Cooperative contributions of Interferon regulatory factor 1 (IRF1) and IRF8 to interferon-γ-mediated cytotoxic effects on oligodendroglial progenitor cells

Background: Administration of exogenous interferon-gamma (IFN gamma) aggravates the symptoms of multiple sclerosis (MS), whereas interferon-beta (IFN beta) is used for treatment of MS patients. We previously demonstrated that IFN gamma induces apoptosis of oligodendroglial progenitor cells (OPCs), suggesting that IFN gamma is more toxic to OPCs than IFN beta. Thus we hypothesized that a difference in expression profiles between IFN gamma-inducible and IFN beta-inducible genes in OPCs would predict the genes responsible for IFN gamma-mediated cytotoxic effects on OPCs. We have tested this hypothesis particularly focusing on the interferon regulatory factors (IRFs) well-known transcription factors up-regulated by IFNs. Methods: Highly pure primary rat OPC cultures were treated with IFN gamma and IFN beta. Cell death and proliferation were assessed by MTT reduction, caspse-3-like proteinase activity, Annexin-V binding, mitochondrial membrane potential, and BrdU-incorporation. Induction of all nine IRFs was comprehensively compared by quantitative PCR between IFN gamma-treated and IFN beta-treated OPCs. IRFs more strongly induced by IFN gamma than by IFN beta were selected, and tested for their ability to induce OPC apoptosis by overexpression and by inhibition by dominant-negative proteins or small interference RNA either in the presence or absence of IFN gamma. Results: Unlike IFN gamma, IFN beta did not induce apoptosis of OPCs. Among nine IRFs, IRF1 and IRF8 were preferentially up-regulated by IFN gamma. In contrast, IRF7 was more robustly induced by IFN beta than by IFN gamma. Overexpressed IRF1 elicited apoptosis of OPCs, and a dominant negative IRF1 protein partially protected OPCs from IFN gamma-induced apoptosis, indicating a substantial contribution of IRF1 to IFN gamma-induced OPC apoptosis. On the other hand, overexpression of IRF8 itself had only marginal proapoptotic effects. However, overexpressed IRF8 enhanced the IFNg-induced cytotoxicity and the proapoptotic effect of overexpressed IRF1, and down-regulation of IRF8 by siRNA partially but significantly reduced preapoptotic cells after treatment with IFN gamma, suggesting that IRF8 cooperatively enhances IFN gamma-induced OPC apoptosis. Conclusions: This study has identified that IRF1 and IRF8 mediate IFN gamma-signaling leading to OPC apoptosis. Therapies targeting at these transcription factors and their target genes could reduce IFN gamma induced OPC loss and thereby enhance remyelination in MS patients.