Identification of soluble 14-3-3∊ as a novel subchondral bone mediator involved in cartilage degradation in osteoarthritis.

OBJECTIVE: Mechanical stress plays an important role in cartilage degradation and subchondral bone remodeling in osteoarthritis (OA). The remodeling of the subchondral bone could initiate cartilage loss in OA through the interplay of bone and cartilage. The aim of this study was to identify soluble mediators released by loaded osteoblasts/osteocytes that could induce the release of catabolic factors by chondrocytes.

METHODS: Murine osteoblasts/osteocytes were subjected to cyclic compression, and then conditioned medium from either compressed (CCM) or uncompressed (UCM) cells was used to stimulate mouse chondrocytes. Chondrocyte expression of matrix metalloproteinase 3 (MMP-3), MMP-13, type II collagen, and aggrecan was assessed by reverse transcription-polymerase chain reaction, Western blotting, and enzyme-linked immunosorbent assay. Soluble mediators released by compressed osteoblasts/osteocytes were identified using iTRAQ (isobaric tags for relative and absolute quantification), a differential secretome analysis. Subchondral bone and cartilage samples were isolated from OA patients, and culture medium conditioned with OA subchondral bone or cartilage was used to stimulate human chondrocytes.

RESULTS: Stimulation of mouse chondrocytes with CCM strongly induced the messenger RNA (mRNA) expression and protein release of MMP-3 and MMP-13 and inhibited the mRNA expression of type II collagen and aggrecan. Differential secretome analysis revealed that 10 proteins were up-regulated in compressed osteoblasts/osteocytes. Among them, soluble 14-3-3∊ (s14-3-3∊) dose-dependently induced the release of catabolic factors by chondrocytes, mimicking the effects of cell compression. Addition of a 14-3-3∊ blocking antibody greatly attenuated the CCM-mediated induction of MMP-3 and MMP-13 expression. Furthermore, in human OA subchondral bone, s14-3-3∊ was strongly released, and in cultures of human OA chondrocytes, s14-3-3∊ stimulated MMP-3 expression.

CONCLUSION: The results of this study identify s14-3-3∊ as a novel soluble mediator critical in the communication between subchondral bone and cartilage in OA. Thus, s14-3-3∊ may be a potential target for future therapeutic or prognostic applications in OA.