Hepatocyte programmed cell death: the trigger for inflammation and fibrosis in metabolic dysfunction-associated steatohepatitis

Abstract

By replacing and removing defective or infected cells, programmed cell death (PCD) contributes to homeostasis maintenance and body development, which is ubiquitously present in mammals and can occur at any time. Besides apoptosis, more novel modalities of PCD have been described recently, such as necroptosis, pyroptosis, ferroptosis, and autophagy-dependent cell death. PCD not only regulates multiple physiological processes, but also participates in the pathogenesis of diverse disorders, including metabolic dysfunction-associated steatotic liver disease (MASLD). MASLD is mainly classified into metabolic dysfunction-associated steatotic liver (MASL) and metabolic dysfunction-associated steatohepatitis (MASH), and the latter putatively progresses to cirrhosis and hepatocellular carcinoma. Owing to increased incidence and obscure etiology of MASH, its management still remains a tremendous challenge. Recently, hepatocyte PCD has been attracted much attention as a potent driver of the pathological progression from MASL to MASH, and some pharmacological agents have been proved to exert their salutary effects on MASH partly via the regulation of the activity of hepatocyte PCD. The current review recapitulates the pathogenesis of different modalities of PCD, clarifies the mechanisms underlying how metabolic disorders in MASLD induce hepatocyte PCD and how hepatocyte PCD contributes to inflammatory and fibrotic progression of MASH, discusses several signaling pathways in hepatocytes governing the execution of PCD, and summarizes some potential pharmacological agents for MASH treatment which exert their therapeutic effects partly via the regulation of hepatocyte PCD. These findings indicate that hepatocyte PCD putatively represents a new therapeutic point of intervention for MASH.

Keywords: hepatic fibrosis; hepatic inflammation; metabolic disorders; metabolic dysfunction-associated steatohepatitis; programmed cell death.

FIGURE 1

The pathophysiology of apoptosis and necroptosis. The apoptotic pathway can be executed by death receptor pathway, mitochondrial pathway, and endoplasmic reticulum pathway. The death receptors located on cellular membrane can be recognized and activated by death ligands, such as FasL and TNFα, which then is able to recruit associated adaptor proteins and assemble the death-inducing signaling complex. This complex leads to the activation of caspase-8, which can activate caspase-3/7 and caspase-6 sequentially, ultimately causing apoptosis. Intrinsic stimulations like oxidative stress cleaves BID to tBid, which can translocate to the mitochondria where it induces MOMP formation and CytC and AIF release. The released CytC activates caspase-9 and caspase-3/7 sequentially, ultimately leading to apoptosis. Moreover, in hepatocytes, caspase-8 and caspase-6 also participate in the cleavage of tBid and the release of CytC, thus strikingly amplifying the death signal initiated by death receptors. ERS also plays a critical role in apoptosis execution. On the one hand, ERS initiates the mitochondrial apoptosis reaction chain by downregulating Bcl-2 via IRE1α-JNK and PERK-eIFα-CHOP axis. On the other hand, ERS activates caspase-12, caspase-9, and caspase-3 sequentially, culminating in apoptosis. When capase-8 is inhibited, the death-inducing signaling complex mentioned above activates RIPK1, which then recruits RIPK3 and phosphorylates MLKL. The phosphorylated MLKL is then translocated to cellular membrane where it undergoes oligomerization and triggers necroptosis. Abbreviations: TNF, tumor necrosis factor; MOMP, mitochondrial outer membrane pore formation; BID, BH3-interacting domain death agonist; CytC, Cytochrome c; ERS, endoplasmic reticulum stress; RIPK1, receptor-interacting protein kinase-1; MLKL, pseudokinase mixed lineage domain-like.

