Loss of caspase-8 protects mice against inflammation-related hepatocarcinogenesis but induces non-apoptotic liver injury

Mediators of necroptosis: from cell death to metabolic regulation

Necroptosis, a programmed cell death mechanism distinct from apoptosis, has garnered attention for its role in various pathological conditions. While initially recognized for its involvement in cell death, recent research has revealed that key necroptotic mediators, including receptor-interacting protein kinases (RIPKs) and mixed lineage kinase domain-like protein (MLKL), possess additional functions that go beyond inducing cell demise. These functions encompass influencing critical aspects of metabolic regulation, such as energy metabolism, glucose homeostasis, and lipid metabolism. Dysregulated necroptosis has been implicated in metabolic diseases, including obesity, diabetes, metabolic dysfunction-associated steatotic liver disease (MASLD) and alcohol-associated liver disease (ALD), contributing to chronic inflammation and tissue damage. This review provides insight into the multifaceted role of necroptosis, encompassing both cell death and these extra-necroptotic functions, in the context of metabolic diseases. Understanding this intricate interplay is crucial for developing targeted therapeutic strategies in diseases that currently lack effective treatments.

Liver receptor homolog-1 (NR5A2) orchestrates hepatic inflammation and TNF-induced cell death

The nuclear receptor liver receptor homolog-1 (LRH-1) has been shown to promote apoptosis resistance in various tissues and disease contexts; however, its role in liver cell death remains unexplored. Hepatocyte-specific deletion of LRH-1 causes mild steatosis and inflammation but unexpectedly shields female mice from tumor necrosis factor (TNF)-induced hepatocyte apoptosis and associated hepatitis. LRH-1-deficient hepatocytes show markedly attenuated estrogen receptor alpha and elevated nuclear factor κB (NF-κB) activity, while LRH-1 overexpression inhibits NF-κB activity. This inhibition relies on direct physical interaction of LRH-1's ligand-binding domain and the Rel homology domain of NF-κB subunit RelA. Mechanistically, increased transcription of anti-apoptotic NF-κB target genes and the proteasomal degradation of pro-apoptotic BCL-2 interacting mediator of cell death prevent mitochondrial apoptosis and ultimately protect mice from TNF-induced liver damage. Collectively, our study emphasizes LRH-1 as a critical, sex-dependent regulator of cell death and inflammation in the healthy and diseased liver.

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.

TNF in the liver: targeting a central player in inflammation

Tumour necrosis factor-α (TNF) is a multifunctional cytokine. First recognized as an endogenous soluble factor that induces necrosis of solid tumours, TNF became increasingly important as pro-inflammatory cytokine being involved in the immunopathogenesis of several autoimmune diseases. In the liver, TNF induces numerous biological responses such as hepatocyte apoptosis and necroptosis, liver inflammation and regeneration, and autoimmunity, but also progression to hepatocellular carcinoma. Considering these multiple functions of TNF in the liver, we propose anti-TNF therapies that specifically target TNF signalling at the level of its specific receptors.

Comparison of Necroptosis With Apoptosis for OVX-Induced Osteoporosis

As one common kind of osteoporosis, postmenopausal osteoporosis (PMOP) is associated with the death and excessive loss of osteocytes. Estrogen deficiency of PMOP can cause osteocyte death by regulating necroptosis and apoptosis, but their roles in POMP have not been compared. In the present study, ovariectomy (OVX)-induced rat and murine long bone osteocyte Y4 (MLO-Y4) cells were used to compare the influence of necroptosis and apoptosis on osteocyte death and bone loss. Benzyloxycarbonyl-Val-Ala-Asp (zVAD) and necrostatin-1 (Nec-1) were used to specifically block cell apoptosis and necroptosis, respectively. OVX rats and MLO-Y4 cells were divided into zVAD group, Nec-1 group, zVAD + Nec-1 group, vehicle, and control group. The tibial plateaus of the rat model were harvested at 8 weeks after OVX and were analyzed by micro–computed tomography, transmission electron microscopy (TEM), the transferase dUTP nick end labeling assay, and western blot. The death of MLO-Y4 was stimulated by TNF-α and was measured by flow cytometry and TEM. The results found that necroptosis and apoptosis were both responsible for the death and excessive loss of osteocytes, as well as bone loss in OVX-induced osteoporosis, and furthermore necroptosis may generate greater impact on the death of osteocytes than apoptosis. Necroptotic death of osteocytes was mainly regulated by the receptor-interacting protein kinase 3 signaling pathway. Collectively, inhibition of necroptosis may produce better efficacy in reducing osteocyte loss than that of apoptosis, and combined blockade of necroptosis and apoptosis provide new insights into preventing and treating PMOP.

Cell Death in Hepatocellular Carcinoma: Pathogenesis and Therapeutic Opportunities

Simple Summary

The progression of liver tumors is highly influenced by the interactions between cancer cells and the surrounding environment, and, consequently, can determine whether the primary tumor regresses, metastasizes, or establishes micrometastases. In the context of liver cancer, cell death is a double-edged sword. On one hand, cell death promotes inflammation, fibrosis, and angiogenesis, which are tightly orchestrated by a variety of resident and infiltrating host cells. On the other hand, targeting cell death in advanced hepatocellular carcinoma could represent an attractive therapeutic approach for limiting tumor growth. Further studies are needed to investigate therapeutic strategies combining current chemotherapies with novel drugs targeting either cell death or the tumor microenvironment.

Abstract

Hepatocellular carcinoma (HCC) is the most prevalent primary liver cancer and the third leading cause of cancer death worldwide. Closely associated with liver inflammation and fibrosis, hepatocyte cell death is a common trigger for acute and chronic liver disease arising from different etiologies, including viral hepatitis, alcohol abuse, and fatty liver. In this review, we discuss the contribution of different types of cell death, including apoptosis, necroptosis, pyroptosis, or autophagy, to the progression of liver disease and the development of HCC. Interestingly, inflammasomes have recently emerged as pivotal innate sensors with a highly pathogenic role in various liver diseases. In this regard, an increased inflammatory response would act as a key element promoting a pro-oncogenic microenvironment that may result not only in tumor growth, but also in the formation of a premetastatic niche. Importantly, nonparenchymal hepatic cells, such as liver sinusoidal endothelial cells, hepatic stellate cells, and hepatic macrophages, play an important role in establishing the tumor microenvironment, stimulating tumorigenesis by paracrine communication through cytokines and/or angiocrine factors. Finally, we update the potential therapeutic options to inhibit tumorigenesis, and we propose different mechanisms to consider in the tumor microenvironment field for HCC resolution.

Hepatocyte and immune cell crosstalk in non-alcoholic fatty liver disease

Introduction: Nonalcoholic fatty liver disease (NAFLD) is the most widespread chronic liver disease in the world. It can evolve into nonalcoholic steatohepatitis (NASH) where inflammation and hepatocyte ballooning are key participants in the determination of this steatotic state.Areas covered: To provide a systematic overview and current understanding of the role of inflammation in NAFLD and its progression to NASH, the function of the cells involved, and the activation pathways of the innate immunity and cell death; resulting in inflammation and chronic liver disease. A PubMed search was made with relevant articles together with relevant references were included for the writing of this review.Expert opinion: Innate and adaptive immunity are the key players in the NAFLD progression; some of the markers presented during NAFLD are also known to be immunity biomarkers. All cells involved in NAFLD and NASH are known to have immunoregulatory properties and their imbalance will completely change the cytokine profile and form a pro-inflammatory microenvironment. It is necessary to fully answer the question of what initiators and metabolic imbalances are particularly important, considering sterile inflammation as the architect of the disease. Due to the shortage of elucidation of NASH progression, we discuss in this review, how inflammation is a key part of this development and we presume the targets should lead to inflammation and oxidative stress treatment.

