The pseudokinase MLKL regulates hepatic insulin sensitivity independently of inflammation

RIPK1 is dispensable for cell death regulation in β-cells during hyperglycemia

OBJECTIVE

Receptor-interacting protein kinase 1 (RIPK1) orchestrates the decision between cell survival and cell death in response to tumor necrosis factor (TNF) and other cytokines. Whereas the scaffolding function of RIPK1 is crucial to prevent TNF-induced apoptosis and necroptosis, its kinase activity is required for necroptosis and partially for apoptosis. Although TNF is a proinflammatory cytokine associated with β-cell loss in diabetes, the mechanism by which TNF induces β-cell demise remains unclear.

METHODS

Here, we dissected the contribution of RIPK1 scaffold versus kinase functions to β-cell death regulation using mice lacking RIPK1 specifically in β-cells (Ripk1β-KO mice) or expressing a kinase-dead version of RIPK1 (Ripk1D138N mice), respectively. These mice were challenged with streptozotocin, a model of autoimmune diabetes. Moreover, Ripk1β-KO mice were further challenged with a high-fat diet to induce hyperglycemia. For mechanistic studies, pancreatic islets were subjected to various killing and sensitising agents.

RESULTS

Inhibition of RIPK1 kinase activity (Ripk1D138N mice) did not affect the onset and progression of hyperglycemia in a type 1 diabetes model. Moreover, the absence of RIPK1 expression in β-cells did not affect normoglycemia under basal conditions or hyperglycemia under diabetic challenges. Ex vivo, primary pancreatic islets are not sensitised to TNF-induced apoptosis and necroptosis in the absence of RIPK1. Intriguingly, we found that pancreatic islets display high levels of the antiapoptotic cellular FLICE-inhibitory protein (cFLIP) and low levels of apoptosis (Caspase-8) and necroptosis (RIPK3) components. Cycloheximide treatment, which led to a reduction in cFLIP levels, rendered primary islets sensitive to TNF-induced cell death which was fully blocked by caspase inhibition.

CONCLUSIONS

Unlike in many other cell types (e.g., epithelial, and immune), RIPK1 is not required for cell death regulation in β-cells under physiological conditions or diabetic challenges. Moreover, in vivo and in vitro evidence suggest that pancreatic β-cells do not undergo necroptosis but mainly caspase-dependent death in response to TNF. Last, our results show that β-cells have a distinct mode of regulation of TNF-cytotoxicity that is independent of RIPK1 and that may be highly dependent on cFLIP.

MLKL promotes hepatocarcinogenesis through inhibition of AMPK-mediated autophagy

The pseudokinase mixed lineage kinase domain-like (MLKL) is an essential component of the activation of the necroptotic pathway. Emerging evidence suggests that MLKL plays a key role in liver disease. However, how MLKL contributes to hepatocarcinogenesis has not been fully elucidated. Herein, we report that MLKL is upregulated in a diethylnitrosamine (DEN)-induced murine HCC model and is associated with human hepatocellular carcinomas. Hepatocyte-specific MLKL knockout suppresses the progression of hepatocarcinogenesis. Conversely, MLKL overexpression aggravates the initiation and progression of DEN-induced HCC. Mechanistic study reveals that deletion of MLKL significantly increases the activation of autophagy, thereby protecting against hepatocarcinogenesis. MLKL directly interacts with AMPKα1 and inhibits its activity independent of its necroptotic function. Mechanistically, MLKL serves as a bridging molecule between AMPKα1 and protein phosphatase 1B (PPM1B), thus enhancing the dephosphorylation of AMPKα1. Consistently, MLKL expression correlates negatively with AMPKα1 phosphorylation in HCC patients. Taken together, our findings highlight MLKL as a novel AMPK gatekeeper that plays key roles in inhibiting autophagy and driving hepatocarcinogenesis, suggesting that the MLKL-AMPKα1 axis is a potential therapeutic target for HCC.

How obesity affects adipocyte turnover

Cellular turnover is fundamental for tissue homeostasis and integrity. Adipocyte turnover, accounting for 4% of the total cellular mass turnover in humans, is essential for adipose tissue homeostasis during metabolic stress. In obesity, an altered adipose tissue microenvironment promotes adipocyte death. To clear dead adipocytes, macrophages are recruited and form a distinctive structure known as crown-like structure; subsequently, new adipocytes are generated from adipose stem and progenitor cells in the adipogenic niche to replace dead adipocytes. Accumulating evidence indicates that adipocyte death, clearance, and adipogenesis are sophisticatedly orchestrated during adipocyte turnover. In this Review, we summarize our current understandings of each step in adipocyte turnover, discussing its key players and regulatory mechanisms.

Probiotics, gut microbiome, and cardiovascular diseases: An update

Cardiovascular diseases (CVD) are one of the most challenging diseases and many factors have been demonstrated to affect their pathogenesis. One of the major factors that affect CVDs, especially atherosclerosis, is the gut microbiota (GM). Genetics play a key role in linking CVDs with GM, in addition to some environmental factors which can be either beneficial or harmful. The interplay between GM and CVDs is complex due to the numerous mechanisms through which microbial components and their metabolites can influence CVDs. Within this interplay, the immune system plays a major role, mainly based on the immunomodulatory effects of microbial dysbiosis and its resulting metabolites. The resulting modulation of chronic inflammatory processes was found to reduce the severity of CVDs and to maintain cardiovascular health. To better understand the specific roles of GM-related metabolites in this interplay, this review presents an updated perspective on gut metabolites related effects on the cardiovascular system, highlighting the possible benefits of probiotics in therapeutic strategies.

Non-Necroptotic Roles of MLKL in Diet-Induced Obesity, Liver Pathology, and Insulin Sensitivity: Insights from a High-Fat, High-Fructose, High-Cholesterol Diet Mouse Model

Chronic inflammation is a key player in metabolic dysfunction-associated fatty liver disease (MAFLD) progression. Necroptosis, an inflammatory cell death pathway, is elevated in MAFLD patients and mouse models, yet its role is unclear due to the diverse mouse models and inhibition strategies. In our study, we inhibited necroptosis by targeting mixed lineage kinase domain-like pseudokinase (MLKL), the terminal effector of necroptosis, in a high-fat, high-fructose, high-cholesterol (HFHFrHC) mouse model of diet-induced MAFLD. Despite the HFHFrHC diet upregulating MLKL (2.5-fold), WT mice livers showed no increase in necroptosis markers or associated proinflammatory cytokines. Surprisingly, Mlkl-/- mice experienced exacerbated liver inflammation without protection from diet-induced liver damage, steatosis, or fibrosis. In contrast, Mlkl+/- mice showed a significant reduction in these parameters that was associated with elevated Pparα and Pparγ levels. Both Mlkl-/- and Mlkl+/- mice on the HFHFrHC diet resisted diet-induced obesity, attributed to the increased beiging, enhanced oxygen consumption, and energy expenditure due to adipose tissue, and exhibited improved insulin sensitivity. These findings highlight the tissue-specific effects of MLKL on the liver and adipose tissue, and they suggest a dose-dependent effect of MLKL on liver pathology.

Sestrin2 maintains hepatic immune homeostasis and redox balance partially via inhibiting RIPK3-mediated necroptosis in metabolic dysfunction-associated steatohepatitis

BACKGROUND & AIMS

Necroptosis, a novel type of programmed cell death, is intricately associated with inflammatory response. Currently, most studies focus on the activation of necroptosis, while the mechanisms underlying the negative regulation of necroptosis remain poorly understood.

METHODS

The effects of sestrin2 (SESN2) overexpression or knockdown on the regulation of necroptosis were assessed in the TNFα/Smac-mimetic/Z-VAD-FMK (T/S/Z)-induced necroptosis model and palmitic acid (PA)-induced lipotoxicity model. Western-blot, co-Immunoprecipitation, Glutathione S-transferase pull-down, and confocal assays were employed to explore the regulatory mechanisms including protein-protein interactions and post-translational modification. Furthermore, we used GSK'872, a specific inhibitor of receptor-interacting serine/threonine-protein kinase (RIPK) 3, to evaluate the relationship between SESN2-related alterations and RIPK3-mediated necroptosis in T/S/Z-induced necroptosis model, PA-induced lipotoxicity model, and high-fat high-cholesterol diet (HFHCD)-induced non-alcoholic steatohepatitis model.

