Daily administration of the GIP-R antagonist (Pro3)GIP in streptozotocin-induced diabetes suggests that insulin-dependent mechanisms are critical to anti-obesity-diabetes actions of (Pro3)GIP

GLP-1/GIP analogues: potential impact in the landscape of obesity pharmacotherapy

INTRODUCTION

: Obesity is recognised as a major healthcare challenge. Following years of slow progress in discovery of safe, effective therapies for weight management, recent approval of the glucagon-like peptide 1 receptor (GLP-1R) mimetics, liraglutide and semaglutide, for obesity has generated considerable excitement. It is anticipated these agents will pave the way for application of tirzepatide, a highly effective glucose-dependent insulinotropic polypeptide receptor (GIPR), GLP-1R co-agonist recently approved for management of type 2 diabetes mellitus.

AREAS COVERED

: Following promising weight loss in obese individuals in Phase III clinical trials, liraglutide and semaglutide were approved for weight management without diabetes. Tirzepatide has attained Fast Track designation for obesity management by the US Food and Drug Association. This narrative review summarises experimental, preclinical and clinical data for these agents and related GLP-1R/GIPR co-agonists, prioritising clinical research published within the last 10 years where possible.

EXPERT OPINION

: GLP-1R mimetics are often discontinued within 24-months, owing to gastrointestinal side-effects, meaning long-term application of these agents in obesity is questioned. Combined GIPR/GLP-1R agonism appears to induce fewer side-effects, indicating GLP-1R/GIPR co-agonists may be more suitable for enduring obesity management. After years of debate, this GIPR-biased GLP-1R/GIPR co-agonist highlights the therapeutic promise of including GIPR modulation for diabetes and obesity therapy.

Glucose-dependent insulinotropic polypeptide counteracts diet-induced obesity along with reduced feeding, elevated plasma leptin and activation of leptin-responsive and proopiomelanocortin neurons in the arcuate nucleus

AIM

To clarify the effects of glucose-dependent insulinotropic polypeptide (GIP) receptor agonists (GIPRAs) on feeding and body weight.

MATERIALS AND METHODS

Acute and subchronic effects of subcutaneous GIPFA-085, a long-acting GIPRA, on blood glucose, food intake, body weight, respiratory exchange ratio and plasma leptin levels were measured in diet-induced obese (DIO) mice and/or functional leptin-deficient ob/ob mice. The effects of GIPFA-085 on the hypothalamic arcuate nucleus (ARC) neurons from lean and DIO mice were studied by measuring cytosolic Ca²⁺ concentration ([Ca²⁺ ]_i ).

RESULTS

Single bolus GIPFA-085 (30, 300 nmol/kg) dose-dependently reduced blood glucose in glucose tolerance tests, elevated plasma leptin levels at 0.5-6 hours and inhibited food intake at 2-24 hours after injection in DIO mice. Daily GIPFA-085 (300 nmol/kg) inhibited food intake and increased fat utilization on day 1, and reduced body weight gain on days 3-12 of treatment in DIO, but not ob/ob, mice. GIPFA-085 increased [Ca²⁺ ]_i in the ARC leptin-responsive and proopiomelanocortin (POMC) neurons. GIPFA-085 and leptin cooperated to increase [Ca²⁺ ]_i in ARC neurons and inhibit food intake.

CONCLUSIONS

GIPFA-085 acutely inhibits feeding and increases lipid utilization, and sustainedly lowers body weight in DIO mice via mechanisms involving rises in leptin and activation of ARC leptin-responsive and POMC neurons. This study highlights the therapeutic potential of GIPRAs for treating obesity and diabetes.

Emetic Response to T-2 Toxin Correspond to Secretion of Glucagon-like Peptide-17–36 Amide and Glucose-Dependent Insulinotropic Polypeptide

