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J. Biol. Chem., Vol. 280, Issue 18, 18517-18524, May 6, 2005

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The RNA-binding Protein IMP-3 Is a Translational Activator of Insulin-like Growth Factor II Leader-3 mRNA during Proliferation of Human K562 Leukemia Cells^*

Baisong Liao, Yan Hu, David J. Herrick¶, and Gary Brewer||

From the Department of Molecular Genetics, Microbiology, and Immunology, Robert Wood Johnson Medical School, University of Medicine and Dentistry of New Jersey, Piscataway, New Jersey 08854 and the ^¶Department of Dermatology and Cutaneous Biology, Thomas Jefferson University, Philadelphia, Pennsylvania 19107

IMP-3, a member of the insulin-like growth factor-II (IGF-II) mRNA-binding protein (IMP) family, is expressed mainly during embryonic development and in some tumors. Thus, IMP-3 is considered to be an oncofetal protein. The functional significance of IMP-3 is not clear. To identify the functions of IMP-3 in target gene expression and cell proliferation, RNA interference was employed to knock down IMP-3 expression. Using human K562 leukemia cells as a model, we show that IMP-3 protein associates with IGF-II leader-3 and leader-4 mRNAs and H19 RNA but not c-myc and -actin mRNAs in vivo by messenger ribonucleoprotein immunoprecipitation analyses. IMP-3 knock down significantly decreased levels of intracellular and secreted IGF-II without affecting IGF-II leader-3, leader-4, c-myc, or -actin mRNA levels and H19 RNA levels compared with the negative control siRNA treatment. Moreover, IMP-3 knock down specifically suppressed translation of chimeric IGF-II leader-3/luciferase mRNA without altering reporter mRNA levels. Together, these results suggest that IMP-3 knock down reduced IGF-II expression by inhibiting translation of IGF-II mRNA. IMP-3 knock down also markedly inhibited cell proliferation. The addition of recombinant human IGF-II peptide to these cells restored cell proliferation rates to normal. IMP-3 and IMP-1, two members of the IMP family with significant structural similarity, appear to have some distinct RNA targets and functions in K562 cells. Thus, we have identified IMP-3 as a translational activator of IGF-II leader-3 mRNA. IMP-3 plays a critical role in regulation of cell proliferation via an IGF-II-dependent pathway in K562 leukemia cells.

Received for publication, January 7, 2005 , and in revised form, February 25, 2005.

^* This work was supported by National Institutes of Health Grant CA094243 (to G. B.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

These authors contributed equally to this work.

^|| To whom correspondence should be addressed: Dept. of Molecular Genetics, Microbiology and Immunology, Robert Wood Johnson Medical School, University of Medicine and Dentistry of New Jersey, 675 Hoes Lane, Piscataway, NJ 08854. Tel.: 732-235-3473; Fax: 732-235-5223; E-mail: brewerga{at}umdnj.edu.

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