Bile Acid Toxicity and Protein Kinases

If the bile acids reach to pathological concentrations due to cholestasis, accumulation of hydrophobic bile acids within the hepatocyte may result in cell death. Thus, hydrophobic bile acids induce apoptosis in hepatocytes, while hydrophilic bile acids increase intracellular adenosine 3',5'-monophosphate (cAMP) levels and activate mitogen-activated protein kinase (MAPK) and phosphoinositide 3-kinase (PI3K) pathways to protect hepatocytes from apoptosis.Two apoptotic pathways have been described in bile acids-induced death. Both are controlled by multiple protein kinase signaling pathways. In mitochondria-controlled pathway, caspase-8 is activated with death domain-independent manner, whereas, Fas-dependent classical pathway involves ligand-independent oligomerization of Fas.Hydrophobic bile acids dose-dependently upregulate the inflammatory response by further stimulating production of inflammatory cytokines. Death receptor-mediated apoptosis is regulated at the cell surface by the receptor expression, at the death-inducing signaling complex (DISC) by expression of procaspase-8, the death receptors Fas-associated death domain (FADD), and cellular FADD-like interleukin 1-beta (IL-1β)-converting enzyme (FLICE) inhibitory protein (cFLIP). Bile acids prevent cFLIP recruitment to the DISC and thereby enhance initiator caspase activation and lead to cholestatic apoptosis. At mitochondria, the expression of B-cell leukemia/lymphoma-2 (Bcl-2) family proteins contribute to apoptosis by regulating mitochondrial cytochrome c release via Bcl-2, Bcl-2 homology 3 (BH3) interacting domain death agonist (Bid), or Bcl-2 associated protein x (Bax). Fas receptor CD95 activation by hydrophobic bile acids is initiated by reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase-dependent reactive oxygen species (ROS) signaling. However, activation of necroptosis by ligands of death receptors requires the kinase activity of receptor interacting protein1 (RIP1), which mediates the activation of RIP3 and mixed lineage kinase domain-like protein (MLKL). In this chapter, mainly the effect of protein kinases signal transduction on the mechanisms of hydrophobic bile acids-induced inflammation, apoptosis, necroptosis and necrosis are discussed.

Necroptosis in Hepatosteatotic Ischaemia-Reperfusion Injury

While liver transplantation remains the sole treatment option for patients with end-stage liver disease, there are numerous limitations to liver transplantation including the scarcity of donor livers and a rise in livers that are unsuitable to transplant such as those with excess steatosis. Fatty livers are susceptible to ischaemia-reperfusion (IR) injury during transplantation and IR injury results in primary graft non-function, graft failure and mortality. Recent studies have described new cell death pathways which differ from the traditional apoptotic pathway. Necroptosis, a regulated form of cell death, has been associated with hepatic IR injury. Receptor-interacting protein kinase 3 (RIPK3) and mixed-lineage kinase domain-like pseudokinase (MLKL) are thought to be instrumental in the execution of necroptosis. The study of hepatic necroptosis and potential therapeutic approaches to attenuate IR injury will be a key factor in improving our knowledge regarding liver transplantation with fatty donor livers. In this review, we focus on the effect of hepatic steatosis during liver transplantation as well as molecular mechanisms of necroptosis and its involvement during liver IR injury. We also discuss the immune responses triggered during necroptosis and examine the utility of necroptosis inhibitors as potential therapeutic approaches to alleviate IR injury.

Multiple roles of caspase-8 in cell death, inflammation, and innate immunity

Caspase-8 is an apical caspase involved in the programmed form of cell death called apoptosis that is critically important for mammalian development and immunity. Apoptosis was historically described as immunologically silent in contrast to other types of programmed cell death such as necroptosis or pyroptosis. Recent reports suggest considerable crosstalk between these different forms of cell death. It is becoming increasingly clear that caspase-8 has many non-apoptotic roles, participating in multiple processes including regulation of necroptosis (mediated by receptor-interacting serine/threonine kinases, RIPK1-RIPK3), inflammatory cytokine expression, inflammasome activation, and cleavage of IL-1β and gasdermin D, and protection against shock and microbial infection. In this review, we discuss the involvement of caspase-8 in cell death and inflammation and highlight its role in innate immune responses and in the relationship between different forms of cell death. Caspase-8 is one of the central components in this type of crosstalk.

Lipid-encapsulated siRNA for hepatocyte-directed treatment of advanced liver disease

Lipid-based RNA nanocarriers have been recently accepted as a novel therapeutic option in humans, thus increasing the therapeutic options for patients. Tailored nanomedicines will enable to treat chronic liver disease (CLD) and end-stage liver cancer, disorders with high mortality and few treatment options. Here, we investigated the curative potential of gene therapy of a key molecule in CLD, the c-Jun N-terminal kinase-2 (Jnk2). Delivery to hepatocytes was achieved using a lipid-based clinically employable siRNA formulation that includes a cationic aminolipid to knockdown Jnk2 (named siJnk2). After assessing the therapeutic potential of siJnk2 treatment, non-invasive imaging demonstrated reduced apoptotic cell death and improved hepatocarcinogenesis was evidenced by improved liver parenchyma as well as ameliorated markers of hepatic damage, reduced fibrogenesis in 1-year-old mice. Strikingly, chronic siJnk2 treatment reduced premalignant nodules, indicative of tumor initiation. Furthermore, siJnk2 treatment led to a significant activation of the immune cell compartment. In conclusion, Jnk2 knockdown in hepatocytes ameliorated hepatitis, fibrogenesis, and initiation of hepatocellular carcinoma (HCC), and hence might be a suitable therapeutic option, to define novel molecular targets for precision medicine in CLD.

Intestinal dysbiosis augments liver disease progression via NLRP3 in a murine model of primary sclerosing cholangitis

OBJECTIVE

There is a striking association between human cholestatic liver disease (CLD) and inflammatory bowel disease. However, the functional implications for intestinal microbiota and inflammasome-mediated innate immune response in CLD remain elusive. Here we investigated the functional role of gut-liver crosstalk for CLD in the murine Mdr2 knockout (Mdr2-/-) model resembling human primary sclerosing cholangitis (PSC).

DESIGN

Male Mdr2-/-, Mdr2-/- crossed with hepatocyte-specific deletion of caspase-8 (Mdr2-/- /Casp8∆hepa) and wild-type (WT) control mice were housed for 8 or 52 weeks, respectively, to characterise the impact of Mdr2 deletion on liver and gut including bile acid and microbiota profiling. To block caspase activation, a pan-caspase inhibitor (IDN-7314) was administered. Finally, the functional role of Mdr2-/- -associated intestinal dysbiosis was studied by microbiota transfer experiments.

RESULTS

Mdr2-/- mice displayed an unfavourable intestinal microbiota signature and pronounced NLRP3 inflammasome activation within the gut-liver axis. Intestinal dysbiosis in Mdr2-/- mice prompted intestinal barrier dysfunction and increased bacterial translocation amplifying the hepatic NLRP3-mediated innate immune response. Transfer of Mdr2-/- microbiota into healthy WT control mice induced significant liver injury in recipient mice, highlighting the causal role of intestinal dysbiosis for disease progression. Strikingly, IDN-7314 dampened inflammasome activation, ameliorated liver injury, reversed serum bile acid profile and cholestasis-associated microbiota signature.

CONCLUSIONS

MDR2-associated cholestasis triggers intestinal dysbiosis. In turn, translocation of endotoxin into the portal vein and subsequent NLRP3 inflammasome activation contribute to higher liver injury. This process does not essentially depend on caspase-8 in hepatocytes, but can be blocked by IDN-7314.

Context-Dependent Role of NF-κB Signaling in Primary Liver Cancer-from Tumor Development to Therapeutic Implications

Chronic inflammatory cell death is a major risk factor for the development of diverse cancers including liver cancer. Herein, disruption of the hepatic microenvironment as well as the immune cell composition are major determinants of malignant transformation and progression in hepatocellular carcinomas (HCC). Considerable research efforts have focused on the identification of predisposing factors that promote induction of an oncogenic field effect within the inflammatory liver microenvironment. Among the most prominent factors involved in this so-called inflammation-fibrosis-cancer axis is the NF-κB pathway. The dominant role of this pathway for malignant transformation and progression in HCC is well documented. Pathway activation is significantly linked to poor prognostic traits as well as stemness characteristics, which places modulation of NF-κB signaling in the focus of therapeutic interventions. However, it is well recognized that the mechanistic importance of the pathway for HCC is highly context and cell type dependent. While constitutive pathway activation in an inflammatory etiological background can significantly promote HCC development and progression, absence of NF-κB signaling in differentiated liver cells also significantly enhances liver cancer development. Thus, therapeutic targeting of NF-κB as well as associated family members may not only exert beneficial effects but also negatively impact viability of healthy hepatocytes and/or cholangiocytes, respectively. The review presented here aims to decipher the complexity and paradoxical functions of NF-κB signaling in primary liver and non-parenchymal cells, as well as the induced molecular alterations that drive HCC development and progression with a particular focus on (immune-) therapeutic interventions.

