Expression of mutant thyroid hormone nuclear receptors in human hepatocellular carcinoma cells

To understand the expression and role of thyroid hormone nuclear receptors (TRs) in hepatocarcinogenesis, we characterized the TRs in 16 human hepatocellular carcinoma (HCC) specimens. The full-length cDNAs for the two TR subtypes, alpha1 and beta1, were cloned from several tumors by reverse transcription-polymerase chain reaction. Southern blot analysis indicated that, in addition to the full-length cDNA, truncated TRalpha1 and TRbeta1 cDNAs were present in nine tumors (53%). In addition, point mutations detected by the mismatch RNase cleavage assay in TRalpha1 and TRbeta1 were found in 65% and 76% of the tumors, respectively. The mutations were confirmed by DNA sequencing. Interestingly, most of the TRalpha1 mutations were in amino acid codons 209-228 and 245-256, two hot-spots in HCC patients. However, no hot-spot was detected in TRbeta1. The expression of TRalpha1 and TRbeta1 proteins was determined in the tissue extracts by western blotting. TRbeta1 protein was expressed or elevated in 10 tumors but not in normal livers, whereas the expression of TRalpha1 was variable among tumors. The mutant TR proteins were translated in vitro, and their hormone- and DNA-binding activities were evaluated. Abnormal binding to the thyroid hormone response elements was observed. The proteins' DNA binding activity was either partially impaired or completely lost. The high prevalence of TR mutations found in the tumors of patients with hepatocellular carcinoma suggests that mutant TRs could play an important role in liver carcinogenesis.