Caspase-2 promotes obesity, the metabolic syndrome and nonalcoholic fatty liver disease

Reengineering of Circularly Permuted Caspase-2 to Enhance Enzyme Stability and Enable Crystallographic Studies

Caspase activation has been linked to several diseases, including cancer, neurodegeneration, and inflammatory conditions, generating interest in targeting this family of proteases for drug development. Caspase-2 (Casp2) in particular has been implicated in Alzheimer's Disease (AD) by cleaving tau protein into fragment Δtau314, which reversibly impairs cognitive and synaptic function. Thus, Casp2 inhibition could be a useful strategy for therapeutic treatment of AD. To that end, we have previously synthesized and characterized various series of peptide and peptidomimetic inhibitors that demonstrate potency and selectivity for Casp2 over caspase-3 (Casp3). Despite promising developments in the design of selective Casp2 inhibitors, low expression yields of Casp2 hinder crystallographic experiments and make structure-based design challenging. The design of circularly permuted (cp) Casp2 increased protein yields considerably; however, this protein could not be crystallized. This article describes the characterization of ten novel cpCasp2 mutants, designed with the goal of increasing stability and facilitating crystallization. Gratifyingly, engineered mutant JF1cpCasp2 displayed high relative stability and was readily crystallizable with the canonical Casp2 inhibitor AcVDVAD-CHO, leading to what we believe to be the first crystal structures of any reverse caspase in the PDB. Moreover, we have reported the structure of JF1cpCasp2 with our recently described Casp2-selective inhibitor MUR-65, which revealed a unique interaction with Arg417 in the binding pocket. Overall, JF1cpCasp2 has proven valuable for structure-based design and expanding understanding of Casp2 inhibition, with potential implications for drug discovery and the development of more selective compounds.

Lipotoxicity-driven metabolic dysfunction-associated steatotic liver disease (MASLD)

Metabolic dysfunction-associated steatotic liver disease (MASLD) encompasses a spectrum of liver lesions, ranging from simple steatosis to metabolic dysfunction-associated steatohepatitis (MASH), that may further progress to cirrhosis. MASLD is estimated to affect more than one third of the general population and it represents a risk factor for end-stage liver failure and liver cancer, substantially contributing to liver-related morbidity and mortality. Although the pathogenesis of MASLD is incompletely understood, it is known to consist of a multifactorial process influenced by extrinsic and intrinsic factors such as metabolic, environmental and demographic features, gut microbiota and genetics. Dysregulation of both extracellular and intracellular lipid composition is known to promote the generation of toxic lipid species, thereby triggering lipotoxicity and cellular stress. These events ultimately lead to the activation of distinct cell death pathways, resulting in inflammation, fibrogenesis and, eventually, carcinogenesis. In this manuscript, we provide a comprehensive review of the role of lipotoxicity during MASLD pathogenesis, discussing the most relevant lipid species and related molecular mechanisms, summarizing the cell type-specific effects and highlighting the most promising putative therapeutic strategies for modulating lipotoxicity and lipid metabolism in MASLD.

The concealed side of caspases: beyond a killer of cells

Since the late 20th century, researchers have known that caspases are a pillar of cell death, particularly apoptosis. However, recent advances in cell biology have unraveled the multiple roles of caspases. These enzymes have an unconventional role in cell proliferation, differentiation, and invasion. As a result, caspase deregulation can fuel the fire of cancer, incite flames of inflammation, flare neurodegenerative disorders, and exacerbate skin pathologies. Several therapeutic approaches toward caspase inhibition have been investigated, but can caspase inhibitors harness the maladaptive effect of these proteases without causing significant side effects? A few studies have exploited caspase induction for cancer or adoptive cell therapies. Here, we provide a compelling picture of caspases, starting with their evolution, their polytomous roles beyond cell death, the flaws of their deregulation, and the merits of targeting them for therapeutic implications. Furthermore, we provide a deeper understanding of the evolution of caspase-related research up to the current era, pinpointing the role of caspases in cell survival and aiding in the development of effective caspase-targeted therapies.

Ally or traitor: the dual role of p62 in caspase-2 regulation

Caspase-2 is a unique and conserved cysteine protease that is involved in several cellular processes, including different forms of cell death, maintenance of genomic stability, and the response to reactive oxygen species. Despite advances in caspase-2 research in recent years, the mechanisms underlying its activation remain largely unclear. Although caspase-2 is activated in the PIDDosome complex, its processing could occur even in the absence of PIDD1 and/or RAIDD, suggesting the existence of an alternative platform for caspase-2 activation. Here, we show that caspase-2 undergoes ubiquitination and interacts with scaffolding protein p62/sequestosome-1 (SQSTM1) under normal conditions and in response to DNA damage. p62 promotes proteasomal but not autophagic caspase-2 degradation as well as its dimerization and activation that triggers the caspase cascade and, subsequently, cell death. Inhibition of p62 expression attenuates cisplatin-induced caspase-2 processing and apoptosis. Notably, the ZZ domain of p62 is critical for caspase-2 binding, whereas the UBA domain is seemingly required to stabilize the p62-caspase-2 complex. Thus, we have uncovered the dual role of p62 in regulating caspase-2 activity: it can foster the degradation of caspase-2 in the proteasome or facilitate its activation by acting as a scaffold platform.

