Bcl2 Suppresses DNA Repair by Enhancing c-Myc Transcriptional Activity

Bcl2 and c-Myc are two major oncogenic proteins that can functionally promote DNA damage, genetic instability, and tumorigenesis. However, the mechanism(s) remains unclear. Nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is the most potent carcinogen contained in cigarette smoke that induces cellular DNA damage. Here we found that Bcl2 potently suppresses the repair of NNK-induced abasic sites of DNA lesions in association with increased c-Myc transcriptional activity. The Bcl2 BH4 domain (amino acids 6-31) was found to bind directly to c-Myc MBII domain (amino acids 106-143), and this interaction is required for Bcl2 to enhance c-Myc transcriptional activity and inhibit DNA repair. In addition to mitochondria, Bcl2 is also expressed in the nucleus, where it co-localizes with c-Myc. Expression of nuclear-targeted Bcl2 enhances c-Myc transcriptional activity with suppression of DNA repair but fails to prolong cell survival. Depletion of c-Myc expression from cells overexpressing Bcl2 significantly accelerates the repair of NNK-induced DNA damage, indicating that c-Myc may be essential for the Bcl2 effect on DNA repair. It is known that apurinic/apyrimidinic endonuclease (APE1) plays a crucial role in the repair of abasic sites of DNA lesions. That overexpression of Bcl2 results in up-regulation of c-Myc and down-regulation of APE1 suggests APE1 may function as the downstream target of Bcl2/c-Myc in the DNA repair machinery. Thus, Bcl2, in addition to its survival function, may also suppress DNA repair in a novel mechanism involving c-Myc and APE1, which may lead to an accumulation of DNA damage in living cells, genetic instability, and tumorigenesis. Bcl2 and c-Myc are two major oncogenic proteins that can functionally promote DNA damage, genetic instability, and tumorigenesis. However, the mechanism(s) remains unclear. Nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is the most potent carcinogen contained in cigarette smoke that induces cellular DNA damage. Here we found that Bcl2 potently suppresses the repair of NNK-induced abasic sites of DNA lesions in association with increased c-Myc transcriptional activity. The Bcl2 BH4 domain (amino acids 6-31) was found to bind directly to c-Myc MBII domain (amino acids 106-143), and this interaction is required for Bcl2 to enhance c-Myc transcriptional activity and inhibit DNA repair. In addition to mitochondria, Bcl2 is also expressed in the nucleus, where it co-localizes with c-Myc. Expression of nuclear-targeted Bcl2 enhances c-Myc transcriptional activity with suppression of DNA repair but fails to prolong cell survival. Depletion of c-Myc expression from cells overexpressing Bcl2 significantly accelerates the repair of NNK-induced DNA damage, indicating that c-Myc may be essential for the Bcl2 effect on DNA repair. It is known that apurinic/apyrimidinic endonuclease (APE1) plays a crucial role in the repair of abasic sites of DNA lesions. That overexpression of Bcl2 results in up-regulation of c-Myc and down-regulation of APE1 suggests APE1 may function as the downstream target of Bcl2/c-Myc in the DNA repair machinery. Thus, Bcl2, in addition to its survival function, may also suppress DNA repair in a novel mechanism involving c-Myc and APE1, which may lead to an accumulation of DNA damage in living cells, genetic instability, and tumorigenesis. Bcl2 was discovered by cloning the t(14,18)(q32;q21) chromosomal breakpoint characteristic of human follicular B-cell lymphoma (1Bakhshi A. Jensen J.P. Goldman P. Wright J.J. McBride O.W. Epstein A.L. Korsmeyer S.J. Cell. 1985; 41: 899-906Abstract Full Text PDF PubMed Scopus (1008) Google Scholar, 2Cleary M.L. Sklar J. Proc. Natl. Acad. Sci. U. S. A. 1985; 82: 7439-7443Crossref PubMed Scopus (832) Google Scholar, 3Tsujimoto Y. Gorham J. Cossman J. Jaffe E. Croce C.M. Science. 1985; 229: 1390-1393Crossref PubMed Scopus (835) Google Scholar). Bcl2 has the unique role of extending cell survival rather than promoting cell proliferation (4Cheng N. Janumyan Y.M. Didion L. Hofwegen C.V. Yang E. Knudson C.M. Oncogene. 2004; 23: 3770-3780Crossref PubMed Scopus (41) Google Scholar). B- and T-cell lineage Bcl2 transgenic mice models demonstrate an oncogenic phenotype (5Linette G.P. Hess J.L. Sentman C.L. Korsmeyer S.J. Blood. 1995; 86: 1255-1260Crossref PubMed Google Scholar, 6McDonnell T.J. Korsmeyer S.J. Nature. 1991; 349: 254-256Crossref PubMed Scopus (764) Google Scholar, 7Strasser A. Harris A.W. Cory S. Oncogene. 1993; 8: 1-9PubMed Google Scholar). For example, Eμ-Bcl2 transgenic mice show an increased incidence of B-cell lymphomas (7Strasser A. Harris A.W. Cory S. Oncogene. 1993; 8: 1-9PubMed Google Scholar), whereas lck promoter-driven Bcl2 in T-cells can develop peripheral T-cell lymphomas (5Linette G.P. Hess J.L. Sentman C.L. Korsmeyer S.J. Blood. 1995; 86: 1255-1260Crossref PubMed Google Scholar). Thus, Bcl2 not only functions as a potent antiapoptotic molecule but also as a cellular proto-oncogene that can promote tumor development. Several reports indicate that Bcl2 can enhance DNA damage and attenuate DNA repair in association with genetic instability (8Chebonnel-Lasserre C. Gauny S. Kronenberg A. Oncogene. 1996; 13: 1489-1497PubMed Google Scholar, 9Kuo M. Shiah S. Wang C. Chuang S. Mol. Pharmacol. 1999; 55: 894-901PubMed Google Scholar, 10Liu Y. Naumovski L. Hanawalt P. Cancer Res. 1997; 57: 1650-1653PubMed Google Scholar, 11Saintigny Y. Dumay A. Lambert S. Lopez B.S. EMBO J. 2001; 20: 2596-2607Crossref PubMed Scopus (81) Google Scholar). However, the mechanism(s) by which Bcl2 regulates DNA damage and repair remains enigmatic. c-Myc is a protooncogene that normally regulates cellular growth and proliferation and in some contexts induces apoptosis (12Evan G.I. Wyllie A.H. M. C.M. Cell. Full Text PDF PubMed Scopus Google Scholar, J. J. Mol. Cell. 2001; PubMed Scopus Google Scholar). of c-Myc is to by promoting cellular proliferation and c-Myc also to by Proc. Na