The beneficial effects of a muscarinic agonist on pancreatic β-cells

The Role of G Protein-Coupled Receptors and Receptor Kinases in Pancreatic β-Cell Function and Diabetes

Type 2 diabetes (T2D) mellitus has emerged as a major global health concern that has accelerated in recent years due to poor diet and lifestyle. Afflicted individuals have high blood glucose levels that stem from the inability of the pancreas to make enough insulin to meet demand. Although medication can help to maintain normal blood glucose levels in individuals with chronic disease, many of these medicines are outdated, have severe side effects, and often become less efficacious over time, necessitating the need for insulin therapy. G protein-coupled receptors (GPCRs) regulate many physiologic processes, including blood glucose levels. In pancreatic β cells, GPCRs regulate β-cell growth, apoptosis, and insulin secretion, which are all critical in maintaining sufficient β-cell mass and insulin output to ensure euglycemia. In recent years, new insights into the signaling of incretin receptors and other GPCRs have underscored the potential of these receptors as desirable targets in the treatment of diabetes. The signaling of these receptors is modulated by GPCR kinases (GRKs) that phosphorylate agonist-activated GPCRs, marking the receptor for arrestin binding and internalization. Interestingly, genome-wide association studies using diabetic patient cohorts link the GRKs and arrestins with T2D. Moreover, recent reports show that GRKs and arrestins expressed in the β cell serve a critical role in the regulation of β-cell function, including β-cell growth and insulin secretion in both GPCR-dependent and -independent pathways. In this review, we describe recent insights into GPCR signaling and the importance of GRK function in modulating β-cell physiology. SIGNIFICANCE STATEMENT: Pancreatic β cells contain a diverse array of G protein-coupled receptors (GPCRs) that have been shown to improve β-cell function and survival, yet only a handful have been successfully targeted in the treatment of diabetes. This review discusses recent advances in our understanding of β-cell GPCR pharmacology and regulation by GPCR kinases while also highlighting the necessity of investigating islet-enriched GPCRs that have largely been unexplored to unveil novel treatment strategies.

LncRNA SPRY4‑IT1 is upregulated and promotes the proliferation of prostate cancer cells under hypoxia in vitro

The incidence and mortality rate of prostate cancer are among the highest for all cancers worldwide; this disease has a high cancer mortality rate in males, following lung cancer. Sprouty4-intron 1 (SPRY4-IT1) has been shown to play a variety of roles in tumors. Our previous study demonstrated that SPRY4-IT1 sponges microRNA-101-3p to promote the proliferation and metastasis of bladder cancer cells by upregulating enhancer of zeste homolog 2 expression; however, the role of SPRY4-IT1 in prostate cancer has not been fully established. In the present study, the expression levels, effects and mechanism of action of SPRY4-IT1 were investigated in prostate cancer tissues and cell lines using reverse transcription-quantitative PCR, western blotting, Cell Counting Kit-8 and flow cytometry assays. The results indicated that SPRY4-IT1 expression was upregulated in prostate cancer tissues and cell lines. Furthermore, hypoxia increased the expression levels of SPRY4-IT1 in prostate cancer cells. Knockdown of SPRY4-IT1 expression led to S-phase arrest, decreased expression levels of the cell cycle-associated proteins CDK2 and cyclin D1. AKT phosphorylation was also reduced by SPRY4-IT1 knockdown. In summary, the findings indicate the elevation of SPRY4-IT1 expression in prostate cancer. Under hypoxic conditions in vitro, SPRY4-IT1 overexpression promoted prostate cancer cell proliferation via a mechanism involving regulation of the cell cycle and the PI3K/AKT signaling pathway. Therefore, it may provide a basis for the development of targeted therapies.

GABA signalling in human pancreatic islets

The pancreatic islets are essential microorgans controlling the glucose level in the blood. The islets consist of different cell types which communicate with each other by means of auto- and paracrine interactions. One of the communication molecules produced by and released within the islets is γ-aminobutyric acid (GABA), a well-known inhibitor of neuronal excitability in the mammalian nervous system. Interestingly, GABA is also present in the blood in the nanomolar concentration range. Thus, GABA can affect not only islet function per se (e.g. hormone secretion) but also interactions between immune cells and the pancreatic islet cells in physiological conditions and in pathological states (particularly in type 1 diabetes). In the last decade the interest in GABA signalling in islets has increased. The broad research scope ranges from fundamental physiological studies at the molecular and cellular level to pathological implications and clinical trials. The aim of this mini-review is to outline the current status of the islet GABA field mostly in relation to human islets, to identify the gaps in the current knowledge and what clinical implications GABA signalling may have in islets.

