ROS PRODUCTION INDUCED BY 3,5-DIIODO-L-THYRONINE: A NEW EXTRANUCLEAR EFFECT FOR THYROID HORMONES?

Extranuclear effects of thyroid hormones are now widely acknowledged. In the last few years a new thyroid hormone has come to the stage, the 3,5-diiodo-L-thyronine (3,5-T2), a deiodination metabolite of L-T3 that is able to mimic some important rapid nongenomical effects of L-T3. High levels of 3,5-T2 have been reported in the case of brain tumors and other non thyroidal illnesses, such as liver cirrhosis, and it appears likely that 3,5-T2 has a particular physiological role. Previous results from our laboratory demonstrated that 3,5-T2 in the physiological range (0.1 nM) mimics L-T3 activation of the Na/H exchanger, increasing intracellular pH in L-6 myoblasts (Incerpi et al., Endocrinology 140:693-689, 1999) and chick embryo hepatocytes (Incerpi et al., Endocrinology, 143:1660-1668, 2002). We here show that 3,5-T2 increase the fluorescence of diclorofluorescein in L-6 myoblasts, probably due to the production of reactive oxygen species (ROS). The increase is partially prevented by superoxide dismutase (SOD) or by treatment of cells with an inhibitor of the plasma membrane NADPH oxidase, that is believed to be an important source of superoxide formation both inside and outside the cells. Experiments with Electron Paramagnetic Resonance spectroscopy, using Cu,Zn SOD as a probe, show that 3,5-T2 over a wide concentration range induces superoxide formation, whereas other analogs of thyroid hormone do not show the same effect. The results also indicate that superoxide is produced on the outside of the cell membrane, probably due to activation of the plasma membrane NADPH oxidase. Experiments are in progress in order to understand whether ROS production represent a physiological signal, possibly a response to a pathological condition.