IFN regulatory factor 8 sensitizes soft tissue sarcoma cells to death receptor-initiated apoptosis via repression of FLICE-like protein expression.
IFN regulatory factor 8 (IRF8) has been shown to suppress tumor development at least partly through regulating apoptosis of tumor cells; however, the molecular mechanisms underlying IRF8 regulation of apoptosis are still not fully understood. Here, we showed that disrupting IRF8 function resulted in inhibition of cytochrome c release, caspase-9 and caspase-3 activation, and poly(ADP-ribose) polymerase cleavage in soft tissue sarcoma (STS) cells. Inhibition of the mitochondrion-dependent apoptosis signaling cascade is apparently due to blockage of caspase-8 and Bid activation. Analysis of signaling events upstream of caspase-8 revealed that disrupting IRF8 function dramatically increases FLIP mRNA stability, resulting in increased IRF8 protein level. Furthermore, primary myeloid cells isolated from IRF8-null mice also exhibited increased FLIP protein level, suggesting that IRF8 might be a general repressor of FLIP. Nuclear IRF8 protein was absent in 92% (55 of 60) of human STS specimens, and 99% (59 of 60) of human STS specimens exhibited FLIP expression, suggesting that the nuclear IRF8 protein level is inversely correlated with FLIP level in vivo. Silencing FLIP expression significantly increased human sarcoma cells to both FasL-induced and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis, and ectopic expression of IRF8 also significantly increased the sensitivity of these human sarcoma cells to FasL- and TRAIL-induced apoptosis. Taken together, our data suggest that IRF8 mediates FLIP expression level to regulate apoptosis and targeting IRF8 expression is a potentially effective therapeutic strategy to sensitize apoptosis-resistant human STS to apoptosis, thereby possibly overcoming chemoresistance of STS, currently a major obstacle in human STS therapy.
Yang
Dafeng
D
Department of Biochemistry, Medical College of Georgia, Augusta, GA 30912, USA.
Wang
Suizhao
S
Brooks
Craig
C
Dong
Zheng
Z
Schoenlein
Patricia V
PV
Kumar
Vijay
V
Ouyang
Xinshou
X
Xiong
Huabao
H
Lahat
Guy
G
Hayes-Jordan
Andrea
A
Lazar
Alexander
A
Pollock
Raphael
R
Lev
Dina
D
Liu
Kebin
K
D000818:Animals; D017209:Apoptosis / Q000187:drug effects / Q000502:physiology*; D053446:CASP8 and FADD-Like Apoptosis Regulating Protein / Q000096:biosynthesis* / Q000235:genetics; D053181:Caspase 8 / Q000378:metabolism; D061945:Caspase Inhibitors; D045744:Cell Line, Tumor; D004789:Enzyme Activation; D053222:Fas Ligand Protein / Q000494:pharmacology; D006801:Humans; D007150:Immunohistochemistry; D050835:Interferon Regulatory Factors / Q000096:biosynthesis*; D051379:Mice; D008928:Mitochondria / Q000187:drug effects / Q000502:physiology*; D012333:RNA, Messenger / Q000096:biosynthesis / Q000235:genetics; D034741:RNA, Small Interfering / Q000235:genetics; D053220:Receptors, TNF-Related Apoptosis-Inducing Ligand / Q000378:metabolism; D012509:Sarcoma / Q000188:drug therapy / Q000235:genetics / Q000378:metabolism* / Q000473:pathology; D012513:Sarcoma, Experimental / Q000188:drug therapy / Q000235:genetics / Q000378:metabolism / Q000473:pathology; D053221:TNF-Related Apoptosis-Inducing Ligand / Q000494:pharmacology*; D000099185:Interferon Regulatory Factor-8
D053446:CASP8 and FADD-Like Apoptosis Regulating Protein; D053181:Caspase 8; D061945:Caspase Inhibitors; D053222:Fas Ligand Protein; D050835:Interferon Regulatory Factors; D012333:RNA, Messenger; D034741:RNA, Small Interfering; D053220:Receptors, TNF-Related Apoptosis-Inducing Ligand; D053221:TNF-Related Apoptosis-Inducing Ligand; D000099185:Interferon Regulatory Factor-8