Mitochondrial localization of APE/Ref-1 in thyroid cells

Mutations of mitochondrial DNA (mtDNA) are associated with different human diseases, including cancer and aging. Reactive oxygen species produced duri ng oxidative phosphorylation are a major source of mtDNA damage. It is not clear, however, whether DNA repair mechanisms, able to abolish effects due to oxidative damage, are present in mitochondria. APE/Ref-1 is a nuclear pr otein possessing both redox activity (by which activates, "in vitro", the D NA-binding functions of several transcription factors) and DNA repair activ ity over apurinic/apyrimidinic sites. Immunohistochemical evidences indicat e that in follicular thyroid cells, APE/Ref-1 is located in both nucleus an d cytoplasm. Electronmicroscopy immunocytochemistry performed in the rat th yroid FRTL-5 cell line, indicates that part of the cytoplasmatic APE/Ref-1 is located in mitochondria. The presence of APE/Ref-1 inside mitochondria i s further demonstrated by western blot analysis after cell fractionation. I n the Kimol cell line (which is derived from FRTL-5, transformed by the Ki- ras oncogene) the amount of mitochondrial APE/Ref-1 is reduced by three to fourfold with respect to the normal FRTL-5 cells. These results suggest tha t: (i) a machinery capable of repairing DNA damaged by oxidative stress is present in mitochondria and (ii) mtDNA repair mechanisms may be impaired du ring cell transformation. (C) 2001 Elsevier Science B.V. All rights reserve d.