Asif, A.; Panjwani, K.; Nair, K.; Smith, P.; Dancan, O.; Crosbourne, I.; DeLuca, J.; Humphrey, T.; Ramos, R. B.; Corr, D. T.; Padilla-Benavides, T.; Barroso, M.

Intracellular iron homeostasis is increasingly recognized as a regulator of cancer cell behavior, but how iron distribution influences extracellular matrix (ECM) organization and invasion remains poorly understood. Here, we show that loss of divalent metal transporter 1 (DMT1/SLC11A2) disrupts intracellular iron homeostasis and promotes cancer cell invasion through an iron-ER-ECM axis. In MDA-MB-231 cells, DMT1 knockout (KO) reduced total iron content but increased the labile iron pool (LIP) in both 2D and 3D culture models, indicating altered intracellular iron distribution. Across transcriptomic and phenotypic readouts, DMT1-dependent effects were more evident in 3D than in 2D models, with DMT1 KO inducing endoplasmic reticulum (ER) stress and impaired collagen/ECM organization. Functionally, the DMT1-loss phenotype was marked by reduced 2D motility, whereas in 3D spheroid models DMT1 KO cells displayed enhanced invasive outgrowth in both Matrigel and collagen matrices. Iron chelation further modulated this phenotype in a DMT1-dependent manner. Pharmacologic induction of ER stress phenocopied the loose spheroid architecture and invasive behavior, supporting ER stress as a mechanistic link between altered iron handling and ECM destabilization. Together, these findings identify intracellular iron distribution, rather than total iron abundance alone, as a determinant of ECM integrity and context-dependent cancer cell invasion. Significance StatementOur study identifies an iron-ER-ECM axis through which intracellular iron homeostasis regulates cancer cell invasion. Total cellular iron content alone is insufficient to predict invasive behavior without considering how iron is distributed within the cell. By preserving intracellular iron homeostasis and ER function, DMT1 supports collagen synthesis and maintains ECM integrity. In contrast, DMT1 loss disrupts these processes, promoting formation of loosely aggregated spheroids and enhanced invasion in 3D tumor models despite reduced total iron levels. These findings challenge the assumption that lowering bulk iron uniformly suppresses invasive phenotypes and instead highlight intracellular iron trafficking as a potential therapeutic target for limiting cancer cell invasion.