| [Background]Hepatocellular carcinoma (HCC) is one of the most malignant digestive systemtumors. Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a recently characterized member of the TNF super family. In contrast to normal primary cells, numerous tumors of diverse tissue origin are susceptible to TRAIL. The known TRAIL death-inducing receptors include DR4 (TRAIL-R1) and DR5 (TRAIL-R2). In addition, two TRAIL receptors that do not signal apoptosis have been characterized and have been shown to inhibit DR4- and DR5-mediated death. They are DcRl (TRAIL-R3) and DcR2 (TRAIL-R4), which both act as decoy receptors and dominant-negative inhibitors of TRAIL-induced cell death. DcRl is a glycosylphosphatidylinositol- anchored protein incapable of intracellular signaling due to the lack of a cytoplasmic tail. DcR2 has a truncated death domain. Although many researchers suggest that the decoy receptors are widely expressed in normal tissue, recent studies have shown high level of DcRl and DcR2 mRNA in surgically resected tumors of the brain and gastrointestinal tract. But their presence does not correlate with resistance to TRAIL, which do not support the original hypothesis that decoy receptor expression is contribute to apoptosis inhibition. Thus, the apoptosis-inducing function of TRAIL is not only determined by the different expression of receptors.[Objective]To investigate the different expression of receptors of TRAIL in human hepatocellular carcinoma and neighboring non-tumor liver tissues and throw light on the biologic roles of TRAIL in HCC.[Methods](1) Tissues of HCC and neighboring non-tumor liver were obtained from patients who underwent hepatic surgery. Total RNA was extracted from surgically resected tissues and then we performed reverse transcriptase-polymerase chain reaction to measure the relative levels of gene expression. (2) Immunohistochemical staining for TRAIL-R1,R2,R3,R4 was performed on surgically resected HCC tissues. (3) SK-Hepl cells were cultured in 24-well plates. Recombinant human TRAIL with or without y-IFN was added, and the cells were incubated for an additional 12 hours. The viability of cells was detected by flow cytometer (FCM).[Results](1) The expression of death receptors, DR4 and DR5, was found frequently in both HCC and normal liver tissues, whereas the expression of decoy receptors was different. The expression levels of DcRl and DcR2 mRNA in neighboring non-tumor livers were higher than those in HCCs (100% vs 67.86%, 100% vs 78.57%, respectively, /K0.01).(2) By immnohistochemistry staining, we found a parallel result for receptors of TRAIL. The expression levels of DcRl and DcR2 protein in neighboring non-tumor livers were higher than those in HCCs (96.15% vs 42. 31%, 100% vs 53. 85% respectively, /K0.01)(3) TRAIL alone, even at a dose of up to 5 p,g/ml, failed to induce significant cytotoxicity or apoptosis in SK-Hepl cell line. However, in the presence of a subtoxic level (500U/mL) of y-IFN, TRAIL-treated cells showed significant apoptosis (15.63%, 36.41%, 44.34%, 44.22%, 56.85%, /X0.01).[Conclusions]TRAIL is a recently identified member of the TNF family and is capable of inducing apoptosis. TRAIL-R1 and R2, which contain cytoplasmic death domain, show wide spread distribution in HCC and neighboring non-tumor livers. Immunohistochemical results revealed that they are mainly located in cytoplasm. The decoy receptor, DcRl and DcR2, showed different expression between HCC and normal livertissues, also found frequently in HCC. Furthermore, we also could not find a positive relation between the expression of protective TRAIL receptors (DcRl and DcR2) and the sensitivity to the TRAIL-induced apoptosis in SK-Hepl cell line. Although TRAIL failed to induce significant apoptosis, our results show that HCC cell was significantly sensitized to TRAIL-induced apoptosis by y-IFN. Combinations of TRAIL and y-IFN may have added anti-tumor activity and have theraputic efficacy in vivo.
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