Urinary macromolecular inhibition of crystal adhesion to renal epithelial cells is impaired in male stone formers

Background. Retention of microcrystals that form in tubular fluid could be a critical event in kidney stone formation. This study was performed to determine if urinary macromolecules from stone-forming (SF) individuals have reduced ability to inhibit crystal adhesion to renal cells. Methods. A first morning whole urine (WU) sample was obtained from 24 SF subjects (17 males and 7 females) and 24 age-, race-, and sex-matched controls (C). An aliquot of urine was centrifuged and an ultrafiltrate (UF) free of macromolecules >10 kD and 10x concentrate (U_conc) were prepared. Results. Supplementing UF with increasing amounts of U_conc to return the macromolecule concentration to 0.25x, 0.5x, or 1 x of baseline progressively decreased crystal binding to cells. This effect was blunted in the male SF group compared to controls (P < 0.05, SF vs. C, for UF plus 0.25 x macromolecules). No difference was apparent in the female groups. In order to identify responsible macromolecule(s), calcium oxalate mono-hydrate (COM) crystals were coated with U_conc and adherent proteins then released and probed by Western blot. Coated COM crystals from male controls contained 3.5-fold more Tamm-Horsfall protein (THP) than SF subjects (P < 0.01). COM crystal coating with other proteins did not consistently differ between the groups. COM crystal coating by urinary prothrombin fragment 1 (UPTF1, P < 0.05) and crystal adhesion inhibitor (CAI) (P = 0.09) correlated with decreased crystal binding to cells, whereas coating with osteopontin (OPN) correlated with increased adhesion tendency (P < 0.05). Conclusion. Urinary macromolecules >10 kD coat COM crystals and block their adhesion to renal cells. This capacity appears to be blunted in male but not female SF individuals. Multiple urinary proteins may play a role in renal cell-urinary crystal interactions, and THP appears to be one of the more important ones.