Tetrandrine, a
bis-benzylisoquinoline alkaloid from the root of Stephania tetrandra, induces apoptosis in human T-cell lines, lung
carcinoma and
hepatoblastoma cells. However, the mechanisms by which
tetrandrine inhibits
tumor cell growth are poorly understood. The purpose of the present study was to investigate the intracellular signaling mechanism of
tetrandrine-induced apoptosis in HepG2 cells. The induction of apoptosis was determined by morphological analysis and
terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling assay. Treatment of cells with
tetrandrine caused the upregulation of p53, downregulation of Bcl-X(L), cleavage of Bid and Bax, and release of
cytochrome c, which were accompanied by activation of
caspases 9, 3 and 8. The activation of
caspases 9 and 3 preceded that of
caspase 8. A broad-spectrum
caspase inhibitor and a
caspase 8-specific inhibitor completely blocked
tetrandrine-induced Bid processing,
cytochrome c release, activation of
caspase 3, and cell death. These findings and data showing the early release of
cytochrome c, cleavage of Bid and downregulation of Bcl-X(L) suggest that the mitochondrial pathway is primarily involved in
tetrandrine-induced apoptosis. The activation of
caspase 8 after early
caspases 9 and 3 activation might act as an amplification loop for activation of upstream signals such as Bid cleavage or
cytochrome c release. These data suggest that
tetrandrine may constitute a plausible therapeutic for
hepatocellular carcinoma.