Cyto-injury factors in urine: a possible mechanism for the development of interstitial cystitis

PURPOSE: In most of our patients with interstitial cystitis (IC), the disease is associated with an increased urothelial permeability whose cause has not been identified. We postulate that both normal urine and the urine of IC patients contains factors capable of injuring the mucosa and causing an increased permeability that would allow urine components to leak into the bladder muscle. To test this hypothesis, we examined fractions of normal urine for toxic effects on bladder smooth muscle and epithelial cells in vitro. In the same in vitro system, we measured the effects of Tamm-Horsfall protein (THP), a normal urinary glycoprotein that may be a scavenger of injurious agents capable of "detoxifying" normal metabolic products. MATERIALS AND METHODS: Human urothelial cells (T24) and rabbit bladder smooth muscle cells were incubated overnight with various fractions prepared from healthy volunteers' urine. The urine fractions of molecular weights >100 Da were incubated overnight with either urothelial or smooth muscle target cells after no treatment or after heating to 56C, preincubation with THP, exposure to heparin, or elution from heparin. Cytotoxicity was determined for each group using a neutral red uptake assay. RESULTS: Urine fractions of molecular weight 500 to 1000 Da were cytotoxic to smooth muscle cells (39%) and urothelial cells (50%). Cytotoxicity levels for THP-treated fractions were significantly lower than those for untreated fractions in both urothelial cells (7% versus 89%, p <0. 001) and smooth muscle cells (8% versus 70%, p <0.01). Fractions exposed to heparin were less cytotoxic to smooth muscle cells (20%) than were untreated fractions (27%). Fractions eluted from heparin were also cytotoxic to urothelial cells (42%). CONCLUSIONS: Normal human urine contains heat labile, cationic components of low molecular weight that bind to heparin. These components, when separated from the bulk of the urinary wastes, are cytotoxic to urothelial cells as well as underlying smooth muscle cells, indicating their potential for causing bladder mucosal injury. The cytotoxic activity can be blocked by the presence of THP. This urinary cytoprotective activity of THP may play an important but unrecognized role in the development of IC.