HMGB1 is a bone-active cytokine

High mobility group box I (HMGB 1) is a chromatin protein that acts as an immunomodulatory cytokine upon active release from myeloid cells. HMGB I is also an alarmin, an endogenous molecule released by dying cells that acts to initiate tissue repair. We have previously reported that osteoclasts and osteoblasts release HMGBI and release by the latter is regulated by parathyroid hormone (PTH), an agent of bone remodeling. A recent study suggests that HMGB I acts as a chemotactic agent to osteoclasts and osteoblasts during endochondral ossification. To explore the potential impact of HMGB I in the bone microenvironment and its mechanism of release by osseous cells, we characterized the effects of recombinant protein (rHMGBI) on multiple murine bone cell preparations that together exhibit the various cell phenotypes present in bone. We also inquired whether apoptotic bone cells release HMGBI. rHMGBI enhanced the RANKL/OPG steady state mRNA ratio and dramatically augmented the release of tumor necrosis factor-alpha (TNF alpha) and interleukin-6 (IL-6) in osteoblastogenic bone marrow stromal cell (BMSC) cultures but not in the calvarial-derived MC3T3-EI cells. Interestingly, rHMGBI promoted GSK-3beta phosphorylation in MC3T3-EI cells but not in BMSCs. Apoptotic bone cells released HMGBI, including MLO-Y4 osteocyte-like cells. MLO-Y4 release of HMGBI was coincident with caspase-3 cleavage. Furthermore, the anti-apoptotic action of PITH on MC3T3-EI cells correlated with the observed decrease in HMGB I release. Our data suggest that apoptotic bone cells release HMGB 1, that within the marrow HMGB I is a bone resorption signal, and that intramembraneous and endochondral osteoblasts exhibit differential responses to this cytokine.