Diabetes associated cell stress and dysfunction: role of mitochondrial and non-mitochondrial ROS production and activity

Abstract

It is now widely accepted, given the current weight of experimental evidence, that reactive oxygen species (ROS) contribute to cell and tissue dysfunction and damage caused by glucolipotoxicity in diabetes. The source of ROS in the insulin secreting pancreatic beta-cells and in the cells which are targets for insulin action has been considered to be the mitochondrial electron transport chain. While this source is undoubtably important, we provide additional information and evidence for NADPH oxidase-dependent generation of ROS both in pancreatic beta-cells and in insulin sensitive cells. While mitochondrial ROS generation may be important for regulation of mitochondrial uncoupling protein (UCP) activity and thus disruption of cellular energy metabolism, the NADPH oxidase associated ROS may alter parameters of signal transduction, insulin secretion, insulin action and cell proliferation or cell death. Thus NADPH oxidase may be a useful target for intervention strategies based on reversing the negative impact of glucolipotoxicity in diabetes.

Figure 1

Relevant sites of production of reactive oxygen species (ROS) and antioxidant systems in a generic cell type ROS can be generated through glucose metabolism in mitochondria (by electron transport chain (ETC) activity) and in the plasma membrane (through NADPH oxidase – NADPH_ox). The main antioxidant enzymes are superoxide dismutase (SOD), glutathione reductase, glutathione peroxidase and catalase.

Figure 3

The central role of reactive oxygen species (ROS) and uncoupling protein (UCP) for the first and second phases of insulin secretion or induction of cell death The normal flux of metabolites (e.g. glucose and fatty acids) through metabolic pathways (e.g. TCA cycle) generates NADH and FADH₂ that are used by the electron transport chain for proton translocation and ATP synthesis. An increased ATP/ADP ratio leads to elevation of intracellular Ca²⁺ and the peak of insulin secretion in the first phase (A). The production of ROS and activation of UCP are associated with high metabolic flux required to maintain the ATP/ADP ratio and to sustain insulin secretion for a prolonged period (B). In this situation the O₂ consumption is elevated. However, as a consequence of sustained ROS production and UCP activation causing excessive H⁺ leak, ATP levels will fall resulting in cell death by apoptosis (C).

Figure 2

Mechanism of insulin secretion stimulated by glucose and fatty acids in pancreaticβ-cells Glucose and fatty acids generate ATP, which promotes closure of the ATP-dependent K⁺ channel leading to cell membrane depolarization. As a consequence, voltage-dependent Ca²⁺ channels are opened, increasing intracellular Ca²⁺ concentration leading to insulin secretion. The NADPH oxidase complex in the plasma membrane is activated through protein kinase C (PKC), which is activated by fatty acid derived signalling molecules. The production of nitric oxide (NO) by inducible nitric oxide synthase (iNOS) is up-regulated by cytokines and fatty acids subsequently impacting on pancreatic β-cell function. CPT-complex, carnitine palmitoyl transferase complex; F-1,6-P, fructose-1,6-diphosphate; F-6-P, fructose-6-phosphate; GLUT-2, glucose transporter-2; GK, glucokinase; G-6-P, glucose-6-phosphate; OAA, oxaloacetic acid; PDX-1, pancreatic duodenal homeobox gene-1; PFK, phosphofructokinase; PKC, protein kinase C; PPAR, peroxisome proliferator-activated receptor; ROS, reactive oxygen species; SG, secretory granule.

Figure 4

Induction of insulin resistance by oxidative stress Prolonged high plasma levels of glucose and free fatty acids (FFA) lead to an increased production of reactive oxygen species (ROS) and reactive nitrogen species (RNS), which induce activation of various stress-activated protein kinases such as JNK, p38, and IKKβ. These kinases have been suggested to phosphorylate serine insulin receptor substrate-1 (IRS-1). In addition, IKKβ leads to activation of NFκB, a transcriptional factor that increases iNOS expression and nitric oxide (NO) production. NO can induce IRS-1 S-nitrosylation. Both serine phosphorylation and S-nitrosylation of IRS-1 have been associated with increased proteosome-dependent degradation of signal transduction associated proteins and suppressed insulin signalling. These effects result in insulin resistance in the liver, skeletal muscle and adipose tissue.