Glucagon-like peptide-1 receptor is present on human hepatocytes and has a direct role in decreasing hepatic steatosis in vitro by modulating elements of the insulin signaling pathway

Exogenous glucagon-like peptide 1 reduces contractions in human colon circular muscle

Glucagon-like peptide 1 (GLP1) is a naturally occurring peptide secreted by intestinal L-cells. Though its primary function is to serve as an incretin, GLP1 reduces gastrointestinal motility. However, only a handful of animal studies have specifically evaluated the influence of GLP1 on colonic motility. Consequently, the aims of this study were to investigate the effects induced by exogenous GLP1, to analyze the mechanism of action, and to verify the presence of GLP1 receptors (GLP1Rs) in human colon circular muscular strips. Organ bath technique, RT-PCR, western blotting, and immunofluorescence were used. In human colon, exogenous GLP1 reduced, in a concentration-dependent manner, the amplitude of the spontaneous contractions without affecting the frequency and the resting basal tone. This inhibitory effect was significantly reduced by exendin (9-39), a GLP1R antagonist, which per se significantly increased the spontaneous mechanical activity. Moreover, it was abolished by tetrodotoxin, a neural blocker, or Nω-nitro-l-arginine - a blocker of neuronal nitric oxide synthase (nNOS). The biomolecular analysis revealed a genic and protein expression of the GLP1R in the human colon. The double-labeling experiments with anti-neurofilament or anti-nNOS showed, for the first time, that immunoreactivity for the GLP1R was expressed in nitrergic neurons of the myenteric plexus. In conclusion, the results of this study suggest that GLP1R is expressed in the human colon and, once activated by exogenous GLP1, mediates an inhibitory effect on large intestine motility through NO neural release.

Exenatide improves hepatic steatosis by enhancing lipid use in adipose tissue in nondiabetic rats

AIM: To investigate the metabolic changes in skeletal muscle and/or adipose tissue in glucagon-like peptide-1-induced improvement of nonalcoholic fatty liver disease (NAFLD).

METHODS: Male Wistar rats were fed either a control diet (control group) or a high-fat diet (HFD). After 4 wk, the HFD-fed rats were subdivided into two groups; one group was injected with exenatide [HFD-Ex(+) group] and the other with saline [HFD-Ex(-) group] every day for 12 wk. The control group received saline and were fed a control diet. Changes in weight gain, energy intake, and oxygen consumption were analyzed. Glucose tolerance tests were performed after 8 wk of treatment. Histological assessments were performed in liver and adipose tissue. RNA expression levels of lipid metabolism related genes were evaluated in liver, skeletal muscle, and adipose tissue.

RESULTS: Exenatide attenuated weight gain [HFD-Ex(-) vs HFD-Ex(+)] and reduced energy intake, which was accompanied by an increase in oxygen consumption and a decrease in the respiratory exchange ratio [HFD-Ex(-) vs HFD-Ex(+)]. However, exenatide did not affect glucose tolerance. Exenatide reduced lipid content in the liver and adipose tissue. Exenatide did not affect the expression of lipid metabolism-related genes in the liver or skeletal muscle. In adipose tissue, exenatide significantly upregulated lipolytic genes, including hormone-sensitive lipase, carnitine palmitoyltransferase-1, long-chain acyl-CoA dehydrogenase, and acyl-CoA oxidase 1 [HFD-Ex(-) vs HFD-Ex(+)]. Exenatide also upregulated catalase and superoxide dismutase 2 [HFD-Ex(-) vs HFD-Ex(+)].

CONCLUSION: In addition to reducing appetite, enhanced lipid use by exenatide in adipose tissue may reduce hepatic lipid content in NAFLD, most likely by decreasing lipid influx into the liver.

Exendin-4 shows no effects on the prostatic index in high-fat-diet-fed rat with benign prostatic hyperplasia by improving insulin resistance

Benign prostatic hyperplasia (BPH) is a prevalent disease globally, and accumulating evidence has indicated an association between BPH, insulin resistance (IR) and diabetes. Exendin-4 is widely used in clinics, which could enhance the proliferation of pancreatic β cells. The ability of exendin-4 to promote tumorigenesis has been of concern, and whether exendin-4 would enhance the propagation of BPH is not fully understood. We aimed to determine whether glucagon-like peptide-1 receptors (GLP-1Rs) were expressed in rat prostate and to determine the effect of exendin-4 on prostate of BPH. Male Wistar rats were used and assigned to six groups: normal diet (ND), high-fat diet (HFD), HFD + exendin-4, HFD + BPH, HFD + BPH + exendin-4 and HFD + BPH + rosiglitazone group. After castration, steroids were injected subcutaneously for 4 weeks to induce BPH. Rats were kept on high-fat diet to induce IR. Treatment groups were treated with exendin-4 and rosiglitazone. Prostatic index and HOMA-IR index were used to evaluate the prostatic hyperplasia status and the degree of IR respectively. The expression of GLP-1R was indicated not only by immunohistochemistry, but also by Western blot analysis. The expression of GLP-1R was significantly higher, and HOMA-IR index and body weight significantly decreased after administration of exendin-4. However, no significant differences in the prostatic index were observed between exendin-4 treatment groups and non-exendin-4 treatment groups. Prostatic index was not influenced by exendin-4 maybe by improving IR and weight loss.

Sympathetic nervous system control of triglyceride metabolism: novel concepts derived from recent studies

Important players in triglyceride (TG) metabolism include the liver (production), white adipose tissue (WAT) (storage), heart and skeletal muscle (combustion to generate ATP), and brown adipose tissue (BAT) (combustion toward heat), the collective action of which determine plasma TG levels. Interestingly, recent evidence points to a prominent role of the hypothalamus in TG metabolism through innervating the liver, WAT, and BAT mainly via sympathetic branches of the autonomic nervous system. Here, we review the recent findings in the area of sympathetic control of TG metabolism. Various neuronal populations, such as neuropeptide Y (NPY)-expressing neurons and melanocortin-expressing neurons, as well as peripherally produced hormones (i.e., GLP-1, leptin, and insulin), modulate sympathetic outflow from the hypothalamus toward target organs and thereby influence peripheral TG metabolism. We conclude that sympathetic stimulation in general increases lipolysis in WAT, enhances VLDL-TG production by the liver, and increases the activity of BAT with respect to lipolysis of TG, followed by combustion of fatty acids toward heat. Moreover, the increased knowledge about the involvement of the neuroendocrine system in TG metabolism presented in this review offers new therapeutic options to fight hypertriglyceridemia by specifically modulating sympathetic nervous system outflow toward liver, BAT, or WAT.

Glucose-induced glucagon-like Peptide 1 secretion is deficient in patients with non-alcoholic fatty liver disease

Background & Aims

The incretins glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are gastrointestinal peptide hormones regulating postprandial insulin release from pancreatic β-cells. GLP-1 agonism is a treatment strategy in Type 2 diabetes and is evaluated in Non-alcoholic fatty liver disease (NAFLD). However, the role of incretins in its pathophysiology is insufficiently understood. Studies in mice suggest improvement of hepatic steatosis by GLP-1 agonism. We determined the secretion of incretins after oral glucose administration in non-diabetic NAFLD patients.

Methods

N = 52 patients (n = 16 NAFLD and n = 36 Non-alcoholic steatohepatitis (NASH) patients) and n = 50 matched healthy controls were included. Standardized oral glucose tolerance test was performed. Glucose, insulin, glucagon, GLP-1 and GIP plasma levels were measured sequentially for 120 minutes after glucose administration.

Results

Glucose induced GLP-1 secretion was significantly decreased in patients compared to controls (p<0.001). In contrast, GIP secretion was unchanged. There was no difference in GLP-1 and GIP secretion between NAFLD and NASH subgroups. All patients were insulin resistant, however HOMA2-IR was highest in the NASH subgroup. Fasting and glucose-induced insulin secretion was higher in NAFLD and NASH compared to controls, while the glucose lowering effect was diminished. Concomitantly, fasting glucagon secretion was significantly elevated in NAFLD and NASH.

Conclusions

Glucose-induced GLP-1 secretion is deficient in patients with NAFLD and NASH. GIP secretion is contrarily preserved. Insulin resistance, with hyperinsulinemia and hyperglucagonemia, is present in all patients, and is more severe in NASH compared to NAFLD. These pathophysiologic findings endorse the current evaluation of GLP-1 agonism for the treatment of NAFLD.

