Negative regulation of the antimetastatic gene nm23-h1 by thyroid hormone receptors

Metastasis of various malignant cells is inversely related to the abundance of the Nm23-H1 protein. The possible role of thyroid hormones in tumor metastasis has now been investigated by examining the effect of T₃ on the expression of the Nm23-H1 gene. Human hepatoma HepG2 cells, in which endogenous thyroid hormone receptor subtype α1 (TRα1) is expressed at a low level, were stably transfected, either with expression plasmids encoding wild-type TRα1 or a dominant negative mutant of TRα1, or with the empty vector (yielding HepG2-Wt, HepG2-Mt, and HepG2-Neo cells, respectively). Immunoblot analysis revealed that exposure of HepG2-Wt and HepG2-Neo cells, but not HepG2-Mt cells, to T₃-induced time-dependent decreases in the abundance of Nm23-H1 messenger RNA and protein, with the extent of these effects correlating with the level of expression of TRα1. An in vitro assay also revealed that T₃ induced a marked increase in the invasive activity of HepG2-Wt cells; it induced a smaller increase in that of HepG2-Neo cells but had no effect on that of HepG2-Mt cells. Finally, the promoter region of Nm23-H1 spanning nucleotides -471 to -437 (relative to the transcriptional initiation site) inhibited the expression of a downstream reporter gene, in a T₃-dependent manner, in COS-1 cells also transfected with an expression plasmid encoding TRα1 or TRβ1. The DNA binding domain of TRβ1 was required for this inhibitory effect. These results indicate that T₃, acting through TRs, inhibits transcription of Nm23-H1, and that this effect is mediated by a negative regulatory element in the promoter region of the gene. Thus, it is possible that T₃ promotes tumor metastasis by inducing down-regulation of Nm23-H1 expression.