Cristina Ripoll

The Estrogenic Effect of Bisphenol A Disrupts Pancreatic β-Cell Function In Vivo and Induces Insulin Resistance

The function of the pancreatic β-cell is the storage and release of insulin, the main hormone involved in blood glucose homeostasis. The results in this article show that the widespread environmental contaminant bisphenol-A (BPA) imitates 17β-estradiol (E2) effects in vivo on blood glucose homeostasis through genomic and nongenomic pathways. The exposure of adult mice to a single low dose (10 μg/kg) of either E2 or BPA induces a rapid decrease in glycemia that correlates with a rise of plasma insulin. Longer exposures to E2 and BPA induce an increase in pancreatic β-cell insulin content in an estrogen-receptor–dependent manner. This effect is visible after 2 days of treatment and starting at doses as low as 10 μg/kg/day. After 4 days of treatment with either E2 or BPA, these mice developed chronic hyperinsulinemia, and their glucose and insulin tolerance tests were altered. These experiments unveil the link between environmental estrogens and insulin resistance. Therefore, either abnormal levels of endogenous estrogens or environmental estrogen exposure enhances the risk of developing type 2 diabetes mellitus, hypertension, and dyslipidemia.

Physiology of the pancreatic α-cell and glucagon secretion: role in glucose homeostasis and diabetes

The secretion of glucagon by pancreatic alpha-cells plays a critical role in the regulation of glycaemia. This hormone counteracts hypoglycaemia and opposes insulin actions by stimulating hepatic glucose synthesis and mobilization, thereby increasing blood glucose concentrations. During the last decade, knowledge of alpha-cell physiology has greatly improved, especially concerning molecular and cellular mechanisms. In this review, we have addressed recent findings on alpha-cell physiology and the regulation of ion channels, electrical activity, calcium signals and glucagon release. Our focus in this review has been the multiple control levels that modulate glucagon secretion from glucose and nutrients to paracrine and neural inputs. Additionally, we have described the glucagon actions on glycaemia and energy metabolism, and discussed their involvement in the pathophysiology of diabetes. Finally, some of the present approaches for diabetes therapy related to alpha-cell function are also discussed in this review. A better understanding of the alpha-cell physiology is necessary for an integral comprehension of the regulation of glucose homeostasis and the development of diabetes.

Low Doses of Bisphenol A and Diethylstilbestrol Impair Ca2+ Signals in Pancreatic α-Cells through a Nonclassical Membrane Estrogen Receptor within Intact Islets of Langerhans

Glucagon, secreted from pancreatic α-cells integrated within the islets of Langerhans, is involved in the regulation of glucose metabolism by enhancing the synthesis and mobilization of glucose in the liver. In addition, it has other extrahepatic effects ranging from lipolysis in adipose tissue to the control of satiety in the central nervous system. In this article, we show that the endocrine disruptors bisphenol A (BPA) and diethylstilbestrol (DES), at a concentration of 10−9 M, suppressed low-glucose–induced intracellular calcium ion ([Ca2+]i) oscillations in α-cells, the signal that triggers glucagon secretion. This action has a rapid onset, and it is reproduced by the impermeable molecule estradiol (E2) conjugated to horseradish peroxidase (E-HRP). Competition studies using E-HRP binding in immunocytochemically identified α-cells indicate that 17β-E2, BPA, and DES share a common membrane-binding site whose pharmacologic profile differs from the classical ER. The effects triggered by BPA, DES, and E2 are blocked by the Gαi- and Gαo-protein inhibitor pertussis toxin, by the guanylate cyclase–specific inhibitor 1H-[1,2,4] oxadiazolo[4,3-a] quinoxalin-1-one, and by the nitric oxide synthase inhibitor N-nitro-l-arginine methyl ester. The effects are reproduced by 8-bromo-guanosine 3′,5′-cyclic monophosphate and suppressed in the presence of the cGMP-dependent protein kinase inhibitor KT-5823. The action of E2, BPA, and DES in pancreatic α-cells may explain some of the effects elicited by endocrine disruptors in the metabolism of glucose and lipid.

