Transforming growth factor-β1 promotes initiation of entheseal pathological bone formation via osteonectin induction in ankylosing spondylitis.

Ankylosing spondylitis (AS) is an inflammatory disease marked by aberrant new bone formation and spinal ankylosis. Transforming growth factor-β1 (TGFβ1) is implicated in osteogenesis, yet its downstream effectors in AS remain unclear. Osteonectin, a glycoprotein secreted by osteoblasts, plays a critical role in bone formation with TGFβ1, but has not been studied in AS. This study aimed to investigate the TGFβ1-osteonectin mediated regulation of osteogenic activity in AS. Serum levels of TGFβ1, osteonectin, and osteocalcin were measured in AS patients and healthy controls. Measured these serum levels were compared with the modified Stoke Ankylosing Spondylitis Spinal Score (mSASSS). Enthesis cells isolated from AS spinal tissues were stimulated with recombinant TGFβ1 in vitro. Proliferation, matrix production, migration, and clonogenicity were evaluated. Immunohistochemical staining of spinal tissues was performed to assess osteonectin expression. AS patients showed significantly elevated serum levels of TGFβ1 and osteonectin compared to healthy controls. Osteonectin expression, but not osteocalcin, was significantly correlated with serum level of TGFβ1. TGFβ1 stimulation enhanced proliferation, collagen deposition, migratory capacity, and colony formation in enthesis cells derived from AS patients. Immunohistochemistry revealed increase of osteonectin expression in periosteal regions of AS tissues. These findings suggest that osteonectin is a TGFβ1-responsive effector involved in early osteoprogenitor activation and may contribute to pathological bone formation in AS. Osteonectin represents a potential biomarker and therapeutic target for early structural progression in AS.