STAT1 can

exert its effect on target DNA either by direct binding or indirectly through the formation of complexes with other transcription factors. We hypothesized that the DNA-binding region of STAT1 may contain a site Midostaurin that is important for the constitutive interaction of STAT1 and the GILT promoter. Therefore, we tested whether known DNA-binding mutants – V426D/T427D,29 E428A/E429S30 and K544A/E545A,31– can alter the activity of the GILT promoter. Our luciferase reporter gene experiment indicated that only V426D/T427D was unable to decrease the activity of the GILT promoter, suggesting that STAT1 binding to DNA is necessary and that residues V426/T427 are the most important for the STAT1 suppressive effect on the ligand-independent activity of the GILT promoter. The mutant V426D/T427D is defective in the IFN-γ-induced STAT1 DNA binding 3-MA to specific GAS sites and shows weakened, non-specific protein–DNA interactions,29 and therefore the implication is that GAS sites remain an important target for STAT1, even in the absence of IFN-γ stimulation. The DAPA confirmed

that indeed the V426D/T427D (Mut 1) mutant cannot bind to GAS-like sites in the GILT promoter in vitro, whereas the K544A/E545A (Mut 3) mutant binds to GAS-like sites, albeit weakly. However, we were unable to show that the mutant E428A/E429S (Mut 2), which suppresses GILT promoter activity as in the WT, binds in vitro to a GAS-like site in the GILT promoter. This apparent discrepancy

may be caused by very weak binding to the GAS site in the GILT promoter that is below the limits of detection by DAPA, and/or perhaps is caused by the binding of this mutant to another, as yet unidentified, transcription factor. The fact that the absence of STAT1 increases the activity of the GILT promoter and GILT protein expression may be caused by competition/interaction of STAT1 with other transcription factors. For example, STAT3 can replace STAT1 in STAT1−/− cells to drive the transcription of certain genes in response Tolmetin to IFN-γ or interleukin-6.41 STAT1 and STAT3 dimers bind selectively to very similar, but not identical, elements27,42 and thus activate different, but overlapping, sets of genes. In addition, Egr-1 (also designated zif268, TIS 8, NFGI-A, Krox 24) has been identified as one of the transcription factors that targets GILT.43 Egr-1 is a member of the immediate-early gene family that includes FOS, JUN and early growth-response genes.44,45 Egr-1 binds to 5′-GCGGGGGCG-3′ consensus sequences within the promoter region of target genes.46 The GILT promoter contains several GC-rich domains in the vicinity of GAS-like sites and it is therefore possible that the binding of Egr-1 and STAT1 to some regions of the GILT promoter are mutually exclusive. The competition for binding to the GILT promoter, if any, remains to be shown.