Telomere shortening impairs organ regeneration by inhibiting cell cycle re-entry of a subpopulation of cells

Genetic analysis of myc and telomerase interactions in vivo

Myc is a transcription factor with pleiotropic effects on tumorigenesis which are likely to be mediated by its target genes. A known Myc transcriptional target is the catalytic subunit of telomerase, Tert. However, the contribution of Tert activation to Myc-induced tumorigenesis in vivo remains unknown. In this study, we addressed the role of telomerase in Myc-induced skin papillomatosis by using compound mice with a switchable Myc gene, Inv-MycERTAM mice, in combination with either telomerase deficiency (Terc-/-) or telomerase overexpression (K5-mTert) in the skin. We first demonstrated that Myc activates telomerase in the skin. With Inv-MycERTAM x Terc-/- mice, we further showed that this telomerase activation is partially required to elicit a full hyperplastic Myc-induced response. The presence of critically short telomeres in late-generation Inv-MycERTAM x Terc-/- mice further reduced the skin lesion induced by Myc. On the other hand, telomerase overexpression in the skin of K5-mTert mice augments Myc-induced hyperplasia in the absence of changes in telomere length, suggesting a direct role of telomerase in the Myc protumorigenic response. Taken together, these results highlight telomerase as a mediator of Myc-induced papillomatosis and suggest telomerase as a putative therapeutic target for Myc-dependent lesions.

Telomeres and telomerase in cancer stem cells

Alterations in telomere dynamics both suppress and facilitate malignant transformation by regulating genomic stability and cell lifespan. Checkpoints induced by telomere dysfunction play a major role in tumour suppression, whereas telomere shortening contributes to the initiation of cancer by inducing chromosomal instability. Since stem cells are exposed to various tumourigenic agents and stresses throughout their lifetime, the ageing stem cell is a major target of malignant transformation. This review summarises our knowledge of telomere length and telomerase activity in stem cells during ageing and carcinogenesis.

Cellular senescence in vivo: its relevance in ageing and cardiovascular disease

In most somatic mammalian cell types extensive replication and various types of cellular insults induce a permanent form of growth arrest called senescence. Senescence has been comprehensively characterised in cell culture, but its occurrence in vivo has only recently started to become appreciated. In this mini-review, we examine the evidence for the occurrence of senescence in vivo, with particular emphasis on the cardiovascular system. We also describe the senescent phenotype and discuss its pathophysiological implications. We examine findings in animal models of ageing and human genetic disorders that argue for and against a role of senescence in age-related pathologies in general and vascular disease in particular.

Premature telomere shortening and impaired regenerative response in hepatocytes of individuals with NAFLD

AIMS

The risk factors associated with poor prognosis of nonalcoholic fatty liver disease (NAFLD) are not fully understood. Our aim was to assess the role of progressive hepatocellular telomere shortening in the clinical course of NAFLD.

METHODS

We measured average telomere lengths in liver tissue samples from 44 patients with NAFLD by quantitative fluorescence in situ hybridization using a telomere-specific probe. Patients in which telomeres measured at least 80% of the lengths of age-matched controls were categorized as group A. Those patients with telomeres measuring less than 80% of the control lengths formed group B.

RESULTS

Within group B, some samples showed a remarkable shortening of hepatocyte telomeres in younger patients, whereas some group A patients showed almost normal telomere lengths until their seventies. Among clinicopathological factors, body mass index (BMI), homeostasis model assessment insulin resistance (HOMA-IR), histological degree of steatosis and intensity of 8-hydroxy-2'-deoxyguanosine (8-OHdG) immunostaining were all significantly higher in group B than in group A. Ki-67 immunohistochemistry demonstrated that group B liver tissues were significantly less proliferative than those from group A, despite no significant difference in the necroinflammatory activities of group A and B samples. In group B patients, the ratios of Ki-67 positive index to alanine aminotransferase value were significantly lower than group A.

CONCLUSIONS

Greater insulin resistance can result in more severe hepatic steatosis among group B patients, leading to an overproduction of reactive oxygen species, which may accelerate telomere erosion. Furthermore the regenerative response of hepatocytes with prominent telomere shortening may be impaired, making these cells vulnerable to the effect of a 'second-hit' insult.

