Opposing effects of prostaglandin E2 receptors EP3 and EP4 on mouse and human β-cell survival and proliferation

Gαz-independent and -dependent Improvements With EPA Supplementation on the Early Type 1 Diabetes Phenotype of NOD Mice

Prostaglandin E2 (PGE2) is a key mediator of inflammation and is derived from the omega-6 polyunsaturated fatty acid, arachidonic acid (AA). In the β-cell, the PGE2 receptor, Prostaglandin EP3 receptor (EP3), is coupled to the unique heterotrimeric G protein alpha subunit, Gɑz to reduce the production of cyclic adenosine monophosphate (cAMP), a key signaling molecule that activates β-cell function, proliferation, and survival pathways. Nonobese diabetic (NOD) mice are a strong model of type 1 diabetes (T1D), and NOD mice lacking Gɑz are protected from hyperglycemia. Therefore, limiting systemic PGE2 production could potentially improve both the inflammatory and β-cell dysfunction phenotype of T1D. Here, we sought to evaluate the effect of eicosapentaenoic acid (EPA) feeding, which limits PGE2 production, on the early T1D phenotype of NOD mice in the presence and absence of Gαz. Wild-type and Gαz knockout NOD mice were fed a control or EPA-enriched diet for 12 weeks, beginning at age 4 to 5 weeks. Oral glucose tolerance, splenic T-cell populations, islet cytokine/chemokine gene expression, islet insulitis, measurements of β-cell mass, and measurements of β-cell function were quantified. EPA diet feeding and Gɑz loss independently improved different aspects of the early NOD T1D phenotype and coordinated to alter the expression of certain cytokine/chemokine genes and enhance incretin-potentiated insulin secretion. Our results shed critical light on the Gαz-dependent and -independent effects of dietary EPA enrichment and provide a rationale for future research into novel pharmacological and dietary adjuvant therapies for T1D.

The prostaglandin E2 EP3 receptor has disparate effects on islet insulin secretion and content in β-cells in a high-fat diet-induced mouse model of obesity

Signaling through prostaglandin E2 EP3 receptor (EP3) actively contributes to the β-cell dysfunction of type 2 diabetes (T2D). In T2D models, full-body EP3 knockout mice have a significantly worse metabolic phenotype than wild-type controls due to hyperphagia and severe insulin resistance resulting from loss of EP3 in extra-pancreatic tissues, masking any potential beneficial effects of EP3 loss in the β cell. We hypothesized β-cell-specific EP3 knockout (EP3 βKO) mice would be protected from high-fat diet (HFD)-induced glucose intolerance, phenocopying mice lacking the EP3 effector, Gαz, which is much more limited in its tissue distribution. When fed a HFD for 16 wk, though, EP3 βKO mice were partially, but not fully, protected from glucose intolerance. In addition, exendin-4, an analog of the incretin hormone, glucagon-like peptide 1, more strongly potentiated glucose-stimulated insulin secretion in islets from both control diet- and HFD-fed EP3 βKO mice as compared with wild-type controls, with no effect of β-cell-specific EP3 loss on islet insulin content or markers of replication and survival. However, after 26 wk of diet feeding, islets from both control diet- and HFD-fed EP3 βKO mice secreted significantly less insulin as a percent of content in response to stimulatory glucose, with or without exendin-4, with elevated total insulin content unrelated to markers of β-cell replication and survival, revealing severe β-cell dysfunction. Our results suggest that EP3 serves a critical role in temporally regulating β-cell function along the progression to T2D and that there exist Gαz-independent mechanisms behind its effects.NEW & NOTEWORTHY The EP3 receptor is a strong inhibitor of β-cell function and replication, suggesting it as a potential therapeutic target for the disease. Yet, EP3 has protective roles in extrapancreatic tissues. To address this, we designed β-cell-specific EP3 knockout mice and subjected them to high-fat diet feeding to induce glucose intolerance. The negative metabolic phenotype of full-body knockout mice was ablated, and EP3 loss improved glucose tolerance, with converse effects on islet insulin secretion and content.

The PTGS2/COX2-PGE2 signaling cascade in inflammation: Pro or anti? A case study with type 1 diabetes mellitus

Prostaglandins are lipid mediators involved in physiological processes, such as constriction or dilation of blood vessels, but also pathophysiological processes, which include inflammation, pain and fever. They are produced by almost all cell types in the organism by activation of Prostaglandin endoperoxide synthases/Cyclooxygenases. The inducible Prostaglandin Endoperoxide Synthase 2/Cyclooxygenase 2 (PTGS2/COX2) plays an important role in pathologies associated with inflammatory signaling. The main product derived from PTGS2/COX2 expression and activation is Prostaglandin E2 (PGE2), which promotes a wide variety of tissue-specific effects, pending environmental inputs. One of the major sources of PGE2 are infiltrating inflammatory cells - the production of this molecule increases drastically in damaged tissues. Immune infiltration is a hallmark of type 1 diabetes mellitus, a multifactorial disease that leads to autoimmune-mediated pancreatic beta cell destruction. Controversial effects for the PTGS2/COX2-PGE2 signaling cascade in pancreatic islet cells subjected to diabetogenic conditions have been reported, allocating PGE2 as both, cause and consequence of inflammation. Herein, we review the main effects of this molecular pathway in a tissue-specific manner, with a special emphasis on beta cell mass protection/destruction and its potential role in the prevention or development of T1DM. We also discuss strategies to target this pathway for future therapies.

What the BTBR/J mouse has taught us about diabetes and diabetic complications

Human and mouse genetics have delivered numerous diabetogenic loci, but it is mainly through the use of animal models that the pathophysiological basis for their contribution to diabetes has been investigated. More than 20 years ago, we serendipidously identified a mouse strain that could serve as a model of obesity-prone type 2 diabetes, the BTBR (Black and Tan Brachyury) mouse (BTBR T+ Itpr3tf/J, 2018) carrying the Lepob mutation. We went on to discover that the BTBR-Lepob mouse is an excellent model of diabetic nephropathy and is now widely used by nephrologists in academia and the pharmaceutical industry. In this review, we describe the motivation for developing this animal model, the many genes identified and the insights about diabetes and diabetes complications derived from >100 studies conducted in this remarkable animal model.

