Covalent Inhibition of SHMT2 by Gambogic Acid Induces Ferroptosis Through Mitochondrial Collapse in Triple‐Negative Breast Cancer

ABSTRACT Triple‐negative breast cancer (TNBC) is an aggressive subtype lacking targeted therapies due to the absence of hormone receptors and HER2 expression, resulting in poor clinical outcomes and limited treatment options. Identifying novel vulnerabilities is therefore critical to advancing TNBC therapeutics. Mitochondrial metabolism has emerged as a key regulator of cancer cell survival and proliferation, with serine hydroxymethyltransferase 2 (SHMT2) playing a central role in mitochondrial one‐carbon metabolism by supplying one‐carbon units for nucleotide biosynthesis and maintaining redox homeostasis. Despite its established importance in cancer metabolism, the functional role and therapeutic potential of SHMT2 in TNBC remain underexplored. Here, we demonstrate that gambogic acid (GA), a natural product with reported anticancer properties, exerts potent and selective cytotoxicity against TNBC cells by covalently targeting SHMT2. GA binds specifically to the critical cysteine residue Cys241, inhibiting SHMT2 enzymatic activity and disrupting mitochondrial function. This leads to bioenergetic collapse, activation of the Nrf2/HO‐1 axis, iron overload, and induction of ferroptosis, a non‐apoptotic form of cell death increasingly recognized for its therapeutic potential. Our integrative chemoproteomic and mechanistic studies reveal a novel SHMT2‐mitochondria‐Nrf2/HO‐1‐ferroptosis axis driving GA's anti‐TNBC activity. Moreover, SHMT2 overexpression in TNBC correlates with tumor aggressiveness and poor prognosis, underscoring its role as a metabolic oncogene and promising drug target. These findings establish GA as a novel covalent SHMT2 inhibitor and provide a new framework for exploiting metabolic vulnerabilities to overcome TNBC treatment resistance.