The mTOR target 4E-BP1 contributes to differential protein expression during normoxia and hypoxia through changes in mRNA translation efficiency.

Hypoxia causes a rapid and sustained inhibition in mRNA translation that is characterized by both a transient phosphorylation of eukaryotic initiation factor 2-alpha (eIF2alpha) and by inhibition of the mRNA cap binding protein eIF4E via activation of two distinct inhibitory proteins, the mammalian target of rapamycin (mTOR) target 4E-BP1 and the eIF4E transporter 4E-T. Although the importance of eIF2alpha phosphorylation during hypoxia has been clearly demonstrated, there is little information on the potential relevance of eIF4E regulation. We generated HeLa cells stably expressing a short hairpin interfering RNA (shRNA) against 4E-BP1 and found that despite efficient knockdown, no significant changes occurred in the overall inhibition of mRNA translation during hypoxia. However, using a proteomics approach we identified seven proteins that were exclusively expressed in the 4E-BP1 knockdown cells during both normoxic and hypoxic conditions. Further investigation of the transcriptional and translational regulation of these genes by quantitative RT-PCR indicated that the loss of 4E-BP1 causes a significant increase in the rate of protein synthesis of S100 calcium-binding protein A4 (S100A4) and transgelin 2. These 4E-BP1 regulated proteins have previously been associated with tumor cell motility, invasion and metastasis and may thus contribute to an adverse tumor phenotype.