Age-dependent alterations of the T cell repertoire and functional diversity of T cells of the aged

The aging immune system is characterized by the contraction of T cell receptor (TCR) diversity and the de novo expression of NKrelated receptors (NKR) on oligoclonal T cells. NKR+ T cells likely represent a secondary immune diversification as a biological adaptation of aging to ensure host defense despite shrinkage of the TCR repertoire. NKRs are expressed in various combinations even among TCR-identical cells, and are capable of triggering effector pathways in either TCR-independent or TCR-dependent fashion. Understanding the biology of NKR+ T cells will be pivotal to the development of strategies to enhance immunity in the elderly.

The expression signature of in vitro senescence resembles mouse but not human aging

A comparison of several microarray datasets from aging human, mouse and rat and datasets from senescent cells from human and mouse shows a similarity between the expression signatures of cellular senescence and aging in mouse but not in humans.

Background

The biological mechanisms that underlie aging have not yet been fully identified. Senescence, a phenomenon occurring in vitro, limits the number of cell divisions in mammalian cell cultures and has been suggested to contribute to aging.

Results

We investigated whether the changes in gene expression that occur during mammalian aging and induction of cellular senescence are similar. We compared changes of gene expression in seven microarray datasets from aging human, mouse and rat, as well as four microarray datasets from senescent cells of man and mouse. The datasets were publicly available or obtained from other laboratories. Correlation measures were used to establish similarities of the expression profiles and gene ontology analyses to identify functional groups of genes that are co-regulated. Robust similarities were established between aging in different species and tissues, indicating that there is an aging transcriptome. Although some cross-species comparisons displayed high correlation, intra-species similarities were more reliable. Similarly, a senescence transcriptome was demonstrated that is conserved across cell types. A similarity between the expression signatures of cellular senescence and aging could be established in mouse, but not in human.

Conclusion

Our study is the first to use microarray data from several studies and laboratories for dissection of a complex biological phenotype. We demonstrate the presence of a mammalian aging transcriptome, and discuss why similarity between cellular senescence and aging is apparent in aging mice only.

Telomerase antagonists GRN163 and GRN163L inhibit tumor growth and increase chemosensitivity of human hepatoma

Most cancer cells have an immortal growth capacity as a consequence of telomerase reactivation. Inhibition of this enzyme leads to increased telomere dysfunction, which limits the proliferative capacity of tumor cells; thus, telomerase inhibition represents a potentially safe and universal target for cancer treatment. We evaluated the potential of two thio-phosphoramidate oligonucleotide inhibitors of telomerase, GRN163 and GRN163L, as drug candidates for the treatment of human hepatoma. GRN163 and GRN163L were tested in preclinical studies using systemic administration to treat flank xenografts of different human hepatoma cell lines (Hep3B and Huh7) in nude mice. The studies showed that both GRN163 and GRN163L inhibited telomerase activity and tumor cell growth in a dose-dependent manner in vitro and in vivo. The potency and efficacy of the lipid-conjugated antagonist, GRN163L, was superior to the nonlipidated parent compound, GRN163. Impaired tumor growth in vivo was associated with critical telomere shortening, induction of telomere dysfunction, reduced rate of cell proliferation, and increased apoptosis in the treatment groups. In vitro, GRN163L administration led to higher prevalence of chromosomal telomere-free ends and DNA damage foci in both hepatoma cell lines. In addition, in vitro chemosensitivity assay showed that pretreatment with GRN163L increased doxorubicin sensitivity of Hep3B. In conclusion, our data support the development of GRN163L, a novel lipidated conjugate of the telomerase inhibitor GRN163, for systemic treatment of human hepatoma. In addition to limiting the proliferative capacity of hepatoma, GRN163L might also increase the sensitivity of this tumor type to conventional chemotherapy.

