Constitutive IDO expression in human cancer is sustained by an autocrine signaling loop involving IL-6, STAT3 and the AHR

// Ulrike M. Litzenburger 1,2,* , Christiane A. Opitz 1,2,3,* , Felix Sahm 2,4,5 , Katharina J. Rauschenbach 1,2 , Saskia Trump 6 , Marcus Winter 6 , Martina Ott 1,2 , Katharina Ochs 1,2 , Christian Lutz 7 , Xiangdong Liu 8 , Natasa Anastasov 9 , Irina Lehmann 6 , Thomas Höfer 10 , Andreas von Deimling 4,5 , Wolfgang Wick 1,11 , and Michael Platten 1,2 1 Department of Neurooncology, Neurology Clinic and National Center for Tumor Diseases University Hospital of Heidelberg, Heidelberg, Germany; 2 Clinical Cooperation Unit Neuroimmunology and Brain Tumor Immunology, German Cancer Research Center (DKFZ), Heidelberg, Germany; 3 Brain Cancer Metabolism Group, German Cancer Research Center (DKFZ), Heidelberg, Germany, 4 Department of Neuropathology, Institute of Pathology, University Hospital of Heidelberg, Germany; 5 Clinical Cooperation Unit Neuropathology, German Cancer Research Center (DKFZ), Heidelberg, Germany; 6 Department for Environmental Immunology, Helmholtz Center for Environmental Research, Leipzig, Germany, 7 Heidelberg Pharma GmbH, Ladenburg, Germany; 8 Incyte Corporation, Experimental Station, Wilmington, Delaware, USA, 9 Institute of Radiation Biology, Helmholtz Center Munich, German Research Center for Environmental Health, Germany; 10 Theoretical Systems Biology, German Cancer Research Center (DKFZ), Heidelberg, Germany; 11 Clinical Cooperation Unit Neurooncology, German Cancer Research Center (DKFZ), Heidelberg, Germany * These authors contributed equally to this work Correspondence: Michael Platten, email: // Christiane Opitz, email: // Keywords : IDO, immunosuppression, autoactivation loop Received : January 13, 2014 Accepted : January 18, 2014 Published : January 20, 2014 Abstract Indoleamine-2,3-dioxygenase (IDO) inhibitors have entered clinical trials based on their ability to restore anti-tumor immunity in preclinical studies. However, the mechanisms leading to constitutive expression of IDO in human tumors are largely unknown. Here we analyzed the pathways mediating constitutive IDO expression in human cancer. IDO-positive tumor cells and tissues showed basal phosphorylation and acetylation of STAT3 as evidenced by western blotting and immunoprecipitation. Inhibition of IL-6 or STAT3 using siRNA and/or pharmacological inhibitors reduced IDO mRNA and protein expression as well as kynurenine formation. In turn, IDO enzymatic activity activated the AHR as shown by the induction of AHR target genes. IDO-mediated AHR activation induced IL-6 expression, while inhibition or knockdown of the AHR reduced IL-6 expression. IDO activity thus sustains its own expression via an autocrine AHR–IL-6–STAT3 signaling loop. Inhibition of the AHR–IL-6–STAT3 signaling loop restored T-cell proliferation in mixed leukocyte reactions performed in the presence of IDO-expressing human cancer cells. Identification of the IDO-AHR-IL-6-STAT3 signaling loop maintaining IDO expression in human cancers reveals novel therapeutic targets for the inhibition of this core pathway promoting immunosuppression of human cancers. The relevance of the IDO-AHR-IL-6-STAT3 transcriptional circuit is underscored by the finding that high expression of its members IDO, STAT3 and the AHR target gene CYP1B1 is associated with reduced relapse-free survival in lung cancer patients.