Leukemic CD34 ؉ immature acute myeloid leukemia (AML) cells express Fas receptor but are frequently resistant to Fas agonistic reagents. Fas plays an important role in T-cell-mediated cytotoxicity, and recently it has been suggested that altered Fas signaling may contribute to drug resistance. Therefore, Fas resistance could be one of the mechanisms by which AML progenitors escape chemotherapy or T-cell-based immune intervention. However, the molecular mechanism of Fas resistance in AML cells has not been identified. Fas signaling can be interrupted at 3 mains levels: Fas clustering, alteration of death-inducing-signalingcomplex (DISC) formation, and effector caspase inhibition of downstream caspase-8. This study shows that in the Fas-resistant CD34 ؉ CD38 ؊ KG1a cells, Fas agonists resulted in Fas aggregation but not in caspase-8 activation, related to a defect in DISC formation. However, pretreatment with chelerythrin, but not with calphostin C, resulted in the restoration of Fas-induced caspase-8 activation and cytotoxicity, suggesting that some atypical protein kinase C (PKC) isoforms contributed to the lack of DISC formation. Indeed, treatment with antisense oligonucleotides directed against PKC and enforced expression of Par-4, a negative regulator of PKC activity, restored Fasinduced caspase-8 activity and apoptosis. Moreover, it was found that PKC interacts with FADD and that PKC immunoextracts prepared from KG1a cells are able to phosphorylate FADD in vitro, whereas this phosphorylation is dramatically reduced in Par-4 transfectant cells. IntroductionFas (APO-1/CD95) is a 45-kd membrane protein that belongs to the tumor necrosis factor (TNF)-nerve growth factor receptor family, a group of type 1 transmembrane receptors. 1 Mutational analysis of Fas and the human TNF receptor (TNFR-1) proteins demonstrates that the cytoplasmic domains share a homologous region necessary to transduce the apoptotic signal. This conserved region of approximately 70 amino acids was, therefore, designated as the death domain (DD). The only known physiological ligand of Fas, Fas-L (CD95L), belongs to the family of TNF-related cytokines. 2 Fas-L is synthesized as a transmembrane molecule, and soluble Fas-L trimers can be generated through processing by a metalloprotease. 3,4 Engagement of Fas by agonistic anti-Fas antibodies or by Fas-L triggers apoptosis in a variety of cell types. However, only membrane-bound or multimerized Fas-L induces cell death. 3,4 Moreover, ligand-dependent activation of Fas death pathway requires the oligomerization of Fas receptor, but ligand-independent activation can occur on Fas aggregation induced by Fas overexpression or treatment with anticancer drugs or radiation. [5][6][7][8][9] Clustering of Fas recruits Fas-associated death domain (FADD)-containing protein, which is a bipartite molecule with a death effector domain (DED) at the amino terminus and a DD at the carboxyl terminus. FADD binds to Fas through a DD-DD interaction and recruits the DED-containing procaspase-8 through a DED-DED inter...
Leukemic CD34+ immature acute myeloid leukemia (AML) cells express Fas receptor but are frequently resistant to Fas agonistic reagents. Fas plays an important role in T-cell–mediated cytotoxicity, and recently it has been suggested that altered Fas signaling may contribute to drug resistance. Therefore, Fas resistance could be one of the mechanisms by which AML progenitors escape chemotherapy or T-cell–based immune intervention. However, the molecular mechanism of Fas resistance in AML cells has not been identified. Fas signaling can be interrupted at 3 mains levels: Fas clustering, alteration of death-inducing-signaling-complex (DISC) formation, and effector caspase inhibition of downstream caspase-8. This study shows that in the Fas-resistant CD34+CD38− KG1a cells, Fas agonists resulted in Fas aggregation but not in caspase-8 activation, related to a defect in DISC formation. However, pretreatment with chelerythrin, but not with calphostin C, resulted in the restoration of Fas-induced caspase-8 activation and cytotoxicity, suggesting that some atypical protein kinase C (PKC) isoforms contributed to the lack of DISC formation. Indeed, treatment with antisense oligonucleotides directed against PKCζ and enforced expression of Par-4, a negative regulator of PKCζ activity, restored Fas-induced caspase-8 activity and apoptosis. Moreover, it was found that PKCζ interacts with FADD and that PKCζ immunoextracts prepared from KG1a cells are able to phosphorylate FADD in vitro, whereas this phosphorylation is dramatically reduced in Par-4 transfectant cells. In conclusion, it is suggested that in AML cells, PKCζ plays an important role in Fas resistance by inhibiting DISC formation, possibly by phosphorylating FADD.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.