Overexpression of Bcl-2 protects human hepatoma cells from Fas-antibody-mediated apoptosis.

BACKGROUND/AIMS: Fas is a cell surface antigen, that triggers apoptosis upon specific ligand or antibody binding. The proto-oncogene bcl-2 prevents apoptosis induced by various treatments. The aim of our study was to evaluate whether Bcl-2 protects hepatoma cells from Fas-mediated apoptosis.

METHODS: Two human cell lines, HCC-T and HepG2 were used. Expression of Fas antigen and Bcl-2 was detected by flow cytometry and Western blotting. Cell viability and apoptotic change were examined after anti-Fas- and antisense oligodeoxynucleotide treatments. Apoptotic cells were detected by nick-end labelling and the TUNEL method. To test if Bcl-2 expression can protect HepG2 cells from Fas-mediated apoptosis, the cells were transduced using retroviral vector, LZBC, designed to coexpress E. coli beta-galactosidase and human Bcl-2. To further confirm the protective effect of Bcl-2 expression against Fas-mediated apoptosis in HepG2, Bcl-2 expressing plasmid vector was produced and a cell line stably expressing Bcl-2 was cloned.

RESULTS: Western blot analysis showed constitutive Bcl-2 expression in HCC-T cells, but not in HepG2 cells. HCC-T was resistant to apoptosis after treatment with an agonist anti-Fas antibody (1 microg/ml for 3 days), whereas 33% of the HepG2 cells were killed by this treatment. Inhibition of Bcl-2 expression by transfection of antisense oligodeoxynucleotides caused spontaneous apoptosis in HCC-T, but not in HepG2 cells, suggesting that Bcl-2 is essential for survival of HCC-T cells, whereas other proteins may substitute for it in HepG2 cells. Following LZBC infection, 10% HepG2 cells were beta-galactosidase-positive by X-gal staining and Bcl-2-positive. In cells surviving after anti-Fas treatment, the proportion of beta-galactosidase-positive cells increased to 50% and the beta-galactosidase activity increased 6-fold, indicating that Bcl-2 expression protected the cells from Fas-mediated apoptosis. In the cloned HepG2 cells stably expressing Bcl-2, the extent of Fas-mediated apoptosis was inversely related to the level of Bcl-2 expression.

CONCLUSION: Bcl-2 confers protection to human hepatoma cells against Fas-mediated apoptosis, and is essential for survival of some, but not all, hepatoma cells.