Thyroid hormones and their receptors in the regulation of cell proliferation.

In the present work, we have reviewed data showing that triiodothyronine and its nuclear receptors modify expression of different genes/proteins involved in cell cycle control beginning from growth factors (such as EGF and TGF-beta), to cell surface receptors (EGFR), as well as proteins acting at the cell membrane (Ras), various transcription factors (c-Fos, c-Myc, E2F1), cyclins, Cip/Kip family of cdk2 inhibitors, and p53 inhibitor Mdm2 (Table 1). We have shown how TRs are also able to modify the fate of a cell, thanks to their ability to form complexes with other transcription factors such as p53 - a key regulator of apoptosis and proliferation. Available data show that the function of thyroid hormones and of their receptors on cell proliferation is not homogenous. In fact, it strongly depends on the cell type, its developmental state (progenitor or differentiated), its patho-physiological state (normal or tumor cell), and the so-called 'cellular context'. Therefore, it is not possible to uniformly recommend T3 treatment or T3 depletion to stop or initiate proliferation of all cell types. Instead, a very individual and careful action should be considered.