Lipocalin-2 expression in hypoxic murine osteocytes enhances RANKL-induced osteoclastogenesis

Osteocytes regulate bone remodeling by interacting with osteoblasts and osteoclasts. Hypoxia influences osteocyte function and has been linked to increased osteoclastogenesis in pathological conditions such as orthodontic tooth movement (OTM) and bone metabolic diseases; however, the molecular mechanisms underlying these effects remain unclear. This study aimed to identify hypoxia-responsive genes in osteocytes and investigate their effects on osteoclastogenesis. Transcriptome analysis of murine long bone osteocyte-Y4 (MLO-Y4) osteocytes cultured under hypoxia (2% O₂) revealed that lipocalin-2 ( Lcn2 ) was the most significantly upregulated gene. Real-time RT-PCR confirmed increased Lcn2 expression and an elevated Rankl /osteoprotegerin ( Opg) ratio. Primary osteocytes were purified from DMP1-Topaz mice showed same hypoxic response. Functional analysis demonstrated that Lcn2 did not directly affect osteoclast precursors. However, it enhanced osteoclastogenesis via osteocytes in co-culture experiments. Western blot analysis demonstrated that LCN2 activated the MAPK signaling pathway in osteocytes. Furthermore, immunohistochemical analysis of hypoxic osteocytes on the compression side of OTM exhibited increased LCN2 expression. These findings suggest that LCN2 is upregulated in osteocytes under hypoxia and promotes osteoclastogenesis by increasing RANKL expression. This study provides new insights into the molecular mechanisms of bone resorption under hypoxic conditions and suggests Lcn2 as a potential therapeutic target for bone metabolic diseases.