Improved enzymatic isolation of fibroblasts for the creation of autologous skin substitutes

The number of medical applications using autologous fibroblasts is increasing rapidly. We investigated thoroughly the procedure to isolate cells from skin using the enzymatic tissue dissociation procedure. Tissue digestion efficiency, cell viability, and yield were investigated in relation to size of tissue fragments, digestion volume to tissue ratio, digestion time, and importance of other protease activities present in Clostridium histolyticum collagenase (CHC) (neutral protease, clostripain, and trypsin). The results showed that digestion was optimal with small tissue fragments (2-3 mm3) and with volumes tissue ratios > or =2 ml/g tissue. For incubations < or =10 h, the digestion efficiency and cell isolation yields were significantly improved by increasing the collagenase, neutral protease, or clostripain activity, whereas trypsin activity had no effects. However, a too high proteolytic activity of one of the proteases present in CHC digestion solution or long exposure times interfered with cell viability and cell culture yields. The optimal range of CHC proteases activities per milliliter digestion solutions was determined for digestions < or =10 h (collagenase 2700-3900 Mandl U/ml, neutral protease 5100-10,000 caseinase U/ml, and clostripain 35-48 BAEE U/ml) and for longer digestions (>14 h) (collagenase 1350- 3000 U/ml, neutral protease 2550-7700 U/ml, and clostripain 18-36 U/ml). Using these conditions, a maximum fibroblast expansion was achieved when isolated cells were seeded at 1 x 10(4) cells/cm2. These results did not only allow selection of optimal CHC batches able to digest dermal tissue with an high cell viability but also significantly increased the fibroblast yields, enabling us to produce autologous dermal tissue in a clinically acceptable time frame of 3 wk.

Telomerase reverse transcriptase mRNA expression in peripheral lymphocytes of patients with chronic HBV and HCV infections

Telomerase activity is present at low levels in peripheral lymphocytes (PL) and is upregulated upon activation, possibly protecting PL from telomere shortening. As decreased telomere length is considered a sign of cellular senescence, telomerase may, therefore, play an important role on immune function, organ regeneration and carcinogenesis. So far, quantification of human telomerase reverse transcriptase levels (hTERT) in PL, has not been reported. We determined hTERT mRNA levels in PL of hepatitis B virus (HBV) and hepatitis C virus (HCV) patients, in an attempt to address whether hTERT transcripts in PL are altered in these viral diseases, which are characterized by immune dysfunction and increased incidence of hepatocarcinogenesis. hTERT mRNA levels in PL of HBV (n = 17), HCV (n = 24) patients and healthy controls (n = 22) were quantified by real-time polymerase chain reaction. We observed significantly lower hTERT mRNA levels in HBV and HCV patients compared with healthy individuals (P < 0.05). hTERT mRNA levels were not associated with the patients' clinical status (inactive, hepatitis and cirrhosis). Also no correlation was observed between hTERT mRNA expression, and HBV and HCV replicative activity. In the inactive group (n = 18) we observed a negative correlation between hTERT mRNA expression and disease duration (rs = -0.52, P < 0.03). We performed for the first time an accurate quantification of hTERT mRNA expression in PL of HBV and HCV patients. The observed low levels of hTERT mRNA expression in the above patients may suggest its involvement in the immunopathogenesis of chronic viral hepatitis.

The cellular level of telomere dysfunction determines induction of senescence or apoptosis in vivo

Telomere dysfunction induces two types of cellular response: cellular senescence and apoptosis. We analysed the extent to which the cellular level of telomere dysfunction and p53 gene status affect these cellular responses in mouse liver using the experimental system of TRF2 inhibition by a dominant-negative version of the protein (TRF2delta B delta M). We show that the level of telomere dysfunction correlates with the level of TRF2delta B delta M protein expression resulting in chromosomal fusions, aberrant mitotic figures and aneuploidy of liver cells. These alterations provoked p53-independent apoptosis, but a strictly p53-dependent senescence response in distinct populations of mouse liver cells depending on the cellular level of TRF2delta B delta M expression. Apoptosis was associated with higher expression of TRF2delta B delta M, whereas cellular senescence was associated with low levels of TRF2delta B delta M) expression. Our data provide experimental evidence that induction of senescence or apoptosis in vivo depends on the cellular level of telomere dysfunction and differentially on p53 gene function.

