Hydrogen peroxide generation and its regulation in FRTL-5 and porcine thyroid cells

Hydrogen peroxide acts as electron acceptor in the oxidative reactions (iodination and coupling) by which the thyroid hormones are formed. Regulation of the generation of H2O2 was studied in monolayer cultures of the FRTL-5 rat thyroid cell line and in primary monolayer cultures of porcine thyroid cells. Both cell types were grown in a medium containing either a six-hormone mixture, including TSH (6H), or a five-hormone mixture without TSH (5H) for 1 to several days before the experiment. The production of H2O2 was measured with the homovanillic acid fluorescence assay and expressed as picomoles of H2O2 formed per min/microgram DNA. In FRTL-5 cells grown in 6H medium, only a weak and varying stimulation of H2O2 production was induced by TSH at high concentration (greater than 10 mU/ml), and no stimulation was seen by TSH at low concentration or by 8-bromo-cAMP, whereas forskolin had a good stimulatory effect. In FRTL-5 cells grown in 5H medium for 1-3 days, all three substances were potent stimulators. In porcine thyrocytes examined in the same way, none of the three presumptive stimulators had any effect in 6H cultures, and only TSH (at high concentration) had a weak effect in 5H medium. ATP, a stimulator of the Ca2+/phosphatidylinositol cascade via a P2-purinergic receptor, had no effect on H2O2 generation in FRTL-5 cells in 6H medium. Phorbol myristate acetate (PMA), a direct activator of protein kinase-C, induced a weak stimulation in these cells. In FRTL-5 cells in 5H medium, both ATP and PMA evoked a strong, and similar, enhancement of H2O2 production. In porcine cells in 6H medium, ATP evoked a moderate and PMA a strong stimulation; the effects in 5H were similar to the corresponding effects in 6H medium. The observations are interpreted to show that in FRTL-5 cells the regulation of H2O2 generation uses both the cAMP cascade and the Ca2+/phosphatidylinositol cascade, whereas in porcine thyrocytes the cAMP route is unimportant. In FRTL-5 cells the Ca2+/phosphatidylinositol cascade may be influenced by the cAMP system.