Abstract
BackgroundEukaryotic initiation factor 4E (eIF4E) is essential for cap-dependent initiation of translation. Cell proliferation is associated with increased activity of eIF4E and elevated expression of eIF4E leads to tumorigenic transformation. Many tumors express very high levels of eIF4E and this may be a critical factor in progression of the disease. In contrast, overexpression of 4EBP, an inhibitor of eIF4E, leads to cell cycle arrest and phenotypic reversion of some transformed cells.ResultsA constitutively active form of 4EBP-1 was inducibly expressed in the human breast cancer cell line MCF7. Induction of constitutively active 4EBP-1 led to cell cycle arrest. This was not associated with a general inhibition of protein synthesis but rather with changes in specific cell cycle regulatory proteins. Cyclin D1 was downregulated while levels of the CDK inhibitor p27Kip1 were increased. The levels of cyclin E and CDK2 were unaffected but the activity of CDK2 was significantly reduced due to increased association with p27Kip1. The increase in p27Kip1 did not reflect changes in p27Kip1 mRNA or degradation rates. Rather, it was associated with enhanced synthesis of the protein, even though 4EBP-1 is expected to inhibit translation. This could be explained, at least in part, by the ability of the p27Kip1 5'-UTR to mediate cap-independent translation, which was also enhanced by expression of constitutively active 4EBP-1.ConclusionsExpression of active 4EBP-1 in MCF7 leads to cell cycle arrest which is associated with downregulation of cyclin D1 and upregulation of p27Kip1. Upregulation of p27Kip1reflects increased synthesis which corresponds to enhanced cap-independent translation through the 5'-UTR of the p27Kip1 mRNA.
Highlights
Eukaryotic initiation factor 4E is essential for cap-dependent initiation of translation
The 4E binding protein (4EBP)-1-5A cDNA was placed under the control of a promoter that is responsive to the insect hormone ponasterone A, an ecdysone analog
The stably transfected MCF7 line (MCF7-4EBP-1-5A) was treated with ponasterone A (pon A) and at various times cells were harvested for analysis of 4EBP-1 expression by Western blotting (Fig. 1A)
Summary
Eukaryotic initiation factor 4E (eIF4E) is essential for cap-dependent initiation of translation. Cell proliferation is associated with increased activity of eIF4E and elevated expression of eIF4E leads to tumorigenic transformation. Most eukaryotic mRNAs are translated through a cap-dependent mechanism of initiation. In this process, eIF4E, which functions by binding the 7-methylguanosine cap, is the rate-limiting factor due to its low abundance [1,2]. In normal cells, elevated eIF4E activity is associated with proliferation and is regulated by multiple mechanisms. An inhibitor of eIF4E, 4E binding protein (4EBP), is regulated by phosphorylation. The net result is that phosphorylation of 4EBP enhances cap-dependent translation initiation. The net result is that phosphorylation of 4EBP enhances cap-dependent translation initiation. 4EBP (page number not for citation purposes)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
