INVOLVEMENT OF 2 REGULATORY ELEMENTS IN INTERFERON-GAMMA-REGULATED EXPRESSION OF HUMAN INDOLEAMINE 2,3-DIOXYGENASE GENE

Interferon (IFN)-gamma-induced expression of indoleamine 2,3-dioxygenase (IDO) gene is implicated in the antimicrobial and antiproliferative effects of IFN-gamma in cell cultures, Earlier studies identified a 96 base pair (bp) regulatory region upstream of the IDO gene that conferred IFN-gamma response to the chloroamphenicol acetyltransferase (CAT) gene linked to herpesvirus thymidine kinase promoter, The IFN-gamma-responsive region was further narrowed to a 67 bp fragment by 3' deletion, This 67 bp fragment contains several sequence elements of potential interest, including a 14 bp sequence homologous to the ISRE sequence found in IFN-alpha-inducible genes and two palindromic sequences (PE I and PE II) homologous to the GAS sequence identified in IFN-gamma-inducible genes, Site-directed mutagenesis studies showed that IFN-gamma-induced expression of IDO-CAT constructs involved cooperation between two elements: the ISRE homolog and the PE II (but not PE I), Either element alone with its flanking sequence was inadequate in conferring an IFN-gamma response to CAT reporter gene, Two IFN-gamma-regulated protein factors interacting with these two elements were identified, The factor binding to the ISRE region was induced with a slower kinetics, required new protein synthesis, and reacted with antibodies to IRF-1. The factor interacting with the PE II region appeared rapidly after treatment with IFN-gamma independently of new protein synthesis, and its binding to DNA probe was blocked by antibodies to p91 factor, reported to bind to GAS element. The results indicate that the IFN-gamma-inducible expression of the human IDO gene differs from that of other IFN-gamma-inducible genes with respect to a complete dependence on two control elements, an ISRE and a GAS element (PE II), which interact with IFN-gamma-regulated factors IRF-1 and p91, The involvement of IRF-1 explains the dependence on new protein synthesis for IFN-gamma-mediated induction of the IDO gene.