Carfilzomib promotes Iodine-125 seed radiation-induced apoptosis, paraptosis, and ferroptosis in esophageal squamous cell carcinoma by aggravating endoplasmic reticulum stress

Iodine-125 (¹²⁵I) seed brachytherapy has been applied to treat various malignant tumors such as esophageal cancer, however, radioresistance can reduce its efficacy. Endoplasmic reticulum stress (ERS) and subsequent unfolded protein response (UPR) is one of the core mechanisms of ¹²⁵I seed radiation-induced cell death, thus aggravating ERS has been considered a promising sensitization strategy. Herein, we show that combination therapy of an irreversible proteasome inhibitor carfilzomib (CFZ) and ¹²⁵I seed radiation displayed strong anti-tumor effect on esophageal squamous cell carcinoma (ESCC). Mechanistically, ERS and UPR regulated multiple cell death modalities induced by the combination therapy, including apoptosis, paraptosis, and ferroptosis. ¹²⁵I seed radiation induced reactive oxygen species (ROS) production, DNA damage, p53 activation, and apoptosis. CFZ promoted ROS production, and augmented ¹²⁵I seed radiation-induced apoptosis via the mitochondrial pathway, which was mediated by the UPR-C/EBP homologous protein (CHOP) pathway and was independent of the p53 pathway. CFZ enhanced ¹²⁵I seed radiation-induced intracellular Ca²⁺ overload, protein ubiquitination, ERS, and UPR, consequently promoting paraptosis. ¹²⁵I seed radiation induced accumulation of intracellular Fe²⁺ and lipid peroxides but upregulated the expression of ferroptosis inhibitors, SLC7A11 and glutathione peroxidase 4 (GPX4). The combination therapy promoted ferroptosis by enhancing the accumulation of intracellular Fe²⁺ and downregulating GPX4 expression. The mouse experiment demonstrated that CFZ can promote the efficacy of ¹²⁵I seed radiation with good tolerance. Our findings suggest that combination therapy of ¹²⁵I seed radiation and CFZ is associated with multiple cell death modalities and may serve as a promising therapeutic strategy for ESCC.