Interstitial cystitis (IC) involves a chronic inflammation of the bladder, which is characterized by pelvic pain and urinary symptoms like frequency and urgency. Though the etiology of IC is not yet known completely, the involvement of mast cells in IC is known.  Tryptophan is metabolized to kynurenine with indoleamine-2,3-dioxygenase (IDO), and this pathway is called the kynurenine pathway. Recently, it has been reported that kynurenine activates mast cells through the aryl hydrocarbon receptor (AhR) [1]. However, the relationship between kynurenine/AhR signaling and IC has not been studied. Thus, we examined the role of the kynurenine/AhR signaling in the pathology of IC using a rat model of cyclophosphamide (CYP)-induced cystitis.

We used 10- to 11-week-old female Wistar/ST rats. First, we examined whether kynurenine pathway was activated in the bladder of cystitis rats. Cystitis was induced by intraperitoneal injection of CYP (150 mg/kg body weight). Control rats were administered with the same volume of saline. At 24 hours after injection, we harvested the bladders and used them for tissue evaluation. We measured the mRNA expression levels of IDO1 in the bladders using real-time polymerase chain reaction and the tryptophan and kynurenine contents of the bladders using ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). Next, we evaluated the effect of 1-methyl-L-tryptophan (MT), an inhibitor of IDO, on cystitis rats. The rats were assigned to four groups: control+vehicle (n=5), control+MT (n=8), CYP+vehicle (n=4), and CYP+MT (n=5). We administered MT to rats in the control+MT group and CYP+MT group, perorally (100 mg/kg/day) for five days, until the injection of CYP or saline. We used hydroxypropyl methylcellulose as the vehicle. At 24 hours after injection, we performed conscious cystometry (80 μL/min). Then, we used other rats to measure the mRNA expression levels of AhR in the bladders (n=3). We used the Student’s t-test for the statistical analysis of the 2 groups and ANOVA and Bonferroni-type multiple t-tests for the analysis of 4 groups.

In the cystitis models, the mRNA expression level of IDO1 in the bladder was significantly higher than that in the control rats (p < 0.01). The kynurenine levels in the urinary tissues from the cystitis rats were also obviously higher than those in the urinary tissues from the control rats (Figure 1). Intercontraction intervals were smaller in the CYP+vehicle group than in the in control+vehicle group. Intercontraction intervals in the CYP+MT group were longer than those in the CYP+vehicle group. Intercontraction intervals in control+MT were slightly longer than those in control+vehicle group (Figure 2AB). There were no differences  in maximum voiding pressure among the four groups. The mRNA expression level of AhR in the CYP+vehicle group was more than that in the control+vehicle group. The mRNA expression level of AhR in CYP+MT was lower than that in the CYP+MT group and similar to that in the control+vehicle group . There were no differences between the control+vehicle and control+MT groups (Figure 2C).

Shortening of intercontraction intervals is known to be one of the symptoms of IC. In this study, these symptoms were observed as in case of the existing reports. In the bladders of the CYP-induced cystitis models, the mRNA expression level of IDO1 was significantly higher than that in the control rats. Similarly, the kynurenine content was higher in the cystitis models than that in the control rats. These data suggested that tryptophan metabolism in kynurenine pathway in the bladder was promoted by CYP-induced cystitis. We then evaluated the effect of MT, an inhibitor of IDO1, on cystitis. Our results suggest that the inhibition of the kynurenine pathway improved the micturition intervals of cystitis rats. Moreover, the mRNA expression level of AhR was higher in the bladders of cystitis rats and normalized by administration of MT. Therefore, kynurenine/AhR signaling might be involved in the symptoms of IC.

In this study, we found that the kynurenine pathway was enhanced in the bladders of cystitis rats, and inhibition of the kynurenine pathway improved the urinary symptoms. Moreover, the kynurenine/AhR signaling was activated in the bladders of cystitis rats and suppressed by MT. Our study suggests that the kynurenine/AhR signaling might be a new target for treatment and prevention of IC.

Wang H, Do DC, Liu J, Wang B, Qu J, Ke X, Luo X, Tang HM, Tang HL, Hu C, Anderson ME, Liu Z, Gao P. Functional role of kynurenine and aryl hydrocarbon receptor axis in chronic rhinosinusitis with nasal polyps. J Allergy Clin Immunol. 2018 Feb;141(2):586-600.