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

J Cell Physiol. 1998 Oct;177(1):174-86. doi: 10.1002/(SICI)1097-4652(199810)177:1<174::AID-JCP18>3.0.CO;2-D.

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.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Adult
  • Cells, Cultured
  • DNA-Binding Proteins / metabolism
  • Fibroblasts / cytology
  • Fibroblasts / enzymology
  • Gene Expression Regulation, Enzymologic / drug effects*
  • Gene Expression Regulation, Enzymologic / immunology
  • Humans
  • Indoleamine-Pyrrole 2,3,-Dioxygenase
  • Infant, Newborn
  • Interferon-gamma / antagonists & inhibitors
  • Interferon-gamma / immunology
  • Interferon-gamma / pharmacology
  • Polymerase Chain Reaction
  • RNA, Messenger / metabolism
  • Skin / cytology
  • Transcription, Genetic / physiology
  • Transfection
  • Transforming Growth Factor beta / pharmacology*
  • Tryptophan / metabolism*
  • Tryptophan Oxygenase / genetics*
  • Tryptophan-tRNA Ligase / genetics*

Substances

  • DNA-Binding Proteins
  • Indoleamine-Pyrrole 2,3,-Dioxygenase
  • RNA, Messenger
  • Transforming Growth Factor beta
  • Interferon-gamma
  • Tryptophan
  • Tryptophan Oxygenase
  • Tryptophan-tRNA Ligase