Posttranscriptional regulation of the prostaglandin E receptor spliced-isoform EP3-γ and its implication in pancreatic β-cell failure

In Type 2 diabetes (T2D), elevated lipid levels have been suggested to contribute to insulin resistance and β-cell dysfunction. We previously reported that the expression of the PGE2 receptor EP3 is elevated in islets of T2D individuals and is preferentially stimulated by palmitate, leading to β-cell failure. The mouse EP3 receptor generates three isoforms by alternative splicing which differ in their C-terminal domain and are referred to as mEP3α, mEP3β, and mEP3γ. We bring evidence that the expression of the mEP3γ isoform is elevated in islets of diabetic db/db mice and is selectively upregulated by palmitate. Specific knockdown of the mEP3γ isoform restores the expression of β-cell-specific genes and rescues MIN6 cells from palmitate-induced dysfunction and apoptosis. This study indicates that palmitate stimulates the expression of the mEP3γ by a posttranscriptional mechanism, compared to the other spliced isoforms, and that the de novo synthesized ceramide plays an important role in FFA-induced mEP3γ expression in β-cells. Moreover, induced levels of mEP3γ mRNA by palmitate or ceramide depend on p38 MAPK activation. Our findings suggest that mEP3γ gene expression is regulated at the posttranscriptional level and defines the EP3 signaling axis as an important pathway mediating β-cell-impaired function and demise.

Harnessing prostaglandin E2 signaling to ameliorate autoimmunity

The etiology of most autoimmune diseases remains unknown; however, shared among them is a disruption of immunoregulation. Prostaglandin lipid signaling molecules possess context-dependent immunoregulatory properties, making their role in autoimmunity difficult to decipher. For example, prostaglandin E2 (PGE2) can function as an immunosuppressive molecule as well as a proinflammatory mediator in different circumstances, contributing to the expansion and activation of T cell subsets associated with autoimmunity. Recently, PGE2 was shown to play important roles in the resolution and post-resolution phases of inflammation, promoting return to tissue homeostasis. We propose that PGE2 plays both proinflammatory and pro-resolutory roles in the etiology of autoimmunity, and that harnessing this signaling pathway during the resolution phase might help prevent autoimmune attack.

Plasma Prostaglandin E2 Metabolite Levels Predict Type 2 Diabetes Status and One-Year Therapeutic Response Independent of Clinical Markers of Inflammation

Over half of patients with type 2 diabetes (T2D) are unable to achieve blood glucose targets despite therapeutic compliance, significantly increasing their risk of long-term complications. Discovering ways to identify and properly treat these individuals is a critical problem in the field. The arachidonic acid metabolite, prostaglandin E₂ (PGE₂), has shown great promise as a biomarker of β-cell dysfunction in T2D. PGE₂ synthesis, secretion, and downstream signaling are all upregulated in pancreatic islets isolated from T2D mice and human organ donors. In these islets, preventing β-cell PGE₂ signaling via a prostaglandin EP3 receptor antagonist significantly improves their glucose-stimulated and hormone-potentiated insulin secretion response. In this clinical cohort study, 167 participants, 35 non-diabetic, and 132 with T2D, were recruited from the University of Wisconsin Hospital and Clinics. At enrollment, a standard set of demographic, biometric, and clinical measurements were performed to quantify obesity status and glucose control. C reactive protein was measured to exclude acute inflammation/illness, and white cell count (WBC), erythrocyte sedimentation rate (ESR), and fasting triglycerides were used as markers of systemic inflammation. Finally, a plasma sample for research was used to determine circulating PGE₂ metabolite (PGEM) levels. At baseline, PGEM levels were not correlated with WBC and triglycerides, only weakly correlated with ESR, and were the strongest predictor of T2D disease status. One year after enrollment, blood glucose management was assessed by chart review, with a clinically-relevant change in hemoglobin A1c (HbA1c) defined as ≥0.5%. PGEM levels were strongly predictive of therapeutic response, independent of age, obesity, glucose control, and systemic inflammation at enrollment. Our results provide strong support for future research in this area.

Circulating exosome-like vesicles of humans with nondiabetic obesity impaired islet β-cell proliferation, which was associated with decreased Omentin-1 protein cargo

The regulation of β-cell mass in the status of nondiabetic obesity remains not well understood. We aimed to investigate the role of circulating exosome-like vesicles (ELVs) isolated from humans with simple obesity in the regulation of islet β-cell mass. Between June 2017 and July 2019, 81 subjects with simple obesity and 102 healthy volunteers with normal weight were recruited. ELVs were isolated by ultra-centrifugation. The proliferations of β-cells and islets were measured by 5-ethynl-2′-deoxyuridine (EdU). Protein components in ELVs were identified by Quantitative Proteomic Analysis and verified by Western blot and ELISA. The role of specific exosomal protein was analyzed by gain-of-function approach in ELVs released by 3T3-L1 preadipocytes. Circulating ELVs from subjects with simple obesity inhibited β-cell proliferation in vitro without affecting its apoptosis, secretion, and inflammation. The protein levels of Rictor and Omentin-1 were downregulated in circulating ELVs from subjects with simple obesity and associated with the obesity-linked pathologic conditions. The ELV-carried Omentin-1 and Omentin-1 protein per se were validated to increase β-cell proliferation and activate Akt signaling pathway. Moreover, Omentin-1 in ELVs was downregulated by insulin. The circulating ELVs may act as a negative regulator for β-cell mass in nondiabetic obesity through inhibiting β-cell proliferation. This effect was associated with downregulated Omentin-1 protein in ELVs. This newly identified ELV-carried protein could be a mediator linking insulin resistance to impaired β-cell proliferation and a new potential target for increasing β-cell mass in obesity and T2DM.

NR5A2/LRH-1 regulates the PTGS2-PGE2-PTGER1 pathway contributing to pancreatic islet survival and function

LRH-1/NR5A2 is implicated in islet morphogenesis postnatally, and its activation using the agonist BL001 protects islets against apoptosis, reverting hyperglycemia in mouse models of Type 1 Diabetes Mellitus. Islet transcriptome profiling revealed that the expression of PTGS2/COX2 is increased by BL001. Herein, we sought to define the role of LRH-1 in postnatal islet morphogenesis and chart the BL001 mode of action conferring beta cell protection. LRH-1 ablation within developing beta cells impeded beta cell proliferation, correlating with mouse growth retardation, weight loss, and hypoglycemia leading to lethality. LRH-1 deletion in adult beta cells abolished the BL001 antidiabetic action, correlating with beta cell destruction and blunted Ptgs2 induction. Islet PTGS2 inactivation led to reduced PGE₂ levels and loss of BL001 protection against cytokines as evidenced by increased cytochrome c release and cleaved-PARP. The PTGER1 antagonist—ONO-8130—negated BL001-mediated islet survival. Our results define the LRH-1/PTGS2/PGE₂/PTGER1 signaling axis as a key pathway mediating BL001 survival properties.

