HIF-1-regulated expression of calreticulin promotes breast tumorigenesis and progression through Wnt/β-catenin pathway activation

Calreticulin (CALR) is a multi-functional protein that participates in various cellular processes, which include calcium homeostasis, cell adhesion, protein folding and cancer progression. However, the role of CALR in breast cancer is unclear. Here we report that CALR is overexpressed in breast cancer compared with normal tissue, and its expression is correlated with patient mortality and stemness indices. CALR expression was increased in mammosphere cultures, CD24-CD44+ subsets and ALDH+ subsets, which are enriched for breast cancer stem cells (BCSCs). Additionally, CALR knockdown led to BCSC depletion, which impaired tumor initiation and metastasis, and enhanced chemosensitivity in vivo. Chromatinimmunoprecipitation and reporter assays revealed that hypoxia-inducible factor 1 (HIF-1) directly activated CALR transcription in hypoxic breast cancer cells. CALR expression was correlated with Wnt/β-catenin pathway activation and an activator of Wnt/β-catenin signaling abrogated the inhibitory effect of CALR knockdown on mammosphere formation. Taken together, our results demonstrate that CALRfacilitates breast cancer progression by promoting the BCSC phenotype through Wnt/β-catenin signaling in a HIF-1-dependent manner, and suggest that CALR may represent a novel target for breast cancer therapy. In this study, we demonstrate that CALR expression is activated by HIF-1 when human BC cells are exposed to hypoxia. Knockdown of CALR expression decreased the number of BCSCs and impaired Wnt/β-catenin pathway activation. CALR expression in human BC biopsies was correlated with patient mortality.