Nonalcoholic fatty liver disease and type 2 diabetes in obese children

Nonalcoholic fatty liver disease (NAFLD) is commonly found in adults and adolescents with type 2 diabetes (T2DM). The cause-effect relations of these 2 conditions are complex and it is difficult to decipher whether one drives the other or vice versa. Genetic predispositions, along with obesity, are probably shared culprits of both. NAFLD may precede the diagnosis of diabetes and play a critical role of driving its development by way of increasing hepatic and whole body insulin resistance. On the other hand, T2DM is associated with hyperinsulinemia, a resistance to some of the effects of gut derived peptides and increased systemic free fatty acids, that can all promote hepatic lipid deposition. Thus, each condition may promote the development of the other and their mutual presence creates a vicious cycle. Upon studying this complex interplay from another angle, reduction of liver fat significantly improves glucose metabolism in patients with T2DM highlighting the tight pathophysiological link between them.

GLP-1-derived nonapeptide GLP-1(28-36)amide represses hepatic gluconeogenic gene expression and improves pyruvate tolerance in high-fat diet-fed mice

Certain "degradation" products of GLP-1 were found to possess beneficial effects on metabolic homeostasis. Here, we investigated the function of the COOH-terminal fragment of GLP-1, the nonapeptide GLP-1(28-36)amide, in hepatic glucose metabolism. C57BL/6 mice fed a high-fat diet (HFD) for 13 wk were injected intraperitoneally with GLP-1(28-36)amide for 6 wk. A significant reduction in body weight gain in response to HFD feeding was observed in GLP-1(28-36)amide-treated mice. GLP-1(28-36)amide administration moderately improved glucose disposal during glucose tolerance test but more drastically attenuated glucose production during pyruvate tolerance test, which was associated with reduced hepatic expression of the gluconeogenic genes Pck1, G6pc, and Ppargc1a. Mice treated with GLP-1(28-36)amide exhibited increased phosphorylation of PKA targets, including cAMP response element-binding protein (CREB), ATF-1, and β-catenin. In primary hepatocytes, GLP-1(28-36)amide reduced glucose production and expression of Pck1, G6pc, and Ppargc1a, which was associated with increased cAMP content and PKA target phosphorylation. These effects were attenuated by PKA inhibition. We suggest that GLP-1(28-36)amide represses hepatic gluconeogenesis involving the activation of components of the cAMP/PKA signaling pathway. This study further confirmed that GLP-1(28-36)amide possesses therapeutic potential for diabetes and other metabolic disorders.

Activation of PPARβ/δ protects pancreatic β cells from palmitate-induced apoptosis by upregulating the expression of GLP-1 receptor

We previously showed that activated peroxisome proliferator-activated receptor (PPAR)β/δ can protect pancreatic β cells against lipotoxic apoptosis. However, the molecular mechanism remained unclear. Glucagon-like peptide-1 receptor (GLP-1R) has been reported to exhibit a protective effect against lipotoxic apoptosis in pancreatic β cells. In the present study, we aimed to investigate the underlying molecular mechanisms that PPARβ/δ activation suppressed apoptosis and improved β cell function impaired by fatty acids, focusing on contribution of GLP-1R. Isolated rat islets and rat insulin-secreting INS-1 cells were treated with the PPARβ/δ agonist GW501516 (GW) in the presence or absence of palmitate (PA) and transfected with siRNA for PPARβ/δ or treated with the PPARβ/δ antagonist GSK0660. Apoptosis was assessed by DNA fragmentation, Hoechst 33342 staining and flow cytometry. GLP-1R expression in INS-1 cells and islets was assayed by immunoblotting, quantitative PCR (qPCR) and immunofluorescence staining. SREBP-1c, Caveolin-1, Akt, Bcl-2, Bcl-xl and caspase-3 expression was measured using immunoblotting and qPCR. Our results showed that PPARβ/δ activation decreased apoptosis in β cells and robustly stimulated GLP-1R expression under lipotoxic conditions. GW enhanced glucose-stimulated insulin secretion (GSIS) impaired by PA through stimulation of GLP-1R expression in β cells. Moreover, SREBP-1c/Caveolin-1 signaling was involved in PPARβ/δ-regulated GLP-1R expression. Finally, GW exerted anti-apoptotic effects via interfering with GLP-1R-dependent Akt/Bcl-2 and Bcl-xl/caspase-3 signaling pathways. Our study suggested that the anti-apoptotic action of GW may involve its transcriptional regulation of GLP-1R, and PPARβ/δ activation may represent a new therapeutic method for protecting pancreatic β cells from lipotoxicity.

Nonalcoholic Fatty liver: a possible new target for type 2 diabetes prevention and treatment

Non-alcoholic fatty liver disease (NAFLD) is the most common liver disorder worldwide. Several lines of evidence have indicated a pathogenic role of insulin resistance, and a strong association with type 2 diabetes (T2MD) and metabolic syndrome. Importantly, NAFLD appears to enhance the risk for T2MD, as well as worsen glycemic control and cardiovascular disease in diabetic patients. In turn, T2MD may promote NAFLD progression. The opportunity to take into account NAFLD in T2MD prevention and care has stimulated several clinical studies in which antidiabetic drugs, such as metformin, thiazolidinediones, GLP-1 analogues and DPP-4 inhibitors have been evaluated in NAFLD patients. In this review, we provide an overview of preclinical and clinical evidences on the possible efficacy of antidiabetic drugs in NAFLD treatment. Overall, available data suggest that metformin has beneficial effects on body weight reduction and metabolic parameters, with uncertain effects on liver histology, while pioglitazone may improve liver histology. Few data, mostly preclinical, are available on DPP4 inhibitors and GLP-1 analogues. The heterogeneity of these studies and the small number of patients do not allow for firm conclusions about treatment guidelines, and further randomized, controlled studies are needed.

Indirect effects of glucagon-like peptide-1 receptor agonist exendin-4 on the peripheral circadian clocks in mice

Circadian clocks in peripheral tissues are powerfully entrained by feeding. The mechanisms underlying this food entrainment remain unclear, although various humoral and neural factors have been reported to affect peripheral clocks. Because glucagon-like peptide-1 (GLP-1), which is rapidly secreted in response to food ingestion, influences multiple humoral and neural signaling pathways, we suggest that GLP-1 plays a role in the food entrainment of peripheral clocks. To test this, we compared the effects of exendin-4, a GLP-1 receptor agonist, on mRNA expression of the clock genes (Clock, Bmal1, Nr1d1, Per1, Per2, and Cry1) with those of refeeding. In addition, we investigated whether exendin-4 could affect the rhythms of the peripheral clocks. In male C57BL/6J mice, although refeeding rapidly (within 2 h) altered mRNA levels of Per1 and Per2 in the liver and that of Per1 in adipose tissue, a single i.p. injection of exendin-4 did not cause such changes. However, unlike the GLP-1 receptor antagonist exendin-(9-39), exendin-4 significantly influenced Per1 mRNA levels in the liver at 12 h after injection. Moreover, pretreatment with exendin-4 affected the rapid-feeding-induced change in Per1 not only in the liver, but also in adipose tissue, without effect on food intake. Furthermore, during light-phase restricted feeding, repeated dosing of exendin-4 at the beginning of the dark phase profoundly influenced both the food intake and daily rhythms of clock gene expression in peripheral tissues. Thus, these results suggest that exendin-4 modulates peripheral clocks via multiple mechanisms different from those of refeeding.

Liraglutide efficacy and action in non-alcoholic steatohepatitis (LEAN): study protocol for a phase II multicentre, double-blinded, randomised, controlled trial

INTRODUCTION

Non-alcoholic steatohepatitis (NASH) is now the commonest cause of chronic liver disease. Despite this, there are no universally accepted pharmacological therapies for NASH. Liraglutide (Victoza), a human glucagon-like peptide-1 (GLP-1) analogue, has been shown to improve weight loss, glycaemic control and liver enzymes in type 2 diabetes. There is currently a lack of prospective-controlled studies investigating the efficacy of GLP-1 analogues in patients with NASH.