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LRH-1 ablation during development impedes neonatal beta cell replication

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LRH-1 knockout in adult beta cells negates BL001-mediated antidiabetic properties

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Islets lacking PTGS2 are refractory to BL001-mediated protection against cytokines

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PTGER1 relays the BL001/LRH-1/PTGS2/PGE2 signaling axis to islet survival

Small-molecule discovery in the pancreatic beta cell

The pancreatic beta cell is the only cell type in the body responsible for insulin secretion, and thus plays a unique role in the control of glucose homeostasis. The loss of beta-cell mass and function plays an important role in both type 1 and type 2 diabetes. Thus, using chemical biology to identify small molecules targeting the beta cell could be an important component to developing future therapeutics for diabetes. This strategy provides an attractive path toward increasing beta-cell numbers in vivo. A regenerative strategy involves enhancing proliferation, differentiation, or neogenesis. On the other hand, protecting beta cells from cell death, or improving maturity and function, could preserve beta-cell mass. Here, we discuss the current state of chemical matter available to study beta-cell regeneration, and how they were discovered.

Effects of Arachidonic Acid and Its Metabolites on Functional Beta-Cell Mass

Arachidonic acid (AA) is a polyunsaturated 20-carbon fatty acid present in phospholipids in the plasma membrane. The three primary pathways by which AA is metabolized are mediated by cyclooxygenase (COX) enzymes, lipoxygenase (LOX) enzymes, and cytochrome P450 (CYP) enzymes. These three pathways produce eicosanoids, lipid signaling molecules that play roles in biological processes such as inflammation, pain, and immune function. Eicosanoids have been demonstrated to play a role in inflammatory, renal, and cardiovascular diseases as well type 1 and type 2 diabetes. Alterations in AA release or AA concentrations have been shown to affect insulin secretion from the pancreatic beta cell, leading to interest in the role of AA and its metabolites in the regulation of beta-cell function and maintenance of beta-cell mass. In this review, we discuss the metabolism of AA by COX, LOX, and CYP, the roles of these enzymes and their metabolites in beta-cell mass and function, and the possibility of targeting these pathways as novel therapies for treating diabetes.

Pharmacological modulation of prostaglandin E2 (PGE2 ) EP receptors improves cardiomyocyte function under hyperglycemic conditions

Type 2 diabetes (T2D) affects >30 million Americans and nearly 70% of individuals with T2D will die from cardiovascular disease (CVD). Circulating levels of the inflammatory signaling lipid, prostaglandin E₂ (PGE₂ ), are elevated in the setting of obesity and T2D and are associated with decreased cardiac function. The EP3 and EP4 PGE₂ receptors have opposing actions in several tissues, including the heart: overexpression of EP3 in cardiomyocytes impairs function, while EP4 overexpression improves function. Here we performed complementary studies in vitro with isolated cardiomyocytes and in vivo using db/db mice, a model of T2D, to analyze the effects of EP3 inhibition or EP4 activation on cardiac function. Using echocardiography, we found that 2 weeks of systemic treatment of db/db mice with 20 mg/kg of EP3 antagonist, beginning at 6 weeks of age, improves ejection fraction and fractional shortening (with no effect on heart rate). We further show that either EP3 blockade or EP4 activation enhances contractility and calcium cycling in isolated mouse cardiomyocytes cultured in both normal and high glucose. Thus, peak [Ca²⁺ ]_I transient amplitude was increased, while time to peak [Ca²⁺ ]_I and [Ca²⁺ ]_I decay were decreased. These data suggest that modulation of EP3 and EP4 activity has beneficial effects on cardiomyocyte contractility and overall heart function.

Pharmacological blockade of the EP3 prostaglandin E2 receptor in the setting of type 2 diabetes enhances β-cell proliferation and identity and relieves oxidative damage

OBJECTIVE

Type 2 diabetes is characterized by hyperglycemia and inflammation. Prostaglandin E2, which signals through four G protein-coupled receptors (EP1-4), is a mediator of inflammation and is upregulated in diabetes. We have shown previously that EP3 receptor blockade promotes β-cell proliferation and survival in isolated mouse and human islets ex vivo. Here, we analyzed whether systemic EP3 blockade could enhance β-cell mass and identity in the setting of type 2 diabetes using mice with a spontaneous mutation in the leptin receptor (Leprdb).

METHODS

Four- or six-week-old, db/+, and db/db male mice were treated with an EP3 antagonist daily for two weeks. Pancreata were analyzed for α-cell and β-cell proliferation and β-cell mass. Islets were isolated for transcriptomic analysis. Selected gene expression changes were validated by immunolabeling of the pancreatic tissue sections.

RESULTS

EP3 blockade increased β-cell mass in db/db mice through enhanced β-cell proliferation. Importantly, there were no effects on α-cell proliferation. EP3 blockade reversed the changes in islet gene expression associated with the db/db phenotype and restored the islet architecture. Expression of the GLP-1 receptor was slightly increased by EP3 antagonist treatment in db/db mice. In addition, the transcription factor nuclear factor E2-related factor 2 (Nrf2) and downstream targets were increased in islets from db/db mice in response to treatment with an EP3 antagonist. The markers of oxidative stress were decreased.

CONCLUSIONS

The current study suggests that EP3 blockade promotes β-cell mass expansion in db/db mice. The beneficial effects of EP3 blockade may be mediated through Nrf2, which has recently emerged as a key mediator in the protection against cellular oxidative damage.

Prostaglandin EP3 receptor signaling is required to prevent insulin hypersecretion and metabolic dysfunction in a non-obese mouse model of insulin resistance

When homozygous for the Leptin^Ob mutation (Ob), Black-and-Tan Brachyury (BTBR) mice become morbidly obese and severely insulin resistant, and by 10 wk of age, frankly diabetic. Previous work has shown prostaglandin EP3 receptor (EP3) expression and activity is upregulated in islets from BTBR-Ob mice as compared with lean controls, actively contributing to their β-cell dysfunction. In this work, we aimed to test the impact of β-cell-specific EP3 loss on the BTBR-Ob phenotype by crossing Ptger3 floxed mice with the rat insulin promoter (RIP)-Cre^Herr driver strain. Instead, germline recombination of the floxed allele in the founder mouse-an event whose prevalence we identified as directly associated with underlying insulin resistance of the background strain-generated a full-body knockout. Full-body EP3 loss provided no diabetes protection to BTBR-Ob mice but, unexpectedly, significantly worsened BTBR-lean insulin resistance and glucose tolerance. This in vivo phenotype was not associated with changes in β-cell fractional area or markers of β-cell replication ex vivo. Instead, EP3-null BTBR-lean islets had essentially uncontrolled insulin hypersecretion. The selective upregulation of constitutively active EP3 splice variants in islets from young, lean BTBR mice as compared with C57BL/6J, where no phenotype of EP3 loss has been observed, provides a potential explanation for the hypersecretion phenotype. In support of this, high islet EP3 expression in Balb/c females versus Balb/c males was fully consistent with their sexually dimorphic metabolic phenotype after loss of EP3-coupled Gα_z protein. Taken together, our findings provide a new dimension to the understanding of EP3 as a critical brake on insulin secretion.NEW & NOTEWORTHY Islet prostaglandin EP3 receptor (EP3) signaling is well known as upregulated in the pathophysiological conditions of type 2 diabetes, contributing to β-cell dysfunction. Unexpected findings in mouse models of non-obese insulin sensitivity and resistance provide a new dimension to our understanding of EP3 as a key modulator of insulin secretion. A previously unknown relationship between mouse insulin resistance and the penetrance of rat insulin promoter-driven germline floxed allele recombination is critical to consider when creating β-cell-specific knockouts.

