The Redox Communication Network as a Regulator of Metabolism

Abstract

Key tissues are dysfunctional in obesity, diabetes, cardiovascular disease, fatty liver and other metabolic diseases. Focus has centered on individual organs as though each was isolated. Attention has been paid to insulin resistance as the key relevant pathosis, particularly insulin receptor signaling. However, many tissues play important roles in synergistically regulating metabolic homeostasis and should be considered part of a network. Our approach identifies redox as an acute regulator of the greater metabolic network. Redox reactions involve the transfer of electrons between two molecules and in this work refer to commonly shared molecules, reflective of energy state, that can readily lose electrons to increase or gain electrons to decrease the oxidation state of molecules including NAD(P), NAD(P)H, and thiols. Metabolism alters such redox molecules to impact metabolic function in many tissues, thus, responding to anabolic and catabolic stimuli appropriately and synergistically. It is also important to consider environmental factors that have arisen or increased in recent decades as putative modifiers of redox and reactive oxygen species (ROS) and thus metabolic state. ROS are highly reactive, controlled by the thiol redox state and influence the function of thousands of proteins. Lactate (L) and pyruvate (P) in cells are present in a ratio of about 10 reflective of the cytosolic NADH to NAD ratio. Equilibrium is maintained in cells because lactate dehydrogenase is highly expressed and near equilibrium. The major source of circulating lactate and pyruvate is muscle, although other tissues also contribute. Acetoacetate (A) is produced primarily by liver mitochondria where β-hydroxybutyrate dehydrogenase is highly expressed, and maintains a ratio of β-hydroxybutyrate (β) to A of about 2, reflective of the mitochondrial NADH to NAD ratio. All four metabolites as well as the thiols, cysteine and glutathione, are transported into and out of cells, due to high expression of relevant transporters. Our model supports regulation of all collaborating metabolic organs through changes in circulating redox metabolites, regardless of whether change was initiated exogenously or by a single organ. Validation of these predictions suggests novel ways to understand function by monitoring and impacting redox state.

Keywords: ROS; adipocytes; energy metabolism; hepatocytes; metabolic regulation; network; redox; β-cells.

FIGURE 1

Illustration of mitochondrial interactions among pyridine nucleotides, ROS and the thiol redox system. SOD, superoxide dismutase; NNT, nicotinamide nucleotide transhydrogenase; GPX, GSH peroxidase; Prx, peroxiredoxin; Trx, thioredoxin; R, reductase. Mitochondrial ROS is produced at high NADH levels. NADH is derived from available fuels and promotes ROS production when cellular ATP levels are sufficient. NNT plays a vital role in ROS removal driven by the proton gradient as indicated by red arrow.

FIGURE 2

Shared citric acid cycle cofactors. Shared factors are highlighted.

FIGURE 3

Illustration of how the intracellular redox state is communicated to the blood stream via common metabolites. The thiol ratio is reflected mainly in the cysteine to cystine ratio but also in the GSH/GSSG ratio. The mitochondrial pyridine nucleotide redox state is reflected in the β/A ratio and regulated mainly by liver mitochondria. The cytosolic pyridine nucleotide redox state is reflected in the L/P ratio and regulated mainly by muscle. All of these redox indicators are readily transported into and out of cells. Bold type indicates reaction is mainly regulated in that tissue.

FIGURE 4

Illustration of the time course of changes in blood metabolites in response to a glucose load. Data are from a single patient. (A) The β/A ratio; (B) the sum of β plus A; (C) the L/P ratio; and (D) the sum of L plus P. Assays were performed on neutralized acid extracts, prepared rapidly after blood samples were taken, and analyzed within 24 h (Williamson and Corkey, 1969, 1979).

FIGURE 5

Example of the changes in β/A ratios in isolated hepatocytes in response to the ketoacids of leucine, valine and isoleucine, carbohydrate- derived, lactate and pyruvate, and the FFA, oleate. Data derived from references (Corkey et al., 1981, 1982).

FIGURE 6

Effect of β-hydroxybutyrate (β-OHB) (A), Iron (B) and ROS removal (A) on stimulation of insulin secretion (A) and ROS generation (B) in clonal pancreatic β-cells (Adimora et al., 2010; Go and Jones, 2010b). Panel A shows insulin secretion at basal glucose in response to 20 mM β-OHB and concentration-dependent reversal with increasing NAC (n-acetyl cysteine). Panel B shows concentration-dependent ROS production from iron at basal and stimulatory glucose.

FIGURE 7

Effect of saccharin on ROS production in clonal pancreatic β-cells (A) and human adipocytes (B). A, Dark circles are vehicle, light circles are 5 mM saccharin. B, Concentration-dependent ROS production in cultured, differentiated human adipocytes in response to increasing concentrations of saccharin. The positive controls are tert-butyl hydroperoxide (t-BOOH) with and without 10 mM saccharin.