Sending the Signal to Bone: How Tumor-Derived EVs Orchestrate Pre-Metastatic Niche Formation and Skeletal Colonization

Abstract

Bone is a preferred site for disseminated tumor cells, yet the molecular mechanisms that prepare the skeletal microenvironment for metastatic colonization are only beginning to be understood. At the heart of this process are extracellular vesicles (EVs), nano-sized, lipid-encapsulated particles secreted by cancer cells and stromal components. This review consolidates current findings that position EVs as key architects of the bone-metastatic niche. We detail the biogenesis of EVs and their organotropic distribution, focusing on how integrin patterns and bone-specific ligands guide vesicle homing to mineralized tissues. We then outline the sequential establishment of the pre-metastatic niche, driven by EV-mediated processes including fibronectin deposition, stromal cell reprogramming, angiogenesis, neurogenesis, metabolic reconfiguration, and immune modulation, specifically, the expansion of myeloid-derived suppressor cells and impaired lymphocyte function. Within the bone microenvironment, tumor-derived EVs carrying microRNAs and proteins shift the balance toward osteoclastogenesis, inhibit osteoblast differentiation, and disrupt osteocyte signaling. These alterations promote osteolytic destruction or aberrant bone formation depending on tumor type. We also highlight cutting-edge imaging modalities and single-EV omics technologies that resolve EV heterogeneity and identify potential biomarkers detectable in plasma and urine. Finally, we explore therapeutic approaches targeting EVs, such as inhibition of nSMase2 or Rab27A, extracorporeal EV clearance, and delivery of engineered, bone-targeted vesicles, while addressing translational challenges and regulatory considerations. This review offers a roadmap for leveraging EV biology in predicting, preventing, and treating skeletal metastases by integrating advances across basic biology, bioengineering, and translational science.

Keywords: EV-targeted therapy; bone metastasis; extracellular vesicles; liquid biopsy biomarkers; osteoclastogenesis; pre-metastatic niche.

Figure 1

The “vicious cycle” of bone metastasis. Disseminated tumor cells that localize to bone initiate a destructive feed-forward loop by secreting osteolytic mediators, mainly parathyroid hormone-related protein (PTHrP), which stimulates osteoblasts to increase expression of receptor activator of nuclear factor-κB ligand (RANKL). RANKL engages its receptor RANK on osteoclasts, promoting their activity. Osteoclast-mediated degradation of the bone matrix releases a reservoir of growth factors, including transforming growth factor-β (TGF-β), insulin-like growth factors (IGFs), and bone morphogenetic proteins (BMPs), which in turn enhance tumor cell proliferation and further PTHrP production. Alkaline phosphatase (ALP) activity in osteoblasts indicates reactive bone formation at the metastatic interface, while osteocytes contribute to the amplification of osteoclastogenic and pro-tumorigenic signaling through interleukins 6 and 9. Collectively, these reciprocal interactions establish a self-sustaining loop in which skeletal degradation and tumor expansion are mutually reinforcing, underscoring the therapeutic imperative to concurrently target tumor cells, RANKL-driven osteoclast activation, and the release of bone-derived growth factors. Created in BioRender. Mohammad, K. (2025) https://BioRender.com/rojtj52.

Figure 2

Biogenesis, bone-tropism, and cellular uptake of tumor-derived extracellular vesicles (EVs). The schematic illustrates (i) intraluminal vesicle (ILV) formation within multivesicular endosomes (MVEs) and subsequent release of exosomes (30–150 nm) alongside larger microvesicles (100–1000 nm) from the plasma membrane of a cancer cell; (ii) the molecular interface between circulating EVs and the mineralized bone matrix, highlighting integrin-mediated binding to collagen, proteoglycans, and other matrix ligands; and (iii) downstream uptake of EVs by osteoclasts, osteoblasts, osteocytes, and bone-resident fibroblasts, which re-program these cells toward a pro-metastatic phenotype. Key structural elements (lipid bilayer, integrins, representative cargo) and bone-matrix components are annotated for clarity. Created in BioRender. Mohammad, K. (2025) https://BioRender.com/kmzro5f.

Figure 3

Systemic actions of tumor-derived EVs (TDEVs) during pre-metastatic niche formation in bone. Starting at the primary tumor, TDEVs enter the bloodstream and home to bone, where their cargo re-educates multiple stromal and immune populations. The diagram summarizes the principal functional effects detected to date: fibroblast activation to cancer-associated fibroblasts (CAFs) with matrix metalloproteinase (MMP) release; endothelial proliferation, tube formation, and vascular leakiness; adipocyte lipolysis and beige/brown trans-differentiation; skewing of hematopoietic stem cells (HSCs) toward osteoclastogenesis; modulation of osteoblast/osteoclast balance via altered RANKL expression; polarization of macrophages to M2/TAMs; suppression of cytotoxic lymphocytes and dendritic cell maturation; and neuro-adrenergic remodeling that favors tumor cell invasion. Arrows indicate the dissemination path (primary tumor → circulation → bone) and up- or downregulation (↑/↓) of each cellular process. Created in BioRender. Mohammad, K. (2025) https://BioRender.com/bobizhh.