Evidence for a GTP-Binding Protein Involved in Interferon-γ Transduction Signal

These studies were designed to investigate the characteristics of the intracellular messengers induced by interferons (IFN-α/β and IFN-γ) after receptor binding. Pretreatment of target cells with V. cholerae toxin, which is known to activate a membrane GTP-binding stimulatory protein (G_s), potentiated the action of IFN-γ, but not of IFN-α/β. By contrast, B. pertussis toxin, which is known to activate the GTP-binding inhibitory protein (G_i), had no effects on the action of both IFN-α/β and IFN-γ. Further support to the involvement of G proteins in IFN-γ transduction signal came from the finding that a nonhydrolizable GTP analog, GTP-γ-S, enhanced in the presence of phorbol esters (PMA) the antiviral and antiproliferative activity of IFN-γ, but not of IFN-α/β. On the other hand, forskolin or PGE₁, known to increase the intracellular cAMP levels by different metabolic pathways, when added together with IFN-γ, significantly potentiated its antiviral and antiproliferative activity. Pretreatment of the cultures with the above drugs completely prevented IFN-γ activity. No effects were observed when forskolin or PGE₁ were used with IFN-α/β. Finally, the modulation of IFN-γ activity by the above drugs was not a consequence of changes in the expression of the specific surface receptors, since [¹²⁵I]IFN-γ binding by pretreated target cells was comparable to that of untreated cultures. Altogether these results demonstrate that the IFN-γ, but not the IFN-α/β, transduction signal is mediated after receptor binding by a G protein with functional characteristics similar to those of the known G_s proteins. Activation of the adenylate cyclase system could be one of the subsequent steps involved in IFN-γ action.