FIGURE 1: Summary of major routes for tryptophan uptake and utilization. Tryptophan is transported by the solute carriers SLC1A5 and SLC7A5. Intracellular tryptophan can be incorporated into proteins or metabolized by the serotonin or the kynurenine pathways. Proteins labelled in blue are transcriptionally upregulated by MYC and as consequence are highly expressed in colon cancer cells. Kynurenine can be exported by cancer cells and cause T-cell inactivation. Intracellular kynurenine binds to and activates the transcription factor AHR, leading to its nuclear translocation. In the nucleus AHR heterodimerizes with ARNT to promote the expression of genes that contain xenobiotic responsive elements (XRE) in their promoters, including genes involved in xenobiotic response and a signature of genes that promote protein synthesis.