The T-2 toxin, a major secondary metabolite of Fusarium Gramineae, is considered a great risk to humans and animals due to its toxicity, such as inducing emesis. The mechanism of emesis is a complex signal involving an imbalance of hormones and neurotransmitters, as well as activity of visceral afferent neurons. The T-2 toxin has been proven to induce emesis and possess the capacity to elevate expressions of intestinal hormones glucagon-like peptide-_17–36 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), both of which are important emetic factors. In addition, the activation of calcium-sensitive receptor (CaSR) and transient receptor potential (TRP) channels are engaged in intestinal hormone release. However, it is unknown whether hormones GLP-1 and GIP mediate T-2 toxin-induced emetic response through activating CaSR and TRP channels. To further assess the mechanism of T-2 toxin-induced emesis, we studied the hypothesis that T-2 toxin-caused emetic response and intestinal hormones GLP-1 and GIP released in mink are associated with activating calcium transduction. Following oral gavage and intraperitoneal injection T-2 toxin, emetic responses were observed in a dose-dependent manner, which notably corresponded to the secretion of GLP-1 and GIP, and were suppressed by pretreatment with respective antagonist Exending_9–39 and Pro3GIP. Additional research found that NPS-2143 (NPS) and ruthenium red (RR), respective antagonists of CaSR and TRP channels, dramatically inhibited both T-2 toxin-induced emesis response and the expression of plasma GLP-1 and GIP. According to these data, we observed that T-2 toxin-induced emetic response corresponds to secretion of GLP-1 and GIP via calcium transduction.

Individual and combined effects of GIP and xenin on differentiation, glucose uptake and lipolysis in 3T3-L1 adipocytes

The incretin hormone glucose-dependent insulinotropic polypeptide (GIP), released postprandially from K-cells, has established actions on adipocytes and lipid metabolism. In addition, xenin, a related peptide hormone also secreted from K-cells after a meal, has postulated effects on energy regulation and lipid turnover. The current study has probed direct individual and combined effects of GIP and xenin on adipocyte function in 3T3-L1 adipocytes, using enzyme-resistant peptide analogues, (d-Ala2)GIP and xenin-25-Gln, and knockdown (KD) of receptors for both peptides. (d-Ala2)GIP stimulated adipocyte differentiation and lipid accumulation in 3T3-L1 adipocytes over 96 h, with xenin-25-Gln evoking similar effects. Combined treatment significantly countered these individual adipogenic effects. Individual receptor KD impaired lipid accumulation and adipocyte differentiation, with combined receptor KD preventing differentiation. (d-Ala2)GIP and xenin-25-Gln increased glycerol release from 3T3-L1 adipocytes, but this lipolytic effect was significantly less apparent with combined treatment. Key adipogenic and lipolytic genes were upregulated by (d-Ala2)GIP or xenin-25-Gln, but not by dual peptide culture. Similarly, both (d-Ala2)GIP and xenin-25-Gln stimulated insulin-induced glucose uptake in 3T3-L1 adipocytes, but this effect was annulled by dual treatment. In conclusion, GIP and xenin possess direct, comparable, lipogenic and lipolytic actions in 3T3-L1 adipocytes. However, effects on lipid metabolism are significantly diminished by combined administration.

Blockade of gastric inhibitory polypeptide (GIP) action as a novel means of countering insulin resistance in the treatment of obesity-diabetes

Gastric inhibitory polypeptide (GIP) is a 42 amino acid hormone secreted from intestinal K-cells in response to nutrient ingestion. Despite a recognised physiological role for GIP as an insulin secretagogue to control postprandial blood glucose levels, growing evidence reveals important actions of GIP on adipocytes and promotion of fat deposition in tissues. As such, blockade of GIP receptor (GIPR) action has been proposed as a means to counter insulin resistance, and improve metabolic status in obesity and related diabetes. In agreement with this, numerous independent observations in animal models support important therapeutic applications of GIPR antagonists in obesity-diabetes. Sustained administration of peptide-based GIPR inhibitors, low molecular weight GIPR antagonists, GIPR neutralising antibodies as well as genetic knockout of GIPR's or vaccination against GIP all demonstrate amelioration of insulin resistance and reduced body weight gain in response to high fat feeding. These observations were consistently associated with decreased accumulation of lipids in peripheral tissues, thereby alleviating insulin resistance. Although the impact of prolonged GIPR inhibition on bone turnover still needs to be determined, evidence to date indicates that GIPR antagonists represent an exciting novel treatment option for obesity-diabetes.

Pharmacological Actions of Glucagon-Like Peptide-1, Gastric Inhibitory Polypeptide, and Glucagon

Glucagon family of peptide hormones is a group of structurally related brain-gut peptides that exert their pleiotropic actions through interactions with unique members of class B1 G protein-coupled receptors (GPCRs). They are key regulators of hormonal homeostasis and are important drug targets for metabolic disorders such as type-2 diabetes mellitus (T2DM), obesity, and dysregulations of the nervous systems such as migraine, anxiety, depression, neurodegeneration, psychiatric disorders, and cardiovascular diseases. The current review aims to provide a detailed overview of the current understanding of the pharmacological actions and therapeutic advances of three members within this family including glucagon-like peptide-1 (GLP-1), gastric inhibitory polypeptide (GIP), and glucagon.