Rapid insulinotropic effect of 17β-estradiol via a plasma membrane receptor

Impaired insulin secretion is a hallmark in both type I and type II diabetic individuals. Whereas type I (insulin‐dependent diabetes mellitus) implies β‐cell destruction, type II (non‐insulin dependent diabetes mellitus), responsible for 75% of diabetic syndromes, involves diminished glucose‐dependent secretion of insulin from pancreatic β‐cells. Although a clear demonstration of a direct effect of 17β‐estradiol on the pancreatic β‐cell is lacking, an in vivo insulinotropic effect has been suggested. In this report we describe the effects of 17β‐estradiol in mouse pancreatic β‐cells. 17β‐Estradiol, at physiological concentrations, closes KATP channels, which are also targets for antidiabetic sulfonylureas, in a rapid and reversible manner. Furthermore, in synergy with glucose, 17β‐estradiol depolarizes the plasma membrane, eliciting electrical activity and intracellular calcium signals, which in turn enhance insulin secretion. These effects occur through a receptor located at the plasma membrane, distinct from the classic cytosolic estrogen receptor. Specific competitive binding and localization of 17β‐estradiol receptors at the plasma membrane was demonstrated using confocal reflective microscopy and immunocytochemistry. Gaining deeper knowledge of the effect induced by 17β‐estradiol may be important in order to better understand the hormonal regulation of insulin secretion and for the treatment of NIDDM.— Nadal, A., Rovira, J. M., Laribi, O., Leon‐Quinto, T., Andreu, E., Ripoll, C., Soria, B. Rapid insulinotropic effect of 17b‐estradiol via a plasma membrane receptor. FASEB J. 12, 1341–1348 (1998)

Low doses of the endocrine disruptor bisphenol-A and the native hormone 17beta-estradiol rapidly activate transcription factor CREB.

Endocrine‐disrupting chemicals (EDCs) are hormone‐like agents present in the environment that alter the endocrine system of wildlife and humans. Most EDCs have potencies far below those of the natural hormone 17β‐E2 when acting through the classic estrogen receptors (ERs). Here, we show that the environmental estrogen Bisphenol‐A and the native hormone 17β‐E2 activate the transcription factor, cAMP‐responsive element binding protein (CREB) with the same potency. Phosphorylated CREB (P‐CREB) was increased after only a 5‐minute application of either BPA or 17β‐E2 in a calcium‐dependent manner. The effect was reproduced by the membrane‐impermeable molecule E2 conjugated to horseradish peroxidase (E‐HRP). The increase in PCREB was not modified by the anti‐estrogen ICI 182,780. Therefore, low‐dose of BPA activates the transcription factor CREB via an alternative mechanism, involving a non‐classical membrane estrogen receptor. Because these effects are elicited at concentrations as low as 10–9 M, this observation is of environmental and public health relevance.

Bisphenol-A acts as a potent estrogen via non-classical estrogen triggered pathways

Bisphenol-A (BPA) is an estrogenic monomer commonly used in the manufacture of numerous consumer products such as food and beverage containers. Widespread human exposure to significant doses of this compound has been reported. Traditionally, BPA has been considered a weak estrogen, based on its lower binding affinity to the nuclear estrogen receptors (ERs) compared to 17-β estradiol (E2) as well as its low transcriptional activity after ERs activation. However, in vivo animal studies have demonstrated that it can interfere with endocrine signaling pathways at low doses during fetal, neonatal or perinatal periods as well as in adulthood. In addition, mounting evidence suggests a variety of pathways through which BPA can elicit cellular responses at very low concentrations with the same or even higher efficiency than E2. Thus, the purpose of the present review is to analyze with substantiated scientific evidence the strong estrogenic activity of BPA when it acts through alternative mechanisms of action at least in certain cell types.

Non‐genomic actions of 17β‐oestradiol in mouse pancreatic β‐cells are mediated by a cGMP‐dependent protein kinase

1 Intracellular calcium concentration ([Ca2+]i) was measured in mouse whole islets of Langerhans using the calcium‐sensitive fluorescent dye Indo‐1. 2 Application of physiological concentrations of 17β‐oestradiol in the presence of a stimulatory glucose concentration (8 mm) potentiated the [Ca2+]i signal in 83 % of islets tested. Potentiation was manifested as either an increase in the frequency or duration of [Ca2+]i oscillations. 3 The effects caused by 17β‐oestradiol were mimicked by the cyclic nucleotide analogues 8‐bromoguanosine‐3′,5′‐cyclic monophosphate (8‐Br‐cGMP) and 8‐bromoadenosine‐3′,5′‐cyclic monophosphate (8‐Br‐cAMP). 4 Direct measurements of both cyclic nucleotides demonstrated that nanomolar concentrations of 17β‐oestradiol in the presence of 8 mm glucose increased cGMP levels, yet cAMP levels were unchanged. The increment in cGMP was similar to that induced by 11 mm glucose. 5 Patch‐clamp recording in intact cells showed that 8‐Br‐cGMP reproduced the inhibitory action of 17β‐oestradiol on ATP‐sensitive K+ (KATP) channel activity. This was not a membrane‐bound effect since it could not be observed in excised patches. 6 The action of 17β‐oestradiol on KATP channel activity was not modified by the specific inhibitor of soluble guanylate cyclase (sGC) LY 83583. This result indicates a likely involvement of a membrane guanylate cyclase (mGC). 7 The rapid decrease in KATP channel activity elicited by 17β‐oestradiol was greatly reduced using Rp‐8‐pCPT‐cGMPS, a specific blocker of cGMP‐dependent protein kinase (PKG). Conversely, Rp‐cAMPS, which inhibits cAMP‐dependent protein kinase (PKA), had little effect. 8 The results presented here indicate that rapid, non‐genomic effects of 17β‐oestradiol after interaction with its binding site at the plasma membrane of pancreatic β‐cells is a cGMP‐dependent phosphorylation process.

