The hepatitis B virus HBx protein inhibits caspase 3 activity

The hepatitis B virus-encoded HBx protein coactivates transcription of vira l and cellular genes, and it is believed to play an important role in hepat itis B virus-related liver cancer. HBx has been shown to alter the coordina ted balance between proliferation and programmed cell death, being able to either induce or block apoptosis, Here, we demonstrate for the first time t hat the HBx is a potent caspase 3 inhibitor. Rat fibroblasts (REV2) and hep atoma cells (Hep) synthesizing the HBx protein were resistant to various ap optotic stimuli such as growth factor depletion, tumor necrosis factor alph a, or anti-Fas antibodies administration. In these cells, HBK prevented DNA fragmentation and cell death in the absence of de novo protein synthesis, with a similar efficiency as the competitive caspase 3 substrates inhibitor s VAD-FMK and DEVD-FMK. Protein extracts obtained from the HBx positive cel ls contained a very low caspase activity, and addition of anti-HBx antibody restored the endogenous caspase activity. To obtain a functional map of th e anti-caspase activity of HBx, various cell lines were established that sy nthesized either N-terminally or C-terminally truncated HBx molecules. Thes e gene dissection experiments revealed that the regions required for the an ti-caspase activity overlap with the two known transactivation domains of H Bx.