Targeting the NR3C1-HSPA8 axis with cucurbitacin E exacerbates endoplasmic reticulum stress and induces stress-mediated cell death in multiple myeloma
Multiple myeloma (MM) remains an incurable hematological malignancy characterized by constitutive high-level immunoglobulin synthesis, which renders MM cells uniquely dependent on endoplasmic reticulum (ER) function and vulnerable to ER stress exacerbation. Through high-throughput screening of 1,000 natural products using an IRE1/XBP1 reporter system, we identified cucurbitacin E (CuE) as a potent activator of the unfolded protein response (UPR) in MM cells. CuE selectively induced cytotoxicity in MM cell lines and primary CD138-positive myeloma cells while sparing non-malignant cells. Mechanistically, transcriptome analysis revealed HSPA8, a master chaperone critical for protein homeostasis, as the most significantly downregulated gene upon CuE treatment. Genetic ablation of HSPA8 recapitulated CuE-induced ER stress and impaired MM cell survival, establishing HSPA8 as a key downstream effector. Integrated computational prediction and experimental validation-including molecular docking, molecular dynamics simulations, DARTS, and CETSA-identified NR3C1 as the direct binding target of CuE. NR3C1 knockout abrogated CuE-induced HSPA8 downregulation, UPR activation, and cytotoxicity, demonstrating that CuE acts through the NR3C1-HSPA8 axis. In vivo, CuE potently suppressed MM xenograft growth in an NR3C1-dependent manner without overt toxicity. Collectively, this study identifies CuE as a novel therapeutic candidate for MM and establishes that pharmacologically targeting the NR3C1-HSPA8 axis to amplify ER stress represents a promising strategy for MM treatment.