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Senescence and Immortality in Hepatocellular Carcinoma View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Bilkent University Institutional Repository Cancer Letters 286 (2009) 103–113 Contents lists available at ScienceDirect Cancer Letters journal homepage: www.elsevier.com/locate/canlet Mini-review Senescence and immortality in hepatocellular carcinoma Mehmet Ozturk a,b,*, Ayca Arslan-Ergul a, Sevgi Bagislar a,b, Serif Senturk a, Haluk Yuzugullu a,b a Department of Molecular Biology and Genetics, Bilkent University, 06800 Ankara, Turkey b Centre de Recherche INSERM-Université Joseph Fourrier U823, Institut Albert Bonniot, 38042 Grenoble, France article info abstract Article history: Cellular senescence is a process leading to terminal growth arrest with characteristic mor- Received 26 March 2008 phological features. This process is mediated by telomere-dependent, oncogene-induced Received in revised form 23 June 2008 and ROS-induced pathways, but persistent DNA damage is the most common cause. Senes- Accepted 29 October 2008 cence arrest is mediated by p16INK4a- and p21Cip1-dependent pathways both leading to ret- inoblastoma protein (pRb) activation. p53 plays a relay role between DNA damage sensing and p21Cip1 activation. pRb arrests the cell cycle by recruiting proliferation genes to facul- tative heterochromatin for permanent silencing. Replicative senescence that occurs in Keywords: hepatocytes in culture and in liver cirrhosis is associated with lack of telomerase activity Liver cancer Senescence and results in telomere shortening. Hepatocellular carcinoma (HCC) cells display inactivat- INK4a Telomeres ing mutations of p53 and epigenetic silencing of p16 . Moreover, they re-express telo- DNA damage merase reverse transcriptase required for telomere maintenance. Thus, senescence bypass p53 and cellular immortality is likely to contribute significantly to HCC development. Onco- p16INK4a gene-induced senescence in premalignant lesions and reversible immortality of cancer Cip1 p21 cells including HCC offer new potentials for tumor prevention and treatment. Retinoblastoma Ó 2008 Elsevier Ireland Ltd. All rights reserved. Cirrhosis Hepatocytes Telomerase reverse transcriptase 1. Introduction senescence markers such as senescence-associated b-galactosidase (SABG), p16INK4A, senescence-associated Senescence is an evolutionary term meaning ‘‘the pro- DNA-damage foci and senescence-associated heterochro- cess of becoming old”; the phase from full maturity to matin foci (for a review see Ref. [3]). This cellular response death characterized by accumulation of metabolic prod- has both beneficial (anti-cancer) and probably deleterious ucts and decreased probability of reproduction or survival (such as tissue aging) effects on the organism. Most of [1]. The term ‘‘cellular senescence” was initially used by our knowledge of cellular senescence is derived from Hayflick and colleagues to define cells that ceased to divide in vitro studies performed with fibroblasts, and some epi- in culture [2]. Today, cellular senescence is recognized as a thelial cells such as mammary epithelial cells. Animal response of proliferating somatic cells to stress and dam- models are increasingly being used to study cellular senes- age from exogenous and endogenous sources. It is charac- cence in vivo. Telomerase-deficient mouse models lacking terized by permanent cell cycle arrest. Senescent cells also RNA subunit (TERCÀ/À) have been very useful in demon- display altered morphology and an altered pattern of gene strating the critical role of telomeres in organ aging and tu- expression, and can be recognized by the presence of mor susceptibility [4]. Other mouse models including tumor suppressor gene-deficient and oncogene-expressing mice were also used extensively. * Corresponding author. Address: Centre de Recherche INSERM-Uni- Compared to other tissues and cancer models, the role versité Joseph Fourrier U823, Institut Albert Bonniot, Grenoble 38000, France. Tel.: +33 (0) 4 76 54 94 10; fax: +33 (0) 4 76 54 94 54. of senescence in liver cells and its implications in hepato- E-mail address: [email protected] (M. Ozturk). cellular carcinogenesis have been less explored. One of 0304-3835/$ - see front matter Ó 2008 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.canlet.2008.10.048 104 M. Ozturk et al. / Cancer Letters 286 (2009) 103–113 the main obstacles is the lack of adequate in vitro systems. chromosome ends. They are also indispensable for proper As hepatocytes can not divide in cell culture, the study of recombination and chromosomal segregation during cell their replicative senescence mechanisms is not easy. Nev- division. Telomeres become shorter with every cell divi- ertheless, these cells are able to quit their quiescent state sion in