Hepatocyte telomere shortening and senescence are general markers of human liver cirrhosis

Telomere and telomerase in chronic liver disease and hepatocarcinoma

The pathogenesis of liver cirrhosis is not completely elucidated. Although in the majority of patients, the risk factors may be identified in B and C viral hepatitis, alcohol intake, drugs or fatty liver disease, there is a small percentage of patients with no apparent risk factors. In addition, the evolution of chronic liver disease is highly heterogeneous from one patient to another. Among patient with identical risk factors, some rapidly progress to cirrhosis and hepatocellular carcinoma (HCC) whereas others have a benign course. Therefore, a genetic predisposition may contribute to the development of cirrhosis and HCC. Evidence supporting the role of genetic factors as a risk for cirrhosis has been accumulating during the past years. In addition to the results from epidemiological studies, polymorphisms studies and data on twins, the concept of telomere shortening as a genetic risk factor for chronic liver disease and HCC has been proposed. Here we review the literature on telomerase mutations, telomere shortening and liver disease including hepatocellular carcinoma.

Growth factors enhance liver regeneration in acute-on-chronic liver failure

Acute-on-chronic liver failure is a distinct syndrome characterized by a rapid progression of liver disease culminating in organ failure and death. The only definitive treatment is liver transplantation. However, there is a possible element of reversibility and hepatic regeneration if the acute insult can be tided over. Exogenously administered growth factors may stimulate hepatocytes, hepatic progenitor cells and bone marrow-derived cells to supplement hepatic regeneration. The proposed review is intended to provide an in-depth analysis of the individual components of hepatic and bone marrow niches and highlight the growing role of various growth factors in liver regeneration in health and in liver failure.

Detection of telomerase reverse transcriptase mRNA in peripheral blood mononuclear cells of patients with liver failure

BACKGROUND

Telomerase activity is closely associated with the expression of human telomerase reverse transcriptase (hTERT) mRNA; although it can be induced in hepatocytes during liver regeneration, its dynamic change in patients with liver failure has remained unclear.

OBJECTIVES

We investigated the variation and significance of hTERT mRNA expression in peripheral blood mononuclear cells (PBMCs) of the patients with liver failure.

PATIENTS AND METHODS

In this clinical experimental study, 76 Chinese patients were enrolled in the study between 2010 and 2012. The level of PBMCs hTERT mRNA was detected by relative quantitative real-time polymerase chain reaction (RT-PCR) in the samples taken before treatment and at seven-day intervals during a 28-day treatment period. The patients were divided into survivor and non-survivor groups according to the 3-months mortality after treatment. The dynamic variation of PBMCs hTERT mRNA was analyzed and its association with prognosis was assessed by the area under the receiver-operating characteristic curve.

RESULTS

The median level of PBMCs hTERT mRNA in survivors increased with treatment time and was significantly higher than the corresponding level in non-survivors after 14 days of treatment (P < 0.001). Subgroup analyses showed that the levels of PBMCs hTERT mRNA were remarkably higher in patients with acute-on-chronic liver failure than in those with chronic liver failure (P < 0.05). In patients with the same clinical type of liver failure, the level was markedly higher in survivors than in non-survivors after 14 days of treatment (P < 0.05); however, the levels were not significantly different between subgroups with different clinical type but the same prognosis. The sensitivity and specificity of PBMCs hTERT mRNA was high in evaluating the prognosis at day 14 and became much higher at days 21 and 28 post treatment. The expression of PBMCs hTERT mRNA had high sensitivity and specificity in evaluating the prognosis as early as day 14 post treatment and was significantly superior to the prognostic value of serum alpha-fetoprotein.

CONCLUSIONS

The expression of PBMCs hTERT mRNA is closely associated with patient outcome, which indicates that hTERT mRNA in PBMCs might be useful as a prognostic biomarker of liver failure.

Q-FISH measurement of hepatocyte telomere lengths in donor liver and graft after pediatric living-donor liver transplantation: donor age affects telomere length sustainability

Along with the increasing need for living-donor liver transplantation (LDLT), the issue of organ shortage has become a serious problem. Therefore, the use of organs from elderly donors has been increasing. While the short-term results of LDLT have greatly improved, problems affecting the long-term outcome of transplant patients remain unsolved. Furthermore, since contradictory data have been reported with regard to the relationship between donor age and LT/LDLT outcome, the question of whether the use of elderly donors influences the long-term outcome of a graft after LT/LDLT remains unsettled. To address whether hepatocyte telomere length reflects the outcome of LDLT, we analyzed the telomere lengths of hepatocytes in informative biopsy samples from 12 paired donors and recipients (grafts) of pediatric LDLT more than 5 years after adult-to-child LDLT because of primary biliary atresia, using quantitative fluorescence in situ hybridization (Q-FISH). The telomere lengths in the paired samples showed a robust relationship between the donor and grafted hepatocytes (r = 0.765, p = 0.0038), demonstrating the feasibility of our Q-FISH method for cell-specific evaluation. While 8 pairs showed no significant difference between the telomere lengths for the donor and the recipient, the other 4 pairs showed significantly shorter telomeres in the recipient than in the donor. Multiple regression analysis revealed that the donors in the latter group were older than those in the former (p = 0.001). Despite the small number of subjects, this pilot study indicates that donor age is a crucial factor affecting telomere length sustainability in hepatocytes after pediatric LDLT, and that the telomeres in grafted livers may be elongated somewhat longer when the grafts are immunologically well controlled.

M2 kupffer cells promote hepatocyte senescence: an IL-6-dependent protective mechanism against alcoholic liver disease

Alcoholic liver disease is a predominant cause of liver-related mortality in Western countries. The early steps of alcohol-induced steatosis and liver injury involve several mechanisms, including inflammation and oxidative stress. The inflammatory process is initiated by polarization of Kupffer cells toward a proinflammatory M1 phenotype, and we recently found that promoting anti-inflammatory M2 Kupffer cell polarization protects against alcohol-induced hepatocyte steatosis and apoptosis. Alcohol-induced oxidative stress is a potential trigger of senescence, and senescent cells exhibit characteristic functional resistance to apoptosis. We sought to evaluate induction of hepatocyte senescence as an early protective mechanism against alcoholic liver disease. Combining in vivo and in vitro studies, we show that M2 macrophages trigger hepatocyte senescence and enhance alcohol-induced hepatocyte senescence, as indicated by increased β-galactosidase activity, elevated CDKN1A mRNA expression, and induction of nuclear p21. We identify IL-6 as the mediator of M2-induced hepatocyte senescence. Senescent hepatocytes display characteristic resistance to apoptosis but also to steatosis, thus arguing for an early protective effect against alcoholic liver disease. These findings further suggest that pharmacologic interventions targeting M2 polarization during the early stages of alcoholic liver disease may represent an attractive strategy for the limitation of inflammation, hepatocyte apoptosis, and steatosis.

