Persistent Oxidative Stress Due to Absence of Uncoupling Protein 2 Associated with Impaired Pancreatic β-Cell Function

Division of Translational Biology, The Hamner Institutes for Health Sciences, Research Triangle Park, North Carolina 27709, USA.
Endocrinology (Impact Factor: 4.5). 02/2009; 150(7):3040-8. DOI: 10.1210/en.2008-1642
Source: PubMed


Uncoupling protein (UCP) 2 is a widely expressed mitochondrial protein whose precise function is still unclear but has been linked to mitochondria-derived reactive oxygen species production. Thus, the chronic absence of UCP2 has the potential to promote persistent reactive oxygen species accumulation and an oxidative stress response. Here, we show that Ucp2-/- mice on three highly congenic (N >10) strain backgrounds (C57BL/6J, A/J, 129/SvImJ), including two independently generated sources of Ucp2-null animals, all exhibit increased oxidative stress. Ucp2-null animals exhibit a decreased ratio of reduced glutathione to its oxidized form in blood and tissues that normally express UCP2, including pancreatic islets. Islets from Ucp2-/- mice exhibit elevated levels of numerous antioxidant enzymes, increased nitrotyrosine and F4/80 staining, but no change in insulin content. Contrary to results in Ucp2-/- mice of mixed 129/B6 strain background, glucose-stimulated insulin secretion in Ucp2-/- islets of each congenic strain was significantly decreased. These data show that the chronic absence of UCP2 causes oxidative stress, including in islets, and is accompanied by impaired glucose-stimulated insulin secretion.

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Available from: Sheila Collins, May 28, 2015
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    • "Given the unresolved molecular function of UCP2, it is perhaps not surprising that the protein's physiological role in beta cells has not yet been established conclusively either. Depending on their genetic background, global Ucp2 knockout mice exhibit either improved glucose tolerance and GSIS [5] [9] or unaltered glucose tolerance [10] and impaired GSIS [9]. Beta-cell-specific ablation of UCP2 leads to glucose-intolerant mice whose pancreatic islets, however, exhibit higher GSIS than their wild type counterparts [11]. "
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    ABSTRACT: We have recently shown that overnight exposure of INS-1E insulinoma cells to palmitate in the presence of high glucose causes defects in both mitochondrial energy metabolism and glucose-stimulated insulin secretion (GSIS). Here we report experiments designed to test the involvement of mitochondrial uncoupling protein-2 (UCP2) in these glucolipotoxic effects. Measuring real-time oxygen consumption in siRNA-transfected INS-1E cells, we show that deleterious effects of palmitate on the glucose sensitivity of mitochondrial respiration and on the coupling efficiency of oxidative phosphorylation are independent of UCP2. Consistently, palmitate impairs GSIS to the same extent in cells with and without UCP2. Furthermore, we knocked down UCP2 in spheroid INS-1E cell clusters (pseudoislets) to test whether or not UCP2 regulates insulin secretion during prolonged glucose exposure. We demonstrate that there are no differences in temporal GSIS kinetics between perifused pseudoislets with and without UCP2. We conclude that UCP2 is not involved in palmitate-induced impairment of GSIS in INS-1E insulinoma cells and is not needed for the amplification of insulin release. These conclusions inform ongoing debate on the disputed biochemical and physiological functions of the beta cell UCP2.
    Full-text · Article · May 2015
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    • ". Indeed, studies involving this original genetic knockout strain demonstrated that UCP2 restricts the insulin secretory capacity of mice fed a high fat diet [16] and that UCP2 mediates beta cell defects caused by free fatty acids [17]. However, work on more recently established Ucp2-deficient mouse strains [18] has suggested a physiological instead of a pathological role for UCP2 as the protein has been attributed a protective function in assisting beta cells to deal with sustained oxidative stress [6]. Such stress is for example encountered after chronic fatty acid exposure [19] [20]. "
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    ABSTRACT: High glucose and fatty acid levels impair pancreatic beta cell function. We have recently shown that palmitate-induced loss of INS-1E insulinoma cells is related to increased reactive oxygen species (ROS) production as both toxic effects are prevented by palmitoleate. Here we show that palmitate-induced ROS are mostly mitochondrial: oxidation of MitoSOX, a mitochondria-targeted superoxide probe, is increased by palmitate, whilst oxidation of the equivalent non-targeted probe is unaffected. Moreover, mitochondrial respiratory inhibition with antimycin A stimulates palmitate-induced MitoSOX oxidation. We also show that palmitate does not change the level of mitochondrial uncoupling protein-2 (UCP2) and that UCP2 knockdown does not affect palmitate-induced MitoSOX oxidation. Palmitoleate does not influence MitoSOX oxidation in INS-1E cells ±UCP2 and largely prevents the palmitate-induced effects. Importantly, UCP2 knockdown amplifies the preventive effect of palmitoleate on palmitate-induced ROS. Consistently, viability effects of palmitate and palmitoleate are similar between cells ±UCP2, but UCP2 knockdown significantly augments the palmitoleate protection against palmitate-induced cell loss at high glucose. We conclude that UCP2 neither mediates palmitate-induced mitochondrial ROS generation and the associated cell loss, nor protects against these deleterious effects. Instead, UCP2 dampens palmitoleate protection against palmitate toxicity. Copyright © 2014 The Authors. Published by Elsevier B.V. All rights reserved.
    Full-text · Article · Nov 2014
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    • "PPAR-δ activation is shown to induce UCP2 expression in several cell lines, such as H9c2 cells, adipose cells, and human muscle cells [26–28]. UCP2 is a widely expressed mitochondrial inner membrane carrier protein that regulates the mitochondrial membrane potential created by the proton gradient across the inner mitochondrial membrane [29]. UCP2 is also thought to play a role in the detoxification of ROS produced by the mitochondria given that mitochondrial production of ROS is dependent on the mitochondrial membrane potential [30, 31]. "
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    ABSTRACT: High-salt diet-induced cardiac hypertrophy and fibrosis are associated with increased reactive oxygen species production. Transient receptor potential vanilloid type 1 (TRPV1), a specific receptor for capsaicin, exerts a protective role in cardiac remodeling that resulted from myocardial infarction, and peroxisome proliferation-activated receptors δ (PPAR-δ) play an important role in metabolic myocardium remodeling. However, it remains unknown whether activation of TRPV1 could alleviate cardiac hypertrophy and fibrosis and the effect of cross-talk between TRPV1 and PPAR-δ on suppressing high-salt diet-generated oxidative stress. In this study, high-salt diet-induced cardiac hypertrophy and fibrosis are characterized by significant enhancement of HW/BW%, LVEDD, and LVESD, decreased FS and EF, and increased collagen deposition. These alterations were associated with downregulation of PPAR-δ, UCP2 expression, upregulation of iNOS production, and increased oxidative/nitrotyrosine stress. These adverse effects of long-term high-salt diet were attenuated by chronic treatment with capsaicin. However, this effect of capsaicin was absent in TRPV1(-/-) mice on a high-salt diet. Our finding suggests that chronic dietary capsaicin consumption attenuates long-term high-salt diet-induced cardiac hypertrophy and fibrosis. This benefit effect is likely to be caused by TRPV1 mediated upregulation of PPAR-δ expression.
    Full-text · Article · Jul 2014 · PPAR Research
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