Necroptotic Cell Death in Liver Transplantation and Underlying Diseases: Mechanisms and Clinical Perspective

Cell death is a natural process for the turnover of aged cells, but it can also arise as a result of pathological conditions. Cell death is recognized as a key feature in both acute and chronic hepatobiliary diseases caused by drug, alcohol, and fat uptake; by viral infection; or after surgical intervention. In the case of chronic disease, cell death can lead to (chronic) secondary inflammation, cirrhosis, and the progression to liver cancer. In liver transplantation, graft preservation and ischemia/reperfusion injury are associated with acute cell death. In both cases, so-called programmed cell death modalities are involved. Several distinct types of programmed cell death have been described of which apoptosis and necroptosis are the most well known. Parenchymal liver cells, including hepatocytes and cholangiocytes, are susceptible to both apoptosis and necroptosis, which are triggered by distinct signal transduction pathways. Apoptosis is dependent on a proteolytic cascade of caspase enzymes, whereas necroptosis induction is caspase-independent. Moreover, different from the "silent" apoptotic cell death, necroptosis can cause a secondary inflammatory cascade, so-called necroinflammation, triggered by the release of various damage-associated molecular patterns (DAMPs). These DAMPs activate the innate immune system, leading to both local and systemic inflammatory responses, which can even cause remote organ failure. Therapeutic targeting of necroptosis by pharmacological inhibitors, such as necrostatin-1, shows variable effects in different disease models.

RIPK1 and death receptor signaling drive biliary damage and early liver tumorigenesis in mice with chronic hepatobiliary injury

Hepatocyte apoptosis is intrinsically linked to chronic liver disease and hepatocarcinogenesis. Conversely, necroptosis of hepatocytes and other liver cell types and its relevance for liver disease is debated. Using liver parenchymal cell (LPC)-specific TGF-beta-activated kinase 1 (TAK1)-deficient (TAK1^LPC-KO) mice, which exhibit spontaneous hepatocellular and biliary damage, hepatitis, and early hepatocarcinogenesis, we have investigated the contribution of apoptosis and necroptosis in hepatocyte and cholangiocyte death and their impact on liver disease progression. Here, we provide in vivo evidence showing that TAK1-deficient cholangiocytes undergo spontaneous necroptosis induced primarily by TNFR1 and dependent on RIPK1 kinase activity, RIPK3, and NEMO. In contrast, TAK1-deficient hepatocytes die by FADD-dependent apoptosis, which is not significantly inhibited by LPC-specific RIPK1 deficiency, inhibition of RIPK1 kinase activity, RIPK3 deficiency or combined LPC-specific deletion of TNFR1, TRAILR, and Fas. Accordingly, normal mouse cholangiocytes can undergo necroptosis, while primary hepatocytes are resistant to it and die exclusively by apoptosis upon treatment with cell death-inducing stimuli in vitro, likely due to the differential expression of RIPK3. Interestingly, the genetic modifications that conferred protection from biliary damage also prevented the spontaneous lethality that was often observed in TAK1^LPC-KO mice. In the presence of chronic hepatocyte apoptosis, preventing biliary damage delayed but did not avert hepatocarcinogenesis. On the contrary, inhibition of hepatocyte apoptosis fully prevented liver tumorigenesis even in mice with extensive biliary damage. Altogether, our results suggest that using RIPK1 kinase activity inhibitors could be therapeutically useful for cholestatic liver disease patients.

Inflammation and Liver Cancer: Molecular Mechanisms and Therapeutic Targets

Hepatocellular carcinoma (HCC) is associated with chronic inflammation and fibrosis arising from different etiologies, including hepatitis B and C and alcoholic and nonalcoholic fatty liver diseases. The inflammatory cytokines tumor necrosis factor-α and interleukin-6 and their downstream targets nuclear factor kappa B (NF-κB), c-Jun N-terminal kinase (JNK), and signal transducer and activator of transcription 3 drive inflammation-associated HCC. Further, while adaptive immunity promotes immune surveillance to eradicate early HCC, adaptive immune cells, such as CD8+ T cells, Th17 cells, and B cells, can also stimulate HCC development. Thus, the role of the hepatic immune system in HCC development is a highly complex topic. This review highlights the role of cytokine signals, NF-κB, JNK, innate and adaptive immunity, and hepatic stellate cells in HCC and discusses whether these pathways could be therapeutic targets. The authors will also discuss cholangiocarcinoma and liver metastasis because biliary inflammation and tumor-associated stroma are essential for cholangiocarcinoma development and because primary tumor-derived inflammatory mediators promote the formation of a "premetastasis niche" in the liver.

Inactivation of caspase 8 in liver parenchymal cells confers protection against murine obstructive cholestasis

BACKGROUND & AIMS

Caspase 8 (CASP8) is the apical initiator caspase in death receptor-mediated apoptosis. Strong evidence for a link between death receptor signaling pathways and cholestasis has recently emerged. Herein, we investigated the role of CASP8-dependent and independent pathways during experimental cholestasis.

METHODS

Liver injury was characterized in a cohort of human sera (n = 28) and biopsies from patients with stage IV primary biliary cholangitis. In parallel, mice with either specific deletion of Casp8 in liver parenchymal cells (Casp8^Δhepa) or hepatocytes (Casp8^Δhep), and mice with constitutive Ripk3 (Ripk3^-/-) deletion, were subjected to surgical ligation of the common bile duct (BDL) from 2 to 28 days. Floxed (Casp8^fl/fl) and Ripk3^+/+ mice were used as controls. Moreover, the pan-caspase inhibitor IDN-7314 was used, and cell death mechanisms were studied in primary isolated hepatocytes.

RESULTS

Overexpression of activated caspase 3, CASP8 and RIPK3 was characteristic of liver explants from patients with primary biliary cholangitis. Twenty-eight days after BDL, Casp8^Δhepamice showed decreased necrotic foci, serum aminotransferase levels and apoptosis along with diminished compensatory proliferation and ductular reaction. These results correlated with a decreased inflammatory profile and ameliorated liver fibrogenesis. A similar phenotype was observed in Ripk3^-/- mice. IDN-7314 treatment decreased CASP8 levels but failed to prevent BDL-induced cholestasis, independently of CASP8 in hepatocytes.

CONCLUSION

These findings show that intervention against CASP8 in liver parenchymal cells - specifically in cholangiocytes - might be a beneficial option for treating obstructive cholestasis, while broad pan-caspase inhibition might trigger undesirable side effects.

LAY SUMMARY

Loss of caspase 8 - a protein involved in programmed cell death - in liver parenchymal cells protects against experimental cholestasis. Therefore, specific pharmacological intervention against caspase 8 might be a valid alternative for the treatment of obstructive cholestasis in the clinic, whereas broad pan-caspase inhibiting drugs might trigger undesirable side effects.

Necroptosis Signaling Pathways in Stroke: From Mechanisms to Therapies

It has been confirmed that apoptosis, autophagy and necrosis are the three major modes of cell death. For a long time, necrosis is regarded as a deranged or accidental cell demise. In recent years, there is evidence showing that necrotic cell death can be a well regulated and orchestrated event, which is also known as programmed cell death or “necroptosis”. Necroptosis can be triggered by a variety of external stimuli and regulated by a caspase-independent pathway. It plays a key role in the pathogenesis of some diseases including neurological diseases. In the past two decades, a variety of studies have revealed that the necroptosis related pathway is activated in stroke, and plays a crucial role in the pathogenesis of stroke. Moreover, necroptosis may serve as a potential target in the therapy of stroke because genetic or pharmacological inhibition of necroptosis has been shown to be neuroprotective in stroke in vitro and in vivo. In this review, we briefly summarize re-cent advances in necroptosis, introduce the mechanism and strategies targeting necroptosis in stroke, and finally propose some issues in the treatment of stroke by targeting necroptosis

RIP Kinases in Liver Cell Death, Inflammation and Cancer

Cell death is intrinsically linked to inflammatory liver disease and cancer development. Recent genetic studies have suggested that receptor-interacting protein kinase (RIPK)1 is implicated in liver disease pathogenesis by regulating caspase-dependent hepatocyte apoptosis induced by tumor necrosis factor (TNF) or other stimuli. In contrast, the contribution of caspase-independent RIPK3/mixed lineage kinase like (MLKL)-mediated hepatocyte necroptosis remains debatable. Hepatocyte apoptosis depends on the balance between RIPK1 prosurvival scaffolding functions and its kinase-activity-mediated proapoptotic function. Several regulatory steps promote the prosurvival role of RIPK1, including phosphorylation and ubiquitination of RIPK1 itself and other molecules involved in RIPK1 signaling. Pharmacological inhibition of liver damage by targeting RIPK1 signaling emerges as a potential therapeutic strategy to prevent chronic liver inflammation and hepatocarcinogenesis.