Telomere dysfunction in peripheral blood mononuclear cells from patients with primary biliary cirrhosis

BACKGROUND

Chromosomal instability in peripheral blood mononuclear cells has a role in the onset of primary biliary cirrhosis. We hypothesized that patients with primary biliary cirrhosis may harbour telomere dysfunction, with consequent chromosomal instability and cellular senescence.

AIM

To evaluate the clinical significance of telomerase activity and telomere length in peripheral blood mononuclear cells from patients with primary biliary cirrhosis.

STUDY DESIGN

In this population-based case control study, 48 women with primary biliary cirrhosis (25 with cirrhosis), 12 with chronic hepatitis C matched by age and severity of disease, and 55 age-matched healthy women were identified. Mononuclear cells from the peripheral blood of patients and controls were isolated. Telomere length and telomerase activity were measured.

RESULTS

Telomere length and telomerase activity did not differ between cases (5.9 ± 1.5 kb) and controls (6.2 ± 1.4 kb, pc=0.164). Telomere shortening and advanced-stage disease strongly correlated with telomerase activity. Patients with advanced disease retained significantly less telomerase activity than those with early-stage disease (0.6 ± 0.9 OD vs. 1.5 ± 3.7 OD, p=0.03). Telomere loss correlated with age, suggesting premature cellular ageing in patients with primary biliary cirrhosis.

CONCLUSION

Our data strongly support the telomere hypothesis of human cirrhosis, indicating that telomere shortening and telomerase activity represent a molecular mechanism in the evolution of human cirrhosis in a selected population of patients.

Effect of cytotoxic chemotherapy on markers of molecular age in patients with breast cancer

BACKGROUND

Senescent cells, which express p16 (INK4a) , accumulate with aging and contribute to age-related pathology. To understand whether cytotoxic agents promote molecular aging, we measured expression of p16 (INK4a) and other senescence markers in breast cancer patients treated with adjuvant chemotherapy.

METHODS

Blood and clinical information were prospectively obtained from 33 women with stage I to III breast cancer at four time points: before anthracycline-based chemotherapy, immediately after anthracycline-based chemotherapy, 3 months after anthracycline-based chemotherapy, and 12 months after anthracycline-based chemotherapy. Expression of senescence markers p16 (INK4a) and ARF mRNA was determined using TaqMan quantitative reverse-transcription polymerase chain reaction in CD3(+) T lymphocytes, telomere length was determined by Southern analysis, and senescence-associated cytokines were determined by enzyme-linked immunosorbent assay. Findings were independently assessed in a cross-sectional cohort of 176 breast cancer survivors enrolled a median of 3.4 years after treatment; 39% previously received chemotherapy. All statistical tests were two-sided.

RESULTS

In prospectively analyzed patients, expression of p16 (INK4a) and ARF increased immediately after chemotherapy and remained elevated 12 months after treatment. Median increase in log2 p16 (INK4a) was 0.81 (interquartile range = 0.28-1.62; Wilcoxon signed-rank P < .001), or a 75% absolute increase in expression, equivalent to the increase observed over 14.7 years of chronological aging. ARF expression was comparably increased (P < .001). Increased expression of p16 (INK4a) and ARF was associated with dose-dense therapy and hematological toxicity. Expression of two senescence-associated cytokines (VEGFA and MCP1) was durably increased by adjuvant chemotherapy. Telomere length was not affected by chemotherapy. In a cross-sectional cohort, prior chemotherapy exposure was independently associated with a log2-increase in p16 (INK4a) expression of 0.57 (repeated measures model, P < .001), comparable with 10.4 years of chronological aging.

CONCLUSIONS

Adjuvant chemotherapy for breast cancer is gerontogenic, inducing cellular senescence in vivo, thereby accelerating molecular aging of hematopoietic tissues.

Two-tier regenerative response in liver failure in humans

Acute and chronic liver failure is associated with high mortality. The enormous regenerative potential of the liver has generated a lot of attention. We undertook this work to assess the two-tier regenerative response in liver failure by immunohistochemistry and to correlate such response with liver histology in acute liver failure (ALF), acute-on-chronic liver failure (ACLF), and decompensated cirrhosis (CHD). Histological examination and immunohistochemical analysis of proliferating hepatocytes and activated hepatic progenitor cells (HPCs) were performed on the liver tissue of patients with ALF (25), ACLF (70), and CHD (70). Comparative analysis of regenerative markers and correlation with histological parameters were done in ALF, ACLF, and CHD. Hepatocytes proliferated significantly more in ALF in comparison to ACLF (p < 0.001) and CHD (p < 0.001). HPC proliferation was significantly higher in ACLF (p < 0.001) and CHD (p < 0.001) than in ALF. ACLF patients showed the highest HPC proliferation and differentiation. Significantly more intermediate hepatocytes were found in ACLF than in ALF and CHD (p < 0.001). Marked parenchymal replacement by fibrosis and/or necrosis correlated significantly with activation of HPC in ACLF (p = 0.01, odds ratio (OR) 4.95) and in CHD (p = 0.05, OR 4.19). The study of liver regeneration in human acute and chronic liver failure suggests that hepatocyte proliferation, providing the first line of regeneration response, is most active in ALF whereas HPC activation, the second line of defense, is more prominent in ACLF. More HPC differentiate to hepatocytes in ACLF than in CHD, reflecting better regenerative potential in ACLF.

Cancer-associated mutations in healthy individuals: assessing the risk of carcinogenesis

Mutations associated with hematopoietic malignancies have been repeatedly identified in healthy individuals. For certain cases, such as the t(14;18) translocation and monoclonal B-cell lymphocytosis, no clear link between the presence of aberrant cells and the later development of cancer has been established. Intriguingly, longitudinal studies suggest that these abnormalities persist for long periods of time in some individuals, but in others are transient in which they disappear completely. Here, we present a mathematical model, based on cellular replication limits, that provides a possible explanation for these seemingly contradictory findings. It proposes that the transient and persistent nature of the phenotypes depends on the stage in the differentiation pathway of a given lineage in which the mutation originates. Our work suggests that cellular replication limits may not only prevent cancer by aborting clonal expansion of cells, but also by influencing the fate of altered but nonneoplastic cells in healthy tissue.