RESULTS

Our findings revealed that SESN2 was upregulated under conditions that induce necroptosis and functioned as a negative regulator of necroptosis. High levels of SESN2 could equipped hepatocytes with the ability to defend against necroptotic inflammation and oxidative stress. Mechanistically, SESN2 interacted with RIPK3 and tuned down necroptosis by inhibiting the phosphorylation of RIPK3, promoting the ubiquitination of RIPK3, and preventing the formation of the RIPK1/RIPK3 necrosome. The depletion of SESN2 resulted in excessive necroptosis, accompanied by increased fat accumulation, inflammation, and oxidative stress in the experimental steatohepatitis model. Blocking necroptosis by GSK'872 reduced the liberation of pro-inflammatory cytokines and reactive oxygen species generation, but not hepatocyte fat deposition, in both PA-treated SESN2 knockout cells and HFHCD-fed SESN2 knockout mice, suggesting that the activation of RIPK3-mediated necroptosis may partially account for the hyperinflammation and excessive oxidative stress induced by SESN2 deficiency.

CONCLUSION

Our results suggested that SESN2 inhibited RIPK3-mediated necroptosis; this regulation is an important for the immune homeostasis and the redox balance in the liver.

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.

Non-Necroptotic Roles of MLKL in Diet-Induced Obesity, Liver Pathology, and Insulin Sensitivity: Insights from a High Fat, High Fructose, High Cholesterol Diet Mouse Model

Chronic inflammation is a key player in metabolic dysfunction-associated fatty liver disease (MAFLD) progression. Necroptosis, an inflammatory cell death pathway, is elevated in MAFLD patients and mouse models, yet its role is unclear due to diverse mouse models and inhibition strategies. In our study, we inhibited necroptosis by targeting mixed lineage kinase domain like pseudokinase (MLKL), the terminal effector of necroptosis, in a high-fat, high-fructose, high-cholesterol (HFHFrHC) mouse model of diet-induced MAFLD mouse model. Despite HFHFrHC diet upregulating MLKL (2.5-fold), WT mice livers showed no increase in necroptosis markers or associated proinflammatory cytokines. Surprisingly, Mlkl -/- mice experienced exacerbated liver inflammation without protection from diet-induced liver damage, steatosis, or fibrosis. In contrast, Mlkl +/- mice showed significant reduction in these parameters that was associated with elevated Pparα and Pparγ levels. Both Mlkl -/- and Mlkl +/- mice on HFHFrHC diet resisted diet-induced obesity, attributed to increased beiging, enhanced oxygen consumption and energy expenditure due to adipose tissue, and exhibited improved insulin sensitivity. These findings highlight the tissue specific effects of MLKL on the liver and adipose tissue, and suggest a dose-dependent effect of MLKL on liver pathology.

Targeting necroptosis in fibrosis

Necroptosis, a type of programmed cell death that resembles necrosis, is now known to depend on a different molecular mechanism from apoptosis, according to several recent studies. Many efforts have reported the possible influence of necroptosis in human disorders and concluded the crucial role in the pathophysiology of various diseases, including liver diseases, renal injuries, cancers, and others. Fibrosis is the most common end-stage pathological cascade of several chronic inflammatory disorders. In this review, we explain the impact of necroptosis and fibrosis, for which necroptosis has been demonstrated to be a contributing factor. We also go over the inhibitors of necroptosis and how they have been applied to fibrosis models. This review helps to clarify the role of necroptosis in fibrosis and will encourage clinical efforts to target this pathway of programmed cell death.

Investigating EGF and PAG1 as necroptosis-related biomarkers for diabetic nephropathy: an in silico and in vitro validation study

The current study aims to understand the mechanisms behind regulated cell death (RCD) in diabetic nephropathy and identify related biomarkers through bioinformatics and experimental validation. Datasets of bulk and single-cell RNA sequencing were obtained from public databases and analyzed using gene set variation analysis (GSVA) with gene sets related to RCD, including autophagy, necroptosis, pyroptosis, apoptosis, and ferroptosis. RCD-related gene biomarkers were identified using weighted gene correlation network analysis (WGCNA). The results were verified through experiments with an independent cohort and in vitro experiments. The GSVA revealed higher necroptosis scores in diabetic nephropathy. Three necroptosis-related biomarkers, EGF, PAG1, and ZFP36, were identified and showed strong diagnostic ability for diabetic kidney disease. In vitro experiments showed high levels of necroptotic markers in HK-2 cells treated with high glucose. Bioinformatics and experimental validation have thus identified EGF and PAG1 as necroptosis-related biomarkers for diabetic nephropathy.

Alterations in gut microbiota and host transcriptome of patients with coronary artery disease

BACKGROUND

Coronary artery disease (CAD) is a widespread heart condition caused by atherosclerosis and influences millions of people worldwide. Early detection of CAD is challenging due to the lack of specific biomarkers. The gut microbiota and host-microbiota interactions have been well documented to affect human health. However, investigation that reveals the role of gut microbes in CAD is still limited. This study aims to uncover the synergistic effects of host genes and gut microbes associated with CAD through integrative genomic analyses.

RESULTS

Herein, we collected 52 fecal and 50 blood samples from CAD patients and matched controls, and performed amplicon and transcriptomic sequencing on these samples, respectively. By comparing CAD patients with health controls, we found that dysregulated gut microbes were significantly associated with CAD. By leveraging the Random Forest method, we found that combining 20 bacteria and 30 gene biomarkers could distinguish CAD patients from health controls with a high performance (AUC = 0.92). We observed that there existed prominent associations of gut microbes with several clinical indices relevant to heart functions. Integration analysis revealed that CAD-relevant gut microbe genus Fusicatenibacter was associated with expression of CAD-risk genes, such as GBP2, MLKL, and CPR65, which is in line with previous evidence (Tang et al., Nat Rev Cardiol 16:137-154, 2019; Kummen et al., J Am Coll Cardiol 71:1184-1186, 2018). In addition, the upregulation of immune-related pathways in CAD patients were identified to be primarily associated with higher abundance of genus Blautia, Eubacterium, Fusicatenibacter, and Monoglobus.

CONCLUSIONS

Our results highlight that dysregulated gut microbes contribute risk to CAD by interacting with host genes. These identified microbes and interacted risk genes may have high potentials as biomarkers for CAD.

Non-apoptotic programmed cell deaths in diabetic pulmonary dysfunction: the new side of advanced glycation end products

Diabetes mellitus (DM) is a chronic metabolic disorder that affects multiple organs and systems, including the pulmonary system. Pulmonary dysfunction in DM patients has been observed and studied for years, but the underlying mechanisms have not been fully understood. In addition to traditional mechanisms such as the production and accumulation of advanced glycation end products (AGEs), angiopathy, tissue glycation, oxidative stress, and systemic inflammation, recent studies have focused on programmed cell deaths (PCDs), especially the non-apoptotic ones, in diabetic pulmonary dysfunction. Non-apoptotic PCDs (NAPCDs) including autophagic cell death, necroptosis, pyroptosis, ferroptosis, and copper-induced cell death have been found to have certain correlations with diabetes and relevant complications. The AGE-AGE receptor (RAGE) axis not only plays an important role in the traditional pathogenesis of diabetes lung disease but also plays an important role in non-apoptotic cell death. In this review, we summarize novel studies about the roles of non-apoptotic PCDs in diabetic pulmonary dysfunction and focus on their interactions with the AGE-RAGE axis.

Multiple forms of cell death: A focus on the PI3K/AKT pathway

Cell death is a natural biological process that occurs in living organisms. Since 1963, extensive research has shed light on the occurrence, progress, and final outcome of cell death. According to different cell phenotypes, it is classified into different types, including apoptosis, pyroptosis, necroptosis, autophagy, ferroptosis, cuproptosis, and so on. However, regardless of the form of cell death, what we ultimately expect is the disappearance of abnormal cells, such as tumor cells, while normal cells survive. As a result, it is vital to investigate the details of cell death, including death triggers, potent regulators, and executioners. Although significant progress has been made in understanding molecular pathways of cell death, many aspects remain unclear because of the complex regulatory networks in cells. Among them, the phosphoinositide-3-kinase (PI3K)/protein kinase B(AKT) pathway is discovered to be a crucial regulator of the cell death process. AKT, as a proto-oncogene, has become a major focus of attention in the medical community due to its role in regulating a multiplicity of cellular functions counting metabolism, immunity, proliferation, survival, transcription, and protein synthesis. Here, we explored the connection between the PI3K/AKT pathway and cell death, aiming to enhance our comprehension of the mechanism underlying this process. Such knowledge may pave the way for the subsequent development of more effective disease treatments, such as finding suitable targets for drug intervention.

