Bile Acid-induced Apoptosis in Hepatocytes Is Caspase-6-dependent

Apoptosis induced by hydrophobic bile acids is thought to contribute to liver injury during cholestasis. Caspase-6 is an executioner caspase that also appears to have regulatory functions in hematopoetic cell lines. We aimed to elucidate the role of caspase-6 in bile acid-induced apoptosis. The major human hydrophobic bile acid, glycochenodeoxycholic acid (GCDCA, 75 mu mol/liter), rapidly induced caspase-6 cleavage in HepG2-Ntcp human hepatoma cells. GCDCA-induced, but not tumor necrosis factor alpha-or etoposide-induced activation of effector caspases-3 and -7 was significantly reduced by 50% in caspase-6-deficient HepG2-Ntcp cells as well as in primary rat hepatocytes pretreated with a caspase-6 inhibitor. Inhibition of caspase-9 reduced GCDCA-induced activation of caspase-6, whereas inhibition of caspase-6 reduced activation of caspase-8 placing caspase-6 between caspase-9 and caspase-8. GCDCA also induced apoptosis in Fas-deficient Hep3B-Ntcp and HuH7-Ntcp hepatoma cells. In addition, GCDCA-induced apoptosis was reduced by 50% in FADD-deficient HepG2-Ntcp cells, whereas apoptosis induced by tumor necrosis factor alpha was reduced by 90%. Collectively, these observations suggest that GCDCA can induce hepatocyte apoptosis in the absence of death receptor signaling, presumably by a compensatory mitochondrial pathway. In conclusion, caspase-6 appears to play an important regulatory role in the promotion of bile acid-induced apoptosis as part of a feedback loop.