Nongenomic effect of thyroid hormone on free-radical production in human polymorphonuclear leukocytes

Over the past few years increasing evidence has suggested the nongenomic effects of thyroid hormone, such as the activation of the signal transduction pathways and the activation of nuclear factor-kappa B by the induction of oxidative stress. The present study was undertaken to investigate the effect of thyroid hormone on human polymorphonuclear leukocytes (PMNLs) which are known as important sources of reactive oxygen species in the circulation. The production of superoxide anion (O-2(-)) and the activity of myeloperoxidase were determined in the presence and absence of several inhibitors of the signalling pathway. L-Thyroxine (T-4), L-3,5,3'-tri-iodothyronine (T-3) and L-3,5-di-iodothyronine (T-2) stimulated O-2(-) production in PMNLs in a dose-dependent manner within a few minutes of addition to cells. Thyroid horinone-stimulated O-2(-) production was partially inhibited by pertussis toxin, an inhibitor of GTP-binding G protein, and was completely abolished by the protein kinase C inhibitors calphostin C and Ro-32-0432, and by a calcium chelator (BAPTA; bis-(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid). Thyroid hormone stimulated myeloperoxidase activity and induced I-125- incorporation into PMNLs. Furthermore, thyroid hormone pre-incubation enhanced O-2(-) production for n-formyl-methionyl-leucyl-phenylalanine (FMLP) stimulation. In conclusion, novel nongenomic actions of thyroid hormone, the induction of superoxide anion production and the stimulation of myeloperoxidase activity in PMNLs were demonstrated. The induction of O-2(-) production requires calcium and is mediated by a pertussis toxin-sensitive G protein via stimulation of protein kinase C(s). These results suggest the existence of a membrane-bound binding site for thyroid hormone in PMNLs and a physiological role for thyroid hormone in the cellular defence mechanisms by stimulating free-radical production.