Modulation of cellular tryptophan metabolism in human fibroblasts by transforming growth factor-β: Selective inhibition of indoleamine 2,3-dioxygenase and tryptophanyl-tRNA synthetase gene expression

Abstract

Alterations in the rate of cellular tryptophan metabolism are involved in mediating important biological activities associated with cytokines and growth factors. Indoleamine 2,3-dioxygenase (IDO) and tryptophanyl-tRNA synthetase are enzymes of tryptophan metabolism whose expression in a variety of cells and tissues is highly inducible by interferon-gamma (IFN-gamma). Transforming growth factor-beta (TGF-beta) antagonizes many cellular responses to IFN-gamma. The interaction of these two cytokines plays an important role in maintaining homeostasis during inflammation and repair. In human skin and synovial fibroblasts in vitro, TGF-beta caused time- and dose-dependent abrogation of IFN-gamma-stimulated expression of IDO and tryptophanyl-tRNA synthetase mRNAs. The inhibition was selective and did not appear to be due to down-regulation of IFN-gamma signaling by TGF-beta. In parallel with its effect on IDO mRNA expression, TGF-beta caused a marked reduction in intracellular IDO protein levels and abrogated IDO activity and tryptophan catabolism in these cells induced by IFN-gamma. IFN-gamma caused a rapid and striking increase in the amount of IDO heterogeneous nuclear pre-mRNA and induced transcription of the IDO gene, as demonstrated by transient transfection assays. TGF-beta partially reversed this stimulation. IFN regulatory factor (IRF)-1 and stat1 are cellular intermediates in IFN signaling. Both are implicated in activation of IDO transcription in response to IFN-gamma. The stimulation by IFN-gamma of IRF-1 protein and mRNA expression was not prevented by treatment of fibroblasts with TGF-beta. Furthermore, gel mobility shift assays indicated that TGF-beta did not inhibit the induction of stat1 and IRF-1 binding activity to their cognate DNA recognition sites in the IDO gene promoter. In contrast, the stability of IDO mRNA transcripts was reduced in fibroblasts treated with TGF-beta, as shown by determination of mRNA half-lives following blockade of transcription with 5,6-dichlorobenzimidazole riboside. The findings indicate that TGF-beta prevents the induction of IDO and tryptophanyl-tRNA synthetase gene expression in fibroblasts. The repression of IDO expression by TGF-beta is mediated at both transcriptional and posttranscriptional levels. These results implicate TGF-beta in the negative regulation of tryptophan metabolism, provide evidence for the molecular basis of this regulation, and indicate that cellular tryptophan metabolism is under tight immunological control.

IFN‐γ‐induced IDO and WRS expression in microglia is differentially regulated by IL‐4

“…As expected from other studies on myeloid cells, IFN-γ induced the expression of IDO mRNA ( Figure 1A). In the presence of IFN-γ, TGF-β decreased and TNF-α increased IDO expression (data not shown), consistent with others' results (Kwidzinski et al 2005;MacKenzie et al 1999;Robinson et al 2003;Yuan et al 1998). However IL-4, which has been reported to inhibit IDO expression in a number of cell types, increased IFN-γ induced IDO expression ( Figure 1B).…”

“…The results from the present study in mouse microglia are consistent with previous reports showing that the expression of IDO and WRS is upregulated by IFN-γ. However, many have reported that IL-4 and IL-13 antagonize the effects of IFN-γ (Chaves et al 2001;Doherty et al 1993;Musso et al 1994;O'Keefe et al 1999;Oswald et al 1992;Yuan et al 1998). In distinct contrast, here we show for the first time that in microglia, IL-4 and IL-13 modulate the expression of these enzymes in a distinct fashion (summarized in Figure 9A, B).…”

“…The antagonistic effects of various cytokines to IFN-γ on IDO expression in fibroblasts as well as the myeloid lineage dendritic cells and monocytes have been reported (Chaves et al 2001;Doherty et al 1993;Musso et al 1994;Yuan et al 1998). However, the effects of antiinflammatory cytokines such as IL-4 and IL-13 on the expression of IFN-γ induced IDO and WRS expression in primary microglia, the resident myeloid cells of the CNS that are involved in various pathological disorders of the brain, have not been previously evaluated.…”

“…Parallel induction of IDO and WRS by IFN-γ has been found in a number of cell types (Boasso et al 2005); Fleckner et al 1995;Rubin et al 1991Yoshida et al 1981. On the other hand, antiinflammatory cytokines can antagonize many cellular responses induced by IFN-γ, including IDO induction (Chaves et al 2001;Doherty et al 1993;Musso et al 1994;Oswald et al 1992;Yuan et al 1998), but the effects on WRS are not as well studied.…”

Serum tryptophan decrease correlates with immune activation and impaired quality of life in colorectal cancer

“…Selective serotonin reuptake inhibitors (SSRI) have been reported to improve depression in advanced cancer (Holland et al, 1998), and in melanoma treated with interferon a (Musselman et al, 2001) since the latter also upregulates IDO (Taylor and Feng, 1991). However, these SSRI antidepressant effects might not depend on IDO-mediated serum tryptophan reduction, and studies of the effect of IDO inhibitors (Yuan et al, 1998) on cancer-related QoL deterioration would be needed to confirm the mechanism suggested by the present findings.…”