Hepatocyte senescence predicts progression in non-alcohol-related fatty liver disease

Evaluating causality of cellular senescence in non-alcoholic fatty liver disease

Cellular senescence is a state of irreversible cell cycle arrest that has important physiological functions. However, cellular senescence is also a hallmark of ageing and has been associated with several pathological conditions. A wide range of factors including genotoxic stress, mitogens and inflammatory cytokines can induce senescence. Phenotypically, senescent cells are characterised by short telomeres, an enlarged nuclear area and damaged genomic and mitochondrial DNA. Secretion of proinflammatory proteins, also known as the senescence-associated secretory phenotype, is a characteristic of senescent cells that is thought to be the main contributor to their disease-inducing properties. In the past decade, the role of cellular senescence in the development of non-alcoholic fatty liver disease (NAFLD) and its progression towards non-alcoholic steatohepatitis (NASH) has garnered significant interest. Until recently, it was suggested that hepatocyte cellular senescence is a mere consequence of the metabolic dysregulation and inflammatory phenomena in fatty liver disease. However, recent work in rodents has suggested that senescence may be a causal factor in NAFLD development. Although causality is yet to be established in humans, current evidence suggests that targeting senescent cells has therapeutic potential for NAFLD. We aim to provide insights into the quality of the evidence supporting a causal role of cellular senescence in the development of NAFLD in rodents and humans. We will elaborate on key cellular and molecular features of senescence and discuss the efficacy and safety of novel senolytic drugs for the treatment or prevention of NAFLD.

The Roles and Pharmacological Effects of FGF21 in Preventing Aging-Associated Metabolic Diseases

With the continuous improvement of living standards but the lack of exercise, aging-associated metabolic diseases such as obesity, type 2 diabetes mellitus (T2DM), and non-alcoholic fatty liver disease (NAFLD) are becoming a lingering dark cloud over society. Studies have found that metabolic disorders are near related to glucose, lipid metabolism, and cellular aging. Fibroblast growth factor 21 (FGF21), a member of the FGFs family, efficiently regulates the homeostasis of metabolism and cellular aging. By activating autophagy genes and improving inflammation, FGF21 indirectly delays cellular aging and directly exerts anti-aging effects by regulating aging genes. FGF21 can also regulate glucose and lipid metabolism by controlling metabolism-related genes, such as adipose triglyceride lipase (ATGL) and acetyl-CoA carboxylase (ACC1). Because FGF21 can regulate metabolism and cellular aging simultaneously, FGF21 analogs and FGF21 receptor agonists are gradually being valued and could become a treatment approach for aging-associated metabolic diseases. However, the mechanism by which FGF21 achieves curative effects is still not known. This review aims to interpret the interactive influence between FGF21, aging, and metabolic diseases and delineate the pharmacology of FGF21, providing theoretical support for further research on FGF21.

Hepatic Senescence Accompanies the Development of NAFLD in Non-Aged Mice Independently of Obesity

Senescence is considered to be a cardinal player in several chronic inflammatory and metabolic pathologies. The two dominant mechanisms of senescence include replicative senescence, predominantly depending on age-induced telomere shortening, and stress-induced senescence, triggered by external or intracellular harmful stimuli. Recent data indicate that hepatocyte senescence is involved in the development of nonalcoholic fatty liver disease (NAFLD). However, previous studies have mainly focused on age-related senescence during NAFLD, in the presence or absence of obesity, while information about whether the phenomenon is characterized by replicative or stress-induced senescence, especially in non-aged organisms, is scarce. Herein, we subjected young mice to two different diet-induced NAFLD models which differed in the presence of obesity. In both models, liver fat accumulation and increased hepatic mRNA expression of steatosis-related genes were accompanied by hepatic senescence, indicated by the increased expression of senescence-associated genes and the presence of a robust hybrid histo-/immunochemical senescence-specific staining in the liver. Surprisingly, telomere length and global DNA methylation did not differ between the steatotic and the control livers, while malondialdehyde, a marker of oxidative stress, was upregulated in the mouse NAFLD livers. These findings suggest that senescence accompanies NAFLD emergence, even in non-aged organisms, and highlight the role of stress-induced senescence during steatosis development independently of obesity.

Study of Cellular Senescence and Vitamin D Deficiency in Nonalcoholic Fatty Liver Disease and The Potential Protective Effect of Vitamin D Supplementation

BACKGROUND

Nonalcoholic fatty liver disease (NAFLD) is a pathological process characterized by excessive hepatic fatty deposition with possible involvement of vitamin D deficiency and cellular senescence. The aim of this study is to investigate the pathophysiologic role of vitamin D deficiency and cellular senescence in NAFLD development. Moreover, it aims to investigate the potential protective role of vitamin D supplementation.

METHODS

This is an experimental Case/Control study. Forty-five male albino rats were enrolled in this study. Animals were divided into four groups: negative and positive control groups (10 for each group), a model of NAFLD (11) and vitamin D-treated NAFLD groups (14). At the end of the experiment, all rats were subjected to the following investigation; biochemical estimation of serum 25 hydroxycholecalciferol, senescence marker protein-30 (SMP-30), lipid profile and calculation of homeostatic model of insulin resistance (HOMA-IR).

RESULTS

NAFLD group shows a significant increase in glucose, insulin levels, and HOMA- IR compared with both normal controls. This finding indicates the intimate association between insulin resistance and NAFLD pathogenesis. Moreover, it was found that NAFLD group shows a significant decrease in SMP-30 level compared with normal controls. While vitamin D-treated NAFLD group shows significant increased SMP-30 and decrease in HOMA-IR in comparison with nontreated NAFLD group.

CONCLUSION

Vitamin D deficiency and increased cellular senescence are key features of NAFLD. Vitamin D supplementation could play a protective role, which needs further investigation including clinical human study.

Senolytic Drugs: Reducing Senescent Cell Viability to Extend Health Span

Senescence is the consequence of a signaling mechanism activated in stressed cells to prevent proliferation of cells with damage. Senescent cells (Sncs) often develop a senescence-associated secretory phenotype to prompt immune clearance, which drives chronic sterile inflammation and plays a causal role in aging and age-related diseases. Sncs accumulate with age and at anatomical sites of disease. Thus, they are regarded as a logical therapeutic target. Senotherapeutics are a new class of drugs that selectively kill Sncs (senolytics) or suppress their disease-causing phenotypes (senomorphics/senostatics). Since 2015, several senolytics went from identification to clinical trial. Preclinical data indicate that senolytics alleviate disease in numerous organs, improve physical function and resilience, and suppress all causes of mortality, even if administered to the aged. Here, we review the evidence that Sncs drive aging and disease, the approaches to identify and optimize senotherapeutics, and the current status of preclinical and clinical testing of senolytics.

Cellular Senescence in Liver Disease and Regeneration

Cellular senescence is an irreversible cell cycle arrest implemented by the cell as a result of stressful insults. Characterized by phenotypic alterations, including secretome changes and genomic instability, senescence is capable of exerting both detrimental and beneficial processes. Accumulating evidence has shown that cellular senescence plays a relevant role in the occurrence and development of liver disease, as a mechanism to contain damage and promote regeneration, but also characterizing the onset and correlating with the extent of damage. The evidence of senescent mechanisms acting on the cell populations of the liver will be described including the role of markers to detect cellular senescence. Overall, this review intends to summarize the role of senescence in liver homeostasis, injury, disease, and regeneration.

Remodeling of whole-body lipid metabolism and a diabetic-like phenotype caused by loss of CDK1 and hepatocyte division

Cell cycle progression and lipid metabolism are well-coordinated processes required for proper cell proliferation. In liver diseases that arise from dysregulated lipid metabolism, hepatocyte proliferation is diminished. To study the outcome of CDK1 loss and blocked hepatocyte proliferation on lipid metabolism and the consequent impact on whole-body physiology, we performed lipidomics, metabolomics, and RNA-seq analyses on a mouse model. We observed reduced triacylglycerides in liver of young mice, caused by oxidative stress that activated FOXO1 to promote the expression of Pnpla2/ATGL. Additionally, we discovered that hepatocytes displayed malfunctioning β-oxidation, reflected by increased acylcarnitines (ACs) and reduced β-hydroxybutyrate. This led to elevated plasma free fatty acids (FFAs), which were transported to the adipose tissue for storage and triggered greater insulin secretion. Upon aging, chronic hyperinsulinemia resulted in insulin resistance and hepatic steatosis through activation of LXR. Here, we demonstrate that loss of hepatocyte proliferation is not only an outcome but also possibly a causative factor for liver pathology.