Chemokine receptor CXCR2 is transactivated by p53 and induces p38-mediated cellular senescence in response to DNA damage

Mammalian cells may undergo permanent growth arrest/senescence when they incur excessive DNA damage. As a key player during DNA damage response (DDR), p53 transactivates an array of target genes that are involved in various cellular processes including the induction of cellular senescence. Chemokine receptor CXCR2 was previously reported to mediate replicative and oncogene-induced senescence in a DDR and p53-dependent manner. Here, we report that CXCR2 is upregulated in various types of cells in response to genotoxic or oxidative stress. Unexpectedly, we found that the upregulation of CXCR2 depends on the function of p53. Like other p53 target genes such as p21, CXCR2 is transactivated by p53. We identified a p53-binding site in the CXCR2 promoter that responds to changes in p53 functional status. Thus, CXCR2 may act downstream of p53. While the senescence-associated secretory phenotype (SASP) exhibits a kinetics that is distinct from that of CXCR2 expression and does not require p53, it reinforces senescence. We further showed that the cellular senescence caused by CXCR2 upregulation is mediated by p38 activation. Our results thus demonstrate CXCR2 as a critical mediator of cellular senescence downstream of p53 in response to DNA damage.

Senescence at a glance

Cellular senescence is a stable proliferation arrest that is associated with extensive cellular remodelling and an altered secretory pathway. Through its numerous inducers that lead to altered gene expression, senescence is able to influence many contrasting functions and pathologies, namely tumour suppression, tumour promotion, wound healing and ageing. As senescence is able to control such important tissue functions, it is now being pinpointed as a possible route for novel therapies. This article and accompanying poster aim to provide a summary of the initiators, pathways and roles of senescence, as well as present examples of senescence and a possible use for senescence in therapy.

Resistin impairs SIRT1 function and induces senescence-associated phenotype in hepatocytes

Resistin is a cysteine-rich secreted protein which significantly inhibits phosphorylation of AMP-activated protein kinase in both human and mouse hepatocytes. It has been demonstrated that resistin plays an important role in inducing hepatic insulin resistance. However, whether resistin has other unknown influences on hepatocytes still remains poorly studied. Here, we show that recombinant resistin protein significantly reduces expression of SIRT1, attenuates the interaction between SIRT1 and PPARα as well as PGC-1α, and increases PGC-1α acetyl-lysine levels in HepG2 cells. In line with this, resistin treatment weakens the association between SIRT1 and major satellite repeats and alters the transcription level of SIRT1 target genes in mouse primary hepatocytes. Resistin treatment also significantly increases senescence-associated β-galactosidase activity in mouse primary hepatocytes and this effect can be eliminated by co-treatment with the SIRT1 agonists resveratrol and nicotinamide mononucleotide. Our findings suggest that resistin is a negative regulator of SIRT1 in both human hepatoma cell line HepG2 and mouse hepatocytes and that it might also play an important role in the development of senescence-associated liver diseases.

From cirrhosis to hepatocellular carcinoma: new molecular insights on inflammation and cellular senescence

Sequential progression from chronic liver disease to fibrosis and to cirrhosis culminates in neoplasia in hepatocellular carcinoma (HCC). The preneoplastic setting of the cirrhotic background provides a conducive environment for cellular transformation. The role of classical inflammation in cirrhosis is widely known, but the exact mechanism linking inflammation and cancer remains elusive. Recent studies have elucidated roles for NF-κB, STAT3 and JNK as possible missing links. In addition, the "inflammasome" (a multiprotein complex and sensor of cellular damage) is a recently identified player in this field. The hallmarks of cirrhosis include necroinflammation, deposition of extracellular matrix and shortening of telomeres, leading to senescence and regeneration. Additionally, the accumulation of genetic/epigenetic changes propels atypical cells toward a malignant phenotype. This review provides recent information on the classical inflammatory pathway, together with a spotlight on inflammasomes and the immunomodulatory role of cellular senescence during the progression from cirrhosis to HCC. Moreover, lacunae in the current knowledge were identified and key questions raised on whether the observed adaptive responses are beneficial or detrimental to tissue homeostasis in a complex organ like liver.

Telomere shortening in human diseases

The discovery of telomeres dates back to the early 20th century. In humans, telomeres are heterochromatic structures with tandem DNA repeats of 5'-TTAGGG-3' at the chromosomal ends. Telomere length varies greatly among species and ranges from 10 to 15 kb in humans. With each cell division, telomeres shorten progressively because of the 'end-replication problem'. Short or dysfunctional telomeres are often recognized as DNA DSBs, triggering cell-cycle arrest and result in cellular senescence or apoptotic cell death. Therefore, telomere shortening serves as an important tumor-suppressive mechanism by limiting cellular proliferative capacity by regulating senescence checkpoint activation. Although telomeres serve as a mitotic clock to cells, they also confer capping on chromosomes, with help from telomere-associated proteins. Over the past decades, many studies of telomere biology have demonstrated that telomeres and telomere-associated proteins are implicated in human genetic diseases. In addition, it has become more apparent that accelerated telomere erosion is associated with a myriad of metabolic and inflammatory diseases. Moreover, critically short or unprotected telomeres are likely to form telomeric fusions, leading to genomic instability, the cornerstone for carcinogenesis. In light of these, this minireview summarizes studies on telomeres and telomere-associated proteins in human diseases. Elucidating the roles of telomeres involved in the mechanisms underlying pathogenesis of these diseases may open up new possibilities for novel molecular targets as well as provide important diagnostic and therapeutic implications.

Hepatoprotective Effects of Swimming Exercise against D-Galactose-Induced Senescence Rat Model

This study investigates whether a 12-week swimming exercise training can prevent liver damage or senescence associated biomarkers in an experimental aging model in rats. Twenty-three male Sprague-Dawley rats were divided into four groups: vehicle treatment with sedentary control (C, n = 6), aging induction with sedentary (A, n = 6), vehicle treatment with swimming exercise (SW, n = 5), and aging induction with swimming exercise (A + SW, n = 6). Rats in groups A and AS received intraperitoneal d-galactose injections (150 mg/kg/day) for 12 weeks to induce aging. Rats in groups SW and A + SW were subjected to swimming exercise training for 12 weeks. Body weight, liver weight, epididymal fat mass, blood biochemistry, and liver pathology were performed at the end of the experiment. Hepatic senescence protein markers such as β-galactosidase, p53, and p21, as well as the inflammatory mediator, IL-6, were examined. The d-galactose-treated rats exhibited increases in AST and γ-GT plasma levels and β-galactosidase protein expression compared to the control group. Swimming exercise significantly reduced BW, epididymal fat mass, γ-GT activity, and p53, p21, and IL-6 protein levels compared to the aging group. These results suggest that a 12-week swimming exercise program suppresses senescence markers and downregulates inflammatory mediator in the liver tissues of d-galactose-induced aging rats.

