The cdk5 Kinase Regulates the STAT3 Transcription Factor to Prevent DNA Damage upon Topoisomerase I Inhibition

Abstract

The STAT3 transcription factors are cytoplasmic proteins that induce gene activation in response to growth factor stimulation. Following tyrosine phosphorylation, STAT3 proteins dimerize, translocate to the nucleus, and activate specific target genes involved in cell-cycle progression. Despite its importance in cancer cells, the molecular mechanisms by which this protein is regulated in response to DNA damage remain to be characterized. In this study, we show that STAT3 is activated in response to topoisomerase I inhibition. Following treatment, STAT3 is phosphorylated on its C-terminal serine 727 residue but not on its tyrosine 705 site. We also show that topoisomerase I inhibition induced the up-regulation of the cdk5 kinase, a protein initially described in neuronal stress responses. In co-immunoprecipitations, cdk5 was found to associate with STAT3, and pulldown experiments indicated that it associates with the C-terminal activation domain of STAT3 upon DNA damage. Importantly, the cdk5-STAT3 pathway reduced DNA damage in response to topoisomerase I inhibition through the up-regulation of Eme1, an endonuclease involved in DNA repair. ChIP experiments indicated that STAT3 can be found associated with the Eme1 promoter when phosphorylated only on its serine 727 residue and not on tyrosine 705. We therefore propose that the cdk5-STAT3 oncogenic pathway plays an important role in the expression of DNA repair genes and that these proteins could be used as predictive markers of tumors that will fail to respond to chemotherapy. The STAT3 transcription factors are cytoplasmic proteins that induce gene activation in response to growth factor stimulation. Following tyrosine phosphorylation, STAT3 proteins dimerize, translocate to the nucleus, and activate specific target genes involved in cell-cycle progression. Despite its importance in cancer cells, the molecular mechanisms by which this protein is regulated in response to DNA damage remain to be characterized. In this study, we show that STAT3 is activated in response to topoisomerase I inhibition. Following treatment, STAT3 is phosphorylated on its C-terminal serine 727 residue but not on its tyrosine 705 site. We also show that topoisomerase I inhibition induced the up-regulation of the cdk5 kinase, a protein initially described in neuronal stress responses. In co-immunoprecipitations, cdk5 was found to associate with STAT3, and pulldown experiments indicated that it associates with the C-terminal activation domain of STAT3 upon DNA damage. Importantly, the cdk5-STAT3 pathway reduced DNA damage in response to topoisomerase I inhibition through the up-regulation of Eme1, an endonuclease involved in DNA repair. ChIP experiments indicated that STAT3 can be found associated with the Eme1 promoter when phosphorylated only on its serine 727 residue and not on tyrosine 705. We therefore propose that the cdk5-STAT3 oncogenic pathway plays an important role in the expression of DNA repair genes and that these proteins could be used as predictive markers of tumors that will fail to respond to chemotherapy. IntroductionSignal transducer and activator of transcription 3 (STAT3) 3The abbreviations used are: STAT3signal transducers and activators of transcription 3ChIPchromatin immunoprecipitationCBPCREB-binding proteincdkcyclin-dependent kinasesiRNAsmall interference RNAPBSphosphate-buffered salineRTreverse transcriptionIL-6interleukin-6FACSfluorescent-activated cell sorting. proteins are cytoplasmic transcription factors that translocate into the nucleus following growth factor stimulation. In contrast to normal cells where its phosphorylation is only transient, constitutive activation of STAT3 has been reported in several primary cancers and tumor cell lines (1Bromberg J. J. Clin. Invest. 