RIPK1 protects hepatocytes from death in Fas-induced hepatitis

IRE1 RNase controls CD95-mediated cell death

Signalling by the Unfolded Protein Response (UPR) or by the Death Receptors (DR) are frequently activated towards pro-tumoral outputs in cancer. Herein, we demonstrate that the UPR sensor IRE1 controls the expression of the DR CD95/Fas, and its cell death-inducing ability. Both genetic and pharmacologic blunting of IRE1 activity increased CD95 expression and exacerbated CD95L-induced cell death in glioblastoma (GB) and Triple-Negative Breast Cancer (TNBC) cell lines. In accordance, CD95 mRNA was identified as a target of Regulated IRE1-Dependent Decay of RNA (RIDD). Whilst CD95 expression is elevated in TNBC and GB human tumours exhibiting low RIDD activity, it is surprisingly lower in XBP1s-low human tumour samples. We show that IRE1 RNase inhibition limited CD95 expression and reduced CD95-mediated hepatic toxicity in mice. In addition, overexpression of XBP1s increased CD95 expression and sensitized GB and TNBC cells to CD95L-induced cell death. Overall, these results demonstrate the tight IRE1-mediated control of CD95-dependent cell death in a dual manner through both RIDD and XBP1s, and they identify a novel link between IRE1 and CD95 signalling.

Gastrodin ameliorates Concanavalin A-induced acute hepatitis via the IL6/JAK2/STAT3 pathway

AIMS

Gastrodin, the main active ingredient of Gastrodia elata Blume, has been shown to protect against many inflammatory diseases. Our study aimed to investigate the anti-inflammatory role of gastrodin in concanavalin A (ConA)-induced acute hepatitis in mice and to explore its precise mechanism.

METHODS

C57BL/6 mice were administered with gastrodin (50 or 100mg/kg) for 3 days prior to intravenous injection of ConA to induce acute autoimmune hepatitis (AIH). Serum aminotransferases levels and cytokine levels were measured. Liver tissue histology was conducted to assess the degree of liver injury. Splenocytes pretreated with gastrodin were stimulated with ConA to observe splenocyte proliferation.

RESULTS

Gastrodin greatly reduced the level of serum aminotransferases, inflammatory cytokine such as IL-6 and TNF-α and histopathological damage in ConA-induced hepatitis. Besides, gastrodin had an inhibitory effect on liver apoptosis, and autophagy. Furthermore, gastrodin inhibited the proliferation of splenocytes in vitro. The protein expression of p-JAK2 and p-STAT3 was markedly affected by gastrodin pretreatment.

CONCLUSIONS

The present study indicated that gastrodin pretreatment exerted protective effects against ConA-induced acute hepatitis, partly through the inhibition of the IL6/JAK2/STAT3 pathway. Further studies are recommended to determine the potential therapeutic role of gastrodin in acute AIH.

RIPK1 in Liver Parenchymal Cells Limits Murine Hepatitis during Acute CCl4-Induced Liver Injury

Some life-threatening acute hepatitis originates from drug-induced liver injury (DILI). Carbon tetrachloride (CCl₄)-induced acute liver injury in mice is the widely used model of choice to study acute DILI, which pathogenesis involves a complex interplay of oxidative stress, necrosis, and apoptosis. Since the receptor interacting protein kinase-1 (RIPK1) is able to direct cell fate towards survival or death, it may potentially affect the pathological process of xenobiotic-induced liver damage. Two different mouse lines, either deficient for Ripk1 specifically in liver parenchymal cells (Ripk1^LPC-KO) or for the kinase activity of RIPK1 (Ripk1^K45A, kinase dead), plus their respective wild-type littermates (Ripk1^fl/fl, Ripk1^wt/wt), were exposed to single toxic doses of CCl₄. This exposure led in similar injury in Ripk1^K45A mice and their littermate controls. However, Ripk1^LPC-KO mice developed more severe symptoms with massive hepatocyte apoptosis as compared to their littermate controls. A pretreatment with a TNF-α receptor decoy exacerbated liver apoptosis in both Ripk1^fl/fl and Ripk1^LPC-KO mice. Besides, a FasL antagonist promoted hepatocyte apoptosis in Ripk1^fl/fl mice but reduced it in Ripk1^LPC-KO mice. Thus, the scaffolding properties of RIPK1 protect hepatocytes from apoptosis during CCl₄ intoxication. TNF-α and FasL emerged as factors promoting hepatocyte survival. These protective effects appeared to be independent of RIPK1, at least in part, for TNF-α, but dependent on RIPK1 for FasL. These new data complete the deciphering of the molecular mechanisms involved in DILI in the context of research on their prevention or cure.

