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Cell cycle progression and proliferation despite 4BP-1 dephosphorylation
1Molecular Cardiology Program, Divisions of Cardiology and Circulatory Physiology, Department of Medicine, Columbia University College of Physicians and Surgeons, New York, New York 10032, USA.
Abstract:
Proliferation and cell cycle progression in response to growth factors require de novo protein synthesis. It has been proposed that binding of the eukaryotic translation initiation factor 4E (eIF-4E) to the inhibitory protein 4BP-1 blocks translation by preventing access of eIF-4G to the 5' cap of the mRNA. The signal for translation initiation is thought to involve phosphorylation of 4BP-1, which causes it to dissociate from eIF-4E and allows eIF-4G to localize to the 5' cap. It has been suggested that the ability of the macrolide antibiotic rapamycin to inhibit 4BP-1 phosphorylation is responsible for the potent antiproliferative property of this drug. We now show that rapamycin-resistant cells exhibited normal proliferation despite dephosphorylation of 4BP-1 that allows it to bind to eIF-4E. Moreover, despite rapamycin-induced dephosphorylation of 4BP-1, eIF-4E-eIF-4G complexes (eIF-4F) were still detected. In contrast, amino acid withdrawal, which caused a similar degree of 4BP-1 dephosphorylation, resulted in dissociation of the eIF-4E-eIF-4G complex. Thus, 4BP-1 dephosphorylation is not equivalent to eIF-4E inactivation and does not explain the antiproliferative property of rapamycin.
Insights
Rapamycin
Area of Science:
- Cell Biology
- Molecular Biology
- Biochemistry
Background:
- Cell proliferation and cell cycle progression depend on new protein synthesis, regulated by translation initiation factors.
- The eukaryotic translation initiation factor 4E (eIF-4E) binding to 4BP-1 inhibits translation by blocking eIF-4G access to mRNA 5' caps.
- Phosphorylation of 4BP-1 is proposed to release eIF-4E, enabling translation initiation.
Purpose of the Study:
- To investigate the role of 4BP-1 dephosphorylation in rapamycin's antiproliferative effects.
- To determine if 4BP-1 dephosphorylation is sufficient to explain rapamycin's antiproliferative properties.
Main Methods:
- Analysis of rapamycin-resistant cells.
- Assessment of 4BP-1 phosphorylation status.
- Detection of eIF-4E-eIF-4G complexes (eIF-4F) under various conditions.
- Comparison of rapamycin treatment with amino acid withdrawal.
Main Results:
- Rapamycin-resistant cells maintained normal proliferation despite 4BP-1 dephosphorylation.
- Rapamycin-induced 4BP-1 dephosphorylation did not prevent the formation of eIF-4E-eIF-4G complexes.
- Amino acid withdrawal caused 4BP-1 dephosphorylation and dissociation of the eIF-4E-eIF-4G complex.
Conclusions:
- 4BP-1 dephosphorylation is not equivalent to eIF-4E inactivation.
- The antiproliferative effect of rapamycin is not solely explained by 4BP-1 dephosphorylation.
- Regulation of translation initiation is more complex than previously suggested.