Induction of apoptosis in porcine thyroid follicles by transforming growth factor beta1 and epidermal growth factor

For thyroid cells in culture DNA fragmentation and morphological changes related to apoptosis were first described in dog thyroid cells after deprivation of serum, epidermal growth factor or thyrotropin. With intact porcine thyroid follicles in three-dimensional culture, the effect of deprivation of growth factors and of incubation with transforming growth factor beta1 (TGF-beta1), epidermal growth factor (EGF), thyrotropin (TSH) or insulin-like growth factor I (IGF-I) on the incidence of apoptosis was studied. Thyroid follicles were embedded in growth factor-depleted Matrigel and cultured in serum-free medium with or without growth factors for 7 days followed by incubation for 4, 24 and 72 h with TGF-beta1 (2 or 5 ng/mL). The percentage of apoptotic cells was determined by direct counting in electron-microscopy. Approximately 1% of apoptotic bodies could be detected in unstimulated follicles. This was unchanged in the presence of TSH (1 mU/mL) or IGF (10 ng/mL) but significantly increased up to 3.99 +/- 1.24% with 2 ng/mL of EGF. After incubation with TGF-beta apoptosis increased dose-dependently to 4.05 +/- 0.67% with 2 ng/mL TGF-beta1 and 5.16 +/- 1.75% with 5 ng/mL TGF-beta1. The incidence of necrotic cells remained constant at about 1 to 2%. Preincubation of follicles with 2 ng/mL of EGF followed by incubation with 5 ng/mL TGF-beta1 increased the rate of apoptic bodies up to 13.19 +/- 1.9%. We conclude that growth factor depletion in thyroid follicles in three-dimensional culture does not lead to apoptosis. TGF-beta1, however, induces apoptosis even in quiescent thyroid follicular cells and is significantly more pronounced in growing thyroid cells. EGF, which is a dedifferentiating growth factor for thyroid cells, also induces apoptosis. As EGF enhances TGF-beta1 mRNA and protein in thyroid follicular cells, the induction of apoptosis by EGF might also be due to TGF-beta1.