The extracellular signal-regulated kinase pathway regulates the phosphorylation of 4E-BP1 at multiple sites

The phorbol ester, 12-O-tetradecanoylphorbol 13-acetate (TPA), a potent stimulator of Erk, leads to the phosphorylation of 4E-BP1 and its dissociation from eIF4E. In contrast to agonists such as insulin, this occurs independently of PKB activation. In this report, we investigate the mechanism by which TPA regulates 4E-BP1 phosphorylation. Treatment of HEK293 cells with TPA was found to result in the phosphorylation of 4E-BP1 at Ser⁶⁴, Thr⁶⁹, and Thr^36/45. The TPA-stimulated phosphorylation of all these sites is sensitive to inhibitors of MEK and to the inhibitor of mTOR, rapamycin, indicating that inputs from both mTOR and MEK are required for the regulation of 4E-BP1 phosphorylation by TPA. Indeed, evidence is presented that mTOR may initially be required for the phosphorylation of Thr⁴⁵ in a priming step, which is necessary for the subsequent phosphorylation of Ser⁶⁴ and Thr⁶⁹ through an Erk-dependent pathway. Overexpression of constitutively active MEK in HEK293 cells resulted both in the phosphorylation of 4E-BP1 at Ser⁶⁴ and Thr^36/45 and its release from eIF4E. In this case, the phosphorylation of these sites was also blocked by inhibitors of MEK or by rapamycin. In conclusion, the Erk pathway, via mechanisms also requiring mTOR, regulates the phosphorylation of multiple sites in 4E-BP1 in vivo and this is sufficient for the release of 4E-BP1 from eIF4E.