BH3-only proteins contribute to steatotic liver ischemia-reperfusion injury

Apoptotic cell death in disease-Current understanding of the NCCD 2023

Apoptosis is a form of regulated cell death (RCD) that involves proteases of the caspase family. Pharmacological and genetic strategies that experimentally inhibit or delay apoptosis in mammalian systems have elucidated the key contribution of this process not only to (post-)embryonic development and adult tissue homeostasis, but also to the etiology of multiple human disorders. Consistent with this notion, while defects in the molecular machinery for apoptotic cell death impair organismal development and promote oncogenesis, the unwarranted activation of apoptosis promotes cell loss and tissue damage in the context of various neurological, cardiovascular, renal, hepatic, infectious, neoplastic and inflammatory conditions. Here, the Nomenclature Committee on Cell Death (NCCD) gathered to critically summarize an abundant pre-clinical literature mechanistically linking the core apoptotic apparatus to organismal homeostasis in the context of disease.

N-Acetylcysteine Reduced Ischemia and Reperfusion Damage Associated with Steatohepatitis in Mice

N-acetylcysteine (NAC) is a pharmacological alternative with great potential for reducing the deleterious effects of surgical procedures on patients with steatohepatitis. We evaluated the effect of NAC on hepatic ischemia/reperfusion (I/R) injury in C57BL/6J mice, 8 weeks-old, weighing 25-30 g, with steatohepatitis induced by a methionine- and choline-deficient (MCD) diet. Groups: MCD group (steatohepatitis), MCD-I/R group (steatohepatitis plus 30 min of 70% liver ischemia and 24 h of reperfusion), MCD-I/R+NAC group (same as MCD-I/R group plus 150 mg/kg NAC 15 min before ischemia), and control group (normal AIN-93M diet). Liver enzymes and histopathology; nitrite and TBARS (thiobarbituric acid reactive substances) levels; pro-inflammatory cytokines; antioxidants enzymes; Nrf2 (nuclear factor erythroid-2-related factor 2) expression; and apoptosis were evaluated. In the group treated with NAC, reductions in inflammatory infiltration; AST (aspartate aminotransferase), nitrite, and TBARS levels; GPx (gutathione peroxidase) activity; cytokines synthesis; and number of apoptotic cells were observed while the GR (glutathione reductase) activity was increased. No differences were observed in Nfr2 expression or in SOD (superoxide dismutase), CAT (catalase), and GST (glutathione S-transferase) activities. Thus, it may be concluded that NAC exerts beneficial effects on mice livers with steatohepatitis submitted to I/R by reducing oxidative stress, inflammatory response, and cell death.

Influence of Fat on Differential Receptor Interacting Serine/Threonine Protein Kinase 1 Activity Leading to Apoptotic Cell Death in Murine Liver Ischemia Reperfusion Injury Through Caspase 8

Current understanding is that receptor interacting serine/threonine protein kinase 1 (RIPK1) can lead to two distinct forms of cell death: RIPK3‐mediated necroptosis or caspase 8 (Casp8)‐mediated apoptosis. Here, we report that RIPK1 signaling is indispensable for protection from hepatocellular injury in a steatotic liver undergoing ischemia reperfusion injury (IRI) but not in the lean liver. In lean liver IRI, RIPK1‐mediated cell death is operational, leading to protection in RIP1 kinase‐dead knock‐in (RIPK1^K45A) mice and necrostatin‐1s (Nec1s)‐treated lean wild‐type (WT) mice. However, when fed a high‐fat diet (HFD), RIPK1^K45A‐treated and Nec1s‐treated WT mice undergoing IRI demonstrate exacerbated hepatocellular injury along with decreased RIPK1 ubiquitylation. Furthermore, we demonstrate that HFD‐fed RIPK3^–/–/Casp8^–/– mice show protection from IRI, but HFD‐fed RIPK3^–/–/Casp8^–/+ mice do not. We also show that blockade of RIPK1 leads to increased Casp8 activity and decreases mitochondrial viability. Conclusion: Although more studies are required, we provide important proof of concept for RIPK1 inhibition leading to distinctive outcomes in lean and steatotic liver undergoing IRI. Considering the rising incidence of nonalcoholic fatty liver disease (NAFLD) in the general population, it will be imperative to address this critical difference when treating patients with RIPK1 inhibitors. This study also presents a new target for drug therapy to prevent hepatocellular injury in NAFLD.

