Inducible nitric oxide synthase in experimental urinary tract infection

Sammanfattning: Urinary tract infection (UTI) is among the most common bacterial infections in humans and the majority are caused by Escherichia coli (E. coli). Abundant evidence indicates that nitric oxide (NO) produced by the inducible NO synthase (iNOS) plays an important role in host defense against infection. This thesis examines iNOS expression in experimental UTI models. The rat and pig bladder urothelium did not express eNOS, nNOS or iNOS under normal conditions. iNOS expression in the rat bladder urothelium was, however, observed after systemic LPS treatment. In experimental bacterial UTI in mice, inflammatory cells expressed both iNOS and COX-2, but uroepithelial cells expressed only iNOS and with a later onset than observed in the inflammatory cells. Increased urinary nitrite levels in E. coli-infected mice coincided in time with the observed iNOS positive inflammatory cells. No difference in bacterial clearance or persistence was noted in bladders and kidneys of wild-type and iNOS deficient mice. When the effect of NO/peroxynitrite on bacterial viability was investigated, it was noted that a uropathogenic E. coli strain was less sensitive to NO-mediated killing than an avirulent E. coli strain. Isolated human uroepithelial cells did not express iNOS when exposed to uropathogenic E. coli or LPS. A combination of IL-1b, TNFa and IFNg was required for iNOS induction in human uroepithelial cells. The signaling pathways leading to iNOS induction depend on the activation of tyrosine kinases, JAK/STAT, PKC, p38 MAPK and NF-kB. Cytokines, but not bacteria, caused nuclear translocation and binding of NF-kB to the iNOS promoter. P fimbriated bacteria and sphingomyelinase inhibited cytokine-induced iNOS expression. Oxidative stress induced by H2O2 had more profound effects on cell viability in uroepithelial cells than NO and nitrogen species. In summary, our results demonstrate activation of iNOS in inflammatory and uroepithelial cells as part of the host defense against bacterial UTI. Wild-type and iNOS deficient mice were equally susceptible to E. coli-induced UTI. iNOS expression in uroepithelial cells was induced by cytokines but not bacteria. The failure of E. coli to cause iNOS induction may be related to insufficient translocation and binding of nuclear NF-kB. Both bacteria and uroepithelial cells were relatively resistant to NO-mediated cytotoxicity.