A Study of Sperm DNA Damage Mechanism Based on miRNA Sequencing

To analyze the differential expression profiles of microRNAs (miRNAs) in spermatozoa of patients with sperm DNA damage and to investigate the role of miRNAs in sperm DNA damage. Male infertility patients with sperm DNA damage who attended the First Affiliated Hospital of Henan University of Chinese Medicine from October 2023 to December 2023 were selected and included in this study as a case group. Fertile healthy men who were seen at the health check-up center during the same period and diagnosed by examination were also included as a control group. Sperm miRNA expression was detected in patients with sperm DNA damage (case group, n = 5) and healthy medical check-ups (control group, n = 5) using high-throughput sequencing technology. The differentially expressed miRNAs between the two groups were bioinformatically analyzed to explore the main biological functions of the target genes. We found that 63 miRNAs were significantly changed in the spermatozoa of patients with sperm DNA damage,|log2 (foldchange)| ≥ 1, p < .05. Gene Ontology (GO) enrichment analysis indicated that these differential miRNAs might be involved in developmental process, anatomical structure development, cellular macromolecule metabolic process, multicellular organism development, system development, and so on. Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis showed that that they mainly affect the PI3K-AKT signaling pathway. The present study suggests that the altered expression of miR-1255a, miR-921, and miR-3156-5p may play an important role in the sperm DNA damage process, and the mechanism may involve the phosphatidylinositol-3'-kinase-AKT (PI3K-AKT) signaling pathway.

Exploring caspase functions in mouse models

Caspases are enzymes with protease activity. Despite being known for more than three decades, caspase investigation still yields surprising and fascinating information. Initially associated with cell death and inflammation, their functions have gradually been revealed to extend beyond, targeting pathways such as cell proliferation, migration, and differentiation. These processes are also associated with disease mechanisms, positioning caspases as potential targets for numerous pathologies including inflammatory, neurological, metabolic, or oncological conditions. While in vitro studies play a crucial role in elucidating molecular pathways, they lack the context of the body's complexity. Therefore, laboratory animals are an indispensable part of successfully understanding and applying caspase networks. This paper aims to summarize and discuss recent knowledge, understanding, and challenges in caspase knock-out mice.

A Review on Caspases: Key Regulators of Biological Activities and Apoptosis

Caspases are proteolytic enzymes that belong to the cysteine protease family and play a crucial role in homeostasis and programmed cell death. Caspases have been broadly classified by their known roles in apoptosis (caspase-3, caspase-6, caspase-7, caspase-8, and caspase-9 in mammals) and in inflammation (caspase-1, caspase-4, caspase-5, and caspase-12 in humans, and caspase-1, caspase-11, and caspase-12 in mice). Caspases involved in apoptosis have been subclassified by their mechanism of action as either initiator caspases (caspase-8 and caspase-9) or executioner caspases (caspase-3, caspase-6, and caspase-7). Caspases that participate in apoptosis are inhibited by proteins known as inhibitors of apoptosis (IAPs). In addition to apoptosis, caspases play a role in necroptosis, pyroptosis, and autophagy, which are non-apoptotic cell death processes. Dysregulation of caspases features prominently in many human diseases, including cancer, autoimmunity, and neurodegenerative disorders, and increasing evidence shows that altering caspase activity can confer therapeutic benefits. This review covers the different types of caspases, their functions, and their physiological and biological activities and roles in different organisms.

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.

Targeting caspase-2 interactions with tau in Alzheimer's disease and related dementias

Targeting amyloid-β plaques and tau tangles has failed to provide effective treatments for Alzheimer's disease and related dementias (ADRD). A more fruitful pathway to ADRD therapeutics may be the development of therapies that target common signaling pathways that disrupt synaptic connections and impede communication between neurons. In this review, we present our characterization of a signaling pathway common to several neurological diseases featuring dementia including Alzheimer's disease, frontotemporal dementia, Lewy body dementia, and Huntington's disease. This signaling pathway features the cleavage of tau by caspase-2 (Casp2) yielding Δtau314 (Casp2/tau/Δtau314). Through a not yet fully delineated mechanism, Δtau314 catalyzes the mislocalization and accumulation of tau to dendritic spines leading to the internalization of AMPA receptors and the concomitant weakening of synaptic transmission. Here, we review the accumulated evidence supporting Casp2 as a druggable target and its importance in ADRD. Additionally, we provide a brief overview of our initial medicinal chemistry explorations aimed at the preparation of novel, brain penetrant Casp2 inhibitors. We anticipate that this review will spark broader interest in Casp2 as a target for restoring synaptic dysfunction in ADRD.

Protective Effect of Vitamin D against Hepatic Molecular Apoptosis Caused by a High-Fat Diet in Rats

The protective effects of vitamin D (VitD) in different diseases were studied. The liver is of great interest, especially with the presence of VitD receptors. A high-fat diet (HFD) is associated with many diseases, including liver injury. Consumption of saturated fatty acids triggers hepatic apoptosis and is associated with increased inflammation. We aimed in this study to investigate the protective effects of VitD on hepatic molecular apoptotic changes in response to an HFD in rats. Forty male Wistar albino rats were used and divided into four groups: control, HFD, control + VitD, and VitD-supplemented HFD (HFD + VitD) groups. After six months, the rats were sacrificed, and the livers were removed. RNA was extracted from liver tissues and used for the quantitative real-time RT-PCR of different genes: B-cell lymphoma/leukemia-2 (BCL2), BCL-2-associated X protein (Bax), Fas cell surface death receptor (FAS), FAS ligand (FASL), and tumor necrosis factor α (TNF-α). The results showed that an HFD increased the expression of the pro-apoptotic genes Bax, FAS, and FASL, and reduced the expression of the anti-apoptotic gene BCL2. Interestingly, a VitD-supplemented HFD significantly increased the BCL2 expression and decreased the expression of all pro-apoptotic genes and TNFα. In conclusion, VitD has a protective role against hepatic molecular apoptotic changes in response to an HFD.

