Involvement of Heat Shock Protein 47 in Osteophyte Formation of Knee Joint Osteoarthritis

Osteophytes contribute to the development and progression of knee osteoarthritis (OA). Although transforming growth factor-β (TGF-β) and bone morphogenic protein-2 (BMP2) are known to induce osteophytes, regulators of osteophyte formation remain elusive. This study aimed to search for molecules that modulate osteophytosis in a mouse knee OA model. Proteomic analysis, followed by immunohistochemistry of osteophyte and articular cartilage, identified heat shock protein 47 (HSP47), a molecular chaperone for procollagens, as a molecule selectively overexpressed by osteophyte fibrocartilaginous cells. The treatment of TGF-β3 and BMP2 to a three-dimensional pellet culture of mouse mesenchymal stem cells caused their differentiation into osteophyte-like cells accompanied with the up-regulation of HSP47. The pellet sizes of stimulated three-dimensional-cultured mesenchymal stem cells were significantly reduced by knockdown of HSP47 or treatment with AK778 (HSP47 inhibitor), because of increased apoptosis. Furthermore, intra-articular AK778 injections suppressed osteophyte formation in a mouse OA model. Importantly, the studies with human samples demonstrated HSP47 overexpression by osteophyte fibrocartilaginous cells in human OA knee joints. Similarly, the overexpression of HSP47 was observed in the TGF-β3- and BMP2-treated human osteophytic cell spheroids as well as the size reduction of spheroids by AK778 treatment. These findings highlight the promoting function of HSP47 in osteophyte formation in OA knee joints and suggest that therapeutic interventions targeting HSP47 may be of clinical value.