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 disea se is associated with an increased urothelial permeability whose cause has not been identified. We postulate that both normal urine and the urine of I C patients contains factors capable of injuring the mucosa and causing an i ncreased permeability that would allow urine components to leak into the bl adder muscle. To test this hypothesis, we examined fractions of normal urin e 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 prot ein (THP), a normal urinary glycoprotein that may be a scavenger of injurio us agents capable of "detoxifying" normal metabolic products. Materials and Methods: Human urothelial cells (T24) and rabbit bladder smoo th muscle cells were incubated overnight with various fractions prepared fr om 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 f or 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 level s for THP-treated fractions were significantly lower than those for untreat ed 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 le ss 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 o f low molecular weight that bind to heparin. These components, when separat ed from the bulk of the urinary wastes, are cytotoxic to urothelial cells a s well as underlying smooth muscle cells, indicating their potential for ca using bladder mucosal injury. The cytotoxic activity can be blocked by the presence of THP. This urinary cytoprotective activity of THP may play an im portant but unrecognized role in the development of IC.