METHODS AND ANALYSIS

Liraglutide efficacy and action in NASH (LEAN) is a phase II, multicentre, double-blinded, placebo-controlled, randomised clinical trial designed to investigate whether a 48-week treatment with 1.8 mg liraglutide will result in improvements in liver histology in patients with NASH. Adult, overweight (body mass index ≥25 kg/m(2)) patients with biopsy-confirmed NASH were assessed for eligibility at five recruitment centres in the UK. Patients who satisfied the eligibility criteria were randomly assigned (1:1) to receive once-daily subcutaneous injections of either 1.8 mg liraglutide or liraglutide-placebo (control). Using A'Hern's single stage phase II methodology (significance level 0.05; power 0.90) and accounting for an estimated 20% withdrawal rate, a minimum of 25 patients were randomised to each treatment group. The primary outcome measure will be centrally assessed using an intention-to-treat analysis of the proportion of evaluable patients achieving an improvement in liver histology between liver biopsies at baseline and after 48 weeks of treatment. Histological improvement will be defined as a combination of the disappearance of active NASH and no worsening in fibrosis.

ETHICS AND DISSEMINATION

The protocol was approved by the National Research Ethics Service (East Midlands-Northampton committee; 10/H0402/32) and the Medicines and Healthcare products Regulatory Agency. Recruitment into the LEAN started in August 2010 and ended in May 2013, with 52 patients randomised. The treatment follow-up of LEAN participants is currently ongoing and is due to finish in July 2014. The findings of this trial will be disseminated through peer-reviewed publications and international presentations.

TRIAL REGISTRATION

clinicaltrials.gov NCT01237119.

Effects of combination therapy with vildagliptin and valsartan in a mouse model of type 2 diabetes

Background

Dipeptidyl peptidase-4 (DPP-4) inhibitors modulate incretin hormones and exert anti-diabetic effects in type 2 diabetes mellitus. Treatment with angiotensin II type 1 receptor blockers (ARB) is a proven successful intervention for hypertension with type 2 diabetes. The present study investigated the combined effects of the DPP-4 inhibitor vildagliptin and the ARB valsartan in a mouse model of type 2 diabetes.

Methods

C57BL/6 J mice fed with high-fat diet (HFD) or db/db mice were treated with placebo, phloridzin (PHZ), vildagliptin alone (ViL), valsartan alone (VaL) or ViL with VaL (ViLVaL) for 8 weeks.

Results

Glucose metabolism was improved in response to PHZ, ViL and ViLVaL in both HFD and db/db mice. Upon glucose challenge, ViLVaL showed the greatest suppression of blood glucose excursions, with increased insulin secretion, in db/db mice. ViLVaL treatment also showed an improvement of insulin sensitivity in db/db mice. Serum inflammatory cytokines were significantly decreased, and adiponectin was highest, in the ViLVaL group. ViLVaL improved insulin signaling and attenuated stress signaling in liver with amelioration of hepatic steatosis due to activated fatty acid oxidation in db/db mice. Furthermore, immunohistochemical analysis of the pancreas revealed that the combination treatment resulted in an increased expression of insulin and PDX-1, and increased insulin content.

Conclusions

The combination therapy of ViL and VaL improves both pancreatic beta-cell function and insulin sensitivity, with a reduction of the inflammatory and cell stress milieu in mouse models of T2DM. Our results suggest that this combination therapy exerts additive or even synergistic benefits to treat T2DM.

The role of gastrointestinal hormones in hepatic lipid metabolism

Hepatocellular accumulation of free fatty acids (FFAs) in the form of triglycerides constitutes the metabolic basis for the development of nonalcoholic fatty liver disease (NAFLD). Recent data demonstrate that excess FFA hepatocyte storage is likely to lead to lipotoxicity and hepatocyte apoptosis. Hence, FFA-mediated hepatocyte injury is a key contributor to the pathogenesis of nonalcoholic steatohepatitis (NASH). Nonalcoholic steatohepatitis, obesity, type 2 diabetes, essential hypertension, and other common medical problems together comprise metabolic syndrome. Evidence suggests that peptide hormones from the L cells of the distal small intestine, which comprise the core of the enteroendocrine system (EES), play two key roles, serving either as incretins, or as mediators of appetite and satiety in the central nervous system. Recent data related to glucagon-like peptide-1 (GLP-1) and other known L-cell hormones have accumulated due to the increasing frequency of bariatric surgery, which increase delivery of bile salts to the hindgut. Bile acids are a key stimulus for the TGR5 receptor of the L cells. Enhanced bile-salt flow and subsequent EES stimulation may be central to elimination of hepatic steatosis following bariatric surgery. Although GLP-1 is a clinically relevant pharmacological analogue that drives pancreatic β-cell insulin output, GLP-1 analogues also have independent benefits via their effects on hepatocellular FFA metabolism. The authors also discuss recent data regarding the role of the major peptides released by the EES, which promote satiety and modulate energy homeostasis and utilization, as well as those that control fat absorption and intestinal permeability. Taken together, elucidating novel functions for EES-related peptides and pharmacologic development of peptide analogues offer potential far-ranging treatment for obesity-related human disease.

Changes in post-prandial glucose and pancreatic hormones, and steady-state insulin and free fatty acids after gastric bypass surgery

BACKGROUND

Changes in the multiple mechanisms that regulate glucose metabolism after gastric bypass (RYGB) are still being unveiled. The objective of this study was to compare the changes of glucose and pancreatic hormones [C-peptide, glucagon, and pancreatic polypeptide (PP)] during a meal tolerance test (MTT) and steady-state insulin and free fatty acid (FFA) concentrations during euglycemic-hyperinsulinemic clamp 14 days and 6 months after RYGB in morbidly obese nondiabetic patients.

METHODS

Two groups were studied at baseline and at 14 days: the RYGB followed by caloric restriction group (RYGB, n = 12) and the equivalent caloric restriction alone group (Diet, n = 10), to control for energy intake and weight loss. The RYGB group was studied again at 6 months to assess the changes after substantial weight loss. During MTT, the early and overall changes in glucose and pancreatic hormone concentrations were determined, and during the clamp, steady-state insulin and FFA concentrations were assessed.

RESULTS

After 14 days, RYGB patients had enhanced postprandial glucose, C-peptide, and glucagon responses, and decreased postprandial PP concentrations. Steady-state insulin concentrations were decreased at 14 days only in RYGB patients, and FFA increased in both groups. Six months after RYGB and substantial weight loss, the decrease in insulin concentrations during clamp persisted, and there were further changes in postprandial glucose and glucagon responses. FFA concentrations during clamp were significantly lower at 6 months, relative to presurgical values.

CONCLUSIONS

In morbidly obese nondiabetic patients, RYGB produces early changes in postmeal glucose, C-peptide, glucagon, and PP responses, and it appears to enhance insulin clearance early after RYGB and improve insulin sensitivity in adipose tissue at 6 months postsurgery. The early changes cannot be explained by caloric restriction alone.

Nonalcoholic fatty liver disease: current issues and novel treatment approaches

Nonalcoholic fatty liver disease (NAFLD) is considered the most common liver disorder in the Western world. It is commonly associated with insulin resistance, obesity, dyslipidaemia, type 2 diabetes mellitus (T2DM) and cardiovascular disease. Nonalcoholic steatohepatitis (NASH) is characterized by steatosis with necroinflammation and eventual fibrosis, which can lead to end-stage liver disease and hepatocellular carcinoma. Its pathogenesis is complex, and involves a state of 'lipotoxicity' in which insulin resistance, with increased free fatty acid release from adipose tissue to the liver, play a key role in the onset of a 'lipotoxic liver disease' and its progression to NASH. The diagnosis of NASH is challenging, as most affected patients are symptom free and the role of routine screening is not clearly established. A complete medical history is important to rule out other causes of fatty liver disease (alcohol abuse, medications, other). Plasma aminotransferase levels and liver ultrasound are helpful in the diagnosis of NAFLD/NASH, but a liver biopsy is often required for a definitive diagnosis. However, there is an active search for plasma biomarkers and imaging techniques that may non-invasively aid in the diagnosis. The treatment of NASH requires a multifaceted approach. The goal is to reverse obesity-associated lipotoxicity and insulin resistance via lifestyle intervention. Although there is no pharmacological agent approved for the treatment of NAFLD, vitamin E (in patients without T2DM) and the thiazolidinedione pioglitazone (in patients with and without T2DM) have shown the most consistent results in randomized controlled trials. This review concentrates on our current understanding of the disease, with a focus on the existing therapeutic approaches and potential future pharmacological developments for NAFLD and NASH.

