Thyroid hormone receptor inhibits hepatoma cell migration through transcriptional activation of Dickkopf 4

Triiodothyronine (T₃) is a potent form of thyroid hormone mediates several physiological processes including cellular growth, development, and differentiation via binding to the nuclear thyroid hormone receptor (TR). Recent studies have demonstrated critical roles of T₃/TR in tumor progression. Moreover, long-term hypothyroidism appears to be associated with the incidence of human hepatocellular carcinoma (HCC), independent of other major HCC risk factors. Dickkopf (DKK) 4, a secreted protein that antagonizes the canonical Wnt signaling pathway, is induced by T₃ at both mRNA and protein levels in HCC cell lines. However, the mechanism underlying T₃-mediated regulation of DKK4 remains unknown. In the present study, the 5' promoter region of DKK4 was serially deleted, and the reporter assay performed to localize the T₃ response element (TRE). Consequently, we identified an atypical direct repeat TRE between nucleotides -1645 and -1629 conferring T₃ responsiveness to the DKK4 gene. This region was further validated using chromatin immunoprecipitation (ChIP) and electrophoretic mobility shift assay (EMSA). Stable DKK4 overexpression in SK-Hep-1 cells suppressed cell invasion and metastatic potential, both in vivo andin vitro, via reduction of matrix metalloproteinase-2 (MMP-2) expression. Our findings collectively suggest that DKK4 upregulated by T₃/TR antagonizes the Wnt signal pathway to suppress tumor cell progression, thus providing new insights into the molecular mechanism underlying thyroid hormone activity in HCC.