The Human Islet: Mini-Organ With Mega-Impact

This review focuses on the human pancreatic islet-including its structure, cell composition, development, function, and dysfunction. After providing a historical timeline of key discoveries about human islets over the past century, we describe new research approaches and technologies that are being used to study human islets and how these are providing insight into human islet physiology and pathophysiology. We also describe changes or adaptations in human islets in response to physiologic challenges such as pregnancy, aging, and insulin resistance and discuss islet changes in human diabetes of many forms. We outline current and future interventions being developed to protect, restore, or replace human islets. The review also highlights unresolved questions about human islets and proposes areas where additional research on human islets is needed.

Discovery of ciliary G protein-coupled receptors regulating pancreatic islet insulin and glucagon secretion

Multiple G protein-coupled receptors (GPCRs) are expressed in pancreatic islet cells, but the majority have unknown functions. We observed specific GPCRs localized to primary cilia, a prominent signaling organelle, in pancreatic α and β cells. Loss of cilia disrupts β-cell endocrine function, but the molecular drivers are unknown. Using functional expression, we identified multiple GPCRs localized to cilia in mouse and human islet α and β cells, including FFAR4, PTGER4, ADRB2, KISS1R, and P2RY14. Free fatty acid receptor 4 (FFAR4) and prostaglandin E receptor 4 (PTGER4) agonists stimulate ciliary cAMP signaling and promote glucagon and insulin secretion by α- and β-cell lines and by mouse and human islets. Transport of GPCRs to primary cilia requires TULP3, whose knockdown in primary human and mouse islets relocalized ciliary FFAR4 and PTGER4 and impaired regulated glucagon or insulin secretion, without affecting ciliary structure. Our findings provide index evidence that regulated hormone secretion by islet α and β cells is controlled by ciliary GPCRs providing new targets for diabetes.

Human Islet Expression Levels of Prostaglandin E2 Synthetic Enzymes, But Not Prostaglandin EP3 Receptor, Are Positively Correlated with Markers of β-Cell Function and Mass in Nondiabetic Obesity

Elevated islet production of prostaglandin E₂ (PGE₂), an arachidonic acid metabolite, and expression of prostaglandin E₂ receptor subtype EP3 (EP3) are well-known contributors to the β-cell dysfunction of type 2 diabetes (T2D). Yet, many of the same pathophysiological conditions exist in obesity, and little is known about how the PGE₂ production and signaling pathway influences nondiabetic β-cell function. In this work, plasma arachidonic acid and PGE₂ metabolite levels were quantified in a cohort of nondiabetic and T2D human subjects to identify their relationship with glycemic control, obesity, and systemic inflammation. In order to link these findings to processes happening at the islet level, cadaveric human islets were subject to gene expression and functional assays. Interleukin-6 (IL-6) and cyclooxygenase-2 (COX-2) mRNA levels, but not those of EP3, positively correlated with donor body mass index (BMI). IL-6 expression also strongly correlated with the expression of COX-2 and other PGE₂ synthetic pathway genes. Insulin secretion assays using an EP3-specific antagonist confirmed functionally relevant upregulation of PGE₂ production. Yet, islets from obese donors were not dysfunctional, secreting just as much insulin in basal and stimulatory conditions as those from nonobese donors as a percent of content. Islet insulin content, on the other hand, was increased with both donor BMI and islet COX-2 expression, while EP3 expression was unaffected. We conclude that upregulated islet PGE₂ production may be part of the β-cell adaption response to obesity and insulin resistance that only becomes dysfunctional when both ligand and receptor are highly expressed in T2D.

New Aspects of Diabetes Research and Therapeutic Development

Both type 1 and type 2 diabetes mellitus are advancing at exponential rates, placing significant burdens on health care networks worldwide. Although traditional pharmacologic therapies such as insulin and oral antidiabetic stalwarts like metformin and the sulfonylureas continue to be used, newer drugs are now on the market targeting novel blood glucose-lowering pathways. Furthermore, exciting new developments in the understanding of beta cell and islet biology are driving the potential for treatments targeting incretin action, islet transplantation with new methods for immunologic protection, and the generation of functional beta cells from stem cells. Here we discuss the mechanistic details underlying past, present, and future diabetes therapies and evaluate their potential to treat and possibly reverse type 1 and 2 diabetes in humans. SIGNIFICANCE STATEMENT: Diabetes mellitus has reached epidemic proportions in the developed and developing world alike. As the last several years have seen many new developments in the field, a new and up to date review of these advances and their careful evaluation will help both clinical and research diabetologists to better understand where the field is currently heading.

The Emerging Role of COX-2, 15-LOX and PPARγ in Metabolic Diseases and Cancer: An Introduction to Novel Multi-target Directed Ligands (MTDLs)

Emerging evidence supports an intertwining framework for the involvement of different inflammatory pathways in a common pathological background for a number of disorders. Of importance are pathways involving arachidonic acid metabolism by cyclooxygenase-2 (COX-2) and 15-lipoxygenase (15-LOX). Both enzyme activities and their products are implicated in a range of pathophysiological processes encompassing metabolic impairment leading to adipose inflammation and the subsequent vascular and neurological disorders, in addition to various pro- and antitumorigenic effects. A further layer of complexity is encountered by the disparate, and often reciprocal, modulatory effect COX-2 and 15-LOX activities and metabolites exert on each other or on other cellular targets, the most prominent of which is peroxisome proliferator-activated receptor gamma (PPARγ). Thus, effective therapeutic intervention with such multifaceted disorders requires the simultaneous modulation of more than one target. Here, we describe the role of COX-2, 15-LOX, and PPARγ in cancer and complications of metabolic disorders, highlight the value of designing multi-target directed ligands (MTDLs) modifying their activity, and summarizing the available literature regarding the rationale and feasibility of design and synthesis of these ligands together with their known biological effects. We speculate on the potential impact of MTDLs in these disorders as well as emphasize the need for structured future effort to translate these early results facilitating the adoption of these, and similar, molecules in clinical research.

