Fas palmitoylation by the palmitoyl acyltransferase DHHC7 regulates Fas stability

Fas (CD95, TNFRSF6) is a prototypic death receptor that belongs to the tumor necrosis factor receptors (TNFRs) superfamily.1 Upon activation by the Fas ligand (FasL, TNFSF6), Fas initiates a signaling cascade that leads to caspases activation and ultimately to cell death.2 For these reasons, Fas was mainly considered as a tumor-suppressor protein. However, accumulating evidences support a significant role for Fas in alternative non-death signaling leading to cell survival, proliferation, epithelial–mesenchymal transition, cancer growth and metastasis in some context.3, 4, 5 Such conditional multi-signaling of Fas has been well demonstrated in colon cancer model.6,7 We demonstrated that Fas and FasL are constitutively modified by S-palmitoylation, a reversible post-translational modification that consists in the addition of a palmitic acid on the cysteine residue through a thiol linkage.8, 9, 10 The palmitoylation of a protein can affect its interactions with its surrounding membrane lipids and proteins, therefore impacting certain properties such as association with specific membrane domains, trafficking between cell compartments or stability.11, 12, 13 The palmitoylation reaction is catalyzed by the conserved DHHC (aspartate–histidine–histidine–cysteine) family of enzymes, which are characterized by the specific DHHC motif required for the palmitoyl acyltransferase (PAT) activity.14 In human, the 23 members of the DHHC family described are localized to the distinct intracellular membrane compartments, mainly the Golgi apparatus and the endoplasmic reticulum where palmitoylation likely occurs.14,15 Since their discovery in 2002, increasing numbers of DHHC-substrate pairs have been identified allowing a deeper comprehension of palmitoylation regulation.16 Fas palmitoylation occurs on an intracellular cysteine adjacent to the transmembrane domain (cysteine 199 in human) and is critical for its ability to trigger cell death.8,9 At the molecular level, we and others already reported that the pro-death role of Fas palmitoylation relies on (i) the formation of supramolecular Fas aggregates9 and (ii) Fas targeting in nanodomains enriched in cholesterol and glycosphingolipids, which is a prerequisite for proper activation of Fas-induced cell death.8,17, 18, 19, 20, 21 In these domains and upon FasL binding, palmitoylation allows Fas to recruit ezrin and actin cytoskeleton that lead to Fas internalization and transmission of death signal.8 We thus aimed at understanding how Fas palmitoylation is regulated. We report here the identification of DHHC7 as a Fas-palmitoylating enzyme and revealed an additional role of Fas palmitoylation. The latter consists in the stabilization of Fas by preventing its degradation through the lysosomal pathway.