Effect of 17beta-estradiol on insulin secretion and cytosolic calcium in Min6 mouse insulinoma cells and human islets of Langerhans

Non-glucose modulators of insulin secretion in healthy humans: (dis)similarities between islet and in vivo studies

Optimal metabolic homeostasis requires precise temporal and quantitative control of insulin secretion. Both in vivo and in vitro studies have often focused on the regulation by glucose although many additional factors including other nutrients, neurotransmitters, hormones and drugs, modulate the secretory function of pancreatic β-cells. This review is based on the analysis of clinical investigations characterizing the effects of non-glucose modulators of insulin secretion in healthy subjects, and of experimental studies testing the same modulators in islets isolated from normal human donors. The aim was to determine whether the information gathered in vitro can reliably be translated to the in vivo situation. The comparison evidenced both convincing similarities and areas of discordance. The lack of coherence generally stems from the use of exceedingly high concentrations of test agents at too high or too low glucose concentrations in vitro, which casts doubts on the physiological relevance of a number of observations made in isolated islets. Future projects resorting to human islets should avoid extreme experimental conditions, such as oversized stimulations or inhibitions of β-cells, which are unlikely to throw light on normal insulin secretion and contribute to the elucidation of its defects.

Examining the association between serum free fatty acids and blood levels of testosterone

BACKGROUND

Multiple studies have uncovered the effects that ingested fat has on human blood levels of testosterone. Yet, few reports have discussed the effect of circulating serum free fatty acids (FFAs). The present study aimed to explore the relationship between serum free fatty acids and blood levels of testosterone.

METHODS

In total, 5719 adults were pooled from the database of the National Health and Nutrition Examination Survey (NHANES) from 2011 to 2012. Based on multivariable-linear regression models, we employed a total of 30 FFAs to interpret the relationship of FFAs with blood levels of testosterone. Two models with covariate adjustments were designated for further evaluation and analysis.

RESULTS

Capric acid [β = -0.014, 95% confidence interval (CI) = -0.023, -0.004, P = 0.005], myristic acid (β = -0.001, 95% CI = -0.001, 0.000, P ≤ 0.001), pentadecanoic acid (β = -0.013, 95% CI = -0.018, -0.008, P ≤ 0.001), margaric acid (β = -0.011, 95% CI = -0.017, -0.005, P ≤ 0.001) and alpha-linolenic acid (β = -0.001, 95% CI = -0.002, 0.000, P = 0.004) in the fully adjusted model were significantly negatively correlated with the testosterone level inh obese men. In the fully adjusted model for the female analysis, myristic acid, pentadecanoic acid, palmitic acid, margaric acid, stearic acid, myristoleic acid, oleic acid, nervonic acid and alpha-linolenic acid were found significantly associated with the testosterone level.

CONCLUSIONS

Our findings indicate a significant negative correlation between serum FFAs and blood levels of testosterone. Furthermore, we reveal the essentiality of serum FFAs and their potential effects on the reduction of testosterone levels.

Tamoxifen Improves Glucose Tolerance in a Delivery-, Sex-, and Strain-Dependent Manner in Mice

Tamoxifen, a selective estrogen-receptor modulator, is widely used in mouse models to temporally control gene expression but is also known to affect body composition. We report that tamoxifen has significant and sustained effects on glucose tolerance, independent of effects on insulin sensitivity, in mice. IP, but not oral, tamoxifen delivery improved glucose tolerance in three inbred mouse strains. The extent and persistence of tamoxifen-induced effects were sex and strain dependent. These findings highlight the need to revise commonly used tamoxifen-based protocols for gene manipulation in mice by including longer chase periods after injection, oral delivery, and the use of tamoxifen-treated littermate controls.

The Role of Estrogens in Pancreatic Islet Physiopathology

In rodent models of insulin-deficient diabetes, 17β-estradiol (E2) protects pancreatic insulin-producing β-cells against oxidative stress, amyloid polypeptide toxicity, gluco-lipotoxicity, and apoptosis. Three estrogen receptors (ERs)-ERα, ERβ, and the G protein-coupled ER (GPER)-have been identified in rodent and human β-cells. This chapter describes recent advances in our understanding of the role of ERs in islet β-cell function, nutrient homeostasis, survival from pro-apoptotic stimuli, and proliferation. We discuss why and how ERs represent potential therapeutic targets for the maintenance of functional β-cell mass.

