Bone metastasis in endocrine-related cancer: unravelling invasion and destruction

Bone is a common and debilitating site for metastatic cancer cell expansion. Skeletal metastasis is a multistage process, with primary stages of circulating tumour cells, progressing to a dormant state in vasculature and bone marrow niches, followed by tumourigenic reactivation, proliferation and finally bone destruction. The frequency of bone metastasis is reconciled in Paget's 'seed and soil' hypothesis, where a conducive microenvironment (bone niche) is essential for cancer cell colonisation. Cancer cells can mimic bone cells (osteomimicry) and interact with the bone marrow's vascular architecture, utilising pathways akin to haematopoietic stem cell expansion. Current research suggests that each phase of bone metastasis is associated with specific gene expression and protein abundance patterns. For example, E-selectin, CXCR-4 and CXCL-12 are crucial for cancer cell homing, dormancy and colonisation of bone tissue. In contrast, different primary cancers appear to have unique staging profiles. In prostate cancer, dormancy is modulated by the CXCR-4/CXCL-12, ANXA2/CXCL12 and GAS6 pathways, while in breast cancer, dormancy involves ERK1/2, p38, MSK1, LIF, BMP-7, TGF-β1/2 and bone resorption factors. Conversely, osteoblastic metastasis in both breast and prostate cancers is characterised by ET-1, Dkk1 suppression and the release of IL-6, MCP-1, VEGF, FGF and IGF, while osteolytic metastasis primarily depends on PTHrP, RANKL, OPG, TGF-β, IGF, TNF-α, IL-1 and IL-7. Understanding the complex molecular mechanisms facilitating cancer cell colonisation and expansion in bone tissues is essential for developing effective treatments to prevent bone metastases. In this review, we discuss current theories linking bone remodelling with bone.