Hepatocellular carcinoma and other malignancies in autoimmune hepatitis

Hepatocellular carcinoma and extrahepatic malignancies can complicate the course of autoimmune hepatitis, and these occurrences may increase in frequency as the survival of patients with cirrhosis is extended and the prospect of new nonstandard immune-modifying intervention is realized. The frequency of hepatocellular carcinoma in patients with autoimmune hepatitis and cirrhosis is 1-9 %, and annual occurrence in patients with cirrhosis is 1.1-1.9 %. The standardized incidence ratio for hepatocellular carcinoma in autoimmune hepatitis is 23.3 (95 % confidence interval (CI) 7.5-54.3) in Sweden, and the standardized mortality ratio for hepatobiliary cancer is 42.3 (95 % CI 20.3-77.9) in New Zealand. The principal risk factor is long-standing cirrhosis, and patients at risk are characterized mainly by cirrhosis for ≥ 10 years, manifestations of portal hypertension, persistent liver inflammation, and immunosuppressive therapy for ≥ 3 years. Multiple molecular disturbances, including the accumulation of senescent hepatocytes because of telomere shortening, step-wise accumulation of chromosomal injuries, and aberrations in transcription factors and genes, may contribute to the risk. Extraheptic malignancies of diverse cell types occur in 5 % in an unpredictable fashion. The standardized incidence ratio is 2.7 (95 % CI 1.8-3.9) in New Zealand, and non-melanoma skin cancers are most common. Outcomes are related to the nature and stage of the tumor at diagnosis. Surveillance recommendations have not been promulgated, but hepatic ultrasonography every six months in patients with cirrhosis is a consideration. Routine health screening measures for other malignancies should be applied diligently.

Genome-wide transcriptional reorganization associated with senescence-to-immortality switch during human hepatocellular carcinogenesis

Senescence is a permanent proliferation arrest in response to cell stress such as DNA damage. It contributes strongly to tissue aging and serves as a major barrier against tumor development. Most tumor cells are believed to bypass the senescence barrier (become “immortal”) by inactivating growth control genes such as TP53 and CDKN2A. They also reactivate telomerase reverse transcriptase. Senescence-to-immortality transition is accompanied by major phenotypic and biochemical changes mediated by genome-wide transcriptional modifications. This appears to happen during hepatocellular carcinoma (HCC) development in patients with liver cirrhosis, however, the accompanying transcriptional changes are virtually unknown. We investigated genome-wide transcriptional changes related to the senescence-to-immortality switch during hepatocellular carcinogenesis. Initially, we performed transcriptome analysis of senescent and immortal clones of Huh7 HCC cell line, and identified genes with significant differential expression to establish a senescence-related gene list. Through the analysis of senescence-related gene expression in different liver tissues we showed that cirrhosis and HCC display expression patterns compatible with senescent and immortal phenotypes, respectively; dysplasia being a transitional state. Gene set enrichment analysis revealed that cirrhosis/senescence-associated genes were preferentially expressed in non-tumor tissues, less malignant tumors, and differentiated or senescent cells. In contrast, HCC/immortality genes were up-regulated in tumor tissues, or more malignant tumors and progenitor cells. In HCC tumors and immortal cells genes involved in DNA repair, cell cycle, telomere extension and branched chain amino acid metabolism were up-regulated, whereas genes involved in cell signaling, as well as in drug, lipid, retinoid and glycolytic metabolism were down-regulated. Based on these distinctive gene expression features we developed a 15-gene hepatocellular immortality signature test that discriminated HCC from cirrhosis with high accuracy. Our findings demonstrate that senescence bypass plays a central role in hepatocellular carcinogenesis engendering systematic changes in the transcription of genes regulating DNA repair, proliferation, differentiation and metabolism.

The role of cellular senescence in the gastrointestinal mucosa

Cellular senescence is a biologically irreversible state of cell-growth arrest that occurs following either a replicative or an oncogenic stimulus. This phenomenon occurs as a response to the presence of premalignant cells and appears to be an important anticancer mechanism that keeps these transformed cells at bay. Many exogenous and endogenous triggers for senescence have been recognized to act via genomic or epigenomic pathways. The most common stimulus for senescence is progressive loss of telomeric DNA, which results in the loss of chromosomal stability and eventual unregulated growth and malignancy. Senescence is activated through an interaction between the p16 and p53 tumor-suppressor genes. Senescent cells can be identified in vitro because they express senescence-associated β-galactosidase, a marker of increased lysosomal activity. Cellular senescence plays an integral role in the prevention and development of both benign and malignant gastrointestinal diseases. The senescence cascade and the cell-cycle checkpoints that dictate the progression and maintenance of senescence are important in all types of gastrointestinal cancers, including pancreatic, liver, gastric, colon, and esophageal cancers. Understanding the pathogenic mechanisms involved in cellular senescence is important for the development of agents targeted toward the treatment of gastrointestinal tumors.

HIV, age, and the severity of hepatitis C virus-related liver disease: a cohort study

BACKGROUND

Persons with HIV infection have been reported to develop age-related diseases at younger ages than those without HIV. Whether this finding is related to HIV infection or failure to control for other risk factors is unknown.

OBJECTIVE

To investigate whether persons with HIV infection develop hepatitis C virus (HCV)-related liver disease at younger ages than similar persons without HIV.

DESIGN

Comparison of the severity of liver fibrosis by age among persons who have HCV with and without HIV followed concurrently in the same protocol.

SETTING

Observational cohort from Baltimore, Maryland, participating in the ALIVE (AIDS Linked to the IntraVenous Experience) study.

PARTICIPANTS

1176 current and former injection drug users with antibodies to HCV.

MEASUREMENTS

Liver fibrosis assessed semiannually from 2006 to 2011 by elastography (FibroScan, Echosens, Paris, France) and using previously validated thresholds for clinically significant fibrosis and cirrhosis; concurrent assessment of medical history, alcohol and illicit drug use, HCV RNA levels, hepatitis B virus surface antigen level, body mass index, and (for those with HIV) CD4+ lymphocyte count and HIV RNA levels.