Bioinformatics functional analysis of let-7a, miR-34a, and miR-199a/b reveals novel insights into immune system pathways and cancer hallmarks for hepatocellular carcinoma

Let-7a, miR-34a, and miR-199 a/b have gained a great attention as master regulators for cellular processes. In particular, these three micro-RNAs act as potential onco-suppressors for hepatocellular carcinoma. Bioinformatics can reveal the functionality of these micro-RNAs through target prediction and functional annotation analysis. In the current study, in silico analysis using innovative servers (miRror Suite, DAVID, miRGator V3.0, GeneTrail) has demonstrated the combinatorial and the individual target genes of these micro-RNAs and further explored their roles in hepatocellular carcinoma progression. There were 87 common target messenger RNAs (p ≤ 0.05) that were predicted to be regulated by the three micro-RNAs using miRror 2.0 target prediction tool. In addition, the functional enrichment analysis of these targets that was performed by DAVID functional annotation and REACTOME tools revealed two major immune-related pathways, eight hepatocellular carcinoma hallmarks-linked pathways, and two pathways that mediate interconnected processes between immune system and hepatocellular carcinoma hallmarks. Moreover, protein-protein interaction network for the predicted common targets was obtained by using STRING database. The individual analysis of target genes and pathways for the three micro-RNAs of interest using miRGator V3.0 and GeneTrail servers revealed some novel predicted target oncogenes such as SOX4, which we validated experimentally, in addition to some regulated pathways of immune system and hepatocarcinogenesis such as insulin signaling pathway and adipocytokine signaling pathway. In general, our results demonstrate that let-7a, miR-34a, and miR-199 a/b have novel interactions in different immune system pathways and major hepatocellular carcinoma hallmarks. Thus, our findings shed more light on the roles of these miRNAs as cancer silencers.

Notch inhibition counteracts Paneth cell death in absence of caspase-8

Opposing activities of Notch and Wnt signaling regulate mucosal barrier homeostasis and differentiation of intestinal epithelial cells. Specifically, Wnt activity is essential for differentiation of secretory cells including Wnt3-producing Paneth cells, whereas Notch signaling strongly promotes generation of absorptive cells. Loss of caspase-8 in intestinal epithelium (casp8^∆int) is associated with fulminant epithelial necroptosis, severe Paneth cell death, secondary intestinal inflammation, and an increase in Notch activity. Here, we found that pharmacological Notch inhibition with dibenzazepine (DBZ) is able to essentially rescue the loss of Paneth cells, deescalate the inflammatory phenotype, and reduce intestinal permeability in casp8^∆int mice. The secretory cell metaplasia in DBZ-treated casp8^∆int animals is proliferative, indicating for Notch activities partially insensitive to gamma-secretase inhibition in a casp8^∆int background. Our data suggest that casp8 acts in the intestinal Notch network.

Exploitation of Apoptotic Regulation in Cancer

Within an organism, environmental stresses can trigger cell death, particularly apoptotic cell death. Apoptotic cells, themselves, are potent regulators of their cellular environment, involved primarily in effecting homeostatic control. Tumors, especially, exist in a dynamic balance of cell proliferation and cell death. This special feature of the tumorous microenvironment—namely, the prominence and persistence of cell death—necessarily entails a magnification of the extrinsic, postmortem effects of dead cells. In both normal and malignant tissues, apoptotic regulation is exerted through immune as well as non-immune mechanisms. Apoptotic cells suppress the repertoire of immune reactivities, both by attenuating innate (especially inflammatory) responses and by abrogating adaptive responses. In addition, apoptotic cells modulate multiple vital cell activities, including survival, proliferation (cell number), and growth (cell size). While the microenvironment of the tumor may contribute to apoptosis, the postmortem effects of apoptotic cells feature prominently in the reciprocal acclimatization between the tumor and its environment. In much the same way that pathogens evade the host’s defenses through exploitation of key aspects of innate and adaptive immunity, cancer cells subvert several normal homeostatic processes, in particular wound healing and organ regeneration, to transform and overtake their environment. In understanding this subversion, it is crucial to view a tumor not simply as a clone of malignant cells, but rather as a complex and highly organized structure in which there exists a multidirectional flow of information between the cancer cells themselves and the multiple other cell types and extracellular matrix components of which the tumor is comprised. Apoptotic cells, therefore, have the unfortunate consequence of facilitating tumorigenesis and tumor survival.

The interplay of IKK, NF-κB and RIPK1 signaling in the regulation of cell death, tissue homeostasis and inflammation

Regulated cell death pathways have important functions in host defense and tissue homeostasis. Studies in genetic mouse models provided evidence that cell death could cause inflammation in different tissues. Inhibition of RIPK3-MLKL-dependent necroptosis by FADD and caspase-8 was identified as a key mechanism preventing inflammation in epithelial barriers. Moreover, the interplay between IKK/NF-κB and RIPK1 signaling was recognized as a critical determinant of tissue homeostasis and inflammation. NEMO was shown to regulate RIPK1 kinase activity-mediated apoptosis by NF-κB-dependent and -independent functions, which are critical for averting chronic tissue injury and inflammation in the intestine and the liver. In addition, RIPK1 was shown to exhibit kinase activity-independent functions that are essential for preventing cell death, maintaining tissue architecture and inhibiting inflammation. In the intestine, RIPK1 acts as a scaffold to prevent epithelial cell apoptosis and preserve tissue integrity. In the skin, RIPK1 functions via its RHIM to counteract ZBP1/DAI-dependent activation of RIPK3-MLKL-dependent necroptosis and inflammation. Collectively, these studies provided evidence that the regulation of cell death signaling plays an important role in the maintenance of tissue homeostasis, and suggested that cell death could be causally involved in the pathogenesis of inflammatory diseases.

The in vivo evidence for regulated necrosis

Necrosis is a hallmark of several widespread diseases or their direct complications. In the past decade, we learned that necrosis can be a regulated process that is potentially druggable. RIPK3- and MLKL-mediated necroptosis represents by far the best studied pathway of regulated necrosis. During necroptosis, the release of damage-associated molecular patterns (DAMPs) drives a phenomenon referred to as necroinflammation, a common consequence of necrosis. However, most studies of regulated necrosis investigated cell lines in vitro in a cell autonomous manner, which represents a non-physiological situation. Conclusions based on such work might not necessarily be transferrable to disease states in which synchronized, non-cell autonomous effects occur. Here, we summarize the current knowledge of the pathophysiological relevance of necroptosis in vivo, and in light of this understanding, we reassess the morphological classification of necrosis that is generally used by pathologists. Along these lines, we discuss the paucity of data implicating necroptosis in human disease. Finally, the in vivo relevance of non-necroptotic forms of necrosis, such as ferroptosis, is addressed.

Inhibition of Caspase-8 does not protect from alcohol-induced liver apoptosis but alleviates alcoholic hepatic steatosis in mice

Hepatic apoptosis is involved in the progression of alcoholic liver disease (ALD). Caspase-8, the apical initiator in death receptor-mediated apoptosis, has been implicated in acute liver injury and in non-alcoholic steatohepatitis. However, the relevance of Caspase-8 in the pathogenesis of ALD remains unclear. In the present study, we investigated the impact of Caspase-8 in human and murine alcohol-induced apoptosis and in ALD. We investigated human samples from ALD patients, primary mouse hepatocytes, and hepatocyte-specific Caspase-8 knockout (Casp8^Δhepa) mice in acute and chronic models of ethanol (EtOH) administration. Caspase-8 activation was detected in liver biopsies from ALD patients, as well as in livers of wild-type (WT) mice after chronic ethanol feeding for 8 weeks using the Lieber-DeCarli model. Lack of Caspase-8 expression in Casp8^Δhepa animals failed to prevent alcohol-induced liver damage and apoptosis. Instead, inhibition of Caspase-8 shifted the ethanol-induced death signals towards pronounced activation of the intrinsic, mitochondria-dependent apoptosis pathway in Casp8^Δhepa livers involving enhanced release of cytochrome c, stronger Caspase-9 activation and specific morphological changes of mitochondria. In vitro and in vivo intervention using a pan-caspase inhibitor markedly attenuated alcohol-induced hepatocyte damage in a Caspase-8-independent manner. Surprisingly, EtOH-fed Casp8^Δhepa mice displayed significantly attenuated steatosis and reduced hepatic triglyceride and free fatty acids content. Caspase-8 is dispensable for alcohol-induced apoptosis, but plays an unexpected role for alcohol-dependent fat metabolism. We provide evidence that simultaneous inhibition of extrinsic and intrinsic apoptosis signaling using pan-caspase inhibitors in vivo might be an optimal approach to treat alcohol-induced liver injury.

