IFN-GAMMA PRIMING UP-REGULATES IFN-STIMULATED-GENE-FACTOR-3 (ISGF3) COMPONENTS, AUGMENTING RESPONSIVENESS OF IFN-RESISTANT MELANOMA-CELLS TO TYPE-I IFNS

IFN-stimulated gene factor 3 (ISGF3) mediates transcriptional activati on of IFN-sensitive genes (ISGs), The component subunits of ISGF3, STA T1 alpha beta, STAT2, and p48-ISGF3 gamma, are tyrosine phosphorylated before their assembly into a complex. Subsequently, the ISGF3 complex is translocated to the nucleus. We have recently established that the responsiveness of human melanoma cell lines to type I IFNs correlates directly with their intracellular levels of ISGF3 components, particu larly STAT1. In the present study, we show that pretreating IFN-resist ant melanoma cell lines with IFN-gamma (IFN-gamma priming) before stim ulation with type I IFN also results in increased levels of ISGF3 comp onents and enhanced DNA-binding activation of ISGF3. In addition, IFN- gamma priming of IFN-resistant melanoma cell lines increased expressio n of type I IFN-induced ISG products, including ISG54, 2'-5'-oligoaden ylate synthase, HLA class I, B7-1, and ICAM-1 Ags, Furthermore, IFN-ga mma priming enhanced the antiviral effect of IFN-beta on the IFN-resis tant melanoma cell line, MM96, These results support a role for IFN-ga mma priming in up-regulating ISGF3, thereby augmenting the responsiven ess of IFN-resistant melanoma cell lines to type I IFN and providing a molecular basis and justification for using sequential IFN therapy, a s proposed by others, to enhance the use of IFNs in the treatment of m elanoma.