Unfolded Protein Response as a Therapeutic Target: 4-(Heptyloxy)phenol Induces Programmed Cell Death in Ewing Sarcoma

Ewing sarcoma is a rare and aggressive pediatric malignancy with limited therapeutic options, particularly for relapsed or refractory cases, highlighting the urgent need for innovative treatment strategies. In this study, we identify endoplasmic reticulum stress (ERS) and the unfolded protein response (UPR) as critical therapeutic vulnerabilities in Ewing sarcoma and introduce 4-(heptyloxy)phenol (AC-45594) as a novel small-molecule agent that exploits these stress pathways. AC-45594 selectively inhibited the growth of Ewing sarcoma cells among 13 cancer and 6 noncancerous cell lines, demonstrating marked tumor specificity. Structure-activity relationship studies revealed that both the phenolic hydroxyl group and an optimal alkoxy chain length (7-9 carbon atoms) are essential for its activity. Mechanistically, AC-45594 induces ERS, activates UPR, and drives a shift from adaptive to terminal stress signaling, culminating in apoptosis of Ewing sarcoma cells. Proteomic and gene expression analyses further supported the selective activation of proapoptotic UPR signaling. These findings establish ERS and UPR as actionable targets in Ewing sarcoma and position AC-45594 as a first-in-class compound capable of selectively inducing stress-driven cell death. This work lays the foundation for a new class of therapeutics targeting maladaptive stress responses in pediatric sarcomas and potentially other hard-to-treat cancers.