Activation of caspase-8 in transforming growth factor-beta-induced apoptosis of human hepatoma cells

Transforming growth factor-beta 1 (TGF-beta 1) has been shown to induce apo ptosis in normal or transformed hepatocytes. To elucidate the biochemical p athways leading to apoptosis induced by TGF-beta 1 in human hepatoma cells (HuH-7), we examined the expression of Bcl-2-related proteins and X-chromos ome-linked inhibitor of apoptosis (XIAP), and activation of the caspase cas cade following TGF-beta 1 treatment. Bcl-xL expression began to decline at 12 hours after TGF-beta 1 treatment and progressively decreased to very low levels in a time-dependent manner. Bar expression showed a little change t hroughout the experiment. On the other hand, activation of caspase-8 was cl early observed at 36 hours after TGF-beta 1 treatment, followed by activati on of caspase-9, and caspase-3 was activated at 48 hours after treatment at which time apoptosis of HuH-7 cells was observed. TGF-beta 1 significantly decreased XIAP expression in HuH-7 cells. Addition of an inhibitor of casp ase-8 or caspase-3 (IETD-FMK or DEVD-CHO) markedly inhibited TGF-beta 1-ind uced apoptosis of HuH-7 cells. Fas/Fas ligand (FasL) interactions in HuH-7 cells were not involved in the apoptotic process. Furthermore, epidermal gr owth factor (FGF) also completely inhibited TGF-beta 1-induced apoptosis of HuH-7 cells by inhibiting activation of the caspase cascade. Our results s uggested that activation of caspase-3 initiated through caspase-8 activatio n is involved in the apoptotic process induced by TGF-beta 1 in HuH-7 cells . Our results also showed that down-regulation of the expression of Bcl-xL and XIAP by TGF-beta 1 may facilitate activation of caspase-3 in these cell s.