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J. Biol. Chem., Vol. 283, Issue 19, 12851-12861, May 9, 2008

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Clusterin Activates Survival through the Phosphatidylinositol 3-Kinase/Akt Pathway^*

From the Biochemistry Department, Institute of Pathology B23, Avenue de l'Hôpital, 3, University of Liège, Liège 4000, Belgium

Clusterin is, in its major form, a secreted heterodimeric disulfide-linked glycoprotein (75–80 kDa). It was first linked to cell death in the rat ventral prostate after androgen deprivation. Recent studies have demonstrated that overexpression of clusterin in prostatic cells protects them against tumor necrosis factor- (TNF)-induced apoptosis. However the details of this survival mechanism remain undefined. Here, we investigate how clusterin prevents cells from undergoing TNF-induced apoptosis. We established a double-stable prostatic cell line for inducible clusterin by using the Tet-On gene expression system. We demonstrated that 50% of the cells overexpressing clusterin escaped from TNF- and actinomycin D-induced cell death. Moreover we demonstrated that the incubation of MLL cells with conditioned medium containing the secreted clusterin or the supplementation of purified clusterin in the extracellular medium decreased the TNF-induced apoptosis significantly. This extracellular action implicates megalin, the putative membrane receptor for clusterin to mediate survival. Indeed clusterin overexpression up-regulated the expression of megalin and induced its phosphorylation in a dose-dependent manner. We interestingly showed that clusterin overexpression is associated with the up-regulation of the phosphorylation of Akt. Activated Akt induced the phosphorylation of Bad and caused a decrease of cytochrome c release. These results enable us to pinpoint one mechanism by which secreted clusterin favors survival in androgen-independent prostate cancer cells, implicating its receptor megalin and Akt survival pathway.

Received for publication, January 16, 2008 , and in revised form, March 5, 2008.

^* This work was supported in part by the Fonds National de la Recherche Scientifique. 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 A beneficiary of a "Télévie" research grant.

² To whom correspondence should be addressed. Tel. 32-43-66-24-24; Fax: 32-43-66-45-85; E-mail: jclosset{at}ulg.ac.be.

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