Calcium oxalate monohydrate crystals are endocytosed by renal epithelial cells and induce proliferation

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Abstract
Monkey kidney epithelial cells of the nontransformed BSC-1 line were used as a model system with which to search for biological responses to urinary crystals commonly found in renal stones. Calcium oxalate monohydrate (COM), the most common urinary crystal, was avidly internalized, initiated DNA synthesis, and stimulated cell multiplication. The increase in DNA synthesis observed after exposure to COM crystals was equivalent in magnitude to that of 10% calf serum, but occurred 8 h later. Maximal stimulation of DNA synthesis by COM was associated with crystal endocytosis by 50% of the cell monolayer. COM crystals also stimulated DNA synthesis and multiplication of canine kidney epithelial cells (MDCK line). As COM stimulated growth of both monkey and canine renal cells but not fibroblasts, the mitogenic effect of this crystal appeared cell-type specific. Hydroxyapatite also enhanced multiplication of BSC-1 cells, whereas brushite, another calcium-containing urinary crystal, did not. In the presence of nephrocalcin (NC), a glycoprotein in normal human urine that inhibits nucleation, aggregation, and growth of COM crystals, the capacity of these crystals to initiate DNA synthesis was blocked. This is the first demonstration that specific calcium-containing urinary crystals can induce proliferation of renal epithelial cells and that NC can inhibit this effect.

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