Identification and characterization of nuclear 3,5,3'-triiodothyronine-binding sites in fetal human lung

Thyroid hormones have been implicated in the prenatal lung development of several species. To investigate the possibility that thyroid hormones could play a role in human lung development, we examined nuclei from fetal human lung for the presence of high affinity T3-binding sites. A single class of high affinity (Kd = 35 +/- 3 pM) binding sites for L-T3 was identified in nuclei from fetuses between 12-19 weeks of gestation. The T3-binding capacity increased 65% between 12-13 weeks (binding capacity, 0.227 and 0.282 fmol/micrograms DNA in two experiments) and 16-19 weeks of gestation [binding capacity, 0.420 +/- 0.014 (SEM) fmol/micrograms DNA; n = 8]. Maximal binding was achieved after 4 h of incubation at 20 C. The half-times for dissociation of the T3-receptor complex were 36 h, 10.5 h, 3 h, and 23 min at 2, 20, 25, and 37 C, respectively. The relative order of potency of thyroid hormone analogs in displacing L-T3 from the receptor was: T3-proprionate greater than 3.3'-5-triiodothyroacetic acid greater than L-T3 greater than D-T3 greater than L-T4 greater than 3,5-diethyl-3'-isopropyl-D,L-thyronine greater rT3 greater than 3,5-dimethyl-3'-isopropyl-L-thyronine. Some receptors were released from the nuclei into the supernatant during incubation (7.8 +/- 0.3% after 4 h at 20 C). This release increased with incubation time and temperature, but was independent of T3 concentration, Ca2+, and gestational age. Incubation at 37 C also inactivated some of the receptors, but T3 provided dose-dependent protection from this loss of binding activity. Our results are consistent with the proposal that nuclear receptors mediate a direct effect of thyroid hormones on developing human lung as early as the second trimester of gestation.