Oncogenic GLI1 and STAT1/3 activation drives immunosuppressive tryptophan/kynurenine metabolism via synergistic induction of IDO1 in skin cancer cells

Pharmacological targeting of Hedgehog (HH)/GLI has proven effective for certain blood, brain and skin cancers including basal cell carcinoma (BCC). However, limited response rates and the development of drug resistance call for improved anti-HH therapies that take into account synergistic crosstalk mechanisms and immune evasion strategies. In previous work, we demonstrated that crosstalk of HH/GLI with pro-inflammatory Interleukin-6 (IL6) signaling drives BCC by promoting tumor cell proliferation [1]. In the present study, we screened for possible mechanisms of cancer immune evasion regulated by synergistic HH-IL6 signaling and identified the immunosuppressive enzyme indoleamine 2,3-dioxygenase 1 (IDO1) as a novel transcriptional target co-regulated by HH-IL6 signaling. Analysis of the cis-regulatory region of IDO1 by chromatin-immunoprecipitation revealed co-occupancy of this region by HH- IL6 induced GLI1 and STAT3 transcription factors along with active chromatin marks at the histone level. Elevated expression of IDO1 in human BCC with high-level HH and IL6 signatures supports the clinical relevance of our mechanistic data. Genetic inhibition of GLI1 expression prevented the induction of IDO1 expression in response to IL6/STAT3 and IFN{gamma}/STAT1 signaling in human melanoma cells. Pharmacological targeting of HH signaling at the level of GLI proteins interfered with IDO1 expression and consequently prevented the production of the immunosuppressive metabolite kynurenine generated by active IDO1 from tryptophan. Further, inhibition of GLI1 enhanced the efficacy of the selective IDO1 inhibitor epacadostat. Of note, inhibition of HH/GLI signaling in melanoma cells not only reduced IDO1 expression but also interfered with the repression of T cell activation by attenuating IDO1/kynurenine-mediated immunosuppression. These data identify the immunosuppressive IDO1-kynurenine pathway as a novel pro-tumorigenic effector of oncogenic HH-IL6 and GLI-STAT cooperation. Our data suggest simultaneous pharmacological targeting of the HH/GLI, JAK/STAT and IDO1- kynurenine axis as rational combination therapy in skin cancers.