Newer perspective on the coupling between glucose-mediated signaling and β-cell functionality

Expression analysis and possible functional roles of semaphorin/plexin/CRMP families in mouse pancreatic islets

Semaphorins were initially identified as axon guidance molecules that were widely expressed and involved in divergent functions in various organs, including neuronal development and immunological processes. Collapsin response mediator proteins (CRMPs) are involved in the intracellular signaling of semaphorin 3A (Sema3a) and are highly expressed in the nervous system. However, the participation of semaphorins or their receptors plexins and CRMPs in the regulation of islet function remains unknown. In this study, we measured the expression of semaphorin, plexin, and CRMP families in mouse islets, and their expression levels were altered by treatment with high glucose or a glucokinase activator (GKA). The expression and phosphorylation of CRMP-2 in islets were upregulated in high-fat diet (HF)-fed obese mice, and the expression of CRMP-2 was downregulated in islets from db/db mice. HF-fed CRMP-2 knockout mice exhibited impaired glucose tolerance. These results indicated that the semaphorin/plexin/CRMP families in mouse islets might be involved in glucose metabolism partly through glucose/glucokinase.

The matricellular protein Fibulin-5 regulates β-cell proliferation in an autocrine/paracrine manner

The matricellular protein Fibulin-5 (Fbln5) is a secreted protein that is essential for elastic fiber formation, and pancreatic islets are usually surrounded by the extracellular matrix (ECM), which includes elastic fibers. However, much uncertainty remains regarding the function of the ECM and its components in β-cells. Here, we describe the role of Fbln5 in β-cell replication. Fbln5 expression was increased upon glucose stimulation in β-cells of mouse and human islets. β-Cell-specific Fbln5-knockout (βFbln5KO) mice exhibit significantly reduced β-cell proliferation in vivo but not in vitro. Secreted extracellular Fbln5 enhances β-cell replication. Fbln5-deficient β-cells exhibit the downregulated expression of the gene encoding Polo-like kinase 1 (PLK1), which is accompanied by ERK-mediated FoxM1 nuclear export. These data suggest that Fbln5 is secreted from β-cells in response to glucose and plays important roles in the appropriate maintenance of β-cell functions in an autocrine or paracrine manner.

Harnessing beta cell regeneration biology for diabetes therapy

The pandemic scale of diabetes mellitus is alarming, its complications remain devastating, and current treatments still pose a major burden on those affected and on the healthcare system as a whole. As the disease emanates from the destruction or dysfunction of insulin-producing pancreatic β-cells, a real cure requires their restoration and protection. An attractive strategy is to regenerate β-cells directly within the pancreas; however, while several approaches for β-cell regeneration have been proposed in the past, clinical translation has proven challenging. This review scrutinizes recent findings in β-cell regeneration and discusses their potential clinical implementation. Hereby, we aim to delineate a path for innovative, targeted therapies to help shift from 'caring for' to 'curing' diabetes.

Association between protein undernutrition and diabetes: Molecular implications in the reduction of insulin secretion

Undernutrition is still a recurring nutritional problem in low and middle-income countries. It is directly associated with the social and economic sphere, but it can also negatively impact the health of the population. In this sense, it is believed that undernourished individuals may be more susceptible to the development of non-communicable diseases, such as diabetes mellitus, throughout life. This hypothesis was postulated and confirmed until today by several studies that demonstrate that experimental models submitted to protein undernutrition present alterations in glycemic homeostasis linked, in part, to the reduction of insulin secretion. Therefore, understanding the changes that lead to a reduction in the secretion of this hormone is essential to prevent the development of diabetes in undernourished individuals. This narrative review aims to describe the main molecular changes already characterized in pancreatic β cells that will contribute to the reduction of insulin secretion in protein undernutrition. So, it will provide new perspectives and targets for postulation and action of therapeutic strategies to improve glycemic homeostasis during this nutritional deficiency.

Protective effects of S100A8 on sepsis mortality: Links to sepsis risk in obesity and diabetes

Obesity and diabetes are independent risk factors for death during sepsis. S100A8, an alarmin, is related to inflammation, obesity, and diabetes. Here, we examine the role of S100A8 in sepsis of obesity and diabetes models. Injection of S100A8 prolongs the survival of septic mice induced by lethal endotoxemia, Escherichia coli injection, or cecal ligation and puncture. S100A8 decrease the LPS-induced expression of proinflammatory cytokines in peritoneal macrophages by inhibiting TLR4-mediated signals in an autocrine manner. db/db, ob/ob, and western diet-fed mice demonstrate reduced upregulation of S100A8 induced by LPS treatment in both serum and peritoneal cells. These mice also show shorter survival after LPS injection, and S100A8 supplementation prolonged the survival. While myelomonocytic cells-specific S100A8-deficient mice (Lyz2 ^cre :S100A8 ^{floxed/floxed} ) exhibit shorter survival after LPS treatment, S100A8 supplementation prolonged the survival. Thus, myelomonocytic cell-derived S100A8 is crucial for protection from sepsis, and S100A8 supplementation improves sepsis, particularly in mice with obesity and diabetes.

