TWEAK and Fn14 expression in osteoarthritis: evidence for TWEAK-induced RANKL release from chondrocytes
ObjectiveA role for TWEAK expression and that of its receptor Fn14 by osteoblasts and synovial tissue has been proposed in the pathogenesis of osteoarthritis (OA). Here, we examined whether the cartilage in OA was also a source and target of TWEAK. DesignArticular cartilage samples from 24 patients undergoing hip or knee replacement surgery for OA were investigated for TWEAK and Fn14 expression by both immunostaining and real-time RT-PCR. Samples were graded according to the Pritzker scale from grade 0 (normal) through increasing levels of cartilage damage (grades 1-3). Human primary chondrocytes isolated from OA cartilage were treated with combinations of recombinant TWEAK and TNF and examined for acute effects on TWEAK, FN14, RANKL and ADAM17 mRNA levels. Released soluble RANKL levels were measured by ELISA. ResultsImmunostaining indicated that the majority (21/24) of OA cartilage samples expressed low levels of TWEAK protein and high levels of Fn14, however, expression did not appear to vary with respect to OA grade. TWEAK mRNA levels were elevated in grade 2 OA cartilage compared to grade 0 (p < 0.05), while Fn14 mRNA levels appeared elevated across all damage grades, significantly in grade 2 samples. Primary chondrocytes treated with TNF, alone or in combination with TWEAK, exhibited early upregulated Fn14 mRNA expression. TWEAK induced RANKL mRNA expression. while both TWEAK and TNF induced the mRNA expression of the RANKL sheddase, ADAM17. Consistent with these findings, TWEAK and TNF both induced soluble RANKL release by chondrocytes. ConclusionsThe expression and regulation of TWEAK and Fn14 is a feature of human OA articular cartilage, suggesting a pathogenetic role. TNF may prime chondrocytes for TWEAK reactivity by upregulating Fn14 expression. The TWEAK and TNF induction of ADAM17 expression and sRANKL release by chondrocytes points to a further disease pathway in human OA.