Soluble Epoxide Hydrolase Inhibition Regulates Septoclast Activity to Promote Long Bone Growth and Fracture Healing by Enhancing Endothelial-to-Mesenchymal Notch Signalling.

Endochondral ossification is essential for the development of appendicular bones, physiological bone remodelling and fracture healing. Recent studies have identified mesenchymal stromal cell-derived FABP5+ septoclasts (SCs) as key mediators for the growth and repair of long bones, particularly in cartilage matrix degradation and growth plate remodelling via the secretion of matrix metalloproteinases. Our previous study has shown that soluble epoxide hydrolase (sEH) inhibitor, 1-trifluoromethoxyphenyl-3-(1-propionylpiperidin-4-yl) urea (TPPU), promotes long bone growth and bone repair by enhancing H-type vessel-coupled osteogenesis. However, whether TPPU treatment regulates SC activity, thereby promoting long-bone growth and fracture healing, remains unclear. Here, our in vitro and in vivo results showed that TPPU treatment promoted long-bone growth in newborn mice and regulated the hypertrophic layer in the growth plate, with a reduced ratio of hypertrophic cartilage (HC) to proliferative cartilage (PC) width. Furthermore, TPPU treatment enhanced SC activity, as evidenced by elevated expression of MMP9 and FABP5 in the metaphysis near the growth plate. Simultaneously, TPPU induced FABP5+ SC-like cells to degrade chondrocytes in co-cultured human umbilical vein endothelial cells (HUVECs) and human dental pulp stem cells (hDPSCs). Mechanistically, TPPU enhanced the crosstalk of co-cultured HUVECs and hDPSCs to activate the NOTCH signalling pathway in hDPSCs by upregulating HIF-1α expression in HUVECs. Furthermore, TPPU enhanced fracture healing by inducing more FABP5+ SCs and MMP9 secretion at the fracture site. Collectively, these findings highlight sEH as a promising therapeutic target that regulates endochondral ossification through inducing SC activity, offering new opportunities for bone development and repair.