Hemeoxygenase-1 maintains bone mass via attenuating a redox imbalance in osteoclast

Heme oxygenase-1 (HO-1) has long been considered to be an endogenous antioxidant. However, the role of HO-1 is highly controversial in developing metabolic diseases. We hypothesized that HO-1 plays a role in maintaining bone mass by alleviating a redox imbalance. We investigated its role in bone remodeling. The absence of HO-1 in mice led to decreased bone mass with elevated activity and number of OCs, as well as higher serum levels of reactive oxygen species (ROS). HO-1, which is constitutively expressed at a high level in osteoclast (OC) precursors, was down-regulated during OC differentiation. HO-1 deficiency in bone marrow macrophages (BMM) in vitro resulted in increased numbers and activity of OCs due to enhanced receptor activator of nuclear factor-κB ligand (RANKL) signaling. This was associated with increased activation of nuclear factor-κB and of nuclear factor of activated T-cells, cytoplasmic 1 along with elevated levels of intracellular calcium and ROS. Decreased bone mass in the absence of HO-1 appears to be mainly due to increased osteoclastogenesis and bone resorption resulting from elevated RANKL signaling in OCs. Our data highlight the potential role of HO-1 in maintaining bone mass by negatively regulating OCs.