Bortezomib Relieves Immune Tolerance in Nasopharyngeal Carcinoma via STAT1 Suppression and Indoleamine 2,3-Dioxygenase Downregulation

Radiotherapy is the primary treatment for nasopharyngeal carcinoma (NPC). Patients with intermediate and advanced stage NPC receiving only radiotherapy have limited survival, so newer immunotherapeutic approaches are sought. The major impediment to better clinical outcomes is tumor immune tolerance. Indoleamine 2,3-dioxygenase (IDO), an IFNγ-inducible enzyme, is a major inducer of immune tolerance during tumor development; therefore, inhibition of the IDO pathway is an important modality for cancer treatment. We show that bortezomib, a proteasomal inhibitor, inhibited the pathways leading to STAT1 and IRF-1 activation, both of which are necessary for IDO expression. Bortezomib downregulated IFNγ-induced IDO expression via inhibition of STAT1 phosphorylation and nuclear translocation, thereby suppressing STAT1-driven IDO transcription in NPC cells. Bortezomib also promoted IκB-α phosphorylation-ubiquitination, which released NF-κB from IκB-α. However, the released NF-κB could not enter the nucleus to conduct its biological effects and accumulated in the cytoplasm. Negative feedback inhibited the transcription of NF-κB, which is important for activating IRF-1 expression. IDO expression is regulated by two important transcription factor binding sites, ISREs, which bind STAT1 and IRF-1, and GASs, which binds STAT1. Bortezomib upregulated IRF-1 protein by inhibiting its proteasome-dependent degradation, but it also inhibited STAT1 phosphorylation, which directly inhibited the activation of GAS and indirectly inhibited the activation of ISRE, which needs both STAT1 and IRF-1. These discoveries provide a mechanism for the antitumor action of bortezomib and have implications for the development of clinical cancer immunotherapy for preventing and treating NPC. Cancer Immunol Res; 5(1); 42–51. ©2016 AACR.