Role of metabolically generated reactive oxygen species for lipotoxicity in pancreatic β‐cells

Abstract

Chronically elevated concentrations of non-esterified fatty acids (NEFAs) in type 2 diabetes may be involved in β-cell dysfunction and apoptosis. It has been shown that long-chain saturated NEFAs exhibit a strong cytotoxic effect upon insulin-producing cells, while short-chain as well as unsaturated NEFAs are well tolerated. Moreover, long-chain unsaturated NEFAs counteract the toxicity of palmitic acid. Reactive oxygen species (ROS) formation and gene expression analyses together with viability assays in different β-cell lines showed that the G-protein-coupled receptors 40 and 120 do not mediate lipotoxicity. This is independent from the role, which these receptors, specifically GPR40, play in the potentiation of glucose-induced insulin secretion by saturated and unsaturated long-chain NEFAs. Long-chain NEFAs are not only metabolized in the mitochondria but also in peroxisomes. In contrast to mitochondrial β-oxidation, the acyl-coenzyme A (CoA) oxidases in the peroxisomes form hydrogen peroxide and not reducing equivalents. As β-cells almost completely lack catalase, they are exceptionally vulnerable to hydrogen peroxide generated in peroxisomes. ROS generation in the respiratory chain is less important because overexpression of catalase and superoxide dismutase in the mitochondria do not provide protection. Thus, peroxisomally generated hydrogen peroxide is the likely ROS that causes pancreatic β-cell dysfunction and ultimately β-cell death. Keywords: β-oxidation, G-protein-coupled receptors, insulin secretion, mitochondria, non-esterified fatty acids, peroxisomes, reactive oxygen species, type 2 diabetes mellitus Date submitted 26 March 2010; date of final acceptance 29 April 2010 IntroductionType 2 diabetes mellitus is a complex metabolic disorder with a dramatically increasing prevalence worldwide [1]. This disorder is characterized by peripheral insulin resistance and pancreatic β-cell dysfunction [2,3], resulting in defective glucose-induced insulin secretion [4][5][6] and ultimately in β-cell loss through apoptosis [7,8]. Hypercaloric Western diets, rich in carbohydrates and saturated fats, are responsible for the manifestation of the metabolic syndrome. This is characterized by dyslipidaemia, hypertension, and obesity, which precede type 2 diabetes manifestation. Accompanying elevated levels of non-esterified fatty acids (NEFAs) [9] can suppress insulin secretion and cause β-cell dysfunction and loss, a phenomenon referred to as lipotoxicity [10,11]. Although lipotoxicity is subject to intensive research and scientific discussion, a conclusive molecular mechanism has not been elucidated. Structural Requirements for LipotoxicityThe effects of NEFAs upon insulin-producing cells are dependent on chain length and degree of saturation [12]. Saturated long-chain NEFAs, such as the physiologically most abundant saturated NEFA palmitic acid, are highly toxic, Correspondence to: Prof. Sigurd Lenzen, Institute of Clinical Biochemistry, Hannover Medical School, 30623 Hannover, Germany. E-mail: lenzen.sigurd@mh-han...

Lipotoxicity disrupts incretin-regulated human β cell connectivity

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The p66Shc redox adaptor protein is induced by saturated fatty acids and mediates lipotoxicity-induced apoptosis in pancreatic beta cells

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