Interferon-gamma signaling in human retinal pigment epithelial cells mediated by STAT1, ICSBP, and IRF-1 transcription factors

PURPOSE. Studies have shown that interferon (IFN)-gamma stimulates expressi on of intercellular adhesion molecule-1 (ICAM-1), major histocompatibility complex (MHC) class II, interleukin (IL)-6, and inducible nitric oxide synt hase and inhibits replication of Toxoplasma gondii in human retinal pigment epithelial (HRPE) cells. The present study was undertaken to investigate t he molecular mechanisms of IFN-gamma action. METHODS. RNA, whole-cell extracts, and nuclear extracts were prepared from HRPE cells cultured in the presence or absence of IFN-gamma. Activation of IFN-gamma-responsive genes was analyzed by electrophoretic mobility shift a ssay, reverse transcription-polymerase chain reaction (RT-PCR), western blo t analysis, and immunoprecipitation. RESULTS. HRPE cells constitutively expressed two members of the IFN regulat ory factor (IRF) family of transcription factors, IRF-1 and IRF-2. After ex posure to IFN-gamma, transcription of IRF-1 and IFN consensus sequence bind ing protein (ICSBP) genes were induced; TRF-2 gene transcription was not up regulated. Activation of IFN-gamma-responsive genes was mediated by tyrosin e phosphorylation of the signal transducer and activator of transcription ( STAT)-1 factor. CONCLUSIONS. This study characterized the IFN-gamma signaling pathway in HR PE cells and identified IRF-1, ICSBP, and tyrosine-phosphorylated STAT1 as mediators of IFN-gamma action in these cells. ICSBP is thought to be exclus ively used in immunologic responses and has previously been detected only i n lymphoid cells. However, the current study shows that ICSBP expression is inducible in HRPE cells, suggesting that it may regulate gene transcriptio n in RPE cells and possibly in other nonimmunologic cell types.