Activation of the phosphatidylinositol 3-kinase (PI3K) signaling pathway is known to protect tumor cells from apoptosis and more specifically from the Fas-mediated apoptotic signal. The antitumoral agent edelfosine sensitizes leukemic cells to death by inducing the redistribution of the apoptotic receptor Fas into plasma membrane subdomains called lipid rafts. Herein, we show that inhibition of the PI3K signal by edelfosine triggers a Fas-mediated apoptotic signal independently of the Fas/FasL interaction. Furthermore, similarly to edelfosine, blockade of the PI3K activity, using specific inhibitors LY294002 and wortmannin, leads to the clustering of Fas whose supramolecular complex is colocalized within the lipid rafts. These findings indicate that the antitumoral agent edelfosine down-modulates the PI3K signal to sensitize tumor cells to death through the redistribution of Fas into large platform of membrane rafts. (Mol Cancer Res 2008;6(4):604 -13)

No abstract

The immune system eliminates infected or transformed cells through the activation of the death receptor CD95. CD95 engagement drives the recruitment of the adaptor protein Fasassociated death domain protein (FADD), which in turn aggregates and activates initiator caspases-8 and -10. The CD95-mediated apoptotic signal relies on the capacity to form the CD95/FADD/caspases complex termed the death-inducing signalling complex (DISC). Cells are classified according to the magnitude of DISC formation as either type I (efficient DISC formation) or type II (inefficient). CD95 localised to lipid rafts in type I cells, whereas the death receptor was excluded from these domains in type II cells. Here, we show that inhibition of both PI3K class IA and serine-threonine kinase Akt in type II cells promoted the redistribution of CD95 into lipid rafts, DISC formation and the initiation of the apoptotic signal. Strikingly, these molecular events took place independently of CD95L and the actin cytoskeleton. Overall, these findings highlight that the oncogenic PI3K/Akt signalling pathway participates in maintaining cells in a type II phenotype by excluding CD95 from lipid rafts. 2368 IntroductionThe death receptor CD95 (Fas/APO1) belongs to the tumour necrosis factor (TNF) receptor family. Although CD95 is ubiquitously expressed, its cognate ligand, CD95L, displays a more restricted expression pattern. This apoptotic ligand is found at the plasma membrane of immune cells [1] and participates in the elimination of transformed and infected cells. A common feature of the death receptors is the presence of an intracellular domain termed the death domain (DD). Binding of CD95L to CD95 triggers the recruitment at the DD level of the adaptor protein Fas-associated death domain protein (FADD), which in turn aggregates proteases called caspase-8 and -10. The close vicinity of these caspases facilitates their activation and the induction of the caspase cascade, culminating in the death of the cell. The CD95/FADD/caspase-8 complex is called the death-inducing signalling complex (DISC) [2]. Investigations of the molecular mechanisms modulating the initial steps of CD95 signalling showed that the redistribution of CD95 into nanometer to micrometer-sized domains of the plasma membrane, termed lipid rafts, enhanced the formation of the DISC and the transmission of the apoptotic signal [3][4][5][6]. In addition, some anti-tumoral drugs eliminate malignant cells through the redistribution of CD95 into lipid rafts and the induction of a CD95L-independent apoptotic signal (e.g. rituximab [7], resveratrol [8,9], edelfosin [3,10] and cisplatin [11]).According to the signalling pathway triggered upon CD95 engagement, cells are classified, both in vivo and in vitro, as type I or type II cells [12]. In this regard, the DISC is efficiently formed in type I cells and the large amount of activated caspase-8 directly activates the executioner caspases-3, -6 and -7, which in turn process various intracellular substrates. On the contrary, type II cells displa...

