Abstract LB-265: IRF8 is an essential transcriptional activator of iNOS although MDSCs up-regulate iNOS expression via an IRF8-independent mechanism

Abstract

Abstract Nitric Oxide Synthase (NOS) is a family of enzymes that catalyzes L-arginine metabolism to generate nitric oxide (NO). There are three main isoforms of NOS, named neuronal NOS (nNOS), endothelial NOS (eNOS), and inducible NOS (iNOS or NOS2). These three distinct isoforms of NOS have different cellular localization, regulation, and catalytic properties. iNOS is the most extensively studied one among these three NOS isoforms. iNOS was originally identified in myeloid cells, and it is known that its expression is often induced after myeloid cell activation by endotoxins or cytokines to generate NO which then acts as a defense effector to suppress invading microorganisms or neoplastic cells. It is known that IFNγ signaling pathway and NF-κB synergistically regulate iNOS expression, and that IFN regulatory factor 8 (IRF8) enhances interaction between IFNγ signaling and NF-κB in regulation of iNOS expression. However, the molecular mechanism underlying iNOS expression regulation in myeloid cell lineage is still not fully understood. We report here that IRF8 is an essential transcription activator of iNOS in myeloid cells since IRF8-deficient myeloid cells lose iNOS expression. Furthermore, a key phenotype of IRF8 KO mice is the mass accumulation of CD11b+Gr1+ myeloid-derived suppressor cells (MDSCs), suggesting that IRF8 is a key suppressor of MDSC differentiation under physiological conditions. On the other hand, accumulation of MDSCs is a hallmark of cancers and iNOS expression is dramatically elevated in tumor-induced MDSCs, despite that tumor-induced MDSCs exhibit diminished IRF8 expression. Furthermore, the IFNγ signaling pathway is impaired and NF-κB subunits are undetectable in tumor-induced MDSCs. Our data therefore indicate that iNOS expression is activated by an IRF8-independent mechanism that is also IFNγ and NF-κB-independent in tumor-induced MDSCs under pathological conditions. At the functional level, we determined that iNOS-KO mice exhibit lower tumor incidence as compared to WT mice. However, the sizes of established tumors are not different between iNOS-KO mice and WT mice. Taken together, our data determine that IRF8 is an essential transcriptional activator of iNOS expression in myeloid cells under physiological conditions, although iNOS expression is activated by an IFNγ-, NF-κB-, and IRF8-independent mechanism in tumor-induced MDSCs under pathological conditions. iNOS acts as a suppressor of tumor cell colonization but not tumor growth. Citation Format: Mohammed M. Ibrahim, Amy V. Paschall, Priscilla S. Redd, Kebin Liu. IRF8 is an essential transcriptional activator of iNOS although MDSCs up-regulate iNOS expression via an IRF8-independent mechanism. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr LB-265.