Cell Sources and Influencing Factors of Liver Regeneration: A Review

Liver regeneration (LR) is a set of complicated mechanisms between cells and molecules in which the processes of initiation, maintenance, and termination of liver repair are regulated. Although LR has been studied extensively, there are still numerous challenges in gaining its full understanding. Cells for LR have a wide range of sources and the feature of plasticity, and regeneration patterns are not the same under different conditions. Many patients undergoing partial hepatectomy develop cirrhosis or steatosis. The changes of LR in these cases are not clear. Many types of cells participate in LR. Hepatocytes, biliary epithelial cells, hepatic progenitor cells, and human liver stem cells can serve as the cell sources for LR. However, different types and degrees of damage trigger the response from the most suitable cells. Exploring the cell sources of LR is of great significance for accelerating recovery of liver function under different pathological patterns and developing a cell therapy strategy to cope with the shortage of donors for liver transplantation. In clinical practice, the background of the liver influences regeneration. Fibrosis and steatosis create different LR microenvironments and signal molecule interaction patterns. In addition, factors such as partial hepatectomy, aging, platelets, nerves, hormones, bile acids, and gut microbiota are widely involved in this process. Understanding the influencing factors of LR has practical value for individualized treatment of patients with liver diseases. In this review, we have summarized recent studies and proposed our views. We discuss cell sources and the influential factors on LR to help in solving clinical problems.

Breed-related expression patterns of Ki67, γH2AX, and p21 during ageing in the canine liver

Cellular senescence is a molecular hallmark of ageing that is associated with multiple pathologies, and DNA damage marker γH2AX, together with cell cycle inhibitor p21, have been used as senescence markers in multiple species including dogs. Idiopathic canine chronic hepatitis has recognised breed-related differences in predisposition and prognosis, but reasons behind this are poorly understood. This retrospective study using archived post mortem tissue aimed to provide insight into liver ageing in 51 microscopically normal canine livers across seven breed categories, including those with and without increased risk of chronic hepatitis. Immunohistochemistry was conducted for γH2AX, p21, and cell proliferation marker Ki67, and the mean number of positive hepatocytes per high power field was determined. All three markers were strongly correlated to each other, but no age-dependent expression was seen in the combined study population. Overall expression levels were low in most dogs, with median values representing less than 1.5% of hepatocytes, but this increased to 20-30% in individual dogs at the upper end of the range. Individual breed differences were noted in two breeds that have increased risk of chronic hepatitis, with English Springer Spaniels having lower expression of Ki67 than other dogs, and Labradors having higher expression of Ki67 and γH2AX than other dogs. These results warrant further investigation in these breeds and highlight a need to validate reliable markers of cellular senescence in dogs.

Senescence in RASopathies, a possible novel contributor to a complex pathophenoype

Senescence is a biological process that induces a permanent cell cycle arrest and a specific gene expression program in response to various stressors. Following studies over the last few decades, the concept of senescence has evolved from an antiproliferative mechanism in cancer (oncogene-induced senescence) to a critical component of physiological processes associated with embryonic development, tissue regeneration, ageing and its associated diseases. In somatic cells, oncogenic mutations in RAS-MAPK pathway genes are associated with oncogene-induced senescence and cancer, while germline mutations in the same pathway are linked to a group of monogenic developmental disorders generally termed RASopathies. Here, we consider that in these disorders, senescence induction may result in opposing outcomes, a tumour protective effect and a possible contributor to a premature ageing phenotype identified in Costello syndrome, which belongs to the RASopathy group. In this review, we will highlight the role of senescence in organismal homeostasis and we will describe the current knowledge about senescence in RASopathies. Additionally, we provide a perspective on examples of experimentally characterised RASopathy mutations that, alone or in combination with various stressors, may also trigger an age-dependent chronic senescence, possibly contributing to the age-dependent worsening of RASopathy pathophenotype and the reduction of lifespan.

Hepatocellular Senescence: Immunosurveillance and Future Senescence-Induced Therapy in Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is the third leading cause of cancer-related deaths worldwide. The lack of effective targeted drugs has become a challenge on treating HCC patients. Cellular senescence is closely linked to the occurrence, development, and therapy of tumor. Induction of cellular senescence and further activation of immune surveillance provides a new strategy to develop HCC targeted drugs, that is, senescence-induced therapy for HCC. Precancerous hepatocytes or HCC cells can be induced into senescent cells, subsequently producing senescence-associated secretory phenotype (SASP) factors. SASP factors recruit and activate various types of immune cells, including T cells, NK cells, macrophages, and their subtypes, which carry out the role of immune surveillance and elimination of senescent cells, ultimately preventing the occurrence of HCC or inhibiting the progression of HCC. Specific interventions in several checkpoints of senescence-mediated therapy will make positive contributions to suppress tumorigenesis and progression of HCC, for instance, by applying small molecular compounds to induce cellular senescence or selecting cytokines/chemokines to activate immunosurveillance, supplementing adoptive immunocytes to remove senescent cells, and screening chemical drugs to induce apoptosis of senescent cells or accelerate clearance of senescent cells. These interventional checkpoints become potential chemotherapeutic targets in senescence-induced therapy for HCC. In this review, we focus on the frontiers of senescence-induced therapy and discuss senescent characteristics of hepatocytes during hepatocarcinogenesis as well as the roles and mechanisms of senescent cell induction and clearance, and cellular senescence-related immunosurveillance during the formation and progression of HCC.

The Dynamic Interplay Between Mast Cells, Aging/Cellular Senescence, and Liver Disease

Mast cells are key players in acute immune responses that are evidenced by degranulation leading to a heightened allergic response. Activation of mast cells can trigger a number of different pathways contributing to metabolic conditions and disease progression. Aging results in irreversible physiological changes affecting all organs, including the liver. The liver undergoes senescence, changes in protein expression, and cell signaling phenotypes during aging, which regulate disease progression. Cellular senescence contributes to the age-related changes. Unsurprisingly, mast cells also undergo age-related changes in number, localization, and activation throughout their lifetime, which adversely affects the etiology and progression of many physiological conditions including liver diseases. In this review, we discuss the role of mast cells during aging, including features of aging (e.g., senescence) in the context of biliary diseases such as primary biliary cholangitis and primary sclerosing cholangitis and nonalcoholic fatty liver disease.

Loss of hepatocyte cell division leads to liver inflammation and fibrosis

The liver possesses a remarkable regenerative capacity based partly on the ability of hepatocytes to re-enter the cell cycle and divide to replace damaged cells. This capability is substantially reduced upon chronic damage, but it is not clear if this is a cause or consequence of liver disease. Here, we investigate whether blocking hepatocyte division using two different mouse models affects physiology as well as clinical liver manifestations like fibrosis and inflammation. We find that in P14 Cdk1Liv-/- mice, where the division of hepatocytes is abolished, polyploidy, DNA damage, and increased p53 signaling are prevalent. Cdk1Liv-/- mice display classical markers of liver damage two weeks after birth, including elevated ALT, ALP, and bilirubin levels, despite the lack of exogenous liver injury. Inflammation was further studied using cytokine arrays, unveiling elevated levels of CCL2, TIMP1, CXCL10, and IL1-Rn in Cdk1Liv-/- liver, which resulted in increased numbers of monocytes. Ablation of CDK2-dependent DNA re-replication and polyploidy in Cdk1Liv-/- mice reversed most of these phenotypes. Overall, our data indicate that blocking hepatocyte division induces biological processes driving the onset of the disease phenotype. It suggests that the decrease in hepatocyte division observed in liver disease may not only be a consequence of fibrosis and inflammation, but also a pathological cue.

Short Telomeres: Cause and Consequence in Liver Disease

Short telomere syndrome is a genetically inherited syndrome resulting in premature telomere shortening. This premature shortening of telomeres can result in hematologic, pulmonary, vascular, gastrointestinal, and hepatic manifestations of disease. Identifying patients with short telomere syndrome can be a clinical challenge due to the multitude of potential manifestations and lack of widely available diagnostic tests. In this review, we will highlight hepatic manifestations of short telomere syndrome with a focus on diagnosis, testing, and potential treatments.

