Background: Gout is caused by the interactions between monosodium urate (MSU) crystals and immune cells. Bone erosion in advanced gout is associated with tophi; lesions comprising of inflammatory cells surrounding collections of MSU crystals, often observed in subchondral bone. This study investigated the direct effects of MSU crystals and indirect effects of MSU crystal-induced inflammation on osteocyte gene expression in vitro.
Methods: For the direct assays, MSU crystals (0.1mg/mL) were added to MLO-Y4 osteocyte-like cells for 1h, 6h and 24h. For the indirect assays, RAW264.7 macrophage-like cells were cultured with MSU crystals (0.5mg/mL) for 24h and conditioned media harvested and filtered. The MSU crystal-exposed conditioned media or conditioned media from untreated RAW264.7 cells (control) was added to MLO-Y4 cultures (40%) for 1h, 6h and 24h. Changes in MLO-Y4 gene expression were examined using real-time PCR. The relationship between osteocytes, MSU crystals and macrophages in erosive gout was examined by polarizing light microscopy and CD68 immunohistochemistry in joint samples obtained from cadaveric donors with crystal-proven gout.
Results: In direct assays, MSU crystals alone did not change E11, connexin43, ORP150, osteocalcin, RANKL or OPG expression in MLO-Y4 cells. Expression of inflammatory mediators was also unchanged. In contrast, addition of conditioned media from MSU crystal-exposed RAW264.7 cells increased E11, connexin43 and RANKL expression, and reduced OPG expression in MLO-Y4 cells. Upregulated expression of inflammatory genes IL-1β, IL-6, IL-8, IL-11, TNF-α and cyclooxygenase-2 was also observed. In histological analysis of joint samples affected by gout, numerous CD68+ macrophages and MSU crystals were identified in proximity to osteocytes within bone.
Conclusions: MSU crystals do not directly affect osteocyte-related gene expression. However, inflammation resulting from interactions between MSU crystals and immune cells may increase osteocyte expression of pro-resorptive and inflammatory mediators, which may affect local bone remodeling in joints affected by gout.