Unravelling innervation of pancreatic islets

The central and peripheral nervous systems play critical roles in regulating pancreatic islet function and glucose metabolism. Over the last century, in vitro and in vivo studies along with examination of human pancreas samples have revealed the structure of islet innervation, investigated the contribution of sympathetic, parasympathetic and sensory neural pathways to glucose control, and begun to determine how the structure and function of pancreatic nerves are disrupted in metabolic disease. Now, state-of-the art techniques such as 3D imaging of pancreatic innervation and targeted in vivo neuromodulation provide further insights into the anatomy and physiological roles of islet innervation. Here, we provide a summary of the published work on the anatomy of pancreatic islet innervation, its roles, and evidence for disordered islet innervation in metabolic disease. Finally, we discuss the possibilities offered by new technologies to increase our knowledge of islet innervation and its contributions to metabolic regulation.

Quercetin prevents insulin dysfunction in hypertensive animals

Angiotensin II induced increase in hypertension enhances oxidative stress and compromises insulin action and pancreatic function. Quercetin-rich foods are beneficial for hypertensive and diabetic animals owing to their antioxidant function. The aim of this study was to evaluate the antioxidant effects of quercetin in hypertensive rats on insulin action, signaling, and secretion. Wistar rats were randomly divided into three groups: sham, hypertensive rats (H), and hypertensive rats supplemented with quercetin (HQ). After three months of initial hypertension, quercetin was administered at 50 mg/kg/day for 30 days. Our results indicate that hypertension and serum lipid peroxidation levels were reduced by quercetin supplementation. We observed increased insulin sensitivity in adipose tissue, corroborating the insulin tolerance test, HOMA index, and improvements in lipid profile. Despite normal insulin secretion at 2.8 and 20 mM of glucose, animals treated with quercetin exhibited increased number of islets per section; increased protein expression of muscarinic receptor type 3, VEGF, and catalase in islets; and hepatic mRNA levels of Ide were normalized. In conclusion, supplementation with quercetin improved insulin action and prevented pancreatic and metabolic dysfunction.

Protective potential of cerium oxide nanoparticles in diabetes mellitus

BACKGROUND

Diabetes mellitus (DM) is a non-communicable metabolic disease which is closely related to excessive oxidative stress after constant exposure to high plasma glucose. Although the current antidiabetic medications are effective in lowering blood glucose, these medications do not prevent or reverse the disease progression. Thus, there is a crucial need to explore new therapeutic interventions that could address this shortcoming. As cerium oxide nanoparticles (CONPs) possess antioxidant property, this agent may be used as a treatment option for the management of DM.

PURPOSE

This review aims to provide a critical evaluation of the pharmacological and antidiabetic effects of CONPs in cell and animal models. The roles of CONPs in attenuating DM complications are also presented in this report.

METHODS

We conducted a literature search in the PubMed database using the keywords "cerium oxide", "cerous oxide", "ceria", "nanoceria", and "diabetes" from inception to December 2020. The inclusion criteria were primary source articles that investigated the role of CONPs in DM and diabetic complications.

RESULTS

We identified 47 articles from the initial search. After the thorough screening, only 31 articles were included in this study. We found that CONPs can attenuate parameters that are related to DM and diabetic complications in various animals and cell culture models.

CONCLUSION

CONPs could potentially be used in the treatment of those with DM and complications caused by the disease.

Intestinal Electrical Stimulation Enhances Release of Postprandial Incretin Hormones Via Cholinergic Mechanisms

INTRODUCTION

Intestinal electrical stimulation (IES) has been reported to reduce body weight and improve glucose tolerance in obese and diabetic rats. Our study aimed to investigate possible IES mechanisms involving incretin hormones using intraduodenal glucose infusion in rats. We hypothesized that the enhanced release of postprandial glucagon-like peptide-1 (GLP-1) at early phase by IES was mediated through neuro/paracrine mechanisms involving the vagal nerve and glucose-dependent insulinotropic peptide (GIP).