Aging and Chronic Liver Disease

Aging increases the incidence of chronic liver disease (CLD), worsens its prognosis, and represents the predominant risk factor for its development at all different stages. The hepatic sinusoid, which is fundamental for maintaining liver homeostasis, is composed by hepatocytes, liver sinusoidal endothelial cells, hepatic stellate cells, and hepatic macrophages. During CLD progression, hepatic cells suffer deregulations in their phenotype, which ultimately lead to disease development. The effects of aging on the hepatic sinusoid phenotype and function are not well understood, nevertheless, studies performed in experimental models of liver diseases and aging demonstrate alterations in all hepatic sinusoidal cells. This review provides an updated description of age-related changes in the hepatic sinusoid and discusses the implications for CLD development and treatment. Lastly, we propose aging as a novel therapeutic target to treat liver diseases and summarize the most promising therapies to prevent or improve CLD and extend healthspan.

Nonphagocytic Activation of NOX2 Is Implicated in Progressive Nonalcoholic Steatohepatitis During Aging

BACKGROUND AND AIMS

Older patients with obesity/type II diabetes mellitus frequently present with advanced NASH. Whether this is due to specific molecular pathways that accelerate fibrosis during aging is unknown. Activation of the Src homology 2 domain-containing collagen-related (Shc) proteins and redox stress have been recognized in aging; however, their link to NASH has not been explored.

APPROACH AND RESULTS

Shc expression increased in livers of older patients with NASH, as assessed by real time quantitative PCR (RT-qPCR) or western blots. Fibrosis, Shc expression, markers of senescence, and nicotinamide adenine dinucleotide phosphate, reduced form oxidases (NOXs) were studied in young/old mice on fast food diet (FFD). To inhibit Shc in old mice, lentiviral (LV)-short hairpin Shc versus control-LV were used during FFD. For hepatocyte-specific effects, floxed (fl/fl) Shc mice on FFD were injected with adeno-associated virus 8-thyroxine-binding globulin-Cre-recombinase versus control. Fibrosis was accelerated in older mice on FFD, and Shc inhibition by LV in older mice or hepatocyte-specific deletion resulted in significantly improved inflammation, reduction in senescence markers in older mice, lipid peroxidation, and fibrosis. To study NOX2 activation, the interaction of p47^phox (NOX2 regulatory subunit) and p52Shc was evaluated by proximity ligation and coimmunoprecipitations. Palmitate-induced p52Shc binding to p47^phox , activating the NOX2 complex, more so at an older age. Kinetics of binding were assessed in Src homology 2 domain (SH2) or phosphotyrosine-binding (PTB) domain deletion mutants by biolayer interferometry, revealing the role of SH2 and the PTB domains. Lastly, an in silico model of p52Shc/p47^phox interaction using RosettaDock was generated.

CONCLUSIONS

Accelerated fibrosis in the aged is modulated by p52Shc/NOX2. We show a pathway for direct activation of the phagocytic NOX2 in hepatocytes by p52Shc binding and activating the p47^phox subunit that results in redox stress and accelerated fibrosis in the aged.

Evaluation of induced biological effects in rats by continuous and natural gamma radiation using a physical simulator

PURPOSE

The effects of radioactivity on human health have been debated for many years but there are still important gaps that need to be addressed especially related to the effects of high natural background radiation on the local population. The beach of Meaípe, in the city of Guarapari (Brazil), emits natural gamma radiation due to the presence of monazite sands. We aimed to investigate the effects of gamma radiation doses on the biological system of wistar rats using a physical simulator of gamma radiation designed using Meaípe monazite sands.

METHODS

Female Wistar rats were divided into three groups, submitted to no radiation (control group) and to continuous radiation levels, one of very high level (20 μSv h^-1) and another of high level (3.6 μSv h^-1). The three group of animals were monitored weekly for 3 months and at the end of the study the animals were sacrificed, and the organs were extracted and weighed for anthropometric, oxidative stress and histological evaluations.

RESULTS

Exposure to radiation released by the monazite sands did not cause anthropometric alterations or blood pressure change in the animals. Similarly, there was no change in the quantification of ovarian follicles between the radiation groups and the control group. There was no difference in the oxidative stress quantification by the thiobarbituric acid reactive substance and advanced oxidation protein product methods in the ovaries. There were no evidenced damages in the structure of the renal tissue. It was observed the presence of granulomas in the hepatic tissue and alterations in the nuclei of the hepatocytes.

CONCLUSIONS

Our results suggest that the continuous exposure of females rats to 3.6 and 20 μSv h^-1 doses of gamma radiation slightly affected the hepatic tissue, but did not alter the histological parameters in the kidneys and ovaries and oxidative stress.

Liver fibrosis score, physical frailty, and the risk of dementia in older adults: The Italian Longitudinal Study on Aging

INTRODUCTION

Liver fibrosis increases progressively with aging and has been associated with poorer cognitive performance in middle-aged and older adults. We investigated the relationships between a non-invasive score for advanced liver fibrosis (non-alcoholic fatty liver disease [NAFLD] fibrosis score [NFS]) and dementia risk. We also assessed physical frailty, a common geriatric condition which is associated to dementia. We tested the joint effects of physical frailty and fibrosis on dementia incidence.

METHODS

A total of 1061 older adults (65 to 84 years), from the Italian Longitudinal Study on Aging, were prospectively evaluated for the risk of dementia in a period between 1992 and 2001. Liver fibrosis was defined according to the NFS. Physical frailty was assessed according to the Fried's criteria. Cox proportional hazards models were used to estimate the short- and long-term risk of overall dementia, associated to the NFS, testing the effect modifier of physical frailty status.

RESULTS

Older adults with only high NFS (F3-F4) did not exhibit a significant increased risk of overall dementia. Over 8 years of follow-up, frail older adults with high NFS had an increased risk of overall dementia (hazard ratio [HR]: 4.23; 95% confidence interval [CI]: 1.22 to 14.70, P = .023). Finally, physically frail older adults with low albumin serum levels (albumin < 4.3 g/dL) and with advanced liver fibrosis (F3-F4 NFS) compared to those with lower liver fibrosis score (F0-F2 NFS) were more likely to have a higher risk of overall dementia in a long term-period (HR: 16.42; 95% CI: 1.44 to 187.67, P = .024).

DISCUSSION

Advanced liver fibrosis (F3-F4 NFS) could be a long-term predictor for overall dementia in people with physical frailty. These findings should encourage a typical geriatric, multidisciplinary assessment which accounts also for the possible co-presence of frail condition in older adults with chronic liver disease and liver fibrosis.

A Senescence-Centric View of Aging: Implications for Longevity and Disease

Cellular senescence is a state of stable cell cycle arrest associated with macromolecular alterations and secretion of proinflammatory cytokines and molecules. From their initial discovery in the 1960s, senescent cells have been hypothesized as potential contributors to the age-associated loss of regenerative potential. Here, we discuss recent evidence that implicates cellular senescence as a central regulatory mechanism of the aging process. We provide a comprehensive overview of age-associated pathologies in which cellular senescence has been implicated. We describe mechanisms by which senescent cells drive aging and diseases, and we discuss updates on exploiting these mechanisms as therapeutic targets. Finally, we critically analyze the use of senotherapeutics and their translation to the clinic, highlighting limitations and suggesting ideas for future applications and developments.

The role of adipose tissue senescence in obesity- and ageing-related metabolic disorders

Adipose tissue as the largest energy reservoir and endocrine organ is essential for maintenance of systemic glucose, lipid and energy homeostasis, but these metabolic functions decline with ageing and obesity. Adipose tissue senescence is one of the common features in obesity and ageing. Although cellular senescence is a defensive mechanism preventing tumorigenesis, its occurrence in adipose tissue causatively induces defective adipogenesis, inflammation, aberrant adipocytokines production and insulin resistance, leading to adipose tissue dysfunction. In addition to these paracrine effects, adipose tissue senescence also triggers systemic inflammation and senescence as well as insulin resistance in the distal metabolic organs, resulting in Type 2 diabetes and other premature physiological declines. Multiple cell types including mature adipocytes, immune cells, endothelial cells and progenitor cells gradually senesce at different levels in different fat depots with ageing and obesity, highlighting the heterogeneity and complexity of adipose tissue senescence. In this review, we discuss the causes and consequences of adipose tissue senescence, and the major cell types responsible for adipose tissue senescence in ageing and obesity. In addition, we summarize the pharmacological approaches and lifestyle intervention targeting adipose tissue senescence for the treatment of obesity- and ageing-related metabolic diseases.