2002; 109: 1139-1142Crossref PubMed Scopus (740) Google Scholar, 2Yu H. Jove R. Nat. Rev. Cancer. 2004; 4: 97-105Crossref PubMed Scopus (1918) Google Scholar, 3Levy D.E. Lee C.K. J. Clin. Invest. 2002; 109: 1143-1148Crossref PubMed Scopus (740) Google Scholar). This abnormal activation is due to oncogenic kinases such as epidermal growth factor receptor, Her2/Neu, src, or bcr-abl, which induce STAT3 activation through phosphorylation of its tyrosine 705 residue (4Bromberg J.F. Horvath C.M. Besser D. Lathem W.W. Darnell Jr., J.E. Mol. Cell. Biol. 1998; 18: 2553-2558Crossref PubMed Scopus (568) Google Scholar). This phosphorylation allows the nuclear translocation and DNA binding of the STAT3 dimer and the up-regulation of several genes involved in cell-cycle and cell survival such as cyclin D1, Myc, or bclxl. The up-regulation of these cancer genes mediates the oncogenic activity of STAT3 and its ability to transform cells (5Bromberg J.F. Wrzeszczynska M.H. Devgan G. Zhao Y. Pestell R.G. Albanese C. Darnell Jr., J.E. Cell. 1999; 98: 295-303Abstract Full Text Full Text PDF PubMed Scopus (2472) Google Scholar).A second phosphorylation occurs on the serine 727 residue of the C-terminal activation domain. It has been proposed that this phosphorylation is necessary for maximal gene activation, because its mutation prevents STAT3 transcriptional function (6Wen Z. Zhong Z. Darnell Jr., J.E. Cell. 1995; 82: 241-250Abstract Full Text PDF PubMed Scopus (1728) Google Scholar). It is believed that this modification favors the recruitment of transcriptional cofactors such as CBP, NcoA, or P-Tefb that binds to the C-terminal domain of the transcription factor (7Giraud S. Bienvenu F. Avril S. Gascan H. Heery D.M. Coqueret O. J. Biol. Chem. 2002; 277: 8004-8011Abstract Full Text Full Text PDF PubMed Scopus (122) Google Scholar, 8Giraud S. Hurlstone A. Avril S. Coqueret O. Oncogene. 2004; 23: 7391-7398Crossref PubMed Scopus (90) Google Scholar, 9Paulson M. Pisharody S. Pan L. Guadagno S. Mui A.L. Levy D.E. J. Biol. Chem. 1999; 274: 25343-25349Abstract Full Text Full Text PDF PubMed Scopus (185) Google Scholar, 10Nakashima K. Yanagisawa M. Arakawa H. Kimura N. Hisatsune T. Kawabata M. Miyazono K. Taga T. Science. 1999; 284: 479-482Crossref PubMed Scopus (720) Google Scholar). However, it remains to be determined if the association of STAT3 with its coactivators is a direct consequence of Ser-727 phosphorylation.Although it was initially believed that the tyrosine phosphorylation is essential for STAT3 activity, several groups have recently reported that specific forms of the transcription factor, which are only phosphorylated on its Ser-727 residue, can induce gene activation. In prostate cancer, Ser-727 phosphorylation is sufficient to activate STAT3 and drive tumorigenesis in the absence of tyrosine 705 activation (11Qin H.R. Kim H.J. Kim J.Y. Hurt E.M. Klarmann G.J. Kawasaki B.T. Duhagon Serrat M.A. Farrar W.L. Cancer Res. 2008; 68: 7736-7741Crossref PubMed Scopus (106) Google Scholar). Elegant results have shown that tyrosine 705 mutants can associate with NF-κB and induce the expression of genes such as mras or met, which are likely to play an important role in cell transformation by STAT3 (12Yang J. Chatterjee-Kishore M. Staugaitis S.M. Nguyen H. Schlessinger K. Levy D.E. Stark G.R. Cancer Res. 2005; 65: 939-947PubMed Google Scholar, 13Yang J. Liao X. Agarwal M.K. Barnes L. Auron P.E. Stark G.R. Genes Dev. 2007; 21: 1396-1408Crossref PubMed Scopus (448) Google Scholar, 14Lee H. Herrmann A. Deng J.H. Kujawski M. Niu G. Li Z. Forman S. Jove R. Pardoll D.M. Yu H. Cancer Cell. 2009; 15: 283-293Abstract Full Text Full Text PDF PubMed Scopus (513) Google Scholar). These results lead to the important conclusion that the influence of STAT3 on cell transformation can be independent of the tyrosine 705 phosphorylation and that this site should not be considered as a unique marker of STAT3 oncogenic