Experimental models of hepatotoxicity related to acute liver failure

Abstract

Acute liver failure can be the consequence of various etiologies, with most cases arising from drug-induced hepatotoxicity in Western countries. Despite advances in this field, the management of acute liver failure continues to be one of the most challenging problems in clinical medicine. The availability of adequate experimental models is of crucial importance to provide a better understanding of this condition and to allow identification of novel drug targets, testing the efficacy of new therapeutic interventions and acting as models for assessing mechanisms of toxicity. Experimental models of hepatotoxicity related to acute liver failure rely on surgical procedures, chemical exposure or viral infection. Each of these models has a number of strengths and weaknesses. This paper specifically reviews commonly used chemical in vivo and in vitro models of hepatotoxicity associated with acute liver failure.

Keywords: Acetaminophen; Acute liver failure; Concanavalin A; D-galactosamine; Fas ligand; Hepatotoxicity.

Figure 1. Chemical structure of acetaminophen/paracetamol

Figure 2. Mechanism of acetaminophen-induced hepatotoxicity

High dose of acetaminophen (APAP) results in the formation of the reactive metabolite N-acetyl-p-benzoquinone imine (NAPQI) by cytochrome P450 (CYP) enzymes in hepatocytes. NAPQI depletes the glutathione (GSH) pool and binds to cysteine sulfhydryl groups of proteins thereby forming APAP-protein adducts. This initiates mitochondrial oxidant stress and peroxynitrite formation. The early oxidant stress triggers activation of mitogen-activated protein kinases (MAPK) that ultimately cause c-jun N-terminal kinase activation (p-JNK), which then translocates to mitochondria to further exacerbate the mitochondrial oxidant stress. The oxidant stress promotes the mitochondrial permeability transition (MPT) pore opening, resulting in the collapse of the membrane potential and cessation of adenosine triphosphate (ATP) synthesis. Early formation of a Bax-based pore in the outer mitochondrial membrane and subsequent swelling of the matrix with rupture of the outer mitochondrial membrane leads to release of intermembrane proteins, such as endonuclease G and apoptosis-inducing factor (AIF), and their translocation to the nucleus with nuclear DNA fragmentation. The massive mitochondrial dysfunction and nuclear DNA damage are the main causes of necrotic cell death.

Figure 3. Chemical structure of D-galactosamine

Figure 4. Pathophysiology of D-galactosamine/endotoxin-based liver injury

Endotoxin (ET) binds to Tolllike receptor 4 (TLR-4) on Kupffer cells, which triggers transcriptional activation of tumor necrosis factor α (TNFα). This results in activation of neutrophils, whereby TNFα is mainly responsible for neutrophil recruitment into the liver sinusoids. Furthermore, TNFα induces various adhesion molecules, such as the intercellular adhesion molecule 1 (ICAM-1), the vascular adhesion molecule 1 (VCAM-1) and selectin chemokines in hepatocytes and endothelial cells. Some of these adhesion molecules are critical for neutrophil extravasation. High dose of D-galactosamine (Gal) depletes the cellular uridine triphosphate and inhibits mRNA synthesis, such as of anti-apoptotic genes, in hepatocytes. This results in activation of the caspase cascade and DNA fragmentation. Caspase activation is also caused by TNFα-induced apoptosis involving the TNFα receptor 1 (TNFα-R1) and caspase activation.

Figure 5. Fas ligand-induced apoptosis in type I and type II cells

Binding of Jo2 or MegaFasL to the Fas ligand receptor (FasR) with a so-called ‘death domain’ (FADD) activates caspase (casp) 8. In type I cells (red arrow), the activation of casp 8 is sufficient to directly activate effector caspase 3. In contrast, in type II cells (blue arrows) casp 8 activation triggers Bid cleavage and truncated Bid (tBid) translocation to mitochondria. Together with Bax and Bad, tBid causes the mitochondrial outer membrane permeabilization with release of cytochrome c. The latter, together with pro-casp 9 and apoptotic protease activating factor 1 (apaf 1), form the apoptosome, which activates casp 9. This leads to drastically enhanced casp 3 activation, followed by induction of apoptosis.