Rapid Regulation of KATP Channel Activity by 17β-Estradiol in Pancreatic β-Cells Involves the Estrogen Receptor β and the Atrial Natriuretic Peptide Receptor

The ATP-sensitive potassium (K(ATP)) channel is a key molecule involved in glucose-stimulated insulin secretion. The activity of this channel regulates beta-cell membrane potential, glucose- induced [Ca(2+)](i) signals, and insulin release. In this study, the rapid effect of physiological concentrations of 17beta-estradiol (E2) on K(ATP) channel activity was studied in intact beta-cells by use of the patch-clamp technique. When cells from wild-type (WT) mice were used, 1 nm E2 rapidly reduced K(ATP) channel activity by 60%. The action of E2 on K(ATP) channel was not modified in beta-cells from ERalpha-/- mice, yet it was significantly reduced in cells from ERbeta-/- mice. The effect of E2 was mimicked by the ERbeta agonist 2,3-bis(4-hydroxyphenyl)-propionitrile (DPN). Activation of ERbeta by DPN enhanced glucose-induced Ca(2+) signals and insulin release. Previous evidence indicated that the acute inhibitory effects of E2 on K(ATP) channel activity involve cyclic GMP and cyclic GMP-dependent protein kinase. In this study, we used beta-cells from mice with genetic ablation of the membrane guanylate cyclase A receptor for atrial natriuretic peptide (also called the atrial natriuretic peptide receptor) (GC-A KO mice) to demonstrate the involvement of this membrane receptor in the rapid E2 actions triggered in beta-cells. E2 rapidly inhibited K(ATP) channel activity and enhanced insulin release in islets from WT mice but not in islets from GC-A KO mice. In addition, DPN reduced K(ATP) channel activity in beta-cells from WT mice, but not in beta-cells from GC-A KO mice. This work unveils a new role for ERbeta as an insulinotropic molecule that may have important physiological and pharmacological implications.

Engineering pancreatic islets.

Abstract. Pancreatic islets are neuroendocrine organs that control blood glucose homeostasis. The precise interplay of a heterogeneous group of cell populations (β, α, δ and PP cells) results in the fine-tuned release of counterbalanced hormones (insulin, glucagon, somatostatin and pancreatic polypeptide respectively). Under the premises of detailed knowledge of the physiological basis underlying this behaviour, two lines of investigation might be inferred: generating computational and operational models to explain and predict this behaviour and engineering islet cells to reconstruct pancreatic endocrine function. Whilst the former is being fuelled by new computational strategies, giving biophysicists the possibility of modelling a system in which new emergent properties appear, the latter is benefiting from the useful tools and strategic knowledge achieved by molecular, cell and developmental biologists. This includes using tumour cell lines, engineering islet cell precursors, knowledge of the mechanisms of differentiation, regeneration and growth and, finally, therapeutic cloning of human tissues. Gaining deep physiological understanding of the basis governing these processes is instrumental for engineering new pancreatic islets.

Role of leptin in the pancreatic β-cell: effects and signaling pathways

Leptin plays an important role in the control of food intake, energy expenditure, metabolism, and body weight. This hormone also has a key function in the regulation of glucose homeostasis. Although leptin acts through central and peripheral mechanisms to modulate glucose metabolism, the pancreatic β-cell of the endocrine pancreas is a critical target of leptin actions. Leptin receptors are present in the β-cell, and their activation directly inhibits insulin secretion from these endocrine cells. The effects of leptin on insulin occur also in the long term, since this hormone inhibits insulin gene expression as well. Additionally, β-cell mass can be affected by leptin through changes in proliferation, apoptosis, or cell size. All these different functions in the β-cell are triggered by leptin as a result of the large diversity of signaling pathways that this hormone is able to activate in the endocrine pancreas. Therefore, leptin can participate in glucose homeostasis owing to different levels of modulation of the pancreatic β-cell population. Furthermore, it has been proposed that alterations in this level of regulation could contribute to the impairment of β-cell function in obesity states. In the present review, we will discuss all these issues with special emphasis on the effects and pathways of leptin signaling in the pancreatic β-cell.