activity.This conclusion also leads to the hypothesis that STAT3 can induce different transcriptional programs, depending on which sites are phosphorylated and certainly on the type of stimulation. Although STAT3 activation is well characterized in response to growth factor stimulation, little is known about its regulation in response to other stimulation such as DNA damage and chemotherapy treatment. Interestingly, several studies have suggested that an abnormal activation of this transcription factor is associated with intrinsic drug resistance (15Barré B. Vigneron A. Perkins N. Roninson I.B. Gamelin E. Coqueret O. Trends Mol. Med. 2007; 13: 4-11Abstract Full Text Full Text PDF PubMed Scopus (102) Google Scholar). STAT3 expression has been associated with resistance to radiation-induced apoptosis (16Sano S. Chan K.S. Kira M. Kataoka K. Takagi S. Tarutani M. Itami S. Kiguchi K. Yokoi M. Sugasawa K. Mori T. Hanaoka F. Takeda J. DiGiovanni J. Cancer Res. 2005; 65: 5720-5729Crossref PubMed Scopus (86) Google Scholar, 17Shen Y. Devgan G. Darnell Jr., J.E. Bromberg J.F. Proc. Natl. Acad. Sci. U.S.A. 2001; 98: 1543-1548Crossref PubMed Scopus (230) Google Scholar, 18Niu G. Wright K.L. Ma Y. Wright G.M. Huang M. Irby R. Briggs J. Karras J. Cress W.D. Pardoll D. Jove R. Chen J. Yu H. Mol. Cell. Biol. 2005; 25: 7432-7440Crossref PubMed Scopus (297) Google Scholar), and it can also confer resistance to Fas or paclitaxel-mediated apoptosis in multiple myeloma and ovarian cancer (19Catlett-Falcone R. Landowski T.H. Oshiro M.M. Turkson J. Levitzki A. Savino R. Ciliberto G. Moscinski L. Fernández-Luna J.L. Nuñez G. Dalton W.S. Jove R. Immunity. 1999; 10: 105-115Abstract Full Text Full Text PDF PubMed Scopus (1440) Google Scholar, 20Duan Z. Foster R. Bell D.A. Mahoney J. Wolak K. Vaidya A. Hampel C. Lee H. Seiden M.V. Clin. Cancer Res. 2006; 12: 5055-5063Crossref PubMed Scopus (211) Google Scholar). Most of the time, escape to drug treatment is related to the STAT3-mediated expression of survival proteins such as bcl-xl or survivin (21Diaz N. Minton S. Cox C. Bowman T. Gritsko T. Garcia R. Eweis I. Wloch M. Livingston S. Seijo E. Cantor A. Lee J.H. Beam C.A. Sullivan D. Jove R. Muro-Cacho C.A. Clin. Cancer Res. 2006; 12: 20-28Crossref PubMed Scopus (203) Google Scholar, 22Gritsko T. Williams A. Turkson J. Kaneko S. Bowman T. Huang M. Nam S. Eweis I. Diaz N. Sullivan D. Yoder S. Enkemann S. Eschrich S. Lee J.H. Beam C.A. Cheng J. Minton S. Muro-Cacho C.A. Jove R. Clin. Cancer Res. 2006; 12: 11-19Crossref PubMed Scopus (448) Google Scholar). In addition, we have recently shown that the epidermal growth factor receptor-src-STAT3 pathway can prevent senescence induction (23Vigneron A. Roninson I.B. Gamelin E. Coqueret O. Cancer Res. 2005; 65: 8927-8935Crossref PubMed Scopus (54) Google Scholar) and activate DNA repair genes (24Vigneron A. Gamelin E. Coqueret O. Cancer Res. 2008; 68: 815-825Crossref PubMed Scopus (46) Google Scholar) to confer resistance to chemotherapy this study, we have characterized the regulation of STAT3 DNA damage. In cell we have found that the transcription factor is phosphorylated on its serine 727 residue following topoisomerase I inhibition and that tyrosine 705 phosphorylation is not In addition, we have also that this phosphorylation is due to the binding of the cdk5 to the transcription cdk5 is a kinase, which was initially characterized in associated with its specific activators this protein plays an important role in neuronal and R. Nat. Rev. Mol. Biol. 2001; PubMed Scopus Google Scholar, X. X. M. X. J. D. J. Z. Full Text Full Text PDF PubMed Scopus Google Scholar, J.H. A. C. 2006; 13: PubMed Scopus Google Scholar). In response to topoisomerase I we have that cdk5 is activated and that it with STAT3 to induce its serine to be involved in the STAT3-mediated regulation of the cyclin D1, and Eme1 Importantly, ChIP that the transcription factor can be found