Receptor-interacting protein kinase-1 ablation in liver parenchymal cells promotes liver fibrosis in murine NASH without affecting other symptoms

Non-alcoholic steatohepatitis (NASH), a chronic liver disease that emerged in industrialized countries, can further progress into liver fibrosis, cirrhosis, and hepatocellular carcinoma. In the next decade, NASH is predicted to become the leading cause of liver transplantation, the only current interventional therapeutic option. Hepatocyte death, triggered by different death ligands, plays key role in its progression. Previously, we showed that the receptor-interacting protein kinase-1 (RIPK1) in hepatocytes exhibits a protective role in ligand-induced death. Now, to decipher the role of RIPK1 in NASH, Ripk1^LPC-KO mice, deficient for RIPK1 only in liver parenchymal cells, and their wild-type littermates (Ripk1^fl/fl) were fed for 3, 5, or 12 weeks with high-fat high-cholesterol diet (HFHCD). The main clinical signs of NASH were analyzed to compare the pathophysiological state established in mice. Most of the symptoms evolved similarly whatever the genotype, whether it was the increase in liver to body weight ratio, the steatosis grade or the worsening of liver damage revealed by serum transaminase levels. In parallel, inflammation markers followed the same kinetics with significant equivalent inductions of cytokines (hepatic mRNA levels and blood cytokine concentrations) and a main peak of hepatic infiltration of immune cells at 3 weeks of HFHCD. Despite this identical inflammatory response, more hepatic fibrosis was significantly evidenced at week 12 in Ripk1^LPC-KO mice. This coincided with over-induced rates of transcripts of genes implied in fibrosis development (Tgfb1, Tgfbi, Timp1, and Timp2) in Ripk1^LPC-KO animals. In conclusion, our results show that RIPK1 in hepatocyte limits the progression of liver fibrosis during NASH.

RIPK3: A New Player in Renal Fibrosis

Chronic kidney disease (CKD) is the end result of a plethora of renal insults, including repeated episodes of acute or toxic kidney injury, glomerular, or diabetic kidney disease. It affects a large number of the population worldwide, resulting in significant personal morbidity and mortality and economic cost to the community. Hence it is appropriate to focus on treatment strategies that interrupt the development of kidney fibrosis, the end result of all forms of CKD, in addition to upstream factors that may be specific to certain diseases. However, the current clinical approach to prevent or manage renal fibrosis remains unsatisfactory. The rising importance of receptor-interacting serine/threonine-protein kinase (RIPK) 3 in the inflammatory response and TGF-β1 signaling is increasingly recognized. We discuss here the biological functions of RIPK3 and its role in the development of renal fibrosis.

Necroptosis signaling in liver diseases: An update

The apoptosis alternate cell death pathways are extensively studied in recent years and their significance has been well recognized. With identification of newer cell death pathways, the therapeutic opportunities to modulate cell death have indeed further extended. Necroptosis, among other apoptosis alternate pathways, has been immensely studied recently in different hepatic disease models. Receptor-interacting protein 1 (RIPK1), RIPK3 and mixed lineage kinase domain like (MLKL) seemed to be the key players to mediate necroptosis pathway. Initially, necroptosis seemed to be following the typical pathway. But recently diverse pathways and outcomes have been observed. With recent studies reporting diverse outcomes, the necroptosis signalling has become a lot more interesting and intricate. The typical RIPK1 signalling followed by RIPK3 and MLKL might not always be strictly followed. Although, necroptosis signalling has been intensively investigated in various disease conditions; however, there is still a need to further elaborate and understand the unique scaffolding and kinase properties and other signalling interactions of necroptosis signalling molecules.

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.

Depletion of RIPK1 in hepatocytes exacerbates liver damage in fulminant viral hepatitis

The protein kinase RIPK1 plays a crucial role at the crossroad of stress-induced signaling pathways that affects cell’s decision to live or die. The present study aimed to define the role of RIPK1 in hepatocytes during fulminant viral hepatitis, a worldwide syndrome mainly observed in hepatitis B virus (HBV) infected patients. Mice deficient for RIPK1, specifically in liver parenchymal cells (Ripk1^LPC-KO) and their wild-type littermates (Ripk1^fl/fl), were challenged by either the murine hepatitis virus type 3 (MHV3) or poly I:C, a synthetic analog of double-stranded RNA mimicking viral pathogen-associated molecular pattern. Ripk1^LPC-KO mice developed more severe symptoms at early stage of the MHV3-induced fulminant hepatitis. Similarly, administration of poly I:C only triggered increase of systemic transaminases in Ripk1^LPC-KO mice, reflecting liver damage through induced apoptosis as illustrated by cleaved-caspase 3 labeling of liver tissue sections. Neutralization of TNF-α or prior depletion of macrophages were able to prevent the appearance of apoptosis of hepatocytes in poly I:C-challenged Ripk1^LPC-KO mice. Moreover, poly I:C never induced direct hepatocyte death in primary culture whatever the murine genotype, while it always stimulated an anti-viral response. Our investigations demonstrated that RIPK1 protects hepatocytes from TNF-α secreted from macrophages during viral induced fulminant hepatitis. These data emphasize the potential worsening risks of an HBV infection in people with polymorphism or homozygous amorphic mutations already described for the RIPK1 gene.

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.

Checkpoints in TNF-Induced Cell Death: Implications in Inflammation and Cancer

Tumor necrosis factor (TNF) is a proinflammatory cytokine that coordinates tissue homeostasis by regulating cytokine production, cell survival, and cell death. However, how life and death decisions are made in response to TNF is poorly understood. Many inflammatory pathologies are now recognized to be driven by aberrant TNF-induced cell death, which, in most circumstances, depends on the kinase Receptor-interacting serine/threonine-protein kinase 1 (RIPK1). Recent advances have identified ubiquitin (Ub)-mediated phosphorylation of RIPK1 as belonging to crucial checkpoints for cell fate in inflammation and infection. A better understanding of these checkpoints might lead to new approaches for the treatment of chronic inflammatory diseases fueled by aberrant RIPK1-induced cell death, and/or reveal novel strategies for anticancer immunotherapies, harnessing the ability of RIPK1 to trigger immunogenic cell death.