MAFA missense mutation causes familial insulinomatosis and diabetes mellitus

We report a disease-causing mutation in the β-cell–enriched MAFA transcription factor. Strikingly, the missense p.Ser64Phe MAFA mutation was associated with either of two distinct phenotypes, multiple insulin-producing neuroendocrine tumors of the pancreas—a condition known as insulinomatosis—or diabetes mellitus, recapitulating the physiological properties of MAFA both as an oncogene and as a key islet β-cell transcription factor. The implication of MAFA in these human phenotypes will provide insights into how this transcription factor regulates human β-cell activity as well as into the mechanisms of Maf-induced tumorigenesis.

The β-cell–enriched MAFA transcription factor plays a central role in regulating glucose-stimulated insulin secretion while also demonstrating oncogenic transformation potential in vitro. No disease-causing MAFA variants have been previously described. We investigated a large pedigree with autosomal dominant inheritance of diabetes mellitus or insulinomatosis, an adult-onset condition of recurrent hyperinsulinemic hypoglycemia caused by multiple insulin-secreting neuroendocrine tumors of the pancreas. Using exome sequencing, we identified a missense MAFA mutation (p.Ser64Phe, c.191C>T) segregating with both phenotypes of insulinomatosis and diabetes. This mutation was also found in a second unrelated family with the same clinical phenotype, while no germline or somatic MAFA mutations were identified in nine patients with sporadic insulinomatosis. In the two families, insulinomatosis presented more frequently in females (eight females/two males) and diabetes more often in males (12 males/four females). Four patients from the index family, including two homozygotes, had a history of congenital cataract and/or glaucoma. The p.Ser64Phe mutation was found to impair phosphorylation within the transactivation domain of MAFA and profoundly increased MAFA protein stability under both high and low glucose concentrations in β-cell lines. In addition, the transactivation potential of p.Ser64Phe MAFA in β-cell lines was enhanced compared with wild-type MAFA. In summary, the p.Ser64Phe missense MAFA mutation leads to familial insulinomatosis or diabetes by impacting MAFA protein stability and transactivation ability. The human phenotypes associated with the p.Ser64Phe MAFA missense mutation reflect both the oncogenic capacity of MAFA and its key role in islet β-cell activity.

Interventions to improve β-cell mass and function

Diabetes mellitus (T2DM) has become an epidemiologically important disease worldwide and is also becoming a great matter of concern due to the effects associated with it like: high morbidity, elevated health care cost and shortened life span. T2DM is a chronic metabolic disease characterized by insulin resistance as well as β-cell dysfunction. It is widely accepted that in the face of insulin resistance, euglycemia can be maintained by increase in pancreatic β-cell mass and insulin secretion. This compensation is largely due to enhanced secretion of insulin by the β-cell mass, which is present initially, and thereby subsequent increases in β-cell mass provide additional insulin secretion. However, the mechanism by which β-cell anatomical plasticity and functional plasticity for insulin secretion is coordinated and executed in different physiological and pathophysiological states is complex and has been poorly understood. As the incidence of T2DM continues to increase at an alarming rate, it is becoming imperative to shift the research focus towards the β-cell physiology where identification of novel pathways that influence the β-cell proliferation and/or contribute to increase insulin secretion has the potential to lead to new therapies for preventing or delaying onset of disease.

Rapid estrogen actions on ion channels: A survey in search for mechanisms

A survey of nearly two hundred reports shows that rapid estrogenic actions can be detected across a range of kinds of estrogens, a range of doses, on a wide range of tissue, cell and ion channel types. Striking is the fact that preparations of estrogenic agents that do not permeate the cell membrane almost always mimic the actions of the estrogenic agents that do permeate the membrane. All kinds of estrogens, ranging from natural ones, through receptor modulators, endocrine disruptors, phytoestrogens, agonists, and antagonists to novel G-1 and STX, have been reported to be effective. For actions on specific types of ion channels, the possibility of opposing actions, in different cases, is the rule, not the exception. With this variety there is no single, specific action mechanism for estrogens per se, although in some cases estrogens can act directly or via some signaling pathways to affect ion channels. We infer that estrogens can bind a large number of substrates/receptors at the membrane surface. As against the variety of subsequent routes of action, this initial step of the estrogen's binding action is the key.