Association of leukocyte telomere length with non-alcoholic fatty liver disease in patients with type 2 diabetes

Background:

Leukocyte telomere has been shown to be related to insulin resistance-related diseases, such as type 2 diabetes mellitus (T2DM) and non-alcoholic fatty liver disease (NAFLD). This cross-sectional study investigated the association of leukocyte telomere length (LTL) with NAFLD in T2DM patients.

Methods:

Clinical features were collected and LTL was measured by Southern blot-based terminal restriction fragment length analysis in 120 T2DM patients without NAFLD and 120 age-matched T2DM patients with NAFLD. NAFLD was clinically defined by manifestations of ultrasonography. The correlation between LTL and clinical and biochemical parameters were analyzed by Pearson correlation or Spearman correlation analysis. Factors for NAFLD in T2DM patients were identified using multiple logistic regressions.

Results:

LTL in T2DM patients with NAFLD were significantly longer than those without NAFLD (6400.2 ± 71.8 base pairs [bp] vs. 6023.7 ± 49.5 bp, P < 0.001), especially when diabetes duration was less than 2 years. Meanwhile, the trend of shorter LTL was associated with the increased diabetes duration in T2DM patient with NAFLD, but not in T2DM patients without NAFLD. Finally, LTL (odds ratio [OR]: 1.001, 95% confidence interval [CI]: 1.000–1.002, P = 0.001), as well as body mass index (OR: 1.314, 95% CI: 1.169–1.477, P < 0.001) and triglycerides (OR: 1.984, 95% CI: 1.432–2.747, P < 0.001), had a significant association with NAFLD status in T2DM patients.

Conclusions:

T2DM patients with NAFLD had a significantly longer LTL than those without NAFLD. The longer LTL was especially evident in the early stage of T2DM, indicating that longer LTL may be used as a biomarker for NAFLD in T2DM patients.

Cellular senescence in vivo: From cells to tissues to pathologies

Senescent cells accumulate during aging in a variety of tissues. Although scarce, they could influence tissue function non-cell-autonomously via secretion of a range of factors in their neighborhood. Recent studies support a role of senescent cells in age-related morbidity, including neurodegenerative diseases, cardiovascular pathologies, cancers, aging-associated nephrological alterations, chronic pulmonary disease and osteoarthritis, indicating that senescent cells could represent an interesting target for therapeutic exploitation across a range of pathophysiological contexts. In this article, we review data available to indicate which cell types can undergo senescence within various mammalian tissue environments and how these processes may contribute to tissue-specific pathologies associated with old age. We also consider markers used to identify senescent cells in vitro and in vivo. The data discussed may serve as an important starting point for an extended definition of molecular and functional characteristics of senescent cells in different organs and may hence promote the development and refinement of targeting strategies aimed at removing senescent cells from aging tissues.

Modifying nutritional substrates induces macrovesicular lipid droplet accumulation and metabolic alterations in a cellular model of hepatic steatosis

BACKGROUND AND AIMS

Nonalcoholic fatty liver disease (NAFLD) begins with steatosis, where a mixed macrovesicular pattern of large and small lipid droplets (LDs) develops. Since in vitro models recapitulating this are limited, the aims of this study were to develop mixed macrovesicular steatosis in immortalized hepatocytes and investigate effects on intracellular metabolism by altering nutritional substrates.

METHODS

Huh7 cells were cultured in 11 mM glucose and 2% human serum (HS) for 7 days before additional sugars and fatty acids (FAs), either with 200 µM FAs (low fat low sugar; LFLS), 5.5 mM fructose + 200 µM FAs (low fat high sugar; LFHS), or 5.5 mM fructose + 800 µM FAs (high fat high sugar; HFHS), were added for 7 days. FA metabolism, lipid droplet characteristics, and transcriptomic signatures were investigated.

RESULTS

Between the LFLS and LFHS conditions, there were few notable differences. In the HFHS condition, intracellular triacylglycerol (TAG) was increased and the LD pattern and distribution was similar to that found in primary steatotic hepatocytes. HFHS-treated cells had lower levels of de novo-derived FAs and secreted larger, TAG-rich lipoprotein particles. RNA sequencing and gene set enrichment analysis showed changes in several pathways including those involved in metabolism and cell cycle.

CONCLUSIONS

Repeated doses of HFHS treatment resulted in a cellular model of NAFLD with a mixed macrovesicular LD pattern and metabolic dysfunction. Since these nutrients have been implicated in the development of NAFLD in humans, the model provides a good physiological basis for studying NAFLD development or regression in vitro.

Cell cycle regulation in NAFLD: when imbalanced metabolism limits cell division

Cell division is essential for organismal growth and tissue homeostasis. It is exceptionally significant in tissues chronically exposed to intrinsic and external damage, like the liver. After decades of studying the regulation of cell cycle by extracellular signals, there are still gaps in our knowledge on how these two interact with metabolic pathways in vivo. Studying the cross-talk of these pathways has direct clinical implications as defects in cell division, signaling pathways, and metabolic homeostasis are frequently observed in liver diseases. In this review, we will focus on recent reports which describe various functions of cell cycle regulators in hepatic homeostasis. We will describe the interplay between the cell cycle and metabolism during liver regeneration after acute and chronic damage. We will focus our attention on non-alcoholic fatty liver disease, especially non-alcoholic steatohepatitis. The global incidence of non-alcoholic fatty liver disease is increasing exponentially. Therefore, understanding the interplay between cell cycle regulators and metabolism may lead to the discovery of novel therapeutic targets amenable to intervention.

Perforin Acts as an Immune Regulator to Prevent the Progression of NAFLD

Non-alcoholic fatty liver disease (NAFLD) is one of the main causes of cirrhosis and major risk factors for hepatocellular carcinoma and liver-related death. Despite substantial clinical and basic research, the pathogenesis of obesity-related NAFLD remains poorly understood. In this study, we show that perforin can act as an immune regulator to prevent the progression of NAFLD. Aged perforin-deficient (Prf^−/−) mice have increased lipid accumulation in the liver compared to WT mice. With high-fat diet (HFD) challenge, Prf^−/− mice have increased liver weight, more severe liver damage, and increased liver inflammation when compared with WT controls. Mechanistic studies revealed that perforin specifically regulates intrinsic IFN-γ production in CD4 T cells, not CD8 T cells. We found that CD4 T cell depletion reduces liver injury and ameliorates the inflammation and metabolic morbidities in Prf^−/− mice. Furthermore, improved liver characteristics in HFD Prf^−/− and IFN-γR^−/− double knockout mice confirmed that IFN-γ is a key factor for mediating perforin regulation of NAFLD progression. Overall, our findings reveal the important regulatory role perforin plays in the progression of obesity-related NAFLD and highlight novel strategies for treating NAFLD.

Ginsenoside Rg1 protects against d-galactose induced fatty liver disease in a mouse model via FOXO1 transcriptional factor

AIMS

Rg1 is the most active component of traditional Chinese medicine ginseng, having anti-aging and anti-oxidative stress features in multiple organs. Cellular senescence of hepatocytes is involved in the progression of a wide spectrum of chronic liver diseases. In this study, we investigated the potential benefits and mechanism of action of Rg1 on aging-driven chronic liver diseases.

MATERIALS AND METHODS

A total of 40 male C57BL/6 mice were randomly divided into four groups: control group; Rg1 group; Rg1+d-gal group; and d-gal group. Blood and liver tissue samples were collected for determination of liver function, biochemical and molecular markers, as well as histopathological investigation.

