Tonic modulation of dog thyrocyte H2O2 generation and I- uptake by thyrotropin through the cyclic adenosine 3',5'-monophosphate cascade.

The dog thyrocyte I- trapping activity and the expression of the genes coding for dog thyrocyte thyroglobulin or thyroid peroxidase are enhanced by TSH through the cAMP cascade and reduced by mitogens such as epidermal growth factor (EGF) or 12-O-tetradecanoylphorbol 13-acetate (TPA). In this work, we investigated whether H2O2 generation (a limiting step of thyroid hormone synthesis) is modulated by chronic treatment of the thyrocyte with TSH or mitogens such as EGF or TPA. We observed that both basal and carbachol- or ionomycin-stimulated H2O2 generation by the dog thyrocyte were concentration and time dependently enhanced by prolonged (12- to 72-h) exposure to TSH. This effect was reproduced by agents that increase the dog thyrocyte cAMP level or that mimic this increase. It was abolished when protein or RNA synthesis was inhibited. By contrast, EGF and TPA concentration and time dependently antagonized the effect of TSH. In addition, chronic exposure to EGF reduced both basal and carbachol- or ionomycin-stimulated H2O2 generation. The effect of TPA was reproduced by another protein kinase-C activating phorbol ester, phorbol dibutyrate, but not by beta-phorbol, an inactive phorbol ester. Modulation of dog thyrocyte H2O2 generation by chronic exposure to TSH or to the mitogens EGF and TPA was totally parallel to the modulation of their 125I- uptake. Taken together our results suggest that H2O2 generation (or at least one of its constituents) is a differentiation characteristic of the dog thyrocyte under tonic control of TSH through the cAMP cascade as iodide transport, thyroid peroxidase, and thyroglobulin.