Oxidative damage of mitochondrial DNA in diabetes and its protection by manganese superoxide dismutase

Mitochondrial biogenesis and the development of diabetic retinopathy

Retinal mitochondria become dysfunctional and their DNA (mtDNA) is damaged in diabetes. The biogenesis of mitochondrial DNA is tightly controlled by nuclear-mitochondrial transcriptional factors, and translocation of transcription factor A (TFAM) to the mitochondria is essential for transcription and replication. Our aim is to investigate the effects of diabetes on nuclear-mitochondrial communication in the retina and its role in the development of retinopathy. Damage of mtDNA, copy number, and biogenesis (PGC1, NRF1, TFAM) were analyzed in the retinas from streptozotocin-diabetic wild-type (WT) and MnSOD transgenic (Tg) mice. Binding between TFAM and chaperone Hsp70 was quantified by coimmunoprecipitation. The key parameters were confirmed in isolated retinal endothelial cells and in the retinas from human donors with diabetic retinopathy. Diabetes in WT mice increased retinal mtDNA damage and decreased copy number. The gene transcripts of PGC1, NRF1, and TFAM were increased, but mitochondrial accumulation of TFAM was significantly decreased, and the binding of Hsp70 and TFAM was subnormal compared to WT nondiabetic mice. However, Tg diabetic mice were protected from retinal mtDNA damage and alterations in mitochondrial biogenesis. In retinal endothelial cells, high glucose decreased the number of mitochondria, as demonstrated by MitoTracker green staining and by electron microscopy, and impaired the transcriptional factors. Similar alterations in biogenesis were observed in the donors with diabetic retinopathy. Thus, retinal mitochondrial biogenesis is under the control of superoxide radicals and is impaired in diabetes, possibly by decreased transport of TFAM to the mitochondria. Modulation of biogenesis by pharmaceutical or molecular means may provide a potential strategy to retard the development/progression of diabetic retinopathy.

Role of mitochondria biogenesis in the metabolic memory associated with the continued progression of diabetic retinopathy and its regulation by lipoic acid

PURPOSE

Termination of hyperglycemia does not arrest the progression of diabetic retinopathy, and retinal mitochondrial DNA (mtDNA) remains damaged, resulting in a continuous cycle of mitochondrial dysfunction. This study is to investigate the role of mitochondria biogenesis (regulated by nuclear mitochondrial signaling) in the metabolic memory phenomenon.

METHODS

Mitochondria DNA copy number, functional integrity, and biogenesis (peroxisome proliferator-activated receptor-γ coactivator-1α [PGC1], nuclear respiratory factor 1 [NRF1], mitochondrial transcriptional factor [TFAM]) were analyzed in the retina from streptozotocin-diabetic rats maintained in poor or good control for 12 months (PC and GC respectively), or in PC for 6 months followed by 6 months of GC (Rev). The effect of direct inhibition of superoxide on prior insult was investigated by supplementing lipoic acid (LA) during their 6 months of GC (R+LA). Binding of TFAM with chaperones (heat shock proteins 70 and 60, Hsp70 and Hsp60 respectively) was quantified by coimmunoprecipitation. The key parameters and the number of mitochondria (by transmission electron microscopy and fluorescence microscopy) were confirmed in isolated retinal endothelial cells.

RESULTS

Six months of GC in the rats in Rev group did not provide any benefit to diabetes-induced decreased mtDNA copy number, increased gene transcripts of PGC1, NRF1, and TFAM, and decreased mitochondrial TFAM. The binding of TFAM with the chaperones remained subnormal. Supplementation of LA (R+LA), however, had a significant beneficial effect on the impaired mitochondria biogenesis, and also on the continued progression of diabetic retinopathy. Similar results of reversal of high glucose insult were observed in isolated retinal endothelial cells.

CONCLUSIONS

Dysregulated mitochondria biogenesis contributes to the metabolic memory, and supplementation of GC with therapies targeted in modulating mitochondria homeostasis has potential in helping diabetic patients retard progression of retinopathy.

Diabetic retinopathy and damage to mitochondrial structure and transport machinery

PURPOSE

Mitochondrial function is controlled by membrane structure. In diabetes, retinal mitochondria are dysfunctional, and reversal of hyperglycemia fails to inhibit such changes. The goal of this study was to use anatomic and molecular biologic techniques to investigate the effect of diabetes on mitochondrial membrane structure.