An atlas and functional analysis of G-protein coupled receptors in human islets of Langerhans

G-protein coupled receptors (GPCRs) regulate hormone secretion from islets of Langerhans, and recently developed therapies for type-2 diabetes target islet GLP-1 receptors. However, the total number of GPCRs expressed by human islets, as well as their function and interactions with drugs, is poorly understood. In this review we have constructed an atlas of all GPCRs expressed by human islets: the 'islet GPCRome'. We have used this atlas to describe how islet GPCRs interact with their endogenous ligands, regulate islet hormone secretion, and interact with drugs known to target GPCRs, with a focus on drug/receptor interactions that may affect insulin secretion. The islet GPCRome consists of 293 GPCRs, a majority of which have unknown effects on insulin, glucagon and somatostatin secretion. The islet GPCRs are activated by 271 different endogenous ligands, at least 131 of which are present in islet cells. A large signalling redundancy was also found, with 119 ligands activating more than one islet receptor. Islet GPCRs are also the targets of a large number of clinically used drugs, and based on their coupling characteristics and effects on receptor signalling we identified 107 drugs predicted to stimulate and 184 drugs predicted to inhibit insulin secretion. The islet GPCRome highlights knowledge gaps in the current understanding of islet GPCR function, and identifies GPCR/ligand/drug interactions that might affect insulin secretion, which are important for understanding the metabolic side effects of drugs. This approach may aid in the design of new safer therapeutic agents with fewer detrimental effects on islet hormone secretion.

Role of ERβ and GPR30 in the endocrine pancreas: A matter of estrogen dose

The endocrine pancreas has emerged as a target for estrogens. The functions of pancreatic α-, β- and δ-cells are modulated by the endogenous hormone, 17β-estradiol (E2). Low physiological concentrations (100pM-1nM) of E2 rapidly decrease the activity of the ATP-sensitive potassium channel (K(ATP)) and enhance glucose-induced insulin release in β-cells in an estrogen receptor β (ERβ)-dependent manner. In addition to the insulinotropic action of ERβ, the newly described estrogen receptor, GPR30, is involved in the insulinotropic effects of high doses of E2 (100nM-5μM). The specific GPR30 agonist G1 also increases insulin secretion in β-cells. Low glucose-induced calcium oscillations and glucagon secretion are suppressed by E2. The effects on glucagon secretion may be mediated by GPR30. Somatostatin release is also decreased by E2 and G1. In this review we summarize all the data published up to date on the rapid insulinotropic effects of estrogens in the endocrine pancreas and propose a model to integrate the estrogen actions mediated through both receptors.

Importance of oestrogen receptors to preserve functional β-cell mass in diabetes

Protecting the functional mass of insulin-producing β cells of the pancreas is a major therapeutic challenge in patients with type 1 (T1DM) or type 2 diabetes mellitus (T2DM). The gonadal hormone 17β-oestradiol (E2) is involved in reproductive, bone, cardiovascular and neuronal physiology. In rodent models of T1DM and T2DM, treatment with E2 protects pancreatic β cells against oxidative stress, amyloid polypeptide toxicity, lipotoxicity and apoptosis. Three oestrogen receptors (ERs)--ERα, ERβ and the G protein-coupled ER (GPER)--have been identified in rodent and human β cells. Whereas activation of ERα enhances glucose-stimulated insulin biosynthesis, reduces islet toxic lipid accumulation and promotes β-cell survival from proapoptotic stimuli, activation of ERβ increases glucose-stimulated insulin secretion. However, activation of GPER protects β cells from apoptosis, raises glucose-stimulated insulin secretion and lipid homeostasis without affecting insulin biosynthesis. Oestrogens are also improving islet engraftment in rodent models of pancreatic islet transplantation. This Review describes developments in the role of ERs in islet insulin biosynthesis and secretion, lipid homeostasis and survival. Moreover, we discuss why and how enhancing ER action in β cells without the undesirable effect of general oestrogen therapy is a therapeutic avenue to preserve functional β-cell mass in patients with diabetes mellitus.

