EXPRESSION OF OSTEOPONTIN, A URINARY INHIBITOR OF STONE MINERAL CRYSTAL-GROWTH, IN RAT-KIDNEY

Cultured mouse kidney cortical cells secrete osteopontin, a bone matri x protein that is also found in urine. Osteopontin is associated with cell proliferation/tumerogenesis and also inhibits kidney stone minera l crystal growth [1]. Using antibodies raised against osteopontin isol ated from the culture medium, we localized osteopontin in normal rat k idney. Fluorescence, confocal and electron microscopy revealed osteopo ntin primarily in cells of the descending thin limb of the loop of Hen le (DTL) and in papillary surface epithelium (PSE) in the area of the calyceal fornix. In situ hybridization with labeled RNA made from a cD NA that contains the entire coding sequence for mouse osteopontin reve aled message at the same sites at which protein was demonstrated by im munocytochemistry. Immunogold labeling was localized to a population o f dense vesicles distinct from lysosomes and endosomes. To examine the turnover of osteopontin, rats were injected with the protein synthesi s inhibitor cyclohexamide, 14 mg/kg, six hours prior to kidney fixatio n. These kidneys no longer demonstrated osteopontin in DTL and the imm unofluorescence in the papillary surface was attenuated. Thus, osteopo ntin is secreted at two sites in the kidney where urine is highly conc entrated in stone mineral constituents. It has a relatively rapid turn over, suggesting that it could be subject to physiological regulation. Osteopontin may be important in the normal endogenous defense against kidney stone formation.