Rac1 contributes to maximal activation of STAT1 and STAT3 in IFN-γ-stimulated rat astrocytes

Rac1 GTPase is implicated as a signaling mediator in various cellular events. In this study, we show that Rac1 contributes to IFN-gamma-induced inflammatory responses in rat astrocytes. We revealed that IFN-gamma rapidly stimulated activation of Rac1 in C6 astroglioma cells by investigating GST-PAK-PBD-binding ability. We also found that Rac1 deficiency led to attenuation of IFN-gamma-responsive transcriptional responses. Compared with levels in control cells, IFN-gamma-induced IFN-gamma-activated sequence promoter activity was markedly reduced in both C6 astroglioma cells and primary astrocytes expressing RacN17, a well-characterized Rac1-negative mutant. The expression of several IFN-gamma-responsive genes, such as MCP-1 and ICAM-1, was also reduced in cells expressing RacN17. Consistent with these observations, IFN-gamma-induced phosphorylation of STAT1 and STAT3 was lower in C6 cells expressing RacN17 (referred to as C6-RacN17) than in control cells. However, there was no difference in expression level of IFN-gammaRalpha subunit and IFN-gamma-induced phosphorylation of JAK1 between C6 control and C6-RacN17 cells. Interestingly, Rac1 appeared to associate with IFN-gammaRalpha and augment the interaction of IFN-gammaR with either STAT1 or STAT3 in response to IFN-gamma. Taken together, we suggest that Rac1 may serve as an auxiliary mediator of IFN-gamma-signaling, at least at the level of STAT activation, thus contributing to maximal activation of IFN-gamma-responsive inflammatory signaling in rat astrocytes.