Hypoxia-activated XBP1s recruits HDAC2-EZH2 to engage epigenetic suppression of ΔNp63α expression and promote breast cancer metastasis independent of HIF1α

Hypoxia is a hallmark of cancer development. However, the molecular mechanisms by which hypoxia promotes tumor metastasis are not fully understood. In this study, we demonstrate that hypoxia promotes breast cancer metastasis through suppression of Delta Np63 alpha in a HIF1 alpha-independent manner. We show that hypoxia-activated XBP1s forms a stable repressor protein complex with HDAC2 and EZH2 to suppress Delta Np63 alpha transcription. Notably, H3K27ac is predominantly occupied on the Delta Np63 promoter under normoxia, while H3K27me3 on the promoter under hypoxia. We show that XBP1s binds to the Delta Np63 promoter to recruit HDAC2 and EZH2 in facilitating the switch of H3K27ac to H3K27me3. Pharmacological inhibition or the knockdown of either HDAC2 or EZH2 leads to increased H3K27ac, accompanied by the reduced H3K27me3 and restoration of Delta Np63 alpha expression suppressed by hypoxia, resulting in inhibition of cell migration. Furthermore, the pharmacological inhibition of IRE1 alpha, but not HIF1 alpha, upregulates Delta Np63 alpha expression in vitro and inhibits tumor metastasis in vivo. Clinical analyses reveal that reduced p63 expression is correlated with the elevated expression of XBP1, HDAC2, or EZH2, and is associated with poor overall survival in human breast cancer patients. Together, these results indicate that hypoxia-activated XBP1s modulates the epigenetic program in suppression of Delta Np63 alpha to promote breast cancer metastasis independent of HIF1 alpha and provides a molecular basis for targeting the XBP1s/HDAC2/EZH2-Delta Np63 alpha axis as a putative strategy in the treatment of breast cancer metastasis.