METHODS

Wistar rats were maintained in poor glycemic control (PC; GHb 11.2%) or good glycemic control (GC; GHb 5.5%) for 12 months or in PC for 6 months, followed by GC for an additional 6 months. The structure of the retinal mitochondria in the microvascular region was evaluated by electron microscopy (TEM) and gene expressions of mitochondrial structure-related proteins by rat mitochondrial PCR array. Representative genes were validated by real-time PCR, and their protein expression by Western blot. The results were confirmed in the retina obtained from human donors with diabetic retinopathy.

RESULTS

TEM showed enlarged mitochondria with partial cristolysis in the retinal microvasculature from PC rats, compared with those from normal rats. Among 84 genes, 6 retinal genes were upregulated and 12 were downregulated. PCR confirmed alternations in the gene expressions of fusion (Mfn2), carrier (Timm44 and Slc25a21), Akt1, and fission proteins (Dnm1l). Protein levels of Mfn2 and Dnm1l were consistent with their mRNA levels, but their mitochondrial abundance was decreased. Reversal of hyperglycemia failed to normalize these changes. Retinas from donors with diabetic retinopathy also presented similar patterns of changes in the gene and protein expressions.

CONCLUSIONS

Mitochondrial structural and transport proteins play an important role in the development of diabetic retinopathy and also in the metabolic memory phenomenon associated with its continued progression.

Clair DK. Manganese superoxide dismutase: guardian of the powerhouse

The mitochondrion is vital for many metabolic pathways in the cell, contributing all or important constituent enzymes for diverse functions such as β-oxidation of fatty acids, the urea cycle, the citric acid cycle, and ATP synthesis. The mitochondrion is also a major site of reactive oxygen species (ROS) production in the cell. Aberrant production of mitochondrial ROS can have dramatic effects on cellular function, in part, due to oxidative modification of key metabolic proteins localized in the mitochondrion. The cell is equipped with myriad antioxidant enzyme systems to combat deleterious ROS production in mitochondria, with the mitochondrial antioxidant enzyme manganese superoxide dismutase (MnSOD) acting as the chief ROS scavenging enzyme in the cell. Factors that affect the expression and/or the activity of MnSOD, resulting in diminished antioxidant capacity of the cell, can have extraordinary consequences on the overall health of the cell by altering mitochondrial metabolic function, leading to the development and progression of numerous diseases. A better understanding of the mechanisms by which MnSOD protects cells from the harmful effects of overproduction of ROS, in particular, the effects of ROS on mitochondrial metabolic enzymes, may contribute to the development of novel treatments for various diseases in which ROS are an important component.

Erythrocyte antioxidant defenses as a potential biomarker of liver mitochondrial status in different oxidative conditions

The need for minimally invasive biomarkers to predict the progression of non-alcoholic fatty liver disease to non-alcoholic steatohepatitis is a priority. Oxidative stress and mitochondrial dysfunction contribute in this physiopathological process. The aim of this study was to analyze the potential role of erythrocytes as surrogate biomarkers of hepatic mitochondrial oxidative status in an animal model under different dietary oxidative conditions. Interestingly, we found that erythrocyte antioxidant status correlated with triglyceride content (p < 0.05-p < 0.001), thiobarbituric acid reactive species levels (p < 0.001) and with liver mitochondrial antioxidant levels (p < 0.001). These data suggest that erythrocyte antioxidant defenses could be used as sensitive and minimally invasive biomarkers of mitochondrial status in diverse oxidative conditions.

Inflammation in diabetic retinopathy

Diabetes causes a number of metabolic and physiologic abnormalities in the retina, but which of these abnormalities contribute to recognized features of diabetic retinopathy (DR) is less clear. Many of the molecular and physiologic abnormalities that have been found to develop in the retina in diabetes are consistent with inflammation. Moreover, a number of anti-inflammatory therapies have been found to significantly inhibit development of different aspects of DR in animal models. Herein, we review the inflammatory mediators and their relationship to early and late DR, and discuss the potential of anti-inflammatory approaches to inhibit development of different stages of the retinopathy. We focus primarily on information derived from in vivo studies, supplementing with information from in vitro studies were important.

