Endocrine Abstracts

Inhibins A and B, peptides belonging to TGF-ß superfamily, have an ubiquitous antagonist action on activin signalling. In thyroid gland, TGF-ß1 and activin mainly exert an inhibitory action on growth and function. In human thyroid, beside the presence of activin β subunits and its transductional elements (ActRI/II, SMAD), there is no evidence for α subunit expression, an essential component for inhibin A/B gathering (α-ßA/α-ßB), nor for its transductional cascade. The identification of mRNA expression for inhibin α subunit and inhibins transductional elements in normal and nodular human thyroid tissue was the aim of this study. We therefore analyzed by competitive RT-PCR 10 samples from goitrous thyroid, 3 of whom from hyperfunctioning nodules, and 3 normal samples as control. Beside the expected mRNA expression for ß subunits we detected for the first time thyroidal expression of α subunit mRNA both in normal and nodular samples, although with a differential pattern. Interestingly, ßB subunit was constantly overexpressed in goitrous tissue, while α subunit levels were increased in hyperfunctioning nodules. We next evaluated the presence of mRNA for ancillary proteins (coreceptor ß-glycan [ßgly], Inhibin Binding Protein [IBP] and Follistatin [FS]) that agonize inhibin binding to ActRII, interfering with activin signalling. ßgly and FS were regularly expressed in all samples, suggesting that human thyroid could be a high affinity target for inhibin. On the contrary, IBP was only expressed in nodular tissue. Since activin signal may be overridden by a specific interaction between inhibin B and IBP, their simultaneous presence suggest a role for this complex in goitrogenesis. These preliminary results show that human thyroid may be a source and a target for inhibin. Also, we suggest that inhibin B/IBP complex may play an autocrine role in the control of thyroid growth.