Kaji H, Hinkle P M
Department of Pharmacology, University of Rochester School of Medicine, New York 14642.
Endocrinology. 1989 Feb;124(2):930-6. doi: 10.1210/endo-124-2-930.
Interactions between thyroid hormone and 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] were examined in a rat pituitary tumor cell line, GH4C1. Cells were incubated in thyroid hormone-depleted medium for 2 days, and specific nuclear binding of [125I]T3 was measured. 1,25-(OH)2D3 decreased nuclear [125I]T3 binding without changing total cellular uptake of [125I]T3. This 1,25-(OH)2D3 effect required 2-3 h to become evident and 24 h to reach a maximum (40-50% of control) and was reversible. Treatment with 1,25-(OH)2D3 for 8 h changed the maximal binding capacity for [125I]T3 from 80.2 +/- 2.9 to 50.3 +/- 6.3 fmol/10(6) cells, whereas Kd was not significantly altered. The decrease in [125I]T3 binding was dose dependent, with an IC50 for 1,25-(OH)2D3 of 1 nM in thyroid hormone-depleted medium. 1,25-(OH)2D3 caused little change in [125I]T3 binding to isolated nuclei, i.e. 1,25-(OH)2D3 does not compete directly with [125I]T3 for binding. It is unlikely that 1,25-(OH)2D3 decreased [125I]T3 binding by increasing the concentration of intracellular free calcium ([Ca2+]i), since 1,25-(OH)2D3 did not change [Ca2+]i in Indo-I-loaded GH4C1 cells. Two major species (6 and 2.6 kilobases) of mRNA for c-erb-A, which have been reported to encode nuclear thyroid hormone receptors, were found by Northern blot analysis, and both were decreased by treatment with 1,25-(OH)2D3 for 8 h. T3 (2 nM) caused a 3-fold increase in GH production over 72 h and 1,25-(OH)2D3 inhibited GH induction by T3, with an IC50 at approximately 1 nM. 1,25-(OH)2D3 stimulated PRL synthesis 5-fold when 10 nM T3 was present, but not when T3 was absent. In summary, 1,25-(OH)2D3 caused a dose-dependent down-regulation of nuclear thyroid hormone receptors at a pretranslational level and diminished GH induction by T3. These results suggest that 1,25-(OH)2D3 inhibits GH synthesis indirectly, at least partly, by attenuating endogenous thyroid hormone action.
在大鼠垂体瘤细胞系GH4C1中研究了甲状腺激素与1,25 - 二羟基维生素D3[1,25-(OH)2D3]之间的相互作用。将细胞在甲状腺激素缺乏的培养基中孵育2天，然后测量[125I]T3的特异性核结合。1,25-(OH)2D3降低了核[125I]T3结合，而不改变细胞对[125I]T3的总摄取。1,25-(OH)2D3的这种作用需要2 - 3小时才变得明显，24小时达到最大值（对照的40 - 50%），并且是可逆的。用1,25-(OH)2D3处理8小时后，[125I]T3的最大结合容量从80.2±2.9变为50.3±6.3 fmol/10(6)细胞，而解离常数（Kd）没有显著改变。[125I]T3结合的减少呈剂量依赖性，在甲状腺激素缺乏的培养基中，1,25-(OH)2D3的半数抑制浓度（IC50）为1 nM。1,25-(OH)2D3对[125I]T3与分离核的结合影响很小，即1,25-(OH)2D3不与[125I]T3直接竞争结合。1,25-(OH)2D3不太可能通过增加细胞内游离钙（[Ca2+]i）浓度来降低[125I]T3结合，因为1,25-(OH)2D3在负载Indo - I的GH4C1细胞中不改变[Ca2+]i。通过Northern印迹分析发现了两种主要的c - erb - A mRNA（6和2.6千碱基），据报道它们编码核甲状腺激素受体，并且用1,25-(OH)2D3处理8小时后两者均减少。T3（2 nM）在72小时内使生长激素（GH）分泌增加3倍，而1,25-(OH)2D3抑制T3诱导的GH分泌，IC50约为1 nM。当存在10 nM T3时，1,25-(OH)2D3刺激催乳素（PRL）合成增加5倍，但不存在T3时则无此作用。总之，1,25-(OH)2D3在翻译前水平引起核甲状腺激素受体的剂量依赖性下调，并减弱T3诱导的GH分泌。这些结果表明，1,25-(OH)2D3至少部分地通过减弱内源性甲状腺激素作用间接抑制GH合成。