Targeting Endothelial Necroptosis Disrupts Profibrotic Endothelial-Hepatic Stellate Cells Crosstalk to Alleviate Liver Fibrosis in Nonalcoholic Steatohepatitis

Chronic liver diseases affect over a billion people worldwide and often lead to fibrosis. Nonalcoholic steatohepatitis (NASH), a disease paralleling a worldwide surge in metabolic syndromes, is characterized by liver fibrosis, and its pathogenesis remains largely unknown, with no effective treatment available. Necroptosis has been implicated in liver fibrosis pathogenesis. However, there is a lack of research on necroptosis specific to certain cell types, particularly the vascular system, in the context of liver fibrosis and NASH. Here, we employed a mouse model of NASH in combination with inducible gene knockout mice to investigate the role of endothelial necroptosis in NASH progression. We found that endothelial cell (EC)-specific knockout of mixed lineage kinase domain-like protein (MLKL), a critical executioner involved in the disruption of cell membranes during necroptosis, alleviated liver fibrosis in the mouse NASH model. Mechanistically, EC-specific deletion of Mlkl mitigated the activation of TGFβ/Smad 2/3 pathway, disrupting the pro-fibrotic crosstalk between endothelial cells and hepatic stellate cells (HSCs). Our findings highlight endothelial MLKL as a promising molecular target for developing therapeutic interventions for NASH.

Analysis of Signaling Pathways of Necroptotic and Pyroptotic Cell Death in the Hearts of Rats With Type 2 Diabetes Mellitus

Diabetes mellitus is known to produce various cell-damaging events and thereby underlie heart dysfunction and remodeling. However, very little is known about its inflammation-associated pathomechanisms due to necrosis-like cell death. For this purpose, we aimed to investigate signaling pathways of necroptosis and pyroptosis, known to produce plasma membrane rupture with the resultant promotion of inflammation. One-year old Zucker diabetic fatty (ZDF) rats did not exhibit significant heart dysfunction as revealed by echocardiographic measurement. On the other hand, there was a decrease in heart rate due to diabetes. Immunoblotting analysis showed that the left ventricles of ZDF rats overexpress neither the main necroptotic proteins including receptor-interacting protein kinase 3 (RIP3) and mixed lineage domain kinase-like pseudokinase (MLKL), nor the pyroptotic regulators including NLR family pyrin domain containing 3 protein (NLRP3), caspase-1, interleukin-1 beta (IL-1beta and the N-terminal gasdermin D (GSDMD-N). On the other hand, the increased activation of the RIP3 kinase due to phosphorylation was found in such hearts. In summary, we showed for the first time that the activation of cardiac RIP3 is upregulated due to disturbances in glucose metabolism which, however, did not proceed to necrosis-like cell death. These data can indicate that the activated RIP3 might also underlie other pleiotropic, non-necroptotic signaling pathways under basal conditions.

Celastrol inhibits necroptosis by attenuating the RIPK1/RIPK3/MLKL pathway and confers protection against acute pancreatitis in mice

Necroptosis is a necrotic form of regulated cell death, which is primarily mediated by the receptor-interacting protein kinase 1 (RIPK1), RIPK3, and mixed lineage kinase domain-like (MLKL) pathway in a caspase-independent manner. Necroptosis has been found to occur in virtually all tissues and diseases evaluated, including pancreatitis. Celastrol, a pentacyclic triterpene extracted from the roots of Tripterygium wilfordii (thunder god vine), possesses potent anti-inflammatory and anti-oxidative activities. Yet, it is unclear whether celastrol has any effects on necroptosis and necroptotic-related diseases. Here we showed that celastrol significantly suppressed necroptosis induced by lipopolysaccharide (LPS) plus pan-caspase inhibitor (IDN-6556) or by tumor-necrosis factor-α in combination with LCL-161 (Smac mimetic) and IDN-6556 (TSI). In these in vitro cellular models, celastrol inhibited the phosphorylation of RIPK1, RIPK3, and MLKL and the formation of necrosome during necroptotic induction, suggesting its possible action on upstream signaling of the necroptotic pathway. Consistent with the known role of mitochondrial dysfunction in necroptosis, we found that celastrol significantly rescued TSI-induced loss of mitochondrial membrane potential. TSI-induced intracellular and mitochondrial reactive oxygen species (mtROS), which are involved in the autophosphorylation of RIPK1 and recruitment of RIPK3, were significantly attenuated by celastrol. Moreover, in a mouse model of acute pancreatitis that is associated with necroptosis, celastrol administration significantly reduced the severity of caerulein-induced acute pancreatitis accompanied by decreased phosphorylation of MLKL in pancreatic tissues. Collectively, celastrol can attenuate the activation of RIPK1/RIPK3/MLKL signaling likely by attenuating mtROS production, thereby inhibiting necroptosis and conferring protection against caerulein-induced pancreatitis in mice.

Necrostatin-1S mitigates type-2 diabetes-associated cognitive decrement and lipotoxicity-induced neuro-microglia changes through p-RIPK-RIPK3-p-MLKL axis

Type-2 diabetes mellitus (T2DM) is associated with neuroinflammation and cognitive decrement. Necroptosis programmed necrosis is emerging as the major contributing factor to central changes. It is best characterized by the upregulation of p-RIPK(Receptor Interacting Kinase), p-RIPK3, and the phosphorylated-MLKL (mixed-lineage kinase domain-like protein). The present study aims to evaluate the neuroprotective effect of Necrostatin (Nec-1S), a p-RIPK inhibitor, on cognitive changes in the experimental T2DM model in C57BL/6 mice and lipotoxicity-induced neuro-microglia changes in neuro2A and BV2 cells. Further, the study also explores whether Nec-1S would restore mitochondrial and autophago-lysosomal function.T2DM was developed in mice by feeding them a high-fat diet (HFD) for 16 weeks and injecting a single dose of streptozotocin (100 mg/kg, i.p) on the 12^th week. Nec-1S was administered for 3 weeks at (10 mg/kg, i.p) once every 3 days. Lipotoxicity was induced in neuro2A, and BV2 cells using 200 µM palmitate/bovine serum albumin conjugate. Nec-1S (50 µM), and GSK-872(10 µM) were further used to explore their relative effect. The neurobehavioral performance was assessed using mazes and task-assisted performance tests. To decipher the hypothesis plasma parameters, western blot, immunofluorescence, microscopy, and quantitative reverse transcription-PCR studies were carried out. The Nec-1S treatment restored cognitive performance and reduced the p-RIPK-p-RIPK3-p-MLKL mediated neuro-microglia changes in the brain and in cells as well, under lipotoxic stress. Nec-1S reduced tau, and amyloid oligomer load. Moreover, Nec-1S restored mitochondrial function and autophago-lysosome clearance. The findings highlight the central impact of metabolic syndrome and how Nes-1S, by acting as a multifaceted agent, improved central functioning.

Macrophage-derived MLKL in alcohol-associated liver disease: Regulation of phagocytosis

BACKGROUND AND AIMS

Mixed lineage kinase domain-like pseudokinase (MLKL), a key terminal effector of necroptosis, also plays a role in intracellular vesicle trafficking that is critical for regulating liver inflammation and injury in alcohol-associated liver disease (ALD). Although receptor interacting protein kinase 3 (Rip3)-/- mice are completely protected from ethanol-induced liver injury, Mlkl-/- mice are only partially protected. Therefore, we hypothesized that cell-specific functions of MLKL may contribute to ethanol-induced injury.

APPROACH AND RESULTS

Bone marrow transplants between Mlkl-/- mice and littermates were conducted to distinguish the role of myeloid versus nonmyeloid Mlkl in the Gao-binge model of ALD. Ethanol-induced hepatic injury, steatosis, and inflammation were exacerbated in Mlkl-/- →wild-type (WT) mice, whereas Mlkl deficiency in nonmyeloid cells (WT→ Mlkl-/- ) had no effect on Gao-binge ethanol-induced injury. Importantly, Mlkl deficiency in myeloid cells exacerbated ethanol-mediated bacterial burden and accumulation of immune cells in livers. Mechanistically, challenging macrophages with lipopolysaccharide (LPS) induced signal transducer and activator of transcription 1-mediated expression and phosphorylation of MLKL, as well as translocation and oligomerization of MLKL to intracellular compartments, including phagosomes and lysosomes but not plasma membrane. Importantly, pharmacological or genetic inhibition of MLKL suppressed the phagocytic capability of primary mouse Kupffer cells (KCs) at baseline and in response to LPS with/without ethanol as well as peripheral monocytes isolated from both healthy controls and patients with alcohol-associated hepatitis. Further, in vivo studies revealed that KCs of Mlkl-/- mice phagocytosed fewer bioparticles than KCs of WT mice.

CONCLUSION

Together, these data indicate that myeloid MLKL restricts ethanol-induced liver inflammation and injury by regulating hepatic immune cell homeostasis and macrophage phagocytosis.