Skp2-dependent degradation of p27kip1 is essential for cell cycle progression

The activity of the SCF(skp2) E3 ligase is required for the proteolytic turnover of several proteins involved in cell cycle control and transcriptional regulation. Loss of skp2 in the mouse leads to a complex phenotype including changes in cell size and DNA content as well as severe proliferation defects. Here we show that the loss of a single skp2 substrate, namely, the cyclin kinase inhibitor p27kip1, reverts the phenotype of skp2 knockout hepatocytes to normal. By comparing the kinetics of p27 turnover and cell cycle progression in skp2 knockout and p27T187A knock-in mice, we define a short period in G1 in which p27 is able to block the cell cycle after the exit from quiescence. Loss of p27 turnover during this period prevents mitotic division and instead leads to compensatory cell growth.

Liver regeneration: from myth to mechanism

The unusual regenerative properties of the liver are a logical adaptation by organisms, as the liver is the main detoxifying organ of the body and is likely to be injured by ingested toxins. The numerous cytokine- and growth-factor-mediated pathways that are involved in regulating liver regeneration are being successfully dissected using molecular and genetic approaches. So what is known about this process at present and which questions remain?

Mechanism of telomere shortening by oxidative stress

We investigated whether oxidative stress, which contributes to aging, accelerates the telomere shortening in human cultured cells. The terminal restriction fragment (TRF) from WI-38 fibroblasts irradiated with UVA (365-nm light) decreased with increasing of the irradiation dose. Furthermore, UVA irradiation dose-dependently increased the formation of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) in both WI-38 fibroblasts and HL-60 cells. In order to clarify the mechanism of the acceleration of telomere shortening, we investigated site-specific DNA damage induced by UVA irradiation in the presence of endogenous photosensitizers using (32)P 5' end-labeled DNA fragments containing telomeric oligonucleotide (TTAGGG)(4). UVA irradiation with riboflavin induced 8-oxodG formation in the DNA fragments containing telomeric sequence, and Fpg protein treatment led to chain cleavages at the central guanine of 5'-GGG-3' in telomere sequence. Human 8-oxodG-DNA glycosylase introduces a chain break in a double-stranded oligonucleotide specifically at an 8-oxodG residue. The amount of 8-oxodG formation in DNA fragment containing telomere sequence [5'-CGC(TTAGGG)(7)CGC-3'] was approximately five times more than that in the DNA fragment containing nontelomere sequence [5'-CGC(TGTGAG)(7)CGC-3']. Furthermore, H(2)O(2) plus Cu(II) caused DNA damage, including 8-oxodG formation, specifically at the GGG sequence in the telomere sequence (5'-TTAGGG-3'). It is concluded that the formation of 8-oxodG at the GGG triplet in telomere sequence induced by oxidative stress could participate in acceleration of telomere shortening.

Telomeres and telomerase: a dual role in hepatocarcinogenesis

Telomere shortening limits the proliferative capacity of primary human cells and restrains the regenerative capacity of organ systems during chronic diseases and aging. Telomere shortening apparently has a dual role in tumor development and progression. On the one hand, it induces chromosomal instability and the initiation of cancer; on the other hand, tumor progression requires stabilization of telomeres. The predominant mechanism of telomere stabilization in tumor cells is the activation of the telomere-synthesizing enzyme telomerase. The potential use of telomerase activators for the treatment of regenerative disorders will ultimately depend on their effects on tumorigenesis. This review focuses on the role of telomere shortening and telomerase in carcinogenesis with a special focus on hepatocellular carcinoma.

