Inhibition of type I collagen mRNA expression independent of tryptophan depletin in interferon-γ-treated human dental fibroblasts

Interferon-γ (IFN-γ) is a pleiotropic cytokine that modulates type I collagen synthesis. In addition, IFN-γ also exerts potent effects on cellular tryptophan levels by inducing the expression of indoleamine 2,3-dioxygenase (IDO) and tryptophanyl-tRNA synthetase. Because recent evidence indicates that IDO-mediated oxidative tryptophan catabolism is important in cellular responses to IFN-γ, we investigated the role of IDO in the IFN-γ-induced modulation of type I collagen gene expression. IFN-γ (≥ 50 U/ml) stimulated IDO expression in human dermal fibroblasts in vitro, resulting in a > 90% depletion of tryptophan in the culture media following incubation for 48 h. Higher concentrations of IFN-γ (≥ 500 U/ml) caused a marked decrease in type I collagen mRNA levels. Time-course studies indicated that maximal induction of IDO mRNA expression in IFN-γ-treated fibroblast cultures (24 h) preceded the maximal decrease in collagen mRNA (96 h). Type I collagen mRNA levels were also markedly and selectively decreased in fibroblasts maintained in tryptophan-depleted cultures. Addition of exogenous tryptophan (up to 2500 μM) to IFN-γ-treated fibroblasts restored 'normal' concentrations of tryptophan in the culture media, but did not abrogate the IFN-γ-induced decrease in collagen mRNA. Addition of the tryptophan metabolite kynurenine, in concentrations similar to those generated in fibroblast cultures following IFN-γ treatment for 48 h, had no significant effect on type I collagen mRNA levels. These results indicate that although IFN-γ causes activation of IDO and enhanced tryptophan catabolism in fibroblast cultures, neither the ensuing tryptophan starvation nor the accumulation of kynurenine in the culture media can fully account for the inhibitory effects of IFN-γ on type I collagen mRNA expression.