MLKL signaling regulates macrophage polarization in acute pancreatitis through CXCL10

Acute pancreatitis (AP) is a disease characterized by local and systemic inflammation with an increasing incidence worldwide. Receptor-interacting serine/threonine protein kinase 3 (RIPK3), mixed-lineage kinase domain-like protein (MLKL), and innate immune cell macrophages have been reported to be involved in the pathogenesis of AP. However, the mechanisms by which RIPK3 and MLKL regulate pancreatic injury, as well as the interactions between injured pancreatic acinar cells and infiltrating macrophages in AP, remain poorly defined. In the present study, experimental pancreatitis was induced in C57BL/6J, Ripk3-/- and Mlkl-/- mice by cerulein plus lipopolysaccharide in vivo, and primary pancreatic acinar cells were also isolated to uncover cellular mechanisms during cerulein stimulation in vitro. The results showed that MLKL and its phosphorylated protein p-MLKL were upregulated in the pancreas of the mouse AP model and cerulein-treated pancreatic acinar cells, independent of its canonical upstream molecule Ripk3, and appeared to function in a cell death-independent manner. Knockout of Mlkl attenuated AP in mice by reducing the polarization of pancreatic macrophages toward the M1 phenotype, and this protective effect was partly achieved by reducing the secretion of CXCL10 from pancreatic acinar cells, whereas knockout of Ripk3 did not. In vitro neutralization of CXCL10 impaired the pro-M1 ability of the conditioned medium of cerulein-treated pancreatic acinar cells, whereas in vivo neutralization of CXCL10 reduced the polarization of pancreatic macrophages toward M1 and the severity of AP in mice. These findings suggested that targeting the MLKL-CXCL10-macrophage axis might be a promising strategy for the treatment of AP.

Cell death and inflammation during obesity: "Know my methods, WAT(son)"

Obesity is a state of low-grade chronic inflammation that causes multiple metabolic diseases. During obesity, signalling via cytokines of the TNF family mediate cell death and inflammation within the adipose tissue, eventually resulting in lipid spill-over, glucotoxicity and insulin resistance. These events ultimately lead to ectopic lipid deposition, glucose intolerance and other metabolic complications with life-threatening consequences. Here we review the literature on how inflammatory responses affect metabolic processes such as energy homeostasis and insulin signalling. This review mainly focuses on the role of cell death in the adipose tissue as a key player in metabolic inflammation.

The web of death: the expanding complexity of necroptotic signaling

The past decade has seen the emergence of the necroptosis programmed cell death pathway as an important contributor to the pathophysiology of myriad diseases. The receptor interacting protein kinase (RIPK)1 and RIPK3, and the pseudokinase executioner protein, mixed lineage kinase domain-like (MLKL), have grown to prominence as the core pathway components. Depending on cellular context, these proteins also serve as integrators of signals, such as post-translational modifications and protein or metabolite interactions, adding layers of complexity to pathway regulation. Here, we describe the emerging picture of the web of proteins that tune necroptotic signal transduction and how these events have diverged across species, presumably owing to selective pressures of pathogens upon the RIPK3-MLKL protein pair.

Inhibition of RIPK1 kinase does not affect diabetes development: β-Cells survive RIPK1 activation

OBJECTIVES

Type 1 diabetes (T1D) is caused by progressive immune-mediated loss of insulin-producing β-cells. Inflammation is detrimental to β-cell function and survival, moreover, both apoptosis and necrosis have been implicated as mechanisms of β-cell loss in T1D. The receptor interacting serine/threonine protein kinase 1 (RIPK1) promotes inflammation by serving as a scaffold for NF-κB and MAPK activation, or by acting as a kinase that triggers apoptosis or necroptosis. It is unclear whether RIPK1 kinase activity is involved in T1D pathology. In the present study, we investigated if absence of RIPK1 activation would affect the susceptibility to immune-mediated diabetes or diet induced obesity (DIO).

METHODS

The RIPK1 knockin mouse line carrying a mutation mimicking serine 25 phosphorylation (Ripk1^S25D/S25D), which abrogates RIPK1 kinase activity, was utilized to assess the in vivo role of RIPK1 in immune-mediated diabetes or diet induced obesity (DIO). In vitro, β-cell death and RIPK1 kinase activity was analysed in conditions known to induce RIPK1-dependent apoptosis/necroptosis.

RESULTS

We demonstrate that Ripk1^S25D/S25D mice presented normal glucose metabolism and β-cell function. Furthermore, immune-mediated diabetes and DIO were not different between Ripk1^S25D/S25D and Ripk1^+/+ mice. Despite strong activation of RIPK1 kinase and other necroptosis effectors (RIPK3 and MLKL) by TNF+BV6+zVAD, no cell death was observed in mouse islets nor human β-cells.

CONCLUSION

Our results contrast recent literature showing that most cell types undergo necroptosis following RIPK1 kinase activation. This peculiarity may reflect an adaptation to the inability of β-cells to proliferate and self-renewal.

A Review of Defatting Strategies for Non-Alcoholic Fatty Liver Disease

Non-alcoholic fatty liver disease is a huge cause of chronic liver failure around the world. This condition has become more prevalent as rates of metabolic syndrome, type 2 diabetes, and obesity have also escalated. The unfortunate outcome for many people is liver cirrhosis that warrants transplantation or being unable to receive a transplant since many livers are discarded due to high levels of steatosis. Over the past several years, however, a great deal of work has gone into understanding the pathophysiology of this disease as well as possible treatment options. This review summarizes various defatting strategies including in vitro use of pharmacologic agents, machine perfusion of extracted livers, and genomic approaches targeting specific proteins. The goal of the field is to reduce the number of necessary transplants and expand the pool of organs available for use.

Early fish domestication affects methylation of key genes involved in the rapid onset of the farmed phenotype

Animal domestication is a process of environmental modulation and artificial selection leading to permanent phenotypic modifications. Recent studies showed that phenotypic changes occur very early in domestication, i.e., within the first generation in captivity, which raises the hypothesis that epigenetic mechanisms may play a critical role on the early onset of the domestic phenotype. In this context, we applied reduced representation bisulphite sequencing to compare methylation profiles between wild Nile tilapia females and their offspring reared under farmed conditions. Approximately 700 differentially methylated CpG sites were found, many of them associated not only with genes involved in muscle growth, immunity, autophagy and diet response but also related to epigenetic mechanisms, such as RNA methylation and histone modifications. This bottom-up approach showed that the phenotypic traits often related to domestic animals (e.g., higher growth rate and different immune status) may be regulated epigenetically and prior to artificial selection on gene sequences. Moreover, it revealed the importance of diet in this process, as reflected by differential methylation patterns in genes critical to fat metabolism. Finally, our study highlighted that the TGF-β1 signalling pathway may regulate and be regulated by several differentially methylated CpG-associated genes. This could be an important and multifunctional component in promoting adaptation of fish to a domestic environment while modulating growth and immunity-related traits.

The necroptosis-inducing pseudokinase mixed lineage kinase domain-like regulates the adipogenic differentiation of pre-adipocytes

Receptor-interacting protein kinase-3 (RIPK3) and mixed lineage kinase domain-like (MLKL) proteins are key regulators of necroptosis, a highly pro-inflammatory mode of cell death, which has been involved in various human diseases. Necroptotic-independent functions of RIPK3 and MLKL also exist, notably in the adipose tissue but remain poorly defined. Using knock-out (KO) cell models, we investigated the role of RIPK3 and MLKL in adipocyte differentiation. Mlkl-KO abolished white adipocyte differentiation via a strong expression of Wnt10b, a ligand of the Wnt/β-catenin pathway, and a downregulation of genes involved in lipid metabolism. This effect was not recapitulated by the ablation of Ripk3. Conversely, Mlkl and Ripk3 deficiencies did not block beige adipocyte differentiation. These findings indicate that RIPK3 and MLKL have distinct roles in adipogenesis. The absence of MLKL blocks the differentiation of white, but not beige, adipocytes highlighting the therapeutic potential of MLKL inhibition in obesity.

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Mlkl deficiency inhibits white, but not beige, adipocyte differentiation

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MLKL deficiency suppresses the expression of master regulators of adipogenesis

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Mlkl deficiency up-regulates Wnt10b expression

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Ripk3 deficiency does not alter white and beige adipocyte differentiation

Targeted therapeutics and novel signaling pathways in non-alcohol-associated fatty liver/steatohepatitis (NAFL/NASH)

Non-alcohol-associated fatty liver/steatohepatitis (NAFL/NASH) has become the leading cause of liver disease worldwide. NASH, an advanced form of NAFL, can be progressive and more susceptible to developing cirrhosis and hepatocellular carcinoma. Currently, lifestyle interventions are the most essential and effective strategies for preventing and controlling NAFL without the development of fibrosis. While there are still limited appropriate drugs specifically to treat NAFL/NASH, growing progress is being seen in elucidating the pathogenesis and identifying therapeutic targets. In this review, we discussed recent developments in etiology and prospective therapeutic targets, as well as pharmacological candidates in pre/clinical trials and patents, with a focus on diabetes, hepatic lipid metabolism, inflammation, and fibrosis. Importantly, growing evidence elucidates that the disruption of the gut-liver axis and microbe-derived metabolites drive the pathogenesis of NAFL/NASH. Extracellular vesicles (EVs) act as a signaling mediator, resulting in lipid accumulation, macrophage and hepatic stellate cell activation, further promoting inflammation and liver fibrosis progression during the development of NAFL/NASH. Targeting gut microbiota or EVs may serve as new strategies for the treatment of NAFL/NASH. Finally, other mechanisms, such as cell therapy and genetic approaches, also have enormous therapeutic potential. Incorporating drugs with different mechanisms and personalized medicine may improve the efficacy to better benefit patients with NAFL/NASH.