Mechanisms of estradiol-induced insulin secretion by the G protein-coupled estrogen receptor GPR30/GPER in pancreatic beta-cells

Sexual dimorphism and supplementation studies suggest an important role for estrogens in the amelioration of glucose intolerance and diabetes. Because little is known regarding the signaling mechanisms involved in estradiol-mediated insulin secretion, we investigated the role of the G protein-coupled receptor 30, now designated G protein-coupled estrogen receptor (GPER), in activating signal transduction cascades in β-cells, leading to secretion of insulin. GPER function in estradiol-induced signaling in the pancreatic β-cell line MIN6 was assessed using small interfering RNA and GPER-selective ligands (G-1 and G15) and in islets isolated from wild-type and GPER knockout mice. GPER is expressed in MIN6 cells, where estradiol and the GPER-selective agonist G-1 mediate calcium mobilization and activation of ERK and phosphatidylinositol 3-kinase. Both estradiol and G-1 induced insulin secretion under low- and high-glucose conditions, which was inhibited by pretreatment with GPER antagonist G15 as well as depletion of GPER by small interfering RNA. Insulin secretion in response to estradiol and G-1 was dependent on epidermal growth factor receptor and ERK activation and further modulated by phosphatidylinositol 3-kinase activity. In islets isolated from wild-type mice, the GPER antagonist G15 inhibited insulin secretion induced by estradiol and G-1, both of which failed to induce insulin secretion in islets obtained from GPER knockout mice. Our results indicate that GPER activation of the epidermal growth factor receptor and ERK in response to estradiol treatment plays a critical role in the secretion of insulin from β-cells. The results of this study suggest that the activation of downstream signaling pathways by the GPER-selective ligand G-1 could represent a novel therapeutic strategy in the treatment of diabetes.

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.

17beta-Estradiol modulates apoptosis in pancreatic beta-cells by specific involvement of the sulfonylurea receptor (SUR) isoform SUR1

Apoptosis of pancreatic beta-cells is an important factor in the pathophysiology of diabetes. Previously, we have shown that the "phytoestrogen" resveratrol can induce beta-cell apoptosis dependent on the expression of sulfonylurea receptor (SUR) 1, the regulatory subunit of pancreatic ATP-sensitive K(+) channels. Here, we investigate whether 17beta-estradiol also influences beta-cell apoptosis in a SUR1-dependent manner. Therefore, islets from wild type or SUR1 knock-out mice, clonal beta-cells, or HEK293 cells expressing different SUR forms were treated with 17beta-estradiol or estrone. Different apoptotic parameters were determined and estrogen binding to SUR was analyzed. In murine islets, 17beta-estradiol treatment resulted in significant apoptotic changes, which in their nature (either apoptotic or anti-apoptotic) were dependent on the age of the animal. These effects were not observed in SUR1 knock-out mice. Furthermore, 17beta-estradiol, which specifically binds to SUR, induced enhanced apoptosis in SUR1-expressing HEK293 cells and clonal beta-cells, whereas apoptosis in recombinant cells expressing SUR2A or SUR2B (cardiac or vascular SUR-isoforms) or sham-transfected control cells was significantly lower. The apoptotic potency of 17beta-estradiol was much higher than that of resveratrol or estrone. SUR1-specific 17beta-estradiol-induced apoptosis was either abolished by the mutation M1289T in transmembrane helix 17 of SUR1 or clearly enhanced by two mutations in nucleotide binding fold 2 (R1379C, R1379L). In conclusion, 17beta-estradiol treatment modulates beta-cell apoptosis under specific involvement of SUR1 in an age-dependent manner. 17beta-Estradiol-induced apoptosis can be influenced by certain SUR1 mutations. These findings may contribute to the understanding of pathophysiological changes in beta-cell mass and could, for instance, provide interesting aspects concerning the etiology of gestational diabetes.

