Altered Tryptophan Catabolism in Placentas From Women With Pre-eclampsia

Abstract

Background: The kynurenine pathway enzymes, breaking down tryptophan, are abundant in placental tissue. These metabolites are involved in immunoregulatory mechanisms, although the role of this pathway in pre-eclampsia (PE) has only begun to be characterized. Here, we determined tryptophan and metabolite levels together with the expression of kynurenine pathway enzymes and inflammatory factors in placental tissue from women with and without PE.

Methods: Thirty-six placentas (18 PE and 18 controls) were analyzed for expression of kynurenine pathway enzymes indoleamine-2,3-dioxygenase (IDO1 and 2), tryptophan-2,3-dioxygenase (TDO), kynurenine-3-mono-oxygenase (KMO) and quinolinate phosphoribosyltransferase (QPRT) as well as interleukin (IL)-1β, IL-6, and serum amyloid A (SAA). Tryptophan and kynurenine content were measured using high-pressure liquid chromatography and quinolinic acid was measured using gas chromatography-mass spectrometry.

Conclusions: Tryptophan content was reduced in placentas from women with PE. There was an increased kynurenine/tryptophan ratio in placentas from women with PE but no significant change in downstream metabolites. We confirmed a reduction in IDO1 expression and found a compensatory increase in TDO expression in placentas from women with PE. SAA was reduced in PE placentas compared with controls. Our data show that tryptophan content and the inflammatory mediator SAA are both compromised in placentas from women with PE. Further studies on the role of tryptophan catabolism and mediators of inflammation in sustaining healthy immunobiological pathways in the placenta are warranted.

Keywords: indoleamine 2,3-dioxygenase; kynurenine pathway; pre-eclampsia; serum amyloid A; tryptophan; tryptophan 2,3-dioxygenase.

Figure 1.

Schematic overview of the Kynurenine pathway, showing the main enzymes and their products. Enzymes are listed in italic font in the figure. ACMSD indicates 2-amino 3-carboxymuconate-6-semialdehyde decarboxylase; 3-HAO, 3-hydroxyanthranilate 3,4-dioxygenase; IDO, indoleamine 2,3-dioxygenase; IDO2, indoleamine 2,3-dioxygenase 2; KATs, kynurenine aminotransferases; KMO, kynurenine 3-monooxygenase; NAD⁺, nicotinamide adenine dinucleotide; QPRT, quinolinate-phosphoribosyltransferase; TDO, tryptophan 2,3-dioxygenase.

Figure 2.

(A) Tryptophan content was reduced in placentas from women with PE compared with healthy controls (nmol/gram tissue, mean ± 2SEM). (B) mRNA expression of SAA was significantly decreased in placentas from women with PE compared with controls (AU, as defined below, median ± confidence interval). (C) The mRNA expression of IDO1 was reduced in placentas from women with PE compared with controls (AU, mean ± 2SEM). (D) TDO expression was increased in the placentas from women with PE vs the healthy controls (AU, mean ± 2SEM). All mRNA data were analyzed via the comparative threshold cycle method as previously described (see Methods); where the relative mRNA expression in arbitrary units (AU) was obtained by normalization against the expression of the housekeeping gene (GAPDH) in each sample. All data shown in (A) to (D) are raw values and the significance level indicated in the figures is based on the statistical analysis, weighted regression as described in methods. IDO indicates indoleamine-2,3-dioxygenase; PE, pre-eclampsia; SAA, serum amyloid A, TDO, tryptophan 2,3-dioxygenase. *P < .05, **P < .01, ***P < .005.

Figure 3.

(A) The mRNA expression of SAA in healthy placental tissue correlates significantly with the activity of the first step of the kynurenine pathway, as indicated by the KYN/TRP ratio (Pearson’s R = 0.63, P < .005). (B) There was no association between the mRNA expression of SAA and the kynurenine pathway activity in placentas from women with PE. KYN/TRP indicates kynurenine/tryptophan; PE, pre-eclampsia; SAA, serum amyloid A.