p53-Independent Regulation of p21 Expression and Senescence by Chk2

The Chk2 kinase is a tumor suppressor and key component of the DNA damage checkpoint response that encompasses cell cycle arrest, apoptosis, and DNA repair. It has also been shown to have a role in replicative senescence resulting from dysfunctional telomeres. Some of these functions are at least partially exerted through activation of the p53 transcription factor. High-level expression of virally transduced Chk2 in A549 human lung carcinoma cells led to arrested proliferation, apoptosis, and senescence. These were accompanied by various molecular events, including p21 (p21) transcriptional induction, consistent with p53 activation. However, Chk2-dependent senescence and p21 transcriptional induction also occurred in p53-defective SK-BR-3 (breast carcinoma) and HaCaT (immortalized keratinocyte) cells. Small interfering RNA–mediated knockdown of p21 in p53-defective cells expressing Chk2 resulted in a decrease in senescent cells. These results revealed a p53-independent role for Chk2 in p21 induction and senescence that may contribute to tumor suppression and genotoxic treatment outcome. (Mol Cancer Res 2005;3(11):627–34) Introduction The Chk2 kinase has a central role in the checkpoint response to DNA damage that leads to cell cycle arrest and apoptosis (1, 2). These components of the checkpoint response are believed to contribute to tumor suppression by curtailing inappropriate proliferation of potentially tumorigenic cells containing damaged DNA. They also contribute to treatment outcome of various genotoxic cancer therapies. Additionally, Chk2 has been described recently to be involved in replicative senescence (3, 4), a cellular process characterized by dysfunctional telomeres and associated with cellular aging that may also contribute to tumor suppression. Senescence can be triggered by various stress signals, including DNA damage (5), through a mechanism that has been called stress or aberrant signaling-induced senescence. Although it resembles replicative senescence in many ways—they share many components in their signaling pathways and display similar cellular phenotypes—stress or aberrant signalinginduced senescence is not associated with cell replication counting or dysfunctional telomeres. At the molecular level, DNA damage–induced senescence in human cells is characterized by p53 activation and subsequent accumulation of the cyclin-dependent kinase inhibitor, p21 (p21), which leads to growth arrest. It shifts the balance of p53 response outcome from cell death through apoptosis or mitotic catastrophe when absent to cell cycle arrest and senescence when present (6). Although DNA damage–induced senescence can be activated in tumor cells through the p53/p21 pathway, it can also occur through alternative mechanisms (5, 7). Thus, p53 and p21 are positive but not essential regulators of replicative and stress-induced senescence (8). Chk2 is a central component of the signal transduction pathway that is activated by DNA damage and dysfunctional telomeres resulting in apoptosis or cell cycle arrest and senescence. Signals emanating from blocked replication or DNA damage trigger the activation of the proximal checkpoint kinases ataxia telangiectasia mutated (ATM) and ataxia telangiectasia and Rad3-related (ATR). When activated, these kinases phosphorylate a wide spectrum of proteins that includes Chk2 (2, 9, 10). Chk2 phosphorylation by ATM at T68 leads to its oligomerization and is followed by an autophosphorylation cascade leading to its full activation (1, 2). Activated Chk2 propagates the DNA damage signal to several effector proteins involved in cell cycle arrest, apoptosis, and DNA repair (1, 2, 10). Similarly, signals emanating from dysfunctional telomere ends result in activation of the checkpoint response, including Chk2 phosphorylation and activation, which leads to a permanent arrest in cell cycling called replicative senescence (3, 4). Chk2-mediated apoptosis and sustained cell cycle arrest are at least partially transduced through the p53 tumor suppressor Received 8/3/05; revised 10/7/05; accepted 10/10/05. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. Note: C-M. Aliouat-Denis and N. Dendouga contributed equally to this work. C-M. Aliouat-Denis is currently at Faculté de Pharmacie, Université de Lille 2, rue du Pr. Laguesse 3, 59 006 Lille, France. U. Steller is currently at PicoRapid Technologie GmbH, Fahrenheitstrasse 1, D-28359 Bremen, Germany. W. Luyten is currently at IriDM, Naamsestraat 59, B-3000 Leuven, Belgium. Requests for reprints: Jorge E. Vialard, Oncology Discovery Research and Early Development, Johnson & Johnson Pharmaceutical Research and Early Development, Turnhoutseweg 30, B-2340 Beerse, Belgium. Phone: 32-14-60-3971; Fax: 32-14-60-5403. E-mail: jvialard@prdbe.jnj. com Copyright D 2005 American Association for Cancer Research. doi:10.1158/1541-7786.MCR-05-0121 Mol Cancer Res 2005;3(11). November 2005 627 Research. on September 27, 2021. © 2005 American Association for Cancer mcr.aacrjournals. org Downloaded from Published OnlineFirst November 10, 2005; DOI: 10.1158/1541-7786.MCR-05-0121