RESULTS

Among 1176 participants with antibodies to HCV, the median age was 49 years and 34% were coinfected with HIV and HCV. Participants contributed 5634 valid liver fibrosis measurements. The prevalence of clinically significant fibrosis without cirrhosis (12.9% vs. 9.5%) and of cirrhosis (19.5% vs. 11.0%) was greater in persons coinfected with HIV and HCV than in those with only HCV (P < 0.001). Increasing age and HIV infection were independently associated with liver fibrosis, as were daily alcohol use, chronic hepatitis B virus infection, body mass index greater than 25 kg/m2, and greater plasma HCV RNA levels. When these factors were kept constant, persons with HIV had liver fibrosis measurements equal to those of persons without HIV, who were, on average, 9.2 years older.

LIMITATION

The process of liver fibrosis began before the study in most persons.

CONCLUSION

In this cohort, persons who have HCV with HIV have liver fibrosis stages similar to those without HIV who are nearly a decade older.

PRIMARY FUNDING SOURCE

National Institute on Drug Abuse.

Hepatic regeneration and the epithelial to mesenchymal transition

Liver injuries are repaired by fibrosis and regeneration. The core stage is the repair response and fibrosis formation as a scar. The cause of overly-responsive scar formation and diminished regeneration, especially in liver fibrosis and cirrhosis, is still unknown. The epithelial to mesenchymal transition (EMT), a previously discovered mechanism, plays an important role in liver fibrosis and tumor metastasis. Recently, EMT has been found to be associated with liver and bile duct cell fibrosis. Analyzing the established models and chronic disease processes, we propose that EMT liver cells may also lose their regenerative capability due to phenotype changes and that the remaining liver cells may quickly lose their regenerative capability in liver fibrosis or cirrhosis. Recognizing these phenotype changes or transition cells may play an important role in targeting therapy to reverse fibrosis not only by disrupting the transition that is necessary to produce the extracellular matrix but also by restoring the regenerative capacity of EMT-like cells.

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

BACKGROUND & AIMS

Models of non-alcohol-related fatty liver disease (NAFLD) reveal features of accelerated ageing, such as impaired regeneration, and an increased risk of hepatocellular carcinoma. The relation between accelerated ageing, disease progression and clinical outcome has not been previously investigated and is the subject of the current study.

METHODS

Liver sections from 70 patients with NAFLD (105 biopsies) and 60 controls were studied for telomere length, nuclear area, DNA damage and cell cycle phase markers, using quantitative fluorescent in situ hybridization and immunohistochemistry.

RESULTS

Hepatocyte telomeres were shorter in NAFLD than controls (p <0.0001). Hepatocytes in NAFLD demonstrated lack of cell cycle progression beyond G1/S phase and high-level expression of p21, the universal cell cycle inhibitor (p=0.001). γ-H(2)AX expression increased with steatosis (p=0.01), indicating DNA damage, and was associated with shorter hepatocyte telomeres (p <0.0001). Hepatocyte p21 expression correlated with fibrosis stage and diabetes mellitus, independently (p <0.001 and p=0.002, respectively). Further analysis revealed that an adverse liver-related outcome was strongly associated with higher hepatocyte p21 expression and greater hepatocyte nuclear area (p=0.02 and p=0.006), but not with telomere length. In paired biopsies, changes in hepatocyte p21 expression and nuclear area mirrored changes in fibrosis stage (p=0.01 and p=0.006, respectively).

CONCLUSIONS

These findings are consistent with hepatocyte senescence and permanent cell cycle arrest in NAFLD. Hepatocyte senescence correlated closely with fibrosis stage, diabetes mellitus, and clinical outcome. Hepatocyte p21 expression could be used as a prognostic marker and for stratification in clinical studies.

Telomere shortening is associated with reduced duodenal HCOFormula secretory but normal gastric acid secretory capacity in aging mice

The incidence of duodenal ulcer, especially Helicobacter pylori-negative duodenal ulcer, strongly increases with age. In humans, telomere length shortening is considered to be one critical factor in cellular senescence and organ survival. In this study, we compared basal and stimulated gastric acid and duodenal HCO(3)(-) secretory rates in aged late-generation (G(3)) telomerase-deficient (mTERC(-/-)) mice, which are characterized by severe telomere dysfunction due to the inability to elongate telomeres during cell division. We found that basal and forskolin-stimulated HCO(3)(-) secretion and short-circuit current (I(sc)) in isolated duodenal mucosa of G(3) mTERC(-/-) mice were markedly reduced compared with age-matched wild-type mice. In contrast, basal and forskolin-stimulated acid secretory rates in isolated G(3) mTERC(-/-) gastric mucosa were not significantly altered. Correspondingly, duodenal mucosa of G(3) mTERC(-/-) mice showed slimming and shortening of villi, whereas gastric mucosal histology was not significantly altered. However, the ratios of cystic fibrosis transmembrane conductance regulator (CFTR) and solute-linked carrier 26 gene family (Slc26a6) mRNA expression in relation to cytokeratin-18 were not altered in duodenal mucosa. The further knockout of p21, which is a downstream effector of telomere shortening-induced senescence, rescued villus atrophy of duodenal mucosa, and basal and forskolin-stimulated duodenal HCO(3)(-) secretion and I(sc) in mTERC(-/-) p21(-/-) double-knockout mice were not different from wild-type controls. In conclusion, genetic ablation of telomerase resulted in p21-dependent duodenal mucosal atrophy and reduced duodenal HCO(3)(-) secretory capacity, whereas gastric morphology and acid secretory function were preserved. This suggests that telomere shortening during aging may result in an imbalance between aggressive and protective secretions against duodenal mucosa and thus predispose to ulcer formation.

Early life stress and telomere length: investigating the connection and possible mechanisms: a critical survey of the evidence base, research methodology and basic biology

How can adverse experiences in early life, such as maltreatment, exert such powerful negative effects on health decades later? The answer may lie in changes to DNA. New research suggests that exposure to stress can accelerate the erosion of DNA segments called telomeres. Shorter telomere length correlates with chronological age and also disease morbidity and mortality. Thus, telomere erosion is a potential mechanism linking childhood stress to health problems later in life. However, an array of mechanistic, methodological, and basic biological questions must be addressed in order to translate telomere discoveries into clinical applications for monitoring health and predicting disease risk. This paper covers the current state of the science and lays out new research directions.

