INTERFERON-RESISTANT HUMAN-MELANOMA CELLS ARE DEFICIENT IN ISGF3 COMPONENTS, STAT1, STAT2, AND P48-ISGF3-GAMMA

The mechanism of IFN resistance was examined in three long-term cell l ines, SK-MEL-28, SK-MEL-3, and MM96, exhibiting significant variation in responsiveness to the antiproliferative and antiviral effects of ty pe to IFNs, The JAK-STAT components involved in IFN signal transductio n were analyzed in detail, After exposure to IFN, activation of the IF N type I receptor-linked tyrosine kinases, JAk-1 and TYK-2, was detect ed at similar levels in both IFN-sensitive and IFN-resistant cell type s, indicating that IFN resistance did not result from a deficiency in signaling at the level of receptor-associated kinase activation. Howev er, analysis of ISGF3 transcription factor components, STAT1, STAT2, a nd p48-ISGF3 gamma, revealed that their expression and activation corr elated with cellular IFN responsiveness. The analysis was extended to also include IFN-sensitive primary melanocytes, three additional IFN-r esistant melanoma cell lines, and seven cell cultures recently establi shed from melanoma patient biopsies, It was consistently observed that the most marked difference in ISGF3 was a lack of STAT1 in the resist ant versus the sensitive cells, Transfection of the IFN-resistant MM96 cell line to express increased levels of STAT1 protein partially rest ored IFN responsiveness in an antiviral assay. We conclude that a defe ct in the level of STAT1 and possibly all three ISGF3 components in IF N-resistant human melanoma cells may be a general phenomenon responsib le for reduced cellular responsiveness of melanomas to IFNs.