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RESEARCH COMMUNICATION
Département de Biochimie, Université de Montréal, Montréal, Québec H3C 3J7, Canada
| Abstract |
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[Keywords: STAT5; RasV12; E2F1; ATM; DNA damage; p53; senescence]
Received August 29, 2006; revised version accepted November 15, 2006.
To investigate the nature of the signals that activate the p53 pathway during senescence, we thought that it would be valuable to compare two oncogenes that activate different signaling pathways. We anticipated that each oncogene might trigger specific pathways, but also that a general cellular stress could signal an oncogenic threat in the cells. Therefore, we generated a novel model of oncogene-induced senescence using a constitutively active allele of STAT5 (ca-STAT5). STAT5A together with STAT5B are transcription factors that mediate cytokine and hormone signals. Their constitutive activation has been observed in several human cancers, and they are oncogenic in cell culture models and transgenic animals (Bowman et al. 2000
). In this study, we report that knocking down ATM expression with a short hairpin RNA (shRNA) blocked p53 induction in response to aberrant STAT5A activation and bypassed the senescence response to this oncogene when the Rb pathway was also inactivated. Knocking down ATM also inhibited E2F1-induced senescence and, in combination with Rb inactivation, suppressed RasV12-induced senescence. In agreement, normal cells expressing ca-STAT5 or RasV12 accumulated DNA damage foci and exhibited a constitutive activation of the DNA damage signaling pathway. Bypassing senescence by inactivating the p53 and the Rb tumor suppressor pathways did not eliminate the DNA damage foci, explaining why they persist in premalignant lesions and may constitute an additional barrier to full oncogenic transformation.
| Results and Discussion |
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To investigate the pathways linking oncogenes to p53 and senescence, we first infected normal human diploid fibroblasts IMR90 or BJ (data not shown) with retroviruses expressing a constitutive allele of STAT5A (ca-STAT5A), also known as STAT5A1*6 (Onishi et al. 1998
) or an empty vector. Ca-STAT5A induced a cell cycle arrest with the characteristics of cellular senescence (Fig. 1A) including low BrdU incorporation and the nuclear accumulation of the p53 tumor suppressor protein (Fig. 1B). In previous studies, we found that the ATM kinase and the DNA damage signaling pathway was required for senescence in response to
-interferon (Moiseeva et al. 2006
). To investigate if a similar pathway was active in cells expressing ca-STAT5A we knocked down ATM expression using an already validated shRNA (shATM) (Fig. 1C; Mukhopadhyay et al. 2005
; Moiseeva et al. 2006
). Cells expressing shATM accumulated much less p53 than cells expressing a control hairpin (Fig. 1D), but did not bypass the senescent response to ca-STAT5A in normal human fibroblasts (Fig. 1E). STAT5A-induced senescence was not bypassed in cells expressing the human papillomavirus oncoproteins E6 or E7, which inactivate the p53 and the Rb pathway, respectively. However, coexpressing both E6 and E7 did circumvent ca-STAT5A-induced senescence (Fig. 1E). In agreement with the ability of shATM to inhibit p53 accumulation in response to ca-STAT5A, combining shATM with E7 but not with E6 also blocked ca-STAT5A-induced senescence (Fig. 1E). In this way, shATM acted genetically as a p53 suppressor. We concluded that either the p53 or the Rb pathway is sufficient for the senescence response to STAT5A, and that the p53 pathway was activated via the DNA damage-responsive kinase ATM.
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-Gal-negative cells that could further propagated with few signs of senescence (Fig. 1E). Taken together, these results suggest that the DNA damage signaling pathway is critical for the senescence response to oncogenes.
STAT5A and RasV12 induce a constitutive activation of the DNA damage signaling pathway
ATM is activated by double-stranded breaks and other forms of damage to DNA (Bakkenist and Kastan 2003
). These lesions can be readily visualized using antibodies against
H2AX, which is a phosphorylated version of the histone H2AX at Ser 139 (dAdda di Fagagna et al. 2003
; Bartkova et al. 2005
; Gorgoulis et al. 2005
). Staining with these antibodies revealed the presence of DNA damage foci in cells that were induced to senesce by oncogenic STAT5A, oncogenic ras, or hydrogen peroxide (Fig. 2A,B).