microRNA-483 Protects Pancreatic β-Cells by Targeting ALDH1A3

Pancreatic β-cell dysfunction is central to the development and progression of type 2 diabetes. Dysregulation of microRNAs (miRNAs) has been associated with pancreatic islet dysfunction in type 2 diabetes. Previous study has shown that miR-483 is expressed relatively higher in β-cells than in α-cells. To explore the physiological function of miR-483, we generated a β-cell-specific knockout mouse model of miR-483. Loss of miR-483 enhances high-fat diet-induced hyperglycemia and glucose intolerance by the attenuation of diet-induced insulin release. Intriguingly, mice with miR-483 deletion exhibited loss of β-cell features, as indicated by elevated expression of aldehyde dehydrogenase family 1, subfamily A3 (Aldh1a3), a marker of β-cell dedifferentiation. Moreover, Aldh1a3 was validated as a direct target of miR-483 and overexpression of miR-483 repressed Aldh1a3 expression. Genetic ablation of miR-483 also induced alterations in blood lipid profile. Collectively, these data suggest that miR-483 is critical in protecting β-cell function by repressing the β-cell disallowed gene Aldh1a3. The dysregulated miR-483 may impair insulin secretion and initiate β-cell dedifferentiation during the development of type 2 diabetes.

Role of 2‑series prostaglandins in the pathogenesis of type 2 diabetes mellitus and non‑alcoholic fatty liver disease (Review)

Nowadays, metabolic syndromes are emerging as global epidemics, whose incidence are increasing annually. However, the efficacy of therapy does not increase proportionately with the increased morbidity. Type 2 diabetes mellitus (T2DM) and non-alcoholic fatty liver disease (NAFLD) are two common metabolic syndromes that are closely associated. The pathogenic mechanisms of T2DM and NAFLD have been studied, and it was revealed that insulin resistance, hyperglycemia, hepatic lipid accumulation and inflammation markedly contribute to the development of these two diseases. The 2-series prostaglandins (PGs), a subgroup of eicosanoids, including PGD₂, PGE₂, PGF_2α and PGI₂, are converted from arachidonic acid catalyzed by the rate-limiting enzymes cyclooxygenases (COXs). Considering their wide distribution in almost every tissue, 2-series PG pathways exert complex and interlinked effects in mediating pancreatic β-cell function and proliferation, insulin sensitivity, fat accumulation and lipolysis, as well as inflammatory processes. Previous studies have revealed that metabolic disturbances, such as hyperglycemia and hyperlipidemia, can be improved by treatment with COX inhibitors. At present, an accumulating number of studies have focused on the roles of 2-series PGs and their metabolites in the pathogenesis of metabolic syndromes, particularly T2DM and NAFLD. In the present review, the role of 2-series PGs in the highly intertwined pathogenic mechanisms of T2DM and NAFLD was discussed, and important therapeutic strategies based on targeting 2-series PG pathways in T2DM and NAFLD treatment were provided.

Metabolic roles of G protein-coupled receptor signaling in obesity and type 2 diabetes

The incidence of obesity and type 2 diabetes (T2D) has been increasing steadily worldwide. It is estimated that by 2045 more than 800 million people will be suffering from diabetes. Despite the advancements in modern medicine, more effective therapies for treating obesity and T2D are needed. G protein-coupled receptors (GPCRs) have emerged as important drug targets for various chronic diseases, including obesity, T2D, and liver diseases. During the past two decades, many laboratories worldwide focused on understanding the role of GPCR signaling in regulating glucose metabolism and energy homeostasis. The information gained from these studies can guide the development of novel therapeutic agents. In this review, we summarize recent studies providing insights into the role of GPCR signaling in peripheral, metabolically important tissues such as pancreas, liver, skeletal muscle, and adipose tissue, focusing primarily on the use of mutant animal models and human data.

Rat prostaglandin EP3 receptor is highly promiscuous and is the sole prostanoid receptor family member that regulates INS-1 (832/3) cell glucose-stimulated insulin secretion

Chronic elevations in fatty acid metabolites termed prostaglandins can be found in circulation and in pancreatic islets from mice or humans with diabetes and have been suggested as contributing to the β‐cell dysfunction of the disease. Two‐series prostaglandins bind to a family of G‐protein‐coupled receptors, each with different biochemical and pharmacological properties. Prostaglandin E receptor (EP) subfamily agonists and antagonists have been shown to influence β‐cell insulin secretion, replication, and/or survival. Here, we define EP3 as the sole prostanoid receptor family member expressed in a rat β‐cell‐derived line that regulates glucose‐stimulated insulin secretion. Several other agonists classically understood as selective for other prostanoid receptor family members also reduce glucose‐stimulated insulin secretion, but these effects are only observed at relatively high concentrations, and, using a well‐characterized EP3‐specific antagonist, are mediated solely by cross‐reactivity with rat EP3. Our findings confirm the critical role of EP3 in regulating β‐cell function, but are also of general interest, as many agonists supposedly selective for other prostanoid receptor family members are also full and efficacious agonists of EP3. Therefore, care must be taken when interpreting experimental results from cells or cell lines that also express EP3.

Maternal exposure to Di-n-butyl phthalate (DBP) aggravate gestational diabetes mellitus via FoxM1 suppression by pSTAT1 signalling

The study focused on the toxicological effect of Di-n-butyl phthalate (DBP) on the expression of Phosphorylated signal transducer and activator of transcription 1 (pSTAT1) -regulated Forkhead box protein M1 (FoxM1), which might provide a new understanding of gestational diabetes mellitus (GDM) development and a potential target for treatment. Streptozotocin (STZ) (40 mg/kg) was introduced in maternal rats by intraperitoneal injection on gestation day 0 (GD 0) in the STZ and STZ + DBP groups. DBP was introduced in maternal rats by oral feeding in the STZ + DBP group over the following 3 days (750 mg/kg/day). The changes in fasting blood glucose level in rats were detected on GD 1 and GD 5. The insulin levels in maternal rats and PIBCs were measured on GD 18. The Oral Glucose Tolerance Test (OGTT) test was performed on GD 18 to check the stability of the GDM model. The primary islet β cells (PIBCs) were established for in vitro experiments. We examined the FoxM1 and pSTAT1 expression in pancreas by immunohistochemistry. Real-time PCR and Western blot were used to detect the pSTAR1 and FoxM1 protein and mRNA gene expression levels in PIBCs. Cell Counting Kit-8 (CCK-8) and flow cytometric analysis was used to test the viability and apoptosis of cells. The results showed that the STZ + DBP group had higher glucose and lower insulin secretion levels than the other groups by both fasting test and OGTT. FoxM1 was significantly suppressed while pSTAT1 was highly expressed after DBP exposure. FoxM1 could be regulated by pSTAT1. DBP can influence the progression of GDM through its toxicological effect, which significantly increases the expression of pSTAT1 and suppresses FoxM1, causing a decline in β cell viability.

