Hypoxic ER stress suppresses β-catenin expression and promotes cooperation between the transcription factors XBP1 and HIF1α for cell survival

Hypoxia occurs in many human solid tumors and activates multiple cellular adaptive-response pathways, including the unfolded protein response (UPR) in the endoplasmic reticulum (ER). Wnt/beta-catenin signaling plays a critical role in tumorigenesis, and beta-catenin has been shown to enhance hypoxia-inducible factor 1 alpha (HIF1 alpha)-activated gene expression, thereby supporting cell survival during hypoxia. However, the molecular interplay between hypoxic ER stress, Wnt/beta-catenin signaling, and HIF1 alpha-mediated gene regulation during hypoxia remains incompletely understood. Here, we report that hypoxic ER stress reduces beta-catenin stability, which, in turn, enhances the activity of spliced X-box-binding protein 1 (XBP1s), a transcription factor and signal transducer of the UPR, in HIF1 alpha-mediated hypoxic responses. We observed that in the RKO colon cancer cell line, which possesses a Wnt-stimulated beta-catenin signaling cascade, increased ER stress during hypoxia is accompanied by a reduction in low-density lipoprotein receptor-related protein 6 (LRP6), and this reduction in LRP6 decreased beta-catenin accumulation and impaired Wnt/beta-catenin signaling. Of note, beta-catenin interacted with both XBP1s and HIF1 alpha, suppressing XBP1s-mediated augmentation of HIF1 alpha target gene expression. Furthermore, Wnt stimulation or beta-catenin overexpression blunted XBP1s-mediated cell survival under hypoxia. Together, these results reveal an unanticipated role for the Wnt/beta-catenin pathway in hindering hypoxic UPR-mediated responses that increase cell survival. Our findings suggest that the molecular cross-talks between hypoxic ER stress, LRP6/beta-catenin signaling, and the HIF1 alpha pathway may represent an unappreciated mechanism that enables some tumor subtypes to survive and grow in hypoxic conditions.