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Naoya Emoto, Toshio Tsushima, Kazuo Shizume, Toshiaki Tanaka, Motoyasu Saji, Yoshito Ohba, Kae Wakai, Mariko Arai and Eiji Ohmura

Abstract. Nb2 cell is a rat lymphoma cell line that responds to lactogens such as prolactin and human growth hormone (hGH) with an increased rate of proliferation. We explored the relationship between the biochemical events induced by hGH and its derivatives and their receptor binding activities. hGH stimulated RNA, DNA and protein synthesis of Nb2 cells as a function of time. Stimulation of RNA and protein was maximal at 2–3 h and 12 h, respectively, after the addition of hGH. DNA synthesis, measured by the rate of [3H]thymidine incorporation, reached a maximum after 18-h incubation with hGH. Stimulation of DNA synthesis was elicited by hGH in a dose-dependent manner between 0.45 and 45 pmol/l. The activity of the 20 K hGH variant in stimulating DNA synthesis was approx 30% of that of hGH. In contrast, S1-hGH, which lacks a sequence of ten amino acids (140–149) of hGH, showed a 3.2-fold greater activity than hGH. F1 (aminoterminal sequence 1–134 of hGH) was only 0.06% as active as hGH, and the activity of F2 (C-terminal 42 amino acid residue of hGH) was less than 0.01%. Both fragment 1–15 and 32–46 were without effect. The relative potencies of these hGH derivatives in stimulating DNA synthesis were similar to their relative abilities to inhibit [125I]hGH binding to lactogenic receptors on Nb2 cell. Nb2 cells provide a suitable model to study the relationship between receptor binding and the biochemical events induced by lactogens.

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Mariko Arai, Toshio Tsushima, Osamu Isozaki, Hiroshi Demura, Kazuo Shizume, Naoya Emoto, Megumi Miyakawa, Yasuko Nozoe, Hitomi Murakami and Eiji Ohmura

Arai M, Tsushima T, Isozaki 0, Demura H, Shizume K, Emoto N, Miyakawa M, Nozoe Y, Murakami H, Ohmura E. Effects of transforming growth factor α (TGF-α) on DNA synthesis and thyrotropin-induced iodine metabolism in cultured porcine thyroid cells. Eur J Endocrinol 1995;132:242–8. ISSN 0804–4643

Transforming growth factor α (TGF-α) is a potent mitogen that is similar structurally to epidermal growth factor (EGF). As EGF is a potent growth stimulator and an inhibitor of iodine metabolism in cultured thyroid cells of several species, we studied whether TGF-α has similar effects using porcine thyroid cells in culture. Recombinant human TGF-α dose-dependently stimulated DNA synthesis of thyroid cells, with maximal stimulation (eight- to ninefold above basal) occurring at 2 nmol/l. The potency was approximately 50% that of mouse EGF and correlated with the ability to compete with EGF for receptor binding, suggesting that the action of TGF-α is mediated by interaction with EGF receptors. When thyroid cells were cultured for 3 days with thyrotropin (TSH) in the presence of TGF-α, TSH-induced iodide uptake was inhibited in a dose-dependent manner. The potency of TGF-α again was approximately 50% that of EGF. Transforming growth factor α did not inhibit TSH-stimulated cAMP production. Moreover, iodide uptake stimulated by either forskolin or 8-bromo-cAMP also was inhibited by TGF-α. Thus, we conclude that TGF-α inhibits TSH-induced iodine metabolism largely by acting at the steps distal to cAMP production. Northern blot analysis revealed expression of TGF-α mRNA in porcine thyroid cells. These observations suggest that TGF-α acts as an autocrine modulator of growth and differentiated functions in porcine thyroid cells.

T Tsushima, Department of Medicine 2, Tokyo Women's Medical College, Kawadacho 8–1, Shinjukuku, Tokyo 162, Japan

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Motoyasu Saji, Osamu Isozaki, Toshio Tsushima, Mariko Arai, Megumi Miyakawa, Yoshito Ohba, Yumi Tsuchiya, Tomohide Sano and Kazuo Shizume

Abstract. The effect of iodide on growth of rat thyroid cells (FRTL-5) was studied. TSH-stimulated cell growth was inhibited by iodide in a concentration-dependent manner, and an effect of iodide was detected at 10−6 mol/l. KClO4 or 1-methylimidazole-2-thiol blocked the effect of iodide, suggesting that iodide uptake and its organification are required to produce the inhibitory effect of iodide on cell growth. Iodide not only decreased TSH-stimulated cAMP production in FRTL-5 cells but also cell growth induced by cAMP. These observations suggest that iodide inhibits TSH-stimulated growth of the cells by attenuating cAMP production and also by acting on the step(s) distal to cAMP generation. The inhibitory effect of iodide was also seen in growth stimulated by insulin, insulin-like growth factor-I or 12-O-tetradecanoyl phorbol 13-acetate, suggesting multiple sites of action of iodide in the process of growth of FRTL-5 cells.