1) in combination with low and high hyperinsulinemic-euglycemic clamps (Experiments 2 and 3), in order to evaluate hepatic and extra-hepatic insulin sensitivity, in adult male Wistar rats. Blood glucose, plasma corticosterone and insulin levels were measured alongside endogenous glucose production and glucose disappearance. Livers were harvested to determine glycogen content. Under basal conditions peripheral administration of Olanzapine induced pronounced hyperglycemia without a significant increase in hepatic glucose production (Experiment 1). The clamp experiments revealed a clear insulin resistance both at hepatic (Experiment 2) and extra-hepatic levels (Experiment 3). The induction of insulin resistance in Experiments 2 and 3 was supported by decreased hepatic glycogen stores in Olanzapine-treated rats. Central administration of Olanzapine, however, did not result in any significant changes in blood glucose, plasma insulin or corticosterone concentrations nor in glucose production. In conclusion, acute intragastric administration of Olanzapine leads to hyperglycemia and insulin resistance in male rats. The metabolic side-effects of Olanzapine appear to be mediated primarily via a peripheral mechanism, and not to have a central origin.

FOXO3a mediates signaling crosstalk that coordinates ubiquitin and atrogin-1/MAFbx expression during glucocorticoid-induced skeletal muscle atrophy

Muscle atrophy is a consequence of chronic diseases (e.g., diabetes) and glucocorticoid-induced insulin resistance that results from enhanced activity of the ubiquitin-proteasome pathway. The PI3K/Akt pathway inhibits the FOXO-mediated transcription of the muscle-specific E3 ligase atrogin-1/MAFbx (AT-1), whereas the MEK/ERK pathway increases Sp1 activity and ubiquitin (UbC) expression. The observations raise a question about how the transcription of these atrogenes is synchronized in atrophic muscle. We tested a signaling model in which FOXO3a mediates crosstalk between the PI3K/Akt and MEK/ERK pathways to coordinate AT-1 and UbC expression. In rat L6 myotubes, dexamethasone (> or = 24 h) reduced insulin receptor substrate (IRS)-1 protein and PI3K/Akt signaling and increased AT-1 mRNA. IRS-2 protein, MEK/ERK signaling, Sp1 phosphorylation, and UbC transcription were simultaneously increased. Knockdown of IRS-1 using small interfering RNA or adenovirus-mediated expression of constitutively activated FOXO3a increased IRS-2 protein, MEK/ERK signaling, and UbC expression. Changes in PI3K/Akt and MEK/ERK signaling were recapitulated in rat muscles undergoing atrophy due to streptozotocin-induced insulin deficiency and concurrently elevated glucocorticoid production. IRS-1 and Akt phosphorylation were decreased, whereas MEK/ERK signaling and expression of IRS-2, UbC and AT-1 were increased. We conclude that FOXO3a mediates a reciprocal communication between the IRS-1/PI3K/Akt and IRS-2/MEK/ERK pathways that coordinates AT-1 and ubiquitin expression during muscle atrophy.