Identification and characterization of an hnRNP E1 translational silencing motif

Non-canonical transforming growth factor beta (TGF beta) signaling through protein kinase B (Akt2) induces phosphorylation of heterogeneous nuclear ribonucleoprotein E1 (hnRNP E1) at serine-43 (p-hnRNP E1). This post-translational modification (PTM) of hnRNP E1 promotes its dissociation from a 3' untranslated region (UTR) nucleic acid regulatory motif, driving epithelial to mesenchymal transition (EMT) and metastasis. We have identified an hnRNP E1 consensus-binding motif and genomically resolved a subset of genes in which it is contained. This study characterizes the binding kinetics of the consensus-binding motif and hnRNP E1, its various K-homology (KH) domains and p-hnRNP E1. Levels of p-hnRNP E1 are highly upregulated in metastatic cancer cells and low in normal epithelial tissue. We show a correlation between this PTM and levels of Akt2 and its activated form, phosphorylated serine-474 (p-Akt2). Using cellular progression models of metastasis, we observed a signature high level of Akt2, p-Akt2 and p-hnRNP E1 protein expression, coupled to a significantly reduced level of total hnRNP E1 in metastatic cells. Genes that are translationally silenced by hnRNP E1 and expressed by its dissociation are highly implicated in the progression of EMT and metastasis. This study provides insight into a non-canonical TGF beta signaling cascade that is responsible for inducing EMT by aberrant expression of hnRNP E1 silenced targets. The relevance of this system in metastatic progression is clearly shown in cellular models by the high abundance of p-hnRNP E1 and low levels of hnRNP E1. New insights provided by the resolution of this molecular mechanism provide targets for therapeutic intervention and give further insight into the role of the TGF beta microenvironment.