5: REST negatively and ISGF3 positively regulates the human STAT1 gene in melanoma
STAT1 plays a pivotal role in signal transduction and transcriptional activation in response to type I and II interferons (IFNs). Regulation of STAT1 expression has significant consequences in human cancer cells, where STAT1 deficiencies have been associated with cellular resistance to type I IFN. Distinct promoter, enhancer and repressor regions have previously been described in the regulatory part of the human STAT1 gene extending as far as the second intron. A putative ISRE sequence in the STAT1 promoter is inducible by type I IFN and binds the IFN- α / β -induced complex, ISGF3. Together with the previously characterized IRF-E/GAS/IRF-E (IGI) motif, these positive regulatory elements provide a means for intracellular amplification of STAT1 expression which is necessary for increasing cell responsiveness to the IFNs. In contrast, the transcriptional repressor REST binds to an RE-1 element in the STAT1 repressor region and in doing so represses transcription from the STAT1 gene regulatory region in melanoma cells lines. Repression significantly decreased in a REST-null cell line. Altering REST function from a transcriptional repressor into an activator as REST-VP16, increased expression from RE-1-targeted reporters. RNA expression of 65 melanoma cell lines by microarray and selected lines with known IFN responsiveness showed significant inverse correlations between STAT1/REST that were related to cellular responses to IFN. Thus REST, through the intronic RE-1 element, provides a means for down-regulating STAT1 expression, affecting melanoma responsiveness to IFN. Intracellular levels of REST may be a useful marker to test for IFN-resistance and as a novel therapeutic target in IFN-resistant melanomas.