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Divisions of Endocrinology and Metabolism, First Department of Medicine, University of Debrecen, Hungary
¹ Department of Microbiology, Medical and Health Science Centre, University of Debrecen, Hungary

(Requests for offprints should be addressed to Z Varga; Email: vargazs{at}ibel.dote.hu)

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-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 (O₂^–) and the activity of myeloperoxidase were determined in the presence and absence of several inhibitors of the signalling pathway. L-Thyroxine (T₄) L-3,5,3'-tri-iodothyronine (T₃) and L-3,5-di-iodothyronine (T₂) stimulated O₂^– production in PMNLs in a dose-dependent manner within a few minutes of addition to cells. Thyroid hormone-stimulated O₂^– 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'-tetra-acetic acid). Thyroid hormone stimulated myeloperoxidase activity and induced ¹²⁵I^– incorporation into PMNLs. Furthermore, thyroid hormone pre-incubation enhanced O₂^– 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 O₂^– 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.

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