Glucokinase as a therapeutic target based on findings from the analysis of mouse models

I investigated mouse models to elucidate the pathophysiology and to establish a new treatment strategy for type 2 diabetes, with a particular focus on glucokinase. The decrease in pancreatic beta-cell function and mass are important factors in the pathophysiology of type 2 diabetes. My group have shown that glucokinase plays an important role in high-fat diet-induced and high-starch diet-induced beta-cell expansion. The findings indicated that the mechanism of short-term high-fat diet-induced beta-cell proliferation involved a glucokinase-independent pathway, suggesting that there are different pathways and mechanisms in the proliferation of pancreatic beta-cells during short-term versus long-term high-fat diets. Because enhancement of glucose signals via glucokinase is important for beta-cell proliferation, it was thought that beta-cell mass would be increased and insulin secretion would be maintained by glucokinase activators. However, sub-chronic administration of a glucokinase activator in db/db mice produced an unsustained hypoglycemic effect and promoted hepatic fat accumulation without changes in beta-cell function and mass. In contrast, my group have shown that inactivating glucokinase in beta-cells prevented beta-cell failure and led to an improvement in glucose tolerance in db/db mice. Regulation of glucokinase activity has an influence on the pathophysiology of type 2 diabetes and can be one of the therapeutic targets.

Hypoglycemic flavonoids from Selaginella tamariscina (P.Beauv.) Spring

Six flavonoids, namely, three undescribed biflavonoids, one undescribed 8-aryl flavonoid, and two known compounds, were isolated from Selaginella tamariscina (P.Beauv.) Spring. The structures and absolute configurations of those undescribed compounds were established by NMR spectroscopy data, HRESIMS analyses and electronic circular dichroism (ECD) analyses. In addition, all the isolates were evaluated for their hypoglycemic activity in HepG2 cells. Involvenflavone H, I, and J significantly increased glucose consumption in both normal and insulin-resistant HepG2 cells. Interestingly, these three compounds can effectively upregulate the protein expression of glucokinase (GCK) and adenylate cyclases (ADCYs). These results suggested that involvenflavone H, I, and J (especially involvenflavone J) may have potent hypoglycemic activity, which also provided promising molecular targets for the treatment of diabetes.

Translational research on human pancreatic β-cell mass expansion for the treatment of diabetes

The structural, functional, and pathological differences between human islets and rodent islets, such as mouse or rat islets, have been clarified, and research using human islets is becoming more important for elucidating the pathophysiology of diabetes and developing therapeutic strategies for diabetes. Increasing the functional human β-cell mass is a feasible method for the treatment of both type 1 and type 2 diabetes. The glucokinase-mediated glucose signaling pathway is known to promote β-cell proliferation not only in rodent models but also in humans. However, little is known about the signaling components of glucose- or glucokinase-mediated signaling pathways. Studies have gradually revealed the involvement of ER stress-related molecules, cell cycle regulators, inflammatory proteins, extracellular matrix proteins, and neurotransmitters in the glucokinase-mediated signaling pathway in β-cells. Unraveling the mechanisms of those molecules in the regulation of human β-cell mass will provide new insights into how the functional β-cell mass can be increased. The human islet distribution program is essential for human islet research. However, human islets for research are only available by import from Europe and America into Japan or Asia. Since Japanese or Asian diabetes patients possibly show different features compared with European or American diabetes patients, studies using human islets from Japanese or Asian populations have become important for the elucidation of pathophysiology in these specific population groups. This review outlines new pathways important for the regulation of human pancreatic β-cell mass and expectations regarding the establishment of a human islet distribution program in Japan.

Mutant Huntingtin Impairs Pancreatic β-cells by Recruiting IRS-2 and Disturbing the PI3K/AKT/FoxO1 Signaling Pathway in Huntington's Disease

Patients with Huntington's disease (HD) have an increased incidence of diabetes. However, the molecular mechanisms of pancreatic β-cell dysfunction have not been entirely clarified. Revealing the pathogenesis of diabetes can provide a novel understanding of the onset and progression of HD, as well as potential clues for the development of new therapeutics. Here, we demonstrated that the mouse pancreatic insulinoma cell line NIT-1 expressing N-terminal mutant huntingtin (mHTT) containing 160 polyglutamine (160Q cells) displayed lower cell proliferative ability than the cells expressing N-terminal wild-type HTT containing 20 polyglutamine (20Q cells). In addition, 160Q cells were more prone to apoptosis and exhibited deficient glucose-stimulated insulin expression and secretion. Furthermore, insulin signaling molecule insulin receptor substrate 2 (IRS-2) expression decreased and was recruited into mHTT aggregates. Consequently, glucose stimulation failed to activate the downstream molecule phosphatidylinositol-3 kinase (PI3K) in 160Q cells, leading to reduced phosphorylation levels of serine-threonine protein kinase AKT and forkhead box protein O1 (FoxO1). These data indicate that activation of the glucose-stimulated PI3K/AKT/FoxO1 signaling pathway is significantly blocked in pancreatic β-cells in HD. Importantly, insulin treatment inhibited the aggregation of mHTT and significantly improved the activation of PI3K/AKT/FoxO1 signaling in 160Q cells. These results suggest that the inhibition of the PI3K/AKT/FoxO1 pathway might be due to the recruitment of IRS-2 into mHTT aggregates in HD β-cells, ultimately contributing to the impairment of pancreatic β-cells. In conclusion, our work provides new insight into the underlying mechanisms of the high incidence of diabetes and abnormal glucose homeostasis in HD.