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Molecular and Cellular Biology, January 2009, p. 559-569, Vol. 29, No. 2
0270-7306/09/$08.00+0     doi:10.1128/MCB.01041-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

Heat Shock Protein Hsp72 Controls Oncogene-Induced Senescence Pathways in Cancer Cells{triangledown}

Vladimir L. Gabai,1,{dagger} Julia A. Yaglom,1,{dagger} Todd Waldman,2 and Michael Y. Sherman1*

Department of Biochemistry, Boston University School of Medicine, Boston, Massachusetts 02118,1 Lombardi Comprehensive Cancer Center, Georgetown University School of Medicine, Washington, DC 200572

Received 2 July 2008/ Returned for modification 11 August 2008/ Accepted 29 October 2008

The heat shock protein Hsp72 is expressed at the elevated levels in various human tumors, and its levels often correlate with poor prognosis. Previously we reported that knockdown of Hsp72 in certain cancer cells, but not in untransformed breast epithelial cells, triggers senescence via p53-dependent and p53-independent mechanisms. Here we demonstrate that the p53-dependent pathway controlled by Hsp72 depends on the oncogenic form of phosphatidylinositol 3-kinase (PI3K). Indeed, upon expression of the oncogenic PI3K, epithelial cells began responding to Hsp72 depletion by activating the p53 pathway. Moreover, in cancer cell lines, activation of the p53 pathway caused by depletion of Hsp72 was dependent on oncogenes that activate the PI3K pathway. On the other hand, the p53-independent senescence pathway controlled by Hsp72 was associated with the Ras oncogene. In this pathway, extracellular signal-regulated kinases (ERKs) were critical for senescence, and Hsp72 controlled the ERK-activating kinase cascade at the level of Raf-1. Importantly, upon Ras expression, untransformed cells started responding to knockdown of Hsp72 by constitutive activation of ERKs, culminating in senescence. Therefore, Hsp72 is intimately involved in suppression of at least two separate senescence signaling pathways that are regulated by distinct oncogenes in transformed cells, which explains why cancer cells become "addicted" to this heat shock protein.

* Corresponding author. Mailing address: Department of Biochemistry, Boston University Medical School, 715 Albany St., Boston, MA 02118. Phone: (617) 638-5971. Fax: (617) 638-5339. E-mail: sherma1{at}bu.edu

{triangledown} Published ahead of print on 10 November 2008.

{dagger} V.L.G. and J.A.Y. contributed equally to this study.

Molecular and Cellular Biology, January 2009, p. 559-569, Vol. 29, No. 2
0270-7306/09/$08.00+0     doi:10.1128/MCB.01041-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.





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