Fig. 1Hypothesis for a mechanism by which excess glucose and saturated free fatty acids (SFAs) affect reactive oxygen species (ROS) generation in adipocytes in early stages of obesity. Excess glucose generates ROS via the pentose phosphate pathway, rather than by overloading mitochondrial oxidative phosphorylation, while SFA generate ROS following activating, Toll-like receptor 4 (TLR4) or lipid rafts. Nicotinamide adenine dinucleotide phosphate (NADPH) oxidase 4 (NOX4) might be the common mediator of ROS generation by both excess glucose and SFAs. ROS generated by both glucose and SFA activate nuclear factor κB (NF-κB). LR, lipid raft; SAA3, serum amyloid A3; MCP-1, monocyte chemoattractant protein-1.
Fig. 2Hypothesis for a mechanism by which mitochondrial β-oxidation of free fatty acid (FFA) from triglyceride stores affects reactive oxygen species (ROS) generation in adipocytes in the late stage of obesity. Uptake of excess glucose and FAA are inhibited, resulting in a decrease of pentose phosphate pathway and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase 4 (NOX4) activity. FFA from fat stores overload mitochondrial oxidative phosphorylation and generate ROS. Mitochondria-derived ROS activate nuclear factor κB. IR, insulin receptor; IRS, insulin receptor substrate; GLUT4, glucose transporter 4; LPL, lipoprotein lipase; FATP, fatty acid transport protein; HSL, hormone-sensitive lipase.
Fig. 3Scheme of all mechanisms by which these three distinct sources of reactive oxygen species (ROS) might affect adipocyte insulin resistance and adipose tissue inflammation. NOX, nicotinamide adenine dinucleotide phosphate (NADPH) oxidase.