Impaired interaction of mutant thyroid hormone receptors associated with human hepatocellular carcinoma with transcriptional coregulators

Thyroid hormone (T-3) exerts its many biological activities through interac tion with specific nuclear receptors (TRs) that function as ligand-dependen t transcription factors at genes that contain a thyroid hormone response el ement (TRE). Mutant TRs have been detected in human hepatocellular carcinom a cell lines and tissue, but their contribution to carcinogenesis has remai ned unclear. The interaction of four such mutant TRs (J7-TR alpha1, J7-TR b eta1, H-TR alpha1, and L-TR alpha1) with transcriptional coregulators has n ow been investigated. With the exception of J7-TR alpha1, which in the abse nce of T-3 exhibited transcriptional silencing activity with a TRE-reporter gene construct in transfected cells, the mutant TRs had little effect (com pared with that of wild-type receptors) on transcriptional activity of the reporter gene in the absence or presence of T-3, of the transcriptional cor epressors SMRT, NCoR or of the transcriptional coactivator SRC. Electrophor etic mobility-shift assays revealed that, in the presence of T-3, the J7-TR beta1 mutant did not interact with SRC, whereas J7-TR alpha1 and H-TR alph a1 exhibited reduced abilities to associate with this coactivator and L-TR alpha1 showed an ability to interact with SRC similar to that of wild-type TR alpha1. The dominant negative activity of the mutant TRs in transfected cells appeared inversely related to the ability of the receptors to interac t with SRC. Whereas J7-TR beta1, H-TR alpha1, and L-TR alpha1 did not inter act with SMRT, and NCoR. J7-TR alpha1 bind to corepressors but failed to di ssociate from them in the presence of T-3. These aberrant interactions betw een the mutant TRs end transcriptional coregulators may contribute to the h ighly variable clinical characteristics of human hepatocellular carcinoma.