Osteocytes and bone tumor niche

Osteocytes are the most abundant cell type of the bone, representing the 95% of all bone cells. Advances in the last decade have established them as dynamic and multifunctional cells, capable of maintaining systemic mineral homeostasis and remodeling bone mass according to environmental requirements. Osteocytes regulate both bone-forming osteoblasts and bone-resorbing osteoclasts, mainly through the receptor activator of nuclear factor kappa B (RANK)/RANK ligand (RANKL) biological pathway and sclerostin, a potent inhibitor of the canonical Wingless-type (Wnt) signaling. Bone metastases are common in patients with advanced cancer, with 65%–75% of patients with breast or prostate cancer and over 35% of patients with lung cancer found to have bone metastases. Furthermore, multiple myeloma is characterized by the presence of osteolytic lesions in up to 80% of patients at diagnosis. Despite the remarkable progress in our understanding of osteocyte biology, there has been a paucity of information regarding the role of osteocytes in the progression of cancer in bone. Recent studies, however, suggest that tumor cells communicate with osteocytes to generate a microenvironment that supports the growth and survival of cancer cells and stimulates bone destruction. The aim of this chapter is to explore the possible role of osteocytes in malignant bone disease, according to all reported information in the current literature.