Camptothecin resistance is determined by the regulation of topoisomerase I degradation mediated by ubiquitin proteasome pathway

// Koji Ando 1,* , Ankur K. Shah 1,* , Vibhu Sachdev 1 , Benjamin P. Kleinstiver 2,3 , Julian Taylor-Parker 1 , Moira M. Welch 2 , Yiheng Hu 4 , Ravi Salgia 5 , Forest M. White 6 , Jeffrey D. Parvin 4 , Al Ozonoff 7 , Lucia E. Rameh 8 , J. Keith Joung 2,3 and Ajit K. Bharti 1 1 Department of Medicine, Division of Hematology Oncology, Boston University School of Medicine, Boston, MA, USA 2 Molecular Pathology Unit, Massachusetts General Hospital, Charlestown, MA, USA 3 Department of Pathology, Harvard Medical School, Boston, MA, USA 4 Department of Biomedical Informatics, The Ohio State University Comprehensive Cancer Center, Columbus, OH, USA 5 Department of Medical Oncology and Therapeutics Research, City of Hope, Duarte , CA, USA 6 Department of Biological Engineering, Koch Institute for Integrative Cancer Research, Massachusetts Institute of Technology, Cambridge, MA, USA 7 Center for Patient Safety and Quality Research, Boston Children’s Hospital, Boston, MA, USA 8 Department of Medicine, Obesity Research Center, Boston University School of Medicine, Boston, MA, USA * These authors have contributed equally to this work Correspondence to: Ajit K Bharti, email: // Keywords : topoisomerase I, BRCA1, DNAPK, PTEN, ubiquitin proteasome pathway Received : February 17, 2017 Accepted : March 03, 2017 Published : March 18, 2017 Abstract Proteasomal degradation of topoisomerase I (topoI) is one of the most remarkable cellular phenomena observed in response to camptothecin (CPT). Importantly, the rate of topoI degradation is linked to CPT resistance. Formation of the topoI-DNA-CPT cleavable complex inhibits DNA re-ligation resulting in DNA-double strand break (DSB). The degradation of topoI marks the first step in the ubiquitin proteasome pathway (UPP) dependent DNA damage response (DDR). Here, we show that the Ku70/Ku80 heterodimer binds with topoI, and that the DNA-dependent protein kinase (DNA-PKcs) phosphorylates topoI on serine 10 (topoI-pS10), which is subsequently ubiquitinated by BRCA1. A higher basal level of topoI-pS10 ensures rapid topoI degradation leading to CPT resistance. Importantly, PTEN regulates DNA-PKcs kinase activity in this pathway and PTEN deletion ensures DNA-PKcs dependent higher topoI-pS10, rapid topoI degradation and CPT resistance.