Le récepteur de mort CD95 appartient à la famille du récepteur tumor necrosis factor (TNF). Ce récepteur est retrouvé muté et non fonctionnel dans les souris Lpr et Lpr cg et chez les patients atteints d'autoimmune lymphoproliferative syndrome de type Ia (ALPS). Ces mutations du récepteur CD95 bloquent le signal apoptotique et entraînent, chez le patient comme chez la souris, une lymphoproliféra-tion, des adénopathies, une splénomégalie, une accumulation d'une population lymphocytaire T CD4 -CD8 -et de l'auto-immunité. Alors que CD95 a été impliqué, dans un premier temps, dans la contraction du nombre de lymphocytes T activés lors de la réponse antitumorale ou infectieuse, il semble en fait jouer un rôle dans la tolérance périphérique et l'élimination des lymphocytes activés de manière chronique par les antigènes du soi de faible affinité. Le ligand de CD95, le CD95L, est exprimé à la surface des lymphocytes T activés et des cellules natural killer (NK) où il joue un rôle important dans l'élimination des cellules tumorales et infectées. L'engagement de CD95 par le CD95L déclenche l'activation de cystéines protéases appelées caspases (cysteine aspartyl-specific proteases). Une de ces caspases, la caspase-12 est retrouvée sous une forme longue (active et ancestrale), principalement dans certaines populations d'origine africaine, et interviendrait dans l'atténuation de la réponse inflammatoire. La pression de sélection responsable de la conservation par ces populations de la forme longue de la caspase-12 reste, à ce jour, inconnue. Mots clés Apoptose · Caspase · Lymphome · Auto-immunitéAbstract The death receptor CD95 belongs to the tumor necrosis factor (TNF) receptor super family. Lpr and Lpr cg mice and patients suffering from autoimmune lymphoproliferative syndrome (ALPS) type Ia exhibit mutated and non-functional CD95 alleles. The patients and the mice display similar phenotypes such as lymphoproliferation, adenopathy, splenomegaly, accumulation of double negative T lymphocytes (B220 + CD4 -CD8 -) and the production of autoimmune antibodies. Although CD95 was initially thought to be involved in the elimination of T lymphocytes activated by tumoral or infectious antigens, recent studies have shown that in fact CD95 plays a pivotal role in peripheral tolerance and the elimination of lymphocytes chronically stimulated by self-antigens. The cognate CD95 ligand, CD95L, is expressed at the membrane of activated lymphocytes and natural killer cells upon which it plays a pivotal role in the elimination of transformed and infected cells. The engagement of CD95 leads to the activation of a family of cysteine proteases termed caspases and the subsequent induction of the apoptotic phenotype. Among these caspases, the long form of caspase-12 has been found only in certain African populations, wherein it may play an essential role in the attenuation of the inflammatory response. To date, the selection pressure responsible for the conservation of the long form of the caspase-12 in these populations remains unknown.

Supplementary Figures S1-S3 from Localization of Fas/CD95 into the Lipid Rafts on Down-Modulation of the Phosphatidylinositol 3-Kinase Signaling Pathway

<div>Abstract<p>Activation of the phosphatidylinositol 3-kinase (PI3K) signaling pathway is known to protect tumor cells from apoptosis and more specifically from the Fas-mediated apoptotic signal. The antitumoral agent edelfosine sensitizes leukemic cells to death by inducing the redistribution of the apoptotic receptor Fas into plasma membrane subdomains called lipid rafts. Herein, we show that inhibition of the PI3K signal by edelfosine triggers a Fas-mediated apoptotic signal independently of the Fas/FasL interaction. Furthermore, similarly to edelfosine, blockade of the PI3K activity, using specific inhibitors LY294002 and wortmannin, leads to the clustering of Fas whose supramolecular complex is colocalized within the lipid rafts. These findings indicate that the antitumoral agent edelfosine down-modulates the PI3K signal to sensitize tumor cells to death through the redistribution of Fas into large platform of membrane rafts. (Mol Cancer Res 2008;6(4):604–13)</p></div>

Supplementary Figures S1-S3 from Localization of Fas/CD95 into the Lipid Rafts on Down-Modulation of the Phosphatidylinositol 3-Kinase Signaling Pathway