RIPK1 protects hepatocytes from death in Fas-induced hepatitis

Hepatocyte death is a central event during liver disease progression, in which immune cells play key roles by activating members of the Tumor Necrosis Factor Receptor Superfamily (TNFRSF), including TNFR1 (TNFRSF1A), Fas (TNFRSF6) and TRAIL-R2 (TNFRSF10B). Receptor Interacting Protein Kinase 1 (RIPK1) emerged as a signaling node downstream of these receptors. In the case of TNFR1, RIPK1 has been demonstrated to paradoxically serve as a scaffold to promote the survival of hepatocytes and as a kinase to kill them. To evaluate whether RIPK1 also protects hepatocytes from death in response to FasL or TRAIL, we took advantage of liver parenchymal cell-specific Ripk1 knockout mice (Ripk1^LPC-KO). We found that Ripk1^LPC-KO mice, as well as primary hepatocytes derived from them, were more susceptible to Fas-mediated apoptosis than their respective WT counterparts. Fas-induced hepatocyte death was independent of TNF-α signaling. Interestingly, while TRAIL administration did not induce hepatitis in Ripk1^LPC-KO mice or in their WT counterparts, its combination with IFN-γ only induced TNF-α dependent apoptosis in the Ripk1^LPC-KO mice. Together, our data demonstrate the protective role of RIPK1 downstream of Fas and highlight the general protective function of RIPK1 in hepatocytes exposed to inflammatory conditions, where TNF-α, FasL and/or TRAIL are present.

Voluntary distance running prevents TNF-mediated liver injury in mice through alterations of the intrahepatic immune milieu

Physical activity confers a broad spectrum of health benefits. Beyond the obvious role in metabolically driven diseases, the role of physical activity in acute liver injury is poorly explored. To study the role of physical activity in acute liver injury, a novel model of voluntary distance running in mice was developed and mice were subjected to acute liver injury induced by N-galactosamine (GalN) and lipopolysaccharide (LPS). Analyses included histological stains, immunoblotting, qRT-PCR and FACS analysis. Voluntary distance running increased to an average of 10.3 km/day after a learning curve. Running lead to a decrease in the absolute numbers of intrahepatic CD4+ T and B lymphocytes and macrophages after 7 weeks. In parallel, hepatic mRNA expression of inflammatory cytokines including IL-6 and IL-1beta, TGF-beta and monocyte chemoattractant protein-1 (MCP-1/CCL2) were suppressed, while TNF-α was not affected by exercise. Likewise, expression of the macrophage-specific antigen F4/80 was downregulated 1.6-fold from exercise. Notably, acute liver injury from GaIN/LPS was significantly blunted following 7 weeks of voluntary exercise as determined by liver histology, a 84.6% reduction of alanine aminotransferase (P<0.01) and a 54.6% reduction of aspartate aminotransferase (P<0.05) compared with sedentary mice. Additionally, proinflammatory cytokines, activation of caspase 3 and JNK were significantly lower, while antiapoptotic protein A20 increased. Voluntary distance running alters the intrahepatic immune phenotype producing an environment that is less susceptible to acute liver injury.

Kinase-independent functions of RIPK1 regulate hepatocyte survival and liver carcinogenesis

The mechanisms that regulate cell death and inflammation play an important role in liver disease and cancer. Receptor-interacting protein kinase 1 (RIPK1) induces apoptosis and necroptosis via kinase-dependent mechanisms and exhibits kinase-independent prosurvival and proinflammatory functions. Here, we have used genetic mouse models to study the role of RIPK1 in liver homeostasis, injury, and cancer. While ablating either RIPK1 or RelA in liver parenchymal cells (LPCs) did not cause spontaneous liver pathology, mice with combined deficiency of RIPK1 and RelA in LPCs showed increased hepatocyte apoptosis and developed spontaneous chronic liver disease and cancer that were independent of TNF receptor 1 (TNFR1) signaling. In contrast, mice with LPC-specific knockout of Ripk1 showed reduced diethylnitrosamine-induced (DEN-induced) liver tumorigenesis that correlated with increased DEN-induced hepatocyte apoptosis. Lack of RIPK1 kinase activity did not inhibit DEN-induced liver tumor formation, showing that kinase-independent functions of RIPK1 promote DEN-induced hepatocarcinogenesis. Moreover, mice lacking both RIPK1 and TNFR1 in LPCs displayed normal tumor formation in response to DEN, demonstrating that RIPK1 deficiency decreases DEN-induced liver tumor formation in a TNFR1-dependent manner. Therefore, these findings indicate that RIPK1 cooperates with NF-κB signaling to prevent TNFR1-independent hepatocyte apoptosis and the development of chronic liver disease and cancer, but acts downstream of TNFR1 signaling to promote DEN-induced liver tumorigenesis.

Hepatic overexpression of cAMP-responsive element modulator α induces a regulatory T-cell response in a murine model of chronic liver disease

OBJECTIVE

Th17 cells are a subset of CD4⁺ T-helper cells characterised by interleukin 17 (IL-17) production, a cytokine that plays a crucial role in inflammation-associated diseases. The cyclic AMP-responsive element modulator-α (CREMα) is a central mediator of T-cell pathogenesis, which contributes to increased IL-17 expression in patients with autoimmune disorders. Since an increased Th17 response is associated with a poor prognosis in patients with chronic liver injury, we investigated the relevance of Th17 cells for chronic liver disease (CLD) and hepatocarcinogenesis.

DESIGN

Transgenic mice overexpressing CREMα were crossed with hepatocyte-specific Nemo knockout mice (Nemo^Δhepa) to generate Nemo^Δhepa/CREMα^Tg mice. The impact of CREMα^Tg on CLD progression was examined. Additionally, soft agar colony formation assays, in vitro studies, adoptive transfer of bone marrow-derived cells (BMDCs) and T cells, and gene arrays in T cells were performed.

RESULTS

8-week-old Nemo^Δhepa/CREMα^Tg mice presented significantly decreased transaminase levels, concomitant with reduced numbers of CD11b⁺ dendritic cells and CD8⁺ T cells. CREMα^Tg overexpression in Nemo^Δhepa mice was associated with significantly reduced hepatic fibrogenesis and carcinogenesis at 52 weeks. Interestingly, hepatic stellate cell-derived retinoic acid induced a regulatory T-cell (Treg) phenotype in CREMα^Tg hepatic T cells. Moreover, simultaneous adoptive transfer of BMDCs and T cells from CREMα^Tg into Nemo^Δhepa mice ameliorated markers of liver injury and hepatitis.

CONCLUSIONS

Our results demonstrate that overexpression of CREMα in T cells changes the inflammatory milieu, attenuating initiation and progression of CLD. Unexpectedly, our study indicates that CREMα transgenic T cells shift chronic inflammation in Nemo^Δhepa livers towards a protective Treg response.

The MLKL Channel in Necroptosis Is an Octamer Formed by Tetramers in a Dyadic Process

Oligomerization of the mixed-lineage kinase domain-like protein (MLKL) is essential for its cation channel function in necroptosis. Here we show that the MLKL channel is an octamer comprising two previously identified tetramers most likely in their side-by-side position. Intermolecule disulfide bonds are present in the tetramer but are not required for octamer assembly and necroptosis. MLKL forms oligomers in the necrosome and is then released from the necrosome before or during its membrane translocation. We identified two MLKL mutants that could not oligomerize into octamers, although they formed a tetramer, and also, one MLKL mutant could spontaneously form a disulfide bond-linked octamer. Subsequent analysis revealed that the tetramers fail to translocate to the plasma membrane and that the MLKL octamer formation depends on α-helices 4 and 5. While MLKL could be detected from outside the cells, its N- and C-terminal ends could not be detected, indicating that the MLKL octamer spans across the plasma membrane, leaving its N and C termini inside the cell. These data allowed us to propose a 180° symmetry model of the MLKL octamer and conclude that the fully assembled MLKL octamers, but not the previously described tetramers, act as effectors of necroptosis.

Questions and controversies: the role of necroptosis in liver disease

Acute and chronic liver injury results in hepatocyte death and turnover. If injury becomes chronic, the continuous cell death and turnover leads to chronic inflammation, fibrosis and ultimately cirrhosis and hepatocellular carcinoma. Controlling liver cell death both in acute injury, to rescue the liver from acute liver failure, and in chronic injury, to curb secondary inflammation and fibrosis, is of paramount importance as a therapeutic strategy. Both apoptosis and necrosis occur in the liver, but the occurrence of necroptosis in the liver and its contribution to liver disease is controversial. Necroptosis is a form of regulated necrosis which occurs in certain cell types when caspases (+/-cIAPs) are inhibited through the RIPK1-RIPK3 activation of MLKL. The occurrence of necroptosis in the liver has recently been examined in multiple liver injury models with conflicting results. The aim of this review is to summarize the published data with an emphasis on the controversies and remaining questions in the field.

