INHIBITION OF TYPE-I COLLAGEN MESSENGER-RNA EXPRESSION INDEPENDENT OFTRYPTOPHAN DEPLETION IN INTERFERON-GAMMA-TREATED HUMAN DERMAL FIBROBLASTS

Interferon-gamma (IFN-gamma) is a pleiotropic cytokine that modulates type I collagen synthesis, In addition, IFN-gamma also exerts potent e ffects on cellular tryptophan levels by inducing the expression of ind oleamine 2,3-dioxygenase (IDO) and tryptophanyl-tRNA synthetase. Becau se recent evidence indicates that IDO-mediated oxidative tryptophan ca tabolism is important in cellular responses to IFN-gamma, we investiga ted the role of IDO in the IFN-gamma-induced modulation of type I coll agen gene expression. IFN-gamma (greater than or equal to 50 U/ml) sti mulated IDO expression in human dermal fibroblasts in vitro, resulting in a >90% depletion of tryptophan in the culture media following incu bation for 48 h. Higher concentrations of IFN-gamma (greater than or e qual to 500 U/ml) caused a marked decrease in type I collagen mRNA lev els, Time-course studies indicated that maximal induction of IDO mRNA expression in IFN-gamma-treated fibroblast cultures (24 h) preceded th e maximal decrease in collagen mRNA (96 h). Type I collagen mRNA level s were also markedly and selectively decreased in fibroblasts maintain ed in tryptophan-depleted cultures. Addition of exogenous tryptophan ( up to 2500 mu M) to IFN-gamma-treated fibroblasts restored ''normal'' concentrations of tryptophan in the culture media, but did not abrogat e the IFN-gamma-induced decrease in collagen mRNA. Addition of the try ptophan metabolite kynurenine, in concentrations similar to those gene rated in fibroblast cultures following IFN-gamma treatment for 48 h, h ad no significant effect on type I collagen mRNA levels, These results indicate that although IFN-gamma causes activation of IDO and enhance d tryptophan catabolism in fibroblast cultures, neither the ensuing tr yptophan starvation nor the accumulation of kynurenine in the culture media can fully account for the inhibitory effects of IFN-gamma on typ e I collagen mRNA expression.