The NF-κB p65 and p50 homodimer cooperate with pSTAT1 to synergistically activate iNOS transcription in cancer cells and myeloid cells (INM1P.440)

Abstract

Abstract Inducible nitric oxide synthase (iNOS) metabolizes L-arginine to produce NO, which was originally identified in myeloid cells as a host defense mechanism against pathogens. However, recent studies have revealed that iNOS is often induced in both tumor and myeloid cells in the tumor microenvironment. Compelling experimental data have shown that iNOS can promote or suppress tumor development in certain cellular conditions. The molecular mechanisms underlying this contrasting function of iNOS is unknown. Because iNOS is often induced by inflammatory signals, it is likely that the opposite functions of iNOS might be controlled by inflammatory signaling pathways. We show here that proinflammatory IFN-γ and NF-κB signals synergistically induce iNOS expression in human colon cancer cells. We further demonstrated that NF-κB directly binds to the NOS2 promoter to regulate iNOS expression. Although the IFN-γ and NF-κB signaling pathways alone are insufficient to induce iNOS expression in myeloid cells, IFN-γ and NF-κB can synergistically induce iNOS expression. We determined that IFN-γ up-regulates IRF8 expression to augment NF-κB induction of iNOS expression. We observed that the p65/p65 and p50/p50 homodimers, not the common canonical p65/p50 heterodimer, directly binds to the nos2 promoter to regulate iNOS expression in myeloid cells. Our results thus determine that the IFN-γ-induced IRF8 acts in concert with the NF-κB p65/p65 and p50/p50 homodimers to regulate iNOS expression.