The authors thank Dr Victor de Jager for assistance with the Rosetta
Resolver software; Dr Ivo Touw for many fruitful discussions and
critical reading of the manuscript; Liu Wing for technical assistance;
Drs Peter Seither, Andreas Weith (Boehringer Ingelheim,
Biberach, Germany), Helmuth Dolznig, Thomas Waerner, and
Sandra Pilat (IMP, Vienna, Austria) for mRNA profiling of
erythroblasts, of which the complete data will be published
elsewhere; Dr Bart Aarts (Erasmus MC, Rotterdam, The Netherlands)
for assistance in confocal scanning microscopy; Dr David Brautigan
(University of Virginia, Charlottesville) for anti-Igbp1 antibodies;
Dr Manfred Boehm (National Institutes of Health/National
Heart, Lung, and Blood Institute, Bethesda, MD) for anti-Uhmk1
antibodies; and Ortho-Biotech (Tilburg, The Netherlands) for their
kind gift of Eprex (erythropoietin).Stem cell factor (SCF)–induced activation
of phosphoinositide-3-kinase (PI3K) is required
for transient amplification of the
erythroblast compartment. PI3K stimulates
the activation of mTOR (target of
rapamycin) and subsequent release of
the cap-binding translation initiation factor
4E (eIF4E) from the 4E-binding protein
4EBP, which controls the recruitment of
structured mRNAs to polysomes. Enhanced
expression of eIF4E renders proliferation
of erythroblasts independent of
PI3K. To investigate which mRNAs are
selectively recruited to polysomes, we
compared SCF-dependent gene expression
between total and polysome-bound
mRNA. This identified 111 genes primarily
subject to translational regulation. For
8 of 9 genes studied in more detail, the
SCF-induced polysome recruitment of
transcripts exceeded 5-fold regulation and
was PI3K-dependent and eIF4E-sensitive,
whereas total mRNA was not affected by
signal transduction. One of the targets,
Immunoglobulin binding protein 1 (Igbp1),
is a regulatory subunit of protein phosphatase
2A (Pp2a) sustaining mTOR signaling.
Constitutive expression of Igbp1
impaired erythroid differentiation, maintained
4EBP and p70S6k phosphorylation,
and enhanced polysome recruitment
of multiple eIF4E-sensitive mRNAs.
Thus, PI3K-dependent polysome recruitment
of Igbp1 acts as a positive feedback
mechanism on translation initiation underscoring
the important regulatory role of
selectivemRNArecruitment to polysomes
in the balance between proliferation and
maturation of erythroblasts. (Blood. 2008;
112:2750-2760)peer-reviewe
