ABSTRACT

We have elucidated the action of tri-iodothyronine (T₃) on the synthesis and secretion of seven major plasma proteins in a human hepatoblastoma cell line, Hep G2, and established an in vitro experimental model of human liver cells for the study of the mechanism of the action of thyroid hormone.

Hep G2 cells cultured in serum-free medium were treated with various concentrations of T₃. During the first 24 h of T₃ treatment, accumulation of α-fetoprotein in the medium was decreased in a dose-dependent manner (10⁻¹¹-10⁻⁸ m), and the inhibitory effect was enhanced during the second 24 h of T₃ treatment. On the other hand, α₁-antitrypsin accumulation in the medium during the second 24 h of hormone treatment was decreased by T₃ (10⁻⁹–10⁻⁸ m), although no change in accumulation was observed during the first 24 h of T₃ treatment.

The newly synthesized [³⁵S]Met-labelled α₁-acid glycoprotein was increased by T₃ and reached 3·4-fold within 37 h of 10⁻⁸ m T₃ treatment. The stimulatory effect increased time-dependently (4·6fold after 61 h). In contrast, the synthesis of α-fetoprotein was reduced to half of that of the control after T₃ treatment for 37 h.

Although the content of newly synthesized [³⁵S]α₁-antitrypsin was not affected by 10⁻⁸m T₃ treatment during 3 days of hormone treatment, the accumulation of α₁-antitrypsin in the medium decreased to 87%; in contrast, total cellular newly synthesized α₁-antitrypsin increased to 105–130% of that of the control. From these results, it is suggested that α₁-antitrypsin secretion might be suppressed by T₃ treatment. However, T₃ did not affect the accumulation of albumin, transferrin, fibronectin and α₂-macroglobulin in the medium throughout the experiments.

It was shown that T₃ has diverse control mechanisms on the synthesis and secretion of plasma proteins in Hep G2 cells: stimulation (α₁-acid glycoprotein) or inhibition (α-fetoprotein) of plasma protein synthesis, and inhibition of protein secretion (α₁-antitrypsin).

ABSTRACT

It is well known that epidermal growth factor (EGF) induces down-regulation of LH receptors and desensitization to gonadotrophin stimulation in gonadal cells, including granulosa cells. In a previous study we showed that EGF receptor levels in rat granulosa cells were increased up to fourfold after 96 h of culture with human GH in the presence of FSH, and the present study has evaluated the action of EGF on these cells. The induced EGF receptors were identical in size to the pre-existing receptors as assessed by affinity labelling with ¹²⁵I-EGF. After 48 h in culture, various amounts of EGF (0·5–10 ng) were added and the cells were cultured for a further 48 h. The addition of EGF caused down-regulation of LH receptors in cells expressing high levels of EGF receptors. However, this down-regulation was less than that in control cells. After the cells were washed, cAMP synthesis in response to human chorionic gonadotrophin (hCG) increased by two to three times the control value and this increase was closely correlated with an increase in EGF receptor content. However, stimulation with cholera toxin or forskolin showed no such augmentation, indicating that it may not be due to quantitative alterations in G proteins and their effector systems. Induction of EGF potentiation required long-term exposure to EGF, for at least more than 24 h. In addition, progesterone synthesis was sensitive to stimulation with lower doses of hCG. These findings indicate that the activation of hGH-induced EGF receptors may potentiate gonadotrophin action in granulosa cells.