The hostile bone niche: physicochemical stress as a driver of tumor adaptation, resistance, and pain

Bone metastases are commonly interpreted through the biochemical “vicious cycle” in which tumor cells, osteoclasts, osteoblast-lineage cells, stromal cells, and immune cells exchange cytokines and growth factors that amplify skeletal colonization. Although this model remains essential, it does not fully explain why bone lesions become highly adaptive, therapy-resistant, painful, and spatially heterogeneous. In this review, we propose that the metastatic bone niche should be viewed as a hostile physicochemical ecosystem in which hypoxia, extracellular acidosis, elevated interstitial pressure, impaired transport, and abnormal matrix mechanics function as active selective pressures. As skeletal lesions expand, vascular disruption and altered remodeling may generate additional stress beyond normal marrow gradients, although the extent of these changes varies by lesion and remains incompletely measured in patients. In parallel, glycolytic tumor metabolism and osteoclast-mediated proton secretion acidify the extracellular space, thereby enhancing invasion, immunosuppression, osteolysis, and nociceptor activation. Pathological remodeling further alters matrix stiffness, mineral architecture, and mechanotransduction, shaping tumor-cell plasticity, dormancy, reactivation, and resistance. These stressors do not act independently; rather, they form reciprocal feedback loops that remodel tumor, bone, vascular, immune, stromal, and neural compartments. We discussed how this integrated framework helps explain metastatic persistence, remodeling imbalance, treatment failure, and cancer-induced bone pain across breast, prostate, lung, and selected primary bone malignancies. Finally, we highlight key knowledge gaps, including the need for spatially resolved measurements of oxygen, pH, pressure, and matrix mechanics in human bone lesions, as well as more faithful experimental models that reproduce the multistressor biology of skeletal metastasis.