Mitochondria and redox signaling in steatohepatitis

Abstract

Alcoholic and nonalcoholic fatty liver diseases are potentially pathological conditions that can progress to steatohepatitis, fibrosis, and cirrhosis. These conditions affect millions of people throughout the world in part through poor lifestyle choices of excess alcohol consumption, overnutrition, and lack of regular physical activity. Abnormal mitochondrial and cellular redox homeostasis has been documented in steatohepatitis and results in alterations of multiple redox-sensitive signaling cascades. Ultimately, these changes in signaling lead to altered enzyme function and transcriptional activities of proteins critical to mitochondrial and cellular function. In this article, we review the current hypotheses linking mitochondrial redox state to the overall pathophysiology of alcoholic and nonalcoholic steatohepatitis and briefly discuss the current therapeutic options under investigation.

FIG. 1.

Cellular FAO. Long chain nonesterified fatty acids are actively transported through the plasma membrane, converted to acyl-CoAs, conveyed through the cytoplasm by fatty acid binding proteins, and transported into the mitochondrial matrix through the action of CPTs. The acyl-CoAs enter the beta-oxidation cycle that produces reducing equivalents as NADH, acetyl-CoAs, and chained shortened acyl-CoAs. The beta-oxidation of acyl-CoAs requires four enzymatic reactions: dehydrogenation by acyl-CoA dehydrogenase (AD), hydration by enoyl-CoA hydratase (2), oxidation by L-β-hydroxyacyl-CoA dehydrogenase (3), and thiolysis by β-ketothiolase(4). The last three enzymatic steps of beta-oxidation are carried out by MTP (2,3,4). The acetyl-CoA produced can then be shuttled to ketogenesis, enter the TCA cycle, or be used in steroidogenesis. CPT, carnitine palmitoyltransferase; TCA, tricarboxylic acid; MTP, mitochondrial trifunctional protein; FAO, fatty acid oxidation.

FIG. 2.

Oxidative phosphorylation. Utilization of reducing equivalents generated through β-oxidation and the TCA cycle to fuel the pumping of hydrogen from the mitochondrial matrix into the intermembrane space by the flow of electrons through the complexes of the ETC and, ultimately, reducing molecular oxygen to water. The movement of hydrogen back into the mitochondrial matrix, down this electrochemical gradient, through Complex V (ATP synthase) provides the necessary energy for the production of ATP. Cyt c, cytochrome c; FADH₂, reduced flavin adenine dinucleotide; NADH, reduced nicotinamide adenine dinucleotide; Ubq, ubiquinone; ETC, electron transport chain; ATP, adenosine triphosphate.

FIG. 3.

Factors affecting mitochondrial dysfunction in steatohepatitis. Observed clustering of interrelated factors linked to mitochondrial antioxidant status and redox nodes and steatohepatitis.

FIG. 4.

Redox nodes in mitochondrial ROS management. Involvement of four redox nodes (GSH/GSSG, Trx(SH)₂/TrxSS, NADP⁺/NADPH, and NAD⁺/NADH) in the reduction of ROS in the mitochondria. In this example, the reduction of H₂O₂, produced through the dismutation of mitochondrial superoxide (O₂⁻) by Mn-SOD, is mediated by several enzymes that require the efficient maintenance of the mitochondrial redox nodes. GPx, glutathione peroxidase; GSH, reduced glutathione; GSHR, glutathione reductase; GSSG, oxidized glutathione; Mn-SOD, manganese-superoxide dismutase; NAD⁺, nicotinamide adenine dinucleotide; NADP⁺, nicotinamide adenine dinucleotide phosphate; NADPH, reduced nicotinamide adenine dinucleotide phosphate; NNT, nicotinamide nucleotide transhydrogenase; Prx, peroxiredoxin; TrxR, thioredoxin reductase; Trx(SH)₂, reduced thioredoxin; TrxSS, oxidized thioredoxin; ROS, reactive oxygen species.

FIG. 5.

Sites of ROS production in steatohepatitis. Cellular localization and enzymes responsible for ROS production observed in steatohepatitis. AOX, acyl-CoA oxidase; CYP, cytochrome P450 enzyme; ER, endoplasmic reticulum; NOX, NADPH oxidase.

FIG. 6.

Mitochondrial ROS production. Under normal conditions, 1%–2% of electrons entering the ETC are transferred to molecular oxygen by Complexes I and III to produce superoxide (O₂⁻). Increases in the mitochondrial electrochemical gradient through uncoupling of the production of reducing equivalents from ATP synthesis results in increased production of O₂⁻. Additionally, any decrease in expression or post-translational modification of ETC components that further decreases the efficiency of electron transport results in increased mitochondrial ROS production.

FIG. 7.

Mitochondrial FAO. The mitochondria represents the site of the majority of oxidation of cellular fatty acids, as acyl-CoAs, and the entry and initiation of fatty acid metabolism is controlled by the activity of CPT. Reduced expression and activity observed in steatohepatitis limits mitochondrial FAO and perpetuates excess cellular lipid levels. Fatty acids within the mitochondria undergo β-oxidation through a 4-enzyme pathway that produces acetyl-CoA and a 2-carbon shortened acyl-CoA that can undergo further oxidation. The produced reducing equivalents are utilized by the ETC, whereas the acetyl-CoA enters the TCA cycle resulting in the production of CO₂ and additional reducing equivalents or reduced to produce ketone bodies. Decreased functioning of the enzymes of β-oxidation by oxidative modification or genetic manipulation has been shown to result in increased steatosis and is present in progressive steatohepatitis. KO, knockout.

FIG. 8.

Ultrastructural changes in steatohepatitis. Representative transmission electron microscopic images of from Otsuka Long-Evans Tokushima Fatty rats (A) with steatohepatitis and healthy controls (B). In image A, the Otsuka Long-Evans Tokushima Fatty liver demonstrates large, perinuclear lipid droplets and a disruption in nuclear integrity.