Naturally produced crystals obtained from kidney stones are less injurious to renal tubular epithelial cells than synthetic crystals

OBJECTIVE: To determine the differences in cell responses to synthetic and biological crystals of calcium oxalate (CaOx) and brushite MATERIALS AND METHODS: Nephrolithiasis depends on crystal retention within the kidneys, often promoted by crystal attachment to the injured renal epithelium; studies often use various crystals that might be injurious to cells and cause the exposure of crystal binding molecules on cell surfaces, thus promoting crystal attachment and retention. The synthetic crystals used in these studies might be more injurious than the biological crystals naturally produced in the kidneys and that form kidney stones. We exposed the renal epithelial cell line NRK 52E in vitro to CaOx or brushite crystals at 67 or 133 μg/cm² for 3 or 6 h. Synthetic crystals were purchased and the biocrystals were obtained by pulverizing CaOx and brushite stones. We determined the release of lactate dehydrogenase (LDH), hydrogen peroxide (H₂O₂) and 8-isoprostane (8-IP), and monocyte chemoattractant protein-1 (MCP-1), as markers of injury, oxidative stress and inflammation, respectively. Cells were also examined after trypan blue staining to determine their membrane integrity. We also examined crystals of CaOx by scanning electron microscopy both in the native state as well as after decalcification. RESULTS: Exposure to both the synthetic and biological crystals resulted in a significant increase in LDH release and trypan blue staining, as a sign of crystal-induced injury. There was increased production of H₂O₂ and 8-IP, suggesting the development of oxidative stress. In addition MCP-1 production was also significantly increased. However, the synthetic crystals caused significantly higher increases in all the indicators than the biological crystals. CONCLUSIONS: These results indicate that even though both synthetic and naturally produced biocrystals invoke a response from the renal epithelial cells, the latter are significantly less injurious and inflammatory. Exposure to low concentrations of these crystals alone might not invoke an inflammatory response, cause the uncovering of crystal binding molecules on epithelial cell surfaces, and promote crystal attachment and retention.