associated with the Eme1 promoter when phosphorylated only on serine We therefore propose that cdk5 the pathway and that this is an important in the response of tumors to topoisomerase I this study, we have found that the STAT3 transcription factor is phosphorylated on its serine C-terminal residue but not on tyrosine 705 upon topoisomerase I inhibition. results that this is due to the activation of the cdk5 kinase, which binds to the C-terminal of domain of the transcription factor to induce its Importantly, cdk5 is involved in the of genes such as and cyclin and in the STAT3-mediated up-regulation of the Eme1 an endonuclease involved in the of In of these we propose that the pathway plays an important role in the response to topoisomerase I inhibition and chemotherapy is well known that STAT3 is activated the following or growth factor stimulation. In this the transcription factor binds to the promoter of several cell genes such as cyclin D1, or to induce expression and activate activation by STAT3 the is due to the phosphorylation of STAT3 on its tyrosine residue, by nuclear translocation and DNA The second phosphorylation of STAT3 on its serine residue allows the of the dimer with transcriptional cofactors such as CBP, NcoA, or However, this pathway is not the only by which proteins are because several results have shown that these transcription factors induce transcription in the absence of tyrosine This was described with when it was shown that this transcription factor can drive the expression of several genes in the absence of tyrosine phosphorylation M. Wright K.L. Stark G.R. J. PubMed Scopus Google Scholar). forms of STAT3 on its tyrosine residue, have shown that these mutants can induce the expression of genes such as and which certainly play an important role in the oncogenic activity of these gene activation is a consequence of the of a that plays a role in cells (12Yang J. Chatterjee-Kishore M. Staugaitis S.M. Nguyen H. Schlessinger K. Levy D.E. Stark G.R. Cancer Res. 2005; 65: 939-947PubMed Google Scholar, 13Yang J. Liao X. Agarwal M.K. Barnes L. Auron P.E. Stark G.R. Genes Dev. 2007; 21: 1396-1408Crossref PubMed Scopus (448) Google Scholar). Most the genes regulated by STAT3 in these are not activated when the transcription factor is phosphorylated on its tyrosine This leads to the important conclusion that the STAT3 transcriptional on its ChIP we have been to STAT3 on the Eme1 promoter when phosphorylated only on its Ser-727 We therefore propose that STAT3 is activated by DNA damage the of the cell and that its serine phosphorylation allows the specific up-regulation of DNA repair genes such as the Eme1 the role of STAT3 in the response to treatment has not been well characterized. it is known that and are regulated following DNA damage. is involved in the and and can associate with repair proteins such as and in response to S.M. J. S. M. Chen A. J. Sci. 2005; PubMed Scopus Google Scholar, M. A. Cancer Res. 2004; PubMed Scopus Google Scholar). In addition, this transcription factor is also phosphorylated in response to topoisomerase M. A. Cancer Res. 2004; PubMed Scopus Google Scholar). has been shown to the expression of this has been to the ability of several oncogenic kinases such as or to induce drug resistance A. C. G. M. G. R. T. Mol. Cell. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar, A. G. I. A. M. J. R. T. Mol. Cell. Biol. 2002; PubMed Scopus Google Scholar). Interestingly, results also that STAT3 plays an important role in the regulation of The of the protein tyrosine a constitutive activation of STAT3 as a consequence of and this leads to with C. P.E. T. Cancer Cell. 2008; Full Text Full Text PDF PubMed Scopus Google Scholar). the STAT3 and DNA repair has not been characterized in this study, because this has been to a expression of cyclin a STAT3 and DNA it is well known that a direct target of STAT3, can induce DNA damage