Background: Gout is the most common form of inflammatory arthritis. It is characterised by the deposition of monosodium urate (MSU) crystals within joints. MSU crystals are found within subchondral bone. Previous research has determined that patients with advanced gout have enhanced osteoclast-mediated bone resorption and impaired osteoblast-mediated bone formation, leading to focal bone erosion. The osteocyte has been shown to be a master regulator of bone remodelling. The aim of this study was to investigate the effects of MSU crystals on osteocyte-like cell viability.
Methods: MSU crystals were prepared by recrystallisation of uric acid. MLO-Y4 osteocyte-like cells were cultured on plastic (2D) or in 3 mg/ml type I collagen gels and cultured with 0.01-0.5 mg/ml MSU crystals or soluble uric acid for 24 hours (day 0). Cells were then washed and MSU crystals or uric acid completely removed. Cell viability was assessed 24 hours and 48 hours after the addition of MSU crystals using MTT and alamarBlue assays.
Results: In 2D cultures, 0.1-0.5 mg/ml MSU crystals significantly reduced MLO-Y4 cell viability by ~70% 24 hours after the addition of MSU crystals (P<0.05). In collagen gels, higher concentrations of MSU crystals (0.3-0.5 mg/ml) reduced MLO-Y4 cell viability by ~30-40% 24 hours after the addition of MSU crystals (P<0.01); whereas lower concentrations of MSU crystals (0.01-0.1 mg/ml) had no effect (P>0.05). However, 48 hours after the addition of MSU crystals, culture with the higher concentrations of MSU crystals (0.3-0.5 mg/ml) resulted in a 100% reduction of MLO-Y4 cell viability (P<0.01). The inhibitory effect on cell viability was specific to MSU crystals, as soluble uric acid did not reduce viability.
Conclusion: These results indicate that MSU crystals are toxic to osteocyte-like cells and that direct crystal-cell interactions are not necessarily required to reduce osteocyte viability. The interactions between MSU crystals and osteocytes may contribute to bone erosion in gout.
Disclosure: The authors declared no competing interests. This work was supported by a University of Auckland FRDF grant (9801/3704255).