Loss of MLKL ameliorates liver fibrosis by inhibiting hepatocyte necroptosis and hepatic stellate cell activation

Background: Liver fibrosis affects millions of people worldwide without an effective treatment. Although multiple cell types in the liver contribute to the fibrogenic process, hepatocyte death is considered to be the trigger. Multiple forms of cell death, including necrosis, apoptosis, and necroptosis, have been reported to co-exist in liver diseases. Mixed lineage kinase domain-like protein (MLKL) is the terminal effector in necroptosis pathway. Although necroptosis has been reported to play an important role in a number of liver diseases, the function of MLKL in liver fibrosis has yet to be unraveled. Methods and Results: Here we report that MLKL level is positively correlated with a number of fibrotic markers in liver samples from both patients with liver fibrosis and animal models. Mlkl deletion in mice significantly reduces clinical symptoms of CCl₄- and bile duct ligation (BDL) -induced liver injury and fibrosis. Further studies indicate that Mlkl^-/- blocks liver fibrosis by reducing hepatocyte necroptosis and hepatic stellate cell (HSC) activation. AAV8-mediated specific knockdown of Mlkl in hepatocytes remarkably alleviates CCl₄-induced liver fibrosis in both preventative and therapeutic ways. Conclusion: Our results show that MLKL-mediated signaling plays an important role in liver damage and fibrosis, and targeting MLKL might be an effective way to treat liver fibrosis.

C1q/tumour necrosis factor-related protein-3 alleviates high-glucose-induced lipid accumulation and necroinflammation in renal tubular cells by activating the adenosine monophosphate-activated protein kinase pathway

Lipid accumulation and progressive necroinflammation play pivotal roles in the development of diabetic nephropathy. C1q tumour necrosis factor-related protein-3 (CTRP3) is an adipokine with pleiotropic functions in cell proliferation, glucose and lipid metabolism, and inflammation. However, the mechanism and involvement of CTRP3 in lipid metabolism and the necroinflammation of renal tubular cells remain unclear. Here, we report that CTRP3 expression decreased in a time- and concentration-dependent manner in high glucose-stimulated HK-2 cells. We noted that the overexpression of CTRP3 or recombinant CTRP3 (rCTRP3) treatment prevented high glucose-induced lipid accumulation by inhibiting the expression of sterol regulatory element-binding protein-1 and increasing the expression of peroxisome proliferator-activated receptor-α and ATP-binding cassette A1. Moreover, the nucleotide-binding oligomerisation domain-like receptor protein 3-mediated inflammatory response and mixed lineage kinase domain-like protein-dependent necroinflammation were inhibited by CTRP3 overexpression or rCTRP3 treatment in HK-2 cells cultured in high glucose. Furthermore, lipotoxicity-induced by palmitic acid was found to be involved in necroinflammation in HK-2 cells, and CTRP3 displayed the same protective effect. CTRP3 also activated the adenosine monophosphate-activated protein kinase (AMPK) pathway, whereas adenine 9-β-D-arabinofuranoside, an AMPK inhibitor, replicated the protective effects of CTRP3. Besides, using kidney biopsies from patients with diabetes, we found that decreased CTRP3 expression was accompanied by increased lipid deposition, as well as the structural and functional injury of renal tubular cells. Our findings demonstrate that CTRP3 affects lipid metabolism and necroinflammation in renal tubular cells via the AMPK signalling pathway. Thus, CTRP3 may be a potential therapeutic target in diabetic renal injury.

Suppression of RIP1 activity via S415 dephosphorylation ameliorates obesity-related hepatic insulin resistance

OBJECTIVE

Receptor-interacting serine/threonine-protein kinase 1 (RIP1) is a well-documented key regulator of TNFα-mediated inflammation, apoptosis, and necroptosis, which contribute to the development of obesity-related metabolic diseases such as nonalcoholic steatohepatitis. However, the mechanism regarding how RIP1 influences obesity-related insulin resistance remains elusive.

METHODS

Primary hepatocytes with necrostatin 1 treatment or RIP1 expression were exposed to palmitic acid (PA), prior to the examination of cellular insulin signaling. Phosphorylation sites of RIP1 were detected by liquid chromatography with tandem mass spectrometry, and RIP1 variants with mutated phosphorylation sites were overexpressed in hepatocytes to identify the specific residue that influenced the RIP1-mediated insulin resistance. Adenovirus expressing RIP1 (S415A) mutant were administered into diet-induced obese mice to assess the effects on insulin sensitivity.

RESULTS

This study uncovered an aberrant increase in RIP1 activity during the development of obesity-induced insulin resistance. Inhibition of RIP1 activity with necrostatin 1 ameliorated PA- or high-fat diet-caused hepatic insulin resistance. With liquid chromatography with tandem mass spectrometry analysis and mutagenesis screening, S415, a novel phosphorylation site of RIP1, was identified to be responsible for RIP1-mediated insulin resistance. Loss-of-function mutation of S415 efficiently blunted RIP1-evoked insulin resistance in PA-treated hepatocytes or diet-induced obese mice.

CONCLUSIONS

These findings highlight the diabetogenic role of RIP1 S415 and propose RIP1 as a promising therapeutic target for type 2 diabetes.

Rare catastrophes and evolutionary legacies: human germline gene variants in MLKL and the necroptosis signalling pathway

Programmed cell death has long been characterised as a key player in the development of human disease. Necroptosis is a lytic form of programmed cell death that is universally mediated by the effector protein mixed lineage kinase domain-like (MLKL), a pseudokinase. MLKL's activating kinase, receptor interacting protein kinase 3 (RIPK3), is itself activated within context specific scaffolds of receptor interacting protein kinase 1 (RIPK1), Z-DNA Binding Protein-1 (ZBP1) or TIR domain-containing adaptor inducing interferon-β (TRIF). These core necroptosis modulating proteins have been comprehensively revealed as potent drivers and suppressors of disease in inbred mouse strains. However, their roles in human disease within the 'real world' of diverse genetic backgrounds, natural infection and environmental challenges remains less well understood. Over 20 unique disease-associated human germline gene variants in this core necroptotic machinery have been reported in the literature and human clinico-genetics databases like ClinVar to date. In this review, we provide an overview of these human gene variants, with an emphasis on those encoding MLKL. These experiments of nature have the potential to not only enrich our understanding of the basic biology of necroptosis, but offer important population level insights into which clinical indications stand to benefit most from necroptosis-targeted drugs.

Liver Fibrosis-From Mechanisms of Injury to Modulation of Disease

The Transregional Collaborative Research Center "Organ Fibrosis: From Mechanisms of Injury to Modulation of Disease" (referred to as SFB/TRR57) was funded for 13 years (2009-2021) by the German Research Council (DFG). This consortium was hosted by the Medical Schools of the RWTH Aachen University and Bonn University in Germany. The SFB/TRR57 implemented combined basic and clinical research to achieve detailed knowledge in three selected key questions: (i) What are the relevant mechanisms and signal pathways required for initiating organ fibrosis? (ii) Which immunological mechanisms and molecules contribute to organ fibrosis? and (iii) How can organ fibrosis be modulated, e.g., by interventional strategies including imaging and pharmacological approaches? In this review we will summarize the liver-related key findings of this consortium gained within the last 12 years on these three aspects of liver fibrogenesis. We will highlight the role of cell death and cell cycle pathways as well as nutritional and iron-related mechanisms for liver fibrosis initiation. Moreover, we will define and characterize the major immune cell compartments relevant for liver fibrogenesis, and finally point to potential signaling pathways and pharmacological targets that turned out to be suitable to develop novel approaches for improved therapy and diagnosis of liver fibrosis. In summary, this review will provide a comprehensive overview about the knowledge on liver fibrogenesis and its potential therapy gained by the SFB/TRR57 consortium within the last decade. The kidney-related research results obtained by the same consortium are highlighted in an article published back-to-back in Frontiers in Medicine.