Gastrointestinal hormones and the regulation of β-cell mass

Type 2 diabetes occurs due to a relative deficit in β-cell mass or function. Glucagon-like peptide 1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP), cholecystokinin (CCK), and gastrin are gastrointestinal hormones that are secreted in response to nutrient intake, regulating digestion, insulin secretion, satiety, and β-cell mass. In this review, we focus upon β-cell mass regulation. β-cell mass expands through β-cell proliferation and islet neogenesis; β-cell mass is lost via apoptosis. GLP-1 and GIP are well-studied gastrointestinal hormones and influence β-cell proliferation, apoptosis, and islet neogenesis. CCK regulates β-cell apoptosis and mitogenesis, and gastrin stimulates islet neogenesis. GLP-1 and GIP bind to G protein-coupled receptors and regulate β-cell mass via multiple signaling pathways. The protein kinase A pathway is central to this process because it directly regulates proliferative and anti-apoptotic genes and transactivates several signaling cascades, including Akt and mitogen-activated protein kinases. However, the signaling pathways downstream of G protein-coupled CCK receptors that influence β-cell mass remain unidentified. Gastrointestinal hormones integrate nutrient signals from the gut to the β-cell, regulating insulin secretion and β-cell mass adaptation.

The role of glucagon- and somatostatin-secreting cells in the regulation of insulin release and beta-cell function in heterotypic pseudoislets

BACKGROUND

Pseudoislet studies have concentrated on single beta-cell lines or a combination of insulin and glucagon-secreting cells, overlooking the potential role of somatostatin in insulin release. This study sought to evaluate a heterotypic pseudoislet model containing insulin- (MIN6), glucagon- (αTC1.9) and somatostatin (TGP52)-secreting cells of mouse origin and to compare these pseudoislets with traditional monolayer preparations.

METHODS

Cellular viability (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide and lactate dehydrogenase assays), proliferation (5-bromo-2-deoxyuridine ELISA), hormone content and functional insulin release in response to a variety of stimuli were measured. Differential expression of E-cadherin, connexin 36 and connexin 43 was assessed by reverse transcriptase-polymerase chain reaction and Western blot to determine a possible role for adherens in insulin release from these pseudoislets.

RESULTS

All pseudoislet cells displayed reduced proliferation coupled with an increase in cell death which may contribute to their static size in culture. While MIN6 and TGP52 cells expressed E-cadherin and showed sustained or improved hormone content when configured as pseudoislets, αTC1.9 lacked E-cadherin and contained less glucagon following pseudoislet formation. MIN6 and αTC1.9 cells expressed connexin 36, but not connexin 43 and TGP52 cells expressed connexin 43 only. In the presence of Alanine, Arginine and glucagon-like peptide-1, heterotypic pseudoislet cultures secreted levels of insulin that were comparable to that of MIN6 pseudoislets. In addition, pseudoislets comprising all three cell lines released more insulin into the surrounding culture medium than MIN6 pseudoislets when studied over a 1-week period.

CONCLUSIONS

The current model may prove useful in studying the role of islet cell interactions in the release of insulin from pancreatic islets.

Comparison of insulin release from MIN6 pseudoislets and pancreatic islets of Langerhans reveals importance of homotypic cell interactions

OBJECTIVES

Cellular communication is required for normal patterns of insulin secretion from β cells. Experiments using isolated islets of Langerhans are hampered by lack of supply and the consuming isolation process. Pseudoislets comprising clonal cells have emerged as an alternative to study islet-cell interactions and insulin secretion. The current study compared MIN6 pseudoislets and freshly isolated mouse islets.

METHODS

Insulin content and release were measured by insulin radioimmunoassay. Reverse transcription polymerase chain reaction and Western blot analysis of adhesion molecule expression were performed on MIN6 monolayers and pseudoislets. MIN6 cellular proliferation and viability were measured by 5-bromo-2-deoxyuridine (BrdU) enzyme-linked immunosorbent assay, 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT), and lactate dehydrogenase assays.

RESULTS

Mouse islets were found to have greater insulin content than pseudoislets. However, insulin release was comparable between the 2 groups. With the use of MIN6 monolayers as a control, the expression of the adhesion molecule E-cadherin and connexin 36 were found to be enhanced in cells cultured as pseudoislets. Moreover, connexin 43 was shown to be absent from MIN6 cells irrespective of configuration. Finally, MIN6 pseudoislets seem able to manage their rate of proliferation with apoptosis resulting in a static size in the culture for extended periods.

