Hypoxia-Inducible Factors Modulate the Stemness and Malignancy of Colon Cancer Cells by Playing Opposite Roles in Canonical Wnt Signaling

This study examined the role played by hypoxia-inducible factors (HIFs) in malignant phenotype maintenance and canonical Wnt signaling. Under normoxia, we determined that both HIF-1a and HIF-2a are expressed in human colon cancer cells but not in their non-malignant counterparts. The stable knockdown of HIF-1a or HIF-2a expression induced negative effects on the malignant phenotype of colon cancer cells, with lactate production, the rate of apoptosis, migration, CXCR4-mediated chemotaxis, and tumorigenic activity all being significantly affected by HIF knockdown and with HIF-1a depletion exerting greater effects. Knockdown of these two HIF transcripts induced different and even opposite effects on b-catenin transcriptional activity in colon cancer cells with different genetic Wnt signaling pathways. In SW480 cells, HIF-2a knockdown did not affect b-catenin levels, increasing the transcriptional activity of b-catenin by inducing its nuclear accumulation, whereas HIF-1a silencing negatively affected the stability and transcriptional activity of b-catenin, inducing its exit from the nuclei and its recruitment to the cell membrane by E-cadherin. In addition, although HIF-1a depletion induced a reversal of the epithelial-to-mesenchymal transition (EMT), HIF-2a silencing altered the expression of the stem cell markers CD44, Oct4, and CD24 and of the differentiation marker CK20 in the opposite direction as HIF-1a silencing. Remarkably, HIF-2a knockdown also enhanced b-catenin transcriptional activity under hypoxia in cells that displayed normal Wnt signaling, suggesting that the gene negatively modulates canonical Wnt signaling in colon cancer cells. Taken together, our result