Hematopoietic inhibition by interferon-γ is partially mediated through interferon regulatory factor-1

Abstract

Biologic responses to cytokines are mediated by intracellular pathways involving induction of signaling and metabolic cascades. Interferon (IFN) regulatory factor-1 (IRF-1) is a major transcription factor induced not only by IFN-γ but also by other cytokines including tumor necrosis factor-α (TNF-α). Possible IRF-1 binding sequence elements have been located in the promoter regions of several genes, including p53, inducible nitric oxide synthase, and cyclin D1. IFN-γ and TNF-α can inhibit hematopoiesis in vitro and have been implicated in the pathophysiology of bone marrow (BM) failure. We investigated whether the inhibitory effects of these cytokines were intracellularly mediated through the expression of IRF-1 or -2 in target cells. In total BM cells, IRF-1 mRNA expression increased after stimulation with IFN-γ and TNF-α; the stronger effect was observed with IFN-γ. In contrast, IRF-2 mRNA expression was constitutive and not altered by cytokine stimulation. By gene amplification, low levels of IRF-1 mRNA were present in unstimulated, highly purified CD34+ cells; on exposure to IFN-γ and TNF- α, amplified IRF-1 mRNA showed a much stronger signal than control. When CD34+ cells were treated with IFN-γ and TNF-α, IRF-1 antisense oligodeoxynucleotide (ODN) partially reversed the suppressive effects on CD34+ cell-derived colony formation by IFN-γ but not those by TNF-α. In parallel experiments, IRF-1 antisense ODN decreased both IRF-1 protein and mRNA expression. The effects of ODN were sequence-specific and concentration- dependent. These results suggest that the inhibitory hematopoietic effects of IFN-γ and TNF-α are mediated by different pathways. For IFN-γ, IRF-1 is involved in the activation of cellular genes responsible for IFN-γ suppressive effects.