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H2AX or pS/TQ epitopes were more numerous than foci labeled by phospho-ATMS1981 or the ATM target phospho-53BP1, suggesting that other PIKKs (PI3 kinase-like kinases) are active in senescent cells. In accord, we detected numerous ATR foci in cells expressing either RasV12 or ca-STAT5A (Supplementary Fig. 4). However, the pattern of ATR staining was different from the pattern induced by replication arrest in cells treated with hydroxyurea (Supplementary Fig. 4), suggesting that the oncogenic pathway to ATR activation does not involve a classic S-phase arrest. This is consistent with the fact that senescence induced by RasV12 (Serrano et al. 1997
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ODDI persist in cells that bypassed senescence
The presence of DNA damage foci characterized both advanced human cancers and premalignant lesions (DiTullio et al. 2002
; Bartkova et al. 2005
; Gorgoulis et al. 2005
). Since those tumors must have already found a way around the senescent barrier, we reasoned that bypassing oncogene-induced senescence might not eliminate ODDI. To investigate this point we used E6 and E7 or E1A to bypass the senescent arrest to both ca-STAT5A and RasV12. As expected, cells expressing E6, E7, and ca-STAT5A, E1A, and ca-STAT5A, E6, E7, and RasV12 or E1A and RasV12 also contained ODDI (Fig. 4). These foci were also labeled by the antibody recognizing phospho-ATM Ser 1981 (Supplementary Fig. 5) and phospho-53BP1 (data not shown), suggesting that ATM is active in cells that escape senescence. A quantitation of phospho-ATM foci in senescent cells and in cells that bypassed senescence with E1A revealed that the average number of foci did not change (Supplementary Fig. 5). Hence, DNA damage signaling persists after bypassing senescence where it may provide additional anti-proliferative barriers. In fact, it is known that bypassing senescence is not sufficient to transform primary cells (Morales et al. 1999
; Dannenberg et al. 2000
; Peeper et al. 2001
). One possibility is that DNA damage foci activate other barriers to halt cellular transformation.
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| Materials and methods |
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IMR90 cells (American Type Culture Collection) and BJ normal fibroblasts (S. Lowe) were cultured in Dulbeccos modified Eagle medium (DMEM, GIBCO) supplemented with 10% fetal bovine serum (FBS, Hyclone) and 1% penicillin G/streptomycin sulfate (GIBCO). pBabe, pBabeRas, pLPC, pLPCE1A, pWZL, and pWZLRas were previously described (Ferbeyre et al. 2000
), and LXSN, LXSNE6, LXSNE7, and LXSNE6/E7 were provided by Dr. D. Galloway. STAT5A1*6 and STAT5B1*6 were subcloned from pMXSTAT5A1*6 and pMXSTAT5B1*6 (Onishi et al. 1998
) into pBabe as EcoRISalI fragments. shRNA against ATM (shATM) was described by Mukhopadhyay et al. (2005)
. Retroviral-mediated gene transfer, BrdU incorporation, cell cycle analysis, senescence assays, and fluorescence microscopy were done as described (Ferbeyre et al. 2000
).
Protein expression analysis
Cells were collected by trypsinization, washed with PBS, lysed in 100 µL of SDS sample buffer (60 mM Tris-HCl at pH 6.8, 10% glycerol, 2% SDS, 5% 2-mercaptoethanol), and boiled for 5 min. Twenty micrograms of total cell protein were separated on SDS-PAGE and transferred to Immobilon-P membranes (Millipore). Primary antibodies used were anti-phospho-Chk1S317 (2344, 1:1000, Cell Signaling), anti-phospho-Chk2T68 (2661, 1:1000, Cell Signaling), anti-phospho-Chk2S19 (2666, 1:1000, Cell Signaling), anti-Chk2 (2662, 1:1000, Cell Signaling), anti-ATM (from M.B. Kastan, St. Jude Childrens Research Hospital, Memphis, TN), and anti-
-tubulin (B-5-1-2, 1:5000, Sigma). Signals were revealed after incubation with anti-mouse or anti-rabbit secondary antibodies coupled to peroxidase (Amersham) by using enhanced chemiluminescence (ECL, Amersham) or Lumi-LightPLUS (Roche). Primary antibodies used for immunofluorescence were anti-
H2AX (JBW301, 1:200, Upstate Biotechnology), anti-53BP1 (Ab-1, 1:200, Oncogene), anti-pS/TQ epitopes (2851, 1:200, Cell Signaling), anti-phospho-ATMS1981 (10H11.E12, 1:200, Cell Signaling), anti-TRF2 (4A794, 1:200, Upstate Biotechnology), anti-p53 mix (1801, 1:100, and 421, 1:100, Santa Cruz Biotechnology), and anti-p53 (CM1, 1:50, Novocastra). Secondary antibodies used were AlexaFluor 488- or AlexaFluor 568-conjugated secondary antibodies (1:1000, Molecular Probes).
| Acknowledgments |
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| Footnotes |
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E-MAIL g.ferbeyre{at}umontreal.ca; FAX (514) 343-2210. ![]()
Supplemental material is available at http://www.genesdev.org.
Article is online at http://www.genesdev.org/cgi/doi/10.1101/gad.1487307
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