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J. Biol. Chem., Vol. 281, Issue 26, 17758-17767, June 30, 2006

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Urokinase Signaling through Its Receptor Protects against Anoikis by Increasing BCL-xL Expression Levels^*

From the Institute of Genetics and Biophysics "A. Buzzati Traverso," Consiglio Nazionale delle Ricerche (CNR), 80131 Naples, Italy

The acquired capabilities of resistance to apoptotic cell death and tissue invasion are considered to be obligate steps in tumor progression. The binding of the serine protease urokinase (uPA) to its receptor (uPAR) plays a central role in the molecular events coordinating tumor cell adhesion, migration, and invasion. Here we investigate whether uPAR signaling may also prevent apoptosis following loss of anchorage (anoikis) or DNA damage. If nontransformed human retinal pigment epithelial cells are pre-exposed to uPA or to its noncatalytic amino-terminal region (residues 1–135), they exhibit a markedly reduced susceptibility to anoikis as well as to UV-induced apoptosis. This anti-apoptotic effect is retained by a uPA-derived synthetic peptide corresponding to the receptor binding domain and is inhibited by anti-uPAR polyclonal antibodies. Furthermore, the stable reduction of uPA or uPAR expression by RNA interference leads to an increased susceptibility to UV-, cisplatin-, and detachment-induced apoptosis. In particular, the level of uPAR expression positively correlates with cell resistance to anoikis. The protective ability of uPA is prevented by UO126, LY294002, by an MAPK targeting small interference RNA, and by a dominant negative Akt variant. Accordingly, incubation of retinal pigment epithelial cells with uPA elicits a time-dependent enhancement of MAPK and phosphatidylinositol 3-kinase activities as well as the transcriptional activation of Bcl-xL anti-apoptotic factor. Vice versa, the silencing of Bcl-xL expression prevents uPA protection from anoikis. In conclusion, the data show that ligand engagement of uPAR promotes cell survival by activating Bcl-xL transcription through the MEK/ERK- and phosphatidylinositol 3-kinase/Akt-dependent pathways.

Received for publication, February 24, 2006 , and in revised form, April 10, 2006.

^* This work was supported by the Italian Association for Cancer Research, by the Association for International Cancer Research, and by European Union Framework Programme 6 Grant LSHC-CT-2003-503297. 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: Institute of Genetics and Biophysics "Adriano Buzzati-Traverso," Via P. Castellino 111, 80131 Naples, Italy. Tel.: 39-081-6132450; Fax: 39-081-6132706; E-mail: stoppell{at}igb.cnr.it.

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