PubMed2000
In vitro studies have demonstrated that cysteine groups present in most of the steroid receptors play an essential role in the steroid binding process as well as in the ability of this superfamily of signaling proteins to function as transcription factors. However, there is poor experimental evidence, if any, which demonstrates that under conditions of oxidative stress the steroid receptors in general, and the mineralocorticoid receptor in particular, are affected in vivo in a similar fashion as has been described for cell-free systems or cells in culture. In the present work we report that when mice are exposed to oxidative stress by treatment with L-buthionine-(S,R)-sulfoximine (L-(S,R)-BSO), a glutathione depleting agent, the aldosterone-dependent mineralocorticoid biological response (measured as sodium retention and potassium elimination) was diminished in a directly proportional manner with respect to the depletion of renal glutathione. Accordingly, the steroid binding capacity of the mineralocorticoid receptor was also abrogated, whereas the receptor protein level remained unchanged. The harmful effects observed in mice after glutathione depletion were efficiently prevented by co-treatment with glutathione monoethyl ester. Similar inhibition in the steroid binding capacity was also generated in vitro by receptor alkylation and receptor oxidation, an effect which was prevented in the presence of reducing agents. Since the glutathione deficit generated in vivo by treatment with L-(S,R)-BSO did not significantly affect other renal proteins which are known to be required for the mineralocorticoid mechanism of action, we suggest that in renal cells a low redox potential exerts drastic and uncompensated inhibition of the receptor-mediated mineralocorticoid biological response. This effect was ascribed to the loss of steroid binding capacity of oxidized receptor, most likely by modification of essential cysteines as supported by experiments where a decreased number of reactive thiols and reduced covalent binding of thiol-reactive ligand were evidenced on immunopurified receptor after in vivo treatment with L-(S,R)-BSO.