POS-431 ALTERED REGULATION OF TRYPTOPHAN METABOLISM AND ARYL HYDROCARBON RECEPTOR DISTRIBUTION IN RODENT POLYCYSTIC KIDNEYS

Abstract

Polycystic kidney disease (PKD) is an inherited disorder characterized by stimulated cell proliferation, increased fluid secretion into cysts, and interstitial inflammation in renal tissue. Many mechanisms of disease progression have been determined in both autosomal dominant and autosomal recessive PKD, but the exact signaling pathways responsible for cystogenesis remain unclear. To explore novel mechanisms of PKD progression, we evaluated altered metabolic activity and associated receptor expression and distribution in the kidneys of 4- and 20-week-old PCK rats, an orthologous model of human autosomal recessive PKD, by mass spectrometry, quantitative analysis, real-time RT-PCR and immunofluorescence. Based on mass spectrometry and quantitative analysis results, the abundances of kynurenine and kynurenic acid were significantly higher in both 4- and 20-week-old PCK kidneys compared with age-matched wild-type kidneys. Since kynurenine and kynurenic acid are endogenous agonists of aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor for activating metabolic enzymes such as those in the cytochrome P450 (CYP) superfamily, the expression of AhR was detected by real-time RT-PCR in the kidneys of wild-type and PCK rats. At 4-weeks of age with early stage PKD, the expression levels of AhR were significantly lower in PKD compared with wild-type kidneys. Intriguingly, its expression level in 20-week-old PCK kidneys with progressive stage PKD was higher compared with wild-type kidneys. Since cysts are derived from medulla in PCK kidneys, the distribution of AhR was observed by immunofluorescence mainly in renal medullary specimens. AhR is known to translocate into the nucleus upon ligand binding, and in normal kidneys at 4 weeks of age, AhR was localized to the nucleus in tubular epithelia, but not at 20 weeks of age in normal kidneys. However, in PCK kidneys at 20 weeks of age, AhR was localized in the nucleus in normal tubular epithelia and in developed cysts, while in dilated renal tubules (which develop into cysts), AhR translocated to the nucleus in PCK kidneys both at 4 and 20 weeks of age. Indolamine-2,3-dioxygenase 2 (IDO2) is an enzyme which catalyzes the conversion from tryptophan to kynurenine/kynurenic acid. In epithelial cells of dilated tubules in PCK kidneys, IDO2 was localized in the cytoplasm, suggesting that IDO2 may have a role in activating the AhR signaling pathway in cystogenesis. The current findings suggest that the progression of PKD may relate to the increased activation of tryptophan metabolism in combination with altered expression and distribution of AhR. Regulation of the kynurenine/kynurenic acid and AhR signaling pathway may have therapeutic potential against disease progression in PKD.