ACTIVATION OF STAT1 AND SUBSEQUENT TRANSCRIPTION OF INDUCIBLE NITRIC-OXIDE SYNTHASE GENE IN C6 GLIOMA-CELLS IS INDEPENDENT OF INTERFERON-GAMMA-INDUCED MAPK ACTIVATION THAT IS MEDIATED BY P21(RAS)

Rat C6 glioma cells have been used to characterize molecular events in volved in the regulation of inducible nitric oxide synthase (iNOS) gen e expression stimulated by interferon-gamma (IFN-gamma) plus lipopolys accharide (LPS). IFNs induce a signaling event which involves activati on of Stat1 transcription factor. Previous studies have shown that IFN s also induce extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK/MAPK) activation. However, the mechanisms by which IFNs stimulate MAPK activation remain elusive. Here we show that in C 6 glioma cells, transiently expressing the dominant-negative form of c -Ha-Ras (Asn-17) abrogated IFN-gamma-induced ERK1 and ERK2 activation. Furthermore, PD98059, a specific MEK1 inhibitor, also blocked this ac tivation. These results indicate that p21(ras) and MEK1 are required f or IFN-gamma-induced ERK1 and ERK2 activation. Recent studies have rep orted that MAPK is responsible for serine phosphorylation of Stat1 whi ch is required for Stat1's DNA binding and maximal transcriptional act ivity. Thus, we examined the role of the Ras-MAPK pathway in Stat1 act ivation and subsequent iNOS induction in C6 glioma cells. Further expe riments showed that neither Asn-17 Ras expression nor concentrations o f PD98059, which completely abrogated IFN-gamma-induced ERK1 and ERK2 activation, affected Stat1 DNA binding activity or iNOS induction, ind icating that the Ras-MAPK pathway does not appear to be involved in th e activation of Stat1 and subsequent iNOS induction in C6 glioma cells . (C) 1997 Federation of European Biochemical Societies.