Mechanisms of acetaminophen-induced liver injury and its implications for therapeutic interventions

Abstract

Acetaminophen (APAP) overdose is the leading cause of drug-induced acute liver failure in many developed countries. Mitochondrial oxidative stress is considered to be the predominant cellular event in APAP-induced liver injury. Accordingly, N-acetyl cysteine, a known scavenger of reactive oxygen species (ROS), is recommended as an effective clinical antidote against APAP-induced acute liver injury (AILI) when it is given at an early phase; however, the narrow therapeutic window limits its use. Hence, the development of novel therapeutic approaches that can offer broadly protective effects against AILI is clearly needed. To this end, it is necessary to better understand the mechanisms of APAP hepatotoxicity. Up to now, in addition to mitochondrial oxidative stress, many other cellular processes, including phase I/phase II metabolism, endoplasmic reticulum stress, autophagy, sterile inflammation, microcirculatory dysfunction, and liver regeneration, have been identified to be involved in the pathogenesis of AILI, providing new targets for developing more effective therapeutic interventions against APAP-induced liver injury. In this review, we summarize intracellular and extracellular events involved in APAP hepatotoxicity, along with emphatic discussions on the possible therapeutic approaches targeting these different cellular events.

Keywords: Acetaminophen; Autophagy; Endoplasmic reticulum stress; Liver regeneration; Mitochondrial oxidative stress; Sterile inflammation.

Fig. 1

Metabolic activation of acetaminophen. 85–90% of APAP is primarily metabolized by phase II conjugating enzymes (mainly UGT and SULT). Only 2% is excreted unchanged in the urine. And approximately 5–9% is metabolized mainly by CYP 2E1 into the highly reactive intermediate metabolite NAPQI. In general, NAPQI is detoxified by conjugating with GSH. However, excessive NAPQI depletes GSH following APAP overdose, leading to formation of APAP-ADs through covalent binding of sulfhydryl groups in cellular proteins. APAP, acetaminophen; UGT, UDP-glucuronosyltransferase; SULT, sulfotransferase; CYP 2E1, Cytochrome P450 2E1; NAPQI, N-acetyl-p-benzoquinone imine; GSH, glutathione; APAP-ADs, APAP protein adducts.

Fig. 2

Intracellular signaling events in acetaminophen hepatotoxicity. Excessive quantities of NAPQI generated by acetaminophen overdose deplete GSH in the cytoplasm, ER and mitochondria, leading to ER stress, mitochondrial oxidative stress and dysfunction. This induces TCA cycle and β-oxidation dysfunction, ATP depletion and the opening of the MPT pore, which, subsequently, leads to the translocation of mitochondrial proteins, such as AIF and Endo G, to the nucleus. This results in nuclear DNA fragmentation and ultimately necrotic cell death. ROS caused by NAPQI activates JNK signaling pathways. Sustained activation of JNK amplifies mitochondrial ROS and forms a self-sustaining activation loop. Nrf2, p53, adiponectin and FGF21 signaling pathways are activated to cope with cellular stress and injury. Similarly, autophagy alleviates AILI through removal of damaged mitochondria and detrimental APAP-ADs. NAPQI, N-acetyl-p-benzoquinone imine; ER, endoplasmic reticulum; TCA, tricarboxylic acid cycle; MPT, mitochondria permeability transition; AIF, apoptosis inducing factor; ROS, reactive oxygen species; JNK, c-Jun N-terminal kinase; AILI, acetaminophen-induced liver injury; APAP-ADs, acetaminophen protein adducts.

Fig. 3

Sterile inflammation and microcirculatory dysfunction induced by acetaminophen hepatotoxicity. DAMPs (including DNA fragments, HMGB1, uric acid and ATP) released from necrotic hepatocytes, transcriptional activate pro-inflammatory cytokines and chemokines in Kupffer cells. This causes neutrophils activation and recruited to the damage area of the liver, resulting in aggravation of hepatocytes necrosis at first, but contributing to liver repair and regeneration at a late stage. Tissue damage caused by APAP activate the coagulation system, concomitant with the generation of thrombin and formation of soluble and insoluble fibrin. Thrombin activates the platelets through downstream PAR-1 and PAR-4 signaling pathways, thereby exacerbates liver injury. During late stage, fibrin induced by thrombin contributes to liver repair through engagement of the leukocyte α_Mβ₂ integrin and subsequent induction of Mmp12. DAMP, damage associated molecular pattern; HMGB1, high mobility group box-1 protein; LSEC, liver sinusoidal endothelial cell; PAR-1, proteinase-activated receptor 1; PAR-4, proteinase-activated receptor 4; Mmp12, matrix metalloproteinases 12.