Interferon-gamma induction of the human leukocyte antigen-E gene is mediated through binding of a complex containing STAT1alpha to a distinct interferon-gamma-responsive element.
Expression of the human major histocompatibility complex (MHC) class I genes has been shown previously to increase at the transcriptional level following exposure to interferon-gamma (IFN-gamma). In this report we have examined the molecular mechanisms involved in the IFN-gamma-induced transcription of the human MHC class I gene, HLA-E. Functional analysis of CAT reporter gene constructs under the control of the HLA-E promoter transfected into U937 cells revealed the presence of a distinct IFN-gamma-responsive element, termed the interferon response region (IRR), that was necessary and sufficient to mediate the response to IFN-gamma. This cis-acting regulatory sequence contains an imperfect inverted repeat; the 5'-half of the IRR resembles the IFN-gamma activation site (GAS), and the 3'-half of the IRR resembles the interferon-stimulated response element (ISRE). Gel mobility shift assays demonstrated that the IRR bound a single, specific, IFN-gamma-induced complex (IRR-AC), which was formed rapidly following treatment with IFN-gamma and was independent of protein synthesis. Competition experiments with GAS and ISRE sequences from other IFN-inducible genes showed that GAS sequences competed for the IRR-AC, whereas ISRE sequences did not compete. Mutational analysis demonstrated that point mutations in either the 5'-half or 3'-half of the IRR prevented binding of the complex and abrogated or markedly reduced the IFN-gamma responsiveness of reporter gene constructs. Supershift analysis revealed that the IRR-AC contains a factor that was recognized by antibodies specific for the protein STAT1alpha (signal transducer and activator of transcription). Taken together, these findings suggest that the mechanism of IFN-gamma-induced transcription of the HLA-E gene is distinct from that of other MHC class I genes.