Potential of incretin-based therapies for non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is becoming an epidemic, paralleling the increased prevalence of obesity and diabetes, which are risk factors. In this review, we present the current pre-clinical evidence showing that GLP-1 analogues and DPP4 inhibitors can improve hepatic steatosis. Although some of the effects could be due to overall improvement in metabolic parameters, there are data to support improvements independent of weight loss, as well as direct effects on the hepatocyte in vitro. Multiple hepatocyte signal transduction pathways appear to be activated by GLP-1 and its analogues, with both AMP-activated protein kinase and Akt proposed to be key players in improving hepatic steatosis. However, it is controversial as to whether the pancreatic-type GLP-1 receptor is present or responsible for conferring the GLP-1 signal in the hepatocyte. In total, the data support the need for more rigorous prospective clinical trials to further investigate the potential of incretin therapies for treatment of NAFLD.

Effects of intraportal exenatide on hepatic glucose metabolism in the conscious dog

Incretins improve glucose metabolism through multiple mechanisms. It remains unclear whether direct hepatic effects are an important part of exenatide's (Ex-4) acute action. Therefore, the objective of this study was to determine the effect of intraportal delivery of Ex-4 on hepatic glucose production and uptake. Fasted conscious dogs were studied during a hyperglycemic clamp in which glucose was infused into the hepatic portal vein. At the same time, portal saline (control; n = 8) or exenatide was infused at low (0.3 pmol·kg⁻¹·min⁻¹, Ex-4-low; n = 5) or high (0.9 pmol·kg⁻¹·min⁻¹, Ex-4-high; n = 8) rates. Arterial plasma glucose levels were maintained at 160 mg/dl during the experimental period. This required a greater rate of glucose infusion in the Ex-4-high group (1.5 ± 0.4, 2.0 ± 0.7, and 3.7 ± 0.7 mg·kg⁻¹·min⁻¹ between 30 and 240 min in the control, Ex-4-low, and Ex-4-high groups, respectively). Plasma insulin levels were elevated by Ex-4 (arterial: 4,745 ± 428, 5,710 ± 355, and 7,262 ± 1,053 μU/ml; hepatic sinusoidal: 14,679 ± 1,700, 15,341 ± 2,208, and 20,445 ± 4,020 μU/ml, 240 min, area under the curve), whereas the suppression of glucagon was nearly maximal in all groups. Although glucose utilization was greater during Ex-4 infusion (5.92 ± 0.53, 6.41 ± 0.57, and 8.12 ± 0.54 mg·kg⁻¹·min⁻¹), when indices of hepatic, muscle, and whole body glucose uptake were expressed relative to circulating insulin concentrations, there was no indication of insulin-independent effects of Ex-4. Thus, this study does not support the notion that Ex-4 generates acute changes in hepatic glucose metabolism through direct effects on the liver.

Nonalcoholic Fatty liver disease, diabetes mellitus and cardiovascular disease: newer data

Nonalcoholic fatty liver disease (NAFLD) is the most common, chronic liver disease worldwide. Within this spectrum, steatosis alone is apparently benign, while nonalcoholic steatohepatitis may progress to cirrhosis and hepatocellular carcinoma. NAFLD is strongly associated with obesity, dyslipidemia, type 2 diabetes mellitus, and cardiovascular disease. The pathogenesis of hepatic steatosis is not clearly known, but its main characteristics are considered insulin resistance, mitochondrial dysfunction, increased free fatty acids reflux from adipose tissue to the liver, hepatocyte lipotoxicity, stimulation of chronic necroinflammation, and fibrogenic response. With recent advances in technology, advanced imaging techniques provide important information for diagnosis. There is a significant research effort in developing noninvasive monitoring of disease progression to fibrosis and response to therapy with potential novel biomarkers, in order to facilitate diagnosis for the detection of advanced cirrhosis and to minimize the need of liver biopsy. The identification of NAFLD should be sought as part of the routine assessment of type 2 diabetics, as sought the microvascular complications and cardiovascular disease, because it is essential for the early diagnosis and proper intervention. Diet, exercise training, and weight loss provide significant clinical benefits and must be considered of first line for treating NAFLD.

Effects of sitagliptin on nonalcoholic fatty liver disease in diet-induced obese rats

BACKGROUND

Studies investigating the effects of dipeptidyl peptidase-4 inhibitors on hepatic steatosis are lacking. We aimed to determine the effects of sitagliptin on nonalcoholic fatty liver disease (NAFLD) in rats with diet-induced obesity.

METHODS

A total of 24 adult female Sprague-Dawley rats, which were 24 weeks old and weighed 199-240 grams, were used. The rats were randomly separated into two groups. The control group (n=6) was fed with standard rat diet; the remaining rats (n=18) were fed with a high-fat diet (HFD) to induce NAFLD. After 12 weeks, rats that were fed with a HFD were randomly separated into two groups: (1) HFD-only group (n=8) was fed with a HFD for an additional 4 weeks, (2) HFD-sitagliptin group (n=10) received sitagliptin (3 mg/kg) for 4 weeks in addition to HFD. At the end of the study (16(th) week), blood samples were drawn from all rats to determine serum glucose, triglyceride, cholesterol, alanine aminotransferase (ALT), and plasma insulin levels. Insulin resistance was determined using the homeostasis model assessment of insulin resistance (HOMA-IR) index. Histopathologic evaluation of liver samples was undertaken.

RESULTS

The HFD-sitagliptin group had significantly lower serum glucose (140.8±18.8 vs. 224.7±20.6 mg/dL, P<0.001), plasma insulin (15.8±4.4 vs. 28.0±5.9 μIU/L, P<0.001), HOMA-IR index (4.9±1.8 vs. 15.9±2.3, P<0.001), serum triglycerides (199.0±108.7 vs. 468.0±370.7 mg/dL, P<0.001), and cholesterol (82.0±26.7 vs. 90.5±7.0, P<0.001) values compared to the HFD-only group. Hepatic steatosis was significantly less (mean score, 1 vs. 2; P<0.001) in the HFD-sitagliptin group compared to the HFD-only group, whereas there was no difference in hepatic inflammation (P=0.057), liver weight (P=0.068), and ALT levels (P=0.232).

CONCLUSION

Sitagliptin may improve hepatic steatosis by increasing insulin sensitivity and improving lipid profiles in rats.

Acute peripheral administration of synthetic human GLP-1 (7-36 amide) decreases circulating IL-6 in obese patients with type 2 diabetes mellitus: a potential role for GLP-1 in modulation of the diabetic pro-inflammatory state?

BACKGROUND

To explore the effects of acute administration of GLP-1 and GIP on circulating levels of key adipocyte-derived hormones and gut-brain peptides with established roles in energy and appetite regulation, modulation of insulin sensitivity and inflammation.

METHODS

Six obese male patients with diet-treated type 2 diabetes (T2DM) and 6 healthy lean subjects were studied. The protocol included 4 experiments for each participant that were carried out in randomised order and comprised: GLP-1 infusion at a rate of 1 pmol/kg/min for 4h, GIP at a rate of 2 pmol/kg/min, GLP-1+GIP and placebo infusion. Plasma leptin, adiponectin, IL-6, insulin, ghrelin and obestatin were measured at baseline, 15, 60, 120, 180 and 240 min following the start of infusion.

RESULTS

Patients with T2DM had higher baseline IL-6 compared with healthy [day of placebo infusion: T2DM IL-6 mean (SEM) 1.3 (0.3) pg/ml vs 0.3 (0.1)pg/ml, p=0.003]. GLP-1 infusion in T2DM was associated with a significant reduction in circulating IL-6 [baseline IL-6 1.2 pg/ml vs IL-6=0.7 at 120 min, p=0.0001; vs IL-6=0.8 at 180 min, p=0.001]. There was no significant change in leptin, adiponectin, ghrelin or obestatin compared to baseline on all 4 experimental days in both groups.

CONCLUSION

Short-term infusion of supraphysiological concentrations of GLP-1 in T2DM results in suppression of IL-6, a key inflammatory mediator strongly linked to development of obesity and T2DM-related insulin resistance. It remains to be confirmed whether GLP-1-based diabetes therapies can impact favourably on cardiovascular outcomes.

