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.