Genuine selective caspase-2 inhibition with new irreversible small peptidomimetics

Caspase-2 (Casp2) is a promising therapeutic target in several human diseases, including nonalcoholic steatohepatitis (NASH) and Alzheimer's disease (AD). However, the design of an active-site-directed inhibitor selective to individual caspase family members is challenging because caspases have extremely similar active sites. Here we present new peptidomimetics derived from the VDVAD pentapeptide structure, harboring non-natural modifications at the P2 position and an irreversible warhead. Enzyme kinetics show that these new compounds, such as LJ2 or its specific isomers LJ2a, and LJ3a, strongly and irreversibly inhibit Casp2 with genuine selectivity. In agreement with the established role of Casp2 in cellular stress responses, LJ2 inhibits cell death induced by microtubule destabilization or hydroxamic acid-based deacetylase inhibition. The most potent peptidomimetic, LJ2a, inhibits human Casp2 with a remarkably high inactivation rate (k3/Ki ~5,500,000 M-1 s-1), and the most selective inhibitor, LJ3a, has close to a 1000 times higher inactivation rate on Casp2 as compared to Casp3. Structural analysis of LJ3a shows that the spatial configuration of Cα at the P2 position determines inhibitor efficacy. In transfected human cell lines overexpressing site-1 protease (S1P), sterol regulatory element-binding protein 2 (SREBP2) and Casp2, LJ2a and LJ3a fully inhibit Casp2-mediated S1P cleavage and thus SREBP2 activation, suggesting a potential to prevent NASH development. Furthermore, in primary hippocampal neurons treated with β-amyloid oligomers, submicromolar concentrations of LJ2a and of LJ3a prevent synapse loss, indicating a potential for further investigations in AD treatment.

A long way to go: caspase inhibitors in clinical use

Caspases are an evolutionary conserved family of cysteine-dependent proteases that are involved in many vital cellular processes including apoptosis, proliferation, differentiation and inflammatory response. Dysregulation of caspase-mediated apoptosis and inflammation has been linked to the pathogenesis of various diseases such as inflammatory diseases, neurological disorders, metabolic diseases, and cancer. Multiple caspase inhibitors have been designed and synthesized as a potential therapeutic tool for the treatment of cell death-related pathologies. However, only a few have progressed to clinical trials because of the consistent challenges faced amongst the different types of caspase inhibitors used for the treatment of the various pathologies, namely an inadequate efficacy, poor target specificity, or adverse side effects. Importantly, a large proportion of this failure lies in the lack of understanding various caspase functions. To overcome the current challenges, further studies on understanding caspase function in a disease model is a fundamental requirement to effectively develop their inhibitors as a treatment for the different pathologies. Therefore, the present review focuses on the descriptive properties and characteristics of caspase inhibitors known to date, and their therapeutic application in animal and clinical studies. In addition, a brief discussion on the achievements, and current challenges faced, are presented in support to providing more perspectives for further development of successful therapeutic caspase inhibitors for various diseases.

Upregulation of miR-96-5p by bone marrow mesenchymal stem cells and their exosomes alleviate non-alcoholic steatohepatitis: Emphasis on caspase-2 signaling inhibition

Non-alcoholic steatohepatitis (NASH) has evolved as the most common and devastating chronic liver disease. This study aimed to explore the underlined mechanism for the therapeutic potentials of bone marrow mesenchymal stem cells (BM-MSCs) and their derived exosomes (BM-MSCs-Exo) in an experimental model of high fat diet (HFD) induced NASH. Rats were fed with HFD for 12 weeks. At the seventh week, BM-MSCs were given at a dose of 1x10⁶ cell i.v., per rat. A total of three doses of BM-MSCs were given per each rat in six weeks. BM-MSCs-Exo were given at a dose of 15, 30 and 120 µg/kg i.v., twice per week for six weeks. Perfect homing to the liver was detected. Beneficial effects were reported to BM-MSCs or BM-MSCs-Exo cotreatment; where the highest anti-steatotic effects were attributed to BM-MSCs-Exo (120 µg/kg) showing significant downregulation of fatty acid synthesis (SREB1, 2, ACC), downregulation in lipid uptake (CD36); accompanied by significant upregulation in fatty acid oxidation (PPARα, CPT1). These events were associated with abrogation of hepatic steatosis and ballooning in HFD-induced NASH. BM-MSCs or BM-MSCs-Exo cotreatment exerted significant anti-apoptotic effects mediated by significant decrease in Bax/Bcl2 ratio. Besides, significant increase in mitochondrial mitophagy genes (Parkin, PINK1, ULK1, BNIP3L, ATG5, ATG7, ATG12) were detected in BM-MSCs or BM-MSCs-Exo cotreated groups. These findings are thought to be modulated through upregulation of miRNA-96-5p which leads to downregulation of its downstream target caspase-2. Being a critical player in NASH development, caspase-2 targeting by miRNA-96-5p could be a promising therapeutic modality to treat NASH.

Hyoscyamus albus nortropane alkaloids reduce hyperglycemia and hyperinsulinemia induced in HepG2 cells through the regulation of SIRT1/NF-kB/JNK pathway

BACKGROUND

Chronic superphysiological glucose and insulin concentrations are known to trigger several tissue and organ failures, including insulin resistance, oxidative stress and chronic low-grade inflammation. Hence, the screening for molecules that may counteract such conditions is essential in current existing therapeutic strategies, thereby the use of medicinal plant derivatives represents a promising axis in this regard.

METHODS

In this study, the effect of a selected traditional medicinal plant, Hyoscyamus albus from which, calystegines have been isolated, was investigated in an experimental model of hyperinsulinemia and hyperglycemia induced on HepG2 cells. The mRNA and protein expression levels of different insulin signaling, gluconeogenic and inflammatory pathway- related molecules were examined. Additionally, cell viability and apoptosis, oxidative stress extent and mitochondrial dysfunctions were assayed using flow cytometric and qRT-PCR techniques.

