genomewide characterization of host-pathogen interactions by ...

Maren Depke
Discussion and Conclusions
Three other induced genes, kynreninase (Kynu), inducible nitric oxide synthase 2 (Nos2), and
tryptophanyl-tRNA synthetase (Wars), found attention. These are closely linked to inflammation
and interferon action.
Kynureninase was reported to be induced in immortalized murine macrophages (cell line
MT2) by IFN-γ, which was interpreted as an activation of the kynurenine pathway leading to the
formation of quinolinic acid from tryptophan since the cells also induced indoleamine 2,3-
dioxygenase (IDO), which catalyzes the first step of tryptophan degradation (Alberati-Giani et al.
1996). Interestingly, BMM after IFN-γ treatment (this study) did not differentially express
indoleamine 2,3-dioxygenase (IDO) which is known to be induced by interferons in many cell
types (Carlin et al. 1989, Taylor MW/Feng 1991, Alberati-Giani et al. 1996). Even more,
expression of Ido1 and Ido2 was low and even absent in several samples. In literature references,
a lack of inducible IDO activity by IFN-γ was observed in murine peritoneal macrophages. These
cells were able to kill pathogens, and the antimicrobial effect was not diminished by addition of
tryptophan like it was observed for monocyte-derived macrophages (Murray HW et al. 1989).
It was reported that the immune defense mechanisms of IDO/tryptophan degradation and
nitric oxide synthase (NOS)/arginine degradation influence each other. Nitric oxide synthase
catalyzes the first, rate-limiting step of arginine degradation to NO and citrulline
(Knowles/Moncada 1994). NO decreases IDO activity in human mononuclear phagocytes by
interfering with the heme iron located at the active site of IDO enzyme (Thomas et al. 1993).
Experiments gave hints for species-specific reactions to IFN-γ since human macrophages induced
IDO and murine macrophages NOS. Nevertheless, in presence of inhibitors of NO synthesis also
murine macrophages exhibited IDO activity (Thomas et al. 1993). NOS inhibitors in combination
with IFN-γ further increased the induction of IDO mRNA in murine macrophages MT2, which was
already induced after IFN-γ treatment alone (Alberati-Giani et al. 1997). Influence of NO on IDO
mRNA could not be detected in the human epithelial cell line RT4, but NO led to accelerated
proteosomal degradation of IDO protein. The authors stated that the transcriptional regulation of
IDO might be species or cell type specific and needs to be furthermore analyzed (Hucke et al.
2004). Not only NO and reactive nitrogen species influence IDO, but there are also hints for
influences in the opposite direction. The intermediate 3-hydroxyanthranilic acid from the
kynurenine pathway of tryptophan degradation was shown to inhibit the enzymatic activity of
iNOS and the iNOS mRNA induction by IFN-γ and LPS in mouse macrophage RAW 264.7 cells. The
decrease in iNOS mRNA level resulted from inhibition of NFκB activation (Sekkaï et al. 1997).
Even when there are still open questions concerning the interplay between IDO and iNOS, and
even when details in their regulation might differ between mouse and human and between the
different cell types, many information support the proposition that in each physiological situation
a cellular decision towards one of the two antimicrobial mechanisms has to be taken since each
mechanism compromises the other. Fittingly, during the treatment of BMM with IFN-γ the
induction of Nos2, inducible nitric oxide synthase 2, was observed in parallel to the lacking
induction/expression of IDO (this study).
Wars is known to be regulated by IFN-γ (Fleckner et al. 1991, Buwitt et al. 1992, Bange et al.
1992, Rubin et al. 1991). The biological function of the induction of this metabolic enzyme in
inflammatory situations is discussed in different ways: 1) protection of the host from self-induced
tryptophan starvation: In an environment of tryptophan degradation, the amino acid would be
saved from IDO activity when complexed to tRNA, not accessible for pathogens, but still available
for host protein synthesis (Boasso et al. 2005, Murray MF 2003). 2) compensation for increased
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