Metal-based anti-diabetic drugs: advances and challenges

The current status and likely future directions of complexes of V(V/IV), Cr(III), Mo(VI), W(VI), Zn(II), Cu(II), and Mn(III) as potential oral drugs against type 2 diabetes are reviewed. We propose a unified model of extra- and intracellular mechanisms of anti-diabetic efficacies of V(V/IV), Mo(VI), W(VI), and Cr(III), centred on high-oxidation-state oxido/peroxido species that inhibit protein tyrosine phosphatases (PTPs) involved in insulin signalling. The postulated oxidative mechanism of anti-diabetic activity of Cr(III) via carcinogenic Cr(VI/V) (which adds to safety concerns) is consistent with recent clinical trials on Cr(III) picolinate, where activity was apparent only in patients with poorly controlled diabetes (high oxidative stress), and the correlation between the anti-diabetic activities and ease of oxidation of Cr(III) supplements and their metabolites in vivo. Zn(II) and Cu(II) anti-diabetics act via different mechanisms and are unlikely to be used as specific anti-diabetics due to their diverse and unpredictable biological activities. Hence, future research directions are likely to centre on enhancing the bioavailability and selectivity of V(V/IV), Mo(VI), or W(VI) drugs. The strategy of potentiating circulating insulin with metal ions has distinct therapeutic advantages over interventions that stimulate the release of more insulin, or use insulin mimetics, because of many adverse side-effects of increased levels of insulin, including increased risks of cancer and cardiovascular diseases.

Glucose oscillations, more than constant high glucose, induce p53 activation and a metabolic memory in human endothelial cells

AIMS/HYPOTHESIS

Damage persists in HUVECs exposed to a constant high glucose concentration long after glucose normalisation, a phenomenon termed 'metabolic memory'. Evaluation of the effects of exposure of HUVECs to oscillating high glucose on the induction of markers of oxidative stress and DNA damage (phospho-γ-histone H2AX and PKCδ) and onset of metabolic memory, and the possible role of the tumour suppressor transcriptional factor p53 is of pivotal interest.

METHODS

HUVECs were incubated for 3 weeks in 5 or 25 mmol/l glucose or oscillating glucose (24 h in 5 mmol/l glucose followed by 24 h in 25 mmol/l glucose) or for 1 week in constant 5 mmol/l glucose after being exposed for 2 weeks to continuous 25 mmol/l high glucose or oscillating glucose. Transcriptional activity of p53 was also evaluated in the first 24 h after high glucose exposure.

RESULTS

High constant glucose upregulated phospho-γ-histone H2AX and protein kinase C (PKC)δ compared with control. Oscillating glucose was even more effective than both normal and constant high glucose. Both constant and oscillating glucose resulted in a memory effect, which was more pronounced in the oscillating condition. Transcriptional activity of p53 peaked 6 h after glucose exposure, showing a predicted oscillatory behaviour.

CONCLUSIONS/INTERPRETATION

Exposure to oscillating glucose was more deleterious than constant high glucose and induced a metabolic memory after glucose normalisation. Hyperactivation of p53 during glucose oscillation might be due to the absence of consistent feedback inhibition during each glucose spike and might account for the worse outcome of this condition.

Epigenetic changes in mitochondrial superoxide dismutase in the retina and the development of diabetic retinopathy

OBJECTIVE

To investigate the role of epigenetic regulation of the manganese superoxide dismutase gene (sod2) in the development of diabetic retinopathy and the metabolic memory phenomenon associated with its continued progression after hyperglycemia is terminated.

RESEARCH DESIGN AND METHODS

Streptozotocin-induced diabetic rats were maintained in poor glycemic control (PC, GHb ∼12%) or in good glycemic control (GC, GHb ~7.0%) for 4 months, or were allowed to maintain PC for 2 months, followed by GC for 2 additional months (PC-Rev). For experimental galactosemia, a group of normal rats were fed a 30% galactose diet for 4 months or for 2 months, followed by a normal diet for 2 additional months. Trimethyl histone H4 lysine 20 (H4K20me3), acetyl histone H3 lysine 9 (H3K9), and nuclear transcriptional factor NF-κB p65 and p50 at the retinal sod2 promoter and enhancer were examined by chromatin immunoprecipitation.

RESULTS

Hyperglycemia (diabetes or galactosemia) increased H4K20me3, acetyl H3K9, and NF-κB p65 at the promoter and enhancer of retinal sod2, upregulated protein and gene expression of SUV420h2, and increased the interactions of acetyl H3K9 and NF-κB p65 to H4K20me3. Reversal of hyperglycemia failed to prevent increases in H4K20me3, acetyl H3K9, and NF-κB p65 at sod2, and sod2 and SUV420h2 continued to be abnormal. Silencing SUV420h2 by its small interfering RNA in retinal endothelial cells prevented a glucose-induced increase in H4K20me3 at the sod2 enhancer and a decrease in sod2 transcripts.