KEY FINDINGS

Rg1 played an anti-aging role in reversing d-galactose induced increase in senescence-associated SA-β-gal staining and p53, p21 protein in hepatocytes of mice and sustained mitochondria homeostasis. Meanwhile, Rg1 protected livers from d-galactose caused abnormal elevation of ALT and AST in serum, hepatic steatosis, reduction in hepatic glucose production, hydrogenic degeneration, inflammatory phenomena including senescence-associated secretory phenotype (SASP) IL-1β, IL-6, MCP-1 elevation and lymphocyte infiltration. Furthermore, Rg1 suppressed drastic elevation in FOXO1 phosphorylation resulting in maintaining FOXO1 protein level in the liver after d-galactose treatment, followed by FOXO1 targeted antioxidase SOD and CAT significant up-regulation concurrent with marked decrease in lipid peroxidation marker MDA.

SIGNIFICANCE

Rg1 exerts pharmaceutic effects of maintaining FOXO1 activity in liver, which enhances anti-oxidation potential of Rg1 to ameliorate SASP and to inhibit inflammation, also promotes metabolic homeostasis, and thus protects livers from senescence induced fatty liver disease. The study provides a potential therapeutic strategy for alleviating chronic liver pathology.

New aspects of hepatic endothelial cells in physiology and nonalcoholic fatty liver disease

The liver is the central metabolic hub for carbohydrate, lipid, and protein metabolism. It is composed of four major types of cells, including hepatocytes, endothelial cells (ECs), Kupffer cells, and stellate cells. Hepatic ECs are highly heterogeneous in both mice and humans, representing the second largest population of cells in liver. The majority of them line hepatic sinusoids known as liver sinusoidal ECs (LSECs). The structure and biology of LSECs and their roles in physiology and liver disease were reviewed recently. Here, we do not give a comprehensive review of LSEC structure, function, or pathophysiology. Instead, we focus on the recent progress in LSEC research and other hepatic ECs in physiology and nonalcoholic fatty liver disease and other hepatic fibrosis-related conditions. We discuss several current areas of interest, including capillarization, scavenger function, autophagy, cellular senescence, paracrine effects, and mechanotransduction. In addition, we summarize the strengths and weaknesses of evidence for the potential role of endothelial-to-mesenchymal transition in liver fibrosis.

The p66shc-mediated Regulation of Hepatocyte Senescence Influences Hepatic Steatosis in Nonalcoholic Fatty Liver Disease

Background

Recent studies have suggested that hepatocyte senescence could contribute to hepatic steatosis and its progression in nonalcoholic fatty liver disease (NAFLD). However, the underlying mechanism causing hepatocyte senescence in this pathological condition is still unclear. A thorough understanding of the mechanism could provide a new target for therapeutic intervention. The purpose of this study was to investigate the role of p66shc in hepatocyte senescence and hepatocyte damage in NAFLD progression.

Material/Methods

We examined the expression levels of hepatic p66shc and senescence markers in rats and humans with NAFLD, and we assessed the effect of p66shc knockdown or overexpression on senescence and steatosis in human liver cells.

Results

In this study, we showed that increased hepatic p66shc expression was consistent with upregulated expression of the following senescence markers in NAFLD rats: heterochromatin protein-1-beta (HP1β), p16, p21, and p53. Furthermore, senescence and steatosis could be induced in hepatoblastoma cell line (HepG2) cells when cells were stimulated with a low concentration of H₂O₂, and this effect was significantly alleviated by knockdown of p66shc. However, overexpression of p66shc could promote senescence and steatosis in L02 cells. Finally, increased hepatic p66shc protein levels correlated with enhanced expression of the senescence marker p21 and mirrored the degree of disease severity in NAFLD patients.

Conclusions

Our findings indicated that the increase in hepatocyte senescence and steatosis in NAFLD may be caused by the upregulation of p66shc expression, implying that strategies for p66shc-mediated regulation of hepatocyte senescence may provide new therapeutic tools for NAFLD.

The aged liver: Beyond cellular senescence

The aging of the population, the increased prevalence of chronic liver diseases in elderly and the need to broaden the list of potential liver donors enjoin us to better understand what is an aged liver. In this review, we provide a brief introduction to cellular senescence, revisit the main morphological and functional modifications of the liver induced by aging, particularly concerning metabolism, immune response and regeneration, and try to elude some of the signalling pathways responsible for these modifications. Finally, we discuss the clinical consequences of aging on chronic liver diseases and the implications of older age for donors and recipients in liver transplantation.

Elimination of Age-Associated Hepatic Steatosis and Correction of Aging Phenotype by Inhibition of cdk4-C/EBPα-p300 Axis

The aging liver is affected by several disorders, including steatosis, that can lead to a decline of liver functions. Here, we present evidence that the cdk4-C/EBPα-p300 axis is a critical regulator of age-associated disorders, including steatosis. We found that patients with non-alcoholic fatty liver disease (NAFLD) have increased levels of cdk4 and that cdk4-resistant C/EBPα-S193A mice do not develop hepatic steatosis with advancing age. Underlying mechanisms include a block in C/EBPα activation and subsequent failure in activation of enzymes involved in the development of NAFLD. Inhibition of cdk4 in aged wild-type (WT) mice by a specific cdk4 inhibitor, PD-0332991, reduces C/EBPα-p300 complexes and eliminates hepatic steatosis. Moreover, the inhibition of cdk4 in aged mice reverses many age-related disorders. Mechanisms of correction include elimination of cellular senescence and alterations in the chromatin structure of hepatocytes. Thus, the inhibition of cdk4 might be considered as a therapeutic approach to correct age-associated liver disorders.

Nguyen et al. show that nuclear elevation of cdk4 leads to age-associated disorders, such as hepatic steatosis, and to age-dependent decline of liver functions and morphology. Elevation of cdk4 changes multiple molecular aspects of liver biology. Inhibition of cdk4 in old mice eliminates hepatic steatosis and corrects age-associated liver disorders.

Ataxia telangiectasia mutated pathway disruption affects hepatic DNA and tissue damage in nonalcoholic fatty liver disease

To overcome the rising burdens of nonalcoholic fatty liver disease, mechanistic linkages in mitochondrial dysfunction, inflammation and hepatic injury are critical. As ataxia telangiectasia mutated (ATM) gene oversees DNA integrity and mitochondrial homeostasis, we analyzed mRNAs and total proteins or phosphoproteins related to ATM gene by arrays in subjects with healthy liver, fatty liver or nonalcoholic steatohepatitis. Functional genomics approaches were used to query DNA damage or cell growth events. The effects of fatty acid-induced toxicity in mitochondrial health, DNA integrity and cell proliferation were validated in HuH-7 cells, including by inhibiting ATM kinase activity or knckdown of its mRNA. In fatty livers, DNA damage and ATM pathway activation was observed. During induced steatosis in HuH-7 cells, lowering of ATM activity produced mitochondrial dysregulation, DNA damage and cell growth inhibition. In livers undergoing steatohepatitis, ATM was depleted with increased hepatic DNA damage and growth-arrest due to cell cycle checkpoint activations. Moreover, molecular signatures of oncogenesis were associated with upstream mechanistic networks directing cell metabolism, inflammation or growth that were either activated (in fatty liver) or inactivated (in steatohepatitis). To compensate for hepatic growth arrest, preoncogenic oval cell populations expressing connexin-43 and/or albumin emerged. These oval cells avoided DNA damage and proliferated actively. We concluded that ATM is a major contributor to the onset and progression of nonalcoholic fatty liver disease. Therefore, specific markers for ATM pathway dysregulation will allow prospective segregation of cohorts for disease susceptibility and progression from steatosis to steatohepatitis. This will offer superior design and evaluation parameters for clinical trials. Restoration of ATM activity with targeted therapies should be appropriate for nonalcoholic fatty liver disease.

The Alpha-1 Antitrypsin Polymer Load Correlates With Hepatocyte Senescence, Fibrosis Stage and Liver-Related Mortality

Background

Alpha-1 antitrypsin deficiency (AATD) is an important, inherited cause of chronic liver disease. Marked variation in fibrosis stages in patients with homozygous deficiency and those factors that determine whether heterozygous carriers develop liver fibrosis, remain unexplained. Murine studies implicate polymerized alpha-1 antitrypsin (AAT) within hepatocytes as pathogenic.

Aims and Methods

The relationship between the quantity of polymerized AAT within hepatocytes (polymer load), stage of hepatic fibrosis and liver-related clinical outcomes (death, evolution to hepatocellular carcinoma, or need for liver transplantation) were investigated using liver tissue from 92 patients at first presentation with either homozygous or heterozygous AATD. Further tissue-based studies were undertaken to determine if polymerized AAT was associated with failure of cell cycle progression, accelerated aging or hepatocyte senescence by immunohistochemical analysis.

