IFN-stimulated gene factor 3 (ISGF3) mediates transcriptional activation of IFN-sensitive genes (ISGs). The component subunits of ISGF3, STAT1αβ, STAT2, and p48-ISGF3γ, 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, particularly STAT1. In the present study, we show that pretreating IFN-resistant melanoma cell lines with IFN-γ (IFN-γ priming) before stimulation with type I IFN also results in increased levels of ISGF3 components and enhanced DNA-binding activation of ISGF3. In addition, IFN-γ priming of IFN-resistant melanoma cell lines increased expression of type I IFN-induced ISG products, including ISG54, 2′-5′-oligoadenylate synthase, HLA class I, B7-1, and ICAM-1 Ags. Furthermore, IFN-γ priming enhanced the antiviral effect of IFN-β on the IFN-resistant melanoma cell line, MM96. These results support a role for IFN-γ priming in up-regulating ISGF3, thereby augmenting the responsiveness of IFN-resistant melanoma cell lines to type I IFN and providing a molecular basis and justification for using sequential IFN therapy, as proposed by others, to enhance the use of IFNs in the treatment of melanoma.
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Wong et al. (1998) studied this question.