Cell death mechanisms in human chronic liver diseases: a far cry from clinical applicability

The liver is constantly exposed to a host of injurious stimuli. This results in hepatocellular death mainly by apoptosis and necrosis, but also due to autophagy, necroptosis, pyroptosis and in some cases by an intricately balanced combination thereof. Overwhelming and continuous cell death in the liver leads to inflammation, fibrosis, cirrhosis, and eventually hepatocellular carcinoma. Although data from various disease models may suggest a specific (predominant) cell death mode for different aetiologies, the clinical reality is not as clear cut. Reliable and non-invasive cell death markers are not available in general practice and assessment of cell death mode to absolute certainty from liver biopsies does not seem feasible, yet. Various aetiologies probably induce different predominant cell death modes within the liver, although the death modes involved may change during disease progression. Moreover, current methods applicable in patients are limited to surrogate markers for apoptosis (M30), and possibly for pyroptosis (IL-1 family) and necro(pto)sis (HMGB1). Although markers for some death modes are not available at all (autophagy), others may not be specific for a cell death mode or might not always definitely indicate dying cells. Physicians need to take care in asserting the presence of cell death. Still the serum-derived markers are valuable tools to assess severity of chronic liver diseases. This review gives a short overview of known hepatocellular cell death modes in various aetiologies of chronic liver disease. Also the limitations of current knowledge in human settings and utilization of surrogate markers for disease assessment are summarized.

RIPK1 protects from TNF-α-mediated liver damage during hepatitis

Cell death of hepatocytes is a prominent characteristic in the pathogenesis of liver disease, while hepatolysis is a starting point of inflammation in hepatitis and loss of hepatic function. However, the precise molecular mechanisms of hepatocyte cell death, the role of the cytokines of hepatic microenvironment and the involvement of intracellular kinases, remain unclear. Tumor necrosis factor alpha (TNF-α) is a key cytokine involved in cell death or survival pathways and the role of RIPK1 has been associated to the TNF-α-dependent signaling pathway. We took advantage of two different deficient mouse lines, the RIPK1 kinase dead knock-in mice (Ripk1^K45A) and the conditional knockout mice lacking RIPK1 only in liver parenchymal cells (Ripk1^LPC-KO), to characterize the role of RIPK1 and TNF-α in hepatitis induced by concanavalin A (ConA). Our results show that RIPK1 is dispensable for liver homeostasis under steady-state conditions but in contrast, RIPK1 kinase activity contributes to caspase-independent cell death induction following ConA injection and RIPK1 also serves as a scaffold, protecting hepatocytes from massive apoptotic cell death in this model. In the Ripk1^LPC-KO mice challenged with ConA, TNF-α triggers apoptosis, responsible for the observed severe hepatitis. Mechanism potentially involves both TNF-independent canonical NF-κB activation, as well as TNF-dependent, but canonical NF-κB-independent mechanisms. In conclusion, our results suggest that RIPK1 kinase activity is a pertinent therapeutic target to protect liver against excessive cell death in liver diseases.

Receptor interacting protein 3 protects mice from high-fat diet-induced liver injury

Multiple pathways of programmed cell death are important in liver homeostasis. Hepatocyte death is associated with progression of nonalcoholic fatty liver disease, and inhibition of apoptosis partially protects against liver injury in response to a high-fat diet (HFD). However, the contribution of necroptosis, a caspase-independent pathway of cell death, to HFD-induced liver injury is not known. Wild-type C57BL/6 and receptor interacting protein (RIP) 3^-/- mice were randomized to chow or HFD. HFD-fed C57BL/6 mice increased expression of RIP3, the master regulator of necroptosis, as well as phosphorylated mixed lineage kinase domain-like, an effector of necroptotic cell death, in liver. HFD did not increase phosphorylated mixed lineage kinase domain-like in RIP3^-/- mice. HFD increased fasting insulin and glucose, as well as glucose intolerance, in C57BL/6 mice. RIP3^-/- mice were glucose-intolerant even on the chow diet; HFD further increased fasting glucose and insulin but not glucose intolerance. HFD also increased hepatic steatosis, plasma alanine aminotransferase activity, inflammation, oxidative stress, and hepatocellular apoptosis in wild-type mice; these responses were exacerbated in RIP3^-/- mice. Importantly, increased inflammation and injury were associated with early indicators of fibrosis in RIP3^-/- compared to C57BL/6 mice. Culture of AML12 hepatocytes with palmitic acid increased cytotoxicity through apoptosis and necrosis. Inhibition of RIP1 with necrostatin-1 or small interfering RNA knockdown of RIP3 reduced palmitic acid-induced cytotoxicity.

CONCLUSION

Absence of RIP3, a key mediator of necroptosis, exacerbated HFD-induced liver injury, associated with increased inflammation and hepatocyte apoptosis, as well as early fibrotic responses; these findings indicate that shifts in the mode of hepatocellular death can influence disease progression and have therapeutic implications because manipulation of hepatocyte cell death pathways is being considered as a target for treatment of nonalcoholic fatty liver disease. (Hepatology 2016;64:1518-1533).

Necrostatin-1 treatment inhibits osteocyte necroptosis and trabecular deterioration in ovariectomized rats

Estrogen (E2) deficiency has been associated with accelerated osteocyte apoptosis. Our previous study showed necroptosis accelerated the loss of osteocytes in E2 deficiency-induced osteoporosis in rats in addition to apoptosis, but the mechanism involved remains. Necroptosis is a caspase-independent form of programmed cell death. In the necroptosis pathway, receptor interaction proteins 1 and 3 (RIP1/3) play vital roles. Necrostatin-1 (Nec-1) has been confirmed to be a specific inhibitor of necroptosis. However, the effect of Nec-1 on postmenopausal osteoporosis remains ambiguous. The aim of this study was to investigate the effect of Nec-1 on osteocytes in ovariectomized (OVX) rats. We found that an increased number of necroptotic osteocytes was related to the production of tumor necrosis factor-alpha (TNF-α) in OVX rats. Treatment with Nec-1 significantly decreased RIP1 and RIP3 expression in OVX rats and inhibited osteocyte necroptosis induced by TNF-α in vitro. Both E2 and Nec-1 treatment markedly ameliorated trabecular bone deterioration. Nec-1 also significantly elevated the levels of bone formation markers and decreased bone resorption markers. These data suggest that the role of Nec-1 on alleviating bone loss might be associated with Nec-1 restraining TNF-α-induced osteocyte necroptosis in rats with E2 deficiency-induced osteoporosis. This process may represent a novel therapeutic strategy for the treatment of postmenopausal osteoporosis.

Activation of necroptosis in human and experimental cholestasis

Cholestasis encompasses liver injury and inflammation. Necroptosis, a necrotic cell death pathway regulated by receptor-interacting protein (RIP) 3, may mediate cell death and inflammation in the liver. We aimed to investigate the role of necroptosis in mediating deleterious processes associated with cholestatic liver disease. Hallmarks of necroptosis were evaluated in liver biopsies of primary biliary cholangitis (PBC) patients and in wild-type and RIP3-deficient (RIP3^−/−) mice subjected to common bile duct ligation (BDL). The functional link between RIP3, heme oxygenase-1 (HO-1) and antioxidant response was investigated in vivo after BDL and in vitro. We demonstrate increased RIP3 expression and mixed lineage kinase domain-like protein (MLKL) phosphorylation in liver samples of human PBC patients, coincident with thioflavin T labeling, suggesting activation of necroptosis. BDL resulted in evident hallmarks of necroptosis, concomitant with progressive bile duct hyperplasia, multifocal necrosis, fibrosis and inflammation. MLKL phosphorylation was increased and insoluble aggregates of RIP3, MLKL and RIP1 formed in BLD liver tissue samples. Furthermore, RIP3 deficiency blocked BDL-induced necroinflammation at 3 and 14 days post-BDL. Serum hepatic enzymes, fibrogenic liver gene expression and oxidative stress decreased in RIP3^−/− mice at 3 days after BDL. However, at 14 days, cholestasis aggravated and fibrosis was not halted. RIP3 deficiency further associated with increased hepatic expression of HO-1 and accumulation of iron in BDL mice. The functional link between HO-1 activity and bile acid toxicity was established in RIP3-deficient primary hepatocytes. Necroptosis is triggered in PBC patients and mediates hepatic necroinflammation in BDL-induced acute cholestasis. Targeting necroptosis may represent a therapeutic strategy for acute cholestasis, although complementary approaches may be required to control progression of chronic cholestatic liver disease.