Diabetes and nonalcoholic Fatty liver disease: a pathogenic duo

Recent data increasingly support a complex interplay between the metabolic condition diabetes mellitus and the pathologically defined nonalcoholic fatty liver disease (NAFLD). NAFLD predicts the development of type 2 diabetes and vice versa, and each condition may serve as a progression factor for the other. Although the association of diabetes and NAFLD is likely to be partly the result of a "common soil," it is also probable that diabetes interacts with NAFLD through specific pathogenic mechanisms. In particular, through interrelated metabolic pathways currently only partly understood, diabetes appears to accelerate the progression of NAFLD to nonalcoholic steatohepatitis, defined by the presence of necroinflammation, with varying degrees of liver fibrosis. In the research setting, obstacles that have made the identification of clinically significant NAFLD, and particularly nonalcoholic steatohepatitis, difficult are being addressed with the use of new imaging techniques combined with risk algorithms derived from peripheral blood profiling. These techniques are likely to be used in the diabetes population in the near future. This review examines the pathogenic links between NAFLD and diabetes by exploring the epidemiological evidence in humans and also through newer animal models. Emerging technology to help screen noninvasively for differing pathological forms of NAFLD and the potential role of preventive and therapeutic approaches for NAFLD in the setting of diabetes are also examined.

Direct effect of GLP-1 infusion on endogenous glucose production in humans

AIMS/HYPOTHESIS

Glucagon-like peptide-1 (GLP-1) lowers glucose levels by potentiating glucose-induced insulin secretion and inhibiting glucagon release. The question of whether GLP-1 exerts direct effects on the liver, independently of the hormonal changes, is controversial. We tested whether an exogenous GLP-1 infusion, designed to achieve physiological postprandial levels, directly affects endogenous glucose production (EGP) under conditions mimicking the fasting state in diabetes.

METHODS

In 14 healthy volunteers, we applied the pancreatic clamp technique, whereby plasma insulin and glucagon levels are clamped using somatostatin and hormone replacement. The clamp was applied in paired, 4 h experiments, during which saline (control) or GLP-1(7-37)amide (0.4 pmol min⁻¹ kg⁻¹) was infused.

RESULTS

During the control study, plasma insulin and glucagon were maintained at basal levels and plasma C-peptide was suppressed, such that plasma glucose rose to a plateau of ~10.5 mmol/l and tracer-determined EGP increased by ~60%. During GLP-1 infusion at matched plasma glucose levels, the rise of EGP from baseline was fully prevented. Lipolysis (as indexed by NEFA concentrations and tracer-determined glycerol rate of appearance) and substrate utilisation (by indirect calorimetry) were similar between control and GLP-1 infusion.

CONCLUSIONS/INTERPRETATION

GLP-1 inhibits EGP under conditions where plasma insulin and glucagon are not allowed to change and glucose concentrations are matched, indicating either a direct effect on hepatocytes or neurally mediated inhibition.

Bile acid transporters and regulatory nuclear receptors in the liver and beyond

Bile acid (BA) transporters are critical for maintenance of the enterohepatic BA circulation where BAs exert their multiple physiological functions including stimulation of bile flow, intestinal absorption of lipophilic nutrients, solubilization and excretion of cholesterol, as well as antimicrobial and metabolic effects. Tight regulation of BA transporters via nuclear receptors is necessary to maintain proper BA homeostasis. Hereditary and acquired defects of BA transporters are involved in the pathogenesis of several hepatobiliary disorders including cholestasis, gallstones, fatty liver disease and liver cancer, but also play a role in intestinal and metabolic disorders beyond the liver. Thus, pharmacological modification of BA transporters and their regulatory nuclear receptors opens novel treatment strategies for a wide range of disorders.

GLP-1 receptor activation indirectly reduces hepatic lipid accumulation but does not attenuate development of atherosclerosis in diabetic male ApoE(-/-) mice

Glucagon-like peptide-1 receptor (GLP-1R) agonists reduce lipid accumulation in peripheral tissues, attenuating atherosclerosis and hepatic steatosis in preclinical studies. We examined whether GLP-1R activation decreases atherosclerosis progression in high-fat diet-fed male ApoE(-/-) mice after administration of streptozotocin and treatment with the long-acting GLP-1R agonist taspoglutide administered once monthly vs. metformin in the drinking water for 12 wk. Taspoglutide did not reduce plaque area or lipid content in the aortic arch or abdominal aorta, and no significant change in aortic macrophage accumulation was detected after taspoglutide or metformin. In contrast, hepatic triglyceride levels were significantly reduced in livers from taspoglutide-treated mice. Both peripheral and intracerebroventricular administration of exendin-4 rapidly decreased plasma triglyceride levels in fasted mice, and taspoglutide therapy in ApoE(-/-) mice modulated the expression of hepatic genes controlling fatty acid uptake and oxidation. We were unable to detect expression of the entire Glp1r coding sequence in macrophages isolated from ApoE(-/-), C57BL/6, and IL10(-/-) mice. Similarly, Glp1r mRNA transcripts were not detected in RNA from isolated murine hepatocytes. Using Western blotting and tissue extracts from Glp1r(+/+) and Glp1r(-/-) mice, and cells transfected with a tagged murine GLP-1R cDNA, we could not validate the sensitivity and specificity of three different GLP-1R antisera commonly used for the detection of GLP-1R protein. Taken together, these findings illustrate divergent actions of GLP-1R agonists on atherosclerosis progression and accumulation of ectopic lipid in ApoE(-/-) mice and highlight the importance of indirect GLP-1R actions for the control of hepatic lipid accumulation.

Liver and diabetes. A vicious circle

The complex and bi-directional relationship linking the liver and diabetes has recently gained intense new interest. This critical review of the published work aims to highlight the most recent basic and clinical data underlying the development of type 2 diabetes, in those with non-alcoholic fatty liver disease. Moreover, the potentially detrimental effects of type 2 diabetes in liver injury are also discussed in each of the two sections of the present paper. Fatty liver and diabetes share insulin resistance as their chief pathogenic determinant. The roles of the hypothalamus, the intestinal microbiome, white adipose tissue and inflammation are discussed in detail. Molecular insights into hepatocyte insulin resistance as the initiator of systemic insulin resistance are also presented with full coverage of the danger of fatty acids. Lipotoxicity, apoptosis, lipoautophagy, endoplasmic reticular stress response and recent developments in genetics are discussed. Closing the circle, special emphasis is given to biochemical pathways and clinical evidence supporting the role of type 2 diabetes as a risk factor for the development of progressive liver disease, including non-alcoholic steatohepatitis, cirrhosis and primary liver cancer. In conclusion, data support non-alcoholic fatty liver disease as a risk factor for the development of type 2 diabetes which is, in turn, a major contributor to progressive liver disease. This pathway leading from fatty liver to type 2 diabetes and back from the latter to the progressive liver disease is a vicious circle.

Safety and efficacy of liraglutide in patients with type 2 diabetes and elevated liver enzymes: individual patient data meta-analysis of the LEAD program

BACKGROUND

Non-alcoholic fatty liver disease has reached epidemic proportions in type 2 diabetes (T2D). Glucagon-like peptide-1 analogues are licensed in T2D, yet little data exist on efficacy and safety in liver injury.

AIM

To assess the safety and efficacy of 26-week liraglutide on liver parameters in comparison with active-placebo.

METHODS

Individual patient data meta-analysis was performed using patient-level data combined from six 26-week, phase-III, randomised controlled T2D trials, which comprise the 'Liraglutide Effect and Action in Diabetes' (LEAD) program. The LEAD-2 sub-study was analysed to assess the effect on CT-measured hepatic steatosis.

RESULTS

Of 4442 patients analysed, 2241 (50.8%) patients had an abnormal ALT at baseline [mean ALT 33.8(14.9) IU/L in females; 47.3(18.3) IU/L in males]. Liraglutide 1.8 mg reduced ALT in these patients vs. placebo (-8.20 vs. -5.01 IU/L; P = 0.003), and was dose-dependent (no significant differences vs. placebo with liraglutide 0.6 or 1.2 mg). This effect was lost after adjusting for liraglutide's reduction in weight (mean ALT difference vs. placebo -1.41 IU/L, P = 0.21) and HbA1c (+0.57 IU/L, P = 0.63). Adverse effects with 1.8 mg liraglutide were similar between patients with and without baseline abnormal ALT. In LEAD-2 sub-study, liraglutide 1.8 mg showed a trend towards improving hepatic steatosis vs. placebo (liver-to-spleen attenuation ratio +0.10 vs. 0.00; P = 0.07). This difference was reduced when correcting for changes in weight (+0.06, P = 0.25) and HbA(1c) (0.00, P = 0.93).

