Relationship of receptor affinity to the modulation of thyroid hormone nuclear receptor levels and growth hormone synthesis by L-triiodothyronine and iodothyronine analogues in cultured GH1 cells.

Abstract

A B S T R A C T We have previously demonstrated that L-triiodothyronine (L-T3) induces an increase in growth hormone synthesis and messenger RNA in cultured GH1 cells, a rat pituitary cell line. In addition to regulating the growth hormone response, L-T3 elicits a time-and dose-dependent reduction in the level of its nuclear receptor, which is a direct function of the occupancy of the receptor binding site. In this study we have compared the relative affinity of L-T3, triiodothyroacetic acid, D-triiodothyronine (D-T3), and L-thyroxine (L-T4) for the receptor with the induction ofthe growth hormone synthesis and the ability of these compounds to elicit a reduction in thyroid hormone nuclear receptor levels. Triiodothyroacetic acid and D-T3 were specifically examined because the biologic effect of these compounds in the intact rat is significantly lower than predicted by their affinity for the receptor using isolated rat liver nuclei in vitro. In intact cells each compound demonstrated an excellent relationship between the relative receptor affinity, the induction of growth hormone production, and the concentration-dependent reduction in nuclear receptor levels. With the exception of D-T3, the relative affinity of iodothyronine was identical for the receptor using intact cells in serum-free media, or isolated GH, cell nuclei in vitro. The apparent receptor affinity of D-T3 with intact cells was 5.5-fold lower than with isolated nuclei, which suggests a decrease in cell entry of D-T3 relative

Regulation of gene expression by thyroid hormone.

“…These relative affinities have been found for receptor in rat brain, liver, and kidney, and for receptor in a variety of cultured cell lines derived from the rat and other species (3,42,44,45). Scatchard analysis of hormone binding to receptor demonstrates linear plots with no evidence for positive or negative cooperativity (46)(47)(48)(49).…”

“…Similar results have been observed with receptor obtained from rat liver nuclei. The affinity of receptors for L-T3, L-T4, and other iodothyronines parallels the iodothyronine biologic potency in cultured cells (42) and in intact animals (44). If the affinity for receptor and the biologic potency of L-T3 is assigned the value of 1, the values for several well-studied iodothyronines are: triiodothyroacetic acid (TRIAC) (…”

“…Evidence to support this notion has been derived from studies in intact animals (2, 33) and cultured cells (3,23,42,43). Several different related strains of growth hormone producing rat pituitary cell lines (GH,, GH3, GH4, and GC) have been shown to be highly effective cell culture models for studying thyroid hormone action.…”

Role of L-thyroxine in nuclear thyroid hormone receptor occupancy and growth hormone production in cultured GC cells.

“…Studies in euthyroid rats suggested that T4 constituted 10-15% of the endogenous iodothyronines specifically associated with thyroid nuclear receptors in liver and kidney (15) but was not detected in anterior pituitary (16,17). T4 was shown to have intrinsic biological activity by Samuels et al (18), who showed that T4 induced growth hormone (GH) production and thyroid nuclear receptor down-regulation in GH, cells in the absence ofappreciable conversion ofT4 to T3. More recently, others have shown that GH1 cells do generate appreciable amounts of T3 from T4 (19).…”

“…Although T4 affects biological activity in anterior pituitary by means ofT3(T4), these reports (16)(17)(18)(19) also suggest that the role of T4 itself in thyroid hormone action in anterior pituitary is not clearly defined. The issue is important for understanding both the action of thyroid hormones under physiologic conditions and during the alterations in plasma thyroid hormone concentrations that occur in patients with nonthyroidal disease.…”