Intermittent Ischemic Preconditioning Protects Against Hepatic Ischemia-Reperfusion Injury and Extensive Hepatectomy in Steatotic Rat Liver

BACKGROUND

Hepatic steatosis causes severe liver damage and has deleterious effects when associated with ischemia-reperfusion mechanisms. Ischemic preconditioning (IPC) protects lean liver against prolonged ischemia by improving micro-circulation and reducing lipid peroxidation. We investigated the effect of intermittent IPC on liver ischemia-reperfusion injury (IRI) and extensive hepatectomy in severe hepatic steatosis.

METHODS

Severe hepatic steatosis was performed by 12-14 weeks of choline-free diet in 108 Wistar rats. We induced 30-minute ischemia-reperfusion manipulations and extensive hepatectomy with or without prior IPC in steatotic livers and after 6 and 24 hours of reperfusion blood transaminases, and IL6, TNFα, NO and Lactate in blood and liver tissue were measured.

RESULTS

Steatotic rats subjected to hepatic ischemia-reperfusion alone after extensive hepatectomy, showed severe liver damage with significantly increased values of AST, ALT, TNFα and Lactate and significantly reduced IL6 and NO, while no one rat survived for more than 29 hours. On the contrary, steatotic rats subjected to intermittent IPC, 24 hours before ischemia-reperfusion, presented increased 30-day survival (67%), lower values of AST, ALT, TNFα and Lactate, and increased IL6 and NO levels. Simple and intermittent IPC manipulations, 1 hour before the IRI and extended hepatectomy, did not prolong survival more than 57 and 98 hours, respectively. Simple IPC, 24 hours before IRI and extended hepatectomy had the lowest possible survival (16.7%).

CONCLUSIONS

Hepatic steatosis and IRI after major liver surgery largely affect morbidity and mortality. Intermittent IPC, 24 hours before IRI and extensive hepatectomy, presents higher 30-day survival and improved liver function parameters.

Mitochondrial Damage-Associated Molecular Patterns (MTDs) Are Released during Hepatic Ischemia Reperfusion and Induce Inflammatory Responses

Ischemia / reperfusion injury (IRI) during the course of liver transplantation enhances the immunogenicity of allografts and thus impacts overall graft outcome. This sterile inflammatory insult is known to activate innate immunity and propagate organ damage through the recognition of damage-associate molecular pattern (DAMP) molecules. The purpose of the present study was to investigate the role of mitochondrial DAMPs (MTDs) in the pathogenesis of hepatic IRI. Using in vitro models we observed that levels of MTDs were significantly higher in both transplantation-associated and warm IR, and that co-culture of MTDs with human and rat hepatocytes significantly increased cell death. MTDs were also released in an in vivo rat model of hepatic IRI and associated with increased secretion of inflammatory cytokines (TNF-α, IL-6, and IL-10) and increased liver injury compared to the sham group. Our results suggest that hepatic IR results in a significant increase of MTDs both in vitro and in vivo suggesting that MTDs may serve as a novel marker in hepatic IRI. Co-culture of MTDs with hepatocytes showed a decrease in cell viability in a concentration dependent manner, which indicates that MTDs is a toxic mediator participating in the pathogenesis of liver IR injury.

Inflammasome activation and function in liver disease

Inflammation contributes to the pathogenesis of most acute and chronic liver diseases. Inflammasomes are multiprotein complexes that can sense danger signals from damaged cells and pathogens and assemble to mediate caspase-1 activation, which proteolytically activates the cytokines IL-1β and IL-18. In contrast to other inflammatory responses, inflammasome activation uniquely requires two signals to induce inflammation, therefore setting an increased threshold. IL-1β, generated upon caspase-1 activation, provides positive feed-forward stimulation for inflammatory cytokines, thereby amplifying inflammation. Inflammasome activation has been studied in different human and experimental liver diseases and has been identified as a major contributor to hepatocyte damage, immune cell activation and amplification of liver inflammation. In this Review, we discuss the different types of inflammasomes, their activation and biological functions in the context of liver injury and disease progression. Specifically, we focus on the triggers of inflammasome activation in alcoholic steatohepatitis and NASH, chronic HCV infection, ischaemia-reperfusion injury and paracetamol-induced liver injury. The application and translation of these discoveries into therapies promises novel approaches in the treatment of inflammation in liver disease.