CONCLUSIONS

Twenty-six weeks' liraglutide 1.8 mg is safe, well tolerated and improves liver enzymes in patients with type 2 diabetes. This effect appears to be mediated by its action on weight loss and glycaemic control.

Exendin-4 reduces glycemia by increasing liver glucokinase activity: an insulin independent effect

Exendin-4 is a stable peptide agonist of GLP-1 receptor that exhibits insulinotropic actions. Some in vivo studies indicated insulin-independent glucoregulatory actions of exendin-4. That finding prompted us to evaluate effects of exendin-4 on liver glucose metabolism. Acute and chronic treatment of exendin-4 resulted in increased hepatic glucokinase activity in db/db mice but not in lean C57 mice. The stimulatory effect of exendin-4 on glucokinase activity was abrogated by exendin 9-39, a GLP-1 antagonist. Exposure of hepatocytes isolated from db/db mice to exendin-4 elicited a rapid increase in cAMP, which was synergized by IBMX, an inhibitor of cAMP degradation. The GLP-1 antagonist, exendin 9-39, has abolished the cAMP generating effects of exendin-4 as well. Furthermore, chronic treatment of exendin-4 in streptozotocin-treated C57 mice resulted in restoration of hepatic glycogen, an indicator of improved glucose metabolism, without apparent changes in serum insulin levels. In conclusion, exendin-4 increased glucokinase enzyme protein and activity in liver via a mechanism parallel to and independent of insulin. Exendin-4-induced increase in hepatic glucokinase activity is more pronounced in the presence of hepatic insulin resistance. This beneficial effect of exendin-4 on liver glucokinase activity may be mediated by GLP-1 receptor.

Improved glycaemia correlates with liver fat reduction in obese, type 2 diabetes, patients given glucagon-like peptide-1 (GLP-1) receptor agonists

Glucagon-like peptide-1 receptor agonists (GLP-1 RA) are effective for obese patients with type 2 diabetes mellitus (T2DM) because they concomitantly target obesity and dysglycaemia. Considering the high prevalence of non-alcoholic fatty liver disease (NAFLD) in patients with T2DM, we determined the impact of 6 months’ GLP-1 RA therapy on intrahepatic lipid (IHL) in obese, T2DM patients with hepatic steatosis, and evaluated the inter-relationship between changes in IHL with those in glycosylated haemoglobin (HbA₁c), body weight, and volume of abdominal visceral and subcutaneous adipose tissue (VAT and SAT). We prospectively studied 25 (12 male) patients, age 50±10 years, BMI 38.4±5.6 kg/m² (mean ± SD) with baseline IHL of 28.2% (16.5 to 43.1%) and HbA₁c of 9.6% (7.9 to 10.7%) (median and interquartile range). Patients treated with metformin and sulphonylureas/DPP-IV inhibitors were given 6 months GLP-1 RA (exenatide, n = 19; liraglutide, n = 6). IHL was quantified by liver proton magnetic resonance spectroscopy (¹H MRS) and VAT and SAT by whole body magnetic resonance imaging (MRI). Treatment was associated with mean weight loss of 5.0 kg (95% CI 3.5,6.5 kg), mean HbA_1c reduction of 1·6% (17 mmol/mol) (0·8,2·4%) and a 42% relative reduction in IHL (−59.3, −16.5%). The relative reduction in IHL correlated with that in HbA₁c (ρ = 0.49; p = 0.01) but was not significantly correlated with that in total body weight, VAT or SAT. The greatest IHL reduction occurred in individuals with highest pre-treatment levels. Mechanistic studies are needed to determine potential direct effects of GLP-1 RA on human liver lipid metabolism.

GLP-1 receptor activation inhibits VLDL production and reverses hepatic steatosis by decreasing hepatic lipogenesis in high-fat-fed APOE*3-Leiden mice

Objective

In addition to improve glucose intolerance, recent studies suggest that glucagon-like peptide-1 (GLP-1) receptor agonism also decreases triglyceride (TG) levels. The aim of this study was to evaluate the effect of GLP-1 receptor agonism on very-low-density lipoprotein (VLDL)-TG production and liver TG metabolism.

Experimental Approach

The GLP-1 peptide analogues CNTO3649 and exendin-4 were continuously administered subcutaneously to high fat diet-fed APOE*3-Leiden transgenic mice. After 4 weeks, hepatic VLDL production, lipid content, and expression profiles of selected genes involved in lipid metabolism were determined.

Results

CNTO3649 and exendin-4 reduced fasting plasma glucose (up to −30% and −28% respectively) and insulin (−43% and −65% respectively). In addition, these agents reduced VLDL-TG production (−36% and −54% respectively) and VLDL-apoB production (−36% and −43% respectively), indicating reduced production of VLDL particles rather than reduced lipidation of apoB. Moreover, they markedly decreased hepatic content of TG (−39% and −55% respectively), cholesterol (−30% and −55% respectively), and phospholipids (−23% and −36% respectively), accompanied by down-regulation of expression of genes involved in hepatic lipogenesis (Srebp-1c, Fasn, Dgat1) and apoB synthesis (Apob).

Conclusion

GLP-1 receptor agonism reduces VLDL production and hepatic steatosis in addition to an improvement of glycemic control. These data suggest that GLP-receptor agonists could reduce hepatic steatosis and ameliorate dyslipidemia in patients with type 2 diabetes mellitus.

Intestinal GATA4 deficiency protects from diet-induced hepatic steatosis

BACKGROUND & AIMS

GATA4, a zinc finger domain transcription factor, is critical for jejunal identity. Mice with an intestine-specific GATA4 deficiency (GATA4iKO) are resistant to diet-induced obesity and insulin resistance. Although they have decreased intestinal lipid absorption, hepatic de novo lipogenesis is inhibited. Here, we investigated dietary lipid-dependent and independent effects on the development of steatosis and fibrosis in GATA4iKO mice.

METHODS

GATA4iKO and control mice were fed a Western-type diet (WTD) or a methionine and choline-deficient diet (MCDD) for 20 and 3 weeks, respectively. Functional effects of GATA4iKO on diet-induced liver steatosis were investigated.

RESULTS

WTD-but not MCDD-fed GATA4iKO mice showed lower hepatic concentrations of triglycerides, free fatty acids, and thiobarbituric acid reactive species and had reduced expression of lipogenic as well as fibrotic genes compared with controls. Reduced nuclear sterol regulatory element-binding protein-1c protein levels were accompanied by lower lipogenic gene expression. Oil red O and Sirius Red staining of liver sections confirmed the observed reduction in hepatic lipid accumulation and fibrosis. Immunohistochemical staining revealed an increased number of jejunal glucagon-like peptide 1 (GLP-1) positive cells in GATA4iKO mice. Consequently, we found enhanced phosphorylation of hepatic AMP-activated protein kinase and acetyl-CoA carboxylase alpha.

CONCLUSIONS

Our results provide strong indications for a protective effect of intestinal GATA4 deficiency on the development of hepatic steatosis and fibrosis via GLP-1, thereby blocking hepatic de novo lipogenesis.

Repression of sterol regulatory element-binding protein 1-c is involved in the protective effects of exendin-4 in pancreatic β-cell line

Exendin-4 (Ex-4), a long-acting agonist of glucagon-like peptide-1 receptor, is a novel anti-diabetic drug that prevents β-cells against various toxicities. However, the mechanism and molecules mediating the protection procession of Ex-4 are not fully understood. We investigated the protective effect of Ex-4 against lipotoxicity, mediated by a repression of sterol regulatory element-binding protein (SREBP)-1c, a regulator of genes expression involved in fat and cholesterol synthesis. To observe the effect of Ex-4, we evaluated glucose-stimulated insulin secretion (GSIS) and apoptosis in the MIN6 pancreatic β-cell line, which were cultured in DMEM medium containing 500 μM palmitate, with or without 10 nM Ex-4. We also examined the roles of SREBP-1c in lipotoxicity model by knockdown with si-RNA. Treatment with Ex-4 improved insulin secretion and survival as well as reduced SREBP-1c expression and activity in palmitate-treated MIN6 cells. This improvement was accompanied with an upregulation of PI3K/Akt signaling pathway, and LY294.002, a specific inhibitor of PI3 kinase, abrogated effects of Ex-4 on insulin secretion. Moreover, SREBP-1c in nuclei was increased by the inhibition of PI3 kinase. Lipotoxic effects of palmitate in the insulin secretion and apoptosis were significantly prevented by SREBP-1 knockdown. In conclusion, Ex-4 protects β-cell against palmitate-induced β-cell dysfunction and apoptosis, by inhibiting SREBP-1c expression and activity through the PI3K/Akt signaling pathway.

