Fibulin-4 reduces extracellular matrix production and suppresses chondrocyte differentiation via DKK1- mediated canonical Wnt/β-catenin signaling

Fibulin-4 is an extracellular matrix (ECM) protein implicated in connective tissue development and elastic fiber formation. However, little is known about the underlying function of Fibulin-4 in cartilage development. The aim of this study was to investigate the role and probable mechanism of Fibulin-4 stimulation in ECM production and chondrocyte differentiation. Fibulin-4 has been observed to be abnormally elevated and matrix metalloproteinases 13 (MMP13) level was highly expressed in human osteoarthritis (OA) chondrocytes. In the chondrogenic cell line ATDC5, Fibulin-4 stimulation profoundly inhibited the expression of ECM gene type II collagen (Col2a1), aggrecan (Acan), and type X collagen (Col10a1). Overexpression of Fibulin-4 attenuated the expression of master transcription factors Sox6, Sox9 and Runx2, but had no effect on the expression of Sox5. Additionally, Fibulin-4 stimulation activated canonical Wnt pathway by reducing the expression of Wnt inhibitor DKK1 but not Sost. Moreover, Fibulin-4 augmented the expression of Wnt/β-catenin signaling target genes like β-catenin and Wnt-3a as well as diminished GSK-3β activation, and DKK1 abolished the effect of Fibulin-4 on chondrocyte differentiation, suggesting that Fibulin-4 is an important regulator of ECM production and chondrocyte differentiation through DKK1-mediated Wnt/β-catenin signaling. Our study provides evidence of a previously unknown link between Fibulin-4 and the canonical Wnt/β-catenin pathway that may contribute to our understanding of the molecular mechanisms of OA.