Agonist-independent Gαz activity negatively regulates beta-cell compensation in a diet-induced obesity model of type 2 diabetes

The inhibitory G protein alpha-subunit (Gα_z) is an important modulator of beta-cell function. Full-body Gα_z-null mice are protected from hyperglycemia and glucose intolerance after long-term high-fat diet (HFD) feeding. In this study, at a time point in the feeding regimen where WT mice are only mildly glucose intolerant, transcriptomics analyses reveal islets from HFD-fed Gα_z KO mice have a dramatically altered gene expression pattern as compared with WT HFD-fed mice, with entire gene pathways not only being more strongly upregulated or downregulated versus control-diet fed groups but actually reversed in direction. Genes involved in the “pancreatic secretion” pathway are the most strongly differentially regulated: a finding that correlates with enhanced islet insulin secretion and decreased glucagon secretion at the study end. The protection of Gα_z-null mice from HFD-induced diabetes is beta-cell autonomous, as beta cell–specific Gα_z-null mice phenocopy the full-body KOs. The glucose-stimulated and incretin-potentiated insulin secretion response of islets from HFD-fed beta cell–specific Gα_z-null mice is significantly improved as compared with islets from HFD-fed WT controls, which, along with no impact of Gα_z loss or HFD feeding on beta-cell proliferation or surrogates of beta-cell mass, supports a secretion-specific mechanism. Gα_z is coupled to the prostaglandin EP3 receptor in pancreatic beta cells. We confirm the EP3γ splice variant has both constitutive and agonist-sensitive activity to inhibit cAMP production and downstream beta-cell function, with both activities being dependent on the presence of beta-cell Gα_z.

International Union of Basic and Clinical Pharmacology. CIX. Differences and Similarities between Human and Rodent Prostaglandin E2 Receptors (EP1-4) and Prostacyclin Receptor (IP): Specific Roles in Pathophysiologic Conditions

Prostaglandins are derived from arachidonic acid metabolism through cyclooxygenase activities. Among prostaglandins (PGs), prostacyclin (PGI2) and PGE2 are strongly involved in the regulation of homeostasis and main physiologic functions. In addition, the synthesis of these two prostaglandins is significantly increased during inflammation. PGI2 and PGE2 exert their biologic actions by binding to their respective receptors, namely prostacyclin receptor (IP) and prostaglandin E2 receptor (EP) 1-4, which belong to the family of G-protein-coupled receptors. IP and EP1-4 receptors are widely distributed in the body and thus play various physiologic and pathophysiologic roles. In this review, we discuss the recent advances in studies using pharmacological approaches, genetically modified animals, and genome-wide association studies regarding the roles of IP and EP1-4 receptors in the immune, cardiovascular, nervous, gastrointestinal, respiratory, genitourinary, and musculoskeletal systems. In particular, we highlight similarities and differences between human and rodents in terms of the specific roles of IP and EP1-4 receptors and their downstream signaling pathways, functions, and activities for each biologic system. We also highlight the potential novel therapeutic benefit of targeting IP and EP1-4 receptors in several diseases based on the scientific advances, animal models, and human studies. SIGNIFICANCE STATEMENT: In this review, we present an update of the pathophysiologic role of the prostacyclin receptor, prostaglandin E2 receptor (EP) 1, EP2, EP3, and EP4 receptors when activated by the two main prostaglandins, namely prostacyclin and prostaglandin E2, produced during inflammatory conditions in human and rodents. In addition, this comparison of the published results in each tissue and/or pathology should facilitate the choice of the most appropriate model for the future studies.

Ins1-Cre and Ins1-CreER Gene Replacement Alleles Are Susceptible To Silencing By DNA Hypermethylation

Targeted gene ablation studies of the endocrine pancreas have long suffered from suboptimal Cre deleter strains. In many cases, Cre lines purportedly specific for beta cells also displayed expression in other islet endocrine cells or in a subset of neurons in the brain. Several pancreas and endocrine Cre lines have experienced silencing or mosaicism over time. In addition, many Cre transgenic constructs were designed to include the hGH mini-gene, which by itself increases beta-cell replication and decreases beta-cell function. More recently, driver lines with Cre or CreER inserted into the Ins1 locus were generated, with the intent of producing β cell-specific Cre lines with faithful recapitulation of insulin expression. These lines were bred in multiple labs to several different mouse lines harboring various lox alleles. In our hands, the ability of the Ins1-Cre and Ins1-CreER lines to delete target genes varied from that originally reported, with both alleles displaying low levels of expression, increased levels of methylation compared to the wild-type allele, and ultimately inefficient or absent target deletion. Thus, caution is warranted in the interpretation of results obtained with these genetic tools, and Cre expression and activity should be monitored regularly when using these lines.

EP4 activation ameliorates liver ischemia/reperfusion injury via ERK1/2‑GSK3β‑dependent MPTP inhibition

Prostaglandin E receptor subtype 4 (EP4) is widely distributed in the heart, but its role in hepatic ischemia/reperfusion (I/R), particularly in mitochondrial permeability transition pore (MPTP) modulation, is yet to be elucidated. In the present study, an EP4 agonist (CAY10598) was used in a rat model to evaluate the effects of EP4 activation on liver I/R and the mechanisms underlying this. I/R insult upregulated hepatic EP4 expression during early reperfusion. In addition, subcutaneous CAY10598 injection prior to the onset of reperfusion significantly increased hepatocyte cAMP concentrations and decreased serum ALT and AST levels and necrotic and apoptotic cell percentages, after 6 h of reperfusion. Moreover, CAY10598 protected mitochondrial morphology, markedly inhibited mitochondrial permeability transition pore (MPTP) opening and decreased liver reactive oxygen species levels. This occurred via activation of the ERK1/2-GSK3β pathway rather than the janus kinase (JAK)2-signal transducers and activators of transcription (STAT)3 pathway, and resulted in prevention of mitochondria-associated cell injury. The MPTP opener carboxyatractyloside (CATR) and the ERK1/2 inhibitor PD98059 also partially reversed the protective effects of CAY10598 on the liver and mitochondria. The current findings indicate that EP4 activation induces ERK1/2-GSK3β signaling and subsequent MPTP inhibition to provide hepatoprotection, and these observations are informative for developing new molecular targets and preventative therapies for I/R in a clinical setting.