CONCLUSIONS

The current study found that MIN6 pseudoislets share many important functional and molecular features with islets of Langerhans.

Secreted phosphoprotein 1 upstream invasive network construction and analysis of lung adenocarcinoma compared with human normal adjacent tissues by integrative biocomputation

The aim of this study is to set up single molecular secreted phosphoprotein 1 (SPP1) upstream invasive network of lung adenocarcinoma. This paper proposed an integrated method based on linear programming and a decomposition procedure with integrated analysis of the significant function cluster using Kappa statistics and fuzzy heuristic clustering. Our study proved that only modules appearing in lung adenocarcinoma include cytokine module (CXCL13, GREM1_2 inhibition), cell adhesion module (COL11A1_2 activation; CDH3 inhibition), and receptor binding module (NMU activation; CXCL13, GREM1_2 inhibition), which increase the invasion of cancer cell. We compared skeletal development, signal, biological regulation, sequence variant modules between human normal adjacent tissues and lung adenocarcinoma. SPP1 skeletal development module appears in human normal adjacent tissues (COL11A1_1 activation; COL10A1 inhibition), whereas in lung adenocarcinoma (COL11A1_2, COL1A2 activation); signal module appears in human normal adjacent tissues (COL11A1_1, CXCL13, MMP11, SPINK1 activation; COL10A1, COL3A1 inhibition), whereas in lung adenocarcinoma (COL11A1_2, COL1A2, MMP12 activation; CDH3, CXCL13, GREM1_2, MMP11, SPINK1 inhibition); biological regulation module appears in human normal adjacent tissues (CXCL13, MKI67, PYCR1 activation; NEK2, SPDEF, TOP2A_2, TOX3_1 inhibition), whereas in lung adenocarcinoma (HMGB3, MKI67, NMU, PYCR1, TOX3_2 activation; CXCL13, SPDEF, TOP2A_2 inhibition); sequence variant module appears in human normal adjacent tissues (COL11A1_1, MKI67, MMP11 activation; ASPM, COL10A1, COL3A1, NEK2, TMPRSS4, TOP2A_2 inhibition), whereas in lung adenocarcinoma (COL11A1_2, COL1A2, HMMR, MKI67, MMP12 activation; ABCC3, ASPM, CDH3, MMP11, TOP2A_2 inhibition). It can be deduced that modules above in human normal adjacent tissues reflect the invasive inhibition of normal cells, whereas in lung adenocarcinoma increase the invasion of cancer cell. Our study of SPP1 upstream invasive network may be useful to identify novel and potentially targets for prognosis and therapy of lung adenocarcinoma.

Therapeutic potential for GIP receptor agonists and antagonists

Glucose-dependent insulinotropic polypeptide (GIP or gastric inhibitory polypeptide) is a 42-amino-acid hormone, secreted from the enteroendocrine K cells, which has insulin-releasing and extrapancreatic glucoregulatory actions. However, the unfavourable pharmacokinetic profile and the weak biological effects of native GIP limit its effectiveness for the treatment of type 2 diabetes. To overcome this, longer-acting GIP agonists exhibiting enzymatic stability and enhanced bioactivity have been generated and successfully tested in animal models of diabetes. Thus, GIP receptor agonists offer one of the newest classes of potential antidiabetic drug. GIP is also known to play a role in lipid metabolism and fat deposition. Accordingly, both genetic and chemical ablation of GIP signalling in mice with obesity-diabetes can protect against, or even reverse many of the obesity-associated metabolic disturbances. Strong parallels exist with the beneficial metabolic effects of Roux-en-Y gastric bypass in obese, insulin-resistant humans that surgically ablates GIP-secreting K cells. The purpose of this article is to highlight the therapeutic potential of GIP-based therapeutics in the treatment of type 2 diabetes and obesity.

Nutrient-dependent secretion of glucose-dependent insulinotropic polypeptide from primary murine K cells

AIMS/HYPOTHESIS

Glucose-dependent insulinotropic polypeptide (GIP) is an incretin hormone with anti-apoptotic effects on the pancreatic beta cell. The aim of this study was to generate transgenic mice with fluorescently labelled GIP-secreting K cells and to use these to investigate pathways by which K cells detect nutrients.

METHODS

Transgenic mice were generated in which the GIP promoter drives the expression of the yellow fluorescent protein Venus. Fluorescent cells were purified by flow cytometry and analysed by quantitative RT-PCR. GIP secretion was assayed in primary cultures of small intestine.

