The dialkyl resorcinol stemphol disrupts calcium homeostasis to trigger programmed immunogenic necrosis in cancer

Stemphol (STP) is a novel druggable phytotoxin triggering mixed apoptotic and non-apoptotic necrotic-like cell death in human acute myeloid leukemia (AML). Use of several chemical inhibitors highlighted that STP-induced non-canonical programmed cell death was Ca²⁺-dependent but independent of caspases, poly (ADP-ribose) polymerase-1, cathepsin, or calpains. Similar to thapsigargin, STP led to increased cytosolic Ca²⁺ levels and computational docking confirmed binding of STP within the thapsigargin binding pocket of the sarco/endoplasmic reticulum (ER) Ca²⁺-ATPase (SERCA). Moreover, the inositol 1,4,5-trisphosphate receptor is implicated in STP-modulated cytosolic Ca²⁺ accumulation leading to ER stress and mitochondrial swelling associated with collapsed cristae as observed by electron microscopy. Confocal fluorescent microscopy allowed identifying mitochondrial Ca²⁺ overload as initiator of STP-induced cell death insensitive to necrostatin-1 or cycloheximide. Finally, we observed that STP-induced necrosis is dependent of mitochondrial permeability transition pore (mPTP) opening. Importantly, the translational immunogenic potential of STP was validated by HMGB1 release of STP-treated AML patient cells. STP reduced colony and in vivo tumor forming potential and impaired the development of AML patient-derived xenografts in zebrafish.