Perspectives in liver redox imbalance: Toxicological and pharmacological aspects underlying iron overloading, nonalcoholic fatty liver disease, and thyroid hormone action.

Oxidative stress is an imbalance between oxidants and antioxidants in favor of the oxidants, leading to a disruption of redox signaling and control, and/or molecular damage altering cellular functions. This redox imbalance may trigger different responses depending on the antioxidant potential of a given cell, the level of reactive oxygen/nitrogen species (ROS/RNS) attained and the time of exposure, with protective effects being induced at low ROS/RNS levels in acute or short-term conditions, and harmful effects after high ROS/RNS exposure in prolonged situations. Relevant conditions underlying liver redox imbalance include iron overload associated with ROS production via Fenton chemistry and the magnitude of the iron labile pool achieved, with low iron exposure inducing protective effects related to nuclear factor-κB, signal transducer and activation of transcription 3, and nuclear factor erythroid-related factor 2 (Nrf2) activation and upregulation of ferritin, hepcidin, acute-phase response and antioxidant components, whereas high iron exposure causes drastic oxidation of biomolecules, mitochondrial dysfunction, and cell death due to necrosis, apoptosis and/or ferroptosis. Redox imbalance in nonalcoholic fatty liver disease (NAFLD) is related to polyunsaturated fatty acid depletion, lipogenic factor sterol regulatory element-binding protein-1c upregulation, fatty acid oxidation-dependent peroxisome proliferator-activated receptor-α downregulation, low antioxidant factor Nrf2 and insulin resistance, a phenomenon that is exacerbated in nonalcoholic steatohepatitis triggering an inflammatory response. Thyroid hormone (T3 ) administration determines liver preconditioning against ischemia-reperfusion injury due to the redox activation of several transcription factors, AMP-activated protein kinase, unfolded protein response and autophagy. High grade liver redox imbalance occurring in severe iron overload is adequately handled by iron chelation, however, that underlying NAFLD/NASH is currently under study in several Phase II and Phase III trials.