EGF activates cytoplasmic FAK to enhance KOR translation in the cytoplasm. (A) Western blot analysis detecting KOR, Grb7, and anti–phospho-tyrosine–immunoprecipitated Grb7 from the cytoplasmic (top) or nuclear (bottom) fractions of P19 cells. FAK, phospho-FAK (pFAK), Flag, Grb7, and actin serve as the controls for transfection and input. RT-qPCR assays of KOR and actin mRNA are shown on the right. Error bars represent SDs. CY, cytoplasmic fraction; NE, nuclear fraction. (B) Western blot analysis of P19 cells transfected as indicated and in the presence or absence of EGF. (C) Model of EGF’s dual action to elevate the level of KOR. Without EGF, nuclear SHP-2 and cytoplasmic FAK are inactive (i–SHP-2 and i-FAK). Upon EGF stimulation, unknown pathways activate nuclear SHP-2 (a–SHP-2), which dephosphorylates nuclear Grb7. Dephosphorylated nuclear Grb7 recruits KOR mRNA and HuR to form a nuclear export complex, thereby shuttling KOR mRNA to the cytoplasm. EGF also activates cytoplasmic FAK (a-FAK), which phosphorylates the preexisting cytoplasmic and nuclear-exported Grb7, releasing KOR mRNA for translation.