The role of arachidonic acid and its metabolites in insulin secretion from human islets of langerhans

The roles played by arachidonic acid and its cyclooxygenase (COX)-generated and lipoxygenase (LOX)-generated metabolites have been studied using rodent islets and insulin-secreting cell lines, but very little is known about COX and LOX isoform expression and the effects of modulation of arachidonic acid generation and metabolism in human islets. We have used RT-PCR to identify mRNAs for cytosolic phospholipase A(2) (cPLA(2)), COX-1, COX-2, 5-LOX, and 12-LOX in isolated human islets. COX-3 and 15-LOX were not expressed by human islets. Perifusion experiments with human islets indicated that PLA(2) inhibition inhibited glucose-stimulated insulin secretion, whereas inhibitors of COX-2 and 12-LOX enzymes enhanced basal insulin secretion and also secretory responses induced by 20 mmol/l glucose or by 50 mumol/l arachidonic acid. Inhibition of COX-1 with 100 mumol/l acetaminophen did not significantly affect glucose-stimulated insulin secretion. These data indicate that the stimulation of insulin secretion from human islets in response to arachidonic acid does not require its metabolism through COX-2 and 5-/12-LOX pathways. The products of COX-2 and LOX activities have been implicated in cytokine-mediated damage of beta-cells, so selective inhibitors of these enzymes would be expected to have a dual protective role in diabetes: they would minimize beta-cell dysfunction while maintaining insulin secretion through enhancing endogenous arachidonic acid levels.

Estrogen receptors in pancreatic tumors

OBJECTIVES

Although a potential role for estrogen receptors (ER) in pancreatic tumors has been debated for many years, the importance of the receptors in these neoplasms remains unknown. Even the expression of the 2 ER isoforms, ER-alpha and ER-beta, in histological subtypes of pancreatic neoplasms is controversial. The aim of the present study was to systematically review the available literature about ER expression in pancreatic tumors and to discuss the potential importance of estrogen signaling in them.

METHODS

We performed a comprehensive literature search and analyzed the results regarding ER expression in pancreatic tumors, with special emphasis on the specificity of the antibodies used for immunohistochemistry.

RESULTS

Many articles have been published investigating the expression of ERs in pancreatic tumors, but the results are inconsistent. Moreover, most studies used antibodies that detected only ER-alpha, not ER-beta. Thus, the expression pattern of ER-beta in pancreatic neoplasm remains especially unclear.

CONCLUSIONS

The lack of detailed studies evaluating the expression of both ER-alpha and ER-beta receptors using isoform-specific antibodies likely contributes to the inconsistency of published results concerning ER expression in pancreatic tumors. Available published evidence suggests that a thorough reexamination of the potential role of ERs in pancreatic neoplasms is warranted.

Direct regulation of insulin secretion by angiotensin II in human islets of Langerhans

AIMS/HYPOTHESIS

This study aimed to identify the expression of angiotensin II receptors in isolated human islets and beta cells and to examine the functional consequences of their activation.

MATERIALS AND METHODS

Single-cell RT-PCR was used to identify whether human islet cells express mRNA for type 1 angiotensin II receptors (AT(1)), and western blotting was used to determine AT(1) protein expression by human islets and MIN6 beta cells. We measured changes in intracellular calcium by microfluorimetry using Fura 2-loaded MIN6 cells and human islet cells. Dynamic insulin secretory responses were determined by RIA following perifusion of human islets and MIN6 cells.

RESULTS

Human islets expressed mRNAs for both the angiotensin precursor, angiotensinogen, and for angiotensin-converting enzyme. In addition, human and mouse beta cells expressed AT(1). These were functionally coupled to increases in intracellular calcium, which occurred at least in part through phospholipase-C-sensitive mechanisms and calcium influx through voltage-operated calcium channels. Short-term exposure of human islets and MIN6 cells to angiotensin II caused a rapid, short-lived initiation of insulin secretion at 2 mmol/l glucose and potentiation of insulin secretion induced by glucose (at 8 and 16.7 mmol/l).

CONCLUSIONS/INTERPRETATION

These data demonstrate that the AT(1) is expressed by beta cells and that angiotensin II effects a short-lived and direct stimulation of human and mouse beta cells to promote insulin secretion, most probably through elevations in intracellular calcium. Locally produced angiotensin II may be important in regulating a coordinated insulin secretory response from beta cells.