Modulatory effects of fibronectin on calcium oxalate crystallization, growth, aggregation, adhesion on renal tubular cells, and invasion through extracellular matrix.

Fibronectin, an extracellular matrix (ECM) protein, has been thought to be involved in pathogenic mechanisms of kidney stone disease, especially calcium oxalate (CaOx) type. Nevertheless, its precise roles in modulation of CaOx crystal remained unclear. We thus performed a systematic evaluation of effects of fibronectin on CaOx monohydrate (COM) crystal (the major causative chemical crystal in kidney stone formation) in various stages of kidney stone pathogenesis, including crystallization, crystal growth, aggregation, adhesion onto renal tubular cells, and invasion through ECM in renal interstitium. The data showed that fibronectin significantly decreased crystallization, growth and adhesive capability of COM crystals in a dose-dependent manner. In contrast, COM crystal aggregation and invasion through ECM migration chamber were significantly enhanced by fibronectin in a dose-dependent fashion. Sequence analysis revealed three calcium-binding and six oxalate-binding domains in fibronectin. Immunofluorescence study confirmed binding of fibronectin to COM crystals. Additionally, calcium- and oxalate-affinity assays confirmed depletion of both calcium and oxalate ions after incubation with fibronectin. Moreover, calcium-saturated and oxalate-saturated forms of fibronectin markedly reduced the modulatory activities of fibronectin on COM crystallization, crystal growth, aggregation, and adhesion onto the cells. These data strongly indicate the dual functions of fibronectin, which serves as an inhibitor for COM crystallization, crystal growth and adhesion onto renal tubular cells, but on the other hand, acts as a promoter for COM crystal aggregation and invasion through ECM. Finally, its COM crystal modulatory activities are most likely mediated through binding with calcium and oxalate ions on the crystals and in their environment.