Empty vectors were used as controls. The plasmids were transfected into WT and Stat1−/− cells using Lipofectamine LTX (Invitrogen). In some cases, luciferase plasmids were co-transfected with various Stat1 constructs,

into Stat1−/− cells. pRL-SV40 (Promega) encoding Renilla luciferase, was co-transfected at a luciferase : firefly ratio of 1:10. Apoptosis inhibitor Whole-cell lysates were prepared 48 hr post-transfection, and the assay was carried out using the dual-reporter luciferase assay kit (Promega). Samples were read on a Berthold luminometer. Luciferase values were normalized to Renilla expression for each sample. Typically, STAT1 regulates gene expression upon stimulation with IFN, but STAT1 has been also implicated in regulating the constitutive expression of several genes.22–25 Thus, we tested whether STAT1 would have an effect on the constitutive expression of GILT. We hypothesized that the lack of STAT1 regulation in Stat1−/− MEFs

would either not affect the constitutive expression of GILT or would decrease it when compared with WT MEFs.22,24Stat1−/− MEFs19,26 and WT MEFs were tested for the expression of GILT by Western blotting. Surprisingly, semiquantitative Western blot analysis of Stat1−/− MEFs showed an increased expression of GILT protein that was not dependent on IFN-γ treatment (Fig. 1a). AG-014699 clinical trial When WT MEFs were treated with IFN-γ, GILT expression was increased (Fig. 1b), whereas the levels of GILT in IFN-γ-treated Stat1−/− MEFs remained unchanged. These MEFs were derived from C57BL/6 mice. The same result was achieved using MEFs derived from CD1 mice (data not shown), therefore excluding the 17-DMAG (Alvespimycin) HCl possibility that this phenotype is specific to this particular fibroblast cell line. Increased expression of GILT protein in Stat1−/− MEFs led to the hypothesis that STAT1 may actually play a negative role in regulating the GILT promoter activity under basal conditions.

To address this possibility, we used the luciferase assay to determine the specific activation of the GILT promoter in WT and Stat1−/− MEFs. The GILT promoter, 772 bp in length, was cloned into the pGL3 basic vector encoding the firefly luciferase reporter gene. The activity of the firefly luciferase reporter gene under control of the GILT promoter in WT cells and in Stat1−/− cells is shown in Fig. 1c. The decreased expression of GILT in unstimulated WT MEFs implies that phosphorylation of STAT1 is not required for the negative regulatory function of STAT1. Therefore, we transfected Stat1−/− cells with alternatively spliced forms of Stat1 (Stat1α and Stat1β), as well as with the phosphorylation-deficient mutants Stat1α-Y701F, Stat1α-S727A and Stat1β -Y701F, and the double mutant Stat1α-YF/SA, along with firefly luciferase plasmids expressing the GILT promoter.