Cellular and molecular mechanisms of liver injury

Abstract

Derangements in apoptosis of liver cells are mechanistically important in the pathogenesis of end-stage liver disease. Vulnerable hepatocytes can undergo apoptosis via an extrinsic, death receptor-mediated pathway, or alternatively intracellular stress can activate the intrinsic pathway of apoptosis. Both pathways converge on mitochondria, and mitochondrial dysfunction is a prerequisite for hepatocyte apoptosis. Persistent apoptosis is a feature of chronic liver diseases, and massive apoptosis is a feature of acute liver diseases. Fibrogenesis is stimulated by ongoing hepatocyte apoptosis, eventually resulting in cirrhosis of the liver in chronic liver diseases. Endothelial cell apoptosis occurs in ischemia-reperfusion injury. Natural killer and natural killer T cells remove virus-infected hepatocytes by death receptor-mediated fibrosis. Lastly, activated stellate cell apoptosis leads to slowing and resolution of apoptosis. This review summarizes recent cellular and molecular advances in the understanding of the injury mechanisms leading to end-stage liver disease.

Figure 1

Extrinsic and intrinsic pathways of hepatocyte apoptosis. The extrinsic pathway is activated by death receptors. Fas or TRAIL (depicted here) bind to their cognate receptors, leading to the formation of the death-inducing signaling complex (DISC), with caspase 8 activation, Bid cleavage, and subsequent mitochondrial permeabilization. Bim activation can also occur downstream of death receptor signaling, leading to Bax activation and mitochondrial permeabilization. The TNF-α signaling pathway also leads to Bid cleavage with lysosomal permeabilization, leading to release of lysosomal contents and mitochondrial permeabilization. The intrinsic pathway of cell death can be initiated by myriad intracellular stressors that can activate the ER stress pathway, lysosomal permeabilization, or JNK activation. These cascades lead to inhibition of the antiapoptotic proteins (Bcl-xL, Bcl-2) and activation of the proapoptotic proteins (Bax, Bim, Bad, Bid). Mitochondrial permeabilization occurs eventually and is required for hepatocyte apoptosis.

Figure 2

The ER stress pathway. The ER membrane contains 3 stress transducers: IRE1, ATF6, and PERK. Their activation is controlled by the ER molecular chaperone immunoglobulin-binding protein of B cells (BiP/GRP78). Release from BiP binding activates the ER stress transducers. IRE1 contains a cytoplasmic kinase domain and an endoribonuclease domain. The latter cleaves XBP1 to spliced XBP1 (sXBP1), leading to transcriptional activation of adaptive molecules to overcome ER stress, such as ER chaperones, lipid synthesis, and increasing ER-associated degradation (ERAD). PERK activation leads to phosphorylation of eukaryotic translation initiation factor 2α, leading to global translation attenuation. ATF6 also transcriptionally activates UPR target genes. These processes collectively function to overcome and correct ER stress. In the face of sustained ER stress, apoptotic machinery becomes activated. PERK leads to selective translation of ATF4, transcription of CHOP, and activation of the apoptotic machinery. CHOP can affect Bim levels and death receptor 5 levels. IRE1 can also activate JNK. Active Bax/Bak can permeabilize the ER membrane, leading to calcium release, activation of calpains, and apoptosis. Growth arrest and DNA damage protein 34 (GADD34) associates with protein phosphatase 1 (PP1), leading to dephosphorylation of eukaryotic translation initiation factor 2α and attenuation of the UPR.

Figure 3

Fas and TRAIL receptor signaling. Fas and TRAIL receptors activate conserved signaling pathways on receptor ligand interactions. FADD binds to the intracellular death domain (DD) containing trimerized receptor. FADD also contains a death effector domain (DED) that leads to activation of caspase 8 by cleavage and homodimerization of procaspase 8. Caspase 8 leads to cleavage of Bid to tBid and downstream mitochondrial permeabilization. Mitochondrial contents, including cytochrome c, SMAC/DIABLO, APAF-1, and endonuclease G, are released, leading to the activation of caspase 3/7. Caspase 3/7 leads to cleavage of cellular proteins and the characteristic apoptotic morphology.

Figure 4

TNF-α signaling. TNF-α binds to the extracellular domain of its cognate receptor, TNFR1. The intracellular portion of TNFR1 contains a death domain (DD). This interacts with the DD of TNFR1-associated death domain protein (TRADD) and recruits receptor interacting protein also via its DD, and TNF receptor–associated factor (TRAF2) via its kinase domain or an intermediate domain. This signal transduction complex is referred to as complex 1. Nuclear factor κB (NF-κB) activation and transient JNK activation occur downstream of complex 1. Active NF-κB translocates to the nucleus, leading to the transcription of antiapoptotic genes. Among its known target genes are cellular FLICE-like inhibitory protein (cFLIP), Bcl-xL, Mcl-1, A1, and XIAP, which regulate apoptosis at multiple levels. The early and transient activation of JNK also promotes survival. Sustained JNK activation requires receptor interacting protein; although the exact pathways are not fully known, oxidative stress is recognized as one mediator of sustained JNK activation. TRADD, receptor interacting protein, and TRAF2 then undergo receptor dissociation, recruit FADD via its DD. FADD also contains a death effector domain (DED) that leads to activation of caspase 8, cleavage of Bid to tBid, and downstream mitochondrial permeabilization.

Figure 5

The central role of hepatocyte apoptosis in liver injury. Vulnerable hepatocytes undergo apoptosis when stressed. Apoptosis can be initiated via Kupffer cell release of TNF-α, leading to activation of JNK. Activated NK and NK T cells release Fas or TRAIL, and interferon gamma, which up-regulates Fas or TRAIL release, leading to death receptor–mediated hepatocyte apoptosis. Hepatocyte apoptosis can also occur via activation of the intrinsic pathway (not shown here). Apoptotic hepatocytes are engulfed Kupffer cells, leading in turn to their activation. Activated Kupffer cells secrete TNF-α, interleukins, and interferon to promote the inflammatory response. They also secrete transforming growth factor β, leading to activation of stellate cells. Stellate cells can also be directly activated by apoptotic bodies. Activated stellate cells secrete collagen type I, leading to liver fibrosis. Attenuation of hepatocyte apoptosis, or forced apoptosis of activated stellate cells, such as with proteasomal inhibitors or TRAIL, can lead to resolution of fibrosis.