RESULTS

Treatment of IR HepG2 cells with calystegines strongly protected the injured cells from apoptosis, oxidative stress and mitochondrial integrity loss. Interestingly, nortropane alkaloids efficiently regulated the impaired glucose metabolism in IR HepG2 cells, through the stimulation of glucose uptake and the modulation of SIRT1/Foxo1/G6PC/mTOR pathway, which is governing the hepatic gluconeogenesis. Furthermore, the alkaloidal extract restored the defective insulin signaling pathway, mainly by promoting the expression of Insr at the mRNA and protein levels. What is more, treated cells exhibited significant mitigated inflammatory response, as evidenced by the modulation and the regulation of the NF- κB/JNK/TLR4 axis and the downstream proinflammatory cytokines recruitment.

CONCLUSION

Overall, the present investigation demonstrates that calystegines from Hyoscyamus albus provide cytoprotection to the HepG2 cells against insulin/glucose induced insulin resistance and apoptosis due to the regulation of SIRT1/Foxo1/G6PC/mTOR and NF-κB/JNK/TLR4 signaling pathways. Video Abstract.

The p53-caspase-2 axis in the cell cycle and DNA damage response

Caspase-2 was discovered almost three decades ago. It was one of the first two mammalian homologs of CED-3, the other being interleukin 1β-converting enzyme (ICE/caspase-1). Despite high similarity with CED-3 and its fly and mammalian counterparts (DRONC and caspase-9, respectively), the function of caspase-2 in apoptosis has remained enigmatic. A number of recent studies suggest that caspase-2 plays an important role in the regulation of p53 in response to cellular stress and DNA damage to prevent the proliferation and accumulation of damaged or aberrant cells. Here, we review these recent observations and their implications in caspase-2-mediated cellular death, senescence, and tumor suppression.

Caspase-2 as a master regulator of genomic stability

Genomic instability underlies genesis and the development of various types of cancer. During tumorigenesis, cancer initiating cells assume a set of features, which allow them to survive and proliferate. Different mutations and chromosomal alterations promote a selection of the most aggressive cancer clones that worsen the prognosis of the disease. Despite that caspase-2 was described as a protease fulfilling an initiator and an effector function in apoptosis, it has recently been discovered to play an important role in the maintenance of genomic integrity and normal chromosome configuration. This protein is able to stabilize p53 and affect the level of transcription factors, which activates cell response to oxidative stress. Here we focus on the discussion on the mechanism(s) of how caspase-2 regulates genomic stability and decreases tumorigenesis.

How does hepatic lipid accumulation lead to lipotoxicity in non-alcoholic fatty liver disease?

BACKGROUND

Non-alcoholic fatty liver disease (NAFLD), characterized as excess lipid accumulation in the liver which is not due to alcohol use, has emerged as one of the major health problems around the world. The dysregulated lipid metabolism creates a lipotoxic environment which promotes the development of NAFLD, especially the progression from simple steatosis (NAFL) to non-alcoholic steatohepatitis (NASH).

PURPOSEAND AIM

This review focuses on the mechanisms of lipid accumulation in the liver, with an emphasis on the metabolic fate of free fatty acids (FFAs) in NAFLD and presents an update on the relevant cellular processes/mechanisms that are involved in lipotoxicity. The changes in the levels of various lipid species that result from the imbalance between lipolysis/lipid uptake/lipogenesis and lipid oxidation/secretion can cause organellar dysfunction, e.g. ER stress, mitochondrial dysfunction, lysosomal dysfunction, JNK activation, secretion of extracellular vesicles (EVs) and aggravate (or be exacerbated by) hypoxia which ultimately lead to cell death. The aim of this review is to provide an overview of how abnormal lipid metabolism leads to lipotoxicity and the cellular mechanisms of lipotoxicity in the context of NAFLD.

Caspase-2 Substrates: To Apoptosis, Cell Cycle Control, and Beyond

Caspase-2 belongs to the caspase family of proteins responsible for essential cellular functions including apoptosis and inflammation. Uniquely, caspase-2 has been identified as a tumor suppressor, but how it regulates this function is still unknown. For many years, caspase-2 has been considered an “orphan” caspase because, although it is able to induce apoptosis, there is an abundance of conflicting evidence that questions its necessity for apoptosis. Recent evidence supports that caspase-2 has non-apoptotic functions in the cell cycle and protection from genomic instability. It is unclear how caspase-2 regulates these opposing functions, which has made the mechanism of tumor suppression by caspase-2 difficult to determine. As a protease, caspase-2 likely exerts its functions by proteolytic cleavage of cellular substrates. This review highlights the known substrates of caspase-2 with a special focus on their functional relevance to caspase-2’s role as a tumor suppressor.

Transplantation of brown adipose tissue up-regulates miR-99a to ameliorate liver metabolic disorders in diabetic mice by targeting NOX4

Nonalcoholic fatty liver disease (NAFLD), main cause of liver damage, is inextricably linked to diabetes. However, there is no specific means to improve the pathology of fatty liver in diabetic patients. Brown adipose tissue (BAT) is an important endocrine organ that secretes adipokines and microRNAs (miRNAs) involved in systemic metabolic regulation. To investigate the effects of BAT transplantation on liver lipid metabolism in diabetic mice, we transplanted BAT from male donor mice into diabetic mice induced by streptozotocin (STZ) combined with high-fat diet (HFD). At 10 weeks after transplantation, BAT transplantation significantly decreased the blood glucose and lipid, downregulated FAS, CD36, Scd1, ACCα, NOX2, NOX4, TGF-β1, FN and COL-1, up-regulated Nrf2, reversed the pathological changes of liver and increased the circulating miR-99a in diabetic mice. To verify whether circulating miR-99a improves oxidative stress by targeting inhibition of NOX4, we used 0.4mM palmitic acid (PA) to treat the LO2 cells. The expression of NOX4 protein was significantly decreased after transfection with miR-99a mimic, and increased after transfection with miR-99a inhibitor. Luciferase reporter assay confirmed that miR-99a could target NOX4 mRNA. These findings clarify the role of miR-99a and NOX4 in liver beneficial effect of BAT transplantation in diabetic mice.

