On the biochemical and molecular mechanisms by which malathion induces dysfunction in pancreatic islets in vivo and in vitro

doi:10.1016/j.pestbp.2013.04.003

Pesticide Biochemistry and Physiology

Volume 106, Issues 1–2, May–June 2013, Pages 51-60

https://doi.org/10.1016/j.pestbp.2013.04.003 Get rights and content

Highlights

•
Pancreatic endocrine system is affected in acute poisoning with malathion.
•
Malathion impairs glucose metabolism by increasing oxidative stress markers.
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Impairment of glucose metabolism is critical in dysfunction of insulin induced by malathion.

Abstract

Recent studies showed that organophosphorus pesticides can increase incidence of metabolic disorders and diabetes. Considering the importance of pancreas in regulating blood glucose, the effect of malathion on essential elements of glucose-stimulated insulin secretion (GSIS) in in vitro and in vivo conditions were evaluated.

After dividing of rats into control and treatment groups, oral glucose tolerance test (OGTT) was examined and then kinetic of glucose as well as pancreatic response to raise blood glucose were evaluated. After isolation of islets from pancreas, its function as well as oxidative stress markers and essential elements of GSIS were examined.

Malathion at dose of 400 mg/kg impaired GTT and increased AUC_0–180 min (P = 0.047) and T_1/2_β of glucose (P = 0.0016), and reduced insulin response (P = 0.005) 30 min after oral administering of glucose. In addition to impaired glucose tolerance, there were significant increases in lipid peroxidation (P < 0.001), carbonyl groups (P = 0.007) and 8-deoxyguanosine (P = 0.011) as the biomarkers of reactive oxygen species (ROS)-mediated damage to lipid, protein and DNA, respectively in islets. In static condition, a remarkable decrease was observed in ratio of insulin release/mM glucose (P < 0.001) and a dramatic increase was seen in ROS formation at all glucose levels. Malathion only reduced ATP/ADP ratio in stimulating concentrations of glucose. Despite the dramatic reduction of glucokinase (GCK) mRNA expression (P = 0.004), the expression of glucose transporter 2 (GLUT2) (P = 0.01) was increased significantly.

Conclusion

Dysfunction of glucose metabolism and impairment of insulin secretion are associated with a depletion of energy and induction of oxidative stress following acute exposure to malathion.

Graphical abstract

Introduction

Malathion is one of the largest selling and useful organophosphorus (OPs) insecticides in the world. The main mechanism of action of this insecticide, like other OPs, is the inhibition of acetylcholinesterase (AChE) activity in the target tissues. There are numerous evidences that most of people have been exposed to OP pesticides in their workplaces, homes or through trace contaminants in food. Epidemiological studies in occupationally-exposed cases and also animal experiments have confirmed the role of OPs in impairment of glucose homostasis and induction of diabetes [1], [2], [3], [4]. On the basis of an epidemiologic study done for the period of 1993–2003 in the US, prevalence of diabetes among OP applicators was mentioned [2]. This capability of OPs was already confirmed in animal models [1]. The change in critical enzymes activity in the metabolic pathways such as glycolysis, glycogenesis, glycogenolysis and gluconeogenesis, adrenal gland, and induction of oxidative stress in liver, muscle and pancreas were found responsible for this disturbing effect of OPs on glucose hemostasis [1].

Pancreas has a critical role in glucose hemostasis by insulin and glucagon secretion. Although OPs appear to be one of the leading causes of pancreatic dysfunction, the involved mechanisms have not been known fully elucidated yet. It has been shown that incubation of isolated islets of rat with different doses of malathion, diminishes secretion of insulin in the presence of basal (2.8 mM) or stimulating (16.7 mM) concentrations of glucose [3]. Change in activity of glutamate dehydrogenase (GDH) and glucokinase (GCK) enzymes have been reported as the mechanisms for glucose and insulin dysregulation by OPs [5], [6]. Formation of free radicals in making the imbalance between oxidant and antioxidants of the Langerhans islets has been reported as the main causes of insulin signaling dysfunction [7].

Glucose is the major physiological stimulus of insulin secretion from pancreatic islets where β-cells catabolize glucose to obtain energy required for biosynthesis and exocytosis of insulin. The β-cells generate the signal for glucose-stimulated insulin secretion (GSIS) dependent on the concentration of glucose. In this pathway, glucose transporter 2 (GLUT2) and GCK are two key regulators of metabolism and energy production of islets [8]. GLUT2 contributes to recognizing of glucose not only by enlightening the metabolic signaling cascade but also by activating specific protein signaling pathway [9]. The GCK as the signal recognition enzyme couples changes in the millimolar blood glucose levels to corresponding signal-generating metabolic flux rates for increasing the ATP/ADP ratio and initiation of glucose-induced insulin secretion [10].

In the previous studies, the role of oxidative damage in blood, liver [11], [12], [13], muscle [14] and brain [15], [16] following exposure to OPs was described. Oxidative stress could be a cause of insulin signaling dysfunction and failure of β-cells. The physiological levels of reactive oxygen species (ROS) are important to maintain various cell functions such as β-cell, but overload of ROS when exceeds the capacity of the antioxidant system induces oxidative stress [17]. Since β-cells have the lowest antioxidant enzymes in comparison to other tissues, they are essentially vulnerable to oxidative-induced damage [18].

In the present study we aimed to identify the biochemical and molecular mechanisms behind insulin insufficiency and oxidative stress on isolated rat pancreatic islets exposed to malathion.