FIGURE 2

The pathophysiology of pyroptosis. NLRs and AIM2 can recognize endogenous DAMPs and exogenous PAMPs, and then recruits pro-capase-1 by interacting with ASC, thus forming an inflammasome, which cleaves pro-caspase-1 to caspase-1. Caspase-1 subsequently leads to the cleavage of GSDMD and releases its N-terminal fragment, namely GSDMD-N, which ultimately results in pyroptosis. Pro-caspase-4/5/11 has the capacity to directly bind to liposome A at the tail of LPS, which promotes the self-oligomerization and self-activation of caspase-4/5/11. The activated caspase-4/5/11 is able to cleave GSDMD to GSDMD-N and induce pyroptosis. Moreover, Pannexin-1, the channel protein on cellular membrane, also can be cleaved by capsase-11, which then mediates the efflux of ATP. The repeated stimulation of ATP is a potent driver for P2X7 channel opening, which triggers the efflux of K⁺, Ca⁺, and Na⁺, thus causing cell swelling and lytic cell death. In addition, K⁺efflux also serves as a contributor to activate NLRP3/ASC/caspase-1 signaling axis, further promoting the cleavage of GSDMD. Besides GSDMD, GSDME also play roles in the pathogenesis of pyroptosis. GSDME is cleaved to GSDME-N by granzyme B and caspase-3, which similarly induces the pore formation in cellular membrane and ultimate pyroptosis. Abbreviations: DAMPs, damaged associated molecular patterns; PAMPs, pathogen-associated molecular patterns; NLRs, Nod-like receptors; AIM2, absent in melanoma-2; GSDMD, gasdermin D.

FIGURE 3

An overview of autophagy. Autophagy is classified into three modalities, namely macro-autophagy, micro-autophagy, and chaperone-mediated autophagy. Macro-autophagy requires autophagosome formation, which can selectively engulf and sequester part of the cellular contents, such as lipid droplets, protein aggregates, and organelles. Then, the autophagosome is fused with the lysosome, where these cellular contents are degraded. Micro-autophagy refers to the direct engulfment and subsequent degradation of cellular constituents by the lysosome. Chaperone-mediated autophagy is specialized in degrading the proteins containing the KFERQ-like motif, which combine with HSPA8, and then bind to LAMP2A receptor on the lysosome and be further translocated to the lysosome where these proteins are degraded.

FIGURE 4

The roles of programmed cell death in the proinflammatory and fibrotic progression in MASH. Hepatic apoptotic bodies derived from apoptotic process stimulate the production of death receptor ligands, such as TNF-α and FasL. TNF-α has the capacity to activate NF-kB and JNK pathways, triggering a series of inflammatory responses. FasL stimulates the secretion of chemokines in macrophages through MyD88 signaling axis. Hepatocyte apoptosis also correlates with the production of TGF-β1 and the release of purinergic ligands, including UDP-glucose and UDP-fructose. Among which, TGF-β1 contributes to liver fibrosis through facilitating extracellular matrix deposition via TGF-β1/Smad axis in HSCs, while UDP-glucose/UDP-fructose recognizes purinergic receptor P2Y14 distributed in HSCs, resulting in the activation of HSCs and liver fibrosis. Low expression of RIPK3 level in hepatocytes and NF-kB activation in MASH may lead to sublethal necroptosis, which expedites the progression of the disease. Pyroptosis induces the release of massive inflammatory cytokines, such as IL-18 and IL-1β, directly promoting inflammatory responses. Moreover, the released NLRP3 inflammasome particles in pyroptosis can be engulfed by HSCs, which results in HSCs activation and α-SMA upregulation, contributing to liver fibrosis. Autophagy deficiency in MASH is unfavorable for the elimination of ERS and dysregulated or impaired mitochondria, thus potentiating ROS generation, which promotes liver inflammation. In addition, hepatocyte autophagy deficiency also contributes to liver fibrosis. Abbreviations: TNF, tumor necrosis factor; MASH, metabolic dysfunction-associated steatohepatitis; HSCs, hepatic stellate cells; NF-kB, nuclear factor-kB; TGF-β1, transforming growth factor-β1; ERS, Endoplasmic reticulum stress; ROS, reactive oxidative stress; RIPK3, receptor-interacting protein kinase 3.