Overexpression of the mTOR alpha4 phosphoprotein activates protein phosphatase 2A and increases Stat1α binding to PIAS1

Alpha4 phosphoprotein in the mTOR pathway is a prolactin (PRL)-downregulated gene product that interacts with the catalytic subunit of serine/threonine protein phosphatase 2A (PP2Ac) in rat Nb2 lymphoma cells. Transient overexpression of alpha4 in COS-1 cells inhibited PRL-inducible interferon-regulatory-1 (IRF-1) promoter activity, but the mechanism underlying this inhibition was not known. The present study showed a stable alpha4–PP2Ac complex that was not dissociated by rapamycin in COS-1 cells. Transient overexpression of alpha4 in COS-1 cells had no effect on endogenous PP2Ac protein levels but significantly increased PP2Ac carboxymethylation and PP2A activity as compared to controls. The increased PP2A activity was accompanied by decreased phosphorylation of eukaryotic initiation factor 4E-binding protein (4E-BP1) but had no effect on Stat phosphorylation. However, overexpressed alpha4 decreased arginine methylation of Stat1α and increased Stat1α binding to the Stat1α-specific inhibitor, PIAS1. In summary, ectopic alpha4 increased PP2A activity in COS-1 cells and this was accompanied by Stat1α hypomethylation and increased Stat1α–PIAS1 association. These events would inhibit Stat action and ultimately inhibit PRL-inducible IRF-1 promoter activity.