Hypoxia upregulates ovarian cancer invasiveness via the binding of HIF-1α to a hypoxia-induced, methylation-free hypoxia response element of S100A4 gene.

Hypoxia is known to play important roles in the development and progression of tumors. We previously demonstrated that S100A4, a critical molecule for metastasis, was upregulated in ovarian cancer cells. Therefore, we examined the mechanisms of the upregulation of S100A4 expression in ovarian carcinoma cells, with particular attention paid to the effects of hypoxia. The expression levels of S100A4 were found to be correlated with the invasiveness of ovarian carcinoma cells in vitro and in vivo, and the upregulation of S100A4 expression was associated with hypomethylation of CpG sites in the first intron of S100A4 in ovarian carcinoma cell lines and tissues. The expression of S100A4 was increased under hypoxia and was associated with elevated invasiveness, which was inhibited by S100A4 small interfering RNA (siRNA). In addition, exposure to hypoxia reduced the methylation of hypoxia-response elements (HRE) of the S100A4 gene in a time-dependent fashion, in association with the increased binding of HIF-1α to a methylation-free HRE in ovarian carcinoma cells. These results indicate that hypoxia-induced hypomethylation plays an essential role in S100A4 overexpression and the epigenetic transformation of ovarian carcinoma cells into the "metastatic phenotype."