Cross-regulation between EGFR and BER pathways in human thyrocytes: role of oxidative stress

Introdution. The main clinical concern about thyroid nodules (TNs) is their possible hidden malignancy. To date mechanisms involved in TNs malignant transition are unresolved. One of the most interesting aspects of thyroid tissue is that although thyroid cells produce physiologically large amounts of H2O2, necessary for hormone synthesis, but hyperproliferation and/or incorrect use of the same, may cause an increase in DNA lesions, that are removed by the Base Excition Repair (BER) system. A link between Epidermal Growth Factor Receptor (EGFR) signaling and DNA repair mechanisms has been documented, however, their cross-regulation is poorly understood in thyroid tissue physiopathology. Since, EGF is a physiological regulator of thyroid functions, this study investigated the effect of oxidative stress on EGFR and BER signalling in a thyroid cell model. Methods. Normal human thyroid cells, Nthy-ori3-1, were treated with H2O2, EGF, LY294002 (LY) and PD98059 (PD) alone and in combination. Goiter tissues were obtained from patients undergoing thyroidectomy. EGFR and BER pathways were analyzed by Western Blot and qRT-PCR in 15 genes involved in BER and/or EGFR signalling. Results. A time-dependent increase of OGG1 and MUTYH gene expression is observed after H2O2 treatment, and reversed by EGF and LY alone or combined with H2O2. LY and PD induced EGFR gene overexpression, alone and combined with H2O2. In addition, OGG1 protein is expressed in quiescent cells, while it is reduced or absent after H2O2 treatments alone or combined with LY and PD. By contrast, in these treatments the EGFR and MAPK proteins are active. To elucidate molecular relationship we perform gene cluster analysis; in summary after H2O2 treatment we observed an increase of expression of BER and antioxidant signalling in contrast to EGF pathway that shows an over expression after treatment with PD alone and combined with H2O2; confirmed by cluster analysis suggesting a feedback control mechanism. Conclusion. Gene expression cluster analysis suggests that ERB and BER cross-talk may be mediated by cJUN/AP1 transcription factor. Our observations present the first evidence that a pronounced stress with H2O2 may induce a cross-regulation between EGF and BER pathways in thyroid epithelial cells.