Activation of the p70 S6 kinase and phosphorylation of the 4E-BP1 repressor of mRNA translation by type I interferons.

The Type I IFN receptor-generated signals required for initiation of mRNA translation and, ultimately, induction of protein products that mediate IFN responses, remain unknown. We have previously shown that IFNalpha and IFNbeta induce phosphorylation of insulin receptor substrate proteins and downstream engagement of the phosphatidylinositol (PI) 3'-kinase pathway. In the present study we provide evidence for the existence of a Type I IFN-dependent signaling cascade activated downstream of PI 3'-kinase, involving p70 S6 kinase. Our data demonstrate that p70 S6K is rapidly phosphorylated on threonine 421 and serine 424 and is activated during treatment of cells with IFNalpha or IFNbeta. Such activation of p70 S6K is blocked by pharmacological inhibitors of the PI 3'-kinase or the FKBP 12-rapamycin-associated protein/mammalian target of rapamycin (FRAP/mTOR). Consistent with this, the Type I IFN-dependent phosphorylation/activation of p70 S6K is defective in embryonic fibroblasts from mice with targeted disruption of the p85alpha and p85beta subunits of the PI 3'-kinase (p85alpha-/-beta-/-). Treatment of sensitive cell lines with IFNalpha or IFNbeta also results in phosphorylation/inactivation of the 4E-BP-1 repressor of mRNA translation. Such 4E-BP1 phosphorylation is also PI3'-kinase-dependent and rapamycin-sensitive, indicating that the Type I IFN-inducible activation of PI3'-kinase and FRAP/mTOR results in dissociation of 4E-BP1 from the eukaryotic initiation factor-4E (eIF4E) complex. Altogether, our data establish that the Type I IFN receptor-activated PI 3'-kinase pathway mediates activation of the p70 S6 kinase and inactivation of 4E-BP1, to regulate mRNA translation and induction of Type I IFN responses.