Expression of TNF-related apoptosis-inducing ligand in human hepatocellular carcinoma

TNF-related apoptosis-inducing ligand (TRAIL), as well as Fas ligand, plays a pivotal role in lymphocyte cytotoxicity and the maintenance of immunological homeostasis in various tissues, but its physiological role in immune evasion of cancer cells remains unknown. We have previously shown strong resistance to TRAIL-induced cytotoxicity in human hepatocellular carcinomas (HCCs). The current study investigates the expression of TRAIL in HCCs. We found that three HCC cells, HepG2, Hep3B and Huh7 cells, constitutively express TRAIL mRNA and protein, as detected by reverse transcriptase PCR and Western blotting. Four of 10 human HCC tissues demonstrated positive staining for TRAIL, whereas non-tumor tissues showed little detectable staining. TRAIL expression on tumor cells was detected by flow cytometry and was dramatically induced after the addition of doxorubicin, a chemotherapeutic agent, or cytokine stimulation with TNF-alpha, IL-1beta or IL-18. This expression was induced principally via the NF-kappaB activation pathway, since IkappaB transfection significantly reduced TRAIL expression. In addition, the expressed TRAIL was functional. The TRAIL on HCC cells induced apoptosis in Jurkat cells that are sensitive to TRAIL-mediated apoptosis, and this process was specifically inhibited by recombinant TRAIL-receptors:Fc which binds to TRAIL. In conclusion, TRAIL expressed on the surface of HCC cells by cytokines or cytostatic drugs might contribute to an alternative mechanism that enables tumors to evade immune surveillance by inducing apoptosis of activated human lymphocytes.