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1 Division of Nephrology, Department of Medicine, University of Texas Health Science Center, South Texas Veterans Health Care System, 2 Geriatrics Research and Education Center, San Antonio, Texas 78229-3900; and 3 Department of Biochemistry, McGill University, Montreal, Quebec, Canada H3Z 2Z3
Protein synthesis is required for renal
hypertrophy, and proximal tubular epithelial cells are an important
cell type involved in this process. We examined IGF-I regulation of
protein synthesis in murine proximal tubular epithelial (MCT) cells. We
focused on initial events in protein translation and the signaling
events involved. Translation of capped mRNAs is under the control of eukaryotic initiation factor 4E (eIF4E). In the resting cell, eIF4E is
normally kept in an inactive state by binding to 4E-BP1, its binding
protein. Phosphorylation of 4E-BP1 results in dissociation of the
eIF4E-4E-BP1 complex allowing eIF4E to initiate peptide synthesis.
IGF-I stimulated protein synthesis, augmented phosphorylation of 4E-BP1
and promoted the dissociation of eIF4E from 4E-BP1. IGF-I stimulated
the activities of phosphatidylinositol (PI) 3-kinase, Akt, and
ERK1/2-type MAPK in MCT cells. IGF-I-induced phosphorylation of 4E-BP1,
dissociation of the 4E-BP1-eIF4E complex, and increase in protein
synthesis required activation of both PI 3-kinase and ERK pathways.
Furthermore, ERK activation by IGF-I was also PI 3-kinase dependent.
Transfection with the Thr37,46
Ala37,46
mutant of 4E-BP1 showed that phosphorylation of Thr37,46
residues was required for IGF-I induction of protein synthesis in MCT
cells. Our observations reveal the importance of initial events in
protein translation in IGF-I-induced protein synthesis in MCT cells and
identify the regulatory signaling pathways involved.
phosphatidylinositol 3-kinase; mitogen-activated protein kinase; protein translation; eukaryotic initiation factor 4E; eIF4E binding protein
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