While the multifunctional proteins of the transforming growth factor-β (TGF-β) family have a potent antiproliferative effect on thyroid follicular cell growth, increased expression of TGF-β in proliferating thyroid cells and in thyroid tumours has recently been described, suggesting a secondary counter-regulatory role of these proteins. We have studied further this apparent paradox in vitro using FRTL-5 cells, 5 continuous cell strains from feline multinodular goitres (MNG) and 29 primary cultures prepared from human MNG.
While dose dependent inhibition of FRTL-5 cell growth was confirmed, a variable fraction of these cells was naturally resistant towards TGF-β1, thus explaining the large interassay variability of growth inhibition (36 to 98% within 2 days, n=19). After 40 days of continuous exposure, FRTL-5 cells became fully refractory towards TGF-β1 inhibition, due to the selective growth of naturally resistant subclones, as demonstrated for example by microscopic observation of three-dimensionally growing collagen-embedded cell clusters. The refractoriness could still be demonstrated even after several cell passages. In addition, 2 out of 5 feline thyroid cell strains obtained from feline MNG and 18 out of 29 primary cultures from human MNG showed a high degree of refractoriness towards TGF-β.
We conclude that constitutively TGF-β resistant cells may occur in thyroid glands and that persistent TGF-β refractoriness may secondarily be acquired. Resistant cells may escape regular growth control mechanisms and hence may contribute to the notorious heterogeneity of thyroid growth and to nodular transformation.
Journal of Endocrinology (1996) 149, 485–496
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