Sustained telomere length in hepatocytes and cholangiocytes with increasing age in normal liver

Telomeres, a validated biomarker of aging, comprise multiple nucleotide repeats capping chromosomes that shorten with each cell cycle until a critical length is achieved, precipitating cell senescence. Only two previous studies focused on the effect of aging in "normal" liver tissue, but these studies were compromised by small sample size, limited age range, tissue derived from individuals with an increased risk of senescence, and the use of liver homogenates. We developed a robust large-volume, four-color quantitative fluorescent in situ hybridization technique to measure telomere length in large numbers of hepatocytes, Kupffer cells, hepatic stellate cells, CD4-positive and CD8-positive lymphocytes, and cholangiocytes. Following validation against the gold standard (Southern blotting), the technique was applied to normal archived paraffin-embedded liver tissue obtained following reperfusion of implanted donor liver. We studied 73 highly selected donors aged 5-79 years with a short medical illness preceding death and no history of liver disease, reperfusion injury, or steatosis and normal graft function 1-year posttransplantation. Cholangiocytes had significantly longer telomeres compared with all other intrahepatic lineages over a wide age range (P < 0.05). Age-related telomere attrition was restricted to sinusoidal cells (i.e., Kupffer cells [P = 0.0054] and stellate cells [P = 0.0001]). Cholangiocytes and hepatocytes showed no age-related telomere shortening.

CONCLUSION

In normal liver and over a broad age range, cholangiocytes have longer telomeres than all other intrahepatic lineages. Age-related telomere length decline is restricted to Kupffer cells and stellate cells.

Role of cellular senescence in hepatic wound healing and carcinogenesis

A state of permanent growth arrest characterises a senescent cell. Both the beneficial and deleterious effects that have accrued in senescent cells are observed in a complex organ, such as the liver. Injury to liver tissues triggers processes of regeneration and associated wound healing. Persistent injury can also lead to the neoplastic state. Recent evidence linked the senescent characteristics of the cells to the beneficial processes of wound healing and tumour surveillance in the liver. On the other hand, the secretory phenotype of senescent cells can also selectively promote undesirable neoplastic progression. In an evolutionary context, a senescent cell can function primarily as an adaptive response featuring the characteristics of altruism, trade-offs and bystander effects. Using the liver cell as a model system, this review focuses on the current knowledge of the role of senescence in these seemingly contradictory cell phenomena.

Telomere length in circulating serum DNA as a novel non-invasive biomarker for cirrhosis: a nested case-control analysis

BACKGROUND & AIMS

Previous studies have indicated that telomere length is associated with altered risk of various tumours including hepatitis B virus (HBV)-related hepatocellular carcinoma. However, the association between telomere length and the risk of cirrhosis has not been reported.

METHODS

In this nested case-control study, we used real-time quantitative PCR to determine the relative telomere length (RTL) in serum DNA samples from 100 HBV-related cirrhosis cases and 100 frequency-matched HBV controls, and evaluated the associations between RTL and cirrhosis risk by logistic regression analyses.

RESULTS

We found that cirrhotic cases had a significantly longer RTL (median, 0.36; range, 0.08-1.87) than non-cirrhotic controls (median, 0.20; range, 0.05-1.11) (P < 0.0001). Compared with subjects with short RTL, those with long RTL had a significantly increased cirrhosis risk [odds ratio, 2.76, 95% confidence interval, 1.50-5.10; P = 0.001]. Quartile analysis further indicated a dose-response effect for this association. Compared with patients with the lowest quartile of RTL, the cirrhosis risk for those with the second, third and highest quartile of RTL was 2.68 (0.91-7.87, P = 0.073), 3.37 (1.32-10.54, P = 0.013) and 6.64 (2.41-18.32, P < 0.0001) respectively (P(trend) <0.0001). Moreover, the area under the receiver operating characteristic curve increased from 0.60 (epidemiological variables) to 0.72 (epidemiological variables plus RTL), with statistically significant difference assessed by bootstrap analysis.

CONCLUSIONS

Our study presents the first epidemiological evidence that RTL in serum DNA could potentially be used as a simple, inexpensive and non-invasive marker of cirrhosis risk, a finding that warrants further investigations in independent retrospective and prospective populations.

Hepatocellular telomere length in biliary atresia measured by Q-FISH

BACKGROUND

Liver transplantation for biliary atresia is indicated whenever a Kasai portoenterostomy is considered unfeasible. However, the timing of liver transplantation in biliary atresia has not been precisely defined. Excessive shortening of hepatocellular telomeres may occur in patients with biliary atresia, and therefore, telomere length could be a predictor of hepatocellular reserve capacity.

METHODS

Hepatic tissues were obtained from 20 patients with biliary atresia who underwent LT and 10 age-matched autopsied individuals (mean age, 1.7 and 1.2 years, respectively). Telomere lengths were measured by Southern blotting and quantitative fluorescence in situ hybridization using the normalized telomere-centromere ratio. The correlation between the normalized telomere-centromere ratio for the hepatocytes in biliary atresia and the pediatric end-stage liver disease score was analyzed.

RESULTS

The median terminal restriction fragment length of the hepatic tissues in biliary atresia was not significantly different from that of the control (p = 0.425), whereas the median normalized telomere-centromere ratio of hepatocytes in biliary atresia was significantly smaller than that of the control (p < 0.001). Regression analysis demonstrated a negative correlation of the normalized telomere-centromere ratio with the pediatric end-stage liver disease score in biliary atresia (p < 0.001).

CONCLUSIONS

Telomere length analysis using quantitative fluorescence in situ hybridization could be an objective indicator of hepatocellular reserve capacity in patients with biliary atresia, and excessive telomere shortening supports the early implementation of liver transplantation.

Telomere shortening and karyotypic alterations in hepatocytes in long-term transplanted human liver allografts

The long-term fate of aged liver allografts in young recipients who received grafts from older donors is unknown. We evaluated graft aging by analyzing hepatocytic telomere length and karyotypic changes. Seventeen pediatric individuals who underwent living-donor liver transplantation for congenital biliary diseases were selected. At a median of 10.4 years post-transplant, ten had tolerated grafts with weaned off immunosuppressants, and seven had idiopathic post-transplantation hepatitis. Fluorescence in situ hybridization was used to evaluate the telomere signal intensity (TI) and karyotypic changes. First, we measured predictive age-dependent TI decline with regression analysis of donor livers. The mean TI at the earliest (within a year) and latest biopsies was significantly lower than the predicted TI of the studied allografts. With univariate analysis, a higher abnormal karyotype ratio in the donor liver was correlated with development of idiopathic post-transplantation hepatitis. With multivariate analysis that included clinical parameters, a greater TI decline at the earliest biopsy was correlated with the development of idiopathic post-transplantation hepatitis. In conclusion, graft aging as measured by TI decline and donor abnormal karyotype ratio was associated with idiopathic post-transplantation hepatitis of long-term transplanted liver allografts.

