Is HMGB1 an osteocyte alarmin?

Abstract

The death of osteocytes, the terminally differentiated cells of the osteoblast lineage that are embedded in bone and regulate remodeling, is significant to both normal and pathological bone resorption. Apoptotic osteocytes putatively release a clarion signal that enhances the development of the bone-resorbing osteoclasts and targets their migration to the breach in the osteocyte network. This phenomenon is thought to underlie normal repair of bone microdamage and contribute to the etiologies of inflammatory bone loss. The chromatin protein high mobility group box 1 protein (HMGB1) has been identified as an "alarmin" in other tissues. An alarmin is an endogenous molecule released by dead and dying cells that alert the innate immune system to damage and the need for tissue repair. Wang and colleagues presented evidence in a landmark 1999 study showing that released HMGB1 is a lethal mediator of sepsis. Extracellular HMGB1 is a ligand for the toll-like receptors (TLRs) and for the receptor for advanced glycation end products (RAGE) all of which amplify inflammation. Recent studies by our lab and others have shown that HMGB1 is a bone-active cytokine. It is released by apoptotic osteoblasts in vitro, including the MLO-Y4 osteocyte-like cells. Extracellular HMGB1 enhances the expression of RANKL, TNFalpha, and IL6 in osteoblastogenic bone marrow stromal cell cultures, and it is chemotactic to osteoclasts. In this prospectus we will review HMGB1 activity at the immune-bone interface and propose a role for HMGB1 as an osteocyte alarmin and mediator of normal remodeling and inflammatory bone loss.