Cell Cycle Progression and Proliferation Despite 4BP-1 Dephosphorylation

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.

ACKNOWLEDGMENTS

We thank J. Blenis for anti-p70^s6k antibody, J. Lawrence for anti-PHAS-I (anti-4BP-1) and anti-eIF-4E antibodies, S. Morley and R. Rhoads for anti-eIF-4G antibodies, V. Richon for MELC and for data on the effects of rapamycin on MELC growth, and J. Hurwitz and D. Cobrinik for critical reading of the manuscript.

This work was supported by the NIH, MDA, and AHA (A.R.M.) and the Richard and Lynne Kaiser Family Foundation and by a grant from the Johnson and Johnson Focused Giving Program. S.O.M. is a recipient of an American Heart Association Clinician Scientist Award and the NY Academy of Medicine Glorney-Raisbeck Fellowship.