Assessing cell and organ senescence biomarkers

A major goal in cancer and aging research is to discriminate the biochemical modifications that happen locally that could account for the healthiness or malignancy of tissues. Senescence is one general antiproliferative cellular process that acts as a strong barrier for cancer progression, playing a crucial role in aging. Here, we focus on the current methods to assess cellular senescence, discriminating the advantages and disadvantages of several senescence biomarkers.

Relative telomere length: a novel non-invasive biomarker for the risk of non-cirrhotic hepatocellular carcinoma in patients with chronic hepatitis B infection

BACKGROUND AND AIMS

Telomere length has emerged as a promising risk predictor of various cancers including hepatocellular carcinoma (HCC). However, the majority of studies in this area measured telomere length in hepatocytes and one in lymphocytes with conflicting results. Moreover, no studies have been reported on using circulating DNA telomere length as a non-invasive HCC biomarker.

METHODS

We conducted a nested case-control study to determine the relative telomere length (RTL) in serum DNA from 140 hepatitis B virus (HBV)-related HCC cases and 280 frequency-matched cancer-free HBV controls.

RESULTS

Cases had a significantly longer RTL (median, 0.31; range, 0.02-2.31) than controls (median, 0.20; range, 0.01-1.60) (P = 0.003). Consistently, longer RTLs conferred a significantly increased HCC risk compared to short RTLs in a univariate logistic regression analysis (odds ratio [OR] = 1.55, 95% confidence interval [CI] = 1.02-2.33, P = 0.038). This association attenuated after multivariate adjustment (OR = 1.40, 95% CI = 0.90-2.19, P = 0.132). In a quartile analysis, a significant dose-response relationship was noted in univariate analysis (P(trend) = 0.017) which was again attenuated in multivariate analysis (P(trend) = 0.079). Further analyses revealed that the significant association between serum RTL and HCC risk was evident in non-cirrhotic (OR = 3.54, 95% CI 1.58-7.93 P = 0.002), but not cirrhotic (OR = 0.95, 95% CI 0.55-1.64, P = 0.860) HBV patients. Moreover, the significantly increased HCC risk conferred by cirrhosis was modulated by RTL with a significant interaction effect (P(interaction) = 0.013).

CONCLUSIONS

RTL in circulating cell-free serum DNA could potentially be used as a novel non-invasive biomarker for non-cirrhotic HCC. Prospective cohort studies are warranted to validate this finding and assess its clinical significance in HCC prevention.

Greater age and hepatocellular aging are independent risk factors for hepatocellular carcinoma arising from non-B non-C non-alcoholic chronic liver disease

We previously reported that hepatocellular aging can be assessed by measuring the nuclear size of hepatocytes. We attempted to elucidate whether this method is useful to identify the high risk group of hepatocellular carcinoma (HCC) in the patients with non-B non-C non-alcoholic liver injury. Fourteen patients with HCC and 78 without HCC, both of whom presented with non-B non-C non-alcoholic chronic liver injury and underwent liver biopsy, were selected. Twelve histologically normal liver tissues were selected as controls. The relative nuclear size (RNS) was calculated as the average nuclear size of the hepatocytes divided by that of lymphocytes. Multiple clinicopathological parameters were studied. The RNS values of normal livers ranged from 1.32 to 2.10, showing a gradual increase in an age-dependent manner. The RNS values of the injured livers without HCC increased after middle age. Univariate analysis identified greater age, existence of diabetes and RNS, as significantly positive contributors and ALT value and the degree of steatosis as negative contributors for the occurrence of HCC. Only age and RNS retained significance in multivariate analysis. All of the HCC patients were older than 50 and showed RNS values higher than 2.00. Therefore, such patients are classified as a high risk group of HCC.

Novel approach to bile duct damage in primary biliary cirrhosis: participation of cellular senescence and autophagy

Primary biliary cirrhosis (PBC) is characterized by antimitochondrial autoantibodies (AMAs) in patients' sera and histologically by chronic nonsuppurative destructive cholangitis in small bile ducts, eventually followed by extensive bile duct loss and biliary cirrhosis. The autoimmune-mediated pathogenesis of bile duct lesions, including the significance of AMAs, triggers of the autoimmune process, and so on remain unclear. We have reported that cellular senescence in biliary epithelial cells (BECs) may be involved in bile duct lesions and that autophagy may precede the process of biliary epithelial senescence in PBC. Interestingly, BECs in damaged bile ducts show characteristicsof cellular senescence and autophagy in PBC. A suspected causative factor of biliary epithelial senescence is oxidative stress. Furthermore, senescent BECs may modulate the microenvironment around bile ducts by expressing various chemokines and cytokines called senescence-associated secretory phenotypes and contribute to the pathogenesis in PBC.

Peroxisome proliferator-activated receptor-γ cross-regulation of signaling events implicated in liver fibrogenesis

Peroxisome proliferator-activated receptor-γ (PPARγ) is a nuclear receptor with transcriptional activity controlling multiple physical and pathological processes. Recently, PPARγ has been implicated in the pathogenesis of liver fibrosis. Its depleted expression has strong associations with the activation and transdifferentiation of hepatic stellate cells, the central event in liver fibrogenesis. Studies over the past decade demonstrate that PPARγ cross-regulates a number of signaling pathways mediated by growth factors and adipokines, and cellular events including apoptosis and senescence. These signaling and cellular events and their molecular interactions with PPARγ system are profoundly involved in liver fibrogenesis. We critically summarize these mechanistic insights into the PPARγ regulation in liver fibrogenesis based on the updated findings in this area. We conclude with a discussion of the impacts of these discoveries on the interpretation of liver fibrogenesis and their potential therapeutic implications. PPARγ activation could be a promising strategy for antifibrotic therapy.