RIPK3-Mediated Necroptosis and Apoptosis Contributes to Renal Tubular Cell Progressive Loss and Chronic Kidney Disease Progression in Rats

Tubulointerstitial fibrosis (TIF) is caused by the progressive loss of renal tubular cells and the consequent replacement of the extracellular matrix. The progressive depletion of renal tubular cells results from apoptosis and necroptosis; however, the relative significance of each of these cell death mechanisms at different stages during the progression of chronic kidney disease (CKD) remains unclear. We sought to explore the mechanisms of renal tubular cell death during the early and intermediate stages of chronic renal damage of subtotal nephrectomied (SNx) rats. The results of tissue histological assays indicated that the numbers of necrotic dying cells and apoptotic cells were significantly higher in kidney tissues derived from a rat model of CKD. In addition, there was a significant increase in necroptosis observed by transmission electron microscopy (TEM) and an increase in the proportion of TUNEL-positive cells in kidney tissues from SNx rats compared with control rats, and necrostatin-1 (Nec-1) could inhibit necroptosis and reduce the proportion of TUNEL-positive cells. More importantly, we observed a significant increase in the incidence of necroptosis compared with apoptosis by TEM in vivo and in vitro and a significant increase in the proportion of TUNEL-positive tubular epithelial cells that did not express caspase-3 compared with those expressing cleaved caspase-3 in vitro. Furthermore, treatment with Nec-1 and zVAD strongly reduced necroptosis- and apoptosis-mediated renal tubular cell death and decreased the levels of blood urea nitrogen and serum creatinine and tubular damage scores of SNx rats. These results suggest that necroptotic cell death plays a more significant role than apoptosis in mediating the loss of renal tubular cells in SNx rats and that effectively blocking both necroptosis and apoptosis improves renal function and tubular damage at early and intermediate stages of CKD.

Zinc inhibits aflatoxin B1-induced cytotoxicity and genotoxicity in human hepatocytes (HepG2 cells)

Aflatoxin B1 (AFB1) has strong carcinogenicity. Consumption of AFB1-contaminated agricultural products and the occurrence of hepatocellular carcinoma have received widespread attention. The aim of this paper was to investigate whether zinc supplementation could inhibit AFB1-induced cytotoxicity and genotoxicity in HepG2 cells and the mechanism of this inhibition. Our data suggest that zinc sources can relieve a certain degree of AFB1-induced cytotoxicity and genotoxicity by protecting against apoptotic body formation and DNA strand breaks, affecting S phase cell cycle arrest, reducing 8-OHdG formation, inhibiting global DNA hypomethylation and regulating gene expression in antioxidation, zinc-association and apoptosis processes. Consequently, zinc stabilizes the integrity of DNA and improves cell survival. These data provides new insights into the protective role of zinc in alleviating AFB1-induced cytotoxicity and mediating epigenetic changes in hepatocytes, demonstrating that zinc sources have detoxification properties in mycotoxin-induced toxicity.

The RIP1-RIP3 Complex Mediates Osteocyte Necroptosis after Ovariectomy in Rats

Osteocyte apoptosis has been reported to play a central role in bone remodeling. In addition to apoptosis, other mechanisms may be involved in osteocyte loss. This study aimed to investigate the effect of necroptosis on osteocytes in ovariectomized (OVX) rats. Ninety-six female Sprague-Dawley rats were randomly divided into an OVX group and a sham group. At 0, 4, 8 and 12 weeks after surgery, specimens from each group (n = 12 each) were harvested. Bone mineral density (BMD) and body weight were measured. Transmission electron microscopy (TEM) and micro-CT were used to observe the changes in cellular morphology and bone microarchitecture induced by estrogen deficiency. Osteocyte apoptosis and necroptosis were evaluated via TUNEL and immunofluorescence staining for active caspase-3. At 8 weeks after ovariectomy, a greater number of osteocytes with typical necrotic morphological features were TUNEL positive but negative for active caspase-3. Western blotting, quantitative real-time PCR and immunofluorescence assessments demonstrated that the levels of receptor-interacting serine/threonine protein kinase 1 (RIP1) and RIP3 in osteocytes were significantly increased at 8 weeks after ovariectomy. These data are the first to suggest that necroptosis accelerates osteocyte loss under conditions of estrogen deficiency-induced osteoporosis in OVX rats. These findings provide evidence of a potential mechanism through which osteocyte necroptosis is associated with postmenopausal osteoporosis.

Multifunctional murrel caspase 1, 2, 3, 8 and 9: Conservation, uniqueness and their pathogen-induced expression pattern

Caspases are evolutionarily conserved proteases which play fundamental role in apoptosis. Invasion of pathogen triggers the activation of caspases-mediated pro-inflammatory and pro-apoptotic pathways, where multifunctional caspases are involved. In striped murrel Channa striatus, epizootic ulcerative syndrome (EUS) causes endemics resulting in huge economic loss. Aphanomyces invadans, an oomycete is the primary causative agent of EUS which further induces secondary bacterial infections especially Aeromonas hydrophila. In order to get insights into the caspase gene family in C. striatus during EUS infection, we performed various physicochemical and structural analyses on the cDNA and protein sequences of five different murrel caspases namely CsCasp 1, 2, 3, 8 and 9. Sequence analysis of murrel caspase proteins showed that in spite of the conserved CASC domain, each caspase embraces some unique features which made them functionally different. Tissue distribution analysis showed that all the murrel caspases are highly expressed in one of the immune organs such as liver, kidney, spleen and blood cells. Further, to understand the role of caspase during EUS infection, modulation in expression of each caspase gene was analysed after inducing fungal and bacterial infection in C. striatus. Pathogen-induced gene expression pattern revealed an interesting fact that the expression of all the caspase genes reached a maximum level at 24 h post-infection (p.i) in case of bacteria, whereas it was 48 h in fungus. However, the initiation of elevated expression differed between each caspase based on their role such as pro-inflammatory, initiator and executioner caspase. Overall, the results suggested that the caspases in murrel are diverse in their structure and function. Here, we discuss the similarities and differences of five different murrel caspases.

NEMO Prevents Steatohepatitis and Hepatocellular Carcinoma by Inhibiting RIPK1 Kinase Activity-Mediated Hepatocyte Apoptosis

IκB kinase/nuclear [corrected] factor κB (IKK/NF-κB) signaling exhibits important yet opposing functions in hepatocarcinogenesis. Mice lacking NEMO in liver parenchymal cells (LPC) spontaneously develop steatohepatitis and hepatocellular carcinoma (HCC) suggesting that NF-κB prevents liver disease and cancer. Here, we show that complete NF-κB inhibition by combined LPC-specific ablation of RelA, c-Rel, and RelB did not phenocopy NEMO deficiency, but constitutively active IKK2-mediated NF-κB activation prevented hepatocellular damage and HCC in NEMO(LPC-KO) mice. Knock-in expression of kinase inactive receptor-interacting protein kinase 1 (RIPK1) prevented hepatocyte apoptosis and HCC, while RIPK1 ablation induced TNFR1-associated death domain protein (TRADD)-dependent hepatocyte apoptosis and liver tumors in NEMO(LPC-KO) mice, revealing distinct kinase-dependent and scaffolding functions of RIPK1. Collectively, these results show that NEMO prevents hepatocarcinogenesis by inhibiting RIPK1 kinase activity-driven hepatocyte apoptosis through NF-κB-dependent and -independent functions.

TNFα in liver fibrosis

Hepatocyte death, inflammation, and liver fibrosis are the hallmarks of chronic liver disease. Tumor necrosis factor-α (TNFα) is an inflammatory cytokine involved in liver inflammation and sustained liver inflammation leads to liver fibrosis. TNFα exerts inflammation, proliferation, and apoptosis. However, the role of TNFα signaling in liver fibrosis is not fully understood. This review highlights the recent findings demonstrating the molecular mechanisms of TNFα and its downstream signaling in liver fibrosis. During the progression of liver fibrosis, hepatic stellate cells play a pivotal role in a dynamic process of production of extracellular matrix proteins and modulation of immune response. Hepatic stellate cells transdifferentiate into activated myofibroblasts in response to damaged hepatocyte-derived mediators and immune cell-derived cytokines/chemokines. Here, we will discuss the role of TNFα in hepatic stellate cell survival and activation and the crosstalk between hepatic stellate cells and hepatocytes or other immune cells, such as macrophages, dendritic cells, and B cells in the development of liver fibrosis.