Results

The AAT polymer load correlated closely with hepatic fibrosis stage and long-term clinical outcome, independent of homozygous or heterozygous status. AAT polymers within hepatocytes correlated closely with failure of cell cycle progression assessed using cell cycle phase markers, accelerated aging manifest as shortened telomeres and other markers consistent with hepatocyte senescence manifest as the presence of nuclear p21 expression and enlarged nuclei. The proportion of p21 positive hepatocytes or hepatocytes with enlarged nuclei correlated with hepatic fibrosis stage and the long-term clinical outcome.

Conclusion

These data suggest that accumulation of AAT polymers within hepatocytes drives senescence. Quantitation of both the AAT polymer load or hepatocyte senescence markers correlated with hepatic fibrosis stage and the long-term clinical outcome. Either or both could be considered markers of disease severity and treatment response in clinical trials.

Chronic Inflammation in Non-Alcoholic Steatohepatitis: Molecular Mechanisms and Therapeutic Strategies

Non-Alcoholic Steatohepatitis (NASH) is the progressive form of Non-Alcoholic Fatty Liver Disease (NAFLD), the main cause of chronic liver complications. The development of NASH is the consequence of aberrant activation of hepatic conventional immune, parenchymal, and endothelial cells in response to inflammatory mediators from the liver, adipose tissue, and gut. Hepatocytes, Kupffer cells and liver sinusoidal endothelial cells contribute to the significant accumulation of bone-marrow derived-macrophages and neutrophils in the liver, a hallmark of NASH. The aberrant activation of these immune cells elicits harmful inflammation and liver injury, leading to NASH progression. In this review, we highlight the processes triggering the recruitment and/or activation of hepatic innate immune cells, with a focus on macrophages, neutrophils, and innate lymphoid cells as well as the contribution of hepatocytes and endothelial cells in driving liver inflammation/fibrosis. On-going studies and preliminary results from global and specific therapeutic strategies to manage this NASH-related inflammation will also be discussed.

The Role of Senescence in the Development of Nonalcoholic Fatty Liver Disease and Progression to Nonalcoholic Steatohepatitis

In recent years, cellular senescence has generated a lot of interest among researchers because of its involvement in both the normal aging process and common human diseases. During senescence, cells undergo alterations that include telomere shortening, nuclear area enlargement, and genomic and mitochondrial DNA damage, leading to irreversible cell cycle arrest, and secretion of proinflammatory cytokines. Evidence suggests that the complex process of senescence is involved in the development of a plethora of chronic diseases including metabolic and inflammatory disorders and tumorigenesis. Recently, several human and animal studies have emphasized the involvement of senescence in the pathogenesis and development of liver steatosis including the progression to nonalcoholic steatohepatitis (NASH) as characterized by the additional emergence of inflammation, hepatocyte ballooning, and liver fibrosis. The development of nonalcoholic fatty liver disease (NAFLD) and its progression to NASH are commonly accompanied by several pathophysiological events including metabolic dysregulation and inflammatory phenomena occurring within the liver that may contribute to or derive from cellular senescence, implying that the latter may be both a stimulus and a consequence of the disease. Conclusion: In this review, we summarize the current literature on the impact of cellular senescence in NAFLD/NASH and discuss the effectiveness and safety of novel senolytic drugs and therapeutic options available to delay or treat the disease. Finally, we identify the open questions and issues to be addressed in the near future.

Increased Glutaminolysis Marks Active Scarring in Nonalcoholic Steatohepatitis Progression

BACKGROUND & AIMS

Nonalcoholic steatohepatitis (NASH) occurs in the context of aberrant metabolism. Glutaminolysis is required for metabolic reprograming of hepatic stellate cells (HSCs) and liver fibrogenesis in mice. However, it is unclear how changes in HSC glutamine metabolism contribute to net changes in hepatic glutaminolytic activity during fibrosis progression, or whether this could be used to track fibrogenic activity in NASH. We postulated that increased HSC glutaminolysis marks active scarring in NASH.

METHODS

Glutaminolysis was assessed in mouse NASH fibrosis models and in NASH patients. Serum and liver levels of glutamine and glutamate and hepatic expression of glutamine transporter/metabolic enzymes were correlated with each other and with fibrosis severity. Glutaminolysis was disrupted in HSCs to examine if this directly influenced fibrogenesis. ¹⁸F-fluoroglutamine positron emission tomography was used to determine how liver glutamine assimilation tracked with hepatic fibrogenic activity in situ.

RESULTS

The serum glutamate/glutamine ratio increased and correlated with its hepatic ratio, myofibroblast content, and fibrosis severity. Healthy livers almost exclusively expressed liver-type glutaminase (Gls2); Gls2 protein localized in zone 1 hepatocytes, whereas glutamine synthase was restricted to zone 3 hepatocytes. In fibrotic livers, Gls2 levels reduced and glutamine synthase zonality was lost, but both Slc1a5 (glutamine transporter) and kidney-type Gls1 were up-regulated; Gls1 protein was restricted to stromal cells and accumulated in fibrotic septa. Hepatocytes did not compensate for decreased Gls2 by inducing Gls1. Limiting glutamine or directly inhibiting GLS1 inhibited growth and fibrogenic activity in cultured human HSCs. Compared with healthy livers, fibrotic livers were ¹⁸F-fluoroglutamine-avid by positron emission tomography, suggesting that glutamine-addicted myofibroblasts drive increased hepatic utilization of glutamine as fibrosis progresses.

CONCLUSIONS

Glutaminolysis is a potential diagnostic marker and therapeutic target during NASH fibrosis progression.

Alpha-1 Antitrypsin Deficiency and Accelerated Aging: A New Model for an Old Disease?

Alpha-1 antitrypsin (AAT) protects the lung by inhibiting neutrophil proteinases, but AAT has many other non-proteolytic functions that are anti-inflammatory, antiviral and homeostatic. Approximately 1 in 1600 to 1 in 5000 people have the homozygous Z mutation, which causes AAT misfolding, accumulation in (predominantly) liver cells and low circulating levels of AAT, leading to AAT deficiency (AATD). AATD is classically a disease of neutrophilic inflammation, with an aggressive and damaging innate immune response contributing to emphysema and other pathologies. AATD is one of the most common genetic disorders but considerably under-recognised. Most patients are diagnosed later in life, by which time they may have accumulated significant lung, liver and multisystem damage. Disease presentation is heterogeneous and not fully explained by deficiency levels alone or exposure to cigarette smoking. This suggests other factors influence AATD-associated pathological processes. Aging itself is associated with organ dysfunction, including emphysema and airflow obstruction, inflammation, altered immune cell responses (termed immunosenescence) and a loss of proteostasis. Many of these processes are present in AATD but at an earlier age and more advanced stage compared with chronological aging alone. Augmentation therapy does not completely abrogate the manifold disease processes present in AATD. New approaches are needed. There is emerging evidence that both age- and AATD-related disease processes are amenable to correction by targeting proteostasis, autophagy, immunosenescence and epigenetic factors. This review explores the impact of the aging process on AATD presentation and discusses novel therapeutic strategies to mitigate low levels of AAT or misfolded AAT in an aging host.

Active vitamin D supplementation alleviates initiation and progression of nonalcoholic fatty liver disease by repressing the p53 pathway

BACKGROUND/AIMS

Recent studies have found vitamin D deficiency promotes fat deposition into the hepatocytes, thus contributing to the development of nonalcoholic fatty liver disease (NAFLD), which is a hepatic manifestation of metabolic syndrome. This study aimed to investigate the potential effects of vitamin D on NAFLD with the involvement of the p53 pathway.

METHODS

Initially, an in vivo high-fat diet (HFD)-induced NAFLD mouse model was established. Then the HFD-induced NAFLD mice were treated with vitamin D. Next, the serum levels of TNF-α, GSH-px and malondialdehyde (MDA) were assessed using ELISA and ROS content was evaluated by flow cytometry, followed by the measurement of expression of Duox1, Duox2, SOD1, SOD2, PRDX1 I, ACC, SREBP1c, MTTP, PPARα, p53, p21 and p16 using RT-qPCR and Western blot analysis. Positive expression of FAS and FASL proteins was measured using immunohistochemistry. TUNEL and Senescence-associated β-galactosidase (SA-β-Gal) staining were subsequently conducted to assess the senescence and apoptosis of hepatocytes.

