Upregulated GDF‐15 in Ankylosing Spondylitis: Pathologic Ossification Promotion through Ligament Fibroblast Osteogenic Differentiation

Objective To investigate the role of growth differentiation factor 15 (GDF‐15) in ankylosing spondylitis (AS). Methods A proteoglycan‐induced arthritis mouse model was established to mimic the pathologic changes of AS. Expression of GDF‐15 in serum and the edge of the vertebral cartilage plate and the spinal entheseal sites in the model group was detected by enzyme‐linked immunosorbent assay and immunohistochemistry. Then primary fibroblasts from the ligaments of patients with femoral neck fracture (healthy controls [HC]) and patients with AS were extracted. Western blotting, alkaline phosphatase (ALP) staining and activity assay, and alizarin red staining were used to detect the osteogenic ability of primary fibroblasts. Results Expression of GDF‐15 was up‐regulated in serum and the edge of the vertebral cartilage plate and the spinal entheseal sites in the model group. In vitro, our results showed that GDF‐15 promoted the osteogenic differentiation of HC and AS fibroblasts. Furthermore, our results showed that in AS fibroblasts, GDF‐15 elevated the expression of osteogenic marker genes ( SP7 , RUNX2 , and COL1 ) as well as p‐glycogen synthase kinase 3β and β‐catenin, which are involved in the Wnt/β‐catenin signaling pathway. This effect of GDF‐15 in AS fibroblasts could be reversed by the inhibitor of the Wnt/β‐catenin signaling pathway, DKK‐1, suggesting that GDF‐15 promoted the osteogenic differentiation of AS fibroblasts via the Wnt/β‐catenin signaling pathway. Furthermore, knockdown of GDF‐15 also suppressed osteogenic differentiation and inhibited Wnt/β‐catenin signaling in AS fibroblasts. Conclusion This study revealed aberrant up‐regulation of GDF‐15 in an AS mouse model and osteogenic effect of GDF‐15 in AS fibroblasts via the Wnt/β‐catenin signaling pathway, which may be one of the mechanisms and therapeutic targets of new bone formation in AS.