Apoptosis might be involved in the reduction of the thyroid cell population in physiopathological situations such as goitre involution and autoimmune deleterious processes. Up to now, little attention has been paid to the apoptotic phenomenon in the normal thyroid gland the specialized metabolism of which is expected to generate reactive oxygen species. Indeed, thyroid cells have the capacity to synthesize H2O2. In this study, we have analyzed the capacity of H2O2 to trigger apoptosis of pig thyrocytes in culture to try to determine whether thyrocytes exhibit a particular resistance to apoptosis induced by an oxidative stress. We show that exposure of thyrocytes cultured as monolayers to exogenous H2O2 induced cell death with characteristics of apoptosis. The effect of H2O2 was concentration-dependent; apoptotic cells were already observed after exposure to 50 micro M H2O2. At high concentrations (millimolar range), H2O2 exerted toxic effects leading to rapid cell disruption. Within the first hour after the onset of exposure to 50-300 micro M H2O2, early signs of apoptosis, i.e. DNA fragmentation, appeared in a low (0.1-1%) but definite fraction of thyrocytes. The proportion of adherent cells exhibiting DNA fragmentation remained fairly constant after 6, 15 and 24 h. During the 24-h period, an increasing number of cells detached from the culture dish and up to 30-40% of cells in suspension displayed apoptotic features. The fraction of cells that lost contact with the culture dish amounted to up to 25% 24 h after addition of 300 micro M H2O2. In conclusion, as reported for other cell types, low H2O2 concentrations are capable of triggering apoptosis in thyrocytes cultured as monolayers. Thyrocytes that undergo apoptosis secondarily lose contact with neighbour cells and the substratum; cell detachment from the monolayer probably happens within 1-2 h after initiation of DNA fragmentation. Our data show that the apoptotic commitment can take place many hours after initiation of the oxidative stress.
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