Thyroid hormone responsiveness in human growth hormone-related genes. Possible correlation with receptor-induced DNA conformational changes.

Triiodothyronine (T3) induces the transcription of the human chorionic somatomammotropin (hCS) promoter transfected into rat pituitary (GC) cells, but does not stimulate the homologous human growth hormone (hGH) promoter. As demonstrated by forward and reverse mutagenesis, this differential T3 responsiveness is due to subtle structural differences in a T3 response element located between nucleotides -64 and -44 of the 5'-flanking DNA of the hGH and hCS promoters. Synthetic hCS(-70/-40) DNA binds thyroid hormone receptors with a 4-fold higher affinity than the corresponding hGH T3 response element, indicating that small differences in receptor binding properties are reflected by major differences in T3 responsiveness. Analysis of circular permutation fragments containing the native hGH and hCS or mutated hCS(-70/-40) sequences demonstrates that the thyroid hormone receptor induces DNA bending. The extent of bending shows a possible correlation with the function of these sequences, suggesting that the receptor-induced changes in DNA conformation may be required for thyroid hormone receptor action.