Oxidative stress plays a significant role in the development of insulin resistance; however, the cellular targets of oxidation that cause insulin resistance have yet to be fully elucidated. Methionine sulfoxide reductases (Msr) reduce oxidized methionine residues, thereby repairing and protecting proteins from oxidation. Recently, several genome-wide analyses have found human obesity to be strongly correlated with polymorphisms near the methionine sulfoxide reductase A (MsrA) locus. In this study, we tested whether modulation of MsrA expression significantly alters the development of obesity and/or insulin resistance in mice. We show that mice lacking MsrA (MsrA−/−) are prone to the development of high fat diet-induced insulin resistance and a reduced physiological insulin response compared to high fat-fed wild type mice. We also show that oxidative stress in C2C12 cell cultures reduces both insulin-stimulated phosphorylation and autophosphorylation of the insulin receptor. Tissues from high fat-fed mice show similar reduction in insulin receptor function and the lack of MsrA further diminishes these functions. Together, these data demonstrate for the first time that MsrA plays a role in the regulation of glucose homeostasis. In addition, these data support a novel hypothesis that obesity-induced insulin resistance is caused in part by reduced function of insulin signaling proteins arising from protein oxidation.
oxidative stress; methionine sulfoxide; diabetes; obesity; glucose homeostasis
Emerging evidence suggests that TLR (Toll-like receptor) 4 and downstream pathways [MAPKs (mitogen-activated protein kinases) and NF-κB (nuclear factor κB)] play an important role in the pathogenesis of insulin resistance. LPS (lipopolysaccharide) and saturated NEFA (non-esterified fatty acids) activate TLR4, and plasma concentrations of these TLR4 ligands are elevated in obesity and Type 2 diabetes. Our goals were to define the role of TLR4 on the insulin resistance caused by LPS and saturated NEFA, and to dissect the independent contribution of LPS and NEFA to the activation of TLR4-driven pathways by employing TAK-242, a specific inhibitor of TLR4. LPS caused robust activation of the MAPK and NF-κB pathways in L6 myotubes, along with impaired insulin signalling and glucose transport. TAK-242 completely prevented the inflammatory response (MAPK and NF-κB activation) caused by LPS, and, in turn, improved LPS-induced insulin resistance. Similar to LPS, stearate strongly activated MAPKs, although stimulation of the NF-κB axis was modest. As seen with LPS, the inflammatory response caused by stearate was accompanied by impaired insulin action. TAK-242 also blunted stearate-induced inflammation; yet, the protective effect conferred by TAK-242 was partial and observed only on MAPKs. Consequently, the insulin resistance caused by stearate was only partially improved by TAK-242. In summary, TAK-242 provides complete and partial protection against LPS- and NEFA-induced inflammation and insulin resistance, respectively. Thus, LPS-induced insulin resistance depends entirely on TLR4, whereas NEFA works through TLR4-dependent and -independent mechanisms to impair insulin action.
endotoxin; mitogen-activated protein kinase (MAPK); nuclear factor κB (NF-κB); saturated non-esterified fatty acid (NEFA); Toll-like receptor 4 (TLR4).; AP-1, activator protein-1; 2-DG, 2-deoxy-D[1,2-3H]glucose; DAG, diacylglycerol; FBS, fetal bovine serum; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; GSK, glycogen synthase kinase; IκB, inhibitory κB; IKK, IκB kinase; IL, interleukin; iNOS, inducible nitric oxide synthase; IRAK, IL-1-receptor-associated kinase; IRS, insulin receptor substrate; JNK, c-Jun N-terminal kinase; LPS, lipopolysaccharide; MAPK, mitogen-activated protein kinase; MCP1, monocyte chemoattractant protein-1; MEMα, minimum essential medium α; MyD88, myeloid differentiation factor 88; NEFA, non-esterified fatty acid(s); NF-κB, nuclear factor κB; NOD2, nucleotide-binding oligomerization domain-2; PKC, protein kinase C; RT–PCR, reverse transcription–PCR; TIR, Toll/IL-1 receptor; TIRAP, TIR domain-containing adaptor protein; TLR, Toll-like receptor; TNF, tumour necrosis factor; TRAF-6, TNF-receptor-associated factor-6
Genetic manipulations of Mn superoxide dismutase (MnSOD), SOD2 expression have demonstrated that altering the level of MnSOD activity is critical for cellular function and life span in invertebrates. In mammals, Sod2 homozygous knockout mice die shortly after birth, and alterations of MnSOD levels are correlated with changes in oxidative damage and in the generation of mitochondrial reactive oxygen species. In this study, we directly tested the effects of overexpressing MnSOD in young (4–6 months) and old (26–28 months) mice on mitochondrial function, levels of oxidative damage or stress, life span, and end-of-life pathology. Our data show that an approximately twofold overexpression of MnSOD throughout life in mice resulted in decreased lipid peroxidation, increased resistance against paraquat-induced oxidative stress, and decreased age-related decline in mitochondrial ATP production. However, this change in MnSOD expression did not alter either life span or age-related pathology.
Oxidative damage; Mn superoxide dismutase; Pathology; Aging
Glutathione peroxidase 4 (Gpx4) is a unique antioxidant enzyme that repairs oxidative damage to biomembranes. In the present study, we examined the effect of Gpx4 on the release of various apoptogenic proteins from mitochondria using transgenic mice overexpressing Gpx4 [Tg(GPX4+/0)] and mice deficient in Gpx4 (Gpx4+/− mice). Diquat exposure triggered apoptosis that occurred through intrinsic pathway and resulted in the mitochondrial release of cytochrome c (cyt. c), Smac/DIABLO, and Omi/HtrA2 in the liver of wild-type (Wt) mice. Liver apoptosis and cyt. c release were suppressed in Tg(GPX4+/0) mice but exacerbated in Gpx4+/− mice; however, neither the Tg(GPX4+/0) nor the Gpx4+/− mice showed any alterations in the levels of Smac/DIABLO or Omi/HtrA2 released from mitochondria. Submitochondrial fractionation data showed that Smac/DIABLO and Omi/HtrA2 existed primarily in the intermembrane space and matrix, while cyt. c and Gpx4 were both associated with inner membrane. In addition, diquat exposure induced cardiolipin peroxidation in the liver of Wt mice; the levels of cardiolipin peroxidation were reduced in Tg(GPX4+/0) mice but elevated in Gpx4+/− mice. These data suggest that Gpx4 differentially regulates apoptogenic protein release due to its inner membrane location in mitochondria and its ability to repair cardiolipin peroxidation.
Gpx4; Phospholipid hydroperoxide glutathione peroxidase 4; Apoptosis; Cardiolipin; Lipid peroxidation; Oxidative stress