Clusterin overexpression protects against western diet-induced obesity and NAFLD

Obesity is a significant risk factor for various metabolic diseases and is closely related to non-alcoholic fatty liver disease (NAFLD) characterized by inflammation and oxidative stress. Clusterin is a multi-functional protein that is up-regulated in the pathogenesis of various metabolic diseases, including obesity and NAFLD. Our previous studies indicated that hepatocyte-specific overexpression of clusterin alleviates methionine choline-deficient (MCD) diet-induced non-alcoholic steatohepatitis (NASH) by activating nuclear factor erythroid 2-related factor 2 (Nrf2). Here we generated transgenic mice with whole-body clusterin overexpression (wCLU-tg) and investigated the role of clusterin in Western diet-induced obesity and NAFLD. We confirmed that obesity parameters and the spectrum of NAFLD of wCLU-tg mice were improved compared to wild type mice. Contrarily, clusterin deficiency deteriorated metabolic disruptions. We also found that clusterin activates target molecules for obesity and NAFLD, namely Nrf2 and AMPK, suggesting that clusterin protects against Western diet-induced obesity and NAFLD by activating Nrf2 and AMPK.

[Cell death mechanisms in non-alcoholic steatohepatitis]

Continuous cell death associated with inflammation is a key trigger of disease progression notably in chronic liver diseases such as non-alcoholic steatohepatitis (NASH). Apoptosis has been studied as a potential target for reducing cell death in NASH. However, recent studies suggest that caspase inhibition is inefficient to treat NASH patients and may aggravate the disease by redirecting cells to alternative mechanisms of cell death. Alternative forms of lytic cell death have recently been identified and are known to induce strong inflammatory responses due to cell membrane permeabilization. Therefore, controlling lytic cell death modes offers new opportunities for potential therapeutic intervention in NASH. This review summarizes the underlying molecular mechanisms of apoptosis and lytic cell death modes, including necroptosis, pyroptosis and ferroptosis, and discusses their relevance in NASH.

Processes exacerbating apoptosis in non-alcoholic steatohepatitis

Non-alcoholic fatty liver disease (NAFLD) is a significant public health concern, owing to its high prevalence, progressive nature and lack of effective medical therapies. NAFLD is a complex and multifactorial disease involving the progressive and concerted action of factors that contribute to the development of liver inflammation and eventually fibrosis. Here, we summarize fundamental molecular mechanisms underlying the pathogenesis of non-alcoholic steatohepatitis (NASH), how they are interrelated and possible translation to clinical applications. We focus on processes triggering and exacerbating apoptotic signalling in the liver of NAFLD patients and their metabolic and pathological implications. Indeed, liver injury and inflammation are cardinal histopathological features of NASH, a duo in which derailment of apoptosis is of paramount importance. In turn, the liver houses a very high number of mitochondria, crucial metabolic unifiers of both extrinsic and intrinsic signals that converge in apoptosis activation. The role of lifestyle options is also dissected, highlighting the management of modifiable risk factors, such as obesity and harmful alcohol consumption, influencing apoptosis signalling in the liver and ultimately NAFLD progression. Integrating NAFLD-associated pathologic mechanisms in the cell death context could provide clues for a more profound understating of the disease and pave the way for novel rational therapies.

Uncovering the PIDDosome and caspase-2 as regulators of organogenesis and cellular differentiation

The PIDDosome is a multiprotein complex that drives activation of caspase-2, an endopeptidase originally implicated in apoptosis. Yet, unlike other caspases involved in cell death and inflammation, caspase-2 seems to exert additional versatile functions unrelated to cell death. These emerging roles range from control of transcription factor activity to ploidy surveillance. Thus, caspase-2 and the PIDDosome act as a critical regulatory unit controlling cellular differentiation processes during organogenesis and regeneration. These newly established functions of the PIDDosome and its downstream effector render its components attractive targets for drug-development aiming to prevent fatty liver diseases, neurodegenerative disorders or osteoporosis.

Caspase-2 and p75 neurotrophin receptor (p75NTR) are involved in the regulation of SREBP and lipid genes in hepatocyte cells

Lipid-induced toxicity is part of several human diseases, but the mechanisms involved are not fully understood. Fatty liver is characterized by the expression of different growth and tissue factors. The neurotrophin, nerve growth factor (NGF) and its pro-form, pro-NGF, are present in fatty liver together with p75 neurotrophin receptor (p75NTR). Stimulation of human Huh7 hepatocyte cells with NGF and pro-NGF induced Sterol-regulator-element-binding protein-2 (SREBP2) activation and increased Low-Density Lipoprotein Receptor (LDLR) expression. We observed that phosphorylation of caspase-2 by p38 MAPK was essential for this regulation involving a caspase-3-mediated cleavage of SREBP2. RNA sequencing showed that several genes involved in lipid metabolism were altered in p75NTR-deficient mouse liver. The same lipogenic genes were downregulated in p75NTR gene-engineered human Huh7 cells and reciprocally upregulated by stimulation of p75NTRs. In the knock-out mice the serum cholesterol and triglyceride levels were reduced, suggesting a physiological role of p75NTRs in whole-body lipid metabolism. Taken together, this study shows that p75NTR signaling influences a network of genes involved in lipid metabolism in liver and hepatocyte cells. Modulation of p75NTR signaling may be a target to consider in various metabolic disorders accompanied by increased lipid accumulation.

