Transient and sustained ERK phosphorylation and nuclear translocation in growth control

Abstract

Growth stimulation and inhibition are both associated with tyrosine phosphorylation. We examined the effects of epidermal growth factor (EGF), a growth stimulant, and compound 5 (Cpd 5), a protein‐tyrosine phosphatase (PTPase) inhibitor, which inhibits the growth of the same Hep3B hepatoma cells. We found that both EGF and Cpd 5 induced tyrosine phosphorylation of EGF receptor (EGFR) and ERK. However, the phosphorylation caused by EGF was transient and that caused by Cpd 5 was prolonged. Furthermore, Cpd 5 action caused a strong nuclear phospho‐ERK signal and induced phospho‐Elk‐1, a nuclear target of ERK activation, in contrast to the weak effects of EGF. An ERK kinase assay demonstrated that ERK activated by Cpd 5 could phosphorylate its physiological substrate, Elk‐1. The MEK inhibitors PD098056 and U0126 abrogated both the induction by Cpd 5 of phospho‐ERK, its nuclear translocation and phospho‐Elk‐1 and also antagonized its growth inhibitory effects. Furthermore, phospho‐ERK phosphatase and phospho‐Elk‐1 activities were lost from nuclear extracts from Cpd 5 treated, but not EGF treated cells. In conclusion, the data show that Cpd 5 causes growth inhibition as a consequence of prolonged ERK and Elk‐1 phosphorylation, likely a result of inhibition of multiple PTPases, including those acting on phospho‐EGFR, on phospho‐ERK, and on phospho‐Elk‐1, in contrast to the kinase driven transient activation resulting from EGF.