The Multicellular Vicious Cycle of Bone Metastasis: Reciprocal Interactions Between Tumor Cells and the Bone Microenvironment

Bone metastasis arises from a complex interplay between disseminated tumor cells and the highly specialized bone microenvironment. This review synthesizes mechanistic insights describing how tumor-derived factors—including PTHrP, interleukins, Wnt modulators, and extracellular vesicles—disrupt physiological bone remodeling to initiate a self-reinforcing vicious cycle. Tumor signals stimulate osteo-blasts, osteocytes, stromal cells, and immune populations to increase RANKL expression and enhance osteoclast differentiation, resulting in pathological bone resorption. Osteoclast-mediated degradation releases matrix-embedded growth factors such as TGF-β, IGFs, and BMPs, which in turn promote tumor proliferation, invasion, immune evasion, and further osteoclastogenic factor production. Meanwhile, os-teoblasts and osteocytes undergo inflammatory reprogramming, adipocytes provide metabolic and cytokine support, and stromal and endothelial cells reshape the metastatic niche to foster tumor survival, dormancy escape, and overt lesion expansion. This ecosystem-wide reciprocity underscores bone metastasis as a dynamic, multicellular process rather than a unidirectional event. Understanding the temporal progression from premetastatic niche formation to dormancy, reactivation, and lesion growth reveals therapeutic opportunities targeting osteoclasts, TGF-β signaling, interleukin networks, EV-mediated communication, and niche-specific cell states. Disrupting multiple arms of the vicious cycle may offer more durable control of metastatic disease.