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 T-3 on the expression of the Nm23-H1 gene. Human hepatoma HepG2 cells, in which endogenous thyroid hormone receptor subtype alpha 1(TR alpha 1) is expressed at a low level, were stably transfected, either with expression plasmids encoding wild-type TR alpha 1 or a dominant negative mutant of TR alpha 1, 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 T-3-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 TR alpha 1. An in vitro assay also revealed that T-3 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-N1 spanning nucleotides -471 to -437 (relative to the transcriptional initiation site) inhibited the expression of a downstream reporter gene, in a T-3-dependent manner, in COS-1 cells also transfected with an expression plasmid encoding TR alpha 1 or TR beta 1. The DNA binding domain of TR beta 1 was required for this inhibitory effect. These results indicate that T-3, 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 T-3 promotes tumor metastasis by inducing down-reglation of Nm23-H1 expression.