Combined inhibition of FLIP and XIAP induces Bax-independent apoptosis in type II colorectal cancer cells.
Death receptors can directly (type I cells) or indirectly induce apoptosis by activating mitochondrial-regulated apoptosis (type II cells). The level of caspase 8 activation is thought to determine whether a cell is type I or II, with type II cells less efficient at activating this caspase following death receptor activation. FLICE-inhibitory protein (FLIP) blocks death receptor-mediated apoptosis by inhibiting caspase 8 activation; therefore, we assessed whether silencing FLIP could convert type II cells into type I. FLIP silencing-induced caspase 8 activation in Bax wild-type and null HCT116 colorectal cancer cells; however, complete caspase 3 processing and apoptosis were only observed in Bax wild-type cells. Bax-null cells were also more resistant to chemotherapy and tumor necrosis factor-related apoptosis inducing ligand and, unlike the Bax wild-type cells, were not sensitized to these agents by FLIP silencing. Further analyses indicated that release of second mitochondrial activator of caspases from mitochondria and subsequent inhibition of X-linked inhibitor of apoptosis protein (XIAP) was required to induce full caspase 3 processing and apoptosis following FLIP silencing. These results indicate that silencing FLIP does not necessarily bypass the requirement for mitochondrial involvement in type II cells. Furthermore, targeting FLIP and XIAP may represent a therapeutic strategy for the treatment of colorectal tumors with defects in mitochondrial-regulated apoptosis.
Wilson
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TR
1Drug Resistance Laboratory, Centre for Cancer Research and Cell Biology, Queen's University Belfast, Belfast, UK.
McEwan
M
M
McLaughlin
K
K
Le Clorennec
C
C
Allen
W L
WL
Fennell
D A
DA
Johnston
P G
PG
Longley
D B
DB
D000970:Antineoplastic Agents / Q000627:therapeutic use; D017209:Apoptosis; D053446:CASP8 and FADD-Like Apoptosis Regulating Protein / Q000037:antagonists & inhibitors* / Q000235:genetics; D053148:Caspase 3 / Q000378:metabolism; D045744:Cell Line, Tumor; D015179:Colorectal Neoplasms / Q000188:drug therapy / Q000378:metabolism / Q000473:pathology*; D020868:Gene Silencing; D006801:Humans; D008928:Mitochondria / Q000201:enzymology; D009367:Neoplasm Staging; D034741:RNA, Small Interfering / Q000235:genetics; D053221:TNF-Related Apoptosis-Inducing Ligand / Q000494:pharmacology; D051636:X-Linked Inhibitor of Apoptosis Protein / Q000037:antagonists & inhibitors* / Q000235:genetics; D051028:bcl-2-Associated X Protein / Q000235:genetics / Q000378:metabolism
D000970:Antineoplastic Agents; D053446:CASP8 and FADD-Like Apoptosis Regulating Protein; D034741:RNA, Small Interfering; D053221:TNF-Related Apoptosis-Inducing Ligand; D051636:X-Linked Inhibitor of Apoptosis Protein; C496866:XIAP protein, human; D051028:bcl-2-Associated X Protein; D053148:Caspase 3