APE/Ref-1 is increased in nuclear fractions of human thyroid hyperfunctioning nodules.

ABSTRACT

Apurinic/apyrimidinic endonuclease APE/Ref-1 is a multifunctional protein provided with DNA repair, transcription-factor regulation and anti-apoptotic activities. We have previously reported that, in thyroid cells, TSH regulates both the synthesis and nuclear translocation of APE/Ref-1. We have also shown that nuclear levels of this protein are reduced both in thyroid carcinoma tissues and cell lines. In the present study, APE/Ref-1 expression and cellular localization were analysed by Western blot in hyperfunctioning thyroid nodules from patients with toxic adenoma and/or toxic multinodular goiter. The total content of APE/Ref-1 protein was increased in the majority of the hyperfunctioning tissues with respect to normal adjacent tissue. There was also an increase in the nuclear levels of APE/Ref-1, suggesting enhanced cytoplasm-to-nucleus translocation of the protein in addition to its increased rate of synthesis. These results demonstrate that the phenomenon of nuclear translocation of APE/Ref-1 hypothesized on the basis of cell culture experiments does actually occur in vivo. Together with previous observations in thyroid carcinomas and tumoral cell lines, our findings suggest a two-stage model of APE/Ref-1 behaviour during malignant thyrocyte transformation an early stage characterized by simple hyperplasia and upregulation of APE/Ref-1 in the nuclear compartment of the cell and a later stage in which nuclear levels of the protein drop to below-normal levels as the cell becomes progressively undifferentiated.