Malic Enzyme 1 Limits Acetaminophen-Induced Liver Injury by Sustaining Redox and Bioenergetic Homeostasis

Background: Acetaminophen (APAP) overdose remains a major cause of acute liver injury. Although N-acetylcysteine (NAC) is the clinically established antidote for APAP toxicity, its efficacy is greatest when administered early, and additional therapeutic strategies are still needed for patients with delayed presentation or progressive injury. Because APAP hepatotoxicity involves coupled disturbances in redox control, mitochondrial performance, and cellular metabolism, metabolic enzymes that sustain NADPH availability may critically influence disease severity. Malic enzyme 1 (ME1), a cytosolic NADPH-generating enzyme, has not been functionally defined in this context. Methods: To determine the contribution of ME1 to APAP-induced liver injury (AILI), we used hepatocyte-specific ME1 knockout mice, hepatic overexpression and reconstitution approaches, primary mouse hepatocytes, and an enzymatically inactive ME1 mutant. Liver injury and associated changes in oxidative stress, mitochondrial function, energy metabolism, autophagic flux, and endoplasmic reticulum (ER) stress were evaluated using biochemical, histological, molecular, and ultrastructural analyses, together with pharmacological interventions. Results: Genetic loss of ME1 did not substantially alter early APAP metabolic activation-related indices, including APAP-protein adduct formation, but markedly increased hepatocellular metabolic vulnerability after APAP challenge. This phenotype was characterized by enhanced lipid peroxidation, impaired mitochondrial polarization, reduced ATP availability, defective autophagic flux, and amplified ER stress, leading to more severe liver damage. In contrast, ME1 overexpression or reconstitution promoted a more adaptive metabolic response and limited tissue injury. These effects depended largely on ME1 catalytic activity, as protection was markedly weakened with the mutant enzyme. Pharmacological analyses further supported the involvement of AMPK/mTOR-associated autophagy regulation and ER stress adaptation in the downstream actions of ME1. Malic acid also partially attenuated APAP-induced hepatotoxicity in vivo and in vitro. Conclusions: ME1 functions as an endogenous metabolic factor that influences the outcome of APAP-induced liver injury. Its catalytic activity supports hepatocyte survival primarily by preserving reductive capacity, bioenergetic balance, and adaptive stress responses, rather than by altering APAP metabolic activation.