and and DNA repair O. M. S. M. G.M. G.M. Mol. Cell. 2002; Full Text Full Text PDF PubMed Scopus Google Scholar). In this study, we these that this transcription factor is activated by in response to topoisomerase I We that this allows the of a STAT3 that the expression of DNA repair genes upon treatment. In of results an essential role of NF-κB in the response to DNA damage S. Perkins Cancer Res. 2006; PubMed Scopus Google Scholar), hypothesis is that genes involved in the response to are by a specific that be activated by It will be to if this binds DNA repair genes as to STAT3 such as or a consequence of DNA repair genes results that the cdk5-STAT3 pathway DNA damage in response to topoisomerase I inhibition. This that these proteins play an essential role in the resistance of cancer cells to chemotherapy. the importance of this oncogenic results have shown that the cdk5-STAT3 pathway plays an essential role in H. Chen J. Biol. Chem. 2007; Full Text Full Text PDF PubMed Scopus Google Scholar). In addition, we and have recently shown that STAT3 prevents the induction of senescence through G. Wright K.L. Ma Y. Wright G.M. Huang M. Irby R. Briggs J. Karras J. Cress W.D. Pardoll D. Jove R. Chen J. Yu H. Mol. Cell. Biol. 2005; 25: 7432-7440Crossref PubMed Scopus (297) Google Scholar, B. Avril S. Coqueret O. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar, C. N. J. C. Oncogene. 1999; 18: PubMed Scopus Google Scholar, F. B. S. Avril S. Coqueret O. Mol. Biol. Cell. 2005; PubMed Scopus Google Scholar). Interestingly, cdk5 is also involved in senescence programs, because this cell through and K. Mol. Cell. Biol. 2004; PubMed Scopus Google Scholar, Mol. Cell. Full Text Full Text PDF PubMed Scopus Google Scholar). It will be to if cdk5 is also involved in the of the pathway by the STAT3 senescence of this and other results H. Chen J. Biol. Chem. 2007; Full Text Full Text PDF PubMed Scopus Google Scholar), we therefore propose that cdk5 plays an important role in cell transformation by the STAT3 it has been proposed that cell transformation an intrinsic resistance to chemotherapy Cell. 2002; Full Text Full Text PDF PubMed Scopus Google Scholar), we that cdk5-STAT3 cancer cells with intrinsic resistance due to Eme1 expression and that this a of cell We propose that the on tumor of the cdk5-STAT3 oncogenic of its phosphorylation and of its target will with a resistance of tumors that will fail to respond to chemotherapy (15Barré B. Vigneron A. Perkins N. Roninson I.B. Gamelin E. Coqueret O. Trends Mol. Med. 2007; 13: 4-11Abstract Full Text Full Text PDF PubMed Scopus (102) Google Scholar, C. E. G.M. H. Clin. Cancer Res. 2007; 13: PubMed Scopus Google Scholar). In addition, we also propose that STAT3 which are as cancer H. Jove R. Nat. Rev. Cancer. 2004; 4: 97-105Crossref PubMed Scopus (1918) Google Scholar, C. E. G.M. H. Clin. Cancer Res. 2007; 13: PubMed Scopus Google Scholar, M. H. M. J. Clin. 2009; PubMed Scopus Google Scholar) should be in in with to DNA repair and the of IntroductionSignal transducer and activator of transcription 3 (STAT3) 3The abbreviations used are: STAT3signal transducers and activators of transcription 3ChIPchromatin immunoprecipitationCBPCREB-binding proteincdkcyclin-dependent kinasesiRNAsmall interference RNAPBSphosphate-buffered salineRTreverse transcriptionIL-6interleukin-6FACSfluorescent-activated cell sorting. proteins are cytoplasmic transcription factors that translocate into the nucleus following growth factor stimulation. In contrast to normal cells where its phosphorylation is only transient, constitutive activation of STAT3 has been reported in several primary cancers and tumor cell lines (1Bromberg J. J. Clin. Invest. 2002; 109: 1139-1142Crossref PubMed Scopus (740) Google Scholar, 2Yu H. Jove R. Nat. Rev. Cancer. 2004; 4: 97-105Crossref PubMed Scopus (1918) Google Scholar, 3Levy D.E. Lee C.K. J. Clin. Invest. 