Fas/CD95 death receptor and lipid rafts: New targets for apoptosis-directed cancer therapy

Induction of apoptosis in cancer chemotherapy. Anti-proliferative cancer chemotherapy engages apoptosis through a multistep pathway. Cells undergo drug-induced damages on DNA or cell cycle, and sense and calibrate these lesions through the presence of “sensors”, which set off signals that eventually trigger apoptosis. Mutation or deletion of sensors in tumors precludes or hampers the triggering of downstream signaling events, which, in turn, can also be inhibited in cancer cells, leading to a high apoptosis threshold and thus providing cancer cells with a high survival capacity in adverse conditions. Drugs targeting directly the apoptotic machinery could hypothetically circumvent these obstacles and lead to rapid demise of cancer cells.

Schematic diagram of the human Fas death receptor. Mature human Fas protein consists of 319 amino acids (aas) with an N-terminal extracellular domain of 157 aas, a short transmembrane region (17 aas) and a C-terminal cytoplasmic domain of 145 aas. Relevant domains for Fas oligomerization and apoptotic activity are shown. An N-terminal extracellular oligomerization domain (NOD) of 49 aas (Arg-1 to Pro-49) responsible for the FasL-independent oligomerization of the receptor. Three cysteine-rich domains (CRD1-Gln31 to Val-67, CRD2-Pro-68 to Cys-111-, and CRD3-Arg-112 to Lys150-) containing four, six and eight Cys residues in each domain, respectively. A cytoplasmic death domain (DD) of 85 aas (Ser-214 to Ile-298) is crucial for apoptotic signaling. The last 15 amino acids (Asp-305 to Val-319) of the Fas amino acid sequence represent a C-terminal inhibitory domain (CID). Domains and membrane are not to scale.

Schematic representation of Fas activation through its aggregation in membrane rafts. Fas molecules are brought together and concentrated in membrane rafts facilitating the formation of DISCs, following protein-protein interactions between Fas-FADD through their respective death domains (DD), and FADD-procaspase-8 through their respective death effector domains (DED). DISC formation leads to activation of unprocessed procaspase-8 by driving its dimerization and autoproteolysis, resulting in the release of mature, active caspase-8 (composed of a p20/p10 heteromer) into the cytoplasm. The asteriks represent the active-site cysteine residues of caspase-8, the dash lines indicate proteolytic processing in trans, and the arrowheads point to the sites of proteolytic cleavage. Actin cytoskeleton through ezrin is involved in the clustering of Fas in lipid rafts.

Selective antitumor action of edelfosine. Normal cells do not take up the drug, and therefore they are spared, whereas cancer cells incorporate edelfosine and undergo apoptosis mediated by intracellular triggering of Fas.

Clustering of Fas death receptor and downstream signaling molecules in lipid rafts in cancer chemotherapy. Edelfosine and Aplidin induce apoptosis in cancer cells through aggregation of Fas, downstream signaling molecules, including FADD, procaspases 8 and 10, JNK and Bid, and actin-linking proteins (ezrin, moesin) in clusters of lipid rafts. Aplidin also induces recruitment of FasL into lipid rafts, facilitating Fas/FasL killing between neighboring cells.

Fas/FasL involvement in cancer chemotherapy. This scheme depicts the three mechanisms by which the Fas/FasL system is involved in cancer chemotherapy: (A) chemotherapeutic drugs induce de novo FasL synthesis and this newly synthesized FasL binds to Fas, killing cancer cells in an autocrine and paracrine manner; (B) chemotherapeutic drugs induce Fas clustering in lipid rafts together with downstream signaling molecules, leading to apoptosis; and (C) chemotherapeutic drugs induce Fas and FasL clustering in lipid rafts together with downstream signaling molecules, and interaction of the corresponding Fas/FasL pairs in neighboring cancer cells lead to their respective apoptotic cell death.

Putative processes involved in the clustering of Fas in lipid rafts. Persistent activation of JNK, increases in ROS or ceramide levels as well as reorganization of the actin cytoskeleton have been suggested to lead to clustering of Fas. Ezrin links Fas with the actin cytoskeleton.