Semisynthetic bile acids: a new therapeutic option for metabolic syndrome

Bile acids are endogenous emulsifiers synthesized from cholesterol having a peculiar amphiphilic structure. Appreciation of their beneficial effects on human health, recognized since ancient times, has expanded enormously since the discovery of their role as signaling molecules. Activation of farnesoid X receptor (FXR) and Takeda G-protein receptor-5 (TGR5) signaling pathways by bile acids, regulating glucose, lipid and energy metabolism, have become attractive avenue for metabolic syndrome treatment. Therefore, extensive effort has been directed into the research and synthesis of bile acid derivatives with improved pharmacokinetic properties and high potency and selectivity for these receptors. Minor modifications in the structure of bile acids and their derivatives may result in fine-tuning modulation of their biological functions, and most importantly, in an evasion of undesired effect. A great number of semisynthetic bile acid analogues have been designed and put in preclinical and clinical settings. Obeticholic acid (INT-747) has achieved the biggest clinical success so far being in use for the treatment of primary biliary cholangitis. This review summarizes and critically evaluates the key chemical modifications of bile acids resulting in development of novel semisynthetic derivatives as well as the current status of their preclinical and clinical evaluation in the treatment of metabolic syndrome, an aspect that is so far lacking in the scientific literature. Taking into account the balance between therapeutic benefits and potential adverse effects associated with specific structure and mechanism of action, recommendations for future studies are proposed.

Fpr2 Deficiency Alleviates Diet-Induced Insulin Resistance Through Reducing Body Weight Gain and Inhibiting Inflammation Mediated by Macrophage Chemotaxis and M1 Polarization

Obesity and related inflammation are critical for the pathogenesis of insulin resistance, but the underlying mechanisms are not fully understood. Formyl peptide receptor 2 (FPR2) plays important roles in host immune responses and inflammation-related diseases. We found that Fpr2 expression was elevated in the white adipose tissue of high-fat diet (HFD)-induced obese mice and db/db mice. The systemic deletion of Fpr2 alleviated HFD-induced obesity, insulin resistance, hyperglycemia, hyperlipidemia, and hepatic steatosis. Furthermore, Fpr2 deletion in HFD-fed mice elevated body temperature, reduced fat mass, and inhibited inflammation by reducing macrophage infiltration and M1 polarization in metabolic tissues. Bone marrow transplantations between wild-type and Fpr2^-/- mice and myeloid-specific Fpr2 deletion demonstrated that Fpr2-expressing myeloid cells exacerbated HFD-induced obesity, insulin resistance, glucose/lipid metabolic disturbances, and inflammation. Mechanistic studies revealed that Fpr2 deletion in HFD-fed mice enhanced energy expenditure probably through increasing thermogenesis in skeletal muscle; serum amyloid A3 and other factors secreted by adipocytes induced macrophage chemotaxis via Fpr2; and Fpr2 deletion suppressed macrophage chemotaxis and lipopolysaccharide-, palmitate-, and interferon-γ-induced macrophage M1 polarization through blocking their signals. Altogether, our studies demonstrate that myeloid Fpr2 plays critical roles in obesity and related metabolic disorders via regulating muscle energy expenditure, macrophage chemotaxis, and M1 polarization.

Potent and selective caspase-2 inhibitor prevents MDM-2 cleavage in reversine-treated colon cancer cells

Most caspases can be positioned unambiguously within the regulated cell death networks of apoptosis and pyroptosis, but the role of caspase-2, a highly conserved protease within the family, remains enigmatic. This is mainly due to lack of selective chemical and biochemical tools for the investigation of this protease. In this study, we used our hybrid combinatorial substrate library (HyCoSuL) approach to broadly profile caspase-2 substrate specificity using peptide scanning libraries. This screen uncovered previously unknown caspase-2 peptidyl substrate preferences, which were further used to develop caspase-2 selective fluorogenic substrates and covalent, irreversible AOMK inhibitors. Finally, we used the champion inhibitor (NH-23-C2) in reversine-treated HCT-116 colon cancer cells to selectively block caspase-2 activity and caspase-2-mediated MDM-2 cleavage. In addition, we showed that NH-23-C2 does not block caspase-3 or caspase-8, which makes it a powerful chemical tool to dissect the true role of caspase-2 in various biological setups.

Developmental origins of nonalcoholic fatty liver disease as a risk factor for exaggerated metabolic and cardiovascular-renal disease

Intrauterine growth restriction (IUGR) is linked to increased risk for chronic disease. Placental ischemia and insufficiency in the mother are implicated in predisposing IUGR offspring to metabolic dysfunction, including hypertension, insulin resistance, abnormalities in glucose homeostasis, and nonalcoholic fatty liver disease (NAFLD). It is unclear whether these metabolic disturbances contribute to the developmental origins of exaggerated cardiovascular-renal disease (CVRD) risk accompanying IUGR. IUGR impacts the pancreas, adipose tissue, and liver, which are hypothesized to program for hepatic insulin resistance and subsequent NAFLD. NAFLD is projected to become the major cause of chronic liver disease and contributor to uncontrolled type 2 diabetes mellitus, which is a leading cause of chronic kidney disease. While NAFLD is increased in experimental models of IUGR, lacking is a full comprehension of the mechanisms responsible for programming of NAFLD and whether this potentiates susceptibility to liver injury. The use of well-established and clinically relevant rodent models, which mimic the clinical characteristics of IUGR, metabolic disturbances, and increased blood pressure in the offspring, will permit investigation into mechanisms linking adverse influences during early life and later chronic health. The purpose of this review is to propose mechanisms, including those proinflammatory in nature, whereby IUGR exacerbates the pathogenesis of NAFLD and how these adverse programmed outcomes contribute to exaggerated CVRD risk. Understanding the etiology of the developmental origins of chronic disease will allow investigators to uncover treatment strategies to intervene in the mother and her offspring to halt the increasing prevalence of metabolic dysfunction and CVRD.

