Regulation of IDO1 expression and the kynurenine pathway in synovial fibroblasts

Abstract

Abstract Background Regulation of tryptophan metabolism along the kynurenine pathway by Indoleamine 2,3-dioxygenase (IDO) has been implicated in immune modulation. The purpose of this study was to investigate the mechanisms underlying IDO1 expression and its consequences on the pro-inflammatory and senescent phenotype of human synovial fibroblasts (SF). Methods Human SF were acutely treated with TNFα or IL6/sIL6R. Expression of genes implicated in tryptophan metabolism (IDO1, IDO2, KYNU and WARS) and the senescence associated secretory phenotype (SASP) (IL6, IL8, MCP1 y MMP3) were quantified by RT-PCR. IDO1 was silenced by lentiviral vectors. Expression of IDO1 protein was determinated by Western Blot. Production of kynurenine, a surrogate marker of IDO1 activity, was quantified by ELISA. Quantitative data were statistically analyzed by Pearson or Spearman correlation coefficient or Mann-Whitney test where appropriate. Results Basal expression of genes involved in Trp metabolism along the kynurenine pathway was comparable in SF regardless of their origin (normal, rheumatoid arthritis and osteoarthritis). Both TNFα and IL6/sIL6R induced Trp degradation and the expression of IDO1, KYNU and WARS, with IL6/sIL6R being more potent, but only IL6/sIL6R was able to induce expression of IDO2. IDO1 silencing specifically promoted an increase in basal IL6 mRNA levels and protein secretion without affecting other SASP components (IL8, MCP1 and MMP3). Conclusion Inflammatory cytokines TNFα and IL6/sIL6R induce the expression of enzymes implicated in the kynurenine pathway in SF. IDO1 negatively regulates IL6 production by SF.