UNIVERSITY OF MALTA
RESEARCH SEMINARS
Abstract form
Title: Translation of IGBP1 mRNA contributes to the regulation of
expansion and differentiation of erythroid progenitors
Presenter: Godfrey Grech B.Sc., M.Phil.
Contact address: Erasmus MC, Department of Hematology, Rotterdam
Tel: 0031 010 408 8300
Fax:
Email: g.grech@erasmusmc.nl
Presentation date: 28th November 2005
(approximately 200-250 words)
Abstract
Erythroid progenitors can be expanded in vitro in the presence of erythropoietin (Epo), stem
cell factor (SCF) and dexamethasone, while they differentiate to enucleated erythrocytes in
presence of Epo only. Our study aims to identify (i) signaling pathways that control
expansion of erythroid progenitors and (ii) genes regulated by these signaling pathways. SCF
strongly activates phosphotidylinositol 3 kinase (PI3K). Since inhibition of PI3K with
LY294002 induces terminal differentiation of erythroid progenitors under Epo and SCF
stimulation, SCF seems to enhance renewal divisions of erythroid progenitors via a PI3Kdependent mechanism. An important PI3K-dependent process in cell proliferation is
regulation of mRNA translation. PI3K controls the activity of mTOR (mammalian target of
rapamycin), whose activation results in phosphorylation of eIF4E (eukaryote Initiation
Factor 4E)-binding protein (4E-BP). Fully phosphorylated 4E-BP releases eIF4E, which can
subsequently bind eIF4G, the scaffolding protein of the eIF4F cap-binding complex. In
particular mRNAs with a structured UTR (untranslated region) require optimal availability
of eIF4E to be translated. SCF, but not Epo can induce full phosphorylation of 4E-BP and
efficient formation of the eIF4F cap-binding complex. Overexpression of eIF4E inhibited
erythroid differentiation, indicating that SCF-induced formation of the eIF4F complex
contributes to progenitor expansion. A major step in mRNA translation controlled by eIF4F
is polysome recruitment. To identify genes whose expression is regulated by signalinginduced polysome recruitment, we compared total and polysome-bound mRNA from factor
deprived and Epo-, SCF- or Epo plus SCF restimulated progenitors on gene-expression
micro-arrays. Candidate genes (17) were selected using ANOVA. Polysome recruitment of
15/17 targets tested so far appeared to be dependent on PI3K activation and eIF4E
expression. Constitutive expression of these targets in erythroid progenitors revealed that
IGBP1 (Immunoglobulin binding protein 1) was able to inhibit erythroid differentiation
comparable to overexpression of eIF4E. IGBP1 binds to and modulates the activity of the
catalytic subunit of PP2A, a major cellular serine/threonine phosphatase, which also
dephosphorylates 4E-BP. Overexpression of IGBP1 does not inhibit 4E-BP
dephosphorylation in absence of factor, but maintains phosphorylation of 4E-BP in presence
of Epo. Hence, we elucidate a mechanism by which the IGBP1/PP2A complex prolongs the
phosphorylation of mTOR targets, which may result in inhibition of erythroid progenitor
differentiation.