Storage lipid studies in tuberculosis reveal that foam cell biogenesis is disease-specific

Foam cells are lipid-laden macrophages that contribute to the inflammation and tissue damage associated with many chronic inflammatory disorders. Although foam cell biogenesis has been extensively studied in atherosclerosis, how these cells form during a chronic infectious disease such as tuberculosis is unknown. Here we report that, unlike the cholesterol-laden cells of atherosclerosis, foam cells in tuberculous lung lesions accumulate triglycerides. Consequently, the biogenesis of foam cells varies with the underlying disease. In vitro mechanistic studies showed that triglyceride accumulation in human macrophages infected with Mycobacterium tuberculosis is mediated by TNF receptor signaling through downstream activation of the caspase cascade and the mammalian target of rapamycin complex 1 (mTORC1). These features are distinct from the known biogenesis of atherogenic foam cells and establish a new paradigm for non-atherogenic foam cell formation. Moreover, they reveal novel targets for disease-specific pharmacological interventions against maladaptive macrophage responses.

To die or not to die: death signaling in nonalcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is an emerging liver disease worldwide. In subset of patients, NAFLD progresses to its advanced form, nonalcoholic steatohepatitis (NASH), which is accompanied with inflammation and fibrosis. Saturated free fatty acid-induced hepatocyte apoptosis is a feature of NASH. Death signaling in NASH does not always result in apoptosis, but can alternatively lead to the survival of cells presenting signs of pro-inflammatory and pro-fibrotic signals. With the current lack of established treatments for NASH, it is important to understand the molecular mechanisms responsible for disease development and progression. This review focuses on the latest findings in hepatocyte death signaling and discusses possible targets for intervention, including caspases, death receptor and c-Jun N-terminal kinase 1 signaling, oxidative stress, and endoplasmic reticulum stress, as well as epigenomic factors.

Dimer-specific immunoprecipitation of active caspase-2 identifies TRAF proteins as novel activators

Caspase-2 has been shown to initiate apoptotic cell death in response to specific intracellular stressors such as DNA damage. However, the molecular mechanisms immediately upstream of its activation are still poorly understood. We combined a caspase-2 bimolecular fluorescence complementation (BiFC) system with fluorophore-specific immunoprecipitation to isolate and study the active caspase-2 dimer and its interactome. Using this technique, we found that tumor necrosis factor receptor-associated factor 2 (TRAF2), as well as TRAF1 and 3, directly binds to the active caspase-2 dimer. TRAF2 in particular is necessary for caspase-2 activation in response to apoptotic cell death stimuli. Furthermore, we found that dimerized caspase-2 is ubiquitylated in a TRAF2-dependent manner at K15, K152, and K153, which in turn stabilizes the active caspase-2 dimer complex, promotes its association with an insoluble cellular fraction, and enhances its activity to fully commit the cell to apoptosis. Together, these data indicate that TRAF2 positively regulates caspase-2 activation and consequent cell death by driving its activation through dimer-stabilizing ubiquitylation.

Caspases in metabolic disease and their therapeutic potential

Caspases, a family of cysteine-dependent aspartate-specific proteases, are central to the maintenance of cellular and organismal homoeostasis by functioning as key mediators of the inflammatory response and/or apoptosis. Both metabolic inflammation and apoptosis play a central role in the pathogenesis of metabolic disease such as obesity and the progression of nonalcoholic steatohepatisis (NASH) to more severe liver disease. Obesity and nonalcoholic fatty liver disease (NAFLD) are the leading global health challenges associated with the development of numerous comorbidities including insulin resistance, type-2 diabetes and early mortality. Despite the high prevalence, current treatment strategies including lifestyle, dietary, pharmaceutical and surgical interventions, are often limited in their efficacy to manage or treat obesity, and there are currently no clinical therapies for NAFLD/NASH. As mediators of inflammation and cell death, caspases are attractive therapeutic targets for the treatment of these metabolic diseases. As such, pan-caspase inhibitors that act by blocking apoptosis have reached phase I/II clinical trials in severe liver disease. However, there is still a lack of knowledge of the specific and differential functions of individual caspases. In addition, cross-talk between alternate cell death pathways is a growing concern for long-term caspase inhibition. Evidence is emerging of the important cell-death-independent, non-apoptotic functions of caspases in metabolic homoeostasis that may be of therapeutic value. Here, we review the current evidence for roles of caspases in metabolic disease and discuss their potential targeting as a therapeutic strategy.

Triggering and resolution of inflammation in NASH

Nonalcoholic steatohepatitis (NASH) is considered the progressive form of nonalcoholic fatty liver disease (NAFLD) and is characterized by liver steatosis, inflammation, hepatocellular injury and different degrees of fibrosis. A central issue in this field relates to the identification of those factors that trigger inflammation, thus fuelling the transition from nonalcoholic fatty liver to NASH. These triggers of liver inflammation might have their origins outside the liver (such as in adipose tissue or the gut) as well as inside the organ (for instance, lipotoxicity, innate immune responses, cell death pathways, mitochondrial dysfunction and endoplasmic reticulum stress), both of which contribute to NASH development. In this Review, we summarize the currently available information on the key upstream triggers of inflammation in NASH. We further delineate the mechanisms by which liver inflammation is resolved and the implications of a defective pro-resolution process. A better knowledge of these mechanisms should help to design targeted therapies able to halt or reverse disease progression.