Hepatocellular telomere shortening correlates with chromosomal instability and the development of human hepatoma

The telomere hypothesis of cancer initiation indicates that telomere shortening initiates cancer by induction of chromosomal instability. To test whether this hypothesis applies to human hepatocellular carcinoma (HCC), we analyzed the telomere length of hepatocytes in cytological smears of fine-needle biopsies of liver tumors from patients with cirrhosis (n = 39). The tumors consisted of 24 HCC and 15 regenerative nodules as diagnosed by combined histological and cytological diagnostics. In addition, we analyzed the telomere length of hepatocytes in HCC and surrounding noncancerous liver tissue within individual patients in another cohort of 10 patients with cirrhosis. Telomere length analysis of hepatocytes was correlated with tumor pathology and ploidy grade of the tumors, which was analyzed by cytophotometry. Telomeres were significantly shortened in hepatocytes of HCC compared to hepatocytes in regenerative nodules or surrounding noncancerous liver tissue. Hepatocyte telomere shortening in HCC was independent of the patient's age. There was no overlap in mean telomere lengths of individual samples when comparing HCC with regenerative nodules or noncancerous surrounding liver. Within the HCC group, telomeres were significantly shorter in hepatocytes of aneuploid tumors compared to diploid tumors. In conclusion, our data suggest that the telomere hypothesis of cancer initiation applies to human HCC and that cell type-specific telomere length analysis might indicate the risk of HCC development.

Mitogen stimulation cooperates with telomere shortening to activate DNA damage responses and senescence signaling

Replicative senescence is induced by critical telomere shortening and limits the proliferation of primary cells to a finite number of divisions. To characterize the activity status of the replicative senescence program in the context of cell cycle activity, we analyzed the senescence phenotypes and signaling pathways in quiescent and growth-stimulated primary human fibroblasts in vitro and liver cells in vivo. This study shows that replicative senescence signaling operates at a low level in cells with shortened telomeres but becomes fully activated when cells are stimulated to enter the cell cycle. This study also shows that the dysfunctional telomeres and nontelomeric DNA lesions in senescent cells do not elicit a DNA damage signal unless the cells are induced to enter the cell cycle by mitogen stimulation. The amplification of senescence signaling and DNA damage responses by mitogen stimulation in cells with shortened telomeres is mediated in part through the MEK/mitogen-activated protein kinase pathway. These findings have implications for the further understanding of replicative senescence and analysis of its role in vivo.

Telomere length is reset during early mammalian embryogenesis

The enzyme telomerase is active in germ cells and early embryonic development and is crucial for the maintenance of telomere length. Whereas the different length of telomeres in germ cells and somatic cells is well documented, information on telomere length regulation during embryogenesis is lacking. In this study, we demonstrate a telomere elongation program at the transition from morula to blastocyst in mice and cattle that establishes a specific telomere length set point during embryogenesis. We show that this process restores telomeres in cloned embryos derived from fibroblasts, regardless of the telomere length of donor nuclei, and that telomere elongation at this stage of embryogenesis is telomerase-dependent because it is abrogated in telomerase-deficient mice. These data demonstrate that early mammalian embryos have a telomerase-dependent genetic program that elongates telomeres to a defined length, possibly required to ensure sufficient telomere reserves for species integrity.

When cells get stressed: an integrative view of cellular senescence

Cells entering a state of senescence undergo a permanent cell cycle arrest, accompanied by a set of functional and morphological changes. Senescence of cells occurs following an extended period of proliferation in culture or in response to various physiologic stresses, yet little is known about the role this phenomenon plays in vivo. The study of senescence has focused largely on its hypothesized role as a barrier to extended cell division, governed by a division-counting mechanism in the form of telomere length. Here, we discuss the biological functions of cellular senescence and suggest that it should be viewed in terms of its role as a general cellular stress response program, rather than strictly as a barrier to unlimited cycles of cell growth and division. We also discuss the relative roles played by telomere shortening and telomere uncapping in the induction of senescence.

When cells get stressed: an integrative view of cellular senescence

Cells entering a state of senescence undergo a permanent cell cycle arrest, accompanied by a set of functional and morphological changes. Senescence of cells occurs following an extended period of proliferation in culture or in response to various physiologic stresses, yet little is known about the role this phenomenon plays in vivo. The study of senescence has focused largely on its hypothesized role as a barrier to extended cell division, governed by a division-counting mechanism in the form of telomere length. Here, we discuss the biological functions of cellular senescence and suggest that it should be viewed in terms of its role as a general cellular stress response program, rather than strictly as a barrier to unlimited cycles of cell growth and division. We also discuss the relative roles played by telomere shortening and telomere uncapping in the induction of senescence.