Ref-1 (called also APE) is a bifunctional protein playing a role in a large variety of cell functions. It is a major member of the DNA base excision repair system. Moreover, through reduction of cysteine residues, Ref-1 controls the activity of several transcription factors. It has been previously demonstrated that TSH up-regulates Ref-1 gene expression in thyroid cells. By using the rat FRTL-5 cell line, we demonstrate that TSH controls Ref-1 intracellular localization. Western blot experiments indicate that addition of TSH to the culture medium increases the Ref-1 cytoplasm-to-nucleus translocation. This phenomenon occurs at early times of TSH stimulation and is not dependent on protein neosynthesis. The Ref-1 cellular compartmentalization was also investigated in human thyroid tumors. A Ref-1 nuclear/cytoplasmic ratio difference between normal and cancerous thyroid tissues was observed. These results suggest that Ref-1 localization may have a critical role in the control of thyroid cell functions.

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