The Role of the HMGB1 C-Terminal Domain in Epithelial–Mesenchymal Transition and Invasion in 2D and 3D MDA-MB-231 Breast Cancer Models

High-mobility group box 1 (HMGB1) is a multifunctional protein that operates both within the nucleus and as an extracellular signaling molecule. Its extracellular activity has been increasingly associated with cancer progression. Emerging evidence suggests that structural modifications of HMGB1, including C-terminal truncation, may alter its biological activity, though the underlying mechanisms remain largely unexplored. Here, we show that HMGB1, which lacks the entire C-terminal acidic tail, is associated with increased cellular plasticity and invasive potential through distinct signaling pathways not strictly dependent on RAGE (Receptor for Advanced Glycation End-product) under the tested conditions. Functional analyses indicate that this truncated form promotes epithelial–mesenchymal transition-related behaviors and activates downstream inflammatory signaling in a context-dependent manner. Notably, pharmacological intervention with metformin effectively suppressed responses to the full-length protein but was less effective against the tail-less variant, underscoring potential therapeutic challenges. These findings suggest an underappreciated regulatory role of the HMGB1 C-terminal domain in tumor aggressiveness.