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Grelet et al. J Cancer Metastasis Treat 2019;5:16 I http://dx.doi.org/10.20517/2394-4722.2018.85 Page 5 of 10

of mu opioid receptor (MOR) mRNA via interaction with ARE RNA-binding protein 1 (AUF1) and poly A
[44]
binding protein (PABP) .

Alternative polyadenylation
The addition of the poly-(A) to the messenger RNA 3’ UTR is a co-transcriptional process occurring in the
nucleus. As 3’ UTRs contain cis elements that are involved in various aspects of mRNA metabolism, 3’ UTR
alternative polyadenylation (APA) can affect post-transcriptional gene regulation considerably in various
ways, including modulation of stability, translation, nuclear export and cellular localization of mRNA. 3’
[45]
UTR-APA can also affect the localization of the encoded protein . Polyadenylation of mRNA is a two-step
process consisting of endonucleolytic cleavage and addition of an untemplated poly(A) tail.

The role for hnRNP E1 in the regulation of alternative polyadenylation has been established in an in vitro-
[46]
transcribed and polyadenylated alpha-globin 3’ UTR assay . Furthermore, a screening study for alternative
polyadenylation utilizing RNA interference (RNAi) identified hnRNP E1 as the second highest factor in the
control of polyadenylation signal usage. The mechanism by which hnRNP E1 modulates polyadenylation has
[47]
yet to be characterized .
Direct control of translation machinery through BAT elements
As we and others have previously demonstrated, the regulation of gene expression at the post-transcriptional
level plays an indispensable role in TGFβ-induced EMT and metastasis [30,34,48-52] . We identified a transcript-
selective translational regulatory pathway in which a ribonucleoprotein (mRNP) complex binds to a
33-nucleotide TGFβ-activated translation (BAT) element in the mRNA 3’ UTR and silences the translation of
a cohort of mesenchymal protein-encoding mRNAs. HnRNP E1 is a critical component of the BAT-binding
[53]
mRNP complex . Silenced mRNAs include Disabled2 (Dab2) and Interleukin-like EMT-inducer (ILEI),
which are involved in mediating EMT [34,51,54,55] . Furthermore, TGFβ activates a kinase cascade terminating
in the phosphorylation of hnRNP E1 protein at Serine 43. This phosphorylation occurs by isoform-2 specific
activation of Akt and induces the release of the mRNP complex from the BAT element. This results in the
reversal of translational silencing of mesenchymal protein-encoding transcripts that are required for EMT.
By using a genome-wide combinatorial approach involving expression, polysome profiling and RIP-Chip
analysis, we have identified the members of the cohort of translationally regulated mRNAs that are induced
[53]
during TGFβ-mediated EMT .

At the molecular level, the eukaryotic elongation factor-1 A1 (eEF1A1) is an important additional functional
component of the mRNP complex. We have previously demonstrated that the BAT element, hnRNP E1
and eEF1A1 form a ternary complex that mediates translational silencing at the translational elongation
[35]
step . In non-stimulated cells exhibiting epithelial characteristics, hnRNP E1 binds to eEF1A1 and blocks
progression of the 80S ribosome by preventing the release of eEF1A1 from the ribosomal A-site following
GTP hydrolysis. EMT induced by either TGFβ or hnRNP E1 knockdown disrupts the mRNP complex,
allowing eEF1A1-mediated translational elongation of mesenchymal transcripts to proceed [34,35] [Figure 2].

This mode of translational regulation represents an unusual case of dependency upon either agonists or
stimuli to upregulate translation through 3’-UTR elements. Thus, the elucidation of this post-transcriptional
regulatory pathway identified an “EMT gene signature” and provided mechanistic information as to how
cell plasticity could be tightly regulated. Taken together, this work underscores the contribution of the non-
phosphorylated hnRNP E1 protein to maintenance of epithelial cell integrity under normal conditions.
During tumor-related epithelial plasticity, hnRNP E1 also acts as the trigger for the reversal of translational
silencing, resulting in a fine-tuned, spatio-temporally controlled increase in mesenchymal protein
expression.