Fabkin Promoted Osteoclasts Mature and Bone Loss in OVX-Induced Osteoporosis Mice

Osteoporosis (OP) is a chronic skeletal disorder characterized by reduced bone mineral density and increased fracture risk, particularly from hip fractures linked to a 20%-24% increased mortality risk within the first year. Current therapies remained inadequate in addressing metabolic drivers, which prompted exploration of novel targets. Fabkin, a recently identified hormonal complex comprising fatty acid-binding protein 4 (FABP4), nucleoside diphosphate kinase (NDPK), and adenosine kinase (ADK), was implicated in metabolic and inflammatory signaling. It was suggested that Fabkin could promote Ca²⁺ influx and the release of inflammatory factors via the PKA pathway. However, its role in bone loss remained unclear. To investigate the role of Fabkin in OP, an ovariectomy (OVX)-induced murine model was used to simulate estrogen-deficient osteoporosis. FABP4-KO mice were generated to disrupt Fabkin formation, and bone microarchitecture was assessed using micro-CT and histological staining. Osteoclastogenesis assays in vitro were conducted using bone marrow-derived macrophages (BMDMs) treated with recombinant Fabkin components. Western blotting and RT-qPCR were performed to analyze key signaling pathways involved in osteoclast differentiation, particularly MAPK and NF-κB. Immunofluorescence analysis revealed an increase in Fabkin expression in the bone marrow of OVX mice. OVX-induced osteoporosis was significantly attenuated in FABP4-KO mice with higher BMD. TRAP staining showed a reduction in osteoclast numbers in FABP4-KO mice. In vitro, Fabkin treatment significantly enhanced osteoclast differentiation and bone resorption, whereas FABP4 deficiency inhibited osteoclast formation. Molecular analysis revealed that Fabkin promoted osteoclastogenesis via the RANKL-induced MAPK and NF-κB signaling pathways. These findings suggested that FABP4 might directly exacerbate osteoclastogenesis by acting as a Fabkin complex instead of a lipid regulator.