CONCLUSIONS

Increased H4K20me3 at sod2 contributes to its downregulation and is important in the development of diabetic retinopathy and in the metabolic memory phenomenon. Targeting epigenetic changes may serve as potential therapeutic targets to retard the development and progression of diabetic retinopathy.

Diabetic retinopathy, superoxide damage and antioxidants

Retinopathy, the leading cause of acquired blindness in young adults, is one of the most feared complications of diabetes, and hyperglycemia is considered as the major trigger for its development. The microvasculature of the retina is constantly bombarded by high glucose, and this insult results in many metabolic, structural and functional changes. Retinal mitochondria become dysfunctional, its DNA is damaged and proteins encoded by its DNA are decreased. The electron transport chain system becomes compromised, further producing superoxide and providing no relief to the retina from a continuous cycle of damage. Although the retina attempts to initiate repair mechanisms by inducing gene expressions of the repair enzymes, their mitochondrial accumulation remains deficient. Understanding the molecular mechanism of mitochondrial damage should help identify therapies to treat/retard this sight threatening complication of diabetes. Our hope is that if the retinal mitochondria are maintained healthy with adjunct therapies, the development and progression of diabetic retinopathy can be inhibited.

Insulin receptor substrate-1 and -2 mediate resistance to glucose-induced caspase-3 activation in human neuroblastoma cells

Hyperglycemia in patients with type 2 diabetes causes multiple neuronal complications, e.g., diabetic polyneuropathy, cognitive decline, and embryonic neural crest defects due to increased apoptosis. Possible mechanisms of neuronal response to increased glucose burden are still a matter of debate. Insulin and insulin-like growth factor-1 (IGF-1) receptor signaling inhibits glucose-induced caspase-3 activation and apoptotic cell death. The insulin receptor substrates (IRS) are intracellular adapter proteins mediating insulin's and IGF-1's intracellular effects. Even though all IRS proteins have similar function and structure, recent data suggest different actions of IRS-1 and IRS-2 in mediating their anti-apoptotic effects in glucose neurotoxicity. We therefore investigated the role of IRS-1/-2 in glucose-induced caspase-3 activation using human neuroblastoma cells. Overexpression of IRS-1 or IRS-2 caused complete resistance to glucose-induced caspase-3 cleavage. Inhibition of PI3-kinase reversed this protective effect of IRS-1 or IRS-2. However, MAP-kinases inhibition had only minor impact. IRS overexpression increased MnSOD abundance as well as BAD phosphorylation while Bim and BAX levels remained unchanged. Since Akt promotes cell survival at least partially via phosphorylation and inhibition of downstream forkhead box-O (FoxO) transcription factors, we generated neuroblastoma cells stably overexpressing a dominant negative mutant of FoxO1 mimicking activation of the insulin/IGF-1 pathway on FoxO-mediated transcription. Using these cells we showed that FoxO1 is not involved in neuronal protection mediated by increased IRS-1/-2 expression. Thus, overexpression of both IRS-1 and IRS-2 induces complete resistance to glucose-induced caspase-3 activation via PI3-kinase mediated BAD phosphorylation and MnSOD expression independent of FoxO1.

Role of mitochondrial DNA damage in the development of diabetic retinopathy, and the metabolic memory phenomenon associated with its progression

Diabetic retinopathy does not halt after hyperglycemia is terminated; the retina continues to experience increased oxidative stress, suggesting a memory phenomenon. Mitochondrial DNA (mtDNA) is highly sensitive to oxidative damage. The goal is to investigate the role of mtDNA damage in the development of diabetic retinopathy, and in the metabolic memory. mtDNA damage and its functional consequences on electron transport chain (ETC) were analyzed in the retina from streptozotocin-diabetic rats maintained in poor control (PC, glycated hemoglobin >11%) for 12 months or PC for 6 months followed by good control (GC, GHb < 6.5%) for 6 months. Diabetes damaged retinal mtDNA and elevated DNA repair enzymes (glycosylase). ETC proteins that were encoded by the mitochondrial genome and the glycosylases were compromised in the mitochondria. Re-institution of GC after 6 months of PC failed to protect mtDNA damage, and ETC proteins remained subnormal. Thus, mtDNA continues to be damaged even after PC is terminated. Although the retina tries to overcome mtDNA damage by inducing glycosylase, they remain deficient in the mitochondria with a compromised ETC system. The process is further exacerbated by subsequent increased mtDNA damage providing no relief to the retina from a continuous cycle of damage, and termination of hyperglycemia fails to arrest the progression of retinopathy.