IGF2 mRNA binding protein p62/IMP2-2 in hepatocellular carcinoma: antiapoptotic action is independent of IGF2/PI3K signaling

Abstract

The insulin-like growth factor II ( IGF2) mRNA binding protein (IMP) p62/IMP2-2, originally isolated from a hepatocellular carcinoma (HCC) patient, induces a steatotic phenotype when overexpressed in mouse livers. Still, p62 transgenic livers do not show liver cell damage but exhibit a pronounced induction of Igf2 and activation of the downstream survival kinase AKT. The aim of this study was to investigate the relation between p62 and IGF2 expression in the human system and to study potential antiapoptotic actions of p62. p62 and IGF2 mRNA levels were assessed by real-time RT-PCR. For knockdown and overexpression experiments, human hepatoma HepG2 and PLC/PRF/5 cells were transfected with siRNA or plasmid DNA. Phosphorylated AKT and ERK1/2 were analyzed by Western blot. Investigations of 32 human HCC tissues showed a strong correlation between p62 and IGF2 expression. Of note, p62 expression was increased markedly in patients with poor outcome. In hepatoma cells overexpression of p62 lowered levels of doxorubicin-induced caspase-3-like activity. Vice versa, knockdown of p62 resulted in increased doxorubicin-induced apoptosis. However, neither PI3K inhibitors nor a neutralizing IGF2 antibody showed any effects. Western blot analysis revealed increased levels of phosphorylated ERK1/2 in hepatoma cells overexpressing p62 and decreased levels in p62 knockdown experiments. When p62-overexpressing cells were treated with ERK1/2 inhibitors, the apoptosis-protecting effect of p62 was completely abrogated. Our data demonstrate that p62 exerts IGF2-independent antiapoptotic action, which is facilitated via phosphorylation of ERK1/2. Furthermore, p62 might serve as a new prognostic marker in HCC.