Inherited liver shunts in dogs elucidate pathways regulating embryonic development and clinical disorders of the portal vein

Congenital disorders of the hepatic portal vasculature are rare in man but occur frequently in certain dog breeds. In dogs, there are two main subtypes: intrahepatic portosystemic shunts, which are considered to stem from defective closure of the embryonic ductus venosus, and extrahepatic shunts, which connect the splanchnic vascular system with the vena cava or vena azygos. Both subtypes result in nearly complete bypass of the liver by the portal blood flow. In both subtypes the development of the smaller branches of the portal vein tree in the liver is impaired and terminal branches delivering portal blood to the liver lobules are often lacking. The clinical signs are due to poor liver growth, development, and function. Patency of the ductus venosus seems to be a digenic trait in Irish wolfhounds, whereas Cairn terriers with extrahepatic portosystemic shunts display a more complex inheritance. The genes involved in these disorders cannot be identified with the sporadic human cases, but in dogs, the genome-wide study of the extrahepatic form is at an advanced stage. The canine disease may lead to the identification of novel genes and pathways cooperating in growth and development of the hepatic portal vein tree. The same pathways likely regulate the development of the vascular system of regenerating livers during liver diseases such as hepatitis and cirrhosis. Therefore, the identification of these molecular pathways may provide a basis for future proregenerative intervention.

Cirrhosis and constitutional telomerase mutations

There is currently a growing body of evidence supporting the role of a genetic predisposition in cirrhosis development. Given that telomere shortening has been associated with fibrosis progression, Calado et al. and Hartmann et al. recently focussed on telomerase gene mutations in human cirrhosis.

New insights into liver regeneration

Even if the Greeks probably anticipated rather than discovered the extraordinary regenerative capacity of the liver with the Prometheus myth, this phenomenon still fascinates scientists nowadays with the same enthusiasm. There are good reasons to decipher this process other than to find an answer to our fantasy of immortality: it could indeed help patients needing large liver resections or living-donor liver transplantation, it could increase our understanding of liver pathology and finally it could enable novel cell-therapy approaches. For decades, most of our knowledge about the mechanisms involved in liver regeneration came from the classic two-thirds partial hepatectomy (PH) model. In this scenario, hepatocytes play the leading role, which raises the question of the simple existence of a stem cell population. Recently however, hepatic progenitor cells come again under the limelight, seeming to play a role in liver physiology and in various liver diseases such as steatosis or cirrhosis. Excellent reviews have recently addressed liver regeneration. Our goal is therefore to focus on recent improvements in the field, highlighting data mostly published in the last two years in order to draw a putative picture of what the future research axes on liver regeneration might look like.

Recurrent hepatitis C and acute allograft rejection: clinicopathologic features with emphasis on the differential diagnosis between these entities

Chronic hepatitis C virus infection is the leading etiology for liver transplantation in the United States. Recurrent hepatitis C occurs nearly universally in these patients and represents a serious posttransplantation complication. Despite the detailed characterization of the histologic features of both recurrent hepatitis C and acute cellular rejection (ACR) over the last decades, the pathologic distinction between these 2 conditions remains one of the greatest diagnostic challenges in liver pathology. An accurate diagnosis, nevertheless, plays an essential role in patient management, as different therapeutic strategies are used for these conditions. In this review, the clinicopathologic features of posttransplantation recurrent hepatitis C and ACR are discussed, with emphasis on distinguishing histopathologic features, morphologic variants, ancillary techniques, and diagnostic pitfalls.

A maternal high-fat diet represses the expression of antioxidant defense genes and induces the cellular senescence pathway in the liver of male offspring rats

Maternal high-fat (HF) diet feeding is associated with increased risk of developing metabolism-related diseases in adult offspring, including chronic liver disease. The present study tested the hypothesis that maternal HF diet leads to a decreased antioxidant defense capacity and causes cellular senescence in liver of adult offspring rats, which might increase risk of developing chronic liver disease. Timed-pregnant Sprague Dawley rats were fed a HF diet (45% of energy from fat) or a control (C) diet (16% of energy from fat) during gestation and lactation. The resulting offspring were fed a C diet after weaning to generate 2 offspring groups: C diet-fed offspring of dams fed C diet (C/C) and C diet-fed offspring of dams fed a HF diet (HF/C). At 12 wk of age, male rats were killed and samples were collected for analysis. Maternal HF diet significantly increased plasma TG and hepatic TBARS concentrations and the size of hepatic lipid droplets in offspring rats. The expression of antioxidant defense genes, such as glutathione peroxidase-1, Cu/Zn superoxide dismutase (Sod1), paraoxonase enzymes (Pon1, Pon2, and Pon3), were significantly lower in the liver of HF/C pups than in C/C pups. The expression of Inhibitor of cyclin dependent Kinase 4a (p16INK4a), a marker of cellular senescence, and cyclooxygenase-2 (Cox2), a proinflammatory marker, was significantly higher in the HF/C offspring group than in the C/C offspring group. Western-blot analysis shows that cyclin D1 and phosphorylated retinoblastoma protein were significantly lower in HF/C offspring than in C/C offspring. The results provide the first evidence to our knowledge that maternal HF diet might alter antioxidant defense capacity and program the p16INK4a-dependent cellular senescence in the liver of adult offspring.

The promoter of human telomerase reverse transcriptase is activated during liver regeneration and hepatocyte proliferation

BACKGROUND & AIMS

Telomerase activity has not been detected in healthy human liver biopsy samples, but it is up-regulated in most human liver tumors. It is not clear whether telomerase is activated in response to acute or chronic liver injury. Telomerase activity is closely associated with expression of its catalytic subunit, telomerase reverse transcriptase (TERT). We analyzed the activity of the human TERT (hTERT) promoter during liver regeneration in vivo and hepatocyte proliferation in vitro.

METHODS

We used hTERTp-lacZ transgenic mice, which contain an 8.0-kilobase pair fragment of the hTERT gene promoter, to study the role of TERT in liver regeneration following partial hepatectomy. As an in vitro model, we used the HepaRG cell line as a new model system for human hepatocyte proliferation and differentiation.

RESULTS

Activity of the hTERT promoter increased significantly after partial hepatectomy; it was also induced in hepatocytes, based on immunohistologic analysis. Similar to the in vivo results, telomerase activity and hTERT expression were up-regulated in proliferating HepaRG cells and repressed in response to growth arrest and differentiation. Promoter mapping revealed that a proximal 0.3-kilobase pair fragment contains all elements necessary for regulation of hTERT in HepaRG cells. We identified E2F2 and E2F7 as transcription factors that control the differential expression of hTERT in proliferating hepatocytes, in vitro and in vivo.

CONCLUSIONS

hTERT is induced in hepatocytes during liver regeneration, indicating a functional role for telomerase in human liver.