RESULTS

HFD-induced mice treated with vitamin D presented with significantly increased GSH-px levels, as well as protein expression of SOD1, SOD2, PRDX1, MTTP and PPARα, but decreased MDA and ROS levels, expression of Duox1, Duox2, ACC, SREBP1c, p53, p21 and p16, positive expression of FAS and FASL proteins as well as impaired senescence and apoptosis of hepatocytes.

CONCLUSION

Active vitamin D supplementation could potentially impede hepatocyte senescence and apoptosis via suppression of the p53 pathway, thus preventing the progression of NAFLD. Our study provides available evidence on the potential clinical utility of vitamin D supplementation in NAFLD.

Senescent hepatocytes enhance natural killer cell activity via the CXCL-10/CXCR3 axis

Cellular senescence and natural killer (NK) cells play an important role in liver diseases. Chemokines, a component of the senescence-associated secretory phenotype, can recruit NK cells and are involved in the development of various liver diseases. The effect of the C-X-C motif chemokine ligand (CXCL)-9, −10, −11/C-X-C motif chemokine receptor (CXCR)3 axis in senescent hepatocytes remains unknown. The chemokines secreted by senescent hepatocytes, the contribution of the CXCL-9, −10, −11/CXCR3 axis to the migration of NK cells, and the effect of senescent hepatocytes on the function of NK cells were investigated in the present study. The results demonstrated significantly increased levels of C-C motif chemokine ligand 2 and CXCL-1, −2 and −10 in the supernatant of senescent AML12 cells. Despite increased mRNA expression of CXCL-9, −10, and −11 in these cells, western blotting revealed significantly enhanced expression of only CXCL-10. The expression of CXCR3 on the surface of NK cells stimulated by senescent AML12 cells was upregulated (fold change, >3). Following incubation with the supernatant of senescent hepatocytes, both CD107a and interferon γ expression in NK cells increased by >2.5-fold. The cytotoxic effect of NK cells was notably higher stimulated by senescent AML12 cells. Chemotaxis and blocking assays demonstrated that the senescent hepatocytes enhanced the migration of NK cells via the CXCL-10/CXCR3 axis. The present study suggests that senescent hepatocytes secrete various chemokines, including CXCL-10, resulting in the upregulation and activation of CXCR3 in NK cells and the enhancement of NK cell migration via the CXCL-10/CXCR3 axis.

Hepatic senescence, the good and the bad

Gradual alterations of cell’s physiology and functions due to age or exposure to various stresses lead to the conversion of normal cells to senescent cells. Once becoming senescent, the cell stops dividing permanently but remains metabolically active. Cellular senescence does not have a single marker but is characterized mainly by a combination of multiple markers, such as, morphological changes, expression of cell cycle inhibitors, senescence associated β-galactosidase activity, and changes in nuclear membrane. When cells in an organ become senescent, the entire organism can be affected. This may occur through the senescence-associated secretory phenotype (SASP). SASP may exert beneficial or harmful effects on the microenvironment of tissues. Research on senescence has become a very exciting field in cell biology since the link between age-related diseases, including cancer, and senescence has been established. The loss of regenerative and homeostatic capacity of the liver over the age is somehow connected to cellular senescence. The major contributors of senescence properties in the liver are hepatocytes and cholangiocytes. Senescent cells in the liver have been implicated in the etiology of chronic liver diseases including cirrhosis and hepatocellular carcinoma and in the interference of liver regeneration. This review summarizes recently reported findings in the understanding of the molecular mechanisms of senescence and its relationship with liver diseases.

New Perspectives in Liver Transplantation: From Regeneration to Bioengineering

Advanced liver diseases have very high morbidity and mortality due to associated complications, and liver transplantation represents the only current therapeutic option. However, due to worldwide donor shortages, new alternative approaches are mandatory for such patients. Regenerative medicine could be the more appropriate answer to this need. Advances in knowledge of physiology of liver regeneration, stem cells, and 3D scaffolds for tissue engineering have accelerated the race towards efficient therapies for liver failure. In this review, we propose an update on liver regeneration, cell-based regenerative medicine and bioengineering alternatives to liver transplantation.

The role of telomeres and telomerase in cirrhosis and liver cancer

Telomerase is a key enzyme for cell survival that prevents telomere shortening and the subsequent cellular senescence that is observed after many rounds of cell division. In contrast, inactivation of telomerase is observed in most cells of the adult liver. Absence of telomerase activity and shortening of telomeres has been implicated in hepatocyte senescence and the development of cirrhosis, a chronic liver disease that can lead to hepatocellular carcinoma (HCC) development. During hepatocarcinogenesis, telomerase reactivation is required to enable the uncontrolled cell proliferation that leads to malignant transformation and HCC development. Part of the telomerase complex, telomerase reverse transcriptase, is encoded by TERT, and several mechanisms of telomerase reactivation have been described in HCC that include somatic TERT promoter mutations, TERT amplification, TERT translocation and viral insertion into the TERT gene. An understanding of the role of telomeres and telomerase in HCC development is important to develop future targeted therapies and improve survival of this disease. In this Review, the roles of telomeres and telomerase in liver carcinogenesis are discussed, in addition to their potential translation to clinical practice as biomarkers and therapeutic targets.

Hypoxia-adaptive pathways: A pharmacological target in fibrotic disease?

Wound healing responses are physiological reactions to injuries and share common characteristics and phases independently of the injured organ or tissue. A major hallmark of wound healing responses is the formation of extra-cellular matrix (ECM), mainly consisting of collagen fibers, to restore the initial organ architecture and function. Overshooting wound healing responses result in unphysiological accumulation of ECM and collagen deposition, a process called fibrosis. Importantly, hypoxia (oxygen demand exceeds supply) plays a significant role during wound healing responses and fibrotic diseases. Under hypoxic conditions, cells activate a gene program, including the stabilization of hypoxia-inducible factors (HIFs), which induces the expression of HIF target genes counteracting hypoxia. In contrast, in normoxia, so-called HIF-prolyl hydroxylases (PHDs) oxygen-dependently hydroxylate HIF-α, which marks it for proteasomal degradation. Importantly, PHDs can be pharmacologically inhibited (PHI) by so-called PHD inhibitors. There is mounting evidence that the HIF-pathway is continuously up-regulated during the development of tissue fibrosis, and that pharmacological (HIFI) or genetic inhibition of HIF can prevent organ fibrosis. By contrast, initial (short-term) activation of the HIF pathway via PHI during wound healing seems to be beneficial in several models of inflammation or acute organ injury. Thus, timing and duration of PHI and HIFI treatment seem to be crucial. In this review, we will highlight the role of hypoxia-adaptive pathways during wound healing responses and development of fibrotic disease. Moreover, we will discuss whether PHI and HIFI might be a promising treatment option in fibrotic disease, and consider putative pitfalls that might result from this approach.

Senescence suppressed proliferation of host hepatocytes is precondition for liver repopulation

In the fumarylacetoacetate hydrolase deficient (Fah^-/-) mouse, massive liver repopulation can be easily obtained after transplanted hepatocytes. Understanding the mechanisms of complete liver repopulation in Fah^-/- mice will be useful for future clinical application. Here, we found that the endogenous hepatocytes in liver of Fah^-/- mice undertook senescence during the time of tyrosinemia symptoms. Increase of senescent hepatocytes in Fah^-/- mice provided proliferative advantage to the transplanted hepatocytes. Importantly, senescent hepatocytes upregulated the expression of extracellular matrix enzyme, contributing to degradation of extracellular matrix components and weakness of cell adhesion and connection. The liver exhibiting a loose architecture provided the space for the engraftment and expansion of transplanted hepatocytes. These findings underscore the underlying mechanisms of completed liver repopulation in Fah^-/- mice. Senescence followed by loose hepatic parenchyma is a preconditioning for liver repopulation, which would be a promising strategy to achieve therapeutic liver repopulation in clinical settings.