somatic cells, because of replication complex’s in vivo and proliferate massively in response to partial inability to copy the ends of linear DNA, which also makes hepatectomy or liver injury [5]. This capacity can be ex- them a ‘‘cell cycle counter” for the cell [14]. Telomeres are plored to study in vivo senescence of hepatocytes using ro- added to the end of chromosomes with a complex contain- dent models. Studies with clinical samples indicate that ing the RNA template TERC and the reverse transcriptase hepatocyte senescence occurs in vivo in patients with TERT [15]. Most somatic cells lack telomerase activity be- chronic hepatitis, cirrhosis and HCC [6–8]. In contrast to cause the expression of TERT is repressed, in contrast to the paucity of studies directly addressing cellular senes- TERC expression. The lack of sufficient TERT expression in cence, the critical role of telomere shortening (as a feature somatic cells is the main cause of telomere shortening dur- associated with replicative senescence) in cirrhosis and ing cell replication. This telomerase activity also helps to HCC development is well established [9]. Telomeres in nor- maintain telomere integrity by telomere capping [15]. mal liver show a consistent but slow shortening during The loss of telomeres has long been considered to be the aging. In contrast, hepatocyte DNA telomere shortening is critical signal for senescence induction. It is now well accelerated in patients with chronic liver disease with known that telomere-dependent senescence is induced shortest telomeres described in cirrhotic liver and HCC. by a change in the protected status of shortened telomeres, Telomerase-deficient mice have also been used elegantly whereby the loss of telomere DNA contributes to this to demonstrate the critical roles of telomerase and telo- change [16]. The loss of telomere protection or any other meres in liver regeneration and experimentally induced cause of telomere dysfunction results in inappropriate cirrhosis [10,11]. A major accomplishment in recent years chromosomal end-to-end fusions through non-homolo- was the demonstration of critical role played by senes- gous end joining or homologous recombination DNA repair cence for the clearance of ras-induced murine liver carci- pathways [17]. These DNA repair pathways are used prin- nomas following p53 restoration [12]. cipally to repair double-strand DNA breaks (DSBs). Thus, it Despite a relatively important progress, the mecha- is highly likely that the open-ended telomere DNA is nisms of hepatocellular senescence and the role of cellular sensed as a DSB by the cell machinery when telomere immortality in HCC remain ill-known issues. As one of the structure becomes dysfunctional. Accordingly, dysfunc- rare tissues with ample clinical data on senescence-related tional telomeres elicit a potent DSB type DNA damage re- aberrations, liver may serve as an excellent model to fur- sponse by recruiting phosphorylated H2AX, 53BP1, NBS1 ther explore the relevance of cellular senescence in human and MDC1 [18]. biology. Moreover, a better understanding of senescence Telomere-dependent senescence is not the only form of and immortality in hepatic tissues may help to develop senescence. At least two other forms of telomere-indepen- new preventive and therapeutic approaches for severe li- dent senescence are presently known: (1) oncogene-in- ver diseases such as cirrhosis and HCC. Here we will review duced senescence; and (2) reactive oxygen species (ROS)- recent progress on senescence and immortality mecha- induced senescence (Fig. 1). nisms with a specific emphasis on hepatocellular Oncogene-induced senescence had initially been identi- carcinogenesis. fied as a response to expression of Ras oncogene in normal cells ([19], for a recent review see [20]). The expression of oncogenic Ras in primary human or rodent cells results in 2. Senescence pathways permanent G1 arrest. The arrest was accompanied by accu- mulation of p53 and p16INK4a, and was phenotypically Cellular senescence has long been considered as a indistinguishable from cellular senescence. This landmark mechanism that limits the number of cell divisions (or observation suggested that the onset of cellular senescence population doublings) in response to progressive telomere does not simply reflect the accumulation of cell divisions, shortening. Most human somatic cells are telomerase-defi- but can be prematurely activated in response to an onco- cient because of the repression of telomerase reverse genic stimulus [19]. In 10 years following this important transcriptase (TERT) expression. Therefore, proliferating discovery, telomere-independent forms of senescence have somatic cells undergo progressive telomere DNA erosion become a new focus of extensive research
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