Kynurenine Negatively Regulates TGFB1 Transcription in Hepatic Stellate Cells through AhR

Abstract

Hepatocyte injury is coupled to the activation of hepatic stellate cells (HSCs), contributing to the pathogenesis of liver fibrosis. In hepatocytes, tryptophan 2,3-dioxygenase (TDO2) is the primary enzyme responsible for the conversion of tryptophan to kynurenine, an endogenous ligand of aryl hydrocarbon receptor (AhR), which may control the activation of HSCs. Transforming growth factor beta (TGF-β) is a key mediator of HSC activation. There are conflicting reports on the effect of AhR on TGF-β regulation depending on cell types and conditions. This study investigated TDO2 expression in hepatocytes, the effect of kynurenine on HSCs, and the role of AhR in TGFB1 transcription in HSCs. When animals were challenged as a single or multiples doses by liver toxicant, TDO2 protein and mRNA levels were both significantly diminished. In GEO database analyses, TDO2 expression was inhibited in patients with alcoholic hepatitis or cirrhosis, suggestive of the inverse correlation between TDO2 level and TGFB1 expression. Kynurenine treatment prevented TGFB1-dependent increase in TGFB1 mRNA in LX-2 cells, a human HSC cell-line, which was reversed by chemical inhibition or knockdown of AhR. Similar results were obtained using TGFB1 promoter reporter analysis. Chromatin immunoprecipitation (ChIP) and TGFB1 promoter reporter assays using putative XRE mutants showed that AhR directly represses TGFB1 transcription by binding to the DNA binding site located in the intronic region. The ability of kynurenine to inhibit HSC activation was corroborated by changes other HSC markers. Conclusions: Kynurenine, a tryptophan metabolite produced primarily by TDO2, inhibits HSC activation, which may result from AhR-mediated negative regulation of TGFB1 gene in HSCs, supporting the concept that kynurenine produced in healthy hepatocytes serves as a paracrine mediator that maintains HSC quiescence.