Regulation of NKT cell-mediated immune responses to tumours and liver inflammation by mitochondrial PGAM5-Drp1 signalling

The receptor-interacting protein kinase 3 (RIPK3) plays crucial roles in programmed necrosis and innate inflammatory responses. However, a little is known about the involvement of RIPK3 in NKT cell-mediated immune responses. Here, we demonstrate that RIPK3 plays an essential role in NKT cell function via activation of the mitochondrial phosphatase phosphoglycerate mutase 5 (PGAM5). RIPK3-mediated activation of PGAM5 promotes the expression of cytokines by facilitating nuclear translocation of NFAT and dephosphorylation of dynamin-related protein 1 (Drp1), a GTPase is essential for mitochondrial homoeostasis. Ripk3(-/-) mice show reduced NKT cell responses to metastatic tumour cells, and both deletion of RIPK3 and pharmacological inhibition of Drp1 protects mice from NKT cell-mediated induction of acute liver damage. Collectively, the results identify a crucial role for RIPK3-PGAM5-Drp1/NFAT signalling in NKT cell activation, and further suggest that RIPK3-PGAM5 signalling may mediate crosstalk between mitochondrial function and immune signalling.

Protective effect of Salvia miltiorrhiza and Carthamus tinctorius extract against lipopolysaccharide-induced liver injury

AIM: To investigate the hepatoprotective effects and mechanisms of an extract of Salvia miltiorrhiza and Carthamus tinctorius in vivo.

METHODS: C57BL/6J mice were randomly assigned to five groups and intraperitoneally administered 0.9% saline, Salvia miltiorrhiza and Carthamus tinctorius extract [Danhong injection (DHI), 0.75 and 3 g/kg mixed extract] or reduced glutathione for injection (RGI, 300 mg/kg) for 30 min before exposure to lipopolysaccharide (LPS, 16 mg/kg). After intraperitoneal LPS stimulation for 90 min or 6 h, the mice were sacrificed by ether anaesthesia, and serum and liver samples were collected. Histological analysis (H&E) and terminal deoxynucleotidyl transferase-mediated dUTP nick end-labelling (TUNEL) staining were performed. Alanine transferase (ALT), aspartate transaminase (AST), total bilirubin (TBil), glutathione-S-transferase (GST), malondialdehyde (MDA), tumour necrosis factor (TNF)-α, interleukin (IL)-6, and caspase-3 levels were measured. Bax, Bcl-2, P-IκBα, IκBα, P-NF-κB p65, and NF-κB p65 protein levels were determined by Western blot. TNF-α, IL-6, caspase-3, Bax and Bcl-2 mRNA expression was measured by real-time reverse transcription-polymerase chain reaction (RT-PCR).

RESULTS: Hematoxylin-eosin staining and TUNEL results suggested that DHI (3 g/kg) treatment alleviated inflammatory and apoptotic (P < 0.01) injury in the liver of mice. DHI treatment dose-dependently blunted the abnormal changes in biochemical parameters such as ALT (72.53 ± 2.83 for 3 g/kg, P < 0.01), AST (76.97 ± 5.00 for 3 g/kg, P < 0.01), TBil (1.17 ± 0.10 for 3 g/kg, P < 0.01), MDA (0.81 ± 0.36 for 3 g/kg, P < 0.01), and GST (358.86 ± 12.09 for 3 g/kg, P < 0.01). Moreover, DHI (3 g/kg) remarkably decreased LPS-induced protein expression of TNF-α (340.55 ± 10.18 for 3 g/kg, P < 0.01), IL-6 (261.34 ± 10.18 for 3 g/kg, P < 0.01), and enzyme activity of caspase-3 (0.93 ± 0.029 for 3 g/kg, P < 0.01). The LPS-induced mRNA expression of TNF-α, IL-6 and caspase-3 was also decreased by DHI. Western blot analysis revealed that DHI antagonised LPS-stimulated decrease of Bcl-2 and increase of Bax protein expression. Furthermore, DHI inhibited LPS-induced IκBα and NF-κB p65 phosphorylation.

CONCLUSION: DHI may be a multi-function protectant against acute hepatic injury in mice through its anti-inflammatory, anti-oxidative and anti-apoptotic activities.

Necroptosis is a key pathogenic event in human and experimental murine models of non-alcoholic steatohepatitis

Hepatocyte cell death, inflammation and oxidative stress constitute key pathogenic mechanisms underlying non-alcoholic fatty liver disease (NAFLD). We aimed to investigate the role of necroptosis in human and experimental NAFLD and its association with tumour necrosis factor α (TNF-α) and oxidative stress. Serum markers of necrosis, liver receptor-interacting protein 3 (RIP3) and phosphorylated mixed lineage kinase domain-like (MLKL) were evaluated in control individuals and patients with NAFLD. C57BL/6 wild-type (WT) or RIP3-deficient (RIP3(-/-)) mice were fed a high-fat choline-deficient (HFCD) or methionine and choline-deficient (MCD) diet, with subsequent histological and biochemical analysis of hepatic damage. In primary murine hepatocytes, necroptosis and oxidative stress were also assessed after necrostatin-1 (Nec-1) treatment or RIP3 silencing. We show that circulating markers of necrosis and TNF-α, as well as liver RIP3 and MLKL phosphorylation were increased in NAFLD. Likewise, RIP3 and MLKL protein levels and TNF-α expression were increased in the liver of HFCD and MCD diet-fed mice. Moreover, RIP3 and MLKL sequestration in the insoluble protein fraction of NASH (non-alcoholic steatohepatitis) mice liver lysates represented an early event during stetatohepatitis progression. Functional studies in primary murine hepatocytes established the association between TNF-α-induced RIP3 expression, activation of necroptosis and oxidative stress. Strikingly, RIP3 deficiency attenuated MCD diet-induced liver injury, steatosis, inflammation, fibrosis and oxidative stress. In conclusion, necroptosis is increased in the liver of NAFLD patients and in experimental models of NASH. Further, TNF-α triggers RIP3-dependent oxidative stress during hepatocyte necroptosis. As such, targeting necroptosis appears to arrest or at least impair NAFLD progression.

Intestinal genetic inactivation of caspase-8 diminishes migration of enterocytes

AIM: To verify the hypothesis that caspase-8 (Casp8), which regulates cellular apoptosis and necroptosis, is critically involved in enterocyte migration.

METHODS: Casp8-silenced Caco2 cells were used in migration assays. In addition, enterocyte-specific Casp8 heterozygous (Casp8^+/∆int) or homozygous knockout mice (Casp8^∆int) were generated by crossing genetically modified mice carrying loxP recombination sites in intron 2 and 4 of the murine Casp8 gene with transgenic animals expressing a cre-transgene under control of the villin promoter in a pure C57/BL6 genetic background. The nucleoside analog BrdU was injected i.p. in male Casp8^+/∆int and Casp8^∆int animals 4 h, 20 h, or 40 h before performing morphometric studies. Locations of anti-BrdU-immunostained cells (cell^max) in at least 50 hemi-crypts of 6 histoanatomically distinct intestinal mucosal regions were numbered and extracted for statistical procedures. For the mice cohort (n = 28), the walking distance of enterocytes was evaluated from cell^max within crypt (n = 57), plateau (n = 19), and villus (n = 172) positions, resulting in a total of 6838 observations. Data analysis was performed by fitting a three-level mixed effects model to the data.

RESULTS: In cell culture experiments with Caco2 cells, Casp8 knockdown efficiency mediated by RNA interference on Casp8 transcripts was 80% controlled as determined by Western blotting. In the scratch assay, migration of Casp8-deleted Caco2 cells was significantly diminished when compared with controls (Casp8^∆scramble and Caco2). In BrdU-labeled Casp8^∆int mice, cell^max locations were found along the hemi-crypts in a lower position than it was for Casp8^+/∆int or control (cre-negative) animals. Statistical data analysis with a three-level mixed effects model revealed that in the six different intestinal locations (distinct segments of the small and large intestine), cell movement between the three mice groups differed widely. Especially in duodenal hemi-crypts, enterocyte movement was different between the groups. At 20 h, duodenal cell^max location was significantly lower in Casp8^∆int (25.67 ± 2.49) than in Casp8^+/∆int (35.67 ± 4.78; P < 0.05) or control littermates (44.33 ± 0.94; P < 0.01).

CONCLUSION: Casp8-dependent migration of enterocytes is likely involved in intestinal physiology and inflammation-related pathophysiology.