c-FLIP knockdown induces ligand-independent DR5-, FADD-, caspase-8-, and caspase-9-dependent apoptosis in breast cancer cells

• Published in: Biochemical pharmacology Vol. 76; no. 12; pp. 1694 - 1704
• Main Authors: Day, Travis W., Huang, Su, Safa, Ahmad R.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier Inc 15-12-2008; Elsevier
• Subjects: Apoptosis; Apoptosis - drug effects; Biological and medical sciences; Breast cancer; Breast Neoplasms - chemistry; Breast Neoplasms - drug therapy; Breast Neoplasms - pathology; c-FLIP; CASP8 and FADD-Like Apoptosis Regulating Protein - antagonists & inhibitors; CASP8 and FADD-Like Apoptosis Regulating Protein - genetics; Caspase 8 - metabolism; Caspase 9 - metabolism; Cell Line, Tumor; Fas-Associated Death Domain Protein - metabolism; Gene Silencing; Gynecology. Andrology. Obstetrics; Humans; Ligands; Mammary gland diseases; Medical sciences; Pharmacology. Drug treatments; Protein Isoforms; Receptors, TNF-Related Apoptosis-Inducing Ligand - metabolism; RNA, Small Interfering - pharmacology; Small interfering RNA (siRNA); TRAIL receptor 5; Tumors; Breast cancer; Small interfering RNA (siRNA); TRAIL receptor 5; c-FLIP; Apoptosis; Breast disease; RNA interference; Cysteine endopeptidases; Death receptor 5; Fas associated death domain protein; FLIP protein; Tumor cell; Biological receptor; Enzyme; Cytokine; siRNA; Malignant tumor; Adaptor protein; Mammary gland diseases; Peptidases; Gene silencing; Tumor necrosis factor; Cell death; Hydrolases; TNF related apoptosis inducing ligand; Cancer
• ISSN: 0006-2952; 1873-2968
• DOI: 10.1016/j.bcp.2008.09.007

Cellular-FLICE inhibitory protein (c-FLIP) is an inhibitor of apoptosis downstream of the death receptors Fas, DR4, and DR5, and is expressed as long (c-FLIP
L) and short (c-FLIP
S) splice forms. We found that the knockdown of c-FLIP using small interfering RNA (siRNA) triggered ligand-independent caspase-8- and -9-dependent spontaneous apoptosis and decreased the proliferation of MCF-7 breast cancer cells. Further analysis revealed that an apoptotic inhibitory complex (AIC) comprised of DR5, FADD, caspase-8, and c-FLIP
L exists in MCF-7 cells, and the absence of c-FLIP
L from this complex induces DR5- and FADD-mediated caspase-8 activation in the death inducing signaling complex (DISC). c-FLIP
S was not detected in the AIC, and using splice form-specific siRNAs we showed that c-FLIP
L but not c-FLIP
S is required to prevent spontaneous death signaling in MCF-7 cells. These results clearly show that c-FLIP
L prevents ligand-independent death signaling and provides direct support for studying c-FLIP as a relevant therapeutic target for breast cancers.