The Role of cIAP1 and XIAP in Apoptosis Induced by Tumor Necrosis Factor Alpha in Esophageal Squamous Cell Carcinoma Cells. 
The inhibitor of apoptosis protein (IAP) family are reported to play important roles in cancer cells evading apoptosis.  However, the significance of their expression in human esophageal squamous cell carcinoma (ESCC) cells remains uncertain.  The present study aimed to investigate the role of the IAP family members in tumor necrosis factor-α (TNF-α)-induced apoptosis of human ESCC cells.  Five human ESCC cell lines were pretreated with TNF-α, cycloheximide (CHX, protein synthesis inhibitor), epoxomicin (proteasome inhibitor).  Apoptosis assay and protein study with Western blot testing were conducted.  Knockdown experiments with IAP siRNA were conducted, and the effect on cell apoptosis was analyzed.  Significant apoptosis was induced in five ESCC cell lines by TNF-α plus CHX stimulation, but not when treated with TNF-α or CHX alone.  The protein expression levels of cIAP1 and XIAP were decreased by treatment with TNF-α in the presence of CHX, and the degree of cIAP1 and XIAP expression decrease was correlated with sensitivity to TNF-α plus CHX-induced apoptosis.  Epoxomicin suppressed TNF-α plus CHX-induced degradation of survivin, cIAP1, and XIAP, in addition to apoptosis.  A caspase inhibitor (z-VAD-fmk) suppressed TNF-α plus CHX-induced apoptosis, but did not suppress degradation of survivin, cIAP1, and XIAP.  Furthermore, cIAP1 or XIAP siRNA transfected cells underwent apoptosis in response to treatment with TNF-α alone.  Double knockdown of both genes resulted in further increased apoptosis.  cIAP1 and XIAP play an essential role in the resistance of ESCC cells against apoptosis.