SAGE-Hindawi Access to Research Journal of Aging Research Volume 2011, Article ID 103253, 14 pages doi:10.4061/2011/103253 Research Article Self-Renewal Signalling in Presenescent Tetraploid IMR90 Cells Anda Huna, 1 Kristine Salmina, 1 Elina Jascenko, 2 Gunars Duburs, 2 Inna Inashkina, 1 and Jekaterina Erenpreisa 1 1 Latvian Biomedical Research and Study Centre, Ratsupites 1, 1067 Riga, Latvia 2 Latvian Institute of Organic Synthesis, Aizkraukles 21, 1006 Riga, Latvia Correspondence should be addressed to Jekaterina Erenpreisa, katrina@biomed.lu.lv Received 15 December 2010; Revised 22 February 2011; Accepted 25 February 2011 Academic Editor: Noam Shomron Copyright © 2011 Anda Huna et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Endopolyploidy and genomic instability are shared features of both stress-induced cellular senescence and malignant growth. Here, we examined these facets in the widely used normal human fibroblast model of senescence, IMR90. At the presenescence stage, a small (2–7%) proportion of cells overcome the 4n-G1 checkpoint, simultaneously inducing self-renewal (NANOG-positivity), the DNA damage response (DDR; γ-H2AX-positive foci), and senescence (p16inka4a- and p21CIP1-positivity) signalling, some cells reach octoploid DNA content and divide. All of these markers initially appear and partially colocalise in the perinucleolar compartment. Further, with development of senescence and accumulation of p16inka4a and p21CIP1, NANOG is downregulated in most cells. The cells increasingly arrest in the 4n-G1 fraction, completely halt divisions and ultimately degenerate. A positive link between DDR, self-renewal, and senescence signalling is initiated in the cells overcoming the tetraploidy barrier, indicating that cellular and molecular context of induced tetraploidy during this period of presenescence is favourable for carcinogenesis. 1. Introduction Cellular senescence is a condition in which the cells remain alive but are unable to proliferate. Premature senescence can be triggered by certain stresses independently of the number of cell divisions or telomere length [1], possibly as a result of protracted DNA damage signalling [2]. Oncogene-induced senescence is thought to behave similarly, driven at the very early stages of tumour development where it serves as a barrier to cancer progression [3]. Subsequent progression to full-blown malignancy is favoured when tumour stem cells acquire further mutations that impair the senescence pathway, for example, mutations in TP53 or CDKN2a [4, 5]. During in vitro culture, human fibroblast cells undergo a presenescence phenomenon whereby they display evidence of chromosome instability (CIN) within an apparently highly heterogenous population with signs of chromosomal dam- age, and the appearance of polyploid interphase cells and their divisions [4, 6–12]. Whereas the frequency of diploid mitotic cells at presenescence is declining, the number of polyploid mitoses increases to a peak before a sharp fall as the cells change to the characteristic flat morphology indicative of replicative senescence [13, 14]. These data stimulated the hypothesis that telomeric loss at senescence may represent a “genetic time bomb” causally involved in both cell senescence and malignant transformation [13, 15]. In is clear that CIN associated with polyploidy at the presenescence stage may substantially increase the mutability and risk of malignant transformation [16–18]. Moreover, there are reports from normal cell cultures of revertant cells escaping senescence by acquiring mutations [19] and their ability to depolyploidise and restart mitoses [9–12, 17]. The features of CIN, including polyploidy, are also characteristic of malignant tumors where the degree of CIN is correlated with aggression [20]. Induced endopolyploidy is a typical response of tumour cells with deficient p53 function to the action of DNA or spindle-damaging agents [21–24]. For a decade, it has been generally accepted that sublethal genotoxic damage to cancer cells associated with anticancer clinical modalities accelerates cellular senescence [1, 25], with concomitant induction of polyploidy as a component. However, we and others have recently shown that the induction of endopolyploidy followed by arrest and subse- quent slippage from a spindle checkpoint is accompanied