The fibrogenic process and the unleashing of acute-on-chronic liver failure

Acute-on-chronic liver failure (ACLF) is a life-threatening condition characterized by a rapid deterioration of previously well-compensated chronic liver diseases. One of the main obstacles in ACLF is the lack of knowledge of the pathogenesis and specific broad-spectrum treatments. An excessive systemic inflammatory response has been proposed to explain the pathogenesis of ACLF; this hypothesis involves stellate cells, which are implicated in many liver homeostatic functions that include vitamin A storage, regulation of sinusoidal blood flow, local inflammation, maintenance of the hepatocyte phenotype and extracellular matrix remodeling. However, when there is damage to the liver, these cells are the main target of the inflammatory stimulus, as a result, the secretion of the extracellular matrix is altered. Activated hepatic stellate cells raise the survival of neutrophils by the stimulation of granulocytes colonies and macrophages, which exacerbates liver inflammation and promotes damage to hepatocytes. Elevation of pathogen-associated molecular patterns is related to liver damage by different pathophysiological mechanisms of decompensation, showing ballooning degeneration and cell death with a predominance of cholestatic infection. Moreover, patients with ACLF present a marked elevation of C-reactive protein together with an elevation of the leukocyte count. Chronic liver disease is a complex pathological state with a heterogeneous pathophysiology in which genetic factors of the host and external triggers interact and culminate in hepatic insufficiency. The better understanding of such interactions should lead to a better comprehension of the disease and to the discovery of new treatment targets that will make acute decompensations preventable and even decrease mortality.

Senescent cells in the development of cardiometabolic disease

PURPOSE OF REVIEW

Senescent cells have recently been identified as key players in the development of metabolic dysfunction. In this review, we will highlight recent developments in this field and discuss the concept of targeting these cells to prevent or treat cardiometabolic diseases.

RECENT FINDINGS

Evidence is accumulating that cellular senescence contributes to adipose tissue dysfunction, presumably through induction of low-grade inflammation and inhibition of adipogenic differentiation leading to insulin resistance and dyslipidaemia. Senescent cells modulate their surroundings through their bioactive secretome and only a relatively small number of senescent cells is sufficient to cause persistent physical dysfunction even in young mice. Proof-of-principle studies showed that selective elimination of senescent cells can prevent or delay the development of cardiometabolic diseases in mice.

SUMMARY

The metabolic consequences of senescent cell accumulation in various tissues are now unravelling and point to new therapeutic opportunities for the treatment of cardiometabolic diseases.

Relationship between Telomere Maintenance and Liver Disease

Previous studies have established a correlation between increasing chronological age and risk of cirrhosis. This pattern raised interest in the role of telomeres and the telomerase complex in the pathogenesis of liver fibrosis and cirrhosis. This review aims to summarize and analyze the current understanding of telomere regulation in hepatocytes and lymphocytes and how this ultimately relates to the development of liver fibrosis. Notably, in chronic viral hepatitis, telomere shortening in hepatocytes and lymphocytes occurs in such a way that may promote further viral replication while also leading to liver damage. However, while telomere shortening occurs in both hepatocytes and lymphocytes and ultimately results in cellular death, the mechanisms of telomere loss appear to be initiated by independent processes. The understanding of telomere maintenance on a hepatic and immune system level in both viral and non-viral etiologies of cirrhosis may open doors to novel therapeutic strategies.

Senescent Hepatocytes in Decompensated Liver Show Reduced UPRMT and Its Key Player, CLPP, Attenuates Senescence In Vitro

BACKGROUND AND AIMS

Non-dividing hepatocytes in end-stage liver disease indicates permanent growth arrest similar to senescence. Identifying senescence in vivo is often challenging and mechanisms inhibiting senescence are poorly understood. In lower organisms mitochondrial unfolded protein response (UPR^MT) helps in increasing longevity; however, its role in senescence and liver disease is poorly understood. Aim of this study was to identify hepatocyte senescence and the role of UPR^MT in cryptogenic cirrhosis.

METHODS

Doxorubicin was used to induce senescence in non-neoplastic hepatocytes (PH5CH8) and hepatoma cells (HepG2 and Huh7). Senescence-associated markers and unfolded protein response was evaluated by fluorescence microscopy, immunoblotting and gene expression. Explants/biopsies from normal, fibrosis, compensated and decompensated cirrhosis without any known etiology were examined for presence of senescence and UPR^MT by immunohistochemistry and gene expression.

RESULTS

Accumulation of senescent hepatocytes in cryptogenic cirrhosis was associated with reduced proliferation, increased expression of γH2AX and p21, together with loss of LaminB1. Dysfunctional mitochondria and compromised UPR^MT were key features of senescent hepatocytes both in vitro and also in decompensated cirrhosis. Intriguingly, compensated cirrhotic liver mounted strong UPR^MT, with high levels of mitochondrial protease, CLPP. Overexpression of CLPP inhibited senescence in vitro, by reducing mitochondrial ROS and altering oxygen consumption.

CONCLUSIONS

Our results implicate a role of hepatocyte senescence in cryptogenic cirrhosis together with a crucial role of UPR^MT in preventing hepatocyte senescence. A compromised UPR^MT may shift the fate of cirrhotic liver toward decompensation by exaggerating hepatocyte senescence. Restoring CLPP levels at least in cell culture appears as a promising strategy in mitohormesis, thereby, preventing senescence and possibly improving hepatocyte function.

Suppressed hepatocyte proliferation via a ROS-HNE-P21 pathway is associated with nicotine- and cotinine-enhanced alcoholic fatty liver in mice

CYP2A5 is a major enzyme responsible for nicotine and cotinine metabolism in mice. Nicotine and cotinine enhance alcoholic fatty liver in wild type (WT) mice but not in CYP2A5 knockout (KO) mice, and reactive oxygen species (ROS) generated during the CYP2A5-mediated metabolism contributes to the enhancing effect. In combination with ethanol, nicotine and cotinine increased lipid peroxidation end product 4-hydroxynonenal (HNE) in WT mice but not in KO mice. In ethanol-fed KO mice, only 5 and 10 genes were regulated by nicotine and cotinine, respectively. However, in ethanol-fed WT mice, 59 and 104 genes were regulated by nicotine and cotinine, respectively, and 7 genes were up-regulated by both nicotine and cotinine. Plin 2 and Cdkn1a are among the 7 genes. Plin2 encodes adipose differentiation-related protein (ADRP), a lipid droplet-associated protein, which was confirmed to be increased by nicotine and cotinine in WT mice but not in KO mice. Cdkn1a encodes P21 and elevated P21 in nuclei was also confirmed. HNE can increase P21 and P21 inhibit cell proliferation. Consistently, hepatocyte proliferation markers proliferating cell nuclear antigen (PCNA) and Ki67 were decreased in WT mice but not in KO mice by nicotine/ethanol and cotinine/ethanol, respectively. These results suggest that inhibition of liver proliferation via a ROS-HNE-P21 pathway is involved in nicotine- and cotinine-enhanced alcoholic fatty liver.

Inflammatory cytokines and change of Th1/Th2 balance as prognostic indicators for hepatocellular carcinoma in patients treated with transarterial chemoembolization

Tumor-associated immune response plays a critical role in cancer pathogenesis. This study evaluated clinical implications of T cell cytokines and regulatory T cells (Tregs) in HCC patients treated with TACE. Whole blood was obtained for analysis of T cell cytokines (IL-1β, IL-2, IL-4, IL-5, IL-6, IL-9, IL-10, IL-12p70, IL-13, IL-17A, IL-22, IFN-γ, and TNF-α) and Tregs from 142 HCC patients. Patients with CTP class A had a significantly lower proportion of detectable IL-4 or IL-6, but a higher proportion of detectable IL-22 than patients with CTP class B/C. IL-6 level was well correlated with tumor stage and undetectable IL-17A was associated with extrahepatic metastasis. The overall survival rate was significantly higher in patients who had undetectable IL-6 or detectable IL-22 than patients who did not. IL-6 among cytokines remained independently predictive factor for survival. Increased IFN-γ/IL-10 ratio and no increase in IL-6 level following TACE were associated with prolonged survival, and baseline Tregs could affect Th1/Th2 balance. T cell cytokines are associated with a variety of clinical aspects of HCC, and IL-6 is the most significant predictor of survival. A shift toward increased Th1 response and no increase in IL-6 level exert favorable immunologic effects on HCC prognosis.