Interferon Regulatory Factor 1 Mediates the Interferon-γ Induction of the Human Immunoproteasome Subunit Multicatalytic Endopeptidase Complex-like 1

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Proteasomes generate antigenic peptides from intracellular proteins for presentation to the immune system by the major histocompatibility complex class I molecules. The antiviral cytokine IFN-gamma alters the catalytic specificity of proteasomes by inducing the synthesis of an alternative set of three proteolytically active proteasome subunits. We have analyzed the mechanism of IFN-gamma induction for the IFN-gamma-induced subunit multicatalytic endopeptidase complex-like 1 (MECL1). The human MECL1 promoter contains two interferon-stimulated response elements (ISREs), generally known to bind members of the interferon regulatory factor (IRF) family. The importance of these elements for IFN-gamma induction of MECL1 was addressed by transfecting an endothelial cell line with MECL1 promoter constructs. By deletions and mutations of the ISRE sequences, we demonstrated that both ISREs were needed for full IFN-gamma induction of the reporter gene. The second (downstream) ISRE was essential for both IFN-gamma-induced and basal transcriptional activity of the promoter. In electrophoretic mobility shift assays, anti-IRF-1 antibodies supershifted an IFN-gamma-induced protein binding specifically to both ISRE sequences, whereas IRF-2 bound the second ISRE before induction. Co-transfection of IRF-1 resulted in induced MECL1 promoter activity in the absence of IFN-gamma. These data indicate that the IFN-gamma induction of human MECL1 is mediated by IFN-gamma-induced IRF-1.