Interplay of Oxidative Stress and Necrosis-like Cell Death in Cardiac Ischemia/Reperfusion Injury: A Focus on Necroptosis

Extensive research work has been carried out to define the exact significance and contribution of regulated necrosis-like cell death program, such as necroptosis to cardiac ischemic injury. This cell damaging process plays a critical role in the pathomechanisms of myocardial infarction (MI) and post-infarction heart failure (HF). Accordingly, it has been documented that the modulation of key molecules of the canonical signaling pathway of necroptosis, involving receptor-interacting protein kinases (RIP1 and RIP3) as well as mixed lineage kinase domain-like pseudokinase (MLKL), elicit cardioprotective effects. This is evidenced by the reduction of the MI-induced infarct size, alleviation of myocardial dysfunction, and adverse cardiac remodeling. In addition to this molecular signaling of necroptosis, the non-canonical pathway, involving Ca2+/calmodulin-dependent protein kinase II (CaMKII)-mediated regulation of mitochondrial permeability transition pore (mPTP) opening, and phosphoglycerate mutase 5 (PGAM5)–dynamin-related protein 1 (Drp-1)-induced mitochondrial fission, has recently been linked to ischemic heart injury. Since MI and HF are characterized by an imbalance between reactive oxygen species production and degradation as well as the occurrence of necroptosis in the heart, it is likely that oxidative stress (OS) may be involved in the mechanisms of this cell death program for inducing cardiac damage. In this review, therefore, several observations from different studies are presented to support this paradigm linking cardiac OS, the canonical and non-canonical pathways of necroptosis, and ischemia-induced injury. It is concluded that a multiple therapeutic approach targeting some specific changes in OS and necroptosis may be beneficial in improving the treatment of ischemic heart disease.

Whole Grain Qingke Attenuates High-Fat Diet-Induced Obesity in Mice With Alterations in Gut Microbiota and Metabolite Profile

Whole grain Qingke (WGQK) displays anti-obesity and lipid-lowering properties; however, the underlying mechanism remains elusive. This study investigated the alteration of gut microbiota composition and metabolite profile induced by WGQK intervention in mice through the integration of 16S ribosomal RNA (rRNA) sequencing and an untargeted metabolomics study. C57BL/6J male mice were fed a normal control diet (NC), high-fat diet (HFD), and HFD plus 30% WGQK (HFD+QK) for 16 weeks. The WGQK intervention decreased body weight gain, glucose tolerance, and serum lipid levels, and alleviated liver function damage induced by HFD. Moreover, WGQK changed gut microbiota composition and enriched specific genera such as Akkermansia, Bifidobacterium, and Lactobacillus. Fecal metabolomics analysis indicated that WGQK enhanced the abundance of tryptophan metabolism-related metabolites (indole, 3-indoleacetic acid, indole acetic acid (IAA), 5-hydroxyindole-3-acetic acid), histidine metabolism-related metabolites (histamine), and some unsaturated fatty acids (oleic acid, 9,10-dihydroxy-12Z-octadecenoic acid, and alpha-linolenic acid). Spearman correlation analysis revealed that these metabolites were negatively correlated with obesity-related parameters and positively correlated with the gut genera enriched by WGQK. Moreover, WGQK promoted the expression of Cholesterol 7α-hydroxylase (CYP7A1) responsible for primary bile acids production, accompanied by a decline in intestinal FXR-FGF15 expression levels. The transcript levels of two genes associated with lipogenesis, such as lipid fatty acid synthase (FAS) and acetyl-CoA carboxylase (ACC) were also decreased in the HFD+QK group. Overall, our results suggest interactions between gut microbial shifts and host amino acid/lipid metabolism, and shed light on the mechanisms underlying the anti-obesity effect of WGQK.

Alternative Mitophagy Protects the Heart Against Obesity-Associated Cardiomyopathy

RATIONALE

Obesity-associated cardiomyopathy characterized by hypertrophy and mitochondrial dysfunction. Mitochondrial quality control mechanisms, including mitophagy, are essential for the maintenance of cardiac function in obesity-associated cardiomyopathy. However, autophagic flux peaks at around 6 weeks of high-fat diet (HFD) consumption and declines thereafter.

OBJECTIVE

We investigated whether mitophagy is activated during the chronic phase of cardiomyopathy associated with obesity (obesity cardiomyopathy) after general autophagy is downregulated and, if so, what the underlying mechanism and the functional significance are.

METHODS AND RESULTS

Mice were fed either a normal diet or a HFD (60 kcal% fat). Mitophagy, evaluated using Mito-Keima, was increased after 3 weeks of HFD consumption and continued to increase after conventional mechanisms of autophagy were inactivated, at least until 24 weeks. HFD consumption time-dependently upregulated both Ser555-phosphorylated Ulk1 (unc-51 like kinase 1) and Rab9 (Ras-related protein Rab-9) in the mitochondrial fraction. Mitochondria were sequestrated by Rab9-positive ring-like structures in cardiomyocytes isolated from mice after 20 weeks of HFD consumption, consistent with the activation of alternative mitophagy. Increases in mitophagy induced by HFD consumption for 20 weeks were abolished in cardiac-specific ulk1 knockout mouse hearts, in which both diastolic and systolic dysfunction were exacerbated. Rab9 S179A knock-in mice, in which alternative mitophagy is selectively suppressed, exhibited impaired mitophagy and more severe cardiac dysfunction than control mice following HFD consumption for 20 weeks. Overexpression of Rab9 in the heart increased mitophagy and protected against cardiac dysfunction during HFD consumption. HFD-induced activation of Rab9-dependent mitophagy was accompanied by upregulation of TFE3 (transcription factor binding to IGHM enhancer 3), which plays an essential role in transcriptional activation of mitophagy.

CONCLUSIONS

Ulk1-Rab9-dependent alternative mitophagy is activated during the chronic phase of HFD consumption and serves as an essential mitochondrial quality control mechanism, thereby protecting the heart against obesity cardiomyopathy.

Intermittent hypoxia aggravates non-alcoholic fatty liver disease via RIPK3-dependent necroptosis-modulated Nrf2/NFκB signaling pathway

AIMS

Hepatocyte necroptosis is a critical event in the progression of non-alcoholic fatty liver disease (NAFLD). Obstructive sleep apnea hypopnea syndrome (OSAHS) and chronic intermittent hypoxia (CIH) may be linked with the pathogenesis and the severity of NAFLD. However, the potential role of necroptosis in OSAHS-associated NAFLD has not been evaluated. The present study investigated whether IH could affect NAFLD progression through promoting receptor-interacting protein kinase-3 (RIPK3)-dependent necroptosis, oxidative stress, and inflammatory response, and further elucidated the underlying molecular mechanisms.

MAIN METHODS

LO2 cells were treated with palmitic acid (PA) and subjected to IH, and necroptosis, oxidative stress, and inflammation were assessed. The high-fat choline-deficient (HFCD)-fed mouse model was also used to assess the effects of CIH in experimental NAFLD in vivo.

KEY FINDINGS

In this study, we found that RIPK3-mediated necroptosis was activated both in the PA plus IH-treated LO2 cells and liver of HFCD/CIH mice, and which could trigger oxidative stress and inflammatory response by decreasing Nrf2 and increasing p-P65. RIPK3 downregulation significantly reduced hepatocyte necroptosis, and ameliorated oxidative stress and inflammation through modulating Nrf2/NFκB pathway in vitro and vivo. Similarly, pretreatment with TBHQ, an activator of Nrf2, effectively blocked the generation of oxidative productions and inflammatory cytokines. In addition, RIPK3 inhibitor GSK-872 or TBHQ administration obviously alleviated hepatic injury, including histology, transaminase activities, and triglyceride contents in vivo.

SIGNIFICANCE

IH aggravates NAFLD via RIPK3-dependent necroptosis-modulated Nrf2/NFκB signaling pathway, and which should be considered as a potential therapeutic strategy for the treatment of NAFLD with OSASH.

Mixed Lineage Kinase Domain-Like Pseudokinase (MLKL) Gene Expression in Human Atherosclerosis with and without Type 2 Diabetes Mellitus

Background

Mixed lineage kinase domain-like pseudokinase (MLKL), one of the main downstream components of the necroptosis or programmed necrosis has recently been demonstrated in advanced atherosclerotic lesions. However, its precise role in the atherosclerosis pathogenesis still requires more elucidation. Our study was set to delineate both the changes in peripheral MLKL gene expression and its influence on disease severity in atherosclerotic patients with and without type 2 diabetes mellitus.

Methods

The study involved 50 patients (20 non-diabetics and 30 diabetics) undergoing coronary artery bypass graft and 20 apparently healthy controls. Taqman RT-PCR was used to quantify MLKL mRNA expression levels, while ELISA was employed to estimate serum insulin and high sensitivity C-reactive protein (hsCRP) levels.

