The role of the unfolded protein response pathway in bone homeostasis and potential therapeutic target in cancer-associated bone disease

The unfolded protein response pathway is an evolutionarily conserved cytoprotective signaling cascade, essential for cell function and survival. Unfolded protein response signaling is tightly integrated with bone cell differentiation and function, and chronic unfolded protein response activation has been identified in bone disease. The unfolded protein response has been found to promote oncogenesis and drug resistance, raising the possibility that unfolded protein response modulators may have activity as anti-cancer agents. Cancer-associated bone disease remains a major cause of morbidity for patients with multiple myeloma or bone-metastatic disease. Understanding the critical role of unfolded protein response signaling in cancer development and metastasis, as well as its role in bone homeostasis, may lead to novel mechanisms by which to target cancer-associated bone disease. In this review, we summarize the current research delineating the roles of the unfolded protein response in bone biology and pathophysiology, and furthermore, review unfolded protein response modulating agents in the contexts of cancer and cancer-associated bone disease.