RESULTS

Expression of Venus in transgenic mice was restricted to K cells, as assessed by immunofluorescence and measurements of the Gip mRNA and GIP protein contents of purified cells. K cells expressed high levels of mRNA for Kir6.2 (also known as Kcnj11), Sur1 (also known as Abcc8), Sglt1 (also known as Slc5a1), and of the G-protein-coupled lipid receptors Gpr40 (also known as Ffar1), Gpr119 and Gpr120. In primary cultures, GIP release was stimulated by glucose, glutamine and linoleic acid, and potentiated by forskolin plus 3-isobutyl-1-methylxanthine (IBMX), but was unaffected by the artificial sweetener sucralose. Secretion was half-maximal at 0.6 mmol/l glucose and partially mimicked by alpha-methylglucopyranoside, suggesting the involvement of SGLT1. Tolbutamide triggered secretion under basal conditions, whereas diazoxide suppressed responses in forskolin/IBMX.

CONCLUSIONS/INTERPRETATION

These transgenic mice and primary culture techniques provide novel opportunities to interrogate the mechanisms of GIP secretion. Glucose-triggered GIP secretion was SGLT1-dependent and modulated by K(ATP) channel activity but not determined by sweet taste receptors. Synergistic stimulation by elevated cAMP and glucose suggests that targeting appropriate G-protein-coupled receptors may provide opportunities to modulate GIP release in vivo.

Expression of human insulin gene wrapped with chitosan nanoparticles in NIH3T3 cells and diabetic rats

AIM

To study the expression of human insulin gene wrapped with chitosan nanoparticles in NIH3T3 cells and diabetic rats.

METHODS

pCMV.Ins, an expression plasmid of the human insulin gene, was constructed. In total, 100 microg pCMV.Ins wrapped with chitosan nanoparticles (chitosan-pCMV.Ins) was transfected to NIH3T3 cells and diabetes rats through lavage and coloclysis, respectively. The transfected cells were grown in Dulbecco's modified Eagle's medium, containing G418, for 72 h after transfection. The clones were selected and continued to grow in G418 medium for 24 d. The expression of human insulin was detected by immunohistochemistry. Human insulin in the culture medium of transfected cells was measured. Fasting blood glucose and plasma human insulin of diabetic rats were measured for 5 d after transfection. RT-PCR and Western blotting were performed to confirm the expression of the human insulin gene in diabetic rats.

RESULTS

Approximately 10% of NIH3T3 cells transfected by chitosan-pCMV.Ins expressed human insulin. Human insulin in the culture medium of NIH3T3 cells transfected by chitosan-pCMV.Ins significantly increased compared with that of the control group (P<0.01). Fasting blood glucose levels of the lavage group and the coloclysis group decreased significantly in 5 d (P<0.01) in comparison, while plasma insulin levels were much higher (P<0.01). The human insulin gene mRNA and human insulin were only detected in the lavage and the coloclysis groups.

CONCLUSION

The human insulin gene can be transfected and expressed successfully by chitosan- pCMV.Ins in NIH3T3 cells and diabetes rats, which indicates that chitosan is a promising, non-viral vector for gene expression.

Gene therapy for type 1 diabetes mellitus in rats by gastrointestinal administration of chitosan nanoparticles containing human insulin gene

AIM: To study the expression of human insulin gene in gastrointestinal tracts of diabetic rats.

METHODS: pCMV.Ins, an expression plasmid of the human insulin gene, wrapped with chitosan nanoparticles, was transfected to the diabetic rats through lavage and coloclysis, respectively. Fasting blood glucose and plasma insulin levels were measured for 7 d. Reverse transcription polymerase chain reaction (RT-PCR) analysis and Western blot analysis were performed to confirm the expression of human insulin gene.

RESULTS: Compared with the control group, the fasting blood glucose levels in the lavage and coloclysis groups were decreased significantly in 4 d (5.63 ± 0.48 mmol/L and 5.07 ± 0.37 mmol/L vs 22.12 ± 1.31 mmol/L, respectively, P < 0.01), while the plasma insulin levels were much higher (32.26 ± 1.81 &mgr;IU/mL and 32.79 ± 1.84 &mgr;IU/mL vs 14.23 ± 1.38 &mgr;IU/mL, respectively, P < 0.01). The human insulin gene mRNA and human insulin were only detected in the lavage and coloclysis groups.

CONCLUSION: Human insulin gene wrapped with chitosan nanoparticles can be successfully transfected to rats through gastrointestinal tract, indicating that chitosan is a promising non-viral vector.