Autoregulation of free radicals via control of uncoupling proteins in beta-cells

Periwal, Vipul

Abstract

Mitochondrial dysfunction can result from several factors, including oxidative stress. Mitochondria are major producers of reactive oxygen species (ROS) through a process that is sensitive to the proton motive force and regulated by scavenging enzymes (SE) and the activation of uncoupling proteins (UCP), which decrease DeltaPsi. The tissue-damaging effects of ROS are hypothesized to underlie many well-defined diseases and clinically-relevant complications, including those associated with diabetes, Parkinson's, Alzheimer's, myopathy, and atherosclerosis. ROS have also been shown to play important signaling roles in mitochondrial biogenesis, longevity, and mitochondrial evolution and adaptation. Therefore, tight regulation of ROS is important to minimize damage without impacting important signaling functions. We have continued our investigations of beta cell mitochondrial activity by adding a cytosolic compartment. Distinguishing the cytosolic and mitochondrial domains, this model integrates the key reactions and pathways in beta-cells. We can simulate beta-cell metabolism in vivo in response to elevated glucose, the primary model input. The effects of cytosolic NADH production and transport on beta-cell metabolism were investigated by disturbing the NADH transport coefficient (95% decreased). Simulation results of normal condition and disturbed transport condition were compared with available experimental data for model validation. The simulated changes of metabolite concentrations (e.g., ATP and NADH) and reaction fluxes (e.g. glucose utilization, TCA cycle) can match available experimental data. Moreover, under normal conditions, cytosolic NADH production can account for 10% vs. 30% of the mitochondrial NADH production at basal state and elevated glucose state, respectively. By inhibiting NADH transport, the cytosolic NADH increased tremendously, but the NADH transport only can account for less than 5% of the mitochondrial NADH production. Mitochondrial membrane potential only had minor increase. In summary, this model helps to quantitatively evaluate the role of NADH transport in beta-cell metabolism and provides a way to improve our understanding of the regulation of GSIS in beta-cells. We have initiated a model of muscle mitochondrial function. Insulin resistance (IR) induced by high-fat diet (HFD) may be attributed to an elevation of oxidative species (ROS) levels in animal studies. The underlying mechanism, however, is unclear. A better understanding of this connection is essential for the treatment of IR and type 2 diabetes. An integrated mathematical model of skeletal muscle mitochondrial metabolism was developed, which can simulate the metabolic responses of muscle mitochondria to various substrates (e.g., pyruvate). Firstly, this model was validated to compare model simulations with experimental data (rates of oxygen consumption and ROS emission) of skeletal muscle derived from normal diet. The mechanism by which HFD can induce changes in oxygen consumption and ROS emission were further investigated by perturbing each of the reaction coefficients of upstream reactions and ROS removal processes for each substrate. The corresponding simulations were compared with experimental data obtained from skeletal muscle obtained from animals on HFD. Skeletal muscle mitochondria derived from HFD animals had higher metabolic activity, evident in the remarkable increases of oxygen consumption and ROS emission rates in response to each substrate. Model simulations suggested that these increases were controlled primarily by the availability of reducing equivalents, which supports the hypothesis that HFD stimulates upstream reactions in skeletal muscle. Specifically, increases in ROS emission were much more sensitive to parameter changes than increases in oxygen consumption, possibly reflecting different regulation mechanisms. As antioxidant protection mechanisms, the effects of ROS removal processes on ROS balance were further investigated. The results suggest that the system of scavenging enzymes has the most important effect on ROS removal. These investigations are ongoing and we are collaborating with experimental colleagues to test the predictions of our models.