Decreased Nuclear Uptake of [125I]Triiodo-L-Thyronine in Fibroblasts from Patients with Peripheral Thyroid Hormone Resistance*

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Abstract
The cellular mechanisms that cause the syndrome of generalized, peripheral, and pituitary resistance to thyroid hormones are unclear. In order to investigate the possibility of a defect at the nuclear receptor for T3, we examined: 1) equilibrium binding of [125I]T3 (64 analyses) to the nuclei of intact cultured fibroblasts from 12 patients with both sporadic and familial generalized or selective pituitary resistance, 17 normal subjects, and two apparently normal siblings from affected families; and 2) kinetics of [125I]T3 nuclear uptake (22 analyses) in intact fibroblasts from 4 generalized resistant patients from two different kindreds, compared to one apparently normal sister of one affected patient and 6 normal subjects. Statistical analysis of the equilibrium binding parameters showed no differences in nuclear binding capacity for T3 among all groups. The equilibrium dissociation constants (Kd) also were not significantly different in fibroblasts from the selective pituitary resistant patients, the normal siblings, and the normal subjects. In contrast, the KdS from the general resistant patients were significantly increased (median = 1.94 × 10−10 M, 5-95% confidence interval = 1.18–2.93 × 10−10 M 1.11, 0.77–1.25 × 10−10 M; P < 0.008). Since there was considerable overlap of the equilibrium binding values among groups, we studied the kinetics of [125I]T3 nuclear uptake in selected cell lines from normal subjects and familial generalized resistant patients. Kinetic analysis of the association curve revealed that, compared to the normal subjects, maximum binding at 10−10 M [125I]T3 was significantly reduced to 50% in two resistant patients, to 10% in another two patients, and to 65% in one apparently normal sibling. The empiric parameter λ that corresponds to the slope at the origin of the curve was not significantly different in any patient compared to normal. The calculated apparent association constant, derived from λ, and the binding capacity obtained at equilibrium, showed differences that were significant for only one of the patients. We conclude that cultured fibroblasts from patients with familial generalized resistance to thyroid hormone display abnormal kinetics of T3 nuclear uptake which provide a convenient in vitro method for the recognition of thyroid hormone resistance at the cell level. Furthermore, because cultured fibroblasts can be grown under controlled experimental conditions, they are a suitable tissue for further study of the molecular basis of these disorders.