In absence of thyrotropin (TSH), FRTL-5 rat thyroid cells stop proliferating and lose the functional characteristics of thyroid tissue. FRTL-5 cells regain their differentiated state and their proliferation activity upon addition of TSH. In this study we investigated the synthesis of histone Hl variants and Hl° in FRTL-5 cells exposed to 10-8 M TSH, two days after TSH withdrawal. TSH induced the synthesis of some Hl variants and Hl°. This effect was already evident six hours after TSH addition, thus well before proliferation, DNA or thyroglobulin synthesis was induced. These data indicate that the induction of Hl° and some Hl variants is an early event after TSH stimulation and may thus be related to the functional differentation of FRTL-5 cells.
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