Modulation of cellular tryptophan metabolism in human fibroblasts by transforming growth factor-beta: selective inhibition of indoleamine 2,3-dioxygenase and tryptophanyl-tRNA synthetase gene expression.
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.
Yuan
W
W
Section of Rheumatology, University of Illinois College of Medicine, Chicago 60607-7171, USA.
Collado-Hidalgo
A
A
Yufit
T
T
Taylor
M
M
Varga
J
J
D000328:Adult; D002478:Cells, Cultured; D004268:DNA-Binding Proteins / Q000378:metabolism; D005347:Fibroblasts / Q000166:cytology / Q000201:enzymology; D015971:Gene Expression Regulation, Enzymologic / Q000187:drug effects* / Q000276:immunology; D006801:Humans; D050503:Indoleamine-Pyrrole 2,3,-Dioxygenase; D007231:Infant, Newborn; D007371:Interferon-gamma / Q000037:antagonists & inhibitors / Q000276:immunology / Q000494:pharmacology; D016133:Polymerase Chain Reaction; D012333:RNA, Messenger / Q000378:metabolism; D012867:Skin / Q000166:cytology; D014158:Transcription, Genetic / Q000502:physiology; D014162:Transfection; D016212:Transforming Growth Factor beta / Q000494:pharmacology*; D014364:Tryptophan / Q000378:metabolism*; D014366:Tryptophan Oxygenase / Q000235:genetics*; D014369:Tryptophan-tRNA Ligase / Q000235:genetics*
D004268:DNA-Binding Proteins; D050503:Indoleamine-Pyrrole 2,3,-Dioxygenase; D012333:RNA, Messenger; D016212:Transforming Growth Factor beta; D007371:Interferon-gamma; D014364:Tryptophan; D014366:Tryptophan Oxygenase; D014369:Tryptophan-tRNA Ligase