δ-Iodolactones decrease epidermal growth factor-induced proliferation and inositol-1,4,5-trisphosphate generation in porcine thyroid follicles—a possible mechanism of growth inhibition by iodide

Abstract

Dugrillon A, Gärtner R. δIodolactones decrease epidermal growth factor-induced proliferation and inositol-1,4,5-trisphosphate generation in porcine thyroid follicles–a possible mechanism of growth inhibition by iodide. Eur J Endocrinol 1995;132:735–43. ISSN 0804–4643 δ-Iodolactones (6-iodo-8,11,14-eicosatrienoic δ-lactone, δ-IL), an iodinated derivative of arachidonic acid, has been shown to be synthesized in thyroid tissue and to inhibit thyroid cell proliferation. It is discussed as a potential mediator of the autoregulatory pathway of iodide in cyclic adenosine-3′,5′-monophosphate (cAMP)- and thyrotropin (TSH)-independent growth. We therefore further localized the action of iodide and of δ-IL in isolated porcine thyroid follicles. Epidermal growth factor (EGF) and 12-O-tetradecanoylphorbol-13-acetate (TPA) dose dependently stimulated thyroid cell proliferation which could be inhibited by staurosporin (0.1–10 nmol/l), Iodide (2.5–40 μmol/l) as well as δ-IL (0.5–2 μmol/l) also dose dependently inhibited EGF- and TPA-induced proliferation. As the calcium ionophor A23187 (100 pmol/l) completely abolished the inhibitory effects of iodide and of δ-IL, this may indicate a mechanism of δ-IL at or proximal to the calcium-dependent activation of protein kinase C. The growth inhibitory effect was restricted to δ-iodolactones when δ-Il was compared to 6-iodo-8,11,14,17-eicosatetraenoic δ-lactone and 5-iodo-7,10,13,16,19-docosapentaenoic γ-lactone, It could not be prevented with propylthiouracil and therefore deiodination and a different iodide action is unlikely. Inositol-1,4,5-trisphosphate (IP3) and cAMP were measured in extracts from isolated porcine thyroid follicles stimulated with EGF 10 ng/ml) or TSH 1.0U/l) revealing comparable kinetics in IP3 generation, while cAMP formation was only stimulated by TSH, δ-Iodolactone (2 μmol/l) only decreased EGF-induced IP3 formation, whereas TSH-induced IP3 and cAMP formation was unchanged. The γ-iodolactone which did not inhibit thyroid cell proliferation, also had no effect on IP 3 generation. These results demonstrate an action of iodide and δ-IL at the calcium-dependent signal transduction modulating thyroid cell proliferation by EGF but not TSH. δ-Iodolactone acts at or proximal to the generation of IP3 induced by EGF, whereas the TSH-dependent signal transduction seems to be unaltered. δ-Iodolactone may therefore be speculated as a specific inhibitory mediator of iodide on growth factor-induced thyroid cell proliferation. Alex Dugrillon, Zentrallabor der Medizinischen Klinik und Poliklinik, University of Heidelberg, Bergheimer Str. 58, D-69115 Heidelberg. Germany