Direct effect of GLP-1 infusion on endogenous glucose production in humans

AIMS/HYPOTHESIS

Glucagon-like peptide-1 (GLP-1) lowers glucose levels by potentiating glucose-induced insulin secretion and inhibiting glucagon release. The question of whether GLP-1 exerts direct effects on the liver, independently of the hormonal changes, is controversial. We tested whether an exogenous GLP-1 infusion, designed to achieve physiological postprandial levels, directly affects endogenous glucose production (EGP) under conditions mimicking the fasting state in diabetes.

METHODS

In 14 healthy volunteers, we applied the pancreatic clamp technique, whereby plasma insulin and glucagon levels are clamped using somatostatin and hormone replacement. The clamp was applied in paired, 4 h experiments, during which saline (control) or GLP-1(7-37)amide (0.4 pmol min⁻¹ kg⁻¹) was infused.

RESULTS

During the control study, plasma insulin and glucagon were maintained at basal levels and plasma C-peptide was suppressed, such that plasma glucose rose to a plateau of ~10.5 mmol/l and tracer-determined EGP increased by ~60%. During GLP-1 infusion at matched plasma glucose levels, the rise of EGP from baseline was fully prevented. Lipolysis (as indexed by NEFA concentrations and tracer-determined glycerol rate of appearance) and substrate utilisation (by indirect calorimetry) were similar between control and GLP-1 infusion.

CONCLUSIONS/INTERPRETATION

GLP-1 inhibits EGP under conditions where plasma insulin and glucagon are not allowed to change and glucose concentrations are matched, indicating either a direct effect on hepatocytes or neurally mediated inhibition.

Perspective: TGR5 (Gpbar-1) in liver physiology and disease

Bile acids are signaling molecules with diverse endocrine functions. Bile acid effects are mediated through the nuclear receptor farnesoid X receptor (FXR), the G-protein coupled receptor TGR5 (Gpbar-1) and various other bile acid sensing molecules. TGR5 is almost ubiquitously expressed and has been detected in different non-parenchymal cells of human and rodent liver. Here, TGR5 has anti-inflammatory, anti-apoptotic and choleretic functions. Mice with targeted deletion of TGR5 are protected from the development of cholesterol gallstones. Administration of specific TGR5 agonists lowers serum and liver triglyceride levels thereby reducing liver steatosis. Furthermore, activation of TGR5 promotes intestinal glucagon-like peptide-1 (GLP-1) release, thereby modulating glucose homeostasis and energy expenditure in brown adipose tissue and skeletal muscle. Additionally, TGR5 exerts anti-inflammatory actions resulting in decreased liver injury in animal models of sepsis. These beneficial effects make TGR5 an attractive therapeutic target for metabolic diseases, such as diabetes, obesity, atherosclerosis and steatohepatitis.

The glucagon-like peptide-1 receptor agonist Exendin 4 has a protective role in ischemic injury of lean and steatotic liver by inhibiting cell death and stimulating lipolysis

Nonalcoholic fatty liver disease is an increasingly prevalent spectrum of conditions characterized by excess fat deposition within hepatocytes. Affected hepatocytes are known to be highly susceptible to ischemic insults, responding to injury with increased cell death, and commensurate liver dysfunction. Numerous clinical circumstances lead to hepatic ischemia. Mechanistically, specific means of reducing hepatic vulnerability to ischemia are of increasing clinical importance. In this study, we demonstrate that the glucagon-like peptide-1 receptor agonist Exendin 4 (Ex4) protects hepatocytes from ischemia reperfusion injury by mitigating necrosis and apoptosis. Importantly, this effect is more pronounced in steatotic livers, with significantly reducing cell death and facilitating the initiation of lipolysis. Ex4 treatment leads to increased lipid droplet fission, and phosphorylation of perilipin and hormone sensitive lipase - all hallmarks of lipolysis. Importantly, the protective effects of Ex4 are seen after a short course of perioperative treatment, potentially making this clinically relevant. Thus, we conclude that Ex4 has a role in protecting lean and fatty livers from ischemic injury. The rapidity of the effect and the clinical availability of Ex4 make this an attractive new therapeutic approach for treating fatty livers at the time of an ischemic insult.

The effectiveness of liraglutide in nonalcoholic fatty liver disease patients with type 2 diabetes mellitus compared to sitagliptin and pioglitazone

Background. Liraglutide leading to improve not only glycaemic control but also liver inflammation in non-alcoholic fatty liver disease (NAFLD) patients. Aims. The aim of this study is to elucidate the effectiveness of liraglutide in NAFLD patients with type 2 diabetes mellitus (T2DM) compared to sitagliptin and pioglitazone. Methods. We retrospectively enrolled 82 Japanese NAFLD patients with T2DM and divided into three groups (liraglutide: N = 26, sitagliptin; N = 36, pioglitazone; N = 20). We compared the baseline characteristics, changes of laboratory data and body weight. Results. At the end of follow-up, ALT, fast blood glucose, and HbA1c level significantly improved among the three groups. AST to platelet ratio significantly decreased in liraglutide group and pioglitazone group. The body weight significantly decreased in liraglutide group (81.8 kg to 78.0 kg, P < 0.01). On the other hands, the body weight significantly increased in pioglitazone group and did not change in sitagliptin group. Multivariate regression analysis indicated that administration of liraglutide as an independent factor of body weight reduction for more than 5% (OR 9.04; 95% CI 1.12–73.1, P = 0.04). Conclusions. Administration of liraglutide improved T2DM but also improvement of liver inflammation, alteration of liver fibrosis, and reduction of body weight.

Pertussis toxin, an inhibitor of G(αi) PCR, inhibits bile acid- and cytokine-induced apoptosis in primary rat hepatocytes

Excessive hepatocyte apoptosis is a common event in acute and chronic liver diseases leading to loss of functional liver tissue. Approaches to prevent apoptosis have therefore high potential for the treatment of liver disease. G-protein coupled receptors (GPCR) play crucial roles in cell fate (proliferation, cell death) and act through heterotrimeric G-proteins. G_αiPCRs have been shown to regulate lipoapoptosis in hepatocytes, but their role in inflammation- or bile acid-induced apoptosis is unknown. Here, we analyzed the effect of inhibiting G_αiPCR function, using pertussis toxin (PT), on bile acid- and cytokine-induced apoptosis in hepatocytes. Primary rat hepatocytes, HepG2-rNtcp cells (human hepatocellular carcinoma cells) or H-4-II-E cells (rat hepatoma cells) were exposed to glycochenodeoxycholic acid (GCDCA) or tumor necrosis factor-α (TNFα)/actinomycin D (ActD). PT (50–200 nmol/L) was added 30 minutes prior to the apoptotic stimulus. Apoptosis (caspase-3 activity, acridine orange staining) and necrosis (sytox green staining) were assessed. PT significantly reduced GCDCA- and TNFα/ActD-induced apoptosis in rat hepatocytes (−60%, p<0.05) in a dose-dependent manner (with no shift to necrosis), but not in HepG2-rNtcp cells or rat H-4-II-E cells. The protective effect of pertussis toxin was independent of the activation of selected cell survival signal transduction pathways, including ERK, p38 MAPK, PI3K and PKC pathways, as specific protein kinase inhibitors did not reverse the protective effects of pertussis toxin in GCDCA-exposed hepatocytes. Conclusion: Pertussis toxin, an inhibitor of G_αiPCRs, protects hepatocytes, but not hepatocellular carcinoma cells, against bile acid- and cytokine-induced apoptosis and has therapeutic potential as primary hepatoprotective drug, as well as adjuvant in anti-cancer therapy.

Dipeptidyl Peptidase IV Inhibitor Improves Insulin Resistance and Steatosis in a Refractory Nonalcoholic Fatty Liver Disease Patient: A Case Report

A 67-year-old Asian woman was referred to Kurume University Hospital due to abnormal liver function tests. She was diagnosed with nonalcoholic fatty liver disease (NAFLD). NAFLD was treated by diet therapy with medication of metformin and pioglitazone; however, NAFLD did not improve. Subsequently, the patient was administered sitagliptin. Although her energy intake and physical activity did not change, her hemoglobin A1c level was decreased from 7.8 to 6.4% 3 months after treatment. Moreover, her serum insulin level and homeostasis model assessment-insulin resistance value were also improved, as was the severity of hepatic steatosis. These findings indicate that sitagliptin may improve insulin resistance and steatosis in patients with refractory NAFLD.

