Whether cells adhere, migrate, or proliferate in response to a particular stimulus often depends on what other signals the cells have previously received. Exposure of cells to interferon-γ (IFN-γ), a proinflammatory cytokine, influences cellular adhesion, migration, and morphology by inducing a specific genetic program and thus altering the cell’s complement of signaling molecules. The guanylate-binding protein family GBP, which are guanosine triphosphatases with the unusual property of hydrolyzing GTP to both GDP and GMP, is transcriptionally induced by IFNs. Messmer-Blust et al. show that in cultured mouse cell lines mGBP-2 mediates the suppression of cell spreading caused by IFN-γ. NIH-3T3 cells and murine B16 melanoma cells exhibited decreased rate of spreading on fibronectin when overexpressing mGBP-2 to amounts similar to those induced by treatment of the cells with IFN-γ for 8 hours. NIH-3T3 cells also exhibited a reduced rate of cell spreading on collagen- or laminin-coated dishes. Cells treated with IFN-γ and in which mGBP-2 was knocked down or cells expressing a mutant form of mGBP-2 presumed to have disrupted GTP binding exhibited a rate of cell spreading similar to control cells, suggesting that the induction of mGBP-2 and its GTP-binding activity is necessary for IFN-γ to affect cell spreading. Less Rac activation occurred in the mGBP-2–overexpressing cells or IFN-γ–treated cells plated on fibronectin compared with control cells, and introduction of a constitutively active Rac1 mutant, but not Cdc42 or RhoA mutants, increased the rate of cell spreading in mGBP-2–expressing cells. The mGBP-2–overexpressing cells also exhibited reduced activation of Rac in response to platelet-derived growth factor (PDGF), as did treatment of nontransfected cells with IFN-γ. Coimmunoprecipitation revealed that mGBP-2, but not the presumed GTP-binding deficient mutant, and the catalytic subunit of phosphoinositide 3-kinase (PI3K) interacted, and cells overexpressing mGBP-2 had reduced phosphorylated Akt in response to plating on fibronectin or exposure to PDGF. The kinase activity of PI3K in cells overexpressing mGBP-2 that were plated on fibronectin was reduced compared with control cells. Thus, by increasing the abundance of mGBP-2, which inhibits PI3K, IFN-γ inhibits growth factor–stimulated signaling through the PI3K pathway and inhibits cell spreading, which also relies on PI3K-mediated stimulation of Rac.
A. F. Messmer-Blust, S. Balasubramanian, V. Y Gorbacheva, J. A. Jeyaratnam, D. J. Vestal, The interferon-γ–induced murine guanylate-binding protein-2 inhibits Rac activation during cell spreading on fibronectin and after platelet-derived growth factor treatment: Role for phosphatidylinositol 3-kinase. Mol. Biol. Cell 21, 2514–2528 (2010). [Abstract] [Full Text]