2002; 109: 1143-1148Crossref PubMed Scopus (740) Google Scholar). This abnormal activation is due to oncogenic kinases such as epidermal growth factor receptor, Her2/Neu, src, or bcr-abl, which induce STAT3 activation through phosphorylation of its tyrosine 705 residue (4Bromberg J.F. Horvath C.M. Besser D. Lathem W.W. Darnell Jr., J.E. Mol. Cell. Biol. 1998; 18: 2553-2558Crossref PubMed Scopus (568) Google Scholar). This phosphorylation allows the nuclear translocation and DNA binding of the STAT3 dimer and the up-regulation of several genes involved in cell-cycle and cell survival such as cyclin D1, Myc, or bclxl. The up-regulation of these cancer genes mediates the oncogenic activity of STAT3 and its ability to transform cells (5Bromberg J.F. Wrzeszczynska M.H. Devgan G. Zhao Y. Pestell R.G. Albanese C. Darnell Jr., J.E. Cell. 1999; 98: 295-303Abstract Full Text Full Text PDF PubMed Scopus (2472) Google Scholar).A second phosphorylation occurs on the serine 727 residue of the C-terminal activation domain. It has been proposed that this phosphorylation is necessary for maximal gene activation, because its mutation prevents STAT3 transcriptional function (6Wen Z. Zhong Z. Darnell Jr., J.E. Cell. 1995; 82: 241-250Abstract Full Text PDF PubMed Scopus (1728) Google Scholar). It is believed that this modification favors the recruitment of transcriptional cofactors such as CBP, NcoA, or P-Tefb that binds to the C-terminal domain of the transcription factor (7Giraud S. Bienvenu F. Avril S. Gascan H. Heery D.M. Coqueret O. J. Biol. Chem. 2002; 277: 8004-8011Abstract Full Text Full Text PDF PubMed Scopus (122) Google Scholar, 8Giraud S. Hurlstone A. Avril S. Coqueret O. Oncogene. 2004; 23: 7391-7398Crossref PubMed Scopus (90) Google Scholar, 9Paulson M. Pisharody S. Pan L. Guadagno S. Mui A.L. Levy D.E. J. Biol. Chem. 1999; 274: 25343-25349Abstract Full Text Full Text PDF PubMed Scopus (185) Google Scholar, 10Nakashima K. Yanagisawa M. Arakawa H. Kimura N. Hisatsune T. Kawabata M. Miyazono K. Taga T. Science. 1999; 284: 479-482Crossref PubMed Scopus (720) Google Scholar). However, it remains to be determined if the association of STAT3 with its coactivators is a direct consequence of Ser-727 phosphorylation.Although it was initially believed that the tyrosine phosphorylation is essential for STAT3 activity, several groups have recently reported that specific forms of the transcription factor, which are only phosphorylated on its Ser-727 residue, can induce gene activation. In prostate cancer, Ser-727 phosphorylation is sufficient to activate STAT3 and drive tumorigenesis in the absence of tyrosine 705 activation (11Qin H.R. Kim H.J. Kim J.Y. Hurt E.M. Klarmann G.J. Kawasaki B.T. Duhagon Serrat M.A. Farrar W.L. Cancer Res. 2008; 68: 7736-7741Crossref PubMed Scopus (106) Google Scholar). Elegant results have shown that tyrosine 705 mutants can associate with NF-κB and induce the expression of genes such as mras or met, which are likely to play an important role in cell transformation by STAT3 (12Yang J. Chatterjee-Kishore M. Staugaitis S.M. Nguyen H. Schlessinger K. Levy D.E. Stark G.R. Cancer Res. 2005; 65: 939-947PubMed Google Scholar, 13Yang J. Liao X. Agarwal M.K. Barnes L. Auron P.E. Stark G.R. Genes Dev. 2007; 21: 1396-1408Crossref PubMed Scopus (448) Google Scholar, 14Lee H. Herrmann A. Deng J.H. Kujawski M. Niu G. Li Z. Forman S. Jove R. Pardoll D.M. Yu H. Cancer Cell. 2009; 15: 283-293Abstract Full Text Full Text PDF PubMed Scopus (513) Google Scholar). These results lead to the important conclusion that the influence of STAT3 on cell transformation can be independent of the tyrosine 705 phosphorylation and that this site should not be considered as a unique marker of STAT3 oncogenic activity.This conclusion also leads to the hypothesis that STAT3 can induce different transcriptional programs, depending on which sites are phosphorylated and certainly on the type of stimulation. Although