GLP-1 Receptor Agonist and Non-Alcoholic Fatty Liver Disease

Non-alcoholic fatty liver disease (NAFLD), one of the most common liver diseases, is caused by the disruption of hepatic lipid homeostasis. It is associated with insulin resistance as seen in type 2 diabetes mellitus. Glucagon-like peptide-1 (GLP-1) is an incretin that increases insulin sensitivity and aids glucose metabolism. In recent in vivo and in vitro studies, GLP-1 presents a novel therapeutic approach against NAFLD by increasing fatty acid oxidation, decreasing lipogenesis, and improving hepatic glucose metabolism. In this report, we provide an overview of the role and mechanism of GLP-1 in relieving NAFLD.

Features, diagnosis, and treatment of nonalcoholic fatty liver disease

As the global incidence of obesity has increased, nonalcoholic fatty liver disease (NAFLD) has become a worldwide health concern. NAFLD occurs in children and adults of all ethnicities and includes isolated fatty liver and nonalcoholic steatohepatitis (NASH). Patients with NASH are at risk for developing cirrhosis, hepatic decompensation, and hepatocellular carcinoma and have increased all-cause mortality. NAFLD is associated with a variety of clinical conditions and is an independent risk factor for hepatocellular carcinoma. The pathogenesis of NAFLD and the specific steps that lead to NASH and advanced fibrosis are not fully understood, although researchers have found that a combination of environmental, genetic, and metabolic factors lead to advanced disease. There have been improvements in noninvasive radiographic methods to diagnose NAFLD, especially for advanced disease. However, liver biopsy is still the standard method of diagnosis for NASH. There are many challenges to treating patients with NASH, and no therapies have been approved by the U.S. Food and Drug Administration; multimodal approaches are being developed and becoming the standard of care. We review pathogenesis and treatment approaches for the West's largest liver-related public health concern.

The role of the Wnt signaling pathway in incretin hormone production and function

Glucose metabolism is tightly controlled by multiple hormones and neurotransmitters in response to nutritional, environmental, and emotional changes. In addition to insulin and glucagon produced by pancreatic islets, two incretin hormones, namely glucagon-like peptide-1 (GLP-1) and gastric inhibitory polypeptide (GIP, also known as glucose-dependent insulinotropic peptide), also play important roles in blood glucose homeostasis. The incretin hormones mainly exert their regulatory effects via their corresponding receptors, which are expressed in pancreatic islets as well as many other extra-pancreatic organs. Recent studies have shown that the genes which encode these two incretin hormones can be regulated by the effectors of the Wnt signaling pathway, including TCF7L2, a transcription factor identified recently by extensive genome wide association studies as an important type 2 diabetes risk gene. Interestingly, TCF7L2 and β-catenin (β-cat), another effector of Wnt signaling pathway, may also mediate the function of the incretin hormones as well as the expression of their receptors in pancreatic β-cells. In this review, we have introduced the incretin hormones and the Wnt signaling pathway, summarized recent findings in the field, and provided our perspectives.

Characterization of glucagon-like peptide 1 receptor (GLP1R) gene in chickens: functional analysis, tissue distribution, and identification of its transcript variants

Glucagon-like peptide 1 (GLP1) receptor plays a critical role in mediating the biological actions of GLP1 in mammals and fish; however, the gene structure, expression, and functionality of GLP1 receptor (GLP1R) remain largely unknown in birds. In this study, the full-length cDNA of chicken GLP1R (cGLP1R) was first cloned from brain tissue by reverse transcription PCR. The putative cGLP1R is 459 amino acids in length and shares high amino acid sequence identity with that of human (79%), rat (80%), and Xenopus (75%). Using a pGL3-CRE luciferase reporter system, we found that cGLP1R expressed in Chinese hamster ovary cells could be potently activated by cGLP1 (EC(50), 0.11 nM) but not by other structurally related peptides, indicating that cGLP1R is a functional receptor specific to cGLP1. Interestingly, in addition to identification of the transcript encoding cGLP1R of 459 amino acids, eight transcript variants, which were generated by alternative mRNA splicing and predicted to encode either C-terminally or N-terminally truncated cGLP1Rs, were also identified from chicken brain or testis. In line with this finding, multiple cGLP1R transcripts were detected to be expressed in most chicken tissues examined, including pancreas, gastrointestinal tract, and various brain regions by reverse transcription PCR. Using the dual-luciferase reporter assay system, we further found that the 5'-flanking region of cGLP1R gene displays promoter activities in cultured HepG2 and HEK293 cells, suggesting that it may control cGLP1R gene transcription in chicken tissues, including nonpancreatic tissues. Taken together, the results from the present study establish a molecular basis to investigate the roles of GLP1 in chickens.

Effects of liraglutide on β-cell-specific glucokinase-deficient neonatal mice

The glucagon-like peptide-1 receptor agonist liraglutide is used to treat diabetes. A hallmark of liraglutide is the glucose-dependent facilitation of insulin secretion from pancreatic β-cells. In β-cells, the glycolytic enzyme glucokinase plays a pivotal role as a glucose sensor. However, the role of glucokinase in the glucose-dependent action of liraglutide remains unknown. We first examined the effects of liraglutide on glucokinase haploinsufficient (Gck(+/-)) mice. Single administration of liraglutide significantly improved glucose tolerance in Gck(+/-) mice without increase of insulin secretion. We also assessed the effects of liraglutide on the survival rates, metabolic parameters, and histology of liver or pancreas of β-cell-specific glucokinase-deficient (Gck(-/-)) newborn mice. Liraglutide reduced the blood glucose levels in Gck(-/-) neonates but failed to prolong survival, and all the mice died within 1 wk. Furthermore, liraglutide did not improve glucose-induced insulin secretion in isolated islets from Gck(-/-) neonates. Liraglutide initially prevented increases in alanine aminotransferase, free fatty acids, and triglycerides in Gck(-/-) neonates but not at 4 d after birth. Liraglutide transiently prevented liver steatosis, with reduced triglyceride contents and elevated glycogen contents in Gck(-/-) neonate livers at 2 d after birth. Liraglutide also protected against reductions in β-cells in Gck(-/-) neonates at 4 d after birth. Taken together, β-cell glucokinase appears to be essential for liraglutide-mediated insulin secretion, but liraglutide may improve glycemic control, steatosis, and β-cell death in a glucokinase-independent fashion.

Insulin concentration modulates hepatic lipid accumulation in mice in part via transcriptional regulation of fatty acid transport proteins

Background

Fatty liver disease (FLD) is commonly associated with insulin resistance and obesity, but interestingly it is also observed at low insulin states, such as prolonged fasting. Thus, we asked whether insulin is an independent modulator of hepatic lipid accumulation.

Methods/Principal Findings

In mice we induced, hypo- and hyperinsulinemia associated FLD by diet induced obesity and streptozotocin treatment, respectively. The mechanism of free fatty acid induced steatosis was studied in cell culture with mouse liver cells under different insulin concentrations, pharmacological phosphoinositol-3-kinase (PI3K) inhibition and siRNA targeted gene knock-down. We found with in vivo and in vitro models that lipid storage is increased, as expected, in both hypo- and hyperinsulinemic states, and that it is mediated by signaling through either insulin receptor substrate (IRS) 1 or 2. As previously reported, IRS-1 was up-regulated at high insulin concentrations, while IRS-2 was increased at low levels of insulin concentration. Relative increase in either of these insulin substrates, was associated with an increase in liver-specific fatty acid transport proteins (FATP) 2&5, and increased lipid storage. Furthermore, utilizing pharmacological PI3K inhibition we found that the IRS-PI3K pathway was necessary for lipogenesis, while FATP responses were mediated via IRS signaling. Data from additional siRNA experiments showed that knock-down of IRSs impacted FATP levels.

Conclusions/Significance

States of perturbed insulin signaling (low-insulin or high-insulin) both lead to increased hepatic lipid storage via FATP and IRS signaling. These novel findings offer a common mechanism of FLD pathogenesis in states of both inadequate (prolonged fasting) and ineffective (obesity) insulin signaling.