METHODS

Fifteen normal male Sprague-Dawley rats chronically implanted with duodenal electrodes for IES, and an intra-duodenum catheter for the infusion of glucose were studied in a series of sessions with IES of different parameters with and without atropine and M3 receptor antagonist. Blood samples were collected via the tail vein for the measurement of blood glucose, and plasma GLP-1, and GIP.

RESULTS

(1) Compared to sham-IES, IES of 0.3 ms reduced blood glucose by 16.5-28.4% between 30 and 120 min (all time points p < 0.05), and IES of 3-ms reduced blood glucose at 60 (12.6%) and 90 min (11.8%). IES of 0.3 ms showed a greater hypoglycemic effect than 3 ms (p = 0.024) at 30 min. (2) IES elevated plasma GLP-1 with 0.3 ms (p = 0.001) and with 3 ms p = 0.03). (3) IES substantially elevated plasma GIP with 0.3 ms (p = 0.002) and with 3 ms (p < 0.001). (4) Pretreatment of atropine and the M3 receptor antagonist 4-DAMP blocked the effects of IES on GLP-1, GIP, and blood glucose.

CONCLUSIONS

IES reduces postprandial blood glucose by enhancing the release of GLP-1 and GIP mediated via the cholinergic mechanism.

Cholinergic-pathway-weakness-associated pancreatic islet dysfunction: a low-protein-diet imprint effect on weaned rat offspring

Currently, metabolic disorders are one of the major health problems worldwide, which have been shown to be related to perinatal nutritional insults, and the autonomic nervous system and endocrine pancreas are pivotal targets of the malprogramming of metabolic function. We aimed to assess glucose–insulin homeostasis and the involvement of cholinergic responsiveness (vagus nerve activity and insulinotropic muscarinic response) in pancreatic islet capacity to secrete insulin in weaned rat offspring whose mothers were undernourished in the first 2 weeks of the suckling phase. At delivery, dams were fed a low-protein (4% protein, LP group) or a normal-protein diet (20.5% protein, NP group) during the first 2 weeks of the suckling period. Litter size was adjusted to six pups per mother, and rats were weaned at 21 days old. Weaned LP rats presented a lean phenotype (P < 0.01); hypoglycaemia, hypoinsulinaemia and hypoleptinaemia (P < 0.05); and normal corticosteronaemia (P > 0.05). In addition, milk insulin levels in mothers of the LP rats were twofold higher than those of mothers of the NP rats (P < 0.001). Regarding glucose–insulin homeostasis, weaned LP rats were glucose-intolerant (P < 0.01) and displayed impaired pancreatic islet insulinotropic function (P < 0.05). The M3 subtype of the muscarinic acetylcholine receptor (M3mAChR) from weaned LP rats was less responsive, and the superior vagus nerve electrical activity was reduced by 30% (P < 0.01). A low-protein diet in the suckling period malprogrammes the vagus nerve to low tonus and impairs muscarinic response in the pancreatic β-cells of weaned rats, which are imprinted to secrete inadequate insulin amounts from an early age.

Beta-cell M3 muscarinic acetylcholine receptors as potential targets for novel antidiabetic drugs

A key feature of type 2 diabetes (T2D) is that beta-cells of the pancreatic islets fail to release sufficient amounts of insulin to overcome peripheral insulin resistance. Glucose-stimulated insulin secretion (GSIS) is regulated by the activity of numerous neurotransmitters, hormones and paracrine factors that act by stimulating specific G protein-coupled receptors expressed by pancreatic beta-cells. Studies with both mouse and human islets suggest that acetylcholine (ACh) acts on beta-cell M₃ muscarinic receptors (M3Rs) to promote GSIS. In mouse islets, beta-cell M3Rs are thought to be activated by ACh released from parasympathetic nerve endings. Interestingly, studies with human pancreatic islets suggest that ACh is synthesized, stored and released by alpha-cells, which, in human pancreatic islets, are intermingled with beta-cells. Independent of the source of pancreatic islet ACh, recent studies indicate that beta-cell M3Rs represent a potential target for drugs capable of promoting insulin release for therapeutic purposes. In this review, we will provide an overview about signaling pathways and molecules that regulate the activity of beta-cell M3Rs. We will also discuss a novel pharmacological strategy to stimulate the activity of these receptors to reduce the metabolic impairments associated with T2D.