Section snippets

Chemicals

All chemicals were purchased from Merck, Darmstate, Germany unless otherwise stated. DL-dithiothreitol (DTT), bovine albumin serum (BSA), adenosine diphosphate (ADP) sodium salt, adenosine triphospahte (ATP) disodium salt, nicotinamide adenine dinucleotide phosphate (NADPH), 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES), tetrabutylammonium hydrogen sulfate (TBAHS), 2′,7′-dichorodihydrofluorescein-diacetate (DCFH-DA), agarose, diethyl dicarbonate (DEPC) and solid phase extraction

Effect on serum AChE activity

Serum AChE activity, as percent of control, is shown in Fig. 1. Administration of malathion at dose of 400 mg/kg reduced AChE activity to 20.5 ± 5% in comparison with controls (P < 0.001). This means that activity of this enzyme was inhibited about 79.5%.

Effect on OGTT and kinetic parameters of glucose

As shown in Fig. 2, no change was observed in the fasting blood glucose between the malathion-treated (M-400) and control groups (P = 0.23). Following OGTT, in all groups, administration of oral glucose caused a rapid increase in blood glucose and

Discussion

In this study we have explored the effects of a single dose of malathion on the pancreatic endocrine function in rats. Following performance of in vivo and in vitro studies, we showed that acute exposure to malathion (400 mg/kg equal to ∼25% LD50) was able to induce a significant changes in glucose kinetic parameters and also GSIS, indicating an early involvement of the pancreas in malathion toxicity. Also, the observed increase in oxidative stress damage and reduction in production of energy in

Acknowledgements

Authors wish to thank Dr Mohsen Amini for consulting in determination of ATP/ADP and also Mr Mehdi Gholami and Mrs Sepideh Karoubi who assisted authors in the lab. Authors also thank the assistance of INSF. This study was the PhD thesis of the first author that was supported by TUMS.

Author’s contribution: A Nili-Ahmadabadi helped in bibliography and design of the study and did the work and drafted the manuscript as his Ph.D thesis. N Pourkhalili contributed in animal laparotomy surgery, islet

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Citation Excerpt :
Another study also demonstrated that exposure to diazinon was associated with oxidative stress and inflammatory response, as shown by the increase of ROS and inflammatory markers (NADPH oxidase, TNFα) levels in adipose tissue. It was also shown that acute malathion exposure led to oxidative stress damage (Nili-Ahmadabadi et al., 2013). Incubation of rat erythrocytes with phosphomidon (Datta et al., 1992) or hepatocytes with dichlorvos (Yamano and Morita, 1992) increased activity of superoxide dismutase and catalase and reduced the activity of glutathione reductase.
Widespread use and the bioaccumulation of pesticides in the environment lead to the contamination of air, water, soil and agricultural resources. A huge body of evidence points to the association between the pesticide exposure and increase in the incidence of chronic diseases, e.g. cancer, birth defects, reproductive disorders, neurodegenerative, cardiovascular and respiratory diseases, developmental disorders, metabolic disorders, chronic renal disorders or autoimmune diseases. Organophosphorus compounds are among the most widely used pesticides. A growing body of evidence is suggesting the potential interdependence between the organophosphorus pesticides (OPs) exposure and risk of obesity and type 2 diabetes mellitus (T2DM). This article reviews the current literature to highlight the latest in vitro and in vivo evidences on the possible influence of OPs on obesity and T2DM development, as well as epidemiological evidence for the metabolic toxicity of OPs in humans. The article also draws attention to the influence of maternal OPs exposure on offspring. Summarized studies suggest that OPs exposure is associated with metabolic changes linked with obesity and T2DM indicated that such exposures may increase risk or vulnerability to other contributory components.

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¹: Equally contributed as correspondence.

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On the biochemical and molecular mechanisms by which malathion induces dysfunction in pancreatic islets in vivo and in vitro

Highlights

Abstract

Conclusion

Graphical abstract

Introduction

Section snippets

Chemicals

Effect on serum AChE activity

Effect on OGTT and kinetic parameters of glucose

Discussion

Acknowledgements

Pestic. Biochem. Physiol.

Pestic. Biochem. Physiol.

Pestic. Biochem. Physiol.

Pestic. Biochem. Physiol.

Mol. Cell. Endocrinol.

Metabolism

Emviron. Toxicol. Pharmacol.

Pestic. Biochem. Physiol.

Free. Radic. Bio. Med.

Toxicol. Appl. Pharmacol.

Life Sci.

J. Biol. Chem.

Biochem. Biophys. Res. Commun.

Environ. Toxicol. Pharmacol.

Chem. Biol. Interact.

Environ. Toxicol. Pharmacol.

Pestic. Biochem. Physiol.

Toxicol. Appl. Pharmacol.

Incident diabetes and pesticide exposure among licensed pesticide applicators: agricultural health study

Am. J. Epidemiol.

Stimulatory effects of Malathion on the key enzymes activities of insulin secretion in langerhans islets, glutamate dehydrogenase and glucokinase

Toxicol. Mech. Methods

Comparative effects of calcium channel blockers, autonomic nervous system blockers and free radical scavengers on diazinon-induced hyposecretion of insulin from isolated islets of langerhans

Arh. Hig. Rada. Toksikol.

Transcriptional regulation of glucose sensors in pancreatic -cells and liver: an update

Sensors

Biochemical evidence for free radical induced lipid peroxidation as a mechanism for subchronic toxicity of malathion in blood and liver of rats

Hum. Exp. Toxicol.