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

Abstract

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.Clusterin, also known as testosterone-repressed message-2, is overexpressed in the rat prostate during castration-induced programmed cell death (1). Clusterin overexpression reaches a maximum 3-4 days post-castration and coincides with the onset of massive cell death (2). The clusterin level also rises in various tissues during cell death responses (3-5). As clusterin is present during apoptosis, it was initially viewed as a cell death inducer, but other studies suggest that clusterin overproduction occurs in resistant cells (6, 7). Thus, clusterin has been described as an anti-apoptotic factor, and it has also been implicated in prostate cancer progression to androgen independence (8). Miyake et al. (9) have demonstrated that the overexpression of clusterin in human androgen-responsive prostate cancer cells LNCaP by stable transfection rendered them highly resistant to androgen ablation, and the introduction of antisense testosterone-repressed message-2 oligodeoxynucleotide therapy in the Shionogi tumor model induces apoptosis and tumor regression. Moreover small interference RNA-mediated clusterin gene silencing in osteosarcoma and prostate cancer induces significant reduction of cellular growth and increases cellular apoptosis (10, 11).Clusterin is a sulfated glycoprotein encoded by a single gene. It has two known isoforms obtained by alternative splicing (12). The first isoform, secretory clusteri...

Clusterin Is a Ligand for Apolipoprotein E Receptor 2 (ApoER2) and Very Low Density Lipoprotein Receptor (VLDLR) and Signals via the Reelin-signaling Pathway

“…In particular, soluble clusterin was described to protect cells from heat shock and TNF-␣ by interfering with the apoptotic pathway (48,49). Detailed studies on the anti-apoptotic effect of clusterin on TNF-␣ and actinomycin-induced cell death revealed that this effect is mediated via the PI3K/Akt pathway, possibly activated via LRP2 (50). LRP2 belongs to the LDL receptor family just as ApoER2 and VLDLR and it remains to be established whether these different effects of clusterin have more in common than the activation of PI3K/Akt.…”

Tamoxifen Suppresses Clusterin Level through Akt Inactivation and Proteasome Degradation in Human Prostate Cancer Cells

“…Recent work has shown that the survival effect of clusterin on rat prostate cells is mediated in part by activation of the PI3K/Akt pathway (Ammar and Closset, 2008). In neuroblastoma cells, the mRNA levels of clusterin were enhanced by B-Myb overexpression, a nuclear transcription factor that recognize the Myb consensus sequence (C/T)AACNG in viral and cellular promoters (Cervellera et al, 2000).…”

From Humans to Experimental Models: The Cytoprotective Role of Clusterin in the Kidney

“…In mice, CLU in normal tissues (heart and kidney) is present as a single protein band at approximately 40 kDa in Western blot analysis in our studies [24,55,56], while in cultured cells from these tissues two protein bands at approximately 60 kDa and 40 kDa are detected [24,55,56], suggesting that mouse CLU probably is also retrotranslocated to the cytosol following exposure to sub lethal stress in culture conditions. A variety of biological activities of intracellular sCLU or cytoplasmic CLU (cCLU) have been reported; it inhibits apoptosis by the interaction with BAX or GRP78 [54,57,58] or promotes cell survival by the activation of Akt and NF-κB pathway [48,59]. It is of much more interest to see that sCLU (~70 kDa) acts as an intracellular chaperone to interact with both ATP7A and ATP7B (Cu-ATPases) and facilitates degradation of misfolded/mislocalized mutant ATP7B [60].…”

“…These studies suggest that sCLU in the blood may serve not only as an inhibitor of the lytic terminal complement cascade, but also as a regulator of lipid transport and local lipid redistribution. Furthermore, addition of sCLU prevents cell apoptosis in cultured cells treated with TNF-α, H 2 O 2 or gentamicin probably by activation of magalinphosphatidylinositol 3-kinase/Akt pathway [47][48][49], and mediates clearance of cellular debris into non-professional phagocytes [50]. Based on all of these observations, it has been proposed that sCLU functions as an extracellular chaperone, a previously unknown qualitycontrol system for protein folding that mediates the recognition and disposal of extracellular misfolded proteins via receptor (i.e.…”