Yang Dafeng, Wang Suizhao, Brooks Craig, Dong Zheng, Schoenlein Patricia V, Kumar Vijay, Ouyang Xinshou, Xiong Huabao, Lahat Guy, Hayes-Jordan Andrea, Lazar Alexander, Pollock Raphael, Lev Dina, Liu Kebin
Department of Biochemistry, Medical College of Georgia, Augusta, GA 30912, USA.
Cancer Res. 2009 Feb 1;69(3):1080-8. doi: 10.1158/0008-5472.CAN-08-2520. Epub 2009 Jan 20.
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.
干扰素调节因子8（IRF8）已被证明至少部分通过调节肿瘤细胞凋亡来抑制肿瘤发展；然而，IRF8调节凋亡的分子机制仍未完全阐明。在此，我们发现破坏IRF8功能会导致软组织肉瘤（STS）细胞中细胞色素c释放、半胱天冬酶-9和半胱天冬酶-3激活以及聚（ADP-核糖）聚合酶裂解受到抑制。线粒体依赖性凋亡信号级联反应的抑制显然是由于半胱天冬酶-8和Bid激活受阻。对半胱天冬酶-8上游信号事件的分析表明，破坏IRF8功能会显著增加FLIP mRNA稳定性，导致IRF8蛋白水平升高。此外，从IRF8基因敲除小鼠分离的原代髓细胞也表现出FLIP蛋白水平升高，表明IRF8可能是FLIP的一般抑制因子。在92%（60个样本中的55个）的人类STS标本中未检测到核IRF8蛋白，99%（60个样本中的59个）的人类STS标本表现出FLIP表达，表明体内核IRF8蛋白水平与FLIP水平呈负相关。沉默FLIP表达可显著增加人肉瘤细胞对FasL诱导的和肿瘤坏死因子相关凋亡诱导配体（TRAIL）诱导的凋亡的敏感性，IRF8的异位表达也显著增加了这些人肉瘤细胞对FasL和TRAIL诱导的凋亡的敏感性。综上所述，我们的数据表明IRF8通过介导FLIP表达水平来调节凋亡，靶向IRF8表达是使抗凋亡的人类STS对凋亡敏感的潜在有效治疗策略，从而可能克服目前人类STS治疗中的主要障碍——STS的化疗耐药性。
