Acetaminophen Toxicity: Novel Insights Into Mechanisms and Future Perspectives

Abstract

Acetaminophen (APAP) overdose is the most common cause of acute liver failure in the US, and decades of intense study of its pathogenesis resulted in the development of the antidote N-acetylcysteine, which facilitates scavenging of the reactive metabolite and is the only treatment in clinical use. However, the narrow therapeutic window of this intervention necessitates a better understanding of the intricacies of APAP-induced liver injury for the development of additional therapeutic approaches that can benefit late-presenting patients. More recent investigations into APAP hepatotoxicity have established the critical role of mitochondrial dysfunction in mediating liver injury as well as clarified mechanisms of APAP-induced hepatocyte cell death. Thus, it is now established that mitochondrial oxidative and nitrosative stress is a key mechanistic feature involved in downstream signaling after APAP overdose. The identification of specific mediators of necrotic cell death further establishes the regulated nature of APAP-induced hepatocyte cell death. In addition, the discovery of the role of mitochondrial dynamics and autophagy in APAP-induced liver injury provides additional insight into the elaborate cell signaling mechanisms involved in the pathogenesis of this important clinical problem. In spite of these new insights into the mechanisms of liver injury, significant controversy still exists on the role of innate immunity in APAP-induced hepatotoxicity.

Figure 1

Mechanism of acetaminophen (APAP)-induced hepatocyte cell death. At high concentrations, APAP in hepatocytes is metabolized by components of the cytochrome P450 system to a reactive intermediate, N-acetyl-p-benzoquinone imine (NAPQI). High concentrations of NAPQI deplete cellular glutathione stores and subsequently form APAP protein adducts, especially on mitochondrial proteins. Components of the electron transport chain such as ATP synthase are affected, which compromises respiratory chain function and enhances generation of free radicals such as superoxide. This reacts with nitric oxide (NO) within the mitochondria to produce highly reactive peroxynitrite, which nitrates mitochondrial proteins such as manganese superoxide dismutase (MnSOD). This compromises mitochondrial antioxidant defenses, causing mitochondrial oxidant stress and oxidation of proteins such as mitochondrial thioredoxin. In the cytosol, oxidation of thioredoxin results in its detachment from its binding partner apoptosis signal-regulating kinase 1 (ASK1), which is then activated. ASK1, along with activated mixed-lineage kinase 3 (MLK3) then activate c-jun N-terminal kinase (JNK) to its phosphorylated form through MKK4 phosphorylation. Phosphorylated JNK translocates to the mitochondria and binds to Sab on the outer mitochondrial membrane, which, through a Src-mediated pathway, further inhibits mitochondrial electron transport. This amplifies mitochondrial oxidant stress, which is further exacerbated by translocation of Bax and glycogen synthase kinase-3β (GSK-3β) from the cytosol to the mitochondria. These events activate the mitochondrial permeability transition, which releases mitochondrial intermembrane proteins such as endonuclease G and apoptosis-inducing factor (AIF), along with cytochrome c and Smac. Translocation of AIF and endonuclease G to the nucleus then induces nuclear DNA fragmentation, which along with activation of receptor-interacting protein kinases 3/1 (RIP3/RIP1) finally induce programmed necrosis.

Figure 2

Sterile inflammation and liver regeneration. A sterile inflammatory response is initiated by release of damage-associated molecular patterns (DAMPs) from necrotic cells. DAMPs activate pattern recognition receptors such as toll-like receptors (TLRs), which induces the formation of cytokines and chemokines and the recruitment of inflammatory cells (see text for details). In APAP-induced liver injury, the preponderance of experimental and clinical evidence suggests that this sterile inflammatory response does not aggravate the original injury but causes the removal of necrotic cells and promotes regeneration (see text for details). HMGB1, high-mobility group box 1 protein; IL, interleukin; MCP-1, monocyte chemoattractant protein-1; mtDNA, mitochondrial DNA; RAGE, receptor for advanced glycation end products; ROS, reactive oxygen species.