title: Dual regulatory roles of human AP-endonuclease (APE1/Ref-1) in CDKN1A/p21 expression.
abstract: The human AP-endonuclease (APE1/Ref-1), an essential multifunctional protein involved in repair of oxidative DNA damage as well as in transcriptional regulation, is often overexpressed in tumor cells. APE1 was earlier shown to stimulate p53's DNA binding and its transactivation function in the expression of cyclin-dependent kinase inhibitor p21 (CDKN1A) gene. Here, we show APE1's stable binding to p53 cis elements which are required for p53-mediated activation of p21 in p53-expressing wild type HCT116 cells. However, surprisingly, we observed APE1-dependent repression of p21 in isogenic p53-null HCT116 cells. Ectopic expression of p53 in the p53-null cells abrogated this repression suggesting that APE1's negative regulatory role in p21 expression is dependent on the p53 status. We then identified APE1's another binding site in p21's proximal promoter region containing cis elements for AP4, a repressor of p21. Interestingly, APE1 and AP4 showed mutual dependence for p21 repression. Moreover, ectopic p53 in p53-null cells inhibited AP4's association with APE1, their binding to the promoter and p21 repression. These results together establish APE1's role as a co-activator or co-repressor of p21 gene, dependent on p53 status. It is thus likely that APE1 overexpression and inactivation of p53, often observed in tumor cells, promote tumor cell proliferation by constitutively downregulating p21 expression.
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Texas Medical Branch, Galveston, Texas, United States of America.
authors.forename: Shiladitya
authors.initials: S
authors.lastname: Sengupta
authors.forename: Sankar
authors.initials: S
authors.lastname: Mitra
authors.forename: Kishor K
authors.initials: KK
authors.lastname: Bhakat
mesh_terms: D001665:Binding Sites / Q000235:genetics; D050759:Cyclin-Dependent Kinase Inhibitor p21 / Q000235:genetics / Q000378:metabolism*; D043603:DNA-(Apurinic or Apyrimidinic Site) Lyase / Q000235:genetics / Q000378:metabolism*; D045325:HCT116 Cells; D006801:Humans; D011401:Promoter Regions, Genetic / Q000235:genetics; D011485:Protein Binding / Q000235:genetics; D016159:Tumor Suppressor Protein p53 / Q000235:genetics / Q000378:metabolism
chemical_list: D050759:Cyclin-Dependent Kinase Inhibitor p21; D016159:Tumor Suppressor Protein p53; C085108:APEX1 protein, human; D043603:DNA-(Apurinic or Apyrimidinic Site) Lyase