Thyroid hormone (T 3 ) mediates cellular growth, development, and differentiation by binding to the nuclear thyroid hormone receptor (TR). Recent studies suggest that long-term hypothyroidism is associated with human hepatocellular carcinoma (HCC) independent from other major HCC risk factors. Dickkopf (DKK) 4, a secreted protein, antagonizes the Wnt signal pathway. In this study, we demonstrate that T 3 may play a suppressor role by inducing DKK4 expression in HCC cells at both the messenger RNA (mRNA) and protein levels. DKK4 was down-regulated in 67.5% of HCC cancerous tissues. The decrease in DKK4 levels was accompanied by a concomitant decrease in TR protein levels in the matched cancerous tissues in 31% of tissues compared by immunoblotting with the adjacent noncancerous tissues. Further, TR and DKK4 expression levels were positively correlated in both normal and cancerous specimens by tissue array analysis. In function assays, stable DKK4 transfected into J7 or HepG2 cells decreased cell invasion in vitro. Conversely, knocking down DKK4 restores cell invasiveness. DKK4-expressing J7 clones showed increased degradation of b-catenin, but down-regulation of CD44, cyclin D1, and c-Jun. To investigate the effect of DKK4 and TR on tumor growth in vivo, we established a xenograft of J7 cells in nude mice. J7-DKK4 and J7-TRa1 overexpressing mice, which displayed growth arrest, lower lung colony formation index, and smaller tumor size than in control mice, supporting an inhibitory role of DKK4 in tumor progression. Conclusion: Taken together, these data suggest that the TR/DKK4/Wnt/b-catenin cascade influences the proliferation and migration of hepatoma cells during the metastasis process and support a tumor suppressor role of the TR. (HEPATOLOGY 2012;55:910-920) T hyroid hormone, 3,3 0 -5-triiodo-l-thyronine (T 3 ), is a potent mediator of many physiological processes including embryonic development, cell differentiation, metabolism, and the regulation of cell proliferation.1,2 The actions of T 3 are mediated by nuclear thyroid hormone receptors (TRs). TRs are ligand-dependent transcription factors that comprise modular functional domains that mediate hormone binding (ligands), DNA binding, receptor homo-and heterodimerization, and interaction with other transcription factors and cofactors.3 TRs are derived from two genes, TRa and TRb, located on human chromosomes 17 and 3, respectively. Transcripts of each of these genes undergo alternative promoter choice to generate TRa1 and TRa2 as well as TRb1 and TRb2 receptor isoforms.
2-4Using a complementary DNA (cDNA) microarray technique, we previously identified 148 genes that are positively regulated by T 3 in a TRa1-overexpressing