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J. Biol. Chem., Vol. 283, Issue 13, 8601-8610, March 28, 2008

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Suppression of Programmed Cell Death 4 (PDCD4) Protein Expression by BCR-ABL-regulated Engagement of the mTOR/p70 S6 Kinase Pathway^*

Nathalie Carayol, Efstratios Katsoulidis, Antonella Sassano, Jessica K. Altman, Brian J. Druker, and Leonidas C. Platanias¹

From the Robert H. Lurie Comprehensive Cancer Center and Division of Hematology/Oncology, Northwestern University Medical School and Jesse Brown Veterans Affairs Medical Center, Chicago, Illinois 60611 and the Howard Hughes Medical Institute and Department of Hematology and Medical Oncology, Oregon Health and Sciences University, Portland, Oregon 97239

There is accumulating evidence that mammalian target of rapamycin (mTOR)-activated pathways play important roles in cell growth and survival of BCR-ABL-transformed cells. We have previously shown that the mTOR/p70 S6 kinase (p70 S6K) pathway is constitutively activated in BCR-ABL transformed cells and that inhibition of BCR-ABL kinase activity by imatinib mesylate abrogates such activation. We now provide evidence for the existence of a novel regulatory mechanism by which BCR-ABL promotes cell proliferation, involving p70 S6K-mediated suppression of expression of programmed cell death 4 (PDCD4), a tumor suppressor protein that acts as an inhibitor of cap-dependent translation by blocking the translation initiation factor eIF4A. Our data also establish that second generation BCR-ABL kinase inhibitors block activation of p70 S6K and downstream engagement of the S6 ribosomal protein in BCR-ABL transformed cells. Moreover, PDCD4 protein expression is up-regulated by inhibition of the BCR-ABL kinase in K562 cells and BaF3/BCR-ABL transfectants, suggesting a mechanism for the generation of the proapoptotic effects of such inhibitors. Knockdown of PDCD4 expression results in reversal of the suppressive effects of nilotinib and imatinib mesylate on leukemic progenitor colony formation, suggesting an important role for this protein in the generation of antileukemic responses. Altogether, our studies identify a novel mechanism by which BCR-ABL may promote leukemic cell growth, involving sequential engagement of the mTOR/p70 S6K pathway and downstream suppression of PDCD4 expression.

Received for publication, September 21, 2007 , and in revised form, January 2, 2008.

^* This work was supported by National Institutes of Health Grants CA77816, CA100579, CA121192, and CA94079 and a merit review grant from the Department of Veterans Affairs. 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.

¹ To whom correspondence should be addressed: Robert H. Lurie Comprehensive Cancer Center, 303 E. Superior St., Lurie 3-107, Chicago, IL 60611. Tel.: 312-503-4267; Fax: 312-908-1372; E-mail: l-platanias{at}northwestern.edu.

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