Updates on Dietary Models of Nonalcoholic Fatty Liver Disease: Current Studies and Insights

Nonalcoholic fatty liver disease (NAFLD) is a disease of increasing interest, as its prevalence is on the rise. NAFLD has been linked to metabolic syndrome, which is becoming more common due to the Western diet. Because NAFLD can lead to cirrhosis and related complications including hepatocellular carcinoma, the increasing prevalence is concerning, and medical therapy aimed at treating NAFLD is of great interest. Researchers studying the effects of medical therapy on NAFLD use dietary mouse models. The two main types of mouse model diets are the methionine- and choline-deficient (MCD) diet and the Western-like diet (WD). Although both induce NAFLD, the mechanisms are very different. We reviewed several studies conducted within the last 5 years that used MCD diet or WD mouse models in order to mimic this disease in a way most similar to humans. The MCD diet inconsistently induces NAFLD and fibrosis and does not completely induce metabolic syndrome. Thus, the clinical significance of the MCD diet is questionable. In contrast, WD mouse models consisting of high fat, cholesterol, and a combination of high-fructose corn syrup, sucrose, fructose, or glucose not only lead to metabolic syndrome but also induce NAFLD with fibrosis, making these choices most suitable for research.

Metabolic profiling of visceral adipose tissue from obese subjects with or without metabolic syndrome

Obesity represents one of the most complex public health challenges and has recently reached epidemic proportions. Obesity is also considered to be primarily responsible for the rising prevalence of metabolic syndrome, defined as the coexistence in the same individual of several risk factors for atherosclerosis, including dyslipidemia, hypertension and hyperglycemia, as well as for cancer. Additionally, the presence of three of the five risk factors (abdominal obesity, low high-density lipoprotein cholesterol, high triglycerides, high fasting glucose and high blood pressure) characterizes metabolic syndrome, which has serious clinical consequences. The current study was conducted in order to identify metabolic differences in visceral adipose tissue (VAT) collected from obese (body mass index 43-48) human subjects who were diagnosed with metabolic syndrome, obese individuals who were metabolically healthy and nonobese healthy controls. Extensive gas chromatography/mass spectrometry (GC/MS) and liquid chromatography/mass spectrometry (LC/MS/MS) analyses were used to obtain the untargeted VAT metabolomic profiles of 481 metabolites belonging to all biochemical pathways. Our results indicated consistent increases in oxidative stress markers from the pathologically obese samples in addition to subtle markers of elevated glucose levels that may be consistent with metabolic syndrome. In the tissue derived from the pathologically obese subjects, there were significantly elevated levels of plasmalogens, which may be increased in response to oxidative changes in addition to changes in glycerolphosphorylcholine, glycerolphosphorylethanolamine glycerolphosphorylserine, ceramides and sphingolipids. These data could be potentially helpful for recognizing new pathways that underlie the metabolic-vascular complications of obesity and may lead to the development of innovative targeted therapies.

Knockout of inositol-requiring enzyme 1α in pro-opiomelanocortin neurons decreases fat mass via increasing energy expenditure

Although numerous functions of inositol-requiring enzyme 1α (IRE1α) have been identified, a role of IRE1α in pro-opiomelanocortin (POMC) neurons in the arcuate nucleus of the hypothalamus is largely unknown. Here, we showed that mice lacking IRE1α specifically in POMC neurons (PIKO) are lean and resistant to high-fat diet-induced obesity and obesity-related insulin resistance, liver steatosis and leptin resistance. Furthermore, PIKO mice had higher energy expenditure, probably due to increased thermogenesis in brown adipose tissue. Additionally, α-melanocyte-stimulating hormone production was increased in the hypothalamus of PIKO mice. These results demonstrate that IRE1α in POMC neurons plays a critical role in the regulation of obesity and obesity-related metabolic disorders. Our results also suggest that IRE1α is not only an endoplasmic reticulum stress sensor, but also a new potential therapeutic target for obesity and obesity-related metabolic diseases.

Caspase-2 deficiency enhances whole-body carbohydrate utilisation and prevents high-fat diet-induced obesity

Caspase-2 has been shown to be involved in metabolic homeostasis. Here, we show that caspase-2 deficiency alters basal energy metabolism by shifting the balance in fuel choice from fatty acid to carbohydrate usage. At 4 weeks of age, whole-body carbohydrate utilisation was increased in Casp2^-/- mice and was maintained into adulthood. By 17 weeks of age, Casp2^-/- mice had reduced white adipose mass, smaller white adipocytes decreased fasting blood glucose and plasma triglycerides but maintained normal insulin levels. When placed on a 12-week high-fat diet (HFD), Casp2^-/- mice resisted the development of obesity, fatty liver, hyperinsulinemia and insulin resistance. In addition, HFD-fed Casp2^-/- mice had reduced white adipocyte hypertrophy, apoptosis and expansion of both subcutaneous and visceral adipose depots. Increased expression of UCP1 and the maintenance of adiponectin levels in white adipose tissue of HFD-fed Casp2^-/- mice indicated increased browning and adipocyte hyperplasia. We found that while the preference for whole-body carbohydrate utilisation was maintained, HFD-fed Casp2^-/- mice were not impaired in their ability to switch to utilising fats as a fuel source. Our findings suggest that caspase-2 impacts basal energy metabolism by regulating adipocyte biology and fat expansion, most likely via a non-apoptotic function. Furthermore, we show that caspase-2 deficiency shifts the balance in fuel choice towards increased carbohydrate utilisation and propose that this is due to mild energy stress. As a consequence, Casp2^-/- mice show an adaptive remodelling of adipose tissue that protects from HFD-induced obesity and improves glucose homeostasis while paradoxically increasing their susceptibility to oxidative stress induced damage and premature ageing.