Results

Compared with the control group, MLKL gene was up regulated significantly in cardiovascular diseases (CVD; p ≤ 0.001). Higher MLKL expression was demonstrated in diabetic CVD group than non-diabetic group (p < 0.05). Correlation studies reported positive associations between MLKL and markers of dyslipidemia, inflammation, and insulin resistance. Multiple regression analysis revealed that FBG levels, hsCRP levels, and total white blood cells count were significant predictors for MLKL levels. Receiver operating characteristic curve showed a significant diagnostic value of MLKL for CVD. Moreover, regression analysis demonstrated that MLKL and hsCRP were independent predicting factors for the severity of CVD.

Conclusion

MLKL is linked to hallmarks of atherosclerosis and could explain increased cardiovascular risk in diabetic patients. Thus, it can be a potential drug target for treatment of atherosclerotic patients.

MLKL promotes cellular differentiation in myeloid leukemia by facilitating the release of G-CSF

The blockade of cellular differentiation represents a hallmark of acute myeloid leukemia (AML), which is largely attributed to the dysfunction of lineage-specific transcription factors controlling cellular differentiation. However, alternative mechanisms of cellular differentiation programs in AML remain largely unexplored. Here we report that mixed lineage kinase domain-like protein (MLKL) contributes to the cellular differentiation of transformed hematopoietic progenitor cells in AML. Using gene-targeted mice, we show that MLKL facilitates the release of granulocyte colony-stimulating factor (G-CSF) by controlling membrane permeabilization in leukemic cells. Mlkl^−/− hematopoietic stem and progenitor cells released reduced amounts of G-CSF while retaining their capacity for CSF3 (G-CSF) mRNA expression, G-CSF protein translation, and G-CSF receptor signaling. MLKL associates with early endosomes and controls G-CSF release from intracellular storage by plasma membrane pore formation, whereas cell death remained unaffected by loss of MLKL. Of note, MLKL expression was significantly reduced in AML patients, specifically in those with a poor-risk AML subtype. Our data provide evidence that MLKL controls myeloid differentiation in AML by controlling the release of G-CSF from leukemic progenitor cells.

The Role of the Key Effector of Necroptotic Cell Death, MLKL, in Mouse Models of Disease

Necroptosis is an inflammatory form of lytic programmed cell death that is thought to have evolved to defend against pathogens. Genetic deletion of the terminal effector protein—MLKL—shows no overt phenotype in the C57BL/6 mouse strain under conventional laboratory housing conditions. Small molecules that inhibit necroptosis by targeting the kinase activity of RIPK1, one of the main upstream conduits to MLKL activation, have shown promise in several murine models of non-infectious disease and in phase II human clinical trials. This has triggered in excess of one billion dollars (USD) in investment into the emerging class of necroptosis blocking drugs, and the potential utility of targeting the terminal effector is being closely scrutinised. Here we review murine models of disease, both genetic deletion and mutation, that investigate the role of MLKL. We summarize a series of examples from several broad disease categories including ischemia reperfusion injury, sterile inflammation, pathogen infection and hematological stress. Elucidating MLKL’s contribution to mouse models of disease is an important first step to identify human indications that stand to benefit most from MLKL-targeted drug therapies.

Targeting programmed cell death in metabolic dysfunction-associated fatty liver disease (MAFLD): a promising new therapy

Most currently recommended therapies for metabolic dysfunction-associated fatty liver disease (MAFLD) involve diet control and exercise therapy. We searched PubMed and compiled the most recent research into possible forms of programmed cell death in MAFLD, including apoptosis, necroptosis, autophagy, pyroptosis and ferroptosis. Here, we summarize the state of knowledge on the signaling mechanisms for each type and, based on their characteristics, discuss how they might be relevant in MAFLD-related pathological mechanisms. Although significant challenges exist in the translation of fundamental science into clinical therapy, this review should provide a theoretical basis for innovative MAFLD clinical treatment plans that target programmed cell death.

A family harboring an MLKL loss of function variant implicates impaired necroptosis in diabetes

Maturity-onset diabetes of the young, MODY, is an autosomal dominant disease with incomplete penetrance. In a family with multiple generations of diabetes and several early onset diabetic siblings, we found the previously reported P33T PDX1 damaging mutation. Interestingly, this substitution was also present in a healthy sibling. In contrast, a second very rare heterozygous damaging mutation in the necroptosis terminal effector, MLKL, was found exclusively in the diabetic family members. Aberrant cell death by necroptosis is a cause of inflammatory diseases and has been widely implicated in human pathologies, but has not yet been attributed functions in diabetes. Here, we report that the MLKL substitution observed in diabetic patients, G316D, results in diminished phosphorylation by its upstream activator, the RIPK3 kinase, and no capacity to reconstitute necroptosis in two distinct MLKL-/- human cell lines. This MLKL mutation may act as a modifier to the P33T PDX1 mutation, and points to a potential role of impairment of necroptosis in diabetes. Our findings highlight the importance of family studies in unraveling MODY's incomplete penetrance, and provide further support for the involvement of dysregulated necroptosis in human disease.

An Endoplasmic Reticulum Stress-MicroRNA-26a Feedback Circuit in NAFLD

BACKGROUND AND AIMS

Endoplasmic reticulum (ER) stress is an adaptive response to excessive ER demand and contributes to the development of numerous diseases, including nonalcoholic fatty liver disease (NAFLD), which is hallmarked by the accumulation of lipid within hepatocytes. However, the underlying mechanisms remain elusive. MicroRNAs (miRNAs) play an indispensable role in various stress responses, but their implications in ER stress have not yet been systemically investigated. In this study, we identify a negative feedback loop consisting of hepatic ER stress and miR-26a in NAFLD pathogenesis.

APPROACH AND RESULTS

Combining miRNA dot blot array and quantitative PCR, we find that miR-26a is specifically induced by ER stress in liver cells. This induction of miR-26a is critical for cells to cope with ER stress. In human hepatoma cells and murine primary hepatocytes, overexpression of miR-26a markedly alleviates chemical-induced ER stress, as well as palmitate-triggered ER stress and lipid accumulation. Conversely, deficiency of miR-26a exhibits opposite effects. Mechanistically, miR-26a directly targets the eukaryotic initiation factor 2α, a core ER stress effector controlling cellular translation. Intriguingly, miR-26a is reduced in the livers of patients with NAFLD. Hepatocyte-specific restoration of miR-26a in mice significantly mitigates high-fat diet-induced ER stress and hepatic steatosis. In contrast, deficiency of miR-26a in mice exacerbates high-fat diet-induced ER stress, lipid accumulation, inflammation and hepatic steatosis.

CONCLUSIONS

Our findings suggest ER stress-induced miR-26a up-regulation as a regulator for hepatic ER stress resolution, and highlight the ER stress/miR-26a/eukaryotic initiation factor 2α cascade as a promising therapeutic strategy for NAFLD.

The potential role of necroptosis in clinical diseases (Review)

As an important type of programmed cell death in addition to apoptosis, necroptosis occurs in a variety of pathophysiological processes, including infections, liver diseases, kidney injury, neurodegenerative diseases, cardiovascular diseases, and human tumors. It can be triggered by a variety of factors, such as tumor necrosis factor receptor and Toll‑like receptor families, intracellular DNA and RNA sensors, and interferon, and is mainly mediated by receptor‑interacting protein kinase 1 (RIP1), RIP3, and mixed lineage kinase domain‑like protein. A better understanding of the mechanism of necroptosis may be useful in the development of novel drugs for necroptosis‑related diseases. In this review, the focus is on the molecular mechanisms of necroptosis, exploring the role of necroptosis in different pathologies, discussing their potential as a novel therapeutic target for disease therapy, and providing suggestions for further study in this area.

Defatting strategies in the current era of liver steatosis

Liver steatosis is emerging as a major cause of chronic liver disease worldwide, mainly due to the increasing rate of obesity, type 2 diabetes, and metabolic syndrome. Because of the increased incidence of liver steatosis, many organs are currently declined for transplantation despite high demand and waiting list mortality. Defatting strategies have recently emerged as a means of rapidly reducing liver steatosis to expand the pool of available organs. This review summarises advances in defatting strategies in experimental and human models of liver steatosis over the last 20 years.

MLKL: Functions beyond serving as the Executioner of Necroptosis

Recently, necroptosis, as a programmed cell death pathway, has drawn much attention as it has been implicated in multiple pathologies, especially in the field of inflammatory diseases. Pseudokinase mixed lineage kinase domain-like protein (MLKL) serves as a terminal-known obligate effector in the process of necroptosis. To date, the majority of research on MLKL has focused on its role in necroptosis, and the prevailing view has been that the sole function of MLKL is to mediate necroptosis. However, increasing evidence indicates that MLKL can serve as a regulator of many diseases via its non-necroptotic functions. These functions of MLKL shed light on its functional complexity and diversity. In this review, we briefly introduce the current state of knowledge regarding the structure of MLKL, necroptosis signaling, as well as cross-linkages among necroptosis and other regulated cell death pathways, and we particularly highlight recent progress related to newly identified functions and inhibitors of MLKL. These discussions promote a better understanding of the role of MLKL in diseases, which will foster efforts to pharmacologically target this molecule in clinical treatments.

