Protection of INS-1 Cells From Free Fatty Acid–Induced Apoptosis by Targeting hOGG1 to Mitochondria

Abstract

Chronic exposure to elevated levels of free fatty acids (FFAs) impairs pancreatic ␤-cell function and contributes to the decline of insulin secretion in type 2 diabetes. Previously, we reported that FFAs caused increased nitric oxide (NO) production, which damaged mitochondrial DNA (mtDNA) and ultimately led to apoptosis in INS-1 cells. To firmly establish the link between FFA-generated mtDNA damage and apoptosis, we stably transfected INS-1 cells with an expression vector containing the gene for the DNA repair enzyme human 8-oxoguanine DNA glycosylase/ apurinic lyase (hOGG1) downstream of the mitochondrial targeting sequence (MTS) from manganese superoxide dismutase. Successful integration of MTS-OGG1 into the INS-1 cellular genome was confirmed by Southern blot analysis. Western blots and enzyme activity assays revealed that hOGG1 was targeted to mitochondria and the recombinant enzyme was active. MTS-OGG1 cells showed a significant decrease in FFA-induced mtDNA damage compared with vector-only transfectants. Additionally, hOGG1 overexpression in mitochondria decreased FFA-induced inhibition of ATP production and protected INS-1 cells from apoptosis. These results indicate that mtDNA damage plays a pivotal role in FFA-induced ␤-cell dysfunction and apoptosis. Therefore, targeting DNA repair enzymes into ␤-cell mitochondria could be a potential therapeutic strategy for preventing or delaying the onset of type 2 diabetes symptoms. Diabetes 55:1022-1028, 2006 C hronic elevation of cellular free fatty acids (FFAs) is associated with both insulin resistance and type 2 diabetes (1,2). Results obtained from a variety of different laboratories suggest that the accumulation of lipid into islet tissue is deleterious to normal ␤-cell function and that this elevation in FFA content ultimately leads to ␤-cell failure and death through a process termed "lipotoxicity." Exposure of ␤-cells to high concentrations of FFAs has been correlated with impaired insulin secretion and changes in the expression of genes involved in the lipogenic and fat oxidation pathways and is considered to be an important factor in the pathogenesis of type 2 diabetes (3,4). Moreover, FFAs have been shown to cause ␤-cell death by both apoptotic and necrotic mechanisms (5,6). Although the exact mechanisms involved in FFA-induced apoptosis and necrosis remain to be clarified, it has been suggested that the ␤-cell dysfunction and death observed in type 2 diabetes may involve exaggerated activation of the inducible form of nitric oxide synthase (iNOS) by FFAs with the consequent generation of excess nitric oxide (NO) (7).Although NO plays a prominent role in regulating many biological functions, a growing body of evidence indicates that at high concentrations, it can also be cytotoxic and mutagenic (8). However, the cellular targets with which NO interacts have not been fully identified. We hypothesized that one of these targets might be mitochondrial DNA (mtDNA). Recently, we reported that FFAs (2:1, oleate:palmitate) caused a rise in NO production that ...

Mitochondrial 8-oxoguanine glycosylase decreases mitochondrial fragmentation and improves mitochondrial function in H9C2 cells under oxidative stress conditions

“…Interestingly, mitochondrial Ogg1 overexpression attenuated XO-induced apoptosis as determined by suppression of caspase-3 activation, by reduced DNA fragmentation, and by blunted appearance of condensed, fragmented nuclei (33). In INS-1 cells, Ogg1 overexpression in mitochondria decreased fatty acid-induced inhibition of ATP production and protected cells from apoptosis (32). A recent study using transgenic mice with cardiac-specific overexpression of active mitochondrial Ogg1 presented reduced cardiac fibrosis following transaortic constriction (41).…”

Oxidative DNA damage in disease—Insights gained from base excision repair glycosylase‐deficient mouse models

“…OGG1 has been shown to have both nuclear and mitochondrial localization. Several cell culture studies have addressed the role of mitochondrial OGG1 on diverse cellular functions, including insulin secretion from INS-1 cells and glucose uptake in C2C12 cells [Rachek et al, 2002;Rachek et al, 2006;Yuzefovych et al, 2012]. Some of the findings from these cell studies have been replicated in a recently developed transgenic mouse model overexpressing a mitochondrial form of human OGG1 [Yuzefovych et al, 2012]; this model is discussed in greater detail below.…”

Erythropoietin: Elucidating new cellular targets that broaden therapeutic strategies

“…For example, free fatty acids, which can lead to reactive oxygen species, have been shown to also contribute to mitochondrial DNA damage and impaired pancreatic β-cell function (Rachek et al, 2006). In patients with type 2 DM, skeletal muscle mitochondria have been described to be smaller than those in control subjects (Kelley et al, 2002).…”