Time for a paradigm shift in treating type 1 diabetes mellitus: coupling inflammation to islet regeneration

Type 1 diabetes mellitus (T1DM) is an autoimmune disease that targets the destruction of islet beta-cells resulting in insulin deficiency, hyperglycemia and death if untreated. Despite advances in medical devices and longer-acting insulin, there is still no robust therapy to substitute and protect beta-cells that are lost in T1DM. Attempts to refrain from the autoimmune attack have failed to achieve glycemic control in patients highlighting the necessity for a paradigm shift in T1DM treatment. Paradoxically, beta-cells are present in T1DM patients indicating a disturbed equilibrium between the immune attack and beta-cell regeneration reminiscent of unresolved wound healing that under normal circumstances progression towards an anti-inflammatory milieu promotes regeneration. Thus, the ultimate T1DM therapy should concomitantly restore immune self-tolerance and replenish the beta-cell mass similar to wound healing. Recently the agonistic activation of the nuclear receptor LRH-1/NR5A2 was shown to induce immune self-tolerance, increase beta-cell survival and promote regeneration through a mechanism of alpha-to-beta cell phenotypic switch. This trans-regeneration process appears to be facilitated by a pancreatic anti-inflammatory environment induced by LRH-1/NR5A2 activation. Herein, we review the literature on the role of LRH1/NR5A2 in immunity and islet physiology and propose that a cross-talk between these cellular compartments is mandatory to achieve therapeutic benefits.

Cardiac injury modulates critical components of prostaglandin E2 signaling during zebrafish heart regeneration

The inability to effectively stimulate cardiomyocyte proliferation remains a principle barrier to regeneration in the adult human heart. A tightly regulated, acute inflammatory response mediated by a range of cell types is required to initiate regenerative processes. Prostaglandin E₂ (PGE₂), a potent lipid signaling molecule induced by inflammation, has been shown to promote regeneration and cell proliferation; however, the dynamics of PGE₂ signaling in the context of heart regeneration remain underexplored. Here, we employ the regeneration-competent zebrafish to characterize components of the PGE₂ signaling circuit following cardiac injury. In the regenerating adult heart, we documented an increase in PGE₂ levels, concurrent with upregulation of cox2a and ptges, two genes critical for PGE₂ synthesis. Furthermore, we identified the epicardium as the most prominent site for cox2a expression, thereby suggesting a role for this tissue as an inflammatory mediator. Injury also drove the opposing expression of PGE₂ receptors, upregulating pro-restorative ptger2a and downregulating the opposing receptor ptger3. Importantly, treatment with pharmacological inhibitors of Cox2 activity suppressed both production of PGE₂, and the proliferation of cardiomyocytes. These results suggest that injury-induced PGE₂ signaling is key to stimulating cardiomyocyte proliferation during regeneration.

The Beta Cell in Type 2 Diabetes

PURPOSE OF REVIEW

This review summarizes the alterations in the β-cell observed in type 2 diabetes (T2D), focusing on changes in β-cell identity and mass and changes associated with metabolism and intracellular signaling.

RECENT FINDINGS

In the setting of T2D, β-cells undergo changes in gene expression, reverting to a more immature state and in some cases transdifferentiating into other islet cell types. Alleviation of metabolic stress, ER stress, and maladaptive prostaglandin signaling could improve β-cell function and survival. The β-cell defects leading to T2D likely differ in different individuals and include variations in β-cell mass, development, β-cell expansion, responses to ER and oxidative stress, insulin production and secretion, and intracellular signaling pathways. The recent recognition that some β-cells undergo dedifferentiation without dying in T2D suggests strategies to revive these cells and rejuvenate their functionality.

The effect of the EP3 antagonist DG-041 on male mice with diet-induced obesity

BACKGROUND/AIMS

The prostaglandin E₂ (PGE₂) EP3 receptor has a multifaceted role in metabolism. Drugs targeting EP3 have been proposed as therapeutics for diabetes; however, studies utilizing global EP3 knockout mice suggest that EP3 blockade increases obesity and insulin resistance. The present studies attempt to determine the effect of acute EP3 antagonist treatment on the diabetic phenotype.

METHODS

DG-041 was confirmed to be a high affinity antagonist at the mouse EP3 receptor by competition radioligand binding and by blockade of EP3-mediated responses. DG-041 pharmacokinetic studies were performed to determine the most efficacious route of administration. Male C57BL/6 × BALB/c (CB6F1) mice were fed diets containing 10%, 45%, or 60% calories from fat to induce obesity. Changes to the metabolic phenotype in these mice were evaluated after one week treatment with DG-041.

RESULTS

Subcutaneous injections of DG-041 at 20 mg/kg blocked the sulprostone-evoked rise in mean arterial pressure confirming the efficacy of this administration regime. Seven day treatment with DG-041 had minimal effect on body composition or glycemic control. DG-041 administration caused a reduction in skeletal muscle triglyceride content while showing a trend toward increased hepatic triglycerides.

CONCLUSION

Short term EP3 administration of DG-041 produced effective blockade of the EP3 receptor and decreased skeletal muscle triglyceride content but had no significant effects on the diabetic phenotype.

Toll-like receptors TLR2 and TLR4 block the replication of pancreatic β cells in diet-induced obesity

Consumption of a high-energy Western diet triggers mild adaptive β cell proliferation to compensate for peripheral insulin resistance; however, the underlying molecular mechanism remains unclear. In the present study we show that the toll-like receptors TLR2 and TLR4 inhibited the diet-induced replication of β cells in mice and humans. The combined, but not the individual, loss of TLR2 and TLR4 increased the replication of β cells, but not that of α cells, leading to enlarged β cell area and hyperinsulinemia in diet-induced obesity. Loss of TLR2 and TLR4 increased the nuclear abundance of the cell cycle regulators cyclin D2 and Cdk4 in a manner dependent on the signaling mediator Erk. These data reveal a regulatory mechanism controlling the proliferation of β cells in diet-induced obesity and suggest that selective targeting of the TLR2/TLR4 pathways may reverse β cell failure in patients with diabetes.