Differential role of MLKL in alcohol-associated and non-alcohol-associated fatty liver diseases in mice and humans

Hepatocellular death contributes to progression of alcohol-associated (ALD-associated) and non-alcohol-associated (NAFL/NASH) liver diseases. However, receptor-interaction protein kinase 3 (RIP3), an intermediate in necroptotic cell death, contributes to injury in murine models of ALD but not NAFL/NASH. We show here that a differential role for mixed-lineage kinase domain-like protein (MLKL), the downstream effector of RIP3, in murine models of ALD versus NAFL/NASH and that RIP1-RIP3-MLKL can be used as biomarkers to distinguish alcohol-associated hepatitis (AH) from NASH. Phospho-MLKL was higher in livers of patients with NASH compared with AH or healthy controls (HCs). MLKL expression, phosphorylation, oligomerization, and translocation to plasma membrane were induced in WT mice fed diets high in fat, fructose, and cholesterol but not in response to Gao-binge (acute on chronic) ethanol exposure. Mlkl-/- mice were not protected from ethanol-induced hepatocellular injury, which was associated with increased expression of chemokines and neutrophil recruitment. Circulating concentrations of RIP1 and RIP3, but not MLKL, distinguished patients with AH from HCs or patients with NASH. Taken together, these data indicate that MLKL is differentially activated in ALD/AH compared with NAFL/NASH in both murine models and patients. Furthermore, plasma RIP1 and RIP3 may be promising biomarkers for distinguishing AH and NASH.

Role of necroptosis in chronic hepatic inflammation and fibrosis in a mouse model of increased oxidative stress

Mice deficient in the antioxidant enzyme Cu/Zn-superoxide dismutase (Sod1^-/- or Sod1KO mice) have increased oxidative stress, show accelerated aging and develop spontaneous hepatocellular carcinoma (HCC) with age. Similar to humans, HCC development in Sod1KO mice progresses from non-alcoholic fatty liver disease (NAFLD) to non-alcoholic steatohepatitis (NASH) with fibrosis, which eventually progresses to HCC. Oxidative stress plays a role in NAFLD to NASH progression, and liver inflammation is the main mechanism that drives the disease progression from NASH to fibrosis. Because necroptosis is a major source of inflammation, we tested the hypothesis that increased necroptosis in the liver plays a role in increased inflammation and fibrosis in Sod1KO mice. Phosphorylation of MLKL (P-MLKL), a well-accepted marker of necroptosis, and expression of MLKL protein were significantly increased in the livers of Sod1KO mice compared to wild type (WT) mice indicating increased necroptosis. Similarly, phosphorylation of RIPK3 and RIPK3 protein levels were also significantly increased. Markers of pro-inflammatory M1 macrophages, NLRP3 inflammasome, and transcript levels of pro-inflammatory cytokines and chemokines, e.g., TNFα, IL-6, IL-1β, and Ccl2 that are associated with human NASH, were significantly increased. Expression of antioxidant enzymes and heat shock proteins, and markers of fibrosis and oncogenic transcription factor STAT3 were also upregulated and autophagy was downregulated in the livers of Sod1KO mice. Short term treatment of Sod1KO mice with necrostatin-1s (Nec-1s), a necroptosis inhibitor, reversed these conditions. Our data show for the first time that necroptosis-mediated inflammation contributes to fibrosis in a mouse model of increased oxidative stress and accelerated aging, that also exhibits progressive HCC development.

MLKL inhibits intestinal tumorigenesis by suppressing STAT3 signaling pathway

Mixed lineage kinase domain-like protein (MLKL) plays an important role in necroptosis, but the role and mechanism of MLKL in intestinal tumorigenesis remain unclear. Here, we found that hematopoietic- and nonhematopoietic-derived MLKL affected intestinal inflammation, but nonhematopoietic-derived MLKL primarily inhibited intestinal tumorigenesis. Loss of MLKL enhanced intestinal regeneration and the susceptibility to intestinal tumorigenesis in Apc^min/+ mice by hyperactivating the Janus kinase 2 (JAK2)/ signal transducer and activator of transcription 3 (STAT3) axis. Furthermore, MLKL deficiency increased interleukin-6 (IL-6) production in dendritic cells. Administration of anti-IL-6R antibody therapy reduced intestinal tumorigenesis in Apc^min/+Mlkl^-/- mice. Notably, low MLKL expression in human colorectal tumors, which enhanced STAT3 activation, was associated with decreased overall survival. Together, our results reveal that MLKL exhibits a suppressive effect during intestinal tumorigenesis by suppressing the IL-6/JAK2/STAT3 signals.

Changes of Necroptosis in Irbesartan Medicated Cardioprotection in Diabetic Rats

BACKGROUND

Diabetic cardiomyopathy (DCM) is strongly linked to microvascular disease, renin-angiotensin system (RAS) activation, cardiac inflammation and cell apoptosis. Irbesartan is an angiotensin II (Ang II) receptor antagonist in RAS system, which inhibited the conversion of Ang I into Ang II, while the specific mechanism is still obscure.

OBJECTIVE

This study aims to investigate the expressions necroptosis RIP1-RIP3-MLKL pathway in myocardium of diabetic rats, and the protective action of irbesartan on myocardial damage.

MATERIALS AND METHODS

In our study, 30 Sprague-Dawley rats were divided into 5 groups: CON4W, high glucose and high caloric (HC4W), diabetes mellitus 4 weeks (DM4W group), diabetes mellitus 8 weeks (DM8W group), and irbesartan diabetes 8 weeks (Ir DM8W group).

RESULTS

We discovered that as diabetes progresses, the rats gradually lost weight, the HW/BW ratio were increased gradually, and the cardiac function became worse accompanied with the aggravation of inflammatory injury. Meanwhile, the myocardial fibers and cells were disordered, and the expression of positive substances, RIP1 and RIP3 increased significantly. The mRNA and protein levels of myocardial RIP1, RIP3 and MLKL were all increased with the progression of DM. After the intervention of irbesartan in diabetic rats, the cardiac function was improved, whereas inflammatory injury and HW/BW ratio were decreased. Also, the myocardial fibrosis injury was attenuated, and the PAS positive substances, RIP1 and RIP3 were significantly decreased. The curative effect of irbesartan was related to decreased myocardial RIP1, RIP3 and MLKL mRNA and protein levels.

CONCLUSION

In conclusion, irbesartan has a cardioprotective effect on the diabetic rats, and its mechanism may be connected with inhibition of RIP1-RIP3-MLKL pathway.

Adipose-specific knockout of ubiquitin-conjugating enzyme E2L6 (Ube2l6) reduces diet-induced obesity, insulin resistance, and hepatic steatosis

Ubiquitin/ISG15-conjugating enzyme E2 L6 (UBE2L6/Ube2l6) catalyzes protein ISGylation and ubiquitylation, post-translational modifications which regulate protein stability. Ube2l6 plays a role in promoting in vitro adipogenesis; however, its mechanism(s) of action and in vivo effects remain unknown. Here, we discovered that UBE2L6 levels were upregulated, and UBE2L6 and adipose triglyceride lipase (ATGL/Atgl) levels were negatively correlated, in white adipose tissue (WAT) from obese humans and obese mice. Therefore, we employed adipose-specific Ube2l6 knockout (Ube2l6^AKO) mice and age-matched Ube2l6^flox/flox controls to assess adipocyte Ube2l6's role in high-fat diet (HFD)-induced obesity, insulin resistance, and hepatic steatosis. HFD-fed Ube2l6^AKO mice displayed lower subcutaneous and visceral WAT mass levels relative to controls. HFD-fed Ube2l6^AKO mice also showed WAT adipocyte hypoplasia and hypotrophy as well as enhanced whole-body metabolic activity relative to controls. Furthermore, glucose intolerance, insulin resistance, compensatory hyperinsulinemia, hypercholesterolemia, and hepatic steatosis were lower in HFD-fed Ube2l6^AKO mice as compared to controls. Mechanistically, we found that Atgl protein expression and Atgl-mediated lipolysis were negatively regulated by Ube2l6's promotion of Atgl protein ubiquitylation. Collectively, adipocyte Ube2l6 functions as a negative regulator of Atgl protein stability and, consequently, promotes HFD-induced obesity, insulin resistance, and hepatic steatosis.