ATF6 promotes colorectal cancer growth and stemness by regulating the Wnt pathway II

The unfolded protein response maintains endoplasmic reticulum (ER) homeostasis by sensing protein-folding stress and orchestrating cellular adaptation via the ER-transmembrane proteins IRE1, PERK and ATF6. Malignant cells can co-opt IRE1 and PERK to sustain growth; however, the importance of ATF6 in cancer remains poorly deciphered. We observed elevated ATF6 transcriptional activity in several cancers including colorectal carcinoma (CRC). Genetic silencing or small molecule inhibition of ATF6 blocked cell cycle progression and reduced viability of several human CRC cell lines in vitro and disrupted tumor progression in vivo. Unexpectedly, ATF6 interference disabled Wnt and Myc signaling and reduced stemness. ATF6 inhibition attenuated growth of organoids derived from malignant but not normal human intestinal tissue, decreasing Wnt-pathway activity and driving cellular differentiation. Wnt-surrogate agonism in a Wnt-dependent CRC organoid restored pathway activity and rescued growth under ATF6 blockade. Our findings identify ATF6 as an unexpected facilitator of oncogenic Wnt signaling in CRC. We identified ATF6, a latent transcription factor involved in the stress response pathway, as a regulator of cellular state in colon cancer. More specifically, genetic interference with ATF6 resulted in colon cancer cell growth inhibition. Bulk RNA-seq data indicated that this process was imparted through loss of stem cell function and a concomitant increase in cellular differentiation. We developed a small molecule inhibitor of ATF6 that primarily acted by promoting ATF6 degradation. This compound displayed cell growth inhibition in various CRC models but had limited impact in normal human colon organoids. To further investigate the impact of ATF6 on cellular state, chromatin accessibility, and cellular differentiation, we performed snRNA analysis contrasting a control arm and SMI arm in 2 technical replicates per group.