The role of Cox-2 and prostaglandin E2 receptor EP3 in pancreatic β-cell death

Elevated levels of lipids, in particular saturated fatty acids, are known to be associated with type 2 diabetes (T2D) and to have a negative effect on β-cell function and survival. We bring new evidence indicating that palmitate up-regulates cyclooxygenase-2 (COX-2) expression levels in human islets and in MIN6 β cells, and that it is elevated in islets isolated from T2D donors. Both small interfering specific cyclooxygenase-2 small interfering RNA (siRNA) or the COX-2 inhibitor celecoxib significantly inhibited apoptosis induced by palmitate. Prostaglandin E₂ (PGE₂), the predominant product of COX-2 enzymatic activity, activates membrane receptors, which are members of the GPCR-family (EP1-EP4). In the present study, elevated expression of the PGE₂ receptor subtype 3 (EP3) receptor was observed in β cells exposed to palmitate and in islets from individuals with T2D. Down-regulation of the pathway using EP3 siRNA or the specific L-798,106 antagonist markedly decreased the levels of palmitate-induced apoptosis. Altogether, our data put forward the COX-2-PGE₂-EP3 pathway as one of the mediators of palmitate-induced apoptosis in β-cells.-Amior, L., Srivastava, R., Nano, R., Bertuzzi, F., Melloul, D. The role of Cox-2 and prostaglandin E₂ receptor EP3 in pancreatic β-cell death.

Increases in bioactive lipids accompany early metabolic changes associated with β-cell expansion in response to short-term high-fat diet

Pancreatic β-cell expansion is a highly regulated metabolic adaptation to increased somatic demands, including obesity and pregnancy; adult β cells otherwise rarely proliferate. We previously showed that high-fat diet (HFD) feeding induces mouse β-cell proliferation in less than 1 wk in the absence of insulin resistance. Here we metabolically profiled tissues from a short-term HFD β-cell expansion mouse model to identify pathways and metabolite changes associated with β-cell proliferation. Mice fed HFD vs. chow diet (CD) showed a 14.3% increase in body weight after 7 days; β-cell proliferation increased 1.75-fold without insulin resistance. Plasma from 1-wk HFD-fed mice induced β-cell proliferation ex vivo. The plasma, as well as liver, skeletal muscle, and bone, were assessed by LC and GC mass-spectrometry for global metabolite changes. Of the 1,283 metabolites detected, 159 showed significant changes [false discovery rate (FDR) < 0.1]. The majority of changes were in liver and muscle. Pathway enrichment analysis revealed key metabolic changes in steroid synthesis and lipid metabolism, including free fatty acids and other bioactive lipids. Other important enrichments included changes in the citric acid cycle and 1-carbon metabolism pathways implicated in DNA methylation. Although the minority of changes were observed in bone and plasma (<20), increased p-cresol sulfate was increased >4 fold in plasma (the largest increase in all tissues), and pantothenate (vitamin B₅) decreased >2-fold. The results suggest that HFD-mediated β-cell expansion is associated with complex, global metabolite changes. The finding could be a significant insight into Type 2 diabetes pathogenesis and potential novel drug targets.

Age-related islet inflammation marks the proliferative decline of pancreatic beta-cells in zebrafish

The pancreatic islet, a cellular community harboring the insulin-producing beta-cells, is known to undergo age-related alterations. However, only a handful of signals associated with aging have been identified. By comparing beta-cells from younger and older zebrafish, here we show that the aging islets exhibit signs of chronic inflammation. These include recruitment of tnfα-expressing macrophages and the activation of NF-kB signaling in beta-cells. Using a transgenic reporter, we show that NF-kB activity is undetectable in juvenile beta-cells, whereas cells from older fish exhibit heterogeneous NF-kB activity. We link this heterogeneity to differences in gene expression and proliferation. Beta-cells with high NF-kB signaling proliferate significantly less compared to their neighbors with low activity. The NF-kB signaling^hi cells also exhibit premature upregulation of socs2, an age-related gene that inhibits beta-cell proliferation. Together, our results show that NF-kB activity marks the asynchronous decline in beta-cell proliferation with advancing age.

Cooperative function of Pdx1 and Oc1 in multipotent pancreatic progenitors impacts postnatal islet maturation and adaptability

The transcription factors pancreatic and duodenal homeobox 1 (Pdx1) and onecut1 (Oc1) are coexpressed in multipotent pancreatic progenitors (MPCs), but their expression patterns diverge in hormone-expressing cells, with Oc1 expression being extinguished in the endocrine lineage and Pdx1 being maintained at high levels in β-cells. We previously demonstrated that cooperative function of these two factors in MPCs is necessary for proper specification and differentiation of pancreatic endocrine cells. In those studies, we observed a persistent decrease in expression of the β-cell maturity factor MafA. We therefore hypothesized that Pdx1 and Oc1 cooperativity in MPCs impacts postnatal β-cell maturation and function. Here our model of Pdx1-Oc1 double heterozygosity was used to investigate the impact of haploinsufficiency for both of these factors on postnatal β-cell maturation, function, and adaptability. Examining mice at postnatal day (P) 14, we observed alterations in pancreatic insulin content in both Pdx1 heterozygotes and double heterozygotes. Gene expression analysis at this age revealed significantly decreased expression of many genes important for glucose-stimulated insulin secretion (e.g., Glut2, Pcsk1/2, Abcc8) exclusively in double heterozygotes. Analysis of P14 islets revealed an increase in the number of mixed islets in double heterozygotes. We predicted that double-heterozygous β-cells would have an impaired ability to respond to stress. Indeed, we observed that β-cell proliferation fails to increase in double heterozygotes in response to either high-fat diet or placental lactogen. We thus report here the importance of cooperation between regulatory factors early in development for postnatal islet maturation and adaptability.

Naringin Ameliorates Streptozotocin-Induced Diabetes through Forkhead Box M1-Mediated Beta Cell Proliferation

BACKGROUND

Adult pancreatic beta cells, though quiescent, can proliferate in response to physiological need. This inherent character is used in exploring the possibilities of expanding the beta cell mass in the treatment of diabetes. Forkhead box M1 (FoxM1) transcription factor is an important regulator in the proliferation and survival of adult beta cell mass. Naringin, a flavanone glycoside, is reported to have antidiabetic activity and exhibited an increase in insulin levels in diabetic animals.

OBJECTIVES

The present study tried to evaluate the role of naringin in the regulation of FoxM1 in the pancreas of diabetic rats and to reascertain its antilipidemic and antioxidant properties.

METHODS

Diabetes was induced in male rats using streptozotocin and treated with naringin (100 mg/kg) orally for 4 and 8 weeks. Serum biochemical parameters, insulin, gene and protein expression of FoxM1, and antioxidant markers in rat pancreas were analyzed.

RESULTS

Naringin administration reduced the blood sugar, urea, creatinine, and cholesterol values and improved the pancreatic antioxidant status in diabetic rats. Naringin-treated diabetic rats showed a significant increase in mRNA and protein expression of FoxM1 compared to the diabetic control rats, indicating regeneration of cells. It also increased the insulin immunopositive cells, indicating functional beta cells.

CONCLUSION

Naringin was found to upregulate the FoxM1 transcription factor in diabetic animals, which influenced the proliferation and functional status of beta cells.