Celastrol induces ferroptosis in multiple myeloma associated with suppression of the squalene synthase-mediated lipid defense axis

Multiple myeloma (MM) remains incurable due to pervasive drug resistance, motivating exploration of non-apoptotic cell death pathways. Squalene synthase (SQS, encoded by FDFT1), the first committed enzyme in sterol biosynthesis within the mevalonate (MVA) pathway, has been linked to ferroptosis defense, but its role in MM is poorly defined. This study aimed to investigate whether SQS modulates celastrol-induced ferroptosis in MM. Four MM cell lines (RPMI-8226, U266, KMS-11, SKO-007) were treated with celastrol. Cell viability, intracellular iron, reactive oxygen species (ROS), malondialdehyde (MDA), and glutathione (GSH) were measured. Ferroptosis inhibitors (ferrostatin-1, deferoxamine mesylate) and lentiviral SQS overexpression were used to assess functional dependence. Bulk RNA-seq data from the MMRF-COMMPASS cohort (n = 764 primary CD138+ plasma cell samples) were analyzed for FDFT1-ferroptosis correlations and pathway activity using GSVA and GSEA. Celastrol reduced viability (24-h IC50: 2.41-4.44 µmol/L) with iron accumulation, ROS elevation, MDA increase, and GSH depletion in all four cell lines. Ferrostatin-1 and deferoxamine partially rescued viability and restored SQS, GPX4, SLC7A11, and FSP1 protein levels. SQS overexpression partially protected RPMI-8226 cells and mitigated iron/MDA accumulation while preserving GSH. In MMRF-COMMPASS, FDFT1-high tumors showed significantly elevated ferroptosis defense pathway activity (GSVA P_adj = 1.37 × 10-6; GSEA NES = 2.13, P_adj = 2.68 × 10-3), although FDFT1 was not an independent prognostic marker (univariate HR = 1.064, P = 0.441; multivariate HR = 1.059, P = 0.482). SQS is a functional ferroptosis defense node in MM. Celastrol-induced ferroptosis is associated with suppression of the MVA-SQS axis, supported by pharmacological, genetic, and clinical transcriptomic evidence. These findings provide a preclinical rationale for targeting SQS-mediated metabolic vulnerability in MM.