STAT3 activation is well characterized in response to growth factor stimulation, little is known about its regulation in response to other stimulation such as DNA damage and chemotherapy treatment. Interestingly, several studies have suggested that an abnormal activation of this transcription factor is associated with intrinsic drug resistance (15Barré B. Vigneron A. Perkins N. Roninson I.B. Gamelin E. Coqueret O. Trends Mol. Med. 2007; 13: 4-11Abstract Full Text Full Text PDF PubMed Scopus (102) Google Scholar). STAT3 expression has been associated with resistance to radiation-induced apoptosis (16Sano S. Chan K.S. Kira M. Kataoka K. Takagi S. Tarutani M. Itami S. Kiguchi K. Yokoi M. Sugasawa K. Mori T. Hanaoka F. Takeda J. DiGiovanni J. Cancer Res. 2005; 65: 5720-5729Crossref PubMed Scopus (86) Google Scholar, 17Shen Y. Devgan G. Darnell Jr., J.E. Bromberg J.F. Proc. Natl. Acad. Sci. U.S.A. 2001; 98: 1543-1548Crossref PubMed Scopus (230) Google Scholar, 18Niu G. Wright K.L. Ma Y. Wright G.M. Huang M. Irby R. Briggs J. Karras J. Cress W.D. Pardoll D. Jove R. Chen J. Yu H. Mol. Cell. Biol. 2005; 25: 7432-7440Crossref PubMed Scopus (297) Google Scholar), and it can also confer resistance to Fas or paclitaxel-mediated apoptosis in multiple myeloma and ovarian cancer (19Catlett-Falcone R. Landowski T.H. Oshiro M.M. Turkson J. Levitzki A. Savino R. Ciliberto G. Moscinski L. Fernández-Luna J.L. Nuñez G. Dalton W.S. Jove R. Immunity. 1999; 10: 105-115Abstract Full Text Full Text PDF PubMed Scopus (1440) Google Scholar, 20Duan Z. Foster R. Bell D.A. Mahoney J. Wolak K. Vaidya A. Hampel C. Lee H. Seiden M.V. Clin. Cancer Res. 2006; 12: 5055-5063Crossref PubMed Scopus (211) Google Scholar). Most of the time, escape to drug treatment is related to the STAT3-mediated expression of survival proteins such as bcl-xl or survivin (21Diaz N. Minton S. Cox C. Bowman T. Gritsko T. Garcia R. Eweis I. Wloch M. Livingston S. Seijo E. Cantor A. Lee J.H. Beam C.A. Sullivan D. Jove R. Muro-Cacho C.A. Clin. Cancer Res. 2006; 12: 20-28Crossref PubMed Scopus (203) Google Scholar, 22Gritsko T. Williams A. Turkson J. Kaneko S. Bowman T. Huang M. Nam S. Eweis I. Diaz N. Sullivan D. Yoder S. Enkemann S. Eschrich S. Lee J.H. Beam C.A. Cheng J. Minton S. Muro-Cacho C.A. Jove R. Clin. Cancer Res. 2006; 12: 11-19Crossref PubMed Scopus (448) Google Scholar). In addition, we have recently shown that the epidermal growth factor receptor-src-STAT3 pathway can prevent senescence induction (23Vigneron A. Roninson I.B. Gamelin E. Coqueret O. Cancer Res. 2005; 65: 8927-8935Crossref PubMed Scopus (54) Google Scholar) and activate DNA repair genes (24Vigneron A. Gamelin E. Coqueret O. Cancer Res. 2008; 68: 815-825Crossref PubMed Scopus (46) Google Scholar) to confer resistance to chemotherapy this study, we have characterized the regulation of STAT3 DNA damage. In cell we have found that the transcription factor is phosphorylated on its serine 727 residue following topoisomerase I inhibition and that tyrosine 705 phosphorylation is not In addition, we have also that this phosphorylation is due to the binding of the cdk5 to the transcription cdk5 is a kinase, which was initially characterized in associated with its specific activators this protein plays an important role in neuronal and R. Nat. Rev. Mol. Biol. 2001; PubMed Scopus Google Scholar, X. X. M. X. J. D. J. Z. Full Text Full Text PDF PubMed Scopus Google Scholar, J.H. A. C. 2006; 13: PubMed Scopus Google Scholar). In response to topoisomerase I we have that cdk5 is activated and that it with STAT3 to induce its serine to be involved in the STAT3-mediated regulation of the cyclin D1, and Eme1 Importantly, ChIP that the transcription factor can be found associated with the Eme1 promoter when phosphorylated only on serine We therefore propose that cdk5 the pathway and that this is an important in the response of tumors to topoisomerase I inhibition.